Controlling the Pore Size of Mesoporous Carbon Thin Films through Thermal and Solvent Annealing

Herein an approach to controlling the pore size of mesoporous carbon thin films from metal‐free polyacrylonitrile‐containing block copolymers is described. A high‐molecular‐weight poly(acrylonitrile‐block‐methyl methacrylate) (PAN‐b‐PMMA) is synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. The authors systematically investigate the self‐assembly behavior of PAN‐b‐PMMA thin films during thermal and solvent annealing, as well as the pore size of mesoporous carbon thin films after pyrolysis. The as‐spin‐coated PAN‐b‐PMMA is microphase‐separated into uniformly spaced globular nanostructures, and these globular nanostructures evolve into various morphologies after thermal or solvent annealing. Surprisingly, through thermal annealing and subsequent pyrolysis of PAN‐b‐PMMA into mesoporous carbon thin films, the pore size and center‐to‐center spacing increase significantly with thermal annealing temperature, different from most block copolymers. In addition, the choice of solvent in solvent annealing strongly influences the block copolymer nanostructure and the pore size of mesoporous carbon thin films. The discoveries herein provide a simple strategy to control the pore size of mesoporous carbon thin films by tuning thermal or solvent annealing conditions, instead of synthesizing a series of block copolymers of various molecular weights and compositions.     

Growth and Properties of Fully Strained SrCoOx (x ≈ 2.8) Thin Films on DyScO3

High quality epitaxial SrCoO_x (oxygen deficient SrCoO₃) thin films were grown on (110) DyScO3 substrates by pulsed laser deposition.The disappearance of half order peaks in X‐ray diffraction as well as the XAS at the O K‐edge indicates an oxygen content of x ≈ 2.8 in the thin films. Magnetization measurements reveal that the specific substrate strain suppresses the ferromagnetism found in the corresponding bulk material and the emergence of an antiferromagnetic‐type spin correlation as predicted by theoretical calculations. Our work demonstrates that the magnetism can be tuned by in‐plane strain in SrCoO_x thin films.     

Epitaxial Growth of Oriented Metalloporphyrin Network Thin Film for Improved Selectivity of Volatile Organic Compounds

This study reports an oriented and homogenous cobalt‐metalloporphyrin network (PIZA‐1) thin film prepared by liquid phase epitaxial (LPE) method. The thickness of the obtained thin films can be well controlled, and their photocurrent properties can also be tuned by LPE cycles or the introduction of conductive guest molecules (tetracyanoquinodimethane and C₆₀) into the PIZA‐1 pores. The study of quartz crystal microbalance adsorption confirms that the PIZA‐1 thin film with [110]‐orientation presents much higher selectivity of benzene over toluene and p‐xylene than that of the PIZA‐1 powder with mixed orientations. These results reveal that the selective adsorption of volatile organic compounds highly depends on the growth orientations of porphyrin‐based metal‐organic framework thin films. Furthermore, the work will provide a new perspective for developing important semiconductive sensing materials with improved selectivity of guest compounds.     

AFM Tip Hammering Nanolithography

Nanolithography at low cost and high speed is made possible by using a vibrating AFM tip in tapping‐mode as a nanohammer to forge polystyrene‐block‐poly(ethylene/butylenes)‐block‐polystyrene triblock copolymer monolayer thin films after annealing to transform their microstructures from as‐cast poorly ordered cylinders into well‐ordered hexagonal spheres. Annealing is accomplished in cyclohexane vapor, a selective solvent for the majority poly(ethylene/butylenes) block. Experimental results demonstrate that such structure‐tailored thin films enable macroscopic AFM tip writing to be performed in their surface; imprinted and embossed patterns can be generated with a sub‐20‐nm line‐width resolution. In addition, it is found that the lithographic patterns generated can be erased within 5 min by thermal annealing at 70 °C, and if necessary the erasion process can be expedited by increasing the annealing temperature.     

