Self-assembled Multilayer Structures Research Articles

Reclaimed lignosulfonate-modified bamboo fibers prepared via layer-by-layer self-assembly to reinforce poly(3-hydroxybutyrate) biocomposite

Fiber-reinforced poly(3-hydroxybutyrate) (PHB) biocomposites have been increasingly applied in various fields, such as building structures, interior decorations, and product packaging, due to their high strength to weight ratios, low thermal conductivities, low carbon footprints, and biodegradabilities. However, poor compatibility between the hydrophilic natural fibers and the thermoplastic biocomposites limits their mechanical strengths. This work proposes a method based on layer-by-layer self-assembly (LbL) to improve the interfacial compatibility between the PHB and bamboo fibers (BFs). Sodium lignosulfonate (SL) derived from pulp and paper industry waste was used as the raw material. Then, long carbon chains were introduced into the SL via the Mannich reaction. A hydrophobic self-assembled multilayer structure (SMS) was constructed on the surfaces of the BFs. The preparation process was environmentally friendly, controllable and simple. The mechanical properties were improved by the modifications, including the tensile strength (44%), flexural strength (13%), and impact strength (41%). The thermal properties of the fiber-reinforced biocomposites were also enhanced. This study presents a novel strategy for high-value utilization of industrial byproducts and the enhancement of biocomposite interfacial compatibility.

Electrocatalytic self-assembled multilayer structures based on thiolated Fc-DAB dendrimers: Determination of heavy metal ions by enzymatic inhibition

The development of sensitive and rapid inhibition-based biosensors for monitoring trace levels of heavy metals such as lead and copper ions in environmental samples is of high interest. In this work, we describe the fabrication of HRP sensitive peroxide biosensors based on ferrocenyl thiolated DAB dendrimers as bonding-layer between electrodeposited and colloidal gold nanoparticles and compare their electrocatalytic properties with those of thiolated DAB homologues. The complete electrochemical, kinetic and analytical characterization of these new heavy metals’ inhibition-biosensors are investigated.

Tethering vapor-phase deposited GLYMO coupling molecules to silane-crosslinked polyethylene surface via plasma grafting approaches

Herein, an attempt was made to deposit and graft GLYMO (3-glycidyloxypropyltrimethoxysilane) self-assembled layers on silane-crosslinked polyethylene (Si-XLPE). The principal objective was to create an anchoring layer on the pristine surface of Si-XLPE to modify its poor surface bonding properties. Since attachment of the GLYMO self-assembled layers to the partially non-polar surface of Si-XLPE seemed challenging, plasma post-irradiation and syn-irradiation grafting methods were utilized separately for activating the substrate and the deposited GLYMO molecules to provide the necessary condition for grafting and fabrication of uniform self-assembled layers. According to surface chemistry analyses, plasma grafting methods appeared to be beneficial tools for enhancing grafting of GLYMO molecules and reducing the self-condensed GLYMO aggregates and its self-assembled multilayer structures. Morphology investigations confirmed the positive role of plasma grafting in decreasing the self-condensed aggregated GLYMO structures on the surface. Moreover, GLYMO plasma grafting flattened the surface owing to the special orientation of the deposited molecules during the grafting and plasma etching. Surface wettability studies, along with chemistry outcomes, demonstrated the tendency of GLYMO molecules to tether to the surface with their trimethoxysilane groups, while their closed epoxide rings aligned to the out of surface. Overall, the plasma post-irradiation grafting approach exhibited higher grafting density.

Flexible and Responsive Chiral Nematic Cellulose Nanocrystal/Poly(ethylene glycol) Composite Films with Uniform and Tunable Structural Color.

