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