Switching plasmon coupling through the formation of dimers from polyaniline-coated gold nanospheres

All-State Switching of the Mie Resonance of Conductive Polyaniline Nanospheres

Polyaniline (PANI), a conductive polymer, is a promising active material for optical switching. In most studies, active switching has so far been realized only between two states, whereas PANI has a total of six states. The optical properties of nanoscale PANI in all six states have remained unclear. Herein we report on all-state switching of the Mie resonance on PANI nanospheres (NSs) and active plasmon switching on PANI-coated Au nanodisks (NDs). All-state switching of differently sized PANI NSs is achieved by proton doping/dedoping and electrochemical methods. Theoretical studies show that the scattering peaks of the individual PANI NSs originate from Mie resonances. All-state switching is further demonstrated on PANI-coated circular Au NDs, where an unprecedentedly large plasmon peak shift of ∼200 nm is realized. Our study not only provides a fundamental understanding of the optical properties of PANI but also opens the probability for developing high-performance dynamic media for active plasmonics.

Electrochemical coating of different conductive polymers on diverse plasmonic metal nanocrystals

Conductive polymers are attracting much attention for realizing active plasmonics on conventional static plasmonic nanostructures because of their variable dielectric functions. Combining organic conductive polymers with inorganic plasmonic nanostructures allows for the creation of active devices, such as active metasurfaces, reconfigurable metalenses and dynamic plasmonic holography. However, the complexity of such a combination, together with the poor control in polymer thickness and morphology, has limited the advancement of active plasmonics. Herein we report on the electrochemical coating of conductive polymers on pre-grown metal nanocrystals. Robust control of the polymer thickness and morphology is accomplished through the variation of the applied electrochemical potential. Various types of conductive polymers are coated on different metal nanocrystals, including Au, Pd and Pt. Active plasmonic color switching and H2O2 sensing are demonstrated with polyaniline-coated Au nanorods.

(Gold nanorod core)/(poly(3,4-ethylene-dioxythiophene) shell) nanostructures and their monolayer arrays for plasmonic switching

(Gold nanorod core)/(poly(3,4-ethylene-dioxythiophene) (PEDOT) shell) nanostructures are prepared by the surfactant-assisted oxidative polymerization of 3,4-ethylene-dioxythiophene on the surface of gold nanorods (NRs). The PEDOT shell exhibits distinct dielectric properties at doped and undoped states, which allows the manipulation of plasmonic responses of the Au nanorod core. The shift in plasmon resonance induced by the dedoping of PEDOT is found to be associated with the overlap between the plasmon resonance band of the core/shell nanostructure and the spectral region where the largest refractive index variation of PEDOT occurs, as well as with the type of the dedopant. Macroscopic two-dimensional (2D) monolayer arrays of core/shell nanostructures with controlled particle number densities are fabricated on indium tin oxide (ITO)-coated glass substrates by electrophoretic deposition. A reversible plasmonic shift of about 70 nm is obtained on the core/shell nanostructure monolayer array with a number density of around 18 particles per μm2. Our design of colloidal (Au nanorod core)/(PEDOT shell) nanostructures and their 2D monolayer arrays paves the way for the fabrication of high-performance plasmonic switches in large-scale practical usages as well as for the preparation of advanced, programmable chromic materials for a broad range of applications, such as smart windows, anti-counterfeiting tags, and medical and environmental sensors.

Multi-interfacial plasmon coupling in multigap (Au/AgAu)@CdS core-shell hybrids for efficient photocatalytic hydrogen generation

Plasmon coupling induced intense light absorption and near-field enhancement have vast potential for high-efficiency photocatalytic applications. Herein, (Au/AgAu)@CdS core-shell hybrids with strong multi-interfacial plasmon coupling were prepared through a convenient strategy for efficient photocatalytic hydrogen generation. Bimetallic Au/AgAu cores with an adjustable number of nanogaps (from one to four) were primarily synthesized by well-controlled multi-cycle galvanic replacement and overgrowth processes. Extinction tests and numerical simulations synergistically revealed that the multigap Au/AgAu hybrids possess a gap-dependent light absorption region and a local electric field owing to the multigap-induced multi-interfacial plasmon coupling. With these characteristics, hetero-photocatalysts prepared by further coating of CdS shells on multigap Au/AgAu cores exhibited a prominent gap-dependent photocatalytic hydrogen production activity from water splitting under light irradiation (λ > 420 nm). It is found that the hydrogen generation rates of multigap (Au/AgAu)@CdS have an exponential improvement compared with that of pure CdS as the number of nanogaps increases. In particular, four-gap (Au/AgAu)@CdS core-shell catalysts displayed the highest hydrogen generation rate, that is 96.1 and 47.2 times those of pure CdS and gapless Au@CdS core-shell hybrids. These improvements can be ascribed to the strong plasmon absorption and near-field enhancement induced by the multi-interfacial plasmon coupling, which can greatly improve the light-harvesting efficiency, offer more plasmonic energy, and boost the generation and separation of electron-hole pairs in the multigap catalysts.

