Search

Strain is extensively used to controllably tailor the electronic properties of materials. In the context of indirect band-gap semiconductors such as silicon, strain lifts the valley degeneracy of the six conduction band minima, and by extension the valley states of electrons bound to phosphorus donors. Here, single phosphorus atoms are embedded in an engineered thin layer of silicon strained to 0.8% and their wave function imaged using spatially resolved spectroscopy. A prevalence of the out-of-plane valleys is confirmed from the real-space images, and a combination of theoretical modelling tools is used to assess how this valley repopulation effect can yield isotropic exchange and tunnel interactions in the xy-plane relevant for atomically precise donor qubit devices. Finally, the residual presence of in-plane valleys is evidenced by a Fourier analysis of both experimental and theoretical images, and atomistic calculations highlight the importance of higher orbital excited states to obtain a precise relationship between valley population and strain. Controlling the valley degree of freedom in engineered strained epilayers provides a new competitive asset for the development of donor-based quantum technologies in silicon.

Artificial synapses are memristor-based devices mimicing biological synapses, and they are used in neuromorphic computing systems that process information in a parallel, energy efficient way and store information in an analog, non-volatile form. The next generation of computing systems are anticipated to use memristive circuits, as they can overcome the shortcomings of the von Neumann computer architecture in which the levels of memory and the CPU are separated, creating a bottleneck that causes energy-loss during information transfer. Memristors are utilized to build Resistive Random Access Memory (RRAM) that allows for multi-level data storage and construction of self-correcting, autonomous learning systems that can solve complex computational tasks that have historically required super-computing hardware. Artificial synapses have received attention since HP Labs fabricated the first practical memristor device. In this review we summarize the working principles, device architectures, fabrication and processing techniques, as well as the strategies for materials selection including binary metal oxide, perovskite, polymer, and organic materials. We also discuss the applications and challenges of using artificial synapses in artificial intelligence tasks such as image recognition, tactile sensing and speech recognition.

© 2019, The Author(s). Understanding and manipulating complex spin texture in multiferroics can offer new perspectives for electric field-controlled spin manipulation. In BiFeO 3 , a well-known room temperature multiferroic, the competition between various exchange interactions manifests itself as non-collinear spin order, i.e., an incommensurate spin cycloid with period 64 nm. We report on the stability and systematic expansion of the length of the spin cycloid in (110)-oriented epitaxial Co-doped BiFeO 3 thin films. Neutron diffraction shows (i) this cycloid, despite its partly out-of-plane canted propagation vector, can be stabilized in thinnest films; (ii) the cycloid length expands significantly with decreasing film thickness; (iii) theory confirms a unique [11 2 ¯] cycloid propagation direction; and (iv) in the temperature dependence the cycloid length expands significantly close to T N . These observations are supported by Monte Carlo simulations based on a first-principles effective Hamiltonian method. Our results therefore offer new opportunities for nanoscale magnonic devices based on complex spin textures.

Recently, spin spiral type multiferroic materials are widely investigated for their unusual properties and potential applications in memory technology, sensors, and field-effect transistors. It is of interest to investigate surface magnetism in iron vanadate (FeV2O4) due to varying physical properties. Here, the temperature-dependent magnetic domain evolution has been systematically investigated by using cryogenic magnetic force microscopy (MFM). We find irregular and asymmetrical magnetic domain coarsening and shrinking as a function of temperature and also different domain evolution between field cooled and zero field cooled MFM. These findings provide an improved understanding of physical properties related to domain structures.

Theoretical investigations on ferroelectric tunnel junctions (FTJs) with asymmetric electrodes and a composite barrier are presented. A large tunneling electroresistance effect exists for the Pt/SrTiO 3 /BaTiO 3 /SrRuO 3 junction; on the other hand, exchange of the dielectric and ferroelectric layer stacking sequence can seriously degrade the performance. These correlations are rationalized by the proposed concept of an asymmetry factor, defined as the ratio between the average barrier heights of FTJs for two opposite polarization orientations. We show that a large asymmetry factor is beneficial to FTJs. This work may provide a way to enhance the performance of FTJs by structure engineering.

