STM images of the edges of the graphene islands. (a) 23 × 23 nm2 STM image of a monolayer on top of the (3 × 3) SiC-reconstructed surface. This image was recorded at a high tunnel voltage (-2.5 V) with a current of 0.2 nA. With such high voltage, only the (3 × 3) SiC surface can be imaged on the bare SiC and through the monolayer. One can still distinguish that the edges of the monolayer are brighter than the center. It corresponds to a height difference of 40 pm. (b) To get a more detailed image of the edges, a 6.6 × 4.6 nm2 STM image was recorded at low tunnel voltage (+10 mV) with a current of 0.1 nA. In these experimental conditions, the graphene lattice can be probed with an atomic resolution. The graphene edge is folded and bent towards the SiC surface. The height of the fold is 50 pm. This particular shape is consistent with the compressive stress deduced from the Raman spectra. This compressive stress is occured by the thermal stress because of the cooling down of the sample after the growth and because the graphene edges seem to be bound to the SiC surface.