The nature of the ferroelectric size effects has been discussed for decades, however, a unified picture is still missing. It seems that interfacial defects play an important role in size effects of ferroelectrics [1]. To investigate this issue we have prepared epitaxial Pb(Zr_xTi_1-x)O₃ nanocrystals with different Zr/Ti ratio (x between 0 and 0.52), in this way providing a lattice mismatch with the SrTiO₃ substrate between ~3.4 % and ~0.2%.
Epitaxial PZT nanoislands were prepared by chemical solution deposition onto conductive (001) single-crystal STO:Nb substrates. This method allowed us to prepare uniform and good quality structures and to tune the size of the final structures (i.e. their height can be between 5 and 25 nm). A cross-sectional high-resolution transmission electron microscopy (HRTEM) analysis showed that the PZT nanoislands grown on STO have a truncated-pyramid morphology. The islands are single-crystalline and single-c-domain. Using HRTEM the nature of the misfit dislocations has been analyzed. It turns out that they extend into a PZT region with dimensions as large as ~4 nm in height and ~8 nm in width. Since ferroelectricity is a co-operative phenomenon of a sufficient number of regular noncentrosymmetric unit cells, this significant ratio points to a potential long-range influence on the ferroelectric properties of the PZT nanoislands. Probing the polarization of a large number of bare nanoislands by piezoresponce force microscopy it was found that the PZT (x=0.52) nanoislands with a height of ~9 show an apparent polarization instability. By contrast, the same PZT islands with a height of ~20 nm show well-developed ferroelectricity, as also do tetragonal, epitaxial PbTiO₃ nanoislands ~9 nm in height and free from misfit dislocations at the interface.
[1] M.-W. Chu, I. Szafraniak, R. Scholz, C. Harnagea, D. Hesse, M. Alexe, U. Goesele, Nature Materials, 3 (2004) 87.

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