STEM and shadow-imaging of biomolecules at 6 eV beam energy

Abstract

The imaging of biomolecules in the lensless, field-emission, point-projection shadow-image electron microscope (PPM), operating between 2 and 100 eV, is reviewed. Radiation damage in this instrument is compared with X-ray and electron microscope imaging (or holography). Two regimes are considered for imaging at low voltage—one in which the inelastic mean free path λ I is much larger than the molecule, and one in which it is much shorter. A summary of measurements of λ I is given for organic films. Recent results from out point-projection instrument are presented from carbon films operating at about 100 V, and at lower resolution from purple membrane and lipid monolayers. A new instrument is described which allows energy filtered imaging, rapid exchange of tip and sample and operation in STEM and STM modes in addition to PPM. First PPM results are shown at 80 eV. A method of reconstruction for these in-line electron holograms (or coherent shadow images) is demonstrated using experimental images. By scanning the field-emission tip using STM methods, an elastically-filtered scanning transmission electron microscope (STEM) image will be obtained at sub-nanometer resolution and lower voltages. By this means the field of view of the PPM may be increased at the highest magnification. The use of solid Xenon at 55 K is proposed as an electron-transparent substrate or matrix for molecules of interest, in view of its favorable properties as a photocathode (long inelastic mean free path) and the reduction in radiation damage which occurs at low temperature. The elimination of elastic scattering in this crystalline substrate is made possible by operating at an electron wavelength greater than twice the largest interplanar spacing.