Nanocomposites based on highly luminescent nanocrystals and semiconducting conjugated polymer for inkjet printing

In this work nanocomposites based on organic-capped semiconductor nanocrystals formed of a core of CdSe coated with a shell of ZnS (CdSe@ZnS), with different sizes, and a semiconducting conjugated polymer, namely poly[(9,9-dihexylfluoren-2,7-diyl)-alt- (2,5-dimethyl-1,4-phenylene)] (PF-DMB) have been investigated. The nanocomposites are prepared by mixing the pre-synthesized components in organic solvents, thereby assisting the dispersion of the organic-coated nano-objects in the polymer host. UV-vis steady state and time-resolved spectroscopy along with (photo)electrochemical techniques have been performed to characterize the obtained materials. The study shows that the embedded nanocrystals increase the PF-DMB stability against oxidation and, at the same time, extend the light harvesting capability to the visible spectral region, thus resulting in detectable photocurrent signals. The nanocomposites have been dispensed by means of a piezo-actuated inkjet system. Such inks present viscosity and surface tension properties well suited for stable and reliable drop-on-demand printing using an inkjet printer. The fabrication of arrays of single-color pixels made of the nanocomposites and micrometers in size has been performed. Confocal and atomic force microscopy have confirmed that inkjet-printed microstructures present the intrinsic emission properties of both the embedded nanocrystals and PF-DMB, resulting in a combined luminescence. Finally, the morphology of the printed pixels is influenced by the embedded nanofillers.     

Inorganic-organic hybrid solar cell: bridging quantum dots to conjugated polymer nanowires

Quantum dots show great promise for fabrication of hybrid bulk heterojunction solar cells with enhanced power conversion efficiency, yet controlling the morphology and interface structure on the nanometer length scale is challenging. Here, we demonstrate quantum dot-based hybrid solar cells with improved electronic interaction between donor and acceptor components, resulting in significant improvement in short-circuit current and open-circuit voltage. CdS quantum dots were bound onto crystalline P3HT nanowires through solvent-assisted grafting and ligand exchange, leading to controlled organic-inorganic phase separation and an improved maximum power conversion efficiency of 4.1% under AM 1.5 solar illumination. Our approach can be applied to a wide range of quantum dots and polymer hybrids and is compatible with solution processing, thereby offering a general scheme for improving the efficiency of nanocrystal hybrid solar cells.     

Effects of surface modification on the fluorescence properties of conjugated polymer/ZnO nanocomposites

We report the effects of surface modification on the fluorescence properties of conjugated polymer/zinc oxide nanocomposites. Zinc oxide nanoparticle surface was partially capped by poly(vinylpyrolidone) (PVP) during hydrolysis of zinc acetate dihydrate in methanol in ambient condition. The ZnO-capped nanoparticles were characterized by XRD and TEM. The fluorescence properties of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV)/ZnO nanocomposites with different mass ratios were studied by steady-state and time-resolved spectroscopy. The results indicate that the surface-modified ZnO nanoparticles are more effective to quench the emission of MEH-PPV by charge transfer process than the non-capped ZnO. The more effective charge transfer in MEH-PPV/ZnO-capped is attributed to the better dispersion of the nanoparticles in MEH-PPV matrix and THF solvent.     

Carbon nanotube–polymer interactions in nanocomposites: A review

The interaction between carbon nanotubes and polymers is critically reviewed. The interfacial characteristics directly influence the efficiency of nanotube reinforcements in improving mechanical, thermal, and electrical properties of the polymer nanocomposite. In this review, various techniques of interaction measurements, including experimental and modelling studies, are described. From the experimental approaches, wetting, spectroscopy and probe microscopy techniques are discussed in detail. Molecular dynamics, coarse grain simulation and density functional theory are also explained as the main modelling approaches in interaction measurement studies. Different methods of interaction improvement, mainly categorized under covalent and noncovalent interactions, are described afterwards. Modelling predictions of nanocomposite properties, such as Young’s modulus, are compared with the experimental results in the literature and the challenges are discussed. Finally, it is concluded that an optimum carbon nanotube–polymer interaction is a key factor towards reaching the full potential of carbon nanotubes in nanocomposites.     

