Band mixing in narrow gap semiconductors

Segment of a periodic table depicting the elements that are molecular/atomic (blue) and that have extended network structures (gray) at STP.

The electron transport through an energy transmission spectrum between two reservoirs with different temperatures and chemical potentials is studied. The heat flow carried by the electrons is obtained. Taking into account the radiative heat leaks between the two electron reservoirs, the performance parameters of the heat pump are derived by numerical calculation. The performance characteristic curves of the heat pump are plotted. The influence of the heat leaks, the position of resonance energy level and the width of the level on the operation performance of the heat pump is analyzed. When the width of resonance energy level is infinitely small, the coefficient of performance may reach the value of Carnot heat pump.

First-principles calculations were carried out within the framework of density functional theory to investigate the influence of phase changes on the electronic and optical properties of CsSnI3. The lattice parameter and band gap of four different phases, i.e., α-, β-, γ-, and δ-phases, of CsSnI3 are estimated by employing different exchange–correlation functionals in order to explore their ability to reproduce geometric and electronic structures adequately. Comparison of the calculated total energies shows the non-perovskite orthorhombic (δ-phase) modification of CsSnI3 is the most stable, followed by the orthorhombic (γ-CsSnI3) perovskite phase. Thermal stability calculations in the form of temperature dependence of entropy as well as the absence of imaginary frequencies in the phonon dispersion diagrams also confirmed the dynamical stability of the δ-CsSnI3. The influence of the structural phase changes on the band gap and Fermi level shifts of CsSnI3 were assessed. Contribution of the electronic states on the formation of the valence and conduction band of four phases of CsSnI3 were determined, which were calculated using various exchange–correlation functionals, including the high-precision hybrid functional HSE06, and compared with available experimental ones. The calculated energy band distribution diagrams showed that all three perovskite modifications of CsSnI3 have direct transitions, while δ-CsSnI3 has an indirect transition. It was found that during the transition from δ- to α-phase, the Fermi level descends to the low energy region (towards the valence band), and the band gap decreases from 2.99 eV to 1.33 eV. During the transition from α- to β-phase, the band gap width again decreases to 1.23 eV and the Fermi level mixes by 1.65 eV towards the conduction band (CB). On the contrary, the band gap increases from β- to γ-phase and the Fermi level shifts by 0.41 eV towards the conduction band. The values of the complex dielectric constant and the refractive index of four phases of CsSnI3 were also calculated.

Efficient calculational techniques for reflection high energy electron diffraction (RHEED) are reported for surfaces with large periodic supercells. A fast Fourier transform approach enables the computer time scaling of a conventional RHEED calculation to be reduced to n2 log (n), where n is the number of diffracted beams used in the calculation. The special technique needed to implement this for arbitrary incident azimuths with symmetry optimization is detailed. A Green's function method is also introduced which is particularly suitable for calculations for imperfect surfaces. This combines the conventional approach to RHEED for dealing with substrate diffraction with a Green's function treatment for an imperfect surface of supercells and has n log (n) time scaling. Techniques for matching the results of the conventional and Green's function treatments at the substrate–surface interface are given. In addition, numerical procedures for solving the resulting equations are described and a selection of illustrative results is presented.

A theory is constructed of the temperature dependence of quantum oscillation phenomena in narrow-gap electronic semiconductors, taking into account the thermal smearing of Landau levels. Oscillations of longitudinal electrical conductivity in narrow-gap electronic semiconductors at various temperatures are studied. An integral expression is obtained for the longitudinal conductivity in narrow-gap electronic semiconductors, taking into account the diffuse broadening of the Landau levels. A formula is obtained for the dependence of the oscillations of longitudinal electrical conductivity on the band gap of narrow-gap semiconductors. The calculation results are compared with experimental data.

