MAKING MATERIALS FOR FUTURE ELECTRONICS

FROM:  NATIONAL SCIENCE FOUNDATION 

Materials for the next generation of electronics and photovoltaics

MacArthur Fellow develops new uses for carbon nanotubes

One of the longstanding problems of working with nanomaterials--substances at the molecular and atomic scale--is controlling their size. When their size changes, their properties also change. This suggests that uniform control over size is critical in order to use them reliably as components in electronics.

Put another way, "if you don't control size, you will have inhomogeneity in performance," says Mark Hersam. "You don't want some of your cell phones to work, and others not."

Hersam, a professor of materials science engineering, chemistry and medicine at Northwestern University, has developed a method to separate nanomaterials by size, therefore providing a consistency in properties otherwise not available. Moreover, the solution came straight from the life sciences--biochemistry, in fact.

The technique, known as density gradient ultracentrifugation, is a decades-old process used to separate biomolecules. The National Science Foundation (NSF)-funded scientist theorized correctly that he could adapt it to separate carbon nanotubes, rolled sheets of graphene (a single atomic layer of hexagonally bonded carbon atoms), long recognized for their potential applications in computers and tablets, smart phones and other portable devices, photovoltaics, batteries and bioimaging.

The technique has proved so successful that Hersam and his team now hold two dozen pending or issued patents, and in 2007 established their own company, NanoIntegris, jump-started with a $150,000 NSF small business grant. The company has been able to scale up production by 10,000-fold, and currently has 700 customers in 40 countries.

"We now have the capacity to produce ten times the worldwide demand for this material," Hersam says.

NSF supports Hersam with a $640,000 individual investigator grant awarded in 2010 for five years. Also, he directs Northwestern's Materials Research Science and Engineering Center (MRSEC), which NSF funds, including support for approximately 30 faculty members/researchers.

Hersam also is a recent recipient of one of this year's prestigious MacArthur fellowships, a $625,000 no-strings-attached award, popularly known as a "genius" grant. These go to talented individuals who have shown extraordinary originality and dedication in their fields, and are meant to encourage beneficiaries to freely explore their interests without fear of risk-taking.

"This will allow us to take more risks in our research, since there are no 'milestones' we have to meet," he says, referring to a frequent requirement of many funders. "I also have a strong interest in teaching, so I will use the funds to influence as many students as possible."

The carbon nanotubes separation process, which Hersam developed, begins with a centrifuge tube. Into that, "we load a water based solution and introduce an additive which allows us to tune the buoyant density of the solution itself," he explains.

"What we create is a gradient in the buoyant density of the aqueous solution, with low density at the top and high density at the bottom," he continues. "We then load the carbon nanotubes and put it into the centrifuge, which drives the nanotubes through the gradient. The nanotubes move through the gradient until their density matches that of the gradient. The result is that the nanotubes form separated bands in the centrifuge tube by density. Since the density of the nanotube is a function of its diameter, this method allows separation by diameter."

One property that distinguishes these materials from traditional semiconductors like silicon is that they are mechanically flexible. "Carbon nanotubes are highly resilient," Hersam says. "That allows us to integrate electronics on flexible substrates, like clothing, shoes, and wrist bands for real time monitoring of biomedical diagnostics and athletic performance. These materials have the right combination of properties to realize wearable electronics."

He and his colleagues also are working on energy technologies, such as solar cells and batteries "that can improve efficiency and reduce the cost of solar cells, and increase the capacity and reduce the charging time of batteries," he says. "The resulting batteries and solar cells are also mechanically flexible, and thus can be integrated with flexible electronics."

They likely even will prove waterproof. "It turns out that carbon nanomaterials are hydrophobic, so water will roll right off of them," he says.

Materials at the nanometer scale now "can realize new properties and combinations of properties that are unprecedented," he adds. "This will not only improve current technologies, but enable new technologies in the future."

-- Marlene Cimons, National Science Foundation
Investigators
Mark Hersam
Monica Olvera
Related Institutions/Organizations
Northwestern University

NSF FUNDS SCIENTIST STUDYING USE OF LIQUID METALS IN ELECTRONICS

FROM:  NATIONAL SCIENCE FOUNDATION 

Changing the shape and function of liquid metal

Researchers study gallium to design adjustable electronic components, including new types of antennas

Gallium is one of the few metals that turns into a liquid at room temperature. When that happens, its surface oxidizes, forming a "skin" over the fluid, almost like a water balloon or a water bed. Years ago, scientists often thought the coating a nuisance. Today they consider it an opportunity.

