Atomistic full-quantum transport model for zigzag graphene nanoribbon-based structures: Complex energy-band method

NASA Astrophysics Data System (ADS)

Chen, Chun-Nan; Luo, Win-Jet; Shyu, Feng-Lin; Chung, Hsien-Ching; Lin, Chiun-Yan; Wu, Jhao-Ying

2018-01-01

Using a non-equilibrium Green’s function framework in combination with the complex energy-band method, an atomistic full-quantum model for solving quantum transport problems for a zigzag-edge graphene nanoribbon (zGNR) structure is proposed. For transport calculations, the mathematical expressions from the theory for zGNR-based device structures are derived in detail. The transport properties of zGNR-based devices are calculated and studied in detail using the proposed method.

Probing dynamic behavior of electric fields and band diagrams in complex semiconductor heterostructures

NASA Astrophysics Data System (ADS)

Turkulets, Yury; Shalish, Ilan

2018-01-01

Modern bandgap engineered electronic devices are typically made of multi-semiconductor multi-layer heterostructures that pose a major challenge to silicon-era characterization methods. As a result, contemporary bandgap engineering relies mostly on simulated band structures that are hardly ever verified experimentally. Here, we present a method that experimentally evaluates bandgap, band offsets, and electric fields, in complex multi-semiconductor layered structures, and it does so simultaneously in all the layers. The method uses a modest optical photocurrent spectroscopy setup at ambient conditions. The results are analyzed using a simple model for electro-absorption. As an example, we apply the method to a typical GaN high electron mobility transistor structure. Measurements under various external electric fields allow us to experimentally construct band diagrams, not only at equilibrium but also under any other working conditions of the device. The electric fields are then used to obtain the charge carrier density and mobility in the quantum well as a function of the gate voltage over the entire range of operating conditions of the device. The principles exemplified here may serve as guidelines for the development of methods for simultaneous characterization of all the layers in complex, multi-semiconductor structures.

Band gap in tubular pillar phononic crystal plate.

PubMed

Shu, Fengfeng; Liu, Yongshun; Wu, Junfeng; Wu, Yihui

2016-09-01

In this paper, a phononic crystal (PC) plate with tubular pillars is presented and investigated. The band structures and mode displacement profiles are calculated by using finite element method. The result shows that a complete band gap opens when the ratio of the pillar height to the plate thickness is about 1.6. However, for classic cylinder pillar structures, a band gap opens when the ratio is equal or greater than 3. A tubular pillar design with a void room in it enhances acoustic multiple scattering and gives rise to the opening of the band gap. In order to verify it, a PC structure with double tubular pillars different in size (one within the other) is introduced and a more than 2times band gap enlargement is observed. Furthermore, the coupling between the resonant mode and the plate mode around the band gap is characterized, as well as the effect of the geometrical parameters on the band gap. The behavior of such structure could be utilized to design a pillar PC with stronger structural stability and to enlarge band gaps. Copyright © 2016 Elsevier B.V. All rights reserved.

Selective Area Band Engineering of Graphene using Cobalt-Mediated Oxidation.

PubMed

Bazylewski, Paul F; Nguyen, Van Luan; Bauer, Robert P C; Hunt, Adrian H; McDermott, Eamon J G; Leedahl, Brett D; Kukharenko, Andrey I; Cholakh, Seif O; Kurmaev, Ernst Z; Blaha, Peter; Moewes, Alexander; Lee, Young Hee; Chang, Gap Soo

2015-10-21

This study reports a scalable and economical method to open a band gap in single layer graphene by deposition of cobalt metal on its surface using physical vapor deposition in high vacuum. At low cobalt thickness, clusters form at impurity sites on the graphene without etching or damaging the graphene. When exposed to oxygen at room temperature, oxygen functional groups form in proportion to the cobalt thickness that modify the graphene band structure. Cobalt/Graphene resulting from this treatment can support a band gap of 0.30 eV, while remaining largely undamaged to preserve its structural and electrical properties. A mechanism of cobalt-mediated band opening is proposed as a two-step process starting with charge transfer from metal to graphene, followed by formation of oxides where cobalt has been deposited. Contributions from the formation of both CoO and oxygen functional groups on graphene affect the electronic structure to open a band gap. This study demonstrates that cobalt-mediated oxidation is a viable method to introduce a band gap into graphene at room temperature that could be applicable in electronics applications.

Selective Area Band Engineering of Graphene using Cobalt-Mediated Oxidation

PubMed Central

Bazylewski, Paul F.; Nguyen, Van Luan; Bauer, Robert P.C.; Hunt, Adrian H.; McDermott, Eamon J. G.; Leedahl, Brett D.; Kukharenko, Andrey I.; Cholakh, Seif O.; Kurmaev, Ernst Z.; Blaha, Peter; Moewes, Alexander; Lee, Young Hee; Chang, Gap Soo

2015-01-01

This study reports a scalable and economical method to open a band gap in single layer graphene by deposition of cobalt metal on its surface using physical vapor deposition in high vacuum. At low cobalt thickness, clusters form at impurity sites on the graphene without etching or damaging the graphene. When exposed to oxygen at room temperature, oxygen functional groups form in proportion to the cobalt thickness that modify the graphene band structure. Cobalt/Graphene resulting from this treatment can support a band gap of 0.30 eV, while remaining largely undamaged to preserve its structural and electrical properties. A mechanism of cobalt-mediated band opening is proposed as a two-step process starting with charge transfer from metal to graphene, followed by formation of oxides where cobalt has been deposited. Contributions from the formation of both CoO and oxygen functional groups on graphene affect the electronic structure to open a band gap. This study demonstrates that cobalt-mediated oxidation is a viable method to introduce a band gap into graphene at room temperature that could be applicable in electronics applications. PMID:26486966

The Development of Layered Photonic Band Gap Structures Using a Micro-Transfer Molding Technique

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sutherland, Kevin Jerome

Photonic band gap (PBG) crystals are periodic dielectric structures that manipulate electromagnetic radiation in a manner similar to semiconductor devices manipulating electrons. Whereas a semiconductor material exhibits an electronic band gap in which electrons cannot exist, similarly, a photonic crystal containing a photonic band gap does not allow the propagation of specific frequencies of electromagnetic radiation. This phenomenon results from the destructive Bragg diffraction interference that a wave propagating at a specific frequency will experience because of the periodic change in dielectric permitivity. This gives rise to a variety of optical applications for improving the efficiency and effectiveness of opto-electronicmore » devices. These applications are reviewed later. Several methods are currently used to fabricate photonic crystals, which are also discussed in detail. This research involves a layer-by-layer micro-transfer molding ({mu}TM) and stacking method to create three-dimensional FCC structures of epoxy or titania. The structures, once reduced significantly in size can be infiltrated with an organic gain media and stacked on a semiconductor to improve the efficiency of an electronically pumped light-emitting diode. Photonic band gap structures have been proven to effectively create a band gap for certain frequencies of electro-magnetic radiation in the microwave and near-infrared ranges. The objective of this research project was originally two-fold: to fabricate a three dimensional (3-D) structure of a size scaled to prohibit electromagnetic propagation within the visible wavelength range, and then to characterize that structure using laser dye emission spectra. As a master mold has not yet been developed for the micro transfer molding technique in the visible range, the research was limited to scaling down the length scale as much as possible with the current available technology and characterizing these structures with other methods.« less

NiO: correlated band structure of a charge-transfer insulator.

PubMed

Kunes, J; Anisimov, V I; Skornyakov, S L; Lukoyanov, A V; Vollhardt, D

2007-10-12

The band structure of the prototypical charge-transfer insulator NiO is computed by using a combination of an ab initio band structure method and the dynamical mean-field theory with a quantum Monte-Carlo impurity solver. Employing a Hamiltonian which includes both Ni d and O p orbitals we find excellent agreement with the energy bands determined from angle-resolved photoemission spectroscopy. This brings an important progress in a long-standing problem of solid-state theory. Most notably we obtain the low-energy Zhang-Rice bands with strongly k-dependent orbital character discussed previously in the context of low-energy model theories.

Vibration isolation design for periodically stiffened shells by the wave finite element method

NASA Astrophysics Data System (ADS)

Hong, Jie; He, Xueqing; Zhang, Dayi; Zhang, Bing; Ma, Yanhong

2018-04-01

Periodically stiffened shell structures are widely used due to their excellent specific strength, in particular for aeronautical and astronautical components. This paper presents an improved Wave Finite Element Method (FEM) that can be employed to predict the band-gap characteristics of stiffened shell structures efficiently. An aero-engine casing, which is a typical periodically stiffened shell structure, was employed to verify the validation and efficiency of the Wave FEM. Good agreement has been found between the Wave FEM and the classical FEM for different boundary conditions. One effective wave selection method based on the Wave FEM has thus been put forward to filter the radial modes of a shell structure. Furthermore, an optimisation strategy by the combination of the Wave FEM and genetic algorithm was presented for periodically stiffened shell structures. The optimal out-of-plane band gap and the mass of the whole structure can be achieved by the optimisation strategy under an aerodynamic load. Results also indicate that geometric parameters of stiffeners can be properly selected that the out-of-plane vibration attenuates significantly in the frequency band of interest. This study can provide valuable references for designing the band gaps of vibration isolation.

Bi-directional evolutionary optimization for photonic band gap structures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Meng, Fei; School of Civil Engineering, Central South University, Changsha 410075; Huang, Xiaodong, E-mail: huang.xiaodong@rmit.edu.au

2015-12-01

Toward an efficient and easy-implement optimization for photonic band gap structures, this paper extends the bi-directional evolutionary structural optimization (BESO) method for maximizing photonic band gaps. Photonic crystals are assumed to be periodically composed of two dielectric materials with the different permittivity. Based on the finite element analysis and sensitivity analysis, BESO starts from a simple initial design without any band gap and gradually re-distributes dielectric materials within the unit cell so that the resulting photonic crystal possesses a maximum band gap between two specified adjacent bands. Numerical examples demonstrated the proposed optimization algorithm can successfully obtain the band gapsmore » from the first to the tenth band for both transverse magnetic and electric polarizations. Some optimized photonic crystals exhibit novel patterns markedly different from traditional designs of photonic crystals.« less

Band gap structures for 2D phononic crystals with composite scatterer

NASA Astrophysics Data System (ADS)

Qi, Xiao-qiao; Li, Tuan-jie; Zhang, Jia-long; Zhang, Zhen; Tang, Ya-qiong

2018-05-01

We investigated the band gap structures in two-dimensional phononic crystals with composite scatterer. The composite scatterers are composed of two materials (Bragg scattering type) or three materials (locally resonance type). The finite element method is used to calculate the band gap structure, eigenmodes and transmission spectrum. The variation of the location and width of band gap are also investigated as a function of material ratio in the scatterer. We have found that the change trends the widest band gap of the two phononic crystals are different as the material ratio changing. In addition to this, there are three complete band gaps at most for the Bragg-scattering-type phononic crystals in the first six bands; however, the locally resonance-type phononic crystals exist only two complete band gap at most in the first six bands. The gap-tuning effect can be controlled by the material ratio in the scatterer.

Band structure analysis of leaky Bloch waves in 2D phononic crystal plates.

PubMed

Mazzotti, Matteo; Miniaci, Marco; Bartoli, Ivan

2017-02-01

A hybrid Finite Element-Plane Wave Expansion method is presented for the band structure analysis of phononic crystal plates with two dimensional lattice that are in contact with acoustic half-spaces. The method enables the computation of both real (propagative) and imaginary (attenuation) components of the Bloch wavenumber at any given frequency. Three numerical applications are presented: a benchmark dispersion analysis for an oil-loaded Titanium isotropic plate, the band structure analysis of a water-loaded Tungsten slab with square cylindrical cavities and a phononic crystal plate composed of Aurum cylinders embedded in an epoxy matrix. Copyright © 2016 Elsevier B.V. All rights reserved.

Acoustic band gaps of the woodpile sonic crystal with the simple cubic lattice

NASA Astrophysics Data System (ADS)

Wu, Liang-Yu; Chen, Lien-Wen

2011-02-01

This study theoretically and experimentally investigates the acoustic band gap of a three-dimensional woodpile sonic crystal. Such crystals are built by blocks or rods that are orthogonally stacked together. The adjacent layers are perpendicular to each other. The woodpile structure is embedded in air background. Their band structures and transmission spectra are calculated using the finite element method with a periodic boundary condition. The dependence of the band gap on the width of the stacked rods is discussed. The deaf bands in the band structure are observed by comparing with the calculated transmission spectra. The experimental transmission spectra for the Γ-X and Γ-X' directions are also presented. The calculated results are compared with the experimental results.

Effect of synthesis method on structure, band gap and surface morphology of delafossite oxides, CuAlO2 and CuFeO2

NASA Astrophysics Data System (ADS)

Shah, Aadil Abass; Azam, Ameer

2018-04-01

In this research work we have reported the synthesis of two different delafossites, CuAlO2 and CuFeO2 by two different synthesis methods viz hydrothermal method and the combustion method. The effect of synthesis on structure, band gap and morphology of the synthesized delafossites was carried out using various techniques. The phase and structure of the synthesized delafossites were studied and confirmed using X-ray diffraction and the crystallite size was calculated. FTIR measurements showed the presence of different stretching modes and functional groups in the synthesized oxides. The surface morphology was studied using the scanning electron microscopy. The band gap of the synthesized delafossite oxides was found to be in the range of 2.8 and 3.3 eV.

Shape effects on the electronic structure and the optical gain of InAsN/GaAs nanostructures: From a quantum lens to a quantum ring

NASA Astrophysics Data System (ADS)

Chen, J.; Fan, W. J.; Xu, Q.; Zhang, X. W.; Li, S. S.; Xia, J. B.

2012-10-01

The electronic structures of self-assembled InAs1-xNx/GaAs nanostructures from quantum lens to quantum rings (QRs) are calculated using the 10-band k.p method and the valence force field (VFF) method. With the variation of shape of the nanostructure and nitrogen (N) content, it shows that the N and the strains can significantly affect the energy levels especially the conduction band because the N resonant state has repulsion interaction with the conduction band due to the band anticrossing (BAC). The structures with N and greater height have smaller transition energy, and the structures with N have greater optical gain due to its overwhelming greater value of factor f+f-1. After analyzing the shape effect, we suggested that the nanostructures with volcano shape are preferred because the maximum optical gain occurs for quantum volcano. With our simulation result, researchers could select quantum dots (QDs) structures to design laser with better performance.

Electronic structure of α-SrB4O7: experiment and theory

NASA Astrophysics Data System (ADS)

Atuchin, V. V.; Kesler, V. G.; Zaitsev, A. I.; Molokeev, M. S.; Aleksandrovsky, A. S.; Kuzubov, A. A.; Ignatova, N. Y.

2013-02-01

The investigation of valence band structure and electronic parameters of constituent element core levels of α-SrB4O7 has been carried out with x-ray photoemission spectroscopy. Optical-quality crystal α-SrB4O7 has been grown by the Czochralski method. Detailed photoemission spectra of the element core levels have been recorded from the powder sample under excitation by nonmonochromatic Al Kα radiation (1486.6 eV). The band structure of α-SrB4O7 has been calculated by ab initio methods and compared to XPS measurements. It has been found that the band structure of α-SrB4O7 is weakly dependent on the Sr-related states.

Electron band structure of the high pressure cubic phase of AlH3

NASA Astrophysics Data System (ADS)

Shi, Hongliang; Zarifi, Niliffar; Yim, Wai-Leung; Tse, J. S.

2012-07-01

The electronic band structure of the cubic Pm3n phase of AlH3 stable above 100 GPa is examined with semi-local, Tran-Blaha modified Becke-Johnson local density approximation (TB-mBJLDA), screened hybrid density functionals and GW methods. The shift of the conduction band to higher energy with increasing pressure is predicted by all methods. However, there are significant differences in detail band structure. In the pressure range from 90 to160 GPa, semi-local, hybrid functional and TB-mBJLDA calculations predicted that AlH3 is a poor metal. In comparison, GW calculations show a gap opening at 160 GPa and AlH3 becomes a small gap semi-conductor. From the trends of the calculated band shifts, it can be concluded that the favourable conditions leading to the nesting of Fermi surfaces predicted by semi-local calculation have disappeared if the exchange term is included. The results highlight the importance of the correction to the exchange energy on the band structure of hydrogen dominant dense metal hydrides at high pressure hydrides and may help to rationalize the absence of superconductivity in AlH3 from experimental measurements.

Theoretical investigation of the structural, electronic, and thermodynamic properties of CdS1-xSex alloys

NASA Astrophysics Data System (ADS)

Long, Debing; Li, Mingkai; Meng, Dongxue; Ahuja, Rajeev; He, Yunbin

2018-03-01

In this work, the structural, electronic, and thermodynamic properties of wurtzite (WZ) and zincblende (ZB) CdS1-xSex alloys are investigated using the density functional theory (DFT) and the cluster expansion method. A special quasirandom structure containing 16 atoms is constructed to calculate the band structures of random alloys. The band gaps of CdS1-xSex alloys are direct and decrease as the Se content increases. The delta self-consistent-field method is applied to correct band gaps that are underestimated by DFT. The band offsets clearly reflect the variation in valence band maxima and conduction band minima, thus providing information useful to the design of relevant quantum well structures. The positive formation enthalpies of both phases imply that CdS1-xSex is an immiscible system and tends to phase separate. The influence of lattice vibrations on the phase diagram is investigated by calculating the phonon density of states. Lattice vibration effects can reduce the critical temperature Tc and increase alloy solid solubilities. This influence is especially significant in the ZB structure. When only chemical interactions are present, the Tc values for WZ- and ZB-CdS1-xSex are 260 K and 249 K, respectively. The lattice vibration enthalpy and entropy lower the Tc to 255 K and 233 K, respectively.

Active Narrow-Band Vibration Isolation of Large Engineering Structures

NASA Technical Reports Server (NTRS)

Rahman, Zahidul; Spanos, John

1994-01-01

We present a narrow-band tracking control method using a variant of the Least Mean Squares (LMS) algorithm to isolate slowly changing periodic disturbances from engineering structures. The advantage of the algorithm is that it has a simple architecture and is relatively easy to implement while it can isolate disturbances on the order of 40-50 dB over decades of frequency band. We also present the results of an experiment conducted on a flexible truss structure. The average disturbance rejection achieved is over 40 dB over the frequency band of 5 Hz to 50 Hz.

Band structure and phonon properties of lithium fluoride at high pressure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Panchal, J. M., E-mail: amitjignesh@yahoo.co.in; Department of Physics, University School of Sciences, Gujarat University, Ahmedabad 380009, Gujarat; Joshi, Mitesh

2016-05-23

High pressure structural and electronic properties of Lithium Fluoride (LiF) have been studied by employing an ab-initio pseudopotential method and a linear response scheme within the density functional theory (DFT) in conjunction with quasi harmonic Debye model. The band structure and electronic density of states conforms that the LiF is stable and is having insulator behavior at ambient as well as at high pressure up to 1 Mbar. Conclusions based on Band structure, phonon dispersion and phonon density of states are outlined.

The Properties of Outer Retinal Band Three Investigated With Adaptive-Optics Optical Coherence Tomography

PubMed Central

Jonnal, Ravi S.; Gorczynska, Iwona; Migacz, Justin V.; Azimipour, Mehdi; Zawadzki, Robert J.; Werner, John S.

2017-01-01

Purpose Optical coherence tomography's (OCT) third outer retinal band has been attributed to the zone of interdigitation between RPE cells and cone outer segments. The purpose of this paper is to investigate the structure of this band with adaptive optics (AO)-OCT. Methods Using AO-OCT, images were obtained from two subjects. Axial structure was characterized by measuring band 3 thickness and separation between bands 2 and 3 in segmented cones. Lateral structure was characterized by correlation of band 3 with band 2 and comparison of their power spectra. Band thickness and separation were also measured in a clinical OCT image of one subject. Results Band 3 thickness ranged from 4.3 to 6.4 μm. Band 2 correlations ranged between 0.35 and 0.41 and power spectra of both bands confirmed peak frequencies that agree with histologic density measurements. In clinical images, band 3 thickness was between 14 and 19 μm. Measurements of AO-OCT of interband distance were lower than our corresponding clinical OCT measurements. Conclusions Band 3 originates from a structure with axial extent similar to a single surface. Correlation with band 2 suggests an origin within the cone photoreceptor. These two observations indicate that band 3 corresponds predominantly to cone outer segment tips (COST). Conventional OCT may overestimate both the thickness of band 3 and outer segment length. PMID:28877320

Effect of elastic strain redistribution on electronic band structures of compressively strained GaInAsP/InP membrane quantum wires

NASA Astrophysics Data System (ADS)

Ferdous, F.; Haque, A.

2007-05-01

The effect of redistribution of elastic strain relaxation on the energy band structures of GaInAsP/InP compressively strained membrane quantum wires fabricated by electron-beam lithography, reactive-ion etching and two-step epitaxial growth is theoretically studied using an 8-band k ṡp method. Anisotropic strain analysis by the finite element method shows that due to etching away the top and the bottom InP clad layers in membrane structures, redistribution of strain occurs. It is found that strain redistribution increases the effective bandgap of membrane quantum wire structures causing a blueshift of the emission frequency. Comparison with effective bandgap calculations neglecting confinement and band mixing demonstrates that neglect of these effects leads to an overestimation of the change in the bandgap. We have also investigated the effect of variation of wire width, barrier strain compensation, number of stacked quantum wire layers, and thickness of the top and the bottom residual InP layers in membrane structures on the change in the effective bandgap of membrane structures.

Nanoscale measurements of unoccupied band dispersion in few-layer graphene.

PubMed

Jobst, Johannes; Kautz, Jaap; Geelen, Daniël; Tromp, Rudolf M; van der Molen, Sense Jan

2015-11-26

The properties of any material are fundamentally determined by its electronic band structure. Each band represents a series of allowed states inside a material, relating electron energy and momentum. The occupied bands, that is, the filled electron states below the Fermi level, can be routinely measured. However, it is remarkably difficult to characterize the empty part of the band structure experimentally. Here, we present direct measurements of unoccupied bands of monolayer, bilayer and trilayer graphene. To obtain these, we introduce a technique based on low-energy electron microscopy. It relies on the dependence of the electron reflectivity on incidence angle and energy and has a spatial resolution ∼10 nm. The method can be easily applied to other nanomaterials such as van der Waals structures that are available in small crystals only.

Photonic band structures solved by a plane-wave-based transfer-matrix method.

PubMed

Li, Zhi-Yuan; Lin, Lan-Lan

2003-04-01

Transfer-matrix methods adopting a plane-wave basis have been routinely used to calculate the scattering of electromagnetic waves by general multilayer gratings and photonic crystal slabs. In this paper we show that this technique, when combined with Bloch's theorem, can be extended to solve the photonic band structure for 2D and 3D photonic crystal structures. Three different eigensolution schemes to solve the traditional band diagrams along high-symmetry lines in the first Brillouin zone of the crystal are discussed. Optimal rules for the Fourier expansion over the dielectric function and electromagnetic fields with discontinuities occurring at the boundary of different material domains have been employed to accelerate the convergence of numerical computation. Application of this method to an important class of 3D layer-by-layer photonic crystals reveals the superior convergency of this different approach over the conventional plane-wave expansion method.

Comparing photonic band structure calculation methods for diamond and pyrochlore crystals.

PubMed

Vermolen, E C M; Thijssen, J H J; Moroz, A; Megens, M; van Blaaderen, A

2009-04-27

The photonic band diagrams of close-packed colloidal diamond and pyrochlore structures, have been studied using Korringa-Kohn-Rostoker (KKR) and plane-wave calculations. In addition, the occurrence of a band gap has been investigated for the binary Laves structures and their constituent large- and small-sphere substructures. It was recently shown that these Laves structures give the possibility to fabricate the diamond and pyrochlore structures by self-organization. The comparison of the two calculation methods opens the possibility to study the validity and the convergence of the results, which have been an issue for diamond-related structures in the past. The KKR calculations systematically give a lower value for the gap width than the plane-wave calculations. This difference can partly be ascribed to a convergence issue in the plane-wave code when a contact point of two spheres coincides with the grid.

Implementation of density functional theory method on object-oriented programming (C++) to calculate energy band structure using the projector augmented wave (PAW)

NASA Astrophysics Data System (ADS)

Alfianto, E.; Rusydi, F.; Aisyah, N. D.; Fadilla, R. N.; Dipojono, H. K.; Martoprawiro, M. A.

2017-05-01

This study implemented DFT method into the C++ programming language with object-oriented programming rules (expressive software). The use of expressive software results in getting a simple programming structure, which is similar to mathematical formula. This will facilitate the scientific community to develop the software. We validate our software by calculating the energy band structure of Silica, Carbon, and Germanium with FCC structure using the Projector Augmented Wave (PAW) method then compare the results to Quantum Espresso calculation’s results. This study shows that the accuracy of the software is 85% compared to Quantum Espresso.

The properties of optimal two-dimensional phononic crystals with different material contrasts

NASA Astrophysics Data System (ADS)

Liu, Zong-Fa; Wu, Bin; He, Cun-Fu

2016-09-01

By modifying the spatial distribution of constituent material phases, phononic crystals (PnCs) can be designed to exhibit band gaps within which sound and vibration cannot propagate. In this paper, the developed topology optimization method (TOM), based on genetic algorithms (GAs) and the finite element method (FEM), is proposed to design two-dimensional (2D) solid PnC structures composed of two contrasting elastic materials. The PnCs have the lowest order band gap that is the third band gap for the coupled mode, the first band gap for the shear mode or the XY 34 Z band gap for the mixed mode. Moreover, the effects of the ratios of contrasting material properties on the optimal layout of unit cells and the corresponding phononic band gaps (PBGs) are investigated. The results indicate that the topology of the optimal PnCs and corresponding band gaps varies with the change of material contrasts. The law can be used for the rapid design of desired PnC structures.

High-spin structures in the 139Pr nucleus

NASA Astrophysics Data System (ADS)

Yeoh, E. Y.; Zhu, S. J.; Wang, J. G.; Xiao, Z. G.; Zhang, M.; Yan, W. H.; Wang, R. S.; Xu, Q.; Wu, X. G.; He, C. Y.; Li, G. S.; Zheng, Y.; Li, C. B.; Cao, X. P.; Hu, S. P.; Yao, S. H.; Yu, B. B.

2012-06-01

Background: 139Pr is located in a transitional region of neutron number close to the N=82 shell. The study of its high-spin states and collective bands is important for systematically understanding the nuclear structural characteristics in this region.Purpose: To investigate the high-spin levels and to search for oblate bands in 139Pr.Methods: The high-spin states of 139Pr have been studied via the reaction 124Sn(19F,4n) at a beam energy of 80 MeV. The experiment was carried out at the HI-13 Tandem Accelerator at the China Institute of Atomic Energy (CIAE). The data analysis was done by using the γ-γ coincidence method.Results: The level scheme of 139Pr has been expanded with spin up to 45/2ℏ. A total of 39 new levels and 45 new transitions are identified. Four collective band structures at high-spin states have been newly established. From systematic analysis, one of the bands is proposed as a double decoupled band; two bands are proposed as oblate bands with γ˜-60∘; another band is suggested as an oblate-triaxial band with γ˜-90∘. The other characteristics for these bands are discussed.Conclusions: A new level scheme in 139Pr has been established and the collective bands at high spin have been identified. The result shows that the strong oblate shape-driving effect is caused by neutrons at the high-spin states in 139Pr.

Manipulation of Dirac cones in intercalated epitaxial graphene

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Minsung; Tringides, Michael C.; Hershberger, Matthew T.

Graphene is an intriguing material in view of its unique Dirac quasi-particles, and the manipulation of its electronic structure is important in material design and applications. Here, we theoretically investigate the electronic band structure of epitaxial graphene on SiC with intercalation of rare earth metal ions (e.g., Yb and Dy) using first-principles calculations. We can use the intercalation to control the coupling of the constituent components (buffer layer, graphene, and substrate), resulting in strong modification of the graphene band structure. We also demonstrate that the metal-intercalated epitaxial graphene has tunable band structures by controlling the energies of Dirac cones asmore » well as the linear and quadratic band dispersion depending on the intercalation layer and density. Thus, the metal intercalation is a viable method to manipulate the electronic band structure of the epitaxial graphene, which can enhance the functional utility and controllability of the material.« less

Manipulation of Dirac cones in intercalated epitaxial graphene

DOE PAGES

Kim, Minsung; Tringides, Michael C.; Hershberger, Matthew T.; ...

2017-07-12

Graphene is an intriguing material in view of its unique Dirac quasi-particles, and the manipulation of its electronic structure is important in material design and applications. Here, we theoretically investigate the electronic band structure of epitaxial graphene on SiC with intercalation of rare earth metal ions (e.g., Yb and Dy) using first-principles calculations. We can use the intercalation to control the coupling of the constituent components (buffer layer, graphene, and substrate), resulting in strong modification of the graphene band structure. We also demonstrate that the metal-intercalated epitaxial graphene has tunable band structures by controlling the energies of Dirac cones asmore » well as the linear and quadratic band dispersion depending on the intercalation layer and density. Thus, the metal intercalation is a viable method to manipulate the electronic band structure of the epitaxial graphene, which can enhance the functional utility and controllability of the material.« less

Effect of background dielectric on TE-polarized photonic bandgap of metallodielectric photonic crystals using Dirichlet-to-Neumann map method.

PubMed

Sedghi, Aliasghar; Rezaei, Behrooz

2016-11-20

Using the Dirichlet-to-Neumann map method, we have calculated the photonic band structure of two-dimensional metallodielectric photonic crystals having the square and triangular lattices of circular metal rods in a dielectric background. We have selected the transverse electric mode of electromagnetic waves, and the resulting band structures showed the existence of photonic bandgap in these structures. We theoretically study the effect of background dielectric on the photonic bandgap.

Band structure of one-dimensional photonic crystal with graphene layers using the Fresnel coefficients method

NASA Astrophysics Data System (ADS)

Jafari, A.; Rahmat, A.

2018-04-01

In this paper, we have calculated the band structure of an instance of one-dimensional photonic crystal (1DPC) composed of double-layered dielectrics via the Fresnel coefficients method. Then, we supposed the addition of a thin layer of graphene to each dielectric layer and the given photonic crystal (PC) composed of dielectric-graphene composites. The effects of graphene layers on the PC band structure were evaluated. We found out that according to the effective medium theory unlike the TE polarization, the electric permittivity of the dielectric layers changed at TM polarization. As such, the band structure of PC for TM polarization changed, too. Moreover, instead of bandgap related to “zero averaged refractive index” an approximately omnidirectional bandgap appeared and a related bandgap to “𝜀 = 0” disappeared. In addition, a new angular gap branch appeared at a new frequency at TM polarization in which the width of gap increased as the angle increased.

Infrared photonic bandgap materials and structures

NASA Astrophysics Data System (ADS)

Sundaram, S. K.; Keller, P. E.; Riley, B. J.; Martinez, J. E.; Johnson, B. R.; Allen, P. J.; Saraf, L. V.; Anheier, N. C., Jr.; Liau, F.

2006-02-01

Three-dimensional periodic dielectric structure can be described by band theory, analogous to electron waves in a crystal. Photonic band gap (PBG) structures were introduced in 1987. The PBG is an energy band in which optical modes, spontaneous emission, and zero-point fluctuations are all absent. It was first theoretically predicted that a three-dimensional photonic crystal could have a complete band gap. E. Yablonovitch built the first three-dimensional photonic crystal (Yablonovite) on microwave length scale, with a complete PBG. In nature, photonic crystals occur as semiprecious opal and the microscopic structures on the wings of some tropical butterflies, which are repeating structures (PBG structure/materials) that inhibit the propagation of some frequencies of light. Pacific Northwest National Laboratory (PNNL) has been developing tunable (between 3.5 and 16 μm) quantum cascade lasers (QCL), chalcogenides, and all other components for an integrated approach to chemical sensing. We have made significant progress in modeling and fabrication of infrared photonic band gap (PBG) materials and structures. We modeled several 2-D designs and defect configurations. Transmission spectra were computed by the Finite Difference Time Domain Method (with FullWAVE TM). The band gaps were computed by the Plane Wave Expansion Method (with BandSOLVE TM). The modeled designs and defects were compared and the best design was identified. On the experimental front, chalcogenide glasses were used as the starting materials. As IIS 3, a common chalcogenide, is an important infrared (IR) transparent material with a variety of potential applications such as IR sensors, waveguides, and photonic crystals. Wet-chemical lithography has been extended to PBG fabrication and challenges identified. An overview of results and challenges will be presented.

Design of tree structured matched wavelet for HRV signals of menstrual cycle.

PubMed

Rawal, Kirti; Saini, B S; Saini, Indu

2016-07-01

An algorithm is presented for designing a new class of wavelets matched to the Heart Rate Variability (HRV) signals of the menstrual cycle. The proposed wavelets are used to find HRV variations between phases of menstrual cycle. The method finds the signal matching characteristics by minimising the shape feature error using Least Mean Square method. The proposed filter banks are used for the decomposition of the HRV signal. For reconstructing the original signal, the tree structure method is used. In this approach, decomposed sub-bands are selected based upon their energy in each sub-band. Thus, instead of using all sub-bands for reconstruction, sub-bands having high energy content are used for the reconstruction of signal. Thus, a lower number of sub-bands are required for reconstruction of the original signal which shows the effectiveness of newly created filter coefficients. Results show that proposed wavelets are able to differentiate HRV variations between phases of the menstrual cycle accurately than standard wavelets.

Band-structure calculations of noble-gas and alkali halide solids using accurate Kohn-Sham potentials with self-interaction correction

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Y.; Krieger, J.B.; Norman, M.R.

1991-11-15

The optimized-effective-potential (OEP) method and a method developed recently by Krieger, Li, and Iafrate (KLI) are applied to the band-structure calculations of noble-gas and alkali halide solids employing the self-interaction-corrected (SIC) local-spin-density (LSD) approximation for the exchange-correlation energy functional. The resulting band gaps from both calculations are found to be in fair agreement with the experimental values. The discrepancies are typically within a few percent with results that are nearly the same as those of previously published orbital-dependent multipotential SIC calculations, whereas the LSD results underestimate the band gaps by as much as 40%. As in the LSD---and it ismore » believed to be the case even for the exact Kohn-Sham potential---both the OEP and KLI predict valence-band widths which are narrower than those of experiment. In all cases, the KLI method yields essentially the same results as the OEP.« less

Modification of the band offset in boronitrene

NASA Astrophysics Data System (ADS)

Obodo, K. O.; Andrew, R. C.; Chetty, N.

2011-10-01

Using density functional methods within the generalized gradient approximation implemented in the Quantum Espresso codes, we modify the band offset in a single layer of boronitrene by substituting a double line of carbon atoms. This effectively introduces a line of dipoles at the interface. We considered various junctions of this system within the zigzag and armchair orientations. Our results show that the “zigzag-short” structure is energetically most stable, with a formation energy of 0.502 eV and with a band offset of 1.51 eV. The “zigzag-long” structure has a band offset of 1.99 eV. The armchair structures are nonpolar, while the zigzag-single structures show a charge accumulation for the C-substituted B and charge depletion for the C-substituted N at the junction. Consequently there is no shifting of the bands.

Three-dimensional ordered particulate structures: Method to retrieve characteristics from photonic band gap data

NASA Astrophysics Data System (ADS)

Miskevich, Alexander A.; Loiko, Valery A.

2015-01-01

A method to retrieve characteristics of ordered particulate structures, such as photonic crystals, is proposed. It is based on the solution of the inverse problem using data on the photonic band gap (PBG). The quasicrystalline approximation (QCA) of the theory of multiple scattering of waves and the transfer matrix method (TMM) are used. Retrieval of the refractive index of particles is demonstrated. Refractive indices of the artificial opal particles are estimated using the published experimental data.

Nanoscale measurements of unoccupied band dispersion in few-layer graphene

PubMed Central

Jobst, Johannes; Kautz, Jaap; Geelen, Daniël; Tromp, Rudolf M.; van der Molen, Sense Jan

2015-01-01

The properties of any material are fundamentally determined by its electronic band structure. Each band represents a series of allowed states inside a material, relating electron energy and momentum. The occupied bands, that is, the filled electron states below the Fermi level, can be routinely measured. However, it is remarkably difficult to characterize the empty part of the band structure experimentally. Here, we present direct measurements of unoccupied bands of monolayer, bilayer and trilayer graphene. To obtain these, we introduce a technique based on low-energy electron microscopy. It relies on the dependence of the electron reflectivity on incidence angle and energy and has a spatial resolution ∼10 nm. The method can be easily applied to other nanomaterials such as van der Waals structures that are available in small crystals only. PMID:26608712

Ab initio electronic structure calculations for metallic intermediate band formation in photovoltaic materials

NASA Astrophysics Data System (ADS)

Wahnón, P.; Tablero, C.

2002-04-01

A metallic isolated band in the middle of the band gap of several III-V semiconductors has been predicted as photovoltaic materials with the possibility of providing substantially enhanced efficiencies. We have investigated the electronic band structures and lattice constants of GanAsmM and GanPmM with M=Sc, Ti, V, and Cr, to identify whether this isolated band is likely to exist by means of accurate calculations. For this task, we use the SIESTA program, an ab initio periodic density-functional method, fully self consistent in the local-density approximation. Norm-conserving, nonlocal pseudopotentials and confined linear combination of atomic orbitals have been used. We have carried out a case study of GanAsmTi and GanPmTi energy-band structure including analyses of the effect of the basis set, fine k-point mesh to ensure numerical convergence, structural parameters, and generalized gradient approximation for exchange and correlation corrections. We find the isolated intermediate band when one Ti atom replaces the position of one As (or P) atom in the crystal structure. For this kind of compound we show that the intermediate band relative position inside the band gap and width are sensitive to the dynamic relaxation of the crystal and the size of the basis set.

Alkali-metal induced band structure deformation investigated by angle-resolved photoemission spectroscopy and first-principles calculations

NASA Astrophysics Data System (ADS)

Ito, S.; Feng, B.; Arita, M.; Someya, T.; Chen, W.-C.; Takayama, A.; Iimori, T.; Namatame, H.; Taniguchi, M.; Cheng, C.-M.; Tang, S.-J.; Komori, F.; Matsuda, I.

2018-04-01

Alkali-metal adsorption on the surface of materials is widely used for in situ surface electron doping, particularly for observing unoccupied band structures by angle-resolved photoemission spectroscopy (ARPES). However, the effects of alkali-metal atoms on the resulting band structures have yet to be fully investigated, owing to difficulties in both experiments and calculations. Here, we combine ARPES measurements on cesium-adsorbed ultrathin bismuth films with first-principles calculations of the electronic charge densities and demonstrate a simple method to evaluate alkali-metal induced band deformation. We reveal that deformation of bismuth surface bands is directly correlated with vertical charge-density profiles at each electronic state of bismuth. In contrast, a change in the quantized bulk bands is well described by a conventional rigid-band-shift picture. We discuss these two aspects of the band deformation holistically, considering spatial distributions of the electronic states and cesium-bismuth hybridization, and provide a prescription for applying alkali-metal adsorption to a wide range of materials.

The narrow pass band filter of tunable 1D phononic crystals with a dielectric elastomer layer

NASA Astrophysics Data System (ADS)

Wu, Liang-Yu; Wu, Mei-Ling; Chen, Lien-Wen

2009-01-01

In this paper, we study the defect bands of a 1D phononic crystal consisting of aluminum (Al) and polymethyl methacrylate (PMMA) layers with a dielectric elastomer (DE) defect layer. The plane wave expansion (PWE) method and supercell calculation are used to calculate the band structure and the defect bands. The transmission spectra are obtained using the finite element method (FEM). Since the thickness of the dielectric elastomer defect layer is controlled by applying an electric voltage, the frequencies of the defect bands can be tuned. A narrow pass band filter can be developed and designed by using the dielectric elastomer.

A novel combination of PBG cell for achieving HPF, BPF, and LPF in an electro-optic system

NASA Astrophysics Data System (ADS)

Tsao, Shyh-Lin; Lee, Wen-Ching

2004-10-01

In this paper, a novel Frequency Division Multiplexer (FDM) using Photonic Band Gap (PBG) cell combination concept circuit is proposed for achieving a 3-band FDM. The preliminary 3-band FDM structure is the combination of three PBG cells. The observable frequency response experimental results are presented. We also simulate and measure all the scattering parameters for the novel 3-band FDM. The disclosed method in this paper demonstrates the possibility for applying photonic bandgap structure in designing a frequency division device.

Research on low-frequency band gap property of a hybrid phononic crystal

NASA Astrophysics Data System (ADS)

Dong, Yake; Yao, Hong; Du, Jun; Zhao, Jingbo; Chao, Ding; Wang, Benchi

2018-05-01

A hybrid phononic crystal has been investigated. The characteristic frequency of XY mode, transmission loss and displacement vector have been calculated by the finite element method. There are Bragg scattering band gap and local resonance band gap in the band structures. We studied the influence factors of band gap. There are many flat bands in the eigenfrequencies curve. There are many flat bands in the curve. The band gap covers a large range in low frequency. The band gaps cover more than 95% below 3000 Hz.

Structural studies and band gap tuning of Cr doped ZnO nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Srinet, Gunjan, E-mail: gunjansrinet@gmail.com; Kumar, Ravindra, E-mail: gunjansrinet@gmail.com; Sajal, Vivek, E-mail: gunjansrinet@gmail.com

2014-04-24

Structural and optical properties of Cr doped ZnO nanoparticles prepared by the thermal decomposition method are presented. X-ray diffraction studies confirmed the substitution of Cr on Zn sites without changing the wurtzite structure of ZnO. Modified form of W-H equations was used to calculate various physical parameters and their variation with Cr doping is discussed. Significant red shift was observed in band gap, i.e., a band gap tuning is achieved by Cr doping which could eventually be useful for optoelectronic applications.

Phonon dispersions, band structures, and dielectric functions of BeO and BeS polymorphs

NASA Astrophysics Data System (ADS)

Wang, Ke-Long; Gao, Shang-Peng

2018-07-01

Structures, phonon dispersions, electronic structures, and dielectric functions of beryllium oxide (BeO) and beryllium sulfide (BeS) polymorphs are investigated by density functional theory and many-body perturbation theory. Phonon calculations indicate that both wurtzite (w-) and zincblende (zb-) structures are dynamically stable for BeO and BeS, whereas rocksalt (rs-) structures for both BeO and BeS have imaginary phonon frequencies and thus are dynamically unstable at zero pressure. Band structures for the 4 dynamically stable phases show that only w-BeO has a direct band gap. Both the one-shot G0W0 and quasiparticle self-consistent GW methods are used to correct band energies at high symmetry k-points. Bethe-Salpeter equation (BSE), which considers Coulomb correlated electron-hole pairs, is employed to deal with the computation of macroscopic dielectric functions. It is shown that BSE calculation, employing scissors operator derived by self-consistent GW method, can give dielectric functions agreeing very well with experimental measurement of w-BeO. Weak anisotropic characters can be observed for w-BeO and w-BeS. Both zb-BeS and w-BeS show high optical transition probabilities within a narrow ultraviolet energy range.

Native multimer analysis of plasma and platelet von Willebrand factor compared to denaturing separation: implication for the interpretation of satellite bands.

PubMed

Hohenstein, Kurt; Griesmacher, Andrea; Weigel, Günter; Golderer, Georg; Ott, Helmut Werner

2011-06-01

Blue native electrophoresis (BNE) was applied to analyze the von Willebrand factor (vWF) multimers in their native state and to present a methodology to perform blue native electrophoresis on human plasma proteins, which has not been done before. The major difference between this method and the commonly used SDS-agarose gel electrophoresis is the lack of satellite bands in the high-resolution native gel. To further analyze this phenomenon, a second dimension was performed under denaturing conditions. Thereby, we obtained a pattern in which each protein sub-unit from the first dimension dissociates into three distinct sub-bands. These bands confirm the triplet structure, which consists of an intermediate band and two satellite bands. By introducing the second dimension, our novel method separates the triplet structure into a higher resolution than the commonly used SDS-agarose gel electrophoresis does. This helps considerably in the classification of ambiguous von Willebrand's disease subtypes. In addition, our method has the additional advantage of being able to resolve the triplet structure of platelet vWF multimers, which has not been identified previously through conventional SDS-agarose electrophoresis multimer analysis. This potential enables us to compare the triplet structure from platelet and plasmatic vWF, and may help to find out whether structural abnormalities concern the vWF molecule in the platelet itself, or whether they are due to the physiological processing of vWF shed into circulation. Owing to its resolution and sensitivity, this native separation technique offers a promising tool for the analysis and detection of von Willebrand disorder, and for the classification of von Willebrand's disease subtypes. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Low-frequency band gap of locally resonant phononic crystals with a dual-base plate.

PubMed

Zuo, Shuguang; Huang, Haidong; Wu, Xudong; Zhang, Minghai; Ni, Tianxin

2018-03-01

To achieve a wider band gap and a lower cut-on frequency, a locally resonant phononic crystal (LRPC) with a dual-base plate is investigated in this paper. Compared with the LRPC with a single plate, the band structure of the LRPC with a dual-base plate is calculated using the method of plane wave expansion and verified by the finite element method. According to the analysis of the band curves of the LRPC with a dual-base plate, the mechanisms are explained. Next, the influences of the thickness of the plates, the stiffness of the springs, the mass of resonators, and the lattice constant are also investigated. The results show that the structural asymmetry between the upper and the lower plate is conducive to reducing the cut-on frequency and broadening the band gap effectively. The results indicate a different approach for the application of LRPC in vibration and noise control.

Graphene-insulator-semiconductor capacitors as superior test structures for photoelectric determination of semiconductor devices band diagrams

NASA Astrophysics Data System (ADS)

Piskorski, K.; Passi, V.; Ruhkopf, J.; Lemme, M. C.; Przewlocki, H. M.

2018-05-01

We report on the advantages of using Graphene-Insulator-Semiconductor (GIS) instead of Metal-Insulator-Semiconductor (MIS) structures in reliable and precise photoelectric determination of the band alignment at the semiconductor-insulator interface and of the insulator band gap determination. Due to the high transparency to light of the graphene gate in GIS structures large photocurrents due to emission of both electrons and holes from the substrate and negligible photocurrents due to emission of carriers from the gate can be obtained, which allows reliable determination of barrier heights for both electrons, Ee and holes, Eh from the semiconductor substrate. Knowing the values of both Ee and Eh allows direct determination of the insulator band gap EG(I). Photoelectric measurements were made of a series of Graphene-SiO2-Si structures and an example is shown of the results obtained in sequential measurements of the same structure giving the following barrier height values: Ee = 4.34 ± 0.01 eV and Eh = 4.70 ± 0.03 eV. Based on this result and results obtained for other structures in the series we conservatively estimate the maximum uncertainty of both barrier heights estimations at ± 0.05 eV. This sets the SiO2 band gap estimation at EG(I) = 7.92 ± 0.1 eV. It is shown that widely different SiO2 band gap values were found by research groups using various determination methods. We hypothesize that these differences are due to different sensitivities of measurement methods used to the existence of the SiO2 valence band tail.

Tunable acoustic metamaterial based on piezoelectric ceramic transducer

NASA Astrophysics Data System (ADS)

Zhu, Xiaohui; Qiao, Jing; Zhang, Guangyu; Zhou, Qiang; Wu, Yingdan; Li, Longqiu

2017-04-01

In this paper, a tunable metamaterial consisting of periodic layers of steel, polyurea and piezoelectric ceramic transducer (PZT) was presented. The PZT layer in this structure was connected to an inductor L. Transfer matrix method was used to calculate the band structure of the sample. It was observed that an extremely narrow stop band was induced by the PZT layer with inductor L. This narrow stop band was attributed to the resonance circuit constituted by the piezoelectric layer, for the piezoelectric layer with electrodes could be seen as a capacitor. Further, homogenization was used to calculate the effective elastic constants of the sample. Results showed that the effective parameters of this structure behaved negative in the narrow stop band. The location of the narrow stop band was in the charge of inductor L, which could be used to design acoustic filters or noise insulators by changing the parameters of structure.

A program for calculating photonic band structures, Green's functions and transmission/reflection coefficients using a non-orthogonal FDTD method

NASA Astrophysics Data System (ADS)

Ward, A. J.; Pendry, J. B.

2000-06-01

In this paper we present an updated version of our ONYX program for calculating photonic band structures using a non-orthogonal finite difference time domain method. This new version employs the same transparent formalism as the first version with the same capabilities for calculating photonic band structures or causal Green's functions but also includes extra subroutines for the calculation of transmission and reflection coefficients. Both the electric and magnetic fields are placed onto a discrete lattice by approximating the spacial and temporal derivatives with finite differences. This results in discrete versions of Maxwell's equations which can be used to integrate the fields forwards in time. The time required for a calculation using this method scales linearly with the number of real space points used in the discretization so the technique is ideally suited to handling systems with large and complicated unit cells.

One-dimensional mercury(II) halide coordination polymers of 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine ligand: Synthesis, crystal structure, spectroscopic and DFT studies

DOE Office of Scientific and Technical Information (OSTI.GOV)

Saghatforoush, Lotfali, E-mail: saghatforoush@gmail.com; Khoshtarkib, Zeinab; Amani, Vahid

2016-01-15

Three new coordination polymers, [Hg(μ-bptz)X{sub 2}]{sub n} (X=Cl (1), Br (2)) and [Hg{sub 2}(μ-bptz)(μ-I){sub 2}I{sub 2}]{sub n} (3) (bptz=3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) were synthesized. X-ray structural analysis indicated that compounds 1 and 2 are composed of one-dimensional (1D) linear chains while the compound 3 has 1D stair-stepped structure. The electronic band structure along with density of states (DOS) calculated by the DFT method indicates that compound 1 and 2 are direct band gap semiconductors; however, compound 3 is an indirect semiconductor. The linear optical properties of the compounds are also calculated by DFT method. According to the DFT calculations, the observed emission bandmore » of the compounds in solid state is due to electron transfer from an excited bptz-π* state (CBs) to the top of VBs. {sup 1}H NMR spectra of the compounds indicate that, in solution phase, the compounds don’t decompose completely. Thermal stability of the compounds is studied using TG, DTA methods. - Graphical abstract: Synthesis, crystal structure and emission spectra of [Hg(μ-bptz)X{sub 2}]{sub n} (X=Cl and Br) and [Hg{sub 2}(μ-bptz)(μ-I){sub 2}I{sub 2}]{sub n} are presented. The electronic band structure and linear optical properties of the compounds are calculated by the DFT method. - Highlights: • Three 1D Hg(II) halide coordination polymers with bptz ligand have been prepared. • The structures of the compounds are determined by single crystal XRD. • DFT calculations show that [Hg(μ-bptz)X{sub 2}]{sub n} (X=Cl and Br) have a direct band gap. • DFT calculations show that [Hg{sub 2}(μ-bptz)(μ-I){sub 2}I{sub 2}]{sub n} has an indirect band gap. • The compounds show an intraligand electron transfer emission band in solid state.« less

Band selection method based on spectrum difference in targets of interest in hyperspectral imagery

NASA Astrophysics Data System (ADS)

Zhang, Xiaohan; Yang, Guang; Yang, Yongbo; Huang, Junhua

2016-10-01

While hyperspectral data shares rich spectrum information, it has numbers of bands with high correlation coefficients, causing great data redundancy. A reasonable band selection is important for subsequent processing. Bands with large amount of information and low correlation should be selected. On this basis, according to the needs of target detection applications, the spectral characteristics of the objects of interest are taken into consideration in this paper, and a new method based on spectrum difference is proposed. Firstly, according to the spectrum differences of targets of interest, a difference matrix which represents the different spectral reflectance of different targets in different bands is structured. By setting a threshold, the bands satisfying the conditions would be left, constituting a subset of bands. Then, the correlation coefficients between bands are calculated and correlation matrix is given. According to the size of the correlation coefficient, the bands can be set into several groups. At last, the conception of normalized variance is used on behalf of the information content of each band. The bands are sorted by the value of its normalized variance. Set needing number of bands, and the optimum band combination solution can be get by these three steps. This method retains the greatest degree of difference between the target of interest and is easy to achieve by computer automatically. Besides, false color image synthesis experiment is carried out using the bands selected by this method as well as other 3 methods to show the performance of method in this paper.

Opening complete band gaps in two dimensional locally resonant phononic crystals

NASA Astrophysics Data System (ADS)

Zhou, Xiaoling; Wang, Longqi

2018-05-01

Locally resonant phononic crystals (LRPCs) which have low frequency band gaps attract a growing attention in both scientific and engineering field recently. Wide complete locally resonant band gaps are the goal for researchers. In this paper, complete band gaps are achieved by carefully designing the geometrical properties of the inclusions in two dimensional LRPCs. The band structures and mechanisms of different types of models are investigated by the finite element method. The translational vibration patterns in both the in-plane and out-of-plane directions contribute to the full band gaps. The frequency response of the finite periodic structures demonstrate the attenuation effects in the complete band gaps. Moreover, it is found that the complete band gaps can be further widened and lowered by increasing the height of the inclusions. The tunable properties by changing the geometrical parameters provide a good way to open wide locally resonant band gaps.

Ab - initio study of rare earth magnesium alloy: TbMg

NASA Astrophysics Data System (ADS)

Kumari, Meena; Yadav, Priya; Nautiyal, Shashank; Verma, U. P.

2018-05-01

The structural, electronic and magnetic properties of TbMg were analyzed by using full-potential linearized augmented plane wave method. This intermetallic is stable in structure CsCl (B2 phase) with space group Pm-3m. In electronic properties, we show the electronic band structure and density of states plots. These plots show that this alloy have metallic character because there is no band gap between the valance band and conduction band at Fermi level. The structural properties, i.e. equilibrium lattice constant, bulk modulus and its pressure derivative, energy and volume show good agreement with available data. In this paper, we also present the total magnetic moment along with the magnetic moment on the atomic and interstitial sites of TbMg intermetallic in B2 phase.

Determination of a natural valence-band offset - The case of HgTe and CdTe

NASA Technical Reports Server (NTRS)

Shih, C. K.; Spicer, W. E.

1987-01-01

A method to determine a natural valence-band offset (NVBO), i.e., the change in the valence-band maximum energy which is intrinsic to the bulk band structures of semiconductors is proposed. The HgTe-CdTe system is used as an example in which it is found that the valence-band maximum of HgTe lies 0.35 + or - 0.06 eV above that of CdTe. The NVBO of 0.35 eV is in good agreement with the X-ray photoemission spectroscopy measurement of the heterojunction offset. The procedure to determine the NVBO between semiconductors, and its implication on the heterojunction band lineup and the electronic structures of semiconductor alloys, are discussed.

Band structures in two-dimensional phononic crystals with periodic Jerusalem cross slot

NASA Astrophysics Data System (ADS)

Li, Yinggang; Chen, Tianning; Wang, Xiaopeng; Yu, Kunpeng; Song, Ruifang

2015-01-01

In this paper, a novel two-dimensional phononic crystal composed of periodic Jerusalem cross slot in air matrix with a square lattice is presented. The dispersion relations and the transmission coefficient spectra are calculated by using the finite element method based on the Bloch theorem. The formation mechanisms of the band gaps are analyzed based on the acoustic mode analysis. Numerical results show that the proposed phononic crystal structure can yield large band gaps in the low-frequency range. The formation mechanism of opening the acoustic band gaps is mainly attributed to the resonance modes of the cavities inside the Jerusalem cross slot structure. Furthermore, the effects of the geometrical parameters on the band gaps are further explored numerically. Results show that the band gaps can be modulated in an extremely large frequency range by the geometry parameters such as the slot length and width. These properties of acoustic waves in the proposed phononic crystals can potentially be applied to optimize band gaps and generate low-frequency filters and waveguides.

Effect of Se concentration on photonic bandgap of 1-D As-S-Se/air multilayers

NASA Astrophysics Data System (ADS)

Singh, Rajpal; Suthar, B.; Bhargava, A.

2018-05-01

The photonic band structure of 1-D chalcogenide photonic crystal consisting of As-S-Se/air multilayered structure is studied. The photonic band structure is calculated using plane wave expansion method. The effect of Se constration on the photonic bandgap is studied. It is found that the photonic bandgap increases with Se-concentration and shows the red shift.

Structure determination and characterization of two rare-earth molybdenum borate compounds: LnMoBO(6) (Ln = La, Ce).

PubMed

Zhao, Dan; Cheng, Wen-Dan; Zhang, Hao; Hang, Shu-Ping; Fang, Ming

2008-07-28

The structural, optical, and electronic properties of two rare-earth molybdenum borate compounds, LnMoBO(6) (Ln = La, Ce), have been investigated by means of single-crystal X-ray diffraction, elemental analyses, and spectral measurements, as well as calculations of energy band structures, density of states, and optical response functions by the density functional method. The title compounds, which crystallize in monoclinic space group P2(1)/c, possess a similar network of interconnected [Ce(2)(MoO(4))(2)](2+) chains and [BO(2)](-) wavy chains. Novel 1D molybdenum oxide chains are contained in their three-dimensional (3D) networks. The calculated results of crystal energy band structure by the density functional theory (DFT) method show that the solid-state compound LaMoBO(6) is a semiconductor with indirect band gaps.

A hybrid phononic crystal for roof application.

PubMed

Wan, Qingmian; Shao, Rong

2017-11-01

Phononic crystal is a type of acoustic material, and the study of phononic crystals has attracted great attention from national research institutions. Meanwhile, noise reduction in the low-frequency range has always encountered difficulties and troubles in the engineering field. In order to obtain a unique and effective low-frequency noise reduction method, in this paper a low frequency noise attenuation system based on phononic crystal structure is proposed and demonstrated. The finite element simulation of the band gap is consistent with the final test results. The effects of structure parameters on the band gaps were studied by changing the structure parameters and the band gaps can be controlled by suitably tuning structure parameters. The structure and results provide a good support for phononic crystal structures engineering application.

Ground-state energies of simple metals

NASA Technical Reports Server (NTRS)

Hammerberg, J.; Ashcroft, N. W.

1974-01-01

A structural expansion for the static ground-state energy of a simple metal is derived. Two methods are presented, one an approach based on single-particle band structure which treats the electron gas as a nonlinear dielectric, the other a more general many-particle analysis using finite-temperature perturbation theory. The two methods are compared, and it is shown in detail how band-structure effects, Fermi-surface distortions, and chemical-potential shifts affect the total energy. These are of special interest in corrections to the total energy beyond third order in the electron-ion interaction and hence to systems where differences in energies for various crystal structures are exceptionally small. Preliminary calculations using these methods for the zero-temperature thermodynamic functions of atomic hydrogen are reported.

Experimental demonstrations in audible frequency range of band gap tunability and negative refraction in two-dimensional sonic crystal.

PubMed

Pichard, Hélène; Richoux, Olivier; Groby, Jean-Philippe

2012-10-01

The propagation of audible acoustic waves in two-dimensional square lattice tunable sonic crystals (SC) made of square cross-section infinitely rigid rods embedded in air is investigated experimentally. The band structure is calculated with the plane wave expansion (PWE) method and compared with experimental measurements carried out on a finite extend structure of 200 cm width, 70 cm depth and 15 cm height. The structure is made of square inclusions of 5 cm side with a periodicity of L = 7.5 cm placed inbetween two rigid plates. The existence of tunable complete band gaps in the audible frequency range is demonstrated experimentally by rotating the scatterers around their vertical axis. Negative refraction is then analyzed by use of the anisotropy of the equi-frequency surface (EFS) in the first band and of a finite difference time domain (FDTD) method. Experimental results finally show negative refraction in the audible frequency range.

Curvature effects on the electronic and transport properties of semiconductor films

NASA Astrophysics Data System (ADS)

Batista, F. F.; Chaves, Andrey; da Costa, D. R.; Farias, G. A.

2018-05-01

Within the effective mass approximation, we study the curvature effects on the electronic and transport properties of semiconductor films. We investigate how the geometry-induced potential resulting exclusively from periodic ripples in the film induces electronic confinement and a superlattice band structure. For fixed curvature parameters, such a confinement can be easily tuned by an external electric field, hence features of the superlattice band structure such as its energy gaps and band curvature can be controlled by an external parameter. We also show that, for some values of curvature and electric field, it is possible to obtain massless Dirac bands for a smooth curved structure. Moreover, we use a wave packet propagation method to demonstrate that the ripples are responsible for a significant inter-sub-band transition, specially for moderate values of the ripple height.

Lamb waves in phononic crystal slabs with square or rectangular symmetries

NASA Astrophysics Data System (ADS)

Brunet, Thomas; Vasseur, Jérôme; Bonello, Bernard; Djafari-Rouhani, Bahram; Hladky-Hennion, Anne-Christine

2008-08-01

We report on both numerical and experimental results showing the occurrence of band gaps for Lamb waves propagating in phononic crystal plates. The structures are made of centered rectangular and square arrays of holes drilled in a silicon plate. A supercell plane wave expansion method is used to calculate the band structures and to predict the position and the magnitude of the gaps. The band structures of phononic crystal slabs are then measured using a laser ultrasonic technique. Lamb waves in the megahertz range and with wave vectors ranging over more than the first two reduced Brillouin zones are investigated.

Band structure of cavity-type hypersonic phononic crystals fabricated by femtosecond laser-induced two-photon polymerization

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rakhymzhanov, A. M.; Utegulov, Z. N., E-mail: zhutegulov@nu.edu.kz, E-mail: fytas@mpip-mainz.mpg.de; Optics Laboratory, National Laboratory Astana, Nazarbayev University, Astana 10000

2016-05-16

The phononic band diagram of a periodic square structure fabricated by femtosecond laser pulse-induced two photon polymerization is recorded by Brillouin light scattering (BLS) at hypersonic (GHz) frequencies and computed by finite element method. The theoretical calculations along the two main symmetry directions quantitatively capture the band diagrams of the air- and liquid-filled structure and moreover represent the BLS intensities. The theory helps identify the observed modes, reveals the origin of the observed bandgaps at the Brillouin zone boundaries, and unravels direction dependent effective medium behavior.

Analyzing the photonic band gaps in two-dimensional plasma photonic crystals with fractal Sierpinski gasket structure based on the Monte Carlo method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Hai-Feng, E-mail: hanlor@163.com; Key Laboratory of Radar Imaging and Microwave Photonics; Liu, Shao-Bin

2016-08-15

In this paper, the properties of photonic band gaps (PBGs) in two types of two-dimensional plasma-dielectric photonic crystals (2D PPCs) under a transverse-magnetic (TM) wave are theoretically investigated by a modified plane wave expansion (PWE) method where Monte Carlo method is introduced. The proposed PWE method can be used to calculate the band structures of 2D PPCs which possess arbitrary-shaped filler and any lattice. The efficiency and convergence of the present method are discussed by a numerical example. The configuration of 2D PPCs is the square lattices with fractal Sierpinski gasket structure whose constituents are homogeneous and isotropic. The type-1more » PPCs is filled with the dielectric cylinders in the plasma background, while its complementary structure is called type-2 PPCs, in which plasma cylinders behave as the fillers in the dielectric background. The calculated results reveal that the enough accuracy and good convergence can be obtained, if the number of random sampling points of Monte Carlo method is large enough. The band structures of two types of PPCs with different fractal orders of Sierpinski gasket structure also are theoretically computed for a comparison. It is demonstrate that the PBGs in higher frequency region are more easily produced in the type-1 PPCs rather than in the type-2 PPCs. Sierpinski gasket structure introduced in the 2D PPCs leads to a larger cutoff frequency, enhances and induces more PBGs in high frequency region. The effects of configurational parameters of two types of PPCs on the PBGs are also investigated in detail. The results show that the PBGs of the PPCs can be easily manipulated by tuning those parameters. The present type-1 PPCs are more suitable to design the tunable compacted devices.« less

A combined representation method for use in band structure calculations. 1: Method

NASA Technical Reports Server (NTRS)

Friedli, C.; Ashcroft, N. W.

1975-01-01

A representation was described whose basis levels combine the important physical aspects of a finite set of plane waves with those of a set of Bloch tight-binding levels. The chosen combination has a particularly simple dependence on the wave vector within the Brillouin Zone, and its use in reducing the standard one-electron band structure problem to the usual secular equation has the advantage that the lattice sums involved in the calculation of the matrix elements are actually independent of the wave vector. For systems with complicated crystal structures, for which the Korringa-Kohn-Rostoker (KKR), Augmented-Plane Wave (APW) and Orthogonalized-Plane Wave (OPW) methods are difficult to apply, the present method leads to results with satisfactory accuracy and convergence.

Electronic structure modifications and band gap narrowing in Zn0.95V0.05O

NASA Astrophysics Data System (ADS)

Ahad, Abdul; Majid, S. S.; Rahman, F.; Shukla, D. K.; Phase, D. M.

2018-04-01

We present here, structural, optical and electronic structure studies on Zn0.95V0.05O, synthesized using solid state method. Rietveld refinement of x-ray diffraction pattern indicates no considerable change in the lattice of doped ZnO. The band gap of doped sample, as calculated by Kubelka-Munk transformed reflectance spectra, has been found reduced compared to pure ZnO. Considerable changes in absorbance in UV-Vis range is observed in doped sample. V doping induced decrease in band gap is supported by x-ray absorption spectroscopy measurements. It is experimentally confirmed that conduction band edge in Zn0.95V0.05O has shifted towards Fermi level than in pure ZnO.

Research on local resonance and Bragg scattering coexistence in phononic crystal

NASA Astrophysics Data System (ADS)

Dong, Yake; Yao, Hong; Du, Jun; Zhao, Jingbo; Jiang, Jiulong

2017-04-01

Based on the finite element method (FEM), characteristics of the local resonance band gap and the Bragg scattering band gap of two periodically-distributed vibrator structures are studied. Conditions of original anti-resonance generation are theoretically derived. The original anti-resonance effect leads to localization of vibration. Factors which influence original anti-resonance band gap are analyzed. The band gap width and the mass ratio between two vibrators are closely correlated to each other. Results show that the original anti-resonance band gap has few influencing factors. In the locally resonant structure, the Bragg scattering band gap is found. The mass density of the elastic medium and the elasticity modulus have an important impact on the Bragg band gap. The coexistence of the two mechanisms makes the band gap larger. The band gap covered 90% of the low frequencies below 2000 Hz. All in all, the research could provide references for studying the low-frequency and broad band gap of phononic crystal.

Analysis of band structure, transmission properties, and dispersion behavior of THz wave in one-dimensional parabolic plasma photonic crystal

DOE Office of Scientific and Technical Information (OSTI.GOV)

Askari, Nasim; Eslami, Esmaeil, E-mail: eeslami@iust.ac.ir; Mirzaie, Reza

2015-11-15

The photonic band gap of obliquely incident terahertz electromagnetic waves in a one-dimensional plasma photonic crystal is studied. The periodic structure consists of lossless dielectric and inhomogeneous plasma with a parabolic density profile. The dispersion relation and the THz wave transmittance are analyzed based on the electromagnetic equations and transfer matrix method. The dependence of effective plasma frequency and photonic band gap characteristics on dielectric and plasma thickness, plasma density, and incident angle are discussed in detail. A theoretical calculation for effective plasma frequency is presented and compared with numerical results. Results of these two methods are in good agreement.

Unipolar Barrier Dual-Band Infrared Detectors

NASA Technical Reports Server (NTRS)

Ting, David Z. (Inventor); Soibel, Alexander (Inventor); Khoshakhlagh, Arezou (Inventor); Gunapala, Sarath (Inventor)

2017-01-01

Dual-band barrier infrared detectors having structures configured to reduce spectral crosstalk between spectral bands and/or enhance quantum efficiency, and methods of their manufacture are provided. In particular, dual-band device structures are provided for constructing high-performance barrier infrared detectors having reduced crosstalk and/or enhance quantum efficiency using novel multi-segmented absorber regions. The novel absorber regions may comprise both p-type and n-type absorber sections. Utilizing such multi-segmented absorbers it is possible to construct any suitable barrier infrared detector having reduced crosstalk, including npBPN, nBPN, pBPN, npBN, npBP, pBN and nBP structures. The pBPN and pBN detector structures have high quantum efficiency and suppresses dark current, but has a smaller etch depth than conventional detectors and does not require a thick bottom contact layer.

Peristalticity-driven banded chemical garden

NASA Astrophysics Data System (ADS)

Pópity-Tóth, É.; Schuszter, G.; Horváth, D.; Tóth, Á.

2018-05-01

Complex structures in nature are often formed by self-assembly. In order to mimic the formation, to enhance the production, or to modify the structures, easy-to-use methods are sought to couple engineering and self-assembly. Chemical-garden-like precipitation reactions are frequently used to study such couplings because of the intrinsic chemical and hydrodynamic interplays. In this work, we present a simple method of applying periodic pressure fluctuations given by a peristaltic pump which can be used to achieve regularly banded precipitate membranes in the copper-phosphate system.

Enhancing the performance of tungsten doped InZnO thin film transistors via sequential ambient annealing

NASA Astrophysics Data System (ADS)

Park, Hyun-Woo; Song, Aeran; Kwon, Sera; Choi, Dukhyun; Kim, Younghak; Jun, Byung-Hyuk; Kim, Han-Ki; Chung, Kwun-Bum

2018-03-01

This study suggests a sequential ambient annealing process as an excellent post-treatment method to enhance the device performance and stability of W (tungsten) doped InZnO thin film transistors (WIZO-TFTs). Sequential ambient annealing at 250 °C significantly enhanced the device performance and stability of WIZO-TFTs, compared with other post-treatment methods, such as air ambient annealing and vacuum ambient annealing at 250 °C. To understand the enhanced device performance and stability of WIZO-TFT with sequential ambient annealing, we investigate the correlations between device performance and stability and electronic structures, such as band alignment, a feature of the conduction band, and band edge states below the conduction band. The enhanced performance of WIZO-TFTs with sequential ambient annealing is related to the modification of the electronic structure. In addition, the dominant mechanism responsible for the enhanced device performance and stability of WIZO-TFTs is considered to be a change in the shallow-level and deep-level band edge states below the conduction band.