Novel Synthesis,Coating, and Networking of Curved Copper Nanowires for Flexible Transparent Conductive Electrodes

In this work, a whole manufacturing process of the curved copper nanowires (CCNs) based flexible transparent conductive electrode (FTCE) is reported with all solution processes, including synthesis, coating, and networking. The CCNs with high purity and good quality are designed and synthesized by a binary polyol coreduction method. In this reaction, volume ratio and reaction time are the significant factors for the successful synthesis. These nanowires have an average 50 nm in width and 25–40 μm range in length with curved structure and high softness. Furthermore, a meniscus‐dragging deposition (MDD) method is used to uniformly coat the well‐dispersed CCNs on the glass or polyethylene terephthalate substrate with a simple process. The optoelectrical property of the CCNs thin films is precisely controlled by applying the MDD method. The FTCE is fabricated by networking of CCNs using solvent‐dipped annealing method with vacuum‐free, transfer‐free, and low‐temperature conditions. To remove the natural oxide layer, the CCNs thin films are reduced by glycerol or NaBH₄ solution at low temperature. As a highly robust FTCE, the CCNs thin film exhibits excellent optoelectrical performance (T = 86.62%, R _s = 99.14 Ω ^?1), flexibility, and durability (R/R ₀ < 1.05 at 2000 bending, 5 mm of bending radius).     

Carbon Nanotubes for Cold Electron Sources

Technological advances in the field of microelectronic fabrication techniques have triggered a great interest in vacuum microelectronics. In contrast to solid‐state microelectronics, which entails scattering‐dominated electron transport in semiconducting solids, vacuum microelectronics relies on the scattering‐free, ballistic motion of electrons in vacuum. Since the first international conference on vacuum microelectronics substantial progress in this field has been made. The first technological devices using micrometer‐sized electron emitting structures are currently being commercialized. Field‐emission flat‐panel displays (FED) seem to be an especially promising competitor to LCD displays. Today there is only one mature technology for producing micro‐gated field‐emission arrays: the Spindt metal‐tip process. The drawbacks of this technology are expensive production, critical lifetime in vacuum, and high operating voltage. Carbon nanotubes (CNT) can be regarded as the potential second‐generation technology to the Spindt metal micro‐tip. In this review we show that the field emission (FE) behavior of CNT can be accurately described by Fowler–Nordheim tunneling and that the field‐enhancement factor β is the most prominent factor. Therefore the FE properties of a CNT thin film can be understood in terms of local field enhancement β(x,y), which can be determined with scanning anode field emission microscopy (SAFEM). To characterize the FE properties of an ensemble of electron emitters we used a statistical approach (as for thin film emitters), where f(β)dβ gives the number of emitters on a unit area with field‐enhancement factors within the interval [β,β + dβ]. We show that the field‐enhancement distribution function f(β) gives an almost complete characterization of the FE properties.     

Graphoepitaxial Directed Self‐Assembly of Polystyrene‐Block‐Polydimethylsiloxane Block Copolymer on Substrates Functionalized with Hexamethyldisilazane to Fabricate Nanoscale Silicon Patterns

In block copolymer (BCP) nanolithography, microphase separated polystyrene‐block‐polydimethylsiloxane (PS‐b‐PDMS) thin films are particularly attractive as they can form small features and the two blocks can be readily differentiated during pattern transfer. However, PS‐b‐PDMS is challenging because the chemical differences in the blocks can result in poor surface‐wetting, poor pattern orientation control and structural instabilities. Usually the interfacial energies at substrate surface are engineered with the use of a hydroxyl‐terminated polydimethylsiloxane (PDMS‐OH) homopolymer brush. Herein, we report a facile, rapid and tuneable molecular functionalization approach using hexamethyldisilazane (HMDS). The work is applied to both planar and topographically patterned substrates and investigation of graphoepitaxial methods for directed self‐assembly and long‐range translational alignment of BCP domains is reported. The hexagonally arranged in‐plane and out‐of‐plane PDMS cylinders structures formed by microphase separation were successfully used as on‐chip etch masks for pattern transfer to the underlying silicon substrate. The molecular approach developed here affords significant advantages when compared to the more usual PDMS‐OH brushes used.     

