Abstract:
The coherence of an electron beam is crucial for the performance of electron microscopy, coherent diffractive imaging, holography, and many other advanced instrumentation methods that rely on the phase coherence of electron waves. In the first part of this presentation, I will talk about several types of thermally stable nanoemitters for electron beams developed in our group. Their lifetimes are long, they can be generated or regenerated by annealing, and their structures remain the same after each regeneration. We have demonstrated that electron beams emitted from noble-metal covered W(111) single-atom tips possess high brightness and full spatial coherence. However, the energy spread is 0.3-0.4 eV, which is not smaller than that of a typical metal cold-field emitter. A niobium nanoemitter, a nano-protrusion with a small flat top region ~1 nm in width grown on the Nb(100) facet, has been prepared. Electron beams emitted from it will probably possess both high spatial and temporal coherence if the emitter is cooled below its superconducting temperature. In the second part of the presentation, I will discuss possible applications of these highly coherent electron sources. Over the past several years, we have been developing low-voltage (80-5000 V) coherent electron imaging techniques. An advantage of this approach is that there is a possibility to achieve diffraction-limited resolution without the need to fabricate a high-quality lens. Coherent diffractive imaging has been successfully demonstrated in optical microscopy and x-ray microscopy. There are relatively fewer experiments in electron microscopy mainly because optical lasers and synchrotron light sources are usually with better coherence than electron sources. Now we have demonstrated full spatial coherence for single-atom electron sources. Thus coherent imaging based on single-atom electron sources is very promising to reach atomic resolution even for nonperiodic structures like biological molecules. Our ultimate goal is to achieve high-contrast and high-spatial-resolution imaging of two-dimensional materials and organic molecules under low-dose conditions.