Adaptation of SERS-Active Materials to Analysis of Certain Molecules By Electroless Corrosive Deposition of Coinage Metals on Silicon Nanostructures

Abstract

Here we present results of our research work on an electroless corrosive deposition of coinage metals on nanostructured silicon. Principal steps of nanostructured silicon fabrication and further deposition of metals allowing growth of metallic films controlled by dopant type of a silicon substrate, morphology of silicon skeleton and electroless deposition regimes are presented. Imprinting of the nanostructured pattern to metallic films by means of the sculpted silicon template opens their new morphological, optical, mechanical and electrical properties, which can be successfully applied in micro- and nanosystems and biomedicine. In the context of the latter, we pay special attention to activity of the metal-coated silicon nanostructures in surface enhanced Raman scattering (SERS) spectroscopy and profits provided both by using silicon templating and electroless deposition of metals. The electroless corrosion is mostly associated with undesirable surface destruction of different materials. Indeed, corrosion can lead to the irreparable harm both in macro- and nanoscaled worlds. Oxidation/degradation of metallic components of massive constructions in aggressive environment are very dangerous as well as corrosion of conductive metallic wires in ICs and MEMS causing further failure of the whole device. Surprisingly, but it has opened an opportunity to turn corrosion effect into positive way in some applications. For instance, since the 90-th of the last century corrosion of monocrystalline silicon (c-Si) in solutions for chemical cleaning, containing trace amounts of Cu ions, has been actively studied [1]. Solutions for chemical cleaning usually contain deionized water and HF. However, the Cu ions can unpredictably appear in solutions from environment on different stages of ICs production. Corrosion of c-Si also takes a place in HF-based solutions containing ions of other coinage metals (Ag, Au, etc.). Understanding mechanism of corrosion has allowed to apply this process for fabrication of the Cu conductive interconnections, which possess improved adhesion to the Si wafers [2]. New wave of interest to this process has been raised due to an opportunity to fabricate metallic nanostructures of variable morphology by electroless corrosive deposition of metals on silicon nanostructures. During our work related to the electroless corrosive deposition of coinage metals on silicon nanostructures we found that this process occurs according to mechanism of island film growth. It was revealed that Si nanocrystallites corrode faster than c-Si because of a great number of the surface states. Metallic atoms were observed to prevalently deposit on an external surface of silicon skeleton based on n-Si because of excess of electrons from dopant atoms. On the other hand, the inner growth of metallic atoms has been found to dominate on the substrates based on p-Si due to absence of dopant atoms supplying electrons. In the last case the reduction of metallic ions to the atomic form is connected only with taking electronts from the silicon atoms. As a result of our systematic investigations we developed a technique of electroless corrosive deposition of Cu, Ag and Au on nanostructured Si, which provides precise control of metallic deposit morphology from quasi-continuous films consisted of metallic nanoparticles to dendrites or nanovoids. The obtained films were found to show activity in the SERS-spectroscopy that is known as an extremely sensitive method of qualitative and quantitative analysis in analytical chemistry, biomedicine, environment and food control, forensics, etc. It was shown that nanosculpted silicon acts not only as a source of electrons for reduction of metal ions and template defining shape, sizes and special location of metallic nanostructures, but also prevents their fast oxidation and coalescence, which usually lead to the inhibition of SERS-activity. SERS-activity of the substrates was observed even for the 2 years aged samples. Morphology of these substrates can be adjusted to detect certain analyte molecules in dependence on their molecular weight including to achieve attomolar detection limit. [1] H. Morinaga, M. Suyama, T. Ohmi, J. Electrochem. Soc. 1994, 141, 2834. [2] J.L. Gole, L.T. Seals, P.T. Lillehei, J. Electrochem. Soc. 2000, 147, 3785–3789.