Will the Real Nanotech Please Stand Up?
Researchers are building devices one molecule at a time. TR sorts the possible form the preposterous.
The grand ballroom of the boston Marriott had been packed with a standingroom-only crowd of several thousand materials scientists eager to hear Richard Smalley’s evening plenary talk on “new devices and materials from carbon.” Afterward, in a nearly empty meeting room at the hotel, the Rice University chemist looks tired and spent as he fields questions. Then suddenly he’s revitalized; he leans forward and focuses intently. The conversation has swung to one of his favorite subjects: how nanotechnology will help save the world.
There are roughly 6 billion people on Earth, Smalley points out on this November night, and research aimed at producing better, cheaper, more efficient materials will be one key to feeding and housing that population as it soars toward an eventual steady state of 10 billion or more. But the limits to how strong, conductive and intricate a material can be “are set at the nanometer scale,” he says. “The dream,” says Smalley, “is to build with that level of finesse, to make it perfect down to the last atom.” This capability, he contends, would bring smaller, more efficient batteries, stronger materials, and vastly improved and cheaper electronics.
These are no ravings from the latest trendy “futurist.” Smalley is one of the country’s most respected chemists, a 1996 Nobel laureate in chemistry, and director of a new $33 million Nanoscale Science and Technology Center at Rice. Nor is he alone. A growing number of researchers share Smalley’s conviction that controlling the structure of materials down to a few atoms or molecules will have an immense impact on everything from computing to medicine. The ability to manipulate matter an atom at a time has been the stuff of science fiction for years. But recent development of high-tech tools, especially probes sensitive enough to both image and move individual atoms and molecules, has begun to turn these fantasies into scientific reality.
During this past year, two groups of researchers have independently fabricated a transistor out of a single carbon molecule. Scientists have built prototype information storage devices with data bits as small as 50 nanometers across. Other researchers have recently made a molecule that rotates, acting as a nanowheel, as well as a rudimentary abacus with single molecules acting as the sliding beads.
These are, admittedly, laboratory novelties. And, in truth, no one really knows what will result from the emerging science. For one thing, while scientists can painstakingly make nanodevices one at a time in the lab, they still must find a rapid-and commercially feasible-way to make millions of them. They also lack reliable methods for integrating nanoscale components. But these first steps provide compelling evidence that it is possible to build working nanodevices-and they have begun to generate considerable hope (along with a fair amount of hype) that Smalley’s dream of building new materials with molecular precision will come true.
what has brought this dream within reach is researchers’ new-found ability to image and manipulate individual atoms. In the early 1980s, physicists at IBM Research in Zurich invented the scanning tunneling microscope (STM), which made it possible for the first time to capture direct images of matter at the atomic scale. This was the discovery that opened up the nanoworld. Relying on the STM and a closely related instrument called an atomic force microscope (AFM), scientists can now directly push atoms and molecules about and prod them into place.
There are two forms of atomic manipulation. One involves physical manipulation to slide atoms around on a metal surface to form 2-D structures. The other approach attempts to fabricate stable structures with atomic resolution by breaking and forming chemical bonds, using the strong electric fields generated by the STM apparatus itself.
These are still exotic laboratory investigations. But for those in corporate and university research labs, the development of these powerful new tools means that “you can go hog-wild in imaging and manipulating entirely new physical structures,” according to Donald Eigler, a physicist at IBM Almaden Research Center in San Jose, Calif. Eigler’s group is, for example, studying the magnetism of several atoms perched on a surface. While the work using STM could eventually lead to advances in computing and magnetic data storage, Eigler is not driven only by practical applications. “What gets me most excited,” he says, “is when I see an aspect of nature that has not been seen before. This is new turf.”
The boundaries of this new turf are still being drawn in a sometimes contentious debate. Most physical scientists report that nanospace is a mysterious place that operates according to its own rules. And even researchers like Smalley who believe the work will eventually pay off in significant benefits for society point out that they are just beginning to understand the physics of the very small and learn how to control behavior in this realm.
A few, however, maintain they have it all but figured out. For nearly two decades, K. Eric Drexler, chairman of the Palo Alto, Calif.-based Foresight Institute, a nonprofit group that aims to promote nanotech, has been describing in precise detail how nanomanufacturing will work-and change the world. Drexler envisions self-replicating nanorobots that mechanically push atoms and molecules together to build a wide array of essential materials. Huge numbers of these nanorobots working together would supply the world’s materials needs at almost no cost, essentially wiping out hunger and ending pollution from conventional factories.
