1. 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

2. Electron Microscopes: Crystallography Crystallography: Arrangement of atoms Crystals have atoms arranged in ordered lattices Amorphous: no ordering of atoms Crystallography affects properties (electrical, strength, etc)

3. 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)

4. 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

5. 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

6. SEM: How it works 1.Electron beam strikes surface and electrons penetrate surface 2.Interactions occur between electrons and sample 3.Electrons and photons emitted from sample 4.Emitted electrons captured on CRT 5.SEM image made from detected electrons

7. SEM: Electron Beam Interactions Valence 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

8. SEM and TEM Instruments http://www.vcbio.science.ru.nl/en/image-gallery/electron/

9. 9 Electron Spectroscopy 1.Electron or photon strikes an unexcited atom (in ground state) 2.Electron from inner shell of atom is ejected 3.Electron from outer shell fills inner shell hole 4.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.

10. 10 Electron Spectroscopy 1.Electron or photon strikes an unexcited atom (in ground state) 2.Electron from inner shell of atom is ejected 3.Electron from outer shell fills inner shell hole 4.Energy is released as X-ray (EDS) or ejection of a third electron (AES) from further shell N&N Fig. 8.12

11. 11 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.

12. 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.”