11.1 Electron microscopes  (Page 2/2)

High-resolution transmission electron microscope (hrtem)

There are high-resolution TEM (HRTEM) which have been built. In fact the resolution is sufficient to show carbon atoms in diamond separated by only 89 picometers and atoms in silicon at 78 picometers. This is at magnifications of 50 million times. The ability to determine the positions of atoms within materials has made the HRTEM a very useful tool for nano-technologies research. It is also very important for the development of semiconductor devices for electronics and photonics.

Transmission electron microscopes produce two-dimensional images.

Scanning electron microscope (sem)

The Scanning Electron Microscope (SEM) produces images by hitting the target with a primary electron beam which then excites the surface of the target. This causes secondary electrons to be emitted from the surface which are then detected. So the electron beam in the SEM is moved (or scanned) across the sample, while detectors build an image from the secondary electrons.

Generally, the transmission electron microscope's resolution is about an order of magnitude better than the SEM resolution. However, because the SEM image relies on surface processes rather than transmission it is able to image bulk samples (unlike optical microscopes and TEM which require the samples to be thin) and has a much greater depth of view, and so can produce images that are a good representation of the 3D structure of the sample.

Disadvantages of an electron microscope

Electron microscopes are expensive to buy and maintain. They are also very sensitive to vibration and external magnetic fields. This means that special facilities are required to house microscopes aimed at achieving high resolutions. Also the targets have to be viewed in vacuum, as the electrons would scatter off the molecules that make up air.

Scanning electron microscope (sem)

Scanning electron microscopes usually image conductive or semi-conductive materials best. A common preparation technique is to coat the target with a several-nanometer layer of conductive material, such as gold, from a sputtering machine; however this process has the potential to disturb delicate samples.

The targets have to be prepared in many ways to give proper detail. This may result in artifacts purely as a result of the treatment. This gives the problem of distinguishing artifacts from material, particularly in biological samples. Scientists maintain that the results from various preparation techniques have been compared, and as there is no reason that they should all produce similar artifacts, it is therefore reasonable to believe that electron microscopy features correlate with living cells.

Uses of electron microscopes

Electron microscopes can be used to study:

• the topography of an object $-$ how its surface looks.
• the morphology of particles making up an object $-$ their shapes and sizes.
• the composition of an object $-$ the elements and compounds that the object is composed of and the relative amounts of them.
• the crystallographic information for crystalline samples $-$ how the atoms are arranged in the object.

End of chapter exercises

1. If the following particles have the same velocity, which has the shortest wavelength: electron, hydrogen atom, lead atom?
2. A bullet weighing 30 g is fired at a velocity of $500\mathrm{m}·{\mathrm{s}}^{-1}$ . What is its wavelength?
3. Calculate the wavelength of an electron which has a kinetic energy of $1.602\times {10}^{-19}$  J.
4. If the wavelength of an electron is ${10}^{-9}$  m what is its velocity?
5. Considering how one calculates wavelength using slits, try to explain why we would not be able to physically observe diffraction of the cricket ball in the first worked example.