The reduction of sizes in ferroic materials, and in particular ferroelectrics, is motivated by an increasing number of their industrial applications, as memory devices with enhanced storage efficiency, energy harvesters with increased anisotropic piezo-responses or photocatalysts improved activities. As a matter of fact, decreasing the thickness/size of such materials for their technological integration requires a deeper knowledge of their behavior as the boundary conditions are fundamentally changed and thus will drastically affect their physical properties. In addition, extrinsic effects including internal strains, vacancies, surface terminations, shapes, dead layers,.... make their investigation much more complex than anticipated.
With the purpose to gain a better understanding of the ferroic functional properties and their relationship to the nanoscale phenomena, transmission electron microscopy (TEM) and scanning TEM (STEM) techniques appear as very powerful tools. Here we will illustrate our findings through two particular cases of ferroic oxides, namely the model multiferroic BiFeO3 and model ferroelectric BaTiO3 synthetized as nanoobjects using different chemical routes. Firstly, we will present our results on the rhombohedral BiFeO3 (Fig. 1) nanoobjects obtained by sol-gel and hydrothermal techniques. By tuning the size of this small band gap ferroelectric, we will discuss our results on multiferroic and photoinduced properties in view of the surface termination and particle size. Secondly, ferroelectric BaTiO3 nano-objects (Fig. 2-4), as nanotori or nanocubes, have been obtained through hydrothermal synthesis in a scalable process. To unveil the topological complex crystal growth that may be involved in the spontaneous nucleation of BaTiO3 as nanotori (Fig. 2) and nanocubes (Fig. 3-4), local structure and atomic arrangement have been investigated. The nanocubes, as a second stage during the BaTiO3 growth, have been investigated by STEM tomography (Fig. 3) and EDX mapping (Fig. 4), showing the high crystalline quality of these objects, with [100]T, [010]T and [001]T terminating planes and presenting a particular internal arrangement containing nanopores. For the nanotori, two arrangements were theoretically predicted. The first one exhibits isotropic radial atomic planes and large strain, while the second is highly anisotropic, with no adaptation of the crystal structure to the torus shape. Our observations indicate that the as-synthesized nanotori grow preferentially with parallel atomic planes.
All these original results revealed at the atomic scale will be discussed in view of the theoretical predictions and the micro and macroscopic ferroic properties measured on these systems.
We acknowledge the French National Research Agency (MATMECA Equipex project) for financial support and D. Delille and A. Carlsson for their first experiments at FEI facilities.