Isotope Effect of Hydrogen Functionalization in Layered Germanane: Implications for Germanane-Based Optoelectronics

Chemical functionalization represents a critical tool for van der Waals (vdW) layered nanomaterials in many aspects, ranging from the monolayer preparation by the solution exfoliation to the modulation of properties. Apart from attaching different chemical groups to one given nanomaterial, much less noticeable but fundamentally attractive is the isotope effect on the functional group, which can in principle tune the physical properties with unconverted chemical behaviors. Here, we report the isotope effect of hydrogen terminations in the layered Ge on lattice vibrations and electronic and atomic structures. We show that the Ge–Ge in-plane vibration responds to the mass variation of hydrogen terminations sensitively in frequencies, providing an indirect path to tune planar phonons through chemical bonds. A significant optical band gap modulation of 40 meV by −H and −D decorations is revealed, and the vdW gap increases by ∼0.3 Å, indicating the modification of layer–layer vdW interactions with isotope effect. The results not only unveil the fundamental isotope effect of hydrogen functionalization but also open up the effective band gap engineering toward germanane-based optoelectronic applications such as photodetectors and photocatalysts.