Effect of Substrate and Morphology on the Relaxation Dynamics of Carriers and Phonons in Graphene

Abstract

Graphene, the two dimensional allotrope of the carbon family, exhibits extremely high electron mobility, thermal conductivity and fascinating ultrafast carrier-carrier and carrier-phonons interactions. However, being merely one atom thick, introducing a substrate or altering the morphological form of graphene can affect both its equilibrium and non-equilibrium dynamics and inevitably influence the performance of graphene-based devices. In the first part of this dissertation, we use fluence and energy dependent ultrafast pump-probe spectroscopy to determine the effect of substrate on the femtosecond transient electron and phonon dynamics of single layer graphene transferred on sapphire, quartz and single crystalline diamond. Using a multi-channel cooling theory involving surface phonons of the substrate, intrinsic optical phonons of graphene and the corresponding competing scattering rates, we proceed to explain the strong substrate-dependent dynamics of graphene observed in our experiments. We stipulate that the sub-nm surface roughness of the studied substrates, enable a strong coupling between the phototexcited carriers in graphene and the surface vibrational modes of the polar substrates. We show that this additional energy relaxation pathway can compete with the intrinsic phonons of graphene to not only reduce the transient electron temperature but also the carrier and optical phonon lifetimes in graphene. In the second part of this dissertation, we introduce a methodology for fabrication of a novel quasi-one dimensional morphology of graphene called curled graphene ribbons (CGR). Our gate dependent scanning photocurrent measurements reveal an astounding two orders of magnitude enhancement in the photocurrent response of CGR which we attributed to the photothermoelectric effect (PTE).Understanding how the equilibrium and non-equilibrium dynamics of carriers and phonons in graphene are altered by the interface or morphology and deciphering the various energy relaxation pathways, will pave the way towards realization of higher performance graphene based electronics and optoelectronics.