Vertical stacking of ultrathin two-dimensional materials via weak van der Waals (vdW) interactions is identified as an important technique for tuning the physical properties and designing viable products for nanoelectronics, spintronics, and renewable energy source applications. The geometry, electronic, and photocatalytic properties of vdW heterostructures of GeC and Janus transition metal dichalcogenides $M\mathrm{SSe}$ ($M$ = Mo, W) monolayers are investigated by performing first-principles calculations. Two different possible models of GeC-$M\mathrm{SSe}$ heterostructures are presented with an alternative order of chalcogen atoms at opposite surfaces in $M\mathrm{SSe}$. The most favorable stacking pattern of both models is dynamically and energetically feasible. A direct type-II band alignment is obtained in both models of understudy heterobilayer systems. The spin orbit coupling (SOC) effect causes considerable Rashba spin splitting in both $M\mathrm{SSe}$ monolayers. In particular, a greater Rashba spin polarization is demonstrated in model 1 (GeC-WSSe) than model 2 (GeC-MoSSe) caused by the alternative order of chalcogen atoms and larger SOC effect of heavier W than Mo atoms, which provides a platform for experimental and theoretical understanding of designing two-dimensional spintronic devices. More interestingly, the appropriate band alignments of model 1 with the standard water redox potentials enable its capability to dissociate water into ${\text{H}}^{+}/{\text{H}}_2$ and ${\text{O}}_2/{\text{H}}_2\text{O}$. In contrast to model 1, model 2 can only oxidize water into ${\text{O}}_2/{\text{H}}_2\text{O}$. The simulated design of GeC-$M\mathrm{SSe}$ is predicted for promising use in future electronic, spintronics, and photocatalytic water splitting.

In this work, we construct an ultrathin graphene/GaS heterostructure and investigate its electronic properties as well as the effect of vertical strain using density functional theory. The calculated results of the equilibrium interlayer spacing (3.356 Å) and the binding energy show that the intrinsic properties of isolated graphene and GaS monolayers can be preserved and the weak van der Waals interactions are dominated in the heterostructures. The van der Waals heterostructure (vdWH) forms an n-type Schottky contact with a small Schottky barrier height of 0.51 eV. This small Schottky barrier height can also be tuned by applying vertical strain. Furthermore, we find that the n-type Schottky contact of the vdWH can be changed to p-type when the interlayer spacing is decreased and exceeded to 2.60 Å. These findings show the great potential application of the graphene/GaS vdWH for designing next generation devices.

Janus monolayers and their van der Waals heterostuctures are investigated by hybrid density functional theory calculations. MoSSe, WSSe, MoSeTe and WSeTe are found to be direct band gap semiconductors. External electric fields are used to transform indirect MoSTe and WSTe to direct band gap semiconductors. MoSSe–WSSe, MoSeTe–WSeTe and MoSTe–WSTe vdW heterostructures are also indirect band gap semiconductors with type-II band alignment. Similar to the corresponding monolayers, in some of the above mentioned vdW heterostructures an external electric field and tensile strain can transform indirect to direct band gaps, while sustaining type-II band alignment. Janus monolayers have lower values of the work function (ϕ) than their vdW heterostructure counterparts. Furthermore, absorption spectra, absorption efficiency, and valence(conduction) band edge potentials are calculated to understand the optical and photocatalytic behavior of these systems. Red and blue shifts are observed in the position of excitonic peaks due to the induced strain in Janus monolayers. Strong device absorption efficiencies (80–90%) are observed for the WSeTe, MoSTe and WSTe monolayers in the visible, infra-red and ultraviolet regions. Energetically favourable band edge positions in Janus monolayers make them suitable for water splitting at zero pH. We find that the MoSSe–WSSe heterostructure and the MoSTe monolayer are promising candidates for water splitting with conduction and valence band edges positioned just outside of the redox interval.

In the present work, we investigate systematically the electronic and optical properties of Janus ZrSSe using first-principles calculations. Our calculations demonstrate that the Janus ZrSSe monolayer is an indirect semiconductor at equilibrium. The band gap of the Janus ZrSSe is 1.341 eV using the Heyd–Scuseria–Ernzerhof hybrid functional, larger than the band gap of ZrSe₂ monolayer and smaller than that of ZrS₂ monolayer. Based on the analysis of the band edge alignment, we confirm that the Janus ZrSSe monolayer possesses photocatalytic activities that can be used in water splitting applications. While strain engineering plays an important role in modulating the electronic properties and optical characteristics of the Janus ZrSSe monolayer, the influence of the external electric field on these properties is negligible. The biaxial strain, ε_b, has significantly changed the band of the Janus ZrSSe monolayer, and particularly, the semiconductor–metal phase transition which occurred at ε_b = 7%. The Janus ZrSSe monolayer can absorb light in both visible and ultraviolet regions. Also, the biaxial strain has shifted the first optical gap of the Janus ZrSSe monolayer. Our findings provide additional information for the prospect of applying the Janus ZrSSe monolayer in nanoelectronic devices, especially in water splitting technology.

Two-dimensional graphene-based van der Waals heterostructures have received considerable interest because of their intriguing characteristics compared with the constituent single-layer two-dimensional materials. Here, we investigate the interfacial characteristics, Schottky contact, and optical performance of $\mathrm{graphene}/{\mathrm{Ga}}_2\mathrm{SSe}$ van der Waals (vdW) heterostructure using first-principles calculations. The effects of stacking patterns, electric gating, and interlayer coupling on the interfacial properties of $\mathrm{graphene}/{\mathrm{Ga}}_2\mathrm{SSe}$ heterostructures are also examined. Our results demonstrate that the Dirac cone of graphene is well preserved at the $\mathrm{\Gamma }$ point in all stacking patterns due to the weak vdW interactions, which keep the heterostructures feasible such that they can be obtained in further experiments. Moreover, depending on the stacking patterns, a small band gap of about 13–17 meV opens in graphene and has a high carrier mobility, indicating that the $\mathrm{graphene}/{\mathrm{Ga}}_2\mathrm{SSe}$ heterostructures are potential candidates for future high-speed nanoelectronic applications. In the ground state, the $\mathrm{graphene}/{\mathrm{Ga}}_2\mathrm{SSe}$ heterostructures form an $n$-type Schottky contact. The transformation from an $n$-type to a $p$-type Schottky contact or to an Ohmic contact can be forced by electric gating or by varying the interlayer coupling. Our findings could provide physical guidance for designing controllable Schottky nanodevices with high electronic and optical performances.

In this work, we perform a first-principle study to investigate the atomic and electronic structures of the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ van der Waals heterostructure (vdWH) as well as its tunable electronic structure via interlayer coupling and an external perpendicular electric field. The ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH is structurally and thermodynamically stable at room temperature. Our results demonstrate that the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH exhibits a semiconducting characteristic with a direct band gap of 1.86/2.66 eV as given by the PBE/HSE06 calculation. This value of band gap conveniently lies in the visible light energy range, thus unraveling the strong optical absorption of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH in the technologically important visible light regime. The band edges of the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH separately from the ${\mathrm{C}}_3{\mathrm{N}}_4$ and ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ layers, thus resulting in a type-II band alignment, which is highly desirable for achieving efficient electron-hole separation. Remarkably, the electronic structure and the band alignment types can be flexibly tuned between type-I and type-II by applying an external electric field, by changing the interlayer distance and by applying the in-plane strain. Our findings reveal the potential of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH as a tunable hybrid material with strong potential in optoelectronic applications.