The handmade SE-ADM imaging system was attached to a field-emission SEM (SU5000, Hitachi High-Tech Corp, Japan) (Fig. 1A). The liquid sample holder was mounted onto the SEM stage and the detector terminal was connected to a pre-amplifier under the holder [13] (link), [14] (link). The electrical signal from the pre-amplifier was fed into the external input of SEM. The SEM images (1280 × 1020 pixels) were captured at 1000–20,000 × magnification with a scanning time of 40 s, a working distance of 7 mm, an EB acceleration voltage of 6–8 kV and a current of 1–10 pA. High-resolution SE-ADM images were processed from the LPF signal and scanning signal using the image-processing toolbox of MATLAB R2021a (Math Works Inc. Natick, MA, USA). The original SE-ADM images were filtered using a 2D Gaussian filter (GF) with a kernel size of 11 × 11 pixels and a radius of 1.2 s. Background subtraction was achieved by subtracting SE-ADM images from the filtered images using a broad GF (400 × 400 pixels, 200σ).
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