| Sažetak (engleski) | Surface-enhanced Raman scattering (SERS) is an exceptionally powerful vibrational spectroscopy technique, which finds wide applications in the identification and structural studies of biological materials and chemical substances. Among the various promising SERS-active substrates, it has been shown that non-expensive, easy-to-fabricate, stable in air, uniform, reproducible, and highly sensitive SERS-active substrates can be produced using noble-metal nanoparticles deposited or grown on the porous silicon surface. Porous silicon (pSi), most commonly obtained on top of a crystalline silicon (cSi) wafer by its anodization in hydrofluoric acid solution, is a versatile nanostructured material known for its many unique optical, chemical, and physical properties and its corresponding usage. By variation of the anodization parameters, porous layers with tunable pore sizes and thicknesses can be produced. Moreover, the obtained vast active surface area has an inherent property to spontaneously reduce metallic ions, which have positive reduction potentials with respect to hydrogen, when immersed in their aqueous solutions. Utilizing this feature, immersion plating has become the most common method to coat pSi with certain noble metals (Ag, Au, Cu) and form SERS-active substrates due to its simplicity, low cost, and, most importantly, control of the substrate morphology by a precise variation of the deposition conditions. So far, SERS measurements on pSi were conducted with excitation wavelengths in the visible spectral range, where the strongest Raman enhancement was expected due to the matching of the laser wavelength with the localized surface plasmon resonance of the metal/pSi substrates. Extending the wavelength range to the near-infrared (NIR) at 1064 nm, despite the loss of sensitivity due to the Raman’s scattering fourth-power dependence on the excitation frequency, has advantages in the absence of resonance-Raman effects for the majority of molecules, the reduction of fluorescent photobleaching and plasmonic heating, as well as the avoidance of possible photodegradation of biological molecules. Although NIR SERS with an excitation at 1064 nm has been demonstrated to be operative during the past 30 years, hitherto metal-coated pSi was not used as an SERS-active substrate for NIR excitation. The main reason is probably the deep penetration of NIR light inside cSi or pSi, hence such an excitation induces the photoluminescence (PL) of the underlying cSi which acts as a mechanical support for the porous layer, due to the energy matching with the cSi band gap, and consequently the SERS signal is concealed with the broad PL peak. More than 20 years ago, pSi multilayers were produced by the utilization of another of its important properties; the fact that already etched porous layers are not affected during the electrochemical anodization, i.e., cSi dissolution occurs only at the etching front, which is the interface between pSi and cSi. Thus, by varying the current density applied during the etch process, the porosity can be modulated in the direction perpendicular to the pSi surface and, as a result, almost any refractive index-depth profile can be realized. This allows the fabrication of a variety of pSi structures with desired optical properties and with a wide range of applications such as omnidirectional mirrors, chemical and biological sensors, waveguides, and biomolecular screening. Among those, periodic structures that can control the propagation of a certain frequency range of light are called porous silicon photonic crystals (PhC pSi). They are characterized by a high reflectivity stopband, which can be tuned to appear anywhere in the predetermined spectral region depending on the appropriate selection of fabrication parameters. The aim of this study is the structural optimization of reproducible SERS substrates for near-infrared (1064 𝑛𝑚) excitation in the form of PhC pSi covered with gold (Au) and silver (Ag) nanostructures. To obtain the SERS effect, PhC pSi with efficient reflectance in the NIR spectral range (~1064 𝑛𝑚) that quenches the cSi substrate band gap PL had to be produced. Here, we report a detailed fabrication of porous silicon rugate filters (pSi RFs), specific type of photonic crystal, and subsequent synthesis of nanostructured Ag and Au coating with appropriate morphology by immersion plating of pSi RFs in Ag and Au salt aqueous solution. Also, in order to match the excitation wavelength with the localized surface plasmon resonance of metal nanoparticle, and therefore to obtain even greater SERS enhancement, gold nanorods were synthesized via seed-mediated method and drop-casted on the surface of PhC pSi. The SERS activity of such substrates was evaluated using aqueous/ethanolic solutions of rhodamine 6G (R6G) and crystal violet (CV) dyes at various concentrations. To our knowledge, this is the first time that the 1064 nm NIR laser excitation is used for obtaining the SERS effect on porous silicon as a substrate. The thesis is divided into 6 chapters. The short theoretical description of the Raman scattering is given in the first chapter. The optical properties of crystalline silicon are described in the second chapter where the greatest emphasis is placed on the processes of photoluminescence and Raman scattering on the silicon crystals. The third chapter systematically describes porous silicon: its history, production, properties and usage. This chapter also gives a thorough theoretical description of porous silicon photonic crystals with additional paragraph regarding their production, properties and applications. The fourth chapter elucidates the amplification of the Raman signal during the process of surface-enhanced Raman scattering. Moreover, the influence of the noble metal nanoparticles with different shapes and sizes on the efficiency of SERS effect is explained. Furthermore, the synthesis of gold nanorods and the deposition of noble metals on the porous silicon surface is detailed. The materials and methods used in this study, including experimental setup for porous silicon photonic crystals production, gold nanorods synthesis, scanning (SEM) and transmission (TEM) electron microscopy, UV-vis-NIR and FT-Raman spectroscopy, are overviewed in the fifth chapter. Finally, in the last, sixth chapter, the results of the research are presented and discussed, and conclusions with possible directions for further scientific work are given. |
