Valence Band Maximum Research Articles

Role of sintering temperature in modulating the charge transport of BiCuSeO thermoelectric system: correlations to the microstructure

The influence of sintering temperature on the thermoelectric (TE) transport of BiCuSeO is reported in the present work, with an aim to optimize the processing conditions for higher TE transport. BiCuSeO samples were synthesized at four different sintering temperatures, viz., 673 K, 773 K, 873 K, and 973 K. A non-degenerate type of conductivity is observed in all the samples at high temperatures, witnessing the thermal activation of the carriers. The Fermi level was positioned below the valence band maximum, thereby exhibiting a p-type degenerate transport in the entire range of temperature. It was observed that the variations of weighted mobility and power factor were found to have identical trends. The highest power factor was noticed at 554 K with a value of 129 μWm−1 K−2 for the sample sintered at 673 K.

First principles study on polarization and piezoelectric properties of group substitution regulated lead-free organic perovskite ferroelectrics

Organic ferroelectrics are desirable for the applications in the field of wearable electronics due to their eco-friendly process-ability, mechanical flexibility, low processing temperatures, and lightweight. In this work, we use five organic groups as substitution for organic cation and study the effects of organic cations on the structural stability, electronic structure, mechanical properties and spontaneous polarization of metal-free perovskite <i>A</i>-NH<sub>4</sub>-(PF<sub>6</sub>)<sub>3</sub> (<i>A</i> = MDABCO, CNDABCO, ODABCO, NODABCO, SHDABCO) through first-principles calculations. Firstly, the stabilities of the five materials are calculated by molecular dynamics simulations, and the energy values of all systems are negative and stable after 500 fs, which demonstrates the stabilities of the five materials at 300 K. The electronic structure calculation shows that the organic perovskite materials have wide band gap with a value of about 7.05 eV. The valence band maximum (VBM) and Cconduction band minimum (CBM) are occupied by different elements, which is conductive to the separation of electrons and holes. We find that organic cations have an important contribution to the spontaneous polarization of materials, with a contribution rate over 50%. The presence of hydrogen atoms in the substituting groups (MDABCO, ODABCO) enhances the hydrogen bond interaction between the organic cations and <inline-formula><tex-math id="Z-20240616143151">\begin{document}${\rm PF}_6^- $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20240385_Z-20240616143151.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20240385_Z-20240616143151.png"/></alternatives></inline-formula> and increases the displacement of the organic cation, resulting in an increase in the contribution of the polarization of the organic cation to the total polarization. In addition, we observe large piezoelectric strain components, the calculated value of <i>d</i><sub>33</sub> is 36.5 pC/N for CNDABCO-NH<sub>4</sub>-(PF<sub>6</sub>)<sub>3</sub>, 32.3 pC/N for SHNDABCO-NH<sub>4</sub>-(PF<sub>6</sub>)<sub>3</sub>, which is larger than the known value of <i>d</i><sub>33</sub> of MDABCO-NH<sub>4</sub>-I<sub>3</sub>(14pC/N). The calculated value of <i>d</i><sub>14</sub> is 57.5 pC/N for ODABCO-NH<sub>4</sub>-(PF<sub>6</sub>)<sub>3</sub>, 27.5 pC/N for NODABCO-NH<sub>4</sub>-(PF<sub>6</sub>)<sub>3</sub>. These components are at a high level among known organic perovskite materials and comparable to many known inorganic crystals. The large value of <i>d</i><sub>14</sub> is found to be closely related to the large value of elastic compliance tensor <i>s</i><sub>44</sub>. The analysis of Young’s modulus and bulk’s modulus shows that these organic perovskite materials have good ductility. These results indicate that these organic materials are excellent candidates for future environmentally friendly piezoelectric materials.

Band alignment of one-dimensional transition-metal dichalcogenide heterotubes.

