Oxide Heterointerfaces Research Articles

Oxygen Ion Pumping across Atomically Designed Oxide Heterointerfaces

AbstractComplex oxide heterointerfaces and heterostructures have demonstrated enormous emergent phenomena over the last decades, attributed to the reconstructions of mis‐matched crystalline structure, polarity, and spin ordering across the heterointerfaces. This work employs the heterostructures of La0.7Sr0.3MnO3 and CaFeO2.5 as model system to demonstrate an interface‐specific oxygen migration/reconstruction across the interfaces due to the mismatched chemical potential, which dramatically influences the ferromagnetic and electronic states of La0.7Sr0.3MnO3 layer. Specifically, the alternative stacking of octahedral (Oh) and tetrahedral (Td) layers in CaFeO2.5 are used to form two distinct heterointerfaces, namely the Oh‐Td and the Oh‐Oh interfaces with the adjacent La0.7Sr0.3MnO3 layer. Interestingly, the oxygen ion migrates toward opposite directions across the interface for these two cases, in which the CaFeO2.5 layer acts as an “oxygen pump” and manipulates the oxygen contents of its adjacent La0.7Sr0.3MnO3 layers. Such manipulation leads to a dramatically changed ferromagnetic transition temperature for the heterostructure with the Oh‐Td and Oh‐Oh interface. This work establishes a feasible and efficient strategy to control the oxygen ionic distribution through atomic‐scale interface design and opens up new opportunities to exploit emergent states at the complex oxide heterostructures through selective oxygen ion evolution.

In-situ construction of integrated transition metals and metal oxides with carbon nanomaterial heterostructures to modulate electron redistribution for boosted water splitting

Heterostructure engineering has been proposed as a promising approach to construct high-efficiency bifunctional electrocatalyst for overall water splitting. Integrated transition metal and metal oxide heterointerfaces with carbon nanomaterials are designed to facilitate electrocatalytic performance toward overall water splitting. Fe/MnO heterostructures are fabricated on graphene using a one-step DC arc plasma technology. In this way, increased electrochemically active specific areas, accelerated charge transfer, and proper interfacial electronic structure can be achieved. Benefiting from the synergistic effect of multiple materials, fabricated Fe/MnO/graphene heterointerfaces can make the redistribution of electrons between heterointerfaces, which effectively optimizes adsorption/desorption energy of the reaction intermediates. Fe/MnO/graphene delivers excellent catalytic activity with only 362 mV and 339 mV overpotential to reach current density of 10 mA cm−2 for OER and HER and exceptional long-term stability in alkaline solution. As electrocatalyst at both the cathode and the anode of an alkaline electrolyzer simultaneously, Fe/MnO/graphene electrocatalyst requires a cell voltage of 1.987 V at a current density of 10 mA cm−2 for overall water splitting. Therefore, this work offers a feasible route to construct hierarchically nanohybrids as bifunctional electrocatalysts by combining transition metal and metal oxide with carbon nanomaterials.

Tuning the Local Environment of Pt Species at CNT@MO2-x (M = Sn and Ce) Heterointerfaces for Boosted Alkaline Hydrogen Evolution.

As the most promising hydrogen evolution reaction (HER) electrocatalysts, platinum (Pt)-based catalysts still struggle with sluggish kinetics and expensive costs in alkaline media. Herein, we accelerate the alkaline hydrogen evolution kinetics by optimizing the local environment of Pt species and metal oxide heterointerfaces. The well-dispersed PtRu bimetallic clusters with adjacent MO2-x (M = Sn and Ce) on carbon nanotubes (PtRu/CNT@MO2-x) are demonstrated to be a potential electrocatalyst for alkaline HER, exhibiting an overpotential of only 75 mV at 100 mA cm-2 in 1 M KOH. The excellent mass activity of 12.3 mA μg-1Pt+Ru and specific activity of 32.0 mA cm-2ECSA at an overpotential of 70 mV are 56 and 64 times higher than those of commercial Pt/C. Experimental and theoretical investigations reveal that the heterointerfaces between Pt clusters and MO2-x can simultaneously promote H2O adsorption and activation, while the modification with Ru further optimizes H adsorption and H2O dissociation energy barriers. Then, the matching kinetics between the accelerated elementary steps achieved superb hydrogen generation in alkaline media. This work provides new insight into catalytic local environment design to simultaneously optimize the elementary steps for obtaining ideal alkaline HER performance.

