Thickness-Dependent Electronic and Optical Properties of Bernal-Stacked Few-Layer Germanane

Few-layer germanane has been successfully fabricated experimentally very recently and shown great potential applications in electronic and optoelectronic devices. In this work, we investigate thickness-dependent electronic and optical properties of Bernal-stacked few-layer germanane within the framework of many-body perturbation theory. Few-layer and bulk germanane are all direct band gap semiconductors, and the band gaps are tunable in a broad range. Strong excitonic effects are observed in few-layer germanane, leading to a distinct red shift in optical absorption spectra, and the exciton binding energy in monolayer germanane can be as large as 750 mV, which is 15 times larger than that of bulk germanane. More importantly, the quasi-particle band gaps, optical gaps, and exciton binding energies rapidly decrease with the increase of the layer and follow a power law of A + B/N-beta (0 < beta < 2) with the stacking layer.