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

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92 Chapter 3 Frequently, liquid nitrogen is used to help in achieving adequate vacuum inside the TEM, through a process known as cryopumping. With a boiling point of 77 K, liquid nitrogen can cool internal surfaces sufficiently to cause water vapor and organic vapors (hydrocarbons) to condense onto the surface. These vapors are therefore removed from the vacuum, so long as the surface remains cold. In a diffusion-pumped vacuum system, liquid nitrogen is often poured into a container (known as a trap) above the DP to minimize “backstreaming” of diffusion-pump vapor into the TEM column. Organic vapors can also enter the vacuum from components such as the rubber o-ring seals. When a surface is irradiated with electrons, hydrocarbon molecules that arrive at the surface can become chemically bonded together to form a solid polymer. This electron-beam contamination may occur on both the top and bottom surfaces of a TEM specimen; the resulting contamination layers cause additional scattering of the electrons, reducing the contrast of the TEM image. To combat this process, liquid nitrogen is added to a decontaminator trap attached to the side of the TEM column. By conduction through metal rods or braiding, it cools metal plates located just above or below the specimen, thereby condensing water and organic vapors and giving a low partial pressure of these components in the immediate vicinity of the specimen.
Chapter 4 TEM SPECIMENS AND IMAGES In the transmitted-light microscope, variation of intensity within an image is caused by differences in the absorption of photons within different regions of the specimen. In the case of a TEM, however, essentially all of the incoming electrons are transmitted through the specimen, provided it is suitably thin. Although not absorbed, these electrons are scattered (deflected in their path) by the atoms of the specimen. To understand the formation and interpretation of TEM images, we need to examine the nature of this scattering process. Many of the concepts involved can be understood by considering just a single “target” atom, which we represent in the simplest way as a central nucleus surrounded by orbiting particles, the atomic electrons; see Fig. 4-1. This is the classical planetary model, first established by Ernest Rutherford from analysis of the scattering of alpha particles. In fact, our analysis will largely follow that of Rutherford. Instead of positive alpha particles, we have negative incident electrons, but the cause of the scattering remains the same, namely the electrostatic (Coulomb) interaction between charged particles. Interaction between the incoming fast electron and an atomic nucleus gives rise to elastic scattering where (as we will see) almost no energy is transferred. Interaction between the fast electron and atomic electrons results in inelastic scattering, in which process the transmitted electron can lose an appreciable amount of energy. We will consider these two types of event in turn, dealing first with the kinematics of scattering, where we consider the momentum and energy of the fast electron, without needing to know the nature of the forces involved. As the nature of the force does not matter, the kinematics of electron scattering employs the same principles of classical mechanics as are used to treat the collision of larger (macroscopic) objects.
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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
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 92 and 93: 84 Chapter 3 diffraction pattern (s
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 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
Page 142 and 143: 134 Chapter 5 Figure 5-7. (a) Secon
Page 144 and 145: 136 Chapter 5 Because each secondar
Page 146 and 147: 138 Chapter 5 Backscattered electro
Page 148 and 149: 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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