Some piezoelectric materials may be ‘fakes’

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The research shows that the PFM technique can present false positives because it not only measures piezoelectricity but also flexoelectricity.

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Irene Arias and Amir Abdollahi, from the Department of Civil and Environmental Engineering and the Numerical Methods Laboratory (LaCàN)

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Gustau Catalán and Neus Domingo, from the Catalan Institute of Nanoscience and Nanotechnology (ICN2), which is a Severo Ochoa centre of excellence on the Universitat Autònoma de Barcelona campus and a member of the Barcelona Institute of Science and Technology.

A team of researchers has discovered that the most widespread technique for characterising piezoelectric properties—the ability of some materials to generate electricity when subjected to mechanical stress—can yield ‘false positives’ due to flexoelectricity. The study, which is the result of the collaboration between the Numerical Methods Laboratory (LaCàN) at the UPC and the Catalan Institute of Nanoscience and Nanotechnology (ICN2), will facilitate research of the application of these materials in pressure sensors and electric nanogenerators.

Apr 12, 2019

Piezoresponse force microscopy (PFM) is the most widespread technique for characterising piezoelectric properties at the nanoscale, i.e. for determining the ability of some materials to generate electricity when they are subjected to mechanical stress and, at the same time, to deform in response to a voltage. Piezoelectricity is used in a wide variety of applications: pregnancy ultrasounds, injection motors, sensors for measuring deformation, actuators and sonars, among others. PFM not only determines whether a material is piezoelectric, but also its degree of piezoelectricity, which is particularly important for applications of these materials in microelectronics and nanotechnology.

Now a team of researchers from the Numerical Methods Laboratory (LaCàN) at the Universitat Politècnica de Catalunya · BarcelonaTech (UPC) and the Catalan Institute of Nanoscience and Nanotechnology (ICN2) has theoretically and experimentally demonstrated that the PFM technique can generate false positives when the piezoelectricity of a material is measured at the nanoscale. The PFM technique consists in applying a voltage to the surface of a material via an electrically conducting tip in an atomic force microscope. The microscopic tip itself detects the deformation of the material in response to voltage; the piezoelectric coefficient is obtained dividing the deformation by the voltage. The researchers show, however, that the application of a voltage with a nanoscopic tip can generate deformations in any material, whether piezoelectric or not. In other words, any material measured with a piezoresponse force microscope gives a non-zero piezoelectric coefficient, even if it is not piezoelectric.

The cause of this curious behaviour is flexoelectricity, a phenomenon that occurs at the nanoscale, whereby all material emits a small voltage when an inhomogeneous pressure is applied to it or, vice versa, it is deformed when an inhomogeneous electric field is applied to it; that is precisely the kind of field generated by microscopic tips. Flexoelectricity may not only make a material look like it is piezoelectric despite not being so, but it can also alter the piezoelectric coefficient of materials that are indeed piezoelectric. This has very important consequences for characterising piezoelectric devices in microelectronics: the results imply that, from now on, measurements made with PFM to characterise the materials in these devices should take into account the effect of flexoelectricity.

“We are studying flexoelectricity from the computational point of view, which involves many fundamental manifestations of physics”, explains the LaCàN researcher Irene Arias, adding that “we have discovered that the PFM technique can present false positives because it not only measures piezoelectricity, i.e. the response to an electric field, but also flexoelectricity. We have developed a model that allows us to quantify these responses and, therefore, to separate the piezoelectric part from the flexoelectric part”.

“Barcelona is today the world capital of research in flexoelectricity. In our city there are three different groups—two of them involved in this study—with flexoelectric projects funded by the European Research Council, the top division in European scientific research. To put it in context: no other region can match it even considering all the rest of the European countries together! This concentration of resources facilitates theoretical-experimental collaborations such as the one reflected in this study, which generate very powerful results”, stressed the ICN2 researcher Gustau Catalán.

The research, which was published in the journal Nature Communications, was conducted by a team of researchers made up of Irene Arias and Amir Abdollahi, from the Department of Civil and Environmental Engineering and the Numerical Methods Laboratory (LaCàN), Gustau Catalán, an ICREA researcher from the Catalan Institute of Nanoscience and Nanotechnology (ICN2), which is a Severo Ochoa centre of excellence on the Universitat Autònoma de Barcelona campus and a member of the Barcelona Institute of Science and Technology, and Neus Domingo, also an ICN2 researcher.