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Surface vs. Bulk Phonons in Off-axis EELS

Acquiring electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) away from optic axis has been exploited to avoid surface phonons near q=0, which leads to delocalized energy loss signal and poor spatial resolution 1 . This approach has also been used for phonon dispersion measurement 2, 3 when high enough momentum resolution is provided. With a large momentum transfer, contribution from surface modes is expected to vanish, leaving bulk phonons the only signal in an off-axis EEL spectrum. However, the small energy difference between surface and bulk phonons at q=0 makes identifying their contribution a non-trivial task 4 . In this work, we have studied phonons in h-BN and SrTiO 3 to re-examine the surface and bulk contributions in phonon spectra by off-axis EELS. The EELS experiments were performed using a monochromated Nion UltraSTEM 200 operating at 60 kV. We have used a convergence half angle of 3 mrad in our experiments. The spatial, energy, and momentum resolutions are about 3 nm, 12 meV, and 0.4 , respectively. With this setup, spatial and momentum resolved EEL spectra are taken from h-BN and SrTiO 3 .