A Solution‐Processable High‐Coloration‐Efficiency Low‐Switching‐Voltage Electrochromic Polymer Based on Polycyclopentadithiophene

A solution‐processable electrochromic (EC) polymer, poly(4,4‐dioctyl‐cyclopenta[2,1‐b:3,4‐b′]‐dithiophene) (PDOCPDT) is prepared by means of chemical oxidative polymerization of the corresponding monomer. The UV‐vis spectrum of the spin‐coated PDOCPDT film displays an absorption maximum of 580 nm. Although the polymer is deep blue in its neutral state, it turns to transparent bluish after being oxidized. PDOCPDT film (thickness 120 nm) exhibits high coloration efficiency (CE)—as high as 932 C cm^–2 at 580 nm, low response time (0.75 s), high optical density (0.75 at 580 nm), and high‐level stability for long term switch (it switches repetitively 1000 times with less than 8 % contrast loss). The electrochemical stability and redox potentials of PDOCPDT films are independent of film thickness (50–180 nm) and active area (up to 2 cm × 2 cm). Nevertheless, optical contrast increases as the film thickness increases, although the CE and response time changes irregularly with the film thickness. The good EC properties combined with the easy film fabrication process make PDOCPDT a notable candidate for application in EC devices. (ECDs) A simple transmissive‐type ECD with good CE using PDOCPDT film as an active layer is also demonstrated.     

Designing Non‐Centrosymmetric Molecular Crystals: Optimal Packing May Be Just One Carbon Away

Molecular nonlinear optical (NLO) crystals feature important advantages compared to inorganic counterparts, such as low dielectric constants, ultrafast response times, and large electro‐optic coefficients. Conjugated push–pull chromophores connecting electron‐donating with accepting groups are often employed in the design of these crystals. However, associated large molecular dipole moments induce antiparallel or centrosymmetric conformations in the solid‐state, which leads to NLO inactivity. The cation–anion hydrogen bond interactions of a hydroxy‐piperidino electron donor group are combined with increased van der Waals volume effects induced by an ethyl modification of the electron‐accepting moiety. This produces non‐centrosymmetric packing in the organic salt EHPSI‐4NBS ((E)‐1‐ethyl‐2‐(4‐(4‐(hydroxymethyl)piperidin‐1‐yl)styryl)‐3,3‐dimethyl‐3H‐indol‐1‐ium 4‐nitrobenzenesulfonate). Converting a methyl group into ethyl changes the packing symmetry in the molecular crystal to switch on NLO activity. This behavior is attributed to the increased size of the ethyl group, which pushes apart the van der Waals contacts of the cation that lead to centrosymmetric packing in the methyl derivative. To test the NLO properties of EHPSI‐4NBS, THz generation experiments are performed at 1200 nm pump wavelength. Spectral amplitude similar to DAST ((E)‐4‐(4‐(dimethylamino)styryl)‐1‐methylpyridin‐1‐ium tosylate) crystal is observed with generation profile from 0 to 3.8 THz.     

Symmetric and Asymmetric Conjugated 3,3′‐Bipyridine Derivatives as a New Class of Third‐Order NLO Chromophores with an Enhanced Non‐resonant,Nonlinear Refractive Index in the Picosecond Range

We report on the synthesis and third‐order nonlinear optical (NLO) properties of new asymmetric (push–pull) and symmetric chromophores based on the 3,3′‐bipyridine core. The nonlinear refraction as well as the linear and nonlinear absorption of these compounds has been studied, in solution, by spectroscopy and picosecond single‐shot Z‐scan measurements. The results are very promising in terms of non‐resonant, nonlinear refractive index in the near infrared, particularly with enhancement of the (nonlinear efficiency/transparency) trade‐off afforded by the symmetrization of the chromophores. A new polymer with this structural design has also been investigated.     

Highly Efficient and Thermally Stable Electro‐Optical Dendrimers for Photonics

After a brief review on electro‐optical (EO) polymers, the recent development of EO dendrimers is summarized. Both single‐ and multiple‐dendron‐modified nonlinear optical (NLO) chromophores in the guest–host polymer systems showed a very significant enhancement of poling efficiency (up to a three‐fold increase) due to the minimization of intermolecular electrostatic interactions among large dipole moment chromophores through the dendritic effect. Moreover, multiple NLO chromophore building blocks can also be placed into a dendrimer to construct a precise molecular architecture with a predetermined chemical composition. The site‐isolation effect, through the encapsulation of NLO moieties with dendrons, can greatly enhance the performance of EO materials. A very large EO coefficient (r₃₃ = 60 pm/V at 1.55 μm) and high temporal stability (85 °C for more than 1000 h) were achieved in a NLO dendrimer (see Figure) through the double‐end functionalization of a three‐dimensional phenyl‐tetracyanobutadienyl (Ph‐TCBD)‐containing NLO chromophore with thermally crosslinkable trifluorovinylether‐containing dendrons.     

