Molecularly Coupled Two‐Dimensional Titanium Oxide and Carbide Sheets for Wearable and High‐Rate Quasi‐Solid‐State Rechargeable Batteries

High‐performance, flexible, and lightweight powering electrodes are urgently needed to meet the increasing interest in deformable electronic devices, particularly those utilizing solid‐state electrolytes and performing at high charging rates, which unfortunately have remained a formidable challenge. Here, by regularly stacking two‐dimensional (2D) titanium oxide and carbide sheets, in which the two kinds of sheets are coupled at the molecular level, a self‐standing electrode is achieved with ideal mechanical durability and excellent electrochemical performance, including superb rate performance (delivering a capacity of 114 mAh g^?1 in 3.4 min) and good cycling stability (remaining >93% after 1000 cycles at 1000 mA g^?1). Profiting from these advantages, a flexible and safe full lithium‐ion battery, employing a poly(ethylene glycol) diamine‐based gel polymer as the electrolyte, possesses an excellent power density of 1412 W kg^?1 while maintaining a high energy density of 59 Wh kg^?1, which outperforms most documented flexible batteries that utilize liquid electrolytes and is even comparable with some cells using coin configurations. Importantly, the performance was well maintained under mechanical deformation and after multiple breaking and self‐healing cycles, demonstrating the feasibility for practical application in wearable powering devices. The results highlight the numerous possibilities for utilizing sheet materials to fabricate wearable electrode materials.     

Structural Determinants of the Sign of the Pyroelectric Effect in Quasi‐Amorphous SrTiO3 Films

The magnitude and direction of the permanent electric polarization in the non‐crystalline, polar phase (termed quasi‐amorphous) of SrTiO₃ in Si\SiO₂\Me\SrTiO₃\Me, (Me = Cr or W), Si\SrRuO₃\SrTiO₃, and Si\SrTiO₃ layered structures were investigated. Three potential sources of the polarization which appears after the material is pulled through a temperature gradient were considered: a) contact potential difference; b) a flexoelectric effect due to a strain gradient caused by substrate curvature; and c) a flexoelectric effect due to the thermally induced strain gradient that develops while pulling through the steep temperature gradient. Measurements show that options a) and b) can be eliminated from consideration. In most cases studied in this (Si\SrTiO₃, Si\SiO₂\Me\SrTiO₃\Me, M = Cr or W) and previous works (Si\BaTiO₃, Si\BaZrO₃), the top surface of the quasi‐amorphous phase acquires a negative charge upon heating. However, in Si\SrRuO₃\SrTiO₃ structures the top surface acquires a positive charge upon heating. On the basis of the difference in the measured expansion of the upper and lower surfaces of the SrTiO₃ layer in the presence and absence of SrRuO₃, we contend that the magnitude and direction of the pyroelectric effect are determined by the out‐of‐plane gradient of the in‐plane strain in the SrTiO₃ layer while pulling through the temperature gradient.     

Catalytic Twist‐Spun Yarns of Nitrogen‐Doped Carbon Nanotubes

The treatment of free‐standing sheets of multiwalled carbon nanotubes (MWNTs) with a NH₃/He plasma results in self‐supporting sheets of aligned N‐doped MWNTs (CN_x). These CN_x sheets can be easily twist spun in the solid state to provide strong CN_x yarns that are knottable, weavable, and sewable. The CN_x yarns exhibit tunable catalytic activity for electrochemically driven oxygen reduction reactions (ORR), as well as specific capacitances (up to 39 F·g^?1) that are 2.6 times higher than for the parent MWNTs. Due to a high degree of nanotube alignment, the CN_x yarns exhibit specific strengths (451 ± 61 MPa·cm³·g^?1) that are three times larger than observed for hybrid CN_x/MWNT biscrolled yarns containing 70 wt.% CN_x in the form of a powder. This difference in mechanical strength arises from substantial differences in yarn morphology, revealed by electron microscopy imaging of yarn cross‐ sections, as well as the absence of a significant strength contribution from CN_x nanotubes in the biscrolled yarns. Finally, the chemical nature and abundance of the incorporated nitrogen within the CN_x nanotubes is studied as function of plasma exposure and annealing processes using X‐ray photoelectron spectroscopy and correlated with catalytic activity.     