Electronic structure of scandium-doped MgB2

NASA Astrophysics Data System (ADS)

de La Peña, Omar; Agrestini, Stefano

2005-03-01

Recently has been reported the synthesis of a new superconducting alloy based on MgB2, where Mg is partially substituted with Sc. In order to analyze the effect of Sc doping on the structural and superconducting properties of Mg1-xScxB2, we have performed a detailed study of the electronic structure for this new diboride. The calculations have been done using the first-principles LAPW method, within the supercell approach for modeling the doping. In this work we report results for the electronic band structure, Fermi surface, and density of states. The effect of the Sc-d orbitals on the structural and electronic properties of Mg1-xScxB2 is analyzed. Increasing the Sc concentration (x) the σ-band is gradually filled, because Sc have one valence electron more than Mg. Interestingly, the analysis of the band structure shows that even for ScB2 the top of the σ-band remain above the Fermi level, nevertheless the σ-band presents high dispersion and has an important contribution of d states. In this way, in addition to the band filling effect, Sc doping gradually reduces the two-dimensional character of the σ- band in Mg1-xScxB2 as a result of increasing the sp(B)-d(Sc) hybridization. This research was partially supported by Consejo Nacional de Ciencia y Tecnolog'ia (CONACYT, M'exico) under Grant. No. 43830-F

A Wide Band Absorbing Material Design Using Band-Pass Frequency Selective Surface

NASA Astrophysics Data System (ADS)

Xu, Yonggang; Xu, Qiang; Liu, Ting; Zheng, Dianliang; Zhou, Li

2018-03-01

Based on the high frequency advantage characteristics of the Fe based absorbing coating, a method for designing the structure of broadband absorbing structure by using frequency selective surface (FSS) is proposed. According to the transmission and reflection characteristic of the different size FSS structure, the frequency variation characteristic was simulated. Secondly, the genetic algorithm was used to optimize the high frequency broadband absorbing materials, including the single and double magnetic layer material. Finally, the absorbing characteristics in iron layer were analyzed as the band pass FSS structure was embedded, the results showed that the band-pass FSS had the influence on widening the absorbing frequency. As the FSS was set as the bottom layer, it was effective to achieve the good absorbing property in low frequency and the high frequency absorbing performance was not weakened, because the band-pass FSS led the low frequency absorption and the high frequency shielding effect. The results of this paper are of guiding significance for designing and manufacturing the broadband absorbing materials.

Nudged elastic band method and density functional theory calculation for finding a local minimum energy pathway of p-benzoquinone and phenol fragmentation in mass spectrometry.

PubMed

Sugimura, Natsuhiko; Igarashi, Yoko; Aoyama, Reiko; Shibue, Toshimichi

2017-02-01

Analysis of the fragmentation pathways of molecules in mass spectrometry gives a fundamental insight into gas-phase ion chemistry. However, the conventional intrinsic reaction coordinates method requires knowledge of the transition states of ion structures in the fragmentation pathways. Herein, we use the nudged elastic band method, using only the initial and final state ion structures in the fragmentation pathways, and report the advantages and limitations of the method. We found a minimum energy path of p-benzoquinone ion fragmentation with two saddle points and one intermediate structure. The primary energy barrier, which corresponded to the cleavage of the C-C bond adjacent to the CO group, was calculated to be 1.50 eV. An additional energy barrier, which corresponded to the cleavage of the CO group, was calculated to be 0.68 eV. We also found an energy barrier of 3.00 eV, which was the rate determining step of the keto-enol tautomerization in CO elimination from the molecular ion of phenol. The nudged elastic band method allowed the determination of a minimum energy path using only the initial and final state ion structures in the fragmentation pathways, and it provided faster than the conventional intrinsic reaction coordinates method. In addition, this method was found to be effective in the analysis of the charge structures of the molecules during the fragmentation in mass spectrometry.

Dipole-allowed direct band gap silicon superlattices

PubMed Central

Oh, Young Jun; Lee, In-Ho; Kim, Sunghyun; Lee, Jooyoung; Chang, Kee Joo

2015-01-01

Silicon is the most popular material used in electronic devices. However, its poor optical properties owing to its indirect band gap nature limit its usage in optoelectronic devices. Here we present the discovery of super-stable pure-silicon superlattice structures that can serve as promising materials for solar cell applications and can lead to the realization of pure Si-based optoelectronic devices. The structures are almost identical to that of bulk Si except that defective layers are intercalated in the diamond lattice. The superlattices exhibit dipole-allowed direct band gaps as well as indirect band gaps, providing ideal conditions for the investigation of a direct-to-indirect band gap transition. The fact that almost all structural portions of the superlattices originate from bulk Si warrants their stability and good lattice matching with bulk Si. Through first-principles molecular dynamics simulations, we confirmed their thermal stability and propose a possible method to synthesize the defective layer through wafer bonding. PMID:26656482

Design of UWB Monopole Antenna with Dual Notched Bands Using One Modified Electromagnetic-Bandgap Structure

PubMed Central

Xu, Ziqiang

2013-01-01

A modified electromagnetic-bandgap (M-EBG) structure and its application to planar monopole ultra-wideband (UWB) antenna are presented. The proposed M-EBG which comprises two strip patch and an edge-located via can perform dual notched bands. By properly designing and placing strip patch near the feedline, the proposed M-EBG not only possesses a simple structure and compact size but also exhibits good band rejection. Moreover, it is easy to tune the dual notched bands by altering the dimensions of the M-EBG. A demonstration antenna with dual band-notched characteristics is designed and fabricated to validate the proposed method. The results show that the proposed antenna can satisfy the requirements of VSWR < 2 over UWB 3.1–10.6 GHz, except for the rejected bands of the world interoperability for microwave access (WiMAX) and the wireless local area network (WLAN) at 3.5 GHz and 5.5 GHz, respectively. PMID:24170984

Nature of the optical band shapes in polymethine dyes and H-aggregates: dozy chaos and excitons. Comparison with dimers, H*- and J-aggregates.

PubMed

Egorov, Vladimir V

2017-05-01

Results on the theoretical explanation of the shape of optical bands in polymethine dyes, their dimers and aggregates are summarized. The theoretical dependence of the shape of optical bands for the dye monomers in the vinylogous series in line with a change in the solvent polarity is considered. A simple physical (analytical) model of the shape of optical absorption bands in H-aggregates of polymethine dyes is developed based on taking the dozy-chaos dynamics of the transient state and the Frenkel exciton effect in the theory of molecular quantum transitions into account. As an example, the details of the experimental shape of one of the known H-bands are well reproduced by this analytical model under the assumption that the main optical chromophore of H-aggregates is a tetramer resulting from the two most probable processes of inelastic binary collisions in sequence: first, monomers between themselves, and then, between the resulting dimers. The obtained results indicate that in contrast with the compact structure of J-aggregates (brickwork structure), the structure of H-aggregates is not the compact pack-of-cards structure, as stated in the literature, but a loose alternate structure. Based on this theoretical model, a simple general (analytical) method for treating the more complex shapes of optical bands in polymethine dyes in comparison with the H-band under consideration is proposed. This method mirrors the physical process of molecular aggregates forming in liquid solutions: aggregates are generated in the most probable processes of inelastic multiple binary collisions between polymethine species generally differing in complexity. The results obtained are given against a background of the theoretical results on the shape of optical bands in polymethine dyes and their aggregates (dimers, H*- and J-aggregates) previously obtained by V.V.E.

Nature of the optical band shapes in polymethine dyes and H-aggregates: dozy chaos and excitons. Comparison with dimers, H*- and J-aggregates

PubMed Central

2017-01-01

Results on the theoretical explanation of the shape of optical bands in polymethine dyes, their dimers and aggregates are summarized. The theoretical dependence of the shape of optical bands for the dye monomers in the vinylogous series in line with a change in the solvent polarity is considered. A simple physical (analytical) model of the shape of optical absorption bands in H-aggregates of polymethine dyes is developed based on taking the dozy-chaos dynamics of the transient state and the Frenkel exciton effect in the theory of molecular quantum transitions into account. As an example, the details of the experimental shape of one of the known H-bands are well reproduced by this analytical model under the assumption that the main optical chromophore of H-aggregates is a tetramer resulting from the two most probable processes of inelastic binary collisions in sequence: first, monomers between themselves, and then, between the resulting dimers. The obtained results indicate that in contrast with the compact structure of J-aggregates (brickwork structure), the structure of H-aggregates is not the compact pack-of-cards structure, as stated in the literature, but a loose alternate structure. Based on this theoretical model, a simple general (analytical) method for treating the more complex shapes of optical bands in polymethine dyes in comparison with the H-band under consideration is proposed. This method mirrors the physical process of molecular aggregates forming in liquid solutions: aggregates are generated in the most probable processes of inelastic multiple binary collisions between polymethine species generally differing in complexity. The results obtained are given against a background of the theoretical results on the shape of optical bands in polymethine dyes and their aggregates (dimers, H*- and J-aggregates) previously obtained by V.V.E. PMID:28572984

Nature of the optical band shapes in polymethine dyes and H-aggregates: dozy chaos and excitons. Comparison with dimers, H*- and J-aggregates

NASA Astrophysics Data System (ADS)

Egorov, Vladimir V.

2017-05-01

Results on the theoretical explanation of the shape of optical bands in polymethine dyes, their dimers and aggregates are summarized. The theoretical dependence of the shape of optical bands for the dye monomers in the vinylogous series in line with a change in the solvent polarity is considered. A simple physical (analytical) model of the shape of optical absorption bands in H-aggregates of polymethine dyes is developed based on taking the dozy-chaos dynamics of the transient state and the Frenkel exciton effect in the theory of molecular quantum transitions into account. As an example, the details of the experimental shape of one of the known H-bands are well reproduced by this analytical model under the assumption that the main optical chromophore of H-aggregates is a tetramer resulting from the two most probable processes of inelastic binary collisions in sequence: first, monomers between themselves, and then, between the resulting dimers. The obtained results indicate that in contrast with the compact structure of J-aggregates (brickwork structure), the structure of H-aggregates is not the compact pack-of-cards structure, as stated in the literature, but a loose alternate structure. Based on this theoretical model, a simple general (analytical) method for treating the more complex shapes of optical bands in polymethine dyes in comparison with the H-band under consideration is proposed. This method mirrors the physical process of molecular aggregates forming in liquid solutions: aggregates are generated in the most probable processes of inelastic multiple binary collisions between polymethine species generally differing in complexity. The results obtained are given against a background of the theoretical results on the shape of optical bands in polymethine dyes and their aggregates (dimers, H*- and J-aggregates) previously obtained by V.V.E.

Structural expansions for the ground state energy of a simple metal

NASA Technical Reports Server (NTRS)

Hammerberg, J.; Ashcroft, N. W.

1973-01-01

A structural expansion for the static ground state energy of a simple metal is derived. An approach based on single particle band structure which treats the electron gas as a non-linear dielectric is presented, along with a more general many particle analysis using finite temperature perturbation theory. The two methods are compared, and it is shown in detail how band-structure effects, Fermi surface distortions, and chemical potential shifts affect the total energy. These are of special interest in corrections to the total energy beyond third order in the electron ion interaction, and hence to systems where differences in energies for various crystal structures are exceptionally small. Preliminary calculations using these methods for the zero temperature thermodynamic functions of atomic hydrogen are reported.

Ultrawide bandgap pentamode metamaterials with an asymmetric double-cone outside profile

NASA Astrophysics Data System (ADS)

Chu, Yangyang; Li, Yucheng; Cai, Chengxin; Liu, Guangshuan; Wang, Zhaohong; Xu, Zhuo

2018-03-01

The band-gap characteristic is an important feature of acoustic metamaterials, which has important theoretical and practical significance in acoustic devices. Pentamode metamaterials (PMs) with phonon band-gap characteristics based on an asymmetric double-cone outside profile are presented and studied in this paper. The phonon band structures of these PMs are calculated by using the finite element method. In addition to the single-mode band-gaps, the complete 3D band-gaps are also obtained by changing the outside profile of the double-cone. Moreover, by adjusting the outside profile and the diameter of the double-cone to reduce the symmetry of the structure, the complete 3D band-gap can be widened. Further parametric analysis is presented to investigate the effect of geometrical parameters on the phonon band-gap property, the numerical simulations show that the maximum relative bandwidth is expanded by 15.14 times through reducing the symmetry of the structure. This study provides a possible way for PMs to control elastic wave propagation in the field of depressing vibration and noise, acoustic filtering and acoustic cloaking.

Quasiparticle energy bands and Fermi surfaces of monolayer NbSe2

NASA Astrophysics Data System (ADS)

Kim, Sejoong; Son, Young-Woo

2017-10-01

A quasiparticle band structure of a single layer 2 H -NbSe2 is reported by using first-principles G W calculation. We show that a self-energy correction increases the width of a partially occupied band and alters its Fermi surface shape when comparing those using conventional mean-field calculation methods. Owing to a broken inversion symmetry in the trigonal prismatic single layer structure, the spin-orbit interaction is included and its impact on the Fermi surface and quasiparticle energy bands are discussed. We also calculate the doping dependent static susceptibilities from the band structures obtained by the mean-field calculation as well as G W calculation with and without spin-orbit interactions. A complete tight-binding model is constructed within the three-band third nearest neighbor hoppings and is shown to reproduce our G W quasiparticle energy bands and Fermi surface very well. Considering variations of the Fermi surface shapes depending on self-energy corrections and spin-orbit interactions, we discuss the formations of charge density wave (CDW) with different dielectric environments and their implications on recent controversial experimental results on CDW transition temperatures.

Electronic structures of Plutonium compounds with the NaCl-type monochalcogenides structure

NASA Astrophysics Data System (ADS)

Maehira, Takahiro; Tatetsu, Yasutomi

2012-12-01

We calculate the energy band structure and the Fermi surface of PuS, PuSe and PuTe by using a self-consistent relativistic linear augmented-plane-wave method with the exchange and correlation potential in a local density approximation. It is found in common that the energy bands in the vicinity of the Fermi level are mainly due to the hybridization between Pu 5/ and monochalcogenide p electrons. The obtained main Fermi surfaces are composed of two hole sheets and one electron sheet, all of which are constructed from the band having the Pu 5/ state and the monochalcogenide p state.

Calculation of the Energy-Band Structure of the Kronig-Penney Model Using the Nearly-Free and Tightly-Bound-Electron Approximations

ERIC Educational Resources Information Center

Wetsel, Grover C., Jr.

1978-01-01

Calculates the energy-band structure of noninteracting electrons in a one-dimensional crystal using exact and approximate methods for a rectangular-well atomic potential. A comparison of the two solutions as a function of potential-well depth and ratio of lattice spacing to well width is presented. (Author/GA)

Large scale phononic metamaterials for seismic isolation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aravantinos-Zafiris, N.; Sigalas, M. M.

In this work, we numerically examine structures that could be characterized as large scale phononic metamaterials. These novel structures could have band gaps in the frequency spectrum of seismic waves when their dimensions are chosen appropriately, thus raising the belief that they could be serious candidates for seismic isolation structures. Different and easy to fabricate structures were examined made from construction materials such as concrete and steel. The well-known finite difference time domain method is used in our calculations in order to calculate the band structures of the proposed metamaterials.

A Multispectral Image Creating Method for a New Airborne Four-Camera System with Different Bandpass Filters

PubMed Central

Li, Hanlun; Zhang, Aiwu; Hu, Shaoxing

2015-01-01

This paper describes an airborne high resolution four-camera multispectral system which mainly consists of four identical monochrome cameras equipped with four interchangeable bandpass filters. For this multispectral system, an automatic multispectral data composing method was proposed. The homography registration model was chosen, and the scale-invariant feature transform (SIFT) and random sample consensus (RANSAC) were used to generate matching points. For the difficult registration problem between visible band images and near-infrared band images in cases lacking manmade objects, we presented an effective method based on the structural characteristics of the system. Experiments show that our method can acquire high quality multispectral images and the band-to-band alignment error of the composed multiple spectral images is less than 2.5 pixels. PMID:26205264

Zak phase and band inversion in dimerized one-dimensional locally resonant metamaterials

NASA Astrophysics Data System (ADS)

Zhu, Weiwei; Ding, Ya-qiong; Ren, Jie; Sun, Yong; Li, Yunhui; Jiang, Haitao; Chen, Hong

2018-05-01

The Zak phase, which refers to Berry's phase picked up by a particle moving across the Brillouin zone, characterizes the topological properties of Bloch bands in a one-dimensional periodic system. Here the Zak phase in dimerized one-dimensional locally resonant metamaterials is investigated. It is found that there are some singular points in the bulk band across which the Bloch states contribute π to the Zak phase, whereas in the rest of the band the contribution is nearly zero. These singular points associated with zero reflection are caused by two different mechanisms: the dimerization-independent antiresonance of each branch and the dimerization-dependent destructive interference in multiple backscattering. The structure undergoes a topological phase-transition point in the band structure where the band inverts, and the Zak phase, which is determined by the numbers of singular points in the bulk band, changes following a shift in dimerization parameter. Finally, the interface state between two dimerized metamaterial structures with different topological properties in the first band gap is demonstrated experimentally. The quasi-one-dimensional configuration of the system allows one to explore topology-inspired new methods and applications on the subwavelength scale.

Optical properties of InGaN grown by MOCVD on sapphire and on bulk GaN

NASA Astrophysics Data System (ADS)

Osinski, Marek; Eliseev, Petr G.; Lee, Jinhyun; Smagley, Vladimir A.; Sugahara, Tamoya; Sakai, Shiro

1999-11-01

Experimental data on photoluminescence of various bulk and quantum-well epitaxial InGaN/GaN structures grown by MOCVD are interpreted in terms of a band-tail model of inhomogeneously broadened radiative recombination. The anomalous temperature-induced blue spectral is shown to result from band-tail recombination under non-degenerate conditions. Significant differences are observed between epilayers grown on sapphire substrates and on GaN substrates prepared by the sublimination method, with no apparent evidence of band tails in homoepitaxial structures, indicating their higher crystalline quality.

Bands dispersion and charge transfer in β-BeH2

NASA Astrophysics Data System (ADS)

Trivedi, D. K.; Galav, K. L.; Joshi, K. B.

2018-04-01

Predictive capabilities of ab-initio method are utilised to explore bands dispersion and charge transfer in β-BeH2. Investigations are carried out using the linear combination of atomic orbitals method at the level of density functional theory. The crystal structure and related parameters are settled by coupling total energy calculations with the Murnaghan equation of state. Electronic bands dispersion from PBE-GGA is reported. The PBE-GGA, and PBE0 hybrid functional, show that β-BeH2 is a direct gap semiconductor with 1.18 and 2.40 eV band gap. The band gap slowly decreases with pressure and beyond l00 GPa overlap of conduction and valence bands at the r point is observed. Charge transfer is studied by means of Mullikan population analysis.

Thermal tuning the reversible optical band gap of self-assembled polystyrene photonic crystals

NASA Astrophysics Data System (ADS)

Vakili Tahami, S. H.; Pourmahdian, S.; Shirkavand Hadavand, B.; Azizi, Z. S.; Tehranchi, M. M.

2016-11-01

Nano-sized polymeric colloidal particles could undergo self-organization into three-dimensional structures to produce desired optical properties. In this research, a facile emulsifier-free emulsion polymerization method was employed to synthesize highly mono-disperse sub-micron polystyrene colloids. A high quality photonic crystal (PhC) structure was prepared by colloidal polystyrene. The reversible thermal tuning effect on photonic band gap position as well as the attenuation of the band gap was investigated in detail. The position of PBG can be tuned from 420 nm to 400 nm by varying the temperature of the PhC structure, reversibly. This reversible effect provides a reconfigurable PhC structure which could be used as thermo-responsive shape memory polymers.

Ab-initio study on electronic properties of rocksalt SnAs

NASA Astrophysics Data System (ADS)

Babariya, Bindiya; Vaghela, M. V.; Gajjar, P. N.

2018-05-01

Within the frame work of Local Density Approximation of Exchange and Correlation, ab-initio method of density functional theory with Abinit code is used to compute electronic energy band structure, density of States and charge density of SnAs in rocksalt phase. Our result after optimization for lattice constant agrees with experimental value within 0.59% deviation. The computed electronic energy bands in high symmetry directions Γ→K→X→Γ→L→X→W→L→U shown metallic nature. The lowest band in the electronic band structure is showing band-gap approximately 1.70 eV from next higher band and no crossing between lowest two bands are seen. The density of states revels p-p orbit hybridization between Sn and As atoms. The spherical contour around Sn and As in the charge density plot represent partly ionic and partly covalent bonding. Fermi surface topology is the resultant effect of the single band crossing along L direction at Ef.

Spherical silicon-shell photonic band gap structures fabricated by laser-assisted chemical vapor deposition

NASA Astrophysics Data System (ADS)

Wang, H.; Yang, Z. Y.; Lu, Y. F.

2007-02-01

Laser-assisted chemical vapor deposition was applied in fabricating three-dimensional (3D) spherical-shell photonic band gap (PBG) structures by depositing silicon shells covering silica particles, which had been self-assembled into 3D colloidal crystals. The colloidal crystals of self-assembled silica particles were formed on silicon substrates using the isothermal heating evaporation approach. A continuous wave Nd:YAG laser (1064nm wavelength) was used to deposit silicon shells by thermally decomposing disilane gas. Periodic silicon-shell/silica-particle PBG structures were obtained. By removing the silica particles enclosed in the silicon shells using hydrofluoric acid, hollow spherical silicon-shell arrays were produced. This technique is capable of fabricating structures with complete photonic band gaps, which is predicted by simulations with the plane wave method. The techniques developed in this study have the potential to flexibly engineer the positions of the PBGs by varying both the silica particle size and the silicon-shell thickness. Ellipsometry was used to investigate the specific photonic band gaps for both structures.

Electronic structure and thermoelectric properties of half-Heusler compounds with eight electron valence count—KScX (X = C and Ge)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ciftci, Yasemin O.; Mahanti, Subhendra D.

Electronic band structure and structural properties of two representative half-Heusler (HH) compounds with 8 electron valence count (VC), KScC and KScGe, have been studied using first principles methods within density functional theory and generalized gradient approximation. These systems differ from the well studied class of HH compounds like ZrNiSn and ZrCoSb which have VC = 18 because of the absence of d electrons of the transition metal atoms Ni and Co. Electronic transport properties such as Seebeck coefficient (S), electrical conductivity (σ), electronic thermal conductivity (κ{sub e}) (the latter two scaled by electronic relaxation time), and the power factor (S{sup 2}σ) havemore » been calculated using semi-classical Boltzmann transport theory within constant relaxation time approximation. Both the compounds are direct band gap semiconductors with band extrema at the X point. Their electronic structures show a mixture of heavy and light bands near the valance band maximum and highly anisotropic conduction and valence bands near the band extrema, desirable features of good thermoelectric. Optimal p- or n-type doping concentrations have been estimated based on thermopower and maximum power factors. The optimum room temperature values of S are ∼1.5 times larger than that of the best room temperature thermoelectric Bi{sub 2}Te{sub 3}. We also discuss the impact of the band structure on deviations from Weidemann-Franz law as one tunes the chemical potential across the band gap.« less

Effect of conduction band non-parabolicity on the optical gain of quantum cascade lasers based on the effective two-band finite difference method

NASA Astrophysics Data System (ADS)

Cho, Gookbin; Kim, Jungho

2017-09-01

We theoretically investigate the effect of conduction band non-parabolicity (NPB) on the optical gain spectrum of quantum cascade lasers (QCLs) using the effective two-band finite difference method. Based on the effective two-band model to consider the NPB effect in the multiple quantum wells (QWs), the wave functions and confined energies of electron states are calculated in two different active-region structures, which correspond to three-QW single-phonon and four-QW double-phonon resonance designs. In addition, intersubband optical dipole moments and polar-optical-phonon scattering times are calculated and compared without and with the conduction band NPB effect. Finally, the calculation results of optical gain spectra are compared in the two QCL structures having the same peak gain wavelength of 8.55 μm. The gain peaks are greatly shifted to longer wavelengths and the overall gain magnitudes are slightly reduced when the NPB effect is considered. Compared with the three-QW active-region design, the redshift of the peak gain is more prominent in the four-QW active-region design, which makes use of higher electronic states for the lasing transition.

Design and experimental verification of a dual-band metamaterial filter

NASA Astrophysics Data System (ADS)

Zhu, Hong-Yang; Yao, Ai-Qin; Zhong, Min

2016-10-01

In this paper, we present the design, simulation, and experimental verification of a dual-band free-standing metamaterial filter operating in a frequency range of 1 THz-30 THz. The proposed structure consists of periodically arranged composite air holes, and exhibits two broad and flat transmission bands. To clarify the effects of the structural parameters on both resonant transmission bands, three sets of experiments are performed. The first resonant transmission band shows a shift towards higher frequency when the side width w 1 of the main air hole is increased. In contrast, the second resonant transmission band displays a shift towards lower frequency when the side width w 2 of the sub-holes is increased, while the first resonant transmission band is unchanged. The measured results indicate that these resonant bands can be modulated individually by simply optimizing the relevant structural parameters (w 1 or w 2) for the required band. In addition, these resonant bands merge into a single resonant band with a bandwidth of 7.7 THz when w 1 and w 2 are optimized simultaneously. The structure proposed in this paper adopts different resonant mechanisms for transmission at different frequencies and thus offers a method to achieve a dual-band and low-loss filter. Project supported by the Doctorate Scientific Research Foundation of Hezhou University, China (Grant No. HZUBS201503), the Promotion of the Basic Ability of Young and Middle-aged Teachers in Universities Project of Guangxi Zhuang Autonomous Region, China (Grant No. KY2016YB453), the Guangxi Colleges and Universities Key Laboratory Symbolic Computation, China, Engineering Data Processing and Mathematical Support Autonomous Discipline Project of Hezhou University, China (Grant No. 2016HZXYSX01).

Density functional theory calculations of III-N based semiconductors with mBJLDA

NASA Astrophysics Data System (ADS)

Gürel, Hikmet Hakan; Akıncı, Özden; Ünlü, Hilmi

2017-02-01

In this work, we present first principles calculations based on a full potential linear augmented plane-wave method (FP-LAPW) to calculate structural and electronic properties of III-V based nitrides such as GaN, AlN, InN in a zinc-blende cubic structure. First principles calculation using the local density approximation (LDA) and generalized gradient approximation (GGA) underestimate the band gap. We proposed a new potential called modified Becke-Johnson local density approximation (MBJLDA) that combines modified Becke-Johnson exchange potential and the LDA correlation potential to get better band gap results compared to experiment. We compared various exchange-correlation potentials (LSDA, GGA, HSE, and MBJLDA) to determine band gaps and structural properties of semiconductors. We show that using MBJLDA density potential gives a better agreement with experimental data for band gaps III-V nitrides based semiconductors.

Morphology and band structure regulation of graphitic carbon nitride microspheres by solvothermal temperature to boost photocatalytic activity

NASA Astrophysics Data System (ADS)

Wang, Shuaijun; Yan, Qingyun; Dong, Pei; Zhao, Chaocheng; Wang, Yongqiang; Liu, Fang; Li, Lin

2018-06-01

Graphitic carbon nitride (g-C3N4) microspheres (CNMS) were fabricated via a solvothermal method by using supramolecular complexes of dicyandiamide and cyanuric chloride as precursors. The effect of solvothermal temperature on the morphology, band structure, and activity was systematically investigated. Structural characterization results indicate that the samples prepared at 180 °C (CNMS-180) and 200 °C (CNMS-200) possess spherical morphology, while irregular bulk particles were obtained at 160 °C (CN-160). In addition, the band gap increased as the solvothermal temperature decreased from 200 to 160 °C. In comparison with CN-160 and CNMS-200, the valence band of CNMS-180 was more positive and thus gives higher photo-oxidation capability. Accordingly, CNMS-180 exhibits higher photocatalytic degradation efficiency on Rhodamine B, stronger photocurrent response, and lower charge transfer resistance. Additionally, CNMS-180 exhibits excellent stability after four runs. This work might provide a guidance for the regulation of morphology and band structure of g-C3N4-based materials prepared at low temperatures.

Electronic band structures and optical properties of type-II superlattice photodetectors with interfacial effect.

PubMed

Qiao, Peng-Fei; Mou, Shin; Chuang, Shun Lien

2012-01-30

The electronic band structures and optical properties of type-II superlattice (T2SL) photodetectors in the mid-infrared (IR) range are investigated. We formulate a rigorous band structure model using the 8-band k · p method to include the conduction and valence band mixing. After solving the 8 × 8 Hamiltonian and deriving explicitly the new momentum matrix elements in terms of envelope functions, optical transition rates are obtained through the Fermi's golden rule under various doping and injection conditions. Optical measurements on T2SL photodetectors are compared with our model and show good agreement. Our modeling results of quantum structures connect directly to the device-level design and simulation. The predicted doping effect is readily applicable to the optimization of photodetectors. We further include interfacial (IF) layers to study the significance of their effect. Optical properties of T2SLs are expected to have a large tunable range by controlling the thickness and material composition of the IF layers. Our model provides an efficient tool for the designs of novel photodetectors.

Crystal growth, electronic structure and optical properties of Sr2Mg(BO3)2

NASA Astrophysics Data System (ADS)

Lv, Xianshun; Wei, Lei; Wang, Xuping; Xu, Jianhua; Yu, Huajian; Hu, Yanyan; Zhang, Huadi; Zhang, Cong; Wang, Jiyang; Li, Qinggang

2018-02-01

Single crystals of Sr2Mg(BO3)2 (SMBO) were grown by Kyropoulos method. X-ray powder diffraction (XRD) analysis, transmission spectrum, thermal properties, band structure, density of states and charge distribution as well as Raman spectra of SMBO were described. The as-grown SMBO crystals show wide transparency range with UV cut-off below 180 nm. A direct band gap of 4.66 eV was obtained from the calculated electronic structure results. The calculated band structure and density of states results indicated the top valence band is determined by O 2p states whereas the low conduction band mainly consists of Sr 5s states. Twelve Raman peaks were observed in the experimental spectrum, fewer than the number predicted by the site group analysis. Raman peaks of SMBO were assigned combining first-principle calculation and site group analysis results. The strongest peak at 917 cm-1 in the experimental spectrum is assigned to symmetric stretching mode A1‧(ν1) of free BO3 units. SMBO is a potential Raman crystal which can be used in deep UV laser frequency conversion.

Tetrahedral 4 α and 12C+α cluster structures in 16O

NASA Astrophysics Data System (ADS)

Kanada-En'yo, Yoshiko

2017-09-01

I have investigated structures of the ground and excited states of 16O with the method of variation after spin-parity projection in the antisymmetrized molecular dynamics model combined with the generator coordinate method of 12C+α cluster. The calculation reasonably reproduces the experimental energy spectra; E 2 , E 3 , E 4 , and I S 1 transitions; and α -decay properties. The formation of 4 α clusters has been confirmed from nucleon degrees of freedom in the AMD model without assuming the existence of any clusters. They form "tetrahedral" 4 α - and 12C+α cluster structures. The 12C+α structure constructs the Kπ=0+ band consisting of the 02+, 21+, and 41+ states and the Kπ=0- band of the 12-, 32-, and 51- states. The 01+, 31-, and 42+ states are assigned to the ground band constructed from the tetrahedral 4 α structure. The 01+ and 31- are approximately interpreted as Td band members with the ideal tetrahedral configuration. The ground-state 4 α correlation plays an important role in the enhancement of the E 3 transition strength to the 31-. The 42+ state is not the ideal Td member but constructed from a distorted tetrahedral 4 α structure. Moreover, significant state mixing of the tetrahedral 4 α and 12C+α cluster structures occurs between 41+ and 42+ states, indicating that the Td configuration of 4 α is rather fragile at Jπ=4+ .

Method using photo-induced and thermal bending of MEMS sensors

DOEpatents

Datskos, Panagiotis G.

2001-01-01

A method for measuring chemical analytes and physical forces by measuring changes in the deflection of a microelectromechanical cantilever structure while it is being irradiated by a light having an energy above the band gap of the structure.

Nondestructive Evaluation of One-Dimensional Periodic Structures by Transmission of Laser-Excited Wide-Band Acoustic Pulses

NASA Astrophysics Data System (ADS)

Karabutov, A. A.; Kozhushko, V. V.; Pelivanov, I. M.; Podymova, N. B.

2001-03-01

The propagation of ultrasound in a one-dimensional model and actual periodic structures (PSs) is studied experimentally by the method of optoacoustic spectroscopy based on the laser thermooptical excitation and wide-band piezodetection of short acoustic pulses. It is shown that the ultrasound transmission spectrum of a PS has stop and pass bands, and the greater the number of layers in the PSs, the deeper the stop bands. The case where the thickness, density, and ultrasound velocity of one or several layers in the PS are modified is studied in detail. In this case, a narrow local maximum of ultrasound transmission appears in the stop band, whose location depends considerably on the position of the "defective" layer in the PS. The experimental data obtained coincide well with the theoretical calculation. The nondestructive evaluation of actual PSs consisting of two epoxy-glued identical aluminum plates is carried out by the optoacoustic method. Such materials are widely used in aircraft industry. It is shown that the ultrasound transmission spectrum for these materials depends considerably on the thickness of the epoxy-glue layer.

The angular electronic band structure and free particle model of aromatic molecules: High-frequency photon-induced ring current

NASA Astrophysics Data System (ADS)

Öncan, Mehmet; Koç, Fatih; Şahin, Mehmet; Köksal, Koray

2017-05-01

This work introduces an analysis of the relationship of first-principles calculations based on DFT method with the results of free particle model for ring-shaped aromatic molecules. However, the main aim of the study is to reveal the angular electronic band structure of the ring-shaped molecules. As in the case of spherical molecules such as fullerene, it is possible to observe a parabolic dispersion of electronic states with the variation of angular quantum number in the planar ring-shaped molecules. This work also discusses the transition probabilities between the occupied and virtual states by analyzing the angular electronic band structure and the possibility of ring currents in the case of spin angular momentum (SAM) or orbital angular momentum (OAM) carrying light. Current study focuses on the benzene molecule to obtain its angular electronic band structure. The obtained electronic band structure can be considered as a useful tool to see the transition probabilities between the electronic states and possible contribution of the states to the ring currents. The photoinduced current due to the transfer of SAM into the benzene molecule has been investigated by using analytical calculations within the frame of time-dependent perturbation theory.

High-Pressure Study of Perovskite-Like Organometal Halide: Band-Gap Narrowing and Structural Evolution of [NH 3 -(CH 2 ) 4 -NH 3 ]CuCl 4

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Qian; Li, Shourui; Wang, Kai

Searching for nontoxic and stable perovskite-like alternatives to lead-based halide perovskites for photovoltaic application is one urgent issue in photoelectricity science. Such exploration inevitably requires an effective method to accurately control both the crystalline and electronic structures. This work applies high pressure to narrow the band gap of perovskite-like organometal halide, [NH 3-(CH 2) 4-NH 3]CuCl 4 (DABCuCl4), through the crystalline-structure tuning. The band gap keeps decreasing below ~12 GPa, involving the shrinkage and distortion of CuCl 4 2–. Inorganic distortion determines both band-gap narrowing and phase transition between 6.4 and 10.5 GPa, and organic chains function as the springmore » cushion, evidenced by the structural transition at ~0.8 GPa. The supporting function of organic chains protects DABCuCl 4 from phase transition and amorphization, which also contributes to the sustaining band-gap narrowing. This work combines crystal structure and macroscopic property together and offers new strategies for the further design and synthesis of hybrid perovskite-like alternatives.« less

Low-bias flat band-stop filter based on velocity modulated gaussian graphene superlattice

NASA Astrophysics Data System (ADS)

Sattari-Esfahlan, S. M.; Shojaei, S.

2018-05-01

Transport properties of biased planar Gaussian graphene superlattice (PGGSL) with Fermi velocity barrier is investigated by transfer matrix method (TMM). It is observed that enlargement of bias voltage over miniband width breaks the miniband to WSLs leads to suppressing resonant tunneling. Transmission spectrum shows flat wide stop-band property controllable by external bias voltage with stop-band width of near 200 meV. The simulations demonstrate that strong velocity barriers prevent tunneling of Dirac electrons leading to controllable enhancement of stop-band width. By increasing ratio of Fermi velocity in barriers to wells υc stop-band width increase. As wide transmission stop-band width (BWT) of filter is tunable from 40 meV to 340 meV is obtained by enhancing ratio of υc from 0.2 to 1.5, respectively. Proposed structure suggests easy tunable wide band-stop electronic filter with a modulated flat stop-band characteristic by height of electrostatic barrier and structural parameters. Robust sensitivity of band width to velocity barrier intensity in certain bias voltages and flat band feature of proposed filter may be opens novel venue in GSL based flat band low noise filters and velocity modulation devices.

Torsional wave band gap properties in a circular plate of a two-dimensional generalized phononic crystal

NASA Astrophysics Data System (ADS)

Zhao, Lei; Shu, Haisheng; Liang, Shanjun; Shi, Xiaona; An, Shuowei; Ren, Wanyue; Zhu, Jie

2018-05-01

The torsional wave band gap properties of a two-dimensional generalized phononic crystal (GPC) are investigated in this paper. The GPC structure considered is consisted of two different materials being arranged with radial and circumferential periodicities simultaneously. Based on the viewpoint of energy distribution and the finite element method, the power flow, energy density, sound intensity vector together with the stress field of the structure excited by torsional load are numerically calculated and discussed. Our results show that, the band gap of Bragg type exists in these two-dimensional composite structures, and the band gap range is mainly determined by radial periodicity while the circumferential periodicity would result in some transmission peaks within the band gap. These peaks are mainly produced by two different mechanisms, the energy leakage occurred in circumferential channels and the excitation of the local eigenmodes of certain scatterers. These results may be useful in torsional vibration control for various rotational parts and components, and in the application of energy harvesting, etc.

Dynamic analysis of periodic vibration suppressors with multiple secondary oscillators

NASA Astrophysics Data System (ADS)

Ma, Jiangang; Sheng, Meiping; Guo, Zhiwei; Qin, Qi

2018-06-01

A periodic vibration suppressor with multiple secondary oscillators is examined in this paper to reduce the low-frequency vibration. The band-gap properties of infinite periodic structure and vibration transmission properties of finite periodic structure attached with secondary oscillators with arbitrary degree of freedom are thoroughly analyzed by the plane-wave-expansion method. A simply supported plate with a periodic rectangular array of vibration suppressors is considered. The dynamic model of this periodic structure is established and the equation of harmonic vibration response is theoretically derived and numerically examined. Compared with the simply supported plate without attached suppressors, the proposed plate can obtain better vibration control, and the vibration response can be effectively reduced in several frequency bands owing to the multiple band-gap property. By analyzing the modal properties of the periodic vibration suppressors, the relationship between modal frequencies and the parameters of spring stiffness and mass is established. With the numerical results, the design guidance of the locally resonant structure with multiple secondary oscillators is proposed to provide practical guidance for application. Finally, a practical periodic specimen is designed and fabricated, and then an experiment is carried out to validate the effectiveness of periodic suppressors in the reality. The results show that the experimental band gaps have a good coincidence with those in the theoretical model, and the low-frequency vibration of the plate with periodic suppressors can be effectively reduced in the tuned band gaps. Both the theoretical results and experimental results prove that the design method is effective and the structure with periodic suppressors has a promising application in engineering.

Electronic structures of of PuX (X=S, Se, Te)

NASA Astrophysics Data System (ADS)

Maehira, Takahiro; Sakai, Eijiro; Tatetsu, Yasutomi

2013-08-01

We have calculated the energy band structures and the Fermi surfaces of PuS, PuSe, and PuTe by using a self-consistent relativistic linear augmented-plane-wave method with the exchange and correlation potential in the local density approximation. In general, the energy bands near the Fermi level are mainly caused by the hybridization between the Pu 5 f and the monochalcogenide p electrons. The obtained main Fermi surfaces consisted of two hole sheets and one electron sheet, which were constructed from the band having both the Pu 5 f state and the monochalcogenide p state.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, Xin; Qiao, Weiye; Li, Yuliang

The structure stabilities and electronic properties are investigated by using ab initio self-consistent-field crystal orbital method based on density functional theory for the one-dimensional (1D) double-wall nanotubes made of n-gon SiO{sub 2} nanotubes encapsulated inside zigzag carbon nanotubes. It is found that formation of the combined systems is energetically favorable when the distance between the two constituents is around the Van der Waals scope. The obtained band structures show that all the combined systems are semiconductors with nonzero energy gaps. The frontier energy bands (the highest occupied band and the lowest unoccupied band) of double-wall nanotubes are mainly derived frommore » the corresponding carbon nanotubes. The mobilities of charge carriers are calculated to be within the range of 10{sup 2}–10{sup 4} cm{sup 2} V{sup −1} s{sup −1} for the hybrid double-wall nanotubes. Young’s moduli are also calculated for the combined systems. For the comparison, geometrical and electronic properties of n-gon SiO{sub 2} nanotubes are also calculated and discussed. - Graphical abstract: Structures and band structures of the optimum 1D Double walls nanotubes. The optimized structures are 3-gon SiO2@(15,0), 5-gon SiO2@(17,0), 6-gon SiO2@(18,0) and 7-gon SiO2@(19,0). - Highlights: • The structure and electronic properties of the 1D n-gon SiO{sub 2}@(m,0)s are studied using SCF-CO method. • The encapsulation of 1D n-gon SiO{sub 2} tubes inside zigzag carbon nanotubes can be energetically favorable. • The 1D n-gon SiO{sub 2}@(m,0)s are all semiconductors. • The mobility of charge carriers and Young’s moduli are calculated.« less

Engel-Vosko GGA calculations of the structural, electronic and optical properties of LiYO2

NASA Astrophysics Data System (ADS)

Muhammad, Nisar; Khan, Afzal; Haidar Khan, Shah; Sajjaj Siraj, Muhammad; Shah, Syed Sarmad Ali; Murtaza, Ghulam

2017-09-01

Structural, electronic and optical properties of lithium yttrium oxide (LiYO2) are investigated using density functional theory (DFT). These calculations are based on full potential linearized augmented plane wave (FP-LAPW) method implemented by WIEN2k. The generalized gradient approximation (GGA) is used as an exchange correlation potential with Perdew-Burk-Ernzerhof (PBE) and Engel-Vosko (EV) as exchange correlation functional. The structural properties are calculated with PBE-GGA as it gives the equilibrium lattice constants very close to the experimental values. While, the band structure and optical properties are calculated with EV-GGA obtain much closer results to their experimental values. Our calculations confirm LiYO2 as large indirect band gap semiconductor having band gap of 5.23 eV exhibiting the characteristics of ultrawide band gap materials showing the properties like higher critical breakdown field, higher temperature operation and higher radiation tolerance. In this article, we report the density of states (DOS) in terms of contribution from s, p, and d-states of the constituent atoms, the band structure, the electronic structure, and the frequency-dependent optical properties of LiYO2. The optical properties presented in this article reveal LiYO2 a suitable candidate for the field of optoelectronic and optical devices.

Photonic band structures of two-dimensional magnetized plasma photonic crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Qi, L.

By using modified plane wave method, photonic band structures of the transverse electric polarization for two types of two-dimensional magnetized plasma photonic crystals are obtained, and influences of the external magnetic field, plasma density, and dielectric materials on the dispersion curves are studied, respectively. Results show that two areas of flat bands appear in the dispersion curves due to the role of external magnetic field, and the higher frequencies of the up and down flat bands are corresponding to the right-circled and left-circled cutoff frequencies, respectively. Adjusting external magnetic field and plasma density can not only control positions of themore » flat bands, but also can control the location and width of the local gap; increasing relative dielectric constant of the dielectric materials makes omni-direction gaps appear.« less

Single-Band and Dual-Band Infrared Detectors

NASA Technical Reports Server (NTRS)

Ting, David Z. (Inventor); Gunapala, Sarath D. (Inventor); Soibel, Alexander (Inventor); Nguyen, Jean (Inventor); Khoshakhlagh, Arezou (Inventor)

2015-01-01

Bias-switchable dual-band infrared detectors and methods of manufacturing such detectors are provided. The infrared detectors are based on a back-to-back heterojunction diode design, where the detector structure consists of, sequentially, a top contact layer, a unipolar hole barrier layer, an absorber layer, a unipolar electron barrier, a second absorber, a second unipolar hole barrier, and a bottom contact layer. In addition, by substantially reducing the width of one of the absorber layers, a single-band infrared detector can also be formed.

Single-Band and Dual-Band Infrared Detectors

NASA Technical Reports Server (NTRS)

Ting, David Z. (Inventor); Gunapala, Sarath D. (Inventor); Soibel, Alexander (Inventor); Nguyen, Jean (Inventor); Khoshakhlagh, Arezou (Inventor)

2017-01-01

Bias-switchable dual-band infrared detectors and methods of manufacturing such detectors are provided. The infrared detectors are based on a back-to-back heterojunction diode design, where the detector structure consists of, sequentially, a top contact layer, a unipolar hole barrier layer, an absorber layer, a unipolar electron barrier, a second absorber, a second unipolar hole barrier, and a bottom contact layer. In addition, by substantially reducing the width of one of the absorber layers, a single-band infrared detector can also be formed.

Lateral energy band profile modulation in tunnel field effect transistors based on gate structure engineering

NASA Astrophysics Data System (ADS)

Cui, Ning; Liang, Renrong; Wang, Jing; Xu, Jun

2012-06-01

Choosing novel materials and structures is important for enhancing the on-state current in tunnel field-effect transistors (TFETs). In this paper, we reveal that the on-state performance of TFETs is mainly determined by the energy band profile of the channel. According to this interpretation, we present a new concept of energy band profile modulation (BPM) achieved with gate structure engineering. It is believed that this approach can be used to suppress the ambipolar effect. Based on this method, a Si TFET device with a symmetrical tri-material-gate (TMG) structure is proposed. Two-dimensional numerical simulations demonstrated that the special band profile in this device can boost on-state performance, and it also suppresses the off-state current induced by the ambipolar effect. These unique advantages are maintained over a wide range of gate lengths and supply voltages. The BPM concept can serve as a guideline for improving the performance of nanoscale TFET devices.

First-principles energy band calculation of Ruddlesden–Popper compound Sr{sub 3}Sn{sub 2}O{sub 7} using modified Becke–Johnson exchange potential

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kamimura, Sunao, E-mail: kamimura-sunao@che.kyutech.ac.jp; National Institute of Advanced Industrial Science and Technology; Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Science, Kyushu University, 6-1 Kasuga Kouen, Kasuga, Fukuoka 816-8580 Japan

The electronic structure of Sr{sub 3}Sn{sub 2}O{sub 7} is evaluated by the scalar-relativistic full potential linearized augmented plane wave (FLAPW+lo) method using the modified Becke–Johnson potential (Tran–Blaha potential) combined with the local density approximation correlation (MBJ–LDA). The fundamental gap between the valence band (VB) and conduction band (CB) is estimated to be 3.96 eV, which is close to the experimental value. Sn 5s states and Sr 4d states are predominant in the lower and upper CB, respectively. On the other hand, the lower VB is mainly composed of Sn 5s, 5p, and O 2p states, while the upper VB mainlymore » consists of O 2p states. These features of the DOS are well reflected by the optical transition between the upper VB and lower CB, as seen in the energy dependence of the dielectric function. Furthermore, the absorption coefficient estimated from the MBJ–LDA is similar to the experimental result. - Graphical abstract: Calculated energy band structure along the symmetry lines of the first BZ of Sr{sub 3}Sn{sub 2}O{sub 7} crystal obtained using the MBJ potential. - Highlights: • Electronic structure of Sr{sub 3}Sn{sub 2}O{sub 7} is calculated on the basis of MBJ–LDA method for the first time. • Band gap of Sr{sub 3}Sn{sub 2}O{sub 7} is determined accurately on the basis of MBJ–LDA method. • The experimental absorption spectrum of Sr{sub 3}Sn{sub 2}O{sub 7} produced by MBJ–LDA is more accurate than that obtained by GGA method.« less

First-principles studies of electric field effects on the electronic structure of trilayer graphene

NASA Astrophysics Data System (ADS)

Wang, Yun-Peng; Li, Xiang-Guo; Fry, James N.; Cheng, Hai-Ping

2016-10-01

A gate electric field is a powerful way to manipulate the physical properties of nanojunctions made of two-dimensional crystals. To simulate field effects on the electronic structure of trilayer graphene, we used density functional theory in combination with the effective screening medium method, which enables us to understand the field-dependent layer-layer interactions and the fundamental physics underlying band gap variations and the resulting band modifications. Two different graphene stacking orders, Bernal (or ABC) and rhombohedral (or ABA), were considered. In addition to confirming the experimentally observed band gap opening in ABC-stacked and the band overlap in ABA-stacked trilayer systems, our results reveal rich physics in these fascinating systems, where layer-layer couplings are present but some characteristics features of single-layer graphene are partially preserved. For ABC stacking, the electric-field-induced band gap size can be tuned by charge doping, while for ABA band the tunable quantity is the band overlap. Our calculations show that the electronic structures of the two stacking orders respond very differently to charge doping. We find that in the ABA stacking hole doping can reopen a band gap in the band-overlapping region, a phenomenon distinctly different from electron doping. The physical origins of the observed behaviors were fully analyzed, and we conclude that the dual-gate configuration greatly enhances the tunability of the trilayer systems.

Effects of the c-Si/a-SiO2 interfacial atomic structure on its band alignment: an ab initio study.

PubMed

Zheng, Fan; Pham, Hieu H; Wang, Lin-Wang

2017-12-13

The crystalline-Si/amorphous-SiO 2 (c-Si/a-SiO 2 ) interface is an important system used in many applications, ranging from transistors to solar cells. The transition region of the c-Si/a-SiO 2 interface plays a critical role in determining the band alignment between the two regions. However, the question of how this interface band offset is affected by the transition region thickness and its local atomic arrangement is yet to be fully investigated. Here, by controlling the parameters of the classical Monte Carlo bond switching algorithm, we have generated the atomic structures of the interfaces with various thicknesses, as well as containing Si at different oxidation states. A hybrid functional method, as shown by our calculations to reproduce the GW and experimental results for bulk Si and SiO 2 , was used to calculate the electronic structure of the heterojunction. This allowed us to study the correlation between the interface band characterization and its atomic structures. We found that although the systems with different thicknesses showed quite different atomic structures near the transition region, the calculated band offset tended to be the same, unaffected by the details of the interfacial structure. Our band offset calculation agrees well with the experimental measurements. This robustness of the interfacial electronic structure to its interfacial atomic details could be another reason for the success of the c-Si/a-SiO 2 interface in Si-based electronic applications. Nevertheless, when a reactive force field is used to generate the a-SiO 2 and c-Si/a-SiO 2 interfaces, the band offset significantly deviates from the experimental values by about 1 eV.

Effects of the c-Si/a-SiO 2 interfacial atomic structure on its band alignment: an ab initio study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zheng, Fan; Pham, Hieu H.; Wang, Lin-Wang

The crystalline-Si/amorphous-SiO 2 (c-Si/a-SiO 2) interface is an important system used in many applications, ranging from transistors to solar cells. The transition region of the c-Si/a-SiO 2 interface plays a critical role in determining the band alignment between the two regions. However, the question of how this interface band offset is affected by the transition region thickness and its local atomic arrangement is yet to be fully investigated. Here in this study, by controlling the parameters of the classical Monte Carlo bond switching algorithm, we have generated the atomic structures of the interfaces with various thicknesses, as well as containingmore » Si at different oxidation states. A hybrid functional method, as shown by our calculations to reproduce the GW and experimental results for bulk Si and SiO 2, was used to calculate the electronic structure of the heterojunction. This allowed us to study the correlation between the interface band characterization and its atomic structures. We found that although the systems with different thicknesses showed quite different atomic structures near the transition region, the calculated band offset tended to be the same, unaffected by the details of the interfacial structure. Our band offset calculation agrees well with the experimental measurements. This robustness of the interfacial electronic structure to its interfacial atomic details could be another reason for the success of the c-Si/a-SiO 2 interface in Si-based electronic applications. Nevertheless, when a reactive force field is used to generate the a-SiO 2 and c-Si/a-SiO 2 interfaces, the band offset significantly deviates from the experimental values by about 1 eV« less

Effects of the c-Si/a-SiO 2 interfacial atomic structure on its band alignment: an ab initio study

DOE PAGES

Zheng, Fan; Pham, Hieu H.; Wang, Lin-Wang

2017-11-13

The crystalline-Si/amorphous-SiO 2 (c-Si/a-SiO 2) interface is an important system used in many applications, ranging from transistors to solar cells. The transition region of the c-Si/a-SiO 2 interface plays a critical role in determining the band alignment between the two regions. However, the question of how this interface band offset is affected by the transition region thickness and its local atomic arrangement is yet to be fully investigated. Here in this study, by controlling the parameters of the classical Monte Carlo bond switching algorithm, we have generated the atomic structures of the interfaces with various thicknesses, as well as containingmore » Si at different oxidation states. A hybrid functional method, as shown by our calculations to reproduce the GW and experimental results for bulk Si and SiO 2, was used to calculate the electronic structure of the heterojunction. This allowed us to study the correlation between the interface band characterization and its atomic structures. We found that although the systems with different thicknesses showed quite different atomic structures near the transition region, the calculated band offset tended to be the same, unaffected by the details of the interfacial structure. Our band offset calculation agrees well with the experimental measurements. This robustness of the interfacial electronic structure to its interfacial atomic details could be another reason for the success of the c-Si/a-SiO 2 interface in Si-based electronic applications. Nevertheless, when a reactive force field is used to generate the a-SiO 2 and c-Si/a-SiO 2 interfaces, the band offset significantly deviates from the experimental values by about 1 eV« less

Photonic band gap properties of one-dimensional Thue-Morse all-dielectric photonic quasicrystal

NASA Astrophysics Data System (ADS)

Yue, Chenxi; Tan, Wei; Liu, Jianjun

2018-05-01

In this paper, the photonic band gap (PBG) properties of one-dimensional (1D) Thue-Morse photonic quasicrystal (PQC) S4 structure are theoretically investigated by using transfer matrix method in Bragg condition. The effects of the center wavelength, relative permittivity and incident angle on PBG properties are elaborately analyzed. Numerical results reveal that, in the case of normal incidence, the symmetry and periodicity properties of the photonic band structure are presented. As the center wavelength increases, the PBG center frequency and PBG width decrease while the photonic band structure is always symmetrical about the central frequency and the photonic band structure repeats periodically in the expanding observation frequency range. With the decrease of relative permittivity contrast, the PBG width and the relative PBG width gradually decreases until PBG disappears while the symmetry of the photonic band structure always exists. In the case of oblique incidence, as the incident angle increases, multiple narrow PBGs gradually merge into a wide PBG for the TE mode while for the TM mode, the number of PBG continuously decreases and eventually disappears, i.e., multiple narrow PBGs become a wide passband for the TM mode. The research results will provide a reference for the choice of the material, the incident angle for the PBG properties and its applications of 1D Thue-Morse PQC.

Self-interaction correction in multiple scattering theory: application to transition metal oxides

DOE Office of Scientific and Technical Information (OSTI.GOV)

Daene, Markus W; Lueders, Martin; Ernst, Arthur

2009-01-01

We apply to transition metal monoxides the self-interaction corrected (SIC) local spin density (LSD) approximation, implemented locally in the multiple scattering theory within the Korringa-Kohn-Rostoker (KKR) band structure method. The calculated electronic structure and in particular magnetic moments and energy gaps are discussed in reference to the earlier SIC results obtained within the LMTO-ASA band structure method, involving transformations between Bloch and Wannier representations to solve the eigenvalue problem and calculate the SIC charge and potential. Since the KKR can be easily extended to treat disordered alloys, by invoking the coherent potential approximation (CPA), in this paper we compare themore » CPA approach and supercell calculations to study the electronic structure of NiO with cation vacancies.« less

Single crystal growth, electronic structure and optical properties of Cs2HgBr4

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Shkumat, P. N.; Parasyuk, O. V.; Fedorchuk, A. O.; Khyzhun, O. Y.

2015-10-01

We report on successful synthesis of high-quality single crystal of cesium mercury tetrabromide, Cs2HgBr4, by using the vertical Bridgman-Stockbarger method as well as on studies of its electronic structure. For the Cs2HgBr4 crystal, we have recorded X-ray photoelectron spectra for both pristine and Ar+ ion-bombarded surfaces. Our data indicate that the Cs2HgBr4 single crystal surface is rather sensitive with respect to Ar+ ion-bombardment. In particular, such a treatment of the Cs2HgBr4 single crystal surface alters its elemental stoichiometry. To explore peculiarities of the energy distribution of total and partial densities of states within the valence band and the conduction band of Cs2HgBr4, we have made band-structure calculations based on density functional theory (DFT) employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package. The APW+lo calculations allow for concluding that the Br 4p states make the major contributions in the upper portion of the valence band, while its lower portion is dominated by contributors of the Hg 5d and Cs 5p states. Further, the main contributors to the bottom of the conduction band of Cs2HgBr4 are the unoccupied Br p and Hg s states. In addition, main optical characteristics of Cs2HgBr4 such as dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity have been explored from the first-principles band-structure calculations.

First-principles study of the structural, electronic and thermal properties of CaLiF3

NASA Astrophysics Data System (ADS)

Chouit, N.; Amara Korba, S.; Slimani, M.; Meradji, H.; Ghemid, S.; Khenata, R.

2013-09-01

Density functional theory calculations have been performed to study the structural, electronic and optical properties of CaLiF3 cubic fluoroperovskite. Our calculations were carried out by means of the full-potential linearized augmented plane-wave method. The exchange-correlation potential is treated by the local density approximation and the generalized gradient approximation (GGA) (Perdew, Burke and Ernzerhof). Moreover, the alternative form of GGA proposed by Engel and Vosko is also used for band structure calculations. The calculated total energy versus volume allows us to obtain structural properties such as the lattice constant (a0), bulk modulus (B0) and pressure derivative of the bulk modulus (B'0 ). Band structure, density of states and band gap pressure coefficients are also given. Our calculations show that CaLiF3 has an indirect band gap (R-Γ). Following the quasi-harmonic Debye model, in which the phononic effects are considered, the temperature and pressure effects on the lattice constant, bulk modulus, thermal expansion coefficient, Debye temperature and heat capacities are calculated.

Topological Classification of Crystalline Insulators through Band Structure Combinatorics

NASA Astrophysics Data System (ADS)

Kruthoff, Jorrit; de Boer, Jan; van Wezel, Jasper; Kane, Charles L.; Slager, Robert-Jan

2017-10-01

We present a method for efficiently enumerating all allowed, topologically distinct, electronic band structures within a given crystal structure in all physically relevant dimensions. The algorithm applies to crystals without time-reversal, particle-hole, chiral, or any other anticommuting or anti-unitary symmetries. The results presented match the mathematical structure underlying the topological classification of these crystals in terms of K -theory and therefore elucidate this abstract mathematical framework from a simple combinatorial perspective. Using a straightforward counting procedure, we classify all allowed topological phases of spinless particles in crystals in class A . Employing this classification, we study transitions between topological phases within class A that are driven by band inversions at high-symmetry points in the first Brillouin zone. This enables us to list all possible types of phase transitions within a given crystal structure and to identify whether or not they give rise to intermediate Weyl semimetallic phases.

A parallel-vector algorithm for rapid structural analysis on high-performance computers

NASA Technical Reports Server (NTRS)

Storaasli, Olaf O.; Nguyen, Duc T.; Agarwal, Tarun K.

1990-01-01

A fast, accurate Choleski method for the solution of symmetric systems of linear equations is presented. This direct method is based on a variable-band storage scheme and takes advantage of column heights to reduce the number of operations in the Choleski factorization. The method employs parallel computation in the outermost DO-loop and vector computation via the 'loop unrolling' technique in the innermost DO-loop. The method avoids computations with zeros outside the column heights, and as an option, zeros inside the band. The close relationship between Choleski and Gauss elimination methods is examined. The minor changes required to convert the Choleski code to a Gauss code to solve non-positive-definite symmetric systems of equations are identified. The results for two large-scale structural analyses performed on supercomputers, demonstrate the accuracy and speed of the method.

A parallel-vector algorithm for rapid structural analysis on high-performance computers

NASA Technical Reports Server (NTRS)

Storaasli, Olaf O.; Nguyen, Duc T.; Agarwal, Tarun K.

1990-01-01

A fast, accurate Choleski method for the solution of symmetric systems of linear equations is presented. This direct method is based on a variable-band storage scheme and takes advantage of column heights to reduce the number of operations in the Choleski factorization. The method employs parallel computation in the outermost DO-loop and vector computation via the loop unrolling technique in the innermost DO-loop. The method avoids computations with zeros outside the column heights, and as an option, zeros inside the band. The close relationship between Choleski and Gauss elimination methods is examined. The minor changes required to convert the Choleski code to a Gauss code to solve non-positive-definite symmetric systems of equations are identified. The results for two large scale structural analyses performed on supercomputers, demonstrate the accuracy and speed of the method.

Periodic dielectric structure for production of photonic band gap and method for fabricating the same

DOEpatents

Ozbay, Ekmel; Tuttle, Gary; Michel, Erick; Ho, Kai-Ming; Biswas, Rana; Chan, Che-Ting; Soukoulis, Costas

1995-01-01

A method for fabricating a periodic dielectric structure which exhibits a photonic band gap. Alignment holes are formed in a wafer of dielectric material having a given crystal orientation. A planar layer of elongate rods is then formed in a section of the wafer. The formation of the rods includes the step of selectively removing the dielectric material of the wafer between the rods. The formation of alignment holes and layers of elongate rods and wafers is then repeated to form a plurality of patterned wafers. A stack of patterned wafers is then formed by rotating each successive wafer with respect to the next-previous wafer, and then placing the successive wafer on the stack. This stacking results in a stack of patterned wafers having a four-layer periodicity exhibiting a photonic band gap.

Decoration of wide bandgap semiconducting materials for enhancing photoelectrochemical efficiency of PEC systems.

NASA Astrophysics Data System (ADS)

Bakranov, N.; Zhabaikhanov, A.; Kudaibergenov, S.; Ibraev, N.

2018-03-01

The production of photoanodes based on wide-band gap materials such as TiO2 is economically viable because of the low cost of synthesis methods. Contrary to economic aspects, wide-band gap semiconductor materials have a significant disadvantage due to low sensitivity to photons of visible light. To increase the photoactive parameters of the material of the electrodes in the visible range, the methods for decorating nanomasses of titanium dioxide by narrow-gap semiconductors are used. One of the most suitable narrow-gap semiconductor materials are CdS and Fe2O3. Controlled deposition of such materials on wide-gap semiconductors allows to regulate both the diffusion time of charge carriers and the band structure of TiO2/Fe2O3 and TiO2/CdS composites. The dimensions of the structure of the photoelectrode material of the cell have a large influence on the characteristics of the photocatalyst created. Thus, in the hematite structures of nanometre dimension, the rate of recombination of charge carriers fades away in comparison with bulk structures. Reducing the size of CdS structures also positively affects the nature of the photocatalytic reaction.

Direct band gap silicon crystals predicted by an inverse design method

NASA Astrophysics Data System (ADS)

Oh, Young Jun; Lee, In-Ho; Lee, Jooyoung; Kim, Sunghyun; Chang, Kee Joo

2015-03-01

Cubic diamond silicon has an indirect band gap and does not absorb or emit light as efficiently as other semiconductors with direct band gaps. Thus, searching for Si crystals with direct band gaps around 1.3 eV is important to realize efficient thin-film solar cells. In this work, we report various crystalline silicon allotropes with direct and quasi-direct band gaps, which are predicted by the inverse design method which combines a conformation space annealing algorithm for global optimization and first-principles density functional calculations. The predicted allotropes exhibit energies less than 0.3 eV per atom and good lattice matches, compared with the diamond structure. The structural stability is examined by performing finite-temperature ab initio molecular dynamics simulations and calculating the phonon spectra. The absorption spectra are obtained by solving the Bethe-Salpeter equation together with the quasiparticle G0W0 approximation. For several allotropes with the band gaps around 1 eV, photovoltaic efficiencies are comparable to those of best-known photovoltaic absorbers such as CuInSe2. This work is supported by the National Research Foundation of Korea (2005-0093845 and 2008-0061987), Samsung Science and Technology Foundation (SSTF-BA1401-08), KIAS Center for Advanced Computation, and KISTI (KSC-2013-C2-040).

Total-energy Assisted Tight-binding Method Based on Local Density Approximation of Density Functional Theory

NASA Astrophysics Data System (ADS)

Fujiwara, Takeo; Nishino, Shinya; Yamamoto, Susumu; Suzuki, Takashi; Ikeda, Minoru; Ohtani, Yasuaki

2018-06-01

A novel tight-binding method is developed, based on the extended Hückel approximation and charge self-consistency, with referring the band structure and the total energy of the local density approximation of the density functional theory. The parameters are so adjusted by computer that the result reproduces the band structure and the total energy, and the algorithm for determining parameters is established. The set of determined parameters is applicable to a variety of crystalline compounds and change of lattice constants, and, in other words, it is transferable. Examples are demonstrated for Si crystals of several crystalline structures varying lattice constants. Since the set of parameters is transferable, the present tight-binding method may be applicable also to molecular dynamics simulations of large-scale systems and long-time dynamical processes.

Crystal structure and phase stability in Fe{sub 1{minus}x}Co{sub x} from AB initio theory

DOE Office of Scientific and Technical Information (OSTI.GOV)

Soederlind, P.; Abrikosov, I.A.; James, P.

1996-06-01

For alloys between Fe and Co, their magnetic properties determine their structure. From the occupation of d states, a phase diagram is expected which depend largely on the spin polarization. A method more elaborate than canonical band models is used to calculate the spin moment and crystal structure energies. This method was the multisublattice generalization of the coherent potential approximation in conjunction with the Linear-Muffin-Tin-Orbital method in the atomic sphere approximation. To treat itinerant magnetism, the Vosko-Wilk-Nusair parameterization was used for the local spin density approximation. The fcc, bcc, and hcp phases were studied as completely random alloys, while themore » {alpha}{prime} phase for off-stoichiometries were considered as partially ordered. Results are compared with experiment and canonical band model.« less

Complex band structures of transition metal dichalcogenide monolayers with spin-orbit coupling effects

NASA Astrophysics Data System (ADS)

Szczęśniak, Dominik; Ennaoui, Ahmed; Ahzi, Saïd

2016-09-01

Recently, the transition metal dichalcogenides have attracted renewed attention due to the potential use of their low-dimensional forms in both nano- and opto-electronics. In such applications, the electronic and transport properties of monolayer transition metal dichalcogenides play a pivotal role. The present paper provides a new insight into these essential properties by studying the complex band structures of popular transition metal dichalcogenide monolayers (MX 2, where M  =  Mo, W; X  =  S, Se, Te) while including spin-orbit coupling effects. The conducted symmetry-based tight-binding calculations show that the analytical continuation from the real band structures to the complex momentum space leads to nonlinear generalized eigenvalue problems. Herein an efficient method for solving such a class of nonlinear problems is presented and yields a complete set of physically relevant eigenvalues. Solutions obtained by this method are characterized and classified into propagating and evanescent states, where the latter states manifest not only monotonic but also oscillatory decay character. It is observed that some of the oscillatory evanescent states create characteristic complex loops at the direct band gap of MX 2 monolayers, where electrons can directly tunnel between the band gap edges. To describe these tunneling currents, decay behavior of electronic states in the forbidden energy region is elucidated and their importance within the ballistic transport regime is briefly discussed.

Propagation of SH waves in an infinite/semi-infinite piezoelectric/piezomagnetic periodically layered structure.

PubMed

Pang, Yu; Liu, Yu-Shan; Liu, Jin-Xi; Feng, Wen-Jie

2016-04-01

In this paper, SH bulk/surface waves propagating in the corresponding infinite/semi-infinite piezoelectric (PE)/piezomagnetic (PM) and PM/PE periodically layered composites are investigated by two methods, the stiffness matrix method and the transfer matrix method. For a semi-infinite PE/PM or PM/PE medium, the free surface is parallel to the layer interface. Both PE and PM materials are assumed to be transversely isotropic solids. Dispersion equations are derived by the stiffness/transfer matrix methods, respectively. The effects of electric-magnetic (ME) boundary conditions at the free surface and the layer thickness ratios on dispersion curves are considered in detail. Numerical examples show that the results calculated by the two methods are the same. The dispersion curves of SH surface waves are below the bulk bands or inside the frequency gaps. The ratio of the layer thickness has an important effect not only on the bulk bands but also on the dispersion curves of SH surface waves. Electric and magnetic boundary conditions, respectively, determine the dispersion curves of SH surface waves for the PE/PM and PM/PE semi-infinite structures. The band structures of SH bulk waves are consistent for the PE/PM and PM/PE structures, however, the dispersive behaviors of SH surface waves are indeed different for the two composites. The realization of the above-mentioned characteristics of SH waves will make it possible to design PE/PM acoustic wave devices with periodical structures and achieve the better performance. Copyright © 2016 Elsevier B.V. All rights reserved.

