High‐Performance Ferroelectric Memory Based on Phase‐Separated Films of Polymer Blends

High‐performance polymer memory is fabricated using blends of ferroelectric poly(vinylidene‐fluoride‐trifluoroethylene) (P(VDF‐TrFE)) and highly insulating poly(p‐phenylene oxide) (PPO). The blend films spontaneously phase separate into amorphous PPO nanospheres embedded in a semicrystalline P(VDF‐TrFE) matrix. Using low molecular weight PPO with high miscibility in a common solvent, i.e., methyl ethyl ketone, blend films are spin cast with extremely low roughness (R_rms ≈ 4.92 nm) and achieve nanoscale phase seperation (PPO domain size < 200 nm). These blend devices display highly improved ferroelectric and dielectric performance with low dielectric losses (<0.2 up to 1 MHz), enhanced thermal stability (up to ≈353 K), excellent fatigue endurance (80% retention after 10⁶ cycles at 1 KHz) and high dielectric breakdown fields (≈360 MV/m).     

1D versus 2D Growth of Soluble Acene Crystals from Soluble Acene/Polymer Blends Governed by a Residual Solvent Reservoir in a Phase‐Separated Polymer Matrix

The growth mechanism of soluble acene is highly dependent on the remaining residual solvent following solution processing. The relationship between the amount of residual solvent and the growth modes of a prototypical soluble acene, 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene) are examined under spin casting TIPS‐pentacene/insulating polymer blends. Changing spin time of the blend solution allows to control the amount of residual solvent, which significantly determines the growth modes of TIPS‐pentacene vertically segregated onto the insulating polymer. In situ observation of crystal growth reveals that excess residual solvent in short spin time induces a convective flow in a drying droplet, thereby resulting in 1D growth of TIPS‐pentacene crystals. On the other hand, optimal amount of residual solvent in a moderate spin time results in 2D growth of TIPS‐pentacene crystals. The well‐developed 2D spherulites allow for higher field‐effect mobility than that of the 1D crystals because of the higher perfectness and coverage of TIPS‐pentacene crystals. The use of other types of soluble acene and insulating polymer only changes the kinetics of crystallization, while the transition of growth mode from 1D to 2D is still observed. This general growth mechanism facilitates the understanding of crystallization behavior of soluble acene for the development of high‐performance organic transistors.     

Generation of Compositional‐Gradient Structures in Biodegradable,Immiscible, Polymer Blends by Intermolecular Hydrogen‐Bonding Interactions

A biodegradable, immiscible poly(butylenes adipate‐co‐butylenes terephthalate) [P(BA‐co‐BT)]/poly(ethylene oxide) (PEO) polymer blend film with compositional gradient in the film‐thickness direction has been successfully prepared in the presence of a low‐molecular‐weight compound 4,4′‐thiodiphenal (TDP), which is used as a miscibility‐enhancing agent. The miscibilities of the P(BA‐co‐BT)/PEO/TDP ternary blend films and the P(BA‐co‐BT)/PEO/TDP gradient film were investigated by differential scanning calorimetry (DSC). The compositional gradient structure of the P(BA‐co‐BT)/PEO/TDP (46/46/8 w/w/w) film has been confirmed by microscopic mapping measurement of Fourier‐transform infrared spectra and dynamic mechanical thermal analysis. We have developed a new strategy for generating gradient‐phase structures in immiscible polymer‐blend systems by homogenization, i.e., adding a third agent that can enhance the miscibility of the two immiscible polymers through simultaneous formation of hydrogen bonds with two component polymers.     

