Theoretical Studies Of Surface Electronic And Vibrational States In Surface-Enhanced Raman Spectroscopy

The greatly enhanced electromagnetic field generated by the plasmon resonance at the surfaces drives various experimental observations of electronic and vibrational excitations near the enhanced area.Especially in recent years,it has been discovered that plasmons generate hot carriers,and the subsequent electron transfer process to the adsorbed molecules leads to an improvement of catalytic performance.However,existing experimental and theoretical investigations have not yet given a consistent understanding of the relationship between the generation of hot carriers and the electronic structure of metal surfaces,and the effect of different bonding interactions between the adsorbate and catalytically active sites on the plasmon-driven electron transfer is a rapidly growing research area.In the plasmon enhanced spectroscopic experiments monitoring surface catalytic reactions,due to the bonding interaction between metal surfaces and molecules,low-frequency vibrational modes emerge from the potential energy surface.Moreover,these vibrational modes are broadly related to the interfacial electron transfer,theoretical study of these soft modes is currently in request.In this thesis,we carry out theoretical investigations in the following three specific issues:We calculated the band structure of the single crystal surface employing the slab model with two-dimensional periodicity,the surface states emerged from the band gaps of the bulk crystal.Projecting the density of states onto local atomic orbitals shows that these discrete surface states are originated from extending the states of free electrons of the bulk band,the surface states are spatially localized in the vicinity of the first layer due to the scattering of specific potential barrier of single crystal surface.The energy gap between the surface states matches well with the photonic energy of plasmonic excitation.Furthermore,we calculated the dielectric function of single crystal surfaces employing the linear response theory within random phase approximation.The results show the optical property strongly corresponds to the surface band structure in the energy range of surface plasmon excitation,while the dielectric response of single crystal surfaces are identical in higher range of excitation energy,indicating the specific surface states are more viable for the hot carrier generation.Combining with tip-enhanced Raman spectroscopy,we constructed catalytically active sites on transition metal surface,taking phenyl isocyanide as the probing molecule that bonds to metal surfaces with the prototypical coordinationbackdonation,we investigate the variation of adsorption interaction on different active sites utilizing the theoretical model and experimental technique with high spatial resolution,which can be characterized by plasmon enhanced vibrational spectroscopy in the gap of nano-structures.Calculations employing density functional theory method were performed to obtain the variation of local density of raw metallic states in the spatial transition from terrace sites to the step edge of defects on the single crystal surface,we discovered the edge sites are expected to have higher chemical activity.Further calculations of bonding states of molecules on the metallic sites revealed that the resonating hybridization is highly enhanced on the step edge.Moreover,the adsorption on step edge promotes a much smaller energy gap between the back-donated bonding states and Fermi level,indicating an energy alignment more viable for the hot electron transfer to the molecule.The interfacial process of hot-carrier transfer to adsorbed molecules and subsequent chemical reactions are broadly coupled with the vibration of soft modes containing the displacement of the metal-molecule bond.Furthermore,the reactants and intermediates of electrocatalysis are polarized by strong electric field,the potential energy surfaces are highly distorted,which identifies the vibrational Stark effect as a versatile experimental technique to monitor the reaction progress in catalysis.We performed theoretical calculation of Raman spectra for carbon monoxide adsorbed on platinum surface,however,the calculated vibrational spectra based on the harmonic approximation exhibit unstable numerical results with the variation of the applied electric field.We solved for the eigenstates of the anharmonic potential energy surfaces employing the discrete variable representation,a stable numerical profile of spectra variation are obtained,especially in the fine-tuning regime of the soft modes,which shifts only a few wavenumbers.In summary,we proposed a new possible pathway of photoelectric process in surface plasmon excitation;the electronic structure of catalytically active sites,the correlation between chemical bonding and vibrational spectra of adsorbed molecules are investigated with material simulation;stable numerical spectra of anharmonic vibration are obtained by optimizing the theoretical method.We expect in the future study,these theoretical proposals could be generalized to a systematic formalism,and could be applied to a greater extent of substantial instances.