NANOTECH AND MOLECULAR ELECTRONICS. Nanotechnology and molecular electronics research seemed to be leading a charmed existence in 2001, with many important advances coming to fruition. As semiconductor devices approach their physical limits, researchers are trying to devise ways to decrease the size of features in microelectronic circuits, and a number of studies were carried out with miniaturization goals in mind.
Another group used cylindrical polymeric micelles as building blocks to fabricate ceramic lines on a semiconductor surface [J. Am. Chem. Soc., 123, 3147 (2001); C&EN, April 2, page 13]. The micelles are made of a block copolymer containing substituted ferrocenylsilane and siloxane units. Chemistry professors Ian Manners and Mitchell A. Winnik and graduate student Jason A. Massey at the University of Toronto found that dissolving the copolymer in n-hexane induces self-assembly into 20-nm-diameter cylindrical micelles, in which polyferrocenylsilane segments form an iron-rich core surrounded by an insulating polysiloxane sheath.
Manners and Winnik then collaborated with chemist Martin Möller, physicist Joachim P. Spatz, and coworkers at the University of Ulm, in Germany, to convert these micelle nanostructures into magnetic ceramic nanopatterns. Manners hopes these ceramic lines will display magnetic, conductive, or semiconductive properties or prove capable of serving as etching resists for producing two-dimensional quantum wires in semiconducting substrates.
Postdoctoral researcher Anat Hatzor and associate chemistry professor Paul S. Weiss at Pennsylvania State University came up with a chemical approach for growing "wires" measuring just a few nanometers in width and positioning them with roughly 1-nm accuracy [Science, 291, 1019 (2001); C&EN, Feb. 12, page 10]. The approach could make it possible to connect functional molecules between electrodes for molecular electronics use.
Weiss also helped direct a study in which single molecules were used as switches. Weiss, chemistry and materials science and engineering professor David L. Allara at Penn State, chemistry professor James M. Tour at Rice University, Houston, and coworkers demonstrated that individual molecules can function as the active elements in electronic switches and that the molecules can remain switched on or off for hours at a time [Science, 292, 2303 (2001); C&EN, June 25, page 11].
The switches--phenylene ethynylene oligomers embedded in highly ordered dodecanethiolate monolayers--toggle slowly between a strongly conducting "on" state and a nonconducting "off" state. And a nitro-derivatized phenylene ethynylene could be switched from on to off by applying an electric field. Potential applications include memory and logic devices.
Tour and applied physics and electrical engineering professor Mark A. Reed at Yale University also showed this year that bundles of several thousand such molecules can be fashioned into functioning random-access-memory devices [Appl. Phys. Lett., 78, 3735 (2001)]. The tiny circuits switch controllably between conductivity states in a way that allows data to be written, read, and erased.
The tiny nanotubes produced in recent years from carbon and other materials have attracted widespread interest among nanotechnology and molecular electronics researchers. Metallic nanotube "contaminants" formed in nanotube synthesis have made it difficult to use nanotubes as semiconductors. However, a group led by Phaedon Avouris, manager of nanometer-scale science and technology at IBM's T. J. Watson Research Center, Yorktown Heights, N.Y., found this year that mixed single-wall nanotubes could be picked clean of metallic tubes in a simple procedure, leaving an intact bundle of semiconducting tubes [Science, 292, 706 (2001); C&EN, April 30, page 13]. Using the technique, the team fashioned from single-wall nanotube ropes an array of field-effect transistors--the first array of transistors made from nanotubes.
Avouris and coworkers also used carbon nanotube bundles to construct the first single-molecule logic gates [Nano Letters, 1, 453 (2001); C&EN, Sept. 3, page 9]. The work shows, he said, that "carbon nanotubes are now the top candidate to replace silicon when current chip features just can't be made any smaller."
When another group placed an aqueous suspension of metallic nanoparticles in an alternating current field between two planar electrodes, the particles assembled into conducting microwires that grew from one electrode to the other [Science, 294, 1082 (2001); C&EN, Nov. 5, page 35]. The wires aren't the nanowire type--their diameters are micrometers across--but the method represents an easy way to create electrical connections in watery environments. The microwires were created by chemical engineers Eric W. Kaler of the University of Delaware and Orlin D. Velev of North Carolina State University and their coworkers, who believe the wires could be used in wet electronic and bioelectronic circuits, including chemical sensors.
