Assessing the environmental and public health impacts of the Deepwater Horizon oil spill is difficult owing to the extreme depth of the blowout and the large volumes of oil released. One strategy for remediation of the plume is to use the intrinsic bioremediation potential of deep-sea microorganisms to degrade the oil. This strategy depends on a number of environmental factors, including a favorable response of indigenous microorganisms to an increased concentration of hydrocarbons and/or dispersant. To study the effects of the spill, researchers collected deep-water samples from across the Gulf of Mexico and analyzed their physical, chemical, and microbiological properties using a variety of techniques, including synchrotron radiation Fourier-transform infrared (SR-FTIR) spectroscopy at ALS Beamline1.4.3. The studies suggest that the plume did indeed stimulate indigenous deep-sea bacteria that are closely related to known petroleum degraders.
The oil slick in the Gulf of Mexico as seen from a helicopter (photo by Rick Loomis, Los Angeles Times, May 6, 2010).
Seventeen deep-water samples were taken between May 25 and June 2, 2010, from across the Gulf of Mexico. The data indicated the presence of an oil plume from 1099 to 1219 m deep at distances of up to 10 km from the wellhead with a southwest current. The average temperature within the plume was 4.7°C and the pressure was 1136 dB. At most plume locations, a slight decrease in oxygen concentration was detected, indicative of microbial respiration and oxygen consumption, as would be expected if the hydrocarbons were being catabolized. Molecular biochemistry analyses revealed that the dispersed oil plume affected microbial cell densities and composition. Cell densities inside the plume were almost two times higher than outside the plume.
The Deepwater Horizon blowout in the Gulf of Mexico on April 20, 2010, resulted in the largest oil spill in the history of the United States. The biological effects and expected fate of the oil are unknown, partly due to the extreme depth and magnitude of this event and partly due to the primary initial mitigation strategy that injected unprecedented quantities of oil dispersant directly at the wellhead (1544 m below the sea surface). Indigenous deep-sea microorganisms that degrade oil could represent a significant natural attenuation mechanism; but this would depend on how native microorganisms respond to an increased concentration of hydrocarbons and/or dispersant at such extreme depths and temperatures (~4°C). A collaboration led by Berkeley Lab researchers here reports that the dispersed hydrocarbon plume stimulated the growth of a type of bacteria that thrives in cold temperatures and at great depths. Infrared spectroscopy at the ALS, with the ability to study microbial processes at the molecular level, provided key pieces of the puzzle.
Read more: Molecular Measurements of the Deep-Sea Oil Plume in the Gulf of Mexico
Hoi-Ying Holman of the Lab’s Earth Sciences Division has won the 2010 David A. Shirley Award for Outstanding Scientific Achievement at the Advanced Light Source. Holman was recognized for her work “pioneering the study of living cells and their response to environmental stimuli using synchrotron-based FTIR (Fourier Transform Infrared) spectromocroscopy.”
The award was announced by David L. Osborn of the ALS’s Users Executive Committee, and was presented at the ALS Users' Meeting banquet, October 14, 2010.
Holman notes that much of the current quantitative understanding of cellular molecular reactions has come from traditional biochemistry experiments that are either averaged over large populations or performed in-vitro with purified bulk biomolecules. Although these approaches have clarified many detailed mechanisms, they are not sufficient to reveal the phenotypic heterogeneities that are known to be present even in a genetically homogeneous population—and that are important in fields ranging from ecology to pathogenesis. The challenge has been to identify those cells of ecological or medical importance within a large population and then track their biochemical reactions in situ in real-time. By coupling infrared (IR) rays from a synchrotron to microscope environmental platforms, Holman has developed a robust and label-free approach to probe the chemical underpinnings of microbiological processes, which enables high-throughput, non-invasive spectroscopic microanalysis.
August 24, 2010
In the aftermath of the explosion of BP’s Deepwater Horizon drilling rig in the Gulf of Mexico, a dispersed oil plume was formed at a depth between 3,600 and 4,000 feet and extending some 10 miles out from the wellhead. An intensive study by scientists with the Lawrence Berkeley National Laboratory (Berkeley Lab) and the Advanced Light Source found that microbial activity, spearheaded by a new and unclassified species, degrades oil much faster than anticipated. This degradation appears to take place without a significant level of oxygen depletion.
||Microbes are degrading oil in the deepwater plume from the BP oil spill in the Gulf, a study by Berkeley Lab researchers has shown. (Image from Hoi-Ying Holman group)
“Our findings show that the influx of oil profoundly altered the microbial community by significantly stimulating deep-sea psychrophilic (cold temperature) gamma-proteobacteria that are closely related to known petroleum-degrading microbes,” says Terry Hazen, a microbial ecologist with Berkeley Lab’s Earth Sciences Division and principal investigator with the Energy Biosciences Institute, who led this study. “This enrichment of psychrophilic petroleum degraders with their rapid oil biodegradation rates appears to be one of the major mechanisms behind the rapid decline of the deepwater dispersed oil plume that has been observed.”