Polymer Template Synthesis of Flexible BaTiO3 Crystal Nanofibers

BaTiO₃ crystals are attractive materials due to their high dielectric properties, but they are brittle and inelastic ceramics, which limits their broader applications in emerging fields, such as flexible electronics. A scalable strategy for the fabrication of ultra‐flexible crystalline BaTiO₃ nanofiber (NF) films by a sol–gel electrospinning method, followed by a brief calcination, is reported. It facilitates the formation of perovskite BaTiO₃ crystals with intricate grain boundaries at a low temperatures by growing them within polymer NF templates. The ceramic films have a polymer‐like softness of 50 mN, a large Young's modulus of 61 MPa, and an elastic strain of 0.9%. Moreover, they have a low density of 28 mg cm^?3 and demonstrate superior softness without fracture after deformation. Piezoelectric sensors fabricated based on these films exhibit a high sensitivity of 80 ms with an output voltage of 1.05 V at a pressure of 100 kPa. This approach allows for the large‐scale fabrication of flexible BaTiO₃ crystal NF films.     

Epitaxy of Single‐Crystalline GaN Film on CMOS‐Compatible Si(100) Substrate Buffered by Graphene

Fabricating single‐crystalline gallium nitride (GaN)‐based devices on a Si(100) substrate, which is compatible with the mainstream complementary metal‐oxide‐semiconductor circuits, is a prerequisite for next‐generation high‐performance electronics and optoelectronics. However, the direct epitaxy of single‐crystalline GaN on a Si(100) substrate remains challenging due to the asymmetric surface domains of Si(100), which can lead to polycrystalline GaN with a two‐domain structure. Here, by utilizing single‐crystalline graphene as a buffer layer, the epitaxy of a single‐crystalline GaN film on a Si(100) substrate is demonstrated. The in situ treatment of graphene with NH₃ can generate sp³ C? N bonds, which then triggers the nucleation of nitrides. The one‐atom‐thick single‐crystalline graphene provides an in‐plane driving force to align all GaN domains to form a single crystal. The nucleation mechanisms and domain evolutions are further clarified by surface science exploration and first‐principle calculations. This work lays the foundation for the integration of GaN‐based devices into Si‐based integrated circuits and also broadens the choice for the epitaxy of nitrides on unconventional amorphous or flexible substrates.     

Atomically Defined Rare‐Earth Scandate Crystal Surfaces

The fabrication of well‐defined, atomically sharp substrate surfaces over a wide range of lattice parameters is reported, which is crucial for atomically regulated epitaxial growth of complex oxide heterostructures. By applying a framework for controlled selective wet etching of complex oxides on the stable rare‐earth scandates (REScO₃), a_pseudocubic = 0.394 – 0.404 nm, the large chemical sensitivity of REScO₃ to basic solutions is exploited, which results in reproducible, single‐terminated surfaces. Time‐of‐flight mass‐spectroscopy measurements show that after wet etching the surfaces are predominantly ScO₂ ‐terminated. Moreover, the morphology study of SrRuO₃ thin‐film growth gives no evidence for mixed termination. Therefore, it is concluded that the REScO₃ surfaces are completely ScO₂ ‐terminated.     

Photoferroelectric Thin Films for Flexible Systems by a Three‐in‐One Solution‐Based Approach

The effective incorporation of (multi)functional oxides into next‐generation flexible electronics systems requires novel fabrication technologies that enable the direct integration of crystalline oxide layers in them. Unfortunately, this is considerably challenging due to the thermal incompatibility between the crystallization temperatures of metal oxides (>600 °C) and the thermal stability of the flexible polymer substrates conventionally used (<400 °C). Here, it is shown that BiFeO₃ thin films can be grown on flexible plastic by solution processing involving three different but complementary strategies to induce the crystallization of the perovskite phase at a lower temperature limit of 325 °C. This “three‐in‐one” approach is based on the synthesis of tailored metal precursors i) with a molecular structure resembling the crystalline structure of the oxide phase, which additionally allows both ii) photochemical and iii) internal combustion reactions taking place in the thin films. The flexible BiFeO₃ thin films obtained from a specifically designed molecular complex with N‐methyldiethanolamine yield a large remnant polarization of 17.5 µC cm^?2, also showing photovoltaic and photocatalytic effects. This result paves the way for the direct integration of an interesting class of oxides with photoferroelectric properties in flexible devices with multiple applications in information and communication technology, and energy.     