Oxygen Vacancy-Driven Heterointerface Breaks the Linear-Scaling Relationship of Intermediates toward Electrocatalytic CO2 Reduction.

Smart metal-metal oxide heterointerface construction holds promising potentials to endow an efficient electron redistribution for electrochemical CO2 reduction reaction (CO2RR). However, inhibited by the intrinsic linear-scaling relationship, the binding energies of competitive intermediates will simultaneously change due to the shifts of electronic energy level, making it difficult to exclusively tailor the binding energies to target intermediates and the final CO2RR performance. Nonetheless, creating specific adsorption sites selective for target intermediates probably breaks the linear-scaling relationship. To verify it, Ag nanoclusters were anchored onto oxygen vacancy-rich CeO2 nanorods (Ag/OV-CeO2) for CO2RR, and it was found that the oxygen vacancy-driven heterointerface could effectively promote CO2RR to CO across the entire potential window, where a maximum CO Faraday efficiency (FE) of 96.3% at -0.9 V and an impressively high CO FE of over 62.3% were achieved at a low overpotential of 390 mV within a flow cell. The experimental and computational results collectively suggested that the oxygen vacancy-driven heterointerfacial charge spillover conferred an optimal electronic structure of Ag and introduced additional adsorption sites exclusively recognizable for *COOH, which, beyond the linear-scaling relationship, enhanced the binding energy to *COOH without hindering *CO desorption, thus resulting in the efficient CO2RR to CO.

Common anion rule in oxide heterointerfaces: Experimental verification by in situ photoemission spectroscopy

The band alignment at the interface is one of the fundamental parameters for designing electronic devices and artificial functional materials. However, there is no firmly established guideline for oxide heterostructures, limiting the functional design of oxide heterostructures. Here, we provide spectral evidence that the band diagram of oxide heterointerfaces is well described by the Zhong and Hansmann scheme based on the common anion rule [Z. Zhong and P. Hansmann, Phys. Rev. X 7, 011023 (2017)]. By utilizing the elemental selectivity of Ti 2p–3d resonant photoemission for the Ti 3d state near the Fermi level, we directly visualize the presence or absence of charge transfer from the overlayer films to SrTiO3 in prototypical heterointerfaces of SrVO3/SrTiO3 and SrNbO3/SrTiO3. It is found that the charge transfer occurs in SrNbO3/SrTiO3 but not in SrVO3/SrTiO3, as predicted by the Zhong and Hansmann scheme, indicating that the presence or absence, as well as the sign and amount, of interfacial charge transfer is predicted by this scheme. Our findings provide guidelines for designing and controlling the functionalities in oxide nanostructures.

2D Tantalum Disulfide Reduction Strategy Customized Ta2O5/rGO Heterointerface Aerogel Toward Boosting Electromagnetic Wave Absorption and Flame Retardancy.

The exceptional and substantial electron affinity, as well as the excellent chemical and thermal stability of transition metal oxides (TMOs), infuse infinite vitality into multifunctional applications, especially in the field of electromagnetic wave (EMW) absorption. Nonetheless, the suboptimal structural mechanical properties and absence of structural regulation continue to hinder the advancement of TMOs-based aerogels. Herein, a novel 2D tantalum disulfide (2H-TaS2) reduction strategy is demonstrated to synthesize Ta2O5/reduced graphene oxide (rGO) heterointerface aerogels with unique characters. As the prerequisite, the defects, interfaces, and configurations of aerogels are regulated by varying the concentration of 2H-TaS2 to ensure the Ta2O5/rGO heterointerface aerogels with appealing EMW absorption properties such as a minimum reflection loss (RLmin) of -61.93dB and an effective absorption bandwidth (EAB) of 8.54GHz (7.80-16.34GHz). This strategy provides valuable insights for designing advanced EMW absorbers. Meanwhile, the aerogel exhibits favorable thermal insulation performance with a value of 36mW m-1 K-1, outstanding fire resistance capability, and exceptional mechanical energy dissipation performance, making it promising for applications in the aerospace industry and consumer electronics devices.

