MerlinEM-Medipix3 Detector in Transmission Electron Microscope—Applications and Opportunities

At the University of Glasgow, experiments with hybrid pixelated counting detectors started with a Medipix2 detector [1]. It was clear that direct electron detection and hardware-based electron counting offers advantageous imaging capabilities. Subsequently, a Medipix3 detector with a Merlin readout system was commercialised as the MerlinEM detector through a collaboration between the University of Glasgow and Quantum Detectors Ltd. The detector has now been applied in multiple experimental configurations. In this talk capabilities in low dose imaging, 4D-STEM, electromagnetic field imaging, ptychography, precession electron diffraction and electron energy loss spectroscopy will be demonstrated.<br/><br/>The MerlinEM detector has been applied to 4D-STEM at the prototype stage to improve magnetic imaging in Lorentz microscopy [2]. It was shown that acquiring a full diffraction pattern for each point in a scan with subsequent gradient correlation processing vastly improves the signal to noise. The sensitivity of this new method allowed imaging of magnetic fields with the highest spatial resolution available on the microscope (&lt; 1 nm field-free imaging on probe corrected Jeol ARM200 cFEG at the University of Glasgow) which was not previously accessible using the standard (annular) quadrant detection.<br/><br/>The benefits of electron counting, and dynamic range binning, can be exploited for electron ptychography. The MerlinEM detector can be used with 18800 fps in 1-bit mode (and faster when partially read-out), to achieve atomic resolution in a very low dose regime [3] and can offer significant advantages for sample stability and charging issues. Moreover, low-dose ptychography with defocused probe can be applied to CryoEM to image biological specimens like viruses [4].<br/><br/>Direct electron detection together with a relatively high number of pixels (256 x 256 or 512 x 512) and high dynamic range (max 24-bit counting) can be used in conjunction for improved results with parallel beam diffraction and precession electron diffraction. A study of very beam sensitive halide perovskite solar cell materials has utilised the features of the detector in the nano-diffraction regime [5].<br/>NanoMEGAS SPRL has integrated the MerlinEM camera into their imaging framework. Direct detection enhances the speed, stability and reliability of the precession diffraction toolkit. We will present the recent results of this work.<br/><br/>Last, Medipix3 in a 4x1 configuration (1024 x 256) offers capabilities as an EELS detector. Two MerlinEELS detectors have been installed at STEM LPS Orsay lab: on a monochromated and Cs corrected Nion HERMES-S 200 (CHROMATEM) microscope and a Cs corrected Nion USTEM200 microscope. Apart from the high DQE and MTF, there are two main advantages of the detector for EELS: high dynamic range (24-bit) for collection of the zero-loss peak area; and an exceptionally high signal to noise ratio for collecting the high loss edges.<br/><br/>[1] Mac Raighne A., et al. "Medipix2 as a highly flexible scanning/imaging detector for transmission electron microscopy." Journal of Instrumentation 6.01 (2011): C01047.<br/>[2] Krajnak M., et al. "Pixelated detectors and improved efficiency for magnetic imaging in STEM differential phase contrast." Ultramicroscopy 165 (2016): 42-50.<br/>[3] O'Leary CM., et al. "Electron Ptychography Using Fast Binary 4D STEM Data." Microscopy and Microanalysis 25.S2 (2019): 1662-1663.<br/>[4] Zhou L., et al. “Low-dose phase retrieval of biological specimens using cryo-electron ptychography.” Nat Commun 11, 2773 (2020).<br/>[5] Doherty TAS., et al. "Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites." Nature 580, 360–366 (2020)