Capping-layer-mediated lattice mismatch and redox reaction in SrTiO3-based bilayers.

It is well known that the traditional two-dimensional electron system (2DES) hosted by the SrTiO3substrate can exhibit diverse electronic states by modifying the capping layer in heterostructures. However, such capping layer engineering is less studied in the SrTiO3-layer-carried 2DES (or bilayer 2DES), which is different from the traditional one on transport properties but more applicable to the thin-film devices. Here, several SrTiO3bilayers are fabricated by growing various crystalline and amorphous oxide capping layers on the epitaxial SrTiO3layers. For the crystalline bilayer 2DES, the monotonical reduction on the interfacial conductance, as well as carrier mobility, is recorded on increasing the lattice mismatch between the capping layers and epitaxial SrTiO3layer. The mobility edge raised by the interfacial disorders is highlighted in the crystalline bilayer 2DES. On the other hand, when increasing the concentration of Al with high oxygen affinity in the capping layer, the amorphous bilayer 2DES becomes more conductive accompanied by the enhanced carrier mobility but almost constant carrier density. This observation cannot be explained by the simple redox-reaction model, and the interfacial charge screening and band bending need to be considered. Moreover, when the capping oxide layers have the same chemical composition but with different forms, the crystalline 2DES with a large lattice mismatch is more insulating than its amorphous counterpart, and vice versa. Our results shed some light on understanding the different dominant role in forming the bilayer 2DES using crystalline and amorphous oxide capping layer, which may be applicable in designing other functional oxide interfaces.

Atomic Origins of Enhanced Ferroelectricity in Nanocolumnar PbTiO3 /PbO Composite Thin Films.

Nanocomposite films hold great promise for multifunctional devices by integrating different functionalities within a single film. The microstructure of the precipitate/secondary phase is an essential element in designing composites' properties. The interphase strain between the matrix and secondary phase is responsible for strain-mediated functionalities, such as magnetoelectric coupling and ferroelectricity. However, a quantitative microstructure-dependent interphase strain characterization has been scarcely studied. Here, it is demonstrated that the PbTiO3 (PTO)/PbO composite system can be prepared in nano-spherical and nanocolumnar configurations by tuning the misfit strain, confirmed by a three-dimensional reconstructive microscopy technique. With the atomic resolution quantitative microscopy with a depth resolution of a few nanometers, it is discovered that the strained region in PTO is much larger and more uniform in nanocolumnar compared to nano-spherical composites, resulting in much enhanced ferroelectric properties. The interphase strain between PbO and PTO in the nanocolumnar structure leads to a giant c/a ratio of 1.20 (bulk value of 1.06), accompanied by a Ti polarization displacement of 0.48 Å and an effective ferroelectric polarization of 241.7 µC cm-2 , three times compared to the bulk value. The quantitative atomic-scale strain and polarization analysis on the interphase strain provides an important guideline for designing ferroelectric nanocomposites.

Microstructure-Dependent Thermal Stability of Super-Tetragonal Nanocomposite Films through In Situ TEM/EELS Study.

Smart microstructure design in nanocomposite films allows us to tailor physical properties such as ferroelectricity and thermal stability to broaden applications of next-generation electronic devices. Here, we study the thermal stability of self-assembled PbTiO3 (PTO)/PbO nanocomposite films with nano-spherical and nanocolumnar microstructures by utilizing an environmental transmission electron microscopy (TEM) combined with electron energy loss spectroscopy (EELS). The real-time study reveals that the microstructure-dependent interphase strain has an effect on the stabilization of the tetragonal phase. Compared to the nano-spherical configuration, the nanocomposite film with the nanocolumnar microstructure can maintain the giant tetragonality of ∼1.20 up to 450 °C, and the tetragonal phase is predicted to be stable at elevated temperatures > 600 °C. Moreover, the temperature-dependent EELS further demonstrates the sensitivity of the chemical bonding of Pb and Ti with O to the PTO lattice distortion, correlating the structural variation and electronic properties at different temperatures. Such in situ heating TEM study provides insights into the thermal stability of nanocomposites with different microstructures and facilitates the advancement of power electronics applications in harsh environments.

Anomalous Lattice Evolution-Mediated Electrical Properties in Transparent KNN-Based Lead-Free Ferroelectric Films.

