Heterogeneous Transparent Ultrathin Films with Tunable‐Color Luminescence Based on the Assembly of Photoactive Organic Molecules and Layered Double Hydroxides

Multicolor luminescent films have great potential for use in optoelectronics, solid‐state light‐emitting materials, and optical devices. This work describes a systematic investigation of the ordered assembly of two‐ (blue/green, blue/orange, red/blue, red/green) and three‐color (blue/red/green) light‐emitting ultrathin films (UTFs) by using different photofunctional anions [bis(N‐methylacridinium)@polyvinylsulfonate ion pairs and anionic derivatives of poly(p‐phenylene), poly(phenylenevinylene), and poly(thiophene)] and Mg‐Al‐layered double hydroxide nanosheets as building blocks. The rational combination of luminescent components affords precise control of the emission wavelengths and intensity, and multicolored luminescent UTFs can be precisely tailored covering most of the visible spectral region. The assembly process of the UTFs and their luminescence properties, as monitored by UV–vis absorption and fluorescence spectroscopy, resulted in a gradual change in luminescence color in the selected light‐emitting spectral region upon increasing the number of deposition cycles. X‐ray diffraction demonstrates that the UTFs are periodic layered structures involving heterogeneous superlattices associated with individual photoactive anion–LDH units. These UTFs also exhibit well‐defined multicolor polarized fluorescence with high polarization anisotropy, and the emissive color changes with polarization direction. Therefore, this work provides a way of fabricating heterogeneous UTFs with tunable‐color luminescence as well as polarized multicolor emission, which have potential applications in the areas of light displays and optoelectronic devices.     

Tubular Titania Nanostructures via Layer‐by‐Layer Self‐Assembly

Nanostructured titania‐polyelectrolyte composite and pure anatase and rutile titania tubes were successfully prepared by layer‐by‐layer (LbL) deposition of a water‐soluble titania precursor, titanium(IV ) bis(ammonium lactato) dihydroxide (TALH) and the oppositely charged poly(ethylenimine) (PEI) to form multilayer films. The tube structure was produced by depositing inside the cylindrical pores of a polycarbonate (PC) membrane template, followed by calcination at various temperatures. The morphology, structure and crystal phase of the titania tubes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD) and UV‐vis absorbance measurements. The as‐prepared anatase titania tubes exhibit very promising photocatalytic properties, demonstrated by the degradation of the azodye methyl orange (MO) as a model molecule. They are also easily separated from the reaction system by simple filtration or centrifugation, allowing for straightforward recycling. The reported strategy provides a simple and versatile technique to fabricate titania based tubular nanostructures, which could easily be extended to prepare tubular structures of other materials and may find application in catalysis, chemical sensing, and nanodevices.     

Pseudobilayer Vesicle Formation via Layer‐by‐Layer Assembly of Hydrophobically Modified Polymers on Sacrificial Substrates

A bilayer of a hydrophobically modified polyelectrolyte, octadecyl poly(acrylamide) (PAAm), sandwiched between the layers of a hydrophilic polyelectrolyte, poly(ethyleneimine) (PEI), is prepared by the sequential electrostatic–hydrophobic–electrostatic‐interaction‐driven self‐assembly on planar and colloid substrates. This process results in a PEI/[PAAm]₂/PEI‐multilayer‐coated substrate. The removal of a PAA/PEI/[PAAm]₂/PEI‐multilayer‐coated decomposable colloidal template produces hollow capsules. Irregular hydrophobic domains of the [PAAm]₂ bilayer in the PEI/[PAAm]₂/PEI‐multilayer capsule are infiltrated with a lipid to obtain a uniform, distinct hydrophobic layer, imparting the capsule with a pseudobilayer vesicle structure.     

Surface Imprinting in Layer‐by‐Layer Nanostructured Films

In this Full Paper, we develop a novel approach for the generation of stable molecularly imprinted sites in polymeric films by combining the layer‐by‐layer (LbL) technique and photochemical crosslinking of the layered structure. After photo‐crosslinking, the imprinted films show high reproducibility and rapid loading and unloading of imprinted sites by the template molecules. Moreover, the competitive adsorption of template molecules and redox labels into the imprinted film using electrochemical methods indicates that the imprinted film has higher affinity for template molecules. We believe this approach may have some advantages over traditional ways of preparing imprinted sites in polymer matrices and it may open a new avenue for the functionalization of LbL films.     

