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A Question from Last Year Final Exam

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1 A Question from Last Year Final Exam
A Question from Last Year Final Exam Recommend an instrumental method that will provide information about the chemical composition and crystal symmetry of precipitates (small black dots of ~0.1m wide) in a polycrystalline sample with micrometer-sized grains as shown below. State your reasons [10 marks] Precipitates see atoms by TEM

2 Limits of OM, SEM, SPM and XRD
G.B. NN core G.B. NN shell BT BT BT-BaTiO3 NN-NaNbO3 G.B.-Grain boundary M.G.J.-multiple grain junction M.G.J. EDS 0.2m NN/BT Lateral resolution: ~m Details of microstructure: e.g., domain structure, chemical inhomogeneity phase distribution, grain boundaries, interfaces, precipitates, dislocations, etc. at~0:30 TEM overview Difference between SEM and TEM Difference between EM and OM

3 Chemical analysis at a nanometer scale in a Transmission Electron Microscope

4 Why TEM? The uniqueness of TEM is the ability to obtain full morphological (grain size, grain boundary and interface, secondary phase and distribution, defects and their nature, etc.), crystallographic, atomic structural and microanalytical such as chemical composition (at nm scale), bonding (distance and angle), electronic structure, coordination number data from the sample. TEM is the most efficient and versatile technique for the characterization of materials. see atoms by TEM

5 Lecture-6 Transmission Electron Microscopy (TEM) Scanning Transmission Electron Microscopy (STEM)
What is a TEM? How it works - gun, lenses, specimen holder Resolution What can a TEM do? Imaging and diffraction Imaging-diffraction and phase contrast Diffraction-Selected area electron diffraction (SAED) and Convergent beam electron diffraction (CBED) Chemical analysis EDS, Electron Energy Loss Spectroscopy (EELS) Energy Filtered Imaging People use the abbreviation TEM both for Transmission Electron Microscope (the instrument) and Transmission Electron Microscopy (the technique of using a Transmission Electron Microscope). The three words transmission, electron and microscope, tell what the instrument will do: Transmission means that we use it to look through things. Electron stands for electrons, the things we use to do microscopy. Microscope is the instrument we use to do microscopy (Microscopy literally means looking at something small). to~2:40 History & applications

6 What is TEM? Lecture-6 Transmission Electron Microscopy (TEM) Scanning Transmission Electron Microscopy (STEM) TEM is an microscopy technique that functions similar to a light microscope, which uses a beam of exited electrons as a light source to provide mophorlogical, compositional and crystallographic information of an ultra thin specimen. The image is formed by the interaction of the electrons transmitted through the specimen, which is then magnified and focused on a fluorescence screen containing a layer of photographic film. What is a TEM? How it works - gun, lenses, specimen stage Resolution People use the abbreviation TEM both for Transmission Electron Microscope (the instrument) and Transmission Electron Microscopy (the technique of using a Transmission Electron Microscope). The three words transmission, electron and microscope, tell what the instrument will do: Transmission means that we use it to look through things. Electron stands for electrons, the things we use to do microscopy. Microscope is the instrument we use to do microscopy (Microscopy literally means looking at something small). ~2:20-2:40 fluorescence screen Milestones of Science: Ernst Ruska and the Electron Microscope at~5:00-7:15 and ~9:03-9:23

7 Comparison of OM and TEM
A better comparison can be made between the TEM and the light microscope. Not only there are lots of similarities in the basic design, they are also used for the same purpose, i.e. microscopy. The main difference between the two instruments shown above appears to be that one is upside down in comparison with the other (the lamp in the light microscope is at the bottom, the electron source in the TEM at the top). That, however, is just a matter of practicalities in the design. There are already two significant differences apparent from the pictures: In the TEM we do not observe the image directly but the electron beam is converted to light by the viewing screen (In the TEM, one surface covered with a phosphor, which is a materials emitting light when struck by electrons, is the view screen and also called the fluorescent screen) and that light passes through a front window and reaches our eye. The inside of the TEM is in vacuum, not air as the light microscope. There are also a lot of differences between a light microscope and a TEM: e.g., instrument weight and size, specimen size, microscope cost, operation easiness, Illumination source, lenses, etc. Principal features of an optical microscope and a transmission electron microscope, drawn to emphasize the similarities of overall design.

