The Sharp End of Nanotechnology

The images produced by the STM have opened up a previously inaccessible microscopic world and have dramatically increased our understanding of surface structure and atomic scale processes. Although, perhaps more important than it's imaging capabilities, the STM also has the ability to manipulate individual atoms on a sample surface, which was demonstrated for the first time at the start of the 1990's and has resulted in some of science's most iconic images: 'IBM' written with individual Xenon atoms on a nickel surface and the 'quantum corrals' which beautifully illustrated the wave nature of the electron. This ability to have total control over the positions of individual atoms has huge technological implications, as Richard Feynman suggested, in his own inimitable style, nearly 50 years ago:

"I am not afraid to consider the final question as to whether, ultimately - in the great future - we can arrange the atoms the way we want; the very atoms, all the way down! What would happen if we could arrange the atoms one by one the way we want them?"

The ability to engineer devices on this sub-nanometre scale goes beyond the realm of what is today usually considered as 'nanotechnology' and is probably more accurately described as 'pico-technology' (a picometre is one thousandth of a nanometre, and one trillionth of a meter). One of the most obvious applications of this new technology is in the drive for faster and smaller processors, and this is a very active area of current research. For example, it has been shown that it is possible for a single molecule (composed of several atoms) to perform an evolution of its electrons. The challenge now therefore is to use atomic scale technologies to build an architecture to enable the exchange of information with a single molecule and between individual molecules that are stuck on a surface. The development of this technology into a real computational device would result in a thousand-fold decrease in size compared to current semiconductor equivalents.

The STM has undoubtedly been very successful in opening up the microworld, however it suffers from one major handicap: that both the tip and the surface must be conductors (in order for the tunnelling current to flow). This prevents the STM being used with insulating surfaces, and rules out many important systems, including biological samples. In fact, for the architecture described above to be fully realised, the molecules must be attached to an insulating substrate. To overcome this problem, the Atomic Force Microscope (AFM) was developed by the same team that invented the STM.

The principle of the AFM is very similar to the STM, no flow of electrons is involved but the force on the atomically sharp tip is used as the imaging signal. The tip in an AFM is attached to the end of a flexible cantilever, and the deflection of this cantilever, which can be measured very accurately, is directly proportional to the force on the tip due to its interaction with the surface atoms. Intuitively, the AFM is 'feeling' the atoms as it scans above the surface.

The most recent and exciting developments in the 'scanning probe' area have been concerned with the AFM, operated in its so-called 'non-contact' mode; where it is now capable of atomic scale resolution on a variety of insulating as well as conducting systems. In fact recent experiments have even managed to resolve sub-atomic structure, the individual atomic orbitals of atoms in the silicon surface can now be seen.

In addition to developing the same single atom manipulation capabilities as the STM, the AFM has now reached the point where it can actually directly measure the force of single chemical bond between two atoms (an atom in the surface and the tip apex atom). This was used very recently in a ground breaking experiment by a group in Osaka, Japan to chemically identify individual atoms of different elements on a surface, a development which takes chemical analysis to a whole new level.

As these remarkable instruments continue to be developed and improved, the world of atoms and molecules is now more accessible and tangible than ever. The work of the 'atom engineers' is only just beginning, and they undoubtedly have a profound effect on the development of technology in the coming decades.

Dr. Tom Trevethan is a researcher at University College London

For more information

Nobelprize.org - The Scanning Tunneling Microscope
http://nobelprize.org/educational_games/physics/microscopes/scanning/index.html

IBM Research - STM Microscope Gallery
http://www.almaden.ibm.com/vis/stm/gallery.html

University of Regensburg STM / AFM Images
http://www.physik.uni-regensburg.de/forschung/giessibl/fjg/imagegallery/afmimages/afmimages_e.shtml