Small Wonders

Now, it is becoming possible to build structures so small that they operate independently of the world ruled by classical physics, devices so tiny that they directly link to the invisible quantum universe that lurks inside everything. And while there are no nanomachines yet in existence, there are nanostructures at sizes ranging from less than a billionth of a meter up to 10, or maybe 100, nanometers. (In this sense, a machine is defined as a device with a definite function, like an engine or microchip; structures are more passive objects and tend to be pieces of potential machines.)

Scientists and industrial corporations are betting that these tiny apparatuses will become the foundations of a new technological order. Public and private money is being heavily pumped into the research of nanoscientists. The intended effect is to revolutionize the classical manufacturing methods currently used by the consumer electronics industry. A technology based on quantum mechanics may not be just around the corner, but is a holy grail for futurist crusaders and down-to-earth experimentalists alike.

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Stanford University assistant professor Thomas W. Kenny is a trained physicist who works as a mechanical engineer. Kenny is comfortable with both classical physics and quantum mechanics. He makes micromachines, also called Micro Electrical Mechanical Systems, or MEMS.

Kenny says that defining the size of what qualifies as nanotechnology depends upon one's point of view. Now that the National Science Foundation, a federal agency, is gearing up to coordinate the spending of hundreds of millions of dollars a year on nanotech research, scientists everywhere have started measuring their experiments in nanometers, hoping to tap the funding flood. Relatively large devices may have teeny components. Conventionally sized silicon transistors, for instance, can be made of layers of chemicals a few nanometers thick. That does not qualify them as nanomachines, of course. Most of the people interviewed for this story agreed that "nanotechnology" best applies to structures with dimensions of less than 100 nanometers. IBM's Don Eigler, the first person to pick up and move an individual atom, suggests the outer limit is less than 10 nanometers.

Kenny says that his micromachines have pieces that are smaller than a micron, which is 1,000 nanometers. Although Kenny avoids describing his work as nanotechnology, it certainly operates at the nanotech frontier. His tiny devices are closer to looking like familiar machines than most nanostructures. Classical physics and engineering work well for designing Kenny's micromachines, but at a certain point quantum mechanics rears its many heads.

Like most experimentalists, Kenny works with a group of graduate students and postdoctoral researchers, and he and his collaborators have found a niche for themselves in academia (and private industry, too). They are measurers of the ineffable. They make machines that quantify infinitesimal physical forces or distances, ranging from wavelengths of light to intercellular tensions in artificial human skins to the incredibly weak magnetic interactions between atoms. In partnership with corporations like IBM, Kenny's team develops what he calls "tool kits" of ultra-ultra-fine sensors and measuring instruments that have astounding applications.

One of these micromachines, called a silicon cantilever, is shaped like a thin diving board about 60 nanometers thick and 25,000 nanometers long. There are multiple uses for this device. Used as the tip of an instrument called an atomic force microscope, for instance, a cantilever functions like the needle of an old-fashioned record player that is translating bumps in grooved wax into electronic frequencies, and then sound. In an atomic force microscope, though, the cantilever bounces over atoms. A computer uses lasers to measure the degree of bounce, translating the bounces into pictures of atoms.

Or the cantilever can be used as a writing instrument. By running a weak electrical current through the cantilever, its narrow tip can "write" on a flat surface, melting nano-sized pits into a soft surface. The pits correspond to zeros and ones -- the "bits" in computer language -- and could one day perform as a "thermomechanical" data storage system for new generations of smaller, faster, more powerful computers.

Cantilever machines come in many sizes and shapes and have many applications. Last year, for instance, one of Kenny's students, Benjamin W. Chui, invented a cantilever that measures forces of pushing and pulling at the same time, or "microfriction." Such a machine is useful in medical research. It can measure, for example, how much force is being exerted by human skin cells as they grow, thereby helping in the design of artificial skin.

Heat and friction are the main obstacles to building ever smaller micromachines. Below a certain size threshold, mechanisms such as ball bearings, gears, and other mechanical architectures drawn from the macroworld cannot be lubricated. The movement of micromachines is, therefore, done by materials designed to flex up and down, as opposed to rotating or sliding. There is a limit, however, to how far down the nanoscale familiar mechanical shapes and classical electronics can function. Somewhere around nanometer-size, quantum mechanical effects appear, and everything changes.

And at the place where quantum order asserts itself, Tom Kenny's micromachines give way to nanostructures.

