Metal Oxide Heterostructures Research Articles

Interfacial spin-glass-like state and exchange bias in epitaxial iridate-manganite heterostructure

Interface engineering in transition metal oxide heterostructures not only gives rise to exotic physical phenomena, but also has potential applications in spintronics. In this study, we demonstrate the appearance of strong interfacial coupling in 5d paramagnetic SrIrO3 and 3d ferromagnetic La0.67Sr0.33MnO3 bilayers, which manifests itself in the dramatic enhancement of coercivity by about 70 times larger than that of the single La0.67Sr0.33MnO3 layer and the observation of an unexpected exchange bias effect up to 40 K. Systematic magnetic analysis further reveals the emergence of an interfacial spin glass state in this 5d-3d oxide bilayer. This interfacial-coupling-induced spin glass state is the underlying origin of the unconventional exchange bias effect rarely observed in paramagnetic/ferromagnetic heterostructures.

P-type Co3O4 nanoarrays decorated on the surface of n-type flower-like WO3 nanosheets for high-performance gas sensing

Metal oxide heterostructures are significant materials for developing and improving various toxic gas detection sensors. However, the gas sensing performance, such as response, selectivity, stability and response/recovery time of the sensors, still need to be optimized for practical applications. Low dimensional heterostructures will improve the gas sensing performance remarkably. In this work, hierarchical cactus-like WO3/Co3O4 nanocomposites were directly deposited on the surface of ceramic tube via a simple two-step process. The p-type Co3O4 nanorods array were grown on the surface of n-type WO3 nanosheets by using chemical bath deposition (CBD) and thus many nanoscale p-n junctions were formed which greatly improved the gas sensing performance. A response value (Ra/Rg) of 6.19 to 1 ppm triethylamine (TEA) was achieved at 240 ℃ by the WO3/Co3O4 nanocomposite with the CBD deposition time of 30 min, which is 3 times higher than that of the pure WO3 nanosheets (Sr = 2.02). And the response/recovery time was about 13 s/152 s, which shows good gas sensing performance to TEA.

Optical characteristics of charge carrier transfer across interfaces between YBa2Cu3O6+δ and La0.7Ca0.3MnO3

Transition metal oxides undergo many different phase transitions when doped with charge carriers. Thanks to recent advances in the synthesis of metal-oxide heterostructures, doping can now be achieved through charge transfer across interfaces. This paper reports spectral ellipsometry measurements on multilayers of superconducting cuprates and ferromagnetic manganates and demonstrates that the optical spectra reveal fingerprints of interfacial charge transfer. Optical spectroscopy can thus serve as a nondestructive probe of the doping level at buried metal-oxide interfaces.

Analysis of the interfacial characteristics of BiVO4/metal oxide heterostructures and its implication on their junction properties.

The formation of heterostructures has proven to be a viable way to achieve high photoelectrochemical water splitting efficiencies with BiVO4 based photoanodes. Especially, cobalt and nickel based oxides are suitable low cost contact materials. However, the exact role of these contact materials is not yet completely understood because of the difficulty to individually quantify the effects of surface passivation, charge carrier separation and catalysis on the efficiency of a heterostructure. In this study, we used photoelectron spectroscopy in combination with in situ thin film deposition to obtain direct information on the interface structure between polycrystalline BiVO4 and NiO, CoOx and Sn-doped In2O3 (ITO). Strong upwards band bending was observed for the BiVO4/NiO and BiVO4/CoOx interfaces without observing chemical changes in BiVO4, while limited band bending and reduction of Bi and V was observed while forming the BiVO4/ITO interface. Thus, the tunability of the Fermi level position within BiVO4 seems to be limited to a certain range. The feasibility of high upwards band bending through junctions with high work function (WF) compounds demonstrate that nickel oxide and cobalt oxide are able to enhance the charge carrier separation in BiVO4. Similar studies could help to identify whether new photoelectrode materials and their heterostructures would be suitable for photoelectrochemical water splitting.

