Conduction Band Minimum Research Articles

Real-space representation of the quasiparticle self-consistent GW self-energy and its application to defect calculations

The quasiparticle self-consistent QS$GW$ approach incorporates the corrections of the quasiparticle energies from their Kohn-Sham density functional theory (DFT) eigenvalues by means of an energy independent and Hermitian self-energy matrix usually given in the basis set of the DFT eigenstates. By expanding these into an atom-centered basis set (specifically here the linearized muffin-tin orbitals) a real space representation of the self-energy corrections becomes possible. We show that this representation is relatively short-ranged. This offers new opportunities to construct the self-energy of a complex system from parts of the system by a cut-and-paste method. Specifically for a point defect, represented in a large supercell, the self-eneregy can be constructed from those of the host and a smaller defect containing cell. The self-energy of the periodic host can be constructed simply from a $GW$ calculation for the primitive cell. We show for the case of the As$_\mathrm{Ga}$ in GaAs that the defect part can already be well represented by a minimal 8 atom cell and allows us to construct the self-energy for a 64 cell in good agreement with direct QS$GW$ calculations for the large cell. Using this approach to an even larger 216 atom cell shows the defect band approaches an isolated defect level. The calculations also allow to identify a second defect band which appears as a resonance near the conduction band minimum. The results on the extracted defect levels agree well with Green's function calculations for an isolated defect and with experimental data.

Z-scheme SnC/HfS2 van der Waals heterojunction increases photocatalytic overall water splitting

Designing a direct Z-scheme system is one of the effective ways to develop a high-efficient photocatalyst. In this paper, we designed the SnC/HfS2 heterojunction and explored its electronic structure and photocatalytic properties for water splitting based on first-principles calculations. Our results suggest that SnC/HfS2 heterostructure is a typical direct Z-scheme heterojunction, which can effectively separate carriers and possesses strong oxidation and reduction capabilities. The valence band maximum of SnC is close to the conduction band minimum of HfS2, which is in favor of the recombination of inter-layer carriers. The very small interlayer band gap and appropriate built-in electric field direction make the migration of electrons and holes along the Z-path. The photo-generated electrons on SnC make the hydrogen evolution reaction happen continuously, while the photo-generated holes on HfS2 make the oxygen evolution reaction happen continuously. The calculation of the reaction energy barrier indicates that the procedure of photocatalytic water splitting on the SnC/HfS2 heterojunction can be spontaneous. Our results show that SnC/HfS2 heterojunction is a potential direct Z-scheme photocatalyst for the overall decomposition of water.

Effects of coexistence of Mo and Zn vacancies with different valence states and interstitial H on the magneto-optical properties of ZnO: First-principles calculations

Research progress has been achieved on the influence of transition metal Mo doping and Zn vacancy coexistence on the magnetic and optical properties of ZnO. However, the interstitial H impurity in the Mo-doped ZnO system has been neglected. In this work, first principles were used to calculate the effects of the coexistence of different valence states of Mo and Zn vacancies and interstitial H on the magneto-optical properties of ZnO under the framework of density functional theory. Magnetic results showed that when Mo and Zn vacancies have +6 and 0 valences, respectively, the Zn22Mo6+O24(VZn0) system without interstitial H shows the largest magnetic moment and strongest magnetism. In the doped system, the local electrons of the spin-polarized O-2p orbital behave as acceptors near the valence band maximum, and the itinerant electrons behave as donors near the conduction band minimum. The itinerant electrons of the O-2p orbital adjacent to the Zn vacancy mainly contribute to the magnetic properties of the Zn22Mo6+O24(VZn0) system, and they are most beneficial to the design and preparation of new ZnO-based dilute magnetic semiconductors. Analysis of optical properties found that the Zn22Mo6+HiO24(VZn0) system with interstitial H have the easiest separation of electron–hole pairs, the highest mobility, the longest relaxation time, and the strongest built-in electric field when the Mo and Zn vacancies have +6 and 0 valences, respectively. Therefore, this system has the longest carrier lifetime and best activity. In addition, the absorption spectrum of the Zn22Mo6+HiO24(VZn0) system shows the most remarkable red shift in the visible and near-infrared regions. When pH = 0/7, the Zn22Mo6+HiO24(VZn0) system shows the strongest photocatalytic reduction ability. Given its carrier lifetime and activity, absorption spectrum, and redox ability, the Zn22Mo6+HiO24(VZn0) system is relatively the best for the design and preparation of new ZnO-based photocatalysts.

