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Controlling the elastic properties of materials is an issue that has been widely investigated because adapting available materials for incoming applications is required. Many efforts have been made in the last decade to make the elastic properties of ferroelectric materials adjustable for their use in high-technology products or processes. Additionally, in this new era of real-time wireless control, wireless devices are expected to replace traditional wired ones. However, there is no published research on visible-light wireless control of the elastic properties of ferroelectric materials. Here, we present the absolute proof the elasticity in polycrystalline BaTiO3 can be reversibly switched and linearly tuned with visible light. Thermal effects are definitively isolated so that the photo-induced phenomenon is unequivocally revealed. The light-induced change of Young’s modulus is originated by the increased screening of the charged domain walls under illumination. These results provide a promising avenue to the development of a new generation of optically driven devices based on elastically photosensitive ferroelectric materials., Postprint (author's final draft)

Although 2D Bi2O2Se plays an important role in the electronics and optoelectronics based on its in-plane property, its out-of-plane electrical transport behavior remains unclear, especially in fabricating vertical devices with high integration density for novel functionality. Here, a solution-processed method is developed to prepare 2D Bi2O2Se with mass production (e.g., hundreds of milliliter scale). The out-of-plane ferroelectric property of 2D Bi2O2Se is observed by piezoresponse force microscopy and the ferroelectric dipole map atom-by-atom at the Bi2O2Se surface, which shows an atomically resolved dipolar displacement of Se ions. The out-of-plane resistant switching property of 2D Bi2O2Se is revealed by conductive atomic force microscopy. Moreover, the electric field on the local polarization of Bi2O2Se is addressed by using ab initio simulations, which shows a broken inversion symmetry along the z-axis of Bi2O2Se. The working mechanism of resistant switching behavior in Bi2O2Se is attributed to the diffusion and shuttle of Se ions. Besides, a controllable wet-assembled method is developed to prepare Bi2O2Se thin film with centimeter scale and explores its application on photodetectors under 808 nm laser light. This work reveals the unique out-of-plane transport behavior of 2D Bi2O2Se, providing the basis for fabricating multifunctional devices with high integration based on this 2D material.

Oxide materials, including perovskite oxides, are well known for hosting a range of properties, such as ferroelectricity and superconductivity, which are significant for both fundamental understanding and technological applications. The interplay among structural, orbital, charge, and spin factors in these oxides provides a platform for studying and manipulating material properties that can be incorporated into devices for daily life, scientific studies, and industry. This motivation drives the development of atomic-level understanding of the phenomena exhibited by oxide materials, helping in the formulation of design principles to achieve desired properties. In this dissertation, I will explore different properties, including ferroelectricity, piezoelectricity, and magnetism, exhibited by various oxide materials with complex crystal structures through first principles density functional theory calculations and group theoretic arguments.Layered perovskites are structures formed by the rearrangement of perovskite structures, with other structural layers interspersed between them. Various families of layered perovskite oxide ferroelectrics demonstrate a coupling between polarization and structural order parameters, particularly involving octahedral rotation distortions. The Aurivillius-phase oxides SrBi2B2O9 (B=Ta, Nb) are recognized for exhibiting such order-parameter couplings and are renowned for their excellent room temperature ferroelectric performance. After providing a brief introduction to the physics of layered oxides in Chapter 1 and the theoretical methods used in our research in Chapter 2, I will discuss the results of our calculations which explore the ferroelectric switching processes of SrBi2B2O9 to identify low-energy switching paths in Chapter 3. This chapter also elucidates the roles of coupled order parameters in determining the complex ferroelectric domain structure in SrBi2 B2 O9.Non-polar structural distortions, like octahedral rotations, present additional degrees of freedom for manipulating ferroelectric properties in Aurivillius phases compared to traditional ferroelectrics such as BaTiO3 and PbTiO3. In Chapter 4, the results of first-principles calculations on the Aurivillius-phase oxide Bi2WO6 are explored. The chapter elucidates the ferroelectric switching mechanism and inves- tigates the coupling of electrical polarization with the isolated spin of a magnetic dopant (Fe3+) in Bi2WO6. It details the change in spin directionality along the ferroelectric switching pathways and emphasizes the roles of crystalline symmetry in such coupling. In addition to demonstrating geometry-driven ferroelectricity, layered perovskites like Ruddlesden-Popper phases also exhibit piezoelectricity. However, there has been minimal study of piezoelectricity in Ruddlesden-Popper phases. Chapter 5 will highlight our efforts to establish a microscopic understanding of piezoelectricity by exploring the roles of the interplay between strain and structural distortions on piezoelectric coefficients in representative Ruddlesden-Popper phases, including Ca3Ti2O7, Sr3Zr2O7, and Sr3Sn2O7. Some layered oxides host intriguing phenomena like frustrated magnetism together with the ferroelectricity. Recent experiments have highlighted the rare earth oxide TbInO3 for its simultaneous manifestation of improper ferroelectricity and frustrated magnetism, making it a potential spin liquid candidate material of interest to the materials science and physics communities. While spin liquids are anticipated to exhibit superconductivity upon suitable doping, the influence of doping on the electrical conductivity of TbInO3 remains an open question. Chapter 6 will unveil the outcomes of our investigations into the doping effect in hexagonal TbInO3. The dissertation will wrap up with a concise exploration of future directions in Chapter 7.

We report the mesomorphic behaviour, dielectric and ferroelectric properties, and computational modelling of binary mixtures containing two bent-core liquid crystals: the so-called NG75-COO (3,4́- Bis[4-(4-n-tetradecyloxybenzoyloxy)benzoyloxy]biphenyl), which forms smectic phases, and IP31- AzB (3,4′ - Bis[4-(4-n-tetradecyloxyphenylazo)benzoyloxy]biphenyl), which forms columnar phases. The phase diagram, assessed by polarised optical microscopy, shows that the binary mixtures retain the mesophase behaviour of the major component, whilst the equimolar mixture displays smectic-type phases. Dielectric and ferroelectric analyses were carried out on samples containing 10%, 50%, and 90% of IP31-AzB (molar %), and we also investigated structure–property correlations by differential functional theory. The NG75-COO/IP31-AzB mixtures undergo strong dielectric and ferroelectric response due to the presence of highly polarisable groups in the bent-core components, particularly at the ester groups. All the mixtures under study exhibit light-responsiveness induced by reversible E-to-Z photoisomerization (trans-to-cis) of the azobenzene group in IP31-AzB, together with an increase in the molecular dipole moment. The potential to tune the phase behaviour of the mixtures, as well as their dielectric and ferroelectric responses, are investigated by light irradiation under different conditions of intensity and temperature.

Está depositada una versión preprint del artículo, The peculiar features of domain walls observed in ferroelectrics make them promising active elements for next-generation non-volatile memories, logic gates and energy-harvesting devices. Although extensive research activity has been devoted recently to making full use of this technological potential, concrete realizations of working nanodevices exploiting these functional properties are yet to be demonstrated. Here, we fabricate a multiferroic tunnel junction based on ferromagnetic La0.7Sr0.3MnO3 electrodes separated by an ultrathin ferroelectric BaTiO3 tunnel barrier, where a head-to-head domain wall is constrained. An electron gas stabilized by oxygen vacancies is confined within the domain wall, displaying discrete quantum-well energy levels. These states assist resonant electron tunnelling processes across the barrier, leading to strong quantum oscillations of the electrical conductance., Ministerio de Economía y Competitividad (España), Comunidad Autónoma de Madrid, United States Department of Energy (DOE), BBVA Foundation, European Research Council (ERC), Red Española de Superconputación (RES), Universite Paris Saclay (DAlembert Program), Centre National de la Recherche Scientifique (CNRS), CeSViMa (Madrid), Depto. de Física de Materiales, Fac. de Ciencias Físicas, TRUE, pub

