  Methyl-substituted Germanane is an emerging material that has been proposed
for novel applications in optoelectronics, photoelectrocatalysis, and
biosensors. It is a two-dimensional semiconductor with a strong above-gap
fluorescence associated with water intercalation. Here, we use time-resolved
photoluminescence spectroscopy to understand the mechanism causing this
fluorescence. We show that it originates from two distinct exciton populations.
Both populations recombine exponentially, accompanied by the thermally
activated transfer of exciton population from the shorter- to the longer-lived
type. The two exciton populations involve different electronic levels and
couple to different phonons. The longer-lived type of exciton migrates within
the disordered energy landscape of localized recombination centers. These
outcomes shed light on the fundamental optical and electronic properties of
functionalized germanane, enabling the groundwork for future applications in
optoelectronics, light-harvesting, and sensing.
