Electronic Structures and Li-Diffusion Properties of Group IV-V Layered Materials: Hexagonal Germanium Phosphide and Germanium Arsenide

Based on density functional theory (DFT) calculations that include an empirical van der Waals interaction, we propose a layered hexagonal phase of bulk GeP and GeAs that is marginally less stable than the monoclinic phase experimentally observed. Both types of monolayers are dynamically stable semiconductors. Application of 2% isotropic stretching along two in-plane directions practically transforms the GeAs monolayer into a direct-gap (= 1.60 eV) material, rendering it useful in optoelectronics. In addition, comparison of effective masses shows that the GeAs monolayer can function better as n-type materials, especially when it is subject to the in-plane strain. Furthermore, a detailed comparison of the activation barriers for the rate-determining steps along the different paths on the GeP surface indicates that the Li atom can diffuse on the surface-4000 times faster than on graphene. Another comparison of the barriers allows us to identify a preferred diffusion path in the interlayer region of bulk hexagonal GeP. The diffusion is expected to occur as fast as in graphite, suggesting that its bulk can be useful as an anode material in lithium ion battery.