First Principle Calculation of Accurate Electronic and Related Properties of Zinc Blende Indium Arsenide (zb-InAs).

We carried out a density functional theory (DFT) study of the electronic and related properties of zinc blende indium arsenide (zb-InAs). These related properties include the total and partial densities of states and electron and hole effective masses. We utilized the local density approximation (LDA) potential of Ceperley and Alder. Instead of the conventional practice of performing self-consistent calculations with a single basis set, albeit judiciously selected, we do several self-consistent calculations with successively augmented basis sets to search for and reach the ground state of the material. As such, our calculations strictly adhere to the conditions of validity of DFT and the results are fully supported by the theory, which explains the agreement between our findings and corresponding, experimental results. Indeed, unlike some 21 previous ab initio DFT calculations that reported zb-InAs band gaps that are negative or zero, we found the room temperature measured value of 0.360 eV. It is a clear achievement to reproduce not only the locations of the peaks in the valence band density of states, but also the measured values of the electron and hole effective masses. This agreement with experimental results underscores not only the correct description of the band gap, but also of the overall structure of the bands, including their curvatures in the vicinities of the conduction band minimum (CBM) and of the valence band maximum (VBM).

Intermolecular Energy Gap-Induced Formation of High-Valent Cobalt Species in CoOOH Surface Layer on Cobalt Sulfides for Efficient Water Oxidation.

Transition metal-based electrocatalysts will undergo surface reconstruction to form active oxyhydroxide-based hybrids, which are regarded as the "true-catalysts" for the oxygen evolution reaction (OER). Much effort has been devoted to understanding the surface reconstruction, but little on identifying the origin of the enhanced performance derived from the substrate effect. Herein, we report the electrochemical synthesis of amorphous CoOOH layers on the surface of various cobalt sulfides (CoSα ), and identify that the reduced intermolecular energy gap (Δinter ) between the valence band maximum (VBM) of CoOOH and the conduction band minimum (CBM) of CoSα can accelerate the formation of OER-active high-valent Co4+ species. The combination of electrochemical and in situ spectroscopic approaches, including cyclic voltammetry (CV), operando electron paramagnetic resonance (EPR) and Raman, reveals that Co species in the CoOOH/Co9 S8 are more readily oxidized to CoO2 /Co9 S8 than in CoOOH and other CoOOH/CoSα . This work provides a new design principle for transition metal-based OER electrocatalysts.

Intermolecular Energy Gap‐Induced Formation of High‐Valent Cobalt Species in CoOOH Surface Layer on Cobalt Sulfides for Efficient Water Oxidation

AbstractTransition metal‐based electrocatalysts will undergo surface reconstruction to form active oxyhydroxide‐based hybrids, which are regarded as the “true‐catalysts” for the oxygen evolution reaction (OER). Much effort has been devoted to understanding the surface reconstruction, but little on identifying the origin of the enhanced performance derived from the substrate effect. Herein, we report the electrochemical synthesis of amorphous CoOOH layers on the surface of various cobalt sulfides (CoSα), and identify that the reduced intermolecular energy gap (Δinter) between the valence band maximum (VBM) of CoOOH and the conduction band minimum (CBM) of CoSαcan accelerate the formation of OER‐active high‐valent Co4+species. The combination of electrochemical and in situ spectroscopic approaches, including cyclic voltammetry (CV), operando electron paramagnetic resonance (EPR) and Raman, reveals that Co species in the CoOOH/Co9S8are more readily oxidized to CoO2/Co9S8than in CoOOH and other CoOOH/CoSα. This work provides a new design principle for transition metal‐based OER electrocatalysts.

Anomalous ambipolar transport in depleted GaAs nanowires

We have used a polarized microluminescence technique to investigate photocarrier charge and spin transport in n-type depleted GaAs nanowires ($ \approx 10^{17}$ cm$^{-3}$ doping level). At 6K, a long-distance tail appears in the luminescence spatial profile, indicative of charge and spin transport, only limited by the length of the NW. This tail is independent on excitation power and temperature. Using a self-consistent calculation based on the drift-diffusion and Poisson equations as well as on photocarrier statistics (Van Roosbroeck model), it is found that this tail is due to photocarrier drift in an internal electric field nearly two orders of magnitude larger than electric fields predicted by the usual ambipolar model. This large electric field appears because of two effects. Firstly, for transport in the spatial fluctuations of the conduction band minimum and valence band maximum, the electron mobility is activated by the internal electric field. This implies, in a counter intuitive way, that the spatial fluctuations favor long distance transport. Secondly, the range of carrier transport is further increased because of the finite NW length, an effect which plays a key role in one-dimensional systems.