It’s a utopian vision that few researchers doing experiments on the nanoscale have bought into. But, not surprisingly, it holds a vast appeal for many others. This notion of nanotechnology has taken on a life of its own. And for a broad audience of technology enthusiasts, as well as for some in the media, it has become the best-known version of the nanotech dream.
That, according to some scientists, is exactly the problem. Drexler’s ideas may have helped create early excitement for nanotech, but after years of hearing grandiose speculations of a brave new nanoworld, researchers say it’s time to let the science overtake the fantasies. “There has been no experimental verification for any of Drexler’s ideas,” says Mark Reed, a nanoelectronics researcher and head of Yale University’s electrical engineering department. “We’re now starting to do the real measurements and demonstrations at that scale to get a realistic view of what can be fabricated and how things work. It’s time for the real nanotech to stand up.”
Some argue that the advent of practical nanotech is already here. It is a modest start. Scientists are not yet building practical electronic devices out of single atoms or molecules-and there are definitely no nanorobots around. But Richard Siegel, a materials scientist at Rensselaer Polytechnic Institute who headed a National Science Foundation-sponsored report last year on nanotech, says controlled synthesis of materials on a nanometer scale has already begun. The report also concluded that a worldwide race to exploit nanomaterials and build nanodevices is well under way, led by numerous university research groups and large industrial labs such as IBM Research, Motorola and Japan’s NEC Fundamental Research.
For now, these materials are mostly made by traditional methods of chemical synthesis, but Siegel says the availability of tools for atomic imaging has begun to enable scientists to make selective nano-structures. Siegel points, for example, to the development of nanocrystalline materials used in the giant magnetoresistance (GMR) devices that have in the past few years dramatically accelerated the pace of improvement in information storage. GMR technology relies on multiple layers of thin films, some only a few atoms thick; the precise layering of these thin films at the molecular level is responsible for the high sensitivity of the device. Siegel argues that “the huge impact of nanotech will come in nanoelectronics.” The nanocrystals used in GMR, he suggests, are “only the tip of that iceberg.”
For those making micrometer-sized devices (now common in advanced electronics and optics), the collision with the nanoscale is rapidly approaching. The expanding field of MEMs (micro-electromechanical machines), which is developing tiny machines to act as everything from microphones to miniature rockets, is also bumping up against the nanoworld and routinely making working parts as small as a few hundred nanometers.
For purists, however, you need to think smaller-much smaller-before you enter the real nanoworld. For these chemists and physicists, it is below about 50 nanometers where the fun begins. In this new arena, forces such as gravity that govern the everyday world rapidly lose their familiar meanings. “Physical intuition fails miserably in the nanoworld. You have to throw away your preconceived notions,” says Reed. “You see all kinds of unusual effects.” For one thing, electrons can go places that, according to classical physics, they can’t be. In some cases, says Reed, “It’s like throwing a tennis ball at a garage door and having the ball pop out the other side.”
This is also where today’s silicon-based electronics begin to fail. On the nanoscale, conventional transistors leak electrons like sieves, and the “dopant” atoms inserted into silicon to control its properties behave like huge, awkward boulders. Yet if the nanoscale poses sharp obstacles to conventional electronic technologies, it also opens up remarkable new possibilities that may leave today’s electronics looking like the Model T.
If electronic devices could be reduced to the size of individual molecules, then the game would be entirely altered. Molecular electronics was proposed in the 1970s by Mark Ratner, who is now at Northwestern University, and Ari Aviram of IBM. For years it remained a tantalizing idea far beyond the abilities of experimentalists. But during the past couple of years, leading-edge researchers have begun making actual wires and components out of single molecules. And now they have begun to make crude devices that actually work.
At Yale, Reed and his coworkers have, for one, made a diode out of several individual organic molecules. The simple diode, which is several nanometers long, is far from being a practical device, says Reed. But, he adds, it’s a first, encouraging step to making transistors and logic devices at that scale.
one key to the advances in molecular electronics could be an exotic molecule called the carbon nanotube. This remarkable carbon structure-discovered by researchers at Japan’s NEC in 1991-is a close chemical cousin of the buckyball, a new form of carbon discovered by Smalley in 1985. But while the buckyball is a soccerball-shaped molecule of 60 carbon atoms, nanotubes are long pipes of a rolled-up sheet of graphite. They’re electrically conducting and have been made into wires only a few nanometers in diameter.
