Efficiency of band edge optical transitions of 2D 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 non-magnetic 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 comparable to or exceeding MoS$_2$. Despite strong in-plane dipole transitions, most two-dimensional materials underperform bulk semiconductors due to the absence of out-of-plane components. To elucidate 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 $d$-$d$ transition as a key mechanism enabling optical responses in transition-metal dichalcogenides. We derived an analytical radiative recombination model incorporating multi-valley effects and found that excitonic corrections are essential for accurate lifetime predictions. Some direct-gap materials exhibit dark excitons as their lowest-energy states, classifying them as quasi-direct band gap semiconductors, which is critical for tuning excitonic recombination dynamics.