Background information - What is transmission electron microscopy?
A transmission electron microscope (TEM) is an analytical tool allowing visualisation and analysis of specimens in the realms of microspace (1 micron/1μm = 10-6m) to nanospace (1 nanometer/nm = 10-9m). The TEM reveals levels of detail and complexity inaccessible by light microscopy because it uses a focused beam of high energy electrons. It allows detailed micro-structural examination through high-resolution and high magnification imaging. It also enables the investigation of crystal structures, specimen orientations and chemical compositions of phases, precipitates and contaminants through diffraction pattern, X-ray and electron-energy analysis.
Transmission electron microscopy is used to produce images from a sample by illuminating the sample with electrons (i.e. the electron beam) within a high vacuum, and detecting the electrons that are transmitted through the sample. Ultimately, using a TEM we can see the columns of atoms present in crystalline samples.
Show the silicon wafer as an
The word “transmission” means “to pass through”. Essentially, the way the transmission electronmicroscope creates a conventional image (usually termed a bright field image) of a sample can be compared to shadow puppetry. Imagine a torch beam shone through a lattice on a window. The light passes through the transparent parts of the window, but is stopped by the lattice bars. On a wall beyond, we see the lattice bars as shadows. The TEM uses a beam of highly energetic electrons instead of light from a torch. On the way through the sample some parts of the material stop or deflect electrons more than other parts. The electrons are collected from below the sample onto a phosphorescent screen or through a camera. In the regions where electrons do not pass through the sample the image is dark. Where electrons are unscattered, the image is brighter, and there are a range of greys in between depending on the way the electrons interact with and are scattered by the sample.
Magnifications of up to 1, 000,000x and resolution below 1 nm are achieved routinely. A scale bar is essential on a TEM image. From this the actual size of structures in the image can be calculated. For more on scale bar use visit: http://scalebar.emu.usyd.edu.au/main.html
Quantitative and qualitative elemental analysis can be provided from features as small as 1 nm. For crystalline phases the crystal structure, lattice constraints and specimen orientation can be determined.
Electrons are negatively charged particles within the atom. Unlike light photons, electrons cannot be focussed by glass lenses; instead, electromagnets are used to focus the electrons.
Advantages of the TEM over a light microscope
The transmission electron microscope (TEM) provides the user with advantages over the light microscope (LM) in three key areas:
- Resolution at high magnification. Resolution can be defined as the smallest distance between two closely opposed points, at which they may be recognized as two separate entities. The best resolution possible in a LM is about 200 nm whereas a typical TEM has a resolution of better than 1 nm.
- Structural information. If the material being viewed has a periodic structure like a crystal then the beam can interact with that structure in such a way that it diffracts. This provides information on crystal structure, symmetry and orientation of materials.
- Microanalysis i.e. the analysis of sample chemical composition can be performed in the TEM.