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Sample records for small silicon clusters

  1. Nucleation of Small Silicon Carbide Dust Clusters in AGB Stars

    NASA Astrophysics Data System (ADS)

    Gobrecht, David; Cristallo, Sergio; Piersanti, Luciano; Bromley, Stefan T.

    2017-05-01

    Silicon carbide (SiC) grains are a major dust component in carbon-rich asymptotic giant branch stars. However, the formation pathways of these grains are not fully understood. We calculate ground states and energetically low-lying structures of (SiC) n , n = 1, 16 clusters by means of simulated annealing and Monte Carlo simulations of seed structures and subsequent quantum-mechanical calculations on the density functional level of theory. We derive the infrared (IR) spectra of these clusters and compare the IR signatures to observational and laboratory data. According to energetic considerations, we evaluate the viability of SiC cluster growth at several densities and temperatures, characterizing various locations and evolutionary states in circumstellar envelopes. We discover new, energetically low-lying structures for Si4C4, Si5C5, Si15C15, and Si16C16 and new ground states for Si10C10 and Si15C15. The clusters with carbon-segregated substructures tend to be more stable by 4-9 eV than their bulk-like isomers with alternating Si-C bonds. However, we find ground states with cage geometries resembling buckminsterfullerens (“bucky-like”) for Si12C12 and Si16C16 and low-lying stable cage structures for n ≥ 12. The latter findings thus indicate a regime of cluster sizes that differ from small clusters as well as from large-scale crystals. Thus—and owing to their stability and geometry—the latter clusters may mark a transition from a quantum-confined cluster regime to a crystalline, solid bulk-material. The calculated vibrational IR spectra of the ground-state SiC clusters show significant emission. They include the 10-13 μm wavelength range and the 11.3 μm feature inferred from laboratory measurements and observations, respectively, although the overall intensities are rather low.

  2. Structural and electronic properties of small silicon clusters

    NASA Astrophysics Data System (ADS)

    Baturin, V. S.; Lepeshkin, S. V.; Magnitskaya, M. V.; Matsko, N. L.; Uspenskii, Yu A.

    2014-05-01

    The atomic structure and electronic spectrum of silicon nanoclusters (Si-ncs) Si7, Si10,Si10H16 and Si10H20 are calculated using the evolutionary algorithm with total energy computed within density functional theory and generalized gradient approximation (DFT-GGA). When analysing the low-energy structures, we pay significant attention to their symmetry and interatomic bond geometry. The candidate structures arising in the process of evolutionary algorithm convergence are also considered and classified by their topology and grouping near local energy minima. Possible ways to improve the convergence of evolutionary computation are discussed. Addressing qualitative criteria for the ground-state atomic structure of Si-ncs, we consider correlations between the density of electronic states and the total energetics of clusters in the ground state and low-energy-isomer configurations.

  3. Vanadium-doped small silicon clusters: Photoelectron spectroscopy and density-functional calculations

    NASA Astrophysics Data System (ADS)

    Xu, Hong-Guang; Zhang, Zeng-Guang; Feng, Yuan; Yuan, Jinyun; Zhao, Yuchao; Zheng, Weijun

    2010-03-01

    Vanadium-doped small silicon clusters, VSin- and VSin- ( n = 3-6), have been studied by anion photoelectron spectroscopy. The vertical detachment energies (VDEs) and adiabatic detachment energies (ADEs) of these clusters were obtained from their photoelectron spectra. We have also conducted density-functional calculations of VSin- and VSin- clusters and determined their structures by comparison of theoretical calculations with experimental results. Our results show that two V atoms in VSin- clusters tend to form a strong V-V bond. VSi6- has D3d symmetry with the six Si atoms forming a chair like six-membered ring similar to the ring in cyclohexane and the two vanadium atoms are joined with a δ bond.

  4. Photoelectron imaging of small silicon cluster anions, Sin- (n=2-7)

    NASA Astrophysics Data System (ADS)

    Peppernick, Samuel J.; Gunaratne, K. D. Dasitha; Sayres, Scott G.; Castleman, A. W.

    2010-01-01

    Photoelectron imaging experiments were conducted on small silicon cluster anions, Sin- (n =2-7), acquired at a photon energy of 3.49 eV (355 nm). Electronic transitions arising from the anion ground states are observed, and the evaluated vertical detachment energies agree well with previous measurements and theoretical calculations. The anisotropy β parameters have also been determined for each unique feature appearing in the photoelectron angular distributions at the employed photon energy. Separate calculations using density functional theory are also undertaken to determine the relative atomic orbital contributions constructing the interrogated highest occupied and low-lying molecular orbitals of a specific cluster. A method to interpret the observed cluster angular distributions, term the β-wave approach, is then implemented which provides quantitative predictions of the anisotropy β parameter for partial wave emission from molecular orbitals partitioned by varying contributions of atomic orbital angular momenta. Highlighted in the β-wave analysis is the ability of discriminating between disparate molecular orbitals from two nearly isoenergetic structural isomers of opposing point group symmetry for the Si4- and Si6- cluster ions, respectively.

  5. Determination of ionization energies of small silicon clusters with vacuum?ultraviolet (VUV) radiation

    SciTech Connect

    Kostko, Oleg; Leone, Stephen R.; Duncan, Michael A.; Ahmed, Musahid

    2009-09-23

    In this work we report on single photon vacuum ultraviolet photoionization of small silicon clusters (n=1-7) produced via laser ablation of Si. The adiabatic ionization energies (AIE) are extracted from experimental photoionization efficiency (PIE) curves with the help of Frank?Condon simulations, used to interpret the shape and onset of the PIE curves. The obtained AIEs are (all energies are in eV): Si (8.13+-0.05), Si2 (7.92+-0.05), Si3 (8.12+-0.05), Si4 (8.2+-0.1), Si5 (7.96+-0.07), Si6 (7.8+-0.1), and Si7 (7.8+-0.1). Most of the experimental AIE values are in good agreement with ab initio electronic structure calculations. To explain observed deviations between the experimental and theoretical AIEs for Si4 and Si6, a theoretical search of different isomers of these species is performed. Electronic structure calculations aid in the interpretation of the a2PIu state of Si2+ dimer in the PIE spectrum. Time dependent density functional theory (TD-DFT) calculations are performed to reveal the energies of electronically excited states in the cations for a number of Si clusters.

  6. A density functional study of small sized silver-doped silicon clusters: Ag2Sin (n = 1-13)

    NASA Astrophysics Data System (ADS)

    Yang, Cai; Hao Jia, Song; Ma, Mao Fen; Zhang, Shuai; Lu, Cheng; Li, Gen Quan

    2015-11-01

    The structures and electronic properties for global minimum geometric structures of small-sized neutral Ag2Sin (n = 1-13) clusters have been investigated using the CALYPSO structure searching method coupled with density functional theory calculations. A great deal of low-energy geometric isomers are optimised at the B3LYP / GENECP theory level. The optimised structures suggest that the ground state Ag2Sin clusters are visibly distorted compared with the corresponding pure silicon clusters and favor a three-dimensional configuration. Starting with Ag2Si12, one Ag atom is fully encapsulated by the Si outer cages. Based on the averaged binding energy, fragmentation energy, second-order energy difference and HOMO-LUMO energy gap, it is seen that Ag2Si2 and Ag2Si5 are tested to be the most stable clusters, and the chemical stabilities of pure Sin+2 clusters can be reduced to some extent after doping two Ag atoms. Additionally, natural population and natural electronic configuration are discussed and the results reveal that charges transfer from the Ag atoms to the silicon frames and the spd hybridisations are present in all Ag2Sin clusters. Lastly, the results of natural bonds show that the Ag-Si bond in Ag2Sin clusters is dominated by small ionic character. Supplementary material in the form of one pdf file available from the Journal web page at http://dx.doi.org/10.1140/epjd/e2015-60404-1

  7. Structure, stability, and dissociation of small ionic silicon oxide clusters [SiO{sub n}{sup +}(n = 3, 4)]: Insight from density functional and topological exploration

    SciTech Connect

    Sen, Kaushik; Ghosh, Deepanwita; Pakhira, Srimanta; Banu, Tahamida; Das, Abhijit K.

    2013-12-21

    The structures, energies, isomerization, and decomposition pathways of small ionic silicon oxide clusters, SiO{sub n}{sup +} (n = 3, 4), on doublet and quartet energy surfaces are investigated by density functional theory. New structural isomers of these ionic clusters have been obtained with this systematic study. The energy ordering of the isomeric cluster ions on doublet spin surface is found to follow the same general trend as that of the neutral ones, while it differs on the quartet surface. Our computational results reveal the energetically most preferred decomposition pathways of the ionic clusters on both spin surfaces. To comprehend the reaction mechanism, bonding evolution theory has also been employed using atoms in molecules formalism. The possible reasons behind the structural deformation of some isomers on quartet surface have also been addressed. Our results are expected to provide important insight into the decomposition mechanism and relative stability of the SiO{sub n}{sup +} clusters on both the energy surfaces.

  8. Small clusters: aerosol precursors

    SciTech Connect

    Castleman, A.W. Jr.; Keesee, R.G.

    1983-01-01

    Studies of the structure, stability, electronic properties, and formation kinetics of small clusters provide information useful in furthering an understanding of nucleation processes, the formation and stability of collodial media, and the nature of surfaces. Using mass spectrometry, coupled with various high pressure ion clustering and molecular beam techniques, the details of the primary clustering steps leading to nucleation from the vapor are obtained by direct observation. This paper is devoted to a review of some recent results on these subjects obtained in the authors' laboratory.

  9. A theoretical study of the structures and electronic transitions of small silicon nitride clusters (SinNm, n + m ⩽ 4)

    NASA Astrophysics Data System (ADS)

    Owusu-Ansah, E.; Wang, Y. M.; Shi, Y. J.

    2016-12-01

    A quantum mechanical study of small silicon nitride clusters (SinNm, n + m ⩽ 4) was performed for both the ground and excited electronic states. The diatomic SiN as well as the tri-atomic SiN2 and Si2N clusters were chosen as our benchmark systems for an extensive investigation of the various methods, functionals and basis sets. Among the methods investigated for the ground-state geometry optimization and vibrational frequency calculations, density functional theory (DFT) with the B3LYP functional was found to be the best performed method overall. For the basis set, 6-311G(d) is chosen for its nice balance between accuracy and efficiency in time. At the B3LYP/6-311G(d) level of theory, the most stable isomers of SiN2, Si2N, SiN3, Si3N and Si2N2, were determined to be the triplet (3Σ-) asymmetric linear isomer (Sisbnd Ndbnd N), the doublet (2Πg) symmetric linear isomer (Sidbnd Ndbnd Si), the doublet (2A‧) nonlinear isomer (Sidbnd Nsbnd Ndbnd N), the doublet (2B1) Y-type isomer, and the singlet (1Σg+) symmetric linear isomer (Sidbnd Ndbnd Ndbnd Si), respectively. Analysis of the relative energies of various stable isomers shows that Nsbnd N bonding is the most favored one and Sisbnd Si bonding is least favored. Due to its good performance in predicting the electronic transitions from the X2Σ+ ground state of SiN to three low-lying excited states A2 Π , B2Σ+ , and D2 Π , TDDFT/B3LYP/6-311G(d) was chosen over EOMCCSD/6-311G(d) for the excited state calculations. The first ten electronic transitions for the most stable isomers of all SinNm (n + m ⩽ 4) clusters were calculated.

  10. Size-selecting effect of water on fluorescent silicon clusters.

    PubMed

    Torricelli, G; Akraiam, A; von Haeften, K

    2011-08-05

    Silicon clusters were produced by gas aggregation in vacuum and co-deposited with water vapour onto a cold target where the water vapour froze. Melting of the ice yielded fluorescent silicon nanoparticles suspended in water which were investigated by photoluminescence spectroscopy (PL) and atomic force microscopy (AFM). The PL spectrum showed a prominent band at 420 nm and other, less intense bands at shorter wavelengths. No fluorescence was observed below 275 nm. The shortest wavelength observed was related to a silicon cluster diameter of 0.9 nm using a simple particle-in-a-box model. Drops of the suspension were also deposited on freshly cleaved HOPG and investigated by AFM. The images showed single and agglomerated clusters with heights of typically 0.6 up to 2 nm. The sizes displayed by our measurements are not correlated to the average sizes that result from gas aggregation, indicating a size-selecting effect of the water suspension. The cluster-cluster interaction in water is governed by repulsion due to thermal energy and attraction due to van der Waals forces. For very small clusters repulsion dominates; at 3 nm diameter the two forces are balanced. We identify this stable phase of small clusters as the origin of exceptionally stable fluorescence.

  11. Small copper-doped silicon clusters CuSin (n = 4-10) and their anions: structures, thermochemistry, and electron affinities.

    PubMed

    Lin, Lin; Yang, Jucai

    2015-06-01

    The structures and energies of copper-doped small silicon clusters CuSi n (n = 4-10) and their anions were investigated systematically using CCSD(T)/aug-cc-pVTZ-DK//MP2/6-31G(2df,p), G4//MP2/6-31G(2df,p), and the B3LYP/6-311+G* basis set. The performance of the methods used for the prediction of energetic and thermodynamic properties was evaluated. Comparing experimental [Xu et al. (2012) J Chem Phys 136:104308] and theoretical calculations, it was concluded that the CCSD(T) results are very accurate and exhibit the best performance; the mean absolute deviation from experimental data was 0.043 eV. The excellent agreement of vertical detachment energy (VDE) between experimental results and CCSD(T) calculations indicates that the ground state structures of CuSi n (-) (n = 4-10) presented in this paper are reliable. For CuSi10, assigning 2.90±0.08 eV to the experimental adiabatic electron affinity (AEA) and 3.90±0.08 eV to the VDE is more reasonable than to 3.46±0.08 eV and 3.62±0.08 eV, respectively, based on the CCSD(T) calculations and the previous photoelectron spectrum of CuSi10 (-) (Xu et al., op. cit.). The AEAs of CuSi n (n = 4-10), excluding CuSi7, are in excellent agreement with experimental data, showing that the ground state structures of CuSi n (n = 4-6, 8-10) reported in this paper are reliable. CuSi10 is suggested to be the smallest endohedral ground state structure. However, adding an additional electron to CuSi10 pulls out the Cu atom from the center location, forming an exohedral ground state structure of CuSi10 (-). The charge transfer and dissociation energy of Cu from CuSi n and their anions determined to examine the nature of bonding and their relative stabilities.

  12. Multiple Aromaticity and Antiaromaticity in Silicon Clusters

    SciTech Connect

    Zhai, Hua JIN.; Kuznetsov, A E.; Boldyrev, Alexander I.; Wang, Lai S.

    2004-12-17

    A series of silicon clusters containing four atoms but with different charge states (Si{sub 4}{sup 2+}, Si{sub 4}, Si{sub 4}{sup 2-}, and NaSi{sub 4}{sup -}) were studied by photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series were interpreted in terms of aromaticity and antiaromaticity, which allowed the prediction of how structures of the four-atom silicon clusters change upon addition or removal of two electrons. It is shown that Si{sub 4}{sup 2+} is square-planar, analogous to the recently discovered aromatic Al{sub 4}{sup 2-} cluster. Upon addition of two electrons, neutral Si{sub 4} becomes {sigma}-antiaromatic and exhibits a rhombus distortion. Adding two more electrons to Si{sub 4} leads to two energetically close structures of Si{sub 4}{sup 2-}: either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of doubly charged Si{sub 4}{sup 2-}, a stabilizing cation (Na{sup +}) was used to produce Si{sub 4}{sup 2-} in the gas phase in the form of Na{sup +}[Si{sub 4}{sup 2-}], which was characterized experimentally by photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na{sup +}[Si{sub 4}{sup 2-}] species is highly unusual.

  13. Multiple Aromaticity and Antiaromaticity in Silicon Clusters

    SciTech Connect

    Zhai, Hua JIN.; Kuznetsov, A E.; Boldyrev, Alexander I.; Wang, Lai S.

    2004-12-10

    A series of silicon clusters four atoms, but with different charge states (Si42+, Si4, Si42-, and NaSi4-), are studied using photoelectron spectroscopy and ab initio calculations. Structure evolution and chemical bonding in this series are interpreted in terms of aromaticity and antiaromaticity, allowing prediction of how structures of how structures of the four-atom silicon cluster change upon addition or reduction of two electrons. It is shown that Si42+ is square planar, analogous to the recently discovered aromatic A142- cluster. Upon addition of two electrons, the neutral Si4 becomes ?-antiaromatic, resulting in a rhombus distortion. Adding two more electrons to Si4 leads to two energetically close structures of Si42-: either a double antiaromatic parallelogram structure or an aromatic system with a butterfly distortion. Because of the electronic instability of the doubly charged Si42-, a stabilizing cation Na+ was used to produce Si42- in the gas phase in the form of Na+ [Si42-], which was characterized experimentally using photoelectron spectroscopy. Multiple antiaromaticity in the parallelogram Na+ [Si42] species is highly unusual in chemistry.

  14. Vibrational Spectra and Structures of Silicon - Clusters

    NASA Astrophysics Data System (ADS)

    Presilla-Marquez, Jose-Domingo

    1992-01-01

    Fourier transform studies of silicon-carbon clusters produced by vaporizing mixtures of silicon and carbon-12 or carbon-13 and quenching the products in argon, have enabled the identification of new vibrational spectra and structures. The far infrared nu_3(b_2) fundamental has been found at 160.4 cm ^{-1} for SiC_2 . Its structure is now described as T-shaped with ionic bonding. In the case of Si_2C, a previously observed vibration at 1188.4 cm^ {-1} has been confirmed as the nu_3(b_2), antisymmetric Si -C stretching mode, and a new vibration at 839.5 cm ^{-1} has been identified as the nu_1(a_1), symmetric Si-Si stretching fundamental. The nu_2(a _1) bending mode has not been observed; however, the nu_2(a_1) + nu_3(b_2) combination band has been identified at 1354.8 cm^{-1} . The results of force constant adjustment calculations confirm the ground state geometry of Si_2 C as a floppy, bent symmetrical structure. Five out of six fundamental modes have been identified for the Si_3C cluster which is observed here for the first time: the symmetric breathing vibration, nu_1(a_1) = 658.2 cm ^{-1}; the Si_ beta-Si_alpha -Si_beta symmetric deformation vibration, nu_2(a_1) = 511.8 cm^{-1}; the Si beta-C-Si_{beta } symmetric deformation vibration, nu_3(a_1) = 309.5 cm ^{-1}; the Si_beta -C antisymmetric stretching vibration, nu_5(b_2) = 1101.4 cm ^{-1}; and the Si_ alpha-Si_beta antisymmetric stretching vibration, nu_6(b _2) = 357.6 cm^{-1} . The fundamental frequencies, relative intensities, and isotopic shifts are in excellent agreement with the results of ab initio calculations predicting a rhomboidal structure of C_{2v} symmetry for Si_3C, with carbon-silicon transannular bonding between the two equivalent Si atoms. Another new silicon-carbon cluster, rhombic Si _2C_2, has been identified and the nu_3(b_ {rm 1u}) = 982.9 and nu _4(b_{rm 2u}) = 382.2 cm^{-1} fundamentals have been assigned. These results are in excellent agreement with ab initio calculations. The first

  15. Precise Fabrication of Silicon Wafers Using Gas Cluster Ion Beams

    SciTech Connect

    Isogai, Hiromichi; Toyoda, Eiji; Izunome, Koji; Kashima, Kazuhiko; Mashita, Takafumi; Toyoda, Noriaki; Yamada, Isao

    2009-03-10

    Precise surface processing of a silicon wafer was studied by using a gas cluster ion beam (GCIB). The damage caused to the silicon surface was strongly dependent on irradiation parameters. The extent of damage varied with the species of source gas and the acceleration voltage (Va) of cluster ions. It also varied with the cluster size and residual gas pressure. The influence of electron acceleration voltage (Ve) used for ionization of a neutral cluster was also investigated. The irradiation damage, such as an amorphous silicon (a-Si) layer, a mixed layer of a-Si and c-Si (transition layer), and surface roughness, was increased with Ve. It is suggested that the increase in the amount of energy per atom was induced by high Ve, because of variation of the cluster size and/or cluster charge. An undamaged smooth surface can be produced by Ar-GCIB irradiation at low Ve and Va.

  16. Determination of the Structures of Silicon and Metal Doped Silicon Clusters

    NASA Astrophysics Data System (ADS)

    Lyon, Jonathan T.; Fielicke, Andre; Janssens, Ewald; Lievens, Peter

    2014-06-01

    Strongly bound clusters are often used as convenient models for bulk material. Silicon clusters are particularly interesting due to their importance in the electronics industry. We perform experimental IR multiple photon dissociation spectroscopy in the gas-phase, which makes use of a free electron laser, and compare the results with that predicted by density functional and MP2 theory calculations. Comparison of the vibrational spectra with that predicted by theoretical calculations for several structural isomers for each cluster size leads to accurate structural assignments. Here, we present our results for silicon clusters, and compare the structures with those of select transition metal doped SinM clusters. Of particular interest is the transition from exohedral to endoheral metal doped silicon clusters and how the transition size changes for different metal dopant atoms. Journal of Chemical Physics 2012, 136, 064301 e.g., ChemPhysChem 2014, 15, 328.

  17. Structures of small bismuth cluster cations

    NASA Astrophysics Data System (ADS)

    Kelting, Rebecca; Baldes, Alexander; Schwarz, Ulrike; Rapps, Thomas; Schooss, Detlef; Weis, Patrick; Neiss, Christian; Weigend, Florian; Kappes, Manfred M.

    2012-04-01

    The structures of bismuth cluster cations in the range between 4 and 14 atoms have been assigned by a combination of gas phase ion mobility and trapped ion electron diffraction measurements together with density functional theory calculations. We find that above 8 atoms the clusters adopt prolate structures with coordination numbers between 3 and 4 and highly directional bonds. These open structures are more like those seen for clusters of semiconducting-in-bulk elements (such as silicon) rather than resembling the compact structures typical for clusters of metallic-in-bulk elements. An accurate description of bismuth clusters at the level of density functional theory, in particular of fragmentation pathways and dissociation energetics, requires taking spin-orbit coupling into account. For n = 11 we infer that low energy isomers can have fragmentation thresholds comparable to their structural interconversion barriers. This gives rise to experimental isomer distributions which are dependent on formation and annealing histories.

  18. Efficient calculation of optical linear response of large silicon clusters.

    NASA Astrophysics Data System (ADS)

    Chang, Gefei; Chang, Yia-Chung

    2005-03-01

    Nanoscale silicon clusters have potential applications as light-emitting devices and bio-sensors. Ab initio calculations of the optical linear response of small-size nanoparticles have been performed via time-dependent density functional theory (TDDFT)^1 and by solving many-body Bethe-Salpeter equations (MBSE)^2,3. We show that the ab initio calculations can be made much more efficient when the nanocluster possess high point group symmetry and symmetrized basis functions are used. This allows us to extend the ab initio calculation to much larger Si clusters (up to a few hundred Si atoms) on a personal computer. The optical linear response of Si nanocluster (passivated with hydrogen) as a function of cluster size is examined. The effect of phosphorus doping of Si nanocluster on its optical properties is also studied.1. Ogũt,S., J. R. Chelikowsky, and S. G. Louie, PRL 80, 3162(1998); Marques, M., A. Castro, and A. Rubio, J. Chem. Phys. 115, 3006(2001). 2. Rohlfing, M., and S. G. Louie, PRL 80, 3320(1998);PRB 62, 4927(2000). 3. Grossman, J. C., M. Rohlfing, L. Mitas, S. G. Louie, and M. L. Cohen,PRL 86, 472(2001).

  19. On the Clustering of Europa's Small Craters

    NASA Technical Reports Server (NTRS)

    Bierhaus, E. B.; Chapman, C. R.; Merline, W. J.

    2001-01-01

    We analyze the spatial distribution of Europa's small craters and find that many are too tightly clustered to result from random, primary impacts. Additional information is contained in the original extended abstract.

  20. Small-Diameter Silicon Nanowire Surfaces

    NASA Astrophysics Data System (ADS)

    Ma, D. D. D.; Lee, C. S.; Au, F. C. K.; Tong, S. Y.; Lee, S. T.

    2003-03-01

    Small-diameter (1 to 7 nanometers) silicon nanowires (SiNWs) were prepared, and their surfaces were removed of oxide and terminated with hydrogen by a hydrofluoric acid dip. Scanning tunneling microscopy (STM) of these SiNWs, performed both in air and in ultrahigh vacuum, revealed atomically resolved images that can be interpreted as hydrogen-terminated Si (111)-(1 × 1) and Si (001)-(1 × 1) surfaces corresponding to SiH3 on Si (111) and SiH2 on Si (001), respectively. These hydrogen-terminated SiNW surfaces seem to be more oxidation-resistant than regular silicon wafer surfaces, because atomically resolved STM images of SiNWs were obtained in air after several days' exposure to the ambient environment. Scanning tunneling spectroscopy measurements were performed on the oxide-removed SiNWs and were used to evaluate the electronic energy gaps. The energy gaps were found to increase with decreasing SiNW diameter from 1.1 electron volts for 7 nanometers to 3.5 electron volts for 1.3 nanometers, in agreement with previous theoretical predictions.

  1. Optical (Hyper) Polarizabilities of Small Silicon Clusters

    DTIC Science & Technology

    1990-11-01

    and R. E. Sinal- ley, Chem. Phys. Lett. 138. 119 (19S7): A. Kasuva and Y. -Nishina. Z. Phys. D 12, 493 (1989). 5. Y. Wang , N. Herron. WV. Mahler and A...Rev. B 37, 6991 (1988); R. Biswas. C. Z. Wang , C. T. Chan, K. NI. Ho and C. NI. Souk- oulis. Phys. Rev. Lett. 63, 1491 (1989); A. D. Nlistriotis. N...Irvine, CA 92717 Professor Frank DiSalvo Professor Roald Hoffmann Department of Chemistry Department of Chemistry Cornell University Cornell University

  2. Field Evaporation of Grounded Arsenic Doped Silicon Clusters

    NASA Astrophysics Data System (ADS)

    Deng, Zexiang; She, Juncong; Li, Zhibing; Wang, Weiliang; Chen, Qiang

    2015-08-01

    We have investigated the field evaporation of grounded arsenic (As) doped silicon (Si) clusters composed of 52 atoms with density functional theory (DFT) to mimic Si nano structures of hundreds of nanometers long standing on a substrate. Six cluster structures with different As doping concentrations and dopant locations are studied. The critical evaporation electric fields are found to be lower for clusters with higher doping concentrations and doping sites closer to the surface. We attribute the difference to the difference in binding energies corresponding to the different As-doping concentrations and to the doping locations. Our theoretical study could shed light on the stability of nano apexes under high electric field.

  3. Magnetic impurities in small metal clusters

    NASA Astrophysics Data System (ADS)

    Pastor, G. M.

    2005-09-01

    [Dedicated to Bernhard Mühlschlegel on the occasion of his 80th birthday]Magnetic impurities in small metallic clusters are investigated in the framework of the Anderson model by using exact diagonalization and geometry optimization methods.The singlet-triplet spin gap E shows a remarkable dependence as a function of band-filling, cluster structure, and impurity position that can be interpreted in terms of the environment-specific conduction-electron spectrum. The low-energy spin excitations involve similar energies as isomerizations. Interesting correlations between cluster structure and magnetic behavior are revealed. Finite-temperature properties such as specific heat, effective impurity moment, and magnetic susceptibility are calculated exactly in the canonical ensemble. A finite-size equivalent of the Kondo effect is identified and its structural dependence is discussed.

  4. Stability and migration of small copper clusters in amorphous dielectrics

    SciTech Connect

    Guzman, David M.; Onofrio, Nicolas; Strachan, Alejandro

    2015-05-21

    We use density functional theory (DFT) to study the thermodynamic stability and migration of copper ions and small clusters embedded in amorphous silicon dioxide. We perform the calculations over an ensemble of statistically independent structures to quantify the role of the intrinsic atomic-level variability in the amorphous matrix affect the properties. The predicted formation energy of a Cu ion in the silica matrix is 2.7 ± 2.4 eV, significantly lower the value for crystalline SiO{sub 2}. Interestingly, we find that Cu clusters of any size are energetically favorable as compared to isolated ions; showing that the formation of metallic clusters does not require overcoming a nucleation barrier as is often assumed. We also find a broad distribution of activation energies for Cu migration, from 0.4 to 1.1 eV. This study provides insights into the stability of nanoscale metallic clusters in silica of interest in electrochemical metallization cell memories and optoelectronics.

  5. Stability and migration of small copper clusters in amorphous dielectrics

    NASA Astrophysics Data System (ADS)

    Guzman, David M.; Onofrio, Nicolas; Strachan, Alejandro

    2015-05-01

    We use density functional theory (DFT) to study the thermodynamic stability and migration of copper ions and small clusters embedded in amorphous silicon dioxide. We perform the calculations over an ensemble of statistically independent structures to quantify the role of the intrinsic atomic-level variability in the amorphous matrix affect the properties. The predicted formation energy of a Cu ion in the silica matrix is 2.7 ± 2.4 eV, significantly lower the value for crystalline SiO2. Interestingly, we find that Cu clusters of any size are energetically favorable as compared to isolated ions; showing that the formation of metallic clusters does not require overcoming a nucleation barrier as is often assumed. We also find a broad distribution of activation energies for Cu migration, from 0.4 to 1.1 eV. This study provides insights into the stability of nanoscale metallic clusters in silica of interest in electrochemical metallization cell memories and optoelectronics.

  6. The structure of small metal clusters

    NASA Technical Reports Server (NTRS)

    Bauschlicher, C. W., Jr.; Pettersson, L. G. M.

    1986-01-01

    One metal atom surrounded by its 12 nearest neighbors is considered for both D(3d) (face-centered cubic-like) and D(3h) (hexagonal close-packed-like) geometries. For Al and Be, the neutral cluster and the positive and negative ions are considered for idealized (all bonds equal) and distorted geometries. The D(3d) geometry is found to be the lowest for Be13, while the D(3h) geometry is lower for Al13. This is the reverse of what is expected based upon the bulk metal structures, Be(hcp) and Al(fcc). Al13 is found to have only small distortions, while Be13 shows large distortions for both the D(3d) and D(3h) geometries. The ions have geometries which are similar to those found for the neutral systems. Both all-electron and effective core potential calculations were carried out on the X13 clusters; the agreement is very good.

  7. Generation of argon-ion mixed silicon plasmas forming argon encapsulated silicon clusters

    NASA Astrophysics Data System (ADS)

    Kaneko, T.; Takaya, H.; Hatakeyama, R.

    2006-12-01

    An inductively coupled argon (Ar) plasma is superimposed on a silicon (Si) plasma generated by an electron beam gun in order to realize the formation of gas-atom encapsulated Si cage clusters. The Si clusters, which are formed and deposited on a substrate, are analyzed by laser-desorption time-of-flight mass spectrometry and are found to have the mass spectra of not only pure Si cluster (Sin; n =1-17) but also Si cluster doped with Ar atom (ArSin; n =10-20) in the case that the large amount of Ar ions is generated in addition to the Si plasma. Together with the analysis of x-ray photoelectron spectroscopy, it is revealed that the Ar atom is included in the Si cluster, forming the structure of endohedral Ar@Sin complexes. Furthermore, the mass spectrum of Ar@Sin indicates the existence of the magic numbered cluster size n =15, 16 similar to the metal encapsulated Si clusters.

  8. Recent advances in small molecule OLED-on-silicon microdisplays

    NASA Astrophysics Data System (ADS)

    Ghosh, Amalkumar P.; Ali, Tariq A.; Khayrullin, Ilyas; Vazan, Fridrich; Prache, Olivier F.; Wacyk, Ihor

    2009-08-01

    High resolution OLED-on-silicon microdisplay technology is unique and challenging since it requires very small subpixel dimensions (~ 2-5 microns). eMagin's OLED microdisplay is based on white top emitter architecture using small molecule organic materials. The devices are fabricated using high Tg materials. The devices are hermetically sealed with vacuum deposited thin film layers. LCD-type color filters are patterned using photolithography methods to generate primary R, G, B colors. Results of recent improvements in the OLED-on-silicon microdisplay technology, with emphasis on efficiencies, lifetimes, grey scale and CIE color coordinates for SVGA and SXGA resolution microdisplays is presented.

  9. The role of dimers in evaporation of small argon clusters.

    PubMed

    Napari, Ismo; Vehkamaki, Hanna

    2004-07-08

    Evaporation of small Lennard-Jones argon clusters has been studied using molecular dynamic simulations. An extensive library of clusters with 4, 5, 6, 11, and 21 atoms has been obtained from an earlier study. Analysis of the evaporation properties of the clusters indicate, that the fraction of dimer evaporations of all evaporation events increases with the total energy of the cluster. The fraction of evaporated dimers from clusters with a constant lifetime is independent of the cluster size for short-lived clusters and increases with cluster size for long-lived clusters. Only a few percent of the clusters which are long lived enough to participate in vapor-liquid nucleation decay by emitting dimers. The mean cluster lifetime as a function of total energy shows the same exponentially decreasing trend for monomer and dimer evaporation channels. The fraction of trimer evaporations is found to be vanishingly small.

  10. Dimensional phase transition in small Yukawa clusters

    SciTech Connect

    Sheridan, T. E.; Wells, K. D.

    2010-01-15

    We investigate the one- to two-dimensional zigzag transition in clusters consisting of a small number of particles interacting through a Yukawa (Debye) potential and confined in a two-dimensional biharmonic potential well. Dusty (complex) plasma clusters with n<=19 monodisperse particles are characterized experimentally for two different confining wells. The well anisotropy is accurately measured, and the Debye shielding parameter is determined from the longitudinal breathing frequency. Debye shielding is shown to be important. A model for this system is used to predict equilibrium particle configurations. The experiment and model exhibit excellent agreement. The critical value of n for the zigzag transition is found to be less than that predicted for an unshielded Coulomb interaction. The zigzag transition is shown to behave as a continuous phase transition from a one-dimensional to a two-dimensional state, where the state variables are the number of particles, the well anisotropy and the Debye shielding parameter. A universal critical exponent for the zigzag transition is identified for transitions caused by varying the Debye shielding parameter.

  11. Annealing studies of cluster defects in ion-implanted silicon using high resolution DLTS

    NASA Astrophysics Data System (ADS)

    Gad, M. A.; Evans-Freeman, J. H.

    2006-12-01

    High resolution Laplace deep level transient spectroscopy (LDLTS) has been applied to investigate the annealing behaviour of small cluster defects in n-type Si. The Si was implanted with either Ge or Si, with energies 1500 keV and 850 keV respectively, and doses of 1 × 10 10 cm -2. The low dose ensured that there was a minimum of carrier removal due to deep defect states after implantation. Defect states in the as-implanted samples were attributed to VO pairs, divacancies and very small interstitial cluster defects, after detailed depth profiling. LDLTS of Ge + and Si + implanted silicon shows that there are three closely spaced deep levels associated with these clusters, with energies in the region of Ec-400 meV. Samples were then isochronally annealed in very small temperature intervals up to 560 K, in situ in our high temperature measurement cryostat, and the LDLTS re-examined as a function of annealing temperature. A new deeper energy level emerges as the cluster-related signal reduces, and it is suggested that this new trap is a major recombination centre, by comparison with current-voltage data.

  12. Studies of silicon cluster--metal atom compound formation in a supersonic molecular beam

    SciTech Connect

    Beck, S.M.

    1987-10-01

    The first observation of a reaction between a metal atom and silicon in a supersonic jet to form metal atom silicon clusters is reported. Using the technique of laser vaporization supersonic expansion with metal carbonyl seeded carrier gas, clusters of the form MSi/sub n/ have been detected by ArF and KrF laser photoionization time-of-flight mass spectrometry. Three transition metals have been investigated, Cr, Mo, and W. The dominant product cluster peaks observed in the mass spectra obtained for all three metals corresponds to identical but remarkable cluster stoichiometries. The dominant peaks have formulas given by MSi/sub n/ where n = 15 and n = 16. The metal--semiconductor clusters are relatively more stable towards photofragmentation than the bare silicon cluster of the same size. The observation of these new species may be relevant to reactions which occur at the interface between a silicon wafer and deposited metals.

  13. Ferromagnetism in Silicon Single Crystals with Positively Charged Vacancy Clusters

    NASA Astrophysics Data System (ADS)

    Liu, Yu; Zhang, Xinghong; Yuan, Quan; Han, Jiecai; Zhou, Shengqiang; Song, Bo

    Defect-induced ferromagnetism provides an alternative for organic and semiconductor spintronics. Here, we investigated the magnetism in Silicon after neutron irradiation and try to correlate the observed magnetism to particular defects in Si. Commercially available p-type Si single crystal wafer is cut into pieces for performing neutron irradiations. The magnetic impurities are ruled out as they can not be detected by secondary ion mass spectroscopy. With positron annihilation lifetime spectroscopy, the positron trapping center corresponding to lifetime 375 ps is assigned to a kind of stable vacancy clusters of hexagonal rings (V6) and its concentration is enhanced by increasing neutron doses. After irradiation, the samples still show strong diamagnetism. The weak ferromagnetic signal in Si after irradiation enhances and then weakens with increasing irradiation doses. The saturation magnetization at room temperature is almost the same as that at 5 K. The X-ray magnetic circular dichroism further provides the direct evidence that Silicon is the origin of this ferromagnetism. Using first-principles calculations, it is found that positively charged V6 brings the spin polarization and the defects have coupling with each other. The work is financially supported by the Helmholtz Postdoc Programme (Initiative and Networking Fund, PD-146).

  14. Performance of silicon pixel detectors at small track incidence angles for the ATLAS Inner Tracker upgrade

    NASA Astrophysics Data System (ADS)

    Viel, Simon; Banerjee, Swagato; Brandt, Gerhard; Carney, Rebecca; Garcia-Sciveres, Maurice; Hard, Andrew Straiton; Kaplan, Laser Seymour; Kashif, Lashkar; Pranko, Aliaksandr; Rieger, Julia; Wolf, Julian; Wu, Sau Lan; Yang, Hongtao

    2016-09-01

    In order to enable the ATLAS experiment to successfully track charged particles produced in high-energy collisions at the High-Luminosity Large Hadron Collider, the current ATLAS Inner Detector will be replaced by the Inner Tracker (ITk), entirely composed of silicon pixel and strip detectors. An extension of the tracking coverage of the ITk to very forward pseudorapidity values is proposed, using pixel modules placed in a long cylindrical layer around the beam pipe. The measurement of long pixel clusters, detected when charged particles cross the silicon sensor at small incidence angles, has potential to significantly improve the tracking efficiency, fake track rejection, and resolution of the ITk in the very forward region. The performance of state-of-the-art pixel modules at small track incidence angles is studied using test beam data collected at SLAC and CERN.

  15. Transportation Cluster Volume 3 [Small Power Sources].

    ERIC Educational Resources Information Center

    Pennsylvania State Dept. of Justice, Harrisburg. Bureau of Correction.

    The document is one of seven volumes of instructional materials developed around a cluster of Transportation Industries. Primarily technical in focus, they are designed to be used in a cluster-concept program and to integrate with a regular General Education Development (G.E.D.) program so that students may attain an employable skill level and a…

  16. Statistical fragmentation of small neutral carbon clusters

    SciTech Connect

    Diaz-Tendero, S.; Alcami, M.; Martin, F.; Hervieux, P.-A.

    2005-03-01

    We present a statistical fragmentation study of the C{sub 5}, C{sub 7}, and C{sub 9} carbon clusters using the Metropolis Monte Carlo and Weisskopf methods. We show that inclusion of several isomeric forms as well as rotational effects is essential to reproduce the experimental observations. We have found that, for cluster excitation energies around 10 eV, several fragmentation channels are efficiently populated, but the dominant one always corresponds to C{sub n-3}/C{sub 3}. For high enough excitation energies, we observe first-order phase transitions corresponding to a complete breakup of the cluster.

  17. Optical properties of silicon clusters deposited on the basal plane of graphite

    NASA Astrophysics Data System (ADS)

    Dinh, L. N.; Chase, L. L.; Balooch, M.; Terminello, L. J.; Tench, R. J.; Wooten, F.

    1994-04-01

    Laser ablation was used to deposit of silicon on highly oriented pyrolytic graphite surfaces in an ultra high-vacuum environment equipped with Auger electron spectroscopy (AES), scanning tunneling microscopy (STM) and luminescence spectroscopy. For deposition of up to several monolayers, post annealing produced silicon clusters, whose size distribution was determined vs annealing time and temperature using STM. Pure silicon clusters ranging from 1 to 10 nm showed no detectable photoluminescence in visible range. Exposure to oxygen at 10(exp -6) Torr and for up to 8 hours showed adsorption on the surface of the clusters without silicon oxide formation and no detectable luminescence. Hydrogen termination of these clusters was accomplished by exposing them to atomic hydrogen beam but did not result in any photoluminescence. Prolonged exposure of these clusters to ambient air, however, resulted in strong photoluminescence spectra with color ranging from red to greenish-blue depending on average cluster size. Auger electron spectra revealed the existence of partially oxidized silicon clusters. This luminescence could be due to either an oxide phase or to changes in electronic structure of the clusters as a result of quantum confinement effect.

  18. Reaction of anionic and cationic silicon clusters with tungsten hexafluoride studied by fourier transform ion cyclotron resonance mass spectrometry

    NASA Astrophysics Data System (ADS)

    Reents, W. D., Jr.; Mandich, M. L.; Bondybey, V. E.

    1986-10-01

    Anionic and cationic silicon clusters react exothermically with WF 6 to give a variety of tungsten and silicon fluorides. All the silicon anionic clusters transfer their charge to WF 6. The electron affinities of Si 2-6 are estimated to be less than the electron affinity of WF 6 (350 kJ/mole (3.6 eV)).

  19. Structural evolution of small ruthenium cluster anions

    SciTech Connect

    Waldt, Eugen; Hehn, Anna-Sophia; Ahlrichs, Reinhart; Kappes, Manfred M.; Schooss, Detlef

    2015-01-14

    The structures of ruthenium cluster anions have been investigated using a combination of trapped ion electron diffraction and density functional theory computations in the size range from eight to twenty atoms. In this size range, three different structural motifs are found: Ru{sub 8}{sup −}–Ru{sub 12}{sup −} have simple cubic structures, Ru{sub 13}{sup −}–Ru{sub 16}{sup −} form double layered hexagonal structures, and larger clusters form close packed motifs. For Ru{sub 17}{sup −}, we find hexagonal close packed stacking, whereas octahedral structures occur for Ru{sub 18}{sup −}–Ru{sub 20}{sup −}. Our calculations also predict simple cubic structures for the smaller clusters Ru{sub 4}{sup −}–Ru{sub 7}{sup −}, which were not accessible to electron diffraction measurements.

  20. Multiplicity and Small Clusters: Theory/Models

    NASA Astrophysics Data System (ADS)

    Parker, Richard

    2017-06-01

    In this talk I will review the recent theory and simulations of binary star and low-mass cluster formation. I will discuss the various formation mechanisms proposed for binary and multiple star formation, and compare recent simulations to the observed properties of binaries. I will describe efforts to distinguish high-order multiples from low-mass star clusters and how their subsequent evolution may sculpt the binary properties observed in the Galactic field. Finally, I will discuss how the missing physics from current simulations may affect comparisons with observations, and highlight future prospects in this area.

  1. Vacuum ultraviolet (VUV) photoionization of small water clusters.

    PubMed

    Belau, Leonid; Wilson, Kevin R; Leone, Stephen R; Ahmed, Musahid

    2007-10-11

    Tunable vacuum ultraviolet (VUV) photoionization studies of water clusters are performed using 10-14 eV synchrotron radiation and analyzed by reflectron time-of-flight (TOF) mass spectrometry. Photoionization efficiency (PIE) curves for protonated water clusters (H2O)(n)H+ are measured with 50 meV energy resolution. The appearance energies of a series of protonated water clusters are determined from the photoionization threshold for clusters composed of up to 79 molecules. These appearance energies represent an upper limit of the adiabatic ionization energy of the corresponding parent neutral water cluster in the supersonic molecular beam. The experimental results show a sharp drop in the appearance energy for the small neutral water clusters (from 12.62 +/- 0.05 to 10.94 +/- 0.06 eV, for H2O and (H2O)4, respectively), followed by a gradual decrease for clusters up to (H2O)23 converging to a value of 10.6 eV (+/-0.2 eV). The dissociation energy to remove a water molecule from the corresponding neutral water cluster is derived through thermodynamic cycles utilizing the dissociation energies of protonated water clusters reported previously in the literature. The experimental results show a gradual decrease of the dissociation energy for removal of one water molecule for small neutral water clusters (3 cluster.

  2. Characterizing the Small Scale Structure in Clusters of Galaxies

    NASA Technical Reports Server (NTRS)

    Forman, William R.

    1999-01-01

    We have applied a wavelet transform analysis to quantify the small-scale (less than a few arcminutes) X-ray structure of clusters of galaxies to several clusters. Three of these clusters show interesting structure. In addition to the small scale structure in surface brightness, we have searched for temperature structure using Advanced Satellite for Cosmology and Astrophysics (ASCA) observations. For A754, we find a filamentary structure, similar to that previously detected in A85. For A119, we find small scale structure associated with both cool and hot regions with a hot filament extending to the northeast. For A1750, one of the first clusters to show substructure from Einstein, we have a rederived the substructure from the ROSAT observations. The ASCA observations show a hot region between the two small scale features - sub-clusters. The hot region shows that the two subclusters are merging and producing a shock wave at the leading edge of the interaction. This analysis had been applied to a sample of 17 clusters to determine the radial distribution of substructure in a sample. The number of detected substructures was rather small and it was decided to select a different cluster sample at larger redshift. This provided a larger radial range over which substructure could be detected. This new cluster sample also includes about 20 clusters and is being analyzed. Finally, we are considering how to compare the substructure parameters of nearby clusters with a more distant sample to see if one can extract cosmological information. Since substructure frequency should depend on the cosmological density parameter, it may be possible to derive new constraints by comparing nearby and distant clusters.

  3. Structural properties of small rhodium clusters

    SciTech Connect

    Soon, Yee Yeen; Yoon, Tiem Leong; Lim, Thong Leng

    2015-04-24

    We report a systematic study of the structural properties of rhodium clusters at the atomistic level. A novel global-minimum search algorithm, known as parallel tempering multicanonical basin hopping plus genetic algorithm (PTMBHGA), is used to obtain the geometrical structures with lowest minima at the semi-empirical level where Gupta potential is used to describe the atomic interaction among the rhodium atoms. These structures are then re-optimized at the density functional theory (DFT) level with exchange-correlation energy approximated by Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA). The structures are optimized for different spin multiplicities. The ones with lowest energies will be taken as ground-state structures. In most cases, we observe only minor changes in the geometry and bond length of the clusters as a result of DFT-level re-optimization. Only in some limited cases, the initial geometries obtained from the PTMBHGA are modified by the re-optimization. The variation of structural properties, such as ground-state geometry, symmetry and binding energy, with respect to the cluster size is studied and agreed well with other results available in the literature.

  4. Geometric and Topological Transitions of Small Clusters of Liquid Particles

    NASA Astrophysics Data System (ADS)

    Giammona, James; Campas, Otger

    The geometry and topology of small particle clusters has been studied in several disciplines due to the fundamental nature of the problem and its relevance to applications. Recent theoretical work can predict observed packings for small numbers of hard, spherical particles, but little is known about how using deformable particles changes the geometry and topology of these clusters. To study this problem, we simulate small clusters of liquid particles using a Langevin approach and obtain the geometrical and topological transitions for clusters of N particles (up to N=7) as the particles' interfacial tension and adhesion energy are varied. As particles become more adhesive and increase their contact angle, we observe well-defined packing transitions in the clusters. For N=5, a topological transition occurs at a critical value of the contact angle. For N=6, we obtain two stable cluster geometries for a given value of the contact angle, namely an 8-faced deltahedron and an octahedron. For N=7, there appears to be a complex landscape of cluster geometries and topologies, with transitions occurring at well-defined values of the contact angle. Our findings can help in the controlled assembly of particular arrangements of small clusters of bubbles or adherent droplets.

  5. Small-scale Conformity of the Virgo Cluster Galaxies

    NASA Astrophysics Data System (ADS)

    Lee, Hye-Ran; Lee, Joon Hyeop; Jeong, Hyunjin; Park, Byeong-Gon

    2016-06-01

    We investigate the small-scale conformity in color between bright galaxies and their faint companions in the Virgo Cluster. Cluster member galaxies are spectroscopically determined using the Extended Virgo Cluster Catalog and the Sloan Digital Sky Survey Data Release 12. We find that the luminosity-weighted mean color of faint galaxies depends on the color of adjacent bright galaxy as well as on the cluster-scale environment (gravitational potential index). From this result for the entire area of the Virgo Cluster, it is not distinguishable whether the small-scale conformity is genuine or if it is artificially produced due to cluster-scale variation of galaxy color. To disentangle this degeneracy, we divide the Virgo Cluster area into three sub-areas so that the cluster-scale environmental dependence is minimized: A1 (central), A2 (intermediate), and A3 (outermost). We find conformity in color between bright galaxies and their faint companions (color-color slope significance S ˜ 2.73σ and correlation coefficient {cc}˜ 0.50) in A2, where the cluster-scale environmental dependence is almost negligible. On the other hand, the conformity is not significant or very marginal (S ˜ 1.75σ and {cc}˜ 0.27) in A1. The conformity is not significant either in A3 (S ˜ 1.59σ and {cc}˜ 0.44), but the sample size is too small in this area. These results are consistent with a scenario in which the small-scale conformity in a cluster is a vestige of infallen groups and these groups lose conformity as they come closer to the cluster center.

  6. Structures and energetics of neutral and ionic silicon-germanium clusters: density functional theory and coupled cluster studies.

    PubMed

    Wang, Yi-Siang; Chao, Sheng D

    2011-03-10

    We have calculated the structural and energetic properties of neutral and ionic (singly charged anionic and cationic) semiconductor binary silicon-germanium clusters Si(m)Ge(n) for s = m + n ≤ 12 using the density functional theory (DFT-B3LYP) and coupled cluster [CCSD(T)] methods with Pople's 6-311++G(3df, 3pd) basis set. Neutral and anionic clusters share similar ground state structures for s = 3-7, independent of the stoichiometry and atom locations, but start to deviate at s = 8. The relative energetic stability of the calculated ground state structures among possible isomers has been analyzed through a bond strength propensity model where the pair interactions of Si-Si, Si-Ge, and Ge-Ge are competing. Electron affinities, ionization potentials, energy gaps between the highest and lowest occupied molecular orbitals (HOMO-LUMO gaps), and cluster mixing energies were calculated and analyzed. Overall, for a fixed s, the vertical ionization potential increases as the number of silicon atoms m increases, while the vertical electron affinity shows a dip at m = 2. As s increases, the ionization potentials increase from s = 2 to s = 3 and then decrease slowly to s = 8. The mixing energies for neutral and ionic clusters are all negative, indicating that the binary clusters are more stable than pure elemental clusters. Except for s = 4 and 8, cationic clusters are more stable than anionic ones and, thus, are more likely to be observed in experiments.

  7. Stable small bubble clusters in two-dimensional foams.

    PubMed

    Zhang, Kai; Kuo, Chin-Chang; See, Nathaniel; O'Hern, Corey; Dennin, Michael

    2017-06-21

    Key features of the mechanical response of amorphous particulate materials, such as foams, emulsions, and granular media, to applied stress are determined by the frequency and size of particle rearrangements that occur as the system transitions from one mechanically stable state to another. This work describes coordinated experimental and computational studies of bubble rafts, which are quasi-two dimensional systems of bubbles confined to the air-water interface. We focus on small mechanically stable clusters of four, five, six, and seven bubbles with two different sizes with diameter ratio σL/σS ≃ 1.4. Focusing on small bubble clusters, which can be viewed as subsystems of a larger system, allows us to investigate the full ensemble of clusters that form, measure the respective frequencies with which the clusters occur, and determine the form of the bubble-bubble interactions. We emphasize several important results. First, for clusters with N > 5 bubbles, we find using discrete element simulations that short-range attractive interactions between bubbles give rise to a larger ensemble of distinct mechanically stable clusters compared to that generated by long-range attractive interactions. The additional clusters in systems with short-range attractions possess larger gaps between pairs of neighboring bubbles on the periphery of the clusters. The ensemble of bubble clusters observed in experiments is similar to the ensemble of clusters with long-range attractive interactions. We also compare the frequency with which each cluster occurs in simulations and experiments. We find that the cluster frequencies are extremely sensitive to the protocol used to generate them and only weakly correlated to the energy of the clusters.

  8. Small Si clusters on surfaces of carbon nanotubes

    SciTech Connect

    Meng, Lijun; Zhang, Kaiwang; Stocks, George Malcolm; Zhong, Jianxin

    2006-01-01

    Structures of small Si clusters, Sin, on surfaces of carbon nanotubes have been studied by molecular dynamics simulation. We show that the lowest-energy structures of Sin are three-dimensional clusters rather than thin Si sheets covering the surface of a nanotube. As n increases from 10 to 30, Sin undergoes structural transitions from a tent-like structure (with nanotube surface as its base) to a cage-like structure (without interior atoms) and further to a spherical compact structure (with interior atoms). Our results are different from the structures of small Si clusters found in a free space without Si-nanotube interaction.

  9. Ionization dynamics of small water clusters: Proton transfer rate

    NASA Astrophysics Data System (ADS)

    Tachikawa, Hiroto; Takada, Tomoya

    2016-08-01

    The surfaces of icy planets and comets are composed of frozen water (H2O), carbon dioxide (CO2), and methane (CH4). These surfaces are irradiated by solar wind and cosmic rays from the interstellar space and they cause ionization of surface molecules. In this report, the effects of ionization of cold water clusters have been investigated using a direct ab initio molecular dynamics (AIMD) method to elucidate the rate of proton transfer (PT) in cations of small water clusters (H2O)n (n = 2-7). After ionization of the water clusters, PT occurred in all the cluster cations, and dissociation of the OH radical occurred for n = 4-7. The time of PT decreased with increasing the cluster size at n = 2-5 and reached a limiting value at n = 6 and 7. The mechanism of the PT process in ionized water clusters was discussed based on the theoretical results.

  10. Distribution and Chemical State of Cu-rich Clusters in Silicon: Preprint

    SciTech Connect

    Buonassisi, T.; Marcus, M. A.; Istratov, A. A.; Heuer, M.; Ciszek, T. F.; Lai, B.; Cai, Z.; Weber, E. R.

    2004-08-01

    the chemical state and distribution of Cu-rich clusters were determined in four different silicon-based materials with varying contamination pathways and degrees of oxygen concentration, including as-grown multicrystalline silicon. In all four samples, Cu3Si was the only chemical state observed. Cu3Si clusters were observed at structural defects within all four materials; XBIC measurements revealed that the presence of Cu3Si corresponds to increased recombination activity. Oxidized Cu compounds are not likely to form in silicon. The +1 eV edge shift in the -XAS absorption spectrum of Cu3Si relative to Cu metal is believed to be an indication of a degree of covalent bonding between Cu atoms and their silicon neighbors.

  11. Structure and stability of a silicon cluster on sequential doping with carbon atoms

    NASA Astrophysics Data System (ADS)

    AzeezullaNazrulla, Mohammed; Joshi, Krati; Israel, S.; Krishnamurty, Sailaja

    2016-02-01

    SiC is a highly stable material in bulk. On the other hand, alloys of silicon and carbon at nanoscale length are interesting from both technological as well fundamental view point and are being currently synthesized by various experimental groups (Truong et. al., 2015 [26]). In the present work, we identify a well-known silicon cluster viz., Si10 and dope it sequentially with carbon atoms. The evolution of electronic structure (spin state and the structural properties) on doping, the charge redistribution and structural properties are analyzed. It is interesting to note that the ground state SiC clusters prefer to be in the lowest spin state. Further, it is seen that carbon atoms are the electron rich centres while silicon atoms are electron deficient in every SiC alloy cluster. The carbon-carbon bond lengths in alloy clusters are equivalent to those seen in fullerene molecules. Interestingly, the carbon atoms tend to aggregate together with silicon atoms surrounding them by donating the charge. As a consequence, very few Si-Si bonds are noted with increasing concentrations of C atoms in a SiC alloy. Physical and chemical stability of doped clusters is studied by carrying out finite temperature behaviour and adsorbing O2 molecule on Si9C and Si8C2 clusters, respectively.

  12. Interference effects in electron scattering from small water clusters

    NASA Astrophysics Data System (ADS)

    Verkhovtsev, Alexey; Ellis-Gibbings, Lilian; Blanco, Francisco; García, Gustavo

    2017-10-01

    The importance of interference effects in electron scattering from complex molecular targets is demonstrated theoretically by an exemplificative case study of small water clusters. Interference contributions arising from all the scattering centres in the target lead to an increase of the elastic differential cross sections at small scattering angles and to the appearance of prominent minima in the cross sections of three-dimensional isomers which resemble structural properties of bulk water. The phenomena which are absent for isolated water molecules but emerge for small water clusters may help to better understand electron-driven processes in condensed aqueous media.

  13. Photoionization dynamics of glycine adsorbed on a silicon cluster: ''On-the-fly'' simulations

    SciTech Connect

    Shemesh, Dorit; Baer, Roi; Seideman, Tamar; Gerber, R. Benny

    2005-05-08

    Dynamics of glycine chemisorbed on the surface of a silicon cluster is studied for a process that involves single-photon ionization, followed by recombination with the electron after a selected time delay. The process is studied by ''on-the-fly'' molecular dynamics simulations, using the semiempirical parametric method number 3 (PM3) potential energy surface. The system is taken to be in the ground state prior to photoionization, and time delays from 5 to 50 fs before the recombination are considered. The time evolution is computed over 10 ps. The main findings are (1) the positive charge after ionization is initially mostly distributed on the silicon cluster. (2) After ionization the major structural changes are on the silicon cluster. These include Si-Si bond breaking and formation and hydrogen transfer between different silicon atoms. (3) The transient ionization event gives rise to dynamical behavior that depends sensitively on the ion state lifetime. Subsequent to 45 fs evolution in the charged state, the glycine molecule starts to rotate on the silicon cluster. Implications of the results to various processes that are induced by transient transition to a charged state are discussed. These include inelastic tunneling in molecular devices, photochemistry on conducting surfaces, and electron-molecule scattering.

  14. Measurement of small molecule diffusion with an optofluidic silicon chip.

    PubMed

    Ryckeboer, Eva; Vierendeels, Jan; Lee, Agnes; Werquin, Sam; Bienstman, Peter; Baets, Roel

    2013-11-21

    In this work we explore the micro-ring resonator platform to study the diffusion-driven mass transport of small molecules within microfluidic channels. The micro-ring resonators are integrated on a silicon-on-insulator photonic chip and combined with microfluidics in poly(dimethylsiloxane) (PDMS). We apply a strong initial gradient in the solute concentration and use the micro-ring resonators to observe how this concentration evolves over time and space. This can be achieved by tracking the optical resonances of multiple micro-rings as they shift with changing solute concentration. Experiments are performed for both glucose and NaCl and at different temperatures. The measured concentration profiles are used to calculate the diffusion coefficient of both glucose and NaCl in water. The good agreement between measurement and theoretical prediction demonstrates the relevance of this method.

  15. Dynamics of small mobile helium clusters near tungsten surfaces

    NASA Astrophysics Data System (ADS)

    Hu, Lin; Hammond, Karl D.; Wirth, Brian D.; Maroudas, Dimitrios

    2014-08-01

    We report the results of a systematic atomic-scale analysis of the dynamics of small mobile helium clusters in tungsten, near tungsten surfaces. These helium clusters are attracted to tungsten surfaces due to an elastic interaction force that drives surface segregation. As the clusters migrate toward the surface, trap mutation and cluster dissociation are activated at rates higher than in the bulk. These kinetic processes are responsible for important structural, morphological, and compositional features in plasma-exposed tungsten, including surface adatoms, near-surface immobile helium-vacancy complexes, and retained helium content. Detailed results are presented for di-helium and tri-helium clusters near low-Miller-index tungsten surfaces.

  16. Ab initio Monte Carlo investigation of small lithium clusters.

    SciTech Connect

    Srinivas, S.

    1999-06-16

    Structural and thermal properties of small lithium clusters are studied using ab initio-based Monte Carlo simulations. The ab initio scheme uses a Hartree-Fock/density functional treatment of the electronic structure combined with a jump-walking Monte Carlo sampling of nuclear configurations. Structural forms of Li{sub 8} and Li{sub 9}{sup +} clusters are obtained and their thermal properties analyzed in terms of probability distributions of the cluster potential energy, average potential energy and configurational heat capacity all considered as a function of the cluster temperature. Details of the gradual evolution with temperature of the structural forms sampled are examined. Temperatures characterizing the onset of structural changes and isomer coexistence are identified for both clusters.

  17. Ultra-small rhenium clusters supported on graphene.

    PubMed

    Miramontes, Orlando; Bonafé, Franco; Santiago, Ulises; Larios-Rodriguez, Eduardo; Velázquez-Salazar, Jesús J; Mariscal, Marcelo M; Yacaman, Miguel José

    2015-03-28

    The adsorption of very small rhenium clusters (2-13 atoms) supported on graphene was studied by high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The atomic structure of the clusters was fully resolved with the aid of density functional theory calculations and STEM simulations. It was found that octahedral and tetrahedral structures work as seeds to obtain more complex morphologies. Finally, a detailed analysis of the electronic structure suggested that a higher catalytic effect can be expected in Re clusters when adsorbed on graphene than in isolated ones.

  18. Ultra-small rhenium clusters supported on graphene

    PubMed Central

    Miramontes, Orlando; Bonafé, Franco; Santiago, Ulises; Larios-Rodriguez, Eduardo; Velázquez-Salazar, Jesús J.; Mariscal, Marcelo M.; Yacaman, Miguel José

    2015-01-01

    The adsorption of very small rhenium clusters (2 – 13 atoms) supported on graphene was studied with high annular dark field - scanning transmission electron microscopy (HAADF-STEM). The atomic structure of the clusters was fully resolved with the aid of density functional calculations and STEM simulations. It was found that octahedral and tetrahedral structures work as seeds to obtain more complex morphologies. Finally, a detailed analysis of the electronic structure suggested that a higher catalytic effect can be expected in Re clusters when adsorbed on graphene than in isolated ones. PMID:25721176

  19. Networks of neuroblastoma cells on porous silicon substrates reveal a small world topology.

    PubMed

    Marinaro, Giovanni; La Rocca, Rosanna; Toma, Andrea; Barberio, Marianna; Cancedda, Laura; Di Fabrizio, Enzo; Decuzzi, Paolo; Gentile, Francesco

    2015-02-01

    The human brain is a tightly interweaving network of neural cells where the complexity of the network is given by the large number of its constituents and its architecture. The topological structure of neurons in the brain translates into its increased computational capabilities, low energy consumption, and nondeterministic functions, which differentiate human behavior from artificial computational schemes. In this manuscript, we fabricated porous silicon chips with a small pore size ranging from 8 to 75 nm and large fractal dimensions up to Df ∼ 2.8. In culturing neuroblastoma N2A cells on the described substrates, we found that those cells adhere more firmly to and proliferate on the porous surfaces compared to the conventional nominally flat silicon substrates, which were used as controls. More importantly, we observed that N2A cells on the porous substrates create highly clustered, small world topology patterns. We conjecture that neurons with a similar architecture may elaborate information more efficiently than in random or regular grids. Moreover, we hypothesize that systems of neurons on nano-scale geometry evolve in time to form networks in which the propagation of information is maximized.

  20. Investigation of dislocation cluster evolution during directional solidification of multicrystalline silicon

    NASA Astrophysics Data System (ADS)

    Oriwol, Daniel; Trempa, Matthias; Sylla, Lamine; Leipner, Hartmut S.

    2017-04-01

    Dislocation clusters are the main crystal defects in multicrystalline silicon and are detrimental for solar cell efficiency. They were formed during the silicon ingot casting due to the relaxation of strain energy. The evolution of the dislocation clusters was studied by means of automated analysing tools of the standard wafer and cell production giving information about the cluster development as a function of the ingot height. Due to the observation of the whole wafer surface the point of view is of macroscopic nature. It was found that the dislocations tend to build clusters of high density which usually expand in diameter as a function of ingot height. According to their structure the dislocation clusters can be divided into light and dense clusters. The appearance of both types shows a clear dependence on the orientation of the grain growth direction. Additionally, a process of annihilation of dislocation clusters during the crystallization has been observed. To complement the macroscopic description, the dislocation clusters were also investigates by TEM. It is shown that the dislocations within the subgrain boundaries are closely arranged. Distances of 40-30 nm were found. These results lead to the conclusion that the dislocation density within the cluster structure is impossible to quantify by means of etch pit counting.

  1. Design and Analysis Considerations for Cluster Randomized Controlled Trials That Have a Small Number of Clusters.

    PubMed

    Deke, John

    2016-10-25

    Cluster randomized controlled trials (CRCTs) often require a large number of clusters in order to detect small effects with high probability. However, there are contexts where it may be possible to design a CRCT with a much smaller number of clusters (10 or fewer) and still detect meaningful effects. The objective is to offer recommendations for best practices in design and analysis for small CRCTs. I use simulations to examine alternative design and analysis approaches. Specifically, I examine (1) which analytic approaches control Type I errors at the desired rate, (2) which design and analytic approaches yield the most power, (3) what is the design effect of spurious correlations, and (4) examples of specific scenarios under which impacts of different sizes can be detected with high probability. I find that (1) mixed effects modeling and using Ordinary Least Squares (OLS) on data aggregated to the cluster level both control the Type I error rate, (2) randomization within blocks is always recommended, but how best to account for blocking through covariate adjustment depends on whether the precision gains offset the degrees of freedom loss, (3) power calculations can be accurate when design effects from small sample, spurious correlations are taken into account, and (4) it is very difficult to detect small effects with just four clusters, but with six or more clusters, there are realistic circumstances under which small effects can be detected with high probability. © The Author(s) 2016.

  2. Interaction of aromatic molecules with small gold clusters

    NASA Astrophysics Data System (ADS)

    Molina, Luis M.; López, María. J.; Alonso, Julio A.

    2017-09-01

    Ab initio density functional simulations have been performed to study the adsorption of aromatic molecules (benzene and toluene) on small Aun clusters. The calculations reveal a strong interaction between gold and π electrons of benzene, accompanied by a small electronic charge transfer from benzene to gold. We report a variety of binding conformations, with varying degrees of contact between the carbon atoms in benzene and the cluster. Therefore, the interaction between the aromatic part of molecules involved in the synthesis of fine chemicals catalyzed by gold must not be neglected, and could play an important role during some reaction stages.

  3. Correlation effects on the properties of small cobalt clusters

    NASA Astrophysics Data System (ADS)

    Hancock, Yvette; Iäs, Mari

    2009-03-01

    Demands for higher-density magnetic storage media and smaller memory devices require atomic-scale magnetic components with stable magnetic properties. One such candidate for this application is a small transition metal cluster. The magnetic properties of transition metal clusters are very sensitive to the geometry of the cluster, the local atomic and structural environments, and to the system size. In this work, the GGA + U DFT approach is used for the first time to study the system properties of small cobalt clusters consisting of 2 to 5 atoms. Previous studies using DFT and tight-binding approaches have been found to overestimate the binding energies, dissociation energies and vibrational frequencies of the clusters against their known experimental values. By including a Hubbard U correction between 2 -- 3 eV, the DFT method can then be fitted to reproduce the experimental results, thereby improving upon previous theoretical descriptions of these systems. The effect of U on the calculated magnetic and structural properties of the clusters is also discussed.

  4. Silicon decorated cone shaped carbon nanotube clusters for lithium ion battery anodes.

    PubMed

    Wang, Wei; Ruiz, Isaac; Ahmed, Kazi; Bay, Hamed Hosseini; George, Aaron S; Wang, Johnny; Butler, John; Ozkan, Mihrimah; Ozkan, Cengiz S

    2014-08-27

    In this work, we report the synthesis of an three-dimensional (3D) cone-shape CNT clusters (CCC) via chemical vapor deposition (CVD) with subsequent inductively coupled plasma (ICP) treatment. An innovative silicon decorated cone-shape CNT clusters (SCCC) is prepared by simply depositing amorphous silicon onto CCC via magnetron sputtering. The seamless connection between silicon decorated CNT cones and graphene facilitates the charge transfer in the system and suggests a binder-free technique of preparing lithium ion battery (LIB) anodes. Lithium ion batteries based on this novel 3D SCCC architecture demonstrates high reversible capacity of 1954 mAh g(-1) and excellent cycling stability (>1200 mAh g(-1) capacity with ≈ 100% coulombic efficiency after 230 cycles). © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  5. Integrated spectral study of small angular diameter galactic open clusters

    NASA Astrophysics Data System (ADS)

    Clariá, J. J.; Ahumada, A. V.; Bica, E.; Pavani, D. B.; Parisi, M. C.

    2017-10-01

    This paper presents flux-calibrated integrated spectra obtained at Complejo Astronómico El Leoncito (CASLEO, Argentina) for a sample of 9 Galactic open clusters of small angular diameter. The spectra cover the optical range (3800-6800 Å), with a resolution of ∼14 Å. With one exception (Ruprecht 158), the selected clusters are projected into the fourth Galactic quadrant (282o < l < 345o) near the Galactic plane (∣b∣ ≤ 9o). We performed simultaneous estimates of foreground interstellar reddening and age by comparing the continuum distribution and line strenghts of the cluster spectra with those of template cluster spectra with known parameters. We thus provide spectroscopic information independent from that derived through color-magnitude diagram studies. We found three clusters (Collinder 249, NGC 4463 and Ruprecht 122) younger than ∼40 Myr, four moderately young ones (BH 92, Harvard 5, Hogg 14 and Pismis 23) with ages within 200-400 Myr, and two intermediate-age ones (Ruprecht 158 and ESO 065-SC07) with ages within 1.0-2.2 Gyr. The derived foreground E(B - V) color excesses vary from around 0.0 in Ruprecht 158 to ∼1.1 in Pismis 23. In general terms, the results obtained show good agreement with previous photometric results. In Ruprecht 158 and BH 92, however, some differences are found between the parameters here obtained and previous values in the literature. Individual spectra of some comparatively bright stars located in the fields of 5 out of the 9 clusters here studied, allowed us to evaluate their membership status. The current cluster sample complements that of 46 open clusters previously studied by our group in an effort to gather a spectral library with several clusters per age bin. The cluster spectral library that we have been building is an important tool to tie studies of resolved and unresolved stellar content.

  6. Craters Clusters on Mars: Atmospheric Dispersion of Small Impactors

    NASA Astrophysics Data System (ADS)

    Melosh, H. J.; McEwen, A. S.

    2007-12-01

    The HiRISE imager aboard the Mars Reconnaissance Orbiter has imaged 18 of the 20 small dark areas that correspond to fresh impact scars on the surface of Mars, reported by Malin et al in 2006 to have appeared after 1999. Operating at a resolution of approximately 30 cm/pixel, HiRISE revealed that the largest of these craters contains eolian ripples and is probably more than just a few years old. The other dark halos often contain clusters of small impact craters with diameters ranging from 1 to 30 m in diameter. About half of these are clusters of 3-10 craters, with one cluster of more than 1,000 craters near 10 m diameter. The dispersion of the clusters is typically less than 100 m, although the largest cluster, whose craters exhibit clear signs of oblique impact, spreads almost 500 m. The size and dispersion of these clusters is consistent with dispersion of incoming meteoroids by atmospheric fragmentation at an altitude of about 20 km. Modeling of the atmospheric fragmentation and impact process shows that the size of the incoming meteoroids is in the range of 0.3 to 1 m (15 m for the largest, older crater), bulk density near 2000 kg/m3 and strength 0.5 to 1 bar, consistent with the properties of terrestrial or Venusian stony impactors. Small crater clusters on Mars are thus consistent with the theory for atmospheric dispersion of impactors observed on Earth and Venus, whose mean dispersions are typically 1 km and 10 km, proportional to the square root of surface atmospheric density.

  7. Stability and Spectra of Small 3He-4He Clusters

    NASA Astrophysics Data System (ADS)

    Navarro, J.; Fantoni, S.; Guardiola, R.; Zuker, A.

    Diffusion Monte Carlo calculations have been systematically performed to analyze the stability of small mixed 3He-4He clusters, as well as their excitation spectra. The picture that emerges is that of systems with strong shell effects whose binding and excitation energies are essentially determined by the monopole properties of an effective Hamiltonian.

  8. On the performance limiting behavior of defect clusters in commercial silicon solar cells

    SciTech Connect

    Sopori, B.L.; Chen, W.; Jones, K.; Gee, J.

    1998-09-01

    The authors report the observation of defect clusters in high-quality, commercial silicon solar cell substrates. The nature of the defect clusters, their mechanism of formation, and precipitation of metallic impurities at the defect clusters are discussed. This defect configuration influences the device performance in a unique way--by primarily degrading the voltage-related parameters. Network modeling is used to show that, in an N/P junction device, these regions act as shunts that dissipate power generated within the cell.

  9. Linear and Nonlinear Optical Properties of Small Silicon Clusters

    DTIC Science & Technology

    1990-08-01

    Wang , N. Herron, W. Mahler and A. Suna, J. Opt. Soc. Am. B 6, 808 (1989). 6. Y. Q. Li, C. C. Sung, R. Inguva and C. M. Bowden, J. Opt. Soc. Am. B 6, 814...J. Tersoff, Phys. Rev. B 37, 6991 (1988); R. Biswas, C. Z. Wang , C. T. Chan, K. M. Ho and C. M. Souk- oulis, Phys. Rev. Lett. 63, 1491 (1989); A. D...IL 60208 Irvine, CA 92717 Professor Frank DiSalvo Professor Roald Hoffmann Department of Chemistry Department of Chemistry Cornell University Cornell

  10. Ion-channeling analysis of boron clusters in silicon

    NASA Astrophysics Data System (ADS)

    Selen, L. J. M.; Janssen, F. J. J.; van IJzendoorn, L. J.; de Voigt, M. J. A.; Theunissen, M. J. J.; Smulders, P. J. M.; Eijkemans, T. J.

