Theoretical Study On Surface-enhanced Raman Scattering Of Molecules And Molecules-Au_n Complex

Due to its high performance,SERS(Surface-enhanced Raman scattering)has been well-developed.The unknown explanation of SERS sets back its development further.In order to fundamentally solve the problem,it is necessary to use theoretical simulation for advantageous processing.In this paper,we use chemical simulation methods to study the enhancement mechanism of Surface-enhanced Raman spectra of 1,4-benzenedithiol(1,4-BDT),pyrazine(Py)and their complex with gold cluster from the aspects of adsorption site,adsorption configuration and size effect.By optimizing calculations,we found that the 1,4-BDT is a non-planar and transconfiguration in non-solvent,a planar and cis-configuration in solvent(water,ethanol).The regular red or blue shifts of the Raman spectra occur in the solvent relative to nonsolvent,and the findings are in agreement with the experimental results.We point out that the strong vibration peak of 425cm-1 in the Raman spectrum of dimer which is built by two 1,4-BDT is belonged to the vibration of the S-S bond,and the Raman resonance enhancement factor is 5 or 6.The absorption spectra of 1,4-BDT-Aun(n=1,3,5)are red shifted with the increase of number of the gold atoms.According to the Raman spectra of 1,4-BDT,1,4-BDT-Au3 and dimer-Au3,we explore the enhanced-factor of the Raman intensity of dimer-Au3 compared with 1,4-BDT.We further study the effect of different strength and direction of the static electric field on the ground state and excited state properties of 1,4-BDT-Au2 complexes by using DFT and TDDFT.The results show that vibrational wavenumber frequency shift and electric field present the firstorder Stark effect,and electric field have a stronger role in the regulation of Raman intensity of the totally symmetric vibration mode than non-totally symmetric vibration mode.The applied electric field can also adjust the excited state energy and the effect of pre-resonance Raman spectroscopy.The charge transfer resonance transitions(metal to molecule)selectively enhance the Raman spectra of some totally and non-totally symmetric vibrational modes in the Raman spectra.According to these researches,we prove that the SERS phenomenon of the molecular level can be adjusted and controlled by the electric field,the conclusions can guide the experimental application of the SERS effect multi-parameter control.For further research in this field,we use DFT and TDDFT to study the SERS effect of Au3-Py("off" mode)and Au3-Py-Au3("on" mode).We find that special split phenomenon of Raman spectrum of the "off" mode through comparing the Raman spectra of Py,Au3-Py and Au3-Py-Au3.And the split phenomenon can be used to detect the formation of single molecule conductance.The absorption spectra of the complexes are red shifted with the increase of the adsorbed gold clusters.We also simulate the Raman spectra of the "on /off" mode under the applied electric field,and explore the relationship between the electric field and the number of charge transfer,how the electric fields affect the Raman intensity and peak position.In this paper,we analyze the related properties of their energy,polarization,energy level distribution,charge density difference and so on,in order to better explain the mechanism of SERS enhancement.