Search

Vertically aligned nanocomposite films hold the promise of making a significant contribution to the performance enhancement and multifunctional integration of flexible thin-film devices but are hindered by the difficulty of direct epitaxial growth on flexible substrates. In this work, freestanding BaTiO3–CeO2 epitaxial nanocomposite membranes were prepared by using water-soluble Sr3Al2O6 as the sacrificial buffer during film deposition. The membrane's lattice parameters as well as their epitaxial relationship, before and after its removal from the single-crystal substrates, were quantitatively measured and analyzed by combining x-ray diffraction and TEM. In addition, synchrotron soft x-ray absorption spectroscopy reveals the discrepancy in the surface and internal electron states of Ce and Ti via comparative studies in total electron yield mode and transmission mode, respectively. This work exemplified that freestanding membranes provide a convenient platform for (1) the study of in-plane lattice information of epitaxial oxide films via TEM and for (2) the comparative study of surface and internal electron band information of epitaxial oxide films. [ABSTRACT FROM AUTHOR]

The generation and control of multiple phases via hydrogen insertion open up avenues for tuning the properties of transition metal oxides. Here, by employing both in situ x-ray diffractions and in situ electrical measurements, we accurately probed the full structural phase transitions during the reversible process of hydrogen insertion into and extraction from the vanadium dioxide lattice. Repeatable switches between the hydrogenated VO2 phases and the pristine VO2 phase were demonstrated, implying potential applications for hydrogen detection/storage and multi-state information memorizers. Moreover, different phases were further discussed by synchrotron x-ray absorption spectroscopy and theoretical first-principles calculations, which reveal that hydrogen insertion greatly affects the filling of the d-band as well as the electrical properties. This work will provide fundamental insight into the comprehensive understanding of hydrogen-induced phase transition in metal oxides and may guide the development of proton-based sensors and devices. [ABSTRACT FROM AUTHOR]

3D crystalline order with 1 nm resolution is observed in aqueous solutions of supramolecular nanotubes containing 94 % water, at concentrations as low as 6 wt%. 50 of star‐like organic ions arrange into supramolecular rings which, in turn, stack on top of each other to form long hollow tubes with 15 nm outer diameter. Cryo‐TEM and X‐ray diffraction show that the parallel nanotubes arrange on a perfect hexagonal lattice. Unexpectedly, fiber diffraction on sheared solutions revealed numerous hkl Bragg reflections on several layer lines indicating longitudinal interlock between the tubes and 3D crystalline order with molecular‐scale details transferred across 10 nm thick layers of water. The observed high 3D order is attributed to long‐range attraction between like‐charged tubes and amplified charge modulation by the extremely high intra‐tube correlation length. [ABSTRACT FROM AUTHOR]

The high demand for flexible spintronics based on multiferroic heterostructures makes growing high‐quality flexible, functional oxides urgently, in which needs to be deposited on lattice‐matched substrates. In this paper, ultraflexible and malleable iron (Fe)/BaTiO3 (BTO) multiferroic heterostructures are demonstrated, showing a perfect crystallinity and hetero‐epitaxial growth. In terms of performance, they indicate good multiferroic properties and excellent bending tunability, as well as obvious magnetoelectric (ME) coupling effect. During the phase transformation from the rhombohedral phase to the orthorhombic phase of BTO layers in the heating process, a large ME coupling coefficient of 120 Oe °C−1 along the out‐of‐plane direction is obtained. This value keeps consistent in the phase‐field simulation of magnetic domain evolution, in which the biaxial compressive strain induced‐magnetoelastic anisotropy facilitates the magnetic easy axis of Fe layers to the [110] or [–1–10] direction. Besides, ultraflexible Fe/BTO heterostructures are found to have a 690 Oe ferromagnetic resonance (FMR) field shift along the out‐of‐plane direction under the flexible tuning (R = 5 mm). This work should pave a way toward flexible spintronic and functional devices with fast speed, portability, and low energy consumption. [ABSTRACT FROM AUTHOR]

Magnetic materials and devices that can be folded and twisted without sacrificing their functional properties are highly desirable for flexible electronic applications in wearable products and implantable systems. In this work, a high‐quality single crystalline freestanding Fe3O4 thin film with strong magnetism has been synthesized by pulsed laser deposition using a water‐dissolvable Sr3Al2O6 sacrificial layer, and the resulting freestanding film, with magnetism confirmed at multiple length scales, is highly flexible with a bending radius as small as 7.18 µm and twist angle as large as 122°, in sharp contrast with bulk magnetite that is quite brittle. When transferred to a polydimethylsiloxane support layer, the Fe3O4 film can be bent with large deformation without affecting its magnetization, demonstrating its robust magnetism. The work thus offers a viable solution for flexible magnetic materials that can be utilized in a range of applications. [ABSTRACT FROM AUTHOR]