8 Structure and Function of TEM
Structure and Function of TEM CM200 (200kV) Column Electron Gun EDS Detector Condenser Lens Objective Lens Specimen Holder SAD Aperture Binocular Magnifying Lenses TV Monitor Camera Chamber Viewing Chamber A fundamental understanding of materials characteristics is required. Understanding starts with characterizing materials from the macro- to the sub-nanoscale: morphology, Crystal structure, chemical composition, interfaces, surfaces and defects. The Transmission Electron Microscope is the single, most powerful instrument for studying the full range of advanced materials. Starting at the overview level of the optical microscope, it proceeds down to the Angstrom level of atomic structure, generating a multitude of signals. These signals include elastically scattered electrons carrying structure information, inelastically scattered electrons and X-rays providing chemical information, and secondary electrons that give surface information. In a typical TEM a static beam of electrons at kV accelerating voltage illuminate a region of an electron transparent specimen which is immersed in the objective lens of the microscope. A number of intermediate lenses are used to project either the image or the diffraction pattern onto a fluorescent screen for observation. The screen is usually lifted and the image formed on photographic film for recording purposes. The most important part of the microscope is the column. That's where the real action is. It starts at the top, in the gun, where the electron beam is generated. The electrons then come down the column and go through the specimen about halfway down the column. Then the electrons go further down and finally reach the bulging thing at the bottom. This is called the projection chamber (the image is projected there). Through the window of the projection chamber the operator looks at the images shown by the microscope on the fluorescent screen. For the rest the microscope consists of supports, electronics (hidden below the table top), vacuum pumps (also invisible), etc. Cost:  $4,000,000 at~2:40-4:40 at~0:30

9 Vacuum The electron microscope is built like a series of vessels connected by pipes and valves separate all the vessels from each other. The vacuum around the specimen is around 10-7 Torr. The vacuum in the gun depends on the type of gun, either around 10-7 Torr (the tungsten or LaB6 gun) or 10-9 Torr (for the Field Emission Gun). The pressure in the projection chamber is usually only 10-5 Torr (and often worse). This pressure is not very good because the projection chamber holds the negatives used to record images. Even though we dry the negatives before putting them in the microscope, they still will give off so many gases that the vacuum in the projection chamber never gets very good. Electron microscopes cannot operate in air for a number of reasons: the electrons in the beam will hit molecules in the air and before the beam has traveled 1 meter, there will be no more electrons in the beam. air is not a very good insulator, so putting a voltage of one hundred or more kilovolts across the small distance between the cathode and the anode in the electron gun would result in an enormous series of discharges (a bit like lighting) instead of a steady electron beam. air contains mainly gases like nitrogen, oxygen and carbon-dioxide, but there are also other molecules. Many of these are hydrocarbons. These hydrocarbons stick to the TEM specimens and fall apart when they are hit by an electron beam. The carbon remains and builds up on the specimen (we call this contamination). This can happen even in vacuum, so in air it would be hopeless - before you have chance of looking at a specimen it is covered in a thick layer of carbon and you can longer look through it.

10 How it works? The Lenses in TEM
at~0:20-0:44 Condenser lenses(two)-control how strongly beam is focused (condensed) onto specimen. At low Mag. spread beam to illuminate a large area, at high Mag. strongly condense beam. Objective lens-focus image (image formation) and contribute most to the magnification and resolution of the image. Magnetic material Running water B Cu coils The picture above shows a TEM lens cut vertically through the center (it is completely round). The grey part is the magnetic material. This makes an almost complete circuit around the blue/orange-yellow part. The magnetic field is concentrated where the circuit is broken (this is called the gap). The rest - the yoke - gives almost no magnetic field to the outside. The copper wires (shown cut through) form the coil through which the current is running. Because the current heats the coil, we have to cool it with water - the blue part. This (cold) water comes in through a pipe, flows around the coil and then out again, taking the heat with it. There is one big advantage to using electro-magnets and electrons over glass lenses and light : we can change the current through the coil, which changes the magnetic field, which in turn changes the strength of the lens. In light microscopy there is only one way of changing magnification. Because you cannot change the shape of a glass lens, you have to exchange lenses (light microscopes therefore normally have a revolving turret with three or four lenses). In the TEM we simply change the current through one or more lenses and we can have an endless number of variations - and thus many more different magnifications. There is also a disadvantage to using magnetic fields instead of glass lenses. A glass lens we can give any shape we want. Normally a lens (as sketched below) has two surfaces that are very close to spherical (the dashed lines), except at the very edges where it doesn't matter because there lens is held in a frame.  Away from the axis (the vertical black dashed line) the field becomes stronger and electron paths will be focussed too strongly, like in the imperfect glass lens above. This magnetic lens therefore has spherical aberration. It is not possible to make magnetic lenses without spherical aberration. And the spherical aberration remains the limitation to the resolution of the TEM. Four lenses form magnification system-determine the magnification of the microscope. Whenever the magnification is changed, the currents through these lenses change. at~5:50-7:00