Interface properties of nonpolar LiAlO2/SrTiO3 heterostructures

Advances in creating metal oxide heterostructures have received a great deal of attention because novel properties like two-dimensional electron gas (2DEG), which was first found in LaAlO3/SrTiO3 interface, while does not exist in the bulk materials. To extend the study on 2DEG, we investigate the electronic property of interesting nonpolar/nonpolar LiAlO2/SrTiO3 heterostructure (also perovskite/nonperovskite oxide heterostructure), with and without oxygen vacancies by first-principles calculations. Two types of interfaces, SrO/LiAlO2 and TiO2/LiAlO2 interfaces, were modeled with the stable stacking configurations. Oxygen vacancies at LiAlO2/SrTiO3 interface induce the carriers in this system. there is no 2DEG behavior detected. Our results show that three-dimensional transport behavior occurs in the perovskite/non-perovskite oxide interface.

SnO2 nanocrystal-Fe2O3 nanorod hybrid structures: an anode material with enhanced lithium storage capacity

Through a two-step hydrothermal method, we synthesized a metal-oxide heterostructure composed of SnO2 nanocrystals (NCs) with a diameter of ~ 6 nm and Fe2O3 nanorods (NRs) with a length of ~ 200 nm and a width of ~ 9 nm. The SnO2 NCs are well dispersed on the surface of Fe2O3 NRs. The effect of Fe2O3 NRs loading amount on the electrochemical performance of nanocomposite is investigated. The nanocomposites with less loading amount of Fe2O3 exhibit better electrochemical performance. The enhanced performance is attributed to the synergistic effect of two components. On one hand, the conversion reaction of SnO2 is facilitated due to the presence of Fe2O3 NRs, resulting in a high capacity. On the other hand, Fe2O3 NRs improve the stability of electrode, promoting the cycling performance. The high loading amount of Fe2O3 NRs renders the aggregation of electrode materials, resulting in poor electrochemical performance. Our results demonstrate that adjusting the loading amount of metal oxides in hybrid structures is an effective strategy to enhance lithium storage capacity.

Dynamic Field Modulation of the Octahedral Framework in Metal Oxide Heterostructures.

Control over the oxygen octahedral framework is widely recognized as key to the design of functional properties in perovskite oxide heterostructures. Although the oxygen octahedral framework can be manipulated during synthesis, the as-grown oxygen octahedra generally remain fixed, preventing the development of adaptive behavior in electronic and ionotronic systems. Here, it is demonstrated that the oxygen octahedral framework can be dynamically and reversibly manipulated by an electric field through the coupling with oxygen vacancies. Studying model WO3 heterostructures during ionic liquid gating with a combination of in situ X-ray scattering and spectroscopy, it is shown that large changes in electronic properties can arise due to the increased flexibility of the octahedral network at high vacancy concentrations. The results describe a generic framework for the construction of dynamic systems and devices with an array of field-tunable properties.

Adsorption of CO2 on Heterostructures of Bi2O3 Nanocluster-Modified TiO2 and the Role of Reduction in Promoting CO2 Activation.

The capture and conversion of CO2 are of significant importance in enabling the production of sustainable fuels, contributing to alleviating greenhouse gas emissions. While there are a number of key steps required to convert CO2, the initial step of adsorption and activation by the catalyst is critical. Well-known metal oxides such as oxidized TiO2 or CeO2 are unable to promote this step. In addressing this difficult problem, a recent experimental work shows the potential for bismuth-containing materials to adsorb and convert CO2, the origin of which is attributed to the role of the bismuth lone pair. In this paper, we present density functional theory (DFT) simulations of enhanced CO2 adsorption on heterostructures composed of extended TiO2 rutile (110) and anatase (101) surfaces modified with Bi2O3 nanoclusters, highlighting in particular the role of heterostructure reduction in activating CO2. These heterostructures show low coordinated Bi sites in the nanoclusters and a valence band edge that is dominated by Bi–O states, typical of the Bi3+ lone pair. The reduction of Bi2O3–TiO2 heterostructures can be facile and produces reduced Bi2+ and Ti3+ species. The interaction of CO2 with this electron-rich, reduced system can produce CO directly, reoxidizing the heterostructure, or form an activated carboxyl species (CO2–) through electron transfer from the reduced heterostructure to CO2. The oxidized Bi2O3–TiO2 heterostructures can adsorb CO2 in carbonate-like adsorption modes, with moderately strong adsorption energies. The hydrogenation of the nanocluster and migration to adsorbed CO2 is feasible with H-migration barriers less than 0.7 eV, but this forms a stable COOH intermediate rather than breaking C–O bonds or producing formate. These results highlight that a reducible metal oxide heterostructure composed of a semiconducting metal oxide modified with suitable metal oxide nanoclusters can activate CO2, potentially overcoming the difficulties associated with the difficult first step in CO2 conversion.