In this letter, a series of vanadium dioxide (VO2) epitaxial films were deliberately deposited on TiO2 substrates with different orientations [(001), (110), and (101)], in an attempt to gain insights into the strained VO2 epitaxial film. We found in-plane [100] and [1-10] directions, obviously anisotropic metal-insulator transition (MIT) in (110)-oriented VO2 films. In combination with synchrotron radiation high-resolution x-ray diffraction characterizations, electronic transport data reveal that the critical temperature of MIT depends on the strain state of the dimeric vanadium atomic chain along the c axis of the rutile phase. The anisotropy of MIT is closely related to the orientation of the VO2 films, which is caused by the varied orientation configuration of V-V atomic chain dimerization in the films. Soft x-ray absorption spectroscopy results further indicate that this anisotropy may be driven by the directional hybridization of O 2p and V 3d orbitals with respect to the orientation of VO2 thin films. The polarization-dependent V L-edge and O K-edge XAS data suggest that the elongation of the apical V-O bond length increases the p-d orbital overlap; thus, the energy level of the d// orbital is raised relative to that of the π* orbital. These anisotropic MIT behaviors will help us to understand how the strain engineering depends on the strain state of vanadium dimers in VO2 films. [ABSTRACT FROM AUTHOR]

We investigated the microstructure of PbTiO3 films epitaxially grown on DyScO3 (110) substrates using synchrotron-based 3D reciprocal space mapping and X-ray reflection interface microscopy. The results show that an in-plane anisotropic mixture domain state (a* + a/c) exists in the PbTiO3 films, where a* domains reside between a/c twin stripe domains. Furthermore, when compared with traditional a domains in a/c twins, the a* domain is found to possess a similar in-plane lattice parameter but a much smaller out-of-plane lattice (∼0.5%). We also discovered a temperature-driven transition between a* and a/c pointing to potential applications in functional devices due to the huge mechanical response (4.85%) and polarization rotation between the a* and c domains. [ABSTRACT FROM AUTHOR]

Understanding and controlling the domain evolution under external stimuli in multiferroic thin films is critical to realizing nanoelectronic devices, including for non-volatile memory, data storage, sensors, and optoelectronics. In this article, we studied the shear-strain effect on the domain evolution with temperature in highly strained BiFeO3 thin films on rhombohedral LaAlO3 substrates using a high-resolution synchrotron X-ray diffraction three dimensional-reciprocal space mapping (3D-RSM) technique. The results revealed significant biaxial, anisotropic, evolution behaviors of the mixed-phase (MC + R′/T′ phases) BiFeO3 ferroelectric domains along the in-plane [100] and [010] axes. These biaxial, anisotropic, evolution behaviors were attributed to the shear-strain-modulated transition pathways of the mixed-phase ferroelectric domains. This viewpoint was further verified in the BiFeO3/LaSrAlO4 (001) system in which no anisotropic evolution behaviors of the mixed-phase domains were found. This work sheds light on the quantitative analysis of domain evolution in multi-domain systems and demonstrates that the shear-strain effect could act as an effective tool to manipulate the domain behavior and control novel functionalities of ferroelectric thin films. [ABSTRACT FROM AUTHOR]

Magnetic compounds with significant and temperature-insensitive magnetoresistance are quite promising for device applications. Here, we intentionally doped different amounts (0.025-0.1) of Co into epitaxial (La0.7Sr0.3)MnO3 thin films. The evolution of electrical, magnetic, magneto-transport, and valence state of this series of Co-doped (La0.7Sr0.3)MnO3 have been systematically characterized. Remarkably, the magnetoresistance in 0.1Co-doped (La0.7Sr0.3)MnO3 can reach significantly 35% within an extremely large temperature window (from 40 to 200 K). Other Co-doped epitaxial (La0.7Sr0.3)MnO3 thin films show similar sharp MR-peaks around Curie temperature as that in (La0.7Sr0.3)MnO3. The evolution of magnetic and magneto-transport properties of Co-(La0.7Sr0.3)MnO3 films can be explained by antiferromagnetic coupling induced by paramagnetic/low-spin Co3+(Co2+) ions in (La0.7Sr0.3)MnO3. Our work demonstrates that doping paramagnetic/low-spin Co into manganites is an effective way to optimize their magneto-transport properties. [ABSTRACT FROM AUTHOR]

