Vibrational spectroscopy in the electron microscope

Abstract

Vibrational spectroscopies using infrared radiation1,2, Raman scattering3, neutrons4, low-energy electrons5 and inelastic electron tunnelling6 are powerful techniques that can analyse bonding arrangements, identify chemical compounds and probe many other important properties of materials. The spatial resolution of these spectroscopies is typically one micrometre or more, although it can reach a few tens of nanometres or even a few ångströms when enhanced by the presence of a sharp metallic tip6,7. If vibrational spectroscopy could be combined with the spatial resolution and flexibility of the transmission electron microscope, it would open up the study of vibrational modes in many different types of nanostructures. Unfortunately, the energy resolution of electron energy loss spectroscopy performed in the electron microscope has until now been too poor to allow such a combination. Recent developments that have improved the attainable energy resolution of electron energy loss spectroscopy in a scanning transmission electron microscope to around ten millielectronvolts now allow vibrational spectroscopy to be carried out in the electron microscope. Here we describe the innovations responsible for the progress, and present examples of applications in inorganic and organic materials, including the detection of hydrogen. We also demonstrate that the vibrational signal has both high- and low-spatial-resolution components, that the first component can be used to map vibrational features at nanometre-level resolution, and that the second component can be used for analysis carried out with the beam positioned just outside the sample—that is, for ‘aloof’ spectroscopy that largely avoids radiation damage.

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Figure 1: Revealing vibrational signals in the electron microscope.
Figure 2: Vibrational spectra from various materials.
Figure 3: Profile showing the spatial variation of the vibrational signal.

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Acknowledgements

We thank A. Howie and J.-C. Idrobo for discussions, W. J. Bowman, J. Bruley, J. H. Butler, V. Domnich, R. A. Haber, Y. Ikuhara, M. R. Libera, D. S. Lowry and V. Nicolosi for provision of samples, J. Mardinly for help with running the instruments, our co-workers at Nion, especially N. J. Bacon, G. J. Corbin, P. J. Cramer, Z. Dellby, R. W. Hayner, P. Hrncirik, P. Phoungphidok, M. C. Sarahan, G. S. Skone, Z. Szilagyi and T. Yoo for help with the construction of the hardware, electronics and software for HERMES, and C. Trevor of Gatan Inc. for an instability-analysing script. We also acknowledge the use of facilities within the LeRoy Eyring Center for Solid State Science at Arizona State University. Financial support for the purchase of the microscopes was provided by National Science Foundation grants DMR MRI 0821796 (Arizona State University) and DMR MRI-R2 959905 (Rutgers University). Department of Energy grant DE-SC0004954 provided support for P.A.C. and microscopy performed at Arizona State University, and Department of Energy grant DE-SC0005132 provided support for P.E.B., M.J.L. and microscopy performed at Rutgers University. Additional support was provided by the Department of Energy (grant DE-SC0007694), the Natural Sciences and Engineering Council of Canada, the UK Engineering and Physical Research Council (capital equipment grant EP/J021156/1), Arizona State University, Rutgers University and Nion Co.

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Authors

Contributions

P.A.C., O.L.K. and P.R. initiated the project, P.A.C., O.L.K. and J.Z. prepared samples, T.A., P.E.B., N.D., O.L.K., M.J.L., T.C.L. and J.Z. obtained electron microscope spectra and images, P.A.C. and E.S. obtained infrared and Raman spectra, T.C.L., N.D., R.F.E. and J.Z. analysed EELS results, P.R. performed theoretical simulations, P.E.B., R.W.C., N.D., R.F.E., O.L.K., T.C.L. and P.R. advised on theoretical interpretation, P.E.B., O.L.K., T.C.L. and E.S. prepared the figures, and O.L.K. and P.R. wrote the paper. All the authors read and commented on the manuscript.

Corresponding authors

Correspondence to Ondrej L. Krivanek or Peter A. Crozier.

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Competing interests

N.D., O.L.K. and T.C.L. have a financial interest in Nion Company.

Extended data figures and tables

Extended Data Figure 1 Analysis of total system instabilities, performed by plotting the position of the ZLP on the EEL spectrometer detector as a function of time.

a, Using the Gatan Enfinium EELS and original power supplies: 50 meV peak-to-peak and 12 meV r.m.s. instability. b, As a but with improved-stability power supplies: 18 meV peak-to-peak and 4.5 meV r.m.s. instability. c, Single magnetic sector EELS with Nion power supplies: 12 meV peak-to-peak and 3 meV r.m.s. instability. Panels a and b show the position of the ZLP on the EELS detector as a function of time, marking each position of the ZLP with a single point; c shows the whole ZLP profile, together with a blue trace that follows the instantaneous position of the centre of the profile.

Extended Data Figure 2 Raman spectrum of epoxy resin.

The major peak is centred on 2,920 cm−1 = 362 meV.

Extended Data Figure 3 Fall-off of the h-BN LO phonon signal in the vacuum, probed out to 300 nm.

a, HAADF image of an edge of a BN particle of nearly constant thickness, with the probed line indicated by a yellow arrow. b, Shown are the intensity of the BN phonon signal peak (at 173 meV) along the probed line (blue diamonds), the Ko model of the phonon signal decay in vacuum (dotted blue line), the exponential model of the same (dotted red line), and approximate sample thickness (green triangles). The edge of the particle is at R = 0.

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Krivanek, O., Lovejoy, T., Dellby, N. et al. Vibrational spectroscopy in the electron microscope. Nature 514, 209–212 (2014). https://doi.org/10.1038/nature13870

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