The size-dependent ferroelectric phase transition in BaTiO₃ nanocrystals probed by surface plasmons.

A new technique for probing the temperature dependence of the dielectric constant of ferroelectric nanocrystals (NCs) using shifts in the localized surface plasmon resonance (LSPR) wavelength of gold nanoparticles attached to the surface of the ferroelectric NCs is demonstrated. This technique can selectively probe the surface of the NCs and was used to study the ferroelectric-to-paraelectric phase transition of barium titanate (BTO) nanocubes in three size regimes of 16 ± 4, 47 ± 11, and 220 ± 140 nm. Temperature-dependent Raman spectroscopy was also applied to probe the whole volume of the NCs. The LSPR-based technique revealed that the ∼16 nm BTO NCs were dominated by surface effects, and as the NC size increased bulk BTO behavior governed. This supports recent propositions about the lack of intrinsic size dependence of the transition temperature. Therefore, the surface chemistry/structure probably affected the ferroelectric behavior rather than finite size effects. A distinct phase transition at the surface characterized by a very long relaxation time was detected by the LSPR-based technique.