Spectroscopic investigations on sol-gel derived organic-inorganic hybrid films for photonics from ormosils and tetrapropylorthotitanate

Titania/ormosils organic-inorganic hybrid films with a thickness of approximately 1.55-μm were prepared as planar waveguides by a single spin-coating process and low temperature heat treatment. Acid-catalyzed solutions, firstly, γ-glycidoxypropyltrimethoxysilane (GLYMO) mixed with tetrapropylorthotitanate, secondly, methyltrimethoxysilane (MTES) mixed with tetrapropylorthotitanate, and thirdly, γ-glycidoxypropyltrimethoxysilane and methyltrimethoxysilane mixed with tetrapropylorthotitanate were used as precursors. Purely inorganic, crack-free and highly transparent silica-titania films with a thickness of more than 0.5-μm were also obtained after a heat treatment temperature of 500 °C. Films were characterized by thermal gravimetric analysis, and certain optical tools such as UV-visible spectra, Fourier transform infrared spectra and Raman spectra. The obtained results indicate that the GLYMO derived films show more shrinkage and the GLYMO and MTES derived films higher refractive index during heating. The hardness and Young's modulus of the films were characterized by a nanoindenter. Hardness as high as 6.83 GPa was obtained in GLYMO and MTES derived film after being heat-treated at 800 °C. It has been proposed that the high hardness of the film could be related to the carbon and high titanium content in the film.     

Photoluminescence and XPS analyses of Mn doped ZnS nanocrystals embedded in sol-gel derived hybrid coatings

Photoluminescence and X-ray photoelectron spectroscopy studies of Mn²⁺ doped ZnS nanocrystals in inorganic-organic hybrid coatings prepared by a sol-gel process are presented. A 25-fold enhancement of photoluminescence was observed after UV irradiation for 6 h in an ambient atmosphere. X-ray photoelectron spectroscopy results indicate a chemical shift of binding energy from ZnS to ZnSO₄ after UV irradiation. X-ray diffraction results show a decrease of ZnS nanocrystal size during UV irradiation. The cause of these phenomena was discussed based on a photochemical reaction on ZnS nanocrystal surface.     

Structure-property relationship in sol-gel derived polymer/silica hybrid nanocomposites prepared at various pH

This paper reports a comparative study on structure-property relationship of acrylic rubber (ACM)/silica, epoxidised natural rubber (ENR)/silica and poly (vinyl alcohol) (PVA)/silica hybrid nanocomposites prepared by sol-gel technique under different pH levels (pH = 1.0–13.0), probably for the first time. The initial concentration of tetraethoxysilane (TEOS) (used as the precursor for silica) was kept at 45 wt%, and tetrahydrofuran (THF) for ACM/silica and ENR/silica while water for PVA/silica were taken as solvents. TEOS to water mole ratio was maintained at 1:2 for the rubber/silica systems to accomplish the sol-gel reaction. The structure of the resultant hybrid composites was determined by using electron microscopy, Fourier Transform infrared spectroscopy and solubility. Dynamic mechanical and mechanical properties were also investigated. The silica particles were found to exist as nanoparticles (average diameter <100 nm) at low pH (≤ 2.0) beyond which these aggregate, although the amount of silica generation was not strictly influenced by the various pH conditions in all the systems. These nanocomposites were optically clear and showed superior mechanical reinforcement over the micro-composites containing aggregated silica structures with lower optical clarity. The nanocomposites exhibited higher storage modulus both at the glassy and the rubbery regions as compared to those micro-composites. The loss tangent peak heights were also minimum and the Tg shifted to higher temperature for those nanocomposites. The maximum improvement of mechanical properties was observed with the PVA/silica nanocomposites due to higher level of interaction between the hydroxyl groups of PVA and the silanol groups of the silica phase.     

Hierarchically structured polymer blends based on silsesquioxane hybrid nanocomposites with quaternary ammonium units for antimicrobial coatings

The paper reports the first study on hierarchical assemblies (nanofibrillar micelles confined within semi-cylindrical shells) with silsesquioxane and quaternary ammonium units obtained through polymer blending intended for antimicrobial/antifungal stone coatings. The formation of hierarchical structures on solid surfaces is due to the multiple intermolecular ionic interactions, intermolecular Van der Waals and hydrophobic interactions acting among the component molecules. Their antimicrobial/antifungal properties toward the Gram-negative bacteria, Escherichia coli, Gram-positive bacteria, Staphylococcus aureus, and Candida albicans fungus were determined in aqueous solution and were found to be strongly dependent of the topographical features of the coating.     

Glass transitions in binary drug + polymer systems

To evaluate miscibility, glass transition temperatures T_g have been determined for two binary polymer (Plasdone S-630 copovidone or Eudragit® E) + drug systems as a function of composition. Each polymer serves for encapsulation of the anti-HIV drug Efavirenz. In both systems the T_g vs. drug concentration diagrams are s-shaped. T_gs of Efavirenz + Plasdone mixtures with drug mass fraction below φ_drug = 0.6 are above linear values. This implies enhanced thermal and mechanical stability—an advantage for the drug encapsulation. In the other system, a strong negative deviation of T_gs is observed over the entire compositional range and explained by positive excess mixing volumes. Several equations are used to represent T_g vs. composition diagrams, but only one (Brostow et al. Mater Lett 2008; 62:3152) provides reliable results.     

