Bottom Of Conduction Band Research Articles

Large‐Gap Quantum Spin Hall State in Double‐Transition‐Metal Homologous Compounds of WSi2N4: A First‐Principles Study

Recently, the MoSi2N4(MoN)4n homologous compounds have been synthesized, extending the MA2Z4 family materials to the superthick MA2Z4(MZ)n form. Herein, a first‐principles study on the double‐transition‐metal W2TiSi2N6 and related group VI–IV M2M′Si2N6 nanosheets is performed. Robust structural stability is affirmed in this W2TiSi2N6 nanosheet from the energetic, mechanical, dynamical, and thermal perspectives. Unlike the WSi2N4 and WSi2N4(WN)n systems, the W2TiSi2N6 nanosheet exhibits a semimetal behavior with the top valence and bottom conduction bands touching each other at the Fermi level. With the inclusion of spin–orbit coupling, a nontrivial band gap is opened in the W2TiSi2N6 nanosheet, which has a sizeable bulk gap of 0.11 eV. The Z2 invariant is 1 and a pair of topologically protected edge states emerges in the gap region, confirming that W2TiSi2N6 nanosheet is a quantum spin Hall (QSH) insulator. Through in‐plane strain engineering, the bulk gap can be further increased to 0.23 eV, which is sufficiently large for the room‐temperature QSH effect. Such a large‐gap QSH state is also present in other group VI–IV M2M′Si2N6 systems with the combinations of MM′ = WZr, MoZr, and MoHf. This study demonstrates that the double‐metal M2M′Si2N6 nanosheets are promising candidates to realize fascinating topological quantum states.

Study on the electronic structures and optical properties of Ca-doped KH2PO4 crystal

The first-principles method has been used to investigate the impact of Ca substitution at K sites on the electrical structures and optical characteristics of KH2PO4 crystals. And we use the FNV correction approach for the finite-size system and the HSE06 hybrid functional to update the defect formation energy and fix the "band-edge" issue. The results of defect formation energy indicate that the defect CaK· is the easiest to form. Crystal properties may be more susceptible to damage. The valence band maximum (VBM) of CaK× is shifted downward and the conduction band minimum (CBM) is shifted downward. The VBM of CaK· and CaK·· is slightly shifted upward, and CBM is almost unchanged. But Ca-induced defect energy levels are responsible for bandgap reduction and conduction band bottom shift. The absorption peaks of PE-KDP and FE-KDP occur at 1.1 eV and 2.05 eV, respectively, when the electron transitions between the defect transition level and CBM. Conversely, the absorption maxima are located at 6.5 eV and 5.4 eV, correspondingly, when the electron makes a jump between the defect transition level and VBM. Stokes redshifts are 0.3 and 0.9 eV. It damages the crystal and lowers the irradiation resistance threshold by generating vibrational energy in the lattice. Avoid CaK for better KDP quality.

Valley-dependent topological phase transition in monolayer ferrovalley materials RuXY (X, Y = F, Cl, Br)

Due to the breaking of the time reversal symmetry and spatial inversion symmetry, hexagonal ferrovalley materials have intrinsic large valley polarization. Model analysis shows that tuning the two different band gaps of valleys can realize phase transitions between ferrovalley semiconductors, half valley metals, and valley-polarized quantum anomalous Hall semiconductors. Through first-principle calculations, monolayer ferrovalley materials RuXY (X, Y = F, Cl, Br), which exhibit valley splitting at the top valence band and the bottom conduction band, are predicted to achieve this valley-dependent topological phase transition. Due to the different orbital proportions of d orbitals, the valley splitting at the top valence band is much greater than that at the bottom conduction band. Strain can regulate the interaction between orbitals, thus producing valley-dependent band inversion, leading to the quantum spin or valley Hall effect. The chiral edge states are demonstrated under appropriate biaxial strain. The topological phase transition is related to the inversion of the band structure and Berry curvatures at K and K′ valleys. These results have certain significance for the design of two-dimensional valley-dependent quantum materials and the application of valleytronic devices.

