▎ 摘　　要

We examine the ultrafast carrier phonon dynamics in graphene flakes with different lateral sizes. By using a size-selective centrifugation technique, we create graphene films with graphene flake sizes in the region of 120-450 nm. The transient transmission exhibits two-step relaxation, which are attributed to carrier thermalization followed by hot phonon cooling. We find that the cooling of the hot phonons proceeds faster, by a factor of three, for the smallest graphene flakes. (C) 2014 AIP Publishing LLC.