A Study Of Chemical Enhancement Effect About The Surface Of Self-assembled Molecules With SERS Activities

Surface-enhanced Raman spectroscopy has incomparable advantage over theother spectrum, such as: ultimate sensitivity, giving vast information of a singlemolecule on the substrate and so on. A wide range of attention have been attracted inacademia since the time of the discovery of SERS. Both electromagnetic enhancement(EM) and chemical enhancement (CM) contribute to SERS intensities, but much morework has focused on the former effect. The EM effect is caused by efficiencyenhancement of electromagnetic field via the excitation of localized surface plasmonpolaritons of the roughened metallic surface. It has nothing to do with the structuresof molecules or the process that the molecules get adsorbed on the metal and makechemical bondings with metal atoms. The enhancement factor (EF) of EM effect canreach as much as104-106. But it can not explain some experimental phenomenons,such as: different molecules with different SERS intensities, the molecules in the hotspots own the largest SERS intensities. For this renson, the CM effect had to beresearched.The CM effect involves in chemical bond, charge transfer, the increase ofmolecule polarizability, the key factor of CM effect is the presence of the strongechemical bond between molecules and metal atoms. It can solve the problem forwhich the EM unable to explain. The thesis will probe the change of CM effect of themolecules with various of structures by surface enhanced Raman spectroscopy. Theconcrete research contents and main conclusions are as follows：(1) The uniform and stable silver nanoparticles (AgNP) were prepared bychemical-reduction method. We used1.4-benzenedithiol (BDT) as probe moleculeand prepared several samples of BDT in solution and Au substrate. Vis the SERSmeasurements and finite-difference time-domain (FDTD) simulations for those BDTsamples, we can recognize the signal of BDT was enhanced vastly with theintroduction of AgNP. And the experimental enhancement factor (EF) was~106forAgNP with the FDTD simulation results. In addition, the vibrational acticity change of BDT in solution and Au substratewas revealed by surface-enhanced Raman spectroscopy. We can recognize that thevibrational acticity make a difference because of BDT with different geometries. Inthe SERS spectrum of symmetrical molecule structures, such as: nBDT-AgNP﹑Au-BDT-AgNP and Au-nBDT-AgNP, the ungerade vibrational modes of16b﹑20a﹑18a and19a did not appear. When the symmetry of molecule structures reduced tounsymmetrical, such as: BDT-AgNP and Au-BDT-PMMA-AgNP, those ungeradevibrational modes appeared again.(2) The key factor of CM effect is the presence of the stronge chemical bondbetween molecules and metal atoms. The CM effect features are dependent not onlyon the structures of molecule but also on the PH and the kinds of SERS substrates.Here, we researched the factor of molecule structures which impact on CM effect.1.4-benzenedithiol﹑thiophenol and1.4-benzenedimethanethiol were prepared andassembled on AgNP, via their SERS measurements, we examined the moleculestructures dependence of the CM effect. The results showed that no CM effect wouldform only if the molecule can make chemical bondings with metal atoms, such asthiophenol and1.4-benzenedithiol on Au substrate. What is more, the intensity of CMeffect would decrease if the molecule groups which may block the process of chargetransfer exist, just as-CH2of1.4-benzenedimethanethiol.