Ryan A. Murdick

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.

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.

Ultrafast electron diffractive voltammetry: General formalism and applications

We present a general formalism of ultrafast diffractive voltammetry approach as a contact-free tool to investigate the ultrafast surface charge dynamics in nanostructured interfaces. As case studies, the photoinduced surface charging processes in oxidized silicon surface and the hot electron dynamics in nanoparticle-decorated interface are examined based on the diffractive voltammetry framework. We identify that the charge redistribution processes appear on the surface, sub-surface, and vacuum levels when driven by intense femtosecond laser pulses. To elucidate the voltammetry contribution from different sources, we perform controlled experiments using shadow imaging techniques and N-particle simulations to aid the investigation of the photovoltage dynamics in the presence of photoemission. We show that voltammetry contribution associated with photoemission has a long decay tail and plays a more significant role in the nanosecond timescale, whereas the ultrafast voltammetry are dominated by local charge transfer, such as surface charging and molecular charge transport at nanostructured interfaces. We also discuss the general applicability of the diffractive voltammetry as an integral part of quantitative ultrafast electron diffraction methodology in researching different types of interfaces having distinctive surface diffraction and boundary conditions.

Light-Induced Charge Carrier Dynamics at Nanostructured Interfaces Investigated by Ultrafast Electron Diffractive Photovoltammetry

We present an ultrafast photovoltammetry framework to investigate the surface charge carrier dynamics at the nanometer scale. This diffraction-based method utilizes the feature-gated nanomaterial diffraction pattern to identify the scattering sites and to deduce the associated charge dynamics from the nanocrystallographic refraction-shift observed in the ultrafast electron diffraction patterns. From applying this methodology on SiO2/Si interface, and surfaces decorated with nanoparticles and water–ice adsorbed layer, we are able to elucidate the localized charge injection, dielectric relaxation, and carrier diffusion, with direct resolution in the charge state and possibly correlated structural dynamics at these interfaces, which are central to nanoelectronics, photovoltaics, and photocatalysis development. These new results highlight the high sensitivity of the interfacial charge transfer to the nanoscale modification, environment, and surface plasmonics enhancement and demonstrate the diffraction-based ultrafast surface voltage probe as a unique and powerful method to resolve the nanometer scale charge carrier dynamics.