Doping single-walled carbon nanotubes with surfactant peptides containing electron-donor substituents and nitrogen heterocycles

We examined the potential of a series of aromatic moieties with different electron-donating ability to alter SWCNT electronic properties. The selected aromatic moieties included p-amino-phenylalanine, 3-[4-(dimethylamino)phenyl]propanoic acid, tyrosine, tryptophan, and 4-pyridylalanine. These moieties contained exocyclic electron-donor substituents, such as amine, dimethylamine, and hydroxyl groups, as well as nitrogen-containing heterocycles, including indole and pyridine. We attached these aromatic molecules to the N-terminus of an amphiphilic surfactant peptide and obtained stable aqueous peptide/SWCNT dispersions. Atomic force microscopy images and ultraviolet-visible-near-infrared spectra revealed that, under the conditions used, all peptides, except for the one with 4-pyridylalanine, disperse SWCNTs well in solution. The local electron density of states of the peptide-coated SWCNTs was examined using scanning tunneling spectroscopy dI/dV plots. All spectra showed an additional peak in the conduction band side very close to the Fermi level, indicating n-type doping of the SWCNTs. The doping behavior was further verified by Raman spectroscopy G-band peak downshifts. Additionally, we found that the location of the heteroatom contributes significantly to the interaction between the aromatic moieties and the SWCNTs.