Using electron microscope methods of a hitherto unknown accuracy, scientists from Forschungszentrum Juelich have succeeded in locally demonstrating polarization in the ferroelectric PbZr0.2Ti0.8O3 and measuring it atom by atom. The broken line forms the boundary of two areas with different electrical polarization marked by the arrows. This is due to the fact that the atoms (Pb: lead; Z: zircon; Ti: titanium; O: oxygen) are displaced from their positions and therefore their electrical charges cannot compensate for each other. On the left, the oxygen atoms are displaced 38 pm downwards, and on the right to the same degree upwards out of the zircon/titanium atomic row. This row itself is displaced vertically by 10 pm from the center line between the lead atoms. In order to write information in applications for data storage, the boundary between these two areas of different polarization directions is displaced to the left or to the right so that only one polarization direction exists in the material. Image: Forschungszentrum Juelich
transmission electron microscopy has taken a great step forward with the introduction of aberration-corrected electron optics. An entirely new generation of instruments enables studies in condensed-matter physics and materials science to be performed at atomic-scale resolution. These new possibilities are meeting the growing demand of nanosciences and nanotechnology for the atomic-scale characterization of materials, nanosynthesized products and devices, and the validation of expected functions. Equipped with electron-energy filters and electron-energy–loss spectrometers, the new instruments allow studies not only of structure but also of elemental composition and chemical bonding. The energy resolution is about 100 milli–electron volts, and the accuracy of spatial measurements has reached a few picometers. However, understanding the results is generally not straightforward and only possible with extensive quantum-mechanical computer calculations.
Knut Urban explains: “This is the beginning of a new physics of materials which enables researchers to determine physical parameters and properties in the nano range through highly precise measurements of the atomic spacings. This will also provide clues on how these properties may be manipulated in order to gain new functions and better functional performance.”
Displacements of a few picometres decide on a whole number of physical properties, which are of eminent importance for technology. Another example is the ferroelectricity of titanates materials. Here, the electrical charges of the individual types of atoms inside the building blocks of crystals, the unit cells, cannot fully compensate for each other as they are not arranged in the necessary symmetry.
Therefore, electric dipoles are formed inside the unit cells, which add up over a larger crystal area to form the so-called polarisation. This is used to write information bits. An example is PbZr0.2Ti0.8O3 which is used in chip cards for data storage. With the aid of new electron optical methods, atomic displacements can be measured atom by atom thus making it possible to determine local polarisation for the first time.