Science

1. Phase Transitions

2. Nanoacarbons

3. Solar Energy Utilization

4. Charge Transport

5. Nanoacoustics

Phase Transitions in Correlated Electron Materials

The correlated electron materials exhibit ordered electronic states caused by competition between different ground states driven by charge, orbital, spin, and lattice degrees of freedom. There are immense practical interests in these materials as electronic orderings give rise to superconductivity, colossal magnetoresistance, and ferroelectricity and magnetism. Femtosecond pump-probe spectroscopy has recently been demonstrated as a powerful tool to study the quasiparticle and collective excitation of electronically ordered states by following the temporal and spectral evolution of the optical response functions. The ability to use photons to transiently alter the local symmetry characters of either charge or orbital orders to induce changes of other degrees of freedoms, such as spin and lattice degrees of freedom to induce phase transitions between ordered states is a fascinating new frontier. By further incorporating atomic scale spatial resolution to the pump-probe studies a crucial new dimension is provided for the phase transformations to be directly characterized, thus offering the needed clarity to link the optically accessible single particle excited states to the hitherto hidden real-space manifestation of collective excitations, thus opening up a new vista of research exploring the excited states that bridge different electronic orders and the dynamical responses of symmetry breaking that underlies the fundamental physics.

Material Transformations on the Nanometer Energy Landscape

The light-induced structural transformation in nanostructures and molecular solids represents a new direction of research exploring the far-from-equilibrium state of matter. Different from the thermal or pressure induced phase transitions, using optical excitation it is possible to transiently change the nature of chemical bonds to enable new structural phase to form that is  inaccessible from thermodynamic preparations.  We aim to elucidate the essential physics by dynamically imaging the topological and charge dynamics central to these phenomena.  Recently, we are interested in  studying the photoinduced structural transformation of nanoparticles (1-50 nm) and layered/interfaced systems. The transformative nature of these finite systems will give us insight into how different material attributes emerge and diverge on the nanometer energy landscape so that new concept of engineering materials for different functions can be formed.  

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Photodynamics of Nanocarbons

The ability of modifying the material properties by photoexcitation derives from the strong interaction between electronic degree of freedom and certain arrangement of atoms at the excited state.  The transient localization of charges due to electronic excitation has the ability to impose concentrated Coulomb forces, dragging charges across the interfaces and sometimes even allowing modification of macroscopic atomic structures. Carbons, with its propensity to form a wide range of bonding networks in various forms and sizes, and appear to subject to influences from the environment to reconstruct their electronic and optical properties, promising applications in optoelectronic and sensing.   However, it is generally difficult to obtain direct insight into the mechanisms via which minute changes in bonding forces could affect the electronic and optical properties. We aim to shed light on the underlying mechanism via simultaneously monitoring the changes both in the electronic and atomic degrees of freedoms following selective photoexcitations in these materials.

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Solar Energy Utilization

Solar photovoltaic (PV) devices are of prime importance as a clean and efficient alternative to electricity generation using fossil fuels. We are exploring two avenues in solar photovoltaics.

1. Thin film materials: Hydrogenated amorphous silicon (a-Si:H) is one of the most important and inexpensive PV materials. A well-known limitation of the material is ‘light-induced degradation’ known as Staebler-Wronski Effect (SWE). The physical origins of SWE are not known, but are believed to be related to the creation of defects and manifest in an array of experiments implying structural changes. A fundamental understanding of microscopic processes in this representative system is a necessary part of the challenge to fully exploit PVs. Novel ultrafast electron diffraction (UED) as well as the transient photovoltaic measurements will be conducted to obtain information on microstructures and dynamics on the time scale (femto - pico seconds) accessible to accurate ab initio simulations. Nonthermal structural dynamics induced by sub-bandgap excitation relevant to defect formation and its annealability will be examined and compared to a thermal annealing process. The short-time radial distribution function analysis of UED data can directly map out changes of the local structures relevant to the degradation near the defect sites. In contrast, an above-gap short wavelength excitation and the longer time dynamics will also be examined; in those cases the annealing behavior will take place. The reversibility of these excitations will be systematically investigated for films prepared under different topographic conditions, such as structures with completely random network, or those embedded with micro- and nano-scale crystalline domains. These structures possess different defect density and carrier mobility that will affect SWE. The structure-function correlation related to these attributes can also be  characterized in situ by studying the transient photovoltaic effects. 

