▎ 摘　　要

Using low-energy electron microscopy (LEEM), we have measured the local concentration of mobile carbon adatoms from which graphene sheets form on a Ru(0001) surface, and simultaneously, the growth rates of individual graphene islands. Graphene crystal growth on Ru differs strikingly from that of two-dimensional metal islands on metals: (i) C adatoms experience a large energy barrier to attaching to graphene step edges, so adatom diffusion does not limit growth. (ii) The supersaturations needed for appreciable growth rates are comparable to those required to nucleate islands. (iii) The growth rate is a highly nonlinear function of supersaturation, with a large activation energy (2.0 +/- 0.1 eV). Our analysis suggests that graphene grows by adding rare clusters of about five atoms rather than adding the abundant monomers ( adatoms). Knowing the growth mechanism and monitoring the supersaturation, we can control the pattern of growing graphene sheets.