Rhinow Daniel
Max-Planck-Institute of Biophysics, Department of Structural Biology, Max-von-Laue-Str. 3, D-60438 Frankfurt, Germany.
Ultramicroscopy. 2016 Jan;160:1-6. doi: 10.1016/j.ultramic.2015.09.003. Epub 2015 Sep 8.
A variety of physical phase plate designs have been developed to maximize phase contrast for weak phase objects in the transmission electron microscope (TEM). Most progress towards application in structural biology has been made with Zernike PPs consisting of a ~30 nm film of amorphous carbon with a central hole. Although problems such as beam-induced deterioration of Zernike PPs remain unsolved, it is likely that thin film phase plates will be applied routinely in TEM of ice-embedded biological specimens in the near future. However, the thick carbon film of thin film PPs dampens high-resolution information, which precludes their use for single-particle electron cryo-microscopy at atomic resolution. In this work, an improved design for a thin film phase plate is proposed, combining the advantages of Zernike PPs and 2D materials, such as graphene. The improved design features a disc of phase-shifting material mounted on an ultrathin support film. The proposed device imparts a phase shift only to electrons scattered to low angles, whereas contrast at high resolution is generated by conventional defocusing. The device maximizes phase contrast at low spatial frequencies, where defocus contrast is limiting, while damping of information at high spatial frequencies is avoided. Experiments demonstrate that the fabrication of such a device is feasible.
为了在透射电子显微镜（TEM）中最大化弱相位物体的相位对比度，已经开发了多种物理相位板设计。在结构生物学应用方面取得的最大进展是使用了由带有中心孔的约30nm非晶碳膜组成的泽尼克相位板（Zernike PPs）。尽管诸如束流引起的泽尼克相位板劣化等问题仍未解决，但薄膜相位板很可能在不久的将来常规应用于冷冻嵌入生物标本的TEM中。然而，薄膜相位板的厚碳膜会抑制高分辨率信息，这使得它们无法用于原子分辨率的单颗粒电子冷冻显微镜。在这项工作中，提出了一种改进的薄膜相位板设计，结合了泽尼克相位板和二维材料（如石墨烯）的优点。改进后的设计特点是将一个相移材料圆盘安装在超薄支撑膜上。所提出的装置仅对散射到低角度的电子施加相移，而高分辨率下的对比度则通过传统的散焦产生。该装置在散焦对比度受限的低空间频率下最大化了相位对比度，同时避免了高空间频率下信息的衰减。实验表明制造这样一种装置是可行的。
