Ferroelectrics as smart mechanical materials

This is the peer reviewed version of the following article: Cordero, K., Domingo Marimon, Neus, Abdollahi, A., Sort Viñas, Jordi, Catalan, G. Ferroelectrics as smart mechanical materials. "Advanced materials", 21 Juliol 2017, vol. 29, núm. 37, p. 1-6, which has been published in final form at DOI: 10.1002/adma.201702210. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.The mechanical properties of materials are insensitive to space inversion, even when they are crystallographically asymmetric. In practice, this means that turning a piezoelectric crystal upside down or switching the polarization of a ferroelectric should not change its mechanical response. Strain gradients, however, introduce an additional source of asymmetry that has mechanical consequences. Using nanoindentation and contact-resonance force microscopy, this study demonstrates that the mechanical response to indentation of a uniaxial ferroelectric (LiNbO3) does change when its polarity is switched, and use this mechanical asymmetry both to quantify its flexoelectricity and to mechanically read the sign of its ferroelectric domains.Peer ReviewedPostprint (author's final draft

Water adsorption, dissociation and oxidation on SrTiO3 and ferroelectric surfaces revealed by ambient pressure X-ray photoelectron spectroscopy

Water dissociation on oxides is of great interest because its fundamental aspects are still not well understood and it has implications in many processes, from ferroelectric polarization screening phenomena to surface catalysis and surface chemistry on oxides. In situ water dissociation and redox processes on metal oxide perovskites which easily expose TiO-terminated surfaces, such as SrTiO, BaTiO or Pb(Zr,Ti)O, are studied by ambient pressure XPS, as a function of water vapour pressure. From the analysis of the O1s spectrum, we determine the presence of different types of oxygen based species, from hydroxyl groups, either bound to Ti and metal sites or lattice oxygen, to different peroxide compounds, and propose a model for the adsorbate layer composition, valid for environmental conditions. From the XPS analysis, we describe the existing surface redox reactions for metal oxide perovskites, occurring at different water vapour pressures. Among them, peroxide species resulting from surface oxidative reactions are correlated with the presence of Ti ions, which are observed to specifically promote surface oxidation and water dissociation as compared to other metals. Finally, surface peroxidation is enhanced by X-ray beam irradiation, leading to a higher coverage of peroxide species after beam overexposure and by ferroelectric polarization, demonstrating the enhancement of the reactivity of the surfaces of ferroelectric materials due to the effect of internal electric fields

Ferroelectrics as smart mechanical materials

The mechanical properties of materials are insensitive to space inversion, even when they are crystallographically asymmetric. In practice, this means that turning a piezoelectric crystal upside down or switching the polarization of a ferroelectric should not change its mechanical response. Strain gradients, however, introduce an additional source of asymmetry that has mechanical consequences. Using nanoindentation and contact-resonance force microscopy, this study demonstrates that the mechanical response to indentation of a uniaxial ferroelectric (LiNbO₃) does change when its polarity is switched, and use this mechanical asymmetry both to quantify its flexoelectricity and to mechanically read the sign of its ferroelectric domains

Switchable tribology of ferroelectrics

Switchable tribological properties of ferroelectrics offer an alternative route to visualize and control ferroelectric domains. Here, we observe the switchable friction and wear behavior of ferroelectrics using a nanoscale scanning probe—down domains have lower friction coefficients and show slower wear rates than up domains and can be used as smart masks. This asymmetry is enabled by flexoelectrically coupled polarization in the up and down domains under a sufficiently high contact force. Moreover, we determine that this polarization-sensitive tribological asymmetry is widely applicable across various ferroelectrics with different chemical compositions and crystalline symmetry. Finally, using this switchable tribology and multi-pass patterning with a domain-based dynamic smart mask, we demonstrate three-dimensional nanostructuring exploiting the asymmetric wear rates of up and down domains, which can, furthermore, be scaled up to technologically relevant (mm–cm) size. These findings demonstrate that ferroelectrics are electrically tunable tribological materials at the nanoscale for versatile applications.Peer ReviewedPostprint (published version

Water affinity and surface charging at the z-Cut and y-Cut LiNbO3 surfaces : an ambient pressure X-ray photoelectron spectroscopy study