    2001-11-01

    We have measured axially channeled Rutherford backscattering spectra of Si1-xGex nanofilms in silicon(001). A step in the yield of the host crystal was found for off-normal axes at the depth of the nanofilm. The step was measured as a function of the angle between the incoming beam and the [011] axis and shows two maxima. It is found that Monte Carlo simulations assuming tetragonal distortion reproduce the experimental results. A universal curve was derived which enables determination of the tetragonal distortion from ion-channeling experiments, for a given film thickness. The results are compared with XRD measurements.

  11. Characteristics of multiprocessing MCNP5 on small personal computer clusters

    SciTech Connect

    Robinson, Sean M.; McConn, Ronald J.; Pagh, Richard T.; Schweppe, John E.; Siciliano, Edward R.

    2006-06-05

    The feasibility and efficiency of performing MCNP5 calculations with a small, heterogeneous computing cluster built from Microsoft® Windows™ Personal Computers (PCs) are explored. The performance increases that may be expected with such clusters are estimated. Our results show that the speed increase from additional slave PCs is nearly linear up to 10 processors. Guidance is given as to the specific advantages of changing various parameters present in the system. Implementing load balancing, and reducing the overhead from the MCNP rendezvous mechanism add to heterogeneous cluster efficiency. Hyper-threading technology and matching the total number of slave processes to the total number of logical processors also yield modest speed increases in the range below 7 processors. Because of the ease of acquisition of heterogeneous desktop computers, and the peak in efficiency at the level of a few physical processors, a strong case is made for the use of small clusters as a tool for producing MCNP5 calculations rapidly, and detailed instructions for constructing such clusters are provided.

  12. Monoxides of small terbium clusters: A density functional theory investigation

    SciTech Connect

    Zhang, G. L.; Yuan, H. K. Chen, H.; Kuang, A. L.; Li, Y.; Wang, J. Z.; Chen, J.

    2014-12-28

    To investigate the effect of oxygen atom on the geometrical structures, electronic, and magnetic properties of small terbium clusters, we carried out the first-principles calculations on Tb{sub n}O (n = 1-14) clusters. The capping of an oxygen atom on one trigonal-facet of Tb{sub n} structures is always favored energetically, which can significantly improve the structural stability. The far-infrared vibrational spectroscopies are found to be different from those of corresponding bare clusters, providing a distinct signal to detect the characteristic structures of Tb{sub n}O clusters. The primary effect of oxygen atom on magnetic properties is to change the magnetic orderings among Tb atoms and to reduce small of local magnetic moments of the O-coordinated Tb atoms, both of which serve as the key reasons for the experimental magnetic evolution of an oscillating behavior. These calculations are consistent with, and help to account for, the experimentally observed magnetic properties of monoxide Tb{sub n}O clusters [C. N. Van Dijk et al., J. Appl. Phys. 107, 09B526 (2010)].

  13. Predicting miscibility of binary liquids from small cluster QCE calculations

    NASA Astrophysics Data System (ADS)

    Ingenmey, Johannes; von Domaros, Michael; Kirchner, Barbara

    2017-04-01

    The quantum cluster equilibrium method is applied to model binary systems of molecular solvents. We minimize the computational effort as well as the experimental input and present the results obtained for the completely miscible acetonitrile/acetone, benzene/acetone, and water/acetone systems, as well as for the hardly miscible water/benzene system. Only clusters of sizes up to n = 3 are applied and these are optimized employing the low-cost functional PBEh-3c. The thermodynamic functions of the pure liquids are in reasonable agreement with experiments. For both non-water containing binary systems, the Gibbs energy of mixing can be reproduced with an accuracy of ≈0.25 kJ/mol. Water containing systems are not sufficiently described by small clusters. The empirical mean-field parameter amf and exclusion volume scaling parameter bxv which depend on the experimental input are approximated by linear interpolation between their neat liquids' reference values. This makes the approach independent from the experimental data of the binary system. Despite the roughness of the approximation as well as the small size of the cluster sets, the approach is able to correctly predict the mixing behavior of all acetone systems. The benzene/water system is correctly predicted to be non-miscible at most mole fractions. A small range at high benzene concentrations (x > 0.8 ) is falsely predicted to be miscible.

  14. Small gold clusters catalyzing the conversion of glycerol to epichlorohydrin.

    PubMed

    Pembere, Anthony M; Yang, Mengzhou; Luo, Zhixun

    2017-10-04

    The conversion of glycerol to epichlorohydrin (GTE) is of great interest because the product is widely used in plastics, rubbers and adhesives, and also contributes to the disposal of the reactant glycerol, a major by-product in biodiesel production. Here we find effective catalysis by small gold clusters for the GTE reaction in water with an enhanced selectivity towards the desired product. Along with natural bond orbital (NBO) analysis rationalizing the donor-acceptor charge-transfer interactions, we illustrate the mechanism for bond activation in the reactants and intermediates over gold cluster catalysts, and present thermodynamically and kinetically favoured reaction pathways for dehydrochlorination in GTE processes.

  15. Dimerization of argon and the properties of its small clusters

    NASA Astrophysics Data System (ADS)

    Titov, S. V.; Serov, S. A.; Ostrovskii, G. M.

    2016-12-01

    Statistical thermodynamic means are used to study the bound state of a small cluster AN (2 ≤ N ≤ 5) of Lennard-Jones particles in a spherical cavity. The statistical sum is calculated by the Monte Carlo method. For the dimer, integration is reduced to quadratures. The integration region contains only phase space points corresponding to the bound cluster state. Dimerization constant 2A = A2 is calculated via the probability of finding a molecule in the bound state using the example of argon.

  16. Structures and stabilities of copper encapsulated within silicon nano-clusters: Cu@Si n ( n = 9-15)

    NASA Astrophysics Data System (ADS)

    Hossain, Delwar; Pittman, Charles U., Jr.; Gwaltney, Steven R.

    2008-01-01

    Density functional electronic-structure calculations were performed for Cu@Si n ( n = 9-15) clusters. The lowest-energy endohedral structure and its stability for each Cu@Si n cluster were determined. The encapsulation of Cu within silicon clusters generates stable neutral Cu@Si n clusters. The binding energies and embedding energies of these clusters indicate that they are likely to be chemically stable. The relative cluster stabilities and other thermodynamic properties alternate with cluster size, with an apparent preference existing for clusters with an even number of Si atoms.

  17. The evolution of dielectric properties of sodium, silicon and argon clusters

    NASA Astrophysics Data System (ADS)

    Jackson, Koblar; Yang, Mingli; Ma, Li; Jellinek, Julius

    2012-02-01

    We used a computational scheme based on site-specific polarizabilities to study the evolution of the dielectric properties of sodium, silicon and argon clusters. In this approach, the total cluster polarizability is decomposed into local dipole (LD) and charge-transfer (CT) parts. The local dipole part measures the redistribution of charge within an atomic volume, while the CT part describes the movement of charge between volumes. We find distinct differences in the relative contributions of the LD and CT components to the total polarizability as a function of cluster size for the different cluster types and relate this to the development of metallic behavior. The method also directly probes the extent of electrostatic screening of the cluster interior to an applied electric field.

  18. Small-scale galaxy clustering in the eagle simulation

    NASA Astrophysics Data System (ADS)

    Artale, M. Celeste; Pedrosa, Susana E.; Trayford, James W.; Theuns, Tom; Farrow, Daniel J.; Norberg, Peder; Zehavi, Idit; Bower, Richard G.; Schaller, Matthieu

    2017-09-01

    We study present-day galaxy clustering in the eagle cosmological hydrodynamical simulation. eagle's galaxy formation parameters were calibrated to reproduce the redshift z = 0.1 galaxy stellar mass function, and the simulation also reproduces galaxy colours well. The simulation volume is too small to correctly sample large-scale fluctuations and we therefore concentrate on scales smaller than a few mega parsecs. We find very good agreement with observed clustering measurements from the Galaxy And Mass Assembly (GAMA) survey, when galaxies are binned by stellar mass, colour or luminosity. However, low-mass red galaxies are clustered too strongly, which is at least partly due to limited numerical resolution. Apart from this limitation, we conclude that eagle galaxies inhabit similar dark matter haloes as observed GAMA galaxies, and that the radial distribution of satellite galaxies, as a function of stellar mass and colour, is similar to that observed as well.

  19. Incremental Model-Based Clustering for Large Datasets With Small Clusters

    DTIC Science & Technology

    2003-12-10

    Paulia Flower Image Data These data describe an RGB (3-band) image of a St. Paulia flower shown in Figure 4. There are 46,656 pixels after background...removal. The small yellow flower centers are particularly eyecatching. Figure 4: RGB Image of a St. Paulia flower . There are 46,656 data pixels after...background removal. Note the small yellow flower centers. Results for the model-based clustering strategies on the flower image of Figure 4 are summarized

  20. Atomic resolution electron microscopy of small metal clusters

    NASA Astrophysics Data System (ADS)

    Bovin, J.-O.; Malm, J.-O.

    1991-03-01

    Atomic resolution imaging of cluster structures has been performed with high resolution transmission electron microscopy (HRTEM). Metal particles of the sizes 1 nanometer to tens of nanometers have been surface profile imaged on different supports; like zeolites, cordierite and amorphous carbon. It is shown that organic ligands in Schmid-clusters coordinated to the metal surface are desorbed or destroyed by the electron beam. Dynamic events on the surfaces and in the bulk of small metal particles have been recorded for small crystals of Au, Pt, Rh and Pb and can be classified under three headings; The smaller the crystals are the faster rearrangements of the crystal structure; “clouds” of atoms existing outside some surfaces are involved in extensive structural rearrangements of the surface or crystal surface growth; localized atom hopping on surfaces during crystal growth and desorption also occurs.

  1. Electronic Principles Governing the Stability and Reactivity of Ligated Metal and Silicon Encapsulated Transition Metal Clusters

    NASA Astrophysics Data System (ADS)

    Abreu, Marissa Baddick

    A thorough understanding of the underlying electronic principles guiding the stability and reactivity of clusters has direct implications for the identification of stable clusters for incorporation into clusters-assembled materials with tunable properties. This work explores the electronic principles governing the stability and reactivity of two types of clusters: ligated metal clusters and silicon encapsulated transition metal clusters. In the first case, the reactivity of iodine-protected aluminum clusters, Al13Ix - (x=0-4) and Al14Iy- (y=0-5), with the protic species methanol was studied. The symmetrical ground states of Al13Ix- showed no reactivity with methanol but reactivity was achieved in a higher energy isomer of Al 13I2- with iodines on adjacent aluminum atoms -- complementary Lewis acid-base active sites were induced on the opposite side of the cluster capable of breaking the O-H bond in methanol. Al 14Iy- (y=2-5) react with methanol, but only at the ligated adatom site. Reaction of methanol with Al14 - and Al14I- showed that ligation of the adatom was necessary for the reaction to occur there -- revealing the concept of a ligand-activated adatom. In the second case, the study focused heavily on CrSi12, a silicon encapsulated transition metal cluster whose stability and the reason for that stability has been debated heavily in the literature. Calculations of the energetic properties of CrSi n (n=6-16) revealed both CrSi12 and CrSi14 to have enhanced stability relative to other clusters; however CrSi12 lacks all the traditional markers of a magic cluster. Molecular orbital analysis of each of these clusters showed the CNFEG model to be inadequate in describing their stability. Because the 3dz2 orbital of Cr is unfilled in CrSi12, this cluster has only 16 effective valence electrons, meaning that the 18-electron rule is not applicable. The moderate stability of CrSi 12 can be accounted for by the crystal-field splitting of the 3d orbitals, which pushes the

  2. Small Al clusters. I - The effect of basis set and correlation on the geometry of small Al clusters

    NASA Technical Reports Server (NTRS)

    Bauschlicher, Charles W., Jr.; Pettersson, Lars G. M.

    1987-01-01

    A detailed study is presented of the basis set requirements and effects of correlation on the geometry and structure of small Al(n) clusters n = 2, 4, and 13. An effective core potential (ECP) is developed from the Huzinaga basis which accurately reproduces the all-electron results. It is found that depolarization functions are very important in computing the bond length, and that the bond contraction obtained is about as large for a 13-atom cluster as for the Al(4) rhombus structure. With d functions on each center in Al(13), a bond distance shorter than the bulk is obtained, as expected. The inclusion of correlation in Al(4) is found to be less important than the addition of polarization functions for determining the bond length, but increases substantially the calculated atomization energy. These calibration calculations allow an accurate treatment for Al(n) clusters to be designed.

  3. Resonant mixing of optical orbital and spin angular momentum by using chiral silicon nanosphere clusters.

    PubMed

    Al-Jarro, Ahmed; Biris, Claudiu G; Panoiu, Nicolae C

    2016-04-04

    We present an in-depth analysis of the resonant intermixing between optical orbital and spin angular momentum of Laguerre-Gaussian (LG) beams, mediated by chiral clusters made of silicon nanospheres. In particular, we establish a relationship between the spin and orbital quantum numbers characterizing the LG beam and the order q of the rotation symmetry group q of the cluster of nanospheres for which resonantly enhanced coupling between the two components of the optical angular momentum is observed. Thus, similar to the case of diffraction grating-mediated transfer of linear momentum between optical beams, we demonstrate that clusters of nanospheres that are invariant to specific rotation transformations can efficiently transfer optical angular momentum between LG beams with different quantum numbers. We also discuss the conditions in which the resonant interaction between LG beams and a chiral cluster of nanospheres leads to the generation of superchiral light.

  4. A theoretical study of the lowest-energy PtPd co-doped silicon clusters: Chirality and fluxional transformation

    NASA Astrophysics Data System (ADS)

    Lv, Peng; Lu, Zhansheng; Yang, Feng; Zhang, Yi; Yang, Xinwei; Xu, Guoliang; Yang, Zongxian

    2017-03-01

    The lowest-energy structures of PtPdSiqn (n = 1- 7; q = 0 , ± 1) clusters are searched based on the PSO algorithm implemented in the CALYPSO code and the first-principle DFT-D computations implemented in DMol3 code. Interestingly, the chirality has been found for the lowest-energy structures of the neutral and charged PtPdSi4, neutral and anionic PtPdSi5, and cationic PtPdSi7 clusters. The first principles molecular dynamics (MD) simulations show that the fluxional transformation between the chiral configurations for neutral PtPdSi4 cluster can take place at 400 K, which is also confirmed by the rather small transformation barrier. The equivalent atoms involved in the bond breaking and formation, as well as the other atoms (modulator), may facilitate the dynamical behavior. The current finding is thus beyond imagination. It is noticeable that the current study provides a potential way to create interesting cluster with chirality and transformation, based on silicon.

  5. Phonon induced spin relaxation times of single donors and donor clusters in silicon

    NASA Astrophysics Data System (ADS)

    Hsueh, Yuling; Buch, Holger; Hollenberg, Lloyd; Simmons, Michelle; Klimeck, Gerhard; Rahman, Rajib

    2014-03-01

    The phonon induced relaxation times (T1) of electron spins bound to single phosphorous (P) donors and P donor clusters in silicon is computed using the atomistic tight-binding method. The electron-phonon Hamiltonian is directly computed from the strain dependent tight-binding Hamiltonian, and the relaxation time is computed from Fermi's Golden Rule using the donor states and the electron-phonon Hamiltonian. The self-consistent Hartree method is used to compute the multi-electron wavefunctions in donor clusters. The method takes into account the full band structure of silicon including the spin-orbit interaction, and captures both valley repopulation and single valley g-factor shifts in a unified framework. The single donor relaxation rate varies proportionally to B5, and is of the order of seconds at B =2T, both in good agreement with experimental single donor data (A. Morello et. al., Nature 467, 687 (2010)). T1 calculations in donor clusters show variations for different electron numbers and donor numbers and locations. The computed T1 in a 4P:5e donor cluster match well with a scanning tunneling microscope patterned P donor cluster (H. Buch et. al., Nature Communications 4, 2017 (2013)).

  6. Phase transformation during silica cluster impact on crystal silicon substrate studied by molecular dynamics simulation

    NASA Astrophysics Data System (ADS)

    Chen, Ruling; Luo, Jianbin; Guo, Dan; Lu, Xinchun

    2008-07-01

    The process of a silica cluster impact on a crystal silicon substrate is studied by molecular dynamics simulation. At the impact loading stage, crystal silicon of the impact zone transforms to a locally ordered molten with increasing the local temperature and pressure of the impact zone. And then the transient molten forms amorphous silicon directly as the local temperature and pressure decrease at the impact unloading stage. Moreover, the phase behavior between the locally ordered molten and amorphous silicon exhibits the reversible structural transition. The transient molten contains not only lots of four-fold atom but also many three- and five-fold atoms. And the five-fold atom is similar to the mixture structure of semi-Si-II and semi-bct5-Si. The structure transformation between five- and four-fold atoms is affected by both pressure and temperature. The structure transformation between three- and four-fold atoms is affected mostly by temperature. The direct structure transformation between five- and three-fold atoms is not observed. Finally, these five- and three-fold atoms are also different from the usual five- and three-fold deficient atoms of amorphous silicon. In addition, according to the change of coordination number of atoms the impact process is divided into six stages: elastic, plastic, hysteresis, phase regressive, adhesion and cooling stages.

  7. Small area silicon diffused junction x-ray detectors

    SciTech Connect

    Walton, J.T.; Pehl, R.H.; Larsh, A.E.

    1981-10-01

    The low temperature performance of silicon diffused junction detectors in the measurement of low energy x-rays is reported. The detectors have an area of 0.04 cm/sup 2/ and a thickness of 100 ..mu..m. The spectral resolutions of these detectors were found to be in close agreement with expected values indicating that the defects introduced by the high temperature processing required in the device fabrication were not deleteriously affecting the detection of low energy x-rays. Device performance over a temperature range of 77 to 150/sup 0/K is given. These detectors were designed to detect low energy x-rays in the presence of minimum ionizing electrons. The successful application of silicon diffused junction technology to x-ray detector fabrication may facilitate the development of other novel silicon x-ray detector designs.

  8. Near-Infrared Spectroscopy of Small Protonated Water Clusters

    NASA Astrophysics Data System (ADS)

    Wagner, J. Philipp; McDonald, David C., II; McCoy, Anne B.; Duncan, Michael A.

    2017-06-01

    Small protonated water clusters and their argon tagged analogues of the general formula H^{+}(H_{2}O)_{n}Ar_{m} have been generated in a pulsed electric discharge source. Clusters containing n=1-8 water molecules were mass-selected and their absorptions in the near-infrared were probed with a tunable Nd/colonYAG pumped OPA/OPA laser system in the region from 4850-7350 cm^{-1}. A doublet corresponding to overtones of the free O-H stretches of the external waters was observed around 7200 cm^{-1} that was continuously decreasing in intensity with increasing cluster size. Broad, mostly featureless absorptions were found around 5300 cm^{-1} associated with stretch/bend combinations and with the hydrogen bonded waters in the core of the clusters. Vibrational assignments were substantiated by comparison to anharmonic frequency computations via second-order vibrational perturbation theory (VPT2) at the MP2/aug-cc-pVTZ level of theory.

  9. Ionization Thresholds of Small Carbon Clusters: Tunable VUVExperiments and Theory

    SciTech Connect

    Belau, Leonid; Wheeler, Steven E.; Ticknor, Brian W.; Ahmed,Musahid; Leone, Stephen R.; Allen, Wesley D.; Schaefer III, Henry F.; Duncan, Michael A.

    2007-07-31

    Small carbon clusters (Cn, n = 2-15) are produced in amolecular beam by pulsed laser vaporization and studied with vacuumultraviolet (VUV) photoionization mass spectrometry. The required VUVradiation in the 8-12 eV range is provided by the Advanced Light Source(ALS) at the Lawrence Berkeley National Laboratory. Mass spectra atvarious ionization energies reveal the qualitative relative abundances ofthe neutral carbon clusters produced. By far the most abundant species isC3. Using the tunability of the ALS, ionization threshold spectra arerecorded for the clusters up to 15 atoms in size. The ionizationthresholds are compared to those measured previously with charge-transferbracketing methods. To interpret the ionization thresholds for differentcluster sizes, new ab initio calculations are carried out on the clustersfor n = 4-10. Geometric structures are optimized at the CCSD(T) levelwith cc-pVTZ (or cc-pVDZ) basis sets, and focal point extrapolations areapplied to both neutral and cation species to determine adiabatic andvertical ionization potentials. The comparison of computed and measuredionization potentials makes it possible to investigate the isomericstructures of the neutral clusters produced in this experiment. Themeasurements are inconclusive for the n = 4-6 species because ofunquenched excited electronic states. However, the data provide evidencefor the prominence of linear structures for the n = 7, 9, 11, 13 speciesand the presence of cyclic C10.

  10. Small-scale, self-propagating combustion realized with on-chip porous silicon.

    PubMed

    Piekiel, Nicholas W; Morris, Christopher J

    2015-05-13

    For small-scale energy applications, energetic materials represent a high energy density source that, in certain cases, can be accessed with a very small amount of energy input. Recent advances in microprocessing techniques allow for the implementation of a porous silicon energetic material onto a crystalline silicon wafer at the microscale; however, combustion at a small length scale remains to be fully investigated, particularly with regards to the limitations of increased relative heat loss during combustion. The present study explores the critical dimensions of an on-chip porous silicon energetic material (porous silicon + sodium perchlorate (NaClO4)) required to propagate combustion. We etched ∼97 μm wide and ∼45 μm deep porous silicon channels that burned at a steady rate of 4.6 m/s, remaining steady across 90° changes in direction. In an effort to minimize the potential on-chip footprint for energetic porous silicon, we also explored the minimum spacing between porous silicon channels. We demonstrated independent burning of porous silicon channels at a spacing of <40 μm. Using this spacing, it was possible to have a flame path length of >0.5 m on a chip surface area of 1.65 cm(2). Smaller porous silicon channels of ∼28 μm wide and ∼14 μm deep were also utilized. These samples propagated combustion, but at times, did so unsteadily. This result may suggest that we are approaching a critical length scale for self-propagating combustion in a porous silicon energetic material.

  11. How do small water clusters bind an excess electron?

    PubMed

    Hammer, Nathan I; Shin, Joong-Won; Headrick, Jeffrey M; Diken, Eric G; Roscioli, Joseph R; Weddle, Gary H; Johnson, Mark A

    2004-10-22

    The arrangement of water molecules around a hydrated electron has eluded explanation for more than 40 years. Here we report sharp vibrational bands for small gas-phase water cluster anions, (H2O)(4-6)- and (D2O)(4-6)-. Analysis of these bands reveals a detailed picture of the diffuse electron-binding site. The electron is closely associated with a single water molecule attached to the supporting network through a double H-bond acceptor motif. The local OH stretching bands of this molecule are dramatically distorted in the pentamer and smaller clusters because the excited vibrational levels are strongly coupled to the electron continuum. The vibration-to-electronic energy transfer rates, as revealed by line shape analysis, are mode-specific and remarkably fast, with the symmetric stretching mode surviving for less than 10 vibrational periods [50 fs in (H2O)4-].

  12. A model potential for acetonitrile: from small clusters to liquid.

    PubMed

    Albertí, M; Amat, A; De Angelis, F; Pirani, F

    2013-06-13

    A portable model potential, representing the intermolecular interaction of acetonitrile with itself and with ions, is proposed. Such model, formulated as a combination of a few effective components, given in terms of the polarizability and dipole moment values of the molecular partners, is here adopted as a building block of the force field of acetonitrile clusters in molecular dynamics simulations. Its reliability is tested by comparing the predicted features for both small ionic and neutral clusters containing acetonitrile with ab initio results and experimental information. Its application to molecular dynamics simulations of liquid acetonitrile and of the solvated Li(+), Na(+), K(+), and I(-), performed at several values of the temperature, discloses an ample and interesting phenomenology, described in an internally consistent way. Such model will be useful to assess the effect of intermolecular interactions on the dynamics of acetonitrile processes occurring in various environments of applied relevance, with emphasis on the dye-sensitized solar cell framework.

  13. Nanothermodynamics of iron clusters: Small clusters, icosahedral and fcc-cuboctahedral structures

    NASA Astrophysics Data System (ADS)

    Angelié, C.; Soudan, J.-M.

    2017-05-01

    The study of the thermodynamics and structures of iron clusters has been carried on, focusing on small clusters and initial icosahedral and fcc-cuboctahedral structures. Two combined tools are used. First, energy intervals are explored by the Monte Carlo algorithm, called σ-mapping, detailed in the work of Soudan et al. [J. Chem. Phys. 135, 144109 (2011), Paper I]. In its flat histogram version, it provides the classical density of states, gp(Ep), in terms of the potential energy of the system. Second, the iron system is described by a potential which is called "corrected EAM" (cEAM), explained in the work of Basire et al. [J. Chem. Phys. 141, 104304 (2014), Paper II]. Small clusters from 3 to 12 atoms in their ground state have been compared first with published Density Functional Theory (DFT) calculations, giving a complete agreement of geometries. The series of 13, 55, 147, and 309 atom icosahedrons is shown to be the most stable form for the cEAM potential. However, the 147 atom cluster has a special behaviour, since decreasing the energy from the liquid zone leads to the irreversible trapping of the cluster in a reproducible amorphous state, 7.38 eV higher in energy than the icosahedron. This behaviour is not observed at the higher size of 309 atoms. The heat capacity of the 55, 147, and 309 atom clusters revealed a pronounced peak in the solid zone, related to a solid-solid transition, prior to the melting peak. The corresponding series of 13, 55, and 147 atom cuboctahedrons has been compared, underscoring the unstability towards the icosahedral structure. This unstability occurs clearly in several steps for the 147 atom cluster, with a sudden transformation at a transition state. This illustrates the concerted icosahedron-cuboctahedron transformation of Buckminster Fuller-Mackay, which is calculated for the cEAM potential. Two other clusters of initial fcc structures with 24 and 38 atoms have been studied, as well as a 302 atom cluster. Each one relaxes

  14. Understanding the structural transformation, stability of medium-sized neutral and charged silicon clusters.

    PubMed

    Ding, Li Ping; Zhang, Fang Hui; Zhu, Yong Sheng; Lu, Cheng; Kuang, Xiao Yu; Lv, Jian; Shao, Peng

    2015-11-03

    The structural and electronic properties for the global minimum structures of medium-sized neutral, anionic and cationic Sin(μ) (n = 20-30, μ = 0, -1 and +1) clusters have been studied using an unbiased CALYPSO structure searching method in conjunction with first-principles calculations. A large number of low-lying isomers are optimized at the B3PW91/6-311 + G* level of theory. Harmonic vibrational analysis has been performed to assure that the optimized geometries are stable. The growth behaviors clearly indicate that a structural transition from the prolate to spherical-like geometries occurs at n = 26 for neutral silicon clusters, n = 27 for anions and n = 25 for cations. These results are in good agreement with the available experimental and theoretical predicted findings. In addition, no significant structural differences are observed between the neutral and cation charged silicon clusters with n = 20-24, both of them favor prolate structures. The HOMO-LUMO gaps and vertical ionization potential patterns indicate that Si22 is the most chemical stable cluster, and its dynamical stability is deeply discussed by the vibrational spectra calculations.

  15. Understanding the structural transformation, stability of medium-sized neutral and charged silicon clusters

    PubMed Central

    Ding, Li Ping; Zhang, Fang Hui; Zhu, Yong Sheng; Lu, Cheng; Kuang, Xiao Yu; Lv, Jian; Shao, Peng

    2015-01-01

    The structural and electronic properties for the global minimum structures of medium-sized neutral, anionic and cationic Sinμ (n = 20–30, μ = 0, −1 and +1) clusters have been studied using an unbiased CALYPSO structure searching method in conjunction with first-principles calculations. A large number of low-lying isomers are optimized at the B3PW91/6-311 + G* level of theory. Harmonic vibrational analysis has been performed to assure that the optimized geometries are stable. The growth behaviors clearly indicate that a structural transition from the prolate to spherical-like geometries occurs at n = 26 for neutral silicon clusters, n = 27 for anions and n = 25 for cations. These results are in good agreement with the available experimental and theoretical predicted findings. In addition, no significant structural differences are observed between the neutral and cation charged silicon clusters with n = 20–24, both of them favor prolate structures. The HOMO-LUMO gaps and vertical ionization potential patterns indicate that Si22 is the most chemical stable cluster, and its dynamical stability is deeply discussed by the vibrational spectra calculations. PMID:26526519

  16. Unbiased structural search of small copper clusters within DFT

    NASA Astrophysics Data System (ADS)

    Cogollo-Olivo, Beatriz H.; Seriani, Nicola; Montoya, Javier A.

    2015-11-01

    The atomic structure of small Cu clusters with 3-6 atoms has been investigated by density functional theory and random search algorithm. New metastable structures have been found that lie merely tens of meV/atom above the corresponding ground state, and could therefore be present at thermodynamic equilibrium at room temperature or slightly above. Moreover, we show that the previously proposed linear configuration for Cu3 is in fact a local maximum of the energy. Finally, we argue that the random search algorithm also provides qualitative information about the attraction basin of each structure in the energy landscape.

  17. {Ni4O4} Cluster Complex to Enhance the Reductive Photocurrent Response on Silicon Nanowire Photocathodes

    PubMed Central

    Mange, Yatin J.; Chandrasekaran, Soundarrajan; Hollingsworth, Nathan; Voelcker, Nicolas H.; Parkin, Ivan P.; Nann, Thomas; Macdonald, Thomas J.

    2017-01-01

    Metal organic {Ni4O4} clusters, known oxidation catalysts, have been shown to provide a valuable route in increasing the photocurrent response on silicon nanowire (SiNW) photocathodes. {Ni4O4} clusters have been paired with SiNWs to form a new photocathode composite for water splitting. Under AM1.5 conditions, the combination of {Ni4O4} clusters with SiNWs gave a current density of −16 mA/cm2, which corresponds to an increase in current density of 60% when compared to bare SiNWs. The composite electrode was fully characterised and shown to be an efficient and stable photocathode for water splitting. PMID:28336866

  18. GW and Bethe-Salpeter study of small water clusters

    SciTech Connect

    Blase, Xavier Boulanger, Paul; Bruneval, Fabien; Fernandez-Serra, Marivi; Duchemin, Ivan

    2016-01-21

    We study within the GW and Bethe-Salpeter many-body perturbation theories the electronic and optical properties of small (H{sub 2}O){sub n} water clusters (n = 1-6). Comparison with high-level CCSD(T) Coupled-Cluster at the Single Double (Triple) levels and ADC(3) Green’s function third order algebraic diagrammatic construction calculations indicates that the standard non-self-consistent G{sub 0}W{sub 0}@PBE or G{sub 0}W{sub 0}@PBE0 approaches significantly underestimate the ionization energy by about 1.1 eV and 0.5 eV, respectively. Consequently, the related Bethe-Salpeter lowest optical excitations are found to be located much too low in energy when building transitions from a non-self-consistent G{sub 0}W{sub 0} description of the quasiparticle spectrum. Simple self-consistent schemes, with update of the eigenvalues only, are shown to provide a weak dependence on the Kohn-Sham starting point and a much better agreement with reference calculations. The present findings rationalize the theory to experiment possible discrepancies observed in previous G{sub 0}W{sub 0} and Bethe-Salpeter studies of bulk water. The increase of the optical gap with increasing cluster size is consistent with the evolution from gas to dense ice or water phases and results from an enhanced screening of the electron-hole interaction.

  19. GW and Bethe-Salpeter study of small water clusters.

    PubMed

    Blase, Xavier; Boulanger, Paul; Bruneval, Fabien; Fernandez-Serra, Marivi; Duchemin, Ivan

    2016-01-21

    We study within the GW and Bethe-Salpeter many-body perturbation theories the electronic and optical properties of small (H2O)n water clusters (n = 1-6). Comparison with high-level CCSD(T) Coupled-Cluster at the Single Double (Triple) levels and ADC(3) Green's function third order algebraic diagrammatic construction calculations indicates that the standard non-self-consistent G0W0@PBE or G0W0@PBE0 approaches significantly underestimate the ionization energy by about 1.1 eV and 0.5 eV, respectively. Consequently, the related Bethe-Salpeter lowest optical excitations are found to be located much too low in energy when building transitions from a non-self-consistent G0W0 description of the quasiparticle spectrum. Simple self-consistent schemes, with update of the eigenvalues only, are shown to provide a weak dependence on the Kohn-Sham starting point and a much better agreement with reference calculations. The present findings rationalize the theory to experiment possible discrepancies observed in previous G0W0 and Bethe-Salpeter studies of bulk water. The increase of the optical gap with increasing cluster size is consistent with the evolution from gas to dense ice or water phases and results from an enhanced screening of the electron-hole interaction.

  20. GW and Bethe-Salpeter study of small water clusters

    NASA Astrophysics Data System (ADS)

    Blase, Xavier; Boulanger, Paul; Bruneval, Fabien; Fernandez-Serra, Marivi; Duchemin, Ivan

    2016-01-01

    We study within the GW and Bethe-Salpeter many-body perturbation theories the electronic and optical properties of small (H2O)n water clusters (n = 1-6). Comparison with high-level CCSD(T) Coupled-Cluster at the Single Double (Triple) levels and ADC(3) Green's function third order algebraic diagrammatic construction calculations indicates that the standard non-self-consistent G0W0@PBE or G0W0@PBE0 approaches significantly underestimate the ionization energy by about 1.1 eV and 0.5 eV, respectively. Consequently, the related Bethe-Salpeter lowest optical excitations are found to be located much too low in energy when building transitions from a non-self-consistent G0W0 description of the quasiparticle spectrum. Simple self-consistent schemes, with update of the eigenvalues only, are shown to provide a weak dependence on the Kohn-Sham starting point and a much better agreement with reference calculations. The present findings rationalize the theory to experiment possible discrepancies observed in previous G0W0 and Bethe-Salpeter studies of bulk water. The increase of the optical gap with increasing cluster size is consistent with the evolution from gas to dense ice or water phases and results from an enhanced screening of the electron-hole interaction.

  1. Between geometry, stability, and polarizability: Density functional theory studies of silicon clusters Sin (n=3-10)

    NASA Astrophysics Data System (ADS)

    Pouchan, Claude; Bégué, Didier; Zhang, Daisy Y.

    2004-09-01

    The relationship between the polarizability, stability, and the geometry of small-size silicon clusters has been investigated by the density functional theory methods. Results obtained at local density approximation/Vosko-Wilk-Nusair and general gradient approximation/BLYP levels with polarized even-tempered basis set of quadruple ζ quality are presented and compared with those obtained by the B3LYP method, as well as with the ab initio results in recent literature. We have found that the polarizability is directly related to the size of the energy gap between symmetry-compatible bonding and antibonding molecular orbitals, but not necessarily to the size of the HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap. Furthermore, we have defined two structural parameters, namely, the averaged Si-Si distances and the standard deviation of the Si-Si distances, which were found to correlate remarkably well with the binding energy of the clusters and the HOMO-LUMO gap, respectively. These straightforward correlations would, therefore, provide a means to predict the physical properties, in particular, the polarizability and the stability, simply based on the structural information of the cluster.

  2. Robustness of cluster synchronous patterns in small-world networks with inter-cluster co-competition balance

    SciTech Connect

    Zhang, Jianbao; Ma, Zhongjun; Chen, Guanrong

    2014-06-15

    All edges in the classical Watts and Strogatz's small-world network model are unweighted and cooperative (positive). By introducing competitive (negative) inter-cluster edges and assigning edge weights to mimic more realistic networks, this paper develops a modified model which possesses co-competitive weighted couplings and cluster structures while maintaining the common small-world network properties of small average shortest path lengths and large clustering coefficients. Based on theoretical analysis, it is proved that the new model with inter-cluster co-competition balance has an important dynamical property of robust cluster synchronous pattern formation. More precisely, clusters will neither merge nor split regardless of adding or deleting nodes and edges, under the condition of inter-cluster co-competition balance. Numerical simulations demonstrate the robustness of the model against the increase of the coupling strength and several topological variations.

  3. Ab initio calculations and rate equation simulations for vacancy and vacancy-oxygen clustering in silicon

    NASA Astrophysics Data System (ADS)

    Kissinger, G.; Dabrowski, J.; Sinno, T.; Yang, Y.; Kot, D.; Sattler, A.

    2017-06-01

    Formation energies of vacancy clusters Vn with (1≤n≤11) in crystalline Si and of their complexes VnOm (1≤m≤14) with oxygen were computed by ab initio density functional theory (DFT) within the generalized gradient approximation (GGA) and used to simulate the process of vacancy agglomeration after rapid thermal annealing (RTA) and also the formation of bulk microdefects (BMDs) in Si. Rate equation modeling of vacancy agglomeration after RTA and subsequent annealing below 1000 °C was carried out and the results were compared for binding energies obtained with different cell size. In Czochralski silicon, the interaction between vacancies and interstitial oxygen has to be taken into account. The BMD formation in Czochralski silicon was modeled by a continuum model comprised of a large system of rate equations, which accounts for the free energy of BMDs as a function of BMD size and shape, temperature and point defect concentrations.

  4. Structural, Electronic, and Mechanical Properties of Chains of Silicon Clusters with Trigonal Bipiramidal Structures

    NASA Astrophysics Data System (ADS)

    Tchernatinsky, Alex; Jayanthi, C. S.; Wu, S. Y.

    2002-03-01

    We report here our investigation of structural, electronic, and mechanical properties of non-trivial silicon chains. These chains are built from elementary blocks of clusters of 5 and 8 silicon atoms, arranged into trigonal bipiramidal structures and periodically continued along the axial direction. The initial configurations are constructed such that there are no dangling bonds associated with any atom along the chains. The stable configurations of the chains are determined by molecular dynamics simulations based on the first-principles method of Sankey and Niklewski [1] using the self-consistency scheme given in Ref. [2]. Mechanical and electronic properties of the resulting stable chains will be presented. 1. O.F.Sankey, and D.J.Niklewski, Phys. Rev. B 40 3979(1989). 2. P.Ordejon, E.Artacho, and J.M.Soler, Phys.Rev. B 53,10441 (1996).

  5. Silicon avalanche photodiode operation and lifetime analysis for small satellites.

    PubMed

    Tan, Yue Chuan; Chandrasekara, Rakhitha; Cheng, Cliff; Ling, Alexander

    2013-07-15

    Silicon avalanche photodiodes (APDs) are sensitive to operating temperature fluctuations and are also susceptible to radiation flux expected in satellite-based quantum experiments. We introduce a low power voltage adjusting mechanism to overcome the effects of in-orbit temperature fluctuations. We also present data on the performance of Si APDs after irradiation (γ-ray and proton beam). Combined with an analysis of expected orbital irradiation, we propose that a Si APD in a 400 km equatorial orbit may operate beyond the lifetime of the satellite.

  6. Growth and properties of silicon heterostructures with buried nanosize Mg2Si clusters

    NASA Astrophysics Data System (ADS)

    Galkin, N. G.; Galkin, K. N.

    2005-06-01

    The technology of solid-phase growth of nanosize islands of magnesium suicide on Si (111) 7x7 with narrow distributions of lateral size and height (60 - 80 and 5 - 7 nanometers, respectively) and density of up to 2x 109 sm-2 is proposed. A 20-50 nm thick Si layer has been grown upon these islands. Basing on the data of AES, EELS, AFM and JR spectroscopy, a conclusion is made that the Mg2Si islands remain in depth of the Si layer. The suggestion is made that sizes, density and crystal structure of the buried magnesium suicide clusters preserves. It is shown, that the system of three as-grown layers of buried clusters has smoother surface than the one layer system. The contribution of the Mg2Si clusters into the dielectric function is observed at the energy 0.8-1.2 eV, it is maximal if the clusters are localized on the silicon surface. It is shown, that with increase of the number of Mg2Si cluster layers their contribution increases into the effective number of electrons per a unit cell and effective dielectric function of the sample.

  7. Phase-shifter using submicron silicon waveguide couplers with ultra-small electro-mechanical actuator.

    PubMed

    Ikeda, Taro; Takahashi, Kazunori; Kanamori, Yoshiaki; Hane, Kazuhiro

    2010-03-29

    Phase shifter is an important part of optical waveguide circuits as used in interferometer. However, it is not always easy to generate a large phase shift in a small region. Here, a variable phase-shifter operating as delay-line of silicon waveguide was designed and fabricated by silicon micromachining. The proposed phase-shifter consists of a freestanding submicron-wide silicon waveguide with two waveguide couplers and an ultrasmall silicon comb-drive actuator. The position of the freestanding waveguide is moved by the actuator to vary the total optical path. Phase-shift was measured in a Mach-Zehnder interferometer to be 3.0pi at the displacement of 1.0 mum at the voltage of 31 V. The dimension of the fabricated device is 50microm wide and 85microm long.

  8. Physical electrostatics of small field emitter arrays/clusters

    NASA Astrophysics Data System (ADS)

    Forbes, Richard G.

    2016-08-01

    This paper aims to improve qualitative understanding of electrostatic influences on apex field enhancement factors (AFEFs) for small field emitter arrays/clusters. Using the "floating sphere at emitter-plate potential" (FSEPP) model, it re-examines the electrostatics and mathematics of three simple systems of identical post-like emitters. For the isolated emitter, various approaches are noted. An adequate approximation is to consider only the effects of sphere charges and (for significantly separated emitters) image charges. For the 2-emitter system, formulas are found for charge-transfer ("charge-blunting") effects and neighbor-field effects, for widely spaced and for "sufficiently closely spaced" emitters. Mutual charge-blunting is always the dominant effect, with a related (negative) fractional AFEF-change δtwo. For sufficiently small emitter spacing c, |δtwo| varies approximately as 1/c; for large spacing, |δtwo| decreases as 1/c3. In a 3-emitter equispaced linear array, differential charge-blunting and differential neighbor-field effects occur, but differential charge-blunting effects are dominant, and cause the "exposed" outer emitters to have higher AFEF (γ0) than the central emitter (γ1). Formulas are found for the exposure ratio Ξ = γ0/γ1, for large and for sufficiently small separations. The FSEPP model for an isolated emitter has accuracy around 30%. Line-charge models (LCMs) are an alternative, but an apparent difficulty with recent LCM implementations is identified. Better descriptions of array electrostatics may involve developing good fitting equations for AFEFs derived from accurate numerical solution of Laplace's equation, perhaps with equation form(s) guided qualitatively by FSEPP-model results. In existing fitting formulas, the AFEF-reduction decreases exponentially as c increases, which is different from the FSEPP-model formulas. This discrepancy needs to be investigated, using systematic Laplace-based simulations and appropriate results

  9. Molecular dynamics simulation studies of gold nano-cluster on silicon (001) surface

    NASA Astrophysics Data System (ADS)

    Sarangi, S. S.; Satyam, P. V.; Nayak, S. K.; Mahanti, S. D.

    2017-02-01

    Classical molecular dynamics simulations with modified embedded atom method model has been carried out to understand the interfacial behavior of a gold(Au) nano-cluster on a silicon(Si) surface. For illustration, a gold nano-cluster (NC) consisting of 108 atoms has been chosen on the Si (001) surface. We have investigated on the process of melting of this NC, the temperature dependent intermixing of Au and Si, and the diffusion of Au atoms into the Si substrate. It has been observed that the NC becomes semi-spherical around the room temperature and the melting process started around 450 K. In order to examine the substrate effect on the melting point temperature of the gold NC, a similar NC has been studied in vacuum. The melting point temperature of this free NC is found to be around 480 K. The observed suppression of melting temperature (by 30 K) for the supported NC can be understood in terms of substrate induced change in morphology. We observe inter-diffusion between gold and silicon atoms for temperatures greater than 650 K.

  10. Molecular dynamics simulation studies of gold nano-cluster on silicon (001) surface

    NASA Astrophysics Data System (ADS)

    Sarangi, S. S.; Satyam, P. V.; Nayak, S. K.; Mahanti, S. D.

    2017-08-01

    Classical molecular dynamics simulations with modified embedded atom method model has been carried out to understand the interfacial behavior of a gold(Au) nano-cluster on a silicon(Si) surface. For illustration, a gold nano-cluster (NC) consisting of 108 atoms has been chosen on the Si (001) surface. We have investigated on the process of melting of this NC, the temperature dependent intermixing of Au and Si, and the diffusion of Au atoms into the Si substrate. It has been observed that the NC becomes semi-spherical around the room temperature and the melting process started around 450 K. In order to examine the substrate effect on the melting point temperature of the gold NC, a similar NC has been studied in vacuum. The melting point temperature of this free NC is found to be around 480 K. The observed suppression of melting temperature (by 30 K) for the supported NC can be understood in terms of substrate induced change in morphology. We observe inter-diffusion between gold and silicon atoms for temperatures greater than 650 K.

  11. Effects of interstitial clustering on transient enhanced diffusion of boron in silicon

    SciTech Connect

    Solmi, S.; Valmorri, S.

    1997-11-01

    A simulation model for boron diffusion which takes into account the aggregation of the excess interstitials in clusters, and subsequently, the dissolution of these defects, is proposed. The interstitial supersaturation and generation rate are determined according to the classical theory of nucleation and growth of particles, in analogy with the precipitation of a new phase in heavily doped silicon. The clusters are considered as precipitates formed by interstitial Si atoms. The B diffusion is modelled on the basis of the dopant-interstitial pair diffusion mechanism. The clusters dissolution during annealing maintains nearly constant, for a long period, the interstitial supersaturation and the related enhancement of the boron diffusion. This gives a good account of the diffusion results over a large range of experimental conditions. Furthermore, this approach describes most of the behavior of the transient enhanced diffusion (TED), like the temperature dependence of the level of the B diffusion enhancement, the dependence of the duration of the phenomenon on implanted dose, and the scarce dependence on the damage distribution in depth. The results of the simulations are compared with experimental data on the kinetics of interstitial cluster dissolution and of B TED.

  12. Photoluminescence study of self-interstitial clusters and extended defects in ion-implanted silicon

    NASA Astrophysics Data System (ADS)

    Giri, P. K.

    2003-12-01

    We report on the photoluminescence (PL) studies of self-interstitial (I) clustering in ion-implanted Si at various stages of post-implantation annealing. Low-temperature PL measurements on as-implanted and low-temperature annealed (up to 450°C) samples show sharp X and W bands at 1200 and 1218 nm which are attributed to I4 and I3 clusters, respectively. Annealing at 600°C shows a drastic change in the PL spectra. In case of high-energy self-ion-implanted samples, 600°C annealing produces several peaks in the range 1250-1400 nm. For longer duration annealing, two broad bands form at 1322 and 1392 nm irrespective of the ion fluence. These PL signatures are attributed to I8 clusters and/or (1 0 0) I-chains, and they are believed to be the precursor of {3 1 1} rod-like defects. For annealing above 600°C and for fluence ⩾1×1013 cm-2, a sharp PL band is observed at 1376 nm and it is attributed to {3 1 1} rod-like defects. At higher fluences, an additional broad band appears in the PL spectrum at ∼1576 nm which is related to residual ion-damage or extended defect formation. These results illustrate the potential of silicon I-clusters as a possible source of light emission from Si.

  13. Effect of particle clustering of silver nanoparticles on ultrathin silicon solar cell

    NASA Astrophysics Data System (ADS)

    Shokeen, Poonam; Jain, Amit; Kapoor, Avinashi; Gupta, Vinay

    2016-07-01

    Particle clustering is a major concern for uniform dispersal of nanoparticles in various deposition procedures. Well separated uniform distribution of metal nanoparticles is essential for effective coupling of surface plasmons. This work experimentally and theoretically, discusses the effect of nanoparticle clustering on the light trapping efficiency of silver nanoparticles. Pulsed laser deposition system has been used for deposition of silver nanoparticles, and substrate heating has been used to promote uniform distribution of nanoparticles. Pre-heated substrate depositions are compared with corresponding post-annealed samples. XRD, FESEM, Photoluminescence and UV-visible spectroscopy have been used to study the variations in their structural and optical properties. Mono-dispersal of silver nanoparticles for pre-heated substrates results in sharper surface plasmon resonance in comparison to post-annealed samples. Mie theory is used to estimate the particle size of the nanoparticles and findings are in accordance with quantitative analysis of FESEM images. Finite-difference time domain technique is used to discuss the effect of particle distribution on an ultrathin film silicon solar cell. Device degradation is observed as a result of clustering of silver nanoparticles. Hence, mono-dispersal of plasmonic nanostructures is important for required results and pre-heated deposition of metal nanoparticles by pulsed laser deposition can effectively solve the problem of particle clustering.

  14. Small silicon, big opportunities: the development and future of colloidally-stable monodisperse silicon nanocrystals.

    PubMed

    Mastronardi, Melanie L; Henderson, Eric J; Puzzo, Daniel P; Ozin, Geoffrey A

    2012-11-14

    Nanomaterials are becoming increasingly widespread in consumer technologies, but there is global concern about the toxicity of nanomaterials to humans and the environment as they move rapidly from the research laboratory to the market place. With this in mind, it makes sense to intensify the nanochemistry community's global research effort on the synthesis and study of nanoparticles that are purportedly "green". One potentially green nanoparticle that seems to be a most promising candidate in this context is silicon, whose appealing optical, optoelectronic, photonic, and biomedical attributes are recently gaining much attention. In this paper, we outline some of our recent contributions to the development of the growing field of silicon nanocrystals (ncSi) in order to stress the importance of continued study of ncSi as a green alternative to the archetypal semiconductor nanocrystals like CdSe, InAs, and PbS. While a variety of developments in synthetic methods, characterization techniques, and applications have been reported in recent years, the ability to prepare colloidally-stable monodisperse ncSi samples may prove to have the largest impact on the field, as it opens the door to study and access the tunable size-dependent properties of ncSi. Here, we summarize our recent contributions in size-separation methods to achieve monodisperse samples, the characterization of size-dependant property trends, the development of ncSi applications, and their potential impact on the promising future of ncSi.

  15. Characterizing the Small Scale Structure in Clusters of Galaxies

    NASA Technical Reports Server (NTRS)

    Forman, William R.

    2001-01-01

    We studied galaxy clusters Abell 119, Abell 754, and Abell 1750, using data from the ASCA and ROSAT satellites. In addition, we completed the paper "Merging Binary Clusters". In this paper we study three prominent bi-modal X-ray clusters: A3528, A1750 and A3395. Since the sub-clusters in these systems have projected separations of 0.93, 1.00 and 0.67 Mpc respectively, we examine their X-ray and optical observations to investigate the dynamics and possible merging of these sub-clusters. Using data taken with ROSAT and ASCA, we analyze the temperature and surface brightness distributions. We also analyze the velocity distributions of the three clusters using new measurements supplemented with previously published data. We examined both the overall cluster properties as well as the two sub-cluster elements in each. These results were then applied to the determination of the overall cluster masses, that demonstrate excellent consistency between the various methods used. While the characteristic parameters of the sub-clusters are typical of isolated objects, our temperature results for the regions between the two sub-clusters clearly confirm the presence of merger activity that is suggested by the surface brightness distributions. These three clusters represent a progression of equal-sized sub-cluster mergers, starting from initial contact to immediately before first core passage.

  16. Nano Vacancy Clusters and Trap Limited Diffusion of Si Interstitials in Silicon

    SciTech Connect

    Prof. Wei-Kan Chu

    2010-05-05

    The objective of this project is to develop a method to characterize nano vacancy clusters and the dynamics of their formation in ion-irradiated silicon. It will impact (1) semiconductor device processing involving ion implantation, and (2) device design concerning irradiation hardness in harsh environments. It also aims to enhance minority participation in research and curricula on emerging materials and ion beam science. Vacancy defects are of scientific and technological importance since they are ubiquitous when the host materials are exposed to particle irradiation. Studies on vacancy clustering in the past decades were mainly theoretical and the approach heavily relied on the total-energy calculation methods. The lack of experimental data is mainly due to the formidable task in measuring the cluster size and density using modern metrological techniques, including transmission electron microscopy and positron annihilation spectroscopy. To surmount these challenges, we proposed a novel approach to tackle the metrological problems on the nano vacancy clusters, especially in determining densities and sizes of the nano vacancies based on the premise that the vacancy-clusters act as diffusion-trapping centers. For a silicon substrate containing vacancyclusters, the diffusion of interstitials (from the surface) can be classified into three phases: (1) an ultrafast phase-I in which the trapping centers have little effect on the diffusion of interstitials; (2) a prolonged phase-II in which the loss rate of interstitials by trapping balances the influx of interstitials from the surface; and (3) a phase-III diffusion in which surface influx of interstitials depletes the trapping centers and interstitials consequently propagate deeper into the bulk. By measuring diffusion profiles of Si interstitials as a function of diffusion time, void sizes and void densities can be obtained through fitting. Experimentally, our approach to characterize voids is realized through three

  17. Fourier Transform Infrared Spectroscopic Studies of Small Pure Carbon Clusters

    NASA Astrophysics Data System (ADS)

    Kranze, Richard Harlow

    1996-01-01

    Fourier transform infrared studies of small pure carbon linear chains have been carried out by trapping the products of the vacuum ultraviolet photolysis of 2 -methyl-1,3,-butadiene and by the high temperature evaporation of graphite, in solid Ar at 10 K. Isotopic data obtained for each of the species studied enabled the unambiguous assignments. Comparison of these assignments with theoretical calculations yielded very good agreement. The results of the presented research have resulted in new information on two pure carbon linear chains, C _5 and C_9, and have cleared up considerable confusion surrounding previous assignments to the linear C_6 carbon cluster. In the course of the C_9 study, however, it was also determined that in certain circumstances extreme care must be taken when making assignments based on isotopic substitutions. The v_4(sigma_{u }) antisymmetric stretch at 1446.6 cm ^{-1} of the linear C_5 carbon cluster has been identified for the first time by the analysis of isotopic data. This assignment agrees well with the predicted frequencies of both Botschwina and Sebald (1989) and Ewing and Shavitt (1992). Several isotopic bands belonging to the v_3(sigma u) antisymmetric stretch were also reassigned as a result of the better resolution available in the present study. Both the v_4(sigma_ {u}) and v_5(sigma u) assignments for the linear C_6 carbon cluster were confirmed through the analysis of isotopic data. Prior to publication of this work, it had been suggested by Neumark et al. (1993) that the assignments particularly for v_5( sigmau) were incorrect on the basis of force constant adjustment calculations. However, publication of the present work with the isotopic evidence, which definitively proved that the proposed assignments were correct, Neumark et al. agreed and concluded their difficulty resulted from the large anharmonicity involved with the v_5(sigma_{u }) mode. A second antisymmetric stretch, v_7( sigmau), for the linear C_9 cluster has

  18. Small iron-carbonyl clusters bearing imidazolium-2-trithioperoxycarboxylate ligands.

    PubMed

    Beltrán, Tomás F; Zaragoza, Guillermo; Delaude, Lionel

    2017-10-14

    The reaction of [Fe2(CO)9] with two representative imidazolium-2-dithiocarboxylate zwitterions derived from common N-heterocyclic carbenes (NHCs) bearing mesityl (IMes) or 2,6-diisopropylphenyl substituents on their nitrogen atoms (IDip) unexpectedly afforded two small bimetallic iron-carbonyl clusters with the generic formula [Fe2(CO)6(μ-κ(2)-S,S'-κ(2)-S,S'-S3C·NHC)]. After a brief optimization of the reaction conditions, these two "sulfur-enriched" products were isolated in low yields. They were fully characterized by IR, NMR, UV/Visible, and ESI-MS techniques, and their molecular structures were determined by single crystal X-ray diffraction analysis. The two compounds adopted a butterfly-type disposition in the solid state, with an [Fe2(CO)6] core bridged by the trithioperoxycarboxylate moiety of the in situ generated NHC·CS3 ligands. Bond lengths recorded for the CS3(-) unit revealed that its negative charge was mostly located on the remote sulfur atom.

  19. Small scale clustering of late forming dark matter

    NASA Astrophysics Data System (ADS)

    Agarwal, S.; Corasaniti, P.-S.; Das, S.; Rasera, Y.

    2015-09-01

    We perform a study of the nonlinear clustering of matter in the late-forming dark matter (LFDM) scenario in which dark matter results from the transition of a nonminimally coupled scalar field from radiation to collisionless matter. A distinct feature of this model is the presence of a damped oscillatory cutoff in the linear matter power spectrum at small scales. We use a suite of high-resolution N-body simulations to study the imprints of LFDM on the nonlinear matter power spectrum, the halo mass and velocity functions and the halo density profiles. The model largely satisfies high-redshift matter power spectrum constraints from Lyman-α forest measurements, while it predicts suppressed abundance of low-mass halos (˜109- 1010 h-1 M⊙ ) at all redshifts compared to a vanilla Λ CDM model. The analysis of the LFDM halo velocity function shows a better agreement than the Λ CDM prediction with the observed abundance of low-velocity galaxies in the local volume. Halos with mass M ≳1011 h-1 M⊙ show minor departures of the density profiles from Λ CDM expectations, while smaller-mass halos are less dense, consistent with the fact that they form later than their Λ CDM counterparts.

  20. Observation of small metal clusters on graphite surface with scanning tunneling microscopy

    NASA Astrophysics Data System (ADS)

    Shen, Jian; Zhu, Changxin; Ma, Zili; Pang, Shijin; Xue, Zengquan

    The motivation for studying the dynamic behavior and morphology of small metal clusters on solid single crystal surface is the desire to understand the physical mechanisms evolving in the initial stages of thin-film growth. In the experiments we have used a scanning tunneling microscope to study the static morphology of small Pt and Ni clusters supported on clean graphite surfaces, as well as the dynamic behaviors of small Pt clusters in an ultrahigh vacuum chamber. The metal deposition was fulfilled by controllable evaporation from ultra-pure superfine metal wires at room temperature in UHV. The STM images of small Pt and Ni clusters on graphite substrates with atomic resolution, as well as a series of STM images reveal some transformation processes of small metal clusters on the solid crystal surfaces, which provide us a better understanding on the procedure of atomic diffusion of metal clusters. All the STM images have been performed at room temperature.

  1. Electronic Structure and Geometries of Small Compound Metal Clusters

    SciTech Connect

    1999-04-14

    During the tenure of the DOE grant DE-FG05-87EI145316 we have concentrated on equilibrium geometries, stability, and the electronic structure of transition metal-carbon clusters (met-cars), clusters designed to mimic the chemistry of atoms, and reactivity of homo-nuclear metal clusters and ions with various reactant molecules. It is difficult to describe all the research the authors have accomplished as they have published 38 papers. In this report, they outline briefly the salient features of their work on the following topics: (1) Designer Clusters: Building Blocks for a New Class of Solids; (2) Atomic Structure, Stability, and Electronic Properties of Metallo-Carbohedrenes; (3) Reactivity of Metal Clusters with H{sub 2} and NO; and (4) Anomalous Spectroscopy of Li{sub 4} Clusters.

  2. Plastic-syringe induced silicone contamination in organic photovoltaic fabrication: implications for small-volume additives.

    PubMed

    Carr, John A; Nalwa, Kanwar S; Mahadevapuram, Rakesh; Chen, Yuqing; Anderegg, James; Chaudhary, Sumit

    2012-06-27

    Herein, the implications of silicone contamination found in solution-processed conjugated polymer solar cells are explored. Similar to a previous work based on molecular cells, we find this contamination as a result of the use of plastic syringes during fabrication. However, in contrast to the molecular case, we find that glass-syringe fabricated devices give superior performance than plastic-syringe fabricated devices in poly(3-hexylthiophene)-based cells. We find that the unintentional silicone addition alters the solution's wettability, which translates to a thinner, less absorbent film on spinning. With many groups studying the effects of small-volume additives, this work should be closely considered as many of these additives may also directly alter the solutions' wettability, or the amount of silicone dissolved off the plastic syringes, or both. Thereby, film thickness, which generally is not reported in detail, can vary significantly from device to device.

  3. Plastic-Syringe Induced Silicone Contamination in Organic Photvoltaic Fabrication: Implications for Small-Volume Additives

    SciTech Connect

    Carr, John A.; Nalwa, Kanwar S.; Mahadevapuram, Rakesh; Chen, Yuqing; Anderegg, James; Chaudhary, Sumit

    2012-05-15

    Herein, the implications of silicone contamination found in solution-processed conjugated polymer solar cells are explored. Similar to a previous work based on molecular cells, we find this contamination as a result of the use of plastic syringes during fabrication. However, in contrast to the molecular case, we find that glass-syringe fabricated devices give superior performance than plastic-syringe fabricated devices in poly(3-hexylthiophene)-based cells. We find that the unintentional silicone addition alters the solution’s wettability, which translates to a thinner, less absorbent film on spinning. With many groups studying the effects of small-volume additives, this work should be closely considered as many of these additives may also directly alter the solutions’ wettability, or the amount of silicone dissolved off the plastic syringes, or both. Thereby, film thickness, which generally is not reported in detail, can vary significantly from device to device.

  4. Properties of small Ar sub N-1 K/+/ ionic clusters

    NASA Technical Reports Server (NTRS)

    Etters, R. D.; Danilowicz, R.; Dugan, J.

    1977-01-01

    A self-consistent formalism is developed that, based upon a many-body potential, dynamically determines the thermodynamic properties of ionic clusters without an a priori designation of the equilibrium structures. Aggregates consisting of a single closed shell K(+) ion and N-1 isoelectronic argon atoms were studied. The clusters form crystallites at low temperatures, and melting transitions and spontaneous dissociations are indicated. The results confirm experimental evidence that shows that ionic clusters become less stable with increasing N. The crystallite structures formed by four different clusters are isosceles triangle, skewed form, octahedron with ion in the middle, and icosahedron with the ion in the middle.

  5. STABILITY OF SMALL SELF-INTERSTITIAL CLUSTERS IN TUNGSTEN

    SciTech Connect

    Setyawan, Wahyu; Nandipati, Giridhar; Kurtz, Richard J.

    2015-12-31

    Density functional theory was employed to explore the stability of interstitial clusters in W up to size seven. For each cluster size, the most stable configuration consists of parallel dumbbells. For clusters larger than size three, parallel dumbbells prefer to form in a multilayer fashion, instead of a planar structure. For size-7 clusters, the most stable configuration is a complete octahedron. The binding energy of a [111] dumbbell to the most stable cluster increases with cluster size, namely 2.49, 3.68, 4.76, 4.82, 5.47, and 6.85 eV for clusters of size 1, 2, 3, 4, 5, and 6, respectively. For a size-2 cluster, collinear dumbbells are still repulsive at the maximum allowable distance of 13.8 Å (the fifth neighbor along [111]). On the other hand, parallel dumbbells are strongly bound together. Two parallel dumbbells in which the axis-to-axis distance is within a cylindrical radius of 5.2 Å still exhibit a considerable binding of 0.28 eV. The most stable cluster in each size will be used to explore interactions with transmutation products.

  6. Structural motifs and stability of small argon-nitrogen clusters

    NASA Astrophysics Data System (ADS)

    Hewage, Jinasena W.; Amar, François G.

    2003-11-01

    The molecular dynamics (MD) simulation method is used to study Arm(N2)n clusters. Using realistic pair potentials for the argon-argon, nitrogen-nitrogen, and argon-nitrogen interactions, the structures and thermodynamics of these clusters are investigated. The initial focus of the study is the series of thirteen particle clusters of Arm(N2)13-m (0⩽m⩽13). These icosahedral argon-nitrogen clusters display systematic changes in energetics when argon is substituted by nitrogen in the central position. The relative stability of argon-centered clusters over nitrogen-centered clusters is further investigated by defining and calculating a "species-centric" order parameter which can be monitored during a MD simulation. These results are interpreted in terms of frustration effects due to anisotropy in the N2-N2 and N2-Ar potentials. The consequences of these observations for cluster stability and for dynamical behavior, such as melting and evaporation, are investigated. The dynamical studies of larger clusters reveal that the mixed clusters evolve towards a structure with an argon core coated by a nitrogen shell.

  7. The microstructure of dislocation clusters in industrial directionally solidified multicrystalline silicon

    SciTech Connect

    Kivambe, Maulid M.; Stokkan, Gaute; Ervik, Torunn; Lohne, Otto; Ryningen, Birgit

    2011-09-15

    The microstructure of commonly occurring dislocation patterns in industrial directionally solidified multicrystalline silicon has been systematically studied by light microscopy, electron backscatter diffraction, and transmission electron microscopy. The work has been focused on dislocation clusters on wafers near the top of cast blocks. In near {l_brace}111{r_brace} grain surface, dislocation arrays parallel to {l_brace}110{r_brace} plane traces are lying in parallel rows of {l_brace}111{r_brace} planes inclined to the surface, in mainly <112>30 deg. orientation. The dislocation configuration suggests that the microstructure may result from a recovery process. The dislocations formed during crystal growth and cooling have undergone transformations at high temperature in order to achieve low energy configurations for minimization of dislocation and crystal energy.

  8. Optimal design of small-diameter silicone chest drain devices.

    PubMed

    Chung, Juhyun; Li, John K-J

    2006-03-01

    To overcome the complications due to the use of noncompliant large diameter conventional chest drain devices, a flexible small diameter chest drain device was designed and simulated based on computational fluid dynamics (CFD) techniques. It was clearly shown that the pressure drop and velocity increase of the most distal drainage holes, which are located near the suction end, are dominant over other drainage holes. A conventional chest drain device with circular side holes showed higher mass flow rate due to larger cross sectional area. It also showed less dependency on the side hole placement compared to open channel, closed cavity chest drain with rectangular side holes. When all holes are opened the conventional chest drain showed 6% increase in flow rate while the open channel, closed cavity drain device showed 47% increase in flow rate reflecting a better design performance. These results provide an insight into the CFD-based optimal design of chest drain devices for potential applications in clinical intraoperative procedures.

  9. Small-Angle Neutron Scattering Measurements of Magnetic Cluster Sizes in Magnetic Recording Disks

    SciTech Connect

    Toney, Michael F

    2003-06-17

    We describe Small Angle Neutron Scattering measurements of the magnetic cluster size distributions for several longitudinal magnetic recording media. We find that the average magnetic cluster size is slightly larger than the average physical grain size, that there is a broad distribution of cluster sizes, and that the cluster size is inversely correlated to the media signal-to-noise ratio. These results show that intergranular magnetic coupling in these media is small and they provide empirical data for the cluster-size distribution that can be incorporated into models of magnetic recording.

  10. Nonlinear effects in infrared action spectroscopy of silicon and vanadium oxide clusters: experiment and kinetic modeling.

    PubMed

    Calvo, Florent; Li, Yejun; Kiawi, Denis M; Bakker, Joost M; Parneix, Pascal; Janssens, Ewald

    2015-10-21

    For structural assignment of gas phase compounds, infrared action spectra are usually compared to computed linear absorption spectra. However, action spectroscopy is highly nonlinear owing to the necessary transfer of the excitation energy and its subsequent redistribution leading to statistical ionization or dissociation. Here, we examine by joint experiment and dedicated modeling how such nonlinear effects affect the spectroscopic features in the case of selected inorganic clusters. Vibrational spectra of neutral silicon clusters are recorded by tunable IR-UV two-color ionization while IR spectra for cationic vanadium oxide clusters are obtained by IR multiphoton absorption followed by dissociation of the bare cluster or of its complex with Xe. Our kinetic modeling accounts for vibrational anharmonicities, for the laser interaction through photon absorption and stimulated emission rates, as well as for the relevant ionization or dissociation rates, all based on input parameters from quantum chemical calculations. Comparison of the measured and calculated spectra indicates an overall agreement as far as trends are concerned, except for the photodissociation of the V3O7(+)-Xe messenger complex, for which anharmonicities are too large and poorly captured by the perturbative anharmonic model. In all systems studied, nonlinear effects are essentially manifested by variations in the intensities as well as spectral broadenings. Differences in some band positions originate from inaccuracies of the quantum chemical data rather than specific nonlinear effects. The simulations further yield information on the average number of photons absorbed, which is otherwise unaccessible information: several to several tens of photons need to be absorbed to observe a band through dissociation, while three to five photons can be sufficient for detection of a band via IR-UV ionization.

  11. Vibration-rotation-tunneling dynamics in small water clusters

    SciTech Connect

    Pugliano, N.

    1992-11-01

    The goal of this work is to characterize the intermolecular vibrations of small water clusters. Using tunable far infrared laser absorption spectroscopy, large amplitude vibration-rotation-tunneling (VRT) dynamics in vibrationally excited states of the water dimer and the water trimer are investigated. This study begins with the measurement of 12 VRT subbands, consisting of approximately 230 transitions, which are assigned to an 82.6 cm[sup [minus]1] intermolecular vibration of the water dimer-d[sub 4]. Each of the VRT subbands originate from K[sub a][double prime]=0 and terminate in either K[sub a][prime]=0 or 1. These data provide a complete characterization of the tunneling dynamics in the vibrationally excited state as well as definitive symmetry labels for all VRT energy levels. Furthermore, an accurate value for the A[prime] rotational constant is found to agree well with its corresponding ground state value. All other excited state rotational constants are fitted, and discussed in terms of the corresponding ground state constants. In this vibration, the quantum tunneling motions are determined to exhibit large dependencies with both the K[sub a][prime] quantum number and the vibrational coordinate, as is evidenced by the measured tunneling splittings. The generalized internal-axis-method treatment which has been developed to model the tunneling dynamics, is considered for the qualitative description of each tunneling pathway, however, the variation of tunneling splittings with vibrational excitation indicate that the high barrier approximation does not appear to be applicable for this vibrational coordinate. The data are consistent with a motion possessing a[prime] symmetry, and the vibration is assigned as the [nu][sub 12] acceptor bending coordinate. This assignment is in agreement with the vibrational symmetry, the resultsof high level ab initio calculations, and preliminary data assigned to the analogous vibration in the D[sub 2]O-DOH isotopomer.

  12. Vibration-rotation-tunneling dynamics in small water clusters

    SciTech Connect

    Pugliano, N.

    1992-11-01

    The goal of this work is to characterize the intermolecular vibrations of small water clusters. Using tunable far infrared laser absorption spectroscopy, large amplitude vibration-rotation-tunneling (VRT) dynamics in vibrationally excited states of the water dimer and the water trimer are investigated. This study begins with the measurement of 12 VRT subbands, consisting of approximately 230 transitions, which are assigned to an 82.6 cm{sup {minus}1} intermolecular vibration of the water dimer-d{sub 4}. Each of the VRT subbands originate from K{sub a}{double_prime}=0 and terminate in either K{sub a}{prime}=0 or 1. These data provide a complete characterization of the tunneling dynamics in the vibrationally excited state as well as definitive symmetry labels for all VRT energy levels. Furthermore, an accurate value for the A{prime} rotational constant is found to agree well with its corresponding ground state value. All other excited state rotational constants are fitted, and discussed in terms of the corresponding ground state constants. In this vibration, the quantum tunneling motions are determined to exhibit large dependencies with both the K{sub a}{prime} quantum number and the vibrational coordinate, as is evidenced by the measured tunneling splittings. The generalized internal-axis-method treatment which has been developed to model the tunneling dynamics, is considered for the qualitative description of each tunneling pathway, however, the variation of tunneling splittings with vibrational excitation indicate that the high barrier approximation does not appear to be applicable for this vibrational coordinate. The data are consistent with a motion possessing a{prime} symmetry, and the vibration is assigned as the {nu}{sub 12} acceptor bending coordinate. This assignment is in agreement with the vibrational symmetry, the resultsof high level ab initio calculations, and preliminary data assigned to the analogous vibration in the D{sub 2}O-DOH isotopomer.

  13. Computer simulation structure and vibrations of small metal cluster on the Cu (111) surface

    NASA Astrophysics Data System (ADS)

    Borisova, Svetlana D.; Rusina, Galina G.

    2015-10-01

    Vibrational properties of the small tetrahedral cluster of Co on the Cu (111) surface are studied by using tight-binding second moment approximation interatomic interaction potentials. It was shown that interaction of the clusters with substrate leads to arising of frustrated translation and frustrated rotation in-plane polarized vibrational modes localized on the cluster atoms. The Co4 cluster on the surface the high frequency modes remain strongly localized and mixed with the nearest neighbor atoms vibrations.

  14. Computer simulation structure and vibrations of small metal cluster on the Cu (111) surface

    SciTech Connect

    Borisova, Svetlana D. Rusina, Galina G.

    2015-10-27

    Vibrational properties of the small tetrahedral cluster of Co on the Cu (111) surface are studied by using tight-binding second moment approximation interatomic interaction potentials. It was shown that interaction of the clusters with substrate leads to arising of frustrated translation and frustrated rotation in-plane polarized vibrational modes localized on the cluster atoms. The Co{sub 4} cluster on the surface the high frequency modes remain strongly localized and mixed with the nearest neighbor atoms vibrations.

  15. Reactivity of small transition-metal clusters with CO

    NASA Astrophysics Data System (ADS)

    Andersson, Mats T.; Gronbeck, H.; Holmgren, L.; Rosen, Arne

    1995-09-01

    The size-dependent reactivity of several transition-metal clusters: Con, Nbn, Rhn, and Wn with CO has been investigated in a cluster beam experiment. The reactions occur at single-collision-like conditions and the results are evaluated in terms of the reaction probability (S) in a collision. For all the four metals, clusters with more than 10 - 15 atoms show a high reaction probability, S >= 0.4, rather independent of size. For smaller Nbn and Wn, the reaction probability is lower, and for Nbn, large variations in the CO reactivity are observed in the n equals 8 - 13 range with a distinct minimum at Nb10. Using an LCAO approach within the local spin density approximation (LSDA) the adsorption of molecular CO on Nbn has also been investigated theoretically. The geometries of the bare clusters were optimized and two different sites for CO were investigated. The discussion is based on a detailed analysis of Nb4. The calculations show that compact structures with high coordination numbers are the most stable ones for the bare Nb clusters and hollow sites, also maximizing the coordination, are preferred for CO adsorption. The calculations indicate that a high CO-Nbn bond strength is obtained for clusters with a high density of states close to the Fermi level and for which the HOMO level has a symmetry that allows for an efficient back-donation of electrons to the 2(pi) *-orbital of CO. A particularly low chemisorption energy was calculated for the Nb10 cluster.

  16. Theoretical studies of the electronic structure of small metal clusters

    NASA Technical Reports Server (NTRS)

    Jordan, K. D.

    1982-01-01

    Theoretical studies of the electronic structure of metal clusters, in particular clusters of Group IIA and IIB atoms were conducted. Early in the project it became clear that electron correlation involving d orbitals plays a more important role in the binding of these clusters than had been previously anticipated. This necessitated that computer codes for calculating two electron integrals and for constructing the resulting CI Hamiltonions be replaced with newer, more efficient procedures. Program modification, interfacing and testing were performed. Results of both plans are reported.

