Invited Paper NS+AS-FrM1
Vibrational Spectroscopy in the Electron Microscope

Friday, October 25, 2019, 8:20 am, Room A222

Vibrational (phonon) spectroscopy using an electron microscope’s primary beam was introduced in 2014, and it has now progressed very significantly. The attainable energy resolution stands at ~5 meV (at 30 keV primary energy), our understanding of the electron-matter interaction has deepened, and several types of new applications have been explored.

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 in h-BN to be imaged with atomic (0.2 nm) resolution [1,2]. In elemental Si, impact scattering is dominant, and it allows atomic resolution to be reached without angular selection [3].

The angular (momentum) distribution of vibrational scattering has also been explored [4-6]. Attainable spatial resolution is then inversely related to the angular resolution. Optical and acoustic branches of vibrational scattering have been mapped in hexagonal and cubic BN, and in graphene and graphite.

Dipole scattering provides another exciting 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 confined electron beam from the sample. In this way, vibrational properties of biological and other “fragile” materials can be probed without significant radiation damage [7], and this may well revolutionize analysis in the electron microscope. The technique has recently been used to detect isotopic substitution: ¹³C vs. ¹²C at a specific site in an amino acid (L-alanine), and to map the distribution of the two species [8].

[1] C. Dwyer et al., Phys. Rev. Letts 117 (2016) 256101

[2] F.S. Hage et al., Phys. Rev. Letts 122 (2019) 016103

[3] K. Venkatraman et al., arXiv:1812.08895 (2018)

[4] F.S. Hage et al., Sci. Adv. 2018;4:eaar7495 1-5

[5] R. Senga et al., arXiv:1812.08294 (2018), Nature (2019, in print)

[6] Lovejoy et al. Proceedings 2019 M&M Conference (in print)

[7] P. Rez et al., Nature Comms 7 (2016) 10945, doi: 10.1038/ncomms10945

[8] J. Hachtel et al., Science 363 (2019) 525–528