2.4 Electron Holography

In conventional HRTEM, the phase and amplitude images of the exit electron wave are mixed non-linearly and convoluted with the phase contrast transfer function. It is usually a rather sophisticated and complex process to retrieve the phase and amplitude images. Electron holography can recover the true object image. There are numerous ways of performing electron holography in TEM and STEM (Cowley, 1992). The most popular technique is off-axis holography. Holography is based on the interference and diffraction properties of waves, thereby producing a true image of an object (including amplitude and phase) without any distortion of the lenses (Fig. 2.14). The development of high-brightness high-coherence electron sources has made it possible to obtain holograms using electron waves in TEM (Gabor, 1949: Tonomura, 1993; Lichte, 1991; Tonomura, et al., 1995).

Figure 2.14: (a) Formation of off-axis electron hologram in TEM using an electrostatic biprism, and (b) Reconstruction of electron hologram using laser diffraction

The phase image can provide the distribution of an electrostatic field around a charged particle as well as the thickness projected potential image of a nanocrystal. The former can be used to extract the charge distribution in the particle (Frost, et al., 1995), and the latter is useful to determine if the particle contains a hard core or an empty cavity (Datye, et al., 1995). This measurement is based on the electron phase shift after transmission through the electrostatic potential of the specimen. The perturbation of the field to the electron wave results in a...