Electronic and optical properties of Si and Ge nanocrystals: An ab initio study

First-principles calculations within density functional theory and many-body perturbation theory have been carried out in order to investigate the structural, electronic and optical properties of undoped and doped silicon nanostructures. We consider Si nanoclusters co-doped with B and P. We find that the electronic band gap is reduced with respect to that of the undoped crystals, suggesting the possibility of impurity based engineering of electronic and optical properties of Si nanocrystals. Finally, motivated by recent suggestions concerning the chance of exploiting Ge dots for photovoltaic nanodevices, we present calculations of the electronic and optical properties of a Ge₃₅H₃₆ nanocrystal, and compare the results with those for the corresponding Si₃₅H₃₆ nanocrystals and the co-doped Si₃₃BPH₃₆.     

Doping in silicon nanocrystals: An ab initio study of the structural, electronic and optical properties

There are experimental evidences that doping control at the nanoscale can significantly modify the optical properties with respect to the pure systems. This is the case of silicon nanocrystals (Si-nc), for which it has been shown that the photoluminescence (PL) peak can be tuned also below the bulk Si band gap by properly controlling the impurities, for example by boron (B) and phosphorus (P) codoping. In this work, we report on an ab initio study of impurity states in Si-nc. We consider B and P substitutional impurities for Si-nc with a diameter up to 2.2 nm. Formation energies (FEs), electronic, optical and structural properties have been determined as a function of the cluster dimension. For both B-doped and P-doped Si-nc the FE increases on decreasing the dimension, showing that the substitutional doping gets progressively more difficult for the smaller nanocrystals. Moreover, subsurface impurity positions result to be the most stable ones. The codoping reduces the FE strongly favoring this process with respect to the simple n-doping or p-doping. Such an effect can be attributed to charge compensation between the donor and the acceptor atoms. Moreover, smaller structural deformations, with respect to n-doped and p-doped cases, localized only around the impurity sites are observed. The band gap and the optical threshold are largely reduced with respect to the undoped Si-nc showing the possibility of an impurity-based engineering of the Si-nc PL properties.     

An ab initio study of the structural,elastic, electronic and optical properties of the newly synthesized nitridoaluminate LiCaAlN2

The structural parameters, elastic constants, electronic structure and optical properties of the recently reported monoclinic quaternary nitridoaluminate LiCaAlN₂ are investigated in detail using the ab initio plane-wave pseudopotential method within the generalized gradient approximation. The calculated equilibrium structural parameters are in excellent agreement with the experimental data, which validate the reliability of the applied theoretical method. The chemical and structural stabilities of LiCaAlN₂ are confirmed by calculating the cohesion energy and enthalpy of formation. Chemical band stiffness is calculated to explain the pressure dependence of the lattice parameters. Through the band structure calculation, LiCaAlN₂ is predicted to be an indirect band gap of 2.725 eV. The charge-carrier effective masses are estimated from the band structure dispersions. The frequency-dependent dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity coefficient and electron energy loss function spectra are calculated for polarized incident light in a wide energy range. Optical spectra exhibit a noticeable anisotropy. Single-crystal and polycrystalline elastic constants and related properties, including isotropic sound velocities and Debye temperatures, are numerically estimated. The calculated elastic constants and elastic compliances are used to analyse and visualize the elastic anisotropy of LiCaAlN₂. The calculated elastic constants demonstrate the mechanical stability and brittle behaviour of the considered material.     

First-principles calculations of the structural,elastic, electronic and optical properties of Si2P2O and Ge2P2O at high pressures

The structures of Si₂P₂O and Ge₂P₂O with the space group Cmc21 are derived. The structural, mechanical, elastic anisotropy, electronic and optoelectronic properties are calculated by first principles calculations based on density functional theory (DFT) with generalized gradient approximation (GGA) at high pressure for Si₂P₂O and Ge₂P₂O. By using the elastic stability criteria, it is shown that their structures are all stable. The phonon dispersion spectra are researched throughout the Brillouin zone as implemented in the CASTEP code, which indicates that the optimized structures are stable dynamically. The brittle/ductile behaviors are assessed in the pressures from 0 GPa to 50 GPa. Our calculations present that the performances of them become ductile with pressure rise. Moreover, the anisotropies of them are discussed by the Young's moduli at different pressure, and the results indicate that the anisotropies of them are obvious. The direct band structures of Ge₂P₂O and the indirect band gap of Si₂P₂O show that Si₂P₂O and Ge₂P₂O present semiconducting character at 0 GPa and 50 GPa. The band structures of Si₂P₂O are changed obviously with the increase of pressure. The total DOS originate mainly from O ‘s’ states, O ‘p’ states, P ‘s’ states and P ‘p’ states and M ‘p’ states (M=Si, Ge). The trends of DOS for Si₂P₂O and Ge₂P₂O display many similarities, and the change of DOS is obviously affected by the pressure for Si₂P₂O. The optoelectronic properties of them are researched. The calculated static dielectric constants, ɛ₁(0), are 3.2 at 0 GPa and 6.4 at 50 GPa for Si₂P₂O, and the values of Ge₂P₂O are 10.2 and 9.2 at 0 GPa and 50 GPa.     

