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Vibrational spectroscopy at atomic resolution with electron impact scattering

Abstract

Atomic vibrations control all thermally activated processes in materials, including diffusion, heat transport, phase transformations and surface chemistry. Recent developments in scanning transmission electron microscopy (STEM) have enabled nanoscale probing of vibrational modes using electron energy-loss spectroscopy (EELS)1,2. Although atomically resolved analysis is routine in STEM, vibrational spectroscopy employing oscillating dipoles yields signals originating from regions tens of nanometres in size, because the scattering angles are only a few microradians3. Recently, it has been shown that energy-filtered images recorded at high scattering angles display atomic resolution4. Here we show, using conventional on-axis EELS, that non-dipole, impact scattering vibrational signals are present, and exhibit atomic resolution. This on-axis signal shows variations in the spectral peak shape and intensity as the electron probe is scanned across an individual atomic column in a Si sample. Although atomic spatial resolution in coherent elastic scattering will complicate the quantitative interpretation of spectra from crystals, the change in peak shape provides compelling evidence that the vibrational EELS excitation process is highly localized. High spatial resolution is also demonstrated in SiO2, an amorphous polar material. Our approach represents an important technical advance that will provide new insights into the local thermal, elastic and kinetic properties of materials.

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Fig. 1: Acquisition geometry and vibrational spectra from silicon.
Fig. 2: Background subtracted spectra and linescan for large collection angle data.
Fig. 3: Background subtracted spectra and linescans for small collection angle data.
Fig. 4: High-resolution vibrational spectroscopy in SiO2.

Data availability

The experimental linescans and simultaneously acquired ADF signals analysed in this work have been uploaded to the Figshare repository. The data can be found at https://doi.org/10.6084/m9.figshare.c.4621640. Files have been uploaded in the easily readable 32-bit.tif image format. The data represented in Figs. 1b, 2c,d, 3b,c and 4a,c are available as Supplementary information files.

Code availability

All of the MATLAB codes used to analyse the data are uploaded to B.D.A.L.’s GitHub page (https://github.com/bdalevin) under the repository titled ‘Background-Modelling-EELS-MATLAB-Code’.

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Acknowledgements

Financial support for K.V., B.D.A.L., P.R. and P.A.C. was provided by the US National Science Foundation (grant no. CHE-1508667) and for B.D.A.L. and P.A.C. by the US Department of Energy (grant no. DE-SC0004954). We also acknowledge the use of (S)TEM at John M. Cowley Center for High Resolution Electron Microscopy in the Eyring Materials Center at Arizona State University. P.A.C. acknowledges stimulating discussions on atomic-resolution vibrational spectroscopy with L. Allen. We acknowledge assistance from A. Singh in the use of Phonopy.

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Contributions

K.V. prepared samples. K.V. and K.M. acquired all experimental vibrational EELS data. B.D.A.L. developed software for spectral processing. K.V. and B.D.A.L. analysed the EELS results. B.D.A.L. performed simulations of convergent beam electron diffraction (CBED) patterns. K.V. performed dielectric theory simulations. P.R. developed and interpreted phonon models. P.A.C. and P.R. initiated the project and were involved in extensive discussions on the interpretation of the results. All authors were active in writing the manuscript.

Corresponding author

Correspondence to Peter A. Crozier.

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The authors declare no competing interests.

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Peer review information Nature Physics thanks Robert Klie and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–6 and references.

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Fig 2c_2_OffColumn

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Fig 2c_3_OnColumn

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Venkatraman, K., Levin, B.D.A., March, K. et al. Vibrational spectroscopy at atomic resolution with electron impact scattering. Nat. Phys. 15, 1237–1241 (2019). https://doi.org/10.1038/s41567-019-0675-5

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