Nanotubes are, both literally and metaphorically, a tunnel between the nano and macroscopic worlds. These structures make possible a long fiber that is only a few atoms wide. On a practical level, says Smalley, batteries might use nanotubes both to shuttle electrons between atoms and to carry a charge centimeters away. “Their great virtue is that they are molecular,” says Smalley. Each nanotube, he says, is “an entity that has its own behavior and integrity.” That means you can push the individual carbon molecules around, like tiny nanologs.
Actually, a nanotube acts a bit more like cooked spaghetti, says Phaedon Avouris, manager of IBM Research’s nanometer-scale science and technology group in Yorktown Heights, N.Y. Each nanotube will stick to a surface and this adhesion is strong enough to maintain any shape you push it into. The adhesion also provides good electrical contact between the nanotube and metal electrodes.
Most recently, Avouris and his coworkers have maneuvered one of these “nanonoodles” to bridge a pair of electrodes and prodded the molecules into rings and letters. The IBM scientists have also made a functional field-effect transistor-a basic electronic device-at room temperature out of a single nanotube.
The successful development of molecular electronics would mean a single chip could hold billions of nanoscale transistors, making a computer orders of magnitude more powerful than today’s machines. It also could mean building tiny and cheap computers that house millions of nano-transistors; such salt-grain-sized computers could be easily and cheaply incorporated into scores of other products-even into “smart” materials.
Nanotechnology could also make possible information-storage devices with immense capacity. Investigators at IBM Research in Zurich, led by physicists Gerd Binnig and Peter Vettiger, are building a micromechanical prototype that uses tiny silicon tips to read and write data bits that are less than 50 nanometers wide. That would translate into hard disks with storage capacities of close to a trillion bytes (terabytes)-a couple of orders of magnitude larger than the hard drives on today’s top-of-the-line PCs. It could also mean small products, the size of a wristwatch, say, that have immense storage capacity.
In their experiments, Binnig and his co-workers use the AFM tip to read nanobits of information on a polymer surface. Using a single tip, however, would mean a process that’s far too slow to be practical. Binnig has therefore wired arrays of more than 1,000 AFM tips that act in parallel. The arrays can rapidly write information by punching tiny divots in the substrate and read the nanobits by detecting the depressions.
Meanwhile, Binnig’s colleagues at IBM Zurich have used the STM to turn out even smaller nanoobjects with clockwork precision. James Gimzewski, an IBM chemist, has built an exquisitely small abacus. Gimzewski used the STM tip as the “finger” to move the abacus beads, which are buckyballs with diameters of less than 1 nanometer.
Gimzewski’s latest invention is a wheel constructed from a propeller-shaped molecule that spins on a tiny, bearing-like structure. Gimzewski says that while the rotating molecule suggests possible future nanomachines, the research remains embryonic. At this point, he says, “if you can get anything to work in the nanoworld, you don’t worry about its practicality. We’re just starting. It’s like children playing with Legos.”
The Zurich work reflects a deeply entrenched-and strongly Swiss-belief in mechanics. Physicist Binnig says, “Mechanics have been overlooked because electronics is so successful. It’s considered old-fashioned.” His device for information storage, however, works more or less like a tiny phonograph needle.
As you explore the nanoworld, he says, mechanical devices become an attractive alternative to electronics.
Binnig says the mechanical approach can be extended well beyond data storage, and that “everything you can do electronically, you can do mechanically.” Electronics are particularly good at guiding energy along precise paths to a well-defined place. But, he says, nanomechanics has an advantage of working with very low power consumption. While a 3-D nanoelectronics device would melt immediately from its own heat, Binnig says, you “could imagine” a 3-D nanomechanical device that would run cool. What’s more, mechanical devices may prove easier than electronics to integrate with biological, optical and chemical systems.
Enter the Hype
it’s somewhere around here that the science starts getting mixed up with science fiction. If you can make a nanowheel, why not a nanogear? A self-powered nanoboat? Why not build a nanorobot to move around the atoms for you?
And while you’re at it, why not make nanorobots that can replicate themselves, making it possible to staff nanofactories capable of piecing together almost anything out of the basic building blocks of atoms? Welcome to molecular manufacturing, as preached by nanoevangelist Drexler. At the core of the Drexlerian vision is a gizmo called an “assembler.” This hypothesized robotic apparatus would work by mechanically positioning atoms into virtually any configuration. If the chemistry between the atoms doesn’t take, the assembler would apply a small mechanical force (Drexler and his followers call it mechanochemistry). Get billions of these assemblers to work in parallel to arrange all the atoms just right-well, then, you can build just about anything you can imagine.
