The XRR measurements on bare liquid copper and the copper surface covered with the CVDgrown graphene layer were performed in situ at T = 1400 K in a reactor for CVD growth on a liquid metal catalyst. Diluted methane gas was used as a precursor and the CH4/H2 ratio was 13×10 -3 (constant flow), 9×10 -3 (injection after accumulation for 30 s followed by a constant flow), and 13×10 -3 (injection after accumulation for 20 s followed by a constant flow) for growths 1, 2, and 3 respectively. More details on the growth conditions of the single-atom-high graphene layer are described elsewhere.