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

We report in situ measurements of four-probe dc resistance ~R! and thermopower ~S! of Csand K-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 Cs doping. In contrast, for K 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 ;120 and ;40 were observed for Csand K-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 K!. In contrast to the anomalously large positive S300 K;140– 150 mV/K observed in pristine SWNT at room temperature, the Cs-doped samples exhibited a small negative S;27 mV/K as expected for an ordinary metal. @S0163-1829~98!52032-2#