Development of metal-assisted chemical etching (MACE) based silicon (Si) nano/microstructures and their applications in surface enhanced Raman spectroscopy (SERS) for chemical sensing

Abstract

Surface-enhanced Raman Scattering (SERS) has emerged as a key research area for its excellent detection capability and fingerprint identification of the various chemicals such as methylene blue (MB), malachite green (MG), melamine etc. Recently, it has drawn tremendous attention for potential application in the food sector, medical sector, security sector etc. Though some progresses have already been achieved in the molecular level detection of chemical species with SERS technique, still more challenges are there such as repeatability, scalability, reproducibility, shelf life. One of the key factors is the development of the SERS substrates for improvements in the SERS signal and detection limit as well. In this regard, significant progress has already been achieved in the fabrication of the SERS-active substrates by using both the top-down and bottom-up approaches. Bottom-up approaches, such chemical synthesis, self-assembly, electrodeposition, dip-coating processes are successfully employed to fabricate SERS-active substrates. On the other hand, the top-down approaches, such as lithography, oblique angle metal film deposition and metal-assisted chemical etching (MACE) and which are mainly used to fabricate various nanostructures, has shown very effective and reproducible SERS-active substrates preparation. However, still preparation of the SERS substrates with enhanced Raman signal and also with higher detection limit have been considered as the main challenges and need to be addressed. Though, several existing approaches such as conventional or unconventional nanofabrication methods are used for the preparation of Si-based SERS substrates by top-down approach, MACE process is emerging as a very simple and cost-effective process for fabrication of the nanostructures and hence, it has gained sufficient attention globally. Thus, the main objective of this present work is to fabricate Si nano/microstructure by using MACE process and then utilizing these Si substrates for detection of the methylene blue (MB).Different nano/microstructures were fabricated on Si by using noble metals catalyst such as gold (Au) and silver (Ag). The effect of different etching parameters such as thermal annealing of the metal catalyst, effect of the concentration of the oxidising agent, those are directly associated with the MACE process were investigated. The experimental results revealed that the thermal annealing process eliminated the pin-holes present in the metal catalyst when 50 nm thick Au acted as metal catalyst and successfully fabricated deeper micro-trenches on Si substrates than the un-annealed counterpart of it. Moreover, it also indicated that higher thickness of the metal catalyst micro-stripes help to fabricate flat bottom type deeper trench than the thinner counterpart. Furthermore, the mechanism for the obtained results are discussed in the thesis. The morphological effect of the metal micro-stripes was also examined and the results indicated that various nano/microstructures could be fabricated by controlling the morphology of the used metal catalyst. As it is very well known fact that the SERS detection limit or enhancement of Raman signal primarily depend on the fabricated nano/microstructures present on the surface of the Si substrate and incorporated with the noble metal (Ag) nanoparticles. The performance of the Si-based SERS substrates was tested for the chemical detection of MB with very low concentrations starting from nM to pM, and the result indicated that nanostructured Si substrate showed better SERS detection than its microstructured counterpart. The probable reasons for such behaviours are also discussed. More studies were also carried out to enhance the detection limit of the MB, since higher the detection limit is better for the usability of these substrates for molecular level detection of different chemical species. Thus, combination of discontinuous Au film and Ag nanoparticles (NPs) were deposited on the nanoporous Si substrates, those were obtained by MACE process and the result suggested that the detection limit of MB can be further improved by depositing discontinuous Au thin film with various thicknesses. It is also noticedfrom the experimental results that up to certain thickness of the Au discontinuous thin films, it helps to improve the limit. On the other hand, higher thickness of the Au discontinuous thin films, degrades the detection limit. Thus, there is a direct correlation between the detection limit and the thickness of the Au discontinuous film on the nanoporous Si substrate. Our experimental findings suggested that the best detection could be achieved when a combination of 30 nm Au discontinuous thin film and Ag NPs were used for preparation of the SERS-active substrates which can detect 10 pM MB. Furthermore, highest enhancement factor of the order of 108 is also achieved with this same combination of Au discontinuous thin film and Ag NPs. Thus, we believe that the prepared SERS-active substrate in this work can have the capability to detect MB at molecular level. We believe that the easy, simple and reusable SERS-active substrates will be more helpful to develop chemical sensors to detect molecular level chemical species other than MB with higher enhancement factor and can be utilized for the food safety, environmental monitoring, security and medical diagnosis in the near future.