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 it 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 it leads to large scattering angles. Selecting the impact scattering (with an aperture in the diffraction plane) allows the vibrational signal from a single impurity atom in a 2D material to be detected, at atomic resolution.

The angular (momentum) distribution of vibrational scattering of fast electrons has also been explored. Optical and acoustic branches of vibrational scattering have been mapped in several materials while maintain a spatial resolution of a few nm, with an angular resolution proportional to 1/(spatial resolution). 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 the analysis of beam-sensitive materials in the electron microscope.