A review on mechanics and mechanical properties of 2D materials—Graphene and beyond

Ferroelectric materials, because of their robust spontaneous electrical polarization, are widely used in various applications. Recent advances in modelling, synthesis and characterization techniques are spurring unprecedented advances in the study of these materials. In this Review, we focus on thin-film ferroelectric materials and, in particular, on the possibility of controlling their properties through the application of strain engineering in conventional and unconventional ways. We explore how the study of ferroelectric materials has expanded our understanding of fundamental effects, enabled the discovery of novel phases and physics, and allowed unprecedented control of materials properties. We discuss several exciting possibilities for the development of new devices, including those in electronic, thermal and photovoltaic applications, and transduction sensors and actuators. We conclude with a brief survey of the different directions that the field may expand to over the coming years. Strain engineering can be used to control the properties of thin-film ferroelectric materials, which are promising for electronic, thermal, photovoltaic and transduction applications. This Review addresses fundamental aspects, novel ways to control materials properties and the development of new ferroelectric-based devices.

https://escholarship.org/content/qt51w3s3s1/qt51w3s3s1.pdf?t=p8f5p7

Thin-film ferroelectric materials and their applications

The flexoelectric effect is the response of electric polarization to a mechanical strain gradient. It can be viewed as a higher-order effect with respect to piezoelectricity, which is the response of polarization to strain itself. However, at the nanoscale, where large strain gradients are expected, the flexoelectric effect becomes appreciable. Besides, in contrast to the piezoelectric effect, flexoelectricity is allowed by symmetry in any material. Due to these qualities flexoelectricity has attracted growing interest during the past decade. Presently, its role in the physics of dielectrics and semiconductors is widely recognized and the effect is viewed as promising for practical applications. On the other hand, the available theoretical and experimental results are rather contradictory, attesting to a limited understanding in the field. This review paper presents a critical analysis of the current knowledge on the flexoelectricity in common solids, excluding organic materials and liquid crystals.

/pdf/fundamentals-of-flexoelectricity-in-solids-49ghtjhntc.pdf

Fundamentals of flexoelectricity in solids

Multiferroic heterostructures can be synthesized by integrating monolithic ferroelectric and magnetic materials, with interfacial coupling between electric polarization and magnetization, through the exchange of elastic, electric, and magnetic energy. Although the nature of the interfaces remains to be unraveled, such cross coupling can be utilized to manipulate the magnetization (or polarization) with an electric (or magnetic) field, known as a converse (or direct) magnetoelectric effect. It can be exploited to significantly improve the performance of or/and add new functionalities to many existing or emerging devices such as memory devices, tunable microwave devices, sensors, etc. The exciting technological potential, along with the rich physical phenomena at the interface, has sparked intensive research on multiferroic heterostructures for more than a decade. Here, we summarize the most recent progresses in the fundamental principles and potential applications of the interface-based magnetoelectric effect in multiferroic heterostructures, and present our perspectives on some key issues that require further study in order to realize their practical device applications.

Multiferroic Heterostructures Integrating Ferroelectric and Magnetic Materials.

We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

/pdf/giant-flexoelectric-effect-in-ferroelectric-epitaxial-thin-24sbqwd74x.pdf

Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

In perovskite oxide ferroelectrics, gradients of lattice strain are known to induce nanoscale topological structures, leading to novel or enhanced functionality. Here, we experimentally detect and theoretically analyze thickness distribution of structural properties in epitaxial ${\mathrm{Pb}}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}$ films grown on (001) $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrates. We show that the relaxation of substrate-imposed stress produces a strain gradient, which leads to the formation of distinct ferroelectric phases as a function of distance from the film-substrate interface within the same film. Charge carriers trapped at phase boundaries stabilize the induced phases and manifest themselves under electric field. Crosstalk between the phases, where polarization may rotate in one phase and invert in the other one, opens perspectives for advanced ferroelectric thin film devices.

Ferroelectric phase transitions induced by a strain gradient

Reference EPFL-ARTICLE-212631doi:10.1063/1.4919883View record in Web of Science Record created on 2015-09-28, modified on 2017-11-27

Erratum: “Upper bounds for flexoelectric coefficients in ferroelectrics” [Appl. Phys. Lett. 104, 082913 (2014)]

The influence of the relative humidity on the domain growth during local switching by a conductive tip of a scanning probe microscope has been studied in Rb:KTP single crystals. The abnormal format...

Influence of Humidity on Local Polarization Reversal in a Rb:KTP Single Crystal

Flexoelectricity is an electromechanical phenomenon that, unlike piezoelectricity, is present in any material. While the largest flexoelectric response is observed in ferroelectrics and related materials, its manifestations in various other materials are of great interest for a broad scope of scientific and industrial tasks. Here, we provide a simple theoretical model for flexoelectricity in metals. This is done in terms of continuum mechanics. Estimates for aluminum are provided, and guidelines for an experimental detection of the phenomenon are formulated. The magnitude for the flexoelectric effect in metals is found to be comparable with that in high-k dielectrics, which makes metals just as good as other materials for applications in flexoelectric sensing and energy harvesting. Besides, in metals, flexoelectricity may be easier to interpret because it is predicted to be free of surface contributions.

Flexoelectricity in metals

The stability of Ising 180-degree domain walls with respect to appearance of chirality is analytically studied in tetragonal perovskite-type ferroelectrics taking into account electro-mechanical coupling. It is shown that the widely used approximation neglecting the elastic effects may lead to qualitatively wrong results. Sufficient condition for the stability of Ising walls is formulated only in terms of elastic and electrostrictive properties, regardless the correlation energy. We demonstrate that elasticity stabilizes Ising walls in tetragonal Pb(Zr1-cTic)O3 and makes chiral walls unfavorable for any composition of the material (factor c), while nonelastic model predicts chirality of 180-degree walls near the morphotropic boundary.

/pdf/impact-of-strain-effects-on-the-stability-of-ising-walls-in-3s1qkrntf4.pdf

P. V. Yudin

Papers

Ferroelectric phase transitions induced by a strain gradient

Erratum: “Upper bounds for flexoelectric coefficients in ferroelectrics” [Appl. Phys. Lett. 104, 082913 (2014)]

Influence of Humidity on Local Polarization Reversal in a Rb:KTP Single Crystal

Flexoelectricity in metals

Impact of Strain Effects on the Stability of Ising Walls in Ferroelectrics