There’s just one problem: Few chemists, physicists or materials scientists see any evidence that this will be possible. Many believers in the Drexlerian vision are computer scientists who delight in simulating how it all will work. They produce elegant molecular models of nanogears and pumps but offer no clear plan for how to actually build such things.
Proponents of molecular manufacturing aren’t deterred by the skepticism of their more mainstream colleagues-although they do concede that their vision will take decades to be realized. Theoretical calculations and computer modeling say it can be done, insists Ralph Merkle, a computer scientist at the Xerox Palo Alto Research Center and a director, with Drexler, of the Foresight Institute. In particular, Merkle defends the two key proposals that have drawn the most fire from other scientists: the suggestion of self-replicating assemblers, and positional control of atoms and molecules to do mechanochemistry.
In self-replication, a molecular computer would direct the construction of a nanorobotic arm to build another computer; this second computer then directs the construction of another tiny computer, and so on. Self-replication is a concept that has been kicking around in computer science for years, says Merkle, and logically it should work. The idea of positional control calls for the robotic arms to precisely place atoms and molecules in a way that they bond, forming whatever you want. As long as you don’t violate any physical laws, Merkle says, this mechanical approach to chemistry makes sense.
But Drexler’s critics point out that chemistry is a very complex process at the molecular level. To play the game of chemistry, says Smalley, means controlling atoms in three dimensions. At each reaction site, atoms feel the influence of a dozen or so neighboring atoms; to do mechanochemistry, you would need to control the motion of each one. For a nanorobot, that would be an inconceivably complicated juggling act. Other highly respected researchers simply dismiss Drexler’s ideas out of hand. Says IBM’s Eigler: “He has had no influence on what goes on in nanoscience. Based on what little I’ve seen, Drexler’s ideas are nanofanciful notions that are not very meaningful.”
in any case, before researchers worry about building nanofactories, they need to figure out a practical way to mass-produce any device on the nanoscale. Some hope to make various exotic forms of lithography (optical lithography is the standard technology used to etch patterns on silicon chips) work below 100 nanometers. But how small-and how fast-lithographic methods could ultimately become is anyone’s guess (see “Chips Go Nano,” p. 55). Likewise, pushing molecules around one at a time using an STM is an exceedingly slow-and difficult-way to make anything. What’s more, once you’re done, you still have only one very tiny object. Building a single computer chip one atom at a time using today’s STM technology would take, according to one estimate, 1,000 years.
One solution is to link up the STM or AFM tips in an array that works in parallel-a nanomechanical assembly line that might appeal to Henry Ford. This is the strategy IBM’s Binnig is taking in his information storage device. And while wiring these tiny arrays and turning them into a working device is a chore, the preliminary research at IBM Zurich and several other labs suggests it just might work.
But many believe the longer-term answer lies in a process called self-assembly. Unlike the Drexlerian construction plan that uses self-replicating nanorobots to move atoms around, self-assembly relies on chemistry to position the pieces of a nanoscale structure, taking advantage of certain molecules’ ability to arrange themselves in complex structures. In chemical terms, self-assembly works because molecules seek the thermodynamic minimum of the structure you want. Think of it as a prefab house that builds itself using chemistry.
But so far, chemists and materials scientists have learned to build only the simplest structures. The feat of assembling specific features in the materials and combining different materials remains a daunting challenge.
The solution to that problem could determine which nanodevices are practical-and how long it takes for them to hit the market. For most applications you would need to fabricate and integrate billions of nanoobjects. And to compete in such areas as information technology, you’ll have to do it very cheaply. That, say many scientists, will require the synthesis prowess of chemistry. “Don’t expect anyone to get to the point where you add ingredients in a beaker and out pops an integrated circuit,” says Yale’s Reed. However, the hope is that self-assembly could eventually place nanoelectronic devices “where you want them,” Reed says.
That will take time. But there are encouraging signs that this approach will work. Self-assembly is, in a sense, where chemistry and materials science-the arts of building actual stuff-meet the physics of the nanoscale. Physics has provided scientists with the means to manipulate nanoobjects and understand the workings of the nanoworld, and now researchers are looking to chemistry and materials science for the next advances that will help turn all this work into a practical technology.
No one really knows where those breakthroughs will come from-or even if they’ll come. But, as the science of the nanoworld grows, the shape of the real possibilities are beginning to emerge from the nanofog.