Graphene is one of the most promising materials for nanoelectronics owing to its unique Dirac cone-like dispersion of the electronic state and high mobility of the charge carriers. However, to facilitate the implementation of the graphene-based devices, an essential change of its electronic structure, a creation of the band gap should controllably be done. Brought about by two fundamentally different mechanisms, a sublattice symmetry breaking or an induced strong spin-orbit interaction, the band gap appearance can drive graphene into a narrow-gap semiconductor or a 2D topological insulator phase, respectively, with both cases being technologically relevant. The later case, characterized by a spin-orbit gap between the valence and conduction bands, can give rise to the spin-polarized topologically protected edge states. Here, we study the effect of the spin-orbit interaction enhancement in graphene placed in contact with a lead monolayer. By means of angle-resolved photoemission spectroscopy, we show that intercalation of the Pb interlayer between the graphene sheet and the Pt(111) surface leads to formation of a gap of ∼200 meV at the Dirac point of graphene. Spin-resolved measurements confirm the splitting to be of a spin-orbit nature, and the measured near-gap spin structure resembles that of the quantum spin Hall state in graphene, proposed by Kane and Mele [ Phys. Rev. Lett. 2005 , 95 , 226801 ]. With a bandstructure tuned in this way, graphene acquires a functionality going beyond its intrinsic properties and becomes more attractive for possible spintronic applications.

基于密度泛函理论, 采用第一性原理赝势平面波方法计算了Co、Cr单掺杂以及Co-Cr共掺杂金红石型TiO2的能带结构、态密度和光学性质. 计算结果表明: 纯金红石的禁带宽度为3.0 eV, Co掺杂金红石型TiO2的带隙为1.21 eV, 导带顶和价带底都位于G点处, 仍为直接带隙, 在价带与导带之间出现了由Co 3d和Ti 3d轨道杂化形成的杂质能级；Cr掺杂金红石型TiO2的直接带隙为0.85 eV, 在价带与导带之间的杂质能级由Cr 3d和Ti 3d轨道杂化轨道构成, 导带和价带都向低能级方向移动；Co-Cr共掺杂, 由于电子的强烈杂化, 使O-2p态和Ti-3d 态向 Co-3d 和 Cr-3d态移动, 使价带顶能级向高能级移动而导带底能级向低能方向移动, 极大地减小了禁带的宽度, 也是共掺杂改性的离子选择依据. 掺杂金红石型TiO2的介电峰、折射率和吸收系数峰都向低能方向移动；在E<2.029 eV的范围内, 纯金红石的e2、k和吸收系数为零, 掺杂后的跃迁强度都大于未掺杂时的跃迁强度, Co-Cr共掺杂的跃迁强度大于Co掺杂及Cr掺杂, 说明Co、Cr共掺杂更能增强电子在低能端的光学跃迁, 具有更佳的可见光催化性能.

The effects of pressure on the crystal structure, elastic properties, and electronic characteristics of Al&lt;sub&gt;4&lt;/sub&gt;In&lt;sub&gt;2&lt;/sub&gt;N&lt;sub&gt;6&lt;/sub&gt; are systematically studied using first-principles density functional theory. The lattice constants of Al&lt;sub&gt;4&lt;/sub&gt;In&lt;sub&gt;2&lt;/sub&gt;N&lt;sub&gt;6&lt;/sub&gt; decrease with the increase of pressure, exhibiting anisotropic compression with greater compressibility along the &lt;i&gt;c&lt;/i&gt;-axis. In terms of mechanical properties, the bulk modulus increases with the increase of pressure, indicating enhanced compressive resistance. Notably, the Vickers hardness decreases with the increase of pressure, indicating that high pressure can induce plastic deformation in Al&lt;sub&gt;4&lt;/sub&gt;In&lt;sub&gt;2&lt;/sub&gt;N&lt;sub&gt;6&lt;/sub&gt;. The calculations of elastic constants and phonon spectra confirm that Al&lt;sub&gt;4&lt;/sub&gt;In&lt;sub&gt;2&lt;/sub&gt;N&lt;sub&gt;6&lt;/sub&gt; retains mechanical and dynamical stability in the pressure range of 0–30 GPa. Electronic structure calculations reveal that Al&lt;sub&gt;4&lt;/sub&gt;In&lt;sub&gt;2&lt;/sub&gt;N&lt;sub&gt;6&lt;/sub&gt; possesses a direct band gap, and non-overlapping conduction and valence bands at the Fermi level. The conduction band has a higher carrier mobility than the valence band. The band gap increases almost linearly with pressure rising from 3.35 eV at 0 GPa to 4.24 eV at 30 GPa, demonstrating significant pressure-induced modulation of the electronic structure. Furthermore, the analysis of differential charge densities reveals that increasing pressure can strengthen the Al-N and In-N bonds in Al&lt;sub&gt;4&lt;/sub&gt;In&lt;sub&gt;2&lt;/sub&gt;N&lt;sub&gt;6&lt;/sub&gt; through shortened interatomic distances and stronger atomic interactions, increasing its compression resistance. In summary, this study not only deepens our understanding of the high-pressure properties of Al&lt;sub&gt;4&lt;/sub&gt;In&lt;sub&gt;2&lt;/sub&gt;N&lt;sub&gt;6&lt;/sub&gt; but also provides theoretical guidance for its application in UV optoelectronics. Pressure-driven modulation of its mechanical and electronic characteristics highlights its potential in efficient high-pressure optoelectronic devices and materials.