Polarization dependence of water adsorption and desorption on LiNbO₃ surfaces was demonstrated using X-ray photoelectron spectroscopy (XPS) carried out in situ under near-ambient conditions. Positive and negative (0001) faces (z-cut) of the same crystal were compared for the same temperature and pressure conditions. Our results indicate a preferential adsorption on the positive face of the crystal with increasing water pressure and also higher desorption temperature of the adsorbed molecular water at the positive face. Adsorption measurements on the (1100) face (y-cut) showed also strong affinity to water, as observed for the z-cut positive surface. We found a direct relation between the capacity of the surface to discharge and/or to screen surface charges and the affinity for water of each face. XPS spectra indicate the presence of OH groups at the surface for all the conditions and surfaces measured

Observation of flat Γ\Gamma moir\'e bands in twisted bilayer WSe2_2

The recent observation of correlated phases in transition metal dichalcogenide moir\'e systems at integer and fractional filling promises new insight into metal-insulator transitions and the unusual states of matter that can emerge near such transitions. Here, we combine real- and momentum-space mapping techniques to study moir\'e superlattice effects in 57.4$^{\circ}$ twisted WSe$_2$ (tWSe$_2$). Our data reveal a split-off flat band that derives from the monolayer $\Gamma$ states. Using advanced data analysis, we directly quantify the moir\'e potential from our data. We further demonstrate that the global valence band maximum in tWSe$_2$ is close in energy to this flat band but derives from the monolayer K-states which show weaker superlattice effects. These results constrain theoretical models and open the perspective that $\Gamma$-valley flat bands might be involved in the correlated physics of twisted WSe$_2$

Hidden magnetic states emergent under electric field, in a room temperature composite magnetoelectric multiferroic

The ability to control a magnetic phase with an electric field is of great current interest for a variety of low power electronics in which the magnetic state is used either for information storage or logic operations. Over the past several years, there has been a considerable amount of research on pathways to control the direction of magnetization with an electric field. More recently, an alternative pathway involving the change of the magnetic state (ferromagnet to antiferromagnet) has been proposed. In this paper, we demonstrate electric field control of the Anomalous Hall Transport in a metamagnetic FeRh thin film, accompanying an antiferromagnet (AFM) to ferromagnet (FM) phase transition. This approach provides us with a pathway to "hide" or "reveal" a given ferromagnetic region at zero magnetic field. By converting the AFM phase into the FM phase, the stray field, and hence sensitivity to external fields, is decreased or eliminated. Using detailed structural analyses of FeRh films of varying crystalline quality and chemical order, we relate the direct nanoscale origins of this memory effect to site disorder as well as variations of the net magnetic anisotropy of FM nuclei. Our work opens pathways toward a new generation of antiferromagnetic - ferromagnetic interactions for spintronics

