Postdoctoral Scholar, California Institute of Technology
The study of atomic motions and bond dilation on the time scale of nuclear motion.
Ultrafast electron diffraction (UED) with atomic scale spatial and temporal resolution is a powerful tool that provides unique windows into the correlated atomic motions, bond dilation, and structural transformation of various materials on the time scale of nuclear motion by recording the transient non-equilibrium diffraction patterns with sub-picosecond temporal resolution following laser excitation. Using UED paradigms that correlate structure, dynamics, and functionality can be established, leading to a full understanding of light-matter interactions. To accomplish this task, a femtosecond laser pulse initiate the dynamics in an atomically-thin sample that are subsequently probed using accelerated electron pulses. Electron probes are sensitive to both nuclei and electrons. Additionally, they have much higher scattering cross sections and remarkably less radiation damage, which makes them ideal to study material structures, especially atomically-thin layers such as graphene and 2D materials. In order to spatially resolve a molecular structure, the wavelength required is sub-angstroms, which can be easily obtained using accelerated electrons.The atoms in a specimen act as scattering centers for incident electrons, and each atom becomes a coherent source of an outgoing spherical wave. The electron diffraction experiment thus becomes a conceptual analog of the well-known multi-slit diffraction experiment, but at the molecular length scale. Thus, UED is an advantageous technique to investigate the movement of atoms in real time.
Abstract: Ultrafast electron diffraction is employed to spatiotemporally visualize the lattice dynamics of 11 nm-thick single-crystal and 2 nm-thick polycrystalline gold nanofilms. Surprisingly, the electron-phonon coupling rates derived from two temperature simulations of the data reveal a faster interaction between electrons and the lattice in the case of the single-crystal sample. We interpret this unexpected behavior as arising from quantum confinement of the electrons in the 2 nm-thick gold nanofilm, as supported by absorption spectra, an effect that counteracts the expected increase in the electron scattering off surfaces and grain boundaries in the polycrystalline materials.
Pub.: 08 Apr '17, Pinned: 28 Jun '17