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Ferroic order is characterized by hystereses with two remanent states and therefore inherently binary. The increasing interest in materials showing non-discrete responses, however, calls for a paradigm shift towards continuously tunable remanent ferroic states. Device integration for oxide nanoelectronics furthermore requires this tunability at the nanoscale. Here we demonstrate that we can arbitrarily set the remanent ferroelectric polarization at nanometric dimensions. We accomplish this in ultrathin epitaxial PbZr 0.52 Ti 0.48 O 3 films featuring a dense pattern of decoupled nanometric 180° domains with a broad coercive-field distribution. This multilevel switching is achieved by driving the system towards the instability at the morphotropic phase boundary. The phase competition near this boundary in combination with epitaxial strain increases the responsiveness to external stimuli and unlocks new degrees of freedom to nano-control the polarization. We highlight the technological benefits of non-binary switching by demonstrating a quasi-continuous tunability of the non-linear optical response and of tunnel electroresistance. Setting any polarization value in ferroelectric thin films is a key step for their implementation in neuromorphic devices. Here, the authors demonstrate continuous modulation of the remanent polarization at the nanoscale in PbZr 0.52 Ti 0.48 O 3 films.

In order to bring the diverse functionalities of transition metal oxides into modern electronics, it is imperative to integrate oxide films with controllable properties onto the silicon platform. Here, we present asymmetric LaMnO 3 /BaTiO 3 /SrTiO 3 superlattices fabricated on silicon with layer thickness control at the unit-cell level. By harnessing the coherent strain between the constituent layers, we overcome the biaxial thermal tension from silicon and stabilize c -axis oriented BaTiO 3 layers with substantially enhanced tetragonality, as revealed by atomically resolved scanning transmission electron microscopy. Optical second harmonic generation measurements signify a predominant out-of-plane polarized state with strongly enhanced net polarization in the tricolor superlattices, as compared to the BaTiO 3 single film and conventional BaTiO 3 /SrTiO 3 superlattice grown on silicon. Meanwhile, this coherent strain in turn suppresses the magnetism of LaMnO 3 as the thickness of BaTiO 3 increases. Our study raises the prospect of designing artificial oxide superlattices on silicon with tailored functionalities. Integrating multifunctional oxides on silicon is highly desirable. Here, the authors present asymmetric BaTiO3 superlattices on silicon exhibiting enhanced out-of-plane polarization by harnessing the interfacial strain and broken inversion symmetry.

Raman spectroscopy enables the non-destructive characterization of chemical composition, crystallinity, defects, or strain in countless materials. However, the Raman response of surfaces or thin films is often weak and obscured by dominant bulk signals. Here we overcome this limitation by placing a transferable porous gold membrane, (PAuM) on the surface of interest. Slot-shaped nanopores in the membrane act as plasmonic antennas and enhance the Raman response of the surface or thin film underneath. Simultaneously, the PAuM suppresses the penetration of the excitation laser into the bulk, efficiently blocking its Raman signal. Using graphene as a model surface, we show that this method increases the surface-to-bulk Raman signal ratio by three orders of magnitude. We find that 90% of the Raman enhancement occurs within the top 2.5 nm of the material, demonstrating truly surface-sensitive Raman scattering. To validate our approach, we quantify the strain in a 12.5 nm thin Silicon film and analyze the surface of a LaNiO 3 thin film. We observe a Raman mode splitting for the LaNiO 3 surface-layer, which is spectroscopic evidence that the surface structure differs from the bulk. These results validate that PAuM gives direct access to Raman signatures of thin films and surfaces. Characterizing surfaces by Raman spectroscopy is limited by the competition of surface and bulk Raman responses. Here the authors use nanoporous plasmonic membranes to enhance surface Raman signals while suppressing bulk contributions.

