Semiempirical Tight Binding Research Articles

Methane cracking on single-wall carbon nanotubes studied by semi-empirical tight binding simulations

The catalytic properties of thermally distorted single wall carbon nanotubes for the methane dissociation reaction have been investigated in the framework of semi-empirical tight- binding molecular dynamics, electronic structure and total energy calculations. It is shown that, if methane molecules and the nanotube are allowed to get closer than the equilibrium physi-sorption distance, the next dissociation reaction step is characterized by a small enthalpy change. Moreover the reactivity hierarchy of the various carbon nanotubes considered is related to the electronic density of states originating from the axial components of the atomic orbitals.

Theoretical Unimolecular Kinetics for CH4 + M ⇄ CH3 + H + M in Eight Baths, M = He, Ne, Ar, Kr, H2, N2, CO, and CH4

Ensembles of classical trajectories are used to study collisional energy transfer in highly vibrationally excited CH(4) for eight bath gases. Several simplifying assumptions for the CH(4) + M interaction potential energy surface are tested against full dimensional direct dynamics trajectory calculations for M = He, Ne, and H(2). The calculated energy transfer averages are confirmed to be sensitive to the shape of the repulsive wall of the intermolecular potential, with an exponential repulsive wall required for quantitative predictions. For the diatomic baths, the usual "separable pairwise" approximation for the interaction potential is unable to describe the orientation dependence of the interaction potential accurately, and the ambiguity in the resulting parametrizations contributes an additional uncertainty to the predicted energy transfer averages of 20-40%. On the other hand, the energy transfer averages are shown to be insensitive to the level of theory used to describe the intramolecular CH(4) potential, with a computationally efficient semiempirical tight binding potential for hydrocarbons performing equally well as an MP2 potential. The relative collisional energy transfer efficiencies of the eight bath gases are discussed and shown to be a function of temperature. The ensemble-averaged energy transferred in deactivating collisions <ΔE(d)> for each bath is used to parametrize a single-exponential-down model for collisional energy transfer in master equation calculations. The predicted decomposition rate coefficients for CH(4) agree well with available experimental rate coefficients for M = He, Ar, Kr, and CH(4). The effect of vibrational anharmonicity on the predicted rate coefficients is considered briefly.

Second harmonic response of the Si(111)7 × 7 surface

Optical second harmonic generation spectra have been experimentally obtained from a clean Si(111) 7 × 7 in two different polarization configurations isolating the rotational anisotropic and isotropic contributions. The energy of the fundamental photon is varied from 0.8 eV to 2.5 eV. For comparison, we also use a microscopic formulation based on the semi-empirical tight binding method to evaluate the nonlinear surface susceptibility tensor χ(2 ω). Good agreement between theory and experiment is obtained with respect to the number of resonances, their position in energy, and surface or bulk character.

Bulk and Surface Properties of Rutile TiO2 from Self-Consistent-Charge Density Functional Tight Binding.

Bulk rutile TiO2 and its (110) surface have been investigated with a computationally efficient semiempirical tight binding method: self-consistent-charge density functional tight binding (SCC-DFTB). Comparisons of energetic, mechanical, and electronic properties are made to density functional theory (DFT) and to experiment to characterize the accuracy of SCC-DFTB for bulk rutile TiO2 and TiO2(110). Despite the fact that the SCC-DFTB parameters for Ti, Ti-Ti, and Ti-O were developed in the context of small biologically relevant Ti containing compounds, SCC-DFTB predicts many properties of bulk TiO2 and the TiO2(110) surface with accuracy similar to local and gradient-corrected DFT. In particular, SCC-DFTB predicts a direct band gap of TiO2 of 2.46 eV, which is in better agreement with experiment, 3.06 eV, than DFT utilizing the local density approximation (LDA), 2.0 eV. SCC-DFTB also performs similar in terms of accuracy as LDA-DFT for the phonon frequencies of the bulk lattice and for the relaxed geometry of the TiO2(110) surface. SCC-DFTB does, however, overestimate the surface energy of TiO2(110) compared to LDA-DFT. Nevertheless, the overall accuracy of SCC-DFTB, which is substantially more computationally efficient than DFT, is encouraging for bulk rutile TiO2 and TiO2(110).

