Magnetic edge-states in nanographene, HNO3-doped nanographene and its residue compounds of nanographene-based nanoporous carbon

Abstract

We investigated the magnetic and electronic properties of nanographene and its charge transfer effect, using near edge X-ray absorption fine structure (NEXAFS), magnetic susceptibility and ESR measurements, and elemental analysis, with the employment of nanoporous carbon, which consists of a three dimensional disordered network of loosely stacked nanographene sheets, in relation to the host–guest interaction with HNO₃ as the electron-accepting guest. The adsorption of electron acceptor HNO₃ decreases the intensity of the edge state peak in NEXAFS as a result of the charge-transfer-induced Fermi energy downshift, in agreement with the decrease in the edge-state spin concentration, and it also induces the structural expansion, which makes the inter-nanographene sheet distance elongated, resulting in weakening of the inter-nanographene-sheet antiferromagnetic interaction as evidenced by the decrease in the Weiss temperature. In addition, the decomposition of HNO₃, which takes place with the electron-rich edge state as an oxidation catalyst, results in the creation of oxygen/nitrogen-containing functional groups bonded to the periphery of the nanographene sheets. Heat-treatment of the HNO₃-ACFs under evacuation desorbs the HNO₃ molecules completely, though a part of the oxygen/nitrogen-containing species remains strongly bonded to the edge even at a high temperature of ∼800 °C, according to NEXAFS and elemental analysis results. These remaining species participate in the charge transfer, modifying the electronic structure as observed with the decrease in the orbital susceptibility and the strengthening of the inter-nanographene-sheet antiferromagnetic interaction.