Band structure and unconventional electronic topology of CoSi

NASA Astrophysics Data System (ADS)

Pshenay-Severin, D. A.; Ivanov, Y. V.; Burkov, A. A.; Burkov, A. T.

2018-04-01

Semimetals with certain crystal symmetries may possess unusual electronic structure topology, distinct from that of the conventional Weyl and Dirac semimetals. Characteristic property of these materials is the existence of band-touching points with multiple (higher than two-fold) degeneracy and nonzero Chern number. CoSi is a representative of this group of materials exhibiting the so-called ‘new fermions’. We report on an ab initio calculation of the electronic structure of CoSi using density functional methods, taking into account the spin-orbit interactions. The linearized \

Energy band structure and electrical properties of Ga-oxide/GaN interface formed by remote oxygen plasma

NASA Astrophysics Data System (ADS)

Yamamoto, Taishi; Taoka, Noriyuki; Ohta, Akio; Truyen, Nguyen Xuan; Yamada, Hisashi; Takahashi, Tokio; Ikeda, Mitsuhisa; Makihara, Katsunori; Nakatsuka, Osamu; Shimizu, Mitsuaki; Miyazaki, Seiichi

2018-06-01

The energy band structure of a Ga-oxide/GaN structure formed by remote oxygen plasma exposure and the electrical interface properties of the GaN metal–oxide–semiconductor (MOS) capacitors with the SiO2/Ga-oxide/GaN structures with postdeposition annealing (PDA) at various temperatures have been investigated. Reflection high-energy electron diffraction and X-ray photoelectron spectroscopy clarified that the formed Ga-oxide layer is neither a single nor polycrystalline phase with high crystallinity. We found that the energy band offsets at the conduction band minimum and at the valence band maximum between the Ga-oxide layer and the GaN surface were 0.4 and 1.2 ± 0.2 eV, respectively. Furthermore, capacitance–voltage (C–V) characteristics revealed that the interface trap density (D it) is lower than the evaluation limit of Terman method without depending on the PDA temperatures, and that the SiO2/Ga-oxide stack can work as a protection layer to maintain the low D it, avoiding the significant decomposition of GaN at the high PDA temperature of 800 °C.

Electronic structure and optical properties of noncentrosymmetric LiGaSe2: Experimental measurements and DFT band structure calculations

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Ananchenko, L. N.; Isaenko, L. I.; Yelisseyev, A. P.; Khyzhun, O. Y.

2017-04-01

We report on measurements of X-ray photoelectron (XP) spectra for pristine and Ar+ ion-irradiated surfaces of LiGaSe2 single crystal grown by Bridgman-Stockbarger method. Electronic structure of the LiGaSe2 compound is studied from a theoretical and experimental viewpoint. In particular, total and partial densities of states of LiGaSe2 are investigated by density functional theory (DFT) calculations employing the augmented plane wave + local orbitals (APW + lo) method and they are verified by data of X-ray spectroscopy measurements. The DFT calculations indicate that the main contributors to the valence band of LiGaSe2 are the Se 4p states, which contribute mainly at the top and in the upper portion of the valence band, with also essential contributions of these states in the lower portion of the band. Other substantial contributions to the valence band of LiGaSe2 emerge from the Ga 4s and Ga 4p states contributing mainly at the lower ant upper portions of the valence band, respectively. With respect to the conduction band, the calculations indicate that its bottom is composed mainly from contributions of the unoccupied Ga s and Se p states. The present calculations are confirmed experimentally when comparing the XP valence-band spectrum of the LiGaS2 single crystal on a common energy scale with the X-ray emission bands representing the energy distribution of the Ga 4p and Se 4p states. Measurements of the fundamental absorption edges at room temperature reveal that bandgap value, Eg, of LiGaSe2 is equal to 3.47 eV and the Eg value increases up to 3.66 eV when decreasing temperature to 80 K. The main optical characteristics of the LiGaSe2 compound are clarified by the DFT calculations.

Emissivity corrected infrared method for imaging anomalous structural heat flows

DOEpatents

Del Grande, Nancy K.; Durbin, Philip F.; Dolan, Kenneth W.; Perkins, Dwight E.

1995-01-01

A method for detecting flaws in structures using dual band infrared radiation. Heat is applied to the structure being evaluated. The structure is scanned for two different wavelengths and data obtained in the form of images. Images are used to remove clutter to form a corrected image. The existence and nature of a flaw is determined by investigating a variety of features.

Anomalous structural disorder and distortion in metal-to-insulator-transition Ti{sub 2}O{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hwang, In-Hui; Jin, Zhenlan; Park, Chang-In

2016-01-07

Mott proposed that impurity bands in corundum-symmetry Ti{sub 2}O{sub 3} at high temperatures caused a collapse in the bandgap. However, the origin of the impurity bands has not yet been clarified. We examine the local structural properties of metal-to-insulator-transition Ti{sub 2}O{sub 3} using in-situ x-ray absorption fine structure (XAFS) measurements at the Ti K edge in the temperature range from 288 to 739 K. The Ti{sub 2}O{sub 3} powder is synthesized by using a chemical reaction method. X-ray diffraction (XRD) measurements from Ti{sub 2}O{sub 3} with a Rietveld refinement demonstrate a single-phased R-3c symmetry without additional distortion. Extended-XAFS combined with XRDmore » reveals a zigzag patterned Ti position and an anomalous structural disorder in Ti-Ti pairs, accompanied by a bond length expansion of the Ti-Ti pairs along the c-axis for T > 450 K. The local structural distortion and disorder of the Ti atoms would induce impurity levels in the band gap between the Ti 3d a{sub 1g} and e{sub g}{sup π} bands, resulting in a collapse of the band gap for T > 450 K.« less

Constructing Repairable Meta-Structures of Ultra-Broad-Band Electromagnetic Absorption from Three-Dimensional Printed Patterned Shells.

PubMed

Song, Wei-Li; Zhou, Zhili; Wang, Li-Chen; Cheng, Xiao-Dong; Chen, Mingji; He, Rujie; Chen, Haosen; Yang, Yazheng; Fang, Daining

2017-12-13

Ultra-broad-band electromagnetic absorption materials and structures are increasingly attractive for their critical role in competing with the advanced broad-band electromagnetic detection systems. Mechanically soft and weak wax-based materials composites are known to be insufficient to serve in practical electromagnetic absorption applications. To break through such barriers, here we developed an innovative strategy to enable the wax-based composites to be robust and repairable meta-structures by employing a three-dimensional (3D) printed polymeric patterned shell. Because of the integrated merits from both the dielectric loss wax-based composites and mechanically robust 3D printed shells, the as-fabricated meta-structures enable bear mechanical collision and compression, coupled with ultra-broad-band absorption (7-40 and 75-110 GHz, reflection loss  smaller than -10 dB) approaching state-of-the-art electromagnetic absorption materials. With the assistance of experiment and simulation methods, the design advantages and mechanism of employing such 3D printed shells for substantially promoting the electromagnetic absorption performance have been demonstrated. Therefore, such universal strategy that could be widely extended to other categories of wax-based composites highlights a smart stage on which high-performance practical multifunction meta-structures with ultra-broad-band electromagnetic absorption could be envisaged.

Generalized thermoelastic wave band gaps in phononic crystals without energy dissipation

NASA Astrophysics Data System (ADS)

Wu, Ying; Yu, Kaiping; Li, Xiao; Zhou, Haotian

2016-01-01

We present a theoretical investigation of the thermoelastic wave propagation in the phononic crystals in the context of Green-Nagdhi theory by taking thermoelastic coupling into account. The thermal field is assumed to be steady. Thermoelastic wave band structures of 3D and 2D are derived by using the plane wave expansion method. For the 2D problem, the anti-plane shear mode is not affected by the temperature difference. Thermoelastic wave bands of the in-plane x-y mode are calculated for lead/silicone rubber, aluminium/silicone rubber, and aurum/silicone rubber phononic crystals. The new findings in the numerical results indicate that the thermoelastic wave bands are composed of the pure elastic wave bands and the thermal wave bands, and that the thermal wave bands can serve as the low boundary of the first band gap when the filling ratio is low. In addition, for the lead/silicone rubber phononic crystals the effects of lattice type (square, rectangle, regular triangle, and hexagon) and inclusion shape (circle, oval, and square) on the normalized thermoelastic bandwidth and the upper/lower gap boundaries are analysed and discussed. It is concluded that their effects on the thermoelastic wave band structure are remarkable.

Thermal tuning on band gaps of 2D phononic crystals considering adhesive layers

NASA Astrophysics Data System (ADS)

Zhou, Xiaoliang; Chen, Jialin; Li, Yuhang; Sun, Yuxin; Xing, Yufeng

2018-02-01

Phononic crystals are very attractive in many applications, such as noise reduction, filters and vibration isolation, due to their special forbidden band gap structures. In the present paper, the investigation of tunable band gaps of 2D phononic crystals with adhesive layers based on thermal changing is conducted. Based on the lumped-mass method, an analytical model of 2D phononic crystals with relatively thin adhesive layers is established, in which the in-plane and out-of-plane modes are both in consideration. The adhesive material is sensitive to temperature so that the band structure can be tuned and controlled by temperature variation. As temperature increases from 20 °C-80 °C, the first band gap shifts to the frequency zone around 10 kHz, which is included by the audible frequency range. The results propose an important guideline for applications, such as noise suppression using the 2D phononic crystals.

Determination of band structure parameters and the quasi-particle gap of CdSe quantum dots by cyclic voltammetry.

PubMed

Inamdar, Shaukatali N; Ingole, Pravin P; Haram, Santosh K

2008-12-01

Band structure parameters such as the conduction band edge, the valence band edge and the quasi-particle gap of diffusing CdSe quantum dots (Q-dots) of various sizes were determined using cyclic voltammetry. These parameters are strongly dependent on the size of the Q-dots. The results obtained from voltammetric measurements are compared to spectroscopic and theoretical data. The fit obtained to the reported calculations based on the semi-empirical pseudopotential method (SEPM)-especially in the strong size-confinement region, is the best reported so far, according to our knowledge. For the smallest CdSe Q-dots, the difference between the quasi-particle gap and the optical band gap gives the electron-hole Coulombic interaction energy (J(e1,h1)). Interband states seen in the photoluminescence spectra were verified with cyclic voltammetry measurements.

Omnidirectional photonic band gap enlarged by one-dimensional ternary unmagnetized plasma photonic crystals based on a new Fibonacci quasiperiodic structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang Haifeng; Nanjing Artillery Academy, Nanjing 211132; Liu Shaobin

2012-11-15

In this paper, an omnidirectional photonic band gap realized by one-dimensional ternary unmagnetized plasma photonic crystals based on a new Fibonacci quasiperiodic structure, which is composed of homogeneous unmagnetized plasma and two kinds of isotropic dielectric, is theoretically studied by the transfer matrix method. It has been shown that such an omnidirectional photonic band gap originates from Bragg gap in contrast to zero-n gap or single negative (negative permittivity or negative permeability) gap, and it is insensitive to the incidence angle and the polarization of electromagnetic wave. From the numerical results, the frequency range and central frequency of omnidirectional photonicmore » band gap can be tuned by the thickness and density of the plasma but cease to change with increasing Fibonacci order. The bandwidth of omnidirectional photonic band gap can be notably enlarged. Moreover, the plasma collision frequency has no effect on the bandwidth of omnidirectional photonic band gap. It is shown that such new structure Fibonacci quasiperiodic one-dimensional ternary plasma photonic crystals have a superior feature in the enhancement of frequency range of omnidirectional photonic band gap compared with the conventional ternary and conventional Fibonacci quasiperiodic ternary plasma photonic crystals.« less

Electronic structure and optical properties of iron based chalcogenide FeX2 (X = S, Se, Te) for photovoltaic applications: a first principle study

NASA Astrophysics Data System (ADS)

Ghosh, Anima; Thangavel, R.

2017-11-01

In present work, the electronic structure and optical properties of the FeX2 (X = S, Se, Te) compounds have been evaluated by the density functional theory based on the scalar-relativistic full potential linear augmented plane wave method via Wien2K. From the total energy calculations, it has been found that all the compounds have direct band nature, which determined by iron 3 d states at valance band edge and anion p dominated at conduction band at Γ-point and the fundamental band gap between the valence band and conduction band are estimated 1.40, 1.02 and 0.88 eV respectively with scissor correction for FeS2, FeSe2 and FeTe2 which are close to the experimental values. The optical properties such as dielectric tensor components and the absorption coefficient of these materials are determined in order to investigate their usefulness in photovoltaic applications.

Electronic structure and optical properties of defect chalcopyrite HgGa2Se4

NASA Astrophysics Data System (ADS)

Gabrelian, B. V.; Lavrentyev, A. A.; Vu, Tuan V.; Parasyuk, O. V.; Khyzhun, O. Y.

2018-01-01

We report on studies from an experimental and theoretical viewpoint of the electronic structure of mercury digallium selenide, HgGa2Se4, a very promising optoelectronic material. In particular, the method of X-ray photoelectron spectroscopy (XPS) was used to evaluate binding energies of the constituent element core electrons and the shape of the valence band for pristine and Ar+-ion bombarded surfaces of HgGa2Se4 single crystal. First principles band-structure calculations were performed in the present work using the augmented plane wave + local orbitals (APW+lo). These calculations indicate that the Se 4p states are the main contributors at the top and in the upper portion of the valence band with slightly smaller contributions of the Ga 4p states in the upper portion of the band as well. Further, the central portion of the valence band is determined mainly by contributions of the Ga 4s states, and the Hg 5d states are the principal contributors to the bottom of the valence band. These theoretical data are in fair agreement when matching on a common energy scale of the X-ray emission bands giving information on the energy distribution of the Se 4p and Ga 4p states and the XPS valence-band spectrum of the HgGa2Se4 crystal. The principal optical constants are elucidated from the DFT calculations.

Polarization and angle insensitive dual-band bandpass frequency selective surface using all-dielectric metamaterials

NASA Astrophysics Data System (ADS)

Yu, Fei; Wang, Jun; Wang, Jiafu; Ma, Hua; Du, Hongliang; Xu, Zhuo; Qu, Shaobo

2016-04-01

In this paper, we demonstrate a dual-band bandpass all-dielectric frequency selective surface (FSS), the building elements of which are high-permittivity ceramic particles rather than metallic patterns. With proper structural design and parameter adjustment, the resonant frequency can be tuned at will. Dual-band bandpass response can be realized due to the coupling between electric and magnetic resonances. As an example, a dual-band bandpass FSS is designed in Ku band, which is composed of two-dimensional periodic arrays of complementary quatrefoil structures (CQS) cut from dielectric plates. Moreover, cylindrical dielectric resonators are introduced and placed in the center of each CQS to broaden the bandwidth and to sharpen the cut-off frequency. Theoretical analysis shows that the bandpass response arises from impedance matching caused by electric and magnetic resonances. In addition, effective electromagnetic parameters and dynamic field distributions are presented to explain the mechanism of impedance matching. The proposed FSS has the merits of polarization independence, stable transmission, and sharp roll-off frequency. The method can also be used to design all-dielectric FSSs with continuum structures at other frequencies.

Interaction of aldolase with actin-containing filaments. Structural studies.

PubMed Central

Stewart, M; Morton, D J; Clarke, F M

1980-01-01

Electron micrographs of the paracrystals formed when fructose bisphosphate aldolase (EC 4.1.2.13) is added to actin-containing filaments were analysed by computer methods so that ultrastructural changes could be correlated with the various stoicheiometries of binding determined in the preceding paper [Walsh, Winzor, Clarke, Masters & Morton (1980) Biochem. J. 186, 89-98]. Paracrystals formed with aldolase and either F-actin or F-actin-tropomyosin have a single light transverse band every 38 nm, which is due to aldolase molecules cross-linking the filaments. In contrast, the paracrystals formed between aldolase and F-actin-tropomyosin-troponin filaments show two transverse bands every 38 nm: a major band, interpreted as aldolase binding to troponin, and a minor band, interpreted as aldolase cross-linking the filaments. The intensity of the minor band varies with Ca2+ concentration, being greatest when the Ca2+ concentration is low. A model for the different paracrystal structures which relates the various patterns and binding stoicheiometries to structural changes in the actin-containing filaments is proposed. Images PLATE 1 PMID:6892771

Synthesis of High-Frequency Ground Motion Using Information Extracted from Low-Frequency Ground Motion

NASA Astrophysics Data System (ADS)

Iwaki, A.; Fujiwara, H.

2012-12-01

Broadband ground motion computations of scenario earthquakes are often based on hybrid methods that are the combinations of deterministic approach in lower frequency band and stochastic approach in higher frequency band. Typical computation methods for low-frequency and high-frequency (LF and HF, respectively) ground motions are the numerical simulations, such as finite-difference and finite-element methods based on three-dimensional velocity structure model, and the stochastic Green's function method, respectively. In such hybrid methods, LF and HF wave fields are generated through two different methods that are completely independent of each other, and are combined at the matching frequency. However, LF and HF wave fields are essentially not independent as long as they are from the same event. In this study, we focus on the relation among acceleration envelopes at different frequency bands, and attempt to synthesize HF ground motion using the information extracted from LF ground motion, aiming to propose a new method for broad-band strong motion prediction. Our study area is Kanto area, Japan. We use the K-NET and KiK-net surface acceleration data and compute RMS envelope at four frequency bands: 0.5-1.0 Hz, 1.0-2.0 Hz, 2.0-4.0 Hz, .0-8.0 Hz, and 8.0-16.0 Hz. Taking the ratio of the envelopes of adjacent bands, we find that the envelope ratios have stable shapes at each site. The empirical envelope-ratio characteristics are combined with low-frequency envelope of the target earthquake to synthesize HF ground motion. We have applied the method to M5-class earthquakes and a M7 target earthquake that occurred in the vicinity of Kanto area, and successfully reproduced the observed HF ground motion of the target earthquake. The method can be applied to a broad band ground motion simulation for a scenario earthquake by combining numerically-computed low-frequency (~1 Hz) ground motion with the empirical envelope ratio characteristics to generate broadband ground motion. The strengths of the proposed method are that: 1) it is based on observed ground motion characteristics, 2) it takes full advantage of precise velocity structure model, and 3) it is simple and easy to apply.

Numerical modelling of Mars supersonic disk-gap-band parachute inflation

NASA Astrophysics Data System (ADS)

Gao, Xinglong; Zhang, Qingbin; Tang, Qiangang

2016-06-01

The transient dynamic behaviour of supersonic disk-gap-band parachutes in a Mars entry environment involving fluid structure interactions is studied. Based on the multi-material Arbitrary Lagrange-Euler method, the coupling dynamic model between a viscous compressible fluid and a flexible large deformation structure of the parachute is solved. The inflation performance of a parachute with a fixed forebody under different flow conditions is analysed. The decelerating parameters of the parachute, including drag area, opening loads, and coefficients, are obtained from the supersonic wind tunnel test data from NASA. Meanwhile, the evolution of the three-dimensional shape of the disk-gap-band parachute during supersonic inflation is presented, and the structural dynamic behaviour of the parachute is predicted. Then, the influence of the presence of the capsule on the flow field of the parachute is investigated, and the wake of unsteady fluid and the distribution of shock wave around the supersonic parachute are presented. Finally, the structural dynamic response of the canopy fabric under high-pressure conditions is comparatively analysed. The results show that the disk-gap-band parachute is well inflated without serious collapse. As the Mach numbers increase from 2.0 to 2.5, the drag coefficients gradually decrease, along with a small decrease in inflation time, which corresponds with test results, and proves the validity of the method proposed in this paper.

Effect of p–d hybridization, structural distortion and cation electronegativity on electronic properties of ZnSnX{sub 2} (X=P, As, Sb) chalcopyrite semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mishra, S.; Ganguli, B., E-mail: biplabg@nitrkl.ac.in

2013-04-15

Significant effects of p–d hybridization, structural distortion and cation-electro-negativity are found on band gap in ZnSnX{sub 2} (X=P, As, Sb). Our study suggests these compounds to be direct band gap semiconductors with band gaps of 1.23, 0.68 and 0.19 eV respectively. Lattice constants, tetragonal distortion (η), anion displacement, bond lengths and bulk moduli are calculated by Density Functional Theory based on Tight binding Linear Muffin-Tin orbital method. Our result of structural properties is in good agreement with the available experimental and other theoretical results. Calculated band gaps also agree well with the experimental works within LDA limitation. Unlike other semiconductorsmore » in the group II–IV–V{sub 2}, there is a reduction in the band gap of 0.22, 0.20 and 0.24 eV respectively in ZnSnX{sub 2} (X=P, As, Sb) due to p–d hybridization. Structural distortion decreases band gap by 0.20, 0.12 and 0.10 eV respectively. We find that cation electronegativity effect is responsible for increasing the band gap relative to their binary analogs GaInP{sub 2}, InGaAs{sub 2} and GaInSb{sub 2} respectively and increment are 0.13, 0.04 and 0.13 eV respectively. - Graphical abstract: One unit cell of ZnSnX{sub 2} (X=P, As, Sb) chalcopyrite semiconductor. Semiconductors ZnSnX{sub 2} (X=P, As, Sb) are found to be direct band gap semiconductors with band gaps 1.23, 0.68 and 0.19 eV respectively. The quantitative estimate of effects of p–d hybridization, structural distortion and cation electronegativity shows band gaps change significantly due to these effects. Highlights: ► ZnSnX{sub 2} (X=P, As, Sb) are direct band gap semiconductors. ► These have band gaps of 1.23 eV, 0.68 eV and 0.19 eV respectively. ► The band gap reduction due to p–d hybridization is 13.41%, 18.51% and 40% respectively. ► Band gap reduction due to structural distortion is 12.12%, 11.11% and 16.66% respectively. ► Band gap increases 8.38%, 3.70% and 21.31% respectively due to cation electronegativity.« less

A three-dimensional structural dissection of Drosophila polytene chromosomes.

PubMed

Urata, Y; Parmelee, S J; Agard, D A; Sedat, J W

1995-10-01

We have analyzed the three-dimensional structural details of Drosophila melanogaster polytene chromosome bands and interbands using three-dimensional light microscopy and a novel method of sample preparation that does not involve flattening or stretching the chromosomes. Bands have been visualized in unfixed chromosomes stained with the DNA specific dye 4,6-Diamidino-2-phenylindole (DAPI). Interbands have been visualized using fixed chromosomes that have been immunostained with an antibody to RNA polymerase II. Additionally, these structures have been analyzed using in situ hybridization with probes from specific genetic loci (Notch and white). Bands are seen to be composed of approximately 36 substructural features that measure 0.2-0.4 micron in diameter. We suggest that these substructural features are in fact longitudinal fibers made up of bundles of chromatids. Band shape can be a reproducible characteristic of a particular band and is dependent on the spatial relationship of these bundles, varying from bands with a uniform distribution of bundles to bands with a peripheral concentration of chromatin. Interbands are composed of bundles of chromatids of a similar size and number as those seen in the bands. The distribution of bundles is similar between a band and the neighboring interband, implying that there is a long range organization to the DNA that includes both the coding and the noncoding portions of genes. Finally, we note that the polytene chromosome has a circular shape when viewed in cross section, whether there are one or two homologs present.

Novel Galvanic Corrosion Inhibitors: Synthesis, Characterization, Fabrication and Testing

DTIC Science & Technology

2007-09-30

have attempted to develop methods based on chemical structural modification to prevent galvanically-induced composite corrosion. [9, 10-12] These...of the two metallopolymers 11 and 12 show characteristic MLCT (metal-to-ligand charge transfer) absorption band of tris(bipyridyl)Ru(II) unit at k...showed absorption band at 450 nm and emission band at 325 nm of tris(bipyridyl)Ru(II) units in its respective UV-vis and fluorescence spectra. Very

The Shock and Vibration Digest. Volume 16, Number 1

DTIC Science & Technology

1984-01-01

investigation of the measure- ment of frequency band average loss factors of structural components for use in the statistical energy analysis method of...stiffness. Matrix methods Key Words: Finite element technique. Statistical energy analysis . Experimental techniques. Framed structures, Com- puter...programs In order to further understand the practical application of the statistical energy analysis , a two section plate-like frame structure is

Comparative study of the pentamodal property of four potential pentamode microstructures

NASA Astrophysics Data System (ADS)

Huang, Yan; Lu, Xuegang; Liang, Gongying; Xu, Zhuo

2017-03-01

In this paper, a numerical comparative study is presented on the pentamodal property of four potential pentamode microstructures (three based on simple cubic and one on body-centered cubic structures) based on phonon band calculations. The finite-element method is employed to calculate the band structures, and the two essential factors of the ratio of bulk modulus B to shear modulus G and the single-mode band gap (SBG) are analyzed to quantitatively evaluate the pentamodal property. The results show that all four structures possess a higher B/G ratio than traditional materials. One of the simple cubic structures exhibits the incomplete SBG, while the three other structures exhibit complete SBG to decouple the compression and shear waves in all propagation directions. Further parametric analyses are presented investigating the effects of geometrical and material parameters on the pentamodal property of these structures. This study provides guidelines for the future design of novel pentamode microstructures possessing a high B/G ratio and a low-frequency broadband SBG.

[Spectrum studies on titania photocatalysts].

PubMed

Su, W; Fu, X; Wei, K; Zhang, H; Lin, H; Wang, X; Li, D

2001-02-01

The nano-sized TiO2 photocatalysts were prepared by sol-gel method and characterized by FTIR spectroscopy, FT-Raman spectroscopy and diffuse reflectance spectroscopy(DRS). Photocatalytic degradation of oleic acid over the TiO2 catalysts was investigated. The result showed that calcination temperature has strong effect on crystal structure, energy band structure, optical adsorption and photocatalytic activity of the TiO2 catalysts. It was found that the TiO2 photocatalyst calcined at 400 degrees C has the best apparent optical adsorption, the biggest band edge position and the highest photoactivity. The effect of calcination temperature on photocatalytic activity of TiO2 catalysts has been ascribed to the changes in structure and optical property of catalyst such as crystal size, content of rutile, residual NO3-, and band-edge position of light adsorption.

A comprehensive study of g-factors, elastic, structural and electronic properties of III-V semiconductors using hybrid-density functional theory

NASA Astrophysics Data System (ADS)

Bastos, Carlos M. O.; Sabino, Fernando P.; Sipahi, Guilherme M.; Da Silva, Juarez L. F.

2018-02-01

Despite the large number of theoretical III-V semiconductor studies reported every year, our atomistic understanding is still limited. The limitations of the theoretical approaches to yield accurate structural and electronic properties on an equal footing, is due to the unphysical self-interaction problem that mainly affects the band gap and spin-orbit splitting (SOC) in semiconductors and, in particular, III-V systems with similar magnitude of the band gap and SOC. In this work, we report a consistent study of the structural and electronic properties of the III-V semiconductors by using the screening hybrid-density functional theory framework, by fitting the α parameters for 12 different III-V compounds, namely, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb, to minimize the deviation between the theoretical and experimental values of the band gap and SOC. Structural relaxation effects were also included. Except for AlP, whose α = 0.127, we obtained α values that ranged from 0.209 to 0.343, which deviate by less than 0.1 from the universal value of 0.25. Our results for the lattice parameter and elastic constants indicate that the fitting of α does not affect those structural parameters when compared with the HSE06 functional, where α = 0.25. Our analysis of the band structure based on the k ṡ p method shows that the effective masses are in agreement with the experimental values, which can be attributed to the simultaneous fitting of the band gap and SOC. Also, we estimate the values of g-factors, extracted directly from the band structure, which are close to experimental results, which indicate that the obtained band structure produced a realistic set of k ṡ p parameters.

Emissivity corrected infrared method for imaging anomalous structural heat flows

DOEpatents

Del Grande, N.K.; Durbin, P.F.; Dolan, K.W.; Perkins, D.E.

1995-08-22

A method for detecting flaws in structures using dual band infrared radiation is disclosed. Heat is applied to the structure being evaluated. The structure is scanned for two different wavelengths and data obtained in the form of images. Images are used to remove clutter to form a corrected image. The existence and nature of a flaw is determined by investigating a variety of features. 1 fig.

Hydrothermal temperature effect on crystal structures, optical properties and electrical conductivity of ZnO nanostructures

NASA Astrophysics Data System (ADS)

Dhafina, Wan Almaz; Salleh, Hasiah; Daud, Mohd Zalani; Ghazali, Mohd Sabri Mohd; Ghazali, Salmah Mohd

2017-09-01

ZnO is an wide direct band gap semiconductor and possess rich family of nanostructures which turned to be a key role in the nanotechnology field of applications. Hydrothermal method was proven to be simple, robust and low cost among the reported methods to synthesize ZnO nanostructures. In this work, the properties of ZnO nanostructures were altered by varying temperatures of hydrothermal process. The changes in term of morphological, crystal structures, optical properties and electrical conductivity were investigated. A drastic change of ZnO nanostructures morphology and decreases of 002 diffraction peak were observed as the hydrothermal temperature increased. The band gap of samples decreased as the size of ZnO nanostructure increased, whereas the electrical conductivity had no influence on the band gap value but more on the morphology of ZnO nanostructures instead.

Periodic dielectric structure for production of photonic band gap and method for fabricating the same

DOEpatents

Ozbay, E.; Tuttle, G.; Michel, E.; Ho, K.M.; Biswas, R.; Chan, C.T.; Soukoulis, C.

1995-04-11

A method is disclosed for fabricating a periodic dielectric structure which exhibits a photonic band gap. Alignment holes are formed in a wafer of dielectric material having a given crystal orientation. A planar layer of elongate rods is then formed in a section of the wafer. The formation of the rods includes the step of selectively removing the dielectric material of the wafer between the rods. The formation of alignment holes and layers of elongate rods and wafers is then repeated to form a plurality of patterned wafers. A stack of patterned wafers is then formed by rotating each successive wafer with respect to the next-previous wafer, and then placing the successive wafer on the stack. This stacking results in a stack of patterned wafers having a four-layer periodicity exhibiting a photonic band gap. 42 figures.

Spin-polarized structural, elastic, electronic and magnetic properties of half-metallic ferromagnetism in V-doped ZnSe

NASA Astrophysics Data System (ADS)

Monir, M. El Amine.; Baltache, H.; Murtaza, G.; Khenata, R.; Ahmed, Waleed K.; Bouhemadou, A.; Omran, S. Bin; Seddik, T.

2015-01-01

Based on first principles spin-polarized density functional theory, the structural, elastic electronic and magnetic properties of Zn1-xVxSe (for x=0.25, 0.50, 0.75) in zinc blende structure have been studied. The investigation was done using the full-potential augmented plane wave method as implemented in WIEN2k code. The exchange-correlation potential was treated with the generalized gradient approximation PBE-GGA for the structural and elastic properties. Moreover, the PBE-GGA+U approximation (where U is the Hubbard correlation terms) is employed to treat the "d" electrons properly. A comparative study between the band structures, electronic structures, total and partial densities of states and local moments calculated within both GGA and GGA+U schemes is presented. The analysis of spin-polarized band structure and density of states shows the half-metallic ferromagnetic character and are also used to determine s(p)-d exchange constants N0α (conduction band) and N0β (valence band) due to Se(4p)-V(3d) hybridization. It has been clearly evidence that the magnetic moment of V is reduced from its free space change value of 3 μB and the minor atomic magnetic moment on Zn and Se are generated.

Monoclinic Tungsten Oxide with {100} Facet Orientation and Tuned Electronic Band Structure for Enhanced Photocatalytic Oxidations.

PubMed

Zhang, Ning; Chen, Chen; Mei, Zongwei; Liu, Xiaohe; Qu, Xiaolei; Li, Yunxiang; Li, Siqi; Qi, Weihong; Zhang, Yuanjian; Ye, Jinhua; Roy, Vellaisamy A L; Ma, Renzhi

2016-04-27

Exploring surface-exposed highly active crystal facets for photocatalytic oxidations is promising in utilizing monoclinic WO3 semiconductor. However, the previously reported highly active facets for monoclinic WO3 were mainly toward enhancing photocatalytic reductions. Here we report that the WO3 with {100} facet orientation and tuned surface electronic band structure can effectively enhance photocatalytic oxidation properties. The {100} faceted WO3 single crystals are synthesized via a facile hydrothermal method. The UV-visible diffuse reflectance, X-ray photoelectron spectroscopy valence band spectra, and photoelectrochemical measurements suggest that the {100} faceted WO3 has a much higher energy level of valence band maximum compared with the normal WO3 crystals without preferred orientation of the crystal face. The density functional theory calculations reveal that the shift of O 2p and W 5d states in {100} face induce a unique band structure. In comparison with the normal WO3, the {100} faceted WO3 exhibits an O2 evolution rate about 5.1 times in water splitting, and also shows an acetone evolution rate of 4.2 times as well as CO2 evolution rate of 3.8 times in gaseous degradation of 2-propanol. This study demonstrates an efficient crystal face engineering route to tune the surface electronic band structure for enhanced photocatalytic oxidations.

Electronic properties and structural phase transition in A4 [M4O4] (A=Li, Na, K and Rb; M=Ag and Cu): A first principles study

NASA Astrophysics Data System (ADS)

Umamaheswari, R.; Yogeswari, M.; Kalpana, G.

2013-02-01

Self-consistent scalar relativistic band structure calculations for AMO (A=Li, Na, K and Rb; M=Ag and Cu) compounds have been performed using the tight-binding linear muffin-tin orbital (TB-LMTO) method within the local density approximation (LDA). At ambient conditions, these compounds are found to crystallize in tetragonal KAgO-type structure with two different space group I-4m2 and I4/mmm. Nowadays, hypothetical structures are being considered to look for new functional materials. AMO compounds have stoichiometry similar to eight-electron half-Heusler materials of type I-I-VI which crystallizes in cubic (C1b) MgAgAs-type structure with space group F-43m. For all these compounds, by interchanging the positions of atoms in the hypothetical cubic structure, three phases (α, β and γ) are formed. The energy-volume relation for these compounds in tetragonal KAgO-type structure and cubic α, β and γ phases of related structure have been obtained. Under ambient conditions these compounds are more stable in tetragonal KAgO-type (I4/mmm) structure. The total energies calculated within the atomic sphere approximation (ASA) were used to determine the ground state properties such as equilibrium lattice parameters, c/a ratio, bulk modulus, cohesive energy and are compared with the available experimental results. The results of the electronic band structure calculations at ambient condition show that LiCuO and NaMO are indirect band gap semiconductors whereas KMO and RbMO are direct band gap semiconductors. At high pressure the band gap decreases and the phenomenon of band overlap metallization occur. Also these compounds undergo structural phase transition from tetragonal I-4m2 phase to cubic α-phase and transition pressures were calculated.

Elastic and viscoelastic effects in rubber/air acoustic band gap structures: A theoretical and experimental study

NASA Astrophysics Data System (ADS)

Merheb, B.; Deymier, P. A.; Jain, M.; Aloshyna-Lesuffleur, M.; Mohanty, S.; Berker, A.; Greger, R. W.

2008-09-01

The transmission of acoustic waves through centimeter-scale elastic and viscoelastic two-dimensional silicone rubber/air phononic crystal structures is investigated theoretically and experimentally. We introduce a finite difference time domain method for two-dimensional elastic and viscoelastic composite structures. Elastic fluid-solid phononic crystals composed of a two-dimensional array of cylindrical air inclusions in a solid rubber matrix, as well as an array of rubber cylinders in an air matrix, are shown to behave similarly to fluid-fluid composite structures. These systems exhibit very wide band gaps in their transmission spectra that extend to frequencies in the audible range of the spectrum. This effect is associated with the very low value of the transverse speed of sound in rubber compared to that of the longitudinal polarization. The difference in transmission between elastic and viscoelastic rubber/air crystals results from attenuation of transmission over a very wide frequency range, leaving only narrow passing bands at very low frequencies. These phononic crystals demonstrate the practical design of elastic or viscoelastic solid rubber/air acoustic band gap sound barriers with small dimensions.

Probing the band structure and local electronic properties of low-dimensional semiconductor structures

NASA Astrophysics Data System (ADS)

Walrath, Jenna Cherie

Low-dimensional semiconductor structures are important for a wide variety of applications, and recent advances in nanoscale fabrication are paving the way for increasingly precise nano-engineering of a wide range of materials. It is therefore essential that the physics of materials at the nanoscale are thoroughly understood to unleash the full potential of nanotechnology, requiring the development of increasingly sophisticated instrumentation and modeling. Of particular interest is the relationship between the local density of states (LDOS) of low-dimensional structures and the band structure and local electronic properties. This dissertation presents the investigation of the band structure, LDOS, and local electronic properties of nanostructures ranging from zero-dimensional (0D) quantum dots (QDs) to two-dimensional (2D) thin films, synthesizing computational and experimental approaches including Poisson-Schrodinger band structure calculations, scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and scanning thermoelectric microscopy (SThEM). A method is presented for quantifying the local Seebeck coefficient (S) with SThEM, using a quasi-3D conversion matrix approach to directly convert temperature gradient-induced voltages S. For a GaAs p-n junction, the resulting S-profile is consistent with that computed using the free carrier concentration profile. This combined computational-experimental approach is expected to enable nanoscale measurements of S across a wide variety of heterostructure interfaces. The local carrier concentration, n, is profiled across epitaxial InAs/GaAs QDs, where SThEM is used to profile the temperature gradient-induced voltage, which is converted to a profile of the local S and finally to an n profile. The S profile is converted to a conduction band-edge profile and compared with Poisson-Schrodinger band-edge simulations. The combined computational-experimental approach suggests a reduced n in the QD center in comparison to that of the 2D alloy layer. The surface composition and band structure of ordered horizontal Sb2Te3 nanowires induced by femtosecond laser irradiation of a thin film are investigated, revealing a band gap modulation between buried Sb2Te3 nanowires and the surrounding insulating material. Finally, STM and STS are used to investigate the band structure of BiSbTe alloys at room temperature, revealing both the Fermi level and Dirac point located inside the bulk bandgap, indicating bulk-like insulating behavior with accessible surface states.

Doping effects in InN/GaN short-period quantum well structures-Theoretical studies based on density functional methods

NASA Astrophysics Data System (ADS)

Strak, Pawel; Kempisty, Pawel; Sakowski, Konrad; Krukowski, Stanislaw

2014-09-01

Density functional theory studies were conducted to determine an influence of the carrier concentration on the optical and electronic properties of InN/GaN superlattice system. The oscillator strength values, energy gaps and the band profiles were obtained. The band profiles were found to be strongly affected for technically possible heavy n-type doping while for p-type doping the carrier influence, both screening and band shift, is negligible. Blue shift of the transition energy between conduction band minima and valence band maxima was observed for high concentrations of both type carriers.

Simulations of wave propagation and disorder in 3D non-close-packed colloidal photonic crystals with low refractive index contrast.

PubMed

Glushko, O; Meisels, R; Kuchar, F

2010-03-29

The plane-wave expansion method (PWEM), the multiple-scattering method (MSM) and the 3D finite-difference time-domain method (FDTD) are applied for simulations of propagation of electromagnetic waves through 3D colloidal photonic crystals. The system investigated is not a "usual" artificial opal with close-packed fcc lattice but a dilute bcc structure which occurs due to long-range repulsive interaction between electrically charged colloidal particles during the growth process. The basic optical properties of non-close-packed colloidal PhCs are explored by examining the band structure and reflection spectra for a bcc lattice of silica spheres in an aqueous medium. Finite size effects and correspondence between the Bragg model, band structure and reflection spectra are discussed. The effects of size, positional and missing-spheres disorder are investigated. In addition, by analyzing the results of experimental work we show that the fabricated structures have reduced plane-to-plane distance probably due to the effect of gravity during growth.

Increasing low frequency sound attenuation using compounded single layer of sonic crystal

NASA Astrophysics Data System (ADS)

Gulia, Preeti; Gupta, Arpan

2018-05-01

Sonic crystals (SC) are man-made periodic structures where sound hard scatterers are arranged in a crystalline manner. SC reduces noise in a particular range of frequencies called as band gap. Sonic crystals have a promising application in noise shielding; however, the application is limited due to the size of structure. Particularly for low frequencies, the structure becomes quite bulky, restricting its practical application. This paper presents a compounded model of SC, which has the same overall area and filling fraction but with increased low frequency sound attenuation. Two cases have been considered, a three layer SC and a compounded single layer SC. Both models have been analyzed using finite element simulation and plane wave expansion method. Band gaps for periodic structures have been obtained using both methods which are in good agreement. Further, sound transmission loss has been evaluated using finite element method. The results demonstrate the use of compounded model of Sonic Crystal for low frequency sound attenuation.

Understanding Asteroid Disruptions Using Very Young Dust Bands

NASA Astrophysics Data System (ADS)

Espy Kehoe, Ashley J.; Kehoe, T. J.; Colwell, J. E.; Dermott, S. F.

2013-10-01

Zodiacal dust bands are structures that result from the dynamical sculpting of the dust particles released in the catastrophic disruption of an asteroid. Partial dust bands are the recently discovered younger siblings of the dust bands, ones that are still forming and due to very recent disruptions within the last few hundred thousand years. During the early stages of formation, these structures retain information on the original catastrophic disruptions that produced them (since the dust has not yet been lost or significantly altered by orbital or collisional decay). The first partial dust band, at about 17 degrees latitude, was revealed using a very precise method of co-adding the IRAS data set. We have shown that these partial dust bands exhibit structure consistent with a forming band, can be used to constrain the original size distribution of the dust produced in the catastrophic disruption of an asteroid, and these very young structures also allow a much better estimate of the total amount of dust released in the disruption. In order to interpret the observations and constrain the parameters of the dust injected into the cloud following an asteroid disruption, we have developed detailed models of the dynamical evolution of the dust that makes up the band. We model the dust velocity distribution resulting from the initial impact and then track the orbital evolution of the dust under the effects of gravitational perturbations from all the planets as well as radiative forces of Poynting-Robertson drag, solar wind drag and radiation pressure and use these results to produce maps of the thermal emission. Through the comparison of our newly completed dynamical evolution models with the coadded observations, we can put constraints on the parameters of dust producing the band. We confirm the source of the band as the very young Emilkowalski cluster ( <250,000 years; Nesvorny et al., 2003) and present our most recent estimates of the size-distribution and cross-sectional area of material in the band and discuss the implications of these constraints on the temporal evolution of the zodiacal cloud and to the structure of the parent asteroid.

FP-LAPW calculations of the elastic, electronic and thermoelectric properties of the filled skutterudite CeRu{sub 4}Sb{sub 12}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shankar, A., E-mail: amitshan2009@gmail.com; Rai, D.P.; Chettri, Sandeep

2016-08-15

We have investigated the electronic structure, elastic and thermoelectric properties of the filled skutterudite CeRu{sub 4}Sb{sub 12} using the density functional theory (DFT). The full potential linearized augmented plane wave (FP-LAPW) method within a framework of the generalized gradient approximation (GGA) approach is used to perform the calculations presented here. The electronic structure calculation suggests an indirect band gap semiconducting nature of the material with energy band gap of 0.08 eV. The analysis of the elastic constants at relaxed positions reveals the ductile nature of the sample material with covalent contribution in the inter-atomic bonding. The narrow band gap semiconductingmore » nature with high value of Seebeck coefficient suggests the possibility of the thermoelectric application of the material. The analysis of the thermal transport properties confirms the result obtained from the energy band structure of the material with high thermopower and dimensionless figure of merit 0.19 at room temperature.« less

Effect of ripple taper on band-gap overlap in a coaxial Bragg structure operating at terahertz frequency

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ding Xueyong; Li Hongfan; Lv Zhensu

Based on the mode-coupling method, numerical analysis is presented to demonstrate the influence of ripple taper on band-gap overlap in a coaxial Bragg structure operating at terahertz frequency. Results show that the interval between the band-gaps of the competing mode and the desired working mode is narrowed by use of positive-taper ripples, but is expanded if negative-taper ripples are employed, and the influence of the negative-taper ripples is obviously more advantageous than the positive-taper ripples; the band-gap overlap of modes can be efficiently separated by use of negative-taper ripples. The residual side-lobes of the frequency response in a coaxial Braggmore » structure with ripple taper also can be effectively suppressed by employing the windowing-function technique. These peculiarities provide potential advantage in constructing a coaxial Bragg cavity with high quality factor for single higher-order-mode operation of a high-power free-electron maser in the terahertz frequency range.« less

X-ray diffraction, crystal structure, and spectral features of the optical susceptibilities of single crystals of the ternary borate oxide lead bismuth tetraoxide, PbBiBO4.

PubMed

Reshak, Ali Hussain; Kityk, I V; Auluck, S; Chen, Xuean

2009-05-14

The all-electron full-potential linearized augmented plane-wave method has been used for an ab initio theoretical study of the band structure, the spectral features of the optical susceptibilities, the density of states, and the electron charge density for PbBiBO4. Our calculations show that the valence-band maximum (VBM) and conduction-band minimum (CBM) are located at the center of the Brillouin zone, resulting in a direct energy gap of about 3.2 eV. We have synthesized the PbBiBO4 crystal by employing a conventional solid-state reaction method. The theoretical calculations in this work are based on the structure built from our measured atomic parameters. We should emphasize that the observed experimental X-ray diffraction (XRD) pattern is in good agreement with the theoretical one, confirming that our structural model is valid. Our calculated bond lengths show excellent agreement with the experimental data. This agreement is attributed to our use of full-potential calculations. The spectral features of the optical susceptibilities show a small positive uniaxial anisotropy.

Effect of Zn doping on structural, optical and thermal properties of CeO2 nanoparticles

NASA Astrophysics Data System (ADS)

Ramasamy, V.; Vijayalakshmi, G.

2015-09-01

The undoped and Zn doped CeO2 nanoparticles were synthesized by chemical precipitation method at room temperature. The undoped and Zn doped CeO2 nanoparticles have been characterized by X-ray powder diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), ultraviolet visible and photoluminescence (PL) spectroscopy, Fourier transform infrared spectroscopy (FTIR) and thermogravimetry and differential thermal analysis (TG-DTA). The cubic fluorite structures of the CeO2 nanoparticles were determined by XRD. The influence of particle size on structural parameters such as lattice parameter (a), inter planar distance (d), dislocation density (δ), microstrain (ε), lattice strain (η) and texture co-efficient (TC) were also determined. The lattice strains were determined by Williamson-Hall plot method. The effect of Zn doping with shifting of the bands were observed by UV-Vis spectroscopy and also their optical band gap were determined. The emission spectra and energy band diagram of the undoped and Zn doped samples were derived from PL spectroscopy. The structural bond vibrations of undoped and Zn doped CeO2 nanoparticles were analyzed by FTIR spectroscopy. The thermal property (weight loss and decomposition) of the sample is observed by TG-DTA curve.

Effect of Long-Period Ordering of the Structure of a Plant on the Initial Stages of Photosynthesis

NASA Astrophysics Data System (ADS)

Korshunov, M. A.; Shabanov, A. V.; Bukhanov, E. R.; Shabanov, V. F.

2018-01-01

Using data on the structure of plant leaves, specific features of light propagation in biophotoniccrystal structures have been established by the transfer matrix method. Splitting of the stopband in two bands has been found. The density of photonic states and the electromagnetic field value have been calculated. The occurrence of two photosystems (splitting of the stopband in two bands), the peculiarity of the long-wavelength quantum yield and its enhancement (Emerson effect), and water dissociation in the soft mode due to an increase in the electromagnetic field on the layers are explained.

RefMoB, a Reflectivity Feature Model-Based Automated Method for Measuring Four Outer Retinal Hyperreflective Bands in Optical Coherence Tomography

PubMed Central

Ross, Douglas H.; Clark, Mark E.; Godara, Pooja; Huisingh, Carrie; McGwin, Gerald; Owsley, Cynthia; Litts, Katie M.; Spaide, Richard F.; Sloan, Kenneth R.; Curcio, Christine A.

2015-01-01

Purpose. To validate a model-driven method (RefMoB) of automatically describing the four outer retinal hyperreflective bands revealed by spectral-domain optical coherence tomography (SDOCT), for comparison with histology of normal macula; to report thickness and position of bands, particularly band 2 (ellipsoid zone [EZ], commonly called IS/OS). Methods. Foveal and superior perifoveal scans of seven SDOCT volumes of five individuals aged 28 to 69 years with healthy maculas were used (seven eyes for validation, five eyes for measurement). RefMoB determines band thickness and position by a multistage procedure that models reflectivities as a summation of Gaussians. Band thickness and positions were compared with those obtained by manual evaluators for the same scans, and compared with an independent published histological dataset. Results. Agreement among manual evaluators was moderate. Relative to manual evaluation, RefMoB reported reduced thickness and vertical shifts in band positions in a band-specific manner for both simulated and empirical data. In foveal and perifoveal scans, band 1 was thick relative to the anatomical external limiting membrane, band 2 aligned with the outer one-third of the anatomical IS ellipsoid, and band 3 (IZ, interdigitation of retinal pigment epithelium and photoreceptors) was cleanly delineated. Conclusions. RefMoB is suitable for automatic description of the location and thickness of the four outer retinal hyperreflective bands. Initial results suggest that band 2 aligns with the outer ellipsoid, thus supporting its recent designation as EZ. Automated and objective delineation of band 3 will help investigations of structural biomarkers of dark-adaptation changes in aging. PMID:26132776

Proposed square spiral microfabrication architecture for large three-dimensional photonic band gap crystals.

PubMed

Toader, O; John, S

2001-05-11

We present a blueprint for a three-dimensional photonic band gap (PBG) material that is amenable to large-scale microfabrication on the optical scale using glancing angle deposition methods. The proposed chiral crystal consists of square spiral posts on a tetragonal lattice. In the case of silicon posts in air (direct structure), the full PBG can be as large as 15% of the gap center frequency, whereas for air posts in a silicon background (inverted structure) the maximum PBG is 24% of the center frequency. This PBG occurs between the fourth and fifth bands of the photon dispersion relation and is very robust to variations (disorder) in the geometrical parameters of the crystal.

Band structure and thermoelectric properties of half-Heusler semiconductors from many-body perturbation theory

NASA Astrophysics Data System (ADS)

Zahedifar, Maedeh; Kratzer, Peter

2018-01-01

Various ab initio approaches to the band structure of A NiSn and A CoSb half-Heusler compounds (A = Ti, Zr, Hf) are compared and their consequences for the prediction of thermoelectric properties are explored. Density functional theory with the generalized-gradient approximation (GGA), as well as the hybrid density functional HSE06 and ab initio many-body perturbation theory in the form of the G W0 approach, are employed. The G W0 calculations confirm the trend of a smaller band gap (0.75 to 1.05 eV) in A NiSn compared to the A CoSb compounds (1.13 to 1.44 eV) already expected from the GGA calculations. While in A NiSn materials the G W0 band gap is 20% to 50% larger than in HSE06, the fundamental gap of A CoSb materials is smaller in G W0 compared to HSE06. This is because G W0 , similar to PBE, locates the valence band maximum at the L point of the Brillouin zone, whereas it is at the Γ point in the HSE06 calculations. The differences are attributed to the observation that the relative positions of the d levels of the transition metal atoms vary among the different methods. Using the calculated band structures and scattering rates taking into account the band effective masses at the extrema, the Seebeck coefficients, thermoelectric power factors, and figures of merit Z T are predicted for all six half-Heusler compounds. Comparable performance is predicted for the n -type A NiSn materials, whereas clear differences are found for the p -type A CoSb materials. Using the most reliable G W0 electronic structure, ZrCoSb is predicted to be the most efficient material with a power factor of up to 0.07 W/(K2 m) at a temperature of 600 K. We find strong variations among the different ab initio methods not only in the prediction of the maximum power factor and Z T value of a given material, but also in comparing different materials to each other, in particular in the p -type thermoelectric materials. Thus we conclude that the most elaborate, but also most costly G W0 method is required to perform a reliable computational search for the optimum material.

A study of the phosphate mineral kapundaite NaCa(Fe3+)4(PO4)4(OH)3ṡ5(H2O) using SEM/EDX and vibrational spectroscopic methods

NASA Astrophysics Data System (ADS)

Frost, Ray L.; López, Andrés; Xi, Yunfei; Scholz, Ricardo

2014-03-01

Vibrational spectroscopy enables subtle details of the molecular structure of kapundaite to be determined. Single crystals of a pure phase from a Brazilian pegmatite were used. Kapundaite is the Fe3+ member of the wardite group. The infrared and Raman spectroscopy were applied to compare the structure of kapundaite with wardite. The Raman spectrum of kapundaite in the 800-1400 cm-1 spectral range shows two intense bands at 1089 and 1114 cm-1 assigned to the ν1PO43- symmetric stretching vibrations. The observation of two bands provides evidence for the non-equivalence of the phosphate units in the kapundaite structure. The infrared spectrum of kapundaite in the 500-1300 cm-1 shows much greater complexity than the Raman spectrum. Strong infrared bands are found at 966, 1003 and 1036 cm-1 and are attributed to the ν1PO43- symmetric stretching mode and ν3PO43- antisymmetric stretching mode. Raman bands in the ν4 out of plane bending modes of the PO43- unit support the concept of non-equivalent phosphate units in the kapundaite structure. In the 2600-3800 cm-1 spectral range, Raman bands for kapundaite are found at 2905, 3151, 3311, 3449 and 3530 cm-1. These bands are broad and are assigned to OH stretching vibrations. Broad infrared bands are also found at 2904, 3105, 3307, 3453 and 3523 cm-1 and are attributed to water. Raman spectroscopy complimented with infrared spectroscopy has enabled aspects of the structure of kapundaite to be ascertained and compared with that of other phosphate minerals.

Origin of shear thickening in semidilute wormlike micellar solutions and evidence of elastic turbulence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Marín-Santibáñez, Benjamín M.; Pérez-González, José, E-mail: jpg@esfm.ipn.mx; Rodríguez-González, Francisco

2014-11-01

The origin of shear thickening in an equimolar semidilute wormlike micellar solution of cetylpyridinium chloride and sodium salicylate was investigated in this work by using Couette rheometry, flow visualization, and capillary Rheo-particle image velocimetry. The use of the combined methods allowed the discovery of gradient shear banding flow occurring from a critical shear stress and consisting of two main bands, one isotropic (transparent) of high viscosity and one structured (turbid) of low viscosity. Mechanical rheometry indicated macroscopic shear thinning behavior in the shear banding regime. However, local velocimetry showed that the turbid band increased its viscosity along with the shearmore » stress, even though barely reached the value of the viscosity of the isotropic phase. This shear band is the precursor of shear induced structures that subsequently give rise to the average increase in viscosity or apparent shear thickening of the solution. Further increase in the shear stress promoted the growing of the turbid band across the flow region and led to destabilization of the shear banding flow independently of the type of rheometer used, as well as to vorticity banding in Couette flow. At last, vorticity banding disappeared and the flow developed elastic turbulence with chaotic dynamics.« less

Quasiparticle semiconductor band structures including spin-orbit interactions.

PubMed

Malone, Brad D; Cohen, Marvin L

2013-03-13

We present first-principles calculations of the quasiparticle band structure of the group IV materials Si and Ge and the group III-V compound semiconductors AlP, AlAs, AlSb, InP, InAs, InSb, GaP, GaAs and GaSb. Calculations are performed using the plane wave pseudopotential method and the 'one-shot' GW method, i.e. G(0)W(0). Quasiparticle band structures, augmented with the effects of spin-orbit, are obtained via a Wannier interpolation of the obtained quasiparticle energies and calculated spin-orbit matrix. Our calculations explicitly treat the shallow semicore states of In and Ga, which are known to be important in the description of the electronic properties, as valence states in the quasiparticle calculation. Our calculated quasiparticle energies, combining both the ab initio evaluation of the electron self-energy and the vector part of the pseudopotential representing the spin-orbit effects, are in generally very good agreement with experimental values. These calculations illustrate the predictive power of the methodology as applied to group IV and III-V semiconductors.

Study of cobalt effect on structural and optical properties of Dy doped ZnO nanoparticles

NASA Astrophysics Data System (ADS)

Kumar, Pawan; Pandey, Praveen C.

2018-05-01

The present study has been carried out to investigate the effect of Co doping on structural and optical properties of Dy doped ZnO nanoparticles. We have prepared pure Zinc oxide, Dy (1%) doped ZnO and Dy (1%) doped ZnO co-doped with Co(2%) with the help of simple sol-gel combustion method. The structural analysis carried out using X-ray diffraction spectra (XRD) indicates substitution of Dy and Co at Zn site of ZnO crystal structure and hexagonal crystal structure without any secondary phase formation in all the samples. The surface morphology was analyzed by transmission electron microscopy (TEM). Absorption study indicates that Dy doping causes a small shift in band edge, while Co co-doping results significant change is absorption edge as well as introduce defect level absorption in the visible region. The band gap of samples decreases due to Dy and Co doping, which can be attributed to defect level formation below the conduction band in the system.

Analysis of the structural, electronic and optic properties of Ni doped MgSiP{sub 2} semiconductor chalcopyrite compound

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kocak, Belgin, E-mail: koakbelgin@gmail.com; Ciftci, Yasemin Oztekin, E-mail: yasemin@gazi.edu.tr

2016-03-25

The structural, electronic band structure and optic properties of the Ni doped MgSiP{sub 2} chalcopyrite compound have been performed by using first-principles method in the density functional theory (DFT) as implemented in Vienna Ab-initio Simulation Package (VASP). The generalized gradient approximation (GGA) in the scheme of Perdew, Burke and Ernzerhof (PBE) is used for the exchange and correlation functional. The present lattice constant (a) follows generally the Vegard’s law. The electronic band structure, total and partial density of states (DOS and PDOS) are calculated. We present data for the frequency dependence of imaginary and real parts of dielectric functions ofmore » Ni doped MgSiP{sub 2}. For further investigation of the optical properties the reflectivity, refractive index, extinction coefficient and electron energy loss function are also predicted. Our obtained results indicate that the lattice constants, electronic band structure and optical properties of this compound are dependent on the substitution concentration of Ni.« less

Defect induced structural inhomogeneity, ultraviolet light emission and near-band-edge photoluminescence broadening in degenerate In2O3 nanowires

NASA Astrophysics Data System (ADS)

Mukherjee, Souvik; Sarkar, Ketaki; Wiederrecht, Gary P.; Schaller, Richard D.; Gosztola, David J.; Stroscio, Michael A.; Dutta, Mitra

2018-04-01

We demonstrate here defect induced changes on the morphology and surface properties of indium oxide (In2O3) nanowires and further study their effects on the near-band-edge (NBE) emission, thereby showing the significant influence of surface states on In2O3 nanostructure based device characteristics for potential optoelectronic applications. In2O3 nanowires with cubic crystal structure (c-In2O3) were synthesized via carbothermal reduction technique using a gold-catalyst-assisted vapor-liquid-solid method. Onset of strong optical absorption could be observed at energies greater than 3.5 eV consistent with highly n-type characteristics due to unintentional doping from oxygen vacancy ({V}{{O}}) defects as confirmed using Raman spectroscopy. A combination of high resolution transmission electron microscopy, x-ray photoelectron spectroscopy and valence band analysis on the nanowire morphology and stoichiometry reveals presence of high-density of {V}{{O}} defects on the surface of the nanowires. As a result, chemisorbed oxygen species can be observed leading to upward band bending at the surface which corresponds to a smaller valence band offset of 2.15 eV. Temperature dependent photoluminescence (PL) spectroscopy was used to study the nature of the defect states and the influence of the surface states on the electronic band structure and NBE emission has been discussed. Our data reveals significant broadening of the NBE PL peak consistent with impurity band broadening leading to band-tailing effect from heavy doping.

Mid-frequency Band Dynamics of Large Space Structures

NASA Technical Reports Server (NTRS)

Coppolino, Robert N.; Adams, Douglas S.

2004-01-01

High and low intensity dynamic environments experienced by a spacecraft during launch and on-orbit operations, respectively, induce structural loads and motions, which are difficult to reliably predict. Structural dynamics in low- and mid-frequency bands are sensitive to component interface uncertainty and non-linearity as evidenced in laboratory testing and flight operations. Analytical tools for prediction of linear system response are not necessarily adequate for reliable prediction of mid-frequency band dynamics and analysis of measured laboratory and flight data. A new MATLAB toolbox, designed to address the key challenges of mid-frequency band dynamics, is introduced in this paper. Finite-element models of major subassemblies are defined following rational frequency-wavelength guidelines. For computational efficiency, these subassemblies are described as linear, component mode models. The complete structural system model is composed of component mode subassemblies and linear or non-linear joint descriptions. Computation and display of structural dynamic responses are accomplished employing well-established, stable numerical methods, modern signal processing procedures and descriptive graphical tools. Parametric sensitivity and Monte-Carlo based system identification tools are used to reconcile models with experimental data and investigate the effects of uncertainties. Models and dynamic responses are exported for employment in applications, such as detailed structural integrity and mechanical-optical-control performance analyses.

Energy bands and acceptor binding energies of GaN

NASA Astrophysics Data System (ADS)

Xia, Jian-Bai; Cheah, K. W.; Wang, Xiao-Liang; Sun, Dian-Zhao; Kong, Mei-Ying

1999-04-01

The energy bands of zinc-blende and wurtzite GaN are calculated with the empirical pseudopotential method, and the pseudopotential parameters for Ga and N atoms are given. The calculated energy bands are in agreement with those obtained by the ab initio method. The effective-mass theory for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN for both structures are given. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor are 24 and 142 meV for the zinc-blende structure, 20 and 131, and 97 meV for the wurtzite structure, respectively, which are consistent with recent experimental results. It is proposed that there are two kinds of acceptor in wurtzite GaN. One kind is the general acceptor such as C, which substitutes N, which satisfies the effective-mass theory. The other kind of acceptor includes Mg, Zn, Cd, etc., the binding energy of these acceptors is deviated from that given by the effective-mass theory. In this report, wurtzite GaN is grown by the molecular-beam epitaxy method, and the photoluminescence spectra were measured. Three main peaks are assigned to the donor-acceptor transitions from two kinds of acceptors. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material.

Band-gap corrected density functional theory calculations for InAs/GaSb type II superlattices

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Jianwei; Zhang, Yong

2014-12-07

We performed pseudopotential based density functional theory (DFT) calculations for GaSb/InAs type II superlattices (T2SLs), with bandgap errors from the local density approximation mitigated by applying an empirical method to correct the bulk bandgaps. Specifically, this work (1) compared the calculated bandgaps with experimental data and non-self-consistent atomistic methods; (2) calculated the T2SL band structures with varying structural parameters; (3) investigated the interfacial effects associated with the no-common-atom heterostructure; and (4) studied the strain effect due to lattice mismatch between the two components. This work demonstrates the feasibility of applying the DFT method to more exotic heterostructures and defect problemsmore » related to this material system.« less

Measurement of locally resonant band gaps in a surface phononic crystal with inverted conical pillars

NASA Astrophysics Data System (ADS)

Hsu, Jin-Chen; Lin, Fan-Shun

2018-07-01

In this paper, we numerically and experimentally study locally resonant (LR) band gaps for surface acoustic waves (SAWs) in a honeycomb array of inverted conical pillars grown on the surface of a 128°YX lithium-niobate substrate. We show that the inverted conical pillars can be used to generate lower LR band gaps below the sound cone. This lowering effect is caused by the increase in the effective pillar mass without increasing the effective stiffness. We employ the finite-element method to calculate the LR band gaps and wideband slanted-finger interdigital transducers to measure the transmission of SAWs. Numerical results show that SAWs are prohibited from propagating through the structure in the lowered LR band gaps. Obvious LR band-gap lowering is observed in the experimental result of a surface phononic crystal with a honeycomb array of inverted conical pillars. The results enable enhanced control over the phononic metamaterial and surface structures, which may have applications in low-frequency waveguiding, acoustic isolation, acoustic absorbers, and acoustic filters.

Zn(x)Cd(1-x)Se nanomultipods with tunable band gaps: synthesis and first-principles calculations.

PubMed

Wei, Hao; Su, Yanjie; Han, Ziyi; Li, Tongtong; Ren, Xinglong; Yang, Zhi; Wei, Liangming; Cong, Fengsong; Zhang, Yafei

2013-06-14

In this paper, we demonstrate that ZnxCd1-xSe nanomultipods can be synthesized via a facile and nontoxic solution-based method. Interesting aspects of composition, morphology and optical properties were deeply explored. The value of Zn/(Zn+Cd) could be altered across the entire range from 0.08 to 0.86 by varying the ratio of cation precursor contents. The band gap energy could be linearly tuned from 1.88 to 2.48 eV with respect to the value of Zn/(Zn+Cd). The experiment also showed that oleylamine played a dominant role in the formation of multipod structure. A possible growth mechanism was further suggested. First-principles calculations of band gap energy and density of states in the Vienna ab initio simulation package code were performed to verify the experimental variation tendency of the band gap. Computational results indicated that dissimilarities of electronic band structures and orbital constitutions determined the tunable band gap of the as-synthesized nanomultipod, which might be promising for versatile applications in relevant areas of solar cells, biomedicine, sensors, catalysts and so on.

A novel method to detect shadows on multispectral images

NASA Astrophysics Data System (ADS)

Daǧlayan Sevim, Hazan; Yardımcı ćetin, Yasemin; Özışık Başkurt, Didem

2016-10-01

Shadowing occurs when the direct light coming from a light source is obstructed by high human made structures, mountains or clouds. Since shadow regions are illuminated only by scattered light, true spectral properties of the objects are not observed in such regions. Therefore, many object classification and change detection problems utilize shadow detection as a preprocessing step. Besides, shadows are useful for obtaining 3D information of the objects such as estimating the height of buildings. With pervasiveness of remote sensing images, shadow detection is ever more important. This study aims to develop a shadow detection method on multispectral images based on the transformation of C1C2C3 space and contribution of NIR bands. The proposed method is tested on Worldview-2 images covering Ankara, Turkey at different times. The new index is used on these 8-band multispectral images with two NIR bands. The method is compared with methods in the literature.

New insight into the shape coexistence and shape evolution of {sup 157}Yb

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xu, C.; Hua, H.; Li, X. Q.

2011-01-15

High-spin states in {sup 157}Yb have been populated in the {sup 144}Sm({sup 16}O,3n){sup 157}Yb fusion-evaporation reaction at a beam energy of 85 MeV. Two rotational bands built on the {nu}f{sub 7/2} and {nu}h{sub 9/2} intrinsic states, respectively, have been established for the first time. The newly observed {nu}f{sub 7/2} band and previously known {nu}i{sub 13/2} band in {sup 157}Yb are discussed in terms of total Routhian surface methods and compared with the structures in the neighboring N = 87 isotones. The structural characters observed in {sup 157}Yb provide evidence for shape coexistence of three distinct shapes: prolate, triaxial, and oblate.more » At higher spins, both the {nu}f{sub 7/2} band and {nu}i{sub 13/2} band in {sup 157}Yb undergo a shape evolution with sizable alignments occurring.« less

Effect of doping on electronic properties of HgSe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nag, Abhinav, E-mail: abhinavn76@gmail.com; Sastri, O. S. K. S., E-mail: sastri.osks@gmail.com; Kumar, Jagdish, E-mail: jagdishphysicist@gmail.com

2016-05-23

First principle study of electronic properties of pure and doped HgSe have been performed using all electron Full Potential Linearized Augmented Plane Wave (FP-LAPW) method using ELK code. The electronic exchange and co-relations are considered using Generalized Gradient Approach (GGA). Lattice parameter, Density of States (DOS) and Band structure calculations have been performed. The total energy curve (Energy vs Lattice parameter), DOS and band structure calculations are in good agreement with the experimental values and those obtained using other DFT codes. The doped material is studied within the Virtual Crystal Approximation (VCA) with doping levels of 10% to 25% ofmore » electrons (hole) per unit cell. Results predict zero band gap in undopedHgSe and bands meet at Fermi level near the symmetry point Γ. For doped HgSe, we found that by electron (hole) doping, the point where conduction and valence bands meet can be shifted below (above) the fermi level.« less

Band gap engineering of N-alloyed Ga{sub 2}O{sub 3} thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Song, Dongyu; Li, Bingsheng, E-mail: libingsheng@hit.edu.cn, E-mail: ashen@ccny.cuny.edu; Sui, Yu

2016-06-15

The authors report the tuning of band gap of GaON ternary alloy in a wide range of 2.75 eV. The samples were prepared by a two-step nitridation method. First, the samples were deposited on 2-inch fused silica substrates by megnetron sputtering with NH{sub 3} and Ar gas for 60 minutes. Then they were annealed in NH{sub 3} ambience at different temperatures. The optical band gap energies are calculated from transmittance measurements. With the increase of nitridation temperature, the band gap gradually decreases from 4.8 eV to 2.05 eV. X-ray diffraction results indicate that as-deposited amorphous samples can crystallize into monoclinicmore » and hexagonal structures after they were annealed in oxygen or ammonia ambience, respectively. The narrowing of the band gap is attributed to the enhanced repulsion of N2p -Ga3d orbits and formation of hexagonal structure.« less

AB INITIO STUDY OF OPTOELECTRONIC PROPERTIES OF SPINEL ZnAl2O4 BEYOND GGA AND LDA

NASA Astrophysics Data System (ADS)

Yousaf, Masood; Saeed, M. A.; Isa, Ahmad Radzi Mat; Rahnamaye Aliabad, H. A.; Noor, N. A.

