Coupled plasmonics for tip enhanced Raman scattering and understanding of quantum effects in plasmonic junctions

摘要

Over the past few decades, plasmonics has emerged as a technology that spans across several fields including laser spectroscopy, material science and solid-state physics. The near-field coupling between light and the surface plasmons of metal nanostructures has transformed single-molecule detection and imaging, tracking of chemical reactions and boosting catalytic efficiencies. Surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS), by taking advantage of the enormous field enhancements arising from the excitation of the localized surface-plasmon resonances (LSPR), have both shown great potential in ultra-sensitive detection. In this dissertation, we focus on the effect of coupled plasmonics in TERS for the detection and nanoscale imaging of biologically relevant molecules such as proteins, in a non-invasive, label-free and selective manner. The other aspect of this dissertation will be devoted to the effect of quantum tunneling in ultra-small plasmonic nano-junctions, as evidenced by its gap plasmon shifts and time-dependent spectral changes.;The thesis will begin with an introduction to coupled plasmonics: reviewing the related backgrounds of plasmonics & plasmonic coupling, a topic of close relevance to the research conducted in this thesis, after which a brief description of SERS and TERS will be given. This will then be followed by the introduction of VSE and its applications in determining local electric fields in various environments. A short survey of literatures that report the quantum effect (electron tunneling) as observed in plasmonic gaps and will be presented at the end of the introduction. The results of the dissertation research can be categorized into two major parts:;Part I is associated with implementing and understanding of the TERS system. TERS, unlike SERS, does not rely on the uncontrollable nanoparticle aggregation for signal enhancement, but uses an illuminated metallic sharp tip to provide additional control (using AFM or STM) over the positioning of the enhanced plasmonic fields. Unfortunately, the field enhancement from a single metal nanoparticle tip is often not enough for detecting Raman signals of most non-resonant molecules. To overcome this, most recent TERS studies resort to plasmonic coupling between the sharp metal tip and a smooth metal film ('gap mode'), creating a nanometer sized cavity in between the gap where the field enhancement is orders of magnitude higher than that of a single metallic tip. So far, gap mode TERS has had great success in the detection and imaging of small molecules. In ideal conditions, sub-molecular resolution has been reported. However, for larger molecules such as lipids, proteins or molecules that cannot be easily transferred to a metal film, 'gap mode' TERS is difficult to implement. Striving to solve this problem, we coupled a sharp metal tip with an addressable nanoparticle probe (functionalized nanoparticles) to achieve both higher signal enhancement and chemical specificity at the same time. We found that by coupling the Au-ball AFM tip to a metal nanoparticle (AuNP) bound to a protein-coated surface can result in Raman signals of the proteins underneath the bound nanoparticle (Chapter 2). In light of this result, we further extended this technique to imaging integrin receptors on fixed cells using cyclic RGD ligand functionalized AuNPs (Chapter 3) and tested the capability of this method in differentiating different mutant variants from the wild-type streptavidin (Chapter 4).;Part II of the thesis reports an electron tunneling effect in sub-nanometer plasmonic junctions as evidenced by the plasmon resonance shifts and nitrile peak shifts observed experimentally (Chapter 5). The electron tunneling effect and non-local effect in plasmonic gaps have been theoretically predicted and experimentally observed to cause blue shifting and broadening of the coupled plasmon mode. This effect can ultimately lead to reduction of the local electric fields due to charge screening when the conductivity becomes large enough. Experimentally the vibrational stark effect (VSE) is reported to monitor local electric fields through stark shifts in specific vibrational modes. Here, we use VSE as a molecular voltmeter to track the changes in nitrile normal modes as the gap size of the nanoparticle-on-mirror (NPoM) junction is modulated from nanometer to sub-nanometer regimes and show that subtle changes in the local electric fields can be related to electron tunneling as confirmed by a blue shift in dark-field scattering.