Transport properties of single-wall carbon nanotubes

We have measured electron transport properties of laser-vaporization (LV) and are-discharge (AD) grown pristine and doped single-wall carbon nanotubes (SWNT). The thermopower (S) of mats of pristine bundles of SWNTs prepared by both methods was found to be anomalously large, S = +40 to +60 mu V/K at 300 K, 10-15 times higher than found in the basal plane of "flat" graphite. Both LV and AD mats exhibited unusual S vs T dependence from 300 down to 10 K, featuring a broad maximum around 100 K superimposed on a metallic (linear) background. Upon chemical doping with either iodine or alkali metals (K, Cs), the magnitude of S (300 K) decreased similar to 4-fold. The sign of S was positive for the iodine-doped and negative for the alkali-metal-doped SWNT samples. Four-probe resistance (R) of the AD pristine SWNT mats increased with cooling within the studied T range from 300 to 10 K, while the LV pristine material exhibited a shallow resistivity minimum at T < 300 K. Chemical doping with both iodine or alkali metals (K, Cs) resulted in a sharp decrease of R (300 K) values by a factor of 40 (iodine, K) to 120 (Cs). The R vs T curves in the iodine-doped SWNT samples became almost T-independent down to 10 K. The optimally Cs-doped SWNT samples exhibited a 120-fold drop in R (300 K) and metallic behavior over the entire range of measurement (80 K < T < 300 K).