Crystallization and Microstructure of Yttria‐Stabilized‐Zirconia Thin Films Deposited by Spray Pyrolysis

The crystallization and microstuctural evolution upon thermal treatment of yttria‐stabilized zirconia (YSZ, Zr_0.85Y_0.15O_1‐δ) thin films deposited by spray pyrolysis at 370 °C are investigated. The as‐deposited YSZ films are mainly amorphous with a few crystallites of 3 nm in diameter and crystallize in the temperature range from 400 °C to 900 °C. Fully crystalline YSZ thin films are obtained after heating to 900 °C or by isothermal dwells for at least 17 h at a temperature as low as 600 °C. Three exothermic heat releasing processes with activation energies are assigned to the crystallization and the oxidation of residuals from the precursor. Microporosity develops during crystallization and mass loss. During crystallization the microstrain decreases from 4% to less than 1%. Simultaneously, the average grain size increases from 3 nm to 10 nm. The tetragonal phase content of the YSZ thin film increases with increasing temperature and isothermal dwell time. Based on these data, gentle processing conditions can be designed for zirconia based thin films, which meet the requirements for Si‐based microfabrication of miniaturized electrochemical devices such as micro‐solid oxide fuel cells or sensors.     

Generation of Self‐Assembled 3D Mesostructured SnO2 Thin Films with Highly Crystalline Frameworks

Crack‐free, mesoporous SnO₂ films with highly crystalline pore walls are obtained by evaporation‐induced self‐assembly using a novel amphiphilic block‐copolymer template (“KLE” type, poly(ethylene‐co‐butylene)‐block‐poly(ethylene oxide)), which leads to well‐defined arrays of contracted spherical mesopores by suitable heat‐treatment procedures. Because of the improved templating properties of these polymers, a facile heat‐treatment procedure can be applied whilst keeping the mesoscopic order intact up to 600–650 °C. The formation mechanism and the mesostructural evolution are investigated by various state‐of‐the‐art techniques, particularly by a specially constructed 2D small‐angle X‐ray scattering setup. It is found that the main benefit from the polymers is the formation of an ordered mesostructure under the drastic conditions of using molecular Sn precursors (SnCl₄), taking advantage of the large segregation strength of these amphiphiles. Furthermore, it is found that the crystallization mechanism is different from other mesostructured metal oxides such as TiO₂. In the case of SnO₂, a significant degree of crystallization (induced by heat treatment) already starts at quite low temperatures, 250–300 °C. Therefore, this study provides a better understanding of the general parameters governing the preparation of mesoporous metal oxides films with crystalline pore walls.     

The Remarkable Thermal Stability of Amorphous In‐Zn‐O Transparent Conductors

Transparent conducting oxides (TCOs) are increasingly critical components in photovoltaic cells, low‐e windows, flat panel displays, electrochromic devices, and flexible electronics. The conventional TCOs, such as Sn‐doped In₂O₃, are crystalline single phase materials. Here, we report on In‐Zn‐O (IZO), a compositionally tunable amorphous TCO with some significantly improved properties. Compositionally graded thin film samples were deposited by co‐sputtering from separate In₂O₃ and ZnO targets onto glass substrates at 100 °C. For the metals composition range of 55–84 cation% indium, the as‐deposited IZO thin films are amorphous, smooth (R_RMS < 0.4 nm), conductive (σ ∼ 3000 Ω⁻¹ · cm⁻¹), and transparent in the visible (T_Vis > 90%). Furthermore, the amorphous IZO thin films demonstrate remarkable functional and structural stability with respect to heating up to 600 °C in either air or argon. Hence, though not completely understood at present, these amorphous materials constitute a new class of fundamentally interesting and technologically important high performance transparent conductors.     

Low‐Temperature‐Processed Printed Metal Oxide Transistors Based on Pure Aqueous Inks

Additive patterning of transparent conducting metal oxides at low temperatures is a critical step in realizing low‐cost transparent electronics for display technology and photovoltaics. In this work, inkjet‐printed metal oxide transistors based on pure aqueous chemistries are presented. These inks readily convert to functional thin films at lower processing temperatures (T ≤ 250 °C) relative to organic solvent‐based oxide inks, facilitating the fabrication of high‐performance transistors with both inkjet‐printed transparent electrodes of aluminum‐doped cadmium oxide (ACO) and semiconductor (InO_x ). The intrinsic fluid properties of these water‐based solutions enable the printing of fine features with coffee‐ring free line profiles and smoother line edges than those formed from organic solvent‐based inks. The influence of low‐temperature annealing on the optical, electrical, and crystallographic properties of the ACO electrodes is investigated, as well as the role of aluminum doping in improving these properties. Finally, the all‐aqueous‐printed thin film transistors (TFTs) with inkjet‐patterned semiconductor (InO_x ) and source/drain (ACO) layers are characterized, which show ideal low contact resistance (R _c < 160 Ω cm) and competitive transistor performance (µ _lin up to 19 cm² V^?1 s^?1, Subthreshold Slope (SS) ≤150 mV dec^?1) with only low‐temperature processing (T ≤ 250 °C).     

