The fantastic progress in the miniaturization of electronic devices that has taken place in the past few decades has largely been made possible by perfecting a century-old technique called lithography. Devices are built up layer by layer, the pattern on each layer being defined by a lithographic “mask”. In this “top-down” approach we start with a large, uniform chunk of semiconducting material and finish with a device that can have features that measure a fraction of a micron across. Forty years ago Richard Feynman proposed an alternative, “bottom-up” approach in which electronic devices would assemble themselves from individual atoms and molecules. Although such “self-assembly” sounds like science fiction, it can work in practice. All the plants and animals in the natural world are the result of self-assembly!
However, despite years of research and enormous scientific and commercial potential, there is still not a single example of a complete, self-assembled electronic device made directly from atoms or molecules. Nevertheless, researchers remain optimistic, heartened by the discovery of new carbon molecules that can act as electrical wires. Now a team of physicists from France, Russia and Germany has shown that these so-called carbon nanotubes can also exhibit zero resistance to electric current at temperatures below 1 K when connected to superconductors (A Yu Kasumov et al. 1999 Science 284 1508).
Carbon nanotubes are a by-product of the discovery of fullerenes in the 1980s. Under the right conditions carbon atoms can assemble spontaneously from the plasma of an arc discharge and form closed meshes shaped like footballs and cigars. The beauty of molecules such as carbon-60 – which has the same pattern of pentagons and hexagons that is found in soccer balls – is in sharp contrast with the atomic chaos from which they form with a surprisingly high yield.
In 1991 Sumio Ijima of the NEC Laboratory in Tsukuba, Japan, discovered tube-like carbon structures that measured only a few nanometres across but were several microns long. These nanotubes can be described as rolled-up two-dimensional graphite sheets. The atoms are arranged in hexagons and the nanotubes are characterized by their diameter and their helicity, which is related to the direction in which the sheet is rolled. In addition to being very strong and flexible, carbon nanotubes also have unusual electronic properties. A specific nanotube can be metallic or semiconducting, depending on its diameter and helicity (see Physics World January 1998).
But what does “metallic” mean at the molecular level? A metallic wire can be broadly defined as a many-electron system which, if held between two metal contacts playing the role of electron reservoirs, has delocalized orbitals at the Fermi level of the contacts. (In a metal the Fermi level is the energy that separates occupied and unoccupied electron states. The electrical behaviour of all materials is largely determined by the behaviour of electrons near the Fermi level.) More precisely, if a bias voltage is applied across the contacts, thereby splitting their Fermi levels, there should be at least a molecular orbital in the “window” between the two Fermi levels that extends from one contact to the other.
Most molecules do not satisfy this criterion. If they are sufficiently regular and rich in mobile electrons, they can have extended orbitals. However, when the molecule is more than a few tens of atoms long an instability almost always occurs. Some bonds shorten, others get longer and a “gap” opens in the energy spectrum at the Fermi energy of the molecule. Inside this gap there are no energy states available to electrons, so the molecule becomes an insulator or a semiconductor. Semiconducting molecules can be “doped” by introducing atoms that add electrons to the system. Although doping can move the Fermi energy to outside the energy gap, the resulting disorder can localize the electrons, which means that they are no longer free to move.
Carbon nanotubes are the only molecules we know of in which this does not happen. The symmetry of nanotubes gives them a unique property: per unit length they have very few mobile electrons at the Fermi energy and lots of bonded electrons. Nanotubes thus have more “bones” than “flesh”, and this electronic rigidity prevents the instability described above.
In 1997 the metallic character of certain nanotubes was verified in experiments. These results immediately suggest another question. If metals such as lead and aluminium become superconductors at low temperatures, can nanotubes also carry electric currents without any losses?
Early experiments could not answer this question. When a current was passed through a nanotube, the voltage drop was dominated by the contact resistance between the nanotubes and the probes supplying the current. This contact resistance was too large to discriminate between a superconducting state, in which the electrons form pairs, and a normal metallic state, in which they are independent. Besides, measurements by Paul McEuen’s group at the University of California at Berkeley and by Cees Dekker’s group at the Delft University of Technology in the Netherlands indicated that the electrons in a nanotube strongly repel one another, which is not favourable for superconductivity.
A more sophisticated way of looking for superconducting behaviour in nanotubes has been developed by Hélène Bouchiat of the Université Paris-Sud in Orsay, and co-workers in Orsay, the Russian Academy of Sciences in Chernogolovka, the University of Montpellier in France and the Max Planck Institute in Stuttgart. Bouchiat and co-workers exploit the so-called proximity effect – an exotic effect in which an ordinary metal can carry a supercurrent when placed between two superconductors. However, this type of superconductivity can be induced over ranges of a few microns at most.
Would nanotubes display this proximity effect? A positive answer would at least demonstrate that electrons do not lose their quantum coherence as they travel along the nanotube. Quantum coherence means that the electrons remain in a well defined quantum state – or superposition of quantum states – as they travel along the nanotube. Although quantum coherence is not directly related to superconductivity, it might be exploited in device applications in the future.
The French-Russian-German team first solved the problem of the contact resistance by dipping the nanotubes in gold that had been melted with a laser beam, a technique pioneered by Alik Kazumov from Chernogolovka. Gold appears to make an excellent solder for nanotubes and the contact resistances are small (about the same as the resistance of a metallic nanotube). Although gold is not a superconductor, electrodes made from tantalum or rhenium and cooled to about 1 K can be used to make the gold superconducting due to the proximity effect.
Therefore the question now is: will the proximity effect also work on the nanotubes? The answer is yes. The team observes supercurrents in single nanotubes and bundles of nanotubes when the apparatus is cooled below the superconducting transition temperature of the contacts. For experiments on bundles of nanotubes the maximum supercurrent agrees with theoretical predictions.
However, the measurements performed on a single nanotube give surprising results: the maximum supercurrent is 40 times larger than expected. It is possible that this effect is due to enhanced superconductivity in the tantalum or rhenium layers caused by the melting of the gold. Other subtle and unexplored interactions between the nanotubes and the contacts might also be responsible. There is also a remote possibility that single-carbon nanotubes are indeed real superconductors, rather than materials that can only carry supercurrents as a result of the proximity effect, but this is thought unlikely at present.
Whatever the explanation, the work of Bouchiat and co-workers demonstrates beautifully the coherent character of electron propagation along the nanotube. Moreover, the experiment confirms that carbon nanotubes really are the ideal molecular wires that, until recently, only existed in the dreams of physicists.
