Dynamic Random Scan Approach of Spectrum Imaging for Temporal Evolution of Spectroscopic Signals

Abstract

The evolution of the scanning modules for scanning transmission electron microscopes (STEM) now makes possible to generate arbitrary scanning patterns, and to approach scanning modes already used in other techniques. The typical limited scanning speed of the probe displacements, with circuits based on magnetic coils, does not necessarily warrant its use in bright-field and annular dark-field (BF/ADF) image modes. This limit does not apply in hyperspectral imaging for which longer pixel dwell-times are required. The use of a randomized sequence of the scanning matrix with a fast beam-blanker can limit dose accumulation effects when investigating irradiation sensitive materials. Unlike standard sequential scanning modes, a pre-defined pattern with fully shuffled raster order can sample the entire frame homogeneously in a given time window, such that the time evolution of the structural and spectroscopic signal can also be followed. Furthermore, inpainting processes can be applied to reconstruct the full image from partial sparse image sets [1]. With regard to sample drift correction by frame averaging [2], a series of reconstructed images obtained with the same pixel number at successive time intervals can provide any structural plus spectral changes occurring within the full hyperspectral image acquisition time [3]. In this contribution, the benefits of using this special randomized scanning mode for electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) experiments in a STEM is demonstrated, and how it enables to recover temporal information, which would be otherwise not feasible.