Molecular plasmonics

Molecular plasmonics, the study of how molecules interact with plasmonic nanostructures, is a new and rapidly expanding field. It is well known that plasmonic nanostructures are capable of harvesting light and concentrating it in the near field (Figure 1). This special behavior results from the collective oscillation of the conduction electrons in a metallic nanostructure, and has enabled a plethora of exciting applications such as plasmon-enhanced solar cells, photonic circuits, superlenses, chemical and biological sensing, and the detection of single molecules. Underlying many of these processes is the ability of plasmon excitation to greatly enhance electromagnetic fields at the particle surface. The Camden group is working to develop new applications of plasmonic nanostructures and to understand fundamental features of the molecule-plasmon couplings underlying these applications.

Surface-Enhanced Nonlinear Spectroscopy

Concurrent with the rapid expansion of plasmonics, the last decade has seen an exponential increase in the exploration and utilization of multiphoton processes. Two-photon transitions, for example, play an essential role in energy up-conversion, all-optical switching, optical data storage, 3D lithography, and biological imaging. While plasmonic nanostructures have been utilized extensively to enhance linear spectroscopies, such as surface enhanced Raman scattering (SERS), almost no effort has been devoted to studying the enhancement of nonlinear spectroscopies, despite their potential impact.10.

Multiphoton processes are different in many respects from their one-photonanalogues as they are nonlinear in the incident field and have complementary selection rules. It is commonlyassumed that the weakness of nonlinear spectroscopies makesthem impracticalformany applications; however, plasmonenhancementprovides a method to overcome this limitation. The Camden group is currently studying a prototypical nonlinear spectroscopy: surface enhanced hyper-Raman scattering.We have recently shown the ability of SEHRS to probe “dark” electronic states, i.e.those that are not one photon accessible, and to extract details of the two-photon molecular properties(Figure 2). We also work closely with theoretical chemists to unravel the details of our spectra. These studies are ongoing and we are excited about the future applications of SEHRS.

Mapping Plasmon Modes

Many applications of plasmonic nanoparticles, of which surface enhanced spectroscopy is just one, require an intimate understanding of the nanoparticle LSPR. Mapping these plasmon modes at the nanometer scale, however, remains a major challenge. Further, the connection between optically driven and electron driven plasmons is not well understood. Electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has emerged as a technique capable of mapping the energy and spatial distribution of plasmon modes at the nanometer scale.Therefore, we are correlating optical and electron microscopies at the single nanoparticle level (Figure 3). This powerful combination of STEM/EELS and optical scattering has the potential to directly image the analyte molecules in surface-enhanced spectroscopy.

Our group also employs computational electrodynamics to simulate scattering and absorption cross-sections of nanoparticles with arbitrary structure. Specifically we are interested in examining the electromagnetic enhancement factors in the gaps (hot-spots) between nanoparticles with different types of structure. Our simulations are performed in conjunction with the experimental findings to better understand properties of the nanoaggregates observed in the experiments.

Developing Projects

Fundamental studies of Single-Molecule SERS (SMSERS): Single molecule SERS scattering (SMSERS) provides the only optical method to measure the vibration spectrum of a single molecule. This technique has tremendous applications in biological and chemical systems. Since the discovery of SMSERS more than a decade ago, considerable efforts have been made to understand the mechanisms and conditions underlying SMSERS. However, there are still many outstanding questions regarding SMSERS and our group is exploring the structural and optical properties of the nanoparticles involved in the observation of a SMSERS.

Ultrasensitive sugar detection with SERS: The large enhancement factors of SERS classify it as a highly sensitive spectroscopy technique; thus, presenting SERS often finds application in detection of biological analytes. The general applicability of SERS, however, is limited by the ability of the analyte molecule to bind specifically to the surface. In collaboration with the Best Group at UTK we are developing sensors to selectively bind sugar molecules for detection.

Recent Collaborators:

Lasse Jensen – Pennsylvania State University, Ab initio calculation of resonance Raman and resonance hyper-Raman intensities.

David Jenkins - University of Tennessee - Synthesis of Ligands for SERS studies

David Masiello - University of Washington - Full wave simulations for EELS

Jill Millstone - University of Pittsburgh - Complex nanoparticle synthesis