Band Edge Research Articles

Multifunctional Phototransistor Based on Double Van der Waals Heterojunction with Reversed Band Edge Bending

AbstractDouble van der Waals heterojunctions (vdWHs) based on 2D materials showcase multifunctional properties, including anti‐ambipolar behavior and polarization‐sensitive photodetection capabilities, providing a new degree of freedom for the development of next‐generation integrated electronics and optoelectronics. Herein, this work reports an anti‐ambipolar transistor with high polarized photosensitivity based on MoS2/Ta2NiS5/WSe2 double vdWH with back‐to‐back type‐I band alignment. It demonstrates a noticeable negative differential transconductance with an ultrahigh peak‐to‐valley ratio of 4.3 × 104 and a bidirectional transconductance variation from 41.6 to −17.5 nS when VD = 2 V. It is ascribed to the effective gate‐modulation of the reversed band edge bending at the double vdWH interfaces. Additionally, the structure benefits from a photogating effect and the anisotropic Ta2NiS5 interlayer, achieving a peak responsivity of 120.58 A W−1 and a decent specific detectivity of 2.65 × 1012 Jones at VD = 2 V and Vg = 5 V and exhibiting an exceptional photocurrent anisotropic ratio of 15.31 via the photovoltaic effect under 635 nm light. These findings not only expand the potential applications of the advanced 2D Ta2NiS5 material but also offer valuable insights for the development of multifunctional optoelectronic devices leveraging 2D double vdWHs.

Operando double-edge high-resolution X-ray absorption spectroscopy study of BiVO4 photoanodes.

High energy resolution fluorescence detected X-ray absorption spectroscopy is a powerful method for probing the electronic structure of functional materials. The X-ray penetration depth and photon-in/photon-out nature of the method allow operando experiments to be performed, in particular in electrochemical cells. Here, operando high-resolution X-ray absorption measurements of a BiVO4 photoanode are reported, simultaneously probing the local electronic states of both cations. Small but significant variations of the spectral lineshapes induced by the applied potential were observed and an explanation in terms of the occupation of electronic states at or near the band edges is proposed.

Plasmon-exciton interactions in ZnO/AuNPs heterostructure film for high photoconductivity

This study investigatesplasmon-exciton interactions within ZnO/AuNPs heterostructure films. Elemental, structural, and morphological analyses validate the successful formation of these films. The observed shift in the plasmonic resonance band and absorption edge otowards the red region the red region is attributed to strong surface plasmon damping influenced by exciton interactions within ZnO. Irradiation with blue light induces plasmonic electrons, generation in the ZnO/AuNPs film, enhancing electrical conductivity through interaction with ZnO excitonic states. UV light irradiating generates the electron-hole pair in the ZnO film and interacts with the free electrons in AuNPs. The ZnO/AuNPs heterostructure films hold promise for applications such as high-performance UV-photodetectors, photoactive layers in solar cells, light-emitting diodes, and energy storage devices.

Janus PtSSe-based van der Waals heterostructures for direct Z-scheme photocatalytic water splitting

The semiconductor photocatalysts based on van der Waals heterostructures (vdWHs) and direct Z-scheme mechanism are viable candidates for water splitting using sunlight. In this study, we exploit the combined effects of the inherent dipole in a Janus PtSSe monolayer and the intrinsic electric field present at the interface of Janus PtSSe-based vdWHs for water splitting. These vdWHs exhibit type-II band edge position, narrow band gap and intrinsic electric fields, establishing them as prototypical direct Z-scheme systems. Remarkably, the density functional theory-based results find the potential drop of ∼0.2–3.2 eV across the interface of vdWHs, promoting the interlayer photogenerated carrier's recombination while suppressing the intralayer charge transfer, which leads to a direct Z-scheme charge transfer pathway with robust redox capabilities. In conjunction with redox reactions, the built-in electric field at the interface generates sufficient driving force for the photogenerated carrier's separation. Also, these Janus PtSSe-based vdWHs possess exceptional abilities in capturing visible light with high solar-to-hydrogen (STH) conversion efficiency. The results obtained from this study are important to designing efficient direct Z-scheme photocatalysts for splitting water under sunlight and offer valuable insights for potential commercial implementations.

