Doping mechanism in single-wall carbon nanotubes studied by optical absorption

We have separately probed the doping behavior of semiconducting and metallic single wall carbon nanotube (SWNT) films, by optical absorption and dc resistance measurements. Either electron acceptors (Br2, I2) or donors (K, Cs) were used as dopants with controlled stoichiometry. Disappearance of the absorption bands at 0.7, 1.2 (assigned to semiconducting SWNT) and 1.8 eV (assigned to metallic SWNT) and the concomitant decrease of resistance due to doping were attributed to electron depletion or filling in specific bands of semiconducting or metallic SWNT. This demonstrates the amphoteric doping behavior of SWNT. Semiconducting and metallic SWNT undergo charge transfer in a specific sequence: initially the transition at 0.7 eV, subsequently around 1.2 eV and finally the feature at 1.8 eV are affected depending on the concentration of the dopant. Changes in the electronic properties are discussed in terms of charge transfer mechanisms in the framework of the rigid-band model.