As part of catalysis—and more precisely of heterogeneous catalysis—heterogeneous photocatalysis is an area of chemistry impacting many reactions as varied as total or mild oxidation reactions, dehydrogenation reactions, metal deposition, hydrogen transfers, etc. These different reactions are mainly facing applications in the field of water and air purification treatments—targeting both chemical pollutants and biological ones, self-decontaminating or self-cleaning products, organic fine chemistry as well as energy-related areas with hydrogen production from water. Relative to the history of catalysis processes, coined by Berzelius at the beginning of nineteenth century, photocatalysis remains a recent discipline, as marked by J.M. Herrmann in its introductory paper of this Special Issue. Basically, photocatalysis differentiates from conventional catalysis by the activation of the catalytic solid, which occurs through photon absorption rather than by thermal activation. This photonic activation thus requires the use of a semiconductor material as catalyst, provided that the radiation wavelengths are greater than its band gap, which corresponds to the energy gap between both conduction and valence bands of the semiconductor. Activating a semiconductor leads to the promotion of an electron from the valence to the conduction band, with the simultaneous creation of a photogenerated hole within the valence band. Further, the transfer of photogenerated charge carriers to the photocatalyst surface allows redox reactions to occur with adsorbed reactants, coming from gas or liquid phase depending on the application. At the catalyst surface, the redox reactions are separated into reduction and oxidative steps, involving on one hand, conduction band electrons and adsorbed electron acceptors such as, e.g., oxygen molecules (eCB +A→A), and on the other hand, valence band holes and adsorbed electron donors such as, e.g., organic molecules or more generally the targeted pollutant (hVB + D→D). Indirect oxidation reactions also occur through the formation of highly oxidative hydroxyl radicals generated by the oxidation of water by holes. Thus, the photocatalysis discipline exists through the ability of a material, in this case a semiconductor (usually TiO2), to simultaneously interact with light and reactants, through both aband adsorption phenomena, respectively. Nowadays, the “Advanced Oxidation Process” (AOP) nature of photocatalysis remains the most exploited, facing applications in the field of water and air purification treatments and self-decontaminating/self-cleaning products. On one hand, the implementation of photocatalysis for environmentally friendly cleaning water or air fits into a sustainable development approach, driven by the search for alternative S. Lacombe (*) Institute for Analytical Sciences and Physico-Chemistry for Environment and Materials (IPREM), UMR 5254 CNRS, University of Pau and Pays de l’Adour, Helioparc : 2 avenue of President ANGOT, 64053 Pau Cedex 09, France e-mail: sylvie.lacombe@univ-pau.fr

