title: Rippling ultrafast dynamics of suspended 2D monolayers, graphene.
abstract: 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.
authors: Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125; jhu@caltech.edu.
authors: Jianbo
authors: J
authors: Hu
authors: Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125.
authors: Giovanni M
authors: GM
authors: Vanacore
authors: Theory and Simulation of Materials and National Centre for Computational Design and Discovery of Novel Materials, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
authors: Andrea
authors: A
authors: Cepellotti
authors: Theory and Simulation of Materials and National Centre for Computational Design and Discovery of Novel Materials, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
authors: Nicola
authors: N
authors: Marzari
authors: Physical Biology Center for Ultrafast Science and Technology, Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125.
authors: Ahmed H
authors: AH
authors: Zewail
keywords: 2D materials; first-principles lattice dynamics; monolayer graphene; rippling dynamics; ultrafast electron diffraction