Extreme magnetoresistance at high-mobility oxide heterointerfaces with dynamic defect tunability

Magnetic field-induced changes in the electrical resistance of materials reveal insights into the fundamental properties governing their electronic and magnetic behavior. Various classes of magnetoresistance have been realized, including giant, colossal, and extraordinary magnetoresistance, each with distinct physical origins. In recent years, extreme magnetoresistance (XMR) has been observed in topological and non-topological materials displaying a non-saturating magnetoresistance reaching 103−108% in magnetic fields up to 60 T. XMR is often intimately linked to a gapless band structure with steep bands and charge compensation. Here, we show that a linear XMR of 80,000% at 15 T and 2 K emerges at the high-mobility interface between the large band-gap oxides γ-Al2O3 and SrTiO3. Despite the chemically and electronically very dissimilar environment, the temperature/field phase diagrams of γ-Al2O3/SrTiO3 bear a striking resemblance to XMR semimetals. By comparing magnetotransport, microscopic current imaging, and momentum-resolved band structures, we conclude that the XMR in γ-Al2O3/SrTiO3 is not strongly linked to the band structure, but arises from weak disorder enforcing a squeezed guiding center motion of electrons. We also present a dynamic XMR self-enhancement through an autonomous redistribution of quasi-mobile oxygen vacancies. Our findings shed new light on XMR and introduce tunability using dynamic defect engineering.

Amorphous/crystal interface modulation of RuCoWOx toward alkaline hydrogen evolution

Oxides have traditionally been considered inert in hydrogen evolution reaction due to their strong adsorption of hydrogen. Here, we successfully prepared a novel RuCoWOx-400 catalyst by strategically adjusting the calcination temperature to manipulate the growth of oxide heterointerfaces. The characterization techniques reveal that RuCoWOx-400 is a porous structure and contains multiple heterogeneous interfaces to enhance catalytic activity. Additionally, the as-synthesized RuCoWOx-400 exhibits low overpotential of 93 mV at 10 mA cm−2 coupled with small Tafel slope of 37.22 mV dec−1 in 1 M KOH. Remarkably, the electrochemical performance of RuCoWOx-400 remains stable for up to 50 h. Moreover, the adsorption and desorption of water at Co, W, and Ru sites coordinate the hydrogen evolution reaction, highlighting the unique role of these elements in enhancing catalytic activity. This study underscores the potential of oxide materials in hydrogen evolution reaction, paving the way for further exploration and optimization in this field.

LaAlO3/SrTiO3 Heterointerface: 20 Years and Beyond

AbstractThis year marks the 20th anniversary of the discovery of LaAlO3/SrTiO3 (LAO/STO) oxide heterointerfaces. Since their discovery, transition metal oxide (TMO) interfaces have emerged as a fascinating and fast‐growing area of research, offering a variety of unique and exotic physical properties which has provided a strong impetus for the rapid advances and actualization of oxide electronics. This review revisits the fundamental mechanisms accounting for the two‐dimensional (2D) conducting interfaces, and how new models proposed to better account for the unique interfacial effects. Recent breakthroughs in the theoretical and experimental domains of oxide interfaces are also discussed including the detection and investigation of 2D quasiparticle. Moving beyond the well‐known LAO/STO interface, this review delves into other systems where unconventional interfacial superconductivity, interfacial magnetism, and spin polarization are dealt with in greater detail. In terms of device applications, this review proceeds with a treatment on the recent developments in domains including field effect transistors and freestanding heterostructure membranes. By emphasizing the opportunities and challenges of integrating oxide interfaces with existing technologies, the review will end off with an outlook projecting the progress and the trajectory of this research domain in the years to come.

