Ferroelectric state and polarization switching behaviour of ultrafine BaTiO3 nanoparticles with large-scale size uniformity

BaTiO3 (BTO) nanoparticles (NPs) with controlled particle sizes of 2.8, 4.5 and 8.1 nm and large-scale size uniformity are fabricated. The structure and polarization switching behaviour of the 4.5 nm BTO NPs are investigated. Structural analysis using the atomic pair distribution function (PDF) technique confirms that the particles are non-centrosymmetric at room temperature, and reveals that the ferroelectric distortions are spatially coherent. With increasing temperature, declined ferroelectric features and responses are observed by Raman scattering and second harmonic generation (SHG), suggesting a diffuse phase transition. After self-assembly into compact thin films, selectively written and read electric polarization of the nanocrystals show direct evidence for the presence of ferroelectricity, demonstrating that the local spatial coherent asymmetric nanoparticles with detectable Ti distortion can lead to sufficient macroscopic polarization in the 4.5 nm BTO NP aggregate. The results indicate that these ultrafine ferroelectric NPs are promising candidate materials for the fabrication of high-density ferroelectric memory devices.