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Time Resolved Electron Energy Loss Spectroscopy as a Tool for Controlling and Monitoring the Early Stages of Electron Beam Induced Transformations

Spatially Resolved Electron Energy Loss Spectroscopy (EELS) in the context of a Scanning Transmission Electron Microscope (STEM) has been demonstrated to be a powerful technique for studying the chemistry and electronic structure of materials at the nanometer scale. Most recent developments on the acquisition system have highly improved the sensitivity of the technique, as clearly demonstrated by selective atoms imaging [1]. The high detention efficiency of the charged coupled device (CCD) camera detector offers new possibilities to the EEL spectroscopy. With the new system, the required acquisition time for recording a satisfactory signal has been brought down to 10 to a few 100 ms for the core loss region (i.e. on the C or N Kedge). These improvements provide the possibility of studying the early stages of beam induced specimen transformation, by monitoring the Energy Loss Near Edge Structures (ELNES) changes using Time Resolved EELS (chrono-spectroscopy mode). Moreover the combination of Time and Spatially Resolved EELS with adapted mathematical methods, like Multivariate Statistical Analysis (MSA), allows us to control and monitor in situ in the electron microscope the electron beam induced transformations. In the chrono-spectroscopy mode, a probe of 0.5 nm in diameter with a current of ca 0.15-0.20 nA is fixed at a specific location on the specimen and a series of spectra are recorded with the dose as variable (with roughly dose = time x 10 e/nm s). In this paper, we illustrate this technique by studying the beam-induced processes in two different systems: