Superconductivity in Freestanding Infinite-Layer Nickelate Membranes.

The observation of superconductivity in infinite-layer nickelates has attracted significant attention due to its potential as a new platform for exploring high-Tc superconductivity. However, thus far, superconductivity has only been observed in epitaxial thin films, which limits the manipulation capabilities and modulation methods compared to two-dimensional exfoliated materials. Given the exceptionally giant strain tunability and stacking capability of freestanding membranes, separating superconducting nickelates from the as-grown substrate is a novel way to engineer the superconductivity and uncover the underlying physics. Herein, we report the synthesis of the superconducting freestanding La0.8Sr0.2NiO2 membranes (TC Zero = 10.6 K), emphasizing the crucial roles of the interface engineering in the precursor phase film growth and the quick transfer process in achieving superconductivity. Our work offers a new versatile platform for investigating the superconductivity in nickelates, such as the pairing symmetry via constructing Josephson tunneling junctions and higher Tc values via high-pressure experiments. This article is protected by copyright. All rights reserved.

In Situ Preparation of Superconducting Infinite-Layer Nickelate Thin Films with Atomically Flat Surface.

Since their discovery, the infinite-layer nickelates have been regarded as an appealing system for gaining deeper insights into high-temperature superconductivity (HTSC). However, the synthesis of superconducting samples has been proven to be challenging. Here, an ultrahigh vacuum (UHV) in situ ${\mathrm{\text{in situ}}}$ reduction method is developed using atomic hydrogen as a reducing agent and is applied in the lanthanum nickelate system. The reduction parameters, including the reduction temperature (TR) and hydrogen pressure (PH), are systematically explored. It is found that the reduction window for achieving superconducting transition is quite wide, reaching nearly 80°C in TR and three orders of magnitude in PH when the reduction time is set to 30 min. And there exists an optimal PH for achieving the highest Tc if both TR and reduction time are fixed. More prominently, as confirmed by atomic force microscopy and scanning transmission electron microscopy, the atomically flat surface can be preserved during the in situ ${\mathrm{\text{in situ}}}$ reduction process, providing advantages over the ex situ ${\mathrm{\text{ex situ}}}$ CaH2 method for surface-sensitive experiments.

Evidence for Anisotropic Superconductivity Beyond Pauli Limit in Infinite-Layer Lanthanum Nickelates.

After being expected to be a promising analog to cuprates for decades, superconductivity has recently been discovered in infinite-layer nickelates, providing new opportunities to explore mechanisms of high-temperature superconductivity. However, in sharp contrast to the single-band and anisotropic superconductivity in cuprates, nickelates exhibit a multi-band electronic structure and an unexpected isotropic superconductivity as reported recently, which challenges the cuprate-like picture in nickelates. Here, it is shown that strong anisotropic magnetotransport behaviors exist in La-based nickelate films with enhanced crystallinity and superconductivity ( T c onset $T_{\rm{c}}^{{\rm{onset}}}$ = 18.8 K, T c zero $T_{\rm{c}}^{{\rm{zero}}}$ = 16.5 K). The upper critical fields are anisotropic and violate the estimated Bardeen-Cooper-Schrieffer (BCS) Pauli limit ( H Pauli , µ = 1 µ B = 1.86 × T c , H = 0 ${H}_{\mathrm{Pauli},\mu =1{\mu}_{B}}=1.86\ensuremath{\times{}}{T}_{\mathrm{c},H=0}$ ) for in-plane magnetic fields. Moreover, the anisotropic superconductivity is further manifested by the cusp-like peak of the angle-dependent Tc and the vortex motion anisotropy under external magnetic fields.

Prominent Size Effects without a Depolarization Field Observed in Ultrathin Ferroelectric Oxide Membranes.