Processing and Low Voltage Switching of Organic Ferroelectric Phase‐Separated Bistable Diodes

The processing of solution‐based binary blends of the ferroelectric random copolymer poly(vinylidene fluoride‐trifluoroethylene) P(VDF‐TrFE) and the semiconducting polymer poly(9,9‐dioctylfluorenyl‐2,7‐diyl) (PFO) applied by spin‐coating and wire‐bar coating is investigated. By systematic variation of blend composition, solvent, and deposition temperature it is shown that much smoother blend films can be obtained than reported thus far. At a low PFO:P(VDF‐TrFE) ratio the blend film consists of disk‐shaped PFO domains embedded in a P(VDF‐TrFE) matrix, while an inverted structure is obtained in case the P(VDF‐TrFE) is the minority component. The microstructure of the phase separated blend films is self‐affine. From this observation and from the domain size distribution it is concluded that the phase separation occurs via spinodal decomposition, irrespectively of blend ratio. This is explained by the strong incompatibility of the two polymers expressed by the binary phase diagram, as constructed from thermal analysis data. Time resolved numerical simulation of the microstructure evolution during de‐mixing qualitatively shows how an elevated deposition temperature has a smoothening effect as a result of the reduction of the repulsion between the blend components. The small roughness allowed the realization of bistable rectifying diodes that switch at low voltages with a yield of 100%. This indicates that memory characteristics can be tailored from the outset while processing parameters can be adjusted according to the phase behavior of the active components.     

Highly Stretchable and Conductive Polymer Material Made from Poly(3,4‐ethylenedioxythiophene) and Polyurethane Elastomers

A highly elastic and stretchable conductive polymer material resulted from blending the conductive polymer poly(3,4‐ethylenedioxythiophene):p‐tosylate and an aliphatic polyurethane elastomer. The blend inherited advantageous properties from its constituents, namely high conductivity of 120 S cm^–1 from its conductive polymer component, and elastomeric mechanical properties resembling those of the polyurethane, including good adhesion to various substrates. Stretching of the blend material by up to 50 % resulted in increased conductivity, while subsequent relaxation to the unstretched state caused a decrease of conductivity compared to the pristine blend. These initial changes in conductivity were reproducible on further cycling between 50 % stretching and the unstretched state for at least 10 cycles. Stretching beyond 50 % resulted in decreasing conductivity of the blend but with substantial conductivity remaining even when stretched by 200 %. Optical, mechanical, and thermal properties of the blend, as well as high resolution electron microscopy of bulk cross‐sections, suggest that the system is a single phase and not two separate phases. Ageing experiments indicate that the material retains substantial conductivity for at least a few years at room temperature.     

Perpendicularly Aligned,Size‐and Spacing‐Controlled Nanocylinders by Molecular‐Weight Adjustment of a Homopolymer Blended in an Asymmetric Triblock Copolymer

Perpendicularly arrayed and size‐controlled nanocylinders have been prepared by simply blending an asymmetric polystyrene‐block‐polyisoprene‐block‐polystyrene triblock copolymer with polystyrene (the minority component) homopolymers of different molecular weights. The preference for perpendicular orientation or hexagonal ordering of the nanocylinders over a large area in the asymmetric block copolymer can be controlled by adjusting the molecular weight of the blended homopolymer, and the perfection of hexagonal ordering of the perpendicular cylinders can be tuned by using a substrate whose surface tension is much different from that of the majority component of the block copolymer. Such highly controlled nanostructured block‐copolymer materials, which have been obtained by a simple method independent of film thickness and interfacial tension between the blocks and the substrates, have wide‐ranging commercial potential, e.g., for use in membranes and nanotemplates with size‐tunable pores, bandgap‐controlled photonic crystals, and other nanotechnological fields demanding a specific nanosize and nanomorphology.     

Charge Transfer Excitons in Bulk Heterojunctions of a Polyfluorene Copolymer and a Fullerene Derivative

The photophysical properties of blends of fluorene copolymer and the fullerene derivative PCBM are analyzed with a particular attention to photovoltaic applications. The properties of the blends are determined by the relative alignment of the HOMO energy levels. In the blend where the HOMO levels of the copolymer and the fullerene are aligned there is not signature of charge stabilization and photovoltaic effect. While in the blend where there is an offset between the HOMO levels the charge stabilization is accompanied by good photovoltaic performances. The photoluminescence spectrum of the latter blend is characterized by the appearance of a new peak at low energy with a lifetime of a few ns that red‐shifts with the increase of the PCBM percentage. The feature is attributed to the emission from a charge‐transfer exciton that is red‐shifted by the change of dielectric constant of the medium.     

