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Scanning Electron Microscopy. Click on the green arrow to return to the previous page
The Scanning Electron Microscope (SEM) was first available as research tool in Universities in the late 1960s and has steadily evolved over the years and its resolution has become much greater such that it is routinely used to study very small structures such as nanoparticles. Because the electron optics utilise pinhole apertures to collimate the electron beam, SEM images possess enormous depth of focus and striking three-dimensional appearance.
Instead of lenses made from glass as used in an optical microscope, the SEM uses magnetic lenses to focus an ultra fine beam of electrons from an electron gun or source and special scanning coils to raster the beam over the surface of a sample placed inside its vacuum specimen chamber. Many samples are rendered electrically conductive by coating them with an ultra thin layer of metal, typically gold using a sputter coater in which argon ions impinge on a gold target and knock off tiny fragments of gold which coat a sample placed below. Such a sputter coater is shown and a spider sputter coated with gold. The electron beam interacts with and is scattered by the atoms in the specimen surface to produce secondary electrons and these are collected using a positively charged collector inside the specimen chamber. Electrons of different energies are also produced as well as dim amounts of visible light and X-rays all of which can be collected to derive information about the specimen being studied. Electrical signals obtained from these interactions are greatly amplified and used to modulate the brightness of a monitor. This used to be a cathode ray tube but these have been replaced by modern technology and the whole instrument is now controlled by software. The magnification obtainable is equal to the ratio of the area scanned on the sample by the electron beam to the area of the monitor used to display the image and can range from as little as 20 times to more than a million exceeding by a factor of 1000 the resolution obtainable from a light microscope.Move the mouse cursor over an image to see a description.