Light-matter interactions in Correlated Electron Materials
In strongly correlated electron materials, the intricate interplay of light, charge, spin, and lattice degrees of freedom leads to complex phase behavior and dynamic response, enabling the ultrafast control of materials by light. The photo-melting of charge density waves and light-induced manipulation of diverse states in Mott insulators, multiferroic ferromagnets, and high-temperature superconductors will be elucidated by combining the strengths of advanced electron and photonic techniques. Ultrafast methods with spatial resolution at the atomic level will take snapshots of the temporal evolution of critical phenomena to disentangle the different degrees of freedom involved and provide insight into their cooperative dynamics. We work with materials collaborators to synthesize functional complex materials down to nanometer scale allowing characterization tools to address correlated states in variable conditions of localization, doping, and confinement to test the tremendous device opportunities of extremely high speed and density. We work with theory collaborators to apply a wide variety of toolsets involving ab initio modeling and molecular dynamics simulation to closely examine the highly controlled and highly constrained experimental data to establish material design principles that guide further synthesis and characterization efforts for validation in an interactive scheme.
Photodynamics of Nanocarbons
The ability to modify the material properties by photoexcitation derives from the strong interaction between the electronic degree of freedom and a 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 their propensity to form a wide range of bonding networks in various forms and sizes, and appear to be subject to influences from the environment to reconstruct their electronic and optical properties, promising applications in optoelectronics 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.
Surface plasmon resonance enhancement in functional nanostructured interfaces
Recently there has been a lot of progress in understanding the surface plasmon resonances (SPR) phenomena associated with metallic nanostructures. By manipulating the composition, shape, and size of plasmonic nanoparticles, it is possible to design nanostructures that interact with the entire solar spectrum and beyond. The unique capacity of plasmonic nanocrystals to concentrate electromagnetic radiations in the proximity of nanocrystal surface and then covert the energy stored in the photon fields into electrical, chemical, and thermal energies is very appealing. Already, there are a surge of applications recently utilizing these features to enhance the performance of photocatalysis, solar cells, and heat-induced selective tissue targeting, enabling a rapidly growing field in nanoscience in the last decade. We will couple the ultrafast surface diffraction and time-of-flight mass spectrometry to provide simultaneous information on the microscopic couplings between the local plasmons and the activated electronic and phononic degrees of freedom. The key reaction coordinates to elucidate are the charge-mediated and thermally mediated channels for enhanced photoreactions. By manipulating the local morphology, we aim to control the field enhancement and prioritize either local charge-transfer-induced or remote resonance-transfer-mediated reaction channels. Currently, there has been no consensus regarding the detailed coupling channels in plasmonics catalysis, but there is a high demand for the technology to improve its efficiency to meet the challenge of mass conversion of solar energy into chemical energy. The ultrafast electron crystallography and voltammetry methodologies offer unique capabilities to simultaneously track the electronic and structural dynamics, which are good matches to these investigations.
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.
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.
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.
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 sandwiched between metallic nanoparticles and the silicon substrate. Aided by density functional theory calculation, the observed structurally correlated molecular transport and charging processes appeared to be sensitive to 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 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.
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 a 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 an efficient thermal dissipator by matching the thermal impedance on the nanometer scale.