The fabrication of responsive photonic structures from cellulose nanocrystals (CNCs) that can operate in the entire visible spectrum is challenging due to the requirements of precise periodic modulation of the pitch size of the self-assembled multilayer structures at the length scale within the wavelength of the visible light. The surface charge density of CNCs is an important factor in controlling the pitch size of the chiral nematic structure of the dried solid CNC films. The assembly of poly(ethylene glycol) (PEG) together with CNCs into smaller chiral nematic domains results in solid films with uniform helical structure upon slow drying. Large, flexible, and flat photonic composite films with uniform structure colors from blue to red are prepared by changing the composition of CNCs and PEG. The CNC/PEG(80/20) composite film demonstrates a reversible and smooth structural color change between green and transparent in response to an increase and decrease of relative humidity between 50% and 100% owing to the reversible swelling and dehydration of the chiral nematic structure. The composite also shows excellent mechanical and thermal properties, complementing the multifunctional property profile.

Novel sandwiched structures in starch/cellulose nanowhiskers (CNWs) composite films

Abstract Novel sandwiched structures of cellulose nanowhiskers (CNWs) were found for the first time at the cross section of fractured starch/CNWs composite films. CNWs were obtained by hydrolysing bleached flax yarns through heating in a concentrated sulfuric acid (60 wt. %) aqueous solution at 55 °C for 60 min. Starch and starch/CNWs composite films were prepared by casting starch and mixtures of starch/CNWs homogenous aqueous suspensions, which was followed by drying at atmosphere. The CNWs’ layers are sandwiched within starch matrices in a parallel direction to nanocomposite film surfaces. The layer thickness increases with an increase in the content of CNWs. The discovery of novel sandwiched structures demonstrates that both the interaction and evaporation rate of the solvent can affect the dispersion and thus play important roles in the nanoparticle dispersion. Such nanocomposite films in the presence of self-assembled multi-layer structures may further improve mechanical and gas barrier properties as a promising material candidate for food packaging applications.

Carbon and nitrogen co-doping self-assembled MoS2 multilayer films

Mo–S–C–N composite films were prepared using reactive magnetron sputtering of graphite and MoS2 targets in argon and nitrogen atmospheres. The effects of carbon/nitrogen co-doping and carbon concentration on the composition, microstructure, mechanical and tribological properties of deposited films have been investigated by various characterization techniques. The results show that the deposited films comprise MoS2 nanocrystalline and amorphous carbon, and the incorporating nitrogen forms Mo-N and C–N chemical bonds. Increasing carbon concentration leads to the increase of sp2 carbon fraction in the films. Furthermore, the high-resolution transmission electron microscopy reveals that a self-assembled multilayer structure with periodicity in the nanometer scale is formed in the Mo–S–C–N film. Benefiting from the composite and self-assembled multilayer structures, the hardness of Mo–S–C–N film deposited at optimized parameter reaches up to 9.76GPa, and corresponding friction experiment indicates that this composite films display low friction coefficient and high wear resistance both in vacuum and ambient air conditions.

Self-Assembled Multilayer Structures of Imidazolium Based Ionic Liquids at Mica Interfaces Are Induced By Confinement and the Presence of Water

Tuning chemical structure and molecular layering of ionic liquids (IL) at solid interfaces offers leverage to design performance of ILs in emerging technologies including super-capacitors, heterogeneous catalysis or lubrication. Recent experimental interpretations suggest that ILs containing cations with long hydrophobic tails form well-ordered bilayers at interfaces. Here we demonstrate that interfacial bilayer formation is insignificant and not an intrinsic quality of hydrophobic ILs. In contrast, bilayer formation is triggered by boundary conditions including confinement and humidity present in the IL. Therefore, we performed force versus distance profiles using atomic force microscopy and the surface forces apparatus. Our results support models of disperse low-density bilayer formation in confined situations, and at high surface charging in the presence of water. Conversely, interfacial structuring of long-chain ILs in dry environments and at low surface charging is disordered and bulk structuring also dominates interface structures in confinement. Our results demonstrate that boundary conditions such as charging, confinement and the presence of impurities drive structure formation of ILs at interfaces, while at low surface charge solvophobic interactions have a much weaker influence on interface structuring than generally believed. These results have important implications for understanding solid/IL interfaces and clarify previous interpretations.