Composition gradients, or dissimilar ferroelectric bilayers, demonstrate colossal electromechanical figures of merit attributed to the motion of ferroelastic domain walls. Yet, mechanistic understanding of polarization switching pathways that drive ferroelastic switching in these systems remains elusive. Here, the crucial roles of strain and electrostatic boundary conditions in ferroelectric bilayer systems are revealed, which underpin their ferroelastic switching dynamics. Using in situ electrical biasing in the transmission electron microscope (TEM), the motion of ferroelastic domain walls is investigated in a tetragonal (T) Pb(Zr,Ti)O3 (PZT)/rhombohedral (R) PZT epitaxial bilayer system. Atomic resolution electron microscopy, in tandem with phase field simulations, indicates that ferroelastic switching is triggered by predominant nucleation at the triple domain junctions located at the interface between the T/R layers. Furthermore, this interfacial nucleation leads to systematic reversable reorientation of ferroelastic domain walls. Deterministic ferroelastic domain switching, driven by the interfacial strain and electrostatic boundary conditions in the ferroelectric bilayer, provides a viable pathway toward novel design of miniaturized energy-efficient electromechanical devices.

The origins of recently reported visible-light photoelectrochemical activity in ZnS–GaP (ZG) multilayer films are investigated using aberration-corrected scanning transmission electron microscopy (STEM). It is revealed that the multilayers carry a large volume fraction of defects, specifically stacking faults and twins, at the interfaces. The defects act as excellent channels for diffusion. For each ZG interface, a ∼5 nm-interdiffused region with an effective chemical composition of a ZnS–GaP solid solution is observed. Previous theoretical calculations have found that ZnS–GaP solid solutions possess a lower band gap than either GaP or ZnS and thus are expected to have better visible-light photo-activity. These findings are thus able to explain the observed commensurate increase in the visible-light photoelectrochemical response with increasing number of ZG layers. This work suggests that interfaces with intentionally designed lattice imperfections and/or intentionally driven interdiffusion leading to local solid solution formation provide a new materials design strategy for achieving efficient visible-light photo-activity.

Here, the deterministic and reversible transformation of nanoscale ferroelectric bubbles into cylindrical domains using a scanning probe microscopy (SPM) approach is demonstrated. The bubble domains—sub-10 nm spheroid topological structures with rotational polarization—can be erased by applying a mechanical force via the SPM tip. Application of an electrical pulse with a specific combination of amplitude and duration can recreate the bubble domain state. This combination of mechanical and electrical passes is essential for realization of reversible transformation as application of only electrical pulses results in complete erasure of the bubble domain state. Effective Hamiltonian-based simulations reproduce phase sequences for both the mechanical and electric passes and confirm the intrinsic nature of these transitions. This simple and effective pathway for switching between various topological defect states may be exploited for emergent devices.

The traditional view of complex oxide heteroepitaxy is that the substrate is merely a passive bystander. Here we show that this is not always the case, in particular for a metal oxide/metal oxide interface during the synthesis of epitaxial nickel oxide (NiO) nanoislands on (001) strontium titanate (SrTiO3-STO) single crystal substrates. We find that the substrate atoms are driven to encapsulate the metal oxide nanocrystal islands - a phenomenon more commonly reported for metal/metal oxide heterogeneous catalysis where the migration of atoms from the support onto the functional nanoparticle system is driven by strong metal-support interactions (SMSI). High-resolution transmission electron microscopy (HRTEM), high angle angular dark field (HAADF) scanning transmission electron microscopy (STEM), and electron tomography (ET) are used to reveal a clear physical displacement of the interface; the atoms from the STO substrate climb and cover the bottom facets of NiO nanoparticles during the synthesis process during which a unique caldera-shaped structure is formed. It is postulated that this phenomenon occurs to lower the work function and surface energy of the system. These results indicate that the SMSI could be important in the metal oxide/metal oxide systems. In this way changes to the substrate-film interface could be exploited by controlling and modifying the properties of nanoscaled functional oxides.