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Hongjie Dai grows self-assembling nanotubes from the bottom up. That's one reason why the China-born physical chemist was recently awarded a $625,000 research fellowship by the David and Lucile Packard Foundation. Paul McEuen says that Dai, an assistant professor of chemistry at Stanford, is one of the world's three top people in nanotech.

Dai, age 34, is certainly a new breed of scientist. His research group works simultaneously in chemistry, physics, engineering, and biology. Yet in some ways Dai is a farmer. His fields are laboratories full of vacuum pumps and super-hot ovens. He grows crops of carbon nanotubes. He fertilizes his crops with methane and other hydrocarbons.

It all started with the buckyball, invented in 1985 by a Nobel Prize-winning team led by Richard E. Smalley of Rice University. Smalley's buckyballs -- short for buckminsterfullerenes, a new element Smalley discovered in his lab -- are incredibly strong molecules made of carbon atoms. Hongjie Dai, and other nanotubeologists, learned how to transform the balls into elongated tubes. At first, the long, thin tubes of strongly bonded carbon atoms grew, noodlelike, in a carbon soup, all hopelessly entangled with each other.

Dai improved on this manufacturing method by learning how to grow the carbon nanotubes symmetrically. He heats up his methane feedstock, dashes in a bit of iron oxide catalyst, and sits back for an hour. Soon arrays of tubes sprout up in compact, orderly bundles, looking for all the world like cities of little world trade centers. This achievement is something on the order of growing millions of soda straws straight upward into outer space.

Courtesy of Hongjie Dai Multiple magnifications of nanotubes. (Lengths are given in microns) click to view larger image

Dai's tubes are, in a sense, the first self-assembling nanomaterial. In the futurist world of K. Eric Drexler, self-assembly means that armies of tiny robots build greater armies of tinier robots, ad infinitum. In the real world, Dai's self-assembly makes use of the same physical processes of attraction and repulsion that make the rainwater on a car windshield bead up in an orderly fashion.

The atom-thin nanostructures that Dai grows have several revolutionary applications, depending on which way the carbon atoms link to each other. In one form, the nanotubes are a metal. In another form, the tubes are a semiconductor. Either way, says Dai, the tubes are 100 times stronger than steel. Used in composite materials, they may one day be capable of making everything from tennis rackets to automobiles and airplane frames.

Hongjie Dai's semiconducting nanotubes can also function as transistors, which means a single tube can be used as a switch to turn flows of electricity on or off. Or, in a quantum sense, the tubes can function as controllable gates through which discrete packages of energy enter and exit. This important function of on-off control lies at the heart of electronics, classical and quantum.

In another atomic pattern, the crystalline tubes become metallic wires -- possibly "ballistic" wires, through which electricity travels almost without losing energy. These extraordinary wires could enable the production of atom-sized transistors and electronic circuits powered by single electrons. These wires are so fine that they can be connected to atom-sized electrodes in electronic circuits measured in angstroms. (There are 10 angstroms in a nanometer.) This interconnectivity means that it may one day be possible to construct the most mind-boggling machine yet imagined by the human brain: the quantum computer.

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Michael F. Crommie moves individual atoms around like some people move poker chips: He slides them one by one into piles. Only he does it with a scanning tunneling microscope. That's why the Physics Department at the University of California at Berkeley recently hired Crommie -- and the rather incredible microscope he put together piece by piece -- away from Boston University.

Crommie, age 37, was born in Southern California, where his father, an aerospace engineer, designed heat shields for the Apollo moon program. Crommie says he "grew up wanting to build spaceships, like Dad." Instead, the younger Crommie ended up going about as far inside space as one can get. Using his scanning tunneling microscope, Crommie finds lone atoms, and then pushes them into geometric structures called quantum corrals.

In Crommie's wild and woolly frontier world, quantum mechanics calls the shots. His microscope doesn't magnify -- it "tunnels." What does that mean? It means that electrons sitting at the tip of the microscope's thin probe do the impossible: They shoot through barriers that the rules of classical physics absolutely forbid an electron to pass.

Imagine a golf ball rolling down a slight slope until it hits a brick wall. Classical physics says that the ball does not have enough energy to pass through the brick wall. But quantum mechanics says that there is an extremely small probability that the golf ball will jump through the wall and continue to roll on the other side. (Although possible, this event is so improbable that it would take several ages of our universe for it to occur.)

