Optical and electronic properties of monoatomic 2D germanene sheets

This work investigates the optical and electronic properties of Germanene – the 2D nanosheet of germanium. Using Time Dependent Density Functional Theory (TDDFT) calculations, various structures with increasing number of germanium atoms were considered to study the effects of size on the properties. The effect of doping with boron (B), phosphorous (P), carbon (C), silicon (Si), and zirconium (Zr) were investigated. All structures were optimized using B3LYP functional and 6-31G basis set. Frequency and energy calculations were carried out to yield the Density of States (DOS), IR and UV-VIS spectra. It was seen that the structures have size tunable spectral response in the ultraviolet spectrum. The wavelengths vary from 225 nm to 310 nm as the sheet size increases. It was also seen that the doping of B, P, C, Si, and Zr, can shift the absorption from UV region to VIS region and reduce the exciton binding energy upto ~60%. There exist disagreements on the exact electronic structure of these novel 2D materials, with different calculations and experiments providing varying answers. In this work we compare two sets of nearest tight-binding parameters - sp3 and sp3d5s* and explore the electronic structure of Hydrogen (H) passivated 2D monoatomic Ge sheets (Germanene). Spin-orbit coupling was included in the calculations. Using the models, the effect of sheet-width (pseudo 1D structures), axis orientation, and effects of uniaxial strain were explored. The physical structures for the unit-cells for the pseudo 1D structures were determined after full-geometry optimization in the DFT package Gaussian. Therefore, our TB models accounted for the deviation from the idealized structures because of buckling perpendicular to the axis of these structures. The more detailed sp3d5s* model gave an indirect bandgap of 1.30557 eV for infinite H-passivated Germanene