Scanning electron diffraction (SED), performed in a (S)TEM, is a powerful technique combining information in reciprocal space and real space to achieve nanoscale crystal cartography of materials structure. SED involves scanning a focused electron beam across a specimen and recording an electron diffraction pattern at each position to yield a 4D dataset comprising a 2D diffraction pattern at every position in the 2D scan region. Obtaining high quality data depends on fast acquisition, large dynamic range, and accurate recording of the location and intensity of diffraction spots. Here, we present SED measurements using the pnCCD (S)TEM camera taking a Ti-Fe-Mo alloy for demonstration. The large number of pixels and high readout speed of this camera enables the recording of high quality diffraction patterns in a short acquisition time. Further, using the various camera operation modes, position and intensity of diffraction spots can be determined precisely.

The pnCCD (S)TEM camera provides fast acquisition of 2D camera images using a direct detecting, radiation hard pnCCD with 264×264 pixels [1]. Routinely, the readout speed is 1000 frames per second (fps) and can be further increased by binning and windowing. For example, with the pnCCD (S)TEM camera, a 256×256 STEM dataset — where a camera image is recorded at each of the 65 536 probe positions — can be recorded in less than 70 s. The camera properties can be changed by modifying the voltages applied to the pnCCD and thus adjusted to the experimental needs [2]. Considering scanning electron diffraction experiments, which are performed at high electron beam intensities, the combination of data recorded in two different camera operation modes allows a comprehensive diffraction pattern analysis with quantitative and spatial information. In the high-charge-handling-capacity (HCHC) mode, up to 16 000 incident electrons per pixel per second can be processed for a primary electron energy of 80 keV and a readout speed of 1000 fps. In the case of higher electron rates where the amount of signal exceeds the charge handling capacity of the affected detector pixels, signal spills over into neighboring pixels. Although diffraction spots broaden, the quantitative information is preserved. In the anti-blooming (AB) mode, the amount of signal exceeding the charge handling capacity is drained from the detector preventing an overflowing of pixels. Thus, the spatial information is preserved. The data can be analysed in a number of ways [3], most simply by plotting the intensity of a subset of pixels as a function of probe position in flexible post-experiment schemes to obtain ‘virtual diffraction images’ or to perform differential phase contrast analysis.

Results are shown (Figure 1) from a Ti(40 at.%)-Fe(20 at.%)-Mo(40 at.%) alloy from which SED data was acquired in an FEI Titan G2 80-200 ChemiSTEM microscope, operated at 200 keV. A diffraction pattern was recorded for each of the 512×512 probe positions using both HCHC and AB modes of the pnCCD (S)TEM camera at a readout speed of 1000 fps. Each dataset was thus acquired with a total acquisition time of less than 5 minutes per STEM dataset. Virtual diffraction images using the AB-mode data were then formed to discriminate the two phases existing in an ultra-fine lamellar microstructure [4] in this Ti-Fe-Mo alloy.

[1] H. Ryll et al, Journal of Instrumentation, in press.

[2] J. Schmidt et al, Journal of Instrumentation 11  (2016), p. P01012

[3] P. Meock et al, Crystal Research and Technology 46 (2011), p.589-606

[4] A.J. Knowles et al, Ti-2015, proceedings of the 13th World Conference on Titanium, in press.

DNJ, RKL & PAM acknowledge: ERC grant 291522-3DIMAGE and EU grant 312483-ESTEEM2.

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