Optical and thermoelectric properties of non-Janus CuI and AgI, and Janus Cu2BrI and Ag2BrI monolayers by many-body perturbation theory

Abstract

In an outstanding experimental advance in the field of two-dimensional nanomaterials, cuprous iodide (CuI) and silver iodide (AgI) monolayers have been grown via a novel graphene encapsulation synthesis approach [K. Mustonen et al., Adv. Mater. 34, 2106922 (2022)]. Inspired by this accomplishment, we conduct first-principles calculations to investigate the elastic, phonon, and electron thermal transport, electronic, and optical properties of the non-Janus CuI and AgI and Janus ${\mathrm{Cu}}_{2}\mathrm{BrI}$ and ${\mathrm{Ag}}_{2}\mathrm{BrI}$ monolayers. Electronic and excitonic optical properties are elaborately studied using the many-body perturbation theory on the basis of $GW$ approximation. Our results indicate that these novel systems are stable but with soft elastic modulus and ultralow lattice thermal conductivity. It is also shown that the studied monolayers are wide-gap semiconductors with exciton binding energies close to 1 eV. The spin-orbit induced band splitting of Janus monolayers are increased more than $100%$ under a uniaxial strain of $3%$, and for non-Janus monolayers, a noticeable increase is observed under a perpendicular electric field. Thermoelectric efficiency of silver-based monolayers is higher than 1.2, making them promising candidates for next-generation thermoelectric devices. The presented first-principles results provide a deep understanding of the stability, thermal transport, and tunable optoelectronic properties of CuI, AgI, ${\mathrm{Cu}}_{2}\mathrm{BrI}$, and ${\mathrm{Ag}}_{2}\mathrm{BrI}$ monolayers, which can serve as a guide for the oncoming studies.