Chemical doping of epitaxial graphene (EG) by organic free radicals (4-amino-2,2,6,6-tetramethyl-1-piperridinyloxy; 4-amino-TEMPO) was investigated using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and high resolution photoemission spectroscopy (HRPES). STM images revealed that the empty density of states near the adsorption site significantly decreased as a result of radical doping. STS indicated that 4-amino-TEMPO radicals acted as n-type dopants on monolayer graphene. The radicals adsorbed onto EG through the nitroxide groups, leaving the amine group unreacted, which was confirmed by the binding energies of N 1s and O 1s core-level spectra. Furthermore, the measured work function changes verified that increased adsorption of the radicals on EG showed n-type doping characteristics.
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Choi et al. (2009) studied this question.
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