Thermal and Plasmonic Stabilization of Silver Nanostructures Using a Bilayer Anchoring Technique

Abstract

In this work, we demonstrate how to suppress the shape instability of silver (Ag) nanotriangular pyramids following high-temperature annealing without a coating or encapsulation, thus producing a more stable optical plasmonic system. Nanosphere lithography (NSL) was used to fabricate large-area arrays of nanotriangular pyramids of Ag on glass substrates. By using a combination of morphology and spectroscopic studies it was found that exposure of this system to high temperatures of 473 K and beyond in air led to a rapid change in nanostructure shape, and thus, the surface area, with a substantial change to the original plasmonic character. On the other hand, NSL nanotriangular pyramids made from bilayers of Ag on Co or Co on Ag showed much smaller changes in shape and area following annealing up to 573 K in air. In the case of pure Ag, the NSL nanotriangular pyramid changed into a more spherical shape with an overall decrease of ∼24% in its surface area following annealing at 573 K. This lead to a large blue shift of over ∼287 nm or ∼39% in the location of the dipolar plasmonic resonance. On the other hand, the triangular shape of Ag was retained in both the metal bilayer cases, with much smaller area changes of ∼10 and ∼9%, for the Ag deposit when on Co and when under Co, respectively. Consequently, the plasmonic shifts were substantially smaller, of ∼65 nm or about 9%, in these bilayer systems. The mechanism for this stabilization was attributed to the higher surface energy of Co and much lower diffusivity of Co as well as Ag on Co that resulted in an anchoring of the Ag shape to its original state. The plasmonic quality factor for the bimetal NSL nanotriangular pyramids also showed substantially improved stability over pure Ag, further indicating that this anchoring approach is a viable pathway to produce pristine Ag surfaces for high-temperature plasmonic applications.

Bimetallic Fe–Ag Nanopyramid Arrays for Optical Communication Applications

“…Angle-Resolved Nanosphere Lithography. To fabricate partially overlapped nanopyramidal metallic nanostructures, the nanosphere lithography technique was used as described by Sandireddy et al 38 First, a clean Petri dish was partially filled from the corner till the central dry area (∼2 cm 2 ) was achieved, as shown in Scheme 1. Then, a droplet of polystyrene (PS) bead solution (beads of diameter 500 nm, 1:8 by volume in ethanol) was placed in the dry area in the Petri dish.…”

“…36,37 Additionally, Ag−Co and Ag−Fe systems have also demonstrated reduced degradation rate of plasmonic signals due to oxidation of silver thus leading to very stable plasmonic behavior over long periods of time. 37,38 Fe 2 O 3 −Au core−shell nanoparticles have been reported to have a high Faraday rotation at resonant frequency ascribed to a spectral overlap of magneto-optical transition to plasmonic resonance. 39 Nanostructured bimetallic−dielectric interfaces have shown increased linear optical transparency due to magnetoplasmonic effects.…”

In Situ Contact Angle Tuning of Silver Nanostructures by Laser Heating in Different Fluid Ambient

“…Ag nanopyramids of thickness ∼20 nm were prepared using nanosphere lithography (NSL) technique. 8 After deposition of silver metal, samples were sonicated in ethanol to remove the mask and leave behind Ag nanopyramids. A 50 μL droplet of water, glycerol, or water−glycerol mixture was placed on the Ag thin film and Ag nanopyramid array made on quartz substrates.…”

“…7 For nanoparticles prepared on substrates, the surface area of a nanoparticle depends on the contact angle made by the nanoparticle, especially when sufficient thermal energy is provided for the nanoparticle to achieve its equilibrium contact angle. 8 Young's equation provides the equilibrium contact angle (θ c eq ) of liquid droplets under mechanical equilibrium when on a smooth solid substrate. The equilibrium condition leads to the well-known relation between the surface energy of liquid droplet (γ L ), the surface energy of the underlying substrate (γ S ), and interfacial energy of the liquid−substrate interface (γ SL ) given by γ S = γ SL + γ L cos(θ c eq ).…”

“…20 The aspect ratio (ratio of height to diameter) of spherical cap nanoparticles are directly related to contact angle on the substrate on which nanostructures are deposited. 8 Recently, it has been reported that optical absorption efficiency can be tuned by contact angle of Ag nanoparticles in plasmonic solar cells. 22 Also, several applications utilizing LSPR and SERS phenomena involve fluids that surround the nanoparticles and contain molecules to be adsorbed or analyzed at room temperatures or at elevated temperatures in catalysis monitoring.…”