Nanometric phonon spectroscopy for diamond and cubic boron nitride

Local phonon properties near heterointerfaces or lattice defects are key for controlling various material properties such as thermal conductivity and thermoelectricity. However, high-spatial-resolution measurements of bulk phonons have hardly been established, even in typical nonpolar and polar bulk materials such as diamond and cubic boron nitride (c-BN), respectively. Here, we demonstrate nanometric measurements of phonon dispersions and mapping using a heterojunction of diamond and c-BN single crystals. To achieve high spatial/energy resolution, we use a monochromated (\ensuremath{\sim}20--30 meV) electron nanoprobe, a high-sensitivity scintillator for counting inelastically scattered electrons, and the Richardson--Lucy deconvolution. The diamond phonon dispersion is first measured under a large-scattering-vector condition, particularly for the \ensuremath{\Gamma} point. Differential scattering cross sections of optical and acoustic-phonon modes in diamond and c-BN are also measured, and the totals of their modes are on the order of ${10}^{\ensuremath{-}5}\phantom{\rule{0.16em}{0ex}}\mathrm{n}{\mathrm{m}}²$ for both crystals.