Highly sensitive electron detector for electron energy loss spectroscopy and zero-loss filtered ptychography

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In November 2019, IRIS Adlershof of Humboldt University of Berlin received an aberration-corrected scanning transmission electron microscope (STEM) equipped with a monochromator and electron energy loss spectrometer (EELS) system. This instrument will be utilized to characterize a large variety of materials systems, including samples produced by partners within the CRC 951 "Hybrid Inorganic/Organic Systems for Opto-Electronics" (HIOS), but also to develop new characterization techniques. The special features of this microscope include its high spatial resolution (< 0.07 nm) and its high spectral resolution in EELS (< 6 meV), making it unique in Germany. EELS spectra are currently acquired using a camera in which electrons landing in one pixel will also generate significant signal in the neighboring pixels (poor point spread function). This leads to a smearing out of the recorded spectra, especially when very weak peaks are being recorded in the vicinity of much stronger peaks. This is commonly the case when characterizing beam-sensitive organic materials by low-loss EELS, where some vibrational peaks are very close to the strong zero-loss peak. More than one year after ordering the new microscope microscope, a direct electron detector became available for its spectrometer. With this detector diffraction patterns and EELS spectra can be recorded without read-out noise and a dynamic range of > 10 Mio : 1 at a frame rate of at least 2250 frames per second, i.e. effectively more than 100 times higher than with the current detector. It can simultaneously record a zero-loss peak with more than 10 Mio counts while still being able to count individual electrons. This detector thus enormously increases the quality of the recorded data and allows the electron dose for the characterization of beam-sensitive materials to be reduced significantly. The ptychography experiments being carried out using this microscope will also benefit greatly from the enhanced speed and sensitivity of this new detector. The enhancement in sensitivity has already been shown to allow for a significant increase in the spatial resolution of ptychography reconstructions, at least for very thin materials.