Nanostructures raise solar cell efficiency

Nanoparticles scatter incoming light
The latest design maximises photon absorption
Researchers are working to develop new devices that could lead to big gains in thin-film solar cell efficiency by increasing both the number of photons thin-film solar cells absorb and the number of excited electrons the same devices collect.
Past approach
In the past, engineers have tried to add quantum wells to thin-film solar cell devices by stacking several quantum-well layers to achieve a high probability of absorption of low-energy photons.
This approach, however, can be counter productive because electron-hole pairs get stuck in the quantum wells, making it impossible for them to generate current for the device.
From the outside, the new optimized devices behave just like traditional thin-film solar cells. But inside, nanostructures enable the solar cells to circumvent an important trade-off that has stymied past attempts to incorporate quantum wells into thin-film solar cells in order to boost device efficiency.
Quantum wells can increase solar cell efficiency by raising photon absorption by lowering the energy band gap.
Thanks to nanostructures that scatter and channel light, University of California, San Diego, electrical engineers are working toward thin-film “single junction” solar cells with the potential for nearly 45 per cent sunlight-to-electricity conversion efficiencies.
“The most recent estimate of the maximum power conversion efficiency — under normal illumination conditions — that one can expect with our new thin-film solar cell approach is approximately 45 per cent.
This is a very large improvement over the 31 per cent maximum theoretical efficiency for today’s solar cells with classic p-n junctions,” said Edward Yu, the Principal Investigator.

The UC San Diego engineers are using nanoparticles to scatter incoming light into paths within the quantum well region — paths that run parallel to the p-n junction. This gives photons more time to be absorbed without having to stack the quantum wells to a thickness that makes it hard for electrons and holes to escape, according to a University of California, San Diego, press release.
Long path
In the UCSD approach, the photons are provided with a long path along the quantum wells and the carriers have a short path to the electrode. This design maximizes photon absorption while minimizing a major drain on device efficiency in solar cells — electron-hole recombination

Nanotech: The Tiny Science Is Big, and Getting Bigger

After decades of hype, speculation, and multimillion-dollar laboratory research, the long-promised nanotechnology revolution is finally coming to a store near you. For proof, check out the transparent sunscreens, spillproof pants, and tennis rackets with extra pop now on sale. Nanotechnology gets its name from the nanometer, a unit of measurement that is one billionth of a meter. A human hair is about 20,000 nanometers thick. Scientists say materials and devices manufactured at the nanoscale promise to change life as we know it.

"I'd say [nanotechnology] has the potential to be truly revolutionary," said Gregory Rorrer, a chemical engineer at Oregon State University in Corvallis. "That's why there's so much interest in it right now."

Rorrer is less than a year into a four-year, 1.3-million-dollar (U.S.) grant from the National Science Foundation to develop a process to produce nanostructured semiconductor materials using single-celled marine organisms called diatoms.

The grant is a small piece of the billions of dollars the United States government is funneling into research and development to spur the nanotechnology revolution.

Lawrence Gasman is the founder of NanoMarkets, a Sterling, Virginia-based nanotech market analysis firm. He said the coming revolution will be akin to the plastics revolution of the 1960s. At that time, plastics transformed everything from kitchen appliances and food containers to housing construction and medical safety.

"What it's really about is applying the latest and greatest in materials science to solving real-world problems," Gasman said. "That's where the money will be made, and that's where [nanotechnology] will change lives."

Nanotech Dream

Scientists are intrigued by materials and devices built at the nanoscale, because they bridge the realm of behavior between Newtonian physics (forces like gravity represented by apples falling out of trees, for example) and quantum mechanics (the laws of physics that apply at very small scales such as those found in atoms). Straddling both realms, nanomaterials have unique properties.

For example, the element germanium glows blue at the nanoscale when energy is applied to it. Rorrer, the OSU chemical engineer, said the property has a host of applications in electronic and medical-imaging technologies.

Medieval stained glass window makers are considered the first nanotechnologists. These craftsmen understood the nanoproperties of gold, which gives glass different colors depending on the concentration of gold nanoparticles in it.

