Self assembled gold and silver nanoparticulates on silicon nanotips as surface enhanced Raman active substrates
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Self assembled gold and silver nanoparticulates on silicon nanotips as surface enhanced Raman active substrates
S. Chattopadhyay*a, H. C. Lo,* K. H. Chen,* C. H. Hsu,** L. C. Chen** * Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei-106, Taiwan ** Center for Condensed Matter Sciences, National Taiwan University, Taipei-106, Taiwan
Abstract:
Silicon nanotips, grown via electron cyclotron resonance plasma chemical vapour deposition, with apex diameters of ~2nm and lengths of 1000 nm and densities of 1011/cm2 were used as a new substrate for surface enhanced Raman spectroscopy. Ion beam sputtered gold and silver self assemble on these substrates as nanoparticulates of 4-10 nm diameter and these metallic nanoparticulates assist in the surface enhancement of Raman signals of analytes. Molecules such as Rhodamine 6G and bis-Pyridyl ethylene of varied concentrations, in the range of 10-6-10-10 M, has been studied on these substrates and enhancements in the range of 106-108 were observed. Introduction: Surface enhanced Raman spectroscopy (SERS) has evolved as a powerful and sensitive tool for detection and identification of a wide range of adsorbate molecules down to the limit of single molecule detection [1,2]. Weak Raman signals can be enhanced in presence of metallic nano or microparticulates by several orders making detection of analytes possible [3]. The enhancement is believed to be predominantly due to two mechanisms, a a
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long range classical electromagnetic (EM) effect and a chemical effect [4]. The respective contributions of these mechanism to the overall SERS enhancement is controversial but the EM effect is believed to be few orders more than that due to the chemical effect.
One important step for widespread application of SERS is the fabrication of proper substrates which are SERS active and provide high enhancement factors. The first substrate on which SERS was observed was electrochemically roughened silver (Ag) electrode [3]. Since then several metallic films and particulates have been tried [5-9]. These substrates generally demonstrate enhancement factor of 103-106. In this paper we report a nanostructured silicon substrate on which silver or gold nanoparticulates, with less than 10 nm diameters, can be self assembled with high densities that exhibit surface enhancement factors in excess of 106.
Experimental A high density electron cyclotron resonance (ECR) plasma discharge of methane, silane (SiH4), methane (CH4), hydrogen (H2) and argon (Ar) gas mixture was used to etch out these silicon nanotips from a bare untreated crystalline silicon wafer. SiC clusters, produced in the gas phase from the SiH4/CH4 plasma, distributes itself on the silicon substrate. Severe plasma etching of the silicon substrate partially masked by silicon carbide nanoparticles results in the uniform array of aligned silicon nanotips with a tip diameter typically in the range of ~2 nm and the tip length of abou
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