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The Vast Area of Small

– Christopher O'Carroll

Physicist Mark Tuominen, polymer scientist Thomas Russell and chemical engineer James Watkins superimposed here on a sacrificial template prepared by postdoc fellow S.H.Kim and grad student M. Misner. (photo by Ben Barnhart) THE WORD IS AN UNGAINLY six-syllable giant – nanotechnology. The objects it refers to are so small that more than a million of them could cluster together on the period at the end of this sentence. Wires thousands of times finer than a human hair. Sieves whose minuscule pores can sift a gas or liquid to sort one molecule from another. Infinitesimal capsules capable of delivering life-saving drugs to precise locations inside the body. These are just a few of the tiny but hugely significant items taking shape in nanotech research labs as scientists perfect the ability to manipulate matter with extraordinary precision on a scale smaller than ever before. “It’s a vast area of research,” says physicist Anthony Dinsmore, one of many faculty members whose work is helping to keep this campus in the vanguard of nanotech research. (Experts in the field habitually abbreviate “nanotechnology” to the less cumbersome “nanotech” or the breezy “nano.”) With scientists from such diverse departments as physics, chemistry, chemical engineering and polymer science and engineering attracting major grants for their nanotech work, it’s a vast area of research in which UMass has surged to an early leadership position. In 1998, the first year that the National Science Foundation (NSF) designated nanotech as a special funding priority, UMass research teams scored two separate NSF nano grants totaling more than $1 million. Since only 24 research projects won NSF support that year, even one grant constituted a major endorsement of an institution’s research excellence, so UMass was especially pleased to receive two. Then in 2001, the same federal agency acknowledged the university’s preeminence again with two nanotech grants totaling $2.25 million. Says physicist Mark Tuominen, one of several researchers who received NSF support in both of those funding cycles, “I think it speaks highly of UMass that in the first call for nano proposals we got two, and in the second major initiative we got two. There’s a lot of good nanoscience going on here, and it’s doing well in a competitive market.” In an era when even inexpensive home computers boast nanosecond processing speeds, it’s pretty widely known that “nano” is a prefix meaning “one billionth.” Nanotechnology involves working with quantities of matter so small that they are measured in nanometers, billionths of a meter. Most of us have seen metrical measuring sticks calibrated in millimeters, thousandths of a meter. In the everyday physical world, outside of science laboratories, a millimeter seems very small indeed, but from a nano perspective, it’s a vast distance. Subdivide a millimeter into a thousand parts and you’re down to the scale of a micron, a millionth of a meter. Then dice a micron into a thousand fractions and you finally find yourself in the nanometer realm. To work with matter on such a small scale – not simply studying its behavior but actually manipulating it to build useful devices and materials with nanometer dimensions – scientists rely on a new generation of research equipment including the atomic force microscope that enables them to visualize atoms and molecules with resolution as fine as an angstrom, a tenth of a nanometer. Last year, one of the country’s largest private foundations presented the university with a $750,000 grant to equip a state-of-the-art nanotech research facility, the W.M. Keck Foundation Nanostructures Laboratory. “We can see the nanoscale like never before,” says polymer scientist Todd Emrick. “Once you see it, once you visualize it, that gets your mind going in terms of new materials development.” Chemical engineer James Watkins explains that the new Keck Laboratory plays a crucial role in enabling nanotech researchers from many different disciplines to coordinate their efforts on multidisciplinary research teams. “Our objectives are different, but we need a common set of tools,” he says. “The Keck Lab provides the infrastructure.” Watkins, whose team received one of the first-round NSF nano grants, exemplifies the interdisciplinary nature of the field. Although he now teaches in the chemical engineering department, he earned his Ph.D. in the department of polymer science and engineering, so he is particularly attuned to the benefits of sharing research expertise across traditional disciplinary boundaries. “Synergy is what makes these projects work,” Tuominen agrees. “By myself as a physicist, I can only do so much. But when I collaborate with chemists and polymer scientists and chemical engineers, we can accomplish so much more.” The tasks that nanotech researchers set themselves present some ferocious challenges. John Donne may have written poetically of “gold to airy thinness beat,” but no set of tools wielded by human hands can pound metal to the airy thinness of a wire less than 20 nanometers in diameter. And if the goal is a surface bristling with a forest of such nanowires, uniformly