Heterointerface MnO2/RuO2 with rich oxygen vacancies for enhanced oxygen evolution in acidic media.

The design and synthesis of oxygen evolution reaction (OER) electrocatalysts that operate efficiently and stably under acidic conditions are important for the preparation of green hydrogen energy. The low intrinsic catalytic activity and poor acid resistance of commercial RuO2 limit its further development, and the construction of heterointerface structures is the most promising strategy to break through the intrinsic activity limitation of electrocatalysts. Herein, we synthesized spherical and oxygen vacancy-rich heterointerface MnO2/RuO2 using morphology control, which promoted the kinetics of the oxygen evolution reaction with the interaction between oxygen vacancies and the oxide heterointerface. MnO2/RuO2 was reported to be an acidic OER catalyst with excellent performance and stability, requiring only an ultra-low overpotential of 181 mV in 0.5 M H2SO4 to achieve a current density of 10 mA cm-2. The catalyst activity remained essentially unchanged in a 140 h stability test with an ultra-high mass activity (858.9 A g-1@ 1.5 V), which was far superior to commercial RuO2 and most previously reported noble metal-based acidic OER catalysts. The experimental results showed that the effect of more oxygen vacancies and the heterointerfaces of manganese ruthenium oxides broke the intrinsic activity limitation, provided more active sites for the OER, accelerated reaction kinetics, and improved the stability of the catalyst. The excellent performance of the catalyst suggests that MnO2/RuO2 provides a new idea for the design and study of heterointerfaces in metal oxide nanomaterials.

Exchange coupling effect at Cuprate-Manganite superlattices driven by dimensionality

The strongly correlated of multi-degrees of freedoms at transition metal oxide heterointerfaces greatly enriches its physical properties and expands the relevant application fields. It has been reported that dimensionality is an effective method to tune the properties of oxide heterostructure. SCO films display structural transformation from planar-type to chain-type with the change of thickness. In this text, the high quality of SCO/LCMO superlattices are deposited by pulsed laser deposition system. And the interfacial exchange coupling effect is effectively manipulated by controlling the dimensionality of SCO layer. Combined with XAS measurement, it is found that the charge transfer would occur at heterointerface. When the SCO layer is thin, the interfacial superexchange coupling supported by charge transfer generates a weak magnetic moment to pin the ferromagnetic LCMO layer. The charge transfer would decrease as the SCO layer is thicken. Meanwhile, the long-range antiferromagnetic order in thicken SCO layer can interacted with LCMO layer and resulted in the exchange bias effect. This experiment confirms the important role of dimensionality to tune properties in multifunctional oxide heterostructure.

Record high room temperature resistance switching in ferroelectric-gated Mott transistors unlocked by interfacial charge engineering

The superior size and power scaling potential of ferroelectric-gated Mott transistors makes them promising building blocks for developing energy-efficient memory and logic applications in the post-Moore’s Law era. The close to metallic carrier density in the Mott channel, however, imposes the bottleneck for achieving substantial field effect modulation via a solid-state gate. Previous studies have focused on optimizing the thickness, charge mobility, and carrier density of single-layer correlated channels, which have only led to moderate resistance switching at room temperature. Here, we report a record high nonvolatile resistance switching ratio of 38,440% at 300 K in a prototype Mott transistor consisting of a ferroelectric PbZr0.2Ti0.8O3 gate and an RNiO3 (R: rare earth)/La0.67Sr0.33MnO3 composite channel. The ultrathin La0.67Sr0.33MnO3 buffer layer not only tailors the carrier density profile in RNiO3 through interfacial charge transfer, as corroborated by first-principles calculations, but also provides an extended screening layer that reduces the depolarization effect in the ferroelectric gate. Our study points to an effective material strategy for the functional design of complex oxide heterointerfaces that harnesses the competing roles of charge in field effect screening and ferroelectric depolarization effects.