Another novel molecular electronic component devised this year is a nanoscale transistor that can be switched between "on" and "off" states with a single electron and works efficiently at room temperature. Professor Cees Dekker and coworkers in the department of applied physics at Delft University of Technology, the Netherlands, formed the room-temperature single-electron transistor (SET) within a single carbon nanotube [Science, 293, 76 (2001); C&EN, July 9, page 10].
And a team of scientists led by Dekker built a multiple-transistor logic circuit using carbon nanotube field-effect transistors (FETs) [Science, 294, 1317 (2001); C&EN, Nov. 12, page 7]. The tiny FETs are individually controllable and can be integrated on a single chip. Prior to this study, nanotube FETs couldn't be turned on and off individually.
An industry group also developed what it believes is the first truly molecular-scale transistor. The device's channel was just one molecule long. Physicist Jan Hendrik Schön and coworkers at Bell Labs created the transistor from monolayers of conjugated organic molecules on a doped silicon substrate [Nature, 413, 713 (2001); C&EN, Oct. 22, page 14]. From two of these transistors, they were able to build a logic gate.
Also this year, Schön and colleagues discovered a plastic superconductor [Nature, 410, 189 (2001); C&EN, March 12, page 14]. They made the discovery when they observed superconductivity in a solution-deposited thin film of poly(3-hexylthiophene) cooled below 2.5 K. "To the best of our knowledge," Schön told C&EN, it's "the first demonstration of superconductivity in an organic polymer film." Potential applications include superconducting electronic and optoelectronic devices.
Chemistry professor Charles M. Lieber and coworkers at Harvard University developed at least three types of nanowire-based electrical components this year. First, they reported what is probably the smallest light-emitting diode (LED) ever made. They created it by crossing two doped semiconductor nanowires and applying a voltage to one of them. Light was emitted where the two nanowires cross [Nature, 409, 66 (2001); C&EN, Jan. 8, page 7]. The current-to-light conversion efficiency was relatively low, but Lieber said he believed it could be improved. He envisions future nano-LEDs that cover the entire visible and near-infrared range, and grids of crossed nanowires that could function as high-resolution LED arrays--for display, holographic, and optical processing applications.
In a third study, Lieber and coworkers used semiconductor nanowires to build FETs, and they performed basic digital computations using logic devices built from nanowire-based FET logic gates [Science, 294, 1313 (2001); C&EN, Nov. 12, page 7]. The team used metal catalysts to prepare p-type (positive-charge-carrying) silicon nanowires and n-type gallium nitride nanowires. They then arranged the wires into circuits from solution by flowing suspensions of nanowires onto a patterned surface.
Meanwhile, another group found that the semiconductor pentacene, a leading candidate for organic electronics applications, can be prepared as thin films. They grew single-crystal grains of pentacene as large as 0.1 mm on a side--nearly 100 times larger than reported previously [Nature, 412, 517 (2001); C&EN, Aug. 6, page 11]. The work was carried out by Frank J. Meyer zu Heringdorf, Mark C. Reuter, and Rudolf M. Tromp at IBM's T. J. Watson Research Center. A researcher in the field commented that the study could make possible the use of vapor deposition to fabricate integrated circuits from single-crystal pentacene circuit elements.
Organic materials can also be used to create photovoltaic devices. In fact, some organic solar cells are beginning to approach the efficiencies of conventional inorganic ones. Visiting scientist J. Devin MacKenzie at the University of Cambridge's Cavendish Laboratory and coworkers found that a crystalline perylene dye and a hexabenzocoronene liquid crystal self-assemble into a highly efficient photovoltaic thin film [Science, 293, 1119 (2001); C&EN, Aug. 13, page 9]. The films are much easier to produce than earlier organic solar materials, which required multiple fabrication steps.
Scientists have also begun to explore attaching organic molecules to single-wall carbon nanotubes as manipulatable handles that can be used to help organize them into arrays. Preliminary success with this strategy was reported independently by two research teams (C&EN, May 7, page 15). One, led by assistant professor of chemistry Hongjie Dai of Stanford University, found a simple technique for noncovalently anchoring aromatic molecules to the sides of single-wall nanotubes [J. Am. Chem. Soc., 123, 3838 (2001)]. The anchored molecules have "tails" to which proteins or a variety of other molecules can be covalently attached. They used this approach to immobilize proteins, DNA, and smaller biomolecules on nanotube sidewalls. Potential applications include miniaturized sensors and nanotube-based electronic components.