Switching plasmonic Fano resonance in gold nanosphere-nanoplate heterodimers

The interference between spectrally overlapping superradiant and subradiant plasmon resonances generates plasmonic Fano resonance, which allows for attractive applications such as electromagnetically induced transparency, light trapping, and refractometric sensing with high figures of merit. The active switching of plasmonic Fano resonance holds great promise in modulating optical signals, dynamically harvesting light energy, and constructing switchable plasmonic sensors. However, structures enabling the active control of plasmonic Fano resonance have rarely been achieved because of the fabrication complexity and cost. Herein we report on the realization of active plasmonic Fano resonance switching on Au nanosphere-nanoplate heterodimers. The active switching is enabled by varying the refractive index of a layer of polyaniline that fills in the gap between the Au nanosphere and the Au nanoplate. A reversible spectral shift of 20 nm is observed on the individual heterodimers during switching. The maximal spectral shift decreases as the interparticle gap distance is enlarged, showing a strong dependence of the spectral shift on the local electric field intensity enhancement in the gap region. This trend agrees with the predicted dependence of the refractive index sensitivity on the local field intensity enhancement. Our results provide insights into the development of plasmonic structures supporting actively switchable Fano resonances, which can lead to new technological applications, such as switchable cloaking and display, dynamic coding of optical signals, color sorting and filtering. The Au heterodimers with polyaniline in the gap can also be applied for the sensing of local environmental parameters such as pH values and heavy metal ions.

Stimuli-Responsive Optical Nanomaterials

Responsive optical nanomaterials that can sense and translate various external stimuli into optical signals, in the forms of observable changes in appearance and variations in spectral line shapes, are among the most active research topics in nanooptics. They are intensively exploited within the regimes of the four classic optical phenomena-diffraction in photonic crystals, absorption of plasmonic nanostructures, as well as color-switching systems, refraction of assembled birefringent nanostructures, and emission of photoluminescent nanomaterials and molecules. Herein, a comprehensive review of these research activities regarding the fundamental principles and practical strategies is provided. Starting with an overview of their substantial developments during the latest three decades, each subtopic discussion is led with fundamental theories that delineate the correlation between nanostructures and optical properties and the delicate research strategies are elaborated with specific attention focused on working principles and optical performances. The unique advantages and inherent limitations of each responsive optical nanoscale platform are summarized, accompanied by empirical criteria that should be met and perspectives on research opportunities where the developments of next-generation responsive optical nanomaterials might be directed.

Optofluidic system based on electrowetting technology for dynamically tunable spectrum absorber

An optofluidic system that uses the electrowetting technology to dynamically control the local surface plasmon resonance of the silver nanoparticle is invented. The silver nanoparticle is initially suspended at the interface of the polar liquid and the non-polar liquid. As the interface morphology changes with the applied voltage, the media distribution surrounding particle is changed accordingly, thus realizing the resonance absorption peak's modulation. The investigation result shows that a wide range of the spectral colors from red to blue can be selectively absorbed just by a single device. Specifically, when the radius of the particle is 50 nm, the wavelength of the absorption peak can be dynamically modulated from 460 nm to 607 nm. This proposed method can be used to design and prepare rapidly adjustable optical elements.

