Beyond graphene: Clean, hydrogenated and halogenated silicene, germanene, stanene, and plumbene

The fascinating electronic and optoelectronic properties of freestanding graphene and the possible inclusion of novel two-dimensional (2D) systems in silicon-based electronics have driven the search for atomic layers consisting of other group-IV elements Si, Ge, Sn, and Pb, which form similar hexagonal lattices and are isoelectronic to graphene. The resulting 2D crystals silicene, germanene, stanene and plumbene, referred as Xenes, but also their functionalized counterparts, e.g. the hydrogenated sheet crystals, named as Xanes, silicane, germanane, and stanane, are in the focus of this review article. In addition, halogenated Xenes are investigated. The consequences of the larger atomic radii on the atomic geometry, the energetic stability, and possible epitaxial preparations are discussed. In the case of honeycomb atomic arrangements, the low-energy electronic excitations are ruled by almost linear bands. Spin-orbit coupling opens small gaps leading to Dirac fermions with finite effective masses. The linear bands give rise to an absorbance of the Xenes determined by the finestructure constant in the long-wavelength regime. While for vanishing photon energies the excitonic influence is still an open question, saddle-point excitons and excitons at M-0 van Hove singularities appear at higher frequencies. After opening substantial fundamental gaps by hydrogenation, the absorption edges of the Xanes, silicane, germanane, and stanane, are dominated by bound excitons with extremely large binding energies. Other chemical functionalizations, but also vertical electric fields, yield electronic structures ranging from topological to trivial insulators. Even a quantum spin Hall phase is predicted at room temperature. The topological character and the possible quantization of the spin Hall conductivity are studied versus gap inversion, chemical functionalization, and Rashba spin-orbit interaction. The drastic changes of the electronic properties of Xenes with chemical functionalization, interaction with the substrate, and external perturbations, open future opportunities for tailoring fundamental properties and, therefore, interesting applications in novel electronic and optoelectronic nanodevices.