David Kirkpatrick

October 19, 2010

Mass producing graphene

News from the University of Houston:

University of Houston professor taking next step with graphene research

The 2010 Nobel Prize in Physics went to the two scientists who first isolated graphene, one-atom-thick crystals of graphite. Now, a researcher with the University of Houston Cullen College of Engineering is trying to develop a method to mass-produce this revolutionary material.

Graphene has several properties that make it different from literally everything else on Earth: it is the first two-dimensional material ever developed; the world’s thinnest and strongest material; the best conductor of heat ever found; a far better conductor of electricity than copper; it is virtually transparent; and is so dense that no gas can pass through it. These properties make graphene a game changer for everything from energy storage devices to flat device displays.

Most importantly, perhaps, is graphene’s potential as a replacement for silicon in computer chips. The properties of graphene would enable the historical growth in computing power to continue for decades to come.

To realize these benefits, though, a way to create plentiful, defect-free graphene must be developed. Qingkai Yu, an assistant research professor with the college’s department of electrical and computer engineering and the university’s Center for Advanced Materials, is developing methods to mass-produce such high-quality graphene.

Yu is using a technology known as chemical vapor deposition. During this process, he heats methane to around 1000 degrees Celsius, breaking the gas down into its building blocks of carbon and hydrogen atoms. The carbon atoms then attach to a metallic surface to form graphene.

“This approach could produce cheap, high-quality graphene on a large scale,” Yu said.

Yu first demonstrated the viability of chemical vapor deposition for graphene creation two years ago in a paper in the journal Applied Physics Letters. He has since continued working to perfect this method.

Yu’s initial research would often result in several layers of graphene stacked together on a nickel surface. He subsequently discovered the effectiveness of copper for graphene creation. Copper has since been adopted by graphene researchers worldwide.

Yu’s work is not finished. The single layers of graphene he is now able to create are formed out of multiple graphene crystals that join together as they grow. The places where these crystals combine, known as the grain boundaries, are defects that limit the usefulness of graphene, particularly as a replacement for silicon-based computer chips.

Yu is attempting to create large layers of graphene that form out of a single crystal.

“You can imagine how important this sort of graphene is,” said Yu. “Semiconductors became a multibillion-dollar industry based on single-crystal silicon and graphene is called the post-silicon-era material. So single-crystal graphene is the Holy Grail for the next age of semiconductors.”

 

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Yu is conducting his research in collaboration with UH Ph.D. students Wei Wu and Zhihua Su as well as postdoctoral researcher Zhihong Liu. These efforts have been supported by the National Science Foundation, the U.S. Department of Defense, the U.S. Department of Energy, SEMATECH and the UH Center for Advanced Materials.

 

September 2, 2010

Food for not so easy thought

Everyone thought the biggest threat from China was the sheer volume of Treasuries held by that nation and the potential stranglehold it has over the U.S. economy. Realistically that has never been a real issue because as such a heavy investor in the U.S. economy, China has a vested interest in our financial sector remaining strong.

Now squeezing us on manufacturing vital elements of computing and electronics by taking complete control over rare earth metals is a different angle of attack altogether. You know the U.S. government is taking this very seriously when it has both the energy department and the DoD on the job.

The release:

China’s monopoly on 17 key elements sets stage for supply crisis

China’s monopoly on the global supply of elements critical for production of computer hard disc drives, hybrid-electric cars, military weapons, and other key products — and its increasingly strict limits on exports — is setting the stage for a crisis in the United States. That’s the topic of the cover story of Chemical & Engineering News (C&EN), ACS’ weekly newsmagazine.

C&EN Senior Editor Mitch Jacoby and Contributing Editor Jessie Jiang explain that the situation involves a family of chemical elements that may soon start to live up to their name, the “rare earths.” China has virtually cornered the global market on them, and produces most of the world’s supply. Since 2005, China has been raising prices and restricting exports, most recently in 2010, fostering a potential supply crisis in the U.S.

The article describes how the U.S. is now responding to this emerging crisis. To boost supplies, for instance, plans are being developed to resume production at the largest U.S. rare-earth mine — Mountain Pass in southern California — which has been dormant since 2002. The U.S. Department of Energy and the Department of Defense are among the government agencies grappling with the problem.

