Scanning Probe Microscopy

Almost thirty years ago, microscopy experienced a paradigm shift. Microscopy had been classically regarded as a method to expand our vision power and, for centuries, had maintained the same methodology: the radiation scattered by an object was collected by a detector and placed at an appreciable distance from it. Although the change of electromagnetic waves to wavelike electrons had resulted in a large increment in resolution, it was not until recently that microscopy as an expansion of our sense of touch has begun to be considered. The idea had been explored as early as eighty years ago by Synge, who discussed it within the context of electromagnetic waves. It was understood that the limits of optical microscopy had already been reached, but Synge pointed out that the Rayleigh-Abbe diffraction limit was only valid if light was collected in the far-field. By the approximation of a small aperture to the source, we can collect light before it experiences diffraction and obtain a resolution as good as the aperture is small. From the beginning, two major difficulties were apparent: that a small aperture would only transmit an infinitesimal amount of light and that there was no known technique to hold the aperture at a distance short enough to avoid diffraction effects. The second problem was solved when the way to manipulate piezoelectric materials was learned. However, it was not an aperture as Synge predicted but a metal probe which was held in close proximity to a sample. It was not the intention to collect light before it diffracted but to collect electrons by virtue of the tunneling quantum effect. Nevertheless, the result was similar to that predicted by Synge, a new technique of microscopy with resolution limited only by the size of the probe: Scanning Tunneling Microscopy. It did not take much time for useful variations to be found: two metals are needed to have quantum tunneling but any two materials experience forces when they are brought close enough to each other, which is the physical principle behind Atomic Force Microscopy. The proposal of Synge was also successfully tried a little later, leading to Scanning Near-Field Optical Microscopy in its different configurations. All these techniques share characteristics and are grouped under the label Scanning Probe Microscopy. Such common characteristics include the piezoelectric control of the position of the probe, the construction of an image pixel by pixel with the help of suitable software and, most important, the collection of information based on an interaction that can be only detected at nanometric distances. Scanning Probe Microscopy is one of the oldest nanotechnologies and will be the support for most of the nanotechnologies that will be developed in the future. In this work we intend to give a general description of this group of technologies, discussing separately their different approaches, their relationships and their prospects for the future.