Chemical Mapping Of A Single Molecule With Sub-nm Resolution By Plasmon Enhanced Raman Scattering

As human exploration into the microscopic world goes deeper and broader, it becomes a highly demanding task to realize high spatial resolution with chemical recognition at the nanoscale. Conventional photon-based optical microscopy is difficult to achieve this target due to the Abbe diffraction limit, whereas electron-based microscopic techniques such as scanning electron microscopy, transmission electron microscopy, and scanning tunneling microscopy (STM) have limited chemical identification capability down to nanoscale, despite their impressive power in providing atomically resolved topographic information. It is true that once combined with spectroscopic techniques, the electron microscopy can provide chemical mapping down to several nanometer for inorganic materials, but evidently not applicable to organic molecules as the high-energy electrons will destroy the molecules. On the other hand, although the local tunneling spectra in STM can reflect to some extent the electronic structures of the sample, it is generally believed that STM is relatively poor in chemical identification since the operational principle of STM is based on the quantum tunneling of free electrons from the metallic tip or sample. By contrast, Raman spectroscopy based on molecular vibrations is generally viewed as a powerful tool for investigating molecular structures because the vibrational’fingerprints’can serve, as important information for molecular identification and structural analysis. Therefore, tip-enhanced Raman scattering (TERS), a technique that combines the advantages of both scanning probe microscopy (SPM) and Raman spectroscopy, offer a promise to not only spatially resolve a single molecule, but also to achieve single-molecule TERS with improved sensitivity and spatial resolution by exploiting the resonant enhancement of the localized surface plasmon field. The dream goal is to achieve chemical mapping with spatial resolution down to a single molecule or even submolecular level by optical means.However, many single-molecule TERS reports up to date are based on spectral fluctuations to claim the single-molecule observation, only a few chemical mapping results are documented in the literature with spatial resolution limited to3-15nm, obviously not suitable for resolving a single organic molecule chemically. There are two major factors responsible for such difficulty. Single-molecule TERS studies require large field enhancement to improve detection sensitivity, which is usually realized by using an intense incident laser in conventional TERS. However, a strong laser field could result in undesired diffusion, desorption and even damage of the molecule, thus affecting the sustainability and stability of TERS measurements. The other factor is the spatial extent of the local surface plasmon field, which is believed to be in the range of5-10nm, presenting an electrodynamical limit to improve the spatial resolution.This thesis is aimed at achieving chemical mapping of a single molecule by TERS. In order to perform clean and reproducible single-molecule TERS measurement, our strategy is to make the TERS instrumentation by combining with an ultrahigh-vacuum and low-temperature STM. Furthermore, by controlling the spectral matching among the plasmon resonance, laser line and molecular vibronic transitions, we introduce the concept of nonlinear stimulated Raman scattering (SRS) into the STM-controlled TERS using a single continuous-wave laser, thus greatly improving the spectral sensitivity and spatial resolution. We demonstrate unprecedented submolecular Raman mapping of a single porphyrin molecule with sub-nm resolution, resolving even the inner structure of a molecule. This STM-controlled spectral matching TERS technique not only provides a new route to perform chemical imaging with sub-nm resolution by optical means in the fields of nanophotonics, surface science and biochemistry, but also opens up a new avenue for studying nonlinear optical processes and photochemistry at the single-molecule scale.This thesis consists of five chapters. It starts from an overview of TERS status, and then presents a brief description on the instrumentation of a custom UHV-LT-TERS system, including the fabrication of plasmonic tips for TERS studies. Finally, the thesis investigates in detail single-molecule TERS on two representative molecules (i.e.,4,4’-bipyridine and H2TBPP porphyrin). Following are brief summaries of each chapter.In Chapter One, we introduce the research background of Raman scattering, surface-enhanced Raman scattering, and TERS. The chapter is concluded with a brief description on the motivations and objectives of this thesis.In Chapter Two, we provide an overview on the instrumentation of various existing TERS setups. After analyzing the advantages and deficiencies of each setup, we describe the construction of our custom side-illumination optical setup that is designed to perform single-molecule TERS experiments by combining with the low-temperature ultrahigh-vacuum (LT-UHV) STM.In Chapter Three, we describe the fabrication procedure of silver tips for TERS experiments, including electrochemical etching and subsequent tip cleaning in UHV. We also highlight the importance of modifying tip status by voltage pulses to tune the resonance mode of the nanocavity Plasmon, which is monitored in situ via the STM induced luminescence technique.In Chapter Four, we investigate the tip-enhanced Raman scattering of a small organic molecule of4,4’-bipyridine in two different tip-substrate combinations, namely, Au-tip on Au(111) and Ag-tip on Ag(111), respectively. The surface self-assembly of4,4’-bipyridine molecules evaporated onto the metal surfaces in vacuum was studied by high-resolution STM images for different coverage at full monolayer and sub-monolayer. After obtaining unambiguous vibrational’fingerprint’ on the molecular island, we proceed to investigate the TERS spectra of single4,4’-bipyridine molecules. The Ag-tip on Ag(111) is found to offer much stronger plasmonic enhancement, allowing us to demonstrate the TERS mapping of a single4,4’-bipyridine molecule.In Chapter Five, inspired by the faint contrast in the TERS mapping within a single4,4’-bipyridine molecule, we then select an optoelectronic porphyrin molecule (H2TBPP) for TERS studies, wishing to resolve the inner molecular structures. This is because that the H2TBPP molecule is relatively large in size and more importantly, features a characteristic four-lobe pattern, which may allow us to carry out stable and controllable single-molecule TERS mapping. We find that the spectral matching between the nanocavity plasmon mode and molecular viboronic transitions plays an important role in improving the signal level and stability of TERS measurements. A nonlinear relationship between the TERS signals and excitation power is observed under the optimal spectral-matching condition. The combination of nonlinear third-order stimulated Raman scattering (SRS) with TERS provides dramatic improvements in the detection sensitivity and spatial resolution. By exploiting such STM-controlled spectral-matching SRS-TERS, we could not only chemically identify a single H2TBPP molecule and its configuration on the surface, but also optically resolve the inner structure of a single molecule down to sub-nm resolution (about0.5nm). These observations justify the feasibility to achieve chemical identification and imaging by optical means at the single-molecule scale. Our findings are both fundamentally important and practically valuable for the exploration of chemical reaction mechanism at the nanoscale, high-resolution biomolecular imaging, and molecular nanotechnology.