Metal nanoparticles are the most frequently used nanostructures in plasmonics. However, besides nanoparticles, metal nanowires feature several advantages for applications. Their elongation offers a larger interaction volume, their resonances can reach higher quality factors, and their mode structure provides better coupling into integrated hybrid dielectric-plasmonic circuits. It is crucial though, to control the distance of the wire to a supporting substrate, to another metal layer or to active materials with sub-nanometer precision. A dielectric coating can be utilized for distance control, but it must not degrade the plasmonic properties. In this paper, we introduce a controlled synthesis and coating approach for silver nanowires to fulfill these demands. We synthesize and characterize silver nanowires of around 70 nm in diameter. These nanowires are coated with nm-sized silica shells using a modified Stöber method to achieve a homogeneous and smooth surface quality. We use transmission electron microscopy, dark-field microscopy and electron-energy loss spectroscopy to study morphology and plasmonic resonances of individual nanowires and quantify the influence of the silica coating. Thorough numerical simulations support the experimental findings showing that the coating does not deteriorate the plasmonic properties and thus introduce silver nanowires as usable building blocks for integrated hybrid plasmonic systems.
Spin-momentum locking is a peculiar effect in the near-field of guided optical or plasmonic modes. It can be utilized to map the spinning or handedness of electromagnetic fields onto the propagation direction. This motivates a method to probe the circular dichroism of an illuminated chiral object. In this work, we demonstrate local, subdiffraction limited chiral coupling of light and propagating surface plasmon polaritons in a self-assembled system of a gold nanoantenna and a silver nanowire. A thin silica shell around the nanowire provides precise distance control and also serves as a host for fluorescent molecules, which indicate the direction of plasmon propagation. We characterize our nanoantenna−nanowire systems comprehensively through correlated electron microscopy, energydispersive X-ray spectroscopy, dark-field, and fluorescence imaging. Three-dimensional numerical simulations support the experimental findings. Besides our measurement of far-field polarization, we estimate sensing capabilities and derive not only a sensitivity of 1 mdeg for the ellipticity of the light field, but also find 10 3 deg cm 2 /dmol for the circular dichroism of an analyte locally introduced in the hot spot of the antenna−wire system. Thorough modeling of a prototypical design predicts on-chip sensing of chiral analytes. This introduces our system as an ultracompact sensor for chiral response far below the diffraction limit.
Bimetallic nanostructures
comprising plasmonic and catalytic components
have recently emerged as a promising approach to generate a new type
of photo-enhanced nanoreactors. Most designs however concentrate on
plasmon-induced charge separation, leaving photo-generated heat as
a side product. This work presents a photoreactor based on Au–Pd
nanorods with an optimized photothermal conversion, which aims to
effectively utilize the photo-generated heat to increase the rate
of Pd-catalyzed reactions. Dumbbell-shaped Au nanorods were fabricated
via a seed-mediated growth method using binary surfactants. Pd clusters
were selectively grown at the tips of the Au nanorods, using the zeta
potential as a new synthetic parameter to indicate the surfactant
remaining on the nanorod surface. The photothermal conversion of the
Au–Pd nanorods was improved with a thin layer of polydopamine
(PDA) or TiO2. As a result, a 60% higher temperature increment
of the dispersion compared to that for bare Au rods at the same light
intensity and particle density could be achieved. The catalytic performance
of the coated particles was then tested using the reduction of 4-nitrophenol
as the model reaction. Under light, the PDA-coated Au–Pd nanorods
exhibited an improved catalytic activity, increasing the reaction
rate by a factor 3. An analysis of the activation energy confirmed
the photoheating effect to be the dominant mechanism accelerating
the reaction. Thus, the increased photothermal heating is responsible
for the reaction acceleration. Interestingly, the same analysis shows
a roughly 10% higher reaction rate for particles under illumination
compared to under dark heating, possibly implying a crucial role of
localized heat gradients at the particle surface. Finally, the coating
thickness was identified as an essential parameter determining the
photothermal conversion efficiency and the reaction acceleration.
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