The ability to sensitively image electric fields is important for understanding many nanoelectronic phenomena, including charge accumulation at surfaces1 and interfaces2 and field distributions in active electronic devices3. A particularly exciting application is the visualization of domain patterns in ferroelectric and nanoferroic materials4,5, owing to their potential in computing and data storage6–8. Here, we use a scanning nitrogen-vacancy (NV) microscope, well known for its use in magnetometry9, to image domain patterns in piezoelectric (Pb[Zr0.2Ti0.8]O3) and improper ferroelectric (YMnO3) materials through their electric fields. Electric field detection is enabled by measuring the Stark shift of the NV spin10,11 using a gradiometric detection scheme12. Analysis of the electric field maps allows us to discriminate between different types of surface charge distributions, as well as to reconstruct maps of the three-dimensional electric field vector and charge density. The ability to measure both stray electric and magnetic fields9,13 under ambient conditions opens opportunities for the study of multiferroic and multifunctional materials and devices8,14.A scanning nitrogen-vacancy microscope is used to image ferroelectric domains in piezoelectric and improper ferroelectric samples with high sensitivity. The technique relies on the nitrogen-vacancy’s Stark shift produced by the samples’ electric field.

Material surfaces encompass structural and chemical discontinuities that often lead to the loss of the property of interest in so-called dead layers. It is particularly problematic in nanoscale oxide electronics, where the integration of strongly correlated materials into devices is obstructed by the thickness threshold required for the emergence of their functionality. Here we report the stabilization of ultrathin out-of-plane ferroelectricity in oxide heterostructures through the design of an artificial flux-closure architecture. Inserting an in-plane-polarized ferroelectric epitaxial buffer provides the continuity of polarization at the interface; despite its insulating nature, we observe the emergence of polarization in our out-of-plane-polarized model of ferroelectric BaTiO3 from the very first unit cell. In BiFeO3, the flux-closure approach stabilizes a 251° domain wall. Its unusual chirality is probably associated with the ferroelectric analogue to the Dzyaloshinskii–Moriya interaction. We, thus, see that in an adaptively engineered geometry, the depolarizing-field-screening properties of an insulator can even surpass those of a metal and be a source of functionality. This could be a useful insight on the road towards the next generation of oxide electronics.Ferroelectric dead layers can form at perovskite interfaces—a major challenge in integrating oxide thin films into devices. Here, by depositing an in-plane-polarized epitaxial buffer layer of Bi5FeTi3O15, out-of-plane polarization is demonstrated in ultrathin films down to the single-unit-cell level.

Increased data storage densities are required for the next generation of nonvolatile random access memories and data storage devices based on ferroelectric materials. Yet, with intensified miniaturization, these devices face a loss of their ferroelectric properties. Therefore, a full microscopic understanding of the impact of the nanoscale defects on the ferroelectric switching dynamics is crucial. However, collecting real-time data at the atomic and nanoscale remains very challenging. In this work, we explore the ferroelectric response of a Pb(Zr0.2Ti0.8)O3 thin film ferroelectric capacitor to electrical biasing in situ in the transmission electron microscope. Using a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and differential phase contrast (DPC)-STEM imaging we unveil the structural and polarization state of the ferroelectric thin film, integrated into a capacitor architecture, before and during biasing. Thus, we can correlate real-time changes in the DPC signal with the presence of misfit dislocations and ferroelastic domains. A reduction in the domain wall velocity of 24% is measured in defective regions of the film when compared to predominantly defect-free regions.

Increased data storage densities are required for the next generation of nonvolatile random access memories and data storage devices based on ferroelectric materials. Yet, with intensified miniaturization, these devices face a loss of their ferroelectric properties. Therefore, a full microscopic understanding of the impact of the nanoscale defects on the ferroelectric switching dynamics is crucial. However, collecting real-time data at the atomic and nanoscale remains very challenging. In this work, we explore the ferroelectric response of a Pb(Zr Ti )O thin film ferroelectric capacitor to electrical biasing in situ in the transmission electron microscope. Using a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and differential phase contrast (DPC)-STEM imaging we unveil the structural and polarization state of the ferroelectric thin film, integrated into a capacitor architecture, before and during biasing. Thus, we can correlate real-time changes in the DPC signal with the presence of misfit dislocations and ferroelastic domains. A reduction in the domain wall velocity of 24% is measured in defective regions of the film when compared to predominantly defect-free regions.

