Built up field emission electron sources, while bright, have limited angle over which they are spatially coherent. For purposes such as electron holography and low-energy electron point source (LEEPS) microscopy, this angular limitation has long been a fundamental stumbling block. In a major advance, Mutus et al. demonstrate a tip which opens the spatial coherence angle to ~14 degrees. The tip is formed by field assisted nitrogen etch of tungsten to expose a single tungsten atom, while enveloping the remainder of the tip in a TiN sheath. The authors claim that the tip is extra-ordinarily stable.
The paper may be found on the arXiv. For an interesting application, see this paper (also on the arXiv).