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Temporal and spatial tuning of the refractive index of optical thin films is desired for flat optics applications. The redistribution of mobile ions in mixed ionic-electronic conductors (MIEC) has been demonstrated to serve as a viable means for achieving optical tuning down to the nanoscale. Here we studied the dynamic range of the optical tuning achievable in the refractive index, in the MIEC oxide – PrxCe1−xO2−δ (PCO), for x = 0.1, 0.2 and 0.4, at 500 °C, by in-situ spectrophotometry. Significant increases in the modulation of both the imaginary and real optical constants in the visible and the adjacent spectra were obtained for increased doping levels. Device employing an electrochemical titration method was implemented to modulate the oxygen concentration, and thereby the optical transmission of PCO. Incorporation of a patterned top electrode allowed for the demonstration of spatial control of PCO thin film properties by in-situ video imaging of the optical switching process. The electrochemically induced optical state is shown to remain non-volatile upon quenching the device to room temperature under applied bias. [ABSTRACT FROM AUTHOR]

The chemical expansion of Pr0.1Ce0.9O2–δ (PCO) and CeO2–δ thin films is investigated in the temperature range between 600 °C and 800 °C by laser Doppler vibrometry (LDV). It enables non-contact determination of nanometer scale changes in film thickness at high temperatures. The present study is the first systematic and detailed investigation of chemical expansion of doped and undoped ceria thin films at temperatures above 650 °C. The thin films were deposited on yttria stabilized zirconia substrates (YSZ), operated as an electrochemical oxygen pump, to periodically adjust the oxygen activity in the films, leading to reversible expansion and contraction of the film. This further leads to stresses in the underlying YSZ substrates, accompanied by bending of the overall devices. Film thickness changes and sample bending are found to reach up to 10 and several hundred nanometers, respectively, at excitation frequencies from 0.1 to 10 Hz and applied voltages from 0–0.75 V for PCO and 0–1 V for ceria. At low frequencies, equilibrium conditions are approached. As a consequence maximum thin-film expansion of PCO is expected due to full reduction of the Pr ions. The lower detection limit for displacements is found to be in the subnanometer range. At 800 °C and an excitation frequency of 1 Hz, the LDV shows a remarkable resolution of 0.3 nm which allows, for example, the characterization of materials with small levels of expansion, such as undoped ceria at high oxygen partial pressure. As the correlation between film expansion and sample bending is obtained through this study, a dimensional change of a free body consisting of the same material can be calculated using the high resolution characteristics of this system. A minimum detectable dimensional change of 5 pm is estimated even under challenging high-temperature conditions at 800 °C opening up opportunities to investigate electro-chemo-mechanical phenomena heretofore impossible to investigate. The expansion data are correlated with previous results on the oxygen nonstoichiometry of PCO thin films, and a defect model for bulk ceria solid solutions is adopted to calculate the cation and anion radii changes in the constrained films during chemical expansion. The constrained films exhibit anisotropic volume expansion with displacements perpendicular to the substrate plane nearly double that of bulk samples. The PCO films used here generate high total displacements of several 100 nm's with high reproducibility. Consequently, PCO films are identified to be a potential core component of high-temperature actuators. They benefit not only from high displacements at temperatures where most piezoelectric materials no longer operate while exhibiting, low voltage operation and low energy consumption. [ABSTRACT FROM AUTHOR]

Enhanced ionic mobility in mixed ionic and electronic conducting solids contributes to improved performance of memristive memory, energy storage and conversion, and catalytic devices. Ionic mobility can be significantly depressed at reduced temperatures, for example, due to defect association and therefore needs to be monitored. Measurements of ionic transport in mixed conductors, however, proves to be difficult due to dominant electronic conductivity. This study examines the impact of different levels of quenched‐in oxygen deficiency on the oxygen vacancy mobility near room temperature. A praseodymium doped ceria (Pr0.1Ce0.9O2–δ) film is grown by pulsed laser deposition and annealed in various oxygen partial pressures to modify its oxygen vacancy concentration. Changes in film non‐stoichiometry are monitored by tracking the optical absorption related to the oxidation state of Pr ions. A 13‐fold increase in ionic mobility at 60 °C for increases in oxygen non‐stoichiometry from 0.032 to 0.042 is detected with negligible changes in migration enthalpy and large changes in pre‐factor. Several factors potentially contributing to the large pre‐factor changes are examined and discussed. Insights into how ionic defect concentration can markedly impact ionic mobility should help in elucidating the origins of variations seen in nanoionic devices. [ABSTRACT FROM AUTHOR]

