Drop-cast Films Research Articles

Solution and surfactant-free growth of supported high index facet SERS active nanoparticles of rhenium by phase demixing

Stable, high-index facet Re nanoparticles have been grown by a solid state synthetic method, negating the need for solutions or surfactants to control seeding, supracrystallization and NP shape. By using mixtures of K[ReO4] and the cyclic triphosphazene [NP(O2C12H8)]3, high-index facet nanoparticles and nanocrystals ∼3 nm in size can be seeded and grown from drop-cast films and powders due to phase demixing of the metallopolymer. NP dispersions are formed directly within a carbon support that liquefies, allowing NP coarsening and ripening, and the eventual formation of a solidified graphitic support filled with crystals. Successful growth of mesoscale supracrystals of Re also occurs from ripening of nucleated NP seeds, incubated within a solidified and partially dewetted solid support that patterns the surface. The supported Re NP dispersions also exhibit surface enhanced Raman scattering within a graphitic matrix.

Photo-Conversion and Structural Properties of the Drop-Cast Films of 6,13-Pentacene Diketone

6,13-pentacene diketone is a unique material which can be converted to pentacene only by photo irradiation. Drop-cast films of this soluble precursor of pentacene were prepared and their thin film properties were studied. The effect of solvent and photo irradiation conditions on photo-conversion behavior and film structures were investigated, compared to spin-cast films. The drop-cast films exhibited large plate-like crystals having a size of several microns, and showed intense peaks in X-ray diffraction. The electrical measurement using a field-effect transistor was also performed.

Intrinsic electrochemical activity of single walled carbon nanotube–Nafion assemblies

The intrinsic electrochemical properties and activity of single walled carbon nanotube (SWNT) network electrodes modified by a drop-cast Nafion film have been determined using the one electron oxidation of ferrocene trimethyl ammonium (FcTMA(+)) as a model redox probe in the Nafion film. Facilitated by the very low transport coefficient of FcTMA(+) in Nafion (apparent diffusion coefficient of 1.8 × 10(-10) cm(2) s(-1)), SWNTs in the 2-D network behave as individual elements, at short (practical) times, each with their own characteristic diffusion, independent of neighbouring sites, and the response is diagnostic of the proportion of SWNTs active in the composite. Data are analysed using candidate models for cases where: (i) electron transfer events only occur at discrete sites along the sidewall (with a defect density typical of chemical vapour deposition SWNTs); (ii) all of the SWNTs in a network are active. The first case predicts currents that are much smaller than seen experimentally, indicating that significant portions of SWNTs are active in the SWNT-Nafion composite. However, the predictions for a fully active SWNT result in higher currents than seen experimentally, indicating that a fraction of SWNTs are not connected and/or that not all SWNTs are wetted completely by the Nafion film to provide full access of the redox mediator to the SWNT surface.

Hierarchical assembly of a molecular material through the amorphous phase and the evolution of its fluorescence emission

Fabrication of an amorphous phase followed by a hierarchy of forms leading to the crystalline state offers a new dimension in the assembly of small molecule based materials. The morphology and extent of crystallinity of the nanostructures in drop-cast thin films of a diaminodicyanoquinodimethane molecule are shown to be tunable through the variation in the composition of the solvent mixture used for drop-casting. The different states of assembly starting from the solvated molecule through amorphous spherical particles, crystalline nanofibers and nano/microcrystals to bulk crystals are shown to be accompanied by a smooth variation of the fluorescence emission, the color evolving from green to blue, and the efficiency increasing steadily with the overall enhancement from the solution to the bulk crystalline state being ∼400 times. Quantum chemical computations on the molecule and its H-bonded dimer and π-dimers provide insight into the impact of intermolecular interactions in the crystalline assemblies on the electronic structure. The current study illustrates a significant departure from the conventional molecules-to-materials transition, opening up new hierarchical pathways of assembly of molecular materials.

