Surface modification of graphene and graphite by nitrogen plasma: Determination of chemical state alterations and assignments by quantitative X-ray photoelectron spectroscopy

Multilayer graphene (MLGR) and its bulk analog, highly oriented pyrolytic graphite (HOPG), were treated by radio frequency activated low pressure N2 gas plasma (at negative bias 0-200 V, for 5-20 min). Surface composition and chemical-state alterations were delineated by X-ray photoelectron spectroscopy (XPS). Covalently bonded nitrogen of 5-15 at% incorporated into the surface. The higher N concentration in MLGR below 100 V is attributed to the larger number of defects. The equal N content at 200 V indicates intensive formation of reactive sites. In-depth distribution of N is restricted to 2-4 monolayers. Model calculation resulted in 23 at% N (at 100 V) in the top graphene layers of HOPG. Three different chemical states of nitrogen (pyridine-type at 398.3 eV, pyrrole- and triazine-type at 399.7 eV and N substituting C in graphite-like network at 400.9 eV) were determined from high-resolution N1s spectral region for all samples. Pyridine and pyrrole-triazine components increase preferentially with increasing bias. Alterations of the C1s and O1s spectra are discussed in a critical approach. The amount of reacted carbon was consistent with that required for the three different nitrogen and oxygen states, thus validating the proposed assignments.