Active control of plasmonic colors: emerging display technologies

In recent years there has been a growing interest in the use of plasmonic nanostructures for color generation, a technology that dates back to ancient times. Plasmonic structural colors have several attractive features but once the structures are prepared the colors are normally fixed. Lately, several concepts have emerged for actively tuning the colors, which opens up for many new potential applications, the most obvious being novel color displays. In this review we summarize recent progress in active control of plasmonic colors and evaluate them with respect to performance criteria for color displays. It is suggested that actively controlled plasmonic colors are generally less interesting for emissive displays but could be useful for new types of electrochromic devices relying on ambient light (electronic paper). Furthermore, there are several other potential applications such as images to be revealed on demand and colorimetric sensors.

Cluster Assemblies Produced by Aggregation of Preformed Ag Clusters in Ionic Liquids

Room-temperature ionic liquids (RTILs) can be used as electrosterical stabilizers for nanoparticles without adding stabilizing agents. However, the nanoparticle stability and its mechanisms are still in discussion. We deposited preformed 2 nm ±0.6 nm silver clusters into the ionic liquid C₄MIM PF₆ using in situ UV/vis absorption to monitor the deposition process. The time- and temperature-dependent cluster aggregation process was studied with ex situ UV/vis absorption spectroscopy analyzed with electrodynamic calculations using generalized Mie theory. On an atomistic level, the sample structure was investigated using EXAFS and a neural network based analysis of XANES. The combination of all methods shows that an aggregation of the original 2 nm clusters without coalescence takes place, which can be controlled or stopped by choosing an appropriate sample temperature. This approach allows the controlled production of chainlike cluster aggregates in RTIL, promising for a number of applications.

Advanced Plasmonic Materials for Dynamic Color Display

Plasmonic structures exhibit promising applications in high-resolution and durable color generation. Research on advanced hybrid plasmonic materials that allow dynamically reconfigurable color control has developed rapidly in recent years. Some of these results may give rise to practically applicable reflective displays in living colors with high performance and low power consumption. They will attract broad interest from display markets, compared with static plasmonic color printing, for example, in applications such as digital signage, full-color electronic paper, and electronic device screens. In this progress report, the most promising recent examples of utilizing advanced plasmonic materials for the realization of dynamic color display are highlighted and put into perspective. The performances, advantages, and disadvantages of different technologies are discussed, with emphasis placed on both the potential and possible limitations of various hybrid materials for dynamic plasmonic color display.

Surface plasmon resonance extension through two-block metal-conducting polymer nanorods

Research on surface plasmon resonance coupling of metallic nanostructures is an important area in the field of plasmonics because distinctive collective optical properties can be realized that are different from the individual constituents. Here we report the localized surface plasmon resonance of hybrid metal-organic nanorods. Colloidal-dispersed Au-PPy nanorods were synthesized as a representative material using a modified electrochemical method, and the collective oscillation properties were systematically investigated by comparing these materials with pure Au nanorods. We observed the extended surface plasmon resonance of a hybrid system. The presence of doped-PPy segments on Au segments induced an enhanced coherent electric field due to the partial contribution of π-electrons on the PPy segment, which led to a red-shifted plasmon feature. Additionally, we demonstrated that surface plasmon resonance extension can be tuned by dopant anions, which demonstrates a way of tuning a dopant-induced plasmonic system.

Localized surface plasmon resonance can be exploited for a range of applications, but remains difficult to tailor in metal-organic nanostructures. Here the authors synthesize gold-polypyrrole nanorods and observe a unique extended surface plasmon resonance, which they find to be tunable through doping.

Controllable oligomerization: defying step-growth kinetics in the polymerization of gold nanoparticles

We report a new method for one-step dimerization of AuNP@PSPAA, which defies the step-growth kinetics and gives a record yield.

Enhanced plasmon coupling of partly embedded gold nanospheres with surrounding silicon

Gold nanospheres (AuNSs) were partly embedded into silicon through metal-assisted chemical etching, producing multiple-dimensional coupling of the plasmon resonances with the induced image charges in the surrounding medium. Rich plasmonic features of such coupling system were revealed by single particle dark-field scattering spectra, characterizing by two splitted multipolar resonances at short wavelength region and a mixed dipolar resonance extending to infrared region. Numerical electrodynamic calculations indicated that the multipolar modes arise from the in-plane and out-of-plane quadrupolar resonances, which are excited by the horizontal and verticle electric field components, respectively, of the incident light owing to the enhanced coupling interaction. As the embedding depth increases, the degree of symmetry breaking in such nanoparticles/substrate system changes, resulting in significantly modified optical response, which supplies a new way to modulate the optical properties of plasmonic nanoparticles.