But if the golf ball were an electron riding a conductive wire, and the brick wall a piece of atom-scale insulation, the seemingly impossible would become probable. Quantum mechanics says there is a definite probability that the electron will jump, or "tunnel," through the insulation-barrier and appear on the other side to continue its journey. The reason that this apparent magic can happen: The barrier is only a few atoms thick, and the mathematics of quantum mechanics says that at the scale of a few atoms, electrons will jump through the insulation a quantifiable percentage of the time. Above a certain thickness, the probability of tunneling falls off dramatically.

And this is why quantum effects can play havoc on electronics at the small scales: If electrons jump willy-nilly through insulating barriers in electronic circuits, the circuits short out. Learning how to control the flight of electrons is one of the principal focuses of nanoscience. Crommie wants his electrons to tunnel only upon command.

At the tip of Crommie's scanning tunneling microscope electrons jump off through space, to atoms resting on a surface. This creates a measurable electrical current. Slight fluctuations in the current are transmitted to Crommie's computer, which turns electrical variations into pictures of atoms.

These atoms do not look anything at all like the classical models of atoms we learned to draw in grade school (that is, tiny solar systems with electrons whizzing in orbits about a nucleus). Crommie's atoms-on-a-surface resemble ball bearings nestled in corrugated egg cartons. What the microscope sees is the electron cloud that surrounds the nucleus of the atom and interacts with other atoms. Pictures of atoms can be used to study their essential properties: how they sit and move, and how they repel and bind to one another.

Courtesy of IBM Research Division Atomic corral with probability waves

The scanning tunneling microscope has another trick, too. The tip of its probe can stick to individual atoms lying on a surface and move them into, say, circles. These are the quantum corrals. The microscope can then take pictures of how electrons confined inside a corral of atoms behave. (More precisely, the pictures are graphic representations of the probability waves created by the ephemeral electrons. The probability waves, which look like ripples in a pond, reflect measurements made by the microscope, and are graphical presentations of the probability of electrons being present in a certain space. Where there are crests in the waves, there are probably more electrons.)

The ability to move individual atoms is key to building nanomachines, not just nanostructures. The first atom-sized machines are likely to be switches, inside which atomic structures function as conductors and insulators, like today's microchips, but many times smaller and more efficient.

Crommie says his group's work is "pretty far ahead of today's industrial applications." But he expects the not-too-distant future to feature devices in which individual atoms function like toggles in a household light switch. The problem with Crommie's nanotechnology right now is that it all takes place at temperatures a few degrees above absolute zero, where the nearly perpetual movement of atoms is stilled. Whereas Tom Kenny's cantilevers, and Hongjie Dai's nanotubes, work at room temperature, Crommie's even smaller structures jitter themselves into smithereens at normal temperatures. Like many of his colleagues, though, Crommie looks to the living body for inspiration. DNA, proteins, and cells of all sorts already function as self-assembling nanoscale machines in animals and plants, and they function at normal temperatures.

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Charles Marcus, nanotechnologist and professor of physics at Stanford, shows off an artificial atom -- a quantum dot. Peering through the lenses of an optical microscope, it is possible to see little gold wires trailing off into nothingness. "Somewhere down there," muses Marcus, "is our little device."

Marcus is a nanotech enthusiast; as such, he believes that scientists should be dreamers. But it is important not to confuse scientific dreaming with the real thing, he opines. Like all nanoscientists, Marcus is aware that the media's perception of nanotech is largely shaped by the Foresight Institute. Marcus says he has nothing against the Foresight Institute's predictions. But ...

"Eric Drexler's book contains some useful engineering formulas. It's just not useful to my research. And I think it's fair to say that the future of nanostuff will be even wilder than Drexler has imagined," Marcus remarks.

Marcus' quantum dots usually live in the bottom of super-cooled refrigerators where electricity and magnetic fields are applied experimentally to test the dots' properties. A quantum dot can be as big as 500 nanometers, but its "walls" are only a few atoms thick. One of the most amazing things about a quantum dot is its ability to "element-shift." By changing the voltage of the electricity flowing through the dot, the artificial atom can mimic any one of the more than 100 elements appearing in the periodic table, such as hydrogen, magnesium, carbon, or potassium. It can also make elements that never yet existed by simply adding extra electrons to the mix.

