Physics, photosynthesis and ‘Green’ solar cells

In a light harvesting quantum photocell, particles of light (photons) can efficiently generate electrons. When two absorbing channels are used, solar power entering the system through the two absorbers (a and b) efficiently generates power in the machine (M). Credit: Nathaniel Gabor and Tamar Melen

A University of California, Riverside assistant professor has combined photosynthesis and physics to make a key discovery that could help make solar cells more efficient. The findings were recently published in the journal Nano Letters.

Nathan Gabor is focused on experimental condensed matter physics, and uses light to probe the fundamental laws of quantum mechanics. But, he got interested in photosynthesis when a question popped into his head in 2010: Why are plants green? He soon discovered that no one really knows.

During the past six years, he sought to help change that by combining his background in physics with a deep dive into biology.

He set out to re-think solar energy conversion by asking the question: can we make materials for solar cells that more efficiently absorb the fluctuating amount of energy from the sun. Plants have evolved to do this, but current affordable solar cells — which are at best 20 percent efficient — do not control these sudden changes in solar power, Gabor said. That results in a lot of wasted energy and helps prevent wide-scale adoption of solar cells as an energy source.

Gabor, and several other UC Riverside physicists, addressed the problem by designing a new type of quantum heat engine photocell, which helps manipulate the flow of energy in solar cells. The design incorporates a heat engine photocell that absorbs photons from the sun and converts the photon energy into electricity.

Surprisingly, the researchers found that the quantum heat engine photocell could regulate solar power conversion without requiring active feedback or adaptive control mechanisms. In conventional photovoltaic technology, which is used on rooftops and solar farms today, fluctuations in solar power must be suppressed by voltage converters and feedback controllers, which dramatically reduce the overall efficiency.

The goal of the UC Riverside teams was to design the simplest photocell that matches the amount of solar power from the sun as close as possible to the average power demand and to suppress energy fluctuations to avoid the accumulation of excess energy.

The researchers compared the two simplest quantum mechanical photocell systems: one in which the photocell absorbed only a single color of light, and the other in which the photocell absorbed two colors. They found that by simply incorporating two photon-absorbing channels, rather than only one, the regulation of energy flow emerges naturally within the photocell.

The basic operating principle is that one channel absorbs at a wavelength for which the average input power is high, while the other absorbs at low power. The photocell switches between high and low power to convert varying levels of solar power into a steady-state output.

When Gabor’s team applied these simple models to the measured solar spectrum on Earth’s surface, they discovered that the absorption of green light, the most radiant portion of the solar power spectrum per unit wavelength, provides no regulatory benefit and should therefore be avoided. They systematically optimized the photocell parameters to reduce solar energy fluctuations, and found that the absorption spectrum looks nearly identical to the absorption spectrum observed in photosynthetic green plants.

The findings led the researchers to propose that natural regulation of energy they found in the quantum heat engine photocell may play a critical role in the photosynthesis in plants, perhaps explaining the predominance of green plants on Earth.

Other researchers have recently found that several molecular structures in plants, including chlorophyll a and b molecules, could be critical in preventing the accumulation of excess energy in plants, which could kill them. The UC Riverside researchers found that the molecular structure of the quantum heat engine photocell they studied is very similar to the structure of photosynthetic molecules that incorporate pairs of chlorophyll.

The hypothesis set out by Gabor and his team is the first to connect quantum mechanical structure to the greenness of plants, and provides a clear set of tests for researchers aiming to verify natural regulation. Equally important, their design allows regulation without active input, a process made possible by the photocell’s quantum mechanical structure.

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Story Source:

Materials provided by University of California – Riverside. Original written by Sean Nealon. Note: Content may be edited for style and length.

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Journal Reference:

1. Trevor B. Arp, Yafis Barlas, Vivek Aji, Nathaniel M. Gabor. Natural Regulation of Energy Flow in a Green Quantum Photocell. Nano Letters, 2016; DOI: 10.1021/acs.nanolett.6b03136

U of California: Nano submarines could change healthcare, says nanoengineer professor

A leading global chemist has come to the Sunshine Coast to discuss how his team is close to creating a successful nano submarine that could revolutionise the healthcare system.

When asked what exactly a “nano submarine” was, University of California San Diego chair of nanoengineering professor Joseph Wang described it as like something taken from the 1966 film Fantastic Voyage, where medical personnel board a submarine were shrunk to microscopic size to travel through the bloodstream of a wounded diplomat and save his life.

Professor Wang said his team was getting closer to the goal of using nano submarines in a variety of ways, minus the shrunken humans and sabotage of the 1966 film.

“It’s like the Fantastic Voyage movie, where you want to improve therapeutic and diagnostic abilities through proper timing and proper location to improve efficiency,” he said.

