Polymer Nanocrystals Research Articles

Hybrid Solar Cells with Bulk Heterojunction Between Conjugated Polymer and PbS Nanocrystals

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Synthesis and Characterization of Copper (I) Chloride (CuCl) Nanocrystals in Conductive Polymer for UV Light Emitters

AbstractIntrinsic γ-Copper (I) Chloride is an ionic I-VII compound semiconductor material with relatively low conductivity. To fabricate an efficient electroluminescent device based on CuCl nanocrystals (NC) the conductivity of the CuCl NC film should be relatively high. In order to improve the conductivity of CuCl films, nanocrystals were embedded in a highly conductive polymer (Polyaniline) and deposited on glass substrates via the spin-coating method. The deposited films were heated at 140°C for durations between 1 and 12 hours in vacuo. The room temperature UV-Vis absorption spectra for all CuCl films showed both Z1,2 and Z3 excitonic absorption features and the absorption intensity increased as the anneal time increased. Room temperature photoluminescence (PL) measurements of the hybrid films reveal very intense Z3 excitonic emission. Room temperature X-ray diffraction (XRD) confirmed the preferential growth of CuCl nanocrystals whose average size is ≈40 nm in the <111> orientation. Resistivity measurements were carried out using a four-point probe system, which confirmed that the resistivity of the composite film was ≈500 Ω/cm. This is an improvement when compared to the vacuum evaporated CuCl thin films.

Investigation of annealing effects on microstructure of hybrid nanocrystal–polymer solar cells by impedance spectroscopy

We have fabricated hybrid bulk heterojunction solar cells with blends of MEH-PPV (poly[2-methoxy,5-(2-ethylhexoxy)-1,4-phenylene vinylene]) and nano-CdS as electron donor and electron acceptor, respectively. The effect of thermal annealing was investigated with impedance spectroscopy (IS) and capacitance–voltage ( C– V) characteristic measurements on devices. The IS and C– V characteristic manifested very different before and after thermal annealing. We performed equivalent circuit to explain the effect of thermal annealing. The thermal treatment of nanocrystal–polymer films is seen to aid in the formation of a continuous network for electron transport between nanorods, and hence improves devices performance. The method based on the IS is available to probe the microstructure of hybrid bulk heterojunction solar cells before and after thermal annealing, and therefore detect the mechanism for the annealing improvements.

Superstructures of PbS nanocrystals in a conjugated polymer and the aligning role ofoxidation

We present a method to directly align PbS nanocrystals in micron-sized superstructureswithin a conjugated polymer. First, lead sulfide nanocrystals are directly synthesized in aMEH-PPV suspension via a single pot, surfactant-free method. Post-synthesis precipitationof the composite solution involving mild oxidation of the nanocrystals results in theformation of nanocrystal–polymer and nanocrystal-oxide superstructures. Detailed TEM isused to study the crystallographic nature of these structures and the roles of polymer andlead sulfate. An epitaxial relationship between lead sulfide and lead sulfate at thenanoscale is shown, giving insight into the oxidation rates of the PbS nanocrystals’facets.

Efficient solution-processed hybrid polymer–nanocrystal near infrared light-emitting devices

We report solution-processed hybrid LEDs, emitting infrared light with enhanced efficiency, based on PbSe nanocrystal (NC) lumophores embedded a conjugated polymer forming the nanocomposite active layer. The effect of device geometry and the NC-polymer blend composition on device performance is studied in detail. Increasing the PbSe NC concentration leads to a substantial decrease in device current, and an increase in efficiency and turn-on voltage. Analysis of the device data indicates that the PbSe NCs act as carrier traps and radiative recombination centers. With optimized concentration of NC emitters and active layer thickness, the devices exhibit an external electroluminescence quantum efficiency of 0.83% at a peak emission wavelength of 1280 nm.

Paramagnetic Point Defects and Charge Carriers in PbS and CdS Nanocrystal Polymer Composites

Using electron paramagnetic resonance we identify point defects in CdS nanocrystals. A spectrum consistent with Cd vacancies in distorted sites is observed, and evidence for S vacancies is presente...