Dynamical tuning of the thermal conductivity in crystals, ¿, is critical for thermal management applications, as well as for energy harvesting and the development of novel phononic devices able to perform logic operations with phonons. Such a desired ¿ control can be achieved in functional materials that experience large structural and phonon variations as a result of field-induced phase transformations. However, this approach is only practical within reduced temperature intervals containing zero-bias phase transition points, since otherwise the necessary driving fields become excessively large and the materials’ performances are detrimentally affected. Here, based on first-principles calculations, we propose an alternative strategy for dynamically tuning ¿ that is operative over broad temperature conditions and realizable in a wide class of materials. By shining light on the archetypal perovskite oxide KNbO3, we predict that ultrafast and reversible ferroelectric-to-paraelectric phase transitions are induced, yielding large and anisotropic ¿ variations (up to ˜30% at T = 300 K). These light-induced thermal transport shifts can take place at temperatures spanning several hundreds of kelvin and are essentially the result of anharmonic effects affecting the phonon lifetimes., Postprint (published version)

Complex-oxide superlattices provide a pathway to numerous emergent phenomena because of the juxtaposition of disparate properties and the strong interfacial interactions in these unit-cell-precise structures. This is particularly true in superlattices of ferroelectric and dielectric materials, wherein new forms of ferroelectricity, exotic dipolar textures, and distinctive domain structures can be produced. Here, relaxor-like behavior, typically associated with the chemical inhomogeneity and complexity of solid solutions, is observed in (BaTiO3 )n /(SrTiO3 )n (n = 4-20 unit cells) superlattices. Dielectric studies and subsequent Vogel-Fulcher analysis show significant frequency dispersion of the dielectric maximum across a range of periodicities, with enhanced dielectric constant and more robust relaxor behavior for smaller period n. Bond-valence molecular-dynamics simulations predict the relaxor-like behavior observed experimentally, and interpretations of the polar patterns via 2D discrete-wavelet transforms in shorter-period superlattices suggest that the relaxor behavior arises from shape variations of the dipolar configurations, in contrast to frozen antipolar stripe domains in longer-period superlattices (n = 16). Moreover, the size and shape of the dipolar configurations are tuned by superlattice periodicity, thus providing a definitive design strategy to use superlattice layering to create relaxor-like behavior which may expand the ability to control desired properties in these complex systems.

We employ synchrotron-based near-field infrared spectroscopy to image the phononic properties of ferroelectric domain walls in hexagonal (h) Lu0.6Sc0.4FeO3, and we compare our findings with a detailed symmetry analysis, lattice dynamics calculations, and prior models of domain-wall structure. Rather than metallic and atomically thin as observed in the rare-earth manganites, ferroelectric walls in h-Lu0.6Sc0.4FeO3 are broad and semiconducting, a finding that we attribute to the presence of an A-site substitution-induced intermediate phase that reduces strain and renders the interior of the domain wall nonpolar. Mixed Lu/Sc occupation on the A site also provides compositional heterogeneity over micron-sized length scales, and we leverage the fact that Lu and Sc cluster in different ratios to demonstrate that the spectral characteristics at the wall are robust even in different compositional regimes. This work opens the door to broadband imaging of physical and chemical heterogeneity in ferroics and represents an important step toward revealing the rich properties of these flexible defect states.

Magnetoelectricity enables a solid-state material to generate electricity under magnetic fields. Most magnetoelectric composites are developed through a strain-mediated route by coupling piezoelectric and magnetostrictive phases. However, the limited availability of high-performance magnetostrictive components has become a constraint for the development of novel magnetoelectric materials. Here, we demonstrate that nanostructured composites of magnetic and pyroelectric materials can generate electrical output, a phenomenon we refer to as the magnetopyroelectric (MPE) effect, which is analogous to the magnetoelectric effect in strain-mediated composite multiferroics. Our composite consists of magnetic iron oxide nanoparticles (IONPs) dispersed in a ferroelectric (and also pyroelectric) poly(vinylidene fluoride–trifluoroethylene) (P(VDF–TrFE)) matrix. Under a high-frequency low-magnitude alternating magnetic field, the IONPs generate heat through hysteresis loss, which stimulates the depolarization process of the pyroelectric polymer. This magnetopyroelectric approach creates a new opportunity to develop magnetoelectric materials for a wide range of applications., Peer Reviewed, Postprint (published version)

The nexus between classic ferroelectricity and the structure of perovskite materials hinges on the concept of lattice disorder. Although the ordered perovskites display short-range displacements of the central cations around their equilibrium points, the lattice disorder dynamically unfolds to generate a myriad of distorted rhombohedral lattices characterized by the hopping of the central cations across <111> directions. It is discovered that the lattice disorder correlates with the emergence of minimum configuration energy <100> pathways for the central cations, resulting in spatially modulated ultrafast polarization nanocluster arrangements that are stabilized by the electric charge defects in the material. Through high-resolution phonon dispersion analyses encompassing molecular dynamics (MD) and density functional theory (DFT) simulations, we provide unequivocal evidence linking the hopping of central cations to the development of diffuse soft phonon modes observed throughout the phase transitions of the perovskite. Through massive MD simulations, we unveil the impact of lattice disorder on the structures of domain walls at finite-temperature vis-à-vis collective activation and deactivation of <100> pathways. Furthermore, our simulations demonstrate the development of hierarchical morphotropic phase boundary (MPB) nanostructures under the combined influence of externally applied pressure and stress relaxation, characterized by sudden emergence of zig-zagged monoclinic arrangements that involve dual <111> shifts of the central cations. These findings have implications for tailoring MPBs in thin-film structures and for the light-induced mobilization of DWs. Avenues are finally uncovered to the exploration of lattice disorder through gradual shear strain application., Peer Reviewed, Postprint (published version)

The effects of flashlamp annealing (FLA) on the quality of ferroelectric HfxZr1–xO2 (HZO) interfaces, integrated on InAs substrates, are evaluated. For the integration of ferroelectric HZO on III-V semiconductors the crystallization via rapid thermal processing (RTP) can severely degrade the HZO/III-V interface. Thermal annealing in the millisecond duration were used in the efforts of reducing diffusion through the HZO film and increase device performance. Electrical characterization showed a lower defect density and improved endurance of the FLA samples compared to the RTP counterparts. The use of multiple low-energy flashes and pre-crystallization during ALD growth was also investigated for further improvement on device performance. Overall, this work provides valuable insight into low thermal budget integration of HZO on III-V semiconductors., We all recognize the feeling of panic when your computer crashes and you realize you forgot to save your progress for the past couple of hours. In this scenario you wait for the computer to boot again and assess what you have lost. Imagine instead the computer booting instantly and you find yourself exactly where you left off. With an emerging technology in computer design this could become the new reality. The basic idea behind the fundamental building block of your electronic devices, known as the transistor, has since its invention in the 1960s been largely unchanged. The design which proved most successful is known as the Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) and is by far the most common design even to this day due to its cheap manufacturing process and its ability to be scaled down to ever smaller devices. However, in the last decade one has started to reach the limits of the MOSFET design, primarily due to limited scalability, and alternative designs are actively being explored. A promising design is the Ferroelectric Tunnel Junction (FTJ) which utilizes the small dimensions of the transistor to its advantage where electrons tunnel through a thin barrier. This barrier is then manipulated to either allow electrons though or not, resulting in the characteristic transistor functionality. In addition to being intrinsically small, the ferroelectric barrier allows the transistor to be toggleable where the state of the transistor is maintained even though power is lost. This ability means that electronic devices utilizing this design does not have to be booted up, as all transistors are already set to the desired state. Additionally, as power is only consumed as transistors are being toggled between states, these devices could reduce power consumption lengthening the battery life across all devices. However, for this to become the new reality of computers, the FTJ design must be improved further to prove as an effective alternative to the MOSFET

Submillimeter-scale electrically driven micromotors have the potential to revolutionize micro-robotic systems, but difficulties in their fabrication processes and unknown physical phenomena prevent their development. In this paper, we propose a submillimeter-scale rotary piezoelectric ultrasonic motor with the smallest stator (0.41 mm × 0.41 mm × 0.25 mm) reported to date. The micromachining technologies enable the creation of tiny components. A micromanipulator that can control a small amount of adhesive of sub-milligram order engenders success in manufacture of the stator. Several experiments clarify the submillimeter-scale physical behaviors, such as the admittance and the quality factor. With appropriate piezoelectric materials, submillimeter-scale ultrasonic motors are built and characterized using a rotor with 0.15 mm diameter. The motor with typical hard PZTs demonstrated maximum torque of 5.4 nNm and a maximum angular velocity of 714 rad/s at an applied voltage with amplitude of 44.8 Vp-p. Furthermore, another motor with single-crystal PMN-PT piezoelectric elements presented the possibility of low-voltage actuation. These submillimeter-scale ultrasonic motors were compared with existing comparable-size micromotors.