“It is like shrinking a big submarine a million times to get the nano-scale submarine.

“We use special nano fabrications to create it.

“You can call it submarine or a nano machine, there are different names for it.”

One nanometer is one-billionth of a meter. To put this into perspective, a strand of human DNA is 2.5 nanometers in diameter while a sheet of paper is about 100,000 nanometers thick.

Professor Wang said the nano submarines could be tailored to specific applications, including diagnosis, treatment and imaging and would use energy within the body’s system to generate its movement.

“It is powered by the blood, by chemical in the blood like glucose, it can autonomously move in blood,” he said.

“This is all part of what we call nano medicine, precision medicine that we use to improve medicines.

“It could improve imaging, diagnosis, treatment, it is multifunctional.”

Professor Wang said there was a fair way to go before human testing could begin, but said the pioneering work could improve drug treatments by providing a more targeted approach.

“Compared to (current) drug delivery, it could take cargo, the drug, and dispose it at the right location, right time and could improve the efficiency of drug,” he said.

Professor Wang was presenting a free public seminar on nano submarines at University of Sunshine Coast’s Innovation Centre at Sippy Downs.

 

University of California / University of Georgia: Integrated Trio of 2D Nanomaterials Unlocks Graphene Electronics Applications

Alexander Balandin (left) and Guanxiong Liu fabricated the voltage-controlled oscillator device in a cleanroom at the UCR’s Center for Nanoscale Science and Engineering (CNSE). Credit: UC Riverside.

Graphene has emerged as one of the most promising two-dimensional crystals, but the future of electronics may include two other nanomaterials, according to a new study by researchers at the

In research published Monday (July 4) in the journal Nature Nanotechnology, the researchers described the integration of three very different two-dimensional (2D) materials to yield a simple, compact, and fast voltage-controlled oscillator (VCO) device. A VCO is an electronic oscillator whose oscillation frequency is controlled by a voltage input.

Titled “An integrated Tantalum Sulfide—Boron Nitride—Graphene Oscillator: A Charge-Density-Wave Device Operating at Room Temperature,” the paper describes the development of the first useful device that exploits the potential of charge-density waves to modulate an electrical current through a 2D material. The new technology could become an ultralow power alternative to conventional silicon-based devices, which are used in thousands of applications from computers to clocks to radios. The thin, flexible nature of the device would make it ideal for use in wearable technologies.

Graphene, a single layer of carbon atoms that exhibits exceptional electrical and thermal conductivities, shows promise as a successor to silicon-based transistors. However, its application has been limited by its inability to function as a semiconductor, which is critical for the ‘on-off’ switching operations performed by electronic components.

To overcome this shortfall, the researchers turned to another 2D nanomaterial, Tantalum Sulfide (TaS2). They showed that voltage-induced changes in the atomic structure of the ‘1T prototype’ of TaS2 enable it to function as an electrical switch at room temperature—a requirement for practical applications.

“There are many charge-density wave materials that have interesting electrical switching properties. However, most of them reveal these properties at very low temperature only. The particular polytype of TaS2 that we used can have abrupt changes in resistance above room temperature. That made a crucial difference,” said Alexander Balandin, UC presidential chair professor of electrical and computer engineering in UCR’s Bourns College of Engineering, who led the research team.

To protect the TaS2 from environmental damage, the researchers coated it with another 2D material, hexagonal boron nitrate, to prevent oxidation. By pairing the boron nitride-capped TaS2 with graphene, the team constructed a three-layer VCO that could pave the way for post-silicon electronics. In the proposed design, graphene functions as an integrated tunable load resistor, which enables precise voltage control of the current and VCO frequency. The prototype UCR devices operated at MHz frequency used in radios, and the extremely fast physical processes that define the device functionality allow for the operation frequency to increase all the way to THz.

Balandin said the integrated system is the first example of a functional voltage-controlled oscillator device comprising 2D materials that operates at room temperature.

“It is difficult to compete with silicon, which has been used and improved for the past 50 years. However, we believe our device shows a unique integration of three very different 2D materials, which utilizes the intrinsic properties of each of these materials. The device can potentially become a low-power alternative to conventional silicon technologies in many different applications,” Balandin said.

The electronic function of graphene envisioned in the proposed 2D device overcomes the problem associated with the absence of the energy band gap, which so far prevented graphene’s use as the transistor channel material. The extremely high thermal conductivity of graphene comes as an additional benefit in the device structure, by facilitating heat removal. The unique heat conduction properties of graphene were experimentally discovered and theoretically explained in 2008 by Balandin’s group at UCR. The Materials Research Society recognized this groundbreaking achievement by awarding Balandin the MRS Medal in 2013.