We compare approaches to measure oxygen surface exchange kinetics, by simultaneous optical transmission relaxation (OTR) and AC-impedance spectroscopy (AC-IS), on the same mixed conducting SrTi0.65Fe0.35O3-x film. Surface exchange coefficients were evaluated as a function of oxygen activity in the film, controlled by gas partial pressure and/or DC bias applied across the ionically conducting yttria-stabilized zirconia substrate. Changes in measured light transmission through the film over time (relaxations) resulted from optical absorption changes in the film corresponding to changes in its oxygen and oxidized Fe (~Fe4+) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The kchem values obtained by OTR were significantly lower than the AC-IS derived kchem values and kq values multiplied by the thermodynamic factor (bulk or thin film), suggesting a possible enhancement in k by the metal current collectors (Pt, Au). Long-term degradation in kchem and kq values obtained by AC-IS was also attributed to deterioration of the porous Pt current collector, while no significant degradation was observed in the optically derived kchem values. The results suggest that, while the current collector might influence measurements by AC-IS, the OTR method offers a continuous, in situ, and contact-free method to measure oxygen exchange kinetics at the native surfaces of thin films. [ABSTRACT FROM AUTHOR]

High precision measurements of oxygen nonstoichiometry δ in thin film metal oxides MaOb±δ at elevated temperatures and controlled oxygen partial pressures pO2 are reported with the aid of resonant microbalances. The resonant microbalances applied here consisted of y-cut langasite (La3Ga5SiO14) and CTGS (Ca3TaGa3Si2O14) piezoelectric resonators, operated in the thickness shear mode at ∼5 MHz. Measurements of variations in δ of Pr0.1Ce0.9O2-δ (PCO) films are reported for the oxygen partial pressure range from 10−8 bar to 0.2 bar at 700 °C, and these results were found to be in good agreement with previously reported oxygen nonstoichiometry δ data derived from chemical capacitance studies. The PCO thin-films were deposited via pulsed laser deposition on both sides of the resonators, whose series resonance frequency was tracked, converted into mass changes and, finally, into nonstoichiometry. The nonstoichiometry was observed to reach a plateau as the oxygen partial pressure dropped below about 10−5 bar, the behavior being attributed to the full reduction of Pr to the trivalent state. These resonators enable stable operation up to temperatures above 1000 °C, thereby maintaining high mass resolution suitable for determining oxygen nonstoichiometry variations in thin films deposited on such resonators. For the given experimental conditions, a mass resolution of ∼50 ng was achieved at 700 °C with the CTGS resonator. [ABSTRACT FROM AUTHOR]

A self-assembled three phase epitaxial nanocomposite film is grown consisting of ≈3 nm diameter fcc metallic Cu nanorods within square prismatic SrO rocksalt nanopillars in a Sr(Ti,Cu)O3- δ perovskite matrix. Each phase has an epitaxial relation to the others. The core-shell-matrix structures are grown on SrTiO3 substrates and can also be integrated onto Si using a thin SrTiO3 buffer. The structure is made by pulsed laser deposition in vacuum from a SrTi0.75Cu0.25O3 target, and formed as a result of the limited solubility of Cu in the perovskite matrix. Wet etching removes the 3 nm diameter Cu nanowires leaving porous SrO pillars. The three-phase nanocomposite film is used as a substrate for growing a second epitaxial nanocomposite consisting of CoFe2O4 spinel pillars in a BiFeO3 perovskite matrix, producing dramatic effects on the structure and magnetic properties of the CoFe2O4. This three-phase vertical nanocomposite provides a complement to the well-known two-phase nanocomposites, and may offer a combination of properties of three different materials as well as additional avenues for strain-mediated coupling within a single film. [ABSTRACT FROM AUTHOR]

