Dramatic Electrochemical Tuning of the Plasmonic Spectrum of a Metal Nanoparticle on a Mirror

Abstract

Enhancement of electric fields in the nanometric gap between plasmonic nanostructures is widely exploited for sensing, enhanced spectroscopies and photochemistry. Plasmonic properties of nanostructures with ultrathin gaps, such as nanoparticles (NPs) positioned on a flat metal surface (a mirror), have attracted much attention in recent years [1]. We studied the effect of electrochemical control on the plasmonic properties of single Au and Ag NPs on a mirror. We found that the coupled plasmonic resonance could be reversibly tuned over a range of tenths of nanometers, more than an order of magnitude higher than expected based just on capacitive charging of the NPs.Our experimental system consisted of single Au or Ag NPs (50 nm – 100 nm) adsorbed on a self-assembled monolayer (SAM) of either polyelectrolytes, cysteamine (CEA) or ethanedithiol (EDT) deposited on an atomically flat 100nm thick Au electrode prepared by template stripping. The NPs were either bare (prepared by H2 reduction) or citrate-stabilized and the system was immersed in a basic solution.Coupling to the mirror electrode resulted in the appearance of several bands in the plasmonic spectrum of individual NPs, as measured by dark-field microspectroscopy (Fig. 1a), and the lowest-energy band could be attributed to the coupled plasmon. Variation of the electrode potential generated a giant, though reversi shift of the spectrum. A plot of the peak position of the coupled plasmon band displayed an S-shaped, phase-transition like behavior (Fig. 1b).This remarkable result could not be observed with individual NPs deposited on non-metallic surfaces, and can therefore be attributed to a dramatic modulation of the coupling between the particles and the mirror with electrode potential.Figure 1. Scattering spectra (a) and variation of the spectral position of the coupled plasmon peak (b) of bare AuNP (ca. 60 nm) on CEA/Au electrode. Inset in (a): Scheme of NP on the mirror.