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NO JIVE IN THIS JAVA: MSE’S LIN BREWING BIG CHANGES IN NANOSTRUCTURES
 

Some read tealeaves, but Zhiqun Lin thinks he can get a better fix on the future of technology by scrutinizing “coffee rings.” For while tealeaf reading is an art form at best, Lin’s work with coffee rings is solidly scientific.

And, apparently, Lin has a few coffee lovers among his colleagues: along with collaborators from Shanghai’s Fudan University and two of his Iowa State graduate students, the assistant professor in the Department of Materials Science and Engineering recently published “Self-Assembly of Gradient Concentric Rings via Solvent Evaporation from a Capillary Bridge” in The American Physical Society’s prestigious Physical Review Letters.

Lithography’s advantages ‘evaporate’

The paper summarizes research Lin feels has the potential to revolutionize the production of nanostructured materials used in devices such as photovoltaic cells, biosensors, and light-emitting diodes. The evaporation process he outlines, Lin says, could result in applications that are far more economical and efficient than the lithographic techniques currently employed to fabricate many nanomaterials.

“Lithography is very time consuming and expensive,” Lin observes. “If you want to get these kinds of concentric rings out of a photo-lithographic or electron beam-lithographic process, you need a clean room, a mask aligner, a UV light source, and possibly a scanning electron microscope. But all I need is one drop of solution and two simple tools.”

Left: digital image of ring pattern formed by deposition of conjugated polymer from 0.075 mg/ml toluene solution in sphere-on-Si geometry. Right: detail of fluorescent ring patterns . As solution moves inward, rings become smaller and height decreases (lower left ).

Using what Lin calls a “stick-slip” technique, the solution is held in place between two substrates—a spherical lens and a flat silicon (Si) substrate. As the solvent evaporates, the conjugated polymer that constitutes the solute is deposited upon both substrates at the outer edges of their surfaces, resulting in a perfectly formed coffee-ring pattern. The sphere-on-Si configuration of the substrates rigorously controls the solvent’s evaporation rate, such that rings deposited successively inside of one another exhibit a similarly high degree of uniformity.

The specific architecture of a given sample—that is, the height of successive rings, as well as the distances between them—is determined by the concentration of the solution and the solvent’s properties, which affect the rate of evaporation and resulting deposition of the polymer solute. Ultimately, the process produces a series of gradient concentric rings that exhibit an unprecedented degree of uniformity rivaling any lithographically engineered surface at a fraction of the cost.

Make that a double espresso
Beyond the techniques outlined in his paper, Lin also is adding an extra shot of “caffeine” to his scientific brew that would serve (quite literally) as an eye-opener for potential optical, optoelectronic, and sensory applications. Along the microscopic rims of the “coffee rings” themselves, Lin is incorporating quantum dots—tiny, highly emissive semiconductors only a few nanometers in diameter. He currently has seed funding from the U.S. Department of Energy’s Ames Laboratory to develop quantum dots tailored with conjugated polymer, which will be distributed uniformly through a porous alumina membrane developed by Lin himself.

“If I can combine the microscopic structure formation of the rings with the self-assembly of nanostructured formations,” Lin says, “I can get hierarchically structured formations. So inside each gradient concentric ring, we’ll have this hexagonal packing of quantum dots whose optical qualities can be predicted or manipulated by quantum mechanics.”

The highly controlled architectures resulting from this process are especially promising for use in devices such as solar cells, whose high cost and relative inefficiency have, up to the present, limited their wider application. By chemically “tethering” quantum dots to conjugated polymer for self-assembly upon concentric gradient rings in a highly controlled interface, Lin hopes to go beyond the current limitations of solar cells and other optoelectronic technologies by making the exchange of electrons among their components far more efficient.

Same process, different brew
Lin characterizes the research field as “very hot” at the moment. Considering the implications, it’s not difficult to see why: in addition to optoelectronic applications, Lin notes, the substitution of an organometallic diblock copolymer in the “coffee ring” evaporation process has the potential to generate microchips with storage capacities a full magnitude greater than those currently available—in short, consumer-grade hard drives with 500 terabytes of storage (that’s half a petabyte), rather than the current standard of 500 gigabytes.

What you would actually do with half a petabyte of digital storage space on your desktop computer? Pour yourself another cup of coffee—and ponder the possibilities.