Tailored Surfaces/Assemblies for Molecular Plasmonics and Plasmonic Molecular Electronics

Molecular plasmonics uses and explores molecule-plasmon interactions on metal nanostructures for spectroscopic, nanophotonic, and nanoelectronic devices. This review focuses on tailored surfaces/assemblies for molecular plasmonics and describes active molecular plasmonic devices in which functional molecules and polymers change their structural, electrical, and/or optical properties in response to external stimuli and that can dynamically tune the plasmonic properties. We also explore an emerging research field combining molecular plasmonics and molecular electronics.

Active Electrochemical Plasmonic Switching on Polyaniline-Coated Gold Nanocrystals

High-performance electrochemical plasmonic switching is realized on both single-particle and ensemble levels by coating polyaniline on colloidal gold nanocrystals through surfactant-assisted oxidative polymerization. Under small applied potentials, the core@shell nanostructures exhibit reversible plasmon shifts as large as 150 nm, a switching time of less than 10 ms, and a high switching stability.

Silver Nanodisk Monolayers with Surface Coverage Gradients for Use as Optical Rulers and Protractors

Colloidal silver nanodisks (AgNDs) are assembled into a monolayer with a coverage density gradient (CDG) on the surface of flat and cylindrical substrates using the Langmuir-Blodgett (LB) technique. Compressing the LB monolayers during transfer to the substrates causes the CDG assembly of the AgNDs. By functionalizing the AgNDs with poly(ethylene glycol), it is possible to control their order inside the LB monolayer assembly by changing the deposition surface pressure. Well-separated AgNDs, 2D aggregates with different numbers of particles, and highly packed 2D arrays are formed as the deposition surface pressure is increased. Localized surface plasmon resonance (LSPR) spectra collected at different separation distances from the highest coverage spot (HCS) of the CDG AgND arrays on a flat substrate are blue-shifted, and the shift increases systematically upon increasing the distance. The relationship among the LSPR peak position, the peak intensity at a fixed wavelength, and the corresponding separation distance from the HCS is fitted exponentially. A similar systematic blue shift in the LSPR spectrum of the CDG AgND monolayer on a cylindrical substrate is obtained when the substrate is rotated at different angles relative to the HCS. The fabricated CDG AgND monolayers can potentially be used for optically measuring distances and angles.

Polyaniline-coated upconversion nanoparticles with upconverting luminescent and photothermal conversion properties for photothermal cancer therapy

In this study, we developed a nanosystem based on upconversion nanoparticles (UCNPs) coated with a layer of polyaniline nanoparticles (PANPs). The UCNP induces upconversion luminescence for imaging and photothermal conversion properties are due to PANPs. In vitro experiments showed that the UCNPs-PANPs were nontoxic to cells even at a high concentration (800 µg mL⁻¹). Blood analysis and histological experiments demonstrated that the UCNPs-PANPs exhibited no apparent toxicity in mice in vivo. Besides their efficacy in photothermal cancer cell ablation, the UCNP-PANP nanosystem was found to achieve an effective in vivo tumor ablation effect after irradiation using an 808 nm laser. These results demonstrate the potential of the hybrid nanocomposites for use in imaging-guided photothermal therapy.

Reversible switching of structural and plasmonic properties of liquid-crystalline gold nanoparticle assemblies

Hybrid materials built of spherical gold nanoparticles with three different sizes covered with (pro)mesogenic molecules have been prepared. Small-angle X-ray diffraction studies showed that after thermal annealing most of the obtained materials formed long-range ordered assemblies. Variation of the (pro)mesogenic ligand architecture enabled us to achieve a switchable material, which could be reversibly reconfigured between 3D long-range ordered structures with tetragonal and face centred cubic symmetries. This structural reconfiguration induces changes to the plasmonic response of the material. This work demonstrates that it is possible to use LC-based self-assembling phenomena to prepare dynamic materials with structural properties important for the development of active plasmonic metamaterials.

Cellulose nanofiber-assisted dispersion of cellulose nanocrystals@polyaniline in water and its conductive films

The cellulose nanofibers as an efficient and environmentally friendly dispersant have been proven to be an efficient way to disperse cellulose nanocrystals@polyaniline. The obtained film show high electrical conductivity.