"We are trying to flip conventional wisdom on its head," says Michael Dickey, associate professor of chemical and bio-molecular engineering at North Carolina State University. "We are taking an old material and using it in a new way."

The National Science Foundation (NSF)-funded scientist is exploring ways to manipulate and modify this liquid metal in order to mold it into functional structures, in electronics, for example, that result in soft, flexible and stretchable and reconfigurable components, such as antennas.

One potential application could be to find a way to put the liquid gallium (actually an alloy of gallium and indium, the latter keeps the gallium from freezing) into already stretchable material in order to provide conductivity.

"If you put aluminum into a rubber band, it will behave mechanically like aluminum," Dickey explains. "But if you put liquid metal into a rubber band, you have the metal conductivity, which you want, but you still have the properties of the rubber band. We are taking advantage of the fact that this metal forms this oxide layer in order to control its shape."

By better understanding the mechanical properties of the liquid gallium, including learning whether it is possible to modify the "skin" itself--by making it stronger or even weaker, for example--it might be possible to design electronic components that can be "adjustable," that is, that can alter their functions as needed, he says.

"If you can change the shape, you can change the function," he says, adding that new types of antennas with this kind of flexibility could transform smartphones, navigation systems and Wi-Fi. "We could develop potentially better antennas for cell phones that can respond to changing conditions."

Other applications could include wearable items, such as watches or medical devices, or in the field of soft robotics. "Most robots that work in factories are made out of stiff materials and aren't good interfacing with humans," he says. "Taken to the extreme, imagine an octopus with large freedom of motion that could perform delicate tasks."

In medicine, "we could see embedded electronics in gloves that a doctor or lab technician might wear, such as feedback sensors or prosthetics, and the doctor wouldn't even know that they were there," he adds.

Dickey is conducting his research under an NSF Faculty Early Career Development (CAREER) award, which he received in 2010. The award supports junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of education and research within the context of the mission of their organization.

"We're trying to understand what is happening at the surface of the metal," he says. "I look at this metal as being like a composite material. It's a metal that forms this oxide on its surface, and the combination is interesting to me."

He and his team are trying to characterize the mechanical properties of the liquid metal, and "how to harness them," he says. "Also, we've been doing some chemical characterization in order to understand what is actually on the surface, and how we can modify it. And we are trying to reconfigure the shape of the metal."

His team is studying the way the metal flows in response to pressure.

"In a sense, the metal behaves like ketchup," he says. "It only flows when the skin breaks, due to application of a critical pressure."

The team harnessed this property to print the metal into freestanding 3D shapes despite the metal being a liquid.

As part of the grant's educational component, the research will be integrated into a new course at NC State for both undergraduate and graduate engineering students. Also, the researchers developed an interactive module that discusses the work within the context of popular movies designed to attract middle school minority children to higher education and careers in science. Among other things, they participate in a summer camp for about 200 middle school youngsters, "where we talk about the research, liquid metal and different fluids."

In preparing for this presentation, in fact, Dickey spoke at length to Gene Warren Jr., who created the special effects for Terminator 2: Judgment Day, in which "the bad guy turns into liquid metal, and then reassembles," Dickey says. "He said that they used liquid metal to do it. We show the kids a clip from the movie and they really get a kick out of it."

-- Marlene Cimons, National Science Foundation
Investigators
Michael Dickey
John Lach
John Muth
Veena Misra
Thomas Jackson
Shekhar Bhansali
Related Institutions/Organizations
North Carolina State University
Related Programs
Engineering Research Centers

3-D CHIPS COULD EXPAND MICROPROCESSOR CAPACITY

FROM:  NATIONAL SCIENCE FOUNDATION 
Scientist developing 3-D chips to expand capacity of microprocessors

Novel design would consume less power and provide higher performance
Many researchers in the field of integrated circuits worry that the fast paced progress of "miniaturization" will start to slow unless they find new ways to expand the capacity of the conventional two-dimensional chips used today in virtually all electronics.

Emre Salman, an assistant professor of computer and electrical engineering at Stony Brook University, is trying to design new technology, circuits and algorithms for the next generation of microprocessors, mobile computing devices and communication chips, in order "to overcome the fundamental limitations of current electronic systems, such as high power consumption," he says.