spaced in an absolutely regular array, no conventional fabrication technique can come close to delivering satisfactory results. UMass nanoresearch teams have responded to such challenges with ingenious solutions that draw on multiple areas of specialization. THE TEAM HEADED BY TUOMINEN and polymer scientist Thomas Russell has been exploring one successful nanoscale fabrication technique that involves the so-called self-assembly properties of certain polymers. Suppose researchers want to build metallic nanowires for magnetic storage applications, or ceramic insulating material with pores just a few nanometers in diameter. Instead of starting with minuscule quantities of metallic or ceramic substances and trying to fashion the desired object a few atoms or molecules at a time, they will use much more easily manipulated polymer materials, plastics, to form what Watkins calls “a sacrificial template” which can then be used as a sort of ultra-miniature mold in which to cast the more durable material. The process is roughly comparable to the “lost wax” technique by which a sculptor shapes the fine details of a statue in a soft material that is subsequently purged from the mold to make way for molten metal. Nanotech fabrication by polymer self-assembly works like this: Researchers choose two polymers that have a tendency to repel each other in much the same way as oil and water. They then insert a chemical bond so that each molecule of polymer A is tied to a molecule of polymer B. Since the two plastics are now thwarted in their desire to separate from each other completely, they do the next best thing. They self-assemble into a pattern that minimizes contact between them. For example, the molecules of one polymer might curl themselves into minute spheres or cylinders, evenly distributed through a matrix of the other polymer. By adjusting the ratio between the two plastics, and tinkering with the lengths of their chain-like polymer molecules, scientists can determine whether the self-assembly will produce cylinders or spheres, and can precisely control the size of those features. Researchers can then coat some suitable surface with a thin film of the resulting mixture, a layer of one polymer studded with fragments of the other arranged in a uniform array of spherical bubbles or cylindrical tubes. A polymer film full of bubbles can provide a template for a material with a regular pattern of nanoscale pores, while ranks of miniature tubes offer a template for a densely packed array of fine wires. To create a set of nanowires with the help of the polymer template, scientists first remove the thin cylindrical columns of plastic inside the tubes. Depending on the chemical properties of the polymers involved, this can be accomplished by exposing the film to light or by treating it with a solvent. One effective solvent for the task is carbon dioxide gas compressed into a so-called supercritical fluid. “A good way to think about a supercritical fluid is that it’s a hybrid between a gas and a liquid,” Watkins explains. “It has densities that are similar to those of liquids, but from a transport point of view, it thinks it’s a gas, so it can easily diffuse into crevices that are 5 to 10 nanometers wide and smaller.” Once the cylindrical channels in the polymer film have been emptied, there are various methods available to fill them up with small quantities of metal to create extraordinarily thin wires. Metal particles can be bonded to other molecules and floated in with the solvent, or scientists can use such time-tested techniques as electroplating or evaporation to fill the cylindrical holes with metal. With this self-assembly procedure, UMass researchers have been able to produce arrays with a density of more than a trillion nanowires per square inch. If those wires are made of a metal that can be magnetized and used for data storage, it becomes possible to store the contents of 25 DVDs on a disk the size of a quarter. “This is not just good stuff, it’s great stuff,” Russell says. “It’s stunning stuff. This is something that has significant impact in the microelectronics field, the magnetic storage industry – it’s going to have significant impact across the board.” Russell has been applying self-assembly techniques to other nanofabrication challenges as well. In collaboration with Dinsmore and Emrick, he has been working on methods to manufacture nanocapsules with potential applications in such fields as medicine and food processing. The capsule construction procedure starts with a vessel containing oil, water and specially formulated particles of matter measuring less than five nanometers in diameter. Researchers then agitate the mixture as a chef might shake a cruet of salad dressing. Because oil and water don’t mix, the water will form itself into small droplets surrounded by oil. The nanoparticles will then self-assemble at the oil/water interface to form a thin spherical shell around each droplet. When slightly larger nanoparticles are added to the liquid, a few of them will tend to bump their smaller brethren off of a droplet and join together to form a distinct patch on the surface of the nanosphere. After the sphere has been removed from the oil, this patch serves as a miniature