  We studied monolayers of noble gas atoms (Xe, Kr, Ar, and Ne) deposited on
individual ultra- clean suspended nanotubes. For this, we recorded the
resonance frequency of the mechanical motion of the nanotube, since it provides
a direct measure of the coverage. The latter is the number of adsorbed atoms
divided by the number of the carbon atoms of the suspended nanotube. Monolayers
formed when the temperature was lowered in a constant pressure of noble gas
atoms. The coverage of Xe monolayers remained constant at 1/6 over a large
temperature range. This finding reveals that Xe monolayers are solid phases
with a triangular atomic arrangement, and are commensurate with the underlying
carbon nanotube. By comparing our measurements to theoretical calculations, we
identify the phases of Ar and Ne monolayers as fluids, and we tentatively
describe Kr monolayers as solid phases. These results underscore that
mechanical resonators made from single nanotubes are excellent probes for
surface science.