While the properties of functional oxide thin films often depend strongly on oxygen stoichiometry, there have been few means available for its control in a reliable and in situ fashion. This work describes the use of DC bias as a means of systematically controlling the stoichiometry of oxide thin films deposited onto yttria-stabilized zirconia substrates. Impedance spectroscopy is performed on the electrochemical cell Pr0.1Ce0.9O2−δ (PCO)/YSZ/Ag for conditions: T = 550 to 700 °C, pO2 = 10−4 to 1 atm, and ΔE = -100 to 100 mV. The DC bias ΔE is used to control the effective pO2 or oxygen activity at the PCO/YSZ interface. The non-stoichiometry ( δ) of the PCO films is calculated from the measured chemical capacitance ( Cchem ). These δ values, when plotted isothermally as a function of effective pO2, established, either by the surrounding gas composition alone, or in combination with applied bias, agree well with each other and to predictions based on a previously determined defect model. These results confirm the suitability of using bias to precisely control δ of thin films in an in situ fashion and simultaneously monitor these changes by measurement of Cchem . Of further interest is the ability to reach effective pO2s as high as 280 atm. [ABSTRACT FROM AUTHOR]

In this paper, we focus on the effect of processing-dependent lattice strain on oxygen ion conductivity in ceria based solid electrolyte thin films. This is of importance for technological applications, such as micro-SOFCs, microbatteries, and resistive RAM memories. The oxygen ion conductivity can be significantly modified by control of lattice strain, to an extent comparable to the effect of doping bulk ceria with cations of different diameters. The interplay of dopant radii, lattice strain, microstrain, anion-cation near order and oxygen ion transport is analyzed experimentally and interpreted with computational results. Key findings include that films annealed at 600 °C exhibit lattice parameters close to those of their bulk counterparts. With increasing anneal temperature, however, the films exhibited substantial compaction with lattice parameters decreasing by as much as nearly 2% (viz, Δd600-1100 °C: -1.7% (Sc+3) > -1.5% (Gd+3) > -1.2% (La+3)) for the annealing temperature range of 600-1100 °C. Remarkably 2/3rd of the lattice parameter change obtained in bulk ceria upon changing the acceptor diameter from the smaller Sc to larger La, can be reproduced by post annealing a film with fixed dopant diameter. While the impact of lattice compaction on defect association/ordering cannot be entirely excluded, DFT computation revealed that the main effect appears to result in an increase in migration energy and consequent drop in ionic conductivity. As a consequence, it is clear that annealing procedures should be held to a minimum to maintain the optimum level of oxygen ion conductivity for energy-related applications. Results reveal also the importance to understand the role of electro-chemo-mechanical coupling that is active in thin film materials. [ABSTRACT FROM AUTHOR]

We demonstrate the impact on thermal conductivity of varying the concentration of oxygen vacancies and reduced cations in Pr0.1Ce0.9O2-δ thin films prepared by pulsed laser deposition. The oxygen vacancy concentration is controlled by varying the oxygen partial pressure between 1×10-4 and 1 atm at 650 °C. Corresponding changes in the oxygen non-stoichiometry (δ) are monitored by detecting the lattice parameters of the films with high-resolution X-ray diffraction, while the thermal properties are characterized by time-domain thermoreflectance measurements. The films are shown to exhibit a variation in oxygen vacancy content, and in the Pr3+/Pr4+ ratio, corresponding to changes in δ from 0.0027 to 0.0364, leading to a reduction in the thermal conductivity from k=6.62±0.61 to 3.82±0.51 W/m-K, respectively. These values agree well with those predicted by the Callaway and von Baeyer model for thermal conductivity in the presence of point imperfections. These results demonstrate the capability of controlling thermal conductivity via control of anion and cation defect concentrations in a given reducible oxide. [ABSTRACT FROM AUTHOR]

While the properties of functional oxide thin films often depend strongly on their oxygen stoichiometry, there have been few ways to extract this information reliably and in situ. In this work, the derivation of the oxygen non-stoichiometry of dense Pr0.1Ce0.9O2−δ thin films from an analysis of chemical capacitance obtained by impedance spectroscopy is described. Measurements are performed on electrochemical cells of the form Pr0.1Ce0.9O2−δ/Y0.16Zr0.84O1.92/Pr0.1Ce0.9O2−δ over the temperature range of 450 to 800 °C and oxygen partial pressure range of 10−5 to 1 atm O2. With the aid of a defect equilibria model, approximations relate chemical capacitance directly to non-stoichiometry, without need for fitting parameters. The calculated non-stoichiometry allows extraction of the thermodynamic constants defining defect generation. General agreement of these constants with bulk values derived by thermogravimetric analysis is found, thereby confirming the suitability of this technique for measuring oxygen non-stoichiometry of thin oxide films. Potential sources of error observed in earlier chemical capacitance studies on perovskite structured oxide films are also discussed. [ABSTRACT FROM AUTHOR]

