A diffuse-interface approximation for surface diffusion including adatoms
NASA Astrophysics Data System (ADS)
Rätz, Andreas; Voigt, A.
2007-01-01
We introduce a diffuse-interface approximation for solving partial differential equations on evolving surfaces. The model of interest is a fourth-order geometric evolution equation for a growing surface with an additional diffusive adatom density on the surface. Such models arise in the description of epitaxial growth, where the surface of interest is the solid-vapour interface. The model allows us to handle complex geometries in an implicit manner, by considering an evolution equation for a phase-field variable describing the surface and an evolution equation for an extended adatom concentration on a time-independent domain. Matched asymptotic analysis shows the formal convergence towards the sharp interface model and numerical results based on adaptive finite elements demonstrate the applicability of the approach.
Molecular dynamics simulation of nanoscale surface diffusion of heterogeneous adatoms clusters
NASA Astrophysics Data System (ADS)
Muhammad, Imran; Fayyaz, Hussain; Muhammad, Rashid; Muhammad, Ismail; Hafeez, Ullah; Yongqing, Cai; M Arshad, Javid; Ejaz, Ahmad; S, A. Ahmad
2016-07-01
Molecular dynamics simulation employing the embedded atom method potential is utilized to investigate nanoscale surface diffusion mechanisms of binary heterogeneous adatoms clusters at 300 K, 500 K, and 700 K. Surface diffusion of heterogeneous adatoms clusters can be vital for the binary island growth on the surface and can be useful for the formation of alloy-based thin film surface through atomic exchange process. The results of the diffusion process show that at 300 K, the diffusion of small adatoms clusters shows hopping, sliding, and shear motion; whereas for large adatoms clusters (hexamer and above), the diffusion is negligible. At 500 K, small adatoms clusters, i.e., dimer, show almost all possible diffusion mechanisms including the atomic exchange process; however no such exchange is observed for adatoms clusters greater than dimer. At 700 K, the exchange mechanism dominates for all types of clusters, where Zr adatoms show maximum tendency and Ag adatoms show minimum or no tendency toward the exchange process. Separation and recombination of one or more adatoms are also observed at 500 K and 700 K. The Ag adatoms also occupy pop-up positions over the adatoms clusters for short intervals. At 700 K, the vacancies are also generated in the vicinity of the adatoms cluster, vacancy formation, filling, and shifting can be observed from the results.
Diffusion of Cu adatoms and dimers on Cu(111) and Ag(111) surfaces
NASA Astrophysics Data System (ADS)
Mińkowski, Marcin; Załuska-Kotur, Magdalena A.
2015-12-01
Diffusion of Cu adatoms and dimers on Cu(111) and Ag(111) surfaces is analyzed based on ab initio surface potentials. Single adatom diffusion is compared with dimer diffusion on both surfaces. Surface geometry makes the adatoms jump alternately between two states in the same way in both systems, whereas dimers undergo more complex diffusion process that combines translational and rotational motion. Small difference in the surface lattice constant between Cu and Ag crystals results in a completely different energy landscape for dimer jumps. As an effect the character of diffusion process changes. Homogeneous Cu dimer diffusion is more difficult and dimers rather rotate within single surface cell, whereas diffusion over Ag surface is faster and happens more smoothly. The temperature dependence of diffusion coefficient and its parameters: energy barrier and prefactor is calculated and compared for both surfaces.
A calculation of the diffusion energies for adatoms on surfaces of F.C.C. metals
NASA Technical Reports Server (NTRS)
Halicioglu, T.; Pound, G. M.
1979-01-01
The activation energies for diffusion were determined for gold, platinum and iridium adatoms on plane and plane PT surfaces and were found to be in good agreement with the measurements reported by Bassett and Webber. The Lennard-Jones pair potentials were used to model the interatomic forces, and relaxation of the substrate atoms in near proximity to the adatom was considered in detail. The present calculations clarify the mechanism of the observed two-dimensional diffusion of platinum and iridium atoms on a plane PT surface. The results are compared with those obtained using Morse potential functions and different relaxation techniques.
Thermal Diffusion Processes in Metal-Tip-Surface Interactions: Contact Formation and Adatom Mobility
NASA Astrophysics Data System (ADS)
Sørensen, Mads R.; Jacobsen, Karsten W.; Jónsson, Hannes
1996-12-01
We have carried out computer simulations to identify and characterize various thermally activated atomic scale processes that can play an important role in room temperature experiments where a metal tip is brought close to a metal surface. We find that contact formation between the tip and the surface can occur by a sequence of atomic hop and exchange processes which become active on a millisecond time scale when the tip is about 3-5 Å from the surface. Adatoms on the surface are stabilized by the presence of the tip and energy barriers for diffusion processes in the region under the tip are reduced. This can cause adatoms to follow the tip as it is moved over the surface.
Naderi, Ebadollah; Nanavati, Sachin; Majumder, Chiranjib; Ghaisas, S. V.
2015-01-15
CdTe is one of the most promising semiconductor for thin-film based solar cells. Here we report a computational study of Cd and Te adatom diffusion on the CdTe (111) A-type (Cd terminated) and B-type (Te terminated) surfaces and their migration paths. The atomic and electronic structure calculations are performed under the DFT formalism and climbing Nudge Elastic Band (cNEB) method has been applied to evaluate the potential barrier of the Te and Cd diffusion. In general the minimum energy site on the surface is labeled as A{sub a} site. In case of Te and Cd on B-type surface, the sub-surface site (a site just below the top surface) is very close in energy to the A site. This is responsible for the subsurface accumulation of adatoms and therefore, expected to influence the defect formation during growth. The diffusion process of adatoms is considered from A{sub a} (occupied) to A{sub a} (empty) site at the nearest distance. We have explored three possible migration paths for the adatom diffusion. The adatom surface interaction is highly dependent on the type of the surface. Typically, Te interaction with both type (5.2 eV for A-type and 3.8 eV for B-type) is stronger than Cd interactions(2.4 eV for B-type and 0.39 eV for A-type). Cd interaction with the A-type surface is very weak. The distinct behavior of the A-type and B-type surfaces perceived in our study explain the need of maintaining the A-type surface during growth for smooth and stoichiometric growth.
NASA Astrophysics Data System (ADS)
Naderi, Ebadollah; Nanavati, Sachin; Majumder, Chiranjib; Ghaisas, S. V.
2015-01-01
CdTe is one of the most promising semiconductor for thin-film based solar cells. Here we report a computational study of Cd and Te adatom diffusion on the CdTe (111) A-type (Cd terminated) and B-type (Te terminated) surfaces and their migration paths. The atomic and electronic structure calculations are performed under the DFT formalism and climbing Nudge Elastic Band (cNEB) method has been applied to evaluate the potential barrier of the Te and Cd diffusion. In general the minimum energy site on the surface is labeled as Aa site. In case of Te and Cd on B-type surface, the sub-surface site (a site just below the top surface) is very close in energy to the A site. This is responsible for the subsurface accumulation of adatoms and therefore, expected to influence the defect formation during growth. The diffusion process of adatoms is considered from Aa (occupied) to Aa (empty) site at the nearest distance. We have explored three possible migration paths for the adatom diffusion. The adatom surface interaction is highly dependent on the type of the surface. Typically, Te interaction with both type (5.2 eV for A-type and 3.8 eV for B-type) is stronger than Cd interactions(2.4 eV for B-type and 0.39 eV for A-type). Cd interaction with the A-type surface is very weak. The distinct behavior of the A-type and B-type surfaces perceived in our study explain the need of maintaining the A-type surface during growth for smooth and stoichiometric growth.
Modelisation de la diffusion sur les surfaces metalliques: De l'adatome aux processus de croissance
NASA Astrophysics Data System (ADS)
Boisvert, Ghyslain
Cette these est consacree a l'etude des processus de diffusion en surface dans le but ultime de comprendre, et de modeliser, la croissance d'une couche mince. L'importance de bien mai triser la croissance est primordiale compte tenu de son role dans la miniaturisation des circuits electroniques. Nous etudions ici les surface des metaux nobles et de ceux de la fin de la serie de transition. Dans un premier temps, nous nous interessons a la diffusion d'un simple adatome sur une surface metallique. Nous avons, entre autres, mis en evidence l'apparition d'une correlation entre evenements successifs lorsque la temperature est comparable a la barriere de diffusion, i.e., la diffusion ne peut pas etre associee a une marche aleatoire. Nous proposons un modele phenomenologique simple qui reproduit bien les resultats des simulations. Ces calculs nous ont aussi permis de montrer que la diffusion obeit a la loi de Meyer-Neldel. Cette loi stipule que, pour un processus active, le prefacteur augmente exponentiellement avec la barriere. En plus, ce travail permet de clarifier l'origine physique de cette loi. En comparant les resultats dynamiques aux resultats statiques, on se rend compte que la barriere extraite des calculs dynamiques est essentiellement la meme que celle obtenue par une approche statique, beaucoup plus simple. On peut donc obtenir cette barriere a l'aide de methodes plus precises, i.e., ab initio, comme la theorie de la fonctionnelle de la densite, qui sont aussi malheureusement beaucoup plus lourdes. C'est ce que nous avons fait pour plusieurs systemes metalliques. Nos resultats avec cette derniere approche se comparent tres bien aux resultats experimentaux. Nous nous sommes attardes plus longuement a la surface (111) du platine. Cette surface regorge de particularites interessantes, comme la forme d'equilibre non-hexagonale des i lots et deux sites d'adsorption differents pour l'adatome. De plus, des calculs ab initio precedents n'ont pas reussi a confirmer la
NASA Astrophysics Data System (ADS)
Zhang, Wei; Gan, Jie; Li, Qian; Gao, Kun; Sun, Jian; Xu, Ning; Ying, Zhifeng; Wu, Jiada
2011-06-01
The self-diffusion dynamics of Cu adatoms on Cu(1 0 0) surface has been studied based on the calculation of the energy barriers for various hopping events using lattice-gas based approach and a modified model. To simplify the description of the interactions and the calculation of the energy barrier, a three-tier hierarchy of description of atomic configurations was conceived in which the active adatom and its nearest atoms were chosen to constitute basic configuration and taken as a whole to study many-body interactions of the atoms in various atomic configurations, whereas the impacts of the next nearest atoms on the diffusion of the active adatom were considered as multi-site interactions. Besides the simple hopping of single adatoms, the movements of dimers and trimers as the results of multiple hopping events have also been examined. Taking into account the hopping events of all adatoms, the stability of atomic configurations has been examined and the evolution of atomic configurations has also been analyzed.
NASA Astrophysics Data System (ADS)
Dieška, Peter; Štich, Ivan
2009-03-01
Ab initio techniques are used to elucidate the lateral noncontact AFM pair exchange of Sn and Ge adatoms on the Ge(111)-c(2×8) surface both at low and room temperatures. Two different processes are considered: (a) tip-assisted surface diffusion and (b) active tip participation via adatom pick-up/deposition processes. The adatom diffusion profiles indicate fairly modest energy barriers between 0.6 and 0.8 eV, which can be further significantly reduced by the tip. However, the diffusion-mediated exchange mechanism is precluded by a large barrier (>1eV) to the (Sn, Ge)-pair exchange. The experimental data are only consistent with a mechanism involving a simultaneous adatom pick-up/deposition and adatom diffusion processes. Simulation results show that, contrary to general belief, the tip apex modification due to the pick-up/deposition processes may not be experimentally noticeable.
Liu, Li; Chen, Zheyuan; Wang, Lei; Polyakova Stolyarova, Elena; Taniguchi, Takashi; Watanabe, Kenji; Hone, James; Flynn, George W; Brus, Louis E
2013-04-25
We examine the nucleation kinetics of Au clusters on graphene and explore the relationship with layer number and underlying supporting substrate of graphene. Using the mean field theory of diffusion-limited aggregation, morphology patterns are semiquantitatively analyzed to obtain Au adatom effective diffusion constants and activation energies. Under specified assumptions, the Au adatom diffusion constant for single-layer graphene supported on SiO2 is ∼50 times smaller than that for hexagonal boron nitride (h-BN)-supported graphene and on the order of 800 times smaller than that for multilayer graphite. Bilayer graphene on SiO2 shows a Au adatom diffusion constant similar to single-layer graphene on h-BN. Scanning probe data show that single-layer graphene is far flatter on h-BN than on SiO2. Two factors are proposed as contributing to the observed lower diffusion constants on single-layer graphene: local surface roughness and homogeneous loss of dispersion/van der Waals electronic stability in multilayers. Graphene Raman spectroscopy shows little charge transfer between Au nanoparticles and graphene. PMID:23121443
Spectral functions of isolated Ce adatoms on paramagnetic surfaces.
Gardonio, S; Wehling, T O; Petaccia, L; Lizzit, S; Vilmercati, P; Goldoni, A; Karolak, M; Lichtenstein, A I; Carbone, C
2011-07-01
We report photoemission experiments revealing the full valence electron spectral function of Ce adatoms on Ag(111), W(110), and Rh(111) surfaces. A transfer of Ce 4f spectral weight from the ionization peak towards the Fermi level is demonstrated upon changing the substrate from Ag(111) to Rh(111). In the intermediate case of Ce on W(110) the ionization peak is found to be split. This evolution of the spectra is explained by means of first-principles theory, which clearly demonstrates that a reliable understanding of magnetic adatoms on metal surfaces requires simultaneous low and high energy spectroscopic information. PMID:21797632
Diffusion and growth of aluminum adatoms on magnesium clusters with hexahedral structure
NASA Astrophysics Data System (ADS)
Dai, Xiongying; Hu, Wangyu; Yang, Jianyu; Chen, Chuanpin
2015-02-01
The surface diffusion and growth of Al atoms on Mg clusters with hexahedral structure was investigated using molecular dynamics simulations. The diffusion pathways and the corresponding energy barriers were determined via the nudged elastic band method. Two diffusion paths from a (0001) facet to a neighboring (1 1 bar 01) facet and between two adjacent (1 1 bar 01) facets were considered. The energy barriers on the (1 1 bar 01) facets and between the two (1 1 bar 01) facets were remarkably increased. As such, the adatom's mobility became limited at low temperatures. The growth of small Al-Mg nanoclusters was modeled via the one-by-one atom deposition technique to form an anomalous core-shell structure. The Mg atoms with lower surface energy and larger atomic radius occupied the core and the Al atoms with higher surface energy and smaller atomic radius occupied the shell.
Co adatoms on Cu surfaces: Ballistic conductance and Kondo temperature
NASA Astrophysics Data System (ADS)
Baruselli, P. P.; Requist, R.; Smogunov, A.; Fabrizio, M.; Tosatti, E.
2015-07-01
The Kondo zero-bias anomaly of Co adatoms probed by scanning tunneling microscopy is known to depend on the height of the tip above the surface, and this dependence is different on different low index Cu surfaces. On the (100) surface, the Kondo temperature first decreases then increases as the tip approaches the adatom, while on the (111) surface it is virtually unaffected. These trends are captured by combined density functional theory and numerical renormalization-group calculations. The adatoms are found to be described by an S =1 Anderson model on both surfaces, and ab initio calculations help identify the symmetry of the active d orbitals. We correctly reproduce the Fano line shape of the zero-bias anomaly for Co/Cu(100) in the tunneling regime but not in the contact regime, where it is probably dependent on the details of the tip and contact geometry. The line shape for Co/Cu(111) is presumably affected by the presence of surface states, which are not included in our method. We also discuss the role of symmetry, which is preserved in our model scattering geometry but most likely broken in experimental conditions.
Effects of Thermal Electronic Excitations on the Diffusion of Oxygen Adatoms on Graphene.
Sun, Tao; Yao, Xinxin; Fabris, Stefano
2016-05-01
We conduct first-principles calculations to study oxygen diffusion on the graphene surface as a function of temperature up to 3000 K. The minimum energy migration path and the corresponding activation energy are determined by the nudged elastic band method with explicit inclusion of thermal electronic excitations. Below 1000 K the activation energy for epoxy oxygen to migrate remains close to its room temperature value (0.72 eV). Above 1000 K the activation energy decreases near linearly with temperature, from 0.70 eV at 1000 K to 0.47 eV at 3000 K. We show that this reduction originates from thermal electronic excitations. In particular, the effect is determined by the large contrasts in the electronic structures of the initial and transition states: the transition state exhibits much larger electronic density of states near the Fermi level and is more susceptible to thermal electronic excitations. The reduction in activation energy leads to appreciable enhancement in the diffusivity of oxygen adatoms. A moderate decrease in the vibrational prefactor, also caused by thermal electronic excitations, does not alter this trend. These findings may facilitate future works to accurately describe the dynamics of O adatoms on graphene at high T, which are critical for determining surface thermodynamic properties such as equilibrium coverage. PMID:27074529
Tunneling spectroscopy of a magnetic adatoms on topological insulator surfaces
NASA Astrophysics Data System (ADS)
Misiorny, M.; Bjerngaard, M.; Paaske, J.
In this communication, we address the question of how the presence of a magnetic impurity on a topological insulator (TI) surface manifests in the inelastic electron tunneling spectroscopy (IETS) when such a system is probed by a STM. For this purpose, we consider a single magnetic adatom with arbitrary spin, whose dynamics is governed by the local magnetic anisotropy. The spin is exchange-coupled to two-dimensional helical surface electrons, corresponding to the surface of a three-dimensional TI like Bi2Se3, with its characteristic hexagonally warped Dirac cone band structure. Employing an effective exchange-tunneling model, we calculate the non-linear differential conductance from a spin-polarized STM tip to the helical substrate, valid in the perturbative regime of weak exchange-tunneling and including the nonequilibrium pumping of the adatom spin states. The interplay between the magnetic anisotropy and the spin-momentum locked surface electrons is shown to give a number of specific imprints in the IETS, which could be investigated by spin-resolved scanning tunneling spectroscopy. M. Misiorny, M. Bjerngaard and J. Paaske, manuscript in preparation Work supported by the Polish Ministry of Science and Education as `Iuventus Plus' project (IP2014 030973) in years 2015-2016.
Electronic Nature of Step-edge Barriers Against Adatom Descent on Transition-metal Surfaces
Mo, Yina; Zhu, Wenguang; Kaxiras, Efthimios; Zhang, Zhenyu
2008-01-01
The activation barriers against adatom migration on terraces and across steps play an essential role in determining the growth morphology of surfaces, interfaces, and thin lms. By studying a series of adatoms on representative transition metal surfaces through extensive rst-principles calculations, we establish a clear correlation between the preferred mechanism and activation energy for adatom descent at a step and the relative degree of electronic shell lling between the adatom and the substrate. We also nd an approximate linear relation between the adatom hopping barriers at step edges and the adatom-surface bonding strength. These results may serve as simple guiding rules for predicting the precise atomic nature of surface morphologies in heteroepitaxial growth such as nanowires.
Control of the crystal structure of InAs nanowires by tuning contributions of adatom diffusion
NASA Astrophysics Data System (ADS)
Huang, Hui; Ren, Xiaomin; Ye, Xian; Guo, Jingwei; Wang, Qi; Zhang, Xia; Cai, Shiwei; Huang, Yongqing
2010-11-01
The dependence of crystal structure on contributions of adatom diffusion (ADD) and precursor direct impingement (DIM) was investigated for vapor-liquid-solid growth of InAs nanowires (NWs). The ADD contributions from the sidewalls and substrate surface can be changed by using GaAs NWs of different length as the basis for growing InAs NWs. We found that pure zinc-blende structure is favored when DIM contributions dominate. Moreover, without changing the NW diameter or growth parameters (such as temperature or V/III ratio), a transition from zinc-blende to wurtzite structure can be realized by increasing the ADD contributions. A nucleation model is proposed in which ADD and DIM contributions play different roles in determining the location and phase of the nucleus.
Mason, Daniel R.; Gramotnev, Dmitri K.; Gramotnev, Galina
2008-09-15
We conduct the detailed numerical investigation of a nanomanipulation and nanofabrication technique--thermal tweezers with dynamic evolution of surface temperature, caused by absorption of interfering laser pulses in a thin metal film or any other absorbing surface. This technique uses random Brownian forces in the presence of strong temperature modulation (surface thermophoresis) for effective manipulation of particles/adatoms with nanoscale resolution. Substantial redistribution of particles on the surface is shown to occur with the typical size of the obtained pattern elements of {approx}100 nm, which is significantly smaller than the wavelength of the incident pulses used (532 nm). It is also demonstrated that thermal tweezers based on surface thermophoresis of particles/adatoms are much more effective in achieving permanent high maximum-to-minimum concentration ratios than bulk thermophoresis, which is explained by the interaction of diffusing particles with the periodic lattice potential on the surface. Typically required pulse regimes including pulse lengths and energies are also determined. The approach is applicable for reproducing any holographically achievable surface patterns, and can thus be used for engineering properties of surfaces including nanopatterning and design of surface metamaterials.
Han, Yong; Evans, James W.
2015-10-27
Large-scale first-principles density functional theory calculations are performed to investigate the adsorption and diffusion of Ru adatoms on monolayer graphene (G) supported on Ru(0001). The G sheet exhibits a periodic moiré-cell superstructure due to lattice mismatch. Within a moiré cell, there are three distinct regions: fcc, hcp, and mound, in which the C6-ring center is above a fcc site, a hcp site, and a surface Ru atom of Ru(0001), respectively. The adsorption energy of a Ru adatom is evaluated at specific sites in these distinct regions. We find the strongest binding at an adsorption site above a C atom inmore » the fcc region, next strongest in the hcp region, then the fcc-hcp boundary (ridge) between these regions, and the weakest binding in the mound region. Behavior is similar to that observed from small-unit-cell calculations of Habenicht et al. [Top. Catal. 57, 69 (2014)], which differ from previous large-scale calculations. We determine the minimum-energy path for local diffusion near the center of the fcc region and obtain a local diffusion barrier of ~0.48 eV. We also estimate a significantly lower local diffusion barrier in the ridge region. These barriers and information on the adsorption energy variation facilitate development of a realistic model for the global potential energy surface for Ru adatoms. Furthermore, this in turn enables simulation studies elucidating diffusion-mediated directed-assembly of Ru nanoclusters during deposition of Ru on G/Ru(0001).« less
Han, Yong; Evans, James W.
2015-10-27
Large-scale first-principles density functional theory calculations are performed to investigate the adsorption and diffusion of Ru adatoms on monolayer graphene (G) supported on Ru(0001). The G sheet exhibits a periodic moiré-cell superstructure due to lattice mismatch. Within a moiré cell, there are three distinct regions: fcc, hcp, and mound, in which the C6-ring center is above a fcc site, a hcp site, and a surface Ru atom of Ru(0001), respectively. The adsorption energy of a Ru adatom is evaluated at specific sites in these distinct regions. We find the strongest binding at an adsorption site above a C atom in the fcc region, next strongest in the hcp region, then the fcc-hcp boundary (ridge) between these regions, and the weakest binding in the mound region. Behavior is similar to that observed from small-unit-cell calculations of Habenicht et al. [Top. Catal. 57, 69 (2014)], which differ from previous large-scale calculations. We determine the minimum-energy path for local diffusion near the center of the fcc region and obtain a local diffusion barrier of ~0.48 eV. We also estimate a significantly lower local diffusion barrier in the ridge region. These barriers and information on the adsorption energy variation facilitate development of a realistic model for the global potential energy surface for Ru adatoms. Furthermore, this in turn enables simulation studies elucidating diffusion-mediated directed-assembly of Ru nanoclusters during deposition of Ru on G/Ru(0001).
Han, Yong; Evans, James W.
2015-10-28
Large-scale first-principles density functional theory calculations are performed to investigate the adsorption and diffusion of Ru adatoms on monolayer graphene (G) supported on Ru(0001). The G sheet exhibits a periodic moiré-cell superstructure due to lattice mismatch. Within a moiré cell, there are three distinct regions: fcc, hcp, and mound, in which the C{sub 6}-ring center is above a fcc site, a hcp site, and a surface Ru atom of Ru(0001), respectively. The adsorption energy of a Ru adatom is evaluated at specific sites in these distinct regions. We find the strongest binding at an adsorption site above a C atom in the fcc region, next strongest in the hcp region, then the fcc-hcp boundary (ridge) between these regions, and the weakest binding in the mound region. Behavior is similar to that observed from small-unit-cell calculations of Habenicht et al. [Top. Catal. 57, 69 (2014)], which differ from previous large-scale calculations. We determine the minimum-energy path for local diffusion near the center of the fcc region and obtain a local diffusion barrier of ∼0.48 eV. We also estimate a significantly lower local diffusion barrier in the ridge region. These barriers and information on the adsorption energy variation facilitate development of a realistic model for the global potential energy surface for Ru adatoms. This in turn enables simulation studies elucidating diffusion-mediated directed-assembly of Ru nanoclusters during deposition of Ru on G/Ru(0001)
NASA Astrophysics Data System (ADS)
Yang, Jianyu; Hu, Wangyu; Wu, Yurong; Dai, Xiongying
2012-06-01
The structure of nickel (Ni), iron (Fe), and magnesium (Mg) adatoms on the aluminum (Al) truncated octahedron is studied using molecular dynamics and the analytic embedded atom method. First, the energy barriers of several typical diffusion processes of Ni, Fe, and Mg adatoms on the Al truncated octahedral cluster were calculated using the nudged elastic band method. The calculated energy barriers were found to be related to the surface energy and atomic radius of the adatom and substrate atom. The result shows that the incorporation of Ni and Fe atoms into Al core easily occurs, and the Mg atom should segregate at the surface of the Al cluster. Thus, the growth of Ni, Fe and Mg on the Al truncated octahedron with 1289 atoms was simulated at several temperatures. In the Ni-Al and Fe-Al cases, the core-shell structure was not obtained. For the Mg-Al system, a good Mg shell on the Al core was found at lower temperatures, and an almost perfect truncated octahedron with more Al shells emerged with an increase in temperature.
N and Ti adatom dynamics on stoichiometric polar TiN(111) surfaces
NASA Astrophysics Data System (ADS)
Sangiovanni, D. G.; Tasnádi, F.; Hultman, L.; Petrov, I.; Greene, J. E.; Chirita, V.
2016-07-01
We use molecular dynamics (MD) based on the modified embedded atom method (MEAM) to determine diffusion coefficients and migration pathways for Ti and N adatoms (Tiad and Nad) on TiN(111). The reliability of the classical model-potential is verified by comparison with density functional theory (DFT) results at 0 K. MD simulations carried out at temperatures between 600 and 1800 K show that both Tiad and Nad favor fcc surface sites and migrate among them by passing through metastable hcp positions. We find that Nad species are considerably more mobile than Tiad on TiN(111); contrary to our previous results on TiN(001). In addition, we show that lattice vibrations at finite temperatures strongly modify the potential energy landscape and result in smaller adatom migration energies, Ea = 1.03 for Tiad and 0.61 eV for Nad, compared to 0 K values Ea0K = 1.55 (Tiad) and 0.79 eV (Nad). We also demonstrate that the inclusion of dipole corrections, neglected in previous DFT calculations, is necessary in order to obtain the correct formation energies for polar surfaces such as TiN(111).
Giant Magnetic Anisotropy of Co, Ru, and Os Adatoms on MgO (001) Surface.
Ou, Xuedong; Wang, Hongbo; Fan, Fengren; Li, Zhengwei; Wu, Hua
2015-12-18
Large magnetic anisotropy energy (MAE) is desirable and critical for nanoscale magnetic devices. Here, using ligand-field level diagrams and density functional calculations, we well explain the very recent discovery [I. G. Rau et al., Science 344, 988 (2014)] that an individual Co adatom on a MgO (001) surface has a large MAE of more than 60 meV. More importantly, we predict that a giant MAE up to 110 meV could be realized for Ru adatoms on MgO (001), and even more for the Os adatoms (208 meV). This is a joint effect of the special ligand field, orbital multiplet, and significant spin-orbit interaction, in the intermediate-spin state of the Ru or Os adatoms on top of the surface oxygens. The giant MAE could provide a route to atomic scale memory. PMID:26722941
Giant Magnetic Anisotropy of Co, Ru, and Os Adatoms on MgO (001) Surface
NASA Astrophysics Data System (ADS)
Ou, Xuedong; Wang, Hongbo; Fan, Fengren; Li, Zhengwei; Wu, Hua
2015-12-01
Large magnetic anisotropy energy (MAE) is desirable and critical for nanoscale magnetic devices. Here, using ligand-field level diagrams and density functional calculations, we well explain the very recent discovery [I. G. Rau et al., Science 344, 988 (2014)] that an individual Co adatom on a MgO (001) surface has a large MAE of more than 60 meV. More importantly, we predict that a giant MAE up to 110 meV could be realized for Ru adatoms on MgO (001), and even more for the Os adatoms (208 meV). This is a joint effect of the special ligand field, orbital multiplet, and significant spin-orbit interaction, in the intermediate-spin state of the Ru or Os adatoms on top of the surface oxygens. The giant MAE could provide a route to atomic scale memory.
Influence of the adatom diffusion on selective growth of GaN nanowire regular arrays
NASA Astrophysics Data System (ADS)
Gotschke, T.; Schumann, T.; Limbach, F.; Stoica, T.; Calarco, R.
2011-03-01
Molecular beam epitaxy (MBE) on patterned Si/AlN/Si(111) substrates was used to obtain regular arrays of uniform-size GaN nanowires (NWs). The silicon top layer has been patterned with e-beam lithography, resulting in uniform arrays of holes with different diameters (dh) and periods (P). While the NW length is almost insensitive to the array parameters, the diameter increases significantly with dh and P till it saturates at P values higher than 800 nm. A diffusion induced model was used to explain the experimental results with an effective diffusion length of the adatoms on the Si, estimated to be about 400 nm.
Reducing the In2O3(111) Surface Results in Ordered Indium Adatoms
Wagner, Margareta; Seiler, Steffen; Meyer, Bernd; Boatner, Lynn A; Schmid, M.; Diebold, U.
2014-01-01
The In2O3(111) surface can be transformed from an oxidized bulk termination to one that is covered by single In adatoms. As each adatom sits at one specific site within the surface unit cell they form a well-ordered (1 1) superstructure. Annealing at 500 C in O2 or in ultrahigh vacuum results in a fully reversible conversion between these two surface terminations; this transformation and intermediate stages were followed with Scanning Tunneling Microscopy (STM). Formation of this novel surface structure under reducing conditions is corroborated by Density Functional Theory (DFT). The reduced adatom-covered and the oxidized In2O3(111) surfaces are expected to exhibit different chemical and electronic properties, which can easily be exploited by the facile and reversible switching between the two terminations.
Surface modifications by field induced diffusion.
