A publication of the Foresight Institute
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The latter half of 2001 saw a number of significant advances in the field of molecular electronics. Academic and government research efforts were pushing the basic technologies forward even as a growing number of private business ventures are seeking ways to develop commercial products. As if in acknowledgment of these advances, the annual Foresight Feynman Prizes in Nanotechnology, for both theoretical and experimental work, went to researchers who are leaders in the field of molecular electronics (see article).
In June, researchers led by Paul S. Weiss at Pennsylvania State University and James M. Tour at Rice University in Houston reported that they have demonstrated single molecules that switch between ON and OFF states, and then hold in a state for hours at a time. The function of their molecular switches is based in part on conformational changes which happen when molecules alter their arrangement by rotation of their atoms around a single bond, effectively changing shape by moving or turning that determine how and when that conductance switching occurs in those molecules.
As described in their report in the 22 June 2001 issue of Science, they tracked over time the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers. The persistence times for isolated and bundled molecules in either the ON or OFF switch state ranged from seconds to tens of hours. When the surrounding matrix is well ordered, the rate at which the inserted molecules switch is low. When the surrounding matrix is poorly ordered, the inserted molecules switch more often. As a result, the team concluded that the switching is a result of conformational changes in the molecules or bundles, rather than electrostatic effects of charge transfer. The moleculescomprised of alternating benzene rings and two carbon atoms with triple bonds between them and a functional group on the central of three ringswere the first single molecules to have their switching documented.
"We essentially tightened the noose around the molecule and showed that once its motion was reduced, switching went way down," says Paul Weiss, associate professor of chemistry at Penn State. "We have not worked out how to make computer architecture or anything close to that, but tackling the very small end, which is our specialty, has been an interesting and exciting project. Our next step is figuring out how to control the molecules' movement between 'on' and 'off.' In bundles of thousands of molecules, our collaborator, Mark Reed in electrical engineering at Yale University and his group, have been able accomplish movement between the states. Our work was the first to show that single molecules could function as switches."
"We have demonstrated that single molecules can switch. So switching and memory could be scaled to the single molecule level one million times smaller than the smallest transistor," said James Tour, a professor of chemistry at Rice University.
Funding for the research was provided by the Army Research Office, the Defense Advanced Research Projects Agency (DARPA), the National Science Foundation (NSF), the Office of Naval Research, and Zyvex Corporation.
For details, see the press release from Penn State. Additional coverage can be found in this research summary from Technology Research Magazine ("Molecule makes mini memory", by C. Sachdev, 15 August 2001)
Molecular electronics was also the focus of an extensive article in the New York Times ("Clever Wiring Harnesses Tiny Switches", by K. Chang, 17 July 2001), providing an overview of recent advances in the field of molecular electronics.
The article highlighted work by Hewlett-Packard Labs. HP was awarded a patent on 3 July 2001 for a wiring strategy that describes how to connect molecular-scale devices by essentially assigning each switch a random marker that allows signals to be routed to it. The method is important because, as the NYT article states, "conventional wires are too wide to attach to such molecular components, and the prospect of trying to hook together billions of components or more is daunting, if not impossible." The article quotes HP Labs research director Stanley Williams: "The current patent really is the blueprint for the research we're going to be doing for the next four years." HP was awarded another patent on a molecular memory device in October 2000.
The article also refers to the work by Weiss and Tour, and notes that "researchers have already constructed the tiniest of components molecules that act as switches and they are now starting to tackle the harder problem: how to wire the tiny switches together into useful devices."
Additional coverage of the molecular electronics work that led to the patent granted to Hewlett-Packard can be found in an HP press release (17 July 2001), which quotes HP's Stan Williams: "We have a strategy to reinvent the integrated circuit with molecular rather than semiconductor components."
Also, an extensive article on the Small Times website ("UCLA team develops molecular switches", by Jayne Fried, 26 October 2001) describes recent work by James Heath and his coworkers at the California NanoSystems Institute (CNSI) to develop working molecular electronics devices. According to the article, they have attached molecular switches on a grid as small as 50 nanometers, a significant step forward in the UCLA effort to build a rudimentary molecular computer. "There's a long way to go," Heath said. "Right now we have circuits with molecules on a grid on normal lithographic wires." The goal is that one day the grid would be assembled with carbon nanotubes.