Dithiopheneindenofluorene (TIF) Semiconducting Polymers with Very High Mobility in Field‐Effect Transistors

The charge‐carrier mobility of organic semiconducting polymers is known to be enhanced when the energetic disorder of the polymer is minimized. Fused, planar aromatic ring structures contribute to reducing the polymer conformational disorder, as demonstrated by polymers containing the indacenodithiophene ( IDT ) repeat unit, which have both a low Urbach energy and a high mobility in thin‐film‐transistor (TFT) devices. Expanding on this design motif, copolymers containing the dithiopheneindenofluorene repeat unit are synthesized, which extends the fused aromatic structure with two additional phenyl rings, further rigidifying the polymer backbone. A range of copolymers are prepared and their electrical properties and thin‐film morphology evaluated, with the co ‐benzothiadiazole polymer having a twofold increase in hole mobility when compared to the IDT analog, reaching values of almost 3 cm² V^?1 s^?1 in bottom‐gate top‐contact organic field‐effect transistors.     

High‐Performance Flexible Photodetectors based on High‐Quality Perovskite Thin Films by a Vapor–Solution Method

Organometal halide perovskites are new light‐harvesting materials for lightweight and flexible optoelectronic devices due to their excellent optoelectronic properties and low‐temperature process capability. However, the preparation of high‐quality perovskite films on flexible substrates has still been a great challenge to date. Here, a novel vapor–solution method is developed to achieve uniform and pinhole‐free organometal halide perovskite films on flexible indium tin oxide/poly(ethylene terephthalate) substrates. Based on the as‐prepared high‐quality perovskite thin films, high‐performance flexible photodetectors (PDs) are constructed, which display a nR value of 81 A W^?1 at a low working voltage of 1 V, three orders higher than that of previously reported flexible perovskite thin‐film PDs. In addition, these flexible PDs exhibit excellent flexural stability and durability under various bending situations with their optoelectronic performance well retained. This breakthrough on the growth of high‐quality perovskite thin films opens up a new avenue to develop high‐performance flexible optoelectronic devices.     

Two Dimensional Nanoarrays of Individual Protein Molecules

Protein molecules on solid surfaces are essential to a number of applications, such as biosensors, biomaterials, and drug delivery. In most approaches for protein immobilization, inter‐molecular distances on the solid surface are not controlled and this may lead to aggregation and crowding. Here, a simple approach to immobilize individual protein molecules in a well‐ordered 2D array is shown, using nanopatterns obtained from a polystyrene‐block‐poly(2‐hydroxyethyl methacrylate) (PS‐b‐PHEMA) diblock copolymer thin film. This water‐stable and protein‐resistant polymer film contains hexagonally ordered PS cylindrical domains in a PHEMA matrix. The PS domains are activated by incorporating alkyne‐functionalized PS and immobilizing azide‐tagged proteins specifically onto each PS domain using “Click” chemistry. The nanometer size of the PS domain dictates that each domain can accommodate no more than one protein molecule, as verified by atomic force microscopy imaging. Immunoassay shows that the amount of specifically bound antibody scales with the number density of individual protein molecules on the 2D nanoarrays.     

Resistive Switching of Individual,Chemically Synthesized TiO2 Nanoparticles

Resistively switching devices are considered promising for next‐generation nonvolatile random‐access memories. Today, such memories are fabricated by means of “top–down approaches” applying thin films sandwiched between nanoscaled electrodes. In contrast, this work presents a “bottom–up approach” disclosing for the first time the resistive switching (RS) of individual TiO₂ nanoparticles (NPs). The NPs, which have sizes of 80 and 350 nm, respectively, are obtained by wet chemical synthesis and thermally treated under oxidizing or vacuum conditions for crystallization, respectively. These NPs are deposited on a Pt/Ir bottom electrode and individual NPs are electrically characterized by means of a nanomanipulator system in situ, in a scanning electron microscope. While amorphous NPs and calcined NPs reveal no switching hysteresis, a very interesting behavior is found for the vacuum‐annealed, crystalline TiO_2–x NPs. These NPs reveal forming‐free RS behavior, dominantly complementary switching (CS) and, to a small degree, bipolar switching (BS) characteristics. In contrast, similarly vacuum‐annealed TiO₂ thin films grown by atomic layer deposition show standard BS behavior under the same conditions. The interesting CS behavior of the TiO_2–x NPs is attributed to the formation of a core–shell‐like structure by re‐oxidation of the reduced NPs as a unique feature.     