Reversible electric-field induced domain switching in ferroelectric thin films gives rise to a large electromechanical coupling. Despite extensive in situ studies confirming a dominant contribution from domain switching, the speed of the domain wall motion had not been discussed enough. In this study, we performed time-resolved measurement of lattice elongation and non-180° domain switching for an epitaxial rhombohedral (111)/(111)-oriented (Pb(Zr0.65Ti0.35)O3 film under nanosecond electric field pulses by means of synchrotron x-ray diffraction. Both lattice elongation and non-180° domain switching due to a 200-ns electric pulse were directly observed from the shift of the 222 diffraction position toward a lower angle and the change in the integrated intensity ratio of 222 to 222 peaks, respectively. The non-180° domain switching also results in an increase of the switchable polarization. Following the removal of the electric field, it is seen that the non-180° domain back switching from 222 to 222 is sluggish compared to the relaxation of the field-induced lattice strain. This is different from the (100)/(001)-oriented tetragonal epitaxial Pb(Zr,Ti)O3 films, in which no obvious delay was detected. These results show the importance of the direct time-resolved response observation of the crystal structure change with the application of a high-speed electric pulse field to understand the frequency dispersion of the ferroelectric and piezoelectric responses of Pb(Zr,Ti)O3 films.

The misfit dislocations formed at heteroepitaxial interfaces create long-ranging strain fields in addition to the epitaxial strain. For systems with strong lattice coupling, such as ferroic oxides, this results in unpredictable and potentially debilitating functionality and device performance. In this work, we use dark-field X-ray microscopy to map the lattice distortions around misfit dislocations in an epitaxial film of bismuth ferrite (BiFeO 3 ), a well-known multiferroic. We demonstrate the ability to precisely quantify weak, long-ranging strain fields and their associated symmetry lowering without modifying the mechanical state of the film. We isolate the screw and edge components of the individual dislocations and show how they result in weak charge heterogeneities via flexoelectric coupling. We show that even systems with small lattice mismatches and additional mechanisms of stress relief (such as mechanical twinning) may still give rise to measurable charge and strain heterogeneities that extend over mesoscopic length scales. This sets more stringent physical limitations on device size, dislocation density, and the achievable degree of lattice mismatch in epitaxial systems.

In the field of solar water splitting, searching for and modifying bulk compositions have been the conventional approaches to enhancing visible-light activity. In this work, manipulation of heterointerfaces in ZnS-GaP multilayer films is demonstrated as a successful alternative approach to achieving visible-light-active photoelectrodes. The photocurrent measured under visible light increases with the increasing number of interfaces for ZnS-GaP multilayer films with the same total thickness, indicating it to be a predominantly interface-driven effect. The activity extends to long wavelengths (650 nm), much longer than those expected for pure ZnS and also longer than those previously reported for GaP. Density functional theory calculations of ZnS-GaP multilayers predict the presence of electronic states associated with atoms at the interfaces between ZnS and GaP that are different from those found within the layers away from the interfaces; these states, formed due to unique bonding environments found at the interfaces, lead to a lowering of the band gap and hence the observed visible-light activity. The presence of these electronic states attributed to the interfaces is confirmed by depth-resolved X-ray photoelectron spectroscopy. Thus, we show that interface engineering is a promising route for overcoming common deficiencies of individual bulk materials caused by both wide band gaps and indirect band gaps and hence enhancing visible-light absorption and photoelectrochemical performance.

Epitaxial mixed-phase bismuth ferrite (BiFeO3, BFO) thin films were successfully synthesized on (001) lanthanum aluminate (LaAlO3, LAO) substrates by a chemical solution deposition (CSD) technique. X-ray diffraction measurements confirm the co-existence of a completely relaxed rhombohedral-like (R′) phase and a strained tetragonal-like (T′) phase. Atomic resolution scanning transmission electron microscopy (STEM) measurements reveal that the T′ and R′ phases in our CSD derived BFO/LAO films are mixed homogeneously at the nanoscale. This is in stark contrast to the typical physical vapor deposition derived mixed-phase BFO thin films, which show R′ phase striations embedded in a T′ phase matrix. This phenomenon is attributed to the specific deposition-nucleation-crystallization-relaxation pathway characteristic of the CSD route. This homogenously mixed-phase still demonstrates the well-known morphotropic phase boundary effect, i.e. superior electromechanical properties compared to either the pure T′ phase or R′ phase constituents themselves. Moreover, the maximum piezoelectric coefficient measured by using nanoscale top electrodes shows surprising insensitivity to the clamping effect from the substrate, thereby offering considerable promise in thin film applications.