  17. Thick Silicon Double-Sided Strip Detectors for Low-Energy Small-Animal SPECT

    PubMed Central

    Shokouhi, Sepideh; McDonald, Benjamin S.; Durko, Heather L.; Fritz, Mark A.; Furenlid, Lars R.; Peterson, Todd E.

    2010-01-01

    This work presents characterization studies of thick silicon double-sided strip detectors for a high-resolution small-animal SPECT. The dimension of these detectors is 60.4 mm × 60.4 mm × 1 mm. There are 1024 strips on each side that give the coordinates of the photon interaction, with each strip processed by a separate ASIC channel. Our measurement shows that intrinsic spatial resolution equivalent to the 59 μm strip pitch is attainable. Good trigger uniformity can be achieved by proper setting of a 4-bit DAC in each ASIC channel to remove trigger threshold variations. This is particularly important for triggering at low energies. The thick silicon DSSD (Double-sided strip detector) shows high potential for small-animal SPECT. PMID:20686626

  18. Evaluation of cluster recovery for small area relative risk models.

    PubMed

    Rotejanaprasert, Chawarat

    2014-12-01

    The analysis of disease risk is often considered via relative risk. The comparison of relative risk estimation methods with "true risk" scenarios has been considered on various occasions. However, there has been little examination of how well competing methods perform when the focus is clustering of risk. In this paper, a simulated evaluation of a range of potential spatial risk models and a range of measures that can be used for (a) cluster goodness of fit, (b) cluster diagnostics are considered. Results suggest that exceedence probability is a poor measure of hot spot clustering because of model dependence, whereas residual-based methods are less model dependent and perform better. Local deviance information criteria measures perform well, but conditional predictive ordinate measures yield a high false positive rate.

  19. United we stand: big roles for small RNA gene clusters.

    PubMed

    Felden, Brice; Paillard, Luc

    2017-02-01

    Prokaryotes and eukaryotes evolved relatively similar RNA-based molecular mechanisms to fight potentially deleterious nucleic acids coming from phages, transposons, or viruses. Short RNAs guide effector complexes toward their targets to be silenced or eliminated. These short immunity RNAs are transcribed from clustered loci. Unexpectedly and strikingly, bacterial and eukaryotic immunity RNA clusters share substantial functional and mechanistic resemblances in fighting nucleic acid intruders.

  20. Electronic and magnetic properties of small rhodium clusters

    SciTech Connect

    Soon, Yee Yeen; Yoon, Tiem Leong; Lim, Thong Leng

    2015-04-24

    We report a theoretical study of the electronic and magnetic properties of rhodium-atomic clusters. The lowest energy structures at the semi-empirical level of rhodium clusters are first obtained from a novel global-minimum search algorithm, known as PTMBHGA, where Gupta potential is used to describe the atomic interaction among the rhodium atoms. The structures are then re-optimized at the density functional theory (DFT) level with exchange-correlation energy approximated by Perdew-Burke-Ernzerhof generalized gradient approximation. For the purpose of calculating the magnetic moment of a given cluster, we calculate the optimized structure as a function of the spin multiplicity within the DFT framework. The resultant magnetic moments with the lowest energies so obtained allow us to work out the magnetic moment as a function of cluster size. Rhodium atomic clusters are found to display a unique variation in the magnetic moment as the cluster size varies. However, Rh{sub 4} and Rh{sub 6} are found to be nonmagnetic. Electronic structures of the magnetic ground-state structures are also investigated within the DFT framework. The results are compared against those based on different theoretical approaches available in the literature.

  1. Dynamics and structural changes of small water clusters on ionization.

    PubMed

    Lee, Han Myoung; Kim, Kwang S

    2013-07-05

    Despite utmost importance in understanding water ionization process, reliable theoretical results of structural changes and molecular dynamics (MD) of water clusters on ionization have hardly been reported yet. Here, we investigate the water cations [(H2O)(n = 2-6)(+)] with density functional theory (DFT), Möller-Plesset second-order perturbation theory (MP2), and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. The complete basis set limits of interaction energies at the CCSD(T) level are reported, and the geometrical structures, electronic properties, and infrared spectra are investigated. The characteristics of structures and spectra of the water cluster cations reflect the formation of the hydronium cation moiety (H3O(+)) and the hydroxyl radical. Although most density functionals fail to predict reasonable energetics of the water cations, some functionals are found to be reliable, in reasonable agreement with high-level ab initio results. To understand the ionization process of water clusters, DFT- and MP2-based Born-Oppenheimer MD (BOMD) simulations are performed on ionization. On ionization, the water clusters tend to have an Eigen-like form with the hydronium cation instead of a Zundel-like form, based on reliable BOMD simulations. For the vertically ionized water hexamer, the relatively stable (H2O)5(+) (5sL4A) cluster tends to form with a detached water molecule (H2O).

  2. Formation and evolution of oxygen-vacancy clusters in lead and tin doped silicon

    NASA Astrophysics Data System (ADS)

    Londos, C. A.; Aliprantis, D.; Sgourou, E. N.; Chroneos, A.; Pochet, P.

    2012-06-01

    Infrared spectroscopy (IR) measurements were used to investigate the effect of lead (Pb), tin (Sn), and (Pb, Sn) codoping on electron radiation-induced defects in silicon (Si). The study was mainly focused on oxygen-vacancy (VOn) clusters and in particular their formation and evolution upon annealing. It was determined that Pb causes a larger reduction in the production of the VO defect than Sn. In (Pb, Sn) co-doped Si isochronal anneals revealed that the evolution of VO increases substantially at ˜170 °C. This is attributed to the release of V from the SnV pair. Interestingly, in the corresponding evolution curves of VO in the Sn- and the Pb-doped samples, this inverse annealing stage is also present for the former while it is not present for the latter. This is attributed to the formation of PbV pairs that do not dissociate below 280 °C. The partial capture of V by Sn in co-doped samples is rationalized through the higher compressive local strain around Pb atoms that leads to a retardation of vacancy diffusion. The conversion of VO to the VO2 defect is substantially reduced in the Pb-doped sample. The evolution curves of VO and VO2 clusters in the isovalent doped Si samples hint the production of VO2 from other mechanisms (i.e., besides VO + Oi → VO2). For larger VOn clusters (n = 3,4), the signals are very weak in the Pb-doped sample, whereas for n ≥ 5, they are not present in the spectra. Conversely, bands related with the VO5 and VOnCs defects are present in the spectra of the Sn-doped and (Pb, Sn) codoped Si.

  3. Laser Spectroscopy of Small Mass Selected Metal Clusters and Complexes

    NASA Astrophysics Data System (ADS)

    Robbins, David Lee

    1995-01-01

    Analysis and characterization of novel complexes, especially those exhibiting weak bonding, is of fundamental and applied importance. Van der Waals forces and other weak bonding forces have attracted much attention recently due to recognition of their broad impact. Weak forces control solvent-solute interaction, govern reaction channels at large internuclear separations, and play key roles in the geometry and activity of biological macromolecules. Direct study of these phenomena is extremely complex, but much may be learned by the study of small and simplified systems. The most simplified solvation system conceivable consists of a single solute solvated by one solvent ligand. Production and isolation of such weakly bound complexes is difficult owing to their instability and short lifetime in ordinary conditions. Advances in several technologies over the past two decades have facilitated their study. Two of the most notable advancements have been in molecular beam and laser technologies. These advanced technologies as well as fundamental physical chemistry techniques are exploited for the production, isolation, and study of silver dimer solvated by single rare gas adducts, the magnesium cation solvated by one nitrogen molecule, and the unsolvated bimetallic, AlAg. Resonant photoionization and photodissociation techniques are used to explore the potential energy surfaces of these complexes. These studies reveal electronic, vibrational, and rotational energy levels which allow the determination of structure and dissociation energies. Success in these investigations is due largely to the metals chosen. Both silver and magnesium have a low density of electronic states, strong accessible electronic transitions, and are amenable to molecular beam experiments. For similar reasons theoretical investigations of these or similar systems have recently been undertaken and are compared to the experimental work. Both of the van der Waals studies are unique. Neither complex had

  4. Track finding in silicon trackers with a small number of layers

    NASA Astrophysics Data System (ADS)

    Frühwirth, Rudolf; Glattauer, Robin; Lettenbichler, Jakob; Mitaroff, Winfried; Nadler, Moritz

    2013-12-01

    We present software based on novel techniques, aiming at track finding in silicon trackers with a small number of layers. The core algorithm is a cellular automaton, followed by a Kalman filter and a Hopfield neural network. The first of two test cases is the forward tracking detector (FTD) of the International Large Detector (ILD) at a future linear collider, which covers the forward and backward regions between beam tube and a TPC. It consists of seven disk-shaped silicon detectors (pixels and strips) on either side. Results presented on simulated events without and with background show that our method performs better than a previous one in terms of efficiency, ghost rate and processing speed. The second test case is the silicon vertex detector (SVD) of the Belle II experiment at the B factory at KEK, which is a new device located between a vertex pixel detector and a central drift chamber. It consists of only four cylindrical layers of silicon strip sensors. The focus of this study is on the reconstruction of tracks with very low momentum that miss the surrounding drift chamber. We present results from simulated data, including ghost hits and hits from the machine background.

  5. Performance evaluation of a very high resolution small animal PET imager using silicon scatter detectors

    NASA Astrophysics Data System (ADS)

    Park, Sang-June; Rogers, W. Leslie; Huh, Sam; Kagan, Harris; Honscheid, Klaus; Burdette, Don; Chesi, Enrico; Lacasta, Carlos; Llosa, Gabriela; Mikuz, Marko; Studen, Andrej; Weilhammer, Peter; Clinthorne, Neal H.

    2007-05-01

    A very high resolution positron emission tomography (PET) scanner for small animal imaging based on the idea of inserting a ring of high-granularity solid-state detectors into a conventional PET scanner is under investigation. A particularly interesting configuration of this concept, which takes the form of a degenerate Compton camera, is shown capable of providing sub-millimeter resolution with good sensitivity. We present a Compton PET system and estimate its performance using a proof-of-concept prototype. A prototype single-slice imaging instrument was constructed with two silicon detectors 1 mm thick, each having 512 1.4 mm × 1.4 mm pads arranged in a 32 × 16 array. The silicon detectors were located edgewise on opposite sides and flanked by two non-position sensitive BGO detectors. The scanner performance was measured for its sensitivity, energy, timing, spatial resolution and resolution uniformity. Using the experimental scanner, energy resolution for the silicon detectors is 1%. However, system energy resolution is dominated by the 23% FWHM BGO resolution. Timing resolution for silicon is 82.1 ns FWHM due to time-walk in trigger devices. Using the scattered photons, time resolution between the BGO detectors is 19.4 ns FWHM. Image resolution of 980 µm FWHM at the center of the field-of-view (FOV) is obtained from a 1D profile of a 0.254 mm diameter 18F line source image reconstructed using the conventional 2D filtered back-projection (FBP). The 0.4 mm gap between two line sources is resolved in the image reconstructed with both FBP and the maximum likelihood expectation maximization (ML-EM) algorithm. The experimental instrument demonstrates sub-millimeter resolution. A prototype having sensitivity high enough for initial small animal images can be used for in vivo studies of small animal models of metabolism, molecular mechanism and the development of new radiotracers.

  6. Optical response of small closed-shell sodium clusters.

    PubMed

    Pal, George; Lefkidis, Georgios; Schneider, Hans Christian; Hübner, Wolfgang

    2010-10-21

    Absorption spectra of closed-shell Na(2), Na(3) (+), Na(4), Na(5) (+), Na(6), Na(7) (+), and Na(8) clusters are calculated using a complex Bethe-Salpeter equation derived using a conserving linear response method. In the framework of a quasiparticle approach, we determine electron-hole correlations in the presence of an external field. The calculated results are in excellent agreement with experimental spectra, and some possible cluster geometries that occur in experiments are analyzed. The position and the broadening of the resonances in the spectra arise from a consistent treatment of the scattering and dephasing contributions in the linear response calculation. Comparison between the experimental and the theoretical results yields information about the cluster geometry, which is not accessible experimentally.

  7. Small Al clusters on the Cu(111) surface: Atomic relaxation and vibrational properties

    NASA Astrophysics Data System (ADS)

    Rusina, G. G.; Borisova, S. D.; Chulkov, E. V.

    2010-11-01

    The relaxation and vibrational properties of both Al clusters and the (111) surface of a copper sub-strate were studied using the interatomic interaction potentials obtained in a tight-binding approximation. The presence of small aluminum clusters led to modification of the vibrational states of the substrate, a shift of the Rayleigh mode, and excitation of new Z-polarized modes. Hybridized modes localized on the cluster adatoms and the neighboring atoms of the substrate were found in the phonon spectrum. The localized dipole-active modes of the cluster and their strong hybridization with vibrations of the substrate points to desorption stability of the tri- and heptaatomic clusters.

  8. Decay processes and radiative cooling of small anionic copper clusters

    NASA Astrophysics Data System (ADS)

    Breitenfeldt, Christian; Blaum, Klaus; Froese, Michael W.; George, Sebastian; Guzmán-Ramírez, Gregorio; Lange, Michael; Menk, Sebastian; Schweikhard, Lutz; Wolf, Andreas

    2016-09-01

    The decay of copper clusters Cun- with size n =4 -7 , produced in a metal ion sputter source, was studied in an electrostatic ion-beam trap. The neutral products after electron emission and fragmentation were monitored for ion storage times of up to a second. The observations indicated the presence of radiative cooling. The energy distributions of the remaining clusters were probed by laser irradiation up to several further seconds of storage time. This defined excitation lead to photoinduced decay signals which, again, showed signs of radiative cooling for Cu6,7 -, not, however, for Cu4,5 -.

  9. A 2D silicon detector array for quality assurance in small field dosimetry: DUO.

    PubMed

    Shukaili, Khalsa Al; Petasecca, Marco; Newall, Matthew; Espinoza, Anthony; Perevertaylo, Vladimir L; Corde, Stéphanie; Lerch, Michael; Rosenfeld, Anatoly B

    2017-02-01

    Nowadays, there are many different applications that use small fields in radiotherapy treatments. The dosimetry of small radiation fields is not trivial due to the problems associated with lateral disequilibrium and source occlusion and requires reliable quality assurance (QA). Ideally such a QA tool should provide high spatial resolution, minimal beam perturbation and real time fast measurements. Many different types of silicon diode arrays are used for QA in radiotherapy; however, their application in small filed dosimetry is limited, in part, due to a lack of spatial resolution. The Center of Medical Radiation Physics (CMRP) has developed a new generation of a monolithic silicon diode array detector that will be useful for small field dosimetry in SRS/SRT. The objective of this study is to characterize a monolithic silicon diode array designed for dosimetry QA in SRS/SRT named DUO that is arranged as two orthogonal 1D arrays with 0.2 mm pitch. DUO is two orthogonal 1D silicon detector arrays in a monolithic crystal. Each orthogonal array contains 253 small pixels with size 0.04 × 0.8 mm(2) and three central pixels are with a size of 0.18 × 0.18 mm(2) each. The detector pitch is 0.2 mm and total active area is 52 × 52 mm(2) . The response of the DUO silicon detector was characterized in terms of dose per pulse, percentage depth dose, and spatial resolution in a radiation field incorporating high gradients. Beam profile of small fields and output factors measured on a Varian 2100EX LINAC in a 6 MV radiation fields of square dimensions and sized from 0.5 × 0.5 cm(2) to 5 × 5 cm(2) . The DUO response was compared under the same conditions with EBT3 films and an ionization chamber. The DUO detector shows a dose per pulse dependence of 5% for a range of dose rates from 2.7 × 10(-4) to 1.2 × 10(-4) Gy/pulse and 23% when the rate is further reduced to 2.8 × 10(-5) Gy/pulse. The percentage depth dose measured to 25 cm depth in solid water phantom beyond the

  10. Magic Numbers in Small Iron Clusters: A First-Principles Study

    SciTech Connect

    Kim, Eunja; Mohrland, Andrew B.; Weck, Philippe F.; Pang, Tao; Czerwinski, Kenneth R.; Tomanek, David

    2014-10-03

    We perform ab initio spin-polarized density functional calculations of Fen aggregates with n ≤ 17 atoms to reveal the origin of the observed magic numbers, which indicate particularly high stability of clusters with 7, 13 and 15 atoms. Our results clarify the controversy regarding the ground state geometry of clusters such as Fe5and indicate that magnetism plays an important role in determining the stability and magic numbers in small iron clusters.

  11. Formation of structure in small lead clusters under thermal effect

    NASA Astrophysics Data System (ADS)

    Baidyshev, V. S.; Gafner, Yu. Ya.

    2016-12-01

    The thermal effect on lead clusters with radii up to 5.5 nm has been investigated by the molecular dynamics method using a modified tight-binding potential TB-SMA. The melting of Pb nanoparticles of these sizes is strictly homogeneous, without the formation of a surface liquidlike layer. The primary fcc phase in the particles is retained upon heating in the overwhelming majority of model experiments. An analysis of the structure formation during crystallization has shown that structures with pentagonal symmetry are preferred for lead clusters in this case. It is noted that an increase in the nanoparticle size leads to the dominance of the dodecahedral structure over the icosahedral one.

  12. Relaxation dynamics in small clusters: A modified Monte Carlo approach

    SciTech Connect

    Pal, Barnana

    2008-02-01

    Relaxation dynamics in two-dimensional atomic clusters consisting of mono-atomic particles interacting through Lennard-Jones (L-J) potential has been investigated using Monte Carlo simulation. A modification of the conventional Metropolis algorithm is proposed to introduce realistic thermal motion of the particles moving in the interacting L-J potential field. The proposed algorithm leads to a quick equilibration from the nonequilibrium cluster configuration in a certain temperature regime, where the relaxation time ({tau}), measured in terms of Monte Carlo Steps (MCS) per particle, vary inversely with the square root of system temperature ({radical}T) and pressure (P); {tau} {proportional_to} (P{radical}T){sup -1}. From this a realistic correlation between MCS and time has been predicted.

  13. Energy efficiency enhancements for semiconductors, communications, sensors and software achieved in cool silicon cluster project

    NASA Astrophysics Data System (ADS)

    Ellinger, Frank; Mikolajick, Thomas; Fettweis, Gerhard; Hentschel, Dieter; Kolodinski, Sabine; Warnecke, Helmut; Reppe, Thomas; Tzschoppe, Christoph; Dohl, Jan; Carta, Corrado; Fritsche, David; Tretter, Gregor; Wiatr, Maciej; Detlef Kronholz, Stefan; Mikalo, Ricardo Pablo; Heinrich, Harald; Paulo, Robert; Wolf, Robert; Hübner, Johannes; Waltsgott, Johannes; Meißner, Klaus; Richter, Robert; Michler, Oliver; Bausinger, Markus; Mehlich, Heiko; Hahmann, Martin; Möller, Henning; Wiemer, Maik; Holland, Hans-Jürgen; Gärtner, Roberto; Schubert, Stefan; Richter, Alexander; Strobel, Axel; Fehske, Albrecht; Cech, Sebastian; Aßmann, Uwe; Pawlak, Andreas; Schröter, Michael; Finger, Wolfgang; Schumann, Stefan; Höppner, Sebastian; Walter, Dennis; Eisenreich, Holger; Schüffny, René

    2013-07-01

    An overview about the German cluster project Cool Silicon aiming at increasing the energy efficiency for semiconductors, communications, sensors and software is presented. Examples for achievements are: 1000 times reduced gate leakage in transistors using high-fc (HKMG) materials compared to conventional poly-gate (SiON) devices at the same technology node; 700 V transistors integrated in standard 0.35 μm CMOS; solar cell efficiencies above 19% at < 200 W/m2 irradiation; 0.99 power factor, 87% efficiency and 0.088 distortion factor for dc supplies; 1 ns synchronization resolution via Ethernet; database accelerators allowing 85% energy savings for servers; adaptive software yielding energy reduction of 73% for e-Commerce applications; processors and corresponding data links with 40% and 70% energy savings, respectively, by adaption of clock frequency and supply voltage in less than 20 ns; clock generator chip with tunable frequency from 83-666 MHz and 0.62-1.6 mW dc power; 90 Gb/s on-chip link over 6 mm and efficiency of 174 fJ/mm; dynamic biasing system doubling efficiency in power amplifiers; 60 GHz BiCMOS frontends with dc power to bandwidth ratio of 0.17 mW/MHz; driver assistance systems reducing energy consumption by 10% in cars Contribution to the Topical Issue “International Semiconductor Conference Dresden-Grenoble - ISCDG 2012”, Edited by Gérard Ghibaudo, Francis Balestra and Simon Deleonibus.

  14. Close proximity electrostatic effect from small clusters of emitters

    NASA Astrophysics Data System (ADS)

    Dall’Agnol, Fernando F.; de Assis, Thiago A.

    2017-10-01

    Using a numerical simulation based on the finite-element technique, this work investigates the field emission properties from clusters of a few emitters at close proximity, by analyzing the properties of the maximum local field enhancement factor (γm ) and the corresponding emission current. At short distances between the emitters, we show the existence of a nonintuitive behavior, which consists of the increasing of γm as the distance c between the emitters decreases. Here we investigate this phenomenon for clusters with 2, 3, 4 and 7 identical emitters and study the influence of the proximity effect in the emission current, considering the role of the aspect ratio of the individual emitters. Importantly, our results show that peripheral emitters with high aspect-ratios in large clusters can, in principle, significantly increase the emitted current as a consequence only of the close proximity electrostatic effect (CPEE). This phenomenon can be seen as a physical mechanism to produce self-oscillations of individual emitters. We discuss new insights for understanding the nature of self-oscillations in emitters based on the CPEE, including applications to nanometric oscillators.

  15. Trapping and diffusion kinetic of hydrogen in carbon-cluster ion-implantation projected range in Czochralski silicon wafers

    NASA Astrophysics Data System (ADS)

    Okuyama, Ryosuke; Masada, Ayumi; Kadono, Takeshi; Hirose, Ryo; Koga, Yoshihiro; Okuda, Hidehiko; Kurita, Kazunari

    2017-02-01

    We investigated the diffusion behavior of hydrogen in a silicon wafer made by a carbon-cluster ion-implantation technique after heat treatment and silicon epitaxial growth. A hydrogen peak was observed after high-temperature heat treatment (>1000 °C) and silicon epitaxial growth by secondary ion mass spectrometry analysis. We also confirmed that the hydrogen peak concentration decreased after epitaxial growth upon additional heat treatment. Such a hydrogen diffusion behavior has not been reported. Thus, we derived the activation energy from the projected range of a carbon cluster, assuming only a dissociation reaction, and obtained an activation energy of 0.76 ± 0.04 eV. This value is extremely close to that for the diffusion of hydrogen molecules located at the tetrahedral interstitial site and hydrogen molecules dissociated from multivacancies. Therefore, we assume that the hydrogen in the carbon-cluster projected range diffuses in the molecular state, and hydrogen remaining in the projected range forms complexes of carbon, oxygen, and vacancies.

  16. Electronic structure transformation in small bare Au clusters as seen by x-ray photoelectron spectroscopy

    NASA Astrophysics Data System (ADS)

    Andersson, T.; Zhang, C.; Björneholm, O.; Mikkelä, M.-H.; Jänkälä, K.; Anin, D.; Urpelainen, S.; Huttula, M.; Tchaplyguine, M.

    2017-01-01

    Free bare gold clusters in the size range from few tens to few hundred atoms (≤1 nm dimensions) have been produced in a beam, and the size-dependent development of their full valence band including the 5d and 6s parts has been mapped ‘on the fly’ by synchrotron-based photoelectron spectroscopy. The Au 4f core level has been also probed, and the cluster-specific Au 4f ionization energies have been used to estimate the cluster size. The recorded in the present work valence spectra of the small clusters are compared with the spectra of the large clusters ( N ∼ 103) created by us using a magnetron-based gas aggregation source. The comparison shows a substantially narrower 5d valence band and the decrease in its splitting for gold clusters in the size range of few hundred atoms and below. Our DFT calculations involving the pseudopotential method show that the 5d band width of the ground state increases with the cluster size and by the size N = 20 becomes comparable with the experimental width of the valence photoelectron spectrum. Similar to the earlier observations on supported clusters we interpret our experimental and theoretical results as due to the undercoordination of a large fraction of atoms in the clusters with N ∼ 102 and below. The consequences of such electronic structure of small gold clusters are discussed in connection with their specific physical and chemical properties related to nanoplasmonics and nanocatalysis.

  17. The role of micro size computing clusters for small physics groups

    NASA Astrophysics Data System (ADS)

    Shevel, A. Y.

    2014-06-01

    A small physics group (3-15 persons) might use a number of computing facilities for the analysis/simulation, developing/testing, teaching. It is discussed different types of computing facilities: collaboration computing facilities, group local computing cluster (including colocation), cloud computing. The author discuss the growing variety of different computing options for small groups and does emphasize the role of the group owned computing cluster of micro size.

  18. Differential adsorption of small molecules in spatially functionalized porous silicon nanostructures.

    PubMed

    Wu, Chia-Chen; Chen, Michelle Y; Sailor, Michael J

    2013-09-17

    An experimental approach to rapidly quantify the relative affinity of a small molecule analyte for two different surfaces is described. The method uses optical measurements of high surface area porous Si thin films that contain two spatially distinct surface chemistries. The chemistries are placed on the walls of the ∼10 nm diameter pores of the porous Si film by means of microdroplet patterning, where a chemical resist is drop-coated on the porous Si sample to define distinct regions across the plane of the chip. In this work, the two chemistries consist of a hydrophilic silicon oxide surface and a hydrophobic methyl-terminated silicon surface. Detection is achieved by simultaneous optical reflectance measurements of both regions, where the reflectance spectrum contains a convolution of the Fabry-Pérot interference spectrum of both the oxide and the methyl-grafted layers. The differential partitioning of a test analyte (2-acetoxybenzoic acid or diphenyl ether) from aqueous solution is determined from the Fourier transform of the optical interference spectrum. The approach is rapid and nondestructive, and it can be performed on a small sample volume as a means to quantify the partition behavior of small molecules.

  19. Clustering of water molecules in ultramicroporous carbon: In-situ small-angle neutron scattering

    SciTech Connect

    Bahadur, Jitendra; Contescu, Cristian I.; Rai, Durgesh K.; Gallego, Nidia C.; Melnichenko, Yuri B.

    2016-10-19

    The adsorption of water is central to most of the applications of microporous carbon as adsorbent material. We report early kinetics of water adsorption in the microporous carbon using in-situ small-angle neutron scattering. It is observed that adsorption of water occurs via cluster formation of molecules. Interestingly, the cluster size remains constant throughout the adsorption process whereas number density of clusters increases with time. The role of surface chemistry of microporous carbon on the early kinetics of adsorption process was also investigated. Lastly, the present study provides direct experimental evidence for cluster assisted adsorption of water molecules in microporous carbon (Do-Do model).

  20. Grain growth of cast-multicrystalline silicon grown from small randomly oriented seed crystal

    NASA Astrophysics Data System (ADS)

    Prakash, Ronit R.; Sekiguchi, Takashi; Jiptner, Karolin; Miyamura, Yoshiji; Chen, Jun; Harada, Hirofumi; Kakimoto, Koichi

    2014-09-01

    Multicrystalline silicon was grown from seeds with small grains of random orientation and the growth mechanism was studied with respect to grain size, shape, boundary character and orientation. The average grain size perpendicular to growth direction increased steadily initially, became constant and then increased steadily again. Grain size parallel to growth direction increased rapidly with growth due to grain elongation in the growth direction. Grain shape with respect to growth direction changed from spherical to columnar with growth. Initially non-CSL grain boundary fraction was very high but decreased with growth as the Σ3 grain boundary fraction increased. A simple model was proposed to explain the results.

  1. Small copper clusters studied by x-ray absorption near-edge structure

    NASA Astrophysics Data System (ADS)

    Oyanagi, H.; Sun, Z. H.; Jiang, Y.; Uehara, M.; Nakamura, H.; Yamashita, K.; Orimoto, Y.; Zhang, L.; Lee, C.; Fukano, A.; Maeda, H.

    2012-04-01

    The local structure of copper nanoparticles grown in organic solution by reducing Cu(II) hexafluoroacetylacetonate [Cu(hfac)2] was studied as-grown by the Cu K-edge x-ray absorption near-edge structure (XANES). Comparison of the experimental XANES spectra with reference materials indicated small copper clusters are formed by ligand-exchange with oleylamine and subsequent reducing by diphenylsilane. The multiple-scattering (MS) calculation for various model clusters consisting of 13-135 atoms suggests that small (13-19 atom) Cu clusters are stabilized without a large deformation.

  2. A prototype of very high resolution small animal PET scanner using silicon pad detectors

    PubMed Central

    Park, Sang-June; Leslie Rogers, W.; Huh, Sam; Kagan, Harris; Honscheid, Klaus; Burdette, Don; Chesi, Enrico; Lacasta, Carlos; Llosa, Gabriela; Mikuz, Marko; Studen, Andrej; Weilhammer, Peter; Clinthorne, Neal H.

    2007-01-01

    A very high resolution small animal positron emission tomograph (PET) which can achieve sub-millimeter spatial resolution is being developed using silicon pad detectors. The prototype PET for a single slice instrument consists of two 1 mm thick silicon pad detectors, each containing a 32 × 16 array of 1.4 mm × 1.4 mm pads read out with four VATAGP3 chips which have 128 channels low-noise self triggering ASIC in each chip, coincidence units, a source turntable and tungsten slice collimator. The silicon detectors were located edgewise on opposite sides of a 4 cm field-of-view to maximize efficiency. Energy resolution is dominated by electronic noise, which is 0.98% (1.38 keV) FWHM at 140.5 keV. Coincidence timing resolution is 82.1 ns FWHM and coincidence efficiency was measured to be 1.04 × 10-3 % from two silicon detectors with annihilation photons of 18F source Image data were acquired and reconstructed using conventional 2-D filtered-back projection (FBP) and a maximum likelihood expectation maximization (ML-EM) method. Image resolution of approximately 1.45 mm FWHM is obtained from 1-D profile of 1.1 mm diameter 18F line source image. Even better resolution can be obtained with smaller detector element sizes. While many challenges remain in scaling up the instrument to useful efficiency including densely packed detectors and significantly improved timing resolution, performance of the test setup in terms of easily achieving submillimeter resolution is compelling. PMID:18084629

  3. Clustering Methods with Qualitative Data: a Mixed-Methods Approach for Prevention Research with Small Samples.

    PubMed

    Henry, David; Dymnicki, Allison B; Mohatt, Nathaniel; Allen, James; Kelly, James G

    2015-10-01

    Qualitative methods potentially add depth to prevention research but can produce large amounts of complex data even with small samples. Studies conducted with culturally distinct samples often produce voluminous qualitative data but may lack sufficient sample sizes for sophisticated quantitative analysis. Currently lacking in mixed-methods research are methods allowing for more fully integrating qualitative and quantitative analysis techniques. Cluster analysis can be applied to coded qualitative data to clarify the findings of prevention studies by aiding efforts to reveal such things as the motives of participants for their actions and the reasons behind counterintuitive findings. By clustering groups of participants with similar profiles of codes in a quantitative analysis, cluster analysis can serve as a key component in mixed-methods research. This article reports two studies. In the first study, we conduct simulations to test the accuracy of cluster assignment using three different clustering methods with binary data as produced when coding qualitative interviews. Results indicated that hierarchical clustering, K-means clustering, and latent class analysis produced similar levels of accuracy with binary data and that the accuracy of these methods did not decrease with samples as small as 50. Whereas the first study explores the feasibility of using common clustering methods with binary data, the second study provides a "real-world" example using data from a qualitative study of community leadership connected with a drug abuse prevention project. We discuss the implications of this approach for conducting prevention research, especially with small samples and culturally distinct communities.

  4. Clustering Methods with Qualitative Data: A Mixed Methods Approach for Prevention Research with Small Samples

    PubMed Central

    Henry, David; Dymnicki, Allison B.; Mohatt, Nathaniel; Allen, James; Kelly, James G.

    2016-01-01

    Qualitative methods potentially add depth to prevention research, but can produce large amounts of complex data even with small samples. Studies conducted with culturally distinct samples often produce voluminous qualitative data, but may lack sufficient sample sizes for sophisticated quantitative analysis. Currently lacking in mixed methods research are methods allowing for more fully integrating qualitative and quantitative analysis techniques. Cluster analysis can be applied to coded qualitative data to clarify the findings of prevention studies by aiding efforts to reveal such things as the motives of participants for their actions and the reasons behind counterintuitive findings. By clustering groups of participants with similar profiles of codes in a quantitative analysis, cluster analysis can serve as a key component in mixed methods research. This article reports two studies. In the first study, we conduct simulations to test the accuracy of cluster assignment using three different clustering methods with binary data as produced when coding qualitative interviews. Results indicated that hierarchical clustering, K-Means clustering, and latent class analysis produced similar levels of accuracy with binary data, and that the accuracy of these methods did not decrease with samples as small as 50. Whereas the first study explores the feasibility of using common clustering methods with binary data, the second study provides a “real-world” example using data from a qualitative study of community leadership connected with a drug abuse prevention project. We discuss the implications of this approach for conducting prevention research, especially with small samples and culturally distinct communities. PMID:25946969

  5. Evolution of metallic screening in small metal clusters probed by PCI-Auger spectroscopy.

    PubMed

    Peters, Sven; Peredkov, Sergey; Balkaya, Baris; Ferretti, Nicoletta; Neeb, Matthias; Eberhardt, Wolfgang

    2010-09-07

    Excitation-energy dependent Auger spectra of small copper clusters supported by a thin silica layer have been measured as function of cluster size. The Auger kinetic energy of the clusters clearly changes with the excess energy of the emitted photoelectron while not for the bulk. The kinetic energy shift is attributed to post-collision interaction (PCI) and exhibits a reduced metallic screening ability of small Cu-clusters. The spectroscopic data reveal an evolution from a long-range Coulomb-like interaction to a short-range "screened" electrostatic interaction within the sub-nm range. The data show that core electron spectroscopy such as PCI-Auger measurements can be used as a general tool to follow the metallic character of supported clusters.

  6. Vibrational properties of small cobalt clusters on the Cu(111) surface

    NASA Astrophysics Data System (ADS)

    Borisova, S. D.; Rusina, G. G.; Eremeev, S. V.; Chulkov, E. V.

    2009-06-01

    Vibrational properties of small cobalt clusters (dimer and trimer) adsorbed on the Cu(111) surface are studied using interatomic interaction potentials obtained in a tight-binding approximation. The complete (lateral and vertical) relaxation of the surface, the local phonon density of states, and the polarization of vibration modes of clusters and atoms of the substrate are discussed. It is shown that the adsorption of small cobalt clusters leads to a local modification of the vibrational properties of the substrate surface and to excitation of new vibration modes localized on both the cluster adatoms and substrate surface atoms. An increase in the cluster size causes a decrease in the intensity of peaks of the local density of states and their broadening and also a shift in the frequencies of the peaks.

  7. Solvation of clyclopentadienyl and substituted clyclopentadienyl radicals in small clusters. III. Pre-reactive clusters

    NASA Astrophysics Data System (ADS)

    Fernandez, J. A.; Yao, J.; Bernstein, E. R.

    1999-03-01

    The hydrogen abstraction reaction between cyclopentadienyl radicals [Xcpd, X=H, CH3(m), F, CN] and substituted methanes (CH4, C2H6, CH3CH2OH, CH3Cl, CH2F2, CHF3, and CH3OH) is studied for the isolated one-to-one van der Waals clusters created in a supersonic expansion. Three different types of fluorescence excitation spectra are characterized for these cluster systems: (1) sharp spectra are observed for some clusters, suggesting no cluster chemistry for either the ground or excited electronic states of Xcpd—CNcpd/CH3Cl, CH2F2, CHF3, CH3OH; (2) broad spectra are observed suggesting initiation of cluster chemistry on the excited state cluster potential energy surface—CNcpd-CH4, Fcpd-CHF2Cl, CHF3; and (3) only a greatly reduced bare radical signal is observed, but no cluster emission can be detected—cpd, mcpd/all substituted methanes, Fcpd-CH2F2, CH3Cl, CH3CH2OH, CH3OH, C2H6, and CNcpd/C2H6, CH3CH2OH. These results, taken together, suggest that the Xcpd radicals undergo an excited electronic state electrophilic hydrogen abstraction reaction with substituted methanes. The radical reactivities are in the order mcpd˜cpd>Fcpd>CNcpd and the substituted methane reactivities are in the order C2H6>C2H5OH>CH4>CH3Cl˜CH3OH>CH2F2>CHF2Cl>CHF3. All Xcpd radicals show intense, sharp spectra with CF4. This indication of an excited state Xcpd radical hydrogen abstraction reaction with substituted methanes is further explored by ab initio quantum chemistry techniques at the (7×7) CASSCF/6-31G (complete active space self-consistent field) and cc-pVDZ levels for cpd-CH4. Calculations confirm the idea that the ground state cluster has a reaction barrier (approximately +170 kJ/mol) and a positive free energy of reaction (˜80 kJ/mol). The excited cpd radical, however, can react with CH4 along a barrierless path to generate substantial hot ground product states (C5H6 and CH3). Experimental data are consistent with an Xcpd-C2H4 addition reaction, as well.

  8. Structure and stability of silicon nanoclusters passivated by hydrogen and oxygen: evolutionary algorithm and first- principles study

    NASA Astrophysics Data System (ADS)

    Baturin, V. S.; Lepeshkin, S. V.; Matsko, N. L.; Uspenskii, Yu A.

    2016-02-01

    We investigate the structural and thermodynamical properties of small silicon clusters. Using the graph theory applied to previously obtained structures of Si10H2m clusters we trace the connection between geometry and passivation degree. The existing data on these clusters and structures of Si10O4n clusters obtained here using evolutionary calculations allowed to analyze the features of Si10H2m clusters in hydrogen atmosphere and Si10O4n clusters in oxygen atmosphere. We have shown the basic differences between structures and thermodynamical properties of silicon clusters, passivated by hydrogen and silicon oxide clusters.

  9. Oxidation Phase Diagram of Small Aluminum Clusters Based on First-Principles Calculations

    NASA Astrophysics Data System (ADS)

    Wang, Ligen; Kuklja, Maija M.

    2009-12-01

    We present a density functional theory study of the structure and properties of Al13 and Al12Ni clusters, oxygen adsorptions on the cluster surfaces, and the completely oxidized clusters. The relative stability of various phases at various oxygen pressures and temperatures is investigated based on the "atomistic thermodynamics" which was previously employed for studying metal surfaces. We construct the two-dimensional (P, T) oxidation phase diagrams for these systems. We find that alloying the Al cluster with Ni does not improve its resistance to oxidation. The present study provides valuable insight into basic behaviors of small Al clusters in the presence of oxygen and a theoretical basis for exploring practical applications of these clusters.

  10. Structures and stabilities of small Co clusters on a Cu(111) surface: A theoretical study

    NASA Astrophysics Data System (ADS)

    Huang, R. Z.; Chen, C.; Li, C. M.; Jiang, C. H.; Zhang, R. J.; Gao, Y.

    2017-10-01

    Structures and relative stabilities of small Con clusters (n = 1-12) on a Cu(111) surface are studied using molecular dynamics simulations. It is shown that the supported clusters are all in two-dimensional island structures of the edges forming square microfacets (A step) and/or triangular microfacets (B step) with the substrate. For non-magic-number clusters, the lowest energy structures are the ones of the edges with more A steps and the most unstable structures are the ones of the edges with only A steps or B steps due to the lattice mismatch of the Con/Cu(111) system. Magic number clusters are truncated triangular or elongated shapes with a closed atomic shell and maximum nearest-neighbor bonds. In addition, the anomalous mobility is found for Co3 and Co6 clusters in the diffusion processes of these clusters. The concerted translation and rotation movements are responsible for their special diffusion behaviors.

  11. Radiotherapy electron beams collimated by small tubular applicators: characterization by silicon and diamond diodes

    NASA Astrophysics Data System (ADS)

    Bagalà, P.; Di Venanzio, C.; Falco, M. D.; Guerra, A. S.; Marinelli, Marco; Milani, E.; Pimpinella, M.; Pompili, F.; Prestopino, G.; Santoni, R.; Tonnetti, A.; Verona, C.; Verona-Rinati, G.

    2013-11-01

    High-energy electron beams generated by linear accelerators, typically in the range 6 to 20 MeV, are used in small field sizes for radiotherapy of localized superficial tumors. Unshielded silicon diodes (Si-D) are commonly considered suitable detectors for relative dose measurements in small electron fields due to their high spatial resolution. Recently, a novel synthetic single crystal diamond diode (SCDD) showed suitable properties for standard electron beams and small photon beams dosimetry. The aim of the present study is twofold: to characterize 6 to 15 MeV small electron beams shaped by using commercial tubular applicators with 2, 3, 4 and 5 cm diameter and to assess the dosimetric performance under such irradiation conditions of the novel SCDD dosimeter by comparison with commercially available dosimeters, namely a Si-D and a plane-parallel ionization chamber. Percentage depth dose curves, beam profiles and output factors (OFs) were measured. A good agreement among the dosimeters was observed in all of the performed measurements. As for the tubular applicators, two main effects were evidenced: (i) OFs larger than unity were measured for a number of field sizes and energies, with values up to about 1.3, that is an output 30% greater than that obtained at the 10 × 10 cm2 reference field; (ii) for each diameter of the tubular applicator a noticeable increase of the OF values was observed with increasing beam energy, up to about 100% in the case of the smaller applicator. This OF behavior is remarkably different from what typically observed for small blocked fields having the same size and energy as those used in this study. OFs for tubular applicators depend considerably on the field size, so interpolation is unadvisable to predict the linear accelerator output for such applicators whereas reliable high-resolution detectors, as the silicon and diamond diodes used in this work allow OF measurements with uncertainties of about 1%.

  12. Diffusion dynamics of small molecules from mesoporous silicon films by real-time optical interferometry

    SciTech Connect

    Mares, Jeremy W.; Weiss, Sharon M.

    2011-09-20

    Time-dependent laser reflectometry measurements are presented as a means to rigorously characterize analyte diffusion dynamics of small molecules from mesoporous silicon (PSi) films for drug delivery and membrane physics applications. Calculations based on inclusion of a spatially and temporally dependent solute concentration profile in a one-dimensional Fickian diffusion flow model are performed to determine the diffusion coefficients for the selected prototypical polar species, sucrose (340 Da), exiting from PSi films. The diffusion properties of the molecules depend on both PSi pore size and film thickness. For films with average pore diameters between 10-30 nm and film thicknesses between 300-900 nm, the sucrose diffusion coefficient can be tuned between approximately 100 and 550 {mu}m{sup 2}/s. Extensions of the real-time measurement and modeling approach for determining the diffusivity of small molecules that strongly interact with and corrode the internal surfaces of PSi films are also discussed.

  13. Age determination of 15 old to intermediate-age small Magellanic cloud star clusters

    SciTech Connect

    Parisi, M. C.; Clariá, J. J.; Piatti, A. E.; Geisler, D.; Leiton, R.; Carraro, G.; Costa, E.; Grocholski, A. J.; Sarajedini, A. E-mail: claria@oac.uncor.edu E-mail: dgeisler@astro-udec.cl E-mail: gcarraro@eso.org E-mail: grocholski@phys.lsu.edu

    2014-04-01

    We present color-magnitude diagrams in the V and I bands for 15 star clusters in the Small Magellanic Cloud (SMC) based on data taken with the Very Large Telescope (VLT, Chile). We selected these clusters from our previous work, wherein we derived cluster radial velocities and metallicities from calcium II infrared triplet (CaT) spectra also taken with the VLT. We discovered that the ages of six of our clusters have been appreciably underestimated by previous studies, which used comparatively small telescopes, graphically illustrating the need for large apertures to obtain reliable ages of old and intermediate-age SMC star clusters. In particular, three of these clusters, L4, L6, and L110, turn out to be among the oldest SMC clusters known, with ages of 7.9 ± 1.1, 8.7 ± 1.2, and 7.6 ± 1.0 Gyr, respectively, helping to fill a possible 'SMC cluster age gap'. Using the current ages and metallicities from Parisi et al., we analyze the age distribution, age gradient, and age-metallicity relation (AMR) of a sample of SMC clusters measured homogeneously. There is a suggestion of bimodality in the age distribution but it does not show a constant slope for the first 4 Gyr, and we find no evidence for an age gradient. Due to the improved ages of our cluster sample, we find that our AMR is now better represented in the intermediate/old period than we had derived in Parisi et al., where we simply took ages available in the literature. Additionally, clusters younger than ∼4 Gyr now show better agreement with the bursting model of Pagel and Tautvaišienė, but we confirm that this model is not a good representation of the AMR during the intermediate/old period. A more complicated model is needed to explain the SMC chemical evolution in that period.

  14. Dating star clusters in the Small Magellanic Cloud by means of integrated spectra

    NASA Astrophysics Data System (ADS)

    Ahumada, A. V.; Clariá, J. J.; Bica, E.; Dutra, C. M.

    2002-10-01

    In this study flux-calibrated integrated spectra in the range (3600-6800) Å are presented for 16 concentrated star clusters in the Small Magellanic Cloud (SMC), approximately half of which constitute unstudied objects. We have estimated ages and foreground interstellar reddening values from the comparison of the line strengths and continuum distribution of the cluster spectra with those of template cluster spectra with known parameters. Most of the sample clusters are young blue clusters (6-50 Myr), while L 28, NGC 643 and L 114 are found to be intermediate-age clusters (1-6 Gyr). One well known SMC cluster (NGC 416) was observed for comparison purposes. The sample includes clusters in the surroundings and main body of the SMC, and the derived foreground reddening values are in the range 0.00 <= E(B-V) <= 0.15. The present data also make up a cluster spectral library at SMC metallicity. Based on observations made at Complejo Astronómico El Leoncito, which is operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba and San Juan, Argentina.

  15. STRUCTURAL PARAMETERS OF SEVEN SMALL MAGELLANIC CLOUD INTERMEDIATE-AGE AND OLD STAR CLUSTERS

    SciTech Connect

    Glatt, Katharina; Grebel, Eva K.; Kayser, Andrea; Gallagher, John S.; Harbeck, Daniel; Nota, Antonella; Sabbi, Elena; Sirianni, Marco; Clementini, Gisella; Tosi, Monica; Da Costa, Gary; Koch, Andreas

    2009-11-15

    We present structural parameters for the seven intermediate-age and old star clusters NGC 121, Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC). We fit King profiles and Elson, Fall, and Freeman profiles to both surface-brightness and star-count data taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. Clusters older than {approx}1 Gyr show a spread in cluster core radii that increases with age, while the youngest clusters have relatively compact cores. No evidence for post-core-collapse clusters was found. We find no correlation between core radius and distance from the SMC center, although consistent with other studies of dwarf galaxies, some relatively old and massive clusters have low densities. The oldest SMC star cluster, the only globular NGC121, is the most elliptical object of the studied clusters. No correlation is seen between ellipticity and distance from the SMC center. The structures of these massive intermediate-age (1-8 Gyr) SMC star clusters thus appear to primarily result from internal evolutionary processes.

  16. Age and metallicity of star clusters in the Small Magellanic Cloud from integrated spectroscopy

    NASA Astrophysics Data System (ADS)

    Dias, Bruno; Coelho, Paula; Kerber, Leandro; Barbuy, Beatriz; Idiart, Thais

    2010-04-01

    Analysis of integrated spectra of star clusters in the Magellanic Clouds can bring important information for studies on the chemical evolution of the Clouds. The aim of the present work is to derive ages and metallicities from integrated spectra of 15 star clusters in the Small Magellanic Cloud (SMC), some of them not studied so far. Making use of a full spectrum fitting technique, we compared the integrated spectra of the sample clusters to three different sets of single stellar population models available in the literature. We derived ages and metallicities for the sample clusters employing the codes STARLIGHT and ULySS. Out of the 15 clusters in our sample, 9 are old/intermediate age clusters and 6 are young clusters. We point out the results for the newly identified as old/intermediate age clusters HW1, NGC 152, Lindsay 3 and 11. We also confirm old ages for NGC 361, NGC 419 and Kron 3, and the oldest well-known SMC cluster NGC 121.

  17. Structural Parameters of Seven Small Magellanic Cloud Intermediate-Age and Old Star Clusters

    NASA Astrophysics Data System (ADS)

    Glatt, Katharina; Grebel, Eva K.; Gallagher, John S., III; Nota, Antonella; Sabbi, Elena; Sirianni, Marco; Clementini, Gisella; Da Costa, Gary; Tosi, Monica; Harbeck, Daniel; Koch, Andreas; Kayser, Andrea

    2009-11-01

    We present structural parameters for the seven intermediate-age and old star clusters NGC 121, Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC). We fit King profiles and Elson, Fall, and Freeman profiles to both surface-brightness and star-count data taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. Clusters older than ~1 Gyr show a spread in cluster core radii that increases with age, while the youngest clusters have relatively compact cores. No evidence for post-core-collapse clusters was found. We find no correlation between core radius and distance from the SMC center, although consistent with other studies of dwarf galaxies, some relatively old and massive clusters have low densities. The oldest SMC star cluster, the only globular NGC121, is the most elliptical object of the studied clusters. No correlation is seen between ellipticity and distance from the SMC center. The structures of these massive intermediate-age (1-8 Gyr) SMC star clusters thus appear to primarily result from internal evolutionary processes. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO-10396.

  18. The Effect of Spin-Dependent Donor Cluster Polarizabilities on the Magnetocapacitance of N-Type Silicon

    NASA Astrophysics Data System (ADS)

    New, David A.

    1984-06-01

    Magnetocapacitance studies have been employed to determine the variation of the donor polarizability contribution to the dielectric constant in n-type silicon with magnetic field, temperature, and donor concentration. Silicon samples with phosphorus concentrations ranging from 9 x 10('16)/cc to 1.0 x 10('18)/cc, as well as Si:As samples with 1.4 x 10('17)donors/cc and 2.5 x 10('18)donors/cc, were studied in the temperature range from 30 mK to 4.2 K at fields up to 19 tesla. Experimental results show the donor contribution varying as B('2) at low fields and B('n) at high fields, with n varying with concentration between .5 and 1.2. The low-field region shows a 1/T('n) dependence with n ranging from 1 to 3, with a much weaker temperature dependence at high fields. The magnitude of the field dependence is larger than expected for diamagnetic effects, isotropic, and smaller in Si:As than in Si:P at comparable concentrations, fields, and temperatures. A donor pair model, utilising spin-dependent polarizabilities, is presented which predicts a low-field (B/T)('2) region, weakening temperature dependence as the field increases, and high-field behavior determined by cluster diamagnetic effects. The calculation is extended to clusters of three donors. The question of donor cluster statistics has been studied using ESR donor cluster spectra in Si:P (3 x 10('16)/cc < N(,D) < 4.8 x 10('17)/cc) and Si:As (1.4 x 10('17)/cc < N(,D) < 1.0 x 10('18)/cc). The ratio of the integrated intensity of the outer hyperfine lines to the total integrated intensity is found to vary exponentially with donor concentration, consistent with the predictions of Poisson statistics. By partitioning a system of computer generated donors, requiring each donor in a cluster interact "strongly" with at least one other donor in the cluster, topological and cluster -cluster interaction effects are explored. Results of ESR studies allow explicit results to be calculated from the cluster polarizability

  19. Mechanism for neuronal spike generation by small and large ion channel clusters

    NASA Astrophysics Data System (ADS)

    Zeng, Shangyou; Jung, Peter

    2004-07-01

    Neuronal action potentials are generated by clusters of ion channels between the Hillock and the first segment. If the clusters comprise a large number of sodium and potassium channels, action potentials are generated if the membrane potential exceeds a threshold of about -55mV . Such behavior is well described by an excitable model such as, for example, the Hodgkin-Huxley equations. In this paper we show through stochastic modeling that if the size of the generating ion channel cluster is small, action potentials are generated regardless of whether the membrane potential is below or above the excitation threshold. Action potential generation is then determined by single-channel kinetics. We further show that this switch in generation mechanism manifests itself in peculiar statistical properties of the generated spike trains at small cluster sizes.

  20. Mechanism for neuronal spike generation by small and large ion channel clusters.

    PubMed

    Zeng, Shangyou; Jung, Peter

    2004-07-01

    Neuronal action potentials are generated by clusters of ion channels between the Hillock and the first segment. If the clusters comprise a large number of sodium and potassium channels, action potentials are generated if the membrane potential exceeds a threshold of about -55 mV. Such behavior is well described by an excitable model such as, for example, the Hodgkin-Huxley equations. In this paper we show through stochastic modeling that if the size of the generating ion channel cluster is small, action potentials are generated regardless of whether the membrane potential is below or above the excitation threshold. Action potential generation is then determined by single-channel kinetics. We further show that this switch in generation mechanism manifests itself in peculiar statistical properties of the generated spike trains at small cluster sizes.

  1. Photoelectron imaging of small aluminum clusters: quantifying s-p hybridization.

    PubMed

    Melko, Joshua J; Castleman, A W

    2013-03-07

    Photoelectron imaging experiments and detailed calculations are conducted on Al(n)(-) clusters (n = 3-6) and a calibration method is developed for connecting experimental observations of photoelectron angular distributions to theoretical predictions. It is shown that this method can be used to quantify the degree to which the molecular orbitals are built from s- or p-like atomic orbitals. The highest occupied molecular orbitals of these small aluminum clusters are found to contain varying degrees of s-p mixing, with Al(3)(-) containing the "most hybridized" orbital and Al(4)(-) containing the "least hybridized" orbital. It is shown experimentally that s-p hybridization is already present for the trimer species and, similar to other properties of small metal clusters, oscillates with cluster size.

  2. Analysis of radiation-induced small Cu particle cluster formation in aqueous CuCl2

    USGS Publications Warehouse

    Jayanetti, Sumedha; Mayanovic, Robert A.; Anderson, Alan J.; Bassett, William A.; Chou, I.-Ming

    2001-01-01

    Radition-induced small Cu particle cluster formation in aqueous CuCl2 was analyzed. It was noticed that nearest neighbor distance increased with the increase in the time of irradiation. This showed that the clusters approached the lattice dimension of bulk copper. As the average cluster size approached its bulk dimensions, an increase in the nearest neighbor coordination number was found with the decrease in the surface to volume ratio. Radiolysis of water by incident x-ray beam led to the reduction of copper ions in the solution to themetallic state.

  3. Rotational dynamics of CO solvated in small He clusters: a quantum Monte Carlo study.

    PubMed

    Cazzato, Paolo; Paolini, Stefano; Moroni, Saverio; Baroni, Stefano

    2004-05-15

    The rotational dynamics of CO single molecules solvated in small He clusters (CO @ HeN) has been studied using reptation quantum Monte Carlo simulations for cluster sizes up to N = 30. Our results are in good agreement with the rotovibrational features of the infrared spectrum recently determined for this system and provide a deep insight into the relation between the structure of the cluster and its dynamics. Simulations for large N also provide a prediction of the effective moment of inertia of CO in the He nanodroplet regime, which has not been measured so far.

  4. Studies of the hydrogenation of small unsaturated molecules using organometallic cluster compounds as catalysts

    SciTech Connect

    Not Available

    1981-01-01

    Progress under contract DE-AC02-78ER04900 and plans for continued investigations of the reactivity of transition metal hydride cluster compounds with small heteronuclear unsaturated molecules are described. Research will include further elucidation of the mechanisms of hydrogen transfer from metal atoms to unsaturated substrates and studies of the structure and bonding of partially hydrogenated substrates coordinated to polynuclear metal centers. Investigations of the reactivity of the bridging hydride ligand and the chemistry of alkyl groups in clusters are planned. The development of heteronuclear clusters as homogeneous CO hydrogenation catalysts will be attempted.

  5. Nature of the interaction between rare gas atoms and transition metal doped silicon clusters: the role of shielding effects.

    PubMed

    Ngan, Vu Thi; Janssens, Ewald; Claes, Pieterjan; Fielicke, André; Nguyen, Minh Tho; Lievens, Peter

    2015-07-21

    Mass spectrometry experiments show an exceptionally weak bonding between Si7Mn(+) and rare gas atoms as compared to other exohedrally transition metal (TM) doped silicon clusters and other SinMn(+) (n = 5-10) sizes. The Si7Mn(+) cluster does not form Ar complexes and the observed fraction of Xe complexes is low. The interaction of two cluster series, SinMn(+) (n = 6-10) and Si7TM(+) (TM = Cr, Mn, Cu, and Zn), with Ar and Xe is investigated by density functional theory calculations. The cluster-rare gas binding is for all clusters, except Si7Mn(+) and Si7Zn(+), predominantly driven by short-range interaction between the TM dopant and the rare gas atoms. A high s-character electron density on the metal atoms in Si7Mn(+) and Si7Zn(+) shields the polarization toward the rare gas atoms and thereby hinders formation of short-range complexes. Overall, both Ar and Xe complexes are similar except that the larger polarizability of Xe leads to larger binding energies.

  6. Arbuscular mycorrhizal fungal communities are phylogenetically clustered at small scales

    PubMed Central

    Horn, Sebastian; Caruso, Tancredi; Verbruggen, Erik; Rillig, Matthias C; Hempel, Stefan

    2014-01-01

    Next-generation sequencing technologies with markers covering the full Glomeromycota phylum were used to uncover phylogenetic community structure of arbuscular mycorrhizal fungi (AMF) associated with Festuca brevipila. The study system was a semi-arid grassland with high plant diversity and a steep environmental gradient in pH, C, N, P and soil water content. The AMF community in roots and rhizosphere soil were analyzed separately and consisted of 74 distinct operational taxonomic units (OTUs) in total. Community-level variance partitioning showed that the role of environmental factors in determining AM species composition was marginal when controlling for spatial autocorrelation at multiple scales. Instead, phylogenetic distance and spatial distance were major correlates of AMF communities: OTUs that were more closely related (and which therefore may have similar traits) were more likely to co-occur. This pattern was insensitive to phylogenetic sampling breadth. Given the minor effects of the environment, we propose that at small scales closely related AMF positively associate through biotic factors such as plant-AMF filtering and interactions within the soil biota. PMID:24824667

  7. Arbuscular mycorrhizal fungal communities are phylogenetically clustered at small scales.

    PubMed

    Horn, Sebastian; Caruso, Tancredi; Verbruggen, Erik; Rillig, Matthias C; Hempel, Stefan

    2014-11-01

    Next-generation sequencing technologies with markers covering the full Glomeromycota phylum were used to uncover phylogenetic community structure of arbuscular mycorrhizal fungi (AMF) associated with Festuca brevipila. The study system was a semi-arid grassland with high plant diversity and a steep environmental gradient in pH, C, N, P and soil water content. The AMF community in roots and rhizosphere soil were analyzed separately and consisted of 74 distinct operational taxonomic units (OTUs) in total. Community-level variance partitioning showed that the role of environmental factors in determining AM species composition was marginal when controlling for spatial autocorrelation at multiple scales. Instead, phylogenetic distance and spatial distance were major correlates of AMF communities: OTUs that were more closely related (and which therefore may have similar traits) were more likely to co-occur. This pattern was insensitive to phylogenetic sampling breadth. Given the minor effects of the environment, we propose that at small scales closely related AMF positively associate through biotic factors such as plant-AMF filtering and interactions within the soil biota.

  8. Big Fish in Small Ponds: massive stars in the low-mass clusters of M83

    SciTech Connect

    Andrews, J. E.; Calzetti, D.; McElwee, Sean; Chandar, R.; Elmegreen, B. G.; Kennicutt, R. C.; Kim, Hwihyun; Krumholz, Mark R.; Lee, J. C.; Whitmore, B.; O'Connell, R. W. E-mail: callzetti@astro.umass.edu

    2014-09-20

    We have used multi-wavelength Hubble Space Telescope WFC3 data of the starbursting spiral galaxy M83 in order to measure variations in the upper end of the stellar initial mass function (uIMF) using the production rate of ionizing photons in unresolved clusters with ages ≤ 8 Myr. As in earlier papers on M51 and NGC 4214, the uIMF in M83 is consistent with a universal IMF, and stochastic sampling of the stellar populations in the ∼<10{sup 3} M {sub ☉} clusters are responsible for any deviations in this universality. The ensemble cluster population, as well as individual clusters, also imply that the most massive star in a cluster does not depend on the cluster mass. In fact, we have found that these small clusters seem to have an over-abundance of ionizing photons when compared to an expected universal or truncated IMF. This also suggests that the presence of massive stars in these clusters does not affect the star formation in a destructive way.

  9. Big Fish in Small Ponds: Massive Stars in the Low-mass Clusters of M83

    NASA Astrophysics Data System (ADS)

    Andrews, J. E.; Calzetti, D.; Chandar, R.; Elmegreen, B. G.; Kennicutt, R. C.; Kim, Hwihyun; Krumholz, Mark R.; Lee, J. C.; McElwee, Sean; O'Connell, R. W.; Whitmore, B.

    2014-09-01

    We have used multi-wavelength Hubble Space Telescope WFC3 data of the starbursting spiral galaxy M83 in order to measure variations in the upper end of the stellar initial mass function (uIMF) using the production rate of ionizing photons in unresolved clusters with ages <= 8 Myr. As in earlier papers on M51 and NGC 4214, the uIMF in M83 is consistent with a universal IMF, and stochastic sampling of the stellar populations in the lap103 M ⊙ clusters are responsible for any deviations in this universality. The ensemble cluster population, as well as individual clusters, also imply that the most massive star in a cluster does not depend on the cluster mass. In fact, we have found that these small clusters seem to have an over-abundance of ionizing photons when compared to an expected universal or truncated IMF. This also suggests that the presence of massive stars in these clusters does not affect the star formation in a destructive way.

  10. Age and metallicity of star clusters in the Small Magellanic Cloud from integrated spectroscopy

    NASA Astrophysics Data System (ADS)

    Dias, B.; Coelho, P.; Barbuy, B.; Kerber, L.; Idiart, T.

    2010-09-01

    Context. Analysis of ages and metallicities of star clusters in the Magellanic Clouds provide information for studies on the chemical evolution of the Clouds and other dwarf irregular galaxies. Aims: The aim is to derive ages and metallicities from integrated spectra of 14 star clusters in the Small Magellanic Cloud, including a few intermediate/old age star clusters. Methods: Making use of a full-spectrum fitting technique, we compared the integrated spectra of the sample clusters to three different sets of single stellar population models, using two fitting codes available in the literature. Results: We derive the ages and metallicities of 9 intermediate/old age clusters, some of them previously unstudied, and 5 young clusters. Conclusions: We point out the interest of the newly identified as intermediate/old age clusters HW1, NGC 152, Lindsay 3, Lindsay 11, and Lindsay 113. We also confirm the old ages of NGC 361, NGC 419, Kron 3, and of the very well-known oldest SMC cluster, NGC 121. Appendix A is only available in electronic form at http://www.aanda.org

  11. Interactions of small platinum clusters with the TiC(001) surface

    SciTech Connect

    Mao, Jianjun; Li, Shasha; Chu, Xingli; Yang, Zongxian

    2015-11-14

    Density functional theory calculations are used to elucidate the interactions of small platinum clusters (Pt{sub n}, n = 1–5) with the TiC(001) surface. The results are analyzed in terms of geometric, energetic, and electronic properties. It is found that a single Pt atom prefers to be adsorbed at the C-top site, while a Pt{sub 2} cluster prefers dimerization and a Pt{sub 3} cluster forms a linear structure on the TiC(001). As for the Pt{sub 4} cluster, the three-dimensional distorted tetrahedral structure and the two-dimensional square structure almost have equal stability. In contrast with the two-dimensional isolated Pt{sub 5} cluster, the adsorbed Pt{sub 5} cluster prefers a three-dimensional structure on TiC(001). Substantial charge transfer takes place from TiC(001) surface to the adsorbed Pt{sub n} clusters, resulting in the negatively charged Pt{sub n} clusters. At last, the d-band centers of the absorbed Pt atoms and their implications in the catalytic activity are discussed.

  12. A Silicon detector system on carbon fiber support at small radius

    SciTech Connect

    Marvin E. Johnson

    2004-04-28

    The design of a silicon detector for a p{bar p} collider experiment will be described. The detector uses a carbon fiber support structure with sensors positioned at small radius with respect to the beam. A brief overview of the mechanical design is given. The emphasis is on the electrical characteristics of the detector. General principles involved in grounding systems with carbon fiber structures will be covered. The electrical characteristics of the carbon fiber support structure will be presented. Test results imply that carbon fiber must be regarded as a conductor for the frequency region of interest of 10 to 100 MHz. No distinction is found between carbon fiber and copper. Performance results on noise due to pick-up through the low mass fine pitch cables carrying the analogue signals and floating metal is discussed.

  13. A detector head design for small-animal PET with silicon photomultipliers (SiPM).

    PubMed

    Moehrs, Sascha; Del Guerra, Alberto; Herbert, Deborah J; Mandelkern, Mark A

    2006-03-07

    Small-animal PET systems are now striving for sub-millimetre resolution. Current systems based upon PSPMTs and finely pixellated scintillators can be pushed to higher resolution, but at the expense of other performance parameters and a rapidly escalating cost. Moreover, depth of interaction (DOI) information is usually difficult to assess in such systems, even though this information is highly desirable to reduce the parallax error, which is often the dominant error for such high-resolution systems. In this study we propose a high-resolution detector head for a small-animal PET imaging system with intrinsic DOI information. Instead of a pixellated scintillator, our design is based upon the classic Anger camera principle, i.e. the head is constructed of modular layers each consisting of a continuous slab of scintillator, viewed by a new type of compact silicon photodetector. The photodetector is the recently developed silicon photomultiplier (SiPM) that as well as being very compact has many other attractive properties: high gain at low bias voltage, excellent single-photoelectron resolution and fast timing. A detector head of about 4 x 4 cm2 in area is proposed, constructed from three modular layers of the type described above. We perform a simulation study, using the Monte Carlo simulation package Geant4. The simulation results are used to optimize the geometry of the detector head and characterize its performance. Additionally, hit estimation algorithms are studied to determine the interaction position of annihilation photons correctly over the whole detector surface. The resulting detector has a nearly uniform efficiency for 511 keV photons of approximately 70% and an intrinsic spatial resolution of less than approximately 0.4 mm full width at half maximum (fwhm).

  14. Measurement of the spectral signature of small carbon clusters at near and far infrared wavelengths

    NASA Technical Reports Server (NTRS)

    Tarter, J.; Saykally, R.

    1991-01-01

    A significant percentage of the carbon inventory of the circumstellar and interstellar media may be in the form of large refractory molecules (or small grains) referred to as carbon clusters. At the small end, uneven numbers of carbon atoms seem to be preferred, whereas above 12 atoms, clusters containing an even number of carbon atoms appear to be preferred in laboratory chemistry. In the lab, the cluster C-60 appears to be a particularly stable form and has been nicknamed Bucky Balls because of its resemblance to a soccer ball and to geodesic domes designed by Buckminster Fuller. In order to investigate the prevalence of these clusters, and their relationship to the polycyclic aromatic hydrocarbons (PAHs) that have become the newest focus of IR astronomy, it is necessary to determine the spectroscopic characteristics of these clusters at near and far infrared wavelengths. Described here is the construction of a near to far IR laser magnetic resonance spectrometer that has been built at the University of California Berkeley in order to detect and characterize these spectra. The equipment produces carbon clusters by laser evaporation of a graphitic target. The clusters are then cooled in a supersonic expansion beam in order to simulate conditions in the interstellar medium (ISM). The expansion beam feeds into the spectrometer chamber and permits concentrations of clusters sufficiently high as to permit ultra-high resolution spectroscopy at near and far IR wavelengths. The first successful demonstration of this apparatus occurred last year when the laboratory studies permitted the observational detection of C-5 in the stellar outflow surrounding IRC+10216 in the near-IR. Current efforts focus on reducing the temperature of the supersonic expansion beam that transport the C clusters evaporated from a graphite target into the spectrometer down to temperatures as low as 1 K.

  15. Effects of manganese doping on the structure evolution of small-sized boron clusters.

    PubMed

    Zhao, Lingquan; Qu, Xin; Wang, Yanchao; Lv, Jian; Zhang, Lijun; Hu, Ziyu; Gu, Guangrui; Ma, Yanming

    2017-07-05

    Atomic doping of clusters is known as an effective approach to stabilize or modify the structures and properties of resulting doped clusters. We herein report the effect of manganese (Mn) doping on the structure evolution of small-sized boron (B) clusters. The global minimum structures of both neutral and charged Mn doped B cluster [Formula: see text] (n  =  10-20 and Q  =  0, ±1) have been proposed through extensive first-principles swarm-intelligence based structure searches. It is found that Mn doping has significantly modified the grow behaviors of B clusters, leading to two novel structural transitions from planar to tubular and then to cage-like B structures in both neutral and charged species. Half-sandwich-type structures are most favorable for small [Formula: see text] (n  ⩽  13) clusters and gradually transform to Mn-centered double-ring tubular structures at [Formula: see text] clusters with superior thermodynamic stabilities compared with their neighbors. Most strikingly, endohedral cages become the ground-state structures for larger [Formula: see text] (n  ⩾  19) clusters, among which [Formula: see text] adopts a highly symmetric structure with superior thermodynamic stability and a large HOMO-LUMO gap of 4.53 eV. The unique stability of the endohedral [Formula: see text] cage is attributed to the geometric fit and formation of 18-electron closed-shell configuration. The results significantly advance our understanding about the structure and bonding of B-based clusters and strongly suggest transition-metal doping as a viable route to synthesize intriguing B-based nanomaterials.

  16. Doping-enhanced hyperpolarizabilities of silicon clusters: A global ab initio and density functional theory study of Si10 (Li, Na, K)n (n = 1, 2) clusters

    NASA Astrophysics Data System (ADS)

    Karamanis, Panaghiotis; Marchal, Remi; Carbonniére, Philippe; Pouchan, Claude

    2011-07-01

    A global theoretical study of the (hyper)polarizabilities of alkali doped Si10 is presented and discussed. First, a detailed picture about the low lying isomers of Si10Li, Si10Na, Si10K, Si10Li2, Si10Na2, and Si10K2 has been obtained in a global manner. Then, the microscopic first (hyper)polarizabilities of the most stable configurations have been determined by means of ab initio methods of high predictive capability such as those based on the Møller-Plesset perturbation and coupled cluster theory, paying extra attention to the (hyper)polarizabilities of the open shell mono-doped systems Si10Li, Si10Na, Si10K, and the influence of spin contamination. These results were used to assess the performance of methods of low computational cost based on density functional theory (DFT) in the reliable computation of these properties in order to proceed with an in-depth study of their evolution as a function of the alkali metal, the cluster composition, and the cluster structure. The most interesting outcomes of the performed (hyper)polarizability study indicate that while alkali doping leaves the per atom polarizability practically unaffected, influences dramatically the hyperpolarizabilities of Si10. The lowest energy structures of the mono-doped clusters are characterized by significantly enhanced hyperpolarizabilities as compared to the analogue neutral or charged bare silicon clusters Si10 and Si11, while, certain patterns governed by the type and the number of the doping agents are followed. The observed hyperpolarizability increase is found to be in close connection with specific cluster to alkali metal charge transfer excited states and to the cluster structures. Moreover, an interesting correlation between the anisotropy of the electron density, and the hyperpolarizabilities of these systems has been observed. Finally, it is important to note that the presented method assessment points out that among the various DFT functionals used in this work, (B3LYP, B3PW91

  17. Doping-enhanced hyperpolarizabilities of silicon clusters: a global ab initio and density functional theory study of Si10 (Li, Na, K)n (n=1, 2) clusters.

    PubMed

    Karamanis, Panaghiotis; Marchal, Remi; Carbonniére, Philippe; Pouchan, Claude

    2011-07-28

    A global theoretical study of the (hyper)polarizabilities of alkali doped Si(10) is presented and discussed. First, a detailed picture about the low lying isomers of Si(10)Li, Si(10)Na, Si(10)K, Si(10)Li(2), Si(10)Na(2), and Si(10)K(2) has been obtained in a global manner. Then, the microscopic first (hyper)polarizabilities of the most stable configurations have been determined by means of ab initio methods of high predictive capability such as those based on the Møller-Plesset perturbation and coupled cluster theory, paying extra attention to the (hyper)polarizabilities of the open shell mono-doped systems Si(10)Li, Si(10)Na, Si(10)K, and the influence of spin contamination. These results were used to assess the performance of methods of low computational cost based on density functional theory (DFT) in the reliable computation of these properties in order to proceed with an in-depth study of their evolution as a function of the alkali metal, the cluster composition, and the cluster structure. The most interesting outcomes of the performed (hyper)polarizability study indicate that while alkali doping leaves the per atom polarizability practically unaffected, influences dramatically the hyperpolarizabilities of Si(10). The lowest energy structures of the mono-doped clusters are characterized by significantly enhanced hyperpolarizabilities as compared to the analogue neutral or charged bare silicon clusters Si(10) and Si(11), while, certain patterns governed by the type and the number of the doping agents are followed. The observed hyperpolarizability increase is found to be in close connection with specific cluster to alkali metal charge transfer excited states and to the cluster structures. Moreover, an interesting correlation between the anisotropy of the electron density, and the hyperpolarizabilities of these systems has been observed. Finally, it is important to note that the presented method assessment points out that among the various DFT functionals used

  18. Theoretical study of successive hydrogenations of small platinum clusters: structure and energetics

    SciTech Connect

    Minot, C.; Bigot, B.; Hariti, A.

    1986-01-22

    The successive hydrogenations, up to saturation, of the most stable small platinum clusters Pt/sub n/ (n = 2-13) are studied by the extended Huckel method with (EHT-SO) and without (EHT) spin-orbit coupling. For each hydrogenation step, a large number of possible structures has been calculated in order to determine the stereochemistry of the best stepwise hydrogenated compounds. The results indicate that the energies of the first successive steps of dihydrogen adsorptions on the small clusters decrease stepwise with the hydrogenation rate, except for a few steps which are discussed. In these conditions, if the hydrogenation reactions are either under thermodynamic control or under kinetic control with activation energies related to the reaction enthalpies, all the clusters of a given size will absorb a first H/sub 2/ molecule before one of them can adsorb a second molecule. Another remarkable point is that the energy of first absorption varies with the cluster size. It shows a peak for Pt/sub 3/ and Pt/sub 4/ that makes the Pt/sub 3/ and Pt/sub 4/ species the most likely clusters to be hydrogenated first in a collection of clusters of different size. 19 references, 9 figures, 5 tables.

