Phase‐Separation‐Induced Micropatterned Polymer Surfaces and Their Applications

We report a route to fabricate micropatterned polymer films with micro‐ or nanometer‐scale surface concavities by spreading polymer solutions on a non‐solvent surface. The route is simple, versatile, highly efficient, low‐cost, and easily accessible. The concavity density of the patterned films is tuned from 10⁶ to 10⁹ features cm^–2, and the concavity size is controlled in the range from several micrometers to less than 100 nm, by changing the film‐forming parameters including the polymer concentration, the temperature of the non‐solvent and the interactions between polymer, solvent, and non‐solvent. We further demonstrate that these concavity‐patterned films have significantly enhanced hydrophobicity, owing to the existence of the surface concavities, and their hydrophobicity could be controlled by the concavity density. These films have been used as templates to successfully fabricate convex‐patterned polymer films, inorganic TiO₂ microparticles, and NaCl nanocrystals. Their other potential applications are also discussed.     

Polymer Thin‐Film Transistor Arrays Patterned by Stamping

We present a process to stamp semiconductor polymers suitable for the parallel fabrication of thin‐film transistor island arrays. This process is compatible with roll‐to‐roll fabrication. When a chemically treated elastomeric stamp is pressed against a substrate previously coated with the polymer solution, a capillary force drives the polymer solution into the stamp recesses. Simultaneously, the raised features of the stamp in contact with the substrate absorb the solvent. The resulting polymer thin film reproduces the pattern of the raised features of the stamp. Features with lateral dimensions as small as 2 μm are faithfully reproduced. We use this stamping process to fabricate arrays of polymer thin‐film transistors (TFTs) using poly(fluorene‐co‐bithiophene) and poly(thiophene) semiconductors.     

UV‐Light‐Driven Immobilization of Surface‐Functionalized Oxide Nanocrystals onto Silicon

TiO₂ nanorods (NRs) and γ‐Fe₂O₃ nanocrystals (NCs) passivated with unsaturated long‐chain carboxylic acids, namely 10‐undecylenic acid (10UDA) and oleic acid (OLEA), are covalently anchored to Si(100) at room temperature by UV‐light‐driven reaction of hydrogenated silicon with the carbon–carbon double bond (–C?C–) moieties of the capping surfactants. The high reactivity of vinyl groups towards Si provides a general tool for attaching particles of both materials via Si–C bonds. Interestingly, TiO₂ NRs were efficiently attached to silicon even when capped by OLEA. This latter finding has been explained by a photocatalytic mechanism involving the primary role of hydroxyl radicals that can be generated upon bandgap TiO₂ photoexcitation with UV light. The increased oxide coverage achievable on Si opens access to further surface manipulation, as demonstrated by the possibility of depositing an additional film of Au nanoparticles onto TiO₂ via TiO₂‐catalyzed visible‐light‐driven reduction of aqueous AuCl₄^– ions. Extensive morphological and chemical characterization of the obtained NC‐functionalized Si substrates is provided to support the effectiveness of proposed photochemical approaches.     

Tuning the Electroluminescence Color in Polymer Light‐Emitting Devices Using the Thiol‐Ene Photoreaction

Laterally patterning the emission color of polymer light‐emitting devices is one of the current technological challenges for their commercialization. Photopatterning is one of the most convenient approaches for the production of closely spaced, differently colored pixels, other than direct‐writing approaches such as ink‐jet printing. The photo‐induced addition of thiols to C?C bonds (the thiol‐ene) reaction is here utilized to achieve photobleaching of poly(phenylenevinylene)‐type polymers. By exploiting the chemical selectivity of the UV‐induced thiol‐ene reaction, the emission color in a guest–host system is tuned from that of the guest to that of the host. It is demonstrated that the presented method can be applied for numerous energy‐transfer systems.     