Olsen, Martin; Hummelgård, Magnus; Olin, Håkan
2012-01-01
By applying a voltage pulse to a scanning tunneling microscope tip the surface under the tip will be modified. We have in this paper taken a closer look at the model of electric field induced surface diffusion of adatoms including the van der Waals force as a contribution in formations of a mound on a surface. The dipole moment of an adatom is the sum of the surface induced dipole moment (which is constant) and the dipole moment due to electric field polarisation which depends on the strength and polarity of the electric field. The electric field is analytically modelled by a point charge over an infinite conducting flat surface. From this we calculate the force that cause adatoms to migrate. The calculated force is small for voltage used, typical 1 pN, but due to thermal vibration adatoms are hopping on the surface and even a small net force can be significant in the drift of adatoms. In this way we obtain a novel formula for a polarity dependent threshold voltage for mound formation on the surface for positive tip. Knowing the voltage of the pulse we then can calculate the radius of the formed mound. A threshold electric field for mound formation of about 2 V/nm is calculated. In addition, we found that van der Waals force is of importance for shorter distances and its contribution to the radial force on the adatoms has to be considered for distances smaller than 1.5 nm for commonly used voltages. PMID:22253894
Surface Modifications by Field Induced Diffusion
Olsen, Martin; Hummelgård, Magnus; Olin, Håkan
2012-01-01
By applying a voltage pulse to a scanning tunneling microscope tip the surface under the tip will be modified. We have in this paper taken a closer look at the model of electric field induced surface diffusion of adatoms including the van der Waals force as a contribution in formations of a mound on a surface. The dipole moment of an adatom is the sum of the surface induced dipole moment (which is constant) and the dipole moment due to electric field polarisation which depends on the strength and polarity of the electric field. The electric field is analytically modelled by a point charge over an infinite conducting flat surface. From this we calculate the force that cause adatoms to migrate. The calculated force is small for voltage used, typical 1 pN, but due to thermal vibration adatoms are hopping on the surface and even a small net force can be significant in the drift of adatoms. In this way we obtain a novel formula for a polarity dependent threshold voltage for mound formation on the surface for positive tip. Knowing the voltage of the pulse we then can calculate the radius of the formed mound. A threshold electric field for mound formation of about 2 V/nm is calculated. In addition, we found that van der Waals force is of importance for shorter distances and its contribution to the radial force on the adatoms has to be considered for distances smaller than 1.5 nm for commonly used voltages. PMID:22253894
NASA Technical Reports Server (NTRS)
Karimi, Majid
1993-01-01
Understanding surface diffusion is essential in understanding surface phenomena, such as crystal growth, thin film growth, corrosion, physisorption, and chemisorption. Because of its importance, various experimental and theoretical efforts have been directed to understand this phenomena. The Field Ion Microscope (FIM) has been the major experimental tool for studying surface diffusion. FIM have been employed by various research groups to study surface diffusion of adatoms. Because of limitations of the FIM, such studies are only limited to a few surfaces: nickel, platinum, aluminum, iridium, tungsten, and rhodium. From the theoretical standpoint, various atomistic simulations are performed to study surface diffusion. In most of these calculations the Embedded Atom Method (EAM) along with the molecular static (MS) simulation are utilized. The EAM is a semi-empirical approach for modeling the interatomic interactions. The MS simulation is a technique for minimizing the total energy of a system of particles with respect to the positions of its particles. One of the objectives of this work is to develop the EAM functions for Cu and use them in conjunction with the molecular static (MS) simulation to study diffusion of a Cu atom on a perfect as well as stepped Cu(100) surfaces. This will provide a test of the validity of the EAM functions on Cu(100) surface and near the stepped environments. In particular, we construct a terrace-ledge-kink (TLK) model and calculate the migration energies of an atom on a terrace, near a ledge site, near a kink site, and going over a descending step. We have also calculated formation energies of an atom on the bare surface, a vacancy in the surface, a stepped surface, and a stepped-kink surface. Our results are compared with the available experimental and theoretical results.
Maksymovych, P.; Sorescu, D.C.; Yates, J.T., Jr.
2006-10-06
Microscopic evidence for Au-adatom-induced self-assembly of alkanethiolate species on the Au(111) surface is presented. Based on STM measurements and density-functional theory calculations, a new model for the low-coverage self-assembled monolayer of alkanethiolate on the Au(111) surface is developed, which involves the adsorbate complexes incorporating Au adatoms. It is also concluded that the Au(111) herringbone reconstruction is lifted by the alkanethiolate self-assembly because the reconstructed surface layer provides reactive Au adatoms that drive self-assembly.
Theoretical probing of inelastic spin-excitations in adatoms on surfaces
NASA Astrophysics Data System (ADS)
Lounis, Samir; Schweflinghaus, Benedikt; Dias, Manuel dos Santos; Bouhassoune, Mohammed; Muniz, Roberto B.; Costa, Antonio T.
2014-12-01
We review our recent work on the simulation, description and prediction of spin-excitations in adatoms and dimers deposited on metallic surfaces. This work done together with Douglas L. Mills, is an extension of his seminal contribution (with Pascal Lederer) published 50 years ago on the spin-dynamics of transition metal impurities embedded in transition metal hosts [Lederer et al. (1967)]. The main predictions of his model were verified experimentally with state of the art inelastic scanning tunneling spectroscopy on adatoms. Our formalism, presented in this review, is based on time-dependent density functional theory, combined with the Korringa-Kohn-Rostoker Green function method. Comparison to experiments is shown and discussed in detail. Our scheme enables the description and prediction of the main characteristics of these excitations, i.e. their resonance frequency, their lifetime and their behavior upon application of external perturbations such as a magnetic field.
Site switch of on-surface oxygen adatoms on Rh(111) -- a DFT study
NASA Astrophysics Data System (ADS)
Ganduglia-Pirovano, M. V.; Scheffler, M.
2000-03-01
Oxygen adatoms on Rh(111) at low coverage are known to occupy fcc adsorption sites, but at higher coverages, when also oxygen occupy subsurface sites, the on-surface adatoms switch from the fcc to the hcp adsorption sites (J. Wider, T. Greber, E. Wetli, T.J. Kreutz, P. Schwaller, J. Osterwalder, Surf. Sci. 417) 301 (1998).. We report density-functional theory calculations to analyze the nature of the surface chemical bond of adsorbed oxygen and how this changes with coverage. This also yields to an understanding of the preferred sites and the site-switch. In this context we argue that the Rh_3d and O_1s core level shifts (CLSs) and the adsorption induced change of the work function provide most useful information. We compute initial- and final-state contributions to the CLSs, and compare our results (geometry as well as CLSs) to recent experimental data (A. Baraldi, S. Lizzit, G. Comelli, G. Paolucci, R. Rosei (unpublished)). The dominant factor stabilizing the fcc-site for O adatoms but the hcp-site upon subsurface O formation is found to be an increased ionic contribution to the binding compared to the less stable hollow sites.
Tunneling anisotropic magnetoresistance effect of single adatoms on a noncollinear magnetic surface.
Caffrey, Nuala M; Schröder, Silke; Ferriani, Paolo; Heinze, Stefan
2014-10-01
The tunneling anisotropic magnetoresistance (TAMR) effect demonstrates the sensitivity of spin-polarized electron transport to the orientation of the magnetization with respect to the crystallographic axes. As the TAMR effect requires only a single magnetic electrode, in contrast to the tunneling magnetoresistance effect, it offers an attractive route to alternative spintronic applications. In this work we consider the TAMR effect at the single-atom limit by investigating the anisotropy of the local density of states (LDOS) in the vacuum above transition-metal adatoms adsorbed on a noncollinear magnetic surface, the monolayer of Mn on W(1 1 0). This surface presents a cycloidal spin spiral ground state with an angle of 173° between neighboring spins and thus allows a quasi-continuous exploration of the angular dependence of the TAMR of adsorbed adatoms using scanning tunneling microscopy. Using first-principle calculations, we investigate the TAMR of Co, Rh and Ir adatoms on Mn/W(1 1 0) and relate our results to the magnetization-direction-dependent changes in the LDOS. The anisotropic effect is found to be enhanced dramatically on the adsorption of heavy transition-metal atoms, with values of up to 50% predicted from our calculations. This effect will be measurable even with a non-magnetic STM tip. PMID:25212899
Formation of Sr adatom chains on SrTiO3 (1 1 0) surface determined by strain
NASA Astrophysics Data System (ADS)
Liang, Yan; Li, Fengmiao; Wang, Weihua; Yang, Hao; Guo, Jiandong
2016-09-01
The adsorption behavior of Sr adatoms on the SrTiO3 (1 1 0)-(4 × 1) reconstructed surface with Ti2O3 vacancies distributed in a superstructure is studied by scanning tunneling microscopy and density functional theory calculations. With the adsorption amount increasing, all the Sr adatoms between adjacent Ti2O3 vacancies are closely packed along the quasi-1D stripes on the surface with a uniform separation from each other. The formation of such adatom chains is determined by the surface strain relief—the local lattice relaxations in response to Sr adatoms and Ti2O3 vacancies are incompatible, leading to the strong repulsive interaction between them. Consequently the distribution of Sr chains follows the long-range order of the growth template with their length tunable in a certain range by evaporation amount.
Formation of Sr adatom chains on SrTiO3 (1 1 0) surface determined by strain.
Liang, Yan; Li, Fengmiao; Wang, Weihua; Yang, Hao; Guo, Jiandong
2016-09-14
The adsorption behavior of Sr adatoms on the SrTiO3 (1 1 0)-(4 × 1) reconstructed surface with Ti2O3 vacancies distributed in a superstructure is studied by scanning tunneling microscopy and density functional theory calculations. With the adsorption amount increasing, all the Sr adatoms between adjacent Ti2O3 vacancies are closely packed along the quasi-1D stripes on the surface with a uniform separation from each other. The formation of such adatom chains is determined by the surface strain relief-the local lattice relaxations in response to Sr adatoms and Ti2O3 vacancies are incompatible, leading to the strong repulsive interaction between them. Consequently the distribution of Sr chains follows the long-range order of the growth template with their length tunable in a certain range by evaporation amount. PMID:27390937
NASA Astrophysics Data System (ADS)
Kundin, J.; Hubert, J.; Emmerich, H.
2009-10-01
Here we extend a phase-field model for epitaxial step-flow growth originally derived by Liu and Metiu to capture the case of different adatom diffusivities at neighboring terraces as well as an arbitrary Ehrlich-Schwoebel (ES) barrier. Our extended model approach bridges the atomic to continuum scale in the sense that it takes into account atomic attachment kinetics in full detail and likewise allows to simulate long range transport processes above the surface efficiently. To verify the model we present a matched asymptotic analysis of the derived model equations, which shows that in a special limit the presented model can be related to the Burton-Cabrera-Frank (BCF) model with different kinds of attachment coefficients at either side of a step edge. We demonstrate the capability of our approach by presenting numerical simulations with an Ehrlich-Schwoebel (ES) barrier, which reproduce the well-known step meandering instability. Thereby we show how mathematical analysis helps to specify and validate a phase-field model and thus contributes to the further development of this modeling approach at the nano- to microscale.
NASA Astrophysics Data System (ADS)
Naderi, Ebadollah; Nanavati, Sachin P.; Majumder, Chiranjib; Ghaisas, S. V.
2014-03-01
In the present work we have calculated using density functional theory (DFT), diffusion barrier potentials on both the CdTe (111) surfaces, Cd terminated (A-type) & Te terminated (B-type). We employ nudge elastic band method (NEB) for obtaining the barrier potentials. The barrier is computed for Cd and for Te adatoms on both A-type and B-type surfaces. We report two energetically favourable positions along the normal to the surface, one above and other below the surface. The one above the surface has binding energy slightly more the one below. According to the results of this work, binding energy (in all cases) for adatoms are reasonable and close to experimental data. The barrier potential for hopping adatoms (Cd and Te) on both the surfaces is less than 0.35 eV. Apart from these most probable sites, there are other at least two sites on both the types of surfaces which are meta stable. We have also computed barriers for hopping to and from these meta stable positions. The present results can shade light on the defect formation mechanism in CdTe thin films during growth. The authors would like to thank C-DAC for the computing time on its PARAM series of supercomputers and DST Govt. of India, for partial funding.
A local view of bonding and diffusion at metal surfaces
Feibelman, P.J.
1996-09-01
First-principles density functional calculations and corresponding experimental results underline the importance of basic chemical concepts, such as coordination, valence saturation and promotion-hybridization energetics, in understanding bonding and diffusion of atoms at and on metal surfaces. Several examples are reviewed, including outer-layer relaxations of clean hcp(0001) surfaces, liquid-metal-embrittlement energetics, separation energies of metal-adatom dimers, concerted substitutional self-diffusion on fcc(001) surfaces, and adsorption and diffusion barrier sites for adatoms near steps.
Selva Chandrasekaran, S.; Murugan, P.; Saravanan, P.; Kamat, S. V.
2015-04-07
First principles calculations are performed on 3d-transition metal atom deposited (0001) surface of SmCo{sub 5} to understand the magnetic properties and the improvement of Curie temperature (T{sub c}). Various atomic sites are examined to identify the energetically feasible adsorption of adatom and it is found that the void site of Co-rich (0001) SmCo{sub 5} surface is the most favourable one to deposit. The surface magnetic moments of various adatom deposited SmCo{sub 5} surfaces are larger than the clean surface except for Cu and Zn. Eventually, the surface exchange coupling of clean and adatom deposited surface is found to increase for Mn, Fe, Co, Ni, and Cu deposited surfaces and this improvement results in the increase in T{sub c} of SmCo{sub 5} slab.
Precise Nanoelectronics with Adatom Chains
NASA Technical Reports Server (NTRS)
Yamada, Toshishige
1999-01-01
Adatom chains on an atomically regulated substrate will be building components in future precise nanoelectronics. Adatoms need to be secured with chemical bonding, but then electronic isolation between the adatom and substrate systems is not guaranteed. A one-dimensional model shows that good isolation with existence of surface states is expected on an s-p crossing substrate such as Si, Ge, or GaAs, reflecting the bulk nature of the substrate. Isolation is better if adatoms are electronically similar to the substrate atoms, and can be manipulated by hydrogenation. Chain structures with group IV adatoms with two chemical bonds, or group III adatoms with one chemical bond, are semiconducting, reflecting the surface nature of the substrate. These structures are unintentionally doped due to the charge transfer across the chemical bonds. Physical properties of adatom chains have to be determined for the unified adatom-substrate system.
Migration characterization of Ga and In adatoms on dielectric surface in selective MOVPE
NASA Astrophysics Data System (ADS)
Chen, Wei-Jie; Han, Xiao-Biao; Lin, Jia-Li; Hu, Guo-Heng; Liu, Ming-Gang; Yang, Yi-Bin; Chen, Jie; Wu, Zhi-Sheng; Liu, Yang; Zhang, Bai-Jun
2015-11-01
Migration characterizations of Ga and In adatoms on the dielectric surface in selective metal organic vapor phase epitaxy (MOVPE) were investigated. In the typical MOVPE environment, the selectivity of growth is preserved for GaN, and the growth rate of GaN micro-pyramids is sensitive to the period of the patterned SiO2 mask. A surface migration induced model was adopted to figure out the effective migration length of Ga adatoms on the dielectric surface. Different from the growth of GaN, the selective area growth of InGaN on the patterned template would induce the deposition of InGaN polycrystalline particles on the patterned SiO2 mask with a long period. It was demonstrated with a scanning electron microscope and energy dispersive spectroscopy that the In adatoms exhibit a shorter migration length on the dielectric surface. Project supported by the National Natural Science Foundation of China (Grant Nos. 61274039 and 51177175), the National Basic Research Program of China (Grant No. 2011CB301903), the Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20110171110021), the International Sci. & Tech. Collaboration Program of China (Grant No. 2012DFG52260), the International Sci. & Tech. Collaboration Program of Guangdong Province, China (Grant No. 2013B051000041), the Science and Technology Plan of Guangdong Province, China (Grant No. 2013B010401013), the National High Technology Research and Development Program of China (Grant No. 2014AA032606), and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics, China (Grant No. IOSKL2014KF17).
Ag adatom and dimer motion on Cu(1 1 0)(1 × 2) missing row surface
NASA Astrophysics Data System (ADS)
Sbiaai, Khalid; Eddiai, Adil; Boughaleb, Yahia; Hajjaji, Abdelowahed; Mazroui, M.'hammed; Kara, Abdelkader
2013-11-01
The diffusion of Ag monomer and dimer on Cu(1 1 0)(1 × 2) missing row reconstructed surface is investigated by using the molecular dynamics simulation based on semi-empirical many-body potentials derived from the embedded atom method. In the case of monomer diffusion, both simple jump and long jump follow an Arrhenius law in a large temperature range (300-500 K) but with a small difference in activation energy. The activation barrier energy for simple jump is found to be (0.27 ± 0.02) eV with corresponding prefactor 5.9 ps-1. However, for double jump we found Ea2 = (0.33 ± 0.05) eV. While in the case of dimer, we found that the adatoms activity is notably reduced at low temperature (T < 400 K) and the diffusion via dissociation-reassociation process is more favoured than the other processes. This finding is in a good agreement with the energetic prediction. The presence of concerted jump and leapfrog process is very rare due to their high corresponding activation energy.
New adatom model for Si(11) 7X7 and Si(111)Ge 5X5 reconstructed surfaces
NASA Technical Reports Server (NTRS)
Chadi, D. J.
1985-01-01
A new adatom model differing from the conventional model by a reconstruction of the substrate is proposed. The new adatom structure provides an explanation for the 7x7 and 5x5 size of the unit cells seen on annealed Si(111) and Si(111)-Ge surfaces, respectively. The model is consistent with structural information from vacuum-tunneling microscopy. It also provides simple explanations for stacking-fault-type features expected from Rutherford backscattering experiments and for similarities in the LEED and photoemission spectra of 2x1 and 7x7 surfaces.
Diffusion of tungsten clusters on tungsten (110) surface
Chen, Dong; Hu, Wangyu; Yang, Jianyu; Deng, Huiqiu; Sun, Lixian; Gao, Fei
2009-04-01
Using molecular dynamics simulation and modified analytic embedded-atom method, we have investigated the self-diffusion of clusters on a tungsten (110) surface. As compared to the linear-chain configuration, the close-packed islands for tungsten clusters containing more than nine adatoms have been predicted to be more stable with the relatively lower binding energies. The migration energies show an interesting and oscillating behavior with increasing cluster size. The tetramer, hexamer and octamer have obviously higher migration energies than the others. The different atomic configurations and diffusion mechanisms have been determined during the diffusion processes. It is clear that the dimer-shearing mechanism occurs inside the hexamer, while it occurs at the periphery of heptamer. The successive hopping mechanism of individual atom is of critical importance in the migration of the clusters containing five or fewer adatoms. In addition, the diffusion of a cluster with nine adatoms is achieved through the changes of the cluster shape.
Activation of water on the TiO{sub 2} (110) surface: The case of Ti adatoms
Miao Meng; Liu Yingchun; Wang Qi; Wu Tao; Huang Liping; Gubbins, Keith E.; Nardelli, Marco Buongiorno
2012-02-14
Using first-principles calculations we have studied the reactions of water over Ti adatoms on the (110) surface of rutile TiO{sub 2}. Our results provide fundamental insights into the microscopic mechanisms that drive this reaction at the atomic level and assess the possibility of using this system to activate the water dissociation reaction. In particular, we show that a single water molecule dissociates exothermically with a small energy barrier of 0.17 eV. After dissociation, both H{sup +} and OH{sup -} ions bind strongly to the Ti adatom, which serves as an effective reactive center on the TiO{sub 2} surface. Finally, clustering of Ti adatoms does not improve the redox activity of the system and results in a slightly higher energy barrier for water dissociation.
Valence fluctuations and empty-state resonance for Fe adatom on a surface
NASA Astrophysics Data System (ADS)
Iskakov, S. N.; Mazurenko, V. V.; Valentyuk, M. V.; Lichtenstein, A. I.
2015-12-01
We report on the formation of the high-energy empty-state resonance in the electronic spectrum of the iron adatom on the Pt(111) surface. By using the combination of the first-principles methods and the finite-temperature exact diagonalization approach, we show that the resonance is the result of the valence fluctuations between atomic configurations of the impurity. Our theoretical finding is fully confirmed by the results of the scanning tunneling microscopy measurements [M. F. Crommie et al., Phys. Rev. B 48, 2851 (1993), 10.1103/PhysRevB.48.2851]. In contrast to the previous theoretical results obtained by using local spin density approximation, the paramagnetic state of the impurity in the experiment is naturally reproduced within our approach. This opens a way for interpretation of STM data collected earlier for metallic surface nanosystems with iron impurities.
NASA Astrophysics Data System (ADS)
Lounis, Samir; Bringer, Andreas; Blügel, Stefan
2010-03-01
Surfaces are an inversion asymmetric environment. In combination with the spin-orbit interaction, surface electrons experience a Rashba effect, which leads to spin-split surface states [1]. Having an adatom on such a surface, surface states scatter at it. Interferences are created from which, surprisingly, the fingerprints of spin-orbit coupling cannot be seen with a scanning tunneling microscope (STM) [2]. Instead of a single adatom, Walls and Heller [3] proposed to use a corral of atoms to create extra spin-orbit coupling related modulations in the charge density. Resting on multiple scattering theory, we propose to visualize such effects using a spin-polarized STM considering either a single magnetic adatom or a corral of magnetic adatoms.[4pt] [1] S. Lashell, B.A. McDougall, E. Jensen, Phys. Rev. Lett.77, 3419 (1996).[0pt] [2] L. Petersen and P. Hedegård, Surf. Sci. 49, 459 (2000).[0pt] [3] J.D. Walls and E.J. Heller, Nano Letters 7, 3377 (2007).
Ishida, H.; Terakura, K. )
1989-12-15
The electronic structure of potassium adatoms on the Si(001)-(2{times}1) surface is studied by first-principles calculations within the local-density-functional theory for a wide range of the K coverage ({Theta}) including low {Theta} values with negligibly small direct interactions among adatoms. The symmetric dimer model is assumed for the substrate Si which is modeled by a ten-layer slab. As possible adsorption sites for K, a raised site on the Si dimer chain and/or a valley site between two dimer chains are considered. The calculations are performed for {Theta} ranging from 1/6 to 1 in units of Si monolayers including the Levine model ({Theta}=(1/2)) and also recently proposed double-layer model ({Theta}=1). No free-electron-like surface band characteristic of the K 4{ital s} state appears in the Si gap even for higher {Theta}. However, the two gap states originating from the Si dangling bonds in the limit of {Theta}{r arrow}0 shift to higher binding energies by {similar to}0.5 eV with increasing {Theta} due to the kinetic energy lowering caused by the delocalization of their wave functions toward the overlayer. This is interpreted as a result of the Si-K hybridization which leads to the polarized covalent Si-K bond. The adatom region is essentially neutral even for lower {Theta} if the charge density is averaged in a K sphere, which implies that the adatom-induced dipole moment to reduce the work function shold be attributed to the adatom polarization due to the Si-K orbital mixing rather than the conventional {Theta}-dependent charge transfer.
Surface-state Mediated Interactions: Analytic and Numerical Results for 3 Adatoms
NASA Astrophysics Data System (ADS)
Hyldgaard, Per; Einstein, T. L.
2002-03-01
When mediated by an isotropic Shockley surface-state band (e.g. on noble-metal (111) surfaces), indirect oscillatory electronic interactions between adsorbed atoms(TLE, in Handbook of Surf. Sci.) 1, ed. W. N. Unertl (Elsevier, 1996). are both strong and (in contrast to bulk-mediated interactions) slowly decaying with separation. Moreover, their simple analytical form(P. Hyldgaard and M. Persson, J. Phys.: Cond. Matt. 12), L13 (2000). has been observed experimentally.(J. Repp et al., PRL 85), 2981 (2000); K. Knorr et al., preprint. The s-wave phase shift needed for analysis is obtained from STM images of standing-wave patterns. The interaction of three adsorbed atoms is the sum of the 3 such pair energies plus a trio contribution from electrons traversing the perimeter, d, of the three-adatom cluster.^2,(T. L. Einstein, Surf. Sci. 84), L497 (1979). This trio contribution has a slightly weaker amplitude than the pair energy and a slightly faster asymptotic envelope decay, d-5/2. It also has a different but well-defined d-dependent oscillation period. The asymptotic description is compared with exact model calculations. It should be observable experimentally. http://www2.physics.umd.edu/ einstein/
Surface diffusion activation energy determination using ion beam microtexturing
NASA Technical Reports Server (NTRS)
Rossnagel, S. M.; Robinson, R. S.
1982-01-01
The activation energy for impurity atom (adatom) surface diffusion can be determined from the temperature dependence of the spacing of sputter cones. These cones are formed on the surface during sputtering while simultaneously adding impurities. The impurities form clusters by means of surface diffusion, and these clusters in turn initiate cone formation. Values are given for the surface diffusion activation energies for various materials on polycrystalline Cu, Al, Pb, Au, and Ni. The values for different impurity species on each of these substrates are approximately independent of impurity species within the experimental uncertainty, suggesting the absence of strong chemical bonding effects on the diffusion.
Macroscopic fluctuation theory and first-passage properties of surface diffusion
NASA Astrophysics Data System (ADS)
Meerson, Baruch; Vilenkin, Arkady
2016-02-01
We investigate nonequilibrium fluctuations of a solid surface governed by the stochastic Mullins-Herring equation with conserved noise. This equation describes surface diffusion of adatoms accompanied by their exchange between the surface and the bulk of the solid, when desorption of adatoms is negligible. Previous works dealt with dynamic scaling behavior of the fluctuating interface. Here we determine the probability that the interface first reaches a large given height at a specified time. We also find the optimal time history of the interface conditional on this nonequilibrium fluctuation. We obtain these results by developing a macroscopic fluctuation theory of surface diffusion.
Glancing-angle ion enhanced surface diffusion on gaAs(001) during molecular beam epitaxy.
DeLuca, P M; Ruthe, K C; Barnett, S A
2001-01-01
We describe the effects of glancing incidence 3-4 keV Ar ion bombardment on homoepitaxial growth on vicinal GaAs(001). The average adatom lifetime on surface terraces, measured during growth using specular ion scattering, decreased monotonically with increasing ion current density. The results indicated that surface diffusivity was increased by the ions. The ion beam also suppressed growth oscillations and decreased the film surface roughness. This indicates a change from two-dimensional island nucleation to step-flow growth due to increased adatom surface diffusivity. A simple model, involving direct momentum transfer from ions to adatoms, is shown to be consistent with the measured enhanced diffusion. PMID:11177806
Oxidation mechanism of formic acid on the bismuth adatom-modified Pt(111) surface.
Perales-Rondón, Juan Victor; Ferre-Vilaplana, Adolfo; Feliu, Juan M; Herrero, Enrique
2014-09-24
In order to improve catalytic processes, elucidation of reaction mechanisms is essential. Here, supported by a combination of experimental and computational results, the oxidation mechanism of formic acid on Pt(111) electrodes modified by the incorporation of bismuth adatoms is revealed. In the proposed model, formic acid is first physisorbed on bismuth and then deprotonated and chemisorbed in formate form, also on bismuth, from which configuration the C-H bond is cleaved, on a neighbor Pt site, yielding CO2. It was found computationally that the activation energy for the C-H bond cleavage step is negligible, which was also verified experimentally. PMID:25188779
NASA Astrophysics Data System (ADS)
Shields, Ashley E.; Santos-Carballal, David; de Leeuw, Nora H.
2016-05-01
Thorium dioxide is of significant research interest for its use as a nuclear fuel, particularly as part of mixed oxide fuels. We present the results of a density functional theory (DFT) study of uranium-substituted thorium dioxide, where we found that increasing levels of uranium substitution increases the covalent nature of the bonding in the bulk ThO2 crystal. Three low Miller index surfaces have been simulated and we propose the Wulff morphology for a ThO2 particle and STM images for the (100), (110), and (111) surfaces studied in this work. We have also calculated the adsorption of a uranium atom and the U adatom is found to absorb strongly on all three surfaces, with particular preference for the less stable (100) and (110) surfaces, thus providing a route to the incorporation of uranium into a growing thoria particle.
Diffusion on strained surfaces
NASA Astrophysics Data System (ADS)
Schroeder, M.; Wolf, D. E.
1997-03-01
The change of diffusion kinetics when elastic fields are present is discussed for diffusion on (001) surfaces of simple cubic, fcc and bcc lattices. All particles interact pairwise with a Lennard-Jones potential. The simple cubic lattice was stabilized by an anisotropic prefactor. It is found that generically compressive strain enhances diffusion whereas tensile strain increases the activation barrier. An approximately linear dependence of the barrier in a wide range of misfits is found. In heteroepitaxy, diffusion on top of large clusters is inhomogeneous and anisotropic. The kinetics close to edges and centers of islands are remarkably different. In many cases changes of binding energies are small compared to those of saddle point energies. Thermodynamic arguments (minimization of free energy) are not appropriate to describe diffusion on strained surfaces in these cases.
NASA Astrophysics Data System (ADS)
Cheng, Yu-Ting; Shan, Tzu-Ray; Devine, Bryce; Lee, Donghwa; Liang, Tao; Hinojosa, Beverly B.; Phillpot, Simon R.; Asthagiri, Aravind; Sinnott, Susan B.
2012-08-01
Cu/ZnO heterogeneous systems are used to catalyze the CO2 hydrogenation to methanol, but questions remain about the nature of the active site and the role of Cu-ZnO interactions in the catalyst performance. The way in which ZnO surfaces support Cu clusters and stabilize their active sites is a key factor for maintaining catalyst activity. Processes such as sintering, alloying and encapsulation may play an important role in the activity of the catalyst but are difficult to model directly with density functional theory (DFT). In this work, we report the development of charge-optimized many-body (COMB) potentials to model the Cu/ZnO system. This potential is then used in conjugation with the dimer method, which uses the first derivative of the potential energy and the initial state of the transition to find saddle points, to examine the migration barriers of Cu adatoms on Cu and ZnO surfaces. These findings are validated against the results of density functional theory (DFT) calculations and published experimental data.
NASA Astrophysics Data System (ADS)
Xie, Yi-Qun; Liu, Qing-Wei; Zhang, Peng; Gan, Fu-Xi; Zhuang, Jun
2008-05-01
Using molecular statistics simulations based on the embedded atom method potential, we investigate the reliability of the lateral manipulation of single Pt adatom on Pt(111) surface with a single-atom tip for different tip heights (tip-surface distance) and tip orientations. In the higher tip-height range, tip orientation has little influence on the reliability of the manipulation, and there is an optimal manipulation reliability in this range. In the lower tip-height range the reliability is sensitive to the tip orientation, suggesting that we can obtain a better manipulation reliability with a proper tip orientation. These results can also be extended to the lateral manipulation of Pd adatom on Pd(111) surface.
Zhao, Renjie; Evans, James W.; Oliveira, Tiago J.
2016-04-08
Here, a discrete version of deposition-diffusion equations appropriate for description of step flow on a vicinal surface is analyzed for a two-dimensional grid of adsorption sites representing the stepped surface and explicitly incorporating kinks along the step edges. Model energetics and kinetics appropriately account for binding of adatoms at steps and kinks, distinct terrace and edge diffusion rates, and possible additional barriers for attachment to steps. Analysis of adatom attachment fluxes as well as limiting values of adatom densities at step edges for nonuniform deposition scenarios allows determination of both permeability and kinetic coefficients. Behavior of these quantities is assessedmore » as a function of key system parameters including kink density, step attachment barriers, and the step edge diffusion rate.« less
Smallest Nanoelectronics with Adatom Chains
NASA Technical Reports Server (NTRS)
Yamada, Toshishige; Saini, Subhash (Technical Monitor)
1998-01-01
This viewgraph presentation is focused on the general aspect of atomic chain electronics that I have been studying. Results have been published before, but are being rederived here using a new physical/mathematical picture/model, which deepens the physical understanding. Precise adatom structures can be used as a template on a regulated surface with no uncertainty.