Block‐Copolymer‐Templated Synthesis of Electroactive RuO2‐Based Mesoporous Thin Films

RuO₂‐based mesoporous thin films of optical quality are synthesized from ruthenium‐peroxo‐based sols using micelle templates made of amphiphilic polystyrene‐polyethylene oxide block copolymers. The mesoporous structure and physical properties of the RuO₂ films (mesoporous volume: 30%; pore diameter: ～30 nm) can be controlled by the careful tuning of both the precursor solution and thermal treatment (150–350 °C). The optimal temperature that allows control of both mesoporosity and nanocristallinity is strongly dependent on the substrate (silicon or fluorine‐doped tin oxide). The structure of the resulting mesoporous films are investigated using X‐ray diffraction, X‐ray photoelectron spectroscopy, and atomic force microscopy. Mesoporous layers are additionally characterized by transmission and scanning electron microscopy and ellipsometry while their electrochemical properties are analyzed via cyclic voltammetry. Thick mesoporous films of ruthenium oxide hydrates, RuO₂ · xH₂O, obtained using a thermal treatment at 280 °C, exhibit capacitances as high as 1000 ± 100 F g^?1 at a scan rate of 10 mV s^?1, indicating their potential application as electrode materials.     

Highly Organized Mesoporous TiO2 Films with Controlled Crystallinity: A Li‐Insertion Study

A study of electrochemical Li insertion combined with structural and textural analysis enabled the identification and quantification of individual crystalline and amorphous phases in mesoporous TiO₂ films prepared by the evaporation‐induced self‐assembly procedure. It was found that the properties of the amphiphilic block copolymers used as templates, namely those of a novel poly(ethylene‐co‐butylene)‐b‐poly(ethylene oxide) polymer (KLE) and commercial Pluronic P123 (HO(CH₂CH₂O)₂₀(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H), decisively influence the physicochemical properties of the resulting films. The KLE‐templated films possess a 3D cubic mesoporous structure and are practically amorphous when calcined at temperatures below 450 °C, but treatment at 550–700 °C provides a pure‐phase (anatase), fully crystalline material with intact mesoporous architecture. The electrochemically determined fraction of crystalline anatase increases from 85 to 100 % for films calcined at 550 and 700 °C, respectively. In contrast, the films prepared using Pluronic P123, which also show a 3D cubic pore arrangement, exhibit almost 50 % crystallinity even at a calcination temperature of 400 °C, and their transformation into a fully crystalline material is accompanied by collapse of the mesoporous texture. Therefore, our study revealed the significance of using suitable block‐copolymer templates for the generation of mesoporous metal oxide films. Coupling of both electrochemical and X‐ray diffraction methods has shown to be highly advisable for the correct interpretation of structure properties, in particular the crystallinity, of such sol–gel derived films.     

Single‐Step Deposition of Au‐ and Pt‐Nanoparticle‐Functionalized Tungsten Oxide Nanoneedles Synthesized Via Aerosol‐Assisted CVD,and Used for Fabrication of Selective Gas Microsensor Arrays

Tungsten oxide nanostructures functionalized with gold or platinum NPs are synthesized and integrated, using a single‐step method via aerosol‐assisted chemical vapour deposition, onto micro‐electromechanical system (MEMS)‐based gas‐sensor platforms. This co‐deposition method is demonstrated to be an effective route to incorporate metal nanoparticles (NP) or combinations of metal NPs into nanostructured materials, resulting in an attractive way of tuning functionality in metal oxides (MOX). The results show variations in electronic and sensing properties of tungsten oxide according to the metal NPs introduced, which are used to discriminate effectively analytes (C₂H₅OH, H₂, and CO) that are present in proton‐exchange fuel cells. Improved sensing characteristics, in particular to H₂, are observed at 250 °C with Pt‐functionalized tungsten oxide films, whereas non‐functionalized tungsten oxide films show responses to low concentrations of CO at low temperatures. Differences in the sensing characteristics of these films are attributed to the different reactivities of metal NPs (Au and Pt), and to the degree of electronic interaction at the MOX/metal NP interface. The method presented in this work has advantages over other methods of integrating nanomaterials and devices, of having fewer processing steps, relatively low processing temperature, and no requirement for substrate pre‐treatment.     