Ferroelectric van der Waals CuInP2S6 possesses intriguing quadruple-well states and negative piezoelectricity. Its technological implementation has been impeded by the relatively low Curie temperature (bulk TC ~42 °C) and the lack of precise domain control. Here we show that CuInP2S6 can be immune to the finite size effect and exhibits enhanced ferroelectricity, piezoelectricity, and polar alignment in the ultrathin limit when interfaced with ferroelectric oxide PbZr0.2Ti0.8O3 films. Piezoresponse force microscopy studies reveal that the polar domains in thin CuInP2S6 fully conform to those of underlying PbZr0.2Ti0.8O3, where the piezoelectric coefficient changes sign and increases sharply with reducing thickness. High temperature in situ domain imaging points to a significantly enhanced TC exceeding 200 ºC for 13 nm CuInP2S6 on PbZr0.2Ti0.8O3. Density functional theory modeling and Monte Carlo simulations show that the enhanced polar alignment and TC can be attributed to interface-mediated structure distortion in CuInP2S6. Our study provides an effective material strategy to engineer the polar properties of CuInP2S6 for flexible nanoelectronic, optoelectronic, and mechanical applications.

The demand to produce clean energy has been increasing over the last years due to the various climate changes, which are strongly afflicting the world. Therefore, it is necessary to implement new and alternative energy sources that contribute for the environment preservation. That is the case, for instance, of novel energy-storage devices, which could contribute for the current demand of clean energies. In fact, most of the system used nowadays are those lead-based materials, which are strongly pollutant and contribute for the environment contamination. In this way, the interest in the development and study of lead-free materials has become a real priority in the scientific community. The objective of the present work is to synthesize and investigate the physical properties of lead-free ferroelectric systems based on BaTiO3 (BT) with technological interest. In particular, the structural and dielectric properties have been investigated as a function of the Sn4+ content, used as a doping element in the BT hosting crystalline structure. The phase transition characteristics have been also analyzed in a wide temperature and frequency range., Postprint (author's final draft)

Photostrictive materials have a growing interest because of their great potential as light-driven actuators, among other optomechanical applications. In this context, the optical control of macroscopic strain in ferroelectrics has recently attracted remarkable attention as an effective alternative to the conventional electric control of strain. Here, a clear correlation between optical absorption and light-induced strain in polycrystalline BaTiO3 is shown. Specifically, the grain size and the sample thickness dependence of optical absorption when the material is irradiated with energy photons lower than the band gap evidence that light absorption at charged domain walls is the core of the observed photo-response in ferroelectrics. The photoinduced electronic reconstruction phenomenon is proposed as the primary physical mechanism for light absorption at charged domain walls. Results open a new pathway to designing ferroelectric-based devices with new functionalities like thickness gradient-based photo-controlled nanoactuators., Postprint (published version)

Optical control of functional properties in ferroic materials is now a highly appealing topic because it may entail different paradigms for future technologies. In ferroelectrics, in particular, controlling the properties with light implies noncontact external control of the material’s functionality, thereby opening a pathway for developing the next generation of photocontrolled devices. Recently, experimental observations have demonstrated that the dielectric permittivity of charged-domain-wall ferroelectrics can be easily modulated by visible light. However, because of the wide band gap of ferroelectric materials, the physical origin of this phenomenon is still controversial. Here, the photoinduced electronic reconstruction mechanism is proposed as the primary light-absorption mechanism in charged domain walls, allowing an understanding of the origin of the visible-light control of dielectric permittivity in ferroelectric materials., Postprint (author's final draft)

The discovery of ferroelectricity in CMOS-compatible oxides, such as doped hafnium oxide, has opened new possibilities for electronics by reviving the use of ferroelectric implementations on modern technology platforms. This thesis presents the ground-up integration of ferroelectric HfO2 on a thermally sensitive III-V nanowire platform leading to the successful implementation of ferroelectric transistors (FeFETs), tunnel junctions (FTJs), and varactors for mm-wave applications. As ferroelectric HfO2 on III-V semiconductors is a nascent technology, a special emphasis is put on the fundamental integration issues and the various engineering challenges facing the technology.The fabrication of metal-oxide-semiconductor (MOS) capacitors is treated as well as the measurement methods developed to investigate the interfacial quality to the narrow bandgap III-V materials using both electrical and operando synchrotron light source techniques. After optimizing both the films and the top electrode, the gate stack is integrated onto vertical InAs nanowires on Si in order to successfully implement FeFETs. Their performance and reliability can be explained from the deeper physical understanding obtained from the capacitor structures.By introducing an InAs/(In)GaAsSb/GaSb heterostructure in the nanowire, a ferroelectric tunnel field effect transistor (ferro-TFET) is fabricated. Based on the ultra-short effective channel created by the band-to-band tunneling process, the localized potential variations induced by single ultra-scaled ferroelectric domains and individual defects are sensed and investigated. By intentionally introducing a gate-source overlap in the ferro-TFET, a non-volatile reconfigurable single-transistor solution for modulating an input signal with diverse modes including signal transmission, phase shift, frequency doubling, and mixing is implemented.Finally, by fabricating scaled ferroelectric MOS capacitors in the front-end with a dedicated and adopted RF and mm-wav

Epitaxial orthorhombic phase La doped HfO2 films are promising for achieving robust ferroelectric polarization without wake-up effect. However, lowering the coercive field is crucial for achieving low-power memory devices. In this work, we have investigated the influence of the La content effect on the structural and ferroelectric properties of epitaxial HfO2 thin films. We show that while the remanent polarization is optimum for 2-5 at. % La-doped HfO2 films, the coercive field is decreased with La doping. The experimental work is supported by density functional theory (DFT) calculations which show that the polarization switching in epitaxial La:HfO2 films can be understood based on the synergetic contribution of the presence of a non-ferroelectric monoclinic phase and the La doping itself that causes a reduction of the nucleation and DW motion energy barriers for the crossing path, which makes it more probable than the non-crossing one.

Antiferroelectricity is a state of matter that has so far eluded a clear-cut definition. Even in the best-known material realization, PbZrO3, the physical nature of the driving force towards an antipolar order has not been settled yet. Here, by building a Landau-like continuum Hamiltonian from first principles via an exact long-wave approach, we reconcile the existing theories in terms of a single physical mechanism. In particular, we find that a formerly overlooked trilinear coupling between tilts, tilt gradients, and polarization provides a surprisingly accurate description of the energetics and structure of the antiferroelectric ground state of PbZrO3. We discuss the relevance of our findings to other ferrielectric and incommensurate polar structures that were recently observed in perovskites.