The epitaxial (K0.49Na0.49Li0.02)(Ta0.2Nb0.8)O3 with 2 wt % MnO2 addition (KNNLT-M) film on the transparent La0.03Ba0.97SnO3-coated LaAlO3 (001) substrate is chosen to investigate how the lattice evolution, as well as the electrical properties, optical bandgap energy, and thermal stability, changes with the growth oxygen pressure [P(O2)]. Compared to the other perovskite oxide films, for example, (La,Ca)MnO3, PbTiO3, and BaTiO3, an anomalous lattice evolution with an increased (decreased) out-of-plane (in-plane) lattice constant was observed in KNNLT-M films as P(O2) increases. Such anomalous lattice evolution can improve the electric properties of KNNLT-M films; for example, the ferroelectricity is significantly optimized and the dielectric constant is enhanced from 451 to 1248 at 1 kHz. The X-ray photoelectron spectra results have demonstrated that high P(O2) can make more K cations to enter the perovskite lattice and the Mn2+/Mn3+ existing in KNNLT can effectively suppress the leakage behavior, thus promoting the electrical nature of KNNLT-M films. The optical measurements show that the KNNLT-M film heterostructures are highly transparent with a maximum transmittance of ∼80%, and both direct and indirect bandgap energies increase with increasing P(O2). Meanwhile, all these KNNLT-M films exhibit good thermal stability with stable ferroelectricity up to the high temperature of at least 125 °C. These results demonstrate that the control of the lattice structure and electrical properties by P(O2) is one of the important prerequisites for the application of KNN-based films.

Ferroelectric Self-Polarization Controlled Magnetic Stratification and Magnetic Coupling in Ultrathin La0.67Sr0.33MnO3 Films.

Multiferroic oxide heterostructures consisting of ferromagnetic and ferroelectric components hold the promise for nonvolatile magnetic control via ferroelectric polarization, advantageous for the low-dissipation spintronics. Modern understanding of the magnetoelectric coupling in these systems involves structural, orbital, and magnetic reconstructions at interfaces. Previous works have long proposed polarization-dependent interfacial magnetic structures; however, direct evidence is still missing, which requires advanced characterization tools with near-atomic-scale spatial resolutions. Here, extensive polarized neutron reflectometry (PNR) studies have determined the magnetic depth profiles of PbZr0.2Ti0.8O3/La0.67Sr0.33MnO3 (PZT/LSMO) bilayers with opposite self-polarizations. When the LSMO is 2-3 nm thick, the bilayers show two magnetic transitions on cooling. However, temperature-dependent magnetization is different below the lower-temperature transition for opposite polarizations. PNR finds that the LSMO splits into two magnetic sublayers, but the inter-sublayer magnetic couplings are of opposite signs for the two polarizations. Near-edge X-ray absorption spectroscopy further shows contrasts in both the Mn valences and the Mn-O bond anisotropy between the two polarizations. This work completes the puzzle for the magnetoelectric coupling model at the PZT/LSMO interface, showing a synergic interplay among multiple degrees of freedom toward emergent functionalities at complex oxide interfaces.

Magnetoelectric Coupling Induced Orbital Reconstruction and Ferromagnetic Insulating State in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 Heterostructures.

Novel phenomena at the ferromagnetic/ferroelectric interface have generated much interest. Here, a ferromagnetic insulating state with the Curie temperature about 268-286 K in PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 heterostructures is induced and modulated by varying the PbZr0.52Ti0.48O3 thickness. An abnormally enlarged c/a ratio in La0.67Sr0.33MnO3 by strain-based coupling effect leads to d3z2-r2 orbital preferable occupancy. This orbital reconstruction modulates effective electron transfer and finally leads to a ferromagnetic insulating state. Valence change induced by charge-based coupling effect could be partially responsible for change in the Curie temperature in the strongly correlated electron system of La1-xSrxMnO3. This work provides a deeper understanding of strain effects near the ferromagnetic/ferroelectric interface, especially in a PbZr1-yTiyO3/La1-xSrxMnO3 heterostructure system.

All-oxide-based synthetic antiferromagnets exhibiting layer-resolved magnetization reversal.

Synthesizing antiferromagnets with correlated oxides has been challenging, owing partly to the markedly degraded ferromagnetism of the magnetic layer at nanoscale thicknesses. Here we report on the engineering of an antiferromagnetic interlayer exchange coupling (AF-IEC) between ultrathin but ferromagnetic La2/3Ca1/3MnO3 layers across an insulating CaRu1/2Ti1/2O3 spacer. The layer-resolved magnetic switching leads to sharp steplike hysteresis loops with magnetization plateaus depending on the repetition number of the stacking bilayers. The magnetization configurations can be switched at moderate fields of hundreds of oersted. Moreover, the AF-IEC can also be realized with an alternative magnetic layer of La2/3Sr1/3MnO3 that possesses a Curie temperature near room temperature. The findings will add functionalities to devices with correlated-oxide interfaces.

