Efficiency of band edge optical transitions of two-dimensional monolayer materials: A high-throughput computational study

We performed high-throughput density functional theory calculations of optical matrix elements between band edges across a diverse set of nonmagnetic two-dimensional monolayers with direct band gaps. Materials were ranked as potential optical emitters, leading to the identification of transition-metal nitrogen halides (ZrNCl, TiNBr, TiNCl) and bismuth chalcohalides (BiTeCl) with optical coupling strengths comparable to or exceeding that of MoS2. Despite strong in-plane dipole transitions, most two-dimensional materials underperform relative to bulk semiconductors due to a combination of factors, including the absence of out-of-plane optical components, reduced orbital degeneracy at the band edges due to lower crystal symmetry, and spin degeneracy lifting at non-time-reversal-invariant k points. To elucidate the nature of interband transitions, we introduced the orbital overlap tensor and established a correlation between anomalous Born effective charges and optical coupling, linking charge redistribution to transition strength. We also identified chalcogen-mediated d-d transitions as a key mechanism enabling strong optical responses in transition-metal dichalcogenides. An analytical model for the radiative recombination coefficient, incorporating multivalley effects, was derived and benchmarked against full first-principles calculations. Excitonic corrections were found to be essential for quantitative agreement with experiment. Notably, some direct-gap monolayers host dark excitons as their lowest-energy excitonic states, rendering them quasidirect band gap semiconductors and underscoring the importance of excitonic effects for tuning light emission properties in two-dimensional materials.