St Paul’s School / Daily Life

Sample preparation

1) What kind of samples can be analysed?

Dry, conductive samples of small size. We can image insulating samples quite well too, and you can bodge a little to get larger samples in.

2) Do they have to be painted or coated or specially dried?

Yes, wet samples must be dried. They can be dessicated in a bell jar, or kept under a heat lamp - you will need to take advice, and possibly experiment, to find the best method for your sample. You cannot paint the sample to make it conducting, but you can paint over parts of the surface to improve the dissipation of charge from the area under observation. You can coat the sample, with a plasma coater. If your charging problem proves intractable then we will send your sample to Imperial to be coated.

3) What techniques can we employ at school?

Drying, in general, unless you discover you need a specialised dessicator for delicate samples. We cannot yet coat the area you want to observe.

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SEM

1) How in general does the SEM work?

Electron wave reflection in place of light waves.

2) What is the source of the electrons?

A filament, like a very specialised version of the filaments in an old filament light bulb. This filament is heated, and it is the thermionic emission of electrons that creates the supply of electrons for acceleration.

3) How are they accelerated?

Using a special anode.

4) How are they focussed?

Electromagnets, and Lorentz forces. The electromagnets are arranged so that they behave like lenses. As lenses they are very poor on their own - like looking through the bottom of a coke bottle. Chromatic aberration correctors and spherical aberration correctors are required to achieve optical equivalent.

5) How wide is the beam after focussing?

The region where electrons hit the surface is called a spot. Spot size will vary depending on the imaging mode. We expect it to be nanometer scale.

6) About how fast are they going after being accelerated?

About 25% of the speed of light.

You can work it out knowing the mass and charge of the electron, and the accelerating voltage - if you remember that 1V=1J/C

7) Why are electrons particularly appropriate to use?

They have a much shorter wavelength than visible light, and so have a much shorter intrinsic resolution limit. They can, however, be much more damaging to delicate samples, especially if they charge easily and do not dissipate or conduct charge away.

8) How are the electrons detected after they have interacted with the sample?

By a set of detectors arranged around the final lens in annular quadrants. These are referred to as "back scatter" detectors, since they detect the electrons that are scattered back from the sample. The back scattering process is Rutherford scattering, the elastic scattering of charged particles by the nucleus, the same process that proved the Rutherford model of the atom was a better model than the plum pudding. It is the Rutherford scattering process that causes the atomic number contrast (more charge means more scattered electrons) and the crystal orientation contrast.

9) How is that information turned into an image?

Data is collected serially from each beam spot location, each of the four back scatter detectors is counting the number of electrons arriving from that spot. In topographic mode, only the detectors on one side are active so if the region is ‘in shadow’ then there are fewer electrons detected hence the pixel on the image will be dark, whereas if it is on the top or on the non-shadow side then there are more electrons detected hence a bright pixel.

10) What determines the maximum magnification of the microscope?

The lens system determines the maximum magnification. The wavelength of the electrons is the intrinsic limit, but in practice they are of the order picometres and so that is not the practical limit. In the SEM it is simply the spot size to which the beam can be focussed. Tungsten filament systems have beam quality issues: the energy resolution and the coherence of the beam. For our machine you don’t need high current, high coherence, or a narrow energy resolution beam. For a field-emission SEM you can get much better resolution as everything from the source to the lens to the detectors are much more finely tunable. In a TEM you can image individual atoms depending on the imaging mode - but understanding and analysing TEM images is non-trivial. 

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Xrays

1) Why are Xrays produced?

The X-rays produced are "characteristic x-rays". This is AS material, found in PH2. The short answer is electrons in the "shells" of atoms in the sample are knocked out, and electrons in higher energy states drop into the empty state, releasing X-rays in the process.

2) Does all the material in the sample produce Xrays?

There is a particular region of the sample around the beam spot that is affected by the beam in such a way as to produce X-rays. In general this region is known as the interaction volume

3) Where are the Xrays detected?

In a silicon drift solid state detector inside the chamber. These solid state detectors, capable of differentiating x-ray energies, are a recent development. Previously a Geiger-Muller tube type detector would have been used. The GM tube had to be cooled with liquid nitrogen due to the short dead time and high x-ray flux, and it  would have had a very fragile mica (phyllosilicate) window. Solid state detectors are higher precision, more compact, and more robust.

4) What kind of detector is used?

Silicon drift solid state detector

5) Do elements within more complicated compounds produce characteristic wavelengths of the pure element?

Yes. The x-ray production process is atomic not molecular. As x-rays are produced mostly from inner shell transitions - at least the greatest intensity (biggest lines) are from the inner shell. You will not get bonding information from EDS/EDAX. It is NOT equivalent to IR spectroscopy.

6) How does the SEM work out the relative abundance of each element?

You might think a standard is required, but Cliff and Lorimer were able to show that a standard is not needed if intensities for two elements are gathered simultaneously and compared. The Cliff-Lorimer relation is used, essentially taking the area under the peaks (typically the K-alpha).

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In practice

1) How do you improve image quality?

In general by careful sample prep and mounting, and good choice of focus, brightness, and contrast. Charging and noise are probably our two biggest problems. Noise is clear, charging will be sample dependent, and can be reduced somewhat by painting part of the top surface and ultimately by plasma coating.

2) How should you best position samples to get the best images?

Keep the sample as flat as possible, and use the sample height tool. The sample height should be about a mm below the bar, or below the white teflon bar in the case of EDX work.

3) Is there any method other than USB to transfer images off the computer?

No.

4)How should you leave the SEM - and can you leave it running over lunch etc if not in use?

Empty, under vacuum, and all switched off.

5) Some elaboration about the EDX could be useful - how reliable is it?

Less reliable at low atomic number, will not see H, or often O, or N. Otherwise it is as reliable as the scan protocols used.

6) How could the students/I accidentally break something irreplacable?

Messing around with the spare filament. Moving the SEM without it being properly stowed for travel.

7) How do I avoid crashing into the filament etc?

The filament is away at the top of the column, but you could crash into the detector if you don't use the sample height tool before mounting your sample into the chamber. Additionally, it is commonplace to have the internal camera on when entering the chamber.

8) What should I do if I can’t get a proper vacuum?

The most likely problem is that the chamber isn't sealing at the door. GENTLY press the top of the door at its corners. Listen for the pump to change tone, this will tell you when the seal is made. If the seal still will not make then it is advisable to check the seal with a gloved hand by carefully running your gloved finger over the rubber seal and the inner face of the door - you are feeling for anything that might prevent the seal forming.

9) The screen has gone completely white - what should I do?

The image? Turn the brightness down. Otherwise, it may be charging, try running in anti-charging mode, or try 5kV instead of 15kV, or prepare your sample more carefully to avoid charging. The screen? Switch the computer off and on again.

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Teaching

1) How do you display on the screen outside?

Switch on the Samsung TV using the central button. Go to Display Settings and set the screen resolution to 1024 768. Set the screen to Duplicate.