Lithium doping effect on structural, electronic and optical properties of KCaF3 fluoroperovskite: DFT insights using GGA-PBE

In order to evaluate the effects of Lithium (Li) doping on the various properties of KCaF3, we present first-principles calculations built on density functional theory (DFT) and employing approximation GGA-PBE. The compound possesses a 2 × 2 × 1 supercell and cubic geometry with space group pm3m. By putting the TDOS and PDOS concepts into practice, the effects of Li-doping on electronic structure and, consequently, optical characteristics have been investigated and evaluated in detail. With the formation of new states in band structure, the substitution of the Lithium (Li) atom for the Calcium (Ca) atom drastically adjusts the electronic band structure. The indirect bandgap of pure KCaF3 was measured to be 5.37 eV. The band gap was slightly shrunk as a result of changes in Li concentration. This causes the indirect band gap of KCaF3 to change to the direct band gap for Li-doped KCaF3. A strong interaction between the Li atom and the atoms around it is seen and the density of states for Li-doped KCaF3 relocates at slightly lower energies. Additionally, there is a noticeable change in the partial density of states of KCaF3 near the conduction band bottom, leading us to believe that Li-doping has an impact on the electronic band structure. Investigations into the optical characteristics of doped and undoped KCaF3 and correlations with its electrical structure are made. The massive difference in optical characteristics and band gap (i.e., indirect to direct) caused by Li-doping makes this substance a desirable candidate for optoelectronic materials.

Decreasing the Carrier Concentration of ZrNiSn: An Opposite Way to the Best N-Type Half-Heusler Thermoelectrics.

N-type ZrNiSn-based alloys reach a record thermoelectric figure of merit zT ≈1.2 by increasing the carrier concentration to 4-5 × 1020 cm-3 . In this work, It is reported that a comparable zT can also be realized in trace Ru-doped ZrNiSn-based alloy at even lower temperature by decreasing the carrier concentration. Compared to the previously reported Co doping, the doping of Ru results in a more effective reduction in carrier concentration, and thus higher Seebeck coefficient, lower electronic thermal conductivity, and enhanced thermoelectric performance. The electronic specific heat coefficient of the ZrNi1- x Rux Sn sample remains constant with increasing Ru content, indicating no obvious change in the density of states effective mass. Theoretical calculations show that the doping of Ru has negligible effect on the bottom of conduction band. The lattice thermal conductivity is further reduced by alloying Ti and Hf at the Zr site, and the bipolar diffusion is suppressed by doping of 0.5 at.% Sb. As a result, Ti0.25 Zr0.5 Hf0.25 Ni0.99 Ru0.01 Sn0.995 Sb0.005 reaches not only a zT value of 1.1 at 773 K but also a record average zT value of 0.8 in 300 to 873 K, demonstrating the effectiveness of trace Ru doping on boosting the thermoelectric performance of ZrNiSn-based alloys.

First-principles study of oxygen-doped perovskite CsPbX3 (X = Cl, Br, I)

The electronic structures of perovskite CsPbX3 (X = Cl, Br, I) and O doped CsPbX3 have been studied by first principles based on density functional theory (DFT). The theoretical calculation results show that O doping leads to narrow band gap width of perovskite CsPbX3. For pure CsPbX3, it is found that the 6p states of Pb determines the position of the conduction band bottom and the p states of halogen determines the position of the valence band top; and for O doped CsPbX3, it shows that the incorporation of O is the fundamental reason for the narrowing of band gap and light absorption edge redshifting compared with that of pure CsPbX3.

Tailoring the Electron and Hole Landé Factors in Lead Halide Perovskite Nanocrystals by Quantum Confinement and Halide Exchange.

The tunability of the optical properties of lead halide perovskite nanocrystals makes them highly appealing for applications. Halide anion exchange and quantum confinement enable tailoring of the band gap. For spintronics, the Landé g-factors of electrons and holes are essential. Using empirical tight-binding and k·p methods, we calculate them for nanocrystals of all-inorganic lead halide perovskites CsPbX3 (X = I, Br, Cl). The hole g-factor band gap dependence follows the universal law found for bulk perovskites, while for electrons, a considerable modification is predicted. Based on the k·p analysis, we conclude that this difference arises from the interaction of the bottom conduction band with the spin-orbit split electron states. These predictions are confirmed experimentally for electron and hole g-factors in CsPbI3 nanocrystals in a glass matrix, measured by time-resolved Faraday ellipticity in a magnetic field at cryogenic temperatures.

Experimental and Theoretical Study on Anisotropic Electron Mobility in 4H‐SiC

Electron mobility parallel to the c‐axis in 4H‐SiC is experimentally determined by Hall effect measurements over wide donor density and temperature ranges (6 × 1014–3 × 1018 cm−3 and 140–600 K), and it is compared with that perpendicular to the c‐axis obtained for the same conditions. Empirical equations for the mobility along both directions are determined as functions of donor density and temperature, which contribute to the simulation and designing of SiC devices. The origin of the mobility anisotropy is discussed, focusing on the electron effective mass anisotropy. For a precise analysis, taking into account the effect of electrons at a higher energy region than the conduction band bottom, an average electron effective mass considering the energy distribution is theoretically calculated from the band structure of SiC. As a result, it is clarified that the electron mobility anisotropy including its temperature dependence is explained by the average electron effective mass anisotropy.

