Interfaces and surfaces are ubiquitous in people’s daily life and they play a pivotal role in many research fields such as molecular electronics, photovoltaics, and environmental chemistry. However, due to the lack of interface-specific probes, there are plenty of questions remain unsolved for interfaces. During the past few decades, thanks to the development of ultrafast pulsed laser technology, nonlinear spectroscopy has merged to be a powerful technique to characterize the properties of interfaces. In this dissertation, several novel nonlinear spectroscopic methods will be discussed and used to perform static as well as kinetic study of interfaces.
First, the major emphasis of this dissertation is to understand the electronic structure and charge transfer dynamics at the interface formed by an organic semiconductor – poly(3-hexylthiophene-2,5-diyl) (P3HT) and a metal – gold or an inorganic semiconductor – silicon. Using the interface specific sum frequency generation spectroscopy (SFG), a method of measuring the band gap of buried interfaces is established. The result demonstrates that the electronic structure at buried interface differs from that of the bulk. By combining the transient absorption spectroscopy and vibrational sum frequency generation (VSFG) spectroscopy, the first dynamical electric-field-induced VSFG signal is observed and for the first time, a spectroscopic evidence of direct electron transfer at complex polymer/metal interfaces is presented. Aided by heterodyne detection, a phase rotation approach has been established to disentangle the pure molecular response from the electronic nonresonance to interfacial charge transfer.
The first fourth order three-dimensional SFG spectroscopy (3D SFG) is introduced and used to measure the interstate vibrational coherences from a Re(diCN-bpy)(CO)3Cl monolayer adsorbed on gold surface. It is learned that the surface attachment induces both homogeneous and inhomogeneous dephasing dynamics of the vibrational mode. However, the coherence is preserved upon surface attachment.
Other than applying to air/solid and solid/solid interfaces, nonlinear spectroscopy is also powerful to learn air/aqueous interfaces. VSFG is used to explore the protein adsorption kinetics at air/salt water interface under environmentally relevant conditions. In combination of surface pressure measurement, a novel “salting up” phenomenon is proposed, and the role of ions is discussed. Moreover, a critical surface coverage needs to be satisfied to induce the conformational change of proteins at the surface.
Overall, nonlinear spectroscopy has been proved to be an ideal candidate for non-destructive and interface specific characterization method. By choosing the appropriate method, both static and kinetic information can be achieved.