Mechanical, optical, and thermoelectric properties of semiconducting ZnIn2X4 (X=S, Se, Te) monolayers

In the latest experimental success in the field of two-dimensional materials, $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{S}}_{4}$ nanosheets with a highly appealing efficiency for photocatalytic hydrogen evolution were synthesized [S. Zhang et al., ACS Nano 15, 15238 (2021)]. Motivated by this accomplishment, herein, we conduct first-principles-based calculations to explore the physical properties of the $\mathrm{Zn}{\mathrm{In}}_{2}{X}_{4}$ ($X$ = S, Se, Te) monolayers. The results confirm the desirable dynamical and mechanical stability of the $\mathrm{Zn}{\mathrm{In}}_{2}{X}_{4}$ monolayers. $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{S}}_{4}$ and $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{Se}}_{4}$ are semiconductors with direct band gaps of 3.94 and 2.77 eV, respectively while $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{Te}}_{4}$ shows an indirect band gap of 1.84 eV. The optical properties achieved from the solution of the Bethe-Salpeter equation predict the exciton binding energy of the $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{S}}_{4}$, $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{Se}}_{4}$, and $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{Te}}_{4}$ monolayers to be 0.51, 0.41, and 0.34 eV, respectively, suggesting the high stability of the excitonic states against thermal dissociation. Using the iterative solutions of the Boltzmann transport equation accelerated by machine learning interatomic potentials, the room-temperature lattice thermal conductivity of the $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{S}}_{4}$, $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{Se}}_{4}$, and $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{Te}}_{4}$ monolayers is predicted to be remarkably low as 5.8, 2.0, and 0.4 W/mK, respectively. Due to the low lattice thermal conductivity, high thermopower, and large figure of merit, we propose the $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{Se}}_{4}$ and $\mathrm{Zn}{\mathrm{In}}_{2}{\mathrm{Te}}_{4}$ monolayers as promising candidates for thermoelectric energy conversion systems. This study provides an extensive vision concerning the intrinsic physical properties of the $\mathrm{Zn}{\mathrm{In}}_{2}{X}_{4}$ nanosheets and highlights their characteristics for energy conversion and optoelectronics applications.