Bio‐Inspired Nanospiky Metal Particles Enable Thin,Flexible, and Thermo‐Responsive Polymer Nanocomposites for Thermal Regulation

Safety issues remain a major obstacle toward large‐scale applications of high‐energy lithium‐ion batteries. Embedding thermo‐responsive polymer switching materials (TRPS) into batteries is a potential strategy to prevent thermal runaway, which is a major cause of battery failures. Here, thin, flexible, highly responsive polymer nanocomposites enabled by bio‐inspired nanospiky metal (Ni) particles are reported. These unique Ni particles are synthesized by a simple aqueous reaction at gram‐scale with controlled surface morphology and composition to optimize electrical properties of the nanocomposites. The Ni particles provide TRPS films with a high room‐temperature conductivity of up to 300 S cm^?1. Such TRPS composite films also have a high rate (<1 s) of resistance switching within a narrow temperature range, good reversibility upon on/off switching, and a tunable switching temperature (T_s; 75 to 170 °C) that can be achieved by tailing their compositions. The small size (≈500 nm) of Ni particles enables ready fabrication of thin and flexible TPRS films with thickness approaching 5 µm or less. These features suggest the great potential of using this new type of responsive polymer composite for more effective battery thermal regulation without sacrificing cell performance.     

Tailoring of LaxSr1‐xCoyFe1‐yO3‐δ Nanostructure by Pulsed Laser Deposition

Pulsed Laser Deposition (PLD) was used to prepare thin films with the nominal composition La_0.58Sr_0.4Co_0.2Fe_0.8O_3‐δ (LSCF). The thin film microstructure was investigated as a function of PLD deposition parameters such as: substrate temperature, ambient gas pressure, target‐to‐substrate distance, laser fluence and frequency. It was found that the ambient gas pressure and the substrate temperature are the key PLD process parameters determining the thin film micro‐ and nanostructure. A map of the LSCF film nanostructures is presented as a function of substrate temperature (25–700 °C) and oxygen background pressure (0.013–0.4 mbar), with film structures ranging from fully dense to highly porous. Fully crystalline, dense, and crack‐free LSCF films with a thickness of 300 nm were obtained at an oxygen pressure lower than 0.13 mbar at a temperature of 600 °C. The obtained knowledge on the structure allows for tailoring of perovskite thin film nanostructure, e.g., for solid oxide fuel cell cathodes. A simple geometrical model is proposed, allowing estimation of the catalytic active surface area of the prepared thin films. It is shown that voids at columnar grain boundaries can result in an increase of the surface area by approximately 25 times, when compared to dense flat films.     

High‐Performance Flexible Thermoelectric Devices Based on All‐Inorganic Hybrid Films for Harvesting Low‐Grade Heat

The fabrication of a flexible thermoelectric (TE) device that contains flexible, all‐inorganic hybrid thin films (p‐type single‐wall carbon nanotubes (SWCNTs)/Sb₂Te₃ and n‐type reduced graphene oxide (RGO)/Bi₂Te₃) is reported. The optimized power factors of the p‐type and n‐type hybrid thin films at ambient temperature are about 55 and 108 µW m^?1 K^?2, respectively. The high performance of these films that are fabricated through the combination of vacuum filtration and annealing can be attributed to their planar orientation and network structure. In addition, a TE device, with 10 couples of legs, shows an output power of 23.6 µW at a temperature gradient of 70 K. A prototype of an integrated photovoltaic‐TE (PV‐TE) device demonstrates the ability to harvest low‐grade “waste” thermal energy from the human body and solar irradiation. The flexible TE and PV‐TE device have great potential in wearable energy harvesting and management.     

Electronic Structure of Low‐Temperature Solution‐Processed Amorphous Metal Oxide Semiconductors for Thin‐Film Transistor Applications

The electronic structure of low temperature, solution‐processed indium–zinc oxide thin‐film transistors is complex and remains insufficiently understood. As commonly observed, high device performance with mobility >1 cm² V^?1 s^?1 is achievable after annealing in air above typically 250 °C but performance decreases rapidly when annealing temperatures ≤200 °C are used. Here, the electronic structure of low temperature, solution‐processed oxide thin films as a function of annealing temperature and environment using a combination of X‐ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and photothermal deflection spectroscopy is investigated. The drop‐off in performance at temperatures ≤200 °C to incomplete conversion of metal hydroxide species into the fully coordinated oxide is attributed. The effect of an additional vacuum annealing step, which is beneficial if performed for short times at low temperatures, but leads to catastrophic device failure if performed at too high temperatures or for too long is also investigated. Evidence is found that during vacuum annealing, the workfunction increases and a large concentration of sub‐bandgap defect states (re)appears. These results demonstrate that good devices can only be achieved in low temperature, solution‐processed oxides if a significant concentration of acceptor states below the conduction band minimum is compensated or passivated by shallow hydrogen and oxygen vacancy‐induced donor levels.     

