Keywords: nanoparticles, barium titanate, ferroelectric
Summary:Ferroelectric materials, which possess a spontaneous and switchable electric polarization below the Curie temperature Tc, are widely used in modern electronics, such as capacitors, nonvolotile memory devices, energy storage applications and nanoelectromechanical systems. As the continuous demand for device miniaturization as well as the rapid development of flexible devices, ferroelectric nanoparticles that served as source materials or fillers to realize energy storage, piezoelectric conversion or memory applications have attracted broad interests in recent years. Ferroelectricity in nanoscale materials is the basis for the design and fabrication of integrated ferroelectric devices. Although a critical ferroelectric size ~2.4 nm of BaTiO3 (BTO) material has been calculated by the first-principles method, no experimental proof of electric polarization has been reported for such small BTO yet. The BTO nanocrystals with particles size from 2.8 nm to 8 nm were prepared by a low-cost, green and scalable sol method. We show that, BTO nanocrystals as small as 2.8 nm are still ferroelectric. Especially, ferroelectric polarization switching image and hysteresis loop show direct evidence for the ferroelectricity of the BTO nanoparticle assembly. With the particle size decreasing from 8.0 to 2.8 nm, the increasing of ferroelectric phases coincides well with the “surface phase” increase from the X-ray photoelectron spectroscopy (XPS) data, showing that surface relaxation is the origin of most non-centrosymmetrical phases in these nanocrystals. Here, we also show high prospect of these soluble ferroelectric nanoparticles in the applications of nanocomposite devices. For instance, as these nanoparticles can achieve a high polarization, we made gradated BTO/PVDF nanocomposite films using the nanocrystal sol (with grain size of 8 nm) and achieved a discharged energy density as high as 19.37 J/cm-3.