Near-field phonon nanoscopy and imaging of structural variants in mixed phase BiFeO₃

Epitaxial mixed phase BiFeO₃ (BFO) can be stabilized in a nanoscale mixture of tetragonal-like and rhombohedral-like polymorphs embedded in a tetragonal-like matrix, which provides a fertile playground for the exploration of fascinating material properties. The nanoscale phonon properties of these structural variants, which are expected to reflect information on sample properties coupled with crystal lattice vibrations, have rarely been investigated at the nanoscale. Far-field infrared studies of BFO are restricted by spatial resolution and deep probing depth, whereas scattering-type scanning near-field optical microscopy (s-SNOM) achieves a resolution of tens of nanometers with minimal optical contribution from the substrate. Here, this work combines nano-Fourier transform infrared spectroscopy (nano-FTIR) and s-SNOM imaging to reveal the minute difference in the phonon response of these structural variants. This work reports on the first direct mid-infrared imaging of such nanoscale phase variants in mixed phase BFO based on their distinct vibrational signatures acquired from the nano-FTIR. The noninvasive optical reading in the infrared can further successfully detect electrical switching of ferroelectric BFO, providing insight into future infrared photoelectric applications. This work demonstrates that scanning near-field techniques are versatile and sensitive for probing the structural and physical properties of nanoscale entities with subtle distinctions.