The uncontrolled oil blowout in the Gulf of Mexico from BP’s deepwater well was the deepest and one of the largest oil leaks in history. The extreme depths in the water column and the magnitude of this event posed a great many questions. In addition, to prevent large amounts of the highly flammable Gulf light crude from reaching the surface, BP deployed an unprecedented quantity of the commercial oil dispersant COREXIT 9500 at the wellhead, creating a plume of micron-sized petroleum particles. Although the environmental effects of COREXIT have been studied in surface water applications for more than a decade, its potential impact and effectiveness in the deep waters of the Gulf marine ecosystem were unknown.
||Analysis with Berkeley Lab’s phyloChip revealed the dominant microbe in the dispersed Gulf of Mexico oil plume was a new species, closely related to members of Oceanospirillales family. (Image from Terry Hazen group)
Analysis by Hazen and his colleagues of microbial genes in the dispersed oil plume revealed a variety of hydrocarbon-degraders, some of which were strongly correlated with the concentration changes of various oil contaminants. Analysis of changes in the oil composition as the plume extended from the wellhead pointed to faster than expected biodegradation rates with the half-life of alkanes ranging from 1.2 to 6.1 days.
“Our findings, which provide the first data ever on microbial activity from a deepwater dispersed oil plume, suggest that a great potential for intrinsic bioremediation of oil plumes exists in the deep-sea,” Hazen says. “These findings also show that psychrophilic oil-degrading microbial populations and their associated microbial communities play a significant role in controlling the ultimate fates and consequences of deep-sea oil plumes in the Gulf of Mexico.”
The results of this research were reported in the journal Science (August 26, 2010 on-line) in a paper titled “Deep-sea oil plume enriches indigenous oil-degrading bacteria.” Co-authoring the paper with Hazen were Eric Dubinsky, Todd DeSantis, Gary Andersen, Yvette Piceno, Navjeet Singh, Janet Jansson, Alexander Probst, Sharon Borglin, Julian Fortney, William Stringfellow, Markus Bill, Mark Conrad, Lauren Tom, Krystle Chavarria, Thana Alusi, Regina Lamendella, Dominique Joyner, Chelsea Spier, Jacob Baelum, Manfred Auer, Marcin Zemla, Romy Chakraborty, Eric Sonnenthal, Patrik D’haeseleer, Hoi-Ying Holman, Shariff Osman, Zhenmei Lu, Joy Van Nostrand, Ye Deng, Jizhong Zhou and Olivia Mason.
||A team of Berkeley Lab researchers led by microbial ecologist Terry Hazen (seated in middle) determined the physical, chemical and microbiological properties of the BP deepwater oil plume in the Gulf of Mexico and found faster than expected biodegradation rates. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)
Hazen and his colleagues began their study on May 25, 2010. At that time, the deep reaches of the Gulf of Mexico were a relatively unexplored microbial habitat, where temperatures hover around 5 degrees Celsius, the pressure is enormous, and there is normally little carbon present.
“We deployed on two ships to determine the physical, chemical and microbiological properties of the deepwater oil plume,” Hazen says. “The oil escaping from the damaged wellhead represented an enormous carbon input to the water column ecosystem and while we suspected that hydrocarbon components in the oil could potentially serve as a carbon substrate for deep-sea microbes, scientific data was needed for informed decisions.”
Hazen, who has studied numerous oil-spill sites in the past, is the leader of the Ecology Department and Center for Environmental Biotechnology at Berkeley Lab’s Earth Sciences Division. He conducted this research under an existing grant he holds with the Energy Biosciences Institute (EBI) to study microbial enhanced hydrocarbon recovery. EBI is a partnership led by the University of California (UC) Berkeley and including Berkeley Lab and the University of Illinois that is funded by a $500 million, 10-year grant from BP.
