  We report the recent progress on the theoretical aspects of monolayer
topological insulators including silicene, germanene and stanene, which are
monolayer honeycomb structures of silicon, germanium and tin, respectively.
They show quantum spin Hall effects in nature due to the spin orbit
interaction. The band gap can be tuned by applying perpendicular electric
field, which induces a topological phase transition. We also analyze the
topological properties of generic honeycomb systems together with the
classification of topological insulators. Phase diagram of topological
insulators and superconductors in honeycomb systems are explicitly determined.
We also investigate topological electronics including a topological
field-effect transistor, the topological Kirchhoff's law and the topological
spin-valleytronics.
