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

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84 Chapter 3 diffraction pattern (solid ray crossing the optic axis) rather than an image of the specimen (dashed ray crossing the axis). Because the objective lens reduces the angles of electrons relative to the optic axis, intermediate-lens aberrations are not of great concern. Therefore, the intermediate can be operated as a weak lens (f � several centimeters) without degrading the image or diffraction pattern. objective objective aperture SAD aperture intermediate lens(es) viewing screen specimen projector object plane projector lens Figure 3-14. Thin-lens ray diagram of the imaging system of a TEM. As usual, the image rotation has been suppressed so that the electron optics can be represented on a flat plane. Image planes are represented by horizontal arrows and diffraction planes by horizontal dots. Rays that lead to a TEM-screen diffraction pattern are identified by the double arrowheads. (Note that the diagram is not to scale: the final image magnification is only 8 in this example).
The Transmission Electron Microscope 85 Projector lens The purpose of the projector lens is to produce an image or a diffraction pattern across the entire TEM screen, with an overall diameter of several centimeters. Because of this requirement, some electrons (such as the solid single-arrow ray in Fig. 3-14) arrive at the screen at a large distance from the optic axis, introducing the possibility of image distortion (Chapter 2). To minimize this effect, the projector is designed to be a strong lens, with a focal length of a few millimeters. Ideally, the final-image diameter is fixed (the image should fill the TEM screen) and the projector operates at a fixed excitation, with a single object distance and magnification v/u � 100. However, in many TEMs the projector-lens strength can be reduced in order to give images of relatively low magnification (< 1000) on the viewing screen. As in the case of light optics, the final-image magnification is the algebraic product of the magnification factors of each of the imaging lenses. TEM screen and camera A phosphor screen is used to convert the electron image to a visible form. It consists of a metal plate coated with a thin layer of powder that fluoresces (emits visible light) under electron bombardment. The traditional phosphor material is zinc sulfide (ZnS) with small amounts of metallic impurity added, although alternative phosphors are available with improved sensitivity (electron/photon conversion efficiency). The phosphor is chosen so that light is emitted in the middle of the spectrum (yellow-green region), to which the human eye is most sensitive. The TEM screen is used mainly for focusing a TEM image or diffraction pattern, and for this purpose, light-optical binoculars are often mounted just outside the viewing window, to provide some additional magnification. The viewing window is made of special glass (high lead content) and is of sufficient thickness to absorb the x-rays that are produced when the electrons deposit their energy at the screen. To permanently record a TEM image or diffraction pattern, photographic film can be used. This film has a layer of a silver halide (AgI and/or AgBr) emulsion, similar to that employed in black-and-white photography; both electrons and photons produce a subtle chemical change that can be amplified by immersion in a developer solution. In regions of high image intensity, the developer reduces the silver halide to metallic silver, which appears dark. The recorded image is a therefore a photographic negative, whose contrast is reversed relative to the image seen on the TEM screen. An optical enlarger can be used to make a positive print on photographic paper, which also has a silver-halide coating.
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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
Page 42 and 43: Electron Optics 33 We can define im
Page 44 and 45: Electron Optics 35 electron source
Page 46 and 47: Electron Optics 37 symmetry and use
Page 48 and 49: Electron Optics 39 As we said earli
Page 50 and 51: Electron Optics 41 2.4 Focusing Pro
Page 52 and 53: Electron Optics 43 � = [e/(8mE0)
Page 54 and 55: Electron Optics 45 image plane. Whe
Page 56 and 57: Electron Optics 47 procedure that i
Page 58 and 59: Electron Optics 49 The basic physic
Page 60 and 61: Electron Optics 51 From Eq. (2.7),
Page 62 and 63: Electron Optics 53 y +V 1 +V 2 -V 2
Page 64 and 65: Electron Optics 55 point from the o
Page 66 and 67: 58 Chapter 3 3.1 The Electron Gun T
Page 68 and 69: 60 Chapter 3 The rate of electron e
Page 70 and 71: 62 Chapter 3 electron emission curr
Page 72 and 73: 64 Chapter 3 As seen from the right
Page 74 and 75: 66 Chapter 3 � r r s r � (a) (b
Page 76 and 77: 68 Chapter 3 Table 3-2. Speed v and
Page 78 and 79: 70 Chapter 3 where G is a large amp
Page 80 and 81: 72 Chapter 3 The second condenser (
Page 82 and 83: 74 Chapter 3 Figure 3-9 summarizes
Page 84 and 85: 76 Chapter 3 resolution (especially
Page 86 and 87: 78 Chapter 3 The major advantage of
Page 88 and 89: 80 Chapter 3 contrast (for a given
Page 90 and 91: 82 Chapter 3 A more correct procedu
Page 94 and 95: 86 Chapter 3 Nowadays, photographic
Page 96 and 97: 88 Chapter 3 A related concept is d
Page 98 and 99: 90 Chapter 3 molecules, imparting a
Page 100 and 101: 92 Chapter 3 Frequently, liquid nit
Page 102 and 103: 94 Chapter 4 -e -e -e -e +Ze Figure
Page 104 and 105: 96 Chapter 4 � (a) elastic z x m
Page 106 and 107: 98 Chapter 4 � b a (a) (b) Figure
Page 108 and 109: 100 Chapter 4 fraction of electrons
Page 110 and 111: 102 Chapter 4 whose composition or
Page 112 and 113: 104 Chapter 4 replica crystallites
Page 114 and 115: 106 Chapter 4 4.5 Diffraction Contr
Page 116 and 117: 108 Chapter 4 remain undiffracted (
Page 118 and 119: 110 Chapter 4 Close examination of
Page 120 and 121: 112 Chapter 4 Unfortunately, the va
Page 122 and 123: 114 Chapter 4 Crystals can also con
Page 124 and 125: 116 Chapter 4 A simple example of p
Page 126 and 127: 118 Chapter 4 High-magnification ph
Page 128 and 129: 120 Chapter 4 At this stage it is c
Page 130 and 131: 122 Chapter 4 All of the above meth
Page 132 and 133: 124 Chapter 4 The procedures outlin
Page 134 and 135: 126 Chapter 5 specimen scan coils o
Page 136 and 137: 128 Chapter 5 functions with m and
Page 138 and 139: 130 Chapter 5 inelastic collisions
Page 140 and 141: 132 Chapter 5 Figure 5-5. Dependenc
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134 Chapter 5 Figure 5-7. (a) Secon
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136 Chapter 5 Because each secondar
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138 Chapter 5 Backscattered electro
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140 Chapter 5 Whereas Ip remains co
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142 Chapter 5 Figure 5-14. Red, gre
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144 Chapter 5 the SE2 and SE3 compo
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146 Chapter 5 just-observable loss
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148 Chapter 5 Section 4-10. Films o
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150 Chapter 5 A backscattered-elect
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152 Chapter 5 One example of this p
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Chapter 6 ANALYTICAL ELECTRON MICRO
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Analytical Electron Microscopy 157
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178 Chapter 7 Figure 7-1. Modified
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180 Chapter 7 7.2 Aberration Correc
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182 Chapter 7 Besides decreasing th
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184 Chapter 7 7.4 Electron Holograp
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186 Chapter 7 There are now many al
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188 Chapter 7 holography could ther
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Appendix MATHEMATICAL DERIVATIONS A
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Mathematical Derivations 193 A.2 Im
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REFERENCES Binnig, G., Rohrer, H.,
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INDEX Aberrations, 3, 28 axial, 44
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Index 199 EELS, 172 Electromagnetic
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Index 201 Principal plane, 32, 80 P
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