The Balandin group also demonstrated the first integrated graphene heat spreaders for high-power transistors and light-emitting diodes. “In those applications, graphene was used exclusively as heat conducting material. Its thermal conductivity was the main property. In the present device, we utilize both electrical and thermal conductivity of graphene,” Balandin added.

Explore further: Hot new material can keep electronics cool: Few atomic layers of graphene reveal unique thermal properties

More information: Guanxiong Liu et al, A charge-density-wave oscillator based on an integrated tantalum disulfide–boron nitride–graphene device operating at room temperature, Nature Nanotechnology (2016). DOI: 10.1038/NNANO.2016.108

Journal reference: Nature Nanotechnology

Provided by: University of California – Riverside

 

Save the Trees – Rewritable Paper – New Discovery by Chemists at University of California?

First developed in China in about the year A.D. 150, paper has many uses, the most common being for writing and printing upon. Indeed, the development and spread of civilization owes much to paper’s use as writing material.

According to some surveys, 90 percent of all information in businesses today is retained on paper, even though the bulk of this printed paper is discarded after just one-time use.

Such waste of paper (and ink cartridges)—not to mention the accompanying environmental problems such as deforestation and chemical pollution to air, water and land—could be curtailed if the paper were “rewritable,” that is, capable of being written on and erased multiple times.

Chemists at the University of California, Riverside have now fabricated in the lab just such novel rewritable paper, one that is based on the color switching property of commercial chemicals called redox dyes. The dye forms the imaging layer of the paper. Printing is achieved by using ultraviolet light to photobleach the dye, except the portions that constitute the text on the paper. The new rewritable paper can be erased and written on more than 20 times with no significant loss in contrast and resolution.

Yadong Yin’s lab at the University of California, Riverside has fabricated novel rewritable paper, one that is based on the color switching property of commercial chemicals called redox dyes. Credit: Yin Lab, UC Riverside. 

“This rewritable paper does not require additional inks for printing, making it both economically and environmentally viable,” said Yadong Yin, a professor of chemistry, whose lab led the research. “It represents an attractive alternative to regular paper in meeting the increasing global needs for sustainability and environmental conservation.”

The rewritable paper is essentially rewritable media in the form of glass or plastic film to which letters and patterns can be repeatedly printed, retained for days, and then erased by simple heating.

The paper comes in three primary colors: blue, red and green, produced by using the commercial redox dyes methylene blue, neutral red and acid green, respectively. Included in the dye are titania nanocrystals (these serve as catalysts) and the thickening agent hydrogen cellulose (HEC). The combination of the dye, catalysts and HEC lends high reversibility and repeatability to the film.

During the writing phase, ultraviolet light reduces the dye to its colorless state. During the erasing phase, re-oxidation of the reduced dye recovers the original color; that is, the imaging material recovers its original color by reacting with ambient oxygen. Heating at 115 C can speed up the reaction so that the erasing process is often completed in less than 10 minutes.

“The printed letters remain legible with high resolution at ambient conditions for more than three days – long enough for practical applications such as reading newspapers,” Yin said. “Better still, our rewritable paper is simple to make, has low production cost, low toxicity and low energy consumption.”

His lab is currently working on a paper version of the rewritable paper.

“Even for this kind of paper, heating to 115 C poses no problem,” Yin said. “In conventional laser printers, paper is already heated to 200 C in order to get toner particles to bond to the paper surface.”

His lab also is working on increasing the cycling number (the number of times the rewritable paper can be printed and erased), with a target of 100, to reduce overall cost. His research team is exploring ways to extend the legibility of the printed texts or images for more than three days to expand their potential uses.

“One way is to develop new photocatalyst nanoparticles that become highly reductive when irradiated by ultraviolet light,” Yin said. “We are exploring, too, the possibility of multi-color printing. The design principle can be extended to various commercial redox dyes to produce rewritable paper capable of showing prints of different colors. All these efforts will help increase the practical applications of the technology.”

He was joined in the study by UC Riverside’s Wenshou Wang (first author of the research paper), Ning Xie and Le He. Wang and Yin conceived and designed the experiments. Wang performed the experiments. Xie and He contributed to sample analysis. Wang and Yin analyzed the results.

The research was funded by a grant to Yin from the U.S. Department of Energy.

This technology has been disclosed and assigned UC case number 2015-250. A provisional patent has been filed and the UCR Office of Technology Commercialization is actively seeking a company to license the technology.

Yin’s lab has recently synthesized colloidal titania nanoparticle catalyst doped with barium ions that enables reversible light-responsive color switching with excellent cycling performance and considerably high switching rate.

“The improved performance is attributed to the more effective removal of the photogenerated oxidative holes that results from barium doping. This leaves more electrons for promoting the reduction of redox dyes,” Yin said.

The finding was recently reported in Angewandte Chemie.