Large‐Size Bulk and Thin‐Film Stilbazolium‐Salt Single Crystals for Nonlinear Optics and THz Generation

We present new stilbazolium salt DSTMS (4‐N,N‐dimethylamino‐4′‐N′‐methyl‐stilbazolium 2,4,6‐trimethylbenzenesulfonate) with both high second‐order nonlinear optical properties and very favorable crystal growth characteristics. We are able to obtain very large area bulk single crystals of more than 3 × 3 × 0.2 cm³ with a high optical quality without using seed crystals by using low‐temperature solution growth. We also demonstrate the growth of single crystalline thin films of DSTMS with an area of up to 6 × 5 mm² and a thickness between 5–30 μm. Nonlinear optical measurements reveal that DSTMS possesses large nonlinear optical susceptibilities with χ₁₁₁⁽²⁾ = (430 ± 40) pm V^–1 at 1.9 μm. Highly efficient generation of broadband THz waves with THz electric field strengths of more than 4 kV cm^–1 using 160 fs laser pump pulses at a wavelength λ = 1.45 μm and DSTMS crystals has been demonstrated.     

Polymer‐Stabilized Chromonic Liquid‐Crystalline Polarizer

Robust coatable polarizer is fabricated by the self‐assembly of lyotropic chromonic liquid crystals and subsequent photo‐polymerizing processes. Their molecular packing structures and optical behaviors are investigated by the combined techniques of microscopy, scattering and spectroscopy. To stabilize the oriented Sunset Yellow FCF (H‐SY) films and to minimize the possible defects generated during and after the coating, acrylic acid (AA) is added to the H‐SY/H₂O solution and photo‐polymerized. Utilizing cross‐polarized optical microscopy, phase behaviors of the H‐SY/H₂O/AA solution are monitored by varying the compositions and temperatures of the solution. Based on the experimental results of two‐dimensional wide angle X‐ray diffraction and selected area electron diffraction, the H‐SY crystalline unit cell is determined to be a monoclinic structure with the dimensions of a = 1.70 nm, b = 1.78 nm, c = 0.68 nm, α = β = 90.0° and γ = 84.5°. The molecular arrangements in the oriented H‐SY films were further confirmed by polarized Fourier‐transform infrared spectroscopy. The polymer‐stabilized H‐SY films show good mechanical and chemical stabilities with a high polarizability. Additionally, patterned polarizers are fabricated by applying a photo‐mask during the photo‐polymerization of AA, which may open new doors for practical applications in electro‐optic devices.     

Self‐Assembly of Tunable Nanocrystal Superlattices Using Poly‐(NIPAM) Spacers

Understanding and controlling 3D nanocrystal self‐assembly is a fundamental challenge in materials science. Assembly enables the unique optical and electronic properties of nanocrystals to be exploited in macroscopic materials, and also opens up the possibility to couple the optical response of nanocrystals to the optical modes of the superlattice. To date, assembly of such nanocrystal superlattices (NCSL) has focussed on fixed, close packed structures with particle separations of just 1–3 nm. To achieve highly crystalline structures with tunable optical response, the nanocrystal interparticle separation needs to be precise and easily variable but >50 nm. Here, we show the preparation of nanocrystal superlattices with spacings of 50–500 nm assembled from gold‐poly‐N‐isopropylacrylamide core‐shell particles and the characterization of their fascinating diffraction behavior by means of UV‐vis spectroscopy. These nanocrystal superlattices exhibit pronounced diffraction in the visible (440‐560 nm) with peak half‐widths of the order of 10 nm. The position of the Bragg peak is simply tuned by adjusting the particle volume fraction. Due to the thermoresponsive nature of the polymer shell, temperature is used to initiate crystallization or melting of the superlattice. Heating and cooling cycles cause highly reversible melting/recrystallization in less than a minute.     

Polymer‐Based Devices for Optical Communications

Polymers are emerging as new alternative materials for optical communication devices. We developed two types of polymer‐based devices for optical communications. One type is for ultra high‐speed signal processing that uses nonlinear optical (NLO) polymers in such devices as electro ‐optic (EO) Mach‐Zëhnder (MZ) modulators and EO 2×2 switches. The other is for WDM optical communications that use low‐loss optical polymers in such devices as 1×2, 2×2, 4‐arrayed 2×2 digital optical switches (DOSs) and 16×16 arrayed waveguide grating (AWG) routers. For these devices, we synthesized a polyetherimide‐disperse red 1 (PEI‐DR1) side chain NLO polymer and a low‐loss optical polymer known as fluorinated polyaryleneethers (FPAE). This paper presents the details of our development of these polymeric photonic devices considering all aspects from materials to packaging.     

