The Physical Properties Of Silicon Nanowire Arrays And And Its SERS Applications

Due to the microstructure and quantum confinement, Silicon nanowire arrays have received intensive interests in recent years for their unique electrical and optical properties and potential for device application as solar cell, biosensors, field emission device and lithium-ion battery. Until now, there have been various methods for synthesizing silicon nanowires (SiNWs), such as laser ablation, chemical vapor deposition, thermal evaporation, molecular beam epitaxy. However, these growth process complicated in fabrication and costly in preparation procedures. It is difficult to realize controlled fabrication of silicon nanowire arrays with controllable crystallographic orientation to the substrate surface. Recently, Peng et al. have successfully prepared large-area oriented SiNWs array on silicon utilizing metal-assisted electroless etching method(MacEtch). MacEtch method enables control of various parameters (e.g., diameter, length, crystallographic orientation, and porosity) of SiNWs by adjusting the etchant concentration and etching time. Lager-area uniform SiNW array were prepared by MacEtch. We put forward the related theoretical model to explain the change of etching direction in MacEtch process. The surface state of SiNWs was also discussed in detail. SiNWs arrays coated with Ag nanoparticles act as efficient surface-enhanced Raman scattering (SERS) substrate, which have exhibited distinct surface-enhanced Raman scattering. The main contents are as follows:(1) SiNWs arrays have been fabricated by a metal-assisted chemical etching method. A layer of amorphous shell of SiOx was naturally generated on the surfaces of SiNWs arrays. The shell of amorphous SiOx and the core of the SiNWs show lattice mismatch, which resulted in the tensile stress at the Si/SiOx interface of SiNWs. The Si dangling bond interface center Pbo is the prominent defect at Si/SiOx interface. The surface modification with copper nanoparticles can decrease the concentration of Pbo centers on the surfaces of SiNWs. Photoluminescence (PL) spectra showed that bule peak at407nm (3ev) and red peak at656nm (1.9ev), which were mainly come from oxygen vacancies and oxygen deficiency at Si/SiOx interface.(2) Three types of Si wafers, which were n-type (100), n-type (110) Si, n-type (111) Si, have been used as starting wafers. Ag nanoparticles with different Length/Radius ratio was deposited on silicon wafer surfaces. Our experiments indicated that the Length/Radius ratio of Ag catalysts show different etching behavior on three types of Si substrates. The etching directions of (110) and (111) Si substrates are found to be influenced by the Length/Radius ratio of Ag catalysts, whereases the metal-assisted etching direction of (100)Si substrate was found to be vertical to the surface of (100)Si substrate. On the basis of morphology-dependent etching, we concluded that the radial and lateral corrosion behavior are coexist during the process of metal-assisted chemical etching, especially as Ag catalysts with huge Length/Radius ratio. The direction of the SiNWs arrays relative to the Si substrate can been controlled by adjusting the Length/Radius ratio of Ag catalysts, and with this way we can also achieved multiple zigzag silicon nanowires.(3) A simple and cost-effective chemical method was introduced to assemble Au nanoparticles on smooth Ag spheres for realizing SERS enhancement by the replacement reaction between chloroauric acid and Ag spheres. In addition, the Ag-Au core-shell spheres were fabricated when a certain amount of chloroauric acid was used in the reaction solution. Ag particles decorated with small Au nanoparticles demonstrated the strongest SERS enhancement, while Ag-Au core-shell spheres showed the weakest enhancement.In our study, we revealed the mechanism of anisotropic etching behavior in MacEtch process and made a detailed study about surface states of silicon nanowires. SERS substrates were prepared with SiNW arrays. In future work, we would expand the application fields of SiNW arrays and make the detailed study of the effect of surface states on the physical properties of SiNW arrays.