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Mechanical control of conductance through a single molecule junction

New options to control nanoelectronic systems may arise from the demonstration that mechanical manipulation can control conductance through single molecule electrode junctions. Research reported in Nature Nanotechnology [abstract] demonstrates that conductance through a single organic molecule (pentaphenylene) bridging two gold electrodes changes by an order of magnitude as the electrodes are manipulated to change the angle between the molecule and the electrode from perpendicular to highly tilted. As the molecule is tilted away from the perpendicular, the pi orbitals of the pentaphenylene begin to overlap with the atomic orbitals of the gold electrodes, changing the energy levels of the molecule and thus the conductance through the molecule. From Physorg.com, based upon an Arizona State University news release “Manipulating molecules for a new breed of electronics“:

When electrical devices are shrunk to a molecular scale, both electrical and mechanical properties of a given molecule become critical. Specific properties may be exploited, depending on the needs of the application. Here, a single molecule is attached at either end to a pair of gold electrodes, forming an electrical circuit, whose current can be measured.

In research appearing in today’s issue of the journal Nature Nanotechnology, Nongjian “NJ” Tao, a researcher at the Biodesign Institute at Arizona State University, has demonstrated a clever way of controlling electrical conductance of a single molecule, by exploiting the molecule’s mechanical properties.
Such control may eventually play a role in the design of ultra-tiny electrical gadgets, created to perform myriad useful tasks, from biological and chemical sensing to improving telecommunications and computer memory.

Tao leads a research team used to dealing with the challenges entailed in creating electrical devices of this size, where quirky effects of the quantum world often dominate device behavior. As Tao explains, one such issue is defining and controlling the electrical conductance of a single molecule, attached to a pair of gold electrodes.

”Some molecules have unusual electromechanical properties, which are unlike silicon-based materials. A molecule can also recognize other molecules via specific interactions.” These unique properties can offer tremendous functional flexibility to designers of nanoscale devices.

In the current research, Tao examines the electromechanical properties of single molecules sandwiched between conducting electrodes. When a voltage is applied, a resulting flow of current can be measured. A particular type of molecule, known as pentaphenylene, was used and its electrical conductance examined.

Tao’s group was able to vary the conductance by as much as an order of magnitude, simply by changing the orientation of the molecule with respect to the electrode surfaces. Specifically, the molecule’s tilt angle was altered, with conductance rising as the distance separating the electrodes decreased, and reaching a maximum when the molecule was poised between the electrodes at 90 degrees.

The reason for the dramatic fluctuation in conductance has to do with the so-called pi orbitals of the electrons making up the molecules, and their interaction with electron orbitals in the attached electrodes. As Tao notes, pi orbitals may be thought of as electron clouds, protruding perpendicularly from either side of the plane of the molecule. When the tilt angle of a molecule trapped between two electrodes is altered, these pi orbitals can come in contact and blend with electron orbitals contained in the gold electrode—a process known as lateral coupling. This lateral coupling of orbitals has the effect of increasing conductance. …

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