The Xiong Group

University of California San Diego

RESEARCH

New materials are revolutionizing our world, from fundamental science, to energy and biological applications. There materials are often composed with complex compositions and interfaces. Consequently, understanding the structure and dynamics of these interfaces is an exciting frontier in science. However, observing ultrafast dynamics on interfaces poses a substantial challenge. The goal of the Xiong group is to develop new spectroscopic techniques for observing ultrafast processes of interfaces, enabling breakthrough technologies. To achieve this goal, the group is combining concepts and techniques from chemistry and physics, in particular, we have focused on pushing the frontier of a powerful interfacial-sensitive spectroscopy, named Sum Frequency Generation (SFG) spectroscopy, by adding new dimensions (in time, frequency and space) to this established technique.

Primarily, our research focuses on several directions fields. First, we want to prove the electronic structure and dynamics of solid/solid interfaces by developing novel spectroscopies, including Electronic SFG spectroscopy, and Transient Electric-Field-Induced Vibrational SFG spectroscopy. Second, we have been revealing the relationship between molecular conformation and dynamics of electrochemical liquid/solid interfaces, using Heterodyne 2D Vibrational SFG spectroscopy. Third, we recently developed spatial-resolved vibrational SFG microscope to image complex molecular self-assembled structures, which reveal micro-size domains with specific molecular packings. Lastly, a recent endeavor has been focused on studying a new hybrid light-matter materials – molecular polariton, by performing the first 2D IR spectroscopy to learn the dynamics and coupling between various states in molecular polariton.

Electronic Structure and Ultrafast Charge Dynamics

Organic photovoltaic (OPV) devices are one of the candidates for next generation electronics, due to their mechanical flexibility and economical manufacturing. A fundamental understanding of the interfacial electronic structure and dynamics of OPV is necessary for the design of more efficient devices.

Traditionally, the electronic states of OPV are approximated by measuring UV-Vis spectra of thin film OPV, which assumes thin film OPV is thin enough to approximate the interfacial electronic structures. A more reliable way to determine the interfacial electronic structure is using interfacial sensitive spectroscopy, such as electronic SFG (ESFG) spectroscopy to directly probe the electronic states at interfaces. Similar to the popular vibrational SFG, ESFG is also a second order nonlinear optical signal, which makes it only survives in non-centrosymmetric media, such as at interfaces. By tuning the two input pulses frequencies and make the sum of them to match the electronic transitions, resonant enhancement occurs in the sum frequency signal, which can be used to determine the interfacial electronic structures. ESFG has a few advantages such as the measurement can be carried out at ambient condition, which makes it a versatile method to probe various interfacial electronic devices.

To probe ultrafast charge dynamics at OPV interfaces and correlate the dynamics with molecular interfacial structures, we developed transient Electric-Field-Induced VSFG spectroscopy. This method takes advantages of electric-field-induced effect to measure the transient current at interfaces due to charge transfer, and at the same time, the vibrational spectral features of VSFG encodes chemical structures at interfaces. We recently used this unique method to reveal direct charge transfer at organic semiconductor and metal interfaces, and learned that a sub-ensemble of organic semiconductor has their electronic orbital delocalized into the metal substrates, which enable direct charge transfers.

Molecular conformation on Electrochemical Interfaces

Electrochemistry is a promising route to facilitate CO2 reduction or H2O splitting, in order to produce chemicals from clean resources. Since the reactions occur at the electrodes after molecules receive charges, a thorough understanding of the relationship of the molecular conformation at interfaces and the charge transfer dynamics between molecules and electrodes could enable us to promote certain conformations to enhance electrochemistry efficiency. Therefore, it is necessary to use an interface sensitive technique to investigate the interfacial molecular conformation and relate it to the charge separation dynamics.

Dr. Xiong pioneered the development of heterodyne two-dimensional Sum Frequency Generation (HD 2D SFG) spectroscopy, which is an interface sensitive multidimensional IR spectroscopy. It not only maintains the interface sensitivity from SFG spectroscopy, but also can unambiguously reveal the condense phase dynamics from 2D IR spectroscopy, such as interface inhomogeneity. We recently showed that HD 2D SFG can be used to determine the subtle local environments, vibrational dynamics and orientation distributions of catalysts on the electrode surfaces. These information plays the key for further engineering surface catalysis for CO2 reduction or H2O splitting.

Imaging Heterogeneous Molecular Self-Assembly Structures and Interfaces

Molecular self-assembly structures, such as sodium dodecyl sulfate @ ß-cyclodextrin, can process unique chemical properties for stimulus materials or water purification. Their properties rely on the structure of these self-assemblies, which can be spatially heterogeneous. A passive phase-stabilized heterodyne SFG microscope was developed in our group to study the micron-size domain of these materials. This new microscope can distinguish different domains with various molecular orientations, which would be lost otherwise in ensemble-averaged spectroscopy. This microscope can be applied to other non-centrosymmetric structures or interfaces, such as heterogeneous catalysts, structural color materials and neurons.

Nonlinear Spectroscopy and Ultrafast Dynamics of Molecular Polaritons

Molecular vibrational-polaritons can be described as delocalized quantum superpositions of molecular vibrations and electromagnetic modes resulting from strong coupling between them. It is anticipated that vibrational-polaritons will open opportunities for new photonic and molecular phenomena, including chemical reactivity control through modified vibrational dynamics, tailored potential energy landscapes, and many other phenomena waiting to be demonstrated. Our group performed the first experimental two-dimensional infrared (2D IR) spectra of molecular vibrational-polaritons. Quantum states of molecular vibrational polaritons, hybrid half-light, half-matter quasiparticles, are studied using ultrafast coherent two-dimensional infrared spectroscopy for the first time. Valuable physical insights such as existence of hidden dark states, and ultrafast interactions between dark states and bright polariton states, are unambiguously revealed. 2D IR signals highlight the impact of molecular anharmonicities which are applicable to virtually all molecular systems. These results not only advance coherent two-dimensional spectroscopy into a new realm, but also have significant implications in harvesting the creation of molecular vibrational polaritons for novel infrared photonic devices, lasing, molecular quantum simulation, as well as new chemistry by tailoring potential energy landscapes.

Funding