Introduction
For the majority of people, the most well known types of microscopes are optical or electron microscopes. Optical microscopes focus a beam of light through a series of glass or quartz lenses for magnification of up to x1000. Electron microscopes create magnified images of samples by focusing an electron beam using magnetic fields produced by electromagnetic lenses composed of wire coils. The electron microscope improved the magnification of images to x100,000. However, both methods generate only two-dimensional images. A new technique was needed for the scientific community to provide accurate information in three dimensions.
This new technique had its genesis in 1981 when G. Binnig and H. Rohrer, from IBM research laboratories, invented a new type of microscope called a scanning tunnelling microscope (STM). For this invention they received the Nobel Prize in 1986. The most impressive feature of this new type of microscope is the extremely high spatial resolution of the order of 0.01 nm that can be achieved. This allows scientists to image, and even to manipulate, individual atoms or molecules of materials. The main difference between this technique and optical and electron microscopes is that there is no need for lenses, light or electron sources. The physical foundation for the STM is known as the tunnelling effect, a quantum mechanical property. The tunnelling effect can be produced by simply applying a voltage between a sharp metallic tip and the investigated surface, both separated by a vacuum barrier. If this vacuum barrier is approximately a few atomic diameters thick, electrons are able to tunnel through it, and a current will flow. This tunnelling current depends exponentially on the barrier distance or height. Therefore, by scanning the tip over the surface at either a constant current or height, the record of the vertical tip motion will reflect the surface topography of the sample.
The success of STM gave birth to a large family of instruments generally referred to as Scanning Probe Microscopes (SPM). Each member of this family uses a different type of interaction or force between the probing tip and the sample. The most widely implemented ones are the STM and the Atomic Force Microscope (AFM). The SPM family works on a principle similar to a record player. A sharp tip (e.g. silicon or silicon nitride in AFM, diamond in a record player) is scanned across the surface (the sample, or the record). The interaction between the tip and the surface is measured and converted into an electrical signal which is processed into interpretable results (three-dimensional image of sample topography, or sound from stereo speakers). However, unlike the record player, the sensing tip of an SPM is raster scanned across the sample (similar in nature to how a television image is produced). In addition to topographic imaging, many modern AFMs have the capability to image via a number of different mechanisms including frictional force, phase contrast, amplitude, adhesion and elasticity. Interactions producing electrostatic, magnetic, and chemical forces can also be mapped as long as the tip is capable of sensing these forces. For example, for magnetic force imaging the tip may be coated with iron or nickel.
