Phillip Blake

Gold nanoparticles reduced in situ and dispersed in polymer thin films: optical and thermal properties.

Optical and thermal activity of plasmon-active nanoparticles in transparent dielectric media is of growing interest in thermal therapies, photovoltaics and optoelectronic components in which localized surface plasmon resonance (LSPR) could play a significant role. This work compares a new method to embed gold nanoparticles (AuNPs) in dense, composite films with an extension of a previously introduced method. Microscopic and spectroscopic properties of the two films are related to thermal behavior induced via laser excitation of LSPR at 532 nm in the optically transparent dielectric. Gold nanoparticles were incorporated into effectively nonporous 680 μm thick polydimethylsiloxane (PDMS) films by (1) direct addition of organic-coated 16 nm nanoparticles; and (2) reduction of hydrogen tetrachloroaurate (TCA) into AuNPs. Power loss at LSPR excitation frequency and steady-state temperature maxima at 100 mW continuous laser irradiation showed corresponding increases with respect to the mass of gold introduced into the PDMS films by either method. Measured rates of temperature increase were higher for organic-coated NP, but higher gold content was achieved by reducing TCA, which resulted in larger overall temperature changes in reduced AuNP films.

Enhanced uniformity in arrays of electroless plated spherical gold nanoparticles using tin presensitization.

Gold nanoparticle arrays created with electroless gold plating provide a unique means of transforming nanocylinders usually formed in electron beam lithography to spherical nanoparticles. Alone, electroless gold plating is not selective to the substrate and results in the formation of a gold film on all exposed surfaces of an electron beam patterned sample, including the electron resist. Undesired gold plating occurred near patterned features on the substrate surface, which was reduced by increasing post-spin-coat cure time. When the electron resist is removed, some nanocylinders break off with the gold film, leaving partial cylinders or holes in the patterned elements. By presensitizing the substrate surface with tin, gold cylinders may be selectively deposited to the substrate surface without forming a film on the electron resist. Tin presensitized arrays were produced with 47.1 +/- 7.4 nm radius gold nanoparticles with an interparticle distance of 646.0 +/- 12.4 nm. Defects from sheared, missing, and redeposited Au particles associated with the resist removal were minimized, resulting in enhanced size and shape uniformity of pillars and arrays. Hollow particles were eliminated, and relative standard deviation in particle size was reduced by 7.4% on average, while elongation was reduced 12.3% when astigmatism was eliminated.

Far-field Fano resonance in nanoring lattices modeled from extracted, point dipole polarizability

Coupling and extinction of light among particles representable as point dipoles can be characterized using the coupled dipole approximation (CDA). The analytic form for dipole polarizability of spheroidal particles supports rapid electrodynamic analysis of nanoparticle lattices using CDA. However, computational expense increases for complex shapes with non-analytical polarizabilities which require discrete dipole (DDA) or higher order approximations. This work shows fast CDA analysis of assembled nanorings is possible using a single dipole nanoring polarizability extrapolated from a DDA calculation by summing contributions from individual polarizable volume elements. Plasmon resonance wavelengths of nanorings obtained using extracted polarizabilities blueshift as wall dimensions-to-inner radius aspect ratio increases, consistent with published theory and experiment. Calculated far-field Fano resonance energy maximum and minimum wavelengths were within 1% of full volume element results. Considering polarizability allows a more complete physical picture of predicting plasmon resonance location than metal dielectric alone. This method reduces time required for calculation of diffractive coupling more than 40 000-fold in ordered nanoring systems for 400–1400 nm incident wavelengths. Extension of this technique beyond nanorings is possible for more complex shapes that exhibit dipolar or quadrupole radiation patterns.

Enhanced Nanoparticle Response From Coupled Dipole Excitation for Plasmon Sensors

Regular lattices of metallic nanoparticles exhibit extraordinary spectral features that arise from electromagnetic coupling between the dipole component of localized surface plasmons and constructive interference from diffracted far-field radiation. The present work introduces this coupled dipole excitation as an additional method to perform refractive index-based sensing using gold nanoparticle arrays. These arrays exhibit an aggregate sensitivity of 31 nm·RIU-1 using the coupled dipole peak in transmission UV-vis spectroscopy. This aggregate sensitivity is in good agreement with values predicted by three models for coupled dipole excitation: an analytical coupled dipole approximation, a discrete dipole approximation, and finite difference time domain. A particle-based sensitivity, S NP , of 389 nm·RIU-1 was determined for a fabricated array. Plasmon sensing based on the coupled dipole excitation in a gold nanoparticle array was possible even when the local surface plasmon signal from individual nanoparticles was indistinguishable from noise. Further increases in sensitivity and signal-to-noise are predicted as coupled dipole excitation parameters are optimized in high-precision fabrication of nanoparticle arrays.

