Design fundamentals of plasmonic substrates for surface-enhanced Raman Scattering Applications
Velez-Calder, Raymond A.
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The use of nanomachining methods capable of reproducible construction of nano-arrayed devices have revolutionized the field of plasmonic sensing through the introduction of rationally engineered designs. Significant strives have been made to fabricate plasmonic platforms with tailored interparticle gaps to improve their performance for surface enhanced Raman scattering (SERS) applications. Attention has now been focused on predictive modelling, such as Finite-Difference Time-Domain (FDTD), a promising tool that can advance the optimization of the SERS substrate design process by simulating the plasmonic response induced by an EM wave. Over time a dichotomy has emerged in the implementation of SERS for analytical applications, the construction of substrates, optimization of interparticle spacing as a mean to optimize electromagnetic field-enhancement at the localized surface plasmon level, and the substrate sensitivity over extended areas to achieve quantitative performance. Furthermore, the figures of merits used to validate the SERS activity can be subject to some scrutiny due to signal enhancement mechanisms that are overlooked. Hereon is discussed several fundamental key aspects for the design of plasmonic substrates for SERS applications. FDTD case studies will be discussed thus demonstrating the importance of models for a successful design of a SERS substrate. FDTD was used to address the plasmonic performance of a hybrid nanoarray sensing platform comprised of hexagons and ellipses. The device had already been fabricated, hence making it impossible to ascertain the contributions of each feature to the SERS experimental data. The modelling data provided valuable insight of the underlying effect that the excitation wavelength had on the substrate. Furthermore, the plasmonic coupling of the features in the hybrid device was demonstrated thereby providing valuable insight of the excellent performance demonstrated by this design at multiple excitation wavelengths. Finally, the SERS substrate enhancement factor (SSEF) for plasmonic Ag/SiO2/Si Disc-on-Pillar arrays of variable pitch were contrasted with the analytical performance for quantitative applications. Experimental data were compared with those from FDTD simulations used in the optimization of the array dimensions. A self-assembled monolayer (SAM) of benzenethiol rendered highly reproducible signals (RSD ~ 4% to 10%) and EF values in the orders of 10^6 to 10^8 for all pitches. A remarkable correlation was observed between the modelling and the SERS experimental data after normalization with the illuminated pillars. Spectra corresponding to rhodamine 6G and 4-aminobenzoic acid demonstrated the advantages of using the more densely-packed DOP arrays (gap = 40 nm) for quantitation in spite that the strongest SSEF was attained for an interpillar gap of 400 nm.