▎ 摘　　要

A trace method for determining the trajectories of single metal atoms dispersed on graphene was developed using scanning transmission electron microscopy with angstrom-order accuracy. This method was applied to platinum (Pt) and gold (Au) single atoms dispersed on free-standing single-layer graphene to observe the difference in their dynamic behaviors under electron beam irradiation. High-angle annular dark-field imaging showed that most of the atoms were present at the nanographene edge, especially in three or higher layers of nanographene, but few on the terrace of graphene. According to trajectory analysis, approximately 70% of the Pt single atoms stayed within one atomic distance of graphene (1.4 angstrom) for the whole observation period (40 s). Therefore, Pt continued to be stably dispersed on graphene as single atoms. On the other hand, more than 40% of Au single atoms traveled a distance exceeding the size of the six-membered ring of graphene (2.8 angstrom) even within the minimum observation interval (4 s). Clearly, Au was confirmed to have a larger mobility than Pt under the same electron beam irradiation conditions, which may be one reason for the difficulty in keeping the single atom state for Au.