More information on the molectronics work at UCLA can be found in Foresight Update #44.
|Lucent Bell Labs researchers Zhenan Bao (left) and Hendrik Schön, along with co-worker Hong Meng, reported they had succeeded in fabricating an individually addressable transistor whose channel consists of just one molecule. Using a new technique, they succeeded in fabricating molecular-scale transistors that can be individually controlled.|
An exciting advance was reported by researchers at Lucent Technologies' Bell Laboratories, where the silicon-based semiconductor transistor was developed in the 1950s. Bell Labs scientists Hendrik Schön, Zhenan Bao, and Hong Meng reported they have created organic transistors with a single-molecule channel length. The research was reported in the 18 October 2001 issue of Nature.
A transistor is a three-electrode semiconductor device, conventionally made of an inorganic semiconductor like silicon. It amplifies electrical signals and acts as an electronic switch. Transistors are essentially voltage-controlled switches. In the OFF state, no current can flow through, which represents a "0" in the binary language of computers. When an electric field is applied from the side, from a third terminal known as a gate electrode, the electronic properties shift and current starts to flow: the ON or "1" position of the switch. The transistor's channel is the space between two of its electrodes that influences the transistor's current output and switching speed.
The amplification and switching properties of the transistors are comparable to those of silicon transistors. "When we tested them, they behaved extremely well as both amplifiers and switches," said Schön, an experimental physicist who was the lead researcher.
The multidisciplinary Bell Labs team used the tiny organic transistors, which are roughly a million times smaller than a grain of sand, to build a voltage inverter an electronic circuit that converts a "0" to a "1" or vice versa.
The main challenges in making molecular-scale transistors are fabricating electrodes that are separated by only a few molecules and attaching electrical contacts to the tiny devices. The Bell Labs researchers were able to overcome these hurdles by using a self-assembly technique and a clever design in which the electrodes were shared by many molecular-scale transistors.
"We solved the contact problem by letting one layer of organic molecules self-assemble on one electrode first, and then placing the second electrode above it," said Bao, an organic chemist. "For the self assembly, we simply make a solution of the organic semiconductor, pour it on the base, and the molecules do the work of finding the electrodes and attaching themselves."
The chemical self-assembly technique is easy and relatively inexpensive and determines the transistor channel length. The channel length of the Bell Labs molecular-scale transistors is 1 to 2 nanometers, less than a tenth the size of any channel that has been created, even with the most advanced lithography techniques.
Commenting on the Bell Labs work, Paul Weiss of Penn State said, "This is a beautiful, simple and clever approach. It circumvents many of the difficulties with other nanofabrication approaches."
The Bell Labs press release, along with videos of a press conference and interviews with the researchers, are available on the Bell Labs website. Additional coverage ran in the New York Times ("Precursor to Tiniest Chip Is Developed", by K. Chang, 18 October 2001). The NYT article quotes James Tour: "It is really, really nice work that will influence the field a lot. They hit on something really big."
While the switching layer in the prototype transistor was only one molecule thick, the Bell Labs researchers had only been able to fabricate these "nanotransistors" as a matrix of a few thousand molecules that worked in tandem. However, they soon followed up these results with an even more startling announcement.
On 8 November, the same team reported they had succeeded in fabricating an individually addressable transistor whose channel consists of just one molecule. Using a new technique, they succeeded in fabricating molecular-scale transistors that can be individually controlled. The research was reported online in the 8 November 2001 edition of Science Express, and in print in the 7 December 2001 issue of Science.
To allow a single molecule to take on the semiconducting channel role, the researchers fabricated a monolayer out of a mixture of semiconducting and insulating molecules in order to isolate an individual semiconducting molecule. By creating a mixture with only about one semiconducting molecule to every 5,000 insulating molecules, the researchers end up with only one semiconducting molecule in the transistor. Using two of the nanotransistors, the Bell Labs scientists built a voltage inverter, a standard electronic circuit module that converts a "0" to a "1" or vice versa, creating a NOT gate for computer logic.
Details are available in the Bell Labs press release. Additional coverage is available in a Reuters News Service article and an Associated Press news story on the New York Times website.
