Opening the germanene monolayer band gap using halogen atoms: An efficient approach studied by first-principles calculations

Similar to graphene, the absence of an energy gap has restricted considerably the germanene monolayer practical applications. In this work, we propose an efficient method to create artificially the band gap of the germanene through the chemical functionalization and doping with halogen atoms. Our density functional theory (DFT)-based calculations indicate the semimetal nature of the pristine single layer with a Dirac cone generated at the K point. The applied methods notice structural distortion, however the good stability remains. Relatively large direct band gaps ranging from 0.416 to 1.596 eV can be obtained by the full- and Janus-decoration as confirmed by the HSE06 hybrid functional, where the Cl-Ge-Cl, Br-Ge-Br, F-Ge-Cl, F-Ge-Br, and Cl-Ge-Br exhibit values suitable for optoelectronic applications. In contrast the half-functionalization produces the metallic nature as induced by the large p electrons of the Ge2 atoms. The standard PBE functional shows that the F-, Cl-, Br-, and I-doping give place to the indirect band gap of 0.052, 0.107, 0.116, and 0.110 eV, respectively. Besides, the analysis of the chemical bonds suggests that the halogen atoms are ionically bonded to the host Ge atoms. We believe that results presented herein may pave a solid way to engineer the germanene monolayer electronic properties at the time of designing its applications in nano-devices.