Explore further: Paper electronics could make health care more accessible

Study results appear online today (Dec. 2) in Nature Communications.

Roots of the Lithium Battery Problem: Berkeley Lab Researchers Find Dendrites Start Below the Surface

The lithium-ion batteries that power our laptops, smartphones and electric vehicles could have significantly higher energy density if their graphite anodes were to be replaced by lithium metal anodes. Hampering this change, however, has been the so-called dendrite problem. Over the course of several battery charge/discharge cycles, particularly when the battery is cycled at a fast rate, microscopic fibers of lithium, called “dendrites,” sprout from the surface of the lithium electrode and spread like kudzu across the electrolyte until they reach the other electrode. An electrical current passing through these dendrites can short-circuit the battery, causing it to rapidly overheat and in some instances catch fire. Efforts to solve the problem by curtailing dendrite growth have met with limited success, perhaps because they’ve just been scratching the surface of the problem.

These 3D reconstructions show how dendritic structures that can short-circuit a battery form deep within a lithium electrode, break through the surface and spread across the electrolyte.

Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered that during the early stages of development, the bulk of dendrite material lies below the surface of the lithium electrode, underneath the electrode/electrolyte interface. Using X-ray microtomography at Berkeley Lab’s Advanced Light Source (ALS), a team led by Nitash Balsara, a faculty scientist with Berkeley Lab’s Materials Sciences Division, observed the seeds of dendrites forming in lithium anodes and growing out into a polymer electrolyte during cycling. It was not until the advanced stages of development that the bulk of dendrite material was in the electrolyte. Balsara and his colleagues suspect that non-conductive contaminants in the lithium anode trigger dendrite nucleation.

Nitash Balsara and Katherine Harry at ALS beamline 8.3.2 where they shed important new light on the dendrite problem in lithium batteries. (Photo by Roy Kaltschmidt)

“Contrary to conventional wisdom, it seems that preventing dendrite formation in polymer electrolytes depends on inhibiting the formation of subsurface dendritic structures in the lithium electrode,” Balsara says. “In showing that dendrites are not simple protrusions emanating from the lithium electrode surface and that subsurface non-conductive contaminants might be the source of dendritic structures, our results provide a clear prescription for the path forward to enabling the widespread use of lithium anodes.”

Balsara, who is a professor of chemical engineering at the University of California (UC) Berkeley, is the corresponding author of a paper describing this research in Nature Materials titled “Detection of subsurface structures underneath dendrites formed on cycled lithium metal electrodes.” Co-authors are Katherine Harry, Daniel Hallinan, Dilworth Parkinson and, Alastair MacDowell.

The tremendous capacity of lithium and the metal’s remarkable ability to move lithium ions and electrodes in and out of an electrode as it cycles through charge/discharge make it an ideal anode material. Until now, researchers have studied the dendrite problem using various forms of electron microscopy. This is the first study to employ microtomography using monochromatic beams of high energy or “hard” X-rays, ranging from 22 to 25 keV, at  ALS beamline 8.3.2. This technique allows non-destructive three-dimensional imaging of solid objects at a resolution of approximately one micron.

“We observed crystalline contaminants in the lithium anode that appeared at the base of every dendrite as a bright speck,” says Katherine Harry, a member of Balsara’s research group and the lead author of the Nature Materials paper. “The lithium foils we used in this study contained a number of elements other than lithium with the most abundant being nitrogen. We can’t say definitively that these contaminants are responsible for dendrite nucleation but we plan to address this issue by conducting in situ X-ray microtomography.”

Balsara and his group also plan further study of the role played by the electrolyte in dendrite growth, and they have begun to investigate ways to eliminate non-conductive impurities from lithium anodes.

This research was funded by the DOE Office of Science.

Tetrapod Quantum Dots Light the Way to Stronger Polymers

Berkeley Lab Researchers Use Fluorescent Tetrapod Quantum Dots to Measure the Mechanical Strength of Polymer Fibers

Fluorescent tetrapod nanocrystals could light the way to the future design of stronger polymer nanocomposites. A team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed an advanced opto-mechanical sensing technique based on tetrapod quantum dots that allows precise measurement of the tensile  strength of polymer fibers with minimal impact on the fiber’s mechanical properties.

In a study led by Paul Alivisatos, Berkeley Lab director and the Larry and Diane Bock Professor of Nanotechnology at the University of California (UC) Berkeley, the research team incorporated into polymer fibers a population of tetrapod quantum dots (tQDs) consisting of a cadmium-selenide (CdSe) core and four cadmium sulfide (CdS) arms. The tQDs were incorporated into the polymer fibers via electrospinning, among today’s leading techniques for processing polymers, in which a large electric field is applied to droplets of polymer solution to create micro- and nano-sized fibers. This is the first known application of electrospinning to tQDs.