Residual thermal desorption studies of Ga adatoms on trenched Si(5 5 12) surface
NASA Astrophysics Data System (ADS)
Kumar, Praveen; Kumar, Mahesh; Shivaprasad, S. M.
2013-10-01
We present here the thermal stability studies of the room temperature adsorbed Ga/Si(5 5 12) interfaces in the monolayer coverage regime, using AES and LEED as in-situ UHV characterization probes. Ga grows in Stranski-Krastanov growth mode at RT on the 2 × 1 reconstructed Si(5 5 12) surface where islands form on top of 2 ML of flat pseudomorphic Ga, yielding a (1 × 1) LEED pattern for coverages of 1.2 ML and above. When this RT adsorbed Ga/Si(5 5 12) interface is annealed at different temperatures, initially the strained Ga adlayers relax by agglomerating into 3D islands on top of a single Ga monolayer with an activation energy of 0.19 eV in the temperature range of 200-300 °C. The remnant Ga monolayer with a sharp (1 × 1) LEED pattern desorbs at temperature >400 °C, yielding the (1 1 2)-6 × 1 and 2 × (3 3 7) sub-monolayer superstructural. Finally at 720 °C Ga completely desorbs from the surface and leaves the clean 2 × 1 reconstructed Si(5 5 12) surface. The studies demonstrate the richness of the atomically trenched high index Si(5 5 12) surface, in obtaining several anisotropic features that can be used as templates to grow self-assembled nanostructures.
NASA Astrophysics Data System (ADS)
Ryabishchenkova, A. G.; Otrokov, M. M.; Kuznetsov, V. M.; Chulkov, E. V.
2015-09-01
Ab initio study of the adsorption, diffusion, and intercalation of alkali metal adatoms on the (0001) step surface of the topological insulator Bi2Se3 has been performed for the case of low coverage. The calculations of the activation energies of diffusion of adatoms on the surface and in van der Waals gaps near steps, as well as the estimate of diffusion lengths, have shown that efficient intercalation through steps is possible only for Li and Na. Data obtained for K, Rb, and Cs atoms indicate that their thermal desorption at high temperatures can occur before intercalation. The results have been discussed in the context of existing experimental data.
Quasi-long range ordered hole-adatoms pairs on SrTiO3(110)-(4x1) surface
NASA Astrophysics Data System (ADS)
Guo, Jiandong; Li, Fengmiao; Wang, Zhiming; Meng, Sheng; Zhang, Zhiqiang
2012-02-01
The surface structure of transition metal oxides (TMOs) has been an important issue for chemistry and photocatalysis. We studied the surface of SrTiO3, which is a wide-gap semiconductor and has been believed useful for photo-induced water splitting. Specifically we focused on the (110) surface that bears intrinsic instability of reconstruction due to the surface polarity. The monophased (4x1)-reconstructed surface was obtained with the treatment of argon ion sputtering followed by annealing in ultra high vacuum. More interestingly, we observed a (4x10) quasi-long range ordered hole-adatom structure. The atomic configuration was identified by both experimentally adsorbing additional Sr atoms and density functional calculations. The ordering of the hole-adatom pairs was robust since its formation effectively released the stress on (4x1)-reconstructed SrTiO3(110) surface. Such a surface with ordered defects served as a good template for the guided growth of noble metal nanoclusters with controlled size and density.
Atomic diffusion on vicinal surfaces: step roughening impact on step permeability
NASA Astrophysics Data System (ADS)
Ranguelov, B.; Michailov, M.
2014-12-01
The problem of mass transport in material science for systems with reduced dimensionality holds special academic and technological attention since the fine diffusion control of adatoms could initiate exotic nanoscale patterning at epitaxial interfaces. The present study brings out important details of the atomic diffusion mechanisms on vicinal surfaces, accounting for the subtle competition between an external field imposed on the migrating adatoms and the roughening of the steps bordering the atomic terraces. The computational model reveals a temperature gap for breakdown of step permeability in the vicinity of the step roughening transition and sheds light on recently observed experimental results for atomic step dynamics on Si surfaces. The present study also demonstrates the extended capability of atomistic models in computer simulations to unravel simultaneous effects, to distinguish between them, and finally to assess their specific contribution to experimentally observed complex physical phenomena.
A computational ab initio study of surface diffusion of sulfur on the CdTe (111) surface
NASA Astrophysics Data System (ADS)
Naderi, Ebadollah; Ghaisas, S. V.
2016-08-01
In order to discern the formation of epitaxial growth of CdS shell over CdTe nanocrystals, kinetics related to the initial stages of the growth of CdS on CdTe is investigated using ab-initio methods. We report diffusion of sulfur adatom on the CdTe (111) A-type (Cd-terminated) and B-type (Te-terminated) surfaces within the density functional theory (DFT). The barriers are computed by applying the climbing Nudge Elastic Band (c-NEB) method. From the results surface hopping emerges as the major mode of diffusion. In addition, there is a distinct contribution from kick-out type diffusion in which a CdTe surface atom is kicked out from its position and is replaced by the diffusing sulfur atom. Also, surface vacancy substitution contributes to the concomitant dynamics. There are sites on the B- type surface that are competitively close in terms of the binding energy to the lowest energy site of epitaxy on the surface. The kick-out process is more likely for B-type surface where a Te atom of the surface is displaced by a sulfur adatom. Further, on the B-type surface, subsurface migration of sulfur is indicated. Furthermore, the binding energies of S on CdTe reveal that on the A-type surface, epitaxial sites provide relatively higher binding energies and barriers than on B-type.
Interaction of cesium adatoms with free-standing graphene and graphene-veiled SiO_{2} surfaces
Weck, Philippe F.; Kim, Eunja; Biedermann, Grant W.
2015-04-21
In this study, the interaction of Cs adatoms with mono- or bi-layered graphene (MLG and BLG), either free-standing or on a SiO_{2} substrate, was investigated using density functional theory. The most stable adsorption sites for Cs are found to be hollow sites on both graphene sheets and graphene-veiled SiO_{2}(0001). In addition, larger dipole moments are created when a MLG-veiled SiO_{2}(0001) substrate is used for adsorption of Cs atoms compared to the adsorption on free-standing MLG, due to charge transfer occurring between the MLG and the SiO_{2} substrate. For the adsorption of Cs on BLG-veiled SiO_{2}(0001) substrate, these differences are smoothed out and the binding energies corresponding to different sites are nearly degenerate; smaller dipole moments created by the Cs adatoms on BLG compared to MLG are also predicted.
Interaction of cesium adatoms with free-standing graphene and graphene-veiled SiO2 surfaces
Weck, Philippe F.; Kim, Eunja; Biedermann, Grant W.
2015-04-21
In this study, the interaction of Cs adatoms with mono- or bi-layered graphene (MLG and BLG), either free-standing or on a SiO2 substrate, was investigated using density functional theory. The most stable adsorption sites for Cs are found to be hollow sites on both graphene sheets and graphene-veiled SiO2(0001). In addition, larger dipole moments are created when a MLG-veiled SiO2(0001) substrate is used for adsorption of Cs atoms compared to the adsorption on free-standing MLG, due to charge transfer occurring between the MLG and the SiO2 substrate. For the adsorption of Cs on BLG-veiled SiO2(0001) substrate, these differences are smoothedmore » out and the binding energies corresponding to different sites are nearly degenerate; smaller dipole moments created by the Cs adatoms on BLG compared to MLG are also predicted.« less
Tellurium adatoms as an in-situ surface probe of (111) two-dimensional domains at platinum surfaces.
Rodríguez, P; Herrero, E; Aldaz, A; Feliu, J M
2006-12-01
Irreversibly adsorbed tellurium has been studied as a probe to quantify ordered domains in platinum electrodes. The surface redox process of adsorbed tellurium on the Pt(111) electrode and Pt(111) stepped surfaces takes place around 0.85 V in a well-defined peak. The behavior of this redox process on the Pt(111) vicinal surfaces indicates that the tellurium atoms involved in the redox process are only those deposited on the (111) terrace sites. Moreover, the corresponding charge density is proportional to the number of sites on (111) ordered domains (terraces) that are, at least, three atoms wide. Hence, this charge density can be used to measure the number of (111) terrace sites on any given platinum sample. Structural information about tellurium adsorption is obtained from atomic-resolution STM images for the Pt(111) and Pt(10, 10, 9) electrodes. A rectangular structure (2 x radical 3) and a compact hexagonal structure (11 x 8) were identified. However, the redox peak for adsorbed tellurium on (100) domains at 1.03 V overlaps with peaks arising from steps and (110) sites. Therefore, it cannot be used without problems for the determination of (100) sites on a platinum sample. On the (100) terraces, the surface structure of the adsorbed tellurium is c(2 x 2), as revealed by STM. Finally, tellurium irreversible adsorption has been used to estimate the number of (111) ordered domains terrace sites on different polycrystalline platinum samples, and the results are compared to those obtained with bismuth irreversible adsorption. PMID:17129000
Surface diffusion during shadow-mask-assisted molecular-beam epitaxy of III-V compounds
Schallenberg, T.; Brunner, K.; Borzenko, T.; Molenkamp, L.W.; Karczewski, G.
2005-07-01
We present a comprehensive discussion of molecular-beam epitaxy of III-V compound semiconductors through shadow masks. Based on model calculations and growth experiments, we examine how the surface diffusion and the incorporation of group-III adatoms depend on the growth configuration, group-III and group-V fluxes, and the crystal orientation. According to a macroscopic diffusion model, gradients of the group-V flux drive the unidirectional migration of group-III adatoms. Although this effect is generally observed in the experiments, the different growth profiles obtained for [110]- and [110]-oriented samples reflect the different roles of A-type and B-type steps in the incorporation of group-III adatoms. We also demonstrate that during the heteroepitaxial growth of InAs, the dissociation of the GaAs substrate is locally enhanced by the incidence of the In beam. This effect can be exploited for shadow-mask-assisted etching on selected areas. In addition, we show how the positions and sizes of III-V nanostructures can be controlled with high precision on a planar substrate by the usage of shadow masks with multiple nanoscale apertures.
Appy, David; Lei, Huaping; Wang, Cai-Zhuang; Tringides, Michael C; Liu, Da-Jiang; Evans, James W; Thiel, Patricia A
2014-08-01
In this article, we review basic information about the interaction of transition metal atoms with the (0 0 0 1) surface of graphite, especially fundamental phenomena related to growth. Those phenomena involve adatom-surface bonding, diffusion, morphology of metal clusters, interactions with steps and sputter-induced defects, condensation, and desorption. General traits emerge which have not been summarized previously. Some of these features are rather surprising when compared with metal-on-metal adsorption and growth. Opportunities for future work are pointed out.
NASA Astrophysics Data System (ADS)
Almeida, S.; Ochoa, E.; Chavez, J. J.; Zhou, X. W.; Zubia, D.
2015-08-01
The surface diffusivity and residence time were calculated by molecular dynamics simulations in order to solve the surface diffusion equations for selective-area growth. The calculations for CdTe/CdS material system were performed in substrates with Cd termination and S termination. The surface diffusivity and residence time were obtained at different temperatures (600 K, 800 K, 1000 K, 1200 K, and 1400 K). The thermal activation energies were extracted from Arrhenius equation for each substrate termination. Thereafter, values obtained by molecular dynamics were used in a surface diffusion model to calculate the surface concentration profile of adatoms. Alternating the surface termination has the potential to achieve nanoscale selective-area growth without the need of a dielectric film as a mask.
NASA Astrophysics Data System (ADS)
Otrokov, Mikhail; Ryabishchenkova, Anastasia; Gosalvez, Miguel Angel; Kuznetsov, Vladimir; Chulkov, Evgueni
We present the results of an ab initio study of the adsorption, diffusion, and intercalation of alkali metal adatoms on the (0001) stepped surface of the topological insulator Bi2Se3 for the case of low coverage. The calculations of the activation energies of the adatoms diffusion on the surface and in the van der Waals gaps near the steps, as well as the estimation of diffusion lengths, show that efficient intercalation through the steps is possible only for Li and Na. Data obtained for K, Rb, and Cs atoms indicate that their thermal desorption at high temperatures can start before intercalation. These results are discussed in the context of the experimental data available.
Spin-transfer torque on a single magnetic adatom
NASA Astrophysics Data System (ADS)
Delgado, Fernando; José Palacios, Juan; Fernández-Rossier, Joaquín
2010-03-01
We theoretically show how the spin orientation of a single magnetic adatom can be controlled by spin polarized electrons in a scanning tunnelling microscope configuration. The underlying physical mechanism is spin assisted inelastic tunnelling. Experiments with Mn adatoms deposited on a Cu2N surface have been reported for non-polarized currents [1-2]. We show that by changing the direction of the applied current, the orientation of the magnetic adatom can be completely reversed on a time scale that ranges from a few nanoseconds to microseconds, depending on bias and temperature. The changes in the adatom magnetization direction are, in turn, reflected in the tunnelling conductance. Therefore, this effect opens the possibility of writing/reading a single spin without the need of a local magnetic field.[4pt] [1] C.F. Hirjibehedin, C. P. Lutz, A. J. Heinrich, Science 312, 1021 (2006).[0pt] [2] C. Hirjibehedin et al., Science 317, 1199 (2007).
Metal intercalation-induced selective adatom mass transport on graphene
Liu, Xiaojie; Wang, Cai -Zhuang; Hupalo, Myron; Lin, Hai -Qing; Ho, Kai -Ming; Thiel, Patricia A.; Tringides, Michael C.
2016-03-29
Recent experiments indicate that metal intercalation is a very effective method to manipulate the graphene-adatom interaction and control metal nanostructure formation on graphene. A key question is mass transport, i.e., how atoms deposited uniformly on graphene populate different areas depending on the local intercalation. Using first-principles calculations, we show that partially intercalated graphene, with a mixture of intercalated and pristine areas, can induce an alternating electric field because of the spatial variations in electron doping, and thus, an oscillatory electrostatic potential. As a result, this alternating field can change normal stochastic adatom diffusion to biased diffusion, leading to selective massmore » transport and consequent nucleation, on either the intercalated or pristine areas, depending on the charge state of the adatoms.« less
Indirect exchange interaction between magnetic adatoms in graphene
NASA Astrophysics Data System (ADS)
Krainov, I. V.; Rozhansky, I. V.; Averkiev, N. S.; Lähderanta, E.
2015-10-01
We present a theoretical study of indirect exchange interaction between magnetic adatoms in graphene. The coupling between the adatoms to a graphene sheet is described in the framework of a tunneling Hamiltonian. We account for the possibility of this coupling being of resonant character if a bound state of the adatom effectively interacts with the continuum of 2D delocalized states in graphene. In this case the indirect exchange between the adatoms mediated by the 2D carriers appears to be substantially enhanced compared to the results known from Ruderman-Kittel-Kasuya-Yosida (RKKY) theory. Moreover, unlike the results of RKKY calculations in the case of resonant exchange, the magnetic coupling between the adatoms sitting over different graphene sublattices does not cancel each other. Thus, for a random distribution of the magnetic adatoms over a graphene surface, a nonzero magnetic interaction is expected. We also suggest the idea of controlling the magnetism by driving the tunnel coupling in and out of resonance by a gate voltage.
NASA Astrophysics Data System (ADS)
Roehl, Jason L.
Diffusion of point defects on crystalline surfaces and in their bulk is an important and ubiquitous phenomenon affecting film quality, electronic properties and device functionality. A complete understanding of these diffusion processes enables one to predict and then control those processes. Such understanding includes knowledge of the structural, energetic and electronic properties of these native and non-native point defect diffusion processes. Direct experimental observation of the phenomenon is difficult and microscopic theories of diffusion mechanisms and pathways abound. Thus, knowing the nature of diffusion processes, of specific point defects in given materials, has been a challenging task for analytical theory as well as experiment. The recent advances in computing technology have been a catalyst for the rise of a third mode of investigation. The advent of tremendous computing power, breakthroughs in algorithmic development in computational applications of electronic density functional theory now enables direct computation of the diffusion process. This thesis demonstrates such a method applied to several different examples of point defect diffusion on the (001) surface of gallium arsenide (GaAs) and the bulk of cadmium telluride (CdTe) and cadmium sulfide (CdS). All results presented in this work are ab initio, total-energy pseudopotential calculations within the local density approximation to density-functional theory. Single particle wavefunctions were expanded in a plane-wave basis and reciprocal space k-point sampling was achieved by Monkhorst-Pack generated k-point grids. Both surface and bulk computations employed a supercell approach using periodic boundary conditions. Ga adatom adsorption and diffusion processes were studied on two reconstructions of the GaAs(001) surface including the c(4x4) and c(4x4)-heterodimer surface reconstructions. On the GaAs(001)- c(4x4) surface reconstruction, two distinct sets of minima and transition sites were
Zhachuk, R. A.; Teys, S. A.; Olshanetsky, B. Z.
2011-12-15
The diffusion of strontium atoms on the Si(111) surface at room temperature has been investigated using scanning tunnel microscopy and simulation carried out in terms of the density functional theory and the Monte Carlo method. It has been found that the reconstruction of a clean silicon surface with a 7 Multiplication-Sign 7 structure has a profound effect on the diffusion process. The average velocity of motion of a strontium atom in a unit cell of the 7 Multiplication-Sign 7 structure has been calculated. The main diffusion paths of a strontium atom and the corresponding activation energies have been determined. It has been demonstrated that the formation of scanning tunnel microscope images of the Si(111)-7 Multiplication-Sign 7 surface with adsorbed strontium atoms is significantly affected by the shift of the electron density from the strontium atom to the nearest neighbor silicon adatoms in the 7 Multiplication-Sign 7 structure.
SURFACE INHOMOGENEITY EFFECTS ON CONVECTIVE DIFFUSION
It is suggested that convectlve scaling, with appropriate extensions, provides the most useful framework for estimating the effects of urban-scale surface inhomogeneities on diffusion in convective conditions. trong contrasts in surface heat flux exist between cropland, forests, ...
Strong correlation effects in theoretical STM studies of magnetic adatoms
NASA Astrophysics Data System (ADS)
Dang, Hung T.; dos Santos Dias, Manuel; Liebsch, Ansgar; Lounis, Samir
2016-03-01
We present a theoretical study for the scanning tunneling microscopy (STM) spectra of surface-supported magnetic nanostructures, incorporating strong correlation effects. As concrete examples, we study Co and Mn adatoms on the Cu(111) surface, which are expected to represent the opposite limits of Kondo physics and local moment behavior, using a combination of density functional theory and both quantum Monte Carlo and exact diagonalization impurity solvers. We examine in detail the effects of temperature T , correlation strength U , and impurity d electron occupancy Nd on the local density of states. We also study the effective coherence energy scale, i.e., the Kondo temperature TK, which can be extracted from the STM spectra. Theoretical STM spectra are computed as a function of STM tip position relative to each adatom. Because of the multiorbital nature of the adatoms, the STM spectra are shown to consist of a complicated superposition of orbital contributions, with different orbital symmetries, self-energies, and Kondo temperatures. For a Mn adatom, which is close to half-filling, the STM spectra are featureless near the Fermi level. On the other hand, the quasiparticle peak for a Co adatom gives rise to strongly position-dependent Fano line shapes.
Phase field model for growth of adatom islands
NASA Astrophysics Data System (ADS)
Yu, Yan-Mei; Liu, Bang-Gui
2005-03-01
We developed a phase-field model for epitaxial growth of 2D/3D adatom islands and self-organized formation of regular nanostripes. A local phase-field variable is introduced to describe adatom islands. The evolution of this phase field is determined by a time-dependent equation coupled to a diffusive transport equation of local adatom density. The limited interlayer diffusion and atomic detachment at steps are included in the model. Applied to real submonolayer epitaxial systems, we reproduce not only the scaling law of the island density but also the experimental size and spatial distribution of the islands. With large coverages of adatoms we obtain not only the 3D mounding islands but also their coarsening and roughening exponents. We explored the self-organized formation of regular arrays of Fe nanostripes on W(110) by the hybrid growth of islands and step flows during the post-deposition annealing. Compared with atomic models and mean-field models, this phase-field model can not only span larger space and time scales while containing the elemental atomic kinetic of epitaxy, but also provide a fine visualized morphology of epitaxial features in 2+1 dimensions. Y. M. Yu and B.-G. Liu, Phys. Rev. E 69, 021601 (2004); Phys. Rev. B 70, 051444 (2004).
Ryabishchenkova, A. G. Otrokov, M. M.; Kuznetsov, V. M.; Chulkov, E. V.
2015-09-15
Ab initio study of the adsorption, diffusion, and intercalation of alkali metal adatoms on the (0001) step surface of the topological insulator Bi{sub 2}Se{sub 3} has been performed for the case of low coverage. The calculations of the activation energies of diffusion of adatoms on the surface and in van der Waals gaps near steps, as well as the estimate of diffusion lengths, have shown that efficient intercalation through steps is possible only for Li and Na. Data obtained for K, Rb, and Cs atoms indicate that their thermal desorption at high temperatures can occur before intercalation. The results have been discussed in the context of existing experimental data.
Surface diffusion studies by optical diffraction techniques
Xiao, X.D.
1992-11-01
The newly developed optical techniques have been combined with either second harmonic (SH) diffraction or linear diffraction off a monolayer adsorbate grating for surface diffusion measurement. Anisotropy of surface diffusion of CO on Ni(l10) was used as a demonstration for the second harmonic dim reaction method. The linear diffraction method, which possesses a much higher sensitivity than the SH diffraction method, was employed to study the effect of adsorbate-adsorbate interaction on CO diffusion on Ni(l10) surface. Results showed that only the short range direct CO-CO orbital overlapping interaction influences CO diffusion but not the long range dipole-dipole and CO-NI-CO interactions. Effects of impurities and defects on surface diffusion were further explored by using linear diffraction method on CO/Ni(110) system. It was found that a few percent S impurity can alter the CO diffusion barrier height to a much higher value through changing the Ni(110) surface. The point defects of Ni(l10) surface seem to speed up CO diffusion significantly. A mechanism with long jumps over multiple lattice distance initiated by CO filled vacancy is proposed to explain the observed defect effect.
Manuel, L.
1996-01-12
The transport of atoms or molecules over surfaces has been an important area of study for several decades now, with its progress generally limited by the available experimental techniques to characterize the phenomena. A number of methods have been developed over the years to measure surface diffusion yet only very few systems have been characterized to this day mainly due to the physical limitations inherent in these available methods. Even the STM with its astonishing atomically-resolved images of the surface has been limited in terms of its capability to determine mass transport properties. This is because the STM is inherently a ``slow`` instrument, i.e., a finite time is needed for signal averaging in order to produce the image. A need exists for additional surface diffusion measurement techniques, ideally ones which are able to study varied systems and measure a wide range of diffusion rates. The STM (especially because of its highly local nature) presents itself as a promising tool to conduct dynamical studies if its poor time resolution during ``normal operation`` can somehow be overcome. The purpose of this dissertation is to introduce a new technique of using the STM to measure adatom mobility on surfaces -- one with a capacity to achieve excellent time resolution.
Diffusion mediated localization on membrane surfaces
NASA Technical Reports Server (NTRS)
Weaver, D. L.
1982-01-01
Using the model of a cell membrane of a spherical surface in which membrane components may diffuse, the rate of localization due to trapping under diffusion control has been estimated by computing an analytical expression for the mean trapping time including the possibilities of a trapping probability less than one and/or the establishment of an equilibrium at the trap boundary.
Nanoscale topography influences polymer surface diffusion.
Wang, Dapeng; He, Chunlin; Stoykovich, Mark P; Schwartz, Daniel K
2015-02-24
Using high-throughput single-molecule tracking, we studied the diffusion of poly(ethylene glycol) chains at the interface between water and a hydrophobic surface patterned with an array of hexagonally arranged nanopillars. Polymer molecules displayed anomalous diffusion; in particular, they exhibited intermittent motion (i.e., immobilization and "hopping") suggestive of continuous-time random walk (CTRW) behavior associated with desorption-mediated surface diffusion. The statistics of the molecular trajectories changed systematically on surfaces with pillars of increasing height, exhibiting motion that was increasingly subdiffusive and with longer waiting times between diffusive steps. The trajectories were well-described by kinetic Monte Carlo simulations of CTRW motion in the presence of randomly distributed permeable obstacles, where the permeability (the main undetermined parameter) was conceptually related to the obstacle height. These findings provide new insights into the mechanisms of interfacial transport in the presence of obstacles and on nanotopographically patterned surfaces. PMID:25621372
Metal Adatoms and Clusters on Ultrathin Zirconia Films
2016-01-01
Nucleation and growth of transition metals on zirconia has been studied by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Since STM requires electrical conductivity, ultrathin ZrO2 films grown by oxidation of Pt3Zr(0001) and Pd3Zr(0001) were used as model systems. DFT studies were performed for single metal adatoms on supported ZrO2 films as well as the (1̅11) surface of monoclinic ZrO2. STM shows decreasing cluster size, indicative of increasing metal–oxide interaction, in the sequence Ag < Pd ≈ Au < Ni ≈ Fe. Ag and Pd nucleate mostly at steps and domain boundaries of ZrO2/Pt3Zr(0001) and form three-dimensional clusters. Deposition of low coverages of Ni and Fe at room temperature leads to a high density of few-atom clusters on the oxide terraces. Weak bonding of Ag to the oxide is demonstrated by removing Ag clusters with the STM tip. DFT calculations for single adatoms show that the metal–oxide interaction strength increases in the sequence Ag < Au < Pd < Ni on monoclinic ZrO2, and Ag ≈ Au < Pd < Ni on the supported ultrathin ZrO2 film. With the exception of Au, metal nucleation and growth on ultrathin zirconia films follow the usual rules: More reactive (more electropositive) metals result in a higher cluster density and wet the surface more strongly than more noble metals. These bind mainly to the oxygen anions of the oxide. Au is an exception because it can bind strongly to the Zr cations. Au diffusion may be impeded by changing its charge state between −1 and +1. We discuss differences between the supported ultrathin zirconia films and the surfaces of bulk ZrO2, such as the possibility of charge transfer to the substrate of the films. Due to their large in-plane lattice constant and the variety of adsorption sites, ZrO2{111} surfaces are more reactive than many other oxygen-terminated oxide surfaces. PMID:27213024
Atomistic simulation of the electronic states of adatoms in monolayer MoS{sub 2}
Chang, Jiwon; Larentis, Stefano; Tutuc, Emanuel; Register, Leonard F.; Banerjee, Sanjay K.
2014-04-07
Using an ab initio density functional theory based electronic structure method, we study the effects of adatoms on the electronic properties of monolayer transition metal dichalcogenide Molybdenum-disulfide (MoS{sub 2}). We consider the 1st (Li, Na, K) and 7th (F, Cl, Br) column atoms and metals (Sc, Ti, Ta, Mo, Pd, Pt, Ag, Au). Three high symmetry sites for the adatom on the surface of monolayer MoS{sub 2} are examined as starting points to search for the most energetically stable configuration for each adatom-monolayer MoS{sub 2} system, as well as the type of associated bonding. For the most stable adatom positions, we characterize the emergence of adatom-induced electronic states including any dopant states.
Misfit dislocations and adatom domain competitions in Cu/Ni (1 1¯ 1¯) heteroepitaxial growth
NASA Astrophysics Data System (ADS)
Zhou, Naigen; Gao, Huajian; Zhou, Lang
2009-04-01
Three-dimensional molecular dynamics simulations of Cu/Ni (1 1bar 1bar) heteroepitaxy were carried out based on the Sutton-Chen EAM potential. It was found that the heteroepitaxial growth leads to the nucleation and competitive growth of FCC and HCP domains in the surface adatom monolayer, leading to a network of misfit dislocations along the domain boundaries. Analyses on surface diffusion energy barriers and energy differences between FCC and HCP domains provide explanations why such domain competition mechanisms and the associated misfit dislocations are expected to play a prevalent role in heteroepitaxial growth.
Stochastic models for surface diffusion of molecules
Shea, Patrick Kreuzer, Hans Jürgen
2014-07-28
We derive a stochastic model for the surface diffusion of molecules, starting from the classical equations of motion for an N-atom molecule on a surface. The equation of motion becomes a generalized Langevin equation for the center of mass of the molecule, with a non-Markovian friction kernel. In the Markov approximation, a standard Langevin equation is recovered, and the effect of the molecular vibrations on the diffusion is seen to lead to an increase in the friction for center of mass motion. This effective friction has a simple form that depends on the curvature of the lowest energy diffusion path in the 3N-dimensional coordinate space. We also find that so long as the intramolecular forces are sufficiently strong, memory effects are usually not significant and the Markov approximation can be employed, resulting in a simple one-dimensional model that can account for the effect of the dynamics of the molecular vibrations on the diffusive motion.
First-principles calculations of gated adatoms on graphene
NASA Astrophysics Data System (ADS)
Chan, Kevin T.; Lee, Hoonkyung; Cohen, Marvin L.
2011-03-01
The two-dimensional surface of graphene is well-suited for adsorption of adatoms or molecules. The application of a gate voltage can be used to precisely control the electron concentration of the adsorbate-graphene system. Such control over electronic properties of adsorbates on graphene might have useful applications in areas such as catalysis and hydrogen storage. In this work, the gating of a variety of adatoms adsorbed on graphene is studied using first-principles calculations. We compute the projected density of states, local electrostatic potential, and charge density of the adatom-graphene system as a function of gate voltage. We demonstrate that adatoms on graphene can be ionized by gating, and that the ionization causes a sharp change in the electrostatic potential. Additional interesting features of our results are also discussed. This work was supported by NSF Grant No. DMR10-1006184 and DOE under Contract No. DE-AC02-05CH11231. Computational resources were provided by the IT Division at LBNL.
Diffusion of tungsten on stepped tungsten surfaces
NASA Astrophysics Data System (ADS)
Choi, D. S.; Kim, S. K.; Gomer, R.
1990-08-01
Self-diffusion of thermally generated tungsten atoms near (123) and (257), on the zone (011)-(112) and on (023), on the zone (011)-(001) of a tungsten field emitter has been investigated by the field-emission fluctuation method, using a rectangular probe in order to investigate diffusion anisotropy. In agreement with earlier findings of Gong and Gomer [J. Chem. Phys. 88 (1988) 1359, 1370] diffusion of single W atoms along and across (011) terraces separated by (011) steps, i.e. step edges running along [111] is essentially isotropic with Ed = 16 kcal, D0 ≈ 10 -4 cm 2 s -1, while atoms can cross (001) oriented steps only with much activation energy: Ed ≈ 35 kcal, D0 = 10 -2 cm -2 s -1. Slow diffusion parallel to steps attributed previously by Gong Chem. Phys. 88 (1988) 1359, 1370] to kink motion was also seen along the zone (011)-(112) but seems more complicated than previously assumed, with several regimes, which may correspond to motions of different kink configurations. Distinct dips in the slow regime diffusion coefficients occurred at 910 K, somewhat higher than the previously seen onset of dips, 875 K, and may indicate roughening, as previously hypothesized. Slow diffusion perpendicular to steps was also seen in this zone and is not fully understood. It may arise from some step components always perpendicular to the short slit dimensions, or may correspond to more complicated surface configurations than the step and terrace pattern on an ideal emitter surface.