One-dimensional (1D) van der Waals (vdW) heterotubes, where different kinds of 1D nanotubes coaxially nest inside each other, offer a flexible platform for promising applications. The various properties of these 1D heterotubes depend on their diameter. Here, we present a systematic theoretical investigation into the structural and electronic properties of two kinds of 1D transition-metal dichalcogenide (TMD) heterotubes. We demonstrate that the thermodynamic stability of 1D heterotubes is determined by their interlayer distance. Additionally, we establish that the band alignment transition changes from type I to type II in 1D TMD heterotubes. We identify two distinct transition mechanisms, originating from the exchange of either the valence band maximum or the conduction band minimum. According to an electrostatic model, the band alignment transition is attributed to the interlayer electric field effect, which depends on the heterotube diameter. The findings in this work provide valuable physical insights into the band alignment transition in 1D heterotubes and are instrumental for their potential applications in nanotechnology.

Visible light-driven molecular oxygen activation for oxidative amidation of alcohols using lead-free metal halide perovskite.

Herein, we report the modulation of the band structures of halide perovskite Cs2CuBr4 by tuning the synthesis methods. The photocatalyst PC-1, synthesized by the hot injection method, has a more negative conduction band minima (CBM) than the photocatalyst PC-2, synthesized at room temperature. As a result, PC-1 can activate molecular O2 more efficiently to initiate the radical-mediated dehydrogenation of alcohols. The more positive valence band maxima (VBM) of PC-1 also facilitates amine oxidation to the corresponding radical. Further, improved charge separation and transport and a decrement in the photogenerated charge carrier recombination have been detected for PC-1 to enhance photocatalytic activity. PC-1 showed improved yields for a series of structurally diverse amides (highest yield = 98%) by oxidative amidation of alcohols under visible light irradiation.

Highly efficient hydrogen production and selective CO2 reduction by the C3N5 photocatalyst using only visible light.

The production of energy sources by metal-free photocatalysts based on graphitic carbon nitride (g-C3N4) has garnered substantial attention. In this study, nitrogen-rich carbon nitride (C3N5) was successfully synthesized through the thermal polycondensation of 3-amino-1,2,4-triazole. The structural and physical characterization has suggested that a portion of the triazine rings, which constitute the structural framework of g-C3N4, may be substituted with five-membered rings in C3N5. Furthermore, the polymerization of C3N5 proceeded more extensively than that of g-C3N4 from melamine precursors. The increased nitrogen content in C3N5 resulted in a heightened number of π-electrons and a narrowed energy bandgap, with the potential of the valence band maximum being negatively shifted. Additionally, photocatalytic assessments encompassing nitro blue tetrazolium reduction, H2 production from triethanolamine aqueous solution, and CO2 reduction in the liquid phase were performed. All findings demonstrated that C3N5 exhibits significantly superior photocatalytic properties compared to g-C3N4. It is particularly noteworthy that C3N5 selectively generates methanol and H2 from oversaturated CO2 solutions under visible light irradiation, while g-C3N4 selectively generates formaldehyde. These outcomes strongly indicate that C3N5 serves as a metal-free, visible-light-responsive photocatalyst, capable of contributing to both the production of renewable energy sources and the reduction of greenhouse effect gases.

Electronic band structure change with structural transition of buckled Au2X monolayers induced by strain.

This study investigates the strain-induced structural transitions of η ↔ θ and the changes in electronic band structures of Au2X (X = S, Se, Te, Si, Ge) and Au4SSe. We focus on Au2S monolayers, which can form multiple meta-stable monolayers theoretically, including η-Au2S, a buckled penta-monolayer composed of a square Au lattice and S adatoms. The θ-Au2S is regarded as a distorted structure of η-Au2S. Based on density functional theory (DFT) calculations using a generalized gradient approximation, the conduction and the valence bands of θ-Au2S intersect at the Γ point, leading to linear dispersion, whereas η-Au2S has a band gap of 1.02 eV. The conduction band minimum depends on the specific Au-Au bond distance, while the valence band maximum depends on both Au-S and Au-Au interactions. The band gap undergoes significant changes during the η ↔ θ phase transition of Au2S induced by applying tensile or compressive in-plane biaxial strain to the lattice. Moreover, substituting S atoms with other elements alters the electronic band structures, resulting in a variety of physical properties without disrupting the fundamental Au lattice network. Therefore, the family of Au2X monolayers holds potential as materials for atomic scale network devices.