Electroluminescence of a single active layer polymer–nanocrystal hybrid light-emittingdiode with inversion symmetry

A hybrid polymer–nanocrystal (NC) light-emitting diode (LED) device with asingle active layer structure is simply fabricated by a spin coating. From ahigh-resolution transmission electron microscopy (HRTEM) study, each PVKpolymer particle is observed to be capped with TPBi molecules and CdSe/ZnSNCs are mainly distributed along the circumference of PVK and TPBi surfaces,resulting in a core–shell polymer–NC hybrid of [CdSe/ZnS]/TPBi/[CdSe/ZnS]/PVK.An Al/[CdSe/ZnS]/TPBi/[CdSe/ZnS]/PVK/indium–tin oxide(ITO)/glass LEDshows electroluminescence (EL) centered at around 585 nm at the forward bias of+10 V, which clearly reveals that CdSe/ZnS NCs existing at the interface between PVK andTPBi act as recombination centers for excitons. In particular, EL can be observed at bothforward bias and reverse bias, and this means that this device with an isotropic distributionof NCs has an inversion symmetry.

Energy Transfer versus Charge Separation in Type-II Hybrid Organic−Inorganic Nanocomposites

Hybrid organic-inorganic nanomaterials have the potential of providing synergetic properties. Blends of semiconductor nanocrystals and conjugated polymers in particular promise novel optoelectronic properties. Effective design of tailored optoelectronic properties requires a deep understanding of the photophysics of these composite materials, which includes charge separation and Dexter and Förster energy transfer. We performed a detailed and quantitative spectroscopic investigation of a type II aligned hybrid system consisting of a blue emitting conducting polymer and CdTe nanocrystals. Although charge separation is expected from the type II alignment, we find a dominant (70% efficiency) energy transfer process. We discuss all possible de-excitation pathways for the excitons in terms of the alignment of energy levels, time scales, and physical geometry of the system. This allows us to conclude that energy transfer occurs via the Förster mechanism and provides a clear guideline for the design of novel hybrid materials.

Electrophoretic Deposition of Star Polymer-Europium Chalcogenide Nanocomposite Films

The electrophoretic deposition of polystyrene/divinylbenzene (PS/DVB) star polymer-europium sulphide (EuS) nanocomposite films from a colloidal suspension is reported. Liquid suspension, containing both the PS/DVB star polymer and EuS nanocrystals were prepared by separately injecting dichloromethane (DCM) based solutions of EuS nanocrystals and of the star polymers, respectively, into a stratified liquid combination of hexane and DCM. Scanning electron microscopy illustrates images of the dependence of surface morphology on nanocrystal concentration of the PS/DVB-EuS star polymer film. These polymer-encased nanocrystal films may be a more practical option for the fabrication of magneto-optical thin film devices, such as optical switches, optical isolators, and optical memories.

Encapsulation of π-Conjugated Polymer Nanocrystals and Their Ordered Array Structure toward Photonic Nanomaterials

Polydiacetylene (PDA) nanocrystals were successfully encapsulated by soap-free copolymerization of styrene monomer and hydrophilic styrene derivative. The resulting polystyrene-encapsulated PDA nanocrystals became almost uniform and spherical and could be arranged conveniently on a patterned substrate using the tapered cell method. These basic processes to fabricate ordered array structure on a substrate are very important toward photonic devices application made up of PDA nanocrystals in the near future. In addition, the formation mechanisms for encapsulation were discussed in detail.

Polymer Crystallization Confined in Hard Spherical Microdomains of Diblock Copolymers

We report dynamic Monte Carlo simulations of crystallization under hard confinement of spherical diblock copolymers. We compared the parallel results of blocks with short chains made by breaking block junctions. We observed that blocks crystallize faster but harvest lower crystallinity than short chains, which is in agreement with recent experiments. We found that the fast crystallization can be assigned to the slightly stretched blocks, and the lowered crystallinity is mainly attributed to the frozen block junctions near interfaces. In addition, homogeneous crystal nucleation at low temperatures switches Avrami indexes of both cases from one (first-order kinetics) to 0.5 and meanwhile ruins uniform crystal orientations inside each microdomain. Our present results may facilitate our better understanding of the fabrication of polymer nanocrystals through self-assembly of diblock copolymers.