Gasman, the nanotech-market analyst, said it wasn't until the several decades ago, that nanotechnology began to take off. The catalyst was the advent of high-powered microscopes and tools that could manipulate materials at the nanoscale. "A lot of these tools for manipulating things at the molecular level, seeing things at the molecular level, only evolved in the last 10 to 15 years," he said.

The original vision of nanoscale applications was put forth in 1977 by Eric Drexler, then an undergraduate at the Massachusetts Institute of Technology in Cambridge. Inspired by the emerging field of genetic engineering, Drexler envisioned tiny machines—assemblers—that could cheaply and quickly build any physical object, starting with raw materials at the molecular level.

Drexler's vision and the subsequent excitement about a molecular-construction boom led some to dub him the godfather of nanotechnology.

Some scientists say molecular assemblers are impossible. Skeptics include Richard Smalley, the director of the Carbon Nanotechnology Laboratory at Rice University in Houston, Texas. Smalley was the recipient of the 1996 Nobel Prize in chemistry for the discovery of closed, hollow cages of carbon atoms known as buckyballs.

In 1989 Drexler co-founded the Foresight Institute in Palo Alto, California, with Christine Peterson. The nanotech think tank defines nanotechnology as the coming ability to build products with atomic precision.

"The goal of nanotech is control of the structure of matter, right down to the individual atoms and molecules," explained Peterson, who serves as the institute's vice president and spokesperson. "This ability could affect the quality of virtually every physical structure, from products we manufacture to our internal organs after surgery."

Today nanoscale materials and devices are built using nanoparticles. A nanoparticle is "essentially a piece of matter with just a couple of hundred atoms associated with it," Rorrer, the OSU chemical engineer, explained. "It's one step above the molecular level."

Today and Tomorrow

Most nanotechnology products currently in the marketplace are primarily for the spring-break crowd. They include sunscreens, clothing, and sporting goods. But researchers say these applications are only a fraction of what's to come.

Gasman, the market analyst, said future materials and devices made at the nanoscale will allow for smaller, faster electronics; more efficient gasoline; cheap, flexible solar panels; and detailed, microscopic images of human cells.

Peterson, meanwhile, has her eyes on nanodevices with moving parts, so-called nanomachines. "The most ambitious goal for these will be nanoscale surgery in medicine, bringing nanolevel, three-dimensional control and drug-style chemical action together for the first time."

Miniaturization to the Max: Nanotech Pioneer Lauded

Machines, medicines, and materials a mere fraction the width of a human hair may one day store trillions of bits of information, detect the onset of cancer, and restore a paralyzed limb. But before the promise of nanotechnology is even modestly met, scientists must first learn how to make three-dimensional structures and tools at a scale much too small for even the deftest robots or nimblest human hands to manipulate.

George Whitesides, a chemist at Harvard University in Cambridge, Massachusetts, is on the job. On November 10 he will be awarded the 2003 Kyoto Prize for Lifetime Achievement in Advanced Technology for, among a myriad of accomplishments, laying the foundation for building nanostructures.

Nanotechnology gets its name from a unit of measurement known as the nanometer, which is one billionth of a meter. To put the dimension in perspective, a human hair is about 100,000 nanometers thick.

Whitesides is being recognized for pioneering advances in material sciences that increased understanding of how molecules can assemble themselves and how such assembly can be applied to building devices that are measured in nanometers.

"Materials are very important—they are the stuff out of which everything is made—but they are often invisible to the user. So they are a foundation for technology, not the visible part," said Whitesides.

The Kyoto Prize, one of three to be awarded in Japan for significant contributions to the scientific, cultural, and spiritual development of humankind, comes with a gold medallion and check worth about U.S. $400,000.

"[I'm] pleased, of course, what else?" said Whitesides about the honor. "But for the research group—this is an award that [goes] to a substantial number of people."

The nod is to about 50 graduate students, postdoctoral researchers, and visitors that work in his laboratory at Harvard, which is one of the largest and best funded nanotechnology-related labs in the U.S., according to Mihail Roco, the senior advisor of nanotechnology at the National Science Foundation in Arlington, Virginia, and leader of the National Nanotechnology Initiative.

Multidisciplinary Area

Roco oversees the more than U.S. $700 million the U.S. government now spends on nanotechnology research each year. He said part of Whitesides' success in obtaining a hefty slice of the pie is the ease with which he moves between the various disciplines that make up nanotechnology.