  19. Small clusters of II-VI materials: ZniOi, i=1-9

    NASA Astrophysics Data System (ADS)

    Matxain, Jon M.; Fowler, Joseph E.; Ugalde, Jesus M.

    2000-11-01

    Improvements in the characterization of II-VI compound-based solar cells, and the recent experimental characterization of small clusters and nanoparticles, make the study of small II-VI clusters very interesting. In this work, the ground states of small ZniOi clusters are studied, where i=1-9. Ringlike structures have been found to be the global minima for clusters as large as i=7, and three-dimensional spheroid structures for larger ones, where i=8 and 9. This is due to the stability of obtuse O-Zn-O angles in the first case, and to the stability gained from higher coordination in the second case. Three-dimensional structures may be envisioned as being built from Zn2O2 and Zn3O3 rings, as was the case for ZnS three-dimensional global minima and other ZnO calculations. Calculated natural orbital charges become larger as the cluster size increases, showing a tendency toward bulk charges.

  20. Small clusters of II-VI materials: ZniSi, i=1-9

    NASA Astrophysics Data System (ADS)

    Matxain, Jon M.; Fowler, Joseph E.; Ugalde, Jesus M.

    2000-05-01

    The improvements in the characterization of II-VI compound-based solar cells and the recent experimental characterization of small clusters and nanoparticles make the study of small II-VI clusters very interesting. In this work, the ground states of small ZniSi clusters are studied, i=1-9. Ringlike structures have been found to be the global minima in the case of the smaller studied clusters, i.e., i=1-5, and three-dimensional spheroid structures for larger ones, i=6-9. This is due to the stability of obtuse S-Zn-S angles in the first case, and to the stability gained from higher coordination in the second case. The three-dimensional structures may be envisioned as being built from Zn2S2 and Zn3S3 rings, the last ring being the building block of the zinc-sulfide crystal structures, both zinc blende and wurtzite. As cluster size increases, the geometry of the Zn3S3 rings is closer to the one of bulk. Moreover, this structural tendency produces trends to bulklike properties in other properties such as cohesive energy and atomic charges.

  1. First-principles simulations of hydrogen peroxide formation catalyzed by small neutral gold clusters.

    PubMed

    Kacprzak, Katarzyna A; Akola, Jaakko; Häkkinen, Hannu

    2009-08-14

    Energetics and dynamical pathways for hydrogen peroxide formation from H(2) and O(2) bound to neutral gold dimers and tetramers have been investigated by applying several strategies: T = 0 K geometry optimizations, constrained Car-Parrinello molecular dynamics simulations at T = 300 K and metadynamics at T = 300 K. The competing reaction channels for water and hydrogen peroxide formation have been found and characterized. In each case, the reaction barriers for Au cluster catalyzed proton transfer are less than 1 eV. Water formation is a competitive reaction channel, and the relative weight of H(2)O and H(2)O(2) products may depend on the chosen Au cluster size. Dynamic simulations demonstrate the significance of the geometric fluxionality of small catalytic Au clusters. These results indicate that neutral Au clusters could work as catalysts in aerobic H(2)O(2) formation in ambient conditions.

  2. Clustering of water molecules in ultramicroporous carbon: In-situ small-angle neutron scattering

    DOE PAGES

    Bahadur, Jitendra; Contescu, Cristian I.; Rai, Durgesh K.; ...

    2016-10-19

    The adsorption of water is central to most of the applications of microporous carbon as adsorbent material. We report early kinetics of water adsorption in the microporous carbon using in-situ small-angle neutron scattering. It is observed that adsorption of water occurs via cluster formation of molecules. Interestingly, the cluster size remains constant throughout the adsorption process whereas number density of clusters increases with time. The role of surface chemistry of microporous carbon on the early kinetics of adsorption process was also investigated. Lastly, the present study provides direct experimental evidence for cluster assisted adsorption of water molecules in microporous carbonmore » (Do-Do model).« less

  3. First estimates of the fundamental parameters of the very small open cluster Ruprecht 1

    NASA Astrophysics Data System (ADS)

    Piatti, A. E.; Clariá, J. J.; Parisi, M. C.; Ahumada, A. V.

    New CCD observations with the Washington C and T1 filters in the field of the open cluster Ruprecht 1 are presented. The cluster turned out to be very small, its linear radius being 2.6 +/- 0.2 pc. Ruprecht 1 is moderately reddened [E(B-V) = 0.25] and moderately young (~ 230 Myr). Heliocentric distances of 1.9 +/- 0.4 kpc and 1.5 +/- 0.3 kpc are determined for Z (metallicity) = 0.02 and 0.08, respectively, although evidence is presented favouring a solar metal content rather than a subsolar one. We compare the cluster properties with tose of known open clusters located within 1 kpc around it.

  4. Quantum chemical study of small AlnBm clusters: Structure and physical properties

    NASA Astrophysics Data System (ADS)

    Loukhovitski, Boris I.; Sharipov, Alexander S.; Starik, Alexander M.

    2017-08-01

    The structure and physical properties, including rotational constants, characteristic vibrational temperatures, collision diameter, dipole moment, static polarizability, the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and formation enthalpy of the different isomeric forms of AlnBm clusters with n + m ⩽ 7 are studied using density functional theory. The search of the structure of isomers has been carried employing multistep hierarchical algorithm. Temperature dependencies of thermodynamic functions, such as enthalpy, entropy, and specific heat capacity, have been determined both for the individual isomers and for the ensembles with equilibrium and frozen compositions for the each class of clusters taking into account the anharmonicity of cluster vibrations and the contribution of their excited electronic states. The prospects of the application of small AlnBm clusters as the components of energetic materials are also considered.

  5. First principles investigations of small bimetallic PdGa clusters as catalysts for hydrogen dissociation

    NASA Astrophysics Data System (ADS)

    Kaul, Indu; Ghosh, Prasenjit

    2017-04-01

    Using first principles density functional theory based calculations, we have studied hydrogen dissociation on sub nanometer bimetallic clusters formed from d-block (Pd) and p-block (Ga) elements in gas phase to explore the feasibility of using them as cheap catalysts for hydrogen dissociation. Our calculations show that the dimers, trimers and tetramers of these clusters are thermodynamically more stable than the pure ones for all Ga concentrations. For a given cluster size, we find that the clusters containing equal amount of Pd and Ga are the most stable ones. In contrast to bulk PdGa, the contribution of Pd-d states to the highest occupied molecular orbitals of the bimetallic clusters are either very small or absent. Study of adsorption of hydrogen molecule on these clusters show that hydrogen binds in an activated form only on the Pd rich clusters. From the calculations of hydrogen dissociation barriers on tetramers of pure Pd, 25% Ga (Pd3Ga) and 50% Ga (Pd2Ga2) we find that Pd3Ga is the most efficient catalyst for hydrogen dissociation with barriers even lower than that on the PdGa surfaces.

  6. A comprehensive photometric study of dynamically evolved small van den Bergh-Hagen open clusters

    NASA Astrophysics Data System (ADS)

    Piatti, Andrés E.

    2016-12-01

    We present results from Johnson UBV, Kron-Cousins RI and Washington CT1T2 photometries for seven van den Bergh-Hagen (vdBH) open clusters, namely, vdBH 1, 10, 31, 72, 87, 92, and 118. The high-quality, multiband photometric data sets were used to trace the cluster stellar density radial profiles and to build colour-magnitude diagrams and colour-colour diagrams from which we estimated their structural parameters and fundamental astrophysical properties. The clusters in our sample cover a wide age range, from ˜60 Myr up to 2.8 Gyr, are of relatively small size (˜1-6 pc) and are placed at distances from the Sun which vary between 1.8 and 6.3 kpc, respectively. We also estimated lower limits for the cluster present-day masses as well as half-mass relaxation times (tr). The resulting values in combination with the structural parameter values suggest that the studied clusters are in advanced stages of their internal dynamical evolution (age/tr ˜ 20-320), possibly in the typical phase of those tidally filled with mass segregation in their core regions. Compared to open clusters in the solar neighbourhood, the seven vdBH clusters are within more massive (˜80-380 M⊙), with higher concentration parameter values (c ˜ 0.75-1.15) and dynamically evolved ones.

  7. Observation of small cluster formation in concentrated monoclonal antibody solutions and its implications to solution viscosity.

    PubMed

    Yearley, Eric J; Godfrin, Paul D; Perevozchikova, Tatiana; Zhang, Hailiang; Falus, Peter; Porcar, Lionel; Nagao, Michihiro; Curtis, Joseph E; Gawande, Pradad; Taing, Rosalynn; Zarraga, Isidro E; Wagner, Norman J; Liu, Yun

    2014-04-15

    Monoclonal antibodies (mAbs) are a major class of biopharmaceuticals. It is hypothesized that some concentrated mAb solutions exhibit formation of a solution phase consisting of reversibly self-associated aggregates (or reversible clusters), which is speculated to be responsible for their distinct solution properties. Here, we report direct observation of reversible clusters in concentrated solutions of mAbs using neutron spin echo. Specifically, a stable mAb solution is studied across a transition from dispersed monomers in dilute solution to clustered states at more concentrated conditions, where clusters of a preferred size are observed. Once mAb clusters have formed, their size, in contrast to that observed in typical globular protein solutions, is observed to remain nearly constant over a wide range of concentrations. Our results not only conclusively establish a clear relationship between the undesirable high viscosity of some mAb solutions and the formation of reversible clusters with extended open structures, but also directly observe self-assembled mAb protein clusters of preferred small finite size similar to that in micelle formation that dominate the properties of concentrated mAb solutions.

  8. Prediction of structures and atomization energies of small silver clusters, (Ag)n, n < 100.

    PubMed

    Chen, Mingyang; Dyer, Jason E; Li, Keijing; Dixon, David A

    2013-08-29

    Neutral silver clusters, Ag(n), were studied using density functional theory (DFT) followed by high level coupled cluster CCSD(T) calculations to determine the low energy isomers for each cluster size for small clusters. The normalized atomization energy, heats of formation, and average bond lengths were calculated for each of the different isomeric forms of the silver clusters. For n = 2-6, the preferred geometry is planar, and the larger n = 7-8 clusters prefer higher symmetry, three-dimensional geometries. The low spin state is predicted to be the ground state for every cluster size. A number of new low energy isomers for the heptamer and octamer were found. Additional larger Ag(n) structures, n < 100, were initially optimized using a tree growth-hybrid genetic algorithm with an embedded atom method (EAM) potential. For n ≤ 20, DFT was used to optimize the geometries. DFT with benchmarked functionals were used to predict that the normalized atomization energies ((AE)s) for Ag(n) start to converge slowly to the bulk at n = 55. The (AE) for Ag99 is predicted to be ~50 kcal/mol.

  9. Structure and properties of small clusters of transition 3d-element oxides

    SciTech Connect

    Popov, A. V.

    2011-03-15

    The results of calculations of the parameters of the equilibrium structures of small clusters of transition 3d-element oxides are presented. The calculations are performed by the spin-unrestricted Hartree-Fock method. The properties of the most stable structures, i.e., those with the lowest total energies among all those obtained, are discussed.

  10. Small Impact Crater Clusters in High Resolution HiRISE Images

    NASA Astrophysics Data System (ADS)

    Ivanov, B. A.; Melosh, H. J.; McEwen, A. S.; HiRISE Team

    2008-03-01

    The HiRISE image scale of 0.25-0.32 meters per pixel allows us for the first time to resolve details of small primary crater fields on Mars. The cluster dispersion is used to estimate strength and density of meteoroids.

  11. Stability and Spectra of Small 3He-4He Clusters

    NASA Astrophysics Data System (ADS)

    Navarro, J.; Fantoni, S.; Guardiola, R.; Zuker, A.

    Diffusion Monte Carlo calculations have been systematically performed to analyze the stability of small mixed 3He-4He clusters, as well as their excitation spectra. The picture that emerges is that of systems with strong shell effects whose binding and excitation energies are essentially determined by the monopole properties of an effective Hamiltonian.

  12. On the growth mechanism of multicrystalline silicon ingots with small grains fabricated using single-layer silicon beads

    NASA Astrophysics Data System (ADS)

    Muramatsu, Tetsurou; Takahashi, Isao; Anandha Babu, G.; Usami, Noritaka

    2017-07-01

    The growth mechanism of multicrystalline silicon ingots in directional solidification using single-layer silicon beads (SLSB) coated with Si3N4 was investigated. The grains in the SLSB-seeded ingot were smaller than those in a nonseeded ingot, but still larger than those in a polycrystalline Si (poly-Si)-seeded ingot. The dislocation density in the SLSB-seeded ingot was lower than that in the nonseeded ingot, but higher than that in the poly-Si-seeded ingot. The minority carrier lifetime mapping showed that a higher production yield was obtained in the SLSB-seeded ingot than in the poly-Si-seeded ingot. Grain refinement by the SLSB-seeding method was associated with the increase in nucleation rate. The increase in the surface area of the bottom of the crucible cannot quantitatively explain the grain refinement. Therefore, it is considered that SLSB may supply many nucleation sites because of their uneven structure.

  13. Silicon Phthalocyanines Axially Disubstituted with Erlotinib toward Small-Molecular-Target-Based Photodynamic Therapy.

    PubMed

    Chen, Juan-Juan; Huang, Yi-Zhen; Song, Mei-Ru; Zhang, Zhi-Hong; Xue, Jin-Ping

    2017-09-21

    Small-molecular-target-based photodynamic therapy-a promising targeted anticancer strategy-was developed by conjugating zinc(II) phthalocyanine with a small-molecular-target-based anticancer drug. To prevent self-aggregation and avoid problems of phthalocyanine isomerization, two silicon phthalocyanines di-substituted axially with erlotinib have been synthesized and fully characterized. These conjugates are present in monomeric form in various solvents as well as culture media. Cell-based experiments showed that these conjugates localize in lysosomes and mitochondria, while maintaining high photodynamic activities (IC50 values as low as 8 nm under a light dose of 1.5 J cm(-2) ). With erlotinib as the targeting moiety, two conjugates were found to exhibit high specificity for EGFR-overexpressing cancer cells. Various poly(ethylene glycol) (PEG) linker lengths were shown to have an effect on the photophysical/photochemical properties and on in vitro phototoxicity. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

  14. Ultra-small photoluminescent silicon-carbide nanocrystals by atmospheric-pressure plasmas.

    PubMed

    Askari, Sadegh; Ul Haq, Atta; Macias-Montero, Manuel; Levchenko, Igor; Yu, Fengjiao; Zhou, Wuzong; Ostrikov, Kostya Ken; Maguire, Paul; Svrcek, Vladimir; Mariotti, Davide

    2016-10-06

    Highly size-controllable synthesis of free-standing perfectly crystalline silicon carbide nanocrystals has been achieved for the first time through a plasma-based bottom-up process. This low-cost, scalable, ligand-free atmospheric pressure technique allows fabrication of ultra-small (down to 1.5 nm) nanocrystals with very low level of surface contamination, leading to fundamental insights into optical properties of the nanocrystals. This is also confirmed by their exceptional photoluminescence emission yield enhanced by more than 5 times by reducing the nanocrystals sizes in the range of 1-5 nm, which is attributed to quantum confinement in ultra-small nanocrystals. This method is potentially scalable and readily extendable to a wide range of other classes of materials. Moreover, this ligand-free process can produce colloidal nanocrystals by direct deposition into liquid, onto biological materials or onto the substrate of choice to form nanocrystal films. Our simple but efficient approach based on non-equilibrium plasma environment is a response to the need of most efficient bottom-up processes in nanosynthesis and nanotechnology.

  15. A Theoretical Study of Structural, Electronic and Vibrational Properties of Small Fluoride Clusters

    NASA Astrophysics Data System (ADS)

    Waters, Kevin; Pandey, Ratnesh; Nigam, Sandeep; He, Haiying; Pingle, Subhash; Pandey, Avinash; Pandey, Ravindra

    2014-03-01

    Alkaline earth metal fluorides are an interesting family of ionic crystals having a wide range of applications in solid state lasers, luminescence, scintillators, to name just a few. In this work, small stoichiometric clusters of (MF2)n (M = Mg, Ca Sr, Ba, n =1-6) were studied for structural, vibrational and electronic properties using first-principles methods based on density functional theory. A clear trend of structural and electronic structure evolution was found for all the alkaline earth metal fluorides when the cluster size n increases from 1 to 6. Our study reveals that these fluoride clusters mimic the bulk-like behavior at the very small size. Among the four series of metal fluorides, however, (MgF2)n clusters stands out to be different in its preference of equilibrium structures owing to the much smaller ionic radius of Mg and the higher degree of covalency in the Mg-F bonding. The calculated binding energy, highest stretching frequency, ionization potential, and HOMO-LUMO gap decrease from MgF2 to BaF2 for the same cluster size. These variations are explained in terms of the change in the ionic radius and the basicity of the metal ions.

  16. Vacuum-Ultraviolet (VUV) Photoionization of Small Methanol and Methanol-Water Clusters

    SciTech Connect

    Kostko, Oleg; Belau, Leonid; Wilson, Kevin R.; Ahmed, Musahid

    2008-04-24

    In this work, we report on the vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters. Clusters of methanol with water are generated via co-expansion of the gas phase constituents in a continuous supersonic jet expansion of methanol and water seeded in Ar. The resulting clusters are investigated by single photon ionization with tunable vacuum-ultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Protonated methanol clusters of the form (CH3OH)nH+(n = 1-12) dominate the mass spectrum below the ionization energy of the methanol monomer. With an increase in water concentration, small amounts of mixed clusters of the form (CH3OH n(H2O)H+ (n = 2-11) are detected. The only unprotonated species observed in this work are the methanol monomer and dimer. Appearance energies are obtained from the photoionization efficiency (PIE) curves for CH3OH+, (CH3OH)2+, (CH3OH)nH+ (n = 1-9), and (CH3OH)n(H2O)H+ (n = 2-9) as a function of photon energy. With an increasein the water content in the molecular beam, there is an enhancement of photoionization intensity for the methanol dimer and protonated methanol monomer at threshold. These results are compared and contrasted to previous experimental observations.

  17. Vacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters

    SciTech Connect

    Ahmed, Musahid; Ahmed, Musahid; Wilson, Kevin R.; Belau, Leonid; Kostko, Oleg

    2008-05-12

    In this work we report on thevacuum-ultraviolet (VUV) photoionization of small methanol and methanol-water clusters. Clusters of methanol with water are generated via co-expansion of the gas phase constituents in a continuous supersonic jet expansion of methanol and water seeded in Ar. The resulting clusters are investigated by single photon ionization with tunable vacuumultraviolet synchrotron radiation and mass analyzed using reflectron mass spectrometry. Protonated methanol clusters of the form (CH3OH)nH + (n=1-12) dominate the mass spectrum below the ionization energy of the methanol monomer. With an increase in water concentration, small amounts of mixed clusters of the form (CH3OH)n(H2O)H + (n=2-11) are detected. The only unprotonated species observed in this work are the methanol monomer and dimer. Appearance energies are obtained from the photoionization efficiency (PIE) curves for CH3OH +, (CH 3OH)2 +, (CH3OH)nH + (n=1-9), and (CH 3OH)n(H2O)H + (n=2-9 ) as a function of photon energy. With an increase in the water content in the molecular beam, there is an enhancement of photoionization intensity for methanol dimer and protonated methanol monomer at threshold. These results are compared and contrasted to previous experimental observations.

  18. The CO oxidation mechanism on small Pd clusters. A theoretical study.

    PubMed

    González-Torres, Julio César; Bertin, Virineya; Poulain, Enrique; Olvera-Neria, Oscar

    2015-11-01

    CO is a pollutant that is removed by oxidation using Pd, Pt or Rh as catalysts in the exhaust pipes of vehicles. Here, a quantum chemistry study on the CO + O2 reaction catalyzed by small Pdn clusters (n ≤ 5) using the PBE/TZ2P/ZORA method is performed. The limiting step in this reaction at low temperature and coverage is the O2 dissociation. Pdn clusters catalyze the O=O bond breaking, reducing the energy barrier from 119 kcal mol(-1) without catalyst to ∼35 kcal mol(-1). The charge transfer from Pd to the O2,ad antibonding orbital weakens, and finally breaks the O─O bond. The CO oxidation takes place by the Eley-Rideal (ER) mechanism or the Langmuir-Hinshelwood (LH) mechanism. The ER mechanism presents an energy barrier of 4.10-7.05 kcal mol(-1) and the formed CO2 is released after the reaction. The LH mechanism also shows barrier energies to produce CO2 (7-15 kcal mol(-1)) but it remains adsorbed on Pd clusters. An additional energy (7-25 kcal mol(-1)) is necessary to desorb CO2 and release the metal site. The triplet multiplicity is the ground states of studied Pdn clusters, with the following order of stability: triplet > singlet > quintet state. Graphical Abstract CO oxidation mechanism on small Pd clusters.

  19. The dimeric nature of bonding in gallium: from small clusters to the α-gallium phase.

    PubMed

    Tonner, Ralf; Gaston, Nicola

    2014-11-28

    We consider the structural similarity of small gallium clusters to the bulk structure of α-gallium, which has been previously described as a molecular metal, via density functional theory-based computations. Previous calculations have shown that the tetramer, the hexamer, and the octamer of gallium are all structurally similar to the α-phase. We perform an analysis of the bonding in these clusters in terms of the molecular orbitals and atoms in molecules description in order to assess whether we can see similarities at these sizes to the bonding pattern, which is ascribed to the co-existence of covalent and metallic bonding in the bulk. The singlet Ga4 and Ga8 clusters can be constructed in a singlet ground state from the Ga-dimers in the first excited triplet state of the Ga2-molecule, the (3)Σg(-) state. Molecular orbital (MO) analysis confirms that the dimer is an essential building block of these small clusters. Comparison of the AIM characteristics of the bonds within the clusters to the bonds in the bulk α-phase supports the identification of the covalent bond in the bulk as related to the (3)Σg(-) state of the dimer.

  20. The nature and role of the gold-krypton interactions in small neutral gold clusters.

    PubMed

    Mancera, Luis A; Benoit, David M

    2015-03-26

    We investigate the nature and role of krypton embedding in small neutral gold clusters. For some of these clusters, we observe a particular site-dependent character of the Kr binding that does not completely follow the criterion of binding at low-coordinated sites, widely accepted for interaction of a noble gas with closed-shell metal systems such as metal surfaces. We aim at understanding the effect of low dimensionality and open-shell electronic structure of the odd-numbered clusters on the noble gas-metal cluster interaction. First, we investigate the role of attractive and repulsive forces, and the frontier molecular orbitals. Second, we investigate the Au-Kr interaction in terms of reactivity and bonding character. We use a reactivity index derived from Fukui formalism, and criteria provided by the electron localization function (ELF), in order to classify the type of bonding. We carry out this study on the minimum energy structures of neutral gold clusters, as obtained using pseudo potential plane-wave density functional theory (DFT). A model is proposed that includes the effect of attractive electrostatic, van der Waals and repulsive forces, together with effects originating from orbital overlap. This satisfactorily explains minimum configurations of the noble gas-gold cluster systems, the site preference of the noble gas atoms, and changes in electronic properties.

  1. Self-Assembly of Silver Metal Clusters of Small Atomicity on Cyclic Peptide Nanotubes.

    PubMed

    Cuerva, Miguel; García-Fandiño, Rebeca; Vázquez-Vázquez, Carlos; López-Quintela, M Arturo; Montenegro, Javier; Granja, Juan R

    2015-11-24

    Subnanometric noble metal clusters, composed by only a few atoms, behave like molecular entities and display magnetic, luminescent and catalytic activities. However, noncovalent interactions of molecular metal clusters, lacking of any ligand or surfactant, have not been seen at work. Theoretically attractive and experimentally discernible, van der Waals forces and noncovalent interactions at the metal/organic interfaces will be crucial to understand and develop the next generation of hybrid nanomaterials. Here, we present experimental and theoretical evidence of noncovalent interactions between subnanometric metal (0) silver clusters and aromatic rings and their application in the preparation of 1D self-assembled hybrid architectures with ditopic peptide nanotubes. Atomic force microscopy, fluorescence experiments, circular dichroism and computational simulations verified the occurrence of these interactions in the clean and mild formation of a novel peptide nanotube and metal cluster hybrid material. The findings reported here confirmed the sensitivity of silver metal clusters of small atomicity toward noncovalent interactions, a concept that could find multiple applications in nanotechnology. We conclude that induced supramolecular forces are optimal candidates for the precise spatial positioning and properties modulation of molecular metal clusters. The reported results herein outline and generalize the possibilities that noncovalent interactions will have in this emerging field.

  2. Ionization thresholds of small carbon clusters: tunable VUV experiments and theory.

    PubMed

    Belau, Leonid; Wheeler, Steven E; Ticknor, Brian W; Ahmed, Musahid; Leone, Stephen R; Allen, Wesley D; Schaefer, Henry F; Duncan, Michael A

    2007-08-22

    Small carbon clusters (Cn, n = 2-15) are produced in a molecular beam by pulsed laser vaporization and studied with vacuum ultraviolet (VUV) photoionization mass spectrometry. The required VUV radiation in the 8-12 eV range is provided by the Advanced Light Source (ALS) at the Lawrence Berkeley National Laboratory. Mass spectra at various ionization energies reveal the qualitative relative abundances of the neutral carbon clusters produced. By far the most abundant species is C3. Using the tunability of the ALS, ionization threshold spectra are recorded for the clusters up to 15 atoms in size. The ionization thresholds are compared to those measured previously with charge-transfer bracketing methods. To interpret the ionization thresholds for different cluster sizes, new ab initio calculations are carried out on the clusters for n = 4-10. Geometric structures are optimized at the CCSD(T) level with cc-pVTZ (or cc-pVDZ) basis sets, and focal point extrapolations are applied to both neutral and cation species to determine adiabatic and vertical ionization potentials. The comparison of computed and measured ionization potentials makes it possible to investigate the isomeric structures of the neutral clusters produced in this experiment. The measurements are inconclusive for the n = 4-6 species because of unquenched excited electronic states. However, the data provide evidence for the prominence of linear structures for the n = 7, 9, 11, 13 species and the presence of cyclic C10.

  3. Quantum monte carlo study of the energetics of small hydrogenated and fluoride lithium clusters.

    PubMed

    Moreira, N L; Brito, B G A; Rabelo, J N Teixeira; Cândido, Ladir

    2016-06-30

    An investigation of the energetics of small lithium clusters doped either with a hydrogen or with a fluorine atom as a function of the number of lithium atoms using fixed-node diffusion quantum Monte Carlo (DMC) simulation is reported. It is found that the binding energy (BE) for the doped clusters increases in absolute values leading to a more stable system than for the pure ones in excellent agreement with available experimental measurements. The BE increases for pure, remains almost constant for hydrogenated, and decreases rapidly toward the bulk lithium for the fluoride as a function of the number of lithium atoms in the clusters. The BE, dissociation energy as well as the second difference in energy display a pronounced odd-even oscillation with the number of lithium atoms. The electron correlation inverts the odd-even oscillation pattern for the doped in comparison with the pure clusters and has an impact of 29%-83% to the BE being higher in the pure cluster followed by the hydrogenated and then by the fluoride. The dissociation energy and the second difference in energy indicate that the doped cluster Li3 H is the most stable whereas among the pure ones the more stable are Li2 , Li4 , and Li6 . The electron correlation energy is crucial for the stabilization of Li3 H. © 2016 Wiley Periodicals, Inc.

  4. Energetics of small clusters of group IB metals (Cu, Ag, and Au) adsorbed on graphene

    NASA Astrophysics Data System (ADS)

    Kaur, Gagandeep; Gupta, Shuchi; Dharamvir, Keya

    2013-06-01

    The 2D structure of graphene maximizes the interaction of adsorbate on the layer. Many experiments have been devised to form stable metallic clusters of different sizes. We study the structure and binding energies of group IB clusters Mn (M=Au, Ag, Cu n=1, 3) adsorbed on graphene using Gupta potential [1] (for M-M interaction) and Lennard-Jones potential [2] (for metal-carbon interaction). The total energy of the system has been obtained by placing each of Mn cluster a certain distance above the graphene sheet at various positions and in various orientations. The minimized energy configurations, for all Mn clusters, lie above the center of a hexagon and parallel to the graphene sheet. Binding energy per atom for Ag and Cu metal clusters are less than those of respective Au indicating the lower stability of Ag/Cu metal-graphene system. Using various energy barriers, we can calculate the energy required to move small cluster from one position of minimum energy to another on graphene.

  5. Distinct Short-Range Order Is Inherent to Small Amorphous Calcium Carbonate Clusters (<2 nm)

    SciTech Connect

    Sun, Shengtong; Chevrier, Daniel M.; Zhang, Peng; Gebauer, Denis; Cölfen, Helmut

    2016-09-09

    Amorphous intermediate phases are vital precursors in the crystallization of many biogenic minerals. While inherent short-range orders have been found in amorphous calcium carbonates (ACCs) relating to different crystalline forms, it has never been clarified experimentally whether such orders already exist in very small clusters less than 2 nm in size. Here, we studied the stability and structure of 10,12-pentacosadiynoic acid (PCDA) protected ACC clusters with a core size of ca. 1.4 nm consisting of only seven CaCO3 units. Ligand concentration and structure are shown to be key factors in stabilizing the ACC clusters. More importantly, even in such small CaCO3 entities, a proto-calcite short-range order can be identified but with a relatively high degree of disorder that arises from the very small size of the CaCO3 core. Our findings support the notion of a structural link between prenucleation clusters, amorphous intermediates, and final crystalline polymorphs, which appears central to the understanding of polymorph selection.

  6. AGE DETERMINATION OF SIX INTERMEDIATE-AGE SMALL MAGELLANIC CLOUD STAR CLUSTERS WITH HST/ACS

    SciTech Connect

    Glatt, Katharina; Kayser, Andrea; Grebel, Eva K.; Sabbi, Elena; Gallagher, John S. III; Harbeck, Daniel; Nota, Antonella; Sirianni, Marco; Clementini, Gisella; Tosi, Monica; Koch, Andreas; Da Costa, Gary

    2008-10-15

    We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color-magnitude diagrams (CMDs) extend {approx}3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC 419 where the Padova isochrones provided the best fit. Using Dartmouth isochrones we derive ages of 7.5 {+-} 0.5 Gyr (Lindsay 1), 6.5 {+-} 0.5 Gyr (Kron 3), 6 {+-} 0.5 Gyr (NGC 339), 6 {+-} 0.5 Gyr (NGC 416), and 6.5 {+-} 0.5 Gyr (Lindsay 38). The CMD of NGC 419 shows several main-sequence turnoffs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC 419. We confirm that the SMC contains several intermediate-age populous star clusters with ages unlike those of the Large Magellanic Cloud and the Milky Way. Interestingly, our intermediate-age star clusters have a metallicity spread of {approx}0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally-concentrated blue straggler star candidates in NGC 416, while these are not present for the other clusters. Using the red clump magnitudes, we find that the closest cluster, NGC 419 ({approx}50 kpc), and the farthest cluster, Lindsay 38 ({approx}67 kpc), have a relative distance of {approx}17 kpc, which confirms the large depth of the SMC. The three oldest SMC clusters (NGC 121, Lindsay 1, and Kron 3) lie in the northwestern

  7. Age Determination of Six Intermediate-Age Small Magellanic Cloud Star Clusters with HST/ACS

    NASA Astrophysics Data System (ADS)

    Glatt, Katharina; Grebel, Eva K.; Sabbi, Elena; Gallagher, John S., III; Nota, Antonella; Sirianni, Marco; Clementini, Gisella; Tosi, Monica; Harbeck, Daniel; Koch, Andreas; Kayser, Andrea; Da Costa, Gary

    2008-10-01

    We present a photometric analysis of the star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, Lindsay 38, and NGC 419 in the Small Magellanic Cloud (SMC), observed with the Hubble Space Telescope Advanced Camera for Surveys (ACS) in the F555W and F814W filters. Our color-magnitude diagrams (CMDs) extend ~3.5 mag deeper than the main-sequence turnoff points, deeper than any previous data. Cluster ages were derived using three different isochrone models: Padova, Teramo, and Dartmouth, which are all available in the ACS photometric system. Fitting observed ridgelines for each cluster, we provide a homogeneous and unique set of low-metallicity, single-age fiducial isochrones. The cluster CMDs are best approximated by the Dartmouth isochrones for all clusters, except for NGC 419 where the Padova isochrones provided the best fit. Using Dartmouth isochrones we derive ages of 7.5 ± 0.5 Gyr (Lindsay 1), 6.5 ± 0.5 Gyr (Kron 3), 6 ± 0.5 Gyr (NGC 339), 6 ± 0.5 Gyr (NGC 416), and 6.5 ± 0.5 Gyr (Lindsay 38). The CMD of NGC 419 shows several main-sequence turnoffs, which belong to the cluster and to the SMC field. We thus derive an age range of 1.2-1.6 Gyr for NGC 419. We confirm that the SMC contains several intermediate-age populous star clusters with ages unlike those of the Large Magellanic Cloud and the Milky Way. Interestingly, our intermediate-age star clusters have a metallicity spread of ~0.6 dex, which demonstrates that the SMC does not have a smooth, monotonic age-metallicity relation. We find an indication for centrally-concentrated blue straggler star candidates in NGC 416, while these are not present for the other clusters. Using the red clump magnitudes, we find that the closest cluster, NGC 419 (~50 kpc), and the farthest cluster, Lindsay 38 (~67 kpc), have a relative distance of ~17 kpc, which confirms the large depth of the SMC. The three oldest SMC clusters (NGC 121, Lindsay 1, and Kron 3) lie in the northwestern part of the SMC, while the youngest (NGC 419

  8. Atomic characterization of Au clusters in vapor-liquid-solid grown silicon nanowires

    SciTech Connect

    Chen, Wanghua; Roca i Cabarrocas, Pere; Pareige, Philippe; Castro, Celia; Xu, Tao; Grandidier, Bruno; Stiévenard, Didier

    2015-09-14

    By correlating atom probe tomography with other conventional microscope techniques (scanning electron microscope, scanning transmission electron microscope, and scanning tunneling microscopy), the distribution and composition of Au clusters in individual vapor-liquid-solid grown Si nanowires is investigated. Taking advantage of the characteristics of atom probe tomography, we have developed a sample preparation method by inclining the sample at certain angle to characterize the nanowire sidewall without using focused ion beam. With three-dimensional atomic scale reconstruction, we provide direct evidence of Au clusters tending to remain on the nanowire sidewall rather than being incorporated into the Si nanowires. Based on the composition measurement of Au clusters (28% ± 1%), we have demonstrated the supersaturation of Si atoms in Au clusters, which supports the hypothesis that Au clusters are formed simultaneously during nanowire growth rather than during the cooling process.

  9. Small-angle neutron scattering data on C{sub 60} clusters in weakly polar solutions of fullerenes

    SciTech Connect

    Tropin, T. V. Avdeev, M. V.; Aksenov, V. L.

    2007-05-15

    Solutions of fullerence C{sub 60} in carbon disulfide CS{sub 2} have been investigated by small-angle neutron scattering. Combination of solubility, contrast, and incoherent scattering make it possible to measure and analyze the relatively small scattering cross section of this system. Along with single fullerene molecules, a small amount of large fullerene clusters (more than 100 A in size) is found in these solutions. The formation of these clusters depends on the procedure of solution preparation. The size distribution functions of clusters are compared with the results of the phenomenological cluster model of fullerene solubility.

  10. Phonon-induced pure-dephasing of luminescence, multiple exciton generation, and fission in silicon clusters

    NASA Astrophysics Data System (ADS)

    Liu, Jin; Neukirch, Amanda J.; Prezhdo, Oleg V.

    2013-10-01

    The size and temperature dependence of the pure-dephasing processes involved in luminescence, multiple exciton generation (MEG), and multiple exciton fission (MEF) are investigated for Sin clusters (n = 5-10, 15) using ab initio molecular dynamics and optical response function theory. The cluster bandgaps correlate with two types of binding energy, indicating that bandgaps can be used to characterize cluster stability. Ranging from 5 to 100 fs, the dephasing times are found to be longest for MEF and shortest for MEG, with luminescence falling in the middle range. Generally, the dephasing is fast, if the orbitals supporting the pair of states involved in the superpositions differ in energy, atomic localization, and number of nodes. The dephasing accelerates with temperature, because more phonon modes are activated, and lower frequency acoustic modes are able to explore the anhamonic part of the potential energy surface. The temperature dependence is stronger for larger clusters, since they possess a wider range of low-frequency anharmonic modes. Our research indicates that rapid dephasing in Si clusters favors generation of independent charge carriers from single and multiple excitons, making the clusters a promising material for photon energy conversion. The simulations of the dephasing processes reported in this work assist in understanding of the exciton evolution pathways in inorganic semiconductor clusters and other nanoscale materials.

  11. Relative dispersion of clustered drifters in a small micro-tidal estuary

    NASA Astrophysics Data System (ADS)

    Suara, Kabir; Chanson, Hubert; Borgas, Michael; Brown, Richard J.

    2017-07-01

    Small tide-dominated estuaries are affected by large scale flow structures which combine with the underlying bed generated smaller scale turbulence to significantly increase the magnitude of horizontal diffusivity. Field estimates of horizontal diffusivity and its associated scales are however rare due to limitations in instrumentation. Data from multiple deployments of low and high resolution clusters of GPS-drifters are used to examine the dynamics of a surface flow in a small micro-tidal estuary through relative dispersion analyses. During the field study, cluster diffusivity, which combines both large- and small-scale processes ranged between, 0.01 and 3.01 m2/s for spreading clusters and, -0.06 and -4.2 m2/s for contracting clusters. Pair-particle dispersion, Dp2, was scale dependent and grew as Dp2 ∼ t1.83 in streamwise and Dp2 ∼ t0.8 in cross-stream directions. At small separation scale, pair-particle (d < 0.5 m) relative diffusivity followed the Richardson's 4/3 power law and became weaker as separation scale increases. Pair-particle diffusivity was described as Kp ∼ d1.01 and Kp ∼ d0.85 in the streamwise and cross-stream directions, respectively for separation scales ranging from 0.1 to 10 m. Two methods were used to identify the mechanism responsible for dispersion within the channel. The results clearly revealed the importance of strain fields (stretching and shearing) in the spreading of particles within a small micro-tidal channel. The work provided input for modelling dispersion of passive particle in shallow micro-tidal estuaries where these were not previously experimentally studied.

  12. Magnetism, structure and chemical order in small CoPd clusters: A first-principles study

    NASA Astrophysics Data System (ADS)

    Mokkath, Junais Habeeb

    2014-01-01

    The structural, electronic and magnetic properties of small ComPdn(N=m+n=8,m=0-N) nanoalloy clusters are studied in the framework of a generalized-gradient approximation to density-functional theory. The optimized cluster structures have a clear tendency to maximize the number of nearest-neighbor CoCo pairs. The magnetic order is found to be ferromagnetic-like (FM) for all the ground-state structures. Antiferromagnetic-like spin arrangements were found in some low-lying isomers. The average magnetic moment per atom μ increases approximately linearly with Co content. A remarkable enhancement of the local Co moments is observed as a result of Pd doping. This is a consequence of the increase in the number of Co d holes, due to CoPd charge transfer, combined with the reduced local coordination. The influence of spin-orbit interactions on the cluster properties is also discussed.

  13. Improvement of small-area, amorphous-silicon thin-film photovoltaics on polymer substrate

    SciTech Connect

    Weber, M.F. . Applied Technologies Lab.)

    1990-02-01

    This report describes a contract to produce, using roll-to-roll deposition on polyamide substrate, a small-area amorphous-silicon p-i-n photovoltaic (PV) cell with an energy conversion efficiency of 10% under air mass 1.5 insolation. Three improvements were attempted to achieve this goal: (1) zinc oxide, a transparent conducting oxide, was used as a top contact; the zinc oxide conductivity was improved to 8--9 ohms/square sheet resistance with less than 8% average optical absorption. (2) The red light response was improved with dielectric enhanced metal reflecting electrodes, which increased the short-circuit current density by more than 1 mA/Cm{sup 2}; a three-layer dielectric mirror coating was also designed that can increase the current density by another 1 mA/cm{sup 2}. (3) Improving the fill factor of the n-i-p (reverse structured) devices was also achieved in a multichamber deposition system. The overall energy conversion efficiency of the PV cell was 8.36%. Major obstacles to higher efficiencies are (1) controlling the thin-film defects that cause electrical shunts in devices fabricated on enhanced reflection electrodes, and (2) controlling impurities and introducing dopant profiles near the p/i interface in a continuous web deposition system.

  14. Improvement of small-area, amorphous-silicon thin-film photovoltaics on polymer substrate

    NASA Astrophysics Data System (ADS)

    Weber, Michael F.

    1990-02-01

    This report describes a contract to produce, using roll-to-roll deposition on polyamide substrate, a small-area amorphous-silicon p-i-n photovoltaic (PV) cell with an energy conversion efficiency of 10 percent under air mass 1.5 insolation. Three improvements were attempted to achieve this goal: (1) zinc oxide, a transparent conducting oxide, was used as a top contact; the zinc oxide conductivity was improved to 8 to 9 ohms/square sheet resistance with less than 8 percent average optical absorption, (2) the red light response was improved with dielectric enhanced metal reflecting electrodes, which increased the short-circuit current density by more than 1 mA/sq cm; a three-layer dielectric mirror coating was also designed that can increase the current density by another 1 mA/sq cm, and (3) improving the fill factor of the n-i-p (reverse structured) devices was also achieved in a multichamber deposition system. The overall energy conversion efficiency of the PV cell was 8.36 percent. Major obstacles to higher efficiencies are: (1) controlling the thin-film defects that cause electrical shunts in devices fabricated on enhanced reflection electrodes, and (2) controlling impurities and introducing dopant profiles near the p/i interface in a continuous web deposition system.

  15. Co-delivery of a hydrophobic small molecule and a hydrophilic peptide by porous silicon nanoparticles.

    PubMed

    Liu, Dongfei; Bimbo, Luis M; Mäkilä, Ermei; Villanova, Francesca; Kaasalainen, Martti; Herranz-Blanco, Barbara; Caramella, Carla M; Lehto, Vesa-Pekka; Salonen, Jarno; Herzig, Karl-Heinz; Hirvonen, Jouni; Santos, Hélder A

    2013-09-10

    Nanoparticulate drug delivery systems offer remarkable opportunities for clinical treatment. However, there are several challenges when they are employed to deliver multiple cargos/payloads, particularly concerning the synchronous delivery of small molecular weight drugs and relatively larger peptides. Since porous silicon (PSi) nanoparticles (NPs) can easily contain high payloads of drugs with various properties, we evaluated their carrier potential in multi-drug delivery for co-loading of the hydrophobic drug indomethacin and the hydrophilic human peptide YY3-36 (PYY3-36). Sequential loading of these two drugs into the PSi NPs enhanced the drug release rate of each drug and also their amount permeated across Caco-2 and Caco-2/HT29 cell monolayers. Regardless of the loading approach used, dual or single, the drug permeation profiles were in good correlation with their drug release behaviour. Furthermore, the permeation studies indicated the critical role of the mucus intestinal layer and the paracellular resistance in the permeation of the therapeutic compounds across the intestinal wall. Loading with PYY3-36 also greatly improved the cytocompatibility of the PSi NPs. Conformational analysis indicated that the PYY3-36 could still display biological activity after release from the PSi NPs and permeation across the intestinal cell monolayers. These results are the first demonstration of the promising potential of PSi NPs for simultaneous multi-drug delivery of both hydrophobic and hydrophilic compounds. Copyright © 2013 Elsevier B.V. All rights reserved.

  16. Small effect of water on upper mantle rheology based on silicon self-diffusion coefficients

    NASA Astrophysics Data System (ADS)

    Fei, H.; Wiedenbeck, M.; Yamazaki, D.; Katsura, T.

    2012-12-01

    Water has been considered to significantly affect the mantle dynamics. In particular, experimental deformation studies [1-4] claimed that even small amount of water enhanced the creep in olivine by orders of magnitude. However, we note that their results are experimental artifact due to a number of limitations: e.g., unavoidable grain boundary sliding when polycrystalline samples were used; limited ranges of water contents due to the limited pressures; several orders higher stress and strain rate than those in nature. High temperature creep of silicate minerals is controlled by silicon self-diffusion. Therefore, measurement of silicon self-diffusion coefficients (DSi) in minerals, which can be performed without these limitations, is an independent way to study the mantle rheology. In this study, we measured DSi in Mg end-member of olivine, namely, forsterite, as a function of water content (CH2O) across a wide range, and concluded that effect of water on upper mantle rheology is very small. Forsterite single crystals were doped with <1 to ~800 μg/g of water at 1600 K, 8 GPa using talc+brucite water sources and graphite buffer. The CH2O in the samples were controlled by the ratio of water sources to graphite. The water doped samples were polished, deposited with 500 nm 29Si enriched Mg2SiO4 thin films, and annealed at 8 GPa, 1600 or 1800 K for diffusion with the same proportion of water sources, which successfully made constant values of CH2O during diffusion annealing. The diffusion profiles were obtained by SIMS. CH2O in the samples were determined by FT-IR before and after diffusion, and also examined by SIMS. Our results yield a relationship: DSi ∝ (CH2O)1/3. This is explained by defect chemistry, where DSi∝[VSi‧‧‧‧]×[VO●●]∝(CH2O)2/3×(CH2O)-1/3=(CH2O)1/3 under the charge neutrality condition of [(OH)O●]=2[VMg‧‧] because both Si and O vacancies are needed for Si ions to diffuse. The water contents exponent (1/3) determined in this study

  17. Electron Impact Ionization and Fragmentation Dynamics of Small Atomic and Molecular Clusters

    NASA Astrophysics Data System (ADS)

    Dorn, Alexander

    2016-09-01

    New ionization and fragmentation reactions emerge if target atoms or molecules are embedded in an environment as it is the case in small clusters or in the condensed phase. These can be intermolecular energy and charge transfer processes or a completely modified fragmentation behavior of the molecular ions. Here we study low energy electron impact induced ionization with a multi-electron and ion imaging spectrometer (reaction microscope) and a supersonic gas jet target which can produce small clusters of various target species. Interatomic reactions are studied for the model system of weakly bound Ar2 dimers. Here, the coincident detection of three electrons and two ions gives detailed insight in interatomic Coulombic decay and radiative charge transfer processes. Such processes were also found in bio-relevant systems like water clusters. We studied pure and water-mixed clusters of tetrahydrofuran (C4H8O, THF) which is the simplest analog of deoxyribose in the DNA backbone. One observation is that ionization of the outermost valence orbital for the monomer leads to stable THF ions. In contrast if THF is bound to another THF or a water molecule the molecular ring breaks. In addition we identify intermolecular Coulombic decay induced by energy transfer from a water molecule ionized in the inner valence shell to the neighboring THF molecule.

  18. What Protects the Core When the Thiolated Gold Cluster is Extremely Small?

    SciTech Connect

    Jiang, Deen; Chen, Wei; Whetten, Robert L; Chen, Zhongfang

    2009-01-01

    The title question is motivated by the fact that extremely small thiolated-gold clusters such as Au{sub 20}(SR){sub 16} have been isolated, but their undetermined structures cannot be fully rationalized by the present knowledge derived from single-crystal structures of larger clusters. One needs to go beyond the linear monomer (RSAuSR) and V-shaped dimer (RSAuSRAuSR) motifs that were found to protect larger clusters. We hypothesize that the U-shaped trimer motif (RSAuSRAuSRAuSR) is required to protect the core of some extremely small thiolated-gold clusters, which have about 20 or fewer Au atoms. We test this hypothesis by proposing structural models for Au{sub 10}(SR){sub 8} based on two trimer motifs protecting a tetrahedral Au{sub 4} core and for Au{sub 20}(SR){sub 16} based on four trimer motifs protecting an Au{sub 8} core.

  19. Guided basin-hopping search of small boron clusters with density functional theory

    SciTech Connect

    Ng, Wei Chun; Yoon, Tiem Leong; Lim, Thong Leng

    2015-04-24

    The search for the ground state structures of Boron clusters has been a difficult computational task due to the unique metalloid nature of Boron atom. Previous research works had overcome the problem in the search of the Boron ground-state structures by adding symmetry constraints prior to the process of locating the local minima in the potential energy surface (PES) of the Boron clusters. In this work, we shown that, with the deployment of a novel computational approach that incorporates density functional theory (DFT) into a guided global optimization search algorithm based on basin-hopping, it is possible to directly locate the local minima of small Boron clusters in the PES at the DFT level. The ground-state structures search algorithm as proposed in this work is initiated randomly and needs not a priori symmetry constraint artificially imposed throughout the search process. Small sized Boron clusters so obtained compare well to the results obtained by similar calculations in the literature. The electronic properties of each structures obtained are calculated within the DFT framework.

  20. Surface collisions of small cluster ions at incident energies 10-102 eV

    NASA Astrophysics Data System (ADS)

    Herman, Zdenek

    2004-04-01

    A short review of the field of collisions of slow cluster ions with surfaces is presented. The main subject of the paper is a survey of results of a series of studies on slow cluster ion collisions with surfaces carried out in Innsbruck. In these studies the incident ions were fullerene multiply-charged ions and small cluster ions of polyatomic molecules (acetone and acetonitrile radical cations, protonated ethanol, clustered by 2-4 monomer molecules). For collisions of multiply-charged fullerene cations C60z+ (z from 1 to 5, incident energies 100-500 eV) only singly-charged product ions were observed which were formed by sequential loss of C2 units. The effect of multiple-charge of the projectile on its fragmentation was less pronounced than expected from a full conversion of electronic energy gained in the surface neutralization process. Fragmentation upon surface collision excitation of acetone, acetonitrile and protonated ethanol cluster ions (incident energies 10-80 eV) was found to follow the unimolecular dissociation kinetics. Formation of protonated monomers in collisions of acetone and acetonitrile stoechiometric clusters resulted both from an intra-cluster reaction and from a reaction of the monomer product ion with the surface hydrogen; it could be rationalized using the double-well potential model. Incident energy dependence of relative abundance of dissociation products from surface collisions of protonated ethanol trimers, dimers and monomers could be mutually rationalized, if the fraction of energy transformed in the surface collision into internal energy was related to one internal degree of freedom. A diagram resembling a break-down pattern of the projectile ion was obtained in this way. All internal degrees of freedom of the projectile ion seemed to be involved in the unimolecular dissociation kinetics.

  1. Technical Note: Angular dependence of a 2D monolithic silicon diode array for small field dosimetry.

    PubMed

    Stansook, Nauljun; Utitsarn, Kananan; Petasecca, Marco; Newall, Matthew K; Duncan, Mitchell; Nitschke, Kym; Carolan, Martin; Metcalfe, Peter; Lerch, Michael L F; Perevertaylo, Vladimir L; Tomé, Wolfgang A; Rosenfeld, Anatoly B

    2017-08-01

    This study aims to investigate the 2D monolithic silicon diode array size of 52 × 52 mm(2) (MP512) angular response. An angular correction method has been developed that improves the accuracy of dose measurement in a small field. The MP512 was placed at the center of a cylindrical phantom, irradiated using 6 MV and 10 MV photons and incrementing the incidence of the beam angle in 15° steps from 0° to 180°, and then in 1° steps between 85° and 95°. The MP512 response was characterized for square field sizes varying between 1 × 1 cm(2) and 10 × 10 cm(2) . The angular correction factor was obtained as the ratio of MP512 response to EBT3 film measured doses as a function of the incidence angle (Ɵ) and was normalized at 0° incidence angle. Beam profiles of the corrected MP512 responses were compared with the EBT3 responses to verify the effectiveness of the method adopted. The intrinsic angular dependence of the MP512 shows maximum relative deviation from the response normalized to 0° of 18.5 ± 0.5% and 15.5 ± 0.5% for 6 MV and 10 MV, respectively, demonstrating that the angular response is sensitive to the energy. In contrast, the variation of angular response is less affected by field size. Comparison of cross-plane profiles measured by the corrected MP512 and EBT3 shows an agreement within ±2% for all field sizes when the beams irradiated the array at 0°, 45°, 135°, and 180° angles of incidence from the normal to the detector plane. At 90° incidence, corresponding to a depth dose measurement, up to a 6% discrepancy was observed for a 1 × 1 cm(2) field of 6 MV. An angular correction factor can be adopted for small field sizes. Measurements discrepancies could be encountered when irradiating with very small fields parallel to the detector plane. Using this approach, the MP512 is shown to be a suitable detector for 2D dose mapping of small field size photon beams. © 2017 American Association of Physicists in Medicine.

  2. Role of spin-orbit interaction in the ultrafast demagnetization of small iron clusters

    NASA Astrophysics Data System (ADS)

    Stamenova, Maria; Simoni, Jacopo; Sanvito, Stefano

    2016-07-01

    The ultrafast demagnetization of small iron clusters initiated by an intense optical excitation is studied from the time-dependent spin density functional theory (TDSDFT). In particular we investigate the effect of the spin-orbit interaction on the onset of the demagnetization process. It is found that demagnetization occurs locally, in the vicinity of the atomic sites, and the initial rate of spin loss, coherent with the laser field, is proportional to the square of the ionic spin-orbit coupling strength λ . A simplified quantum spin model comprising spin-orbit interaction and a time-dependent magnetic field is found to be the minimal model able to reproduce our ab initio results. The model predicts the λ2 dependence of the onset rate of demagnetization when it is solved either analytically for the small t regime, or numerically integrated in the time domain. Our findings are supported by additional TDSDFT simulations of clusters made of Co and Ni.

  3. Density functional study of the interaction of carbon monoxide with small neutral and charged silver clusters.

    PubMed

    Zhou, Jia; Li, Zhen-Hua; Wang, Wen-Ning; Fan, Kang-Nian

    2006-06-08

    CO adsorption on small neutral, anionic, and cationic silver clusters Ag(n) (n = 1-7) has been studied with use of the PW91PW91 density functional theory (DFT) method. The adsorption of CO on-top site, among various possible sites, is energetically preferred irrespective of the charge state of the silver cluster. The cationic silver clusters generally have a greater tendency to adsorb CO than the anionic and neutral silver ones, except for n = 3 and 4, and the binding energies reach a local minimum at n = 5. The binding energies on the neutral clusters, instead, reach a local maximum at n = 3, which is about 0.87 eV, probably large enough to be captured in the experiments. Binding of CO to the silver clusters is generally weaker than that to the copper and gold counterparts at the same size and charge state. This is due to the weaker orbital interaction between silver and CO, which is caused by the larger atomic radius of the silver atom. In contrast, Au atoms with a larger nuclear charge but a similar atomic radius to silver owing to the lanthanide contraction are able to have a stronger interaction with CO.

  4. IS THE YOUNG STAR CLUSTER NGC 376 DISSOLVING IN THE FIELD OF THE SMALL MAGELLANIC CLOUD?

    SciTech Connect

    Sabbi, E.; Nota, A.; Smith, L. J.; Tosi, M.; Cignoni, M.; Gallagher, J.

    2011-09-20

    We use deep images acquired with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope in the filters F555W and F814W to characterize the properties of NGC 376, a young star cluster located in the wing of the Small Magellanic Cloud. Using isochrone fitting we derive for NGC 376 an age of 28 {+-} 7 Myr, in good agreement with previous studies. The high spatial resolution ACS data allow us to determine the center of gravity of the cluster and to construct extended surface brightness and radial density profiles. Neither of these profiles can be fitted with a theoretical model, suggesting that the cluster is not in virial equilibrium. Considering the young age of the cluster, we speculate that the distortion of the radial profiles may be the result of the rapid gas dispersal that follows the initial phase of star formation (SF). The cluster shows clear evidence of dynamical mass segregation. From the properties of the radial profiles and the present-day mass function we conclude that NGC 376 appears to have already lost nearly 90% of its initial stellar mass, probably as a consequence of the sudden gas dispersal that follows the early phase of SF.

  5. Infrared Communications for Small Spacecraft: From a Wireless Bus to Cluster Concepts

    NASA Technical Reports Server (NTRS)

    Webb, Suzanne C.; Schneider, Wolfger; Darrin, M. Ann G.; Boone, Bradley G.; Luers, Philip J.; Day, John H. (Technical Monitor)

    2001-01-01

    Nanosatellites operating singly or in clusters are anticipated for future space science missions. To implement this new communications paradigm, we are approaching cluster communications by first developing an infrared (IR) intra-craft wireless bus capability, following initially the MIL-STD-1553B protocol. Benefits of an IR wireless bus are low mass, size, power, and cost, simplicity of implementation, ease of use, minimum EMI, and efficient and reliable data transfer. Our goals are to maximize the reliable link margin in order to afford greater flexibility in receiver placement, which will ease technology insertion. We have developed a concept demonstration using a high-speed visible-band silicon PIN photodiode and a high-efficiency visible LED operating at a data rate up to 4 Mb/sec. In designing an internal IR wireless bus, we have characterized various candidate materials, emitters, and geometries, assuming a single reflection. Thus, we have measured the bidirectional reflectance distribution function (BRDF) for five different materials characteristic of typical spacecraft structures, which range from nearly Lambertian to highly specular. We have fit our data to empirical BRDF functions and modeled the detected irradiance anywhere in the plane of incidence for a divergent (LED) emitter. We have also determined the angular limits on the link geometry to remain within the required bit error rate by determining the received signal-to-noise ratio (SNR) for minimum values of irradiance received at the detector.

  6. Integrated spectral properties of 22 small angular diameter galactic open clusters

    NASA Astrophysics Data System (ADS)

    Ahumada, A. V.; Clariá, J. J.; Bica, E.

    2007-10-01

    Aims:Flux-calibrated integrated spectra of a sample of 22 Galactic open clusters of small angular diameter are presented. With one exception (ESO 429-SC2), all objects have Galactic longitudes in the range 208° < l < 33°. The spectra cover the range ≈3600-6800 Å, with a resolution of ≈14 Å. The properties of the present cluster sample are compared with those of well-studied clusters located in two 90° sectors, centred at l = 257° and l = 347°. The dissolution rate of Galactic open clusters in these two sectors is examined. Methods: Using the equivalent widths of the Balmer lines and comparing line intensities and continuum distribution of the cluster spectra with those of template cluster spectra with known properties, we derive both foreground reddening values and ages. Thus, we provide information independent of that determined through colour-magnitude diagrams. Results: The derived E(B-V) values for the whole sample vary from 0.0 in ESO 445-SC74 to 1.90 in Pismis 24, while the ages range from ~3 Myr (NGC 6604 and BH 151) to ~3.5 Gyr (Ruprecht 2). For six clusters (Dolidze 34, ESO 429-SC2, ESO 445-SC74, Ruprecht 2, BH 151 and Hogg 9) the foreground E(B-V) colour excesses and ages are determined for the first time. The results obtained for the remaining clusters show, in general terms, good agreement with previous photometric results. Conclusions: The age and reddening distributions of the present sample match those of known clusters in the two selected Galactic sectors. The present results would favour a major dissolution rate of star clusters in these two sectors. Two new solar-metallicity templates are defined corresponding to the age groups of (4-5) Myr and 30 Myr among those of Piatti et al. (2002, MNRAS, 335, 233). The Piatti et al. templates of 20 Myr and (3-4) Gyr are here redefined. Based on observations made at Complejo Astronómico El Leoncito, which is operated under agreement between the Consejo Nacional de Investigaciones Científicas y T

  7. Atomic structure, alloying behavior, and magnetism in small Fe-Pt clusters

    NASA Astrophysics Data System (ADS)

    Chittari, Bheema Lingam; Kumar, Vijay

    2015-09-01

    We report results of the atomic structure, alloying behavior, and magnetism in F emP tn(m +n =2 -10 ) clusters using projector augmented wave (PAW) pseudopotential method and spin-polarized generalized gradient approximation (GGA) for the exchange-correlation energy. These results are compared with those obtained by using HCTH exchange-correlation functional and LANL2DZ basis set in the Gaussian program and the overall trends are found to be similar. As in bulk Fe-Pt alloys, clusters with equal composition of Fe and Pt have the largest binding energy and the largest heat of nanoalloy formation for a given number of atoms in the cluster. There are some deviations due to the different symmetries in clusters and in cases where the total number of atoms is odd. The lowest energy isomers tend to maximize bonds between unlike atoms with Fe (Pt) atoms occupying high (low) coordination sites in the core (surface) of the cluster. The binding energy, heat of formation, and the second order difference of the total energy show F e2P t2 , F e4P t4 , and F e4P t6 clusters to be the most stable ones among the different clusters we have studied. The magnetic moments on Fe atoms are high in Pt-rich clusters as well as in small Fe-rich clusters and decrease as the aggregation of Fe atoms and the cluster size increases. The maximum value of the magnetic moments on Fe atoms is ˜3.8 μB , whereas for Pt atoms it is 1 μB. These are quite high compared with the values for bulk Fe as well as bulk FePt and F e3Pt phases while bulk Pt is nonmagnetic. There is significant charge transfer from those Fe atoms that interact directly with Pt atoms. We discuss the hybridization between the electronic states of Pt and Fe atoms as well as the variation in the magnetic moments on Fe and Pt atoms. Our results provide insight into the understanding of the nanoalloy behavior of Fe-Pt and we hope that this would help to design Fe based nanoalloys and their assemblies with high magnetic moments for

  8. Cooperative network clustering and task allocation for heterogeneous small satellite network

    NASA Astrophysics Data System (ADS)

    Qin, Jing

    The research of small satellite has emerged as a hot topic in recent years because of its economical prospects and convenience in launching and design. Due to the size and energy constraints of small satellites, forming a small satellite network(SSN) in which all the satellites cooperate with each other to finish tasks is an efficient and effective way to utilize them. In this dissertation, I designed and evaluated a weight based dominating set clustering algorithm, which efficiently organizes the satellites into stable clusters. The traditional clustering algorithms of large monolithic satellite networks, such as formation flying and satellite swarm, are often limited on automatic formation of clusters. Therefore, a novel Distributed Weight based Dominating Set(DWDS) clustering algorithm is designed to address the clustering problems in the stochastically deployed SSNs. Considering the unique features of small satellites, this algorithm is able to form the clusters efficiently and stably. In this algorithm, satellites are separated into different groups according to their spatial characteristics. A minimum dominating set is chosen as the candidate cluster head set based on their weights, which is a weighted combination of residual energy and connection degree. Then the cluster heads admit new neighbors that accept their invitations into the cluster, until the maximum cluster size is reached. Evaluated by the simulation results, in a SSN with 200 to 800 nodes, the algorithm is able to efficiently cluster more than 90% of nodes in 3 seconds. The Deadline Based Resource Balancing (DBRB) task allocation algorithm is designed for efficient task allocations in heterogeneous LEO small satellite networks. In the task allocation process, the dispatcher needs to consider the deadlines of the tasks as well as the residue energy of different resources for best energy utilization. We assume the tasks adopt a Map-Reduce framework, in which a task can consist of multiple

  9. Study of small carbon and semiconductor clusters using negative ion threshold photodetachment spectroscopy

    SciTech Connect

    Arnold, Caroline Chick

    1994-08-01

    The bonding and electronics of several small carbon and semiconductor clusters containing less than ten atoms are probed using negative ion threshold photodetachment (zero electron kinetic energy, or ZEKE) spectroscopy. ZEKE spectroscopy is a particularly advantageous technique for small cluster study, as it combines mass selection with good spectroscopic resolution. The ground and low-lying electronic states of small clusters in general can be accessed by detaching an electron from the ground anion state. The clusters studied using this technique and described in this work are C6-/C6, Sin-/Sin (n = 2, 3, 4), Ge2-/Ge2, In2P-/In2P,InP2-/InP2, and Ga2As-. The total photodetachment cross sections of several other small carbon clusters and the ZEKE spectrum of the I-•CH3I SN2 reaction complex are also presented to illustrate the versatility of the experimental apparatus. Clusters with so few atoms do not exhibit bulk properties. However, each specie exhibits bonding properties that relate to the type of bonding found in the bulk. C6, as has been predicted, exhibits a linear cumulenic structure, where double bonds connect all six carbon atoms. This double bonding reflects how important π bonding is in certain phases of pure carbon (graphite and fullerenes). The symmetric stretch frequencies observed in the C6- spectra, however, are in poor agreement with the calculated values. Also observed as sharp structure in total photodetachment cross section scans was an excited anion state bound by only ~40 cm-1 relative to the detachment continuum. This excited anion state appears to be a valence bound state, possible because of the high electron affinity of C6, and the open shell of the anion.

  10. Evaluation of Matrix9 silicon photomultiplier array for small-animal PET.

    PubMed

    Du, Junwei; Schmall, Jeffrey P; Yang, Yongfeng; Di, Kun; Roncali, Emilie; Mitchell, Gregory S; Buckley, Steve; Jackson, Carl; Cherry, Simon R

    2015-02-01

    The MatrixSL-9-30035-OEM (Matrix9) from SensL is a large-area silicon photomultiplier (SiPM) photodetector module consisting of a 3 × 3 array of 4 × 4 element SiPM arrays (total of 144 SiPM pixels) and incorporates SensL's front-end electronics board and coincidence board. Each SiPM pixel measures 3.16 × 3.16 mm(2) and the total size of the detector head is 47.8 × 46.3 mm(2). Using 8 × 8 polished LSO/LYSO arrays (pitch 1.5 mm) the performance of this detector system (SiPM array and readout electronics) was evaluated with a view for its eventual use in small-animal positron emission tomography (PET). Measurements of noise, signal, signal-to-noise ratio, energy resolution, flood histogram quality, timing resolution, and array trigger error were obtained at different bias voltages (28.0-32.5 V in 0.5 V intervals) and at different temperatures (5 °C-25 °C in 5 °C degree steps) to find the optimal operating conditions. The best measured signal-to-noise ratio and flood histogram quality for 511 keV gamma photons were obtained at a bias voltage of 30.0 V and a temperature of 5 °C. The energy resolution and timing resolution under these conditions were 14.2% ± 0.1% and 4.2 ± 0.1 ns, respectively. The flood histograms show that all the crystals in the 1.5 mm pitch LSO array can be clearly identified and that smaller crystal pitches can also be resolved. Flood histogram quality was also calculated using different center of gravity based positioning algorithms. Improved and more robust results were achieved using the local 9 pixels for positioning along with an energy offset calibration. To evaluate the front-end detector readout, and multiplexing efficiency, an array trigger error metric is introduced and measured at different lower energy thresholds. Using a lower energy threshold greater than 150 keV effectively eliminates any mispositioning between SiPM arrays. In summary, the Matrix9 detector system can resolve high-resolution scintillator arrays

  11. Evaluation of Matrix9 silicon photomultiplier array for small-animal PET

    PubMed Central

    Du, Junwei; Schmall, Jeffrey P.; Yang, Yongfeng; Di, Kun; Roncali, Emilie; Mitchell, Gregory S.; Buckley, Steve; Jackson, Carl; Cherry, Simon R.

    2015-01-01

    Purpose: The MatrixSL-9-30035-OEM (Matrix9) from SensL is a large-area silicon photomultiplier (SiPM) photodetector module consisting of a 3 × 3 array of 4 × 4 element SiPM arrays (total of 144 SiPM pixels) and incorporates SensL’s front-end electronics board and coincidence board. Each SiPM pixel measures 3.16 × 3.16 mm2 and the total size of the detector head is 47.8 × 46.3 mm2. Using 8 × 8 polished LSO/LYSO arrays (pitch 1.5 mm) the performance of this detector system (SiPM array and readout electronics) was evaluated with a view for its eventual use in small-animal positron emission tomography (PET). Methods: Measurements of noise, signal, signal-to-noise ratio, energy resolution, flood histogram quality, timing resolution, and array trigger error were obtained at different bias voltages (28.0–32.5 V in 0.5 V intervals) and at different temperatures (5 °C–25 °C in 5 °C degree steps) to find the optimal operating conditions. Results: The best measured signal-to-noise ratio and flood histogram quality for 511 keV gamma photons were obtained at a bias voltage of 30.0 V and a temperature of 5 °C. The energy resolution and timing resolution under these conditions were 14.2% ± 0.1% and 4.2 ± 0.1 ns, respectively. The flood histograms show that all the crystals in the 1.5 mm pitch LSO array can be clearly identified and that smaller crystal pitches can also be resolved. Flood histogram quality was also calculated using different center of gravity based positioning algorithms. Improved and more robust results were achieved using the local 9 pixels for positioning along with an energy offset calibration. To evaluate the front-end detector readout, and multiplexing efficiency, an array trigger error metric is introduced and measured at different lower energy thresholds. Using a lower energy threshold greater than 150 keV effectively eliminates any mispositioning between SiPM arrays. Conclusions: In summary, the Matrix9 detector system can resolve

  12. Increased risk of type I errors in cluster randomised trials with small or medium numbers of clusters: a review, reanalysis, and simulation study.

    PubMed

    Kahan, Brennan C; Forbes, Gordon; Ali, Yunus; Jairath, Vipul; Bremner, Stephen; Harhay, Michael O; Hooper, Richard; Wright, Neil; Eldridge, Sandra M; Leyrat, Clémence

    2016-09-06

    Cluster randomised trials (CRTs) are commonly analysed using mixed-effects models or generalised estimating equations (GEEs). However, these analyses do not always perform well with the small number of clusters typical of most CRTs. They can lead to increased risk of a type I error (finding a statistically significant treatment effect when it does not exist) if appropriate corrections are not used. We conducted a small simulation study to evaluate the impact of using small-sample corrections for mixed-effects models or GEEs in CRTs with a small number of clusters. We then reanalysed data from TRIGGER, a CRT with six clusters, to determine the effect of using an inappropriate analysis method in practice. Finally, we reviewed 100 CRTs previously identified by a search on PubMed in order to assess whether trials were using appropriate methods of analysis. Trials were classified as at risk of an increased type I error rate if they did not report using an analysis method which accounted for clustering, or if they had fewer than 40 clusters and performed an individual-level analysis without reporting the use of an appropriate small-sample correction. Our simulation study found that using mixed-effects models or GEEs without an appropriate correction led to inflated type I error rates, even for as many as 70 clusters. Conversely, using small-sample corrections provided correct type I error rates across all scenarios. Reanalysis of the TRIGGER trial found that inappropriate methods of analysis gave much smaller P values (P ≤ 0.01) than appropriate methods (P = 0.04-0.15). In our review, of the 99 trials that reported the number of clusters, 64 (65 %) were at risk of an increased type I error rate; 14 trials did not report using an analysis method which accounted for clustering, and 50 trials with fewer than 40 clusters performed an individual-level analysis without reporting the use of an appropriate correction. CRTs with a small or medium number of clusters are

  13. Effects of incident energy and angle on carbon cluster ions implantation on silicon substrate: a molecular dynamics study

    NASA Astrophysics Data System (ADS)

    Wei, Ye; Sang, Shengbo; Zhou, Bing; Deng, Xiao; Chai, Jing; Ji, Jianlong; Ge, Yang; Huo, Yuanliang; Zhang, Wendong

    2017-09-01

    Carbon cluster ion implantation is an important technique in fabricating functional devices at micro/nanoscale. In this work, a numerical model is constructed for implantation and implemented with a cutting-edge molecular dynamics method. A series of simulations with varying incident energies and incident angles is performed for incidence on silicon substrate and correlated effects are compared in detail. Meanwhile, the behavior of the cluster during implantation is also examined under elevated temperatures. By mapping the nanoscopic morphology with variable parameters, numerical formalism is proposed to explain the different impacts on phrase transition and surface pattern formation. Particularly, implantation efficiency (IE) is computed and further used to evaluate the performance of the overall process. The calculated results could be properly adopted as the theoretical basis for designing nano-structures and adjusting devices’ properties. Project supported by the National Natural Science Foundation of China (Nos. 51622507, 61471255, 61474079, 61403273, 51502193, 51205273), the Natural Science Foundation of Shanxi (Nos. 201601D021057, 201603D421035), the Youth Foundation Project of Shanxi Province (Nos. 2015021097), the Doctoral Fund of MOE of China (No. 20131402110013), the National High Technology Research and Development Program of China (No. 2015AA042601), and the Specialized Project in Public Welfare from The Ministry of Water Resources of China (Nos. 1261530110110).

  14. BINARY QUASARS IN THE SLOAN DIGITAL SKY SURVEY: EVIDENCE FOR EXCESS CLUSTERING ON SMALL SCALES

    SciTech Connect

    Hennawi, J F; Strauss, M A; Oguri, M; Inada, N; Richards, G T; Pindor, B; Schneider, D P; Becker, R H; Gregg, M D; Hall, P B; Johnston, D E; Fan, X; Burles, S; Schlegel, D J; Gunn, J E; Lupton, R; Bahcall, N A; Brunner, R J; Brinkman, J

    2005-11-10

    We present a sample of 218 new quasar pairs with proper transverse separations R{sub prop} < 1 h{sup -1} Mpc over the redshift range 0.5 < z < 3.0, discovered from an extensive follow up campaign to find companions around the Sloan Digital Sky Survey and 2dF Quasar Redshift Survey quasars. This sample includes 26 new binary quasars with separations R{sub prop} < 50 h{sup -1} kpc ({theta} < 10''), more than doubling the number of such systems known. We define a statistical sample of binaries selected with homogeneous criteria and compute its selection function, taking into account sources of incompleteness. The first measurement of the quasar correlation function on scales 10 h{sup -1} kpc < R{sub prop} < 400 h{sup -1} kpc is presented. For R{sub prop} {approx}< 40 h{sup -1} kpc, we detect an order of magnitude excess clustering over the expectation from the large scale (R{sub prop} {approx}> 3 h{sup -1} Mpc) quasar correlation function, extrapolated down as a power law to the separations probed by our binaries. The excess grows to {approx}30 at R{sub prop} {approx} 10 h{sup -1} kpc, and provides compelling evidence that the quasar autocorrelation function gets progressively steeper on sub-Mpc scales. This small scale excess can likely be attributed to dissipative interaction events which trigger quasar activity in rich environments. Recent small scale measurements of galaxy clustering and quasar-galaxy clustering are reviewed and discussed in relation to our measurement of small scale quasar clustering.

