Fig. 2. De-wetting of a thick film and island formation.

a A thick water film prepared at 100 K has low helium reflectivity, indicating a rough surface (time < 200 s). The temperature is increased to 110 K, between 100 and 300 s, and the reflectivity rises later, as the graphene substrate is revealed (time > 600 s)—see main text. b and c are cartoons indicating the morphology of the surface at 100 and 110 K respectively, with water in blue and the graphene/Ni substrate in grey/green. In both cases it is probably amorphous solid water that forms. At 110 K de-wetting of the surface occurs to reveal pristine graphene. The scale bar gives a qualitative indication of the island separation (see Supplementary Note 1). d De-wetting is confirmed as helium diffraction from the graphene film, before water adsorption (grey dashed curve), is identical to that from the thick water film after heating to 110 K (red curve). Both curves are measured at 110 K with an incident beam energy, E_i = 8 meV, plotted with different ordinate limits on the left- and right-hand side. The hexagon shows the principle symmetry directions of the surface Brillouin zone.