The quantum dot's chameleon quality occurs because it "traps" electrons inside its structure. Depending on how many electrons it traps, it roughly assumes the characteristics of an element. It is easy to speculate about the future use of artificial atoms as manufacturing materials, once they are released from their super-low temperature cages. But using quantum dots as switches and components in electrical circuits could also be the basis of a new kind of quantum computing, says Marcus. Such quantum machines, also known as nanocomputers, would make today's most powerful computers look like prehistoric counting sticks.

If the basic paradox of chaos and order can be overcome.

Quantum mechanics' uncertainty principle says that before an atomic particle is measured, it exists in all possible states, all superimposed on one another. An electron, for example, is best described by what physicists term a "probability wave function," a mathematical expression that describes the chance that the electron is traveling at a range of speeds over a range of places. Once you measure its speed or position, quantum reality decoheres, the indefinite wave function "collapses," and either its speed or its position becomes definite. But not both at once.

Marcus says, "Once a measurement has been made, then all of the possible ways that things could have come out vanish, leaving only the way in which things did come out."

Re-enter the quantum dot, which connects the classical and quantum worlds. Inside the dot, electrons can be trapped and controlled for certain amounts of time. The theory of quantum computing shows that if information is stored in the dot's trapped electrons, before the electrons are measured all of the superimposed possibilities form an ultra- complex database.

If quantum computing comes about, less space will hold more information.

Think of it this way: Today's transistors, or microswitches, can be controllably switched to either state 0 or state 1 -- the either/or phenomenon that makes electronic computing possible. The 0s and 1s are coded bits of classical information. Computing capacity depends on how many switches can be built and interconnected.

In quantum computers, quantum bits, or "qubits," can be in both state 0 and state 1 at the same time, superimposed on one another. Theoretically, the ability to create databases of qubits and connect to them will shrink computers and increase their powers of calculation astronomically. But in the present, qubits have not been realized, because it is impossible to access the data without causing decoherence to set in, which destroys the information. And what is the use of storing information in the quantum universe, when attempts to access it import chaos and destroy the data?

Despite this almost ontological problem, Charles Marcus and many of his nanotechnologist colleagues believe that further experimentation with quantum dots could well lead to the development of a quantum computer. In the meantime, Marcus is also working on fabricating quantum wires to connect the quantum universe to the classical world. In that pursuit, he's in a collective that includes Hongjie Dai, Mike Crommie, Paul McEuen, and thousands of other experimentalists.

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Paul McEuen shows a visitor his lab. "My mom was disappointed. She thought it would be full of beakers and Dr. Frankenstein stuff," he grins. It looks like a weekend hobbyist's basement full of water heaters, gaffer's tape, and abandoned screwdrivers. But the water heaters are $250,000 refrigerators full of liquid helium and quantum dots.

It costs a lot of money to do nanotech, which is why university labs are umbilically tied to the U.S. government. The National Science Foundation is heading up a task force of scientist-bureaucrats from NASA, the Department of Defense, the Department of Energy, and several other federal agencies; this group is trying to control developments in nanotechnology, and, to this end, the U.S. government is planning to spend hundreds of millions of dollars on basic nanoscience research over the next two years. It is likely that nanotech manufacturing will become profitable once it passes the research stage. That's why the labs of multinational cybercorporations like IBM and Raychem are also heavily invested in nanotech research.

Those involved in nanotechnology regularly express a degree of social consciousness often missing in experimentalists. McEuen does not necessarily believe that nanotechnology will solve humanity's problems; he does hope that as biology, chemistry, and physics continue to intersect in the pursuit of nanosolutions, human beings will connect more deeply with their environment. "If everybody lives the way we live here," he says, "the planet is doomed. We'll run out of raw materials and kill everything."

But the technology of the infinitesimal is amoral. It is a tool that spans two viewpoints of reality -- classical physics and quantum mechanics -- with wonderful power. The results of nanoinvention -- which will likely include powerful weapons applications, as well as, one can hope, more benign and useful devices -- will change how the world operates its machines. But it cannot change how people operate in the world.

In 1995, the Rand Corp., a government-linked think tank located in Santa Monica, published a study on the potential of nanotechnology. The Rand paper relied heavily on the writings of K. Eric Drexler and the Foresight Institute.

The Rand Corp.'s authors concluded that nanotechnology would best be used to "take advantage of indigenous resources found on asteroids, comets, or planets for mining; defending Earth against impacts; or tools to assist extensive colonization of the solar system on a reasonable time scale." There was no mention of ending world hunger.