Exciton Dynamics in Functionalized Germanane*

Monoelemental 2D materials are fast emerging alongside the much better-characterized transition metal dichalcogenide semiconductors due to high electron mobility, a wide range of achievable band gaps, and the possibility to tune their morphology and physical properties via functionalization. Methyl-substituted Germanane GeCH₃ is a two-dimensional semiconductor with strong above-gap photoluminescence connected to water intercalation.  Here, we use time-resolved fluorescence spectroscopy at different temperatures to unravel the origin of this photoluminescence. We find two different populations of bright excitons localized at recombination centers within puddles of intercalated water. The two exciton populations involve different electronic levels and couple to different phonons. They both recombine exponentially, together with a thermally activated conversion from the shorter- to the longer-lived species. The latter diffuses within the energy distribution of recombination centers. We expect these results to spawn further research on the exciton dynamics, charge separation, and many-body physics in monoelemental 2d semiconductors that will underpin future applications in optoelectronics, light-harvesting, and sensing.