Molecular Switching Moves Closer to Reality
A solution to the long-sought fabled molecular switch appears to have been found by researchers at Michigan Technological University.
Alexander E. Braun, Senior Editor -- Semiconductor International, 8/5/2008 7:54:00 AM
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Researchers have already demonstrated conduction, rectification and switching in metal-molecule junction devices. Among all of these, the demonstration of a single-molecule switch with a negative differential resistance (NDR) feature has drawn considerable attention and effort. NDR, which is a little-known phenomenon, is described as an increase followed by a decrease in current, with the steady increase in applied bias. Almost since its existence was realized, several research groups, in and out of academia, have been working on the problem of unraveling the behavior of NDR in a molecular junction.
Such an understanding has been expected to revolutionize the field of molecular electronics, because it would enable the use of molecules to replace the current generation of transistors. Conceivably, this would make it possible to fit more than a trillion switches onto a 1 cm2 chip. In 1999, a team at Yale University published the description of such a switch, but unfortunately nobody was able to replicate their results or explain how it worked if, indeed, it worked.
It now appears that this goal has been attained by Michigan Technological University (Houghton) physicist Ranjit Pati and his research team. According to them, they have been able to develop a computer model that explains what the elusive mechanism behind the much sought after single molecular switch is and how it functions. They place the origin of NDR in a strongly coupled single-molecule metal junction. To work out the complexities of the subject, they modeled a first-principles quantum transport calculation in a feterpyridine linker molecule sandwiched between a pair of gold electrodes. Upon increasing the applied bias, the researchers found that a new phase in the broken symmetry wavefunction of the molecule emerges from the mixing of occupied and unoccupied molecular orbitals. As a consequence, a non-linear change in the coupling between the molecule and lead is evolved, resulting in NDR. According to the researchers, this model can also be used to explain NDR in other classes of metal-molecule junction devices.
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| Schematic of the strongly coupled single molecule gold junction. (Source: Michigan Technological University) |
Pati and his group were extremely surprised at what their analysis of the NDR revealed. “We had developed this complex model of an organometallic molecule firmly bound between two gold electrodes,” he said. “As we expected, when the current was turned on, it increased along with the voltage, rising to the insignificant level of 142 µA. The twist came when it suddenly, and counterintuitively, dropped because of NDR.”
This occurred because according to the model, up until the 142 µA tipping point, the molecule’s cloud of electrons had been orbiting around their nucleus in perfect equilibrium. However, when this system was bombarded with higher voltage, that steady state fell apart and, pushed by the resulting quantum phase transition, the electrons were immediately forced into a completely different equilibrium. “We never imagined that this would happen,” Pati said. “We were all excited by this unexpected and beautiful result!”
The significance of the Michigan University group’s work and its results is that a molecule that exhibits two different phases when subjected to electrical fields should be able to serve very well as a switch, with one phase being the “zero” and the other the “one,” enabling the production of computers of unheard-of performance. Now, with the model that was developed, this capability promises to leave the realm of blue sky science and become a technology.
Pati and his group are currently working with other scientists to test the model experimentally, but there are no reasons to believe that the model will not prove to be accurate.
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