Quantum mechanical switching at room temperature
Electron waves cancel each other out due to diversion in molecule
Electron waves can reinforce each other but they can also cancel each other out. The latter is called destructive interference. Previous measurements on microstructures at low temperatures had shown that this can result in poorer conduction. Scientists from Leiden University and the University of Groningen - including FOM focusgroup leader Prof.dr. Kees Hummelen en former FOM-oio Dr. Sense Jan van der Molen - together with Danish colleagues have now found experimental evidence that this effect can also take place at room temperature. For this the team made use of the fact that very small systems (such as molecules) continue to behave quantum mechanically even at room temperature. The article was published on Sunday 25 March in Nature Nanotechnology.
The team of chemists and physicists built five different very small molecules to test the electrical conductance (see figure right). With their experiment they demonstrated that fundamentally new possibilities arise if molecules are used as tiny electronic wires. They made three molecules (right) with a linear conjugated structure. These molecules allow electrons to pass through the molecule via a single dominant route. This gave rise to a current if the molecule was subjected to a potential difference. The two other molecules (left) are cross-conjugated: they have a double carbon-oxygen bond symmetrically around the middle. This provides two possible routes for an electron through the molecule: one straight through and one with a diversion via one of the oxygen branches (see figure below).
Anti-phase
The theory is: the electron behaves as a wave. This wave can split up into several smaller waves that each take their own route through the cross-conjugated molecule. These smaller waves reunite at the end of the molecule. If the waves meet exactly in anti-phase (the one wave rises just as the other falls) then they cancel each other out. Then no (or far fewer) electrons can move through the molecule and so the current is either small tor non-existent.
In the case of two cross-conjugated molecules the measurements were found to agree very well with this theory. The fact that electron waves can cancel each other out resulting in a poorer conduction was already known for larger structures at very low temperatures. By using small molecules – about two nanometres long – the researchers have now successfully demonstrated the principle at room temperature.
On and off
In their article the researchers show that destructive quantum interference can be switched on and off by selecting the right chemical branch in the middle of the molecule. A branch with oxygen (double bonds) results in destructive interference and therefore a poor conduction; a branch with hydrogen (single bond) does not result in destructive interference. Moreover, the difference between the two was strikingly large: about a factor of 100 in the conductance.
Is it possible to make practical electronic applications with molecules as the working element? That is still an open question because molecular conductance also has certain disadvantages, especially with respect to stability. As a following step the team is now examining if it is possible to switch the resistance of a molecule by turning the interference on and off. The chemists in Groningen have already synthesised a candidate molecule for this.
The research was carried out by chemists Hennie Valkenier and Kees Hummelen from the University of Groningen, theoretical physicists Troels Markussen and Kristian Thygesen from the Technical University of Denmark and experimental physicists Constant Guédon and Sense Jan van der Molen from Leiden University. The research was partly funded by the Netherlands Organisation for Scientific Research.
Press release of the Netherlands Organisation for Scientific Research (NWO).