2. Nanoparticle PVs: Exploratory effort will also be made to use nanoparticles and interlace them with molecular wires. Quantum confinement effects and molecular adaptive transport will be utilized to enhance the efficiency of photovoltaics. 

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Ultrafast Molecular Transport

We have established a method to conduct research on molecular transport combining measurements of transient photovoltaic effect and ultrafast molecular structural responses. We have observed the molecular thermal and electronic transports through a molecular nanowire sandwitched between metallic nanoparticles and silicon substrate. Aided by density functional theory calculation, the observed structurally correlated molecular transport and charging processes appeared to be sensitive to the interfacial charging and molecular orbital alignments. These processes were found to be completely reversible on the nanosecond time scale, thus promising for device applications. We are currently engaging in varying the wire length and the molecular conductance (conducting, semiconducting, and insulating) to explore a possible general theme of molecular transport phenomena (tunneling vs. hopping transport). Added to the concurrent research is our unique capability of monitoring the molecular structures during transport on the unprecedented ultrafast time scale.

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Nanoacoustics

Acoustic phenomena on the nanometer length scale are unique and can be used to define local thermal energy dissipation and investigate the strain profile in nanomechanical materials. Under different driven conditions, coherent wave-like dynamics and energy dependent driven deformations are studied, reflecting the carrier-lattice interactions and hot phonon effects on the nanometer scale. Our theoretical effort focused on elucidating the different behaviors in the bulk as well as multilayer structures. The modeling was based on a localized thermal bath model – the charge carriers, phonon bath and environments were treated as subsystems with independent temperature descriptions. These predictions were compared with our experimental findings. In the fs-ps regime, the ‘thermal’ prediction based on the 'Two-Temperature Model', which treated only one unified temperature for the lattice, was found to be insufficient. In fact, our diffraction experiments determined two types of temperature – one, defined locally as the ‘vibrational’ energy, can be estimated from the loss of Bragg peak intensity due to fluctuation in different lattice planes; and the other one, defined globally for the probed slab as the ‘strained’ energy, could be determined by the overall expansion or contraction of the lattices. Both of them were not in thermal equilibrium conditions in the short time.  We are investigating different scenarios that could account for such behaviors. In new studies, emphasis will aim at elucidating the confinement effects. Although the mechanism for carrier-phonon interaction and transport of heat in continuous media has been extensively studied by optical pump-probe techniques, it is still relatively unclear the effects of interfaces and the restrictions imposed by the nanoboundaries – particularly for the study of vibrational energy couplings between different components.  These ‘nuances’ are unavoidable in the nanometer scale, and will have important contributions towards understanding the conduction and the dissipation from molecular and nanoscale devices. The nature of vibronic couplings on the nanoscale is to be elucidated via simultaneous structure and spectroscopy determinations. The current two-temperature model will be expanded to include the nanoscaled confinement effects. Wave-like behaviors will be considered in the framework including energy oscillations between the electronic and atomic subsystems – pertaining to the nanoconfinement. The long-range ‘temperature’ and local ‘temperature’ as defined from our experiments will be separately treated theoretically in describing the energetic profile.  The conclusions of this study may provide the basis for engineering a novel transient high temperature interface for novel reactions, for thermal energy concentration using coherent scattering of phonons in nanostructured materials, and for fabricating efficient thermal dissipator by matching the thermal impedance on the nanometer scale.

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