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Polaritons enable subwavelength confinement and highly anisotropic flows of light over a wide spectral range, holding the promise for applications in modern nanophotonic and optoelectronic devices. However, to fully realize their practical application potential, facile methods enabling nanoscale active control of polaritons are needed. Here, we introduce a hybrid polaritonic-oxide heterostructure platform consisting of van der Waals crystals, such as hexagonal boron nitride (hBN) or alpha-phase molybdenum trioxide (α-MoO3), transferred on nanoscale oxygen vacancy patterns on the surface of prototypical correlated perovskite oxide SmNiO3 (SNO). Using a combination of scanning probe microscopy and infrared nanoimaging techniques, we demonstrate nanoscale real-time reconfigurability of complex hyperbolic phonon polaritons patterned at the nanoscale with unmatched resolution. Hydrogenation and temperature modulation allow spatially localized conductivity modulation of SNO nanoscale patterns, enabling robust real-time modulation and nanoscale reconfiguration of hyperbolic polaritons. Our work paves the way towards nanoscale programmable metasurface engineering as a new paradigm for reconfigurable nanophotonic applications facilitated by a hybrid material platform exploiting extreme light-matter interactions in polaritonic systems.

Author(s): Chiu, IT; Lee, MH; Cheng, S; Zhang, S; Heki, L; Zhang, Z; Mohtashami, Y; Lapa, PN; Feng, M; Shafer, P; N'Diaye, AT; Mehta, A; Schuller, JA; Galli, G; Ramanathan, S; Zhu, Y; Schuller, IK; Takamura, Y | Abstract: Topotactic transformations involve structural changes between related crystal structures due to a loss or gain of material while retaining a crystallographic relationship. The perovskite oxide La0.7Sr0.3CoO3 (LSCO) is an ideal system for investigating phase transformations due to its high oxygen vacancy conductivity, relatively low oxygen vacancy formation energy, and strong coupling of the magnetic and electronic properties to the oxygen stoichiometry. While the transition between cobaltite perovskite and brownmillerite (BM) phases has been widely reported, further reduction beyond the BM phase lacks systematic studies. In this paper, we study the evolution of the physical properties of LSCO thin films upon exposure to highly reducing environments. We observe the rarely reported crystalline Ruddlesden-Popper phase, which involves the loss of both oxygen anions and cobalt cations upon annealing where the cobalt is found as isolated Co ions or Co nanoparticles. First-principles calculations confirm that the concurrent loss of oxygen and cobalt ions is thermodynamically possible through an intermediary BM phase. The strong correlation of the magnetic and electronic properties to the crystal structure highlights the potential of utilizing ion migration as a basis for emerging applications such as neuromorphic computing.

Nano-membrane tri-gate β-gallium oxide (β-Ga2O3) field-effect transistors (FETs) on SiO2/Si substrate fabricated via exfoliation have been demonstrated for the first time. By employing electron beam lithography, the minimum-sized features can be defined with the footprint channel width of 50 nm. For high-quality interface between β-Ga2O3 and gate dielectric, atomic layer-deposited 15 nm thick aluminum oxide (Al2O3) was utilized with tri-methyl-aluminum (TMA) self-cleaning surface treatment. The fabricated devices demonstrate extremely low subthreshold slope (SS) of 61 mV dec−1, high drain current (I DS) ON/OFF ratio of 1.5 × 109, and negligible transfer characteristic hysteresis. We also experimentally demonstrated robustness of these devices with current–voltage (I–V) characteristics measured at temperatures up to 400 °C.

The open circuit voltage of solid oxide fuel cells (SOFCs), made with thin film electrolytes of yttria-stabilized cubic zirconia, often lies below the Nernstian value, which has been prescribed to processing defects. The effect persists in films fabricated by different methods and even when their thickness ranges from nanometers to micrometers, which is well beyond the electron direct tunneling length scale. Here we explain this phenomenon in terms of an alternative mechanism, a flash transition, where yttria stabilized zirconia can partially transition into an electronic state; the transition depends on (i) the resistance of the electrolyte, (ii) the electric field and (iii) the temperature. The variability in the open circuit voltage is subscribed to processing-related variability in the resistivity of the films, which has a strong influence on the flash transition.

Point defects, such as oxygen vacancies, control the physical properties of complex oxides, relevant in active areas of research from superconductivity to resistive memory to catalysis. In most oxide semiconductors, electrons that are associated with oxygen vacancies occupy the conduction band, leading to an increase in the electrical conductivity. Here we demonstrate, in contrast, that in the correlated-electron perovskite rare-earth nickelates, RNiO(3) (R is a rare-earth element such as Sm or Nd), electrons associated with oxygen vacancies strongly localize, leading to a dramatic decrease in the electrical conductivity by several orders of magnitude. This unusual behavior is found to stem from the combination of crystal field splitting and filling-controlled Mott–Hubbard electron–electron correlations in the Ni 3d orbitals. Furthermore, we show the distribution of oxygen vacancies in NdNiO(3) can be controlled via an electric field, leading to analog resistance switching behavior. This study demonstrates the potential of nickelates as testbeds to better understand emergent physics in oxide heterostructures as well as candidate systems in the emerging fields of artificial intelligence.

