Transport properties of alkali-metal-doped single-wall carbon nanotubes

We report irt situ measurements of four-probe de resistance (R) and thermopower (S) of Cs- and Ii-doped single-wall carbon nanotube (SWNT) mats as a function of a doping and temperature (T). With increasing dopant exposure, the mat resistance has been found to first decrease and then increase, exhibiting a minimum for optimal Ca doping. In contrast, for Ii doping, the mat resistance decreased monotonically and saturated. This unexpected result suggests that the diameter of the alkali-metal ion plays a role in the transport properties of the tube bundles. A doping-induced decrease in R by factors of similar to 120 and similar to 40 were observed for Cs- and Ii-doped SWNT mats, respectively. The low-temperature upturn of R(T) observed in all pristine SWNT samples was progressively suppressed with increased K doping. The optimally Cs-doped sample exhibited a positive dR/dT over the entire range of measurement (80 K<T<300 Ii). In contrast to the anomalously large positive S(300 K)similar to +40- + 50 mu V/K observed in pristine SWNT at room temperature, the Cs-doped samples exhibited a small negative S similar to -7 mu V/K as expected for an ordinary metal.