While the work demonstrates that a single molecule can function as the active component of a transistor, many challenges remain. "Producing separate gates will be challenging; we're looking towards more complex molecules, which consist of different parts" that would function as the five parts of a transistor, said Schön. Another challenge is wiring the single molecules together in the precise arrangements needed to make up circuits, he added.
In October, a collaborative research team from the University of Arizona and Motorola, Inc. announced results that may in part address the problem of connecting molecular electronics components. In a press release, they reported they have devised a method to measure the electrical conductivity of a single molecule using contacts bonded to the two ends of an octanedithiol molecule. Many previous efforts to characterize possible molecular wires and other molectronic components have given variable results because the contacts were often simple mechanical contacts, not chemically-bonded connections. In their report in the 19 October 2001 issue of Science, the UA/Motorola team describe a method for creating through-bond electrical contacts with very small (2 nanometer) gold particles bonded to single molecules and the achievement of reproducible measurements of the molecules' conductivity.
In the past few months, significant advances were also announced from research efforts to adapt carbon nanotubes into molecular electronic devices. Not content with their success with organic molecule switches (see above), James Tour of Rice University and Paul Weiss of Pennsylvania State University announced in June they had developed a new technique for attaching groups of atoms to the sides of carbon nanotubes, creating compounds with extraordinary strength and conductivity. The work was described in the Journal of the American Chemical Society (v123:6536).
Their work suggests the functionalized carbon nanotubes could be used for making electronic circuits that are far tinier than today's silicon-based circuitry. Doing so will require chemically hooking carbon nanotubes to other microscopic electronic components, comments Weiss. One of the functional groups that the Rice researchers successfully attached to carbon nanotubes has exhibited both memory and switching behaviors necessary for electronic devices, says Tour. The researchers are investigating whether a nanotube and its functional groups retain their desirable strength, conductivity, and chemical traits after they're combined.
Also in June, researchers in the Netherlands reported they had created a single-electron transistor (SET) made from a single carbon nanotube, whose minute size and low-energy requirements should make it an ideal device for molecular computers. The Dutch nanotube single electron transistor, the first to operate efficiently at room temperature, was described in the 29 June 2001 issue of Science.
"We've added yet another important piece to the toolbox for molecular electronics," said Cees Dekker of Delft University of Technology in the Netherlands in a press release. Dekker and his colleagues started with a single carbon nanotube, and used the tip of an atomic force microscope to create sharp bends, or buckles, in the tube. These buckles worked as the barriers, only allowing single electrons through under the right voltages. The whole device was only 1 nm wide and 20 nanometers long.
In August, a research team led by Dekker announced that, using a scanning electron probe, they had imaged the undulations of electron waves in carbon nanotubes. In addition to illuminating basic properties of electron conduction in nanotubes, their results also confirm theoretical predictions that electrons in metallic nanotubes moved along two different electron "bands" that can interfere with each other. A member of the research team said it may be possible to manipulate these electrons to make them interfere with each other and create a circuit. The work was reported in the 9 August 2001 issue of Nature. Additional details are available from an article from the Nature Science Update website.
Also in August, researchers at IBM announced they have created and demonstrated the world's first logic-performing computer circuit within a single molecule, according to an IBM press release. The device, based on a carbon nanotube, functions as a voltage inverter and thus acts as a NOT gate one of the three fundamental binary logic circuits that are the basis for digital computers. They encoded the entire inverter logic function along the length of a single carbon nanotube, forming the world's first single-molecule logic circuit.
The achievement was announced on 26 August 2001 at the 222nd National Meeting of the American Chemical Society (ACS) held in Chicago. The full research paper describing the device is available in the online ACS journal, Nano Letters ("Carbon Nanotube Inter- and Intramolecular Logic Gates")
In April 2001, the same IBM team became the first to develop a technique to produce arrays of carbon nanotube transistors, bypassing the need to separate metallic and semiconducting nanotubes. The team used these nanotube transistors to make the NOT circuit.
Two important papers on the use of carbon nanotubes to form electronic devices and circuits appeared in the 9 November 2001 issue of Science.
Cees Dekker and co-workers at the University of Delft in the Netherlands reported they have demonstrated logic circuits with field-effect transistors based on single carbon nanotubes. The transistors show favorable device characteristics such as high gain, a large on-off ratio, and room-temperature operation. The team was also able to demonstrate one-, two-, and three-transistor circuits that exhibit a range of digital logic operations, such as an inverter, a logic NOR, a static random-access memory cell, and an ac ring oscillator.