Results in the Science paper are based on the analysis of more than 200 samples collected from 17 deepwater sites between May 25 and June 2, 2010. The infrared beamline at LBNL's Advanced Light Source was utilized in the analysis of samples using Fourier-transform infrared spectromicroscopy. Sample analysis was boosted by the use of the latest edition of the award-winning Berkeley Lab PhyloChip – a unique credit card-sized DNA-based microarray that can be used to quickly, accurately and comprehensively detect the presence of up to 50,000 different species of bacteria and archaea in a single sample from any environmental source, without the need of culturing. Use of the Phylochip enabled Hazen and his colleagues to determine that the dominant microbe in the oil plume is a new species, closely related to members of Oceanospirillales family, particularly Oleispirea antarctica and Oceaniserpentilla haliotis.
||Berkeley Lab researchers collected more than 200 samples from 17 deepwater sites around the damaged BP wellhead in the Gulf of Mexico between May 25 and June 2, 2010. (Image from Terry Hazen group)
Hazen and his colleagues attribute the faster than expected rates of oil biodegradation at the 5 degrees Celsius temperature in part to the nature of Gulf light crude, which contains a large volatile component that is more biodegradable. The use of the COREXIT dispersant may have also accelerated biodegradation because of the small size of the oil particles and the low overall concentrations of oil in the plume. In addition, frequent episodic oil leaks from natural seeps in the Gulf seabed may have led to adaptations over long periods of time by the deep-sea microbial community that speed up hydrocarbon degradation rates.
One of the concerns raised about microbial degradation of the oil in a deepwater plume is that the microbes would also be consuming large portions of oxygen in the plume, creating so-called “dead-zones” in the water column where life cannot be sustained. In their study, the Berkeley Lab researchers found that oxygen saturation outside the plume was 67-percent while within the plume it was 59-percent.
“The low concentrations of iron in seawater may have prevented oxygen concentrations dropping more precipitously from biodegradation demand on the petroleum, since many hydrocarbon-degrading enzymes have iron as a component,” Hazen says. “There’s not enough iron to form more of these enzymes, which would degrade the carbon faster but also consume more oxygen.”
Other news stories about this work: New York Times, New York Times, Discovery News, Washington Post, Ars technica, CBS, Wall Street Journal, MS-NBC, ABC, ScienceNow, SF Chronicle, National Public Radio, CNN, Physics Today, and the Associated Press.
This work was also presented to the National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling on September 27, 2010. The commission was established by President Obama in April. See the video on C-SPAN.
Researchers Image Zinnia Cells Down to Nanoscale; Potential Aid for Lignocellulosic Biofuel Production
|A xylem cell with fluorescent lignocellulose bands as the major feature. Click to enlarge.
A team from Lawrence Livermore National Laboratory led by Michael Thelen, in collaboration with researchers from Lawrence Berkeley National Lab and the National Renewable Energy Laboratory, has examined the chemical and structural organization of the plant cell wall in Zinnia elegans tracheary elements (TEs). Tracheary elements are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts.
Using three different microscopy methods (AFM, synchrotron radiation-based FTIR spectromicroscopy, and fluorescence microscopy using a cellulose-specific CBM) in conjunction with chemical extraction of wall components, the team was able to visualize single cells in detail down to the nanoscale, cellular substructures, fine-scale organization of the cell wall, and the chemical composition of these cells, indicating that they contain an abundance of lignocellulose.
The Zinnia TE culture system proved ideal for observing the structure and chemical composition of the cell wall because it comprises a single homogeneous cell type, representing a simpler system compared to plant tissues, which may contain multiple cell types. —Lacayo et al.
|Source: Lacayo et al. Click to enlarge.
In addition to providing insights in plant biology, such studies using Zinnia TEs could prove especially productive in assessing cell wall responses to enzymatic and microbial degradation, thus aiding current efforts in lignocellulosic biofuel production, the researchers suggested.
An open-access paper on their work was published 30 June in the journal Plant Physiology.
The leaves of Zinnia seedlings provide a rich source of single cells that are dark green with chloroplasts and can be cultured in liquid for several days at a time. During the culturing process, the cells change in shape to resemble the tube-like cells that carry water from roots to leaves. Known as xylem, these cells hold the bulk of cellulose and lignin in plants, which are both major targets of recent biofuel research.
The basic idea is that cellulose is a polymer of sugars, which if released by enzymes, can be converted into alcohols and other chemicals used in alternative fuel production. But for this to happen efficiently, we need to find ways to see how this is proceeding at several spatial scales. —Michael Thelen
|Image showing a substructure of the cell wall (ring), and the detailed organization of lignocellulose in the cell wall. Click to enlarge.
