How To Reach C&ENACS Membership Number


September 30, 2002
Volume 80, Number 39
CENEAR 80 39 pp. 38-43
ISSN 0009-2347

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FURTHER ADVANCES have come from transistor design modifications. Earlier this year, for example, the IBM group reported that their so-called top-gated nanotube FETs outperform state-of-the-art silicon FETs in terms of switching rate and the amount of current they can carry per width of conductor. One key difference between the latest design and earlier designs is that the silicon wafers that support the FETs do not function as gates. Instead, the gate is fabricated above the nanotube--allowing all FETs in contact with the silicon wafer to be switched independently. In addition, the new FET design benefits from switching voltages that are an order of magnitude lower than those needed to switch older FETs [Appl. Phys. Lett., 80, 3817, (2002)].

Just recently, the IBM team developed a catalyst-free procedure for preparing single-walled carbon nanotubes. Conventional methods for preparing single-walled tubes, such as discharge and vapor deposition techniques, rely on particles of nickel and cobalt or other catalytic transition metals. A problem with those procedures is that the metal particles left behind in the products disturb the nanotubes' electrical properties, forcing scientists to take complex purification measures.

But now Avouris and coworkers have shown that single-walled nanotubes can be prepared from SiC in the absence of metal catalysts at temperatures above 1,500 °C. The method produces nanotubes that are 1.2 to 1.6 nm wide and are aligned along the face of the support. Other researchers have reported techniques for growing vertically aligned products, which they dub nanotube forests. But nanotubes that grow horizontally are amenable to standard vapor deposition methods and may be connected in parallel to lower their electrical resistance [Nano Lett., published online, Sept. 24,].

Despite the recent advances, Avouris asserts that nanotube FETs remain "far from optimized." Improvements could be made by using thinner insulators with higher dielectric constants, he suggests. "But what's really needed is a better understanding of the mechanism of electrical switching in nanotube FETs."

Carbon nanotubes aren't the only game in town. Nanowires made of semiconductors such as silicon, gallium arsenide, and indium phosphide are being investigated as candidate materials for nanoelectronics. The field is advancing rapidly as researchers are making fast progress in synthesis, device fabrication, and testing.

Skeptics say that the proposed applications are exaggerated and that reports are full of "hype."

Although Harvard University chemistry professor Charles M. Lieber has published widely on carbon nanotubes, semiconducting nanowires figure prominently into his research program. The tiny wires are versatile building blocks that can be used in a bottom-up approach to constructing nanoelectronic circuits, Lieber notes, because the size, structure, and functional properties of nanowires can be controlled readily via synthesis procedures.

In 1998, Lieber and coworkers described a vapor-liquid-solid synthesis method in which laser light is used to ablate nanometer-sized metal clusters that serve as nucleation centers and catalysts for nanowire growth. The Harvard researchers used the technique to prepare uniform single-crystalline nanowires of silicon and germanium with diameters as small as 3 nm and lengths up to 30 mm. Since that time, the procedure has been used to prepare nanowires with even smaller diameters, and it's been extended to a wide variety of materials. Examples include III-V semiconductors such as GaAs and II-VI semiconductors such as ZnSe.

Recently, a number of research groups boosted the complexity of materials that can be prepared by the cluster-nucleation method. The teams prepared modulated structures--nanowires composed of dissimilar segments. The two-tone materials are made by turning the supplies of reactants on and off during synthesis with pulsed lasers or by other methods. These "heterostructured" products open the door to new sophisticated applications, such as terahertz-frequency photon emitters.

Lieber, graduate student Mark S. Gudiksen, and coworkers used modulation methods to prepare nanowires with 21 alternating segments of GaAs and GaP. And they prepared Si and InP nanowires with modulated doping, such that the wires were endowed with alternating p- and n-type regions. Using similar methods, Peidong Yang, an assistant chemistry professor at the University of California, Berkeley, prepared Si-SiGe nanowires. And Lars Samuelson of Lund University, in Sweden, synthesized InAs-InP nanowires (C&EN, Feb. 11, page 7).