A new research field of functional materials and device physics is rising that combines ionic transport with charge carrier modulation to realize emergent physical properties and discovery of metastable phases. The paradigm for enabling function extends far beyond carrier accumulation or depletion in band semiconductors or simply moving ions through an insulating electrolyte. Rather, by carefully selecting electronically or structurally fragile materials, one can collapse or open band gaps via extreme ionic dopant concentration, or reconfigure their entire crystal structure to create new phases. Electron–electron and electron–lattice interactions can be coupled or controlled independently in such systems via electric fields without thermal constraints by use of ionic dopants. The unifying theme across these studies is to introduce ions and electrons via electric fields through interfaces, with electrochemistry playing a dominant role. In this review, we briefly summarize this nascent field of iontronics and discuss principal results to date with examples from binary and complex oxides as well as selected 2D materials systems. We conclude the review by highlighting gaps in fundamental scientific understanding and prospects for the use of such novel devices in future electronic, photonic and energy technologies.

Using a variety of density functional theory (DFT) methods, we present a systematic computational screening effort to analyze the chemical and radiation stability for a large number of anions and cations that constitute room-temperature ionic liquids (RTILs). We compute various electronic properties such as the HOMO–LUMO gap, the ionization potential, and the electron affinities for a large library of ions (42 cations and 42 anions). The theoretical analysis provides the most comprehensive characterization of the chemical and radiation stability of individual ions in RTILs to date. Our calculations reveal that cation stability is closely related to constituent alkyl chain length and branching, whereas the anion stability is mostly dictated by ion size and electronegativity. Furthermore, these calculations show that the ωB97XD functional is the most internally consistent for predicting the chemical and radiation stability. These calculations establish a chemical stability database and a theoretical procedu...

Refractory carbides possess metal-like electronic and catalytic properties, which make them interesting candidates for anodes in solid oxide fuel cells. However, significant challenges include phase instability due to electrochemical potential gradient driven oxidation. This requires an understanding of both the chemical thermodynamics in operating environments along with direct measurement of the catalytic activity in fuel mixtures. Here, we present an experimental study on nanostructured WC as an anode for solid oxide fuel cells operating at 300–500 °C. This is enabled by combining calculated thermochemical equilibria validated against experiments at the material level and in fuel cell devices combined with flow reactor studies on fuel-selective catalytic activity directly at working anode interfaces. With an optimized anode microstructure and hydrogen–methane fuel mixtures, WC anode-based solid oxide fuel cells are shown to achieve a near-ideal open circuit voltage of 1.1 V at 500 °C.

Fuel cells convert chemical energy directly into electrical energy with high efficiencies and environmental benefits, as compared with traditional heat engines. Yttria-stabilized zirconia is perhaps the material with the most potential as an electrolyte in solid oxide fuel cells (SOFCs), owing to its stability and near-unity ionic transference number. Although there exist materials with superior ionic conductivity, they are often limited by their ability to suppress electronic leakage when exposed to the reducing environment at the fuel interface. Such electronic leakage reduces fuel cell power output and the associated chemo-mechanical stresses can also lead to catastrophic fracture of electrolyte membranes. Here we depart from traditional electrolyte design that relies on cation substitution to sustain ionic conduction. Instead, we use a perovskite nickelate as an electrolyte with high initial ionic and electronic conductivity. Since many such oxides are also correlated electron systems, we can suppress the electronic conduction through a filling-controlled Mott transition induced by spontaneous hydrogen incorporation. Using such a nickelate as the electrolyte in free-standing membrane geometry, we demonstrate a low-temperature micro-fabricated SOFC with high performance. The ionic conductivity of the nickelate perovskite is comparable to the best-performing solid electrolytes in the same temperature range, with a very low activation energy. The results present a design strategy for high-performance materials exhibiting emergent properties arising from strong electron correlations.

A fundamental understanding of coupled electrochemical processes including metal dissolution, structural evolution and solvation dynamics at the atomic level is of interest to corrosion research and electrochemistry in general. Using molecular dynamics (MD) simulations based on a reactive force field (ReaxFF), we evaluate the impact of non-stoichiometry in a model system of copper oxide passive films on the local fluctuation of the chloride ion density and structure and dynamics of interfacial water layers. We investigate (a) the interplay of oxygen content in the passive oxide film and the solvation dynamics of halide ions in the aqueous interfacial layers during breakdown of the oxide film, and (b) their combined effects on the dissolution kinetics of copper and adsorption of chloride ions on the copper-oxide surface. We demonstrate that the solvation behavior, particularly near the oxide/aqueous medium interface, is strongly correlated with the interfacial chloride ion concentration, which in turn is influenced by the oxygen stoichiometry in the passive oxide. Residence probability and hydrogen-bond correlations show that water present in the aqueous media forms ordered layers on oxide films with high oxygen content; and as the oxygen content is reduced, this order gets disrupted due to increased chloride ion adsorption. Interfacial molecular order is, therefore, strongly correlated with stoichiometry of the passive oxide film.

