Gregory T. Forcherio

Optical attenuation of plasmonic nanocomposites within photonic devices

Plasmonic nanostructures enable microscopic optical manipulation such as light trapping in photonic devices. However, integration of embedded nanostructures into photonic devices has been limited by tractability of nanoscale and microscale descriptions in device architectures. This work uses a linear algebraic model to distinguish geometric optical responses of nanoparticles integrated into dielectric substrates interacting with macroscopic back-reflectors from absorptive and nonlinear plasmonic effects. Measured transmission, reflection, and attenuation (losses) from ceramic and polymer composites supporting two- and three-dimensional distributions of gold nanoparticles, respectively, are predictable using the model. A unique equilateral display format correlates geometric optical behavior and attenuation to nanoparticle density and back-reflector opacity, allowing intuitive, visual specification of density and opacity necessary to obtain a particular optical performance. The model and display format are useful for facile design and integration of plasmonic nanostructures into photonic devices for light manipulation.

Asymmetric reduction of gold nanoparticles into thermoplasmonic polydimethylsiloxane thin films.

Polymer thin films containing gold nanoparticles (AuNPs) are of growing interest in photovoltaics, biomedicine, optics, and nanoelectromechanical systems (NEMs). This work has identified conditions to rapidly reduce aqueous hydrogen tetrachloroaurate (TCA) that is diffusing into one exposed interface of a partially cured polydimethylsiloxane (PDMS) thin film into AuNPs. Nanospheroids, irregular gold (Au) networks, and micrometer-sized Au conglomerates were formed in a ∼5 μm layer at dissolved TCA contents of 0.005, 0.05, and 0.5 mass percent, respectively. Multiscale morphological, optical, and thermal properties of the resulting asymmetric AuNP-PDMS thin films were characterized. Reduction of TCA diffusing into the interface of partially cured PDMS film increased AuNP content, robustness, and scalability relative to laminar preparation of asymmetric AuNP-PDMS thin films. Optical attenuation and thermoplasmonic film temperature due to incident resonant irradiation increased in linear proportion to the order of magnitude increases in TCA content, from 0.005 to 0.05 to 0.5 mass percent. At the highest TCA content (0.05 mass percent), an asymmetric PDMS film 52-μm-thick with a 7 μm AuNP-containing layer was produced. It attenuated 85% of 18 mW of incident radiation and raised the local temperature to 54.5 °C above ambient. This represented an increase of 3 to 230-fold in photon-to-heat efficiency over previous thermoplasmonic AuNP-containing systems.

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.

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.

Geometric optics of gold nanoparticle-polydimethylsiloxane thin film systems

Interest in optical properties of plasmonic nanoparticles embedded in transparent dielectrics is growing due to potential uses in biomedicine, sustainable energy, and manufacturing. This work evaluates geometric optics in polymer thin films containing mono- or polydisperse gold nanoparticles (AuNP) using a compact linear algebraic sum. Reflection and transmission from polydimethylsiloxane (PDMS) films containing uniformly- or assymetrically-distributed monodisperse or polydisperse AuNPs decreased with AuNP morphological isotropy and particle density. In PDMS, monodisperse AuNPs increased optical attenuation linearly with gold content, while polydisperse AuNPs reduced from hydrogen tetrachloroaurate (TCA) increased optical attenuation in proportion to order-of-magnitude rises in gold content. Polydisperse AuNP concentrated asymmetrically at one film interface exhibited higher attenuation. Cumulative optical responses from AuNP-PDMS films paired with another film or reflective element were within 0.04 units on average from values predicted for transmission, reflection, or attenuation using linear algebra. These results support design of NP-containing dielectric films to integrate into biochemical, microelectromechanical, and optoelectronic devices and systems.

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.

