Fabrication and characterisation of metal/porous silicon hybrid structures as SERS substrates
2017-02-23T04:26:02Z (GMT) by
This thesis presents the results of a study on the fabrication and utilisation of SERS (Surface Enhanced Raman Spectroscopy) substrates based on porous silicon (PSi) decorated with two nanostructured noble metals. Two novel hybrid structures (Au / PSi and Ag / PSi) were successfully developed to provide highly sensitive and reliable substrates for SERS measurements. To gain a good understanding of the new materials, microstructural and chemical properties of the substrates (PSi) were characterised by using electron (scanning and transmission) microscopy and Raman spectroscopy. The thesis comprises three parts and each part consists of two chapters. Part I reviews the available literature on the PSi fabrication methods, properties, characterisation techniques and its applications and also highlights existing gaps needing further exploration. Characterisation of PSi by electron microscopy techniques and Raman spectroscopy are the areas of most focus. It also describes all the fabrication and characterisation techniques used in this study. Part II focuses on the fabrication and characterisation of PSi by the mentioned techniques, while Part III presents the results of the SERS measurements performed on PSi layers decorated with noble metal nanoparticles and their fabrication process. The first chapter presents a brief introduction to silicon micro- and optoelectronics, the emphasis on the universal need for the silicon nanostructures with altered optical properties for the contemporary optoelectronic circuits and large area integration technology. This includes demonstration of the role of silicon nanostructures and in particular, PSi properties, fabrication methods and applications. This chapter also provides a brief introduction to Raman spectroscopy as a tool for the characterisation of silicon nanostructures. Also, the state of the art of the use of PSi as SERS substrate is reviewed. Chapter 2 details the techniques and methods used in this study for the preparation and characterisation of the samples. This chapter is sectioned in two parts with the first part dealing with the PSi fabrication techniques and instrumentation, while the second part focuses on the characterisation and preparation techniques used in the length of this study. This includes a brief introduction to the basic science and instrumentation of each technique including PSi fabrication, electron microscopy techniques, Raman spectroscopy, surface enhanced Raman spectroscopy (SERS), focused ion beam (FIB), as well as the selected experimental conditions. The results of the detailed characterisation of meso-PSi using electron microscopy (EM) techniques and Raman spectroscopy are presented in part II of this thesis comprising two chapters. In Chapter 3, the detailed microstructure and surface chemistry of meso-PSi are presented. Three distinct classes of pore morphology were identified as a function of fabrication parameters for their ability to provide the right pore size and morphology for desired applications. For the first time, the surface chemistry of the different classes were identified and correlated with the pore morphology. Chapter 4 presents the results of a detailed Raman spectroscopy study of meso-PSi, further supported with electron microscopic images. This chapter also discusses the usage of the phonon confinement model (PCM) to estimate the crystallite size of the silicon nanostructures. The study reveals that Raman spectroscopy should be used very carefully to obtain reliable data on the microstructure of the sample as the laser irradiation can modify the surface of the nanostructures permanently. It is further shown that local heating of the samples with Raman laser is predominantly responsible for the inconsistent data reported in the literature. The suitable condition for the use of phonon confinement model (PCM) for the estimation of the crystallite size of PSi samples is also argued. It is shown that, to avoid errors in the estimations, a great deal of care in selecting the Raman measurement conditions is necessary. The use of PSi layers as SERS substrate is further explored in Part III. First silver (Chapter 5) and then gold (Chapter 6) were deposited on different PSi substrates including meso, micro and macro-PSi. The depositions were achieved by using a simple but novel wet chemical method based on galvanic displacement processes. The silver particles crystallised with a dendritic shape and were characterised as single crystals. All the substrates demonstrated an excellent Raman signal enhancement. However, macro-PSi outperformed the other substrate types. The silver nano-dendrites were further characterised by transmission and scanning electron microscopes (TEM and SEM) to reveal their crystalline and structural properties. After successfully studying their enhancement capability, they were used for the detection of three different antibiotics in concentrations down to micromolar levels. The results of the antibiotic sensing are presented in Chapter 5. Deposition of gold on the surface of macro-PSi produced gold crystallites with very sharp tips, which are described as gold nanothorns (Chapter 6). The gold nanothorns were the result of reducing gold chloride with hydrofluoric acid on the surface of PSi. Their characteristic shape was associated with excellent SERS properties, in particular resulting in a strong signal enhancement in the Raman spectra of crystal violet (a typical dye for SERS). The substrate’s enhancement factor was calculated with care and demonstrated the ability to achieve single molecular detection for this substrate. The gold nanothorns were further characterised by SEM, TEM, XRD and UV-Vis and their single crystallinity, characteristic sizes (i.e length, tip radius and angle) and optical properties were revealed.