The electronic states induced by misfit dislocations at zincblende CdTe/CdS, CdS/ZnS, and InP/GaP heterointerfaces and their impacts on the valence band offsets are investigated using first-principles calculations. The (100) and (110) heterointerfaces including perfect edge dislocations with a Burgers vector of $\frac{a}{2}[1\overline{1}0]$ and a line vector of [001] are considered for each system. Two types of dislocation core structures are found: The ``closed'' core has four-membered rings only and contains threefold and fivefold coordinated cations, whereas the ``open'' core has ten-membered rings that involve threefold coordinated atoms. The closed core forms at the (110) interfaces and is energetically more favorable than the open core. The characteristics of dislocation-induced electronic states are heavily dependent on the system and core structure, but all have localized states in the valence and conduction bands. The localized states in the valence band mostly have anion-orbital characteristics, whereas those in the conduction band mainly have cation-orbital characteristics. Some, but not all, dislocation cores also induce electronic states in the gap between the higher of the valence band maxima and the lower of the conduction band minima in the two phases that constitute the interface. The explicit treatment of misfit dislocations changes the valence band offset between the regions at a distance of \ensuremath{\sim}1 nm or more from the heterointerface by typically \ensuremath{\sim}0.1 eV.

Introduction:The use of Density Functional Theory (DFT) simulations to predict the chemical-physical properties of nanostructured compounds and heterogeneous interactions (solid-gas) has become almost essential today, both to predict the properties of technologically advanced nanomaterials and to overcome the limits of experimental characterizations.Specifically, the use of DFT calculations can likewise be used in chemoresistive gas sensor applications, to investigate the physical-chemical properties of nanostructured semiconductors and their possible catalytic activity. Among the various gas sensing materials used, the Metal Oxide Semiconductors (MOXS) are the most investigated since the excellent sensitivity, good stability and low cost of production [1,2]. Nevertheless, besides all of these advantages they present also some drawbacks such as signal drift over the time, lack of selectivity and high operating temperature.Investigators have tried to address these problems in several manners, including the synthesis of solid solutions composed of different metal oxides, doping these materials with deferent kinds of elements and also with controlled concentrations of oxygen vacancies. The latter is a field still little explored, but preliminary experimental and theoretical works highlighted interesting results [3,4].When the sensor is exposed to a reducing (oxidizing) gas, the gas molecules interact with the chemisorbed oxygen species from the surrounding atmosphere and then decrease (increase) the resistance of a n-type based gas sensor, and vice versa for a p-type semiconductor. This change in resistance is due to the exchange of electrons between the surface and the conduction band. For a stoichiometric semiconductor, the electrons involved in this reaction that are in the conduction band are coming from the valence band after heating the material, in this case they need a high energy to blow up from the valence band to the conduction band. But when one introduces some defects, new energy level will be created and then they will be a new electrons source, thus the energy that the sensor needs to be thermoactivated decrease; that is energy that electrons need to jump to the conduction band. Scope and Methods: The purpose of this work is to investigate the impact of oxygen vacancies inside MOXS on their physical-chemical properties in general and on the electrons exchange in particular. Indeed, the creation of oxygen vacancies influences the physical-chemical properties of metal oxides. i.e. The bridging oxygen vacancies lead to the formation of energy level in the band gap region (Fig.1), these energy levels will behave as electrons donors that decrease the energy (ε1) needed for the electrons to be in the conduction band. While creation of oxygen vacancies from in-plane positions drives to the formation of impurity levels located slightly below the conduction band minimum (CBM), in this case the energy (ε2) that they need to jump to the conduction band is lower than ε1. Therefore, we can say that in-plane oxygen vacancies can overcome the disadvantage of high working temperatures for MOXS; as much as the electrons resource is close to the conduction band, as much as the ionization of these electron levels will be easy and then enhance the reactions (adsorption process) on the surface of the active element at low temperature.Tin dioxide is a typical n-type semiconducting material with a wide band gap of 3.6 eV. It has attracted the attention of many researchers because of its broad spectrum of physicochemical properties. It has also been used in several fields such as optoelectronic devices, electrocatalysis, ceramics and gas sensors. It is the semiconductor most studied as active element for the production of chemoresistive gas sensors. Then, it represents the best candidate for the proposed innovative work.In this work, DFT was used to compute the physical-chemical properties of the considered material. The Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA), which improve the LDA results taking into account the electron density gradient due to the non-homogeneity of the electron density, are the principal basis of the exchange-correlation functional within the framework of DFT. In this work, the GGA parameterized PBE (Perdew-Burke-Ernzerhof) has been used for structural properties prediction. This approximation has demonstrated, beside the LDA, their ability to predict the structural properties of materials. Nevertheless, their applications underestimate the band gap energy of materials compared to experimental results. To enhance the results concerning the band gap the Tran-Blaha’s modified Becke-Johnson exchange potential model (TB-mBJ) has been applied, this approximation has proved its ability to predict the band gap of materials.A series of first-principles studies has been carried out using the Full Potential Linearized Augmented Plane Wave (FPLAPW) method within the framework of the DFT, as implemented in the Wien2k code. This method is the most effective method to solve the Khon Sham equation.

