The Scanning Electron Microscope

The Scanning Electron Microscope (SEM)

In order to understand the properties of the metal we need to examine it's structures on all length scales, right down to the atomic. To do this we use powerful microscopes known as electron microscopes to look at the metal up close. On the Engineering Atoms stand we have an example of a Scanning Electron Microscope that you can use yourself to look different metals up close. (If you want to see what the metals look like follow this link). But how do these microscopes work? Below is a schematic showing what is taking place inside the microscope:

Electron Gun:  Electrons are produced by either heating a filament, or by applying a strong electric field to it.  Filaments are often made of tungsten, and have a very sharp tip so that a very narrow stream of electrons is emitted.

Anode:  The electrons are accelerated down the column of the microscope by the anode. The anode is negatively charged so repels the negatively charged electrons, forcing them to accelerate.

Magnetic lenses:  The electrons are focused onto the sample by magnetic lenses.  These ensure a very narrow beam of electrons hits the sample.

Sample:  The sample scatters the electrons.  The amount of scattering (and the number detected at the detector) can depend on many factors, including:  sample height, sample chemistry, and sample crystal structure.  X-rays are also produced by the electrons hitting the atoms in the sample.  We can analyse these X-rays to tell us which elements exist in the sample. 

Detector:  The beam is scanned across the sample, and the signal from every point detected.  There are many different types of detector that can detect electrons with different energies travelling in different directions.  Using different detectors gives us different information about our sample. 

To have a look at some real images produced on a Scanning Electron Microscope take a look here!