Mechanism for Borophene Phase Transition on Substrate

Borophene on the mostly used Ag(111) substrate undergoes a perplexing transition from the <i>v</i>_1/6 to the <i>v</i>_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. <i>Ab initio</i> calculations show that atoms in the <i>v</i>_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 <i>v</i>_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 <i>v</i>_1/6 and <i>v</i>_1/5 phases. These findings can inform experimental efforts to achieve structure- and layer-controllable borophene synthesis.