Giant Plasmon Resonance Shift Using Poly(3,4-ethylenedioxythiophene) Electrochemical Switching

Advances in electrochromic device technology through the exploitation of nanophotonic and nanoplasmonic effects

Research regarding electrochromic (EC) materials, such materials that change their color upon application of an electrochemical stimulus, has been conducted for centuries. However, most recently, increasing efforts have been put into developing novel solutions to utilize these on-off switching materials in advanced nanoplasmonic and nanophotonic devices. Due to the significant change in dielectric properties of oxides such as WO₃, NiO, Mn₂O₃ and conducting polymers like PEDOT:PSS and PANI, EC materials have transcended beyond simple smart window applications and are now found in plasmonic devices for full-color displays and enhanced modulation transmission and photonic devices with ultra-high on-off ratios and sensing abilities. Advancements in nanophotonic ECDs have further decreased EC switching speed by several orders of magnitude, allowing integration in real-time measurement and lab-on-chip applications. The EC nature of such nanoscale devices promises low energy consumption with low operating voltages paired with bistability and long lifetimes. We summarize these novel approaches to EC device design, lay out the current short comings and draw a path forward for future utilization.

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Smart Modulators Based on Electric Field-Triggering of Surface Plasmon-Polariton for Active Plasmonics

Design and properties of a plasmonic modulator in situ tunable by electric field are presented. Our design comprises the creation of periodic surface pattern on the surface of an elastic polymer supported by a piezo-substrate by excimer laser irradiation and subsequent selective coverage by silver by tilted angle vacuum evaporation. The structure creation was confirmed by AFM and FIB-SEM techniques. An external electric field is used for fine control of the polymer pattern amplitude, which tends to decrease with increasing voltage. As a result, surface plasmon-polariton excitation is quenched, leading to the less pronounced structure of plasmon response. This quenching was checked using UV-Vis spectroscopy and SERS measurements, and confirmed by numerical simulation. All methods prove the proposed functionality of the structures enabling the creation smart plasmonic materials for a very broad range of advanced optical applications.

Long-Range Plasmon-Induced Anisotropic Growth of an Organic Semiconductor between Isotropic Gold Nanoparticles

Plasmon-induced diazonium reduction was used to graft an organic semiconductor, namely oligo(bisthienylbenzene) (BTB), onto square arrays of gold nanoparticles (NPs) of various diameters. Grafting was evidenced by scanning electron microscopy (SEM) measurements by the extinction spectra of the localized surface plasmon resonance, as well as by Raman and energy dispersive X-ray (EDX) spectroscopies. We show that BTB is selectively deposited around the NPs. The thickness of the layer increases with increasing irradiation time and reaches a limit which depends on the size of the NPs with the thicker organic layers being generated for smaller NPs. Under polarized irradiation, BTB growth is strongly anisotropic. Starting from arrays with square gratings and spherical NPs, long-range plasmon-induced anisotropic growth makes it possible to generate in the direction of the polarized light, lines, columns, or lines and columns of NPs connected by an organic semiconductor. These results demonstrate that the growth is due to hot electrons.

A stable naked-eye colorimetric sensor for monitoring release of extracellular gamma-aminobutyric acid (GABA) neurotransmitter from SH-SY5Y cells

A novel optical γ-aminobutyric acid (GABA)-based sensor was developed on interacting thiol compounds and o-phthalaldehyde (OPA) to form thiacetal compounds. Then, the thiacetal interacts with the GABA molecule to form an isoindole compound. The effects of four thiol compounds on the stability of the resulting isoindole compound were assessed. The 2-mercaptoethanol, "one of the most used derivatizing agents," is unexpectedly the least stable; while, 16-mercaptohexadecanoic acid resulted in the most durable isoindole compound. The developed sensor showed the capability for detecting GABA within a wide concentration range spanning from 500 nmol L^-1 to 100 µmol L^-1. The detection limit was about 330 nmol L^-1, which indicated the high sensitivity of the developed sensor compared with those previously reported. The findings illustrated the ability to detect GABA at the physiological pH (pH = 7.4) without adjusting the pH value, opening the door for real applications. Furthermore, the sensor could detect various GABA concentrations in human serum with good recovery percentages (98% to 101.4%). In addition, this assay was applied to monitor GABA release from the SH-SY5Y cell line to convert glutamate into GABA. This result indicates the capability of the proposed assay for visually monitoring the release of GABA neurotransmitters.