11 How it works? Image Formation in TEM
A disc of metal Control brightness, convergence under in over focus focus focus First electrons encounter the condenser lenses, which concentrate the electron beam and bring it to a point of focus some way above the specimen plane. The condenser system contains a small physical aperture in the form of a disc of metal (Pt or Mo) with a precisely circular hole at its center. This aperture controls both the intensity and the angle of convergence of the electron beam. Aperture size, together with the precise level of focus of the condenser lenses, form the operator's control of 'brightness' in the final image. When the electrons encounter the specimen, one of three things can happen. They may pass through it unimpeded. They may be scattered without loss of energy (elastic scattering). Or they may be inelastically scattered; this involves an exchange of energy between the electron beam and the specimen, and may cause the emission from the specimen of secondary electrons or X-rays. Image contrast in the TEM depends on preventing the inelastically scattered electrons from contributing to the image. Their exclusion is accomplished by a second small aperture just below the specimen. This is the objective aperture and, again, the operator usually has a choice of three aperture sizes, which comprise the microscope's contrast control. The specimen is surrounded by the objective lens, which magnifies the image of the specimen to the order of x 50. Changes to the current flowing through the objective lens constitute the microscope's 'focus' control. The magnified image from the objective lens is further enlarged by two or more lenses below the specimen. They are called the intermediate lens and the projector lens. Both the absolute and relative excitations of these lenses are controlled electronically by a simple 'magnification' control on the microscope's operating console. Most modern TEMs can be set to give a final magnification anywhere in the range x1,000 - x500,000. Finally, electrons leave the projector lens and strike a screen coated with fluorescent material. The operator sees a continuous image of the specimen, usually green in color, through a viewing window. Fine adjustments to the image are made while viewing it with an externally mounted, low-power binocular. The operator can control brightness, focus, magnification and to some extent contrast. In addition, the operator can improve image quality by careful adjustments of astigmatism correction controls. However, most of the characteristics of the image are predetermined by techniques used to prepare the specimen. A good operator can get the best out of a specimen, but it needs a well-prepared specimen to produce an excellent image. Control contrast Schematic of the Optics of a TEM at~3:00-4:45

12 Why Electrons? Resolution
In expression for the resolution (Rayleigh’s Criterion) r = 0.61/nsin Green Light ~400nm n~1.7 oil immersion r~150nm (0.15m) Electrons -wavelength, =[1.5/(V+10-6V2)]1/2 nm V-accelerating voltage, n-refractive index -aperture of objective lens, very small in TEM  sin  and so r=0.61/ ~0.1 radians 200kV Electrons ~0.0025nm n~1 (vacuum) r~0.02nm (0.2Å) 1/10th size of an atom! UNREALISTIC! WHY? -beam convergence 0.1 radians ~ 5.5o

13 Resolution Limited by Lens Aberrations
Chromatic aberration is caused by the variation of the electron energy and thus electrons are not monochromatic. point is imaged as a disk. Spherical aberration is caused by the lens field acting inhomogeneously on the off-axis rays. For thin specimen, contribution of chromatic aberration can be ignored. So For HREM works, very thin specimen is required. The thinner, the better for HREM. Usually, t should be smaller than 50nm, depending on accelerating voltage used. Let us consider beam and specimen interaction. rmin0.91(Cs3)1/4 Practical resolution of microscope. Cs–coefficient of spherical aberration of lens (~mm) point is imaged as a disk.

14 Beam and Specimen Interaction
(EDS) BF DF HREM Imaging SAED & CBED diffraction (EELS)

15 Scanning Transmission Electron Microscopy
(STEM) JEOL 2000FX Analytical Electron Microscope In STEM, the electron beam is rastered (scan coil) across the surface of a sample in a similar manner to SEM, however, the sample is a thin TEM section and the diffraction contrast image is collected on a solid-state (ADF) detector. Scanning beam specimen                                HAADF Detector ADF BF ADF DF BF Both TEM and STEM specimens must be transparent to electron beam and Thus specimens must be very thin. STEM detector or EELS HAADF-high angle annular dark-field Scanning transmission electron holography microscope


17 Specimen Holder Rotation, tilting, heating, cooling and straining beam
                         Specimen Holder Rotation, tilting, heating, cooling and straining beam holder a split polepiece objective lens Double tilt heating Heating and straining Twin specimen holder at~0:56-1:42

