▎ 摘　　要

Contamination on graphene is a critical problem that has limited the applications of graphene despite its innate versatility. However, the structures of such contamination remain unclear. In this study, we investigated the structure of a contaminated region growing on free-standing monolayer graphene in situ at room temperature using an aberration-corrected scanning transmission electron microscope. Energy-dispersive X-ray spectroscopy showed that carbon atoms were the dominant element of the growing contamination. We quantitatively confirmed that the growing contamination consisted of several layers of nanoscale graphene flakes on pristine single-layer graphene. Electron energy-loss spectroscopy indicated that the hexagonal carbon lattice deforms considerably from the pristine one at the higher layers. Analysis of the three-dimensional atomic structure revealed that the atoms in the third layer were the suppliers of the second layer at the step edge. In addition, we found that the lattice constant of pristine graphene in the region of the first layer was larger than that of the bulk graphite crystals, and that it gradually decreased as the second layer grew on the first layer. The findings of this paper will be helpful not only for the control of nanographene but also for a basic understanding of the intrinsic features of graphene. (C) 2018 Elsevier Ltd. All rights reserved.