First principles study on the structural,electronic and phonon properties of monolayer B2Si

Using first principles density functional theory, we predict a monolayer B₂Si structure with space group Pmm2 in the present work. This structure is confirmed to be dynamically stable. Based on the plane wave pseudopotential approach, the charge density, electron localization function, density of states, energy band, phonon property and thermal conductivity of Pmm2-B₂Si are systematically studied. It is interesting that the sp² hybridization and coordination bond of Si are found in Pmm2-B₂Si, which is the most important factor for its structural stability. The density of states and energy band analysis reveals that Pmm2-B₂Si is metallic because of the partial occupied Si 3pz and B 2pz states. Moreover, the acoustic-optical coupling is important for phonon transport in Pmm2-B₂Si, and the contribution of optical modes to the lattice thermal conductivity along the [100] and [010] directions is 13% and 12%, respectively. This study gives a fundamental understanding of the structural, electronic and phonon properties in Pmm2-B₂Si.     

Mn掺杂Fe2B晶体结构、力学性能及电子结构的第一性原理研究

基于密度泛函理论的第一性原理计算方法,本文系统地研究了Fe8-xMnxB4 (x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, 6, 7 ,8)的晶体结构、机械性能和电子结构。计算得到Fe2B的晶格常数与实验值相符,所有相都具有良好的热力学稳定性和机械稳定性。随着Mn掺杂浓度逐渐增大,Fe8-xMnxB4的各向异性先减弱后增强,Fe7.75Mn0.25B4的各向异性最弱。当Mn掺杂浓度较低时,Fe8-xMnxB4的硬度略微降低,韧性增强。除了Fe7Mn1B4、Fe6Mn2B4、Fe5Mn3B4、Fe4Mn4B4之外,其余的Fe8-xMnxB4相的韧性均比Fe2B好。由电子结构可以发现,Fe8-xMnxB4的力学性能主要由Fe-B键或Mn-B键决定。Mn掺杂到Fe2B中会使得B-B共价键增强,Fe2B的本征脆性得到改善,同时Fe2B的磁性不断减弱。     

Theory prediction of the electronic structure,mechanical and thermodynamic properties of Ca5Pd6Ge6 under pressure

The electronic structures, mechanical and thermodynamic properties of Ca₅Pd₆Ge₆ under pressure have been investigated via the first-principles calculations. The optimised lattice constant was in good agreement with the experimental data. Resulting from the Ca-3d, Ge-4p and Pd-4d states contribution, at the Fermi level, the Ca₅Pd₆Ge₆ exhibits metallic behaviour. The elastic constants were calculated, and the result implies that Ca₅Pd₆Ge₆ was mechanically stable below 50?GPa. The polycrystalline modulus increases almost linearly with pressure. The B/G ratio indicated that Ca₅Pd₆Ge₆ was brittle, and the brittle to ductile transition occurs at 2.5?GPa. Furthermore, the Debye temperature θ_D, the minimum thermal conductivity K was obtained. Finally, the isochoric heat capacity C_v and entropy S were evaluated by the quasi-harmonic Debye model in consideration for the temperature effect.     

An ab initio investigation of vibrational,thermodynamic, and optical properties of Sc_2AlC MAX compound

The structural vibrational, thermodynamical, and optical properties of potentially technologically important, weakly coupled MAX compound, Sc2 Al C are calculated using density functional theory(DFT). The structural properties of Sc_2AlC are compared with the results reported earlier. The vibrational, thermodynamical, and optical properties are theoretically estimated for the first time. The phonon dispersion curve is calculated and the dynamical stability of this compound is investigated. The optical and acoustic modes are observed clearly. We calculate the Helmholtz free energy(F), internal energy(E), entropy(S), and specific heat capacity(Cv) from the phonon density of states. Various optical parameters are also calculated. The reflectance spectrum shows that this compound has the potential to be used as an efficient solar reflector.     

