Surface Structure Determination by LEED Surface: outermost 3 5 layers, 10 Low Energy Electron Diffra

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Surface Structure Determination by LEED
Surface outermost 3 5 layers, 10?Low
Energy Electron DiffractionElectron energy 10
400 eV For Advanced Solid State Physics
965 --- Spring 2004
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Outline

• Experimental apparatus
• 2D crystallography and elastic diffraction
• Diffraction pattern and I-V curve

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Fig. 1 Schematic diagram of LEED optics
including the different elements of the electron
gun, screen, and grids.
Source Website
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Experimental Apparatus

• Video-LEED
• LEED optics
• Control Unit
• Computer
• CCD camera

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What is the diffraction principle?What
information will be obtained?
Source Website
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2-D Crystallography

• Atomic structure Bravais lattice basis.
• Reciprocal Lattice
• , where and are
  direct lattice vectors and reciprocal lattice
  vectors, respectively.
• The reciprocal lattice remains the symmetry of
  the direct lattice.

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5 possible 2D Bravais lattices
Source Website
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Elastic Diffraction
Source Website

• Kinematic scattering under breaking of normal
  translational symmetry.
• Laue equations
• Establishment of Ewald sphere
• http//www.chembio.uoguelph.ca/educmat/chm729/reci
  p/9surew.htm

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Diffraction pattern and I-V curve

• Two aspects
• Diffraction pattern to determine the real-space
  lattice
• I-V curves to determine the atomic position in
  the unit cell

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Bismuth(111) diffraction pattern
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Energy (eV)Bi(111) I-V curve for beam(1-1) at
different temperatures
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• Why not X-ray?
• Reflection coefficient
• Too small reflectivity of X-ray.
• Surface sensitivity
• Strong penetration of X-ray is not appropriate
  for being a surface probe.
• Cu Ka X-ray is of 1.542 Å.

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Challenges

• Complex surface structure
• Reconstruction, relaxation
• Defects
• Adsorbate
• Inelastic scattering
• Multiple Scattering due to strong interaction
  between electrons and ion-cores
• Hundreds of surface structures were observed,
  but much less than number of bulk structures by
  X-ray diffraction.

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• Danke!

Merci!
Thanks!
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