"He is certainly good in chemistry—his original interest—but he moves very easily to fluidic devices, systems engineering, electronics, and many other fields," said Roco. "This is part of his success and recognition."

Whitesides received his bachelor's degree in chemistry from Harvard in 1960 and his Ph.D. in 1964 from the California Institute of Technology. Prior to joining Harvard's chemistry department in 1982, he was a member of the faculty at the Massachusetts Institute of Technology. While a chemist by training, Whitesides said success in nanotechnology requires a solid grasp of all the core sciences. For example, he said that biology is a master at making nanomachines such as the light-harvesting apparatus of green plants, and thus it is important to understand biology so as to understand nature's designs.

"I would say that we need chemistry to make things, biology to teach lessons about what to make, materials science to use the materials, and physics to measure the properties," he said. "It's a multidisciplinary area."

Self Assembly

One of Whitesides' seminal works is a 1989 paper published in the Journal of the American Chemical Society in which he describes how to control the self-assembly of a single layer of molecules, called a monolayer.

"The reason it is important and recognized is that it is a foundation of more complex systems," said Roco. For example, in theory three dimensional structures could be built in layers like rows of bricks stacked on one another to build a house.

The self-assembled monolayer (SAM) process allows scientists to choose the chemical composition of the monolayer, thus choosing the properties of the surface it creates, and control how the molecules self-arrange.

"Self-assembled monolayers are highly ordered monolayers—the molecules are arranged in a pattern with each, at least within a region, in the same orientation. That is, they are crystalline or close to it," said Whitesides. "Because they are regular and because they are very easily made they are essentially ideal as a system with which to study surfaces."

The science of surfaces is central to Whitesides' work. He considers them a form of matter distinct from liquids, solids, and gases. They are what give everything shape and determine properties such as whether or not an object is resistant to water.

"Surfaces are very important in many technologies, especially in the technologies of small things [like] microsystems and nanosystems," said Whitesides.

Whitesides recently advanced his self-assembled monolayer (SAM) technology to develop soft lithography, which is a set of techniques based on the same principles as a mold for an automobile or a rubber "confidential" stamp for an envelope, to make micro and nanostructures.

The technique produces structures similar to those made by photolithography, which is the basic technology for making microelectric devices such as semiconductors.

"Soft lithography is just beginning its development and I do not think it will take over from photolith for high performance microelectronics. But it is much less expensive than photolith and it can handle a larger range of classes of materials," said Whitesides.

The drug industry, for example, is using soft lithography to make tools that will help in the development of new drugs. Researchers are also adapting the technique to make a new class of organic microelectronics.

In the meantime, Whitesides will continue to pioneer advances in materials science. "I like discovery and I like problem solving," he said. "It's fun and sometimes useful."

Can Art Make Nanotechnology Easier to Understand?

The old adage "seeing is believing" hardly applies to nanoscience, which operates on a scale of atoms and molecules. So how do you make something so miniscule and abstract appear real to the ordinary eye?

Why not through art?

A new exhibition at the Los Angeles County Museum of Art, called "nano," merges the art and the atom. Through art-making exhibits, visitors can experience what it's like to move molecules and manipulate atoms one by one.

The project, which was created by a team of nanoscience, media arts, and humanities experts from the University of California, Los Angeles (UCLA), allows visitors to experience nanotechnology by sensing it, even when they can't see it.

"This new science is about a shift in our perception of reality from a purely visual culture to one based on sensing and connectivity," said Victoria Vesna, chair of the UCLA Department of Design/Media Arts, who spearheaded the project with James Gimzewski, a UCLA chemistry professor and nanoscience pioneer.

From Viewing to Sensing

The word nano is Greek for "dwarf." A nanometer is one billionth of a meter. To better understand the miniscule scale of this science, consider this: The average thickness of a human hair is 50,000 nanometers.

The conceptual underpinnings of the science were introduced in 1959 by the Nobel Prize-winning physicist Richard Feynman in a lecture titled "There is Plenty of Room at the Bottom." But it wasn't until the early 1970s that Japanese engineer Norio Taniguchi first proposed the term nanotechnology.

