i K. Raman

Direct Observation of Optically Induced Transient Structures in Graphite Using Ultrafast Electron Crystallography

We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of < or =21 mJ/cm2, lattice vibrations are observed to thermalize on a time scale of approximately 8 ps. At higher fluences approaching the damage threshold, lattice vibration amplitudes saturate. Following a marked initial contraction, graphite is driven nonthermally into a transient state with sp3-like character, forming interlayer bonds. Using ab initio density functional calculations, we trace the governing mechanism back to electronic structure changes following the photoexcitation.

Dynamics of Size-Selected Gold Nanoparticles Studied by Ultrafast Electron Nanocrystallography

We report the studies of ultrafast electron nanocrystallography on size-selected Au nanoparticles (2-20 nm) supported on a molecular interface. Reversible surface melting, melting, and recrystallization were investigated with dynamical full-profile radial distribution functions determined with subpicosecond and picometer accuracies. In an ultrafast photoinduced melting, the nanoparticles are driven to a nonequilibrium transformation, characterized by the initial lattice deformations, nonequilibrium electron-phonon coupling, and, upon melting, the collective bonding and debonding, transforming nanocrystals into shelled nanoliquids. The displasive structural excitation at premelting and the coherent transformation with crystal/liquid coexistence during photomelting differ from the reciprocal behavior of recrystallization, where a hot lattice forms from liquid and then thermally contracts. The degree of structural change and the thermodynamics of melting are found to depend on the size of nanoparticle.

The development and applications of ultrafast electron nanocrystallography.

We review the development of ultrafast electron nanocrystallography as a method for investigating structural dynamics for nanoscale materials and interfaces. Its sensitivity and resolution are demonstrated in the studies of surface melting of gold nanocrystals, nonequilibrium transformation of graphite into reversible diamond-like intermediates, and molecular scale charge dynamics, showing a versatility for not only determining the structures, but also the charge and energy redistribution at interfaces. A quantitative scheme for 3D retrieval of atomic structures is demonstrated with few-particle (<1,000) sensitivity, establishing this nanocrystallographic method as a tool for directly visualizing dynamics within isolated nanomaterials with atomic scale spatio-temporal resolution.

Ultrafast imaging of photoelectron packets generated from graphite surface

We present an electron projection imaging method to study the ultrafast evolution of photoelectron density distribution and transient fields near the surface. The dynamical profile of the photoelectrons from graphite reveals an origin of a thermionic emission, followed by an adiabatic process leading to electron acceleration and cooling before a freely expanding cloud is established. The hot electron emission is found to couple with a surface charge dipole layer formation, with a sheet density several orders of magnitude higher than that of the vacuum emitted cloud.

Photovoltage dynamics of the hydroxylated Si(111) surface investigated by ultrafast electron diffraction

We present a novel method to measure transient photovoltage at nanointerfaces using ultrafast electron diffraction. In particular, we report our results on the photoinduced electronic excitations and their ensuing relaxations in a hydroxyl-terminated silicon surface, a standard substrate for fabricating molecular electronics interfaces. The transient surface voltage is determined by observing Coulomb refraction changes induced by the modified space-charge barrier within a selectively probed volume by femtosecond electron pulses. The results are in agreement with ultrafast photoemission studies of surface state charging, suggesting a charge relaxation mechanism closely coupled to the carrier dynamics near the surface that can be described by a drift-diffusion model. This study demonstrates a newly implemented ultrafast diffraction method for investigating interfacial processes, with both charge and structure resolution.

Electronically driven fragmentation of silver nanocrystals revealed by ultrafast electron crystallography.

We report an ultrafast electron diffraction study of silver nanocrystals under surface plasmon resonance excitation, leading to a concerted fragmentation. By examining simultaneously transient structural, thermal, and Coulombic signatures of the prefragmented state, an electronically driven nonthermal fragmentation scenario is proposed.

Progress of Dynamical Nanomaterial Imaging Using Ultrashort Electron Pulses

A driving force behind the development of ultrafast electron crystallography and microscopy is the recognition that there are a broad range of phase change materials that respond to photoexcitation in unusual pathways compared to those responding to applying pressure, heat, and chemical doping. Since the morphology of the nanomaterials play a role in determining their optical responses, the ability to resolve atomic structure and the transient dynamics that lead to various phase change states are crucial for these studies[1]. First, we will discuss the recent progress at MSU of using ultrafast electron crystallography and related methods to investigate nanomaterial assemblies[2]. We show that the dynamical responses of materials to a short-pulse (fs) laser excitation are multisphased, involving progressive transformations, from the initial femtosecond electronic excitation to ps and ns structural changes. In the study of the optically induced fragmentation of Ag nanocrystals excited at surface plasmon resonance, we find that the dominant dynamical feature in the prefragmentation stage is a defect-mediated instability growth, creating sub-nanocrystalline domains with hot surface and relatively cold core, based on diffraction refinements using a large supercell model[2], as shown in Fig. 1. From the defect density growth process, we suggest that the fragementaion is percolative, starting from valence modification as evidenced by creating topological defects, which are initially seeded in Ag through the strong nonlinear coupling between SPR and interband transition, causing chemical bonds to rupture. These defects will later percolates into larger stripes, thus causing the nanoparticles to fracture. Such creation and growth of topological defects, which can persist on the phonon timescales, could be common in nonequilibrium photoinduced structural phase transition. Further improving the temporal resolution to the phonon timescale (10-100fs) will allow more definitive traces of the initial nonlinear photoassisted electron-phonon coupling that seeds the reactions to be resolved.