Integration of nanoparticles into thin films is essential for the development of functional materials, studies of fundamental interfacial processes, and exploitation of inherent properties from the particles themselves. In this work, we systematically investigate the process of incorporation of silver nanocubes into thin polystyrene films at temperatures just above the polymer glass transition. The process of nanocrystal incorporation can be precisely monitored via far-field spectroscopy to observe the response of spatially resolved hybrid plasmon modes. Each plasmon resonance has a distinct dynamic range and maximum sensitivity forming a complementary set of nanorulers that operates over a distance comparable to the edge length of the cubes. The approach explored in this work is a general robust method for the development of long-range polychromatic nanorulers.
A novel technique for increasing the sensitivity of tilted fibre Bragg grating (TFBG) based refractometers is presented. The TFBG sensor was coated with chemically synthesized silver nanowires ~100 nm in diameter and several micrometres in length. A 3.5-fold increase in sensor sensitivity was obtained relative to the uncoated TFBG sensor. This increase is associated with the excitation of surface plasmons by orthogonally polarized fibre cladding modes at wavelengths near 1.5 μm. Refractometric information is extracted from the sensor via the strong polarization dependence of the grating resonances using a Jones matrix analysis of the transmission spectrum of the fibre.
Plasmonic properties, such as refractive index sensitivity (RIS), surface enhancement of the Raman signal (SERS), fluorescence quenching, and photocatalytic activity, of monolayers of weakly interacting monodisperse silver nanocubes were qualitatively modified in a very well controlled manner by supporting them on thin silicon films with varying thickness. Such fine tunability is made possible by the strong dependence of the nanocube dipolar (D) and quadrupolar (Q) plasmon mode hybridization on the refractive index of the supporting substrate. By increasing the Si film thickness from zero to ~25 nm we were able to "shift" the D resonance mode by up to 200 nm for ~80 nm cubes without significantly affecting the Q mode. The silicon supported nanocubes showed a significant improvement in RIS via the Q mode with a figure of merit greater than 6.5 and about an order of magnitude enhancement of the SERS signal due to the stronger electric field created by the D mode. Such substrates also showed a ~10 times decrease in rhodamine 6G fluorescence as well as the rates of amorphous carbon formation. The study proposes a new way to design and engineer plasmonic nanostructures.
The
utilization of substrate–particle interactions provides
a route to tuning the optical properties of strongly coupled supported
plasmonic nanoparticles. In this work fine control over interparticle
and particle–substrate interactions is demonstrated using Langmuir–Blodgett
monolayers of silver nanocubes deposited onto titanium oxide (TiO
x
) thin films of varying thickness. By using
two Raman reporters, a Rhodamine-B (RhB) and benzenethiol (BT), surface-enhanced
Raman spectroscopy (SERS) independently examines electromagnetic (EM)
enhancement at the substrate/nanocube interface (RhB) and at the surface
of the cubes, where the label is predominately 20–40 nm away
from the dielectric substrate (BT). For RhB the SERS enhancement factor
(EF) drops as much as an order of magnitude on 20 nm TiO
x
with respect to glass. However, for BT, a maximum
SERS EF of (2.5 ± 0.4) × 105 was observed on
TiO
x
compared to (1.5 ± 0.4) ×
105 on glass, an increase of 60%. Control over the organization
of the nanocube monolayer reveals that maximum enhancement occurs
in small, discrete clusters of nanocubes as opposed to large aggregates.
Fine control over the optical properties and near-field EM distribution
of coupled nanostructures can be accomplished through tuning of the
dielectric properties of the substrate yielding a route to optimizing
properties for field-enhanced plasmonic applications.
Colloidal suspensions and films of aluminum oxide afford long-term stability to silver nanoparticles and play an important role in their light driven shape conversion.
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