Enhancement of Raman Spectroscopy by Self-organized WO₃-Ag Porous Nanostructure

Abstract

This thesis reports the preparation, formation and analysis of the anodic WO₃ and WO₃-Ag for Raman signal enhancement. Raman spectroscopy detection is an important technique for analysis of organic compounds. Currently there are two types of Raman spectroscopy enhancement methods, including surface-enhanced Raman spectroscopy (SERS) and photo-induced enhanced Raman spectroscopy (PIERS), aiming to measure the molecules in solutions even at very low concentration. The signals from SERS substrate that has the noble-metal nanoparticles are much higher than that of ordinary Raman spectroscopy, while the PIERS exhibits even larger signal than SERS by the interaction between plasmonic nanoparticles and a photo-activated substrate. Tungsten oxide is a n-type semiconductor with a small band gap which has the advantages of being able to harvest visible light. In this research, anodization of tungsten (W) metal has been used to synthesize nanostructured WO₃. Anodization is a simple and easy electrochemical process with an anode, cathode, electrolyte and DC electric supply, and can be converted to mass production. The morphology of fabricated WO₃ is nanopores, and the microanalysis indicated it contains tungsten, oxygen with an atom ratio approximately of 1 to 3. The efficiency of substrates with different treatments on the enhancement of the Raman signals is investigated. The preparation of WO₃ films with relatively uniform and regular morphology can be achieved with the optimized experimental condition by anodization of W metal sheets. In order to obtain the strong Raman signals on the substrate, the fabricated WO₃ is modified with silver (Ag) nanoparticles to form WO₃-Ag substrate. There are three different types of substrates for the investigation of the Raman signal enhancement: (1) the annealed WO₃ substrate, (2) the annealed WO₃ coated with Ag nanoparticles, and (3) the annealed WO₃ coated with Ag particles followed by a thermal treatment called "dewetting". RhB dye was used in solution as the test material. The most effective substrate for the Raman enhancement is the WO₃-Ag substrate prepared through anodization, magnetron sputtering and thermal dewetting (Substrate 3). The enhancement of RhB Raman signal by the Substrate (3) was almost 200 times stronger than the annealed WO₃ (Substrate 1)