Plasmonic Gas Sensing Using Nanocube Patch Antennas

Abstract

of interest and be as sensitive as possible to small changes in the local environment. While individual particle resonances can be effectively tuned through size, shape and material choice, [ 2,12 ] coupled modes between two or more nanoparticles have been shown to exhibit far greater electric fi eld enhancements and produce more sensitive detectors. [13][14][15][16][17][18] However, these are extremely diffi cult to fabricate, with many top-down lithographic and FIBbased examples being unrealistic for scaling up, and bottom-up techniques that generally produce randomly oriented particle pairs with no control over the alignment, which is key as the resonances of such dimers are highly sensitive to the polarization of incident illumination. Henzie et al. made some important progress in this fi eld by using a gravity-driven technique to assemble nanoparticles in shallow pits in a substrate, which allows excellent control over orientation and positioning of any number of particle sets. [ 19 ] Another elegant solution to the fabrication problem is the placement of a metallic particle above a metal sheet separated by a thin spacer. A "mirror particle" is excited in the metal plane, which produces a strong coupling effect without having to align two separate particles, drastically simplifying the production process. [ 22 ] There is a growing body of work investigating the properties of silver nanocubes (NCs) above metal fi lms, to tune their scattering properties, [ 21 ] produce controlled-refl ectance surfaces, [ 22 ] and to control and enhance the radiative processes of fl uorophores within the spacer material, [ 23,24 ] as well as theoretical studies looking to build a complete description of the structure. [ 25,26 ] This system can be considered an optical patch antenna, where gap plasmon modes are supported between the metal sheet and the NC. [ 21 ] The resonant modes of these antennae are extremely sensitive to the properties of the spacer layer separating the NC and the silver sheet, and any environmental changes that infl uence the refractive index (RI), or more importantly the thickness of the chosen material will produce signifi cant changes in the plasmon resonance. As long as spacer materials which expand in response to a given analyte are available, this design can be utilized to detect any number of chemicals. This forms the basis for a new type of sensor, which can operate equally well in a gaseous or liquid setting (depending on spacer choice) and that is scalable from the sub-wavelength scale (the footprint of one nanocube) up to a large-surface area metamaterial.In this paper, we demonstrate the operation of the nanocube patch antenna system as a gas sensor for the fi rst time, usingThe ability of individual nanocube patch antennas, consisting of a silver nanocube separated from an Ag sheet by a thin fl uoropolymer spacer, to act as subwavelength sensing elements is demonstrated. An increase in relative humidity (RH) causes the spacer to expand, which alters the resonance of the plasmon cavity mode fo...

Broad electrical tuning of plasmonic nanoantennas at visible frequencies

“…In turn, these modifications to the thickness and dielectric constant of the material in the gap between the metal film and the silver nanocubes lead to a modified plasmon resonance of the sample structure. 21,40 As seen from Fig. 3, a negative bias voltage is observed to induce a much larger shift in the plasmon resonance than a positive bias.…”

“…This can be explained by a negative bias causing de-swelling of the polyelectrolyte layers, and thus a red shift of the resonance, whereas a positive bias causes swelling of the polyelectrolyte layers, and thus a blue shift of the resonance. 28,40,41 As mentioned earlier, the relationship between the gap thickness and the plasmon resonance of the nanopatch antennas has previously been characterized and a non-linear dependence was observed. 9,28,40 At small gap sizes of less than $5 nm, the plasmon resonance has been observed to be extremely sensitive to changes in the gap size whereas at larger gaps, the plasmon resonance is much less sensitive.…”

“…28,40,41 As mentioned earlier, the relationship between the gap thickness and the plasmon resonance of the nanopatch antennas has previously been characterized and a non-linear dependence was observed. 9,28,40 At small gap sizes of less than $5 nm, the plasmon resonance has been observed to be extremely sensitive to changes in the gap size whereas at larger gaps, the plasmon resonance is much less sensitive. This is consistent with our observations here where de-swelling of the polyelectrolyte layers induces a larger shift in the plasmon resonance than swelling.…”

“…9 Furthermore, modifications to the index of refraction of the material in the gap region may also modify the plasmon resonance; however, recent studies have shown this effect to be much less significant than changes in the gap thickness. 40 Based on these observations, the tuning of the plasmon resonance observed in our experiment is most likely caused by the swelling and de-swelling of the polyelectrolyte layers depending on the polarity of the electric field and the highly concentrated ionic liquid in combination with a modified dielectric constant of the spacer layer. In turn, these modifications to the thickness and dielectric constant of the material in the gap between the metal film and the silver nanocubes lead to a modified plasmon resonance of the sample structure.…”

Tunable dark plasmons in a metallic nanocube dimer: toward ultimate sensitivity nanoplasmonic sensors

“…Similar structures include nanocube-over-mirror,26,45,46 nanopatch on substrate24,47 and end-to-end nanoparticle with flat terminals,[48][49][50][51][52] etc. The charge plots show the mode at 701 nm (i) and 729 nm (ii).…”