Exploring the electronic and topological properties of single-layer VP2X2Y2 (XY= AlS, BTe, GaS): Valley polarization and quantum anomalous Hall state

The concept of manipulating the valley degree of freedom has garnered growing attention in both fundamental scientific research and emerging applications. In this study, first-principles calculations are performed on the electronic properties and structural stability of single-layer (SL) VP2X2Y2 (XY = AlS, GaS and BTe). All three materials have been identified as ferromagnetic semiconductors, with their conduction band minimum situated at the K point. VP2Al2S2 and VP2Ga2S2, in particular, possess easy out-of-plane magnetization, thus facilitating a pronounced valley polarization observed in the bottom conduction band. Applying compressive strain can further modulate the orbital character of the bands as well as the valley polarization, leading to a transition from the bottom conduction band to the top valence band. Intriguingly, the strained SL VP2Al2S2 and VP2Ga2S2 exhibit a quantum anomalous Hall state with a nonzero Chern number of C=1, which is further evidenced by a quantized Hall conductance and a chiral gapless edge state connecting the valence and conduction bands. Thus, the unique electronic and topological properties of SL VP2X2Y2 suggest potential applications in the fields of electronics, spintronics, and valleytronics.

Computational predictions of silver nitrate AgNO3 using HSE03: An ab initio investigation

A [Formula: see text] and 765[Formula: see text]eV (sufficient to converge the results) have been used to explore the electronic, optical, mechanical, and vibrational properties using ab initio code CASTEP with hybrid functionals. A wide indirect F–Z bandgap energy of 3.77[Formula: see text]eV has been reported. Our partial density of states plot further shows the valence band is made of O 2p and Ag 4d electron orbitals, while Ag S orbitals and N O 2p states contribute to the conduction band bottom (CBM), among the upper conduction band consisting of Ag 4p, O 2[Formula: see text] and N 2p electron orbitals. In our optical data, a high absorption coefficient of [Formula: see text] cm[Formula: see text] has been found, and a relatively low 20% reflectivity simply indicates a high absorption of the material. Although our mechanical data cannot determine the material ductility/brittleness with the B/G ratio, we can report a Poisson’s ratio value of −2.04 in this work. On the other hand, the phonon dispersion and the density of the phonon state plot report may indicate the mechanical instability of the system. The negative Poisson’s ratio (NPR) we provide may hint at the possibility of using AgNO3 as a promising anode in a group I/II elements (Mg, K, Na)-ion battery.

Novel B-site substituted KCuTa3-xNbxO9 solid solution photocatalysts with modulated band structure for visible-light-driven hydrogen evolution

Perovskite-like metal oxides (PLMOs), featuring unique structural and optical properties, exhibit great potential in photocatalytic water splitting field. However, the wide bandgap and strong carrier recombination severely suppress their photocatalytic hydrogen production activity. Thus, design and development of novel PLMO photocatalyst with extended photo-response range and enhanced photo-generated charge separation/transport efficiency remains an ongoing challenge. Herein, a series of novel B-site substituted KCuTa3-xNbxO9 solid solution photocatalysts were synthesized via a simple solid-state reaction method. With an increased content of Nb, a distinct red-shifted of the optical absorption edge of KCuTa3-xNbxO9 solid solution was observed, leading to a decreased bandgap (from 2.69 to 1.91 eV), and a positive shift of the conduction band bottom (from −0.54 to −0.49 eV vs RHE). All of the Nb-substituted KCuTa3O9 solid solutions exhibit enhanced separation efficiency of photoinduced charge carriers, which leads to increased hydrogen evolution activity, among which KCuTa0.75Nb2.25O9 exhibits the highest hydrogen evolution rate of 2.16 μmol h−1 under the visible light irradiation (λ > 420 nm), which is approximately 7-fold higher than that of the pure KCuTa3O9. This study demonstrates the potential of modulating band structure through constructing solid solutions for efficient perovskite-like metal oxides photocatalysis.

