Scanning electron microscopy. The scanning electron microscope (SEM) was one of the first techniques described for assessing the structural changes of dental hard tissues (Xxxxx and Xxxxxx, 1967) and is a technique that is still widely used today in research into dental erosion (Xxxxxxxxx et al., 2003; Xxxxxxx et al., 1996). The SEM uses a very fine beam of electrons, which is scanned across the surface of the sample as a raster of parallel contiguous lines. Upon hitting the sample surface the electrons are either reflected as backscattered electrons or secondary electrons are generated by the interaction of the primary electrons with the sample surface. The number of secondary electrons depends on the surface topography or nature of the sample. These secondary electrons are collected, amplified and analysed before modulating the beam of a cathode ray, scanned in sympathy with the scanning beam. The resulting image resembles that seen through an optical lens but at a much higher resolution and a greater depth of field. The ultimate resolving power of a SEM is dependent on the dimensions of the probe beam which in turn is controlled by diffraction at the final aperture, chromatic aberration and the size of the electron source. A resolution of up to 1 nm is achievable with a field emission system and an in-lens detector. A typical SEM can achieve magnifications of x400000. The magnification is dependent on the excitation of the scan coils, as modified by any residual magnetic or stray fields. The magnification also depends sensitively upon the working distance between the lens and the sample (Xxxxxxx, 2008; Xxxxxxx et al., 2000). For conventional SEM, the sample surface must be coated with a material that is electrically conductive, in order to prevent a reduction in image quality due to accumulation of negative electrostatic charge. These so called charging effects result in artefacts which appear on microscope images as irregular, featureless bright patches, or streaks, and are generally accompanied by loss in resolution (Xxxxxxxx and Xxx Xxxx, 1971). The coating material is usually gold and therefore a disadvantage of this technique is that the samples will be irreversibly altered during the desiccation and sputtering process. Alternatively casts can be made from the samples if their loss/destruction is unacceptable, however the replication technique results in a degree of dimensional inaccuracies and loss of information (Xxxxx Xx Xxx - Xxxxxxxxx et al.; Xxxxx et al., 2010). This has ...