Properties of size-tuned PbS nanocrystals stabilized in a polymer template

Nanoclusters of PbS embedded in polymer matrices have recently been shown to have interesting optical properties with the capability of tuning the effective band gap over a wide spectral range. The results of a systematic investigation of the preparation and characterization of size-tuned PbS nanocrystals stabilized in the polymer nanotemplate of Nafion and their size-dependent physical properties are presented in this paper. These nanocrystals exhibit large characteristic blueshift in optical absorption from the bulk absorption onset value of 3020 nm. XRD and HRTEM measurements indicate the presence of PbS nanocrystals in the size range of 2-6 nm, in the regime of strong quantum confinement. Thermal and electrical properties of the polymer are found to be influenced to a great extent by the embedded PbS nanocrystals.     

Training effect on damping capacity in Fe–20 mass% Mn binary alloy

Two-dimensional finite element analysis together with stream function and neural network models are employed to determine thermo-mechanical behavior during hot strip rolling of AA5083. An appropriate velocity field and stream function is first determined using the rule of volume constancy and upper bound theorem and then temperature field within the metal is predicted by means of a two-dimensional conduction–convection model. In order to consider the effect of flow stress and its dependence on temperature, strain and strain rate, a neural network model is also employed in the analysis. Based on the performed tensile tests, two different neural network models are constructed one for smooth yielding and the other one for the serrated flow. Then, the ANN models are coupled with the thermo-mechanical analysis. In the next step, by combination of the predicted temperature, strain and strain rates fields and the experimental data achieved from the tensile tests, the occurence of dynamic strain ageing during hot rolling is predicted. The model predictions are then compared with the experimental data and good agreement is observed between the two sets of results that shows the validity of the proposed model.     

Effect of acrylic copolymer and terpolymer composition on the properties of in-situ polymer/silica hybrid nanocomposites

Acrylic copolymer- and 5% acrylic acid (AA) modified terpolymer-silica hybrid nanocomposites were synthesized by free radical bulk polymerization of ethyl acrylate (EA), butyl acrylate (BA) and acrylic acid (AA) with simultaneous generation of silica from tetraethoxysilane by sol-gel reaction. The pure polymers were analyzed by using Fourier Transform infrared (FTIR) spectroscopy and solid state nuclear magnetic resonance (NMR) spectroscopy. The hybrid samples were characterized by scanning electron microscopy (SEM), FTIR spectroscopy, NMR spectroscopy, dynamic mechanical, mechanical and thermal properties. SEM images confirmed the presence of nanosilica particles within the polymer matrices, whose dispersion and particle size distribution and visual appearance were dependent on the relative polarity (hydrophilicity) of the polymer matrices and the concentration of the filler. There was no evidence of strong chemical interaction between the polymers and the dispersed silica phase, as confirmed from the FTIR results. Terpolymer-silica hybrids demonstrated superior mechanical properties compared to the copolymer-silica hybrids. They also showed higher dynamic storage modulus and positive shift in the loss tangent peaks. The thermal stability of the nanocomposites was marginally higher which was possibly due to the dipolar interaction at the organic-inorganic interface.     

Spectral narrowing phenomena in the emission from a conjugated polymer

We report a study of the morphology dependent emission properties of a liquid crystalline polyfluorene in solid thin films. Spectral narrowing (SN) is observed in spin coated films, but after thermal treatment of the same films the SN is no longer observed. For films which are spin coated on a rubbed polyimide alignment layer, the situation is different. There is then no influence of thermal treatment on the observation of SN. However, specific combinations of the pump polarization direction, film alignment direction and the long axis of the rectangular excitation area now play an important role. The spectral location of SN peaks also varies with film morphology and for different regions of the same sample. The occurrence of SN is strongly dependent on film morphology and the excitation configuration.     

Tracking of single charge carriers in a conjugated polymer nanoparticle

The motion of individual charge carriers in organic nanostructures was tracked by fluorescence microscopy. A twinkling effect is observed in fluorescence microscopy of single conjugated polymer nanoparticles, that is, small displacements in the fluorescence spot of single nanoparticles of the conjugated polymer PFBT are observed over time. There is evidence that superquenching by the charge carrier induces a dark spot in the nanoparticle, which moves with the carrier, resulting in the observed displacements in the fluorescence. Zero-field mobilities of individual charge carriers consistent with highly trapped polarons were obtained from tracking experiments.     

