Scientists have succeeded in creation of one of the most efficient molecular conductors ever studied: a 16-carbon atoms chain conducted electricity more than ten thousand times better than the similar structures used before. These results open up new possibilities for the development of atomic-scale electronic components and are approaching the creation of devices in which individual chains of atoms act as conductors. This breakthrough is based on work of the scientists from the University of Liverpool in the United Kingdom and the University of Western Australia.
Linear chains of carbon atoms have long been considered one of the most promising materials for atomic-scale electronics. They are viewed as a possible realization of carbine, which is a rare and as yet poorly understood form of carbon, consisting of a one-dimensional chain of atoms. Theoretical calculations predict the unique mechanical and electrical properties for carbine. However, in practice, it is extremely difficult to study. Such structures are unstable and easily destroyed, so scientists usually have to protect their ends with special chemical groups. The problem is that these protective groups simultaneously alter the carbon chain’s intrinsic properties and prevent scientists from observing its true behavior.
The authors of the new study found a way to get over this limitation: they synthesized linear carbon chains ranging from 4 to 16 atoms in length, temporarily stabilized by gold compounds. Then, using a scanning tunneling microscope, they formed molecular contacts between two gold electrodes in which the protective groups were removed, leaving the carbon chain directly connected to the metal. The result was a nearly perfect molecular conductor consisting only of carbon and gold.
The measurements revealed an unexpected pattern. For short chains, conductivity decreased as the length increased, as it typically happens in semiconductor materials. However, starting at approximately 12 carbon atoms, the situation changed dramatically. Longer chains not only stopped losing their conductivity but even began to conduct electricity more efficiently.
The results for a 16-carbon atoms chain were particularly impressive. Its conductivity exceeded similar molecular conductors with protective groups by more than ten thousand times. Moreover, this structure, which was over two nanometers long, could conduct currents of more than 40 microamperes, one of the best results among known molecular conductors of a comparable size.
To understand the reason for this behavior, the scientists made theoretical calculations and conducted spectroscopic studies. It turned out that as the chain length increases, its internal structure changes. In short chains, the single and triple bonds alternate between carbon atoms, which gives the material semiconductor properties. However, interaction with gold electrodes leads to redistribution of electron density, which gradually smooth out the differences between the bonds. As a result, the structure begins to resemble cumulene – a linear carbon form in which all the bonds are virtually identical, and the electrons can move much more freely.
It is this transition that explains an unusual behavior of long chains. If short structures behave like conventional molecular semiconductors, longer ones begin to exhibit properties similar to metal conductors. In effect, the researchers have, for the first time, observed how within the same system a transition from a semiconducting state to a nearly metallic one occurs simply by increasing the carbon chain length.
Thus, the results not only confirm many theoretical predictions about the carbine properties but also likely pave the way for creation of new atomic-scale electronic components. In the future, such carbon chains might be used as ultra-miniature conductors, interconnects for quantum devices, and other elements of nanoelectronics, requiring the most efficient transmission of electrical signals over extremely short distances.