Optical properties of thin GaSe/<Emphasis Type="Italic">n</Emphasis>-Si(111) films

Morphological and optical studies (ellipsometry and reflectance spectroscopy in the ranges 400–750 nm and 1.4–25 μm) of thin GaSe films fabricated by thermal evaporation on the n-Si (111) single-crystal substrates are reported. The film thickness was 15–60 nm. It is established that, in the initial stage of growth, the growth of GaSe on the n-Si (111) substrates occurs via formation of islands (three-dimensional growth). It is shown that, as the thickness increases, the physical parameters of the film change and the films approach single crystals in crystalline and energy band structure. For films with a thickness of 60 nm, the reflectance band peak is attributed to indirect optical transitions enhanced by reflection from the film-substrate interface. From the results of optical studies, quantum effects in the surface region of the thin films are conjectured.     

Octupolar Films with Large Second Harmonic Generation and Electro‐Optical Effects

Thin films that benefit from efficient octupolar molecular packing are prepared for second harmonic generation (SHG) and electro‐optic (EO) applications. The films are composed of 1,3,5‐tricyano‐2, 4,6‐tris(p‐diethylaminostyryl)benzene (TTB) in a ploymethylmetacrylate (PMMA) matrix on aluminum/BK7 glass (Al/BK7) and polyimide/indium tin oxide (PI/ITO) substrates. Octupolar films prepared on both substrates display polycrystalline and cylindrical domains. The molecular orientation, SHG efficiencies, and EO coefficients of the crystalline domains are measured. In the cylinders, the molecular crystal planes are oriented perpendicularly to the major cylinder axis, whereas in the polycrystals, the planes are randomly oriented. While both structures exhibit high and stable SHG and EO efficiencies, the cylinders, in particular, exhibited a very large SHG, a large EO coefficient, and high thermal stability; these characteristics will be useful in second order nonlinear optical applications.     

Second-harmonic generation from CuInSe2 thin films:influence of the substrate-epilayer lattice mismatch

We investigate the second-order nonlinear optical (NLO) properties of tetragonal chalcopyrite-structure CuInSe₂ thin films using optical second-harmonic generation at a fundamental wavelength of 790 nm. An approximate doubling of the second-order optical nonlinearity of the CuInSe₂ thin films is achieved through changing the substrate from GaAs(001) to In_0.29Ga_0.71As(001), thereby reducing the lattice mismatch from 2.2% to 0.2%. This observation suggests that minimizing the lattice mismatch is a key requirement for the growth of high-quality NLO active chalcopyrite structure thin films and is in contrast to some other NLO active thin-film systems     

Synthesis of Optical‐Quality Single‐Crystal β‐BaB2O4 Microwires and Nanowires

The synthesis of optical quality β‐barium borate microwires and nanowires (MNWs) is reported using an organic‐free hydrothermal method with BaCl₂·6H₂O, NaOH, and H₃BO₃ as source materials, and assisted with post‐annealing. As‐synthesized MNWs, with diameters ranging from 500 nm to 2 μm and lengths up to several hundred micrometers, show good optical‐waveguiding capabilities. Based on evanescent coupling between a single BBO MNW waveguide and a fiber taper, propagation losses of 0.30 dB μm^?1 (at 532 nm) and 0.21 dB μm^?1 (at 671 nm) are evaluated, respectively. An evident second‐harmonic generation (SHG) signal at 532 nm with a measured conversion efficiency of about 8.4% is observed when excited by waveguided 1064 nm, picosecond laser pulses within a BBO MNW with a length of the order of 100 μm. The dependence of the SHG conversion efficiency on the MNW diameter is also investigated. These results show a much‐higher SHG efficiency for BBO single‐crystal MNWs compared with bulk crystal, which suggests potential applications in future micro‐/nanoscale nonlinear optical applications such as optical modulation and frequency conversion.     

