A theoretical study on the formation mechanism and the sum-frequency generation spectra of hydrogenated graphene

Hydrogenated graphene (H-Gra) has garnered significant attention as a promising two-dimensional material, with its structural characteristics and formation mechanisms explored through various experimental and theoretical approaches, including sum-frequency generation (SFG) spectroscopy. Despite these efforts, the interpretation of SFG spectra remains contentious. In this study, we employ density functional theory to systematically investigate the stable configurations, adsorption energies, and electronic structures of graphene with varying numbers (n = 1-6) of adsorbed hydrogen atoms. Our results reveal that hydrogen atoms preferentially gather together as n increases, and the average adsorption energy per hydrogen atom is higher for even-numbered configurations than for odd-numbered ones. Furthermore, first-principles simulations of the SFG spectra of H-Gra uncover contributions from C-H stretching modes beyond the well-known symmetric stretching modes (υpsym and υosym). Specifically, additional modes, including υ^s, υH3sym, and υH4sym, corresponding to one, two, and three C-H bond stretchings, respectively, were identified. This work elucidates the formation mechanism of H-Gra via hydrogen gathering and provides insights into its SFG spectral features, offering potential guidance for its efficient synthesis and characterization.