Determination of Precise Redox Properties of Oxygen-Doped Single-Walled Carbon Nanotubes Based on in Situ Photoluminescence Electrochemistry

Single-walled carbon nanotubes doped with a limited amount of oxygen (O-doped SWNTs) are expected to be novel materials due to the appearance of red-shifted new emission and enhancement of the luminescence quantum yields compared to those of pristine SWNTs, which are of importance for the development of high performance biosensors, imaging materials, and optical devices. The appearance of the new optical properties is due to the change in the electronic states induced by the oxygen doping (O-doping) of the SWNTs, thus quantitative analysis of the electronic states of the O-doped SWNTs is crucial. In this study, we have successfully determined the precise electronic states of the O-doped SWNTs based on the in situ photoluminescence (PL) electrochemical method. The measurements revealed the presence of at least two distinct O-doping sites with unique optical and electrochemical properties for all four studied chiralities. The electrochemical measurements also showed that shifts in the valence and conduction band resulting from the O doping are on the order of 0.02-0.03 eV, which is much lower than the red shift of the photoluminescence peak. This behavior agrees with the theoretical simulations using the density functional based tight binding (DFTB) method. This study suggests that the doped sites on the SWNTs act as a neutral quantum dot trapping exciton generated on the tubes.