Durable write-once optical disk memories for 800 nm using silverisland films

Durable write-once (WO) optical disk media have been demonstrated by using silver island films with a protection layer of fluorine-contained polymers. The reflectance of the silver island films is high at the wavelength of 800 nm and can be greatly lowered by laser irradiation, owing to the thermal deformation or the islands. Laser writing has been experimentally confirmed at almost the same line speed as that of compact discs (CDs). Therefore it is expected that a CD-compatible write-once optical disc can be realised     

Preparation and Characterization of a pH‐ and Thermally Responsive Poly(N‐isopropylacrylamide‐co‐acrylic acid)/Porous SiO2 Hybrid

A multifunctional nanohybrid composed of a pH‐ and thermoresponsive hydrogel, poly(N‐isopropylacrylamide‐co‐acrylic acid) [poly(NIPAM‐co‐AAc)], is synthesized in situ within the mesopores of an oxidized porous Si template. The hybrid is characterized by electron microscopy and by thin film optical interference spectroscopy. The optical reflectivity spectrum of the hybrid displays Fabry–Pérot fringes characteristic of thin film optical interference, enabling direct, real‐time observation of the pH‐induced swelling, and volume phase transitions associated with the confined poly(NIPAM‐co‐AAc) hydrogel. The optical response correlates to the percentage of AAc contained within the hydrogel, with a maximum change observed for samples containing 20% AAc. The swelling kinetics of the hydrogel are significantly altered due to the nanoscale confinement, displaying a more rapid response to pH or heating stimuli relative to bulk polymer films. The inclusion of AAc dramatically alters the thermoresponsiveness of the hybrid at pH 7, effectively eliminating the lower critical solution temperature (LCST). The observed changes in the optical reflectivity spectrum are interpreted in terms of changes in the dielectric composition and morphology of the hybrids.     

Block‐Copolymer‐Templated Synthesis of Electroactive RuO2‐Based Mesoporous Thin Films

RuO₂‐based mesoporous thin films of optical quality are synthesized from ruthenium‐peroxo‐based sols using micelle templates made of amphiphilic polystyrene‐polyethylene oxide block copolymers. The mesoporous structure and physical properties of the RuO₂ films (mesoporous volume: 30%; pore diameter: ～30 nm) can be controlled by the careful tuning of both the precursor solution and thermal treatment (150–350 °C). The optimal temperature that allows control of both mesoporosity and nanocristallinity is strongly dependent on the substrate (silicon or fluorine‐doped tin oxide). The structure of the resulting mesoporous films are investigated using X‐ray diffraction, X‐ray photoelectron spectroscopy, and atomic force microscopy. Mesoporous layers are additionally characterized by transmission and scanning electron microscopy and ellipsometry while their electrochemical properties are analyzed via cyclic voltammetry. Thick mesoporous films of ruthenium oxide hydrates, RuO₂ · xH₂O, obtained using a thermal treatment at 280 °C, exhibit capacitances as high as 1000 ± 100 F g^?1 at a scan rate of 10 mV s^?1, indicating their potential application as electrode materials.     

Nanometer‐Sized Copper Sulfide Hollow Spheres with Strong Optical‐Limiting Properties

CuS semiconductor nanometer‐sized hollow spheres are successfully synthesized by using a soft‐template method. A possible growth mechanism is proposed. The linear optical property of the CuS hollow spheres is examined by means of photoluminescence spectroscopy at room temperature. The optical‐limiting (OL) property of these nanostructures is characterized by using a nanosecond Q‐switched YAG laser and an optical parametric oscillator pumped with Surelite‐III. A strong OL response is detected for the CuS hollow spheres in both visible and near infrared (NIR) spectral ranges, which makes these promising materials for applications such as the protection of human eyes or as optical sensors for high‐power laser irradiation. The OL mechanism of the CuS hollow‐sphere nanostructure may be the combination of free‐carrier absorption (FCA) and nonlinear scattering.     

The Effect of Nanoscale Confinement on the Collective Electron Transport Behavior in Au Nanoparticles Self‐Assembled in a Nanostructured Polystyrene‐block‐poly(4‐vinylpyridine) Diblock Copolymer Ultra‐thin Film

This study involves the collective electron transport behavior of sequestered Au nanoparticles in a nanostructured polystyrene‐block‐poly(4‐vinylpyridine). The monolayer thin films (ca. 30 nm) consisting of Au nanoparticles self‐assembled in the 30‐nm spherical poly(4‐vinylpyridine) domains of an polystyrene‐block‐poly(4‐vinylpyridine) diblock copolymer were prepared. From the current‐voltage characteristics of these thin films, the collective electron transport behavior of Au nanoparticles sequestered in the spherical poly(4‐vinylpyridine) nanodomains was found to be dictated by Coulomb blockade and was quasi one‐dimensional, as opposed to the three‐dimensional behavior displayed by Au nanoparticles that had been dispersed randomly in homo‐poly(4‐vinylpyridine). The threshold voltage of these composite increased linearly upon increasing the inter‐nanoparticle distance. The electron tunneling rate constant in the case of Au nanoparticles confined in poly(4‐vinylpyridine) nanodomains is eight times larger than that in the randomly distributed case and it increases upon increasing the amount of Au nanoparticles. This phenomenon indicates that manipulating the spatial arrangement of metal nanoparticles by diblock copolymer can potentially create electronic devices with higher performance.     