  15. Comparing methods of analysing datasets with small clusters: case studies using four paediatric datasets.

    PubMed

    Marston, Louise; Peacock, Janet L; Yu, Keming; Brocklehurst, Peter; Calvert, Sandra A; Greenough, Anne; Marlow, Neil

    2009-07-01

    Studies of prematurely born infants contain a relatively large percentage of multiple births, so the resulting data have a hierarchical structure with small clusters of size 1, 2 or 3. Ignoring the clustering may lead to incorrect inferences. The aim of this study was to compare statistical methods which can be used to analyse such data: generalised estimating equations, multilevel models, multiple linear regression and logistic regression. Four datasets which differed in total size and in percentage of multiple births (n = 254, multiple 18%; n = 176, multiple 9%; n = 10 098, multiple 3%; n = 1585, multiple 8%) were analysed. With the continuous outcome, two-level models produced similar results in the larger dataset, while generalised least squares multilevel modelling (ML GLS 'xtreg' in Stata) and maximum likelihood multilevel modelling (ML MLE 'xtmixed' in Stata) produced divergent estimates using the smaller dataset. For the dichotomous outcome, most methods, except generalised least squares multilevel modelling (ML GH 'xtlogit' in Stata) gave similar odds ratios and 95% confidence intervals within datasets. For the continuous outcome, our results suggest using multilevel modelling. We conclude that generalised least squares multilevel modelling (ML GLS 'xtreg' in Stata) and maximum likelihood multilevel modelling (ML MLE 'xtmixed' in Stata) should be used with caution when the dataset is small. Where the outcome is dichotomous and there is a relatively large percentage of non-independent data, it is recommended that these are accounted for in analyses using logistic regression with adjusted standard errors or multilevel modelling. If, however, the dataset has a small percentage of clusters greater than size 1 (e.g. a population dataset of children where there are few multiples) there appears to be less need to adjust for clustering.

  16. Photoelectron spectra of small LaOn- clusters: decreasing electron affinity upon increasing the number of oxygen atoms

    NASA Astrophysics Data System (ADS)

    Klingeler, R.; Lüttgens, G.; Pontius, N.; Rochow, R.; Bechthold, P. S.; Neeb, M.; Eberhardt, W.

    We present mass selected photoelectron spectra of small lanthanum oxide cluster anions LaOn- (n=1-5) which have been generated in a laser vaporization cluster source. The electron affinity of the lanthanum oxide clusters drops continuously with the number of chemisorbed oxygen atoms as revealed from the anion photoelectron spectra. The decreasing electron affinity behaves contrary to several other metal oxide clusters. The geometry of some of the measured clusters are discussed in comparsion with configuration interaction and density functional calculations using a Gaussian94 program package.

  17. Density functional study of the oxidation of small neutral and charged silver clusters.

    PubMed

    Pereiro, M; Botana, J; Baldomir, D; Serantes, D; Arias, J E

    2010-04-01

    We have studied the energetic and structural stability of the interaction of molecular oxygen with small neutral, anionic and cationic silver clusters, Ag(n) (3 < or = n < or = < 8). The calculations have been carried out using a linear combination of atomic Gaussian-type orbitals within the density functional theory as it is implemented in the demon-ks3.5 code. The O2 molecule has been placed in different positions surrounding the cluster, in order to increase the configurational space of the structural minima. We have found that the oxidized cation and neutral clusters undergo a 2D-3D structural transition even before than the nonoxidized counterparts. Moreover, our results show that the adsorption energies on the cationic and neutral silver oxide clusters manifest an odd-even alternation pattern. Likewise, the average magnetic moment of the O2 radical in the charged and neutral silver environment tends to be greater than the charged and neutral bare diatomic oxygen molecule.

  18. Hydrolysis of Sulfur Dioxide in Small Clusters of Sulfuric Acid: Mechanistic and Kinetic Study.

    PubMed

    Liu, Jingjing; Fang, Sheng; Wang, Zhixiu; Yi, Wencai; Tao, Fu-Ming; Liu, Jing-Yao

    2015-11-17

    The deposition and hydrolysis reaction of SO2 + H2O in small clusters of sulfuric acid and water are studied by theoretical calculations of the molecular clusters SO2-(H2SO4)n-(H2O)m (m = 1,2; n = 1,2). Sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO2 as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid (SO2 + H2O + H2SO4 - H2O) has the lowest energy barrier of 3.83 kcal/mol, in which the cluster H2SO4-(H2O)2 forms initially at the entrance channel. The energy barriers for the three hydrolysis reactions are in the order SO2 + (H2SO4)-H2O > SO2 + (H2SO4)2-H2O > SO2 + H2SO4-H2O. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid (or dimer) clusters than the corresponding reactant (monohydrated SO2). Consequently, sulfuric acid promotes the hydrolysis of SO2 both kinetically and thermodynamically. Kinetics simulations have been performed to study the importance of these reactions in the reduction of atmospheric SO2. The results will give a new insight on how the pre-existing aerosols catalyze the hydrolysis of SO2, leading to the formation and growth of new particles.

  19. Internal electric fields in small water clusters [(H2O)n; n = 2-6].

    PubMed

    Sen, Saumik; Boda, Manjusha; Venkat Lata, S; Naresh Patwari, G

    2016-06-22

    The electric field experienced by a water molecule within a water cluster depends on its position relative to the rest of the water molecules. The stabilization energies and the red-shifts in the donor O-H stretching vibrations in the water clusters increase with the cluster size concomitant with the increase in the electric field experienced by the donor O-H of a particular water molecule due to the hydrogen bonding network. The red-shifts in O-H stretching frequencies show a spread of about ±100 cm(-1) against the corresponding electric fields. Deviations from linearity were marked in the region of 100-160 MV cm(-1), which can be attributed to the strain in the hydrogen bonding network, especially for structures with DDAA and DDA motifs. The linear Stark effect holds up to 200 MV cm(-1) of internal electric field for the average red-shifts in the O-H stretching frequencies, with a Stark tuning rate of 2.4 cm(-1) (MV cm(-1))(-1), suggesting the validity of the classical model in small water clusters.

  20. Multiple Populations in the Old and Massive Small Magellanic Cloud Globular Cluster NGC 121

    NASA Astrophysics Data System (ADS)

    Dalessandro, E.; Lapenna, E.; Mucciarelli, A.; Origlia, L.; Ferraro, F. R.; Lanzoni, B.

    2016-10-01

    We used a combination of optical and near-UV Hubble Space Telescope photometry and FLAMES/ESO-VLT high-resolution spectroscopy to characterize the stellar content of the old and massive globular cluster (GC) NGC 121 in the Small Magellanic Cloud (SMC). We report on the detection of multiple stellar populations, the first case in the SMC stellar cluster system. This result enforces the emerging scenario in which the presence of multiple stellar populations is a distinctive-feature of old and massive GCs regardless of the environment, as far as the light-element distribution is concerned. We find that second-generation (SG) stars are more centrally concentrated than first-generation (FG) ones. More interestingly, at odds with what is typically observed in Galactic GCs, we find that NGC 121 is the only cluster so far to be dominated by FG stars that account for more than 65% of the total cluster mass. In the framework where GCs were born with 90%-95% of FG stars, this observational finding would suggest that either NGC 121 experienced a milder stellar mass-loss with respect to Galactic GCs or it formed a smaller fraction of SG stars. Based on observations collected with NASA/ESA HST, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555 and collected at the ESO-VLT under the program 086.D-0665.

  1. On the calculations of the vibrational Raman spectra of small water clusters

    NASA Astrophysics Data System (ADS)

    Cybulski, Hubert; Sadlej, Joanna

    2007-12-01

    The static and frequency-dependent Raman intensities for small water clusters ((H 2O) n, n = 2-8) were calculated by means of the DFT(B3LYP)/d-aug-cc-pVDZ method. The obtained intensities were analyzed in order to find a correlation between the topology of a cluster and its calculated Raman spectrum. A change from 170 for water dimer to 677 Å 4 u -1 for hexamer-ring cluster reveals a tendency of a practically linear increase in the Raman intensity of the lowest-frequency symmetric OH stretching mode with a number of water molecules in cyclic clusters (including dimer). The stretching vibrations of the three-coordinated single donor-double acceptor (DAA) molecules are marked by higher Raman intensities than the double donor-single acceptor (DDA) molecules. The simple average Raman intensity over the stretching modes of the DAA molecules yields 110 Å 4 u -1 while the corresponding value for the DDA molecules is 70 Å 4 u -1. The Raman intensities calculated at 351.1 nm are larger by 10-40% than the static ones. Previously suggested in literature origin of the shoulder on the blue side of the Raman spectrum of liquid water as arising from the water molecules whose protons are not engaged to hydrogen bond can be confirmed by our calculations.

  2. Theoretical study of IR and photoelectron spectra of small gallium-arsenide clusters

    SciTech Connect

    Pouchan, Claude; Marchal, Rémi; Hayashi, Shinsuke

    2015-01-22

    Relative stabilities of small Ga{sub n}As{sub m} clusters, as well as their structural electronic and vibrational properties, were computed and analysed using a CCSD(T) reference method since experimental data in this area are sparse or unknown. With the aim of investigating larger clusters, we explored several DFT functionals and basis sets able to mimic the reliable CCSD(T) approach. Among them, the PBE0/SBKJC+sp,d appears as the most efficient to describe the structural and vibrational properties since average differences of about 0.042Å and 5.1cm{sup −1} were obtained for bond lengths and fundamental vibrational frequencies, respectively for the first small clusters [1] of the series found from our GSAM method [2]. As further test, this model is used in order to investigate and revisit an experimental IR spectrum of Ga{sub n}As{sub m} mixture previously published by Li et al. [3]. More complicated is the difficulty which arises in the electronic description due to the presence of numerous low lying electronic states nearly degenerated to correctly describe the electronic structure. The case of Ga{sub 2}As will be discussed and the photoelectron spectra of the Ga{sub 2}As anion reanalyzed on the ground of our calculations [4] comparatively to the experimental spectra obtained by Neumark and co-workers [5].

  3. A Quick Test on Rotation Period Clustering for the Small Members of the Koronis Family

    NASA Astrophysics Data System (ADS)

    Chang, Chan-Kao; Lin, Hsing-Wen; Ip, Wing-Huen

    2016-01-01

    Rotation period clustering in prograde/retrograde rotators might be the preliminary indication of the Slivan state in the Koronis family as a result of the Yarkovsky-O’Keefe-Radzievskii-Paddack effect. We follow the general scenario of dispersion in the semimajor axis of the asteroid family members to separate prograde and retrograde rotators in the Koronis family. From the available rotation periods obtained from PTF/iPTF, we were unable to find the rotation period clustering of objects with H ≳ 12 mag in the Koronis family. This could be the result of the intermittent collisional process of small asteroids (D ≲ 20 km) which leads to astray Yarkovsky drifting. Measurement of the pole orientations of our sample will verify our preliminary result and validate our method.

  4. Theoretical study on stable small clusters of oxalic acid with ammonia and water.

    PubMed

    Weber, Kevin H; Liu, Qian; Tao, Fu-Ming

    2014-02-27

    Thermodynamically stable small clusters of oxalic acid (CO2H)2, ammonia (NH3), and water (H2O) are studied through quantum chemical calculations. The (CO2H)2-NH3 core system with up to three waters of hydration was examined by B3LYP density functional theory and MP2 molecular orbital theory with the aug-cc-pVDZ basis set. The (CO2H)2-NH3 core complexes are observed to hydrogen bond strongly and should be found in appreciably significant concentrations in the atmosphere. Subsequent hydration of the (CO2H)2-NH3 core, however, is found to be somewhat prohibitive under ambient conditions. Relative populations of the examined clusters are predicted and the binding patterns detailed. Atmospheric implications related to new particle formations are discussed.

  5. Method and apparatus for detecting and/or imaging clusters of small scattering centers in the body

    DOEpatents

    Perez-Mendez, V.; Sommer, F.G.

    1982-07-13

    An ultrasonic method and apparatus are provided for detecting and imaging clusters of small scattering centers in the breast wherein periodic pulses are applied to an ultrasound emitting transducer and projected into the body, thereafter being received by at least one receiving transducer positioned to receive scattering from the scattering center clusters. The signals are processed to provide an image showing cluster extent and location. 6 figs.

  6. Method and apparatus for detecting and/or imaging clusters of small scattering centers in the body

    DOEpatents

    Perez-Mendez, Victor; Sommer, Frank G.

    1982-01-01

    An ultrasonic method and apparatus are provided for detecting and imaging clusters of small scattering centers in the breast wherein periodic pulses are applied to an ultrasound emitting transducer and projected into the body, thereafter being received by at least one receiving transducer positioned to receive scattering from the scattering center clusters. The signals are processed to provide an image showing cluster extent and location.

  7. On the calculations of the nuclear spin spin coupling constants in small water clusters

    NASA Astrophysics Data System (ADS)

    Cybulski, Hubert; Pecul, Magdalena; Sadlej, Joanna

    2006-08-01

    The calculations of the nuclear spin-spin coupling constants were carried out for small water clusters (H 2O) n, n = 2-6, 12, and 17, using density functional theory (DFT) and second-order polarization propagator method (SOPPA). A wide range of different standard and modified basis sets was tested to enable the choice of the possibly smallest and most flexible basis set. The changes in the oxygen-proton coupling constants upon the cluster formation between the nuclei involved in hydrogen bonding cover a range of ca. 13 Hz. The range of the calculated changes in intramolecular 1JOH couplings shows that the simple model of rigid water clusters seems to be sufficient to reproduce properly the sign and to estimate the magnitude of the gas-to-liquid shift. The sign of the complexation-induced changes in the intramolecular 2JHH coupling constant is different for molecules with a different coordination number. While the sign is positive for the molecules of the single donor-single acceptor (DA) and single donor-double acceptor (DAA) types, it is negative for the double donor-single acceptor (DDA) molecules. In the four-coordinated double donor-double acceptor (DDAA) molecules the sign of Δ 2JHH varies. The hydrogen-bond transmitted intermolecular coupling constants are substantial: 1hJOH spans the range from 2.8 to 8.4 Hz while 2hJOO varies from -0.6 to 7.5 Hz. The average intermolecular 1hJOH coupling constant decays slowly with the H⋯O distance in the cyclic clusters n = 2-6. The average 2hJOO coupling decreases exponentially with the O⋯O separation for the cyclic clusters n = 2-6.

  8. Morphologies and ages of star cluster pairs and multiplets in the Small Magellanic Cloud

    NASA Astrophysics Data System (ADS)

    de Oliveira, M. R.; Dutra, C. M.; Bica, E.; Dottori, H.

    2000-10-01

    An isophotal atlas of 75 star cluster pairs and multiplets in the Small Magellanic Cloud is presented, comprising 176 objects. They are concentrated in the SMC main body. The isophotal contours were made from Digitized Sky Survey* images and showed relevant structural features possibly related to interactions in about 25% of the sample. Previous N-body simulations indicate that such shapes could be due to tidal tails, bridges or common envelopes. The diameter ratio between the members of a pair is preferentially in the range 1 - 2, with a peak at 1. The projected separation is in the range ~ 3 - 22 pc with a pronounced peak at ~ 13 pc. For 91 objects it was possible to derive ages from Colour-Magnitude Diagrams using the OGLE-II photometric survey. The cluster multiplets in general occur in OB stellar associations and/or HII region complexes. This indicates a common origin and suggests that multiplets coalesce into pairs or single clusters in a short time scale. Pairs in the SMC appear to be mostly coeval and consequently captures are a rare phenomenon. We find evidence that star cluster pairs and multiplets may have had an important role in the dynamical history of clusters presently seen as large single objects. The images in this study are based on photographic data obtained using the UK Schmidt Telescope, which was operated by the Royal Observatory Edinburgh, with funding from the UK Science and Engineering Research Council, until 1988 June, and thereafter by the Anglo-Australian Observatory. Original plate material is copyright by the Royal Observatory Edinburgh and the Anglo-Australian Observatory. The plates were processed into the present compressed digital form with their permission. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166.

  9. Highly p-doped regions in silicon solar cells quantitatively analyzed by small angle beveling and micro-Raman spectroscopy

    NASA Astrophysics Data System (ADS)

    Becker, M.; Gösele, U.; Hofmann, A.; Christiansen, S.

    2009-10-01

    Highly p-doped regions in multicrystalline silicon solar cells, such as the back surface field region, are analyzed by means of small angle beveling and micro-Raman spectroscopy. Small angle beveling and subsequent Secco etching are used to enhance the lateral resolution of the micro-Raman spectroscopic measurements and to investigate the microstructure of the back surface field region in detail. The position-dependent analysis of the free carrier concentrations within the back surface field region is based on the Raman specific Fano resonances. The Raman spectroscopic measurement results are compared to results obtained from electrochemical capacitance-voltage measurements, which allows a subsequent calibration of the Raman data for the quantitative analysis of the free carrier concentrations within the highly p-doped regions of silicon solar cells and other devices. Our investigations show that the free carrier as well as the dopant concentration profiles within the back surface field region exhibit a nearly step-functional shape instead of the extended gradient shape which the electrochemical capacitance-voltage measurements suggest. Moreover, we show that the shape of the back surface field is often influenced by grain boundaries and other defects that occur in multicrystalline silicon wafers.

  10. The Very Small Scale Clustering of SDSS-II and SDSS-III Galaxies

    NASA Astrophysics Data System (ADS)

    Piscionere, Jennifer

    2015-01-01

    We measure the angular clustering of galaxies from the Sloan Digital Sky Survey Data Release 7 in order to probe the spatial distribution of satellite galaxies within their dark matter halos. Specifically, we measure the angular correlation function on very small scales (7 - 320‧‧) in a range of luminosity threshold samples (absolute r-band magnitudes of -18 up to -21) that are constructed from the subset of SDSS that has been spectroscopically observed more than once (the so-called plate overlap region). We choose to measure angular clustering in this reduced survey footprint in order to minimize the effects of fiber collision incompleteness, which are otherwise substantial on these small scales. We model our clustering measurements using a fully numerical halo model that populates dark matter halos in N-body simulations to create realistic mock galaxy catalogs. The model has free parameters that specify both the number and spatial distribution of galaxies within their host halos. We adopt a flexible density profile for the spatial distribution of satellite galaxies that is similar to the dark matter Navarro-Frenk-White (NFW) profile, except that the inner slope is allowed to vary. We find that the angular clustering of our most luminous samples (Mr < -20 and -21) suggests that luminous satellite galaxies have substantially steeper inner density profiles than NFW. Lower luminosity samples are less constraining, however, and are consistent with satellite galaxies having shallow density profiles. Our results confirm the findings of Watson et al. (2012) while using different clustering measurements and modeling methodology. With the new SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS; Dawson et al., 2013), we can measure how the same class of galaxy evolves over time. The BOSS CMASS sample is of roughly constant stellar mass and number density out to z ˜ 0.6. The clustering of these samples appears to evolve very little with redshift, and each of the

  11. In situ investigation of the mobility of small gold clusters on cleaved MgO surfaces

    NASA Technical Reports Server (NTRS)

    Metois, J. J.; Heinemann, K.; Poppa, H.

    1976-01-01

    The mobility of small clusters of gold (about 10 A in diameter) on electron-beam-cleaved MgO surfaces was studied by in situ transmission electron microscopy under controlled vacuum and temperature conditions. During the first 10 min following a deposition at room temperature, over 10 per cent of the crystallites moved over short distances (about 20 A) discontinuously, with a velocity greater than 150 A/sec. Eighty per cent of the mobility events were characterized by the avoidance of proximity of other crystallites, and this was tentatively explained as the result of repulsive elastic forces between the interacting crystallites.

  12. Magnetic properties of iron adatoms and small iron clusters on Ag(1 0 0)

    NASA Astrophysics Data System (ADS)

    Lazarovits, B.; Szunyogh, L.; Weinberger, P.

    2002-02-01

    A Green's function embedding technique based on the fully relativistic spin-polarized Screened Korringa-Kohn-Rostoker method is used to calculate the electronic and magnetic properties of magnetic nanostructures. Strongly enhanced spin and orbital moments are obtained for an Fe adatom and for small clusters of Fe on a Ag(1 0 0) surface. As a consequence, for an Fe adatom a magnetic anisotropy energy is found that is about 10 times larger than for an Fe monolayer. Furthermore, the exchange coupling energy between two Fe adatoms is calculated in terms of the force theorem, showing a very rapid decay with increasing distance.

  13. Evaluation of Matrix9 silicon photomultiplier array for small-animal PET

    SciTech Connect

    Du, Junwei Schmall, Jeffrey P.; Yang, Yongfeng; Di, Kun; Roncali, Emilie; Mitchell, Gregory S.; Buckley, Steve; Jackson, Carl; Cherry, Simon R.

    2015-02-15

    Purpose: The MatrixSL-9-30035-OEM (Matrix9) from SensL is a large-area silicon photomultiplier (SiPM) photodetector module consisting of a 3 × 3 array of 4 × 4 element SiPM arrays (total of 144 SiPM pixels) and incorporates SensL’s front-end electronics board and coincidence board. Each SiPM pixel measures 3.16 × 3.16 mm{sup 2} and the total size of the detector head is 47.8 × 46.3 mm{sup 2}. Using 8 × 8 polished LSO/LYSO arrays (pitch 1.5 mm) the performance of this detector system (SiPM array and readout electronics) was evaluated with a view for its eventual use in small-animal positron emission tomography (PET). Methods: Measurements of noise, signal, signal-to-noise ratio, energy resolution, flood histogram quality, timing resolution, and array trigger error were obtained at different bias voltages (28.0–32.5 V in 0.5 V intervals) and at different temperatures (5 °C–25 °C in 5 °C degree steps) to find the optimal operating conditions. Results: The best measured signal-to-noise ratio and flood histogram quality for 511 keV gamma photons were obtained at a bias voltage of 30.0 V and a temperature of 5 °C. The energy resolution and timing resolution under these conditions were 14.2% ± 0.1% and 4.2 ± 0.1 ns, respectively. The flood histograms show that all the crystals in the 1.5 mm pitch LSO array can be clearly identified and that smaller crystal pitches can also be resolved. Flood histogram quality was also calculated using different center of gravity based positioning algorithms. Improved and more robust results were achieved using the local 9 pixels for positioning along with an energy offset calibration. To evaluate the front-end detector readout, and multiplexing efficiency, an array trigger error metric is introduced and measured at different lower energy thresholds. Using a lower energy threshold greater than 150 keV effectively eliminates any mispositioning between SiPM arrays. Conclusions: In summary, the Matrix9 detector system

  14. Defect engineering of the oxygen-vacancy clusters formation in electron irradiated silicon by isovalent doping: An infrared perspective

    NASA Astrophysics Data System (ADS)

    Londos, C. A.; Sgourou, E. N.; Chroneos, A.

    2012-12-01

    Infrared spectroscopy was used to study the production and evolution of oxygen-vacancy (VOn for n = 1, 2, 3 and VmO for m = 1, 2, 3) clusters, in electron-irradiated Czochralski silicon (Cz-Si) samples, doped with isovalent dopants. It was determined that the production of the VO pair is enhanced in Ge-doped Si but is suppressed in Sn and Pb-doped Si. The phenomenon is discussed in terms of the competition between isovalent dopants and oxygen atoms in capturing vacancies in the course of irradiation. In the case of Ge, only transient GeV pairs form, leading finally to an increase of the VO production. Conversely, for Sn and Pb the corresponding pairs with vacancies are stable, having an opposite impact on the formation of VO pairs. Regarding V2O and V3O clusters, our measurements indicate that Ge doping enhances their formation, although Sn and Pb dopants suppress it. Similar arguments as those for the VO pair could be put forward, based on the effect of isovalent impurities on the availability of vacancies. Additionally, it was found that the conversion ratio of VO to VO2 decreases as the covalent radius of the isovalent dopant increases. These results are discussed in terms of the local strains introduced by the isovalent dopants in the Si lattice. These local strains affect the balance of the intrinsic defects created as a result of irradiation, as well as the balance between the two main reactions (VO + Oi → VO2 and VO + SiI → Oi) participating in the VO annealing, leading finally to a decrease of the VO2 production. The larger the covalent radius of the isovalent dopant (rGe < rSn < rPb), the larger the introduced strains in the lattice and then the less the VO2 formation in accordance with our experimental results. Interestingly, an opposite trend was observed for the conversion ratio of VO2 to VO3. The phenomenon is attributed to the enhanced diffusivity of oxygen impurity as a result of the presence of isovalent dopants, leading to an enhanced formation

  15. The changes in small metal cluster size with adsorption Be13Xn, X = H, O, S, Cl and F

    NASA Technical Reports Server (NTRS)

    Bauschlicher, C. W., Jr.

    1985-01-01

    The Be-Be and Be-adsorbate distances are optimized for small Be13Xn clusters,s assuming only two independent degrees of freedom. Results for chemisorption into the three-fold hollows are quite similar to those found for small metal clusters on supports. It is predicted that Cl and F will have the same effect on the metal-metal bond lengths, and so will O and S.

  16. Energy and field size dependence of a silicon diode designed for small-field dosimetry.

    PubMed

    Yarahmadi, Mehran; Wegener, Sonja; Sauer, Otto A

    2017-05-01

    To investigate the energy dependence/spectral sensitivity of silicon diodes designed for small-field dosimetry and obtain response factors (RFs) for arbitrary photon spectra using Monte Carlo (MC) simulations. The EGSnrc user-code DOSRZnrc was used to calculate the dose deposition in water and in the active volume of a stereotactic diode field detector (SFD). Then, the RFs of the SFD were calculated for several circular field sizes and energies at 5 cm depth in water. Several low-energy photon spectra (mean energy 55 to 200 keV), as well as Co-60 radiation (mean energy 1.25 MeV) and a 6 MV Elekta Synergy beam (mean energy 2.9 MeV), in 10 × 10 cm(2) field size were used to validate the MC calculations, using a simple beam model. The RFs of the SFD detector for a 6 MV Elekta Synergy linac photon beam in different field sizes were calculated. These were also measured with EBT3 Gafchromic film and the SFD detector. For the reference field size, differences between measured and calculated RFs were less than 5% at mean energies below 1 MeV and less than 1% at energies above 1 MeV. The calculated RFs for a 6 MV Elekta Synergy linac photon beam as a function of different field sizes showed a good agreement between the measurements and previously reported results. This agreement was within 2% for all considered field sizes. While at high photon energies, the change of response of the SFD is marginal, whereas it is extreme at low energies. Therefore, it is desirable to benchmark response calculations also in the low-energy domain. Our results, with a simple beam model and geometry, indicate that a validation of the simulations by experimental results is achievable. The present work provides a comprehensive table that can be used to calculate SFD detector response factors depending on both, field size and photon energy. © 2017 American Association of Physicists in Medicine.

  17. Origin of complex impact craters on native oxide coated silicon surfaces

    NASA Astrophysics Data System (ADS)

    Samela, Juha; Nordlund, Kai; Popok, Vladimir N.; Campbell, Eleanor E. B.

    2008-02-01

    Crater structures induced by impact of keV-energy Arn+ cluster ions on silicon surfaces are measured with atomic force microscopy. Complex crater structures consisting of a central hillock and outer rim are observed more often on targets covered with a native silicon oxide layer than on targets without the oxide layer. To explain the formation of these complex crater structures, classical molecular dynamics simulations of Ar cluster impacts on oxide coated silicon surfaces, as well as on bulk amorphous silica, amorphous Si, and crystalline Si substrates, are carried out. The diameter of the simulated hillock structures in the silicon oxide layer is in agreement with the experimental results, but the simulations cannot directly explain the height of hillocks and the outer rim structures when the oxide coated silicon substrate is free of defects. However, in simulations of 5keV /atom Ar12 cluster impacts, transient displacements of the amorphous silicon or silicon oxide substrate surfaces are induced in an approximately 50nm wide area surrounding the impact point. In silicon oxide, the transient displacements induce small topographical changes on the surface in the vicinity of the central hillock. The comparison of cluster stopping mechanisms in the various silicon oxide and silicon structures shows that the largest lateral momentum is induced in the silicon oxide layer during the impact; thus, the transient displacements on the surface are stronger than in the other substrates. This can be a reason for the higher frequency of occurrence of the complex craters on oxide coated silicon.

  18. Transient Clustering of Reaction Intermediates during Wet Etching of Silicon Nanostructures.

    PubMed

    Aabdin, Zainul; Xu, Xiu Mei; Sen, Soumyo; Anand, Utkarsh; Král, Petr; Holsteyns, Frank; Mirsaidov, Utkur

    2017-05-10

    Wet chemical etching is a key process in fabricating silicon (Si) nanostructures. Currently, wet etching of Si is proposed to occur through the reaction of surface Si atoms with etchant molecules, forming etch intermediates that dissolve directly into the bulk etchant solution. Here, using in situ transmission electron microscopy (TEM), we follow the nanoscale wet etch dynamics of amorphous Si (a-Si) nanopillars in real-time and show that intermediates generated during alkaline wet etching first aggregate as nanoclusters on the Si surface and then detach from the surface before dissolving in the etchant solution. Molecular dynamics simulations reveal that the molecules of etch intermediates remain weakly bound to the hydroxylated Si surface during the etching and aggregate into nanoclusters via surface diffusion instead of directly diffusing into the etchant solution. We confirmed this model experimentally by suppressing the formation of nanoclusters of etch intermediates on the Si surfaces by shielding the hydroxylated Si sites with large ions. These results suggest that the interaction of etch intermediates with etching surfaces controls the solubility of reaction intermediates and is an important parameter in fabricating densely packed clean 3D nanostructures for future generation microelectronics.

  19. Photodissociation of HCl and small (HCl)m complexes in and on large Arn clusters

    NASA Astrophysics Data System (ADS)

    Nahler, N. Hendrik; Fárník, Michal; Buck, Udo; Vach, Holger; Gerber, R. Benny

    2004-07-01

    Photodissociation experiments were carried out at 193 nm for single HCl molecules which are adsorbed on the surface of large Arn clusters and small (HCl)m complexes which are embedded in the interior of these clusters. For the surface case the size dependence is measured for the average sizes =140-1000. No cage exit events are observed in agreement with the substitutional position of the molecule deeply buried in the outermost shell. This result is confirmed by a molecular dynamics simulation of the pickup process under realistic conditions concerning the experiment and the interaction potentials. The calculations of the dissociation process employ the surface hopping model. For the embedded case the average sizes covered are =3 and 6 and =8-248. The kinetic energy of the H atom fragments is measured exhibiting peaks at zero and around 2.0 eV which mark completely caged and unperturbed fragments, respectively. The ratio of theses peaks strongly depends on the cluster size and agrees well with theoretical predictions for one and two closed icosahedral shells, in which the nonadiabatic coupling of all states was accounted for.

  20. Large-scale confinement and small-scale clustering of floating particles in stratified turbulence

    NASA Astrophysics Data System (ADS)

    Sozza, A.; De Lillo, F.; Musacchio, S.; Boffetta, G.

    2016-09-01

    We derive a simple model, valid within the Boussinesq approximation, for the dynamics of small buoyant particles in stratified turbulence, in the presence of a mean linear density profile. By means of extensive direct numerical simulations, we investigate the statistical distribution of particles as a function of the two dimensionless parameters of the problem. We find that vertical confinement of particles is mainly ruled by the degree of stratification, with a weak dependence on the particle properties. In contrast, small-scale fractal clustering is found to depend on the particles relaxation time and is only slightly dependent on the flow stratification. The implications of our findings for the formation of thin phytoplankton layers are discussed.

  1. Scale invariant SURF detector and automatic clustering segmentation for infrared small targets detection

    NASA Astrophysics Data System (ADS)

    Zhang, Haiying; Bai, Jiaojiao; Li, Zhengjie; Liu, Yan; Liu, Kunhong

    2017-06-01

    The detection and discrimination of infrared small dim targets is a challenge in automatic target recognition (ATR), because there is no salient information of size, shape and texture. Many researchers focus on mining more discriminative information of targets in temporal-spatial. However, such information may not be available with the change of imaging environments, and the targets size and intensity keep changing in different imaging distance. So in this paper, we propose a novel research scheme using density-based clustering and backtracking strategy. In this scheme, the speeded up robust feature (SURF) detector is applied to capture candidate targets in single frame at first. And then, these points are mapped into one frame, so that target traces form a local aggregation pattern. In order to isolate the targets from noises, a newly proposed density-based clustering algorithm, fast search and find of density peak (FSFDP for short), is employed to cluster targets by the spatial intensive distribution. Two important factors of the algorithm, percent and γ , are exploited fully to determine the clustering scale automatically, so as to extract the trace with highest clutter suppression ratio. And at the final step, a backtracking algorithm is designed to detect and discriminate target trace as well as to eliminate clutter. The consistence and continuity of the short-time target trajectory in temporal-spatial is incorporated into the bounding function to speed up the pruning. Compared with several state-of-arts methods, our algorithm is more effective for the dim targets with lower signal-to clutter ratio (SCR). Furthermore, it avoids constructing the candidate target trajectory searching space, so its time complexity is limited to a polynomial level. The extensive experimental results show that it has superior performance in probability of detection (Pd) and false alarm suppressing rate aiming at variety of complex backgrounds.

  2. The structures of small gold cluster anions as determined by a combination of ion mobility measurements and density functional calculations

    NASA Astrophysics Data System (ADS)

    Furche, Filipp; Ahlrichs, Reinhart; Weis, Patrick; Jacob, Christoph; Gilb, Stefan; Bierweiler, Thomas; Kappes, Manfred M.

    2002-10-01

    A combined experimental and theoretical study of small gold cluster anions is performed. The experimental effort consists of ion mobility measurements that lead to the assignment of the collision cross sections for the different cluster sizes at room temperature. The theoretical study is based on ab initio molecular dynamics calculations with the goal to find energetically favorable candidate structures. By comparison of the theoretical results with the measured collision cross sections as well as vertical detachment energies (VDEs) from the literature, we assign structures for the small Aun- ions (n<13) and locate the transition from planar to three-dimensional structures. While a unique assignment based on the observed VDEs alone is generally not possible, the collision cross sections provide a direct and rather sensitive measure of the cluster structure. In contrast to what was expected from other metal clusters and previous theoretical studies, the structural transition occurs at an unusually large cluster size of twelve atoms.

  3. Clustering Information of Non-Sampled Area in Small Area Estimation of Poverty Indicators

    NASA Astrophysics Data System (ADS)

    Sundara, V. Y.; Kurnia, A.; Sadik, K.

    2017-03-01

    Empirical Bayes (EB) is one of indirect estimates methods which used to estimate parameters in small area. Molina and Rao has been used this method for estimates nonlinear small area parameter based on a nested error model. Problems occur when this method is used to estimate parameter of non-sampled area which is solely based on synthetic model which ignore the area effects. This paper proposed an approach to clustering area effects of auxiliary variable by assuming that there are similarities among particular area. A simulation study was presented to demonstrate the proposed approach. All estimations were evaluated based on the relative bias and relative root mean squares error. The result of simulation showed that proposed approach can improve the ability of model to estimate non-sampled area. The proposed model was applied to estimate poverty indicators at sub-districts level in regency and city of Bogor, West Java, Indonesia. The result of case study, relative root mean squares error prediction of empirical Bayes with information cluster is smaller than synthetic model.

  4. A Parallel Multigrid Solver for High Frequency Electromagnetic Field Analyses with Small-scale PC Cluster

    NASA Astrophysics Data System (ADS)

    Yosui, Kuniaki; Iwashita, Takeshi; Mori, Michiya; Kobayashi, Eiichi

    Finite element analyses of electromagnetic field are commonly used for designing of various electronic devices. The scale of the analyses becomes larger and larger, therefore, a fast linear solver is needed to solve linear equations arising from the finite element method. Since a multigrid solver is the fastest linear solver for these problems, parallelization of a multigrid solver is a quite useful approach. From the viewpoint of industrial applications, an effective usage of a small-scale PC cluster is important due to initial cost for introducing parallel computers. In this paper, a distributed parallel multigrid solver for a small-scale PC cluster is developed. In high frequency electromagnetic field analyses, a special block Gauss-Seidel smoother is used for the multigrid solver instead of general smoothers such as Gauss-Seidel smoother or Jacobi smoother in order to improve a convergence rate. The block multicolor ordering technique is applied to parallelize the smoother. A numerical exsample shows that a 3.7-fold speed-up in computational time and a 3.0-fold increase in the scale of the analysis were attained when the number of CPU was increased from one to five.

  5. Variable size small targets detection using density-based clustering combined with backtracking strategy

    NASA Astrophysics Data System (ADS)

    Zhang, Haiying; Lin, Yonggui; Xiao, Fangxiong

    2016-10-01

    The series problem of infrared small target detection in heavy clutter is a challenging work in active vision. During different imaging environments the size and gray intensity of target will keep changing which lead to unstable detection. Focus on mining more robust feature of small targets and following the sequential detection framework, we propose a novel research scheme based on density-based clustering and backtracking strategy in this paper. First, point of interest is extracted by the speeded up robust feature (SURF) detector for its better performance in digging features invariant to uniform scaling, orientation and illumination changes. Second, due to the local aggregation property of target trajectory in space, a new proposed density-based clustering method is introduced to segment the target trajectory, so that the target detection problem is transformed into the extract the target trajectory. Then, In order to keep the integral and independence of the trace as much as possible, two factors: percent and are exploited to help deciding the clustering granularity. Later, the backtracking strategy is adopted to search for the target trajectory with pruning function on the basis of the consistence and continuity of the short-time target trajectory in temporal-spatial. Extended experiments show the validity of our method. Compared with the data association methods executed on the huge candidate trajectory space, the time-consuming is reduced obviously. Additional, the feature detection is more stable for the use of SURF and the false alarm suppression rate is superior to most baseline and state-of-arts methods.

  6. Fingerprints of carbon, nitrogen, and silicon isotopes in small interstellar SiC grains from the Murchison meteorite

    NASA Technical Reports Server (NTRS)

    Hoppe, Peter; Geiss, Johannes; Buehler, Fritz; Neuenschwander, Juerg; Amari, Sachiko; Lewis, Roy S.

    1993-01-01

    We report ion microprobe determinations of the carbon, nitrogen, and silicon isotopic compositions of small SiC grains from the Murchison CM2 chondrite. Analyses were made on samples containing variable numbers of grains and on 14 individual grains. In some cases the multiple-grain sample compositions were probably dominated by only one or two grains. Total ranges observed are given. Only a few grains show values near the range limits. Both the total ranges of carbon and nitrogen isotopic compositions, and even the narrower ranges typical for the majority of the grains, are similar to those observed for larger SiC grains. Two rare components appear to be present in the smaller-size fraction, one characterized by C-12/C-13 about 12-16 and the other by very heavy nitrogen. The carbon and nitrogen isotopic compositions qualitatively may reflect hydrostatic H-burning via the CNO cycle and He-burning in red giants, as well as explosive H-burning in novae. The silicon isotopic compositions of most grains qualitatively show what is the signature of He-burning. The silicon isotopic composition of one grain, however, suggests a different process.

  7. Physico-chemical treatment of wastewater from clusters of small scale cotton textile units.

    PubMed

    Pathe, P P; Biswas, A K; Rao, N N; Kaul, S N

    2005-03-01

    Small scale industries can not own individual wastewater treatment facility due to non-availability of land and skilled manpower for operation and maintenance of wastewater treatment plants. A centralized wastewater treatment facility for clusters of small scale industries is appropriate. This concept is gaining popularity in recent years. In India, various textile process operations are undertaken by individual small scale units. The wastewater generated at these units is conveyed to a common effluent treatment facility comprising of equalization, flocculation-clarification, activated sludge process, secondary clarification and finally discharge into inland surface water bodies. The wastewater from small scale cotton textile processing units was highly coloured and alkaline with average BOD and COD concentration of 205 and 790 mg l(-1), respectively. Due to the presence of several dyes, particularly reactive dyes, the biological treatment is often found less effective. Therefore, applicability of various physico-chemical treatment methods needs to be investigated in pursuit of an alternative to biological treatment of textile wastewater. A physico-chemical treatment scheme, involving chemical coagulation-sedimentation, dual media filtration, activated carbon adsorption followed by chemical oxidation was investigated in this paper. The quality of final treated wastewater in terms of BOD and COD was 18-24 and 230-240 mg l(-1), respectively through this scheme. A scheme of treatment comprising coagulation-sedimentation, dual media filtration, activated carbon, chemical oxidation may be considered as an alternative to biological treatment of textile wastewater.

  8. WFPC2 Observations of Star Clusters in the Magellanic Clouds. Report 2; The Oldest Star Clusters in the Small Magellanic Cloud

    NASA Technical Reports Server (NTRS)

    Mighell, Kenneth J.; Sarajedini, Ata; French, Rica S.

    1998-01-01

    We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) observations in F45OW ( approximately B) and F555W (approximately V) of the intermediate-age populous star clusters NGC 121, NGC 339, NGC 361, NGC 416, and Kron 3 in the Small Magellanic Cloud. We use published photometry of two other SMC populous star clusters, Lindsay 1 and Lindsay 113, to investigate the age sequence of these seven populous star clusters in order to improve our understanding of the formation chronology of the SMC. We analyzed the V vs B-V and M(sub V) vs (B-V)(sub 0) color-magnitude diagrams of these populous Small Magellanic Cloud star clusters using a variety of techniques and determined their ages, metallicities, and reddenings. These new data enable us to improve the age-metallicity relation of star clusters in the Small Magellanic Cloud. In particular, we find that a closed-box continuous star-formation model does not reproduce the age-metallicity relation adequately. However, a theoretical model punctuated by bursts of star formation is in better agreement with the observational data presented herein.

  9. Modeling optical properties of silicon clusters by first principles: From a few atoms to large nanocrystals

    SciTech Connect

    Nurbawono, Argo; Liu, Shuanglong; Zhang, Chun

    2015-04-21

    Time dependent density functional tight binding (TDDFTB) method is implemented with sparse matrix techniques and improved parallelization algorithms. The method is employed to calculate the optical properties of various Si nanocrystals (NCs). The calculated light absorption spectra of small Si NCs from TDDFTB were found to be comparable with many body perturbation methods utilizing planewave basis sets. For large Si NCs (more than a thousand atoms) that are beyond the reach of conventional approaches, the TDDFTB method is able to produce reasonable results that are consistent with prior experiments. We also employed the method to study the effects of surface chemistry on the optical properties of large Si NCs. We learned that the optical properties of Si NCs can be manipulated with small molecule passivations such as methyl, hydroxyl, amino, and fluorine. In general, the shifts and profiles in the absorption spectra can be tuned with suitably chosen passivants.

  10. Encapsulating Metal Clusters and Acid Sites within Small Voids: Synthetic Strategies and Catalytic Consequences

    NASA Astrophysics Data System (ADS)

    Goel, Sarika

    The selective encapsulation of metal clusters within zeolites can be used to prepare clusters that are uniform in diameter and to protect them against sintering and contact with feed impurities, while concurrently allowing active sites to select reactants based on their molecular size, thus conferring enzyme-like specificity to chemical catalysis. The apertures in small and medium-pore zeolites preclude the use of post-synthetic protocols to encapsulate the relevant metal precursors because cationic or anionic precursors with their charge-balancing double layer and gaseous complexes cannot diffuse through their windows or channels. We have developed general strategies to encapsulate metal clusters within small-pore zeolites by using metal precursors stabilized by ammonia or organic amine ligands, which stabilize metal precursors against their premature precipitation at the high temperature and pH conditions required for the hydrothermal synthesis of the target zeolite structures and favor interactions between metal precursors and incipient aluminosilicate nuclei during the self-assembly of microporous frameworks. When synthesis temperatures were higher than 400 K, available ligands were unable to prevent the premature precipitation of the metal precursors. In such cases, encapsulation was achieved instead via interzeolite transformations after successfully encapsulating metal precursors or clusters via post-synthesis exchange or ligand protection into parent zeolites and subsequently converting them into the target structures while retaining the encapsulated clusters or precursors. Such strategies led to the successful selective encapsulation of a wide range of metal clusters (Pt, Pd, Ru, Rh, Ir, Re, and Ag) within small-pore (SOD (sodalite), LTA (Linde type A (zeolite A)), GIS (gismondine), and ANA (analcime)) and medium-pore (MFI (ZSM-5)) zeolites. These protocols provide novel and diverse mechanism-based strategies for the design of catalysts with protected

  11. Modelling galaxy clustering on small scales to tighten constraints on dark energy and modified gravity

    NASA Astrophysics Data System (ADS)

    Wang, Yun

    2017-01-01

    We present a new approach to measuring cosmic expansion history and growth rate of large-scale structure using the anisotropic two-dimensional galaxy correlation function (2DCF) measured from data; it makes use of the empirical modelling of small-scale galaxy clustering derived from numerical simulations by Zheng et al. We validate this method using mock catalogues, before applying it to the analysis of the CMASS sample from the Sloan Digital Sky Survey Data Release 10 of the Baryon Oscillation Spectroscopic Survey. We find that this method enables accurate and precise measurements of cosmic expansion history and growth rate of large-scale structure. Modelling the 2DCF fully including non-linear effects and redshift space distortions in the scale range of 16-144 h-1 Mpc, we find H(0.57)rs(zd)/c = 0.0459 ± 0.0006, DA(0.57)/rs(zd) = 9.011 ± 0.073, and fg(0.57)σ8(0.57) = 0.476 ± 0.050, which correspond to precisions of 1.3 per cent, 0.8 per cent, and 10.5 per cent, respectively. We have defined rs(zd) to be the sound horizon at the drag epoch computed using a simple integral, fg(z) as the growth rate at redshift z, and σ8(z) as the matter power spectrum normalization on 8 h-1 Mpc scale at z. We find that neglecting the small-scale information significantly weakens the constraints on H(z) and DA(z), and leads to a biased estimate of fg(z). Our results indicate that we can significantly tighten constraints on dark energy and modified gravity by reliably modelling small-scale galaxy clustering.

  12. THE CLUSTERED NATURE OF STAR FORMATION. PRE-MAIN-SEQUENCE CLUSTERS IN THE STAR-FORMING REGION NGC 602/N90 IN THE SMALL MAGELLANIC CLOUD

    SciTech Connect

    Gouliermis, Dimitrios A.; Gennaro, Mario; Schmeja, Stefan; Dolphin, Andrew E.; Tognelli, Emanuele; Prada Moroni, Pier Giorgio

    2012-03-20

    Located at the tip of the wing of the Small Magellanic Cloud (SMC), the star-forming region NGC 602/N90 is characterized by the H II nebular ring N90 and the young cluster of pre-main-sequence (PMS) and early-type main-sequence stars NGC 602, located in the central area of the ring. We present a thorough cluster analysis of the stellar sample identified with Hubble Space Telescope/Advanced Camera for Surveys in the region. We show that apart from the central cluster low-mass PMS stars are congregated in 13 additional small, compact sub-clusters at the periphery of NGC 602, identified in terms of their higher stellar density with respect to the average background density derived from star counts. We find that the spatial distribution of the PMS stars is bimodal, with an unusually large fraction ({approx}60%) of the total population being clustered, while the remaining is diffusely distributed in the intercluster area, covering the whole central part of the region. From the corresponding color-magnitude diagrams we disentangle an age difference of {approx}2.5 Myr between NGC 602 and the compact sub-clusters, which appear younger, on the basis of comparison of the brighter PMS stars with evolutionary models, which we accurately calculated for the metal abundance of the SMC. The diffuse PMS population appears to host stars as old as those in NGC 602. Almost all detected PMS sub-clusters appear to be centrally concentrated. When the complete PMS stellar sample, including both clustered and diffused stars, is considered in our cluster analysis, it appears as a single centrally concentrated stellar agglomeration, covering the whole central area of the region. Considering also the hot massive stars of the system, we find evidence that this agglomeration is hierarchically structured. Based on our findings, we propose a scenario according to which the region NGC 602/N90 experiences an active clustered star formation for the last {approx}5 Myr. The central cluster NGC 602 was

  13. Clustering and layering of In adatoms on low and high index silicon surfaces: A comparative study

    NASA Astrophysics Data System (ADS)

    Kuyyalil, Jithesh; Govind; Kumar, Mahesh; Shivaprasad, S. M.

    2010-10-01

    Using the morphological differences of low and high index surfaces as templates for metal growth, several low dimensional overlayer structures with novel structural and electronic properties can be formed. We present here a first report on submonolayer adsorption and residual thermal desorption studies of In adatoms on reconstructed high index Si (5 5 12)-2 × 1 surface and compare it with the observations on planar Si (111)-7 × 7 surface. The study is done by using in-situ Ultra High Vacuum surface sensitive probes like Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These conventional wide area techniques provide an understanding of atomistic issues involved in the evolution of the interface. We have observed an anomalous growth mode during adsorption at room temperature (RT) above 2ML, which includes adatom layering and clustering on Si (111) surface. This is also manifested during the desorption experiments on both surfaces, and the subtle differences on the two surfaces are discussed. The observation of LEED pattern during the adsorption process shows formation of different superstructural phases on Si (111)-7 × 7 surface. On Si (5 5 12) 2 × 1 surface we observe the sequential 2× (225), 2× (337) and 2× (113) facet formation during adsorption/desorption, which include quasi 1D-nanowire/chain structures. A combination of lattice strain effects, presence of step-edge barrier and quantum size effects are employed to speculate the differences in adsorption and desorption.

  14. Quantitative Analysis of Clustered DNA Damages Induced by Silicon Beams of Different Kinetic Energy

    SciTech Connect

    Keszenman D. J.; Keszenman, D.J.; Bennett, P.V.; Sutherland, B.M.; Wilson, P.F.

    2013-05-14

    Humans may b exposed to highly energetic charged particle radiation as a result of medical treatments, occupational activitie or accidental events. In recent years, our increasing presence and burgeoning interest in space exploration beyond low Earth orbit has led to a large increase in the research of the biological effects ofcharged particle radiation typical of that encountered in the space radiation environment. The study of the effects of these types of radiation qualities in terms ofDNA damage induction and repair is fundamental to understand mechanisms both underlying their greater biological effectiveness as we)) as the short and long term risks of health effects such as carcinogenesis, degen rative diseases and premature aging. Charged particle radiation induces a variety of DNA alterations, notably bistranded clustered damages, defined as two or more closely-opposed strand break , oxidized bases or abasic sites within a few helical turns. The induction of such highly complex DNA damage enhances the probability of incorrect or incomplete repair and thus constitutes greater potential for genomic instability, cell death and transformation.

  15. Sensitized erbium emission from silicon-rich nitride/silicon superlattice structures

    SciTech Connect

    Dal Negro, L.; Li, R.; Warga, J.; Basu, S. N.

    2008-05-05

    Erbium-doped silicon-rich nitride/silicon superlattice structures were fabricated by direct magnetron cosputtering deposition on Si substrates. Rapid thermal annealing resulted in the nucleation of small amorphous Si clusters, which efficiently sensitize 1.54 {mu}m emission via a nanosecond-fast nonresonant energy transfer process, providing an alternative route toward the fabrication of Si-compatible devices based on Er sensitization.

  16. Computer Molecular Dynamics Simulation Study of Isomerization and Melting of Small Alkali-Halide Clusters.

    NASA Astrophysics Data System (ADS)

    Luo, Jia

    1987-09-01

    In this study we have systematically investigated, using Molecular Dynamics simulations, the energetics, structure and dynamics of alkali-halide clusters of variable sizes ((NaCl)_{n}, for n = 4, 16 and 108) over a wide temperature range. Our simulations show that the very nature of the phase transformation and the underlying physical processes involved, depend on the size of the system. We conclude that at the lower end of the size spectrum, the phase space of the system is characterized by a small number of stable configuration (solid isomers) between which the system transforms in a diffusionless manner, with temperature dependent rates and branching ratios. As the system size is increased the number of accessible conformers increases leading to a hierarchical kinetics of isomerization events which exhibits itself as a broadening of the transition region. For these small clusters coexistence is between solid isomers rather than inter-phase (solid-liquid) coexistence. The latter develops for clusters of sufficient size, characterized by a dense spectrum of accessible states, separated by thermally surmountable barriers. Under these circumstances conventional melting is observed as a sharp transition, the separation of time-scales for inter-well and intra -well dynamics ceases, and true solid-liquid coexistence is found. In the Methods section of the text, we describe the potential model, the computer program algorithm, the molecular dynamics running initial conditions, and the method of temperature control. In the Results section, we discuss and demonstrate the structures of alkali-halide clusters as a function of temperature and the curves of the caloric equation of state; the time evolution of the kinetic energy and the distribution of short time average of the kinetic energy; the coordination number function and the function n(r), which is related to the pair correlation function, as a function of temperature and size; the mean square displacement, the

  17. Small Launch Vehicle Design Approaches: Clustered Cores Compared with Multi-Stage Inline Concepts

    NASA Technical Reports Server (NTRS)

    Waters, Eric D.; Beers, Benjamin; Esther, Elizabeth; Philips, Alan; Threet, Grady E., Jr.

    2013-01-01

    In an effort to better define small launch vehicle design options two approaches were investigated from the small launch vehicle trade space. The primary focus was to evaluate a clustered common core design against a purpose built inline vehicle. Both designs focused on liquid oxygen (LOX) and rocket propellant grade kerosene (RP-1) stages with the terminal stage later evaluated as a LOX/methane (CH4) stage. A series of performance optimization runs were done in order to minimize gross liftoff weight (GLOW) including alternative thrust levels, delivery altitude for payload, vehicle length to diameter ratio, alternative engine feed systems, re-evaluation of mass growth allowances, passive versus active guidance systems, and rail and tower launch methods. Additionally manufacturability, cost, and operations also play a large role in the benefits and detriments for each design. Presented here is the Advanced Concepts Office's Earth to Orbit Launch Team methodology and high level discussion of the performance trades and trends of both small launch vehicle solutions along with design philosophies that shaped both concepts. Without putting forth a decree stating one approach is better than the other; this discussion is meant to educate the community at large and let the reader determine which architecture is truly the most economical; since each path has such a unique set of limitations and potential payoffs.

  18. Photoabsorption spectra of small cationic xenon clusters from time-dependent density functional theory.

    PubMed

    Oliveira, Micael J T; Nogueira, Fernando; Marques, Miguel A L; Rubio, Angel

    2009-12-07

    Upon ionization, rare-gas (like Ar and Xe) clusters shift their absorption spectrum from the ultraviolet to the visible. This happens as bonding becomes much stronger due to the removal of an electron from a strongly antibonding orbital. In this article, we study the absorption spectrum of small cationic xenon clusters (Xe(n) (+), with n=3,...,35) by means of time-dependent density functional theory. These calculations include relativistic effects through the use of relativistic j-dependent pseudopotentials in a two-spinor formulation of the Kohn-Sham equations. The peak positions in our calculated spectra are in fairly good agreement with experiment and confirm that absorption is mainly due to a charged linear core composed of 3, 4, or 5 Xe atoms where the positive charge is localized. However, we find large deviations concerning the oscillator strengths, which can be partially explained by the unsatisfactory treatment of exchange in common density functionals. Furthermore, we find that adequate ground-state geometries are necessary for the correct prediction of the qualitative features of the spectra.

  19. Photoabsorption spectra of small cationic xenon clusters from time-dependent density functional theory

    SciTech Connect

    Oliveira, Micael J. T.; Nogueira, Fernando; Marques, Miguel A. L.; Rubio, Angel

    2009-12-07

    Upon ionization, rare-gas (like Ar and Xe) clusters shift their absorption spectrum from the ultraviolet to the visible. This happens as bonding becomes much stronger due to the removal of an electron from a strongly antibonding orbital. In this article, we study the absorption spectrum of small cationic xenon clusters (Xe{sub n}{sup +}, with n=3,...,35) by means of time-dependent density functional theory. These calculations include relativistic effects through the use of relativistic j-dependent pseudopotentials in a two-spinor formulation of the Kohn-Sham equations. The peak positions in our calculated spectra are in fairly good agreement with experiment and confirm that absorption is mainly due to a charged linear core composed of 3, 4, or 5 Xe atoms where the positive charge is localized. However, we find large deviations concerning the oscillator strengths, which can be partially explained by the unsatisfactory treatment of exchange in common density functionals. Furthermore, we find that adequate ground-state geometries are necessary for the correct prediction of the qualitative features of the spectra.

  20. Theoretical investigation on isomer formation probability and free energy of small C clusters

    NASA Astrophysics Data System (ADS)

    Lin, Zheng-Zhe

    2015-06-01

    Molecular dynamics simulations and free energy calculations are employed to investigate the evolution, formation probability, detailed balance, and isomerization rate of small C cluster isomer at 2500 K. For C10, the isomer formation probability predicted by free energy is in good agreement with molecular dynamics simulation. However, for C20, C30, and C36, the formation probabilities predicted by free energy are not in agreement with molecular dynamics simulations. Although the cluster systems are in equilibrium, detailed balance is not reached. Such results may be attributed to high transformation barriers between cage, bowl, and sheet isomers. In summary, for mesoscopic nanosystems the free energy criterion, which commonly holds for macroscopic systems in dynamic equilibrium, may not provide a good prediction for isomer formation probability. New theoretical criterion should be further investigated for predicting the isomer formation probability of a mesoscopic nanosystem. Project supported by the National Natural Science Foundation of China (Grant No. 11304239) and the Fundamental Research Funds for the Central Universities.

  1. Auditory disturbances promote temporal clustering of yawning and stretching in small groups of budgerigars (Melopsittacus undulatus).

    PubMed

    Miller, Michael L; Gallup, Andrew C; Vogel, Andrea R; Clark, Anne B

    2012-08-01

    Yawning may serve both social and nonsocial functions. When budgerigars (Melopsittacus undulatus) are briefly held, simulating capture by a predator, the temporal pattern of yawning changes. When this species is observed in a naturalistic setting (undisturbed flock), yawning and also stretching, a related behavior, are mildly contagious. On the basis of these findings, we hypothesized that a stressful event would be followed by the clustering of these behaviors in a group of birds, which may be facilitated both by a standard pattern of responding to a startling stressor and also contagion. In this study, we measured yawning and stretching in 4-bird groups following a nonspecific stressor (loud white noise) for a period of 1 hr, determining whether auditory disturbances alter the timing and frequency of these behaviors. Our results show that stretching, and to a lesser degree yawning, were nonrandomly clumped in time following the auditory disturbances, indicating that the temporal clustering is sensitive to, and enhanced by, environmental stressors while in small groups. No decrease in yawning such as found after handling stress was observed immediately after the loud noise but a similar increase in yawning 20 min after was observed. Future research is required to tease apart the roles of behavioral contagion and a time-setting effect following a startle in this species. This research is of interest because of the potential role that temporal clumping of yawning and stretching could play in both the collective detection of, and response to, local disturbances or predation threats. 2012 APA, all rights reserved

  2. Imaging bond breaking and vibrational energy transfer in small water containing clusters

    NASA Astrophysics Data System (ADS)

    Samanta, Amit K.; Ch'ng, Lee C.; Reisler, Hanna

    2013-06-01

    This letter presents a brief overview of our recent experimental studies of state-to-state vibrational predissociation (VP) dynamics of small hydrogen bonded (H-bonded) clusters following vibrational excitation. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) are used to determine accurate bond dissociation energies (D0) of (H2O)2, (H2O)3, HCl-H2O and NH3-H2O. Pair-correlated product energy distributions from the VP of these complexes are also presented and compared to theoretical models. Further insights into mechanisms are obtained from the recent quasi-classical trajectory (QCT) calculations of Bowman and coworkers. The D0 values for (H2O)2 and (H2O)3 are in very good agreement with recent calculated values, and the results are used to estimate the contributions of cooperative interactions to the H-bonding network.

  3. The VMC survey - XV. The Small Magellanic Cloud-Bridge connection history as traced by their star cluster populations

    NASA Astrophysics Data System (ADS)

    Piatti, Andrés E.; de Grijs, Richard; Rubele, Stefano; Cioni, Maria-Rosa L.; Ripepi, Vincenzo; Kerber, Leandro

    2015-06-01

    We present results based on YJKs photometry of star clusters located in the outermost, eastern region of the Small Magellanic Cloud (SMC). We analysed a total of 51 catalogued clusters whose colour-magnitude diagrams (CMDs), having been cleaned from field-star contamination, were used to assess the clusters' reality and estimate ages of the genuine systems. Based on CMD analysis, 15 catalogued clusters were found to be possible non-genuine aggregates. We investigated the properties of 80 per cent of the catalogued clusters in this part of the SMC by enlarging our sample with previously obtained cluster ages, adopting a homogeneous scale for all. Their spatial distribution suggests that the oldest clusters, log(t yr-1) ≥ 9.6, are in general located at greater distances to the galaxy's centre than their younger counterparts - 9.0 ≤ log(t yr-1) ≤ 9.4 - while two excesses of clusters are seen at log(t yr-1) ˜ 9.2 and log(t yr-1) ˜ 9.7. We found a trail of younger clusters which follow the wing/bridge components. This long spatial sequence does not only harbour very young clusters, log(t yr-1) ˜ 7.3, but it also hosts some of intermediate ages, log(t yr-1) ˜ 9.1. The derived cluster and field-star formation frequencies as a function of age are different. The most surprising feature is an observed excess of clusters with ages of log(t yr-1) < 9.0, which could have been induced by interactions with the LMC.

  4. Replacement equivalence of H- and argon in small (Ar)nH- clusters from optimized structure calculations

    NASA Astrophysics Data System (ADS)

    Sebastianelli, F.; Baccarelli, I.; Di Paola, C.; Gianturco, F. A.

    2004-08-01

    The structural properties of some of the smaller ionic clusters of argon atoms containing the atomic impurity H-, ArnH- with n from 2 up to 7, are examined using different modeling for the interactions within each cluster and by employing different theoretical treatments, both classical and quantum, for the energetics. The same calculations are also carried out for the corresponding neutral homogeneous clusters Arn+1. The results of the calculations, the physical reliability of the interactions modeling, and the similarities and the difference between the anionic and the neutral complexes are discussed in some detail. The emerging picture shows that, due to specific features of the employed atom-atom potentials, the ArnH- and Arn+1 clusters present very similar structures, where the H- dopant substitutes for one of the outer Ar atoms but does not undergo as yet solvation within such small clusters.

  5. Diffusion of small clusters on metal (100) surfaces: Exact master-equation analysis for lattice-gas models

    SciTech Connect

    Sanchez, J.R.; Evans, J.W.

    1999-01-01

    Exact results are presented for the surface diffusion of small two-dimensional clusters, the constituent atoms of which are commensurate with a square lattice of adsorption sites. Cluster motion is due to the hopping of atoms along the cluster perimeter with various rates. We apply the formalism of Titulaer and Deutch [J. Chem. Phys. {bold 77}, 472 (1982)], which describes evolution in reciprocal space via a linear master equation with dimension equal to the number of cluster configurations. We focus on the regime of rapid hopping of atoms along straight close-packed edges, where certain subsets of configurations cycle rapidly between each other. Each such subset is treated as a single quasiconfiguration, thereby reducing the dimension of the evolution equation, simplifying the analysis, and elucidating limiting behavior. We also discuss the influence of concerted atom motions on the diffusion of tetramers and larger clusters. {copyright} {ital 1999} {ital The American Physical Society}

  6. The binding of CO molecule with small Wn(n = 2-9) clusters: a DFT investigation

    NASA Astrophysics Data System (ADS)

    Sun, Xiyuan; Du, Jiguang

    2014-08-01

    The hybrid DFT functional has been utilized to investigate CO adsorption on small W n ( n = 2-9) clusters. The reactivity-selectivity descriptor Δf( r) is proved to be an appropriate tool for predicting the local adsorption site. Our results indicate that the binding of CO to tungsten cluster prefers terminal adsorption mode except for W3 trimer. The Wiberg bond index can predict the interaction between W atoms and CO molecule to a good accuracy using a linear fit. The dependency on number of adsorbate and charge state of host clusters is also investigated. The high CO coverage seriously weakens the W-CO interaction. The anion cluster reveals the stronger interaction with CO molecule than cation and neutral ones. Moreover, the interesting coverage-dependence is revealed for anion cluster.

  7. Doping porous silicon with erbium: pores filling as a method to limit the Er-clustering effects and increasing its light emission.

    PubMed

    Mula, Guido; Printemps, Tony; Licitra, Christophe; Sogne, Elisa; D'Acapito, Francesco; Gambacorti, Narciso; Sestu, Nicola; Saba, Michele; Pinna, Elisa; Chiriu, Daniele; Ricci, Pier Carlo; Casu, Alberto; Quochi, Francesco; Mura, Andrea; Bongiovanni, Giovanni; Falqui, Andrea

    2017-07-20

    Er clustering plays a major role in hindering sufficient optical gain in Er-doped Si materials. For porous Si, the long-standing failure to govern the clustering has been attributed to insufficient knowledge of the several, concomitant and complex processes occurring during the electrochemical Er-doping. We propose here an alternative road to solve the issue: instead of looking for an equilibrium between Er content and light emission using 1-2% Er, we propose to significantly increase the electrochemical doping level to reach the filling the porous silicon pores with luminescent Er-rich material. To better understand the intricate and superposing phenomena of this process, we exploit an original approach based on needle electron tomography, EXAFS and photoluminescence. Needle electron tomography surprisingly shows a heterogeneous distribution of Er content in the silicon thin pores that until now couldn't be revealed by the sole use of scanning electron microscopy compositional mapping. Besides, while showing that pore filling leads to enhanced photoluminescence emission, we demonstrate that the latter is originated from both erbium oxide and silicate. These results give a much deeper understanding of the photoluminescence origin down to nanoscale and could lead to novel approaches focused on noteworthy enhancement of Er-related photoluminescence in porous silicon.

  8. Very Small Scale Clustering and Merger Rate of Luminous Red Galaxies

    NASA Astrophysics Data System (ADS)

    Masjedi, Morad; Hogg, David W.; Cool, Richard J.; Eisenstein, Daniel J.; Blanton, Michael R.; Zehavi, Idit; Berlind, Andreas A.; Bell, Eric F.; Schneider, Donald P.; Warren, Michael S.; Brinkmann, Jon

    2006-06-01

    We present the small-scale (0.01 Mpcsmall scales to be explained in current versions of the halo model for galaxy clustering. We infer an LRG-LRG merger rate of <~0.6×104 Gyr-1 Gpc-3 for this sample. This result suggests that the LRG-LRG mergers are not the main mode of mass growth for LRGs at z<0.36.

  9. Structural selection and amorphization of small Ni-Ti bimetallic clusters.

    PubMed

    Liu, H B; Canizal, G; Schabes-Retchkiman, P S; Ascencio, J A

    2006-06-29

    Classical molecular dynamics simulation is used for structural thermodynamic analysis of Ni-Ti bimetallic clusters. Experimental observation for the nanoclusters synthesized by the bioreduction method is used to consolidate the conclusion. The results demonstrate that Ni-Ti nanoclusters as small as 2-3 nm are not energetically favorable for common ordered geometrical arrangements such as cuboctahedron, decahedron, and icosahedron, though they can be synthesized experimentally. For the elemental distribution, Ni and Ti tend to aggregate separately. In the cases under study, eutectic-like and Ni-core/Ti-shell structures can keep their basic shape and elemental distribution during long periods of relaxation at room temperature. For other cases such as solid solution and Ti-core/Ni-shell, the structures amorphized and the elements tend to distribute uniformly even though they are at temperatures as low as room temperature. Experimental evidence was obtained by the analysis of biosynthesized nanoparticles using transmission electron microscopy techniques. This allowed determination of the partial amorphized structures of small bimetallic particles with cubic and multiple twined-like structures.

  10. CA II TRIPLET SPECTROSCOPY OF SMALL MAGELLANIC CLOUD RED GIANTS. III. ABUNDANCES AND VELOCITIES FOR A SAMPLE OF 14 CLUSTERS

    SciTech Connect

    Parisi, M. C.; Clariá, J. J.; Marcionni, N.; Geisler, D.; Villanova, S.; Sarajedini, A.; Grocholski, A. J. E-mail: claria@oac.uncor.edu E-mail: dgeisler@astro-udec.cl E-mail: ata@astro.ufl.edu

    2015-05-15

    We obtained spectra of red giants in 15 Small Magellanic Cloud (SMC) clusters in the region of the Ca ii lines with FORS2 on the Very Large Telescope. We determined the mean metallicity and radial velocity with mean errors of 0.05 dex and 2.6 km s{sup −1}, respectively, from a mean of 6.5 members per cluster. One cluster (B113) was too young for a reliable metallicity determination and was excluded from the sample. We combined the sample studied here with 15 clusters previously studied by us using the same technique, and with 7 clusters whose metallicities determined by other authors are on a scale similar to ours. This compilation of 36 clusters is the largest SMC cluster sample currently available with accurate and homogeneously determined metallicities. We found a high probability that the metallicity distribution is bimodal, with potential peaks at −1.1 and −0.8 dex. Our data show no strong evidence of a metallicity gradient in the SMC clusters, somewhat at odds with recent evidence from Ca ii triplet spectra of a large sample of field stars. This may be revealing possible differences in the chemical history of clusters and field stars. Our clusters show a significant dispersion of metallicities, whatever age is considered, which could be reflecting the lack of a unique age–metallicity relation in this galaxy. None of the chemical evolution models currently available in the literature satisfactorily represents the global chemical enrichment processes of SMC clusters.

  11. Reducing Capsule Based on Electron Programming: Versatile Synthesizer for Size-Controlled Ultra-Small Metal Clusters.

    PubMed

    Kambe, Tetsuya; Imaoka, Takane; Yamamoto, Kimihisa

    2016-11-07

    Controlled reducing capsules with a specific number of reducing electrons were achieved by appropriately placed BH3 units in the dendritic polyphenylazomethines (DPAs). Using the 1:1 coordination fashion on their basic branches with radius affinity gradient, the 4th generation DPA (DPAG4) possessing four BH3 units in the central positions was prepared as a template synthesizer for size-controlled ultra-small metal clusters. This was well-demonstrated by reduction of Ag, Pt, and other metal ions resulting in monodispersed ultra-small clusters.

  12. Pd Diffusion on MgO(100): The Role of Defects and Small Cluster Mobilit.

    SciTech Connect

    Xu, Lijun; Henkelman, Graeme A.; Campbell, Charles T.; Jonsson, Hannes

    2006-02-09

    The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Density functional theory is used to explore the energy landscape of Pd atoms adsorbed on the terrace of MgO(10 0) and at oxygen vacancy sites. Saddle point finding methods reveal that small Pd clusters diffuse on the terrace in interesting ways. The monomer and dimer diffuse via single atom hops between oxygen sites with barriers of 0.34 eV and 0.43 eV respectively. The trimer and tetramer, however, form 3D clusters by overcoming a 2D–3D transition barrier of less than 60 meV. The trimer diffuses along the surface either by a walking or flipping motion, with comparable barriers of ca. 0.5 eV. The tetramer rolls along the terrace with a lower barrier of 0.42 eV. Soft rotational modes at the saddle point lead to an anomalously high prefactor of 1.3 · 1014 s!1 for tetramer diffusion. This prefactor is two order of magnitude higher than for monomer diffusion, making the tetramer the fastest diffusing species on the terrace at all temperatures for which diffusion is active (above 200 K). Neutral oxygen vacancy sites are found to bind Pd monomers with a 2.63 eV stronger binding energy than the terrace. A second Pd atom, however, binds to this trapped monomer with a smaller energy of 0.56 eV, so that dimers at defects dissociate on a time scale of milliseconds at room temperature. Larger clusters bind more strongly at defects. Trimers and tetramers dissociate from monomer-bound-defects at elevated temperatures of ca. 600 K. These species are also mobile on the terrace, suggesting they are important for the ripening observed at P600 K during Pd vapor deposition on MgO(100) by Haas et al. [G. Haas, A. Menck, H. Brune, J.V. Barth, J.A. Venables, K. Kern, Phys. Rev. B 61 (2000) 11105].

  13. The problem of the structure (state of helium) in small He{sub N}-CO clusters

    SciTech Connect

    Potapov, A. V. Panfilov, V. A.; Surin, L. A.; Dumesh, B. S.

    2010-11-15

    A second-order perturbation theory, developed for calculating the energy levels of the He-CO binary complex, is applied to small He{sub N}-CO clusters with N = 2-4, the helium atoms being considered as a single bound object. The interaction potential between the CO molecule and HeN is represented as a linear expansion in Legendre polynomials, in which the free rotation limit is chosen as the zero approximation and the angular dependence of the interaction is considered as a small perturbation. By fitting calculated rotational transitions to experimental values it was possible to determine the optimal parameters of the potential and to achieve good agreement (to within less than 1%) between calculated and experimental energy levels. As a result, the shape of the angular anisotropy of the interaction potential is obtained for various clusters. It turns out that the minimum of the potential energy is smoothly shifted from an angle between the axes of the CO molecule and the cluster of {theta} = 100{sup o} in He-CO to {theta} = 180{sup o} (the oxygen end) in He{sub 3}-CO and He{sub 4}-CO clusters. Under the assumption that the distribution of helium atoms with respect to the cluster axis is cylindrically symmetric, the structure of the cluster can be represented as a pyramid with the CO molecule at the vertex.

  14. Effect of intermodular connection on fast sparse synchronization in clustered small-world neural networks

    NASA Astrophysics Data System (ADS)

    Kim, Sang-Yoon; Lim, Woochang

    2015-11-01

    We consider a clustered network with small-world subnetworks of inhibitory fast spiking interneurons and investigate the effect of intermodular connection on the emergence of fast sparsely synchronized rhythms by varying both the intermodular coupling strength Jinter and the average number of intermodular links per interneuron Msyn(inter ). In contrast to the case of nonclustered networks, two kinds of sparsely synchronized states such as modular and global synchronization are found. For the case of modular sparse synchronization, the population behavior reveals the modular structure, because the intramodular dynamics of subnetworks make some mismatching. On the other hand, in the case of global sparse synchronization, the population behavior is globally identical, independently of the cluster structure, because the intramodular dynamics of subnetworks make perfect matching. We introduce a realistic cross-correlation modularity measure, representing the matching degree between the instantaneous subpopulation spike rates of the subnetworks, and examine whether the sparse synchronization is global or modular. Depending on its magnitude, the intermodular coupling strength Jinter seems to play "dual" roles for the pacing between spikes in each subnetwork. For large Jinter, due to strong inhibition it plays a destructive role to "spoil" the pacing between spikes, while for small Jinter it plays a constructive role to "favor" the pacing between spikes. Through competition between the constructive and the destructive roles of Jinter, there exists an intermediate optimal Jinter at which the pacing degree between spikes becomes maximal. In contrast, the average number of intermodular links per interneuron Msyn(inter ) seems to play a role just to favor the pacing between spikes. With increasing Msyn(inter ), the pacing degree between spikes increases monotonically thanks to the increase in the degree of effectiveness of global communication between spikes. Furthermore, we

  15. Is gold actor or spectator in the reaction of small AunPd{m/+} clusters with O2?

    NASA Astrophysics Data System (ADS)

    Lang, Sandra M.; Frank, Anja; Fleischer, Irene; Bernhardt, Thorsten M.

