  In recent years, silicene, germanene, and stanene have received considerable
attention due to their possibilities to show a spin Hall effect. Nanoribbons
made of these materials are expected to have topologically protected states.
  In this work, we study the electronic properties of nanotubes made of Si, Ge,
Sn, and functionalized Sn. The main difference between these materials and
graphene is the relevance of spin-orbit interaction. The lack of edge states in
a seamless tube eliminates the possibility to find a topological edge state.
The spin-orbit interaction breaks the degeneracy of Dirac's cones and
eliminates the chances of finding a metal nanotube. As a consequence, this
transforms all nanotubes with spin-orbit interaction in trivial band
insulators.
  We focus our attention on two features. First, we study the energy band gap
as a function of the diameter of the nanotubes. Then, we concentrate on
controlling the band gap of a nanotube by applying an external radial electric
field.
