Image generation in scanning electron microscopes begins by forming an extremely thin beam of high energy electrons which are digitally rastered across the surface of the sample being examined. When the incident electrons encounter the sample surface they stimulate radiation of X-rays and emission of backscattered and secondary electrons. The yield of electrons from each point in the scan is measured and amplified by a charged scintillator linked to a photomultiplier. The resulting electronic signal is transferred to imaging software using the same scan raster but with position coordinates multiplied by the required magnification ratio. Assembly of a monochrome image from these data points provides a two-dimensional map of the intensity of the electrons emitted from the scanned area. Bright sections of the resulting image are from points and zones high electron yield, often from thin parts of the sample, but the presence of chemical elements with strong electron emission also adds intensity. Non-conductive samples are usually coated with a thin layer of metal such as gold or platinum to provide acceptable imaging conditions. Magnifications of up to 500,000 times make it possible to photograph features as small as clusters of atoms. At the same time the microscope characteristics provide great depth of field enabling geometrically complex samples to be visualised in detail.
The SEM images below have been selected to show contrasting microscopic features in two natural forms of silica:
The first set of images are of a mass of minute crystals of druz quartz lining a cavity (vug) in a hydrothermal sinter deposit from an ancient hot spring in the Coromandel Range1. The arrays of terminated hexagonal prisms have their geometric form dictated by the chemistry and physics of silica crystallisation.
The second group of SEM images are of the silica-based shells (frustules) of diatoms retrieved from the Hauraki Gulf2. These single-celled algae construct their complex housings from a strong composite of amorphous silica and organic polymer. Frustules, with elaborate 3-D detail evident down to nanometre scale, are biologically fabricated using genetically coded instructions passed down the generations of these successful organisms.