Self-assembly of a set of hydrophilic–solvophobic–hydrophobic coil–rod–coil molecules based on perylene diimide

The self-assembly and the resulting morphology of a set of asymmetrically substituted perylene diimide is discussed. We synthesized perylene diimides with hydrophilic Jeffamine® (PEO/PPO co-oligomer) attached to the imide nitrogen on one side and (hydrophobic) alkyl chains of different lengths on the other. Although studies on asymmetrically substituted perylene diimides have been reported by various authors, both side chains in this work are linear and we discuss the effect of the length of the hydrophobic alkyl side chain on the self-assembly in water and aqueous mixtures and the resulting morphology. We find that self-assembly occurs in water in a range of concentrations (from 10(-7) to 10(-4) M), and the mode of packing of the H-stacked molecules changes with concentration for the propyl substitution. Although self-assembly does not occur in non-aqueous solvents such as acetone, it does upon addition of water. While other authors observed nano-belts and nano-fibers with asymmetrically substituted perylene diimides, the morphology of drop-cast films in our work consists of folded sheets and hollow tubes, of a few microns in diameter. Such folding in a non-chiral system such as the present case is attributed to predominant asymmetry of interactions between the molecules in the three directions, as we discussed in our previous work on an hydrogen bonded system (S. Khanna, M. K. Khan and P. Sundararajan, Langmuir, 2009, 25, 13183-13193).

Fabrication of Large-scale Drop-cast Films of π-conjugated Polymers with Floating-film Transfer Method

Fabrication of Large-scale Drop-cast Films of π-conjugated Polymers with Floating-film Transfer Method

Analysis of mobile ionic impurities in polyvinylalcohol thin films by thermal discharge current and dielectric impedance spectroscopy

Polyvinylalcohol (PVA) is a water soluble polymer frequently applied in the field of organic electronics for insulating thin film layers. By-products of PVA synthesis are sodium acetate ions which contaminate the polymer material and can impinge on the electronic performance when applied as interlayer dielectrics in thin film transistors. Uncontrollable voltage instabilities and unwanted hysteresis effects are regularly reported with PVA devices. An understanding of these effects require knowledge about the electronic dynamics of the ionic impurities and their influence on the dielectric properties of PVA. Respective data, which are largely unknown, are being presented in this work. Experimental investigations were performed from room temperature to 125°C on drop-cast PVA films of three different quality grades. Data from thermal discharge current (TDC) measurements, polarization experiments, and dielectric impedance spectroscopy concurrently show evidence of mobile ionic carriers. Results from TDC measurements indicate the existence of an intrinsic, build-in electric field of pristine PVA films. The field is caused by asymmetric ionic double layer formation at the two different film-interfaces (substrate/PVA and PVA/air). The mobile ions cause strong electrode polarization effects which dominate dielectric impedance spectra. From a quantitative electrode polarization analysis of isothermal impedance spectra temperature dependent values for the concentration, the mobility and conductivity together with characteristic relaxation times of the mobile carriers are given. Also shown are temperature dependent results for the dc-permittivity and the electronic resistivity. The obtained results demonstrate the feasibility to partly remove contaminants from a PVA solution by dialysis cleaning. Such a cleaning procedure reduces the values of ion concentration, conductivity and relaxation frequency.

Wet Chemical Method for Making Graphene-like Films from Carbon Black

Reduction of strongly oxidized carbon black by hydrazine hydrate yields water-insoluble graphene-like sheets that undergo to self-assembling in thin film on surfaces after drying. The height of a drop-casted graphene-like film was determined by atomic force microscopy (AFM) to be around 20 nm, corresponding to approximately 25 graphene-like layers. The oxidized carbon black and the corresponding reduced form were carefully characterized.

