Electron Diffraction (ED) in the
transmissions electron microscope
A.E. Gunnæs
MENA3100 V08
JEOL 2010F in the Research park
A.E. Gunnæs
MENA3100 V08
Basic TEM
Electron source (HV= 200kV)
Apertures
Magnetic lenses
Sample holder
Vacuum in the column
better than 10-6 Pa
Fluorescence screen
Film box
Pedals for tilting the sample
A.E. Gunnæs
MENA3100 V08
The lenses and apertures in a TEM
ΔU= ~100-1000 kV
Filament
λ= h/(2meV)0.5
Electron source:
(NB non rel. Expr)
Anode
●Tungsten, W
1. and 2. condenser lenses
Condenser aperture
● LaB6
● FEG
Sample
Objective lens
Objective aperture
Recording media:
Selected area aperture
(diffraction aperture)
Intermediate lenses
●Film
● Imaging plates
● CCD camera
Projector lens
Image
A.E. Gunnæs
Fluorescence Screen
(ZnS or ZnS/CdS powder)
MENA3100 V08
●TV camera
c
Simplified ray diagram
b
a
Parallel incoming electron beam
3,8 Å
Si
Sample
Convergent beam
-Discs in diffraction
-Micro diffraction
-STEM mode
-Chemical analysis
1,1 nm
PowderCell 2.0
Objective lense
Diffraction plane
(back focal plane)
Image plane
A.E. Gunnæs
MENA3100 V08
Electron diffraction
Elastic scattered electrons
Inelastic scattered electrons
Only the direction of v is changing.
(Bragg scattering)
Direction and magnitude of v change.
Elastic scattering is due to Coulomb interaction
between the incident electrons and the electric
charge of the electron clouds and the nucleus.
(Rutherford scattering).
The elastic scattering is due to the average
position of the atoms in the lattice.
Energy is transferred to electrons and atoms
in the sample.
-It is due to the movements of the atoms
around their average position in the lattice.
- It give rise to a diffuse background in the
diffraction patterns.
Reflections satisfying Braggs law:
2dsinθ=nλ
Electrons interacts 100-1000 times stronger with matter than X-rays
-more absorption (need thin samples)
-can detect weak reflections not observed with x-rays
A.E. Gunnæs
MENA3100 V08
Sample preparation
TEM grids
3 mm
A.E. Gunnæs
MENA3100 V08
Sample holders
Sample positions
Cooling
Standard
Heating
A.E. Gunnæs
MENA3100 V08