Organic molecule switches like a transistor
Researchers at the University of Alberta have successfully demonstrated a single-molecule switch and transistor.
"There's no longer a question of whether a single molecule can be used as a switch; we have shown that it can be done," professor Robert Wolkow said. "Also, we have demonstrated how you can get two electrodes to act like the three electrodes normally associated with a transistor. In particular, we have shown that a chargeable atom can act as a gate using the same electrode that is also acting as a source."
But there's a caveat: "We don't have any kind of realistic temporal control of the switch yet," Wolkow said. "Right now, it takes minutes to turn it on and off."
Working with post-doctoral fellows Paul Piva and Stanislav Dogel, as well as graduate student Janik Zikovsky, Wolkow's team placed a single organic molecule on a silicon substrate so that the molecule acted as the transistor channel, with the substrate acting as a back gate for switching. The work was performed in cooperation with staff scientists and their post-doctoral assistants at the National Institute for Nanotechnology, which is a part of the National Research Council of Canada, as well as with professor Werner Hofer of the Surface Science Research Centre at Britain's University of Liverpool.
Other research groups have been working to achieve uncontested demonstrations of single-molecule transistors, but their results have not been rock solid, according to Wolkow. The problem has been trying to guarantee that the electrodes connecting to the molecule are in fact switching it. Molecules are so small that it is difficult to attach electrodes to them in a verifiable manner, Wolkow said.
That's where the Alberta team's two-electrode approach comes into play. Ordinarily, Wolkow said, "electrodes attached to a single molecule are just too small to get a gate near enough to operate efficiently, which limits how big a field you can drive onto the molecule. What we have is a trick to make two electrodes act like three, since our single atom is at a different potential—approximately 1V—than the rest of the silicon surface, even if it is charged with just a single electron and is just angstroms away from the other atoms around it."
Wolkow's group has been working with many organic molecules, learning how to bond them to silicon substrates and get them to line up into rows. But the current demonstration is the first to inject electrons into the molecule.
"We use an ordinary silicon substrate that is hydrogen-terminated," Wolkow said. "By removing a single hydrogen atom, we create a dangling bond, to which we attach a single monomer molecule called styrene." The styrene molecule is attached to the surface with a carbon-to-silicon bond.
"The trick is that as the styrene molecule becomes bonded to the surface, it attracts another hydrogen atom from the silicon surface, thus creating a second dangling bond, which is at a different potential than all the other silicon atoms on the surface. Thus, by changing the potential on the whole surface, we can cause that one silicon atom to be charged or not with a single electron."
The other contact to the molecule—the drain—is the tip of a scanning tunneling microscope (STM).
"We demonstrated we can control current through the styrene molecule in the same way that a field-effect transistor works, with the substrate acting as the source and the STM tip acting as the drain," Wolkow said.
For the future, Wolkow wants to form a line of organic molecules, with the first one attached to on-chip electrodes that will act as the source and drain.
- R. Colin Johnson