New energy materials are revolutionizing our world. In particularly, there are many interfaces in these  energy materials often dictate their effieicys.  Consequently, understanding the structure and dynamics of these interfaces is one of the most exciting frontiers 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, including multidimensional spectroscopy, surface sensitive spectroscopy, and high harmonic generation.

Primarily, our research focuses on two fields. First, understanding the electronic structure of solid/solid by developing and using Electronic Sum Frequency Generation spectroscopy. Second, revealing the relationship between molecular conformation and charge seperation dynamics in nano energy materials by Heterodyne 2D Vibrational Sum Frequency Generation spectroscopy.

Electronic Sum Frequency Generation spectroscopy

Organic photovoltaic (OPV) devices are one of the candidates for next generation electronics, due to their mechanical flexibility and economical manufacturing. One of the unsolved issues of OPV is the electronic structure at interfaces. A fundamental understanding of the interfacial electronic structure of OPV is necessary for the design more efficient devices.Traditionally, the electronic states of OPV are determined by measuing UV-Vis spectra of  thin film OPV, which assumes thin film OPV is thin enough to approximate the interfacial electronic structures. 

ESFG Fig.1 Pulse sequence and Feynman Diagram of ESFG.
The resonance enhancement is achieved by having the sum of frequencies of near IR and 800nm beams to match the electronic transitions.
A more reliable way to determine the interfacial electronic structure is using interfacial sensitive spectroscopy, such as electronic sum frequency generation (ESFG) spectroscopy to directly probe the electronic states at interfaces.  Simialar to the popular vibrational  sum frequency generation  spectroscopy, ESFG is also a second order nonlinear optical  signal, which makes it  only survive  in non-centrosymmetric media, such as  at  interfaces. By tuning the  two input  pulses frequencyies and make the sume of them to match the electornic 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 versitale method to probe various interfacial electornic devices.

Heterodyne SFG and 2D SFG to study impact of molecular conformation on charge dynamics
Electrochemistry is a promising route to faciliate  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 thourough understanding of the relationship of the molecular conformation at interfaces and the charge transfer dynamics between moelcules and electrodes could enable us to promote certain conformations to enhance electrochemistry effcieincy. Therefore, it is necessary to use an interface sensitive technique to invesitgate the interfacial molecular conformation and relate it to the charge seperation dynamics. 
2D SFG_pulse sequence
Figure 2. Left, pulse sequene of 2D SFG. Red: Mid-IR, Blue: visible, Orange: Local Oscillator. Right, HD 2D SFG spectrum of CO2 reduction catalysts on Au surface
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 unambigiously reveal the condense phase dynamics from 2D IR spectroscopy, such as interface inhomogeneity. We recently showed that HD 2D SFG can be used to determien the subtle local environments of catalysts on the electrode surfaces. By using Heterodyned SFG and 2D SFG spectroscopy, we can also determien the molecular conformation on the interfaces. Combining this with time-resolved dynamics, we can identify the conformations that promote charge seperation.