Low‐Duty‐Ratio Patterning on a Heterogeneous Surface

A thick film of patterned heterogeneous surface is introduced, in which the surface heterogeneity arises due to a difference in the characteristics of interfacial potential. When a liquid film is coated onto such a surface, the film is unstable only on certain parts of the surface and, consequently, dewetting takes place. In the other parts, the film is stable and no dewetting takes place. This heterogeneous surface is utilized for patterning low‐duty‐ratio patterns, which has proved challenging for any mold‐based unconventional lithographies. Only the natural forces associated with capillarity and dewetting are responsible for the patterning. As a result, defect‐free and uniform patterning is possible over a large area.     

Functional Hydrogel Surfaces: Binding Kinesin‐Based Molecular Motor Proteins to Selected Patterned Sites

Hydrogel microstructures with micrometer‐scale topography and controllable functionality have great potential for numerous nanobiotechnology applications including, for example, three‐dimensional structures that exhibit controlled interactions with proteins and cells. Taking advantage of the strong affinity of histidine (His) residues for metal‐ion–nitrilotriacetic acid (NTA) complexes, we have chemically modified hydrogels to enable protein immobilization with retention of activity by incorporating 2‐methacrylamidobutyl nitrilotriacetic acid, an NTA‐containing monomer that can be copolymerized with a series of monomers to form NTA‐containing hydrogels. By varying the NTA‐monomer composition in the hydrogels, it is possible to control the amount of protein bound to the hydrogel surface. The retention of biological activity was demonstrated by microtubule gliding assays. Normally, hydrogels are resistant to protein binding, but we have selected these materials because of their porous nature. Bringing together hydrogel functionalization and soft‐lithography patterning techniques, it was possible to create a hybrid hydrogel superstructure that possesses binding specificity to His‐tagged protein in selected sites. This type of surface and microstructure is not only advantageous for motor protein integration, but it can also be generally applied to the formation of His‐tagged molecules for sensors and biochip applications.     

Lossless,Passive Transportation of Low Surface Tension Liquids Induced by Patterned Omniphobic Liquidlike Polymer Brushes

Surfaces enabling directional liquid transportation are of great interest for a wide range of applications such as water collection, microfluidics, and heat transfer systems. Surfaces capable of lossless, long-range passive transportation of low surface tension (LST) liquids using wettability patterned, liquidlike coatings with minimal contact angle hysteresis are reported. Lossless LST droplet travel distances over 150 mm are achieved, enabled by a two-phase transportation mechanism: morphological transformation from a bulge to a channel shape, followed by directional transportation along the asymmetrical wedge-shaped channel. The developed surfaces can split, merge, and precisely transport various low-surface tension liquids, including alcohols, alkanes, and solvents. The developed transportation strategy can also enhance LST liquid dropwise condensation through continuous removal of the condensate, even on horizontally positioned surfaces without the assistance of gravity.     

Fabrication of Robust Biomolecular Patterns by Reactive Microcontact Printing on N‐Hydroxysuccinimide Ester‐Containing Polymer Films

The fabrication of robust biomolecule microarrays by reactive microcontact printing (μCP) on spin‐coated thin films of poly(N‐hydroxysuccinimidyl methacrylate) (PNHSMA) on oxidized silicon and glass is described. The approach combines the advantages of activated polymer thin films as coupling layers, characterized by high reactivity and high molecular loading, with the versatility and flexibility of soft lithography. The transfer of amino end‐functionalized poly(ethylene glycol) (PEG) from oxidized poly(dimethylsiloxane) elastomer stamps to PNHSMA films is shown by Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, fluorescence microscopy, and ellipsometry measurements to result in covalent coupling and identical grafting densities, as found previously for coupling from solution. The PEG‐protected areas effectively inhibit the adsorption of fluoresceinamine, bovine serum albumin, as well as 25‐mer DNA, while the unreacted N‐hydroxysuccinimidyl methacrylate ester groups retain their reactivity towards primary amino groups. Biomolecule microarrays have been thus conveniently fabricated in a two‐step procedure. The hybridization of target DNA to immobilized probe DNA in micropatterns proves the concept of reactive μCP on activated polymer films for obtaining robust platforms for biomolecule immobilization and screening.     