STOP METALING A catalyst-free synthesis procedure developed at IBM produces aligned single-walled carbon nanotubes (vertical lines) from SiC without residual metal impurities.

TO EVALUATE the usefulness of the tiny wires in nanoscale electronics, the Harvard researchers fashioned the wires into test circuits that were fabricated using lithography methods and then studied the circuits' electrical properties. According to Lieber, Si nanowire FETs, for example, perform remarkably well. The devices exhibit large on/off ratios (>105)--a requirement for effective switches--and charge-carrier mobilities that exceed those measured in conventional silicon devices.

In addition to constructing FETs from nanowires, Lieber's team has built and tested photodetectors, biological and chemical sensors, and more complex devices, including light-emitting diodes (LEDs) and logic gates. Most of the papers in which the results are reported were published in just the past 18 months.

In the chemical sensing study, Lieber, graduate students Yi Cui and Qingqiao Wei, and Harvard assistant chemistry professor Hongkun Park found that by functionalizing boron-doped Si nanowires, they could prepare efficient sensors. Modifying the wires with an amino-silane compound, for example, produced pH sensors that are linear over a large dynamic range. And they observed that biotin-modified wires can be used to detect picomolar levels of streptavidin. In addition, the Harvard chemists found that the wires could be used to detect proteins reversibly in real time [Science, 293, 1289 (2001)].

By crossing an n-type and a p-type InP nanowire and then attaching electrodes to the wires lithographically, Lieber and coworkers made a nanometer-sized junction--at the point of overlap--that behaved as a current rectifier and an LED [Nature, 409, 66 (2001)]. Based on the study, Lieber says that more complex devices can be constructed from semiconducting nanowires simply by crossing them and forming tic-tac-toe patterns. But taking advantage of the crossed-nanowire idea requires developing a method for preparing the patterns efficiently and reproducibly.

By combining surface patterning techniques and microfluidic alignment methods, the Harvard group developed the needed procedure. Their idea is to flow nanowires from a suspension into tiny parallel channels in a mold, in a layer-by-layer fashion, while rotating the mold orientation--and hence, nanowire flow direction--between layers [Science, 291, 630 (2001)].

With the crossed-nanowire method for making p-n junctions firmly in hand, Lieber and coworkers were well on their way toward making computational logic circuits. Near the end of 2001, the group published a study in Science showing that their procedures could be used to make arrays of nanowire FETs configured as AND, OR, and NOR logic gates. Lieber stresses that in crossed-nanowire structures, such as the Harvard logic gates, the width and length of the FET channels are controlled by synthesis and assembly--not lithography--unlike today's microelectronic components.


Only a few years ago, many scientists sat through molecular electronics presentations thinking that the far-fetched nanocomputing goals would never be reached.

TEAM EFFORT Molecular electronics research at UCLA brings together organic synthesis and device measurements through the efforts of (standing, from left) Jeppesen, Stoddart, Heath, and Yi Luo, and (kneeling, from left) Tseng, Kristen Beverly, and Paul Celestre.
THESE FABRICATION concepts make it possible "to envision a straightforward path to nanometer-scale electronics of the future," Lieber says. His vision, which is supported by experiments conducted by his group in the past several months, includes a nanoscale logic device known as a decoder that can address individual elements in crossed-wire arrays.

Using a handful of molecules--or even just one of them--to make nanoscale switches or other devices is an idea that's gathered momentum in recent times. In 1999, researchers working with chemistry professors James R. Heath and J. Fraser Stoddart of the University of California, Los Angeles, demonstrated that simple circuits--fuses--could be made from a layer of [2]rotaxane molecules. Then a team headed by Rice University chemistry professor James M. Tour and Yale University electrical engineer Mark A. Reed reported success in making reversible switches using benzene thiol molecules.