Investigation of electronic and optical properties of PbxSn1-xO2 for optoelectronic applications: A TB-mBJ DFT approach

A comprehensive study of electronic and optical properties of PbxSn1-xO2 (x = 0.125, 0.25 and 0.375) have been performed utilizing PBE-GGA + TB-mBJ scheme employing WIEN2K package under DFT formalism. The lattice constant parameters obtained for PbxSn1-xO2 are observed to be invariant with rutile SnO2. The DoS (Density of States) profile of PbxSn1-xO2 states a dominant impact of partial states of Pb along the conduction band. A red shift of conduction band edge has been observed with increasing Pb incorporation in PbxSn1-xO2 supercells. The contribution due to partial states of O is supreme as compared to Pb along the valence band, as also encountered in SnO2. The nature of bandstructure is in parity with DoS profile. With increased incorporation of Pb atoms in PbxSn1-xO2 supercells, a significant lowering of conduction band minimum occurs thereby resulting in reduced band gap. The nature of reduction of optical band gap values is in parity with the electronic band structure. The optical parameter characteristic curves undergo redshift due to increased Pb incorporation in PbxSn1-xO2 supercells. Distinct peak values in absorption characteristic curve claims the material to have direct band gap. Higher value of static refractive index causes dispersion in the material. The real (imaginary) part of dielectric constant is consistent with the refractive index (extinction coefficient) characteristic curves. The reflection coefficient curve indicates lossy nature of the material. The reflection coefficient curve contradicts the loss coefficient characteristic curve. Hence, the consequences of this investigation claim the competency of PbxSn1-xO2 material for optoelectronic applications.

Nitrogen Decoration of Basal-Plane Dislocations in 4H - SiC

Basal-plane dislocations (BPDs) pose a great challenge to the reliability of bipolar power devices based on the 4H silicon carbide (4H-SiC). It is well established that heavy nitrogen (N) doping promotes the conversion of BPDs to threading edge dislocations (TEDs) and improves the reliability of 4H-SiC-based bipolar power devices. However, the interaction between N and BPDs, and the effect of N on the electronic properties of BPDs are still ambiguous, which significantly hinder the understanding on the electron-transport mechanism of 4H-SiC-based bipolar power devices. Combining molten-alkali etching and the Kelvin probe force microscopy (KPFM) analysis, we demonstrate that BPDs create acceptor-like states in undoped 4H-SiC, while acting as donors in N-doped 4H-SiC. First-principles calculations verify that BPDs create occupied defect states above the valence band maximum (VBM) and unoccupied defect states under the conduction-band minimum (CBM) of undoped 4H-SiC. The electron transfer from the defect states of intrinsic defects and native impurities to the unoccupied defect states of BPDs gives rise to the acceptor-like behavior of BPDs in undoped 4H-SiC. Defect formation energies indicate that N atoms can spontaneously decorate BPDs during the N doping of 4H-SiC. The binding between N and BPD is strong against decomposition. The accumulation of N dopants at the core of BPDs results in the accumulation of donor-like states at the core of BPDs in N-doped 4H-SiC. This work not only enriches the understanding on the electronic behavior of BPDs in N-doped 4H-SiC, but also helps understand the electron transport mechanism of 4H-SiC-based bipolar power devices.

Efficient perovskite solar cells with low J-V hysteretic behavior based on mesoporous Sn-doped TiO2 electron extraction layer

Organic-inorganic halide perovskite solar cells (PSCs) have recently become a promising and potentially commercially-ready photovoltaic device. However, the hysteresis effect is one of the most significant problems hindering this progress. The main reasons can be attributed to the lower electron mobility of TiO2, causing the imbalance of electron and hole flux. The shallow trap states close to the conduction band are formed by oxygen vacancy, leading to charge recombination. Therefore, elements doping is a valuable strategy for improving electron properties and passivating defects. Herein, we proposed Sn as a dopant in mesoporous structure TiO2 (meso-Sn:TiO2), causing the conduction band minimum and valence band maximum to be slightly upward shifted. The defect states decreased, and the carrier extraction improved. The PSCs with meso-Sn:TiO2 electron extraction layer improved the power conversion efficiency from 16.86% to 20.55%, enhanced the fill factor from 76.62% to 81.72%, and reduced hysteresis index from 0.16 to 0.03. This study proves that the Sn dopant is an effective strategy to improve the photovoltaic performance of PSCs.

