Synthesis, Characterization And Optical Properties Of Nanostructured Silicon

Due to the size-dependent quantum confinement, Si nanowires arrays (SiNAs), Si nanowires (SiNWs) and Si quantum dots (SiQDs) have received intensive interests in recent years for their valuable electrical and optical properties, as well as their potential applications in nanoscale electronic and photonic devices and biological and medical area. Size-tunable nanostructured Si with better crystallinity and morphology and property is critical for further applications. However, systematical researches on convenient synthesis of nanostructured Si with demanded character are in serious requirement. In this paper, we focus our research on SiNAs, SiNWs and SiQDs.We fabricated aligned Si nanowires arrays by chemical etching crystal Si wafers using HF and H₂O₂ etching solution. Silver nanostructures were obtained via an electroless metal deposition process in aqueous HF and AgNO₃ solution. As can be seen in SEM images, the morphology of the etched Si substrates depends strongly on the concentration of the etching solution and the microstructure of the silver deposited on Si wafer. It's believed that the morphology of the SiNAs was highly determined by the microstructure of the nanoscale silver and the concentration of the etching solution.For SiNWs, it is important to make bulk-scale SiNWs with high crystallization, uniform size distribution and good optical properties. Here we develop a novel thermal evaporation method via a new-style high frequency induction furnace. A large quantity of products with different colors was deposited on the graphite substrate clearly by altering the parameters such as ratio of the raw materials, heating rate and carrier gas. The light yellow sponge-like products were characterized by XRD and TEM, which confirm the nanowires were the crystalline diamond cubic Si. HRTEM reveals that SiNWs capped with an oxide sheath were of high crystallinity, and the oxide sheath can be peeling off in aqueous HF solution without changing the crystallinity of the SiNWs core. FTIR spectra demonstrate the surface of SiNWs was capped with Si-O band, which is in accordance with the HRTEM results. PL spectra show that the optical property of SiNWs can be tuned by the size and a sharp blue-shift in PL can be obtained when the size of SiNWs were reduced in aqueous HF solution.Increasing interest has been attracted on the synthesis of SiQDs. Poor stability and water solubility of SiQDs with uncontrollable size have slowed their further applications in biochemistry and microelectronics. In order to alleviate these problems, much more attention has been focused on optimizing the synthesis process and surface functionalization of the SiQDs. Here we synthesized bulk scale of SiQDs by taking SiNWs as the raw materials via a simple chemical etching way. The crucial step was to etch the SiNWs in aqueous etching solution controllably. TEM and HRTEM images show that the SiQDs with uniform size distribution are of great crystallinity, and the diameter of SiQDs was about 4nm, which is close to the Bohr exciton radius. The exact changes in the transformation of SiQDs from SiNWs have shown obviously through TEM images. We proposed a growth model of SiQDs based on the morphological transformation, and discussed the growth mechanism of the SiQDs grown from SiNWs. FTIR spectra reveal clearly that the surface of the SiQDs was oxide terminated after being treated with HNO3 solution, and the stability of Si-O band is better than that of Si-H. The optical show that SiQDs terminated with oxide possess of good solubility and high stability in water, and blue-shift can be seen in optical spectra due to the quantum effect. Such water soluble SiQDs with uniform size distribution and stable optical property will have extensive application in biological areas.