A 2D ferroelectric vortex pattern in twisted BaTiO3 freestanding layers

Imaging Electron Beam-Sensitive Twisted Hybrid Perovskite Bilayers at One-Angstrom Resolution

Twisted halide perovskite bilayers, a type of moiré material, show square moiré patterns with exciting optical properties. Atomic-scale structure analysis and its correlation with properties are difficult to achieve due to the extreme sensitivity of organic-inorganic halide perovskites to the illuminated electron beam in conventional/scanning transmission electron microscopy. Here, we developed a low-dose exit wave reconstruction methodology with a real-space resolution of one angstrom at ∼50 e/Å2, which recovers the phase information on the moiré fringes in CH3NH3PbI3 (MAPbI3) twisted perovskite bilayers at atomic scale, enabling detailed structural analysis of defects and corresponding strain distribution in such moiré materials. This work provides an atomic-level understanding of electron beam-sensitive twisted bilayer materials.

Lattice-mismatched and twisted multi-layered materials for efficient solar cells

We argue that alternating-layer structures of lattice mismatched or misaligned (twisted) atomically-thin layers should be expected to be more efficient absorbers of the broad-spectrum of solar radiation than the bulk material of each individual layer. In such mismatched layer-structures the conduction and valence bands of the bulk material, split into multiple minibands separated by minigaps confined to a small-size emerging Brillouin zone due to band-folding. We extended the Shockley-Queisser approach to calculate the photovoltaic efficiency for a band split into minibands of bandwidth ΔEand mini-gaps δGto model the case when such structures are used as solar cells. We find a significant efficiency enhancement due to impact ionization processes, especially in the limit of small but non-zero δG, and a dramatic increase when fully concentrated Sun-light is used.

Spontaneous curvature in two-dimensional van der Waals heterostructures

Two-dimensional (2D) van der Waals heterostructures consist of different 2D crystals with diverse properties, constituting the cornerstone of the new generation of 2D electronic devices. Yet interfaces in heterostructures inevitably break bulk symmetry and structural continuity, resulting in delicate atomic rearrangements and novel electronic structures. In this paper, we predict that 2D interfaces undergo "spontaneous curvature", which means when two flat 2D layers approach each other, they inevitably experience out-of-plane curvature. Based on deep-learning-assisted large-scale molecular dynamics simulations, we observe significant out-of-plane displacements up to 3.8 Å in graphene/BN bilayers induced by curvature, producing a stable hexagonal moiré pattern, which agrees well with experimentally observations. Additionally, the out-of-plane flexibility of 2D crystals enables the propagation of curvature throughout the system, thereby influencing the mechanical properties of the heterostructure. These findings offer fundamental insights into the atomic structure in 2D van der Waals heterostructures and pave the way for their applications in devices.

Enhanced Bulk Photovoltaic Effect of Single-Domain Freestanding BiFeO3 Membranes

The bulk photovoltaic effect (BPVE), which uniquely exists in non-centrosymmetric materials, has been received extensive attention recently due to its potential to overcome the theoretical Shockley-Queisser limit in traditional p-n junction solar cells. Here, freestanding single-domain BiFeO3 membranes are exfoliated from miscut SrTiO3 substrates by dissolving Sr3Al2O6 sacrificial layers, and transferred on SiO2/Si substrates. This study finds that the freestanding BiFeO3 membranes maintain the single-domain structure and exhibits the significantly enhanced bulk photovoltaic response (≈200% enhancement), compared to the strained BiFeO3 films. The comprehensive atomic imaging analyses manifest that the freestanding BiFeO3 membrane demonstrates the bigger noncentral ion (Fe) displacement, which results in the larger in-plane ferroelectric polarization and substantial increase in the BPVE photocurrent. This work not only provides an effective approach to enhance the BPVE of ferroelectric oxide films, but also can potentially promote the exploration of BPVE of oxide membranes integrated with silicon-based or 2D electronics.

