Applications Of Nanocavity Plasmons In Tip Enhanced Raman Spectroscopy And Scanning Tunneling Microscope Induced Molecular Emission

Tip enhanced Raman spectroscopy(TERS)and scanning tunneling microscope(STM)induced molecular emission are two important research tools to study the interactions and conversions among the molecules,the electrons,the photons,the phonons and the plasmons at nanoscale.In this thesis,we first apply the TERS technique to experimentally study the adsorption behaviors of the single molecules on the metal surface owing to the enhancement and local response of the Raman signals from the single molecules in the plasmonic nanocavity.Then,the interaction between the nanocavity plasmon(NCP)and the single molecules is studied theoretically by using the quantum master equtioan method,and the effect of the nanocavity plasmon in the STM induced molecular emission is futher explored.In Chapter One.we introduced the basic principle and backgrounds related to the works in this thesis.First,the basic knowledge of the surface plasmons are briefly introduced including the surface plasmon polariton(SPP)and the localized surface plasmon(LSP).Then,based on the concepts of STIM and NCP.we introduced the basic principles of TERS and STM induced molecular emission.Finally,an outline of the main works in the thesis is brought out.In Chapter Two,by the enhancement of the single-molecule spectral signals from the nanocavity plasmon,the adsorption behavior of the reductive porphyrin molecules on silver surface is studied by using the TERS technique.By analyzing the STM topological images and TERS spectra,we find three distinctive adsorption types of the reductive porphyrin molecules on bare metal surface.According to the publications reported before and the atomically resolved STM images of the silver crystal surface where the molecules are adsorbed,these three respective TERS spectra can be attributed to three adsorption configurations of the reductive porphyrin molecules on the hollow site,bridge site and top site on the silver surface,respectively.Furthermore,by manipulating the three different absorbing types by STM and comparing the corresponding STM images and the TERS spectra,we found that if the manipulation is performed at the molecular center,the second and third types of the molecules can be both transformed into the first type.While if the molecular lobes are manipulated,the conformations of the first type and the second type molecules can be changed where some lobes are pressed to be lower than the others in the molecule.We also measured the TERS spectrum at varying locations inside the single molecule,and it is found that the TERS spectra on different sites including the molecular center,lobe,and gap are all different.In the end,by measuring the TERS spectra of the reductive porphyrin molecules on sodium chloride(NaCl)grown on the silver surface,we found that the TERS spectrum on the NaCI surface are different from that on the silver surface,which is more similar to the powder Raman spectra,indicating that the sodium chloride can decouple the molecule from the silver to some extent by decreasing the interactions between the molecule and the silver substrate.All the above TERS measurements corresponding to different adsorption configurations have fully demonstrated the high locality of the nanocavity plasmon and the resulted ultra-high spatial resolution of the TERS technique.In order to further study the interaction between the nanocavity plasmon and the single molecule,in Chapter Three,we study theoretically different roles of nanocavity plasmons in STM induced light emission upon selective initial excitation of molecules or plasmons by using a microscopic quantum model.The time evolution and spectroscopic properties of the emission from the coupled plasmon-molecule system in each case are studied using time-dependent quantum mater equations.When the STM tip is placed on the molecule to ensure direct carrier injection induced molecular excitation,the major role of the plasmons is to enhance the molecular emission via increasing its radiative decay rate,resulting in sharp molecule-specific emission peaks.On the other hand,when the STM tip is located in close proximity to the edge of the molecule but without direct carrier injection into the molecule,the role of the plasmon-molecule coupling is to cause the destructive interferences between the two quantum objects,leading to the occurrence of Fano dips around the energy of the molecular exciton in the plasmonic emission spectra.In the end,we also discussed the influence of the molecular radiation rate on the spectral lineshapes of the molecular emission.