In dieser Arbeit werden ferroelektrische Phasenübergänge in verspannten (K,Na)NbO3-Schichten erstmals mit Hilfe temperaturabhängiger dreidimensionaler Röntgenbeugung untersucht. Der Fokus liegt auf stark anisotrop verspannten Dünnschichten, die bei Raumtemperatur ein geordnetes Fischgräten-Domänenmuster mit einer periodischen Anordnung von monoklinen a1a2/MC-Phasen aufweisen. Bei Erhöhung der Temperatur durchlaufen die (K,Na)NbO3-Dünnschichten einen ferroelektrischen Phasenübergang in die orthorhombische Hochtemperaturphase, die sich durch regelmäßige, alternierenden a1/a2-Streifendomänen mit ausschließlich lateraler Polarisation auszeichnet. In-plane Röntgenmessungen zeigen, dass die Filmeinheitszellen eine kleine Verzerrung in der Ebene erfahren, was zur Bildung von vier verschiedenen Einheitszellvarianten und damit vier verschiedenen (Super-)Domänenvarianten führt. Durch den Vergleich von Röntgenbeugungsmessungen verschiedener Bragg-Reflexe an Filmen mit unterschiedlicher Schichtdicke ist es möglich, die spezifischen Beugungsmerkmale zu unterscheiden und sie den einzelnen Phasen zuzuordnen. Mit Hilfe von in situ temperaturabhängiger Röntgenbeugung ist es daher möglich, die Details des Phasenübergangs vom Fischgräten in das Streifen-Domänenmuster aufzudecken. Es zeigt sich, dass dieser sich über einen großen Temperaturbereich erstreckt und in mehreren Schritten vollzieht. Die Beobachtung von Phasenkoexistenz innerhalb des Übergangs und einer thermischen Hysterese in der Phasenübergangstemperatur lassen auf einen Phasenübergang erster Art schließen. Zudem hängt die Phasenübergangstemperatur stark von der Kaliumkonzentration x in der KxNa1-xNbO3-Dünnschicht ab und kann durch eine Änderung von x=0,95 (stärker kompressiv verspannt) auf x=0,8 (stärker tensil verspannt) um etwa 60 K erhöht werden. Darüber hinaus ist dies die erste Studie, die experimentell beobachtete dreidimensionale Domänenanordnungen direkt mit Berechnungen aus Phasenfeldsimulationen vergleicht., In this work, ferroelectric phase transitions in strained (K,Na)NbO3 films are studied for the first time using in situ temperature-dependent three-dimensional X-ray diffraction. The focus lies on strongly anisotropically strained thin films, which exhibit a well-ordered herringbone domain pattern with a periodic arrangement of monoclinic a1a2/MC phases at room temperature. Upon increasing temperatures, the (K,Na)NbO3 thin films undergo a ferroelectric phase transition to the orthorhombic high-temperature phase, which is characterized by a regular pattern of alternating a1/a2 stripe domains with pure lateral polarization. In-plane X-ray measurements show that the film unit cells undergo a small in-plane distortion, leading to the formation of four different unit cell variants and thus four different (super)domain variants. By comparing X-ray diffraction measurements of different Bragg reflections of films with different film thicknesses, it is possible to distinguish the specific diffraction features and assign them to the individual phases observed at the different temperatures. Using in situ temperature-dependent X-ray diffraction, it is therefore possible to reveal the details of the phase transition from the a1a2/MC herringbone to the a1/a2 stripe domain pattern. It is shown to extend over a wide temperature range and to occur in several steps. The observation of phase coexistence within the transition and a thermal hysteresis in the phase transition temperature suggests a first-order type phase transition. Moreover, the phase transition temperature strongly depends on the molar concentration of potassium x in the KxNa1-xNbO3 thin film and can be increased by about 60 K by changing x=0.95 (more compressively strained) to x=0.8 (more tensile strained). Furthermore, this is the first study to directly compare experimentally observed three-dimensional domain arrangements with calculations from phase field simulations.

In recent years, two-dimensional (2D) van der Waals (vdW) materials have aroused much interest for their unique structural, thermal, optical, and electronic properties and have become a hot topic in condensed matter physics and material science. Many research methods, including scanning tunneling microscopy (STM), transmission electron microscopy (TEM), optical and transport measurements, have been used to investigate these unique properties. Among them, STM stands out as a powerful characterization tool with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. This dissertation focuses on scanning tunneling microscopy and spectroscopy (STM/S) investigation of 2D metal chalcogenides and heterostructures. The first part of the dissertation focuses on the continuous interface in WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer agree with previously reported values for MoS2 monolayer and MoS2/WS2 heterobilayer on SiO2 fabricated through the mechanical exfoliation method. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD. The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers with thickness ranging from single to ~20 layers stabilized at the beta phase with a superstructure at room temperature. After cooling down to around 180 K, the beta phase converted to a more stable beta' phase that was distinct from previously reported phases in 2D In2Se3. The kinetics of the reversible thermally driven beta

Electrically controlled magnetism in two-dimensional (2D) multiferroics is highly desirable for both fundamental research and the future development of low-power nanodevices. Herein, inspired by the recently experimentally realized 2D antiferromagnetic MnPSe3 [ Nat. Nanotechnol. 2021, 16 (7), 782] and guided by a heteromagnetic structural design, we engineer strong magnetoelectric coupling in a hydrogen-intercalated 2D MnPSe3 bilayer. Hydrogen functionalization breaks the centrosymmetry of bilayer MnPSe3, leading to out-of-plane ferroelectricity. Moreover, there is a phase transition from antiferromagnetic semiconductor to ferromagnetic half-metal in the H-bonded MnPSe3 layer, while the other remains antiferromagnetic and semiconducting. When reversing the electrical polarization, the intercalated H atom can flip between the top and bottom layers with an ultralow switching barrier, which allows one to tune the magnetic order and conductivity of the individual layers via an external electric field. Our results pave a new avenue to realize strong magnetoelectric coupling in single-phase multiferroic material. The ferroelectricity-controlled magnetic phase transition and half-metallicity offer promising applications in nanoscale spintronics such as electrically written and magnetically read memories.

The transmission electron microscope (TEM) provides multiple modes to study structure-property relationships in materials. In particular, quantum materials, whose properties may be affected by subtle alterations to atomic configuration, require such a valuable instrument for illuminating these relationships. Scanning transmission electron microscopy (STEM) and convergent beam electron diffraction (CBED) are two techniques within the TEM that may be utilized to examine the atomic structure and crystal symmetries of materials. Cadmium arsenide (Cd3As2) is classified as a topological semimetal due to its linear band crossings in the bulk electronic band structure and surface states. Different publications in the past have proposed competing centrosymmetric and non-centrosymmetric crystal structures for Cd3As2, an important distinction that has major implications on the expected band structure and underlying physics. Due to the complicated crystal structure of Cd3As2, as well as the limitations in x-ray diffraction techniques in identifying an inversion center, particularly in thin films, we used CBED to determine the exact point group symmetries of epitaxially grown Cd3As2, finding it to possess an inversion center. Because Cd3As2 possesses fourfold rotational symmetry and an inversion center, it can be classified as a Dirac semimetal. We sought to topologically tune Cd3As2 by introducing biaxial epitaxial stress, producing tensile and compressively stressed films. Using CBED, we found that fourfold rotational symmetry was lost in compressively strained, (112)-oriented Cd3As2, promoting the opening of a band gap and a topological insulator state. We also found that inversion symmetry was broken in tensile strained, (001)-oriented Cd3As2, which may lead to band splitting away from the node and suggest previous studies proposing a non-centrosymmetric crystal structure may have been influenced by residual strain. Strontium titanate (SrTiO3) is an incipient ferroelectric m

Modern room temperature ferroelectrics/piezoelectrics significantly impact advanced nanoelectronics than conventional chemical compounds. Changes in crystallinity modulation, long-range order of atoms in metalloids permits the design of novel materials. The ferroelectric like nature of a single element (selenium, Se) is demonstrated via in-plane (E perpendicular to(ar) to the Se helical chains in micro-rod (MR)) and out-of-plane (E parallel to(el) to the Se helical chains in MR) polarization. Atomic electron microscopy shows large stacks of covalently bound Se atoms in a c-axis orientation for tip bias voltage-dependent switchable domains with a 180 degrees phase and butterfly displacement curves. The single crystalline Se MR has a high in-plane piezoelectric coefficient of 30 pm/V relative to polycrystalline samples due to larger grains, crystal imperfections in MR, and tuned helical chains. The energy conversion of a single Se-MR demonstrated via d(13), d(12) (or d(15)) piezoelectric modes.

