Optical spectroscopy study on the effect of hydrogen adsorption on graphene

The effect of increasing hydrogen adsorption on graphene is investigated using optical transmission spectroscopy over a wide spectrum, from the far-infrared (FIR) to the UV domain. For low hydrogen concentration, the absorption intensities of the interband transitions occurring in the Dirac band (mid-IR and visible) and the M-point van Hove singularity (UV) decrease with increasing hydrogen coverage. This H-coverage dependent spectral change is quantified successfully using the effective medium theory. For highest hydrogen coverage, the optical absorbance decrease culminates in an actual band-gap opening of more than 6 eV. These measurements provide experimental confirmation of predicted large values of direct bandgap transitions in one-sided hydrogenated graphene. Finally, the optical conductivity in the Far-IR regime is behaving in a non-Drude type manner along with the hydrogenation, implying H- induced localization of the free Dirac π electrons.