Synthesis, Morphology, and Sensory Applications of Multifunctional Rod–Coil–Coil Triblock Copolymers and Their Electrospun Nanofibers

We report the synthesis, morphology, and applications of conjugated rod-coil-coil triblock copolymers, polyfluorene-block-poly(N-isopropylacrylamide)-block-poly(N-methylolacrylamide) (PF-b-PNIPAAm-b-PNMA), prepared by atom transfer radical polymerization first and followed by click coupling reaction. The blocks of PF, PNIPAAm, and PNMA were designed for fluorescent probing, hydrophilic thermo-responsive and chemically cross-linking, respectively. In the following, the electrospun (ES) nanofibers of PF-b-PNIPAAm-b-PNMA were prepared in pure water using a single-capillary spinneret. The SAXS and TEM results suggested the lamellar structure of the PF-b-PNIPAAm-b-PNMA along the fiber axis. These obtained nanofibers showed outstanding wettability and dimension stability in the aqueous solution, and resulted in a reversible on/off transition on photoluminescence as the temperatures varied. Furthermore, the high surface/volume ratio of the ES nanofibers efficiently enhanced the temperature-sensitivity and responsive speed compared to those of the drop-cast film. The results indicated that the ES nanofibers of the conjugated rod-coil block copolymers would have potential applications for multifunctional sensory devices.

Preclusion of nano scale self-assembly in block-selective non-aqueous solvents for rod–coil and coil–rod–coil macromolecular surfactants based on perylene tetracarboxylic diimide

Self-assembly of amphiphilic molecules ranging from simple surfactants to block copolymers in a solvent depends on one part of the molecule (one block in block copolymers) being soluble, and the other not. The aggregation of the insoluble segment in the block-selective solvent leads to the self assembly. In this paper, we describe a system of amphiphilic rod–coil and coil–rod–coil molecules, which do not show self assembly in block-selective non-aqueous solvents. We prepared rod–coil molecules based on hydrophilic propylene oxide/ethylene oxide copolymer (PO–EO copolymer) (Jeffamine®) as the flexible segment and photo-conducting large aromatic perylenediimide (PTCDI) as the rod. PO–EO copolymer was attached either to one side of PTCDI (MJ–PTCDI) or both sides (DJ–PTCDI). The former can be considered an inverse macromolecular surfactant, since the tail is hydrophilic and the head is hydrophobic. The DJ–PTCDI is a pseudo Gemini surfactant. Because of the presence of the chromophore, UV–Vis and fluorescent spectra could be used to study the self assembly of these amphiphilic rod coil polymers in solution. PTCDI forms π-interaction mediated aggregates in aqueous solution and these are H-stacked in MJ–PTCDI and J-stacked in DJ–PTCDI. Variable temperature UV and NMR spectra show that the assembly is stable over a large temperature range in water. The aggregates are also stable up to a pH of 12. However, when a non-aqueous solvent is used, no aggregation occurs. This is attributed to the “solvation” of the π-system of the PTCDI. With the addition of water, such solvation seems to be interrupted and aggregation occurs when water becomes a major component. We find that the mole percentage of the aggregates in acetone/water mixtures increases almost linearly with the concentration of water, providing a route to control the extent of aggregation of the chromophores. Due to the long, waxy PO–EO copolymer, MJ–PTCDI and DJ–PTCDI do not show liquid crystalline behavior or nanorod morphology, which were seen with short side chains. The optical microscopy of the bulk material shows aggregated crystals of PTCDI in the waxy matrix, showing that even in the presence of PO–EO copolymer, the molecular assembly of PTCDI takes place in the bulk. Secondary assembly was seen, in that upon ageing of the aqueous solutions, the drop cast films show that the spherical aggregates one-dimensionally coalesced into long fibers. Although UV–Vis spectra indicated no aggregation in non-aqueous solvents, drop-cast films of these solutions show needle-like aggregates and Lego-like assemblies.