2012-12-01

Electronic band structure and optical parameters of ZnAl2O4 are investigated by first-principles technique based on a new potential approximation, known as modified Becke-Johnson (mBJ). This method describes the excited states of insulators and semiconductors more accurately The recent direct band gap result by EV-GGA is underestimated by about 15% compared to our band gap value using mBJ-GGA. The value of the band gap of ZnAl2O4 decreases as follows: Eg(mBJ-GGA/LDA) > Eg(GGA) > Eg(LDA). The band structure base optical parametric quantities (dielectric constant, index of refraction, reflectivity and optical conductivity) are also calculated, and their variations with energy range are discussed. The first critical point (optical absorption's edge) in ZnAl2O4 occurs at about 5.26 eV in case of mBJ. This study about the optoelectronic properties indicates that ZnAl2O4 can be used in optical devices.

Investigation the effect of lattice angle on the band gap width in 3D phononic crystals with rhombohedral(I) lattice

NASA Astrophysics Data System (ADS)

Salehi, H.; Aryadoust, M.; Shoushtari, M. Zargar

2014-07-01

In this paper, the propagation of acoustic waves in the phononic crystal of 3D with rhombohedral(I) lattice is studied theoretically. The crystal composite constituted of nickel spheres embedded in epoxy. The calculations of the band structure and density of states are performed with the plane wave expansion method in the irreducible part of Brillouin zone. In the present work, we have investigated the effect of lattice angle on the band structure and width of the band gap rhombohedral(I) lattice in the irreducible part of the first Brillouin zone and its planes separately. The results show that more than one complete band gape are formed in the four planes of the irreducible part. The most complete band gaps are formed in the (111) plane and the widest complete band gap in (443) with an angle greater than 80. So, if the sound passes through the (111) and (443) planes for the lattice angle close to 90, the crystal phononic displays the excellent insulation behavior. Moreover, in the other planes, the lattice angle does not affect on the width and the number of band gaps. Also, for the filling fraction 5 %, the widest complete band gap is formed. These results are consistent with the effect of symmetry on the band gap width, because the (111) plane has the most symmetry.

Bunch-Kaufman factorization for real symmetric indefinite banded matrices

NASA Technical Reports Server (NTRS)

Jones, Mark T.; Patrick, Merrell L.

1989-01-01

The Bunch-Kaufman algorithm for factoring symmetric indefinite matrices was rejected for banded matrices because it destroys the banded structure of the matrix. Herein, it is shown that for a subclass of real symmetric matrices which arise in solving the generalized eigenvalue problem using Lanczos's method, the Bunch-Kaufman algorithm does not result in major destruction of the bandwidth. Space time complexities of the algorithm are given and used to show that the Bunch-Kaufman algorithm is a significant improvement over LU factorization.

Importance of conduction electron correlation in a Kondo lattice, Ce₂CoSi₃.

PubMed

Patil, Swapnil; Pandey, Sudhir K; Medicherla, V R R; Singh, R S; Bindu, R; Sampathkumaran, E V; Maiti, Kalobaran

2010-06-30

Kondo systems are usually described by the interaction of the correlation induced local moments with the highly itinerant conduction electrons. Here, we study the role of electron correlations among conduction electrons in the electronic structure of a Kondo lattice compound, Ce₂CoSi₃, using high resolution photoemission spectroscopy and ab initio band structure calculations, where Co 3d electrons contribute in the conduction band. High energy resolution employed in the measurements helped to reveal the signatures of Ce 4f states derived Kondo resonance features at the Fermi level and the dominance of Co 3d contributions at higher binding energies in the conduction band. The lineshape of the experimental Co 3d band is found to be significantly different from that obtained from the band structure calculations within the local density approximations, LDA. Consideration of electron-electron Coulomb repulsion, U, among Co 3d electrons within the LDA + U method leads to a better representation of experimental results. The signature of an electron correlation induced satellite feature is also observed in the Co 2p core level spectrum. These results clearly demonstrate the importance of the electron correlation among conduction electrons in deriving the microscopic description of such Kondo systems.

3-D phononic crystals with ultra-wide band gaps

PubMed Central

Lu, Yan; Yang, Yang; Guest, James K.; Srivastava, Ankit

2017-01-01

In this paper gradient based topology optimization (TO) is used to discover 3-D phononic structures that exhibit ultra-wide normalized all-angle all-mode band gaps. The challenging computational task of repeated 3-D phononic band-structure evaluations is accomplished by a combination of a fast mixed variational eigenvalue solver and distributed Graphic Processing Unit (GPU) parallel computations. The TO algorithm utilizes the material distribution-based approach and a gradient-based optimizer. The design sensitivity for the mixed variational eigenvalue problem is derived using the adjoint method and is implemented through highly efficient vectorization techniques. We present optimized results for two-material simple cubic (SC), body centered cubic (BCC), and face centered cubic (FCC) crystal structures and show that in each of these cases different initial designs converge to single inclusion network topologies within their corresponding primitive cells. The optimized results show that large phononic stop bands for bulk wave propagation can be achieved at lower than close packed spherical configurations leading to lighter unit cells. For tungsten carbide - epoxy crystals we identify all angle all mode normalized stop bands exceeding 100%, which is larger than what is possible with only spherical inclusions. PMID:28233812

3-D phononic crystals with ultra-wide band gaps.

PubMed

Lu, Yan; Yang, Yang; Guest, James K; Srivastava, Ankit

2017-02-24

In this paper gradient based topology optimization (TO) is used to discover 3-D phononic structures that exhibit ultra-wide normalized all-angle all-mode band gaps. The challenging computational task of repeated 3-D phononic band-structure evaluations is accomplished by a combination of a fast mixed variational eigenvalue solver and distributed Graphic Processing Unit (GPU) parallel computations. The TO algorithm utilizes the material distribution-based approach and a gradient-based optimizer. The design sensitivity for the mixed variational eigenvalue problem is derived using the adjoint method and is implemented through highly efficient vectorization techniques. We present optimized results for two-material simple cubic (SC), body centered cubic (BCC), and face centered cubic (FCC) crystal structures and show that in each of these cases different initial designs converge to single inclusion network topologies within their corresponding primitive cells. The optimized results show that large phononic stop bands for bulk wave propagation can be achieved at lower than close packed spherical configurations leading to lighter unit cells. For tungsten carbide - epoxy crystals we identify all angle all mode normalized stop bands exceeding 100%, which is larger than what is possible with only spherical inclusions.

Estimation of photonic band gap in the hollow core cylindrical multilayer structure

NASA Astrophysics Data System (ADS)

Chourasia, Ritesh Kumar; Singh, Vivek

2018-04-01

The propagation characteristic of two hollow core cylindrical multilayer structures having high and low refractive index contrast of cladding regions have been studied and compared at two design wavelengths i.e. 1550 nm and 632.8 nm. With the help of transfer matrix method a relation between the incoming light wave and outgoing light wave has been developed using the boundary matching technique. In high refractive index contrast, small numbers of layers are sufficient to provide perfect band gap in both design wavelengths. The spectral position and width of band gap is highly depending on the optical path of incident light in all considered cases. For sensing application, the sensitivity of waveguide can be obtained either by monitoring the width of photonic band gap or by monitoring the spectral shift of photonic band gap. Change in the width of photonic band gap with the core refractive index is larger in high refractive index contrast of cladding materials. However, in the case of monitoring the spectral shift of band gap, the obtained sensitivity is large for low refractive index contrast of cladding materials and further it increases with increase of design wavelength.

The size effect to O2- -Ce4+ charge transfer emission and band gap structure of Sr2 CeO4.

PubMed

Wang, Wenjun; Pan, Yu; Zhang, Wenying; Liu, Xiaoguang; Li, Ling

2018-04-24

Sr 2 CeO 4 phosphors with different crystalline sizes were synthesized by the sol-gel method or the solid-state reaction. Their crystalline size, luminescence intensity of O 2- -Ce 4+ charge transfer and energy gaps were obtained through the characterization by X-ray diffraction, photoluminescence spectra, as well as UV-visible diffuse reflectance measurements. An inverse relationship between photoluminescence (PL) spectra and crystalline size was observed when the heating temperature was from 1000°C to 1300°C. In addition, band energy calculated for all samples showed that a reaction temperature of 1200°C for the solid-state method and 1100°C for sol-gel method gave the largest values, which corresponded with the smallest crystalline size. Correlation between PL intensity and crystalline size showed an inverse relationship. Band structure, density of states and partial density of states of the crystal were calculated to analyze the mechanism using the cambrige sequential total energy package (CASTEP) module integrated with Materials Studio software. Copyright © 2018 John Wiley & Sons, Ltd.

Density Measurements in Air by Optically Exciting the Cordes Bands of I2

NASA Technical Reports Server (NTRS)

Balla, R. Jeffrey; Exton, Reginald J.

2000-01-01

We describe an optical method based on laser-induced fluorescence for obtaining instantaneous measurements of density along a line in low-density air seeded with I2. The Cordes bands of I2 (D(sup 1)sigma(sup +, sub u)) left arrow X(sup 1)sigma(sup +, sub g)) are excited with a tunable ArF excimer laser. air densities in the range (0.1-6.5) x 10(exp 17) cm(exp -3) are measured over 295-583 K using the density-dependent emission ratio of two emission bands of I2; the 340 nm bands and the diffuse-structured McLennan bands near 320 nm.

Spectroscopic and quantum chemical study of the structure of a new paramagnetic dimeric palladium(II,III) complex with creatine

NASA Astrophysics Data System (ADS)

Mitewa, Mariana; Enchev, Venelin; Bakalova, Tatyana

2002-05-01

The structure and coordination mode of the newly synthesized dimeric paramagnetic Pd(II,III) complex are studied using magneto-chemical, EPR and IR spectroscopic methods. In order to perform reliable assignment of the IR bands, the structure and IR spectrum of the free creatine were calculated using ab initio method. For calculation of the configuration of its deprotonated and doubly deprotonated forms the semiempirical AM1 method was used.

The effects of the chemical composition and strain on the electronic properties of GaSb/InAs core-shell nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ning, Feng; Wang, Dan; Tang, Li-Ming, E-mail: lmtang@hnu.edu.cn

2014-09-07

The effects of the chemical composition and strain on the electronic properties of [111] zinc-blende (ZB) and [0001] wurtzite (WZ) GaSb/InAs core-shell nanowires (NWs) with different core diameters and shell thicknesses are studied using first-principles methods. The band structures of the [111] ZB GaSb/InAs core-shell NWs underwent a noticeable type-I/II band alignment transition, associated with a direct-to-indirect band gap transition under a compressive uniaxial strain. The band structures of the [0001] WZ GaSb/InAs core-shell NWs preserved the direct band gap under either compressive or tensile uniaxial strains. In addition, the band gaps and the effective masses of the carriers couldmore » be tuned by their composition. For the core-shell NWs with a fixed GaSb-core size, the band gaps decreased linearly with an increasing InAs-shell thickness, caused by the significant downshift of the conduction bands. For the [111] ZB GaSb/InAs core-shell NWs, the calculated effective masses indicated that the transport properties could be changed from hole-dominated conduction to electron-dominated conduction by changing the InAs-shell thickness.« less

Numerical study on characteristic of two-dimensional metal/dielectric photonic crystals

NASA Astrophysics Data System (ADS)

Zong, Yi-Xin; Xia, Jian-Bai; Wu, Hai-Bin

2017-04-01

An improved plan-wave expansion method is adopted to theoretically study the photonic band diagrams of two-dimensional (2D) metal/dielectric photonic crystals. Based on the photonic band structures, the dependence of flat bands and photonic bandgaps on two parameters (dielectric constant and filling factor) are investigated for two types of 2D metal/dielectric (M/D) photonic crystals, hole and cylinder photonic crystals. The simulation results show that band structures are affected greatly by these two parameters. Flat bands and bandgaps can be easily obtained by tuning these parameters and the bandgap width may reach to the maximum at certain parameters. It is worth noting that the hole-type photonic crystals show more bandgaps than the corresponding cylinder ones, and the frequency ranges of bandgaps also depend strongly on these parameters. Besides, the photonic crystals containing metallic medium can obtain more modulation of photonic bands, band gaps, and large effective refractive index, etc. than the dielectric/dielectric ones. According to the numerical results, the needs of optical devices for flat bands and bandgaps can be met by selecting the suitable geometry and material parameters. Project supported by the National Basic Research Program of China (Grant No. 2011CB922200) and the National Natural Science Foundation of China (Grant No. 605210010).

Extension of photonic band gap in one-dimensional ternary metal-dielectric photonic crystal

NASA Astrophysics Data System (ADS)

Pandey, G. N.; Thapa, Khem B.

2018-05-01

In this paper, the photonic band gap structure in the visible and near infrared for a ternary metal dielectric photonic crystal has been theoretically investigated. At the normal incidence, the high reflectance range can be significantly enlarged at a thicker metal film. The transmission of the structure containing Cu has large compared to the other metals like Al and Ag metals. The transmission properties of the metal are dependent upon the value of the plasma frequency. In this paper we consider the effect of the variation of the thickness of the metal on the reflection bands of ternary metallic-dielectric photonic crystal (MDPC). Finally we find that the enlargement of band gap in MDPC is due to the addition of increase of the thickness of metallic film at normal incidence. All the theoretical calculations are made based on the transfer matrix method together with the Drude model of metal.

Multicolor emission from intermediate band semiconductor ZnO 1-xSe x

DOE Office of Scientific and Technical Information (OSTI.GOV)

Welna, M.; Baranowski, M.; Linhart, W. M.

Photoluminescence and photomodulated reflectivity measurements of ZnOSe alloys are used to demonstrate a splitting of the valence band due to the band anticrossing interaction between localized Se states and the extended valence band states of the host ZnO matrix. A strong multiband emission associated with optical transitions from the conduction band to lower E - and upper E + valence subbands has been observed at room temperature. The composition dependence of the optical transition energies is well explained by the electronic band structure calculated using the kp method combined with the band anticrossing model. The observation of the multiband emissionmore » is possible because of relatively long recombination lifetimes. Longer than 1 ns lifetimes for holes photoexcited to the lower valence subband offer a potential of using the alloy as an intermediate band semiconductor for solar power conversion applications.« less

Multicolor emission from intermediate band semiconductor ZnO 1-xSe x

DOE PAGES

Welna, M.; Baranowski, M.; Linhart, W. M.; ...

2017-03-13

Photoluminescence and photomodulated reflectivity measurements of ZnOSe alloys are used to demonstrate a splitting of the valence band due to the band anticrossing interaction between localized Se states and the extended valence band states of the host ZnO matrix. A strong multiband emission associated with optical transitions from the conduction band to lower E - and upper E + valence subbands has been observed at room temperature. The composition dependence of the optical transition energies is well explained by the electronic band structure calculated using the kp method combined with the band anticrossing model. The observation of the multiband emissionmore » is possible because of relatively long recombination lifetimes. Longer than 1 ns lifetimes for holes photoexcited to the lower valence subband offer a potential of using the alloy as an intermediate band semiconductor for solar power conversion applications.« less

Band gap narrowing in n-type and p-type 3C-, 2H-, 4H-, 6H-SiC, and Si

NASA Astrophysics Data System (ADS)

Persson, C.; Lindefelt, U.; Sernelius, B. E.

1999-10-01

Doping-induced energy shifts of the conduction band minimum and the valence band maximum have been calculated for n-type and p-type 3C-, 2H-, 4H-, 6H-SiC, and Si. The narrowing of the fundamental band gap and of the optical band gap are presented as functions of ionized impurity concentration. The calculations go beyond the common parabolic treatments of the ground state energy dispersion by using energy dispersion and overlap integrals from band structure calculations. The nonparabolic valence band curvatures influence strongly the energy shifts especially in p-type materials. The utilized method is based on a zero-temperature Green's function formalism within the random phase approximation with local field correction according to Hubbard. We have parametrized the shifts of the conduction and the valence bands and made comparisons with recently published results from a semi-empirical model.

A physical parameter method for the design of broad-band X-ray imaging systems to do coronal plasma diagnostics

NASA Technical Reports Server (NTRS)

Kahler, S.; Krieger, A. S.

1978-01-01

The technique commonly used for the analysis of data from broad-band X-ray imaging systems for plasma diagnostics is the filter ratio method. This requires the use of two or more broad-band filters to derive temperatures and line-of-sight emission integrals or emission measure distributions as a function of temperature. Here an alternative analytical approach is proposed in which the temperature response of the imaging system is matched to the physical parameter being investigated. The temperature response of a system designed to measure the total radiated power along the line of sight of any coronal structure is calculated. Other examples are discussed.

Study of surface and bulk electronic structure of II-VI semiconductor nanocrystals using Cu as a nanosensor.

PubMed

Grandhi, G Krishnamurthy; Tomar, Renu; Viswanatha, Ranjani

2012-11-27

Efficiency of the quantum dots based solar cells relies on charge transfer at the interface and hence on the relative alignment of the energy levels between materials. Despite a high demand to obtain size specific band offsets, very few studies exist where meticulous methods like photoelectron spectroscopy are used. However, semiconductor charging during measurements could result in indirect and possibly inaccurate measurements due to shift in valence and conduction band position. Here, in this report, we devise a novel method to study the band offsets by associating an atomic like state with the conduction band and hence obtaining an internal standard. This is achieved by doping copper in semiconductor nanocrystals, leading to the development of a characteristic intragap Cu-related emission feature assigned to the transition from the conduction band to the atomic-like Cu d state. Using this transition we determine the relative band alignment of II-VI semiconductor nanocrystals as a function of size in the below 10 nm size regime. The results are in excellent agreement with the available photoelectron spectroscopy data as well as the theoretical data. We further use this technique to study the excitonic band edge variation as a function of temperature in CdSe nanocrystals. Additionally, surface electronic structure of CdSe nanocrystals have been studied using quantitative measurements of absolute quantum yield and PL decay studies of the Cu related emission and the excitonic emission. The role of TOP and oleic acid as surface passivating ligand molecules has been studied for the first time.

Research Update: Emerging chalcostibite absorbers for thin-film solar cells

DOE PAGES

de Souza Lucas, Francisco Willian; Zakutayev, Andriy

2018-06-04

Copper antimony chalcogenides CuSbCh 2 (Ch=S, Se) are an emerging family of absorbers studied for thin-film solar cells. These non-toxic and Earth-abundant materials show a layered low-dimensional chalcostibite crystal structure, leading to interesting optoelectronic properties for applications in photovoltaic (PV) devices. This research update describes the CuSbCh 2 crystallographic structures, synthesis methods, competing phases, band structures, optoelectronic properties, point defects, carrier dynamics, and interface band offsets, based on experimental and theoretical data. Correlations between these absorber properties and PV device performance are discussed, and opportunities for further increase in the efficiency of the chalcostibite PV devices are highlighted.

Research Update: Emerging chalcostibite absorbers for thin-film solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

de Souza Lucas, Francisco Willian; Zakutayev, Andriy

Copper antimony chalcogenides CuSbCh 2 (Ch=S, Se) are an emerging family of absorbers studied for thin-film solar cells. These non-toxic and Earth-abundant materials show a layered low-dimensional chalcostibite crystal structure, leading to interesting optoelectronic properties for applications in photovoltaic (PV) devices. This research update describes the CuSbCh 2 crystallographic structures, synthesis methods, competing phases, band structures, optoelectronic properties, point defects, carrier dynamics, and interface band offsets, based on experimental and theoretical data. Correlations between these absorber properties and PV device performance are discussed, and opportunities for further increase in the efficiency of the chalcostibite PV devices are highlighted.

Computational research on lithium ion battery materials

NASA Astrophysics Data System (ADS)

Tang, Ping

Crystals of LiFePO4 and related materials have recently received a lot of attention due to their very promising use as cathodes in rechargeable lithium ion batteries. This thesis studied the electronic structures of FePO 4 and LiMPO4, where M=Mn, Fe, Co and Ni within the framework of density-functional theory. The first study compared the electronic structures of the LiMPO 4 and FePO4 materials in their electrochemically active olivine form, using the LAPW (linear augmented plane wave) method [1]. A comparison of results for various spin configurations suggested that the ferromagnetic configuration can serve as a useful approximation for studying general features of these systems. The partial densities of states for the LiMPO4 materials are remarkably similar to each other, showing the transition metal 3d states forming narrow bands above the O 2p band. By contrast, in absence of Li, the majority spin transition metal 3d states are well-hybridized with the O 2p band in FePO4. The second study compared the electronic structures of FePO4 in several crystal structures including an olivine, monoclinic, quartz-like, and CrVO4-like form [2,3]. For this work, in addition to the LAPW method, PAW (Projector Augmented Wave) [4], and PWscf (plane-wave pseudopotential) [5] methods were used. By carefully adjusting the computational parameters, very similar results were achieved for the three independent computational methods. Results for the relative stability of the four crystal structures are reported. In addition, partial densities of state analyses show qualitative information about the crystal field splittings and bond hybridizations and help rationalize the understanding of the electrochemical and stability properties of these materials.

V-doped SnS2: a new intermediate band material for a better use of the solar spectrum.

PubMed

Wahnón, Perla; Conesa, José C; Palacios, Pablo; Lucena, Raquel; Aguilera, Irene; Seminovski, Yohanna; Fresno, Fernando

2011-12-07

Intermediate band materials can boost photovoltaic efficiency through an increase in photocurrent without photovoltage degradation thanks to the use of two sub-bandgap photons to achieve a full electronic transition from the valence band to the conduction band of a semiconductor structure. After having reported in previous works several transition metal-substituted semiconductors as able to achieve the electronic structure needed for this scheme, we propose at present carrying out this substitution in sulfides that have bandgaps of around 2.0 eV and containing octahedrally coordinated cations such as In or Sn. Specifically, the electronic structure of layered SnS(2) with Sn partially substituted by vanadium is examined here with first principles quantum methods and seen to give favourable characteristics in this respect. The synthesis of this material in nanocrystalline powder form is then undertaken and achieved using solvothermal chemical methods. The insertion of vanadium in SnS(2) is found to produce an absorption spectrum in the UV-Vis-NIR range that displays a new sub-bandgap feature in agreement with the quantum calculations. A photocatalytic reaction-based test verifies that this sub-bandgap absorption produces highly mobile electrons and holes in the material that may be used for the solar energy conversion, giving experimental support to the quantum calculations predictions.

Tunneling calculations for GaAs-Al(x)Ga(1-x) as graded band-gap sawtooth superlattices. Thesis

NASA Technical Reports Server (NTRS)

Forrest, Kathrine A.; Meijer, Paul H. E.

1991-01-01

Quantum mechanical tunneling calculations for sawtooth (linearly graded band-gap) and step-barrier AlGaAs superlattices were performed by means of a transfer matrix method, within the effective mass approximation. The transmission coefficient and tunneling current versus applied voltage were computed for several representative structures. Particular consideration was given to effective mass variations. The tunneling properties of step and sawtooth superlattices show some qualitative similarities. Both structures exhibit resonant tunneling, however, because they deform differently under applied fields, the J-V curves differ.

Some optical properties of one dimensional annular photonic crystal with plasma frequency

NASA Astrophysics Data System (ADS)

Pandeya, G. N.; Thapa, Khem B.

2018-05-01

This paper presents the reflection bands, photonic band gaps, of the one-dimensional annul photonic crystal (APC) containing double negative (DNG) metamaterials and air. The proposed annular structure consists of the alternate layers of dispersive DNG material and air immersed in free space. The reflectance properties of the APC by employing the transfer matrix method (TMM) in the cylindrical waves for TE polarization is studied theoretically. In addition of this, we have also studied the effect of plasma frequency on the reflection behavior of the considered annular structure.

Electronic structure and optical properties of Cs2HgCl4: DFT calculations and X-ray photoelectron spectroscopy measurements

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Parasyuk, O. V.; Fedorchuk, A. O.; Khyzhun, O. Y.

2016-10-01

A high-quality single crystal of cesium mercury tetrabromide, Cs2HgCl4, was synthesized by using the vertical Bridgman-Stockbarger method and its electronic structure was studied from both experimental and theoretical viewpoints. In particular, X-ray photoelectron spectra were measured for both pristine and Ar+ ion-bombarded Cs2HgCl4 single crystal surfaces. The present XPS measurements indicate that the Cs2HgCl4 single crystal surface is sensitive with respect to Ar+ ion-bombardment: such a treatment changes substantially its elemental stoichiometry. With the aim of exploring total and partial densities of states within the valence band and conduction band regions of the Cs2HgCl4 compound, band-structure calculations based on density functional theory (DFT) using the augmented plane wave + local orbitals (APW + lo) method as incorporated within the WIEN2k package are performed. The calculations indicate that the Cl 3p states are the principal contributors in the upper portion of the valence band, while the Hg 5d and Cs 5p states dominate in its lower portion. In addition, the calculations allow for concluding that the unoccupied Cl p and Hg s states are the main contributors to the bottom of the conduction band. Furthermore, main optical characteristics of Cs2HgCl4, namely dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity, are elucidated based on the DFT calculations.

Electronic structures of filled tetrahedral semiconductors LiMgN and LiZnN: conduction band distortion

NASA Astrophysics Data System (ADS)

Yu, L. H.; Yao, K. L.; Liu, Z. L.

2004-12-01

The band structures of the filled tetrahedral semiconductors LiMgN and LiZnN, viewed as the zinc-blende (MgN) - and (ZnN) - lattices partially filled with He-like Li + ion interstitials, were studied using the full-potential linearized augmented plane wave method (FP-LAPW) within density functional theory. The conduction band distortions of LiMgN and LiZnN, compared to their “parent” zinc-blende analog AlN and GaN, are discussed. It was found that the insertion of Li + ions at the interstitial sites near the cation or anion pushes the conduction band minimum of the X point in the Brillouin zone upward, relative to that of the Γ point, for both (MgN) - and (ZnN) - lattices (the valence band maximum is at Γ for AlN, GaN, LiMgN, and LiZnN), which provides a method to convert a zinc-blende indirect gap semiconductor into a direct gap material, but the conduction band distortion of the β phase (Li + near the cation) is quite stronger than that of the α phase (Li + near the anion). The total energy calculations show the α phase to be more stable than the β phase for both LiMgN and LiZnN. The Li-N and Mg-N bonds exhibit a strong ionic character, whereas the Zn-N bond has a strong covalent character in LiMgN and LiZnN.

Defect induced structural inhomogeneity, ultraviolet light emission and near-band-edge photoluminescence broadening in degenerate In 2 O 3 nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mukherjee, Souvik; Sarkar, Ketaki; Wiederrecht, Gary P.

We demonstrate here defect induced changes on the morphology and surface properties of indium oxide (In2O3) nanowires and further study their effects on the near-band-edge (NBE) emission, thereby showing the significant influence of surface states on In2O3 nanostructure based device characteristics for potential optoelectronic applications. In2O3 nanowires with cubic crystal structure (c-In2O3) were synthesized via carbothermal reduction technique using a gold-catalyst-assisted vapor–liquid–solid method. Onset of strong optical absorption could be observed at energies greater than 3.5 eV consistent with highly n-type characteristics due to unintentional doping from oxygen vacancy (VO) defects as confirmed using Raman spectroscopy. A combination of highmore » resolution transmission electron microscopy, x-ray photoelectron spectroscopy and valence band analysis on the nanowire morphology and stoichiometry reveals presence of high-density of VO defects on the surface of the nanowires. As a result, chemisorbed oxygen species can be observed leading to upward band bending at the surface which corresponds to a smaller valence band offset of 2.15 eV. Temperature dependent photoluminescence (PL) spectroscopy was used to study the nature of the defect states and the influence of the surface states on the electronic band structure and NBE emission has been discussed. Our data reveals significant broadening of the NBE PL peak consistent with impurity band broadening leading to band-tailing effect from heavy doping.« less

Defect induced structural inhomogeneity, ultraviolet light emission and near-band-edge photoluminescence broadening in degenerate In2O3 nanowires.

PubMed

Mukherjee, Souvik; Sarkar, Ketaki; Wiederrecht, Gary P; Schaller, Richard D; Gosztola, David J; Stroscio, Michael A; Dutta, Mitra

2018-04-27

We demonstrate here defect induced changes on the morphology and surface properties of indium oxide (In 2 O 3 ) nanowires and further study their effects on the near-band-edge (NBE) emission, thereby showing the significant influence of surface states on In 2 O 3 nanostructure based device characteristics for potential optoelectronic applications. In 2 O 3 nanowires with cubic crystal structure (c-In 2 O 3 ) were synthesized via carbothermal reduction technique using a gold-catalyst-assisted vapor-liquid-solid method. Onset of strong optical absorption could be observed at energies greater than 3.5 eV consistent with highly n-type characteristics due to unintentional doping from oxygen vacancy [Formula: see text] defects as confirmed using Raman spectroscopy. A combination of high resolution transmission electron microscopy, x-ray photoelectron spectroscopy and valence band analysis on the nanowire morphology and stoichiometry reveals presence of high-density of [Formula: see text] defects on the surface of the nanowires. As a result, chemisorbed oxygen species can be observed leading to upward band bending at the surface which corresponds to a smaller valence band offset of 2.15 eV. Temperature dependent photoluminescence (PL) spectroscopy was used to study the nature of the defect states and the influence of the surface states on the electronic band structure and NBE emission has been discussed. Our data reveals significant broadening of the NBE PL peak consistent with impurity band broadening leading to band-tailing effect from heavy doping.

Band Gaps for Elastic Wave Propagation in a Periodic Composite Beam Structure Incorporating Microstructure and Surface Energy Effects

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, G. Y.; Gao, X. -L.; Bishop, J. E.

Here, a new model for determining band gaps for elastic wave propagation in a periodic composite beam structure is developed using a non-classical Bernoulli–Euler beam model that incorporates the microstructure, surface energy and rotational inertia effects. The Bloch theorem and transfer matrix method for periodic structures are employed in the formulation. The new model reduces to the classical elasticity-based model when both the microstructure and surface energy effects are not considered. The band gaps predicted by the new model depend on the microstructure and surface elasticity of each constituent material, the unit cell size, the rotational inertia, and the volumemore » fraction. To quantitatively illustrate the effects of these factors, a parametric study is conducted. The numerical results reveal that the band gap predicted by the current non-classical model is always larger than that predicted by the classical model when the beam thickness is very small, but the difference is diminishing as the thickness becomes large. Also, it is found that the first frequency for producing the band gap and the band gap size decrease with the increase of the unit cell length according to both the current and classical models. In addition, it is observed that the effect of the rotational inertia is larger when the exciting frequency is higher and the unit cell length is smaller. Furthermore, it is seen that the volume fraction has a significant effect on the band gap size, and large band gaps can be obtained by tailoring the volume fraction and material parameters.« less

Band Gaps for Elastic Wave Propagation in a Periodic Composite Beam Structure Incorporating Microstructure and Surface Energy Effects

DOE PAGES

Zhang, G. Y.; Gao, X. -L.; Bishop, J. E.; ...

2017-11-20

Here, a new model for determining band gaps for elastic wave propagation in a periodic composite beam structure is developed using a non-classical Bernoulli–Euler beam model that incorporates the microstructure, surface energy and rotational inertia effects. The Bloch theorem and transfer matrix method for periodic structures are employed in the formulation. The new model reduces to the classical elasticity-based model when both the microstructure and surface energy effects are not considered. The band gaps predicted by the new model depend on the microstructure and surface elasticity of each constituent material, the unit cell size, the rotational inertia, and the volumemore » fraction. To quantitatively illustrate the effects of these factors, a parametric study is conducted. The numerical results reveal that the band gap predicted by the current non-classical model is always larger than that predicted by the classical model when the beam thickness is very small, but the difference is diminishing as the thickness becomes large. Also, it is found that the first frequency for producing the band gap and the band gap size decrease with the increase of the unit cell length according to both the current and classical models. In addition, it is observed that the effect of the rotational inertia is larger when the exciting frequency is higher and the unit cell length is smaller. Furthermore, it is seen that the volume fraction has a significant effect on the band gap size, and large band gaps can be obtained by tailoring the volume fraction and material parameters.« less

Electromagnetic-field amplification in finite one-dimensional photonic crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gorelik, V. S.; Kapaev, V. V., E-mail: kapaev@sci.lebedev.ru

2016-09-15

The electromagnetic-field distribution in a finite one-dimensional photonic crystal is studied using the numerical solution of Maxwell’s equations by the transfer-matrix method. The dependence of the transmission coefficient T on the period d (or the wavelength λ) has the characteristic form with M–1 (M is the number of periods in the structure) maxima with T = 1 in the allowed band of an infinite crystal and zero values in the forbidden band. The field-modulus distribution E(x) in the structure for parameters that correspond to the transmission maxima closest to the boundaries of forbidden bands has maxima at the center ofmore » the structure; the value at the maximum considerably exceeds the incident-field strength. For the number of periods M ~ 50, more than an order of magnitude increase in the field amplification is observed. The numerical results are interpreted with an analytic theory constructed by representing the solution in the form of a linear combination of counterpropagating Floquet modes in a periodic structure.« less

Omnidirectional polarization insensitive tunable absorption in graphene metamaterial of nanodisk structure

NASA Astrophysics Data System (ADS)

Ning, Renxia; Bao, Jie; Jiao, Zheng; Xu, Yuan

2015-11-01

Tunable absorption based on graphene metamaterial with nanodisk structure at near-infrared frequency was investigated using the finite difference time domain method. The absorption of the nanodisk structure which consisting of Au-MgF2-graphene-Au-polyimide (from bottom to top) can be tuned by the chemical potential of graphene at certain diameter of nanodisk. The permittivity of graphene is discussed with different chemical potential to obtain tunable absorption. It is shown that the increased value of the chemical potential of graphene can lead to blue-shifted of the absorption peaks and the values decreased. Moreover, dual-band and triple-band absorption can be achieved for resonance frequencies at normal incidence. Compared with diameter of nanodisks, the multilayer structure shows multi-band absorber, and an omnidirectional absorption at 195.25 THz is insensitive to TE/TM polarization. This omnidirectional polarization insensitive absorption may be applied by optical communications such as optical absorber, near infrared stealth, and filter.

GW quasiparticle energy study of ternary tetradymite Bi2Te2Se and Bi2Te2S thin films

NASA Astrophysics Data System (ADS)

Shuaibu, Alhassan; Rahman, Md. Mahmudur; Zainuddin, Hishamuddin; Talib, Zainal Abidin; Muhida, Rifki

2015-04-01

In this work, we have evaluated the quasiparticle energies of ternary tetradymite Bi2Te2Se and Bi2Te2S using first-principles calculation within the G0W0 methods. We have also performed a broad convergence tests in order to investigate the quasiparticle corrections to the structural parameters and to the semi core d electrons in both of the compounds. For each case, we have calculated the many-body corrections within a one-shot GW method of the compounds. Our results have shown that for Bi2Te2Se the GW corrections increase the band gap to almost 10%, and for specific atomic positions, the band structure shows a close value to the experimental one. For Bi2Te2S, despite increase in the band gap due to the GW corrections, possibility of bulk resistivity that can be significant for photovoltaic applications was observed.

Molecular structure, natural bond analysis, vibrational and electronic spectra, surface enhanced Raman scattering and Mulliken atomic charges of the normal modes of [Mn(DDTC)2] complex.

PubMed

Téllez S, Claudio A; Costa, Anilton C; Mondragón, M A; Ferreira, Glaucio B; Versiane, O; Rangel, J L; Lima, G Müller; Martin, A A

2016-12-05

Theoretical and experimental bands have been assigned for the Fourier Transform Infrared and Raman spectra of the bis(diethyldithiocarbamate)Mn(II) complex, [Mn(DDTC)2]. The calculations have been based on the DFT/B3LYP method, second derivative spectra and band deconvolution analysis. The UV-vis experimental spectra were measured in acetonitrile solution, and the calculated electronic spectrum was obtained using the TD/B3LYP method with 6-311G(d, p) basis set for all atoms. Charge transfer bands and those d-d spin forbidden were assigned in the UV-vis spectrum. The natural bond orbital analysis was carried out using the DFT/B3LYP method and the Mn(II) hybridization leading to the planar geometry of the framework was discussed. Surface enhanced Raman scattering (SERS) was also performed. Mulliken charges of the normal modes were obtained and related to the SERS enhanced bands. Copyright © 2016 Elsevier B.V. All rights reserved.

A variable circular-plot method for estimated bird numbers

USGS Publications Warehouse

Reynolds, R.T.; Scott, J.M.; Nussbaum, R.A.

1980-01-01

A bird census method is presented that is designed for tall, structurally complex vegetation types, and rugged terrain. With this method the observer counts all birds seen or heard around a station, and estimates the horizontal distance from the station to each bird. Count periods at stations vary according to the avian community and structural complexity of the vegetation. The density of each species is determined by inspecting a histogram of the number of individuals per unit area in concentric bands of predetermined widths about the stations, choosing the band (with outside radius x) where the density begins to decline, and summing the number of individuals counted within the circle of radius x and dividing by the area (Bx2). Although all observations beyond radius x are rejected with this procedure, coefficients of maximum distance.

Structural and optical characterization of NiSe film grown by screen-printing method

NASA Astrophysics Data System (ADS)

Sharma, Kapil; Sharma, D. K.; Dwivedi, D. K.; Kumar, Vipin

2018-05-01

In present investigation NiSe films were grown by economical screen-printing method. Optimum conditions for growing good quality screen-printed films were found. The films were characterized for their structural and optical properties. The polycrystalline nature of films with hexagonal structure was confirmed through XRD analysis. Direct type of optical band gap of 1.75 eV for the NiSe film was confirmed by optical characterization.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Suhara, Tadahiro; Kanada-En'yo, Yoshiko

Structures of excited states in {sup 14}C are investigated with a method of {beta}-{gamma} constraint antisymmetrized molecular dynamics in combination with the generator coordinate method. Various excited states with the developed 3{alpha}-cluster core structures are suggested in positive- and negative-parity states. In the positive-parity states, triaxial deformed and linear-chain structures are found to construct excited bands. Interestingly, {sup 10}Be+{alpha} correlation is found in the cluster states above the {sup 10}Be+{alpha} threshold energy.

Band Gap Tuning in 2D Layered Materials by Angular Rotation.

PubMed

Polanco-Gonzalez, Javier; Carranco-Rodríguez, Jesús Alfredo; Enríquez-Carrejo, José L; Mani-Gonzalez, Pierre G; Domínguez-Esquivel, José Manuel; Ramos, Manuel

2017-02-08

We present a series of computer-assisted high-resolution transmission electron (HRTEM) simulations to determine Moiré patters by induced twisting effects between slabs at rotational angles of 3°, 5°, 8°, and 16°, for molybdenum disulfide, graphene, tungsten disulfide, and tungsten selenide layered materials. In order to investigate the electronic structure, a series of numerical simulations using density functional methods (DFT) methods was completed using Cambridge serial total energy package (CASTEP) with a generalized gradient approximation to determine both the band structure and density of states on honeycomb-like new superlattices. Our results indicated metallic transitions when the rotation approached 8° with respect to each other laminates for most of the two-dimensional systems that were analyzed.

Band Gap Tuning in 2D Layered Materials by Angular Rotation

PubMed Central

Polanco-Gonzalez, Javier; Carranco-Rodríguez, Jesús Alfredo; Enríquez-Carrejo, José L.; Mani-Gonzalez, Pierre G.; Domínguez-Esquivel, José Manuel; Ramos, Manuel

2017-01-01

We present a series of computer-assisted high-resolution transmission electron (HRTEM) simulations to determine Moiré patters by induced twisting effects between slabs at rotational angles of 3°, 5°, 8°, and 16°, for molybdenum disulfide, graphene, tungsten disulfide, and tungsten selenide layered materials. In order to investigate the electronic structure, a series of numerical simulations using density functional methods (DFT) methods was completed using Cambridge serial total energy package (CASTEP) with a generalized gradient approximation to determine both the band structure and density of states on honeycomb-like new superlattices. Our results indicated metallic transitions when the rotation approached 8° with respect to each other laminates for most of the two-dimensional systems that were analyzed. PMID:28772507

Electronic structure and optical properties of Cs2HgI4: Experimental study and band-structure DFT calculations

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Shkumat, P. N.; Myronchuk, G. L.; Khvyshchun, M.; Fedorchuk, A. O.; Parasyuk, O. V.; Khyzhun, O. Y.

2015-04-01

High-quality single crystal of cesium mercury tetraiodide, Cs2HgI4, has been synthesized by the vertical Bridgman-Stockbarger method and its crystal structure has been refined. In addition, electronic structure and optical properties of Cs2HgI4 have been studied. For the crystal under study, X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated surfaces have been measured. The present X-ray photoelectron spectroscopy (XPS) results indicate that the Cs2HgI4 single crystal surface is very sensitive with respect to Ar+ ion-irradiation. In particular, Ar+ bombardment of the single crystal surface alters the elemental stoichiometry of the Cs2HgI4 surface. To elucidate peculiarities of the energy distribution of the electronic states within the valence-band and conduction-band regions of the Cs2HgI4 compound, we have performed first-principles band-structure calculations based on density functional theory (DFT) as incorporated in the WIEN2k package. Total and partial densities of states for Cs2HgI4 have been calculated. The DFT calculations reveal that the I p states make the major contributions in the upper portion of the valence band, while the Hg d, Cs p and I s states are the dominant contributors in its lower portion. Temperature dependence of the light absorption coefficient and specific electrical conductivity has been explored for Cs2HgI4 in the temperature range of 77-300 K. Main optical characteristics of the Cs2HgI4 compound have been elucidated by the first-principles calculations.

Magnetic field role on the structure and optical response of photonic crystals based on ferrofluids containing Co{sub 0.25}Zn{sub 0.75}Fe{sub 2}O{sub 4} nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

López, J., E-mail: javier.lopez@correounivalle.edu.co; González, Luz E.; Quiñonez, M. F.

2014-05-21

Ferrofluids based on magnetic Co{sub 0.25}Zn{sub 0.75}Fe{sub 2}O{sub 4} ferrite nanoparticles were prepared by co-precipitation method from aqueous salt solutions of Co (II), ZnSO{sub 4}, and Fe (III) in an alkaline medium. Ferrofluids placed in an external magnetic field show properties that make them interesting as magneto-controllable soft photonic crystals. Morphological and structural characterizations of the samples were obtained from Scanning Electron Microscopy and Transmission Electron Microscopy studies. Magnetic properties were investigated with the aid of a vibrating sample magnetometer at room temperature. Herein, the Co{sub 0.25}Zn{sub 0.75}Fe{sub 2}O{sub 4} samples showed superparamagnetic behavior, according to hysteresis loop results. Takingmore » in mind that the Co-Zn ferrite hysteresis loop is very small, our magnetic nanoparticles can be considered soft magnetic material with interesting technological applications. In addition, by using the plane-wave expansion method, we studied the photonic band structure of 2D photonic crystals made of ferrofluids with the same nanoparticles. Previous experimental results show that a magnetic field applied perpendicular to the ferrofluid plane agglomerates the magnetic nanoparticles in parallel rods to form a hexagonal 2D photonic crystal. We calculated the photonic band structure of photonic crystals by means of the effective refractive index of the magnetic fluid, basing the study on the Maxwell-Garnett theory, finding that the photonic band structure does not present any band gaps under the action of applied magnetic field strengths used in our experimental conditions.« less

Subcutaneous Fascial Bands—A Qualitative and Morphometric Analysis

PubMed Central

Li, Weihui; Ahn, Andrew C.

2011-01-01

Background Although fascial bands within the subcutaneous (SQ) layer are commonly seen in ultrasound images, little is known about their functional role, much less their structural characteristics. This study's objective is to describe the morphological features of SQ fascial bands and to systematically evaluate the bands using image analyses tools and morphometric measures. Methods In 28 healthy volunteers, ultrasound images were obtained at three body locations: the lateral aspect of the upper arm, medial aspect of the thigh and posterior aspect of lower leg. Using image analytical techniques, the total SQ band area, fascial band number, fascial band thickness, and SQ zone (layer) thickness were determined. In addition, the SQ spatial coherence was calculated based on the eigenvalues associated with the largest and smallest eigenvectors of the images. Results Fascial bands at these sites were contiguous with the dermis and the epimysium forming an interconnected network within the subcutaneous tissue. Subcutaneous blood vessels were also frequently encased by these fascial bands. The total SQ fascial band area was greater at the thigh and calf compared to the arm and was unrelated to SQ layer (zone) thickness. The thigh was associated with highest average number of fascial bands while calf was associated with the greatest average fascial band thickness. Across body regions, greater SQ zone thickness was associated with thinner fascial bands. SQ coherence was significantly associated with SQ zone thickness and body location (calf with statistically greater coherence compared to arm). Conclusion Fascial bands are structural bridges that mechanically link the skin, subcutaneous layer, and deeper muscle layers. This cohesive network also encases subcutaneous vessels and may indirectly mediate blood flow. The quantity and morphological characteristics of the SQ fascial band may reflect the composite mechanical forces experienced by the body part. PMID:21931632

Effect of Hydrostatic Pressure on the Structural, Electronic and Optical Properties of SnS2 with a Cubic Structure: The DFT Approach

NASA Astrophysics Data System (ADS)

Bakhshayeshi, A.; Taghavi Mendi, R.; Majidiyan Sarmazdeh, M.

2018-02-01

Recently, a cubic structure of polymorphic SnS2 has been synthesized experimentally, which is stable at room temperature. In this paper, we calculated some structural, electronic and optical properties of the cubic SnS2 structure based on the full potential-linearized augmented plane waves method. We also studied the effect of hydrostatic pressure on the physical properties of the cubic SnS2 structure. Structural results show that the compressibility of the cubic SnS2 phase is greater than its trigonal phase and the compressibility decreases with increasing pressure. Investigations of the electronic properties indicate that pressure changes the density of states and the energy band gap increases with increasing pressure. The variation of energy band gap versus pressure is almost linear. We concluded that cubic SnS2 is a semiconductor with an indirect energy band gap, like its trigonal phase. The optical calculations revealed that the dielectric constant decreases with increasing pressure, and the width of the forbidden energy interval increases for electromagnetic wave propagation. Moreover, plasmonic energy and refractive index are changed with increasing pressure.

Control of fault shearing on the fabric of a syn-tectonic granite : magnetic fabric and crystallographic preferred orientation (CPO) of quartz input

NASA Astrophysics Data System (ADS)

Walter, Bastien; Géraud, Yves; Diraison, Marc; Oliot, Emilien

2013-04-01

The late-Miocene monzogranitic pluton of Porto Azzurro (PA) on Elba Island (Italy), was emplaced in the footwall of the N-S striking Zuccale Fault (ZF), a Low-Angle Normal Fault (LANF). In the Barbarossa outcrop, this poorly exposed pluton shows few NNE-SSW and WNW-ESE striking shear bands, respectively moderately dipping eastward and steeply dipping northward, which appear to be associated to the brittle fracturation, and no clear relationship between all these structures and the ZF is described. In order to get information about possible relationship between these shear bands, brittle structures and prior fabric of this igneous stock, and about the timing of formation of these ductile deformations relative to the pluton emplacement, rock fabrics were studied on samples taken both inside and outside of one of these shear bands. The magnetic fabric was analyzed with anisotropy of magnetic susceptibility measurements (AMS), and the crystallographic preferred orientations of dynamically recrystallized quartz were measured with the electron back-scattered diffraction (EBSD) method. Quartz CPOs are directly compared, after EBSD data processing, with the macroscopic ductile structures orientation, according to the geographical North. The pooling of data of these two methods reveals two distinct petrofabrics within the Barbarossa monzogranite. The first fabric, with a low dip angle, is identified only on samples taken outside of the influence of the shear bands. Orientation of paramagnetic minerals, with biotite as the main magnetic mineral carrier, and quartz CPOs are consistent, pervasive within the whole outcrop and are linked to the eastward extension produced by the LANF Zuccale Fault. This fabric suggests that the dynamic of the magmatic supplies during emplacement of the pluton of PA was controlled by the LANF's extension, and confirms this magmatic intrusion to be likely syn-tectonic. The second fabric is identified close or within the studied shear bands with a similar orientation to them. Our data show that these ductile structures impose a local new tectonic fabric overprinting the pre-existing one. The common re-orientation of the magnetic minerals, of the recrystallized quartz and of the brittle structures suggest a strain localization and a continuous strain process localized along stain bands from late-magmatic flowing, highlighted by biotite orientation, then during shear bands activation, at temperature around 350-400° C. Finally, these structures would have remained active through the ductile-brittle transition, leading to the localized intense fracturation of the Barbarossa outcrop.

Effect of alkaline earth modifier on the optical and structural properties of Cu2+ doped phosphate glasses as a bandpass filter

NASA Astrophysics Data System (ADS)

Farouk, M.; Samir, A.; El Okr, M.

2018-02-01

Glasses of composition [16RO-3Al2O3sbnd 6CuOsbnd 20Na2Osbnd 55P2O5], where R is the alkaline earth (R = Mg, Ca, Sr and Ba mol. %), were prepared by conventional melt quenching technique. The glass samples were characterized by X-ray diffraction, infrared spectroscopy, and spectrophotometer. XRD patterns show no sharp peaks indicating the non-crystalline nature of the prepared glasses. The density and molar volume of the glass systems were determined in order to study their structures. These results revealed that addition of alkaline earth elements leads to the formation of non-bridging oxygens (NBOs) and expands (opens up) the structure. The infrared spectra were analyzed to quantify the present phosphate groups. The optical absorption spectra of Cu2+ ions show the characteristic broadband single of Cu2+ ions in octahedral symmetry. The band gap was estimated following two methodologies. The first method considers the band edge of the transmission, while the second approach relays on the estimated values of the optical constants. A decent agreement for the band gap values using the two methods was obtained.

Facile synthesis of diverse graphene nanomeshes based on simultaneous regulation of pore size and surface structure

NASA Astrophysics Data System (ADS)

Zhang, Jia; Song, Huaibing; Zeng, Dawen; Wang, Hao; Qin, Ziyu; Xu, Keng; Pang, Aimin; Xie, Changsheng

2016-08-01

Recently, graphene nanomesh (GNM) has attracted great attentions due to its unique porous structure, abundant active sites, finite band gap and possesses potential applications in the fields of electronics, gas sensor/storage, catalysis, etc. Therefore, diverse GNMs with different physical and chemical properties are required urgently to meet different applications. Herein we demonstrate a facile synthetic method based on the famous Fenton reaction to prepare GNM, by using economically fabricated graphene oxide (GO) as a starting material. By precisely controlling the reaction time, simultaneous regulation of pore size from 2.9 to 11.1 nm and surface structure can be realized. Ultimately, diverse GNMs with tunable band gap and work function can be obtained. Specially, the band gap decreases from 4.5-2.3 eV for GO, which is an insulator, to 3.9-1.24 eV for GNM-5 h, which approaches to a semiconductor. The dual nature of electrophilic addition and oxidizability of HO• is responsible for this controllable synthesis. This efficient, low-cost, inherently scalable synthetic method is suitable for provide diverse and optional GNMs, and may be generalized to a universal technique.

Synthesis of fluorinated graphene oxide by using an easy one-pot deoxyfluorination reaction.

PubMed

Aguilar-Bolados, Héctor; Contreras-Cid, Ahirton; Yazdani-Pedram, Mehrdad; Acosta-Villavicencio, Gabriela; Flores, Marcos; Fuentealba, Pablo; Neira-Carrillo, Andrónico; Verdejo, Raquel; López-Manchado, Miguel A

2018-08-15

The fluorination of two types of graphene oxides conducted by an easy and scalable deoxyfluorination reaction is reported. This reaction was carried out using diethylaminodifluorosulfinium tetrafluoroborate, a stable compound and an efficient reagent for replacing oxygenated functional groups of graphene oxide by fluoride. The graphene oxide produced by the Hummers' method (GOH) showed lower reactivity than that produced by the Brodie's method (GOB). X-ray photoelectron spectroscopy indicated that the highest fluorination degree achieved was 4.7 at.% when GOB was used, and the CF character corresponds to semi-ionic bonds. Additionally, a partial reduction of GO was concomitant with the functionalization reaction. The deoxyfluorination reaction changed the crystalline structure of GO, favoring the reconstruction of Csp 2 structure of the graphene lattice and reducing the number of stacked layers. The fluorination led to the modification of the electronic band structure of this material, increasing the band gap from 2.05 eV for GOB to 3.88 eV for fluorinated GOB, while for GOH the low flurionation led to a slight increase of the band gap, from 3.48 eV to 3.57 eV. Copyright © 2018 Elsevier Inc. All rights reserved.

Facile synthesis of diverse graphene nanomeshes based on simultaneous regulation of pore size and surface structure.

PubMed

Zhang, Jia; Song, Huaibing; Zeng, Dawen; Wang, Hao; Qin, Ziyu; Xu, Keng; Pang, Aimin; Xie, Changsheng

2016-08-26

Recently, graphene nanomesh (GNM) has attracted great attentions due to its unique porous structure, abundant active sites, finite band gap and possesses potential applications in the fields of electronics, gas sensor/storage, catalysis, etc. Therefore, diverse GNMs with different physical and chemical properties are required urgently to meet different applications. Herein we demonstrate a facile synthetic method based on the famous Fenton reaction to prepare GNM, by using economically fabricated graphene oxide (GO) as a starting material. By precisely controlling the reaction time, simultaneous regulation of pore size from 2.9 to 11.1 nm and surface structure can be realized. Ultimately, diverse GNMs with tunable band gap and work function can be obtained. Specially, the band gap decreases from 4.5-2.3 eV for GO, which is an insulator, to 3.9-1.24 eV for GNM-5 h, which approaches to a semiconductor. The dual nature of electrophilic addition and oxidizability of HO(•) is responsible for this controllable synthesis. This efficient, low-cost, inherently scalable synthetic method is suitable for provide diverse and optional GNMs, and may be generalized to a universal technique.

Fabrication, characterization and applications of iron selenide

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hussain, Raja Azadar, E-mail: hussainazadar@yahoo.com; Badshah, Amin; Lal, Bhajan

This review article presents fabrication of FeSe by solid state reactions, solution chemistry routes, chemical vapor deposition, spray pyrolysis and chemical vapor transport. Different properties and applications such as crystal structure and phase transition, band structure, spectroscopy, superconductivity, photocatalytic activity, electrochemical sensing, and fuel cell activity of FeSe have been discussed. - Graphical abstract: Iron selenide can be synthesized by solid state reactions, chemical vapor deposition, solution chemistry routes, chemical vapor transport and spray pyrolysis. - Highlights: • Different fabrication methods of iron selenide (FeSe) have been reviewed. • Crystal structure, band structure and spectroscopy of FeSe have been discussed.more » • Superconducting, catalytic and fuel cell application of FeSe have been presented.« less

Developing Structure-Property Relationships in Branched Wormlike Micelles via Advanced Rheological and Neutron Scattering Techniques

NASA Astrophysics Data System (ADS)

Calabrese, Michelle A.

Surfactant wormlike micelles (WLMs) are of particular scientific interest due to their ability to branch, break, and reform under shear, which can lead to shear banding flow instabilities. The tunable self-assembly of WLMs makes them ubiquitous in applications ranging from consumer products to energy recovery fluids. Altering the topology of WLMs by inducing branching provides a microstructural pathway to design and optimize the flow properties for such targeted applications. The goal of this thesis is to understand the role of micellar branching on the resulting equilibrium and non-equilibrium properties, while advancing instrumentation and analysis methods in rheology and neutron scattering. The degree of branching in the mixed cationic/anionic surfactant solutions is controlled by the addition of sodium tosylate. The equilibrium properties are characterized via small angle neutron scattering (SANS), linear viscoelastic rheology, neutron spin echo, and dynamic light scattering. Combining rheology with spatiotemporally-resolved SANS enables unambiguous identification of non-equilibrium rheological and scattering signatures of branching and shear banding. The nonlinear WLM response is characterized via flow-SANS under steady shear, shear startup, and large amplitude oscillatory shear. New methods of time-resolved data analysis are developed, which improve experimental resolution by several-fold. Shear-induced orientation is a complex function of branching level, radial position, and deformation type. The structural mechanisms behind shear band formation are elucidated for steady and dynamic flows, which depend on branching level. Shear banding disappears at high branching levels for all deformation types. These responses are used to validate constitutive modeling predictions of dynamic shear banding for the first time. Finally, quantitative metrics to predict shear banding from rheology or flow-induced orientation are developed. Together, advanced rheological and neutron techniques provide a platform for creating structure-property relationships that predict flow and structural phenomena in WLMs and other soft materials. These methods have enabled characteristic differences in linear versus branched WLMs to be determined. This research is part of a broader effort to characterize branching in polymers and self-assembled systems, and may aid in the formulation of WLMs for specific applications. Finally, this work provides a basis for testing and developing microstructure-based constitutive equations that incorporate micellar breakage and branching.

Dual-Band Operation of a Microstrip Patch Antenna on a Duroid 5870 Substrate for Ku- and K-Bands

PubMed Central

Islam, M. M.; Islam, M. T.; Faruque, M. R. I.

2013-01-01

The dual-band operation of a microstrip patch antenna on a Duroid 5870 substrate for Ku- and K-bands is presented. The fabrication of the proposed antenna is performed with slots and a Duroid 5870 dielectric substrate and is excited by a 50 Ω microstrip transmission line. A high-frequency structural simulator (HFSS) is used which is based on the finite element method (FEM) in this research. The measured impedance bandwidth (2 : 1 VSWR) achieved is 1.07 GHz (15.93 GHz–14.86 GHz) on the lower band and 0.94 GHz (20.67–19.73 GHz) on the upper band. A stable omnidirectional radiation pattern is observed in the operating frequency band. The proposed prototype antenna behavior is discussed in terms of the comparisons of the measured and simulated results. PMID:24385878

Dual-band operation of a microstrip patch antenna on a Duroid 5870 substrate for Ku- and K-bands.

PubMed

Islam, M M; Islam, M T; Faruque, M R I

2013-01-01

The dual-band operation of a microstrip patch antenna on a Duroid 5870 substrate for Ku- and K-bands is presented. The fabrication of the proposed antenna is performed with slots and a Duroid 5870 dielectric substrate and is excited by a 50 Ω microstrip transmission line. A high-frequency structural simulator (HFSS) is used which is based on the finite element method (FEM) in this research. The measured impedance bandwidth (2 : 1 VSWR) achieved is 1.07 GHz (15.93 GHz-14.86 GHz) on the lower band and 0.94 GHz (20.67-19.73 GHz) on the upper band. A stable omnidirectional radiation pattern is observed in the operating frequency band. The proposed prototype antenna behavior is discussed in terms of the comparisons of the measured and simulated results.

DFT study on the crystal, electronic and magnetic structures of tantalum based double perovskite oxides Ba2MTaO6 (M = Cr, Mn, Fe) via GGA and GGA + U

NASA Astrophysics Data System (ADS)

Saad, H.-E.; Musa, M.; Elhag, Ahmed

2018-06-01

In this paper, we study the crystal, electronic and magnetic structures of three tantalum based double perovskite oxides Ba2MTaO6 (M = Cr, Mn, Fe). All calculations were performed using the full-potential linear augmented plane-wave (PF-LAPW) method based on the first-principles density functional theory (DFT). For the exchange correlation potential, the generalized gradient approximation (GGA) and GGA plus on-site Coulomb parameter (GGA + U) were employed. The structural optimization reveals that the three compounds are stable in cubic structure (space group Fm-3m; tilt system a0a0a0). The band structure, density of states (DOS), charge density and spin magnetic moments were calculated and analyzed in details. By analysis the band structure and DOS, Ba2MTaO6 exhibits an insulating behavior (M = Cr, Fe) and a half-metallic (HM) nature (M = Mn). GGA + U method yields quite accurate results for the band-gap (Eg) as compared with GGA. We found that all three compounds have stable ferromagnetic (FM) ground state within GGA and GGA + U calculations. The M3+ (3d) ions contribute the majority in the total spin magnetic-moments, while, the empty T5+ (5d) ions carry very small induced magnetic moment via the M (3d)-O (2p)-Ta (5d) hybridization.

Subwavelength and directional control of flexural waves in zone-folding induced topological plates

NASA Astrophysics Data System (ADS)

Chaunsali, Rajesh; Chen, Chun-Wei; Yang, Jinkyu

2018-02-01

Inspired by the quantum spin Hall effect shown by topological insulators, we propose a plate structure that can be used to demonstrate the pseudospin Hall effect for flexural waves. The system consists of a thin plate with periodically arranged resonators mounted on its top surface. We extend a technique based on the plane-wave expansion method to identify a double Dirac cone emerging due to the zone-folding in frequency band structures. This particular design allows us to move the double Dirac cone to a lower frequency than the resonating frequency of local resonators. We then manipulate the pattern of local resonators to open subwavelength Bragg band gaps that are topologically distinct. Building on this method, we verify numerically that a waveguide at an interface between two topologically distinct resonating plate structures can be used for guiding low-frequency, spin-dependent one-way flexural waves along a desired path with bends.

Elimination of the asymmetric modes in a Ka-band super overmoded coaxial Cerenkov oscillator

NASA Astrophysics Data System (ADS)

Bai, Zhen; Zhang, Jun; Zhong, Huihuang; Zhao, Xuelong; Yang, Fuxiang

2017-12-01

The issue of asymmetric modes output of a Ka-band super overmoded coaxial Cerenkov oscillator is analyzed in this paper. Due to serious passband overlapping in a super overmoded coaxial slow wave structure (SWS), the asymmetric competition mode EH11 can hardly be suppressed thoroughly by the methods adopted in moderately overmoded devices, especially in the startup of oscillation. If the output structures reflect the asymmetric modes, the asymmetric mode competition in SWS will be aggravated and the normal operation state will be destroyed. In order to solve this problem, a taper waveguide is inserted at a specific position to achieve the destructive interference of the reflected TM11, and a special support structure is designed to avoid reflection of TE11. With these methods, asymmetric mode competition can be successfully eliminated, and the oscillator is capable of achieving a steady fundamental mode operation performance.

Computational investigations of the band structure, and thermodynamic and optical features of thorium-based oxide ThGeO4 using the full-potential linearized augmented plane-wave plus local orbital approach

NASA Astrophysics Data System (ADS)

Chiker, F.; Khachai, H.; Mathieu, C.; Bin-Omran, S.; Kada, Belkacem; Sun, Xiao-Wei; Sandeep; Rai, D. P.; Khenata, R.

2018-05-01

In this study, first-principles investigations were performed using the full-potential linearized augmented plane-wave method of the structural and optoelectronic properties of thorium germinate (ThGeO4), a high-K dielectric material. Under ambient conditions, the structural properties calculated for ThGeO4 in the zircon phase were in excellent agreement with the available experimental data. Furthermore, using the modified Becke -Johnson correction method, the calculated band gaps and optical constants accurately described this compound. Finally, the thermal properties were predicted over a temperature range of 0-700 K and pressures up to 11 GPa using the quasi-harmonic Debye model, where the variations in the heat capacity, primitive cell volume, and thermal expansion coefficients were determined successfully.

Attenuated Total Reflection Fourier Transform Infrared (ATR FT-IR) Spectroscopy as an Analytical Method to Investigate the Secondary Structure of a Model Protein Embedded in Solid Lipid Matrices.

PubMed

Zeeshan, Farrukh; Tabbassum, Misbah; Jorgensen, Lene; Medlicott, Natalie J

2018-02-01

Protein drugs may encounter conformational perturbations during the formulation processing of lipid-based solid dosage forms. In aqueous protein solutions, attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy can investigate these conformational changes following the subtraction of spectral interference of solvent with protein amide I bands. However, in solid dosage forms, the possible spectral contribution of lipid carriers to protein amide I band may be an obstacle to determine conformational alterations. The objective of this study was to develop an ATR FT-IR spectroscopic method for the analysis of protein secondary structure embedded in solid lipid matrices. Bovine serum albumin (BSA) was chosen as a model protein, while Precirol AT05 (glycerol palmitostearate, melting point 58 ℃) was employed as the model lipid matrix. Bovine serum albumin was incorporated into lipid using physical mixing, melting and mixing, or wet granulation mixing methods. Attenuated total reflection FT-IR spectroscopy and size exclusion chromatography (SEC) were performed for the analysis of BSA secondary structure and its dissolution in aqueous media, respectively. The results showed significant interference of Precirol ATO5 with BSA amide I band which was subtracted up to 90% w/w lipid content to analyze BSA secondary structure. In addition, ATR FT-IR spectroscopy also detected thermally denatured BSA solid alone and in the presence of lipid matrix indicating its suitability for the detection of denatured protein solids in lipid matrices. Despite being in the solid state, conformational changes occurred to BSA upon incorporation into solid lipid matrices. However, the extent of these conformational alterations was found to be dependent on the mixing method employed as indicated by area overlap calculations. For instance, the melting and mixing method imparted negligible effect on BSA secondary structure, whereas the wet granulation mixing method promoted more changes. Size exclusion chromatography analysis depicted the complete dissolution of BSA in the aqueous media employed in the wet granulation method. In conclusion, an ATR FT-IR spectroscopic method was successfully developed to investigate BSA secondary structure in solid lipid matrices following the subtraction of lipid spectral interference. The ATR FT-IR spectroscopy could further be applied to investigate the secondary structure perturbations of therapeutic proteins during their formulation development.

Oxygen holes and hybridization in the bismuthates

NASA Astrophysics Data System (ADS)

Khazraie, Arash; Foyevtsova, Kateryna; Elfimov, Ilya; Sawatzky, George A.

2018-02-01

Motivated by the recently renewed interest in the superconducting bismuth perovskites, we investigate the electronic structure of the parent compounds A BiO3 (A = Sr, Ba) using ab initio methods and tight-binding (TB) modeling. We use the density functional theory (DFT) in the local density approximation (LDA) to understand the role of various interactions in shaping the A BiO3 band structure near the Fermi level. It is established that interatomic hybridization involving Bi-6 s and O-2 p orbitals plays the most important role. Based on our DFT calculations, we derive a minimal TB model and demonstrate that it can describe the properties of the band structure as a function of lattice distortions, such as the opening of a charge gap with the onset of the breathing distortion and the associated condensation of holes onto a1 g-symmetric molecular orbitals formed by the O-2 pσ orbitals on collapsed octahedra. We also derive a single band model involving the hopping of an extended molecular orbital involving both Bi-6 s and a linear combination of six O-2 p orbitals which provides a very good description of the dispersion and band gaps of the low energy scale bands straddling the chemical potential.

Statistical Frequency-Dependent Analysis of Trial-to-Trial Variability in Single Time Series by Recurrence Plots.

PubMed

Tošić, Tamara; Sellers, Kristin K; Fröhlich, Flavio; Fedotenkova, Mariia; Beim Graben, Peter; Hutt, Axel

2015-01-01

For decades, research in neuroscience has supported the hypothesis that brain dynamics exhibits recurrent metastable states connected by transients, which together encode fundamental neural information processing. To understand the system's dynamics it is important to detect such recurrence domains, but it is challenging to extract them from experimental neuroscience datasets due to the large trial-to-trial variability. The proposed methodology extracts recurrent metastable states in univariate time series by transforming datasets into their time-frequency representations and computing recurrence plots based on instantaneous spectral power values in various frequency bands. Additionally, a new statistical inference analysis compares different trial recurrence plots with corresponding surrogates to obtain statistically significant recurrent structures. This combination of methods is validated by applying it to two artificial datasets. In a final study of visually-evoked Local Field Potentials in partially anesthetized ferrets, the methodology is able to reveal recurrence structures of neural responses with trial-to-trial variability. Focusing on different frequency bands, the δ-band activity is much less recurrent than α-band activity. Moreover, α-activity is susceptible to pre-stimuli, while δ-activity is much less sensitive to pre-stimuli. This difference in recurrence structures in different frequency bands indicates diverse underlying information processing steps in the brain.

Statistical Frequency-Dependent Analysis of Trial-to-Trial Variability in Single Time Series by Recurrence Plots

PubMed Central

Tošić, Tamara; Sellers, Kristin K.; Fröhlich, Flavio; Fedotenkova, Mariia; beim Graben, Peter; Hutt, Axel

2016-01-01

For decades, research in neuroscience has supported the hypothesis that brain dynamics exhibits recurrent metastable states connected by transients, which together encode fundamental neural information processing. To understand the system's dynamics it is important to detect such recurrence domains, but it is challenging to extract them from experimental neuroscience datasets due to the large trial-to-trial variability. The proposed methodology extracts recurrent metastable states in univariate time series by transforming datasets into their time-frequency representations and computing recurrence plots based on instantaneous spectral power values in various frequency bands. Additionally, a new statistical inference analysis compares different trial recurrence plots with corresponding surrogates to obtain statistically significant recurrent structures. This combination of methods is validated by applying it to two artificial datasets. In a final study of visually-evoked Local Field Potentials in partially anesthetized ferrets, the methodology is able to reveal recurrence structures of neural responses with trial-to-trial variability. Focusing on different frequency bands, the δ-band activity is much less recurrent than α-band activity. Moreover, α-activity is susceptible to pre-stimuli, while δ-activity is much less sensitive to pre-stimuli. This difference in recurrence structures in different frequency bands indicates diverse underlying information processing steps in the brain. PMID:26834580

Experiment and density functional theory analyses of GdTaO4 single crystal

NASA Astrophysics Data System (ADS)

Ding, Shoujun; Kinross, Ashlie; Wang, Xiaofei; Yang, Huajun; Zhang, Qingli; Liu, Wenpeng; Sun, Dunlu

2018-05-01

GdTaO4 is a type of excellent materials that can be used as scintillation, laser matrix as well as self-activated phosphor has generated significant interest. Whereas its band structure, electronic structure and optical properties are still need elucidation. To solve this intriguing problem, high-quality GdTaO4 single crystal (M-type) was grown successfully using Czochralski method. Its structure as well as optical properties was determined in experiment. Moreover, a systematic theoretical calculation based on the density function theory methods were performed on M-type and M‧-type GdTaO4 and their band structure, density of state as well as optical properties were obtained. Combine with the performed experiment results, the calculated results were proved with high reliability. Hence, the calculated results obtained in this work could provide a deep understanding of GdTaO4 material, which also useful for the further investigation on GdTaO4 material.

