The Electron Microscope - 3.2 Flashcards
What are the two main advantages of an electron microscope?
- The electron beam has a very short wavelength and the microscope can therefore resolve objects well - it has a high resolving power.
- As electrons are negatively charged, the beam can be focused using electromagnets.
What are the two types of electron microscope?
- The transmission electron microscope (TEM)
- The scanning electron microscope (SEM)
What does the TEM consist of?
An election gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet.
How does the TEM work?
In a TEM, the beam passes through a thin section of the specimen. Parts of this specimen absorb electrons and therefore appear dark. Other parts of the specimen allow the electrons to pass through and so appear bright. An image is produced on a screen and this can then be photographed to give a photomicrograph.
What is the resolving power of the TEM?
0.1 nm
Why can the resolving power of the TEM not always be achieved?
- Difficulties preparing the specimen limit the resolution that can be achieved.
- A higher energy electron beam is required and this may destroy the specimen.
What are the main limitations of the TEM?
- The whole system must be in a vacuum and therefore living specimens cannot be observed.
- A complex ‘staining’ process is required and even then the image is not in colour.
- The specimen must be extremely thin.
- The image may contain artefacts. Artefacts are things that result from the way the specimen is prepared. Artefacts may appear on the finished photomicrograph but are not part of the natural specimen. It is therefore not always easy to be sure that what we see on a photomicrograph really exists in that form.
In the TEM, the specimen must be extremely thin to allow electrons to penetrate, what does this result in?
A flat, 2D image.
How can we overcome the fact that the TEM only produces a 2D image?
We can partly get over this by taking a series of sections through a specimen. We can then build up a 3D image of the specimen by looking at the series of photomicrographs produced. However, this is a slow and complicated process.
What are the benefits of the SEM over the TEM?
-Specimens don’t need to be extremely thin as electrons do not penetrate.
How does the SEM work?
The SEM directs a beam of electrons onto the surface of the specimen from above, rather Tham penetrating it from below. The beam is then passed back and forth across a portion of the specimen in a regular pattern. The electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the specimen surface.
What does the SEM produce?
A 3D image, produced by computer analysis of the pattern off scattered electrons and secondary electrons produced
Why is the SEM less useful than the TEM?
The basic SEM has a lower resolving power than a TEM, around 20nm, but is still ten times better than a light microscope.
What is the resolving power of the SEM?
Around 20nm
What is resolution?
The smallest separation at which two separate objects/points can be distinguished.
What is the resolution limited by?
The wavelength of light used.