Strain Coupling of Conversion-type Fe3 O4 Thin Films for Lithium Ion Batteries.

Lithiation/delithiation induces significant stresses and strains into the electrodes for lithium ion batteries, which can severely degrade their cycling performance. Moreover, this electrochemically induced strain can interact with the local strain existing at solid-solid interfaces. It is not clear how this interaction affects the lithiation mechanism. The effect of this coupling on the lithiation kinetics in epitaxial Fe3 O4 thin film on a Nb-doped SrTiO3 substrate is investigated. In situ and ex situ transmission electron microscopy (TEM) results show that the lithiation is suppressed by the compressive interfacial strain. At the interface between the film and substrate, the existence of Lix Fe3 O4 rock-salt phase during lithiation consequently restrains the film from delamination. 2D phase-field simulation verifies the effect of strain. This work provides critical insights of understanding the solid-solid interfaces of conversion-type electrodes.

Heteroepitaxy of Fe3O4/Muscovite: A New Perspective for Flexible Spintronics.

Spintronics has captured a lot of attention since it was proposed. It has been triggering numerous research groups to make their efforts on pursuing spin-related electronic devices. Recently, flexible and wearable devices are in a high demand due to their outstanding potential in practical applications. In order to introduce spintronics into the realm of flexible devices, we demonstrate that it is feasible to grow epitaxial Fe3O4 film, a promising candidate for realizing spintronic devices based on tunneling magnetoresistance, on flexible muscovite. In this study, the heteroepitaxy of Fe3O4/muscovite is characterized by X-ray diffraction, high-resolution transmission electron microscopy, and Raman spectroscopy. The chemical composition and magnetic feature are investigated by a combination of X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism. The electrical and magnetic properties are examined to show the preservation of the primitive properties of Fe3O4. Furthermore, various bending tests are performed to show the tunability of functionalities and to confirm that the heterostructures retain the physical properties under repeated cycles. These results illustrate that the Fe3O4/muscovite heterostructure can be a potential candidate for the applications in flexible spintronics.

Interfacial Control of Ferromagnetism in Ultrathin La0.67Ca0.33MnO3 Sandwiched between CaRu1-xTixO3 (x = 0-0.8) Epilayers.

Controlling functionalities in oxide heterostructures remains challenging for the rather complex interfacial interactions. Here, by modifying the interface properties with chemical doping, we achieve a nontrivial control over the ferromagnetism in ultrathin La0.67Ca0.33MnO3 (LCMO) layer sandwiched between CaRu1-xTixO3 [CRTO(x)] epilayers. The Ti doping suppresses the interfacial electron transfer from CRTO(x) to LCMO side; as a result, a steadily decreased Curie temperature with increasing x, from 262 K at x = 0 to 186 K at x = 0.8, is observed for the structures with LCMO fixed at 3.2 nm. Moreover, for more insulating CRTO(x ≥ 0.5), the electron confinement induces an interfacial Mn-eg(x2-y2) orbital order in LCMO which further attenuates the ferromagnetism. Also, in order to characterize the heterointerfaces, for the first time the doping- and thickness-dependent metal-insulator transitions in CRTO(x) films are examined. Our results demonstrate that the LCMO/CRTO(x) heterostructure could be a model system for investigating the interfacial multiple interactions in correlated oxides.

Metal-free cycloaddition to synthesize naphtho[2,3-d][1,2,3]triazole-4,9-diones.

A metal-free domino [3 + 2] cycloaddition is reported to construct naphtho[2,3-d][1,2,3]triazole-4,9-dione derivatives and provide an alternative approach to the azide-alkyne cycloadditions. The key features are easily available starting materials, mild reaction conditions, a good atom economy, eco-friendly characteristics and a broad substrate scope with high yields.

Electric-field-modulated nonvolatile resistance switching in VO2/PMN-PT(111) heterostructures.

The electric-field-modulated resistance switching in VO2 thin films grown on piezoelectric (111)-0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-PT) substrates has been investigated. Large relative change in resistance (10.7%) was observed in VO2/PMN-PT(111) hererostructures at room temperature. For a substrate with a given polarization direction, stable resistive states of VO2 films can be realized even when the applied electric fields are removed from the heterostructures. By sweeping electric fields across the heterostructure appropriately, multiple resistive states can be achieved. These stable resistive states result from the different stable remnant strain states of substrate, which is related to the rearrangements of ferroelectric domain structures in PMN-PT(111) substrate. The resistance switching tuned by electric field in our work may have potential applications for novel electronic devices.