Polymer crystallization under nano-confinement of droplets studied by molecular simulations

Fabrication of polymer nano-crystals proceeds usually through hierarchical ordering of the different-scale structures. Nano-scale patterns are produced first, which serve as a spatial template for subsequent polymer crystallization under nano-confinement. We begin with a survey of the effects of nano-confinement on polymer crystallization, mainly on the basis of the knowledge obtained from molecular simulations. After that, we report dynamic Monte Carlo simulations of polymer crystallization confined in nano-droplets. We observed that the shape of droplets on a solid substrate appears as a pancake, and both initiation and development of crystallization are depressed with the decrease of droplet size. Surface-induced crystal nucleation guides the dominant edge-on crystal orientation at high temperatures; however, its contribution to nucleation rates is not much greater than crystal nucleation in the volume of the droplet. At low temperatures, edge-on crystals are frequent at both substrate/polymer and polymer/air interfaces. In conclusion, molecular simulations can shed light on the microscopic mechanisms of polymer crystallization under nano-confinement.

Effect of surface passivating ligand on structural and optoelectronic properties of polymer : CdSe quantum dot composites

We demonstrate the effect of surface passivation of cadmium selenide quantum dots (CdSe QDs) (∼5–7 nm) by tri-n-octylphosphene-oxide (TOPO) and oleic acid (OA) on the structural and optoelectronic properties of their respective polymer : CdSe composites by dispersing them in poly(2-methoxy-5(2-ethylhexyloxy)-1,4-phenylinevinylene) and poly(3-hexylthiophene) polymers. It has been found that OA passivated-QDs (∼7 nm), as compared with TOPO passivated CdSe QDs (∼5 nm), are of (i) high quality that provide better steric stability against coagulation, homogeneity and photostability to their respective polymer : CdSe composites, (ii) show low value of Stern–Volmer quenching constant (KSV) calculated from photoluminescence quenching measurements. These effects have been attributed to (i) CdSe(OA) (∼7 nm) particles having relatively smaller surface energies compared with CdSe(TOPO) (∼5 nm) particles thus showing lesser quenching capabilities (ii) dominance of respective processes of photoinduced Förster energy transfer between host polymer (donors) and guest CdSe nanocrystals (acceptors) in polymer : CdSe(OA) composites and charge transfer in polymer : CdSe(TOPO) composites.

Preparation and Characterization of Poly(vinylpyrrolidone)/Zirconium Oxide Hybrids by Using Inorganic Nanocrystals

Hybridization of amphiphilic polymer and inorganic nanocrystal was examined aiming for improvement of thermal and optical properties of the organic polymer used. Optically transparent and homogeneous hybrids of poly(vinylpyrrolidone) (PVP) and zirconium oxide nanocrystals (ZrO2-NCs) were prepared through a water/methanol solution. Hydrogen bonding interactions between amide groups of PVP and Zr-OH groups from ZrO2-NC were confirmed in the hybrids obtained. Thermal properties of the hybrids were improved with an increase of the ZrO2-NC content. In this research, ZrO2–NCs in PVP matrix showed fine dispersion state. As expected, refractive index of the hybrids increased with ZrO2-NC concentration and the refractive index variations follow the Lorentz–Lorenz relation.