Tuning the Emitting Color of Organic Light‐Emitting Diodes Through Photochemically Induced Transformations: Towards Single‐Layer,Patterned, Full‐Color Displays and White‐Lighting Applications

Photochemically induced emission tuning for the definition of pixels emitting the three primary colors, red, green, blue (RGB), in a single conducting polymeric layer is investigated. The approach proposed is based on an acid‐induced emission shift of the (1‐[4‐(dimethylamino)phenyl]‐6‐phenylhexatriene) (DMA‐DPH) green emitter and acid‐induced quenching of the red fluorescent emitter (4‐dimethylamino‐4′‐nitrostilbene) (DANS). The two emitters are dispersed in the wide bandgap conducting polymer poly(9‐vinylcarbazole) (PVK), along with a photoacid generator (PAG). In the unexposed film areas, red emission is observed because of efficient energy transfer from PVK and DMA‐DPH to DANS. Exposure of selected areas of the film at different doses results in quenching of the red emitter's fluorescence and the formation of green, blue, or even other color‐emitting pixels, depending on the exposure dose and the relative concentrations of the different compounds in the film. Organic light‐emitting diodes having the PVK polymer containing the appropriate amounts of DMA‐DPH, DANS, and PAG as the emitting layer are fabricated and electroluminescence spectra are recorded. The time stability of induced emission spectrum changes and the color stability during device operation are also examined, and the first encouraging results are obtained.     

High Performance Polymer Light‐Emitting Diodes Fabricated by a Low Temperature Lamination Process

We report on the successful demonstration of high performance polymer light‐emitting diodes (PLEDs) using a low temperature, plastic lamination process. Blue‐ and red‐emitting PLEDs were fabricated by laminating different luminescent polymers and organic compounds together to form the active media. This unique approach eliminates the issue of organic solvent compatibility with the organic layers for fabricating multi‐layer PLEDs. In addition, a template activated surface process (TAS) has been successfully applied to generate an optimum interface for the low temperature lamination process. Atomic force microscopy analysis reveals a distinct difference in the surfaces created by the TAS and the spin‐coating process. This observation coupled with the device data confirms the importance of the activated interface in the lamination process.     

Internal Field Screening in Polymer Light‐Emitting Diodes

We use electromodulation spectroscopy and modeling studies to probe the electric‐field distribution in polyfluorene‐based polymer light‐emitting diodes containing poly(3,4‐ethylenedioxythiophene) poly(styrene sulfonate). The bulk internal field is shown to be zero under ordinary operating conditions, with trapped electrons close to the anode fully screening the bulk semiconductor from the external field. The effect has far‐reaching implications for the understanding and optimization of organic devices.     

Highly Efficient Pure‐White‐Light‐Emitting Diodes from a Single Polymer: Polyfluorene with Naphthalimide Moieties

Light‐emitting diodes exhibiting efficient pure‐white‐light electroluminescence have been successfully developed by using a single polymer: polyfluorene derivatives with 1,8‐naphthalimide chromophores chemically doped onto the polyfluorene backbones. By adjusting the emission wavelength of the 1,8‐naphthalimide components and optimizing the relative content of 1,8‐naphthalimide derivatives in the resulting polymers, white‐light electroluminescence from a single polymer, as opposed to a polymer blend, has been obtained in a device with a configuration of indium tin oxide/poly(3,4‐ethylenedioxythiophene)(50 nm)/polymer(80 nm)/Ca(10 nm)/Al(100 nm). The device exhibits Commission Internationale de l'Eclairage coordinates of (0.32,0.36), a maximum brightness of 11 900 cd m^–2, a current efficiency of 3.8 cd A^–1, a power efficiency of 2.0 lm W^–1, an external quantum efficiency of 1.50 %, and quite stable color coordinates at different driving voltages, even at high luminances of over 5000 cd m^–2.     