Observation of a Charge Transfer State in Low‐Bandgap Polymer/Fullerene Blend Systems by Photoluminescence and Electroluminescence Studies

The presence of charge transfer states generated by the interaction between the fullerene acceptor PCBM and two alternating copolymers of fluorene with donor–acceptor–donor comonomers are reported; the generation leads to modifications in the polymer bandgap and electronic structure. In one of polymer/fullerene blends, the driving force for photocurrent generation, i.e., the gap between the lowest unoccupied molecular orbitals of the donor and acceptor, is only 0.1 eV, but photocurrent is generated. It is shown that the presence of a charge transfer state is more important than the driving force. The charge transfer states are visible through new emission peaks in the photoluminescence spectra and through electroluminescence at a forward bias. The photoluminescence can be quenched under reverse bias, and can be directly correlated to the mechanism of photocurrent generation. The excited charge transfer state is easily dissociated into free charge carriers, and is an important intermediate state through which free charge carriers are generated.     

Energy Landscape of Vertically Anisotropic Polymer Blend Films toward Highly Efficient Polymer Light‐Emitting Diodes (PLEDs)

A blend of two hole‐dominant polymers is created and used as the light emissive layer in light‐emitting diodes to achieve high luminous efficiency up to 22 cd A^?1. The polymer blend F8_1?xSY_x is based on poly(9,9‐dioctylfluorene) (F8) and poly(para‐phenylene vinylene) derivative superyellow (SY). The blend system exhibits a preferential vertical concentration distribution. The resulting energy landscape modifies the overall charge transport behavior of the blend emissive layer. The large difference between the highest unoccupied molecular orbital levels of F8 (5.8 eV) and SY (5.3 eV) introduces hole traps at SY sites within the F8 polymer matrix. This slows down the hole mobility and facilitates a balance between the transport behavior of both the charge carriers. The balance due to such energy landscape facilitates efficient formation of excitons within the emission zone well away from the cathode and minimizes the surface quenching effects. By bringing the light‐emission zone in the middle of the F8_1?xSY_x film, the bulk of the film is exploited for the light emission. Due to the charge trapping nature of SY molecules in F8 matrix and pushing the emission zone in the center, the radiative recombination rate also increases, resulting in excellent device performance.     

Correlating Emissive Non‐Geminate Charge Recombination with Photocurrent Generation Efficiency in Polymer/Perylene Diimide Organic Photovoltaic Blend Films

Evidence for a correlation between the dynamics of emissive non‐geminate charge recombination within organic photovoltaic (OPV) blend films and the photocurrent generation efficiency of the corresponding blend‐based solar cells is presented. Two model OPV systems that consist of binary blends of electron acceptor N′‐bis(1‐ethylpropyl)‐3,4,9,10‐perylene tetracarboxy diimide (PDI) with either poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) or poly(9,9‐dioctylindenofluorene‐co‐benzothiadiazole) (PIF8BT) as electron donor are studied. For the F8BT:PDI and PIF8BT:PDI devices photocurrent generation efficiency is shown to be related to the PDI crystallinity. In contrast to the F8BT:PDI system, thermal annealing of the PIF8BT:PDI layer at 90 °C has a positive impact on the photocurrent generation efficiency and yields a corresponding increase in PL quenching. The devices of both blends have a strongly reduced photocurrent on higher temperature annealing at 120 °C. Delayed luminescence spectroscopy suggests that the improved efficiency of photocurrent generation for the 90 °C annealed PIF8BT:PDI layer is a result of optimized transport of the photogenerated charge‐carriers as well as of enhanced PL quenching due to the maintenance of optimized polymer/PDI interfaces. The studies propose that charge transport in the blend films can be indirectly monitored from the recombination dynamics of free carriers that cause the delayed luminescence. For the F8BT:PDI and PIF8BT:PDI blend films these dynamics are best described by a power‐law decay function and are found to be temperature dependent.     

