Symmetry Engineering of Epitaxial Hf0.5Zr0.5O2 Ultrathin Films.

Robust ferroelectricity in HfO2-based ultrathin films has the potential to revolutionize nonvolatile memory applications in nanoscale electronic devices because of their compatibility with the existing Si technology. However, to fully exploit the potential of ferroelectric HfO2-based thin films, it is crucial to develop strategies for the controlled stabilization of various HfO2-based polymorphs in nanoscale heterostructures. This study demonstrates how substrate-orientation-induced anisotropic strain can engineer the crystal symmetry, structural domain morphology, and growth orientation of ultrathin Hf0.5Zr0.5O2 (HZO) films. Epitaxial ultrathin HZO films were grown on the heterostructures of (001)- and (110)-oriented La2/3Sr1/3MnO3/SrTiO3 (LSMO/STO) substrate. Various structural analyses revealed that the (110)-oriented substrate promotes a higher degree of structural order (crystallinity) with improved stability of the (111)-oriented orthorhombic phase (Pca21) of HZO. Conversely, the (001)-oriented substrate not only induces a distorted orthorhombic structure but also facilitates the partial stabilization of nonpolar phases. Electrical measurements revealed robust ferroelectric properties in epitaxial thin films without any wake-up effect, where the well-ordered crystal symmetry stabilized by STO(110) facilitated better ferroelectric characteristics. This study suggests that tuning the epitaxial growth of ferroelectric HZO through substrate orientation can improve the stability of the metastable ferroelectric orthorhombic phase and thereby offer a better understanding of device applications.

Self-Oxidation Resistance of the Curved Surface of Achromatic Copper.

Copper surfaces that exhibit a wide range of achromatic colors while still metallic have not been studied, despite advancements in antireflection coatings. A series of achromatic copper films grown with [111] preferred orientation by depositing 3D porous nanostructures is introduced via coherent/incoherent atomic sputtering epitaxy. The porous copper nanostructures self-regulate the giant oxidation resistance by constructing a curved surface that generates a series of monoatomic steps, followed by shrinkage of the lattice spacing of one or two surface layers. First-principles calculations confirm that these structural components cooperatively increase the energy barrier against oxygen penetration. The achromaticity of the single-crystalline porous copper films is systematically tuned by geometrical parameters such as pore size distribution and 3D linkage. The optimized achromatic copper films with high oxidation resistance show an unusual switching effect between superhydrophilicity and superhydrophobicity. The tailored 3D porous nanostructures can be a candidate material for numerous applications, such as antireflection coatings, microfluidic devices, droplet tweezers, and reversible wettability switches.

Freestanding Oxide Membranes for Epitaxial Ferroelectric Heterojunctions.

Since facile routes to fabricate freestanding oxide membranes were previously established, tremendous efforts have been made to further improve their crystallinity, and fascinating physical properties have been also reported in heterointegrated freestanding membranes. Here, we demonstrate our synthetic recipe to manufacture highly crystalline perovskite SrRuO3 freestanding membranes using new infinite-layer perovskite SrCuO2 sacrificial layers. To accomplish this, SrRuO3/SrCuO2 bilayer thin films are epitaxially grown on SrTiO3 (001) substrates, and the topmost SrRuO3 layer is chemically exfoliated by etching the SrCuO2 template layer. The as-exfoliated SrRuO3 membranes are mechanically transferred to various nonoxide substrates for the subsequent BaTiO3 film growth. Finally, freestanding heteroepitaxial junctions of ferroelectric BaTiO3 and metallic SrRuO3 are realized, exhibiting robust ferroelectricity. Intriguingly, the enhancement of piezoelectric responses is identified in freestanding BaTiO3/SrRuO3 heterojunctions with mixed ferroelectric domain states. Our approaches will offer more opportunities to develop heteroepitaxial freestanding oxide membranes with high crystallinity and enhanced functionality.

Ultrahigh dielectric permittivity in oxide ceramics by hydrogenation.