Self-Assembled Multilayer Structure and Enhanced Thermochromic Performance of Spinodally Decomposed TiO2–VO2 Thin Film

Composite films of VO2-TiO2 were deposited on sapphire (11-20) substrate by cosputtering method. Self-assembled well-ordered multilayer structure with alternating Ti- and V-rich epitaxial thin layer was obtained by thermal annealing via a spinodal decomposition mechanism. The structured thermochromic films demonstrate superior optical modulation upon phase transition, with significantly reduced transition temperature. The results provide a facile and novel approach to fabricate smart structures with excellent performance.

Elaboration, structural, vibrational and optical investigation of a two-dimensional self-assembled organic–inorganic hybrid compound

Single crystals of a hybrid organic/inorganic material with the formula (C4N3H16)Cl[CuCl4] were elaborated and studied by X-ray diffraction, and photoluminescence. The crystals consist of a self-assembled multilayer structure with a Pnam space group. The structure is built up from the staking of infinite two-dimensional layers of CuCl6 corner-sharing octahedra, separated by organic (C4N3H16)3+ chains. Such a structure may be regarded as a multi quantum well system, in which CuCl6 layers act as semiconductor wells and the organic molecules act as insulator barriersFurthermore, the room temperature IR and Raman spectra of the title compound were recorded and analyzed. For optical investigations, thin films have been prepared by spin-coating method from the ethanol solution of the material. Optical absorption spectra shows characteristic absorptions of CuCl-based layered perovskite centered at 300 and 380nm, whereas the photoluminescence spectra shows a bleu intense emission around 420nm, associated to radiative recombination of confined excitons in the CuCl6 Quantum wells.

Methanol crossover reduction by Nafion modification via layer-by-layer self-assembly techniques

Layer-by-layer (LbL) self-assembly of polyelectrolyte bilayers on the methanol permeability and proton conductivity of Nafion membranes is investigated using polycations PDDA (poly(diallyldimethylammonium chloride)) and PAH (poly(allylamine hydrochloride)) and polyanions PAMP (poly(2-acrylamido-2-methyl-1-propanesulfonic acid)), PAZO (poly(1-(4-(3-carboxy-4-hydroxyphenylazo) benzene sulfonamido)-1,2-ethanediyl, sodium salt)), PSS (poly(sodium styrene sulfonate)) and PAA (poly(acrylic acid)). The formation of polyelectrolyte multilayers on Nafion membranes is confirmed by AFM and UV–visible spectroscopy. The lowest methanol permeability is observed on the self-assembled PDDA-PAA and PAH-PAA bilayers with the exponential growth process. The observed exponential growth process of polyelectrolyte multilayers with PAA polyanion is most likely related to small monomeric block of PAA, resulting in low steric hindrance and high flexibility and mobility of the ionomers and thus promoting the interdiffusion of PAA during the self-assembly. PDDA polycation shows a much better ability to block methanol crossover in comparison with that of PAH polycation. This study shows the importance of LbL self-assembled multilayer structure on the proton conductivity and methanol crossover properties of modified Nafion membranes for application in direct methanol fuel cells (DMFCs).

Electrochemical Analysis of Metal Complex Wires and Their Long-range Electron Transport Properties

The bottom-up methods have recently attracted much attention as the fabrication techniques of nanoscale devices. Self-assembled monolayer (SAM) can be a basis to construct nanoscale structures, and many researchers have challenged to fabricate multilayer systems with self assembly. Among a variety of the preparation methods of self-assembled multilayer structures, the stepwise coordination is one of the most attractive methods because it allows us to construct complex oligomer wires quantitatively and regularly using very simple processes. Our main research targets are the fabrication of π-conjugated bis(terpyridine) complex oligomer wires and their electron transport properties. Linear and branched wires were constructed by the stepwise coordination method, and their electron transport mechanism was analyzed by potential-step chronoamperometry. Additionally, it was found that bis(terpyridine) complex oligomer wires have the excellent long-range electron transport ability and we can control it by changing combination of surface anchoring molecules, metal ions and bridging ligands.

