Hydrogen functionalisation of transition metal dichalcogenide monolayers from first principles

Two-dimensional transition metal dichalcogenide (TMD) monolayers are of great importance due to their unique properties and potential applications in fields such as electrocatalysis and optoelectronics. Although functionalisation of MoS2 has been recently studied through computation and experimentation, TMDs, such as MoSe2, WSe2, and WS2, have remained relatively unaddressed for chemical functionalisation. This study examines the effects brought about by the covalent functionalisation of MoSe2, WSe2, and WS2 by hydrogenation through first-principles density functional theory calculations. In particular, we examined the relationship between phase stability and surface functionalisation by comparing the stability of the 1T- and 2H phase at various hydrogen coverages. We found that the 1T phase became more stable than the 2H phase after a cross-over coverage: approximately 13, 11, and 18%, respectively, for MoSe2, WSe2, and WS2. The highest stability was achieved at close to 50% hydrogen coverage for the 1T phase. At this coverage, the 2H- to 1T-phase transition was found to be kinetically facile. We also found that the band gap of all three TMDs in the 1T phase can be tuned by varying the number of hydrogen coverage. This work shows that chemical functionalisation such as hydrogenation can be generally applied to tune the phase stability and electronic properties of TMD monolayers.