Multiferroics are materials that electrically polarize when subjected to a magnetic field and magnetize under the action of an electric field. In composites, the multiferroic effect is achieved by mixing of ferromagnetic (FM) and ferroelectric (FE) particles. The FM particles are prone to magnetostriction (field-induced deformation), whereas the FE particles display piezoelectricity (electrically polarize under mechanical stress). In solid composites, where the FM and FE grains are in tight contact, the combination of these effects directly leads to multiferroic behavior. In the present work, we considered the FM/FE composites with soft polymer bases, where the particles of alternative kinds are remote from one another. In these systems, the multiferroic coupling is different and more complicated in comparison with the solid ones as it is essentially mediated by an electromagnetically neutral matrix. When either of the fields, magnetic or electric, acts on the ‘akin’ particles (FM or FE) it causes their displacement and by that perturbs the particle elastic environments. The induced mechanical stresses spread over the matrix and inevitably affect the particles of an alternative kind. Therefore, magnetization causes an electric response (due to the piezoeffect in FE) whereas electric polarization might entail a magnetic response (due to the magnetostriction effect in FM). A numerical model accounting for the multiferroic behavior of a polymer composite of the above-described type is proposed and confirmed experimentally on a polymer-based dispersion of iron and lead zirconate micron-size particles. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Nonvolatile memories are in increasing demand as the world moves toward information digitization. The ferroelectric materials offer a promising alternative for this. Since the existing perovskite materials have various flaws, including incompatibility with complementary metal-oxide-semiconductor processes in memory applications, the discovery of new optimized FE thin films was necessary. In 2011, the disclosure of ferroelectricity in hafnia (HfO$_2$) reignited interest in ferroelectric memory devices because this material is well integrated with CMOS technology. Although the reporting of ferroelectricity in HfO$_2$ has been a decade, researchers are still enthralled by this material's properties as well as its possible applications. The ferroelectricity in HfO$_{2}$ has been attributed to the orthorhombic phase with spacegroup $Pca2_1$. This phase is believed to be the metastable phase of the system. Many experimental and theoretical research groups joined the effort to understand the root causes for the stability of this ferroelectric phase of HfO$_{2}$ by considering the role of the surface energy effects, chemical dopants, local strain, oxygen vacancies. However, the understanding was not conclusive. In this part of this work, we will present our first-principles results, predicting a situation where the ferroelectric phase becomes the thermodynamic ground state in the presence of a ordered dopant forming layers. Since the main focus was on understanding and optimizing the ferroelectricity in HfO$_{2}$, we observed that the electro-mechanical response of the system has garnered comparatively less attention. The recent discovery of the negative longitudinal piezoelectric effect in HfO$_2$ has challenged our thinking about piezoelectricity, which was molded by what we know about ferroelectric perovskites. In this work, we will discuss the atomistic underpinnings behind the negative longitudianl piezoelectric effect in HfO$_{2}$. We will also discuss the behavior of

The transmission electron microscope (TEM) provides multiple modes to study structure-property relationships in materials. In particular, quantum materials, whose properties may be affected by subtle alterations to atomic configuration, require such a valuable instrument for illuminating these relationships. Scanning transmission electron microscopy (STEM) and convergent beam electron diffraction (CBED) are two techniques within the TEM that may be utilized to examine the atomic structure and crystal symmetries of materials. Cadmium arsenide (Cd3As2) is classified as a topological semimetal due to its linear band crossings in the bulk electronic band structure and surface states. Different publications in the past have proposed competing centrosymmetric and non-centrosymmetric crystal structures for Cd3As2, an important distinction that has major implications on the expected band structure and underlying physics. Due to the complicated crystal structure of Cd3As2, as well as the limitations in x-ray diffraction techniques in identifying an inversion center, particularly in thin films, we used CBED to determine the exact point group symmetries of epitaxially grown Cd3As2, finding it to possess an inversion center. Because Cd3As2 possesses fourfold rotational symmetry and an inversion center, it can be classified as a Dirac semimetal. We sought to topologically tune Cd3As2 by introducing biaxial epitaxial stress, producing tensile and compressively stressed films. Using CBED, we found that fourfold rotational symmetry was lost in compressively strained, (112)-oriented Cd3As2, promoting the opening of a band gap and a topological insulator state. We also found that inversion symmetry was broken in tensile strained, (001)-oriented Cd3As2, which may lead to band splitting away from the node and suggest previous studies proposing a non-centrosymmetric crystal structure may have been influenced by residual strain. Strontium titanate (SrTiO3) is an incipient ferroelectric m

Although superconductivity in strontium titanate (SrTiO3) was discovered in the 1960s, the origin of superconductivity in SrTiO3 has not been completely understood. Moreover, its unusual superconducting characteristics have made SrTiO3 a fascinating playground to potentially study multiple aspects of unconventional superconductivity. For example, SrTiO3 is one of the most dilute superconductors where standard Bardeen-Cooper-Schrieffer (BCS theory) is believed to be no longer valid. Furthermore, the presence of superconductivity and ferroelectricity and their interplay possibly leads to an unconventional pairing mechanism in SrTiO3. In this dissertation, high-quality, doped, ferroelectric, compressively strained SrTiO3 thin films were grown using hybrid molecular beam epitaxy (MBE) to unveil unique superconducting properties. First, we present unconventional characteristics of relatively thick (160-180 nm) strained SrTiO3 films. In some strained films, we show signatures of unusual superconducting states such as in-plane critical fields far above the Pauli limiting field and second harmonic resistances. The study of superconductivity in ultra-thin SrTiO3 films is hindered by strong surface carrier depletion, which makes thin films insulating. We solve this issue by introducing thin (~10 nm) EuTiO3 capping layers. We show that the EuTiO3 capping layer effectively prevents strong carrier depletion in SrTiO3 films and that a capped 40 nm-thick SrTiO3 film becomes superconducting. We utilize the EuTiO3 capping layer to elucidate the connection between superconductivity and ferroelectricity in SrTiO3 films. Systematic film thickness study shows that superconductivity and ferroelectricity are suppressed simultaneously in the SrTiO3 films of thicknesses less than 40 nm. We speculate that this result implies that broken inversion symmetry (spin-orbit coupling) plays an important role in the superconductivity of strained SrTiO3 films.

Nonvolatile memories are in increasing demand as the world moves toward information digitization. The ferroelectric materials offer a promising alternative for this. Since the existing perovskite materials have various flaws, including incompatibility with complementary metal-oxide-semiconductor processes in memory applications, the discovery of new optimized FE thin films was necessary. In 2011, the disclosure of ferroelectricity in hafnia (HfO$_2$) reignited interest in ferroelectric memory devices because this material is well integrated with CMOS technology. Although the reporting of ferroelectricity in HfO$_2$ has been a decade, researchers are still enthralled by this material's properties as well as its possible applications. The ferroelectricity in HfO$_{2}$ has been attributed to the orthorhombic phase with spacegroup $Pca2_1$. This phase is believed to be the metastable phase of the system. Many experimental and theoretical research groups joined the effort to understand the root causes for the stability of this ferroelectric phase of HfO$_{2}$ by considering the role of the surface energy effects, chemical dopants, local strain, oxygen vacancies. However, the understanding was not conclusive. In this part of this work, we will present our first-principles results, predicting a situation where the ferroelectric phase becomes the thermodynamic ground state in the presence of a ordered dopant forming layers. Since the main focus was on understanding and optimizing the ferroelectricity in HfO$_{2}$, we observed that the electro-mechanical response of the system has garnered comparatively less attention. The recent discovery of the negative longitudinal piezoelectric effect in HfO$_2$ has challenged our thinking about piezoelectricity, which was molded by what we know about ferroelectric perovskites. In this work, we will discuss the atomistic underpinnings behind the negative longitudianl piezoelectric effect in HfO$_{2}$. We will also discuss the behavior of

Strontium titanate (STO), with a wide spectrum of emergent properties such as ferroelectricity and superconductivity, has received significant attention in the community of strongly correlated materials. In the strain-free STO film grown on the SrRuO3 buffer layer, the existing polar nanoregions can facilitate room-temperature ferroelectricity when the STO film thickness approaches 10 nm. Here we show that around this thickness scale, the freestanding STO films without the influence of a substrate show the tetragonal structure at room temperature, contrasting with the cubic structure seen in bulk form. The spectroscopic measurements reveal the modified Ti-O orbital hybridization that causes the Ti ion to deviate from its nominal 4+ valency (3d0 configuration) with excess delocalized 3d electrons. Additionally, the Ti ion in TiO6 octahedron exhibits an off-center displacement. The inherent symmetry lowering in ultrathin freestanding films offers an alternative way to achieve tunable electronic structures that are of paramount importance for future technological applications.

