The investigation of multi-reflection images from small crystallites using dark-field electron microscopy

An investigation of multi-reflection dark-field imaging with the conventional transmission microscope shows that diffraction data can be obtained from small crystalline particles less than 100 Å in diameter. Particles down to a few unit cells in diameter reveal their crystal habit giving the planar spacing and symmetry for a given orientation. In the case of Au, single crystallites as small as 18 Å, in diameter gave good data. This method uses magnifications intermediate between diffraction patterns and high-resolution images, so large fields of particles are imaged with low beam intensities and hence minimized radiation dose and specimen damage. The multi-reflection technique also yields much lower background signals from the support films than selected-area diffraction and allows even faint diffraction images of individual crystallites to be observed. Au crystallites on amorphous carbon films were employed to demonstrate several features of the method. Lattice spacings as small as 2 and 1.4 Å, were measured by observing the shift of the images at two defocus values with known difference. Several individual particles produced multiple diffraction images with high degrees of symmetry (100, 110 and 111 orientations). Useful information is obtained from the shifts in individual diffraction images that are functions of spherical aberration and defocus and the fine structure in the images similar to bend contours caused by beam divergence. The multi-reflection technique was used to study individual iron cores of ferritin molecules. For single-crystal cores, six and fourfold crystal symmetries were observed with lattice spacings consistent with spacings of 2.55 and 2 Å, reported by X-ray and electron diffraction. Many iron cores were found to be either poly crystalline or amorphous structures, with single-crystal cores being rather rare, implying that the core is a poorly crystallized structure.