Rashba states localized to InSe layer in InSe/GaTe(InTe) heterostructure

The zero potential gradient in 2D III-VI chalcogenides NX (N = Ga, In; X = S, Se, Te) monolayers hinders the Rashba spin splitting (RSS), which limits their application in spintronics. Here, we construct InSe/GaTe and InSe/InTe van der Waals (vdW) heterostructures and investigate their Rashba SOC effect on the basis of first-principles calculations. The occurrence of internal electric field in the heterostructures induces spin splitting conduction band minimum (CBM). Rashba characteristic of the spin splitting can be confirmed through the band structures and spin textures. The projected energy bands suggest that the RSS state in both heterostructures mainly roots in InSe layers. Furthermore, in-plane strain, vertical strain, and the external electric field are all proved to be effective methods to regulate the RSS. When an in-plane strain of 6% is exerted, the Rashba coefficient can become as high as 1.33 eV Å and 1.26 eV Å for InSe/GaTe and InSe/InTe, respectively. The occurrence of controllable RSS at CBM makes the both heterostructures become potential candidate materials for spintronic devices.

Effects of unique band structure of h-BN probed by photocurrent excitation spectroscopy

By employing a photocurrent excitation spectroscopy measurement, a direct bandgap of ∼6.46 eV has been resolved for the first time in thick B-10 enriched h-BN films. Together with previous band calculations, an unconventional energy diagram has been constructed to capture the unique features of h-BN: h-BN has a minimum direct bandgap of ∼6.5 eV and a bandgap of ∼6.1 eV which is indirect with the conduction band minimum (CBM) at M-point and valence band maximum (VBM) at K-point in the Brillouin zone, and the energy levels of the donor and acceptor impurities are measured relative to CBM and VBM, respectively.

Novel hydrogen-doped SrSnO3 perovskite with excellent optoelectronic properties as a potential photocatalyst for water splitting

Developing and designing novel electrodes for photocatalytic water splitting using computational analysis has become a crucial interest recently through bulk and surface calculations of the investigated materials. Doping wide band gap metal oxides has proven to be an efficient method for optical properties enhancement and band gap engineering. Herein, first-principles calculations were employed to investigate the possibility to engineer the optical and structural properties of SrSnO3 perovskite as a potential catalyst for photo-driven hydrogen production. Specifically, the synergistic effect of hydrogen doping and oxygen vacancies (OV) on the optoelectronic properties of SrSnO3 was for the first time investigated and discussed in detail. The interstitial hydrogen defects (Hi) are energetically favorable compared with the substitutional hydrogen defects. Mono- and co-hydrogen occupied oxygen vacancies sites were further examined. Interstitial hydrogen doping was found to introduce a shallow defect state below the conduction band minimum (CBM) forming a band tailing and increasing the dielectric constant. Thus, it could be used in gate dielectric applications. The created defect states upon doping were found to depend directly on the defect site and the defect concentration. At high concentration of oxygen vacancies defect, the HOV-OV structural configuration showed localized and shallow defect states with a band gap of 1.3 eV below the CBM. It also considerably increased the dielectric constant with optical absorption enhancement, compared to the pristine SrSnO3 counterpart. With optimum Gibbs free energy of hydrogen evolution reaction (HER) and theoretical band gap straddling of the oxygen and hydrogen evolution potentials, low exciton binding energy, and high permittivity, the HOV-OV structure is an ideal novel candidate catalyst for photocatalytic water splitting.

Stress and Defect Effects on Electron Transport Properties at SnO2/Perovskite Interfaces: A First-Principles Insight.

The structural and electronic properties of interfaces play an important role in the stability and functionality of solar cell devices. Experiments indicate that the SnO2/perovskite interfaces always show superior electron transport efficiency and high structural stability even though there exists a larger lattice mismatch. Aiming at solving the puzzles, we have performed density-functional theory calculations to investigate the electronic characteristics of the SnO2/perovskite interfaces with various stresses and defects. The results prove that the PbI2/SnO2 interfaces have better structural stability and superior characteristics for the electron transport. The tensile stress could move the conduction band minimum (CBM) of CH3NH3PbI3 upward, while the compressive stress could move the CBM of SnO2 downward. By taking into account the stress effect, the CBM offset is 0.07 eV at the PbI2/SnO2 interface and 0.28 eV at the MAI/SnO2 interface. Moreover, our calculations classify VI and Ii at the PbI2/SnO2 interface and Sn–I, Ii and Sni at the MAI/SnO2 interface as harmful defects. The Ii defects are the most easily formed harmful defects and should be avoided at both interfaces. The calculated results are in agreement with the available experimental observations. The present work provides a theoretical basis for improving the stability and photovoltaic performance of the perovskite solar cells.