Electrolyte gating of complex oxides enables investigation of electronic phase boundaries and collective response to strong electric fields. The origin of large conductance modulations and associated emergent properties in such field effect structures is a matter of intense study due to competing contributions from electrostatic (charge accumu- lation) and electrochemical (crystal chemistry changes) effects. Vanadium dioxide (VO2) is a prototypical correlated insulator that shows an insulator-to-metal transition at ∼67 °C and recent studies have noted a vast range of electronic effects in electric double-layer transistors (EDLT). In this study, we demonstrate that the response of electrolyte gated VO2 devices can be deterministically controlled by inserting a monolayer of graphene at the oxide−electrolyte interface. Several electrolytes as well as dopants (such as lithium ions and protons) were employed in EDL transistors to show that graphene serves as an inert barrier that successfully protects the oxide surface from chemical reactions. This monolayer interface has a striking effect on resistance modulation in the vanadium dioxide transistor channel up to several orders of magnitude and enables retention of the insulating phase. The studies allow new insights into the response of correlated insulators in EDLTs and inform design of correlated oxide−2D heterostructures for electronics and sensors.

Materials that undergo an electronic phase change in a reversible manner open up new directions for research in light-matter interactions and photonic devices. The highly tunable dielectric properties, along with the spatial control of insulating and metallic domains, create photonic-crystal-like environments to control light propagation. In this brief overview, recent advances in photonics that utilize solid-state phase-change systems such as vanadium dioxide (VO2) and chalcogenides are discussed. The controllable optical properties such as the refractive index and the optical conductance between two distinct phases in these materials pave the way for switchable photonic devices with memory. Furthermore, the intermediate states with the coexistence of two phases in the vicinity of phase transition in these materials behave as a tunable metamaterial with extraordinary optical properties that is promising for applications such as perfect absorbers.

Finding ways to tune their states is the key to using correlated electron systems in device applications. The authors show how to cleanly generate an extremely oxygen-starved environment, which leads to the collapse of the insulating ground state in VO${}_{2}$, a prototypical correlated oxide. A nonvolatile metallic phase with unique optical properties is stabilized down to 2 K by oxygen vacancies, and the insulating phase plus intermediate-resistance states can be accessed reversibly via ``oxygen breathing''.

Thin film solid oxide fuel cells incorporating vanadium oxide anodes having open circuit potential of 1 V with hydrogen fuel have been realized. The as-deposited anode stoichiometry was varied by choice of growth conditions of vanadium oxide and a striking correlation to fuel cell performance (open circuit potential and peak power density) is observed. Possible mechanisms leading to the experimental observations based on calculated thermodynamic phase stability under fuel cell operating environments, spectroscopic analysis of the anodes and strain-related arguments are presented.

Interrogation of conduction mechanisms in quasi two-dimensional (2D) brittle or highly resistive materials that are in a suspended form presents a formidable experimental challenge. We report high temperature conductivity studies on substrate-free self-supported nano-crystalline ceria membranes up to 800 K utilizing a layer transfer method that enables four-terminal carrier transport studies under extreme environments. Utilizing such structures we observe increasing conductivity with oxygen partial pressure directly opposing the behavior of thin film devices ‘clamped’ by the substrate. We illustrate that the relaxed nature of free standing membranes, and increased surface to volume ratio enable a more sensitive electrical response to oxygen adsorption on the surface which could have implications for their use in oxygen storage devices, solid oxide fuel cells, and chemical sensors.

Professor Arthur Nowick made seminal contributions to the areas of ionic conduction mechanisms in crystalline and disordered systems. An area of emerging interest in the solid state ionics community is investigating conduction in the mesoscopic regime. With free standing membranes, one can probe low-dimensional effects such as confinement without interference from substrates. Membranes have potential relevance to solid state devices that benefit from reduced ionic resistance, for example sensors and solid oxide fuel cells. Membranes with varying lateral dimensions have been previously reported in literature; however, understanding of stress interactions in suspended oxide structures is in early stages. In this paper, we demonstrate self-supported, i.e. in the absence of any additional mechanical support layers, square and circular membranes of 100 nm thick yttria-doped zirconia (YDZ) having side length and diameters of 0.15–2 mm. The buckled membrane shape is intimately linked to the fabrication processes arising from dry versus wet etching protocols. Geometrical considerations associated with buckling and stability are discussed. Thin film solid oxide fuel cells utilizing circular membranes are fabricated, exhibiting open circuit voltages between 0.8 and 1 V that correlate with membrane size and exhibit a total power output on the order of several mW. These results contribute to advancing experimental techniques to fabricate free standing oxide membranes for fundamental and applied studies pertaining to ionic and electronic conduction.