Enhanced photoluminescence in graphene wrapped hydrophobic zinc oxide

This article reports the synthesis of zinc oxide (ZnO) rod like structures over different substrates. The developed ZnO was further wrapped with graphene sheet and thus the hybrid was formed.The samples were characterized by X-ray diffraction, electron microscopy, UV–Vis and Fourier transformed infrared spectroscopy.After graphene wrapping the surface energy of ZnO reduced resulting enhanced hydrophobicity. Also, graphene inclusion enhances the photoluminescence intensity of the underlying ZnO for both band edge and defect induced transition.This enhancement in the PL intensity may be assigned to the supply of electron from the graphene to ZnO or surface plasmon resonance effect.

Sensing at the Nanoscale Using Nitrogen-Vacancy Centers in Diamond: A Model for a Quantum Pressure Sensor.

The sensing of stress under harsh environmental conditions with high resolution has critical importance for a range of applications including earth's subsurface scanning, geological CO2 storage monitoring, and mineral and resource recovery. Using a first-principles density functional theory (DFT) approach combined with the theoretical modelling of the low-energy Hamiltonian, here, we investigate a novel approach to detect unprecedented levels of pressure by taking advantage of the solid-state electronic spin of nitrogen-vacancy (NV) centers in diamond. We computationally explore the effect of strain on the defect band edges and band gaps by varying the lattice parameters of a diamond supercell hosting a single NV center. A low-energy Hamiltonian is developed that includes the effect of stress on the energy level of a ±1 spin manifold at the ground state. By quantifying the energy level shift and split, we predict pressure sensing of up to 0.3 MPa/Hz using the experimentally measured spin dephasing time. We show the superiority of the quantum sensing approach over traditional optical sensing techniques by discussing our results from DFT and theoretical modelling for the frequency shift per unit pressure. Importantly, we propose a quantum manometer that could be useful to measure earth's subsurface vibrations as well as for pressure detection and monitoring in high-temperature superconductivity studies and in material sciences. Our results open avenues for the development of a sensing technology with high sensitivity and resolution under extreme pressure limits that potentially has a wider applicability than the existing pressure sensing technologies.

Environmental-Friendly Ag2S QD-Multilayer MoSe2 van Der Waals Heterostructure for High-Performance Broadband Photodetection.

Broadband photodetectors have drawn intensive attention owing to their wide application prospects in optical communication, imaging, astronomy, and so on. Two-dimensional transition-metal dichalcogenides (TMDs) are considered as highly potential candidates for photodetection applications, benefiting from their excellent photoelectric properties. However, most of the photodetectors based on TMDs suffer from low performance in the near-infrared (NIR) region due to the weak optical absorption efficiency near their absorption band edge, which severely constrains their usage for broadband optoelectronics. Here, by taking advantage of the high absorption coefficient and environment-friendly property of Ag2S quantum dots (QDs), the hybrid of multilayer MoSe2/Ag2S QDs is demonstrated with a high-performance broadband photodetection capability (532-1270 nm). The favorable energy band alignment of MoSe2/Ag2S QDs facilitates effective separation and collection of photogenerated carriers, and the heterostructure device exhibits significant enhancement of performance compared to the bare MoSe2 device. High responsivity, detectivity, and external quantum efficiency of 25.5 A/W, 1.45 × 1011 Jones, and 1070% are obtained at a low working voltage of 1 V under 980 nm illumination. The responsivity of the device can reach up to 1.2 A/W at 1270 nm wavelength, which is competitive to the commercial NIR photodetectors. Meanwhile, broadband imaging capability is demonstrated. Our work may open up a facile and eco-friendly approach to construct high-performance broadband photodetectors for next-generation compact optoelectronic applications.

Ab-initio study of strain-tunable g-GaN/BN nanoheterostructure for optoelectronic and photocatalytic applications.