Electrical properties of silicon dioxide films fabricated at 700°C. II: Low pressure hydride deposition

We have investigated the effects of different annealing treatments on silicon dioxide films produced from the reaction of dichlorosilane and nitrous oxide at 700° C. The electrical quality of these LPCVD films was evaluated by measuring oxide charge and interface trap densities on metal oxide semiconductor (MOS) capacitors. These densities were measured before and after avalanche injection of electron currents into the oxide films. The results of these studies were as follows. (1) The LPCVD oxide films required a post deposition anneal at 1000° C to produce as-grown charge densities similar to those of a standarddry thermal oxide grown and annealed at 1000° C. (2) Post-injection charge densities of LPCVD films given a post deposition anneal at 1000°C were an order of magnitude greater than those of the standard dry thermal oxide. (3) Different annealing treatments produced a series of dominant electron trapping centers in the oxide bulk₁₇ with capture cross sections ranging from 10⁻¹⁴ cm² to 10⁻¹⁷ cm². (4) The electron traps in the LPCVD oxides films were similar to those previously observed in standardwet thermal oxides grown and annealed above 1000° C.     

Direct Synthesis of Large‐Scale WTe2 Thin Films with Low Thermal Conductivity

Large‐scale, polycrystalline WTe₂ thin films are synthesized by atmospheric chemical vapor reaction of W metal films with Te vapor catalyzed by H₂Te intermediates, paving a way to understanding the synthesis mechanism for low bonding energy tellurides and toward synthesis of single‐crystalline telluride nanoflakes. Through‐plane and in‐plane thermal conductivities of single‐crystal WTe₂ flakes and polycrystalline WTe₂ thin films are measured for the first time. Nanoscale grains and disorder in WTe₂ thin films suppress the in‐plane thermal conductivity of WTe₂ greatly, which is at least 7.5 times lower than that of the single‐crystalline flakes.     

Simple Method to Obtain Large‐Size Single‐Crystalline Oxide Sheets

Synthetic techniques to prepare large‐size, flexible, and high‐quality single‐crystalline sheets of transition metal oxides are crucial to developing low‐energy consumption devices. One promising way is a lift‐off and transfer technique using a heterostructure of polymer supporting oxide and Sr₃Al₂O₆ (SAO) layers grown on a single‐crystalline substrate. By removing the water‐soluble SAO and the supporting layers, the oxide sheet is obtained. Although some ferroelectric flexible sheets are prepared by this method, a simpler method for obtaining large‐size sheets is required. Herein, a lift‐off and transfer method is proposed without a supporting layer. With this simple method, single‐crystalline SrRuO₃ and BaTiO₃ flexible sheets with a lateral size of a few millimeters are successfully prepared. The SrRuO₃ sheet exhibits high crystallinity and conductivity. Meanwhile, the ferroelectricity of the BaTiO₃ sheet is successfully observed via polarization hysteresis loop measurements. In addition to the simplicity, this method has low costs and the substrate is reusable. Accordingly, the proposed method could enhance the development of various kinds of large‐size functional oxide sheets.     

Low temperature preparation of piezoelectric thin films by ultraviolet-assisted rapid thermal processing

Ca- and La-modified lead titanate ferroelectric thin films were prepared by a sol–gel method from photosensitive solutions and with an ultraviolet (UV)-assisted rapid thermal processor, including an arrangement of excimer lamps. The diol-based route was used in the preparation of the precursor solutions, whose UV absorption and thermal decomposition were determined. Multiple deposition and crystallization steps of the deposited solution were used to promote preferential orientation.A comparative ferro and piezoelectric study of films prepared by conventional rapid thermal processing at 650°C and films prepared at 550°C with an intermediate step of UV irradiation at 250°C will be presented to assess the value of these films for their use in integrated piezoelectric sensors and microelectromechanical systems.Piezoelectric d₃₃ and e₃₁ coefficients were determined by double-beam laser interferometry and by the direct piezoelectric effect on cantilevers, respectively. The effect of the substrate and processing method on the preferential crystalline orientation of the films and the corresponding piezoelectric properties will be reported. The role of the composition will also be discussed.     