Control of Structural Distortions in Transition‐Metal Oxide Films through Oxygen Displacement at the Heterointerface

Structural distortions in the oxygen octahedral network in transition‐metal oxides play crucial roles in yielding a broad spectrum of functional properties, and precise control of such distortions is a key for developing future oxide‐based electronics. Here, it is shown that the displacement of apical oxygen atom shared between the octahedra at the heterointerface is a determining parameter for these distortions and consequently for control of structural and electronic phases of a strained oxide film. The present analysis by complementary annular dark‐ and bright‐field imaging in aberration‐corrected scanning transmission electron microscopy reveals that structural phase differences in strained monoclinic and tetragonal SrRuO₃ films grown on GdScO₃ substrates result from relaxation of the octahedral tilt, associated with changes in the in‐plane displacement of the apical oxygen atom at the heterointerface. It is further demonstrated that octahedral distortions and magnetrotransport properties of the SrRuO₃ films can be controlled by interface engineering of the oxygen displacement. This provides a further degree of freedom for manipulating structural and electronic properties in strained films, allowing the design of novel oxide‐based heterostructures.     

High‐Rate Continuous Synthesis of Nanocrystalline Perovskites and Metal Oxides in a Colliding Vapor Stream of Microdroplets

A high‐rate, continuous synthesis of functional nanomaterials using a home engineered reactor is reported. The reactor is able to produce low‐cost, kilogram‐scale BaTiO₃ nanopowders with a nanocrystalline particle size less than 30 nm at mild temperatures (<100 °C) and ambient pressure. Nebulization and collision of warm microdroplets (60–80 °C) of Ba(OH)₂ and Ti(O‐nBu)₄ very quickly result in total hydrolysis and subsequent conversion to BaTiO₃, yielding 1.3 kg/day of high purity, highly crystalline nanoparticles (25–30 nm). This synthesis procedure also enables high‐rate production of TiO₂ anatase (2.9 kg/day). It therefore provides a general platform for processing and scaling up of functional inorganic nanomaterials under very mild conditions.     

Self‐Assembly of π‐Conjugated Amphiphiles: Free Standing,Ordered Sheets with Enhanced Mobility

Oligo(p‐phenylenevinylenes) (OPVs) with amphiphilic character are synthesized and their self‐assembly characteristics studied. Careful studies point at two morphologically different states of assemblies, with one being two dimensional sheets and the other as rolled tubes. This is also the first time that self‐assembled sheets are achieved for OPVs. Morphological and photo‐physical studies reveal a unique aggregate to aggregate transition between rolled tubes and two dimensional sheets, which is outlined as a more thermodynamic aggregate. The thermodynamic aggregate (2D sheet) is better ordered and consists of chromophores that are better excitonically coupled. The mobilities of these aggregates are also studied for a field effect transistor device and as expected sheets supersede rolled tubes by a couple of orders. More interestingly, the mobility values obtained for the well ordered chromophores in sheets is three orders higher than any other self‐assembled OPV previously reported. It is hypothesized that the better π interactions enforced by the amphiphilic design and the resultant supramolecular organization is a prime factor for such a remarkable rise in mobilities.     

Self‐Assembly of Flexible Free‐Standing 3D Porous MoS2‐Reduced Graphene Oxide Structure for High‐Performance Lithium‐Ion Batteries

Flexible freestanding electrodes are highly desired to realize wearable/flexible batteries as required for the design and production of flexible electronic devices. Here, the excellent electrochemical performance and inherent flexibility of atomically thin 2D MoS₂ along with the self‐assembly properties of liquid crystalline graphene oxide (LCGO) dispersion are exploited to fabricate a porous anode for high‐performance lithium ion batteries. Flexible, free‐standing MoS₂–reduced graphene oxide (MG) film with a 3D porous structure is fabricated via a facile spontaneous self‐assembly process and subsequent freeze‐drying. This is the first report of a one‐pot self‐assembly, gelation, and subsequent reduction of MoS₂/LCGO composite to form a flexible, high performance electrode for charge storage. The gelation process occurs directly in the mixed dispersion of MoS₂ and LCGO nanosheets at a low temperature (70 °C) and normal atmosphere (1 atm). The MG film with 75 wt% of MoS₂ exhibits a high reversible capacity of 800 mAh g^?1 at a current density of 100 mA g^?1. It also demonstrates excellent rate capability, and excellent cycling stability with no capacity drop over 500 charge/discharge cycles at a current density of 400 mA g^?1.     