Spectroscopic analysis of poly(methyl methacrylate)/alumina polymer nanocomposites prepared by in situ (bulk) polymerization

Poly(methyl methacrylate) polymer nanocomposites were prepared by in situ bulk free radical polymerization. To ensure high-quality dispersion of the oxide nanoparticles, some composites were prepared from nanoparticles predispersed in propylene glycol methyl ether acetate (PGMEA). The degree to which this additional dispersing medium interacted with the aluminum oxide nanoparticle was studied by attenuated total reflectance (ATR-FTIR), which confirmed secondary bonding and ionic interaction across the particle/dispersing medium interface. Additionally, the effect of the dispersing medium and the nanoparticles on the degree of monomer conversion was determined by Raman spectroscopy. In the presence of oxide nanoparticles alone, the active surface of the nanoparticles traps propagating radicals which significantly reduces monomer conversion. Conversely, the degree of monomer conversion is enhanced in composites containing predispersing nanoparticles, apparently by passivation/functionalization of the oxide surface by the PGMEA.     

Charge conduction and photogeneration process in hybrid materials of conjugated polymer and dye-sensitized TiO2 thin-film device

A thin-film hybrid structure consisting of dye-sensitized TiO₂ in combination with polypyrrole (PPy) is developed over indium tin oxide (ITO)-coated glass substrate. The steady-state photoconductivity and current–voltage (J–V) characteristics were recorded in the dark as well as under illumination for the fabricated device having the structure ITO/PPy/TY+TiO₂/Al. The current–voltage (J–V) characteristics of the device in the dark are explained by the formation of percolation networks of nano-particles between the electrodes. The device becomes symmetric at high voltage suggesting the electron injection from the electrodes into the conduction band of TiO₂ under both bias (reverse and forward) directions and the respective current is controled by charge carrier transport. Photoaction spectra of the device prove that the photogeneration of charge carriers is significantly enhanced and spectrally broadened only if electron transfer from the polymers to the dye is possible. It is also found that the increase in concentration of nano-particles changes the spectral shape of the hybrid structure as well as the photoresponse. These results show that a significant photoresponse can be achieved in hybrid materials of dye-sensitized TiO₂ nano-particles dispersed in conducting polymer.     

EPR study of ZnS: Mn2+ nanocrystals in Pyrex glasses

ZnS: Mn2+ nanocrystals embedded in Pyrex glasses were spectrally studied using EPR and photoluminescence techniques. Photoluminescence (PL) and excitation (PLE) spectra revealed that manganese impurities can be classified as two types of luminescent centers, i.e., occupying substitutional sites (Mn2+)sub or interstitial sites (Mn2+)int. Three types of manganese sites of (Mn2+)sub, (Mn2+)int, and Mn clusters were identified by the EPR spectra. An increase of the g1 factor and hyperfine structure (HFS) constant with decreasing sizes of nanocrystals was observed. The increase was attributed to a hybridization of the s-p state of ZnS and the d state of manganese ions enhanced by quantum confinement effects or surface states.     

TiO<Subscript>2</Subscript> nanocrystals grown on graphene as advanced photocatalytic hybrid materials

A graphene/TiO₂ nanocrystals hybrid has been successfully prepared by directly growing TiO₂ nanocrystals on graphene oxide (GO) sheets. The direct growth of the nanocrystals on GO sheets was achieved by a two-step method, in which TiO₂ was first coated on GO sheets by hydrolysis and crystallized into anatase nanocrystals by hydrothermal treatment in the second step. Slow hydrolysis induced by the use of EtOH/H₂O mixed solvent and addition of H₂SO₄ facilitates the selective growth of TiO₂ on GO and suppresses growth of free TiO₂ in solution. The method offers easy access to the GO/TiO₂ nanocrystals hybrid with a uniform coating and strong interactions between TiO₂ and the underlying GO sheets. The strong coupling gives advanced hybrid materials with various applications including photocatalysis. The prepared graphene/TiO₂ nanocrystals hybrid has superior photocatalytic activity to other TiO₂ materials in the degradation of rhodamine B, showing an impressive three-fold photocatalytic enhancement over P25. It is expected that the hybrid material could also be promising for various other applications including lithium ion batteries, where strong electrical coupling to TiO₂ nanoparticles is essential.     

Photoinduced polarization investigations in 75 MeV oxygen ion irradiated kapton-H polyimide

The photoinduced polarization in 75 MeV oxygen ion irradiated (fluence: 1.8 × 10¹¹, 1.8 × 10¹² and 1.8 × 10¹³ ions/cm²) kapton-H polyimide has been investigated by analyzing charge decay characteristics for different polarizing parameters viz. electric fields (40 to 600 kV/cm), temperature (40 to 250 °C) and illumination intensity (1200 to 2800 1x). The fields induced as well as thermal ionization of excitons under illumination are the main causes which provide photopolarization. The charge decay spectra reveal the presence of both shallow and deep trapping sites in pristine and irradiated kapton-H polyimide. The variation in the photopolarization with fluence shows the occurrence of secondary radiation induced crystallinity (SRIC). The SRIC is also responsible for the increase in initial current (I_0P) with intensity of illumination in low fluence irradiated samples. A decrease in I_0P with intensity of illumination in high fluence irradiated samples has been associated to the conversion of trapping sites into recombination centers.