Abstract
Herein we describe the fabrication and characterization of Ag and Au bimetallic plasmonic crystals as a system that exhibits improved capabilities for quantitative, bulk refractive index (RI) sensing and surface-enhanced Raman spectroscopy (SERS) as compared to monometallic plasmonic crystals of similar form. The sensing optics, which are bimetallic plasmonic crystals consisting of sequential nanoscale layers of Ag coated by Au, are chemically stable and useful for quantitative, multispectral, refractive index and spectroscopic chemical sensing. Compared to previously reported homometallic devices, the results presented herein illustrate improvements in performance that stem from the distinctive plasmonic features and strong localized electric fields produced by the Ag and Au layers, which are optimized in terms of metal thickness and geometric features. Finite-difference time-domain (FDTD) simulations theoretically verify the nature of the multimode plasmonic resonances generated by the devices and allow for a better understanding of the enhancements in multispectral refractive index and SERS-based sensing. Taken together, these results demonstrate a robust and potentially useful new platform for chemical/spectroscopic sensing.
Highlights
Studies of surface plasmons have attracted significant attention due to the diverse range of applications and processes in which they can be exploited
This results in an evanescent decaying electric field that extends from the metal surface for ≈100–200 nm, or it can manifest as a localized surface plasmon resonance at the surface of a metal nanostructure
A specific contrast compared to freely diffusing nanoparticles in solution, sensing on plasmonic crystals provides specific advantages, being well-suited for use as chemoresponsive/multispectral imaging optics as well as not being subject to temporal instabilities that arise from interparticle interactions mediated via operando sensitive attributes of surface charge
Summary
Studies of surface plasmons have attracted significant attention due to the diverse range of applications and processes in which they can be exploited. We used bimetallic layers to engineer the plasmonic response of the device over a wider wavelength range, here exploiting the unique plasmonic modes of Ag in the visible range to supplement and enhance those provided by Au. This distinctive PC design further serves to compensate for the chemical instability of Ag by coating it with thin films of Au. To explore the variation in function resulting from the double layer film structure, we used five different ratios of Ag and Au while maintaining a constant overall metal film thickness on the SOG nanohole arrays.
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