Single‐Crystalline p‐Type Zn3As2 Nanowires for Field‐Effect Transistors and Visible‐Light Photodetectors on Rigid and Flexible Substrates

Zn₃As₂ is an important p‐type semiconductor with the merit of high effective mobility. The synthesis of single‐crystalline Zn₃As₂ nanowires (NWs) via a simple chemical vapor deposition method is reported. High‐performance single Zn₃As₂ NW field‐effect transistors (FETs) on rigid SiO₂/Si substrates and visible‐light photodetectors on rigid and flexible substrates are fabricated and studied. As‐fabricated single‐NW FETs exhibit typical p‐type transistor characteristics with the features of high mobility (305.5 cm² V^?1 s^?1) and a high I_on/I_off ratio (10⁵). Single‐NW photodetectors on SiO₂/Si substrate show good sensitivity to visible light. Using the contact printing process, large‐scale ordered Zn₃As₂ NW arrays are successfully assembled on SiO₂/Si substrate to prepare NW thin‐film transistors and photodetectors. The NW‐array photodetectors on rigid SiO₂/Si substrate and flexible PET substrate exhibit enhanced optoelectronic performance compared with the single‐NW devices. The results reveal that the p‐type Zn₃As₂ NWs have important applications in future electronic and optoelectronic devices.     

Mesoporous Co3O4 and CoO@C Topotactically Transformed from Chrysanthemum‐like Co(CO3)0.5(OH)·0.11H2O and Their Lithium‐Storage Properties

In this work, a novel hydrothermal route is developed to synthesize cobalt carbonate hydroxide, Co(CO₃)_0.5(OH)·0.11H₂O. In this method, sodium chloride salt is utilized to organize single‐crystalline nanowires into a chrysanthemum‐like hierarchical assembly. The morphological evolution process of this organized product is investigated by examining different reaction intermediates during the synthesis. The growth and thus the final assembly of the Co(CO₃)_0.5(OH)·0.11H₂O can be finely tuned by selecting preparative parameters, such as the molar ratio of the starting chemicals, the additives, the reaction time and the temperature. Using the flower‐like Co(CO₃)_0.5(OH)·0.11H₂O as a solid precursor, quasi‐single‐crystalline mesoporous Co₃O₄ nanowire arrays are prepared via thermal decomposition in air. Furthermore, carbon can be added onto the spinel oxide by a chemical‐vapor‐deposition method using acetylene, which leads to the generation of carbon‐sheathed CoO nanowire arrays (CoO@C). Through comparing and analyzing the crystal structures, the resultant products and their high crystallinity can be explained by a sequential topotactic transformation of the respective precursors. The electrochemical performances of the typical cobalt oxide products are also evaluated. It is demonstrated that tuning of the surface texture and the pore size of the Co₃O₄ products is very important in lithium‐ion‐battery applications. The carbon‐decorated CoO nanowire arrays exhibit an excellent cyclic performance with nearly 100% capacity retention in a testing range of 70 cycles. Therefore, this CoO@C nanocomposite can be considered to be an attractive candidate as an anode material for further investigation.     

Energetics of Donor‐Doping,Metal Vacancies,and Oxygen‐Loss in A‐Site Rare‐Earth‐Doped BaTiO3

The energetics of La‐doping in BaTiO₃ are reported for both (electronic) donor‐doping with the creation of Ti³⁺ cations and ionic doping with the creation of Ti vacancies. The experiments (for samples prepared in air) and simulations demonstrate that ionic doping is the preferred mechanism for all concentrations of La‐doping. The apparent disagreement with electrical conduction of these ionic doped samples is explained by subsequent oxygen‐loss, which leads to the creation of Ti³⁺ cations. Simulations show that oxygen‐loss is much more favorable in the ionic‐doped system than undoped BaTiO₃ due to the unique local structure created around the defect site. These findings resolve the so‐called “donor‐doping” anomaly in BaTiO₃ and explain the source of semiconductivity in positive temperature coefficient of resistance (PTCR) BaTiO₃ thermistors.     

Nanoscale Refractive Index Tuning of Siloxane‐Based Self‐Assembled Electro‐Optic Superlattices

The refractive indices of self‐assembled organic electro‐optic superlattices can be tuned by intercalating high‐Z optically transparent group 13 metal oxide sheets into the structures during the self‐assembly process. Microstructurally regular acentricity and sizable electro‐optic responses are retained in this straightforward synthetic procedure. This “one‐pot” all wet‐chemistry approach involves: i) layer‐by‐layer covalent self‐assembly of intrinsically acentric multilayers of high‐hyperpolarizability chromophores on inorganic oxide substrates, ii) protecting group cleavage to generate a large density of reactive surface hydroxyl sites, iii) self‐limiting capping of each chromophore layer with octachlorotrisiloxane, iv) deposition of metal oxide sheets derived from THF solutions of Ga(OⁱC₃H₇)₃ or In(OⁱC₃H₇)₃, and v) covalent capping of the resulting superlattices.     