Ferroelastic domain switching significantly affects piezoelectric properties in ferroelectric materials. The ferroelastic domain switching and the lattice deformation of both a-domains and c-domains under an applied electric field were investigated using in-situ synchrotron X-ray diffraction in conjunction with a high-speed pulse generator set up for epitaxial (100)/(001)-oriented tetragonal Pb(Zr0.4Ti0.6)O3 (PZT) films grown on (100) c SrRuO3//(100)KTaO3 substrates. The 004 peak (c-domain) position shifts to a lower 2θ angle, which demonstrates the elongation of the c-axis lattice parameter of the c-domain under an applied electric field. In contrast, the 400 peak (a-domain) shifts in the opposite direction (higher angle), thus indicating a decrease in the a-axis lattice parameter of the a-domain. 90° domain switching from (100) to (001) orientations (from a-domain to c-domain) was observed by a change in the intensities of the 400 and 004 diffraction peaks by applying a high-speed pulsed electric field 200 ns in width. This change also accompanied a tilt in the angles of each domain from the substrate surface normal direction. This behaviour proved that the 90° domain switched within 40 ns under a high-speed pulsed electric field. Direct observation of such high-speed switching opens the way to design piezo-MEMS devices for high-frequency operation.

Measuring the diffusion of ions and vacancies at nanometer length scales is crucial to understanding fundamental mechanisms driving technologies as diverse as batteries, fuel cells, and memristors; yet such measurements remain extremely challenging. Here, we employ a multimodal scanning probe microscopy (SPM) technique to explore the interplay between electronic, elastic, and ionic processes via first-order reversal curve I-V measurements in conjunction with electrochemical strain microscopy (ESM). The technique is employed to investigate the diffusion of oxygen vacancies in model epitaxial nickel oxide (NiO) nanocrystals with resistive switching characteristics. Results indicate that opening of the ESM hysteresis loop is strongly correlated with changes to the resonant frequency, hinting that elastic changes stem from the motion of oxygen (or cation) vacancies in the probed volume of the SPM tip. These changes are further correlated to the current measured on each nanostructure, which shows a hysteresis loop opening at larger (∼2.5 V) voltage windows, suggesting the threshold field for vacancy migration. This study highlights the utility of local multimodal SPM in determining functional and chemical changes in nanoscale volumes in nanostructured NiO, with potential use to explore a wide variety of materials including phase-change memories and memristive devices in combination with site-correlated chemical imaging tools.

Observation of a new type of nanoscale ferroelectric domains, termed as “bubble domains”—laterally confined spheroids of sub-10 nm size with local dipoles self-aligned in a direction opposite to the macroscopic polarization of a surrounding ferroelectric matrix—is reported. The bubble domains appear in ultrathin epitaxial PbZr0.2Ti0.8O3/SrTiO3/PbZr0.2Ti0.8O3 ferroelectric sandwich structures due to the interplay between charge and lattice degrees of freedom. The existence of the bubble domains is revealed by high-resolution piezoresponse force microscopy (PFM), and is corroborated by aberration-corrected atomic-resolution scanning transmission electron microscopy mapping of the polarization displacements. An incommensurate phase and symmetry breaking is found within these domains resulting in local polarization rotation and hence impart a mixed Néel–Bloch-like character to the bubble domain walls. PFM hysteresis loops for the bubble domains reveal that they undergo an irreversible phase transition to cylindrical domains under the electric field, accompanied by a transient rise in the electromechanical response. The observations are in agreement with ab-initio-based calculations, which reveal a very narrow window of electrical and elastic parameters that allow the existence of bubble domains. The findings highlight the richness of polar topologies possible in ultrathin ferroelectric structures and bring forward the prospect of emergent functionalities due to topological transitions.