Quantum Plasmonics: Energy Transport Through Plasmonic Gap

At the interfaces of metal and dielectric materials, strong light-matter interactions excite surface plasmons; this allows electromagnetic field confinement and enhancement on the sub-wavelength scale. Such phenomena have attracted considerable interest in the field of exotic material-based nanophotonic research, with potential applications including nonlinear spectroscopies, information processing, single-molecule sensing, organic-molecule devices, and plasmon chemistry. These innovative plasmonics-based technologies can meet the ever-increasing demands for speed and capacity in nanoscale devices, offering ultrasensitive detection capabilities and low-power operations. Size scaling from the nanometer to sub-nanometer ranges is consistently researched; as a result, the quantum behavior of localized surface plasmons, as well as those of matter, nonlocality, and quantum electron tunneling is investigated using an innovative nanofabrication and chemical functionalization approach, thereby opening a new era of quantum plasmonics. This new field enables the ultimate miniaturization of photonic components and provides extreme limits on light-matter interactions, permitting energy transport across the extremely small plasmonic gap. In this review, a comprehensive overview of the recent developments of quantum plasmonic resonators with particular focus on novel materials is presented. By exploring the novel gap materials in quantum regime, the potential quantum technology applications are also searched for and mapped out.

Plasmonic Photoelectrochemistry: In View of Hot Carriers

Utilizing plasmon-generated hot carriers to drive chemical reactions has emerged as a popular topic in solar photocatalysis. However, a complete description of the underlying mechanism of hot-carrier transfer in photochemical processes remains elusive, particularly for those involving hot holes. Photoelectrochemistry enables to localize hot holes on photoanodes and hot electrons on photocathodes and thus offers an approach to separately explore the hole-transfer dynamics and electron-transfer dynamics. This review summarizes a comprehensive understanding of both hot-hole and hot-electron transfers from photoelectrochemical studies on plasmonic electrodes. Additionally, working principles and applications of spectroelectrochemistry are discussed for plasmonic materials. It is concluded that photoelectrochemistry provides a powerful toolbox to gain mechanistic insights into plasmonic photocatalysis.

(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.

Tunable electrochromic behavior of titanium-based MXenes

Two-dimensional transition metal carbides, nitrides and carbonitrides, popular by the name MXenes, are a promising class of materials as they exhibit intriguing optical, optoelectronic and electrochemical properties. Taking advantage of their metallic conductivity and hydrophilicity, titanium carbide MXenes (Ti3C2Tx and others) are used to fabricate solution processable transparent conducting electrodes (TCEs) for the design of three-electrode electrochromic cells. However, the tunable electrochromic behavior of various titanium-based MXene compositions across the entire visible spectrum has not yet been demonstrated. Here, we investigate the intrinsic electrochromic properties of titanium-based MXenes, Ti3C2Tx, Ti3CNTx, Ti2CTx, and Ti1.6Nb0.4CTx, where individual MXenes serve as a transparent conducting, electrochromic, and plasmonic material layer. Plasmonic extinction bands for Ti3C2Tx, Ti2CTx and Ti1.6Nb0.4CTx are centered at 800, 550 and 480 nm, which are electrochemically tunable to 630, 470 and 410 nm, respectively, whereas Ti3CNTx shows a reversible change in transmittance in the wide visible range. Additionally, the switching rates of MXene electrodes with no additional transparent conductor electrodes are estimated and correlated with the respective electrical figure of merit values. This study demonstrates that MXene-based electrochromic cells are tunable in the entire visible spectrum and suggests the potential of the MXene family of materials in optoelectronic, plasmonic, and photonic applications, such as tunable visible optical filters and modulators, to name a few.

