Strain Gradient Elasticity in SrTiO3 Membranes: Bending versus Stretching

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.

Elastic Properties of Low-Dimensional Single-Crystalline Dielectric Oxides through Controlled Large-Area Wrinkle Generation

Freestanding single-crystalline SrTiO3 membranes, as high-κ dielectrics, hold significant promise as the gate dielectric in two-dimensional (2D) flexible electronics. Nevertheless, the mechanical properties of the SrTiO3 membranes, such as elasticity, remain a critical piece of the puzzle to adequately address the viability of their applications in flexible devices. Here, we report statistical analysis on plane-strain effective Young's modulus of large-area SrTiO3 membranes (5 × 5 mm2) over a series of thicknesses (from 6.5 to 32.2 nm), taking advantage of a highly efficient buckling-based method, which reveals its evident thickness-dependent behavior ranging from 46.01 to 227.17 GPa. Based on microscopic and theoretical results, we elucidate these thickness-dependent behaviors and statistical data deviation with a bilayer model, which consists of a surface layer and a bulk-like layer. The analytical results show that the ∼3.1 nm surface layer has a significant elastic softening compared to the bulk-like layer, while the extracted modulus of the bulk-like layer shows a variation of ∼40 GPa. This variation is considered as a combined contribution from oxygen deficiency presenting in SrTiO3 membranes, and the alignment between applied strain and the crystal orientation. Upon comparison of the extracted elastic properties and electrostatic control capability to those of other typical gate dielectrics, the superior performance of single-crystalline SrTiO3 membranes has been revealed in the context of flexible gate dielectrics, indicating the significant potential of their application in high-performance flexible 2D electronics.

Delamination-Assisted Ultrafast Wrinkle Formation in a Freestanding Film

Freestanding films provide a versatile platform for materials engineering thanks to additional structural motifs not found in films with a substrate. A ubiquitous example is wrinkles, yet little is known about how they can develop over as fast as a few picoseconds due to a lack of experimental probes to visualize their dynamics in real time on the nanoscopic scale. Here, we use time-resolved electron diffraction to directly observe light-activated wrinkling formation in freestanding La2/3Ca1/3MnO3 films. Via a "lock-in" analysis of oscillations in the diffraction peak position, intensity, and width, we quantitatively reconstructed how wrinkles develop on the time scale of lattice vibration. Contrary to the common assumption of fixed boundary conditions, we found that wrinkle development is associated with ultrafast delamination at the film boundaries. Our work provides a generic protocol to quantify wrinkling dynamics in freestanding films and highlights the importance of the film-substrate interaction in determining the properties of freestanding structures.

Lamellar Fluctuations Melt Ferroelectricity

We consider a standard Ginzburg-Landau model of a ferroelectric whose electrical polarization is coupled to gradients of elastic strain. At the harmonic level, such flexoelectric interaction is known to hybridize acoustic and optic phonon modes and lead to phases with modulated lattice structures that precede the state with spontaneously broken inversion symmetry. Here, we use the self-consistent phonon approximation to calculate the effects of thermal and quantum polarization fluctuations on the bare hybridized modes to show that such long-range modulated order is unstable at all temperatures. We discuss the implications for the nearly ferroelectric SrTiO_{3} and KTaO_{3}, and we propose that these systems are melted versions of an underlying modulated state that is dominated by nonzero momentum thermal fluctuations except at the very lowest temperatures.

Size-Induced Ferroelectricity in Antiferroelectric Oxide Membranes

Despite extensive studies on size effects in ferroelectrics, how structures and properties evolve in antiferroelectrics with reduced dimensions still remains elusive. Given the enormous potential of utilizing antiferroelectrics for high-energy-density storage applications, understanding their size effects will provide key information for optimizing device performances at small scales. Here, the fundamental intrinsic size dependence of antiferroelectricity in lead-free NaNbO₃ membranes is investigated. Via a wide range of experimental and theoretical approaches, an intriguing antiferroelectric-to-ferroelectric transition upon reducing membrane thickness is probed. This size effect leads to a ferroelectric single-phase below 40 nm, as well as a mixed-phase state with ferroelectric and antiferroelectric orders coexisting above this critical thickness. Furthermore, it is shown that the antiferroelectric and ferroelectric orders are electrically switchable. First-principle calculations further reveal that the observed transition is driven by the structural distortion arising from the membrane surface. This work provides direct experimental evidence for intrinsic size-driven scaling in antiferroelectrics and demonstrates enormous potential of utilizing size effects to drive emergent properties in environmentally benign lead-free oxides with the membrane platform.