Abstract: Determination of the ionic and electronic contributions to the total conductivity in mixed ionic–electronic conductors (MIEC) is central to understanding their properties, particularly in nanostructured ionic solids. The Hebb–Wagner blocking technique, commonly used to deconvolute ionic and electronic contributions in bulk MIECs, is susceptible to misinterpretation when applied to thin films. In this work, microfabricated electronic blocking electrodes consisting of porous Pt on dense thin yttria-stabilized zirconia (YSZ) films were applied to nanocrystalline CeO2 thin films. The validity of the blocking structure was expressly considered with respect to alternate current and gas phase reaction pathways, with criteria developed to aid in identifying spurious effects. The ionic partial conductivity in nanocrystalline CeO2 thin films was confirmed to be pO2-independent while the electronic partial conductivity was found to be pO2 dependent with a power dependence of −0.31±0.02. These results are compared with theoretical predictions of extrinsically-compensated ceria and previous results on bulk nanocrystalline ceria. [Copyright &y& Elsevier]

SrTi1-xFexO3-δ (STF) model cathodes, with compositions x = 0.35 and 0.5, prepared as dense films with a well-defined area and thickness on top of a single-crystal yttria stabilized zirconia substrate by pulsed layer deposition at 700°C were investigated by electrochemical impedance spectroscopy as a function of electrode geometry, temperature, and oxygen partial pressure. The STF cathode was observed to exhibit typical mixed ionic-electronic behavior with the electrode reaction occurring over the full electrode surface area rather than being limited to the triple-phase boundary. The electrode impedance was observed to be independent of electrode thickness and inversely proportional to the square of the electrode diameter pointing to surface exchange limited kinetics. Furthermore, a Ce0.9Gd0.1O2-δ interlayer was found to have no effect on the electrode impedance. Values for the surface exchange coefficient, k, were calculated and found to be comparable in magnitude to those exhibited by other popular mixed ionic-electronic conductors, such as (La,Sr)(Co,Fe)O3, thereby, confirming the suitability of STF as a model-mixed conducting cathode material. The observed trends are discussed in relation to the known defect and transport properties of STF. [ABSTRACT FROM AUTHOR]

Pentacene organic thin-film transistors (OTFTs) have demonstrated the highest performance among TFTs with an organic semiconductor channel. High operating voltages (20-100 V), stemming from poor capacitive coupling between gate electrode and channel, are a major limitation, particularly for portable battery-powered device applications. OTFTs fabricated on flexible polymer substrates, often characterized by rough surfaces, benefit from the use of high-K dielectrics given the ability to accommodate thicker films which ensure the pinhole-free and good coverage without need to increase operating voltage. As we demonstrate, pyrochlore structured thin films can provide the requisite high dielectric constant coupled with excellent leakage current characteristics, while remaining compatible with the processing requirements of flexible OTFTs. The introduction of an extremely thin parylene film between the BZN dielectric and the pentacene semiconductor markedly shifts the threshold voltage, making it possible to fabricate both enhancement (E) and depletion (D) TFTs. We report the successful fabrication of low-voltage (<2 V) organic transistors and depletion-load inverter using a 200-nm-thick pyrochlore gate dielectric, Bi1.5Zn1.0Nb1.5O7 (BZN), prepared by a room temperature process. The inverters with depletion load were successfully operated under 5 V with excellent noise margin. [ABSTRACT FROM AUTHOR]

ZnO thin film micro-arrays with integrated heaters were fabricated onto silicon wafers. The atmosphere-dependent electrical response of ZnO films sputtered under controlled Ar/O2 ratios onto micro-arrays was examined in response to the following gases: CO, H2, and NO2. The influence of processing conditions on the physical and electrical properties of ZnO films was investigated. The deposition conditions were found to control film morphology, while subsequent annealing at 700 °C in air had only minor effects on the microstructure. ZnO films prepared with high O2/Ar ratios during sputtering show better sensitivity to H2, NO2 and CO with sensitivity to NO2 particularly high at low temperatures. The ac impedance measurements were used to identify the two major components of the total sensor resistance including Schottky barriers at the Pt–ZnO interfaces and a dc bias-induced constriction resistance within the ZnO film. Changes induced in the impedance spectra due to dc bias pre-treatments are hypothesized to result from field-induced modifications in gas coverage of the sensor films. [Copyright &y& Elsevier]

