Electron waves can reinforce each other, but they can also cancel each other out. This is known as destructive interference. Earlier analysis of micro structures revealed that this can result in poorer conduction at low temperatures. Now a team including researchers from Leiden University and the University of Groningen have found experimental evidence that this effect can also occur at room temperature. The team took advantage of the fact that very small systems (such as molecules) behave quantum mechanically even at room temperature. The article was published in Nature Nanotechnology on 25 March 2012.
The team of chemists and physicists built five different very small molecules and tested them for electrical conduction (see diagram on the right). Their experiment demonstrates that fundamental new opportunities arise if molecules are used as electric wires. They built three molecules (right) with a linear conjugated structure. These molecules transfer electrons through one dominant route in the molecule. If the molecule is connected to a power source a current is produced. The two other molecules (left) are cross-conjugated: they have a carbon-oxygen double bond grouped symmetrically around the centre of the molecule. This results in two alternative routes for an electron to pass through the molecule: one that goes straight ahead and one that is diverted to one of the oxygen branches (see diagram below).
The theory is as follows: the electron behaves like a wave which can divide into several smaller waves. Each wave finds its own route through the cross-conjugated molecule and they reunite on the other side. If the waves are exactly anti-phased (the one rises at exactly the same moment as the other falls), they will cancel each other out. This means that no (or in any case far fewer) electrons can pass through the molecule and so the current will be weak or non-existent.
The results of the analysis of the two cross-conjugated molecules clearly upheld this theory. The fact that electron waves can cancel each other out and that this leads to poorer conduction had already been proven for larger structures at very low temperatures. By using smaller molecules – some two nanometres long – the researchers have been able to prove that they theory is valid at room temperature as well.
In their article, the researchers demonstrate how the destructive quantum interference can be turned on and off by selecting the right chemical branch in the centre of the molecule. A branch with double-bonded oxygen results in destructive interference and so poorer conduction, while a branch with single-bonded hydrogen does not produce destructive interference. Moreover, the difference between the two was surprisingly large: the conduction rate changed by approximately a factor of 100.
Could practical electrical applications be built with molecules as active elements? This remains to be seen. Molecular conduction also has disadvantages, particularly where it concerns stability. The next step for the team is to try to switch the resistance of a molecule by turning the interference on and off. The chemists in Groningen have already synthesized a candidate molecule for this purpose.
The research was performed by the chemists Hennie Valkenier and Kees Hummelen of the University of Groningen, theoretical physicists Troels Markussen and Kristian Thygesen of the Technical University of Denmark and experimental physicists Constant Guédon and Sense Jan van der Molen of Leiden University. The research was partially funded by the Netherlands Organisation for Scientific Research (NWO).
Contact: Prof. Kees Hummelen
Article: Observation of quantum interference in molecular charge transport. Nature Nanotechnology, online: 25 March 2012. DOI:10.1038/NNANO.2012.37
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