Vibrational spectroscopy in the electron microscope

Vibrational spectroscopy in the electron microscope (EM) has progressed remarkably since it was introduced 6 years ago (Nature 514 (2014) 209).

Phonons can be excited by fast electrons in two fundamentally different ways: by dipole scattering, which is similar to exciting the sample by infrared light, and by impact scattering, which bears a closer resemblance to neutron scattering. Dipole scattering occurs only in polar materials, and is characterized by small scattering angles (~0.1 mrad) and interaction distances of tens of nanometers. Impact scattering involves a direct interaction between the fast electron and an atomic nucleus, and leads to large scattering angles. Selecting the impact scattering (with an aperture in the diffraction plane) allows the vibrational signal to be imaged in materials such as h-BN with atomic resolution (0.2 nm).

The angular (momentum) distribution of vibrational scattering of fast electrons has also been explored. Attainable spatial resolution is then inversely related to the angular resolution. Optical and acoustic branches of vibrational scattering have been mapped in several materials while maintain a spatial resolution of a few nm. A change in the acoustic phonon signal has been observed at a single lattice defect in SiC, making it likely that the influence of lattice defects on thermal transport will be properly elucidated at last.

Dipole scattering provides another exciting experimental possibility: probing the sample from a small distance, by “aloof spectroscopy”. This approach limits the maximum energy that can be transferred to the sample with significant probability as 1/b, where b is the distance of the focused electron beam from the sample. In this way, vibrational properties of biological and other “fragile” materials can be probed without significant radiation damage, presently with about 5 meV energy resolution. This promises to revolutionize analysis in the electron microscope.