Electronic and optical properties of hydrogenated group-IV multilayer materials

Hydrogenated group-IV layered materials are semiconducting forms of silicene, germanene and stanene. We systematically studied the evolution of the structural, electronic and optical properties of these 2D materials as a function of the number of layers. We verify that the exfoliation energy increases upon the increase of the atomic number (Si → Sn) of the group-IV material. We show that silicane, independent of the number of layers, is an indirect band gap (Γ-M) material. This behavior is different from both germanane and stanane, which are direct band gap (Γ point) semiconductors. The calculated optical spectra show, for all systems, a red shift in the absorption edges and an enhanced absorption of the visible light for the in-plane (α_‖) component upon the increase in the number of layers and, also as a function of the increasing atomic number. Our findings also indicate that: (i) (XH₂)_m(YH₂)_n vdW heterostructures will always present a type-I band alignment for X = Si and Y = Ge or Sn, whereas (ii) for X = Ge and Y = Sn, the band alignment can be tuned (type-I ↔ type-II) by the number of layers (m,n).