Computational Screening of 2D Transition Metal Halides for Optical Applications: The Role of Excitonic Effects

High Resolution Image Download MS PowerPoint Slide Transition metal halides (TMHs) have recently emerged as a promising class of two-dimensional (2D) materials for optoelectronic applications. However, a deep atomistic understanding of excitonic effects, arising in TMHs from the interplay between quantum confinement and diminished dielectric screening, is required for the design of future technological applications. In this work, we investigate 2D TMHs by combining density functional theory atomistic simulations, tight-binding models, and the solution of the Bethe–Salpeter equation. The selected 2D TMHs were obtained from the computational 2D materials database (C2DB) based on specific criteria to identify compounds with promising potential for optoelectronic applications. Our calculated lattice parameters are in agreement with those reported in both the C2DB database and the literature. However, deviations in electronic properties are observed for a few systems due to particularities in the implementation of spin–orbit coupling and hybrid functionals. We also predict exciton binding energies ranging from 0.23 to 0.81 eV, with TiI 2 in the space group P 6 ̅m 2 exhibiting unusual excitonic insulator behavior. Excitons significantly impact optical absorption by reducing the maximum absorption coefficient and shifting the absorption peak, all of which were quantified throughout the manuscript. Furthermore, when evaluating potential heterojunctions using Anderson’s rule, we find that excitonic effects modify 90 out of 276 possible heterojunctions, decreasing the number of type II heterojunctions, which are particularly relevant for optoelectronic applications.