Diffusion and Surface Reaction in Heterogeneous Catalysis
ERIC Educational Resources Information Center
Baiker, A.; Richarz, W.
1978-01-01
Ethylene hydrogenation on a platinum catalyst, electrolytically applied to a tube wall, is a good system for the study of the interactions between diffusion and surface reaction in heterogeneous catalysis. Theoretical background, apparatus, procedure, and student performance of this experiment are discussed. (BB)
Sputtering at grazing ion incidence: Influence of adatom islands
Rosandi, Yudi; Redinger, Alex; Michely, Thomas; Urbassek, Herbert M.
2010-09-15
When energetic ions impinge at grazing incidence onto an atomically flat terrace, they will not sputter. However, when adatom islands (containing N atoms) are deposited on the surface, they induce sputtering. We investigate this effect for the specific case of 83 deg. -incident 5 keV Ar ions on a Pt (111) surface by means of molecular-dynamics simulation and experiment. We find that - for constant coverage {Theta} - the sputter yield has a maximum at island sizes of N congruent with 10-20. A detailed picture explaining the decline of the sputter yield toward larger and smaller island sizes is worked out. Our simulation results are compared with dedicated sputtering experiments, in which a coverage of {Theta}=0.09 of Pt adatoms are deposited onto the Pt (111) surface and form islands with a broad distribution around a most probable size of N congruent with 20.
Diffusion of silver over atomically clean silicon surfaces
Dolbak, A. E. Ol'shanetskii, B. Z.
2013-06-15
The diffusion of silver the (111), (100), and (110) silicon surfaces is studied by Auger electron spectroscopy and low-energy electron diffraction. The mechanisms of diffusion over the (111) and (110) surfaces are revealed, and the temperature dependences of diffusion coefficients are measured. An anisotropy of silver diffusion over the (110) surface is detected.
Numerical Computation of Diffusion on a Surface
Schwartz, Peter; Adalsteinsson, David; Colella, Phillip; Arkin, Adam Paul; Onsum, Matthew
2005-02-24
We present a numerical method for computing diffusive transport on a surface derived from image data. Our underlying discretization method uses a Cartesian grid embedded boundary method for computing the volume transport in region consisting of all points a small distance from the surface. We obtain a representation of this region from image data using a front propagation computation based on level set methods for solving the Hamilton-Jacobi and eikonal equations. We demonstrate that the method is second-order accurate in space and time, and is capable of computing solutions on complex surface geometries obtained from image data of cells.
Diffusing colloidal probes of cell surfaces.
Duncan, Gregg A; Fairbrother, D Howard; Bevan, Michael A
2016-05-25
Measurements and analyses are reported to quantify dynamic and equilibrium interactions between colloidal particles and live cell surfaces using dark field video microscopy. Two-dimensional trajectories of micron-sized polyethylene glycol (PEG)-coated silica colloids relative to adherent epithelial breast cancer cell perimeters are determined allowing measurement of position dependent diffusivities and interaction potentials. PEG was chosen as the material system of interest to assess non-specific interactions with cell surfaces and establishes a basis for investigation of specific interactions in future studies. Analysis of measured potential energies on cell surfaces reveals the spatial dependence in cell topography. With the measured cell topography and models for particle-cell surface hydrodynamic interactions, excellent agreement is obtained between theoretical and measured colloidal transport on cell surfaces. Quantitative analyses of association lifetimes showed that PEG coatings act to stabilize colloids above the cell surface through net repulsive, steric interactions. Our results demonstrate a self-consistent analysis of diffusing colloidal probe interactions due to conservative and non-conservative forces to characterize biophysical cell surface properties. PMID:27117575
Surface diffusion of xenon on Pt(111)
NASA Astrophysics Data System (ADS)
Meixner, D. Laurence; George, Steven M.
1993-06-01
The surface diffusion of xenon on the Pt(111) surface was investigated using laser induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The surface diffusion coefficient at 80 K decreased dramatically from D=8×10-7 cm2/s at θ=0.05θs to approximately D=2×10-8 cm2/s at θ=θs, where θs denotes the saturation coverage at 85 K, corresponding to a commensurate monolayer coverage of 5.0×1014 xenon atoms/cm2. This coverage dependence was consistent with attractive interactions between the adsorbed xenon atoms and the existence of two-dimensional condensed phases of xenon on Pt(111). The kinetic parameters for surface diffusion at θ=θs were Edif=1.3±0.1 kcal/mol and D0=1.1×10-4±0.2 cm2/s. The magnitude of Edif at θ=θs represented the combined effect of the intrinsic corrugation of the adsorbate-surface potential and attractive interactions between the adsorbed xenon atoms. LITD experiments at θ=0.25 θs revealed diffusion kinetic parameters of Edif=1.2±0.2 kcal/mol and D0=3.4×10-4±0.5 cm2/s. The constant Edif at low and high coverage was attributed to the ``breakaway'' of xenon atoms from the edges of condensed phase xenon islands. The coverage dependence of the surface diffusion coefficient for Xe/Pt(111) was explained by a multiple site diffusion mechanism, where collisions with xenon islands limit diffusional motion. Thermal desorption kinetics for xenon on Pt(111) were determined using TPD experiments. Using the variation of heating rates method, the desorption parameters were Edes=6.6±0.2 kcal/mol and νdes=1.3×1013±0.4 s-1, in good agreement with previous studies. The xenon TPD peak shifted to higher temperature versus initial coverage at a fixed heating rate, providing further evidence for attractive interactions between the adsorbed xenon atoms.
Silver diffusion over silicon surfaces with adsorbed tin atoms
Dolbak, A. E. Olshanetskii, B. Z.
2015-02-15
Silver diffusion over the (111), (100), and (110) surfaces of silicon with preliminarily adsorbed tin atoms is studied by Auger electron spectroscopy and low-energy electron diffraction. Diffusion is observed only on the surface of Si(111)-2√3 × 2√3-Sn. The diffusion mechanism is established. It is found that the diffusion coefficient depends on the concentration of diffusing atoms. The diffusion coefficient decreases with increasing silver concentration, while the activation energy and the preexponential factor increase.
Surface modification by subsurface pressure induced diffusion
Zimmermann, Claus G.
2012-01-23
Polycrystalline Ag, covered with a nm thin siloxane layer, was irradiated with ultraviolet light in vacuum at 500 K. Ag particles of different aspect ratios, 50-1000 nm in size, formed on the surface, including a small fraction of nanorods. Pressurized water vapor bubbles are created in the subsurface region by hydrogen radicals photo-chemically released by the siloxane layer. They provide the driving force for a diffusive material flux along grain boundaries to the surface. This mechanism was modeled and found to agree with the experimental timescale: approximately 300 h are required for a 1000 nm particle to form.
Kwon, Oh-Hun; Park, Hyunjin; Seo, Sang-Won; Na, Duk L.; Lee, Jong-Min
2015-01-01
The mean diffusivity (MD) value has been used to describe microstructural properties in Diffusion Tensor Imaging (DTI) in cortical gray matter (GM). Recently, researchers have applied a cortical surface generated from the T1-weighted volume. When the DTI data are analyzed using the cortical surface, it is important to assign an accurate MD value from the volume space to the vertex of the cortical surface, considering the anatomical correspondence between the DTI and the T1-weighted image. Previous studies usually sampled the MD value using the nearest-neighbor (NN) method or Linear method, even though there are geometric distortions in diffusion-weighted volumes. Here we introduce a Surface Guided Diffusion Mapping (SGDM) method to compensate for such geometric distortions. We compared our SGDM method with results using NN and Linear methods by investigating differences in the sampled MD value. We also projected the tissue classification results of non-diffusion-weighted volumes to the cortical midsurface. The CSF probability values provided by the SGDM method were lower than those produced by the NN and Linear methods. The MD values provided by the NN and Linear methods were significantly greater than those of the SGDM method in regions suffering from geometric distortion. These results indicate that the NN and Linear methods assigned the MD value in the CSF region to the cortical midsurface (GM region). Our results suggest that the SGDM method is an effective way to correct such mapping errors. PMID:26236180
Diffusion-limited aggregation on curved surfaces
NASA Astrophysics Data System (ADS)
Choi, J.; Crowdy, D.; Bazant, M. Z.
2010-08-01
We develop a general theory of transport-limited aggregation phenomena occurring on curved surfaces, based on stochastic iterated conformal maps and conformal projections to the complex plane. To illustrate the theory, we use stereographic projections to simulate diffusion-limited aggregation (DLA) on surfaces of constant Gaussian curvature, including the sphere (K>0) and the pseudo-sphere (K<0), which approximate "bumps" and "saddles" in smooth surfaces, respectively. Although the curvature affects the global morphology of the aggregates, the fractal dimension (in the curved metric) is remarkably insensitive to curvature, as long as the particle size is much smaller than the radius of curvature. We conjecture that all aggregates grown by conformally invariant transport on curved surfaces have the same fractal dimension as DLA in the plane. Our simulations suggest, however, that the multifractal dimensions increase from hyperbolic (K<0) to elliptic (K>0) geometry, which we attribute to curvature-dependent screening of tip branching.
Dance of Adatom Islands: Brownian Motion, Scaling and Reshaping
NASA Astrophysics Data System (ADS)
Metiu, Horia; Weakliem, Paul; Bogicevic, Alex; Liu, Shudun
1998-03-01
Dynamics of adatom islands of sizes 17 to 2000 is studied by means of kinetic Monte Carlo simulation. Structures of the islands, especially their relation to the sizes of the islands, are examined in great detail. These information provides us a better understanding of how the diffusion constants of these islands scale with the sizes of the islands. Our earlier prediction that the scaling exponents depend on both the temperature and the systems have recently been confirmed by STM measurements. A simple picture for the Brownian motion of large islands will be presented.
Diffusion limited aggregation. The role of surface diffusion
NASA Astrophysics Data System (ADS)
García-Ruiz, Juan M.; Otálora, Fermín
1991-11-01
We present a growth model in which the hitting particles are able to diffuse to more stable growth sites in the perimeter of a cluster growing by diffusion limited aggregation. By tuning the diffusion path Ls, the morphological output - from disordered fractal to perfect single crystals - can be controlled. Instabilities appear when the mean length of the crystal faces Lf are greater than 2 Ls.
Electron Doping of Ultrathin Black Phosphorus with Cu Adatoms.
Koenig, Steven P; Doganov, Rostislav A; Seixas, Leandro; Carvalho, Alexandra; Tan, Jun You; Watanabe, Kenji; Taniguchi, Takashi; Yakovlev, Nikolai; Castro Neto, Antonio H; Özyilmaz, Barbaros
2016-04-13
Few-layer black phosphorus is a monatomic two-dimensional crystal with a direct band gap that has high carrier mobility for both holes and electrons. Similarly to other layered atomic crystals, like graphene or layered transition metal dichalcogenides, the transport behavior of few-layer black phosphorus is sensitive to surface impurities, adsorbates, and adatoms. Here we study the effect of Cu adatoms onto few-layer black phosphorus by characterizing few-layer black phosphorus field effect devices and by performing first-principles calculations. We find that the addition of Cu adatoms can be used to controllably n-dope few layer black phosphorus, thereby lowering the threshold voltage for n-type conduction without degrading the transport properties. We demonstrate a scalable 2D material-based complementary inverter which utilizes a boron nitride gate dielectric, a graphite gate, and a single bP crystal for both the p- and n-channels. The inverter operates at matched input and output voltages, exhibits a gain of 46, and does not require different contact metals or local electrostatic gating. PMID:26938106
Surface Properties of PEMFC Gas Diffusion Layers
WoodIII, David L; Rulison, Christopher; Borup, Rodney
2010-01-01
The wetting properties of PEMFC Gas Diffusion Layers (GDLs) were quantified by surface characterization measurements and modeling of material properties. Single-fiber contact-angle and surface energy (both Zisman and Owens-Wendt) data of a wide spectrum of GDL types is presented to delineate the effects of hydrophobic post-processing treatments. Modeling of the basic sessile-drop contact angle demonstrates that this value only gives a fraction of the total picture of interfacial wetting physics. Polar forces are shown to contribute 10-20 less than dispersive forces to the composite wetting of GDLs. Internal water contact angles obtained from Owens-Wendt analysis were measured at 13-19 higher than their single-fiber counterparts. An inverse relationship was found between internal contact angle and both Owens-Wendt surface energy and % polarity of the GDL. The most sophisticated PEMFC mathematical models use either experimentally measured capillary pressures or the standard Young-Laplace capillary-pressure equation. Based on the results of the Owens-Wendt analysis, an advancement to the Young-Laplace equation is proposed for use in these mathematical models, which utilizes only solid surface energies and fractional surface coverage of fluoropolymer. Capillary constants for the spectrum of analyzed GDLs are presented for the same purpose.
Simulation study of asymmetric aggregation behavior of adatoms on Si(1 1 1)7×7
NASA Astrophysics Data System (ADS)
Wang, Daimu; Huang, Yinsheng; Wu, Yanning
2010-12-01
We perform detailed simulations of the nucleation and growth of adatoms on the Si(1 1 1)7×7 reconstructed surface based on a modified cooperative diffusion model, and analyze the effect of different asymmetric diffusion and aggregation mechanisms on the growth behavior. We demonstrate that the asymmetry controlled by the difference of effective binding energies between faulted and unfaulted half cells has a strong influence on the preference of occupation of faulted half cells at higher temperatures for both irreversible and partially reversible aggregation. Using the occupation preference as a measure of cluster ordering, the simulation clearly shows growth mode transition from random nucleation at low temperatures to ordered self-organized growth at high temperatures in the case of partially reversible aggregation.
Metals on graphene: correlation between adatom adsorption behavior and growth morphology
Liu, Xiaojie; Wang, Cai-Zhuang; Hupalo, Myron; Lu, Wencai; Tringides, Michael C.; Yao, Yongxin; Ho, Kai-Ming
2012-05-19
We present a systematic study of metal adatom adsorption on graphene by ab initio calculations. The calculations cover alkali metals, sp-simple metals, 3d and group 10 transition metals, noble metals, as well as rare earth metals. The correlation between the adatom adsorption properties and the growth morphology of the metals on graphene is also investigated. We show that the growth morphology is related to the ratio of the metal adsorption energy to its bulk cohesive energy (E(a)/E(c)) and the diffusion barrier (ΔE) of the metal adatom on graphene. Charge transfer, electric dipole and magnetic moments, and graphene lattice distortion induced by metal adsorption would also affect the growth morphologies of the metal islands. We also show that most of the metal nanostructures on graphene would be thermally stable against coarsening.
A Relation for Nanodroplet Diffusion on Smooth Surfaces
Li, Chu; Huang, Jizu; Li, Zhigang
2016-01-01
In this work, we study the diffusion of nanodroplets on smooth surfaces through molecular dynamics (MD) simulations and theoretical analyses. Molecular dynamics simulations show that nanodroplet surface diffusion is different from that of single molecules and solid particles. The dependence of nanodroplet diffusion coefficient on temperature undergoes a transition from linear to nonlinear as the surface wettability is weakened due to the coupling of temperature and surface energy. We also develop a simple relation for the diffusion coefficient by using the contact angle and contact radius of the droplet. It works well for a wide range of surface wettabilities and different sized nanodroplets, as confirmed by MD simulations. PMID:27215471
A Relation for Nanodroplet Diffusion on Smooth Surfaces.
Li, Chu; Huang, Jizu; Li, Zhigang
2016-01-01
In this work, we study the diffusion of nanodroplets on smooth surfaces through molecular dynamics (MD) simulations and theoretical analyses. Molecular dynamics simulations show that nanodroplet surface diffusion is different from that of single molecules and solid particles. The dependence of nanodroplet diffusion coefficient on temperature undergoes a transition from linear to nonlinear as the surface wettability is weakened due to the coupling of temperature and surface energy. We also develop a simple relation for the diffusion coefficient by using the contact angle and contact radius of the droplet. It works well for a wide range of surface wettabilities and different sized nanodroplets, as confirmed by MD simulations. PMID:27215471
Hopping Domain Wall Induced by Paired Adatoms on an Atomic Wire: Si(111)-(5×2)-Au
NASA Astrophysics Data System (ADS)
Kang, Pil-Gyu; Jeong, Hojin; Yeom, Han Woong
2008-04-01
We observed an inhomogeneous fluctuation along one-dimensional atomic wires self-assembled on a Si(111) surface using scanning tunneling microscopy. The fluctuation exhibits dynamic behavior at room temperature and is observed only in a specific geometric condition; the spacing between two neighboring adatom defects is discommensurate with the wire lattice. Upon cooling, the dynamic fluctuation freezes to show the existence of an atomic-scale dislocation or domain wall induced by such “unfavorably” paired adatoms. The microscopic characteristics of the dynamic fluctuation are explained in terms of a hopping solitonic domain wall, and a local potential for this motion imposed by the adatoms is quantified.
Surface diffusion in reversed-phase liquid chromatography
Miyabe, Kanji; Guiochon, Georges A
2010-01-01
More than 40 years ago, Giddings pointed out in 'Dynamics of Chromatography' that surface diffusion should become an important research topic in the kinetics of chromatographic phenomena. However, few studies on surface diffusion in adsorbents used in chromatography were published since then. Most scientists use ordinary rate equations to study mass transfer kinetics in chromatography. They take no account of surface diffusion and overlook the significant contributions of this mass transfer process to chromatographic behavior and to column efficiency at high mobile phase flow rate. Only recently did the significance of surface diffusion in separation processes begin to be recognized in connection with the development of new techniques of fast flow, high efficiency chromatography. In this review, we revisit the reports on experimental data on surface diffusion and introduce a surface-restricted molecular diffusion model, derived as a first approximation for the mechanism of surface diffusion, on the basis of the absolute rate theory. We also explain how this model accounts for many intrinsic characteristics of surface diffusion that cannot properly be explained by the conventional models of surface diffusion.
Surface diffusion and surface crystal growth of tris-naphthyl benzene glasses
NASA Astrophysics Data System (ADS)
Ruan, Shigang; Zhang, Wei; Sun, Ye; Ediger, M. D.; Yu, Lian
2016-08-01
Surface self-diffusion coefficients of α,α,β-tris-naphthyl benzene (TNB) glasses have been measured using the method of surface grating decay. For 1000 nm wavelength gratings, the decay occurs by viscous flow at temperatures above Tg + 15 K, where Tg is the glass transition temperature (347 K), and by surface diffusion at lower temperatures. Surface diffusion of TNB is vastly faster than bulk diffusion, by a factor of 107 at Tg. Comparing TNB with other molecular glasses, each evaluated at its own Tg, we find that surface diffusion has a greater system-to-system variation than bulk diffusion, slowing down with increasing molecular size and intermolecular hydrogen bonding. Experimentally determined surface diffusion coefficients are in reasonable agreement with those from simulations and theoretical predictions. TNB and other molecular glasses show fast crystal growth on the free surface and the growth velocity is nearly proportional to the surface diffusion coefficient, indicating that the process is supported by surface mobility.
Edge effects on the characteristics of uranium diffusion on graphene and graphene nanoribbons
NASA Astrophysics Data System (ADS)
Cheng, Cheng; Han, Han; Ren, Cui-Lan; Wang, Chang-Ying; Shao, Kuan; Huai, Ping
2016-08-01
The first principles density-functional theoretical calculations of U adatom adsorption and diffusion on a planar graphene and quasi-one-dimensional graphene nanoribbons (GNRs) are performed. An energetic preference is found for U adatom diffusing to the hollow sites of both graphene and GNRs surface. A number of U distinctive diffusion paths either perpendicular or parallel to the ribbon growth direction are examined. The edge effects are evidenced by the calculated energy barriers of U adatom diffusion on armchair and zigzag nanoribbons surfaces. The calculation results indicate that the diffusion of U adatom from the inner site toward the edge site is a feasible process, particularly in zigzag GNR. It is viable to control the initial morphology of nuclear carbon material to retard the diffusion and concentration of nuclides. Project supported by the International S & T Cooperation Program of China (Grant No. 2014DFG60230), the National Natural Science Foundation of China (Grant Nos. 91326105, 21306220, and 21501189), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA02040104).
Spiral Surface Growth without Desorption
NASA Astrophysics Data System (ADS)
Karma, Alain; Plapp, Mathis
1998-11-01
Spiral surface growth is well understood in the limit where the step motion is controlled by the local supersaturation of adatoms near the spiral ridge. In epitaxial thin-film growth, however, spirals can form in a step-flow regime where desorption of adatoms is negligible and the ridge dynamics is governed by the nonlocal diffusion field of adatoms on the whole surface. We investigate this limit numerically using a phase-field formulation of the Burton-Cabrera-Frank model, as well as analytically. Quantitative predictions, which differ strikingly from those of the local limit, are made for the selected step spacing as a function of the deposition flux, as well as for the dependence of the relaxation time to steady-state growth on the screw dislocation density.
Mechanisms of Si and Ge diffusion on surfactant terminated (111) silicon and germanium surfaces
NASA Astrophysics Data System (ADS)
Zhachuk, R.; Coutinho, J.
2016-05-01
Surfactant mediated growth of Ge layers and formation of small Ge clusters on Si(111) are promising assemblage processes with envisioned applications in areas such as nanoelectronics or photovoltaics. They critically depend on migration of Si and Ge adatoms on surfactant terminated Si(111) and Ge(111) surfaces. We address Si and Ge adsorption and migration on surfactant (Bi, Sb) terminated (111) surfaces of Si and strained Ge by comprehensively mapping potential energy surfaces using density functional calculations. The main migration paths are identified and corresponding energy barriers are reported. It is shown that the energy barrier for adatom migration through Bi or Sb surfactant trimers (by actually breaking the trimers) is virtually degenerate to the mechanism involving traveling of the adatom around the same surfactant structures. We also find a low-energy anchoring site that is suggested to act as a nucleation structure and to trigger the clustering process. These results suggest a fundamentally new picture for the whole Si(111)sbnd Bi(Sb) epitaxial process.
Magnetism of C adatoms on BN nanostructures: implications for functional nanodevices.
Li, Jia; Zhou, Gang; Chen, Ying; Gu, Bing-Lin; Duan, Wenhui
2009-02-11
Spin-polarized density functional calculations reveal that magnetism can be induced by carbon adatoms on boron nitride nanotubes (BNNTs) and BN hexagonal sheets. As a result of the localization of impurity states, these hybrid sp-electron systems are spin-polarized, with a local magnetic moment of 2.0 mu(B) per C adatom regardless of the tube diameter and the bonding between the C atom and the BNNTs/BN sheets. An analysis of orbital hybridization indicates that two valence electrons participate in the bonding and the remaining two electrons of the C adatom are confined at the adsorption site and contribute to the magnetism accordingly. The effective interaction distance between the C-induced magnetic moments is evaluated. In terms of the diffusion barrier and the adsorption energy of C adatoms on the BN nanotubes/sheets, a fabrication method for BN-C-based functional nanodevices is proposed, and a series of virtual building blocks for functional devices are illustrated. PMID:19154109
Effects of extrinsic point defects in phosphorene: B, C, N, O, and F adatoms
Wang, Gaoxue E-mail: pandey@mtu.edu Pandey, Ravindra E-mail: pandey@mtu.edu; Karna, Shashi P. E-mail: pandey@mtu.edu
2015-04-27
Phosphorene is emerging as a promising 2D semiconducting material with a direct band gap and high carrier mobility. In this paper, we examine the role of the extrinsic point defects including surface adatoms in modifying the electronic properties of phosphorene using density functional theory. The surface adatoms considered are B, C, N, O, and F with a [He] core electronic configuration. Our calculations show that B and C, with electronegativity close to P, prefer to break the sp{sup 3} bonds of phosphorene and reside at the interstitial sites in the 2D lattice by forming sp{sup 2} like bonds with the native atoms. On the other hand, N, O, and F, which are more electronegative than P, prefer the surface sites by attracting the lone pairs of phosphorene. B, N, and F adsorption will also introduce local magnetic moment to the lattice. Moreover, B, C, N, and F adatoms will modify the band gap of phosphorene, yielding metallic transverse tunneling characters. Oxygen does not modify the band gap of phosphorene, and a diode like tunneling behavior is observed. Our results therefore offer a possible route to tailor the electronic and magnetic properties of phosphorene by the adatom functionalization and provide the physical insights of the environmental sensitivity of phosphorene, which will be helpful to experimentalists in evaluating the performance and aging effects of phosphorene-based electronic devices.
Effective diffusion in the region between two surfaces
NASA Astrophysics Data System (ADS)
Valero Valdes, Carlos
2016-08-01
The purpose of this paper is to provide a formula for the effective diffusion operator obtained by projecting the three-dimensional diffusion equation onto a two-dimensional plane, assuming reflective boundary conditions at two surfaces in three-dimensional space and a finite transversal stabilization rate.
Effective diffusion in the region between two surfaces.
Valero Valdes, Carlos
2016-08-01
The purpose of this paper is to provide a formula for the effective diffusion operator obtained by projecting the three-dimensional diffusion equation onto a two-dimensional plane, assuming reflective boundary conditions at two surfaces in three-dimensional space and a finite transversal stabilization rate. PMID:27627260
Ion beam microtexturing and enhanced surface diffusion
NASA Technical Reports Server (NTRS)
Robinson, R. S.
1982-01-01
Ion beam interactions with solid surfaces are discussed with particular emphasis on microtexturing induced by the deliberate deposition of controllable amounts of an impurity material onto a solid surface while simultaneously sputtering the surface with an ion beam. Experimental study of the optical properties of microtextured surfaces is described. Measurements of both absorptance as a function of wavelength and emissivity are presented. A computer code is described that models the sputtering and ion reflection processes involved in microtexture formation.
Effect of bulk aging on surface diffusion of glasses
NASA Astrophysics Data System (ADS)
Brian, Caleb W.; Zhu, Lei; Yu, Lian
2014-02-01
The effect of physical aging on surface diffusion has been determined for two organic glasses, Indomethacin and Nifedipine. The two systems exhibit similar aging kinetics typical of organic glasses. Surface diffusivity remains unchanged despite significant bulk aging that nearly equilibrates the systems and increases the bulk relaxation time by orders of magnitude. The finding is relevant for understanding the stability of amorphous materials and the formation of low-energy glasses by vapor deposition.
Curved and diffuse interface effects on the nuclear surface tension
NASA Astrophysics Data System (ADS)
Kolomietz, V. M.; Lukyanov, S. V.; Sanzhur, A. I.
2012-08-01
We redefine the surface tension coefficient for a nuclear Fermi-liquid drop with a finite diffuse layer. Following the Gibbs-Tolman concept, we introduce the equimolar radius Re of the droplet surface at which the surface tension is applied and the radius of tension surface Rs which provides the minimum of the surface tension coefficient σ. This procedure allows us to derive both the surface tension and the corresponding curvature correction (Tolman length) correctly for the curved and diffuse interface. We point out that the curvature correction depends significantly on the finite diffuse interface. We show that Tolman's length ξ is negative for a nuclear Fermi-liquid drop. The value of the Tolman length is only slightly sensitive to the Skyrme force parametrization and equals ξ=-0.36 fm.
Hydrogen Bonding Slows Down Surface Diffusion of Molecular Glasses.
Chen, Yinshan; Zhang, Wei; Yu, Lian
2016-08-18
Surface-grating decay has been measured for three organic glasses with extensive hydrogen bonding: sorbitol, maltitol, and maltose. For 1000 nm wavelength gratings, the decay occurs by viscous flow in the entire range of temperature studied, covering the viscosity range 10(5)-10(11) Pa s, whereas under the same conditions, the decay mechanism transitions from viscous flow to surface diffusion for organic glasses of similar molecular sizes but with no or limited hydrogen bonding. These results indicate that extensive hydrogen bonding slows down surface diffusion in organic glasses. This effect arises because molecules can preserve hydrogen bonding even near the surface so that the loss of nearest neighbors does not translate into a proportional decrease of the kinetic barrier for diffusion. This explanation is consistent with a strong correlation between liquid fragility and the surface enhancement of diffusion, both reporting resistance of a liquid to dynamic excitation. Slow surface diffusion is expected to hinder any processes that rely on surface transport, for example, surface crystal growth and formation of stable glasses by vapor deposition. PMID:27404465
Diffusion on (110) Surface of Molecular Crystal Pentaerythritol Tetranitrate
Wang, J; Golfinopoulos, T; Gee, R H; Huang, H
2007-01-25
Using classical molecular dynamics simulations, we investigate the diffusion mechanisms of admolecules on the (110) surface of molecular crystal pentaerythritol tetranitrate. Our results show that (1) admolecules are stable at off lattice sites, (2) admolecules diffuse along close-packed [1{bar 1}1] and [{bar 1}11] directions, and (3) admolecules detach from the surface at 350K and above. Based on the number of diffusion jumps as a function of temperature, we estimate the jump frequency to be v=1.14 x 10{sup 12} e{sup -0.08eV/kT} per second.
Chern mosaic: Topology of chiral superconductivity on ferromagnetic adatom lattices
NASA Astrophysics Data System (ADS)
Röntynen, Joel; Ojanen, Teemu
2016-03-01
In this work, we will explore the properties of superconducting surfaces decorated by two-dimensional ferromagnetic adatom lattices. As discovered recently [Röntynen and Ojanen, Phys. Rev. Lett. 114, 236803 (2015), 10.1103/PhysRevLett.114.236803], in the presence of a Rashba spin-orbit coupling these systems may support topological superconductivity with complex phase diagrams and high Chern numbers. We show how the long-range hopping nature of the effective low-energy theory generically gives rise to a phase diagram covered by a Chern mosaic, a rich pattern of topological phases with large Chern numbers. We study different lattice geometries and the dependence of energy gaps and abundance of different phases as a function of system parameters. Our findings establish the studied system as one of the richest platforms for topological matter known to date.
A Multiscale Analysis of Diffusions on Rapidly Varying Surfaces
NASA Astrophysics Data System (ADS)
Duncan, A. B.; Elliott, C. M.; Pavliotis, G. A.; Stuart, A. M.
2015-04-01
Lateral diffusion of molecules on surfaces plays a very important role in various biological processes, including lipid transport across the cell membrane, synaptic transmission, and other phenomena such as exo- and endocytosis, signal transduction, chemotaxis, and cell growth. In many cases, the surfaces can possess spatial inhomogeneities and/or be rapidly changing shape. Using a generalization of the model for a thermally excited Helfrich elastic membrane, we consider the problem of lateral diffusion on quasi-planar surfaces, possessing both spatial and temporal fluctuations. Using results from homogenization theory, we show that, under the assumption of scale separation between the characteristic length and timescales of the membrane fluctuations and the characteristic scale of the diffusing particle, the lateral diffusion process can be well approximated by a Brownian motion on the plane with constant diffusion tensor that depends on a highly nonlinear way on the detailed properties of the surface. The effective diffusion tensor will depend on the relative scales of the spatial and temporal fluctuations, and for different scaling regimes, we prove the existence of a macroscopic limit in each case.
