1 / 42

Limits of OM, SEM, SPM and XRD

G.B. NN. Limits of OM, SEM, SPM and XRD. core. G.B. NN. shell. BT. BT. BT-BaTiO 3 NN-NaNbO 3 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,

valentine-glover
Download Presentation

Limits of OM, SEM, SPM and XRD

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. G.B. NN Limits of OM, SEM, SPM and XRD 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.

  2. Chemical analysis at a nanometer scale

  3. 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.

  4. An Introduction to Transmission Electron Microscopy (TEM)/Scanning TransmissionElectron Microscopy (STEM) • What is a TEM? • How it works - gun, lenses, specimen stage • 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

  5. Basic features of A Modern 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 Cost:  $4,000,000

  6. A simple analogue

  7. A better comparison

  8. 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.

  9. Electron Beam Source

  10. The Lenses in TEM 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. B Four lenses form magnification system-determine the magnification of the microscope. Whenever the magnification is changed, the currents through these lenses change.

  11. Image Formation in TEM Control brightness, convergence binocular Control contrast screen negatives Ray Diagram for a TEM

  12. Why Electrons? Resolution! In the expression for the resolution (Rayleigh’s Criterion) r = 0.61/nsin -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 Green Light 200kV Electrons ~400nm ~0.0025nm n~1.7 oil immersion n~1 (vacuum) r~150nm (0.15m) r~0.02nm (0.2Å) 1/10th size of an atom! UNREALISTIC! WHY?

  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. 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 STEM detector or EELS HAADF-high angle annular dark-field

  16. 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 4 5 3 • 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.

  17. 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.

  18. Rotation, tilting, heating, cooling and straining Specimen Holder beam holder a split polepiece objective lens Double tilt heating Heating and straining Twin specimen holder

  19. Specimen Holder with Electrical Feedthroughs

  20. Imaging BF and DF imaging HREM

  21. BF & DF Imaging – Diffraction Contrast DDFCDF OA OA Beam tilt crystal C-film amorphous D D T-transmitted D-diffracted Objective aperture T Objective aperture T DF image BF image Hole in OA C-film C-film crystal crystal Diffraction + mass-thicknessContrast

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

  23. Bright Field (BF) and Dark Filed (DF) Imaging BF imaging-only transmitted beam is allowed to pass objective aperture to form images. mass-thickness contrast Incident beam BF specimen beam diffracted 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)

  24. Phase Contrast ImagingHigh Resolution Electron Microscopy (HREM) T 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.

  25. Geometry for e-diffraction Electron Diffraction  Bragg’s Law:l = 2dsin e- l=0.037Å (at 100kV) =0.26o if d=4Å dhkl Specimen foil l = 2d L 2 r/L=sin2 as  0 r/L = 2 r/L = l/d or r = lLx r T D Reciprocal lattice 1 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

  26. 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 is used in reciprocal lattices.

  27. 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 d10 [10] b d01 [01] Reciprocal space: Unit cell vectors:a*,b* magnitude direction a* 1/d10b b* 1/d01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.

  28. 3-D Reciprocal Lattice Real space: Unit cell vectors: a,b,c magnitude direction a d100 [100] b d010 [010] c d001 [001] Reciprocal space: Unit cell vectors:a*,b* magnitude direction a* 1/d100b and c b* 1/d010aand c c* 1/d001aand 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.

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

  30. 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.

  31. Why are there so many spots?The Ewald Sphere and Diffraction Pattern High energy electron case Reciprocal lattice k – wave vector lkl = 1/  – wavelength of electron

  32. Ewald’s Sphere Ewald’s sphere is built for interpreting diffraction lkl=1/ Ewald circle C incident beam diffracted beam 2 kd H ki g G 130 0 CG-C0=0G or kd-ki=g Laue equation Wherever a reciprocal lattice point touches the circle, e.g., at G, Bragg's Law is obeyed and a diffracted beam will occur. At H, no diffraction.

  33. Construction of Ewald’s Sphere • Ewald’s sphere is built for interpreting diffraction patterns and it shows which sets of planes are at (or close to) their Bragg angle for diffraction to occur. • Incident wave is represented by a reciprocal vector k (lkl=1/ and points in the direction of wave). • Construct a circle with radius 1/ (i.e., k), which passes through origin of reciprocal lattice, 0. • Wherever a reciprocal point touches the circle, Bragg’s law is obeyed and a diffracted beam will occur. • C0–incident beam and CG–diffracted beam. The angle between C0 and CG must be 2. • 0G is the reciprocal vector g130 and has magnitude of 1/d130. • 0G/2=lklsin,  0G=2/sin,  1/d130=2/sin130 •  =2d130sin130

  34. Ewald’s Sphere Construction in 3D SOLZ In a single crystal only a few sets of planes are oriented at their Bragg angle at any one time. FOLZ ZOLZ Polycrystalline sample ZOLZ, FOLZ and SOLZ are zero order, first order and second order Laue zone, respectively.

  35. parallelbeam 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 SAED Selected Area Electron Diffraction aperture specimen Objective lens Diffraction pattern Back focal plane SAD aperture

  36. Focusing SAED Pattern at Fixed Screen by changing magnetic lens strength specimen lens screen Diffracted beam Transmitted beam Spot pattern SAED (ZOLZ) gives 2-D information

  37. SAED Patterns of Single Crystal, Polycrystalline and Amorphous Samples a 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.

  38. Diffraction Spot Intensity Spot intensity: Ihkl lFhkll2 Fhkl - Structure Factor N Fhkl =  fn exp[2i(hu+kv+lw)] n=1 fn – atomic scattering factor fnZ, sin/ h,k,l are Miller indices and u,v,w fractional coordinates

  39. Indexing Diffraction Pattern-ratio technique Any 2-D section of a reciprocal lattice can be defined by two vectors so only need to index 2 spots. 1.Choose one spot to be the origin and measure r1 2.measure the spacing of a second spot r2 3.measure the angle,  4.prepare a table giving the ratios of the spacings of permitted diffraction planes in the known structure 5.take measured ratio r1/r2 and locate a value close to this in the table 6.assign more widely-spaced plane (lower indices) to the shorter r value 7.calculate angle between pair of planes of the type you have indexed 8.if measured  agrees with one of possible value, accept indexing. if not, revisit the table and select another possible pair of planes 9.finish indexing the pattern by vector addition.

  40. Indexing Electron Diffraction Patterns If we know the index for two diffraction spots It is possible to index the rest of the spots by Using vector addition as shown. Every spots Can be reached by a combination of these two Vectors.

  41. Zone Axis of A SAED Pattern Take any two g vectors, g1 (h1k1l1) and g2 (h2k2l2) from the diffraction patter. The zone axis is given by the vector product: The zone axis is parallel to the electron beam direction.

  42. _ [013] Indexing-example Cubic 1.Choose T as the origin, r1-7.75mm 2.r2-12.87mm 3.~72O 4.Get a table giving relative reciprocal lattice spacings 5.r2/r1=1.66 gives several possible pair of planes in the table 131 (311)? r2  r1 200 T r=L/d r2/r1=d1/d2 6.From the table of interplanar angle in cubic, ~72O gives only one matched pair of planes, {100} (or {200}) and {311} for a face-centered lattice. {100} diffraction is not allowed in a face-centered structure. 7.Calculating interplanar angles leads to (131) or (113) and (200) since angle between (311) and (200) is 25.2O. 8.Zone axis of pattern: r1 x r2 = [013] for (131) and (200) pair _

More Related