Jeffrey A. Geldmeier

Light-Responsive Plasmonic Arrays Consisting of Silver Nanocubes and a Photoisomerizable Matrix

We report on the synthesis of novel branched organic-inorganic azo-polyhedral oligomeric silsesquioxane (POSS) conjugates (Azo-POSS) and their use as a stable active medium to induce reversible plasmonic modulations of embedded metal nanostructures. A dense monolayer of silver nanocubes was deposited on a quartz substrate using the Langmuir-Blodgett technique and subsequently coated with an ultrathin Azo-POSS layer. The reversible light-induced photoisomerization between the trans and cis states of the azobenzene-terminated branched POSS material results in significant changes in the refractive index (up to 0.17) at a wavelength of 380 nm. We observed that the pronounced and reversible change in the surrounding refractive index results in a corresponding hypsochromic plasmonic shift of 6 nm in the plasmonic band of the embedded silver nanocubes. The reversible tuning of the plasmonic modes of noble-metal nanostructures using a variable-refractive-index medium opens up the possibility of fabricating photoactive, hybrid, ultrathin coatings with robust, real-time, photoinitiated responses for prospective applications in photoactive materials that can be reversibly tuned by light illumination.

Remote Giant Multispectral Plasmonic Shifts of Labile Hinged Nanorod Array via Magnetic Field

We report a remotely mediated and fast responsive plasmonic-magnetic nanorod array with extremely large variability in optical appearance (up to 100 nm shifts in scattering maxima) and concurrently for multiple wavelengths in a broad range from UV-vis to near-infrared (at 450, 550, and 670 nm) with an external magnetic field with variable direction. The observed phenomenon demonstrates a rapid, wide-range response controlled via a noninvasive remote stimulus. The remotely controlled system suggested here is a magnetic field-directed assembly of an ordered monolayer array of unipolar oriented magnetic-plasmonic nickel-gold nanorods flexibly hinged to a sticky substrate. The unique geometry of the mobile nanorod array allows for the instant alteration of the surface plasmon polariton modes in the gold segment of the controllably tilting nanorods. This design demonstrates the utility of hybrid bimetallic nanoparticles and gives a novel approach to the design of fast-acting, remotely controlled color-changing nanomaterials for sensing and interfacial transport.

Design of Hybrid Electrochromic Materials with Large Electrical Modulation of Plasmonic Resonances

We present a rational approach to fabricating plasmonically active hybrid polymer-metal nanomaterials with electrochemical tunability of the localized surface plasmon resonances (LSPRs) of noble metal nanostructures embedded in an electroactive polymer matrix. The key requirement for being able to significantly modulate the LSPR band position is a close overlap between the refractive index change [Δn(λ)] of a stimuli-responsive polymeric matrix and the intrinsic LSPR bands. For this purpose, gold nanorods with a controlled aspect ratio, synthesized to provide high refractive index sensitivity while maintaining good oxidative stability, were combined with a solution-processable electroactive and electrochromic polymer (ECP): alkoxy-substituted poly(3,4-propylenedioxythiophene) [PProDOT(CH2OEtHx)2]. Spectral characteristics of the ECP, in particular the Δn(λ) variation, were evaluated as the material was switched between oxidized and reduced states. We fabricated ultrathin plasmonic electrochromic hybrid films consisting of gold nanorods and ECP that exhibited a large, stable, and reversible LSPR modulation of up to 25-30 nm with an applied electrical potential. Finite-difference time-domain (FDTD) simulations confirm a good match between the experimentally measured refractive index change in the ECP and the plasmonic response during electrochemical modulations.

The effect of plasmon resonance coupling in P3HT-coated silver nanodisk monolayers on their optical sensitivity

