Skip to main content

Towards size effects in nanosized ferroelectrics - fabrication of nanocrystals by self-assembling methods

Final Activity Report Summary - NANOFERRO (Towards size effects in nanosized ferroelectrics - fabrication of nanocrystals by self-assembling methods)

The main goal of this work was the preparation of nanosized ferroelectric crystals by self-assembling methods. These approaches offered inexpensive fabrication of structures with size below 100 nm, and several non-conventional routes were applied to fulfil this task. This project focussed on the most important materials for future devices, such as barium titanate (BaTiO3), BiFeO3, PbTiO3 and lead zirconate titanate (PZT) that possessed perovskite crystallographic structures. They were expected to play an important role in fields such as sensors, actuators, memory devices and optics.

The most successful strategy based on the concept of structural and microstructural instability of ultrathin films was carefully studied as part of the project. There was already several evidence that the properties of nanostructures were closely related to misfit dislocations caused by lattice mismatch. Therefore, the suggestion that the formation process of islands could also be defect or strain-dependent was experimentally investigated. The broad choice of substrate and PZT nanocrystal composition provided lattice mismatch ranging between 8 % and 0.2 % approximately. The formation process of nano-islands was carefully investigated by atomic force microscopy, electron microscopy and X-ray diffraction. Moreover, a multi-step deposition procedure was introduced in order to laterally control the crystals' dimension.

The other route of ferroelectric nano-powders preparation used mechano-chemical synthesis. In contrast to the classical solid-state reaction, high energy milling reduced the particle sizes and increased in the contact area of reactant particles; thus, the reaction could proceed without diffusion through the product layer, i.e. the ferroelectric formation occurred at lower temperatures.

The X-ray diffraction demonstrated the perovskite structure of the powder directly after the room temperature synthesis. In addition, it was found that:
1. the powder consisted of loosely packed grains with a broad size distribution, between a few nm and 45 nm;
2. the grains of sizes larger than about 30 nm exhibited well-developed crystalline structure; and
3. the Raman lines of nanopowder exhibited a conspicuous broadening in comparison to Raman lines of the bulk material and their frequencies shifted.