Ultrathin fast-ion conducting oxide membranes are of broad interest to a range of energy conversion technologies. We demonstrate a low-temperature (30 °C) process for controlling internal stress in an archetypal fast-ion conductor, crystalline Y2O3-doped ZrO2 (YDZ), which allows us to form stable suspended nanomembranes akin to those fabricated at high temperature (550 °C). Such a low-temperature synthesis method then enables us to monolithically integrate the suspended oxide-ion conducting membranes onto polyimide (Kapton by DuPont), a polymer with vastly different physical properties than that of a ceramic. Integrated functional heterostructure solid oxide fuel cells operable below the glass transition temperature of the polymer are demonstrated. Our results describe a mechanistic low-temperature processing route for forming stable multifunctional membrane structures, applicable to the realization of various energy conversion and sensing devices and structural skins for miniature autonomous systems.

Direct utilization of hydrocarbons in low temperature solid oxide fuel cells is of growing interest in the landscape of alternative energy technologies. Here, we report on performance of self-supported micro-solid oxide fuel cells (μSOFCs) with ruthenium (Ru) nano-porous thin film anodes operating in natural gas and methane. The μSOFCs consist of 8 mol% yttria-stabilized zirconia thin film electrolytes, porous platinum cathodes and porous Ru anodes, and were tested with dry natural gas and methane as fuels and air as the oxidant. At 500 °C, comparable power densities of 410 mW cm −2 and 440 mW cm −2 were obtained with dry natural gas and methane, respectively. In weakly humidified natural gas, open circuit voltage of 0.95 V at 530 °C with peak power density of 800 mW cm −2 was realized. The μSOFC was continuously operated at constant voltage of 0.7 V with methane, where quasi-periodic oscillatory behavior of the performance was observed. Through post-operation XPS studies it was found that the oxidation state of Ru anode surfaces significantly differs depending on the fuel used, oxidation being enhanced with methane or natural gas. The nature of the oscillation is discussed based on the transition in surface oxygen coverage states and electro-catalytic activity of Ru anodes.

This proceedings manuscript describes representative results of thin film micro-solid oxide fuel cells (micro-SOFCs) utilizing nano-porous ruthenium (Ru) and Ru – gadolinia doped ceria (CGO) nano-composite anodes, investigated for operation with various fuels. 8 mol% yttria-stabilized zirconia (8YSZ) and compositionally graded CGO-8YSZ thin films were fabricated on silicon substrates as self-supported electrolytes. Performance of devices integrating such electrodes and electrolytes are presented with methane and natural gas as fuels. Oscillations in fuel cell current were observed when nano-porous Ru and Ru-CGO anode micro-SOFCs were operated at constant voltage with methane. Degradation mechanisms of pure metallic electrodes are then compared to composite electrodes at the intermediate temperature range of 300 - 500 °C. New synthesis methods to realize percolating ultra-thin composite electrodes is highlighted as a promising research direction.

We demonstrate bipolar switching with high OFF/ON resistance ratios (>104) in Pt/vanadium oxide/Cu structures deposited entirely at room temperature. The SET (RESET) process occurs when negative (positive) bias is applied to the top Cu electrode. The vanadium oxide (VOx) films are amorphous and close to the vanadium pentoxide stoichiometry. We also investigated Cu/VOx/W structures, reversing the position of the Cu electrode, and found the same polarity dependence with respect to the top and bottom electrodes, which suggests that the bipolar nature is linked to the VOx layer itself. Bipolar switching can be observed at 100 °C, indicating that it not due to a temperature-induced metal–insulator transition of a vanadium dioxide second phase. We discuss how ionic drift can lead to the bipolar electrical behavior of our junctions, similar to those observed in devices based on several other defective oxides. Such low-temperature processed oxide switches could be of relevance to back-end or package integration processing schemes.

We investigate the orientation transition and strain relaxation in oxygenated epitaxial strontium-doped lanthanum nickelate, La1.875Sr0.125NiO4+δ (LSNO) thin films grown on single crystal strontium titanate, SrTiO3 (STO) substrates. Structural evolution has been studied as a function of film thickness by x-ray diffraction, pole figure analysis, and transmission electron microscopy (TEM). The LSNO layer grows epitaxial (OR1) with respect to the STO substrate with an orientation (001)OR1//(001)STO and OR1// STO. This orientation is maintained up to approximately 15 nm as observed from TEM, at which point it undergoes reorientation and lattice mismatch strain relaxation. The growth continues with a new orientation (OR2) as (100)OR2//(001)OR1 and with OR2// OR1 and OR2// OR1 with respect to the epitaxial LSNO layer. We consider possible mechanisms in detail leading to these phenomena given that the potassium nickel fluoride (K2NiF4)-structured lattice is able to accommodate excess oxygen interstitials and corresponding changes in the Ni valence state. By investigating the phase space of deposition parameters, we experimentally identify key factors leading to the reorientation phenomena.

The effect of electric field assisted annealing on the microstructure, composition and ionic conductivity properties in CeO2/YSZ oxide heterostructures have been investigated using molecular dynamics simulations. Amorphization–recrystallization steps were performed with and without external electric field of strength 10 MV/cm along three different orientations: in-plane (YZ), normal (X) and 45° resultant (XY) with respect to the oxide heterointerfaces. The microstructural and compositional differences at the interfaces and in the interior of the oxide heterolayers were evaluated and were found to show a clear correlation with the orientations of the applied field. In particular, the XY configuration displayed a compressive lattice strain of ∼9% along with a reduced oxygen vacancy concentration when compared to the others. Ionic density profiles suggest pronounced segregation (∼60% higher compared to the average value in the interior) of yttrium ions closer to the YSZ/CeO2 interface for the XY configuratio...