18 Specimen Holder with Electrical Feedthroughs

19 Specimen Preparation-Destructive
Dispersing crystals or powders on a carbon film on a grid 3mm Making a semiconductor specimen with a Focused Ion Beam (FIB) 1 2 3 4 5 The high-resolution TEM image of a small part of an asbestos fibre shows the crystal structure of the asbestos.The long direction of the fiber is horizontal in this image. The vertical distance between one row of light 'balls' to the next is 0.98 nanometers, the distance between the balls is 0.53 nanometers. The visibility of such distances is an indication of the resolution of the microscope (the smaller the distances visible, the better the resolution). FIB is very similar to a SEM. The essential difference is that the electron beam is replaced by a beam of ions. a failure is located and a strip of Pt is placed as a protective cover. On one side of the strip a trench is milled out with the FIM. The same is done on the other side of the strip (visible structure). The strip is milled on both sides and then the sides connecting the strip to the wafer are cut through. The strip is tilted, cut at the bottom and deposited on a TEM grid. Preparing specimen

20 Specimen Preparation-2
Ion-milling a ceramic Ar (4-6keV, 1mm A) 3mm Ultrasonic cut grind Dimple center part of disk to ~5-10m ion-mill until a hole appears in disk Jet-polishing metal - + a thin stream of acid A disk is mounted in a jet-polishing machine and is electropolished until a small hole is made. Cut into disks and grind Drill a 3mm cylinder Ultramicrotomy-using a (diamond) knife blade Mainly for sectioning biological materials. To avoid ion-milling damage ultramicrotome can also be used to prepare ceramic TEM specimens. TEM specimen preparation

21 What can a TEM do? Imaging
BF and DF imaging HREM Objective Aperture (OA) BF - Bright Field DF - Dark Field

22 BF & DF Imaging – Diffraction Contrast
DDF CDF OA OA Beam tilt crystal C-film amorphous                               D D T-transmitted D-diffracted                                                                                        Objective aperture T Objective aperture T BF image DF image Hole in OA C-film C-film crystal crystal Diffraction contrast imaging-use either a non-diffracted or diffracted beam and remove all other beams from the image by the use of an objective aperture. There are two ways to form a DF image. One is displacive DF, or DDF, as shown. Another one is centered DF, or CDF, i.e., tilt beam and thus only diffracted beams pass through the center hole of the objective aperture. Diffraction + mass-thickness Contrast

23 Diffraction, Thickness and Mass Contrast
BF images Weak diffraction thicker thinner 2 Strong diffraction G.B. 8 7 thickness fringes 1 3 thickness Disk specimen 6 . . . . . . . . . . . . . . . . . . .. . . . . 4 . Low mass High mass 5 S S T S T 8 grains are in different orientations or different diffraction conditions Bright Dark

24 BF and DF Imaging Incident beam BF imaging-only transmitted beam is allowed to pass objective aperture to form images. mass-thickness contrast BF specimen diffracted beam                                DF imaging only diffracted beams are allowed to pass the aperture to form images. Particles in Al-Cu Alloy. thin platelets ll e Vertical, dark Particles e.                                DF transmitted beam objective aperture DF hole in objective aperture(10-100m)

25 Phase Contrast Imaging High Resolution Electron Microscopy (HREM)
D Si BN                                Objective aperture Electron diffraction pattern recorded From both BN film on Si substrate. Use a large objective aperture. Phases and intensities of diffracted and transmitted beams are combined to form a phase contrast image. Phase contrast imaging-use all of the diffracted and non-diffracted beams (by using a large objective aperture or none at all) and add them back together, phase and intensity to form a phase contrast image. Electron diffraction.

26 http://www. matter. org. uk/diffraction/electron/electron_diffraction
Electron Diffraction Geometry for e-diffraction Bragg’s Law: l=2dhklsinhkl e- =[1.5/(V+10-6V2)]1/2 nm l=0.037Å (at 100kV) =0.26o if d=4Å dhkl Specimen foil e-beam Zone axis of crystal l = 2d L 2 e-beam is almost parallel to {hkl} r/L=sin2 as   0 r/L = 2 r/L = l/d or r = lLx sample crystal X-ray r polycrystal T D Reciprocal lattice 1 Due to short wavelength, diffraction angle in TEM is very small. Diffraction angle in diagram is exaggerated. Value of dhkl can be obtained by measuring rhkl. d L -camera length r -distance between T and D spots 1/d -reciprocal of interplanar distance(Å-1) SAED –selected area electron diffraction hkl [hkl] SAED pattern at~3:00-3:34

27 g – reciprocal lattice vector
A reciprocal lattice is another way of view a crystal lattice and is used to understand diffraction patterns. A dimension of 1/d (Å-1) is used in reciprocal lattices. g – reciprocal lattice vector

28 How to build a reciprocal lattice?

29 2-D Reciprocal Lattices
2-D Reciprocal Lattices For every real lattice there is an equivalent  reciprocal lattice. Real space: Unit cell vectors: a,b d-spacing direction a d [10] b d [01] Reciprocal space: Unit cell vectors: a*,b* magnitude direction a* 1/d  b b* 1/d  a A reciprocal lattice can be built using reciprocal vectors. Both the real and reciprocal construc-tions show the same lattice, using different but equivalent descriptions. [01] [10] (10) b* a* 01 (01) 02 10 11 12 20 21 22 Note: each point in the reciprocal lattice represents a set of planes.

