Thesis-PDF - IAP/TU Wien
Thesis-PDF - IAP/TU Wien
Thesis-PDF - IAP/TU Wien
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Chapter 3<br />
Atomic Force Microscopy (AFM)<br />
3.1 Introduction to Scanning Probe Microscopy<br />
Nanoscale science and technology strongly depend on the ability to investigate and<br />
manipulate structures down to the atomic scale. When the Scanning Tunneling<br />
Microscope (STM) was invented in 1981 by G. Binning, H. Rohrer, Ch. Gerber<br />
and E. Weibel ([31]) it was for the first time possible to directly acquire threedimensional<br />
(3D) images of solid surfaces with atomic resolution.<br />
In STM an electric potential difference is applied between the STM scanning<br />
tip and the sample surface. Although the vacuum between these two represents<br />
an ideal potential barrier, electrons can "tunnel through" 1 with a probability that<br />
depends exponentially on the distance between tip and surface. The lateral resolution<br />
is typically less than 1 nm and vertically less than 0.1 nm - sufficiently precise<br />
to define the position of single atoms.<br />
In 1986, only five years after the invention of STM which generally is applicable<br />
only to conducting samples the Atomic Force Microscope (AFM) was invented by<br />
G. Binnig, C. F. Quate and Ch. Gerber ([32]). Originally this kind of Scanning<br />
Probe Microscope (SPM) was developed for measuring ultrasmall forces (less than<br />
1 This so-called "tunnel-effect" is made possible because of quantum physical properties of<br />
electrons. They can behave both like particles and like waves and can be described by solutions<br />
to the so-called Schroedinger equation. These waves can penetrate a potential barrier that<br />
would be otherwise a forbidden area if the particle was described in the classical way. The<br />
particle can therefore "tunnel through" the forbidden area. One of the earliest cases of tunneling<br />
was explained for the alpha decay of heavy atomic nuclei in 1928. Many interesting tunneling<br />
phenomena in solids were anticipated but it had been difficult until 1960 to reconcile theory and<br />
experiment.<br />
The principle of electron tunneling in normal and superconducting state had been effectively<br />
demonstrated in 1960 by I. Giaever when he observed current between two metallic parts separated<br />
by a thin layer of oxide ([33]). The ability to measure a tunneling current in these<br />
experiments gave very direct evidence that electrons could indeed penetrate a potential barrier<br />
and behave like a wave according to quantum mechanics.<br />
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