Scan Engine

Here we report on work by Marcel Tencé et al. on real-time EELS hyperspectral imaging, using the Quantum Detectors Scan Engine in conjunction with the MerlinEELS direct electron detector, taking advantage of high-speed acquisition and noiseless readout. This detector enables hyperspectral images to be recorded in sequence while adjusting the scanned area between each spectrum to remain on the same area of the sample. To achieve this, the group used cross-correlations between successive HAADF images acquired simultaneously with the EELS hyperspectral images to estimate the drift occurring during each scan.

This approach eliminates the need for a separately defined drift-calculation image as is commonly used at present, but which often presents difficulties since a suitable region is not always available.

Figure 1 shows an HAADF image and elemental maps of the Sc L edge (ca. 400 eV) and Dy M edge (ca. 1300 eV) signals from a conventional hyperspectral acquisition without drift correction on an ScDyO3 crystal, oriented along the [100] pseudo-cubic axis. The dwell time was 20 ms, giving a total acquisition time of 444 s. Considerable drift occurred, producing distortions in the images.

Figure 2 shows the equivalent results using their approach with the Quantum Detectors Scan Engine. Here, 100 hyperspectral acquisitions, each with a nominal pixel dwell time of 200 μs, were collected from the same area and summed. The total signal acquisition time is thus the same as in Figure 1 and the count levels were similar. However, the resulting image and maps are essentially free of drift distortion. This method is the way forward for EELS hyperspectral imaging.