Abstract
Current generation electron monochromators employed as attachments to scanning transmission electron microscopes (STEM) offer the ability to obtain vibrational information from materials using electron energy-loss spectroscopy (EELS). We show here that in crystals, long- and short-wavelength phonon modes can be probed simultaneously with on-axis vibrational STEM EELS. The long-wavelength phonons are probed via dipole scattering, while the short-wavelength modes are probed via impact scattering of the incident electrons. 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 employing convergence angles that encompass multiple Brillouin zone boundaries enhances the short-wavelength phonon contribution to the vibrational energy-loss spectrum much more than that achieved by employing collection angles that encompass multiple Brillouin zone boundaries. 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.
Highlights
Electron monochromators have undergone significant improvements since they were first designed by Boersch et al (1962), with current generation monochromators being employed as attachments to scanning transmission electron microscopes (STEM) that can routinely form 1 Å probes (Krivanek et al, 2003, 2018)
This allows to differentiate between spectral features that arise from dipole interaction and those arising from impact scattering
A typical background-subtracted vibrational energy-loss spectrum from a h-BN nanoparticle recorded with α = 10 mrad and β = 10 mrad is shown in Figure 2a
Summary
Electron monochromators have undergone significant improvements since they were first designed by Boersch et al (1962), with current generation monochromators being employed as attachments to scanning transmission electron microscopes (STEM) that can routinely form 1 Å probes (Krivanek et al, 2003, 2018). When combined with high-resolution electron spectrometers (Lovejoy et al, 2018), these monochromators enable an energy resolution of up to 4.2 meV at 30 kV accelerating voltage (Krivanek et al, 2019). Such unprecedented improvement in energy resolution has resulted in performing vibrational spectroscopy with a 1 Å STEM probe using electron energy-loss spectroscopy (EELS). Even in the forward direction, the electron beam can excite vibrational modes that do not involve bond polarization (and cannot be strongly excited by dipole scattering), Downloaded from https://www.cambridge.org/core.
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