Two-dimensional germanane and germanane ribbons: density functional calculation of structural, electronic, optical and transport properties and the role of defects

We have performed first principles calculations combined with non-equilibrium Green's function to study the structural, electronic, optical and transport properties of two-dimensional germanane and germanane ribbons. More importantly, the defect influences on the properties of the germanane-based nanostructures have been investigated. The presence of single hydrogen vacancy induces ferromagnetism to the nonmagnetic pristine germanane according to spontaneous magnetization, while the formation of the dumbbell structure induced by Ge adatom only reduces the electronic band gap. Both H-monovacancy and dumbbell contained defective germanane nanostructures are thermally stable at room temperature. The optical property calculations revealed that the pristine germanane sheet has significant light absorption of the solar spectrum, and the presence of the H-monovacancy and dumbbell defects in the germanane led to redshift and blueshift of the light adsorption peak, respectively. Moreover, both zigzag-and armchair-germanane nanoribbons (zGeNRs and aGeNRs) are nonmagnetic semiconductors with a direct band gap at the G-point, and their band gaps are monotonously reduced with increasing width. Our quantum transport calculations have shown different transport behaviors that depend on the GeNRs' edge topology. While the aGeNRs attain a magnetic moment by introducing H monovacancy, it is unlikely to achieve large magnetic moments in germanane via controlling the shape of the H-vacancy cluster since the dehydrogenated nanostructures prefer nonmagnetic characteristics after atomic reconstruction. These calculated results suggest that the germanane has not only suitable transmission gap and light adsorption, but also directionally dependent electron transport, making it an excellent candidate for potential application in the fields of nanoelectronics and optoelectronics.