The fabrication of the porous silicon templates, the growth of the Co and Ag nanostructures onto porous silicon templates by electrodeposition method, structural and optical characterization

In this thesis, firstly porous silicon (PSi) templates were obtained on phosphor-doped and double-sided polished n-type Si (100) wafers with a resistivity of 1-10 Ωcm. The PSi samples were synthesized by anodization method in a solution of HF-ethanol consisting of 10%, 20% and 30% HF concentrations, at a current density in the range of 20-100 mA/cm2, at constant 30-min anodization. The effect of current density and HF concentration on the structural and optical properties of PSi samples was investigated by SEM, X-SEM, gravimetric method, Raman and photoluminescence spectroscopy. HF concentration determines the current density region over which the macro/mesopores formed, while the current density has important effects on the pore diameters, porosity, layer thicknesses and residual stress. Raman analyses have shown that the TO peak position of the PSi samples shifts to lower wavenumbers compared to that of the crystal silicon in the 20-60 mA/cm2 current density region, and the Raman shifts increase with increasing current density. It has been also found a broader and more asymmetrical appearance in the shape of peak and a remarkable decrease in the size of the Si nanocrystallites occur in the 20-60 mA/cm2 current density region. PL analyses have also revealed that a significantly increase in the PL peak intensity and a remarkable blue shift in the PL peak positions occur in the same current density region. These changes are explained in terms of the QCE model, based on the phenomenon of optical phonons confinement (quantum confinement effect) and the residual stress in the PSi structure. However, at current densities of 80 and 100 mA/cm2, Raman shifts and PL intensities were found to decrease due to the increase in the size of the nanocrystallites, indicating that the QCE effect cannot occur. We have decided to use the sample, with 20% HF concentration and the current density of 60 mA/cm2, owing to its the largest Raman shift and the highest PL intensity or light-emitting property, as a PSi template/or substrate in the growth of Co, Ag and Co/Ag nanostructures. Our second important goal in this thesis is to grow cobalt, silver and cobalt/silver metallic nanostructures in/on PSi templates by electrodeposition method, respectively, to improve the photoluminescence properties of the PSi sample/template. The fabrication of PSi/Co samples was carried out by electrochemical deposition method at different current densities ranging 0.1-0.5 mA/cm2 and deposition times of 5-60 min. From SEM analyses, it was found that the shape of Co nanoparticles changes from spherical structure (t≤15 min) to nano-flake structure (t≥15 min) with increasing deposition time and that their diameters increase with increasing current density. Raman analyses have indicated that a new peak originates from Si-Co-O bonds at 640-665 cm-1 in PSi/Co samples. It has been also found that in the range of 5-10 min, the peak position of Si TO phonon mode of the PSi/Co samples shifts to a longer wavenumber, and that the shape of peak becomes more asymmetric and the particle diameters decrease to less than 6 nm, accordingly, PL intensity increased about 4 times compared to that of the PSi sample. These changes have been explained in terms of the QCE model and the recombination mechanisms caused by surface states in comparison with that of the reference PSi. In the range of 20-60 min, it has been observed that a remarkable increase (4-5 times) in the diameter of Si nanocrystallites, compared to that of the PSi, and Raman peak shape symmetrized and the PL intensities decreased and quenched. These changes have indicated that QCE cannot occur and accordingly, these samples cannot have a light-emitting property. As a result, we may expect that the samples with 10 min at 0.2 and 0.3 mA cm2 with the remarkable PL property will make a significant contribution to the LED applications and the literature. Silver deposition on the PSi templates via electrodeposition method was carried out at three different concentrations of AgNO3 (1, 5 and 10 mM), at a current density in the range of 0.1-0.8 mA/cm2 and at four different deposition times (2, 3, 10 and 15 min). According to the SEM analyses, at 1 mM AgNO3, the Ag deposition is in the form of Ag nanoparticle clusters with a small amount of short nanorod/dendrite of Ag nanostructures, but for 5 mM AgNO3, nanoparticle clusters with a longer and thicker nanowire/dendrite occur. At 10 mM AgNO3, Ag nanostructures were formed only as spherical microparticles at the whole current densities and deposition times. Raman analyses have exhibited that five new peaks located at 190 and 230 cm-1, 470 cm-1, 1351 and 1580 cm-1 are originated from the metallic Ag bonds, Ag-O stretching bond, the Ag-O (Ag2O) bonds, respectively. It was observed that the intensity of Ag-O peaks around 1350 and 1580 cm-1 significantly increase, with increasing AgNO3 concentration and deposition time. PL analyses have shown that for 1 and 5 mM AgNO3, at short deposition times (t≤3 min) and low current densities (≤0.3 mA/cm2) PSi/Ag samples have 2-2.5 times stronger PL activity than both those of the reference PSi and all the other PSi/Ag samples. It was found that the PL activity is completely quenched for the samples, with 10 mM AgNO3 at the whole current density and deposition times. This study reveals that PSi/Ag samples, having significant PL property may be suitable for the LED applications, while samples, with intense Raman peaks, may be a candidate for the sensor applications. In the last chapter of this thesis, PSi/Co/Ag samples were synthesized. In the fabrication of PSi/Co/Ag samples, 0.1M CoSO4 and 0.4M H3BO3 values were kept constant, while AgNO3 concentrations, current densities and deposition times were selected to be 1, 5 and 10 mM; 0.2 and 0.3 mA/cm2; 2, 5, 8 and 10 mins, respectively. SEM analyses have revealed that at 1 and 5 mM AgNO3, firstly Co nanoparticle clusters appear on the surface, and then Ag nanoparticles formed on these Co clusters. It was observed that at 10 mM AgNO3, only Ag microparticles and/or the thick Ag film are formed. Raman analyses have shown that in addition to the peaks observed in the PSi, PSi/Co and PSi/Ag samples, a new peak, belonging to CoAg3 phase appeared at 875 cm-1, as confirmed by XRD analyses. Raman analyses have also shown that the intensity of Ag/Ag-O peaks, located at 1351 and 1590 cm-1 considerably increased (by a factor of 5-6), with increasing AgNO3 concentration and deposition time. This remarkable increase may be a result of three different Co-Ag structures (nanoparticles, dendrite and microparticles) and should be explained by the electromagnetic (EM) enhancement mechanism. PL analyses have revealed that the PL peak intensity of the samples of 1 mM AgNO3, with short deposition times, is slightly more intense than that of PSi sample, but considerably small in comparison with those of the PSi/Ag and PSi/Co samples, but for the samples, with 5 and 10 mM AgNO3, it is completely quenched over the whole current density and deposition time ranges. PSi/Co/Ag samples may contribute to sensor applications as SERS active materials, owing to their considerable intense Raman peak intensity, with respect to the PSi/Ag and PSi/Co samples.