Materials exhibiting multiple stable phases can be used as functional components in electronic and optical applications if the phase transition is controllable. Group-VI transition metal dichalcogenides (TMDs, MX$_2$, where M=Mo, W and X = S, Se) are known to undergo charge induced transitions from semi-conducting H phases to metallic T phases. However, it is difficult to experimentally decouple the effect of composition-dependent energy barriers from indirect effects related to alkali metal-induced exfoliation. Here, using first-principles calculations, we study the energetics of transition between the different structural polytypes of four group-VI TMDs upon lithium adsorption. The calculated barriers provide an explanation for the high proportion of H phase remaining in alkali-treated MoS$_2$ compared to other group-VI monolayers. Likewise, the high proportion of metallic phase in WS$_2$ monolayers after alkali treatment can be explained by a high barrier to revert back to the H phase once in a neutral state.
Published in: "arXiv Material Science".