TiO2‐Based Composite Nanotube Arrays Prepared via Layer‐by‐Layer Assembly

A simple and versatile technique has been developed to prepare TiO₂ and TiO₂‐based composite (TiO₂–CdS and TiO₂–Au) nanotube arrays. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy‐dispersive X‐ray (EDX) analysis, X‐ray diffraction (XRD), thermogravimetric analysis (TGA), UV‐vis spectroscopy, and photoluminescence (PL) spectroscopy are used to characterize their morphology, structure, composition, and properties. The TiO₂–CdS nanotubes contained many TiO₂ and CdS quantum dots and exhibited a novel PL band in the blue‐wavelength range. The reported strategy will be useful for fabricating nanoparticle–nanoparticle composite nanostructure arrays, which are suitable for applications in catalysis, chemical sensors, nanoelectrodes, and nanodevices.     

Layer‐by‐Layer All‐Inorganic Quantum‐Dot‐Based LEDs: A Simple Procedure with Robust Performance

A novel all‐inorganic electroluminescent device is demonstrated based on highly luminescent CdTe nanocrystals intercalated within a laminar hydrotalcite‐like structure. The laminar scaffold acts to both support and distribute the CdTe nanocrystals. The device is synthesized using simple wet chemical processes at room temperature in ambient conditions. It has high thermal stability, operating continuously up to 90 °C, and a maximum efficiency at J = 0.12 A cm^?2. The device is targeted at the automotive industry.     

Stiff and Transparent Multilayer Thin Films Prepared Through Hydrogen‐Bonding Layer‐by‐Layer Assembly of Graphene and Polymer

Due to their exceptional orientation of 2D nanofillers, layer‐by‐layer (LbL) assembled polymer/graphene oxide thin films exhibit unmatched mechanical performance relative to any conventionally produced counterparts with similar composition. Unprecedented mechanical property improvement, by replacing graphene oxide with pristine graphene, is demonstrated in this work. Polyvinylpyrrolidone‐stabilized graphene platelets are alternately deposited with poly(acrylic acid) using hydrogen bonding assisted LbL assembly. Transmission electron microscopy imaging and the Halpin‐Tsai model are used to demonstrate, for the first time, that intact graphene can be processed from water to generate polymer nanocomposite thin films with simultaneous parallel‐alignment, high packing density, and exfoliation. A multilayer thin film with only 3.9 vol% of highly exfoliated, and structurally intact graphene, increases the elastic modulus (E) of a polymer multilayer thin film by 322% (from 1.41 to 4.81 GPa), while maintaining visible light transmittance of ≈90%. This is one of the greatest improvements in elastic modulus ever reported for a graphene‐filled polymer nanocomposite with a glassy (E > 1 GPa) matrix. The technique described here provides a powerful new tool to improve nanocomposite properties (mechanical, gas transport, etc.) that can be universally applied to a variety of polymer matrices and 2D nanoplatelets.     

Enhanced Electrochromism with Rapid Growth Layer‐by‐Layer Assembly of Polyelectrolyte Complexes

In this work, a facile method to deposit fast growing electrochromic multilayer films with enhanced electrochemical properties using layer‐by‐layer (LbL) self‐assembly of complex polyelectrolyte is demonstrated. Two linear polymers, poly(acrylic acid) (PAA) and polyethylenimine (PEI), are used to formulate stable complexes under specific pH to prepare polyaniline (PANI)/PAA‐PEI multilayer films via LbL deposition. By introducing polymeric complexes as building blocks, [PANI/PAA‐PEI]_n films grow much faster compared with [PANI/PAA]_n films, which are deposited under the same condition. Unlike the compact [PANI/PAA]_n films, [PANI/PAA‐PEI]_n films exhibit porous structure that is beneficial to the electrochemical process and leads to improved electrochromic properties. An enhanced optical modulation of 30% is achieved with [PANI/PAA‐PEI]₃₀ films at 630 nm compared with the lower optical modulation of 11% measured from [PANI/PAA]₃₀ films. The switching time of [PANI/PAA‐PEI]₃₀ films is only half of that of [PANI/PAA]₃₀ films, which indicates a faster redox process. Utilizing polyelectrolyte complexes as building blocks is a promising approach to prepare fast growing LbL films for high performance electrochemical device applications.     