We have investigated the effect of texture on in-plane (IPP) and out-of plane (OPP) polarizations of pulsed-laser-deposited BaTiO3 thin films grown on Pt and La0.5Sr0.5CoO3 (LSCO) buffered Pt electrodes. The OPP and IPP polarizations were observed by piezoresponse force microscopy (PFM) for three-dimensional polarization analyses in conjunction with conventional diffraction methods using x-ray diffraction and reflection high energy electron diffraction measurements. BaTiO3 films grown on Pt electrodes exhibited highly (101) preferred orientation with higher IPP component whereas BaTiO3 film grown on LSCO/Pt electrodes showed (001) and (101) orientations with higher OPP component. Measured effective d33 values of BaTiO3 films deposited on Pt and LSCO/Pt electrodes were 14.3 and 54.0 pm/V, respectively. Local piezoelectric strain loops obtained by OPP and IPP-PFM showed that piezoelectric properties were strongly related to film orientation. [ABSTRACT FROM AUTHOR]

In this letter, we report on increased tunability of Ba0.6Sr0.4TiO3 (BST) thin films by use of Ta2O5 and TiO2 films as buffer layers between BST and Si substrates. Ta2O5 and TiO2 butler layers were grown by atomic layer deposition (ALD) onto Si substrates followed by pulsed laser deposition of Ba0.6Sr0.4TiO3 thin films onto the buffer layers. The randomly oriented BST films deposited on TiO2/Si substrates exhibited a broader grain size distribution than the (110) textured BST films grown on Ta2O5/Si substrates. At an applied voltage of 10 V, the BST films grown on Ta2O5/Si and TiO2/Si substrates showed much enhanced tunability values of 53.1% and 72.9%, respectively, as compared to the 20.7% value obtained with BST films grown on MgO single crystal substrates. Successful integration of BST low voltage microwave tunable devices onto Si substrates thus appears possible with the aid of ALD grown Ta2O5 or TiO2 butler layers. [ABSTRACT FROM AUTHOR]

Abstract The effects of lanthanum doping on the transport properties and oxygen exchange kinetics of La x (Ce 0.9 Pr 0.1) 1−x O 2−δ (x = 0, 0.1 and 0.2) have been investigated by electrical conductivity on bulk samples and optical relaxation on thin films respectively. Lanthanum substitution was observed to increase the magnitude of ionic conductivity to values comparable to that of single substituted La x Ce 1−x O 2−δ , with a maximum for a La content of x = 0.1, reaching σ = 9.5 × 10−3 S·cm−1 at 600 °C and a decrease in activation energy from ≈1 eV for Ce 0.9 Pr 0.1 O 2−δ down to ≈0.75 eV for the lanthanum substituted compositions. At the same time, a significant electronic contribution to the total conductivity, estimated to be on the order of 10%, supports significant mixed ionic-electronic conductivity. Substitution with lanthanum increases the absolute values of the oxygen surface exchange coefficient from k chem = 6 × 10−7 cm·s−1 for Ce 0.9 Pr 0.1 O 2−δ to k chem = 1 × 10−6 cm·s−1 for La 0.2 (Ce 0.9 Pr 0.1) 0.8 O 2−δ at 550 °C. Moreover, one observes significant increases in activation energy from 0.7 eV for Ce 0.9 Pr 0.1 O 2−δ to ≈1 eV for La substituted compositions, suggesting a change in the rate-limiting step. Possible sources for the change in the rate-limiting step are suggested. The composition with the highest La content of x = 0.2, although not exhibiting the highest ionic conductivity, shows the highest surface exchange rates, is suggested to result from enhanced segregation of catalytically active lanthanum to the surface. Highlights • La doping in PCO increases ionic conductivity. • La doping in PCO pin praseodymium into its 4+ state. • Oxygen surface exchange coefficient is measured by optical transmission relaxation. • La doping enhances the oxygen surface exchange coefficient. • La doping changes the rate limiting step of the oxygen surface exchange. [ABSTRACT FROM AUTHOR]

Highly porous Pt thin films, with nano-scale porosity, were fabricated by reactive sputtering. The strategy involved deposition of thin film PtO x at room temperature, followed by the subsequent decomposition of the oxide by rapid heat treatment. The resulting films exhibited percolating Pt networks infiltrated with interconnected nanosized pores, critical for superior solid oxide fuel cell cathode performance. This approach is particularly attractive for micro-fabricated solid oxide fuel cells, since it enables fabrication of the entire cell stack (anode/electrolyte/cathode) within the sputtering chamber, without breaking vacuum. In this work, the morphological, crystallographic and chemical properties of the porous electrode were systematically varied by control of deposition conditions. Oxygen reduction reaction kinetics were investigated by means of electrochemical impedance spectroscopy, demonstrating the critical role of nano-pores in achieving satisfactory micro-SOFC cathode performance. [ABSTRACT FROM AUTHOR]