Electron states and bonding in titanium sulfide

The electronic structure of titanium sulfide is studied in a series of molecular orbital and band structure calculations using the first-principles Hartree-Fock-Slater model. The results are correlated with optical data and with X-ray emission and absorption, and compared with recent semiempirical tight binding models.

Semi-empirical tight binding modelling of CdSTe/CdTe, ZnSSe/ZnSe and ZnSSe/ CdSe heterostructures

We present a semi-empirical sp 3s ⁎ tight binding model to calculate the effects of alloy composition and strain on electronic band structure of Cd and Zn based group II–VI heterostructures for photovoltaic devices. The semi-empirical sp 3 s ⁎ TB model Hamiltonian includes second nearest neighbor interactions and spin-orbit splitting of p-states. Bond lengths and atomic energies of cation and anion forming ternary semiconductors are taken as nonlinear function of composition. The 2NN sp 3 s ⁎ tight binding model calculations are compared with those of the package program WIEN2k which uses the generalized gradient approximation (GGA) and local spin density approximation (LSDA) based scaling law for the scissor operator for the self energy corrections to the DFT energy band gaps of semiconductors. We found that both the GGA and LSDA corrected WIEN 2k simulations and 2NN sp 3 s ⁎ TB model accurately reproduces the band gaps and both the valence band and conduction band dispersion curves at Γ, X and L high symmetry points of Brillouin zone, also in good agreement with experiment.

Torsional potentials and full-dimensional simulation of electronic absorption and fluorescence spectra of para-phenylene oligomers using the semiempirical self-consistent charge density-functional tight binding approach

A systematic study on the structural properties of para-phenylene oligomers based on the self-consistent charge density-functional tight binding approach (SCC-DFTB) and its time-dependent (TD) version is presented. Our goal is to investigate the applicability of DFTB for the present class of compounds and to use its computational efficiency for on-the-fly dynamics calculations and to perform in this way simulations of absorption and fluorescence spectra. For this purpose geometry optimizations have been performed for the ground state and for the electronically lowest excited state of oligomers containing two to seven aromatic rings. The torsional potential curves have been computed for para-biphenyl and para-terphenyl in the ground and lowest excited state. Agreement with previously computed DFT results is quite encouraging and DFTB seems to be well suited for the treatment of the class of conjugated pi systems investigated in this work. The intrachain vibrational broadening of absorption and emission spectra computed from dynamics simulations are presented and compared with experimental spectra.

Theoretical study of the optical response of the adsorption of Sb on the GaAs(110) surface

AbstractA theoretical study of the optical response of the adsorption of Sb atoms on the clean GaAs(110) surface is presented. The reflectance anisotropy spectroscopy (RAS) spectrum is calculated and analysed as a function of Sbcoverage. To capture some of the possible dynamical processes of Sb sub‐monolayer growth, a 1 × 3 unit cell is considered whose equilibrium geometry has been obtained by performing an ab‐initio pseudopotential calculation with the use of density functional theory within the local density approximation. The RAS is calculated via the ab‐initio scheme and also by using a semiempirical tight binding formalism. Some trends in the RAS spectra are identified as the clean surface is partially covered by Sb until the one monolayer coverage is achieved. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Tight binding modeling of CdSe/ZnS and CdZnS/CdS II–VI heterostructures for solar cells: Role of d-orbitals

We present a semiempirical tight binding model with sp 3 d 5 basis set to investigate the alloy composition and strain effects on the electronic band structure of group II–VI heterostructures for solar cells. The model Hamiltonian includes nearest neighbor interactions and spin-orbit splitting of p-states. Bond lengths and atomic energies of cation and anion forming compound semiconductors are taken as nonlinear function of composition. Using this scheme, we investigated the composition and strain effects on electronic band structure in Cd 1 − x Zn x S/CdS and CdSe/ZnS heterostructures. We found that the sp 3 d 5 TB model accurately reproduces the band gaps and both the valence band and conduction band dispersion curves at Γ, X and L high symmetry points at the edge of Brillouin zone. A good agreement is found between model predictions and experiment for nonlinear variation of band gaps at Γ, X and L symmetry points.