Photoconductivity of a Low‐Bandgap Conjugated Polymer

The photoconductive properties of a novel low‐bandgap conjugated polymer, poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole)], PCPDTBT, with an optical energy gap of E_g ～ 1.5 eV, have been studied. The results of photoluminescence and photoconductivity measurements indicate efficient electron transfer from PCPDTBT to PCBM ([6,6]‐phenyl‐C61 butyric acid methyl ester, a fullerene derivative), where PCPDTBT acts as the electron donor and PCBM as the electron acceptor. Electron‐transfer facilitates charge separation and results in prolonged carrier lifetime, as observed by fast (t > 100 ps) transient photoconductivity measurements. The photoresponsivities of PCPDTBT and PCPDTBT:PCBM are comparable to those of poly(3‐hexylthiophene), P3HT, and P3HT:PCBM, respectively. Moreover, the spectral sensitivity of PCPDTBT:PCBM extends significantly deeper into the infrared, to 900 nm, than that of P3HT. The potential of PCPDTBT as a material for high‐efficiency polymer solar cells is discussed.     

Photophysical Properties of a Series of Poly(ladder‐type phenylene)s

The photophysical properties, i.e., the fluorescence and phosphorescence of a series of blue light‐emitting poly(ladder‐type phenylene)s have been investigated employing continuous‐wave (cw) and time‐resolved photoluminescence (PL) spectroscopy in solid state and dilute solution. The chemically well‐defined polymers vary from two to five bridged phenyl‐rings per monomer unit bearing aryl‐ or alkyl‐substitution at the bridge‐head carbon atoms. It has been found that the fluorescence energy of the polymers and of the corresponding monomers deviates from a simple 1/N dependence, if the number N of bridged‐phenylene rings is increased beyond a certain limit. Time‐resolved fluorescence spectroscopy on thin films showed that apart from the blue fluorescence of the polymers an additional lower energy emission feature exists, which cannot be assigned to keto‐defects and which seems to be an inherent solid state property of this class of materials. Delayed time‐resolved photoluminescence spectroscopy allowed the detection of phosphorescence energies and lifetimes for all investigated polymers. Photoinduced absorption spectroscopy on thin films showed that the triplet‐triplet absorption red‐shifts with increasing monomer length but reaches a constant value for polymers with N ≥ 4. Amplification of light via amplified spontaneous emission (ASE) from thin film slab waveguide structures could be demonstrated for all ladder‐type polymers but the onset threshold value for ASE varies significantly with the polymer structure.     

Photoluminescence and Electroluminescence from Tris(8‐ hydroxyquinoline)aluminum Nanowires Prepared by Adsorbent‐Assisted Physical Vapor Deposition

Single‐crystalline nanowires are successfully prepared from a small organic functional molecule, tris(8‐hydroxyquinoline)aluminum (Alq₃), by an adsorbent‐assisted physical‐vapor‐deposition method. The introduction of adsorbents can decrease the sublimation temperature of Alq₃, and slow the weight‐loss process markedly, which is proven to be indispensable in improving the uniformity of the as‐prepared Alq₃ nanowires. Measurements of the optical properties reveal that the absorption spectra of the Alq₃ nanowires show an obvious blue‐shift with decreasing diameter. The photoluminescence vibrational fine structure emerges and becomes pronounced with increasing excitation energy, which is attributed to the ordered orientation of the Alq₃ molecules in the nanowires. Furthermore, the Alq₃ nanowires are fabricated into an electroluminescent device, which has an obvious size‐dependent performance.     

