CHAPTER 8(Chapter 11 in text)

Characterization of Nanomaterials

Global Methods of Characterization

SURFACE AREASURFACE AREA

Assuming spherical particle shape,Surface are per gram (specific surface area), A is given by:

DD

DA

66 23

Assuming a density of 3.5g/cm3

BET (Brunauer, Emmett and Teller)

What if it is possible to coat the individual particles with a monolayer of gas molecules? If we knew how much of the gas was adsorbed (physisorped), we can measure the surface area.

X-ray DiffractionX-ray Diffraction

sin2dn FOR CONSTRUCTIVE INTERFERENCE

Bragg’s LawBragg’s Law

X-ray striking atom = interaction with electrons

vibrate with frequency of x-ray

re-radiate X-rays of the same frequency (scattered)

Constructive interference when angle of incidence = angle of reflection

Titania

cosb

D

Constant (0.89-1.39)

Peak width at half height

ELECTRON MICROSCOPY

Lowest resolved feature is half of the wavelength of the illuminating waves

In light optical systems: (short wavelengths applied 400nm)

This leads to 200nm min resolved features.

Electron waves have much shorter wavelengths that can be controlled by the accelerating voltage.

Selects electrons of narrow energy band)

Focuses electrons at the specimen

Picture enlarged

Limit electron beam (can adjust brightness and contrast)

Some in elastic scatter electrons lose energy to below allowed amount, hence chromatic correction.

Field emission point source accelerated to required level (200-300KeV)

Primary electrons hit specimen and majority pass through the sample (after elastic scattering or diffraction). Electrons interacting with specimen transfer parts of their energy to electrons in the specimen, resulting in the emission of:

- Secondary electrons- X-rays- Auger electrons- Inelastic scattered electrons passing through the specimen

Electrons hit the atom and lose energy Which in turn excites atom and emit x-ray photon

Both characteristic of the specimen

Auger electron: For low atomic number (Z) specimens, emitted x-ray photon pushes out electron of low energy, electron has energy characteristic of the atom.

The higher the energy of the primary x-ray photon (equivalent to higher atomic number), the lower the probability of emission of Auger electron.

THE END