Effects of structural order in the pristine state on the thermoelectric power-factor of doped PBTTT films

The thermoelectric property of doped films of a thermotropic liquid-crystalline polythiophene poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) PBTTT-C14 was studied for the first time. Drop-casted films of PBTTT-C14, annealed or not, were doped by nitrosonium phosphate NOPF6 and their thermoelectric performance was compared. Power factor of the doped films was increased by a factor of three at room temperature when pristine films were thermally annealed. Both the electrical conductivity and the Seebeck coefficient of doped films benefited from the annealing process. Films with and without thermal annealing were characterized by ultraviolet–visible near-infrared (UV–vis NIR) spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) both before and after being doped. Improvement in structure order caused by annealing was verified in pristine films. Furthermore, the ordered structure in annealed films was largely retained after the doping treatment. The higher structure order in pristine films with annealing and the preservation of structure feature in the annealed films throughout the doping process were suggested to lead to higher bulk mobility which accounted for the enhanced power factor.

Film processing characteristics of nano gold suitable for conductive application on flexible substrates

In spite of large number of studies and wide use of thiol encapsulated gold nanoparticles, the mechanism of their transformation to thin gold films for conductive applications is not as yet well understood. In order to understand and optimize the process of conversion of nanoparticle based ink for printing on plastic substrates, we synthesize and study thiolated Au nanoparticles, with average size of 2nm, but with differing carbon chain length viz. butane (AuC4), hexane (AuC6) and octane (AuC8). The link between the properties of Au nanoparticle and its transformation from nonconductive gold nanoparticle ink to conductive gold film is studied using a variety of techniques such as thermo gravimetric analysis (TGA), X-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy and electrical conductivity measurements. A combined study of the shape of solution TGA and differential thermal analysis indicates occurrence of two distinct processes corresponding to disentanglement and debonding of thiol chains preceding the sintering of nanoparticles. The lowest sintering temperature is observed to be approximately 155°C for chain length C4, and hence AuC4 on polyethylene terephthalate substrates is studied in detail. Though XRD peaks of thick drop-cast films on polyethylene terephthalate substrate show increasing peak intensity with annealing temperature as expected, for spin coated thin films, in contrast, the peak intensity decreases with increase in annealing temperature. Electrical conductivity of the thin films is comparable to bulk gold after conversion, but decreases with increase in annealing temperature demonstrating the usefulness of insights obtained in the study for optimization of annealing schedules.

Planar-localized surface plasmon resonance device by block-copolymer and nanoimprint lithography fabrication methods

The authors report on the integration of delocalized surface plasmon resonances (SPRs) and localized surface plasmon resonances (LSPRs) on a single device. The submicron SPR device was fabricated with nanoimprint lithography (NIL). Gold nanoparticles for LSPR generation were created and deposited via three methods and analyzed with rhodamine 6 G and surface-enhanced Raman spectroscopy (SERS). Compared to drop-cast and thin film annealing methods, gold nanoparticles fabricated from a diblock-copolymer NIL template produced the most significant effect on the charge-transfer component of the SERS enhancement mechanism due to near-field interactions at the 10 nm inter-particle separation region. The authors also report a 26% enhancement of optical resonance with an integrated SPR-LSPR plasmonic device consisting of a two-dimensional submicron aluminum grating fully coupled with gold nanoparticles measuring 20.4 nm in diameter in a water medium. If the 2D aluminum grating were coupled to an optimized nanoparticle SERS device fabricated from a DBCP NIL template, the coupled nanoparticle-grating device could exhibit an even higher enhancement and optical resonance performance.

Predictably tuning the frontier molecular orbital energy levels of panchromatic low band gap BODIPY-based conjugated polymers

Here we report the synthesis, electrochemical properties, and optical properties of five novel π-conjugated alternating copolymers based on the BODIPY core. These polymers were synthesized via the Sonogashira polymerization reaction and contain BODIPY units alternating with comonomers such as 9,9-bis(2-ethylhexyl)-9H-fluorene (FL), 9-(2-ethylhexyl)-9H-carbazole (CBz), 2,2′-bithiophene (BT), 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) and 4-(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (DTP). These comonomers were rationally chosen based on their gas phase ionization potential (IP) values estimated by density functional theory (DFT) calculations. Cyclic voltammetry on drop-cast thin films, as well as solutions of these polymers, revealed that the highest occupied molecular orbitals (HOMOs) of the resulting polymers correlated well with the ionization potentials (donor strength) of the comonomers. On the contrary, the lowest unoccupied molecular orbital (LUMO) energy levels of all copolymers were fairly invariant, independent of the comonomer used. This suggests that the BODIPY moiety provides the primary influence on the LUMO levels of the polymer. In addition to the experimentally determined HOMO/LUMO energy levels bearing good correlation with theoretical estimates, all polymers were found to possess broad absorption spectra covering the entire visible range, thus making them truly panchromatic. These polymers provide us with a toolset to tune the frontier molecular orbital energy levels, while retaining the low band gap and broad absorption of these polymers.