Two‐Dimensional Mott Insulators in SrVO3 Ultrathin Films

Strongly correlated oxides that undergo a metal‐insulator transition (MIT) are a subject of great current interest for their potential application to future electronics as switches and sensors. Recent advances in thin film technology have opened up new avenues to tailor MIT for novel devices beyond conventional CMOS scaling. Here, dimensional‐crossover‐driven MITs are demonstrated in high‐quality epitaxial SrVO₃ (SVO) thin films grown by a pulsed electron‐beam deposition technique. Thick SVO films (∼25 nm) exhibit metallic behavior with the electrical resistivity following the T² law corresponding to a Fermi liquid system. A temperature driven MIT is induced in SVO ultrathin films with thicknesses below 6.5 nm. The transition temperature T_MIT is at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and 205 K for the 3 nm film. The emergence of the observed MIT can be attributed to the dimensional crossover from a three‐dimensional metal to a two‐dimensional Mott insulator, as the resulting reduction in the effective bandwidth W opens a band gap at the Fermi level. The magneto‐transport study of the SVO ultrathin films also confirm the observed MIT is due to the electron‐electron interactions other than disorder‐induced localization.     

Templated Sub‐100‐nm‐Thick Double‐Gyroid Structure from Si‐Containing Block Copolymer Thin Films

The directed self‐assembly of diblock copolymer chains (poly(1,1‐dimethyl silacyclobutane)‐block‐polystyrene, PDMSB‐b‐PS) into a thin film double gyroid structure is described. A decrease of the kinetics of a typical double‐wave pattern formation is reported within the 3D‐nanostructure when the film thickness on mesas is lower than the gyroid unit cell. However, optimization of the solvent‐vapor annealing process results in very large grains (over 10 µm²) with specific orientation (i.e., parallel to the air substrate) and direction (i.e., along the groove direction) of the characteristic (211) plane, demonstrated by templating sub‐100‐nm‐thick PDMSB‐b‐PS films.     

Phase Transition and Superconductivity Enhancement in Se‐Substituted MoTe2 Thin Films

Consecutively tailoring few‐layer transition metal dichalcogenides MX₂ from 2H to T_d phase may realize the long‐sought topological superconductivity in a single material system by incorporating superconductivity and the quantum spin Hall effect together. Here, this study demonstrates that a consecutive structural phase transition from T_d to 1T′ to 2H polytype can be realized by increasing the Se concentration in Se‐substituted MoTe₂ thin films. More importantly, the Se‐substitution is found to dramatically enhance the superconductivity of the MoTe₂ thin film, which is interpreted as the introduction of two‐band superconductivity. The chemical‐constituent‐induced phase transition offers a new strategy to study the s^+? superconductivity and the possible topological superconductivity, as well as to develop phase‐sensitive devices based on MX₂ materials.     

Coated and Printed Perovskites for Photovoltaic Applications

Hybrid organic–inorganic metal halide perovskite semiconductors provide opportunities and challenges for the fabrication of low‐cost thin‐film photovoltaic devices. The opportunities are clear: the power conversion efficiency (PCE) of small‐area perovskite photovoltaics has surpassed many established thin‐film technologies. However, the large‐scale solution‐based deposition of perovskite layers introduces challenges. To form perovskite layers, precursor solutions are coated or printed and these must then be crystallized into the perovskite structure. The nucleation and crystal growth must be controlled during film formation and subsequent treatments in order to obtain high‐quality, pin‐hole‐free films over large areas. A great deal of understanding regarding material engineering during the perovskite film formation process has been gained through spin‐coating studies. Based on this, significant progress has been made on transferring material engineering strategies to processes capable of scale‐up, such as blade coating, spray coating, inkjet printing, screen printing, relief printing, and gravure printing. Here, an overview is provided of the strategies that led to devices deposited by these scalable techniques with PCEs as high as 21%. Finally, the opportunities to fully close the shrinking gap to record spin‐coated solar cells and to scale these efficiencies to large areas are highlighted.     

