Electrotunable Nanoplasmonics for Amplified Surface Enhanced Raman Spectroscopy

Abstract

Tuning the properties of optical metamaterials in real time is one of the grand challenges of photonics. Being able to do so will enable a class of adaptive photonic materials for use in applications such as surface enhanced Raman spectroscopy and reflectors/absorbers. One strategy to achieving this goal is based on the electrovariable self-assembly and disassembly of two-dimensional nanoparticle arrays at a metal | liquid interface. As expected, the structure results in plasmonic coupling between NPs in the array but perhaps as importantly between the array and the metal surface. In such a system, the density of the nanoparticle array can be reversibly controlled by the variation of electrode potential. Theory suggests that due to a collective plasmon-coupling effect less than 1 V variation of electrode potential can give rise to a dramatic simultaneous change in optical reflectivity from ∼93% to ∼1% and the amplification of the SERS signal by up to 5 orders of magnitude. This is experimentally demonstrated using a platform based on the voltage-controlled assembly of 40 nm Au-nanoparticle arrays at a TiN/Ag electrode in contact with an aqueous electrolyte. We show that all the physics underpinning the behavior of this platform works precisely as suggested by the proposed theory, setting the electrochemical nanoplasmonics as a promising direction in photonics research.

Electrochemical metamaterials

“…Within the acquired potential window, the array was never denser than the one corresponding to 5 nm inter-NP surface-to-surface separation. But even then, as was found in a later paper [71], for large NPs (40 nm in diameter), almost 100% quenching of reflectance was observed at the wavelength of reflection minimum. All the experimentally obtained spectra were fitted to the theory, through the wavelength of the reflection dip and its depth, with one fitting parameter-the average distance between NPs.…”

“…Those effects have been demonstrated in Ref. [71]. Again, varying electrode potential altered the average distance between the NPs with their negatively charged ligands; more positive electrode polarisation created a denser array of NPs, and amplified the SERS signal.…”

“…What was shown in Ref. 71 is that one can manipulate the structure of such arrays, 'electroadsorbing'/'electrodesorbing' them onto/from solid electrodes, at our will and in real time, and how it thereby affects the SERS signal, demonstrating a crucial role of electromagnetic enhancement mechanism [75].…”

“…Further densification of NP array was making this dependence nonlinear, thus demonstrating clear emergence of hot spots not only underneath NPs but also in between NPs, when they come closer to each other, and, plasmon resonance wise, start 'talking with each other'! In the same work [71], measurements of the Raman signals were paralleled by simple reflectivity measurements. The latter allowed to retrieve the average distances between NPs, control the proximity of the measured and calculated reflections spectra (with full success, again), and with thus evaluated distances between NPs in the monolayer to calculate the enhancement of Raman intensity.…”

Electrochemically driven dynamic plasmonics

“…Dynamic plasmonics enabled by nanoparticle assembly/disassembly can generate the application of tunable surface-enhanced Raman spectroscopy for trace chemical detection. [112][113][114] More importantly, electrochemical metal deposition/stripping-enabled dynamic plasmonics can serve as the operando monitoring platform for observing the change of metal morphologies during deposition and stripping processes, which is particularly important for the metal anode of metal batteries.…”