Tyler Ueltschi and Yue Qi (Van Duyne Group) - CaSTL Seminar

Thursday, May 9, 2019 - 13:00
2201 Natural Science II @ UCI | ZOOM MEETING BROADCAST
Tyler Ueltschi and Yue Qi, Northwestern University, Van Duyne Group
Event Title: 
1PM: Towards Understanding Molecule-Plasmon Interactions on Ultrafast Timescales | 1:30PM - Directly Observing Plasmon-Driven Oxidation of Ferrocyanide using Surface-Enhanced Raman Spectroscopy
CaSTL Center - Prof. Ara Apkarian, Director | Prof. Richard Van Duyne, Northwestern University


1:oo PM

Tyler Ueltschi - Northwestern University (Van Duyne Group)

Title: Towards Understanding Molecule-Plasmon Interactions on Ultrafast Timescales

Abstract: Time-resolved surface-enhanced coherent Raman scattering (tr-SE-CRS) techniques are ideally suited for directly observing molecule-plasmon interactions. Specifically, current development of surface-enhanced femtosecond stimulated Raman scattering (SE-FSRS) experiments has allowed for vibrational characterization of molecules on plasmonic substrates with a spectral resolution of 10 cm-1, while providing an instrument response time as low as 50 femtoseconds. To demonstrate feasibility of time resolved SE-FSRS experiments, as well as characterize the plasmon-molecule interactions in a non-reactive regime, the following experiment was performed. A colloidal solution of gold nanosphere oligomers functionalized with a bypyridine reporter molecule and encapsulated in a silica shell is exposed to a 532 nm actinic pulse (100 fs) to pump the monomer plasmon resonance of the substrate, then at a given time delay (0 – 100 ps) the local environment is probed with the FSRS pulse pair consisting of a 800 nm Raman pump (1 ps) and a broadband Raman probe (100 fs). This induces a non-equilibrium electron distribution that decays on the femtosecond timescale resulting in population of the low-lying phonon modes of the nanoparticle oligomers. The result is in an initial broadening of the localized surface plasmon (LSP), which then modulates due to the expansion and contraction induced by the phonon modes of the nanoparticles. From the perspective of the molecular reporter, the environment is rapidly transitioning from one with a highly delocalized electron distribution to one with a modulating nanogap, effectively changing the coupling of the molecule with the far-field radiation. Both the rapidly changing electronic environment as well as the interaction of molecules within this environment can be observed through SE-FSRS, providing a first step toward fully understanding plasmon-molecule interactions on ultrafast timescales.


Bio: Tyler Ueltschi is a fourth-year Chemistry PhD student from Prof. Richard Van Duyne’s research group at Northwestern University. His passion for spectroscopy started as an undergraduate at the University of Puget Sound when he got the opportunity to do research at the Pacific Northwest National Laboratory with his advisor Prof. Amanda Mifflin and mentors Dr. Patrick El-Khoury and Dr. Hong-fei Wang. Here, his initial experiences with Sum Frequency Generation (SFG), tip-enhanced Raman Spectroscopy (TERS), and computational chemistry spawned a deep interest in vibrational spectroscopy. Now, as a PhD candidate, his research focusses on studying molecule-plasmon interactions on ultrafast timescales. By using femtosecond and/or picosecond pulses, non-equilibrium electron distributions can be driven at plasmonic nanoparticle junctions and through time-resolved Raman spectroscopy techniques the vibrational signature of molecules within this environment can be observed. Development of these methods has allowed for further fundamental investigation of plasmon driven chemistry




Yue Qi -Northwestern University (Van Duyne Group)

Title: Directly Observing Plasmon-Driven Oxidation of Ferrocyanide using Surface-Enhanced Raman Spectroscopy

Abstract: In this talk, I would like to show the direct observation and characterization of plasmon-driven oxidation of ferrocyanide on gold nanosphere oligomers with surface-enhanced Raman spectroscopy (SERS). By comparing to corresponding electrochemical SERS results, we are able to understand the plasmon mediated oxidation that occurs to the surface species under optical pumping in cw pump-probe SERS experiments. With the rapid progression of this reaction at relatively low laser powers and the ease to identify both the reactant and the product species with lack of undesirable byproducts, this reaction is likely to become a model system for fundamental studies on plasmon-driven charge transfer reactions.

Bio: Yue Qi is a second-year chemistry graduate student from Van Duyne group. Her research in Van Duyne group mainly involves investigating plasmon-driven processes with Raman spectroscopy. Prior to Northwestern, Yue received her B.S. degree in chemistry from Peking University, China. Her undergraduate research mainly focused on the design and synthesis of inorganic energy-storage materials.