We report on the optical properties of silver nanodisk (Ag ND) Langmuir Blodgett monolayers that were transferred to substrates in different coupling regimes. Ag ND monolayers deposited in the liquid expanded–gaseous (Le–G) phase demonstrated individual plasmon resonance behavior while monolayers deposited in the liquid condensed–liquid expanded (Lc–Le) and solid–liquid condensed (S–Lc) phases exhibited plasmon coupling between closely packed adjacent nanoparticles, which caused a red shift in their localized surface plasmon resonance (LSPR) spectra. The initial presence of excess polyvinylpyrrolidone (PVP) surfactant micelles on the Ag ND monolayers could be eliminated by first compressing the monolayers to high surface pressures, resulting in blue shifted extinction spectra and increased sensitivity as micelles depleted into the subphase. Ag ND monolayers were then used in conjunction with a conjugated poly(3-hexylthiophene-2,5-diyl) (P3HT) medium to reversibly modulate the LSPR by changing the local refractive index around the nanoparticles. Ultimately, a high reversible LSPR shift of 27 nm was observed with an applied electropotential of ±500 mV to the P3HT-coated Ag ND monolayer. A high refractive index sensitivity (RIS) of 141 nm per RIU was found for monolayers deposited in the Lc–Le phase due to an increase in hot spot formation.

Plasmonic Nanogels for Unclonable Optical Tagging

We demonstrate the fabrication of novel functional gel coatings with randomized physical and chemical patterns that enable dual encoding ability to realize unclonable optical tags. This design is based on swelling-mediated massive reconstruction of an ultrathin responsive gelatinous polymer film uniformly adsorbed with plasmonic nanostructures into a randomized network of interacting folds, resulting in bright electromagnetic hotspots within the folds. We reveal a strong correlation between the topology and near-field electromagnetic field enhancement due to the intimate contact between two plasmonic surfaces within the folds, each of them representing a unique combination of local topography and chemical distribution caused by the formation of electromagnetic hotspots. Because of the efficient trapping of the Raman reporters within the uniquely distributed electromagnetic hotspots, the surface enhanced Raman scattering enhancement from the morphed plasmonic gel was found to be nearly 40 times higher compared to that from the pristine plasmonic gel. Harnessing the nondeterministic nature of the folds, the folded plasmonic gel can be employed as a multidimensional (with dual topo-chemical encoding) optical taggant for prospective anticounterfeiting applications. Such novel optical tags based on the spontaneous folding process are virtually impossible to replicate because of the combination of nondeterministic physical patterns and chemical encoding.

Electrochromic tuning of transparent gold nanorods with poly[(3,4-propylenedioxy)pyrrole] shells in the near-infrared region

We report on electrochemically tunable hybrid nanostructures composed of gold nanorods encapsulated within directly polymerized poly[(3,4-propylenedioxy)pyrrole] (PProDOP) nanoshells with controlled nanoscale thicknesses. This system displays narrow visible-near infrared absorption bands upon applying a variable electric potential due to the remarkable transmissivity of PProDOP at various oxidation states. The PProDOP shells were synthesized by in situ chemical oxidative polymerization using a mild oxidizing agent. The PProDOP demonstrated outstanding electrochemical performance, such as reversible electroactivity, high transmissivity in the visible range at various oxidation states, as well as a low oxidation potential (−1.06 V vs. Fc/Fc+). We suggest that the stable reversible modulation of the observed plasmonic response of the gold nanorods was caused by the variation of the refractive index of PProDOP shells at different oxidation states as shown by spectroscopic ellipsometry and confirmed by finite-difference time-domain (FDTD) simulations. A surface plasmon resonance (LSPR) band of gold nanorods at 800 nm was shifted reversibly by 24 nm by multiple cycling of the electric potential. Overall, these core–shell structures with electrochemical plasmonic tunability in the near-infrared region allow for tailoring of the optical and electrochemical properties of pre-programmed plasmon responses for active control of colorimetric appearance not just across the visible range but also toward the near-infrared.

Dewetting-Induced Photoluminescent Enhancement of Poly(lauryl methacrylate)/Quantum Dot Thin Films

A new method for enhancing photoluminescence from quantum dot (QD)/polymer nanocomposite films is proposed. Poly(lauryl methacrylate) (PLMA) thin films containing embedded QDs are intentionally allowed to undergo dewetting on substrates by exposure to a nonsolvent vapor. After controlled dewetting, films exhibited typical dewetting morphologies with increased amounts of scattering that served to outcouple photoluminescence from the film and reduce internal light propagation within the film. Up to a 5-fold enhancement of the film emission was achieved depending on material factors such as the initial film thickness and QD concentration within the film. An increase in initial film thickness was shown to increase the dewetted maximum feature size and its characteristic length until a critical thickness was reached where dewetting became inhibited. A unique light exposure-based photopatterning method is also presented for the creation of high contrast emissive patterns as guided by spatially controlled dewetting.