Other groups have also been investigating the possibilities of single-molecule systems. For example, a team at UC Berkeley including Park (now at Harvard), Paul L. McEuen (now at Cornell), and A. Paul Alivisatos, trapped a single C60 molecule in a tiny gap between a pair of electrodes to study electronic conduction mechanisms [Nature, 407, 57 (2000)]. And just recently, a single molecule of a transition metal-organic complex--containing either one Co atom or two V atoms--was turned into a single-molecule transistor in independent studies by Park and McEuen (C&EN, June 17, page 4).

Before molecular devices can be wired up and tested, organic chemists need to synthesize the molecules that drive them. In the UCLA collaboration, Stoddart's group prepares the compounds and Heath's group turns them into circuit components. To make the molecules switchable, Stoddart's group designs them so that one segment of a molecule can move (rotate or translate) relative to another segment.

Stoddart notes that, contrary to remarks made by many researchers, the molecules are not difficult to prepare nowadays. But it took some years to build up the necessary expertise in molecular recognition concepts, p-donor/p-acceptor interactions, and other subjects. He adds that his group has synthesized "hundreds, if not thousands" of catenane and rotaxane analogs since 1989.

In 2000, the UCLA team demonstrated a reusable switch based on a [2]catenane--a molecule with a pair of interlocked rings. The switching state of the redox-active molecule was shown to change reversibly, depending on an applied potential. Electrostatic repulsion drives rotation of one ring relative to the other between open and closed states.

MAGNITUDE A series of micrographs in which each image is magnified roughly 10 times more than the previous one (left to right) reveals details of a new HP Labs molecule-based 64-bit prototype memory and logic chip. Using a newly patented procedure, researchers patterned a silicon wafer with 625 memory devices (one shown at top right). At the center of each memory circuit, an 8 3 8 array of crossed metal nanowire electrodes (bottom row) sandwiches roughly 1,000 rotaxane molecules forming 64 bits of nonvolatile memory.

LOOKING TO IMPROVE device performance by tailoring the molecules, Jan O. Jeppesen, a visiting graduate student in Stoddart's lab, and postdoctoral associate Hsian-Rong Tseng prepared various rotaxane analogs of the catenane that brought success in 2000. The series of amphiphilic bistable [2]rotaxanes are dumbbell-shaped molecules capped with large stoppers that are threaded through a cyclophane ring. As with the [2]catenanes, the ring's position relative to the other piece of the molecule is controlled by redox interactions.

Heath and coworkers constructed circuits with the new molecules using a layer-by-layer deposition method to fabricate tic-tac-toe structures with a few thousand molecules situated at each intersection. The team used this fabrication method to prepare 64-bit random-access memory circuits [ChemPhysChem, 3, 519 (2002)]. And Hewlett-Packard Labs announced earlier this month that R. Stanley Williams, Yong Chen, and coworkers have also succeeded in using the UCLA [2]rotaxane molecules to fabricate a 64-bit memory circuit.

Only a few years ago, many scientists sat through molecular electronics presentations thinking that the far-fetched nanocomputing goals would never be reached. But the flurry of demonstration results in just the past two years appears to have decreased the number of nonbelievers. Yet some naysayers remain. Just recently, a California professor was furious about a C&EN story on molecular electronics and wrote to the editor, exclaiming, "In reality, there is no such topic!"

The professor has a few surprises in store. In reality, molecular and nanoelectronics are bustling fields. Large numbers of commercial products may not be available in the near future, but as the vibrant research activity reveals, they're coming.

MODULAR MOLECULES UCLA chemists use synthesis techniques to prepare multipart molecules with customized structures and properties for nanoelectronic devices. [2]Catenanes (left) and [2]rotaxanes (center and right) can be switched on and off depending upon redox state, which determines where in the molecule a cyclophane ring (blue) resides.
Links to videos of the following processes can be found on C&EN Online,

* Switching of a catenane

* Switching of a rotaxane in a device setting

* Construction of a crossbar device

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