Physical Chemistry

Surface Science & Spectroscopy

Surfaces often show unique physical and chemical properties which are different than that of the bulk. The study, understanding and control of these properties is of fundamental importance in both academia and industry. However, surface typically contains only a fraction of molecules than that in the bulk; consequently, the surface specific signal is overwhelmed by the bulk contribution in most scientific techniques. Furthermore, buried surfaces such as solid-solid interfaces are hard to reach. I am interested in using novel techniques such as Sum Frequency Generation (SFG) spectroscopy, to study the surface properties and their molecular origins for multiple systems of interest including Self-Assembled Monolayers, Organic Semiconductors, chemically terminated silicon surfaces, etc. Our recent work includes the study of molecular arrangement and orientation on Rubrene single-crystal surface using Reflection High-Energy Electron Diffraction (RHEED) and SFG. This molecular characterization is fundamental in understanding the interaction between organic semiconductor - electrolytes for cutting edge devices such as flexible LCDs. Preprint can be provided upon request.

Super-resolution Imaging with Chemical Sensitivity using SFG

The variation of chemical and physical properties across the surface can have profound effects on catalysis, oxidation, wetting, and many other important processes. Chemically sensitive SFG microscopy has proven to be an extremely useful tool for imaging surface chemistry. However, the spatial resolution practically achievable in different SFG microscopies is still in micrometer length scales due to diffraction limit of the mid-IR lasers often used in these systems. I am interested in achieving super-resolution SFG imaging. At Baldelli Surface lab (University of Houston), I have combined the Ground State Depletion (GSD) principle with structured pump-IR; in this scheme, a donut shaped pump-IR is utilized to deplete the SFG relevant vibrational ground state of the chemical surface. The dark center of this optical vortex leaves the ground state population intact in an area that is smaller than the focal spot size. This spot is then probed with a pair of visible and mid-IR beams to generate an SFG signal. Therefore, the SFG signal is generated from a relatively sharper spot than that dictated by the Abbe diffraction limit. The proof of concept experiments using surface hydrogen on silicon show a 3-fold resolution improvement. The preprint can be provided upon request.