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In the present study, the effects of the strain engineering and electric field on electronic properties of the GaS monolayer are investigated by ab initio investigations. Our calculated results demonstrate that the GaS monolayer is a semi-conductor with a large indirect bandgap of 2.568 eV at the equilibrium. In the presence of a biaxial strain, the band structure of the GaS monolayer, especially the conduction band, changes significantly. However, while the effect of compressive strain on the energy gap of the GaS monolayer is quite weak, its energy gap depends strongly on the tensile strain. On the other hand, external electric fields can cause the semiconductor–metal transition in the monolayer. Being able to control electronic properties, especially the occurrence of the semiconductor–metal phase transition, makes the GaS monolayer a prospective material for nanoelectromechanical and nanospintronic applications.

Constructing van der Waals heterostructures (vdWHs) based on different two-dimensional materials could afford many interesting properties, which may not hold for single-layered materials. In this study, we design a novel vdWH-GaSe/MoSe2 and investigate its electronic properties using first-principles calculations. It has a type-II band alignment with an indirect bandgap. Moreover, we found that the band alignment transformation of the GaSe/MoSe2 vdWH from type-II to type-I can be realized by decreasing the interlayer distance or by applying a positive electric field. Our findings could provide fundamental insights into the GaSe/MoSe2 vdWH for designing high-performance optoelectronic nanodevices.

Strain effects on the electronic and optical properties of monolayer SnSe is studied by APW + lo method in DFT framework. The applied strains cause direct-indirect transition of SnSe band gap which is mainly constructed by s / p hybridization. The armchair e ac and zigzag e zz reduce the unstrained band gap of 1.05 eV down to 0 eV at 12% compression, but at 12% tension, the band gap decreases to 0.726–0.804 eV. The band gap always increases under biaxial strain e b at 12% compression to 12% tension. We observe an enhancement of real e 1 ( ω ) and imaginary e 2 ( ω ) parts of dielectric function by 14%–30% of magnitude, wider peak distribution to infrared and ultra-violet regions, and appearance of new peaks in the e 1 ( ω ) and e 2 ( ω ) spectrums. As a consequence, the light absorption α ( ω ) is significantly enhanced in the ultra-violet region and the absorption even starts at lower energy at infrared region.

In the present work, we consider electronic properties of WX2 (X = S, Se) monolayers under a biaxial strain e b using the first principles study. Our calculations indicate that, at equilibrium, the WS2 and WSe2 monolayers are semiconductors with a direct band gap of respectively 1.800 eV and 1.566 eV while their bulk structures are indirect semiconductors. The electronic properties of the WX2 monolayers are very sensitive with the biaxial strain, especially compression strain. The biaxial strain e b is the cause of the band gap of the WX2 monolayers and especially the semiconductor-metal phase transition has occurred in the WS2 monolayer at e b = - 10 % . In addition, the direct-indirect band gap transition was observed in both WS2 and WSe2 monolayers at a certain elongation of biaxial strain e b . The phase transitions in these monolayers can be very useful for their applications in nanoelectromechanical devices.

In this work, we consider systematically the influence of uniaxial strain and external electric field E on electronic properties of a monolayer InSe using ab initio approach based on density functional theory. Our calculations indicate that the monolayer InSe has a medium indirect energy bandgap of 1.38 eV at equilibrium. The calculated results also demonstrate that we can adjust the bandgap of the monolayer InSe by strain engineering or electric field. The bandgap of the monolayer InSe changes dramatically when the uniaxial strain is applied. Especially, under the compressed uniaxial strain, an indirect–direct bandgap transition has been observed at certain elongations. Within the electric field magnitude E range from 0 to 5 V/nm, the calculated results show that the negative electric field changes the bandgap of the monolayer InSe up to 23% while the positive electric field effect on its bandgap is negligible.

In this work, making use of density functional theory (DFT) computations, we systematically investigate the effect of biaxial strain engineering and external electric field applied perpendicular to the layers on the band gaps and electronic properties of monolayer MoS 2 . The direct-to-indirect band gaps and semiconductor-to-metal transition are observed in monolayer MoS 2 when strain and electric field are applied in our calculation. We show that when the biaxial strain and external electric field are introduced, the electronic properties including band gaps of monolayer MoS 2 can be reduced to zero. Our results provide many useful insights for the wide applications of monolayer MoS 2 in electronics and optoelectronics.

Electronic structure, optical, and photocatalytic properties of SnS, SnSe and their van der Waals heterostructures are investigated by first-principle calculations. Thermal stability confirmed that SnS, SnSe and SnS-SnSe van der Waals heterostructure are thermodynamically stable. The calculated band structure shows that SnS, SnSe and SnS-SnSe van der Waals heterostructure are indirect band nature while the heterostructure are confirmed for type-II band alignment. Bader charge analysis shows that the charges are transfer from SnS layer to SnSe layer. Furthermore, absorption spectra are calculated to understand the optical behavior of these systems, where the lowest energy transitions are lies in visible region. The valence and conduction band edges straddle the standard redox potentials in SnS, SnSe and their van der Waals heterostructures van der Waals heterostructures, making them promising candidates for water splitting in the acidic solution.

Constructing van der Waals heterostructures can be considered as an effective strategy for generating new physical phenomena that merit for high-efficiency nanodevices. Here, we construct the PbI 2 / SnSe 2 heterostructure using first-principles calculations and explore its electronic structure and optical performance as well as the effects of electric fields. The PbI 2 / SnSe 2 heterostructure owns an indirect band gap and exhibits type-I band alignment, making it promising candidate for light emission applications. Furthermore, the optical absorption of the PbI 2 / SnSe 2 heterostructure in both the visible light and ultra-violet regions is enhanced as compared with that of the PbI 2 and SnSe 2 monolayers. Furthermore, in the PbI 2 / SnSe 2 heterostructure, the electric fields can convert the type-I to type-II band alignment and tune the transition from semiconductor to metal. Our results provide useful guidance for practical application in high-efficiency nanodevices.

We investigate the electronic, photocatalytic and optical properties of a fully hydrogenated indium nitride H–InN–H monolayer under biaxial strain e b and external electric field E using density functional theory. Our findings demonstrate that the H–InN–H monolayer is a semiconductor with an indirect energy gap of 2.591 eV. Under a biaxial strain or electric field, the indirect-direct band gap transition can occur and its band gap depends dramatically on the e b and E. Our analysis of band edge alignment shows that the H–InN–H monolayer can possess photocatalytic activity for water splitting when an electric field or biaxial strain is applied. The optical characteristics of the H–InN–H monolayer depends greatly on the strain. The first optical gap of the H–InN–H monolayer is at the incident energy light of 3.320 eV and the tensile strain causes the first optical gap to shift towards the visible light region.

In this work, we investigate the electronic and optical properties of monolayer GeSe and the possibility of enhancement the photocatalytic activities for the water splitting of monolayer GeSe through strain engineering using first-principles calculations. Our calculations indicate that monolayer GeSe is a semiconductor with a moderate indirect gap of 1.13 eV at equilibrium and we can control its band gap by biaxial strain. In the presence of biaxial strain e b , the semiconductor-metal phase transition happens at large compressive strain of - 10 % and the indirect-direct gap transition occurs at e b = 4 % . The optical spectrum of monolayer GeSe are highly anisotropic and biaxial strain can increase the absorption coefficient of monolayer GeSe up to about 6 × 10 5 cm−1. Our calculations demonstrate that monolayer GeSe possesses photocatalytic properties for water splitting at e b = 5 % and we can enhance its photocatalytic activity by strain.