Interpretable and Efficient Interferometric Contrast in Scanning Transmission Electron Microscopy with a Diffraction-Grating Beam Splitter

Letter

Interpretable and Efficient Interferometric Contrast in Scanning Transmission Electron Microscopy with a Diffraction-Grating Beam Splitter

Tyler R. Harvey, Fehmi S. Yasin, Jordan J. Chess, Jordan S. Pierce, Roberto M. S. dos Reis, Vasfi Burak Özdöl, Peter Ercius, Jim Ciston, Wenchun Feng, Nicholas A. Kotov, Benjamin J. McMorran, and Colin Ophus

Phys. Rev. Applied 10, 061001 – Published 26 December 2018

Abstract

Efficient imaging of biomolecules, two-dimensional materials, and electromagnetic fields depends on retrieval of the phase of transmitted electrons. We demonstrate a method to measure phase in a scanning transmission electron microscope (STEM) using a nanofabricated diffraction grating to produce multiple probe beams. The measured phase is more interpretable than phase-contrast scanning transmission electron microscopy techniques without an off-axis reference wave, and the resolution could surpass that of off-axis electron holography. We apply this technique, called STEM holography, to image nanoparticles, carbon substrates, and electric fields. The contrast observed in experiments agrees well with contrast predicted in simulations.

Received 6 August 2018
Revised 5 November 2018

DOI:https://doi.org/10.1103/PhysRevApplied.10.061001

Physics Subject Headings (PhySH)

Electron beams & optics

Electron diffraction Holography Interferometry Scanning transmission electron microscopy

Atomic, Molecular & OpticalCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Tyler R. Harvey^1,2, Fehmi S. Yasin¹, Jordan J. Chess¹, Jordan S. Pierce¹, Roberto M. S. dos Reis³, Vasfi Burak Özdöl³, Peter Ercius³, Jim Ciston³, Wenchun Feng^4,5, Nicholas A. Kotov^4,6, Benjamin J. McMorran^1,*, and Colin Ophus³

¹Department of Physics, University of Oregon, Eugene, Oregon 97403, USA
²Georg-August-Universität Göttingen, D-37077 Göttingen, Germany
³National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁴Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
⁵US Food and Drug Administration, Silver Spring, Maryland 20993, USA
⁶Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA

^*mcmorran@uoregon.edu

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Issue

Vol. 10, Iss. 6 — December 2018

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Images

Figure 1
(a) A schematic of off-axis electron holography with a biprism. One plane wave is passed through the specimen (brown) and an electrostatic biprism (black dot) interferes this wave with a second plane wave passed through vacuum. (b) A schematic of STEM holography. A diffraction grating in the condenser system produces multiple beams at the specimen (brown). An aperture (black) admits one beam that interacted with the specimen and one passed through vacuum. The projector system combines these beams into a hologram.
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Figure 2
(a) Measured interference fringes formed by two beams in vacuum. (b) An enlargement of the region in (a) highlighted by a white rectangle (same color bar). (c) A line profile (black) with a 95% confidence interval (gray) of interference fringes in the center of (a). (d) A micrograph of the beams used for the experiment. The beam separation is $| x_{0} | = 120 nm$ .
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Figure 3
A comparison of the calculated phase-contrast transfer functions for several phase-contrast STEM techniques. Unlike MIDI-STEM [18], PMIDI-STEM [16], and ptychography [17], STEM holography produces efficient contrast as the spatial frequency approaches zero (see (10)).
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Figure 4
A comparison of micrographs recorded by STEM holography and ADF-STEM on lacey carbon and semiconducting nanoparticles. (a),(d) The amplitude measured by STEM holography. (b),(e) The phase measured by STEM holography; the line profiles in (g) and (h) are taken along the white arrow and averaged over the width of the box. (c),(f) The simultaneously acquired ADF. (g) A comparison of the phase (red) with the ADF (blue) on a lacey carbon edge. Inset: an enlargement to show the noise levels. (h) The same as (g) on the edge of a semiconducting nanoparticle.
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Figure 5
An Au nanoparticle with a reference beam on a uniform ultrathin carbon substrate. (a) The amplitude from STEM holography. (b) The phase from STEM holography. (c) The phase (color) and amplitude (brightness) shown together offer more information than either alone. The color wheel maximum brightness corresponds to the maximum amplitude in the image and black corresponds to zero amplitude. (d) The simultaneously acquired ADF signal.
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Figure 6
The simulated STEM-holography dataset with Au nanoparticles on a carbon wedge and a reference beam in vacuum. (a) The amplitude. (b) The phase. (c) The ADF from the same dataset. (d) The projected potential used to generate the dataset.
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