Local order in undercooled liquid metals: A tight binding molecular dynamics approach

We use constant (zero) pressure molecular dynamics based on a semi-empirical tight binding model to simulate the structures of liquid Ni, Cu, Pd and Au in the stable and undercooled states. We compare our data with the available experiments (neutron scattering for Ni, EXAFS for Pd). We analyze the structure in terms of Voronoi polyhedra and use the neighbors defined by this analysis to perform a Common Neighbors Analysis. We show that a small but significant difference in the number of bonds with five common neighbors exists between undercooled Ni and the other elements. We question the possible experimental observation of such a difference and its relevance to the issue of the existence of an icosahedral short range order in the liquid.

Magnetic properties of Pd atomic clusters from different theoretical approaches

We report a comparative study of the magnetic properties of free-standing PdN clusters (2 ≤ N ≤ 21) obtained through two different theoretical approaches that are extensively employed in electronic structure calculations: a semi-empirical Tight-Binding (TB) model and an ab-initio DFT pseudopotential model. Conclusions are drawn about the reliability of the TB model for the investigation of the electronic structure and magnetic properties of such complex 4d Transition Metals (TM) systems and we compare the results with previous systematic DFT calculations and comment on some available experiments in the literature. PACS. 75.75.+a Magnetic properties of nanostructures - 36.40.Cg Electronic and magnetic properties of clusters - 75.50.-y Studies of specific magnetic materials

Semiempirical tight binding modeling of Cd based II–VI heterostructures for solar cells

Realization of the full potentials of group II–VI heterostructures for photovoltaic device applications require reliable and precise predictive band structure models that are consistent with the fundamental principles of solid state physics. In this article, the electronic band structure of Cd based group II–VI ternary/binary heterostructures are calculated using the second nearest neighbor (2 nn) sp 3 s ⁎ semi-empirical tight binding formalism including the spin-orbit coupling. Using this scheme, we investigated the interface strain effects on band structure, such as band gaps and band offsets, in pseudomorphic Cd 1− x Zn x Te / CdTe and Cd 1− x Zn x S / CdS heterostructures for the entire composition range (0 ≤ x ≤ 1). It is gratifying to report that there is excellent agreement between the model predictions and experiment. The model should be useful in designing the group II–VI based heterostructure solar cells for optimum performance.

The Role of Local Fields in the Optical Properties of Silicon Nanocrystals

ABSTRACTIn this paper we discuss the role of local fields in the optical properties of silicon nanocrystals. Using a semiempirical tight binding approach, local field effects are included into the linear response theory, going beyond the standard independent particle approximation. The results show that local field effects give an important contribution to the optical properties of silicon nanocrystals, leading to a strong suppression of the absorption in the visible spectral range. This effect is attributed to the classical surface polarization contribution. A comparison between the atomistic tight binding approach and a classical dielectric model shows that the dielectric model gives reasonable results not only for large, but even for small silicon nanocrystals.

Electronic structure and reflectance anisotropy spectrum ofInAs(110)

The electronic structure of the $\mathrm{InAs}(110)$ surface is investigated using several theoretical tools: semiempirical tightbinding, density functional theory, and perturbative many-body theory within the GW approximation. Comparison is made with available photoemission data. Moreover, we calculate the optical properties of this surface and perform reflectance anisotropy experiments. The tight-binding and especially the GW approximations provide a very good description of the optical spectra and allow for a more detailed analysis of the origin of the spectral features.

Spin-orbit effects on reflectance anisotropy spectroscopy

We study the effects of the spin-orbit coupling on the reflectance anisotropy spectroscopy of semiconductor surfaces. A microscopic formulation based on a semi-empirical tight binding approach which includes spin-orbit interaction is used and theoretical results are obtained for clean $(110)1\ifmmode\times\else\texttimes\fi{}1$ surfaces of GaAs, InAs, and InSb. We show that the changes due to the spin-orbit coupling on the reflectance anisotropy gives spectra that compares rather well with the experimental results.

Modelling of bandgap and band offset properties in III-N related heterostructures

Reliable and precise knowledge about the strain and composition effects on the electronic properties is crucial for the optimization of III-nitride based heterostructures for electronic and optoelectronic device applications. Using the nearest neighbor sp 3 s ∗ semi-empirical tight binding formalism, we investigated the composition effects on bandgaps and offsets in pseudomorphic InGaN/GaN and GaAsN/GaAs heterostructures with zinc-blende structures. The strain effects are incorporated afterwards in a semi-empirical way. The model should be useful in understanding the effects of composition and strain on heterostructure energy band properties.