Textile‐Based Flexible Tactile Force Sensor Sheet

The next generation of electronics will include human‐interactive flexible sensor sheets to monitor health. One approach is to realize practical macroscale low‐cost sensor arrays to monitor pressure distribution and health conditions without directly attaching a device onto the body. However, practical requirements such as reliability, scalability, and washability are not often discussed as most studies focus on the sensing sensitivity and validations. This study demonstrates an all textile‐based tactile force sensor sheet that covers the above requirements. By considering the device design and materials, high reliability/repeatability (≈250 000 cycles at ≈5 kPa) and washability are realized. These are important factors for practical applications for human‐interactive macroscale sensor sheets. In addition to the fundamental characteristics, pressure distribution mapping and respiration rate monitoring are confirmed by placing the sensor sheets on a bed, chair, and floor.     

Work‐Function Engineering of Graphene Anode by Bis(trifluoromethanesulfonyl)amide Doping for Efficient Polymer Light‐Emitting Diodes

Graphene has been considered to be a potential alternative transparent and flexible electrode for replacing commercially available indium tin oxide (ITO) anode. However, the relatively high sheet resistance and low work function of graphene compared with ITO limit the application of graphene as an anode for organic or polymer light‐emitting diodes (OLEDs or PLEDs). Here, flexible PLEDs made by using bis(trifluoromethanesulfonyl)amide (TFSA, [CF₃SO₂]₂NH) doped graphene anodes are demonstrated to have low sheet resistance and high work function. The graphene is easily doped with TFSA by means of a simple spin‐coating process. After TFSA doping, the sheet resistance of the TFSA‐doped five‐layer graphene, with optical transmittance of ≈88%, is as low as ≈90 Ω sq^?1. The maximum current efficiency and power efficiency of the PLED fabricated on the TFSA‐doped graphene anode are 9.6 cd A^?1 and 10.5 lm W^?1, respectively; these values are markedly higher than those of the PLED fabricated on pristine graphene anode and comparable to those of an ITO anode.     

Template‐Based Growth of Various Oxide Nanorods by Sol–Gel Electrophoresis

The ability to form oxide nanorods is of great interest in a number of areas. In this paper, we report the template‐based growth of nanorods of several oxide ceramics, formed by means of a combination of sol–gel processing and electrophoretic deposition. Both single metal oxides (TiO₂, SiO₂) and complex oxides (BaTiO₃, Sr₂Nb₂O₇, and Pb(Zr_0.52Ti_0.48)O₃) have been grown by this method. Uniformly sized nanorods of about 125–200 nm in diameter and 10 μm in length were grown over large areas with near unidirectional alignment. Desired stoichiometric chemical composition and crystal structure of the oxide nanorods was readily achieved by an appropriate procedure of sol preparation, with a heat treatment (700 °C for 15 min) for crystallization and densification.     

Functional and Well‐Defined β‐Sheet‐Assembled Porous Spherical Shells by Surface‐Guided Peptide Formation

Polypeptides have attracted widespread attention as building blocks for complex materials due to their ability to form higher‐ordered structures such as β‐sheets. However, the ability to precisely control the formation of well‐defined β‐sheet‐assembled materials remains challenging as β‐sheet formation tends to lead to ill‐defined and unprocessable aggregates. This work reports a simple, rapid, and robust strategy to form well‐defined peptide β‐sheet‐assembled shells (i.e., hollow spheres) by employing surface‐initiated N‐carboxyanhydride ring‐opening polymerization under a highly efficient surface‐driven approach. The concept is demonstrated by the preparation of enzyme‐degradable rigid shell architectures composed of H‐bonded poly(L‐valine) (PVal) grafts with porous and sponge‐like surface morphology. The porous PVal‐shells exhibit a remarkable and unprecedented ability to non‐covalently entrap metal nanoparticles, proteins, drug molecules, and biorelevant polymers, which could potentially lead to a diverse range of biodegradable and functional platforms for applications ranging from therapeutic delivery to organic catalysis.     

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Atomic‐layer‐deposited aluminium oxide (Al₂O₃) is applied as rear‐surface‐passivating dielectric layer to passivated emitter and rear cell (PERC)‐type crystalline silicon (c‐Si) solar cells. The excellent passivation of low‐resistivity p‐type silicon by the negative‐charge‐dielectric Al₂O₃ is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al₂O₃ film covered by a 200 nm plasma‐enhanced‐chemical‐vapour‐deposited silicon oxide (SiO_x) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single‐layer of Al₂O₃, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.