Gavrilenko, V. I.; Wu, R. Q.; Downer, M. C.; Ekerdt, J. G.; Lim, D.; Parkinson, P.
2001-04-15
We present calculated second-harmonic-generation (SHG) spectra of the Si(001) surface based on a first-principles description of eigenvalues and eigenvectors using ab initio pseudopotentials. We also present SHG spectra for Ge-covered Si(001). The theoretical results explain all essential features of recent experimental SHG spectra of the Si(001)-(2x1) surface with low coverages of hydrogen and/or germanium, which alter the E{sub 1} resonance in contrasting ways. The strong adatom specificity of the spectra results from redistribution of the adatom-related electronic states on the surface.
Drastic effect of V film orientation on the Fe adatoms magnetism
NASA Astrophysics Data System (ADS)
Yartseva, N. S.; Yartsev, S. V.; Demangeat, C.
2015-11-01
Effect of surface orientation of nonmagnetic bed material on magnetic properties of the peculiar magnetic adatoms groups (MAGs) is found by simulation. Here we present the results of periodic Anderson model calculations for MAGs on V. The MAGs are formed of the Fe adatoms arranged in triangles, ovals, or short chains and placed over V substrate with (001) or (110) surface orientation. It is shown that magnetism of the Fe-MAGs on V(001) surface can be totally suppressed by the V surroundings, whereas the V(110) surface orientation results in magnetization of the Fe-MAGs and onset of noncollinear atomic moments distribution. Noncollinearity strictly depends on symmetry of the Fe-MAG.
Surface reaction and pore diffusion in flow-tube reactors
NASA Technical Reports Server (NTRS)
Keyser, Leon F.; Moore, Steven B.; Leu, Ming-Taun
1991-01-01
The interaction of gas diffusion with surface reaction in porous solids is discussed and applied specifically to heterogeneous rate measurements in flow-tube reactors. External diffusion to the outer surface of a reactive solid, internal diffusion within the pores, surface reaction, and laminar flow are considered. A procedure is developed to correct observed surface rate constants for the interaction of these processes. Measured surface areas and bulk densities are used to construct a semiempirical model for porous diffusion in vapor-formed HNO3-H2O ices which are used to simulate polar stratospheric cloud surfaces. The model is tested experimentally by varying the thickness of these ices from about 15 to 120 microns. The results are consistent with the model predictions and show that the HNO3-H2O ices used are highly porous, and the internal surface must be considered in calculating kinetic parameters from observed loss rates. The best fit of the data yields a tortuosity factor of 3.3 +/-1.1 for the ice substrates.
Germanium nanowire growth controlled by surface diffusion effects
Schmidtbauer, Jan; Bansen, Roman; Heimburger, Robert; Teubner, Thomas; Boeck, Torsten; Fornari, Roberto
2012-07-23
Germanium nanowires (NWs) were grown onto Ge(111) substrates by the vapor-liquid-solid process using gold droplets. The growth was carried out in a molecular beam epitaxy chamber at substrate temperatures between 370 Degree-Sign C and 510 Degree-Sign C. The resulting nanowire growth rate turns out to be highly dependent on the substrate temperature exhibiting the maximum at T = 430 Degree-Sign C. The temperature dependence of growth rate can be attributed to surface diffusion both along the substrate and nanowire sidewalls. Analyzing the diffusive material transport yields a diffusion length of 126 nm at a substrate temperature of 430 Degree-Sign C.
II. Inhibited Diffusion Driven Surface Transmutations
NASA Astrophysics Data System (ADS)
Chubb, Talbot A.
2006-02-01
This paper is the second of a set of three papers dealing with the role of coherent partitioning as a common element in Low Energy Nuclear Reactions (LENR), by which is meant cold-fusion related processes. This paper discusses the first step in a sequence of four steps that seem to be necessary to explain Iwamura 2-α-addition surface transmutations. Three concepts are examined: salt-metal interface states, sequential tunneling that transitions D+ ions from localized interstitial to Bloch form, and the general applicability of 2-dimensional vs. 3-dimensional symmetry hosting networks.
Studies of surface diffusion by second harmonic fluctuation spectroscopy
Zhao, Xiaolin; Goh, M.C.; Subrahmanyan, S.; Eisenthal, K.B. )
1990-05-03
The authors have shown how the fluctuations in the signal from surface second harmonic generation can be utilized for the study of a heterogeneous surface such as palmitic acid (C{sub 15}H{sub 31}COOH) spread on the air/water interface, under conditions of gas-liquid coexistence. The authors report observations of time-correlated fluctuations in the SH signal, with decay constant of approximately 6 s. This is attributed to motions of the liquidlike clusters of palmitic acid. If the motion is diffusive, a diffusion constant of about 10{sup {minus}8} cm{sup 2}/s is estimated for these clusters.
Effects of surface diffusion on high temperature selective emitters.
Peykov, Daniel; Yeng, Yi Xiang; Celanovic, Ivan; Joannopoulos, John D; Schuh, Christopher A
2015-04-20
Using morphological and optical simulations of 1D tantalum photonic crystals at 1200K, surface diffusion was determined to gradually reduce the efficiency of selective emitters. This was attributed to shifting resonance peaks and declining emissivity caused by changes to the cavity dimensions and the aperture width. Decreasing the structure's curvature through larger periods and smaller cavity widths, as well as generating smoother transitions in curvature through the introduction of rounded cavities, was found to alleviate this degradation. An optimized structure, that shows both high efficiency selective emissivity and resistance to surface diffusion, was presented. PMID:25969039
In situ laser reflectance measurement of diffuse surfaces.
Chan, W S; Khan, S U
1978-08-01
Report is made on an in situ method of laser reflectance measurement of diffuse surfaces by using a GaAs laser and off-the-shelf optical components not involving radiation integration over a hemisphere as with most conventional reflectometers. The design features and limitations are described. Several diffuse surfaces were evaluated by this method, and the reflectance results obtained were in good agreement with those determined by the method of integrating sphere that used MgCO(3) surface as a standard. The main advantages of this method are: the elimination of the need of a surface standard; the avoidance of having the surfaces in close contact with the measuring equipment; the accuracy better than 10%; and the relatively straightforward alignment. PMID:20203783
Control of gold surface diffusion on si nanowires.
den Hertog, Martien I; Rouviere, Jean-Luc; Dhalluin, Florian; Desré, Pierre J; Gentile, Pascal; Ferret, Pierre; Oehler, Fabrice; Baron, Thiery
2008-05-01
Silicon nanowires (NW) were grown by the vapor-liquid-solid mechanism using gold as the catalyst and silane as the precursor. Gold from the catalyst particle can diffuse over the wire sidewalls, resulting in gold clusters decorating the wire sidewalls. The presence or absence of gold clusters was observed either by high angle annular darkfield scanning transmission electron microscopy images or by scanning electron microscopy. We find that the gold surface diffusion can be controlled by two growth parameters, the silane partial pressure and the growth temperature, and that the wire diameter also affects gold diffusion. Gold clusters are not present on the NW side walls for high silane partial pressure, low temperature, and small NW diameters. The absence or presence of gold on the NW sidewall has an effect on the sidewall morphology. Different models are qualitatively discussed. The main physical effect governing gold diffusion seems to be the adsorption of silane on the NW sidewalls. PMID:18422363
Atomic diffusion in laser surface modified AISI H13 steel
NASA Astrophysics Data System (ADS)
Aqida, S. N.; Brabazon, D.; Naher, S.
2013-07-01
This paper presents a laser surface modification process of AISI H13 steel using 0.09 and 0.4 mm of laser spot sizes with an aim to increase surface hardness and investigate elements diffusion in laser modified surface. A Rofin DC-015 diffusion-cooled CO2 slab laser was used to process AISI H13 steel samples. Samples of 10 mm diameter were sectioned to 100 mm length in order to process a predefined circumferential area. The parameters selected for examination were laser peak power, pulse repetition frequency (PRF), and overlap percentage. The hardness properties were tested at 981 mN force. Metallographic study and energy dispersive X-ray spectroscopy (EDXS) were performed to observe presence of elements and their distribution in the sample surface. Maximum hardness achieved in the modified surface was 1017 HV0.1. Change of elements composition in the modified layer region was detected in the laser modified samples. Diffusion possibly occurred for C, Cr, Cu, Ni, and S elements. The potential found for increase in surface hardness represents an important method to sustain tooling life. The EDXS findings signify understanding of processing parameters effect on the modified surface composition.
Complexity and diffusion of magnetic flux surfaces in anisotropic turbulence
Servidio, S.; Matthaeus, W. H.; Wan, M.; Rappazzo, A. F.; Ruffolo, D.; Oughton, S.
2014-04-10
The complexity of magnetic flux surfaces is investigated analytically and numerically in static homogeneous magnetic turbulence. Magnetic surfaces are computed to large distances in magnetic fields derived from a reduced magnetohydrodynamic model. The question addressed is whether one can define magnetic surfaces over large distances when turbulence is present. Using a flux surface spectral analysis, we show that magnetic surfaces become complex at small scales, experiencing an exponential thinning that is quantified here. The computation of a flux surface is of either exponential or nondeterministic polynomial complexity, which has the conceptual implication that global identification of magnetic flux surfaces and flux exchange, e.g., in magnetic reconnection, can be intractable in three dimensions. The coarse-grained large-scale magnetic flux experiences diffusive behavior. The link between the diffusion of the coarse-grained flux and field-line random walk is established explicitly through multiple scale analysis. The Kubo number controls both large and small scale limits. These results have consequences for interpreting processes such as magnetic reconnection and field-line diffusion in astrophysical plasmas.
Diffusion in Entangled and Surface Modified Polymer Systems
NASA Astrophysics Data System (ADS)
Tead, Stanley Fromm
Ion beam analysis techniques were used to measure the concentration vs. depth profiles of deuterium labelled polymer molecules in unlabeled and/or chemically dissimilar polymer melts, for several polymer systems. In the first polymer system, the tracer diffusion coefficient of deuterated polystyrene (d-PS) molecules was measured in polystyrene (PS) matrices which were blends of high molecular weight (volume fraction phi) and low molecular weight PS. The dependence of the d-PS diffusion coefficient on phi was quantitatively predicted by the models of reptation and constraint release. In the second polymer system, diffusion of polystyrenes was studied with ring shaped chains substituted (instead of linear ones) in the tracer and/or matrix roles. The diffusion of linear tracers into ring matrices was nearly identical to linear tracer diffusion in linear matrices, a result not predicted by any current theories. Dry etching of polystyrene by four different ion and plasma methods crosslinked the exposed surface monolayer, immobilizing it and reducing its permeability to diffusion by unetched tracer molecules. The integrated thickness of the immobile layer is decreased for an increased ratio of chain scission to crosslinking. The ratio is smallest for reactive ion beam etching, intermediate for reactive ion etching, and largest for the pure plasma techniques. Diffusion was investigated in systems of the polymer polyimide (PI), produced by the imidization of polyamic acid at a temperature T_{rm i }. The effects of thermal processing (imidization) of the polymer and exposure to solvents were studied. The diffusion of deuterated polyamic acid in PI was reduced to negligible levels for T_{rm i}'s at or above 200^circ C; purely thermally activated diffusion (in the absence of solvents) was not seen for any combination of annealing temperatures up to 400^circ C. Ion beam analysis methods were developed to measure the kinetics and depth dependence of the imidization reaction in
Creep rate induced by surface diffusion of porous media
NASA Astrophysics Data System (ADS)
Wang, Y. C.; Li, Y. D.; Wang, X.
2016-01-01
Holes in materials can cause improved or unique performance of the material when the sizes, shapes, and orientation of holes as well as grains are controlled in materials. In the paper, a computational method for creep rate induced by hole surface diffusion of porous materials is presented. The driven force for diffusional mass transport along the hole surface is the surface diffusion energy of hole and the strain energy acting on the surface, which is obtained from rigorous elastic theory. In order to apply the present solution to the realistic porous materials the scale effect is considered by using finite element method based on two-dimensional unit cell for porous materials under uniaxial tension.
NASA Astrophysics Data System (ADS)
Gosálvez, Miguel A.; Otrokov, Mikhail M.; Ferrando, Nestor; Ryabishchenkova, Anastasia G.; Ayuela, Andres; Echenique, Pedro M.; Chulkov, Evgueni V.
2016-02-01
This is the first of two papers that introduce a general expression for the tracer diffusivity in complex, periodic energy landscapes with M distinct hop rates in one-, two-, and three-dimensional diluted systems (low-coverage, single-tracer limit). The present report focuses on the analysis of diffusion in systems where the end sites of the hops are located symmetrically with respect to the hop origins (symmetric hops), as encountered in many ideal surfaces and bulk materials. For diffusion in two dimensions, a number of formulas are presented for complex combinations of the different hops in systems with triangular, rectangular, and square symmetry. The formulas provide values in excellent agreement with kinetic Monte Carlo simulations, concluding that the diffusion coefficient can be directly determined from the proposed expressions without performing the simulations. Based on the diffusion barriers obtained from first-principles calculations and a physically meaningful estimate of the attempt frequencies, the proposed formulas are used to analyze the diffusion of Cu, Ag, and Rb adatoms on the surface and within the van der Waals (vdW) gap of a model topological insulator, Bi2Se3 . Considering the possibility of adsorbate intercalation from the terraces to the vdW gaps at morphological steps, we infer that, at low coverage and room temperature, (i) a majority of the Rb atoms bounce back at the steps and remain on the terraces, (ii) Cu atoms mostly intercalate into the vdW gap, the remaining fraction staying at the steps, and (iii) Ag atoms essentially accumulate at the steps and gradually intercalate into the vdW gap. These conclusions are in good qualitative agreement with previous experiments. The companion report (M. A. Gosálvez et al., Phys. Rev. B, submitted] extends the present study to the description of systems that contain asymmetric hops.
Surface self-diffusion of silicon during high temperature annealing
Acosta-Alba, Pablo E.; Kononchuk, Oleg; Gourdel, Christophe; Claverie, Alain
2014-04-07
The atomic-scale mechanisms driving thermally activated self-diffusion on silicon surfaces are investigated by atomic force microscopy. The evolution of surface topography is quantified over a large spatial bandwidth by means of the Power Spectral Density functions. We propose a parametric model, based on the Mullins-Herring (M-H) diffusion equation, to describe the evolution of the surface topography of silicon during thermal annealing. Usually, a stochastic term is introduced into the M-H model in order to describe intrinsic random fluctuations of the system. In this work, we add two stochastic terms describing the surface thermal fluctuations and the oxidation-evaporation phenomenon. Using this extended model, surface evolution during thermal annealing in reducing atmosphere can be predicted for temperatures above the roughening transition. A very good agreement between experimental and theoretical data describing roughness evolution and self-diffusion phenomenon is obtained. The physical origin and time-evolution of these stochastic terms are discussed. Finally, using this model, we explore the limitations of the smoothening of the silicon surfaces by rapid thermal annealing.
Surface self-diffusion of silicon during high temperature annealing
NASA Astrophysics Data System (ADS)
Acosta-Alba, Pablo E.; Kononchuk, Oleg; Gourdel, Christophe; Claverie, Alain
2014-04-01
The atomic-scale mechanisms driving thermally activated self-diffusion on silicon surfaces are investigated by atomic force microscopy. The evolution of surface topography is quantified over a large spatial bandwidth by means of the Power Spectral Density functions. We propose a parametric model, based on the Mullins-Herring (M-H) diffusion equation, to describe the evolution of the surface topography of silicon during thermal annealing. Usually, a stochastic term is introduced into the M-H model in order to describe intrinsic random fluctuations of the system. In this work, we add two stochastic terms describing the surface thermal fluctuations and the oxidation-evaporation phenomenon. Using this extended model, surface evolution during thermal annealing in reducing atmosphere can be predicted for temperatures above the roughening transition. A very good agreement between experimental and theoretical data describing roughness evolution and self-diffusion phenomenon is obtained. The physical origin and time-evolution of these stochastic terms are discussed. Finally, using this model, we explore the limitations of the smoothening of the silicon surfaces by rapid thermal annealing.
Echo thresholds for reflections from acoustically diffusive architectural surfaces.
Robinson, Philip W; Walther, Andreas; Faller, Christof; Braasch, Jonas
2013-10-01
When sound reflects from an irregular architectural surface, it spreads spatially and temporally. Extensive research has been devoted to prediction and measurement of diffusion, but less has focused on its perceptual effects. This paper examines the effect of temporal diffusion on echo threshold. There are several notable differences between the waveform of a reflection identical to the direct sound and one from an architectural surface. The onset and offset are damped and the energy is spread in time; hence, the reflection response has a lower peak amplitude, and is decorrelated from the direct sound. The perceptual consequences of these differences are previously undocumented. Echo threshold tests are conducted with speech and music signals, using direct sound and a simulated reflection that is either identical to the direct sound or has various degrees of diffusion. Results indicate that for a speech signal, diffuse reflections are less easily detectable as a separate auditory event than specular reflections of the same total energy. For a music signal, no differences are observed between the echo thresholds for reflections with and without temporal diffusion. Additionally, echo thresholds are found to be shorter for speech than for music, and shorter for spatialized than for diotic presentation of signals. PMID:24116414
Diffusion and surface excess of a confined nanoswimmer dispersion
NASA Astrophysics Data System (ADS)
Xiao, Song; Wang, Zhengjia; Chen, Hsuan-Yi; Sheng, Yu-Jane; Tsao, Heng-Kwong
2014-11-01
The diffusivity and surface excess of nanoswimmers which are confined in two plates with the separation H are explored by dissipative particle dynamics. Both mean squared displacement and velocity autocorrelation function methods are used to study the diffusive behavior of nanoswimmers with the Brownian diffusivity D0 and the results obtained from both methods are consistent. The active diffusivity of confined nanoswimmers (D - D0) depends on the wall separation, swimming speed va, and run time τ. Our simulation results show that (D-D0)/va2τ is a function of vaτ/H. The reduction in the diffusivity of active colloids is more significant than that of passive particles. The distribution of nanoswimmers between two parallel walls is acquired and two regions can be identified. The accumulation of nanoswimmers near walls is quantitatively described by the surface excess Γ. It is found that Γ grows as the nanoswimmer concentration cb, swimming speed va, and run time τ are increased. The coupling between the ballistic trajectory of nanoswimmers and the walls results in nanoswimmer accumulation. The simulation outcomes indicate that Γ/Hcb is a function of H/vaτ.
NASA Astrophysics Data System (ADS)
Etzkorn, M.; Hirjibehedin, C. F.; Lehnert, A.; Ouazi, S.; Rusponi, S.; Stepanow, S.; Gambardella, P.; Tieg, C.; Thakur, P.; Lichtenstein, A. I.; Shick, A. B.; Loth, S.; Heinrich, A. J.; Brune, H.
2015-11-01
We report on the magnetic properties of Fe and Co adatoms on a Cu2N /Cu(100 ) -c (2 ×2 ) surface investigated by x-ray magnetic dichroism measurements and density functional theory (DFT) calculations including the local coulomb interaction. We compare these results with properties formerly deduced from STM spin excitation spectroscopy (SES) performed on the individual adatoms. In particular we focus on the values of the local magnetic moments determined by XMCD compared to the expectation values derived from the description of the SES data. The angular dependence of the projected magnetic moments along the magnetic field, as measured by XMCD, can be understood on the basis of the SES Hamiltonian. In agreement with DFT, the XMCD measurements show large orbital contributions to the total magnetic moment for both magnetic adatoms.
Fano fingerprints of Majoranas in Kitaev dimers of superconducting adatoms
NASA Astrophysics Data System (ADS)
Dessotti, F. A.; Ricco, L. S.; Marques, Y.; Machado, R. S.; Guessi, L. H.; Figueira, M. S.; de Souza, M.; Seridonio, A. C.
2016-09-01
We investigate theoretically a Fano interferometer composed by STM and AFM tips close to a Kitaev dimer of superconducting adatoms, in which the adatom placed under the AFM tip, encloses a pair of Majorana fermions (MFs). For the binding energy Δ of the Cooper pair delocalized into the adatoms under the tips coincident with the tunneling amplitude t between them, namely Δ=t, we find that only one MF beneath the AFM tip hybridizes with the adatom coupled to the STM tips. As a result, a gate invariance feature emerges: the Fano profile of the transmittance rises as an invariant quantity depending upon the STM tips Fermi energy, due to the symmetric swap in the gate potential of the AFM tip.
Surfactant-Modified Diffusion on Transition-Metal Surfaces
FEIBELMAN,PETER J.; KELLOGG,GARY LEE
1999-12-01
Wanting to convert surface impurities from a nuisance to a systematically applicable nano-fabrication tool, we have sought to understand how such impurities affect self-diffusion on transition-metal surfaces. Our field-ion microscope experiments reveal that in the presence of surface hydrogen, self-diffusion on Rh(100) is promoted, while on Pt(100), not only is it inhibited, but its mechanism changes. First-principles calculations aimed at learning how oxygen fosters perfect layerwise growth on a growing Pt(111) crystal contradict the idea in the literature that it does so by directly promoting transport over Pt island boundaries. The discovery that its real effect is to burn off adventitious adsorbed carbon monoxide demonstrates the predictive value of state-of-the-art calculation methods.
Diffusion and Phase Transformations of Transition Metals on Silicon Surfaces
NASA Astrophysics Data System (ADS)
Lee, Ming-Yi.
The role of surface diffusion and surface phase reaction kinetics of nickel (Ni) and cobalt (Co) on Si(111) and Si(100) are investigated under Ultra High Vacuum (UHV) conditions using Auger Spectroscopy (AES), Reflection High Electron Energy Diffraction (RHEED) and surface X-ray diffraction. The surface segregation phenomenon and the formation conditions for Si(111)-sqrt{19 } x sqrt{19}- rm R+/-23.4^circ phase (hereafter called sqrt{19}) for Ni/Si(111) are studied by RHEED and AES. Quench cooling induces surface segregation which restores the total accumulated dose of Ni to two surfaces of the wafer. The coverage dependence of phases thus produced follows: 7 x 7 to 1 x 1-RC(0.05Ml) to sqrt{19} (0.16Ml) then to B-type NiSi_2. It is found that there are 3 Ni atoms in the sqrt{19 } unit cell. A "race" of bulk diffusion versus surface diffusion for Ni in/on Si(111) is studied by depositing a laterally confined dot of metal on one side of the double side polished and UHV cleaned Si wafer and then measuring the lateral Auger profile on the reverse side following annealing and quenching. Ni reaches the far side of the wafer at temperatures as low as 500C via bulk diffusion with no measurable contribution from the surface paths, which are short-circuited by numerous, fast bulk paths. Similar results are found for Ni and Co on Si(111) and Si(100). The diffusivity and solid solubility calculated from the experiments are close to the bulk values known from the literature. In addition, the thermal stability, phase transformation and different dissolution mechanisms of sqrt {19} and 1 x 1-RC surface phases of Ni/Si(111) are carefully examined. The activation energies of these processes are compared on an Arrhenius plot. These are discussed in terms of the migration and formation mechanisms involved in these phase transformations. An energy level diagram is used to summarize the atomistic kinetics.
Si adatoms as catalyst for the growth of monolayer Al film on Si(111)
NASA Astrophysics Data System (ADS)
Teng, Jing; Zhang, Lixin; Wu, Kehui; Jiang, Ying; Guo, Jiandong; Guo, Qinlin; Ebert, Philipp; Sakurai, Toshio; Wang, Enge
2010-03-01
Recently, we reported the growth of atomically smooth Al(111) films on Si(111) with continuously controllable thickness down to the extreme level of 1 ML. Here, we study the underlying unexpected Si adatom-mediated clustering-melting mechanism by scanning tunneling microscopy and by the first-principles calculations. The Si adatoms in the initial Si(111)3x3-Al surface act as seeds to form SiAl2 clusters. The clusters are then transformed into Al(111)1x1 by incorporating further incoming Al atoms and spontaneously releasing the Si atoms, which then participate in the next cycle of the process. As a result, a two-dimensional growth of monolayer Al(111) is achieved.
Gallium surface diffusion on GaAs (001) surfaces measured by crystallization dynamics of Ga droplets
Bietti, Sergio Somaschini, Claudio; Esposito, Luca; Sanguinetti, Stefano; Fedorov, Alexey
2014-09-21
We present accurate measurements of Ga cation surface diffusion on GaAs surfaces. The measurement method relies on atomic force microscopy measurement of the morphology of nano–disks that evolve, under group V supply, from nanoscale group III droplets, earlier deposited on the substrate surface. The dependence of the radius of such nano-droplets on crystallization conditions gives direct access to Ga diffusion length. We found an activation energy for Ga on GaAs(001) diffusion E{sub A}=1.31±0.15 eV, a diffusivity prefactor of D₀=0.53(×2.1±1) cm² s⁻¹ that we compare with the values present in literature. The obtained results permit to better understand the fundamental physics governing the motion of group III ad–atoms on III–V crystal surfaces and the fabrication of designable nanostructures.
Cholesterol enhances surface water diffusion of phospholipid bilayers
Cheng, Chi-Yuan; Kausik, Ravinath; Han, Songi; Olijve, Luuk L. C.
2014-12-14
Elucidating the physical effect of cholesterol (Chol) on biological membranes is necessary towards rationalizing their structural and functional role in cell membranes. One of the debated questions is the role of hydration water in Chol-embedding lipid membranes, for which only little direct experimental data are available. Here, we study the hydration dynamics in a series of Chol-rich and depleted bilayer systems using an approach termed {sup 1}H Overhauser dynamic nuclear polarization (ODNP) NMR relaxometry that enables the sensitive and selective determination of water diffusion within 5–10 Å of a nitroxide-based spin label, positioned off the surface of the polar headgroups or within the nonpolar core of lipid membranes. The Chol-rich membrane systems were prepared from mixtures of Chol, dipalmitoyl phosphatidylcholine and/or dioctadecyl phosphatidylcholine lipid that are known to form liquid-ordered, raft-like, domains. Our data reveal that the translational diffusion of local water on the surface and within the hydrocarbon volume of the bilayer is significantly altered, but in opposite directions: accelerated on the membrane surface and dramatically slowed in the bilayer interior with increasing Chol content. Electron paramagnetic resonance (EPR) lineshape analysis shows looser packing of lipid headgroups and concurrently tighter packing in the bilayer core with increasing Chol content, with the effects peaking at lipid compositions reported to form lipid rafts. The complementary capability of ODNP and EPR to site-specifically probe the hydration dynamics and lipid ordering in lipid membrane systems extends the current understanding of how Chol may regulate biological processes. One possible role of Chol is the facilitation of interactions between biological constituents and the lipid membrane through the weakening or disruption of strong hydrogen-bond networks of the surface hydration layers that otherwise exert stronger repulsive forces, as reflected in
Cholesterol enhances surface water diffusion of phospholipid bilayers
NASA Astrophysics Data System (ADS)
Cheng, Chi-Yuan; Olijve, Luuk L. C.; Kausik, Ravinath; Han, Songi
2014-12-01
Elucidating the physical effect of cholesterol (Chol) on biological membranes is necessary towards rationalizing their structural and functional role in cell membranes. One of the debated questions is the role of hydration water in Chol-embedding lipid membranes, for which only little direct experimental data are available. Here, we study the hydration dynamics in a series of Chol-rich and depleted bilayer systems using an approach termed 1H Overhauser dynamic nuclear polarization (ODNP) NMR relaxometry that enables the sensitive and selective determination of water diffusion within 5-10 Å of a nitroxide-based spin label, positioned off the surface of the polar headgroups or within the nonpolar core of lipid membranes. The Chol-rich membrane systems were prepared from mixtures of Chol, dipalmitoyl phosphatidylcholine and/or dioctadecyl phosphatidylcholine lipid that are known to form liquid-ordered, raft-like, domains. Our data reveal that the translational diffusion of local water on the surface and within the hydrocarbon volume of the bilayer is significantly altered, but in opposite directions: accelerated on the membrane surface and dramatically slowed in the bilayer interior with increasing Chol content. Electron paramagnetic resonance (EPR) lineshape analysis shows looser packing of lipid headgroups and concurrently tighter packing in the bilayer core with increasing Chol content, with the effects peaking at lipid compositions reported to form lipid rafts. The complementary capability of ODNP and EPR to site-specifically probe the hydration dynamics and lipid ordering in lipid membrane systems extends the current understanding of how Chol may regulate biological processes. One possible role of Chol is the facilitation of interactions between biological constituents and the lipid membrane through the weakening or disruption of strong hydrogen-bond networks of the surface hydration layers that otherwise exert stronger repulsive forces, as reflected in faster
Cluster Nucleation and Growth from a Highly Supersaturated Adatom Phase: Silver on Magnetite
2014-01-01
The atomic-scale mechanisms underlying the growth of Ag on the (√2×√2)R45°-Fe3O4(001) surface were studied using scanning tunneling microscopy and density functional theory based calculations. For coverages up to 0.5 ML, Ag adatoms populate the surface exclusively; agglomeration into nanoparticles occurs only with the lifting of the reconstruction at 720 K. Above 0.5 ML, Ag clusters nucleate spontaneously and grow at the expense of the surrounding material with mild annealing. This unusual behavior results from a kinetic barrier associated with the (√2×√2)R45° reconstruction, which prevents adatoms from transitioning to the thermodynamically favorable 3D phase. The barrier is identified as the large separation between stable adsorption sites, which prevents homogeneous cluster nucleation and the instability of the Ag dimer against decay to two adatoms. Since the system is dominated by kinetics as long as the (√2×√2)R45° reconstruction exists, the growth is not well described by the traditional growth modes. It can be understood, however, as the result of supersaturation within an adsorption template system. PMID:24945923
Fractal Aspects and Diffusion In The Ocean Surface
NASA Astrophysics Data System (ADS)
Redondo, J. M.; Gade, M.
An important aspect of observations of the ocean surface by satellite is the ability of the sensors, both passive (Colour or IR) or active (SAR), to detect dinamical features that analysed spatially and temporally may be used to model and predict the complex topology of the ocean eddies and currents. Several ways of evaluating the horizontal turbulent diffusivities from the geometrical characteristics of the flow are presented, discussing the relevance of Richardson's law in different weather conditions. Multi- fractal aspects related to the surface roughness of the ocean, and the integral or corre- lation length-scales are used to identify several distinct features and thus, to provide better estimates of local turbulent diffusion in the ocean.
Correlated diffuse x-ray scattering from periodically nanostructured surfaces
NASA Astrophysics Data System (ADS)
Soltwisch, V.; Haase, A.; Wernecke, J.; Probst, J.; Schoengen, M.; Burger, S.; Krumrey, M.; Scholze, F.
2016-07-01
Laterally periodic nanostructures were investigated with grazing incidence small angle x-ray scattering. To support an improved reconstruction of nanostructured surface geometries, we investigated the origin of the contributions to the diffuse scattering pattern which is correlated to the surface roughness. Resonant diffuse scattering leads to a palmlike structure of intensity sheets. Dynamic scattering generates the so-called Yoneda band caused by a resonant scatter enhancement at the critical angle of total reflection and higher-order Yoneda bands originating from a subsequent diffraction of the Yoneda enhanced scattering at the grating. Our explanations are supported by modeling using a solver for the time-harmonic Maxwell's equations based on the finite-element method.
