Conduction Band Minimum Research Articles

Emerging 2D materials for tunneling field effect transistors

This work focuses on understanding the electronic properties of materials to enhance the performance of Tunnel Field Effect Transistor (TFET) through Density Functional Theory (DFT) simulations. Material selection prefers a p-type material with in-plane high density of state (DOS) (and low out-of-plane effective mass, m*, where defined for many layer systems), and high valence band maxima (VBM) energy stacked with an n-type material with low conduction band minimum (CBM) energy (large electron affinity (EA)) that creates a broken or nearly broken band alignment and has low lattice mismatch. SnSe2 is well-suited for an n-type 2D material due to high EA, while WSe2, Black phosphorous (BP) and SnSe are explored for p-type materials. Bilayers consisting of monolayers of WSe2 and SnSe2 show a staggered but nearly broken band alignment (gap of 24 meV) and a high valence band DOS for WSe2. BP-SnSe2 shows a broken band alignment and benefits from a low lattice mismatch. SnSe-SnSe2 shows the highest chemical stability, an optimal performance in terms of DOS of SnSe, tunability with an external field, and high VBM that also leads to a broken band alignment.

Single-layer ZnGaInS4: Desirable bandgap and high carrier separation efficiency for optoelectronics

Two-dimensional optoelectronic materials with desirable bandgaps and high carrier separation efficiency are in urgent need. Herein, single-layer ZnGaInS4 is designed and thoroughly studied by first-principles calculations. Our calculations reveal the excellent stability of single-layer ZnGaInS4, and the small cleavage energy demonstrates the feasibility to exfoliate single-layer ZnGaInS4 from bulk counterparts. At the HSE06 level, single-layer ZnGaInS4 has a moderate bandgap of 2.05 eV, and its valence band maximum and conduction band minimum are spatially separated, thus impeding the electron-hole pair formation and enhancing the photoelectronic performance. In the visible region, single-layer ZnGaInS4 exhibits an optical absorption coefficient of ∼ 105 cm−1. Notably, single-layer ZnGaInS4 possesses high electron carrier mobility and low hole carrier mobility, further affirming the effective carrier separation. Another attractive characteristic is that single-layer ZnGaInS4 has anisotropic hole mobility. The desirable bandgap and high separation efficiency of generated carriers, together with abundant optical absorption, promise single-layer ZnGaInS4 is a hopeful candidate in atomic-thick optoelectronic devices.

La4Ga2S8O3: A Rare-Earth Gallium Oxysulfide with Disulfide Ions.

Band gap engineering using multiple anions is an established approach to novel photocatalysts that exhibit suitable band gap energies for water splitting and high photocorrosion resistance. However, few studies have been conducted on photocatalysts with polyanions, including polychalcogenide ions. Here, we present a new quaternary gallium oxysulfide with disulfide pairs (S2)2-, La4Ga2S8O3, grown out of a KI molten salt. Single-crystal X-ray diffraction analysis revealed that the oxysulfide crystallizes in the orthorhombic space group Pbcn with lattice constants of a = 18.3330(6) Å, b = 13.0590(5) Å, and c = 5.9022(3) Å. In the crystal structure, the GaS4-based zigzag chains and OLa4-based fluorite-like strips are independently arranged in two dimensions, which alternately stack via the disulfide pairs along the third direction. The oxysulfide is a direct-type semiconductor with a band gap of 2.45 eV. First-principles calculations combined with X-ray photoemission spectroscopy measurements show that S 3p states derived from the disulfide pairs dominate the valence band maximum and conduction band minimum, and these band-edge positions are suitable for the oxidation and reduction of water. Our comprehensive study based on the electronic structure suggests that the disulfide pairs make La4Ga2S8O3 a potential photocatalyst for water splitting under visible-light irradiation.

