UC San Diego

New materials are fundamental for modern industry. Novel materials are often composed with complex compositions and interfaces. Therefore, understanding the structure and dynamics of these new materials is important. In this thesis, we apply advanced spectroscopy, such as pump-probe spectroscopy, two-dimensional infrared (2D IR) spectroscopy and transient vibrational sum frequency generation spectroscopy (VSFG) to study the structure and dynamic information of some novel materials.

In chapter 1, we performed 2DIR spectroscopy to understand the structure of new chemical compound. We report a chromium complex mediator, Cr(CO)3(η6-naphthalene), that mediates the traditional thermal dienyne cyclization at ambient temperature. Successful cycloaromatizations were observed to proceed to moderate yield in coordination. We investigate mechanism and kinetic of this novel reaction by Fourier-transform infrared spectroscopy (FT-IR). Static 2D IR spectroscopy was performed to analysis the intermediate. After studying the structure information of new chemical compound, we noticed that dynamic process can help us understand the system parameters changing over time. In chapter 2, we performed a pump-probe spectroscopy to understand the cation rotation in a strain induced -formamidinium lead iodide (-FAPbI3) system. We apply strain engineering to tetragonal FAPbI3 using both theoretical simulations and experimental techniques. Ferroelectricity is then observed and proved on the strained -FAPbI3 thin film. By thermally stimulated depolarization current (TSDC) measurement, the polarization of the epitaxial strain -FAPbI3 thin film is demonstrated. Through pump-probe spectroscopy measurement and X-ray Photoelectron Spectroscopy (XPS) measurement, we demonstrate that the ferroelectricity generated in the strained -FAPbI3 owes to the inorganic framework atom displacement generated from the lead-iodide bonds with neglectable influence of freely-rotational FA+ molecules under room temperature. After study the dynamic of novel material, we found out that molecule conformation at interface greatly influence the performance of materials. Therefore, in chapter 3, we develop a surface/interface sensitive technique called time-resolved electric-field-induced sum-frequency generation (Tr EFI SFG) and develop time-resolved electric-field-induced heterodyne sum frequency (Tr EFI-HD SFG) spectroscopy to follow the charge transfer process at interfaces. Currently, we study the interface which composed of acceptor polymer (BBL) and donor polymer (P3HT). We plan to extract the implicit molecular dynamics by transient HD VSFG. Currently, we are trying to confirm that the direct charge transfer mechanism and strong acceptor-donor interactions can form at the interface. Second, we will try to resolve molecule specific dynamics by performing phase rotation and data retrieval methods on time-dependent HD SFG spectra.