Rippling ultrafast dynamics of suspended 2D monolayers, graphene
Here, using ultrafast electron crystallography (UEC), we report the observation of rippling dynamics in suspended monolayer graphene, the prototypical and most-studied 2D material. The high scattering cross-section for electron/matter interaction, the atomic-scale spatial resolution, and the ultrafast temporal resolution of UEC represent the key elements that make this technique a unique tool for the dynamic investigation of 2D materials, and nanostructures in general. We find that, at early time after the ultrafast optical excitation, graphene undergoes a lattice expansion on a time scale of 5 ps, which is due to the excitation of short-wavelength in-plane acoustic phonon modes that stretch the graphene plane. On a longer time scale, a slower thermal contraction with a time constant of 50 ps is observed and associated with the excitation of out-of-plane phonon modes, which drive the lattice toward thermal equilibrium with the well-known negative thermal expansion coefficient of graphene. From our results and first-principles lattice dynamics and out-of-equilibrium relaxation calculations, we quantitatively elucidate the deformation dynamics of the graphene unit cell.
Additional Information© 2016 National Academy of Sciences. Edited by Jacqueline K. Barton, California Institute of Technology, Pasadena, CA, and approved September 6, 2016 (received for review July 6, 2016). Published online before print October 10, 2016. The authors gratefully acknowledge the assistance of Dr. B. Chen and Dr. X. Fu for TEM characterization and Dr. B. Liao for helpful discussion. This work was supported by the Air Force Office of Scientific Research in the Center for Physical Biology at California Institute of Technology funded by the Gordon and Betty Moore Foundation, the Swiss National Science Foundation through Grant 200021_143636, the Max Planck–Ecole Polytechnique Fédérale de Lausanne (EPFL) Center for Molecular Nanoscience and Technology, and the Swiss National Supercomputing Center (CSCS). Author contributions: J.H., G.M.V., and A.H.Z. designed research; J.H., G.M.V., A.C., N.M., and A.H.Z. performed research; J.H., G.M.V., A.C., N.M., and A.H.Z. analyzed data; and J.H., G.M.V., A.C., N.M., and A.H.Z. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission.
Published - PNAS-2016-Hu-E6555-61.pdf