Adsorption Induced Indirect-to-Direct Band Gap Transition in Monolayer Blue Phosphorus

In this work, we systematically studied adsorption induced indirect-to-direct band gap transition in monolayer blue phosphorus from first-principles calculations by combining one-shot GW approximation and the Bethe-Salpeter equation. Our results revealed that surface adsorption (i.e., O₂, −OH, −COOH, and −CN) strongly modifies the conduction and valence band edges, resulting in an indirect-to-direct band gap transition. More importantly, the direct band gap can be dramatically tuned by either the in-plane strain or the coverage ratio of adsorbates, which enables monolayer blue phosphorus to efficiently adsorb visible light. The mechanism of strain effect and surface adsorption on band gap tuning was deeply discussed. Moreover, our results clearly showed that the adsorbates have an important influence on the exciton binding energies (EBE), while the coverage of adsorbates play a crucial role in the linear scaling behavior between EBE and quasi-particle band gap. Our findings suggest that monolayer blue phosphorus has potential applications in electro-optical devices

Oxidation-Induced Topological Phase Transition in Monolayer 1T′-WTe₂

Monolayer (ML) tungsten ditelluride (WTe₂) is a well-known quantum spin Hall (QSH) insulator with topologically protected gapless edge states, thus promising dissipationless electronic devices. However, experimental findings exhibit the fast oxidation of ML WTe₂ in ambient conditions. To reveal the changes of topological properties of WTe₂ arising from oxidation, we systematically study the surface oxidation reaction of ML 1T′-WTe₂ using first-principles calculations. The calculated results indicate that the fast oxidation of WTe₂ originates from the existence of H₂O in air, which significantly promotes the oxidation of ML 1T′-WTe₂. More importantly, this low-coverage oxidized WTe₂ loses its topological features and is changed into a trivial insulator. Furthermore, we propose a fully oxidized ML WTe₂ that can still possess the QSH insulator states. The topological phase transition induced by oxidation provides exotic insight into understanding the topological features of layered transition-metal dichalcogenide materials