Ceria-Supported Vinylpyridine Polymers: Synthesis, Characterization and Application in Catalysis

A simple, one-step synthesis of 4-vinylpyridine polymer crosslinked with 2% divinylbenzene [poly(4vp-dvb)] anchored with ceria nanocrystals via in situ reactions that involve simultaneous formation of the polymer and ceria nanocrystals is presented. The polymer support is formed by free radical suspension polymerization of the monomers, while ceria nanocrystals are generated from CeCl3·7H2O by the homogeneous precipitation method in a single pot. The polymer supports were characterized by XRD, TEM, SEM, ESCA, and TGA and then tested for their ability as heterogeneous recyclable catalysts in a Biginelli reaction. The catalytic activities of these supports were found to be far superior to the two-step conventional one and simultaneous microporous bead counterparts. Reusability is an added advantage over the other regularly used cerium-based catalysts in aqueous media. The significant increase in the catalytic activity is due to the higher Ce/N ratio and predominance of well-defined and more reactive exposed planes of ceria nanocrystals on the new supports. Such catalysts are of great importance due to environmental, practical and economical concerns.

Crystalline Polymer Ultrathin Films from Mesoscopic Precursors

Thin and ultrathin crystalline polymer films are of funda mental scientific as well as practical interest concerning their structures and properties. 6] For the preparation of ultrathin ( 0.1 mm) films the polymer is applied to the substrate in a non ordered state, usually as a dilute solution in an organic solvent. To overcome the intra and intermolecular interac tions responsible for crystalline order in the solid polymer, and render it soluble, high temperatures are often required. Crystalline order only forms on the substrate during solvent removal or cooling. As a different approach to ultrathin films of crystalline polymers we report on their preparation at room temperature from aqueous dispersions of prefabricated polymer nanocrystals functioning as building blocks (Scheme 1). The very small particle size, in combination with the basic phenomenon that the amorphous regions are located at the surface exclusively in polymer single crystals, results in an efficient interaction between particles in films.

Liquid/Liquid Interfacial Polymerization To Grow Single Crystalline Nanoneedles of Various Conducting Polymers

Single crystalline nanoneedles of polyaniline (PANI) and polypyrrole (PPY) were synthesized using an interfacial polymerization for the first time. The interfacial crystallization of conductive polymers at the liquid/liquid interface allowed PANI and PPY polymers to form single crystalline nanocrystals in a rice-like shape in the dimensions of 63 nm x 12 nm for PANI and 70 nm x 20 nm for PPY. Those crystalline nanoneedles displayed a fast conductance switching in the time scale of milliseconds. An important growth condition necessary to yield highly crystalline conductive polymers was the extended crystallization time at the liquid/liquid interfaces to increase the degree of crystallization. As compared to other interfacial polymerization methods, lower concentrations of monomer and oxidant solutions were employed to further extend the crystallization time. While other interfacial growth of conducting polymers yielded noncrystalline polymer fibers, our interfacial method produced single crystalline nanocrystals of conductive polymers. We recently reported the liquid/liquid interfacial synthesis of conducting PEDOT nanocrystals; however, this liquid/liquid interfacial method needs to be extended to other conductive polymer nanocrystal syntheses in order to demonstrate that our technique could be applied as the general fabrication procedure for the single crystalline conducting polymer growth. In this report, we showed that the liquid/liquid interfacial crystallization could yield PANI nanocrystals and PPY nanocrystals, other important conductive polymers, in addition to PEDOT nanocrystals. The resulting crystalline polymers have a fast conductance switching time between the insulating and conducting states on the order of milliseconds. This technique will be useful to synthesize conducting polymers via oxidative coupling processes in a single crystal state, which is extremely difficult to achieve by other synthetic methods.

Hybrid Solar Cells from a Blend of Poly(3‐hexylthiophene) and Ligand‐Capped TiO2 Nanorods

