▎ 摘　　要

We elucidate how graphene bilayers form on Ir(111). Low-energy electron diffraction (LEED) reveals that the two graphene layers are not always rotationally aligned., Monitoring this misalignment during growth shows that second layer islands nucleate between the existing layer and the substrate. This mechanism occurs both when C segregates from the Ir and when elemental C is deposited from above, Low-energy electron microscopy (LEEM) and angle resolved photoemission spectroscopy (ARPES) show that second layer nucleation occurs preferentially under the first-layer rotational variants that are more weakly bound to the substrate. New layer nucleation tends to occur inhomogeneously at substrate defects. Thus new layer nucleation should be rapid on substrates that weakly graphene, making growth unstable toward mound formation initiated at substrate defects. In contrast stronger binding permits layer-by-layer growth, as for Ru(0001). ARPES shows that bilayer graphene has two slightly p-doped pi-bands. The work function of bilayer graphene Is dominated by the orientation of the bottom layer.