Fast Imaging with Inelastically Scattered Electrons by Off-Axis Chromatic Confocal Electron Microscopy

Abstract

We introduce off-axis chromatic scanning confocal electron microscopy, a technique for fast mapping of inelastically scattered electrons in a scanning transmission electron microscope without a spectrometer. The off-axis confocal mode enables the inelastically scattered electrons to be chromatically dispersed both parallel and perpendicular to the optic axis. This enables electrons with different energy losses to be separated and detected in the image plane, enabling efficient energy filtering in a confocal mode with an integrating detector. We describe the experimental configuration and demonstrate the method with nanoscale core-loss chemical mapping of silver (M4,5) in an aluminium-silver alloy and atomic scale imaging of the low intensity core-loss La (M4,5@840 eV) signal in LaB6. Scan rates up to 2 orders of magnitude faster than conventional methods were used, enabling a corresponding reduction in radiation dose and increase in the field of view. If coupled with the enhanced depth and lateral resolution of the incoherent confocal configuration, this offers an approach for nanoscale three-dimensional chemical mapping.

Seeing structural evolution of organic molecular nano-crystallites using 4D scanning confocal electron diffraction (4D-SCED)

“…The chromatic defocus of inelastically scattered electrons with energy loss of carbon K -edge (Δ E = 285 eV) is Δ f C = C c Δ E / E 0 ≈ 3.5 μ m for E 0 = 200 keV and typical chromatic aberration coefficient C C = 2.5 mm 35 , in the same order to the defocus applied in SCED. In SCED, we use on-axis illumination therefore the chromatically defocused disks are concentric to the diffraction spots 36 , thus do not influence the accuracy to locate the Bragg spots. Nevertheless, this results in a reduced diffraction SBR, with the degree of reduction additionally depending on sample thickness.…”

Four-Dimensional Scanning Transmission Electron Microscopy (4D-STEM): From Scanning Nanodiffraction to Ptychography and Beyond

“…d: Double aberration-corrected images of electron probe in real-space after passing through crystallizing gold, from Etheridge et al (2011). e: Diagram showing how off-axis SCEM can be used to measure inelastic scattering, experimental comparison of on- and off-axis SCEM imaging of carbon with 300 eV edge, compared with EELS measurements of the same edge, adapted from Zheng et al (2014). …”

“…The same group also demonstrated direct measurements of chromatic aberration coefficients in STEM in Zheng & Etheridge (2013). Another follow-up paper by Zheng et al (2014) introduced the concept of “off-axis” SCEM, which is shown in Figure 11e. By tilting the incident STEM probe and varying the post-scattering defocus, they use the chromatic aberration of the system in order to create a 3D dispersion which allows for spectroscopic measurements of the STEM probe without a spectrometer (i.e., without energy dispersive electron optics).…”

Quantitative Scanning Transmission Electron Microscopy for Materials Science: Imaging, Diffraction, Spectroscopy, and Tomography

“…Phase plates can also be used to produce vortex STEM probes (8), approximately linear phase contrast transfer functions (9, 10), or multiple beams for STEM holography (12) and to correct probe aberrations (11) or add phase diversity to ptychographic reconstructions (158). Some STEM experiments use nonstandard optical setups such as a confocal configuration, which can be used to measure 3D sample information (159), to measure the intensity distribution of STEM probes in real space (160), or to characterize inelastic scattering (161). 4D-SCED has recently been extended to orientation mapping of beam-sensitive organic samples (138).…”