Speakers: Gustau Catalan (Centre for Investigations in Nanoscience and Nanotechnology (CIN2), CSIC Barcelona, Spain)
Place: Přednáškový sál Fyzikálního ústavu AV ČR Na Slovance
Presented in English
Organisers:
Department of Dielectrics
Some materials generate polarization when they are subject to a homogeneous deformation. Such materials are called piezoelectric and find many everyday uses in sensors, actuators and energy harvesters. The exist another type of electromechanical coupling, called flexoelectricity, between strain gradients and polarization. Such coupling is far more general than piezoelectricity, as it affects all dielectrics and not just those with a non-centrosymmetric space group. Though this effect is more general and has been known since the sixties, it was largely ignored for decades because it is generally quite small in magnitude.
This situation, however, has begun to change in the past few years. Firstly, large flexoelectric coefficients have been measured in high-permittivity materials. Second, and perhaps most important, strain gradients can be huge at the nanoscale, so that flexoelectricity can have a considerable impact in nanoscopic devices such as ferroelectric thin films. In fact, the flexoelectric polarization can be bigger than theferroelectric/piezoelectric one in the 10nm scale.
In this talk I will start with a quick overview of the physics and salient features of flexoelectricity, and then move on to our most recent results towards applications of this phenomenon. In particular, I want to emphasize how
(i) flexoelectricity can be used as a means to enhance piezoelectricity at the nanoscale, taking advantage of the large flexoelectric polarizations generated by engineered strain gradients. This could be of potential use in energy harvesting applications [1].
(ii) flexoelectricity can also be used in information technologies as a means to switch ferroelectric polarization in thin film ferroelectric memories using a small mechanical force (<1microNewton) delivered by the sharp tip of an Atomic force microscope. This flexoelectric writing mechanism removes the need to apply any voltage to the sample for writing the information state, thus removing problems associated with electric fatigue, leakage and dielectric breakdown [2].
[1] G. Catalan, A. Lubk, A. H. G. Vlooswijk, E. Snoeck, C. Magen, A. Janssens, G. Rispens, G. Rijnders, D. H. A. Blank and B. Noheda; Flexoelectric rotation of polarization in ferroelectric thin films, Nature Materials 10, 963 (2011).
[2] H. Lu, C.-W. Bark, D. Esque de los Ojos, J. Alcala, C. B. Eom, G. Catalan, A. Gruverman, Mechanical Writing of Ferroelectric Polarization, Science 336, 59-61 (2012).
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