Investigation And Applications Of Epitaxial Growth Of Silver Nanoparticles On Silicon Nanowire Surface

Silicon nanowires (SiNWs) have been attracted much attention in recent years due to their excellent optical and electronic properties,that have been widely used in the field of nanoelectronics,optoelectronics,solar cells, Li-ion batteries, as well as bio-and chemical sensors.In order to improve performances of SiNW-based divices or expand application of SiNWs further, SiNWs are usually combined with other materials. As Ag nanoparticles (AgNPs) have properties of low-cost, ease of preparation, excellent conductivity and plasmonic scattering effect, the composite of AgNPs and SiNWs has been intensely investigated in the field of Li-ion batteries, plasmonic solar cells and photoelectrochemical cells. Electroless chemical deposition, thermal evaporation, e-beam evaporation, and sol-gel method are often used to fabricate AgNP/SiNW array composite. However, these methods are not inadequate, some of which can only form nonuniform or sparse AgNPs, and some methods are too complicated.Herein, a brand new and convenient method to fabricate AgNP/SiNW array composite has been demonstrated. A simple annealing process of SiNWs etched by metal-assisted chemical etching (MaCE) method using Ag particles as catalysts is demanded to fabricate the composite. As this is a new fabrication method of Ag/Si composite, we have made a detaied investigation on growth mechanism, and its optical and molecular detecting properties. Some remarkable results have been achieved, as follows:(1) Epitaxial growth of AgNPs on the surface of SiNWs has been achieved by annealing SiNWs fabricated by MaCE using Ag particles as the catalysts. The nucleation of Ag on the surface of SiNWs is realized by direct decomposition of AgmO or the reaction between AgmO and Si depending on the annealing temperature. Along with the formation of AgNPs, a thin layer of SiO2 is formed on the outside surface of AgNPs, which increases with an increase in the annealing temperature. It has been found that uniform Ag nanoislands with a high density of 3700μm-2 epitaxially grow only in a temperature window between 500 and 600℃. The ambient-air epitaxial growth of AgNPs on the surface of SiNWs offers great flexibility in designing ideal metal contacts and for Schottky barrier formation, and for studying the electronic, magnetic and optical properties of the Si/Ag system.(2) The surface roughness of SiNWs would affect epitaxial nucleation and growth of Ag. With the increasing in etching temperature, the surface of SiNWs becomes rougher, affecting epitaxial nucleation of Ag more seriously, leading to a decreasing density of AgNPs. When the etching tempeture increases from 20℃ to 50℃, the density of AgNPs decreases from 3750μm-2 to 231μm-2.(3) The size of catalysts has a great effect on the surface roughness of SiNWs. The as-fabricated short SiNWs can be divied into two parts:rough upper part and smooth lower part. After annealing, few AgNPs can be found on the upper part, while dense AgNPs on the lower part. An obvious boundary line exists between these two parts, which decreases with increasing in size of catalysts. Growth of uniform AgNPs with high density (1000μm-2) on silicon wafer has been realized.(4) A highly stable 3D substrate for surface enhanced Raman scattering (SERS) has been prepared by surface migration of Ag on the surface of SiNWs. Along with the formation of AgNPs, a thin layer of compact SiO2 caps is formed on AgNPs, improving the stability of the SERS substrate significantly by suppressing aggregation of AgNPs during their long term storage in ambient air. The thin SiO2 layer, which protects the Ag from degradation, can be easily and quickly removed just prior to analysis of optimal SERS performance. It also has been demonstrated that the SERS substrates have high uniformity and reproducibility, which is beneficial for their applications.(5) The composite structure of SiO2@AgNPs@p-SiNWs based on SiNWs produced by MaCE method has been designed to realize the significant reflection suppression over a broad wavelength range (300-2500nm). Especially, the reflectivity of the structure even below 0.3% at a wide range of 620-1950 nm can be achieved. It also has been demonstrated that SiO2 capers play a dominant role in the significant reflection suppression of the composite structure.After a systematic research, we have a better understanding on the epitaxial growth of Ag on surface of SiNWs, the effects of silicon substrates on growth of AgNPs, and optical and molecular detecting properties of AgNP/SiNW array composite. Hereafter, we can further investigate the applications of this composite in other fields.