On-demand nanoengineering of in-plane ferroelectric topologies

Hierarchical assemblies of ferroelectric nanodomains, so-called super-domains, can exhibit exotic morphologies that lead to distinct behaviours. Controlling these super-domains reliably is critical for realizing states with desired functional properties. Here we reveal the super-switching mechanism by using a biased atomic force microscopy tip, that is, the switching of the in-plane super-domains, of a model ferroelectric Pb0.6Sr0.4TiO3. We demonstrate that the writing process is dominated by a super-domain nucleation and stabilization process. A complex scanning-probe trajectory enables on-demand formation of intricate centre-divergent, centre-convergent and flux-closure polar structures. Correlative piezoresponse force microscopy and optical spectroscopy confirm the topological nature and tunability of the emergent structures. The precise and versatile nanolithography in a ferroic material and the stability of the generated structures, also validated by phase-field modelling, suggests potential for reliable multi-state nanodevice architectures and, thereby, an alternative route for the creation of tunable topological structures for applications in neuromorphic circuits.

Polar vortex hidden in twisted bilayers of paraelectric SrTiO3

Polar topologies, such as vortex and skyrmion, have attracted significant interest due to their unique physical properties and promising applications in high-density memory devices. To date, all known polar vortices are present in or induced by ferroelectric materials. In this study, we find polar vortex arrays in paraelectric SrTiO3. Using multislice electron ptychography, the evolution of vorticity along the vortex axis is revealed in twisted bilayers of SrTiO3 with deep-sub-angstrom resolution and one picometer accuracy. The surprising finding of polar vortices in a paraelectric crystal opens up opportunities for polarization physics and corresponding new devices.

From oxide epitaxy to freestanding membranes: Opportunities and challenges

Motivated by the growing demand to integrate functional oxides with dissimilar materials, numerous studies have been undertaken to detach a functional oxide film from its original substrate, effectively forming a membrane, which can then be affixed to the desired host material. This review article is centered on the synthesis of functional oxide membranes, encompassing various approaches to their synthesis, exfoliation, and transfer techniques. First, we explore the characteristics of thin-film growth techniques with emphasis on molecular beam epitaxy. We then examine the fundamental principles and pivotal factors underlying three key approaches of creating membranes: (i) chemical lift-off, (ii) the two-dimensional layer-assisted lift-off, and (iii) spalling. We review the methods of exfoliation and transfer for each approach. Last, we provide an outlook into the future of oxide membranes, highlighting their applications and emerging properties.

Stacking-Engineered Ferroelectricity and Multiferroic Order in van der Waals Magnets

Two-dimensional (2D) materials that exhibit spontaneous magnetization, polarization, or strain (referred to as ferroics) have the potential to revolutionize nanotechnology by enhancing the multifunctionality of nanoscale devices. However, multiferroic order is difficult to achieve, requiring complicated coupling between electron and spin degrees of freedom. We propose a universal method to engineer multiferroics from van der Waals magnets by taking advantage of the fact that changing the stacking between 2D layers can break inversion symmetry, resulting in ferroelectricity as well as magnetoelectric coupling. We illustrate this concept using first-principles calculations in bilayer NiI_{2}, which can be made ferroelectric upon rotating two adjacent layers by 180° with respect to the bulk stacking. Furthermore, we discover a novel strong magnetoelectric coupling between the interlayer spin order and interfacial electronic polarization. Our approach is not only general but also systematic and can enable the discovery of a wide variety of 2D multiferroics with strong magnetoelectric coupling.

Moiré polar vortex, flat bands, and Lieb lattice in twisted bilayer BaTiO3

Through first-principles calculations based on density functional theory, we investigate the crystal and electronic structures of twisted bilayer BaTiO3. Our findings reveal that large stacking fault energy leads to a chiral in-plane vortex pattern that was recently observed in experiments. We also found nonzero out-of-plane local dipole moments, indicating that the strong interlayer interaction might offer a promising strategy to stabilize ferroelectric order in the two-dimensional limit. The vortex pattern in the twisted BaTiO3 bilayer supports localized electronic states with quasi-flat bands, associated with the interlayer hybridization of oxygen pz orbitals. We found that the associated bandwidth reaches a minimum at ∼19∘ twisting, configuring the largest magic angle in moiré systems reported so far. Further, the moiré vortex pattern bears a notable resemblance to two interpenetrating Lieb lattices and the corresponding tight-binding model provides a comprehensive description of the evolution the moiré bands with twist angle and reveals the topological nature.

