Plasmon enhanced photocatalysis
Understanding the interaction of molecules with metal surfaces is fundamental for advancing heterogeneous catalysis, chemical sensing, spectroscopy and many other surface related applications. Investigation of the surface-molecule interaction and the reactivity requires detection of molecular signals with monolayer and single molecule sensitivity, which is difficult to achieve using well established traditional analytical techniques. In this research, we take advantage of the intense surface electromagnetic field of localized surface plasmon resonances of metal nanoparticles to induce photochemical reactions as well as to enhance Raman scattering signal of reactant and product species as seen in the Figure on the top. This approach affords single molecule detection sensitivity, allowing us to obtain in-depth mechanistic understanding, for example, to figure out the roles of hot electron transfer versus plasmon pumped adsorbate intramolecular electronic excitation in driving the photochemical reactions. The plasmon-molecule interaction can also be exploited to investigate molecular adsorption properties and to determine the electronic transition energy of the adsorbate. More information is available in our publications listed below.
Gap plasmons and molecular polaritons
According to classical electromagnetic theory (CET), the field enhancement in the gap between two plasmonic nanoparticles increases continuously with decreasing gap length until the nanoparticles make conductive contact. However, for small gap lengths (smaller than a nanometer), the optical response of the coupled system is expected to deviate from CET prediction due to charge transfer between the nanoparticles (quantum tunneling effect) before the nanoparticles actually make physical contact. Experimental validation of the quantum tunneling effect requires controlling the gap lengths with angstrom accuracy, which is a monumental fabrication challenge. The fabrication challenge is significantly simplified by coupling the plasmonic nanoparticles to a metal film, where the dipole-image dipole interaction mimics the dipole-dipole interaction between two nanoparticles. We use nanoparticle on metal film system to investigate the evolution of the plasmonic optical response from capacitive to the quantum tunneling limit (as results in the figure on the top show) and enhance light-matter interaction by embedding molecular and semiconductor systems in the plasmonic nanocavity. Similarly, we study plasmon enhanced charge carrier generation and diffusion processes by coupling the plasmonic nanoparticles with semiconductors. More information can be obtained following the links below.
Interparticle near-field interaction
When photonic and plasmonic nanoparticles are brought in close proximity (within the near-field interaction regime), their collective optical response can be very different from that of the individual components. This distance and interface dependent interaction can be exploited for tailoring materials functionalities, for example, to increase light-matter interaction for photocatalytic, spectroscopic and photovoltaic applications and to create entangled quantum states. In our group, we use scattering type scanning near-field optical microscopy (s-SNOM) to probe the near-field properties of coupled nanoparticles. s-SNOM can achieve a few nanometers spatial resolution (as illustrated in the figure on the top) independent of excitation wavelength. Using this high spatial resolution capability, we study plasmon-plasmon and plasmon-exciton interactions by controlling the coupling distances and interface properties by innovating various advanced fabrication processes. For more information, please follow the link below.
Nanoscale infrared chemical imaging
The fact that the spatial resolution of s-SNOM is independent of excitation wavelength makes it a suitable analytical tool for nanoscale chemical imaging of a multicomponent chemical systems using infrared radiation that matches the chemical bond vibration or "finger print" spectral regions of molecules. We use this approach to investigate chemical phase separation, charge localization and thermal effects in polymer blends that includes semiconducting polymers relevant for organic photovoltaics. The images on the top is for thin film of binary components, and the result shows that the two components are clearly resolved in the s-SNOM (chemical) image although it is not possible to discern the phase separation in the simultaneously recorded topographic image. More information about the principle and sensitivity of the s-SNOM infrared chemical imaging technique can be obtained following the link below.