Self‐Assembled Silver–Gold Nanoisland Films on Glass for SERS Applications

Abstract

We present a study of resonant optical properties of gold-protected silver nanoisland films. Silver nanoislands were grown on a glass substrate using out-diffusion technique, the growth was followed by the deposition of nanometer-thick gold coatings. Scanning electron microscopy and optical spectroscopy were used to characterize morphology and extinction spectra of the grown combined silver-gold nanostructures. Micro Raman spectroscopy of the combined nanoislands has demonstrated their signal enhancement factor exceeding that one of the initial silver nanoislands.Nowadays Surface Enhanced Raman Spectroscopy (SERS) is in a wide use because of its high sensitivity which allows molecular fingerprinting of chemical and biological species. [1,2] The high sensitivity at Raman exciting wavelength close to Surface Plasmon Resonance (SPR) wavelength of plasmonic structures provides SERS applicability in various analytical and sensing devices for both commercial and laboratory needs. Commercial SERS substrates widely use metal nanoparticles deposited on glass slides, and numerous efforts were made to improve Raman signal enhancement by metal nanoparticles via varying their shape, composition and structure. By now, among other metals silver has demonstrated the highest Raman enhancement factor. [1] This has stimulated the design of core-shell nanostructures, silver being the shell material, in particular silvercovered gold nanostructures, [3][4][5][6] in which a silver layer allowed increasing Raman sensitivity of the systems. Contrary, goldcovered silver nanoparticles showed lower sensitivity, [4] except for the cases of relatively big, $50 nm, colloidal silver nanoparticles chemically covered with gold layer, and pin-holes behaved as hot-spots providing additional enhancement of Raman signal. [7] However, silver nanoparticles suffer from high chemical reactivity which can result in their degradation due to the reactions with atmospheric species. [8,9] This high reactivity can also cause destruction of biological objects under analysis. [10][11][12] The latter is also evidenced by registered toxicity of silver nanoparticles. [13] That is why efforts aimed to protect silver nanostructures via depositing chemically inert layers, in spite of resulted decrease in Raman enhancement, were performed. [14,15] We have recently demonstrated [16] out-diffused self-assembled silver nanoisland films capable of exploitation in SERS. In the present study, we show that deposition of a thin, a few nanometers, gold coating results in the formation of bimetallic core-shell nanostructures. Dependence of the SPR spectral positions on the thickness of the gold coating provides tuning the SPR to specific wavelengths, which allows increasing SERS signal. These results obtained for ultra-thin gold films differ from the data obtained for chemically synthesized gold-covered silver nanoparticles with quite thick gold coatings. In the latter case, plasmonic properties of the nanoparticles are mainly defined by gold shells or by essential mixing of go...

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