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Description
PbTiO$_3$ is a material that exhibits a bulk paraelectric-ferroelectric phase transition at a critical temperature T$_c$ of 765 K, with an electric switchable polarisation that develops along the c-axis mostly due to ionic displacements. Theoretical studies in PbTiO$_3$ thin films have revealed complex phase diagrams with different configurations of domains - regions with uniform polarisation [1]. It has been demonstrated that it is possible to control the intrinsic domain pattern in terms of size and shape by tailoring the electrostatic boundary conditions, the film thickness, the deposition temperature, and the epitaxial strain of the substrate. In a Pb$_x$Sr$_{1-x}$TiO$_3$ solid solution, there is also the possibility to adjust the critical temperature T$_c$ and to tune the domain configuration and domain walls by varying the composition (x) [2]. Moreover, PbTiO$_3$/SrTiO$_3$ and Pb$_x$Sr$_{1-x}$TiO$_3$/SrTiO$_3$ superlattices can exhibit more complex domain structures and novel phenomena like negative capacitance [3].
The objective of this research is to study the domain structures and domain walls in PbTiO$_3$ thin films as a function of thickness and deposition temperature on various substrates. We also explore Pb$_x$Sr$_{1-x}$TiO$_3$ solid solutions with various compositions, as well as more complex structures, such as bilayers, heterostructures and superlattices.
In the case of PbTiO$_3$ heterostructures, epitaxially strained on (110)$_o$-oriented DyScO$_3$ substrates, with bottom and top SrRuO$_3$ electrodes, the domain configuration in the PbTiO$_3$ film was found to depend on the PbTiO$_3$ thickness. This evolution was studied using a combination of atomic force microscopy, laboratory and synchrotron x-ray diffraction and high resolution scanning transmission electron microscopy [4]. Above a critical value of PbTiO$_3$ thickness, a modulation in the structure of the top SrRuO$_3$ electrode has been observed, opening the possibility of domain engineering via structural coupling to ferroelastic domains [5].
Moreover, piezoresponse force microscopy analysis of similar PbTiO$_3$ heterostructures without the top SrRuO$_3$ electrodes have been performed for direct imaging of the local polarisation and further investigation of the domain configuration.
References:
[1] Schlom et al., Annu. Rev. Mater. Res., Vol. 37, no. March, pp. 589–626 (2007).
[2] Fernandez-Peña et al., APL Mater., Vol. 4, no. 8, pp. 1–8 (2016).
[3] Zubko et al., Nature, Vol. 534, no. 7608, pp. 524–528 (2016).
[4] Lichtensteiger et al., APL Mater. 11, 061126 (2023).
[5] Lichtensteiger et al., APL Mater. 11, 101110 (2023).
