Kiseok Chang

The nature of photoinduced phase transition and metastable states in vanadium dioxide

Photoinduced threshold switching processes that lead to bistability and the formation of metastable phases in photoinduced phase transition of VO2 are elucidated through ultrafast electron diffraction and diffusive scattering techniques with varying excitation wavelengths. We uncover two distinct regimes of the dynamical phase change: a nearly instantaneous crossover into an intermediate state and its decay led by lattice instabilities over 10 ps timescales. The structure of this intermediate state is identified to be monoclinic, but more akin to M2 rather than M1 based on structure refinements. The extinction of all major monoclinic features within just a few picoseconds at the above-threshold-level (~20%) photoexcitations and the distinct dynamics in diffusive scattering that represents medium-range atomic fluctuations at two photon wavelengths strongly suggest a density-driven and nonthermal pathway for the initial process of the photoinduced phase transition. These results highlight the critical roles of electron correlations and lattice instabilities in driving and controlling phase transformations far from equilibrium.

Nanoconfinement effects on the reversibility of hydrogen storage in ammonia borane: A first-principles study

We investigate atomistic mechanisms governing hydrogen release and uptake processes in ammonia borane (AB) within the framework of the density functional theory. In order to determine the most favorable pathways for the thermal inter-conversion between AB and polyaminoborane plus H(2), we calculate potential energy surfaces for the corresponding reactions. We explore the possibility of enclosing AB in narrow carbon nanotubes to limit the formation of undesirable side-products such as the cyclic compound borazine, which hinder subsequent rehydrogenation of the system. We also explore the effects of nanoconfinement on the possible rehydrogenation pathways of AB and suggest the use of photoexcitation as a means to achieve dehydrogenation of AB at low temperatures.

Transforming carbon nanotubes by silylation: An ab initio study

We use ab initio density functional calculations to study the chemical functionalization of single-wall carbon nanotubes and graphene monolayers by silyl (SiH(3)) radicals and hydrogen. We find that silyl radicals form strong covalent bonds with graphene and nanotube walls, causing local structural relaxations that enhance the s p(3) character of these graphitic nanostructures. Silylation transforms all carbon nanotubes into semiconductors, independent of their chirality. Calculated vibrational spectra suggest that specific frequency shifts can be used as a signature of successful silylation.

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.

Active control of bright electron beams with RF optics for femtosecond microscopy

A frontier challenge in implementing femtosecond electron microscopy is to gain precise optical control of intense beams to mitigate collective space charge effects for significantly improving the throughput. Here, we explore the flexible uses of an RF cavity as a longitudinal lens in a high-intensity beam column for condensing the electron beams both temporally and spectrally, relevant to the design of ultrafast electron microscopy. Through the introduction of a novel atomic grating approach for characterization of electron bunch phase space and control optics, we elucidate the principles for predicting and controlling the phase space dynamics to reach optimal compressions at various electron densities and generating conditions. We provide strategies to identify high-brightness modes, achieving ∼100 fs and ∼1 eV resolutions with 106 electrons per bunch, and establish the scaling of performance for different bunch charges. These results benchmark the sensitivity and resolution from the fundamental beam brightness perspective and also validate the adaptive optics concept to enable delicate control of the density-dependent phase space structures to optimize the performance, including delivering ultrashort, monochromatic, high-dose, or coherent electron bunches.

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.

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.