Mechanism for Borophene Phase Transition on Substrate

Borophene on the mostly used Ag(111) substrate undergoes a perplexing transition from the v_1/6 to the v_1/5 phases as temperature increases, but the underlying mechanism remains elusive, hindering fine control over borophene synthesis. Here, we propose that the phase transition is driven by a critical synergy between in-plane migration of boron atoms and their sinking into the substrate. Ab initio calculations show that atoms in the v_1/6 phase can migrate to form patches with higher coordination numbers, where atoms are easier to sink into the substrate. Atomic sinking further promotes boron migration, locally nucleating v_1/5 domains that eventually expand into a perfect sheet via iterative sinkings and migrations. This temperature-driven, stepwise transition is substantiated by machine-learning-assisted molecular dynamics simulations with an enhanced sampling technique. Moreover, our simulations rationalize the experimental temperature window for synthesizing a series of intermixed v_1/6 and v_1/5 phases. These findings can inform experimental efforts to achieve structure- and layer-controllable borophene synthesis.