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ARTICLE FOR IMMEDIATE RELEASE “Securing the Supply of Rare Earths”

This story is available at http://pubs.acs.org/cen/coverstory/88/8835cover.html

September 2, 2009

Magnetic graphene

Graphene news from Virginia Commonwealth University:

Researchers design new graphene-based, nano-material with magnetic properties

A possible pathway to simply synthesize ferromagnetic graphene

Ferromagnetic Graphone Sheet. Puru Jena/VCU.

An international team of researchers has designed a new graphite-based, magnetic nano-material that acts as a semiconductor and could help material scientists create the next generation of electronic devices like microchips.

The team of researchers from Virginia Commonwealth University; Peking University in Beijing, China; the Chinese Academy of Science in Shanghai, China; and Tohoku University in Sedai, Japan; used theoretical computer modeling to design the new material they called graphone, which is derived from an existing material known as graphene.

Graphene, created by scientists five years ago, is 200 times stronger than steel, its electrons are highly mobile and it has unique optical and transport properties. Some experts believe that graphene may be more versatile than carbon nanotubes, and the ability to make graphene magnetic adds to its potential for novel applications in spintronics. Spintronics is a process using electron spin to synthesize new devices for memory and data processing.

Although graphene’s properties can be significantly modified by introducing defects and by saturating with hydrogen, it has been very difficult for scientists to manipulate the structure to make it magnetic.

“The new material we are predicting – graphone – makes graphene magnetic simply by controlling the amount of hydrogen coverage – basically, how much hydrogen is put on graphene. It avoids previous difficulties associated with the synthesis of magnetic graphene,” said Puru Jena, Ph.D., distinguished professor in the VCU Department of Physics.

“There are many possibilities for engineering new functional materials simply by changing their composition and structure. Our findings may guide researchers in the future to discover this material in the laboratory and to explore its potential technological applications,” said Jena.

“One of the important impacts of this research is that semi-hydrogenation provides us a very unique way to tailor magnetism. The resulting ferromagnetic graphone sheet will have unprecedented possibilities for the applications of graphene-based materials,” said Qiang Sun, Ph.D., research associate professor with the VCU team.

The study appeared online Aug. 31 in the journal Nano Letters, a publication of the American Chemical Society. The work was supported by a grant from the National Natural Science Foundation of China, The National Science Foundation and by the U.S. Department of Energy. Read the article abstract here.

The first author of this paper is Jian Zhou, a Ph.D. student at Peking University. The other authors include Qian Wang, Ph.D., a research associate professor at VCU; Xiaoshuan Chen, Ph.D., a professor at the Shanghai Institute of Technical Physics; and Yoshiyuki Kawazoe, Ph.D.,  a professor at Tohoku University.

About VCU and the VCU Medical Center:


Virginia Commonwealth University is the largest university in Virginia with national and international rankings in sponsored research. Located on two downtown campuses in Richmond, VCU enrolls 32,000 students in 205 certificate and degree programs in the arts, sciences and humanities. Sixty-five of the programs are unique in Virginia, many of them crossing the disciplines of VCU’s 15 schools and one college. MCV Hospitals and the health sciences schools of Virginia Commonwealth University compose the VCU Medical Center, one of the nation’s leading academic medical centers. For more, see www.vcu.edu.

August 6, 2009

Practical solar power

This release is really more of an article on making solar power practical than it is an announcement of news. It’s an interesting read on solar.

The release:

Bringing solar power to the masses

On a 104-degree Friday in July when sunlight bathed The University of Arizona campus, doctoral student Dio Placencia sat before a noisy vacuum chamber in the Chemical Sciences Building trying to advance the renewable energy revolution.

As a member of UA professor Neal R. Armstrong’s research group, Placencia conducts research aimed at creating a thin, flexible organic solar cell that could power a tent or keep a car charged between trips to work and back home again.

He’s passionate about renewable energy and says it’s a waste that so little solar has been incorporated into society. “I have a little flat panel that I walk around with,” Placencia said. “I usually put that on my backpack, and I charge my cell phone when I’m walking to school.”

The sun is clean and free. “Here it is,” he said. “Why not use it?”