Distance Dependence of the Mn-Mn Exchange Interaction in IV-VI Semimagnetic Semiconductors

In the present paper we calculate the exchange interaction between two manganese ions in IV-VI semiconductors with the rocksalt structure. The method of calculations is based on the fourth order perturbation theory with respect to hybridization between band states and localized d orbitals of Mn ions. This hybridization is described by three Harrison integrals: V p d σ , V p d π , and V s d σ . The band states of IV-VI semiconductor are obtained from the semiempirical tight binding model built from s and p orbitals of cations and anions. The resulting exchange term in the Hamiltonian is of the form - Σ i , j = x y z J i j S 1 i S 2 j , however nondiagonal terms of the exchange integral tenso J i > not = j are very small. The dependence of J i j on the Mn-Mn distance is non-monotonic. We also discuss the influence of the local crystal deformations on the exchange integral.

Model for the effects of surface disorder on reflectance anisotropy spectroscopy

We present a simple model for surface disordering and study its consequences on reflectance anisotropy spectroscopy (RAS). Large unit cells are taken as a basis to introduce disorder by means of displacing the atoms from their equilibrium positions. Different displacement schemes are employed and ensemble averages are calculated in order to study the influence on the optical response. For reasons of computing time the semiempirical tight binding approach with an ${\mathrm{sp}}^{3}{s}^{*}$ basis is used to calculate the microscopic dielectric response of the surface, through which RAS is obtained. The well studied clean GaAs(110)(1\ifmmode\times\else\texttimes\fi{}1) surface is used as an example. We find that the spectral RAS structures of GaAs(110)(1\ifmmode\times\else\texttimes\fi{}1) follow a well defined behavior as a function of disorder and that a rather small number of surface atoms are needed only at equilibrium positions to produce already the RAS signal specific for the fully ordered surface reconstruction. This is a consequence of nearest neighbor interactions dominating the polarizability response and explains the fast dynamic response of RAS as compared to the diffraction methods which are limited by their large coherence length (low energy and reflection high energy electron diffraction).

Many-body levels of optically excited and multiply charged InAs nanocrystals modeled by semiempirical tight binding

Many-body levels of optically excited and multiply charged InAs nanocrystals are studied with the semiempirical tight-binding model. Single-particle levels of unstrained spherical InAs nanocrystals are described by the sp 3 d 5 s* nearest-neighbor tight-binding model including spin-orbit coupling. For optically excited InAs nanocrystals, first-order corrections of electron-hole Coulomb and exchange interaction to exciton levels and the oscillator strengths of the exciton levels determine several low-lying, bright-exciton levels. The origin of the large oscillator strengths of the bright exciton levels is explained by the analysis of dominant angular momenta of exciton envelope functions. Good agreement with photoluminescence excitation experiments is achieved for the size dependence of the three lowest bright-exciton energies of InAs nanocrystals with radius larger than 20 A. For multiply charged InAs nanocrystals, polarization of the nanocrystal environment is approximated by modeling the environment with a uniform dielectric medium. This polarization model incorporated into the tight-binding model provides a reasonable description of electron and hole addition energies in scanning tunneling spectroscopy experiments.

Computer simulation of liquid semiconductors

We discuss two examples of computer simulation of liquid semiconductors by two different techniques. Both examples are concerned with the relationship between thermodynamic properties and the atomic structure in the liquid state. By means of ab initio molecular dynamics we analyze the atomic structure of liquid Ge 15Te 85 eutectic alloys. We show that the changes observed in the experimental total structure factor S( q) are located in the GeTe partial structure factor, the TeTe partial structure factor remaining essentially unaltered. Using a semi-empirical tight binding approach, coupled with Gibbs ensemble and constant pressure Monte Carlo calculations, we can calculate the liquid vapor equilibrium of selenium. We obtain a liquid-vapor equilibrium curve and a critical point in the correct range of magnitude and an atomic structure of the liquid phase in good agreement with the experimental data.