Scattering and phase contrast

Abstract

Elastic scattering through angles larger than the objective aperture causes absorption of the electron at the objective diaphragm and a decrease of transmitted intensity. This scattering contrast can be explained by particle optics. The exponential decrease of transmission with increasing specimen thickness can be used for quantitative determination of mass thickness or the total mass of an amorphous particle, for example. The zero-loss mode of electron spectroscopic imaging allows us to increase the contrast by removing inelastically scattered electrons; alternatively, the contrast can be increased by energy filtering at higher energy losses. The superposition of the electron waves at the image plane results in interference effects and causes phase contrast, which depends on defocusing and spherical aberration, on the objective aperture, and also on the particular illumination conditions. It is possible to characterize the imaging process independently of the specimen structure by introducing the contrast-transfer function, which describes how individual spatial frequencies of the Fourier spectrum are modified by the imaging process. The contrast-transfer function of the normal bright-field mode alternates in sign and decreases at high spatial frequencies owing to the partial temporal and spatial coherence. Methods of suppressing the change in sign, by hollow-cone illumination, for example, have been proposed. The idea of holography as an image-restoration method, originally proposed by Gabor for electron microscopy, was for many years impeded by the imperfect coherence of the electron beam. With the introduction of fieldemission guns, holography can now be employed in electron microscopy for the quantitative measurement of phase shifts at the atomic scale. Many different methods can be employed for analog or digital image restoration and for the alignment of image structures in a series of micrographs. A tilt series can be used for tomography, especially for low-dose exposures of biomacromolecules. The phase shift caused by magnetic fields inside ferromagnetic domains can be exploited to image the magnetic structure of thin films or small particles; this is known as Lorentz microscopy. © 2008 Springer Science+Business Media, LLC All rights reserved.