Voltage-Induced Dependence of Raman-Active Modes in Single-Wall Carbon Nanotube Thin Films

We report on electrical Raman measurements in transparent and conducting single-wall carbon nanotube (SWNT) thin films. Application of external voltage results in downshifts of the D and G modes and in reduction of their intensity. The intensities of the radial breathing modes increase with external electric field related to the application of the external voltage in metallic SWNTs, while decreasing in semiconducting SWNTs. A model explaining the phenomenon in terms of both direct and indirect (Joule heating) effects of the field is proposed. Our work rules out the elimination of large amounts of metallic SWNTs in thin film transistors using high field pulses. Our results support the existence of Kohn anomalies in the Raman-active optical branches of metallic graphitic materials.