    2013-01-01

    The reactivity of free binary gold-palladium clusters (AuPd2+, Au2Pd+, Au2Pd2+, and Au2Pd3+) toward molecular oxygen was investigated in an ion trap experiment under multi-collision conditions and compared to the reactivities of bare Aun+ and Pdm+ (n, m = 2 - 5) clusters. Reaction kinetics measurements revealed that the reaction rate is mainly determined by the number of palladium atoms in the clusters and only weakly influenced by additional gold atoms. The same holds true for the observed reaction product distributions. Most interestingly, the most reactive cluster ions Pd3+, Au2Pd3+, and Pd5+ exhibit a strong preference to form tetroxide products, AunPdmO4+. In addition, employing temperature dependent mass spectrometry, a second adsorption species consisting of several weakly bound oxygen molecules was identified for all investigated palladium containing clusters which is, however, only formed at cryogenic temperatures. All these observations suggest that the gold atoms largely act upon a spectator role in the reaction of the binary clusters. Nevertheless, a rough estimation of the relative O2 binding energies via statistical rate theory indicates that the addition of gold to the Pdn+ clusters decreases the O2-cluster interaction strength, although the reaction rate stays constant. This effect in the binary clusters may be of importance to a potential activation and dissociation of the adsorbed O2 molecules. ISSPIC 16 - 16th International Symposium on Small Particles and Inorganic Clusters, edited by Kristiaan Temst, Margriet J. Van Bael, Ewald Janssens, H.-G. Boyen and Françoise Remacle.

  16. Quantum model of a solid-state spin qubit: Ni cluster on a silicon surface by the generalized spin Hamiltonian and X-ray absorption spectroscopy investigations

    NASA Astrophysics Data System (ADS)

    Farberovich, Oleg V.; Mazalova, Victoria L.; Soldatov, Alexander V.

    2015-11-01

    We present here the quantum model of a Ni solid-state electron spin qubit on a silicon surface with the use of a density-functional scheme for the calculation of the exchange integrals in the non-collinear spin configurations in the generalized spin Hamiltonian (GSH) with the anisotropic exchange coupling parameters linking the nickel ions with a silicon substrate. In this model the interaction of a spin qubit with substrate is considered in GSH at the calculation of exchange integrals Jij of the nanosystem Ni7-Si in the one-electron approach taking into account chemical bonds of all Si-atoms of a substrate (environment) with atoms of the Ni7-cluster. The energy pattern was found from the effective GSH Hamiltonian acting in the restricted spin space of the Ni ions by the application of the irreducible tensor operators (ITO) technique. In this paper we offer the model of the quantum solid-state N-spin qubit based on the studying of the spin structure and the spin-dynamics simulations of the 3d-metal Ni clusters on the silicon surface. The solution of the problem of the entanglement between spin states in the N-spin systems is becoming more interesting when considering clusters or molecules with a spectral gap in their density of states. For quantifying the distribution of the entanglement between the individual spin eigenvalues (modes) in the spin structure of the N-spin system we use the density of entanglement (DOE). In this study we have developed and used the advanced high-precision numerical techniques to accurately assess the details of the decoherence process governing the dynamics of the N-spin qubits interacting with a silicon surface. We have studied the Rabi oscillations to evaluate the N-spin qubits system as a function of the time and the magnetic field. We have observed the stabilized Rabi oscillations and have stabilized the quantum dynamical qubit state and Rabi driving after a fixed time (0.327 μs). The comparison of the energy pattern with the

  17. Silicon nanowires with high-k hafnium oxide dielectrics for sensitive detection of small nucleic acid oligomers.

    PubMed

    Dorvel, Brian R; Reddy, Bobby; Go, Jonghyun; Duarte Guevara, Carlos; Salm, Eric; Alam, Muhammad Ashraful; Bashir, Rashid

    2012-07-24

    Nanobiosensors based on silicon nanowire field effect transistors offer advantages of low cost, label-free detection, and potential for massive parallelization. As a result, these sensors have often been suggested as an attractive option for applications in point-of-care (POC) medical diagnostics. Unfortunately, a number of performance issues, such as gate leakage and current instability due to fluid contact, have prevented widespread adoption of the technology for routine use. High-k dielectrics, such as hafnium oxide (HfO(2)), have the known ability to address these challenges by passivating the exposed surfaces against destabilizing concerns of ion transport. With these fundamental stability issues addressed, a promising target for POC diagnostics and SiNWFETs has been small oligonucleotides, more specifically, microRNA (miRNA). MicroRNAs are small RNA oligonucleotides which bind to mRNAs, causing translational repression of proteins, gene silencing, and expressions are typically altered in several forms of cancer. In this paper, we describe a process for fabricating stable HfO(2) dielectric-based silicon nanowires for biosensing applications. Here we demonstrate sensing of single-stranded DNA analogues to their microRNA cousins using miR-10b and miR-21 as templates, both known to be upregulated in breast cancer. We characterize the effect of surface functionalization on device performance using the miR-10b DNA analogue as the target sequence and different molecular weight poly-l-lysine as the functionalization layer. By optimizing the surface functionalization and fabrication protocol, we were able to achieve <100 fM detection levels of the miR-10b DNA analogue, with a theoretical limit of detection of 1 fM. Moreover, the noncomplementary DNA target strand, based on miR-21, showed very little response, indicating a highly sensitive and highly selective biosensing platform.

  18. Silicon Nanowires with High-k Hafnium Oxide Dielectrics for Sensitive Detection of Small Nucleic Acid Oligomers

    PubMed Central

    Dorvel, Brian R.; Reddy, Bobby; Go, Jonghyun; Guevara, Carlos Duarte; Salm, Eric; Alam, Muhammad Ashraful; Bashir, Rashid

    2012-01-01

    Nanobiosensors based on silicon nanowire field effect transistors offer advantages of low cost, label-free detection, and potential for massive parallelization. As a result, these sensors have often been suggested as an attractive option for applications in Point-of-care (POC) medical diagnostics. Unfortunately, a number of performance issues such as gate leakage and current instability due to fluid contact, have prevented widespread adoption of the technology for routine use. High-k dielectrics, such as hafnium oxide (HfO2), have the known ability to address these challenges by passivating the exposed surfaces against destabilizing concerns of ion transport. With these fundamental stability issues addressed, a promising target for POC diagnostics and SiNWFET’s has been small oligonucleotides, more specifically microRNA (miRNA). MicroRNA’s are small RNA oligonucleotides which bind to messenger RNA’s, causing translational repression of proteins, gene silencing, and expressions are typically altered in several forms of cancer. In this paper, we describe a process for fabricating stable HfO2 dielectric based silicon nanowires for biosensing applications. Here we demonstrate sensing of single stranded DNA analogues to their microRNA cousins using miR-10b and miR-21 as templates, both known to be upregulated in breast cancer. We characterize the effect of surface functionalization on device performance using the miR-10b DNA analogue as the target sequence and different molecular weight poly-l-lysine as the functionalization layer. By optimizing the surface functionalization and fabrication protocol, we were able to achieve <100fM detection levels of miR-10b DNA analogue, with a theoretical limit of detection of 1fM. Moreover, the non-complementary DNA target strand, based on miR-21, showed very little response, indicating a highly sensitive and highly selective biosensing platform. PMID:22695179

  19. Basic parameters of three star clusters in the Small Magellanic Cloud: Kron 11, Kron 63 and NGC 121

    NASA Astrophysics Data System (ADS)

    Baume, G.; Noël, N. E. D.; Costa, E.; Carraro, G.; Méndez, R. A.; Pedreros, M. H.

    2008-11-01

    We present observations for three star clusters, Kron 11, Kron 63 and NGC 121, in the Small Magellanic Cloud. We have studied their structure and derived their fundamental parameters by means of their luminosity functions, their colour magnitude diagrams and the Padova suite of isochrones. NGC 121 is a well-studied object, for which we confirm previous evidence about its old age and low-metal content, and have found that it is an undergoing mass segregation. Kron 11 and Kron 63 are poorly populated clusters which had never been studied so far. Kron 11 is several gigayears younger than NGC 121, while Kron 63 is basically a very young star aggregate. Both clusters are immersed in dense stellar fields which share the same population properties, suggesting that in their cases, cluster ages are consistent with typical ages of field stars.

  20. Duplication of small segments within the major breakpoint cluster region in chronic myelogenous leukemia.

    PubMed

    Litz, C E; McClure, J S; Copenhaver, C M; Brunning, R D

    1993-03-15

    The t(9;22) in chronic myelogenous leukemia (CML) may be reciprocal or, in a minority of cases, may result in an extensive deletion of a portion of the major breakpoint cluster region (M-bcr) of the BCR. This report provides evidence of the duplication of small segments within the M-bcr in a small group of patients with CML. Southern blots of Bgl II and Bgl II/BamHI double-digested DNA from the blood or bone marrow of 46 patients with CML were probed with a 5' 1.4-kb Taq I/HindIII M-bcr probe and a 3' 2-kb HindIII/BamHI M-bcr probe. In three patients, rearrangements were noted with both probes in Bgl II-digested DNA, but were not present in Bgl II/BamHI-digested DNA with either probe. Southern analysis of DNA samples double-digested with Bgl II and BspHI from two of these three cases showed no rearrangements with either probe; the M-bcr BspHI site is located 26 bp 3' of the BamHI site in the second intron of the M-bcr. The presence of a rearranged M-bcr with both probes in Bgl II-digested DNA and the lack of rearrangement in Bgl II/BamHI and Bgl II/BspHI double-digested DNA suggest the presence of M-bcr BamHI and BspHI sites on both 9q+ chromosome (9q+) and the Philadelphia chromosome (Ph). This implies a duplication of at least the 26-bp M-bcr BamHI/BspHI fragment in these two samples. Sequence data from one of these two cases confirmed the M-bcr breakpoints to be staggered; the Ph M-bcr breakpoint occurred 258 bp downstream from the 9q+ M-bcr breakpoint. It is concluded that a duplication of small segments within the M-bcr occurs in a small group of patients with CML, which may lead to pseudogermline patterns on Southern blot. Such a duplication may provide insight into the mechanism of some chromosomal translocations in neoplasia.

  1. Effect of Spin Multiplicity in O2 Adsorption and Dissociation on Small Bimetallic AuAg Clusters.

    PubMed

    García-Cruz, Raúl; Poulain, Enrique; Hernández-Pérez, Isaías; Reyes-Nava, Juan A; González-Torres, Julio C; Rubio-Ponce, A; Olvera-Neria, Oscar

    2017-08-17

    To dispose of atomic oxygen, it is necessary the O2 activation; however, an energy barrier must be overcome to break the O-O bond. This work presents theoretical calculations of the O2 adsorption and dissociation on small pure Aun and Agm and bimetallic AunAgm (n + m ≤ 6) clusters using the density functional theory (DFT) and the zeroth-order regular approximation (ZORA) to explicitly include scalar relativistic effects. The most stable AunAgm clusters contain a higher concentration of Au with Ag atoms located in the center of the cluster. The O2 adsorption energy on pure and bimetallic clusters and the ensuing geometries depend on the spin multiplicity of the system. For a doublet multiplicity, O2 is adsorbed in a bridge configuration, whereas for a triplet only one O-metal bond is formed. The charge transfer from metal toward O2 occupies the σ*O-O antibonding natural bond orbital, which weakens the oxygen bond. The Au3 ((2)A) cluster presents the lowest activation energy to dissociate O2, whereas the opposite applies to the AuAg ((3)A) system. In the O2 activation, bimetallic clusters are not as active as pure Aun clusters due to the charge donated by Ag atoms being shared between O2 and Au atoms.

  2. Prediction of Structures and Atomization Energies of Small Silver Clusters, (Ag)n, n < 100

    SciTech Connect

    Chen, Mingyang; Dyer, Jason E.; Li, Keijing; Dixon, David A.

    2013-08-29

    Neutral silver clusters, Agn, were studied using density functional theory (DFT) followed by high level coupled cluster CCSD(T) calculations to determine the low energy isomers for each cluster size for small clusters. The normalized atomization energy, heats of formation, and average bond lengths were calculated for each of the different isomeric forms of the silver clusters. For n = 2–6, the preferred geometry is planar, and the larger n = 7–8 clusters prefer higher symmetry, three-dimensional geometries. The low spin state is predicted to be the ground state for every cluster size. A number of new low energy isomers for the heptamer and octamer were found. Additional larger Agn structures, n < 100, were initially optimized using a tree growth-hybrid genetic algorithm with an embedded atom method (EAM) potential. Finally, for n ≤ 20, DFT was used to optimize the geometries. DFT with benchmarked functionals were used to predict that the normalized atomization energies ((AE)s) for Agn start to converge slowly to the bulk at n = 55. The (AE) for Ag99 is predicted to be ~50 kcal/mol.

  3. Structural and electronic properties of small silver-sulfur clusters: A density functional study

    NASA Astrophysics Data System (ADS)

    Li, Yan-Fang; Li, Yang; Li, Ying; Tan, Jia-Jin; Li, Hui-Li

    2016-10-01

    Density functional theory calculations have been performed to systematically investigate the structural and electronic properties of neutral and anionic AgnSm (2≤n+m≤6) clusters. The results show that the ground-state structures of neutral clusters are different from those of anionic clusters. Theoretical electron detachment energies (both vertical and adiabatic) are compared with the experimental measurements to verify the ground states of silver-sulfur clusters obtained in the present study. For both neutral and anionic systems, the highest occupied-lowest unoccupied molecular orbital energy gaps exhibit an odd-even oscillation as a function of the cluster size. In addition, the natural population analysis reveals that the charges transfer from Ag atoms to S atoms in AgnSm clusters, and the extra electron of AgnSm- clusters is mainly localized on the 3p subshells of S atoms.

  4. First-principles insights into interaction of Au with small Co clusters

    NASA Astrophysics Data System (ADS)

    Aghajani, Mahdieh; Javad Hashemifar, S.; Akbarzadeh, Hadi

    2014-08-01

    The effects of Au doping on structural, electronic, magnetic, and dynamical properties of the Con nano-clusters are investigated by using density functional computations and numeric atom-centered orbitals method. The Au doping that enhances stability of the planar structures of the Con clusters over their 3-dimensional structures is discussed. Doping with the nonmagnetic Au atom is found to decrease the spin moments of the Con clusters. The calculated binding energies and their various derivatives are used to investigate the structural stability of the doped clusters. The electronic energy levels of the clusters are accurately determined and discussed after applying the many body based GW correction. The vibrational and infrared spectra of the doped clusters are calculated to provide spectroscopic finger print of these clusters.

  5. Small Sample Performance of Bias-corrected Sandwich Estimators for Cluster-Randomized Trials with Binary Outcomes

    PubMed Central

    Li, Peng; Redden, David T.

    2014-01-01

    SUMMARY The sandwich estimator in generalized estimating equations (GEE) approach underestimates the true variance in small samples and consequently results in inflated type I error rates in hypothesis testing. This fact limits the application of the GEE in cluster-randomized trials (CRTs) with few clusters. Under various CRT scenarios with correlated binary outcomes, we evaluate the small sample properties of the GEE Wald tests using bias-corrected sandwich estimators. Our results suggest that the GEE Wald z test should be avoided in the analyses of CRTs with few clusters even when bias-corrected sandwich estimators are used. With t-distribution approximation, the Kauermann and Carroll (KC)-correction can keep the test size to nominal levels even when the number of clusters is as low as 10, and is robust to the moderate variation of the cluster sizes. However, in cases with large variations in cluster sizes, the Fay and Graubard (FG)-correction should be used instead. Furthermore, we derive a formula to calculate the power and minimum total number of clusters one needs using the t test and KC-correction for the CRTs with binary outcomes. The power levels as predicted by the proposed formula agree well with the empirical powers from the simulations. The proposed methods are illustrated using real CRT data. We conclude that with appropriate control of type I error rates under small sample sizes, we recommend the use of GEE approach in CRTs with binary outcomes due to fewer assumptions and robustness to the misspecification of the covariance structure. PMID:25345738

  6. Porous silicon Bloch surface and sub-surface wave structure for simultaneous detection of small and large molecules

    NASA Astrophysics Data System (ADS)

    Rodriguez, Gilberto A.; Lonai, John D.; Mernaugh, Raymond L.; Weiss, Sharon M.

    2014-08-01

    A porous silicon (PSi) Bloch surface wave (BSW) and Bloch sub-surface wave (BSSW) composite biosensor is designed and used for the size-selective detection of both small and large molecules. The BSW/BSSW structure consists of a periodic stack of high and low refractive index PSi layers and a reduced optical thickness surface layer that gives rise to a BSW with an evanescent tail that extends above the surface to enable the detection of large surface-bound molecules. Small molecules were detected in the sensor by the BSSW, which is a large electric field intensity spatially localized to a desired region of the Bragg mirror and is generated by the implementation of a step or gradient refractive index profile within the Bragg mirror. The step and gradient BSW/BSSW sensors are designed to maximize both resonance reflectance intensity and sensitivity to large molecules. Size-selective detection of large molecules including latex nanospheres and the M13KO7 bacteriophage as well as small chemical linker molecules is reported.

  7. Stochastic dynamics of small ensembles of non-processive molecular motors: the parallel cluster model.

    PubMed

    Erdmann, Thorsten; Albert, Philipp J; Schwarz, Ulrich S

    2013-11-07

    Non-processive molecular motors have to work together in ensembles in order to generate appreciable levels of force or movement. In skeletal muscle, for example, hundreds of myosin II molecules cooperate in thick filaments. In non-muscle cells, by contrast, small groups with few tens of non-muscle myosin II motors contribute to essential cellular processes such as transport, shape changes, or mechanosensing. Here we introduce a detailed and analytically tractable model for this important situation. Using a three-state crossbridge model for the myosin II motor cycle and exploiting the assumptions of fast power stroke kinetics and equal load sharing between motors in equivalent states, we reduce the stochastic reaction network to a one-step master equation for the binding and unbinding dynamics (parallel cluster model) and derive the rules for ensemble movement. We find that for constant external load, ensemble dynamics is strongly shaped by the catch bond character of myosin II, which leads to an increase of the fraction of bound motors under load and thus to firm attachment even for small ensembles. This adaptation to load results in a concave force-velocity relation described by a Hill relation. For external load provided by a linear spring, myosin II ensembles dynamically adjust themselves towards an isometric state with constant average position and load. The dynamics of the ensembles is now determined mainly by the distribution of motors over the different kinds of bound states. For increasing stiffness of the external spring, there is a sharp transition beyond which myosin II can no longer perform the power stroke. Slow unbinding from the pre-power-stroke state protects the ensembles against detachment.

  8. Stochastic dynamics of small ensembles of non-processive molecular motors: The parallel cluster model

    SciTech Connect

    Erdmann, Thorsten; Albert, Philipp J.; Schwarz, Ulrich S.

    2013-11-07

    Non-processive molecular motors have to work together in ensembles in order to generate appreciable levels of force or movement. In skeletal muscle, for example, hundreds of myosin II molecules cooperate in thick filaments. In non-muscle cells, by contrast, small groups with few tens of non-muscle myosin II motors contribute to essential cellular processes such as transport, shape changes, or mechanosensing. Here we introduce a detailed and analytically tractable model for this important situation. Using a three-state crossbridge model for the myosin II motor cycle and exploiting the assumptions of fast power stroke kinetics and equal load sharing between motors in equivalent states, we reduce the stochastic reaction network to a one-step master equation for the binding and unbinding dynamics (parallel cluster model) and derive the rules for ensemble movement. We find that for constant external load, ensemble dynamics is strongly shaped by the catch bond character of myosin II, which leads to an increase of the fraction of bound motors under load and thus to firm attachment even for small ensembles. This adaptation to load results in a concave force-velocity relation described by a Hill relation. For external load provided by a linear spring, myosin II ensembles dynamically adjust themselves towards an isometric state with constant average position and load. The dynamics of the ensembles is now determined mainly by the distribution of motors over the different kinds of bound states. For increasing stiffness of the external spring, there is a sharp transition beyond which myosin II can no longer perform the power stroke. Slow unbinding from the pre-power-stroke state protects the ensembles against detachment.

  9. Stochastic dynamics of small ensembles of non-processive molecular motors: The parallel cluster model

    NASA Astrophysics Data System (ADS)

    Erdmann, Thorsten; Albert, Philipp J.; Schwarz, Ulrich S.

    2013-11-01

    Non-processive molecular motors have to work together in ensembles in order to generate appreciable levels of force or movement. In skeletal muscle, for example, hundreds of myosin II molecules cooperate in thick filaments. In non-muscle cells, by contrast, small groups with few tens of non-muscle myosin II motors contribute to essential cellular processes such as transport, shape changes, or mechanosensing. Here we introduce a detailed and analytically tractable model for this important situation. Using a three-state crossbridge model for the myosin II motor cycle and exploiting the assumptions of fast power stroke kinetics and equal load sharing between motors in equivalent states, we reduce the stochastic reaction network to a one-step master equation for the binding and unbinding dynamics (parallel cluster model) and derive the rules for ensemble movement. We find that for constant external load, ensemble dynamics is strongly shaped by the catch bond character of myosin II, which leads to an increase of the fraction of bound motors under load and thus to firm attachment even for small ensembles. This adaptation to load results in a concave force-velocity relation described by a Hill relation. For external load provided by a linear spring, myosin II ensembles dynamically adjust themselves towards an isometric state with constant average position and load. The dynamics of the ensembles is now determined mainly by the distribution of motors over the different kinds of bound states. For increasing stiffness of the external spring, there is a sharp transition beyond which myosin II can no longer perform the power stroke. Slow unbinding from the pre-power-stroke state protects the ensembles against detachment.

  10. Molecular geometry and polarizability of small cadmium selenide clusters from all-electron ab initio and Density Functional Theory calculations.

    PubMed

    Karamanis, Panaghiotis; Maroulis, George; Pouchan, Claude

    2006-02-21

    We have calculated molecular geometries and electric polarizabilities for small cadmium selenide clusters. Our calculations were performed with conventional ab initio and density functional theory methods and Gaussian-type basis sets especially designed for (CdSe)(n). We find that the dipole polarizability per atom converges rapidly to the bulk value.

  11. Kinetic-theory predictions of clustering instabilities in granular flows: beyond the small-Knudsen-number regime

    SciTech Connect

    Mitrano, Peter P.; Zenk, John R.; Benyahia, Sofiane; Galvin, Janine E.; Dahl, Steven R.; Hrenya, Christine M.

    2013-12-04

    In this work we quantitatively assess, via instabilities, a Navier–Stokes-order (small- Knudsen-number) continuum model based on the kinetic theory analogy and applied to inelastic spheres in a homogeneous cooling system. Dissipative collisions are known to give rise to instabilities, namely velocity vortices and particle clusters, for sufficiently large domains. We compare predictions for the critical length scales required for particle clustering obtained from transient simulations using the continuum model with molecular dynamics (MD) simulations. The agreement between continuum simulations and MD simulations is excellent, particularly given the presence of well-developed velocity vortices at the onset of clustering. More specifically, spatial mapping of the local velocity-field Knudsen numbers (Knu) at the time of cluster detection reveals Knu » 1 due to the presence of large velocity gradients associated with vortices. Although kinetic-theory-based continuum models are based on a small- Kn (i.e. small-gradient) assumption, our findings suggest that, similar to molecular gases, Navier–Stokes-order (small-Kn) theories are surprisingly accurate outside their expected range of validity.

  12. Small-Scale Interstellar Structure Toward the Open Cluster CHI Persei-Fuse II

    NASA Technical Reports Server (NTRS)

    Sonneborn, George (Technical Monitor); Friedman, Scott

    2003-01-01

    The purpose of this study was to measure the physical conditions of gas along sight lines toward 6 stars in the core Chi Persei open cluster. These sight lines traverse gas in both the Orion and Perseus spiral arms of the Galaxy, at distances of 500 and 2000 pc, respectively. The stars have angular separations ranging from 45 to 280 arcsec; 60 arcsec corresponds to linear distances of 0.15 and 0.6 pc in the two arms. Thus, abundance variations in these observations would constitute evidence for small-scale variations in the properties of the interstellar medium. Ground-based Na I observations at high resolution (approx. 15 km/sec) toward 172 stars (including the 6 in this study) in the double open cluster h and Chi Persei have revealed complex spatial variation. These variations are especially evident in the gas at velocities of -40 and -55 km/sec, corresponding to the Perseus spiral arm. 21 cm observations of HI emission using the Low Resolution DRAO Survey, with a 12-arcmin beam, also show variations. Averaging the Na I apparent optical depth profiles of neighboring sight lines in order to mimic such a beam size reduces the variation, as compared to the individual Na I measurements, but still show variations larger than seen in the 21 cm profiles. Na I is not the dominant ionization state of Na in the interstellar medium. Thus, it is possible that the variations seen really trace physical structures in the interstellar medium, or they may simply result from variations in the radiation field seen by the gas, or be due to some other environmental circumstance. To distinguish among these possibilities in the present study we obtained FUSE spectra toward the 6 targets in order to measure the molecular hydrogen absorption profiles along these sight lines. The higher J states of H2 are populated by the ambient W radiation field, and thus can provide insight into the environment affecting the gas. If both the high and low J states reveal absorption line profiles with

  13. Emergence of small-world anatomical networks in self-organizing clustered neuronal cultures.

    PubMed

    de Santos-Sierra, Daniel; Sendiña-Nadal, Irene; Leyva, Inmaculada; Almendral, Juan A; Anava, Sarit; Ayali, Amir; Papo, David; Boccaletti, Stefano

    2014-01-01

    In vitro primary cultures of dissociated invertebrate neurons from locust ganglia are used to experimentally investigate the morphological evolution of assemblies of living neurons, as they self-organize from collections of separated cells into elaborated, clustered, networks. At all the different stages of the culture's development, identification of neurons' and neurites' location by means of a dedicated software allows to ultimately extract an adjacency matrix from each image of the culture. In turn, a systematic statistical analysis of a group of topological observables grants us the possibility of quantifying and tracking the progression of the main network's characteristics during the self-organization process of the culture. Our results point to the existence of a particular state corresponding to a small-world network configuration, in which several relevant graph's micro- and meso-scale properties emerge. Finally, we identify the main physical processes ruling the culture's morphological transformations, and embed them into a simplified growth model qualitatively reproducing the overall set of experimental observations.

  14. Emergence of Small-World Anatomical Networks in Self-Organizing Clustered Neuronal Cultures

    PubMed Central

    de Santos-Sierra, Daniel; Sendiña-Nadal, Irene; Leyva, Inmaculada; Almendral, Juan A.; Anava, Sarit; Ayali, Amir; Papo, David; Boccaletti, Stefano

    2014-01-01

    In vitro primary cultures of dissociated invertebrate neurons from locust ganglia are used to experimentally investigate the morphological evolution of assemblies of living neurons, as they self-organize from collections of separated cells into elaborated, clustered, networks. At all the different stages of the culture's development, identification of neurons' and neurites' location by means of a dedicated software allows to ultimately extract an adjacency matrix from each image of the culture. In turn, a systematic statistical analysis of a group of topological observables grants us the possibility of quantifying and tracking the progression of the main network's characteristics during the self-organization process of the culture. Our results point to the existence of a particular state corresponding to a small-world network configuration, in which several relevant graph's micro- and meso-scale properties emerge. Finally, we identify the main physical processes ruling the culture's morphological transformations, and embed them into a simplified growth model qualitatively reproducing the overall set of experimental observations. PMID:24489675

  15. [Dynamic study of small metallic clusters]; Estudio Dinamico de Pequenos Agregados Metalicos

    SciTech Connect

    Lopez, M.J.; Jellinek, J.

    1995-12-31

    We present a brief introduction to computer simulation techniques (particularly to classical molecular dynamics) and their application to the study of the thermodynamic properties of a material system. The basic concepts are illustrated in the study of structural and energetic properties such as the liquid-solid transition and the fragmentation of small clusters of nickel. [Espanol] Presentamos una breve introducci{acute o}n de las t{acute e}cnicas de simulaci{acute o}n por ordenador (en particular de la Din{acute a}mica Molecular cl{acute a}sica) y de su aplicaci{acute o}n al estudio de las propiedades termodin{acute a}micas de un sistema material. Los conceptos b{acute a}sicos se ilustran en el estudio de las propieades estructurales y energ{acute e}ticas, as{acute i} como de la transici{acute o}n de fase s{acute o}lido-l{acute i}quido y de las fragmentaciones de peque{tilde n}os agregados de n{acute i}quel.

  16. Clustering on very small scales from a large, complete sample of confirmed quasar pairs

    NASA Astrophysics Data System (ADS)

    Eftekharzadeh, Sarah; Myers, Adam D.; Djorgovski, Stanislav G.; Graham, Matthew J.; Hennawi, Joseph F.; Mahabal, Ashish A.; Richards, Gordon T.

    2016-06-01

    We present by far the largest sample of spectroscopically confirmed binaryquasars with proper transverse separations of 17.0 ≤ Rprop ≤ 36.6 h-1 kpc. Our sample, whichis an order-of-magnitude larger than previous samples, is selected from Sloan Digital Sky Survey (SDSS) imaging over an area corresponding to the SDSS 6th data release (DR6). Our quasars are targeted using a Kernel Density Estimation technique (KDE), and confirmed using long-slit spectroscopy on a range of facilities.Our most complete sub-sample of 44 binary quasars with g<20.85, extends across angular scales of 2.9" < Δθ < 6.3", and is targeted from a parent sample that would be equivalent to a full spectroscopic survey of nearly 300,000 quasars.We determine the projected correlation function of quasars (\\bar Wp) over proper transverse scales of 17.0 ≤ Rprop ≤ 36.6 h-1 kpc, and also in 4 bins of scale within this complete range.To investigate the redshift evolution of quasar clustering on small scales, we make the first self-consistent measurement of the projected quasar correlation function in 4 bins of redshift over 0.4 ≤ z ≤ 2.3.

  17. Stability of small molecular clusters modelled with stochastic and deterministic dynamics

    NASA Astrophysics Data System (ADS)

    Natarajan, Sukina

    This investigation concerns the transition pathway of the condensation phase transition. Under certain conditions condensation is initiated by nucleation events, which are driven by fluctuations or instabilities in the vapour phase. This involves the spontaneous formation of groups of particles, which we refer to as clusters. The clusters have a highly unstable nature and exist momentarily, before breaking up. This makes them difficult to study experimentally and model mathematically, in comparison to larger more stable systems. The aim of this study is to explore the stability of these tiny molecular clusters that exist momentarily within their environment, in terms of the time taken for the cluster to lose particles (decay). To do this we employ a microscopic cluster model of n-nonane molecules in which the cluster is treated in isolation from the vapour particles that would normally surround it. The interactions between cluster particles are modelled using empirical potentials. The cluster's dynamics is modelled using deterministic molecular dynamics simulations. The simulations generate a time evolved trajectory of all the positions, velocities and forces of all the atoms in the cluster. The process of cluster decay in n-nonane clusters is modelled using a Langevin interpretation of the decay mechanism. This treatment views cluster decay as a process of single particle escape from a confining potential of mean force, driven by a particle's interactions with the surrounding cluster particles. The motion of a cluster particle is modelled using a Langevin equation, which is parameterised using the MD generated data in order to extract the decay related parameters. The decay parameters are used to evaluate an Arrhenius type equation for the kinetic decay rate. This is used to calculate the mean timescale of cluster decay for n-nonane clusters, which we refer to as the mean cluster lifetime. We compare the dynamically generated lifetimes calculated from the model to

  18. A Versatile Readout System for Small to Medium Scale Gaseous and Silicon Detectors

    NASA Astrophysics Data System (ADS)

    Calvet, Denis

    2014-02-01

    This paper presents the design and performance of a readout system for gaseous and silicon detectors built for the Minos nuclear physics experiment. A major constraint was to provide a multi-thousand channel, high performance readout system with low manpower effort and tight cost. This was achieved by the re-use of some earlier ASIC and front-end card (FEC) developments, the design of a new digital readout card, called the Feminos, and the use of commercial off-the-shelf components. The proposed system fully exploits the capability of the existing 72-channel AFTER chip designed for the T2K experiment and allows seamless migration to the 64-channel AGET chip, a pin-compatible evolution under production by the GET collaboration. The Feminos is a low complexity card designed to read out a FEC equipped with four AFTER chips (T2K model) or a newly assembled FEC populated with four AGET chips. The trigger clock module (TCM) is a synchronization board that allows system scaling up to 6912 channels with 24 Feminos and FECs, a commercial Gigabit Ethernet switch, and a data acquisition PC. The design of the Feminos hardware, firmware and embedded software are detailed and it is explained how high performance, rapid development and low cost were reached. System operation and data acquisition throughput scaling with multiple Feminos are investigated.

  19. Pulse shape discrimination at low energies with a double sided, small-pitch strip silicon detector

    NASA Astrophysics Data System (ADS)

    Genolini, B.; Le Crom, B.; Assié, M.; Dueñas, J. A.; Mengoni, D.; Ancelin, S.; Beaumel, D.; Blumenfeld, Y.; Chabot, M.; de Séréville, N.; Faul, T.; Guillot, J.; Hammache, F.; Jallat, A.; Le Ven, V.; Martel, I.; Rauly, E.; Suzuki, D.

    2013-12-01

    We achieved particle separation of proton, deuteron and triton at energies ranging between 2 and 10 MeV by the processing of digitized signals obtained with a double-sided, 485-μm pitch strip, 500-μm thick neutron transmutation doped (nTD) silicon detector. We produced the light charged particles in a nuclear reaction induced by a 34-MeV beam of 7Li impinging on a 12C target. We analyzed offline the signals with the goal of a simplified, possibly analog, front-end electronics in mind for the processing of the 15,000 channels of the new state-of-the-art detectors for low energy nuclear physics like GASPARD, HYDE and TRACE which should make use of such methods. At the optimum bias, using the current amplitude versus charge correlation, we obtain a separation of 3 FWHM between protons and deuteron, and 2 FWHM between deuteron and triton at energies as low as 2 MeV; with a square bipolar filter, suited for simple implementation, we separate them by 4.3 and 2.7 FWHM respectively at 5 MeV.

  20. Evidence that small proteins translocate through silicon nitride pores in a folded conformation

    NASA Astrophysics Data System (ADS)

    Stefureac, Radu I.; Trivedi, Dhruti; Marziali, Andre; Lee, Jeremy S.

    2010-11-01

    The interaction of three proteins (histidine-containing phosphocarrier protein, HPr, calmodulin, CaM, and maltose binding protein, MBP) with synthetic silicon nitride (SiNx) membranes has been studied. The proteins which have a net negative charge were electrophoretically driven into pores of 7 and 5 nm diameter with a nominal length of 15 nm. The % blockade current and event duration were measured at three different voltages. For a translocation event it was expected that the % block would be constant with voltage whilst the event duration would decrease with increasing voltage. On the basis of these criteria, we deduce that MBP whose largest dimension is 6.5 nm does not translocate whereas up to 40% of CaM molecules can translocate the 7 nm pore as can a majority of HPr molecules, with some translocations being observed for the 5 nm pore. For translocation events the magnitude of the % blockade current is consistent with a folded conformation of the proteins surrounded by a hydration shell of 0.5-1.0 nm.

  1. Summary abstract: studies of small clusters. Relationship to understanding nucleation and surfaces

    SciTech Connect

    Castleman, A.W. Jr.; Holland, P.M.; Kay, B.D.; Keesee, R.G.; Maerk, T.D.; Peterson, K.I.; Schelling, F.J.; Upschulte, B.

    1981-03-01

    Using high-pressure mass spectrometric technique, details of the primary clustering steps leading to the nucleation of bulk liquid from the vapor are obtained by direct observation of the equilibrium cluster distribution. Results for a large number of systems have been determined, including clusters of neutral inorganic molecules and clusters of different molecules about ions. Determination of enthalpy and entropy values from these data allow one to extract information on the strengths of bonds and orientation of molecules within these clusters through correlation with the results of classical and molecular theories. Observations of mixed cluster species are proving to be of interest in the study of solvation phenomena, and because of their similarity to entities observed in surface desorption studies, are also relevant to surface science. In addition, microclusters are good prototypes for elucidating the details of ordering and structures analogous to those existing at interfaces. This paper is devoted to a review of results.

  2. Monte Carlo Simulations of Small Benzene Clusters: Structure, Dynamics, Diffusion and Phase Changes.

    NASA Astrophysics Data System (ADS)

    Dulles, Frederic John

    The structure, melting, and in some cases, the freezing transitions of benzene clusters containing 7, 12, 13, 14, 19, 55, and 128 molecules, were investigated by isothermal Metropolis Monte Carlo simulations, using a potential function developed by Shi. Analyses included caloric curves, pair correlation functions, and indices of translational and rotational self-diffusion based on a simple random-walk model that eliminated the dependence on the arbitrary MC step-size. The relative utility of different procedures for calculating the Lindemann index (delta) for melting was investigated. Transitions analogous to melting and freezing occurred over a range of temperatures. Clusters of benzene, like those of argon, exhibit special stability at the magic number 13. Solid-like clusters with 19 and fewer molecules had poly-tetrahedral structures related to the icosahedral framework of the 13-molecule cluster. The structure of the 13-molecule cluster differed from published structures based on different and probably less accurate potential functions. The low temperature form possessed six energetically equivalent isomers. Benzene cluster's melting points decreased linearly with the inverse of the clusters' radii which permitted estimations of the solid-liquid interfacial free energy and the bulk melting point implict in the Shi potential. Vibrational and librational motions of the molecules in solid-like clusters were found to be freer than in the bulk. Both in-plane and out-of-plane rotational diffusion began at lower temperatures than translational diffusion with in -plane motion being the least constrained. Surfaces began to melt before the interiors. Upon cooling, liquid-like clusters 19-molecules and smaller in size readily relaxed into their solid-like stable low temperature forms while the molten 128-molecule cluster remained amorphous. The structures and melting behavior of benzene clusters were compared with those of argon and TeF_6 clusters, and found to be more

  3. A theoretical study on small iridium clusters: structural evolution, electronic and magnetic properties, and reactivity predictors.

    PubMed

    Du, Jiguang; Sun, Xiyuan; Chen, Jun; Jiang, Gang

    2010-12-16

    The structural, electronic, and magnetic properties of iridium clusters with sizes of n = 2-15 are investigated by employing the generalized gradient approximation of density functional theory. Simple cube evolution pattern is revealed for Ir(2-15) clusters, as predicted by previous reports. It is remarkable that for Ir(10), Ir(11) clusters, new generated isomers with higher stabilities relative to those reported in previous studies are obtained. The even-sized clusters are more stable than the odd-sized species. The Ir-Ir bonds in the cubic Ir(8) and Ir(12) clusters, which are considered as the basic units in the structural evolution present covalent character. Starting from n = 8, the magnetic moments of Ir(n) clusters decrease sharply. The moments of magnetic clusters show 5d characters. The reactive site selectivity of studied clusters with n = 5-15 is analyzed with condensed Fukui function. The capped atoms in certain clusters (Ir(9), Ir(10), Ir(11), and Ir(13)) generally show extraordinary activity for both nucleophilic and electrophilic attack.

  4. Metallicities and radial velocities of two stellar clusters located in the outer regions of the Small Magellanic Cloud

    NASA Astrophysics Data System (ADS)

    Gramajo, L. V.; Parisi, M. C.; Clariá, J. J.; Geisler, D.; Vásquez, S.; Da Costa, G.; Grebel, E. K.

    2016-08-01

    We studied near-infrared spectra of red giant stars in two Small Magellanic Cloud (SMC) clusters. We used the Caii lines to measure radial velocities as well as the equivalent widths of these lines to determine metallicity. The two studied clusters (L32 and L38) are projected on the outer regions of the SMC so they are particularly interesting to examine the possible existence of a change of sign in the metallicity gradient in the outer regions, as suggested by a recent study.

  5. The VMC Survey - XXI. New star cluster candidates discovered from infrared photometry in the Small Magellanic Cloud

    NASA Astrophysics Data System (ADS)

    Piatti, Andrés E.; Ivanov, Valentin D.; Rubele, Stefano; Marconi, Marcella; Ripepi, Vincenzo; Cioni, Maria-Rosa L.; Oliveira, Joana M.; Bekki, Kenji

    2016-07-01

    We report the first search for new star clusters performed using the VISTA near-infrared YJKs Magellanic Clouds survey (VMC) data sets. We chose a pilot field of ˜0.4 deg2 located in the South-west of the Small Magelllanic Cloud bar, where the star field is among the densest and highest reddened region in the galaxy. In order to devise an appropriate automatic procedure we made use of dimensions and stellar densities observed in the VMC data sets of the known clusters in this area. We executed different kernel density estimations over a sample of more than 358 000 stars with magnitudes measured in the three YJKs filters. We analysed the new cluster candidates whose colour-magnitude diagrams (CMDs), cleaned from field star contamination, were used to assess the clusters' reality and estimate reddenings and ages of the genuine systems. As a result 38 objects (≈ a 55 per cent increase in the known star clusters located in the surveyed field) of 0.15-0.40 arcmin (2.6-7.0 pc) in radius resulted to have near-infrared CMD features which resemble those of star clusters of young to moderate intermediate age (log(t yr-1) ˜7.5-9.0). Most of the new star cluster candidates are hardly recognizable in optical images without the help of a sound star field decontaminated CMD analysis. For highly reddened star cluster candidates (E(B - V) ≥ 0.6 mag) the VMC data sets were necessary in order to recognize them.

  6. The THz/FIR Spectrum of Small Water Clusters in Helium Nanodroplets

    NASA Astrophysics Data System (ADS)

    Schwaab, Gerhard; Schwan, Raffael; Mani, Devendra; Pal, Nitish; Dey, Arghya; Redlich, Britta; van der Meer, Lex; Havenith, Martina

    2017-06-01

    The microscopic properties of water that are relevant for bulk solvation processes are still not fully understood. Here, we combine mass selective Helium nanodroplet spectroscopy with the powerful Terahertz (THz) and far-infrared (FIR) capabilities of the free electron laser facility FELIX to study the fingerprint of small neutral water clusters in the wavelength range from 90-900\\wn. Helium nanodroplets are a gentle, superfluid matrix and allow aggregation of pre-cooled moieties at ultra-cold temperatures (0.37 K). The fast cooling rate allows in some cases to stabilize not only the global minimum structure but also local minimum structures. The FELIX facility in Nijmegen provides narrowband (Δν / ν=0.5%) pulsed radation covering the frequency range from 80-3300 \\wn. We used a repetition rate of 10 Hz and typical pulse energies from 10 mJ at the 90\\wn and 40 mJ at 900\\wn. This corresponds to average powers of 100-400 mW far beyond those available using other radiation sources in this frequency range. The observed spectrum is exceptionally rich and includes lines that are close to or below our resolution limit. By mass selective detection and by varying the pickup pressure, we were able to identify contributions from dimer, trimer, tetramer and pentamer. The number of resonances indicates stabilization of at least two trimer structures in He nanodroplets. A comparison with theoretical predictions is on the way. We are confident that our experiments will contribute to understand the very special behavior of water in a bottom up approach.

  7. Velocity bias from the small-scale clustering of SDSS-III BOSS galaxies

    NASA Astrophysics Data System (ADS)

    Guo, Hong; Zheng, Zheng; Zehavi, Idit; Dawson, Kyle; Skibba, Ramin A.; Tinker, Jeremy L.; Weinberg, David H.; White, Martin; Schneider, Donald P.

    2015-01-01

    We present the measurements and modelling of the projected and redshift-space clustering of CMASS galaxies in the Sloan Digital Sky Survey-III Baryon Oscillation Spectroscopic Survey Data Release 11. For a volume-limited luminous red galaxy sample in the redshift range of 0.48 < z < 0.55, we perform halo occupation distribution modelling of the small- and intermediate-scale (0.1-60 h-1 Mpc) projected and redshift-space two-point correlation functions, with an accurate model built on high-resolution N-body simulations. To interpret the measured redshift-space distortions, the distribution of galaxy velocities must differ from that of the dark matter inside haloes of ˜1013-1014 h-1 M⊙, i.e. the data require the existence of galaxy velocity bias. Most notably, central galaxies on average are not at rest with respect to the core of their host haloes, but rather move around it with a 1D velocity dispersion of 0.22^{+0.03}_{-0.04} times that of the dark matter, implying a spatial offset from the centre at the level of ≲1 per cent of the halo virial radius. The luminous satellite galaxies move more slowly than the dark matter, with velocities 0.86^{+0.08}_{-0.03} times those of the dark matter, which suggests that the velocity and spatial distributions of these satellites cannot both be unbiased. The constraints mainly arise from the Fingers-of-God effect at non-linear scales and the smoothing to the Kaiser effect in the translinear regime; the robustness of the results is demonstrated by a variety of tests. We discuss the implications of the existence of galaxy velocity bias for investigations of galaxy formation and cosmology.

  8. Stimulated blue emission in reconstituted films of ultra-small silicon nanoparticles

    NASA Astrophysics Data System (ADS)

    Nayfeh, M.; Barry, N.; Therrien, J.; Akcakir, O.; Gratton, E.; Belomoin, G.

    2000-03-01

    We dispersed electro-chemical etched Si into a colloid of enriched blue emitting ultra-small (1 nm in diameter) nano particles, and reconstituted it into micro crystallites or uniform films on device quality Si. When the film is excited by near-infrared two photon process at 780 nm, we observe an extremely strong blue band that exhibits a sharp threshold near 106 W/cm2, rising by many orders of magnitude, beyond which a low power dependence sets in. We also observe directed blue beams between faces of the micro crystallites. The results are analyzed in terms of population inversion and stimulated emission in quantum confinement-induced Si-Si dimer phase, found only on ultra-small Si nanoparticles.

  9. Structural and electronic properties of small neutral (MgO)n clusters

    NASA Astrophysics Data System (ADS)

    de La Puente, E.; Aguado, A.; Ayuela, A.; López, J. M.

    1997-09-01

    Ab initio perturbed ion calculations are reported for neutral stoichiometric (MgO)n (n<=13) clusters. A great number of isomer structures are identified and studied. For the isomers of (MgO)n (n<=7) clusters, a full geometrical relaxation is considered. Correlation corrections are included for all cluster sizes using the Coulomb-Hartree-Fock model proposed by Clementi [IBM J. Res. Dev. 9, 2 (1965)]. The results obtained compare favorably with the experimental data and other previous theoretical studies. The inclusion of correlation is crucial in order to achieve a good description of these systems. We find a number of important isomers that allow us to interpret the experimental magic numbers without the assumption of structures based on (MgO)3 subunits. Finally, as an electronic property, the variations in the cluster ionization potential with the cluster size are studied and related to the structural isomer properties.

  10. Solvation of cyclopentadienyl and substituted cyclopentadienyl radicals in small clusters. I. Nonpolar solvents

    NASA Astrophysics Data System (ADS)

    Fernandez, J. A.; Yao, J.; Bernstein, E. R.

    1999-03-01

    Cyclopentadienyl (cpd), methylcpd (mcpd), fluorocpd (Fcpd), and cyanocpd (CNcpd) are generated photolytically, cooled in a supersonic expansion, and clustered with nonpolar solvents. The solvents employed are Ar, N2, CH4, CF4, and C2F6. These radicals and their clusters are studied by a number of laser spectroscopic techniques: Fluorescence excitation (FE), hole burning (HB), and mass resolved excitation (MRE) spectroscopies, and excited state lifetime studies. The radical D1←D0 transition is observed for these systems: The radical to cluster spectroscopic shifts for the clusters are quite large, typically 4 to 5 times those found for stable aromatic species and other radicals. Calculations of cluster structure are carried out for these systems using parameterized potential energy functions. Cluster geometries are similar for all clusters with the solvent placed over the cpd ring and the center-of-mass of the solvent displaced toward the substituent. The calculated cluster spectroscopic shifts are in reasonable agreement with the observed ones for N2 and CF4 with all radicals, but not for C2F6 with the radicals. The Xcpd/Ar data are sacrificed to generate excited state potential parameters for these systems. CH4 is suggested to react with all but the CNcpd radical and may begin to react even with CNcpd. van der Waals vibrations are calculated for these clusters in the harmonic approximation for both D1 and D0 electronic states; calculated van der Waals vibrational energies are employed to assign major cluster vibronic features in the observed spectra.

  11. Adsorption behavior and electronic properties of Pdn (n ≤ 10) clusters on silicon carbide nanotubes: a first-principles study.

    PubMed

    Wang, Jianguang; Ma, Li; Wang, Guanghou

    2013-02-27

    First-principles calculations have been carried out to investigate the adsorption of Pd(n) (n ≤ 10) clusters on the single-walled (8, 0) and (5, 5) SiC nanotubes (SiCNTs). We find that the Pd(n) clusters can be stably adsorbed on the outer surfaces of both SiCNTs through an exothermic adsorption process. The adsorption energies of the Pd(n) clusters on the (8, 0) SiCNT are generally larger than those of clusters on the (5, 5) SiCNT. The number of bonds between the Pd(n) clusters and the SiCNTs increases with increasing cluster size. The Pd atoms adjacent to the SiCNTs adsorb preferentially on the bridge sites over an axial Si-C bond. The adsorption leads to elongation of the Pd-Pd bond lengths and structural reconstruction for the Pd(n) clusters. Moreover, the adsorbed Pd(n) clusters show two-layered structures at the cluster size n ≥ 4. We also find that the adsorbed Pd(n) clusters induce some impurity states within the band gap of the pristine SiCNTs and the strong pd hybridization near the Fermi level, thereby reducing the band gap. The charge transfer from the SiCNTs to the Pd atoms that occurs is observed for all the systems considered. Due to the strong interactions between the Pd(n) clusters and the SiCNTs, most adsorbed Pd(n) clusters exhibit zero magnetic moment.

  12. PRESENT-DAY MASS FUNCTION OF SIX SMALL MAGELLANIC CLOUD INTERMEDIATE-AGE AND OLD STAR CLUSTERS

    SciTech Connect

    Glatt, Katharina; Grebel, Eva K.; Jordi, Katrin; Gallagher, John S. III; Harbeck, Daniel; Da Costa, Gary; Clementini, Gisella; Tosi, Monica; Nota, Antonella; Sabbi, Elena; Sirianni, Marco

    2011-08-15

    We determined the present-day mass functions (PDMFs) of the five intermediate-age star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, and Lindsay 38 and the old star cluster NGC 121 in the Small Magellanic Cloud (SMC) based on observations with the Hubble Space Telescope Advanced Camera for Surveys. The global PDMFs are well matched by Salpeter-like power laws from their main-sequence turnoffs to {approx}0.6 M{sub sun} with a power-law exponent {alpha} ranging from 1.51 {+-} 0.11 (Lindsay 1) to 2.29 {+-} 0.15 (NGC 339). We derive total stellar masses of {approx}10{sup 5} M{sub sun}, except for Lindsay 38, whose mass is of the order of {approx}10{sup 4} M{sub sun}. Differences between the PDMFs most likely reflect the varying stages of dynamical evolution of the clusters. These SMC clusters do not follow the {alpha} versus concentration parameter c correlation as found for Galactic globular clusters of similar mass. This might be an age effect or due to their location in a galaxy where bulge and disk crossings do not play a role. No correlation is found between {alpha} and the cluster core and tidal radii (r{sub c} and r{sub t} , respectively), the half-light radii r{sub h} , age, central surface brightness, metallicity, and galactocentric radius r{sub gc}. All six clusters mass-segregated to different degrees. The two clusters Lindsay 1 and Kron 3 barely show signs for mass segregation, but have low-mass star deficient global PDMFs and might be the remnants of star clusters whose outer parts were stripped. A trend exists between the degree of mass segregation and the ratio age/relaxation time t{sub r,h}, which indicates the stage of dynamical evolution for a cluster. Our data thus suggest that the SMC clusters in the present sample had a range of initial densities and presumably different amounts of mass loss that led to different rates of dynamical evolution. The clusters' positions in the r{sub h,m}/r{sub t} versus r{sub 0}/r{sub h,m} plane imply that all of the

  13. Present-day Mass Function of Six Small Magellanic Cloud Intermediate-age and Old Star Clusters

    NASA Astrophysics Data System (ADS)

    Glatt, Katharina; Grebel, Eva K.; Jordi, Katrin; Gallagher, John S., III; Da Costa, Gary; Clementini, Gisella; Tosi, Monica; Harbeck, Daniel; Nota, Antonella; Sabbi, Elena; Sirianni, Marco

    2011-08-01

    We determined the present-day mass functions (PDMFs) of the five intermediate-age star clusters Lindsay 1, Kron 3, NGC 339, NGC 416, and Lindsay 38 and the old star cluster NGC 121 in the Small Magellanic Cloud (SMC) based on observations with the Hubble Space Telescope Advanced Camera for Surveys. The global PDMFs are well matched by Salpeter-like power laws from their main-sequence turnoffs to ~0.6 M sun with a power-law exponent α ranging from 1.51 ± 0.11 (Lindsay 1) to 2.29 ± 0.15 (NGC 339). We derive total stellar masses of ~105 M sun, except for Lindsay 38, whose mass is of the order of ~104 M sun. Differences between the PDMFs most likely reflect the varying stages of dynamical evolution of the clusters. These SMC clusters do not follow the α versus concentration parameter c correlation as found for Galactic globular clusters of similar mass. This might be an age effect or due to their location in a galaxy where bulge and disk crossings do not play a role. No correlation is found between α and the cluster core and tidal radii (rc and rt , respectively), the half-light radii rh , age, central surface brightness, metallicity, and galactocentric radius r gc. All six clusters mass-segregated to different degrees. The two clusters Lindsay 1 and Kron 3 barely show signs for mass segregation, but have low-mass star deficient global PDMFs and might be the remnants of star clusters whose outer parts were stripped. A trend exists between the degree of mass segregation and the ratio age/relaxation time t r, h , which indicates the stage of dynamical evolution for a cluster. Our data thus suggest that the SMC clusters in the present sample had a range of initial densities and presumably different amounts of mass loss that led to different rates of dynamical evolution. The clusters' positions in the r h, m /rt versus r 0/r h, m plane imply that all of the clusters are tidally filled. Our SMC clusters with projected distances larger than 3 kpc from the SMC center

  14. Breakdown of the Hume-Rothery Rules in Sub-Nanometer-Sized Ta-Containing Bimetallic Small Clusters

    NASA Astrophysics Data System (ADS)

    Miyajima, Ken; Fukushima, Naoya; Himeno, Hidenori; Yamada, Akira; Mafuné, Fumitaka

    2009-11-01

    The Hume-Rothery rules are empirical rules to predict the solid solubility of metals. We examined whether the rules hold for sub-nanometer-sized small particles. We prepared bimetallic cluster ions in the gas phase by a double laser ablation technique. Taking advantage of the magic compositions of the bimetallic cluster ions relating to the distinguished stabilities, the coalescence or the segregation of Ta and another element in the sub-nanometer-sized clusters was discussed. It was found that W, Nb, and Mo readily coalesce with Ta, while Ag, Al, Au, Co, Cu, Fe, Hf, Ni, Pt, Ti, and V are segregated from Ta. On the basis of these results, we concluded that the Hume-Rothery rules do not hold for sub-nanometer-sized particles.

  15. Monte Carlo simulations of growth/decay rate constant ratios for small methanol clusters: Application to nucleation data analysis

    NASA Astrophysics Data System (ADS)

    Hale, Barbara; Wilemski, Gerald; Viets, Aaron

    2013-05-01

    The Bennett Monte Carlo technique and the potential of van Leeuwen and Smit are used to calculate growth/decay rate constant ratios for small model methanol clusters at 220K, 240K and 260K. Temperature scaling properties of the rate constant ratios are demonstrated at these temperatures. The Monte Carlo results are used to study heat release from subcritical cluster formation in adiabatic nucleation rate measurements and to determine corrected final temperatures and supersaturation ratios for the methanol data of Strey, Wagner, and Schmeling. The corrected T and S values provide experimental rates with improved scaling properties. Nucleation rates are also calculated from the Monte Carlo free energy differences for the model methanol clusters and demonstrate the same scaling.

  16. Optimisation of output factor measurements using the Magic Plate 512 silicon dosimeter array in small megavoltage photon fields

    NASA Astrophysics Data System (ADS)

    Utitsarn, K.; Alrowaili, Z. A.; Stansook, N.; Lerch, M.; Petasecca, M.; Carolan, M.; Rosenfeld, A.

    2017-01-01

    We evaluate the impact of an air gap and optimization of this air gap for the MP512 silicon detector array when operated in dosimetry mode for small photon field measurements in solid water. We present output factor measurements for 6MV and 10 MV photon beams with the square field sizes ranging from 0.5 to 10 cm2. The size of the air gap above the MP512 detector was changed from 0.5, 1.0, 1.2, 2.0 and 2.6 mm. We compare the output factors measurements of the MP512 with EBT3 film and the MOSkin dosimeter. For the two photon energies investigated, we find that the output factor measured by the MP512 reduce with increasing air gap and reducing of field size. The reduction in output factor is most pronounced for the 0.5 and 1 cm2 field sizes. The air gap of 0.5 mm and 1.2 mm showed good agreement with the EBT3 film and MOSkin output factor for 6 and 10 MV photon fields, respectively. The negligible effect on dosimetry for the field sizes larger than 4x4 cm2 demonstrates that the electronic disequilibrium caused by small air gap only influences the dosimetry measurements for small fields. The study shows that the output factor reduction is enhanced by increasing of air gap and demonstrates that the optimal air gap for the MP512 at 6 and 10 MV photon fields is 0.5mm.

  17. Structure, energetic and phase transition of small nickel-palladium heterogeneous clusters

    NASA Astrophysics Data System (ADS)

    Hewage, Jinasena W.

    2014-06-01

    Molecular dynamics simulation (MD) with Sutton-Chen potential for palladium-palladium, nickel-nickel and palladium-nickel interactions has been used to generate the minimum energy structures and to study the thermodynamic and dynamic properties of mixed transition metal cluster motifs of Ni n Pd(13- n) for n ≤ 13. Thirteen particle icosahedral clusters of neat palladium and nickel atoms were first reproduced accordingly with the results in literature. Then in the palladium icosahedra, each palladium atom has been successively replaced by nickel atom. Calculation is repeated for both palladium-centered and nickel-centered clusters. It is found that the nickel-centered clusters are more stable than the palladium-centered clusters and cohesive energy increases along the palladium end to nickel end. Phase transition of each cluster from one end-species to the other end-species is studied by means of caloric curve, root mean square bond fluctuation and heat capacity. Trend in variation of melting temperature is opposite to the energy trend. Palladium-centered cluster shows a premelting at low temperature due to the solid-solid structural transition. Species-centric order parameters developed by Hewage and Amar is used to understand the dynamic behavior in the solid-solid transition of palladium-centered cluster to more stable nickel-centered cluster (premelting). This species-centric order parameter calculation further confirmed the stability of nickel-centered species over those of palladium-centered species and solid-solid structural transition at low temperature.

  18. Theoretical studies of the global minima and polarizabilities of small lithium clusters

    SciTech Connect

    Hu, Hanshi; Zhao, Ya-Fan; Hammond, Jeffrey R.; Bylaska, Eric J.; Apra, Edoardo; van Dam, Hubertus JJ; Li, Jun; Govind, Niranjan; Kowalski, Karol

    2016-01-16

    Lithium clusters Lin (n=1-20) have been investigated with density functional theory (DFT) and coupled—cluster (CC) methods. The global-minimum structures are located via an improved basin---hopping algorithm and the lowest energy Lin isomers are confirmed with DFT geometry optimizations, CCSD(T) energy calculations, and by comparing simulated and experimental polarizabilities. The tetrahedral Li4 structure is found to be the basic building block of lithium clusters Lin (n=6-20). Simulated polarizabilities, including thermal effects at room temperature, are in good agreement with measured isotropic polarizabilities.

  19. Theoretical studies of the global minima and polarizabilities of small lithium clusters

    NASA Astrophysics Data System (ADS)

    Hu, Han-Shi; Zhao, Ya-Fan; Hammond, Jeff R.; Bylaska, Eric J.; Aprà, Edoardo; van Dam, Hubertus J. J.; Li, Jun; Govind, Niranjan; Kowalski, Karol

    2016-01-01

    Lithium clusters Lin (n = 1⿿20) have been investigated with density functional theory (DFT) and coupled-cluster (CC) methods. The global minima are located via an improved basin-hopping algorithm. Simulated polarizabilities are in good agreement with the measured data generally. The simulated polarizabilities for Li6, Li12 and Li19 are in reasonable agreement when thermal effects are included, except the Li3 cluster. A linear correlation for the inverse relationship between the CCSD calculated polarizabilities and ionization potential (IP) has been reported to have the linear coefficient of 0.996, which further strengthens our simulations.

  20. Cluster information of non-sampled area in small area estimation of poverty indicators using Empirical Bayes

    NASA Astrophysics Data System (ADS)

    Sundara, Vinny Yuliani; Sadik, Kusman; Kurnia, Anang

    2017-03-01

    Survey is one of data collection method which sampling of individual units from a population. However, national survey only provides limited information which impacts on low precision in small area level. In fact, when the area is not selected as sample unit, estimation cannot be made. Therefore, small area estimation method is required to solve this problem. One of model-based estimation methods is empirical Bayes which has been widely used to estimate parameter in small area, even in non-sampled area. Yet, problems occur when this method is used to estimate parameter of non-sampled area which is solely based on synthetic model which ignore the area effects. This paper proposed an approach to cluster area effects of auxiliary variable by assuming that there are similar among particular area. Direct estimates in several sub-districts in regency and city of Bogor are zero because no household which are under poverty in the sample that selected from these sub-districts. Empirical Bayes method is used to get the estimates are not zero. Empirical Bayes method on FGT poverty measures both Molina & Rao and information clusters have the same estimates in the sub-districts selected as samples, but have different estimates on non-sampled sub-districts. Empirical Bayes methods with information cluster has smaller coefficient of variation. Empirical Bayes method with cluster information is better than empirical Bayes methods without cluster information on non-sampled sub-districts in regency and city of Bogor in terms of coefficient of variation.

  1. The Color and Stellar Mass Dependence of Small-scale Galaxy Clustering in SDSS-III BOSS

    NASA Astrophysics Data System (ADS)

    Law-Smith, Jamie; Eisenstein, Daniel J.

    2017-02-01

    We measure the color and stellar mass dependence of clustering in spectroscopic galaxies at 0.6 < z < 0.65 using data from the Baryon Oscillation Spectroscopic Survey component of the Sloan Digital Sky Survey. We greatly increase the statistical precision of our clustering measurements by using the cross-correlation of 66,657 spectroscopic galaxies to a sample of 6.6 million fainter photometric galaxies. The clustering amplitude w(R) is measured as the ratio of the mean excess number of photometric galaxies found within a specified radius annulus around a spectroscopic galaxy to that from a random photometric galaxy distribution. We recover many of the familiar trends at high signal-to-noise ratio. We find the ratio of the clustering amplitudes of red and blue massive galaxies to be {w}{red}/{w}{blue}=1.92+/- 0.11 in our smallest annulus of 75–125 kpc. At our largest radii (2–4 Mpc), we find {w}{red}/{w}{blue}=1.24+/- 0.05. Red galaxies therefore have denser environments than their blue counterparts at z ∼ 0.625, and this effect increases with decreasing radius. Irrespective of color, we find that w(R) does not obey a simple power-law relation with radius, showing a dip around 1 Mpc. Holding stellar mass fixed, we find a clear differentiation between clustering in red and blue galaxies, showing that clustering is not solely determined by stellar mass. Holding color fixed, we find that clustering increases with stellar mass, especially for red galaxies at small scales (more than a factor of 2 effect over 0.75 dex in stellar mass).

  2. Hartree-Fock calculation of the differential photoionization cross sections of small Li clusters

    SciTech Connect

    Galitskiy, S. A.; Artemyev, A. N.; Jänkälä, K.; Lagutin, B. M.; Demekhin, Ph. V.

    2015-01-21

    Cross sections and angular distribution parameters for the single-photon ionization of all electron orbitals of Li{sub 2−8} are systematically computed in a broad interval of the photoelectron kinetic energies for the energetically most stable geometry of each cluster. Calculations of the partial photoelectron continuum waves in clusters are carried out by the single center method within the Hartree-Fock approximation. We study photoionization cross sections per one electron and analyze in some details general trends in the photoionization of inner and outer shells with respect to the size and geometry of a cluster. The present differential cross sections computed for Li{sub 2} are in a good agreement with the available theoretical data, whereas those computed for Li{sub 3−8} clusters can be considered as theoretical predictions.

  3. Many-body forces and electron correlation in small metal clusters

    NASA Astrophysics Data System (ADS)

    Kaplan, Ilya G.; Hernández-Cobos, Jorge; Ortega-Blake, Iván; Novaro, Octavio

    1996-04-01

    The many-body decomposition of the interaction energy for BeN and LiN (N=2 to 4) clusters is calculated in two approximations: the self-consistent-field method and the Mo/ller-Plesset perturbation theory up to the fourth order. This allows us to estimate the electron-correlation contributions to the many-body forces. The explicit expressions for these contributions in the perturbation theory formalism are obtained. We present a comparative analysis of the role of electron correlations in the BeN and LiN cluster formations and in the many-body interactions in these clusters. As follows from our results, the contribution of electron correlation to many-body interactions is essential for both the BeN and LiN clusters, especially for the latter ones, where nonadditivities are surprisingly large.

  4. A full-configuration-interaction nuclear orbital approach and application for small doped He clusters

    SciTech Connect

    Lara-Castells, M. P. de Aguirre, N. F. Delgado-Barrio, G. Villarreal, P.; Mitrushchenkov, A. O.

    2015-01-22

    An efficient full-configuration-interaction 'nuclear orbital' treatment was developed as a benchmark quantum-chemistry-like method to calculate, ground and excited, fermionic 'solvent' wave-functions and applied to {sup 3}He{sub N} clusters with atomic or molecular impurities [J. Chem. Phys. (Communication) 125, 221101 (2006)]. The main difficulty in handling doped {sup 3}He{sub N} clusters lies in the Fermi-Dirac nuclear statistics, the wide amplitudes of the He-dopant and He-He motions, and the hard-core He-He interaction at short distances. This paper overviews the theoretical approach and its recent applications to energetic, structural and spectroscopic aspects of different dopant-{sup 3}He{sub N} clusters. Preliminary results by using the latest version of the FCI-NO computational implementation, to bosonic Cl{sub 2}(X)-({sup 4}He){sub N} clusters, are also shown.

  5. Hartree-Fock calculation of the differential photoionization cross sections of small Li clusters.

    PubMed

    Galitskiy, S A; Artemyev, A N; Jänkälä, K; Lagutin, B M; Demekhin, Ph V

    2015-01-21

    Cross sections and angular distribution parameters for the single-photon ionization of all electron orbitals of Li2-8 are systematically computed in a broad interval of the photoelectron kinetic energies for the energetically most stable geometry of each cluster. Calculations of the partial photoelectron continuum waves in clusters are carried out by the single center method within the Hartree-Fock approximation. We study photoionization cross sections per one electron and analyze in some details general trends in the photoionization of inner and outer shells with respect to the size and geometry of a cluster. The present differential cross sections computed for Li2 are in a good agreement with the available theoretical data, whereas those computed for Li3-8 clusters can be considered as theoretical predictions.

  6. Geometric isotope effects on small chloride ion water clusters with path integral molecular dynamics simulations

    NASA Astrophysics Data System (ADS)

    Wang, Qi; Suzuki, Kimichi; Nagashima, Umpei; Tachikawa, Masanori; Yan, Shiwei

    2013-11-01

    The geometric isotope effects on the structures of hydrated chloride ionic hydrogen bonded clusters are explored by carrying out path integral molecular dynamics simulations. First, an outer shell coordinate is selected to display the rearrangement of single and multi hydration shell cluster structures. Next, to show the competition of intramolecular and intermolecular nuclear quantum effects, the intramolecular OH∗ stretching and intermolecular ion-water wagging motions are studied for single and multi shell structures, respectively. The results indicate that the intermolecular nuclear quantum effects stabilize the ionic hydrogen bonds in single shell structures, while they are destabilized through the competition with intramolecular nuclear quantum effects in multi shell structures. In addition, the correlations between ion-water stretching motion and other cluster vibrational coordinates are discussed. The results indicate that the intermolecular nuclear quantum effects on the cluster structures are strongly related to the cooperation of the water-water hydrogen bond interactions.

  7. NEW TESTS FOR DISRUPTION MECHANISMS OF STAR CLUSTERS: THE LARGE AND SMALL MAGELLANIC CLOUDS

    SciTech Connect

    Chandar, Rupali; Fall, S. Michael; Whitmore, Bradley C. E-mail: fall@stsci.ed

    2010-03-10

    We compare the observed bivariate distribution of masses (M) and ages (tau) of star clusters in the Large Magellanic Cloud (LMC) with the predicted distributions g(M, tau) from three idealized models for the disruption of star clusters: (1) sudden mass-dependent disruption, (2) gradual mass-dependent disruption, and (3) gradual mass-independent disruption. The model with mass-independent disruption provides a good, first-order description of these cluster populations, with g(M, tau) {proportional_to} M {sup beta}tau{sup g}amma, beta = -1.8 +- 0.2 and gamma = -0.8 +- 0.2, at least for clusters with ages tau {approx}< 10{sup 9} yr and masses M {approx}> 10{sup 3} M{sub sun} (more specifically, tau {approx}< 10{sup 7}(M/10{sup 2} M{sub sun}){sup 1.3} yr). This model predicts that the clusters should have a power-law luminosity function, dN/dL {proportional_to} L {sup -1.8}, in agreement with observations. The first two models, on the other hand, fare poorly when describing the observations, refuting previous claims that mass-dependent disruption of star clusters is observed in the LMC over the studied M-tau domain. Clusters in the SMC can be described by the same g(M, tau) distribution as for the LMC, but with smaller samples and hence larger uncertainties. The successful g(M, tau) model for clusters in the Magellanic Clouds is virtually the same as the one for clusters in the merging Antennae galaxies, but extends the domain of validity to lower masses and to older ages. This indicates that the dominant disruption processes are similar in these very different galaxies over at least tau {approx}< 10{sup 8} yr and possibly tau {approx}< 10{sup 9} yr. The mass functions for young clusters in the LMC are power laws, while that for ancient globular clusters is peaked. We show that the observed shapes of these mass functions are consistent with expectations from the simple evaporation model presented by McLaughlin and Fall.

  8. An analysis of mortality inventory tally using large plots compared to tally using small plot clusters

    Treesearch

    Vernon J. LaBau; John W. Hazard

    2000-01-01

    During an inventory to assess spruce bark beetle impact on the Kenai Peninsula in south-central Alaska, 5-year mortality estimates were made for all growing-stock trees on 0.6 ha areas, on 0.4 ha areas, and on a cluster of four 1/60-ha subplots. The analysis of the results of the comparison between cluster data and the larger plot data highlighted some of the problems...

  9. Optimization of large amorphous silicon and silica structures for molecular dynamics simulations of energetic impacts

    NASA Astrophysics Data System (ADS)

    Samela, Juha; Norris, Scott A.; Nordlund, Kai; Aziz, Michael J.

    2011-07-01

    A practical method to create optimized amorphous silicon and silica structures for molecular dynamics simulations is developed and tested. The method is based on the Wooten, Winer, and Weaire algorithm and combination of small optimized blocks to larger structures. The method makes possible to perform simulations of either very large cluster hypervelocity impacts on amorphous targets or small displacements induced by low energy ion impacts in silicon.

  10. Structures of small Pd Pt bimetallic clusters by Monte Carlo simulation

    NASA Astrophysics Data System (ADS)

    Cheng, Daojian; Huang, Shiping; Wang, Wenchuan

    2006-11-01

    Segregation phenomena of Pd-Pt bimetallic clusters with icosahedral and decahedral structures are investigated by using Monte Carlo method based on the second-moment approximation of the tight-binding (TB-SMA) potentials. The simulation results indicate that the Pd atoms generally lie on the surface of the smaller clusters. The three-shell onion-like structures are observed in 55-atom Pd-Pt bimetallic clusters, in which a single Pd atom is located in the center, and the Pt atoms are in the middle shell, while the Pd atoms are enriched on the surface. With the increase of Pd mole fraction in 55-atom Pd-Pt bimetallic clusters, the Pd atoms occupy the vertices of clusters first, then edge and center sites, and finally the interior shell. It is noticed that some decahedral structures can be transformed into the icosahedron-like structure at 300 and 500 K. Comparisons are made with previous experiments and theoretical studies of Pd-Pt bimetallic clusters.

  11. The search for multiple populations in Magellanic Cloud clusters - I. Two stellar populations in the Small Magellanic Cloud globular cluster NGC 121

    NASA Astrophysics Data System (ADS)

    Niederhofer, F.; Bastian, N.; Kozhurina-Platais, V.; Larsen, S.; Salaris, M.; Dalessandro, E.; Mucciarelli, A.; Cabrera-Ziri, I.; Cordero, M.; Geisler, D.; Hilker, M.; Hollyhead, K.; Kacharov, N.; Lardo, C.; Li, C.; Mackey, D.; Platais, I.

    2017-01-01

    We started a photometric survey using the WFC3/UVIS instrument onboard the Hubble Space Telescope to search for multiple populations within Magellanic Cloud star clusters at various ages. In this paper, we introduce this survey. As first results of this programme, we also present multiband photometric observations of NGC 121 in different filters taken with the WFC3/UVIS and ACS/WFC instruments. We analyse the colour-magnitude diagram (CMD) of NGC 121, which is the only `classical' globular cluster within the Small Magellanic Cloud. Thereby, we use the pseudo-colour CF336W,F438W,F343N = (F336W - F438W) - (F438W - F343N) to separate populations with different C and N abundances. We show that the red giant branch splits up in two distinct populations when using this colour combination. NGC 121 thus appears to be similar to Galactic globular clusters in hosting multiple populations. The fraction of enriched stars (N rich, C poor) in NGC 121 is about 32 per cent ± 3 per cent, which is lower than the median fraction found in Milky Way globular clusters. The enriched population seems to be more centrally concentrated compared to the primordial one. These results are consistent with the recent results by Dalessandro et al. The morphology of the horizontal branch in a CMD using the optical filters F555W and F814W is best produced by a population with a spread in helium of ΔY = 0.025±0.005.

  12. Effect of small versus large clusters of fish school on the yield of a purse-seine small pelagic fishery including a marine protected area.