Tris(8‐hydroxyquinoline‐5‐sulfonate)aluminum Intercalated Mg–Al Layered Double Hydroxide with Blue Luminescence by Hydrothermal Synthesis

Blue luminescent hybrid materials (DDS–AQS(x%)/LDH) are successfully prepared by co‐intercalating tris(8‐hydroxyquinoline‐5‐sulfonate)aluminum anions (AQS^3?) and dodecyl sulfonate (DDS^?) with different molar ratios into Mg–Al layered double hydroxides (LDHs) by the hydrothermal and solution co‐precipitation methods. A film of the material on a quartz substrate is obtained by the solvent evaporation method. The results show the blue luminescence is remarkably different from the pristine Na₃AQS, which has cyan luminescence (ca. 450–470 nm vs. 495 nm). Furthermore, the hydrothermal product of DDS–AQS(66.67%)/LDH exhibits optimal luminous intensity and a significantly enhanced fluorescence lifetime. Nuclear magnetic resonance and Fourier‐transform infrared spectroscopy indicate that the cyan–blue luminescence transition is due to the isomerization of meridianal to facial AQS via ligand flip caused by a host–guest electrostatic interaction, in combination with the dispersion and pre‐intercalation effect of DDS. The hydrothermal conditions can promote a more ordered alignment of the intercalated fac‐AQS compared with alignment in the solution state, and the rigid LDHs environment can confine the internal mobility of AQS to keep the facial configuration stable. This stability allows a facile preparation of large amounts of blue luminous powder/film, which is a new type of inorganic–organic hybrid photofunctional material.     

Layer‐by‐Layer Fabrication and Characterization of Gold‐Nanoparticle/Myoglobin Nanocomposite Films

Multilayer thin films of ～ 7 nm diameter gold nanoparticles (GNPs) linked with horse heart myoglobin (Mb) are fabricated, for the first time, by layer‐by‐layer (LbL) assembly on glass slides, and silicon and plastic substrates. The GNP/Mb nanocomposite films show sharp surface plasmon resonance (SPR) absorption bands that are used to follow the LbL growth of the film and to determine the kinetics of GNP adsorption on the Mb‐modified surface. The GNP/Mb nanocomposite films are characterized using atomic force microscopy, transmission electron microscopy, polarized UV‐vis spectroscopy, and spectroscopic ellipsometry. The GNPs in the multilayer films are spatially separated from one another, and interparticle interactions remain in the film, making it optically anisotropic. The GNP/Mb nanocomposite films are stable in air at temperatures up to 100 °C, and can withstand successive immersions in strongly acidic and basic solutions. The SPR absorption band of the GNP/Mb nanocomposite film in air exhibits a red‐shift in the wavelength maximum and an increase in the maximum absorbance relative to that in water. This result, which is in contrast to that observed with a GNP monolayer on an aminosilane‐functionalized substrate, suggests the shrinkage in air and swelling in water of Mb molecules embedded in the nanocomposite film.     

Room‐Temperature Metallic Fusion‐Induced Layer‐by‐Layer Assembly for Highly Flexible Electrode Applications

To fabricate flexible electrodes, conventional silver (Ag) nanomaterials have been deposited onto flexible substrates, but the formed electrodes display limited electrical conductivity due to residual bulky organic ligands, and thus postsintering processes are required to improve the electrical conductivity. Herein, an entirely different approach is introduced to produce highly flexible electrodes with bulk metal–like electrical conductivity: the room‐temperature metallic fusion of multilayered silver nanoparticles (NPs). Synthesized tetraoctylammonium thiosulfate (TOAS)‐stabilized Ag NPs are deposited onto flexible substrates by layer‐by‐layer assembly involving a perfect ligand‐exchange reaction between bulky TOAS ligands and small tris(2‐aminoethyl)amine linkers. The introduced small linkers substantially reduce the separation distance between neighboring Ag NPs. This shortened interparticle distance, combined with the low cohesive energy of Ag NPs, strongly induces metallic fusion between the close‐packed Ag NPs at room temperature without additional treatments, resulting in a high electrical conductivity of ≈1.60 × 10⁵ S cm^?1 (bulk Ag: ≈6.30 × 10⁵ S cm^?1). Furthermore, depositing the TOAS–Ag NPs onto cellulose papers through this approach can convert the insulating substrates into highly flexible and conductive papers that can be used as 3D current collectors for energy‐storage devices.     

