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Inducing clear ferroelectricity in the quantum paraelectric SrTiO 3 is important for triggering methods to discover hidden phases in condensed matter physics. Several methods such as isotope substitution and freestanding membranes could introduce ferroelectricity in SrTiO 3 toward nonvolatile memory applications. However, the stable transformation from quantum paraelectric SrTiO 3 to ferroelectricity SrTiO 3 at room temperature still remains challenging. Here, we used multiple nano-engineering in (SrTiO 3 ) 0.65 /(CeO 2 ) 0.35 films to achieve an emergent room-temperature ferroelectricity. It is shown that the CeO 2 nanocolumns impose large out-of-plane strains and induce Sr/O deficiency in the SrTiO 3 matrix to form a clear tetragonal structure, which leads to an apparent room-temperature ferroelectric polarization up to 2.5 μC/cm 2 . In collaboration with density functional theory calculations, it is proposed that the compressive strains combined with elemental deficiency give rise to local redistribution of charge density and orbital order, which induce emergent tetragonality of the strained SrTiO 3 . Our work thus paves a pathway for architecting functional systems in perovskite oxides using a multiple nano-design.

Ferroelectric ultrathin films have great potential in electronic devices and device miniaturization with the innovation of technology. In the process of product commercialization, understanding the domain evolution and topological properties of ferroelectrics is a prerequisite for high-density storage devices. In this work, a series of ultrathin PbTiO 3 (PTO) films with varying thicknesses were deposited on cubic KTaO 3 substrates by pulsed laser deposition and were researched by Cs-corrected scanning transmission electron microscopy (STEM), reciprocal space mapping (RSM), and piezoresponse force microscopy (PFM). RSM experiments indicate the existence of a / c domains and show that the lattice constant varies continuously, which is further confirmed by atomic-scale STEM imaging. Diffraction contrast analysis clarifies that with the decrease in PTO film thickness, the critical thickness for the formation of a / c domains could be missing. When the thickness of PTO films is less than 6 nm, the domain configurations in the ultrathin PTO films are the coexistence of a / c domains and bowl-like topological structures, where the latter ones were identified as convergent and divergent types of meron. In addition, abundant 90° charged domain walls in these ultrathin PTO films were identified. PFM studies reveal clear ferroelectric properties for these ultrathin PTO films. These results may shed light on further understanding the domain evolution and topological properties in ultrathin ferroelectric PTO films.