pubs.acs.org/NanoLett
Ultrafast Transient Absorption Microscopy Studies of Carrier Dynamics in Epitaxial Graphene Libai Huang,*,† Gregory V. Hartland,‡ Li-Qiang Chu,§ Luxmi,⊥ Randall M. Feenstra,⊥ Chuanxin Lian,| Kristof Tahy,| and Huili Xing| †
Notre Dame Radiation Laboratory, ‡ Department of Chemistry and Biochemistry, § Department of Chemical and Biomolecular Engineering, and | Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556-5670 and ⊥ Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 ABSTRACT Transient absorption microscopy was employed to image charge carrier dynamics in epitaxial multilayer graphene. The carrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxation times were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limiting value of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial heterogeneity in the dynamics was observed due to differences in coupling between graphene layers and the substrate. KEYWORDS Transient absorption imaging, epitaxial graphene, carrier dynamics, electron-phonon coupling
G
raphene is a two-dimensional material with a single atomic layer of carbon atoms arranged in a hexagonal lattice. Due to its unique structure, graphene exhibits unusual optical and electronic properties.1,2 In particular, the ballistic nature of carrier transport in graphene makes it highly desirable for applications such as nanoscale field effect transistors and single-electron transistors.3 Recent research efforts have successfully produced graphene by mechanical exfoliation,2 epitaxial growth,4 and chemical synthesis.5-7 Epitaxial growth is a promising approach for applications, as it has the ability to prepare graphene on a large scale and supported on a substrate. In this process, vacuum graphitization of SiC at high temperatures results in 1-40 layers of graphene.4 Despite the multilayer structure, evidence suggests that these samples have many of the characteristics of single-layer graphene.8 Energy exchange between the electrons and phonons is particularly important to electron transport, and understanding this process will be vital for the realization of future graphene-based electronics. Transient absorption spectroscopy is a powerful tool to probe energy relaxation of photoexcited carriers and has been applied extensively to closely related carbon nanotubes.9-11 There have been several reports on the ultrafast dynamics of charge carriers in graphene, using optical pump-probe12-14 and terahertz techniques.15,16 However, understanding of the carrier relaxation pathways is far from complete. For example, most experiments see a fast decay of several hundred femtosec-
onds, followed by a slower picosecond decay. The slower decay has been attributed to minority carrier recombination16 or coupling between electrons and acoustic phonons.17 There is also debate about whether the dynamics are carrier concentration dependent.15 Furthermore, almost all measurements reported thus far have been carried out with low spatial resolution. However, epitaxially grown graphene is highly inhomogeneous, with variations in the sample thickness occurring over length scale of a few micrometers.4 This means that these measurements integrated over a distribution of numbers of layers, making it difficult to interpret how the number of layers affects the dynamics.14 In addition to variation in thickness, recent Raman measurement also revealed Raman peak shifts resulting from inhomogeneity in the graphene/substrate interaction.18,19 It is not clear how substrate interactions and doping17 affect the dynamics. To pave the road for electronic devices based on epitaxial graphene, characterization methods with high spatial resolution are needed to understand these effects. In this Letter, we report transient absorption microscopy as a novel tool to characterize graphene and to interrogate the charge carrier dynamics. This technique has the ability to directly image carrier dynamics with a diffraction-limited spatial resolution. The intensity of the transient absorption signal is shown to correlate with the number of graphene layers. The carrier cooling exhibits a biexponential decay, consisting of an instrument-response-limited fast decay time τ1 (