Ultrahigh-Density Polar Vortex Lattice in Square-Shaped Moiré Bilayers of Lead Chalcogenides

Nanoscale exotic polar topological structures, such as vortices and skyrmions, hold promise for next-generation electronic devices, yet their spontaneous formation in 2D van der Waals (vdW) materials remains quite challenging. Herein, we demonstrate from first-principles that ultrahigh-density polar vortices emerge in the square moiré bilayer formed by twisting two layers of centrosymmetric PbS with the D4h point group. The emerged ferroelectricity arises from the inherent complex strain associated with the twisted structures, and the resulting electron polarization is much greater than that obtained in sliding ferroelectricity. Notably, the engineered strain patterns are characterized by peculiar inhomogeneous in-plane fields with a checkerboard distribution of uniaxial tension. This nanoscale nonuniform strain produces an ultrahigh-density vortex polarization lattice. The results from our study not only reveals a new mechanism for electric polarization and polar topologies in moiré bilayers but also provides opportunities for designing 2D ultrahigh-density electric devices.

Chiral Inorganic Polar BaTiO3/BaCO3 Nanohybrids with Spin Selection for Asymmetric Photocatalysis

Chirality-dependent photocatalysis is an emerging domain that leverages unique chiral light-matter interactions for enabling asymmetric catalysis driven by spin polarization induced by circularly polarized light selection. Herein, we synthesize chiral inorganic polar BaTiO3/BaCO3 nanohybrids for asymmetric photocatalysis via a hydrothermal method employing chiral glucose as a structural inducer. When excited by opposite circularly polarized light, the same material exhibits significant asymmetric catalysis, while enantiomers present an opposite polarization preference. More importantly, the preferred circularly polarized light undergoes reversal with reversal of the CD signal. Systematic experimental results demonstrate that more photogenerated carriers are generated in chiral semiconductors under suitable circularly polarized light irradiation, including more spin-polarized electrons, which inhibits the recombination of electron-hole pairs and promotes the activation of oxygen molecules into reactive oxygen species, thus inducing this asymmetric photocatalytic feature. This study provides valuable insights for the development of highly efficient polarization-sensitive chiral perovskite nanostructures as promising candidates for next-generation, multifunctional chiral device applications.

Electron-Beam Writing of Atomic-Scale Reconstructions at Oxide Interfaces

The epitaxial growth of complex oxides enables the production of high-quality films, yet substrate choice is restricted to certain symmetry and lattice parameters, thereby limiting the technological applications of epitaxial oxides. In comparison, the development of free-standing oxide membranes gives opportunities to create novel heterostructures by nonepitaxial stacking of membranes, opening new possibilities for materials design. Here, we introduce a method for writing, with atomic precision, ionically bonded crystalline materials across the gap between an oxide membrane and a carrier substrate. The process involves a thermal pretreatment, followed by localized exposure to the raster scan of a scanning transmission electron microscopy (STEM) beam. STEM imaging and electron energy-loss spectroscopy show that we achieve atomically sharp interface reconstructions between a 30-nm-thick SrTiO3 membrane and a niobium-doped SrTiO3(001)-oriented carrier substrate. These findings indicate new strategies for fabricating synthetic heterostructures with novel structural and electronic properties.

Route to Enhancing Remote Epitaxy of Perovskite Complex Oxide Thin Films

Remote epitaxy is taking center stage in creating freestanding complex oxide thin films with high crystallinity that could serve as an ideal building block for stacking artificial heterostructures with distinctive functionalities. However, there exist technical challenges, particularly in the remote epitaxy of perovskite oxides associated with their harsh growth environments, making the graphene interlayer difficult to survive. Transferred graphene, typically used for creating a remote epitaxy template, poses limitations in ensuring the yield of perovskite films, especially when pulsed laser deposition (PLD) growth is carried out, since graphene degradation can be easily observed. Here, we employ spectroscopic ellipsometry to determine the critical factors that damage the integrity of graphene during PLD by tracking the change in optical properties of graphene in situ. To mitigate the issues observed in the PLD process, we propose an alternative growth strategy based on molecular beam epitaxy to produce single-crystalline perovskite membranes.