Water diffusion on TiO2 anatase surface
NASA Astrophysics Data System (ADS)
Agosta, L.; Gala, F.; Zollo, G.
2015-06-01
Compatibility between biological molecules and inorganic materials, such as crystalline metal oxides, is strongly dependent on the selectivity properties and the adhesion processes at the interface between the two systems. Among the many different aspects that affect the adsorption processes of peptides or proteins onto inorganic surfaces, such as the charge state of the amino acids, the peptide 3D structure, the surface roughness, the presence of vacancies or defects on and below the surface, a key role is certainly played by the water solvent whose molecules mediate the interaction. Then the surface hydration pattern may strongly affect the adsorption behavior of biological molecules. For the particular case of (101) anatase TiO2 surface that has a fundamental importance in the interaction of biocompatible nano-devices with biological environment, it was shown, both theoretically and experimentally, that various hydration patterns are close in energy and that the water molecules are mobile at as low temperature values as 190 K. Then it is important to understand the dynamical behavior of first hydration layer of the (101) anatase surface. As a first approach to this problem, density functional calculations are used to investigate water diffusion on the (101) anatase TiO2 surface by sampling the potential energy surface of water molecules of the first hydration layer thus calculating the water molecule migration energy along some relevant diffusion paths on the (101) surface. The measured activation energy of water migration seems in contrast with the observed surface mobility of the water molecules that, as a consequence could be explained invoking a strong role of the entropic term in the context of the transition state theory.
Sodium diffusion through amorphous silica surfaces: a molecular dynamics study.
Rarivomanantsoa, Michaël; Jund, Philippe; Jullien, Rémi
2004-03-01
We have studied the diffusion inside the silica network of sodium atoms initially located outside the surfaces of an amorphous silica film. We have focused our attention on structural and dynamical quantities, and we have found that the local environment of the sodium atoms is close to the local environment of the sodium atoms inside bulk sodo-silicate glasses obtained by quench. This is in agreement with recent experimental results. PMID:15267353
Rapid diffusion of magic-size islands by combined glide and vacancy mechanisms
Perez, D; Voter, A F; Uche, O U; Hamilton, J C
2009-01-01
Using molecular dynamics, nudged elastic band, and embedded atom methods, we show that certain 2D Ag islands undergo extremely rapid one-dimensional diffusion on Cu(001) surfaces. Indeed, below 300K, hopping rates for 'magic-size' islands are orders of magnitude faster than hopping rates for single Ag adatoms. This rapid diffusion requires both the c(10 x 2) hexagonally-packed superstructure typical of Ag on Cu(001) and appropriate 'magic-sizes' for the islands. The novel highly-cooperative diffusion mechanism presented here couples vacancy diffusion with simultaneous core glide.
A localized meshless method for diffusion on folded surfaces
NASA Astrophysics Data System (ADS)
Cheung, Ka Chun; Ling, Leevan; Ruuth, Steven J.
2015-09-01
Partial differential equations (PDEs) on surfaces arise in a variety of application areas including biological systems, medical imaging, fluid dynamics, mathematical physics, image processing and computer graphics. In this paper, we propose a radial basis function (RBF) discretization of the closest point method. The corresponding localized meshless method may be used to approximate diffusion on smooth or folded surfaces. Our method has the benefit of having an a priori error bound in terms of percentage of the norm of the solution. A stable solver is used to avoid the ill-conditioning that arises when the radial basis functions (RBFs) become flat.
Cleanliness evaluation of rough surfaces with diffuse IR reflectance
NASA Technical Reports Server (NTRS)
Pearson, L. H.
1995-01-01
Contamination on bonding surfaces has been determined to be a primary cause for degraded bond strength in certain solid rocket motor bondlines. Hydrocarbon and silicone based organic contaminants that are airborne or directly introduced to a surface are a significant source of contamination. Diffuse infrared (IR) reflectance has historically been used as an effective technique for detection of organic contaminants, however, common laboratory methods involving the use of a Fourier transform IR spectrometer (FTIR) are impractical for inspecting the large bonding surface areas found on solid rocket motors. Optical methods involving the use of acousto-optic tunable filters and fixed bandpass optical filters are recommended for increased data acquisition speed. Testing and signal analysis methods are presented which provide for simultaneous measurement of contamination concentration and roughness level on rough metal surfaces contaminated with hydrocarbons.
Magnetic adatoms as memory bits: A quantum master equation analysis
NASA Astrophysics Data System (ADS)
Karlewski, Christian; Marthaler, Michael; Märkl, Tobias; Balashov, Timofey; Wulfhekel, Wulf; Schön, Gerd
2015-06-01
Due to underlying symmetries, the ground states of magnetic adatoms may be highly stable, which opens perspectives for application as single-atom memory. A specific example is a single holmium atom (with J =8 ) on a platinum (111) surface for which exceptionally long lifetimes were observed in recent scanning tunneling microscopy studies. For control and read-out, the atom must be coupled to electronic contacts. Hence the spin dynamics of the system is governed by a quantum master equation. Our analysis shows that, in general, it cannot be reduced to a classical master equation in the basis of the unperturbed crystal-field Hamiltonian. Rather, depending on parameters and control fields, "environment-induced superselection" principles choose the appropriate set of basis states, which in turn determines the specific relaxation channels and lifetimes. Our simulations suggest that in ideal situations the lifetimes should be even longer than observed in the experiment. We, therefore, investigate the influence of various perturbations. We also study the initialization process of the state of the Ho atom by applied voltage pulses and conclude that fast, high fidelity preparation, on a 100 -ns time scale, should be possible.
NASA Astrophysics Data System (ADS)
Bhatia, Bhawna; Sholl, David S.
2005-05-01
Plane-wave density functional theory calculations were performed to investigate the binding and diffusion of hydrogen on three flat Ni surfaces, Ni(100), Ni(110), and Ni(111), and two stepped Ni surfaces, Ni(210) and Ni(531). On each surface, the favored adsorption sites were identified by considering the energy and stability of various binding sites and zero-point energy corrections were computed. Binding energies are compared with experimental and theoretical results from the literature. Good agreement with experimental and previous theoretical data is found. At surface coverages where adsorbate-adsorbate interactions are relatively weak, the binding energy of H is similar on the five Ni surfaces studied. Favorable binding energies are observed for stable surface sites, while subsurface sites have unfavorable values relative to the gas phase molecular hydrogen. Minimum energy paths for hydrogen diffusion on Ni surfaces and into subsurface sites were constructed.
Interferometry on diffuse surfaces in high-velocity measurements
NASA Astrophysics Data System (ADS)
Pronin, A.; Gupta, V.
1993-08-01
An interferometer is presented which is capable of measuring the free-surface velocities and displacements of both specular and diffuse surfaces. The setup utilizes a previously used principle of producing a virtual image of one mirror at the same distance from the photodiode as the second mirror of the interferometer, albeit with considerable simplification. It is shown that use of a He-Ne laser of only 5-mW power can produce high contrast displacement fringes from surfaces of materials with nonuniform microstructure, including composites. Substrates of carbon-carbon composites and polycrystalline alumina with nonuniform microstructure on the scale of 5-10 μm, and with peak velocities up to 150 m/s were considered. An experimental strategy which allows one to covert the optical setup to either a velocity or a displacement interferometer is also discussed. It is further shown that use of a fast photodiode and a high-speed digitizer with a 5-ps rise time provides a time resolution of 0.2 ns for recording the displacement fringes, and allows measuring free surface velocities up to 800 m/s. This is demonstrated by measuring such transient surface velocities with rise times of 1 ns on a specular Si surface. In all the experiments reported here, the surface velocities were produced by the reflection of a stress wave, which in turn was generated on the back surface of the substrate, using a Nd:YAG laser pulse.
Horizontal advection, diffusion and plankton spectra at the sea surface.
NASA Astrophysics Data System (ADS)
Bracco, A.; Clayton, S.; Pasquero, C.
2009-04-01
Plankton patchiness is ubiquitous in the oceans, and various physical and biological processes have been proposed as its generating mechanisms. However, a coherent statement on the problem is missing, due to both a small number of suitable observations and to an incomplete understanding of the properties of reactive tracers in turbulent media. Abraham (1998) suggested that horizontal advection may be the dominant process behind the observed distributions of phytoplankton and zooplankton, acting to mix tracers with longer reaction times (Rt) down to smaller scales. Conversely, Mahadevan and Campbell (2002) attributed the relative distributions of sea surface temperature and phytoplankton to small scale upwelling, where tracers with longer Rt are able to homogenize more than those with shorter reaction times. Neither of the above mechanisms can explain simultaneously the (relative) spectral slopes of temperature, phytoplankton and zooplankton. Here, with a simple advection model and a large suite of numerical experiments, we concentrate on some of the physical processes influencing the relative distributions of tracers at the ocean surface, and we investigate: 1) the impact of the spatial scale of tracer supply; 2) the role played by coherent eddies on the distribution of tracers with different Rt; 3) the role of diffusion (so far neglected). We show that diffusion determines the distribution of temperature, regardless of the nature of the forcing. We also find that coherent structures together with differential diffusion of tracers with different Rt impact the tracer distributions. This may help in understanding the highly variable nature of observed plankton spectra.
Diffusion on a Curved Surface Coupled to Diffusion in the Volume: Application to Cell Biology
Novak, Igor L.; Gao, Fei; Choi, Yung-Sze; Resasco, Diana; Schaff, James C.; Slepchenko, Boris M.
2007-01-01
An algorithm is presented for solving a diffusion equation on a curved surface coupled to diffusion in the volume, a problem often arising in cell biology. It applies to pixilated surfaces obtained from experimental images and performs at low computational cost. In the method, the Laplace-Beltrami operator is approximated locally by the Laplacian on the tangential plane and then a finite volume discretization scheme based on a Voronoi decomposition is applied. Convergence studies show that mass conservation built in the discretization scheme and cancellation of sampling error ensure convergence of the solution in space with an order between 1 and 2. The method is applied to a cell-biological problem where a signaling molecule, G-protein Rac, cycles between the cytoplasm and cell membrane thus coupling its diffusion in the membrane to that in the cell interior. Simulations on realistic cell geometry are performed to validate, and determine the accuracy of, a recently proposed simplified quantitative analysis of fluorescence loss in photobleaching. The method is implemented within the Virtual Cell computational framework freely accessible at www.vcell.org. PMID:18836520
Unusual diffusing regimes caused by different adsorbing surfaces.
Guimarães, Veridiana G; Ribeiro, Haroldo V; Li, Quan; Evangelista, Luiz R; Lenzi, Ervin K; Zola, Rafael S
2015-03-01
A confined liquid with dispersed neutral particles is theoretically studied when the limiting surfaces present different dynamics for the adsorption-desorption phenomena. The investigation considers different non-singular kernels in the kinetic equations at the walls, where the suitable choice of the kernel can account for the relative importance of physisorption or chemisorption. We find that even a small difference in the adsorption-desorption rate of one surface (relative to the other) can drastically affect the behavior of the whole system. The surface and bulk densities and the dispersion are calculated when several scenarios are considered and anomalous-like behaviors are found. The approach described here is closely related to experimental situations, and can be applied in several contexts such as dielectric relaxation, diffusion-controlled relaxation in liquids, liquid crystals, and amorphous polymers. PMID:25633342
NASA Astrophysics Data System (ADS)
Akiyama, Toru; Takemoto, Yoshitaka; Nakamura, Kohji; Ito, Tomonori
2016-05-01
The initial growth processes on semipolar AlN(11\\bar{2}2) surfaces, such as adsorption behavior of Al adatoms, are investigated on the basis of ab initio calculations and kinetic Monte Carlo (MC) simulations. By using surface phase diagrams, which are obtained by comparing the adsorption energy from ab initio calculations with gas-phase chemical potentials, we find that the adsorption of Al adatoms under H-poor condition is much easier than that under H-rich condition. Furthermore, our kinetic MC simulations demonstrate that the surface lifetime (diffusion length) of Al adatoms under H-poor condition is found to be four (two) orders of magnitude larger than that under H-rich condition. These differences implie that the growth under H-poor condition is much faster than that under H-rich condition, qualitatively consistent with the experimental results.
Bulk and surface controlled diffusion of fission gas atoms
Andersson, Anders D.
2012-08-09
{sub 2{+-}x}, which compare favorably to available experiments. This is an extension of previous work [13]. In particular, it applies improved chemistry models for the UO{sub 2{+-}x} nonstoichiometry and its impact on the fission gas activation energies. The derivation of these models follows the approach that used in our recent study of uranium vacancy diffusion in UO{sub 2} [14]. Also, based on the calculated DFT data we analyze vacancy enhanced diffusion mechanisms in the intermediate temperature regime. In addition to vacancy enhanced diffusion we investigate species transport on the (111) UO{sub 2} surface. This is motivated by the formation of small voids partially filled with fission gas atoms (bubbles) in UO{sub 2} under irradiation, for which surface diffusion could be the rate-limiting transport step. Diffusion of such bubbles constitutes an alternative mechanism for mass transport in these materials.
Hammond, Karl D.; Wirth, Brian D.
2014-10-14
We present atomistic simulations that show the effect of surface orientation on helium depth distributions and surface feature formation as a result of low-energy helium plasma exposure. We find a pronounced effect of surface orientation on the initial depth of implanted helium ions, as well as a difference in reflection and helium retention across different surface orientations. Our results indicate that single helium interstitials are sufficient to induce the formation of adatom/substitutional helium pairs under certain highly corrugated tungsten surfaces, such as (1 1 1)-orientations, leading to the formation of a relatively concentrated layer of immobile helium immediately below the surface. The energies involved for helium-induced adatom formation on (1 1 1) and (2 1 1) surfaces are exoergic for even a single adatom very close to the surface, while (0 0 1) and (0 1 1) surfaces require two or even three helium atoms in a cluster before a substitutional helium cluster and adatom will form with reasonable probability. This phenomenon results in much higher initial helium retention during helium plasma exposure to (1 1 1) and (2 1 1) tungsten surfaces than is observed for (0 0 1) or (0 1 1) surfaces and is much higher than can be attributed to differences in the initial depth distributions alone. The layer thus formed may serve as nucleation sites for further bubble formation and growth or as a source of material embrittlement or fatigue, which may have implications for the formation of tungsten “fuzz” in plasma-facing divertors for magnetic-confinement nuclear fusion reactors and/or the lifetime of such divertors.
Disorder induced topological transition in graphene with random adatoms
NASA Astrophysics Data System (ADS)
Castro, Eduardo; López-Sancho, María; Vozmediano, María
2015-03-01
Abstract One of the first proposals for a two-dimensional topological insulator was made for graphene, the so called Kane-Mele model, but the very low spin-orbit coupling makes this phase undetectable. It has been suggested that randomly depositing certain heavy adatoms can amplify the effect by many orders, and that a dilute concentration should be enough to open a detectable topological gap. Still lacking, however, is a precise determination of the critical density of random adatoms based in the evolution of the topological index. Based in a finite size analysis of the topological index as a function of the density of randomly distributed adatoms, and also on the localization properties of the system accessed through the Lyapunov exponent, we not only determine the critical density but also establish the nature of this peculiar topological transition. EC acknowledge the financial support of FCT-Portugal through Grant No. EXPL/FIS-NAN/1720/2013.
Graphene surface emitting terahertz laser: Diffusion pumping concept
Davoyan, Arthur R.; Morozov, Mikhail Yu.; Popov, Vyacheslav V.; Satou, Akira; Otsuji, Taiichi
2013-12-16
We suggest a concept of a tunable graphene-based terahertz (THz) surface emitting laser with diffusion pumping. We employ significant difference in the electronic energy gap of graphene and a typical wide-gap semiconductor, and demonstrate that carriers generated in the semiconductor can be efficiently captured by graphene resulting in population inversion and corresponding THz lasing from graphene. We develop design principles for such a laser and estimate its performance. We predict up to 50 W/cm{sup 2} terahertz power output for 100 kW/cm{sup 2} pump power at frequency around 10 THz at room temperature.
NASA Astrophysics Data System (ADS)
Jeong, Hojin; Yeom, Han Woong; Jeong, Sukmin
2008-06-01
Using a first-principles calculation method, we study the changes in the atomic and electronic structures of the Si(111)3×3-Ag surface (hereafter 3-Ag ) via doping of extra Ag adatoms. We present a structural model for the adatom-induced 21×21 superstructure (21-Ag) , which has three Ag adatoms immersed into the substrate Ag layer within a unit cell. The present structural model reproduces well the measured scanning-tunneling-microscopy images as well as the electronic band structure measured by angle-resolved photoelectron spectroscopy. We find out that the complex band structure seen on the 21-Ag phase basically arises from the band folding of the original surface bands of 3-Ag . The extra Ag adatoms doped on 3-Ag modify only the band alignment without any additional adatom-induced surface state. The almost unoccupied two-dimensional free-electron-like band, generally called S1 , at pristine 3-Ag is gradually filled and shifted downward with an increase in the dopant coverage. As this shifted S1 band crosses other surface bands, it loses its free-electron nature.
Surface Diffusion of Single Polymer Chain Using Molecular Dynamics SIMULATION*
NASA Astrophysics Data System (ADS)
Desai, Tapan; Keblinski, Pawel; Kumar, Sanat; Granick, Steve
2004-05-01
Results of recent experiments on polymer chains adsorbed from dilute solution at solid-liquid interface show the power scaling law dependence of the chain diffusivity, D, as a function of the degree of polymerization, N, D ˜ N^3/2. By contrast, DNA molecules bound to fluid cationic lipid bilayers follows Rouse dynamics with D ˜ N^1. We used molecular dynamics simulations to gain an understanding of these dissimilar scaling behaviors. Our model systems contain chains comprised of N monomers connected by anharmonic springs described by the finite extendible nonlinear elastic, FENE potential, embedded into a solvent of N=1 monomers. Two types of simulations we performed: (i) the chain is confined to two dimensions, (ii) the three dimensional chain in the solvent is confined between two solids plates. With randomly placed impenetrable obstacles on the surface, the diffusion of 2D chains exhibits, D ˜ N^3/2 behavior, when the chain radius of gyration, Rg, is larger than half the distance between obstacles, and D ˜ N^1 for shorter chains. In the presence of an athermal solvent, the scaling exponent is 0.75 due to hydrodynamic forces, for the two-dimensional system. We will also discuss the nature of dynamic adsorption transition and effects of hydrodynamics forces on chain diffusion for the three-dimensional simulations.
Surface Viscosity, Diffusion, and Intermonolayer Friction: Simulating Sheared Amphiphilic Bilayers
Shkulipa, S. A.; den Otter, W. K.; Briels, W. J.
2005-01-01
The flow properties of an amphiphilic bilayer are studied in molecular dynamics simulations, by exposing a coarse grained model bilayer to two shear flows directed along the bilayer surface. The first field, with a vorticity perpendicular to the bilayer, induces a regular shear deformation, allowing a direct calculation of the surface viscosity. In experiments this property is measured indirectly, by relating it to the diffusion coefficient of a tracer particle through the Saffman-Einstein expression. The current calculations provide an independent test of this relation. The second flow field, with a vorticity parallel to the bilayer, causes the two constituent monolayers to slide past one another, yielding the interlayer friction coefficient. PMID:15894643
Huang, Wen-Kai; Zhang, Kai-Wen; Yang, Chao-Long; Ding, Haifeng; Wan, Xiangang; Li, Shao-Chun; Evans, James W; Han, Yong
2016-07-13
Heteroepitaxial structures based on Bi2Te3-type topological insulators (TIs) exhibit exotic quantum phenomena. For optimal characterization of these phenomena, it is desirable to control the interface structure during film growth on such TIs. In this process, adatom mobility is a key factor. We demonstrate that Pb mobility on the Bi2Te3(111) surface can be modified by the engineering local strain, ε, which is induced around the point-like defects intrinsically forming in the Bi2Te3(111) thin film grown on a Si(111)-7 × 7 substrate. Scanning tunneling microscopy observations of Pb adatom and cluster distributions and first-principles density functional theory (DFT) analyses of the adsorption energy and diffusion barrier Ed of Pb adatom on Bi2Te3(111) surface show a significant influence of ε. Surprisingly, Ed reveals a cusp-like dependence on ε due to a bifurcation in the position of the stable adsorption site at the critical tensile strain εc ≈ 0.8%. This constitutes a very different strain-dependence of diffusivity from all previous studies focusing on conventional metal or semiconductor surfaces. Kinetic Monte Carlo simulations of Pb deposition, diffusion, and irreversible aggregation incorporating the DFT results reveal adatom and cluster distributions compatible with our experimental observations. PMID:27302741
Tunneling Microscopy of Dynamical Processes on the LEAD/GERMANIUM(111) Surface
NASA Astrophysics Data System (ADS)
Hwang, Ing-Shouh
Knowledge about atomic scale motions is essential for understanding dynamical phenomena on surfaces, such as diffusion, phase transitions, and epitaxial growth. This report describes the results of a study of dynamical processes on the Pb/Ge(111) surface using a scanning tunneling microscope (STM). Individual Pb atom diffUsion and concerted atomic motions on the Ge(111) surface are observed in real time. We also study a structural surface phase transformation at elevated temperatures. At very low Pb coverage, migration of individual Pb atoms is observed in the Ge(111)-c(2 x 8) surface near room temperature. The activation energy of this migration can be measured by analyzing a large number of individual atomic motions from room temperature to 80^ circC. The Pb diffusion is found to occur mainly along the (011) adatom row direction of the c(2 x 8) reconstruction. About half of the adatom migrations are "long jumps". We also observe the formation and annihilation of metastable structural surface excitations, which occur much less often than Pb diffusion. They involve a number of adatoms in the same row moving in concert along the row direction like beads on an abacus. This "adatom row shift" may be responsible for the anisotropy of the Pb atom diffusion. It also provides a new mechanism for atomic transport on crystal surfaces and can explain several structural phenomena associated with the Ge(111) surface. At high coverage, a one monolayer Pb/Ge(111) undergoes a reversible phase transformation from sqrt{3} x sqrt{3 }R30^circ to 1 x 1 at about 180 ^circC. Atomic structures of both high and low temperature phases are resolved, which reveals an order-order transition. Spatial and temporal fluctuations are exposed just above the transition temperature. In addition, the influence of surface strain, phase boundaries, and finite size domains are found to play an important role in the phase transformation.
Surface diffusion of molecular glasses: Material dependence and impact on physical stability
NASA Astrophysics Data System (ADS)
Ruan, Shigang; Zhang, Wei; Yu, Lian
Surface diffusion coefficients have been measured for molecular glasses tris-naphthylbenzene (TNB) and PMMA oligomers by surface grating decay. Surface diffusion on TNB is vastly faster than bulk diffusion, by a factor of 107 at Tg, while the process is very slow on PMMA. Along with the previous results on o - terphenyl, nifedipine, indomethacin, and polystyrene oligomers, we find that surface diffusion slows down with increasing molecular size and intermolecular forces, whereas bulk diffusion has a weaker material dependence. The molecular glasses studied show fast crystal growth on the free surface. A general correlation is observed between the coefficient of surface diffusion and the velocity of surface crystal growth, indicating surface crystallization is supported by surface mobility. (Zhu, L., et al. Phys. Rev. Lett. 106 (2011): 256103; Zhang, W., et al. J. Phys. Chem. B 119 (2015): 5071-5078) Nsf.
Generating a Simulated Fluid Flow Over an Aircraft Surface Using Anisotropic Diffusion
NASA Technical Reports Server (NTRS)
Rodriguez, David L. (Inventor); Sturdza, Peter (Inventor)
2013-01-01
A fluid-flow simulation over a computer-generated aircraft surface is generated using a diffusion technique. The surface is comprised of a surface mesh of polygons. A boundary-layer fluid property is obtained for a subset of the polygons of the surface mesh. A pressure-gradient vector is determined for a selected polygon, the selected polygon belonging to the surface mesh but not one of the subset of polygons. A maximum and minimum diffusion rate is determined along directions determined using a pressure gradient vector corresponding to the selected polygon. A diffusion-path vector is defined between a point in the selected polygon and a neighboring point in a neighboring polygon. An updated fluid property is determined for the selected polygon using a variable diffusion rate, the variable diffusion rate based on the minimum diffusion rate, maximum diffusion rate, and angular difference between the diffusion-path vector and the pressure-gradient vector.
Most current electrostatic surface complexation models describing ionic binding at the particle/water interface rely on the use of Poisson - Boltzmann (PB) theory for relating diffuse layer charge densities to diffuse layer electrostatic potentials. PB theory is known to contain ...
NASA Astrophysics Data System (ADS)
Zhao, Xiaofeng
1993-01-01
The surface segregation in a binary mixture of polymer due to surface energy difference or end-grafting is studied. The surface energy difference induced segregation is compared with the mean-field theory. The end-labeling of chains strengthens the ability of the chain to bind to the surface, and the stretching of the end grafted chains is proved to be a controlling fact limiting the brush density. The structure of a chain near a surface needs to be known in order to make more quantitative analysis. Such knowledge is currently not available although a reflecting surface model is proposed. We also studied the mutual-diffusion of compatible linear chains. It is observed that the broadening of the interfacial width scales as the 1/4 power of the diffusion time for a time scale much longer than the reptation time. It is speculated that the anomalous behavior is either due to small molecular residue in the sample or due to the long chain nature of the polymer itself. If the former possibility can be ruled out, the validity of applying the reptation model to polymer diffusion over small distances might be under challenge. Dynamic secondary ion mass spectrometry (SIMS) is an established technique and its application in polymer science has been around for quite some time. However, the quantitative application in depth profiling was so far not very successful. The technique is reviewed and procedures that ensure correct extraction of depth profiles from raw SIMS data are discussed.
Spin-spin correlations of magnetic adatoms on graphene
NASA Astrophysics Data System (ADS)
Güçlü, A. D.; Bulut, Nejat
2015-03-01
We study the interaction between two magnetic adatom impurities in graphene using the Anderson model. The two-impurity Anderson Hamiltonian is solved numerically by using the quantum Monte Carlo technique. We find that the interimpurity spin susceptibility is strongly enhanced at low temperatures, significantly diverging from the well-known Ruderman-Kittel-Kasuya-Yoshida result which decays as R-3.
NASA Astrophysics Data System (ADS)
Schraknepper, H.; De Souza, R. A.
2016-02-01
Two different physical processes, (i) fast grain-boundary diffusion (FGBD) of oxygen and (ii) hindered oxygen diffusion in a surface space-charge layer, yield oxygen isotope diffusion profiles in a similar form. Two features are observed, with the short, sharp profile close to the surface being followed by a longer, shallower profile. In this study, we develop a procedure for deciding which of the two descriptions applies to experimentally measured profiles. Specifically, we solve Fick's second law, using finite-element simulations, to obtain oxygen isotope diffusion profiles for the two cases. Each set of profiles is then analysed in terms of the competing description. In this manner, we derive falsifiable conditions that allow physical processes to be assigned unambiguously to the two features of such isotope profiles. Applying these conditions to experimental profiles for SrTiO3 single crystals published in the literature, we find that FGBD is an invalid model for describing the diffusion processes.
Generating a Simulated Fluid Flow over a Surface Using Anisotropic Diffusion
NASA Technical Reports Server (NTRS)
Rodriguez, David L. (Inventor); Sturdza, Peter (Inventor)
2016-01-01
A fluid-flow simulation over a computer-generated surface is generated using a diffusion technique. The surface is comprised of a surface mesh of polygons. A boundary-layer fluid property is obtained for a subset of the polygons of the surface mesh. A gradient vector is determined for a selected polygon, the selected polygon belonging to the surface mesh but not one of the subset of polygons. A maximum and minimum diffusion rate is determined along directions determined using the gradient vector corresponding to the selected polygon. A diffusion-path vector is defined between a point in the selected polygon and a neighboring point in a neighboring polygon. An updated fluid property is determined for the selected polygon using a variable diffusion rate, the variable diffusion rate based on the minimum diffusion rate, maximum diffusion rate, and the gradient vector.
Vauth, Sebastian; Mayr, S. G.
2007-06-01
Surface viscous flow, surface diffusion, and ballistic effects have recently been discussed as possible atomic-scale mechanisms to explain the dramatic smoothing reactions observed during keV ion bombardment of amorphous surfaces. By employing multiscale modeling, viz. a combination of molecular dynamics and continuum rate equations, we compare the relevance of the individual processes at room temperature. This is achieved by calculating diffusion constants, viscosities, and lateral transport due to momentum transfer. Depending on the surface structure size, we find the dominance of surface viscous flow or ballistic effects. The findings are found to be valid for both strong and fragile glasses, as represented by amorphous Si and CuTi, respectively.
A model for diffuse and global irradiation on horizontal surfaces
Jain, P.C. )
1990-01-01
The intensity of the direct radiation and the diffuse radiation at any time on a horizontal surface are each expressed as fractions of the intensity of the extraterrestrial radiation. Using these and assuming a random distribution of the bright sunshine hours and not too wide variations in the values of the transmission coefficients, a number of relations for estimating the global and the diffuse irradiation are derived. Two of the relations derived, including the Angstroem correlation for estimating the global irradiation, are already known empirically while several new correlations have been derived. The relations derived in this paper are: (i) H{sub d}/H{sub o} = a{sub 1} + b{sub 1} (S/S{sub o}); (ii) H/H{sub o} = A{sub 2} + b{sub 2} (S/S{sub o}); (iii) H{sub D}/H{sub o} = a{sub 3} + b{sub 3} (H/H{sub o}); (iv) H{sub D}/H = a{sub 4} + b{sub 4} (h{sub o}/) (v) H/(H{minus}H{sub D}) = a{sub 5} + b{sub 5} (S{sub o}/S); (vi) H{sub D}/(H{minus}H{sub D}) = A{sub 6} + b{sub 6} (S{sub o}/S); (vii) H/H{sub D} = a{sub 7} + b{sub 7} (S/S{sub o}); (viii) H/H{sub D} = A{sub 1} + A{sub 2} (S/S{sub o}) + A{sub 3} (S/S{sub o}){sup 2}. The study identifies three independent basic parameters and the constants appearing in the various equations as simple functions of these three basic parameters. This provides unification and inter-relationships between the various constants. Experimental data for the diffuse irradiation, the global irradiation and the bright sunshine duration for Macerata (Italy), Salisbury and Bulawayo (Zimbabwe) is found to show good correlation for the linear eqns (i) to (vii), and the nature and the interrelationships of the constants is found to be as predicted by theory.