Electrochemical Switching of Plasmonic Colors Based on Polyaniline-Coated Plasmonic Nanocrystals

Plasmonic color generation has attracted much research interest because of the unique optical properties of plasmonic nanocrystals that are promising for chromatic applications, such as flat-panel displays, smart windows, and wearable devices. Low-cost, monodisperse plasmonic nanocrystals supporting strong localized surface plasmon resonances are favorable for the generation of plasmonic colors. However, many implementations so far have either a single static state or complexities in the particle alignment and switching mechanism for generating multiple displaying states. Herein, we report on a facile and robust approach for realizing the electrochemical switching of plasmonic colors out of colloidal plasmonic nanocrystals. The metal nanocrystals are coated with a layer of polyaniline, whose refractive index and optical absorption are reversibly switched through the variation of an applied electrochemical potential. The change in refractive index and optical absorption results in the modulation of the plasmonic scattering intensity with a depth of 11 dB. The electrochemical switching process is fast (∼5 ms) and stable (over 1000 switching cycles). A device configuration is further demonstrated for switching plasmonic color patterns in a transparent electrochemical device, which is made from indium tin oxide electrodes and a polyvinyl alcohol solid electrolyte. Our control of plasmonic colors provides a favorable platform for engineering low-cost and high-performance miniaturized optical devices.

Dynamic Tuning of Gap Plasmon Resonances Using a Solid-State Electrochromic Device

Plasmonic antennas and metasurfaces can effectively control light-matter interactions, and this facilitates a deterministic design of optical materials properties, including structural color. However, these optical properties are generally fixed after synthesis and fabrication, while many modern-day optics applications require active, low-power, and nonvolatile tuning. These needs have spurred broad research activities aimed at identifying materials and resonant structures capable of achieving large, dynamic changes in optical properties, especially in the challenging visible spectral range. In this work, we demonstrate dynamic tuning of polarization-dependent gap plasmon resonators that contain the electrochromic oxide WO₃. Its refractive index in the visible changes continuously from n = 2.1 to 1.9 upon electrochemical lithium insertion and removal in a solid-state device. By incorporating WO₃ into a gap plasmon resonator, the resonant wavelength can be shifted continuously and reversibly by up to 58 nm with less than 2 V electrochemical bias voltage. The resonator can remain in a tuned state for tens of minutes under open circuit conditions.

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.

Electrical detection of plasmon-induced isomerization in molecule-nanoparticle network devices

We use a network of molecularly linked gold nanoparticles (NPSAN: nanoparticle self-assembled network) to demonstrate the electrical detection (conductance variation) of plasmon-induced isomerization (PII) of azobenzene derivatives (azobenzene bithiophene: AzBT). We show that PII is more efficient in a 3D-like NPSAN (cluster-NPSAN) than in a purely two-dimensional NPSAN (i.e., a monolayer of AzBT functionalized Au NPs). By comparison with the usual optical (UV-visible light) isomerization of AzBT, PII shows faster (a factor > ∼10) isomerization kinetics. Possible PII mechanisms are discussed: electric field-induced isomerization, two-phonon process, and plasmon-induced resonance energy transfer (PIRET), the latter being the most likely.

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.

Cross Stacking of Nanopatterned PEDOT Films for Use as Soft Electrodes

Cross stacking of nanopatterned conductive polymer film was explored using a sacrificial soft template made of nanopatterned polystyrene (PS) film as a guide for nanopatterned conductive polymer film. For use as a conductive film, the PS pattern was filled with poly(3,4-ethylenedioxythiophene) (PEDOT), and then completely removed, to generate single-patterned PEDOT (SPDOT) film having a conductivity of 1079 S/cm, which was comparable to the pristine unpatterned PEDOT (UPDOT) film on a glass slide. SPDOT layers were stacked across each other to form double-layered (DPDOT) and multiple-layered patterned PEDOT film on a glass slide or polymeric substrate. The patterned PEDOT film showed enhanced optical and electrochemical activity; specifically as compared to the UPDOT film on a glass slide, the DPDOT film showed an increase in reflectance and an enhanced electrochemically active surface by 23.4% and 32.8%, respectively. The patterned PEDOT film on a polymer substrate showed high bendability up to being completely folded and maintained its conductivity for over 10 000 cycles of bending. The patterned PEDOT layers were applied to dye-sensitized solar cells (DSSCs) as a transparent conductive oxide (TCO)-free counter electrode. An N719-based DSSC with a DPDOT film recorded a photoconversion efficiency of 7.54%, which is one of the highest values among the TCO-free DSSCs based on a PEDOT counter electrode.

