Temperature dependent local structure coherence of surface-modified BaTiO3 nanocubes

Surface functionalized barium titanate (BaTiO3) nanocrystals have been explored for highly tunable chemical and electronic properties, potentially of use in ceramic-polymer composites for flexible ferroelectric device applications, directed synthesis of ferroelectric thin films or other nano-architectures, and other potential applications. The detailed temperature dependent local structure evolution of BaTiO3 nanocubes capped with nonpolar oleic acid (OA) and polar tetrafluoroborate (BF4-) ligands are investigated using in situ synchrotron X-ray diffraction and pair distribution function (PDF) analysis, in conjunction with piezoresponse force microscopy (PFM) and Ba-137 nuclear magnetic resonance (NMR) spectroscopy measurements. Diffraction analysis reveals that nanocubes capped by polar BF4- ligands undergo sharper ferroelectric to paraelectric phase transitions than nanocubes capped with nonpolar OA ligands, with the smallest similar to 12 nm nanocubes displaying no transition. Local non-centrosymmetric symmetry is observed by PDF analysis and confirmed by NMR, persisting across the phase transition temperature. Local distortion analysis, manifested in tetragonality (c/a) and Ti off-centering (z(Ti)) parameters, reveals distinct temperature and length-scale dependencies with particle size and capping group. Ferroelectric order is increased by polar BF4- ligands, which is corroborated by an enhancement of PFM response.