Alkali semi-metal films and method and apparatus for fabricating them

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bhandari, Harish B.; Nagarkar, Vivek V.; Ovechkina, Olena E.

Methods and systems for fabricating a film, such as, for example, a photocathode, having a tailored band structure and thin-film components that can be tailored for specific applications, such as, for example photocathode having a high quantum efficiency, and simple components fabricated by those methods.

Manganese silicide nanowires on Si(001).

PubMed

Liu, H J; Owen, J H G; Miki, K; Renner, Ch

2011-05-04

A method for promoting the growth of manganese silicide nanowires on Si(001) at 450 °C is described. The anisotropic surface stress generated by bismuth nanolines blocks the formation of embedded structures and stabilizes the nucleation of manganese silicide islands which grow in a preferred direction, forming nanowires with a band gap of approximately 0.6 eV, matching the reported band gap of MnSi(1.7). This method may also provide a means to form silicide nanowires of other metals where they do not otherwise form. © 2011 IOP Publishing Ltd

Multiphonon contribution to the polaron formation in cuprates with strong electron correlations and strong electron-phonon interaction

NASA Astrophysics Data System (ADS)

Ovchinnikov, Sergey G.; Makarov, Ilya A.; Kozlov, Peter A.

2017-03-01

In this work dependences of the electron band structure and spectral function in the HTSC cuprates on magnitude of electron-phonon interaction (EPI) and temperature are investigated. We use three-band p-d model with diagonal and offdiagonal EPI with breathing and buckling phonon mode in the frameworks of polaronic version of the generalized tight binding (GTB) method. The polaronic quasiparticle excitation in the system with EPI within this approach is formed by a hybridization of the local multiphonon Franck-Condon excitations with lower and upper Hubbard bands. Increasing EPI leads to transfer of spectral weight to high-energy multiphonon excitations and broadening of the spectral function. Temperature effects are taken into account by occupation numbers of local excited polaronic states and variations in the magnitude of spin-spin correlation functions. Increasing the temperature results in band structure reconstruction, spectral weight redistribution, broadening of the spectral function peak at the top of the valence band and the decreasing of the peak intensity. The effect of EPI with two phonon modes on the polaron spectral function is discussed.

Nuclear Structure in China 2010

NASA Astrophysics Data System (ADS)

Bai, Hong-Bo; Meng, Jie; Zhao, En-Guang; Zhou, Shan-Gui

2011-08-01

Personal view on nuclear physics research / Jie Meng -- High-spin level structures in [symbol]Zr / X. P. Cao ... [et al.] -- Constraining the symmetry energy from the neutron skin thickness of tin isotopes / Lie-Wen Chen ... [et al.] -- Wobbling rotation in atomic nuclei / Y. S. Chen and Zao-Chun Gao -- The mixing of scalar mesons and the possible nonstrange dibaryons / L. R. Dai ... [et al.] -- Net baryon productions and gluon saturation in the SPS, RHIC and LHC energy regions / Sheng-Qin Feng -- Production of heavy isotopes with collisions between two actinide nuclides / Z. Q. Feng ... [et al.] -- The projected configuration interaction method / Zao-Chun Gao and Yong-Shou Chen -- Applications of Nilsson mean-field plus extended pairing model to rare-earth nuclei / Xin Guan ... [et al.] -- Complex scaling method and the resonant states / Jian-You Guo ... [et al.] -- Probing the equation of state by deep sub-barrier fusion reactions / Hong-Jun Hao and Jun-Long Tian -- Doublet structure study in A[symbol]105 mass region / C. Y. He ... [et al.] -- Rotational bands in transfermium nuclei / X. T. He -- Shape coexistence and shape evolution [symbol]Yb / H. Hua ... [et al.] -- Multistep shell model method in the complex energy plane / R. J. Liotta -- The evolution of protoneutron stars with kaon condensate / Ang Li -- High spin structures in the [symbol]Lu nucleus / Li Cong-Bo ... [et al.] -- Nuclear stopping and equation of state / QingFeng Li and Ying Yuan -- Covariant description of the low-lying states in neutron-deficient Kr isotopes / Z. X. Li ... [et al.] -- Isospin corrections for superallowed [symbol] transitions / HaoZhao Liang ... [et al.] -- The positive-parity band structures in [symbol]Ag / C. Liu ... [et al.] -- New band structures in odd-odd [symbol]I and [symbol]I / Liu GongYe ... [et al.] -- The sd-pair shell model and interacting boson model / Yan-An Luo ... [et al.] -- Cross-section distributions of fragments in the calcium isotopes projectile fragmentation at the intermediate energy / C. W. Ma ... [et al.].Systematic study of spin assignment and dynamic moment of inertia of high-j intruder band in [symbol]In / K. Y. Ma ... [et al.] -- Signals of diproton emission from the three-body breakup channel of [symbol]Al and [symbol]Mg / Ma Yu-Gang ... [et al.] -- Uncertainties of Th/Eu and Th/Hf chronometers from nucleus masses / Z. M. Niu ... [et al.] -- The chiral doublet bands with [symbol] configuration in A[symbol]100 mass region / B. Qi ... [et al.] -- [symbol] formation probabilities in nuclei and pairing collectivity / Chong Qi -- A theoretical prospective on triggered gamma emission from [symbol]Hf[symbol] isomer / ShuiFa Shen ... [et al.] -- Study of nuclear giant resonances using a Fermi-liquid method / Bao-Xi Sun -- Rotational bands in doubly odd [symbol]Sb / D. P. Sun ... [et al.] -- The study of the neutron N=90 nuclei / W. X. Teng ... [et al.] -- Dynamical modes and mechanisms in ternary reaction of [symbol]Au+[symbol]Au / Jun-Long Tian ... [et al.] -- Dynamical study of X(3872) as a D[symbol] molecular state / B. Wang ... [et al.] -- Super-heavy stability island with a semi-empirical nuclear mass formula / N. Wang ... [et al.] -- Pseudospin partner bands in [symbol]Sb / S. Y. Wang ... [et al.] -- Study of elastic resonance scattering at CIAE / Y. B. Wang ... [et al.] -- Systematic study of survival probability of excited superheavy nuclei / C. J. Xia ... [et al.] -- Angular momentum projection of the Nilsson mean-field plus nearest-orbit pairing interaction model / Ming-Xia Xie ... [et al.] -- Possible shape coexistence for [symbol]Sm in a reflection-asymmetric relativistic mean-field approach / W. Zhang ... [et al.] -- Nuclear pairing reduction due to rotation and blocking / Zhen-Hua Zhang -- Nucleon pair approximation of the shell model: a review and perspective / Y. M. Zhao ... [et al.] -- Band structures in doubly odd [symbol]I / Y. Zheng ... [et al.] -- Lifetimes of high spin states in [symbol]Ag / Y. Zheng ... [et al.] -- Effect of tensor interaction on the shell structure of superheavy nuclei / Xian-Rong Zhou ... [et al.].

DFTB Parameters for the Periodic Table: Part 1, Electronic Structure.

PubMed

Wahiduzzaman, Mohammad; Oliveira, Augusto F; Philipsen, Pier; Zhechkov, Lyuben; van Lenthe, Erik; Witek, Henryk A; Heine, Thomas

2013-09-10

A parametrization scheme for the electronic part of the density-functional based tight-binding (DFTB) method that covers the periodic table is presented. A semiautomatic parametrization scheme has been developed that uses Kohn-Sham energies and band structure curvatures of real and fictitious homoatomic crystal structures as reference data. A confinement potential is used to tighten the Kohn-Sham orbitals, which includes two free parameters that are used to optimize the performance of the method. The method is tested on more than 100 systems and shows excellent overall performance.

Investigation of the quaternary structure of an ABC transporter in living cells using spectrally resolved resonance energy transfer

NASA Astrophysics Data System (ADS)

Singh, Deo Raj

Forster resonance energy transfer (FRET) has become an important tool to study proteins inside living cells. It has been used to explore membrane protein folding and dynamics, determine stoichiometry and geometry of protein complexes, and measure the distance between two molecules. In this dissertation, we use a method based on FRET and optical micro-spectroscopy (OptiMiS) technology, developed in our lab, to probe the structure of dynamic (as opposed to static) protein complexes in living cells. We use this method to determine the association stoichiometry and quaternary structure of an ABC transporter in living cells. Specifically, the transporter we investigate originates from the pathogen Pseudomonas aeruginosa, which is a Gram-negative bacterium with several virulence factors, lipopolysaccharides being one of them. This pathogen coexpresses two unique forms of lipopolysaccharides on its surface, the A- and B-bands. The A-band polysaccharides, synthesized in the cytoplasm, are translocated into the periplasm through an ATP-binding-cassette (ABC) transporter consisting of a transmembranar protein, Wzm, and a nucleotide-binding protein, Wzt. In P. aeruginosa, all of the biochemical studies of A-band LPS are concentrated on the stages of the synthesis and ligation of polysaccharides (PSs), leaving the export stage involving ABC transporter unexplored. The mode of PS export through ABC transporters is still unknown. This difficulty is due to the lack of information about sub-unit composition and structure of this bi-component ABC transporter. Using the FRET-OptiMiS combination method developed by our lab, we found that Wzt forms a rhombus-shaped homo-tetramer which becomes a square upon co-expression with Wzm, and that Wzm forms a square-shaped homo-tetramer both in the presence and absence of Wzt. Based on these results, we propose a structural model for the double-tetramer complex formed by the bi-component ABC transporter in living cells. An understanding of the structure and behavior of this ABC transporter will help develop antibiotics targeting the biosynthesis of the A-band LPS endotoxin.

Babinet's principle and the band structure of surface waves on patterned metal arrays

NASA Astrophysics Data System (ADS)

Edmunds, J. D.; Taylor, M. C.; Hibbins, A. P.; Sambles, J. R.; Youngs, I. J.

2010-05-01

The microwave response of an array of square metal patches and its complementary structure, an array of square holes, has been experimentally studied. The resonant phenomena, which yield either enhanced transmission or reflection, are attributed to the excitation of diffractively coupled surface waves. The band structure of these surface modes has been quantified for both p-(transverse magnetic) and s-(transverse electric) polarized radiation and is found to be dependent on the periodicity of the electric and magnetic fields on resonance. The results are in excellent accord with predictions from finite element method modeling and the electromagnetic form of Babinet's principle [Babinet, C. R. Acad. Sci. 4, 638 (1837)].

The dependence of the tunneling characteristic on the electronic energy bands and the carrier’s states of Graphene superlattice

NASA Astrophysics Data System (ADS)

Yang, C. H.; Shen, G. Z.; Ao, Z. M.; Xu, Y. W.

2016-09-01

Using the transfer matrix method, the carrier tunneling properties in graphene superlattice generated by the Thue-Morse sequence and Kolakoski sequence are investigated. The positions and strength of the transmission can be modulated by the barrier structures, the incident energy and angle, the height and width of the potential. These carriers tunneling characteristic can be understood from the energy band structures in the corresponding superlattice systems and the carrier’s states in well/barriers. The transmission peaks above the critical incident angle rely on the carrier’s resonance in the well regions. The structural diversity can modulate the electronic and transport properties, thus expanding its applications.

Computational predictions of the new Gallium nitride nanoporous structures

NASA Astrophysics Data System (ADS)

Lien, Le Thi Hong; Tuoc, Vu Ngoc; Duong, Do Thi; Thu Huyen, Nguyen

2018-05-01

Nanoporous structural prediction is emerging area of research because of their advantages for a wide range of materials science and technology applications in opto-electronics, environment, sensors, shape-selective and bio-catalysis, to name just a few. We propose a computationally and technically feasible approach for predicting Gallium nitride nanoporous structures with hollows at the nano scale. The designed porous structures are studied with computations using the density functional tight binding (DFTB) and conventional density functional theory methods, revealing a variety of promising mechanical and electronic properties, which can potentially find future realistic applications. Their stability is discussed by means of the free energy computed within the lattice-dynamics approach. Our calculations also indicate that all the reported hollow structures are wide band gap semiconductors in the same fashion with their parent’s bulk stable phase. The electronic band structures of these nanoporous structures are finally examined in detail.

Near-edge band structures and band gaps of Cu-based semiconductors predicted by the modified Becke-Johnson potential plus an on-site Coulomb U

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Yubo; Zhang, Jiawei; Wang, Youwei

Diamond-like Cu-based multinary semiconductors are a rich family of materials that hold promise in a wide range of applications. Unfortunately, accurate theoretical understanding of the electronic properties of these materials is hindered by the involvement of Cu d electrons. Density functional theory (DFT) based calculations using the local density approximation or generalized gradient approximation often give qualitative wrong electronic properties of these materials, especially for narrow-gap systems. The modified Becke-Johnson (mBJ) method has been shown to be a promising alternative to more elaborate theory such as the GW approximation for fast materials screening and predictions. However, straightforward applications of themore » mBJ method to these materials still encounter significant difficulties because of the insufficient treatment of the localized d electrons. We show that combining the promise of mBJ potential and the spirit of the well-established DFT + U method leads to a much improved description of the electronic structures, including the most challenging narrow-gap systems. A survey of the band gaps of about 20 Cu-based semiconductors calculated using the mBJ + U method shows that the results agree with reliable values to within ±0.2 eV.« less

Improved target detection by IR dual-band image fusion

NASA Astrophysics Data System (ADS)

Adomeit, U.; Ebert, R.

2009-09-01

Dual-band thermal imagers acquire information simultaneously in both the 8-12 μm (long-wave infrared, LWIR) and the 3-5 μm (mid-wave infrared, MWIR) spectral range. Compared to single-band thermal imagers they are expected to have several advantages in military applications. These advantages include the opportunity to use the best band for given atmospheric conditions (e. g. cold climate: LWIR, hot and humid climate: MWIR), the potential to better detect camouflaged targets and an improved discrimination between targets and decoys. Most of these advantages have not yet been verified and/or quantified. It is expected that image fusion allows better exploitation of the information content available with dual-band imagers especially with respect to detection of targets. We have developed a method for dual-band image fusion based on the apparent temperature differences in the two bands. This method showed promising results in laboratory tests. In order to evaluate its performance under operational conditions we conducted a field trial in an area with high thermal clutter. In such areas, targets are hardly to detect in single-band images because they vanish in the clutter structure. The image data collected in this field trial was used for a perception experiment. This perception experiment showed an enhanced target detection range and reduced false alarm rate for the fused images compared to the single-band images.

Theoretical band structure of the superconducting antiperovskite oxide Sr3-xSnO

NASA Astrophysics Data System (ADS)

Ikeda, Atsutoshi; Fukumoto, Toshiyuki; Oudah, Mohamed; Hausmann, Jan Niklas; Yonezawa, Shingo; Kobayashi, Shingo; Sato, Masatoshi; Tassel, Cédric; Takeiri, Fumitaka; Takatsu, Hiroshi; Kageyama, Hiroshi; Maeno, Yoshiteru

2018-05-01

In order to investigate the position of the strontium deficiency in superconductive Sr3-xSnO, we synthesized and measured X-ray-diffraction patterns of Sr3-xSnO (x ∼ 0.5). Because no clear peaks originating from superstructures were observed, strontium deficiency is most likely to be randomly distributed. We also performed first-principles band-structure calculations on Sr3-xSnO (x = 0, 0.5) using two methods: full-potential linearized-augmented plane-wave plus local orbitals method and the Korringa-Kohn-Rostoker Green function method combined with the coherent potential approximation. We revealed that the Fermi energy of Sr3-xSnO in case of x ∼ 0.5 is about 0.8 eV below the original Fermi energy of the stoichiometric Sr3SnO, where the mixing of the valence p and conduction d orbitals are considered to be small.

Design of ultra compact polarization splitter based on complete photonic band gap

NASA Astrophysics Data System (ADS)

Sinha, R. K.; Nagpal, Yogita

2005-11-01

Certain select structures in photonic crystals (PhCs) exhibit complete photonic band gap i.e. a frequency region where the photonic band gaps for both polarizations (i.e. transverse electric and transverse magnetic modes) exist and overlap. One of the most fundamental applications of the photonic band gap structures is the design of photonic crystal waveguides, which can be made by inserting linear defects in the photonic crystal structures. By setting closely two parallel 2D PhC waveguides, a directional waveguide coupler can be designed, which can be used to design a polarization splitter. In this paper we design a polarization splitter in a photonic crystal structure composed of two dimensional honeycomb pattern of dielectric rods in air. This photonic crystal structure exhibits a complete photonic band gap that extends from λ = 1.49 μm to λ = 1.61 μm, where lambda is the wavelength in free space, providing a large bandwidth of 120 nm. A polarization splitter can be made by designing a polarization selective coupler. The coupling lengths at various wavelengths for both polarizations have been calculated using the Finite Difference Time Domain method. It has been shown that the coupling length, for TE polarization is much smaller as compared to that for the TM polarization. This principle is used to design a polarization splitter of length 32 μm at λ = 1.55 μm. Further, the spectral response of the extinction ratios for both polarizations in the two waveguides at propagation distance of 32 μm has been studied.

Size-induced axial band structure and directional flow of a ternary-size granular material in a 3-D horizontal rotating drum

NASA Astrophysics Data System (ADS)

Yang, Shiliang; Sun, Yuhao; Ma, Honghe; Chew, Jia Wei

2018-05-01

Differences in the material property of the granular material induce segregation which inevitably influences both natural and industrial processes. To understand the dynamical segregation behavior, the band structure, and also the spatial redistribution of particles induced by the size differences of the particles, a ternary-size granular mixture in a three-dimensional rotating drum operating in the rolling flow regime is numerically simulated using the discrete element method. The results demonstrate that (i) the axial bands of the medium particles are spatially sandwiched in between those of the large and small ones; (ii) the total mass in the active and passive regions is a global parameter independent of segregation; (iii) nearly one-third of all the particles are in the active region, with the small particles having the highest mass fraction; (iv) the axial bands initially appear near the end wall, then become wider and purer in the particular species with time as more axial bands form toward the axial center; and (v) the medium particle type exhibits segregation later and has the narrowest axial bandwidth and least purity in the bands. Compared to the binary-size system, the presence of the medium particle type slightly increases the total mass in the active region, leads to larger mass fractions of the small and large particle types in the active region, and enhances the axial segregation in the system. The results obtained in the current work provide valuable insights regarding size segregation, and band structure and formation in the rotating drum with polydisperse particles.

Structuring β-Ga2O3 photonic crystal photocatalyst for efficient degradation of organic pollutants.

PubMed

Li, Xiaofang; Zhen, Xiuzheng; Meng, Sugang; Xian, Jiangjun; Shao, Yu; Fu, Xianzhi; Li, Danzhen

2013-09-03

Coupling photocatalysts with photonic crystals structure is based on the unique property of photonic crystals in confining, controlling, and manipulating the incident photons. This combination enhances the light absorption in photocatalysts and thus greatly improves their photocatalytic performance. In this study, Ga2O3 photonic crystals with well-arranged skeleton structures were prepared via a dip-coating infiltration method. The positions of the electronic band absorption for Ga2O3 photonic crystals could be made to locate on the red edge, on the blue edge, and away from the edge of their photonic band gaps by changing the pore sizes of the samples, respectively. Particularly, the electronic band absorption of the Ga2O3 photonic crystal with a pore size of 135 nm was enhanced more than other samples by making it locate on the red edge of its photonic band gap, which was confirmed by the higher instantaneous photocurrent and photocatalytic activity for the degradation of various organic pollutants under ultraviolet light irradiation. Furthermore, the degradation mechanism over Ga2O3 photonic crystals was discussed. The design of Ga2O3 photonic crystals presents a prospective application of photonic crystals in photocatalysis to address light harvesting and quantum efficiency problems through manipulating photons or constructing photonic crystal structure as groundwork.

Multi-channel unidirectional transmission of phononic crystal heterojunctions

NASA Astrophysics Data System (ADS)

Xu, Zhenlong; Tong, Jie; Wu, Fugen

2018-02-01

Two square steel columns are arranged in air to form two-dimensional square lattice phononic crystals (PNCs). Two PNCs can be combined into a non-orthogonal 45∘ heterojunction when the difference in the directional band gaps of the two PNC types is utilized. The finite element method is used to calculate the acoustic band structure, the heterogeneous junction transmission characteristics, acoustic field distribution, and many others. Results show that a non-orthogonal PNC heterojunction can produce a multi-channel unidirectional transmission of acoustic waves. With the square scatterer rotated, the heterojunction can select a frequency band for unidirectional transmission performance. This capability is particularly useful for constructing acoustic diodes with wide-bands and high-efficiency unidirectional transmission characteristics.

Electronic structure of LiCoO2 thin films: A combined photoemission spectroscopy and density functional theory study

NASA Astrophysics Data System (ADS)

Ensling, David; Thissen, Andreas; Laubach, Stefan; Schmidt, Peter C.; Jaegermann, Wolfram

2010-11-01

The electronic properties of LiCoO2 have been studied by theoretical band-structure calculations (using density functional theory) and experimental methods (photoemission). Synchrotron-induced photoelectron spectroscopy, resonant photoemission spectroscopy (ResPES), and soft x-ray absorption (XAS) have been applied to investigate the electronic structure of both occupied and unoccupied states. High-quality PES spectra were obtained from stoichiometric and highly crystalline LiCoO2 thin films deposited “in situ” by rf magnetron sputtering. An experimental approach of separating oxygen- and cobalt-derived (final) states by ResPES in the valence-band region is presented. The procedure takes advantage of an antiresonant behavior of cobalt-derived states at the 3p-3d excitation threshold. Information about the unoccupied density of states has been obtained by OK XAS. The structure of the CoL absorption edge is compared to semiempirical charge-transfer multiplet calculations. The experimental results are furthermore compared with band-structure calculations considering three different exchange potentials [generalized gradient approximation (GGA), using a nonlocal Hubbard U (GGA+U) and using a hybrid functional (Becke, three-parameter, Lee-Yang-Parr [B3LYP])]. For these different approaches total density of states and partial valence-band density of states have been investigated. The best qualitative agreement with experimental results has been obtained by using a GGA+U functional with U=2.9eV .

Automated preparation method for colloidal crystal arrays of monodisperse and binary colloid mixtures by contact printing with a pintool plotter.

PubMed

Burkert, Klaus; Neumann, Thomas; Wang, Jianjun; Jonas, Ulrich; Knoll, Wolfgang; Ottleben, Holger

2007-03-13

Photonic crystals and photonic band gap materials with periodic variation of the dielectric constant in the submicrometer range exhibit unique optical properties such as opalescence, optical stop bands, and photonic band gaps. As such, they represent attractive materials for the active elements in sensor arrays. Colloidal crystals, which are 3D gratings leading to Bragg diffraction, are one potential precursor of such optical materials. They have gained particular interest in many technological areas as a result of their specific properties and ease of fabrication. Although basic techniques for the preparation of regular patterns of colloidal crystals on structured substrates by self-assembly of mesoscopic particles are known, the efficient fabrication of colloidal crystal arrays by simple contact printing has not yet been reported. In this article, we present a spotting technique used to produce a microarray comprising up to 9600 single addressable sensor fields of colloidal crystal structures with dimensions down to 100 mum on a microfabricated substrate in different formats. Both monodisperse colloidal crystals and binary colloidal crystal systems were prepared by contact printing of polystyrene particles in aqueous suspension. The array morphology was characterized by optical light microscopy and scanning electron microscopy, which revealed regularly ordered crystalline structures for both systems. In the case of binary crystals, the influence of the concentration ratio of the large and small particles in the printing suspension on the obtained crystal structure was investigated. The optical properties of the colloidal crystal arrays were characterized by reflection spectroscopy. To examine the stop bands of the colloidal crystal arrays in a high-throughput fashion, an optical setup based on a CCD camera was realized that allowed the simultaneous readout of all of the reflection spectra of several thousand sensor fields per array in parallel. In agreement with Bragg's relation, the investigated arrays exhibited strong opalescence and stop bands in the expected wavelength range, confirming the successful formation of highly ordered colloidal crystals. Furthermore, a narrow distribution of wavelength-dependent stop bands across the sensor array was achieved, demonstrating the capability of producing highly reproducible crystal spots by the contact printing method with a pintool plotter.

Electronic properties of ZnPSe3-MoS2 Van der Waals heterostructure

NASA Astrophysics Data System (ADS)

Sharma, Munish; Kumar, Ashok; Ahluwalia, P. K.

2018-04-01

We present a comparative study of electronic properties of ZnPSe3-MoS2 heterostructure using GGA-PBE functional and DFT-D2 method within the framework of density functional theory (DFT). Electronic band structure for the considered heterostructure shows a direct band gap semiconducting character. A decrease in band gap is observed with the heterostructuring as compared to their constituent pristine monolayers. The alignment of valance band maxima and conduction band minima on different layers in heterostructure indicate the physical separation of charge carriers. A work function of 5.31 eV has been calculated for ZnPSe3-MoS2 heterostructure. These results provide a physical basis for the potential applications of these ZnPSe3-MoS2 heterostructure in optoelectronic devices.

Inverse Band Structure Design via Materials Database Screening: Application to Square Planar Thermoelectrics

DOE PAGES

Isaacs, Eric B.; Wolverton, Chris

2018-02-26

Electronic band structure contains a wealth of information on the electronic properties of a solid and is routinely computed. However, the more difficult problem of designing a solid with a desired band structure is an outstanding challenge. In order to address this inverse band structure design problem, we devise an approach using materials database screening with materials attributes based on the constituent elements, nominal electron count, crystal structure, and thermodynamics. Our strategy is tested in the context of thermoelectric materials, for which a targeted band structure containing both flat and dispersive components with respect to crystal momentum is highly desirable.more » We screen for thermodynamically stable or metastable compounds containing d 8 transition metals coordinated by anions in a square planar geometry in order to mimic the properties of recently identified oxide thermoelectrics with such a band structure. In doing so, we identify 157 compounds out of a total of over half a million candidates. After further screening based on electronic band gap and structural anisotropy, we explicitly compute the band structures for the several of the candidates in order to validate the approach. We successfully find two new oxide systems that achieve the targeted band structure. Electronic transport calculations on these two compounds, Ba 2PdO 3 and La 4PdO 7, confirm promising thermoelectric power factor behavior for the compounds. This methodology is easily adapted to other targeted band structures and should be widely applicable to a variety of design problems.« less

Inverse Band Structure Design via Materials Database Screening: Application to Square Planar Thermoelectrics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Isaacs, Eric B.; Wolverton, Chris

Electronic band structure contains a wealth of information on the electronic properties of a solid and is routinely computed. However, the more difficult problem of designing a solid with a desired band structure is an outstanding challenge. In order to address this inverse band structure design problem, we devise an approach using materials database screening with materials attributes based on the constituent elements, nominal electron count, crystal structure, and thermodynamics. Our strategy is tested in the context of thermoelectric materials, for which a targeted band structure containing both flat and dispersive components with respect to crystal momentum is highly desirable.more » We screen for thermodynamically stable or metastable compounds containing d 8 transition metals coordinated by anions in a square planar geometry in order to mimic the properties of recently identified oxide thermoelectrics with such a band structure. In doing so, we identify 157 compounds out of a total of over half a million candidates. After further screening based on electronic band gap and structural anisotropy, we explicitly compute the band structures for the several of the candidates in order to validate the approach. We successfully find two new oxide systems that achieve the targeted band structure. Electronic transport calculations on these two compounds, Ba 2PdO 3 and La 4PdO 7, confirm promising thermoelectric power factor behavior for the compounds. This methodology is easily adapted to other targeted band structures and should be widely applicable to a variety of design problems.« less

Comparative study of Ni and Cu doped ZnO nanoparticles: Structural and optical properties

NASA Astrophysics Data System (ADS)

Thakur, Shaveta; Thakur, Samita; Sharma, Jyoti; Kumar, Sanjay

2018-05-01

Nanoparticles of undoped and doped (0.1 M Ni2+ and Cu2+) ZnO are synthesized using chemical precipitation method. The crystallite size, morphology, chemical bonding and optical properties of as prepared nanoparticles are determined by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and UV-visible spectra. XRD analysis shows that the prepared samples are single phase and have hexagonal wurtzite structure. The crystallite size of the doped and undoped nanoparticles is determined using Scherrer method. The crystallite size is found to be increased with concentration of nickel and copper. All stretching and vibrational bands are observed at their specific positions through FTIR. The increase in band gap can be attributed to the different chemical nature of dopant and host cation.

First principles study of pressure induced polymorphic phase transition in KNO3

NASA Astrophysics Data System (ADS)

Yedukondalu, N.; Vaitheeswaran, G.

2015-06-01

We report the structural, elastic, electronic, and vibrational properties of polymorphic phases II and III of KNO3 based on density functional theory (DFT). Using semi-empirical dispersion correction (DFT-D2) method, we predicted the correct thermodynamic ground state of KNO3 and the obtained ground state properties of the polymorphs are in good agreement with the experiments. We further used this method to calculate the elastic constants, IR and Raman spectra, vibrational frequencies and their assignment of these polymorphs. The calculated Tran Blaha-modified Becke Johnson (TB-mBJ) electronic structure shows that both the polymorphic phases are direct band gap insulators with mixed ionic and covalent bonding. Also the TB-mBJ band gaps are improved over standard DFT functionals which are comparable with the available experiments.

Border Structure of Intercalary Heterochromatin Bands of Drosophila melanogaster Polytene Chromosomes.

PubMed

Khoroshko, V A; Zykova, T Yu; Popova, O O; Zhimulev, I F

2018-03-01

The precise genomic localization of the borders of 62 intercalary heterochromatin bands in Drosophila polytene chromosomes was determined. A new type of bands containing chromatin of different states was identified. This type is a combination of the gray band and the intercalary heterochromatin band, creating a genetic structure that with a light microscope is identified as a continuous band. The border structure of such bands includes the coding regions of genes with ubiquitous activity.

Synthesis, structural, electronic and linear electro-optical features of new quaternary Ag{sub 2}Ga{sub 2}SiS{sub 6} compound

DOE Office of Scientific and Technical Information (OSTI.GOV)

Piasecki, M., E-mail: m.piasecki@ajd.czest.pl; Myronchuk, G.L.; Parasyuk, O.V.

For the first time phase equilibria and phase diagram of the AgGaS{sub 2}–SiS{sub 2} system were successfully explored by differential thermal and X-ray phase analysis methods. Crystal structure of low-temperature (LT) modification of Ag{sub 2}Ga{sub 2}SiS{sub 6} (LРў- Ag{sub 2}Ga{sub 2}SiS{sub 6}) was studied by X-ray powder method and it belongs to tetragonal space group I-42d, with unit cell parameters a=5.7164(4) Å, c=9.8023(7) Å, V=320.32(7) Å{sup 3}. Additional details regarding the crystal structure exploration are available at the web page Fachinformationszentrum Karlsruhe. X-ray photoelectron core-level and valence-band spectra were measured for pristine LРў- Ag{sub 2}Ga{sub 2}SiS{sub 6} crystal surface. Inmore » addition, the X-ray photoelectron valence-band spectrum of LРў-Ag{sub 2}Ga{sub 2}SiS{sub 6} was matched on a common energy scale with the X-ray emission S Kβ{sub 1,3} and Ga Kβ{sub 2} bands, which give information on the energy distribution of the S 3p and Ga 4p states, respectively. The presented X-ray spectroscopy results indicate that the valence S p and Ga p atomic states contribute mainly to the upper and central parts of the valence band of LРў-Ag{sub 2}Ga{sub 2}SiS{sub 6}, respectively, with a less significant contribution also to other valence-band regions. Band gap energy was estimated by measuring the quantum energy in the spectral range of the fundamental absorption. We have found that energy gap Eg is equal to 2.35 eV at 300 K. LT-Ag{sub 2}Ga{sub 2}SiS{sub 6} is a photosensitive material and reveals two spectral maxima on the curve of spectral photoconductivity spectra at λ{sub max1} =590 nm and λ{sub max2} =860 nm. Additionally, linear electro-optical effect of LT-Ag{sub 2}Ga{sub 2}SiS{sub 6} for the wavelengths of a cw He-Ne laser at 1150 nm was explored. - Graphical abstract: Manuscript present the technology of growth and investigation of properties a new quaternary compound Ag{sub 2}Ga{sub 2}SiS{sub 6} including the detailed study of the phase equilibria in the AgGaS{sub 2}–SiS{sub 2} system to determine the physico-chemical conditions of its formation. We examined crystal structure of the compound by X-ray powder method, its electronic structure by XPS method. Finally we have found these crystals like promising material for nonlinear optical, electrooptical and piezoelectric applications. - Highlights: • Phase diagram of the AgGaS{sub 2}–SiS{sub 2} system was successfully described at first time. • Crystal Ag{sub 2}Ga{sub 2}SiS{sub 6} was grown and its structure was determined. • Electronic structure and chemical bonding of the Ag{sub 2}Ga{sub 2}SiS{sub 6} crystal were examined by. • X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES) methods. • Optical and photoelectrical properties were investigated.« less

Wavefunction Properties and Electronic Band Structures of High-Mobility Semiconductor Nanosheet MoS2

NASA Astrophysics Data System (ADS)

Baik, Seung Su; Lee, Hee Sung; Im, Seongil; Choi, Hyoung Joon; Ccsaemp Team; Edl Team

2014-03-01

Molybdenum disulfide (MoS2) nanosheet is regarded as one of the most promising alternatives to the current semiconductors due to its significant band-gap and electron-mobility enhancement upon exfoliating. To elucidate such thickness-dependent properties, we have studied the electronic band structures of bulk and monolayer MoS2 by using the first-principles density-functional method as implemented in the SIESTA code. Based on the wavefunction analyses at the conduction band minimum (CBM) points, we have investigated possible origins of mobility difference between bulk and monolayer MoS2. We provide formation energies of substitutional impurities at the Mo and S sites, and discuss feasible electron sources which may induce a significant difference in the carrier lifetime. This work was supported by NRF of Korea (Grant Nos. 2009-0079462 and 2011-0018306), Nano-Material Technology Development Program (2012M3a7B4034985), and KISTI supercomputing center (Project No. KSC-2013-C3-008). Center for Computational Studies of Advanced Electronic Material Properties.

Hybrid density functional theory band structure engineering in hematite

NASA Astrophysics Data System (ADS)

Pozun, Zachary D.; Henkelman, Graeme

2011-06-01

We present a hybrid density functional theory (DFT) study of doping effects in α-Fe2O3, hematite. Standard DFT underestimates the band gap by roughly 75% and incorrectly identifies hematite as a Mott-Hubbard insulator. Hybrid DFT accurately predicts the proper structural, magnetic, and electronic properties of hematite and, unlike the DFT+U method, does not contain d-electron specific empirical parameters. We find that using a screened functional that smoothly transitions from 12% exact exchange at short ranges to standard DFT at long range accurately reproduces the experimental band gap and other material properties. We then show that the antiferromagnetic symmetry in the pure α-Fe2O3 crystal is broken by all dopants and that the ligand field theory correctly predicts local magnetic moments on the dopants. We characterize the resulting band gaps for hematite doped by transition metals and the p-block post-transition metals. The specific case of Pd doping is investigated in order to correlate calculated doping energies and optical properties with experimentally observed photocatalytic behavior.

Strong Correlation and Topological States in Orbital-Active Dirac Materials

NASA Astrophysics Data System (ADS)

Xu, Shenglong; Wu, Congjun

Two dimensional Dirac materials, starting with graphene, have drawn tremendous research interests in the past decade. Instead of focusing on the pz orbital as in graphene, we go a step further and study its two orbitals counterpart, namely the px and py orbitals on a honeycomb lattice. The model applies to both optical lattices and several solid state systems including organic material, fluoridated tin film, BiX/SBX (X=H.F.CI.Br). In the band structure, besides the well known Dirac points in the graphene band structure, the orbital degrees of freedom give rise to flat bands as well as quadratic band touching points. These new features provide an even wider playground for searching exotic states of matter. With help of mean field theory and functional renormalization group (FRG) method, we explore the effects of interaction on the system and investigate the consequential interesting states such as ferromagnetism, Wigner crystallization, quantum anomalous Hall states and f-wave superconductivity.

All-dielectric band stop filter at terahertz frequencies

NASA Astrophysics Data System (ADS)

Yin, Shan; Chen, Lin

2018-01-01

We design all-dielectric band stop filters with silicon subwavelength rod and block arrays at terahertz frequencies. Supporting magnetic dipole resonances originated from the Mia resonance, the all-dielectric filters can modulate the working band by simply varying the structural geometry, while eliminating the ohmic loss induced by the traditional metallic metamaterials and uninvolved with the complicated mechanism. The nature of the resonance in the silicon arrays is clarified, which is attributed to the destructive interference between the directly transmitted waves and the waves emitted from the magnetic dipole resonances, and the resonance frequency is determined by the dielectric structure. By particularly designing the geometrical parameters, the profile of the transmission spectrum can be tailored, and the step-like band edge can be obtained. The all-dielectric filters can realize 93% modulation of the transmission within 0.04 THz, and maintain the bandwidth of 0.05 THz. This work provides a method to develop THz functional devices, such as filters, switches and sensors.

Theoretical prediction of the band offsets at the ZnO/anatase TiO2 and GaN/ZnO heterojunctions using the self-consistent ab initio DFT/GGA-1/2 method.

PubMed

Fang, D Q; Zhang, S L

2016-01-07

The band offsets of the ZnO/anatase TiO2 and GaN/ZnO heterojunctions are calculated using the density functional theory/generalized gradient approximation (DFT/GGA)-1/2 method, which takes into account the self-energy corrections and can give an approximate description to the quasiparticle characteristics of the electronic structure of semiconductors. We present the results of the ionization potential (IP)-based and interfacial offset-based band alignments. In the interfacial offset-based band alignment, to get the natural band offset, we use the surface calculations to estimate the change of reference level due to the interfacial strain. Based on the interface models and GGA-1/2 calculations, we find that the valence band maximum and conduction band minimum of ZnO, respectively, lie 0.64 eV and 0.57 eV above those of anatase TiO2, while lie 0.84 eV and 1.09 eV below those of GaN, which agree well with the experimental data. However, a large discrepancy exists between the IP-based band offset and the calculated natural band offset, the mechanism of which is discussed. Our results clarify band alignment of the ZnO/anatase TiO2 heterojunction and show good agreement with the GW calculations for the GaN/ZnO heterojunction.

Multispectral and Panchromatic used Enhancement Resolution and Study Effective Enhancement on Supervised and Unsupervised Classification Land – Cover

NASA Astrophysics Data System (ADS)

Salman, S. S.; Abbas, W. A.

2018-05-01

The goal of the study is to support analysis Enhancement of Resolution and study effect on classification methods on bands spectral information of specific and quantitative approaches. In this study introduce a method to enhancement resolution Landsat 8 of combining the bands spectral of 30 meters resolution with panchromatic band 8 of 15 meters resolution, because of importance multispectral imagery to extracting land - cover. Classification methods used in this study to classify several lands -covers recorded from OLI- 8 imagery. Two methods of Data mining can be classified as either supervised or unsupervised. In supervised methods, there is a particular predefined target, that means the algorithm learn which values of the target are associated with which values of the predictor sample. K-nearest neighbors and maximum likelihood algorithms examine in this work as supervised methods. In other hand, no sample identified as target in unsupervised methods, the algorithm of data extraction searches for structure and patterns between all the variables, represented by Fuzzy C-mean clustering method as one of the unsupervised methods, NDVI vegetation index used to compare the results of classification method, the percent of dense vegetation in maximum likelihood method give a best results.

Localized diabatization applied to excitons in molecular crystals

NASA Astrophysics Data System (ADS)

Jin, Zuxin; Subotnik, Joseph E.

2017-06-01

Traditional ab initio electronic structure calculations of periodic systems yield delocalized eigenstates that should be understood as adiabatic states. For example, excitons are bands of extended states which superimpose localized excitations on every lattice site. However, in general, in order to study the effects of nuclear motion on exciton transport, it is standard to work with a localized description of excitons, especially in a hopping regime; even in a band regime, a localized description can be helpful. To extract localized excitons from a band requires essentially a diabatization procedure. In this paper, three distinct methods are proposed for such localized diabatization: (i) a simple projection method, (ii) a more general Pipek-Mezey localization scheme, and (iii) a variant of Boys diabatization. Approaches (i) and (ii) require localized, single-particle Wannier orbitals, while approach (iii) has no such dependence. These methods should be very useful for studying energy transfer through solids with ab initio calculations.

[TLC-SERS study on evodiamine in evodia rutaecarpa].

PubMed

Zhang, Jin-zhi; Wang, Yuan; Chen, Hui; Shao, Hui-bo

2007-05-01

A new method for analyzing the ingredients of evodiamine (EV), rutaecarpine (RU), hydroxyevodiamine (HYD), evodiamide (ED), dihydrorutaecarpine (DRU) and 14-formyldihydrorutaecarpine (FDRU) in evodia rutaecarpa using high performance thin layer chromatography (TLC) and surface enhanced Raman spectroscopy (SERS) technique is reported. The character of this method is that standard samples are not needed. The results show that the characteristic spectral bands of EV, RU, HYD, and ED can be obtained from the TLC spot with microgramme of sample. The spectral band at 1562 cm(-1) was obtained with great enhancement. Molecule absorbed in surface silver sol by nr electrons in ring. The spectral bands of EV, RU, HYD and ED are obviously different due to their differences in structure. The TLC and SERS techniques standard samples are a convenient and speedy method to analyze chemical ingredients with high sensitivity for the study of the Chinese traditional medicine.

Doping of Semiconducting Atomic Chains

NASA Technical Reports Server (NTRS)

Toshishige, Yamada; Kutler, Paul (Technical Monitor)

1997-01-01

Due to the rapid progress in atom manipulation technology, atomic chain electronics would not be a dream, where foreign atoms are placed on a substrate to form a chain, and its electronic properties are designed by controlling the lattice constant d. It has been shown theoretically that a Si atomic chain is metallic regardless of d and that a Mg atomic chain is semiconducting or insulating with a band gap modified with d. For electronic applications, it is essential to establish a method to dope a semiconducting chain, which is to control the Fermi energy position without altering the original band structure. If we replace some of the chain atoms with dopant atoms randomly, the electrons will see random potential along the chain and will be localized strongly in space (Anderson localization). However, if we replace periodically, although the electrons can spread over the chain, there will generally appear new bands and band gaps reflecting the new periodicity of dopant atoms. This will change the original band structure significantly. In order to overcome this dilemma, we may place a dopant atom beside the chain at every N lattice periods (N > 1). Because of the periodic arrangement of dopant atoms, we can avoid the unwanted Anderson localization. Moreover, since the dopant atoms do not constitute the chain, the overlap interaction between them is minimized, and the band structure modification can be made smallest. Some tight-binding results will be discussed to demonstrate the present idea.

The electronic structure of RbTiOPO4 and the effects of the A-site cation substitution in KTiOPO4-family crystals

NASA Astrophysics Data System (ADS)

Atuchin, V. V.; Kesler, V. G.; Meng, Guangsi; Lin, Z. S.

2012-10-01

The electronic structure of RbTiOPO4 has been investigated with x-ray photoemission spectroscopy. Detailed photoemission spectra of the element core levels have been recorded under excitation by nonmonochromatic Al Kα radiation (1486.6 eV). The chemical bonding parameters are compared to those reported for complex titanates and phosphates. The band structures of KTiOPO4, RbTiOPO4, K0.535R0.465TiOPO4 and TlTiOPO4 have been calculated by ab initio methods and compared to available experimental results. It is found that the band structure of KTP-type phosphate crystals is weakly dependent on the nature of the A-site (A=K, Rb, Tl) element.

Electronic structure and magnetic properties of Pr-Co intermetallics: ab initio FP-LAPW calculations and correlation with experiments

NASA Astrophysics Data System (ADS)

Bakkari, Karim; Fersi, Riadh; Kebir Hlil, El; Bessais, Lotfi; Thabet Mliki, Najeh

2018-03-01

First-principle calculations combining density functional theory and the full-potential linearized augmented plane wave (FP-LAPW) method are performed to investigate the electronic and magnetic structure of Pr2Co7 in its two polymorphic forms, (2:7 H) and (2:7 R), for the first time. This type of calculation was also performed for PrCo5 and PrCo2 intermetallics. We have computed the valence density of states separately for spin-up and spin-down states in order to investigate the electronic band structure. This is governed by the strong contribution of the partial DOS of 3d-Co bands compared to the partial DOS of the 4f-Pr bands. Such a high ferromagnetic state is discussed in terms of the strong spin polarization observed in the total DOS. The magnetic moments carried by the Co and Pr atoms located in several sites for all compounds are computed. These results mainly indicate that cobalt atoms make a dominant contribution to the magnetic moments. The notable difference in the atomic moments of Pr and Co atoms between different structural slabs is explained in terms of the magnetic characteristics of the PrCo2 and PrCo5 compounds and the local chemical environments of the Pr and Co atoms in different structural slabs of Pr2Co7. From spin-polarized calculations we have simulated the 3d and 4f band population to estimate the local magnetic moments. These results are in accordance with the magnetic moments calculated using the FP-LAPW method. In addition, the exchange interactions J ij are calculated and used as input for M(T) simulations. Involving the data obtained from the electronic structure calculations, the appropriate Padé Table is applied to simulate the magnetization M(T) and to estimate the mean-field Curie temperature. We report a fairly good agreement between the ab initio calculation of magnetization and Curie temperature with the experimental data.

Simulation Analysis of a Strip Dipole Excited Electromagnetic Band-Gap (EBG) Structure

DTIC Science & Technology

2015-07-01

unlimited. 13. SUPPLEMENTARY NOTES 14. ABSTRACT The design of an Electromagnetic Band-Gap (EBG) for a particular antenna application is...summarized. Discussion is provided on interpretation of the EBG reflection coefficient phase for antenna applications and on the range of the EBG bandwidth...It is determined for antenna applications that the reflection phase and current design methods may be misleading. Instead, it is opinioned and shown

Normal mode analysis of isotopic shifts in Raman spectrum of TNT-d5

NASA Astrophysics Data System (ADS)

Liu, Yuemin; Tzeng, Nianfeng; Liu, Yucheng; Junk, Thomas

2017-09-01

A combined experimental-computational study was conducted on the Raman spectrum of TNT-d5 in the present study. It was found that among the 24 hybrid density functional theory (DFT) methods, O3LYP, tHCTHhyb, and B3LYP simulations yielded the strongest Raman bands which were closest to those measured from experiments. Simulations of hybrid DFT methods did not show that deuterium replacements alter orientations of 2- and 6-nitro with respect to phenyl ring, considering a larger size of the methyl group. However, the deuterium replacements apparently changed the reduced masses for all deuterium related vibrations. Although no difference of structural parameters was shown between TNT and its deuterated analogue, discrepancy was indicated in vibrational zero energy from our simulations. O3LYP simulation exhibited 24 deuterium involved vibrations, which were coupled into seven Raman bands of TNT-d5. This phenomenon can account for the experimental Raman band shifts or split of TNT-d5 when compared with the corresponding bands of TNT. The present study and its outcomes provide in-depth microchemical insights of Raman characteristics of TNT and may facilitate the design of nano-structures of SERS substrates for detection of TNT and its degradation products. All intensities displayed in this study were calculated from numerical simulations.

Structural and luminescence properties of self-yellow emitting undoped and (Ca, Ba, Sr)-doped Zn2V2O7 phosphors synthesized by combustion method

NASA Astrophysics Data System (ADS)

Foka, Kewele E.; Dejene, Birhanu F.; Koao, Lehlohonolo F.; Swart, Hendrik C.

2018-04-01

A self-activated yellow emitting Zn2V2O7 was synthesized by combustion method. The influence of the processing parameters such as synthesis temperature and dopants concentration on the structure, morphology and luminescence properties was investigated. The X-ray diffraction (XRD) analysis confirmed that the samples have a tetragonal structure and no significant structural change was observed in varying both the synthesis temperature and the dopants concentration. The estimated average crystallite size was 78 nm for the undoped samples synthesized at different temperatures and 77 nm for the doped samples. Scanning electron microscope (SEM) images showed agglomerated hexagonal-shaped particles with straight edges at low temperatures and the shape of the particles changed to cylindrical structures at moderate temperatures. At higher temperatures, the morphology changed completely. However, the morphologies of the doped samples looked alike. The photoluminescence (PL) of the product exhibited broad emission bands ranging from 400 to 800 nm. The best luminescence intensity was observed for the undoped Zn2V2O7 samples and those synthesized at 600 ℃ . Any further increase in synthesis temperature, type and concentration of dopants led to a decrease in the luminescence intensity. The broad band emission peak of Zn2V2O7 consisted of two broad bands corresponding to emissions from the Em1 (3T2→1A1) and Em2 (3T1→1A1) transitions.

Electronic transport properties of Ti-impurity band in Si

NASA Astrophysics Data System (ADS)

Olea, J.; González-Díaz, G.; Pastor, D.; Mártil, I.

2009-04-01

In this paper we show that pulsed laser melted high dose implantation of Ti in Si, above the Mott transition, produces an impurity band (IB) in this semiconductor. Using the van der Pauw method and Hall effect measurements we find strong laminated conductivity at the implanted layer and a temperature dependent decoupling between the Ti implanted layer (TIL) and the substrate. The conduction mechanism from the TIL to the substrate shows blocking characteristics that could be well explained through IB theory. Using the ATLAS code we can estimate the energetic position of the IB at 0.36 eV from the conduction band, the density of holes in this band which is closely related to the Ti atomic density and the hole mobility in this band. Band diagrams of the structure at low and high temperatures are also simulated in the ATLAS framework. The simulation obtained is fully coherent with experimental results.

Theoretical aspects of photonic band gap in 1D nano structure of LN: MgLN periodic layer

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sisodia, Namita, E-mail: namitasisodiya@gmail.com

2015-06-24

By using the transfer matrix method, we have analyzed the photonic band gap properties in a periodic layer of LN:MgLN medium. The Width of alternate layers of LN and MgLN is in the range of hundred nanometers. The birefringent and ferroelectric properties of the medium (i.e ordinary, extraordinary refractive indices and electric dipole moment) is given due considerations in the formulation of photonic band gap. Effect of electronic transition dipole moment of the medium on photonic band gap is also taken into account. We find that photonic band gap can be modified by the variation in the ratio of themore » width of two medium. We explain our findings by obtaining numerical values and the effect on the photonic band gap due to variation in the ratio of alternate medium is shown graphically.« less

Mott Transition of MnO under Pressure: A Comparison of Correlated Band Theories

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kasinathan, Deepa; Kunes, Jan; Koepernik, K

The electronic structure, magnetic moment, and volume collapse of MnO under pressure are obtained from four different correlated band theory methods; local density approximation+Hubbard U (LDA+U), pseudopotential self-interaction correction (pseudo-SIC), the hybrid functional (combined local exchange plus Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each method treats correlation among the five Mn 3d orbitals (per spin), including their hybridization with three O 2p orbitals in the valence bands and their changes with pressure. The focus is on comparison of the methods for rock salt MnO (neglecting the observed transition to the NiAs structure in the 90-100 GPamore » range). Each method predicts a first-order volume collapse, but with variation in the predicted volume and critical pressure. Accompanying the volume collapse is a moment collapse, which for all methods is from high-spin to low-spin ((5/2){yields}(1/2)), not to nonmagnetic as the simplest scenario would have. The specific manner in which the transition occurs varies considerably among the methods: pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives insulator-to-insulator and the hybrid method gives an insulator-to-semimetal transition. Projected densities of states above and below the transition are presented for each of the methods and used to analyze the character of each transition. In some cases the rhombohedral symmetry of the antiferromagnetically ordered phase clearly influences the character of the transition.« less

Design and implementation of a hybrid sub-band acoustic echo canceller (AEC)

NASA Astrophysics Data System (ADS)

Bai, Mingsian R.; Yang, Cheng-Ken; Hur, Ker-Nan

2009-04-01

An efficient method is presented for implementing an acoustic echo canceller (AEC) that makes use of hybrid sub-band approach. The hybrid system is comprised of a fixed processor and an adaptive filter in each sub-band. The AEC aims at reducing the echo resulting from the acoustic feedback in loudspeaker-enclosure-microphone (LEM) systems such as teleconferencing and hands-free systems. In order to cancel the acoustical echo efficiently, various processing architectures including fixed filters, hybrid processors, and sub-band structure are investigated. A double-talk detector is incorporated into the proposed AEC to prevent the adaptive filter from diverging in double-talk situations. A de-correlation filter is also used alongside sub-band processing in order to enhance the performance and efficiency of AEC. All algorithms are implemented and verified on the platform of a fixed-point digital signal processor (DSP). The AECs are evaluated in terms of cancellation performance and computation complexity. In addition, listening tests are conducted to assess the subjective performance of the AECs. From the results, the proposed hybrid sub-band AEC was found to be the most effective among all methods in terms of echo reduction and timbral quality.

Structural and optical properties of co-precipitated copper doped zinc oxide

NASA Astrophysics Data System (ADS)

Pandey, Devendra K.; Modi, Anchit; Pandey, Padmini; Gaur, N. K.

2018-05-01

We have synthesized pure and copper doped zinc oxide Zn1-xO:Cux (x = 0, 0.03) powder by wet chemical co-precipitation method followed by sintering of the co-precipitated amorphous phase powder at 450°C for 4 hours. The experiment is performed to recognize the effect of nominal doping of transition metal over the structural, morphological and optical properties. The structural parameters are observed by using Rietveld refinement of X-ray diffraction data which clearly represents that Cu ion is perfectly incorporated at the Zn site with minimal distortions within the lattice. The crystallite size is estimated by Debye-Scherrer and Hall-Williamson formulation. The particle morphology and size is determined with scanning electron microscopic (SEM) technique. The band gap and optical measurements are carried out with UV-visible absorption and photoluminescence (PL) spectroscopic technique, respectively. Enhanced PL spectral response is observed for ZnO:Cu along with non-radiative transitions from conduction band to valence band. The energy levels near the conduction band that are commonly involved in the optoelectronic transitions in the UV-region are traced by using absorption and luminescence spectral graphs.

Energy band gap and spectroscopic studies in Mn{sub 1-x}Cu{sub x}WO{sub 4} (0 ≤ x ≤ 0.125)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mal, Priyanath; Rambabu, P.; Turpu, G. R.

2016-05-06

A study on the effect of nonmagnetic Cu{sup 2+} substitution at Mn{sup 2+} site on the structural and energy band gap of the MnWO{sub 4} is reported. Convenient solid state reaction route has been adopted for the synthesis of Mn{sub 1-x}Cu{sub x}WO{sub 4}. X-ray diffraction (XRD) pattern showed high crystalline quality of the prepared samples. Raman spectroscopic studies were carried out to understand the structural aspects of the doping. 15 Raman active modes were identified out of 18, predicted for wolframite type monoclinic structure of MnWO{sub 4}. UV-visible diffuse reflectance spectra were recorded and analyzed to get energy band gapmore » of the studied system and are found in the range of 2.5 eV to 2.04 eV with a systematic decrease with the increase in Cu{sup 2+} concentration. Energy band gap values are verified by Density Functional Theory calculations based on projector augmented wave (PAW) method. The calculated values are in good agreement with the experimental data.« less

Band-gap engineering and comparative investigation of Ti2Nb10O29 photocatalysts obtained by Various synthetic routes

NASA Astrophysics Data System (ADS)

Xie, Meiling; Zhu, Hekai; Fang, Minghao; Huang, Zhaohui; Liu, Yan'gai; Wu, Xiaowen

2018-03-01

Ti2Nb10O29 photocatalysts were successfully synthesized by three different methods. Ti2Nb10O29 fabricated by the solvothermal method (ST-TNO) exhibited unique microspheres compared to the larger irregular particles observed for the samples annealed in air (Air-TNO) and Ar (Ar-TNO). X-ray Photoelectron Spectroscopy (XPS) results revealed that a partial reduction process from Ti4+ into Ti3+ occurs in Ar-TNO, because of the introduction of oxygen defects. Ar-TNO exhibited visible-light absorption with a band gap of 2.85 eV, while the absorption edges of Air-TNO and ST-TNO were approximately 400 nm. Under UV light irradiation (λ < 420 nm), Ar-TNO exhibited a photocatalytic activity 2.1 times greater than that of Air-TNO, corresponding to the highest activity. The results indicated that the preparation method is crucial for determining the band gap and photocatalytic activity of semiconductors. Moreover, the novel semiconductor photocatalyst can be further applied for constructing the heterojunction and designing the band structure.

Tight-binding analysis of Si and GaAs ultrathin bodies with subatomic wave-function resolution

NASA Astrophysics Data System (ADS)

Tan, Yaohua P.; Povolotskyi, Michael; Kubis, Tillmann; Boykin, Timothy B.; Klimeck, Gerhard

2015-08-01

Empirical tight-binding (ETB) methods are widely used in atomistic device simulations. Traditional ways of generating the ETB parameters rely on direct fitting to bulk experiments or theoretical electronic bands. However, ETB calculations based on existing parameters lead to unphysical results in ultrasmall structures like the As-terminated GaAs ultrathin bodies (UTBs). In this work, it is shown that more transferable ETB parameters with a short interaction range can be obtained by a process of mapping ab initio bands and wave functions to ETB models. This process enables the calibration of not only the ETB energy bands but also the ETB wave functions with corresponding ab initio calculations. Based on the mapping process, ETB models of Si and GaAs are parameterized with respect to hybrid functional calculations. Highly localized ETB basis functions are obtained. Both the ETB energy bands and wave functions with subatomic resolution of UTBs show good agreement with the corresponding hybrid functional calculations. The ETB methods can then be used to explain realistically extended devices in nonequilibrium that cannot be tackled with ab initio methods.

First-principle calculations of structural, electronic, optical, elastic and thermal properties of MgXAs2 (X=Si, Ge) compounds

NASA Astrophysics Data System (ADS)

Cheddadi, S.; Boubendira, K.; Meradji, H.; Ghemid, S.; Hassan, F. El Haj; Lakel, S.; Khenata, R.

2017-12-01

First-principle calculations on the structural, electronic, optical, elastic and thermal properties of the chalcopyrite MgXAs2 (X=Si, Ge) have been performed within the density functional theory (DFT) using the full-potential linearized augmented plane wave (FP-LAPW) method. The obtained equilibrium structural parameters are in good agreement with the available experimental data and theoretical results. The calculated band structures reveal a direct energy band gap for the interested compounds. The predicted band gaps using the modified Becke-Johnson (mBJ) exchange approximation are in fairly good agreement with the experimental data. The optical constants such as the dielectric function, refractive index, and the extinction coefficient are calculated and analysed. The independent elastic parameters namely, C_{11}, C_{12}, C_{13}, C_{33}, C_{44} and C_{66 } are evaluated. The effects of temperature and pressure on some macroscopic properties of MgSiAs2 and MgGeAs2 are predicted using the quasiharmonic Debye model in which the lattice vibrations are taken into account.

Band structures in a two-dimensional phononic crystal with rotational multiple scatterers

NASA Astrophysics Data System (ADS)

Song, Ailing; Wang, Xiaopeng; Chen, Tianning; Wan, Lele

2017-03-01

In this paper, the acoustic wave propagation in a two-dimensional phononic crystal composed of rotational multiple scatterers is investigated. The dispersion relationships, the transmission spectra and the acoustic modes are calculated by using finite element method. In contrast to the system composed of square tubes, there exist a low-frequency resonant bandgap and two wide Bragg bandgaps in the proposed structure, and the transmission spectra coincide with band structures. Specially, the first bandgap is based on locally resonant mechanism, and the simulation results agree well with the results of electrical circuit analogy. Additionally, increasing the rotation angle can remarkably influence the band structures due to the transfer of sound pressure between the internal and external cavities in low-order modes, and the redistribution of sound pressure in high-order modes. Wider bandgaps are obtained in arrays composed of finite unit cells with different rotation angles. The analysis results provide a good reference for tuning and obtaining wide bandgaps, and hence exploring the potential applications of the proposed phononic crystal in low-frequency noise insulation.

Dislocation blocking by AlGaN hot electron injecting layer in the epitaxial growth of GaN terahertz Gunn diode

NASA Astrophysics Data System (ADS)

Li, Liang; Yang, Lin'an; Zhang, Jincheng; Hao, Yue

2013-09-01

This paper reports an efficient method to improve the crystal quality of GaN Gunn diode with AlGaN hot electron injecting layer (HEI). An evident reduction of screw dislocation and edge dislocation densities is achieved by the strain management and the enhanced lateral growth in high temperature grown AlGaN HEI layer. Compared with the top hot electron injecting layer (THEI) structure, the bottom hot electron injecting layer (BHEI) structure enhances the crystal quality of transit region due to the growth sequence modulation of HEI layer. A high Hall mobility of 2934 cm2/Vs at 77 K, a nearly flat downtrend of Hall mobility at the temperature ranging from 300 to 573 K, a low intensity of ratio of yellow luminescence band to band edge emission, a narrow band edge emission line-width, and a smooth surface morphology are observed for the BHEI structural epitaxy of Gunn diode, which indicates that AlGaN BHEI structure is a promising candidate for fabrication of GaN Gunn diodes in terahertz regime.

Designing Semiconductor Heterostructures Using Digitally Accessible Electronic-Structure Data

NASA Astrophysics Data System (ADS)

Shapera, Ethan; Schleife, Andre

Semiconductor sandwich structures, so-called heterojunctions, are at the heart of modern applications with tremendous societal impact: Light-emitting diodes shape the future of lighting and solar cells are promising for renewable energy. However, their computer-based design is hampered by the high cost of electronic structure techniques used to select materials based on alignment of valence and conduction bands and to evaluate excited state properties. We describe, validate, and demonstrate an open source Python framework which rapidly screens existing online databases and user-provided data to find combinations of suitable, previously fabricated materials for optoelectronic applications. The branch point energy aligns valence and conduction bands of different materials, requiring only the bulk density functional theory band structure. We train machine learning algorithms to predict the dielectric constant, electron mobility, and hole mobility with material descriptors available in online databases. Using CdSe and InP as emitting layers for LEDs and CH3NH3PbI3 and nanoparticle PbS as absorbers for solar cells, we demonstrate our broadly applicable, automated method.

Phononic Band Gaps in 2D Quadratic and 3D Cubic Cellular Structures

PubMed Central

Warmuth, Franziska; Körner, Carolin

2015-01-01

The static and dynamic mechanical behaviour of cellular materials can be designed by the architecture of the underlying unit cell. In this paper, the phononic band structure of 2D and 3D cellular structures is investigated. It is shown how the geometry of the unit cell influences the band structure and eventually leads to full band gaps. The mechanism leading to full band gaps is elucidated. Based on this knowledge, a 3D cellular structure with a broad full band gap is identified. Furthermore, the dependence of the width of the gap on the geometry parameters of the unit cell is presented. PMID:28793713

Ultra-High Gradient S-band Linac for Laboratory and Industrial Applications

NASA Astrophysics Data System (ADS)

Faillace, L.; Agustsson, R.; Dolgashev, V.; Frigola, P.; Murokh, A.; Rosenzweig, J.; Yakimenko, V.

2010-11-01

A strong demand for high gradient structures arises from the limited real estate available for linear accelerators. RadiaBeam Technologies is developing a Doubled Energy Compact Accelerator (DECA) structure: an S-band standing wave electron linac designed to operate at accelerating gradients of up to 50 MV/m. In this paper, we present the radio-frequency design of the DECA S-band accelerating structure, operating at 2.856 GHz in the π-mode. The structure design is heavily influenced by NLC collaboration experience with ultra high gradient X-band structures; S-band, however, is chosen to take advantage of commonly available high power S-band klystrons.

Phononic Band Gaps in 2D Quadratic and 3D Cubic Cellular Structures.

PubMed

Warmuth, Franziska; Körner, Carolin

2015-12-02

The static and dynamic mechanical behaviour of cellular materials can be designed by the architecture of the underlying unit cell. In this paper, the phononic band structure of 2D and 3D cellular structures is investigated. It is shown how the geometry of the unit cell influences the band structure and eventually leads to full band gaps. The mechanism leading to full band gaps is elucidated. Based on this knowledge, a 3D cellular structure with a broad full band gap is identified. Furthermore, the dependence of the width of the gap on the geometry parameters of the unit cell is presented.

4f fine-structure levels as the dominant error in the electronic structures of binary lanthanide oxides.

PubMed

Huang, Bolong

2016-04-05

The ground-state 4f fine-structure levels in the intrinsic optical transition gaps between the 2p and 5d orbitals of lanthanide sesquioxides (Ln2 O3 , Ln = La…Lu) were calculated by a two-way crossover search for the U parameters for DFT + U calculations. The original 4f-shell potential perturbation in the linear response method were reformulated within the constraint volume of the given solids. The band structures were also calculated. This method yields nearly constant optical transition gaps between Ln-5d and O-2p orbitals, with magnitudes of 5.3 to 5.5 eV. This result verifies that the error in the band structure calculations for Ln2 O3 is dominated by the inaccuracies in the predicted 4f levels in the 2p-5d transition gaps, which strongly and non-linearly depend on the on-site Hubbard U. The relationship between the 4f occupancies and Hubbard U is non-monotonic and is entirely different from that for materials with 3d or 4d orbitals, such as transition metal oxides. This new linear response DFT + U method can provide a simpler understanding of the electronic structure of Ln2 O3 and enables a quick examination of the electronic structures of lanthanide solids before hybrid functional or GW calculations. © 2015 Wiley Periodicals, Inc.

Analysis of photonic band gap in novel piezoelectric photonic crystal

NASA Astrophysics Data System (ADS)

Malar Kodi, A.; Doni Pon, V.; Joseph Wilson, K. S.

2018-03-01

The transmission properties of one-dimensional novel photonic crystal having silver-doped novel piezoelectric superlattice and air as the two constituent layers have been investigated by means of transfer matrix method. By changing the appropriate thickness of the layers and filling factor of nanocomposite system, the variation in the photonic band gap can be studied. It is found that the photonic band gap increases with the filling factor of the metal nanocomposite and with the thickness of the layer. These structures possess unique characteristics enabling one to operate as optical waveguides, selective filters, optical switches, integrated piezoelectric microactuators, etc.

Appropriate IMFs associated with cepstrum and envelope analysis for ball-bearing fault diagnosis

NASA Astrophysics Data System (ADS)

Tsao, Wen-Chang; Pan, Min-Chun

2014-03-01

The traditional envelope analysis is an effective method for the fault detection of rolling bearings. However, all the resonant frequency bands must be examined during the bearing-fault detection process. To handle the above deficiency, this paper proposes using the empirical mode decomposition (EMD) to select a proper intrinsic mode function (IMF) for the subsequent detection tools; here both envelope analysis and cepstrum analysis are employed and compared. By virtue of the band-pass filtering nature of EMD, the resonant frequency bands of structure to be measured are captured in the IMFs. As impulses arising from rolling elements striking bearing faults modulate with structure resonance, proper IMFs potentially enable to characterize fault signatures. In the study, faulty ball bearings are used to justify the proposed method, and comparisons with the traditional envelope analysis are made. Post the use of IMFs highlighting faultybearing features, the performance of using envelope analysis and cepstrum analysis to single out bearing faults is objectively compared and addressed; it is noted that generally envelope analysis offers better performance.

GW quasiparticle energy study of ternary tetradymite Bi{sub 2}Te{sub 2}Se and Bi{sub 2}Te{sub 2}S thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shuaibu, Alhassan; Department of Physics, Faculty of Science, Nigerian Defence Academy, P.M., 2109. Kaduna Nigeria; Rahman, Md. Mahmudur

In this work, we have evaluated the quasiparticle energies of ternary tetradymite Bi{sub 2}Te{sub 2}Se and Bi{sub 2}Te{sub 2}S using first-principles calculation within the G{sub 0}W{sub 0} methods. We have also performed a broad convergence tests in order to investigate the quasiparticle corrections to the structural parameters and to the semi core d electrons in both of the compounds. For each case, we have calculated the many-body corrections within a one-shot GW method of the compounds. Our results have shown that for Bi{sub 2}Te{sub 2}Se the GW corrections increase the band gap to almost 10%, and for specific atomic positions,more » the band structure shows a close value to the experimental one. For Bi{sub 2}Te{sub 2}S, despite increase in the band gap due to the GW corrections, possibility of bulk resistivity that can be significant for photovoltaic applications was observed.« less

Theoretical reconsideration of antiferromagnetic Fermi surfaces in URu2Su2

NASA Astrophysics Data System (ADS)

Yamagami, Hiroshi

2011-01-01

In an itinerant 5f-band model, the antiferromagnetic (AFM) Fermi surfaces of URu2Si2 are reconsidered using a relativistic LAPW method within a local spin-density approximation, especially taking into account the lattice parameters dependent on pressures. The reduction of the z-coordinate of the Si sites results in the effect of flattening the Ru-Si layers of URu2Si2 crystal structure, thus weakening a hybridization/mixing between the U-5f and Ru-4d states in the band structure. Consequently the 5f bands around the Fermi level are more flat in the dispersion with decreasing the z-coordinate, thus producing three closed Fermi surfaces like "curing-stone", "rugby-ball " and "ball". The origins of de Haas-van Alphen branches can be qualitatively interpreted from the obtained AFM Fermi surfaces.