Measuring Surface Diffusion of Organic Glasses Using Tobacco Mosaic Virus as Probe Nanoparticles
NASA Astrophysics Data System (ADS)
Zhang, Yue; Potter, Richard; Fakhraai, Zahra
Recent studies have shown that diffusion on the surface of organic glasses can be many orders of magnitude faster than bulk diffusion, with lower activation barrier. Developing new probes that can readily measure the diffusion at the surface of an organic glass can help study the effect of chemical structure and molecule's size on the enhanced surface diffusion. In this study, surface diffusion coefficient of molecular glass (TPD) is measured using tobacco mosaic virus (TMV) as probe particles. TMV is placed on the surface of bulk TPD films. The evolution of the meniscus formed around TMV, driven by curvature gradient, is probed at various temperatures. TMV has a well-defined cylindrical shape, with a large aspect ratio (18 nm wide, 300 nm long). As such, the shape of the meniscus around the center of TMV is semi-one dimensional. Based on the self-similarity nature of surface diffusion flow in one dimension, the surface diffusion coefficient and its temperature dependence are measured. It is found that the surface diffusion is greatly enhanced and has weak temperature dependence compared to bulk counterpart, consistent with previous studies, showing that TMV probes serve as an efficient method of measuring surface diffusion. NSF-CAREER DMR-1350044.
Inactivation of Escherichia coli on PTFE surfaces by diffuse coplanar surface barrier discharge
NASA Astrophysics Data System (ADS)
Tučeková, Zlata; Koval'ová, Zuzana; Zahoranová, Anna; Machala, Zdenko; Černák, Mirko
2016-08-01
The non-equilibrium plasma of diffuse coplanar surface barrier discharge (DCSBD) was tested for decontamination of bacteria Escherichia coli on polymer surfaces. We investigated the optical parameters of DCSBD plasma generated in synthetic air with different relative humidity. Our study was provided to estimate the main plasma components active during the DCSBD plasma degradation of E. coli contamination prepared on polytetrafluoroethylene (PTFE, Teflon) surface, in ambient air at atmospheric pressure. The DCSBD plasma was characterized by means of electrical measurements and optical emission spectroscopy. The inactivation of E. coli bacteria was evaluated by standard microbiological cultivation (CFU plate counting). The experimental results of the germicidal efficiency obtained for short plasma exposure times proved the effectiveness of DCSBD plasma for the polymer surface decontamination. Contribution to the topical issue "6th Central European Symposium on Plasma Chemistry (CESPC-6)", edited by Nicolas Gherardi, Ester Marotta and Cristina Paradisi
Zero-Point Spin-Fluctuations of Single Adatoms.
Ibañez-Azpiroz, Julen; Dos Santos Dias, Manuel; Blügel, Stefan; Lounis, Samir
2016-07-13
Stabilizing the magnetic signal of single adatoms is a crucial step toward their successful usage in widespread technological applications such as high-density magnetic data storage devices. The quantum mechanical nature of these tiny objects, however, introduces intrinsic zero-point spin-fluctuations that tend to destabilize the local magnetic moment of interest by dwindling the magnetic anisotropy potential barrier even at absolute zero temperature. Here, we elucidate the origins and quantify the effect of the fundamental ingredients determining the magnitude of the fluctuations, namely, the (i) local magnetic moment, (ii) spin-orbit coupling, and (iii) electron-hole Stoner excitations. Based on a systematic first-principles study of 3d and 4d adatoms, we demonstrate that the transverse contribution of the fluctuations is comparable in size to the magnetic moment itself, leading to a remarkable ≳50% reduction of the magnetic anisotropy energy. Our analysis gives rise to a comprehensible diagram relating the fluctuation magnitude to characteristic features of adatoms, providing practical guidelines for designing magnetically stable nanomagnets with minimal quantum fluctuations. PMID:27248465
Floquet bound states around defects and adatoms in graphene
NASA Astrophysics Data System (ADS)
Lovey, D. A.; Usaj, Gonzalo; Foa Torres, L. E. F.; Balseiro, C. A.
2016-06-01
Recent studies have focused on laser-induced gaps in graphene which have been shown to have a topological origin, thereby hosting robust states at the sample edges. While the focus has remained mainly on these topological chiral edge states, the Floquet bound states around defects lack a detailed study. In this paper we present such a study covering large defects of different shape and also vacancy-like defects and adatoms at the dynamical gap at ℏ Ω /2 (ℏ Ω being the photon energy). Our results, based on analytical calculations as well as numerics for full tight-binding models, show that the bound states are chiral and appear in a number which grows with the defect size. Furthermore, while the bound states exist regardless of the type of the defect's edge termination (zigzag, armchair, mixed), the spectrum is strongly dependent on it. In the case of top adatoms, the bound state quasienergies depend on the adatoms energy. The appearance of such bound states might open the door to the presence of topological effects on the bulk transport properties of dirty graphene.
Surface reaction and bulk diffusion of tritium in SUS-316 stainless steel
NASA Astrophysics Data System (ADS)
Sugisaki, M.; Furuya, H.; Ueki, H.; Ejima, S.
1985-08-01
Diffusivities of tritium in SUS-316 stainless steel were measured by a gas-absorption method in the temperature range from 603 to 853 K. The measurements were carried out on clean and modified surfaces, which were carefully prepared by well characterized treatments. The diffusion behavior of the clean surface is successfully described by a simple diffusion equation and the bulk diffusion coefficient of tritium was determined as D(cm 2/s) = 4.2 × 10 2 exp( {-64}/{( {RT}/{kJ}) }) . The diffusion process for the modified surface was not explained by a simple diffusion equation but by a diffusion equation having an induction period. The microscopic explanation of the induction period is given.
Diffusing colloids in the vicinity of a surface: Anomalous yet Brownian diffusion?
NASA Astrophysics Data System (ADS)
Ignacio, Maxime; Chubynsky, Mykyta V.; Slater, Gary W.
Anomalous yet Brownian diffusion refers to a process with a linear mean-square displacement coexisting with a non-Gaussian Displacement Distribution (DispD). Chubynsky and Slater proposed a model of this phenomenon in which the diffusion coefficient varies randomly in time (''diffusing diffusivity''). Recently, Bechhoefer's group has showed experimentally that diffusion of colloids near a wall exhibits non-Gaussian DispD with exponential tails. Due to hydrodynamic interactions, the diffusivity D (r) is space-dependent and therefore varies in time as the particle moves in space. Qualitatively, the experimental results agree with the predictions of the diffusing diffusivity model. However, the two situations differ in details. First, space-dependent diffusivity implies the possibility of different interpretations of the stochastic term in the overdamped Langevin equation (i.e. the ``Ito-Stratonovich dilemma''). Second, in the system of Bechhoefer et al., there is an external potential due to gravity and the electrostatic repulsion from the wall. Using Lattice Monte Carlo simulations, we explore the role of these effects.
Gianfrancesco, Anthony G.; Tselev, Alexander; Baddorf, Arthur P.; Kalinin, Sergei V.; Vasudevan, Rama K.
2015-10-22
The controlled growth of epitaxial films of complex oxides requires an atomistic understanding of key parameters determining final film morphology, such as termination dependence on adatom diffusion, and height of the Ehrlich–Schwoebel (ES) barrier. In this study, through an in situ scanning tunneling microscopy study of mixed-terminated La_{5/8}Ca_{3/8}MnO_{3} (LCMO) films, we image adatoms and observe pile-up at island edges. Image analysis allows determination of the population of adatoms at the edge of islands and fractions on A-site and B-site terminations. A simple Monte-Carlo model, simulating the random walk of adatoms on a sinusoidal potential landscape using Boltzmann statistics is used to reproduce the experimental data, and provides an estimate of the ES barrier as ~0.18 ± 0.04 eV at T = 1023 K, similar to those of metal adatoms on metallic surfaces. In conclusion, these studies highlight the utility of in situ imaging, in combination with basic Monte-Carlo methods, in elucidating the factors which control the final film growth in complex oxides.
Gianfrancesco, Anthony G.; Tselev, Alexander; Baddorf, Arthur P.; Kalinin, Sergei V.; Vasudevan, Rama K.
2015-10-22
The controlled growth of epitaxial films of complex oxides requires an atomistic understanding of key parameters determining final film morphology, such as termination dependence on adatom diffusion, and height of the Ehrlich–Schwoebel (ES) barrier. In this study, through an in situ scanning tunneling microscopy study of mixed-terminated La5/8Ca3/8MnO3 (LCMO) films, we image adatoms and observe pile-up at island edges. Image analysis allows determination of the population of adatoms at the edge of islands and fractions on A-site and B-site terminations. A simple Monte-Carlo model, simulating the random walk of adatoms on a sinusoidal potential landscape using Boltzmann statistics is usedmore » to reproduce the experimental data, and provides an estimate of the ES barrier as ~0.18 ± 0.04 eV at T = 1023 K, similar to those of metal adatoms on metallic surfaces. In conclusion, these studies highlight the utility of in situ imaging, in combination with basic Monte-Carlo methods, in elucidating the factors which control the final film growth in complex oxides.« less
NASA Astrophysics Data System (ADS)
Gianfrancesco, Anthony G.; Tselev, Alexander; Baddorf, Arthur P.; Kalinin, Sergei V.; Vasudevan, Rama K.
2015-11-01
The controlled growth of epitaxial films of complex oxides requires an atomistic understanding of key parameters determining final film morphology, such as termination dependence on adatom diffusion, and height of the Ehrlich-Schwoebel (ES) barrier. Here, through an in situ scanning tunneling microscopy study of mixed-terminated La5/8Ca3/8MnO3 (LCMO) films, we image adatoms and observe pile-up at island edges. Image analysis allows determination of the population of adatoms at the edge of islands and fractions on A-site and B-site terminations. A simple Monte-Carlo model, simulating the random walk of adatoms on a sinusoidal potential landscape using Boltzmann statistics is used to reproduce the experimental data, and provides an estimate of the ES barrier as ˜0.18 ± 0.04 eV at T = 1023 K, similar to those of metal adatoms on metallic surfaces. These studies highlight the utility of in situ imaging, in combination with basic Monte-Carlo methods, in elucidating the factors which control the final film growth in complex oxides.
Gianfrancesco, Anthony G; Tselev, Alexander; Baddorf, Arthur P; Kalinin, Sergei V; Vasudevan, Rama K
2015-11-13
The controlled growth of epitaxial films of complex oxides requires an atomistic understanding of key parameters determining final film morphology, such as termination dependence on adatom diffusion, and height of the Ehrlich-Schwoebel (ES) barrier. Here, through an in situ scanning tunneling microscopy study of mixed-terminated La5/8Ca3/8MnO3 (LCMO) films, we image adatoms and observe pile-up at island edges. Image analysis allows determination of the population of adatoms at the edge of islands and fractions on A-site and B-site terminations. A simple Monte-Carlo model, simulating the random walk of adatoms on a sinusoidal potential landscape using Boltzmann statistics is used to reproduce the experimental data, and provides an estimate of the ES barrier as ∼0.18 ± 0.04 eV at T = 1023 K, similar to those of metal adatoms on metallic surfaces. These studies highlight the utility of in situ imaging, in combination with basic Monte-Carlo methods, in elucidating the factors which control the final film growth in complex oxides. PMID:26489518
Stability analysis and simulations of coupled bulk-surface reaction–diffusion systems
Madzvamuse, Anotida; Chung, Andy H. W.; Venkataraman, Chandrasekhar
2015-01-01
In this article, we formulate new models for coupled systems of bulk-surface reaction–diffusion equations on stationary volumes. The bulk reaction–diffusion equations are coupled to the surface reaction–diffusion equations through linear Robin-type boundary conditions. We then state and prove the necessary conditions for diffusion-driven instability for the coupled system. Owing to the nature of the coupling between bulk and surface dynamics, we are able to decouple the stability analysis of the bulk and surface dynamics. Under a suitable choice of model parameter values, the bulk reaction–diffusion system can induce patterning on the surface independent of whether the surface reaction–diffusion system produces or not, patterning. On the other hand, the surface reaction–diffusion system cannot generate patterns everywhere in the bulk in the absence of patterning from the bulk reaction–diffusion system. For this case, patterns can be induced only in regions close to the surface membrane. Various numerical experiments are presented to support our theoretical findings. Our most revealing numerical result is that, Robin-type boundary conditions seem to introduce a boundary layer coupling the bulk and surface dynamics. PMID:25792948
Surface migration and volume diffusion in the AgGaSe2-Ag2Se system
NASA Technical Reports Server (NTRS)
Kim, N.-H.; Feigelson, R. S.; Route, R. K.
1992-01-01
Surface migration and volume diffusion in the Ag2Se-AgGaSe2 system were investigated using reactive diffusion couples which were analyzed by X-ray diffraction, optical microscopy, and electron probe microanalysis. The surface diffusivities of all mobile species are found to be much larger than volume diffusivities. The results of the study suggest that Se moves together with Ag and Ga to maintain binary (Ag2Se and Ga2Se3) stoichiometry and electroneutrality. The dominance of surface migration kinetics can account for the uniform annihilation of second-phase precipitates during heat treatments.
NASA Astrophysics Data System (ADS)
Teobaldi, G.; Tanimura, K.; Shluger, A. L.
2010-11-01
The geometries, formation energies, and diffusion barriers of surface and subsurface intrinsic defects in graphite are calculated using spin-polarized density-functional theory and the generalized gradient approximation with a semiempirical van der Waals (vdW) correction for dispersion interactions. The calculated formation energies and diffusion barriers of subsurface interstitial (I) atoms deviate qualitatively and quantitatively from those of surface adatoms. The same trend is found also for subsurface and adatom clusters (I2,I3) . In spite of the semiquantitative agreement on the optimized geometries, the formation energies and diffusion barriers of surface and subsurface vacancies (V), divacancies (VV), and intimate (I-V) Frenkel pairs differ significantly from the values for the analogous defects in the bulk of graphite. This suggests limited transferability of the bulk and subsurface defect models to the surface of graphite. These findings are rationalized in terms of the balance between the covalent and vdW interaction terms at the surface, subsurface, and bulk of graphite. Finally, pairing of individual defects (adatoms, I and V) is calculated to be energetically advantageous both on the surface and in the subsurface regions. This process is shown to either saturate residual dangling bonds or produce singlet spin states, thus contributing to the quenching of residual spin polarization from damaged graphite surfaces.
Orbital signatures of Fano-Kondo line shapes in STM adatom spectroscopy
NASA Astrophysics Data System (ADS)
Frank, Sebastian; Jacob, David
2015-12-01
We investigate the orbital origin of the Fano-Kondo line shapes measured in STM spectroscopy of magnetic adatoms on metal substrates. To this end we calculate the low-bias tunnel spectra of a Co adatom on the (001) and (111) Cu surfaces with our density functional theory-based ab initio transport scheme augmented by local correlations. In order to associate different d orbitals with different Fano line shapes we only correlate individual 3 d orbitals instead of the full Co 3 d shell. We find that Kondo peaks arising in different d levels indeed give rise to different Fano features in the conductance spectra. Hence, the shape of measured Fano features allows us to draw some conclusions about the orbital responsible for the Kondo resonance, although the actual shape is also influenced by temperature, effective interaction, and charge fluctuations. Comparison with a simplified model shows that line shapes are mostly the result of interference between tunneling paths through the correlated d orbital and the s p -type orbitals on the Co atom. Very importantly, the amplitudes of the Fano features vary strongly among orbitals, with the 3 z2 orbital featuring by far the largest amplitude due to its strong direct coupling to the s -type conduction electrons.
Controlled surface diffusion in plasma-enhanced chemical vapor deposition of GaN nanowires.
Hou, Wen Chi; Hong, Franklin Chau-Nan
2009-02-01
This study investigates the growth of GaN nanowires by controlling the surface diffusion of Ga species on sapphire in a plasma-enhanced chemical vapor deposition (CVD) system. Under nitrogen-rich growth conditions, Ga has a tendency to adsorb on the substrate surface diffusing to nanowires to contribute to their growth. The significance of surface diffusion on the growth of nanowires is dependent on the environment of the nanowire on the substrate surface as well as the gas phase species and compositions. Under nitrogen-rich growth conditions, the growth rate is strongly dependent on the surface diffusion of gallium, but the addition of 5% hydrogen in nitrogen plasma instantly diminishes the surface diffusion effect. Gallium desorbs easily from the surface by reaction with hydrogen. On the other hand, under gallium-rich growth conditions, nanowire growth is shown to be dominated by the gas phase deposition, with negligible contribution from surface diffusion. This is the first study reporting the inhibition of surface diffusion effects by hydrogen addition, which can be useful in tailoring the growth and characteristics of nanowires. Without any evidence of direct deposition on the nanowire surface, gallium and nitrogen are shown to dissolve into the catalyst for growing the nanowires at 900 degrees C. PMID:19417353
NASA Astrophysics Data System (ADS)
Renedo, Javier; Cuerno, Rodolfo; Castro, Mario; Muñoz-García, Javier
2016-04-01
Ion-beam sputtering (IBS) is a cost-effective technique able to produce ordered nanopatterns on the surfaces of different materials. To date, most theoretical studies of this process have focused on systems which become amorphous under irradiation, e.g., semiconductors at room temperature. Thus, in spite of the large amount of experimental work on metals, or more recently on semiconductors at high temperatures, such experimental contexts have received relatively little theoretical attention. These systems are characterized by transport mechanisms, e.g., surface diffusion, which are anisotropic as a reflection of the crystalline structure not being overruled by the irradiation. Here, we generalize a previous continuum theory of IBS at normal incidence, in order to account for anisotropic surface diffusion. We explore systematically our generalized model in order to understand the role of anisotropy in the space-ordering properties of the resulting patterns. In particular, we derive a height equation which predicts morphological transitions among hexagonal and rectangular patterns as a function of system parameters and employ an angular correlation function to assess these pattern symmetries. By suitably choosing experimental conditions, it is found that one might be able to experimentally control the type of order displayed by the patterns produced.
NASA Astrophysics Data System (ADS)
Beszeda, I.; Beke, D. L.
Basic parameters of surface mass transport - the surface diffusion length of adatoms, λs, the surface diffusion coefficient, Ds', and the surface reaction rate coefficient, βs', of Cu on alumina are determined in the temperature range 1048-1198 K. Measuring simultaneously the time dependence of the effective thickness, Heff(t), the lateral shift of the boundary, y(t) of beaded films (BF) and using vapour pressure data we concluded that the process is controlled by surface reaction at the perimeters of beads. Supposing Arrhenius-type temperature dependence for Ds', βs' and λs the activation energies and preexponential factors have been calculated.
Special report, diffuse reflectivity of the lunar surface
NASA Technical Reports Server (NTRS)
Fastie, W. G.
1972-01-01
The far ultraviolet diffuse reflectivity of samples of lunar dust material is determined. Equipment for measuring the diffuse reflectivity of materials (e.g. paint samples) is already in existence and requires only minor modification for the proposed experiment which will include the measurement of the polarizing properties of the lunar samples. Measurements can be made as a function of both illumination angle and angle of observation.
NASA Technical Reports Server (NTRS)
Chow, M.; Houska, C. R.
1980-01-01
Solutions are given for one-dimensional diffusion problems with a time varying surface composition and also a composition dependent diffusion coefficient. The most general solution does not require special mathematical functions to fit the variation in surface composition or D(C). In another solution, a series expansion may be used to fit the time dependent surface concentration. These solutions make use of iterative calculations that converge rapidly and are highly stable. Computer times are much shorter than that required for finite difference calculations and can efficiently make use of interactive graphics terminals. Existing gas carburization data were used to provide an illustration of an iterative approach with a time varying carbon composition at the free surface.
Wang, P.S.; Wittberg, T.N.; Keil, R.G.
1982-01-01
TiO/sub 2/ films of about 1000 A were grown onto titanium foils anodically under galvanostatic conditions at 20 mA/cm/sup 2/ in saturated aqueous solutions of ammonium tetraborate. The samples were then aged at 450, 500, and 550/sup 0/C, and oxygen diffusion was observed by Auger electron spectroscopy (AES) profilings. The oxygen diffusivities were calculated by Fick's Second Law, using the Boltzmann-Matano solution, to be 9.4 x 10/sup -17/, 2.6 x 10/sup -16/, and 1.2 x 10/sup -15/ cm/sup 2//sec at 450, 500, and 550/sup 0/C, respectively. The diffusivities obtained by this method were also compared with those obtained using an exact solution to Fick's Second Law. The activation energy was calculated to be 30 kcal/mole.
Ag out-surface diffusion in crystalline SiC with an effective SiO2 diffusion barrier
Xue, H; Xiao, H Y; Zhu, Z; Shutthanandan, V; Snead, L; Boatner, Lynn A; Weber, William J.; Zhang, Yanwen
2015-01-01
For applications of tristructural isotropic (TRISO) fuel particles in high temperature reactors, release of radioactive Ag isotope (110mAg) through the SiC coating layer is a safety concern. To understand the diffusion mechanism, Ag ion implantations near the surface and in the bulk were performed by utilizing different ion energies and energy-degrader foils. High temperature annealing was carried out on the as-irradiated samples to study the possible out-surface diffusion. Before and after annealing, Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) measurements were employed to obtain the elemental profiles of the implanted samples. The results suggest little migration ofmore » buried Ag in the bulk, and an out-diffusion of the implanted Ag in the near-surface region of single crystal SiC. It is also found that a SiO2 layer, which was formed during annealing, may serve as an effective barrier to reduce or prevent Ag out diffusion through the SiC coating layer.« less
Ag Out-surface Diffusion In Crystalline SiC With An Effective SiO2 Diffusion Barrier
Xue, H.; Xiao, Haiyan Y.; Zhu, Zihua; Shutthanandan, V.; Snead, Lance L.; Boatner, Lynn A.; Weber, William J.; Zhang, Y.
2015-09-01
For applications of tristructural isotropic (TRISO) fuel particles in high temperature reactors, release of radioactive Ag isotope (110mAg) through the SiC coating layer is a safety concern. To understand the diffusion mechanism, Ag ion implantations near the surface and in the bulk were performed by utilizing different ion energies and energy-degrader foils. High temperature annealing was carried out on the as-irradiated samples to study the possible out-surface diffusion. Before and after annealing, Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) measurements were employed to obtain the elemental profiles of the implanted samples. The results suggest little migration of buried Ag in the bulk, and an out-diffusion of the implanted Ag in the near-surface region of single crystal SiC. It is also found that a SiO2 layer, which was formed during annealing, may serve as an effective barrier to reduce or prevent Ag out diffusion through the SiC coating layer.
Direct visualization of surface-plasmon bandgaps in the diffuse background of metallic gratings.
Depine, Ricardo A; Ledesma, Silvia
2004-10-01
When a surface plasmon propagates along a microrough grating, it interacts with the periodic plus the random roughness and emits light into the diffuse background, which can present intensity maxima called diffuse light bands. We reexamine previous studies on these bands within the framework of recent studies on photonic surfaces and show that the phenomenon of diffuse light provides an experimental technique for directly imaging the dispersion relation of surface plasmons, including the gap that, under appropriate circumstances, opens in the reciprocal grating space. PMID:15524359
Effect of high rotor pressure-surface diffusion on performance of a transonic turbine
NASA Technical Reports Server (NTRS)
Miser, James W; Stewart, Warner L; Monroe, Daniel E
1955-01-01
The subject turbine was investigated to determine the effect of high rotor pressure-surface diffusion on turbine performance. A comparison of the subject turbine with the most efficient transonic turbine in the present series of investigations showed that the efficiency of the subject turbine was almost as high, the suction-surface diffusion parameter was about the same, and the solidity was reduced by 36 percent. Because the loss per blade increased greatly with an increase in pressure-surface diffusion, the latter is also considered to be an important design consideration.
Senyuk, Bohdan; Glugla, David; Smalyukh, Ivan I
2013-12-01
We study translational and rotational diffusion of anisotropic gold nanoparticles (NPs) dispersed in the bulk of a nematic liquid crystal fluid host. Experimental data reveal strong anisotropy of translational diffusion with respect to the uniform far-field director, which is dependent on shape and surface functionalization of colloids as well as on their ground-state alignment. For example, elongated NPs aligned parallel to the far-field director translationally diffuse more rapidly along the director whereas diffusion of NPs oriented normal to the director is faster in the direction perpendicular to it while they are also undergoing elasticity-constrained rotational diffusion. To understand physical origins of these rich diffusion properties of anisotropic nanocolloids in uniaxially anisotropic nematic fluid media, we compare them to diffusion of prolate and oblate ellipsoidal particles in isotropic fluids as well as to diffusion of shape-isotropic particles in nematic fluids. We also show that surface functionalization of NPs with photosensitive azobenzene groups allows for in situ control of their diffusivity through trans-cis isomerization that changes surface anchoring. PMID:24483468
Disturbance-driven Hillslope Diffusion Scales and Values Clarified by Extant Surface Roughness
NASA Astrophysics Data System (ADS)
Doane, T.; Furbish, D. J.
2015-12-01
In low-relief landscapes, the hillslope diffusion equation approximates the rate of topographic evolution due to disturbance driven sediment transport. Whereas this expression is appealing and performs well, the physical meaning of a hillslope diffusivity remains unclear. Here, a study of the disturbances that redistribute sediment on hillslopes clarifies a physical interpretation. We conceptualize the cumulative hillslope diffusivity, which is a rate constant for topographic degradation at large scales, as the aggregate of all surface disturbing processes. A numerical model that generates pit and mound topography from tree throw events illustrates this idea. Using the diffusion equation, we model the degradation of pits and mounds by all smaller scale disturbances. However, when examined at a larger scale, the effective hillslope diffusivity is composed of the small scale diffusivity plus the effect of tree throw. We also present a method to determine the background hillslope diffusivity using the extant hillslope roughness and rates of roughness production. Numerical simulations show that the variance of the surface roughness of a hillslope as introduced by pit and mound topography reaches a steady state when the rate of variance production (tree throw) is constant. The magnitude of the steady state variance is a function of variance production and decay (small scale diffusivity), so there is an opportunity to determine hillslope diffusivity values if the rates of variance production are known. This method yields estimates of the modern hillslope diffusivity which are useful in problems involving the human and climate change time scales.
Protein diffusion and long-term adsorption states at charged solid surfaces.
Kubiak-Ossowska, Karina; Mulheran, Paul A
2012-11-01
The diffusion pathways of lysozyme adsorbed to a model charged ionic surface are studied using fully atomistic steered molecular dynamics simulation. The simulations start from existing protein adsorption trajectories, where it has been found that one particular residue, Arg128 at the N,C-terminal face, plays a crucial role in anchoring the lysozyme to the surface [Langmuir 2010 , 26 , 15954 - 15965]. We first investigate the desorption pathway for the protein by pulling the Arg128 side chain away from the surface in the normal direction, and its subsequent readsorption, before studying diffusion pathways by pulling the Arg128 side chain parallel to the surface. We find that the orientation of this side chain plays a decisive role in the diffusion process. Initially, it is oriented normal to the surface, aligning in the electrostatic field of the surface during the adsorption process, but after resorption it lies parallel to the surface, being unable to return to its original orientation due to geometric constraints arising from structured water layers at the surface. Diffusion from this alternative adsorption state has a lower energy barrier of ∼0.9 eV, associated with breaking hydrogen bonds along the pathway, in reasonable agreement with the barrier inferred from previous experimental observation of lysozyme surface clustering. These results show the importance of studying protein diffusion alongside adsorption to gain full insight into the formation of protein clusters and films, essential steps in the future development of functionalized surfaces. PMID:23062108
Tuning molecule-substrate coupling via deposition of metal adatoms
NASA Astrophysics Data System (ADS)
Yang, Hung-Hsiang; Tsai, Hsu-Han; Ying, Chi-Feng; Yang, Tsung-Han; Kaun, Chao-Cheng; Chen, Chun-hsien; Lin, Minn-Tsong
2015-11-01
Organic-inorganic hybrids constitute an important class of functional materials. The fundamentals at the molecular levels are, however, relatively unexplored. PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) is a colorant and extensively applied in organic-based optoelectronic devices. PTCDA/Cu(111) and Fe-PTCDA/Cu(111) metal-organic hybrid monolayers are studied by low temperature scanning tunneling microscopy and spectroscopy (STS) and density functional theory (DFT). The former exhibits Moiré pattern-modulated molecular density of states while the latter adapts a commensurate adlattice. Both imaging and spectroscopic results suggest a strong hybridization between PTCDA molecules and Cu(111) substrate. Weak PTCDA-Cu(111) coupling can be obtained by the introduction of Fe adatoms. Compared to PTCDA/Cu(111), STS spectra of Fe-PTCDA/Cu(111) exhibit a higher energy and sharper features of the frontier orbitals. Together with the DFT results, we found that the PTCDA-Cu(111) coupling is attenuated by the presence of Fe adatoms and Fe-PTCDA coordination.
Tuning molecule-substrate coupling via deposition of metal adatoms.
Yang, Hung-Hsiang; Tsai, Hsu-Han; Ying, Chi-Feng; Yang, Tsung-Han; Kaun, Chao-Cheng; Chen, Chun-Hsien; Lin, Minn-Tsong
2015-11-14
Organic-inorganic hybrids constitute an important class of functional materials. The fundamentals at the molecular levels are, however, relatively unexplored. PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride) is a colorant and extensively applied in organic-based optoelectronic devices. PTCDA/Cu(111) and Fe-PTCDA/Cu(111) metal-organic hybrid monolayers are studied by low temperature scanning tunneling microscopy and spectroscopy (STS) and density functional theory (DFT). The former exhibits Moiré pattern-modulated molecular density of states while the latter adapts a commensurate adlattice. Both imaging and spectroscopic results suggest a strong hybridization between PTCDA molecules and Cu(111) substrate. Weak PTCDA-Cu(111) coupling can be obtained by the introduction of Fe adatoms. Compared to PTCDA/Cu(111), STS spectra of Fe-PTCDA/Cu(111) exhibit a higher energy and sharper features of the frontier orbitals. Together with the DFT results, we found that the PTCDA-Cu(111) coupling is attenuated by the presence of Fe adatoms and Fe-PTCDA coordination. PMID:26567677
Differential column batch reactor (DCBR) experiments in organic-free water were conducted for the following volatile organic compounds (VOCs): trichloroethene, tetrachloroethene, cis-1,2 dichlorethene, and toluene. Surface diffusion was required to explain the rate of uptake for ...
Specific ions modulate diffusion dynamics of hydration water on lipid membrane surfaces.
Song, Jinsuk; Franck, John; Pincus, Philip; Kim, Mahn Won; Han, Songi
2014-02-12
Effects of specific ions on the local translational diffusion of water near large hydrophilic lipid vesicle surfaces were measured by Overhauser dynamic nuclear polarization (ODNP). ODNP relies on an unpaired electron spin-containing probe located at molecular or surface sites to report on the dynamics of water protons within ~10 Å from the spin probe, which give rise to spectral densities for electron-proton cross-relaxation processes in the 10 GHz regime. This pushes nuclear magnetic resonance relaxometry to more than an order of magnitude higher frequencies than conventionally feasible, permitting the measurement of water moving with picosecond to subnanosecond correlation times. Diffusion of water within ~10 Å of, i.e., up to ~3 water layers around the spin probes located on hydrophilic lipid vesicle surfaces is ~5 times retarded compared to the bulk water translational diffusion. This directly reflects on the activation barrier for surface water diffusion, i.e., how tightly water is bound to the hydrophilic surface and surrounding waters. We find this value to be modulated by the presence of specific ions in solution, with its order following the known Hofmeister series. While a molecular description of how ions affect the hydration structure at the hydrophilic surface remains to be answered, the finding that Hofmeister ions directly modulate the surface water diffusivity implies that the strength of the hydrogen bond network of surface hydration water is directly modulated on hydrophilic surfaces. PMID:24456096
Crystal surface integrity and diffusion measurements on Earth and planetary materials
NASA Astrophysics Data System (ADS)
Watson, E. B.; Cherniak, D. J.; Thomas, J. B.; Hanchar, J. M.; Wirth, R.