Internal strain-driven bond manipulation and band engineering in Bi2-x Sb x YO4Cl photocatalysts with triple fluorite layers.

In extended solid-state materials, the manipulation of chemical bonds through redox reactions often leads to the emergence of interesting properties, such as unconventional superconductivity, which can be achieved by adjusting the Fermi level through, e.g., intercalation and pressure. Here, we demonstrate that the internal 'biaxial strain' in tri-layered fluorite oxychloride photocatalysts can regulate bond formation and cleavage without redox processes. We achieve this by synthesizing the isovalent solid solution Bi2-x Sb x YO4Cl, which undergoes a structural phase transition from the ideal Bi2YO4Cl structure to the Sb2YO4Cl structure with (Bi,Sb)4O8 rings. Initially, substitution of smaller Sb induces expected lattice contraction, but further substitution beyond x > 0.6 triggers an unusual lattice expansion before the phase transition at x = 1.5. Detailed analysis reveals structural instability at high x values, characterized by Sb-O underbonding, which is attributed to tensile strain exerted from the inner Y sublayer to the outer (Bi,Sb)O sublayer within the triple fluorite block - a concept well-recognized in thin film studies. This concept also explains the formation of zigzag Bi-O chains in Bi2MO4Cl (M = Bi, La). The Sb substitution in Bi2-x Sb x YO4Cl elevates the valence band maximum, resulting in a minimized bandgap of 2.1 eV around x = 0.6, which is significantly smaller than those typically observed in oxychlorides, allowing the absorption of a wider range of light wavelengths. Given the predominance of materials with a double fluorite layer in previous studies, our findings highlight the potential of compounds endowed with triple or thicker fluorite layers as a novel platform for band engineering that utilizes biaxial strain from the inner layer(s) to finely control their electronic structures.

Compositional effects on structural, electronic, elastic, piezoelectric and dielectric properties of GaInN alloys: a first-principles study.

We conduct a comprehensive theoretical analysis of wurtzite GaxIn1-xN ternary alloys, focusing on their structural, electronic, elastic, piezoelectric, and dielectric properties through rigorous first-principles calculations. Our investigation systematically explores the influence of varying Ga composition (x = 0%, 25%, 50%, 75%, 100%) on the alloy properties. Remarkably, we observe a distinctive non-linear correlation between the band gap and Ga concentration, attributable to unique slopes in the absolute positions of the valence band maximum and conduction band minimum with respect to Ga concentration. Our effective band structure analysis reveals the meticulous preservation of Bloch characters near band extrema, minimizing charge carrier scattering. Furthermore, we scrutinize deviations from linear Vegard-like dependence in elastic, piezoelectric, and dielectric constants. Additionally, our calculations encompass various optical properties, including absorption coefficient, reflectivity, refractive index, energy loss function, and extinction coefficient. We analyze their trends with photon energy, providing valuable insights into the optical behavior of GaxIn1-xN alloys. Our results, in excellent agreement with available experimental data, significantly contribute to a deeper understanding of the alloys' electronic properties. This study offers valuable insights that may illuminate potential applications of GaxIn1-xN alloys in diverse technological fields.

Deciphering the bridge oxygen vacancy-induced cascading charge effect for electrochemical ammonia synthesis.

Oxygen vacancy engineering has recently been gaining much interest as the charging effect it induces in a material can be used for varied applications. Usually, semiconductor materials act poorly in electrocatalysis, particularly in the nitrogen reduction reaction (NRR), owing to their inherent charge deficit and huge band gap. Vacancy introduction can be a viable material engineering route to make use of these materials for the NRR. However, a detailed investigation of the vacancy-type and its role for the structural reorientation and charge redistribution of a material is lagging in the field of NRRs. This work thus focuses on the synthesis of oxygen vacancy-engineered SnO2 with a gradual structural transformation from in-plane (iov) to bridge-type oxygen vacancy (bov) density. Consequently, the electron occupancy of the sp3d hybrid orbital changes, leading to an upshifted valence band maxima towards the Fermi level. This has a profound effect on the nature of N2 adsorption and the extent of NN bond polarization. Sn atoms adjacent to the bov are found to have a fair density of dangling charges that accomplish the NRR process at a comparatively low overpotential and determine the binding strength of the intermediates on the active site. The obscured yet stable reaction intermediates are thereby identified with in situ ATR-IR studies. A restricted hydrogen evolution reaction Faradaic on the Sn-site (favored over O-atoms) results in a Faradaic efficiency of 48.5%, which is better than that reported in all the literature reports on SnO2 for the NRR. This study thus unveils sufficient insights into the role of oxygen vacancies in a crystal as well as electronic structural alteration of SnO2 and the effect of active sites on the rate kinetics of the NRR.