The increasing miniaturization of electronics requires a better understanding of material properties at the nanoscale. Many studies have shown that there is a ferroelectric size limit in oxides, below which the ferroelectricity will be strongly suppressed due to the depolarization field, and whether such a limit still exists in the absence of the depolarization field remains unclear. Here, by applying uniaxial strain, we obtain pure in-plane polarized ferroelectricity in ultrathin SrTiO_{3} membranes, providing a clean system with high tunability to explore ferroelectric size effects especially the thickness-dependent ferroelectric instability with no depolarization field. Surprisingly, the domain size, ferroelectric transition temperature, and critical strain for room-temperature ferroelectricity all exhibit significant thickness dependence. These results indicate that the stability of ferroelectricity is suppressed (enhanced) by increasing the surface or bulk ratio (strain), which can be explained by considering the thickness-dependent dipole-dipole interactions within the transverse Ising model. Our study provides new insights into ferroelectric size effects and sheds light on the applications of ferroelectric thin films in nanoelectronics.

Tuning Piezoelectricity via Thermal Annealing at a Freestanding Ferroelectric Membrane.

Tuning the ferroelectric domain structure by a combination of elastic and electrostatic engineering provides an effective route for enhanced piezoelectricity. However, for epitaxial thin films, the clamping effect imposed by the substrate does not allow aftergrowth tuning and also limits the electromechanical response. In contrast, freestanding membranes, which are free of substrate constraints, enable the tuning of a subtle balance between elastic and electrostatic energies, giving new platforms for enhanced and tunable functionalities. Here, highly tunable piezoelectricity is demonstrated in freestanding PbTiO3 membranes, by varying the ferroelectric domain structures from c-dominated to c/a and a domains via aftergrowth thermal treatment. Significantly, the piezoelectric coefficient of the c/a domain structure is enhanced by a factor of 2.5 compared with typical c domain PbTiO3. This work presents a new strategy to manipulate the piezoelectricity in ferroelectric membranes, highlighting their great potential for nano actuators, transducers, sensors and other NEMS device applications.

A TiN@C core-shell support for improving Pt catalyst corrosion resistance.

The corrosion of the support in proton-exchange membrane fuel cells (PEMFCs) is a major obstacle to their development. In this study, we combined the excellent corrosion resistance and strong metal-support interaction (SMSI) provided by titanium nitride (TiN) with the excellent conductivity of carbon to construct a TiN@C composite support composed of a TiN core and a porous carbon nanolayer shell. The composite TiN@C support exhibited a higher corrosion resistance than the carbon support during testing at 1.2 V (vs. RHE) for 400 h. Based on X-ray photoelectron spectroscopy and density functional theory calculations, the improved corrosion resistance originated from the excellent corrosion resistance of titanium nitride itself and SMSI between Pt and N in TiN. Overall, the high corrosion resistance of the TiN@C support can significantly improve PEMFC durability.

Enhanced polarization and abnormal flexural deformation in bent freestanding perovskite oxides.

Recent realizations of ultrathin freestanding perovskite oxides offer a unique platform to probe novel properties in two-dimensional oxides. Here, we observe a giant flexoelectric response in freestanding BiFeO3 and SrTiO3 in their bent state arising from strain gradients up to 3.5 × 107 m-1, suggesting a promising approach for realizing ultra-large polarizations. Additionally, a substantial change in membrane thickness is discovered in bent freestanding BiFeO3, which implies an unusual bending-expansion/shrinkage effect in the ferroelectric membrane that has never been seen before in crystalline materials. Our theoretical model reveals that this unprecedented flexural deformation within the membrane is attributable to a flexoelectricity-piezoelectricity interplay. The finding unveils intriguing nanoscale electromechanical properties and provides guidance for their practical applications in flexible nanoelectromechanical systems.

Nonvolatile ferroelectric domain wall memory integrated on silicon.

Ferroelectric domain wall memories have been proposed as a promising candidate for nonvolatile memories, given their intriguing advantages including low energy consumption and high-density integration. Perovskite oxides possess superior ferroelectric prosperities but perovskite-based domain wall memory integrated on silicon has rarely been reported due to the technical challenges in the sample preparation. Here, we demonstrate a domain wall memory prototype utilizing freestanding BaTiO3 membranes transferred onto silicon. While as-grown BaTiO3 films on (001) SrTiO3 substrate are purely c-axis polarized, we find they exhibit distinct in-plane multidomain structures after released from the substrate and integrated onto silicon due to the collective effects from depolarizing field and strain relaxation. Based on the strong in-plane ferroelectricity, conductive domain walls with reading currents up to nanoampere are observed and can be both created and erased artificially, highlighting the great potential of the integration of perovskite oxides with silicon for ferroelectric domain wall memories.