Simultaneous large band gaps and localization of electromagnetic and elastic waves in defect-free quasicrystals.

PubMed

Yu, Tianbao; Wang, Zhong; Liu, Wenxing; Wang, Tongbiao; Liu, Nianhua; Liao, Qinghua

2016-04-18

We report numerically large and complete photonic and phononic band gaps that simultaneously exist in eight-fold phoxonic quasicrystals (PhXQCs). PhXQCs can possess simultaneous photonic and phononic band gaps over a wide range of geometric parameters. Abundant localized modes can be achieved in defect-free PhXQCs for all photonic and phononic polarizations. These defect-free localized modes exhibit multiform spatial distributions and can confine simultaneously electromagnetic and elastic waves in a large area, thereby providing rich selectivity and enlarging the interaction space of optical and elastic waves. The simulated results based on finite element method show that quasiperiodic structures formed of both solid rods in air and holes in solid materials can simultaneously confine and tailor electromagnetic and elastic waves; these structures showed advantages over the periodic counterparts.

Pressure-induced magneto-structural transition in iron via a modified solid-state nudged elastic band method

NASA Astrophysics Data System (ADS)

Zarkevich, Nikolai A.; Johnson, Duane D.

2015-03-01

Materials under pressure may exhibit critical electronic and structural transitions that affect equation of states, as known for superconductors and the magneto-structural transformations of iron with both geophysical and planetary implications. While experiments often use constant-pressure (diamond-anvil cell, DAC) measurements, many theoretical results address a constant-volume transitions, which avoid issues with magnetic collapse but cannot be directly compared to experiment. We establish a modified solid-state nudge elastic band (MSS-NEB) method to handle magnetic systems that may exhibit moment (and volume) collapse during transformation. We apply it to the pressure-induced transformation in iron between the low-pressure body-centered cubic (bcc) and the high-pressure hexagonal close-packed (hcp) phases, find the bcc-hcp equilibrium coexistence pressure and a transitional pathway, and compare to shock and DAC experiments. We use methods developed with support by the U.S. Department of Energy (DE-FG02-03ER46026 and DE-AC02-07CH11358). Ames Laboratory is operated for the DOE by Iowa State University under contract DE-AC02-07CH11358.

Electronic structure of lanthanide scandates

NASA Astrophysics Data System (ADS)

Mizzi, Christopher A.; Koirala, Pratik; Marks, Laurence D.

2018-02-01

X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and density functional theory calculations were used to study the electronic structure of three lanthanide scandates: GdSc O3,TbSc O3 , and DySc O3 . X-ray photoelectron spectra simulated from first-principles calculations using a combination of on-site hybrid and GGA +U methods were found to be in good agreement with experimental x-ray photoelectron spectra. The hybrid method was used to model the ground state electronic structure and the GGA +U method accounted for the shift of valence state energies due to photoelectron emission via a Slater-Janak transition state approach. From these results, the lanthanide scandate valence bands were determined to be composed of Ln 4 f ,O 2 p , and Sc 3 d states, in agreement with previous work. However, contrary to previous work the minority Ln 4 f states were found to be located closer to, and in some cases at, the valence band maximum. This suggests that minority Ln 4 f electrons may play a larger role in lanthanide scandate properties than previously thought.

Synthesis and characterization of Y2O3 nano-material: An experimental and theoretical study

NASA Astrophysics Data System (ADS)

Ahmad, Sheeraz; Faizan, Mohd; Ahmad, Shabbir; Ikram, Mohd

2018-04-01

We made an attempt to synthesize pure Y2O3 nanomaterial by using the sol-gel method followed by annealing at 600°C and 900°C. The synthesized Y2O3 nanoparticle was characterized by using XRD, FTIR, and UV-Vis spectroscopy. The structural refinement was performed using FULLPROF software by the Rietveld method. The refinement parameters such as lattice constant, atomic position, occupancy, R-factor and goodness of fit (χ2) were calculated. The nanoparticle has a single phase cubic structure with Ia -3 space group. The main absorption band in FTIR spectra centered at 560 cm-1 is attributed to Y-O vibration while the broadband at 3450 cm-1 arises due to O-H vibration. The band gap was obtained from the reflectance spectra using the K-M function F(R∞). The optimized structural parameters and UV-Vis spectrum were calculated using DFT and TD-DFT/B3LYP methods in bulk phase of Y2O3 and compared with experimental UV-Vis spectra in nanophase.

Strain distribution and band structure of InAs/GaAs quantum ring superlattice

NASA Astrophysics Data System (ADS)

Mughnetsyan, Vram; Kirakosyan, Albert

2017-12-01

The elastic strain distribution and the band structure of InAs/GaAs one-layer quantum ring superlattice with square symmetry has been considered in this work. The Green's function formalism based on the method of inclusions has been implied to calculate the components of the strain tensor, while the combination of Green's function method with the Fourier transformation to momentum space in Pikus-Bir Hamiltonian has been used for obtaining the miniband energy dispersion surfaces via the exact diagonalization procedure. The dependencies of the strain tensor components on spatial coordinates are compared with ones for single quantum ring and are in good agreement with previously obtained results for cylindrical quantum disks. It is shown that strain significantly affects the miniband structure of the superlattice and has contribution to the degeneracy lifting effect due to heavy hole-light hole coupling. The demonstrated method is simple and provides reasonable results for comparatively small Hamiltonian matrix. The obtained results may be useful for further investigation and construction of novel devices based on quantum ring superlattices.

Intrinsic properties of high-spin band structures in triaxial nuclei

NASA Astrophysics Data System (ADS)

Jehangir, S.; Bhat, G. H.; Sheikh, J. A.; Palit, R.; Ganai, P. A.

2017-12-01

The band structures of 68,70Ge, 128,130,132,134Ce and 132,134,136,138Nd are investigated using the triaxial projected shell model (TPSM) approach. These nuclei depict forking of the ground-state band into several s-bands and in some cases, both the lowest two observed s-bands depict neutron or proton character. It was discussed in our earlier work that this anomalous behaviour can be explained by considering γ-bands based on two-quasiparticle configurations. As the parent band and the γ-band built on it have the same intrinsic structure, g-factors of the two bands are expected to be similar. In the present work, we have undertaken a detailed investigation of g-factors for the excited band structures of the studied nuclei and the available data for a few high-spin states are shown to be in fair agreement with the predicted values.

Structure-activity relationship of the ionic cocrystal: 5-amino-2-naphthalene sulfonate·ammonium ions for pharmaceutical applications

NASA Astrophysics Data System (ADS)

Sangeetha, M.; Mathammal, R.

2018-02-01

The ionic cocrystals of 5-amino-2-naphthalene sulfonate · ammonium ions (ANSA-ṡNH4+) were grown under slow evaporation method and examined in detail for pharmaceutical applications. The crystal structure and intermolecular interactions were studied from the single X-ray diffraction analysis and the Hirshfeld surfaces. The 2D fingerprint plots displayed the inter-contacts possible in the ionic crystal. Computational DFT method was established to determine the structural, physical and chemical properties. The molecular geometries obtained from the X-ray studies were compared with the optimized geometrical parameters calculated using DFT/6-31 + G(d,p) method. The band gap energy calculated from the UV-Visible spectral analysis and the HOMO-LUMO energy gap are compared. The theoretical UV-Visible calculations helped in determining the type of electronic transition taking place in the title molecule. The maximum absorption bands and transitions involved in the molecule represented the drug reaction possible. Non-linear optical properties were characterized from SHG efficiency measurements experimentally and the NLO parameters are also calculated from the optimized structure. The reactive sites within the molecule are detailed from the MEP surface maps. The molecular docking studies evident the structure-activity of the ionic cocrystal for anti-cancer drug property.

Metal-like Band Structures of Ultrathin Si {111} and {112} Surface Layers Revealed through Density Functional Theory Calculations.

PubMed

Tan, Chih-Shan; Huang, Michael H

2017-09-04

Density functional theory calculations have been performed on Si (100), (110), (111), and (112) planes with tunable number of planes for evaluation of their band structures and density of states profiles. The purpose is to see whether silicon can exhibit facet-dependent properties derived from the presence of a thin surface layer having different band structures. No changes have been observed for single to multiple layers of Si (100) and (110) planes with a consistent band gap between the valence band and the conduction band. However, for 1, 2, 4, and 5 Si (111) and (112) planes, metal-like band structures were obtained with continuous density of states going from the valence band to the conduction band. For 3, 6, and more Si (111) planes, as well as 3 and 6 Si (112) planes, the same band structure as that seen for Si (100) and (110) planes has been obtained. Thus, beyond a layer thickness of five Si (111) planes at ≈1.6 nm, normal semiconductor behavior can be expected. The emergence of metal-like band structures for the Si (111) and (112) planes are related to variation in Si-Si bond length and bond distortion plus 3s and 3p orbital electron contributions in the band structure. This work predicts possession of facet-dependent electrical properties of silicon with consequences in FinFET transistor design. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Structural studies of polytene chromosomes and bone implant coatings: Raman microspectroscopy and atomic force microscopy

NASA Astrophysics Data System (ADS)

de Grauw, Kees

Raman microscopy and atomic force microscopy (AFM) are used for the investigation of the composition and structure of the banding patterns of polytene chromosomes and of hydroxyapatite bone-implant coatings. For Raman microspectroscopy two new measuring methods are introduced: line-scan Raman and Low-wavenumber Raman microspectroscopy. A transparent and easy to use model to predict the depth resolution of a confocal microscope is described. A Chevron-type of filter set was developed for simultaneous measurements of Stokes and anti-Stokes Raman scattering close to the exciting laser frequency. Bands of polytene chromosomes appeared to contain a higher concentration of DNA and proteins compared to interbands. AFM measurements revealed that bands consist of a densely packed chromatin structure and are hardly affected by stretching of the chromosome. Interbands have a more open chromatin structure and are more accessible to solvent molecules. For the study of bone implant coatings Raman micro spectroscopy appeared to provide an easy, non- destructive, way to obtain information about the apatite structure and the degree of crystallinity. It was shown that the degree of crystallinity was constant over coatings produced by plasma spraying while the material density did vary.

Electronic structure of LiGaS 2

NASA Astrophysics Data System (ADS)

Atuchin, V. V.; Isaenko, L. I.; Kesler, V. G.; Lobanov, S.; Huang, H.; Lin, Z. S.

2009-04-01

X-ray photoelectron spectroscopy (XPS) measurement has been performed to determine the valence band structure of LiGaS 2 crystals. The experimental measurement is compared with the electronic structure obtained from the density functional calculations. It is found that the Ga 3d states in the XPS spectrum are much higher than the calculated results. In order to eliminate this discrepancy, the LDA+ U method is employed and reasonable agreement is achieved. Further calculations show that the difference of the linear and nonlinear optical coefficients between LDA and LDA+ U calculations is negligibly small, indicating that the Ga 3d states are actually independent of the excited properties of LiGaS 2 crystals since they are located at a very deep position in the valence bands.

Computational study of dye adsorption onto Brookite TiO2 surfaces for the applications in dye sensitized solar cells

NASA Astrophysics Data System (ADS)

Maluta, N. E.; Dima, R. S.; Nemudzivhadi, H.; Maphanga, R. R.; Sankaran

2017-10-01

The theoretical and computational studies of dye sensitized solar cells (DSSCs) can contribute to a deeper understanding of these type of solar cells. In the current study the density functional theory (DFT) is used to understand the electronic properties of low index brookite (1 0 0) surface doped with ruthenium. The structural optimizations, band structure, and electronic density of states of doped and undoped titanium dioxide (TiO2) brookite surface was performed using the first-principles calculations based on DFT emplotying a plane-wave pseudopotential method. The generalized gradient approximation (GGA) was used in the scheme of Perdew-Burke-Ernzerhof (PBE) to describe the exchange-correlation functional. All calculations were carried out with CASTEP (Cambridge Sequential Total Energy Package) code in Materials Studio of Accelrys Inc. The two different doping methods employed in the current work are, doping by replacement and adsorption. The overlap among the Ruthenium (Ru) 3d, Titanium (Ti) 3d, and Oxygen (O) 2p states enhance photocatalytic activity in the visible light region. The adsorption method shows that an equilibrium position is reached for ruthenium element after optimization. All the methods show that the TiO2 brookite (1 0 0) surface reduces its band gap after been doped with the ruthenium element. From the two techniques used, the total energy of the doped structures show that they are energetically favorable, with the band gap being reduced to 0.263 eV compared to 2.376 eV of the pure system.

Wave propagation in axially moving periodic strings

NASA Astrophysics Data System (ADS)

Sorokin, Vladislav S.; Thomsen, Jon Juel

2017-04-01

The paper deals with analytically studying transverse waves propagation in an axially moving string with periodically modulated cross section. The structure effectively models various relevant technological systems, e.g. belts, thread lines, band saws, etc., and, in particular, roller chain drives for diesel engines by capturing both their spatial periodicity and axial motion. The Method of Varying Amplitudes is employed in the analysis. It is shown that the compound wave traveling in the axially moving periodic string comprises many components with different frequencies and wavenumbers. This is in contrast to non-moving periodic structures, for which all components of the corresponding compound wave feature the same frequency. Due to this "multi-frequency" character of the wave motion, the conventional notion of frequency band-gaps appears to be not applicable for the moving periodic strings. Thus, for such structures, by frequency band-gaps it is proposed to understand frequency ranges in which the primary component of the compound wave attenuates. Such frequency band-gaps can be present for a moving periodic string, but only if its axial velocity is lower than the transverse wave speed, and, the higher the axial velocity, the narrower the frequency band-gaps. The revealed effects could be of potential importance for applications, e.g. they indicate that due to spatial inhomogeneity, oscillations of axially moving periodic chains always involve a multitude of frequencies.

Comparing direct and iterative equation solvers in a large structural analysis software system

NASA Technical Reports Server (NTRS)

Poole, E. L.

1991-01-01

Two direct Choleski equation solvers and two iterative preconditioned conjugate gradient (PCG) equation solvers used in a large structural analysis software system are described. The two direct solvers are implementations of the Choleski method for variable-band matrix storage and sparse matrix storage. The two iterative PCG solvers include the Jacobi conjugate gradient method and an incomplete Choleski conjugate gradient method. The performance of the direct and iterative solvers is compared by solving several representative structural analysis problems. Some key factors affecting the performance of the iterative solvers relative to the direct solvers are identified.

Band registration of tuneable frame format hyperspectral UAV imagers in complex scenes

NASA Astrophysics Data System (ADS)

Honkavaara, Eija; Rosnell, Tomi; Oliveira, Raquel; Tommaselli, Antonio

2017-12-01

A recent revolution in miniaturised sensor technology has provided markets with novel hyperspectral imagers operating in the frame format principle. In the case of unmanned aerial vehicle (UAV) based remote sensing, the frame format technology is highly attractive in comparison to the commonly utilised pushbroom scanning technology, because it offers better stability and the possibility to capture stereoscopic data sets, bringing an opportunity for 3D hyperspectral object reconstruction. Tuneable filters are one of the approaches for capturing multi- or hyperspectral frame images. The individual bands are not aligned when operating a sensor based on tuneable filters from a mobile platform, such as UAV, because the full spectrum recording is carried out in the time-sequential principle. The objective of this investigation was to study the aspects of band registration of an imager based on tuneable filters and to develop a rigorous and efficient approach for band registration in complex 3D scenes, such as forests. The method first determines the orientations of selected reference bands and reconstructs the 3D scene using structure-from-motion and dense image matching technologies. The bands, without orientation, are then matched to the oriented bands accounting the 3D scene to provide exterior orientations, and afterwards, hyperspectral orthomosaics, or hyperspectral point clouds, are calculated. The uncertainty aspects of the novel approach were studied. An empirical assessment was carried out in a forested environment using hyperspectral images captured with a hyperspectral 2D frame format camera, based on a tuneable Fabry-Pérot interferometer (FPI) on board a multicopter and supported by a high spatial resolution consumer colour camera. A theoretical assessment showed that the method was capable of providing band registration accuracy better than 0.5-pixel size. The empirical assessment proved the performance and showed that, with the novel method, most parts of the band misalignments were less than the pixel size. Furthermore, it was shown that the performance of the band alignment was dependent on the spatial distance from the reference band.

Hybrid transfer-matrix FDTD method for layered periodic structures.

PubMed

Deinega, Alexei; Belousov, Sergei; Valuev, Ilya

2009-03-15

A hybrid transfer-matrix finite-difference time-domain (FDTD) method is proposed for modeling the optical properties of finite-width planar periodic structures. This method can also be applied for calculation of the photonic bands in infinite photonic crystals. We describe the procedure of evaluating the transfer-matrix elements by a special numerical FDTD simulation. The accuracy of the new method is tested by comparing computed transmission spectra of a 32-layered photonic crystal composed of spherical or ellipsoidal scatterers with the results of direct FDTD and layer-multiple-scattering calculations.

Band structures in fractal grading porous phononic crystals

NASA Astrophysics Data System (ADS)

Wang, Kai; Liu, Ying; Liang, Tianshu; Wang, Bin

2018-05-01

In this paper, a new grading porous structure is introduced based on a Sierpinski triangle routine, and wave propagation in this fractal grading porous phononic crystal is investigated. The influences of fractal hierarchy and porosity on the band structures in fractal graidng porous phononic crystals are clarified. Vibration modes of unit cell at absolute band gap edges are given to manifest formation mechanism of absolute band gaps. The results show that absolute band gaps are easy to form in fractal structures comparatively to the normal ones with the same porosity. Structures with higher fractal hierarchies benefit multiple wider absolute band gaps. This work provides useful guidance in design of fractal porous phononic crystals.

Pressure effects on band structures in dense lithium

NASA Astrophysics Data System (ADS)

Goto, Naoyuki; Nagara, Hitose

2012-07-01

We studied the change of the band structures in some structures of Li predicted at high pressures, using GGA and GW calculations. The width of the 1s band coming from the 1s electron of Li shows broadening by the pressurization, which is the normal behavior of bands at high pressure. The width of the band just below the Fermi level decreases by the pressurization, which is an opposite behavior to the normal bands. The character of this narrowing band is mostly p-like with a little s-like portion. The band gaps in some structures are really observed even by the GGA calculations. The gaps by the GW calculations increase to about 1.5 times the GGA values. Generally the one-shot GW calculation (diagonal only calculations) gives more reliable values than the GGA, but it may fail to predict band gaps for the case where band dispersion shows complex crossing near the Fermi level. There remains some structures for which GW calculations with off-diagonal elements taken into account are needed to identify the phase to be metallic or semiconducting.

Study on the Effect of Secondary Banded Structure on the Fatigue Property of Non-Quenched and Tempered Micro Alloyed Steel

NASA Astrophysics Data System (ADS)

Yajie, Cheng; Qingliang, Liao; Yue, Zhang

Due to composition segregation and cooling speed, streamline or banded structure were often obtained in the thermal forming parts along the direction of parts forming. Generally speaking, banded structure doesn't decrease the longitudinal mechanical properties, so the secondary banded structure can't get enough attention. The effect of secondary banded structure on the fatigue properties of micro alloyed DG20Mn and 35CrMo steel was investigated using the axial tensile fatigue test of stress ratio of 0.1. The result shows that secondary banded structure was obtained in the center of the steel parts, because of the composition segregation and the lower cooling rate in center part of steel. Secondary banded structure has no significant effect on axial tensile properties of both DG20Mn and 35CrMo, but decreases the axial tensile fatigue performance of DG20Mn steel. This study suggests that under the high cyclic tensile stress, multi-source damage cracks in steel initiated by large strain of pearlite of secondary banded structure, which is larger than damage strain, is the major factor of the decrease of fatigue life of steel.

Enhanced optical transmission through a star-shaped bull's eye at dual resonant-bands in UV and the visible spectral range.

PubMed

Nazari, Tavakol; Khazaeinezhad, Reza; Jung, Woohyun; Joo, Boram; Kong, Byung-Joo; Oh, Kyunghwan

2015-07-13

Dual resonant bands in UV and the visible range were simultaneously observed in the enhanced optical transmission (EOT) through star-shaped plasmonic structures. EOTs through four types of polygonal bull's eyes with a star aperture surrounded by the concentric star grooves were analyzed and compared for 3, 4, 5, and 6 corners, using finite difference time domain (FDTD) method. In contrast to plasmonic resonances in the visible range, the UV-band resonance intensity was found to scale with the number of corners, which is related with higher order multipole interactions. Spectral positions and relative intensities of the dual resonances were analyzed parametrically to find optimal conditions to maximize EOT in UV-visible dual bands.

High-Performance Anti-Retransmission Deception Jamming Utilizing Range Direction Multiple Input and Multiple Output (MIMO) Synthetic Aperture Radar (SAR).

PubMed

Wang, Ruijia; Chen, Jie; Wang, Xing; Sun, Bing

2017-01-09

Retransmission deception jamming seriously degrades the Synthetic Aperture Radar (SAR) detection efficiency and can mislead SAR image interpretation by forming false targets. In order to suppress retransmission deception jamming, this paper proposes a novel multiple input and multiple output (MIMO) SAR structure range direction MIMO SAR, whose multiple channel antennas are vertical to the azimuth. First, based on the multiple channels of range direction MIMO SAR, the orthogonal frequency division multiplexing (OFDM) linear frequency modulation (LFM) signal was adopted as the transmission signal of each channel, which is defined as a sub-band signal. This sub-band signal corresponds to the transmission channel. Then, all of the sub-band signals are modulated with random initial phases and concurrently transmitted. The signal form is more complex and difficult to intercept. Next, the echoes of the sub-band signal are utilized to synthesize a wide band signal after preprocessing. The proposed method will increase the signal to interference ratio and peak amplitude ratio of the signal to resist retransmission deception jamming. Finally, well-focused SAR imagery is obtained using a conventional imaging method where the retransmission deception jamming strength is degraded and defocused. Simulations demonstrated the effectiveness of the proposed method.

High-Performance Anti-Retransmission Deception Jamming Utilizing Range Direction Multiple Input and Multiple Output (MIMO) Synthetic Aperture Radar (SAR)

PubMed Central

Wang, Ruijia; Chen, Jie; Wang, Xing; Sun, Bing

2017-01-01

Retransmission deception jamming seriously degrades the Synthetic Aperture Radar (SAR) detection efficiency and can mislead SAR image interpretation by forming false targets. In order to suppress retransmission deception jamming, this paper proposes a novel multiple input and multiple output (MIMO) SAR structure range direction MIMO SAR, whose multiple channel antennas are vertical to the azimuth. First, based on the multiple channels of range direction MIMO SAR, the orthogonal frequency division multiplexing (OFDM) linear frequency modulation (LFM) signal was adopted as the transmission signal of each channel, which is defined as a sub-band signal. This sub-band signal corresponds to the transmission channel. Then, all of the sub-band signals are modulated with random initial phases and concurrently transmitted. The signal form is more complex and difficult to intercept. Next, the echoes of the sub-band signal are utilized to synthesize a wide band signal after preprocessing. The proposed method will increase the signal to interference ratio and peak amplitude ratio of the signal to resist retransmission deception jamming. Finally, well-focused SAR imagery is obtained using a conventional imaging method where the retransmission deception jamming strength is degraded and defocused. Simulations demonstrated the effectiveness of the proposed method. PMID:28075367

Impact of Fe doping on the electronic structure of SrTiO3 thin films determined by resonant photoemission

NASA Astrophysics Data System (ADS)

Kubacki, J.; Kajewski, D.; Goraus, J.; Szot, K.; Koehl, A.; Lenser, Ch.; Dittmann, R.; Szade, J.

2018-04-01

Epitaxial thin films of Fe doped SrTiO3 have been studied by the use of resonant photoemission. This technique allowed us to identify contributions of the Fe and Ti originating electronic states to the valence band. Two valence states of iron Fe2+ and Fe3+, detected on the base of x-ray absorption studies spectra, appeared to form quite different contributions to the valence band of SrTiO3. The electronic states within the in-gap region can be attributed to Fe and Ti ions. The Fe2+ originating states which can be connected to the presence of oxygen vacancies form a broad band reaching binding energies of about 0.5 eV below the conduction band, while Fe3+ states form in the gap a sharp feature localized just above the top of the valence band. These structures were also confirmed by calculations performed with the use of the FP-LAPW/APW+lo method including Coulomb correlations within the d shell. It has been shown that Fe doping induced Ti originating states in the energy gap which can be related to the hybridization of Ti and Fe 3d orbitals.

Electrical, structural and optical properties of tellurium thin films on silicon substrate

NASA Astrophysics Data System (ADS)

Arora, Swati; Vijay, Y. K.

2018-05-01

Tellurium (Te) thin films of various thicknesses (200nm, 275nm, 350nm & 500nm) were prepared on Silicon (Si) using thermal evaporation at vacuum of 10-5 torr. It is observed that the resistivity decreases exponentially with the Increases Temperature. A direct band gap between 0.368 eV to 0.395 eV is obtained at different temperatures with Four Probe Method which shows that when we increase the thickness of material the band gap will exponentially decreases. Samples were analysed through X-ray diffraction and atomic force microscopy to attain complete and reliable micro structural in order.

Modification of WS2 nanosheets with controllable layers via oxygen ion irradiation

NASA Astrophysics Data System (ADS)

Song, Honglian; Yu, Xiaofei; Chen, Ming; Qiao, Mei; Wang, Tiejun; Zhang, Jing; Liu, Yong; Liu, Peng; Wang, Xuelin

2018-05-01

As one kind of two-dimensional materials, WS2 nanosheets have drawn much attention with different kinds of research methods. Yet ion irradiation method was barely used for WS2 nanosheets. In this paper, the structure, composition and optical band gap (Eg) of the multilayer WS2 films deposited by chemical vapor deposition (CVD) method on sapphire substrates before and after oxygen ion irradiation with different energy and fluences were studied. Precise tailored layer-structures and a controllable optical band gap of WS2 nanosheets were achieved after oxygen ion irradiation. The results shows higher energy oxygen irradiation changed the shape from triangular shaped grains to irregular rectangle shape but did not change 2H-WS2 phase structure. The intensity of E2g1 (Г) and A1g (Г) modes decreased and have small shifts after oxygen ion irradiation. The peak frequency difference between the E2g1 (Г) and A1g (Г) modes (Δω) decreased after oxygen ion irradiation, and this result indicates the number of layers decreased after oxygen ion irradiation. The Eg decreased with the increase of the energy and the fluence of oxygen ions. The number of layers, thickness and optical band gap changed after ion irradiation with different ion fluences and energies. The results proposed a new strategy for precise control of multilayer nanosheets and demonstrated the high applicability of ion irradiation in super-capacitors, field effect transistors and other applications.

Empirical optimization of DFT  +  U and HSE for the band structure of ZnO.

PubMed

Bashyal, Keshab; Pyles, Christopher K; Afroosheh, Sajjad; Lamichhane, Aneer; Zayak, Alexey T

2018-02-14

ZnO is a well-known wide band gap semiconductor with promising potential for applications in optoelectronics, transparent electronics, and spintronics. Computational simulations based on the density functional theory (DFT) play an important role in the research of ZnO, but the standard functionals, like Perdew-Burke-Erzenhof, result in largely underestimated values of the band gap and the binding energies of the Zn 3d electrons. Methods like DFT  +  U and hybrid functionals are meant to remedy the weaknesses of plain DFT. However, both methods are not parameter-free. Direct comparison with experimental data is the best way to optimize the computational parameters. X-ray photoemission spectroscopy (XPS) is commonly considered as a benchmark for the computed electronic densities of states. In this work, both DFT  +  U and HSE methods were parametrized to fit almost exactly the binding energies of electrons in ZnO obtained by XPS. The optimized parameterizations of DFT  +  U and HSE lead to significantly worse results in reproducing the ion-clamped static dielectric tensor, compared to standard high-level calculations, including GW, which in turn yield a perfect match for the dielectric tensor. The failure of our XPS-based optimization reveals the fact that XPS does not report the ground state electronic structure for ZnO and should not be used for benchmarking ground state electronic structure calculations.

Empirical optimization of DFT  +  U and HSE for the band structure of ZnO

NASA Astrophysics Data System (ADS)

Bashyal, Keshab; Pyles, Christopher K.; Afroosheh, Sajjad; Lamichhane, Aneer; Zayak, Alexey T.

2018-02-01

ZnO is a well-known wide band gap semiconductor with promising potential for applications in optoelectronics, transparent electronics, and spintronics. Computational simulations based on the density functional theory (DFT) play an important role in the research of ZnO, but the standard functionals, like Perdew-Burke-Erzenhof, result in largely underestimated values of the band gap and the binding energies of the Zn3d electrons. Methods like DFT  +  U and hybrid functionals are meant to remedy the weaknesses of plain DFT. However, both methods are not parameter-free. Direct comparison with experimental data is the best way to optimize the computational parameters. X-ray photoemission spectroscopy (XPS) is commonly considered as a benchmark for the computed electronic densities of states. In this work, both DFT  +  U and HSE methods were parametrized to fit almost exactly the binding energies of electrons in ZnO obtained by XPS. The optimized parameterizations of DFT  +  U and HSE lead to significantly worse results in reproducing the ion-clamped static dielectric tensor, compared to standard high-level calculations, including GW, which in turn yield a perfect match for the dielectric tensor. The failure of our XPS-based optimization reveals the fact that XPS does not report the ground state electronic structure for ZnO and should not be used for benchmarking ground state electronic structure calculations.

Vibration band gaps for elastic metamaterial rods using wave finite element method

NASA Astrophysics Data System (ADS)

Nobrega, E. D.; Gautier, F.; Pelat, A.; Dos Santos, J. M. C.

2016-10-01

Band gaps in elastic metamaterial rods with spatial periodic distribution and periodically attached local resonators are investigated. New techniques to analyze metamaterial systems are using a combination of analytical or numerical method with wave propagation. One of them, called here wave spectral element method (WSEM), consists of combining the spectral element method (SEM) with Floquet-Bloch's theorem. A modern methodology called wave finite element method (WFEM), developed to calculate dynamic behavior in periodic acoustic and structural systems, utilizes a similar approach where SEM is substituted by the conventional finite element method (FEM). In this paper, it is proposed to use WFEM to calculate band gaps in elastic metamaterial rods with spatial periodic distribution and periodically attached local resonators of multi-degree-of-freedom (M-DOF). Simulated examples with band gaps generated by Bragg scattering and local resonators are calculated by WFEM and verified with WSEM, which is used as a reference method. Results are presented in the form of attenuation constant, vibration transmittance and frequency response function (FRF). For all cases, WFEM and WSEM results are in agreement, provided that the number of elements used in WFEM is sufficient to convergence. An experimental test was conducted with a real elastic metamaterial rod, manufactured with plastic in a 3D printer, without local resonance-type effect. The experimental results for the metamaterial rod with band gaps generated by Bragg scattering are compared with the simulated ones. Both numerical methods (WSEM and WFEM) can localize the band gap position and width very close to the experimental results. A hybrid approach combining WFEM with the commercial finite element software ANSYS is proposed to model complex metamaterial systems. Two examples illustrating its efficiency and accuracy to model an elastic metamaterial rod unit-cell using 1D simple rod element and 3D solid element are demonstrated and the results present good approximation to the experimental data.

Automatic allograft bone selection through band registration and its application to distal femur.

PubMed

Zhang, Yu; Qiu, Lei; Li, Fengzan; Zhang, Qing; Zhang, Li; Niu, Xiaohui

2017-09-01

Clinical reports suggest that large bone defects could be effectively restored by allograft bone transplantation, where allograft bone selection acts an important role. Besides, there is a huge demand for developing the automatic allograft bone selection methods, as the automatic methods could greatly improve the management efficiency of the large bone banks. Although several automatic methods have been presented to select the most suitable allograft bone from the massive allograft bone bank, these methods still suffer from inaccuracy. In this paper, we propose an effective allograft bone selection method without using the contralateral bones. Firstly, the allograft bone is globally aligned to the recipient bone by surface registration. Then, the global alignment is further refined through band registration. The band, defined as the recipient points within the lifted and lowered cutting planes, could involve more local structure of the defected segment. Therefore, our method could achieve robust alignment and high registration accuracy of the allograft and recipient. Moreover, the existing contour method and surface method could be unified into one framework under our method by adjusting the lift and lower distances of the cutting planes. Finally, our method has been validated on the database of distal femurs. The experimental results indicate that our method outperforms the surface method and contour method.

Investigation of microstructural and optical properties of La0.8Ca0.2FeO3 nanostructure synthesized via gel combustion method

NASA Astrophysics Data System (ADS)

Naseem, Swaleha; Ali, S. Asad; Khan, Wasi; Khan, Shakeel

2018-05-01

Ca substituted LaFeO3 orthoferrite nanostructure perovskite has been synthesized by gel combustion method using citric acid as a fuel. The structural and optical properties were investigated by various tools. The structural analysis through Rietveld refinement of the XRD data revealed single phase of orthorhombic structure in R-3c space group of the sample without presence of any other impurity phase. Scanning electron microscopy (SEM) image exhibits non-uniform distribution of the nanoparticles in agglomerated form. The purity of the sample and stoichiometric ratio of the elements were established through energy dispersive x-ray spectroscopy (EDS). FTIR spectroscopy measurement predicts the presence of various band relation of the chemical species of Ca with LaFeO3. Optical properties were explored through UV-visible absorption spectroscopy that showed absorption edge at 347 nm and energy band gap was estimated as 3.47eV using Tauc's relation.

Crystal and electronic structure of copper sulfides

NASA Astrophysics Data System (ADS)

Lukashev, Pavel

Copper sulfides with different copper concentration exist in mineral form ranging from CuS to Cu2S. Among these, chalcosite Cu 2S, and digenite Cu1.8S were the subject of extensive research for decades mainly because of their use as the absorber in photovoltaic cells. Yet; their electronic structure is poorly understood because their crystal structure is complex. Most of the results published so far report the semiconducting nature of these compounds with the energy band gap being in the range of 0.84 to 1.9 eV. The crystal structure consists of a close-packed lattice of S with mobile Cu occupying various types of interstitial sites with a statistical distribution depending on temperature. In this thesis we present the first computational study of their electronic band structure. Initially, we investigated the simpler antifluorite structure. Both local density approximation (LDA) and self-consistent quasiparticle GW calculations with the full-potential linearized muffin-tin orbital method give a semimetallic band structure. Inspection of the nature of the bands shows that the lowest conduction band is mainly Cu-s-like except right near the center of the Brillouin zone where a Cu-s-like state lies about 1 eV below the valence band maximum. Significantly, in GW calculations, this state shifts up by several 0.1 eV but not sufficiently to open a gap. A random distortion of the Cu atoms from the perfect antifluorite positions is found to break the degeneracy of the d state at the Gamma-point and thus opens up a small gap of about 0.1 eV in LDA. As our next step we constructed supercell models for the cubic and hexagonal phases with the Cu positions determined by a weighted random number generator. The low temperature monoclinic phase was also studied. The computed total energies of these structures follow the same order as the reported phases with increasing temperatures. All these models gave similar small band gaps of order 0.1-0.2 eV. However, their conduction band is now mainly s-like and addition of an expected Cu-s level shift opens the gap to about 0.5 eV. Some simpler hexagonal model structures gave slightly larger band gap but were found to be unrealistic. The optical absorption data all show a strong intraband absorption with a minimum in absorption at about 1 eV. Our calculations suggest a significantly lower gap of order 0.5 eV with low absorption cross section, the true nature of which is masked by the free carrier absorption. As part of our study of the related Cu-compounds, we analyzed the quasiparticle effects beyond LDA obtained from a GW calculation on the effective masses and Kohn-Luttinger hamiltonian parameters for CuBr.

Theoretical investigation of the structural stabilities, optoelectronic properties and thermodynamic characteristics of GaPxSb1-x ternary alloys

NASA Astrophysics Data System (ADS)

Oumelaz, F.; Nemiri, O.; Boumaza, A.; Ghemid, S.; Meradji, H.; Bin Omran, S.; El Haj Hassan, F.; Rai, D. P.; Khenata, R.

2018-06-01

In this theoretical study, we have investigated the structural, phase transition, electronic, thermodynamic and optical properties of GaPxSb1-x ternary alloys. Our calculations are performed with the WIEN2k code based on density functional theory using the full-potential linearized augmented plane wave method. For the electron exchange-correlation potential, a generalized gradient approximation within Wu-Cohen scheme is considered. The recently developed Tran-Blaha modified Becke-Johnson potential has also been used to improve the underestimated band gap. The structural properties, including the lattice constants, the bulk moduli and their pressure derivatives are in very good agreement with the available experimental data and theoretical results. Several structural phase transitions were studied here to establish the stable structure and to predict the phase transition under hydrostatic pressure. The computed transition pressure (Pt) of the material of our interest from the zinc blende (B3) to the rock salt (B1) phase has been determined and found to agree well with the experimental and theoretical data. The calculated band structure shows that GaSb binary compound and the ternary alloys are direct band gap semiconductors. Optical parameters such as the dielectric constants and the refractive indices are calculated and analyzed. The thermodynamic results are also interpreted and analyzed.

Complex band structure and electronic transmission eigenchannels

NASA Astrophysics Data System (ADS)

Jensen, Anders; Strange, Mikkel; Smidstrup, Søren; Stokbro, Kurt; Solomon, Gemma C.; Reuter, Matthew G.

2017-12-01

It is natural to characterize materials in transport junctions by their conductance length dependence, β. Theoretical estimations of β are made employing two primary theories: complex band structure and density functional theory (DFT) Landauer transport. It has previously been shown that the β value derived from total Landauer transmission can be related to the β value from the smallest |ki| complex band; however, it is an open question whether there is a deeper relationship between the two. Here we probe the details of the relationship between transmission and complex band structure, in this case individual eigenchannel transmissions and different complex bands. We present calculations of decay constants for the two most conductive states as determined by complex band structure and standard DFT Landauer transport calculations for one semi-conductor and two molecular junctions. The molecular junctions show that both the length dependence of the total transmission and the individual transmission eigenvalues can be, almost always, found through the complex band structure. The complex band structure of the semi-conducting material, however, does not predict the length dependence of the total transmission but only of the individual channels, at some k-points, due to multiple channels contributing to transmission. We also observe instances of vertical bands, some of which are the smallest |ki| complex bands, that do not contribute to transport. By understanding the deeper relationship between complex bands and individual transmission eigenchannels, we can make a general statement about when the previously accepted wisdom linking transmission and complex band structure will fail, namely, when multiple channels contribute significantly to the transmission.

Band Anticrossing in Highly Mismatched Compound Semiconductor Alloys

NASA Technical Reports Server (NTRS)

Yu, Kin Man; Wu, J.; Walukiewicz, W.; Ager, J. W.; Haller, E. E.; Miotkowski, I.; Su, Ching-Hua; Curreri, Peter A. (Technical Monitor)

2001-01-01

Compound semiconductor alloys in which metallic anions are partially replaced with more electronegative isoelectronic atoms have recently attracted significant attention. Group IIIN(sub x)V(sub 1-x) alloys with a small amount of the electronegative N substituting more metallic column V elements has been the most extensively studied class of such Highly Mismatched Alloys (HMAs). We have shown that many of the unusual properties of the IIIN(sub x)V(sub 1-x) alloys can be well explained by the Band Anticrossing (BAC) model that describes the electronic structure in terms of an interaction between highly localized levels of substitutional N and the extended states of the host semiconductor matrix. Most recently the BAC model has been also used to explain similar modifications of the electronic band structure observed in Te-rich ZnS(sub x)Te(sub 1-x) and ZnSe(sub y)Te(sub 1-y) alloys. To date studies of HMAs have been limited to materials with relatively small concentrations of highly electronegative atoms. Here we report investigations of the electronic structure of ZnSe(sub y)Te(sub 1-y) alloys in the entire composition range, y between 0 and 1. The samples used in this study are bulk ZnSe(sub y)Te(sub 1-y) crystals grown by either a modified Bridgman method or by physical vapor transport. Photomodulated reflection (PR) spectroscopy was used to measure the composition dependence of optical transitions from the valence band edge and from the spin-orbit split off band to the conduction band. The pressure dependence of the band gap was measured using optical absorption in a diamond anvil cell. We find that the energy of the spin-orbit split off valence band edge does not depend on composition and is located at about 3 eV below the conduction band edge of ZnSe. On the Te-rich side the pressure and the composition dependence of the optical transitions are well explained by the BAC model which describes the downward shift of the conduction band edge in terms of the interaction between localized Se states and the conduction band. On the other hand we show that the large band gap reduction observed on the Se-rich side of the alloy system is a result of an interaction between the localized Te level and the valence bands. This interaction leads to the formation of a Te-like valence band edge that strongly interacts with the light hole valence band. Calculations based on a modified k(sup dot)p model account for the reduction of the band gap and the large increase of the spin-orbit splitting observed in Se-rich ZnSe(sub y)Te(sub 1-y) alloys. We will also discuss the importance of these new results for understanding of the electronic structure and band offsets in other highly mismatched alloy systems.

Band Anticrossing in Highly Mismatched Compound Semiconductor Alloys

NASA Technical Reports Server (NTRS)

Yu, Kin Man; Wu, J.; Walukiewicz, W.; Ager, J. W.; Haller, E. E.; Miotkowski, I.; Ramdas, A.; Su, Ching-Hua; Whitaker, Ann F. (Technical Monitor)

2001-01-01

Compound semiconductor alloys in which metallic anions are partially replaced with more electronegative isoelectronic atoms have recently attracted significant attention. Group IIIN(x)V(1-x), alloys with a small amount of the electronegative N substituting more metallic column V elements has been the most extensively studied class of such Highly Mismatched Alloys (HMAs). We have shown that many of the unusual properties of the IIIN(x),V(1-x) alloys can be well explained by the Band Anticrossing (BAC) model that describes the electronic structure in terms of an interaction between highly localized levels of substitutional N and the extended states of the host semiconductor matrix. Most recently the BAC model has been also used to explain similar modifications of the electronic band structure observed in Te-rich ZnS(x)Te(l-x) and ZnSe(Y)Te(1-y) alloys. To date studies of HMAs have been limited to materials with relatively small concentrations of highly electronegative atoms. Here we report investigations of the electronic structure of ZnSe(y)Te(1-y) alloys in the entire composition range, 0 less than or equal to y less than or equal to 1. The samples used in this study are bulk ZnSe(y)Te(1-y) crystals grown by either a modified Bridgman method or by physical vapor transport. Photomodulated reflection (PR) spectroscopy was used to measure the composition dependence of optical transitions from the valence band edge and from the spin-orbit split off band to the conduction band. The pressure dependence of the band gap was measured using optical absorption in a diamond anvil cell. We find that the energy of the spin-orbit split off valence band edge does not depend on composition and is located at about 3 eV below the conduction band edge of ZnSe. On the Te-rich side the pressure and the composition dependence of the optical transitions are well explained by the BAC model which describes the downward shift of the conduction band edge in terms of the interaction between localized Se states and the conduction band. On the other hand we show that the large band gap reduction observed on the Se-rich side of the alloy system is a result of an interaction between the localized Te level and the valence bands. This interaction leads to the formation of a Te-like valence band edge that strongly interacts with the light hole valence band. Calculations based on a modified k p model account for the reduction of the band gap and the large increase of the spin-orbit splitting observed in Se-rich ZnSe(y)Te(l-y) alloys. We will also discuss the importance of these new results for understanding of the electronic structure and band offsets in other highly mismatched alloy systems.

Carbon phosphide monolayers with superior carrier mobility

NASA Astrophysics Data System (ADS)

Wang, Gaoxue; Pandey, Ravindra; Karna, Shashi P.

2016-04-01

Two dimensional (2D) materials with a finite band gap and high carrier mobility are sought after materials from both fundamental and technological perspectives. In this paper, we present the results based on the particle swarm optimization method and density functional theory which predict three geometrically different phases of the carbon phosphide (CP) monolayer consisting of sp2 hybridized C atoms and sp3 hybridized P atoms in hexagonal networks. Two of the phases, referred to as α-CP and β-CP with puckered or buckled surfaces are semiconducting with highly anisotropic electronic and mechanical properties. More remarkably, they have the lightest electrons and holes among the known 2D semiconductors, yielding superior carrier mobility. The γ-CP has a distorted hexagonal network and exhibits a semi-metallic behavior with Dirac cones. These theoretical findings suggest that the binary CP monolayer is a yet unexplored 2D material holding great promise for applications in high-performance electronics and optoelectronics.Two dimensional (2D) materials with a finite band gap and high carrier mobility are sought after materials from both fundamental and technological perspectives. In this paper, we present the results based on the particle swarm optimization method and density functional theory which predict three geometrically different phases of the carbon phosphide (CP) monolayer consisting of sp2 hybridized C atoms and sp3 hybridized P atoms in hexagonal networks. Two of the phases, referred to as α-CP and β-CP with puckered or buckled surfaces are semiconducting with highly anisotropic electronic and mechanical properties. More remarkably, they have the lightest electrons and holes among the known 2D semiconductors, yielding superior carrier mobility. The γ-CP has a distorted hexagonal network and exhibits a semi-metallic behavior with Dirac cones. These theoretical findings suggest that the binary CP monolayer is a yet unexplored 2D material holding great promise for applications in high-performance electronics and optoelectronics. Electronic supplementary information (ESI) available: Fig. S1 cohesive energy and structure of the CP monolayer with various stoichiometric compositions obtained using CALYPSO, Fig. S2 history of CALYPSO steps and structure of the CP monolayer, Fig. S3 phonon dispersion with DFT-D2 functional, Fig. S4 band structure for β-CP using the DFT-PBE and DFT-D2 functional forms, Fig. S5 strain energy curves, Fig. S6 projected band structure for α-CP, Fig. S7 projected band structure for β-CP, Fig. S8 projected band structure for γ-CP, Fig. S9 band structures obtained with the GGA-PBE and HSE06 functional; Table S1 lattice parameters with the DFT-D2 functional form; Video S1 AIMD simulation of α-CP at 300 K, Video S2 AIMD simulation of β-CP at 300 K, Video S3 AIMD simulation of γ-CP at 300 K. See DOI: 10.1039/c6nr00498a

Design and analysis of InN - In0.25Ga0.75N single quantum well laser for short distance communication wavelength

NASA Astrophysics Data System (ADS)

Polash, Md. Mobarak Hossain; Alam, M. Shah; Biswas, Saumya

2018-03-01

A single quantum well semiconductor laser based on wurtzite-nitride is designed and analyzed for short distance communication wavelength (at around 1300 nm). The laser structure has 12 Å well layer of InN, 15 Å barrier layer of In0.25Ga0.75N, and 54 Å separate confinement heterostructure layer of GaN. To calculate the electronic characteristics of the structure, a self-consistent method is used where Hamiltonian with effective mass approximation is solved for conduction band while six-bands Hamiltonian matrix with k · p formalism including the polarization effect, valence-band mixing effect, and strain effect is solved for valence band. The interband optical transition elements, optical gain, differential gain, radiative current density, spontaneous emission rate, and threshold characteristics have been calculated. The wave function overlap integral is found to be 45.93% for TE-polarized structure. Also, the spontaneous emission rate is found to be 6.57 × 1027 s - 1 cm - 3 eV - 1 at 1288.21 nm with the carrier density of 5 × 1019 cm - 3. Furthermore, the radiative current density and the radiative recombination rate are found to be 121.92 A cm - 2 and 6.35 × 1027 s - 1 cm - 3, respectively, while the TE-polarized optical gain of the structure is 3872.1 cm - 1 at 1301.7 nm.

Structural influence in the interaction of cysteine with five coordinated copper complexes: Theoretical and experimental studies

NASA Astrophysics Data System (ADS)

Huerta-Aguilar, Carlos Alberto; Thangarasu, Pandiyan; Mora, Jesús Gracia

2018-04-01

Copper complexes of N,N,N‧,N‧-tetrakis(pyridyl-2-ylmethyl)-1,2-diaminoethane (L1) and N,N,N‧,N‧-tetrakis(pyridyl-2-ylmethyl)-1,3-diaminopropane (L2) prepared were characterized completely by different analytical methods. The X-structure of the complexes shows that Cu(II) presents in trigonal bi-pyramidal (TBP) geometry, consisting with the electronic spectra where two visible bands corresponding to five coordinated structure were observed. Thus TD-DFT was used to analyze the orbital contribution to the electronic transitions for the visible bands. Furthermore, the interaction of cysteine with the complexes was spectrally studied, and the results were explained through DFT analysis, observing that the geometrical parameters and oxidation state of metal ions play a vital role in the binding of cysteine with copper ion. It appears that the TBP structure is being changed into octahedral geometry during the addition of cysteine to the complexes as two bands (from complex) is turned to a broad band in visible region, signifying the occupation of cysteine molecule at sixth position of octahedral geometry. In the molecular orbital analysis, the existence of a strong overlapping of HOMOs (from cysteine) with LUMOs of Cu ion was observed. The total energy of the systems calculated by DFT shows that cysteine binds favorably with copper (I) than that with Cu(II).

First-Principles Study of the Jahn-Teller Distortion in the Ti1-XVXH2 and Zr1-XNbxH2 Alloys

NASA Astrophysics Data System (ADS)

Quijano, Ramiro; de Coss, Romeo; Singh, David

2008-03-01

The transition metal dihydrides TiH2 and ZrH2 present the fluorite structure (CaF2) at high temperature but undergoes a tetragonal distortion with c/a<1 at low temperature. Electronic band structure calculations have shown that TiH2 and ZrH2 in the cubic phase display a very flat band at the Fermi level. Thus the low temperature tetragonal distortion has been associated to a Jahn-Teller effect. In order to understand the role of band filling in controlling the structural instability of the transition metal dihydrides, we have performed a first-principles total energy study of the Ti1-XVxH2 and Zr1-xNbxH2 alloys. The calculations were performed using FP-LAPW method within the (DFT) and we use the GGA for exchange correlation functional energy. The critical concentration for which the Jahn-Teller effect is suppressed, was determined from the evolution of the tetragonal-cubic energy barrier. We discuss the electronic mechanism of the structural-instability, in terms of the band filling. From the obtained results we conclude that the tetragonal distortion in TiH2 and ZrH2 is not produced only by a Jahn-Teller Effect. This research was supported by Consejo Nacional de Ciencia y Tecnolog'ia (Conacyt) under Grant No. 43830-F.

Morphology-dependent optical absorption and conduction properties of photoelectrochemical photocatalysts for H2 production: A case study

NASA Astrophysics Data System (ADS)

Huda, Muhammad N.; Turner, John A.

2010-06-01

Efficient photoelectrochemical H2 production by solar irradiation depends not only on the photocatalyst's band gap and its band-edge positions but also on the detailed electronic nature of the bands, such as the localization or delocalization of the band edges and their orbital characteristics. These determine the carrier transport properties, reactivity, light absorption strength, etc. and significantly impact the material's efficiency as a photoconverter. The localization or delocalization of the band edges may arise either due to the orbital nature of the bands or the structural morphology of the material. A recent experimental report on a photocatalyst based on s /p orbitals showed very poor performance for H2 production despite the delocalized nature of the s /p bands as compared to the d-bands of transition metal oxides. It is then important to examine whether this poor performance is inherent to these materials or rather arises from some experimental limitations. A theoretical analysis by first-principle methods is well suited to shed light on this question.

The effect of group IIIA metal ion dopants on the photocatalytic activities of nanocrystalline Sr0.25H1.5Ta2O6·H2O.

PubMed

Liang, Shijing; Zhu, Shuying; Zhu, Jia; Chen, Yan; Zhang, Yongfan; Wu, Ling

2012-01-21

A series of group IIIA metal ion electron acceptors doped into Sr(0.25)H(1.5)Ta(2)O(6)·H(2)O (HST) samples have been prepared by an impregnation and calcination method for the first time. The samples are characterized by XRD, TEM, DRS and XPS. The variations in the electronic structure and photoelectric response after metal ion doping are investigated by theoretical calculations and photocurrent experiments, respectively. Results show that the metal ions can be efficiently incorporated into the HST crystal structure, which is reflected in the lattice contraction. Meanwhile, the photoabsorption edges of the metal-doped HST samples are red shifted to a longer wavelength. Taking into account the ionic radii and electronegativities of the dopants, as well as the XRD and XPS results, it is concluded that Ta(5+) ions may be partially substituted by the Al(3+) and Ga(3+) ions in the framework, while In(3+) ions are the favourable substitutes for Sr(2+) sites in the cavity. The first-principles DFT calculations confirm that the variation of the band structure is sensitive to the type of group IIIA metal ion. Introducing the dopant only at the Ta site induces an obvious variation in the band structure and the band gap becomes narrow. Meanwhile, an ''extra step'' appeared in the band gap, which can trap photogenerated electrons from the valance band (VB) and could enhance the charge mobility and the photocurrent. For the photocatalytic degradation of methyl orange in an aqueous solution and in benzene in the gas phase, the doped samples show superior photocatalytic activities compared with both undoped samples and TiO(2). The enhanced photocatalytic activities can be well explained by their electronic structure, photoabsorption performance, photoelectric response, and the concentration of the active species. Due to the fact that Ga ion doping can create an acceptor impurity level and change the electronic band, efficiently narrowing the band gap, the Ga-doped sample shows the highest photocatalytic activity.

Titanium α-ω phase transformation pathway and a predicted metastable structure

DOE PAGES

Zarkevich, Nickolai A.; Johnson, Duane D.

2016-01-15

A titanium is a highly utilized metal for structural lightweighting and its phases, transformation pathways (transition states), and structures have scientific and industrial importance. Using a proper solid-state nudged elastic band method employing two climbing images combined with density functional theory DFT + U methods for accurate energetics, we detail the pressure-induced α (ductile) to ω (brittle) transformation at the coexistence pressure. We also find two transition states along the minimal-enthalpy path and discover a metastable body-centered orthorhombic structure, with stable phonons, a lower density than the end-point phases, and decreasing stability with increasing pressure.

Towards band structure and band offset engineering of monolayer Mo(1-x)W(x)S2 via Strain

NASA Astrophysics Data System (ADS)

Kim, Joon-Seok; Ahmad, Rafia; Pandey, Tribhuwan; Rai, Amritesh; Feng, Simin; Yang, Jing; Lin, Zhong; Terrones, Mauricio; Banerjee, Sanjay K.; Singh, Abhishek K.; Akinwande, Deji; Lin, Jung-Fu

2018-01-01

Semiconducting transition metal dichalcogenides (TMDs) demonstrate a wide range of optoelectronic properties due to their diverse elemental compositions, and are promising candidates for next-generation optoelectronics and energy harvesting devices. However, effective band offset engineering is required to implement practical structures with desirable functionalities. Here, we explore the pressure-induced band structure evolution of monolayer WS2 and Mo0.5W0.5S2 using hydrostatic compressive strain applied in a diamond anvil cell (DAC) apparatus and theoretical calculations, in order to study the modulation of band structure and explore the possibility of band alignment engineering through different compositions. Higher W composition in Mo(1-x)W(x)S2 contributes to a greater pressure-sensitivity of direct band gap opening, with a maximum value of 54 meV GPa-1 in WS2. Interestingly, while the conduction band minima (CBMs) remains largely unchanged after the rapid gap increase, valence band maxima (VBMs) significantly rise above the initial values. It is suggested that the pressure- and composition-engineering could introduce a wide variety of band alignments including type I, type II, and type III heterojunctions, and allow to construct precise structures with desirable functionalities. No structural transition is observed during the pressure experiments, implying the pressure could provide selective modulation of band offset.

A Theoretical Structure of High School Concert Band Performance

ERIC Educational Resources Information Center

Bergee, Martin J.

2015-01-01

This study used exploratory (EFA) and confirmatory factor analysis (CFA) to verify a theoretical structure for high school concert band performance and to test that structure for viability, generality, and invariance. A total of 101 university students enrolled in two different bands rated two high school band performances (a "first"…

Some recent experimental results related to nuclear chirality

NASA Astrophysics Data System (ADS)

Timár, J.; Kuti, I.; Sohler, D.; Starosta, K.; Koike, T.; Paul, E. S.

2014-09-01

Detailed band structures of three chiral-candidate nuclei, 134Pr, 132La and 103Rh have been studied. The aim of the study was twofold. First, to try to explore the reasons behind the contradiction between the theoretically predicted chirality in these nuclei and the recently observed fingerprints that suggest non-chiral interpretation for the previous chiral candidate band doublets. Second, to search for multiple chiral bands of different types in these nuclei. In 134Pr a new πh11/2vh11/2 band has been observed besides the previously known chiral-candidate πh11/2vh11/2 doublet. This new band and the yrare πh11/2vh11/2 band show the expected features of a chiral doublet structure. This fact combined with the observed similarity between the band structures of 134Pr and 132La suggests that chirality might exist in these nuclei. The detailed study of the 103Rh band structure resulted in the observation of two new chiral-doublet looking structures besides the previously known one. This is indicative of possible existence of multiple chiral doublet structure in this nucleus.

Ab-initio calculation of electronic structure and optical properties of AB-stacked bilayer α-graphyne

NASA Astrophysics Data System (ADS)

Behzad, Somayeh

2016-09-01

Monolayer α-graphyne is a new two-dimensional carbon allotrope with many special features. In this work the electronic properties of AA- and AB-stacked bilayers of this material and then the optical properties are studied, using first principle plane wave method. The electronic spectrum has two Dirac cones for AA stacked bilayer α-graphyne. For AB-stacked bilayer, the interlayer interaction changes the linear bands into parabolic bands. The optical spectra of the most stable AB-stacked bilayer closely resemble to that of the monolayer, except for small shifts of peak positions and increasing of their intensity. For AB-stacked bilayer, a pronounced peak has been found at low energies under the perpendicular polarization. This peak can be clearly ascribed to the transitions at the Dirac point as a result of the small degeneracy lift in the band structure.

Energies of the X- and L-valleys in In{sub 0.53}Ga{sub 0.47}As from electronic structure calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Greene-Diniz, Gabriel; Greer, J. C.; Fischetti, M. V.

2016-02-07

Several theoretical electronic structure methods are applied to study the relative energies of the minima of the X- and L-conduction-band satellite valleys of In{sub x}Ga{sub 1−x}As with x = 0.53. This III-V semiconductor is a contender as a replacement for silicon in high-performance n-type metal-oxide-semiconductor transistors. The energy of the low-lying valleys relative to the conduction-band edge governs the population of channel carriers as the transistor is brought into inversion, hence determining current drive and switching properties at gate voltages above threshold. The calculations indicate that the position of the L- and X-valley minima are ∼1 eV and ∼1.2 eV, respectively, higher in energymore » with respect to the conduction-band minimum at the Γ-point.« less

Effect of hydrostatic pressure and uniaxial strain on the electronic structure of Pb 1-xSn xTe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Geilhufe, Matthias; Nayak, Sanjeev K.; Thomas, Stefan

2015-12-09

The electronic structure of Pb 1–xSn xTe is studied by using the relativistic Korringa-Kohn-Rostoker Green function method in the framework of density functional theory. For all concentrations x, Pb 1–xSn xTe is a direct semiconductor with a narrow band gap. In contrast to pure lead telluride, tin telluride shows an inverted band characteristic close to the Fermi energy. It will be shown that this particular property can be tuned, first, by alloying PbTe and SnTe and, second, by applying hydrostatic pressure or uniaxial strain. Furthermore, the magnitude of strain needed to switch between the regular and inverted band gap canmore » be tuned by the alloy composition. In conclusion, there is a range of potential usage of Pb 1–xSn xTe for spintronic applications.« less

Structural complexity and wide application of two-dimensional S/O type antimonene

NASA Astrophysics Data System (ADS)

Li, T. T.; He, C.; Zhang, W. X.

2018-05-01

Inspired by stable two-dimensional antimonene phases, two new allotropes (S/O and tricycle) antimonenes have been predicted by first-principles calculations in this paper. S/O type antimonene possesses remarkably thermodynamical and dynamical stability, which are comparable to that of buckled type antimonene. The results indicate that S/O type antimonene is a direct band gap semiconductor with a band gap of 2.314 eV and the electronic properties could be effectively tuned by the in-plane strain. In order to explore the potential application, the mechanical properties and optical properties of S/O type antimonene are also extensively studied. It is found the S/O type antimonene is an anisotropic material by the method of analyzing the linear Poisson's ratios and the phonon band structure. These systematical analyses show that S/O type antimonene is a new 2D material with tunable electronic properties, excellent mechanical and optical properties.

Structural damages observed in state buildings after Simav/Turkey earthquake occurred on 19 May 2011

NASA Astrophysics Data System (ADS)

Tama, Y. S.

2012-08-01

Different levels of damages occurred in state buildings, especially in educational facilities, during the Simav earthquake (ML=5.7) on 19 May 2011. A site survey was carried out in the area after the earthquake, where six state buildings were examined in detail. The results of the survey showed that main reasons for the formation of damages in these buildings are the use of low strength concrete, insufficient reinforcement, inappropriate detailing, and low-quality workmanship. The investigated buildings were also evaluated by P25-rapid assessment method. The method demonstrates that two of the buildings in question are in "high risk band"; the other two fall into "detailed evaluation band", and the rest are in the "low risk band". This figure also matches with the damages observed in the site survey.

Transparent ceramic photo-optical semiconductor high power switches

DOEpatents

Werne, Roger W.; Sullivan, James S.; Landingham, Richard L.

2016-01-19

A photoconductive semiconductor switch according to one embodiment includes a structure of sintered nanoparticles of a high band gap material exhibiting a lower electrical resistance when excited by light relative to an electrical resistance thereof when not exposed to the light. A method according to one embodiment includes creating a mixture comprising particles, at least one dopant, and at least one solvent; adding the mixture to a mold; forming a green structure in the mold; and sintering the green structure to form a transparent ceramic. Additional system, methods and products are also presented.

Electronic Structures of Free-Standing Nanowires made from Indirect Bandgap Semiconductor Gallium Phosphide

PubMed Central

Liao, Gaohua; Luo, Ning; Chen, Ke-Qiu; Xu, H. Q.

2016-01-01

We present a theoretical study of the electronic structures of freestanding nanowires made from gallium phosphide (GaP)—a III-V semiconductor with an indirect bulk bandgap. We consider [001]-oriented GaP nanowires with square and rectangular cross sections, and [111]-oriented GaP nanowires with hexagonal cross sections. Based on tight binding models, both the band structures and wave functions of the nanowires are calculated. For the [001]-oriented GaP nanowires, the bands show anti-crossing structures, while the bands of the [111]-oriented nanowires display crossing structures. Two minima are observed in the conduction bands, while the maximum of the valence bands is always at the Γ-point. Using double group theory, we analyze the symmetry properties of the lowest conduction band states and highest valence band states of GaP nanowires with different sizes and directions. The band state wave functions of the lowest conduction bands and the highest valence bands of the nanowires are evaluated by spatial probability distributions. For practical use, we fit the confinement energies of the electrons and holes in the nanowires to obtain an empirical formula. PMID:27307081

Band Structure Characteristics of Nacreous Composite Materials with Various Defects

NASA Astrophysics Data System (ADS)

Yin, J.; Zhang, S.; Zhang, H. W.; Chen, B. S.

2016-06-01

Nacreous composite materials have excellent mechanical properties, such as high strength, high toughness, and wide phononic band gap. In order to research band structure characteristics of nacreous composite materials with various defects, supercell models with the Brick-and-Mortar microstructure are considered. An efficient multi-level substructure algorithm is employed to discuss the band structure. Furthermore, two common systems with point and line defects and varied material parameters are discussed. In addition, band structures concerning straight and deflected crack defects are calculated by changing the shear modulus of the mortar. Finally, the sensitivity of band structures to the random material distribution is presented by considering different volume ratios of the brick. The results reveal that the first band gap of a nacreous composite material is insensitive to defects under certain conditions. It will be of great value to the design and synthesis of new nacreous composite materials for better dynamic properties.

Influence of chromium hyperdoping on the electronic structure of CH3NH3PbI3 perovskite: a first-principles insight.

PubMed

García, Gregorio; Palacios, Pablo; Menéndez-Proupin, Eduardo; Montero-Alejo, Ana L; Conesa, José C; Wahnón, Perla

2018-02-06

Organic-inorganic hybrid halide perovskites compounds are emerging as new materials with great potential for efficient solar cells. This paper explores the possibility of increasing their photovoltaic efficiency through sub-bandgap absorption by way of the in gap band (IGB) concept. Thus, we assess the formation of an in gap band as well as its effect on the absorption features of Organic-inorganic hybrid halide perovskites CH 3 NH 3 PbI 3 (MAPI). For this task, we use density functional theory (DFT) as well as many-body perturbation methods along to spin-orbit coupling (SOC) to study structural, energetic and electronic properties of partially Cr-substituted MAPI perovskites (CH 3 NH 3 Pb 1-x Cr x I 3 ). Our results reveal that Cr replacement does not lead to an important cell distortion, while the energetic of the substitution process evidences the possibility of obtaining Cr-substituted perovskite. The analysis of the electronic structure shows that Cr 3d-orbitals induce new electronic states in the host semiconductor bandgap, which fulfill the requirements to be considered as an IGB. Precise many-body perturbation methods in G 0 W 0 approach provided an accurate description on the electronic structures as well as the position of the IGB. In short, Pb replacement by Cr could be useful for improved absorption features through new sub-bandgap transitions across the in gap band.

Effects of impurity adsorption on topological surface states of Bi2Te3

NASA Astrophysics Data System (ADS)

Shati, Khaqan; Arshad Farhan, M.; Selva Chandrasekaran, S.; Shim, Ji Hoon; Lee, Geunsik

2017-08-01

Electronic structures of Bi2Te3 with adsorption of Rb, In, Ga and Au atoms are studied by using the first-principle method, focusing on the effect of non-magnetic impurities on the topologically protected surface states. Upon monolayer formation, the bulk conduction band is moved down to the Fermi level with a significant Rashba splitting due to n-doping behavior with band modification details depending on the adatom chemistry. Our study shows the robustness of the intrinsic spin-momentum coupled surface band and emergence of a new similar one, which could provide helpful insight for developing novel spintronic devices.

Fabrication of photonic band gap materials

DOEpatents

Constant, Kristen; Subramania, Ganapathi S.; Biswas, Rana; Ho, Kai-Ming

2002-01-15

A method for forming a periodic dielectric structure exhibiting photonic band gap effects includes forming a slurry of a nano-crystalline ceramic dielectric or semiconductor material and monodisperse polymer microspheres, depositing a film of the slurry on a substrate, drying the film, and calcining the film to remove the polymer microspheres therefrom. The film may be cold-pressed after drying and prior to calcining. The ceramic dielectric or semiconductor material may be titania, and the polymer microspheres may be polystyrene microspheres.

Synthesis of Nanocrystalline SnOx (x = 1–2) Thin Film Using a Chemical Bath Deposition Method with Improved Deposition Time, Temperature and pH

PubMed Central

Ebrahimiasl, Saeideh; Yunus, Wan Md. Zin Wan; Kassim, Anuar; Zainal, Zulkarnain

2011-01-01

Nanocrystalline SnOx (x = 1–2) thin films were prepared on glass substrates by a simple chemical bath deposition method. Triethanolamine was used as complexing agent to decrease time and temperature of deposition and shift the pH of the solution to the noncorrosive region. The films were characterized for composition, surface morphology, structure and optical properties. X-ray diffraction analysis confirms that SnOx thin films consist of a polycrystalline structure with an average grain size of 36 nm. Atomic force microscopy studies show a uniform grain distribution without pinholes. The elemental composition was evaluated by energy dispersive X-ray spectroscopy. The average O/Sn atomic percentage ratio is 1.72. Band gap energy and optical transition were determined from optical absorbance data. The film was found to exhibit direct and indirect transitions in the visible spectrum with band gap values of about 3.9 and 3.7 eV, respectively. The optical transmittance in the visible region is 82%. The SnOx nanocrystals exhibit an ultraviolet emission band centered at 392 nm in the vicinity of the band edge, which is attributed to the well-known exciton transition in SnOx. Photosensitivity was detected in the positive region under illumination with white light. PMID:22163690

Vibrational studies on (E)-1-((pyridine-2-yl)methylene)semicarbazide using experimental and theoretical method

NASA Astrophysics Data System (ADS)

Subashchandrabose, S.; Ramesh Babu, N.; Saleem, H.; Syed Ali Padusha, M.

2015-08-01

The (E)-1-((pyridine-2-yl)methylene)semicarbazide (PMSC) was synthesized. The experimental and theoretical study on molecular structure and vibrational spectra were carried out. The FT-IR (400-4000 cm-1), FT-Raman (50-3500 cm-1) and UV-Vis (200-500 nm) spectra of PMSC were recorded. The geometric structure, conformational analysis, vibrational wavenumbers of PMSC in the ground state have been calculated using B3LYP method of 6-311++G(d,p) basis set. The complete vibrational assignments were made on the basis of TED, calculated by SQM method. The Non-linear optical activity was measured by means of first order hyperpolarizability calculation and π-electrons of conjugative bond in the molecule. The intra-molecular charge transfer, mode hyperconjugative interaction and molecular stabilization energies were calculated. The band gap energies between occupied and unoccupied molecular orbitals were analyzed; it proposes lesser band gap with more reactivity. To understand the electronic properties of this molecule the Mulliken charges were also calculated.

