Figure 4: Origin of the C and O atoms and controllable O/C composition and bandgap energy in the GO sheets.

(a,b) Zero-loss filtered bright-field cross-sectional TEM image (a) of the Cu foil after DAS at 240 °C for 120 min (using an energy slit width of 10 eV to select the zero-loss peak) and the corresponding C map (b). The yellow rectangles denote the same location in the TEM images (scale bars, 0.2 μm). (c) Changes in the elemental composition of C⁻ and O⁻ containing species, based on TOF-SIMS analysis of the surfaces of an as-received Cu foil, an annealed Cu foil at 790 °C in ambient hydrogen, and a GO-covered Cu foil that was grown at 240 °C for 120 min. (d) Schematic diagram of GO growth mechanism during DAS process using solid C; C atoms began to diffuse through the Cu along the GBs of the foil while trapping the O species dissolved in the Cu, leading to the predominant diffusion of C–O molecules over Cu as well as through GBs. (e) Changes in the O/C composition and the calculated bandgap energy in the GO sheets that were formed on a Cu surface under three different conditions. The GO sheets were synthesized at 240 °C for 60 min with the Cu foil annealed at 790 °C for 20 min (type I); at 240 °C for 120 min with the Cu foil annealed at 790 °C for 20 min (type II); and at 240 °C for 120 min with the Cu foil annealed at 790 °C for 260 min (type III). Error bars are the standard deviation of the bandgap energy measured in five different samples grown under the same condition.