Growth Modulation of Super-Tetragonal PbTiO₃ Thin Films with Self-Assembled Nanocolumn Structures

Although the epitaxial strain due to lattice mismatch between films and substrates can lead to improved properties and emergent functionalities, the limited choices of substrates and strain relaxation have hindered the further tuning of thin films ferroelectricity by conventional strain engineering via electro-elastic coupling. Here, epitaxial thin films composed of self-assembled arrays of Pb_xTi_yO_x+2y (lead-rich) nanocolumns embedded in a PbTiO₃ matrix by laser ablation of single lead titanate ceramic targets are fabricated. The c-axis lattice parameter of the whole nanostructure can reach up to 4.7 Å and the out-of-plane to in-plane lattice parameter ratio of 1.2 can be achieved, based on which spontaneous polarization is also estimated according to the coupling between tetragonality and polarization for ferroelectrics. The value of tetragonality can be well tuned by the growth conditions, such as the growth temperatures. Moreover, single lead titanate film composed of multiple tetragonalities can also be reached. This work provides a general approach to regulation of lattice states and functionalities beyond their present levels via elastic coupling between the self-assembled nanostructures in three dimensions.