Transmission Electron Microscopy

Question 1

sample requirements

Answer 1

Dry (inside TEM is vacuum - liquids will evap/may cause damage)
Conductive (electrons have to be able to pass through sample)
Non-magnetic (strongly magnetic samples interfere with the electron beam/may be pulled off sample holder)
Stable under e beam (if it damages quickly, can’t analyse)
Thin (to allow e to pass through ~100nm)

Question 2

Bright field imaging

Answer 2

Collect the lightly scattered electrons
Objects that scatter the electrons more will be darker

Question 3

Dark field imaging

Answer 3

Collect the heavily scattered electrons
Objects that scatter the electrons more will be brighter

Question 4

REASONS for mass-thickness contrast

diffraction contrast

Answer 4

MASS
- atoms of high atomic number have high nuclear charge
- high atomic number (heavy) samples scatter e- more

CHARGE
- e- takes longer to travel through thick samples
- the longer the e- is in the sample, the more chance it will scatter

DIFFRACTION
- if the sample is crystalline > orientation dictates scattering

Question 5

Applications of TEM imaging

Answer 5

• particle size
• surface coating
• complex particle structure > biomed app
• cell structure > impact of bioengineered structure on biology
• particle uptake in cells
• microstructure/layering > components of a transistor, quality control and manufacturing
• grain size and shape > polycrystalline material grains

Question 6

HRTEM

Answer 6

High Resolution TEM
- image of atomic planes and their arrangement
- MASS/THICKNESS does NOT apply, contrast is due to complicated physics
- Fourier transform (FT)> to show frequencies in the image, determine atomic composition
- applications: crystal structure, or amorphous, imperfections, coatings (!)

Question 7

STEM

Answer 7

Scanning TEM
Focus beam into small probe, scan it across, collect transmitted AND scattered e- at each point
multiple detectors!

Question 8

STEM detectors

Answer 8

Bright Field
- small scattering angles
- heavy/thick things are darker
Dark Field
- medium scattering angles
- heavy/thick things are brighter
High Angle Annular Dark Field
- large scattering angles
- heavy/thick things are brighter
- NO DIFFRACTION EFFECTS

Question 9

Electron tomography (TEM techniques)

Answer 9

Acquire a series of images at different tilts
Combine 2D images with software to make 3D image