  The high-bias electrical transport properties of suspended metallic
single-walled carbon nanotubes (SWNTs) are investigated at various temperatures
in vacuum, in various gases and when coated with molecular solids. It is
revealed that non-equilibrium optical phonon effects in suspended nanotubes
decrease as the ambient temperature increases. Gas molecules surrounding
suspended SWNTs assist the relaxation of hot phonons and afford enhanced
current flow along nanotubes. Molecular solids of carbon dioxide frozen onto
suspended SWNTs quench the non-equilibrium phonon effect. The discovery of
strong environmental effects on high current transport in nanotubes is
important to high performance nanoelectronics applications of 1D nanowires in
general.