Preparation of Self‐Organized Mesoscale Polymer Patterns on a Solid Substrate: Continuous Pattern Formation from a Receding Meniscus

Regular polymer patterns are formed from casting a dilute polymer solution on a solid substrate. Dissipative structures, e.g., convection patterns, fingering instabilities, and so on, are formed in the evaporation process of casting polymer films. Controlled production and manufacturing of patterned polymer films can be achieved when the evaporating solution edge, especially the meniscus region on the casting substrate, is formed under controlled casting conditions. In this report, we describe a computer‐controlled apparatus which has two precisely manipulated sliding glass plates. A narrow, thin liquid film of polymer solution with a receding meniscus is continuously supplied from a small gap between two glass plates (one sliding and the other stationary), and a patterned polymer film is subsequently formed on the stationary substrate from the evaporating solution edge. Several types of polymer patterns from various polymers are reproducibly prepared by changing preparation conditions such as sliding speed and polymer concentration.     

UV‐vis‐Induced Vitrification of a Molecular Crystal

A charge‐transfer complex of 2,5‐dimethyl‐N,N′‐dicyanoquinonediimine (DM) with silver (crystalline Ag(DM)₂, defined as α) is irreversibly transformed by UV‐vis illumination. Depending on the illumination conditions, three new types of solids (defined as γ, δ, and ?) with different structural and physical properties are obtained and examined by a variety of analytical techniques, including solid‐state, high‐resolution, cross‐polarization magic angle spinning (CP‐MAS) ¹³C NMR, elemental analysis (EA), mass spectrometry (MS), X‐ray absorption fine structure (XAFS), and powder X‐ray diffraction (XRD). The CP‐MAS, EA, MS, and XAFS results indicate that compound γ is a glass state of Ag(DM)₂. The transformation from crystalline (α) to amorphous (γ) solid Ag(DM)₂ is an irreversible exothermic glass transition (glass‐transition temperature 155.2 °C; ΔH = –126.8 kJ mol^–1), which implies that the glass form is thermodynamically more stable than the crystalline form. Compound δ (Ag(DM)_1.5) consists of silver nanoparticles (diameter (7 ± 2) nm ) dispersed in a glassy matrix of neutral DM molecules. The ?N–CN–Ag coordination bonds of the α form are not maintained in the δ form. Decomposition of α by intense illumination results in a white solid (?), identified as being composed of silver nanoparticles (diameter (60 ± 10) nm). Physical and spectroscopic (XAFS) measurements, together with XRD analysis, indicate that the silver nanoparticles in both δ and ? are crystalline with lattice parameters similar to bulk silver; however, the magnetic susceptibilities differ from bulk silver.     

Doping‐Induced Charge Trapping in Organic Light‐Emitting Devices

By using pyran‐containing donor–acceptor dyes as doping molecules in organic light‐emitting devices (OLEDs), we scrutinize the effects of charge trapping and polarization induced by the guest molecules in the electro‐active host material. Laser dyes 4‐(dicyanomethylene)‐2‐methyl‐6‐[2‐(julolidin‐9‐yl)phenyl]ethenyl]‐4H‐pyran (DCM2) and the novel 4‐(dicyanomethylene)‐2‐methyl‐6‐{2‐[(4‐diphenylamino)phenyl]ethenyl}‐4H‐pyran (DCM‐TPA) are used as model compounds. The emission color of these polar dyes depends strongly on doping concentration, which we have attributed to polarization effects induced by the doping molecules themselves. Their frontier orbital energy levels are situated within the bandgap of the tris(8‐hydroxyquinoline)aluminum (Alq₃) host matrix and allow the investigation of either electron trapping or both electron and hole trapping. In the case of DCM‐TPA doping, we were able to show that electron trapping leads to a partial shift of the recombination zone out of the doped Alq₃ region. To impede charge‐recombination processes taking place in the undoped host matrix, a charge‐blocking layer efficiently confines the recombination zone inside the doped zone and gives rise to increased luminous efficiency. For a doping concentration of 1 wt.‐% we obtain a maximum luminous efficiency of 10.4 cd A^–1. At this doping concentration, the yellow emission spectrum shows excellent color saturation with CIE chromaticity coordinates x, y of 0.49 and 0.50, respectively. In the case of DCM2 the recombination zone is much less affected for the same doping concentrations, which is ascribed to the fact that both electrons and holes are being trapped. The experimental findings are corroborated with a numerical simulation of the doped multilayer devices.     