Self-Assembly of Nanocrystalline Structures from Freestanding Oxide Membranes

The exploration of crystalline nanostructures enhances the understanding of quantum phenomena occurring in spatially confined quantum matter and may lead to functional materials with unforeseen applications. A novel route to fabricating nanocrystalline oxide structures of exceptional quality is presented. This is achieved by utilizing a self-assembly process of ultrathin membranes composed of the desired oxide. The thermally induced self-assembly of nanocrystalline structures is driven by dewetting the oxide membranes once they are lifted off and transferred onto sapphire surfaces. In three successive steps, the process provides nanovoids, nanowires, and nanocrystals. Regardless of substrate orientation, the nanostructures are highly anisotropic in shape due to material retraction favoring low-index crystalline lattice directions of the membranes. The orientation of the nanostructures is provided precisely by the crystal lattice of the transferred membrane. The microstructure of the nanocrystals exhibits exceptional quality, characterized by a pristine crystal structure and uniform stoichiometry, both maintained all the way down to the well-developed crystalline facets. The demonstrated self-assembly process holds the potential to improve the understanding of surface diffusion phenomena at the interface of materials, which is important for advancing epitaxial growth technology and paves the way to fabricating crystalline nanostructures by the transfer and self-assembly of membranes.

Abnormal In-Plane Thermomechanical Behavior of Two-Dimensional Hybrid Organic-Inorganic Perovskites

The implementation of two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) in semiconductor device applications will have to accommodate the co-existence of strain and temperature stressors and requires a thorough understanding of the thermomechanical behavior of 2D HOIPs. This will mitigate thermomechanical stability issues and improve the durability of the devices, especially when one considers the high susceptibility of 2D HOIPs to temperature due to their soft nature. Here, we employ atomic force microscopy (AFM) stretching of suspended membranes to measure the temperature dependence of the in-plane Young's modulus (E_∥) of model Ruddlesden-Popper 2D HOIPs with a general formula of (CH₃(CH₂)₃NH₃)₂(CH₃NH₃)_n-1Pb_nI_3n+1 (here, n = 1, 3, or 5). We find that E_∥ values of these 2D HOIPs exhibit a prominent non-monotonic dependence on temperature, particularly an abnormal thermal stiffening behavior (nearly 40% change in E_∥) starting around the order-disorder transition temperature of the butylammonium spacer molecules, which is significantly different from the thermomechanical behavior expected from their 3D counterpart (CH₃NH₃PbI₃) or other low-dimensional material systems. Further raising the temperature eventually reverses the trend to thermal softening. The magnitude of the thermally induced change in E_∥ is also much higher in 2D HOIPs than in their 3D analogs. Our results can shed light on the structural origin of the thermomechanical behavior and provide needed guidance to design 2D HOIPs with desired thermomechanical properties to meet the application needs.

Flexoelectric enhanced film for an ultrahigh tunable piezoelectric-like effect

Recent advancements in electromechanical coupling effects enable electromechanical materials in soft and stretchable formats, offering unique opportunities for biomimetic applications. However, high electromechanical performance and mechanical elasticity hardly coexist in soft materials. Flexoelectricity, an electromechanical coupling between strain gradient and electric polarization, possesses great potential of strain gradient engineering and material design in soft elastomeric materials. In this work, we report a flexoelectric enhanced elastomer-based film (FEEF) with both high electromechanical capability and stretchability. The integrated strategies with biaxial pre-stretch, crosslinking density of the elastomer along with nanoparticle size, particle filling ratio and electric field charging lead to an enhanced flexoelectricity by two orders of magnitude. Furthermore, this FEEF reveals an ultrahigh electromechanical performance by flexoelectric enhancement with its mechanical design. As a representative demonstration, an ultrahigh piezoelectric-like sensing array is fabricated for multifunctional sensing applications in strain, force and vibration, verifying an equivalent piezoelectric coefficient d₃₃ value as high as 1.42 × 10⁴ pC N^-1, and an average d₃₃ value of 4.23 × 10³ pC N^-1 at a large-scale deformation range. This proposed ultra-high piezoelectric-like effect with its approach is anticipated to provide a possibility for highly tunable piezoelectric-like effect by enhanced flexoelectricity and mechanical design in elastomeric materials.