Insights into the Impact of Fe-Doped CsPbCl3 Perovskites on Stability and Luminescence: A First-Principles Investigation

The transition metal-doped CsPbX3 family is regarded as promising materials to be applied in optoelectronic devices. However, the understanding of the underlying luminescence mechanism is still limited. In our study, the mechanism of dual emission for Fe-doped CsPbCl3 is determined from the first-principles methods. Fe doping can cause lattice compression and octahedral deformation and importantly improve the stability of CsPbCl3 well. The spin-included band structure calculation shows that the Fe concentration mainly modulates the energy level position of eg↓ (Fe 3d spin-down state). The eg↓ location in the band gap and near the conduction band minimum can induce efficient energy transfer from the conduction band minimum to the eg↓ state, which leads to the electronic transition eg↓ → t2g↓ of the Fe ion further. The experimental dual emission is confirmed by the calculated band gaps between band edges and between eg↓ and t2g↓ states (Fe 3d spin-down state). According to the analysis of density of states, the eg↓ and t2g↓ states are broad in the larger octahedral deformation, which suggests the experimental broadband emission of the Fe ion. The optical absorption of CsPbCl3 is improved and blue-shifted upon Fe doping, also in agreement with experiments. Our findings provide the understanding for designing high-performance doped perovskite optoelectronic devices.

Structural and electronic properties of the Te-rich ZnOxTe1-x

The structural and electronic properties of the Te-rich ZnOxTe1-x are calculated via first-principles calculations. It is found that the lattice constant exhibits a decreasing trend and it has a negative bowing coefficient. The bandgap narrowing in the Te-rich range is by reason of the introduction of the O level in the bandgap, the downward movement of the conduction band minimum (CBM) and the upward movement of the valence band maximum (VBM). The band crossing is due to that the bottom of the Zn-4 s states in the conduction band is lowered after more O atoms are bonded with Zn atoms. In order to solve the problem of the description for the CBM due to the band crossing in the Te-rich range, the energy which the initial lowered Zn-4 s states correspond to in the modified band anticrossing (BAC) model choose the same value with that of the initial O level which the O-2 s states introduce. Additionally, the combination of the modified BAC model and the standard bowing equation is proposed to describe the bandgap energy in the Te-rich range. It is found that the model in this work can give a good description.

The origin of shifted fermi level and improved thermoelectric performance of monolayer BiCuSeO under pressures

The layered BiCuSeO has been widely investigated because of high thermoelectric (TE) performance. However, there is few reports about the TE property of two-dimensional (2D) BiCuSeO so far. In this work, it is found that the Fermi level passes through the conduction band of [Bi2O2]2+ layer and the valence band of [Cu2Se2]2- layer simultaneously. Therefore, there is no band gap appeared for monolayer BiCuSeO, which would lead to very poor TE performance. First-principles calculation shows that negative hydrostatic pressure could weaken the intra-layer bonding and enhance the inter-layer coupling. Thus, the Fermi level would gradually get out of conduction and valence bands along with the increased pressure. To improve the TE performance of monolayer BiCuSeO, negative hydrostatic pressures are applied and successfully increased its band gap values from 0 eV to 0.85 eV. At the same time, an obvious band convergence near conduction band minimum (CBM) is found under the −2.22 GPa pressure. Besides, the band convergence near the Fermi level under negative pressure will greatly improve the TE performance of n- and p-type monolayer BiCuSeO.

A family of two-dimensional semiconductors with transition metal Kagome lattice, large power factor and ultralow lattice thermal conductivity

