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Motorola Labs and Arizona State University Researchers Develop Method for Electrically Testing Single or Small Bundles of Electronic Molecules

Significant contribution to nanotechnology addresses the need to reliably screen single molecules

TEMPE, Ariz. -- November 20, 2002 -- Researchers at Motorola Labs (NYSE: MOT) and Arizona State University (ASU) are developing new methods to further the efforts of one of today’s most promising fields – nanotechnology. The pursuit of nanotechnology by researchers and businesses around the world is being driven by the need to build ever smaller electronic devices and increase circuit density, coupled with the daunting task and rising cost of further miniaturizing devices via conventional semiconductor fabrication.

While it was thought improbable just 10 years ago, in the past few years scientists have discovered that individual molecules can perform the same functions as transistors, conductors and other electronic components of current technology. This has sparked significant interest in the potential of molecular electronics – the use of molecules as ultra-small electronic devices. However, the abundance in variety of molecules means that rapid and reliable methods need to be developed for screening the electrical properties of molecules. Earlier this year, researchers at Motorola Labs reported on advances in this area by successfully creating custom electronic molecules and using a hybrid assembly technique to interconnect two separate arrays of molecules for electrical testing.

Now, in a joint paper published in Applied Physics Letters (2002, volume 81, 3043-3045), Motorola Labs researchers, Adam M. Rawlett, Theresa J. Hopson, Larry A. Nagahara and Raymond Tsui, with Ganesh Ramachandran and Professor Stuart Lindsay, ASU, Department of Physics and Astronomy, describe developing a more definitive method for electrically testing single or small bundles of electronic molecules.

Previously, a number of researchers have reported on custom-synthesized organic molecules that appear to exhibit electronic functionality, such as switching and memory at room temperature. However, these measurements were typically performed using test structures containing a thousand or more molecules in parallel, and so it has not been possible to know if the data indicated the true electronic properties of a single molecule or some collective intermolecular characteristics. This inability to rapidly and easily screen a single molecule in a defined and reliable manner has been viewed as one major obstacle in the development of molecular electronics. Furthermore, most of the reported measurements involve molecule/metal (contact) interfaces that are different at the two ends of each molecule. The deposition of evaporated metal to form the top contact does not result in a chemical bond to the molecule such as that typically found in the bottom contact. This could introduce asymmetries in the physical and electronic characteristics of the molecule being tested. There have been experiments on direct and symmetrical measurements of a simple single molecule via a mechanical break junction, but such a measurement is difficult to execute and characterize.

Motorola Labs/ASU Approach to Measurement via Conducting Atomic Force Microscopy

Motorola Labs and ASU have now developed a more definitive method of electrically testing single or small bundles of electronic molecules. Two-nanometer long molecules were custom-synthesized to selectively form chemical bonds to a gold substrate at one end and to two-nanometer diameter gold nanoparticles at the other end (one nanometer is 1 billionth of a meter). The conducting tip of an atomic force microscope (AFM) was used to image and then to make a top electrical contact to the molecules. By taking particular care to ensure the AFM tip is in contact with a capping nanoparticle, the current flow in a small bundle of one to five molecules can be measured as a function of the applied bias at room temperature. The current-voltage curve of the test molecule exhibits non-linearity in the form of negative differential resistance (NDR) while that of a control molecule does not. The result indicated that the NDR previously observed in “bulk” measurements of similar molecules exists down to the single/few molecules level, and hints at the ultimate scaling limit that can be achieved with molecule-based electronics. These results represent a significant contribution toward this area of nanotechnology.

About Molecular Electronics
Today’s transistors are so small that a processor containing 40 million of them is no larger than a penny. Shrinking a current transistor to the size of a nanometer-scale molecule would allow today’s processor to occupy an area smaller than a pinhead. Furthermore, molecules are highly uniform in nature, which is very advantageous for the fabrication of ultra-dense, low-power integrated circuits. Additionally, organic and inorganic molecules can be synthesized with unique chemical, physical and biological properties that could be used to promote their self-assembly to one another and to specific surfaces, and to perform functions that can provide memory and logic operations.

Conventional semiconductor devices are fabricated from the “top down,” starting with wafers on which small components are fabricated and interconnected using lithography and other processing techniques. Molecular electronics may enable a "bottom-up" approach, in which the starting components are nanometer-scale in nature and are chemically self-assembled into devices and circuits.

The molecular components currently studied at Motorola Labs include carbon nanotubes with diameters as small as one nanometer and custom-synthesized organic molecules. The carbon nanotubes have been shown to exhibit electrical properties similar to that of conventional silicon transistors. The custom organic molecules, on the other hand, show highly non-linear current-voltage characteristics such as NDR or on/off switching that can be used in device applications.

In addition to working with ASU, Motorola Labs is also collaborating with other universities in a research project on molecular electronics funded by the U. S. Defense Advanced Research Projects Agency (DARPA). The research group at Motorola Labs has previously published work on the controlled placement and electrical characterization of carbon nanotubes and metallic nanoparticles.

About Motorola
Motorola Labs is the research arm of Motorola, Inc., with a strong, global team of scientists and engineers focused on discovering and developing new materials, technologies, architectures, algorithms and processes for future systems, products and product enhancements. Motorola also actively licenses technologies developed in the Labs. For more information on Motorola Labs, visit: www.motorola.com/labs.

Motorola, Inc. (NYSE:MOT) is a global leader in providing integrated communications and embedded electronic solutions. Sales in 2001 were $30 billion. For more information, visit: www.motorola.com.

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