9 October 2012 Large-area Ag nanoparticle arrays for plasmonically enhanced Raman scattering
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Conducting nanoparticles with plasmon resonances create local, nanoscopic field enhancements that boost an analyte molecule’s surface-averaged Raman scattering cross-section orders of magnitude above the bulk Raman cross-section by an amount known as the enhancement factor (EF). Demonstrations of single-molecule sensitivity with EF ~ 1013 have been reported from small “hot spots” (e.g., regions of enhanced electromagnetic near fields) on specialized substrates, but realistic chemical sensing requires high average EF over large substrates for practical sampling.1 By using simple wet chemical methods, NSRDEC scientists have fabricated large-area arrays of novel, highly conducting, anisotropic Ag and Al nanoparticles. The nanoparticles adhere to an ultrathin layer of poly-4(vinyl pyridine), and are anchored by submicron coating of poly-methyl methacrylate on glass and SiO2-coated Si substrates. The average interparticle spacing is determined by the dilution of the nanoparticle-water suspension. We present surface-enhanced Raman spectroscopy (SERS), spectrophotometry, and microscopy data from these nanoparticle arrays, model this data and the nanoscopic field enhancement, and determine the SERS EF. We compare the observed absorption resonances and SERS EF with those predicted by finite difference time domain modeling of the nanoscale fields and optical properties, and find good agreement between measured and calculated reflectivity, achieving EF ~ 106 for benzenethiol adsorbed onto a monolayer array of 120 nm Ag nanoparticles over an area of ~ 0.5 cm2. We discuss a way forward to increase SERS EF to 107 with large-area samples assembled using chemical methods, by using spiky Ag “nano-urchins” with very large predicted field enhancements.
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R. M. Osgood, R. M. Osgood, P. D. Stenhouse, P. D. Stenhouse, C. E. Quigley, C. E. Quigley, M. L. Hoey, M. L. Hoey, J. B. Carlson, J. B. Carlson, K. Lopez, K. Lopez, S. Kooi, S. Kooi, L. Armstrong, L. Armstrong, C. Haines, C. Haines, D. Kapoor, D. Kapoor, M. Cotlet, M. Cotlet, } "Large-area Ag nanoparticle arrays for plasmonically enhanced Raman scattering", Proc. SPIE 8457, Plasmonics: Metallic Nanostructures and Their Optical Properties X, 845738 (9 October 2012); doi: 10.1117/12.930006; https://doi.org/10.1117/12.930006

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