Top 10 Recent Achievements
in Nanoelectronics

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This list represents our view of the most important and innovative research developments that are breaking down conceptual and physical barriers to hasten the arrival of practical, widely available nanoelectronic devices and computers.

Fabrication and demonstration of conducting molecular wires.
Fabrication of a self-assembled molecular electronic circuit array.
Invention of the Quantum Dot Cell and "wireless" electronic computing.
Fabrication and testing of "quantum corrals".
Construction and demonstration of the "Nanomanipulator".
"Printing" of nanostructures using self-assembling molecular monolayers.
Formation of the ULTRA Electronics Research Program at DARPA.
Research on fabricating "hybrid" nanoelectronic-microelectronic logic.
Room-temperature manipulation of molecules with an STM.
Progress toward arrays of micro-STMs and micro-AFMs.

We welcome comments, suggestions, and questions about this list. Please e-mail them to mailto:%20nanotech@mitre.org

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Despite the achievements enumerated above, and numerous others, there are still many challenges to be overcome before nanoelectronic devices become practical and widely available.

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Fabrication and demonstration of conducting molecular wires.

Professor James Tour of the University of South Carolina, along with Professors David Allara and Paul Weiss of Penn State University, have fabricated and demonstrated a functioning molecular wire. Their nanometer-scale wire consists of a single "chain" molecule with its end adsorbed to the surface of a gold lead that is covered by an insulating monolayer of other molecules. The molecular wire completed a circuit between the gold lead and the tip of a scanning-tunneling electron microscope (STM) probe. A number of groups previously have synthesized structures proposed as molecular wires, but never before has any of these been demonstrated to conduct electricity. Therefore, the Tour-Allara-Weiss experiment is a giant step on the road to truly nanometer-scale molecular electronics. For more details of this advance, see: (1) "Are Molecular Wires Conducting?", Science vol. 271, 1705-1707, 22 March 1996 and/or (2) "Single molecular wire shown to be conductive," Chemical and Engineering News, 25 March 1996, p.7.)

In an unpublished report of even more recent work, Prof. Tour, working in collaboration with Prof Mark Reed of the electrical Engineering Department at Yale University, suggests that there may have been a successful demonstration of conduction by a single molecule affixed to at its opposite ends to two gold leads. Members of Prof. Reed's group presently are testing these candidate molecular electronic devices synthesized by Prof Tour. They are stretching one molecule at a time between gold leads in a vacuum to determine if the molecule exhibits the "step" in the current-potential plot that is characteristic of quantum-effect devices. If these difficult and daring experiments are successful, it will be a development of the highest importance.


The Tour-Reed investigations are pointing the way to a molecular-scale electronic transistor or switch. Obviously, with a combination of molecular wires (from Tour-Allara-Weiss) and molecular transistors (from descendants of Tour-Reed), one would have the components necessary for a nanometer-scale molecular electronic logic circuit, like a logic gate. However the assembly and testing of such a device still would be a research challenge, even if one had all the components. One possible route to the assembly of such a device is to use an STM (or AFM) to arrange the molecular components on a surface, in the manner that was demonstrated recently by IBM-Zurich. That is why that IBM advance is of such importance.

All of this very important and innovative research in molecular electronics is motivated by the visionary work of Ari Aviram of the International Business Machines (IBM) Watson Research Laboratory, Professor Mark Ratner of Northwestern University, as well as the efforts of the late Forrest Carter of the Naval Research Laboratory in Washington, DC.

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An interdisciplinary team of scientists and engineers at Purdue University have fabricated, via self-assembly techniques, a regular, extended array of functioning molecular-scale circuit elements. The Purdue team included Ronald P. Andres, Jeffery D. Bielefeld, Jason I. Henderson, David B. Janes, Venkat R. Kolagunta, Clifford P. Kubiak, William J. Mahoney, Richard G. Osifchin, as well as Ronald Reifenberger. This array consists of a two-dimensional lattice of many, many 2 nanometer diameter gold balls, each one of which is affixed individually to the top ends of several molecular wires of the type invented by Prof. James Tour. The bottoms of these wires are affixed to a gold film supported on a nonconducting substrate. By placing the tip of a scanning tunneling electron microscope over the tops of individual gold balls, it has been demonstrated that each unit of gold balls and molecular wires can conduct a current. The current passes vertically from the underlying gold film, through the molecular wires and into the gold balls. Aspects of this exciting development are reported in the 20 September 1996 issue of the journal Science,, pp. 1690-1693. The work was supported by the Army Research Office. This work demonstrates for the first time an extended, heterogeneous structure containing functioning molecular-scale circuit elements. It also demonstrates a new type of multi-stage manufacturing process for such structures, based upon the principles of chemical self-assembly. The Purdue group now plans to attempt to cross link selected neighboring gold balls using molecular wires, to show that current can be passed horizontally through the extended structure.

