title: Imaging transient melting of a nanocrystal using an X-ray laser.
abstract: There is a fundamental interest in studying photoinduced dynamics in nanoparticles and nanostructures as it provides insight into their mechanical and thermal properties out of equilibrium and during phase transitions. Nanoparticles can display significantly different properties from the bulk, which is due to the interplay between their size, morphology, crystallinity, defect concentration, and surface properties. Particularly interesting scenarios arise when nanoparticles undergo phase transitions, such as melting induced by an optical laser. Current theoretical evidence suggests that nanoparticles can undergo reversible nonhomogenous melting with the formation of a core-shell structure consisting of a liquid outer layer. To date, studies from ensembles of nanoparticles have tentatively suggested that such mechanisms are present. Here we demonstrate imaging transient melting and softening of the acoustic phonon modes of an individual gold nanocrystal, using an X-ray free electron laser. The results demonstrate that the transient melting is reversible and nonhomogenous, consistent with a core-shell model of melting. The results have implications for understanding transient processes in nanoparticles and determining their elastic properties as they undergo phase transitions.
authors.affiliation: London Centre for Nanotechnology, University College London, London WC1E 6BT, United Kingdom; Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, CA 94025; Center for Free-Electron Laser Science, Deutsches Elektronensynchrotron, 22607 Hamburg, Germany; jn2clark@slac.stanford.edu.
authors.forename: Jesse N
authors.initials: JN
authors.lastname: Clark
authors.affiliation: London Centre for Nanotechnology, University College London, London WC1E 6BT, United Kingdom;
authors.forename: Loren
authors.initials: L
authors.lastname: Beitra
authors.affiliation: London Centre for Nanotechnology, University College London, London WC1E 6BT, United Kingdom;
authors.forename: Gang
authors.initials: G
authors.lastname: Xiong
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;
authors.forename: David M
authors.initials: DM
authors.lastname: Fritz
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;
authors.forename: Henrik T
authors.initials: HT
authors.lastname: Lemke
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;
authors.forename: Diling
authors.initials: D
authors.lastname: Zhu
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;
authors.forename: Matthieu
authors.initials: M
authors.lastname: Chollet
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;
authors.forename: Garth J
authors.initials: GJ
authors.lastname: Williams
authors.affiliation: Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;
authors.forename: Marc M
authors.initials: MM
authors.lastname: Messerschmidt
authors.affiliation: Australian Research Council Centre of Excellence for Advanced Molecular Imaging, La Trobe University, Bundoora, VIC 3086, Australia;
authors.forename: Brian
authors.initials: B
authors.lastname: Abbey
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439;
authors.forename: Ross J
authors.initials: RJ
authors.lastname: Harder
authors.affiliation: Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, United Kingdom; Research Complex at Harwell, Didcot, Oxfordshire OX11 0DE, United Kingdom;
authors.forename: Alexander M
authors.initials: AM
authors.lastname: Korsunsky
authors.affiliation: Department of Physics, Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom.
authors.forename: Justin S
authors.initials: JS
authors.lastname: Wark
authors.affiliation: Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, CA 94025;
authors.forename: David A
authors.initials: DA
authors.lastname: Reis
authors.affiliation: London Centre for Nanotechnology, University College London, London WC1E 6BT, United Kingdom; Research Complex at Harwell, Didcot, Oxfordshire OX11 0DE, United Kingdom;
authors.forename: Ian K
authors.initials: IK
authors.lastname: Robinson
keywords: X-ray laser; coherent diffraction; phase transition; pump-probe; ultrafast imaging