Layer number dependence of carrier lifetime in graphenes observed using time-resolved mid-infrared luminescence

Femtosecond luminescence of 1-, 2-, and 6-8-layer graphenes as well as graphite (n = infinity) is studied in the near- to mid-infrared region, using a luminescence up-conversion method. The luminescence decay profiles show non-exponential behaviors within several picoseconds. Compared to graphite, 1- and 2-layer graphenes exhibit a remarkable reduction of lifetime at 0.3 eV. A two-temperature model that considers an effective Fermi energy successfully reproduces the decay profiles. On the basis of the Fermi energy (0.2 eV) of 1-layer graphene obtained from the photoemission experiment, we ascribe the layer number dependence of lifetime to the effective Fermi energy difference. (C) 2015 Elsevier B.V. All rights reserved.