Ultra-thin oxide films are of interest in energy conversion technologies such as low temperature solid oxide fuel cells and permeation membranes. Understanding their thermo-mechanical stability is an important problem. Edge clamped, self-supported thin film membranes show hierarchical wrinkles; with the largest wavelengths in the center, while smaller ones arise near the clamped boundary; correspondingly the largest strains, with tensile stress comparable to the residual stress, are in the vicinity of the clamped boundary. Our results can be understood by simple scaling arguments and are valid for membranes in the post-buckling regime far from threshold. We confirm the validity of our analysis by quantitative experimental comparison to self-supported, square micro-machined yttria-stabilized zirconia membranes of edge length 160 μm fabricated by lithography. The modeling and experiments combined provide a foundation for designing failure resistant self-supported membranes of interest to energy conversion. We show this by utilizing such membranes to fabricate thin film solid oxide fuel cells and demonstrate power generation utilizing natural gas as fuel at ∼400 °C.

Oxidation of iron surfaces and oxide growth mechanisms have been studied using reactive molecular dynamics. Oxide growth kinetics on Fe(100), (110), and (111) surface orientations has been investigated at various temperatures and/or an external electric field. The oxide growth kinetics decreases in the order of (110), (111), and (100) surfaces at 300 K over 1 ns timescale while higher temperature increases the oxidation rate. The oxidation rate shows a transition after an initial high rate, implying that the oxide formation mechanism evolves, with iron cation re-ordering. In early stages of surface oxide growth, oxygen transport through iron interstitial sites is dominant, yielding non-stoichiometric wüstite characteristics. The dominant oxygen inward transport decreases as the oxide thickens, evolving into more stoichiometric oxide phases such as wüstite or hematite. This also suggests that cation outward transport increases correspondingly. In addition to oxidation kinetics simulations, formed oxide layers have been relaxed in the range of 600-1500 K to investigate diffusion characteristics, fitting these results into an Arrhenius relation. The activation energy of oxygen diffusion in oxide layers formed on Fe(100), (110), and (111) surfaces was estimated to be 0.32, 0.26, and 0.28 eV, respectively. Comparison between our modeling results and literature data is then discussed. An external electric field (10 MV cm(-1)) facilitates initial oxidation kinetics by promoting oxygen transport through iron lattice interstitial sites, but reaches self-limiting thickness, showing that similar oxide formation stages are maintained when cation transport increases. The effect of the external electric field on iron oxide structure, composition, and oxide activation energy is found to be minimal, whereas cation outward migration is slightly promoted.

Phase change materials such as chalcogenides and complex oxides possess electronic structures that are widely tunable via external stimuli. Under the right triggers, these can be volatile (threshold switches) or non-volatile (memory). I will present an overview of correlated oxide systems where one can create sub-wavelength dielectric environments on-demand. The role of materials synthesis and their influence in photonic device performance will be considered.

Ruthenium and gadolinia-doped ceria (Ru–CGO) composite nano-crystalline thin film anodes are demonstrated for the first time in self-supported micro-solid oxide fuel cells (μSOFCs) for direct methane utilization, targeting development of microstructurally stable anodes under operation. Nano-composite thin film anodes are synthesized by co-sputtering from Ru metal and CGO oxide targets. Detailed electrical, crystalline and microstructural analyses were carried out on composite films to characterize properties as a function of Ru/CGO relative fraction. Microstructural stability of the composite film is compared to Ru metal film from conductivity and grain size behavior during 500 °C thermal treatments. Stress-relaxed composite thin film anodes were developed on self-supported μSOFCs with yttria-stabilized zirconia thin film electrolytes and porous platinum cathodes. μSOFCs were tested with room temperature humidified methane as fuel and air as the oxidant, exhibited an open circuit voltage of 0.97 V and a peak power density of 275 mW cm −2 at 485 °C. Microstructural stability of Ru–CGO composite anodes relative to metal electrodes is discussed. The results suggest a general approach to synthesis of functional metal-oxide nano-composite thin films for electrodes in fuel cells and related energy conversion devices.

Samarium nickelate (SmNiO 3 ) is a correlated oxide that exhibits a metal–insulator transition (MIT) above room temperature and is of interest for advanced electronics and optoelectronics. However, studies on SmNiO 3 thin films have been limited to date, in part due to well-known difficulties in stabilizing the Ni 3+ valence state during growth, which are manifested in non-reproducible electrical characteristics. In this work, we show that stable epitaxial SmNiO 3 thin films can be grown by rf magnetron sputtering without extreme post-deposition annealing conditions using relatively high growth pressure (>200 mTorr). At low growth pressure, SmNiO 3 is insulating and undergoes an irreversible MIT at ∼430 K. As pressure is increased, films become metallic across a large temperature range from 100 to 420 K. At high pressure, films are insulating again but with a reversible and stable MIT at ∼400 K. Phase transition properties can be continuously tuned by control of the sputtering pressure.

