Point-projection microscopy is an electron microscopy technique that uses a sharp metallic tip as a point source of electrons to project a magnified image of a sample, without any additional electron-optical lens elements. At sufficiently large magnification the projected image becomes an in-line hologram, which can be inverted to retrieve a real-space image of the sample object. Due to the use of low-energy electrons (typically <200 eV) this technique was shown to be capable of imaging a single elementary charge adsorbed on graphene, making it a sensitive probe of electric fields at the nanometer scale [1]. Moreover, photo-emitting the imaging electrons with an ultrafast pulsed laser enables the extension of PPM to the femtosecond domain, where it has demonstrated it can visualize the ultrafast dynamics of charge carriers with a combined spatial and temporal resolution of better than 100 nm and 30 fs, respectively [2,3].
Early publications claimed it would be possible to achieve atomic scale resolution with this technique, but so far the best reported spatial resolution is 7-8 Å. The lack of atomic resolution was mainly attributed to environmental disturbances, i.e. mechanical vibrations. However, using a semi-classical model we show that there are inherent resolution limits due to the combined effect of the finite spatial coherence length of the emitted electrons and electron-optical aberrations. The semi-classical model introduced here provides a general framework for simulating the wave-optical properties of (photo)electron sources used in electron microscopy, and evaluating the effect of the spatial coherence length, source geometry and aberrations on the emitted electron beam.
[1] T. Latychevskaia, et al., Nano Letters, 16(9), 5469–5474, (2016).
[2] M. Müller, et al., Nat. Comm., 5, 5292 (2014).
[3] F. Krečinić, et al., arXiv:1803.01766, (2018).

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