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High energy electron beams can now be routinely focused to 1-2 {\AA} andoffer the ability to obtain vibrational information from materials usingmonochromated electron energy-loss spectroscopy (EELS) in a scanningtransmission electron microscope (STEM). Here it is shown that long and shortwavelength phonons can be probed simultaneously with on-axis vibrational STEMEELS. The advantage of using focused electron beams is that the shortwavelength phonons are probed via impact scattering while the long-wavelengthmodes are probed via dipole scattering. The localized character of theshort-wavelength modes is demonstrated by scanning the electron beam across theedge of a hexagonal boron nitride nanoparticle. It is found that the conditionrequired for high spatial resolution STEM imaging also correlates with thecondition to enhance the short-wavelength phonon contribution to thevibrational energy-loss spectrum. Probing short-wavelength phonons at highspatial resolution with on-axis vibrational STEM EELS will help develop afundamental connection between vibrational excitations and bonding arrangementsat atomic scale heterogeneities in materials.

Simultaneous excitation of phonons at the center and boundaries of Brillouin zones with high energy electron beams