  The band structure of fully hydrogenated Si nanosheets and nanotubes are
elucidated by the use of an empirical tight-binding model. The hydrogenated Si
sheet is a semiconductor with indirect band gap of about 2.2 eV. The symmetries
of the wave functions allow us to explain the origin of the gap. We predict
that, for certain chiralities, hydrogenated Si nanotubes represent a new type
of semiconductor, one with co-existing direct and indirect gaps of exactly the
same magnitude. This behavior is different from the Hamada rule established for
non-hydrogenated carbon and silicon nanotubes. Comparison to an ab initio
calculation is made.
