Hot phonon decay in supported and suspended exfoliated graphene

Near infrared pump-probe spectroscopy has been used to measure the ultrafast dynamics of photoexcited charge carriers in monolayer and multilayer graphene. We observe two decay processes occurring on 100-fs and 2-ps time scales. The first is attributed to the rapid electron-phonon thermalization in the system. The second time scale is found to be due to the slow decay of hot phonons. Using a simple theoretical model we calculate the hot phonon decay rate and show that it is significantly faster in monolayer flakes than in multilayer ones. We observe this enhanced decay rate in both supported and suspended flakes and thereby demonstrate that it has an intrinsic origin. Possible decay mechanisms, such as flexural phonons, that could cause such an enhancement are discussed.