Physical Principles of Electron Microscopy: An Introduction to TEM ...

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186 Chapter 7 There are now many alternative holographic schemes, the most popular being off-axis holography (Voelkl et al., 1999). In this method, an electron biprism is used to combine the object wave, representing electrons that pass through the specimen, with a reference wave formed from electrons that pass beyond an edge of the specimen (Fig. 7-6). The biprism consists of a thin conducting wire (< 1 �m diameter) maintained at a positive potential (up to a few hundred volts), usually inserted at the selected-area-aperture plane of a field-emission TEM. The electric field of the biprism bends the electron trajectories (Fig.7-6) and produces a hologram containing sinusoidal fringes, running parallel to the wire, that are modulated by information about the specimen; see Fig. 7-7. Highly astigmatic illumination is used to maximize the size of the coherence patch where the interfering wavefronts overlap. W hologram specimen objective Figure 7-6. Principle of off-axis electron holography. Electrons that pass through the sample are combined with those of an off-axis beam that is attracted toward a positively biased wire W, whose axis runs perpendicular to the diagram. W is the electron-optical equivalent of a glass biprism, as indicated by the dashed lines.
Recent Developments 187 Figure 7-7. Off-axis low-magnification (objective lens off) electron holograms of a sub-�m permalloy structure supported on a 50-nm-thick SiN membrane. Note the fringe bending corresponding to areas with strong change in electron phase (for example, near the edges of the permalloy film). Although some of this bending is electrostatic in nature (due to the “inner potential” of the permalloy), most of it results from the internal magnetization (change in magnetic potential). The holograms were recorded using a JEOL 3000F field-emission TEM. Courtesy of M. Malac, M. Schofield, and Y. Zhu, Brookhaven National Laboratory. Rather than using optical reconstruction, it is more convenient to transfer the hologram (using a CCD camera) into a computer, which can simulate the effect of optical processing by running a program that reconstructs the complex image wave. This process involves calculating the two-dimensional Fourier transform of the hologram intensity (essentially its frequency spectrum) and selecting an off-axis portion of the transform, representing modulation of the sinusoidal fringe pattern. Taking an inverse Fourier transform of this selected data yields two images: the real and imaginary parts of the processed hologram (representing the cosine and sine harmonics), from which the phase and amplitude of the electron exit wave (i.e., the wave at the exit surface of the specimen) can be calculated. The phase image is particularly valuable, as it can reveal changes in magnetic or chemical potential in the specimen. In most ferromagnetic specimens, fringe shifts are mainly due to changes in magnetic potential associated with the internal magnetic field. Electron holography therefore supplements the information obtained from Fresnel and Foucaoult imaging and in fact is more quantitative. In non-magnetic specimens, where only electrostatic effects contribute to the shift of the holographic fringes, the phase image can be processed to yield a map of the local (chemical) potential; see Fig. 7-8. Electron
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Physical Principles of Electron Mic
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Ray F. Egerton Department of Physic
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Contents Preface xi 1. An Introduct
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Contents ix 7. Recent Developments
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xii Preface textbooks, such as Will
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2 1.1 Limitations of the Human Eye
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4 Chapter 1 parallel beam of light
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6 Chapter 1 Figure 1-3. One of the
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8 Chapter 1 Figure 1-5. Light-micro
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10 Chapter 1 Figure 1-7. Scanning t
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12 Chapter 1 Figure 1-8. Early phot
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14 Chapter 1 Figure 1-10. JEOL tran
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16 Chapter 1 hydrated. But high-ene
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18 Chapter 1 Figure 1-14. Scanning
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20 Chapter 1 Figure 1-16. Photograp
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22 Chapter 1 visible-light photons
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24 Chapter 1 Typically, the STM hea
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Chapter 2 ELECTRON OPTICS Chapter 1
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Electron Optics 29 a c b object len
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Electron Optics 31 � a n 2 ~1.5
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Electron Optics 33 We can define im
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Electron Optics 35 electron source
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Electron Optics 37 symmetry and use
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Electron Optics 39 As we said earli
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Electron Optics 41 2.4 Focusing Pro
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Electron Optics 43 � = [e/(8mE0)
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Electron Optics 45 image plane. Whe
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Electron Optics 47 procedure that i
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Electron Optics 49 The basic physic
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Electron Optics 51 From Eq. (2.7),
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Electron Optics 53 y +V 1 +V 2 -V 2
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Electron Optics 55 point from the o
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58 Chapter 3 3.1 The Electron Gun T
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60 Chapter 3 The rate of electron e
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62 Chapter 3 electron emission curr
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64 Chapter 3 As seen from the right
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66 Chapter 3 � r r s r � (a) (b
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68 Chapter 3 Table 3-2. Speed v and
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70 Chapter 3 where G is a large amp
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72 Chapter 3 The second condenser (
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74 Chapter 3 Figure 3-9 summarizes
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76 Chapter 3 resolution (especially
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78 Chapter 3 The major advantage of
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80 Chapter 3 contrast (for a given
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82 Chapter 3 A more correct procedu
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84 Chapter 3 diffraction pattern (s
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86 Chapter 3 Nowadays, photographic
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88 Chapter 3 A related concept is d
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90 Chapter 3 molecules, imparting a
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92 Chapter 3 Frequently, liquid nit
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94 Chapter 4 -e -e -e -e +Ze Figure
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96 Chapter 4 � (a) elastic z x m
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98 Chapter 4 � b a (a) (b) Figure
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100 Chapter 4 fraction of electrons
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102 Chapter 4 whose composition or
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104 Chapter 4 replica crystallites
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106 Chapter 4 4.5 Diffraction Contr
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108 Chapter 4 remain undiffracted (
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110 Chapter 4 Close examination of
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112 Chapter 4 Unfortunately, the va
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114 Chapter 4 Crystals can also con
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116 Chapter 4 A simple example of p
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118 Chapter 4 High-magnification ph
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120 Chapter 4 At this stage it is c
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122 Chapter 4 All of the above meth
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124 Chapter 4 The procedures outlin
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126 Chapter 5 specimen scan coils o
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128 Chapter 5 functions with m and
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130 Chapter 5 inelastic collisions
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132 Chapter 5 Figure 5-5. Dependenc
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Page 154 and 155: 146 Chapter 5 just-observable loss
Page 156 and 157: 148 Chapter 5 Section 4-10. Films o
Page 158 and 159: 150 Chapter 5 A backscattered-elect
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Page 162 and 163: Chapter 6 ANALYTICAL ELECTRON MICRO
Page 164 and 165: Analytical Electron Microscopy 157
Page 184 and 185: 178 Chapter 7 Figure 7-1. Modified
Page 186 and 187: 180 Chapter 7 7.2 Aberration Correc
Page 188 and 189: 182 Chapter 7 Besides decreasing th
Page 190 and 191: 184 Chapter 7 7.4 Electron Holograp
Page 194 and 195: 188 Chapter 7 holography could ther
Page 196 and 197: Appendix MATHEMATICAL DERIVATIONS A
Page 198 and 199: Mathematical Derivations 193 A.2 Im
Page 200 and 201: REFERENCES Binnig, G., Rohrer, H.,
Page 202 and 203: INDEX Aberrations, 3, 28 axial, 44
Page 204 and 205: Index 199 EELS, 172 Electromagnetic
Page 206 and 207: Index 201 Principal plane, 32, 80 P
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Physical Principles of Electron Microscopy: An Introduction to TEM ...

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