  The search or design of silicon nanostructures similar to their carbon
analogues has attracted great interest recently. In this work, density
functional calculations are performed to systematically study a series of
finite and infinite hydrogenated cluster-assembled silicon nanotubes (SiNTs).
It is found that stable one-dimensional SiNTs with formula
$Si_{m(3k+1)}H_{2m(k+1)}$ can be constructed by proper assembly of hydrogenated
fullerene-like silicon clusters $Si_{4m}H_{4m}$. The stability is first
demonstrated by the large cohesive energies and HOMO-LUMO gaps. Among all such
silicon nanotubes, the ones built from $Si_{20}H_{20}$($m=5$) and
$Si_{24}H_{24}$($m=6$) are the most stable due to the silicon bond angles that
are most close to the bulk $sp^{3}$ type in these structures. Thermostability
analysis further verifies that such tubes may well exist at room temperature.
Finally, both finite nanotubes and infinite nanotubes show a large energy gap.
A direct-indirect-direct band gap transition has been revealed with the
increase of the tube radius. The existence of direct band gap may make them
potential building blocks for electronic and optoelectronic devices.