Two-Dimensional Excitons and Photoluminescence Properties of the Organic/Inorganic (4-FC6H4C2H4NH3)2[PbI4] Nanomaterial

Single crystals of a hybrid organic/inorganic material with the formula (4-FC6H4C2H4NH3)2[PbI4] were synthesized and studied by X-ray diffraction, ellipsometry, and photoluminescence. The crystals consist of a self-assembled multilayer structure with a P21/a space group. The inorganic semiconductor sublattice is built up from corner-sharing PbI6 octahedra forming infinite two-dimensional quantum wells. The organic (4-FC6H4C2H4NH3)+ chains form the insulator barriers. A strong room temperature photoluminescence emission, due to excitons confined in the PbI quantum wells, is found around 2.34 eV. The exciton binding energy 540 meV is determined from the temperature dependence of the photoluminescence intensity and using a modified Arrhenius model, which accounts for the two-dimensional electronic structure. Calculations of the exciton binding energy based on a variational method indicate that the dielectric confinement effect is enhanced by the low value of the organic barrier dielectric constant. Moreover, it is shown that the temperature variation of the photoluminescence peak energy is well described by Varshni’s model, thus indicating the free character of the two-dimensional excitons. The temperature dependence of the photoluminescence line width is also analyzed and interpreted in terms of homogeneous broadening due to exciton−phonons interaction.

Degradation of a C/CrC PVD coating after annealing in Ar+H2 at 700°C studied by Raman spectroscopy and transmission electron microscopy

The thermal stability of a hydrogen free C/CrC coating was the focus of this study. The coating was deposited by unbalanced magnetron sputtering of graphite and Cr metal targets in a non-reactive argon atmosphere at high ion irradiation conditions. The coating possessed a nanocomposite structure with amorphous carbon embedded in a metastable NaCl (B1) structure CrC matrix. The nanometre amorphous carbon clusters formed layers within the CrC matrix, producing a self-assembled multilayer structure. In this paper, degradation of the multilayer nanocomposite structure and NaCl (B1) structure CrC phase was evaluated by annealing at 700°C in Ar+5%H2 atmosphere for 30 minutes. Microstructures of the as-deposited and annealed coating were characterised using Raman spectroscopy and cross-sectional transmission electron microscopy (TEM) coupled with electron energy filtered mapping. Raman spectroscopy suggested the presence of graphitic carbon in the coating after annealing, together with a trace of Cr2O3 associated with coating growth defects. TEM investigation of the cross sections of annealed coating revealed regions of C enrichment at the very top (∼40 nm) and bottom (∼10 nm) of the coating, which was confirmed by Raman spectroscopy. However, the central region of the coating retained its composite C/CrC multilayer structure. Tentative mechanisms of C enrichment are proposed. The advantages and limitations of Raman spectroscopy and TEM techniques in studying C/CrC coating are also discussed.

Polymorphism of Long-Chain Alkane-α,ω-Diols with an Even Number of Carbon Atoms

Crystal structures of long-chain alkane-α,ω-diols with an even number of carbon atoms from 20 to 24 are determined. The molecular structures are similar to those of the lower homologues with an even number of carbon atoms analyzed previously; however, the methylene chain packings are different from each other. In the newly analyzed structure, it is found that the crystal structure type has an advantage in the close packing for the methylene part with increasing hydrophobic interactions. Crystal structures of a different polymorphic form in the even number of carbon atoms from 16 to 24 are also determined. The molecular and crystal structures are similar to those of an odd number of carbon atoms analyzed previously. However, the structures are slightly different from self-assembled multilayer structures whose models are derived from a grazing incidence synchrotron X-ray diffraction data. The carbon number starting to exhibit a polymorphic form coincides with that starting to show a rotator phase which are observed just below their melting points. From the viewpoint of the epitaxial growth at the surface and of the calculated density, it is found that the crystal structure of the polymorphic form has an advantage in exhibiting the rotator phase.