 

Fluorescent tetrapod quantum dots or tQDs (brown) serve as stress probes that allow precise measurement of polymer fiber tensile strength with minimal impact on mechanical properties. Inserts show relaxed tQDs (upper) and stressed tQDs (lower).

“The electrospinning process allowed us to put an enormous amount of tQDs, up to 20-percent by weight, into the fibers with minimal effects on the polymer’s bulk mechanical properties,” Alivisatos says. “The tQDs are capable of fluorescently monitoring not only simple uniaxial stress, but stress relaxation and behavior under cyclic varying loads. Furthermore, the tQDs are elastic and recoverable, and undergo no permanent change in sensing ability even upon many cycles of loading to failure.”

Alivisatos is the corresponding author of a paper describing this research in the journal NANO Letters titled “Tetrapod Nanocrystals as Fluorescent Stress Probes of Electrospun Nanocomposites.” Coauthors were Shilpa Raja, Andrew Olson, Kari Thorkelsson, Andrew Luong, Lillian Hsueh, Guoqing Chang, Bernd Gludovatz, Liwei Lin, Ting Xu and Robert Ritchie.

From left, Andrew Olson, Shilpa Raja and Andrew Luong are members of Paul Alivisatos’s research group who used electrospinning to incorporate tetrapod quantum dot stress probes into polymer fibers. (Photo by Roy Kaltschmidt)

Polymer nanocomposites are polymers that contain fillers of nanoparticles dispersed throughout the polymer matrix. Exhibiting a wide range of enhanced mechanical properties, these materials have great potential for a broad range of biomedical and material applications. However, rational design has been hampered by a lack of detailed understanding of how they respond to stress at the micro- and nanoscale.

“Understanding the interface between the polymer and the nanofiller and how stresses are transferred across that barrier are critical in reproducibly synthesizing composites,” Alivisatos says. “All of the established techniques for providing this information have drawbacks, including altering the molecular-level composition and structure of the polymer and potentially weakening mechanical properties such as toughness. It has therefore been of considerable interest to develop optical luminescent stress-sensing nanoparticles and  find a way to embed them inside polymer fibers with minimal impact on the mechanical properties that are being sensed.”

The Berkeley Lab researchers met this challenge by combining semiconductor tQDs of CdSe/CdS, which were developed in an earlier study by Alivisatos and his research group, with electrospinning. The CdSe/CdS tQDs are exceptionally well-suited as nanoscale stress sensors because an applied stress will bend the arms of the tetrapods, causing a shift in the color of their fluorescence. The large electric field used in electrospinning results in a uniform dispersal of tQD aggregates throughout the polymer matrix, thereby minimizing the formation of stress concentrations that would act to degrade the mechanical properties of the polymer. Electrospinning also provided a much stronger bond between the polymer fibers and the tQDs than a previous diffusion-based technique for using tQDs as stress probes that was reported two years ago by Alivisatos and his group. Much higher concentrations of tQDs could also be a achieved with electrospinning rather than diffusion.

When stress was applied to the polymer nanocomposites, elastic and plastic regions of deformation were easily observed as a shift in the fluorescence of the tQDs even at low particle concentrations. As particle concentrations were increased, a  greater fluorescence shift per unit strain was observed. The tQDs acted as non-perturbing probes that tests proved were not adversely affecting the mechanical properties of the polymer fibers in any significant way.

“We performed mechanical tests using a traditional tensile testing machine with all of our types of polymer fibers,” says Shilpa Raja, a lead author of the Nano Letters paper along with Andrew Olson, both members of Alivisatos’ research group. “While the tQDs undoubtedly change the composition of the fiber – it is no longer pure polylactic acid but instead a composite – we found that the mechanical properties of the composite and crystallinity of the polymer phase show minimal change.”

The research team believes their tQD probes should prove valuable for a variety of biological, imaging and materials engineering applications.

“A big advantage in the development of new polymer nanocomposites would be to use tQDs to monitor stress build-ups prior to material failure to see how the material was failing before it actually broke apart,” says co-lead author Olson. “The tQDs could also help in the development of new smart materials by providing insight into why a composite either never exhibited a desired nanoparticle property or stopped exhibiting it during deformation from normal usage.”

For biological applications, the tQD is responsive to forces on the nanoNewton scale, which is the amount of force exerted by living cells as they move around within the body. A prime example of this is metastasizing cancer cells that move through the surrounding extracellular matrix. Other cells that exert force include the fibroblasts that help repair wounds, and cardiomyocytes, the muscle cells in the heart that beat.

“All of these types of cells are known to exert nanoNewton forces, but it is very difficult to measure them,” Raja says.

