Defect and chemical analysis in the TEM

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THE UNIVERSITY Defect and chemical analysis in the TEM Defect and chemical analysis in the TEM I.P.

Chemical analysis: PEELS Centre for Electron Microscopy OF BIRMINGHAM Defect and chemical analysis in

$Chemical analysis: PEELS Mode 2 (imaging) Diffraction contrast Phase contrast Contrast mechanisms in$

1 THE UNIVERSITY Defect and chemical analysis in the TEM Defect and chemical analysis in the TEM I.P. Jones. Chemical analysis: EDX. Chemical analysis: PEELS Centre for Electron Microscopy OF BIRMINGHAM Defect and chemical analysis in the TEM Mode 1 (SAD). Chemical analysis: EDX. Chemical analysis: PEELS Mode 2 (imaging) Diffraction contrast Phase contrast Contrast mechanisms in the TEM Secondary defects only absorption contrast strain contrast Two beam conditions Origin of contrast: movement of atoms perpendicular to reflecting planes. Movement of atoms within planes gives no change of intensity (contrast)

$diffraction contrast R$ s fringes Sometimes

2 Stacking fault Analysing stacking faults using diffraction contrast R Stacking faults Stacking fault fringes Sometimes stacking faults disappear.. g! R = n Stacking faults and APB in Fe Ge

$Why stacking faults are invisible when g.r = integer : 9 10 Analysing stacking faults using diffraction contrast g.r = 0 9 10 g.$ r = 1.25 g.r = 1 Note that: 1. When g and R are perpendicular ( g.

r = 1.25 g.r = 1 Note that: 1. When g and R are perpendicular ( g.

r = 1 are exactly equivalent, as are g.r = 0.25 and g.r = 1.25: g.r is only defined modulo 1. The usual range chosen for g.r is -0.

3 Why stacking faults are invisible when g.r = integer : 9 10 Analysing stacking faults using diffraction contrast g.r = g.r = 0.25 Use a series of two beam conditions g.r = 0.5 g.r = reflecting planes give diffraction contrast Only displacements perpendicular to the 9 g.r = 1.25 g.r = 1 Note that: 1. When g and R are perpendicular ( g.r = 0) the stacking fault has no effect on the reflecting planes: it is invisible. 2. g.r = 0 and g.r = 1 are exactly equivalent, as are g.r = 0.25 and g.r = 1.25: g.r is only defined modulo 1. The usual range chosen for g.r is -0.5 to Other aspects of SF contrast: (i) Nature of outer fringes gives sign of g.r (ii) Similarity BF-DF gives top/bottom of foil. Screw dislocation Analysing dislocations using diffraction contrast

$Analysing dislocations using diffraction contrast Edge dislocation Dislocation defined by Burgers vector b and line$ bxu contrast) u measured by Stereographic projection Burgers vector Stereoviewer Tomography Electropolishing g.

bxu contrast) u measured by Stereographic projection Burgers vector Stereoviewer Tomography Electropolishing g.

Longitudinal section of the 10% cold rolled Interstitial free steel.

4 Analysing dislocations using diffraction contrast Edge dislocation Dislocation defined by Burgers vector b and line direction u b: most displacement parallel to b, so invisibility for g.b = 0 (edge dislocations give weak g.bxu contrast) u measured by Stereographic projection Burgers vector Stereoviewer Tomography Electropolishing g.b analysis in Al-2%Mg TEM foil prepared by Electropolishing. Longitudinal section of the 10% cold rolled Interstitial free steel. Burgers vector analysis with the help of a computer To make the images narrower and show more detail of the dislocation core, dark field weak beam imaging is used. The weak beam is a diffracted beam, several Bragg angles away from its Bragg position.

Defect and chemical analysis in the TEM.

Chemical analysis: PEELS Analysing dislocations

contrast The principles are the same, but the

A Burgers circuit in TiAl (M. Aindow et al. Int.

b = 1 [ 1 ] 6 6 Total b = 1 [ 211] (Aindow et al.

5 Defect and chemical analysis in the TEM. Chemical analysis: EDX. Chemical analysis: PEELS Analysing dislocations using phase contrast Defect analysis: phase contrast The principles are the same, but the way they are exploited is rather different. A Burgers circuit in TiAl (M. Aindow et al. Int. Sc. 12, 29 (200) Projected b = 1 [ 1 ] Projected b = 1 [ 1 ] 6 6 Total b = 1 [ 211] (Aindow et al. Int. Sc. 12, 29 (200.) Thin γ lamellae in a TiAl alloy (Zhang et al. Acta Mat. 52, 191 (200)

6 Defect and chemical analysis in the TEM Chemical analysis: EDX. Chemical analysis: EDX. Chemical analysis: PEELS Same as SEM except: 1. No WDX. 2. Quantification: ratios only (no single element standards). Calculation or standards. 1. Spatial resolution controlled by beam rather than kv and Z. Surfaces usually have wrong composition. [010] [001] Stacking fault P concentration, wt.% P profile !! Measurement 1 " Measurement 2 # Measurement! " # # "! "!!!!!! " " # # " # # # # " " " # "! # " #! nm X-ray profile across a stacking fault in Nb Al Mo and Cr concentration, wt.% Distance, nm Mo Profile Cr Profile Distance, nm Image of grain boundary and variation in P, Cr and Mo concentration across the boundary in 2.25Cr1Mo steel. Quantmap. Beam FWHM ~ 1nm (Ding, Rong, and Knott) High Resolution - when counts are limited Drift detectors in the TEM: fast maps Size of the detector is limiting, for example, for: Small specimens High spatial resolution

7 DT Tran Exploiting high count rates when mapping SDD sensors can deal with high count rates where generated This can often be very useful in TEM! Defect and chemical analysis in the TEM. Chemical analysis: EDX. Chemical analysis: PEELS Chemical mapping of a 6MB DRAM in a TEM using SDD Schematic diagram of PEELS Schematic diagram of EELS Spectrum

Typical spectrum from an MgO grain Examples of EELS

Low energy region Examples of EELS Low energy region

Determination of Ni valence i. Size of edges ii.

8 Typical spectrum from an MgO grain Examples of EELS analysis i. Size of edges ii. Shape of edges iii. Low energy region Examples of EELS analysis i. Size of edges ii. Shape of edges iii. Low energy region (from Williams and Carter) Examples of EELS analysis Determination of Ni valence i. Size of edges ii. Shape of edges iii. Low energy region

9 Examples of EELS analysis Intensity (arb.units) Ce + Ce + Intensity (arb.units) From CePd From CePd i. Size of edges ii. Shape of edges iii. Low energy region Energy Loss (ev) Energy Loss (ev) Edges from standards. Ce + from CePO.xH 2 O and Ce + from CeO 2. Edges from Pd Ce and Pd Ce. Five steps to get the dielectric function and DOS Interband transition strength of matrix and Σ1 grain boundary Bulk and many grain boundaries Σ1 grain boundary A : O2p-Ti; A: O2p-Ti; B: O2p-Ti; C: O2p-Sr; D: O2p-Sr; E: Srp-Ti; E : Srp- Ti d t 2g d t 2g d e g d e g d d e g d t 2g The end

Imaging with Diffraction Contrast

$Imaging with Diffraction Contrast$ Imaging with Diffraction Contrast Duncan Alexander EPFL-CIME 1 Introduction When you study crystalline samples TEM image contrast is dominated by diffraction contrast. An objective aperture to select either