This proceedings manuscript describes in brief representative results on porous Ru anodes incorporated into thin film solid oxide fuel cells (SOFCs). Thin film of 8 mol% yttria-stabilized zirconia (YSZ) was fabricated as free-standing electrolytes, with porous platinum (Pt) and porous Ru deposited as cathode and anode electrodes, respectively. μSOFCs were tested with methane as the fuel and air as the oxidant, exhibiting an open circuit voltage of 0.71 V and a peak power density of 450 mW/cm2 at 500 °C. Anode electrodes were investigated for structural change and was found that morphology evolution in anode electrodes were strongly dependent on the fuels used. Auger electron spectroscopy was carried out on Ru anodes before and after methane operation, and revealed minimal amount of carbon existing on Ru surface, possibly being reaction intermediates. On-going studies on Ru-CGO composite anodes are discussed.

Thermomechanical challenges place restrictions on the choice of fast ion conductors that may be implemented as free standing electrolyte membranes for low temperature solid oxide fuel cells. In order to expand the possible choices, mechanical and chemical stability constraints must be taken into consideration. Here, we present a method to utilize the mechanical stability of a ZrO 2 based electrolyte for this application. Facile low temperature synthesis of solid solution (Y 2 O 3 ) 0.08 (ZrO 2 ) 0.92 –(Gd 2 O 3 ) 0.1 (CeO 2 ) 0.9 free standing electrolytes by co-sputtering is demonstrated. Fuel cells integrating these nanoscale electrolytes show power output of over 1000 mW cm −2 at 510 °C and are thermomechanically robust. The results demonstrate a general route for low temperature synthesis of nanoscale functional oxide alloys for thin film solid oxide fuel cells.

Compositional stability of various vanadium oxides and oxide growth on vanadium surfaces have been studied using reactive molecular dynamics simulation methods. Vanadium dioxide (VO 2 ), sesquioxide (V 2 O 3 ), pentoxide (V 2 O 5 ), and hexavanadium tridecaoxide (V 6 O 13 ) are studied in bulk crystalline and thin film structures, investigating charge distribution and pair distribution functions of particle interactions. The stability is estimated to be pentoxide, hexavanadium tridecaoxide, sesquioxide, and dioxide respectively in decreasing order in thin film structures. We then analyze oxide growth kinetics on vanadium (100) and (110) surfaces. The oxidation rate, stoichiometry, charge distribution, and the effect of surface orientation on kinetic phenomena are noted. In the early stages of surface oxidation of our simulation configurations, sesquioxide is found to be the dominant component. The modeling and simulation results are compared with experiments where available.

We report on growth and physical properties of vanadium dioxide (VO 2 ) films on model conducting oxide underlayers (Nb-doped SrTiO 3 and RuO 2 buffered TiO 2 single crystals). The VO 2 films, synthesized by rf sputtering, are highly textured as seen from X-ray diffraction. The VO 2 film grown on Nb doped SrTiO 3 shows over two orders of magnitude metal–insulator transition, while VO 2 film on RuO 2 buffered TiO 2 shows a smaller resistance change but with an interesting two step transition. X-ray photoelectron spectroscopy has been performed as a function of depth on both sets of structures to provide mechanistic understanding of the transition characteristics. We then investigate voltage-driven transition in the VO 2 films grown on Nb-doped SrTiO 3 substrate as a function of temperature. The present study contributes to efforts towards correlated oxide electronics utilizing phase transitions.

We investigate oxidation and oxide growth on single-crystal copper surfaces using reactive molecular dynamics simulation. The kinetics of surface oxide growth are strongly correlated with the microstructure of the metal substrates. Simulating oxide layer growth along the (100), (110), and (111) orientations of crystalline copper, oxidation characteristics are investigated at temperatures of 300 K and 600 K. The oxidation kinetics are found to strongly depend on the surface orientation, ambient temperature, and surface defects. The effect of surface morphology on oxidation characteristics is analyzed by comparing oxygen adsorption on various sites and the structure factor. The surface oxide formed on (100) retains the initial crystal structure in the 300–600 K range. The (100) surface shows the highest oxidation rate at both temperature conditions but saturates, facilitating oxygen adsorption on hollow sites. The oxidation kinetics of the (100) orientation are found to be not significantly affected by surf...

Films of semiconductor quantum dots (QDs) are promising for lighting technologies, but controlling how current flows through QD films remains a challenge. A new design for a QD light-emitting device that uses atomic layer deposition to fill the interstices between QDs with insulating oxide is introduced. It funnels current through the QDs themselves, thus increasing the light emission yield.