Although scientists knew that matter existed on the nano scale, they were unable to analyze it through microscopes. When magnified ten thousand times in a lens-based microscope, an image starts to go fuzzy; at 100,000 times, it's blank.

The breakthrough came in 1981, when scientists at the IBM laboratories in Zürich, Switzerland, invented the Scanning Tunneling Microscope, which for the first time looked at the topography of atoms that cannot be seen.

The technology marked a paradigm shift in how scientists analyze miniscule matter, allowing them to record shape by tactile sensing instead of viewing it, much like a blind man reading Braille, only on the atomic scale.

But most people's understanding of nanotechnology remains limited. Recently, some artists have suggested that a perceptual shift has to take place in our minds if we want to comprehend the work of nanoscience

Nanotechnology's Big Future

A tsunami is unnoticeable in the open ocean—a long, low wave whose power becomes clear only when it reaches shore and breaks. Technological revolutions travel with the same stealth. Spotting the wave while it's still crossing the ocean is tricky, which explains why so few of us are aware of the one that's approaching. Nanotechnology has been around for two decades, but the first wave of applications is only now beginning to break. As it does, it will affect everything from the batteries we use to the pants we wear to the way we treat cancer.

The main thing to know about nanotechnology is that it's small. Really small. Nano, a prefix that means "dwarf" in Greek, is shorthand for nanometer, one-billionth of a meter: a distance so minute that comparing it to anything in the regular world is a bit of a joke. This comma, for instance, spans about half a million nanometers. To put it another way, a nanometer is the amount a man's beard grows in the time it takes him to lift a razor to his face.

Nanotechnology matters because familiar materials begin to develop odd properties when they're nanosize. Tear a piece of aluminum foil into tiny strips, and it will still behave like aluminum—even afer the strips have become so small that you need a microscope to see them. But keep chopping them smaller, and at some point—20 to 30 nanometers, in this case,—the pieces can explode. Not all nanosize materials change properties so usefully (there's talk of adding nano-aluminum to rocket fuel), but the fact that some do is a boon. With them, scientists can engineer a cornucopia of exotic new materials, such as plastic that conducts electricity and coatings that prevent iron from rusting. It's like you shrink a cat and keep shrinking it, and then at some point, all at once, it turns into a dog.

Substances behave magically at the nanoscale because that's where the essential properties of matter are determined. Arrange calcium carbonate molecules in a sawtooth pattern, for instance, and you get fragile, crumbly chalk. Stack the same molecules like bricks, and they help form the layers of the tough, iridescent shell of an abalone.

It's a tantalizing idea: creating a material with ideal properties by customizing its atomic structure. Scientists have already developed rarefied tools, such as the scanning tunneling microscope, capable of viewing and moving individual atoms via an exquisitely honed tip just one atom wide.

"Nano's going to be like the invention of plastic," says Paul Alivisatos, associate director of physical sciences at Lawrence Berkeley national Laboratory's new nanofabrication center. "It'll be everywhere: in the scalpels doctors use for surgery and in the fabrics we wear." Alivisatos already owns a pair of stain-resistant nanopants from the Gap, made from fibers treated with fluorinated nanopolymer. "I spilled coffee on them this morning, and it rolled right off."

On a table in a lab at Rice University, André Gobin, a graduate student, is working with two slices of raw chicken. He nudges the slices together so they touch and dribbles greenish liquid along the seam. The liquid is a solution of nanoshells: minuscule silica beads covered, in this case, with gold. Switching on an infrared laser, Gobin deftly traces the beam down the length of the green line. Tweezing the chicken up, he dangles what is now a single piece of meat.

Someday soon surgeons may be able to use a nanoshell treatment like this to reconnect veins that have been cut during surgery. "One of the hardest things a doctor has to do during a kidney or heart transplant is reattach cut arteries," says Gobin. "They have to sew the ends together with tiny stitches. Leaks are a big problem." With Gobin's nanoshell solution a surgeon could simply meld the two ends and get a perfect seal. It would make grafting veins as easy as soldering wire.