The β‐to‐γ Transition in BiFeO3: A Powder Neutron Diffraction Study

High‐temperature powder neutron diffraction experiments are conducted around the reported β–γ phase transition (～930 °C) in BiFeO₃. The results demonstrate that while a small volume contraction is observed at the transition temperature, consistent with an insulator–metal transition, both the β‐ and γ‐phase of BiFeO₃ exhibit orthorhombic symmetry; i.e., no further increase of symmetry occurs during this transition. The γ‐orthorhombic phase is observed to persist up to a temperature of approximately 950 °C before complete decomposition into Bi₂Fe₄O₉ (and liquid Bi₂O₃), which subsequently begins to decompose at approximately 960 °C.     

Highly Dispersed Mixed Zirconia and Hafnia Nanoparticles in a Silica Matrix: First Example of a ZrO2–HfO2–SiO2 Ternary Oxide System

ZrO₂ and HfO₂ nanoparticles are homogeneously dispersed in SiO₂ matrices (supported film and bulk powders) by copolymerization of two oxozirconium and oxohafnium clusters (M₄O₂(OMc)₁₂, M = Zr, Hf; OMc = OC(O)–C(CH₃)?CH₂) with (methacryloxypropyl)trimethoxysilane (MAPTMS, (CH2?C(CH₃)C(O)O)–(CH₂)₃Si(OCH₃)₃). After calcination (at a temperature ≥800 °C), a silica matrix with homogeneously distributed MO₂ nanocrystallites is obtained. This route yields a spatially homogeneous dispersion of the metal precursors inside the silica matrix, which is maintained during calcination. The composition of the films and the powders is studied before and after calcination by using Fourier transform infrared (FTIR) analysis, X‐ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS). The local environment of the metal atoms in one of the calcined samples is investigated by using X‐ray Absorption Fine Structure (XAFS) spectroscopy. Through X‐ray diffraction (XRD) the crystallization of Hf and Zr oxides is seen at temperatures higher than those expected for the pure oxides, and transmission electron microscopy (TEM) shows the presence of well‐distributed and isolated crystalline oxide nanoparticles (5–10 nm).     

Substrate‐Dependent Spin–Orbit Coupling in Hybrid Perovskite Thin Films

Solution‐processing hybrid metal halide perovskites are promising materials for developing flexible thin‐film devices. This work reports the substrate effects on the spin–orbit coupling (SOC) in perovskite films through thermal expansion under thermal annealing. X‐ray diffraction (XRD) measurements show that using a flexible polyethylene naphthalate (PEN) substrate introduces a smaller mechanical strain in perovskite MAPbI_3?xCl_x films, as compared to conventional glass substrates. Interestingly, the linear/circular photoexcitation‐modulated photocurrent studies find that decreasing mechanical strain gives rise to a weaker orbit–orbit interaction toward decreasing the SOC in the MAPbI_3?xCl_x films prepared on flexible PEN substrates relative to rigid glass substrates. Simultaneously, decreasing the mechanical strain causes a reduction in the internal magnetic parameter inside the MAPbI_3?xCl_x films, providing further evidence to show that introducing mechanical strain can affect the SOC in hybrid perovskite films upon using flexible substrates toward developing flexible perovskite thin‐film devices. Furthermore, thermal admittance spectroscopy indicates that the trap states are increased in the perovskite films prepared on flexible PEN substrates as compared to glass substrates. Consequently, PEN and rigid glass substrates lead to shorter and longer photoluminescence lifetimes, respectively. Clearly, these findings provide an insightful understanding on substrate effects on optoelectronic properties in flexible perovskite thin‐film devices.     

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.     

Polyarylene Networks via Bergman Cyclopolymerization of Bis‐ortho‐diynyl Arenes

Bis‐ortho‐diynylarene (BODA) monomers, prepared from common bisphenols in three high yielding steps, undergo free‐radical‐mediated thermal polymerization via an initial Bergman cyclo‐rearrangement. Polymerization is carried out at 210 °C in solution or neat with large pre‐vitrification melt windows (4–5 h) to form branched oligomers containing reactive pendant and terminal aryldiynes. Melt‐ and solution‐processable oligomers with weight‐average molecular weight M_w = 3000–24 000 g mol^–1 can be coated as a thin film or molded using soft lithography techniques. Subsequent curing to 450 °C affords network polymers with no detectable glass transition temperatures below 400 °C and thermal stability ranging from 0.5–1.5 % h^–1 isothermal weight loss measured at 450 °C under nitrogen. Heating to 900–1000 °C gives semiconductive glassy carbon in high yield. BODA monomer synthesis, network characterization and kinetics, processability, thin‐film photoluminescence, and thermal properties are described.