Use of Ultrananocrystalline Diamond as a Phase-contrast Aperture Material

Boron-doped “ultrananocrystalline diamond” (UNCD), available as a 2 μm-thick layer on a “diamond on insulator” wafer (Advanced Diamond Technologies, Romeoville, IL), is investigated as a potential material for making devices that can be placed in the back focal plane of the objective lens of a TEM, the goal being to produce high contrast for in-focus images of weak phase objects. The resistance of diamond to knock-on damage when irradiated, even at 300 keV, is an attractive feature. In addition, diamond (indeed, carbon in general) is not expected to form a native oxide that is thicker than a monolayer. The ultrananocrystalline form of diamond is favored over a microcrystalline form because unwanted, “patchy” variations in surface potential (due to differences in the work-function for different crystal facets) are expected to decay more rapidly, i.e. on a length scale comparable to the domain size. We believe, however, that care must be taken to compensate for the contact potential (Galvani potential) between the UNCD device layer and the supporting (boron-doped) silicon wafer (handle).