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 Cs- and 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 Cs- and 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 $S_{300\mathrm{K}}\sim +40\mathrm{}–+50\mathrm{}\mu \mathrm{V}/\mathrm{K}$ observed in pristine SWNT at room temperature, the Cs-doped samples exhibited a small negative $S\sim -7\mu \mathrm{V}/\mathrm{K}$ as expected for an ordinary metal.