Open issues and future challenges

The unsolved and emergent research topics are clearly presented in this chapter. Certain of them will be worthy of the systematic investigations, such as, the germanium-/silicon-related solar cells, batteries and intermetallics, and the reliable establishment of the tight-binding models/the generalized tight binding model in the absence/presence of external fields. For example, it is almost impossible to synthesize and examine all the quasi-stable crystal structures of bilayer group-IV materials except for graphene, mainly owing to very complex intralayer/interlayer multi-orbital hybridizations, prominent buckling structures, and ferromagnetic/anti-ferromagnetic spin configurations. The VASP simulations are available in fully exploring the various ground state energies and the featured electronic, optical and magnetic properties. However, the stacking-dependent tight-binding models cannot be obtained from the well-fitting with the VASP calculations, since the low-lying valence and conduction bands are predicted to exhibit the abnormal energy dispersion relations. How to efficiently deal with the diverse quasi-particle behaviors in non-graphene group-IV materials will become the near-future studying focuses, esfpecially for silicon- and germanium-related semiconductors.