A Green's function formulation of the [formula omitted] theory in the presence of spin-orbit interaction and magnetic field: Application to the electronic structure and related properties of w-GaN

The electronic and optical properties of InAs/GaSb core-shell nanowires are investigated within the effective mass $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ approach. These systems have a broken band gap, which results in spatially separated confinement of electrons and holes. We investigated these structures for different sizes of the InAs and GaSb core and shell radius. We found that for certain configurations, the conduction band states penetrate into the valence band states resulting in a negative band gap (${E}_{g}l0$), which leads to a conduction band ground state that lies below the valence band ground state at the $\ensuremath{\Gamma}$ point. For certain core-shell wires, only one conduction band state penetrates into the valence band and in this case, a minigap $\ensuremath{\Delta}$ opens up away from the $\ensuremath{\Gamma}$ point and as a consequence the electronic properties of the nanowire now depend on both ${E}_{g}$ and $\ensuremath{\Delta}$ values.

There is considerable current effort to discover new thermoelectric materials with a high figure of merit Z. Some of these new materials are narrow-gap semiconductors with rather complex crystal structures. In this paper we discuss the results of electronic structure calculations in two classes of such systems. The first class consists of BaBiTe3, a structural and chemical derivative of the well-studied Bi2Te3. Similarities and differences in the band structures of these two systems are discussed. The second class consists of half-Heusler or “stuffed”-NaCl compounds MNiX, where M is Y, La, Lu, Yb, and X is a pnictogen; As, Sb, Bi. To understand the physical reason behind the energy gap formation, we compare the electronic structure of YNiSb with that of an isoelectronic system ZrNiSn, another isostructural compound of thermoelectric interest. These calculations were carried out within density functional theory (in generalized gradient approximation) using self-consistent full-potential LAPW method. Energy gaps and effective masses associated with the conduction band minimum and valence band maximum have been calculated and these quantities have been used to estimate transport properties. Large room temperature thermopower values in Bi2Te3 and BaBiTe3 can be understood in terms of multiple conduction and valence band extrema whereas similar large values in ZrNiSn and other half-Heusler compounds can be ascribed to large electron and hole effective mass.

Good approximations for the radiative transition rates, conduction band to heavy-hole band and conduction band to light-hole band, which are continuous over a wide range of temperature and electron and hole Fermi levels (degenerate and non-degenerate) and in which photon multiplication may be included, are developed for direct gap III - V semiconductors. They are based on the band structure and wavefunctions of the simple isotropic three-band approximation to Kane's band structure and depend solely on the effective masses at the zone centre, the energy gap and the dielectric constant. They are compared with more accurate calculations based on the diagonalization of the matrix of the four-band model with higher and lower bands taken into account by perturbation theory. Application to (0.0 < x < 0.25) and (0.17 < x < 0.26) shows that over the whole range of materials, Fermi levels (-0.25 to 0.40 eV) and temperatures (80 - 400 K) the agreement is never worse than 20% and usually considerably better. Results are given in detail for InSb and . It is also shown that in degenerate material, when the penetration of the hole Fermi level into the valence band is greater than that of the electron Fermi level into the conduction band, the transition rate to the light-hole band can exceed that to the heavy-hole band. These approximations are eminently suitable for use in modelling semiconductor effects and devices.