Independently, a team led by chemistry professors J. Fraser Stoddart and James R. Heath of the University of California, Los Angeles, produced bundles of single-wall nanotubes that appear to have a conjugated polymer wrapped helically around them [Angew. Chem. Int. Ed., 40, 1721 (2001)]. By sonicating these complexes, the UCLA chemists were able to produce stable nanotube suspensions. Researchers have been struggling to find ways to make nanotubes soluble--or at least suspendable in solution--to make it easier to incorporate them into electrical components.
Chemistry professors Eiichi Nakamura at the University of Tokyo and Benjamin Chu at the State University of New York, Stony Brook, and coworkers reported that the potassium salt of pentaphenylfullerene can assemble into vesicles--hollow bilayer shells, each composed of about 13,000 modified C60 derivatives [Science, 291, 1944 (2001); C&EN, March 12, page 12]. The work could lead to new bilayer systems (such as membranes) with interesting and well-defined properties.
And at the Max Planck Institute of Solid-State Research, physicist Oliver G. Schmidt and colleagues devised a method that creates nanotubes and other nano-objects out of a wide range of materials [Nature, 410, 168 (2001) and Adv. Mat., 13, 756 (2001)]. In this technique, thin films can roll and fold up into nanotubes when a thin epitaxial layer is released from its substrate. The nanotubes' wall thickness and positioning on surfaces is controllable. Such accurately positioned tubes could be used as nanopipelines for fluid transport, the researchers suggest.
In yet another study that could lead to tailored nanostructures, professor of materials science and engineering Laurence D. Marks and graduate student Erman Bengu at Northwestern University developed a technique to prepare boron nitride (BN) analogs of carbon nanotubes [Phys. Rev. Lett., 86, 2385 (2001); C&EN, March 19, page 9]. The nanotubes are made by depositing boron and nitrogen ions on a hot, electrically biased tungsten surface. Marks points out that BN is "far more resistant to oxidation than carbon and therefore suited for high-temperature applications in which carbon nanostructures would burn." And unlike carbon tubes, BN nanotubes are expected to be semiconducting.
Nanotubes typically have been prepared as entangled and poorly ordered mats in which the nanotubes vary in diameter and structure (chirality) and are difficult to separate one from another. But professor of chemistry James K. Gimzewski at UCLA and coworkers at IBM Zurich Research Laboratory, Rüschlikon, Switzerland, in collaboration with engineering professor Mark E. Welland's group at the University of Cambridge, prepared perfectly ordered arrays of single-wall carbon nanotubes with identical diameter and chirality [Science, 292, 1136 (2001); C&EN, April 16, page 6]. The synthesis involved alternately evaporating C60 and nickel through holes in a membrane under vacuum and then heating the resulting deposits in a magnetic field. Potential applications include composite materials, hydrogen storage, nanoelectronic components, and gas sensors.
Another group grew the narrowest molybdenum disulfide nanotubes ever reported. Earlier reports had described MoS2 nanotubes with diameters as small as 15 nm. But tubes created by Maja Remskar and Ales Mrzel at Jozef Stefan Institute in Ljubljana, Slovenia, and coworkers were all just under 1 nm across [Science, 292, 479 (2001); C&EN, April 23, page 13]. The single-wall nanotubes were grown from MoS2 powder, with a small amount of C60 added as a growth promoter. Potential applications include gas mixture separations.
In the materials area, ferromagnetic behavior at and above room temperature in a form of polymeric C60 was detected serendipitously by semiconductor physicist Tatiana L. Makarova of Ioffe Physico-Technical Institute, St. Petersburg, Russia, and coworkers at five other labs [Nature, 413, 716 (2001); C&EN, Oct. 22, page 10]. Only a handful of metal-free magnets had been discovered before, and those materials exhibited magnetic behavior only at very low temperatures. Potential applications include advanced electrical insulators.
And very recently, chemistry professors Chad A. Mirkin and George C. Schatz and coworkers at Northwestern University reported a new method for using fluorescent light to convert silver nanospheres into triangular nanoprisms [Science, 294, 1901 (2001); C&EN, Dec. 3, page 10]. The nanoprisms have interesting light absorption, light scattering, and other optical properties, suggesting potential applications as diagnostic biological labels and in light-emitting diodes. In addition, engineering professor Gehan Amaratunga, research associate Manish Chhowalla, and coworkers at the University of Cambridge developed an arc discharge method for generating carbon-based multilayer nano-onions that could be useful as lubricants [Nature, 414, 506 (2001); C&EN, Dec. 3, page 35].
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