Highly Hydrogenated Monolayer Graphene with Wide Band Gap Opening

Abstract

A thorough spectroscopic characterisation of highly hydrogenated monolayer graphene transferred on nickel grids is herein reported. The hydrogen bonding was found to be favoured for a more distorted graphene lattice. With X ray photoemission spectroscopy on the C 1s core-level, a 100$\%$ $sp^3$ distortion was observed after the hydrogenation of a more $sp^3$-like defected graphene, while a flatter, more $sp^2$-arranged, graphene reached a 62$\%$ $sp^3$ saturation. Electron energy loss spectroscopy showed the $π$-plasmon excitation quenching for the 100$\%$ $sp^3$ sample and a significant reduction for the other one. The high loading levels of hydrogenation led to the opening of a wide optical band gap (6.3 and 6.2 eV). The C-H stretching vibrational mode was also observed, as a direct footprint of graphene hydrogenation. Finally, valence band measurements of the 62$\%$ saturated sample suggest the coexistence of one-side and two-side hydrogenation morphologies.

Figures (6)

• Figure 1: XPS C 1s spectra are reported on the top panel for sample A acquired at 0 kL,120 kL and 320 kL (left-to-right), on the bottom panel for sample B at 0 kL,140 kL and 260 kL (left-to-right). The black dots represent the experimental data, the red solid line is the sum of the fitting curves, the grey dashed line is the background and the fit profiles follow the colour code reported in the legends.
• Figure 2: Area ratio $sp^2$/($sp^2$+$sp^3$) (blue) and $sp^3$/($sp^2$+$sp^3$) (green) as a function of the H dose for sample A (a) and for sample B (b). The green dashed line in (a) represents the exponential fit of the curve associated to the $sp^3$ area and the green solid line the saturation level resulted from the fit.
• Figure 3: EELS spectra at increasing H dose are reported on the left for sample A acquired at 0 kL,120 kL and 240 kL (dark to light blue) and in the middle panel for sample B at 0 kL,140 kL and 260 kL (dark to light blue). On the right, a comparison between the EELS spectrum measured on an Ni TEM grid without graphene ("Empty grid") in black and on sample B after 260 kL of H dose in light blue is shown. Two lines at 6 and 3.4 eV were added as guides for the eye.
• Figure 4: EELS spectra of the vibrational region measured on sample A (left) and B (right) before and after the exposure to atomic hydrogen at saturation (320 kL for sample A and 260 kL for sample B). The spectra before hydrogenation are shown in dark blue and in light blue after exposure. A dashed line at 350 meV is also added as a guide for the eye.
• Figure 5: Square of the EELS spectra measured for sample A at 240 kL (left) and sample B at 260 kL (right). In both the plots, the dashed line represents the straight line drawn in the region of the transition onset.