BACKGROUND THEORY AND TERMINOLOGY FOR

ELECTRON MICROSCOPY

FOR CyberSTEM PRESENTATIONS

Feeding tube from a moth under the scanning electron microscope

ScanningElectronMicroscope

What is scale all about?

Resolution (not magnification!) is the ability to separate two objects optically

Unresolved

Partially resolved

Resolved

Remember that there are 1000 micrometers (µm) in 1 mm and 1000 nanometers (nm) in 1 µm.

The human eye can separate 0.2 mm at a normal viewing distance of 25 cm

The light microscope can separate 0.2 µm (0.002mm) depending on wavelength of light used

Electrons have a smaller wavelength than light therefore provide the highest resolving power – about 2 nm (0.000002mm)

With enough resolution we can magnify an object many millions of times and still see new detail

This is why we use electron microscopes

If you magnified your thumb nail just 10,000 times it would be about the size of a football pitch.

For example think of the size of Suncorp Stadium in Brisbane

The Scanning Electron Microscope is analogous to the stereo binocular light microscope because it looks at surfaces rather than through the specimen.

Beam passes down the microscope column

Electron beam now tends to diverge

But is converged by electromagnetic lenses

Cross section of electromagnetic lenses

Electron beam produced here

Sample

Diagram of Scanning Electron Microscope or SEMin cross section - the electrons are in green

Electromagnetic Lenses

An electromagnetic lens is essentially soft iron core wrapped in wire

As we increase the current in the wire we increase the strength of the magnetic field

Recall the right hand rule electron will move in a helical path spiralling towards the centre of the magnetic field

Electron beam – Specimen Interaction. Note the two types of electrons produced.

Electrons from the focused beam interact with the sample to produce a spray of electrons up from the sample. These come in two types – either secondary electrons or backscattered electrons.

As the beam travels across (scans across) the sample the spray of electrons is then collected little by little and forms the image of our sample on a computer screen.

We can look more closely at these two types of electrons because we use them for different purposes.

+

-

Inelastic scattering

+

-

Elastic scattering

Energy of electron from beam is lost to atom

An incoming electron rebounds back out (as a backscattered electron)

A new electron is knocked out (as a secondary electron)

Example of an image using a scanning electron microscope and secondary electrons

Here the contrast of these grains is all quite similar.We get a three-dimensional image of the surfaces.

Grain containing titanium so it is whiter

Grain containing of silica so it is darker

Example of an image using a scanning electron microscope and backscattered electrons

Here the differing contrast of the grains tells us about composition

So how does this work – telling composition from backscattered electrons?

The higher the atomic number of the atoms the more backscattered electrons are ‘bounced back’ out

This makes the image brighter for the larger atoms

Titanium – Atomic Number 22

Silica – Atomic Number 14

+

-

Inelastic scattering

If the yellow electron falls back again to the inner ring, that is to a lower energy state or valence, then a burst of X-ray energy is given off that equals this loss.

This is a characteristic packet of energy and can tell us what element we are dealing with

Understanding compositional analysis using X-rays and the scanning electron microscope

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

keV

006

0

150

300

450

600

750

900

1050

1200

Cou

nts

CKa

OKa

ClKa

Characteristic chlorine peak

Characteristic carbon peak

Energy of packetsin thousands of electron volts

Amount of packets Characteristic oxygen peak

EDS output from X-rays

Using X-rays to investigate composition in this way is called Energy Dispersive Spectroscopy (EDS) since it produces a spectrum graph

We can get quite detailed information about mass and atomic percentages in materials from EDS

phi-rho-z Method Standardless Quantitative AnalysisFitting Coefficient : 0.4050Element (keV) mass% Error% At% Compound mass% Cation K C K 0.277 65.88 0.08 74.01 75.5733 O K 0.525 28.12 0.72 23.71 34.1444Cl K 2.621 6.00 0.20 2.28 13.7857Total 100.00 100.00