“We’ve done preliminary studies in which we have shown that cardiomyocytes on top of a layer of tQDs can be induced to beat and the tQD layer will show fluorescent shifts in places where the cells are beating. This could be extended to a more biologically-relevant environment in order to study the effects of chemicals and drugs on the metastasis of cancer cells.”

Another exciting potential application is the use of tQDs to make smart polymer nanocomposites that can sense when they have cracks or are about to fracture and can strengthen themselves in response.

“With our technique we are combining two fields that are usually separate and have never been combined on the nanoscale, optical sensing and polymer nanocomposite mechanical tunability,” Raja says. “As the tetrapods are incredibly strong, orders of magnitude stronger than typical polymers, ultimately they can make for stronger interfaces that can self-report impending fracture.”

This research was primarily supported by the DOE Office of Science.

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Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit the Office of Science website at science.energy.gov/.

 

Additional Information

For more about the research of Paul Alivisatos go here

Berkeley Lab Researchers Discover Universal Law For Light Absorption In 2D Semiconductors

From solar cells to optoelectronic sensors to lasers and imaging devices, many of today’s semiconductor technologies hinge upon the absorption of light. Absorption is especially critical for nano-sized structures at the interface between two energy barriers called quantum wells, in which the movement of charge carriers is confined to two-dimensions. Now, for the first time, a simple law of light absorption for 2D semiconductors has been demonstrated.

Working with ultrathin membranes of the semiconductor indium arsenide, a team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has discovered a quantum unit of photon absorption, which they have dubbed “AQ,” that should be general to all 2D semiconductors, including compound semiconductors of the III-V family that are favored for solar films and optoelectronic devices. This discovery not only provides new insight into the optical properties of 2D semiconductors and quantum wells, it should also open doors to exotic new optoelectronic and photonic technologies.

“We used free-standing indium arsenide membranes down to three nanometers in thickness as a model material system to accurately probe the absorption properties of 2D semiconductors as a function of membrane thickness and electron band structure,” says Ali Javey, a faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor of electrical engineering and computer science at the University of California (UC) Berkeley. “We discovered that the magnitude of step-wise absorptance in these materials is independent of thickness and band structure details.”

Javey is one of two corresponding authors of a paper describing this research in the Proceedings of the National Academy of Sciences (PNAS). The paper is titled “Quantum of optical absorption in two-dimensional semiconductors.” Eli Yablonovitch, an electrical engineer who also holds joint appointments with Berkeley Lab and UC Berkeley, is the other corresponding author. Co-authors are Hui Fang, Hans Bechtel, Elena Plis, Michael Martin and Sanjay Krishna.

Previous work has shown that graphene, a two-dimensional sheet of carbon, has a universal value of light absorption. Javey, Yablonovitch and their colleagues have now found that a similar generalized law applies to all 2D semiconductors. This discovery was made possible by a unique process that Javey and his research group developed in which thin films of indium arsenide are transferred onto an optically transparent substrate, in this case calcium fluoride.

“This provided us with ultrathin membranes of indium arsenide, only a few unit cells in thickness, that absorb light on a substrate that absorbed no light,” Javey says. “We were then able to investigate the optical absorption properties of membranes that ranged in thickness from three to 19 nanometers as a function of band structure and thickness.”

Using the Fourier transform infrared spectroscopy (FTIR) capabilities of Beamline 1.4.3 at Berkeley Lab’s Advanced Light Source, a DOE national user facility, Javey, Yablonovitch and their co-authors measured the magnitude of light absorptance in the transition from one electronic band to the next at room temperature. They observed a discrete stepwise increase at each transition from indium arsenide membranes with an AQ value of approximately 1.7-percent per step.

“This absorption law appears to be universal for all 2D semiconductor systems,” says Yablonovitch. “Our results add to the basic understanding of electron–photon interactions under strong quantum confinement and provide a unique insight toward the use of 2D semiconductors for novel photonic and optoelectronic applications.”

This research was supported by DOE’s Office of Science and the National Science Foundation.

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more information, visit http://www.lbl.gov.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

The Advanced Light Source is a third-generation synchrotron light source producing light in the x-ray region of the spectrum that is a billion times brighter than the sun. A DOE national user facility, the ALS attracts scientists from around the world and supports its users in doing outstanding science in a safe environment. The Advanced Light Source is a third-generation synchrotron light source producing light in the x-ray region of the spectrum that is a billion times brighter than the sun. A DOE national user facility, the ALS attracts scientists from around the world and supports its users in doing outstanding science in a safe environment. For more information, visit http://www.als.lbl.gov/.

SOURCE: The U.S. Department of Energy

Printing Ultrafast Graphene Chips for Flexible Electronics

Futurists are always talking about how flexible electronics will change our lives in amazing ways, but we’ve yet to see anything mind-blowing come to market. A team of scientists from the University of Texas in Austin, however, think they’ve found the key to changing that: ultrafast graphene transistors printed on flexible plastic.

