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

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 ($M_{4,5}$) in an aluminium-silver alloy and atomic scale imaging of the low intensity core-loss La ($M_{4,5}@840\mathrm{eV}$) signal in ${\mathrm{LaB}}_6$. 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.

Figure 1

Optical diagram for (a) off-axis and (b) on-axis chromatic scanning confocal electron microscopy. $E_0=\text{primary}\text{beam}\text{energy}$ and $E_c=\text{confocal}\text{energy}$.

Figure 2

(a)–(b) Schematic of the energy dependence of the intensity distribution in the detector plane for (a) off-axis and (b) on-axis SCEM. $E_1$ indicates electrons of an arbitrary energy less than $E_0$. (c)–(d) Probe image after scattering from an amorphous carbon film with the carbon K edge, $\mathrm{\Delta }E=300\mathrm{eV}$, focused at the confocal point (arrow) for (c) off-axis and (d) on-axis SCEM. (e)–(f) Corresponding EEL spectrum (measured with a postcolumn spectrometer) collected with an entrance aperture [white circle in (c)–(d)] centered on the confocal point for (e) off-axis and (f) on-axis SCEM.

Figure 3

Nanoscale off-axis SCEM chemical mapping of Ag precipitates in an Al-Ag alloy. (a)–(c) Off-axis SCEM images with energy window centered at 324.9, 355.5, and 442.2 eV, respectively. (d)–(f) The corresponding energy loss spectra that is intercepted by the detector. (g) A conventional STEM-EEL spectrum of this Ag-Al alloy with the energy windows used in (a)–(c) overlayed for comparison. (h) The background subtracted signal generated by applying the three window technique to the images in (a) to (c). [No spectrometer is used in (a)–(c) or (h)].

Figure 4

Atomic resolution off-axis SCEM map of lanthanum $M_{4,5}$ ($\sim 840\mathrm{eV}$) core loss electrons in ${\mathrm{LaB}}_6$. (a) Conventional STEM-EELS spectrum of the $M_{4,5}$ edge in ${\mathrm{LaB}}_6$ for comparison. (b) Off-axis SCEM spectrum collected with an effective entrance aperture of 5.68 nm. (c) Atomic resolution off-axis SCEM image using the La $M_{4,5}$ edge (no spectrometer is used).