BACKGROUND THEORY AND TERMINOLOGY FOR ELECTRON MICROSCOPY FOR CyberSTEM PRESENTATIONS
Feeding tube from a moth under the scanning electron microscope
Scanning Electron Microscope 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 ( mm)
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 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.
+ - 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
Characteristic chlorine peak Characteristic carbon peak Energy of packets in 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 Analysis Fitting Coefficient : Element (keV) mass% Error% At% Compound mass% Cation K C K O K Cl K Total