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see atoms by TEM A Question from Last Year.

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Presentation on theme: "see atoms by TEM A Question from Last Year."— Presentation transcript:

1 see atoms by TEM 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

2 Limits of OM, SEM, SPM and XRD BT NN BT NN EDS G.B. BT-BaTiO 3 NN-NaNbO 3 G.B.-Grain boundary M.G.J.-multiple grain junction Lateral resolution: ~m Details of microstructure: e.g., domain structure, chemical inhomogeneity phase distribution, grain boundaries, interfaces, precipitates, dislocations, etc. NN/BT M.G.J. 0.2m core shell

3 Chemical analysis at a nanometer scale in a Transmission Electron Microscope (TEM)

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 to~2:40 History & applications

6 Lecture-6 Transmission Electron Microscopy (TEM) Scanning Transmission Electron Microscopy (STEM) What is a TEM? How it works - gun, lenses, specimen stage Resolution ~2:20-2:40 fluorescence screen What is TEM?  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. 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 Principal features of an optical microscope and a transmission electron microscope, drawn to emphasize the similarities of overall design.

8 Electron Gun EDS Detector Condenser Lens Specimen HolderObjective Lens Magnifying Lenses CM200 (200kV) SAD Aperture TV Monitor Viewing Chamber Camera Chamber Cost:  $4,000,000 Column Binocular at~2:40-4:40 Structure and Function of TEM 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 Torr. The vacuum in the gun depends on the type of gun, either around Torr (the tungsten or LaB 6 gun) or Torr (for the Field Emission Gun). The pressure in the projection chamber is usually only 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.

10 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. Four lenses form magnification system- determine the magnification of the microscope. Whenever the magnification is changed, the currents through these lenses change. B at~5:50-7:00 How it works? The Lenses in TEM Running water Cu coils Magnetic material at~0:20-0:44

11 Schematic of the Optics of a TEM Control brightness, convergence Control contrast How it works? Image Formation in TEM A disc of metal under in over focus focus focus at~3:00-4:45

12 Why Electrons? Resolution -wavelength, =[1.5/(V V 2 )] 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) 1/10 th size of an atom! UNREALISTIC! WHY? 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 0.1 radians ~ 5.5 o  -beam convergence

13 Resolution Limited by Lens Aberrations  point is imaged as a disk. Spherical aberration is caused by the lens field acting inhomogeneously on the off-axis rays.  point is imaged Chromatic aberration is caused by the variation of the electron energy and thus electrons are not monochromatic. r min 0.91(C s 3 ) 1/4 Practical resolution of microscope. C s –coefficient of spherical aberration of lens (~mm) as a disk.

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

15 Scanning Transmission Electron Microscopy 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. JEOL 2000FX Analytical Electron Microscope STEM detector or EELS HAADF Detector HAADF-high angle annular dark-field Scanning beam specimen BFADF BF DF (STEM) Scanning transmission electron holography microscope

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

18 Specimen Holder with Electrical Feedthroughs at~3:00-3:34

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.a failure is located and a strip of Pt is placed as a protective cover. 2.On one side of the strip a trench is milled out with the FIM. 3.The same is done on the other side of the strip (visible structure). 4.The strip is milled on both sides and then the sides connecting the strip to the wafer are cut through. 5.The strip is tilted, cut at the bottom and deposited on a TEM grid specimen

20 Specimen Preparation-2 Ion-milling a ceramic 3mm Ultrasonic cut grind Dimple center part of disk to ~5-10m ion-mill until a hole appears in disk Ar (4-6keV, 1mm A) Jet-polishing metal Drill a 3mm cylinder Cut into disks and grind A disk is mounted in a jet-polishing machine and is electropolished until a small hole is made. a thin stream of acid + - 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. 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 Objective aperture C-film amorphous crystal D T BF image C-film crystal D T C-film crystal DF image Diffraction + mass-thickness Contrast Objective aperture DDF CDF Beam tilt T-transmitted D-diffracted Hole in OA OA

23 Diffraction, Thickness and Mass Contrast Disk specimen thickness thinner thicker G.B High mass Low mass TT SS S Bright Dark Strong diffraction Weak diffraction 8 grains are in different orientations or different diffraction conditions thickness fringes BF images

24 BF and DF Imaging Incident beam specimen transmitted beam diffracted beam objective aperture hole in objective aperture(10-100m) BF imaging- only transmitted beam is allowed to pass objective aperture to form images. mass-thickness contrast BF DF 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.

25 Phase Contrast Imaging High Resolution Electron Microscopy (HREM) Use a large objective aperture. Phases and intensities of diffracted and transmitted beams are combined to form a phase contrast image. T D Si Objective aperture Electron diffraction pattern recorded From both BN film on Si substrate. BN

26 Electron Diffraction Specimen foil T D  e-e- L 2 r d hkl [hkl] SAED pattern L -camera length r -distance between T and D spots 1/d -reciprocal of interplanar distance(Å -1 ) SAED –selected area electron diffraction Geometry for e-diffraction Bragg’s Law: =2d hkl sin hkl =0.037Å (at 100kV)  =0.26 o if d=4Å = 2d r/L=sin2  as   0 r/L = 2  r/L = /d or r = Lx 1 d hkl Reciprocal lattice X-ray crystal polycrystal =[1.5/(V V 2 )] 1/2 nm e-beam is almost parallel to {hkl} e-beam Zone axis of crystal sample at~3:00-3:34

27 Reciprocal Lattice 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

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29 2-D Reciprocal Lattices Real space: Unit cell vectors: a,b d-spacing direction a d 10 [10] b d 01 [01] Reciprocal space: Unit cell vectors: a*,b* magnitude direction a* 1/d 10  b b* 1/d 01  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) (01) Note: each point in the reciprocal lattice represents a set of planes. a* b* For every real lattice there is an equivalent reciprocal lattice.

30 3-D Reciprocal Lattice Real space: Unit cell vectors: a,b,c magnitude direction a d 100 [100] b d 010 [010] c d 001 [001] Reciprocal space: Unit cell vectors: a*,b* magnitude direction a* 1/d 100  b and c b* 1/d 010  a and c c* 1/d 001  a and b 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. Orthorhombic

31 Lattice Vectors 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 In a crystal of any structure, g hkl is normal to the (hkl) plane and has a length inversely proportional to the interplanar spacing of the planes. (111) - d [111] -

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

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

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

36 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. SAD aperture Virtual aperture specimen Objective lens Diffraction pattern Back focal plane Selected Area Electron Diffraction parallel beam

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

38 SAED Patterns of Single Crystal, Polycrystalline and Amorphous Samples abc a.Single crystal Fe (BCC) thin film-[001] b.Polycrystalline thin film of Pd 2 Si c.Amorphous thin film of Pd 2 Si. The diffuse halo is indicative of scattering from an amorphous material. r1r1 r2r

39 Diffraction Spot Intensity Spot intensity: I hkl  lF hkl l 2 F hkl - Structure Factor F hkl =  f j exp[2i(hu+kv+lw)] N j=1 f j – atomic scattering factor f j  Z, sin/ h,k,l are Miller indices and u,v,w fractional coordinates

40 (311)? _ [013] _ SAED

41 the table SAED d hkl = L/r hkl 50nm SAED Patterns

42 TEM Convergent beam electron diffraction (CBED) Chemical analysis EDS, Electron Energy Loss Spectroscopy (EELS) Energy Filtered Imaging Secondary Ion Mass Spectroscopy (SIMS) Next Lecture


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