  In this work we report new silicon and germanium tubular nanostructures with
no corresponding stable carbon analogues. The electronic and mechanical
properties of these new tubes were investigated through ab initio methods. Our
results show that the structures have lower energy than their corresponding
nanoribbon structures and are stable up to high temperatures (500 and 1000 K,
for silicon and germanium tubes, respectively). Both tubes are semiconducting
with small indirect band gaps, which can be significantly altered by both
compressive and tensile strains. Large bandgap variations of almost 50% were
observed for strain rates as small as 3%, suggesting possible applications in
sensor devices. They also present high Young's modulus values (0.25 and 0.15
TPa, respectively). TEM images were simulated to help the identification of
these new structures.