Two-dimensional (2D) nanoheterostructures of materials, integrating various phase or materials into a single nanosheet have stimulated large-scale research interest for designing novel two dimensional devices. In contemporary analysis present work, we examined the structural and electronic properties of the isolated 2D BN and GaN monolayers. We have investigated the structural stability and optoelectronic and photocatalytic response of the g-GaN/BN nanoheterostructure along with its response to strain. Nanoheterostructure g-GaN/BN is predicted to be a direct bandgap semiconductor with wide gap of 4.45eV, whose value can be effectively modulated by applied strain ( , ranging from 4.55 ( = - 4%) to 3.58eV ( = 8%). We also discovered that the tensile strain of 8% can substantially tune the direct bandgap of nanoheterostructure to indirect band gap nature. Even more important, the biaxial tensile strain engineering accentuates an enhancement of optical absorption in the UV region, broadening the light harvesting of the g-GaN/BN nanoheterostructure with the shifting of first absorption peak from 4.64 ( = - 4%) to 3.71eV ( = 8%). Furthermore, strain-tuned band edge potentials arrangement perfectly fits the water reduction and oxidation redox potentials. Our findings portend that the g-GaN/BN nanoheterostructure has application in prospective nanoscale optoelectronic devices and photocatalytic hydrogen evolution system. First principles calculations in this study are performed using density functional theory. Generalized gradient approximation within PBEsol functional employed to address the electron-electron exchange-correlation effects. For avoiding periodic interactions between the layers, we have inserted a vacuum region of thickness 10Å in the z-direction. For ensuring the convergence accuracy of the computed results, convergence criteria of the iteration process is set to be 0.0001eV. Local modified Becke-Johnson, a semi local functional, is applied for calculating electronic and optical properties for more accuracy of results. As in layered 2D nanoheterostructure, a factual depiction of the van der Waals interactions cannot be provided by conventional DFT techniques. Accordingly, in order to incorporate these interactions, we had employed the dispersion correction method of Grimme's.

First-principles study of layer-dependent band alignment and work function in MoS[formula omitted] nanoflakes

MoS2 is renowned for its intriguing electronic and optical properties, which exhibit variations dependent on the number of layers and the atomic arrangement within its layers. In this work, we employ first-principles theory calculations to systematically investigate how interlayer spacing and layer number impact the electronic properties of MoS2 nanoflakes. The primary focus of this work is to comprehend the layer-dependent behaviors of the band gap, work function, and band alignment of MoS2. Our findings reveal that the MoS2 band gap experiences a reduction as the number of layers increases, following a power-law relationship, while the work function rises with an increasing layer count. Moreover, both band gap and work function increase with the expansion of the interlayer spacing in MoS2 layers. This study suggests that the layer-dependent characteristics of band edges and work functions in few-layer MoS2 offer valuable opportunities for the manipulation of Schottky barriers, ultimately enhancing carrier injection efficiency.

Mott Gap Filling by Doping Electrons through Depositing One Sub-Monolayer Thin Film of Rb on Ca2CuO2Cl2

Understanding the doping evolution from a Mott insulator to a superconductor probably holds the key to resolve the mystery of unconventional superconductivity in copper oxides. To elucidate the evolution of the electronic state starting from the Mott insulator, we dose the surface of the parent phase Ca2CuO2Cl2 by depositing Rb atoms, which are supposed to donate electrons to the CuO2 planes underneath. We successfully achieved the Rb sub-monolayer thin films in forming the square lattice. The scanning tunneling microscopy or spectroscopy measurements on the surface show that the Fermi energy is pinned within the Mott gap but close to the edge of the charge transfer band. In addition, an in-gap state appears at the bottom of the upper Hubbard band (UHB), and the Mott gap will be significantly diminished. Combined with the Cl defect and the Rb adatom/cluster results, the electron doping is likely to increase the spectra weight of the UHB for the double occupancy. Our results provide information to understand the electron doping to the parent compound of cuprates.

Femtosecond optical studies of the primary charge separation reactions in far-red photosystem II from Synechococcus sp. PCC 7335

Primary processes of light energy conversion by Photosystem II (PSII) were studied using femtosecond broadband pump-probe absorption difference spectroscopy. Transient absorption changes of core complexes isolated from the cyanobacterium Synechococcus sp. PCC 7335 grown under far-red light (FRL-PSII) were compared with the canonical Chl a containing spinach PSII core complexes upon excitation into the red edge of the Qy band. Absorption changes of FRL-PSII were monitored at 278 K in the 400–800 nm spectral range on a timescale of 0.1–500 ps upon selective excitation at 740 nm of four chlorophyll (Chl) f molecules in the light harvesting antenna, or of one Chl d molecule at the ChlD1 position in the reaction center (RC) upon pumping at 710 nm. Numerical analysis of absorption changes and assessment of the energy levels of the presumed ion-radical states made it possible to identify PD1+ChlD1− as the predominant primary charge-separated radical pair, the formation of which upon selective excitation of Chl d has an apparent time of ∼1.6 ps. Electron transfer to the secondary acceptor pheophytin PheoD1 has an apparent time of ∼7 ps with a variety of excitation wavelengths. The energy redistribution between Chl a and Chl f in the antenna occurs within 1 ps, whereas the energy migration from Chl f to the RC occurs mostly with lifetimes of 60 and 400 ps. Potentiometric analysis suggests that in canonical PSII, PD1+ChlD1− can be partially formed from the excited (PD1ChlD1)* state.