Ultrathin Piezoelectric Resonators Based on Graphene and Free-Standing Single-Crystal BaTiO3

Suspended piezoelectric thin films are key elements enabling high-frequency filtering in telecommunication devices. To meet the requirements of next-generation electronics, it is essential to reduce device thickness for reaching higher resonance frequencies. Here, the high-quality mechanical and electrical properties of graphene electrodes are combined with the strong piezoelectric performance of the free-standing complex oxide, BaTiO₃ (BTO), to create ultrathin piezoelectric resonators. It is demonstrated that the device can be brought into mechanical resonance by piezoelectric actuation. By sweeping the DC bias voltage on the top graphene electrode, the BTO membrane is switched between the two poled ferroelectric states. Remarkably, ferroelectric hysteresis is also observed in the resonance frequency, magnitude and Q-factor of the first membrane mode. In the bulk acoustic mode, the device vibrates at 233 GHz. This work demonstrates the potential of combining van der Waals materials with complex oxides for next-generation electronics, which not only opens up opportunities for increasing filter frequencies, but also enables reconfiguration by poling, via ferroelectric memory effect.

Freestanding complex-oxide membranes

Complex oxides show a vast range of functional responses, unparalleled within the inorganic solids realm, making them promising materials for applications as varied as next-generation field-effect transistors, spintronic devices, electro-optic modulators, pyroelectric detectors, or oxygen reduction catalysts. Their stability in ambient conditions, chemical versatility, and large susceptibility to minute structural and electronic modifications make them ideal subjects of study to discover emergent phenomena and to generate novel functionalities for next-generation devices. Recent advances in the synthesis of single-crystal, freestanding complex oxide membranes provide an unprecedented opportunity to study these materials in a nearly-ideal system (e.g. free of mechanical/thermal interaction with substrates) as well as expanding the range of tools for tweaking their order parameters (i.e. (anti-)ferromagnetic, (anti-)ferroelectric, ferroelastic), and increasing the possibility of achieving novel heterointegration approaches (including interfacing dissimilar materials) by avoiding the chemical, structural, or thermal constraints in synthesis processes. Here, we review the recent developments in the fabrication and characterization of complex-oxide membranes and discuss their potential for unraveling novel physicochemical phenomena at the nanoscale and for further exploiting their functionalities in technologically relevant devices.

Thin-Film Ferroelectrics

Over the last 30 years, the study of ferroelectric oxides has been revolutionized by the implementation of epitaxial-thin-film-based studies, which have driven many advances in the understanding of ferroelectric physics and the realization of novel polar structures and functionalities. New questions have motivated the development of advanced synthesis, characterization, and simulations of epitaxial thin films and, in turn, have provided new insights and applications across the micro-, meso-, and macroscopic length scales. This review traces the evolution of ferroelectric thin-film research through the early days developing understanding of the roles of size and strain on ferroelectrics to the present day, where such understanding is used to create complex hierarchical domain structures, novel polar topologies, and controlled chemical and defect profiles. The extension of epitaxial techniques, coupled with advances in high-throughput simulations, now stands to accelerate the discovery and study of new ferroelectric materials. Coming hand-in-hand with these new materials is new understanding and control of ferroelectric functionalities. Today, researchers are actively working to apply these lessons in a number of applications, including novel memory and logic architectures, as well as a host of energy conversion devices.

Self-Sealing Complex Oxide Resonators

Although 2D materials hold great potential for next-generation pressure sensors, recent studies revealed that gases permeate along the membrane-surface interface, necessitating additional sealing procedures. In this work, we demonstrate the use of free-standing complex oxides as self-sealing membranes that allow the reference cavity beneath to be sealed by a simple anneal. To test the hermeticity, we study the gas permeation time constants in nanomechanical resonators made from SrRuO₃ and SrTiO₃ membranes suspended over SiO₂/Si cavities which show an improvement up to 4 orders of magnitude in the permeation time constant after annealing the devices. Similar devices fabricated on Si₃N₄/Si do not show such improvements, suggesting that the adhesion increase over SiO₂ is mediated by oxygen bonds that are formed at the SiO₂/complex oxide interface during the self-sealing anneal. Picosecond ultrasonics measurements confirm the improvement in the adhesion by 70% after annealing.

Highly Confined Phonon Polaritons in Monolayers of Perovskite Oxides

Two-dimensional (2D) materials are able to strongly confine light hybridized with collective excitations of atoms, enabling electric-field enhancements and novel spectroscopic applications. Recently, freestanding monolayers of perovskite oxides have been synthesized, which possess highly infrared-active phonon modes and a complex interplay of competing interactions. Here, we show that this new class of 2D materials exhibits highly confined phonon polaritons by evaluating central figures of merit for phonon polaritons in the tetragonal phases of the 2D perovskites SrTiO₃, KTaO₃, and LiNbO₃, using density functional theory calculations. Specifically, we compute the 2D phonon-polariton dispersions, the propagation-quality, confinement, and deceleration factors, and we show that they are comparable to those found in the prototypical 2D dielectric hexagonal boron nitride. Our results suggest that monolayers of perovskite oxides are promising candidates for polaritonic platforms that enable new possibilities in terms of tunability and spectral ranges.