Gold nanoparticle-polydimethylsiloxane films reflect light internally by optical diffraction and Mie scattering

Optical properties of polymer films embedded with plasmonic nanoparticles (NPs) are important in many implementations. In this work, optical extinction by polydimethylsiloxane (PDMS) films containing gold (Au) NPs was enhanced at resonance compared to AuNPs in suspensions, Beer?Lambert law, or Mie theory by internal reflection due to optical diffraction in 16 nm AuNP?PDMS films and Mie scattering in 76 nm AuNP?PDMS films. Resonant extinction per AuNP for 16 nm AuNPs with negligible resonant Mie scattering was enhanced up to 1.5-fold at interparticle separation (i.e., Wigner?Seitz radii) comparable to incident wavelength. It was attributable to diffraction through apertures formed by overlapping electric fields of adjacent, resonantly excited AuNPs at Wigner?Seitz radii equal to or less than incident wavelengths. Resonant extinction per AuNP for strongly Mie scattering 76 nm AuNPs was enhanced up to 1.3-fold at Wigner?Seitz radii four or more times greater than incident wavelength. Enhanced light trapping from diffraction and/or scattering is relevant to optoelectronic, biomedical, and catalytic activity of substrates embedded with NPs.

Plasmonic extinction in gold nanoparticle-polymer films as film thickness and nanoparticle separation decrease below resonant wavelength

Abstract. Plasmonic nanoparticles embedded in polymer films enhance optoelectronic properties of photovoltaics, sensors, and interconnects. This work examined optical extinction of polymer films containing randomly dispersed gold nanoparticles (AuNP) with negligible Rayleigh scattering cross-sections at particle separations and film thicknesses less than (sub-) to greater than (super-) the localized surface plasmon resonant (LSPR) wavelength, λLSPR. Optical extinction followed opposite trends in sub- and superwavelength films on a per nanoparticle basis. In ∼70-nm-thick polyvinylpyrrolidone films containing 16 nm AuNP, measured resonant extinction per particle decreased as particle separation decreased from ∼130 to 76 nm, consistent with trends from Maxwell Garnett effective medium theory and coupled dipole approximation. In ∼1-mm-thick polydimethylsiloxane films containing 16-nm AuNP, resonant extinction per particle plateaued at particle separations ≥λLSPR, then increased as particle separation radius decreased from ∼514 to 408 nm. Contributions from isolated particles, interparticle interactions and heterogeneities in sub- and super-λLSPR films containing AuNP at sub-λLSPR separations were examined. Characterizing optoplasmonics of thin polymer films embedded with plasmonic NP supports rational development of optoelectronic, biomedical, and catalytic activity using these nanocomposites.

Production of monolayer-rich gold-decorated 2H–WS 2 nanosheets by defect engineering

Chemical functionalization of low-dimensional nanostructures has evolved as powerful tool to tailor the materials’ properties on demand. For two-dimensional transition metal dichalcogenides, functionalization strategies are mostly limited to the metallic 1T-polytype with only few examples showing a successful derivatization of the semiconducting 2H-polytype. Here, we describe that liquid-exfoliated WS2 undergoes a spontaneous redox reaction with AuCl3. We propose that thiol groups at edges and defects sites reduce the AuCl3 to Au0 and are in turn oxidized to disulfides. As a result of the reaction, Au nanoparticles nucleate predominantly at edges with tuneable nanoparticle size and density. The drastic changes in nanosheet mass obtained after high loading with Au nanoparticles can be exploited to enrich the dispersions in laterally large, monolayered nanosheets by simple centrifugation. The optical properties (for example photoluminescence) of the monolayers remain pristine, while the electrocatalytic activity towards the hydrogen evolution reaction is significantly improved.Chemical functionalization: Au-decorated WS 2 sheets show enhanced catalytic activityDefect engineering of WS2 nanosheets via redox chemistry in liquid phase yields enhanced catalytic activity and monolayer enrichment. A team led by Claudia Backes at Ruprecht-Karls University demonstrated that liquid-phase exfoliated WS2 undergoes a spontaneous redox reaction with AuCl3, whereby thiol groups occurring at edges and defect sites reduce the AuCl3 to Au0. The reaction causes Au nanoparticles to nucleate at WS2 edges, and in turn such Au nanoparticle loading determines a substantial change in the nanosheet mass. As the Au decoration preferentially occurs at the edges of incompletely exfoliated WS2 flakes, the dispersions can be further enriched with monolayers by means of centrifugation. While the optical properties of Au-decorated WS2 sheets remain unaltered, their electrocatalytic activity is highly enhanced, showing promise for applications in hydrogen evolution reactions.