Journal Article

Volta potential phase plate for in-focus phase contrast transmission electron microscopy

Plitzko, Jürgen M., Baumeister, Wolfgang, Khoshouei, Maryam, Danev, Radostin, Buijsse, Bart

2014

Overview

Significance Biological electron cryomicroscopy is limited by the radiation sensitivity of the samples and the consequent need to minimize exposure to the beam. This, in turn, results in low-contrast images with a poor signal-to-noise ratio. The current practice to improve phase contrast by defocusing results in contrast transfer functions necessitating image restoration to provide interpretable data. Phase plates enable in-focus phase contrast, but the existing ones, including the thin film Zernike-type phase plate, suffer from severe limitations, such as a short usable life span, fringing artifacts, and problems in using them in automated data acquisition procedures. The Volta phase plate presented here solves those problems and has the potential to become a practical solution for in-focus phase contrast in transmission electron microscopy. We describe a phase plate for transmission electron microscopy taking advantage of a hitherto-unknown phenomenon, namely a beam-induced Volta potential on the surface of a continuous thin film. The Volta potential is negative, indicating that it is not caused by beam-induced electrostatic charging. The film must be heated to ∼200 °C to prevent contamination and enable the Volta potential effect. The phase shift is created “on the fly” by the central diffraction beam eliminating the need for precise phase plate alignment. Images acquired with the Volta phase plate (VPP) show higher contrast and unlike Zernike phase plate images no fringing artifacts. Following installation into the microscope, the VPP has an initial settling time of about a week after which the phase shift behavior becomes stable. The VPP has a long service life and has been used for more than 6 mo without noticeable degradation in performance. The mechanism underlying the VPP is the same as the one responsible for the degradation over time of the performance of thin-film Zernike phase plates, but in the VPP it is used in a constructive way. The exact physics and/or chemistry behind the process causing the Volta potential are not fully understood, but experimental evidence suggests that radiation-induced surface modification combined with a chemical equilibrium between the surface and residual gases in the vacuum play an important role.

• Publisher: National Academy of Sciences,National Acad Sciences
• Subject: Beam currents / Biological Sciences / Carbon / Contamination / cryo-electron microscopy / Diffraction / Electrical phases / Electrons / Electrostatics / Image contrast / longevity / Microscopes / Phase contrast imaging / Phase shift / Physical Sciences / Radiation / Thin films / Transmission electron microscopy