Characterization of interface states in AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with HfO2 gate dielectric grown by atomic layer deposition

The impact of oxidation agent and post-metallization annealing (PMA) on the quality of oxide-semiconductor interface in AlGaN/GaN metal-oxide semiconductor heterostructure field-effect transistors (MOS-HFETs) with HfO2 grown by atomic layer deposition (ALD) was investigated. About six orders of magnitude lower gate leakage current (∼10−7 mA/mm at −10 V) was observed for MOS-HFETs with HfO2 gate oxide grown by ALD using ozone oxidation agent (O-HfO2), compared with grown using water oxidation agent (T-HfO2). In addition, the output current frequency dispersion was effectively reduced for O-HfO2 by applying PMA performed at 400 °C in N2 ambient, where the current frequency dispersion measured using 100 ns-long pulses was reduced from 40% for as-deposited gate oxide to only 10% for devices after applied PMA. Frequency dispersion was found to be consistent with density of oxide/semiconductor interface states (Dit) determined near the semiconductor conduction and valence band edge using the capacitance-voltage curves measured at different temperatures (CV-T) and photo-assisted capacitance transient (photo-C-t) technique, respectively. MOS-HFET structures with O-HfO2 showed about one order of magnitude lower Dit near the semiconductor valence band compared with structures with T-HfO2 (2 × 1011 compared with 1012 eV−1 cm−2 at EC-E = 2.5 eV, respectively). Moreover, for devices with O-HfO2, Dit was further reduced in entire energy gap as a result of PMA performed at 400 °C. The increased Dit for T-HfO2 oxides was attributed to Hf-Hf bonds at the HfO2/GaN interface, as deduced X-ray photoelectron spectroscopy (XPS) analysis. In contrast, formation of HfHf bonds was negligible in as-deposited O-HfO2 oxide films, while PMA at 400 °C led to reduction of the hydroxyl group observed by XPS.

Control of charge order melting through local memristive migration of oxygen vacancies

The colossal magnetoresistance (CMR) in perovskite manganites and the resistive switching (RS) effect in metal-oxide heterostructures have both attracted intensive attention in the past decades. Up to date, however, there has been surprisingly little effort to study the CMR phenomena by employing a memristive switch or by integrating the CMR and memristive properties in a single RS device. Here, we report a memristive control of the melting of the antiferromagnetic charge ordered (AFM-CO) state in ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3\ensuremath{-}\ensuremath{\delta}}$ epitaxial films. We show that an in situ electrotailoring of the boundary condition, which results in layers of oxygen vacancies at the metal-oxide interface, can not only suppress the critical magnetic field for the AFM-CO state melting in the interfacial memristive domain, but also promote the one in the common pristine domain of the RS device in the high and low resistive states. Our study thereby highlights the pivotal roles of functional oxygen vacancies and their dynamics in strong correlation physics and electronics.

High Efficiency Light-Emitting Transistor with Vertical Metal-Oxide Heterostructure.

The monolithic integration of light-emission with a standard logic transistor is a much-desired multifunctionality. Here, a high-efficiency light-emitting transistor (LET) employing an inorganic quantum dots (QDs) emitter and a laser-annealed vertical metal-oxide heterostructure is reported. The experimental results show that the peak efficiency and luminance of this QDs LET (QLET) are 11% and 8000 cdm-2 , respectively at a monochromatic emitting light wavelength of 585 nm. As far as it is known, these are among the highest values ever achieved for LETs. More importantly, the QLET exhibits an ultrahigh electron mobility of up to 25 cm2 V-1 S-1 , a lower efficiency roll-off (7% at high 3000 cdm-2 ), and excellent stability with long-duration gate stress switching cycles. Additionally, this approach is compatible with those used in conventional large-area silicon electronic manufacturing and can enable a scalable and cost-effective procedure for future integrated versatile displays and lighting applications.