Understanding interfacial conductance in non-stoichimetric Ca x Ta y O3−δ /SrTiO3

In search of novel conducting oxide heterointerfaces, we previously uncovered an distinctive quasi two-dimensional electron gas (q-2DEG) type behaviour in non-stoichimetric CaxTayO3-δ/SrTiO3 heterostructure. However, the underlying mechanism remained enigmatic. In this study, we delve into the intricate interplay of growth conditions, stoichiometry, and transport properties of these heterostructures. Using (Ca0.5TaO3)2 Target and the pulsed laser deposition technique, we grow the epitaxial thin films while systematically varying growth parameters, inculding laser energy density, oxygen pressures, and post-deposition annealing. Structural analysis unveiled a notable presence of oxygen vacancies in the as-grown films, while annealed samples exhibited an oxygen surplus. Building upon these findings, our comprehensive charge transport measurements revealed that while oxygen vacancies do contribute to conductivity, the polar catastrophe model takes precedence as the primary source of interfacial conductance in these heterostructures. This study provides valueable insights into the behavior of these innovative heterostructures, paving the way for future advancements in the field.

Enhanced Rashba Spin Orbit Coupling and Magnetic Behavior at Oxide Heterointerfaces by Optical Gating.

Complex oxide heterointerfaces have been a hot research spot due to their rich physical phenomena and broad quantum coherence that respond to multiple external stimuli. Among these external stimuli, light is a very powerful one to manipulate properties such as carrier density and spin characteristics. However, achieving a light-magnetic correlation is in high demand for multifield responding devices, and its intrinsic mechanism remains unclear. Here, by illuminating Nd0.86Sr0.14Al0.86Ni0.14O3-SrTiO3 heterointerfaces using 360 nm light, we observe a series of interesting physical phenomena, like enhanced magnetoresistance (MR). More interestingly, a band splitting and strong Rashba spin-orbit coupling (SOC) effect occur after illumination, accompanied by a magnetic feature and thus leading to an anomalous Hall effect (AHE). Upon optical gating, the magnetism can be caused by Rashba SOC induced spin-orbit torque (SOT). The work will be sure to have great importance in both theoretical studies and all-oxide devices.

Controlled Electronic and Magnetic Landscape in Self-Assembled Complex Oxide Heterostructures.

Complex oxide heterointerfaces contain a rich playground of novel physical properties and functionalities, which give rise to emerging technologies. Among designing and controlling the functional properties of complex oxide film heterostructures, vertically aligned nanostructure (VAN) films using a self-assembling bottom-up deposition method presents great promise in terms of structural flexibility and property tunability. Here, the bottom-up self-assembly is extended to a new approach using a mixture containing a 2Dlayer-by-layer film growth, followed by a 3D VAN film growth. In this work, the two-phase nanocomposite thin films are based on LaAlO3 :LaBO3 , grown on a lattice-mismatched SrTiO3001 (001) single crystal.The 2D-to-3D transient structural assembly is primarily controlled by the composition ratio, leading to the coexistence of multiple interfacial properties, 2D electron gas, and magnetic anisotropy. This approach provides multidimensional film heterostructures which enrich the emergent phenomena for multifunctional applications.

Suppression of conductivity by 1 uc buffer layer at LAO/STO interface

The conducting properties of oxide heterointerface can be significantly affected by buffer layers, gate voltage, capping layer, etc.; sometimes, a monolayer is enough to show a drastic change in these properties. Herein, we report that the insertion of one unit-cell (uc) of CaMnO3 material as a buffer layer at conducting LaAlO3/SrTiO3 interface turns it into an insulator. The presence of MnO2-layer in CaMnO3 uc at the interface blocks the charge transfer to SrTiO3 which makes the interface conducting as explained by the polar catastrophe model. The study may help to resolve the puzzle of the origin of conductivity at oxide heterostructure interfaces.