Plasmon-Induced Conductance Switching of an Electroactive Conjugated Polymer Nanojunction

A plasmonic molecular electronic device, consisting of poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires bridging an ultramicroelectrode and an indium tin oxide (ITO) substrate covered by gold nanoparticles (Au NPs), has been developed. Light irradiation of this device has a dramatic impact on its conductance. Polymer strands, maintained electrochemically in their oxidized, conducting state, reversibly switch to their insulating state upon irradiation by visible-wavelength light, resulting in a sharp decrease in the conductance. The high-conductance state is restored when the light is turned off. Switching depends on the wavelength and the intensity of the incident light. It is due to reversible reduction of the nanosized region of PEDOT nanowires in contact with a gold NP and is attributed to plasmon-induced hot-electron injection into the PEDOT. The high/low conductance ratio can be as great as 1000, and switching requires low light intensity (220 W/m²). These results could open the way to the design of a new family of optoelectronic switches.

Design and characterization of a plasmonic Doppler grating for azimuthal angle-resolved surface plasmon resonances

We present a two-dimensional plasmonic Doppler grating (PDG) for broadband and azimuthal angle-resolved nanophotonic applications. The PDG consists of a set of non-concentric circular rings mimicking the wavefronts of a moving point source that exhibits the Doppler effect and thereby offers a continuous azimuthal angle-dependent lattice momentum for photon-plasmon coupling. The center and span of the working frequency window are fully designable for optimal performance in specific applications. We detail the design, fabrication and optical characterization of the PDG. The design of the Doppler grating provides a general platform for in-plane angle-resolved nanophotonic applications.

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.

Colloidal Synthesis and Applications of Plasmonic Metal Nanoparticles

Plasmonic metal nanoparticles attract intense research attention because of their fascinating surface plasmon resonance properties and their potential applications in diverse fields. Here, some of the recent research efforts on the synthesis and applications of plasmonic metal nanoparticles are highlighted. Starting from the colloidal synthesis of metal nanoparticles, various shaped silver and gold nanostructures are discussed. The applications of plasmonic nanoparticles in photocatalysis, surface-enhanced Raman spectroscopy (SERS), and devices are used as excellent examples showcasing the advantages of these nanoparticles. The report closes with a brief summary and discussion on the challenges and future direction in this research field.

Multifunctional Indium Tin Oxide Electrode Generated by Unusual Surface Modification

The indium tin oxide (ITO) material has been widely used in various scientific fields and has been successfully implemented in several devices. Herein, the electrochemical reduction of ITO electrode in an organic electrolytic solution containing alkali metal, NaI, or redox molecule, N-(ferrocenylmethyl) imidazolium iodide, was investigated. The reduced ITO surfaces were investigated by X-ray photoelectron spectroscopy and grazing incident XRD demonstrating the presence of the electrolyte cation inside the material. Reversibility of this process after re-oxidation was evidenced by XPS. Using a redox molecule based ionic liquid as supporting electrolyte leads to fellow electrochemically the intercalation process. As a result, modified ITO containing ferrocenyl imidazolium was easily generated. This reduction process occurs at mild reducing potential around -1.8 V and causes for higher reducing potential a drastic morphological change accompanied with a decrease of the electrode conductivity at the macroscopic scale. Finally, the self-reducing power of the reduced ITO phase was used to initiate the spontaneous reduction of silver ions leading to the growth of Ag nanoparticles. As a result, transparent and multifunctional active ITO surfaces were generated bearing redox active molecules inside the material and Ag nanoparticles onto the surface.

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.

High-contrast and fast electrochromic switching enabled by plasmonics

With vibrant colours and simple, room-temperature processing methods, electrochromic polymers have attracted attention as active materials for flexible, low-power-consuming devices. However, slow switching speeds in devices realized to date, as well as the complexity of having to combine several distinct polymers to achieve a full-colour gamut, have limited electrochromic materials to niche applications. Here we achieve fast, high-contrast electrochromic switching by significantly enhancing the interaction of light—propagating as deep-subwavelength-confined surface plasmon polaritons through arrays of metallic nanoslits, with an electrochromic polymer—present as an ultra-thin coating on the slit sidewalls. The switchable configuration retains the short temporal charge-diffusion characteristics of thin electrochromic films, while maintaining the high optical contrast associated with thicker electrochromic coatings. We further demonstrate that by controlling the pitch of the nanoslit arrays, it is possible to achieve a full-colour response with high contrast and fast switching speeds, while relying on just one electrochromic polymer.

Slow switching speeds in device configurations have severely limited the applications of electrochromic materials. Here, Xu et al. use plasmonic nanoslit arrays and demonstrate fast, high-contrast, monochromatic and full-colour electrochromic switching using two different electrochromic polymers.

