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.

Figure 1

Differential reflectivity (normalized at $t=0$) for a ten-layer sample as a function of the probe delay at different probe energies. (Inset) The electronic band structure of graphene at the K point of the Brillouin zone with the relevant pump and probe photon energies shown.

Figure 2

(a) Differential reflectivity as a function of the probe delay for samples of different thickness, measured using a probe wavelength of 1120 nm. (b) Extracted phonon lifetimes as a function of layer number. Averaged values per layer are also plotted with the standard deviation as an error bar. The dotted line is a guide for the eye.

Figure 3

(a) Differential reflectivity for suspended mono- and few-layer graphene. (b) Extracted phonon lifetimes for mono- and few-layer suspended graphene plotted alongside the averaged values extracted from the supported flakes. (c) An atomic force micrograph of a suspended flake covering a 2.5-$\mu$m, 290-nm-deep hole.