Boosting dielectric permittivity representing electrical polarizability of dielectric materials has been considered a keystone for achieving scientific breakthroughs as well as technological advances in various multifunctional devices. Here, we demonstrate sizable enhancements of low-frequency dielectric responses in oxygen-deficient oxide ceramics through specific treatments under humid environments. Ultrahigh dielectric permittivity (~5.2 × 106 at 1 Hz) is achieved by hydrogenation, when Ni-substituted BaTiO3 ceramics are exposed to high humidity. Intriguingly, thermal annealing can restore the dielectric on-state (exhibiting huge polarizability in the treated ceramics) to the initial dielectric off-state (displaying low polarizability of ~103 in the pristine ceramics after sintering). The conversion between these two dielectric states via the ambient environment-mediated treatments and the successive application of external stimuli allows us to realize reversible control of dielectric relaxation characteristics in oxide ceramics. Conceptually, our findings are of practical interest for applications to highly efficient dielectric-based humidity sensors.

Machine-learning-assisted analysis of transition metal dichalcogenide thin-film growth.

In situ reflective high-energy electron diffraction (RHEED) is widely used to monitor the surface crystalline state during thin-film growth by molecular beam epitaxy (MBE) and pulsed laser deposition. With the recent development of machine learning (ML), ML-assisted analysis of RHEED videos aids in interpreting the complete RHEED data of oxide thin films. The quantitative analysis of RHEED data allows us to characterize and categorize the growth modes step by step, and extract hidden knowledge of the epitaxial film growth process. In this study, we employed the ML-assisted RHEED analysis method to investigate the growth of 2D thin films of transition metal dichalcogenides (ReSe2) on graphene substrates by MBE. Principal component analysis (PCA) and K-means clustering were used to separate statistically important patterns and visualize the trend of pattern evolution without any notable loss of information. Using the modified PCA, we could monitor the diffraction intensity of solely the ReSe2 layers by filtering out the substrate contribution. These findings demonstrate that ML analysis can be successfully employed to examine and understand the film-growth dynamics of 2D materials. Further, the ML-based method can pave the way for the development of advanced real-time monitoring and autonomous material synthesis techniques.

Reversibly Controlled Ternary Polar States and Ferroelectric Bias Promoted by Boosting Square-Tensile-Strain.

Interaction between dipoles often emerges intriguing physical phenomena, such as exchange bias in the magnetic heterostructures and magnetoelectric effect in multiferroics, which lead to advances in multifunctional heterostructures. However, the defect-dipole tends to be considered the undesired to deteriorate the electronic functionality. Here, deterministic switching between the ferroelectric and the pinched states by exploiting a new substrate of cubic perovskite, BaZrO3 is reported, which boosts the square-tensile-strain to BaTiO3 and promotes four-variants in-plane spontaneous polarization with oxygen vacancy creation. First-principles calculations propose a complex of an oxygen vacancy and two Ti3+ ions coins a charge-neutral defect-dipole. Cooperative control of the defect-dipole and the spontaneous polarization reveals ternary in-plane polar states characterized by biased/pinched hysteresis loops. Furthermore, it is experimentally demonstrated that three electrically controlled polar-ordering states lead to switchable and nonvolatile dielectric states for application of nondestructive electro-dielectric memory. This discovery opens a new route to develop functional materials via manipulating defect-dipoles and offers a novel platform to advance heteroepitaxy beyond the prevalent perovskite substrates.

Sequential Growth of Vertical Transition-Metal Dichalcogenide Heterostructures on Rollable Aluminum Foil.

