Thesis-PDF - IAP/TU Wien

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