Charles Lieber and his research team at Harvard reported a "bottom-up" approach in which functional device elements and element arrays are assembled from solution through the use of electronically well-defined semiconductor nanowire building blocks. Crossed nanowire junctions and junction arrays can be assembled with controllable electrical characteristics. The junctions can be used to create integrated nanoscale field-effect transistor arrays with nanowires as both the conducting channel and gate electrode. Nanowire junction arrays have been configured as OR, AND, and NOR logic-gate structures with substantial gain and have been used to implement basic computation.
Additional details are available in an article from the New York Times ("Nanowires May Lead to Superfast Computer Chips", 9 November 2001) and an item on the Nature Science Update website ("A little logic goes a long way", by Philip Ball, 9 November 2001).
Lieber was recipient of the 2001 Feynman Prize in Nanotechnology for experimental work; the prize for theoretical work went to Mark Ratner, another pioneering figure in the field of molecular electronics (see article).
|Foresight Update 47 - Table of Contents|
The Ninth Foresight Conference on Molecular Nanotechnology brought together leaders in the field of molecular nanotechnology for its annual comprehensive overview of research and development in the field. The Conference was held 8-11 November 2001 in Santa Clara, California.
Foresight's annual technical conference is a meeting of scientists and technologists working in fields leading toward molecular nanotechnology: thorough three-dimensional structural control of materials and devices at the molecular level.
This year's conference featured 37 invited and contributed speakers and 66 poster presenters. Over 500 people registered for the conference (503 to be exact) which represented a 20% increase over the previously historic high number that registered for the 8th annual conference held in Bethesda, MD, in 2000. This year's attendance was 77% higher than the registration for the 7th Foresight conference in 1999, which was also held in Silicon Valley.
Attendees included those from universities, government laboratories, Fortune 500 companies, non-Fortune 500 companies, VC firms, legal firms, foundations, and publishing companies, as well as interested individuals. The largest percentage (over a third of the attendees) came from universities. About 15% of the attendees came from outside the United States, with about equal numbers from Europe and Asian/Pacific countries. The largest number of European attendees came from the United Kingdom (10) and from the Asia-Pacific region, the largest number of attendees came from Japan (13), followed by Korea.
The number of registrants from for-profit business, venture capital, and attorney related firms increased substantially over 2000. In 1999, there were almost no attendees from these type organizations (or at least none who identified themselves as such).
This year's new additions to the conference programming were well received. Over 150 people attended the Nanotechnology Patent Roundtable on Thursday afternoon (Nov. 8) where John Doll, Rolf Hille, and Bruce M. Kislik, Group Directors from the United States Patent and Trademark Office, spoke. And over 250 attendees participated in the Venture Capital for Nanotechnology Panel Discussion held on Friday afternoon (Nov. 9)
The conference covered a wide range of topics relevant to the pursuit of molecular control.
In addition, the 2001 Annual Feynman Prizes in Nanotechnology were awarded during a special evening banquet session. The annual Feynman Prizes are sponsored by the Foresight Institute to recognize recent achievements that contribute to the development of nanotechnology, and to encourage and accelerate that development.
Also presented were the 2001 Distinguished Student Award, and the 2001 Communications Award. For details, see the story in this issue.
|Foresight Update 47 - Table of Contents|
Foresight's web-based forum for breaking news and discussions is back online. The site suffered from problems with user access and lengthy stretches of downtime, in part to allow for an upgrade to the Slash v2.0 software. The site also suffered a series of hack attacks that included infiltration of the site server that caused significant damage. Fortunately, the damage was reparable, and thanks to the efforts of Senior Associate and site administrator Dave Krieger, as well as Foresight chief information officer Ben Harper, the site has been restored and is once again secure and stable. Now that everything is running smoothly again, we encourage all members of the Foresight community to resume visting the site as often as possible we're posting new items daily in our attempts to keep up with the ever-increasing flow of information about nanotechnology and other issues of concern to Foresight and its members. Even more importantly, we encourage all of you to submit any items you'd like to share. We're sure we're not catching everything that we'd like to.
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From Foresight Update 47, originally published 31 December 2001.