Fiber/Biprism Filament in Möllenstedt–Düker Biprism
for Off-axis Electron Holography
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The charged wire in Möllenstedt–Düker biprism in off-axis electron holography system can be made from a gold-coated quartz (glass) fiber or a tungsten wire at diameters of 500 nm to 1 µm. 

Lehmann and Lichte [1] described the detail of the procedure to produce very thin glass fibers, which were mounted on a specially designed selected area aperture, at University of Tübingen. A quartz rod was fist melted over a hydrogen flame, and then pulled apart to produce a small filament in diameter of 50–100 mm. This filament was then pulled away from the tip by putting the filament into the upward-jetting gas flame, resulting in a slightly bigger filament in diameter of 300–500 nm. This glass wire was then coated with a thin layer of Pt-Ir to enhance adhesion between the quartz filament and the subsequent thicker layer of Au. The coated Au layer was used to have good electrical conduction. The Au-coated filament was finally mounted on the selected area aperture holder by using silver glue. Note that in reality, the Au coated biprism filament can be damaged by the electron beam.

Zheng et al. [4] used a thin platinum (Pt) wire in diameter of 0.6 µm and in length of 5 mm as the electrostatic biprism. The biprism was mounted in an individual retractable holder close to the first image plane of the objective lens.

Unlike the negligible distortions in most CTEM analyses, distortions extremely affect off-axis electron holograms as the phase information is encoded in the bending of the interference fringes [1-3]. The origin of the distortions in off-axis electron holograms are projector lenses, local variations in TEM specimen thickness, charging of the biprism filament, and shear distortion of the fiber optic of the CCD camera. [1]









[1] Michael Lehmann and Hannes Lichte, Tutorial on Off-Axis Electron Holography, Microsc. Microanal. 8, 447–466, 2002.
[2] Rau, W.-D., Lichte, H., Völkl, E. & Weierstall, U. (1991). Real-time reconstruction of electron-off-axis holograms recorded with a high pixel CCD camera. J Comput-Assist Microsc 3, 51–63.
[3] Völkl, E. & Lehmann, M. (1999). The reconstruction of off-axis electron holograms. In Introduction to Electron Holography, Völkl, E., Allard, L.F. & Joy, D.C. (Eds.), pp. 125–151. New York: Kluwer Academics/Plenum Publishers.
[4] C. L. Zheng, K. Scheerschmidt, H. Kirmse, I. Häusler, W. Neumann, Imaging of three-dimensional (Si,Ge) nanostructures by off-axis electron holography, Ultramicroscopy 124 (2013) 108–116.




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