Two-dimensional (2D) materials hold promising application potential in the future, so it is worthy to explore new 2D materials with fascinating structures and functionals. Here we predict a series of 15 stable 2D semiconductors, A2B3C4 (A = K, Rb, Cs; B = Ni, Pd, Pt; C = S, Se), using first-principles calculations. The feasibility of mechanical exfoliation from their bulk phases has been confirmed by low cleavage energies. Interestingly, a perfect Kagome lattice formed by transition metal Ni/Pd/Pt atoms have been revealed in these fifteen monolayers. All A2B3C4 monolayers exhibit indirect band features (1.82 eV to 2.76 eV). More encouragingly, and it is theoretically proven, based on constant relaxation time (CRT) approximation, that the A2B3C4 monolayers possess large n-type power factors (1.77 ∼ 2.10 × 103 μWm-1K−2), which is mainly contributed from the loop formed by the conduction band minima in different directions in energy band structure. Furthermore, ultralow lattice thermal conductivity (0.20 Wm-1K−1 to 3.04 Wm-1K−1) has been confirmed in 2D A2B3C4 at room temperature. Finally, the thermoelectric figure of merit (ZT) of A2B3C4 monolayers has been evaluated based on CRT approximation. These interesting findings make A2B3C4 monolayers promising candidates for low-dimensional electronic devices.

Enhancement of CsPbBr3 Perovskite Single-Crystal Photodetector Performance via Zr Ion Implantation

All-inorganic halide perovskite single crystals have attracted significant attention in optoelectronic fields on account of their outstanding optoelectronic properties. Photodetectors (PDs) based on CsPbBr3 single crystal exhibit high responsivity (R) and external quantum efficiency (EQE) due to the direct optical band gap, large optical absorption throughout the visible spectrum range, and long-term stability. Herein, we prepared the centimeter-size CsPbBr3 perovskite single crystal and utilized Zr ion implantation to modify the photoelectric properties of single crystals. The optical band gap of the CsPbBr3 single crystal decreases from 2.24 to 2.12 eV, and the absorption intensity increases after Zr ion implantation. The R and EQE of the CsPbBr3 single crystal photodetector modified by Zr ion implantation are 1.0 A W–1 and 294%, respectively, with a light intensity of 3.8 mW cm–2 and a 5 V bias under 425 nm illumination, which are ten times larger than that of the pristine devices. Meanwhile, the first-principles calculation results show that the introduction of Zr ions decreases the band gap and that the Zr 4d orbit contributes to the conduction band minimum (CBM), which facilitates carrier transport in Zr ion implanted CsPbBr3 single crystals. The results show that Zr ion implantation is an effective strategy to improve the performance of the CsPbBr3 single-crystal-based PDs.

Microwave-assisted synthesis of nanostructured tin sulfide as a highly efficient visible-light active photocatalyst: Characterization, applications, and theoretical studies

In this study, nanostructured tin sulfide was synthesized using a facile microwave-assisted method. The as-prepared sample was further characterized via X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS), UV–Visible Diffuse Reflectance Spectroscopy (DRS), Brunauer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH) and Mott-Schottky analyses. Based on the results, the synthesized SnS is an n-type semiconductor and has a direct band gap of 1.32 eV. Photocatalytic activity of nanostructured SnS was then evaluated through degrading methylene blue (MB) under visible light in a cylindrical batch reactor. The results show that this highly efficient photocatalyst can degrade almost up to 83.06% of 14 mg/L pollutant within 10 min, which can be attributed to its relatively high specific surface area of 52.819 m2/g. In addition, the photocatalyst performance was studied from the isothermic, thermodynamic, and kinetic points of view. The results suggest that Fritz-Schlunder along with Baudu are the most appropriate models to describe the adsorption process (the first step of photochemical reactions), as they have the lowest error functions and highest correlation coefficients. Based on kinetic studies results, there is a good agreement between the pseudo-second-order adsorption kinetic model and our experimental data. Additionally, the corresponding structural parameters of SnS were compared by size-strain plot (SSP), and Williamson-Hall methods. Furthermore, a density functional theory (DFT) approach was conducted to get more insight into the electronic and structural properties of the SnS. In terms of the band structure, 1.35 eV of band gap energy (Eg) is obtained, which perfectly matches with the results of characterization tests (1.32 eV). Finally, the partial density of states curves indicate that the valence band maximum (VBM) is mainly generated from 3pS, 5sSn, and 5pSn states while the conduction band minimum (CBM) is made of 5pSn states. Consequently, the spin-up and spin-down curves are found to be highly symmetric, indicating the non-magnetic property of tin sulfide.

