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

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