The Properties of Outer Retinal Band Three Investigated With Adaptive-Optics Optical Coherence Tomography.

PubMed

Jonnal, Ravi S; Gorczynska, Iwona; Migacz, Justin V; Azimipour, Mehdi; Zawadzki, Robert J; Werner, John S

2017-09-01

Optical coherence tomography's (OCT) third outer retinal band has been attributed to the zone of interdigitation between RPE cells and cone outer segments. The purpose of this paper is to investigate the structure of this band with adaptive optics (AO)-OCT. Using AO-OCT, images were obtained from two subjects. Axial structure was characterized by measuring band 3 thickness and separation between bands 2 and 3 in segmented cones. Lateral structure was characterized by correlation of band 3 with band 2 and comparison of their power spectra. Band thickness and separation were also measured in a clinical OCT image of one subject. Band 3 thickness ranged from 4.3 to 6.4 μm. Band 2 correlations ranged between 0.35 and 0.41 and power spectra of both bands confirmed peak frequencies that agree with histologic density measurements. In clinical images, band 3 thickness was between 14 and 19 μm. Measurements of AO-OCT of interband distance were lower than our corresponding clinical OCT measurements. Band 3 originates from a structure with axial extent similar to a single surface. Correlation with band 2 suggests an origin within the cone photoreceptor. These two observations indicate that band 3 corresponds predominantly to cone outer segment tips (COST). Conventional OCT may overestimate both the thickness of band 3 and outer segment length.

A simplified approach to the band gap correction of defect formation energies: Al, Ga, and In-doped ZnO

NASA Astrophysics Data System (ADS)

Saniz, R.; Xu, Y.; Matsubara, M.; Amini, M. N.; Dixit, H.; Lamoen, D.; Partoens, B.

2013-01-01

The calculation of defect levels in semiconductors within a density functional theory approach suffers greatly from the band gap problem. We propose a band gap correction scheme that is based on the separation of energy differences in electron addition and relaxation energies. We show that it can predict defect levels with a reasonable accuracy, particularly in the case of defects with conduction band character, and yet is simple and computationally economical. We apply this method to ZnO doped with group III elements (Al, Ga, In). As expected from experiment, the results indicate that Zn substitutional doping is preferred over interstitial doping in Al, Ga, and In-doped ZnO, under both zinc-rich and oxygen-rich conditions. Further, all three dopants act as shallow donors, with the +1 charge state having the most advantageous formation energy. Also, doping effects on the electronic structure of ZnO are sufficiently mild so as to affect little the fundamental band gap and lowest conduction bands dispersion, which secures their n-type transparent conducting behavior. A comparison with the extrapolation method based on LDA+U calculations and with the Heyd-Scuseria-Ernzerhof hybrid functional (HSE) shows the reliability of the proposed scheme in predicting the thermodynamic transition levels in shallow donor systems.

Origins of Fermi-level pinning on GaN and InN polar and nonpolar surfaces

NASA Astrophysics Data System (ADS)

Segev, D.; Van de Walle, C. G.

2006-10-01

Using band structure and total energy methods, we study the atomic and electronic structures of the polar (+c and - c plane) and nonpolar (a and m plane) surfaces of GaN and InN. We identify two distinct microscopic origins for Fermi-level pinning on GaN and InN, depending on surface stoichiometry and surface polarity. At moderate Ga/N ratios unoccupied gallium dangling bonds pin the Fermi level on n-type GaN at 0.5 0.7 eV below the conduction-band minimum. Under highly Ga-rich conditions metallic Ga adlayers lead to Fermi-level pinning at 1.8 eV above the valence-band maximum. We also explain the source of the intrinsic electron accumulation that has been universally observed on polar InN surfaces. It is caused by In-In bonds leading to occupied surface states above the conduction-band minimum. We predict that such a charge accumulation will be absent on the nonpolar surfaces of InN, when prepared under specific conditions.

Composite 3D-printed metastructures for low-frequency and broadband vibration absorption

NASA Astrophysics Data System (ADS)

Matlack, Kathryn H.; Bauhofer, Anton; Krödel, Sebastian; Palermo, Antonio; Daraio, Chiara

2016-07-01

Architected materials that control elastic wave propagation are essential in vibration mitigation and sound attenuation. Phononic crystals and acoustic metamaterials use band-gap engineering to forbid certain frequencies from propagating through a material. However, existing solutions are limited in the low-frequency regimes and in their bandwidth of operation because they require impractical sizes and masses. Here, we present a class of materials (labeled elastic metastructures) that supports the formation of wide and low-frequency band gaps, while simultaneously reducing their global mass. To achieve these properties, the metastructures combine local resonances with structural modes of a periodic architected lattice. Whereas the band gaps in these metastructures are induced by Bragg scattering mechanisms, their key feature is that the band-gap size and frequency range can be controlled and broadened through local resonances, which are linked to changes in the lattice geometry. We demonstrate these principles experimentally, using advanced additive manufacturing methods, and inform our designs using finite-element simulations. This design strategy has a broad range of applications, including control of structural vibrations, noise, and shock mitigation.

Robust and adaptive band-to-band image transform of UAS miniature multi-lens multispectral camera

NASA Astrophysics Data System (ADS)

Jhan, Jyun-Ping; Rau, Jiann-Yeou; Haala, Norbert

2018-03-01

Utilizing miniature multispectral (MS) or hyperspectral (HS) cameras by mounting them on an Unmanned Aerial System (UAS) has the benefits of convenience and flexibility to collect remote sensing imagery for precision agriculture, vegetation monitoring, and environment investigation applications. Most miniature MS cameras adopt a multi-lens structure to record discrete MS bands of visible and invisible information. The differences in lens distortion, mounting positions, and viewing angles among lenses mean that the acquired original MS images have significant band misregistration errors. We have developed a Robust and Adaptive Band-to-Band Image Transform (RABBIT) method for dealing with the band co-registration of various types of miniature multi-lens multispectral cameras (Mini-MSCs) to obtain band co-registered MS imagery for remote sensing applications. The RABBIT utilizes modified projective transformation (MPT) to transfer the multiple image geometry of a multi-lens imaging system to one sensor geometry, and combines this with a robust and adaptive correction (RAC) procedure to correct several systematic errors and to obtain sub-pixel accuracy. This study applies three state-of-the-art Mini-MSCs to evaluate the RABBIT method's performance, specifically the Tetracam Miniature Multiple Camera Array (MiniMCA), Micasense RedEdge, and Parrot Sequoia. Six MS datasets acquired at different target distances and dates, and locations are also applied to prove its reliability and applicability. Results prove that RABBIT is feasible for different types of Mini-MSCs with accurate, robust, and rapid image processing efficiency.

Flattening and manipulation of the electronic structure of h-BN/Rh(111) nanomesh upon Sn intercalation

NASA Astrophysics Data System (ADS)

Sugiyama, Yuya; Bernard, Carlo; Okuyama, Yuma; Ideta, Shin-ichiro; Tanaka, Kiyohisa; Greber, Thomas; Hirahara, Toru

2018-06-01

We have deposited Sn on corrugated hexagonal boron nitride (h-BN) nanomeshs formed on Rh(111) and found that Sn atoms are intercalated between h-BN and Rh, flattening the h-BN. Our reflection high-energy electron diffraction (RHEED) analysis showed that the average in-plane lattice constant of h-BN increases due to the loss of the corrugation. Furthermore, electronic structure measurements based on angle-resolved photoemission spectroscopy (ARPES) showed that the h-BN π band width increases significantly while the σ band width does not change as much. These behaviors were partly different from previous reports on the intercalation of h-BN/Rh system. Our results offer a novel, simple method to control the electronic structure of h-BN.

Ultrawide low frequency band gap of phononic crystal in nacreous composite material

NASA Astrophysics Data System (ADS)

Yin, J.; Huang, J.; Zhang, S.; Zhang, H. W.; Chen, B. S.

2014-06-01

The band structure of a nacreous composite material is studied by two proposed models, where an ultrawide low frequency band gap is observed. The first model (tension-shear chain model) with two phases including brick and mortar is investigated to describe the wave propagation in the nacreous composite material, and the dispersion relation is calculated by transfer matrix method and Bloch theorem. The results show that the frequency ranges of the pass bands are quite narrow, because a special tension-shear chain motion in the nacreous composite material is formed by some very slow modes. Furthermore, the second model (two-dimensional finite element model) is presented to investigate its band gap by a multi-level substructure scheme. Our findings will be of great value to the design and synthesis of vibration isolation materials in a wide and low frequency range. Finally, the transmission characteristics are calculated to verify the results.

Manipulating sonic band gaps at will: vibrational density of states in three-dimensional acoustic metamaterial composites

NASA Astrophysics Data System (ADS)

Terao, Takamichi

2018-04-01

Vibrational properties of elastic composites containing a mass-in-mass microstructure embedded in a solid matrix are numerically studied. Using a lattice model, we investigate the vibrational density of states in three-dimensional composite structures where resonant particles are randomly dispersed. By dispersing such particles in the system, a sonic band gap appears. It is confirmed that this band gap can be introduced in a desired frequency regime by changing the parameters of resonant particles and the frequency width of this band gap can be controlled by varying the concentration of the resonant particles to be dispersed. In addition, multiple sonic band gaps can be realized using different species of resonant particles. These results enable us to suggest an alternative method to fabricate devices that can inhibit the propagation of elastic waves with specific frequencies using acoustic metamaterials.

Effective Medium Ratio Obeying Wideband Left-Handed Miniaturized Meta-atoms for Multi-band Applications

NASA Astrophysics Data System (ADS)

Hossain, Mohammad Jakir; Faruque, Mohammad Rashed Iqbal; Islam, Mohammad Tariqul

2017-12-01

In this paper, a miniaturized wideband left-handed (LH) meta-atom based on planar modified multiple hexagonal split ring resonators was designed, simulated, fabricated and tested that can maintain a left-handed property. An analysis and comparison of the different array structures were performed that obtained better effective medium ratio (EMR) and wideband (5.54 GHz) for multi band operations in the microwave regime. Finite-difference time-domain (FDTD) method based Computer Simulation Technology was implemented to design the meta-atom. The meta-atom showed multi-band response in conjunction with wideband and LH property over the certain frequency bands in the microwave spectra. The EMR was considerably improved compared to previously reported meta-atoms. The measured results showed good agreement with the simulated results. The dimensions, S-parameters and EMR parameters of the proposed miniaturized LH meta-atom are appropriate for L-, S-, C-, X-, and Ku-band applications.

Effective Medium Ratio Obeying Wideband Left-Handed Miniaturized Meta-atoms for Multi-band Applications

NASA Astrophysics Data System (ADS)

Hossain, Mohammad Jakir; Faruque, Mohammad Rashed Iqbal; Islam, Mohammad Tariqul

2018-03-01

In this paper, a miniaturized wideband left-handed (LH) meta-atom based on planar modified multiple hexagonal split ring resonators was designed, simulated, fabricated and tested that can maintain a left-handed property. An analysis and comparison of the different array structures were performed that obtained better effective medium ratio (EMR) and wideband (5.54 GHz) for multi band operations in the microwave regime. Finite-difference time-domain (FDTD) method based Computer Simulation Technology was implemented to design the meta-atom. The meta-atom showed multi-band response in conjunction with wideband and LH property over the certain frequency bands in the microwave spectra. The EMR was considerably improved compared to previously reported meta-atoms. The measured results showed good agreement with the simulated results. The dimensions, S-parameters and EMR parameters of the proposed miniaturized LH meta-atom are appropriate for L-, S-, C-, X-, and Ku-band applications.

First principles study on the electronic structures and transport properties of armchair/zigzag edge hybridized graphene nanoribbons

NASA Astrophysics Data System (ADS)

Yi, Xiuying; Long, Mengqiu; Liu, Anhua; Li, Mingjun; Xu, Hui

2018-05-01

Graphene nanoribbons (GNRs) can be mainly classified into armchair graphene nanoribbons (aGNRs) and zigzag graphene nanoribbons (zGNRs) by different edge chiral directions. In this work, by introducing Stone-Wales defects on the edges of the V-shaped aGNRs, we propose a kind of armchair/zigzag edge hybridized GNRs (a/zHGNRs) and using the density functional theory and the nonequilibrium Green's function method, the band structures and electronic transport properties of the a/zHGNRs have been calculated. Our results show that an indirect bandgap appears in the band structures of the a/zHGNRs, which is very different from the direct bandgap of aGNRs and gapless of zGNRs. We also find that the valance band is mainly derived from the armchair partial atoms on the hybridized edge, while the conduction band comes mainly from the zigzag partial atoms of the hybridized edge. Meanwhile, the bandgap also oscillates with a period of three when the ribbon width increases. In addition, our quantum transport calculations show that there is a remarkable transition between the semiconductor and the metal with different ribbon widths in the a/zHGNRs devices, and the corresponding physical analysis is given.

Synthesis, crystal structure, photoluminescence and electrochemical properties of a sandwiched Ni2Ce complex

NASA Astrophysics Data System (ADS)

Güngör, Seyit Ali; Kose, Muhammet

2017-12-01

In this study, a Ni2Ce complex [(NiL)2Ce(NO3)2](NO3) was synthesized and characterized by spectroscopic and analytical methods. The structure of the complex was determined by single crystal X-ray diffraction study. In the structure of the complex, a Ce(III) ion is sandwiched between the two NiL units, which are virtually parallel to each other. The Ce(III) center is 12-coordinate, surrounded by 12 oxygen atoms; four are from phenolic groups, four from methoxy groups, and four from two bidentate nitrate ligands. Hirshfeld surface analysis was used to evaluate the inter-molecular interactions within the crystal packing. The complex molecules are linked by H⋯ONO2 interactions. The largest contribution is H⋯O/O⋯H with 41.6% contribution and followed by H⋯H contacts with 39.1%. The complex showed an excitation band in the range of 510-580 nm. A band in the range of 520-580 nm observed in the emission spectrum almost completely overlapped. This suggests that the band in the emission spectrum of the complex is not the actual fluorescence emission and is assigned to the Rayleigh scattering band. Electrochemical and thermal behaviours of the complex were also investigated.

Synthesis, Structural, Thermal, and Electronic Properties of Palmierite-Related Double Molybdate α-Cs2Pb(MoO4)2.

PubMed

Solodovnikov, Sergey F; Atuchin, Victor V; Solodovnikova, Zoya A; Khyzhun, Oleg Y; Danylenko, Mykola I; Pishchur, Denis P; Plyusnin, Pavel E; Pugachev, Alexey M; Gavrilova, Tatiana A; Yelisseyev, Alexander P; Reshak, Ali H; Alahmed, Zeyad A; Habubi, Nadir F

2017-03-20

Cs 2 Pb(MoO 4 ) 2 crystals were prepared by crystallization from their own melt, and the crystal structure has been studied in detail. At 296 K, the molybdate crystallizes in the low-temperature α-form and has a monoclinic palmierite-related superstructure (space group C2/m, a = 2.13755(13) nm, b = 1.23123(8) nm, c = 1.68024(10) nm, β = 115.037(2)°, Z = 16) possessing the largest unit cell volume, 4.0066(4) nm 3 , among lead-containing palmierites. The compound undergoes a distortive phase transition at 635 K and incongruently melts at 943 K. The electronic structure of α-Cs 2 Pb(MoO 4 ) 2 was explored by using X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy methods. For α-Cs 2 Pb(MoO 4 ) 2 , the photoelectron core-level and valence-band spectra and the XES band representing the energy distribution of Mo 4d and O 2p states were recorded. Our results allow one to conclude that the Mo 4d and O 2p states contribute mainly to the central part and at the top of the valence band, respectively, with also significant contributions throughout the whole valence-band region of the molybdate under consideration.

Investigation of silicate mineral sanidine by vibrational and NMR spectroscopic methods

NASA Astrophysics Data System (ADS)

Anbalagan, G.; Sankari, G.; Ponnusamy, S.; kumar, R. Thilak; Gunasekaran, S.

2009-10-01

Sanidine, a variety of feldspar minerals has been investigated through optical absorption, vibrational (IR and Raman), EPR and NMR spectroscopic techniques. The principal reflections occurring at the d-spacings, 3.2892, 3.2431, 2.9022 and 2.6041 Å confirm the presence of sanidine structure in the mineral. Sanidine shows five prominent characteristic infrared absorption bands in the region 1200-950, 770-720, 590-540 and 650-640 cm -1. The Raman spectrum shows the strongest band at 512 cm -1 characteristic of the feldspar structure, which contains four membered rings of tetrahedra. The UV-vis-NIR absorption spectrum had strong absorption features at 6757, 5780 and 5181 cm -1 due to the combination of fundamental OH- stretching. The bands at 11236 and 8196 cm -1and the strong, well-defined band at (30303 cm -1 attest the presence of Fe 2+ and Fe 3+, respectively, in the sample. The signals at g = 4.3 and 3.7 are interpreted in terms of Fe 3+ at two distinct tetrahedral positions Tl and T2 of the monoclinic crystal structure The 29Si NMR spectrum shows two peaks at -97 and -101 ppm corresponding to T2 and T1, respectively, and one peak in 27Al NMR for Al(IV).

Investigation of silicate mineral sanidine by vibrational and NMR spectroscopic methods.

PubMed

Anbalagan, G; Sankari, G; Ponnusamy, S; Kumar, R Thilak; Gunasekaran, S

2009-10-01

Sanidine, a variety of feldspar minerals has been investigated through optical absorption, vibrational (IR and Raman), EPR and NMR spectroscopic techniques. The principal reflections occurring at the d-spacings, 3.2892, 3.2431, 2.9022 and 2.6041 A confirm the presence of sanidine structure in the mineral. Sanidine shows five prominent characteristic infrared absorption bands in the region 1200-950, 770-720, 590-540 and 650-640 cm(-1). The Raman spectrum shows the strongest band at 512 cm(-1) characteristic of the feldspar structure, which contains four membered rings of tetrahedra. The UV-vis-NIR absorption spectrum had strong absorption features at 6757, 5780 and 5181 cm(-1) due to the combination of fundamental OH- stretching. The bands at 11236 and 8196 cm(-1)and the strong, well-defined band at (30303 cm(-1) attest the presence of Fe(2+) and Fe(3+), respectively, in the sample. The signals at g = 4.3 and 3.7 are interpreted in terms of Fe(3+) at two distinct tetrahedral positions Tl and T2 of the monoclinic crystal structure The (29)Si NMR spectrum shows two peaks at -97 and -101 ppm corresponding to T2 and T1, respectively, and one peak in (27)Al NMR for Al(IV).

Tunneling current in HfO2 and Hf0.5Zr0.5O2-based ferroelectric tunnel junction

NASA Astrophysics Data System (ADS)

Dong, Zhipeng; Cao, Xi; Wu, Tong; Guo, Jing

2018-03-01

Ferroelectric tunnel junctions (FTJs) have been intensively explored for future low power data storage and information processing applications. Among various ferroelectric (FE) materials studied, HfO2 and H0.5Zr0.5O2 (HZO) have the advantage of CMOS process compatibility. The validity of the simple effective mass approximation, for describing the tunneling process in these materials, is examined by computing the complex band structure from ab initio simulations. The results show that the simple effective mass approximation is insufficient to describe the tunneling current in HfO2 and HZO materials, and quantitative accurate descriptions of the complex band structures are indispensable for calculation of the tunneling current. A compact k . p Hamiltonian is parameterized to and validated by ab initio complex band structures, which provides a method for efficiently and accurately computing the tunneling current in HfO2 and HZO. The device characteristics of a metal/FE/metal structure and a metal/FE/semiconductor (M-F-S) structure are investigated by using the non-equilibrium Green's function formalism with the parameterized effective Hamiltonian. The result shows that the M-F-S structure offers a larger resistance window due to an extra barrier in the semiconductor region at off-state. A FTJ utilizing M-F-S structure is beneficial for memory design.

Broadening of effective photonic band gaps in biological chiral structures: From intrinsic narrow band gaps to broad band reflection spectra

NASA Astrophysics Data System (ADS)

Vargas, W. E.; Hernández-Jiménez, M.; Libby, E.; Azofeifa, D. E.; Solis, Á.; Barboza-Aguilar, C.

2015-09-01

Under normal illumination with non-polarized light, reflection spectra of the cuticle of golden-like and red Chrysina aurigans scarabs show a structured broad band of left-handed circularly polarized light. The polarization of the reflected light is attributed to a Bouligand-type left-handed chiral structure found through the scarab's cuticle. By considering these twisted structures as one-dimensional photonic crystals, a novel approach is developed from the dispersion relation of circularly polarized electromagnetic waves traveling through chiral media, to show how the broad band characterizing these spectra arises from an intrinsic narrow photonic band gap whose spectral position moves through visible and near-infrared wavelengths.

Band-gap bowing and p-type doping of (Zn, Mg, Be)O wide-gap semiconductor alloys: a first-principles study

NASA Astrophysics Data System (ADS)

Shi, H.-L.; Duan, Y.

2008-12-01

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p- d repulsion. The NO acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.

Theory of Band Warping and its Effects on Thermoelectronic Transport Properties

NASA Astrophysics Data System (ADS)

Mecholsky, Nicholas; Resca, Lorenzo; Pegg, Ian; Fornari, Marco

2015-03-01

Transport properties of materials depend upon features of band structures near extrema in the BZ. Such features are generally described in terms of quadratic expansions and effective masses. Such expansions, however, are permissible only under strict conditions that are sometimes violated by materials. Suggestive terms such as ``band warping'' have been used to refer to such situations and ad hoc methods have been developed to treat them. We develop a generally applicable theory, based on radial expansions, and a corresponding definition of angular effective mass which also accounts for effects of band non-parabolicity and anisotropy. Further, we develop precise procedures to evaluate band warping quantitatively and as an example we analyze the warping features of valence bands in silicon using first-principles calculations and we compare those with semi-empirical models. We use our theory to generalize derivations of transport coefficients for cases of either single or multiple electronic bands, with either quadratically expansible or warped energy surfaces. We introduce the transport-equivalent ellipsoid and illustrate the drastic effects that band warping can induce on thermoelectric properties using multi-band models. Vitreous State Laboratory and Samsung's GRO program.

Probing the magnetic field dependence of the light hole transition in GaAs/AlGaAs quantum wells using optically pumped NMR

NASA Astrophysics Data System (ADS)

Willmering, Matthew M.; Sesti, Erika L.; Hayes, Sophia E.; Wood, Ryan M.; Bowers, Clifford R.; Thapa, Sunil K.; Stanton, Christopher J.; Reyes, Arneil P.; Kuhns, Philip; McGill, Stephen

2018-02-01

Optically pumped NMR (OPNMR) of the NMR-active Ga/7169 species has been shown to be a unique method to probe electronic energy bands in GaAs, with sensitivity to the light hole-to-conduction band transition. This transition is often obscured in other optical measurements such as magnetoabsorption. Using OPNMR, we exploit the hyperfine interaction between conduction band electrons (and their spin states) and nuclear spins, which are detected through phase-sensitive radio-frequency (NMR) spectroscopy. Measurements were made over a range of external magnetic fields (B0) in two different labs with separate experimental setups to obtain the magnetic field dependence of the light hole-to-conduction band transition energy. In addition, k .p theory was used to interpret the experimental results, mapping out this specific transition's magnetic field dependence in an AlGaAs/GaAs quantum well. The combination of theory and experiment point to a mixing of valence bands at a field of approximately B0=4.7 T, swapping the dominant character of the absorption transition and, thus, explaining the magnetic field dependence. Lastly, the experimental dependence of the light hole-to-conduction band transition energy on B0 is found to be less steep compared to the calculated trend, indicating that inclusion of additional effects may be necessary to accurately model the spin-split band structure. The additional insight gained by Ga/7169 OPNMR about the light hole states will facilitate future testing of more complex band structure models.

Structural predictions for Correlated Electron Materials Using the Functional Dynamical Mean Field Theory Approach

NASA Astrophysics Data System (ADS)

Haule, Kristjan

2018-04-01

The Dynamical Mean Field Theory (DMFT) in combination with the band structure methods has been able to address reach physics of correlated materials, such as the fluctuating local moments, spin and orbital fluctuations, atomic multiplet physics and band formation on equal footing. Recently it is getting increasingly recognized that more predictive ab-initio theory of correlated systems needs to also address the feedback effect of the correlated electronic structure on the ionic positions, as the metal-insulator transition is almost always accompanied with considerable structural distortions. We will review recently developed extension of merger between the Density Functional Theory (DFT) and DMFT method, dubbed DFT+ embedded DMFT (DFT+eDMFT), whichsuccessfully addresses this challenge. It is based on the stationary Luttinger-Ward functional to minimize the numerical error, it subtracts the exact double-counting of DFT and DMFT, and implements self-consistent forces on all atoms in the unit cell. In a few examples, we will also show how the method elucidated the important feedback effect of correlations on crystal structure in rare earth nickelates to explain the mechanism of the metal-insulator transition. The method showed that such feedback effect is also essential to understand the dynamic stability of the high-temperature body-centered cubic phase of elemental iron, and in particular it predicted strong enhancement of the electron-phonon coupling over DFT values in FeSe, which was very recently verified by pioneering time-domain experiment.

Tailoring Dirac Fermions in Molecular Graphene

NASA Astrophysics Data System (ADS)

Gomes, Kenjiro K.; Mar, Warren; Ko, Wonhee; Camp, Charlie D.; Rastawicki, Dominik K.; Guinea, Francisco; Manoharan, Hari C.

2012-02-01

The dynamics of electrons in solids is tied to the band structure created by a periodic atomic potential. The design of artificial lattices, assembled through atomic manipulation, opens the door to engineer electronic band structure and to create novel quantum states. We present scanning tunneling spectroscopic measurements of a nanoassembled honeycomb lattice displaying a Dirac fermion band structure. The artificial lattice is created by atomic manipulation of single CO molecules with the scanning tunneling microscope on the surface of Cu(111). The periodic potential generated by the assembled CO molecules reshapes the band structure of the two-dimensional electron gas, present as a surface state of Cu(111), into a ``molecular graphene'' system. We create local defects in the lattice to observe the quasiparticle interference patterns that unveil the underlying band structure. We present direct comparison between the tunneling data, first-principles calculations of the band structure, and tight-binding models.

Synthesis, structural and semiconducting properties of Ba(Cu1/3 Sb2/3)O3-PbTiO3 solid solutions

NASA Astrophysics Data System (ADS)

Singh, Chandra Bhal; Kumar, Dinesh; Prashant, Verma, Narendra Kumar; Singh, Akhilesh Kumar

2018-05-01

We report the synthesis and properties of a new solid solution 0.05Ba(Cu1/3Sb2/3)O3-0.95PbTiO3 (BCS-PT) which shows the semiconducting properties. In this study, we have designed new perovskite-type (ABO3) solid solution of BCS-PT that have tunable optical band gap. BCS-PT compounds were prepared by conventional solid-state reaction method and their structural, micro-structural and optical properties were analyzed. The calcination temperature for BCS-PT solid solutions has been optimized to obtain a phase pure system. The Reitveld analysis of X-ray data show that all samples crystallize in tetragonal crystal structure with space group P4mm. X-ray investigation revealed that increase in calcination temperature led to increase of lattice parameter `a' while `c' parameter value lowered. The band gap of PbTiO3 is reduced from 3.2 eV to 2.8 eV with BCS doping and with increasing calcination temperature it further reduces to 2.56 eV. The reduced band gap indicated that the compounds are semiconducting and can be used for photovoltaic device applications.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Quennet, Marcel, E-mail: marcel.quennet@fu-berlin.de; Institut für Chemie und Biochemie, Freie Universität Berlin, Takustraße 3, 14195 Berlin; Ritscher, Anna

In this work the Cu/Zn order-disorder transition in Cu{sub 2}ZnSnS{sub 4} kesterites on Wyckoff positions 2c and 2d was investigated by a structural and electronic analysis in theory and experiment. For experimental investigations stoichiometric samples with different Cu/Zn order, annealed in the temperature range of 473–623 K and afterwards quenched, were used. The optical gaps were determined using the Derivation of Absorption Spectrum Fitting (DASF) method. Furthermore, the order-disorder transition was examined by DFT calculations for a closer analysis of the origins of the reduced band gap, showing a good agreement with experimental data with respect to structural and electronicmore » properties. Our studies show a slight increase of lattice parameter c in the kesterite lattice with increasing disorder. Additionally, a reduced band gap was observed with increasing disorder, which is an effect of newly occurring binding motifs in the disordered kesterite structure. - Highlights: • Experimental and theoretical investigation on the order-disorder transition in kesterites. • Slight enlargements of lattice constants due to disorder in experiment and theory. • Strong band gap fluctuations with decreasing order. • Electronic structure deviations due to changing binding motifs. • Disorder as possible main source of low open-circuit voltages.« less

Pass-Band Characteristics of an L-Shaped Waveguide in a Diamond Structure Photonic Crystal

NASA Astrophysics Data System (ADS)

Chen, Shibin; Ma, Jingcun; Yao, Yunshi; Liu, Xin; Lin, Ping

2018-06-01

The conduction characteristics of a L-shaped waveguide in a diamond structure photonic crystal is investigated in this paper. The waveguides were fabricated with titanium dioxide ceramic via 3-D printing and sintering. The effects of the position and size of line defects on the transmission characteristics are first simulated using a finite-difference time-domain method. The simulated results show that, when the length of the rectangular defect equals the lattice constant, multiple extended modes are generated. When the centers of the single unit cell of the diamond structure and the line defect waveguide coincide, higher transmission efficiency in the line defect can be achieved. In addition, the corner of the L-shaped waveguide was optimized to reduce reflection loss at the turning point using the arc transition of the large diameter. Our experimental results indicate that L-shaped waveguides with an optimized photonic band gap structure and high-K materials can produce a pass-band between 13.8 GHz and 14.4 GHz and increase transmission efficiency. The computed results agree with the experimental results. Our results may help the integration of microwave devices in the future and possibly enable new applications of photonic crystals.

Hong Kong secondary school students' attitudes towards science: a study of structural models and gender differences

NASA Astrophysics Data System (ADS)

Wan, Zhi Hong; Lee, John Chi Kin

2017-03-01

This study explored two under-researched areas on students' attitudes towards science, that is, the structural models representing these attitudes and the role played by school bands in moderating the gender differences in such attitudes. The participants were 360 ninth graders in Hong Kong from 3 school bands. The structural equation modelling method was adopted to compare four hypothetical models for students' attitudes towards science. Results reflect that (i) the data supported the three-factor structure of the behavioural domain of students' attitudes towards science; (ii) the four lower level dimensions of the attitudes towards science (i.e. value of science to society, self-concept in science, anxiety towards science and enjoyment of science) could be further integrated into broader categories; (iii) male students demonstrated significantly more positive attitudes towards science in five dimensions (i.e. self-concept in science, enjoyment in science, learning science in and outside the classroom and future participation) and (iv) school bands played a prominent moderating role in gender differences in students' attitudes towards science. Implications for studying and developing students' attitudes towards science are discussed in the paper.

Localized diabatization applied to excitons in molecular crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jin, Zuxin; Subotnik, Joseph E.

Traditional ab initio electronic structure calculations of periodic systems yield delocalized eigenstates that should be understood as adiabatic states. For example, excitons are bands of extended states which superimpose localized excitations on every lattice site. However, in general, in order to study the effects of nuclear motion on exciton transport, it is standard to work with a localized description of excitons, especially in a hopping regime; even in a band regime, a localized description can be helpful. To extract localized excitons from a band requires essentially a diabatization procedure. In this paper, three distinct methods are proposed for such localizedmore » diabatization: (i) a simple projection method, (ii) a more general Pipek-Mezey localization scheme, and (iii) a variant of Boys diabatization. Approaches (i) and (ii) require localized, single-particle Wannier orbitals, while approach (iii) has no such dependence. Lastly, these methods should be very useful for studying energy transfer through solids with ab initio calculations.« less

Localized diabatization applied to excitons in molecular crystals

DOE PAGES

Jin, Zuxin; Subotnik, Joseph E.

2017-06-28

Traditional ab initio electronic structure calculations of periodic systems yield delocalized eigenstates that should be understood as adiabatic states. For example, excitons are bands of extended states which superimpose localized excitations on every lattice site. However, in general, in order to study the effects of nuclear motion on exciton transport, it is standard to work with a localized description of excitons, especially in a hopping regime; even in a band regime, a localized description can be helpful. To extract localized excitons from a band requires essentially a diabatization procedure. In this paper, three distinct methods are proposed for such localizedmore » diabatization: (i) a simple projection method, (ii) a more general Pipek-Mezey localization scheme, and (iii) a variant of Boys diabatization. Approaches (i) and (ii) require localized, single-particle Wannier orbitals, while approach (iii) has no such dependence. Lastly, these methods should be very useful for studying energy transfer through solids with ab initio calculations.« less

Effect of filling factor on photonic bandgap of chalcogenide photonic crystal

NASA Astrophysics Data System (ADS)

Singh, Rajpal; Suthar, B.; Bhargava, A.

2018-05-01

In the present work, the photonic band structure of 1-D chalcogenide photonic crystal of As2S3/air multilayered structure is calculated using the plane wave expansion method. The study is extended to investigate the effect of filling factor on the photonic bandgap. The increase of bandgap is explained in the study.

Classification of visible and infrared hyperspectral images based on image segmentation and edge-preserving filtering

NASA Astrophysics Data System (ADS)

Cui, Binge; Ma, Xiudan; Xie, Xiaoyun; Ren, Guangbo; Ma, Yi

2017-03-01

The classification of hyperspectral images with a few labeled samples is a major challenge which is difficult to meet unless some spatial characteristics can be exploited. In this study, we proposed a novel spectral-spatial hyperspectral image classification method that exploited spatial autocorrelation of hyperspectral images. First, image segmentation is performed on the hyperspectral image to assign each pixel to a homogeneous region. Second, the visible and infrared bands of hyperspectral image are partitioned into multiple subsets of adjacent bands, and each subset is merged into one band. Recursive edge-preserving filtering is performed on each merged band which utilizes the spectral information of neighborhood pixels. Third, the resulting spectral and spatial feature band set is classified using the SVM classifier. Finally, bilateral filtering is performed to remove "salt-and-pepper" noise in the classification result. To preserve the spatial structure of hyperspectral image, edge-preserving filtering is applied independently before and after the classification process. Experimental results on different hyperspectral images prove that the proposed spectral-spatial classification approach is robust and offers more classification accuracy than state-of-the-art methods when the number of labeled samples is small.

Characteristics of strain-sensitive photonic crystal cavities in a flexible substrate.

PubMed

No, You-Shin; Choi, Jae-Hyuck; Kim, Kyoung-Ho; Park, Hong-Gyu

2016-11-14

High-index semiconductor photonic crystal (PhC) cavities in a flexible substrate support strong and tunable optical resonances that can be used for highly sensitive and spatially localized detection of mechanical deformations in physical systems. Here, we report theoretical studies and fundamental understandings of resonant behavior of an optical mode excited in strain-sensitive rod-type PhC cavities consisting of high-index dielectric nanorods embedded in a low-index flexible polymer substrate. Using the three-dimensional finite-difference time-domain simulation method, we calculated two-dimensional transverse-electric-like photonic band diagrams and the three-dimensional dispersion surfaces near the first Γ-point band edge of unidirectionally strained PhCs. A broken rotational symmetry in the PhCs modifies the photonic band structures and results in the asymmetric distributions and different levels of changes in normalized frequencies near the first Γ-point band edge in the reciprocal space, which consequently reveals strain-dependent directional optical losses and selected emission patterns. The calculated electric fields, resonant wavelengths, and quality factors of the band-edge modes in the strained PhCs show an excellent agreement with the results of qualitative analysis of modified dispersion surfaces. Furthermore, polarization-resolved time-averaged Poynting vectors exhibit characteristic dipole-like emission patterns with preferentially selected linear polarizations, originating from the asymmetric band structures in the strained PhCs.

Study of optical reflectance properties in 1D annular photonic crystal containing double negative (DNG) metamaterials

NASA Astrophysics Data System (ADS)

Srivastava, Sanjeev K.; Aghajamali, Alireza

2016-05-01

Theoretical investigation of photonic band gaps or reflection bands in one-dimensional annular photonic crystal (APC) containing double negative (DNG) metamaterials and air has been presented. The proposed structure consists of the alternate layers of dispersive DNG material and air immersed in free space. In order to study photonic band gaps we obtain the reflectance spectrum of the annular PC by employing the transfer matrix method (TMM) in the cylindrical waves for both TE and TM polarizations. In this work we study the effect of azimuthal mode number (m) and starting radius (ρ0) on the three band gaps viz. zero averaged refractive index (zero-nbar) gap, zero permittivity (zero- ε) and zero permeability (zero- μ) gaps. It is found that for m ≥ 1 , zero- μ gap appears in TE mode and zero- ε gap appears in TM mode. The width of both zero- μ and zero- ε gap increases by increasing m values, but the enhancement of zero- μ gap is more appreciable. Also, the effect of ρ0 on the three band gaps (reflection bands) of annular PC structure at the given m-number has been studied, for both TE and TM polarizations. The result shows that in both polarizations zero- ε and zero- μ gaps decreases when ρ0 increases, whereas zero-nbar gap remains invariant.

Temperature control of the ultra-short laser pulse compression in a one-dimensional photonic band gap structure with nematic liquid crystal as a defect layer

NASA Astrophysics Data System (ADS)

Shiri, Ramin; Safari, Ebrahim; Bananej, Alireza

2018-04-01

We investigate numerically the controllable chirped pulse compression in a one-dimensional photonic structure containing a nematic liquid crystal defect layer using the temperature dependent refractive index of the liquid crystal. We consider the structure under irradiation by near-infrared ultra-short laser pulses polarized parallel to the liquid crystal director at a normal angle of incidence. It is found that the dispersion behaviour and consequently the compression ability of the system can be changed in a controlled manner due to the variation in the defect temperature. When the temperature increased from 290 to 305 K, the transmitted pulse duration decreased from 75 to 42 fs in the middle of the structure, correspondingly. As a result, a novel low-loss tunable pulse compressor with a really compact size and high compression factor is achieved. The so-called transfer matrix method is utilized for numerical simulations of the band structure and reflection/transmission spectra of the structure under investigation.

Density Functional Theory Calculations Revealing Metal-like Band Structures for Ultrathin Ge {111} and {211} Surface Layers.

PubMed

Tan, Chih-Shan; Huang, Michael Hsuan-Yi

2018-05-21

To find out if germanium should also possess facet-dependent electrical conductivity properties, surface state density functional theory (DFT) calculations were performed on 1-6 layers of Ge (100), (110), (111), and (211) planes. Tunable Ge (100) and (110) planes always present the same semiconducting band structure with a band gap of 0.67 eV expected of bulk germanium. In contrast, 1, 2, 4, and 5 layers of Ge (111) and (211) plane models show metal-like band structures with continuous density of states (DOS) throughout the entire band. For 3 and 6 layers of Ge (111) and (211) plane models, the normal semiconducting band structure was obtained. The plane layers with metal-like band structures also show Ge-Ge bond length deviations and bond distortions, as well as significantly different 4s and 4p frontier orbital electron count and their relative percentages integrated over the valence and conduction bands from those of the semiconducting state. These differences should contribute to strikingly dissimilar band structures. The calculation results suggest observation of facet-dependent electrical conductivity properties of germanium materials, and transistors made of germanium may also need to consider the facet effects with shrinking dimensions approaching 3 nm. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Insight into the structural, electronic, elastic and optical properties of the alkali hydride compounds, XH (X = Rb and Cs)

NASA Astrophysics Data System (ADS)

Jaradat, Raed; Abu-Jafar, Mohammed; Abdelraziq, Issam; Mousa, Ahmad; Ouahrani, Tarik; Khenata, Rabah

2018-04-01

The equilibrium structural parameters, electronic and optical properties of the alkali hydrides RbH and CsH compounds in rock-salt (RS) and cesium chloride (CsCl) structures have been studied using the full-potential linearized augmented plane-wave (FP-LAPW) method. Wu and Cohen generalized gradient approximation (WC-GGA) was used for the exchange-correlation potential to compute the equilibrium structural parameters, such as the lattice constant (a0), the bulk modulus (B) and bulk modulus first order pressure derivative (B'). In addition to the WC-GGA, the modified Becke Johnson (mBJ) scheme has been also used to overcome the underestimation of the band gap energies. RbH and CsH compounds are found to be semiconductors (wide energy-band gap) using the WC-GGA method, while they are insulators using the mBJ-GGA method. Elastic constants, mechanical and thermodynamic properties were obtained by using the IRelast package. RbH and CsH compounds at ambient pressure are mechanically stable in RS and CsCl structures; they satisfy the Born mechanical stability criteria. Elastic constants (Cij), bulk modulus (B), shear modulus (S) and Debye temperatures (θD) of RbH and CsH compounds decrease as the alkali radius increases. The RS structure of these compounds at ambient conditions is mechanically stronger than CsCl structure. RbH and CsH in RS and CsCl structures are suitable as dielectric compounds. The wide direct energy band gap for these compounds make them promising compounds for optoelectronic UV device applications. Both RbH and CsH have a wide absorption region, on the other hand RbH absorption is very huge compared to the CsH absorption, RbH is an excellent absorbent material, maximum absorption regions are located in the middle ultraviolet (MUV) region and far ultraviolet (FUV) region. The absorption coefficient α (w), imaginary part of the dielectric constant ɛ2(w) and the extinction coefficient k(w) vary in the same way. The present calculated results are in good agreement with the experimental data, indicating the high accuracy of the performed calculations and reliability of the obtained results.

[Spectral studies on nano-sized titania photocatalysts prepared by different drying methods].

PubMed

Ye, Zhao; Zhang, Han-hui; Pan, Hai-bo; Pan, Hong-qing

2002-12-01

Nano-sized TiO2 photocatalysts were prepared by drying the ethanol gel of titanium tetrabutoxide through natural state, supercritical ethanol, supercritical carbon dioxide drying methods and characterized by XRD, FTIR spectroscopy, FT-Raman spectroscopy and fluorescent spectroscopy, respectively. We regard degradation of rhodamine B by photocatalyst as a model reaction, and compare photocatalytic activities of samples obtained. The experimental results show that different drying methods have strong effect on crystal structure, energy band structure, optical adsorption property, surface quality and photocatalytic activity, TiO2 photocatalyst prepared by supercritical carbon dioxide drying method has superior photocatalytic activity.

Transport characteristics of a ZnMgO/ZnO hetero junction and the effect of temperature and Mg content

NASA Astrophysics Data System (ADS)

Uslu, Salih; Yarar, Zeki

2017-02-01

The Ensemble Monte Carlo method is used to calculate the transport characteristics of two dimensional electron gas (2DEG) at a ZnMgO/ZnO hetero structure. The spontaneous and piezoelectric polarizations are considered and there is no intentional doping in either material. Numerical Schrödinger and Poisson equations are solved self consistently to obtain the scattering rates of various scattering mechanisms. The density of carriers, each energy sub bands, potential profile and corresponding wave functions are obtained from the self consistent calculations. The self consistent sub band wave functions of acoustic and optic phonon scattering and interface roughness scattering are used in Monte Carlo method to obtain transport characteristics at ZnMgO/ZnO junction. Two dimensional electron gas confined to ZnMgO/ZnO hetero structure is studied and the effect of temperature and Mg content are investigated.

Indium oxide inverse opal films synthesized by structure replication method

NASA Astrophysics Data System (ADS)

Amrehn, Sabrina; Berghoff, Daniel; Nikitin, Andreas; Reichelt, Matthias; Wu, Xia; Meier, Torsten; Wagner, Thorsten

2016-04-01

We present the synthesis of indium oxide (In2O3) inverse opal films with photonic stop bands in the visible range by a structure replication method. Artificial opal films made of poly(methyl methacrylate) (PMMA) spheres are utilized as template. The opal films are deposited via sedimentation facilitated by ultrasonication, and then impregnated by indium nitrate solution, which is thermally converted to In2O3 after drying. The quality of the resulting inverse opal film depends on many parameters; in this study the water content of the indium nitrate/PMMA composite after drying is investigated. Comparison of the reflectance spectra recorded by vis-spectroscopy with simulated data shows a good agreement between the peak position and calculated stop band positions for the inverse opals. This synthesis is less complex and highly efficient compared to most other techniques and is suitable for use in many applications.

Microwave-assisted synthesis and characterization of nickel ferrite nanoparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Carpenter, Gopal; Sen, Ravindra; Gupta, Nitish, E-mail: nitish.nidhi75@gmail.com

2015-08-28

Nickel ferrite nanoparticles (NiFe{sub 2}O{sub 4}) were successfully prepared by microwave-assisted combustion method (MWAC) using citric Electron acid as a chelating agent. NiFe{sub 2}O{sub 4} nanoparticles were characterized by X-ray diffraction (XRD) pattern, Scanning Microscopy (SEM), Fourier transform infrared (FTIR) and UV-Visible techniques. XRD analysis revealed that NiFe{sub 2}O{sub 4} nanoparticles have spinel cubic structure with the average crystalline size of 26.38 nm. SEM analysis revealed random and porous structural morphology of particles and FTIR showed absorption bands related to octahedral and tetrahedral sites, in the range 400–600cm{sup −1} which strongly favor the formation of NiFe{sub 2}O{sub 4} nanoparticles. The opticalmore » band gap is determined by UV Visible method and found to be 5.4 eV.« less

Multipole surface plasmons in metallic nanohole arrays

NASA Astrophysics Data System (ADS)

Nishida, Munehiro; Hatakenaka, Noriyuki; Kadoya, Yutaka

2015-06-01

The quasibound electromagnetic modes for the arrays of nanoholes perforated in thin gold film are analyzed both numerically by the rigorous coupled wave analysis (RCWA) method and semianalytically by the coupled mode method. It is shown that when the size of the nanohole occupies a large portion of the unit cell, the surface plasmon polaritons (SPPs) at both sides of the film are combined by the higher order waveguide modes of the holes to produce multipole surface plasmons: coupled surface plasmon modes with multipole texture on the elec-tric field distributions. Further, it is revealed that the multipole texture either enhances or suppresses the couplings between SPPs depending on their diffraction orders and also causes band inversion and reconstruction in the coupled SPP band structure. Due to the multipole nature of the quasibound modes, multiple dark modes coexist to produce a variety of Fano resonance structures on the transmission and reflection spectra.

Electrical transport engineering of semiconductor superlattice structures

NASA Astrophysics Data System (ADS)

Shokri, Aliasghar

2014-04-01

We investigate the influence of doping concentration on band structures of electrons and electrical transmission in a typical aperiodic semiconductor superlattice consisting of quantum well and barrier layers, theoretically. For this purpose, we assume that each unit cell of the superlattice contains alternately two types of material GaAs (as a well) and GaAlAs (as a barrier) with six sublayers of two materials. Our calculations are based on the generalized Kronig-Penny (KP) model and the transfer matrix method within the framework of the parabolic conductance band effective mass approximation in the coherent regime. This model reduces the numerical calculation time and enables us to use the transfer matrix method to investigate transport in the superlattices. We show that by varying the doping concentration and geometrical parameters, one can easily block the transmission of the electrons. The numerical results may be useful in designing of nanoenergy filter devices.

New designs of a complete set of Photonic Crystals logic gates

NASA Astrophysics Data System (ADS)

Hussein, Hussein M. E.; Ali, Tamer A.; Rafat, Nadia H.

2018-03-01

In this paper, we introduce new designs of all-optical OR, AND, XOR, NOT, NOR, NAND and XNOR logic gates based on the interference effect. The designs are built using 2D square lattice Photonic Crystal (PhC) structure of dielectric rods embedded in air background. The lattice constant, a, and the rod radius, r, are designed to achieve maximum operating range of frequencies using the gap map. We use the Plane Wave Expansion (PWE) method to obtain the band structure and the gap map of the proposed designs. The operating wavelengths achieve a wide band range that varies between 1266.9 nm and 1996 nm with center wavelength at 1550 nm. The Finite-Difference Time-Domain (FDTD) method is used to study the field behavior inside the PhC gates. The gates satisfy their truth tables with reasonable power contrast ratio between logic '1' and logic '0'.

Photonic band gap structure simulator

DOEpatents

Chen, Chiping; Shapiro, Michael A.; Smirnova, Evgenya I.; Temkin, Richard J.; Sirigiri, Jagadishwar R.

2006-10-03

A system and method for designing photonic band gap structures. The system and method provide a user with the capability to produce a model of a two-dimensional array of conductors corresponding to a unit cell. The model involves a linear equation. Boundary conditions representative of conditions at the boundary of the unit cell are applied to a solution of the Helmholtz equation defined for the unit cell. The linear equation can be approximated by a Hermitian matrix. An eigenvalue of the Helmholtz equation is calculated. One computation approach involves calculating finite differences. The model can include a symmetry element, such as a center of inversion, a rotation axis, and a mirror plane. A graphical user interface is provided for the user's convenience. A display is provided to display to a user the calculated eigenvalue, corresponding to a photonic energy level in the Brilloin zone of the unit cell.

Fourier transform infrared and Raman spectroscopic study of the effect of the thermal treatment and extraction methods on the characteristics of ayocote bean starches.

PubMed

Bernardino-Nicanor, Aurea; Acosta-García, Gerardo; Güemes-Vera, Norma; Montañez-Soto, José Luis; de Los Ángeles Vivar-Vera, María; González-Cruz, Leopoldo

2017-03-01

Starches isolated from four ayocote bean varieties were modified by thermal treatment to determinate the effect of the treatment on the structural changes of ayocote bean starch. Scanning electron microscopy indicates that the starch granules have oval and round shapes, with heterogeneous sizes and fractures when the extraction method is used. The presence of new bands at 2850 and 1560 cm -1 in the FT-IR spectra showed that the thermal treatment of ayocote beans induced an interaction between the protein or lipid and the amylose or amylopectin, while the sharpest band at 3400 cm -1 indicated a dehydration process in the starch granule in addition to the presence of the band at 1260 cm -1 , indicating the product of the retrogradation process. The thermal treatment reduced the crystallinity as well as short-range order. Raman spectroscopy revealed that acute changes occurred in the polysaccharide bonds after thermal treatment. This study showed that the thermal treatment affected the structural properties of ayocote bean starches, the interactions of the lipids and proteins with starch molecules and the retrogradation process of starch.

Influence of the aggregate state on band structure and optical properties of C60 computed with different methods

NASA Astrophysics Data System (ADS)

Pal, Amrita; Arabnejad, Saeid; Yamashita, Koichi; Manzhos, Sergei

2018-05-01

C60 and C60 based molecules are efficient acceptors and electron transport layers for planar perovskite solar cells. While properties of these molecules are well studied by ab initio methods, those of solid C60, specifically its optical absorption properties, are not. We present a combined density functional theory-Density Functional Tight Binding (DFTB) study of the effect of solid state packing on the band structure and optical absorption of C60. The valence and conduction band edge energies of solid C60 differ on the order of 0.1 eV from single molecule frontier orbital energies. We show that calculations of optical properties using linear response time dependent-DFT(B) or the imaginary part of the dielectric constant (dipole approximation) can result in unrealistically large redshifts in the presence of intermolecular interactions compared to available experimental data. We show that optical spectra computed from the frequency-dependent real polarizability can better reproduce the effect of C60 aggregation on optical absorption, specifically with a generalized gradient approximation functional, and may be more suited to study effects of molecular aggregation.

The fcc - bcc structural transition: I. A band theoretical study for Li, K, Rb, Ca, Sr, and the transition metals Ti and V

NASA Astrophysics Data System (ADS)

Sliwko, V. L.; Mohn, P.; Schwarz, K.; Blaha, P.

1996-02-01

Employing a high-precision band structure method (FP LAPW - full potential linearized augmented plane wave) we calculate the total energy variation along the tetragonal distortion path connecting the body centred cubic (bcc) and the face centred cubic (fcc) structures. The total energy along this Bain transformation is calculated, varying c/a and volume, providing a first-principles energy surface which has two minima as a function of c/a. These are shallow and occur for the sp metals at the two cubic structures, while Ti (V) has a minimum at fcc (bcc) but a saddle point (i.e. a minimum in volume and a maximum with respect to c/a) at the other cubic structure. These features can be analysed in terms of an interplay between the Madelung contribution and the band energies. Our total energy results allow us to calculate the elastic constants 0953-8984/8/7/006/img1 and 0953-8984/8/7/006/img2 and to study the influence of pressure on the phase stability. These energy surfaces will be used in part II of this paper to investigate finite-temperature effects by mapping them to a Landau - Ginzburg expansion.

Effect of oxygen vacancies on the electronic and optical properties of tungsten oxide from first principles calculations

NASA Astrophysics Data System (ADS)

Mehmood, Faisal; Pachter, Ruth; Murphy, Neil R.; Johnson, Walter E.; Ramana, Chintalapalle V.

2016-12-01

In this work, we investigated theoretically the role of oxygen vacancies on the electronic and optical properties of cubic, γ-monoclinic, and tetragonal phases of tungsten oxide (WO3) thin films. Following the examination of structural properties and stability of the bulk tungsten oxide polymorphs, we analyzed band structures and optical properties, applying density functional theory (DFT) and GW (Green's (G) function approximation with screened Coulomb interaction (W)) methods. Careful benchmarking of calculated band gaps demonstrated the importance of using a range-separated functional, where results for the pristine room temperature γ-monoclinic structure indicated agreement with experiment. Further, modulation of the band gap for WO3 structures with oxygen vacancies was quantified. Dielectric functions for cubic WO3, calculated at both the single-particle, essentially time-dependent DFT, as well as many-body GW-Bethe-Salpeter equation levels, indicated agreement with experimental data for pristine WO3. Interestingly, we found that introducing oxygen vacancies caused appearance of lower energy absorptions. A smaller refractive index was indicated in the defective WO3 structures. These predictions could lead to further experiments aimed at tuning the optical properties of WO3 by introducing oxygen vacancies, particularly for the lower energy spectral region.

Theoretical study of platonic crystals with periodically structured N-beam resonators

NASA Astrophysics Data System (ADS)

Gao, Penglin; Climente, Alfonso; Sánchez-Dehesa, José; Wu, Linzhi

2018-03-01

A multiple scattering theory is applied to study the properties of flexural waves propagating in a plate with periodically structured N-beam resonators. Each resonator consists of a circular hole containing an inner disk connected to background plate with N rectangular beams. The Bloch theorem is employed to obtain the band structure of a two-dimensional lattice containing a single resonator per unit cell. Also, a numerical algorithm has been developed to get the transmittance through resonator slabs infinitely long in the direction perpendicular to the incident wave. For the numerical validation, a square lattice of 2-beam resonators has been comprehensively analyzed. Its band structure exhibits several flat bands, indicating the existence of local resonances embedded in the structure. Particularly, the one featured as the fundamental mode of the inner disk opens a bandgap at low frequencies. This mode has been fully described in terms of a simple spring-mass model. As a practical application of the results obtained, a homogenization approach has been employed to design a focusing lens for flexural waves, where the index gradient is obtained by adjusting the orientation of the resonators beams. Numerical experiments performed within the framework of a three-dimensional finite element method have been employed to discuss the accuracy of the models described here.

Computational and spectroscopic studies of a new Schiff base 3-hydroxy-4-methoxybenzylidene(2-hydroxyphenyl)amine and molecular structure of its corresponding zwitterionic form.

PubMed

Habibi, Mohammad Hossein; Shojaee, Elahe; Ranjbar, Mahnaz; Memarian, Hamid Reza; Kanayama, Akihiko; Suzuki, Takayoshi

2013-03-15

Computational and spectroscopic properties of a novel Schiff base compound, 3-hydroxy-4-methoxybenzylidene(2-hydroxyphenyl)amine were studied. The crystal structures of the title compound and its corresponding zwitterionic form were analyzed by X-ray diffraction. The presence of N-H, C-O and C=N stretching vibrations in IR spectrum strongly suggest that the title compound has zwitterionic form in the solid state. Molecular geometry of the title compound in the ground state has been calculated using the density functional method (DFT) at B3LYP 6-31++G(d,p) basis set and was compared with the experimental data. The calculated results of the title compound show that the optimized geometry can well reproduce the crystal structure. The molecule shows absorption bands at 345 and 363 nm in EtOH. The shoulder-shaped bands at 415 nm can be assigned to n→π(*) transitions. The absorption band is observed at 285 nm in EtOH corresponds to the π→π(*) transitions. Copyright © 2012 Elsevier B.V. All rights reserved.

High thermoelectric performances of monolayer SnSe allotropes.

PubMed

Hu, Zi-Yu; Li, Kai-Yue; Lu, Yong; Huang, Yan; Shao, Xiao-Hong

2017-10-26

α-SnSe is one of the most promising thermoelectric materials with low thermal conductivity and a high power factor. Since the thermoelectric properties of a material have a strong dependence on its crystal structure, we study the energetic and thermoelectric properties of four new monolayer phases of SnSe (β, γ, δ and ε) together with α-SnSe using the ab initio density functional theory method. The calculated electronic structures show that all five phases are semiconductors with different band gaps. The α, β, γ, and δ phases have an indirect band gap with the hybridization of sp 2 orbitals, whereas the ε phase has a direct band with the hybridization of sp 3 orbitals. The thermoelectric transport properties and coefficients are obtained from the electronic structure using semi-classical Boltzmann theory, and the results indicate that the four new phases of SnSe (β, γ, δ and ε) all have better thermoelectric properties compared with the reported α phase. The predicted ZT value for the β-SnSe phase is 2.06 at 300 K, suggesting that it has great potential for novel thermoelectric applications.

X-ray photoemission spectroscopy investigation of CaTiO{sub 3}:Eu for luminescence property: effect of Eu{sup 3+} ion

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Kaichen; Zhao, Baijun; Gao, Lu, E-mail: gaolu@jlu.edu.cn

2016-06-15

Graphical abstract: The influence on the photoluminescent performance due to the electronic structure change in Eu-doped CaTiO{sub 3} of the specific core-level and valence band spectrum via X-ray photoemission spectroscopy were characterized. - Highlights: • Single phase CaTiO{sub 3} and CaTiO{sub 3}: Eu crystals were prepared under mild hydrothermal method. • Crystal structure, doping level and the relations to their luminescent property were discussed. • Charge compensation mechanism was discussed via valance band spectrum by XPS. - Abstract: Charge compensation of on-site Eu 4f–5d transition that determines the luminescent performance was confirmed with valance band spectrum. Influence of photoelectrons frommore » CaTiO{sub 3}: Eu to the corresponding luminescent performance was discussed based on the crystal structure, doping level and the relations to their luminescent property. This paper is important to further optimize the luminescent performance for improving the efficiency and reducing the cost in light emitting diode industry.« less

Transmission properties of one-dimensional ternary plasma photonic crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shiveshwari, Laxmi; Awasthi, S. K.

2015-09-15

Omnidirectional photonic band gaps (PBGs) are found in one-dimensional ternary plasma photonic crystals (PPC) composed of single negative metamaterials. The band characteristics and transmission properties are investigated through the transfer matrix method. We show that the proposed structure can trap light in three-dimensional space due to the elimination of Brewster's angle transmission resonance allowing the existence of complete PBG. The results are discussed in terms of incident angle, layer thickness, dielectric constant of the dielectric material, and number of unit cells (N) for TE and TM polarizations. It is seen that PBG characteristics is apparent even in an N ≥ 2 system,more » which is weakly sensitive to the incident angle and completely insensitive to the polarization. Finite PPC could be used for multichannel transmission filter without introducing any defect in the geometry. We show that the locations of the multichannel transmission peaks are in the allowed band of the infinite structure. The structure can work as a single or multichannel filter by varying the number of unit cells. Binary PPC can also work as a polarization sensitive tunable filter.« less

Electronic structure of germanium selenide investigated using ultra-violet photo-electron spectroscopy

NASA Astrophysics Data System (ADS)

Mishra, P.; Lohani, H.; Kundu, A. K.; Patel, R.; Solanki, G. K.; Menon, Krishnakumar S. R.; Sekhar, B. R.

2015-07-01

The valence band electronic structure of GeSe single crystals has been investigated using angle resolved photoemission spectroscopy (ARPES) and x-ray photoelectron spectroscopy. The experimentally observed bands from ARPES, match qualitatively with our LDA-based band structure calculations along the Γ-Z, Γ-Y and Γ-T symmetry directions. The valence band maximum occurs nearly midway along the Γ-Z direction, at a binding energy of -0.5 eV, substantiating the indirect band gap of GeSe. Non-dispersive features associated with surface states and indirect transitions have been observed. The difference in hybridization of Se and Ge 4p orbitals leads to the variation of dispersion along the three symmetry directions. The predominance of the Se 4pz orbitals, evidenced from theoretical calculations, may be the cause for highly dispersive bands along the Γ-T direction. Detailed electronic structure analysis reveals the significance of the cation-anion 4p orbitals hybridization in the valence band dispersion of IV-VI semiconductors. This is the first comprehensive report of the electronic structure of a GeSe single crystal using ARPES in conjugation with theoretical band structure analysis.

Synthesis, structural, electronic and linear electro-optical features of new quaternary Ag2Ga2SiS6 compound

NASA Astrophysics Data System (ADS)

Piasecki, M.; Myronchuk, G. L.; Parasyuk, O. V.; Khyzhun, O. Y.; Fedorchuk, A. O.; Pavlyuk, V. V.; Kozer, V. R.; Sachanyuk, V. P.; El-Naggar, A. M.; Albassam, A. A.; Jedryka, J.; Kityk, I. V.

2017-02-01

For the first time phase equilibria and phase diagram of the AgGaS2-SiS2 system were successfully explored by differential thermal and X-ray phase analysis methods. Crystal structure of low-temperature (LT) modification of Ag2Ga2SiS6 (LT- Ag2Ga2SiS6) was studied by X-ray powder method and it belongs to tetragonal space group I-42d, with unit cell parameters a=5.7164(4) Å, c=9.8023(7) Å, V=320.32(7) Å3. Additional details regarding the crystal structure exploration are available at the web page Fachinformationszentrum Karlsruhe. X-ray photoelectron core-level and valence-band spectra were measured for pristine LT- Ag2Ga2SiS6 crystal surface. In addition, the X-ray photoelectron valence-band spectrum of LT-Ag2Ga2SiS6 was matched on a common energy scale with the X-ray emission S Kβ1,3 and Ga Kβ2 bands, which give information on the energy distribution of the S 3p and Ga 4p states, respectively. The presented X-ray spectroscopy results indicate that the valence S p and Ga p atomic states contribute mainly to the upper and central parts of the valence band of LT-Ag2Ga2SiS6, respectively, with a less significant contribution also to other valence-band regions. Band gap energy was estimated by measuring the quantum energy in the spectral range of the fundamental absorption. We have found that energy gap Eg is equal to 2.35 eV at 300 K. LT-Ag2Ga2SiS6 is a photosensitive material and reveals two spectral maxima on the curve of spectral photoconductivity spectra at λmax1 =590 nm and λmax2 =860 nm. Additionally, linear electro-optical effect of LT-Ag2Ga2SiS6 for the wavelengths of a cw He-Ne laser at 1150 nm was explored.

Efficient computation of photonic crystal waveguide modes with dispersive material.

PubMed

Schmidt, Kersten; Kappeler, Roman

2010-03-29

The optimization of PhC waveguides is a key issue for successfully designing PhC devices. Since this design task is computationally expensive, efficient methods are demanded. The available codes for computing photonic bands are also applied to PhC waveguides. They are reliable but not very efficient, which is even more pronounced for dispersive material. We present a method based on higher order finite elements with curved cells, which allows to solve for the band structure taking directly into account the dispersiveness of the materials. This is accomplished by reformulating the wave equations as a linear eigenproblem in the complex wave-vectors k. For this method, we demonstrate the high efficiency for the computation of guided PhC waveguide modes by a convergence analysis.

DFT with larger supercells explains the band gap formation in the antiferromagnetic and paramagnetic phases of the Mott insulators MnO, FeO, CoO, and NiO

NASA Astrophysics Data System (ADS)

Zunger, Alex; Trimarchi, Giancarlo

The existence of large band gaps both in the antiferromagnetic (AFM) and the paramagnetic (PM) phases of the classic Mott insulators MnO, FeO, CoO, and NiO has traditionally been discussed in terms of theoretical methods requiring both (i) simple (often primitive) unit cells and (ii) correlated-electron methodologies. We show that if condition (i) is avoided (by using supercells, such as PM special quasi-random structures, in which chemically identical atoms can have different local environments), then even without condition (ii) one can describe the gaps and moments within a single-determinant DFT band structure approach. In this approach gapping is caused by basic structure, magnetism, and bonding effects underlying DFT, not via dynamic correlation (absent from DFT). As long as correlation is simplistically considered as ``anything that DFT does not get right'', gap formation in the AFM and PM phases is not due to correlation. This result defines the minimal theoretical methods needed to explain gapping and points to the possibility that some transition-metal oxides generally considered to have localized electrons detrimental to transport, could, in fact, rejoin the family of electronic semiconductors, to the benefit of a carrier transport technologies. A. Z. supported by DOE-OS-BES-MSE, Grant DE-FG02-13ER46959.

Effect of titanium on the structural and optical property of NiO nano powders

NASA Astrophysics Data System (ADS)

Amin, Ruhul; Mishra, Prashant; Khatun, Nasima; Ayaz, Saniya; Srivastava, Tulika; Sen, Somaditya

2018-05-01

Nickel Oxide (NiO) and Ti doped NiO nanoparticles were prepared by sol-gel auto combustion method. Powder x-ray diffraction (PXRD) structural studies revealed face centered cubic (FCC) structure of the NiO nanopowders. The crystallite size decreased with Ti incorporation. UV-Vis spectroscopy carried out in diffused reflectance mode revealed decrease in band gap with increment in Urbach energy with doping.

Mapping Glauconite Unites with Using Remote Sensing Techniques in North East of Iran

NASA Astrophysics Data System (ADS)

Ahmadirouhani, R.; Samiee, S.

2014-10-01

Glauconite is a greenish ferric-iron silicate mineral with micaceous structure, characteristically formed in shallow marine environments. Glauconite has been used as a pigmentation agent for oil paint, contaminants remover in environmental studies and a source of potassium in plant fertilizers, and other industries. Koppeh-dagh basin is extended in Iran, Afghanistan and Turkmenistan countries and Glauconite units exist in this basin. In this research for enhancing and mapping glauconitic units in Koppeh-dagh structural zone in north east of Iran, remote sensing techniques such as Spectral Angle Mapper classification (SAM), band ratio and band composition methods on SPOT, ASTER and Landsat data in 3 steps were applied.

The Electronic Structure of the Cs/ n-GaN(0001) Nano-Interface

NASA Astrophysics Data System (ADS)

Benemanskaya, G. V.; Lapushkin, M. N.; Marchenko, D. E.; Timoshnev, S. N.

2018-03-01

Electronic structures of the n-GaN(0001) surface and Cs/ n-GaN(0001) interface with submonolayer Cs coverages were studied for the first time in situ by the photoelectron spectroscopy (PES) method. The spectra of photoemission from the valence band, surface electron states, and core levels (Ga 3 d, Cs 4 d, Cs 5 p) under synchrotron excitation were measured in a range of photon energies within 50-150 eV. Evolution of the spectrum of surface states near the valence-band maximum was revealed by PES during the adsorption of Cs atoms. A metallic character of the Cs/ n-GaN(0001) nano-interface is demonstrated.

Integrative interactive visualization of crystal structure, band structure, and Brillouin zone

NASA Astrophysics Data System (ADS)

Hanson, Robert; Hinke, Ben; van Koevering, Matthew; Oses, Corey; Toher, Cormac; Hicks, David; Gossett, Eric; Plata Ramos, Jose; Curtarolo, Stefano; Aflow Collaboration

The AFLOW library is an open-access database for high throughput ab-initio calculations that serves as a resource for the dissemination of computational results in the area of materials science. Our project aims to create an interactive web-based visualization of any structure in the AFLOW database that has associate band structure data in a way that allows novel simultaneous exploration of the crystal structure, band structure, and Brillouin zone. Interactivity is obtained using two synchronized JSmol implementations, one for the crystal structure and one for the Brillouin zone, along with a D3-based band-structure diagram produced on the fly from data obtained from the AFLOW database. The current website portal (http://aflowlib.mems.duke.edu/users/jmolers/matt/website) allows interactive access and visualization of crystal structure, Brillouin zone and band structure for more than 55,000 inorganic crystal structures. This work was supported by the US Navy Office of Naval Research through a Broad Area Announcement administered by Duke University.

The Sternheimer-GW method and the spectral signatures of plasmonic polarons

NASA Astrophysics Data System (ADS)

Giustino, Feliciano

During the past three decades the GW method has emerged among the most promising electronic structure techniques for predictive calculations of quasiparticle band structures. In order to simplify the GW work-flow while at the same time improving the calculation accuracy, we developed the Sternheimer-GW method. In Sternheimer-GW both the screened Coulomb interaction and the electron Green's function are evaluated by using exclusively occupied Kohn-Sham states, as in density-functional perturbation theory. In this talk I will review the basics of Sternheimer-GW, and I will discuss two recent applications to semiconductors and superconductors. In the case of semiconductors we calculated complete energy- and momentum-resolved spectral functions by combining Sternheimer-GW with the cumulant expansion approach. This study revealed the existence of band structure replicas which arise from electron-plasmon interactions. In the case of superconductors we calculated the Coulomb pseudo-potential from first principles, and combined this approach with the Eliashberg theory of the superconducting critical temperature. This work was supported by the Leverhulme Trust (RL-2012-001), the European Research Council (EU FP7/ERC 239578), the UK Engineering and Physical Sciences Research Council (EP/J009857/1), and the Graphene Flagship (EU FP7/604391).

