Structural and electronic properties of two-dimensional hydrogenated Xenes

Structural and electronic properties of pristine two-dimensional group IV Xenes (X=C, Si, Ge, Sn, Pb) and hydrogenated Xenes are studied, using density functional theory (DFT) calculations with and without spin-orbit coupling (SOC). The pristine hexagonal monolayer Xenes show buckled structure upon relaxation except graphene. The buckling $\delta$ increases linearly from graphene to plumbene. The bond angle $\theta$ between the X atoms is correlated with the degree of hybridization.% $D The band structures without SOC of group-IV Xenes are semi-metallic. However, inclusion of SOC mainly opens the bandgap at the Dirac point. The degree of hybridization and bandgap opening due to SOC are mainly explained in terms of the contribution of $p_{\mathrm{x}}/p_{\mathrm{y}}$ and $p_{z}$ electronic states at Fermi level. Semi hydrogenation (SH) leads to enhanced buckling in all Xenes which indicate a tendency towards more $sp^3$ like structures. The electronic structures of SH Xenes do not show Dirac cones. Incorporating SOC in the non-spin polarized band structure calculations results into splitting of the states. On the other hand, spin polarized band structures show magnetism with magnetic moment of 1.0~$\mu_{\rm{B}}$ and all SH Xenes are magnetic semiconductor except SH plumbene. Full hydrogenation vanishes buckling upon relaxation and the structure becomes planar implying $sp^2$-like hybridization. The band structures for FH Xenes turns out to be semiconducting and the Dirac cones also disappear. The bandgap changes from indirect to direct at FH stanene, while FH plumbene turns out to be semi-metallic. No spin polarized states are observed. The effect of SOC does not instigate any noteworthy changes in the bandgap for FH graphene to FH stanene. On the other hand, the SOC gives rise to bandgap of 0.47 eV in FH plumbene, which is otherwise a semi-metal.