The heat treatment effect on the electronic and magnetic structures of a disordered network of nanographene sheets has been investigated by in situ measurements of X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), and electrical conductance, together with temperature-programmed desorption measurements. Oxygen-containing functional groups bonded to nanographene edges in the pristine sample are almost completely decomposed under heat treatment up to 1300-1500 K, resulting in the formation of edges primarily terminated by hydrogen. The removal of the oxygen-containing groups enhances the conductance owing to the decrease in the electron transport barriers between nanographene sheets. Heat treatment above 1500 K removes also the hydrogen atoms from the edges, promoting the successive fusion of nanographene sheets at the expense of edges. The decrease in the π* peak width in NEXAFS indicates the progress of the fusion reaction, that is, the extension of the π-conjugation, which agrees with the increase in the orbital susceptibility previously reported. The fusion leads to the formation of local π/sp(2) bridges between nanographene sheets and brings about an insulator-to-metal transition at 1500-1600 K, at which the bridge network becomes infinite. As for the magnetism, the intensity of the edge state peak in NEXAFS, which corresponds to the number of the spin-polarized edge states, decreases above 1500 K, though the effective edge-state spin density per edge state starts decreasing at approximately 200 K lower than the temperature of the edge state peak change. This disagreement indicates the development of antiferromagnetic short range ordering as a precursor of a spin glass state near the insulator-metal transition, at which the random network of inter-nanographene-sheet exchange interactions strengthened with the formation of the π/sp(2) bridges becomes infinite.

We investigated the device characteristics of a graphene field effect transistor (FET) of which interfaces were controlled by a self-assembled monolayer (SAM). Electrical transport measurements together with Raman spectroscopy characterizations for bilayer graphene (BLG) and single layer graphene (SLG) on micro-patterned SAM (mp-SAM), respectively, elucidate spatial carrier modulations on the graphene sheets driven by mp-SAM. The SLG-mp-SAM-FET device exhibits unconventional graphene p-n junction characteristics depending on the polarity of source-drain voltage. The observed characteristics can be interpreted as a velocity saturation of hole carriers coupled with polaron states, of which phonon energy is around 30 meV, on the SAM molecules at the graphene p-n junction. The SAM-based micro fabrication techniques presented in this report not only provide a spatial control of electronic properties for graphene but also lend a new perspective in the understanding of graphene-substrate interface based molecular self-assembled systems.

Abstract

We investigated the thermal oxidation process of nanographene using activated carbon fibers (ACFs) by thermogravimetry (TG), X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and electrical conductance measurements. The oxidation process started from the edge of nanographene with the formation of phenol (-OH) or ether (C-O-C) groups attached to edge carbon atoms, as verified by the XPS and NEXAFS results. While the TG results indicated a decrease in the size of the nanographene sheet during the oxidation process, the intensity of the edge-state peak, i.e., the signature of the zigzag edge, decreased in the C K-edge NEXAFS spectra. This suggests that the zigzag edge preferentially reacted with oxygen and that the nanographene terminated with the thermodynamically unstable zigzag edges converted to one terminated with stable armchair edges. As the oxidation temperature increased, the activation energy for the electron hopping transport governed by the Coulomb gap variable range hopping between the nanographene sheets increased, and the tunneling barrier decreased. This change can be understood on the basis of the decrease in the size of the nanographene sheets together with the preferential etching of nanographene edges and the decrease in the inter-nanographene-sheet distance.