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."
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Affecting Evolution and Extinction
Professor Anthony D. Barnosky, seen here in a cave searching for fossils, is also a curator at UC Berkeley's Museum of Paleontology.
Every so often, a huge number of species on Earth are wiped out relatively quickly. The last time a large extinction event occurred, between 50,000 and 10,000 years ago, two-thirds of large mammals were swept into the dustbin of history. Why? UC Berkeley paleontologist Anthony Barnosky sifts through the fossil record to understand how environmental changes can cause mammals to move, evolve, and sometimes die off. His research could even help reveal whether we're headed for another mass extinction.
"Rather than doing experiments, I use the natural experiments that have already taken place on Earth and study their remains," says Barnosky, a professor of Integrative Biology. "By interpreting the information in the fossil record, we can say something about how ongoing changes now are going to affect the ecosystems of the future."
Barnosky's work can be categorized into two areas based on the timescale that the environmental shifts occur. The first are climate changes that take place over thousands of years. The aim there, he says, is to differentiate between effects of climate change that are natural, and those that could be harbingers of a bigger problem. That way, the researchers can better determine how much impact, say, global warming, as a form of human-induced climate change, is having on mammals.
"Is part of being a species the fact that you move around in response to climate change and it's no big deal?" Barnosky says. "I'm trying to establish a natural baseline of how much communities change in response to climate change in the past."
To do that, Barnosky and his research group dig deep into the sediment where they can find a recorded history of climate change. Comparing that data with the fossil record reveals how communities of mammals may have shifted as the climate changed. Late last year, Barnosky used this approach to investigate the cause of large mammal extinctions in the late Pleistocene period, 50,000 to 10,000 years ago.
Historically, scientists have thought that human populations of the time over-hunted, killing off animals such as mammoths, ground sloths, native American horses, and camels. However, Barnosky and his colleagues discovered that human impact wasn't the sole cause of the extinctions. Rather, climate change combined with the over-hunting was a "one-two punch" leading to the extinction, he says. The big concern, Barnosky says, is that the state of the planet then is not so different from today.
A jumble of fossil mammoth bones being excavated from a fossil site in southeastern Washington State. Mammoths, which looked like shaggy elephants, were hunted by prehistoric humans who arrived in North America about 11.4 thousand years ago, as shown by the close intermingling of spear-points with fossil mammal skeletons. However, they also went extinct coincident with climate change in areas where significant human presence has not been shown, such as Alaska, and coincident with climate change in areas where they had coexisted with humans for hundreds of thousands of years, such as parts of Europe. (Anthony D. Barnosky photo)
"We've ramped everything up," he says. "Global warming has never been faster and human populations are exploding exponentially. Realistically, I think the ecosystem will change pretty dramatically."
The second thread of Barnosky's research runs through the fossil record over millions of years rather than thousands. This is the time it takes the physical landscape to transform as mountain ranges push up from the Earth's surface and valleys form between them. Barnosky examines how these long-term physiographic changes correspond with evolutionary transformations.
"I'm interested in whether the formation of new species is driven solely by interactions between different species, like an arms race between predator and prey, or whether it's also caused by external changes in the physical environment," he says.
Again, the researchers look at geological records of how topography and climate of a particular region has changed over millions of years. Then they dive into their Miocene Mammal Mapping Project, a massive database they're building of all the fossil records of mammal species from the Western United States from about 5 million years ago to 30 million years ago. The database is integrated with layers of Geographic Information Science maps of the Western U.S.
"We can now trace how species move around through time and space," Barnosky says. "By comparing those patterns with the geologic information and the various expectations from evolution theory, you begin to get some answers."
For example, there is controversy among evolutionary biologists as to how much climate change impacts the evolutionary process. The thought is that as glaciers grow and melt, mammal populations will be separated in different ways, leading to speciation. To investigate the matter, Barnosky analyzed information in his database and compared that with other published information about mammal species that lived within the last 1.8 million years, a period when the climate shifted every 100,000 years or so. His research suggests that climate changes can indeed influence evolution, but only if the change lasts for at least a million years.
"All of this research is about coming up with better methods to understand the natural patterns of biodiversity," Barnosky says. "That way, we can really know when it's time to worry."
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From Carbon Cycles to Climate Models
In November of last year, Inez Fung won a Scientific American 50 award, recognizing leadership in shaping science and technology.
