BACKGROUND THEORY AND
TERMINOLOGY FOR
ELECTRON MICROSCOPY
FOR CyberSTEM
PRESENTATIONS
Feeding tube from a moth under the
scanning electron microscope
What is scale all about?
Scanning
Electron
Microscope
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.
Electron beam
produced here
Cross section of
electromagnetic
lenses
Sample
Beam passes down the
microscope column
Electron beam now tends to
diverge
But is converged by
electromagnetic lenses
Diagram of Scanning Electron Microscope or SEM
in 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.
A new electron is
knocked out (as a
secondary electron)
-
+
An incoming electron
rebounds back out (as a
backscattered electron)
-
+
Energy of electron from
beam is lost to atom
Inelastic scattering
Elastic scattering
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.
Example of an image using a scanning electron
microscope and backscattered electrons
Grain containing
of silica so it is
darker
Here the differing contrast of the
grains tells us about composition
Grain containing
titanium so it is
whiter
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
Understanding compositional analysis using X-rays
and the scanning electron microscope
-
+
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
EDS output from X-rays
Amount of
packets
1050
900
CKa
1200
Characteristic carbon peak
006
Characteristic oxygen peak
Characteristic chlorine peak
ClKa
600
OKa
Counts
750
450
300
150
0
0.00
1.00
2.00
3.00
4.00
5.00
keV
Energy of packets
in thousands of electron volts
6.00
7.00
8.00
9.00
10.00
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 Analysis
Fitting Coefficient : 0.4050
Element
(keV)
mass% Error%
At% Compound
C K
0.277
65.88
0.08
74.01
O K
0.525
28.12
0.72
23.71
Cl K
2.621
6.00
0.20
2.28
Total
100.00
100.00
mass%
Cation
K
75.5733
34.1444
13.7857