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by Profs. Craig Lent and Wolfgang Porod at the University of Notre Dame. Lent and Porod got the innovative idea of making a nanometer-scale "two-state device" or switch out of a cruciform arrangement of five quantum dots. They showed by quantum-mechanical modeling and simulation that two or more neighboring cruciform switches or "cells" could interact in such a way as to send a signal along a line of cells without any current flowing. These innovative wireless quantum cellular automata are a new idea which has generated much investigation and discussion of method to circumvent well-known scaling problems and fundamental limitations associated with more conventional designs for very-small electronic circuits. Prof. Gary Bernstein, Director of the Microelectronics Laboratory at the University of Notre Dame Electrical Engineering Department is collaborating with Lent and Porod to fabricate quantum dot cells and wireless electronic logic structures based upon them. Profs. Alexander Korotkov and Konstantin Likharev of the State University of New York at Stony Brook have suggested innovative variants of the Lent-Porod wireless logic to remedy some of the limitations of the original scheme.

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by an investigative team led by D. M. Eigler at the International Business Machines(IBM) Almaden Research Laboratory in northern California. Quantum corrals are primitive nanometer-scale devices for the manipulation of electronic charge on the surface of a solid. They consist of enclosures only 2 to 5 nanometers across that are formed from only a few dozen atoms placed in position one at a time, arduously, with a scanning-tunneling electron microscope (STM).

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a virtual environment linked to a live scanning-tunneling electron microscope (STM) experiment. Developed jointly by investigators at the University of North Carolina Department of Computer Science and investigators at the Chemistry Department at the University of California at Los Angeles (UCLA), it allows experimenters to see, "touch," and "feel" atoms.

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These developments are as a result of research conducted by George Whitesides Group at Harvard University and by Steven Chou's Group at the University of Minnesota. This work provides a foundation for simpler, and potentially inexpensive methods for circumventing the much discussed "point-one" barrier--the fundamental limits encountered by conventional UV-visible lithography that prevent it from being used in fabricating structures with features less than approximately 0.1 microns (100 nanometers). (Note that useful frequencies of UV light have wavelengths of 200 to 350 nanometers.) Most other proposed methods for nanofabrication require very much more equipment and investment. The large costs have slowed research and development. (See, for example, the article "Toward 'Point One'", by Gary Stix, in Scientific American, February 1995, pp. 90-95.) The Whitesides-Chou approaches using self-assembling molecular monolayers (SAMs) may speed industrial applications of nanofabrication. (See, for example, George Whitesides article "Self-assembling Materials" in Scientific American, September 1995, pp. 146-149.) Recent work by Whitesides' group has shown how even 3-dimensional nanostructures might be printed relatively easily, while Chou's group has made very innovative applications of the self-assembly techniques to the nanofabrication of structures for ultra-dense magnetic mass storage media.

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at the Defense Advanced Research Projects Agency (DARPA). Founded originally by Dr. Jane Alexander, and now managed by Lt. Col. Gernot Pomrenke, the visionary ULTRA program initiates and funds numerous projects for the exploration and development of ultra-small, ultra-low-power, and ultra-fast electronic computers. The ULTRA Program has been responsible for many of the most innovative and important recent advances in nanoelectronics and nanocomputing.

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in the Nanoelectronics Group at the Texas Instruments Corporation. Drs. Alan Seabaugh, Gary Frazier, and their collaborators are embedding nanometer-scale quantum devices on a chip amidst more conventional microelectronic logic. These techniques were first proposed in the 1980s by Federico Capasso and his collaborators of AT&T Bell Laboratories. "Hybrid logic" is an important transitional step that has the potential to increase enormously the logic density of a chip, while retaining some of the strengths and reliability of conventional microelectronics. This is another simple but blockbusting idea that promises to accelerate the arrival of much more densely integrated electronic computers.

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with a scanning-tunneling electron microscope (STM) by investigators at the IBM-Zurich Research Laboratory.

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Prof. Noel MacDonald of the Cornell University Department of Electrical Engineering has reported and shown pictures of an array of microelectromechanical STMs fabricated on a chip. Analogous work on micro-AFMs (atomic force microscopes) is reported to be underway in the laboratory of Prof. Calvin Quate of Stanford University. Prof. MacDonald has focused his efforts on applying the micro-STM array on ultra-dense data storage. However, it should also be applicable to mass fabrication of nanostructures, which is the application envision by Prof. Quate for his micro-AFMs. Advances to date on this front are a very significant step in the direction of tools for the mass precision manufacture of nanostructures via "mechanosynthesis". Ultimately, if micro-chips containing arrays of proximal probes can themselves be mass manufactured, the way computer chips are today, one can envision them being used as an accessory to desktop workstations everywhere. Then, one might see widespread, mass distributed manufacturing of nanostructures and nanodevices.

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Despite the achievements enumerated above, and numerous others, there are still many challenges to be overcome before nanoelectronic devices become practical and widely available.

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