©2022 American Physical Society, Light-induced ferroelectric domain wall motion turns out to be a promising phenomenon to de- velop new photo-controlled devices. However, the physical origin of this ligh-matter coupling when material is irradiated with visible light remains unclear. Here, a phenomenological model predicting the motion of charged domain walls (CDWs) is developed. The photo-induced electronic reconstruc- tion mechanism is proposed as the primary absorption mechanism, leading to a linear dependence for the polarization perturbation with the light intensity. Domain walls motion is then driven by the energetic difference between domains in a CDW array, such that the macroscopic polarization can be easily tuned., Postprint (author's final draft)

Densely woven highly crystallized biocompatible sodium-potassium niobate Na0.35K0.65NbO3 fibers with an average diameter of 100-200 nm and several hundreds of microns in length were sintered by the sol-gel calcination-assisted electrospinning technique. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM) confirmed preferential cube-on-cube [001] orientation of nanocrystals within the fiber's body, separated by a low angle grain boundary. The Williamson-Hall method was employed to analyze the broadening of XRD reflections and to accurately determine the size and intrinsic strain of nanocrystal fiber aggregates. The main objective of this article is to test the potential capacity of direct XRD analysis to noninvasively control crystallite size and lattice distortion in core-shell coaxial nanofibers., QC 20220531

In recent years, two-dimensional (2D) van der Waals (vdW) materials have aroused much interest for their unique structural, thermal, optical, and electronic properties and have become a hot topic in condensed matter physics and material science. Many research methods, including scanning tunneling microscopy (STM), transmission electron microscopy (TEM), optical and transport measurements, have been used to investigate these unique properties. Among them, STM stands out as a powerful characterization tool with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. This dissertation focuses on scanning tunneling microscopy and spectroscopy (STM/S) investigation of 2D metal chalcogenides and heterostructures. The first part of the dissertation focuses on the continuous interface in WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer agree with previously reported values for MoS2 monolayer and MoS2/WS2 heterobilayer on SiO2 fabricated through the mechanical exfoliation method. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD. The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers with thickness ranging from single to ~20 layers stabilized at the beta phase with a superstructure at room temperature. After cooling down to around 180 K, the beta phase converted to a more stable beta' phase that was distinct from previously reported phases in 2D In2Se3. The kinetics of the reversible thermally driven beta

As the limits of transistor feature size scaling is reaching its saturation, new innovations are needed to prevent performance loss. High performance transistors on the III/V semiconductor platform implemented with ferroelectric oxides might in the future satisfy this demand. But the integration of ferroelectrics on materials such as InAs degrades the substrate and other vital components from the high processing temperatures required to achieve orthorhombic crystallinity. In this master thesis, new engineering strategies have been explored to improve the thermal budget of ferroelectric hafnium zirconium oxide (HZO) implemented on InAs. This was realized through integration of atomic layer deposition (ALD) grown oxides in primarily metal-oxide-semiconductor capacitor (MOSCAP) devices using conventional nano-processing techniques. The MOSCAPs were later characterized by standard current-voltage (IV) measurements and the positive-up negative-down (PUND) technique in order to determine the oxide quality, and especially any ferroelectric behaviour. Ferroelectricity was found in both MOSCAP and metal-insulator-metal capacitor (MIMCAP) structures with the highest as-deposited remanent polarization on InAs at 6.6 µC/cm2 and 12.2 µC/cm2 in the MIM stack. The highest remanent polarization among the MOSCAPs was found in a scaled sample at 6.4 nm, which outperformed samples with similar, but thicker oxides. IV-sweeps also revealed suspected ferroelectric tunnel junction (FTJ)-like behaviour in MOS devices. An annealing study was conducted to investigate if enhanced remanent polarization could be achieved by performing RTP on as-deposited samples. This yielded a remanent polarization of 14.5 µC/cm2 after post-process anneal at 450 °C for 300 s. As-deposited ferroelectric HZO offers new paths for integration of the emerging material in temperature sensitive processes, but requires further research to maximize its potential., En kall eftermiddag den 23 december 1947 hände något fantastiskt på ett laboratorium i New Jersey, USA. Forskare vid Bell laboratoriet demonstrerade för första gången en funktionell transistor! Transistorn är en elektrisk komponent som har tre terminaler där man kan ansluta andra komponenter. Om man leder en ström eller spänning genom ett par av terminalerna kan man kontrollera strömmen genom ett annat par av terminaler. Detta låter som vilken annan elektrisk komponent som helst och inte ens forskarna vid Bell laboratoriet kunde ha förutspått att de hade kommit med århundradets uppfinning. Transistorn är komponenten som utför beräkningarna i nästan all modern teknik. I allt från mobiltelefonen till bilen, från torktumlaren till kaffemaskinen sitter transistorn och tuggar ettor och nollor medans du njuter av morgonens espresso. Sedan dess uppfinning har transistor tagit många vägar i sin utveckling. Arkitekturen har förändrats drastiskt och de har mer än något annat blivit mindre. I den moderna processorn som driver din dator finns hundratals miljoner transistorer, men processorn i sig är bara några få kvadratcentimeter stor! Detta är möjlig för att i takt med att samhället har strävat efter snabbare, effektivare och mindre teknik har en enorm halvledarindustri växt fram. Halvledaren är den typ av material som möjliggör transistorn. Dessa är elektriska isolatorer (de leder inte ström) som kan göras ledande (som en metall). Både halvledarindustrin och forskare har försökt möta den stora efterfrågan på bättre transistorer genom att utveckla avancerad teknik som låter oss manipulera och bygga med material på nanoskalan, alltså mindre än en tusendel av ett hårstrå. Detta har lett till otroligt små transistorer. Men idag har vi börjat stöta på ett fundamentalt problem: vi närmar oss samma skala som enskilda atomer! Detta gör att man måste ta hänsyn till bland annat så kallade kvantfysikaliska fenomen. För att fortsätta förbättra tekniken behövs det nya strategier, som att

Relaxor ferroelectrics exhibit both static and dynamic local structural order which controls their frequency-dependent electrical properties. A combination of advanced scattering and microscopy techniques have been used recently to determine the local structure of relaxors. To complement these, here we show an approach to electrical property measurements which identifies local dipoles whose switching is co-operative, temperature-dependent and responsible for the observed dispersion in dielectric properties. Impedance measurements and equivalent circuit analysis of a canonical relaxor, Pb(Mg1/3Nb2/3)O3 single crystal, over the ranges 180–1050¿K and 100¿Hz–1¿MHz, show that incorporation of a single constant phase element into the equivalent circuit used to fit experimental data is able to account fully for the dispersions that characterise the relaxor response, over this frequency range. This allows parametrisation of the relaxor behaviour, gives increased understanding of the relaxation mechanisms responsible and forms the basis for modifying and controlling relaxor characteristics., Peer Reviewed, Postprint (published version)

In ferroelectric materials, the spontaneous polarization can be reversibly switched by an applied electric field, which makes them attractive for applications in non-volatile memories and other electro-active devices. However, the complex domain structure and domain-wall motion in these materials remains not fully understood. Here, we show a new method for three-dimensional optical tomography to study the domain structure and domain-wall motion in ferroelectrics. Our new method provides a new tool for studying the structure and the dynamics of domain and domain walls, that can be used to optimize the performance of ferroelectric devices.

Solid solutions of BiFeO3 and BaTiO3 are promising lead-free piezoelectric materials, especially around the morphotropic phase boundary at 0.67BiFeO3-0.33BaTiO3. Still, these materials are challenged by phase instability and limited understanding of the processing-properties relationship. Here, we investigate mechanochemical activation and the use of BaTiO3 as seed particles for the 0.67BiFeO3-0.33BaTiO3 phase. Contrary to expectations from seeding in lead-based perovskites, the BaTiO3 seeds do not promote the 0.67BiFeO3-0.33BaTiO3 perovskite phase neither during the mechanochemical activation nor the subsequent sintering, but cause an inhomogeneous structure with remnant BaTiO3. This results in ceramics with weaker low-field piezoelectric response than that of the unseeded route, but with higher field-induced strain, even up to 150 °C. Both routes produce ceramics of high density and without significant secondary phases visible by X-ray diffraction. This demonstrates the advantage of mechanochemical activation and the possibility to tailor the piezoelectric response of 0.67BiFeO3-0.33BaTiO3 through the processing route.