Graphene is amazing. Or at least, it could be. Made from a layer of carbon one-atom thick, it’s the strongest material in the world, it’s… Read…

   9 Incredible Uses for Graphene

Graphene is amazing stuff for a lot different reasons. One reason is that it’s the perfect material for chip-making, and conventional graphene chips have broken several electronic speed records. In the past, however, attempts to put graphene transistors on flexible materials have caused that speed to take a dive. Not with this new method.

Indeed, the chips from Texas clock in at a record-breaking 25-gigahertz. The MIT Technology Review explains the manufacturing process:

To make the transistors, the researchers first fabricate all the non-graphene-containing structures—the electrodes and gates that will be used to switch the transistors on and off—on sheets of plastic. Separately, they grow large sheets of graphene on metal, then peel it off and transfer it to complete the devices. …

The graphene transistors are not only speedy but robust. The devices still work after being soaked in water, and they’re flexible enough to be folded up.

And things are only getting better. Earlier this week we learned about a cutting edge technique for making graphene chips developed by a team of researchers from the University of California.

All we need now is a company to take the plunge and start bringing some of this next level technology to market. And you thought Liquidmetal was cool !!     [Technology Review]

READ MORE …

Scientists Just Figured Out How to Make Lightning-Fast Graphene CPUs

Graphene has the power to change computing forever by making the fastest transistors ever. In theory. We just haven’t figured out how yet. Sound familiar? Fortunately, scientists have just taken a big step closer to making graphene transistors work for real.

Graphene transistors aren’t just fast; they’re lightning fast. The speediest one to date clocked in at some 427 GHz. That’s orders of magnitude more than what you can tease out of today’s processors.  The problem with graphene transistors, though, is that they aren’t particularly good at turning off. They don’t turn off at all actually, which makes it hard to use them as switches.

http://gizmodo.com/scientists-just-figured-out-how-to-make-lightning-fast-1177727488

New Nanomaterial Increases Yield of Solar Cells

 

  6 hours ago

Linked quantum dots – In the new nanomaterial two or more electrons jump across the band gap as a consequence of just a single light particle (arrow with waves) being absorbed. Using special molecules the researchers have strongly linked the …more

Researchers from the FOM Foundation, Delft University of Technology, Toyota Motor Europe and the University of California have developed a nanostructure with which they can make solar cells highly efficient. The researchers published their findings on 23 August 2Researchers from the FOM Foundation, Delft University of Technology, Toyota Motor Europe and the University of California have developed a nanostructure with which they can make solar cells highly efficient. The researchers published their findings on 23 August 2013 in the online edition of Nature Communications.

Smart nanostructures can increase the yield of solar cells. An international team of researchers including physicists from the FOM Foundation, Delft University of Technology and Toyota, have now optimised the nanostructures so that the solar cell provides more electricity and loses less energy in the form of heat.

Solar cells

A conventional solar cell contains a layer of silicon. When sunlight falls on this layer, electrons in the silicon absorb the energy of the light particles (photons). Using this energy the electrons jump across a ‘band gap‘, as a result of which they can freely move and electricity flows.

The yield of a solar cell is optimised if the photon energy is equal to the band gap of silicon. Sunlight, however, contains many photons with energies greater than the band gap. The excess energy is lost as heat, which limits the yield of a conventional solar cell.

Nanospheres

Several years ago the researchers from Delft University of Technology, as well as other physicists, demonstrated that the excess energy could still be put to good use. In small spheres of a semiconducting material the excess energy enables extra electrons to jump across the band gap. These nanospheres, the so-called quantum dots, have a diameter of just one ten thousandth of a human hair.

If a light particle enables an electron in a quantum dot to cross the band gap, the electron moves around in the dot. That ensures that the electron collides with other electrons that subsequently jump across the band gap as well. As a result of this process a single photon can mobilize several electrons thereby multiplying the amount of current produced.

Contact between quantum dots

However, up until now the problem was that the electrons remained trapped in their quantum dots and so could not contribute to the current in the solar cell. That was due to the large molecules that stabilize the surface of quantum dots. These large molecules hinder the electrons jumping from one quantum dot to the next and so no current flows.

In the new design, the researchers replaced the large molecules with small molecules and filled the empty space between the quantum dots with aluminium oxide. This led to far more contact between the quantum dots allowing the electrons to move freely.

Yield

Using laser spectroscopy the physicists saw that a single photon indeed caused the release of several electrons in the material containing linked quantum dots. All of the electrons that jumped across the band gap moved freely around in the material. As a result of this the theoretical yield of solar cells containing such materials rises to 45%, which is more than 10% higher than a conventional solar cell.