AbstractHybrid bulk heterojunction solar cells based on nanocrystalline TiO2 (nc‐TiO2) nanorods capped with trioctylphosphine oxide (TOPO) and regioregular poly(3‐hexylthiophene) (P3HT) are processed from solution and characterized in order to relate the device function (optical absorption, charge separation, and transport and photovoltaic properties) to active‐layer properties and device parameters. Annealing the blend films is found to greatly improve the polymer–metal oxide interaction at the nc‐TiO2/P3HT interface, resulting in a six‐fold increase of the charge separation yield and improved photovoltaic device performance under simulated solar illumination. In addition, the influence of the organic ligand at the nc‐TiO2 particle surface is found to be crucial for charge separation. Ligand‐exchange procedures applied on the TOPO‐capped nc‐TiO2 nanorods with an amphiphilic ruthenium‐based dye are found to further improve the charge‐separation yield at the polymer–nanocrystal interface. However, the poor photocurrents generated in the hybrid blend devices, before and after ligand exchange, suggest that transport within or between nanoparticles limits performance. By comparison with other donor–acceptor bulk heterojunction systems, we conclude that charge transport in the nc‐TiO2:P3HT blend films is limited by the presence of an intrinsic trap distribution mainly associated with the nc‐TiO2 particles.

Effect of Au Nanocrystals Embedded in Conductive Polymer on Non-volatile Memory Window

AbstractRecently, non-volatile polymer memories have been researched as a next generation of non-volatile memory because of its simple structure and easy fabrication process. We found that two types of non-volatile polymer memory have different I-V behavior. First Polymer non-volatile memory with metal / oxide / polymer / metal structure But Polymer non-volatile memory embedded Au Nano-crystal shows different I-V behavior. Polymer non-volatile memory shows NDR(Negative Differential Resistance) Region after threshold voltage and low to high current path at increasing positive and negative bias. We can observe NDR(Negative Differential Resistance) Region on Polymer non-volatile memory embedded Au Nano crystal. We fabricated devices three different type to confirm difference Polymer non-volatile memory with metal / polymer / metal structure, metal / oxide / polymer / metal structure and Au nano-crystal embedded Polymer non-volatile memory. First we fabricated Polymer non-volatile memory with metal / PVK(Poly-n-vinyl carbarzole) / metal structure. first type of device shows ohmic I-V behavior. Second type of polymer non-volatile memory has oxide layer between metal and polymer layer. Oxide layer made by O2 plasma treatment(100W RF power, 100SCCM O2 gas flow) after metal layer deposited. Second type of device has same structure as first device except oxide layer. Second type of device shows I-V behavior similar to Resistive Memory. Resistive non-volatile memory shows low to high current path at increasing positive bias and high to low current path at increasing negative bias. I-V behaviors of second device due to effect of oxide layer between metal and polymer layer. Third type of polymer non-volatile memory we embed Au nano-crystal layer in polymer layer. Au nano-crystal layer embedded by curing process. We deposit 5nm Au layer after spin coated PVK(Poly-n-vinyl carbarzole) layer and curing at 300¡É. We can observe NDR(Negative Differential Resistance) Region and different I-V behaviors with other type of device. Finally we fabricated polymer non-volatile memory embedded au nano-crystal by dispersion method to confirm effect of au nano-crystal. We report difference I-V behaviors polymer non-volatile memory with metal / polymer / metal structure and polymer non-volatile memory embedded au nano-crystals

Variable morphology of PTFE-like polymer nanocrystals fabricated by oriented plasma polymerization at atmospheric pressure

Rapid formation of PTFE-like polymer nanocrystals was achieved by a novel synthesis method—oriented plasma polymerization (OPP) at atmospheric pressure. The entire process was completed within a short period of time ranging between a few seconds to several minutes through dielectric barrier discharge (DBD) at atmospheric pressure. The surface morphology of the coated organic crystal film was observed through scanning electronic microscope (SEM) and different morphology nanocrystals were found such as nanorods and nanotubes. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed the single-crystalline phase of these nanocrystals. The sizes of the nanosingle crystals were from 10 nm to 1 μm. The effects of discharge conditions such as discharge time, ratio of the monomer to carrier gas and power on the nanocrystalline morphology and crystallinity were investigated. As a result, the physical morphology and structures could be controlled through the conditions of the oriented plasma polymerization to some extent. This novel polymerization method opened a new way to nanofabricate polymeric crystallines fast and effectively.