Analysis of effect of bulk vacancy defect on secondary electron emission characteristics of Al<sub>2</sub>O<sub>3</sub>

Based on the combination of the first-principles and Monte Carlo method, the effect of vacancy defect on secondary electron characteristic of Al<sub>2</sub>O<sub>3</sub> is studied in this work. The density functional theory (DFT) calculation results show that the band structure changes when the vacancy defects exist. The existence of Al vacancy defects results in a decrease in band gap from 5.88 to 5.28 eV, and in Fermi level below the energy of the valence band maximum as well. Besides, the elastic mean free paths and inelastic mean free paths of electrons in different crystal structures are also obtained. The comparison shows that the inelastic mean free path of electrons in Al<sub>2</sub>O<sub>3</sub> with O vacancy defects is much larger than those of Al<sub>2</sub>O<sub>3</sub> without defects and Al<sub>2</sub>O<sub>3</sub> with Al vacancy defects. When the energy of electrons is smaller than 50 eV, the inelastic mean free path of electrons in Al<sub>2</sub>O<sub>3</sub> without defects is longer than that in Al<sub>2</sub>O<sub>3</sub> with Al vacancy defects. The elastic mean free path of electrons slightly increases when the vacancy defects exist, and the elastic mean free path of electrons in Al<sub>2</sub>O<sub>3</sub> with Al vacancy defects is the largest. In order to investigate the secondary electron emission characteristics under different vacancy defect ratios, an optimized Monte Carlo algorithm is proposed. When the ratio between O vacancy defect and Al vacancy defect increases, the simulation results show that the maximum value of secondary electron yield decreases with the ratio of vacancy defect increasing. The existence of O vacancy defects increases the probability of inelastic scattering of electrons, so electrons are difficult to emit from the surface. As a result, comparing with Al vacancy defect, the SEY of Al<sub>2</sub>O<sub>3</sub> decreases greatly under the same ratio of O vacancy defect.

Polytelluride square planar chain-induced anharmonicity results in ultralow thermal conductivity and high thermoelectric efficiency in Al2Te5 monolayers.

Two-dimensional (2D) metal chalcogenides provide rich ground for the development of nanoscale thermoelectrics, although achieving optimal thermoelectric efficiency is still a challenge. Here, we leverage the unique chemistry of tellurium (Te), renowned for its hypervalent bonding and catenation abilities, to tackle this challenge as manifested in Al2Te3 and Al2Te5 monolayers. While the former forms a straightforward covalent Al-Te network, the latter adopts a more intricate bonding mechanism, enabled by eccentric features of Te chemistry, to maintain charge balance. In Al2Te5, a square planar chain (SPC) known as polytelluride [Te3]2- is neutralized by the covalently bonded [Al2Te2]2+ framework. The hypervalent nature of Te results in bizarre Born effective charges of 7 and -4 for adjacent Te atoms within the square planar chain, a feature that induces significant anharmonicity in Al2Te5 monolayers. Enhanced anharmonic lattice vibrations and the accordion pattern bestow glass-like, strongly anisotropic thermal conductivity to the Al2Te5 monolayer. The calculated κL values of 1.8 and 0.5 W m-1 K-1 along the a- and b-axes at 600 K are one order of magnitude lower than those of Al2Te3, and even lower than monolayers that contain heavy cations like Bi2Te3. Moreover, the tellurium chain dominates the valence band maximum and conduction band minimum of Al2Te5, leading to a high valley degeneracy of 10, and thus a high power factor and figure of merit (zT). Using rigorous first-principles calculations of electron relaxation time, the estimated hole-doped and electron-doped zT of, respectively, 1.5 and 0.5 at 600 K is achieved for Al2Te5. The pioneering zT of Al2Te5 compared to that of Al2Te3 is rooted merely in its amorphous-like lattice thermal transport and its polytelluride chain. These findings underscore the importance of aluminum telluride and polymeric-based inorganic compounds as practical and cost-effective thermoelectric materials, pending further experimental validation.