Roles of concentration-dependent Cu doping behaviors on the thermoelectric properties of n-type Mg3Sb1.5Bi0.5

Large Seebeck coefficients induced by high degeneracy of conduction band minimum, and low intrinsic lattice thermal conductivity originated from large lattice vibrational anharmonicity render Mg3Sb2 as a promising n-type thermoelectric material. Herein, we demonstrated unique concentration-dependent occupation behaviors of Cu in Mg3.4Sb1.5Bi0.49Te0.01 matrix, evidenced by structural characterization and transport property measurements. It is found that Cu atoms prefer to enter the interstitial lattice sites in Mg3Sb2 host with low doping level (Mg3.4Sb1.5Bi0.49Te0.01 + x% Cu, x < 0.3%), acting as donors for providing additional electrons without deteriorating the carrier mobility. When x is larger than 0.3%, the excessive Cu atoms are inclined to substitute Mg atoms, yielding acceptors to decrease the electron concentration. As a result, the electrical conductivity of the Mg3.4Sb1.5Bi0.49Te0.01 + 0.3% Cu sample reaches 2.3 × 104 S/m at 300 K, increasing by 300% compared with that of the pristine sample. The figure of merit zT values in the whole measured temperature range are significantly improved by the synergetic improvement of power factor and reduction of thermal conductivity. An average zT ∼1.07 from 323 K to 773 K has been achieved for the Mg3.4Sb1.5Bi0.49Te0.01 + 0.3% Cu sample, which is about 30% higher than that of the Mg3.4Sb1.5Bi0.49Te0.01 sample.

Electronic Impurity Doping of a 2D Hybrid Lead Iodide Perovskite by Bi and Sn

Control over conductivity and carrier type (electrons and holes) defines semiconductors. A primary approach to target carrier concentrations involves introducing a small population of aliovalent impurity dopant atoms. In a combined synthetic and computational study, we assess impurity doping by introducing Bi and Sn into the prototype 2D Ruddlesden-Popper hybrid perovskite phenylethylammonium lead iodide (PEA2PbI4). Experimentally, we demonstrate that Bi and Sn can achieve n- and p-type doping, respectively, but the doping efficiency is low. Simulations show that Bi introduces a deep defect energy level (∼0.5 eV below the conduction band minimum) that contributes to the low doping efficiency, but, to reproduce the low doping efficiency observed experimentally, an acceptor level must also be present that limits n-type doping. Experiments find that Sn achieves p-dopant behavior and simulations suggest that this occurs through the additional oxidation of Sn defects. We also study how substitutional Bi incorporation can be controlled by tuning the electrochemical environment during synthesis. First-principles impurity doping simulations can be challenging; typical dopant concentrations constitute less than 0.01% of the atoms, necessitating large supercells, while a high level of theory is needed to capture the electronic levels. We demonstrate simulations of complex defect-containing unit cells that include up to 3383 atoms, employing spin-orbit coupled hybrid density functional theory. While p- and n-type behavior can be achieved with Sn and Bi, simulations and experiments provide concrete directions where future efforts must be focused to achieve higher doping efficiency.Received 5 December 2022Revised 28 April 2023Accepted 4 May 2023DOI:https://doi.org/10.1103/PRXEnergy.2.023010Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the National Renewable Energy Laboratory (NREL) Library, part of a national laboratory of the U.S. Department of Energy.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEnergy materialsSolar energyEnergy Science & Technology

Strain‐Tunable Photocatalytic Performance in Two‐Dimensional Janus In2S2X (X=Se,Te) Monolayer as Photocatalyst for CO2 Reduction