First-principles calculations to investigate structural, electronic and optical properties of ternary copper halides AlCumXn (A = K, Rb, Cs; X = Cl, Br, I)

Low-dimensional ternary metal halides have recently attracted much attention due to their excellent photoelectric performance is of significant interests. Considering the remarkable photoelectric properties of some copper - based halides, herein, we investigate the structural, electronic and optical properties of 3D, 1D and 0D structured ternary copper halides, including ACuX3, ACu2X3, A2CuX3, A4CuX6, and A3Cu2X5 using the first-principles. Their binding energies (Eb) and formation energies (Ef) were calculated, which confirmed the thermodynamic stability of these compounds. Most of these halides are direct band gap semiconductors. The valence band tops of these five type materials are mainly provided by Cu-d orbitals and halide X-p orbitals, while the conduction band bottoms are mainly provided by Cu-s and X-p orbitals. In addition,these ternary copper halides show significant light absorption characteristics, especially in the visible light range. Our results show AlCumXn (A = K, Rb, Cs; X = Cl, Br, I) would have wide photoelectric application potential.

Effect of Y-doping on switching mechanisms and impedance spectroscopy of HfO x -based RRAM devices

Y-doping can effectively improve the performance of HfO x -based resistive random-access memory (RRAM) devices, but the underlying physical mechanism of Y-doping affecting the performance of HfO x -based memristors is still missing and unclear. Although impedance spectroscopy (IS) has been widely used to investigate impedance characteristics and switching mechanisms of RRAM devices, there is less IS analysis on Y-doped HfO x -based RRAM devices as well as devices at different temperatures. Here, the effect of Y-doping on the switching mechanism of HfO x -based RRAM devices with a Ti/HfO x /Pt structure were reported using current–voltage characteristics and IS. The results indicated that doping Y into HfO x films could decrease the forming/operate voltage and improve the RS uniform. Both doped and undoped HfO x -based RRAM devices obeyed the oxygen vacancies (V O) conductive filament model along the grain boundary (GB). Additionally, the GB resistive activation energy of the Y-doped device was inferior to that of the undoped device. It exhibited a shift of the V O trap level towards the conduction band bottom after Y-doping in the HfO x film, which was the main reason for the improved RS performance.

Multiple-site Ag doping in Bi[formula omitted]Se[formula omitted]: Compositional crossover from substitution to intercalation as revealed by photoelectron diffraction and X-ray fluorescence holography

The local structure around Ag atoms in Ag-doped Bi2Se3 (AgxBi2−ySe3) was investigated by photoelectron diffraction and X-ray fluorescence holography to understand the manner of Ag atom doping. At a low Ag concentration (x=0.05), photoelectron diffraction indicated that Ag atoms occupied Bi substitution sites. However, a mere accumulation of holes via Ag substitution for Bi fails to explain the observed variation previously reported in the transport properties of Ag-doped Bi2Se3 with different x values. In particular, simple Ag substitution for Bi fails to explain the pinning of the Fermi level at the bottom of conduction band, as suggested by the observed transport properties. In the case of a high Ag concentration (x=0.2), photoelectron diffraction suggested that the Ag atoms occupied not only the substitutional Bi site but also multiple interstitial sites, namely, the octahedral site in the van der Waals interlayer and the interstitial site in the Se layer. Ag 3d photoelectron spectra revealed that the Ag atoms had the same oxidation state, +1, regardless of the type of occupied site. Furthermore, X-ray fluorescence holography was employed for a model-free local structural analysis that refined the exact locations of Ag atoms in the Bi2Se3 crystal lattice. The behavioral crossover documented herein from hole doping at the substitutional site to electron doping at multiple sites reasonably explains the dependence of electronic structures and transport properties of Ag-doped Bi2Se3 on dopant concentration.

Spectromicroscopy Studies of Silicon Nanowires Array Covered by Tin Oxide Layers.

The composition and atomic and electronic structure of a silicon nanowire (SiNW) array coated with tin oxide are studied at the spectromicroscopic level. SiNWs are covered from top to down with a wide bandgap tin oxide layer using a metal-organic chemical vapor deposition technique. Results obtained via scanning electron microscopy and X-ray diffraction showed that tin-oxide nanocrystals, 20nm in size, form a continuous and highly developed surface with a complex phase composition responsible for the observed electronic structure transformation. The "one spot" combination, containing a chemically sensitive morphology and spectroscopic data, is examined via photoemission electron microscopy in the X-ray absorption near-edge structure spectroscopy (XANES) mode. The observed spectromicroscopy results showed that the entire SiNW surface is covered with a tin(IV) oxide layer and traces of tin(II) oxide and metallic tin phases. The deviation from stoichiometric SnO2 leads to the formation of the density of states sub-band in the atop tin oxide layer bandgap close to the bottom of the SnO2 conduction band. These observations open up the possibility of the precise surface electronic structures estimation using photo-electron microscopy in XANES mode.

