Optical and thermoelectric properties of new Janus ZnMN2 (M = Ge, Sn, Si and N = S, Se, Te) monolayers: a first-principles study.

Thermoelectric materials have received great interest because they directly tap into the vast reserves of currently underused thermal energy, in an environmentally friendly manner. In this work, we investigated the electronic, optical and thermoelectric properties of novel ZnMN2 (M = Ge, Sn, Si and N = S, Se, Te) monolayers by performing density functional theory calculations. The dynamic and thermal stabilities of ZnMN2 (M = Ge, Sn, Si and N = S, Se, Te) monolayers were confirmed by their phonon band structures and ab initio molecular dynamics (AIMD) simulations, which showed that all the studied monolayers are stable. Calculated electronic band structures showed that ZnSiTe2, ZnGeSe2, and ZnSnTe2 have a direct band gap, while the remaining monolayers have an indirect band gap. Optical properties in terms of the imaginary part of the dielectric function have also been investigated, which showed that all the first excitonic peaks lie in the visible region. Transport coefficients, such as the Seebeck coefficient (S), electrical conductivity (σ) and power factor (PF) were calculated using the Boltzmann theory and plotted against chemical potential. The results demonstrated that the peak values of the p-type region for the PF are greater than those of the n-type region. Notably, ZnSiTe2 exhibits a large PF due to its smaller Seebeck coefficient and higher electrical conductivity compared to ZnSnS2, indicating that it is a promising candidate for thermoelectric applications. Our findings reveal that ZnMN2 (M = Ge, Sn, Si and N = S, Se, Te) monolayers open up new possibilities for optoelectronics and thermoelectric device applications.

Stacking effects in van der Waals heterostructures of blueP and Janus XYO (X = Ti, Zr, Hf: Y = S, Se) monolayers.

Using first-principles calculations, the geometry, electronic structure, optical and photocatalytic performance of blueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers and their corresponding van der Waal heterostructures in three possible stacking patterns, are investigated. BlueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers are indirect bandgap semiconductors. A tensile strain of 8(10)% leads to TiSeO(ZrSeO) monolayers transitioning to a direct bandgap of 1.30(1.61) eV. The calculated binding energy and AIMD simulation show that unstrained(strained) blueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers and their heterostructures are thermodynamically stable. Similar to the corresponding monolayers, blueP-XYO (X = Ti, Zr, Hf: Y = S, Se) vdW heterostructures in three possible stacking patterns are indirect bandgap semiconductors with staggered band alignment, except blueP-TiSeO vdW heterostructure, which signifies straddling band alignment. Absorption spectra show that optical transitions are dominated by excitons for blueP and XYO (X = Ti, Zr, Hf; Y = S, Se) monolayers and the corresponding vdW heterostructures. Both E VB and E CB in TiSO, ZrSO, ZrSeO and HfSO monolayers achieve energetically favorable positions, and therefore, are suitable for water splitting at pH = 0, while TiSeO and HfSeO monolayers showed good response for reduction and fail to oxidise water. All studied vdW heterostructures also show good response to any produced O2, while specific stacking reduces H+ to H2.

First-principles study of the electronic structures and optical and photocatalytic performances of van der Waals heterostructures of SiS, P and SiC monolayers.

Designing van der Waals (vdW) heterostructures of two-dimensional materials is an efficient way to realize amazing properties as well as open up opportunities for applications in solar energy conversion, nanoelectronic and optoelectronic devices. The electronic structures and optical and photocatalytic properties of SiS, P and SiC van der Waals (vdW) heterostructures are investigated by (hybrid) first-principles calculations. Both binding energy and thermal stability spectra calculations confirm the stability of these heterostructures. Similar to the corresponding parent monolayers, SiS-P (SiS-SiC) vdW heterostructures are found to be indirect type-II bandgap semiconductors. Furthermore, absorption spectra are calculated to understand the optical behavior of these systems, where the lowest energy transitions lie in the visible region. The valence and conduction band edges straddle the standard redox potentials of SiS, P and SiC vdW heterostructures, making them promising candidates for water splitting in acidic solution.