In order to bring the diverse functionalities of transition metal oxides into modern electronics, it is imperative to integrate oxide films with controllable properties onto the silicon platform. Here, we present asymmetric LaMnO /BaTiO /SrTiO superlattices fabricated on silicon with layer thickness control at the unit-cell level. By harnessing the coherent strain between the constituent layers, we overcome the biaxial thermal tension from silicon and stabilize c-axis oriented BaTiO layers with substantially enhanced tetragonality, as revealed by atomically resolved scanning transmission electron microscopy. Optical second harmonic generation measurements signify a predominant out-of-plane polarized state with strongly enhanced net polarization in the tricolor superlattices, as compared to the BaTiO single film and conventional BaTiO /SrTiO superlattice grown on silicon. Meanwhile, this coherent strain in turn suppresses the magnetism of LaMnO as the thickness of BaTiO increases. Our study raises the prospect of designing artificial oxide superlattices on silicon with tailored functionalities.

The ability to sensitively image electric fields is important for understanding many nanoelectronic phenomena, including charge accumulation at surfaces and interfaces and field distributions in active electronic devices. A particularly exciting application is the visualization of domain patterns in ferroelectric and nanoferroic materials owing to their potential in computing and data storage. Here, we use a scanning nitrogen-vacancy (NV) microscope, well known for its use in magnetometry, to image domain patterns in piezoelectric (Pb[Zr\\(_{x}\\)Ti\\(_{1-x}\\)]O\\(_{3}\\)) and improper ferroelectric (YMnO\\(_{3}\\)) materials through their electric fields. Electric field detection is enabled by measuring the Stark shift of the NV spin using a gradiometric detection scheme. Analysis of the electric field maps allows us to discriminate between different types of surface charge distributions, as well as to reconstruct maps of the three-dimensional electric field vector and charge density. The ability to measure both stray electric and magnetic fields under ambient conditions opens exciting opportunities for the study of multiferroic and multifunctional materials and devices.

Many technologies require highly-oriented and textured functional thin films. The most common synthe-sis approaches use on-axis sputter geometries. However, in some scenarios, on-axis sputtering is not feasible. During ionized physical vapor deposition (PVD), in contrast to conventional PVD, the film-forming species can be accelerated onto the growing film using substrate-bias potentials. This increas-es the ad-atom mobility, but also deflects the trajectory of ions towards the substrate increasing the texture of the growing film. However, potential gas-ion incorporation in the films limits the feasibility of such approaches for the deposition of defect-sensitive materials. In this work, we report on the oblique-angle deposition of highly c-axis oriented AlN (0002) films, enabled by reactive metal-ion syn-chronized HiPIMS. The effect of critical deposition parameters, such as the magnetic configuration, ion kinetic energies and substrate biasing are investigated. The films deposited using HiPIMS show a more pronounced texture and orientation compared to DCMS films. We find that combining the HiPIMS dep-ositions with a moderate substrate bias of -30 V is sufficient to improve the crystalline quality and tex-ture of the films significantly. To reduce process-gas incorporation, and the formation of point defects, the negative substrate-bias potential is synchronized to the Al-rich fraction of each HiPIMS pulse. This leads to reduced Ar-Ion incorporation and improves the structural properties. The films also show uni-form polarization of the grains making this synthesis route suitable for piezoelectric applications. While the compressive stress in the films is still high, the results demonstrate, that synchronized HiPIMS can yield promising results for the synthesis under oblique-angle deposition conditions - even with low substrate-bias potentials.