Controlling the efficiency of electron transport across oxide interfaces is essential for numerous emerging technologies including advanced photovoltaics and light emitting devices. This work illuminates the connections between granular structure, defect chemistry, and the work function of a technologically important transparent conductor, ZnO:Al. Visual evidence is provided for a model of grain boundary oxidation in the form of nanometer-scale heterogeneity in the contact potential between grains and grain boundaries, a phenomenon referred to as electronic granularity. By correlating scanning probe data with photoemission spectroscopy we relate electronic granularity to defect chemistry and, importantly, account for the overall trends in work function. The resulting physical picture connects heterogeneity at the nanoscale to macroscopic properties, informs the design of transparent electrodes, and may be broadly relevant to granular oxide conductors.

Methane-fueled thin film micro-solid oxide fuel cells (μSOFCs) based on palladium (Pd) anodes are discussed in this article. The μSOFCs are composed of porous platinum (Pt) cathodes, 8 mol.% yttria-stabilized zirconia (YSZ) ultrathin electrolytes, and Pd anodes – specifically, a Pt/YSZ/Pd heterostructure synthesized by physical vapor deposition. The Pt/YSZ/Pd μSOFCs exhibit a power density of 385 mW cm−2 and an open circuit voltage of 0.77 V at 550 °C. A detailed study on synthesis, microstructure and functional properties of the nanoporous Pd films is presented. Possible anodic methane reactions, carbon deposition on Pd anodes, and carbon suppression approaches are discussed. The results are of potential relevance to advancing low temperature micro-fuel cell technology using hydrocarbon fuels.

We report on single phase polycrystalline Ba0.5Sr0.5Co0.8Fe0.2O3 thin films grown on (1 0 0) (Y2O3)0.08(ZrO2)0.92 substrates by radio frequency sputtering. Detailed studies on electrical conductivity as a function of temperature up to 500 °C are carried out for films in the 20–100 nm thickness range. Free-standing thin film micro-solid oxide fuel cells utilizing nanostructured Ba0.5Sr0.5Co0.8Fe0.2O3 cathodes are fabricated and tested for the first time. A maximum power density of 35 mW cm−2 at 520 °C was obtained with Ba0.5Sr0.5Co0.8Fe0.2O3/(Y2O3)0.08(ZrO2)0.92/Pt micro-solid oxide fuel cells. These results indicate the significance of microstructure on electrical properties of Ba0.5Sr0.5Co0.8Fe0.2O3 and present the first successful thin film micro-solid oxide fuel cells integrating Ba0.5Sr0.5Co0.8Fe0.2O3. We anticipate these results to be of relevance in advancing micro-solid oxide fuel cells for reduced temperature operation with dense oxide cathodes.

A one-step wet chemistry route has been explored to synthesize hollow hydroxyl titanium oxalate nanoscale spheres under mild experimental conditions. The hollow spheres were ∼200 nm in diameter, with a shell thickness of ∼30 nm. The nanospheres were formed by smaller aggregated colloidal subunits. The influence of temperature and solvent on the structure of the nanospheres was investigated. The formation of hollow interiors in the nanospheres may be rationalized by Ostwald ripening mechanism. Simple thermal treatment topotactically transformed the chemical composition into anatase TiO2. The high-order hollow porous spherical structure was preserved, with smaller crystalline anatase TiO2 nanoparticles as building units. Dense hydroxyl titanium oxalate nanospheres and their corresponding non-hollow porous anatase TiO2 nanospheres were also successfully achieved in suitable reaction conditions. The method and procedure reported herein may be extended in principle for the fabrication of other functional materials.

The use of oxide fuel cells and other solid-state ionic devices in energy applications is limited by their requirement for elevated operating temperatures, typically above 800°C (ref. 1). Thin-film membranes allow low-temperature operation by reducing the ohmic resistance of the electrolytes. However, although proof-of-concept thin-film devices have been demonstrated, scaling up remains a significant challenge because large-area membranes less than ~ 100 nm thick are susceptible to mechanical failure. Here, we report that nanoscale yttria-stabilized zirconia membranes with lateral dimensions on the scale of millimetres or centimetres can be made thermomechanically stable by depositing metallic grids on them to function as mechanical supports. We combine such a membrane with a nanostructured dense oxide cathode to make a thin-film solid-oxide fuel cell that can achieve a power density of 155 mW cm⁻² at 510 °C. We also report a total power output of more than 20 mW from a single fuel-cell chip. Our large-area membranes could also be relevant to electrochemical energy applications such as gas separation, hydrogen production and permeation membranes.

By employing variable-charge molecular dynamics, surface oxide film growth on aluminum−nickel−iron alloys has been studied at 300 and 600 K. The dynamics of oxidation and oxide growth is strongly d...

Symmetric, free standing thin film micro-solid oxide fuel cells of Pt/Y0.16Zr0.84O1.92/Pt structure are studied in this report. The role of parameters such as electrolyte thickness and electrode porosity on fuel cell power output was investigated. A peak power density of 1037 mW cm−2 was exhibited with an optimized structure consisting of nano-porous Pt electrodes and 100 nm thick Y0.16Zr0.84O1.92 with an open circuit voltage of 0.968 V at 500 °C using H2 as fuel and standing air as the oxidant. A twelve hour test of these devices indicates that overall performance shows extreme sensitivity to microstructural changes in the pure metallic electrodes. Results presented herein enable mechanistic routes to performance optimization, provide device stability data and are relevant to advancing micro-fuel cells for portable energy.