Effect of flexoelectricity on the nano-mechanical properties of ferroelectrics

Los materiales ferroeléctricos pueden tener diferentes respuestas electromecánicas, por ejemplo la piezoelectricidad, polarización inducida cuando hay deformación homogénea, y la flexoelectricidad, polarización inducida cuando hay deformación inhomogénea. Dado que la flexoelectricidad está relacionada con los gradientes de deformación, a la nanoescala su efecto es tan o más grande que la piezoelectricidad. La investigación desarrollada en ésta tesis se enfoca en estudiar la interacción entre estas dos propiedades cuando compiten y/o cuando colaboran entre ellas, y de cómo ésta interacción afecta las propiedades mecánicas de los ferroeléctricos. Hasta ahora se ha creído que las propiedades mecánicas son invariantes con respecto a al espacio de inversión, es decir que medirlas en una cara o en la opuesta no debería cambiar su valor. Sin embargo, ésta tesis demuestra que, en presencia de gradientes de deformación, ésta simetría se rompe, ya que tanto las propiedades mecánicas como la respuesta mecánica de los ferroeléctricos depende del signo de su polarización. Éste resultado representa un cambio en la teoría establecida hasta ahora y ofrece un nuevo camino para explorar en la física de fractura de sólidos. Esta tesis está distribuida de la siguiente manera: El capítulo 1 es una introducción a la física de las propiedades mecánicas, la piezoelectricidad y la flexoelectricidad, mientras que el capítulo 2 describe las técnicas experimentales utilizadas para realizar las medidas de las propiedades mecánicas y las respuestas mecánicas requeridas para el proyecto. En el capítulo 3, se midió y analizó las propiedades mecánicas de cristales ferroeléctricos de LiNbO3 con la polarización perpendicular a la superficie y en direcciones opuesta, empleando la técnica de nanoindentación. La inversión de la polarización fue realizada de dos maneras distintas (1) manualmente, es decir, girando el cristal 180º para acceder a la cara opuesta del mismo, y (2) utilizando un cristal periódicamente polarizado, de ésta manera se tuvo acceso a polarizaciones opuestas desde una misma cara. Se observó que, independientemente del método de inversión, todas las propiedades mecánicas son asimétricas con respecto al espacio de inversión. En el capítulo 4, a partir de la ecuación libre de los ferroeléctricos, se desarrolló un modelo para determinar el coeficiente de flexoacoplamiento empleando únicamente las propiedades mecánicas del material. A partir de éste modelo y los datos obtenidos en el capítulo 3, se obtuvo que el valor de dicho coeficiente para LiNbO3 ~ 40 V, un valor más realista que el medido por el método estándar e incluso más cercano al predicho por Kogan y Tagantsev. En el capítulo 5, el objetivo era estudiar el efecto de la flexoelectricidad en la propagación de grietas y la tenacidad de factura en cristales ferroeléctricos de RKTP con la polarización alineada en el plano. Se realizaron grietas paralelas, antiparalelas y perpendiculares a la polarización y Se demostró que la propagación de la grietas esta intrínsecamente relacionado con la dirección de polarización en la que se propaga, ya que la flexoelectricidad disminuye la tenacidad de fractura cuando es paralela a la polarización ferroeléctrica, y por ende las grietas son mas largas. En el capítulo 6, se plantea una posible aplicación como consecuencia de la asimetría en las propiedades mecánicas del capítulo 3: leer la polarización solamente por medios mecánicos. Para probar éste nuevo concepto, se utilizó CRF en el cristal periódicamente polarizado, obteniendo una lectura en concordancia con los resultados del capítulo 3. Además se mostró que al disminuir el volumen ferroeléctrico, es decir con películas delgadas, la resolución de lectura se ve incrementada considerablemente. Finalmente en el capítulo 7 se concluye ésta tesis y plantean las posibles líneas de trabajos futuros.Ferroelectric materials can present various electromechanical responses. These include electrostriction (strain proportional to the square of the electric field) piezoelectricity (polarization induced by a strain), and flexoelectricity (polarization induced by a strain gradient). Since flexoelectricity is proportional to the strain gradients, and these can grow in inverse proportion to the size, at the nanoscale flexoelectricity can be as big as or greater than piezoelectricity. The research developed in this thesis focuses on studying the interaction between these two properties in ferroelectrics, and specifically on how this interaction affects the mechanical properties of ferroelectrics. Until now it has been believed that the mechanical properties are invariant with respect to space inversion, that is to say that measuring them on one side or on the opposite side of a crystal should not change their value, even when the material in question is non-centrosymmetric (piezoelectric or ferroelectric). However, this thesis shows that, in the presence of strain gradients, mechanical inversion symmetry breaks down: the mechanical response of ferroelectrics depends not just on the orientation but also on the sign of their polarization. This result represents a paradigm shift in the physics of solid state mechanics and fracture physics, and opens up new and interesting functional concepts such as mechanical reading of polarization. This thesis is distributed as follows: Chapter 1 is an introduction to the physics of mechanical properties, piezoelectricity and flexoelectricity, while Chapter 2 describes the experimental techniques used in the project for measuring mechanical and electromechanical properties. Chapter 3 describes the characterization and analysis of the mechanical properties of LiNbO3 ferroelectric crystals with polarization perpendicular to the surface, using the nanoindentation technique. The properties were measured for opposite polarization signs, and the inversion of the polarization was done in two different ways: (1) manually, that is, turning the crystal 180º to access the opposite side of it, and (2) using a periodically polarized crystal, so that opposite polarizations can be accessed on the same face. It was observed that, regardless of the “switching” method, all the mechanical responses are asymmetric with respect to space inversion. The causes and consequences of this discovery are discussed. Chapter 4 derives a free-energy model to quantitatively relate the asymmetry of mechanical responses to the flexocoupling coefficient. Using this model and the experimental results of chapter 3, the flexocoupling coefficient of LiNbO3 was calculated using only the mechanical measurements of the material. The value obtained for LiNbO3 is f ~ 40 V. This is a more realistic value than that measured by the standard electromechanical method, and is close to the theoretical value predicted by the theories of Kogan and Tagantsev. The conclussion of this chapter is that mechanical methods not only allow measuring flexocoupling coefficients, but they are quantiatively advantageous when dealing with polar materials where spurious piezoelectricity can artificially enhance the results obtained by conventional electromechanical means. In Chapter 5, the objective was to study the effect of flexoelectricity on the propagation of cracks and the fracture toughness in ferroelectric crystals with polarization aligned in the plane. The material used for this study was a crystal of Rb-KTiOPO4 (R-KTP) with two antiparallel domains in the plane. Using indentation, sets of cracks were opened in the parallel, antiparallel and perpendicular to the polarization. The results showed unambiguously that the propagation of said cracks is asymmetric and intrinsically related to the direction of polarization: flexoelectricity decreases the fracture tenacity when it is parallel to the ferroelectric polarization, thus yielding longer cracks parallel to the polar direction than antiparallel to it. The term "cracking diode" was coined to denominate this effect. Chapter 6 describes the concept demonstration of one possible application of the asymmetry in mechanical properties reported in Chapter 3: read the sign of ferroelectric polarization by purely mechanical means and in a non-destructive way. To demonstrate this new concept, Contact Resonance Frequency Microscopy was used in the periodically poled crystal, obtaining a reading in accordance with the results of chapter 3, namely, that the contact stiffness of down-polarized domains is higher than that of up-polarized domains. It was also shown that, owing to the inverse size dependence of flexoelectricity, working with films results in a considerable resolution increase.. This demonstrates that, by exploiting the interaction between flexoelectricity and ferroelectricity, it is not only possible to mechanically write a ferroelectric memory, but also to mechanically read it. Finally, in chapter 7 this thesis is concluded with a summary of all the results and their consequences