Specifically, the National Science Foundation (NSF)-funded scientist is working on developing three-dimensional integration, an emerging technology that would vertically stack multiple wafers, a technique with the potential to enhance significantly the capability of the current two-dimensional chips.

"Today's typical electronic system on a circuit board consists of multiple chips connected with wires that are at the millimeter and centimeter scale," he explains. "These bulky connections not only slow down the circuit, but also consume power and reduce the reliability of the system."

In 3-D integration technology, on the other hand, those discrete chips, now called tiers, are stacked on top of each other before they are packaged, he says. "The entire 3-D system is placed in a single package," he says. "Vertical connections that achieve communication among the tiers are now in the micrometer scale, and getting even shorter with advances in 3-D manufacturing technology, thereby consuming less power and providing more performance. Essentially, 3-D technology enables higher and heterogeneous integration at a smaller form factor."

This goal, however, faces any number of challenges. "This expansion comes with a variety of difficulties," says Salman, who also directs Stony Brook's Nanoscale Circuits and Systems (NanoCAS) Laboratory. "For example, it is highly challenging to ensure that the diverse planes of a 3-D chip work in harmony as a single entity."

He points out that many scientists have been working on wafer level 3-D integration for more than a decade. However, "the primary emphasis has been on high performance and somewhat homogeneous chips, such as microprocessors," he says.

On the other hand, citing the 2011 edition of the International Technology Roadmap for Semiconductors (ITRS), an important guide for researchers in the field, "the third phase and long term application of 3-D technology includes highly heterogeneous integration, where sensing and communication planes are stacked with conventional data processing and memory planes," he says.

This means that a single 3-D chip will be able detect data from environment, then process and store this data using advanced algorithms, and then wirelessly transmit these data to a remote center, he says.

Unlike the dominant existing research, this relatively long term application has become his team's primary focus, an approach with the potential to enlarge the three-dimensional domain from high performance computing to relatively low power systems-on-chip (SoCs). These low power SoCs have capabilities beyond the boundaries of traditional general purpose processors, since they integrate multiple functions, including sensing, processing, storage and communication into a single 3-D chip, he says.

"Numerous applications exist in health care, energy efficient mobile computing, and environmental control, since a smaller form factor can be achieved at lower power while offering significant computing resources," he says. "Our fundamental objective is to develop a reliable 3-D analysis and design platform for these applications which will host future electronics systems that are increasingly more portable, can interact with the environment, consume low power, yet still offer significant computing capability."

He is conducting his research under an NSF Faculty Early Career Development (CAREER) award. The award supports junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of education and research within the context of the mission of their organization. NSF is funding his work with $453,809 over five years.

"We are developing design methodologies to reliably distribute power to each tier of a 3-D chip," he says. "We are also exploring novel circuit topologies for 3-D power management, thereby increasing energy efficiency. We are investigating various noise coupling paths within a 3-D system, and finding ways to protect sensitive transistors from noise. All of these activities serve for the common goal of improving power, signal and sensing integrity of a heterogeneous 3-D chip."

At the NanoCAS lab, their workstations are equipped with the latest electronic design automation software that allows the researchers to verify their algorithms, models and design methodologies. "We primarily rely on these state-of-the-art IC simulation tools that the semiconductor industry uses to design and verify their chips," he says.

As part of the grant's educational component, Salman plans to integrate these research activities at the secondary, undergraduate and graduate levels, and will involve the NanoCAS lab in an engineering summer camp for high-school students organized at Stony Brook. The program, organized jointly by the department of electrical and computer engineering and the student branch of IEEE (the Institute of Electrical and Electronics Engineers, a world-wide professional association) at Stony Brook, consists of a two-week residential camp at the university campus.

"While the primary goal is to introduce motivated high school students to the field of electrical engineering through theoretical classes and hands-on projects, the students also have an opportunity to learn and experience the university campus life," Salman says.

"At the NanoCAS lab, we offer an experimental course on fiber optic communication," he adds. "The course starts with an interesting history of communication technologies from prehistoric times to modern day. The students then learn the fundamentals of optical voice link and design their own communication link by soldering electronic components. We believe that an efficient link between education and research is essential for the advancement of science and technology to prevail."