Characteristics of a silicon Schottky photodetector produced from colloidal gold nanoparticles

A solution-processed silicon Schottky photodetector fabricated by drop cast film of colloidal gold nanoparticles (NPs) onto n-type monocrystalline silicon wafer is demonstrated. The colloidal gold nanoparticles were synthesised by pulsed Nd:YAG laser ablation of gold target in water at room temperature. Optical absorption and transmission electron microscope (TEM) investigation of the produced gold nanoparticles were carried out. TEM investigation showed synthesised gold particle size of 30 nm. The Schottky junction showed good rectification and photoresponse characteristics. The Schottky barrier height of Au NPs Si contact was calculated from current voltage characteristics and found to be 0.92 eV. The photodetector figures of merit are demonstrated and discussed.

Diffusion-limited aggregation in dry nanocrystal films

ABSTRACTWe demonstrate that drop cast films of colloidal nanocrystals containing excess surfactants develop aggregation over several weeks after solvent evaporation. Fingered structures, typical of diffusion-limited aggregation, are created because of the residual mobility that nanocrystals retain in the film. We are able to control the aggregation through the concentration of surfactant molecules and the drying temperature of the films.

Synthesis, Characterization, and OFET and OLED Properties of π-Extended Ladder-Type Heteroacenes Based on Indolodibenzothiophene

Abstract Ladder-type heteroacenes based on indolodibenzothiophene have been synthesized. The route involved a regioselective twofold intramolecular acid-induced cyclization of the central bis(butylsulfinyl)phenyl unit onto the adjacent carbazole units, followed by dealkylation to give diindolobenzobisbenzothiophene (DIBBBT) derivatives. Importantly, this sequence proceeded efficiently in the presence of the chloro substituents which provided reactive sites for further π-extension of the DIBBBT core. The chloro derivative underwent twofold palladium-catalyzed Suzuki–Miyaura reactions with phenylboronic acid or 4-n-octylphenylboronic acid. Compounds have been characterized by 1H NMR spectroscopy, mass spectrometry, elemental analysis, optical spectroscopy, and solution electrochemical studies. Organic field-effect transistors (OFETs) have been constructed based on drop-cast thin films and the performances as p-type semiconductor are presented. The chloro derivative was also copolymerized with 9,9-dioctyl-2,7-fluorenylenediboronic acid bis(1,3-propanediol) ester to give the alternating copolymer which exhibits blue photoluminescence in solution and blue-green electroluminescence in a solution processed organic light-emitting diode (OLED).

In vivo biocompatibility and in vitro characterization of poly-lactide-co-glycolide structures containing levetiracetam, for the treatment of epilepsy

Epilepsy is a chronic neurological disorder characterized by recurrent seizures, and is highly resistant to medication with up to 40% of patients continuing to experience seizures whilst taking oral antiepileptic drugs. Recent research suggests that this may be due to abnormalities in the blood-brain barrier, which prevent the passage of therapeutic substances into the brain. We sought to develop a drug delivery material that could be implanted within the brain at the origin of the seizures to release antiepileptic drugs locally and avoid the blood brain barrier. We produced poly-lactide-co-glycolide drop-cast films and wet-spun fibers loaded with the novel antiepileptic drug Levetiracetam, and investigated their morphology, in vitro drug release characteristics, and brain biocompatibility in adult rats. The best performing structures released Levetiracetam constantly for at least 5 months in vitro, and were found to be highly brain biocompatible following month-long implantations in the motor cortex of adult rats. These results demonstrate the potential of polymer-based drug delivery devices in the treatment of epilepsy and warrant their investigation in animal models of focal epilepsy.