High-density switchable skyrmion-like polar nanodomains integrated on silicon.

Topological domains in ferroelectrics1-5 have received much attention recently owing to their novel functionalities and potential applications6,7 in electronic devices. So far, however, such topological polar structures have been observed only in superlattices grown on oxide substrates, which limits their applications in silicon-based electronics. Here we report the realization of room-temperature skyrmion-like polar nanodomains in lead titanate/strontium titanate bilayers transferred onto silicon. Moreover, an external electric field can reversibly switch these nanodomains into the other type of polar texture, which substantially modifies their resistive behaviours. The polar-configuration-modulated resistance is ascribed to the distinct band bending and charge carrier distribution in the core of the two types of polar texture. The integration of high-density (more than 200 gigabits per square inch) switchable skyrmion-like polar nanodomains on silicon may enable non-volatile memory applications using topological polar structures in oxides.

Giant Thermal Transport Tuning at a Metal/Ferroelectric Interface.

Interfacial thermal transport plays a prominent role in the thermal management of nanoscale objects and is of fundamental importance for basic research and nanodevices. At metal/insulator interfaces, a configuration commonly found in electronic devices, heat transport strongly depends upon the effective energy transfer from thermalized electrons in the metal to the phonons in the insulator. However, the mechanism of interfacial electron-phonon coupling and thermal transport at metal/insulator interfaces is not well understood. Here, the observation of a substantial enhancement of the interfacial thermal resistance and the important role of surface charges at the metal/ferroelectric interface in an Al/BiFeO3 membrane are reported. By applying uniaxial strain, the interfacial thermal resistance can be varied substantially (up to an order of magnitude), which is attributed to the renormalized interfacial electron-phonon coupling caused by the charge redistribution at the interface due to the polarization rotation. These results imply that surface charges at a metal/insulator interface can substantially enhance the interfacial electron-phonon-mediated thermal coupling, providing a new route to optimize the thermal transport performance in next-generation nanodevices, power electronics, and thermal logic devices.

Rewritable High-Mobility Electrons in Oxide Heterostructure of Layered Perovskite/Perovskite.

Perovskite oxide SrTiO3 can be electron-doped and exhibits high mobility by introducing oxygen vacancies or dopants such as Nb or La. A reversible after-growth tuning of high mobility carriers in SrTiO3 is highly desired for the applications in high-speed electronic devices. Here, we report the observation of tunable high-mobility electrons in layered perovskite/perovskite (Srn+1TinO3n+1/SrTiO3) heterostructure. By use of Srn+1TinO3n+1 as the oxygen diffusion barrier, the oxygen vacancy concentration near the interface can be reversibly engineered by high-temperature annealing or infrared laser heating. Because of the identical elemental compositions (Sr, Ti, and O) throughout the whole heterostructure, interfacial ionic intermixing is absent, giving rise to an extremely high mobility (exceeding 55000 cm2 V-1 s-1 at 2 K) in this type of oxide heterostructure. This layered perovskite/perovskite heterostructure provides a promising platform for reconfigurable high-speed electronic devices.

Freestanding crystalline oxide perovskites down to the monolayer limit.

Two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides reveal the electronic phases that emerge when a bulk crystal is reduced to a monolayer1-4. Transition-metal oxide perovskites host a variety of correlated electronic phases5-12, so similar behaviour in monolayer materials based on transition-metal oxide perovskites would open the door to a rich spectrum of exotic 2D correlated phases that have not yet been explored. Here we report the fabrication of freestanding perovskite films with high crystalline quality almost down to a single unit cell. Using a recently developed method based on water-soluble Sr3Al2O6 as the sacrificial buffer layer13,14 we synthesize freestanding SrTiO3 and BiFeO3 ultrathin films by reactive molecular beam epitaxy and transfer them to diverse substrates, in particular crystalline silicon wafers and holey carbon films. We find that freestanding BiFeO3 films exhibit unexpected and giant tetragonality and polarization when approaching the 2D limit. Our results demonstrate the absence of a critical thickness for stabilizing the crystalline order in the freestanding ultrathin oxide films. The ability to synthesize and transfer crystalline freestanding perovskite films without any thickness limitation onto any desired substrate creates opportunities for research into 2D correlated phases and interfacial phenomena that have not previously been technically possible.

