A Wide Optical-Gap in Fully $s{p}^3$-Like Hydrogenated Monolayer Graphene

Abstract

A comprehensive spectroscopic characterisation of two highly hydrogenated monolayer graphene samples transferred onto nickel grids is reported. With X ray photoemission spectroscopy on the C 1s core-level, a 100$\%$ $s{p}^3$ profile was observed upon hydrogenation of a more $s{p}^3$-like initially defected graphene, while a flatter, more $s{p}^2$-arranged, graphene reached a 62$\%$ $s{p}^3$ saturation. Low-energy reflection electron energy-loss spectroscopy (EELS) corroborates these findings through the $\pi$-plasmon excitation quenching for the fully $s{p}^3$ sample and a significant reduction for the partially converted one. The extreme surface sensitivity of low-energy reflection EELS enables extraction of the optical band gap of the hydrogenated layer even on a metallic support, yielding values of 6.3 and 6.2 eV for the two samples. The C--H stretching vibrational mode is also resolved, providing a direct fingerprint of graphene--hydrogen bonding. Finally, valence-band measurements of the $62\%$ saturated sample suggest the coexistence of one-sided and two-sided hydrogenation morphologies.