Studies of intrinsic hot phonon dynamics in suspended graphene by transient absorption microscopy.

Correlated transient absorption and atomic force microscopy (AFM) measurements have been performed for monolayer graphene, both free-standing and supported on a glass substrate. The AFM images allow us to locate regions of the suspended graphene. The transient absorption traces show a fast instrument response limited decay, followed by a slower intensity dependent decay. The fast decay is assigned to a combination of coupling between the excited charge carriers and the optical phonon modes of graphene and the substrate, and diffusion of the charge carrier out of the probe region. The slow decay is due to the hot phonon effect and reflects the lifetime of the intrinsic optical phonons of graphene. The time constant for the slow decay is longer for suspended graphene compared to substrate-supported graphene. This is attributed to interactions between the excited charge carriers and the surface optical phonon modes of the substrate, which supplies an additional relaxation channel for supported graphene.