In dieser Arbeit werden ferroelektrische Phasenübergänge in verspannten (K,Na)NbO3-Schichten erstmals mit Hilfe temperaturabhängiger dreidimensionaler Röntgenbeugung untersucht. Der Fokus liegt auf stark anisotrop verspannten Dünnschichten, die bei Raumtemperatur ein geordnetes Fischgräten-Domänenmuster mit einer periodischen Anordnung von monoklinen a1a2/MC-Phasen aufweisen. Bei Erhöhung der Temperatur durchlaufen die (K,Na)NbO3-Dünnschichten einen ferroelektrischen Phasenübergang in die orthorhombische Hochtemperaturphase, die sich durch regelmäßige, alternierenden a1/a2-Streifendomänen mit ausschließlich lateraler Polarisation auszeichnet. In-plane Röntgenmessungen zeigen, dass die Filmeinheitszellen eine kleine Verzerrung in der Ebene erfahren, was zur Bildung von vier verschiedenen Einheitszellvarianten und damit vier verschiedenen (Super-)Domänenvarianten führt. Durch den Vergleich von Röntgenbeugungsmessungen verschiedener Bragg-Reflexe an Filmen mit unterschiedlicher Schichtdicke ist es möglich, die spezifischen Beugungsmerkmale zu unterscheiden und sie den einzelnen Phasen zuzuordnen. Mit Hilfe von in situ temperaturabhängiger Röntgenbeugung ist es daher möglich, die Details des Phasenübergangs vom Fischgräten in das Streifen-Domänenmuster aufzudecken. Es zeigt sich, dass dieser sich über einen großen Temperaturbereich erstreckt und in mehreren Schritten vollzieht. Die Beobachtung von Phasenkoexistenz innerhalb des Übergangs und einer thermischen Hysterese in der Phasenübergangstemperatur lassen auf einen Phasenübergang erster Art schließen. Zudem hängt die Phasenübergangstemperatur stark von der Kaliumkonzentration x in der KxNa1-xNbO3-Dünnschicht ab und kann durch eine Änderung von x=0,95 (stärker kompressiv verspannt) auf x=0,8 (stärker tensil verspannt) um etwa 60 K erhöht werden. Darüber hinaus ist dies die erste Studie, die experimentell beobachtete dreidimensionale Domänenanordnungen direkt mit Berechnungen aus Phasenfeldsimulationen vergleicht., In this work, ferroelectric phase transitions in strained (K,Na)NbO3 films are studied for the first time using in situ temperature-dependent three-dimensional X-ray diffraction. The focus lies on strongly anisotropically strained thin films, which exhibit a well-ordered herringbone domain pattern with a periodic arrangement of monoclinic a1a2/MC phases at room temperature. Upon increasing temperatures, the (K,Na)NbO3 thin films undergo a ferroelectric phase transition to the orthorhombic high-temperature phase, which is characterized by a regular pattern of alternating a1/a2 stripe domains with pure lateral polarization. In-plane X-ray measurements show that the film unit cells undergo a small in-plane distortion, leading to the formation of four different unit cell variants and thus four different (super)domain variants. By comparing X-ray diffraction measurements of different Bragg reflections of films with different film thicknesses, it is possible to distinguish the specific diffraction features and assign them to the individual phases observed at the different temperatures. Using in situ temperature-dependent X-ray diffraction, it is therefore possible to reveal the details of the phase transition from the a1a2/MC herringbone to the a1/a2 stripe domain pattern. It is shown to extend over a wide temperature range and to occur in several steps. The observation of phase coexistence within the transition and a thermal hysteresis in the phase transition temperature suggests a first-order type phase transition. Moreover, the phase transition temperature strongly depends on the molar concentration of potassium x in the KxNa1-xNbO3 thin film and can be increased by about 60 K by changing x=0.95 (more compressively strained) to x=0.8 (more tensile strained). Furthermore, this is the first study to directly compare experimentally observed three-dimensional domain arrangements with calculations from phase field simulations.

The mechanism of the gate-field-induced metal-to-insulator transition of the electrons at the interface of SrTiO3 with LaAlO3 or AlOx is of great current interest. Here, we show with infrared ellipsometry and confocal Raman spectroscopy that an important role is played by a polar lattice distortion that is non-collinear, highly asymmetric and hysteretic with respect to the gate field. The anomalous behavior and the large lateral component of the underlying local electric field is explained in terms of the interplay between the oxygen vacancies, that tend to migrate and form extended clusters at the antiferrodistortive domain boundaries, and the interfacial electrons, which get trapped/detrapped at the oxygen vacancy clusters under a positive/negative gate bias. Our findings open new perspectives for the defect engineering of lateral devices with strongly enhanced and hysteretic local electric fields that can be manipulated with various parameters, like strain, temperature, or photons.

While nature provides a plethora of perovskite materials, only a few exhibit large ferroelectricity and possibly multiferroicity. The majority of perovskite materials have the nonpolar CaTiO_{3}(CTO) structure, limiting the scope of their applications. Based on the effective Hamiltonian model as well as first-principles calculations, we propose a general thin-film design method to stabilize the functional BiFeO_{3}(BFO)-type structure, which is a common metastable structure in widespread CTO-type perovskite oxides. It is found that the improper antiferroelectricity in CTO-type perovskite and ferroelectricity in BFO-type perovskite have distinct dependences on mechanical and electric boundary conditions, both of which involve oxygen octahedral rotation and tilt. The above difference can be used to stabilize the highly polar BFO-type structure in many CTO-type perovskite materials.

Flash sintering is arousing growing interest because high-density ceramics can be obtained at lower temperatures and shorter dwell times than conventional sintering. However, not only temperature and dwell times should be controlled during flash sintering but also parameters such as the electric field and electric current should be considered. Controlling all the parameters during the processing allows comprehensive control of the microstructure and, consequently, functional properties can be improved. In this work, it is evidenced that an exhaustive control of the flash electric current is a crucial factor for tailoring the microstructure of BaTiO3 ceramics. The results reveal that the most suitable way to control the sintering process is by using non-linear current profiles because better densification and improved grain growth is achieved. Although the results focus on BaTiO3, this work offers a new pathway to tailor the microstructure of flash sintered ceramics, which may be extended to other materials., Peer Reviewed, Postprint (published version)

Macroscopic descriptions of ferroelectrics have an obvious appeal in terms of efficiency and physical intuition. Their predictive power, however, has often been thwarted by the lack of a systematic procedure to extract the relevant materials parameters from the microscopics. Here we address this limitation by establishing an unambiguous two-way mapping between spatially inhomogeneous fields and discrete lattice modes. This yields a natural treatment of gradient couplings in the macroscopic regime via a long-wavelength expansion of the crystal Hamiltonian. Our analysis reveals an inherent arbitrariness in both the flexoelectric and polarization gradient coefficients, which we ascribe to a translational freedom in the definition of the polar distortion pattern. Remarkably, such arbitrariness cancels out in all physically measurable properties (relaxed atomic structure and energetics) derived from the model, pointing to a generalized translational covariance in the continuum description of inhomogeneous ferroelectric structures. We demonstrate our claims with extensive numerical tests on 180° domain walls in common ferroelectric perovskites, finding excellent agreement between the continuum model and direct first-principles calculations.