Modified Oxygen Defect Chemistry at Transition Metal Oxide Heterostructures Probed by Hard X-ray Photoelectron Spectroscopy and X-ray Diffraction

Transition metal oxide heterostructures are interesting due to the distinctly different properties that can arise from their interfaces, such as superconductivity, high catalytic activity, and magnetism. Oxygen point defects can play an important role at these interfaces in inducing potentially novel properties. The design of oxide heterostructures in which the oxygen defects are manipulated to attain specific functionalities requires the ability to resolve the state and concentration of local oxygen defects across buried interfaces. In this work, we utilized a novel combination of hard X-ray photoelectron spectroscopy (HAXPES) and high resolution X-ray diffraction (HRXRD) to probe the local oxygen defect distribution across the buried interfaces of oxide heterolayers. This approach provides a nondestructive way to qualitatively probe locally the oxygen defects in transition metal oxide heterostructures. We studied two trilayer structures as model systems: the La0.8Sr0.2CoO3−δ/(La0.5Sr0.5)2CoO4−δ/La0.8Sr0...

Interfacial Engineering of Hierarchical Transition Metal Oxide Heterostructures for Highly Sensitive Sensing of Hydrogen Peroxide.

Hydrogen peroxide (H2 O2 ) is a major messenger molecule in cellular signal transduction. Direct detection of H2 O2 in complex environments provides the capability to illuminate its various biological functions. With this in mind, a novel electrochemical approach is here proposed by integrating a series of CoO nanostructures on CuO backbone at electrode interfaces. High-resolution transmission electron microscopy (HRTEM), X-ray diffraction, and X-ray photoelectron spectroscopy demonstrate successful formation of core-shell CuO-CoO hetero-nanostructures. Theoretical calculations further confirm energy-favorable adsorption of H2 O2 on surface sites of CuO-CoO heterostructures. Contributing to the efficient electron transfer path and enhanced capture of H2 O2 in the unique leaf-like CuO-CoO hierarchical 3D interface, an optimal biosensor-based CuO-CoO-2.5 h electrode exhibits an ultrahigh sensitivity (6349 µA m m-1 cm-2 ), excellent selectivity, and a wide detection range for H2 O2 , and is capable of monitoring endogenous H2 O2 derived from human lung carcinoma cells A549. The synergistic effects for enhanced H2 O2 adsorption in integrated CuO-CoO nanostructures and performance of the sensor suggest a potential for exploring pathological and physiological roles of reactive oxygen species like H2 O2 in biological systems.

Atomic Observation of Solid-State Reaction in Fe/ZnO Bilayer

Transition metal oxides are of great interest due to their wide physical characteristics. Transition metal oxide heterostructure could even combine the functions of different metal oxides to perform superior properties. Therefore, the study of heterostructure formation is essential. In this work, the formation of metal oxide heterostruture through diffusional reaction was observed via in-situ transmission electron microscope (TEM). Single-crystal ZnO thin film and Fe metal were annealed at 650∘C, and then Fe3O4/ZnO heterostucture were formed by solid-state diffusion. The HRTEM results and corresponding FFT-DP demonstrated the reaction taking place along the basal plane of ZnO {0001} while Fe3O4 forms along planes {111}, which are both the closest-packing plane. Moreover, Fe3O4 twin defects were observed which composed of (111) spinel-law twin boundaries. The nanofabricated method provides a novel approach to produce transition metal oxides heterostructures for their wide range of applications.

Enhancement of water splitting by controlling the amount of vacancies with varying vacuum level in the synthesis system of SnO2-x/In2O3-y heterostructure as photocatalyst

Photocatalyic water splitting with metal oxide heterostructure as photocatalysts has been a valuable and efficient hydrogen production method in recent years. Previous studies have shown that oxygen vacancies formed in photoelectrochemical reactions play an important part in the efficiency enhancement. However, the relation between the oxygen vacancy formation and the vacuum level of synthesis system has not been investigated. In this work, SnO2-x/In2O3-y heterostructures (SIH), as water splitting photocatalysts, were prepared in the synthesis systems with different vacuum levels. A series of in situ transmission electron microscope (TEM) observation had been carried out, observing the detailed changes during SIH formation, basically following the thermodynamic rules. X-ray photoelectron spectroscopy, photoluminescence measurements and in situ TEM observations indicate that the amount of oxygen vacancies increase in SIH synthesized in ultra-high vacuum (UHV) system compared to SIH formed in a low vacuum furnace. The observed 60% higher hydrogen production efficiency in SIH formed in UHV compared with SIH formed in furnace ambient is attributed to presence of the more abundant oxygen vacancies. The results indicate that an optimized heterostructured photocatalyst can be designed by controlling the vacuum level in the synthesis process.