Loading ultracold gases in topological Floquet bands: the fate of current and center-of-mass responses

NASA Astrophysics Data System (ADS)

Dauphin, Alexandre; Tran, Duc-Thanh; Lewenstein, Maciej; Goldman, Nathan

2017-06-01

Topological band structures can be designed by subjecting lattice systems to time-periodic modulations, as was proposed for irradiated graphene, and recently demonstrated in two-dimensional (2D) ultracold gases and photonic crystals. However, changing the topological nature of Floquet Bloch bands from trivial to non-trivial, by progressively launching the time-modulation, is necessarily accompanied with gap-closing processes: this has important consequences for the loading of particles into a target Floquet band with non-trivial topology, and hence, on the subsequent measurements. In this work, we analyse how such loading sequences can be optimized in view of probing the topology of 2D Floquet bands through transport measurements. In particular, we demonstrate the robustness of center-of-mass responses, as compared to current responses, which present important irregularities due to an interplay between the micro-motion of the drive and inter-band interference effects. The results presented in this work illustrate how probing the center-of-mass displacement of atomic clouds offers a reliable method to detect the topology of Floquet bands, after realistic loading sequences.

Self-interaction corrected LDA + U investigations of BiFeO3 properties: plane-wave pseudopotential method

NASA Astrophysics Data System (ADS)

Yaakob, M. K.; Taib, M. F. M.; Lu, L.; Hassan, O. H.; Yahya, M. Z. A.

2015-11-01

The structural, electronic, elastic, and optical properties of BiFeO3 were investigated using the first-principles calculation based on the local density approximation plus U (LDA + U) method in the frame of plane-wave pseudopotential density functional theory. The application of self-interaction corrected LDA + U method improved the accuracy of the calculated properties. Results of structural, electronic, elastic, and optical properties of BiFeO3, calculated using the LDA + U method were in good agreement with other calculation and experimental data; the optimized choice of on-site Coulomb repulsion U was 3 eV for the treatment of strong electronic localized Fe 3d electrons. Based on the calculated band structure and density of states, the on-site Coulomb repulsion U had a significant effect on the hybridized O 2p and Fe 3d states at the valence and the conduction band. Moreover, the elastic stiffness tensor, the longitudinal and shear wave velocities, bulk modulus, Poisson’s ratio, and the Debye temperature were calculated for U = 0, 3, and 6 eV. The elastic stiffness tensor, bulk modulus, sound velocities, and Debye temperature of BiFeO3 consistently decreased with the increase of the U value.

Nondestructive detection of pork quality based on dual-band VIS/NIR spectroscopy

NASA Astrophysics Data System (ADS)

Wang, Wenxiu; Peng, Yankun; Li, Yongyu; Tang, Xiuying; Liu, Yuanyuan

2015-05-01

With the continuous development of living standards and the relative change of dietary structure, consumers' rising and persistent demand for better quality of meat is emphasized. Colour, pH value, and cooking loss are important quality attributes when evaluating meat. To realize nondestructive detection of multi-parameter of meat quality simultaneously is popular in production and processing of meat and meat products. The objectives of this research were to compare the effectiveness of two bands for rapid nondestructive and simultaneous detection of pork quality attributes. Reflectance spectra of 60 chilled pork samples were collected from a dual-band visible/near-infrared spectroscopy system which covered 350-1100 nm and 1000-2600 nm. Then colour, pH value and cooking loss were determined by standard methods as reference values. Standard normal variables transform (SNVT) was employed to eliminate the spectral noise. A spectrum connection method was put forward for effective integration of the dual-band spectrum to make full use of the whole efficient information. Partial least squares regression (PLSR) and Principal component analysis (PCA) were applied to establish prediction models using based on single-band spectrum and dual-band spectrum, respectively. The experimental results showed that the PLSR model based on dual-band spectral information was superior to the models based on single band spectral information with lower root means quare error (RMSE) and higher accuracy. The PLSR model based on dual-band (use the overlapping part of first band) yielded the best prediction result with correlation coefficient of validation (Rv) of 0.9469, 0.9495, 0.9180, 0.9054 and 0.8789 for L*, a*, b*, pH value and cooking loss, respectively. This mainly because dual-band spectrum can provide sufficient and comprehensive information which reflected the quality attributes. Data fusion from dual-band spectrum could significantly improve pork quality parameters prediction performance. The research also indicated that multi-band spectral information fusion has potential to comprehensively evaluate other quality and safety attributes of pork.

Electronic structures of [001]- and [111]-oriented InSb and GaSb free-standing nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liao, Gaohua; Department of Applied Physics and Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082; Luo, Ning

We report on a theoretical study of the electronic structures of InSb and GaSb nanowires oriented along the [001] and [111] crystallographic directions. The nanowires are described by atomistic, tight-binding models, including spin-orbit interaction. The band structures and the wave functions of the nanowires are calculated by means of a Lanczos iteration algorithm. For the [001]-oriented InSb and GaSb nanowires, the systems with both square and rectangular cross sections are considered. Here, it is found that all the energy bands are doubly degenerate. Although the lowest conduction bands in these nanowires show good parabolic dispersions, the top valence bands showmore » rich and complex structures. In particular, the topmost valence bands of the nanowires with a square cross section show a double maximum structure. In the nanowires with a rectangular cross section, this double maximum structure is suppressed, and the top valence bands gradually develop into parabolic bands as the aspect ratio of the cross section is increased. For the [111]-oriented InSb and GaSb nanowires, the systems with hexagonal cross sections are considered. It is found that all the bands at the Γ-point are again doubly degenerate. However, some of them will split into non-degenerate bands when the wave vector moves away from the Γ-point. Although the lowest conduction bands again show good parabolic dispersions, the topmost valence bands do not show the double maximum structure. Instead, they show a single maximum structure with its maximum at a wave vector slightly away from the Γ-point. The wave functions of the band states near the band gaps of the [001]- and [111]-oriented InSb and GaSb nanowires are also calculated and are presented in terms of probability distributions in the cross sections. It is found that although the probability distributions of the band states in the [001]-oriented nanowires with a rectangular cross section could be qualitatively described by one-band effective mass theory, the probability distributions of the band states in the [001]-oriented nanowires with a square cross section and the [111]-oriented nanowires with a hexagonal cross section show characteristic patterns with symmetries closely related to the irreducible representations of the relevant double point groups and, in general, go beyond the prediction of a simple one-band effective mass theory. We also investigate the effects of quantum confinement on the band structures of the [001]- and [111]-oriented InSb and GaSb nanowires and present an empirical formula for the description of quantization energies of the band edge states in the nanowires, which could be used to estimate the enhancement of the band gaps of the nanowires as a result of quantum confinement. The size dependencies of the electron and hole effective masses in these nanowires are also investigated and discussed.« less

Electronic structures of [001]- and [111]-oriented InSb and GaSb free-standing nanowires

NASA Astrophysics Data System (ADS)

Liao, Gaohua; Luo, Ning; Yang, Zhihu; Chen, Keqiu; Xu, H. Q.

2015-09-01

We report on a theoretical study of the electronic structures of InSb and GaSb nanowires oriented along the [001] and [111] crystallographic directions. The nanowires are described by atomistic, tight-binding models, including spin-orbit interaction. The band structures and the wave functions of the nanowires are calculated by means of a Lanczos iteration algorithm. For the [001]-oriented InSb and GaSb nanowires, the systems with both square and rectangular cross sections are considered. Here, it is found that all the energy bands are doubly degenerate. Although the lowest conduction bands in these nanowires show good parabolic dispersions, the top valence bands show rich and complex structures. In particular, the topmost valence bands of the nanowires with a square cross section show a double maximum structure. In the nanowires with a rectangular cross section, this double maximum structure is suppressed, and the top valence bands gradually develop into parabolic bands as the aspect ratio of the cross section is increased. For the [111]-oriented InSb and GaSb nanowires, the systems with hexagonal cross sections are considered. It is found that all the bands at the Γ-point are again doubly degenerate. However, some of them will split into non-degenerate bands when the wave vector moves away from the Γ-point. Although the lowest conduction bands again show good parabolic dispersions, the topmost valence bands do not show the double maximum structure. Instead, they show a single maximum structure with its maximum at a wave vector slightly away from the Γ-point. The wave functions of the band states near the band gaps of the [001]- and [111]-oriented InSb and GaSb nanowires are also calculated and are presented in terms of probability distributions in the cross sections. It is found that although the probability distributions of the band states in the [001]-oriented nanowires with a rectangular cross section could be qualitatively described by one-band effective mass theory, the probability distributions of the band states in the [001]-oriented nanowires with a square cross section and the [111]-oriented nanowires with a hexagonal cross section show characteristic patterns with symmetries closely related to the irreducible representations of the relevant double point groups and, in general, go beyond the prediction of a simple one-band effective mass theory. We also investigate the effects of quantum confinement on the band structures of the [001]- and [111]-oriented InSb and GaSb nanowires and present an empirical formula for the description of quantization energies of the band edge states in the nanowires, which could be used to estimate the enhancement of the band gaps of the nanowires as a result of quantum confinement. The size dependencies of the electron and hole effective masses in these nanowires are also investigated and discussed.

Spectroscopic investigation of the electronic structure of yttria-stabilized zirconia

NASA Astrophysics Data System (ADS)

Götsch, Thomas; Bertel, Erminald; Menzel, Alexander; Stöger-Pollach, Michael; Penner, Simon

2018-03-01

The electronic structure and optical properties of yttria-stabilized zirconia are investigated as a function of the yttria content using multiple experimental and theoretical methods, including electron energy-loss spectroscopy, Kramers-Kronig analysis to obtain the optical parameters, photoelectron spectroscopy, and density functional theory. It is shown that many properties, including the band gaps, the crystal field splitting, the so-called defect gap between acceptor (YZr') and donor (VO••) states, as well as the index of refraction in the visible range exhibit the same "zig-zag-like" trend as the unit cell height does, showing the influence of an increased yttria content as well as of the tetragonal-cubic phase transition between 8 mol % and 20 mol %Y2O3 . Also, with Čerenkov spectroscopy (CS), a new technique is presented, providing information complementary to electron energy-loss spectroscopy. In CS, the Čerenkov radiation emitted inside the TEM is used to measure the onset of optical absorption. The apparent absorption edges in the Čerenkov spectra correspond to the energetic difference between the disorder states close to the valence band and the oxygen-vacancy-related electronic states within the band gap. Theoretical computations corroborate this assignment: they find both, the acceptor states and the donor states, at the expected energies in the band structures for diverse yttria concentrations. In the end, a schematic electronic structure diagram of the area around the band gap is constructed, including the chemical potential of the electrons obtained from photoelectron spectroscopy. The latter reveal that tetragonal YSZ corresponds to a p -type semiconductor, whereas the cubic samples exhibit n -type semiconductor properties.

Aromatic Cluster Sensor of Protein Folding: Near-UV Electronic Circular Dichroism Bands Assigned to Fold Compactness.

PubMed

Farkas, Viktor; Jákli, Imre; Tóth, Gábor K; Perczel, András

2016-09-19

Both far- and near-UV electronic circular dichroism (ECD) spectra have bands sensitive to thermal unfolding of Trp and Tyr residues containing proteins. Beside spectral changes at 222 nm reporting secondary structural variations (far-UV range), L b bands (near-UV range) are applicable as 3D-fold sensors of protein's core structure. In this study we show that both L b (Tyr) and L b (Trp) ECD bands could be used as sensors of fold compactness. ECD is a relative method and thus requires NMR referencing and cross-validation, also provided here. The ensemble of 204 ECD spectra of Trp-cage miniproteins is analysed as a training set for "calibrating" Trp↔Tyr folded systems of known NMR structure. While in the far-UV ECD spectra changes are linear as a function of the temperature, near-UV ECD data indicate a non-linear and thus, cooperative unfolding mechanism of these proteins. Ensemble of ECD spectra deconvoluted gives both conformational weights and insight to a protein folding↔unfolding mechanism. We found that the L b 293 band is reporting on the 3D-structure compactness. In addition, the pure near-UV ECD spectrum of the unfolded state is described here for the first time. Thus, ECD folding information now validated can be applied with confidence in a large thermal window (5≤T≤85 °C) compared to NMR for studying the unfolding of Trp↔Tyr residue pairs. In conclusion, folding propensities of important proteins (RNA polymerase II, ubiquitin protein ligase, tryptase-inhibitor etc.) can now be analysed with higher confidence. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

3D contour fluorescence spectroscopy with Brus model: Determination of size and band gap of double stranded DNA templated silver nanoclusters

NASA Astrophysics Data System (ADS)

Kamalraj, Devaraj; Yuvaraj, Selvaraj; Yoganand, Coimbatore Paramasivam; Jaffer, Syed S.

2018-01-01

Here, we propose a new synthetic methodology for silver nanocluster preparation by using a double stranded-DNA (ds-DNA) template which no one has reported yet. A new calculative method was formulated to determine the size of the nanocluster and their band gaps by using steady state 3D contour fluorescence technique with Brus model. Generally, the structure and size of the nanoclusters determine by using High Resolution Transmission Electron Microscopy (HR-TEM). Before imaging the samples by using HR-TEM, they are introduced to drying process which causes aggregation and forms bigger polycrystalline particles. It takes long time duration and expensive methodology. In this current methodology, we found out the size and band gap of the nanocluster in the liquid form without any polycrystalline aggregation for which 3D contour fluorescence technique was used as an alternative approach to the HR-TEM method.

Molecular and electronic structure of Re 2Br 4(PMe 3) 4

DOE PAGES

Johnstone, Erik V.; Poineau, Frederic; Todorova, Tanya K.; ...

2016-07-07

The dinuclear rhenium(II) complex Re 2Br 4(PMe 3) 4 was prepared from the reduction of [Re 2Br 8] 2– with ( n-Bu 4N)BH 4 in the presence of PMe 3 in propanol. The complex was characterized by single-crystal X-ray diffraction (SCXRD) and UV–visible spectroscopy. It crystallizes in the monoclinic C2/c space group and is isostructural with its molybdenum and technetium analogues. The Re–Re distance (2.2521(3) Å) is slightly longer than the one in Re 2Cl 4(PMe 3) 4 (2.247(1) Å). The molecular and electronic structure of Re 2X 4(PMe 3) 4 (X = Cl, Br) were studied by multiconfigurational quantummore » chemical methods. The computed ground-state geometry is in excellent agreement with the experimental structure determined by SCXRD. The calculated total bond order (2.75) is consistent with the presence of an electron-rich triple bond and is similar to the one found for Re 2Cl 4(PMe 3) 4. The electronic absorption spectrum of Re 2Br 4(PMe 3) 4 was recorded in benzene and shows a series of low-intensity bands in the range 10 000–26 000 cm –1. The absorption bands were assigned based on calculations of the excitation energies with the multireference wave functions followed by second-order perturbation theory using the CASSCF/CASPT2 method. As a result, calculations predict that the lowest energy band corresponds to the δ* → σ* transition, while the next higher energy bands were attributed to the δ* → π*, δ → σ*, and δ → π* transitions.« less

Structural, thermal and optical absorption features of heavy metal oxides doped tellurite rich glasses

NASA Astrophysics Data System (ADS)

Kaky, Kawa M.; Lakshminarayana, G.; Baki, S. O.; Kityk, I. V.; Taufiq-Yap, Y. H.; Mahdi, M. A.

In order to improve tellurite glass stability to be applicable for optical fiber amplifier applications, glasses with the composition of (70 - x)TeO2. (10)ZnO. (10)WO3. (5)Na2O. (5)TiO2. (x)Bi2O3 (x = 1, 2, 3, 4, and 5 mol%) have been produced and characterized using the related methods. Structural properties were investigated using X-ray diffraction (XRD) which confirms the non-crystalline structure and scanning electron microscopy (SEM) micrographs also confirm the XRD results. The energy dispersive X-ray (EDX) analysis profiles show that all the mentioned elements are present in the prepared glasses. Following the IR spectra, all the tellurium bonds such as stretching vibrations of TeO4 tbp and TeO3/TeO3+1 unit are revealed. Raman spectra confirm the presence of different functional groups, actually, it shows bands mainly in four spectral regions: R1 (65-150) cm-1, R2 (280-550) cm-1, R3 (880-950) cm-1 and R4 (916-926) cm-1 and the identified bands are assigned to respective molecular groups. The thermal study was carried out using Differential scanning calorimetry (DSC) which indicates good thermal stability of the synthesized glasses with increasing Bi concentration. From the optical absorption spectra, we evaluated cut-off edge wavelengths and found increasing cutoff wavelength with an increase in Bi2O3 concentration. In the UV-Visible region, optical band gap energy and allowed transitions were investigated using three methods; direct, indirect, and absorption spectrum fitting (ASF), and band gaps from indirect and ASF were matched.

Fourier transform infrared spectral evidences for protein conformational changes in immature cataractous human lens capsules accelerated by myopia and/or systemic hypertension

NASA Astrophysics Data System (ADS)

Lin, Shan-Yang; Lee, Shui-Mei; Li, Mei-Jane; Liang, Run-Chu

1997-08-01

The possible changes in protein structures of the cataractous human lens capsules of the immature patients with myopia and/or systemic hypertension have been investigated using Fourier transform infrared (FT-IR) microspectroscopy. Second-derivative and deconvolution methods have been applied to obtain the position of the overlapping components of the amide I band and assign them to different secondary structures. Changes in the protein secondary structure and composition of amide I band were estimated quantitatively from Fourier self-deconvolution and curve fitting algorithms. The results indicate that myopia and/or systemic hypertension were found to significantly modify the protein secondary structure of the cataractous human lens capsules to increase the β-type structure and random coil and decrease the α-helix structure. Myopia-induced conformational change in triple helix structure was more pronounced. In conclusion, myopia and/or systemic hypertension seem to modify the conformation of the protein structures in cataractous human lens capsule to change ionic permeation through lens capsule to accelerate the cataract formation of senile patients.

Enhancement of electron correlation due to the molecular dimerization in organic superconductors β -(BDA-TTP )2X (X =I3, SbF6)

NASA Astrophysics Data System (ADS)

Aizawa, Hirohito; Kuroki, Kazuhiko; Yamada, Jun-ichi

2015-10-01

We perform a first-principles band calculation for quasi-two-dimensional organic superconductors β -(BDA -TTP) 2I3 and β -(BDA -TTP) 2SbF6. The first-principles band structures between the I3 and SbF6 salts are apparently different. We construct a tight-binding model for each material which accurately reproduces the first-principles band structure. The obtained transfer energies give the differences as follows: (i) larger dimerization in the I3 salt than the SbF6 salt, and (ii) different signs and directions of the interstacking transfer energies. To decompose the origin of the difference into the dimerization and the interstacking transfer energies, we adopt a simplified model by eliminating the dimerization effect and focus only on the difference caused by the interstacking transfer energies. From the analysis using the simplified model, we find that the difference of the band structure comes mainly from the strength of the dimerization. To compare the strength of the electron correlation having roots in the band structure, we calculate the physical properties originating from the effect of the electron correlation such as the spin susceptibility applying the two-particle self-consistent method. We find that the maximum value of the spin susceptibility for the I3 salt is larger than that of the SbF6 salt. Hypothetically decreasing the dimerization within the model of the I3 salt, the spin susceptibility takes almost the same value as that of the SbF6 salt for the same magnitude of the dimerization. We expect that the different ground state between the I3 and SbF6 salt mainly comes from the strength of the dimerization which is apparently masked in the band calculation along a particular k path.

Stabilizing benzene-like planar N6 rings to form a single atomic honeycomb BeN3 sheet with high carrier mobility.

PubMed

Li, Xiaoyin; Zhang, Shunhong; Zhang, Cunzhi; Wang, Qian

2018-01-18

It is a longstanding quest to use the planar N 6 ring as a structural unit to build stable atomic sheets. However, unlike C 6 H 6 , the neutral N 6 ring is unstable due to the strong repulsion of the lone-pair of electrons. Using first-principles calculations and the global structure search method, we show that the N 6 unit can be stabilized by the linkage of Be atoms, forming a h-BeN 3 honeycomb monolayer, in which the geometry and the π-molecular orbitals of the N 6 rings are well kept. This sheet is not only energetically, dynamically and thermally stable, but also can withstand high temperatures up to 1000 K. Band structure calculation combined with a group theory analysis and a tight-binding model uncover that h-BeN 3 has a π-band dominated band structure with an indirect band gap of 1.67 eV. While it possesses a direct band gap of 2.07 eV at the Γ point lying in the photon energy region of visual light, its interband dipole transition is symmetrically allowed so that electrons can be excited by photons free of phonons. Based on deformation potential theory, a systematic study of the transport properties reveals that the h-BeN 3 sheet possesses a high carrier mobility of ∼10 3 cm 2 V -1 s -1 , superior to the extensively studied transition metal dichalcogenide monolayers. We further demonstrate that this sheet can be rolled up into either zigzag or armchair nanotubes. These nanotubes are also dynamically stable, and are all direct band gap semiconductors with carrier mobility comparable to that of their 2D counterparts, regardless of their chirality and diameter. The robust stability and novel electronic and transport properties of the h-BeN 3 sheet and its tubular derivatives endow them with great potential for applications in nanoelectronic devices.

Shuttle Ku-band and S-band communications implementation study

NASA Technical Reports Server (NTRS)

Dodds, J. G.; Huth, G. K.; Nilsen, P. W.; Polydoros, A.; Simon, M. K.; Weber, C. L.

1980-01-01

Various aspects of the shuttle orbiter S-band network communication system, the S-band payload communication system, and the Ku-band communication system are considered. A method is proposed for obtaining more accurate S-band antenna patterns of the actual shuttle orbiter vehicle during flight because the preliminary antenna patterns using mock-ups are not realistic that they do not include the effects of additional appendages such as wings and tail structures. The Ku-band communication system is discussed especially the TDRS antenna pointing accuracy with respect to the orbiter and the modifications required and resulting performance characteristics of the convolutionally encoded high data rate return link to maintain bit synchronizer lock on the ground. The TDRS user constraints on data bit clock jitter and data asymmetry on unbalanced QPSK with noisy phase references are included. The S-band payload communication system study is outlined including the advantages and experimental results of a peak regulator design built and evaluated by Axiomatrix for the bent-pipe link versus the existing RMS-type regulator. The nominal sweep rate for the deep-space transponder of 250 Hz/s, and effects of phase noise on the performance of a communication system are analyzed.

Effect of thickness on surface morphology, optical and humidity sensing properties of RF magnetron sputtered CCTO thin films

NASA Astrophysics Data System (ADS)

Ahmadipour, Mohsen; Ain, Mohd Fadzil; Ahmad, Zainal Arifin

2016-11-01

In this study, calcium copper titanate (CCTO) thin films were deposited on ITO substrates successfully by radio frequency (RF) magnetron sputtering method in argon atmosphere. The CCTO thin films present a polycrystalline, uniform and porous structure. The surface morphology, optical and humidity sensing properties of the synthesized CCTO thin films have been studied by X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), UV-vis spectrophotometer and current-voltage (I-V) analysis. XRD and AFM confirmed that the intensity of peaks and pore size of CCTO thin films were enhanced by increasing the thin films. Tauc plot method was adopted to estimate the optical band gaps. The surface structure and energy band gaps of the deposited films were affected by film thickness. Energy band gap of the layers were 3.76 eV, 3.68 eV and 3.5 eV for 200 nm, 400 nm, and 600 nm CCTO thin films layer, respectively. The humidity sensing properties were measured by using direct current (DC) analysis method. The response times were 12 s, 22 s, and 35 s while the recovery times were 500 s, 600 s, and 650 s for 200 nm, 400 nm, and 600 nm CCTO thin films, respectively at humidity range of 30-90% relative humidity (RH).

Data-driven signal-resolving approaches of infrared spectra to explore the macroscopic and microscopic spatial distribution of organic and inorganic compounds in plant.

PubMed

Chen, Jian-bo; Sun, Su-qin; Zhou, Qun

2015-07-01

The nondestructive and label-free infrared (IR) spectroscopy is a direct tool to characterize the spatial distribution of organic and inorganic compounds in plant. Since plant samples are usually complex mixtures, signal-resolving methods are necessary to find the spectral features of compounds of interest in the signal-overlapped IR spectra. In this research, two approaches using existing data-driven signal-resolving methods are proposed to interpret the IR spectra of plant samples. If the number of spectra is small, "tri-step identification" can enhance the spectral resolution to separate and identify the overlapped bands. First, the envelope bands of the original spectrum are interpreted according to the spectra-structure correlations. Then the spectrum is differentiated to resolve the underlying peaks in each envelope band. Finally, two-dimensional correlation spectroscopy is used to enhance the spectral resolution further. For a large number of spectra, "tri-step decomposition" can resolve the spectra by multivariate methods to obtain the structural and semi-quantitative information about the chemical components. Principal component analysis is used first to explore the existing signal types without any prior knowledge. Then the spectra are decomposed by self-modeling curve resolution methods to estimate the spectra and contents of significant chemical components. At last, targeted methods such as partial least squares target can explore the content profiles of specific components sensitively. As an example, the macroscopic and microscopic distribution of eugenol and calcium oxalate in the bud of clove is studied.

Revisiting Photoemission and Inverse Photoemission Spectra of Nickel Oxide from First Principles: Implications for Solar Energy Conversion

PubMed Central

2015-01-01

We use two different ab initio quantum mechanics methods, complete active space self-consistent field theory applied to electrostatically embedded clusters and periodic many-body G0W0 calculations, to reanalyze the states formed in nickel(II) oxide upon electron addition and ionization. In agreement with interpretations of earlier measurements, we find that the valence and conduction band edges consist of oxygen and nickel states, respectively. However, contrary to conventional wisdom, we find that the oxygen states of the valence band edge are localized whereas the nickel states at the conduction band edge are delocalized. We argue that these characteristics may lead to low electron–hole recombination and relatively efficient electron transport, which, coupled with band gap engineering, could produce higher solar energy conversion efficiency compared to that of other transition-metal oxides. Both methods find a photoemission/inverse-photoemission gap of 3.6–3.9 eV, in good agreement with the experimental range, lending credence to our analysis of the electronic structure of NiO. PMID:24689856

Fast Bayesian approach for modal identification using free vibration data, Part I - Most probable value

NASA Astrophysics Data System (ADS)

Zhang, Feng-Liang; Ni, Yan-Chun; Au, Siu-Kui; Lam, Heung-Fai

2016-03-01

The identification of modal properties from field testing of civil engineering structures is becoming economically viable, thanks to the advent of modern sensor and data acquisition technology. Its demand is driven by innovative structural designs and increased performance requirements of dynamic-prone structures that call for a close cross-checking or monitoring of their dynamic properties and responses. Existing instrumentation capabilities and modal identification techniques allow structures to be tested under free vibration, forced vibration (known input) or ambient vibration (unknown broadband loading). These tests can be considered complementary rather than competing as they are based on different modeling assumptions in the identification model and have different implications on costs and benefits. Uncertainty arises naturally in the dynamic testing of structures due to measurement noise, sensor alignment error, modeling error, etc. This is especially relevant in field vibration tests because the test condition in the field environment can hardly be controlled. In this work, a Bayesian statistical approach is developed for modal identification using the free vibration response of structures. A frequency domain formulation is proposed that makes statistical inference based on the Fast Fourier Transform (FFT) of the data in a selected frequency band. This significantly simplifies the identification model because only the modes dominating the frequency band need to be included. It also legitimately ignores the information in the excluded frequency bands that are either irrelevant or difficult to model, thereby significantly reducing modeling error risk. The posterior probability density function (PDF) of the modal parameters is derived rigorously from modeling assumptions and Bayesian probability logic. Computational difficulties associated with calculating the posterior statistics, including the most probable value (MPV) and the posterior covariance matrix, are addressed. Fast computational algorithms for determining the MPV are proposed so that the method can be practically implemented. In the companion paper (Part II), analytical formulae are derived for the posterior covariance matrix so that it can be evaluated without resorting to finite difference method. The proposed method is verified using synthetic data. It is also applied to modal identification of full-scale field structures.

Structure of the Odd-Odd Nucleus {sup 188}Re

DOE Office of Scientific and Technical Information (OSTI.GOV)

Balodis, M.; Berzins, J.; Simonova, L.

2009-01-28

Thermal neutron capture gamma-ray spectra for {sup 187}Re(n,{gamma}){sup 188}Re reaction were measured. Singles and coincidence spectra were detected in order to develop the level scheme. The evaluation is in progress, of which the first results are obtained from the analysis of coincidence spectra, allowing to check the level scheme below 500 keV excitation energy. Seven low-energy negative parity bands are developed in order to find better energies for rotational levels. With a good confidence, a few positive parity bands are developed as well. Rotor plus two quasiparticle model calculations, employing effective matrix element method are performed for the system ofmore » six negative parity rotational bands.« less

Structural, electronic and magnetic properties of metal thiophosphate InPS4

NASA Astrophysics Data System (ADS)

Rajpoot, Priyanka; Nayak, Vikas; Kumari, Meena; Yadav, Priya; Nautiyal, Shashank; Verma, U. P.

2017-05-01

The non-centrosymmetric crystal, InPS4, has been investigated by means of density functional theory (DFT). In this paper we have calculated the structural parameters, electronic band structures, density of states plot and magnetic properties using full potential linearized augmented plane wave (FP-LAPW) method. The exchange correlation has been solved employing the generalised gradient approximation due to Perdew-Burke-Ernzerhof. The calculations are performed both without spin as well as spin polarized. The results show that InPS4 is an indirect band gap semiconductor with (N-Г) energy gap of 2.32eV (without spin) and 1.86eV in spin up and down channels.The obtained lattice parameters and energy gap agree well with the experimental results. Our reported magnetic moment results show that the property of InPS4is nonmagnetic.

Multi-band Electronic Structure of Ferromagnetic CeRuPO

NASA Astrophysics Data System (ADS)

Takahashi, Masaya; Ootsuki, Daiki; Horio, Masafumi; Arita, Masashi; Namatame, Hirofumi; Taniguchi, Masaki; Saini, Naurang L.; Sugawara, Hitoshi; Mizokawa, Takashi

2018-04-01

We have studied the multi-band electronic structure of ferromagnetic CeRuPO (TC = 15 K) by means of angle-resolved photoemission spectroscopy (ARPES). The ARPES results show that three hole bands exist around the zone center and two of them cross the Fermi level (EF). Around the zone corner, two electron bands are observed and cross EF. These hole and electron bands, which can be assigned to the Ru 4d bands, are basically consistent with the band-structure calculation including their orbital characters. However, one of the electron bands with Ru 4d 3z2 - r2 character is strongly renormalized indicating correlation effect due to hybridization with the Ce 4f orbitals. The Ru 4d 3z2 - r2 band changes across TC suggesting that the out-of-plane 3z2 - r2 orbital channel plays essential roles in the ferromagnetism.

Partially filled intermediate band of Cr-doped GaN films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sonoda, S.

2012-05-14

We investigated the band structure of sputtered Cr-doped GaN (GaCrN) films using optical absorption, photoelectron yield spectroscopy, and charge transport measurements. It was found that an additional energy band is formed in the intrinsic band gap of GaN upon Cr doping, and that charge carriers in the material move in the inserted band. Prototype solar cells showed enhanced short circuit current and open circuit voltage in the n-GaN/GaCrN/p-GaN structure compared to the GaCrN/p-GaN structure, which validates the proposed concept of an intermediate-band solar cell.

Electronic structure-sunlight driven water splitting activity correlation of (Zn1-yGay)(O1-zNz).

PubMed

RajaAmbal, Sivaraman; Yadav, A K; Jha, S N; Bhattacharyya, D; Gopinath, Chinnnakonda S

2014-11-21

(Zn1-yGay)(O1-zNz) (y≤ 0.10; z≤ 0.15) solid solutions have been investigated for their electronic structure and visible light photocatalytic activity, and a correlation was found between them. (Zn1-yGay)(O1-zNz) with ZnO as the major component have been synthesized by a solution combustion method in 10 minutes using simple raw materials. The local structures of Zn K edge and Ga K edge, and changes in the chemical environment with the incorporation of Ga and N in ZnO were determined by EXAFS study. EXAFS and XRD results suggested the dissolution of GaN in the ZnO lattice. The homogeneity of the solid solution was demonstrated from HRTEM studies. Photoluminescence studies revealed the creation of a new band at the top of the ZnO valence band (VB), and thus the broadening of the VB of (Zn1-yGay)(O1-zNz) or a decrease in the band gap was attributed to the origin of visible light absorption. UV-Vis spectral studies showed light absorption up to 550 nm, which directly supports the VB broadening. Predominant oxygen vacancies and high photocorrosion observed for ZnO were fully suppressed for (Zn1-yGay)(O1-zNz), indicating the minimization of defects, and thus more sustainability under irradiation conditions. The bare solid solution exhibited reasonable and promising activity for solar hydrogen evolution and photoelectrochemical current generation at 0 V. The present work explained factors such as the preparation method, single phase structure with the stabilization of integral parts, homogeneity in the structure, compensation of oxygen vacancies, and suppression of the density of recombination centres that play a pivotal role in realizing solar energy harvesting.

Bridging the gap between atomic microstructure and electronic properties of alloys: The case of (In,Ga)N

NASA Astrophysics Data System (ADS)

Chan, J. A.; Liu, J. Z.; Zunger, Alex

2010-07-01

The atomic microstructure of alloys is rarely perfectly random, instead exhibiting differently shaped precipitates, clusters, zigzag chains, etc. While it is expected that such microstructural features will affect the electronic structures (carrier localization and band gaps), theoretical studies have, until now, been restricted to investigate either perfectly random or artificial “guessed” microstructural features. In this paper, we simulate the alloy microstructures in thermodynamic equilibrium using the static Monte Carlo method and study their electronic structures explicitly using a pseudopotential supercell approach. In this way, we can bridge atomic microstructures with their electronic properties. We derive the atomic microstructures of InGaN using (i) density-functional theory total energies of ˜50 ordered structures to construct a (ii) multibody cluster expansion, including strain effects to which we have applied (iii) static Monte Carlo simulations of systems consisting of over 27000 atoms to determine the equilibrium atomic microstructures. We study two types of alloy thermodynamic behavior: (a) under lattice incoherent conditions, the formation enthalpies are positive and thus the alloy system phase-separates below the miscibility-gap temperature TMG , (b) under lattice coherent conditions, the formation enthalpies can be negative and thus the alloy system exhibits ordering tendency. The microstructure is analyzed in terms of structural motifs (e.g., zigzag chains and InnGa4-nN tetrahedral clusters). The corresponding electronic structure, calculated with the empirical pseudopotentials method, is analyzed in terms of band-edge energies and wave-function localization. We find that the disordered alloys have no electronic localization but significant hole localization, while below the miscibility gap under the incoherent conditions, In-rich precipitates lead to strong electron and hole localization and a reduction in the band gap.

Extended Fenske-Hall LCAO MO Calculations for Mixed Methylene Dihalides

NASA Astrophysics Data System (ADS)

Ziemann, Hartmut; Paulun, Manfred

1988-10-01

The electronic structure of mixed methylene dihalides CH2XY (X, Y = F, Cl, Br. I) has been studied using extended Fenske-Hall LCAO MO method. The comparison with available photoelec­tron spectra confirmes previous assignments of all bands with binding energies <100 eV. The electronic structure changes occurring upon varying the halogen substituents are discussed.

The Marvels of Electromagnetic Band Gap (EBG) Structures

DTIC Science & Technology

2003-11-01

terminology of "Electromagnetic conference papers and journal articles dealing with Band- gaps (EBG)". Recently, many researchers the characterizations...Band Gap (EBG) Structures 9 utilized to reduce the mutual coupling between Structures: An FDTD/Prony Technique elements of antenna arrays. based on the...Band- Gap of several patents. He has had pioneering research contributions in diverse areas of electromagnetics,Snteructure", Dymposiget o l 21 IE 48

Modeling of Photonic Band Gap Crystals and Applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

El-Kady, Ihab Fathy

In this work, the authors have undertaken a theoretical approach to the complex problem of modeling the flow of electromagnetic waves in photonic crystals. The focus is to address the feasibility of using the exciting phenomena of photonic gaps (PBG) in actual applications. The authors start by providing analytical derivations of the computational electromagnetic methods used in their work. They also present a detailed explanation of the physics underlying each approach, as well as a comparative study of the strengths and weaknesses of each method. The Plane Wave expansion, Transfer Matrix, and Finite Difference time Domain Methods are addressed. Theymore » also introduce a new theoretical approach, the Modal Expansion Method. They then shift the attention to actual applications. They begin with a discussion of 2D photonic crystal wave guides. The structure addressed consists of a 2D hexagonal structure of air cylinders in a layered dielectric background. Comparison with the performance of a conventional guide is made, as well as suggestions for enhancing it. The studies provide an upper theoretical limit on the performance of such guides, as they assumed no crystal imperfections and non-absorbing media. Next, they study 3D metallic PBG materials at near infrared and optical wavelengths. The main objective is to study the importance of absorption in the metal and the suitability of observing photonic band gaps in such structures. They study simple cubic structures where the metallic scatters are either cubes or interconnected metallic rods. Several metals are studied (aluminum, gold, copper, and silver). The effect of topology is addressed and isolated metallic cubes are found to be less lossy than the connected rod structures. The results reveal that the best performance is obtained by choosing metals with a large negative real part of the dielectric function, together with a relatively small imaginary part. Finally, they point out a new direction in photonic crystal research that involves the interplay of metallic-PBG rejection and photonic band edge absorption. They propose that an absolute metallic-PBG may be used to suppress the infrared part of the blackbody emission and, emit its energy only through a sharp absorption band. Potential applications of this new PBG mechanism include highly efficient incandescent lamps and enhanced thermophotovoltaic energy conversion. The suggested lamp would be able to recycle the energy that would otherwise go into the unwanted heat associated with usual lamps, into light emitted in the visible spectrum. It is estimated this would increase the efficiency over conventional lamps by about 40%.« less

Characterization of dipole defects in MgAl2O4 spinel

NASA Astrophysics Data System (ADS)

Carvalhaes, R. P. M.; da Rocha, M. S. F.; de Souza, S. S.; Blak, A. R.

2005-01-01

Dipole defects in gamma-irradiated and thermal treated MgAl2O4 samples have been studied through thermally stimulated depolarisation currents(TSDC) technique and computer modelling methods. The presence of TSDC bands varies from sample to sample and some crystals do not present any band. The origin of these bands has been investigated in several different samples. In the spectra of spinels showing TSDC peaks, three bands at 130K, 160K and 320K are observed. The peaks at 130K and 160K have been attributed to dipole defects. After 1200kGy of gamma irradiation the broad band at 320K dislocates to 290K and increases ten times. Pulsed thermal treatments between 350K and 470K produce a progressive reduction of the peak area and a shift in the peak position back to 320K. A detailed analysis of the curve indicates the possibility of a superposition of peaks. Gamma irradiation restores the 320K TSDC peak. Taking into account optical absorption(OA) and electron paramagnetic resonance(EPR) results, the thermal reduction of the 320K TSDC band was attributed to V-type centres as a result of hole trapping at tetrahedral and octahedral cation vacancies. Computer modelling methods, based on lattice energy and defect minimisation, were applied to identify dipole defects that occur in these crystals. The calculations were made in normal and inverse spinel structures, doped with Cr, Co, Mn and Fe in order to justify the presence of dipole bands.

Band alignment of atomic layer deposited MgO/Zn0.8Al0.2O heterointerface determined by charge corrected X-ray photoelectron spectroscopy

NASA Astrophysics Data System (ADS)

Yan, Baojun; Liu, Shulin; Yang, Yuzhen; Heng, Yuekun

2016-05-01

Pure magnesium (MgO) and zinc oxide doped with aluminum oxide (Zn0.8Al0.2O) were prepared via atomic layer deposition. We have studied the structure and band gap of bulk Zn0.8Al0.2O material by X-ray diffractometer (XRD) and Tauc method, and the band offsets and alignment of atomic layer deposited MgO/Zn0.8Al0.2O heterointerface were investigated systematically using X-ray photoelectron spectroscopy (XPS) in this study. Different methodologies, such as neutralizing electron gun, the use of C 1s peak recalibration and zero charging method, were applied to recover the actual position of the core levels in insulator materials which were easily influenced by differential charging phenomena. Schematic band alignment diagram, valence band offset (ΔEV) and conduction band offset (ΔEC) for the interface of the MgO/Zn0.8Al0.2O heterostructure have been constructed. An accurate value of ΔEV = 0.72 ± 0.11 eV was obtained from various combinations of core levels of heterojunction with varied MgO thickness. Given the experimental band gaps of 7.83 eV for MgO and 5.29 eV for Zn0.8Al0.2O, a type-II heterojunction with a ΔEC of 3.26 ± 0.11 eV was found. Band offsets and alignment studies of these heterojunctions are important for gaining deep consideration to the design of various optoelectronic devices based on such heterointerface.

A Case Study Of Organic Dirac Materials -

NASA Astrophysics Data System (ADS)

Commeau, Benjamin; Geilhufe, Matthias; Fernando, Gayanath; Balatsky, Alexander

Dirac Materials are characterized by linear band crossings within the electronic band structure. Most research of Dirac materials has been dedicated towards inorganic materials, e.g., binary chalcogenides as toplogical insulators, the Weyl semimetal TaAs or graphene. The purpose of this study is to investigate the formation of Dirac points in organic materials under pressure and mechanical strain. We study multiple structural phases of the organic charge-transfer salt (BEDT-TTF)2I3. We numerically calculate the relaxed band structure near the Fermi level along different k-space directions. Once the relaxed ion structure is obtained, we pick different cell parameters to shrink and investigate the changes in the band structure. We discuss band structure degeneracies protected by crystalline and other symmetries, if any. Quantum Espresso and VASP codes were used to calculate and validate our results.

Multi-shell spherical GaAs /AlxGa1-x As quantum dot shells-size distribution as a mechanism to generate intermediate band energy levels

NASA Astrophysics Data System (ADS)

Rodríguez-Magdaleno, K. A.; Pérez-Álvarez, R.; Martínez-Orozco, J. C.; Pernas-Salomón, R.

2017-04-01

In this work the generation of an intermediate band of energy levels from multi-shell spherical GaAs /AlxGa1-x As quantum dot shells-size distribution is reported. Within the effective mass approximation the electronic structure of a GaAs spherical quantum-dot surrounded by one, two and three shells is studied in detail using a numerically stable transfer matrix method. We found that a shells-size distribution characterized by continuously wider GaAs domains is a suitable mechanism to generate the intermediate band whose width is also dependent on the Aluminium concentration x. Our results suggest that this effective mechanism can be used for the design of wider intermediate band than reported in other quantum systems with possible solar cells enhanced performance.

Electronic properties of BN-doped bilayer graphene and graphyne in the presence of electric field

NASA Astrophysics Data System (ADS)

Majidi, R.; Karami, A. R.

2013-11-01

In the present paper, we have used density functional theory to study electronic properties of bilayer graphene and graphyne doped with B and N impurities in the presence of electric field. It has been demonstrated that a band gap is opened in the band structures of the bilayer graphene and graphyne by B and N doping. We have also investigated influence of electric field on the electronic properties of BN-doped bilayer graphene and graphyne. It is found that the band gaps induced by B and N impurities are increased by applying electric field. Our results reveal that doping with B and N, and applying electric field are an effective method to open and control a band gap which is useful to design carbon-based next-generation electronic devices.

Experimental and first-principles study of photoluminescent and optical properties of Na-doped CuAlO2: the role of the NaAl-2Na i complex

NASA Astrophysics Data System (ADS)

Liu, Ruijian; Li, Yongfeng; Yao, Bin; Ding, Zhanhui; Deng, Rui; Zhang, Ligong; Zhao, Haifeng; Liu, Lei

2015-08-01

We report that a band-tail emission at 3.08 eV, lower than near-band-edge energy, is observed in photoluminescence measurements of bulk Na-doped CuAlO2. The band-tail emission is attributed to Na-related defects. Electronic structure calculations based on the first-principles method demonstrate that the donor-acceptor compensated complex of NaAl-2Na i in Na-doped CuAlO2 plays a key role in leading to the band-tail emission and bandgap narrowing. Furthermore, Hall effect measurements indicates that the hole concentration in CuAlO2 is independent on Na doping, which is well understood by the donor-acceptor compensation effect of NaAl-2Na i complex.

Negative refraction in metamaterials based on dielectric spherical particles

NASA Astrophysics Data System (ADS)

Huang, T. C.; Wang, B. X.; Zhao, C. Y.

2018-07-01

Negative refraction (NR) metamaterials are featured with unique physical properties and potential to realize full control of electromagnetic waves, which have attracted much attention since the last decade. However, few researches focus on the realization of three-dimensional dielectric NR metamaterials in optic frequency, and the current design methods need further development. In this paper, a three-dimensional all-dielectric NR metamaterial with two NR bands has been realized based on proper excitation of electric and magnetic multipoles. It is also predicted that the coupling of magnetic dipole and electric dipole can lead to the NR bands in near-infrared frequencies, and NR in the visible frequencies can be achieved by the coupling of magnetic quadrupole and electric dipole. Band structures and equal-frequency surfaces of proposed metamaterial arranged in the periodic cubic lattice are solved by adopting the plane wave expansion method, and then the results verify the existence of these two NR frequency bands in periodic metamaterials. In this way, the characteristic parameters such as transmission and absorption of light in two NR bands are also analyzed. In the meantime, the finite-deference time-domain method is used to intuitively display the phenomenon of NR and investigate the effects of disorder in particle arrangement. Besides, it is found that the proposed metamaterials have fine robustness to the disorder in particle arrangement, and these two NR bands can be tuned by adjusting volume fraction. In brief, this work provides means for preliminary designing, profound analysis and intuitively exhibition of NR metamaterials based on dielectric particles.

Efficient band structure modulations in two-dimensional MnPSe3/CrSiTe3 van der Waals heterostructures

NASA Astrophysics Data System (ADS)

Pei, Qi; Wang, Xiaocha; Zou, Jijun; Mi, Wenbo

2018-05-01

As a research upsurge, van der Waals (vdW) heterostructures give rise to numerous combined merits and novel applications in nanoelectronics fields. Here, we systematically investigate the electronic structure of MnPSe3/CrSiTe3 vdW heterostructures with various stacking patterns. Then, particular attention of this work is paid on the band structure modulations in MnPSe3/CrSiTe3 vdW heterostructures via biaxial strain or electric field. Under a tensile strain, the relative band edge positions of heterostructures transform from type-I (nested) to type-II (staggered). The relocation of conduction band minimum also brings about a transition from indirect to direct band gap. Under a compressive strain, the electronic properties change from semiconducting to metallic. The physical mechanism of strain-dependent band structure may be ascribed to the shifts of the energy bands impelled by different superposition of atomic orbitals. Meanwhile, our calculations manifest that band gap values of MnPSe3/CrSiTe3 heterostructures are insensitive to the electric field. Even so, by applying a suitable intensity of negative electric field, the band alignment transition from type-I to type-II can also be realized. The efficient band structure modulations via external factors endow MnPSe3/CrSiTe3 heterostructures with great potential in novel applications, such as strain sensors, photocatalysis, spintronic and photoelectronic devices.

Efficient band structure modulations in two-dimensional MnPSe3/CrSiTe3 van der Waals heterostructures.

PubMed

Pei, Qi; Wang, Xiaocha; Zou, Jijun; Mi, Wenbo

2018-05-25

As a research upsurge, van der Waals (vdW) heterostructures give rise to numerous combined merits and novel applications in nanoelectronics fields. Here, we systematically investigate the electronic structure of MnPSe 3 /CrSiTe 3 vdW heterostructures with various stacking patterns. Then, particular attention of this work is paid on the band structure modulations in MnPSe 3 /CrSiTe 3 vdW heterostructures via biaxial strain or electric field. Under a tensile strain, the relative band edge positions of heterostructures transform from type-I (nested) to type-II (staggered). The relocation of conduction band minimum also brings about a transition from indirect to direct band gap. Under a compressive strain, the electronic properties change from semiconducting to metallic. The physical mechanism of strain-dependent band structure may be ascribed to the shifts of the energy bands impelled by different superposition of atomic orbitals. Meanwhile, our calculations manifest that band gap values of MnPSe 3 /CrSiTe 3 heterostructures are insensitive to the electric field. Even so, by applying a suitable intensity of negative electric field, the band alignment transition from type-I to type-II can also be realized. The efficient band structure modulations via external factors endow MnPSe 3 /CrSiTe 3 heterostructures with great potential in novel applications, such as strain sensors, photocatalysis, spintronic and photoelectronic devices.

Alcohol sensing over O+E+S+C+L+U transmission band based on porous cored octagonal photonic crystal fiber

NASA Astrophysics Data System (ADS)

Paul, Bikash Kumar; Islam, Md. Shadidul; Ahmed, Kawsar; Asaduzzaman, Sayed

2017-06-01

A micro structure porous cored octagonal photonic crystal fiber (P-OPCF) has been proposed to sense aqueous analysts (alcohol series) over a wavelength range of 0.80 μm to 2.0 μm. By implementing a full vectorial finite element method (FEM), the numerical simulation on the proposed O-PCF has been analyzed. Numerical investigation shows that high sensitivity can be gained by changing the structural parameters. The obtained result shows the sensitivities of 66.78%, 67.66%, 68.34%, 68.72%, and 69.09%, and the confinement losses of 2.42×10-10 dB/m, 3.28×10-11 dB/m, 1.21×10-6 dB/m, 4.79×10-10 dB/m, and 4.99×10-9 dB/m at the 1.33 μm wavelength for methanol, ethanol, propanol, butanol, and pentanol, respectively can satisfy the condition of much legibility to install an optical system. The effects of the varying core and cladding diameters, pitch distance, operating wavelength, and effective refractive index are also reported here. It reflects that a significant sensitivity and low confinement loss can be achieved by the proposed P-OPCF. The proposed P-OPCF also covers the wavelength band (O+E+S+C+L+U). The investigation also exhibits that the sensitivity increases when the wavelength increases like S O-band< S E-band < S S-band < S C-band < S L-band < S U-band. This research observation has much pellucidity which has remarkable impact on the field of optical fiber sensor.

Enabling direct silicene integration in electronics: First principles study of silicene on NiSi 2(111)

DOE PAGES

Cheng, Jian -Yih; Chan, Maria K. Y.; Lilley, Carmen M.

2016-09-26

Silicene on metal silicides poses promise for direct integration of silicene into electronic devices. The details of the metal silicide-silicene interface, however, may have significant effects on the electronic properties. In this work, the electronic properties of silicene on NiSi 2(111) and hydrogenated NiSi 2(111) (H:NiSi 2) substrates, as well as hydrogenated silicene (H:silicene) on a NiSi 2(111) substrate, were simulated using first principles methods. The preferred Si surface termination of NiSi 2 was determined through surface energy calculations, and the band structure and density of states (DOS) were calculated for the two-dimensional silicene and H:silicene layers. Hydrogenating NiSi 2more » lowered the binding energy between silicene and the substrate and resulting in partial decoupling of the electronic properties. Relaxed silicene on H:NiSi 2 showed a small band gap opening of 0.14 eV. Silicene on H:NiSi 2 also had a calculated electron effective mass of 0.08m 0 and Fermi velocity of 0.39×10 6 m/s, which are similar to the values for freestanding silicene. H:silicene on NiSi 2 retained its band structure and DOS compared to freestanding H:silicene. The band gap of H:silciene on NiSi 2 was 1.97 eV and is similar to freestanding H:silicene band gap of 2 eV. As a result, this research showed that hydrogenation may be a viable method for decoupling a silicene layer from a NiSi 2(111) substrate to tune its electronic properties.« less

A Green's function formulation of the k→ ·p→ theory in the presence of spin-orbit interaction and magnetic field: Application to the electronic structure and related properties of w-GaN

NASA Astrophysics Data System (ADS)

Shadangi, Subrat K.; Mishra, Sambit R.; Tripathi, Gouri S.

2018-01-01

We use a Green's function perturbation formalism in the presence of an applied magnetic field and spin-orbit effects in the effective mass representation (EMR). The lack of lattice translational symmetry of the vector potential in the presence of the magnetic field is considered by redefining the Green's function in terms of the Peierls' phase factor. The equation of motion of the Green's function as a function of a magnetic wave vector was solved using perturbation theory, leading to expressions for the effective mass and the g-factor. We study the electronic structure of wurtzite GaN theoretically using the resulting k→ ·π→ method, where k→ is the electronic wave vector and π→ is the relativistic momentum operator by considering the conduction band edge and three valence bands. The k→ ·π→ Hamiltonians for the conduction band edge and the valence bands are diagonalized, considering the conduction band and one valence band at a time. We obtain electron and hole dispersions. Effects of other bands are considered by using perturbation theory. Resulting dispersions agree with the results of other calculations. In order to study the effective mass and the g-factor, we use the eigenvalues and eigenfunctions obtained after the diagonalization. Our results for the effective masses and the g-factors agree fairly well with available theoretical and experimental results, Temperature dependence of both the electronic effective mass and g-factor is studied and trends obtained agree with the existing experimental data.

Electronic and transport properties of zigzag carbon nanotubes with the presence of periodical antidot and boron/nitride doping defects

NASA Astrophysics Data System (ADS)

Zoghi, Milad; Yazdanpanah Goharrizi, Arash; Mirjalili, Seyed Mohammad; Kabir, M. Z.

2018-06-01

Electronic and transport properties of Carbon nanotubes (CNTs) are affected by the presence of physical or chemical defects in their structures. In this paper, we present novel platforms of defected zigzag CNTs (Z-CNTs) in which two topologies of antidot and Boron/Nitride (BN) doping defects are periodically imposed throughout the length of perfect tubes. Using the tight binding model and the non-equilibrium Green’s function method, it is realized that the quantum confinement of Z-CNTs is modified by the presence of such defects. This new quantum confinement results in the appearance of mini bands and mini gaps in the transmission spectra, as well as a modified band structure and band gap size. The modified band gap could be either larger or smaller than the intrinsic band gap of a perfect tube, which is determined by the category of Z-CNT. The in-depth analysis shows that the size of the modified band gap is the function of several factors consisting of: the radii of tube (D r), the distance between adjacent defects (d d), the utilized defect topology, and the kind of defect (antidot or BN doping). Furthermore, taking advantage of the tunable band gap size of Z-CNT with the presence of periodical defects, new platforms of defect-based Z-CNT resonant tunneling diode (RTD) are proposed for the first time. Our calculations demonstrate the apparition of resonances in transmission spectra and the negative differential resistance in the I-V characteristics for such RTD platforms.

Tunable photonic crystals with partial bandgaps from blue phase colloidal crystals and dielectric-doped blue phases.

PubMed

Stimulak, Mitja; Ravnik, Miha

2014-09-07

Blue phase colloidal crystals and dielectric nanoparticle/polymer doped blue phases are demonstrated to combine multiple components with different symmetries in one photonic material, creating a photonic crystal with variable and micro-controllable photonic band structure. In this composite photonic material, one contribution to the band structure is determined by the 3D periodic birefringent orientational profile of the blue phases, whereas the second contribution emerges from the regular array of the colloidal particles or from the dielectric/nanoparticle-doped defect network. Using the planewave expansion method, optical photonic bands of the blue phase I and II colloidal crystals and related nanoparticle/polymer doped blue phases are calculated, and then compared to blue phases with no particles and to face-centred-cubic and body-centred-cubic colloidal crystals in isotropic background. We find opening of local band gaps at particular points of Brillouin zone for blue phase colloidal crystals, where there were none in blue phases without particles or dopants. Particle size and filling fraction of the blue phase defect network are demonstrated as parameters that can directly tune the optical bands and local band gaps. In the blue phase I colloidal crystal with an additionally doped defect network, interestingly, we find an indirect total band gap (with the exception of one point) at the entire edge of SC irreducible zone. Finally, this work demonstrates the role of combining multiple - by symmetry - differently organised components in one photonic crystal material, which offers a novel approach towards tunable soft matter photonic materials.

Synthesis and spectroscopic study of CdS nanoparticles using hydrothermal method

NASA Astrophysics Data System (ADS)

AL-Mamoori, Mohammed H. K.; Mahdi, Dunia K.; Al-Shrefi, Saif M.

2018-05-01

In this work, cadmium sulfide nanoparticles (powder) with diameter 50.8 nm was prepared using hydrothermal method. The structural and optical properties of CdS nanoparticles was studied by X-ray diffraction, FESEM, EDS, FTIR, UV-Diffuse Reflectance spectroscopy and Photoluminance spectrum. X-ray diffraction reveal the formation the purity of prepared phase of CdS particles with hexagonal wurtzite structure with particle size 31.8nm by using sheerer equation. The energy dispersion scattering (EDS) examination explains that the sample is composed of a large amount of Cd and S which are exactly CdS nanoparticles and there is a very small trace of (Zn) and (O) element observed because of there is a small pollutions in the measurement place of samples. FESEM shows the spherical shape of nanoparticles with around 50.8 nm diameter. The optical absorption spectral study identified the red shift of the sample in comparison to bulk ZnO in three dimensions. Photoluminance spectrum (PL) at room temperature showed that there are two luminescence peaks at 433.14 nm and 518.21nm. Samples demonstrate a sharp emission band at around 433.18 nm, which is attributed to the typical exciton luminescence. The broad band at 518.21nm which were attributed to the trapped luminescence. The green emission band at 518.21 nm was associated with the emission due to electronic transition from the conduction band to an accepter level due to interstitial sulphur ion.

Electronic structure and defect properties of selenophosphate Pb2P2Se6 for γ-ray detection

NASA Astrophysics Data System (ADS)

Kontsevoi, Oleg Y.; Im, Jino; Wessels, Bruce W.; Kanatzidis, Mercouri G.; Freeman, Arthur J.

Heavy metal chalco-phosphate Pb2P2Se6 has shown a significant promise as an X-ray and γ-ray detector material. To assess the fundamental physical properties important for its performance as detector, theoretical calculations were performed for the electronic structure, band gaps, electron and hole effective masses, and static dielectric constants. The calculations were based on first-principles density functional theory (DFT) and employ the highly precise full potential linearized augmented plane wave method and the projector augmented wave method and include nonlocal exchange-correlation functionals to overcome the band gap underestimation in DFT calculations. The calculations show that Pb2P2Se6 is an indirect band gap material with the calculated band gap of 2.0 eV, has small effective masses, which could result in a good carrier mobility-lifetime product μτ , and a very high static dielectric constant, which could lead to high mobility of carriers by screening of charged scattering centers. We further investigated a large set of native defects in Pb2P2Se6 to determine the optimal growth conditions for application as γ-ray detectors. The results suggest that the prevalent intrinsic defects are selenium vacancies, followed by lead vacancies, then phosphorus vacancies and antisite defects. The effect of various chemical environments on defect properties was examined and the optimal conditions for material synthesis were suggested. Supported by DHS (Grant No. 2014-DN-077-ARI086-01).

Analogs of solid nanoparticles as precursors of aromatic hydrocarbons

NASA Astrophysics Data System (ADS)

Gadallah, K. A. K.; Mutschke, H.; Jäger, C.

2013-06-01

Context. Aromatic =CH and C=C vibrational bands have been observed within shocked interstellar regions, indicating the presence of aromatic emission carriers such as PAHs, which may have been created from adjacent molecular cloud material by interaction with a shock front. Aims: We investigate the evolution of the aromatic =CH and C=C vibrational modes at 3.3 and 6.2 μm wavelength in heated HAC materials, PAHs and mixed PAHs and HACs, respectively, aiming at an explanation of the evolution of carbonaceous dust grains in the shocked regions. Methods: Materials used in these analogs (HAC and PAH materials) were prepared by the laser ablation and the laser pyrolysis methods, respectively. The transmission electron microscopy (TEM) in high-resolution mode was used as an analytical technique to characterize the aromatic layers in HACs. Spectroscopic analysis was prformed in the mid-IR range. Results: A remarkable destruction of aliphatic structures in HACs has been observed with the thermal processing, while aromatic structures become dominating by increasing the diameters of the graphene layers. The aromatic bands at 3.3 and 6.2 μm, observed in the laboratory spectra of PAHs and of the combination of the PAHs and HAC materials, are also clearly observed in the spectrum of the heated HACs. These bands agree with those of aromatic bands observed in astronomical observations. Conclusions: Aromatization of HACs could be a pre-stage in the decomposition process of hydrocarbons that form PAH-clusters in such hot interstellar medium.

Tunable multi-band absorption in metasurface of graphene ribbons based on composite structure

NASA Astrophysics Data System (ADS)

Ning, Renxia; Jiao, Zheng; Bao, Jie

2017-05-01

A tunable multiband absorption based on a graphene metasurface of composite structure at mid-infrared frequency was investigated by the finite difference time domain method. The composite structure were composed of graphene ribbons and a gold-MgF2 layer which was sandwiched in between two dielectric slabs. The permittivity of graphene is discussed with different chemical potential to obtain tunable absorption. And the absorption of the composite structure can be tuned by the chemical potential of graphene at certain frequencies. The impedance matching was used to study the perfect absorption of the structure in our paper. The results show that multi-band absorption can be obtained and some absorption peaks of the composite structure can be tuned through the changing not only of the width of graphene ribbons and gaps, but also the dielectric and the chemical potential of graphene. However, another peak was hardly changed by parameters due to a different resonant mechanism in proposed structure. This flexibily tunable multiband absorption may be applied to optical communications such as optical absorbers, mid infrared stealth devices and filters.

Quasiparticle band gap of organic-inorganic hybrid perovskites: Crystal structure, spin-orbit coupling, and self-energy effects

NASA Astrophysics Data System (ADS)

Gao, Weiwei; Gao, Xiang; Abtew, Tesfaye A.; Sun, Yi-Yang; Zhang, Shengbai; Zhang, Peihong

2016-02-01

The quasiparticle band gap is one of the most important materials properties for photovoltaic applications. Often the band gap of a photovoltaic material is determined (and can be controlled) by various factors, complicating predictive materials optimization. An in-depth understanding of how these factors affect the size of the gap will provide valuable guidance for new materials discovery. Here we report a comprehensive investigation on the band gap formation mechanism in organic-inorganic hybrid perovskites by decoupling various contributing factors which ultimately determine their electronic structure and quasiparticle band gap. Major factors, namely, quasiparticle self-energy, spin-orbit coupling, and structural distortions due to the presence of organic molecules, and their influences on the quasiparticle band structure of organic-inorganic hybrid perovskites are illustrated. We find that although methylammonium cations do not contribute directly to the electronic states near band edges, they play an important role in defining the band gap by introducing structural distortions and controlling the overall lattice constants. The spin-orbit coupling effects drastically reduce the electron and hole effective masses in these systems, which is beneficial for high carrier mobilities and small exciton binding energies.

An enhanced sine dwell method as applied to the Galileo core structure modal survey

NASA Technical Reports Server (NTRS)

Smith, Kenneth S.; Trubert, Marc

1990-01-01

An incremental modal survey performed in 1988 on the core structure of the Galileo spacecraft with its adapters with the purpose of assessing the dynamics of the new portions of the structure is considered. Emphasis is placed on the enhancements of the sine dwell method employed in the test. For each mode, response data is acquired at 32 frequencies in a narrow band enclosing the resonance, utilizing the SWIFT technique. It is pointed out that due to the simplicity of the data processing involved, the diagnostic and modal-parameter data is available within several minutes after data acquisition; however, compared with straight curve-fitting approaches, the method requires more time for data acquisition.

Redox levels in aqueous solution: Effect of van der Waals interactions and hybrid functionals.

PubMed

Ambrosio, Francesco; Miceli, Giacomo; Pasquarello, Alfredo

2015-12-28

We investigate redox levels in aqueous solution using a combination of ab initio molecular dynamics (MD) simulations and thermodynamic integration methods. The molecular dynamics are performed with both the semilocal Perdew-Burke-Ernzerhof functional and a nonlocal functional (rVV10) accounting for van der Waals (vdW) interactions. The band edges are determined through three different schemes, namely, from the energy of the highest occupied and of the lowest unoccupied Kohn-Sham states, from total-energy differences, and from a linear extrapolation of the density of states. It is shown that the latter does not depend on the system size while the former two are subject to significant finite-size effects. For the redox levels, we provide a formulation in analogy to the definition of charge transition levels for defects in crystalline materials. We consider the H(+)/H2 level defining the standard hydrogen electrode, the OH(-)/OH(∗) level corresponding to the oxidation of the hydroxyl ion, and the H2O/OH(∗) level for the dehydrogenation of water. In spite of the large structural modifications induced in liquid water, vdW interactions do not lead to any significant structural effect on the calculated band gap and band edges. The effect on the redox levels is also small since the solvation properties of ionic species are little affected by vdW interactions. Since the electronic properties are not significantly affected by the underlying structural properties, it is justified to perform hybrid functional calculations on the configurations of our MD simulations. The redox levels calculated as a function of the fraction α of Fock exchange are found to remain constant, reproducing a general behavior previously observed for charge transition levels of defects. Comparison with experimental values shows very good agreement. At variance, the band edges and the band gap evolve linearly with α. For α ≃ 0.40, we achieve a band gap, band-edge positions, and redox levels in overall good agreement with experiment.

Redox levels in aqueous solution: Effect of van der Waals interactions and hybrid functionals

NASA Astrophysics Data System (ADS)

Ambrosio, Francesco; Miceli, Giacomo; Pasquarello, Alfredo

2015-12-01

We investigate redox levels in aqueous solution using a combination of ab initio molecular dynamics (MD) simulations and thermodynamic integration methods. The molecular dynamics are performed with both the semilocal Perdew-Burke-Ernzerhof functional and a nonlocal functional (rVV10) accounting for van der Waals (vdW) interactions. The band edges are determined through three different schemes, namely, from the energy of the highest occupied and of the lowest unoccupied Kohn-Sham states, from total-energy differences, and from a linear extrapolation of the density of states. It is shown that the latter does not depend on the system size while the former two are subject to significant finite-size effects. For the redox levels, we provide a formulation in analogy to the definition of charge transition levels for defects in crystalline materials. We consider the H+/H2 level defining the standard hydrogen electrode, the OH-/OH∗ level corresponding to the oxidation of the hydroxyl ion, and the H2O/OH∗ level for the dehydrogenation of water. In spite of the large structural modifications induced in liquid water, vdW interactions do not lead to any significant structural effect on the calculated band gap and band edges. The effect on the redox levels is also small since the solvation properties of ionic species are little affected by vdW interactions. Since the electronic properties are not significantly affected by the underlying structural properties, it is justified to perform hybrid functional calculations on the configurations of our MD simulations. The redox levels calculated as a function of the fraction α of Fock exchange are found to remain constant, reproducing a general behavior previously observed for charge transition levels of defects. Comparison with experimental values shows very good agreement. At variance, the band edges and the band gap evolve linearly with α. For α ≃ 0.40, we achieve a band gap, band-edge positions, and redox levels in overall good agreement with experiment.

Evolution of band topology by competing band overlap and spin-orbit coupling: Twin Dirac cones in Ba3SnO as a prototype

NASA Astrophysics Data System (ADS)

Kariyado, Toshikaze; Ogata, Masao

2017-11-01

We theoretically demonstrate how competition between band inversion and spin-orbit coupling (SOC) results in nontrivial evolution of band topology, taking antiperovskite Ba3SnO as a prototype material. A key observation is that when the band inversion dominates over SOC, there appear "twin" Dirac cones in the band structure. Due to the twin Dirac cones, the band shows highly peculiar structure in which the upper cone of one of the twin continuously transforms to the lower cone of the other. Interestingly, the relative size of the band inversion and SOC is controlled in this series of antiperovskite A3E O by substitution of A (Ca, Sr, Ba) and/or E (Sn, Pb) atoms. Analysis of an effective model shows that the emergence of twin Dirac cones is general, which makes our argument a promising starting point for finding a singular band structure induced by the competing band inversion and SOC.

SLD-MOSCNT: A new MOSCNT with step-linear doping profile in the source and drain regions

NASA Astrophysics Data System (ADS)

Tahne, Behrooz Abdi; Naderi, Ali

2017-01-01

In this paper, a new structure, step-linear doping MOSCNT (SLD-MOSCNT), is proposed to improve the performance of basic MOSCNTs. The basic structure suffers from band to band tunneling (BTBT). We show that using SLD profile for source and drain regions increases the horizontal distance between valence and conduction bands at gate to source/drain junction which reduces BTBT probability. SLD performance is compared with other similar structures which have recently been proposed to reduce BTBT such as MOSCNT with lightly-doped drain and source (LDDS), and with double-light doping in source and drain regions (DLD). The obtained results using a nonequilibrium Green’s function (NEGF) method show that the SLD-MOSCNT has the lowest leakage current, power consumption and delay time, and the highest current ratio and voltage gain. The ambipolar conduction in the proposed structure is very low and can be neglected. In addition, these structures can improve short-channel effects. Also, the investigation of cutoff frequency of the different structures shows that the SLD has the highest cutoff frequency. Device performance has been investigated for gate length from 8 to 20 nm which demonstrates all discussions regarding the superiority of the proposed structure are also valid for different channel lengths. This improvement is more significant especially for channel length less than 12 nm. Therefore, the SLD can be considered as a candidate to be used in the applications with high speed and low power consumption.