The polymers, collectively called lignocellulose, are very insoluble, resistant to common chemicals and mechanical breakage, and are a superior substance for providing strength and structure to plants.
The detailed three-dimensional molecular cell wall structure of plants remains poorly understood.
The capability to image plant cell surfaces at the nanometer scale, together with the corresponding chemical composition, could significantly enhance our understanding of cell wall molecular architecture. A high resolution structural model is crucial for the successful implementation of new approaches for conversion of biomass to liquid fuels. —Alex Malkin, a member of the LLNL team who is an expert in atomic force microscopy.
To make fuels from plant biomass requires a thorough understanding of the organization of cell walls before determining the best methods for cell wall deconstruction into its components. Catherine Lacayo, a postdoctoral scientist working with Thelen and Malkin, has taken the first steps toward a comprehensive approach.
She came up with techniques that reveal the inner structure of cell walls in these single xylem cells, which represent about 70% of the cellulose in plants that can be used in fuel processing.
The research is supported by the Department of Energy Genome Sciences Program through the Office of Biological and Environmental Research, and the DOE’s BioEnergy Research Centers in Emeryville and Oak Ridge. It will appear in the September issue of Plant Physiology.
Catherine I. Lacayo, Alexander J. Malkin, Hoi-Ying N. Holman, Liang Chen, Shi-You Ding, Mona S. Hwang and Michael P. Thelen (2010) Imaging Cell Wall Architecture in Single Zinnia elegans Tracheary Elements. Plant Physiologydoi: 10.1104/pp.110.155242
By Dan Krotz
One of nature’s most gripping feats of survival is now better understood. For the first time, scientists from the U.S. Department of Energy’s Lawrence Berkeley National Laboratory observed the chemical changes in individual cells that enable them to survive conditions that should kill them.
The team tracked the chemical changes in Desulfovibrio vulgaris, which is a single-cell bacterium that normally can only exist in an oxygen-free environment. They exposed the cells to the most hostile of conditions — air — and watched as some cells temporarily survived by initiating a well-orchestrated sequence of chemical events.
A survivor's tale revealed for the first time: Infrared spectromicroscopy analyses of D. vulgaris during oxygen-stress adaptive response depict a well-orchestrated series of chemical changes, such as spikes in levels of metabolites and hydrated sulfur clusters. The graph tracks real-time spectral changes of D. vulgaris as they transition from an oxygen-free to oxygenated environment.
Until now, scientists have not been able to monitor, at a molecular level, the chemical changes in individual cells as they survive extreme conditions. The ability to watch this Herculean adaptation to stress, from such an up-close and real-time vantage, gives scientists an improved way to study adaptive responses in a range of microbes, such as disease-causing pathogens and microbes that play a role in photosynthesis, energy production, and geochemical phenomena. Their work was recently published online in the journal Proceedings of the National Academy of Sciences.
“We can now follow chemical changes in living bacteria as they respond to extreme environments. This opens up a new window into how bacteria adapt and carry out some of life’s most important processes,” says Hoi-Ying Holman, a staff scientist in Berkeley Lab’s Earth Sciences Division.
To achieve this milestone, the team used the Advanced Light Source, a synchrotron and national user facility located at Berkeley Lab that generates bright light for scientific research. In a pioneering approach, they used a beamline equipped with a high-resolution infrared microscope to detect the molecular signatures of a cell’s biochemical and metabolic activity, such as spikes in levels of free radicals and organic acids.
Specifically, the infrared microscope tracks the instantaneous response of hydrogen bond structures in cellular water as their immediate surroundings change. The spectral measurements indicate changes in hydrogen bonds, which in turn indicate changes in the presence of ions and various molecules such as radicals and metabolites.
First, the team exposed D. vulgaris to their much-preferred oxygen-free environment. They then exposed the same group of cells to air and monitored them for several hours.
The foreign environment proved too much for many cells; they died due to a toxic accumulation of free radicals. But some cells with adequate stores of energy survived.
Another look at survival. Infrared spectromicroscopy reveals major chemical changes within cells, whereas the above video, obtained using visible-light imaging, only shows a slight change in cellular morphology: Air-exposed bacteria are slightly larger, and they cannot complete their cell division as indicated by their very elongated or chain-like shape.
And for the first time, thanks to the high-sensitivity synchrotron infrared beamline, the scientists watched as the surviving cells unleashed a series of metabolic changes that enabled them to endure in the presence of oxygen, like a fish out of water.
“We monitored several molecules simultaneously in the same bacteria, and watched their metabolic response to stress and extreme conditions,” says Holman. “We found that multiple chemical processes allow them to adapt.”