Near‐Field Lithography by Two‐Photon Induced Photocleavage of Organic Monolayers

We prove that the enhanced electromagnetic near‐field around metallic nanostructures can be used for localized two‐photon induced activation of surfaces, obtaining a defined chemical pattern with nanometric resolution. Gold nanoparticles (Au‐NP) are deposited on glass slides that were modified with a polysiloxane layer containing a nitroveratrylcarbonyl (NVoc) photoremovable group. Upon illumination with a femtosecond laser, the NVoc entity is removed. Due to the electromagnetic field enhancement of the nanoparticles, the threshold of this process is lowered in the nm‐scale vicinity of the metal structures. Upon cleavage, an amine functional group is released, which can be used to site‐selectively bind species with complementary chemical functionality on the surface. This method can be utilized for sub‐wavelength chemical structuring.     

Synthesis and Room‐Temperature Ultraviolet Photoluminescence Properties of Zirconia Nanowires

This paper reports the synthesis of tetragonal zirconia nanowires using template method. An as‐prepared sample was characterized by scanning and transmission electron microscopy. It was found that the as‐prepared materials were tetragonal zirconia nanowires with average diameters of ca. 80 nm and length of over 10 μm. The Raman spectrum showed peaks at 120, 461, and 629 cm^–1, which are attributed to the E_g, E_g, and B_1g phonon modes of the tetragonal zirconia structure, respectively. The UV‐vis absorption spectrum showed an absorption peak at 232.5 nm (5.33 eV in photon energy). Photoluminescence (PL) spectra of zirconia nanowires showed a strong emission peak at ca. 388 nm at room temperature, which is attributed to the ionized oxygen vacancy in the zirconia nanowires system.     

Facile Preparation of a Patterned,Aminated Polymer Surface by UV‐Light‐Induced Surface Aminolysis

Introducing amine functional groups on polymer surfaces is extremely important for studying various processes that involve polymer surfaces. We report a novel and extremely simple method for preparing a tertiary‐amine‐terminated poly(ethylene terephthalate) (PET) surface by using a UV‐light‐induced surface aminolysis reaction. X‐ray photoelectron spectroscopy and attenuated total‐reflection infrared spectroscopy give direct evidence of the incorporation of tertiary amine functionalities and the possible reaction mechanism behind this technique. Tertiary amines are easily protonated, so we have developed an extremely simple method for immobilizing and patterning biomolecules on a soft surface by the electrostatic self‐assembly of proteins, such as immunoglobulin (IgG) and horseradish peroxide (HRP), onto a patterned, aminated surface. An enzyme–substrate reaction, which is followed optically by observing the resulting precipitation on the surface, is used to reveal the patterned immobilization of HRP, where 3‐amino‐9‐ethylcarbazole, as a substrate for HRP, is deposited on the aminated surface after HRP adsorption. Fluorescein isothiocyanate‐labeled IgG (FITC‐IgG) has been immobilized electrostatically onto the ordered aminated spots, and the fluorescence intensity ratio of the IgG‐immobilized region (inside the spot) to the background (outside the spot) is about 5:1, as calculated from a fluorescence image and fluorescence spectra obtained by microlaser confocal Raman spectroscopy. We have found that the background intensity is mainly caused by the autofluorescence of virgin PET, and after subtracting this value from the measured intensity inside and outside the spot, respectively, a much higher intensity ratio between the spot and the background is obtained (about 22:1). The patterned immobilization of FITC‐IgG has been further proven by examining the change in intensity inside the spot after photobleaching the fluorophore.     

Preparation of Near‐IR Fluorescent Nanoparticles for Fluorescence‐Anisotropy‐Based Immunoagglutination Assay in Whole Blood