AbstractStrain engineering has been widely used in the study of two‐dimensional semiconductors in recent years, because it can improve catalytic performance by changing the structural characteristics of materials. In this work, we systematically investigate the effects of biaxial strain on the electronic structure, band‐edge positions, and optical absorption of perfect Janus In2S2X (X=Se, Te) particles and vacancy In2S2X (V‐In2S2X). Biaxial strain enables In2S2X to achieve a transition between indirect and direct band gaps. Tensile strain reduces the band‐edge potential of the conduction band minimum (CBM), but compressive strain increases it and then tends to stabilize the material. In V‐In2S2X, the tensile strain makes the top bands of the bandgap overlap with the CBM and moves the bottom band of the bandgap down, reducing the bandgap, which makes the light absorption range redshift. Most importantly, we demonstrate that V‐In2S2X obtains excellent photocatalytic CO2 reduction performance under 4 % tensile strain, broadening its practical applications in many fields.

Novel octa-graphene-like structures based on GaP and GaAs.

The discovery of graphene gave way to the search for new two-dimensional structures. In this regard, octa-graphene is a carbon allotrope consisting of 4- and 8-membered rings in a single planar sheet, drawing the research community's attention to study their inorganic analogs. Considering the promising properties of octa-graphene-like structures and the role of GaAs and GaP in semiconductor physics, this study aims to propose, for the first time, two novel inorganics buckled nanosheets based on the octa-graphene structure, the octa-GaAs and octa-GaP. This work investigated the structural, electronic, and vibrational properties of these novel octa-graphene-based materials. The octa-GaP and octa-GaAs have an indirect band gap transition with a valence band maximum between M and Г points and a conduction band minimum at Г point with energy of 3.05 eV and 2.56 eV, respectively. The QTAIMC analysis indicates that both structures have incipient covalent in their bonds. The vibrational analysis demonstrates the occurrence of ΓRaman = 6Ag + 6Bg and ΓRaman = 12A' + 12B″ for octa-GaP and octa-GaAs, respectively. The symmetry reduction of octa-GaAs leads to activating inactive modes observed in the octa-GaP structure. The frontier crystalline orbitals are composed by Ga(px) and P(py and pz) orbitals for octa-GaP and Ga(px and py) and As(s, py, and pz) for octa-GaAs in the valence bands while in the conduction bands by Ga(py, pz, and s) for both compounds and P(px and pz) and As(py). The phonon bands demonstrate the absence of the negative frequency modes and the structural stability of these new nanosheets. This report aims to reveal the fundamental properties of both newfound materials for stimulating experimental research groups in the search for synthesis routes to obtain this structure. This work used the DFT/B3LYP approach implemented in the CRYSTAL17 computational package. Ga, As, and P atomic centers were described by triple-zeta valence with polarization (TZVP) basis set. The vibrational analysis was carried out via coupled-perturbed Hartree-Fock/Kohn Sham (CPHF/KS) method, and the chemical bonds were evaluated via the quantum theory of atoms in molecules and crystals (QTAIMC).

DFT and experimental investigations on the photocatalytic mechanism of C3N4-MQDs/BWO heterojunction for Tetracycline degradation

BackgroundThe development of efficient antibiotic degradation catalysts has attracted much attention from researchers, but little is known about the intrinsic relationship between the geometry, electronic structure, and photocatalytic activity of the catalysts. MethodsHerein, a magnetic recoverable C3N4-MQDs/BWO heterojunction was successful synthesized by combining C3N4 magnetic quantum dots with Bi2WO6 nanosheets through a hydrothermal method. Significant FindingsThe prepared CMBW-2 (C3N4-MQDs/BWO with 2% C3N4-MQDs) was applied to degraded TC (tetracycline) solution of 20mg·L−1 under visible light irradiation, presenting 100% conversion within 120 min, and the reaction rate is 13.8 times and 2.3 times that of Bi2WO6 and g-C3N4/Bi2WO6, respectively. Its stability tests show an unchanged activity after five cycles for a long 600 min visible light irradiation. Various characterization results confirm that the good catalytic performance originates from the formation of a heterojunction to facilitate the separation of generated electron–hole pairs, while its larger specific surface area provides more active adsorption sites. DFT results show that the Bi2WO6(001)/C3N4(002) heterostructure is a staggered band arrangement structure whose VBM (valance band maximum) is mainly inhabited by atomic orbitals of Bi2WO6, while its CBM (conduction band minimum) is mainly inhabited by atomic orbitals of C3N4(002). Based on DFT and experimental results, an indirect Z-scheme is proposed, in which Fe3O4 nanoparticles are regarded as an electronic conductive bridge to realize rapid electron transfer. Moreover, the C3N4-MQDs/BWO heterojunction has high potential for practical applications in the removal of emerging contaminants in sewerage because of its excellent stability and sufficient magnetism.

