Researchers discover a potential on-off switch for nanoelectronics

Researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and Columbia University are studying how electrons flow through a molecular junction (a nanometer scale circuit element that contacts gold atoms with a single molecule).

 

Their findings reveal the electrical resistance through this junction can be turned 'on' and 'off' simply by pushing and pulling the junction. This feature could be used as a switch in nanoscale electronic devices.

 

However, designing a molecular junction requires understanding of how the intrinsic properties of a molecule or junction are actually connected to its measured resistance. This is not possible to determine by experiments at this stage and theory can be valuable in helping to understand resistance measurements.

 

Traditional electronic devices have electrons that diffuse through circuits in a well understood manner. At nanoscale, electrons can travel by a mechanism called quantum tunneling which means that particles can disappear through an energy barrier and suddenly appear on the other side without expending energy.

 

Tracking this "tunneling" of electrons through individual molecules in nanoscale devices has proved difficult and experiments performed have reported conductance results that varied from theoretical predictions by an order of magnitude or more.

 

Previously it was shown that using a scanning tunneling microscope (STM) with a gold tip that was continuously dipped into a gold surface containing a solution of molecules could produce a single molecular strand. When this strand breaks, nearby molecules can hop into the gap between strands and contact the gold electrode causing a sudden change in conductance. Using this technique, researchers found that the conductance of molecules containing amines (a group of molecules related to ammonia) in contact with gold electrodes could be reliably measured.

 

Using results from these experiments it was found that some electron correlation effects were missing from the proposed theory. After adjustment of the theoretical approach the researchers studied the conductance of a junction between gold electrodes and bipyridine (a benzene like ring molecule containing nitrogen). Experimental data showed two stable conductive states and it was hypothesized these were due to different structures being present within the junction. The theory proposed that these two conductive states were as a result of the junctions between the two gold molecules being arranged vertically and at sandwiched angles.

 

Further results suggest that if the bipyridine bonded at an angle more current could flow compared to it bonding vertically. In the STM experiment, when the strand first breaks a bipyridine molecule fills the gap. However as it is a large molecule it bonds at an angle. As the gap increases the bipyridine molecule is able to jump to the vertical configuration resulting in an abrupt fall in conductance.

 

Further work has demonstrated that pushing and pulling this bipyridine junction can repeatedly alter the conductance and hence create a mechanical switch with well defined "on" and "off" states.

The researchers hope to refine and apply their theoretical framework to more complex molecular junctions for study of systems promising for solar energy conversion, such as organic photovoltaics.

 

 

The scientists believe that this is the first step in developing new and improved electronic devices.

 

Reference: U.S. Department of Energy's Lawrence Berkeley National Laboratory.

 

For more attend Printed Electronics Europe 2009 .