Thickness-Dependent Interface Polarity in Infinite-Layer Nickelate Superlattices

The interface polarity plays a vital role in the physical properties of oxide heterointerfaces because it can cause specific modifications of the electronic and atomic structure. Reconstruction due to the strong polarity of the NdNiO2/SrTiO3 interface in recently discovered superconducting nickelate films may play an important role, as no superconductivity has been observed in the bulk. By employing four-dimensional scanning transmission electron microscopy and electron energy-loss spectroscopy, we studied effects of oxygen distribution, polyhedral distortion, elemental intermixing, and dimensionality in NdNiO2/SrTiO3 superlattices grown on SrTiO3 (001) substrates. Oxygen distribution maps show a gradual variation of the oxygen content in the nickelate layer. Remarkably, we demonstrate thickness-dependent interface reconstruction due to a polar discontinuity. An average cation displacement of ∼0.025 nm at interfaces in 8NdNiO2/4SrTiO3 superlattices is twice larger than that in 4NdNiO2/2SrTiO3 superlattices. Our results provide insights into the understanding of reconstructions at NdNiO2/SrTiO3 polar interfaces.

Strain-gradient-modulated Kondo effect at the LaAlO3/SrTiO3 heterointerface

Modulation of Kondo phenomenon in two-dimensional materials is of great significance. Unfortunately, the strain effect, one of the effective regulation methods, is usually lacking in previously studied Kondo systems. Here we show that a tensile or compressive strain gradient induced by a three-point bending apparatus has a prominent impact on Kondo effect of the two dimensional electron gas at the LaAlO3/SrTiO3 heterointerface. Under the tensile strain gradient, Kondo temperature (TK) decreases. In contrast, TK increases under the compressive strain gradient. At the same time, the temperature at which the resistance has a minimum, Tmin, shows the same strain gradient dependence as TK. The present study may pave the way for manipulating many-body effects at oxide heterointerface.

Unusual Mn oxidation state distribution in the vicinity of the tensile-strained interface between CaMnO3−δ and La0.7Ca0.3MnO3 layers

Oxide perovskite materials with heterointerfaces are important structures with applications such as electronic devices. The functionality of these materials depends on many factors, such as the charge, structure, and presence of defects at the interface. Thus, understanding the properties of interfaces and their effects on material function is important in the design and optimization of functional materials. In this study, the interplay among the Mn oxidation state distribution, the presence of oxygen vacancies (VOs), and the structure of the interface is investigated in the heterointerface between CaMnO3−δ and La0.7Ca0.3MnO3 layers by using electron energy loss spectroscopy combined with scanning transmission electron microscopy. Unlike the expectation that the Mn oxidation state distribution is controlled by the distribution of cations intermixing at the interface, it is dominantly influenced by the presence of VOs when the substrate gives tensile stress to it. As a result, the tensile-strained heterointerface shows an anomalously sharp reduction in the Mn oxidation state at the interface. This result suggests that VOs and strain are two essential ingredients to consider for the understanding of oxidation state distribution at interfaces. This study provides insights into the nature of various oxide heterointerfaces.

Carrier tuning of 2D electron gas in field-effect devices based on Al2O3/ZnO heterostructures.

Two-dimensional electron gas (2DEG) formed at oxide heterointerfaces via atomic layer deposition (ALD) has attracted considerable interest toward fascinating electron-related physics and electronic device applications. The employment of oxide-based 2DEG in a confined channel in field-effect transistors (FETs) holds great promise for advanced electronic devices due to its high mobility, spatial confinement, and tunable conductivity. In this work, a 2DEG FET based on the Al2O3/ZnO heterostructure is fabricated with an optimized channel carrier density and oxide thickness. The carrier transport in the bulk and the oxide interface dominantly governed by percolation conduction, optical phonon scattering, and grain boundary scattering is comparatively studied through oxygen annealing and thickness engineering. A tunable carrier density from 4 × 1011 cm-2 to 2 × 1014 cm-2 is achieved with a maximum Hall mobility of ∼62 cm2 V-1 s-1. The electron distribution associated with the annealing process of the ZnO underlayer and the interface reaction during Al2O3 deposition are found to have an impact on the electrical characteristics of the devices. The fabricated Al2O3/ZnO-based 2DEG FET exhibits an on/off ratio over 108, a subthreshold swing of 224 mV dec-1, and a field-effect mobility of 5.7 cm2 V-1 s-1 which can be promising for advanced oxide thin film-containing device and system applications.