2016-09-01
Characterization of diffusion behavior in minerals is key to providing quantitative constraints on the ages and thermal histories of Earth and planetary materials. Laboratory experiments are a vital source of the needed diffusion measurements, but these can pose challenges because the length scales of diffusion achievable in a laboratory time are commonly less than 1 μm. An effective strategy for dealing with this challenge is to conduct experiments involving inward diffusion of the element of interest from a surface source, followed by quantification of the resulting diffusive-uptake profile using a high-resolution depth-profiling technique such as Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA), or ion microprobe (SIMS). The value of data from such experiments is crucially dependent on the assumption that diffusion in the near-surface of the sample is representative of diffusion in the bulk material. Historical arguments suggest that the very process of preparing a polished surface for diffusion studies introduces defects-in the form of dislocations and cracks-in the outermost micrometer of the sample that make this region fundamentally different from the bulk crystal in terms of its diffusion properties. Extensive indirect evidence suggests that, in fact, the near-surface region of carefully prepared samples is no different from the bulk crystal in terms of its diffusion properties. A direct confirmation of this conclusion is nevertheless clearly important. Here we use transmission electron microscopy to confirm that the near-surface regions of olivine, quartz and feldspar crystals prepared using careful polishing protocols contain no features that could plausibly affect diffusion. This finding does not preclude damage to the mineral structure from other techniques used in diffusion studies (e.g., ion implantation), but even in this case the role of possible structural damage can be objectively assessed and controlled. While all
Light adatoms influences on electronic structures of the two-dimensional arsenene nanosheets
NASA Astrophysics Data System (ADS)
Li, Yang; Xia, Congxin; Wang, Tianxing; Tan, Xiaoming; Zhao, Xu; Wei, Shuyi
2016-03-01
Gray arsenic monolayer named as arsenene is a new kind of two-dimensional (2D) semiconductor material. Herein, we focus on the electronic structures of the light atoms (such as B, C, N, O, F) adsorbed arsenene nanosheets by using first-principles calculations. The results show that most adatoms prefer to occupy the bridge site on the arsenene nanosheets except for the C adatom which prefer to valley site. The defect states can be found in the middle gap of the F adsorbed arsenene nanosheets, and N adatom can induce the n-type doping in the system. Moreover, O adatom has negligible effects on its electronic structures. In addition, B, C, N and F adatoms can induce the magnetism in the arsenene nanosheets.
NASA Astrophysics Data System (ADS)
Li, Zhi-Peng; Tok, Eng Soon; Foo, Yong Lim
2010-10-01
In situ ultrahigh vacuum transmission electron microscopy was employed to observe the dynamics of both the coalescence and decay of two-dimensional Fe13Ge8 islands on an atomically smooth Ge(001) surface. Quasi-one-dimensional (1D) island decay behavior was observed on the isotropic Ge(001) surface where annealing caused the islands to shrink significantly faster in one direction. The shrinkage in the perpendicular direction was minimal in comparison. This is contrary to Ostwald ripening for isotropic surfaces, as well as for the quasi-1D island decay model for anisotropic surfaces. During annealing, this island decay behavior occurred by the attachment/detachment limiting mechanism, which is based on an analysis of adatom concentration differences. Therefore, this quasi-1D island decay process is driven by adatom concentration differences between coalescing and decaying islands. Island decay occurs along the large diffusive flux channel between islands and is affected by the local environment.
Anomalous behavior of the coherent light diffusion by a tilted translucent rough surface: part I
NASA Astrophysics Data System (ADS)
Rebollo, M. A.; Perez Quintian, F.; Hogert, Elsa N.; Landau, Monica R.; Gaggioli, Nestor G.
1996-02-01
When a translucent rough surface is illuminated, light is diffused in different directions. The envelope the intensity distribution is called diffusion curve. As the diffuser is rotated with respect to the incident beam, the diffuser curve changes its appearance: the maximum suffers a displacement and changes its shape. Some authors have studied this phenomenon, but none of them explained it properly. In this work we make an additional contribution to address the problem, showing experimentally that the maximum displacement depends on the incident angle and the diffuser ratio T/(sigma) . We compare our experimental results with those that can be calculated with the reformulated Beckmann's theory. We could observe important agreements and differences. For example, Beckmann's theory predicts that the diffusion results are asymmetric, while our measured results are indefectibly symmetric.
Reflection of diffuse light from dielectric one-dimensional rough surfaces.
González-Alcalde, Alma K; Méndez, Eugenio R; Terán, Emiliano; Cuppo, Fabio L S; Olivares, J A; García-Valenzuela, Augusto
2016-03-01
We study the reflection of diffuse light from 1D randomly rough dielectric interfaces. Results for the reflectance under diffuse illumination are obtained by rigorous numerical simulations and then contrasted with those obtained for flat surfaces. We also explore the possibility of using perturbation theories and conclude that they are limited for this type of study. Numerical techniques based on Kirchhoff approximation and reduced Rayleigh equations yield better results. We find that, depending on the refractive index contrast and nature of the irregularities, the roughness can increase or decrease the diffuse reflectance of the surface. PMID:26974906
Franco, Luís F M; Castier, Marcelo; Economou, Ioannis G
2016-08-28
Applying classical molecular dynamics simulations, we calculate the parallel self-diffusion coefficients of different fluids (methane, nitrogen, and carbon dioxide) confined between two {101̄4} calcite crystal planes. We have observed that the molecules close to the calcite surface diffuse differently in distinct directions. This anisotropic behavior of the self-diffusion coefficient is investigated for different temperatures and pore sizes. The ion arrangement in the calcite crystal and the strong interactions between the fluid particles and the calcite surface may explain the anisotropy in this transport property. PMID:27586936
Degenerate mobilities in phase field models are insufficient to capture surface diffusion
NASA Astrophysics Data System (ADS)
Lee, Alpha A.; Münch, Andreas; Süli, Endre
2015-08-01
Phase field models frequently provide insight into phase transitions and are robust numerical tools to solve free boundary problems corresponding to the motion of interfaces. A body of prior literature suggests that interface motion via surface diffusion is the long-time, sharp interface limit of microscopic phase field models such as the Cahn-Hilliard equation with a degenerate mobility function. Contrary to this conventional wisdom, we show that the long-time behaviour of degenerate Cahn-Hilliard equation with a polynomial free energy undergoes coarsening, reflecting the presence of bulk diffusion, rather than pure surface diffusion. This reveals an important limitation of phase field models that are frequently used to model surface diffusion.
Influence of Chemical Composition of Mg Alloys on Surface Alloying by Diffusion Coating
NASA Astrophysics Data System (ADS)
Hirmke, J.; Zhang, M.-X.; St John, D. H.
2012-05-01
A recently developed technique of surface alloying by diffusion-coating has been used to produce coatings on Mg alloys with various Al and Zn contents. The experimental results show that both Al and Zn solutes in the alloy promote the diffusion of alloying elements through grain refinement of the substrate alloys and through reduction of diffusion active energy because of the reduction of melting temperature of the alloys. Therefore, the efficiency of surface alloying increases by diffusion coating. Thick, dense, uniform, and continuous layers of intermetallic compounds, which consist of a τ-phase layer and a β-phase layer, can be produced on the surface of various Mg alloys. The intermetallic compound layers not only have microhardness values that are 4 to 6 times higher than the substrate but also provide effective protection of the Mg alloys from corrosion in 5 pct NaCl solution at room temperature.
Effect of Surface Preparation on CLAM/CLAM Hot Isostatic Pressing diffusion bonding joints
NASA Astrophysics Data System (ADS)
Li, C.; Huang, Q.; Zhang, P.
2009-04-01
Surface preparation is essential for the Hot Isostatic Pressing (HIP) diffusion bonding of RAFM steels. Hot Isostatic Pressing (HIP) diffusion bonding experiments on China Low Activation Martensitic (CLAM) steel was performed to study the effect of surface preparation. A few approaches such as hand lapping, dry-milling and grinding etc., were used to prepare the faying surfaces of the HIP joints. Different sealing techniques were used as well. The HIP parameters were 150 MPa/3 h/1150 °C. After post HIP heat treatment (PHHT), the tensile and Charpy impact tests were carried out. The results showed that hand lapping was not suitable to prepare the faying surfaces of HIP diffusion bonding specimens although the surface roughness by hand lapping was very low.
Using Self-Similarity to Simulate Meniscus Evolution Around TMV Due to Surface Diffusion
NASA Astrophysics Data System (ADS)
Potter, Richard; Zhang, Yue; Fakhraai, Zahra
It has been hypothesized that enhanced surface diffusion allows the formation of stable molecular glasses during physical vapor deposition. The improved properties of these glasses, such as increased density and kinetic stability can help improve material properties in pioneering fields of technology such as organic electronics and pharmaceutical drug delivery. While surface diffusion has been measured previously on the surfaces of organic glasses, direct measurements on the surface of vapor-deposited stable glasses has proven more challenging. This research focuses on a straightforward method for measuring the surface diffusion coefficients of molecular glasses through the use of tobacco mosaic virus (TMV) nanorods as probe particles. In conjunction, mathematical models based on the thin film equation were used to simulate fast meniscus formation around the nanorods on the glassy surface. The evolution of the meniscus is self-similar, which allows quick quantification of the diffusion coefficient, by solving the time evolution for a single experiment. Experimental data were compared and fit to these simulations to derive a quantity for the surface diffusion coefficient, Ds. Nsf-CAREER DMR-1350044.
Self-assembled molecular corrals on a semiconductor surface
NASA Astrophysics Data System (ADS)
Dobrin, S.; Harikumar, K. R.; Jones, R. V.; Li, N.; McNab, I. R.; Polanyi, J. C.; Sloan, P. A.; Waqar, Z.; Yang, J.(S. Y.); Ayissi, S.; Hofer, W. A.
2006-03-01
Nano-corrals for capturing surface electrons are of interest in molecular electronics. Here we show that haloalkane molecules, e.g., 1-chlorododecane, physisorbed on Si(1 1 1)-(7 × 7) self-assemble to form dimers stable to 100 °C which corral silicon adatoms. Corral size is shown to be governed by the haloalkane chain-length. Spectroscopic and theoretical evidence shows that the haloalkane dimer induces electron transfer to the corralled adatom, shifting its energy levels by ˜1 eV. Isolation of a labile pre-cursor points to a model for corral formation which combines mobility with immobility; monomers diffusing in a mobile vertical state meet and convert to the immobile horizontal dimers constituting the corrals.
Chern mosaic: topology of chiral superconductivity in ferromagnetic adatom lattices
NASA Astrophysics Data System (ADS)
Rontynen, Joel; Ojanen, Teemu
Recent experiments have demonstrated signatures of Majorana bound states in ferromagnetic atomic chains. We show that similar systems, extended to two dimensional geometry, may support chiral topological superconductivity with large Chern numbers. Our observation is based on the fact that magnetic adatoms on an s-wave superconductor bind subgap Shiba states, which can hybridize and form subgap energy bands with nontrivial topology. Such a Shiba lattice supports long-range hopping, leading to a complex, mosaic-like phase diagram with large Chern numbers. We analyze the incidence of different Chern numbers phases and the size of their energy gaps for various lattice geometries. Our findings reveal the studied system as one of the riches platforms of topological matter known to date. The authors acknowledge the Finnish Cultural Foundation and the Academy of Finland for support.
NASA Astrophysics Data System (ADS)
Verlinden, Pierre; Van de Wiele, Fernand
1985-03-01
A method is proposed for measuring the diffusion length and surface recombination velocity of Interdigitated Back Contact (IBC) solar cells by means of a simple linear regression on experimental quantum efficiency values versus the inverse of the absorption coefficient. This method is extended to the case of Front Surface Field (FSF) solar cells. Under certain conditions, the real or the effective surface recombination velocity may be measured.
Diffusion of low molecular weight siloxane from bulk to surface
Homma, H.; Kuroyagi, T.; Mirley, C.L.; Ronzello, J.; Boggs, S.A.
1996-12-31
Silicone-based materials for outdoor insulators have the advantage that low molecular weight (LMW) components migrate through the material and coat the surface, thereby restoring hydrophobicity over a period of hours. By measuring the infrared (IR) absorption of siloxane migrating to the silicone surface through a thin carbon coating, the aspect of the LMW siloxane migration was observed as a real time plot and the time constant of the migration was calculated. According to the time dependence of IR-absorbance, the migration mostly saturated within only 12 hours after the carbon coating was applied. Also, the time constant showed a dependence on the concentration of added filler in the silicone samples.
Magnetism of Adatom on Bilayer Graphene and its Control: A First-principles Perspective
NASA Astrophysics Data System (ADS)
Saha-Dasgupta, Tanusri; Nafday, Dhani
2014-03-01
We present first-principles investigation of the electronic and magnetic properties of adatom on bilayer graphene within the framework of density functional theory. In particular, we study the influence of an applied gate-voltage which modifies the electronic states of the bilayer graphene as well as shifts the adatom energy states relative to that of the graphene energy states. Our study carried out for a choice of three different adatoms, Na, Cu and Fe, shows that the nature of adatom-graphene bonding evolves from ionic to covalent, in moving from alkali metal, Na to transition metal, Cu or Fe. This leads to the formation of magnetic moments in the latter cases (Cu, Fe) and its absence in the former (Na). Application of an external electric field to bilayer graphene, completely changes the scenario, switching on a magnetic moment for Na adatom, and switching off the magnetic moments for Cu, and Fe adatoms. Our results have important implications for fundamental studies of controlled adatom magnetism and spintronics application in nanotechnology. The authors thank Ministry of Earth Science and Department of Science and Technology, India for financial support.
Effects of adatoms and physisorbed molecules on the physical properties of antimonene
NASA Astrophysics Data System (ADS)
Üzengi Aktürk, O.; Aktürk, E.; Ciraci, S.
2016-01-01
A recent study predicted that a 2D single layer of antimony in buckled honeycomb as well as asymmetric washboard structures, named antimonene, is stable at high temperature and displays semiconducting properties. Based on first-principles, spin-polarized density functional calculations, we investigated chemisorption of selected adatoms and physisorption of molecules on two antimonene phases. Since adspecies-adspecies interaction is minimized by using large supercells, our results mimic the effects of isolated, single adatoms or molecules. We found that molecules such as H2,O2, and H2O neither form strong chemical bonds nor dissociate; they are physisorbed with a weak binding energy without affecting the properties of antimonene. The adatoms, such as H, Li, B, C, N, O, Al, In, Si, P, Cl, Ti, As, and Sb, are chemisorbed with significant binding energy, whereby the atomic and electronic structures are modified locally. Boron and carbon adatoms are implemented into buckled antimonene crystal leading to a local reconstruction of the crystal. Nitrogen gives rise to Stone-Wales type defects. The localized states originating from adatoms give rise to diversity of electronic structure. The lowest conduction and highest valence bands of antimonene in asymmetric washboard structures have very high curvature. Once combined with adatom states, these bands offer a variety of features. Specific adatoms lead to spin polarization, attain magnetic moments, and can attribute a half-metallic character to antimonene.
Radar scattering from a diffuse vegetation layer over a smooth surface
NASA Technical Reports Server (NTRS)
Engheta, N.; Elachi, C.
1982-01-01
A simple model is presented for the oblique backscatter and bistatic scatter from a smooth surface overlain by a diffuse layer. Only single scattering in the diffuse layer is taken into account. The model analysis shows that the combination of volume scattering and oblique reflection at the surface may increase appreciably the waves scattering. The scattering strongly depends on the properties of the smooth surface. These results support some of the observations made with the Seasat spaceborne imaging radar over flooded regions with heavy vegetation cover.
On the flow of non-axisymmetric perturbations of cylinders via surface diffusion
NASA Astrophysics Data System (ADS)
LeCrone, Jeremy; Simonett, Gieri
2016-03-01
We study the surface diffusion flow acting on a class of general (non-axisymmetric) perturbations of cylinders Cr in IR3. Using tools from parabolic theory on uniformly regular manifolds, and maximal regularity, we establish existence and uniqueness of solutions to surface diffusion flow starting from (spatially-unbounded) surfaces defined over Cr via scalar height functions which are uniformly bounded away from the central cylindrical axis. Additionally, we show that Cr is normally stable with respect to 2π-axially-periodic perturbations if the radius r > 1, and unstable if 0 < r < 1. Stability is also shown to hold in settings with axial Neumann boundary conditions.
Effects of rainfall and surface flow on chemical diffusion from soil to runoff water
Technology Transfer Automated Retrieval System (TEKTRAN)
Although basic processes of diffusion and convection have been used to quantify chemical transport from soil to surface runoff, there are little research results actually showing how these processes were affected by rainfall and surface flow. We developed a laboratory flow cell and a sequence of exp...
Effects of angular dependence of surface diffuseness in deformed nuclei on Coulomb barrier
NASA Astrophysics Data System (ADS)
Adamian, G. G.; Antonenko, N. V.; Malov, L. A.; Scamps, G.; Lacroix, D.
2014-09-01
The angular dependence of surface diffuseness is further discussed. The results of self-consistent calculations are compared with those obtained with the phenomenological mean-field potential. The rather simple parametrizations are suggested. The effects of surface polarization and hexadecapole deformation on the height of the Coulomb barrier are revealed.
Anisotropic diffusion of tensor fields for fold shape analysis on surfaces.
Boucher, Maxime; Evans, Alan; Siddiqi, Kaleem
2011-01-01
The folding pattern of the human cortical surface is organized in a coherent set of troughs and ridges, which mark important anatomical demarcations that are similar across subjects. Cortical surface shape is often analyzed in the literature using isotropic diffusion, a strategy that is questionable because many anatomical regions are known to follow the direction of folds. This paper introduces anisotropic diffusion kernels to follow neighboring fold directions on surfaces, extending recent literature on enhancing curve-like patterns in images. A second contribution is to map deformations that affect sulcal length, i.e., are parallel to neighboring folds, with other deformations that affect sulcal length, within the diffusion process. Using the proposed method, we demonstrate anisotropic shape differences of the cortical surface associated with aging in a database of 95 healthy subjects, such as a contraction of the cingulate sulcus, shorter gyri in the temporal lobe and a contraction in the frontal lobe. PMID:21761663
NASA Astrophysics Data System (ADS)
Toušek, J.; Toušková, J.; Remeš, Z.; Čermák, J.; Kousal, J.; Kindl, D.; Kuřitka, I.
2012-11-01
Novelized method of the surface photovoltage (SPV) measurement convenient for evaluation of exciton diffusion length and thickness of the space charge region (SCR) in organic semiconductors is applied to poly[2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) polymer. Exciton diffusion length and thickness of the SCR was found. The experiment is complemented by measurements of surface potential by the Kelvin probe force microscopy yielding the work function and concentration of free holes. The latter value is much lower than the concentration of ionized states determined from the thickness of the space charge region, which can be ascribed to the presence of traps.
Study on surface photovoltage measurement of long diffusion length silicon: Simulation results
NASA Astrophysics Data System (ADS)
Anttila, O. J.; Hahn, S. K.
1993-07-01
The limitations of surface photovoltage method (SPV), using the analytical solution for the minority-carrier distribution, are studied in detail. The principal source of error in thin wafers of long diffusion length material is the back surface recombination. The possibility of measuring large diffusion lengths in a reliable manner is discussed. With proper back surface passivation, diffusion lengths as long as several times the sample thickness can be measured. Evaluation of minor amounts of iron contamination in p-type material is possible even if the back surface recombination velocity is high, but a correction factor is required for the traditionally used relationship to convert the observed diffusion length change to iron concentration. The value of the correction factor varies between 1 and 1.5 for high back surface recombination velocity; the magnitude depends on the ratio between wafer thickness and the diffusion length of the minority carriers. The detection limit for iron is in the 109-1011 at/cm3 range, depending on the quality of the instrumentation and on the sample preparation; low back surface recombination gives better sensitivity. The use of SPV to measure the denuded zone width in precipitated material is briefly analyzed. It is shown that in heavily precipitated material an SPV measurement underestimates the denuded zone width, and the result will be very sensitive to the diffusion length in the bulk of the sample. Finally, the sensitivity of the measured diffusion length to different sources of errors and requirements for the sample preparation are discussed. The use of constant-flux method introduces some potential errors, which can be avoided through the use of constant bias illumination during the measurement.
Özarslan, Evren; Nevo, Uri; Basser, Peter J.
2008-01-01
In MRI, macroscopic boundaries lead to a diffusion-related increase in signal intensity near them—an effect commonly referred to as edge-enhancement. In diffusion-weighted imaging protocols where the signal attenuation due to diffusion results predominantly from the application of magnetic field gradients, edge-enhancement will depend on the orientation of these diffusion gradients. The resulting diffusion anisotropy can be exploited to map the direction normal to the macroscopic boundary. Simulations suggest that the hypothesized anisotropy may be within observable limits even when the voxel contains no boundary itself—hence, the name remote-anisotropy. Moreover, for certain experimental parameters there may be significant phase cancellations within the voxel that may lead to an edge detraction effect. When this is avoided, the eigenvector corresponding to the smallest eigenvalue of the diffusion tensor obtained from diffusion-tensor imaging can be used to create surface-normal maps conveniently. Experiments performed on simple geometric constructs as well as real tissue demonstrate the feasibility of using the edge-enhancement mechanism to map orientations orthogonal to macroscopic surfaces, which may be used to assess the integrity of tissue and organ boundaries noninvasively. PMID:18065457
NASA Astrophysics Data System (ADS)
Robinson, Philip W.
This thesis addresses the effect of reflections from diffusive architectural surfaces on the perception of echoes and on auditory spatial resolution. Diffusive architectural surfaces play an important role in performance venue design for architectural expression and proper sound distribution. Extensive research has been devoted to the prediction and measurement of the spatial dispersion. However, previous psychoacoustic research on perception of reflections and the precedence effect has focused on specular reflections. This study compares the echo threshold of specular reflections, against those for reflections from realistic architectural surfaces, and against synthesized reflections that isolate individual qualities of reflections from diffusive surfaces, namely temporal dispersion and spectral coloration. In particular, the activation of the precedence effect, as indicated by the echo threshold is measured. Perceptual tests are conducted with direct sound, and simulated or measured reflections with varying temporal dispersion. The threshold for reflections from diffusive architectural surfaces is found to be comparable to that of a specular re ection of similar energy rather than similar amplitude. This is surprising because the amplitude of the dispersed re ection is highly attenuated, and onset cues are reduced. This effect indicates that the auditory system is integrating re ection response energy dispersed over many milliseconds into a single stream. Studies on the effect of a single diffuse reflection are then extended to a full architectural enclosure with various surface properties. This research utilizes auralizations from measured and simulated performance venues to investigate spatial discrimination of multiple acoustic sources in rooms. It is found that discriminating the lateral arrangement of two sources is possible at narrower separation angles when reflections come from at rather than diffusive surfaces. Additionally, subjective impressions are
Dynamics of surface thermal expansion and diffusivity using two-color reflection transient gratings
Pennington, D.M.; Harris, C.B.
1993-02-01
We report ultrafast measurements of the dynamic thermal expansion of a surface and the temperature dependent surface thermal diffusivity using a two-color reflection transient grating technique. Studies were performed on p-type, n-type, and undoped GaAs(100) samples at several temperatures. Using a 75 fs ultraviolet probe with visible excitation beams, the electronic effects that dominate single color experiments become negligible; thus surface expansion due to heating and the subsequent contraction caused by cooling provide the dominant influence on the diffracted probe. The diffracted signal was composed of two components, thermal expansion of the surface and heat flow away from the surface, allowing the determination of the rate of expansion as well as the surface thermal diffusivity. At room temperature a signal rise due to thermal expansion was observed, corresponding to a maximum average displacement of {approx} 1 {angstrom} at 32 ps. Large fringe spacings were used, thus the dominant contributions to the signal were expansion and diffusion perpendicular to the surface. Values for the surface thermal diffusivity of GaAs were measured and found to be in reasonable agreement with bulk values above 50{degrees}K. Below 50{degrees}K, the diffusivity at the surface was more than an order of magnitude slower than in the bulk due to increased phonon boundary scattering. Comparison of the results with a straightforward thermal model yields good agreement over a range of temperatures (12--300{degrees}K). The applicability and advantages of the transient grating technique for studying photothermal and photoacoustic phenomena are discussed.
Evans, J. W.; Bartelt, M. C.
2001-06-15
We consider the irreversible nucleation and growth of two-dimensional islands during submonolayer deposition in the regime of large island sizes. A quasihydrodynamic analysis of rate equations for island densities yields an ordinary differential equation (ODE) for the scaling function describing the island size distribution. This ODE involves the scaling function for the dependence on island size of {open_quotes}capture numbers{close_quotes} describing the aggregation of diffusing adatoms. The latter is determined via a quasihydrodynamic analysis of rate equations for the areas of {open_quotes}capture zones{close_quotes} surrounding islands. Alternatively, a more complicated analysis yields a partial differential equation (PDE) for the scaling function describing the joint probability distribution for island sizes and capture zone areas. Then, applying a moment analysis to this PDE, we obtain refined versions of the above ODE{close_quote}s, together with a third equation for the variance of the cell area distribution (for islands of a given size). The key nontrivial input to the above equations is a detailed characterization of nucleation. We analyze these equations for a general formulation of nucleation, as well as for an idealized picture considered previously, wherein nucleated islands have capture zones lying completely within those of existing islands.
Ku, Bon Ki; Kulkarni, Pramod
2015-01-01
We compare different approaches to measure surface area of aerosol agglomerates. The objective was to compare field methods, such as mobility and diffusion charging based approaches, with laboratory approach, such as Brunauer, Emmett, Teller (BET) method used for bulk powder samples. To allow intercomparison of various surface area measurements, we defined ‘geometric surface area’ of agglomerates (assuming agglomerates are made up of ideal spheres), and compared various surface area measurements to the geometric surface area. Four different approaches for measuring surface area of agglomerate particles in the size range of 60–350 nm were compared using (i) diffusion charging-based sensors from three different manufacturers, (ii) mobility diameter of an agglomerate, (iii) mobility diameter of an agglomerate assuming a linear chain morphology with uniform primary particle size, and (iv) surface area estimation based on tandem mobility–mass measurement and microscopy. Our results indicate that the tandem mobility–mass measurement, which can be applied directly to airborne particles unlike the BET method, agrees well with the BET method. It was also shown that the three diffusion charging-based surface area measurements of silver agglomerates were similar within a factor of 2 and were lower than those obtained from the tandem mobility–mass and microscopy method by a factor of 3–10 in the size range studied. Surface area estimated using the mobility diameter depended on the structure or morphology of the agglomerate with significant underestimation at high fractal dimensions approaching 3. PMID:26692585
Jump rates for surface diffusion of large molecules from first principles
Shea, Patrick Kreuzer, Hans Jürgen
2015-04-21
We apply a recently developed stochastic model for the surface diffusion of large molecules to calculate jump rates for 9,10-dithioanthracene on a Cu(111) surface. The necessary input parameters for the stochastic model are calculated from first principles using density functional theory (DFT). We find that the inclusion of van der Waals corrections to the DFT energies is critical to obtain good agreement with experimental results for the adsorption geometry and energy barrier for diffusion. The predictions for jump rates in our model are in excellent agreement with measured values and show a marked improvement over transition state theory (TST). We find that the jump rate prefactor is reduced by an order of magnitude from the TST estimate due to frictional damping resulting from energy exchange with surface phonons, as well as a rotational mode of the diffusing molecule.
Exploratory laboratory study of lateral turbulent diffusion at the surface of an alluvial channel
Sayre, William W.; Chamberlain, A.R.
1964-01-01
In natural streams turbulent diffusion is one of the principal mechanisms by which liquid and suspended-particulate contaminants are dispersed in the flow. A knowledge of turbulence characteristics is therefore essential in predicting the dispersal rates of contaminants in streams. In this study the theory of diffusion by continuous movements for homogeneous turbulence is applied to lateral diffusion at the surface of an open channel in which there is uniform flow. An exploratory-laboratory investigation was conducted in which the lateral dispersion at the water surface of a sand-Led flume was studied by measuring the lateral spread from a point source of small floating polyethylene articles. The experiment was restricted to a single set of low and channel geometry conditions. The results of the study indicate that with certain restrictions lateral dispersion in alluvial channels may be successfully described by the theory of diffusion by continuous movements. The experiment demonstrates a means for evaluating the lateral diffusion coefficient and also methods for quantitatively estimating fundamental turbulence properties, such as the intensity and the Lagrangian integral scale of turbulence in an alluvial channel. The experimental results show that with increasing distance from the source the coefficient of lateral turbulent diffusion increases initially but tends toward a constant limiting value. This result is in accordance with turbulent diffusion theory. Indications are that the distance downstream from the source required for the diffusion coefficient to reach its limiting value is actually very small when compared to the length scale of most diffusion phenomena in natural streams which are of practical interest.
Diffused surface water problems and a current of anomalous remedies
Smillie, P.L.
1986-01-01
The long-term changes in drainage patterns caused by mineral development may have an even greater impact on the environment and on landowners than the effects of subsidence. A review of the legal theories applied to drainage law and the judicial remedies they allow reveals a need for theories which are flexible for the injured landowner while offering reasonable standards of conduct for the developer. There is also a need for legislation to recognize existing judicial theories governing surface water disputes. There is a growing demand for a unified approach to water law, but no consensus on which approach to follow. A first step would be to acknowledge that each approach has its advantages and shortcomings. Correcting the disparities within the judicial and administrative forums and working to improve both systems is essential.
NASA Astrophysics Data System (ADS)
Hahne, Susanne; Ikonomov, Julian; Sokolowski, Moritz; Maass, Philipp
2013-02-01
Methods of determining surface diffusion coefficients of molecules from signal fluctuations of a locally fixed probe are revisited and refined. Particular emphasis is put on the influence of the molecule's extent. In addition to the formerly introduced autocorrelation method and residence time method, we develop a further method based on the distribution of intervals between successive peaks in the signal. The theoretical findings are applied to scanning tunneling microscopy measurements of copper phthalocyanine (CuPc) molecules on the Ag(100) surface. We discuss advantages and disadvantages of each method and suggest a combination to obtain accurate results for diffusion coefficients.
NASA Astrophysics Data System (ADS)
Erofeev, Mikhail V.; Shulepov, Mikhail A.; Tarasenko, Victor F.
2015-12-01
The paper presents the results of an examination of aluminum samples exposed to runaway electron preionized diffuse discharges in air, nitrogen, and argon at atmospheric pressure. The changes in the chemical composition, structure, and hardness of the aluminum surface layers caused by the action of the discharge were investigated. It has been found that the oxygen and carbon concentrations in the surface layers depend on the number of discharge pulses and on the chemical composition of the working gas. The goal of the study was to find possible uses of runaway electron preionized diffuse discharges in research and industry.