Raman Spectroscopy Of 4-mercaptopyridine Adsorbed On Semiconductor Oxide Surfaces

Since its initial discovery, Surface-Enhanced Raman Scattering (SERS) has been studied diligently and has exhibited extraordinary potential for both life science and nanoscience. Though SERS has various advantages in studies of different fields, most SERS experiments have been essentially restricted on molecules adsorbed on metallic surfaces, especially surfaces of noble metals such as Ag and Au. Only a few reports describing SERS experiments on nonmetallic surfaces are available. However, the enhancement mechanism for SERS in semiconductor materials, which may be different from that of noble metals, is confusing.Different from noble metals, oxides can offer varieties of reaction centers which significantly enrich the ability to adsorb molecules so that the oxide surfaces represent an interesting depositional plat form for a large number of anchors such as carboxiylic, phosphonic and sulphonic acids. Both SERS and normal Raman spectroscopy are very sensitive to the small changes of molecular vibration. Therefore, the objective of this work was to use SERS and Raman spectroscopy to study the interaction between probe molecules and substrates. Due to special molecular structures of 4-MPy, it is sensitive to adsorptive properties of the substrates. So 4-MPy is chosen to investigate surfaces of semiconductor oxides. It may distinctly demonstrate the behavior of semiconductor oxides contacts and play an important role in understanding the enhancement mechanism in semiconductor materials.Our study is outlined as follows:1. The remarkable sensitivity of SERS for the study of adsorbed 4-MPy on a ZnO surface. New bands in the SERS spectra of 4-MPy have appeared, which were not previously reported in the molecule-metal system. The calculation of DFT for both 4-MPy and its dihydrate has been done to understand the molecule vibration. The 4-MPy molecules adsorbed onto the surface of ZnO are detailly studied through UV-Vis spectroscopy. We proposed two interaction modes between 4-MPy molecules and ZnO.2. To study Raman Spectra of 4-MPy adsorbed onto the surface of PbO, the investigation is done by means of Raman spectroscopy. PbO has a layered crystal structure and exists in two polymorphic forms: a tetragonal phase of red color,α-PbO, and an orthorhombic phase of yellow color,β-PbO. We first obtain the different Raman Spectra of 4-MPy adsorbed onto the surfaces of the two different crystal phases. UV-vis spectra are carried out to understand the interaction manner between 4-MPy and PbO. The assignment of Raman spectra is supported by theoretical spectra derived from quantum-chemical calculations. On the basis of the spectrum assignment, the common vibrations for all investigated systems are pertinent to each of them. Futhermore, the interactions between molecules and PbO were proposed. 3. Based on SERS and UV-vis spectra of the ZnO-4-MPy system, we illustrate a schematic of Charge-Transfer (CT) mechanis. The mechanism of the enhancement is simply discussed and we consider that the CT mechanism mainly account for the enhancement. Thought it is difficult to investigate the real CT mechanism between molecules and semiconductors, this simple understanding would be helpful for future research work.