Smart Component for Switching of Plasmon Resonance by External Electric Field

A new approach for preparation of active plasmonic component with capability to switch on/off localized surface plasmon resonance (LSPR) by piezoelectric effect is described. Polyvinylidene fluoride (PVDF) was patterned by polarized KrF excimer laser beam. The polarization was perpendicular to polymer orientation introduced during the poling procedure. Consequently the silver nanoclusters were sputtered onto the polymer surface. Application of an external electric field leads to polymer stretching and surface smoothening. Simultaneously, silver clusters are elongated and interconnected; this process leads to dramatic decrease of surface resistance and complete quenching of plasmon related absorption.

Extremely robust and post-functionalizable gold nanoparticles coated with calix[4]arenes via metal–carbon bonds

Gold nanoparticles stabilized with a thin layer of post-functionalizable calix[4]arenes were prepared through the reductive grafting of a calix[4]arene-tetra-diazonium salt.

Highly Sensitive Electro-Plasmonic Switches Based on Fivefold Stellate Polyhedral Gold Nanoparticles

Electron-photon coupling in metal nanostructures has raised a new trend for active plasmonic switch devices in both fundamental understanding and technological applications. However, low sensitivity switches with an on/off ratio less than 5 have restricted applications. In this work, an electrically modulated plasmonic switch based on a surface-enhanced Raman spectroscopy (SERS) system with a single fivefold stellate polyhedral gold nanoparticle (FSPAuNP) is reported. The reversible switch of the SERS signal shows high sensitivity with an on/off ratio larger than 30. Such a high on/off ratio arises primarily from the plasmonic resonance shift of the FSPAuNP with the incident laser due to the altered free electron density on the nanoparticle under an applied electrochemical potential. This highly sensitive electro-plasmonic switch may enable further development of plasmonic devices.

Highly Resolved Nanostructured PEDOT on Large Areas by Nanosphere Lithography and Electrodeposition

Poly(ethylenedioxythiophene) (PEDOT) films were electrodeposited galvanostatically from an EDOT/sodium dodecyl sulfate solution in water, through a carboxylated polystyrene template monolayer self-assembled on ITO, after which the template was dissolved away in tetrahydrofuran. Analysis of the films by scanning electron microscopy and atomic force microscopy reveals large-area PEDOT honeycomb structures. The morphology of these structures was varied electrochemically, as the effective thickness and, surprisingly, the shape of the honeycomb arrangement depend on the polymerization time. Using nanospheres of 1 μm diameter and charge densities between 12 and 30 mC cm(-2) for electrodeposition generates PEDOT hexagons with very thin rectilinear walls 30-35 nm-thick and 800 nm-long, whereas at higher charge densities, circular bowls are created with 60 nm walls separating adjacent bowls; triangular areas as small as 0.02 μm(2) develop at the intersection of three nanospheres. These morphologies are specific to the use of carboxylated PS spheres and a water-based solution with a surfactant in the galvanostatic electrodeposition mode. Using smaller nanospheres, i.e. 500 nm in diameter, makes it possible to reach PEDOT hexagons with rectilinear walls as small as 11-17 nm-thick and 300 nm-long; circular bowls with 25-35 nm walls separating adjacent bowls and triangular areas as small as 0.003 μm(2) can also be generated. The wettabilities of the surfaces depend markedly on the pore depth of the PEDOT nanostructure, with contact angles going from 82° to 130° with increasing pore size. Finally these nanostructured PEDOT electrodes were used in Grätzel-type dye-sensitized solar cells (DSSCs) as Pt-free counter-electrodes, with an increase in the yield from 7.0 (bulk PEDOT) to 8.1%.

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

Active modulation of the plasmon coupling in homodimers of polyaniline (PANI)-coated Au nanospheres is achieved by changing the proton-doping state of the PANI shell. Such a PANI-enabled modulation of the plasmon coupling in the dimers gives rise to remarkable spectral shifts, which show an exponential dependence on the interparticle gap distance. For the dimer with a 10 nm PANI shell thickness and a 0.5 nm gap distance, the shift of the stronger scattering peak in response to the active modulation reaches 231 nm. Electrodynamic simulations reveal that the shift of the dipolar bonding plasmon mode dominates the position variation of the stronger scattering peak for the dimers with different gap distances. Moreover, the quadrupolar bonding plasmon mode can be turned on and off by controlling the proton-doping state of the dimers with gap distances of less than ∼3 nm. These results are of high importance for fundamentally understanding the sensitivity of coupled plasmon resonance modes to the dielectric environment, as well as for designing active plasmonic devices with high modulation performances.

