Electron Microscopes

• Used to count individual atoms

What can electron microscopes tell us?• Morphology– Size and shape

• Topography– Surface features (roughness, texture, hardness)

• Crystallography– Organization of atoms in a lattice

Electron Microscopes: CrystallographyCrystallography:• Arrangement of atoms• Crystals have atoms arranged in ordered lattices• Amorphous: no ordering of atoms

Crystallography affects properties (electrical, strength, etc)

Microscopes: History• Light microscopes– 500 X to 1000 X magnification– Resolution of 0.2 mm– Limits reached by early 1930s

• Electron Microscopes– Use focused beam of electrons instead of light

• Transmission Electron Microscope (TEM)• Scanning Electron Microscope (SEM)

Scanning Electron Microscopy (SEM)• Provides information about:– Topography of sample or structure– Chemical composition near the surface of sample

• Magnification: ~10X to 300,000X

• Resolution– Nanometer scale– Dependent on:• wavelength of electrons ()• Numerical aperture of lens system (NA)

– Electron gathering ability of the objective– Electron providing ability of the condenser

R 2NA

SEM Instrument• Electron beam– Spot size ~5 nm– Energy ~200 - 50,000 eV (electron volts)– Rastered over surface of sample

• Emitted electrons collected on a cathode ray tube (CRT) to produce SEM images

SEM: How it works

1. Electron beam strikes surface and electrons penetrate surface

2. Interactions occur between electrons and sample3. Electrons and photons emitted from sample4. Emitted electrons captured on CRT5. SEM image made from detected electrons

SEM: Electron Beam InteractionsValence electrons

– Inelastic scattering: Energy transferred to atomic electron– If atomic electron has high enough energy can be emitted from

sample– “Secondary electron” if energy of emitted electron <50 eV

Atomic nuclei– “Backscattered electrons” – Elastic scattering: e- bounce off with same amount of energy– Atoms with high atomic numbers cause more backscattering

Core electrons– Core electron ejected from sample; atom becomes excited– To return to ground state, x-ray photon or Auger electron emitted

SEM and TEM Instruments

http://www.vcbio.science.ru.nl/en/image-gallery/electron/

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Electron Spectroscopy1. Electron or photon strikes an unexcited atom (in ground

state)2. Electron from inner shell of atom is ejected3. Electron from outer shell fills inner shell hole4. Energy is released as X-ray or ejection of a third electron

from further shell• X-ray: Energy Dispersive X-ray Spectroscopy (EDS)• Electron: Auger Electron Spectroscopy (AES)

Emitted energy is characteristic of a specific type of atom because each atom has its own unique electronic structure and energy levels.

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Electron Spectroscopy

1. Electron or photon strikes an unexcited atom (in ground state)2. Electron from inner shell of atom is ejected3. Electron from outer shell fills inner shell hole4. Energy is released as X-ray (EDS) or ejection of a third electron (AES)

from further shell

N&N Fig. 8.12

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Summary of Electron Spectroscopy

• AES is a surface analytical technique– detected electrons are emitted from surface

layers less than 1.5 nm deep. To study deeper, must etch away top layer of atoms and perform AES again.

• AES can broadly detect almost all elements• EDS can only detect elements with atomic

number greater than 11.• EDS can be used for quantitative analysis of

chemical compositions.

A recent SEM image taken at the University of Michigan…

http://www.nanobama.com/

“Each face is made of approximately 150 million tiny carbon nanotubes; that's about how many Americans voted in the 2008 presidential election.”