Nanodevices that self-assemble

Imagine unrolling an electronic newspaper that's automaticallyupdated via the Internet. Or cheap roof shingles that double as solarpanels. These are just two technologies that could become possible withthe advent of plastic electronics made from tiny components thatassemble themselves.

"When you're talking about plastics, you'retalking about petroleum byproducts, so these devices could be quiteinexpensive," says Segalman. "They could also be very lightweight andflexible, leading to all kinds of new uses."

At the most basic level, Segalman is interestedin the structure and patterning of functional polymers, such asconductive forms of plastic. For these functional polymers to form thebasis of plastic electronics, the molecules must be assembled intospecific structures that provide the desired electrical properties. Theshape of the basic structures determine how bright the devices mightglow or how well they convert sunlight into electrical energy. Theproblem is that it's very difficult and inefficient to "build"structures at the nanoscale. (A nanometer is one-billionth of a meter.)Segalman's approach is to spur the polymers into assembling themselves.

To do that, Segalman uses chemical processes tocreate small "block copolymers," molecular chains. Imagine that a "red"string is joined end-to-end with a "blue" one. Chemically, the "red"half and "blue" half of the new longer string repel each other.

"When they're put together, they self-assemble into astructure with the reds on the outside and the blues on the inside,"Segalman explains. "We try to harness that kind of effect to make thestructures that we want."

Many researchers have developedsimilar self-assembly techniques, she adds, but usually using moretraditional polymers like polystyrene, the stuff of plastic drinkingcups. The challenge with conductive polymers is that they're much morefinicky, often clumping together in unexpected ways. Recently,Segalman's research group has started to develop a thermodynamic phasediagram, a "rule book" of sorts for self-assembly.

"Therules say the chemistry equivalent of things like, 'If you make apolymer that looks like this, and you heat it to this temperature, thisis what the end structure will look like," Segalman explains.

Inone experiment, the researchers demonstrated a method to self-assemblea device that could be a component in a future flexible screen. Today'sOrganic Light Emitting Diodes (OLEDs), like those in the displays ofsome newer mobile phones, are still rather costly to produce. That'sbecause they're fabricated in a multi-step process by sandwiching manylayers of materials together.

Segalman's technique is todeposit all of the materials at once and allowing them to self-assembleinto the desired layers. The result is a device that's not only easierto process, but is also likely to produce light more efficiently.

"Weonly make widgets to prove a point," Segalman says. "Our real goal isunderstanding the polymer physics. At the nanometer length scale, wecan't touch or feel to engineer things. So we spend a lot of timethinking about how to control the system in other ways, how to play theright tricks to get something to self-assemble the way we want it to."

Asthey suss out the rules for controlling nanoscale self-assembly, theresearchers are also developing techniques to characterize theirstructures. The aim, of course, is to understand how the structureaffects the properties of what they've built. Indeed, the two effortsmust go hand-in-hand if the researchers hope to generalize theirtechniques for broader use.

"We passed our first hurdle,which is showing we can control the self-assembly of structures" shesays. "So now we're approaching our next hurdle, which is showing whythese structures are important."

Source: sciencematters.berkeley.eduAdded: 15 February 2006