Across the University, professors, researchers, students and others involved in policy planning and economic analysis are working to make that question moot. In a region noted for abundant sunlight, they are chipping away at problems like how to employ solar at the utility-generating plant level, how to harness it to charge the newly indispensable products of the day – cell phones, MP3 players, laptops – what to do at night and when clouds halt the energy giveaway from the sky.

The research proceeds in labs amid state-of-the-art equipment funded by multimillion-dollar federal grants. It’s the product of students’ hunches and long careers spent unlocking the mysteries of science. Along the way, students are being immersed in a nascent industry that many hope will be the economic engine of the next decade.

“Looking at renewable energy is a perfect place to emphasize that we don’t know where the next breakthrough is going to be,” said Leslie P. Tolbert, UA vice president for research, graduate studies and economic development. “Somewhere in a lab someplace, there’s somebody figuring out a whole new way to capture sunlight. In fact, there are many people doing that. And even they are depending on knowing that there is, behind them, a cadre of basic science researchers producing new information that will feed their thoughts.”

Armstrong, a professor of chemistry and optical sciences at the UA, occasionally teaches freshman chemistry. He decided one day near the end of the semester to try to make the material even more relevant. “I said to myself, well, lithium ion batteries in my cell phone, in my iPod,” – his daughter had given him one – “I wonder how much coal we burn to charge those guys up at the end of the day. Because that’s one of the big drivers for portable power, to get all this stuff off the grid.” After making some very conservative calculations, he arrived at an answer, which he shared with the class: “You burn about a quarter of a pound of coal per charge of your lithium ion battery, and you generate about half a pound of CO2 per charge, per battery, per day …. The room got really quiet.”

The next time, he intends to calculate how much coal is burned per Twitter tweet.

“It really is chilling,” Armstrong said. “You start doing the math and thinking about the number of consumer electronic devices that you and I have added to our lives in the last decade that I charge up typically once every night – my laptop computer and my cell phone. Then you start thinking about, ‘What if I do buy an electric car, and I come home at night and plug that sucker in,’ and you do the same thing. We’ll shut this grid down in no time.”

In April, the U.S. Department of Energy announced it was funding Armstrong’s Center for Interface Science as one of 46 Energy Frontier Research Centers. The mission of these centers, which will receive $2 million to $5 million a year for five years, is “to address current fundamental scientific roadblocks to clean energy and energy security,” according to the DOE.

Ever since Armstrong was a graduate student during the first Arab oil embargo in 1973, he’s experienced a succession of government distress calls over energy. One such emergency led him to discover the work of Heinz Gerischer and Frank Willig in Germany. They had figured out how to adsorb dye molecules to the surface of oxides and split water with light from the sun. “I thought, ‘That’s it. That’s what I’m going to do my career on.'”

He moved to the UA in 1978, attracted by a program in photo-thermal solar energy conversion. In the 1980s, with gas cheap and plentiful again, solar went back on the back burner.

The next call came about four years ago. “DOE was beginning to sense that the tides were about to shift again, big-time,” Armstrong said. “And they were really concerned that they didn’t know what to do – how to present this to Congress in a way that would lead to new funding and which would have a rationale associated with it so that by the middle of this century we had someplace to go.”

Armstrong realized it was time to come back to the problem that he wanted to work on 30 years before. “This time, we were really well-equipped,” he said. “We’ve learned how to image molecules at the molecular level, we’ve learned how to measure energies of incredibly thin films, we’ve learned how to make devices, we’ve collaborated with physicists and material sciences and that sort of thing, we’ve done a lot of interesting other stuff and I suddenly realized I could bring it all back together here.”

In his office, he displays a sample of his work: a 1-inch square of glass on which is deposited a thin film of indium tin oxide, a conducting transparent oxide commonly found in display technologies like computer screens. On top of that is a thin film of organic dyes. The last layer is an aluminum electrode.

“You’d have a roll of plastic with these cells laid out on it,” he explained. “The idea is for you to go to Target or something like that and buy this roll of plastic and roll it out. It’s got two wires connected to it, and you plug in your battery or your laptop and charge it up.”