Graphene-supported 2D transition metal oxide heterostructures

New emerging graphene-supported 2D transition metal oxide heterostructures are attracting interest for high-efficiency energy storage and energy conversion devices.

A highly sensitive and fast graphene nanoribbon/CsPbBr3 quantum dot phototransistor with enhanced vertical metal oxide heterostructures.

Although recent breakthroughs in reported graphene-based phototransistors with embedded quantum dots (QDs) have definitely been astonishing, there are still some obstacles in their practical use with regard to their electrical and optical performances. We show that through optimization of the vertical graphene nanoribbon (GNR)/QD/IGZO heterostructure and the ultrahigh efficiency of CsPbBr3 QDs, it is possible to significantly increase the on/off ratio (>103), the subthreshold slope (S.S., 0.9 V dec-1), the device's field effect mobility (μFET, 13 cm-1 V-1 S-1) and other electrical properties. Subsequently, on the basis of the extra optical-electrical characterization, we attribute the enhanced photosensitivity (800), the accelerated detecting speed (141 μs) and the high detectivity (7.5 × 1014 cm Hz1/2 W-1) to the vertical heterostructure associated with the optimized GNR component. To further demonstrate this enhancement phenomenon, the mechanism and theory mode of this vertical heterostructure are analyzed and exploited in this letter. This research indicates that a highly sensitive and fast phototransistor can be realized using the novel GNR/QD/IGZO vertical heterostructure and the long diffusion length of the perovskite QD photosensing component.

Exposing high-energy surfaces by rapid-anneal solid phase epitaxy

The functional design of transition metal oxide heterostructures depends critically on the growth of atomically flat epitaxial thin films. Often, improved functionality is expected for heterostructures and surfaces with orientations that do not have the lowest surface free energy. For example, crystal faces with a high surface free energy, such as rutile (001) planes, frequently exhibit higher catalytic activities but are correspondingly harder to synthesize due to faceting transitions. Here we propose a broadly applicable rapid-anneal solid phase epitaxial synthesis approach for the creation of nanometer thin, high surface free energy oxide heterostructures that are atomically flat. We demonstrate its efficacy by synthesizing atomically flat epitaxial RuO2(001) and TiO2(001) model systems. The former have a superior oxygen evolution activity, quantified by their lower onset potential and higher current density, relative to that of more common RuO2(110) films.

Transistors: Large‐Area Schottky Barrier Transistors Based on Vertically Stacked Graphene–Metal Oxide Heterostructures (Adv. Funct. Mater. 30/2017)

Transistors: Large‐Area Schottky Barrier Transistors Based on Vertically Stacked Graphene–Metal Oxide Heterostructures (Adv. Funct. Mater. 30/2017)

Anchoring Ultrafine ZnFe2O4/C Nanoparticles on 3D ZnFe2O4 Nanoflakes for Boosting Cycle Stability and Energy Density of Flexible Asymmetric Supercapacitor.

Heterostructure-based metal oxide thin films are recognized as the leading material for new generation, high-performance, stable, and flexible supercapacitors. However, morphologies, like nanoflakes, nanotubes, nanorods, and so forth, have been found to suffer from issues related to poor cycle stability and energy density. Thus, to circumvent these problems, herein, we have developed a low-cost, high surface area, and environmentally benign self-assembled ZnFe2O4 nanoflake@ZnFe2O4/C nanoparticle heterostructure electrode via anchoring ZnFe2O4 and carbon nanoparticles using an in situ biomediated green rotational chemical bath deposition approach for the first time. The synthesized ZnFe2O4 nanoflake@ZnFe2O4/C nanoparticle heterostructure thin films demonstrate an excellent specific capacitance of 1884 F g-1 at a current density of 5 mA cm-2. Additionally, all solid-state flexible asymmetric supercapacitor devices were designed on the basis of ZnFe2O4 nanoflake@ZnFe2O4/C nanoparticle heterostructures as the negative electrode and reduced graphene oxide and energy density of 81 Wh kg-1 at a power density of 3.9 kW kg-1. Similarly, the asymmetric device exhibits ultralong cycle stability of 35 000 cycles by losing only 2% capacitance. The excellent performance of the device is attributed to the self-assembled organization of the heterostructures. Moreover, the in situ biomediated green strategy is also applicable for the synthesis of other metal oxide and carbon-based heterostructure electrodes.