Holography, electron

The hologram is subsequently illuminated by a collimated laser beam and an exact three-dimensional image is produced in a diffracted beam. An additional image, called a conjugate image , is also produced with the same amplitude, but the opposite phase. 

In electron interferometry, there are often cases where great precision is required, for example, to measure the thickness distribution in atomic dimensions or to observe microscopic electromagnetic fields. To achieve such precision, phase amplification techniques peculiar to holography have been developed and used. Using these techniques, phase shifts as small as 1/100 of the wavelength can be detected [2.5]. 

Hologram Lens Aperture Fig. 2.2. Optical reconstruction system for interference microscopy. 

When a parallel electron beam is incident to an electromagnetic field, the electron beam is deflected, or phase shifted. The phase shift AiS/his calculated from the Schrodinger equation as. 

When many holograms are formed at different ineident angles of the eleetron beam transmitted onto a speeimen, a three-dimensional view of an objeet ean be obtained numerieally using methods similar to those of X-ray eomputer tomography [2.12]. An arbitrary view seen from any direetion, or any eross-seetion ean be obtained. An example of two views of latex spheres reeon-strueted from 24 different holograms is shown in Fig. 2.5. 

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Electron Holography applied to Size-Controlled Nanocatalysts... 

E. Volkl, L. F. Allard, D. C. Joy (eds.). Introduction to Electron Holography, Kluver Academic/Plenum Publishers, New york, 1999. 

A. Tonomura, Electron Holography, Springer Series in Optical Sciences, Springer, Berlin, 1999. 

CNTs are also valuable as field emitters because they have a small virtual source size [30], a high brightness, and a small positive temperature coefficient of resistance [31]. The latter means that they can run hot under high emission currents, but not go into thermal runaway. Emission from nanotubes can be visualized by electron holography in a TEM [32],... 

Several other techniques, such as electron holography (Lichte, 1986) and convergent beam electron diffraction (CBED) have also been developed for structure analysis. CBED can provide information not only on the lattice parameters and the S5mimetry of crystals, but also accurate structure-factor amplitudes and phases (Hoier et al, 1993). Accurate structure factor determination by CBED can provide information on the location of valence electrons. However, it is more favourable for thick crystals (> 500 A) with small unit cells (< 10 A). Structure analysis by CBED has been summarized in two review articles (Spence, 1993 Tanaka, 1994). 

Equations (8.27) and (8.28) indicate that d>(0) will tend to be more positive for a crystal containing heavier atoms. This is confirmed by experimental measurements of (0) using electron-beam techniques. Measurements by electron holography, for example, give the following values for a number of crystals Si,... 

Figure 6. (Left) Schematic showing basic TEM configuration for off-axis electron holography and (b) photograph showing electron microscope suitably equipped for recording holograms of magnetic materials.

Li et al. made extraordinarily perfect artificial c-axis twist bicrystal junctions.[3] These junctions were extensively characterized using high resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy, and low energy electron diffraction, etc., and the results were compared with computer simulations. [4] More recently, off-axis electron holography provided compelling evidence of the remarkable atomic perfection and reproducibility of the twist junctions.[5]... 

Key words Metal organic vapor phase epitaxy (MOVPE), dislocations, electrical activity of dislocations, transmission electron microscopy, electron holography... 

The present study is focused on dislocations in ZnO epilayers grown by MOVPE. The distribution of edge, mixed and screw dislocations was analyzed for layers grown under different MOVPE conditions using TEM. Electron holography was applied to study the electrical activity of threading dislocations. 

Electron holography is a TEM technique that allows the retrieval of the phase of the electron wave. This provides the possibility to study the interaction of the incident electrons with the electrostatic potential of a charged dislocation with high spatial resolution. Fig.6 indicates the... 

Figure 6. Geometry used for electron holography investigations the electron beam is oriented along the z direction and the dislocation along y perpendicular to the electron-beam direction.

On the basis of Eq.(2), a comparison of the phase shift measured by electron holography and the calculated Ac ) yields the line charge q. Figure 7 shows the evaluated amplitude and phase of an electron hologram for the transmitted beam. The dislocation line direction is indicated by the dashed line in the amplitude image. Due to the noise, the position of the dislocation is visible only by a weak dark contrast in the phase image. 

The coherent emission property has recently been demonstrated and utilized to produce electron holography images [173]. This is an important result since the coherence was much higher than from standard tungsten tips so that nanotube tips may replace tungsten tips in high resolution electron microscopy applications. 

For many years efforts have been made to collect more detailed information about the 3rd dimension in TEM. For thin samples (< 50 nm) through-focus series [6] and electron holography [7,8] have been applied successfully. In this paper we mainly restrict the discussion to electron tomography (ET) that can be applied to thicker samples (50-500 nm). For an overview of the several modes of 3D-TEM that elaborates on the microscopy aspects we refer to an earlier review [4], Here we will summarize the principle and practical aspects of ET for the study of molecular sieves. 

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