Intrinsic ultralow lattice thermal conductivity in lead-free halide perovskites Cs3Bi2X9 (X = Br, I).

Lead-free halide perovskites have recently garnered significant attention due to their rich structural diversity and exceptionally ultralow lattice thermal conductivity (κL). Here, we employ first-principles calculations in conjunction with self-consistent phonon theory and Boltzmann transport equations to investigate the crystal structure, electronic structure, mechanical properties, and κLs of two typical vacancy-ordered halide perovskites, denoted with the general formula Cs3Bi2X9 (X = Br, I). Ultralow κLs of 0.401 and 0.262 W mK-1 at 300 K are predicted for Cs3Bi2Br9 and Cs3Bi2I9, respectively. Our findings reveal that the ultralow κLs are mainly associated with the Cs rattling-like motion, vibrations of halide polyhedral frameworks, and strong scattering in the acoustic and low-frequency optical phonon branches. The structural analysis indicates that these phonon dynamic properties are closely relevant to the bonding hierarchy. The presence of the extended Bi-X antibonding states at the valence band maximum contributes to the soft elastic lattice and low phonon group velocities. Compared to Cs3Bi2Br9, the face-sharing feature and weaker bond strength in Cs3Bi2I9 lead to a softer elasticity modulus and stronger anharmonicity. Additionally, we demonstrate the presence of wave-like κC in Cs3Bi2X9 by evaluating the coherent contribution. Our work provides the physical microscopic mechanisms of the wave-like κC in two typical lead-free halide perovskites, which are beneficial to designing intrinsic materials with the feature of ultralow κL.

Rashba and Dresselhaus effects in doped methylammonium lead halide perovskite MAPbI3.

Inorganic-organic lead halide perovskites, particularly methylammonium lead halide (MAPbI3) perovskite, have been regarded as promising materials for optoelectronics and spintronics. However, the practical applications of these perovskites are limited by lead toxicity and instability under air and pressure. This study investigates the substitution of Pb with Sn and Ge in cubic MAPbI3 perovskite. The properties of the resulting hybrid perovskites are compared using state-of-the-art first-principles-based methodologies, viz., density functional theory (DFT) with generalized gradient approximation (PBE) and hybrid functional (HSE06), in conjunction with spin-orbit coupling (SOC). Here, we mainly study the Rashba-Dresselhaus (RD) effect, which arises due to two major mechanisms: (i) the breaking of inversion symmetry (static and dynamic) and (ii) SOC, originating from the presence of heavy elements. We find significant spin-splitting effects in the conduction band minimum and valence band maximum for hybrid perovskites. To gain a deeper understanding of the observed spin-splitting, the spin textures are analyzed, and Rashba coefficients are calculated. We find that the Dresselhaus effect comes into play in substituted hybrid structures in addition to the usual Rashba effect observed in the pristine compound. Additionally, we observe that the strength of Rashba spin-splitting is substantially tuned by the application of uniaxial strain (±5%). Moreover, certain hybrid perovskites exhibit mechanical stability and ductility, making them potential candidates in perovskite-based optoelectronics and spintronics applications.

Enhancing the energetic and magnetic stability of atomic hydrogen chemisorbed on graphene using (non)compensated B-N pairs.