The scientists studied D. vulgaris because the bacterium, which is among a class of bacteria that reduce sulfate, plays a critical role in many important geochemical processes such as element and nutrient cycling in soils. It also assists in bioremediation and may someday be used to aid energy production and carbon sequestration efforts.
D. vulgaris also intrigues scientists because it is an obligate anaerobe — meaning it can’t survive in the presence of oxygen — yet it participates in many geochemical processes in which oxygen levels fluctuate. For example, it thrives in algae mats, which produce very high concentrations of oxygen during the day.
Scientists have puzzled over this riddle for years. They’ve studied the bacterium’s gene expression, which provides valuable clues to how it adapts. But, as Holman explains, scientists must also understand the ever-changing chemistry inside a cell in order to fully understand its gene expression and adaptive-response pathways.
Scientists have also teased out the chemical changes in the bacterium by studying different cells at various stages in a population of cells. This is not the same as studying the same cell over time, however.
“We are studying the same individuals as opposed to studying the population. We want to watch the same cells over time and not rely on the assumption that all cells within a population behave the same,” says Holman. “This work, which represents a new way to study adaptive response in individual cells, is made possible by the great progress we’ve made in synchrotron infrared cellular imaging.”
“Real-time molecular monitoring of chemical environment in obligate anaerobes during oxygen adaptive response” by Hoi-Ying Holman, Eleanor Wozei, Zhang Lin, Luis Comolli, David Ball, Sharon Borglin, Matthew Fields, Terry Hazen, and Kenneth Downing was published June 16, 2009 in the online early edition of the Proceedings of the National Academy of Sciences.
This work was supported by the U.S. Department of Energy Office of Biological and Environmental Research’s Structural Biology Program. The work is part of the Virtual Institute for Microbial Stress and Survival which is based at Berkeley Lab and supported by DOE’s Genomics Program: GTL.
A tunable graphene bandgap opens the way to nanoelectronics and nanophotonics
Graphene’s electrical properties include electrons so mobile they travel at near light speed. But without a bandgap, graphene’s promise for electronics and photonics can’t be realized. Now researchers can precisely tune a bandgap in bilayer graphene from zero to the infrared.
One of the most unusual features of single-layer graphene (top) is that its conical conduction and valence bands meet at a point – it has no bandgap. Symmetrical bilayer graphene (middle) also lacks a bandgap. Electrical fields (arrows) introduce asymmetry into the bilayer structure (bottom), yielding a bandgap (Δ) that can be selectively tuned.
BERKELEY, CA – Graphene is the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. But there’s a catch: graphene has no bandgap.
“Having no bandgap greatly limits graphene’s uses in electronics,” says Feng Wang of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, where he is a member of the Materials Sciences Division. “For one thing, you can build field-effect transistors with graphene, but if there’s no bandgap you can’t turn them off! If you could achieve a graphene bandgap, however, you should be able to make very good transistors.”
Wang, who is also an assistant professor in the Department of Physics at the University of California at Berkeley, has achieved just that. He and his colleagues have engineered a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts (250 meV, or .25 eV).
Moreover, their experiment was conducted at room temperature, requiring no refrigeration of the device. Among the applications made possible by this breakthrough are new kinds of nanotransistors and – because of its narrow bandgap – nano-LEDs and other nanoscale optical devices in the infrared range.
The researchers describe their work in the June 11 issue of Nature.
Constructing a bilayer graphene transistor
As with monolayer graphene, whose carbon atoms are arranged in “chickenwire” configuration, bilayer graphene – which consists of two graphene layers lying one on the other – also has a zero bandgap and thus behaves like a metal. But a bandgap can be introduced if the mirror-like symmetry of the two layers is disturbed; the material then behaves like a semiconductor.
Previously, in 2006, researchers at Berkeley Lab’s Advanced Light Source (ALS) observed a bandgap in bilayer graphene in which one of the layers was chemically doped by adsorbed metal atoms. But such chemical doping is uncontrolled and not compatible with device applications.
“Creating and especially controlling a bandgap in bilayer graphene has been an outstanding goal,” says Wang. “Unfortunately chemical doping is difficult to control.”
Researchers then tried to tune the bilayer graphene bandgap by doping the substrate electrically instead of chemically, using a perpendicularly applied, continuously tunable electrical field. But when such a field is applied with a single gate (electrode), the bilayer becomes insulating only at temperatures below one degree Kelvin, near absolute zero – suggesting a bandgap value much lower than predicted by theory.
