Preparation, Study Of Ⅱ-Ⅵ Semiconductor Nanoparticles As Enhanced Raman Scattering Substrates

Enhanced Raman spectroscopy can be a powerfully analytical tool for sensitive and selective molecular identification on rough surfaces. Previous study showed rough surfaces have been essentially restricted to the surfaces of noble and transitional metal. This research will extend the enhanced Raman scattering effect from metal to semiconductor fields. Semiconductor nanostructures are currently considered as potential building blocks for nanodevices. The performance and reliability of these devices strongly depend on the surface and interfacial properties of the constituent nanomaterials. Therefore, this study points to the very promising future of using enhanced Raman spectroscopy for understanding surface properties of semiconductor, chemisorption and reactions mechanism of molecules in some practical systems. This work focus on studying II-VI semiconductor nanoparticles as enhanced Raman scattering substrates. Our study is outlined as follows:Firstly, we prepared ZnO nanocrystals with different sizes. We have been able to observe the enhanced Raman scattering from 4-Mpy molecules adsorbed on ZnO nanocrystals, which display 103 enhancement relative to 4-Mpy in solution. The chemical enhancement is most likely responsible for the observed enhancement. Size effects on enhancement of ZnO nanocrystals and excitation wavelength-dependent behavior are clearly observed. Compared with the Raman signals from metals and semiconductor, the particular features of semiconductor substrate which are different from metal substrate are revealed.In order to demonstrate that semiconductor nanocrystals can generate enhanced Raman scattering further, we synthesized 4-Mpy surface-functionalized CdTe quantum dots with the size about 3 nm. The Raman signal of 4-Mpy adsorbed on CdTe quantum dots shows a 104 enhancement compared with that of bulk 4-Mpy. The observed spectrum includes the CdTe LO phonon mode as well as several Raman lines enhanced due to 4-Mpy molecule to the quantum dot surface. A charge-transfer mechanism is most likely responsible for the observed enhancement, since plasmon resonances are ruled out. Further observations include strong perturbation of the absorption spectrum of the quantum dots as well as suppression of the quantum dot photoluminescence due to the adsorbed molecule. In addition, the Raman spectrum of 4-Mpy adsorbed on CdTe quantum dots is considerably different from that on silver colloids due to the special size effect. Furthermore, we prepared ZnS, CdS, CuO nanocrystals, and evaluated their enhancement ability. In particular, we confirm that as-prepared CuO samples are only composed of high purity CuO nanocrystals, which excluded the possibility of existence of Cu.Finally, we prepared Ag/CdTe nanocomposite via self-organization process by electrostatic interaction between positively charged CdTe nanocrystals and negatively charged Ag nanoparticles, and examined their optical properties. The optical properties of Ag/CdTe nanocomposite are dependent on the mixing ratio of both nanoparticles. Photoluminescence of CdTe nanocrystals undergoes considerably quenching if CdTe nanocrystals are in excess, and SERS spectra of BVPP absorbed on Ag colloid became stronger if Ag nanoparticles are in excess. Nevertheless, SERS spectra for composite only exhibit the signals of the CdS nanocrystals, which reflected that prolonged refluxing during the synthesis leads to a partial hydrolysis of the thiols and to form mixed CdTe(S) nanocrystal, similar to CdTe/CdS core/shell structure.