Chapter 16: Electron Diffraction

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Presentation transcript:

Chapter 16: Electron Diffraction Crystal Structure and Crystallography of Materials Chapter 16: Electron Diffraction

Electron Diffraction: a) Accelerate through known potential ⇒ highly monochromatic b) light, negatively changed particles ⇒ can be focused easily with magnetic lens c) Easily detected ⇒ visible on fluorescent screen recorded on photographic film d) High current densities ⇒ W-filament, LaB6-filament e) Easily scattered (fe ≈ 104 fx-ray) ⇒ Absorption is strong ⇒ Need thin samples (specimen preparation is difficult) f) Not bulk diffraction: scattering occurs only thin part of the sample

Electron Wave Generation: Thermionic emission Field-effect emission where c is the speed of light, which is ~3 x 108 m/s. Therefore, the wavelength at 100 keV, 200 keV, and 300 keV in electron microscopes is 3.70 pm, 2.51 pm and 1.96 pm, respectively.

Ewald sphere construction: ∙ λ → small → Large Ewald circle Thin sample (hkl) r*hkl 1/λ 2θ

Electron Diffraction in TEM: λ = 0.0037Å for 100KeV In TEM, l is small while d is constant for material, therefore, has small Bragg angle Therefore, accuracy of d-spacing determination is not as good as X-ray techniques.

Electron Diffraction in TEM: r*hkl 1/λ 2θ rhkl L T D Photo

Electron Diffraction in TEM: Alternatively, Electron diffraction pattern is a direct representation of reciprocal lattice Single crystal Polycrystal

Phase Identification using Diffraction Pattern in TEM:

Phase Identification using Diffraction Pattern in TEM:

Convergent Beam Electron Diffraction: Space group determination accurate determination of lattice parameter 3-D information Schematic diagram to show the formation of convergent-beam electron diffraction patterns

FOLZ and HOLZ:

FOLZ and HOLZ:

FOLZ and HOLZ:

Low Energy Electron Diffraction: Adsorption chemistry e- : 40 ~ 120eV LEED Diffraction Phenomena 1) wavelength 2) scattering 00 10 20 01 02

High Resolution Image (Phase Contrast Image):

High Resolution Image (Phase Contrast Image):

High Resolution Image (Phase Contrast Image):

High Resolution Image (Phase Contrast Image):

High Resolution Image (Phase Contrast Image):

Crystal Structure and Crystallography THE END of Crystal Structure and Crystallography Thanks !!!!