High-sensitivity strain gauges based on single wires of close-packed 14 nm colloidal gold nanoparticles are obtained by a novel variant of convective self-assembly (CSA). This CSA mode named stop-and-go CSA enables the fabrication of nanoparticle wires only a few micrometers wide, separated by distances that can be easily tuned over tens to hundreds of micrometers. Nanoparticle wires are obtained in a single step by direct deposition of nanoparticles from suspensions onto flexible polyethylene terephthalate films, without any lithographic prepatterning. When connected between two electrodes, such single nanoparticle wires function as miniature resistive strain gauges. The high sensitivity, repeatability, and robustness demonstrated by these single-wire strain gauges make them extremely promising for integration into micro-electromechanical systems or for high-resolution strain mapping.
Noble metal films deposited over two-dimensional arrays of polystyrene nanospheres constitute a confirmed class of efficient and cost-effective substrates for surface enhanced Raman scattering (SERS). In this paper, we perform scanning confocal SERS microscopy to investigate the spatial (lateral) variations of the SERS enhancements on gold films over nanospheres (AuFoN) substrates. By constructing SERS imaging maps with a resolution down to the diffraction limit, the local SERS efficiency is found to vary on two different scales. First, the local SERS efficiency is periodically modulated (intensity ratios of 2−3) by the periodic AuFoN surface topography (as demonstrated by correlation with atomic force microscopy imaging of the same sample area); second, randomly distributed SERS hot-spots are observed, at which the SERS intensity is 1 to 2 orders of magnitude larger than at adjacent regions. Furthermore, these hot-spots exhibit fluctuating behavior, characteristic of single-molecule SERS sensitivity. These results are particularly useful for furthering current understanding of SERS on AuFoN substrates. More generally, the SERS maps provide a direct visual demonstration that in SERS only a fraction of the metallic surface yields the major part of the SERS scattering. The evidence of clear correlations between SERS enhancement and topography can be relevant for the characterization of ordered noble-metal plasmonic structures.
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