Selective Growth of Metal‐Free Metallic and Semiconducting Single‐Wall Carbon Nanotubes

A major obstacle for the use of single‐wall carbon nanotubes (SWCNTs) in electronic devices is their mixture of different types of electrical conductivity that strongly depends on their helical structure. The existence of metal impurities as a residue of a metallic growth catalyst may also lower the performance of SWCNT‐based devices. Here, it is shown that by using silicon oxide (SiO_x) nanoparticles as a catalyst, metal‐free semiconducting and metallic SWCNTs can be selectively synthesized by the chemical vapor deposition of ethanol. It is found that control over the nanoparticle size and the content of oxygen in the SiO_x catalyst plays a key role in the selective growth of SWCNTs. Furthermore, by using the as‐grown semiconducting and metallic SWCNTs as the channel material and source/drain electrodes, respectively, all‐SWCNT thin‐film transistors are fabricated to demonstrate the remarkable potential of these SWCNTs for electronic devices.     

Cover Picture: High‐Performance Organic Single‐Crystal Transistors on Flexible Substrates (Adv. Mater. 17/2006)

The cover shows a comparison of thin and thick rubrene single crystals where the flexibility of the thin rubrene crystals is clearly illustrated. On p. 2320, Yang, Bao, and co‐workers report that high performance flexible transistors on plastic substrates fabricated by using these rubrene “thin‐film” single‐crystals demonstrate mobility as high as 4.6 cm² Vs^–1 and ON/OFF ratios of approximately 10⁶.     

A locking‐free nine‐DOF triangular plate element with incompatible approximation

In this paper, a simple locking‐free triangular plate element, labeled here as Mindlin‐type triangular plate element with nine degrees of freedom (MTP9), is presented. The element employs an incompatible approximation with nine degrees of freedom (DOFs) independent of the nodes and the shape of the triangle to define the displacements u/v/w(which is similar to a general solid element) along the x/y/z axes. It is free of shear locking, has a proper rank, and provides stable solutions for thick and thin plates. Moreover, the paper provides a new way to develop simple and efficient locking‐free thick–thin‐plate/shell elements. A variety of numerical examples demonstrate the convergence, accuracy, and robustness of the present element MTP9. Copyright © 2016 John Wiley & Sons, Ltd.     

A crack propagation criterion based on ΔCTOD measured with 2D‐digital image correlation technique

The fatigue cracks growth rate of a forged HSLA steel (AISI 4130) was investigated using thin single edge notch tensile specimen to simulate the crack development on a diesel train crankshafts. The effect of load ratio, R, was investigated at room temperature. Fatigue fracture surfaces were examined by scanning electron microscopy. An approach based on the crack tip opening displacement range (ΔCTOD) was proposed as fatigue crack propagation criterion. ΔCTOD measurements were carried out using 2D‐digital image correlation techniques. J‐integral values were estimated using ΔCTOD. Under test conditions investigated, it was found that the use of ΔCTOD as a fatigue crack growth driving force parameter is relevant and could describe the crack propagation behaviour, under different load ratio R.     

Enhanced Field Emission from a Carbon Nanotube Array Coated with a Hexagonal Boron Nitride Thin Film

A high‐quality field emission electron source made of a highly ordered array of carbon nanotubes (CNTs) coated with a thin film of hexagonal boron nitride (h‐BN) is fabricated using a simple and scalable method. This method offers the benefit of reproducibility, as well as the simplicity, safety, and low cost inherent in using B₂O₃ as the boron precursor. Results measured using h‐BN‐coated CNT arrays are compared with uncoated control arrays. The optimal thickness of the h‐BN film is found to be 3 nm. As a result of the incorporation of h‐BN, the turn‐on field is found to decrease from 4.11 to 1.36 V μm^?1, which can be explained by the significantly lower emission barrier that is achieved due to the negative electron affinity of h‐BN. Meanwhile, the total emission current is observed to increase from 1.6 to 3.7 mA, due to a mechanism that limits the self‐current of any individual emitting tip. This phenomenon also leads to improved emission stability and uniformity. In addition, the lifetime of the arrays is improved as well. The h‐BN‐coated CNT array‐based field emitters proposed in this work may open new paths for the development of future high‐performance vacuum electronic devices.