Extraction Technique for the Conduction Band Minimum Energy in Amorphous Indium–Gallium–Zinc–Oxide Thin Film Transistors

The conduction band minimum energy in amorphous oxide semiconductor-based thin film transistors (AOS TFTs) is a key parameter governing the accurate extraction of energy distribution for the subgap density-of-states (DOSs) and carrier mobility. We report a technique for extraction of the gate voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {CBM}}{)}$ </tex-math></inline-formula> and corresponding energy ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F},{\text {CBM}}}$ </tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {C}-{\text {EREF}}}{)}$ </tex-math></inline-formula> for the quasi-Fermi level ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F}}{)}$ </tex-math></inline-formula> equal to the conduction band minimum ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {C}}{)}$ </tex-math></inline-formula> as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {CBM}}$ </tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {GS}}$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F}}$ </tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {C}}{)}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F},{\text {CBM}}}$ </tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F}}$ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {GS}}$ </tex-math></inline-formula> = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {CBM}}{)}$ </tex-math></inline-formula> . In order to confirm this technique through optoelectronic experimental data, amorphous indium–gallium–zinc–oxide (a-IGZO)-based thin film transistor was irradiated with various wavelengths and power, and obtained <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {CBM}}$ </tex-math></inline-formula> = 7.1 V and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {F} {\text {CBM}}}$ </tex-math></inline-formula> = 71 meV in the dark state. This technique is expected to be useful in the accurate characterization of the subgap DOS and the effective mobility in AOS TFTs through a simple and effective extraction process.

Tailoring the infrared resonances of sulfide perovskites

Perovskites are promising materials for photovoltaic and optoelectronic applications. In this study, we systematically investigated the electronic and optical characteristics of sulfide perovskites using first-principles density functional theory (DFT) and time-dependent DFT calculations. The calculated results demonstrate the sensitive dependence of the optical properties of sulfide perovskites on the crystal symmetry as well as the spin orbit coupling. Particularly, they result in changes in electronic states close to the Fermi level and photoabsorption anisotropy. As a key finding, sulfide perovskites are characterized by an intense and focused optical signature in the infrared region of the electromagnetic spectrum, crucial for infrared LEDs and optical communication. The spin-orbit coupling modifies the valence band maxima and conduction band minima and considerably boosts the infrared resonance. In addition to the fundamental interest, I anticipate that the results of this work help in the development of active optical devices based on sulfide perovskites.

Curvature-controlled band alignment transition in 1D van der Waals heterostructures

The effect of curvature on the band alignment of one-dimensional (1D) van der Waals (vdW) transition metal dichalcogenide (TMDC) heterostructures is studied by comprehensive first-principles calculations. We find that, as the diameter of a TMDC nanotube decreases, the combined effect of curvature-induced flexoelectricity and circumferential tensile strain causes a rapid lowering of the conduction band minimum, whereas the valence band maximum exhibits an initial lowering before rising. As individual TMDC nanotubes form coaxial heterostructures, the concerted effect of diameter-dependent band-edge levels and intertube coupling via flexovoltage can result in a transition of intertube band alignment from Type II to Type I in multiple heterostructural systems, including large-diameter MoSe2@WS2, MoTe2@MoSe2, and MoTe2@WS2 heterostructures. These results lay down a foundation for the rational design of 1D vdW heterostructures.

