N-type doping effect of single-walled carbon nanotubes with aromatic amines

We investigated the chemical doping of the single-walled carbon nanotubes (SWCNTs) networks by a treatment with aromatic amines. Adsorption and intercalation of amine molecules in bundled SWCNTs leads to typical n-type doping observed already for alkali metals. The electron donation to SWCNTs is demonstrated by the X-ray-induced photoelectron spectra (XPS), where the carbon C 1s peak observed at 284.4eV for the sp² carbon in pristine samples is shifted by up to 0.3eV to higher binding energy upon chemical treatment. The development of a Breit-Wigner-Fano component on the lower energy side of the G^- mode in the Raman spectrum as well as a shift of the G⁺ to lower frequency provide evidence for charge accumulation in the nanotube π system, and indication for the n-type doping. The spectroscopic changes are accompanied by the modification of the electrical properties of the SWCNTs. A reduction of conductivity depends on the doping level and implies the decreasing concentration of the charge carriers in the naturally p-doped tubes. Comparing the two selected n-type dopants, the tetramethyl-p-phenylenediamine, shows more pronounced changes in the XPS and the Raman spectra than tetramethylpyrazine, indicating that the sp³ hybridization of nitrogen in the amine groups attached to phenyl ring is much more effective in interaction with the tube π system than the sp² hybridization of nitrogen in the aromatic pyrazine ring.