Origin of the discrepancy between the fundamental and optical gaps and native defects in two dimensional ultra-wide bandgap semiconductor: Gallium thiophosphate

Ultra-wide bandgap (UWBG) semiconductors have great potential for high-power electronics, radio frequency electronics, deep ultraviolet optoelectronic devices, and quantum information technology. Recently, the two-dimensional UWBG GaPS4 was first applied to the solar-blind photodetector in experiments, which was found to have remarkable performance, such as high responsivity, high quantum efficiency, etc., and promising applications in optoelectronic devices. However, the knowledge of monolayer (ML) GaPS4 for us is quite limited, which hinders its design and application in optoelectronic devices. Here, we focus on the properties of electronic structure and intrinsic defects in ML GaPS4 by first-principles calculations. We confirmed that the fundamental gap of ML GaPS4 is 3.87 eV, while the optical gap is 4.22 eV. This discrepancy can be attributed to the inversion symmetry of its structure, which limits the dipole transitions from valence band edges to conduction band edges. Furthermore, we found that intrinsic defects are neither efficient p-type nor n-type dopants in ML GaPS4, which is consistent with experimental observations. Our results also show that if one expects to achieve p-type ML GaPS4 by selecting the appropriate dopant, P-rich conditions should be avoided for the growth process, while for achieving n-type doping, S-rich growth conditions are inappropriate. This is because due to the low strain energy, PS(c)+ has very low formation energy, which leads to the Fermi levels (EF) pinning at 0.35 eV above the valence band maximum and is not beneficial to achieve p-type ML GaPS4 under the P-rich conditions; the large lattice relaxation largely lowers the formation energy of SGa−, which causes the EF pinning at 0.72 eV below the conduction band minimum and severely prevents ML GaPS4 from being n-type doping under the S-rich conditions. Our studies of these fundamental physical properties will be useful for future applications of ML GaPS4 in optoelectronic devices.

Elastic, electronic, optical and thermoelectric properties of the novel Zintl-phase Ba2ZnP2

We report and discuss the results of a detailed first-principles calculations of the structural, elastic, electronic, optical and thermoelectric properties of the new Zintl phase dibarium zinc diphosphide Ba2ZnP2. The calculated structural parameters using the GGA-PBEsol functional are in excellent agreement with the available experimental counterparts. From the monocrystalline elastic constants numerically estimated through the stress-strain technique, a set of related properties, viz., mechanical stability, elastic anisotropy, brittle/ductile character, anisotropic sound velocities, polycrystalline elastic moduli, including isotropic bulk modulus, shear modulus, Young's modulus, Poisson's ratio, average sound velocity and Debye temperature, are deduced. The electronic and optical properties are investigated through the state-of-the art FP-(L)APW + lo method with the accurate TB-mBJ potential. Ba2ZnP2 is an indirect semiconductor with a gap of 1.24 eV. The charge-carrier effective masses are calculated. The valence band maximum is less dispersive than the conduction band minimum. The microscopic origins of the electronic states composing the energy bands are determined via the PDOS diagrams. Topological analysis of the charge density shows that a covalent character is dominantly ruling the Zn–P bond inside the block ZnP4, while an ionic bonding is mainly ruling the bond between the cation Ba and the polyanion ZnP4. Frequency-dependent macroscopic linear optical functions are predicted in a wide energy range 0–30 eV. Within the visible spectra, the calculated magnitude of the absorption coefficient, reflectivity and refractive index are in the ranges ∼4−35×104cm−1, 29−36% and 3.18−3.47, respectively. The semi-classical Boltzmann transport theory within the constant relaxation time approximation is used to study the thermoelectric properties. The title compound has a figure of merit of ∼1.77 at 300 K, which makes it a potential candidate for thermoelectric applications.

Electronic Band Tuning and Multivalley Raman Scattering in Monolayer Transition Metal Dichalcogenides at High Pressures.