Electric Field-Manipulated Optical Chirality in Ferroelectric Vortex Domains

Manipulating optical chirality via electric fields has garnered considerable attention in the realm of both fundamental physics and practical applications. Chiral ferroelectrics, characterized by their inherent optical chirality and switchable spontaneous polarization, are emerging as a promising platform for electronic-photonic integrated circuits applications. Unlike organics with chiral carbon centers, integrating chirality into technologically mature inorganic ferroelectrics has posed a long-standing challenge. Here, the successful introduction of chirality is reported into self-assembly La-doped BiFeO3 nanoislands, which exhibit ferroelectric vortex domains. By employing synergistic experimental techniques with piezoresponse force microscopy and nonlinear optical second-harmonic generation probes, a clear correlation between chirality and polarization configuration within these ferroelectric nanoislands is established. Furthermore, the deterministic control of ferroelectric vortex domains and chirality is demonstrated by applying electric fields, enabling reversible and nonvolatile generation and elimination of optically chiral signals. These findings significantly expand the repertoire of field-controllable chiral systems and lay the groundwork for the development of innovative ferroelectric optoelectronic devices.

Dynamical Manipulation of Polar Topologies from Acoustic Phonon Excitations

Since the recent discovery of polar topologies, a recurrent question has been how to remotely tune them. Many efforts have focused on the pumping of polar optical phonons from optical methods, but with limited success, as only switching between specific phases has been achieved so far. Additionally, the correlation between optical pulse characteristics and the resulting phase is poorly understood. Here, we propose an alternative approach and demonstrate the deterministic and dynamical tailoring of polar topologies using acoustic phonon excitations. Our second-principles simulations reveal that by pumping specific longitudinal and transverse acoustic phonons, various topological textures can be induced in materials like BaTiO3 or PbTiO3. This method leverages the strong coupling between polarization and strain in these materials, enabling predictable and dynamic control of polar patterns. Our findings open up an alternative possibility for the manipulation of polar textures, suggesting a promising research direction.

The fabrication of freestanding complex oxide membranes: Can we avoid using water?

Recent advances in fabricating scalable two-dimensional or freestanding functional materials have shown promise for their use in modern silicon-based electronics and future technologies. A growing interest is in creating freestanding complex oxide membranes using new methods like epitaxial lift-off and mechanical exfoliation to enhance their quality and integrity. Despite these advances, it remains challenging to consistently produce high-quality freestanding oxide membranes on a large scale for practical use. This perspective paper provides an overview of release-and-transfer techniques for fabricating freestanding single-crystalline complex oxide layers, which are initially grown epitaxially. Specifically, we systematically explore the advantages and disadvantages of water-assisted exfoliation of freestanding oxide layers, which have been widely adopted using a water-soluble sacrificial layer in recent years. Furthermore, we compare this approach with other methods to navigate future directions in oxide layer transfer technology, considering material selections, fabrication processes, and functionalization strategies.

Graphical abstract

Seeking a water-free epitaxial lift-off process: (1) Growth: An epitaxial film is grown on a sacrificial layer placed on a single-crystal substrate (top left). (2) Release: The film is released from the substrate without using a water-based sacrificial layer (top right). (3) Transfer: The film is transferred onto a new platform (bottom right). (4) Support Removal: A polymer support is removed from the film (bottom left).

Polar and quasicrystal vortex observed in twisted-bilayer molybdenum disulfide

We report the observation of an electric field in twisted-bilayer molybdenum disulfide (MoS2) and elucidate its correlation with local polar domains using four-dimensional scanning transmission electron microscopy (4D-STEM) and first-principles calculations. We reveal the emergence of in-plane topological vortices within the periodic moiré patterns for both commensurate structures at small twist angles and the incommensurate quasicrystal structure that occurs at a 30° twist. The large-angle twist leads to mosaic chiral vortex patterns with tunable characteristics. A twisted quasicrystal bilayer, characterized by its 12-fold rotational symmetry, hosts complex vortex patterns and can be manipulated by picometer-scale interlayer displacement. Our findings highlight that twisting 2D bilayers is a versatile strategy for tailoring local electric polar vortices.