Crossover from negative to positive magnetoresistance in Sr2CrWO6/Sr2Fe10/9Mo8/9O6 superlattices.

Sr2CrWO6/Sr2Fe10/9Mo8/9O6 (SCWO/SFMO) superlattices with 4, 6, 7, 10 periods (abbreviated as S-1, S-2, S-3, and S-4) were prepared on (0 0 1) SrTiO3 (STO) substrates by pulsed laser deposition. All superlattices show macroscopic ferromagnetic behavior, and the magnetization increases with increasing period. The S-1 superlattice demonstrates semiconductor-like temperature-dependent resistivity in the whole measuring temperature range and negative magnetoresistance of -5.3% at 2 K with 2 T magnetic field, while the other superlattices illustrate metallic behaviors and increasing positive magnetoresistance of 223.1%, 275.4%, and 766.1% under the same conditions. This work not only provides a feasible way to tune the MR effect in magnetic perovskite oxides, but also may stimulate further work on artificially micro-structured thin films with designable magnetic properties.

An ultrathin surface-nitrided porous titanium sheet as a current collector-free sulfur host for high-gravimetric-capacity lithium-sulfur batteries.

A novel cathode structure consisting of ultrathin and freestanding surface-nitrided porous titanium (SNPT) sheets, was designed for high-gravimetric-capacity Li-S batteries. This unusual cathode combines the sulfur host and the current collector together, increasing the sulfur mass ratio by over 20% compared with routine cathodes, resulting in excellent cycling performance with an initial capacity of 1325 mA h g-1 and a coulombic efficiency of over 99% at 1.0C.

Effect of Substrate symmetry on the dendrite morphology of MoS2 Film synthesized by CVD.

In van der Waals epitaxial growth, the substrate plays a particularly important role in the crystal morphology. Here, we synthesized MoS2 by chemical vapour deposition on silicon carbide (SiC). The obtained MoS2 dendritic crystals show six-fold symmetry, which are different from the conventional triangular shapes on SiO2 substrate and from those with three-fold symmetry on SrTiO3 substrate. Interestingly, these MoS2 dendritic crystals on SiC exhibit an average fractal dimension 1.76, which is slightly larger than the classical Diffusion-limited-Aggregation fractal dimension 1.66. The first principle calculation indicates that the six-fold symmetry of the dendritic MoS2 is determined by the lattice symmetry of SiC. To further demonstrating the substrate effect, we break the natural six-fold lattice symmetry of SiC (0001) into groove arrays through etching the substrate. And then we successfully synthesized cross-type dendritic crystal MoS2 with two-fold symmetry. Its average fractal dimension 1.83 is slightly larger than the fractal dimension 1.76 of the previous MoS2 dendrite with six-fold symmetry. In a word, the symmetry of SiC substrate determined the symmetry and the fractal dimension of the dendritic MoS2. This work provides one possibility of inducing the growth orientation of dendritic crystals through controlling the substrate surface symmetry artificially.

Giant positive magnetoresistance in half-metallic double-perovskite Sr2CrWO6 thin films.

Magnetoresistance (MR) is the magnetic field-induced change of electrical resistance. The MR effect not only has wide applications in hard drivers and sensors but also is a long-standing scientific issue for complex interactions. Ferromagnetic/ferrimagnetic oxides generally show negative MR due to the magnetic field-induced spin order. We report the unusually giant positive MR up to 17,200% (at 2 K and 7 T) in 12-nm Sr2CrWO6 thin films, which show metallic behavior with high carrier density of up to 2.26 × 1028 m-3 and high mobility of 5.66 × 104 cm2 V-1 s-1. The possible mechanism is that the external magnetic field suppresses the long-range antiferromagnetic order to form short-range antiferromagnetic fluctuations, which enhance electronic scattering and lead to the giant positive MR. The high mobility may also have contributions to the positive MR. These results not only experimentally confirm that the giant positive MR can be realized in oxides but also open up new opportunities for developing and understanding the giant positive MR in oxides.

