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.* E-mail: hdai@stanford.edu.
2The effect of environment on the electrical transport properties of small structures is an interesting topic with implications to a wide range of applications, from high performance nanoscale electronic devices and interconnects to nanosensors. Thus far, it has been largely unexplored how extrinsic factors affect high field electron transport in 1D nanostructures, and how to exploit them to tune and manipulate the high-current carrying abilities of 1D materials. As an extreme example, it has been shown recently 1 that freely suspended SWNTs in vacuum (in an isolated environment) display negative differential conductance (NDC) and drastically reduced current levels compared to nanotubes lying on solid substrates, 2,3 caused by substantial self-heating and scattering by non-equilibrium optical phonons (OPs) in the suspended isolated SWNTs. and always observed their hallmark negative differential conductance (NDC) behavior at high biases (Fig. 2a). 1 Interestingly, the I-V curves taken at different T 0 tend to converge in the high bias regime, and NDC appears less pronounced at higher T 0 (Fig. 2a). To understand the I-V data, we adopt and extend a recently introduced electro-thermal model for suspended SWNTs. 1 Briefly, the resistance of a SWNT under self-heating can be written aswhere R c is the contact resistance and where κ th is the SWNT thermal conductivity, p'= I 2 (R-R c )/L is the Joule heating per unit length, A=πdb is the cross-sectional area (b~0.34 nm: tube wall thickness) and g is the net heat loss by radiation and heat conduction per unit length. With κ th vs. T as fitting parameters, we solve Eqs. (1) and (2) iteratively until the temperature profile at each point along the SWNT converges within 0.1 K, while obtaining the best current fit with experimental data, at every bias. This temperature profile corresponds to AC phonons 4 (T ac ), which are the main heat carriers in the temperature range considered. 7 We capture the non-equilibrium OP effects in suspended SWNTs with an effective OP temperaturewhere the non-equilibrium phonon coefficient α > 0. The physical picture is that OP phonons emitted by hot electrons relax by decay into AC modes (T op to T ac ) that are subsequently carried out of the tube through the contacts (T ac to...