Minimizing electrostatic charging of an aperture used to produce in-focus phase contrast in the TEM

Abstract

Microfabricated devices designed to provide phase contrast in the transmission electron microscope must be free of phase distortions caused by unexpected electrostatic effects. We find that such phase distortions occur even when a device is heated to 300 °C during use in order to avoid the formation of polymerized, carbonaceous contamination. Remaining factors that could cause unwanted phase distortions include patchy variations in the work function of a clean metal surface, radiation-induced formation of a localized oxide layer, and creation of a contact potential between an irradiated area and the surround due to radiation-induced structural changes. We show that coating a microfabricated device with evaporated carbon apparently eliminates the problem of patchy variation in the work function. Furthermore, we show that a carbon-coated titanium device is superior to a carbon-coated gold device, with respect to radiation-induced electrostatic effects. A carbon-coated, hybrid double-sideband/single-sideband aperture is used to record in-focus, cryo-EM images of monolayer crystals of streptavidin. Images showing no systematic phase error due to charging are achievable under conditions of low-dose data collection. The contrast in such in-focus images is sufficient that one can readily see individual streptavidin tetramer molecules. Nevertheless, these carbon-coated devices perform well for only a limited length of time, and the cause of failure is not yet understood.

Towards an optimum design for electrostatic phase plates

“…Although argon injection did not reduce image artefacts caused by BPPs, it immediately and significantly modified the charging pattern. The experiments demonstrate that physical and chemical surface properties of PPs are critically important for PP charging, in accordance with other work [15]. Ionized argon molecules directly interact with trapped charges on the BPP surface, clean the BPP surface, or even modify the surface structure.…”

“…Numerically reproduced charging artefacts were stronger than previously assumed [1,19], with phase offsets as large as 15π in the case of both charged supporting rods and a charged surface around the central electrode ring. Empirical approaches to reduce charging, such as coating of PPs with a thin film of amorphous carbon [15,19], dissipating trapped charges Fig. 13.…”

“…As described above, charging limits the routine application of BPPs in practice. Coating PPs with a thin layer of amorphous carbon reduces charging considerably, as has been shown for aperture-type [15] and single coaxial PP devices [19]. After coating an electrostatic PP with amorphous carbon, carbon needs to be removed from the short-circuited electrodes.…”

“…Non-reproducible charging is a severe problem even with the simple design of thin-film PPs. Although the physical origins of phase-plate charging are not fully understood, empirical approaches to reduce charging of PPs have been at least partially successful [1,15,16]. However, charging of electrostatic PPs has not been addressed in detail.…”

Generalization of the Matsumoto–Tonomura approximation for the phase shift within an open aperture

“…An extreme example is shown in Figure 5 of (Glaeser et al, 2013), where the FFT of an image shows concentric oscillations of contrast that are centered at the point where the focused electron beam had been intentionally allowed to touch the edge of the aperture. Another extreme example is shown in Figure 5 of (Marko et al, 2011), where the FFT of an image shows a radial dependence that is strongly different from what can be accounted for by defocus, astigmatism, and spherical aberration.…”