Three dimensional vectorial imaging of surface phonons

TL;DR

This paper introduces a novel electron microscopy technique to visualize and reconstruct the complete three-dimensional vectorial surface phonon fields of nanoscale objects, providing new insights into their electromagnetic properties.

Contribution

It demonstrates the first direct 3D vectorial imaging of surface phonons in nanostructures using a monochromated electron beam in a scanning transmission electron microscope.

Findings

Reconstructed 3D surface phonon fields of MgO nanocubes.

Visualized phonon signatures as a function of position, energy-loss, and tilt.

Revealed effects invisible to conventional mapping methods.

Abstract

While phonons and their related properties have been studied comprehensively in bulk materials, a thorough understanding of surface phonons for nanoscale objects remains elusive. Infra-red imaging methods with photons or electrons exist, which can map these vibrational excitations down to the individual atom scale. However, no technique so far was able to directly reveal the complete three-dimensional vectorial picture of the phononic electromagnetic density of states from a nanostructured object. Using a highly monochromated electron beam in a scanning transmission electron microscope, we could visualize varying phonon signatures from nanoscale MgO cubes both as a function of the beam position, energy-loss and tilt angle. The vibrational responses were described in terms of well-defined eigenmodes and were used to tomographically reconstruct the phononic surface fields of the object, disclosing effects invisible by conventional mapping. Surface phonon applications rely critically on the way the electromagnetic field is spatially and vectorially shaped, strongly influencing the optical, thermal and electronic behavior of the materials. Such 3D information therefore promises novel insights in nanoscale physical phenomena and is invaluable to the design and optimization of nanostructures for fascinating new uses.