Band edges positions prediction of the of Ag nanocluster-decorated titania surfaces and their relationship to NO and NO2 interaction from first-principles calculations

Metal nanoclusters deposited on oxides have been widely used in photocatalysis playing an important role in the design of model catalysts with applications in heterogeneous catalysis. In particular, we are interested in the potential activity of these cluster-supported systems for the removal of nitrogen oxides either by possible catalytic reduction and/or by their adsorption. In this work, using first-principles methods, we evaluate the main characteristics of Agn (n = 1–4) nanoclusters isolated and deposited on anatase TiO2(101) and rutile TiO2(110) surfaces. Our results indicate that they are preferably adsorbed on rutile surface. The different formation energy at each surface can be explained using a Bader charge analysis. Particularly for Ag4 the lowest formation energy is obtained for tetrahedral geometry while the isolated Ag4 geometry is planar. Small silver deposits placed superficially on titania surfaces modify its electronic structures and improve the conduction band edges positions for possible NO reduction. Band edges positions with respect to the vacuum potential have been studied. The comparison of the conduction band minimum with the reduction potentials of NO/N2O and N2O/N2 shows that they are higher, being Ag3 on rutile and Ag1, Ag2 and Ag4P on anatase better for NO reduction. To complete the analysis, the calculation of work function, energy gap, ionization energy and electron affinity are relevant since they allow the location of semiconductor band edges at point of zero charge. Finally, the adsorption of nitrogen oxides is studied where the NO2 adsorption is favored over NO.

Electronic properties of InBixP1-x alloy in the P-rich range calculated using first-principles calculations

First-principles calculations are carried out to explore the structural and electronic properties of the P-rich InBixP1-x versus bismuth fraction. It is found that the bowing coefficient for the lattice constant is minor (b=0.042 Å) due to the structural relaxation effect of the bond lengths in InBixP1-x. In the P-rich range, the InBixP1-x alloy has a direct bandgap. The result for the bond length shows that the average bond length in the InBixP1-x alloy becomes long with increasing bismuth fraction, resulting in weakening the total p−p coupling interaction and the total s−s coupling interaction. The descending of the conduction band minimum (CBM) is due to the weakened total p−p coupling interaction while the ascending of the valence band maximum (VBM) is owing to the weakened total s−s coupling interaction. Because of the descending of the CBM and the ascending of the VBM, the bandgap energy is shrunk. It is also found that the initial bismuth level is introduced below the VBM of InP due to the large strain of the In-Bi bond. In order to represent the bandgap energy in the P-rich range, the combination of a linear equation and the fusion-repelling model is proposed. It can offer an accurate description by setting V=1.39 eV2 and γ=1.61 eV. Besides, the positive formation energy shows that the Bi incorporation in InP is energetically unfavorable. This work is helpful for understanding the structural and electronic properties of the P-rich InBixP1-x alloy.

Surface defects related to polishing cycle in ß-Ga2O3 crystals grown by floating zone

We report an investigation on ß-Ga2O3 Schottky barrier diodes performed on substrates grown by floating-zone method using structural characterization techniques (secondary-ion mass spectrometry, inductively coupled plasma-mass spectroscopy, and atomic force microscopy) and electrical measurements (current-voltage, capacitance-voltage, Hall effect, and capacitance deep-level transient spectroscopy). Four distinct electron trap levels labeled ES, E1, E2, and E3 were found in the range of 1 eV below the Ga2O3 conduction band minimum. Among them, E1, E2, and E3 show signatures similar to those reported in the literature for Czochralski and edge-defined film-fed grown ß-Ga2O3 substrates. Trap ES was found near the surface, and we could establish a link between this defect and the damage induced by the substrate polishing technique. The level related to ES was identified at ∼0.31 eV below the conduction band minimum. An energy band above 0.31 eV was also detected and is associated with states at the metal–semiconductor interface. We demonstrated that the interface states and surface deep traps are not uniformly distributed on the ß-Ga2O3 surface. Furthermore, they contribute to the reverse leakage current and the on-state conduction degradation of the diodes.