Although much of nanotechnology's promise remains unrealized, investment in the field is booming. The U.S. government allocated more than a billion dollars to nanotechnology research in 2005—more than twice what it spent on sequencing the human genome when that project was at its height. Japans and the European Union have spent similar amounts, and even smaller countries are hurrying to get a foot in the door. A Korean company has used nanosilver-based antibacterials in refrigerator interiors. The same material can be incorporated in bandages. The hopes is the same on all fronts: to get the jump on a growing global market that the National Science Foundation estimates will be worth a trillion dollars by 2015.

One reason for the rapid global spread of nanotechnology is that the entry cost is comparatively low. Countries that missed out on the computer revolution because they lacked the capital to build vast, high-tech factories that make silicon chips are less likely to miss the nanotech wave.

"It's science you can do in a beaker," says Stephen Empedocles, vice president of Nanosys, a company that's developing cheap solar nanostructures. Traditionally the manufacture of solar-energy cells has required a multimillion-dollar fabrication facility that cooks sheets of glass at extremely high temperatures until the atoms order themselves into a receptive latticework. Solar nanostructures, on the other hand, grow like rock candy. You can "mix them up in a beaker with a hundred dollars' worth of starter chemicals," Empedocles says, and then paint them on window glass to turn an entire building into a solar-energy generator. Or, they might be embedded in the plastic body of a cell phone or a laptop computer.

For a hundred dollars, in fact, anyone can buy nanoparticles—specifically a gram (.04 ounce) of carbon nanotubes—online. Place the order, and you'll receive a small ziplock bag of what looks like soot tucked inside a cardboard FedEx envelope along with some safety instructions. (They recommend gloves to keep the carbon slivers off the skin and a respirator to keep the tiny black specks from entering the lungs.)

There's not much you can do at home with a thimbleful of carbon nanotubes. But some of their mysteries are revealed in another Rice University lab, where Matteo Pasquali holds up a test tube containing a few dark threads so stiff that they seem to have been starched and ironed. These are fibers spun from carbon nanotubes—several billion of them—which, in theory, should be stronger than Kevlar, the material in bulletproof vests.

For now, however, the threads are only about as tough as the acrylic found in an ordinary sweater. The reason the threads are weak, Pasquali believes, is because some portion of the billion nanotubes bundled together have hidden breaks. A photo taken through a microscope shows fibers that look like pale gray hairs, some perfectly straight, others frayed and curling. "We have split ends," Pasquali says with a sigh. "We need a nanotube conditioner."

Carbon has proved a useful element in nanotechnology. One of the science's building blocks is a molecule that contains 60 carbon atoms arranged in a sphere. A molecule of C₆₀ looks like the geodesic dome invented by Buckminster Fuller, thus its nickname: buckyball.

Richard Smalley and colleagues discovered the buckyball in 1985, and in 1996 Smalley and two others earned a Nobel Prize in chemistry for the deed. Until his recent death, Smalley was a bucky fanatic. He renovated his house, close to the Rice University campus in Houston, with a glass skylight shaped like half a buckyball, with precisely proportioned steel struts representing the bonds between atoms.

Smalley was openly proselytical about the merits of buckyballs and a particular fan of their relatives, carbon nanotubes. ("Fifty to a hundred times stronger than steel and one-sixth the weight!" he often pronounced as though reporting the achievements of a precocious child.)

Because of their light, stiff composition, merely sprinkling carbon nanotubes into epoxy strengthens the glue by more than 30 percent. The tubes have also begun turning up in high-end sporting equipment. They strengthen tennis rackets, mountain-bike handlebars, frames for racing bikes, and golf-club shafts. Carbon nantubes also show promise for use in transparent conductive films for displays on computers, cell phones, PDAs, and automatic teller machines.

Smalley was also an ardent advocate of nanotubes as a solution to the world's impending engery crisis. His plan was to replace old copper and aluminum power lines with wires spun from carbon nanotubes. Nanotubes can carry far more current than traditional metal wires—over a billion amps of current per square centimeter (0.16 square inches)—and, unlike metal wires, they lose very little of that energy as heat. In theory, the nanotube power lines would carry electricity over thousands of miles. Rather than relying on local coal-fired power plants, cities could use energy generated by giant solar farms in deserts or by wind farms off coasts. "This is the great getting-up morning of nano," Smally said. "If Mother Nature allows it, we could restring the electrical grid of the world."

