Scanning Probe Microscopy


Schematic operation of a SPM

The control of matter at molecular scale is one of the most important scientific challenges in the last thirty years[7].

Such control makes necessary the use of imaging and characterization tools capable of quantifying modifications and processes at very small scale. Visualization and characterization of surfaces at nanoscale have been the tasks that scanning probe microscopes have focused on.

The first SPM was introduced by Binning and Rohrer en 1982 [1], and its significance was acknowledge when in 1986 they were aawarded the Nobel prize in Physics [7] by their description at atomic scale of a 7x7 structure of silicon using the first version of a Scanning Tunneling Microscope [8]. The need for an instrument that could perform local spectroscopy at a very small scale was the initial motivation that preluded the invention of the STM [7]. Its versatility was discovered when they performed potentiometry and scanning nanolithography using the same set up [9].

In 1986 Binning et. al. developed the first Atomic Force Microscope based in their previous idea. This new instrument allowed them to measure forces down to 1uN between two molecular sized structures[]. With this instrument came the versatility of characterizing conductive and non conductive materials. Its predecessor STM was limited to conducting surfaces as will become clear in the STM dedicated section.

A SPM forms images of surfaces using a physical probe that scans a specimen in a controlled fashion. When the probe scans the sample they 'feel' a mutual interaction that can be measured using appropriate transducers. An image of the surface is obtained by mechanically moving a probe in a raster scan of the specimen, line by line, and recording the probe-surface interaction as a function of position. Alternatively, a control system can be placed so that the interaction between the probe and the sample is kept constant, and the signal used to position the sample respect to the probe (x,y,z) is recorded thus providing a topographical map of the specimen's surface.

Scanning probe microscopes can generally image several interactions simultaneously. They differ in which of theseinteractions is used to obtain an image. Every type of interaction is generally called a mode.cSPM were possible thanks to the ability of ability ofpiezoelectric materials to perform as actuators that execute motions with a precision and accuracy at the atomic level.



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