Direct Observation of Core–Shell Structures in Individual Lead Titanate Ferroelectric Nanostructures by Tip‐Enhanced Refractive Index Mapping

Abstract

Ferroelectrics undergo a size-driven phase transition at the nanoscale below which the spontaneous polarization, their defining property, irrevocably ceases. This threshold often referred to as the superparaelectric limit has tremendous technological relevance in an era of progressing integration. Just as the balance of short-range elastic and long-range electrostatic ordering in bulk, the critical size depends on temperature. Room-temperature tip-enhanced Raman spectroscopy (TERS) imaging of individual lead titanate (PbTiO 3 ) nanoislands is reported with a spatial resolution of ≈3 nm. Monitoring the spectral shift of the gold-tip enhanced luminescence, which depends on the local refractive index, images grains composing the nanoislands. The wavelength of the enhanced luminescence shifts between the grains and their boundaries indicating the predicted core-shell structure of ferroelectric and paraelectric phase. The shear force configuration rules out the distance dependence of capacitive plasmonic coupling between tip and substrate as the origin of the observed shift. As the reported temperature-changes in nonresonant TERS do not account for noticeable thermal effects, the underlying, even though weak, tip-enhanced Raman spectrum of the grain core reflects PbTiO 3 close to the ferroelectric-to-paraelectric phase transition which is primarily related to the finite size of the grains.

Topography-induced variations of localized surface plasmon resonance in tip-enhanced Raman configuration

“…These studies were performed with a plasmonic gold tip in TERS configuration. [9][10][11] In this configuration, a laser beam of specific wavelength was focused at the apex of a gold tip, which in turn exhibited a detectable signal of its own. This tip signal is the result of two contributions.…”