Transmission Electron Microscopy (TEM)
The transmission electron microscope (TEM) generates an electron beam that passes through the sample and the interacts between the electrons and the atoms. These interactions can be used to observe features such as the crystal structure and features, the chemical composition of the sample as well as morphologic information of the sample. The beam of electrons from the electron gun is focused into a small, thin, coherent beam by the use of the condenser lens. This beam is restricted by the condenser aperture, which excludes high angle electrons. The beam then strikes the specimen and parts of it are transmitted depending upon the thickness and electron transparency of the specimen. By selectively converging and diverging these electrons with an electron lens, the enlarged images are formed on a fluorescent surface which is positioned below the beam and specimen.
Scanning Electron Microscopy (SEM)
In scanning electron microscopy, electrons are accelerated towards a sample with significant amounts of kinetic energy. Following the deceleration of these electrons in the solid samples, they give rise to signals as a result of electron-sample interactions. Secondary electrons are a part of these signals that are produced, and are used in the production of SEM images. Backscattered electrons may also be used in the production of the SEM image alongside the secondary electrons. Other signals that are produced include characteristic X-rays, cathodoluminescence, specimen current and transmitted electrons.
Atomic Force Microscopy (AFM)
The principle used in the imaging using an atomic force microscope involves the use of a micro machined cantilever with a sharp tip. This tip is subjected to attractive or repulsive forces/interactions depending on the distance between the atoms and the tip. These forces/interactions are recorded by measuring the degree of deflection of the cantilever. This detection is facilitated by a laser beam that is reflected off the back of the cantilever onto a photo sensitive photo detector. During the scanning of a sample, a tube-shaped scanner located under the sample moves the sample in the horizontal direction (X-Y) and in the vertical direction (Z). This motion causes the sample to be scanned line by line, while the photo sensitive photo detector signal is used to control the vertical movement of the scanner via a feedback loop as the cantilever moves across the sample. The AFM is capable of obtaining measurements from conductors, non-conductors as well as some liquids without delicate sample preparation.
Optical Microscopy (OM)
Optical microscopy allows small features of a sample to be analysed in detail. Optical microscopy can be used for initial examination and characterisation of samples in order to plan the next steps for work or to confirm observations prior to proceeding to more detailed investigations by other laboratory techniques such as SEM/EDX. Apart from the traditional bright field microscopy technique, these microscopes are capable of performing fluorescence microscopy, dark field microscopy and polarising microscopy. In fluorescence microscopy, the specimen is illuminated with light of a specific wavelength which is absorbed by fluorophores in the specimen which emit longer wavelengths. These wavelengths are then detected by the microscope to generate an image. Dark field microscopy is a microscopy technique which excludes the unscattered beam of light following the illumination of a sample. It is a useful technique in the observation of biological specimens.
