Amphoteric doping of single-wall carbon-nanotube thin films as probed by optical absorption spectroscopy

We have separately probed the doping behavior of semiconducting (S) and metallic (M) single-wall carbon-nanotube (SWNT) films, by optical absorption and dc resistance (R) measurements. Either electron acceptors $({\mathrm{Br}}_2,$ ${\mathrm{I}}_2)$ or donors (K, Cs) were used as dopants with controlled stoichiometry. Disappearance of absorption bands at 0.68, 1.2, and 1.8 eV, and concomitant decrease of R by doping have been attributed to electron depletion from or filling to specific bands in S- or M-SWNT’s, demonstrating their amphoteric doping behavior. Changes in the electronic properties are discussed in terms of the charge-transfer mechanism in the framework of the rigid-band model.