Semiconductor–metal transition in Bi2Se3 caused by impurity doping

Doping a typical topological insulator, Bi2Se3, with Ag impurity causes a semiconductor–metal (S-M) transition at 35 K. To deepen the understanding of this phenomenon, structural and transport properties of Ag-doped Bi2Se3 were studied. Single-crystal X-ray diffraction (SC-XRD) showed no structural transitions but slight shrinkage of the lattice, indicating no structural origin of the transition. To better understand electronic properties of Ag-doped Bi2Se3, extended analyses of Hall effect and electric-field effect were carried out. Hall effect measurements revealed that the reduction of resistance was accompanied by increases in not only carrier density but carrier mobility. The field-effect mobility is different for positive and negative gate voltages, indicating that the EF is located at around the bottom of the bulk conduction band (BCB) and that the carrier mobility in the bulk is larger than that at the bottom surface at all temperatures. The pinning of the EF at the BCB is found to be a key issue to induce the S-M transition, because the transition can be caused by depinning of the EF or the crossover between the bulk and the top surface transport.

Bandgap engineering strategy through chemical strain and oxygen vacancies in super-tetragonal BiFeO3 epitaxial films

Tetragonality increase and bandgap reduction in super-tetragonal BFO-based films are simultaneously obtained through chemical strain and oxygen vacancies, due to the reduced the bottom of conduction band confirmed by DFT calculations.

Improved photocatalytic performance of Nb4TeO12 particles by the surface depositing α-Bi2O3

Mixed-metal oxides composed of Nb5+(d0) and Te4+(p-) cations generally exhibit excellent optoelectronic properties due to the stereoactive lone pair of electrons in the structure. In this work, a mixed-metal niobate tellurite Nb4TeO12 is prepared using solid state reaction method. The p-n junctions α-Bi2O3/Nb4TeO12 are prepared via soaking the as-prepared particles and post annealing treatments. Nb4TeO12 has indirect electronic transition characteristic with a band energy of 3.01 eV. The bottom of conduction band is − 0.55 eV and the top of valance band is + 2.46 eV. Surface deposits of α-Bi2O3 induce the recrystallization on the particle surface. The microstructure, electrical impedance, and photocurrent properties are analyzed. The p-n junction can significantly delay the recombination between photo-created h+/e− pairs. As a consequence, significant improvement of photocatalysis in p-n junction α-Bi2O3/Nb4TeO12 was realized. The rate constants of as-prepared Nb4TeO12 and p-n junction α-Bi2O3/Nb4TeO12 (30/100) are 4.21 × 10−3 min−3 and 1.82 × 10−2 min−1, respectively. The experimental results show that deposits of α-Bi2O3 on the surface is effective to improve the surface light absorption and photochemical properties of the Nb5+−Te4+ oxide semiconductors.

Broad intrinsic luminescence properties in Nb4TeO12 mixed-metal niobate tellurite

Mixed-metal oxides composed of Nb5+(d0) and Te4+(p-) cations generally exhibit excellent optoelectronic properties due to the stereoactive lone pair of electrons in the structure. In this work, a mixed-metal niobate tellurite Nb4TeO12 was prepared using solid state reaction method. The polycrystalline samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), optical absorption, photoluminescence (PL) and dynamic properties. This sample shows characteristic indirect electronic transitions with an interband energy of 3.01 eV. The bottom of conduction band and the top of valence band are +0.37 eV and +3.38 eV, respectively. Using a Nd:YAG pulsed laser as the excitation light source, the intrinsic luminescence of Nb4TeO12 was investigated over a wide temperature range of T = 10–300 K. The broad band emission peaking at 513 nm corresponds to the self-activating luminescence of the niobate octahedral group [NbO6]7−. The luminescence decay curve did not depend on the laser excitation energy, and it showed a non-exponential characteristic with two decay lifetime components. The experiments also preliminarily studied the performance of the luminescence application of Nb4TeO12.

First principles investigation of electronic structure, elasticity, thermodynamic properties and lattice thermal conductivity of Mg2V2O7

In this paper, the electronic structure, elasticity, thermodynamic property and lattice thermal conductivity of Mg2V2O7 were investigated via theoretical simulation. The electronic structure shows that Mg2V2O7 is a direct semiconductor with electronic band gap value of 2.94 eV. The top of valence band is mainly dominant by O-p, while the bottom of conduction band is mostly occupied by V-d and O-p. The obtained elastic tensor and phonon dispersion suggest that the phase of Mg2V2O7 is dynamically stable. Calculated mechanical parameter (bulk modulus and Young‘s modulus) and thermodynamical properties demonstrate that Mg2V2O7 might be a potential candidate as oxide-based electronic device under high temperature and pressure. We hope that the research will be contributed to the application of Mg2V2O7.