MEL‐type Pure‐Silica Zeolite Nanocrystals Prepared by an Evaporation‐Assisted Two‐Stage Synthesis Method as Ultra‐Low‐k Materials

A MEL‐type pure‐silica zeolite (PSZ), prepared by spin‐on of nanoparticle suspensions, has been shown to be a promising ultra‐low‐dielectric‐constant (k) material because of its high mechanical strength, hydrophobicity, and chemical stability. In our previous works, a two‐stage synthesis method was used to synthesize a MEL‐zeolite nanoparticle suspension, in which both nanocrystal yield and particle size of the zeolite suspension increased with increasing synthesis time. For instance, at a crystal yield of 63%, the particle size is 80 nm, which has proved to be too large because it introduces a number of problems for the spin‐on films, including large surface roughness, surface striations, and large mesopores. In the current study, the two‐stage synthesis method is modified into an evaporation‐assisted two‐stage method by adding a solvent‐evaporation process between the two thermal‐treatment steps. The modified method can yield much smaller particle sizes (e.g., 14 vs. 80 nm) while maintaining the same nanocrystal yields as the two‐stage synthesis. Furthermore, the nanoparticle suspensions from the evaporation‐assisted two‐stage synthesis show a bimodal particle size distribution. The primary nanoparticles are around 14 nm in size and are stable in the final suspension with 60% solvent evaporation. The factors that affect nanocrystal synthesis are discussed, including the concentration, pH value, and viscosity. Spin‐on films prepared by using suspensions synthesized this way have no striations and improved elastic modulus (9.67 ± 1.48 GPa vs. 7.82 ± 1.30 GPa), as well as a similar k value (1.91 ± 0.09 vs. 1.89 ± 0.08) to the previous two‐stage synthesized films.     

Highly Stable Graphene‐Based Multilayer Films Immobilized via Covalent Bonds and Their Applications in Organic Field‐Effect Transistors

Highly stable graphene oxide (GO)‐based multilayered ultrathin films can be covalently immobilized on solid supports through a covalent‐based method. It is demonstrated that when (3‐aminopropyl) trimethoxysilane (APTMS), which works as a covalent cross‐linking agent, and GO nanosheets are assembled in an layer‐by‐layer (LBL) manner, GO nanosheets can be covalently grafted on the solid substrate successfully to produce uniform multilayered (APTMS/GO)N films over large‐area surfaces. Compared with conventional noncovalent LBL films constructed by electrostatic interactions, those assembled using this covalent‐based method display much higher stability and reproducibility. Upon thermal annealing‐induced reduction of the covalent (APTMS/GO)N films, the obtained reduced GO (RGO) films, (APTMS/RGO)N, preserve their basic structural characteristics. It is also shown that the as‐prepared covalent (APTMS/RGO)N multilayer films can be used as highly stable source/drain electrodes in organic field‐effect transistors (OFETs). When the number of bilayers of the (APTMS/RGO)N film exceeds 2 (ca. 2.7 nm), the OFETs based on (APTMS/RGO)N electrodes display much better electrical performance than devices based on 40 nm Au electrodes. The covalent protocol proposed may open up new opportunities for the construction of graphene‐based ultrathin films with excellent stability and reproducibility, which are desired for practical applications that require withstanding of multistep post‐production processes.     

All‐Polymer Bistable Resistive Memory Device Based on Nanoscale Phase‐Separated PCBM‐Ferroelectric Blends

All polymer nonvolatile bistable memory devices are fabricated from blends of ferroelectric poly(vinylidenefluoride–trifluoroethylene (P(VDF‐TrFE)) and n‐type semiconducting [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM). The nanoscale phase separated films consist of PCBM domains that extend from bottom to top electrode, surrounded by a ferroelectric P(VDF‐TrFE) matrix. Highly conducting poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) polymer electrodes are used to engineer band offsets at the interfaces. The devices display resistive switching behavior due to modulation of this injection barrier. With careful optimization of the solvent and processing conditions, it is possible to spin cast very smooth blend films (R_rms ≈ 7.94 nm) and with good reproducibility. The devices exhibit high I_on/I_off ratios (≈3 × 10³), low read voltages (≈5 V), excellent dielectric response at high frequencies (?_r ≈ 8.3 at 1 MHz), and excellent retention characteristics up to 10 000 s.