THz nanocrystal acoustic vibrations from ZrO2 3D supercrystals†
We report sharp low-frequency Raman spectral features of supercrystals synthesized via the “benzyl alcohol route” and consisting of either yttrium-stabilized or pure zirconia (ZrO2) nanocrystals. In situ formed benzoate species control the nanocrystal growth and act as organic glue leading to the assembly of the nanocrystals in highly ordered 3D supercrystals. We attribute some Raman peaks to THz acoustic vibrations of individual nanocrystals which are only weakly coupled due to the strong acoustic mismatch between the capping ligands and the nanocrystals. Peak positions are consistent with nanocrystal sizes estimated from transmission electron microscopy and X-ray powder diffraction studies. Narrow size distribution, together with uniform morphological and structural features of the nanocrystals, leads to sharp Raman peaks in the THz spectral region. After removing the ligands by annealing, the acoustic vibrations of the nanocrystals couple resulting in an increase of their frequency. These findings suggest that 3D supercrystals of zirconia nanocrystals can be used for manipulating THz phonons.