Engineering Metal Halide Perovskite Nanocrystals with BODIPY Dyes for Photosensitization and Photocatalytic Applications.

The sensitization of surface-anchored organic dyes on semiconductor nanocrystals through energy transfer mechanisms has received increasing attention owing to their potential applications in photodynamic therapy, photocatalysis, and photon upconversion. Here, we investigate the sensitization mechanisms through visible-light excitation of two nanohybrids based on CsPbBr3 perovskite nanocrystals (NC) functionalized with borondipyrromethene (BODIPY) dyes, specifically 8-(4-carboxyphenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP) and 8-(4-carboxyphenyl)-2,6-diiodo-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (I2-BDP), named as NC@BDP and NC@I2-BDP, respectively. The ability of I2-BDP dyes to extract hot hole carriers from the perovskite nanocrystals is comprehensively investigated by combining steady-state and time-resolved fluorescence as well as femtosecond transient absorption spectroscopy with spectroelectrochemistry and quantum chemical theoretical calculations, which together provide a complete overview of the phenomena that take place in the nanohybrid. Förster resonance energy transfer (FRET) dominates (82%) the photosensitization of the singlet excited state of BDP in the NC@BDP nanohybrid with a rate constant of 3.8 ± 0.2 × 1010 s-1, while charge transfer (64%) mediated by an ultrafast charge transfer rate constant of 1.00 ± 0.08 × 1012 s-1 from hot states and hole transfer from the band edge is found to be mainly responsible for the photosensitization of the triplet excited state of I2-BDP in the NC@I2-BDP nanohybrid. These findings suggest that the NC@I2-BDP nanohybrid is a unique energy transfer photocatalyst for oxidizing α-terpinene to ascaridole through singlet oxygen formation.

Exploring the microscopic effects of surrounding matrices (HfO2 and SiO2) on CdSe/ZnS and ZnS/CdSe cylindrical core/shell quantum dot for microelectronic transport applications

The primary goal of this study is to comprehensively describe the physical impact of surrounding dielectrix oxides on the electronic and optical properties of a confined electron within CdSe/ZnS and ZnS/CdSe cylindrical core/shell quantum dot (CCSQD). The effective mass approximation and the compact density matrix approach have been employed to compute the energy eigenvalues as well as the optical absorption coefficients (OACs) and the refractive index changes (RICs). Our calculations revealed that the presence of enveloping oxides acts as an additional confinement tool altering the electron energy spectrum. The ZnS/CdSe CCSQD exhibited more pronounced response and the exploitation of the local electric factor can effectively enhance the optical nonlinearity of OACs and RICs. We demonstrated that confined electrons within the ZnS/CdSe CCSQD are subjected to an intense interaction at the microscopic scale with the host medium. In addition, saturation effects in the nonlinear computed coefficients have been meticulously analyzed in terms of size and dielectric surroundings. Our findings demonstrated that electronic band edge states are heavily impacted by the sharp dielectric mismatch occurring at QD/matrix interface which could be exploited for the creation of novel optoelectronic devices in the future.

Band-structure tunability via modulation of planar buckling in ZnO monolayer: Manifestation in optoelectronic and photocatalytic properties

Two-dimensional ZnO materials, with their exceptional characteristics, hold great potential for novel nanoelectronic devices and catalysts. However, the strategy of modulating the electronic properties is still ongoing. To address this challenge, we conducted first-principles calculations to systematically investigate the structural stability, electronic properties and tunability of photocatalytic performance in ZnO monolayers with different planar buckling. The results indicate that ZnO monolayers with different buckling exhibit dynamical and thermal stability. The band edge positions of ZnO monolayers could be effectively controlled by planar buckling, and ZnO monolayers with suitable buckling can serve as potential candidates for visible-light driven water splitting for hydrogen generation. Furthermore, the narrowed band gaps in ZnO monolayers with increasing of planar buckling suggest their potential for use in the design of optoelectronic devices in nanoscale systems. In addition, the stability of buckled ZnO monolayers on Ag/Pt substrates and planar ZnO monolayer on semiconducting blue phosphorus (BlueP) are explored. Interestingly, Ag and Pt metal substrates have been found to be suitable for the growth of buckled ZnO monolayers. While BlueP monolayer is believed to serve as an excellent substrate for the growth of planar ZnO monolayers. These findings offer new strategy for designing planar or buckled ZnO monolayers with potential applications in futuristic nano-optoelectric and photocatalysis systems.