Substituted p-phenylene ethynylene trimers as fluorescent sensors for nitroaromatic explosives

New sensory materials based on p-phenylene ethynylene trimers integrating calix[4]arene receptors ( CALIX-PET) and tert-butylphenol ( TBP-PET) moieties have been synthesized and their sensitivity and selectivity for the detection of nitroaromatic compounds (NACs) such as nitrobenzene (NB), 2,4-dinitrotoluene (2,4-DNT), 2,4,6-trinitrotoluene (TNT) and picric acid (PA) investigated in fluid phase and solid-state. It was found that both fluorophores displayed high sensitivities toward NACs detection in solution as evaluated by the Stern–Volmer formalism. For all the tested explosives, the ratio of fluorescence intensities ( F 0/ F) is a linear function of the quencher concentration only after appropriate correction of fluorescence quenching data for inner-filter effects. The quenching efficiencies for CALIX-PET and TBP-PET follow the order PA ≫ TNT > DNT > NB, which correlate well with the quenchers electron affinities as evaluated from their LUMOs energies thereby suggesting a photoinduced electron transfer as the dominant mechanism of fluorescence quenching. The selectivity of these sensors was checked against exemplar interferents possessing differentiated electronic properties (benzoic acid, 2,4-dichlorophenol and benzoquinone) and reduced quenching activity was detected. The quenching efficiencies and response times of the two fluorophores in the solid-state toward NB, 2,4-DNT and TNT vapors were evaluated through steady-state fluorescence quenching experiments with the materials dispersed in polymeric matrices or as neat films. The most significant fluorescence quenching responses were achieved for drop-casted films of TBP-PET upon exposure to nitroaromatics.

A Precursor-Limited Nanoparticle Coalescence Pathway for Tuning the Thickness of Laterally-Uniform Colloidal Nanosheets: The Case of SnSe

The availability of high-quality colloidal nanosheets underpins a diverse range of applications and investigations into dimension-dependent physical properties. To facilitate this, synthetic methods that yield single-crystal colloidal nanosheets with regular shapes, uniform lateral dimensions, and tunable thicknesses are critically important. Most strategies that yield colloidal nanosheets achieve some, but not all, of these morphological characteristics. Here, we describe a synthetic pathway that generates colloidal nanosheets of SnSe with uniform lateral dimensions and tunable thicknesses. SnSe represents an excellent prototype system for studying the formation of colloidal nanosheets because of its layered crystal structure and the growing interest in its potential application as an absorption layer in low-cost photovoltaic devices. Freestanding colloidal SnSe nanosheets were synthesized by slowly heating a one-pot reaction mixture of SnCl(2), oleylamine, trioctylphosphine selenide (TOP-Se), and hexamethyldisilazane (HMDS) to 240 °C. The SnSe nanostructures adopt a uniform square-like morphology with lateral dimensions of approximately 500 nm × 500 nm, and the average nanosheet thicknesses can be tuned from approximately 10 to 40 nm by adjusting the concentrations of the SnCl(2) and TOP-Se reagents. Aliquot studies reveal fundamental insights into how the nanosheets form: they first "grow out" laterally via coalescence of individual nanoparticle building blocks to yield a single-crystal nanosheet template and then "grow up" vertically (through nanoparticle attachment to the nanosheet template) in a pseudo layer-by-layer fashion. Vertical growth is therefore limited, and can be controlled, by reagent concentration. Drop-cast films of the SnSe nanosheets are photoactive and have a bandgap of approximately 1 eV. These studies, demonstrated for SnSe but potentially applicable to other systems, establish a straightforward pathway for tuning the thicknesses of colloidal nanosheets while maintaining lateral uniformity.