Self-assembled lamellar structures with functionalized single wall carbon nanotubes

Highly ordered self-assembled multi-layer structures with denatured collagen wrapped single wall carbon nanotubes and surfactant systems were obtained through bioinspired methodology.

Local Three‐Dimensional Visualization of Nanoparticle Assemblies

High-angle annular dark-field (HAADF) imaging technique in the scanning transmission electron microscope has been exploited to study self-assembled multilayer structures of Au/Ag nanoparticles. The HAADF image intensity depends monotonically on the mass and thickness of the sample. Various film thickness between one to four monolayers can be easily distinguished by evaluating the contrast (see Figure).

Side Chain Length Dependence of Optical Properties of Polyanions Based on Poly( p-phenylenevinylene) and Their Self-Assembled Multilayer Structures

Polyanions based on carboxy-substituted poly( p-phenylenevinylene)s (PPVs) with various side chain lengths have been synthesized, and their optical properties investigated. Alternating periodic multilayer structures consisting of PPV and PPV polyanions have been fabricated by a self-assembly method on quartz or indium–tin–oxide-coated glass substrates. The dependence of the optical properties on the side chain length of carboxylic substituents or alkoxy substituents has been discussed.

Photoinduced electron transfer in multilayer self-assembled structures of porphyrins and porphyrin–fullerene dyads on ITO

A new strategy for constructing well-ordered, self-assembled multilayer structures of photoactive donor–acceptor systems has been developed. In this approach indium–tin oxide (ITO) electrodes were modified with successive self-assembled monolayers of Zn porphyrins (ZnP) and free-base porphyrin–fullerene (H2P–C60) dyads to obtain oriented triad configurations. All the layers were attached with two molecular linkers to achieve an organized composition. The multilayer structures were found to enhance the photocurrent and photovoltage generation compared to monolayers of the same compounds. Particularly, combining a densely packed ZnP layer with the highly oriented H2P–C60 dyad layer resulted in 280 times higher photovoltage response than measured for a monolayer of the dyad. Furthermore, both photovoltage and photocurrent were generated in the desired direction pursued with the sample orientations and, as a result of the organization, almost ideal current–voltage curves were formed.

Recrystallization prospects for freestanding low-temperature GaN grown using ZnO buffer layers

Remote plasma enhanced-laser-induced chemical vapor deposition was used to grow gallium nitride films on zinc oxide buffer layers deposited by atomic layer epitaxy on soda lime glass. Freestanding layers of gallium nitride were processed by etching away the substrate and ZnO buffer layer. The n-type carrier mobility for the GaN on ZnO/soda lime glass was found to be similar to the highest values achieved on pure silica, and was accompanied by high carrier concentration. As-grown polycrystalline materials were recrystallized at low temperature (below the 570°C gallium nitride growth temperature). This recrystallization process greatly improved the film structure with a self-assembled multilayer structure evident in the oxygen-rich surface layer of the films that had undergone the process.

Microring light-emitting devices with self-assembled multilayer structures based on poly( p-phenylene vinylene)s

Periodic multilayer structures of poly(p-phenylene vinylene)s have been fabricated by a self-assembly method on flat surfaces and round surfaces of optical fibers. Alternating multilayers consisting of poly(1,4-(2-(5-carboxypentyloxy)-5-methoxyphenylene)vinylene) and poly(p-phenylene vinylene) have been adsorbed onto the positively charged substrates and the optical properties have been studied. The periodic multilayers with microring geometry have also been fabricated around the quartz fibers, and the optical properties and yellow electroluminescence from a light-emitting device with microring geometry have been investigated.