Abstract: Yttria-doped zirconia (YDZ) thin films with nanometric sized grains were prepared by reactive RF sputtering and their oxygen ion conductivities were systematically measured as a function of yttria doping with levels in the range 0.5–9.1mol.% Y2O3. Enhanced oxygen ion conductivities, as derived from impedance spectra, were observed when compared with values reported for bulk YSZ. Furthermore, the peak conductivity for the YDZ films was observed to occur at considerably reduced yttria levels, i.e., at 6.5mol.% Y2O3 (for T >∼400°C) and at 3.2mol.% Y2O3 (for T <∼300°C) vs. 9mol.% Y2O3 in bulk YSZ. Based on an analysis of the Raman spectra, these results are believed to result from the extended meta-stability of the cubic phase to reduced yttria levels at nanometric grain sizes. [Copyright &y& Elsevier]

Abstract: Dense, nanocrystalline gadolinia doped ceria (CGO) thin films were prepared by sputtering on a variety of substrates. The CGO films were electrochemically characterized with the aid of interdigitated Pt electrodes by impedance spectroscopy as a function of operating temperature (175–340°C), CGO film thickness (157, 233, 315nm), deposition temperature (room temperature, 300, 500°C), and Pt electrode pattern geometry (line pattern spacing 10, 15, 25, 50, 100μm). The different parameter variations enabled the identification of five contributions to the impedance spectra attributed, respectively, to wiring and contacts, grain, grain boundary, current constriction at the triple phase boundary, and electrode effects. The ionic conductivity of the CGO films were characterized by the expression σ =(A/T)exp(−E A/kT), with A =108.66±0.72 Scm−1 K and E A =0.97±0.07eV with magnitude higher than values reported for both bulk and thin film CGO previously reported in the literature. The electrochemical impedance of the Pt electrodes was found to decrease markedly for CGO films grown at reduced temperatures. Explanations for these observations are presented. [Copyright &y& Elsevier]

Abstract: Thin film microstructured La1−x Sr x CoO3 (LSCO) electrodes for micro-SOFC application and kinetic studies were characterized by 4 probe electrical conductivity and electrochemical impedance spectroscopy (EIS). The electrical conductivity was analyzed between RT and 700 °C and was found to be strongly dependent on the thin film deposition parameters, in particular on the oxygen partial pressure during deposition. The highest conductivity measured at 600 °C was 700 S/cm. EIS was carried out on different substrate materials. SiO2 was found to be the most promising substrate material with respect to chemical and electrical inertness. Four main contributions were observed by EIS and were attributed respectively to the series resistance of the LSCO, to the grain boundary impedance of the CGO electrolyte and to LSCO electrode reactions. [Copyright &y& Elsevier]

Abstract: Recently, SrTi0.65Fe0.35O3−δ (STF-35) has received considerable attention for use as a resistive oxygen sensor due to its unusual temperature-independent conductivity above 700°C and pO2 >1Pa. While promising as an alternative to zirconia-based amperometric sensors in lean-burn engines, issues of chemical stability, reliability, and response time must be addressed, as well as achieving an improved scientific understanding of this material. This work focuses on the electrical properties and stability of STF thin films grown via pulsed laser deposition (PLD), which allows for control of crystallinity, microstructure and film morphology, features of relevance for gas sensing. STF-35 films were grown on both single crystal sapphire and MgO substrates. After annealing for 5h at 1200°C in ambient atmosphere, the sapphire reacted with the film to form SrAl12O19. No such reaction occurred with the MgO substrate, though RBS data indicates that the film was depleted of Sr and Fe near the interface. The temperature dependence of the electrical conductivity of thin film specimens was characterized with a microprobe station outfitted with a hot stage capable of reaching temperatures of greater than 900°C. The electrical conductivity of the thin films is at least one order of magnitude lower than the reported data on bulk STF-35 and in contrast to bulk STF-35, shows an activated behavior with activation energies in the range of 0.15–0.22eV above 500°C. The modified behavior of the films is ascribed to Fe depletion in the vicinity of the substrate–film interface. [Copyright &y& Elsevier]