In this pioneering study for identifying atomic scale magnetic moment, a single hydrogen atom chemisorbed on pristine graphene exhibits distinct spin polarization. Using first-principles calculations and analyses, we demonstrate that the binding between a H adsorbate and a C substrate is substantially enhanced via compensated B-N pairs embedded into graphene. Surprisingly, the interaction can be further enhanced via non-compensated B-N pair doping. Our established prototype of orbital intercoupling between H 1s and hybridized pz of gapped band edges gives an insight into the enhancing mechanism. For compensated B-N doping, the conduction band minimum (CBM) is pushed upward, which induces stronger interaction between the H 1s and hybridized pz orbitals of the CBM. For non-compensated B-N doping, the orbital interaction occurs between H 1s and hybridized pz orbitals of valence band maximum, thus further lowering the resulting bonding energy due to the enlarged gap. This significantly enhanced interaction between H and C atoms agrees well with the results of charge localization at the gapped band edges. More importantly, the corresponding magnetic moments can be well maintained or even enhanced in both doping; here, one more H atom is needed for non-compensated doping, where its electron occupies the empty CBM. Our findings might provide an effective and practical way to enhance the energetic and magnetic stability of atomic scale magnetic moment on graphene and extensively expand the conception of non-compensated doping.

Intermediate bands and p-orbital tuning of the band structure of BiFeO3 for photovoltaic application.

BiFeO3 is a lead-free inorganic ferroelectric material that holds promise for photovoltaic applications. However, its larger band gap poses a challenge for efficient sunlight absorption, limiting its efficiency in such applications. In this work, nitrogen, which has lower electronegativity compared to oxygen, was selected to tune the band gap of BiFeO3. It was discovered that the band gap of BiFeO3-xNx (x = 0.125, 0.25, 0.375, and 0.5) was reduced from the original value of 3.1 eV for the parent phase to 1.45-1.77 eV, which originated from the intermediate bands caused by the stronger interaction between Fe 3d and N 2p and the valence band maximum shifted towards higher energy levels caused by the introduction of N 2p. In particular, the band gaps of 1.45 and 1.49 eV for BiFeO2.75N0.25 and BiFeO2.625N0.375, respectively, were closer to the optimal band gap of approximately 1.4 eV for photovoltaic materials. Accompanied with the reduction in the band gap, the optical absorption coefficient of N-substituted BiFeO3 in the visible range was evidently increased. Noticeably, N-substituted BiFeO3 showed good ferroelectricity with a decreased band gap and increased optical absorption. The presented results thus demonstrate that N substitution is a promising way to improve the ferroelectric photovoltaic properties of BiFeO3.

Unusual defect properties of the one-dimensional photovoltaic semiconductor selenium.

Defect tolerance is crucial in photovoltaic absorbers. Here we report that trigonal selenium (t-Se) exhibits a perovskite-like antibonding valence band maximum arising from Se p-p coupling. This results in the shallow nature of dominant Se vacancy defects. We further reveal the unique defect self-healing characteristic of Se intrinsic point defects.

Enhanced electron transport and optical properties of experimentally synthesized monolayer Si9C15: a comprehensive DFT study for nanoelectronics and photocatalytic applications.

Two-dimensional silicon-carbide (SixCy) materials stand out for their compatibility with current silicon-based technologies, offering unique advantages in nanoelectronics and photocatalysis. In this study, we employ density functional theory and nonequilibrium Green's function methods to investigate the electronic properties, electron transport characteristics, and optoelectronic qualities of experimentally synthesized monolayer Si9C15. Utilizing the modified deformation potential theory formula, we unveil Si9C15's significant directional anisotropy in electron mobility (706.42 cm2 V-1 s-1) compared to holes (432.84 cm2 V-1 s-1) in the a direction. The electrical transport calculations reveal that configurations with a 3 nm channel length demonstrate an ON state when biased, reaching a peak current of 150 nA. Moreover, this maximum current value escalates to 200 nA under tensile strain, marking an increase of approximately 100 times compared to the 5 nm channel, which remains in an OFF state. Si9C15 exhibits high light absorption coefficients (∼105 cm-1) and suitable band edge positions for water splitting at pH 0-7. Applying 1-5% tensile strain can tune the conduction band minimum and valence band maximum closer to the standard redox potentials, enhancing photocatalytic water splitting efficiency. Remarkably, under illumination at pH 0 and 7, Si9C15 can spontaneously catalyze water splitting, demonstrating its potential as a highly efficient photocatalyst. Our findings emphasize the importance of strain control and device length optimization for performance enhancement in nanoelectronics and renewable energy applications, positioning Si9C15 as a promising material for high-performance field-effect transistors and photocatalytic water splitting.

