Electron microscopy - part 3 ( Electron/specimen interaction)

Electron Specimen interaction

TEM has a higher resolution (anything reflected is SEM, scanning electron microscopy) (anything transmitted is TEM, Transmitted electron micrsocpy).

Auger Electron spectroscopy (AES)

The interaction volume depends on the following factors:

• Atomic number of the material being examined.

• Accelerating voltage being used.

• Angle of incidence for the electron beam.

High Z = cause deceleration of primary electrons.

(Referring to the analysis of gold = 2 and a ½ nanometres)

Reaction Volume

The range of an electron, r, the maximum straight-line distance between where an electron enters and its final resting place, for a given E_o is given by:

Electron- specimen interaction

Reaction Volume

To observe a medulla in a hair sample on an electron microscope, you need to slice the hair in half.

K shell = 1S orbital

(Cover atomic absorption and the Jablonski diagram)

SECONDARY ELECTRONS (SE1)

• This ionized electron or SE that leaves the atom has a very small kinetic energy (5eV).

• Due to their low energy, 5eV, only SE that are very near the surface (<10 nm) can exit the sample and be examined.

• Production of SE is very topography related.

BACKSCATTERED ELECTRONS (BSE)

• The production of BSE varies directly with atomic number.

• This differing production rates causes higher atomic number elements to appear brighter than lower atomic number elements.

• This interaction is utilized to differentiate parts of the specimen that have different average atomic number.

• The radius of a hemispherical region (R) from which backscattered electrons are produced is:

electrons micrograph of money

BACKSCATTERED ELECTRONS (BSE)

• SE2- Elastically scattered electrons (BSE) generate SE as they leave the surface.

• SE3 - BSE collide with the SEM equipment in range of the specimen to produce SE.

• X-rays have a characteristic energy which is unique to the element.

• Provide compositional information about the specimen.

• Compositional information of this type collected in a relatively large area (~1-2 µm in diameter).

X-RAY - Bremsstrahlung (Braking radiation)

• question on X-ray analysis in exam!

X-ray Lines

EDX = energy dispersive x-ray.

X-ray lines

The relationship between atomic number (Z) and characteristic x-ray wavelength (λ) was established by Henry G.J. Moseley in 1914:

where, 

C is a constant for a given x-ray spectral line and σ is called the shielding constant.

put equation into straight line (y=mx+c)

= you you plot z as y and x as 1/the root of lambda

EDX Spectrum

EDX Maps of Money

sample preparation

For conventional imaging in the SEM, specimens must be:

• Electrically conductive.

• Electrically grounded.

• Clean.

• Non-conductive specimens are usually coated.

• ultrathin coating of Au, Au/Pd alloy, Pt, Os, Ir, W, Cr or C (graphite).

• Use sputter coating or CVD.

Environmental SEM

• Non-conducting specimens causes the accumulation of electrostatic charge at the surface.

lines in the image are evidence of charging, can be overcome by environmental SEM.

sample charging: extreme cases

Environmental SEM

• Environmental SEM instruments place the specimen in a relatively high pressure (low vacuum) chamber.

• The high pressure region around the sample in the ESEM neutralizes charge and amplifies the SE1 signal.

• gas is introduced, tiny amount, the ion bumps into gas on its way out, this causes a chemical reaction (ionisation of gas molecule) (light can be generated) (the gas generates a new electron).

• cannot do high resolution in environmental mode.

• (steps to follow)

Manmade textile

un-coated

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