Youngwook Park

october, 2024

08oct1:00 pm2:00 pmYoungwook ParkFritz Haber Institute of the Max Planck Society1:00 pm - 2:00 pm KST

Event Details

Youngwook Park

Affiliation: Fritz Haber Institute of the Max Planck Society

Research Interests: Physical chemistry, molecular physics

Title: Atomic-precision control of photoreactions on surfaces
using laser-coupled STM

Abstract: Scanning probe microscopes (SPMs) have been widely used for monitoring and controlling the local physics and chemistry of materials and molecules at the atomic scale. However, when it comes to photo-induced processes, atomic scale manipulation remains largely unexplored. This is primarily due to the intrinsic discrepancy between optical (μm) and molecular (nm) length scales, which hinders the confinement of photochemistry to the single molecule level. Localized surface plasmons at the SPM junction enable shrinking the photo-excitation to within several tens of nanometers [1]. Using a vanishing gap [2] or employing a pico-cavity [3] has further improved the spatial confinement of the electromagnetic field, driving photoreactions with submolecular precision. We build on these approaches and explore beyond them, especially focusing on systems of chemical importance. We used a low temperature, ultrahigh vacuum scanning tunneling microscope (STM) whose junction is coupled with continuous-wave laser irradiation. Photoreactions are monitored by changes in conductance as well as in tip-enhanced Raman scattering (TERS). In this talk, I will introduce two of our recent endeavors. The first topic concerns a single-molecule photoswitch that operates at a metal–semiconductor junction [4]. A single perylenetetracarboxylic dianhydride (PTCDA) molecule, junctioned between an Ag tip and a Si(111) surface, acts as a conformational switch under laser irradiation, resulting in bistable conductance. The switch involves the reversible breaking and formation of chemical bonds at the molecule–surface interface. Surface plasmons localized at the junction are crucial for triggering the photoreaction The switching behavior is systematically controlled by a 10 picometer-scale vertical motion of the plasmonic tip. The silicon surface, serving as a non- plasmonic counterpart, contributes to this remarkable sensitivity through the formation of strong bonds with the molecule and the inhibition of plasmon generation on the surface. The second part will cover the site-selective photo-dissociation of hydrogen bonds. Triphenylene-2,6,10-tricarboxylic acid (TTC) molecules form two-dimensional honeycomb network structures on an Au(111) surface that are stable at room temperature. In the network, the molecules are hydrogen-bonded via carboxylic groups. In the near-field regime, laser irradiation dissociates the hydrogen bonds, inducing the rearrangement of molecular networks. The photo- induced dissociation of the hydrogen bonds is localized to a few molecules underneath an Ag tip, with minimal impact on neighboring molecules. In contrast, a PtIr tip cannot induce the reaction, suggesting that localized plasmons at the junction contribute to the process. The photoreaction shows a sharp dependence on the wavelength of the incident laser, which is a unique feature of the molecular network.

 

Time

(Tuesday) 1:00 pm - 2:00 pm

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