Theoretical Simulatio Of Tip-Plasmon Enhanced Single-Molecule Spectroscopy

Surface plasmon enhanced (especially tip enhanced) single-molecule spectroscopy is a powerful technique for the investigation of the optical properties of molecules at the single-molecule scale. In recent years, our group has developed a combined system of scanning tunneling microscope (STM) with single photon detector and demonstrated controllable STM induced luminescence (STML) from molecules. The fluorescence modes can be selected by tunning the plasmon resonance frequencies of the nanocavity between the STM tip and substrate. Moreover, we also studied the Raman spectrum at the single-molecule scale by using the tip enhanced Raman scattering (TERS) technique. This technique could even reveal the configuration differences of adsorbed single molecules and enable the chemical mapping of a single molecule with sub-molecular spatical resolution. The obtained spatial mapping reveals typical sub-molecular feature together with a "fingerprint" spectrum for each pixel, with unprecedented spatial resolution below1nm. In this thesis, we combine the first-principles calculations with electrodynamics so as to theoretically simulate and understand the experimental phenomena observed in the single-molecule Raman spectroscopy and STM induced luminescence.In Chapter One, the surface plasmonics and single-molecule spectroscopy were briefly introduced, which provides basic knowledge for understanding the theories adopted in the subsequent chapters. The principle and development of the tip enhanced single-molecule spectroscopy technique were discussed especially. The chapter is concluded with an outline of the thesis and its content.In Chapter Two, four types of selection rules were deduced from the quantum theory of molecular optical response for single-molecule Raman scattering process:the transition selection rule related with vibrational levels of molecules, the symmetry selection rule related with the symmetry of molecules, the spatial selection rule related with the incident-detection configurations, and the surface selection rule related with the axial tip plasmons, in which the surface selection rule is the main part of this thesis.In Chapter Three, the surface selection rule was applied to the TERS study of the single H2TBPP molecule. By comparing with the experimental results, the differences in Raman spectra between single H2TBPP molecules and powder samples were analyzed and explained from the surface selection rule. The differences in TERS spectra of single molecules with different adsorption cinfigurations were also related with the surface selection rule. Moreover, the chemical mapping of single molecules was also simulated from the density functional theory. The stimulated Raman scattering associated with a nonlinear optical process were introcuded to explain the sub-nanometer spatial resolution in the single-molecule TERS mapping.In Chapter Four, we investigate the TERS spectra of small4,4’-bipyridine molecules on Ag(111). The TERS spectra for both isolated single molecules at the step edges and molecular islands on the terraces show similar peak shifts relative to the powder Raman spectrum, but there exists sharp contrast in the relative peak intensities as well. By combining with theoretical simulations, the spectral shifts are attributed mainly to the charge transfer between the bipyridine molecules and Ag substrate. On the other hand, the sharp intensity contrast can be ascribed to different local molecular environments when taking into account the surface Raman selection rule.In Chapter Five, the STM induced luminescence from an isoalted single molecule was preliminarily studied. The concept of transition density was introduced and the excitation rate of STML was deduced from first-principles calculations. The image charge method was adopted to obtain the fluorescend mapping simulations and for preliminary understanding of the STML process on the single-molecule level.