Fundamental understanding of materials properties requires the study and development of defect chemical reactions and models. In-depth analysis of the defect chemistry of metal oxides, however, has generally been limited to the oxygen deficient regime. Here, we present a defect chemical study covering both oxygen deficient and excess regimes using donor-doped layered cuprate (La 1.85 Ce 0.15 CuO 4+δ) thin films over a wide range of oxygen activity levels. Unlike perovskite- or fluorite-structured oxides, layered cuprates can readily accommodate oxygen interstitials as well as vacancies. The scope of the defect chemical study was extended to include highly oxidizing conditions equivalent to many orders of magnitude higher oxygen activities than ambient pressure. This was achieved by electrochemical pumping of oxygen through an ionically conducting yttria-stabilized zirconia (YSZ) substrate with oxygen nonstoichiometry values derived from in situ chemical capacitance measurements between 500°C and 650°C. The defect chemical model was correlated with in-plane conductivity at different oxygen pumping levels in the temperature regime of 275~350°C. Thermodynamic parameters - thermal band gap, enthalpy of reduction and anion Frenkel-pair formation - were derived from a defect chemical analysis. The approach taken here can be extended to other layered oxides and opens up the possibility of accessing much broader oxygen activity limits and thereby an extended range of defect regimes and neighboring phases. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract The effects of strain on the ionic conductivity of rare-earth substituted CeO 2 have been extensively studied, but the results have been inconsistent and focused upon the 'optimised' conductors such as Gd or Sm substituted CeO 2 where defect association is minimised. By thermally annealing epitaxial films deposited by pulsed laser deposition, we varied the strain systematically, whilst avoiding any influence from interfacial or grain boundary effects. The activation energy of the in-plane conductivity was found to increase with increasing compressive biaxial strain, which was quantitatively in excellent agreement with previous computational and experimental studies. These results provide a much needed quantitative consensus on the effects of lattice strain on ionic transport. Furthermore, we demonstrate that the change in the activation energy for Yb-substituted CeO 2 is around three times that for Gd or La substitutions for the same applied strain, indicating the important role played by defect association. These results have significant implications for ionic transport at reduced or ambient temperatures, where changes in conductivity due to strain may be several orders of magnitude larger for 'non-optimised' conductors compared with 'optimised' conductors. We rationalise our results by considering the defect-defect interactions in these materials and through force-field calculations. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

While oxygen defect equilibrium in functional oxide thin films has a strong impact on device performance and longevity, few methods exist for evaluating it in situ. In this study, simultaneous in situ optical transmission and electrochemical impedance spectroscopy measurements are demonstrated as powerful and convenient means for this purpose, utilizing PrxCe1-xO2-δ (PCO) with multiple oxidation states for Pr (3+ and 4+) as a model system. The Pr4+ color center optical absorptivity increased linearly with the Pr4+ concentration, each independently derived from optical and chemical capacitance measurements, respectively, at elevated temperatures (550-700°C) and under controlled pO2 (10-4-1 atm), validating the use of optical absorption as a convenient means for monitoring defect concentrations and oxygen nonstoichiometry. The extracted extinction coefficient for Pr4+ (εPr4+ = 5.86 ± 0.12 × 10-18 cm² with y-axis intercept of 927.5 ± 207.3 cm-1) can now be utilized to study defect equilibria of PCO, and by extension other relevant oxide films, by optical means alone. This enables defect characterization at reduced temperatures where other characterization techniques, for example, chemical capacitance, may not be feasible. [ABSTRACT FROM AUTHOR]

The chemical expansion of thin-film praseodymium‑cerium oxide solid solutions (Pr 0.1 Ce 0.9 O 2−δ , PCO10) is investigated at temperatures ranging from 500 °C to 800 °C and oxygen activities down to about 10−22. Differential Laser Doppler Vibrometry (D-LDV) in combination with periodic electrochemical pumping of oxygen is used to detect very small displacements of the sample surface. With decreasing pumping frequency, the film approaches chemical equilibrium, which is nearly reached at 700 °C below 0.01 Hz. D-LDV enables direct measurement of film thickness changes as well as the bending of the underlying substrates due electrochemically induced lateral mechanical stress. The substrate bending is found to be more than an order of magnitude larger than the film thickness change, increasing the detection limit by the same factor. The thickness change of the thin film is calculated from the measured displacement of the sample surface and compared with directly measured values. Thin-film literature data available for low oxygen activities up to 650 °C agree with our measurements. In addition, the thickness change of the film is determined from the displacement at different sample positions, which validates previous measurements. Finally, thin film chemical expansion is found to be larger by a factor of about 2 than the corresponding bulk value in agreement with predictions from a mechanical model that the out-of-plane expansion of a constrained film is increased. • Electrochemical adjustment of oxygen activity in the films. • Contact-less detection of chemical expansion in thin films up to 750 °C. • Differential Laser-Doppler Vibrometry reduced noise at low frequencies. • Detection of nm-scale motions at 0.001 Hz. • Strain in the films is found to be about twice as large as in bulk samples. [ABSTRACT FROM AUTHOR]

