| Since its inception in the late1970s, surface-enhanced Raman scattering (SERS) has proven to be a resilient and intriguing subject, engendering thousands of articles and along with these considerable controversy as well. SERS has been a growing area of interest in recent years due to its potential as a reliable, high-resolution detection technique for extremely minute quantities of target molecules. In this thesis, we systematically studied the SERS performance based on semiconductor/metal composite nanostructure.We used a simple two-step hydrothermal approach to engineer ZnO nanoflowers evenly coated with silver nanoparticles (NPs) and demonstrated their efficiency as organic molecule detectors in surface enhanced Raman Scattering (SERS). This kind of3-dimensional (3D) hierarchical substrate demonstrates high SERS sensitivity to rhodamine6G (R6G), with enhancement factors (EF) up to109. The SERS maps collected by point-by-point show that a few fortuitous fractions of the substrates can yield "hot spots", which get super-high spectra intensity even at relatively lower concentration of the detected organic molecule. Amplified by this combination of hierarchical nanostructured semiconductor and metal, the Raman enhancement capacity was improved compared to only Ag NPs. This can be ascribed to the intense electromagnetic field at the junction spots between Ag NPs, the charge transfer of semiconductor-molecule-metal assembly and the potential of the3D hierarchical nanostructure for creating more of the adsorption sites, increasing the generation probability of hot spots necessary to SERS.Multifunctional TiO2/Ag composite nanowires were fabricated via a hydrothermal method, with Ag NPs precipitating on the surface of TiO2nanowires. This hierarchical1-demensional (ID) nanostructure can be used as an SERS substrates with high sensitivity, detecting the R6G over a wide range of low concentrations (from1×10-6to1×10-12M). The enhancement factor of the TiO2/Ag composite nanowires is approximately1.94×108. This is attributed to physical SERS enhancement based on the interaction of the transition moment of an adsorbed molecule with the electric field of a surface plasmon resonance and the chemical SERS enhancement mechanism with additional electrical transport properties within these assemblies. In addition, the substrates can be self-cleaned under a ultraviolet (UV) light due to the superior photocatalytic capacity of TiO2/Ag composite nanostructure, making the recycle use of SERS substrates closer to reality. With both the evident SERS performance and high efficiency of photocatalytic capacity, such TiO2/Ag composite nanowires demonstrate considerable potential in chemical sensing of organic pollutants.Currently, two possible contributions to the enhancement factor have been recognized:(ⅰ) the surface plasmon resonance in the metal nanoparticle,(ⅱ) a charge-transfer resonance involving transfer of electrons between the molecule and the metal or semiconductor. These two components are often treated as independently contributing to the overall effect, with the implication that by properly choosing the experimental parameters, another can be ignored. Although varying experimental conditions can influence the relative degree to which each resonance influences the total enhancement, higher enhancements can often be obtained by combining two resonances. Furthermore, the relative enhancements of individual spectral peaks, and therefore the appearance of the spectrum, depend crucially on the exact extent to which each resonance makes a contribution.In summary, the work in this thesis is a combination of traditional metal SERS-active substrate with novel semiconductor SERS-active substrate, building the composite system for their synergetic contribution to SERS. These kinds of SERS-active nanomaterials have great potential in designing novel highly sensitive SERS substrates for the development of SERS-based sensing devices with a broad range of applications. |
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