Spectroelectrochemical determination of the conduction band level of mesoporous titanium dioxide semiconductor in diverse reaction media

AbstractThe conduction band edge (ECB) energy levels of a mesoporous TiO2 film were investigated in aprotic dimethylformamide (DMF) and aqueous media to explore the influence of various proton additives and electron donors. Spectroelectrolysis (SE) technique was employed to measure the ECB in non‐aqueous solvents, revealing a significantly negative ECB in dry non‐aqueous solvents compared to values obtained in aqueous solvents. In non‐aqueous DMF, the ECB of TiO2 was found to be highly dependent on the proton‐donating additive due to the establishment of a proton adsorption–desorption equilibrium, reaching –(1.42–1.94) V versus SCE. The introduction of a well‐known electron donor, 1,3‐dimethyl‐2‐phenyl‐1,3‐dihydrobenzimidazole (BIH) led to a slight positive shift in the TiO2 ECB, indicating that the electron donor contributes to the to the CB energetics of the mesoporous TiO2 films. Under aqueous conditions and in the presence of various electron donors (TEA, TEOA, ascorbic acid, EDTA•2Na, and sodium ascorbate), the proton‐donating/accepting capacity of the additive significantly influenced the pH of the aqueous solvent, causing changes in the TiO2 ECB across the electrolyte solution. This study emphasizes the crucial role of proton additives and electron donors in influencing the CB energetics of mesoporous TiO2 electrodes and aims to determine the extent to which the ECB of TiO2 can shift as a result of the adsorption and intercalation of protons on its surface. The findings obtained herein contribute to the understanding of the energy efficiency of TiO2‐mediated photocatalytic systems in various solvent environments.

Indirect control of band gaps by manipulating local atomic environments using solid solutions and co-doping

The ability to tune band gaps of semiconductors is important for many optoelectronics applications including photocatalysis. A common approach to this is doping, but this often has the disadvantage of introducing defect states in the electronic structure that can result in poor charge mobility and increased recombination losses. In this work, density functional theory calculations are used to understand how co-doping and solid solution formation can allow tuning of semiconductor band gaps through indirect effects. The addition of ZnS to GaP alters the local environments of the Ga and P atoms, resulting in shifts in the energies of the P and Ga states that form the valence and conduction band edges, and hence changes the band gap without altering which atoms form the band edges, providing an explanation for previous experimental observations. Similarly, N doping of ZnO is known from previous experimental work to reduce the band gap and increase visible-light absorption; here we show that, when co-doped with Al, the Al changes the local environment of the N atoms, providing further control of the band gap without introducing new states within the band gap or at the band edges, while also providing an energetically more favourable state than N-doped ZnO. Replacing Al with elements of different electronegativity is an additional tool for band gap tuning, since the different electronegativities correspond to different effects on the N local environment. The consistency in the parameters identified here that control the band gaps across the various systems studied indicates some general concepts that can be applied in tuning the band gaps of semiconductors, without or only minimally affecting charge mobility.

Photocatalytic activity in graded off-valent cations substituted NaNbO3

This study investigates the impact of off-valent doping on the photocatalytic properties of NaNbO3 concerning the degradation of Methylene Blue. Compositions with x values of 0.00 (representing pure NaNbO3, denoted as NBO) and 0.05 within the material system Na1-xAxNbO3 (where A is K1+, Ba2+, La3+, abbreviated as K-NBO, Ba-NBO, and La-NBO respectively) were synthesized using the conventional solid-state reaction method. The UV–visible analysis revealed a decrease in the band gap for samples K-NBO and Ba-NBO, while an increase was observed for sample La-NBO. Raman modes of lower wave numbers merged and shifted towards the higher wave number side. The determination of valence band edge and conduction band edge involved computational analysis based on XPS survey scans, and the band gap energy values were derived from UV–Visible spectroscopy results. Examining the band diagram of the samples (NBO, K-NBO, Ba-NBO, and La-NBO) in conjunction with the highest occupied molecular orbital and lowest unoccupied molecular orbital levels of MB dye provided insights into potential degradation mechanisms. Photocatalytic dye degradation experiments for Methylene Blue demonstrated that doping increased the degradation efficiency of samples K-NBO, Ba-NBO, and La-NBO compared to NBO. Among all NaNbO3 based prepared samples, Ba-NBO exhibited the highest degradation efficiency of 96%, however slightly less than the reference sample P25 TiO2.