The electronic structure of the azabenzenes an ab initio MO-SCF-LCAO study

Ab initio MO-SCF-LCAO calculations have been carried out for benzene and the azabenzenes pyridine, pyridazine, pyrimidine, pyrazine, s-triazine and s-tetrazine, and the results are presented.The general structure of the molecular orbitals is discussed and a classification scheme referring to an ideal case of cylindrical symmetry is described. Orbital energies, empirically corrected for reorganization and correlation, have been used to make assignments for the photoelectron spectra. A number of molecular properties have been computed and are discussed in relation to available experimental information.The electrostatic potential around each molecule is presented in the form of contour maps. Each nitrogen lone pair gives rise to a region of negative potential. The depths of these minima give some information about the relation between electronic structure and basicity. The potential in regions of high π-electron density is negative only in benzene and pyridine. This fact can be correlated with the increasing resistance against direct electrophilic substitution for the azabenzenes as compared to benzene itself.The calculations have been performed using contracted Gaussian functions as a basis. Four s- and two p-type atomic orbitals were used for carbon and nitrogen, whereas for hydrogen two s- and one pσ-type functions were used.     

Structural, electronic, vibrational, and elastic properties of SWCNTs doped with B and N: an ab initio study

A microscopic model is developed in order to analyse the effects of dissipations on single-photon transport in a coupled cavity array where one of the cavities is coupled to a three-level atom and both cavities and the three-level atom are coupled to an external environment. By employing the quasi-boson approach, the single-photon transmission and reflection amplitudes are found exactly for the Ξ-type, V-type and Λ-type three-level atoms. We focus on the dissipation properties in the case of the Λ-type system. Comparing the dissipative case with the nodissipative one, it can be found that the dissipations of the cavities and the Λ-type three-level atom significantly affect the transmission amplitude of single-photon transport. Whether the atom is in tune with the resonant frequency of the cavity or not, incomplete reflection is mostly caused by atom dissipation near the middle dip of the single-photon transport spectrum, while reduced transmission appears to be mainly controlled by cavity dissipation. Dissipations broaden the line width of the single photon transport spectrum.     

Phase stability,mechanical and thermodynamic properties of orthorhombic and trigonal MgSiN2: an ab initio study

Structural stability and mechanical and thermodynamic properties of the orthorhombic and trigonal MgSiN₂ polymorphs (or-MgSiN₂ and tr-MgSiN₂) were investigated through density functional theory and quasi-harmonic Debye model (QHDM). Our calculations show that or-MgSiN₂ is energetically the stable polymorph at low pressure, in agreement with previous experimental and theoretical study. Under pressure, a crystallographic transition from the orthorhombic structure to the trigonal one occurs around 25, 17.45 and 19.05 GPa as obtained from the generalized gradient approximation of Perdew-Wang (GGA-PW91), the generalized gradient approximation parameterized recently by Perdew et al (GGA-PBEsol) and the local density approximation developed by Ceperley and Alder and parameterized by Perdew and Zunger (LDA-CAPZ), respectively. Single-crystalline and polycrystalline elastic constants and related properties, namely Vickers hardness, acoustic Grüneisen parameter, minimum thermal conductivity, isotropic sound velocities and Debye temperature, were numerically estimated for both or-MgSiN₂ and tr-MgSiN₂. We have showed that the hardness of tr-MgSiN₂ is comparable to that of the harder materials like c-BN and B₆O. Temperature and pressure dependencies of volume, bulk modulus, thermal expansion, Grüneisen parameter, heat capacities and Debye temperature were investigated using QHDM.     

Quantum mechanical ab initio calculations of the structural,electronic and optical properties of bulk gold nitrides

In the present work, the atomic and the electronic structures of Au₃N, AuN and AuN₂ are investigated using first-principles density-functional theory (DFT). We studied cohesive energy vs. volume data for a wide range of possible structures of these nitrides. Obtained data were fitted to a Birch-Murnaghan third-order equation of state (EOS) so as to identify the most likely candidates for the true crystal structure in this subset of the infinite parameter space, and to determine their equilibrium structural parameters. The analysis of the electronic properties was achieved by the calculations of the band structure and the total and partial density of states (DOS). Some possible pressure-induced structural phase transitions have been pointed out. Further, we carried out GW₀ calculations within the random-phase approximation (RPA) to the dielectric tensor to investigate the optical spectra of the experimentally suggested modification: Au₃N(D0₉). Obtained results are compared with experiment and with some available previous calculations.     

Electronic structural and bulk properties of ScSe: ab initio study

Electronic, structural and bulk properties of scandium selenide, ScSe have been reported in the present paper. These properties have been studied using first principle calculations as well as the interionic potential model modified with covalency effect. The Gibbs free energy and enthalpy calculations show that present compound undergoes a structural phase transition from the NaCl-type structure to the CsCl-type structure. The stability of the present compound is discussed in terms of electronic band structure and density of states. The calculated equilibrium structural parameters are in a good agreement with the available experimental results.