Electron‐Enhanced Hole Injection in Blue Polyfluorene‐Based Polymer Light‐Emitting Diodes

It has recently been reported that, after electrical conditioning, an ohmic hole contact is formed in poly(9,9‐dioctylfluorene) (PFO)‐based polymer light‐emitting diodes (PLED), despite the large hole‐injection barrier obtained with a poly(styrene sulfonic acid)‐doped poly(3,4‐ethylenedioxythiophene) (PEDOT:PSS) anode. We demonstrate that the initial current at low voltages in a PEDOT:PSS/PFO‐based PLED is electron dominated. The voltage at which the hole injection is enhanced strongly depends on the electron‐transport properties of the device, which can be modified by the replacement of reactive end groups by monomers in the synthesis. Our measurements reveal that the switching voltage of the PLED is governed by the electron concentration at the PEDOT:PSS/PFO contact. The switching effect in PFO is only observed for a PEDOT:PSS hole contact and not for other anodes such as indium tin oxide or Ag.     

A Blue‐Light‐Emitting Polymer with a Rigid Backbone for Enhanced Color Stability

Despite the promising expectations of poly(fluorene) (PF)‐type materials as efficient blue‐light‐emitting polymers, the devices based on these materials are not yet fully utilized. Under prolonged operation of the devices, the PF‐type materials undergo degradation with the appearance of a long‐range emission around 2.2–2.3 eV. As a consequence, the emissive color changes from blue to green with a decrease in the device efficiency. Here, an innovative approach that leads to a new blue‐emitting polymer with remarkable color stability is reported. By modifying the chemical structure of PF to inhibit the formation of keto defects, it is demonstrated that the devices exhibit excellent color stability. This new blue‐emitting polymer, poly(2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta‐[def]phenanthrene)) (PCPP), emits a stabilized, efficient blue electroluminescence without exhibiting any peak in the long‐wavelength region even after prolonged operation of the devices in air.     

Manipulating the Motion of Gold Aggregates Using Stimulus‐Responsive Patterned Polymer Brushes as a Motor

An important goal and major challenge of material science and nanotechnology is building nanomotors for manipulating the motion of nanoparticles (NPs). Here, it is demonstrated that patterned, stimulus‐responsive polymer brush microstructures can be used as motor arrays to manipulate the movement of gold NP aggregates in response to external stimuli that induce a conformational change in the brushes as the driving force. The motion of NP aggregates in the out‐of‐plane direction is achieved with displacements ranging from nanometers to sub‐micrometers. These patterned polymer‐brush microstructures can find applications as efficient motor arrays and nanosensors, and benefit the design of more complex nanodevices.     

Investigation on the Low Luminous Efficiency in a Polymer Light‐Emitting Diode with a High Work‐function Cathode by Soft Contact Lamination**

This article reports the main origin of the low luminescent efficiency in hole‐dominant polymer light‐emitting diodes by controlling the hole injection and by chemically modifying the cathode by molecular monolayers. Since molecular modification of the top electrode is impossible when one deposits the electrode using a vacuum deposition method, this study was performed using a soft contact lamination technique to form electrical contacts on top of the emissive layer. The top electrode was chemically modified with an alkane thiol self‐assembled monolayer (SAM) to act as an interfacial spacer layer between the emitting layer and the cathode. Herein, it is reported that, contrary to common belief, a high device quantum efficiency can be achieved from the dominantly hole‐transporting device with a high work‐function cathode (like Au) by facilitating more hole injection from the anode in the device with low population of exciton quenching channels near the cathode.