Decisive role of preparation technique on the structural, electrical and magnetic properties of vanadium doped ZnO nanoparticles

1% vanadium doped ZnO powders have been successfully synthesized via two different preparation techniques viz., glycine assisted combustion method and mechanical attrition method to understand their impact on the structure-property relation. X-ray Diffraction study validates the Wurtzite phase formation of ZnO with hexagonal structure. X-ray diffraction peak discloses the marked role of size and strain component. Evidently, the shift of lattice strain with the preparation techniques from compressive to tensile nature show the importance of structure property relations in the ZnO compounds. SEM micrographs depict the formation of densely populated nanograins with networking like structures which is characteristics of combustion synthesized product whereas as top down approach exhibits the presence of coarse grains amidst the broken sea of fine particles. Selected area electron diffraction (SAED) images display ring and bright circular spot pattern with changes in method of preparation, which further clarifies the distinct nature of investigated ZnO:V particles. Vibrational spectral study gives clear evidence on the structural disorder as deduced from the changes associated with longitudinal optical mode phonon scattering. Interestingly, the UV spectral study divulges the increment in optical band gap as well as a strong visible light absorption (about 30 %) for milled ZnO:V over their chemically prepared counterparts. Photoluminescence spectra of milled V doped ZnO upheld the structural disorder taken place with the suppression of Near Band Edge (NBE) transition and the shifting of broad asymmetric defect emission band towards the lower wavelength regime. Impedance study enlightens the crucial role of synthesis method with 5 times enhanced grain boundary resistance along with suppression of grain contribution in the milled samples. Magnetic study highlights the remarkable distinction of preparation route which alters the ferromagnetic ordering in the combustion synthesized samples to conventional diamagnetic signature of the host ZnO in the milled samples.

Computational insights into the electronic structure of functionalized monolayer hBN supercapacitor electrodes

In this study, we examined a collection of functionalized hBN monolayers’ electronic structure and quantum capacitance. Different ad-atom adsorption techniques have been used to functionalize the hBN monolayer. Density functional theory calculations are performed to precisely determine the electrical structure of an ad-atom doped hBN monolayer with a range of doping concentrations. The quantum capacitance of every functionalized system was then calculated. Above 500 μF/cm2, a noticeable quantum capacitance has been seen. Our calculations demonstrate that ad-atom doping at hollow position of hBN honeycomb lattice greatly increases the quantum capacitance of hBN monolayer. The microscopic cause of this system’s increased quantum capacitance has been investigated. Our DFT-based calculation shows that the system has a very large quantum capacitance due to the creation of new electronic states in the vicinity of the band edge and the Fermi level shift caused by the ad-atom adsorption.

Conduction mechanism and shallow donor defects in Nb-doped β-Ga2O3 single crystals

Electrical properties, electronic defects, and photoluminescence (PL) of Nb-doped β-Ga2O3 crystals grown by the floating zone method have been studied in the temperature range from 10 to 350 K. The activation energies of shallow and deep traps were obtained and compared with their counterparts. The 0.25-Nb-doped Ga2O3 crystal has a metallic behavior at room temperature and a semiconducting behavior at low temperatures. This behavior was interpreted by the quantum corrections to the electrical resistivity resulting from the weak localization and Coulomb interaction. In addition, the 0.25-Nb-doped Ga2O3 crystal has a shallow donor ionization energy of 0.03 eV, indicating that it requires less energy to enable n-type conductivity. Hence, Nb is a good choice for improving the electrical conductivity of the β-Ga2O3 crystal. Three defect trap states were found at EC-0.81 eV, EC-0.49 eV, and EC-0.12 eV below the conduction band edge. The PL spectra of the undoped sample show two characteristic bands in the UV and blue regions under the excitation wavelength of 256 nm. On the other hand, in the Nb-doped β-Ga2O3 sample, a novel UV emission band with a shorter wavelength was observed.