door that can be popped open to drain out the water and insert some other contents, and to deliver those contents with great precision. In determining how to take advantage of matter’s self-assembly tendencies, scientists can exercise a great deal of control over the properties of the finished nanocapsule. For example, Dinsmore says, they can work with nanoparticles of edible, FDA-approved materials to create capsules that food companies can use to deliver small, precisely targeted doses of nutrients or flavorings. Or they can select particles with certain sizes and bonding properties so that the capsules will resemble miniature mesh cages with gaps of a specific diameter. Such capsules might be used to deliver individual insulin-secreting cells inside the body of a diabetes patient. The gaps in the mesh would be large enough to let beneficial cells and molecules pass through, but small enough to screen out immune system cells trying to attack the foreign invader. Or researchers might be able to design capsules with chemical affinity for the distinctive surface of a cancer cell. Such nanocapsules could deliver microscopic doses of chemotherapy drugs without collateral damage to nearby healthy cells. Some of these adventures in nanotechnology are still at the basic research stage, laying the groundwork for applications that may lie many years in the future. There are other areas of research, however, that use new nano capabilities to refine or enhance technologies that have been in use for many decades. One example can be seen in the field of zeolite research. Zeolites are minerals composed primarily of silicon, oxygen and aluminum, with the atoms arranged in crystalline lattices shot through with pores that range from 10 nanometers in diameter to less than one. There are approximately 30 to 40 naturally occurring types of zeolites, and another 150 synthetics that scientists have developed over the years. Distinguished from one another by, among other factors, the size and shape of their pores, these crystals are used for such purposes as water purification and oil and gas refining. In some cases, zeolites extract a particular molecule from a mixture because that molecule fits into the pores better than other components of the mixture. In certain applications, such as refining petroleum into gasoline and other products, it is not just the pore structure but also the chemical properties of the atoms in the surrounding lattice that contribute to the desired effect. Large hydrocarbon molecules crack into smaller, more useful pieces as they react with the component chemicals arrayed around the zeolite pores. For many decades, it has been common to use beds of powdered zeolite for refining applications. Chemical engineer Michael Tsapatsis and chemist Scott Auerbach are two leaders of a UMass research team that is using the tools of nanotechnology to explore an alternative zeolite form, membranes instead of powder. Auerbach explains that some mixtures being filtered through zeolite powder exhibit “transport resistance” that slows down the process. For certain zeolite applications, replacing a mass of individual particles with a continuous thin membrane is an ideal solution. Today, nanotech researchers have the tools and techniques to pursue that solution by studying the fine structure of zeolites with a degree of precision that facilitates greater understanding of membrane growth. “Although zeolites have been around for 50 years in the commercial and technology arena, we could not manipulate them as a thin film,” Tspatsis says. “Now is the time that this can be done.” For example, when Tsapatsis seeds a surface with fragments of zeolite crystal to start a membrane growing, he has to be able to study the places where different segments of the thin film come together, to see whether the lattice closes up perfectly at the junction or whether some of the pores end up with ragged edges where they should be surrounded by a uniform crystalline structure. With the atomic force microscope and other tools of nanotech research, he is able to look at the smallest details of his zeolite membranes, thereby greatly enhancing his ability to control their growth and achieve the desired pore structure. Improved gas and oil refining technologies, new drug delivery systems, vastly enhanced magnetic storage capacity – the sheer diversity of nanotech research efforts can be a bit bewildering. And there are projects underway in many other areas as well. Nanoresearchers are exploring the design of smaller and faster microprocessor chips, high-resolution display screens as thin and flexible as a sheet of paper or fabric, optical devices with components keyed to many different wavelengths in a single beam of light, sensing devices that can detect the most infinitesimal quantities of toxic chemicals. “Ask 10 different scientists what nanotechnology means, and you’ll get 10 different answers,” Auerbach says. Most of them would agree, though, with Russell’s exuberant assessment of the frontiers that nano research has opened up. “The possibilities,” he declares, “are limitless.”

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The Vast Area of Small

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