Theoretical Study On Surface-enhanced Raman Scattering Mechanism Of Several Alloy Substrates

Raman scattering spectroscopy plays an important role in studying the spectroscopic properties and electronic structures of molecules.The location,intensity and shape of the Raman peaks are highly related to the geometric structure and chemical structure of molecules.Surface enhanced Raman scattering(SERS)refers to the phenomenon that the Raman scattering signals of probe molecules are significantly enhanced when they are adsorbed on or near the surface of nanostructures.At present,the widely accepted SERS enhancement mechanism mainly includes the electromagnetic enhancement mechanism(EM)and chemical enhancement mechanism(CM).SERS overcomes the drawback of normal Raman scattering(NRS)about weak signal and thus is widely used in the fields of life science,organic chemistry,catalytic chemistry and environmental detection.Since the factors that affect Raman scattering intensity are plentiful and complicated,it is difficult to distinguish the contribution of different SERS enhancement mechanisms in experiments.However,the theoretical study can further analyze the microcosmic enhancement mechanism of SERS on the basis of the experimental research.In this paper,the quantum chemistry method is used to simulate the SERS spectra and absorption spectra of molecules adsorbed on several alloy substrates.A method is used to calculate surface-enhanced resonance Raman scattering(SERRS)intensity from the geometrical derivatives of the frequency-dependent polarizability,and the contributions of the EM and CM are analyzed by employing a method of visualization on the electronic transition orbital and natural transition orbital.The factors influencing SERS enhancement are systematically investigated,including the composition of the substrate,charge transfer in different forms,adsorption sites,applied electric fields and coupling effects.This work mainly focused on the following three parts:1.In order to explore the coupling effect on SERS signals,we theoretically investigated the charge transfer enhancement mechanism in SERS of pyrazine adsorbed on Ac@Au7 clusters by using Ac@Au7-pyrazine-Ac@Au7 as a model.The calculation results of coupling model Ac@Au7-pyrazine-Ac@Au7 under different adsorption methods are discussed and compared with the simulation results of the uncoupling model pyrazine-Ac@Au7.The results show that the coupling model can significantly enhance the charge transfer between molecules and metals and increase the SERS intensity by one order of magnitude than the uncoupled model.In addition,the change of coupling ways can also result in nonnegligible effects on the SERS signals of this system.2.In order to explore the enhancement mechanism in SERS of probe molecule adsorbed on a new gold-aluminium alloy SERS substrate,the SERS spectra and absorption spectra of model pyrazine-Au5Al5 have been simulated,and the contributions of the EM and CM are analyzed by employing a method of visualization on the electronic transition orbital.Under the excitation of incident light at different wavelengths,the SERS signals of pyrazine-AusAls model can be significantly enhanced through the charge transfer enhancement mechanism or electromagnetic field enhancement mechanism.Moreover,all of the four pyrazine-AusAls complexes in this study have strong SERRS signals in the ultraviolet region.Hence,gold-aluminium alloy clusters is good candidate for ultraviolet SERS material.In addition,the optical properties of the system can be tuned by altering the binding sites of the probe molecule to the substrate cluster.3.In order to further explore the microscopic enhancement mechanism of gold-silver alloy SERS substrate,the influence of substrate composition and applied electric field on SERS have been systematically studied with the model of pyridine-AumAgn(m+n=6).The SERS spectra and absorption spectra of pyridine molecule adsorbed on five different gold-silver alloy clusters are simulated and compared with the two pure metal substrate models with similar configuration,and the CM and EM in this system are discussed systematically.Then the influence of the applied electric field on SERS enhancement mechanism of Au-Ag alloy substrate is investigated by using the Au3Ag3 cluster as SERS substrate.The applied electric field changes the structure and polarizability of the complex in the ground state,which affects the intensity and position of the static Raman peaks of pyridine-Au3Ag3.The energy and intensity of the charge transfer transitions and intrametallic transitions varies with the electric field change,thus affecting the enhancement factors of chemical enhancement and electromagnetic field enhancement.In addition,the intensity of some specific Raman vibration modes can be selectively influenced by changing the direction of electric field.