Nanoscale Analysis of Phonons, Polaritons, and Molecular Vibrations in Complex Materials*

Electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) is a powerful tool that allows for direct nanoscale access to the localization of collective excitations in complex materials. With modern monochromators the energy resolution of EELS can reach down into the mid-infrared regime providing access to infrared quasiparticles such as phonons, phonon-polaritons, and molecular vibrations while retaining an Ångstrom scale probe. This unprecedented combination of spatial and spectral resolution enables a host of new experiments that just a decade ago would’ve been impossible.

 

Here, I will present several examples of how to use monochromated STEM-EELS in complex system to achieve results that would not be accessible without both aspects of the simultaneous ultrahigh spatial/spectral resolution. In hexagonal boron-nitride, I will show how the dispersion of highly confined hyperbolic phonon polaritons is modified by nanoscale heterogeneity. In oxides superlattices, I will show how emergent phonons are modified by the superlattice period to correlate atomic structure directly to the phonon response and the macroscopic properties of the material.  Lastly, in cross sections of the vascular system of the cucumber, I will identify localized signatures of the molecular vibrational modes to demonstrate how we can perform robust nanoscale vibrational spectroscopy in beam-sensitive whole-cell biological specimens.