    PubMed

    Hieu, Nguyen Trong; Brochier, Timothée; Tri, Nguyen-Huu; Auger, Pierre; Brehmer, Patrice

    2014-09-01

    We consider a fishery model with two sites: (1) a marine protected area (MPA) where fishing is prohibited and (2) an area where the fish population is harvested. We assume that fish can migrate from MPA to fishing area at a very fast time scale and fish spatial organisation can change from small to large clusters of school at a fast time scale. The growth of the fish population and the catch are assumed to occur at a slow time scale. The complete model is a system of five ordinary differential equations with three time scales. We take advantage of the time scales using aggregation of variables methods to derive a reduced model governing the total fish density and fishing effort at the slow time scale. We analyze this aggregated model and show that under some conditions, there exists an equilibrium corresponding to a sustainable fishery. Our results suggest that in small pelagic fisheries the yield is maximum for a fish population distributed among both small and large clusters of school.

  13. Spectroscopic Study of Local Interactions of Platinum in Small [CexOy]Ptx' - Clusters

    NASA Astrophysics Data System (ADS)

    Ray, Manisha; Kafader, Jared O.; Chick Jarrold, Caroline

    2016-06-01

    Cerium oxide is a good ionic conductor, and the conductivity can be enhanced with oxygen vacancies and doping. This conductivity may play an important role in the enhancement of noble or coinage metal toward the water-gas shift reaction when supported by cerium oxide. The ceria-supported platinum catalyst in particular has received much attention because of higher activity at lower temperatures (LT) compared to the most common commercial LT-WGS catalyst. We have used a combination of anion photoelectron spectroscopy and density functional theory calculations to study the interesting molecular and electronic structures and properties of cluster models of ceria-supported platinum. [CexOy]Ptx' - (x,x'=1,2 ; y≤2x') clusters exhibit evidence of ionic bonding possible because of the high electron affinity of Pt and the low ionization potential of cerium oxide clusters. In addition, Pt- is a common daughter ion resulting from photodissociation of [CexOy]Ptx' - clusters. Finally, several of the anion and neutral clusters have profoundly different structures. These features may play a role in the enhancement of catalytic activity toward the water-gas shift reaction.

  14. CO near the Pleiades: Encounter of a star cluster with a small molecular cloud

    NASA Technical Reports Server (NTRS)

    Bally, J.; White, R. E.

    1986-01-01

    Although there is a large amount of interstellar matter near the Pleiades star cluster, the observed dust and gas is not a remnant of the placental molecular cloud from which the star cluster was formed. Carbon monoxide (CO) associated with the visible reflection nebulae was discovered by Cohen (1975). Its radial velocity differs from that of the cluster by many times the cluster escape velocity, which implies that the cloud-cluster association is the result of a chance encounter. This circumstance and the proximity of the Pleiades to the sun creates an unique opportunity for study of interstellar processes at high spatial resolution. To study the molecular component of the gas, a 1.7 square degree field was mapped with the AT&T Bell Laboratories 7-meter antenna (1.7' beam) on a 1' grid in the J=1.0 C(12)O line, obtaining over 6,000 spectra with 50 kHz resolution. The cloud core was mapped in the J=1-0 line of C(13)O. Further observations include an unsuccessful search for CS (J=2-1) at AT&T BL, and some C(12)O J=2-1 spectra obtained at the Millimeter Wave Observatory of the University of Texas.

  15. Quantum chemical study of small BnCm cluster structures and their physical properties

    NASA Astrophysics Data System (ADS)

    Sharipov, Alexander S.; Loukhovitski, Boris I.; Starik, Alexander M.

    2015-09-01

    Different isomeric forms of BnCm clusters with n = 0, ..., 5, m = 0, ..., 5 with the isomerization energy up to 5 eV have been identified by using the multi-step heuristic algorithm based on semiempirical, ab initio and density functional theory calculations. Physical properties, such as rotational constants and characteristic vibrational temperatures, collision diameter, enthalpy of formation, cohesive energy, dipole moment, static isotropic polarizability and magnetic moment of different isomeric forms have been obtained with the usage of density functional theory. It has been revealed that the electric properties of clusters depend on their structure. It was found that the isomers with linear structure contribute mostly to the average polarizability of the ensemble of the isomeric forms of given class of clusters. Temperature-dependent thermodynamic properties of clusters including specific heat capacity and entropy were calculated taking into account the contribution of excited electronic states and possible isomeric forms in the anharmonic oscillator approximation for vibrational degrees of freedom. It was shown that the effect of structural isomers on the thermodynamic properties of the Boltzmann ensemble of clusters can be significant. Supplementary material in the form of one zip file available from the Journal web page at http://dx.doi.org/10.1140/epjd/e2015-60308-0

  16. Embryonic stem cell-based screen for small molecules: cluster analysis reveals four response patterns in developing neural cells.

    PubMed

    Kern, I; Xu, R; Julien, S; Suter, D M; Preynat-Seauve, O; Baquié, M; Poncet, A; Combescure, C; Stoppini, L; Thriel, C V; Krause, Karl-Heinz

    2013-01-01

    Neural differentiation of embryonic stem cells (ESC) is considered a promising model to perform in vitro testing for neuroactive and neurotoxic compounds. We studied the potential of a dual reporter murine ESC line to identify bioactive and/or toxic compounds. This line expressed firefly luciferase under the control of the neural cell-specific tubulin alpha promoter (TUBA1A), and renilla luciferase under the control of the ubiquitous translation elongation factor 1-alpha-1 (EEF1A1) promoter. During neural differentiation, TUBA1A activity increased, while EEF1A1 activity decreased. We first validated our test system using the known neurotoxin methyl mercury. This compound altered expression of both reporter genes, with ESC-derived neural precursors being affected at markedly lower concentrations than undifferentiated ESCs. Analysis of a library of 1040 bioactive compounds picked up 127 compounds with altered EEF1A1 and/or TUBA1A promoter activity, which were classified in 4 clusters. Cluster 1 (low EEF1A1 and TUBA1A) was the largest cluster, containing many cytostatic drugs, as well as known neurodevelopmental toxicants, psychotropic drugs and endocrine disruptors. Cluster 2 (high EEF1A1, stable TUBA1A) was limited to three sulfonamides. Cluster 3 (high EEF1A1 and TUBA1A) was small, but markedly enriched in neuroactive and neurotoxic compounds. Cluster 4 (stable EEF1A1, high TUBA1A) was heterogeneous, containing endocrine disruptors, neurotoxic and cytostatic drugs. The dual reporter gene assay described here might be a useful addition to in vitro drug testing panels. Our two-dimensional testing strategy provides information on complex response patterns, which could not be achieved by a single marker approach.

  17. Space-time stick-breaking processes for small area disease cluster estimation.

    PubMed

    Hossain, Md Monir; Lawson, Andrew B; Cai, Bo; Choi, Jungsoon; Liu, Jihong; Kirby, Russell S

    2013-03-01

    We propose a space-time stick-breaking process for the disease cluster estimation. The dependencies for spatial and temporal effects are introduced by using space-time covariate dependent kernel stick-breaking processes. We compared this model with the space-time standard random effect model by checking each model's ability in terms of cluster detection of various shapes and sizes. This comparison was made for simulated data where the true risks were known. For the simulated data, we have observed that space-time stick-breaking process performs better in detecting medium- and high-risk clusters. For the real data, county specific low birth weight incidences for the state of South Carolina for the years 1997-2007, we have illustrated how the proposed model can be used to find grouping of counties of higher incidence rate.

  18. Space-time stick-breaking processes for small area disease cluster estimation

    PubMed Central

    Lawson, Andrew B.; Cai, Bo; Choi, Jungsoon; Liu, Jihong; Kirby, Russell S.

    2012-01-01

    We propose a space-time stick-breaking process for the disease cluster estimation. The dependencies for spatial and temporal effects are introduced by using space-time covariate dependent kernel stick-breaking processes. We compared this model with the space-time standard random effect model by checking each model’s ability in terms of cluster detection of various shapes and sizes. This comparison was made for simulated data where the true risks were known. For the simulated data, we have observed that space-time stick-breaking process performs better in detecting medium- and high-risk clusters. For the real data, county specific low birth weight incidences for the state of South Carolina for the years 1997–2007, we have illustrated how the proposed model can be used to find grouping of counties of higher incidence rate. PMID:23869181

  19. The hydrogen storage properties of Na decorated small boron cluster B6Na8

    NASA Astrophysics Data System (ADS)

    Tang, Chunmei; Wang, Zhiguo; Zhang, Xue; Wen, Ninghua

    2016-09-01

    The binding energy of the Na atoms to the hollow sites of the B6 cage is larger than the experimental cohesive energy of bulk Na, so the clustering of Na atoms can be avoided. The polarization interaction dominates the adsorption of H2 by the B6Na8 cluster. The Na-coated B6Na8sbnd B8sbnd B6Na8 complex with the dispersive Na atoms and four H2 molecules adsorbed per Na can serve as better building blocks of polymers than the (B6Na8)2 dimer. These findings suggest a new route to design cluster-assembled hydrogen storage materials based on sp2-terminated boron chains.

  20. Communication. Kinetics of scavenging of small, nucleating clusters. First nucleation theorem and sum rules

    DOE PAGES

    Malila, Jussi; McGraw, Robert; Laaksonen, Ari; ...

    2015-01-07

    Despite recent advances in monitoring nucleation from a vapor at close-to-molecular resolution, the identity of the critical cluster, forming the bottleneck for the nucleation process, remains elusive. During past twenty years, the first nucleation theorem has been often used to extract the size of the critical cluster from nucleation rate measurements. However, derivations of the first nucleation theorem invoke certain questionable assumptions that may fail, e.g., in the case of atmospheric new particle formation, including absence of subcritical cluster losses and heterogeneous nucleation on pre-existing nanoparticles. Here we extend the kinetic derivation of the first nucleation theorem to give amore » general framework to include such processes, yielding sum rules connecting the size dependent particle formation and loss rates to the corresponding loss-free nucleation rate and the apparent critical size from a naïve application of the first nucleation theorem that neglects them.« less

  1. Density functional study of structural, electronic, and optical properties of small bimetallic ruthenium-copper clusters.

    PubMed

    Karagiannis, Efstathios E; Kefalidis, Christos E; Petrakopoulou, Ioanna; Tsipis, Constantinos A

    2011-05-01

    The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Cu(n)Ru(m)](+/0/-) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 nonlocal hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Cu(n Ru(m)](+/0/-) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and assignments of all principal electronic transitions were made and interpreted in terms of contribution from specific molecular orbital excitations. Copyright © 2010 Wiley Periodicals, Inc.

  2. An ecohealth assessment of poultry production clusters (PPCs) for the livelihood and biosecurity improvement of small poultry producers in Asia.

    PubMed

    Wang, Libin; Basuno, Edi; Nguyen, Tuan; Aengwanich, Worapol; Ilham, Nyak; Li, Xiaoyun

    2015-01-01

    Poultry production cluster (PPC) programs are key strategies in many Asian countries to engage small commercial poultry producers in high-value production chains and to control infectious poultry diseases. This study assessed the multiple impacts of PPCs through a transdisciplinary ecohealth approach in four Asian countries, and drew the implications for small producers to improve their livelihoods and reduce the risk of spreading infectious diseases in the poultry sector. The data collection combined both quantitative and qualitative methods. It comprised: formal structured household survey questionnaires, measuring the biosecurity level of poultry farms with a biosecurity score card; and key informant interviews. Descriptive statistics were used to process the quantitative data and a content analysis was used to process the qualitative data. This research found that poultry farms in clusters do not necessarily have better economic performance than those outside PPCs. Many farmers in PPCs only consider them to be an advantage for expanding the scale of their poultry operations and improving household incomes, and they are less concerned about-and have limited capacities to-enhancing biosecurity and environmental management. We measured the biosecurity level of farms in PPCs through a 14-item checklist and found that biosecurity is generally very low across all sample sites. The increased flies, mosquitoes, rats, and smells in and around PPCs not only pollute the environment, but also cause social conflicts with the surrounding communities. This research concluded that a poultry cluster, mainly driven by economic objectives, is not necessarily a superior model for the control of infectious diseases. The level of biosecurity in PPCs was found to be low. Given the intensity of poultry operations in PPCs (farms are densely packed into clusters), and the close proximity to residential areas of some PPCs, the risk of spreading infectious diseases, in fact, increases

  3. Influence of Cr doping on the stability and structure of small cobalt oxide clusters

    SciTech Connect

    Tung, Nguyen Thanh; Lievens, Peter; Janssens, Ewald; Tam, Nguyen Minh; Nguyen, Minh Tho

    2014-07-28

    The stability of mass-selected pure cobalt oxide and chromium doped cobalt oxide cluster cations, Co{sub n}O{sub m}{sup +} and Co{sub n−1}CrO{sub m}{sup +} (n = 2, 3; m = 2–6 and n = 4; m = 3–8), has been investigated using photodissociation mass spectrometry. Oxygen-rich Co{sub n}O{sub m}{sup +} clusters (m ⩾ n + 1 for n = 2, 4 and m ⩾ n + 2 for n = 3) prefer to photodissociate via the loss of an oxygen molecule, whereas oxygen poorer clusters favor the evaporation of oxygen atoms. Substituting a single Co atom by a single Cr atom alters the dissociation behavior. All investigated Co{sub n−1}CrO{sub m}{sup +} clusters, except CoCrO{sub 2}{sup +} and CoCrO{sub 3}{sup +}, prefer to decay by eliminating a neutral oxygen molecule. Co{sub 2}O{sub 2}{sup +}, Co{sub 4}O{sub 3}{sup +}, Co{sub 4}O{sub 4}{sup +}, and CoCrO{sub 2}{sup +} are found to be relatively difficult to dissociate and appear as fragmentation product of several larger clusters, suggesting that they are particularly stable. The geometric structures of pure and Cr doped cobalt oxide species are studied using density functional theory calculations. Dissociation energies for different evaporation channels are calculated and compared with the experimental observations. The influence of the dopant atom on the structure and the stability of the clusters is discussed.

  4. Photon-Induced Thermal Desorption of CO from Small Metal-Carbonyl Clusters

    NASA Astrophysics Data System (ADS)

    Lüttgens, G.; Pontius, N.; Bechthold, P. S.; Neeb, M.; Eberhardt, W.

    2002-02-01

    Thermal CO desorption from photoexcited free metal-carbonyl clusters has been resolved in real time using two-color pump-probe photoelectron spectroscopy. Sequential energy dissipation steps between the initial photoexcitation and the final desorption event, e.g., electron relaxation and thermalization, have been resolved for Au2(CO)- and Pt2(CO)-5. The desorption rates for the two clusters differ considerably due to the different numbers of vibrational degrees of freedom. The unimolecular CO-desorption thresholds of Au2(CO)- and Pt2(CO)-5 have been approximated by means of a statistical Rice-Ramsperger-Kassel calculation using the experimentally derived desorption rate constants.

  5. Vibrations of small cobalt clusters on low-index surfaces of copper: Tight-binding simulations

    NASA Astrophysics Data System (ADS)

    Borisova, S. D.; Eremeev, S. V.; Rusina, G. G.; Stepanyuk, V. S.; Bruno, P.; Chulkov, E. V.

    2008-08-01

    Vibrational properties (frequencies, polarizations, and lifetimes) of a single adatom, dimer, and trimer of Co on low-index Cu surfaces, Cu(111), Cu(001), and Cu(110) are studied by using tight-binding second moment approximation interatomic interaction potentials. We show that structural and vibrational properties of the Co clusters strongly depend on the substrate orientation. The longest lifetimes of 1-2.5 ps have been found for high-frequency z -polarized vibrations in all the Co clusters considered. The shortest lifetimes of 0.1-0.8 ps have been obtained for low-frequency horizontal (frustrated translation) vibrational modes.

  6. Structure, rotational dynamics, and superfluidity of small OCS-doped He clusters.

    PubMed

    Moroni, Saverio; Sarsa, Antonio; Fantoni, Stefano; Schmidt, Kevin E; Baroni, Stefano

    2003-04-11

    The structural and dynamical properties of carbonyl sulfide (OCS) molecules solvated in helium clusters are studied using reptation quantum Monte Carlo, for cluster sizes n=3-20 He atoms. Computer simulations allow us to establish a relation between the rotational spectrum of the solvated molecule and the structure of the He solvent, and of both with the onset of superfluidity. Our results agree with a recent spectroscopic study of this system and provide a more complex and detailed microscopic picture of this system than inferred from experiments.

  7. Hydrogen-bond cooperative effects in small cyclic water clusters as revealed by the interacting quantum atoms approach.

    PubMed

    Guevara-Vela, José Manuel; Chávez-Calvillo, Rodrigo; García-Revilla, Marco; Hernández-Trujillo, Jesús; Christiansen, Ove; Francisco, Evelio; Martín Pendás, Angel; Rocha-Rinza, Tomás

    2013-10-11

    The cooperative effects of hydrogen bonding in small water clusters (H2 O)n (n=3-6) have been studied by using the partition of the electronic energy in accordance with the interacting quantum atoms (IQA) approach. The IQA energy splitting is complemented by a topological analysis of the electron density (ρ(r)) compliant with the quantum theory of atoms-in-molecules (QTAIM) and the calculation of electrostatic interactions by using one- and two-electron integrals, thereby avoiding convergence issues inherent to a multipolar expansion. The results show that the cooperative effects of hydrogen bonding in small water clusters arise from a compromise between: 1) the deformation energy (i.e., the energy necessary to modify the electron density and the configuration of the nuclei of the isolated water molecules to those within the water clusters), and 2) the interaction energy (Eint ) of these contorted molecules in (H2 O)n . Whereas the magnitude of both deformation and interaction energies is enhanced as water molecules are added to the system, the augmentation of the latter becomes dominant when the size of the cluster is increased. In addition, the electrostatic, classic, and exchange components of Eint for a pair of water molecules in the cluster (H2 O)n-1 become more attractive when a new H2 O unit is incorporated to generate the system (H2 O)n with the last-mentioned contribution being consistently the most important part of Eint throughout the hydrogen bonds under consideration. This is opposed to the traditional view, which regards hydrogen bonding in water as an electrostatically driven interaction. Overall, the trends of the delocalization indices, δ(Ω,Ω'), the QTAIM atomic charges, the topology of ρ(r), and the IQA results altogether show how polarization, charge transfer, electrostatics, and covalency contribute to the cooperative effects of hydrogen bonding in small water clusters. It is our hope that the analysis presented in this paper could

  8. Simultaneous Multiparametric PET/MRI with Silicon Photomultiplier PET and Ultra-High-Field MRI for Small-Animal Imaging.

    PubMed

    Ko, Guen Bae; Yoon, Hyun Suk; Kim, Kyeong Yun; Lee, Min Sun; Yang, Bo Yeun; Jeong, Jae Min; Lee, Dong Soo; Song, In Chan; Kim, Seok-Ki; Kim, Daehong; Lee, Jae Sung

    2016-08-01

    Visualization of biologic processes at molecular and cellular levels has revolutionized the understanding and treatment of human diseases. However, no single biomedical imaging modality provides complete information, resulting in the emergence of multimodal approaches. Combining state-of-the-art PET and MRI technologies without loss of system performance and overall image quality can provide opportunities for new scientific and clinical innovations. Here, we present a multiparametric PET/MR imager based on a small-animal dedicated, high-performance, silicon photomultiplier (SiPM) PET system and a 7-T MR scanner. A SiPM-based PET insert that has the peak sensitivity of 3.4% and center volumetric resolution of 1.92/0.53 mm(3) (filtered backprojection/ordered-subset expectation maximization) was developed. The SiPM PET insert was placed between the mouse body transceiver coil and gradient coil of a 7-T small-animal MRI scanner for simultaneous PET/MRI. Mutual interference between the MRI and SiPM PET systems was evaluated using various MR pulse sequences. A cylindric corn oil phantom was scanned to assess the effects of the SiPM PET on the MR image acquisition. To assess the influence of MRI on the PET imaging functions, several PET performance indicators including scintillation pulse shape, flood image quality, energy spectrum, counting rate, and phantom image quality were evaluated with and without the application of MR pulse sequences. Simultaneous mouse PET/MRI studies were also performed to demonstrate the potential and usefulness of the multiparametric PET/MRI in preclinical applications. Excellent performance and stability of the PET system were demonstrated, and the PET/MRI combination did not result in significant image quality degradation of either modality. Finally, simultaneous PET/MRI studies in mice demonstrated the feasibility of the developed system for evaluating the biochemical and cellular changes in a brain tumor model and facilitating the

  9. Dynamical calculations of charge-transfer-to-solvent excited states of small I- (CH3CN)n clusters.

    PubMed

    Takayanagi, Toshiyuki

    2006-06-08

    Relaxation dynamics of photoexcited charge-transfer-to-solvent (CTTS) states for the I(-)(CH(3)CN)(n) (n = 2 and 3) clusters has been theoretically studied using electronic structure methods. First, we have calculated several lowest singlet and triplet potential energy surfaces using the multireference configuration interaction method. It was found that the character of the singlet CTTS excited-state potential surfaces is very similar to that of the triplet CTTS states. Due to a small singlet-triplet splitting, the lowest triplet potential energy surface was used as a good model to understand the dynamics of the photoexcited singlet CTTS states. We have carried out direct molecular dynamics simulations on the lowest triplet surface at the B3LYP level. When an I(-) anion is exteriorly solvated by CH(3)CN molecules, we found that the (CH(3)CN)(n)(-) anion cluster is effectively produced. In addition, when the I(-) anion is placed in the interior in I(-)(CH(3)CN)(n) clusters, photoexcitation gives an acetonitrile monomer anion plus neutral monomers. However, if the initial geometric configuration is distorted from the minimum structure, we also found that the (CH(3)CN)(2)(-) anion cluster, where an excess electron is internally trapped, is formed via I(-)(CH(3)CN)(2) + hnu --> I + (CH(3)CN)(2)(-) process.

  10. Physical properties of small water clusters in low and moderate electric fields

    NASA Astrophysics Data System (ADS)

    Acosta-Gutiérrez, S.; Hernández-Rojas, J.; Bretón, J.; Llorente, J. M. Gomez; Wales, D. J.

    2011-09-01

    Likely candidates for the lowest minima of water clusters (H2O)N for N ⩽ 20 interacting with a uniform electric field strength in the range E ⩽ 0.6 V/Å have been identified using basin-hopping global optimization. Two water-water model potentials were considered, namely TIP4P and the polarizable Dang-Chang potential. The two models produce some consistent results but also exhibit significant differences. The cluster internal energy and dipole moment indicate two varieties of topological transition in the structure of the global minimum as the field strength is increased. The first takes place at low field strengths (0.1 V/Åcluster stretches along the field direction, the larger clusters (N > 10) usually forming helical structures.

  11. Structure, fragmentation patterns, and magnetic properties of small nickel oxide clusters.

    PubMed

    Aguilera-Del-Toro, R H; Aguilera-Granja, F; Balbás, L C; Vega, A

    2017-01-25

    We report a comprehensive theoretical study of the structural and electronic properties of neutral and charged nickel oxide clusters, NinOm(0/±) (n = 3-8 and m = 1-10), in the context of recent experiments of photodissociation and Ion Mobility Mass Spectrometry. By means of density functional theory calculations in the generalized gradient approximation for exchange and correlation, we determined the putative ground states as well as the low-energy structural- and spin-isomers which were then used to explore the favorable fragmentation channels of the nickel oxide cationic clusters, and the resulting most abundant products, in good qualitative agreement with photodissociation measurements. Apart from stoichiometries different from those of their nickel oxide macroscopic counterparts, we found a tendency to form compact Ni subclusters, with reentrance of low-coordinated structures close to the equiatomic Ni-O concentration, taking the form of alternating Ni-O rings in the smaller sizes, in good qualitative agreement with Ion Mobility Mass Spectrometry measurements. This structural pattern is manifested in a drop of the total spin magnetic moment close to the equiatomic concentration due to the formation of antiparallel magnetic couplings. Although antiparallel couplings are found to a more or less extent in most clusters, especially in the oxygen rich phase, we identified certain clusters of special interest in the context of magnetic grains because of their large total magnetic moment and abundance. There are even some nickel oxide clusters with a higher total moment than their pure Ni counterparts, due to parallel magnetic couplings and the contribution of the oxygen atoms to the total moment.

  12. Detection of the YORP Effect for Small Asteroids in the Karin Cluster

    NASA Astrophysics Data System (ADS)

    Carruba, V.; Nesvorný, D.; Vokrouhlický, D.

    2016-06-01

    The Karin cluster is a young asteroid family thought to have formed only ≃ 5.75 Myr ago. The young age can be demonstrated by numerically integrating the orbits of Karin cluster members backward in time and showing the convergence of the perihelion and nodal longitudes (as well as other orbital elements). Previous work has pointed out that the convergence is not ideal if the backward integration only accounts for the gravitational perturbations from the solar system planets. It improves when the thermal radiation force known as the Yarkovsky effect is accounted for. This argument can be used to estimate the spin obliquities of the Karin cluster members. Here we take advantage of the fast growing membership of the Karin cluster and show that the obliquity distribution of diameter D≃ 1{--}2 km Karin asteroids is bimodal, as expected if the YORP effect acted to move obliquities toward extreme values (0° or 180°). The measured magnitude of the effect is consistent with the standard YORP model. The surface thermal conductivity is inferred to be 0.07-0.2 W m-1 K-1 (thermal inertia ≃ 300{--}500 J m-2 K-1 s{}-1/2). We find that the strength of the YORP effect is roughly ≃ 0.7 of the nominal strength obtained for a collection of random Gaussian spheroids. These results are consistent with a surface composed of rough, rocky regolith. The obliquity values predicted here for 480 members of the Karin cluster can be validated by the light-curve inversion method.

  13. Ab initio GW quasiparticle energies of small sodium clusters by an all-electron mixed-basis approach

    NASA Astrophysics Data System (ADS)

    Ishii, Soh; Ohno, Kaoru; Kawazoe, Yoshiyuki; Louie, Steven G.

    2001-04-01

    A state-of-the-art GW calculation is carried out for small sodium clusters, Na2, Na4, Na6, and Na8. The quasiparticle energies are evaluated by employing an ab initio GW code based on an all-electron mixed-basis approach, which uses both plane waves and atomic orbitals as basis functions. The calculated ionization potential and the electron affinity are in excellent agreement with available experimental data. The exchange and correlation parts to the electron self-energy within the GW approximation are presented from the viewpoint of their size dependence. In addition, the effect of the off-diagonal elements of the self-energy corrections to the local-density-approximation exchange-correlation potential is discussed. Na2 and Na8 have a larger energy gap than Na4 and Na6, consistent with the fact that they are magic number clusters.

  14. Magnetism and exchange interaction of small rare-earth clusters; Tb as a representative

    PubMed Central

    Peters, Lars; Ghosh, Saurabh; Sanyal, Biplab; van Dijk, Chris; Bowlan, John; de Heer, Walt; Delin, Anna; Di Marco, Igor; Eriksson, Olle; Katsnelson, Mikhail I.; Johansson, Börje; Kirilyuk, Andrei

    2016-01-01

    Here we follow, both experimentally and theoretically, the development of magnetism in Tb clusters from the atomic limit, adding one atom at a time. The exchange interaction is, surprisingly, observed to drastically increase compared to that of bulk, and to exhibit irregular oscillations as a function of the interatomic distance. From electronic structure theory we find that the theoretical magnetic moments oscillate with cluster size in exact agreement with experimental data. Unlike the bulk, the oscillation is not caused by the RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a competition between wave-function overlap of the 5d shell and the on-site exchange interaction, which leads to a competition between ferromagnetic double-exchange and antiferromagnetic super-exchange. This understanding opens up new ways to tune the magnetic properties of rare-earth based magnets with nano-sized building blocks. PMID:26795239

  15. Magnetism and exchange interaction of small rare-earth clusters; Tb as a representative.

    PubMed

    Peters, Lars; Ghosh, Saurabh; Sanyal, Biplab; van Dijk, Chris; Bowlan, John; de Heer, Walt; Delin, Anna; Di Marco, Igor; Eriksson, Olle; Katsnelson, Mikhail I; Johansson, Börje; Kirilyuk, Andrei

    2016-01-22

    Here we follow, both experimentally and theoretically, the development of magnetism in Tb clusters from the atomic limit, adding one atom at a time. The exchange interaction is, surprisingly, observed to drastically increase compared to that of bulk, and to exhibit irregular oscillations as a function of the interatomic distance. From electronic structure theory we find that the theoretical magnetic moments oscillate with cluster size in exact agreement with experimental data. Unlike the bulk, the oscillation is not caused by the RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a competition between wave-function overlap of the 5d shell and the on-site exchange interaction, which leads to a competition between ferromagnetic double-exchange and antiferromagnetic super-exchange. This understanding opens up new ways to tune the magnetic properties of rare-earth based magnets with nano-sized building blocks.

  16. The electronic structures of small Ni(n) (n=2-4) clusters and their interactions with ethylene and triplet oxygen: a theoretical study.

    PubMed

    Pakiari, Ali H; Pahlavan, Farideh

    2014-12-15

    Density functional theory (DFT) calculations of small nickel clusters and their interacting systems are carried out using the BLYP and B97-2 methods, after DFT calibration. All bare nickel clusters in this study have high multiplicities and are paramagnetic. Our results for the interactions between ethylene and oxygen with Ni(n) (n=2-4) clusters at different adsorption modes show that for ethylene, π-orientation is preferred, and that oxygen adsorption in a bridge mode is stronger than on-top coordination. Vibrational frequency analysis reveals that the vibrational modes of ethylene π-coordinated to nickel clusters converge toward the corresponding value for surface-bound ethylene, as the cluster size increases from two to four, showing that finite clusters can be used as localized models for ligand adsorption on nickel surfaces. We also calculate DFT global reactivity descriptors, chemical potential and hardness, and use these to predict the relative stability and reactivity of each bare cluster.

  17. Quantum chemical calculation of the equilibrium structures of small metal atom clusters

    NASA Technical Reports Server (NTRS)

    Kahn, L. R.

    1982-01-01

    Metal atom clusters are studied based on the application of ab initio quantum mechanical approaches. Because these large 'molecular' systems pose special practical computational problems in the application of the quantum mechanical methods, there is a special need to find simplifying techniques that do not compromise the reliability of the calculations. Research is therefore directed towards various aspects of the implementation of the effective core potential technique for the removal of the metal atom core electrons from the calculations.

  18. Small clusters of fast-growing trees enhance forest structure on restored bottomland sites

    USGS Publications Warehouse

    Twedt, D.J.

    2006-01-01

    Despite the diversity of trees in bottomland forests, restoration on bottomland sites is often initiated by planting only a few species of slow-growing, hard mast?producing trees. Although successful at establishing trees, these young forests are slow to develop vertical structure, which is a key predictor of forest bird colonization. Furthermore, when natural seed sources are few, restored sites may be depauperate in woody species. To increase richness of woody species, maximum tree height, and total stem density, I supplemented traditional plantings on each of 40 bottomland restoration sites by planting 96 Eastern cottonwood (Populus deltoides) and American sycamore (Platanus occidentalis) in eight clusters of 12 trees. First year survival of cottonwood stem cuttings (25%) and sycamore seedlings (47%) was poor, but survival increased when afforded protection from competition with weeds. After five growing seasons, 165 of these 320 supplemental tree clusters had at least one surviving tree. Vegetation surrounding surviving clusters of supplemental trees harbored a greater number of woody species, increased stem density, and greater maximum tree height than was found on paired restoration sites without supplemental trees. These increases were primarily accounted for by the supplemental trees.

  19. Synthesis and catalytic properties of metal clusters encapsulated within small-pore (SOD, GIS, ANA) zeolites.

    PubMed

    Goel, Sarika; Wu, Zhijie; Zones, Stacey I; Iglesia, Enrique

    2012-10-24

    The synthesis protocols for encapsulation of metal clusters reported here expand the diversity in catalytic chemistries made possible by the ability of microporous solids to select reactants, transition states, and products on the basis of their molecular size. We report a synthesis strategy for the encapsulation of noble metals and their oxides within SOD (Sodalite, 0.28 nm × 0.28 nm), GIS (Gismondine, 0.45 nm × 0.31 nm), and ANA (Analcime, 0.42 nm × 0.16 nm) zeolites. Encapsulation was achieved via direct hydrothermal synthesis for SOD and GIS using metal precursors stabilized by ammonia or organic amine ligands, which prevent their decomposition or precipitation as colloidal hydroxides at the conditions of hydrothermal synthesis (<380 K) and favor interactions between metal precursors and incipient aluminosilicate nuclei during self-assembly of microporous frameworks. The synthesis of ANA requires higher crystallization temperatures (~415 K) and high pH (>12), thereby causing precipitation of even ligand-stabilized metal precursors as hydroxides. As a result, encapsulation was achieved by the recrystallization of metal clusters containing GIS into ANA, which retained these metal clusters within voids throughout the GIS-ANA transformation.

  20. Rotational structure of small 4He clusters seeded with HF, HCl, and HBr molecules.

    PubMed

    Ramilowski, Jordan A; Mikosz, Aleksandra A; Farrelly, David; Fajín, José Luis Cagide; Fernandez, Berta

    2007-12-13

    Diffusion Monte Carlo calculations are performed for ground and excited rotational states of HX(4He)N, complexes with Ncluster size and potential anisotropy. Nodal surfaces of the many-body wave function are computed approximately by making an adiabatic Born-Oppenheimer-like separation of radial and angular degrees of freedom of the cluster. This procedure is extended to include radial dependencies in the adiabatic nodal function. We predict that the observed decrease in the gas-phase rotational constants for HCl and HBr in a 4He nanodroplet will be smaller than that observed for HF, despite HF's having the largest (by far) gas-phase rotational constant of the three molecules. This suggests that the specifics of the solvation dynamics of a molecule in a 4He cluster are the result of a delicate interplay between the magnitude of the gas-phase rotational constant of the molecule and the anisotropic contributions to the atom-molecule potential energy.

  1. Theoretical study of the electronic states of small cationic niobium clusters, Nbn+ (n=3-5)

    NASA Astrophysics Data System (ADS)

    Majumdar, D.; Balasubramanian, K.

    2001-07-01

    Geometries and energy separations of the various low-lying electronic states of Nbn+ (n=3-5) clusters with different structural arrangements have been investigated. The complete active space multiconfiguration self-consistent field (CASMCSCF) method followed by the multireference singles plus doubles configuration interaction (MRSDCI) that included up to 13 million configuration spin functions have been used to compute several electronic states of these clusters. A 5A2 isosceles triangle geometry in C2v symmetry and a 2A' pyramid structure in Cs symmetry are computed as the ground states of Nb3+ and Nb4+ clusters, respectively. In the case of Nb5+, a 1A' state of distorted edge-capped tetrahedral structure (in Cs symmetry) was found to be the ground state. We also compared our MRSDCI results with density functional calculations. The dissociation and atomization energies have been calculated at the MRSDCI level and the results have been found to be in agreement with experimental findings.

  2. Mice deficient for a small cluster of Piwi-interacting RNAs implicate Piwi-interacting RNAs in transposon control.

    PubMed

    Xu, Mingang; You, Yun; Hunsicker, Patricia; Hori, Tamaki; Small, Chris; Griswold, Michael D; Hecht, Norman B

    2008-07-01

    The mammalian testis expresses a class of small noncoding RNAs that interact with mammalian PIWI proteins. In mice, the PIWI-interacting RNAs (piRNAs) partner with mammalian PIWI proteins, PIWIL1 and PIWIL2, also known as MIWI and MILI, to maintain transposon silencing in the germline genome. Here, we demonstrate that inactivation of Nct1/2, two noncoding RNAs encoding piRNAs, leads to derepression of LINE-1 (L1) but does not affect mouse viability, spermatogenesis, testicular gene expression, or fertility. These findings indicate that piRNAs from a cluster on chromosome 2 are necessary to maintain transposon silencing.

  3. Jet-Cooled High Resolution Infrared Spectroscopy of Small Van Der Waals SF_6 Clusters

    NASA Astrophysics Data System (ADS)

    Asselin, Pierre; Boudon, Vincent; Potapov, Alexey; Bruel, Laurent; Gaveau, Marc-André; Mons, Michel

    2016-06-01

    Using a pulsed slit nozzle multipass absorption spectrometer with a tunable quantum cascade laser we investigated van der Waals clusters involving sulfur hexafluoride in the spectral range near the νb{3} stretching vibration. Different sized homo-complexes were generated in a planar supersonic expansion with typically 0,5 % SF_6 diluted in 6 bar He. Firstly, several rotationally resolved parallel and perpendicular bands of (SF_6)_2, at 934,0 and 956,1 wn (#1 structure) in agreement with Takami et al. but also one band at 933,6 wn (#2 structure) never observed previously, were analyzed in light of a recent theoretical study predicting three nearly isoenergetic isomers of D2d, C2h and C_2 symmetry for the dimer. Furthermore, some broader bands were detected around 938 and 964 wn and assigned to (SF_6)_3 and (SF_6)_4 clusters on the grounds of concentration effects and/or ab initio calculations. Lastly, with 0,5 % rare gas Rg (Rg = Ne, Ar, Kr and Xe) added to the SF_6:He gas mixture, a series of van der Waals (SF_6)_2-Rg hetero-trimers were observed, which display a remarkable linear dependence of the vibrational shift with the polarizability of the rare gas atom provided that the initial SF_6 dimer structure is #2 . In the same time no transitions belonging to the binary complexes SF_6-Rg were found near the νb{3} monomer band. This result suggests a complex thermodynamics within the pulsed supersonic expansion leading to the preponderance of (SF_6)_2-Rg clusters over SF_6-Rg binary systems. R. D. Urban and M. Takami, J. Chem. Phys. 103, 9132 (1995). T. Vazhappily, A. Marjolin and K. D. Jordan, J. Phys. Chem. B, DOI: 10.1021/acs.jpcb.5b09419 (2015).

  4. Photoelectron spectroscopy and ab initio calculations of small Si{sub n}S{sub m}{sup −} (n = 1,2; m = 1–4) clusters

    SciTech Connect

    Xu, Xi-Ling; Deng, Xiao-Jiao; Xu, Hong-Guang; Zheng, Wei-Jun

    2014-09-28

    Binary cluster anions composed of silicon and sulfur elements, Si{sub n}S{sub m}{sup −} (n = 1,2; m = 1-4), were investigated by using photoelectron spectroscopy and ab initio calculations. The vertical detachment energies and the adiabatic detachment energies of these clusters were obtained from their photoelectron spectra. The electron affinity of SiS molecule is determined to be 0.477 ± 0.040 eV. The results show that the most stable structures of the anionic and neutral Si{sub n}S{sub m} (n = 1,2; m = 1-4) clusters prefer to adopt planar configurations except that the structures of Si{sub 2}S{sub 4}{sup −} and Si{sub 2}S{sub 2} are slightly bent.

  5. Small Al clusters. II - Structure and binding in Al(n) (n = 2-6, 13)

    NASA Technical Reports Server (NTRS)

    Pettersson, Lars G. M.; Bauschlicher, Charles W., Jr.; Halicioglu, Timur

    1987-01-01

    The structure and stability of aluminum clusters containing up to six atoms have been studied using correlated wave functions and extended basis sets. The lowest energy structure is planar for Al4 and Al5, but three dimensional for Al6. The icosahedral, hcp, fcc, and two planar structures of Al13 were considered at the SCF level. The lowest energy structure is the icosahedron, but the planar structures are fairly low lying even in this case. A simplified description using two- and three-body interaction potentials is found to agree well with the ab initio structures and binding energies.

  6. Negative ion productions in high velocity collision between small carbon clusters and Helium atom target

    NASA Astrophysics Data System (ADS)

    M, Chabot; K, Béroff; T, Pino; G, Féraud; N, Dothi; Padellec A, Le; G, Martinet; S, Bouneau; Y, Carpentier

    2012-11-01

    We measured absolute double capture cross section of Cn+ ions (n=1,5) colliding, at 2.3 and 2.6 a.u velocities, with an Helium target atom and the branching ratios of fragmentation of the so formed electronically excited anions Cn-*. We also measured absolute cross section for the electronic attachment on neutral Cn clusters colliding at same velocities with He atom. This is to our knowledge the first measurement of neutral-neutral charge exchange in high velocity collision.

  7. Small Al clusters. II - Structure and binding in Al(n) (n = 2-6, 13)

    NASA Technical Reports Server (NTRS)

    Pettersson, Lars G. M.; Bauschlicher, Charles W., Jr.; Halicioglu, Timur

    1987-01-01

    The structure and stability of aluminum clusters containing up to six atoms have been studied using correlated wave functions and extended basis sets. The lowest energy structure is planar for Al4 and Al5, but three dimensional for Al6. The icosahedral, hcp, fcc, and two planar structures of Al13 were considered at the SCF level. The lowest energy structure is the icosahedron, but the planar structures are fairly low lying even in this case. A simplified description using two- and three-body interaction potentials is found to agree well with the ab initio structures and binding energies.

  8. A theoretical equation of state for detonation products with chemical equilibrium composition of the surface of small carbon clusters

    NASA Astrophysics Data System (ADS)

    Shaw, M. Sam

    2000-04-01

    A theoretical equation of state for detonation products is described and compared with various data. A perturbation theory approach is used for the mixture of molecular fluids that is based on Monte Carlo simulations. The solid carbon is characterized as small clusters rather than as a bulk material. The free energy of the clusters is modeled as primarily additive contributions such as vibrational modes, bond strengths, and effective volumes. Up to 30% of the atoms in a diamond cluster are on the surface with dangling bonds capped by various groups composed of C, H, N, and O from the background fluid. A counting term similar to ideal entropy of mixing is also found for the surface composition. Competition between the U, TS, and PV terms in the Gibbs free energy leads to dramatic shifts in the surface composition in some regions. This in turn leads to shifts in the background fluid mixture composition and anomalous behavior in the total EOS. This behavior in PBX 9502 is in good agreement with recent data on release isentropes from overdriven states (1).

  9. Comparing denominator degrees of freedom approximations for the generalized linear mixed model in analyzing binary outcome in small sample cluster-randomized trials.

    PubMed

    Li, Peng; Redden, David T

    2015-04-23

    Small number of clusters and large variation of cluster sizes commonly exist in cluster-randomized trials (CRTs) and are often the critical factors affecting the validity and efficiency of statistical analyses. F tests are commonly used in the generalized linear mixed model (GLMM) to test intervention effects in CRTs. The most challenging issue for the approximate Wald F test is the estimation of the denominator degrees of freedom (DDF). Some DDF approximation methods have been proposed, but their small sample performances in analysing binary outcomes in CRTs with few heterogeneous clusters are not well studied. The small sample performances of five DDF approximations for the F test are compared and contrasted under CRT frameworks with simulations. Specifically, we illustrate how the intraclass correlation (ICC), sample size, and the variation of cluster sizes affect the type I error and statistical power when different DDF approximation methods in GLMM are used to test intervention effect in CRTs with binary outcomes. The results are also illustrated using a real CRT dataset. Our simulation results suggest that the Between-Within method maintains the nominal type I error rates even when the total number of clusters is as low as 10 and is robust to the variation of the cluster sizes. The Residual and Containment methods have inflated type I error rates when the cluster number is small (<30) and the inflation becomes more severe with increased variation in cluster sizes. In contrast, the Satterthwaite and Kenward-Roger methods can provide tests with very conservative type I error rates when the total cluster number is small (<30) and the conservativeness becomes more severe as variation in cluster sizes increases. Our simulations also suggest that the Between-Within method is statistically more powerful than the Satterthwaite or Kenward-Roger method in analysing CRTs with heterogeneous cluster sizes, especially when the cluster number is small. We conclude that the

  10. BOHENDI@FELIX: Probing the Far-Infrared Fingerprint of Small Clusters in Helium Nanodroplets with a Free Electron Laser

    NASA Astrophysics Data System (ADS)

    Schwaab, Gerhard; Schwan, Raffael; Mani, Devendra; Dey, Arghya; Fischer, Theo; Kaufmann, Matin; Redlich, Britta; van der Meer, Lex; Havenith, Martina

    2016-06-01

    Recently, we have installed a helium nanodroplet machine [1,2] at the free electron beamline FELIX in Nijmegen. The current setup allows to study neutral molecules and molecular complexes in the full spectral range from 500--3000 cm-1. First proof of principle experiments using the strong absorber SF_6 were used to verify the overall alignment between helium nanodroplet beam and the FELIX radiation source. Applications so far included the study of small water clusters and the investigation of microsolvation of small solutes. These results will be presented and compared to recent theoretical predictions of the Bowman group.[3] [1] K. von Haeften et al., Phys. Rev. B. 73, 054502 (2006) [2] Choi et al., Int. Rev. Phys. Chem. 25, 15 (2006) [3] Samantha et al., Acc. Chem. Res. 47, 2700 (2014)

  11. Optical absorption of small copper clusters in neon: Cu(n), (n = 1-9).

    PubMed

    Lecoultre, S; Rydlo, A; Félix, C; Buttet, J; Gilb, S; Harbich, W

    2011-02-21

    We present optical absorption spectra in the UV-visible range (1.6 eV < ℏω < 5.5 eV) of mass selected neutral copper clusters Cu(n)(n = 1-9) embedded in a solid neon matrix at 7 K. The atom and the dimer have already been measured in neon matrices, while the absorption spectra for sizes between Cu(3) and Cu(9) are entirely (n = 6-9) or in great part new. They show a higher complexity and a larger number of transitions distributed over the whole energy range compared to similar sizes of silver clusters. The experimental spectra are compared to the time dependent density functional theory (TD-DFT) implemented in the TURBOMOLE package. The analysis indicates that for energies larger than 3 eV the transitions are mainly issued from d-type states; however, the TD-DFT scheme does not reproduce well the detailed structure of the absorption spectra. Below 3 eV the agreement for transitions issued from s-type states is better.

  12. Direct observation of small cluster mobility and ripening. [during annealing of metal films on amorphous substrates

    NASA Technical Reports Server (NTRS)

    Heinemann, K.; Poppa, H.

    1975-01-01

    Direct evidence is reported for the simultaneous occurrence of Ostwald ripening and short-distance cluster mobility during annealing of discontinuous metal films on clean amorphous substrates. The annealing characteristics of very thin particulate deposits of silver on amorphized clean surfaces of single crystalline thin graphite substrates were studied by in-situ transmission electron microscopy (TEM) under controlled environmental conditions (residual gas pressure of 10 to the minus 9th power torr) in the temperature range from 25 to 450 C. Sputter cleaning of the substrate surface, metal deposition, and annealing were monitored by TEM observation. Pseudostereographic presentation of micrographs in different annealing stages, the observation of the annealing behavior at cast shadow edges, and measurements with an electronic image analyzing system were employed to aid the visual perception and the analysis of changes in deposit structure recorded during annealing. Slow Ostwald ripening was found to occur in the entire temperature range, but the overriding surface transport mechanism was short-distance cluster mobility.

  13. First-principles investigation of the dissociation and coupling of methane on small copper clusters: Interplay of collision dynamics and geometric and electronic effects

    NASA Astrophysics Data System (ADS)

    Varghese, Jithin J.; Mushrif, Samir H.

    2015-05-01

    Small metal clusters exhibit unique size and morphology dependent catalytic activity. The search for alternate minimum energy pathways and catalysts to transform methane to more useful chemicals and carbon nanomaterials led us to investigate collision induced dissociation of methane on small Cu clusters. We report here for the first time, the free energy barriers for the collision induced activation, dissociation, and coupling of methane on small Cu clusters (Cun where n = 2-12) using ab initio molecular dynamics and metadynamics simulations. The collision induced activation of the stretching and bending vibrations of methane significantly reduces the free energy barrier for its dissociation. Increase in the cluster size reduces the barrier for dissociation of methane due to the corresponding increase in delocalisation of electron density within the cluster, as demonstrated using the electron localisation function topology analysis. This enables higher probability of favourable alignment of the C-H stretching vibration of methane towards regions of high electron density within the cluster and makes higher number of sites available for the chemisorption of CH3 and H upon dissociation. These characteristics contribute in lowering the barrier for dissociation of methane. Distortion and reorganisation of cluster geometry due to high temperature collision dynamics disturb electron delocalisation within them and increase the barrier for dissociation. Coupling reactions of CHx (x = 1-3) species and recombination of H with CHx have free energy barriers significantly lower than complete dehydrogenation of methane to carbon. Thus, competition favours the former reactions at high hydrogen saturation on the clusters.

  14. Adsorption and Formation of Small Na Clusters on Pristine and Double-Vacancy Graphene for Anodes of Na-Ion Batteries.

    PubMed

    Liang, Zhicong; Fan, Xiaofeng; Zheng, Weitao; Singh, David J

    2017-05-24

    Layered carbon is a likely anode material for Na-ion batteries (NIBs). Graphitic carbon has a low capacity of approximately 35 (mA h)/g due to the formation of NaC64. Using first-principles methods including van der Waals interactions, we analyze the adsorption of Na ions and clusters on graphene in the context of anodes. The interaction between Na ions and graphene is found to be weak. Small Na clusters are not stable on the surface of pristine graphene in the electrochemical environment of NIBs. However, we find that Na ions and clusters can be stored effectively on defected graphene that has double vacancies. In addition, the adsorption energy of small Na clusters near a double vacancy is found to decrease with increasing cluster size. With high concentrations of vacancies the capacity of Na on defective graphene is found to be as much as 10-30 times higher than that of graphitic carbon.

  15. Structure, defects, and strain in silicon-silicon oxide interfaces

    SciTech Connect

    Kovačević, Goran Pivac, Branko

    2014-01-28

    The structure of the interfaces between silicon and silicon-oxide is responsible for proper functioning of MOSFET devices while defects in the interface can deteriorate this function and lead to their failure. In this paper we modeled this interface and characterized its defects and strain. MD simulations were used for reconstructing interfaces into a thermodynamically stable configuration. In all modeled interfaces, defects were found in the form of three-coordinated silicon atom, five coordinated silicon atom, threefold-coordinated oxygen atom, or displaced oxygen atom. Three-coordinated oxygen atom can be created if dangling bonds on silicon are close enough. The structure and stability of three-coordinated silicon atoms (P{sub b} defect) depend on the charge as well as on the electric field across the interface. The negatively charged P{sub b} defect is the most stable one, but the electric field resulting from the interface reduces that stability. Interfaces with large differences in periodic constants of silicon and silicon oxide can be stabilized by buckling of silicon layer. The mechanical stress resulted from the interface between silicon and silicon oxide is greater in the silicon oxide layer. Ab initio modeling of clusters representing silicon and silicon oxide shows about three time larger susceptibility to strain in silicon oxide than in silicon if exposed to the same deformation.

  16. Effect of subsurface Ti-interstitials on the bonding of small gold clusters on rutile TiO(2)(110).

    PubMed

    Madsen, Georg K H; Hammer, Bjørk

    2009-01-28

    The density functional theory is used to examine the electronic structure of small Au clusters, supported on rutile TiO(2)(110) surfaces having subsurface Ti-interstitials. The interstitials reduce the surface and we find that negatively charged gold clusters are stabilized with respect to the stoichiometric surface. The behavior of the open-shell gold clusters can be rationalized in terms of the highest occupied molecular orbitals and the resulting electron affinities. The relative stabilities of closed-shell gold clusters led to recent disagreements in the literature. We show that they are very dependent on the density functional used. As expected, a redshift in the CO stretch vibration is calculated for CO adsorbed on a negatively charged cluster. Somewhat surprisingly a larger redshift is found for CO adsorbed on an overall positively charged Au(3) cluster. This is explained by CO being a local probe of the individual Au charges and one Au atom having an electron accumulation.

  17. Communication: Relativistic Fock-space coupled cluster study of small building blocks of larger uranium complexes

    SciTech Connect

    Tecmer, Paweł Visscher, Lucas; Severo Pereira Gomes, André; Knecht, Stefan

    2014-07-28

    We present a study of the electronic structure of the [UO{sub 2}]{sup +}, [UO{sub 2}]{sup 2} {sup +}, [UO{sub 2}]{sup 3} {sup +}, NUO, [NUO]{sup +}, [NUO]{sup 2} {sup +}, [NUN]{sup −}, NUN, and [NUN]{sup +} molecules with the intermediate Hamiltonian Fock-space coupled cluster method. The accuracy of mean-field approaches based on the eXact-2-Component Hamiltonian to incorporate spin–orbit coupling and Gaunt interactions are compared to results obtained with the Dirac–Coulomb Hamiltonian. Furthermore, we assess the reliability of calculations employing approximate density functionals in describing electronic spectra and quantities useful in rationalizing Uranium (VI) species reactivity (hardness, electronegativity, and electrophilicity)

  18. Quantum chemical calculation of the equilibrium structures of small metal atom clusters

    NASA Technical Reports Server (NTRS)

    Kahn, L. R.

    1981-01-01

    The application of ab initio quantum mechanical approaches in the study of metal atom clusters requires simplifying techniques that do not compromise the reliability of the calculations. Various aspects of the implementation of the effective core potential (ECP) technique for the removal of the metal atom core electrons from the calculation were examined. The ECP molecular integral formulae were modified to bring out the shell characteristics as a first step towards fulfilling the increasing need to speed up the computation of the ECP integrals. Work on the relationships among the derivatives of the molecular integrals that extends some of the techniques pioneered by Komornicki for the calculation of the gradients of the electronic energy was completed and a formulation of the ECP approach that quite naturally unifies the various state-of-the-art "shape- and Hamiltonian-consistent" techniques was discovered.

  19. Complexes of DNA bases and Watson-Crick base pairs with small neutral gold clusters.

    PubMed

    Kryachko, E S; Remacle, F

    2005-12-08

    The nature of the DNA-gold interaction determines and differentiates the affinity of the nucleobases (adenine, thymine, guanine, and cytosine) to gold. Our preliminary computational study [Kryachko, E. S.; Remacle, F. Nano Lett. 2005, 5, 735] demonstrates that two major bonding factors govern this interaction: the anchoring, either of the Au-N or Au-O type, and the nonconventional N-H...Au hydrogen bonding. In this paper, we offer insight into the nature of nucleobase-gold interactions and provide a detailed characterization of their different facets, i.e., geometrical, energetic, and spectroscopic aspects; the gold cluster size and gold coordination effects; proton affinity; and deprotonation energy. We then investigate how the Watson-Crick DNA pairing patterns are modulated by the nucleobase-gold interaction. We do so in terms of the proton affinities and deprotonation energies of those proton acceptors and proton donors which are involved in the interbase hydrogen bondings. A variety of properties of the most stable Watson-Crick [A x T]-Au3 and [G x C]-Au3 hybridized complexes are described and compared with the isolated Watson-Crick A x T and G x C ones. It is shown that enlarging the gold cluster size to Au6 results in a rather short gold-gold bond in the Watson-Crick interbase region of the [G x C]-Au6 complex that bridges the G x C pair and thus leads to a significant strengthening of G x C pairing.

  20. Industrial Education. Mini-Course Cluster: Bikes, Electricity, Small Engines. [Grade 9].

    ERIC Educational Resources Information Center

    Parma City School District, OH.

    Part of a series of curriculum guides dealing with industrial education in junior high schools, this guide provides three units to be used in a one semester course in grade 9 on the subjects of bikes, electricity, and small engines. The section on bicycles is divided into two parts, mechanical and power (i.e. motorcycles) and covers the topics of…

  1. Industrial Education. Mini-Course Cluster: Bikes, Electricity, Small Engines. [Grade 9].

    ERIC Educational Resources Information Center

    Parma City School District, OH.

    Part of a series of curriculum guides dealing with industrial education in junior high schools, this guide provides three units to be used in a one semester course in grade 9 on the subjects of bikes, electricity, and small engines. The section on bicycles is divided into two parts, mechanical and power (i.e. motorcycles) and covers the topics of…

  2. Extended Hückel cluster calculations of the L 2,3VV auger transition in silicon

    NASA Astrophysics Data System (ADS)

    Vidal, R.; Passeggi, M. C. G.; Goldberg, E. C.; Ferrón, J.

    1988-07-01

    We report an independent-electron model calculation of the L 2,3VV Auger lineshape for an ideal Si(100) surface. Valence states were calculated for a cluster of Si atoms using the extended Hückel approximation. Atomic Auger matrix elements were evaluated using the Clementi wave functions for the Si atoms and a Coulombian wave for the continuum wave function. Fairly good agreement between theoretical results and experimental data is found when the presence of the free surface is taken into account.

  3. The structural landscape in 14-vertex clusters of silicon, M@Si14: when two bonding paradigms collide.

    PubMed

    Jin, Xiao; Arcisauskaite, Vaida; McGrady, John E

    2017-09-12

    The structural chemistry of the title clusters has been the source of controversy in the computational literature because the identity of the most stable structure appears to be pathologically dependent on the chosen theoretical model. The candidate structures include a D3h-symmetric 'fullerene-like' isomer with 3-connected vertices (A), an 'arachno' architecture (B) and an octahedral isomer with high vertex connectivities typical of 'closo' electron-deficient clusters (C). The key to understanding these apparently very different structures is the fact that they make use of the limited electron density available from the endohedral metal in very different ways. Early in the transition series the favoured structure is the one that maximises transfer of electron density from the electropositive metal to the cage whereas for later metals it is the one that minimises repulsions with the increasingly core-like d electrons. The varying role of the d electrons across the transition series leads directly to strong functional dependency, and hence to the controversy in the literature.

  4. Graphene-supported small transition-metal clusters: A density functional theory investigation within van der Waals corrections

    NASA Astrophysics Data System (ADS)

    Rêgo, Celso R. C.; Tereshchuk, Polina; Oliveira, Luiz N.; Da Silva, Juarez L. F.

    2017-06-01

    Transition-metal nanoparticles adsorbed on graphene are of great interest due to the unique catalytic and magnetic properties resulting from nanoparticles-graphene interactions. Comparison between the physical properties of such systems and those of the same nanoparticles in the gas phase is especially important. Here we report a systematic density functional investigation of the structural, energetic, and magnetic properties of small Nin, Pdn, and Ptn clusters, comprising from n =1 to 6 atoms, in the gas phase and adsorbed on a graphene monolayer. Our results show that the Ni adatom binds to the graphene hollow site, with -1.47 -meV adsorption energy, while Pd and Pt prefer the bridge sites, with -1.14 - and -1.62 -meV adsorption energies, respectively. This difference is determined by a competition between quantum and classical forces. Ni2 and Pt2 dimers bind perpendicularly on hollow and bridge sites, respectively, while Pd2 lies parallel to the graphene sheet, with each adatom on a bridge site. For larger TMn (TM = Ni , Pd , Pt ; n =3 -6 ) clusters, either two or three atoms bind to bridge graphene sites. In almost all cases the adsorbed clusters retain their gas-phase structures. The exceptions are Ni5 and Pt4, which acquire more compact structures with effective coordination number 12 and 19 % larger than in the gas phase, respectively. As the number of atoms grows, the cluster binds more weakly to the graphene, while its binding energy mounts up. Van der Waals corrections to the plain density functional theory (DFT) total energy raise the adsorption energy, but leave the cluster structure unchanged, in the gas phase or upon adsorption. Bader charge analysis shows that adsorption causes minor charge redistribution: the TM atoms bound to C atoms become positively charged, while the remaining metal atoms acquire negative charge. We have derived an approximate analytical expression for the local densities of states for the d orbitals of Ni , Pd , and Pt adatoms

  5. A study of the young cluster NGC 330 in the Small Magellanic Cloud.

    NASA Astrophysics Data System (ADS)

    Chiosi, C.; Vallenari, A.; Bressan, A.; Deng, L.; Ortolani, S.

    1995-01-01

    In this study we analyze the Colour-Magnitude Diagram (CMD) and the Luminosity Function (LF) of the SMC cluster NGC 330 on the basis of the new BV CCD photometry obtained by Vallenari et al. (1994) and the effective temperatures and bolometric magnitudes of a few supergiant stars taken from Caloi et al. (1993) and Stothers & Chin (1992a,b). The main goal of this study is to infer the kind of mixing taking place in stellar interiors. To this aim we make use of three grids of stellar tracks calculated with different schemes of mixing, namely classical semiconvection (Bressan et al. 1993), full overshoot (Bressan et al. 1993; Fagotto et al. 1994), and diffusive overshoot (Deng 1994; Deng et al. 1994). All the models are fairly homogeneous as far as the remaining input physics is concerned (opacity, nuclear reactions, mass loss rates), and are calculated with the new radiative opacities of Iglesias et al. (1992). We adopt the colour excess E_B-V_=0.06 and the true distance modulus (m-M)_0_=18.85. The metallicity of the cluster is highly uncertain: the observational determinations in fact go from Z=0.0003 to Z=0.006. The analysis of the CMD and LF is made by means of isochrones and synthetic CMDs. These latter allow us to match the CMD and the LF and to get an estimate of the slope x of the initial mass function (IMF) at the same time. With the assumed colour excess and distance modulus, NGC 330 turns out to possess a metallicity slightly lower than Z=0.008 and an age a few 10^7^yr. However, we find that a significant spread in the age is present. The CMD is compatible with ages in the range 1 to 2.5x10^7^yr for semiconvective models, and 1 to 4.8x10^7^yr for full and diffusive overshoot models. Contrary to what claimed by Caloi et al. (1993) and Stothers & Chin (1992a,b), no clear indication arises about the mixing scheme. Indeed, neither the analysis of the integrated LF nor the location of the blue supergiant stars in the theoretical HRD are able to cast light on

  6. Three-dimensional evaluation of gettering ability for oxygen atoms at small-angle tilt boundaries in Czochralski-grown silicon crystals

    SciTech Connect

    Ohno, Yutaka Inoue, Kaihei; Fujiwara, Kozo; Deura, Momoko; Kutsukake, Kentaro; Yonenaga, Ichiro; Shimizu, Yasuo; Inoue, Koji; Ebisawa, Naoki; Nagai, Yasuyoshi

    2015-06-22

    Three-dimensional distribution of oxygen atoms at small-angle tilt boundaries (SATBs) in Czochralski-grown p-type silicon ingots was investigated by atom probe tomography combined with transmission electron microscopy. Oxygen gettering along edge dislocations composing SATBs, post crystal growth, was observed. The gettering ability of SATBs would depend both on the dislocation strain and on the dislocation density. Oxygen atoms would agglomerate in the atomic sites under the tensile hydrostatic stress larger than about 2.0 GPa induced by the dislocations. It was suggested that the density of the atomic sites, depending on the tilt angle of SATBs, determined the gettering ability of SATBs.

  7. Lack of small-scale clustering in 21-cm intensity maps crossed with 2dF galaxy densities at z ~ 0.08

    NASA Astrophysics Data System (ADS)

    Anderson, Christopher; Luciw, Nicholas; Li, Yi-Chao; Kuo, Cheng-Yu; Yadav, Jaswant; Masui, Kiyoshi; Chang, Tzu-Ching; Chen, Xuelei; Oppermann, Niels; Pen, Ue-Li; Timbie, Peter T.

    2017-06-01

    I report results from 21-cm intensity maps acquired from the Parkes radio telescope and cross-correlated with galaxy maps from the 2dF galaxy survey. The data span the redshift range 0.057small scales, around k~1.0 hMpc^-1, compared to the 2dF auto-power spectrum and the theoretical dark matter power spectrum. This indicates either a lack of clustering of neutral hydrogen (HI) at small scales or a small correlation coefficient between optical galaxies and HI, or some combination of the two. A lack of small scale HI clustering would be qualitatively similar to measurements of HI-selected galaxy clustering from the HIPASS and ALFALFA surveys at slightly lower redshifts.

  8. A path-integral Monte Carlo study of a small cluster: The Ar trimer

    NASA Astrophysics Data System (ADS)

    Pérez de Tudela, R.; Márquez-Mijares, M.; González-Lezana, T.; Roncero, O.; Miret-Artés, S.; Delgado-Barrio, G.; Villarreal, P.

    2010-06-01

    The Ar3 system has been studied between T =0 K and T =40 K by means of a path-integral Monte Carlo (PIMC) method. The behavior of the average energy in terms of the temperature has been explained by comparison with results obtained with the thermal averaged rovibrational spectra estimated via: (i) a quantum mechanical method based on distributed Gaussian functions for the interparticle distances and (ii) an analytical model which precisely accounts for the participation of the dissociative continua Ar2+Ar and Ar+Ar+Ar. Beyond T ˜20 K, the system explores floppier configurations than the rigid equilateral geometry, as linear and Ar-Ar2-like arrangements, and fragmentates around T ˜40 K. A careful investigation of the specific heat in terms of a confining radius in the PIMC calculation seems to discard a proper phase transition as in larger clusters, in apparent contradiction with previous reports of precise values for a liquid-gas transition. The onset of this noticeable change in the dynamics of the trimer occurs, however, at a remarkably low value of the temperature in comparison with Arn systems formed with more Ar atoms. Quantum mechanical effects are found of relevance at T ≤15 K, with both energies and radial distributions obtained with a quantum PIMC deviating from the corresponding classical results, thus precluding exclusively classical approaches for a precise description of the system at this low temperature range.

  9. Spontaneous decay of small copper-cluster anions Cun- (n =3 -6 ), on long time scales

    NASA Astrophysics Data System (ADS)

    Hansen, K.; Stockett, M. H.; Kaminska, M.; Nascimento, R. F.; Anderson, E. K.; Gatchell, M.; Chartkunchand, K. C.; Eklund, G.; Zettergren, H.; Schmidt, H. T.; Cederquist, H.

    2017-02-01

    We have measured the spontaneous neutral particle emission from copper-cluster anions (Cun- , n =3 -6 ) stored at cryogenic temperatures in one of the electrostatic ion storage rings of the Double ElectroStatic Ion Ring ExpEriment facility at Stockholm University. The measured rate of emission from the stored Cu3- ions follows a single power-law decay for about 1 ms but then decreases much more rapidly with time. The latter behavior may be due to a decrease in the density of available final states in Cu3 as the excitation energies of the decaying ions approach the electron detachment threshold. The emissions from Cu4- , Cu5- , and Cu6- are well described by sums of two power laws that are quenched by radiative cooling of the stored ions with characteristic times between a few and hundreds of milliseconds. We relate these two-component behaviors to populations of stored ions with higher and lower angular momenta. In a separate experiment, we studied the laser-induced decay of Cu6- ions that were excited by 1.13- or 1.45-eV photons after 46 ms of storage.

  10. Characterization of silicon-carbon clusters by infrared laser spectroscopy. The nu 1 band of SiC4

    NASA Technical Reports Server (NTRS)

    Van Orden, A.; Provencal, R. A.; Giesen, T. F.; Saykally, R. J.

    1995-01-01

    The nu 1 fundamental vibration of linear SiC4 has been observed by infrared diode laser spectroscopy of a supersonic cluster beam. Twenty-four rovibrational transitions were measured in the spectral region of 2094.6 to 2097.1 cm-1, the rotational temperature was 10 K. A combined least-squares fit of these transitions with previously reported microwave data yielded the following molecular constants: nu 1 = 2095.45806(37) cm-1, B" = 0.051161131(52) cm-1, and B' = 0.0509157(96) cm-1. These results are compared to vibrational spectroscopy measurements of SiC4 trapped in a solid Ar matrix and to ab initio calculations.

  11. Efficient calculation of critical eigenvalue clusters in the small signal stability analysis of large power systems

    SciTech Connect

    Angelidis, G.; Semlyen, A.

    1995-02-01

    The paper presents a methodology for the calculation of a selected set of eigenvalues, considered critical in the small signal stability analysis of power systems. It analyzes several alternatives for refining a preliminary rough solution obtained by subspace interactions. These alternatives range from constant-matrix iterative refinement to Newton`s method. Due to an adaptive solution strategy, the overall algorithm is very robust. Newton`s method is much faster than existing approaches. The performance of these methods is demonstrated on several test systems.

  12. Molecular theory for the phase equilibria and cluster distribution of associating fluids with small bond angles.

    PubMed

    Marshall, Bennett D; Chapman, Walter G

    2013-08-07

    We develop a new theory for associating fluids with multiple association sites. The theory accounts for small bond angle effects such as steric hindrance, ring formation, and double bonding. The theory is validated against Monte Carlo simulations for the case of a fluid of patchy colloid particles with three patches and is found to be very accurate. Once validated, the theory is applied to study the phase diagram of a fluid composed of three patch colloids. It is found that bond angle has a significant effect on the phase diagram and the very existence of a liquid-vapor transition.

  13. Ab initio study of the structures and electronic states of small neutral and ionic DABCO--Ar(n) clusters.

    PubMed

    Mathivon, Kevin; Linguerri, Roberto; Hochlaf, Majdi

    2014-03-01

    In the present theoretical work, we investigated the stationary points (minima and transition states) on the ground state potential energy surfaces of neutral and ionic 1,4-diazabicyclo[2.2.2]octane (DABCO)--Ar(n)⁰,⁺¹ (n = 1-4) clusters. As established in our systematic work on DABCO--Ar cluster (Mathivon et al., J Chem Phys 139:164306, 2013), the (R)MP2/aug-cc-pVDZ level is accurate enough for validating the prediction of stable forms. For n = 1 and 2, further computations at the MP2/aug-cc-pVTZ level confirm these assumptions. We show that some of the already known isomers of these heteroclusters derived using lower levels of theory are not realistic. More interestingly, our work reveals that DABCO is subject to slight deformations when binding to a small number of Ar atoms. Moreover, we computed the potential energy surfaces of the lowest singlet electronic states of DABCO--Ar(n)(n = 1-3) and of DABCO⁺--Ar(n)(n = 1-3), and the transition moments for the Sp(p = 1-3) ← S0 neutral transitions. These electronic states are found to be Rydberg in nature. The shape of their potentials is mainly repulsive with slight stabilization in the S2 potentials. Finally, the effects of microsolvation of DABCO in Ar clusters in ground and electronic excited states are discussed. The photophysical and photochemical dynamics of these electronic states may be complex.

  14. Solid State Digital Propulsion "Cluster Thrusters" For Small Satellites Using High Performance Electrically Controlled Extinguishable Solid Propellants (ECESP)

    NASA Technical Reports Server (NTRS)

    Sawka, Wayne N.; Katzakian, Arthur; Grix, Charles

    2005-01-01

    Electrically controlled extinguishable solid propellants (ESCSP) are capable of multiple ignitions, extinguishments and throttle control by the application of electrical power. Both core and end burning no moving parts ECESP grains/motors to three inches in diameter have now been tested. Ongoing research has led to a newer family of even higher performance ECESP providing up to 10% higher performance, manufacturing ease, and significantly higher electrical conduction. The high conductivity was not found to be desirable for larger motors; however it is ideal for downward scaling to micro and pico- propulsion applications with a web thickness of less than 0.125 inch/ diameter. As a solid solution propellant, this ECESP is molecularly uniform, having no granular structure. Because of this homogeneity and workable viscosity it can be directly cast into thin layers or vacuum cast into complex geometries. Both coaxial and grain stacks have been demonstrated. Combining individual propellant coaxial grains and/or grain stacks together form three-dimensional arrays yield modular cluster thrusters. Adoption of fabless manufacturing methods and standards from the electronics industry will provide custom, highly reproducible micro-propulsion arrays and clusters at low costs. These stack and cluster thruster designs provide a small footprint saving spacecraft surface area for solar panels and/or experiments. The simplicity of these thrusters will enable their broad use on micro-pico satellites for primary propulsion, ACS and formation flying applications. Larger spacecraft may find uses for ECESP thrusters on extended booms, on-orbit refueling, pneumatic actuators, and gas generators.

  15. Accurate structures and binding energies for small water clusters: The water trimer

    SciTech Connect

    Nielsen, I.M.; Seidl, E.T.; Janssen, C.L.

    1999-05-01

    The global minimum on the water trimer potential energy surface has been investigated by means of second-order Mo/ller-Plesset (MP2) perturbation theory employing the series of correlation-consistent basis sets aug-cc-pVXZ (X = D, T, Q, 5, 6), the largest of which contains 1329 basis functions. Definitive predictions are made for the binding energy and equilibrium structure, and improved values are presented for the harmonic vibrational frequencies. A value of 15.82{plus_minus}0.05 kcal mol{sup {minus}1} is advanced for the infinite basis set frozen core MP2 binding energy, obtained by extrapolation of MP2 correlation energies computed at the aug-cc-pVQZ MP2 geometry. Inclusion of core correlation, using the aug-cc-pCV5Z basis set, has been found to increase the binding energy by 0.08 kcal mol{sup {minus}1}, and after consideration of core correlation and higher-order correlation effects, the classical binding energy for the water trimer is estimated to be 15.9{plus_minus}0.2 kcal mol{sup {minus}1}. A zero-point vibrational correction of {minus}5.43 kcal mol{sup {minus}1} has been computed from aug-cc-pVTZ MP2 harmonic vibrational frequencies. The accuracy of different computational schemes for obtaining the binding energies of the water dimer and trimer has been investigated, and computationally feasible methods are suggested for obtaining accurate structures and binding energies for larger water clusters.{copyright} {ital 1999 American Institute of Physics.}

  16. CLUSTERS OF SMALL ERUPTIVE FLARES PRODUCED BY MAGNETIC RECONNECTION IN THE SUN

    SciTech Connect

    Archontis, V.; Hansteen, V.

    2014-06-10

    We report on the formation of small solar flares produced by patchy magnetic reconnection between interacting magnetic loops. A three-dimensional (3D) magnetohydrodynamic (MHD) numerical experiment was performed, where a uniform magnetic flux sheet was injected into a fully developed convective layer. The gradual emergence of the field into the solar atmosphere results in a network of magnetic loops, which interact dynamically forming current layers at their interfaces. The formation and ejection of plasmoids out of the current layers leads to patchy reconnection and the spontaneous formation of several small (size ≈1-2 Mm) flares. We find that these flares are short-lived (30 s–3 minutes) bursts of energy in the range O(10{sup 25}-10{sup 27}) erg, which is basically the nanoflare-microflare range. Their persistent formation and co-operative action and evolution leads to recurrent emission of fast EUV/X-ray jets and considerable plasma heating in the active corona.