We report on the enhancement in impedance and the break-down behavior of room temperature deposited single and multi-layer metal/oxide films of comparable thicknesses. Aluminum–alumina and chromium–chrome oxide thin film structures grown by sputtering were explored as model systems in this study. Electrochemical impedance spectroscopy measurements showed that the resistance of the structure increases with an increase in the number of metal/oxide bi-layers while the total thickness was kept constant. The resistance of the multi-layer system was over an order of magnitude higher than the single bi-layer for an equivalent total thickness for the case of thin film Al–Al2O3. In addition, potentiodynamic polarization measurements exhibited a consistent and reproducible improvement in the break-down potential with an increase in the number of layers. Similar results are obtained for the case of the chromium–chromium oxide bi-layer system as well. Combination of interfaces and lateral current spreading in the multi-layer structure leads to superior overall break-down resistance suggesting the concept of in-situ sacrificial layers for corrosion protection.

The ability to controllably tune the work function of transparent conductors is essential for optimizing the efficiency of thin film solar photovoltaics. Here we use Kelvin force microscopy on lithographically patterned ZnO:Al thin films to measure the dependence of the work function on oxygen content during synthesis. Films were synthesized by low temperature reactive sputtering from a Zn:Al 1.2 wt% target. At optimal oxygen content the resistivity is 3×10−4 Ω cm with nearly 90% optical transmission in the 400–1100 nm wavelength range relevant to solar photovoltaics. We find that the expected relationship between band filling and work function breaks down in the vicinity of optimal oxygen stoichiometry. For oxygen-rich ZnO:Al films we measure large work functions close to 5 eV. Our results suggest a method for fabricating transparent oxide electron conductors with large, tunable work functions that could be of relevance in designing electrodes for solid state energy conversion technologies.

Functional all-oxide thin film micro-solid oxide fuel cells (μSOFCs) that are free of platinum (Pt) are discussed in this report. The μSOFCs, with widths of 160 μm, consist of thin film La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3 (LSCF) as both the anode and cathode and Y 0.08 Zr 0.92 O 1.96 (YSZ) as the electrolyte. Open circuit voltage and peak power density at 545 °C are 0.18 V and 210 μW cm −2 , respectively. The LSCF anodes show good lattice and microstructure stability and do not form reaction products with YSZ. The all-oxide μSOFCs endure long-term stability testing at 500 °C for over 100 h, as manifested by stable membrane morphology and crack-free microstructure.

Microstructure and stresses in dense La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) ultra-thin films have been investigated to increase the physical thickness of crack-free cathodes and active area of thermo-mechanically robust micro-solid oxide fuel cell (μSOFC) membranes. Processing protocols employ low deposition rates to create a highly granular nanocrystalline microstructure in LSCF thin films and high substrate temperatures to produce linear temperature-dependent stress evolution that is dominated by compressive stresses in μSOFC membranes. Insight and trade-off on the synthesis are revealed by probing microstructure evolution and electrical conductivity in LSCF thin films, in addition to in situ monitoring of membrane deformation while measuring μSOFC performance at varying temperatures. From these studies, we were able to successfully fabricate failure-resistant square μSOFC (LSCF/YSZ/Pt) membranes with width of 250 μm and crack-free cathodes with thickness of ∼70 nm. Peak power density of ∼120 mW cm−2 and open circuit voltage of ∼0.6 V at 560 °C were achieved on a μSOFC array chip containing ten such membranes. Mechanisms affecting fuel cell performance are discussed. Our results provide fundamental insight to pathways of microstructure and stress engineering of ultra-thin, dense oxide cathodes and μSOFC membranes.

The synthesis of high performance nanostructured oxide electrodes is critical to advancement of energy technologies such as intermediate temperature solid oxide fuel cells. In this communication, we demonstrate that photo-excitation during crystallization of nanostructured 60-nm-thick La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3− δ films leads to a significant improvement in electrical conductivity. Crystallization kinetics is also enhanced by photo-excitation while the crystallization onset temperature remains similar.

The Molecular beam synthesis and characterization are reported for Y2O3 thin films grown on Al2O3 (0001) substrate. The Y2O3 layer was highly oriented in the [111] direction with predominant orientation relations (111) Y2O3 ‖ (0001) Al2O3 and [110] Y2O3 ‖ [2110] Al2O3, corresponding to a lattice mismatch of ∼20% at the interface. No significant interfacial layers were found at the Y2O3/Al2O3 interface and the large lattice misfit was accommodated by formation of stacking faults, dislocations and secondary orientation in the Y2O3 layer. A La2O3 interlayer improved the quality of the Y2O3 films. Full width at half maximum (FWHM) of the Y2O3 (222) peak decreased from 3.12° to 1.43° and the defect density in the Y2O3 layer was significantly reduced. These results may be relevant in the broader context of designing oxide heterolayers with controlled microstructures.