Nature of the metallic and in-gap states in Ni-doped SrTiO3

Epitaxial thin films of SrTiO3(100) doped with 6% and 12% Ni are studied with resonant angle-resolved photoelectron spectroscopy at the Ti and Ni L2,3-edges. We find that the Ni doping shifts the valence band of n-doped pristine SrTiO3 toward the Fermi level (in the direction of p-doping) and reducing the bandgap. In the Ti t2g-derived mobile electron system (MES), the Ni doping depopulates the out-of-plane dxz/yz-derived bands, transforming the MES to two-dimensional and progressively reduces the electron density embedded in the in-plane dxy-derived bands as reflected in their Fermi momentum. Furthermore, the Ti and Ni L2,3-edge resonant photoemission is used to identify the Ni 3d impurity state in the vicinity of the valence-band maximum and decipher the full spectrum of the in-gap states originating from the Ni atoms, Ti atoms, and from their hybridized orbitals. Our experimental information about the dependence of the valence bands, MES, and in-gap states in Ni-doped SrTiO3 may help the development of this material toward its device applications associated with the reduced optical bandgap.

Fabrication of N-doped TiO2 and TiN nanorods by NH3 treatment: Evolution of morphology, structures, composition and optical properties

We investigate the evolution of morphology, electronic and crystalline structures, elemental composition, and optical properties of TiO2 nanorods (NRs) treated in NH3 in the temperature range of 400–1100 °C using both experiments and theoretical calculations. The results show that N-doped TiO2 NRs are obtained in the temperature range of 400–550 °C, in which the formation of the N substitutional defects is more energetically favorable than the N interstitial defects. Our theoretical calculations reveal that the formation of N substitutional defects causes a significant bandgap narrowing, which arises from two different mechanisms: the upward shift of the valence band maximum for a low doping concentration and the downward shift of the conduction band minimum for a high doping concentration. Experimentally, a bandgap in the range of 2.22–2.41 eV can be achieved for N-doped TiO2, which enables the visible-light absorption and activates the visible-light photocatalytic activity of the TiO2 NRs. The nitridation starts from 700 °C and proceeds rapidly in the temperature range of 800–900 °C. However, complete nitridation is achieved when the temperature exceeds 1000 °C. The rutile phase is found to be more prone to nitridation than the anatase phase. During nitridation, the TiO2 NRs experience significant changes in morphology, structure, and optical properties. The latter is strongly affected by the remaining O concentration in the materials.

Hole diffusion effect on the minority trap detection and non-ideal behavior of NiO/β-Ga2O3 heterojunction

A NiO/β-Ga2O3 heterojunction was fabricated by sputtering a highly p-doped NiO layer onto β-Ga2O3. This heterojunction showed a low leakage current and a high turn-on voltage (Von) compared to a Ni/β-Ga2O3 Schottky barrier diode. The extracted Von from the NiO/β-Ga2O3 heterojunction's forward current–voltage characteristics was ∼1.64 V, which was lower than the extracted built-in potential voltage (Vbi) obtained from the capacitance–voltage curve. To explain this difference, deep level transient spectroscopy and Laplace-deep level transient spectroscopy were employed to study majority and minority traps in β-Ga2O3 films. A minority trap was detected near the surface of β-Ga2O3 under a reverse bias of −1 V but was not observed at −4 V, indicating its dependence on hole injection density. Using Silvaco TCAD, the hole diffusion length from P+-NiO to β-Ga2O3 was determined to be 0.15 μm in equilibrium, which is increased with increasing forward voltage. This finding explained why the trap level was not detected at a large reverse bias. Moreover, hole diffusion from NiO into β-Ga2O3 significantly affected the β-Ga2O3 surface band bending and impacted transport mechanisms. It was noted that the energy difference between the conduction band minimum (CBM) of β-Ga2O3 and the valence band maximum (VBM) of NiO was reduced to 1.60 eV, which closely matched the extracted Von value. This supported the dominance of direct band-to-band tunneling of electrons from the CBM of β-Ga2O3 to the VBM of NiO under forward bias voltage.