Strain engineering is a popular approach for manipulating material properties through modifying crystal structures and/or electron-lattice interactions. In this work, we used a phase transition material, vanadium dioxide (VO) as an active and reconfigurable substrate to generate a controllable strain. Amorphous ferromagnetic Ni thin films were deposited on the (010)-VO/(0001)-AlO substrates. It is observed that the magnetic moments of the Ni thin film were modulated by interfacing it with the VO thin film. We observed a hysteresis in the magnetic moment-temperature curves in the vicinity of the metal-insulator transition of the VO thin films at a low magnetic field bias (< 30 Oe). This result can be attributed to magnetoelastic coupling between the Ni and VO thin films through a reversible structural phase transition. As the bias field was increased, the hysteresis was suppressed and eventually disappeared. The competition between the magnetoelastic energy and external Zeeman energy can explain these behaviors. Our observations suggest a promising approach to dynamically controlling the properties of functional thin films by strain engineering. [ABSTRACT FROM AUTHOR]

Electric-field control of the anomalous Hall effect (AHE) was investigated in Fe40Pt60/(001)-Pb(Mg1/3Nb2/3)0.67Ti0.33O3 (FePt/PMN-PT) multiferroic heterostructures at room temperature. It was observed that a very large Hall resistivity change of up to 23.9% was produced using electric fields under a magnetic field bias of 100 Oe. A pulsed electric field sequence was used to generate nonvolatile strain to manipulate the Hall resistivity. Two corresponding nonvolatile states with distinct Hall resistivities were achieved after the electric fields were removed, thus enabling the encoding of binary information for memory applications. These results demonstrate that the Hall resistivity can be reversibly switched in a nonvolatile manner using programmable electric fields. Two remanent magnetic states that were created by electric-field-induced piezo-strain from the PMN-PT were attributed to the nonvolatile and reversible properties of the AHE. This work suggests that a low-energy-consumption-based approach can be used to create nonvolatile resistance states for spintronic devices based on electric-field control of the AHE. [ABSTRACT FROM AUTHOR]

Large π-conjugated conductive two-dimensional (2D) metal–organic frameworks (MOFs) via the bottom-up approach have recently emerged as novel and interesting 2D materials. For the first time, herein we demonstrate the design and successful bottom-up fabrication of a novel noble-metal-free nickel phthalocyanine-based 2D MOF (NiPc–MOF) for highly efficient water oxidation catalysis. This NiPc–MOF can be easily grown on various substrates. The thin film deposited on FTO showed high catalytic oxygen evolution reaction (OER) activity with a very low onset potential (＜1.48 V, overpotential ＜ 0.25 V), high mass activity (883.3 A g−1), and excellent catalytic durability. To the best of our knowledge, this present 2D MOF film catalyst represents the best OER catalytic activity among molecular catalyst-based materials reported so far. This work represents the first synthesis of a nickel phthalocyanine-based 2D MOF material and application of the π-conjugated 2D MOF for water oxidation. [ABSTRACT FROM AUTHOR]

The metal–insulator transition (MIT) behavior in vanadium dioxide (VO2) epitaxial film is known to be dramatically affected by interfacial stress due to lattice mismatching. For the VO2/TiO2 (001) system, there exists a considerable strain in ultra‐thin VO2 thin film, which shows a lower Tc value close to room temperature. As the VO2 epitaxial film grows thicker layer‐by‐layer along the “bottom‐up” route, the strain will be gradually relaxed and Tc will increase as well, until the MIT behavior becomes the same as that of bulk material with a Tc of about 68 °C. Whereas, in this study, we find that the VO2/TiO2 (001) film thinned by “top‐down” wet‐etching shows an abnormal variation in MIT, which accompanies the potential relaxation of film strain with thinning. It is observed that even when the strained VO2 film is etched up to several nanometers, the MIT persists, and Tc will increase up to that of bulk material, showing the trend to a stress‐free ultra‐thin VO2 film. The current findings demonstrate a facial chemical‐etching way to change interfacial strain and modulate the phase transition behavior of ultrathinVO2 films, which can also be applied to other strained oxide films. [ABSTRACT FROM AUTHOR]

Tuning the surface strain of heterogeneous catalysts represents a powerful strategy to engineer their catalytic properties by altering the electronic structures. However, a clear and systematic understanding of strain effect in electrochemical reduction of carbon dioxide is still lacking, which restricts the use of surface strain as a tool to optimize the performance of electrocatalysts. Herein, we demonstrate the strain effect in electrochemical reduction of CO2 by using Pd octahedra and icosahedra with similar sizes as a well-defined platform. The Pd icosahedra/C catalyst shows a maximum Faradaic efficiency for CO production of 91.1 % at −0.8 V versus reversible hydrogen electrode (vs. RHE), 1.7-fold higher than the maximum Faradaic efficiency of Pd octahedra/C catalyst at −0.7 V (vs. RHE). The combination of molecular dynamic simulations and density functional theory calculations reveals that the tensile strain on the surface of icosahedra boosts the catalytic activity by shifting up the d-band center and thus strengthening the adsorption of key intermediate COOH*. This strain effect was further verified directly by the surface valence-band photoemission spectra and electrochemical analysis. [ABSTRACT FROM AUTHOR]