Efficiency of band edge optical transitions of 2D monolayer materials: A high-throughput computational study

TL;DR

This study uses high-throughput DFT calculations to evaluate optical transitions in 2D monolayers, identifying promising materials and elucidating mechanisms affecting their optical performance.

Contribution

It introduces the orbital overlap tensor and correlates Born effective charges with optical coupling, advancing understanding of optical transitions in 2D materials.

Findings

Identified transition-metal nitrogen halides and bismuth chalcohalides with high optical coupling.

Most 2D materials underperform bulk semiconductors due to lack of out-of-plane components.

Chalcogen-mediated $d$-$d$ transitions are key for optical responses.

Abstract

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.