While we go about our daily lives, our planet's atmosphere—land, oceans, and ice—are interacting in an incredibly complex dance. The Earth is a closed system, where these subtle interactions have a global impact on the climate. UC Berkeley professor Inez Fung constructs incredibly complex computer simulations of the climate. By uncovering the myriad forces behind climate change, Fung's research is revealing how our actions today may have dire consequences for the world of tomorrow.
Scientists have long known that the Earth is getting warmer, explains Fung, a professor in the Department of Earth and Planetary Science and Environmental Science, Policy, and Management. This warming is tied to changes in the Earth's atmosphere, specifically the increase of carbon dioxide and other greenhouse gases. The gases trap energy from the sun so that it warms the surface of the planet. Enhancing the greenhouse affect, through the burning of fossil fuels for instance, changes the energy cycle, leading to global warming.
"There's a rogues gallery of these atmospheric species, greenhouse gases like carbon dioxide and methane, that affect the energy cycle and climate," says Fung, co-director of the new Berkeley Institute of the Environment and former director of the Berkeley Atmospheric Sciences Center. "I'm hitting them one-by-one to understand what determines their concentration in the atmosphere, why that's changing, and how."
Fung's experiments take place inside interactive mathematical simulations, what she calls "huge, monster climate models" that can take weeks or months to run on the world's fastest supercomputers. Some of the data that's fed into the simulations come from atmospheric field observations, actual information gathered about the climate at a particular moment. The trick though is modeling how those inputs change.
Most famously, Fung and her colleagues modeled the carbon cycle, how carbon dioxide moves in and out of the atmosphere. Previous calculations included the fact that humans burning fossil fuel at a certain rate will boost carbon dioxide levels in the atmosphere.
"That just a forcing function though," Fung says. "To really understand the carbon cycle, you have to look at the circulation, biology and chemistry of the oceans where the carbon dioxide goes, the photosynthesis of plants as they breath carbon dioxide, the decomposition of plants, and many other forces."
A graphic depiction of the Earth's carbon cycle, highlighting various sources and sinks for carbon. (courtesy the researcher)
More than 15 years ago, Fung's atmospheric model suggested that the land biosphere, in addition to the oceans, have been acting as carbon sinks, repositories for the fossil fuel carbon. Fung's model "prediction" has since stimulated new programs in terrestrial ecology.
Indeed, Fung studies the whole shebang. Last month Fung, UC Berkeley integrative biologist Todd Dawson, and their colleagues, reported that trees are much more involved in carbon uptake and atmospheric cooling than previously believed. A study in the Amazonian forest showed that the roots shift water deep in the ground in such a way that they "pull more carbon dioxide from the atmosphere as they conduct more photosynthesis" even during the dry season, Dawson says.
The trouble is that there's a limit to how much carbon dioxide the world's plants can handle. Right now, plants and oceans absorb about half of the CO2 that's generated from the burning of fossil fuels. Last year, Fung's climate model indicated that in the next fifty years or so, the "breathing biosphere" may be overwhelmed.
Not only is there a metabolic limit to the process, but during periods of drought caused by a warming climate, the plants breathe less in an effort to save water. This affects the levels of CO2 in the atmosphere, creating a feedback loop in the system. And after plants die, their decomposition by microbes in the soil also play a part in the carbon cycle.
"If you don't look at decomposition, it's like looking at your income without considering your expenses," Fung says. "You have to think about the whole life cycle across the entire biosphere."
Simultaneously, the oceans' capabilities as a CO2 sink are hampered. Normally, turbulence shifts the CO2 deep into the ocean where it can't be sucked up into the atmosphere. But as the climate warms, the ocean becomes more stratified, making it difficult for CO2 to be driven to the depths.
"From the biosphere to the oceans, the warming feeds the warming," Fung says.
The question is how much. If things continue as they are, the global temperature may increase by more than two degrees Fahrenheit in the next 50 years or so. The consequences of such an increase are not entirely predictable. Still, Fung says, the model indicates that if fossil fuel CO2 emission is reduced, the plants and oceans could continue to support the carbon cycle as it currently exists.
"I'm not a policy maker, but I think my research could influence policy decisions, and personal decisions, about the kind of future we want," Fung says.
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