A little history

Over the centuries mankind has always been attracted by the observation of the infinitely distant and the infinitesimal small. Inventing the magnifying glass (XV^th century) and later the optical microscope (XVII_^th cent.), a hidden and fascinating world became visible: the matter at small scales. In classical microscopy the resolution is however limited to the wavelength of the radiation: about half a micrometer for visible light. This frontier further challenged the human inventiveness...

After the discovery of wave mechanics in 1923 by L. de Broglie, it appeared that the particles constituting matter – electrons, neutrons, protons – actually behave like waves. With this insight, E. Ruska and M. Knoll replaced in 1932 the light source of an optical microscope by an electron source. The electron microscope was invented. An electron beam is scanned over the surface and the image is constructed from the detection of the diffused electrons.

Electron microscope images of the eye of a fly. One single eye facet is about 3 microns in diameter.

STM image, one nanometer wide, showing the regular arrangement of carbon atoms at the surface of graphite.

Since electrons have a wavelength 3-5 orders of magnitude smaller than visible light, the surface of an object can be imaged with much more details. Today it is a very powerful laboratory instrument used in various fields such as biology, material science, microelectronics and physics. The most instruments have a resolution of about 0.2 nanometers (one nanometer [nm] = one millionth of a millimeter) thus reaching the atomic scale.

Scanning tunneling microscopy

In 1981, G. Binnig and H. Rohrer from IBM research laboratory in Rüschlikon invented a new type of imaging instrument, the scanning tunneling microscope (STM). It is nowadays a widespread surface science technique owing its popularity to the wide range of possible applications, a simple concept, and the ability to obtain a direct real space image of conducting surfaces. Its most striking feature is the extremely high spatial resolution of the order of 0.01 nanometer that can be achieved, allowing to image and even to manipulate individual atoms.
The main difference between this technique and the ones mentioned above is that there is no need for any lenses, light or electron sources.

It is the tunnel effect, a quantum mechanical property, that provides the physical foundation for this technique. All you need is to apply a small voltage between a sharp metallic tip and the investigated surface, both separated by a vacuum barrier (see left sketch). If this barrier is about a few atomic diameters thick, electrons are able to “tunnel” through it and a current will flow. Controlling this tunneling current is the difficult challenge achieved by G. Binnig and H. Rohrer and rewarded with the Nobel Prize in 1986. The current depends exponentially on the barrier distance. Hence, by scanning the tip over the surface at a constant current or barrier distance, the record of the vertical tip motion will reflect the surface topography.

Moreover, this instrument can be used as a spectroscopic tool, allowing to investigate the electron distribution in a conducting material. This powerful mode called scanning tunneling spectroscopy can for example differentiate a normal from a superconducting region within a sample, as demonstrated in the scientific article of MaNEP newsletter issue no.3 (PDF, 136 Ko).

In summary, the STM provides an eye and a nose at the nanometer level, and this is not all of it !

The smallest swiss cross of the world is 25 nm wide and has been manufactured by etching trenches into a surface of a superconductor by STM. (A. Takagi, PhD thesis, Geneva).

In the example shown on the figure, the instrument can in appropriate conditions be used to modify or etch matter. Indeed, the last figure demonstrates that the STM can be used as a nanomechanical shovel !

The success of this technique rapidly gave birth to a large family of instruments generally referred to as scanning probe microscopes. Each member of the family uses a different type of interaction between the probing tip and the sample. The most popular ones are the STM, the atomic force microscope and the scanning near-field optical microscope.