As Tao explains, the intriguing result is a byproduct of the laws of quantum mechanics, which dictate the behavior of matter at the tiniest scales: "The conductivity of a single molecule is not simply inversely proportional to length. It depends on the energy level alignment."
In the metal leads of the electrodes, electrons can move about freely but when they come to an interfacein this case, a molecule that sits in the junction between electrodesthey have to overcome an energy barrier. The height of this energy barrier is critical to how readily electrons can pass through the molecule. By applying a mechanical force to the molecule, the barrier is lowered, improving conductance.
"Theoretically, people have thought of this as a possibility, but this is a demonstration that it really happens," Tao says. "If you stretch the molecule and geometrically increase the length, it energetically lowers the barrier so electrons can easily go through. If you think in optical terms, it becomes more transparent to electrons."
The reason for this has to do with a property known as force-induced resonant tunneling. This occurs when the molecular energy moves closer to the Fermi level of the electrodesthat is, toward the region of optimal conductance. (See figure 1) Thus, as the molecule is stretched, it causes a decrease in the tunneling energy barrier.
For the experiments, Tao's group used 1,4'-Benzenedithiol, the most widely studied entity for molecular electronics. Further experiments demonstrated that the transport of electrons through the molecule underwent a corresponding decrease as the distance between the electrodes was reduced, causing the molecule's geometry to shift from a stretched condition to a relaxed or squeezed state. "We have to do this thousands of times to be sure the effect is robust and reproducible."
In addition to the discovery's practical importance, the new data show close agreement with theor
|Contact: Joseph Caspermeyer|
Arizona State University