Getting the most out of a post-column EELS spectrometer on a TEM/STEM by optimising the optical coupling

Ray tracing is used to find improved set-ups of the projector system of a JEOL ARM 200CF TEM/STEM for use in coupling it to a Gatan 965 Quantum ER EELS system and to explain their performance. The system has a probe aberration corrector but no image corrector. With the latter, the problem would be more challenging. The agreement between the calculated performance and that found experimentally is excellent. At 200kV and using the 2.5mm Quantum entrance aperture, the energy range over which the collection angle changes by a maximum of 5% from that at zero loss has been increased from 1.2keV to 4.7keV. At lower accelerating voltages, these energy ranges are lower e.g. at 80kV they are 0.5keV and 2.0keV respectively. The key factors giving the improvement are an increase in the energy-loss at which the projector cross-over goes to infinity and a reduction of the combination aberrations that occur in a lens stack. As well as improving the energy-loss range, the new set-ups reduce spectrum artefacts and minimise the motion of the diffraction pattern at low STEM magnification for electrons that have lost energy. Even if making the pivot points conjugate with the film plane gives no motion for zero-loss electrons, there will be motion for those electrons that have lost energy, leading to a false sense of security when performing spectrum imaging at low magnifications. De-scanning of the probe after the objective lens is a better way of dealing with this problem

A comparison of a direct electron detector and a high-speed video camera for a scanning precession electron diffraction phase and orientation mapping

A scanning precession electron diffraction system has been integrated with a direct electron detector to allow the collection of improved quality diffraction patterns. This has been used on a two-phase α–β titanium alloy (Timetal® 575) for phase and orientation mapping using an existing pattern-matching algorithm and has been compared to the commonly used detector system, which consisted of a high-speed video-camera imaging the small phosphor focusing screen. Noise is appreciably lower with the direct electron detector, and this is especially noticeable further from the diffraction pattern center where the real electron scattering is reduced and both diffraction spots and inelastic scattering between spots are weaker. The results for orientation mapping are a significant improvement in phase and orientation indexing reliability, especially of fine nanoscale laths of α-Ti, where the weak diffracted signal is rather lost in the noise for the optically coupled camera. This was done at a dose of ~19 e−/Å2, and there is clearly a prospect for reducing the current further while still producing indexable patterns. This opens the way for precession diffraction phase and orientation mapping of radiation-sensitive crystalline materials

Development of aberration corrected differential phase contrast (DPC) STEM

In this article we demonstrate that aberration correction for STEM probes has been achieved for field-free Lorentz STEM imaging of magnetic samples, and that, an order of magnitude improvement of spatial resolution has been obtained. We believe, that our achieved <1 nm spatial resolution is currently the best in the world for direct imaging of magnetic structure by electron microscopy

Mineralogy and petrology of fine‐grained samples recovered from the asteroid (162173) Ryugu

Samples returned from the carbonaceous asteroid (162173) Ryugu by the Hayabusa2 mission revealed that Ryugu is composed of materials consistent with CI chondrites and some types of space weathering. We report detailed mineralogy of the fine-grained Ryugu samples allocated to our “Sand” team and report additional space weathering features found on the grains. The dominant mineralogy is composed of a fine-grained mixture of Mg-rich saponite and serpentine, magnetite, pyrrhotite, pentlandite, dolomite, and Fe-bearing magnesite. These grains have mineralogy comparable to that of CI chondrites, showing severe aqueous alteration but lacking ferrihydrite and sulfate. These results are similar to previous works on large Ryugu grains. In addition to the major minerals, we also find many minerals that are rare or have not been reported among CI chondrites. Accessory minerals identified are hydroxyapatite, Mg-Na phosphate, olivine, low-Ca pyroxene, Mg-Al spinel, chromite, manganochromite, eskolaite, ilmenite, cubanite, polydymite, transjordanite, schreibersite, calcite, moissanite, and poorly crystalline phyllosilicate. We also show scanning transmission electron microscope and scanning electron microscope compositional maps and images of some space-weathered grains and severely heated and melted grains. Although our mineralogical results are consistent with that of millimeter-sized grains, the fine-grained fraction is best suited to investigate impact-induced space weathering