Strong micro-dielectric environment effect on the band gaps of (n, m)single-walled carbon nanotubes

The electronic states of carbon nanotubes are one of the most fundamental properties of the nanotubes. We now describe the finding that the band gaps of (n,m)SWNTs are strongly affected by the change in microdielectric environments around the isolated nanotubes. In situ photoluminescence (PL) spectroelectrochemistry of the films containing 15 isolated (n,m)single-walled carbon nanotubes (SWNTs) cast on ITO electrodes in organic solvents including DMSO, acetonitirile, DMF, THF, and chloroform was completed and then the oxidation and reduction potentials, and band gaps (ΔE_electr) of the (n,m)SWNTs in the solvents were determined. We have discovered that the ΔE_electr of the (n,m)SWNTs become greater as the solvent dielectric constants decreased, which is in sharp contrast to the optical band gaps (ΔE_opt) that show virtually no solvent dependence. Such a strong solvent dependence of the electrochemical band gaps is due to the difference in the solvation energy of the charged SWNTs produced during the electrochemical processes. The ΔE_electr of both mod types of the SWNTs, mod = 1 and mod = 2, linearly increased versus the reciprocal of the tube diameter, which agrees with the theory. Moreover, the states of the π-electrons in the SWNTs were evaluated from the dependence of the band gaps on the diameter of the SWNTs. Furthermore, the states of the π-electrons on the sidewalls of the SWNTs were evaluated using the γ₀ values, a parameter representing the measure of the stability or the degree of delocalization of π-electrons in the sidewall of the SWNTs, and revealed that the γ₀ values of the mod = 1 and mod = 2 SWNTs increased with a decrease in the dielectric constants of the solvents in the range of 38-79. This study has enabled us to understand the essential electronic properties of the carbon nanotubes.