A class of novel core/shell near‐IR fluorescent nanoparticles have been prepared through co‐hydrolysis of a hydrophobic silicon alkoxide, hexadecyltrimethoxysilane, and tetraethyl orthosilicate as the dye‐doped core, followed by the formation of a hydrophilic shell via hydrolysis of tetraethyl orthosilicate in a water‐in‐oil microemulsion. The co‐hydrolysis of hexadecyltrimethoxysilane and tetraethyl orthosilicate produces a highly hydrophobic core for the entrapment of a low‐cost near‐IR fluorescence dye, methylene blue. Experimental investigation of this particular core/shell nanoparticle in comparison with conventional dye‐doped silica nanoparticles demonstrates that the hydrophobic core enables the doped dye to exhibit enhanced fluorescence and show improved stability to dye leaching and exogenous quenchers. In contrast to rhodamine B doped silica nanoparticles, the near‐IR fluorescent nanoparticles also show negligible background fluorescence and low inner‐filtration interference in complex biological systems such as whole blood. This advantage is utilized for the development of an immunoagglutination assay method based on fluorescence‐anisotropy measurement for the detection of alpha fetoprotein (AFP) in whole‐blood samples. The results reveal that increase in fluorescence anisotropy is linearly correlated with AFP concentration in the range 1.9–51.9 ng mL^–1.     

Aligned Polythiophene and its Blend Film by Direct‐Writing for Anisotropic Charge Transport

A combination of patterning and film alignment techniques helps to build multi‐order polymer architecture for application in flexible electronics. A direct‐writing method is employed using microcapillary arrays to prepare semiconducting polymer films with both optical and electrical anisotropy. Not only aligned poly(3‐butylthiophene) (P3BT) nanowires in neat P3BT films, but also aligned P3BT nanowires within a polystyrene (PS) matrix are obtained, which yields an aligned semiconductor/insulator polymer blend with anisotropic charge transport. The field‐effect transistor (FET) mobilities/threshold voltages from both vertical and parallel to alignment directions as well as their dependence on blending ratio are studied. The increased mobility of P3BT/PS blends, as compared with neat P3BT, is observed in both vertical and parallel directions. Using this alignment method, FET mobility and threshold voltage of the semiconductor/insulator polymer blends are comprehensively tuned, from which a digital inverter with gain up to 80 is realized. Therefore, this work not only helps understanding the charge transport mechanism in semiconducting/insulating polymer blends, but also provides an effective approach towards high‐performance field‐effect transistors with tunable mobility and threshold voltage.     

Organic Field‐Effect Devices as Tool to Characterize the Bipolar Transport in Polymer‐Fullerene Blends: The Case of P3HT‐PCBM

In organic bulk heterojunction solar cells (oBHJ) the blend morphology in combination with the charge transport properties of the individual components controls the extracted photocurrent. The organic field‐effect transistor (OFET) has been proved as a powerful instrument to evaluate the unipolar carrier transport properties in a wide range of cases. In our work we extend the OFET concept to the evaluation of the bipolar transport properties in polymer‐fullerenes blends and propose a method to improve the accuracy of the evaluation. The method is based on capacitance–voltage (C–V) measurements on MOS structures prepared on the same blends and delivers complementary information on the bulk heterojunction to the one obtained with FETs. The relevance for photovoltaic applications is investigated through the correlation between the current–voltage behavior of solar cells and the bipolar mobility for composites with varying polymer molecular weight and processed from different solvents. In particular the transport features of solar cells produced from o‐Xylene (oX), a non chlorinated solvent more suitable to production requirements, have been compared to the one of devices cast from Chlorobenzene (CB) solution. For the P3HT‐PCBM blend a consistent correlation between the mobility and the electrical fill factor and power performance was found. A significant asymmetry in the bipolar carrier mobility, together with low electron mobility dependent on the M_w value, affects the performances of thick o‐Xylene cast devices. In the case of devices processed from Chlorobenzene the slower carrier has higher mobility and the small electrical losses detected are eventually more related to the formation of space‐charge and eventually to surface recombination. This results in an efficient charge collection that is almost thickness independent. We report a dependence of the slow‐carrier type (electrons or holes) and their mobility on the specific combination of molecular weight and solvent. The mobility data and the solar cell performance coherently fit to the prediction of a device model only based on the drift of carriers under the built‐in electric field originated in the donor‐acceptor oBHJ.