Study Of Semiconductor-based And Transitional Metal Ions Doped Semiconductor-based SERS Substrates And The Enhancement Mechanism

Surface-enhanced Raman scattering (SERS) has been widely used as a powerfultool for structural analysis and supersensitive detection in a variety of fields frombiomaterials to solid states. Due to its convenient operation and spectroscopicmultiplexing, SERS is considered to be a promising universal method for singlemolecule detection. But there are still some obstacles in SERS application such as lowstability, moderate repeatability and narrow range of SERS substrates. Inconsideration of range of SERS substrates, the mechanism of novel substrates is notclear and research is ongoing. Up to now, only few substrates such as noble metalsand transition metals have been well studied and satisfied for application underpreferred conditions. Other substrates, mainly nonmetallic substrates, have not beeninvestigated as extensively as noble metals and transition metals. 1. Raman Investigation of Nanosized TiO2: Effect of Crystallite Size andQuantum ConfinementThe influence of the TiO2particle size on the enhanced Raman spectroscopyproperties was systematically investigated on the nanometer-size scale. We report onthe enhanced Raman spectrum of4-mercaptobenzoic acid adsorbed on TiO2nanoparticles. The results present in this study highlight the major findings that theintensities of both the molecular lines and the phonon modes of TiO2are stronglysize-dependent. The TiO2crystallite size estimated using the Scherrer equation wasvaried from6.8-14.2nm, as a function of crystal size there exist a large increase inintensity, with a maximum near10.9nm and a subsequent decline at larger sizes.Moreover, we investigate the quantum confinement effects between TiO2and theadsorbed molecules and attribute this to a charge-transfer resonance which isresponsible for the Raman enhancement.2. SERS Study of Molecules Adsorbed on Co-doped ZnO NanoparticlesTransition metal doped semiconductor nanoparticles (NPs) were well-studied fortheir optical and catalytic properties but seldom studied by surface-enhanced Ramanscattering (SERS). In this paper, transition metal doped semiconductor NPs wereinvestigated for their SERS property. Co-doped (0.5%,1%,3%and5%) ZnO(Co-ZnO) NPs and and Ni-doped (1%,3%and5%) ZnO NPs were synthesized andstudied. When4-mercaptobenzoic acid was used as probing molecule, significantSERS signals were obtained on all the samples. Moreover, it is very interesting toobserve a relationship between ions doping concentration and enhancement of SERSsignals. SERS intensities firstly increased with doping concentration, then decreasedwith further increased doping concentration. Charge-transfer (CT) is considered to bethe main contribution on this phenomenon. Different CT ratios from substrates tomolecules induced different intensities of SERS signals. In our experiments, thecrystalline defects of ions doped ZnO NPs caused by dopant affected CT ratios. Apossible mechanism of CT from valance band of ions doped ZnO NPs to lowerunoccupied molecular orbital of molecules via energy of surface states is suggested. X-ray photoelectron spectra, UV-vis spectra, and Raman spectra were used tocharacterize the structure and defects in ions doped ZnO NPs.3. Effects of Mn,Cu,Co and Mg doping on Surface Enhanced RamanScattering properties of TiO2NanoparticlesTiO2and transitional ions doped TiO2(Mn,Cu,Co and Mg) NPs weresynthesized by a sol-hydrothermal method for Surface Enhanced Raman Spectroscopystudy.When trasitional metal ions doped in the TiO2lattice, the SERS signal increasedfirstly and then decreased when the ions doping concentration increased. Thecharge-transfer mechansim play an important role in the SERS phenomenon.Meanwhile, we also studied the relationship between the SERS intensity changingtedency and the kind of the doping ion. The possible mechanism for the phenomena isalso discussed. This study opens up a new dimension for the family of SERSsubstrates.