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Structure of thin films by electron diffraction

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Structure of thin films by electron diffraction. J nos L. L b r ... Least-square fitting of whole-pattern. Fitting parameters: Scale-factor. Atomic positions ... – PowerPoint PPT presentation

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Title: Structure of thin films by electron diffraction


1
Structure of thin films by electron diffraction
  • János L. Lábár

2
Usage of diffraction data in structure
determination
  • Identifying known structures
  • Solving unknown structures
  • Structure determination
  • Unit cell dimensions
  • Space group symmetry
  • Unit cell content (atoms and their appr.
    coordinates)
  • Structure refinement
  • More accurate atomic coordinates
  • Validation of the structure (attainable match)

3
Structure determination
  • Periodic functions ? Fourier coefficients
  • Amplitude diffraction ? the phase problem
  • Phase real space (HRTEM, fragment)
  • reciprocal space (Direct methods)
  • Single crystal diffraction
  • X-rays, neutrons ? electrons
  • Powder diffraction
  • X-rays, neutrons ? electrons

4
Single crystal diffraction
  • Tilting experiments
  • Identification of reflections indexing
  • Unit cell dimensions
  • Space group symmetry (XRD, SAED, CBED)
  • Integration of individual intensities
  • Background
  • Phases (real ? reciprocal space)
  • Dynamic intensities in SAED

5
Single crystal diffraction
  • XRD
  • Up to 2000 atoms in the asymmetric unit
  • Up to 100 atoms guaranteed success
  • Rule of thumb refl gt 10 atoms
  • SAED
  • CRISP, ELD ? Direct methods (EDM)
  • Dynamic intensities in SIR97
  • Up to 30 atoms in the asymmetric unit
  • Size, image

6
Powder diffraction
  • Collapse of 3D into 1D
  • Types
  • Equivalent reflections, multiplicity
  • Exact overlap e.g. (43l) ? (50l) in tetragonal
  • Accidental within instrumental resolution
  • Indexing programs
  • Peak decomposition
  • La Bail
  • Pawley

7
Powder diffraction
  • Degree of overlap Resolution
  • Background
  • Instability negative peaks / oscillating int.
  • XRD ( refinement from neutrons)
  • Synchrotron 60 atoms in asymmetric unit
  • Laboratory ? 30 atoms in asymmetric unit
  • Neutron better for refinement
  • SAED instrumental resolution limit, BKG

8
Powder diffraction SAED resolution (peak width)
  • Beam convergence
  • Elliptical distortion
  • OL spherical aberration ? size of selected area

9
Powder diffraction SAED elliptical distortion
10
Powder diffraction SAED spherical aberration
11
Powder diffraction Pattern decomposition with
ProcessDiffraction
  • Background
  • Normal, log-Normal
  • Polynomial, Spline
  • Peak shapes
  • Gaussian, Lorentzian
  • Pseudo-Voigt
  • Global minimum
  • Downhill SIMPLEX
  • Manual control
  • Example Al Ge SAED on film
  • Large crystal Al Gaussian
  • Small crystal Ge Lorentzian

12
Pattern decomposition with ProcessDiffraction
13
Structure refinement The Rietveld method
  • Start from assumed structure
  • Least-square fitting of whole-pattern
  • Fitting parameters
  • Scale-factor
  • Atomic positions
  • Temperature factors
  • Cell parameters
  • Peak shape parameters (instrumenal ? sample)
  • Background
  • Additional peaks (phase)

14
The Rietveld method
  • Most known structures from Rietveld refinement
  • Scaling factors ? Quantitative phase analysis
    (volume fractions)
  • Neutrons no angle dependence ? best for
    refinement
  • Resolution (peak width) is less important ? SAED
    can also be used efficiently for refinement
  • SAED Cell parameters ? camera length

15
Quantitative phase analysis for nanocrystalline
thin films from SAED
  • Example
  • 100 Å Al 100 Å NiO
  • Measured volume fraction by ProcessDiffraction
    51 Al 49 NiO
  • Fitted parameters peak parameters, L, scaling
    factors, DW

16
Structure refinement from SAED
  • Integrated intensities
  • ELD
  • ProcessDiffraction
  • Refinement
  • FullProf
  • ProcessDiffraction
  • Simple example TiO2 Anatase
  • Selection of origin ? transform z before compare

17
Structure refinement with ProcessDiffraction
  • Structure definition modul
  • Checks coordinates ? site symmetry
  • Options modul checks
  • if selected site is refinable (variation of
    coordinate value does not change site symmetry)
  • If selected options are reasonable
  • Cross-checking for nanocrystalline samples
  • Pair correlation function (different models ?
    measured)

18
ProcessDiffraction Options for refinement
19
Structure refinement with ProcessDiffraction
  • Example Anatase 4 nm powder
  • Acceptable match
  • Refined position of oxygen z0.217
  • Compare to
  • z0.2064 (Pearsons)
  • z0.2094 (Weirich transformed)

20
Is the example result acceptable?Independent test
  • Pair correlation function
  • Measured
  • Calculated for both structures
  • Refined result is in agreement with g(r)

21
Remarks to refinement
  • Nanocrystalline films are strained
  • Exact shape and size of the background is
    ambiguous in electron diffraction
  • Refined position is also a function of refined
    cell dimensions (accurate calibration of camera
    length)

22
Conclusions structure of thin films by electron
diffraction
  • Phase identification from both single crystal and
    powder patterns
  • Quantitative phase analysis from nanocrystalline
    powder patterns
  • Structure determination from single crystal
    patterns (SAED, CBED)
  • Structure refinement from nanocrystalline powder
    patterns
  • Limits are still to be examined
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