Theoretical Study On The Optical Properties Of Single-Molecules Confined In Metallic Nanocavities

Optical measurements based on scanning tunneling microscope(STM)have developed rapidly in recent years and have enabled the detection of the optical properties of singlemolecules at the nanometer or even sub-nanometer level.In single-molecule measurements,the molecule has to be placed in a nanocavity formed by metallic electrodes to enhance its response.As a result,the detected signal could be affected by many factors such as localized surface plasmon supported in the nanocavity and the specific molecule-electrode couplings which are not considered in traditional theories.Therefore,it is important to develop theoretical and computational methods to analyze and interpret the results of such experiments.In single-molecule measurements,the molecule has to be placed in a nanocavity formed by metallic electrodes to enhance its response.As a result,the detected signal could be affected by many factors such as localized surface plasmon supported in the nanocavity and the specific molecule-electrode couplings which are not considered in traditional theories.Therefore,it is important to develop theoretical and computational methods to analyze and interpret the results of such experiments.Moreover,single-molecule experiments are often quite demanding on the measurement conditions which made it difficult to achieve.Thus,to investigate the possible new phenomena and/or new measurement schemes from a theoretical point of view could also be helpful for the further development of the fields.In this thesis,we studied the optical responses of single-molecule localized in metal nanocavities by combining density functional theory calculations and theoretical and computational methods developed for this type of systems.A new method for the measurement of vibronic coupling effect in molecular excited states as well as a new approach for the design of the single-molecule optical switch based on the in situ electric field in metallic nanojunctions were proposed.We also gave a detailed analysis of the fine structures in the single-molecule electroluminescence(EL)spectra of a porphyrin derivative.The main contents of the thesis are as follows:Firstly,the optical responses of single-molecules under the effect of spatially confined plasmonic field generated in the STM tip-substrate nanocavity were studied based on confined field theory.A new method,named tip-enhanced fluorescence excitation(TEFE),was proposed for the visualization of the excited state vibronic coupling effect in real space.As an example,the TEFE spectra and images of a porphine molecule were simulated at firstprinciples level.Calculation results show that the TEFE images of porphine are dependent on both the vibrational modes and excited states,which enables the real-space imaging of the excited state and vibration specific vibronic couplings.It was further demonstrated that the combinational transitions consisting of a Herzberg-Teller(HT)active mode and a FranckCondon(FC)active mode actually have the same pattern to the corresponding images of the involved HT modes.The intensity ratio between these two types of images is equal to the Huang-Rhys factors of the FC modes.Such an interesting result indicates that TEFE is not only capable of visually identifying different types of vibrational transitions,but also enables the direct measurement of the FC activity of molecular vibrations in a given excited state.Secondly,the EL properties of a fused porphyrin copolymer(fused-H2P)molecule were systematically studied by combined first-principle calculations and FC/HT simulations.By comparing the calculated vibrationally-resolved fluorescence spectrum with the experimental spectrum,it was found that the vibrational fine structures in the EL of fused-H2P mainly originates from the 0-1 transition of the total symmetric modes,which explains good correspondence between the measured EL spectrum and the calculated Raman spectrum in previous works.It was found that the asymmetric profile of the 0-0 peak in the EL spectrum of fused-H2P was mainly attributed to the coupling between the molecular exciton and the low frequency vibrations of the terthiophene chains that connects the molecule and the metal electrode.Finally,the STM induced EL properties of a double-decker molecule with throughspace charge transfer excited states were studied at the first-principles level.It was found that the in situ electric field between the STM tip and substrate generated by external bias voltages can cause a significant change to the excitation energy of the through-space charge transfer excited state due to the Stark effect.Such an interesting property can lead to the change of the order of such normally dark states and the bright non-charge-transfer excited state and eventually affect the luminescent of the molecule.Further simulations show that,the luminescence of the molecule can be switched between bright and dark by simply changing the bias polarity or the height of the STM tip under experimentally accessible conditions,which supplies a unique method for the design of controllable single-molecule optical switches.