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.

Surface plasmon resonance properties of silver nanoparticle 2D sheets on metal gratings

Grating-coupled propagating surface plasmons associated with silver-nanoparticle 2D crystalline sheets exhibit sensitive plasmonic resonance tuning. Multilayered silver-nanoparticle 2D crystalline sheets are fabricated on gold or silver grating surfaces by the Langmuir- Blodgett method. We show that the deposition of Ag crystalline nanosheets on Au or Ag grating surfaces causes a drastic change in propagating surface plasmon resonance (SPR) both in angle measurements at fixed wavelengths and in fixed incident-angle mode by irradiation of white light. The dielectric constant of the multilayered silver nanosheet is estimated by a rigorous coupled-wave analysis. We find that the dielectric constant drastically increases as the number of silver-nanosheet layers increases. The experimentally obtained SP dispersions of Ag crystalline nanosheets on Au and Ag gratings are compared with the calculated SP dispersion curves. The drastic change in the surface plasmon resonance caused by the deposition of Ag-nanoparticle 2D crystalline sheets on metal grating surfaces suggests the potential for applications in highly sensitive sensors or for plasmonic devices requiring greatly enhanced electric fields.

(Gold nanorod core)/(polyaniline shell) plasmonic switches with large plasmon shifts and modulation depths

(Gold nanorod core)/(polyaniline shell) nanostructures are prepared for functioning as active plasmonic switches. The single core/shell nanostructures exhibit a remarkable switching performance, with the modulation depth and scattering peak shift reaching 10 dB and 100 nm, respectively. The nanostructures are also deposited on substrates to form macroscale monolayers with remarkable ensemble plasmonic switching performances.

Near-infrared active metamaterials and their applications in tunable surface-enhanced Raman scattering

By utilizing the phase change properties of vanadium dioxide (VO2), we have demonstrated the tuning of the electric and magnetic modes of split ring resonators (SRRs) simultaneously within the near IR range. The electric resonance wavelength is blue-shift about 73 nm while the magnetic resonance mode is red-shifted about 126 nm during the phase transition from insulating to metallic phases. Due to the hysteresis phenomenon of VO2 phase transition, both the electric and magnetic modes shifts are hysteretic. In addition to the frequency shift, the magnetic mode has a trend to vanish due to the fact that the metallic phase VO2 has the tendency to short the gap of SRR. We have also demonstrated the application of this active metamaterials in tunable surface-enhanced Raman scattering (SERS), for a fixed excitation laser wavelength, the Raman intensity can be altered significantly by tuning the electric mode frequency of SRR, which is accomplished by controlling the phase of VO2 with an accurate temperature control.

Detailed mechanism for the orthogonal polarization switching of gold nanorod plasmons

In this work, we describe an electro-optic material capable of orthogonally switching the polarization of the localized surface plasmon resonance scattering of single gold nanorods, independent of their orientation. Liquid crystal samples are prepared in a sandwich configuration with electrodes arranged so that an applied voltage induces alignment-switching of the liquid crystal molecules covering individual gold nanorods. Due to the birefringence of the nematic liquid crystal, the reorientation in the nematic director alignment causes a change in the output polarization of the scattered light. We propose the underlying mechanism to be based on a homogeneous nematic to twisted nematic phase transition and provide support for it via Jones calculus by modelling the effect of ideally aligned homogeneous nematic and twisted nematic phases on polarized light transmitted through the sample. In the model, we include the effects of sample thickness and surface plasmon resonance wavelength, expressed in terms of the phase retardation, χ, on the observed output polarization. We find four distinctively different trends for the output polarization as a function of the incident polarization as χ is varied. Two of these cases provide reproducible orthogonal polarization switching of the surface plasmon resonance while maintaining a high degree of polarization. These results are verified experimentally with liquid crystal cells of different thicknesses. The deviation of the experimental samples from ideal behaviour can be explained by the inherent variations in the surface plasmon resonance maximum and local cell thickness.