The central material of interest for this thesis is strontium titanate (SrTiO3). Doped SrTiO3 exhibits a wide range of remarkable properties. The focus of this work is to investigate its ferroelectric and superconducting behavior. We utilize scanning transmission electron microscopy (STEM) to understand the role of the nanoscale SrTiO3 structure in these properties. SrTiO3 is an incipient ferroelectric in unstrained, pure form, but easily becomes ferroelectric when subjected to small perturbations. Understanding the nature of the ferroelectric transition in SrTiO3 is essential as it can play an important role in other properties. A hallmark of order-disorder transitions is the formation of polar domains above the Curie temperature. These polar regions percolate below the Curie temperature to form a long-range ordered ferroelectric state. Using high-angle annular dark-field STEM and analyzing off-centering of Ti columns, we show local polar regions at room temperature in compressively strained SrTiO3 films, highlighting the order-disorder nature of the ferroelectric transition in this material. Next, we focus on understanding the competition between mobile carriers and polar crystal distortions. Elucidating the nature of this competition is of great interest for polar superconductors, which have attracted significant interest for their potential to host unconventional superconducting states. We observe a systematic suppression of the nanodomains and ferroelectricity with increasing amount of Sm dopant atoms. The itinerant electrons screen polar distortions and disrupt the nanodomains above the Curie temperature. The results provide direct evidence that long-range Coulomb interactions, already present in the paraelectric phase, are driving the ferroelectric transition and are becoming increasingly short-ranged with doping. Moreover, we show that the disorder caused by the dopant atoms themselves presents a second contribution to the destabilization of the ferroelectri

Diese Arbeit beschäftigt sich mit den elektrischen Eigenschaften der resistiven Schaltmaterialien SrTiO3 und NbO2. Im ersten Teil werden NbO2 (001)-Dünnschichten untersucht. Bisher sind die für NbO2-Dünnschichten in der isolierenden Phase gemessenen spezifische Widerstände um einen Faktor von 200 niedriger als der in NbO2-Einkristallen gemessene 10 kΩ cm Widerstand. In dieser Arbeit wird der spezifische Widerstand von NbO2-Dünnschichten auf 945 Ω cm erhöht. Es wird gezeigt, dass leitfähige Perkolationspfade entlang der Korngrenzen für die Abnahme des spezifischen Widerstandes verantwortlich sind. Durch temperaturabhängige Leitfähigkeitsmessungen wurden Defektzustände identifiziert, die für die Verringerung des spezifischen Widerstandes gegenüber dem theoretischen Wert verantwortlich sind. Im zweiten Teil wird der Einfluss des Ti-Antisite Defekts auf das resistive Schalten in SrTiO3 Dünnschichten untersucht, welche mit metallorganischer Dampfphasenepitaxie gezüchtet wurden. Dabei werden sowohl stoichiometrische als auch Strontium defizitäre Schichten untersucht. Es wird über temperaturabhängige Permittivitätsmessungen gezeigt, dass durch Kristalldefekte die weiche Phononenmode gestört wird und bei stark strontiumverarmten Schichten polare Nanoregionen gebildet werden, was auf die Bildung des TiSr Defekts zurückgeführt wurde. Darüber hinaus wird gezeigt, dass stark strontiumdefiziente SrTiO3 -Schichten ein stabiles resistives Schalten mit einem Ein-Aus-Verhältnis von 2e7 bei 10 K aufweisen, während stöchiometrische Dünnschichten kein stabiles Schalten zeigen. Es wird ein diodenartiger Transportmechanismus, der im hochohmigen Zustand auf Schottkyemission beruht und ihm niederohmigen Zustand durch defektassistierten Tunnelstrom dominiert wird, identifiziert. Daraus wurde ein neues Modell für das resistive Schalten, basierend auf dem TiSr Defekt und der induzierten Ferroelektrizität, entwickelt., In this work, the impact of crystal defects on the resistive switching materials SrTiO3 and NbO2 is investigated. The work is divided into two parts. In the first part, NbO2 (001) thin films are studied. So far, resistivities measured for NbO2 thin films in the insulating phase are by a factor of 200 lower than the 10 kΩ cm resistivity measured in NbO2 single crystals. To make this material applicable for resistive switching, the resistivity in the insulating phase has to be increased to effectively block the current in the high resistive state. Throughout the investigations presented in this work, the resistivity of NbO2 thin films is increased to 945 Ω cm. It is shown that conductive percolation paths along the grain boundaries are responsible for the decrease in resistivity. Temperature-dependent conductivity measurements identified defect states responsible for the reduction in resistivity from the theoretical value. In the second part of this work, the influence of the Ti anti-site defect on resistive switching in SrTiO3 thin films grown by metal-organic vapor phase epitaxy is studied. Both stoichiometric and strontium deficient thin films are studied. It is shown via temperature-dependent permittivity measurements that crystal defects harden the soft phonon mode and polar nano regions are formed in highly strontium deficient films, which was attributed to the formation of Ti antisite defects. In addition, highly strontium deficient SrTiO3 films are shown to exhibit stable resistive switching with an on-off ratio of 2e7 at 10 K, whereas stoichiometric thin-films do not show stable switching. A diode-like transport mechanism based on Schottky emission in the high-resistance state and dominated by defect-assisted tunneling current in the low-resistance state is identified. From this, a new model for resistive switching based on the Ti antisite defect and the induced ferroelectricity is developed.

Forward domain growth representing one of the main stages of domain switching is studied for isolated domains and domain arrays appearing as a result of tip-induced switching on the non-polar cuts of lithium niobate crystals. Formation of the wedge-like domains with a high aspect ratio and charged domain walls is observed. The domain growth in the area with a negligible external field is considered in terms of the kinetic approach based on analogy with crystal growth. The domain wall motion by step generation and propagation of the charged kinks is discussed. It is proposed that the switching field contains the inputs of the external field produced by a biased scanning probe microscope tip, the depolarization field produced by charged kinks, and the screening fields. According to the simulation results of the field distribution, the forward growth is caused by the step generation near the tip and the kink propagation induced by the depolarization field produced by the kinks. Scanning with the biased tip creates self-assembled domain arrays with several modes of domain length alteration: doubling, quadrupling, and chaotic. The statistical characterization of the arrays proves their high ordering. The array is formed under the influence of the depolarization field produced by three neighboring domains. The proposed mechanism can be applied for forward domain growth during switching on the polar cuts as well. In this case, the steps on the domain wall are generated on the polar surface, whereas the domain elongates by kink motion in the field produced by the charged kinks. © 2021 Author(s).

Ferroelectric materials are promising for solar energy conversion due to the unique spontaneous polarization effect, leading to effective control of electron-hole recombination and potentially high power conversion efficiency of perovskite solar cells. Herein, the ultrathin ferroelectric hafnia alumina (HfAlO) films were obtained by atomic layer deposition and applied to modify the electron transport layer of PSCs. The XPS and AFM characterizations indicate that the Hf0.39Al0.10O films feature smooth, dense, and compact surface morphology. For the devices incorporating the ultrathin HfAlO films, the oriented ferroelectric effect-induced spontaneous polarization and internal electric field are crucial factors to enhance the charge separation during charge transfer. Accordingly, the short circuit current, open-circuit voltage, and power conversion efficiency (PCE) are increased by 11.8%, 9.0%, and 29.8%, respectively. Moreover, the HfAlO incorporating devices retained ~ 80% of the original PCE after 500 h ageing, much better than that of the pristine devices showing the retention of only ~ 50% of the original PCE.

The recent emergence of multiaxial molecular ferroelectrics opens up a new route toward technological evolution in the next-generation flexible/wearable device applications. However, a fundamental understanding of multiaxial ferroelectricity and polarization switching at the microscopic level in these materials is still missing. Herein, we study a high-temperature multiaxial perovskite ferroelectric (Me3NOH)2[KCo(CN)6] (TMC-4) that exhibits a bond-switching phase transition at 417 K with notable piezoelectricity and spontaneous polarization in the ferroelectric phase. The cleavage and reformation of coordination bonds and hydrogen bonds during the bond-switching transition all contribute to a large entropy change of 178.79 J K-1 kg-1 at the phase transition. Using piezoresponse force microscopy (PFM), we observed diverse ferroelectric domain structures and provide evidence for both 180° and non-180° domain switching and their possible effect on the functional properties of molecular ferroelectrics. The results provide an insight into the multiaxial ferroelectricity of TMC-4 at the microscopic level enabling its further use in device applications. © 2021 The Royal Society of Chemistry.