Nanoring structure, spacing, and local dielectric sensitivity for plasmonic resonances in Fano resonant square lattices

Lattices of plasmonic nanorings with particular geometries exhibit singular, tunable resonance features in the infrared. This work examined effects of nanoring inner radius, wall thickness, and lattice constant on the spectral response of single nanorings and in Fano resonant square lattices, combining use of the discrete and coupled dipole approximations. Increasing nanoring inner radius red-shifted and broadened the localized surface plasmon resonance (LSPR), while wall thickness modulated the LSPR wavelength and decreased absorption relative to scattering. The square lattice constant was tuned to observe diffractively-coupled lattice resonances, which increased resonant extinction 4.3-fold over the single-ring LSPR through Fano resonance. Refractive index sensitivities of 760 and 1075 nm RIU(-1) were computed for the plasmon and lattice resonances of an optimized nanoring lattice. Sensitivity of an optimal nanoring lattice to a local change in dielectric, useful for sensing applications, was 4 to 5 times higher than for isolated nanorings or non-coupling arrays. This was attributable to the Fano line-shape in far-field diffractive coupling with near-field LSPR.

Diffraction in nanoparticle lattices increases sensitivity of localized surface plasmon resonance to refractive index changes

Abstract. Comparing predicted and measured spectra from isolated and ordered nanoparticles (NPs) indicates that ordering NPs into lattices can blueshift the localized surface plasmon resonant (LSPR) spectral feature and increase its wavelength sensitivity to local changes in refractive index. This occurs at lattice constants at or above the resonant wavelength. Numerical analysis indicates its results from effects of diffractive modes on LSPR features that are distinct from Fano resonances, which arise separately due to coupling between diffractive modes and localized plasmons. Refractive index sensitivity of the aggregate LSPR peak from NPs in a square lattice (314  nm RIU−1) was 5.8-fold higher than a comparable peak from random NPs (54  nm RIU−1). Measured sensitivities of Fano resonance features in two ordered samples were 127% and 312%, respectively, of the highest LSPR sensitivity from a random assembly of NPs.

Photothermal response of the plasmonic nanoconglomerates in films assembled by electroless plating

Photothermal transduction of light to heat by evaporated and electroless plated gold (Au) films has been compared. Bare film was compared to film decorated by an ordered lattice of Au nanocylinders. The effects of plasmonic absorption of incident light, heat dissipation in the substrate, and interfacial effects between Au nanoparticles and the substrate were evaluated. Differences in the photothermal response of the films emerged due to interactions between these effects. Significant photothermal transduction was achieved by 30–40 nm Au grains as well as by conglomerate nanocylinders assembled from Au grains. An electroless Au film decorated with conglomerate Au nanocylinders ordered into a hexagonal array enhanced attenuation by 22% and increased light-to-heat conversion by 26%. This was attributed to photon-plasmon coupling. An evaporated Au film of 57 nm thickness attenuated 30% of incident light, compared to 45% attenuation for the electroless film of 35 nm thickness. The evaporated film had a photothermal response of 280 °C per watt of incident light in contrast to 1400 °C per watt for the electroless film.

Rate-Limited Electroless Gold Thin Film Growth: A Real-Time Study

Time-resolved, in situ spectroscopy of electroless (EL) gold (Au) films combined with electron microscopy showed that the deposition rate increased up to two-fold on surfaces swept by the bulk flow of adjacent fluid at Reynolds numbers less than 1.0, compared to batch immersion. Deposition rates from 5.0 to 9.0 nm/min and thicknesses of the EL Au film from 20 to 100 nm, respectively, increased predictably with flow rate at conditions when the deposition was limited primarily by Fickian diffusion. Time-frames were identified for metal island nucleation, growth, and subsequent film development during EL Au deposition by real-time UV-visible spectroscopy of photoluminescence (PL) and surface plasmon features of nanoscale metal deposits. Film thicknesses measured by scanning electron microscopy and X-ray photoelectron spectroscopy paired with real-time optical spectroscopy of kinetic aspects of plasmon and PL optical features indicated that Au film deposition on surfaces swept by a steady flow of adjacent fluid can be primarily diffusion limited.

Fabrication of regular arrays of gold nanospheres by thermal transformation of electroless-plated films

Rectangular lattices of gold nanospheres have been fabricated by thermally annealing Au nanopillars and nanocylinders deposited via electroless plating onto indium-tin-oxide glass substrates in a novel method. The substrates were patterned using e-beam lithography, and particle size and shape were controlled by adjusting the thickness of the poly(methylmethacrylate) mask, e-beam power, and electroless plating parameters. Nanostructures produced by this electroless plating method exhibited greater coalescence than sputtered gold films. Attachment of electroless-plated structures to indium-tin-oxide substrates was stable to stringent thermal, solvent, and electromagnetic exposures. This facile and versatile method is applicable to the fabrication of regular metal nanoparticle array platforms for improved optical and plasmonic features in sensing and imaging devices.

Tapered optical fibers designed for surface plasmon resonance phase matching.

Combining a modified two-step chemical etch method with equations to predict etch parameters and photon-plasmon phase-matching resulted in single-mode tapered optical fibers (TOFs) to optimize electromagnetic field enhancement. The phase-matching equation was used to identify the angle of incidence near the TOF cutoff radius at which surface plasmon resonance (SPR) is maximized. The axisymmetric Young-Laplace equation was used to predict the angle of incidence from the fabrication of a TOF via chemical etching. An optimal cone angle of 20.0 degrees , angles of incidence averaging (81.6 +/- 1.9) degrees , and tip diameters of (80.0 +/- 14.1) nm were achieved through a two-step etching process. These TOF characteristics maximize SPR excitation and field enhancement. The refractive index for optimized SPR excitation in the fabricated TOFs at a wavelength of 650 nm was found to be 1.343.