A plasmonic Fano switch

Plasmonic clusters can support Fano resonances, where the line shape characteristics are controlled by cluster geometry. Here we show that clusters with a hemicircular central disk surrounded by a circular ring of closely spaced, coupled nanodisks yield Fano-like and non-Fano-like spectra for orthogonal incident polarization orientations. When this structure is incorporated into an uniquely broadband, liquid crystal device geometry, the entire Fano resonance spectrum can be switched on and off in a voltage-dependent manner. A reversible transition between the Fano-like and non-Fano-like spectra is induced by relatively low (∼6 V) applied voltages, resulting in a complete on/off switching of the transparency window.

Electrochemical fabrication of highly stable redox-active nanojunctions

Redox-gated molecular junctions were obtained starting with a relatively large gap between two electrodes, in the micrometer range, followed by electrochemical polymerization of aniline. Polyaniline (PANI) grows from the tip side until it bridges the two electrodes. The resulting junctions were characterized electrochemically by following the variation of the tip-substrate current as a function of the electrochemical gate potential for various bias voltages and by recording their I(V) characteristics. The two electrodes make contact through PANI wires, and microjunctions with conductances around 10(-3) S were obtained. On the basis of a similar setup, PANI nanojunctions with conductances between 10(-7) and 10(-8) S were made, where the current appears to be controlled by fewer than 10 oligoaniline strands. Despite the small number of strands connecting the two electrodes, the junctions are highly stable even when several successive potential sweeps are performed. Comparison of the conductance measured in the oxidized and reduced states leads to an on/off ratio of about 70-100, which is higher than that reported for a single aniline heptamer bridging two electrodes, highlighting the interest of connecting a few tens of molecules using the scanning electrochemical microscopy (SECM) configuration. In some cases, the switching of the PANI takes place in several individual conductance steps close to that obtained for a single oligoaniline. Finally, starting with a microjunction and mechanically withdrawing the tip shrinks it down to the nanometer scale and makes it possible to reach the regime where the conductance is controlled by a limited number of strands. This work presents an easy method for making redox-gated nanojunctions and for probing the conductance of a few oligoanilines despite an initially large tip-substrate gap.

Active modulation of nanorod plasmons

Confining visible light to nanoscale dimensions has become possible with surface plasmons. Many plasmonic elements have already been realized. Nanorods, for example, function as efficient optical antennas. However, active control of the plasmonic response remains a roadblock for building optical analogues of electronic circuits. We present a new approach to modulate the polarized scattering intensities of individual gold nanorods by 100% using liquid crystals with applied voltages as low as 4 V. This novel effect is based on the transition from a homogeneous to a twisted nematic phase of the liquid crystal covering the nanorods. With our method it will be possible to actively control optical antennas as well as other plasmonic elements.

An integrated electrochromic nanoplasmonic optical switch

We demonstrate an electrochemically driven optical switch based on absorption modulation of surface plasmon polaritons (SPPs) propagating in a metallic nanoslit waveguide containing nanocrystals of electrochromic Prussian Blue dye. Optical transmission modulation of ∼96% is achieved by electrochemically switching the dye between its oxidized and reduced states using voltages below 1 V. High spatial overlap and long interaction length between the SPP and the active material are achieved by preferential growth of PB nanocrystals on the nanoslit sidewalls. The resulting orthogonalization between the directions of light propagation and that of charge transport from the electrolyte to ultrathin active material inside the nanoslit waveguide offers significant promise for the realization of electrochromic devices with record switching speeds.

Incident-angle-modulated molecular plasmonic switches: a case of weak exciton-plasmon coupling

We have designed an angularly tunable plasmonic system that consists of Au nanodisks in combination with molecules of photoswitchable resonance, spiropyran, to gain new insights into weak exciton-plasmon couplings. In the weak exciton-plasmon coupling regime, switching molecular resonance can induce localized surface plasmon resonance (LSPR) peak shifts due to the change in the refractive index of the molecular materials. On the basis of the angle-resolved spectroscopic study of the nanodisk-spiropyran system both with and without UV irradiation, we reveal an unusual "zigzag" curve for the LSPR peak shifts (due to the photoswitching of the molecular resonance) as a function of the original LSPR peak wavelength. A further theoretical analysis attributes the "zigzag" curve to two significant competing effects that depend on the incident angle of the probe light: plasmon-enhanced molecular resonance absorption and LSPR sensitivity to the surroundings' refractive index.