Flexoelectricity at the nanoscale: switchable mechanical properties of ferroelectrics

Resumen del póster presentado a la 10th Conferencia Fuerzas y Túnel, celebrada en Girona (España) del 5 al 7 de septiembre de 2016.Electrical and mechanical stimulus induced by the tip of an Atomic Force Microscope (AFM) is the basis for the generation and detection of different types of phenomenologies; from imaging ferroelectric ordering of thin films or single crystals to piezoelectric characterization. Moreover, AFM tip can generate flexoelectric fields which can be applied to mechanical writing of ferroelectric polarization. The mechanical properties of materials are believed to be invariant with respect to space inversion, even for non-centrosymmetric materials, such as ferroelectrics, because all the magnitudes involved (stress, strain, and elastic constants) are described by even parity tensors. The standard theory, however, does not take into account the effect of flexoelectricity on structural properties. Flexoelectricity is a coupling between polarization and strain gradients, and it can be huge around crack tips, so it should be important in fracture phenomena. Our recent work, using the nanoindentation technique and PFM images, provide evidences that this spatial inversion symmetry is broken in ferroelectric materials. In this presentation, I will give an overview on flexoelectric induced effects in PFM. I will show how the nanoindentation technique enables us to mobilize the flexoelectric effect around the sharp indenter tip while simultaneously probing the mechanical properties of the material. The measurements on a stoichiometric single crystal Lithium Niobate (SLN) and a periodic poled Lithium Niobate (PPLN) indicate that the energy dissipation, fracture toughness, and mechanical properties are asymmetric in respect to the sign of the polarization. The PFM images of the indented areas show domain switching in positively poled regions, resulting in different polarization patterns in 180 antiparallel domains. We show that this new physical phenomenon is enabled by the interplay between ferroelectricity and flexoelectricity. Aside from the fundamental importance of this new insight, the predicted asymmetry may also find uses in smart nanodevices and coatings with switchable mechanical properties. Finally, I will also give evidences of inverse flexoelectric effects as induced by the strong electric field gradients emanating from a sharp PFM tip, which are present in the measurements performed in all dielectric materials.Peer reviewe