Says Wang, “With these results it was hard to understand exactly what was happening electronically, or why.”
Wang and his colleagues made two key decisions that led to their successful attempt to introduce and determine a bandgap in bilayer graphene. The first was to build a two-gated bilayer device, fabricated by Yuanbo Zhang and Tsung-Ta Tang of the UC Berkeley Department of Physics, which allowed the team to independently adjust the electronic bandgap and the charge doping.
The device was a dual-gated field-effect transistor (FET), a type of transistor that controls the flow of electrons from a source to a drain with electric fields shaped by the gate electrodes. Their nano-FET used a silicon substrate as the bottom gate, with a thin insulating layer of silicon dioxide between it and the stacked graphene layers. A transparent layer of aluminum oxide (sapphire) lay over the graphene bilayer; on top of that was the top gate, made of platinum.
At left, a microscope image looking down through the bilayer-graphene field-effect transistor. Diagram at right identifies the elements.
The other key decision the researchers made was to get a better grasp of what was really going on in the device as they varied the voltage. Rather than try to measure the bandgap by measuring the device’s electrical resistance, or transport, they decided to measure its optical transmission.
“The problem with transport measurements is that they are too sensitive to defects,” says Wang. “A tiny amount of impurity or defect doping can create a big change in the resistance of the graphene and mask the intrinsic behavior of the material. That’s why we decided to go with optical measurements at the Advanced Light Source.”
Using infrared beamline 1.4 at the ALS, under the direction of ALS physicist Michael Martin and Zhao Hao of the Earth Sciences Division, Wang and his colleagues were able to send a tight beam of synchrotron light, focused on the graphene layers, right through the device. As the researchers tuned the electrical fields by precisely varying the voltage of the gate electrodes, they were able to measure variations in the light absorbed by the gated graphene layers. The absorption peak in each spectrum provided a direct measurement of the bandgap at each gate voltage.
“In principle we could have used a tunable laser to measure the optical transmission, but the 1.4 beamline is very bright and can be focused down to the diffraction limit – an important consideration when the graphene-flake target is so small,” Wang says. “Also, compared to a laser, the beamline provides a wider range of frequencies all at once, so we don’t have to painstakingly tune to each absorption frequency we’re trying to measure.”
The malleable electronic structure of bilayer graphene
The results from the ALS measurements were obtained with relative ease and efficiency, and showed that by independently manipulating the voltage of the two gates, the researchers could control two important parameters, the size of the bandgap and the degree of doping of the graphene bilayer. In essence, they created a virtual semiconductor from a material that is not inherently a semiconductor at all.
In ordinary semiconductors, the gap between the conduction band (unoccupied by electrons) the valence band (occupied by electrons) is finite, and fixed by the crystalline structure of the material. In bilayer graphene, however, as Wang’s team demonstrated, the bandgap is variable and can be controlled by an electrical field. Although a pristine graphene bilayer has zero bandgap and conducts like a metal, a gated bilayer can have a bandgap as big as 250 milli-electron volts and behave like a semiconductor.
With precision control of its bandgap over a wide range, plus independent manipulation of its electronic states through electrical doping, dual-gated bilayer graphene becomes a remarkably flexible tool for nanoscale electronic devices.
Wang emphasizes that these first experiments are only the beginning. “The electrical performance of our demonstration device is still limited, and there are many routes to improvement, for example through extra measures to purify the substrate.”
Nevertheless, he says, “We’ve demonstrated that we can arbitrarily change the bandgap in bilayer graphene from zero to 250 milli-electron volts at room temperature, which is remarkable in itself and shows the potential of bilayer graphene for nanoelectronics. This is a narrower bandgap than common semiconductors like silicon or gallium arsenide, and it could enable new kinds of optoelectronic devices for generating, amplifying, and detecting infrared light.”
“Direct observation of a widely tunable bandgap in bilayer graphene,” by Yuanbo Zhang, Tsung-Ta Tang, Caglar Girit, Zhao Hao, Michael C. Martin, Alex Zettl, Michael F. Crommie, Y. Ron Shen, and Feng Wang, appears in the June 11, 2009 issue of Nature. Zhang, Tang, and Girit are members of UC Berkeley’s Department of Physics, in the groups of Professors Crommie, Shen, and Zettl respectively; Zettl, Crommie, and Shen are also members of Berkeley Lab’s Materials Sciences Division.
This work was supported by the U.S. Department of Energy’s Office of Science, Office of Basic Energy Sciences.
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