  We observed strongly size-dependent viscoelasticity in amorphous SiO2 and Si
nanotubes with shell thickness down to ~8 nm. A core-shell model shows that a
~1 nm thick fluid-like surface layer has a significant effect on the mechanical
behavior of nanotubes and matches well with our experimental results.
Surprising, the surface layer exhibits a room temperature viscosity equivalent
to that of bulk glass above 1000 C. Additionally, a low activation energy
extracted from temperature dependent creep tests indicates that the viscous
flow in the surface layer is due to bond motion/switching, instead of bond
breaking. These findings unambiguously show the presence of a fluid-like
surface layer and elucidate its role on dynamic mechanical behavior in
nanoscale inorganic glass.
