Theoretical Studies Of The Influence Factors On Surface Enhanced Raman Scattering

Surface enhanced Raman scattering (SERS) effect is a phenomenon that the Raman signal of molecule is enhanced when it adsorbs on or is close to the surface of nanostructures. Since SERS can overcome the shortcomings of low sensitivity in normal Raman scattering, it is widely applied in many areas such as surface science, analytical science and biological science. There are two generally accepted enhancement mechanism at present, which are electromagnetic enhancement mechanism and chemical enhancement, respectively. The issues that affect SERS are plentiful and complicated, thus it is difficult to determine the chemical contribution to the SERS in experiments. Theoretical study can provide more microcosmic and detailed exploration of chemical enhancement. This thesis mainly focuses on chemical enhancement of SERS in different systems by calculating their SERS and absorption spectra using density functional theory (DFT) and time-dependent DFT (TDDFT) method. The issues that affect chemical enhancement in SERS including the charge transfer in different forms, external electric field, graphene as substrate and doped effect, are discussed in details as follows:1. The influence of charge transfer (CT) in in different forms on chemical enhancement in SERS is investigated by calculating the SERS and surface-enhanced resonance Raman Scattering (SERRS) spectra of the 1,4-Benzenedithiol molecule in the junction of two Au3 clusters. In order to investigate the contribution of charge transfer (CT) enhancement, the wavelengths of incident light are chosen to be at resonance with four representative excited states, which correspond to CT in four different forms. Compared with SERS spectrum. SERRS spectra are enhanced enormously with distinct enhancement factors, which can be attributed to CT resonance in different forms.2. The influence of a static external electric field on SERS is investigated by exploring the Raman spectra and excited state properties of Pyridine-Au20 complex under an exteral electric field. This field slightly changes the equilibrium geometry and polarisabilities, which results in shifted vibration frequencies and selectively enhanced Raman intensities. Further, the energy level of CT transition is tuned by electric field to match or mismatch the resonance condition, leading to the modification of enhancement factor for Raman signal. At the same time, the intensities of vibration modes are sensitive to the orientation of the field. The positive and negative electric field enhances and suppresses some vibrational mode, respectively.3. The influence of graphene and Bboron-doped graphene substrate on SERS is investigated with the Ag2-PATP-Au2 junction adsorbed on graphene and Boron-doped graphene as study model. The CT mechanism in this system is dicussed in details. The interactions between the graphene and junction result in charge redistribution on the junction, and then the changes of static polarizabilities, which directly influence the enhancement of normal Raman spectra. The absorption spectra show that the graphene and Boron-doped graphene induce some CT excited states in the visible and infrared regions. When the energy of incident light is close to the energy of these CT excited states, these electronic transitions will be excited, leading to the enhancement of pre-resonant Raman scattering (pre-RRS) spectra. In pre-RRS spectra, the B-doped model is of stronger Raman intensities, since it produces more CT excited states with intense oscillator strength near the incident light than the graphene model. The charge difference densities (CDDs) method was employed to directly visualize the CT from graphene sheet to PATP molecule.