A comparative first-principles study of the electronic and excitonic properties of 2H-CrX2 (X = S, Se, Te) monolayers

Abstract

Recently, Habib et al. [Nanoscale 11, 20123 (2019)] achieved the first successful synthesis of a two-dimensional (2D) CrS₂ monolayer via chemical vapor deposition, paving the way for the exploration of Cr-based layered materials with exceptional properties. Using first-principles calculations combined with a semi-empirical van der Waals dispersion correction, we have investigated the electronic and excitonic properties of CrX₂ (X = S, Se, Te). Our results predict that the 2H phase of CrX₂ is a direct bandgap semiconductor with a gap ranging from 1.5 eV to 0.8 eV. We observe an increase in electron mobility and a decrease in hole mobility as X varies from S to Te, with values on the order of several hundred cm${}^2$/Vs. This behavior arises from the opposing trends in the deformation potential constants of the valence band compared to those of the conduction band. The exciton binding energy decreases progressively from 0.42 eV in CrS₂ to 0.25 eV in CrTe₂, suggesting a weakening of excitonic effects with the increasing atomic number of the chalcogen element. This trend is associated with increased dielectric screening as X transitions from S to Te. Notably, the relationship between exciton binding energy and bandgap exhibits a linear dependence, described by $E_b=\alpha E_g+\beta$, with $\alpha =0.258$ and $\beta =0.033$. The strong excitonic effects and tunable electronic properties of 2H-CrX₂ monolayers suggest their potential for a range of optoelectronic applications, including LEDs, photodetectors, and solar cells, while also opening up avenues for optimizing performance in quantum emitters and photovoltaics through substrate engineering or external fields.