Vertical van der Waals heterostructures (vdWhs), which are made by layer-by-layer stacking of two-dimensional (2D) materials, offer great opportunities for the development of extraordinary physics and devices such as topological superconductivity, robust quantum Hall phenomenon, electron-hole pair condensation, Coulomb drag, and tunneling devices. However, the size of vdWhs is still limited to the order of a few micrometers, which restricts the large-scale roll-to-roll processes for industrial applications. Herein, we report the sequential growth of a 14 in. vertical vdWhs on a rollable Al foil via chemical vapor deposition. By supplying chalcogen precursors to liquid transition-metal precursor-coated Al foils, we grew a wide range of individual 2D transition-metal dichalcogenide (TMD) films, including MoS2, VS2, ReS2, WS2, SnS2, WSe2, and vanadium-doped MoS2. Additionally, by repeating the growth process, we successfully achieved the layer-by-layer growth of ReS2/MoS2 and SnS2/ReS2/MoS2 vdWhs. The chemically inert Al native oxide layer inhibits the diffusion of chalcogen and metal atoms into Al foils, allowing for the growth of diverse TMDs and their vdWhs. The conductive Al substrate enables the effective use of vdWhs/Al as a hydrogen evolution reaction electrocatalyst with a transfer-free process. This work provides a robust route for the commercialization of 2D TMDs and their vdWhs at a low cost.

Highly enhanced ferroelectricity in HfO2-based ferroelectric thin film by light ion bombardment.

Continuous advancement in nonvolatile and morphotropic beyond-Moore electronic devices requires integration of ferroelectric and semiconductor materials. The emergence of hafnium oxide (HfO2)-based ferroelectrics that are compatible with atomic-layer deposition has opened interesting and promising avenues of research. However, the origins of ferroelectricity and pathways to controlling it in HfO2 are still mysterious. We demonstrate that local helium (He) implantation can activate ferroelectricity in these materials. The possible competing mechanisms, including He ion-induced molar volume changes, vacancy redistribution, vacancy generation, and activation of vacancy mobility, are analyzed. These findings both reveal the origins of ferroelectricity in this system and open pathways for nanoengineered binary ferroelectrics.

Hybrid Deep Learning Crystallographic Mapping of Polymorphic Phases in Polycrystalline Hf0.5 Zr0.5 O2 Thin Films.

By controlling the configuration of polymorphic phases in high-k Hf0.5 Zr0.5 O2 thin films, new functionalities such as persistent ferroelectricity at an extremely small scale can be exploited. To bolster the technological progress and fundamental understanding of phase stabilization (or transition) and switching behavior in the research area, efficient and reliable mapping of the crystal symmetry encompassing the whole scale of thin films is an urgent requisite. Atomic-scale observation with electron microscopy can provide decisive information for discriminating structures with similar symmetries. However, it often demands multiple/multiscale analysis for cross-validation with other techniques, such as X-ray diffraction, due to the limited range of observation. Herein, an efficient and automated methodology for large-scale mapping of the crystal symmetries in polycrystalline Hf0.5 Zr0.5 O2 thin films is developed using scanning probe-based diffraction and a hybrid deep convolutional neural network at a 2 nm2 resolution. The results for the doped hafnia films are fully proven to be compatible with atomic structures revealed by microscopy imaging, not requiring intensive human input for interpretation.

Contribution of the Sub-Surface to Electrocatalytic Activity in Atomically Precise La0.7 Sr0.3 MnO3 Heterostructures.

Electrocatalytic reactions are known to take place at the catalyst/electrolyte interface. Whereas recent studies of size-dependent activity in nanoparticles and thickness-dependent activity of thin films imply that the sub-surface layers of a catalyst can contribute to the catalytic activity as well, most of these studies consider actual modification of the surfaces. In this study, the role of catalytically active sub-surface layers was investigated by employing atomic-scale thickness control of the La0.7 Sr0.3 MnO3 (LSMO) films and heterostructures, without altering the catalyst/electrolyte interface. The activity toward the oxygen evolution reaction (OER) shows a non-monotonic thickness dependence in the LSMO films and a continuous screening effect in LSMO/SrRuO3 heterostructures. The observation leads to the definition of an "electrochemically-relevant depth" on the order of 10 unit cells. This study on the electrocatalytic activity of epitaxial heterostructures provides new insight in designing efficient electrocatalytic nanomaterials and core-shell architectures.