“The grand total in terms of the thickness is about 400 nanometers, which is one ten-thousandth the thickness of a human hair. And yet, shine a light on it and you get electricity out of it. Now we’d like it to be a bit thicker. We have to keep them thin in order to get all of the electrical charge out of the device. But if you think about this as a sandwich structure, we’ve made this incredibly thin sandwich and then each of the layers in contact with each other have to be just right in terms of the chemical composition, the orientation of the molecules, how well they adhere to each of the underlying surfaces. And if I go in and change just one molecule layer, the composition – that’s at the level of 1 nanometer in thickness – I can take a good device and turn it into a bad device; I can take a bad device and turn it into a good device. That’s the kind of level of control that we need. And we don’t fully understand it.”

But the equipment available now – optical microscopes capable of imaging individual molecules and revealing their electrical properties and spatial orientation – are helping his team understand. His goal is to figure out how to have the molecules arrange themselves – every time – in a way to produce lots of electricity. “They have to all line up like little soldiers,” he said.

“We have to give you a technology that is going to look like an ink, like a blue ink, that you can spray down on one of these surfaces and the molecules at the nanometer level are going to say, ‘OK, we’re going to get organized this way,’ and in doing so, when I put that top electrode on and shine a light, I’ll get lots and lots of electricity out of there,” Armstrong said.

A high vacuum photoelectron spectrometer allows them to build each molecular layer, moving it within the vacuum to study it, and then continue with another molecular layer. Other tools, like a silicon microtip, which looks like a tiny phonograph needle, can be positioned to +/- 0.01 nanometers. “Well inside the diameter of a molecule,” he said. Bouncing a laser off the back of the tip yields an image. Passing current through the tip, they can map the electrical properties of molecules. All this can help them build a template to create the ideal array of the molecule assemblies.

Erin Ratcliff joined the team as a postdoctoral electrochemist with a doctorate from Iowa State. “My background wasn’t in solar cells at all,” she said. “I had to come here and had to learn everything, where grad students get it from Day One at the UA.”

She spoke of the business school curve, resembling a hockey stick, when progress begins to accelerate rapidly. “We’re right at the magic moment when the hockey stick starts to take off, when you go from flat to hockey stick. We’re right there. It’s exciting to read the literature and hope that, yes, we will take off. It will be exciting to look back and say ‘I was there for that.'”

 

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June 12, 2009

Graphene and tunable semiconductors

A double dose of graphene news for tonight.

The release:

Tunable semiconductors possible with hot new material called graphene

Tunable bandgap means tunable transistors, LEDs and lasers

Berkeley — Today’s transistors and light emitting diodes (LED) are based on silicon and gallium arsenide semiconductors, which have fixed electronic and optical properties.

Now, University of California, Berkeley, researchers have shown that a form of carbon called graphene has an electronic structure that can be controlled by an electrical field, an effect that can be exploited to make tunable electronic and photonic devices.

While such properties were predicted for a double layer of graphene, this is the first demonstration that bilayer graphene exhibits an electric field-induced, broadly tunable bandgap, according to principal author Feng Wang, UC Berkeley assistant professor of physics.

The bandgap of a material is the energy difference between electrons residing in the two most important states of a material – valence band states and conduction band states – and it determines the electrical and optical properties of the material.

“The real breakthrough in materials science is that for the first time you can use an electric field to close the bandgap and open the bandgap. No other material can do this, only bilayer graphene,” Wang said.

Because tuning the bandgap of bilayer graphene can turn it from a metal into a semiconductor, a single millimeter-square sheet of bilayer graphene could potentially hold millions of differently tuned electronic devices that can be reconfigured at will, he said.

Wang, post-doctoral fellow Yuanbo Zhang, graduate student Tsung-Ta Tang and their UC Berkeley and Lawrence Berkeley National Laboratory (LBNL) colleagues report their success in the June 11 issue of Nature.

“The fundamental difference between a metal and a semiconductor is this bandgap, which allows us to create semiconducting devices,” said coauthor Michael Crommie, UC Berkeley professor of physics. “The ability to simply put a material between two electrodes, apply an electric field and change the bandgap is a huge deal and a major advance in condensed matter physics, because it means that in a device configuration we can change the bandgap on the fly by sending an electrical signal to the material.”

Graphene is a sheet of carbon atoms, each atom chemically bonded to its three neighbors to produce a hexagonal array that looks a lot like chicken wire. Since it was first isolated from graphite, the material in pencil lead, in 2004, it has been a hot topic of research, in part because solid state theory predicts unusual electronic properties, including a high electron mobility more than 10 times that of silicon.