Not everyone is so sure. Carbon nanotubes come in three types. They all conduct electricity, but only one does it especially well. And so far no one has come up with a way to make those nanotubes very long. Right now, the longest electricity-conducting nanotube in existence measures a fraction of an inch.

At the root of the problem is the fact that there are two ways to make nanoparticles: "top down," where a bulk material gets chopped down into nanosize bits, and "bottom up," where molecules grow under controlled conditions, as in crystals, and then snap together into particular configurations based on their charge and molecular chemistry.

Bottom-up constructions—which long carbon nanotubes would require—are where the real power of nano lies. But they're also far more complicated, subject to all the laws of bonding that limit the ways atoms and molecules can be arranged. Getting carbon to curl into a perfectly aligned tube rather than a thick, twisted scroll is exceedingly complex.

Scientists are still relatively ham-fisted when it comes to the finer points of bottom-up assembly, particularly compared with a far more prolific nanofactory: the human body.

The human body makes quick and constant work of assembling raw materials like calcium and keratin to create elaborate structures like bones and skin. Compared with the work a blood cell does, scientists are "pretty much inept," admits Jim Heath, a Caltech chemist who is developing nanoscale sensors capable of detecting and diagnosing cancers. "But we're learning. We've come a long way in the past two years."

Heath's goal is to identify cancers early, when they are still just a few thousand cells strong and far easier to treat. Unlike HIV or malaria, which produce unique antibodies identifiable from a simple blood test, cancers are difficult to spot. Nonetheless, they do leave what Heath calls a fingerprint: a change in the number and type of proteins that regularly circulate in the blood.

Determining which combination of proteins makes up the unique signature of a particular cancer is an ongoing project. "To diagnose one cancer reliably in early stages, we probably need to measure 20 or 25 different proteins," Heath says. "So to develop a test that would identify 20 different cancers, we'd need about 500 measurements. And we would want to be able to do that easily, with just a finger prick of blood."

Heath has already developed nanosize sensors called nanowires that can electronically detect a few protein molecules along with other biochemical markers that are early signs of cancer. Heath's strategy is to coat a collection of nanowires with different compounds, each of which binds to one particular marker. When the marker, which can be a protein, an antibody, or DNA, latches on, it changes the conductivity of the nanowire, creating a tiny but measurable alteration in current. Heath has combined tens of thousands of these sensors onto a single chip, which allows him to detect cancer-signifying molecules in blood while their concentration is still low. The chips also allow him to identify what types of cancer are present. Currently, heath reports, his chip can detect between 20 and 30 relevant biomolecules. He plans to begin using the chip to detect brain cancers this summer.

Richard Smalley was one scientist who followed Heath's progress carefully. Smalley's non-Hodgkin's lymphoma was a relatively slow-moving cancer, but even when he was in remission, between a hundred million and billion cancer cells circulated in his body (a number that doctors consider relatively low).

One of the advantages of treating cancer in an early stage is that the cells are less likely to have mutated and become resistant. Drug resistance is one of the trickiest things about cancer, which adapts so rapidly that medications can rarely keep up. "You don't want a killing mechanism to be fancy," Smalley said. It needs to be fast and thorough.

But targeting a brute-force treatment is difficult, says Jennifer West, a bioengineer who is treating tumors in mice using gold nanoshells. Difficult because things that kill cancer cells typically kill healthy cells as well. "That's what we'd like to avoid," West says, Her approach relies on the fact that tumors grow blood vessels so quickly to keep up with the rapidly multiplying tumor cells that they don't have time to knit tightly and instead leak like rusted pipes. West's gold nanoshells are about 120 nanometers in diameter—a cancer cell is 170 times bigger. So the nanoshells are minute enough to seep through the cracks in the tumor capillaries and become lodged in the tumor.

To kill the tumor, West activates the shells with infrared rays that pass harmlessly through the skin but heat the gold, killing the adjacent tumor cells. Because the cancer cells die, they don't develop the resistance that can plague drug-based cures.