Transition metal dichalcogenides (TMDs) possess spin-valley locking and spin-split K/K' valleys, which have led to many fascinating physical phenomena. However, the electronic structure of TMDs also exhibits other conduction band minima with similar properties, the Q/Q' valleys. The intervalley K-Q scattering enables interesting physical phenomena, including multivalley superconductivity, but those effects are typically hindered in monolayer TMDs due to the large K-Q energy difference (ΔEKQ). To unlock elusive multivalley phenomena in monolayer TMDs, it is desirable to reduce ΔEKQ, while being able to sensitively probe the valley shifts and the multivalley scattering processes. Here, we use high pressure to tune the electronic properties of monolayer MoS2 and WSe2 and probe K-Q crossing and multivalley scattering via double-resonance Raman (DRR) scattering. In both systems, we observed a pressure-induced enhancement of the double-resonance LA and 2LA Raman bands, which can be attributed to a band gap opening and ΔEKQ decrease. First-principles calculations and photoluminescence measurements corroborate this scenario. In our analysis, we also addressed the multivalley nature of the DRR bands for WSe2. Our work establishes the DRR 2LA and LA bands as sensitive probes of strain-induced modifications to the electronic structure of TMDs. Conversely, their intensity could potentially be used to monitor the presence of compressive or tensile strain in TMDs. Furthermore, the ability to probe K-K' and K-Q scattering as a function of strain shall advance our understanding of different multivalley phenomena in TMDs such as superconductivity, valley coherence, and valley transport.

Computational Prediction of Graphdiyne-Supported Three-Atom Single-Cluster Catalysts

Computational Prediction of Graphdiyne-Supported Three-Atom Single-Cluster Catalysts

Synthesis of Hydride-Doped Perovskite Stannate with Visible Light Absorption Capability.

Narrow-gap semiconductors with visible light absorption capability have attracted attention as photofunctional materials. H--doped BaSn0.7Y0.3O3-δ containing Sn(II) species was recently reported to absorb visible light up to 600 nm, which represents the first demonstration of oxyhydride-based visible-light-absorbers. In the present study, a more detailed investigation was made to obtain information on the synthesis and properties of H--doped perovskite-type stannate with respect to the A-site cation of the material and the preparation conditions. H--doped ASn0.7Y0.3O3-δ (A = Ba, Ba0.5Sr0.5, and Sr) obtained by the reaction of ASn0.7Y0.3O3-δ precursors with CaH2 at 773 K under vacuum conditions was shown to have almost the same bandgap (ca. 2.1 eV), regardless of the A-site cation. Physicochemical measurements and theoretical calculations revealed that the identical bandgaps of H--doped ASn0.7Y0.3O3-δ are due to the simultaneous shift of the midgap states composed of Sn2+ with the conduction band minimum. Experimental results also indicated that the appropriate preparation conditions with respect to Y3+-substitution and the temperature for the synthesis of the ASn0.7Y0.3O3-δ precursors were essential to obtain H--doped products that have a low density of defects.

Study of band alignment type in Janus HfSe2/Ga2SeS and HfSeS/GaSe heterostructures

Motivated by the successful preparations and excellent properties of Janus monolayers, we construct the van der Waals (vdW) heterostructures HfSe2/Ga2SeS, and HfSeS/GaSe. Their electronic features and the type of band alignment are investigated. We find that the HfSe2/Ga2SeS exhibits the similar electronic properties to HfSe2/GaSe. Both heterostructures own type-I band alignment, in which both valence band maximum (VBM) and conduction band minimum (CBM) are provided by HfSe2 layer. However, the HfSeS/GaSe heterostructure owns type-II band alignment, in which the electrons and holes are located within the HfSeS layer and the layer of GaSe, respectively. This means that the HfSeS/GaSe heterostructure can facilitate the effective separation of the photo-generated hole and electron pairs. The band offsets of valence bands are 0.09 eV, 0.26 eV and 0.17 eV for HfSe2/GaSe, HfSe2/Ga2SeS, and HfSeS/GaSe heterostructures. The small band offsets indicate the band alignment can easily be modulated by some external means. Thus, the external electric field and interlayer coupling effect are considered to modulate the electronic properties of those heterostructures. They can simultaneously tune the band gap and induce the band alignment transition between type-I and type-II. These results indicate that HfSe2/GaSe and Janus HfSe2/GaSe heterostructures have great applications in nanoelectronic device.