© 2017 The Author(s). The valence band (VB) electronic structure and VB alignments at heterointerfaces of strained epitaxial stannate ASnO3(A=Ca, Sr, and Ba) thin films are characterized using in situ X-ray and ultraviolet photoelectron spectroscopies, with band gaps evaluated using spectroscopic ellipsometry. Scanning transmission electron microscopy with geometric phase analysis is used to resolve strain at atomic resolution. The VB electronic structure is strain state dependent in a manner that correlated with a directional change in Sn-O bond lengths with strain. However, VB offsets are found not to vary significantly with strain, which resulted in ascribing most of the difference in band alignment, due to a change in the band gaps with strain, to the conduction band edge. Our results reveal significant strain tuning of conduction band offsets using epitaxial buffer layers, with strain-induced offset differences as large as 0.6 eV possible for SrSnO3. Such large conduction band offset tunability through elastic strain control may provide a pathway to minimize the loss of charge confinement in 2-dimensional electron gases and enhance the performance of photoelectrochemical stannate-based devices.

Magnonic devices that utilize electric control of spin waves mediated by complex spin textures are an emerging direction in spintronics research. Room-temperature multiferroic materials, such as bismuth ferrite (BiFeO3), would be ideal candidates for this purpose. To realize magnonic devices, a robust long-range spin cycloid with well-known direction is desired, since it is a prerequisite for the magnetoelectric coupling. Despite extensive investigation, the stabilization of a large-scale uniform spin cycloid in nanoscale (100 nm) thin BiFeO3 films has not been accomplished. Here, we demonstrate cycloidal spin order in 100 nm BiFeO3 thin films through the careful choice of crystallographic orientation, and control of the electrostatic and strain boundary conditions. Neutron diffraction, in conjunction with X-ray diffraction, reveals an incommensurate spin cycloid with a unique [11] propagation direction. While this direction is different from bulk BiFeO3, the cycloid length and Neél temperature remain equivalent to bulk at room temperature.

Giant electromechanical response viaferroelastic domain switching is achieved in epitaxial (001) ferroelectric tetragonal (T) PbZr0.3Ti0.7O3/rhombohedral (R) PbZr0.55Ti0.45O3 bilayers, grown on La0.67Sr0.33MnO3 buffered SrTiO3 substrates. X-ray diffraction and transmission electron microscopy show that the domain structure of the T films is tuned as a function of its thickness, from a fully a1/a2-domains (30 nm thick T layer) to a three domain stress-free c/a1/c/a2 polytwin state (100 nm thick T layer). A large switchable polarization is found up to 65 μC cm-2. Quantitative piezoelectric force microscopy reveals enhanced piezoelectric coefficients, with d33 coefficients ranging from 250 to 350 pm V-1, which is up to seven times higher than the nominal PbZrxTi1-xO3 thin film values. These are attributed to the motion of nanoscale ferroelastic domains. Fatigue testing proves that these domains are reversible and repeatable up to 107 cycles. In-situ X-ray synchrotron measurements reveal that the ferroelastic domain switching is promoted by a pulsating strain effect imposed by the R layer. The study reports a fundamental understanding of the origin of giant piezoelectric coefficients in epitaxial ferroelectric bilayers.

The route to phase-pure BiFeO3 (BFO) ceramics with excellent ferroelectric and electromechanical properties is severely impeded by difficulties associated with the perovskite phase stability during synthesis. This has meant that dopants and solid solutions with BFO have been investigated as a means of not only improving the functional properties, but also of improving the perovskite phase formation of BFO-based ceramics. The present work focuses on Sm-modified BFO ceramics of composition Bi0.88Sm0.12FeO3. The polarization and strain behaviors were investigated as a function of the phase composition, microstructure, and chemical composition. Addition of Sm reduces the susceptibility of the BFO perovskite to phase degradation by Si impurities. Si was observed to react into Sm-rich grains dispersed within the microstructure, with no large increases in the amount of bismuth-parasitic phases, namely Bi25FeO39 and Bi2Fe4O9. These as-prepared ceramics exhibited robust polarization behavior showing maximum remnant polarizations of ∼40 to 50 μC/cm2. The electric-fieldinduced strain showed an appreciable stability in terms of the driving field frequency with maximum peak-to-peak strains of ∼0.3% and a coercive field of ∼130 kV/cm.