However, the property that makes it a good conductor – its zero bandgap – also means that it’s always on.

“To make any electronic device, like a transistor, you need to be able to turn it on or off,” Zhang said. “But in graphene, though you have high electron mobility and you can modulate the conductance, you can’t turn it off to make an effective transistor.”

Semiconductors, for example, can be turned off because of a finite bandgap between the valence and conduction electron bands.

While a single layer of graphene has a zero bandgap, two layers of graphene together theoretically should have a variable bandgap controlled by an electrical field, Wang said. Previous experiments on bilayer graphene, however, have failed to demonstrate the predicted bandgap structure, possibly because of impurities. Researchers obtain graphene with a very low-tech method: They take graphite, like that in pencil lead, smear it over a surface, cover with Scotch tape and rip it off. The tape shears the graphite, which is just billions of layers of graphene, to produce single- as well as multi-layered graphene.

Wang, Zhang, Tang and their colleagues decided to construct bilayer graphene with two voltage gates instead of one. When the gate electrodes were attached to the top and bottom of the bilayer and electrical connections (a source and drain) made at the edges of the bilayer sheets, the researchers were able to open up and tune a bandgap merely by varying the gating voltages.

The team also showed that it can change another critical property of graphene, its Fermi energy, that is, the maximum energy of occupied electron states, which controls the electron density in the material.

“With top and bottom gates on bilayer graphene, you can independently control the two most important parameters in a semiconductor: You can change the electronic structure to vary the bandgap continuously, and independently control electron doping by varying the Fermi level,” Wang said.

Because of charge impurities and defects in current devices, the graphene’s electronic properties do not reflect the intrinsic graphene properties. Instead, the researchers took advantage of the optical properties of bandgap materials: If you shine light of just the right color on the material, valence electrons will absorb the light and jump over the bandgap.

In the case of graphene, the maximum bandgap the researchers could produce was 250 milli-electron volts (meV). (In comparison, the semiconductors germanium and silicon have about 740 and 1,200 meV bandgaps, respectively.) Putting the bilayer graphene in a high intensity infrared beam produced by LBNL’s Advanced Light Source (ALS), the researchers saw absorption at the predicted bandgap energies, confirming its tunability.

Because the zero to 250 meV bandgap range allows graphene to be tuned continuously from a metal to a semiconductor, the researchers foresee turning a single sheet of bilayer graphene into a dynamic integrated electronic device with millions of gates deposited on the top and bottom.

“All you need is just a bunch of gates at all positions, and you can change any location to be either a metal or a semiconductor, that is, either a lead to conduct electrons or a transistor,” Zhang said. “So basically, you don’t fabricate any circuit to begin with, and then by applying gate voltages, you can achieve any circuit you want. This gives you extreme flexibility.”

“That would be the dream in the future,” Wang said.

Depending on the lithography technique used, the size of each gate could be much smaller than one micron – a millionth of a meter – allowing millions of separate electronic devices on a millimeter-square piece of bilayer graphene.

Wang and Zhang also foresee optical applications, because the zero-250 meV bandgap means graphene LEDs would emit frequencies anywhere in the far- to mid-infrared range. Ultimately, it could even be used for lasing materials generating light at frequencies from the terahertz to the infrared.

“It is very difficult to find materials that generate light in the infrared, not to mention a tunable light source,” Wang said.

Crommie noted, too, that solid state physicists will have a field day studying the unusual properties of bilayer graphene. For one thing, electrons in monolayer graphene appear to behave as if they have no mass and move like particles of light – photons. In tunable bilayer graphene, the electrons suddenly act as if they have masses that vary with the bandgap.

“This is not just a technological advance, it also opens the door to some really new and potentially interesting physics,” Crommie said.

 

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Wang, Zhang, Tang and their colleagues continue to explore graphene’s electronic properties and possible electronic devices.

Their coauthors are Crommie, Alex Zettl and Y. Ron Shen, UC Berkeley professors of physics; physics post-doctoral fellow Caglar Girit; and Zhao Hao and Michael C. Martin of LBNL’s ALS Division. Zhang is a Miller Post-doctoral Fellow at UC Berkeley.

The work was supported by the U.S. Department of Energy.