Volume 3, Issue 18 February 2006
Nanodevices That Assemble Themselves
Rachel Segalman joined the UC Berkeley faculty in Spring 2004 after completing a postdoctoral fellowship at the Université Louis Pasteur in France.
Imagine unrolling an electronic newspaper that's automatically updated via the Internet. Or cheap roof shingles that double as solar panels. These are just two technologies that could become possible with the advent of plastic electronics made from tiny components that assemble themselves. UC Berkeley chemical engineer Rachel Segalman is conducting the fundamental research that could help make this nanoscale dream a reality. In December, Segalman's efforts earned her a National Science Foundation CAREER Award.
"When you're talking about plastics, you're talking about petroleum byproducts, so these devices could be quite inexpensive," says Segalman. "They could also be very lightweight and flexible, leading to all kinds of new uses."
At the most basic level, Segalman is interested in the structure and patterning of functional polymers, such as conductive forms of plastic. For these functional polymers to form the basis of plastic electronics, the molecules must be assembled into specific structures that provide the desired electrical properties. The shape of the basic structures determine how bright the devices might glow or how well they convert sunlight into electrical energy. The problem 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 to create 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.
Transmission Electron Micrograph of a self-assembling conjugated block copolymer. This block copolymer self-assembles into nanoscale lamellae, fine layers of different alternating materials.
"When they're put together, they self-assemble into a structure with the reds on the outside and the blues on the inside," Segalman explains. "We try to harness that kind of effect to make the structures that we want."
Many researchers have developed similar self-assembly techniques, she adds, but usually using more traditional polymers like polystyrene, the stuff of plastic drinking cups. The challenge with conductive polymers is that they're much more finicky, often clumping together in unexpected ways. Recently, Segalman's research group has started to develop a thermodynamic phase diagram, a "rule book" of sorts for self-assembly.
"The rules say the chemistry equivalent of things like, 'If you make a polymer that looks like this, and you heat it to this temperature, this is what the end structure will look like," Segalman explains.
In one experiment, the researchers demonstrated a method to self-assemble a device that could be a component in a future flexible screen. Today's Organic Light Emitting Diodes (OLEDs), like those in the displays of some newer mobile phones, are still rather costly to produce. That's because they're fabricated in a multi-step process by sandwiching many layers of materials together.
Segalman's technique is to deposit all of the materials at once and allowing them to self-assemble into the desired layers. The result is a device that's not only easier to process, but is also likely to produce light more efficiently.
"We only make widgets to prove a point," Segalman says. "Our real goal is understanding the polymer physics. At the nanometer length scale, we can't touch or feel to engineer things. So we spend a lot of time thinking about how to control the system in other ways, how to play the right tricks to get something to self-assemble the way we want it to."
As they suss out the rules for controlling nanoscale self-assembly, the researchers are also developing techniques to characterize their structures. The aim, of course, is to understand how the structure affects the properties of what they've built. Indeed, the two efforts must go hand-in-hand if the researchers hope to generalize their techniques for broader use.
"We passed our first hurdle, which is showing we can control the self-assembly of structures" she says. "So now we're approaching our next hurdle, which is showing why these structures are important."