30 3-D Reciprocal Lattice Real space: Unit cell vectors: a,b,c magnitude direction a d [100] b d [010] c d [001] Reciprocal space: Unit cell vectors: a*,b* a* 1/d  b and c b* 1/d  a and c c* 1/d  a and b Orthorhombic Note: as volume of unit cell in real space increases the volume of unit cell in reciprocal space decreases, and vice versa. a*,b* and c* are parallel to corresponding a,b and c, and this is only true for the unit cells of cubic, tetragonal and orthorhmbic crystal systems.

31 Lattice Vectors r=ua+vb+wc g*=ha*+kb*+lc* Real space lattice vector
corresponds to directions in crystal and it can be defined as: r=ua+vb+wc a,b and c are unit cell vectors, u,v and w are components of the direction index [uvw]. A reciprocal lattice vector can be written as: g*=ha*+kb*+lc* a*,b* and c* are reciprocal unit vectors, and h,k and l are the Miller indices of the plane (hkl).

32 Effect of Spacing of planes in Real Space on Length of Reciprocal Vector, g
- [111] - (111) - d111 In a crystal of any structure, ghkl is normal to the (hkl) plane and has a length inversely proportional to the interplanar spacing of the planes.

33 Why are there so many spots? Ewald Sphere and Diffraction Pattern
How to build the Ewald’s Sphere? SAED pattern XRD pattern Reciprocal Lattice k – wave vector lkl = 1/  – wavelength of electron

34 A set of real lattice planes
Reciprocal Lattice The Ewald Sphere and Diffraction Pattern Ewald Sphere Construction 1/ A set of real lattice planes D T k – wave vector lkl = 1/  – wavelength of electron Bragg’s Law

35 SAED R R=1/ Why there are so many diffraction spots in ED? XRD R=1/

36 parallel beam SAED A TEM technique to reduce both the area and intensity of the beam contributing to a diffraction pattern by the insertion of an aperture into the image plane of the objective lens. This produces a virtual diaphragm in the plane of the specimen. Virtual Selected Area Electron Diffraction aperture specimen Objective lens Diffraction pattern Back focal plane SAD aperture

37 Focusing SAED Pattern at Fixed Screen
by changing magnetic lens strength specimen lens Diffracted beam screen Transmitted beam SAED gives 2-D information Spot pattern

38 SAED Patterns of Single Crystal, Polycrystalline and Amorphous Samples
b c 020 110 200 r1 r2 Single crystal Fe (BCC) thin film-[001] Polycrystalline thin film of Pd2Si Amorphous thin film of Pd2Si. The diffuse halo is indicative of scattering from an amorphous material.

39 Diffraction Spot Intensity
Spot intensity: Ihkl  lFhkll2 Fhkl - Structure Factor N Fhkl =  fj exp[2i(hu+kv+lw)] j=1 fj – atomic scattering factor fj  Z, sin/ F depends on atomic arrangement in a unit cell and orientation of specimen. h,k,l are Miller indices and u,v,w fractional coordinates

40 _ [013] 131 (311)? 200 Place the tables giving relative reciprocal lattice spacings and interplanar angles in cubic and equation for calculating interplanar angle on reflective projector. Using SAED patterns to determine lattice parameters accuracy ~ 1%. Also phase identification. But CBED will provide much more information about The sample than SAED. SAED _

41 SAED Patterns dhkl = lL/rhkl SAED the table 50nm

42 Next Lecture TEM Convergent beam electron diffraction (CBED)
Chemical analysis EDS, Electron Energy Loss Spectroscopy (EELS) Energy Filtered Imaging Secondary Ion Mass Spectroscopy (SIMS) People use the abbreviation TEM both for Transmission Electron Microscope (the instrument) and Transmission Electron Microscopy (the technique of using a Transmission Electron Microscope). The three words transmission, electron and microscope, tell what the instrument will do: Transmission means that we use it to look through things. Electron stands for electrons, the things we use to do microscopy. Microscope is the instrument we use to do microscopy (Microscopy literally means looking at something small).

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