Room-Temperature Multiferroics and Thermal Conductivity of 0.85BiFe1-2xTixMgxO3-0.15CaTiO3 Epitaxial Thin Films (x = 0.1 and 0.2).

Thin films of 0.85BiFe1-2xTixMgxO3-0.15CaTiO3 (x = 0.1 and 0.2, abbreviated to C-1 and C-2, respectively) have been fabricated on (001) SrTiO3 substrate with and without a conductive La0.7Sr0.3MnO3 buffer layer. The X-ray θ-2θ and ϕ scans, atomic force microscopy, and cross-sectional transmission electron microscopy confirm the (001) epitaxial nature of the thin films with very high growth quality. Both the C-1 and C-2 thin films show well-shaped magnetization-magnetic field hysteresis at room temperature, with enhanced switchable magnetization values of 145.3 and 42.5 emu/cm3, respectively. The polarization-electric loops and piezoresponse force microscopy measurements confirm the room-temperature ferroelectric nature of both films. However, the C-1 films illustrate a relatively weak ferroelectric behavior and the poled states are easy to relax, whereas the C-2 films show a relatively better ferroelectric behavior with stable poled states. More interestingly, the room-temperature thermal conductivity of C-1 and C-2 films are measured to be 1.10 and 0.77 W/(m·K), respectively. These self-consistent multiferroic properties and thermal conductivities are discussed by considering the composition-dependent content and migration of Fe-induced electrons and/or charged point defects. This study not only provides multifunctional materials with excellent room-temperature magnetic, ferroelectric, and thermal conductivity properties but may also stimulate further work to develop BiFeO3-based materials with unusual multifunctional properties.

Silicon Photoanodes Partially Covered by Ni@Ni(OH)2 Core-Shell Particles for Photoelectrochemical Water Oxidation.

Two obstacles hindering solar energy conversion by photoelectrochemical (PEC) water-splitting devices are the charge separation and the transport efficiency at the photoanode-electrolyte interface region. Herein, core-shell-structured Ni@Ni(OH)2 nanoparticles were electrodeposited on the surface of an n-type Si photoanode. The Schottky barrier between Ni and Si is sensitive to the thickness of the Ni(OH)2 shell. The photovoltage output of the photoanode increases with increasing thickness of the Ni(OH)2 shell, and is influenced by interactions between Ni and Ni(OH)2 , the electrolyte screening effect, and the p-type nature of the Ni(OH)2 layer. Ni@Ni(OH)2 core-shell nanoparticles with appropriate shell thicknesses coupled to n-type Si photoanodes promote the separation of photogenerated carriers and improve the charge-injection efficiency to nearly 100 %. An onset potential of 1.03 V versus reversible hydrogen electrode (RHE) and a saturated current density of 36.4 mA cm-2 was obtained for the assembly.

Metal-insulator transition in SrIrO3 with strong spin-orbit interaction.

The thickness-dependent metal-insulator transition is observed in meta-stable orthorhombic SrIrO3 thin films synthesized by pulsed laser deposition. SrIrO3 films with thicknesses less than 3 nm demonstrate insulating behaviour, whereas those thicker than 4 nm exhibit metallic conductivity at high temperature, and insulating-like behaviour at low temperature. Weak/Anderson localization is mainly responsible for the observed thickness-dependent metal-insulator transition in SrIrO3 films. Temperature-dependent resistance fitting shows that electrical-conductivity carriers are mainly scattered by the electron-boson interaction rather than the electron-electron interaction. Analysis of the magneto-conductance proves that the spin-orbit interaction plays a crucial role in the magneto-conductance property of SrIrO3.