Moreover, because the nanoshells lodge only in the tumor and are nontoxic unless activated by infrared light, West expects her treatment to be nearly side-effect free—particularly compared with treatments like chemotherapy and radiation. As part of the FDA approval process, West has injected mice with increasingly large doses of nanoshells. Not a single mouse has died. "We injected mice with increasingly large doses of nanoshells. Not a single mouse has died. "We haven't even been able to induce any adverse effects," she says with a shrug. "If we had injected these mice with the same amount of table salt, they would have keeled over long ago.

Unfortunately, the very thing that makes nanoshells such a promising therapy—their ability to move easily through the body and to interact with different cells—is a downside when it comes to the problem of nanoparticle pollution.

In 2004 Eva Oberdörster, a toxicologist at Sothern Methodist University in Dallas, reported that largemouth bass exposed to water containing buckyballs at a concentration of 500 parts per billion suffered brain damage. And people are similarly vulnerable. After exposing lab-grown human skin and liver cells to an even weaker solution—a mere 20 parts per billion—Rice University chemist Vicki Colvin found that fully half the exposed cells died.

Results like these are troubling, in part because of the rapidly growing number of products already on the market that contain nanoparticles. "With nanomaterials, it's not enough to look at the properties of the bulk material," Colvin warns. "Whether you're working with gold or lead, the toxicity will be hard to predict." There is some evidence, for example, that the nanoscale particles of titanium dioxide used in sunscreen, depending on the way they are nanosized, can produce high amounts of free radicals when exposed to sunlight. Free radicals can damage cells, making some more likely to turn cancerous.

Colvin's concern is that companies are currently optimizing their particles for processability rather than for human health. A recent study found that buckyballs could be made less toxic fairly easily—by attaching inert molecules known as hydroxyl groups. The more hydroxyl groups attached, the less dangerous the buckyballs became. For the most thoroughly coated, the safe exposure level went up by a factor of ten thousand.

But it's hard to get funding for this kind of research, Colvin says. "Funding managers want a sexy story at the end of the day. They want to be able to say that they're helping to cure cancer. It doesn't sound as glorious when your finding is that a certain particle you were hoping to use ought to have hydroxyl groups put on it in order to be safe."

Still, researchers are making important advances. They are finding new ways to use nanosize sensors in water purification systems that will filter everything from bacteria to industrial pollutants like arsenic. The key feature of the new filters is the fact that nanoparticles have a vast amount of surface area for their weight: One ounce (28.3 grams) of nanobeads, for instance, contains a staggering 300,000 square feet (27,871 square meters) of surface area. Because the chemical reactions that neutralize pollutants take place on the surface of the beads, the greater the available area, the more effective the filter.

The potential impact of nanofilters is substantial. Many regions in China have drinking water that contains dangerously high levels of arsenic and other industrial pollutants. Because of this, Colvin predicts that Asia will be a test bed for point-of-use water treatment systems that utilize nanoparticles to eliminate toxic chemicals. "Right now, nanoscale iron is a bit too expensive to be used to treat wastewater," she says. "But it's the best way to clean up concentrated arsenic, and I expect the cost will come down soon."

Because nanotech applications are so potentially useful, Colvin doesn't think research should be stopped, or even slowed. But she does think that a larger proportion of government money should be directed toward safety and related questions—like whether nanoparticles could accumulate undetected in the water and food chains.

Such safety issues are key, given the speed with which the nanotech tsunami is moving. Corporations will invest more than four billion dollars in nanotech this year alone, and a recent nanotech conference in Japan drew a crowd of 30,000.

Meanwhile, commercial applications continue to spread. Homeowners now have the option of installing windows manufactured by PPG Industries, a company that uses nanoscale particles of titanium dioxide to make glass that doesn't streak and never needs washing. Food companies have begun experimenting with nanopackaging that changes color when food spoils or contains bacteria like E. coli. The prefix has even trickled over into popular culture, where it's the advertising hit du jour, with GM hawking a "nano" Hummer, and Apple its iPod Nano digital music player.

"What's amazing is how quickly this is evolving," Colvin says. "Even ten years ago, a lot of these applications would have seemed pretty unrealistic."

The boom left Richard Smalley downright nostalgic: "Nano is a baby that's all growed up," he mused shortly before his death. Perhaps, but we've still got some interesting years ahead.