The mixed ionic and electronic conductor Sr(Ti,Fe)O 3 - α (STF) exhibits fast oxygen surface exchange kinetics, with electron concentration potentially playing a key role. The effect of La donor doping on electron concentration, Fermi level, and overall defect chemistry of STF is investigated on Sr 1 - y La y Ti 0.65 Fe 0.35 O 3 - α (LSTF, 0 ≤ y ≤ 0.5) thin films. Defect chemical modeling, optical absorption, and electrical conductivity measurements in oxidizing conditions indicate compensation of donors by an increase in oxygen and electron concentrations, increase in Fermi level, decrease in oxygen vacancy and hole concentrations, and formation of cation vacancies (for [La] > [Fe]). The surface exchange coefficient, measured by impedance spectroscopy, decreased with increasing donor concentration, suggesting that oxygen exchange kinetics in LSTF are limited by low oxygen vacancy and/or hole concentrations, rather than electron transfer. [ABSTRACT FROM AUTHOR]

Abstract: SnO2 hollow nanofibers (HNFs) with diameters of 300–500nm were fabricated via electrospinning of templating nanofibers and RF-sputtering of SnO2 thin overlayers (15–20nm thick and comparable grain size) followed by heat-treatment at 450°C. Gas sensors using these HNFs exhibited n-type gas sensing characteristics and an enhanced gas response to 2ppm NO2 (R gas /R air =81.4) as compared to that (R gas /R air =19.9) of planar SnO2 thin films. The enhanced response to NO2 is attributed to the greater accessible active area and the greater space charge modulation depth associated with the hollow thin walled structures. [Copyright &y& Elsevier]

Abstract: Chemical and physical synthesis routes were combined to prepare macroporous TiO2 thin films by RF sputtering onto poly methyl methacrylate (PMMA) microsphere templated substrates. The organic template was removed by thermal decomposition at 450°C leaving a thin layer of TiO2 anatase hollow hemispheres with diameter commensurate with that of the original PMMA microspheres (800nm). This unique morphology increased active surface area while reducing the deleterious interfacial effects between film and substrate leading to substantially enhanced NO2 sensitivity. [Copyright &y& Elsevier]

• Pr 0.1 Ce 0.9 O 2 (PCO) films were fabricated by pulsed laser deposition (PLD). • The YSZ-based sensor using a PCO sensing electrode with a thickness of 2200 nm showed large response to 50 ppm toluene. • Interfaces between PCO/gas in the sensing electrode contributed to enhance the toluene response. • The sensor certainly detected 0.25 ppm toluene in dry air. Pr 0.1 Ce 0.9 O 2 (PCO) films, with columnar structure containing voids, were fabricated by pulsed laser deposition (PLD) and integrated into YSZ-based potentiometric gas sensor as a sensing electrodes (SEs). Sensors with PCO film thicknesses of 550, 1100 and 2200 nm, fabricated by controlling the number of PLD shots of 750, 1500 and 3000 respectively, were examined over the temperature range of 450–600 °C. The sensor output voltage, E, of all sensors showed increasing negative change with increasing toluene concentration. The sensor with the thickest PCO SE film (thickness: 2200 nm) showed a much larger toluene response at 450 °C than the others (PCO-SE thickness: 550, 1100 nm). Because PCO is a mixed ionic and electronic conductor and the morphology of SE was porous, the electrochemical oxidation reaction is viewed as occurring at the double phase boundaries – DPBs (interfaces between PCO/gas) as well as triple phase boundaries – TPBs (interfaces of PCO/YSZ/gas). Because a fraction of the toluene is catalytically oxidized in part during gas diffusion within the SE films, especially at lower operating temperatures, the much larger toluene response can be explained by the electrochemical oxidation reaction of the partially oxidized products together with toluene at the TPBs and DPBs. [ABSTRACT FROM AUTHOR]