Covalent Layer‐by‐Layer Assembly of Conjugated Polymers and CdSe Nanoparticles: Multilayer Structure and Photovoltaic Properties

Hybrid thin films of conjugated polymers and CdSe nanoparticles have been fabricated by using a layer‐by‐layer (LbL) approach driven by covalent coupling reactions. This method permits facile covalent crosslinking of the polymer/nanoparticle interlayers in common organic solvents, which provides a general route for preparing robust and uniform functional thin films. The deposition process is linearly related to the number of bilayers as monitored by UV‐vis absorption spectroscopy and ellipsometry. Characterization of the multilayer structures has been carried out by fluorescence spectroscopy, X‐ray photoelectron spectroscopy (XPS), and grazing‐angle Fourier‐transform infrared spectroscopy (FTIR). Techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM) have also been used. A preliminary application of the hybrid films in the development of organic photovoltaics is presented. Upon illumination with white light at 10 mW cm^–2, the self‐assembled multilayer films exhibit steady photocurrent responses with an overall optical‐to‐electrical power conversion efficiency of 0.71 %.     

Charge Storage Capability in Nanoarchitectures of V2O5/Chitosan/Poly(ethylene oxide) Produced Using the Layer‐by‐Layer Technique

The electrochemical and electrochromic properties of layer‐by‐layer nanoarchitectures of V₂O₅/chitosan and V₂O₅ alternated with a blend of poly(ethylene oxide) (PEO) and chitosan have been examined. Using a blend was important, since multilayers of PEO/V₂O₅ could not be built. The number of electrochemically active V₂O₅ sites was estimated to be around 3.4 × 10^–8 mol cm^–2 and 4.4 × 10^–8 mol cm^–2 for V₂O₅/chitosan and V₂O₅/blend, respectively, based on the UV‐vis absorbance attributed to the intervalence V⁴⁺→V⁵⁺ transfer. A pronounced effect from PEO was observed in the migration/diffusion process, according to cyclic voltammetry and impedance spectroscopy data. The charges injected were 3.29 mC cm^–2 and 8.02 mC cm^–2 for V₂O₅/chitosan and V₂O₅/blend, respectively, at 20 mV s^–1. For V₂O₅/blend, the chronopotentiometric curves show that x in Li_xV₂O₅ is about 1.77. Evidence of enhanced ionic transport was provided by the Fourier transform infrared (FTIR) spectrum, which indicated lithium complexation by PEO and formation of a larger amorphous phase of PEO within the V₂O₅ matrix. The importance of these results for the production of Li secondary microbatteries and electrochromic devices is discussed.     

CsPbX3 Quantum Dots for Lighting and Displays: Room‐Temperature Synthesis,Photoluminescence Superiorities,Underlying Origins and White Light‐Emitting Diodes

Recently, Kovalenko and co‐workers and Li and co‐workers developed CsPbX₃ (X = Cl, Br, I) inorganic perovskite quantum dots (IPQDs), which exhibited ultrahigh photoluminescence (PL) quantum yields (QYs), low‐threshold lasing, and multicolor electroluminescence. However, the usual synthesis needs high temperature, inert gas protection, and localized injection operation, which are severely against applications. Moreover, the so unexpectedly high QYs are very confusing. Here, for the first time, the IPQDs' room‐temperature (RT) synthesis, superior PL, underlying origins and potentials in lighting and displays are reported. The synthesis is designed according to supersaturated recrystallization (SR), which is operated at RT, within few seconds, free from inert gas and injection operation. Although formed at RT, IPQDs' PLs have QYs of 80%, 95%, 70%, and FWHMs of 35, 20, and 18 nm for red, green, and blue emissions. As to the origins, the observed 40 meV exciton binding energy, halogen self‐passivation effect, and CsPbX₃@X quantum‐well band alignment are proposed to guarantee the excitons generation and high‐rate radiative recombination at RT. Moreover, such superior optical merits endow them with promising potentials in lighting and displays, which are primarily demonstrated by the white light‐emitting diodes with tunable color temperature and wide color gamut.