Interface Design beyond Epitaxy: Oxide Heterostructures Comprising Symmetry-Forbidden Interfaces

Epitaxial growth of thin-film heterostructures is generally considered the most successful procedure to obtain interfaces of excellent structural and electronic quality between 3D materials. However, these interfaces can only join material systems with crystal lattices of matching symmetries and lattice constants. This article presents a novel category of interfaces, the fabrication of which is membrane-based and does not require epitaxial growth. These interfaces therefore overcome the limitations imposed by epitaxy. Leveraging the additional degrees of freedom gained, atomically clean interfaces are demonstrated between threefold symmetric sapphire and fourfold symmetric SrTiO3. Atomic-resolution imaging reveals structurally well-defined interfaces with a novel moiré-type reconstruction.

Tunable Multistate Ferroelectricity of Unit-Cell-Thick BaTiO3 Revived by a Ferroelectric SnS Monolayer via Interfacial Sliding

Stabilization of multiple polarization states at the atomic scale is pivotal for realizing high-density memory devices beyond prevailing bistable ferroelectric architectures. Here, we show that two-dimensional ferroelectric SnS or GeSe is able to revive and stabilize the ferroelectric order of three-dimensional ferroelectric BaTiO3, even when the latter is thinned to one unit cell in thickness. The underlying mechanism for overcoming the conventional detrimental critical thickness effect is attributed to facile interfacial inversion symmetry breaking by robust in-plane polarization of SnS or GeSe. Furthermore, when invoking interlayer sliding, we can stabilize multiple polarization states and achieve efficient interstate switching in the heterostructures, accompanied by dynamical ferroelectric skyrmionic excitations. When invoking sliding and twisting, the moiré domains exhibit nontrivial polar vortexes, which can be laterally displaced via different sliding schemes. These findings provide an intuitive avenue for simultaneously overcoming the standing critical thickness issue in bulk ferroelectrics and weak polarization issue in sliding ferroelectricity.

Atomic Evolution Mechanism and Suppression of Edge Threading Dislocations in Nitride Remote Heteroepitaxy

The majority of dislocations in nitride epilayers are edge threading dislocations (TDs), which diminish the performance of nitride devices. However, it is extremely difficult to reduce the edge TDs due to the lack of available slip systems. Here, we systematically investigate the formation mechanism of edge TDs and find that besides originating at the coalescence boundaries, these dislocations are also closely related to geometrical misfit dislocations at the interface. Based on this understanding, we propose a novel strategy to reduce the edge TD density of the GaN epilayer by nearly 1 order of magnitude via graphene-assisted remote heteroepitaxy. The first-principles calculations confirm that the insertion of graphene dramatically reduces the energy barrier required for interfacial sliding, which promotes a new strain release channel. This work provides a unique approach to directly suppress the formation of edge TDs at the source, thereby facilitating the enhanced performance of photoelectronic and electronic devices.

Revealing the three-dimensional arrangement of polar topology in nanoparticles

In the early 2000s, low dimensional ferroelectric systems were predicted to have topologically nontrivial polar structures, such as vortices or skyrmions, depending on mechanical or electrical boundary conditions. A few variants of these structures have been experimentally observed in thin film model systems, where they are engineered by balancing electrostatic charge and elastic distortion energies. However, the measurement and classification of topological textures for general ferroelectric nanostructures have remained elusive, as it requires mapping the local polarization at the atomic scale in three dimensions. Here we unveil topological polar structures in ferroelectric BaTiO3 nanoparticles via atomic electron tomography, which enables us to reconstruct the full three-dimensional arrangement of cation atoms at an individual atom level. Our three-dimensional polarization maps reveal clear topological orderings, along with evidence of size-dependent topological transitions from a single vortex structure to multiple vortices, consistent with theoretical predictions. The discovery of the predicted topological polar ordering in nanoscale ferroelectrics, independent of epitaxial strain, widens the research perspective and offers potential for practical applications utilizing contact-free switchable toroidal moments.