The resolution of images formed using scanning transmission electron microscopes (STEMs) is no longer limited by optical limits of the microscope, but instead by sample damage during acquisition. The image is formed by a highly focused beam of electrons being scanned across the specimen, with diffraction intensities recorded at each probe position. However, the beam can also cause localised heating and rearrangement of the atomic structure – and it is this movement that ultimately limits the image quality.
Electron-beam-induced specimen damage is particularly severe for weakly-bound compounds, such as battery materials, photovoltaics or pharmaceuticals. The inability to visualise the atomic structure of these materials easily is a severe impediment to research progress in their respective fields. Overcoming the beam-damage roadblock would have a profound impact across many scientific disciplines. This can be achieved by significantly reducing the number of electrons required to form an image. The mechanics of image formation in STEMs is largely unchanged since their first demonstration 80 years ago: the probe is formed by illuminating a circular aperture with a planar electron wave, brought to a focus on the sample and raster scanned. Portions of the scattered intensity are collected to determine the intensity of the pixel associated with each probe position. Electron detectors have developed significantly in recent years - while the probe-forming apertures have received less attention. A circular aperture creates a probe with broad tails, and an image with only weak contrast, thus requiring many electrons to achieve good signal-to-noise. I have previously developed methods to reshape the electron beam to generate angular momentum. In this work, I will apply related methods to increase the image contrast by intelligent shaping of the wave front. This will reduce the required electron dose, and thus enable atomic resolution STEM imaging of beam sensitive materials.
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