  17. Diffusion of small Ni and Cu clusters on Ni (111): Application of SLKMC-II*

    NASA Astrophysics Data System (ADS)

    Shah, Syed; Nandipati, Giridhar; Rahman, Talat

    2013-03-01

    We have examined the diffusion of small Ni and Cu islands (consisting of up to 10 atoms) on the Ni(111) surface using a self-learning kinetic Monte Carlo (SLKMC-II) method with an improved pattern-recognition scheme that allows inclusion of both fcc and hcp sites in the simulations. In an SLKMC simulation a database holds information about the local neighborhood of an atom and associated processes that is accumulated on-the-fly, as the simulation proceeds. The activation energy barriers for the identified diffusion processes were calculated using semi-empirical interaction potential based on the embedded-atom method. Although a variety of concerted, multi-atom and single-atom processes were automatically revealed in our simulations, we found that these small islands diffuse primarily via concerted motion. We report diffusion coefficients for each island size at several temperatures, and from the Arrhenius plot extract the size-dependent effective diffusion barrier for these islands. Our evaluation of the occurrence frequency of processes most responsible for the diffusion of island of a specific size reveal several that are not accessible in SLKMC-I or in short time-scale MD simulations. We also provide results of extending SLKMC-II to examine epitaxial growth in these systems. DOE Grant DE-FG02-07ER46354

  18. Reassessing the Stroke Belt: Using Small Area Spatial Statistics to Identify Clusters of High Stroke Mortality in the United States.

    PubMed

    Karp, David N; Wolff, Catherine S; Wiebe, Douglas J; Branas, Charles C; Carr, Brendan G; Mullen, Michael T

    2016-07-01

    The stroke belt is described as an 8-state region with high stroke mortality across the southeastern United States. Using spatial statistics, we identified clusters of high stroke mortality (hot spots) and adjacent areas of low stroke mortality (cool spots) for US counties and evaluated for regional differences in county-level risk factors. A cross-sectional study of stroke mortality was conducted using Multiple Cause of Death data (Centers for Disease Control and Prevention) to compute age-adjusted adult stroke mortality rates for US counties. Local indicators of spatial association statistics were used for hot-spot mapping. County-level variables were compared between hot and cool spots. Between 2008 and 2010, there were 393 121 stroke-related deaths. Median age-adjusted adult stroke mortality was 61.7 per 100 000 persons (interquartile range=51.4-74.7). We identified 705 hot-spot counties (22.4%) and 234 cool-spot counties (7.5%); 44.5% of hot-spot counties were located outside of the stroke belt. Hot spots had greater proportions of black residents, higher rates of unemployment, chronic disease, and healthcare utilization, and lower median income and educational attainment. Clusters of high stroke mortality exist beyond the 8-state stroke belt, and variation exists within the stroke belt. Reconsideration of the stroke belt definition and increased attention to local determinants of health underlying small area regional variability could inform targeted healthcare interventions. © 2016 American Heart Association, Inc.

  19. The fragmentation dynamics of small Cs(CsI)n+ cluster ions under low-energy multiple collision conditions

    NASA Astrophysics Data System (ADS)

    Herzschuh, Rainer; Drewello, Thomas

    2004-04-01

    The collision-induced dissociations of small caesium iodide cluster ions of the type Cs(CsI)n+ where n=3-7, have been investigated under low-energy multiple collision conditions. The collisions were performed in the rf-only quadrupole of a BEqQ hybrid mass spectrometer. Breakdown graphs of selected parent ions were obtained by varying the laboratory collision energy in the range of 0-400 eV. The fragmentation dynamic established under these conditions provides a link between the well-known decay behaviour occurring unimolecularly and the dissociations following high energy (keV) collisional activation. Of particular interest is the observation that the energy-dependent dissociation pattern supplies support for the occurrence of one-step fission reactions, featuring the evaporation of presumably intact (CsI)n neutrals as opposed to a sequential decay via nCsI losses. The breakdown graphs thus provide a valuable tool to enhance insight into the fragmentation mechanism of these clusters.

  20. Thermodynamics of small alkali metal halide cluster ions: comparison of classical molecular simulations with experiment and quantum chemistry.

    PubMed

    Vlcek, Lukas; Uhlik, Filip; Moucka, Filip; Nezbeda, Ivo; Chialvo, Ariel A

    2015-01-22

    We evaluate the ability of selected classical molecular models to describe the thermodynamic and structural aspects of gas-phase hydration of alkali metal halide ions and the formation of small water clusters. To understand the effect of many-body interactions (polarization) and charge penetration effects on the accuracy of a force field, we perform Monte Carlo simulations with three rigid water models using different functional forms to account for these effects: (i) point charge nonpolarizable SPC/E, (ii) Drude point charge polarizable SWM4-DP, and (iii) Drude Gaussian charge polarizable BK3. Model predictions are compared with experimental Gibbs free energies and enthalpies of ion hydration, and with microscopic structural properties obtained from quantum DFT calculations. We find that all three models provide comparable predictions for pure water clusters and cation hydration but differ significantly in their description of anion hydration. None of the investigated classical force fields can consistently and quantitatively reproduce the experimental gas-phase hydration thermodynamics. The outcome of this study highlights the relation between the functional form that describes the effective intermolecular interactions and the accuracy of the resulting ion hydration properties.

  1. Quantum chemical study of the interaction of elemental Hg with small neutral, anionic and cationic Au{sub n} (n = 1–6) clusters

    SciTech Connect

    Siddiqui, Shamoon Ahmad; Bouarissa, Nadir; Rasheed, Tabish; Al-Assiri, M.S.

    2013-03-15

    Graphical abstract: Binding energies as a function of cluster size for Au{sub n}Hg, Au{sub n}Hg{sup +} and Au{sub n}Hg{sup −} complexes. Highlights: ► Hg adsorption of neutral and charged Au{sub n} (n = 1–6) clusters has been discussed. ► Size and charged state of cluster significantly affect the Hg adsorption. ► Transfer of electron mainly found from s orbital of Hg to s orbital of Au. - Abstract: Adsorption of elemental mercury (Hg) on small neutral, cationic and anionic gold clusters (Au{sub n}, n = 1–6) has been studied by using the density functional theory (DFT). Results of this investigation show that frontier molecular orbital theory is a useful tool to predict the selectivity of Hg adsorption. It is found that adsorption of Hg on neutral, cationic and anionic Au{sub n} (n = 1–6) clusters are thermodynamically favorable. The binding energies of Hg on the cationic Au{sub n} clusters are greater than those on the neutral and anionic clusters. Natural bond orbital (NBO) analysis indicates that the flow of electrons in the neutral and charged clusters is mainly due to the s orbitals of Hg and Au. Results of NBO analysis also indicate that the binding energy of Hg with Au{sub n} clusters is directly proportional to the charge transfer, i.e. greater is the charge transfer, higher is the binding energy.

  2. Density-functional calculations of the structure and electronic and magnetic properties of small yttrium clusters Yn (n=2-17)

    NASA Astrophysics Data System (ADS)

    Yuan, H. K.; Chen, H.; Kuang, A. L.; Ahmed, A. S.; Xiong, Z. H.

    2007-05-01

    The all-electron spin-polarized generalized gradient approximation to the density-functional theory is used to determine the binding energies, ground-state structures, electronic structures, and magnetic properties of the Yn clusters (n⩽17) . The structural evolution of yttrium clusters, which favors a compact and icosahedral structural growth pattern, is elucidated and compared with the other group-III elemental clusters. The results show that clusters with n=7,13 are more stable than their respective neighbors. Furthermore, the maxima of magnetism at n=8 and n=13 observed experimentally are well described and the magnetic moments for most yttrium clusters are quite small except for Y6 , Y8 , and Y12-Y14 . Particularly, the regular icosahedron structure with a giant moment of 19μB is favored for the Y13 cluster. The similar magnetic features of the scandium and yttrium clusters shown in experiments can be attributed to a common structural motif for these two series of clusters. A change of magnetic ordering from ferromagnetic to antiferromagnetic is observed at n=7 , the exception being the systems Yn with n=8,13,14 which are found to be ferromagnetic. In addition, the calculated ionization potentials are in good agreement with the experimental results, which imply that the predictions of the ground-state geometries of those clusters are accurate.

  3. Calculation of the structural parameters of small cadmium selenide clusters, (CdSe)n, n = 1,2,3

    NASA Astrophysics Data System (ADS)

    Alselawe, A. I. A.; Gopir, G.; Anas, M. M.

    2016-11-01

    The structural parameters of small cadmium selenide clusters (CdSe)n, for n=1,2, and 3 were studied. The calculations were performed using the pseudopotential and planewave basis sets, within the density functional theory. These parameters for geometry optimization can be used as elementary information for further investigations. The total energy calculations for the three linear structures of (CdSe)2 showed that these isomers are close in their stability. The rhombus structure (CdSe)2 planar is more stable isomer. The bond distances for the different structures show that the distances decrease with increasing the atoms in the structure. The calculated bond angles were close to other relevant studies.

  4. Effect of translational and angular momentum conservation on energy equipartition in microcanonical equilibrium in small clusters

    NASA Astrophysics Data System (ADS)

    Niiyama, Tomoaki; Shimizu, Yasushi; Kobayashi, Taizo R.; Okushima, Teruaki; Ikeda, Kensuke S.

    2009-05-01

    We investigate numerically and analytically the effects of conservation of total translational and angular momentum on the distribution of kinetic energy among particles in microcanonical particle systems with small number of degrees of freedom, specifically microclusters. Molecular dynamics simulations of microclusters with constant total energy and momenta, using Lennard-Jones, Morse, and Coulomb plus Born-Mayer-type potentials, show that the distribution of kinetic energy among particles can be inhomogeneous and depend on particle mass and position even in thermal equilibrium. Statistical analysis using a microcanonical measure taking into account of the additional conserved quantities gives theoretical expressions for kinetic energy as a function of the mass and position of a particle with only O(1/N2) deviation from the Maxwell-Boltzmann distribution. These expressions fit numerical results well. Finally, we propose an intuitive interpretation for the inhomogeneity of the kinetic energy distributions.

  5. Silicon monohydride clusters Si(n)H (n = 4-10) and their anions: structures, thermochemistry, and electron affinities.

    PubMed

    Yang, JuCai; Bai, Xue; Li, ChunPing; Xu, WenGuo

    2005-06-30

    The molecular structures, electron affinities, and dissociation energies of the Si(n)H/Si(n)H- (n = 4-10) species have been examined via five hybrid and pure density functional theory (DFT) methods. The basis set used in this work is of double-zeta plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. The three different types of neutral-anion energy separations presented in this work are the adiabatic electron affinity (EA(ad)), the vertical electron affinity (EA(vert)), and the vertical detachment energy (VDE). The first Si-H dissociation energies, D(e)(Si(n)H --> Si(n) + H) for neutral Si(n)H and D(e)(Si(n)H- --> Si(n)- + H) for anionic Si(n)H- species, have also been reported. The structures of the ground states of these clusters are traditional H-Si single-bond forms. The ground-state geometries of Si5H, Si6H, Si8H, and Si9H predicted by the DFT methods are different from previous calculations, such as those obtained by Car-Parrinello molecular dynamics and nonorthogonal tight-binding molecular dynamics schemes. The most reliable EA(ad) values obtained at the B3LYP level of theory are 2.59 (Si4H), 2.84 (Si5H), 2.86 (Si6H), 3.19 (Si7H), 3.14 (Si8H), 3.36 (Si9H), and 3.56 (Si10H) eV. The first dissociation energies (Si(n)H --> Si(n) + H) predicted by all of these methods are 2.20-2.29 (Si4H), 2.30-2.83 (Si5H), 2.12-2.41 (Si6H), 1.75-2.03 (Si7H), 2.41-2.72 (Si8H), 1.86-2.11 (Si9H), and 1.92-2.27 (Si10H) eV. For the negatively charged ion clusters (Si(n)H- --> Si(n)- + H), the dissociation energies predicted are 2.56-2.69 (Si4H-), 2.80-3.01 (Si5H-), 2.86-3.06 (Si6H-), 2.80-3.03 (Si7H-), 2.69-2.92 (Si8H-), 2.92-3.18 (Si9H-), and 2.89-3.25 (Si10H-) eV.

  6. Reversible capture of small molecules on bimetallaborane clusters: synthesis, structural characterization, and photophysical aspects.

    PubMed

    Bould, Jonathan; Baše, Tomáš; Londesborough, Michael G S; Oro, Luis A; Macías, Ramón; Kennedy, John D; Kubát, Pavel; Fuciman, Marcel; Polívka, Tomáš; Lang, Kamil

    2011-08-15

    Metallaborane compounds containing two adjacent metal atoms, [(PMe(2)Ph)(4)MM'B(10)H(10)] (where MM' = Pt(2), 1; PtPd, 7; Pd(2), 8), have been synthesized, and their propensity to sequester O(2), CO, and SO(2) and to then release them under pulsed and continuous irradiation are described. Only [(PMe(2)Ph)(4)Pt(2)B(10)H(10)], 1, undergoes reversible binding of O(2) to form [(PMe(2)Ph)(4)(O(2))Pt(2)B(10)H(10)] 3, but solutions of 1, 7, and 8 all quantitatively take up CO across their metal-metal vectors to form [(PMe(2)Ph)(4)(CO)Pt(2)B(10)H(10)] 4, [(PMe(2)Ph)(4)(CO)PtPdB(10)H(10)] 10, and [(PMe(2)Ph)(4)(CO)Pd(2)B(10)H(10)] 11, respectively. Crystallographically determined interatomic M-M distances and infrared CO stretching frequencies show that the CO molecule is bound progressively more weakly in the sequence {PtPt} > {PtPd} > {PdPd}. Similarly, SO(2) forms [(PMe(2)Ph)(4)(SO(2))Pt(2)B(10)H(10)] 5, [(PMe(2)Ph)(4)(SO(2))PtPdB(10)H(10)] 12, and [(PMe(2)Ph)(4)(SO(2))Pd(2)B(10)H(10)] 13 with progressively weaker binding of the SO(2) molecule. The uptake and release of gas molecules are accompanied by changes in their absorption spectra. Nanosecond transient absorption spectroscopy clearly shows that the O(2) and CO molecules are liberated from the bimetallic binding site with high quantum yields of about 0.6. For 3, in addition to dioxygen release in the triplet ground state, singlet oxygen O(2)((1)Δ(g)) was also detected with a quantum yield <0.01. In most cases, the release and rebinding of the gas molecules can be cycled with little photodegradation of the compounds. Femtosecond transient absorption spectroscopy further reveals that the photorelease of the O(2) and CO molecules, from 3 and 4 respectively, is an ultrafast process taking place on a time scale of tens of picoseconds. For SO(2), the release is even faster (<1 ps), but only in the case of mixed metal PtPd adducts, most probably because of the metal-metal bonding asymmetry in the mixed metal clusters

  7. Thermochemistry and electronic structure of small boron clusters (B(n), n = 5-13) and their anions.

    PubMed

    Truong, Ba Tai; Grant, Daniel J; Nguyen, Minh Tho; Dixon, David A

    2010-01-21

    Thermochemical parameters of a set of small-sized neutral (B(n)) and anionic (B(n)(-)) boron clusters, with n = 5-13, were determined using coupled-cluster theory CCSD(T) calculations with the aug-cc-pVnZ (n = D, T, and Q) basis sets extrapolated to the complete basis set limit (CBS) plus addition corrections and/or G3B3 calculations. Enthalpies of formation, adiabatic electron affinities (EA), vertical (VDE), and adiabatic (ADE) detachment energies were evaluated. Our calculated EAs are in good agreement with recent experiments (values in eV): B(5) (CBS, 2.29; G3B3, 2.48; exptl., 2.33 +/- 0.02), B(6) (CBS, 2.59; G3B3, 3.23; exptl., 3.01 +/- 0.04), B(7) (CBS, 2.62; G3B3, 2.67; exptl., 2.55 +/- 0.05), B(8) (CBS, 3.02; G3B3, 3.11; exptl., 3.02 +/- 0.02), B(9) (G3B3, 3.03; exptl., 3.39 +/- 0.06), B(10) (G3B3, 2.85; exptl., 2.88 +/- 0.09), B(11) (G3B4, 3.48;, exptl., 3.43 +/- 0.01), B(12) (G3B3, 2.33; exptl., 2.21 +/- 0.04), and B(13) (G3B3, 3.62; exptl., 3.78 +/- 0.02). The difference between the calculated adiabatic electron affinity and the adiabatic detachment energy for B(6) is due to the fact that the geometry of the anion is not that of the ground-state neutral. The calculated adiabatic detachment energies to the (3)A(u), C(2h) and (1)A(g), D(2h) excited states of B(6), which have geometries similar to the (1)A(g), D(2h) state of B(6)(-), are 2.93 and 3.06 eV, in excellent agreement with experiment. The VDEs were also well reproduced by the calculations. Partitioning of the electron localization functions into pi and sigma components allows probing of the partial and local delocalization in global nonaromatic systems. The larger clusters appear to exhibit multiple aromaticity. The binding energies per atom vary in a parallel manner for both neutral and anionic series and approach the experimental value for the heat of atomization of B. The resonance energies and the normalized resonance energies are convenient indices to quantify the stabilization of a cluster

  8. Improved multicrystalline silicon ingot quality using single layer silicon beads coated with silicon nitride as seed layer

    NASA Astrophysics Data System (ADS)

    babu, G. Anandha; Takahashi, Isao; Matsushima, Satoru; Usami, Noritaka

    2016-05-01

    We propose to utilize single layer silicon beads (SLSB) coated with silicon nitride as cost-effective seed layer to grow high-quality multicrystalline silicon (mc-Si) ingot. The texture structure of silicon nitride provides a large number of nucleation sites for the fine grain formation at the bottom of the crucible. No special care is needed to prevent seed melting, which would lead to decrease of red zone owing to decrease of feedstock melting time. As we expected, mc-Si ingot seeded with SLSB was found to consist of small, different grain orientations, more uniform grain distribution, high percentage of random grain boundaries, less twin boundaries, and low density of dislocation clusters compared with conventional mc-Si ingot grown under identical growth conditions. These results show that the SLSB seeded mc-Si ingot has enhanced ingot quality. The correlation between grain boundary structure and defect structure as well as the reason responsible for dislocation clusters reduction in SLSB seeded mc-Si wafer are also discussed.

  9. Small-volume potentiometric titrations: EPR investigations of Fe-S cluster N2 in mitochondrial complex I.

    PubMed

    Wright, John J; Salvadori, Enrico; Bridges, Hannah R; Hirst, Judy; Roessler, Maxie M

    2016-09-01

    EPR-based potentiometric titrations are a well-established method for determining the reduction potentials of cofactors in large and complex proteins with at least one EPR-active state. However, such titrations require large amounts of protein. Here, we report a new method that requires an order of magnitude less protein than previously described methods, and that provides EPR samples suitable for measurements at both X- and Q-band microwave frequencies. We demonstrate our method by determining the reduction potential of the terminal [4Fe-4S] cluster (N2) in the intramolecular electron-transfer relay in mammalian respiratory complex I. The value determined by our method, Em7=-158mV, is precise, reproducible, and consistent with previously reported values. Our small-volume potentiometric titration method will facilitate detailed investigations of EPR-active centres in non-abundant and refractory proteins that can only be prepared in small quantities. Copyright © 2016 The Authors. Published by Elsevier Inc. All rights reserved.

  10. Ca II Triplet Spectroscopy of Small Magellanic Cloud Red Giants. IV. Abundances for a Large Sample of Field Stars and Comparison with the Cluster Sample

    NASA Astrophysics Data System (ADS)

    Parisi, M. C.; Geisler, D.; Carraro, G.; Clariá, J. J.; Villanova, S.; Gramajo, L. V.; Sarajedini, A.; Grocholski, A. J.

    2016-09-01

    This paper represents a major step forward in the systematic and homogeneous study of Small Magellanic Cloud (SMC) star clusters and field stars carried out by applying the calcium triplet technique. We present in this work the radial velocity and metallicity of approximately 400 red giant stars in 15 SMC fields, with typical errors of about 7 km s-1 and 0.16 dex, respectively. We added to this information our previously determined metallicity values for 29 clusters and approximately 350 field stars using the identical techniques. Using this enlarged sample, we analyze the metallicity distribution and gradient in this galaxy. We also compare the chemical properties of the clusters and of their surrounding fields. We find a number of surprising results. While the clusters, taken as a whole, show no strong evidence for a metallicity gradient (MG), the field stars exhibit a clear negative gradient in the inner region of the SMC, consistent with the recent results of Dobbie et al. For distances to the center of the galaxy less than 4°, field stars show a considerably smaller metallicity dispersion than that of the clusters. However, in the external SMC regions, clusters and field stars exhibit similar metallicity dispersions. Moreover, in the inner region of the SMC, clusters appear to be concentrated in two groups: one more metal-poor and another more metal-rich than field stars. Individually considered, neither cluster group presents an MG. Most surprisingly, the MG for both stellar populations (clusters and field stars) appears to reverse sign in the outer regions of the SMC. The difference between the cluster metallicity and the mean metallicity of the surrounding field stars turns out to be a strong function of the cluster metallicity. These results could be indicating different chemical evolution histories for these two SMC stellar populations. They could also indicate variations in the chemical behavior of the SMC in its internal and external regions.

  11. First-principles investigation of the dissociation and coupling of methane on small copper clusters: Interplay of collision dynamics and geometric and electronic effects

    SciTech Connect

    Varghese, Jithin J.; Mushrif, Samir H.

    2015-05-14

    Small metal clusters exhibit unique size and morphology dependent catalytic activity. The search for alternate minimum energy pathways and catalysts to transform methane to more useful chemicals and carbon nanomaterials led us to investigate collision induced dissociation of methane on small Cu clusters. We report here for the first time, the free energy barriers for the collision induced activation, dissociation, and coupling of methane on small Cu clusters (Cu{sub n} where n = 2–12) using ab initio molecular dynamics and metadynamics simulations. The collision induced activation of the stretching and bending vibrations of methane significantly reduces the free energy barrier for its dissociation. Increase in the cluster size reduces the barrier for dissociation of methane due to the corresponding increase in delocalisation of electron density within the cluster, as demonstrated using the electron localisation function topology analysis. This enables higher probability of favourable alignment of the C–H stretching vibration of methane towards regions of high electron density within the cluster and makes higher number of sites available for the chemisorption of CH{sub 3} and H upon dissociation. These characteristics contribute in lowering the barrier for dissociation of methane. Distortion and reorganisation of cluster geometry due to high temperature collision dynamics disturb electron delocalisation within them and increase the barrier for dissociation. Coupling reactions of CH{sub x} (x = 1–3) species and recombination of H with CH{sub x} have free energy barriers significantly lower than complete dehydrogenation of methane to carbon. Thus, competition favours the former reactions at high hydrogen saturation on the clusters.

  12. When does a star cluster become a multiple star system? - I. Lifetimes of equal-mass small-N systems

    NASA Astrophysics Data System (ADS)

    Leigh, Nathan W. C.; Shara, Michael M.; Geller, Aaron M.

    2016-06-01

    What is the difference between a long-lived unstable (or quasi-stable) multiple star system and a bona fide star cluster? In this paper, we present a possible framework to address this question, by studying the distributions of disruption times for chaotic gravitational encounters as a function of the number of interacting particles. To this end, we perform a series of numerical scattering experiments with the FEWBODY code, to calculate the distributions of disruption times as a function of both the particle number N and the virial coefficient k. The subsequent distributions are fit with a physically motivated function, consisting of an initial exponential decay followed by a very slowly decreasing tail at long encounter times due to long-lived quasi-stable encounters. We find three primary features characteristic of the calculated distributions of disruption times. These are as follows: (1) the system half-life increases with increasing particle number, (2) the fraction of long-lived quasi-stable encounters increases with increasing particle number and (3) both the system half-life and the fraction of quasi-stable encounters increase with decreasing virial coefficient. We discuss the significance of our results for collisional dynamics, and consider the extrapolation of our results to larger-N systems. We suggest that this could potentially offer a clear and unambiguous distinction between star clusters and (stable or quasi-stable) multiple star systems. Although we are limited by very-small-number statistics, our results tentatively suggest that (for our assumptions) this transition occurs at a critical particle number of order 100.

  13. First-principle study of quantum confinement effect on small sized silicon quantum dots using density-functional theory

    SciTech Connect

    Anas, M. M.; Othman, A. P.; Gopir, G.

    2014-09-03

    Density functional theory (DFT), as a first-principle approach has successfully been implemented to study nanoscale material. Here, DFT by numerical basis-set was used to study the quantum confinement effect as well as electronic properties of silicon quantum dots (Si-QDs) in ground state condition. Selection of quantum dot models were studied intensively before choosing the right structure for simulation. Next, the computational result were used to examine and deduce the electronic properties and its density of state (DOS) for 14 spherical Si-QDs ranging in size up to ∼ 2 nm in diameter. The energy gap was also deduced from the HOMO-LUMO results. The atomistic model of each silicon QDs was constructed by repeating its crystal unit cell of face-centered cubic (FCC) structure, and reconstructed until the spherical shape obtained. The core structure shows tetrahedral (T{sub d}) symmetry structure. It was found that the model need to be passivated, and hence it was noticed that the confinement effect was more pronounced. The model was optimized using Quasi-Newton method for each size of Si-QDs to get relaxed structure before it was simulated. In this model the exchange-correlation potential (V{sub xc}) of the electrons was treated by Local Density Approximation (LDA) functional and Perdew-Zunger (PZ) functional.

  14. Pd diffusion on MgO(1 0 0): The role of defects and small cluster mobility

    NASA Astrophysics Data System (ADS)

    Xu, Lijun; Henkelman, Graeme; Campbell, Charles T.; Jónsson, Hannes

    2006-03-01

    Density functional theory is used to explore the energy landscape of Pd atoms adsorbed on the terrace of MgO(1 0 0) and at oxygen vacancy sites. Saddle point finding methods reveal that small Pd clusters diffuse on the terrace in interesting ways. The monomer and dimer diffuse via single atom hops between oxygen sites with barriers of 0.34 eV and 0.43 eV respectively. The trimer and tetramer, however, form 3D clusters by overcoming a 2D-3D transition barrier of less than 60 meV. The trimer diffuses along the surface either by a walking or flipping motion, with comparable barriers of ca. 0.5 eV. The tetramer rolls along the terrace with a lower barrier of 0.42 eV. Soft rotational modes at the saddle point lead to an anomalously high prefactor of 1.3 × 10 14 s -1 for tetramer diffusion. This prefactor is two order of magnitude higher than for monomer diffusion, making the tetramer the fastest diffusing species on the terrace at all temperatures for which diffusion is active (above 200 K). Neutral oxygen vacancy sites are found to bind Pd monomers with a 2.63 eV stronger binding energy than the terrace. A second Pd atom, however, binds to this trapped monomer with a smaller energy of 0.56 eV, so that dimers at defects dissociate on a time scale of milliseconds at room temperature. Larger clusters bind more strongly at defects. Trimers and tetramers dissociate from monomer-bound-defects at elevated temperatures of ca. 600 K. These species are also mobile on the terrace, suggesting they are important for the ripening observed at ⩾600 K during Pd vapor deposition on MgO(1 0 0) by Haas et al. [G. Haas, A. Menck, H. Brune, J.V. Barth, J.A. Venables, K. Kern, Phys. Rev. B 61 (2000) 11105].

  15. Coexistence of interacting ferromagnetic clusters and small antiferromagnetic clusters in La0.5Ba0.5CoO3

    NASA Astrophysics Data System (ADS)

    Kumar, Devendra; Banerjee, A.

    2013-05-01

    We report detailed dc magnetization and linear and nonlinear ac susceptibility measurements on the hole doped disordered cobaltite La0.5Ba0.5CoO3. Our results show that the magnetically ordered state of the system consists of coexisting non-ferromagnetic phases along with percolating ferromagnetic clusters. The percolating ferromagnetic clusters possibly start a magnetic ordering at the Curie temperature of 201.5(5) K. The non-ferromagnetic phases mainly consist of antiferromagnetic clusters with size smaller than the ferromagnetic clusters. Below the Curie temperature the system exhibits an irreversibility in the field cooled and zero field cooled magnetization and a frequency dependence in the peak of ac susceptibility. These dynamical features indicate the possible coexistence of spin-glass phase along with ferromagnetic clusters similar to La1-xSrxCoO3 (x ≥ 0.18), but the absence of field divergence in the third harmonic of ac susceptibility and zero field cooled memory clearly rule out any such possibility. We argue that the spin-glass phase in La1-xSrxCoO3 (x ≥ 0.18) is associated with the presence of incommensurate antiferromagnetic ordering in non-ferromagnetic phases, which is absent in La0.5Ba0.5CoO3. Our analysis shows that the observed dynamical features in La0.5Ba0.5CoO3 may be due to progressive thermal blocking of ferromagnetic clusters, which is further confirmed by Wohlfarth’s model of superparamagnetism. The frequency dependence of the peak of ac susceptibility obeys the Vogel-Fulcher law with τ0 ≈ 10-9 s. This together with the existence of an AT-line in H-T space indicates the presence of significant inter-cluster interaction among these ferromagnetic clusters.

  16. Formation and crystallization of silicon nanoclusters in SiN{sub x}:H films using femtosecond pulsed laser annealings

    SciTech Connect

    Korchagina, T. T. Volodin, V. A.; Chichkov, B. N.

    2010-12-15

    SiN{sub x}:H films of different compositions grown on glass and silicon substrates using plasma-chemical vapor deposition at a temperature of 380 deg. C have been subjected to pulsed laser annealings. The treatments are performed using titanium-sapphire laser radiation with a wavelength of 800 nm and a pulse duration of 30 fs. Structural changes in the films are studied using Raman spectroscopy. Amorphous silicon nanoclusters are detected in as-grown films with molar fractions of excess silicon of {approx}1/5 and larger. Conditions required for pulsed crystallization of nanoclusters were determined. According to the Raman data, no silicon clusters were detected in as-grown films with a small amount of excess silicon (x > 1.25). Pulsed treatments resulted in the formation of silicon nanoclusters 1-2 nm in size in these films.

  17. Infrared spectroscopy of small protonated water clusters, H(+)(H2O)n (n = 2-5): isomers, argon tagging, and deuteration.

    PubMed

    Douberly, G E; Walters, R S; Cui, J; Jordan, K D; Duncan, M A

    2010-04-08

    Infrared photodissociation spectroscopy is reported for mass-selected H(+)(H(2)O)(n) complexes and their deuterated analogues with and without argon "tagging." H(+)(H(2)O)(n)Ar(m) and D(+)(D(2)O)(n)Ar(m) complexes are studied in the O-H (O-D) stretching region for clusters in the small size range (n = 2-5). Upon infrared excitation, these clusters fragment by the loss of either argon atoms or one or more intact water molecules. Their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water and in the hydrogen bonding region. Experimental studies are complemented by computational work that explores the isomeric structures, their energetics and vibrational spectra. The addition of an argon atom is essential to obtain photodissociation for the n = 2-3 complexes, and specific inclusion of the argon in calculations is necessary to reproduce the measured spectra. For n = 3-5, spectra are obtained both with and without argon. The added argon atom allows selection of a subset of colder clusters and it increases the photodissociation yield. Although most of these clusters have more than one possible isomeric structure, the spectra measured correspond to a single isomer that is computed to be the most stable. Deuteration in these small cluster sizes leads to expected lowering of frequencies, but the spectra indicate the presence of the same single most-stable isomer for each cluster size.

  18. A study of area clustering using factor analysis in small area estimation (An analysis of per capita expenditures of subdistricts level in regency and municipality of Bogor)

    NASA Astrophysics Data System (ADS)

    Wahyudi, Notodiputro, Khairil Anwar; Kurnia, Anang; Anisa, Rahma

    2016-02-01

    Empirical Best Linear Unbiased Prediction (EBLUP) is one of indirect estimating methods which used to estimate parameters of small areas. EBLUP methods works in using auxiliary variables of area while adding the area random effects. In estimating non-sampled area, the standard EBLUP can no longer be used due to no information of area random effects. To obtain more proper estimation methods for non sampled area, the standard EBLUP model has to be modified by adding cluster information. The aim of this research was to study clustering methods using factor analysis by means of simulation, provide better cluster information. The criteria used to evaluate the goodness of fit of the methods in the simulation study were the mean percentage of clustering accuracy. The results of the simulation study showed the use of factor analysis in clustering has increased the average percentage of accuracy particularly when using Ward method. The method was taken into account to estimate the per capita expenditures based on Small Area Estimation (SAE) techniques. The method was eventually used to estimate the per capita expenditures from SUSENAS and the quality of the estimates was measured by RMSE. This research has shown that the standard-modified EBLUP model provided with factor analysis better estimates when compared with standard EBLUP model and the standard-modified EBLUP without the factor analysis. Moreover, it was also shown that the clustering information is important in estimating non sampled area.

  19. Investigating the significance of zero-point motion in small molecular clusters of sulphuric acid and water

    SciTech Connect

    Stinson, Jake L.; Kathmann, Shawn M.; Ford, Ian J.

    2014-01-14

    The nucleation of particles from trace gases in the atmosphere is an important source of cloud condensation nuclei (CCN), and these are vital for the formation of clouds in view of the high supersaturations required for homogeneous water droplet nucleation. The methods of quantum chemistry have increasingly been employed to model nucleation due to their high accuracy and efficiency in calculating configurational energies; and nucleation rates can be obtained from the associated free energies of particle formation. However, even in such advanced approaches, it is typically assumed that the nuclei have a classical nature, which is questionable for some systems. The importance of zero-point motion (also known as quantum nuclear dynamics) in modelling small clusters of sulphuric acid and water is tested here using the path integral molecular dynamics (PIMD) method at the density functional theory (DFT) level of theory. We observe a small zero-point effect on the the equilibrium structures of certain clusters. One configuration is found to display a bimodal behaviour at 300 K in contrast to the stable ionised state suggested from a zero temperature classical geometry optimisation. The general effect of zero-point motion is to promote the extent of proton transfer with respect to classical behaviour. We thank Prof. Angelos Michaelides and his group in University College London (UCL) for practical advice and helpful discussions. This work benefited from interactions with the Thomas Young Centre through seminar and discussions involving the PIMD method. SMK was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. JLS and IJF were supported by the IMPACT scheme at UCL and by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. We are grateful for use of the UCL Legion High Performance Computing Facility and the

  20. Application of a convergent, composite coupled cluster approach to bound state, adiabatic electron affinities in atoms and small molecules.

    PubMed

    Feller, David

    2016-01-07

    Benchmark quality adiabatic electron affinities for a collection of atoms and small molecules were obtained with the Feller-Peterson-Dixon composite coupled cluster theory method. Prior applications of this method demonstrated its ability to accurately predict atomization energies/heats of formation for more than 170 molecules. In the current work, the 1-particle expansion involved very large correlation consistent basis sets, ranging up to aug-cc-pV9Z (aug-cc-pV10Z for H and H2), with the goal of minimizing the residual basis set truncation error that must otherwise be approximated with extrapolation formulas. The n-particle expansion begins with coupled cluster calculations through iterative single and double excitations plus a quasiperturbative treatment of "connected" triple excitations (CCSD(T)) pushed to the complete basis set limit followed by CCSDT, CCSDTQ, or CCSDTQ5 corrections. Due to the small size of the systems examined here, it was possible in many cases to extend the n-particle expansion to the full configuration interaction wave function limit. Additional, smaller corrections associated with core/valence correlation, scalar relativity, anharmonic zero point vibrational energies, and non-adiabatic effects were also included. The overall root mean square (RMS) deviation was 0.005 eV (0.12 kcal/mol). This level of agreement was comparable to what was found with molecular heats of formation. A 95% confidence level corresponds to roughly twice the RMS value or 0.01 eV. While the atomic electron affinities are known experimentally to high accuracy, the molecular values are less certain. This contributes to the difficulty of gauging the accuracy of the theoretical results. A limited number of electron affinities were determined with the explicitly correlated CCSD(T)-F12b method. After extending the VnZ-F12 orbital basis sets with additional diffuse functions, the F12b method was found to accurately reproduce the best F/F(-) value obtained with standard

  1. Application of a convergent, composite coupled cluster approach to bound state, adiabatic electron affinities in atoms and small molecules

    NASA Astrophysics Data System (ADS)

    Feller, David

    2016-01-01

    Benchmark quality adiabatic electron affinities for a collection of atoms and small molecules were obtained with the Feller-Peterson-Dixon composite coupled cluster theory method. Prior applications of this method demonstrated its ability to accurately predict atomization energies/heats of formation for more than 170 molecules. In the current work, the 1-particle expansion involved very large correlation consistent basis sets, ranging up to aug-cc-pV9Z (aug-cc-pV10Z for H and H2), with the goal of minimizing the residual basis set truncation error that must otherwise be approximated with extrapolation formulas. The n-particle expansion begins with coupled cluster calculations through iterative single and double excitations plus a quasiperturbative treatment of "connected" triple excitations (CCSD(T)) pushed to the complete basis set limit followed by CCSDT, CCSDTQ, or CCSDTQ5 corrections. Due to the small size of the systems examined here, it was possible in many cases to extend the n-particle expansion to the full configuration interaction wave function limit. Additional, smaller corrections associated with core/valence correlation, scalar relativity, anharmonic zero point vibrational energies, and non-adiabatic effects were also included. The overall root mean square (RMS) deviation was 0.005 eV (0.12 kcal/mol). This level of agreement was comparable to what was found with molecular heats of formation. A 95% confidence level corresponds to roughly twice the RMS value or 0.01 eV. While the atomic electron affinities are known experimentally to high accuracy, the molecular values are less certain. This contributes to the difficulty of gauging the accuracy of the theoretical results. A limited number of electron affinities were determined with the explicitly correlated CCSD(T)-F12b method. After extending the VnZ-F12 orbital basis sets with additional diffuse functions, the F12b method was found to accurately reproduce the best F/F- value obtained with standard

  2. SU-E-T-163: Characterization of a Novel High Resolution 1D Silicon Monolithic Array for Small Field Commissioning and Quality Assurance

    SciTech Connect

    Bisello, F; McGlade, J; Wang, P; Kralik, J; Kosterin, P; Mooij, R; Solberg, T; Menichelli, D; Celi, J

    2015-06-15

    Purpose: To study the suitability of a novel 1D silicon monolithic array for dosimetry of small radiation fields and for QA of high dose gradient treatment modalities (IMRT and SBRT). Methods: A 1D array composed of 4 monolithic silicon modules of 64 mm length and 1 mm pixel pitch was developed by IBA Dosimetry. Measurements were carried out for 6MV and 15MV photons on two commercial different linacs (TrueBeam and Clinac iX, Varian Medical Systems, Palo Alto, CA) and for a CyberKnife G4 (Accuray Inc., Sunnyvale, CA). The 1D array was used to measure output factors (OF), profiles and off axis correction factors (OACF) for the Iris CyberKnife variable collimator (5–60 mm). In addition, dose profiles (at the isocenter plane) were measured for multiple IMRT and SBRT treatment plans and compared with those obtained using EDR2radiographic film (Carestream Health, Rochester NY), a commercial 2D diode array and with the dose distribution calculated using a commercial TPS (Eclipse, Varian Medical Systems, Palo Alto, CA). Results: Due to the small pixel pitch of the detector, IMRT and SBRT plan profiles deviate from film measurements by less than 2%. Similarly, the 1D array exhibits better performance than the 2D diode array due to the larger (7 mm) pitch of that device. Iris collimator OFs measured using the 1D silicon array are in good agreement with the commissioning values obtained using a commercial stereotactic diode as well as with published data. Maximum deviations are < 3% for the smallest field (5 and 7.5mm) and below 1% for all other dimensions. Conclusion: We have demonstrated good performances of the array for commissioning of small photon fields and in patient QA, compared with diodes and film typically used in these clinical applications. The technology compares favorably with existing commercial solutions The presenting author is founded by a Marie Curie Early Initial Training Network Fellowship of the European Communitys Seventh Framework Programme under

  3. Microstructured silicon radiation detector

    DOEpatents

    Okandan, Murat; Derzon, Mark S.; Draper, Bruce L.

    2017-03-14

    A radiation detector comprises a silicon body in which are defined vertical pores filled with a converter material and situated within silicon depletion regions. One or more charge-collection electrodes are arranged to collect current generated when secondary particles enter the silicon body through walls of the pores. The pores are disposed in low-density clusters, have a majority pore thickness of 5 .mu.m or less, and have a majority aspect ratio, defined as the ratio of pore depth to pore thickness, of at least 10.

  4. Small-angle neutron scattering analysis of Mn–C clusters in high-manganese 18Mn–0.6C steel

    SciTech Connect

    Kang, Mihyun; Shin, Eunjoo; Woo, Wanchuck; Lee, Young-Kook

    2014-10-15

    Nanometer-scale particles (Mn–C clusters) were analyzed quantitatively using small-angle neutron scattering in 18Mn–0.6C (wt.%) austenite high-manganese steel. The size, number, and volume fraction of the particles were determined as a function of strain (0, 5, 15, 30, 45, 50%) at different temperatures (25 and 100 °C). The diameter of the cluster ranges from 2 to 14 nm in the matrix. The total volume fraction of the cluster significantly increases from 2.7 × 10{sup −6} to 8.7 × 10{sup −6} as the strain increases. Such clustering phenomenon is correlated to the serration behavior under loading in high-manganese steels. - Highlights: • Show Mn-C clustering as function of strain in 18Mn-0.6C TWIP steel. • Determine the size, number, and volume fraction of clusters quantitatively. • Compare the clustering behavior at 25 and 100 °C.

  5. Effect of quenching temperature and size on atom movement and local structural change for small copper clusters containing 51-54 atoms during quenching processes

    NASA Astrophysics Data System (ADS)

    Zhang, L.; Fan, Q. N.

    2016-01-01

    Structural changes are sensitive to the atom number for the small size clusters. However, it is hardly predicted for the effects of quenching temperature and contained atom number on the atom movements of these clusters with the modification of a removing or adding atom. In this paper, we demonstrate the formation of many topologically non-equivalent Cu clusters containing 51-54 atoms during quenching processes by means of atomistic simulations. By modifying annealing temperature, different pathways are observed. The simulation results show that the quenching temperature has large effect on the atom movements and the scenario of the formation and growth of local structures in the clusters is greatly different for the four clusters only with one atom difference. When the quenching temperature is high, most atoms in the clusters move individually. In the meantime, changes in the atom packing can be observed in these clusters. Low quenching temperature is helpful to slow down the atom movements and form the structures on icosahedral geometry.

  6. An analysis of the optimal multiobjective inventory clustering decision with small quantity and great variety inventory by applying a DPSO.

    PubMed

    Wang, Shen-Tsu; Li, Meng-Hua

    2014-01-01

    When an enterprise has thousands of varieties in its inventory, the use of a single management method could not be a feasible approach. A better way to manage this problem would be to categorise inventory items into several clusters according to inventory decisions and to use different management methods for managing different clusters. The present study applies DPSO (dynamic particle swarm optimisation) to a problem of clustering of inventory items. Without the requirement of prior inventory knowledge, inventory items are automatically clustered into near optimal clustering number. The obtained clustering results should satisfy the inventory objective equation, which consists of different objectives such as total cost, backorder rate, demand relevance, and inventory turnover rate. This study integrates the above four objectives into a multiobjective equation, and inputs the actual inventory items of the enterprise into DPSO. In comparison with other clustering methods, the proposed method can consider different objectives and obtain an overall better solution to obtain better convergence results and inventory decisions.

  7. Small RNA profiling and characterization of piRNA clusters in the adult testes of the common marmoset, a model primate.

    PubMed

    Hirano, Takamasa; Iwasaki, Yuka W; Lin, Zachary Yu-Ching; Imamura, Masanori; Seki, Naomi M; Sasaki, Erika; Saito, Kuniaki; Okano, Hideyuki; Siomi, Mikiko C; Siomi, Haruhiko

    2014-08-01

    Small RNAs mediate gene silencing by binding Argonaute/Piwi proteins to regulate target RNAs. Here, we describe small RNA profiling of the adult testes of Callithrix jacchus, the common marmoset. The most abundant class of small RNAs in the adult testis was piRNAs, although 353 novel miRNAs but few endo-siRNAs were also identified. MARWI, a marmoset homolog of mouse MIWI and a very abundant PIWI in adult testes, associates with piRNAs that show characteristics of mouse pachytene piRNAs. As in other mammals, most marmoset piRNAs are derived from conserved clustered regions in the genome, which are annotated as intergenic regions. However, unlike in mice, marmoset piRNA clusters are also found on the X chromosome, suggesting escape from meiotic sex chromosome inactivation by the X-linked clusters. Some of the piRNA clusters identified contain antisense-orientated pseudogenes, suggesting the possibility that pseudogene-derived piRNAs may regulate parental functional protein-coding genes. More piRNAs map to transposable element (TE) subfamilies when they have copies in piRNA clusters. In addition, the strand bias observed for piRNAs mapped to each TE subfamily correlates with the polarity of copies inserted in clusters. These findings suggest that pachytene piRNA clusters determine the abundance and strand-bias of TE-derived piRNAs, may regulate protein-coding genes via pseudogene-derived piRNAs, and may even play roles in meiosis in the adult marmoset testis.

  8. On the Electronic and Atomic Structures of Small Au-N(-) (N=4-14) Clusters: A Photoelectron Spectroscopy and Density-Functional Study

    SciTech Connect

    Hakkinen, Hannu; Yoon, Bokwon; Landman, Uzi; Li, Xi; Zhai, Hua-Jin; Wang, Lai S.

    2003-10-17

    We report a joint experimental and theoretical study of the electronic and atomic structures of small gold clusters with up to 14 atoms. Well-resolved photoelectron spectra were obtained for Au-N(-) (N= 1-14) at several photon energies. Even-odd alternations were observed, where the even-sized clusters (except Au-10(-)) exhibit an energy gap between the lowest binding energy peak and the rest of the spectrum, indicating that all the neutral even-sized clusters have closed shells. The Au-10(-) spectrum reveals the existence of isomers, with the ground-state cluster exhibiting an extremely high electron binding energy. Evidence of multiple isomers was also observed in the spectra of N= 4, 8, 12, and 13. The structures of the gold cluster anions in the range N= 4-14 were investigated using first-principles simulations. A striking feature of the anionic clusters in this range is the occurrence of planar ground-state structures, which were predicted in earlier theoretical studies (Hakkinen, H.; et al. Phys. Rev. Lett. 2002, 89, 033401) and observed in ion-mobility experiments (Furche, F.; et al. J. Chem. Phys. 2002, 117, 6982) and the existence of close-lying isomers. The calculated electron detachment energies and density of states were compared with the measured data, which confirmed the ground-state structures of the anions. It is found that the main isomers observed experimentally indeed consist of planar clusters up to Au-12(-), Whereas for Au-13(-) and Au-14(-) the theoretical results from three-dimensional isomers agree better with the experiment, providing further support for the 2D to 3D structural transition at Au-12(-), as concluded from previous ion mobility experiments. We also find that small neutral clusters exhibit a tendency to form two-dimensional structures up to a size of 13 atoms.

  9. Wavelet transform analysis of the small-scale X-ray structure of the cluster Abell 1367

    NASA Technical Reports Server (NTRS)

    Grebeney, S. A.; Forman, W.; Jones, C.; Murray, S.

    1995-01-01

    We have developed a new technique based on a wavelet transform analysis to quantify the small-scale (less than a few arcminutes) X-ray structure of clusters of galaxies. We apply this technique to the ROSAT position sensitive proportional counter (PSPC) and Einstein high-resolution imager (HRI) images of the central region of the cluster Abell 1367 to detect sources embedded within the diffuse intracluster medium. In addition to detecting sources and determining their fluxes and positions, we show that the wavelet analysis allows a characterization of the sources extents. In particular, the wavelet scale at which a given source achieves a maximum signal-to-noise ratio in the wavelet images provides an estimate of the angular extent of the source. To account for the widely varying point response of the ROSAT PSPC as a function of off-axis angle requires a quantitative measurement of the source size and a comparison to a calibration derived from the analysis of a Deep Survey image. Therefore, we assume that each source could be described as an isotropic two-dimensional Gaussian and used the wavelet amplitudes, at different scales, to determine the equivalent Gaussian Full Width Half-Maximum (FWHM) (and its uncertainty) appropriate for each source. In our analysis of the ROSAT PSPC image, we detect 31 X-ray sources above the diffuse cluster emission (within a radius of 24 min), 16 of which are apparently associated with cluster galaxies and two with serendipitous, background quasars. We find that the angular extents of 11 sources exceed the nominal width of the PSPC point-spread function. Four of these extended sources were previously detected by Bechtold et al. (1983) as 1 sec scale features using the Einstein HRI. The same wavelet analysis technique was applied to the Einstein HRI image. We detect 28 sources in the HRI image, of which nine are extended. Eight of the extended sources correspond to sources previously detected by Bechtold et al. Overall, using both the

  10. Wavelet transform analysis of the small-scale X-ray structure of the cluster Abell 1367

    NASA Technical Reports Server (NTRS)

    Grebeney, S. A.; Forman, W.; Jones, C.; Murray, S.

    1995-01-01

    We have developed a new technique based on a wavelet transform analysis to quantify the small-scale (less than a few arcminutes) X-ray structure of clusters of galaxies. We apply this technique to the ROSAT position sensitive proportional counter (PSPC) and Einstein high-resolution imager (HRI) images of the central region of the cluster Abell 1367 to detect sources embedded within the diffuse intracluster medium. In addition to detecting sources and determining their fluxes and positions, we show that the wavelet analysis allows a characterization of the sources extents. In particular, the wavelet scale at which a given source achieves a maximum signal-to-noise ratio in the wavelet images provides an estimate of the angular extent of the source. To account for the widely varying point response of the ROSAT PSPC as a function of off-axis angle requires a quantitative measurement of the source size and a comparison to a calibration derived from the analysis of a Deep Survey image. Therefore, we assume that each source could be described as an isotropic two-dimensional Gaussian and used the wavelet amplitudes, at different scales, to determine the equivalent Gaussian Full Width Half-Maximum (FWHM) (and its uncertainty) appropriate for each source. In our analysis of the ROSAT PSPC image, we detect 31 X-ray sources above the diffuse cluster emission (within a radius of 24 min), 16 of which are apparently associated with cluster galaxies and two with serendipitous, background quasars. We find that the angular extents of 11 sources exceed the nominal width of the PSPC point-spread function. Four of these extended sources were previously detected by Bechtold et al. (1983) as 1 sec scale features using the Einstein HRI. The same wavelet analysis technique was applied to the Einstein HRI image. We detect 28 sources in the HRI image, of which nine are extended. Eight of the extended sources correspond to sources previously detected by Bechtold et al. Overall, using both the

  11. Investigating the significance of zero-point motion in small molecular clusters of sulphuric acid and water

    SciTech Connect

    Stinson, Jake L. Ford, Ian J.; Kathmann, Shawn M.

    2014-01-14

    The nucleation of particles from trace gases in the atmosphere is an important source of cloud condensation nuclei, and these are vital for the formation of clouds in view of the high supersaturations required for homogeneous water droplet nucleation. The methods of quantum chemistry have increasingly been employed to model nucleation due to their high accuracy and efficiency in calculating configurational energies; and nucleation rates can be obtained from the associated free energies of particle formation. However, even in such advanced approaches, it is typically assumed that the nuclei have a classical nature, which is questionable for some systems. The importance of zero-point motion (also known as quantum nuclear dynamics) in modelling small clusters of sulphuric acid and water is tested here using the path integral molecular dynamics method at the density functional level of theory. The general effect of zero-point motion is to distort the mean structure slightly, and to promote the extent of proton transfer with respect to classical behaviour. In a particular configuration of one sulphuric acid molecule with three waters, the range of positions explored by a proton between a sulphuric acid and a water molecule at 300 K (a broad range in contrast to the confinement suggested by geometry optimisation at 0 K) is clearly affected by the inclusion of zero point motion, and similar effects are observed for other configurations.

  12. Investigating the significance of zero-point motion in small molecular clusters of sulphuric acid and water.

    PubMed

    Stinson, Jake L; Kathmann, Shawn M; Ford, Ian J

    2014-01-14

    The nucleation of particles from trace gases in the atmosphere is an important source of cloud condensation nuclei, and these are vital for the formation of clouds in view of the high supersaturations required for homogeneous water droplet nucleation. The methods of quantum chemistry have increasingly been employed to model nucleation due to their high accuracy and efficiency in calculating configurational energies; and nucleation rates can be obtained from the associated free energies of particle formation. However, even in such advanced approaches, it is typically assumed that the nuclei have a classical nature, which is questionable for some systems. The importance of zero-point motion (also known as quantum nuclear dynamics) in modelling small clusters of sulphuric acid and water is tested here using the path integral molecular dynamics method at the density functional level of theory. The general effect of zero-point motion is to distort the mean structure slightly, and to promote the extent of proton transfer with respect to classical behaviour. In a particular configuration of one sulphuric acid molecule with three waters, the range of positions explored by a proton between a sulphuric acid and a water molecule at 300 K (a broad range in contrast to the confinement suggested by geometry optimisation at 0 K) is clearly affected by the inclusion of zero point motion, and similar effects are observed for other configurations.

  13. Antibiotic discovery throughout the Small World Initiative: A molecular strategy to identify biosynthetic gene clusters involved in antagonistic activity.

    PubMed

    Davis, Elizabeth; Sloan, Tyler; Aurelius, Krista; Barbour, Angela; Bodey, Elijah; Clark, Brigette; Dennis, Celeste; Drown, Rachel; Fleming, Megan; Humbert, Allison; Glasgo, Elizabeth; Kerns, Trent; Lingro, Kelly; McMillin, MacKenzie; Meyer, Aaron; Pope, Breanna; Stalevicz, April; Steffen, Brittney; Steindl, Austin; Williams, Carolyn; Wimberley, Carmen; Zenas, Robert; Butela, Kristen; Wildschutte, Hans

    2017-01-22

    The emergence of bacterial pathogens resistant to all known antibiotics is a global health crisis. Adding to this problem is that major pharmaceutical companies have shifted away from antibiotic discovery due to low profitability. As a result, the pipeline of new antibiotics is essentially dry and many bacteria now resist the effects of most commonly used drugs. To address this global health concern, citizen science through the Small World Initiative (SWI) was formed in 2012. As part of SWI, students isolate bacteria from their local environments, characterize the strains, and assay for antibiotic production. During the 2015 fall semester at Bowling Green State University, students isolated 77 soil-derived bacteria and genetically characterized strains using the 16S rRNA gene, identified strains exhibiting antagonistic activity, and performed an expanded SWI workflow using transposon mutagenesis to identify a biosynthetic gene cluster involved in toxigenic compound production. We identified one mutant with loss of antagonistic activity and through subsequent whole-genome sequencing and linker-mediated PCR identified a 24.9 kb biosynthetic gene locus likely involved in inhibitory activity in that mutant. Further assessment against human pathogens demonstrated the inhibition of Bacillus cereus, Listeria monocytogenes, and methicillin-resistant Staphylococcus aureus in the presence of this compound, thus supporting our molecular strategy as an effective research pipeline for SWI antibiotic discovery and genetic characterization.

  14. A cluster randomized trial of alcohol prevention in small businesses: a cascade model of help seeking and risk reduction.

    PubMed

    Reynolds, G Shawn; Bennett, Joel B

    2015-01-01

    The current study adapted two workplace substance abuse prevention programs and tested a conceptual model of workplace training effects on help seeking and alcohol consumption. Questionnaires were collected 1 month before, 1 month after, and 6 months within a cluster randomized field experiment. Texas small businesses in construction, transportation, and service industries. A total of 1510 employees from 45 businesses were randomly assigned to receive no training or one of the interventions. The interventions were 4-hour on-the-job classroom trainings that encouraged healthy lifestyles and seeking professional help (e.g., from the Employee Assistance Program [EAP]). The Team Awareness Program focused on peer referral and team building. The Choices in Health Promotion Program delivered various health topics based on a needs assessment. Questionnaires measured help-seeking attitudes and behavior, frequency of drinking alcohol, and job-related incidents. Mixed-model repeated-measures analyses of covariance were computed. Relative to the control group, training was associated with significantly greater reductions in drinking frequency, willingness to seek help, and seeking help from the EAP. After including help-seeking attitudes as a covariate, the correlation between training and help seeking becomes nonsignificant. Help-seeking behavior was not correlated with drinking frequency. Training improved help-seeking attitudes and behaviors and decreased alcohol risks. The reductions in drinking alcohol were directly correlated with training and independent from help seeking.

  15. First-principles study on stability, and growth strategies of small AlnZr (n=1-9) clusters

    NASA Astrophysics Data System (ADS)

    Li, Zhi; Zhou, Zhonghao; Wang, Hongbin; Li, Shengli; Zhao, Zhen

    2016-09-01

    The geometries, relative stability as well as growth strategies of the AlnZr (n=1-9) clusters are investigated with spin polarized density functional theory: BLYP. The results reveal that the AlnZr clusters are more likely to form the dense accumulation structures than the AlN (N=1-10) clusters. The average binding energies of AlnZr are higher than those of AlN clusters. The AlnZr (n=3, 5, and 7) clusters are more stable than others by the differences of the total binding energies. Mülliken population analysis for the AlnZr clusters shows that the electron's adsorption ability of Zr is slightly lower than that of Al except for AlZr cluster. Local peaks of the HOMO-LUMO gap curve are found at n=3, 5, and 7. The reaction energies of AlnZr are higher, which means that AlnZr clusters are easier to react with Al clusters. Zr atom preferential reacts with Al2 cluster. Local peaks of the magnetic dipole moments are found at n=2, 5, and 8.

  16. The chemistry of nitrogen oxides on small size-selected cobalt clusters, Co{sub n}{sup +}

    SciTech Connect

    Anderson, Marie L.; Lacz, Agnieszka; Drewello, Thomas; Derrick, Peter J.; Woodruff, D. Phil; Mackenzie, Stuart R.

    2009-02-14

    Fourier transform ion cyclotron resonance mass spectrometry has been employed to study the reactions of gas-phase cationic cobalt clusters, Co{sub n}{sup +} (n=4-30), with nitric oxide, NO, and nitrous oxide, N{sub 2}O, under single collision conditions. Isolation of the initial cluster permits detailed investigation of fragmentation channels which characterize the reactions of all but the largest clusters studied. In reaction with N{sub 2}O, most clusters generate the monoxides Co{sub n}O{sup +} without fragmentation, cobalt atom loss accompanying only subsequent reactions. By contrast, chemisorption of even a single NO molecule is accompanied by fragmentation of the cluster. The measured rate coefficients for the Co{sub n}{sup +}+N{sub 2}O reaction as a function of cluster size are significantly smaller than those calculated using the surface charge capture model, while for NO the rates are comparable. The reactions have been studied under high coverage conditions by storing clusters for extended periods to permit multiple reactions to occur. This leads to interesting chemistry on the surface of the cluster resulting in the formation of stable oxide clusters and/or the decomposition of nitric oxide on the cluster with the resulting loss of molecular nitrogen.

  17. Endohedral beryllium atoms in germanium clusters with eight and fewer vertices: how small can a cluster be and still encapsulate a central atom?

    PubMed

    Uţă, M M; King, R B

    2012-05-31

    Structures of the beryllium-centered germanium clusters Be@Ge(n)(z) (n = 8, 7, 6; z = -4, -2, 0, +2) have been investigated by density functional theory to provide some insight regarding the smallest metal cluster that can encapsulate an interstitial atom. The lowest energy structures of the eight-vertex Be@Ge(8)(z) clusters (z = -4, -2, 0, +2) all have the Be atom at the center of a closed polyhedron, namely, a D(4d) square antiprism for Be@Ge(8)(4-), a D(2d) bisdisphenoid for Be@Ge(8)(2-), an ideal O(h) cube for Be@Ge(8), and a C(2v) distorted cube for Be@Ge(8)(2+). The Be-centered cubic structures predicted for Be@Ge(8) and Be@Ge(8)(2+) differ from the previously predicted lowest energy structures for the isoelectronic Ge(8)(2-) and Ge(8). This appears to be related to the larger internal volume of the cube relative to other closed eight-vertex polyhedra. The lowest energy structures for the smaller seven- and six-vertex clusters Be@Ge(n)(z) (n = 7, 6; z = -4, -2, 0, +2) no longer have the Be atom at the center of a closed Ge(n) polyhedron. Instead, either the Ge(n) polyhedron has opened up to provide a larger volume for the Be atom or the Be atom has migrated to the surface of the polyhedron. However, higher energy structures are found in which the Be atom is located at the center of a Ge(n) (n = 7, 6) polyhedron. Examples of such structures are a centered C(2v) capped trigonal prismatic structure for Be@Ge(7)(2-), a centered D(5h) pentagonal bipyramidal structure for Be@Ge(7), a centered D(3h) trigonal prismatic structure for Be@Ge(6)(4-), and a centered octahedral structure for Be@Ge(6). Cluster buildup reactions of the type Be@Ge(n)(z) + Ge(2) → Be@Ge(n+2)(z) (n = 6, 8; z = -4, -2, 0, +2) are all predicted to be highly exothermic. This suggests that interstitial clusters having an endohedral atom inside a bare post transition element polyhedron with eight or fewer vertices are less than the optimum size. This is consistent with the experimental observation

  18. Silicone metalization

    SciTech Connect

    Maghribi, Mariam N.; Krulevitch, Peter; Hamilton, Julie

    2008-12-09

    A system for providing metal features on silicone comprising providing a silicone layer on a matrix and providing a metal layer on the silicone layer. An electronic apparatus can be produced by the system. The electronic apparatus comprises a silicone body and metal features on the silicone body that provide an electronic device.

  19. Silicone metalization

    SciTech Connect

    Maghribi, Mariam N.; Krulevitch, Peter; Hamilton, Julie

    2006-12-05

    A system for providing metal features on silicone comprising providing a silicone layer on a matrix and providing a metal layer on the silicone layer. An electronic apparatus can be produced by the system. The electronic apparatus comprises a silicone body and metal features on the silicone body that provide an electronic device.

  20. Silicone metalization

    DOEpatents

    Maghribi, Mariam N.; Krulevitch, Peter; Hamilton, Julie

    2008-12-09

    A system for providing metal features on silicone comprising providing a silicone layer on a matrix and providing a metal layer on the silicone layer. An electronic apparatus can be produced by the system. The electronic apparatus comprises a silicone body and metal features on the silicone body that provide an electronic device.

  1. Heavy Metal Content in Airborne Dust of Childhood Leukemia Cluster Areas: Even Small Towns Have Air Pollutants

    NASA Astrophysics Data System (ADS)

    Sheppard, P. R.; Witten, M. L.

    2004-12-01

    Currently in the US, there are at least two ongoing clusters of childhood leukemia, where the incidence rate over the last several years has exceeded the national norm. In Fallon, Nevada, a town of 8,000 people, 16 children have been diagnosed with leukemia since 1995, three of whom have died. In Sierra Vista, Arizona, a town of 38,000 people, 12 children have been diagnosed since 1998, two of whom have died. A possible third cluster of childhood leukemia and other cancers is being monitored in Elk Grove, California, a suburb of Sacramento. For the purpose of characterizing the heavy metal content of airborne dust of these three communities, total suspended particulate samples were collected from each town as well as from nearby towns that could be considered as control comparisons. Sampling was done using portable high-volume blowers and glass- or quartz-fiber filter media. Filters were measured for elemental concentrations using inductively coupled plasma mass spectroscopy. To date, our most notable results are from the Nevada region. Compared to other control towns in the region, Fallon had significantly more tungsten in its airborne dust. Uranium was also higher in dust of Fallon than in other control towns. Uranium is a known health hazard, though it is not necessarily specifically related to childhood leukemia. The role of tungsten in childhood leukemia has not been widely studied. However, other research has identified tungsten exposure as an environmental concern in Fallon. A CDC study of human tissue samples from Fallon has shown high tungsten levels in people of Fallon, and a USGS study of drinking water in Fallon also has shown high tungsten there. Tree-ring research on selected trees has shown high tungsten values in recent rings compared to earlier rings. While these multiple indications of tungsten in the Fallon environment do not directly lead to the conclusion that tungsten causes leukemia, they do combine to suggest that biomedical research on the

  2. Evaluation of Potential Locations for Siting Small Modular Reactors near Federal Energy Clusters to Support Federal Clean Energy Goals

    SciTech Connect

    Belles, Randy J.; Omitaomu, Olufemi A.

    2014-09-01

    Geographic information systems (GIS) technology was applied to analyze federal energy demand across the contiguous US. Several federal energy clusters were previously identified, including Hampton Roads, Virginia, which was subsequently studied in detail. This study provides an analysis of three additional diverse federal energy clusters. The analysis shows that there are potential sites in various federal energy clusters that could be evaluated further for placement of an integral pressurized-water reactor (iPWR) to support meeting federal clean energy goals.

  3. Study of the interplay between N-graphene defects and small Pd clusters for enhanced hydrogen storage via a spill-over mechanism.

    PubMed

    Rangel, E; Sansores, E; Vallejo, E; Hernández-Hernández, A; López-Pérez, P A

    2016-12-07

    The hydrogen spill-over mechanism was studied by applying Density Functional Theory. We used small palladium clusters to act as the catalyst supported on the substrate (comprised of pyridinic and pyrrolic nitrogen doped graphene), in order to study hydrogen dissociation, migration and diffusion. Charge transfer and strong binding between the catalyst and the substrate lead to dissociated states of H2 and prevent clusters from detaching and coalescing. In dissociated cases of H2 on Pd clusters, energy barriers below 0.6 eV were found. Likewise, concerning hydrogen migration from the catalyst to the substrate, energy barrier values of 0.8 eV (pyridinic defect) and 0.5 eV (pyrrolic defect) were apparent in the case of the Pd4 cluster at full hydrogen saturation. This indicates that hydrogen dissociation and migration may occur spontaneously at room temperature. This result shows that the interaction between the defects and the small metal clusters may explain the role that defects play in hydrogen migration from the catalyst to the substrate. Subsequently, it was found that thermal desorption does not limit chemisorbed hydrogen diffusion on the substrate. This work may thus help to determine experimental strategies with the capacity to enhance hydrogen storage.

  4. Sowing the seeds of massive black holes in small galaxies: Young clusters as the building blocks of ultracompact dwarf galaxies

    SciTech Connect

    Amaro-Seoane, Pau; Konstantinidis, Symeon; Freitag, Marc Dewi; Coleman Miller, M.; Rasio, Frederic A. E-mail: simos@ari.uni-heidelberg.de E-mail: miller@astro.umd.edu

    2014-02-20

    Interacting galaxies often have complexes of hundreds of young stellar clusters of individual masses ∼10{sup 4}-10{sup 6} M {sub ☉} in regions that are a few hundred parsecs across. These cluster complexes interact dynamically, and their coalescence is a candidate for the origin of some ultracompact dwarf galaxies. Individual clusters with short relaxation times are candidates for the production of intermediate-mass black holes of a few hundred solar masses, via runaway stellar collisions prior to the first supernovae in a cluster. It is therefore possible that a cluster complex hosts multiple intermediate-mass black holes that may be ejected from their individual clusters due to mergers or binary processes, but bound to the complex as a whole. Here we explore the dynamical interaction between initially free-flying massive black holes and clusters in an evolving cluster complex. We find that, after hitting some clusters, it is plausible that the massive black hole will be captured in an ultracompact dwarf forming near the center of the complex. In the process, the hole typically triggers electromagnetic flares via stellar disruptions, and is also likely to be a prominent source of gravitational radiation for the advanced ground-based detectors LIGO and VIRGO. We also discuss other implications of this scenario, notably that the central black hole could be considerably larger than expected in other formation scenarios for ultracompact dwarfs.

  5. Effects of excitation energy on the autodetachment lifetimes of small iodide-doped ROH clusters (R═H-, CH3-, CH3CH(2)-).

    PubMed

    Yandell, Margaret A; Young, Ryan M; King, Sarah B; Neumark, Daniel M

    2012-03-22

    The effect of excitation energy on the lifetimes of the charge-transfer-to-solvent (CTTS) states of small (4 ≤ n ≤ 10) iodide-doped water and alcohol clusters was explored using femtosecond time-resolved photoelectron imaging. Excitation of the CTTS state at wavelengths ranging from 272 to 238 nm leads to the formation of the I···(ROH)(n)(-) (R═H-, CH(3)-, and CH(3)CH(2)-) species, which can be thought of as a vibrationally excited bare solvent cluster anion perturbed by an iodine atom. Autodetachment lifetimes for alcohol-containing clusters range from 1 to 71 ps, while water clusters survive for hundreds of ps in this size range. Autodetachment lifetimes were observed to decrease significantly with increasing excitation energy for a particular number and type of solvent molecules. The application of Klots' model for thermionic emission from clusters to I(-)(H(2)O)(5) and I(-)(CH(3)OH)(7) qualitatively reproduces experimental trends and reveals a high sensitivity to energy parametrization while remaining relatively insensitive to the number of vibrational modes. Experimental and computational results therefore suggest that the rate of electron emission is primarily determined by the energetics of the cluster system rather than by details of molecular structure. © 2011 American Chemical Society

  6. Global minimum structures, stability and electronic properties of small NixSny (x + y ≤ 5) bimetallic clusters; a DFT study

    NASA Astrophysics Data System (ADS)

    Sosa-Hernández, Elisa Marina; Montejano-Carrizales, Juan Martin; Alvarado-Leyva, Pedro Gilberto

    2016-10-01

    We report DFT calculations about the global minimum structures, stability and electronic properties of small free Ni x Sn y nanoalloys ( x + y ≤ 5), by using the free SIESTA code. Our results show that the optimized structures of these binary nanoalloys prefer geometries with high coordination and show significant variations as compared to lower energies structures of the pure clusters. The excess energy reveals a favorable mixing of the constituent atoms for all the clusters studied here. The electronic behavior is analyzed through the ionization potential, electron affinity and hardness.

  7. [Spatial heterogeneity of surface soil mineral components in a small catchment in Karst peak-cluster depression area, South China].

    PubMed

    Gao, Peng; Fu, Tong-Gang; Wang, Ke-Lin; Chen, Hong-Song; Zeng, Fu-Ping

    2013-11-01

    A total of 163 soil samples (0-20 cm layer) were collected from the grid sampling plots (80 m x 80 m) in Huanjiang Observation and Research Station of Karst Ecosystem in a small catchment in Karst cluster-peak depression area, South China. By using classical statistics and geostatistics, the spatial heterogeneity of mineral components (SiO2, Fe2O3, CaO, MgO, Al2O3, MnO, and TiO2) in the soils were studied. The contents of the seven soil mineral components in the study area differed greatly, being in the order of SiO2 > Al2O3 > CaO > MgO > Fe2O3 > TiO2 > MnO, and the variance coefficients also varied obviously, in the order of CaO > MgO > Fe2O3 > TiO2 > SiO2 > Al2O3 > MnO. The seven mineral components accounted for 69.4% of the total soil mass. The spatial patterns and the fittest models of the seven soil mineral components differed from each other. All the seven soil mineral components had a strong spatial autocorrelation, with shorter variation ranges and stronger spatial dependence. The Kriging contour maps indicated that the distribution patterns of soil SiO2, Fe2O3, Al2O3, MnO, and TiO2 were similar, being higher in south and east, lower in north and west, higher in depression, and lower in slope, while the distribution patterns of soil CaO and MgO were in adverse. Natural conditions (vegetation, bare rock rate, slope degree, and slope aspect, etc. ) and human disturbance were the most important factors affecting the spatial patterns of the soil mineral components.

  8. Clusters of Small Clumps Can Explain the Peculiar Properties of Giant Clumps in High-redshift Galaxies

    NASA Astrophysics Data System (ADS)

    Behrendt, M.; Burkert, A.; Schartmann, M.

    2016-03-01

    Giant clumps are a characteristic feature of observed high-redshift disk galaxies. We propose that these kiloparsec-sized clumps have a complex substructure and are the result of many smaller clumps self-organizing themselves into clump clusters (CCs). This bottom-up scenario is in contrast to the common top-down view that these giant clumps form first and then sub-fragment. Using a high-resolution hydrodynamical simulation of an isolated, fragmented massive gas disk and mimicking the observations from Genzel et al. at z ˜ 2, we find remarkable agreement in many details. The CCs appear as single entities of sizes {R}{HWHM} ≃ \\0.9-1.4 kpc and masses ˜(1.5-3) × \\quad {10}9 {M}⊙ , representative of high-z observations. They are organized in a ring around the center of the galaxy. The origin of the observed clumps’ high intrinsic velocity dispersion {σ }{intrinsic} ≃ \\50-100 {km} {{{s}}}-1 is fully explained by the internal irregular motions of their substructure in our simulation. No additional energy input, e.g., via stellar feedback, is necessary. Furthermore, in agreement with observations, we find a small velocity gradient {V}{grad}\\quad ≃ 8-27 {km} {{{s}}}-1 {{kpc}}-1 along the CCs in the beam-smeared velocity residual maps, which corresponds to net prograde and retrograde rotation with respect to the rotation of the galactic disk. The CC scenario could have strong implications for the internal evolution, lifetimes, and the migration timescales of the observed giant clumps, bulge growth, and active galactic nucleus activity, stellar feedback, and the chemical enrichment history of galactic disks.

  9. Spin-orbit effect into isomerization barrier of small gold Clusters. Oh ↔ D2h Fluxionality of the Au62+ cluster Investigated by relativistic methods

    NASA Astrophysics Data System (ADS)

    Muñoz-Castro, Alvaro; Paez-Hernandez, Dayan; Arratia-Perez, Ramiro

    2017-09-01

    The Oh-[Au6]2+ cluster exhibits an open-shell 1s21p2 which trend to a more stable D2h isomer in 31.5 kcal/mol, as observed in the experimental [Au6{P(C6H4Me-o)pH2}6] cluster. By taking into account the spin-orbit coupling (SOC) in Oh-[Au6]2+, a resulting 1s1/221p1/22 closed-shell superatomic configuration is obtained stabilizing such structure by about 14.7 kcal/mol, decreasing the isomerization barrier. Thus, the spin-orbit term favors the Oh ↔ D2h conformation rearrangement depicting a decrease in the calculated energy difference between both conformations, an interesting consequence which is not obtained in the hypothetical lighter counterparts.

  10. The Hueckel Model for Small Metal Clusters. 4. Orbital Properties and Cohesive Energies for Model Clusters of Up to Several Hundred Atoms

    DTIC Science & Technology

    1989-12-01

    Cohesive Energies for Model Clusters of Up to Several Hundred Atoms by D. M. Lindsay, Youqi Wang and Thomas F. George Prepared for Publication in... Wang , Thomas F. George 13a. TYPE OF REPORT 13b. TIME COVERED 114. DATE OF REPORT (Year, Month, Day) 15. PAGE COUNT FROM TO December 1989 34 16... Wang y ........ Department of Chemistry California Institute of Technology Pasadena, California 91125 Dist Thomas F. George (hi Departments of Chemistry