This more efficient type of solar cell is easy to produce: the structure of linked nanospheres can be applied to the solar cell as a type of layered paint. Consequently the new solar cells will not only be more efficient but also cheaper than conventional cells.

The Dutch researchers now want to work with international partners to produce complete solar cells using this design.

Read more at: http://phys.org/news/2013-08-nanomaterial-yield-solar-cells.html#jCpn mobilise several electrons thereby multiplying the amount of current produced.

 

Read more at: http://phys.org/news/2013-08-nanomaterial-yield-solar-cells.html#jCp

Researchers recreate elusive phenomenon with artificial nucle: Atomic Collapse

The first experimental observation of a quantum mechanical phenomenon that was predicted nearly 70 years ago holds important implications for the future of graphene-based electronic devices. Working with microscopic artificial atomic nuclei fabricated on graphene, a collaboration of researchers led by scientists with the U.S. Department of Energy‘s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have imaged the “atomic collapse” states theorized to occur around super-large atomic nuclei.

“Atomic collapse is one of the holy grails of graphene research, as well as a holy grail of atomic and nuclear physics,” says Michael Crommie, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and UC Berkeley‘s Physics Department. “While this work represents a very nice confirmation of basic relativistic quantum mechanics predictions made many decades ago, it is also highly relevant for future nanoscale devices where electrical charge is concentrated into very small areas.”

Crommie is the corresponding author of a paper describing this work in the journal Science. The paper is titled “Observing Atomic Collapse Resonances in Artificial Nuclei on Graphene.” Co-authors are Yang Wang, Dillon Wong, Andrey Shytov, Victor Brar, Sangkook Choi, Qiong Wu, Hsin-Zon Tsai, William Regan, Alex Zettl, Roland Kawakami, Steven Louie, and Leonid Levitov. Originating from the ideas of quantum mechanics pioneer Paul Dirac, atomic collapse theory holds that when the positive electrical charge of a super-heavy atomic nucleus surpasses a critical threshold, the resulting strong Coulomb field causes a negatively charged electron to populate a state where the electron spirals down to the nucleus and then spirals away again, emitting a positron (a positively–charged electron) in the process. This highly unusual electronic state is a significant departure from what happens in a typical atom, where electrons occupy stable circular orbits around the nucleus. “Nuclear physicists have tried to observe atomic collapse for many decades, but they never unambiguously saw the effect because it is so hard to make and maintain the necessary super-large nuclei,” Crommie says. “Graphene has given us the opportunity to see a condensed matter analog of this behavior, since the extraordinary relativistic nature of electrons in graphene yields a much smaller nuclear charge threshold for creating the special supercritical nuclei that will exhibit atomic collapse behavior.”

Perhaps no other material is currently generating as much excitement for new electronic technologies as graphene, sheets of pure carbon just one atom thick through which electrons can freely race 100 times faster than they move through silicon. Electrons moving through graphene’s two-dimensional layer of carbon atoms, which are arranged in a hexagonally patterned honeycomb lattice, perfectly mimic the behavior of highly relativistic charged particles with no mass. Superthin, superstrong, superflexible, and superfast as an electrical conductor, graphene has been touted as a potential wonder material for a host of electronic applications, starting with ultrafast transistors.

EnlargeNonrelativistic electrons orbiting a subcritical nucleus exhibit the traditional circular Bohr orbit of atomic physics. But when the charge on a nucleus exceeds the critical value, Zc, the semiclassical electron trajectory is predicted to spiral in toward the nucleus, then spiral away, a novel electronic state known as “atomic collapse.” Artificial nuclei composed of three or more calcium dimers on graphene exhibit this behavior as graphene’s electrons move in the supercritical Coulomb potential. Credit: Michael Crommie, Lawrence Berkeley National LaboratoryIn recent years scientists predicted that highly-charged impurities in graphene should exhibit a unique electronic resonance – a build-up of electrons partially localized in space and energy – corresponding to the atomic collapse state of super-large atomic nuclei.

Last summer Crommie’s team set the stage for experimentally verifying this prediction by confirming that graphene’s electrons in the vicinity of charged atoms follow the rules of relativistic quantum mechanics. However, the charge on the atoms in that study was not yet large enough to see the elusive atomic collapse. “Those results, however, were encouraging and indicated that we should be able to see the same atomic physics with highly charged impurities in graphene as the atomic collapse physics predicted for isolated atoms with highly charged nuclei,” Crommie says. “That is to say, we should see an electron exhibiting a semiclassical inward spiral trajectory and a novel quantum mechanical state that is partially electron-like near the nucleus and partially hole-like far from the nucleus.

For graphene we talk about ‘holes’ instead of the positrons discussed by nuclear physicists.” To test this idea, Crommie and his research group used a specially equipped scanning tunneling microscope (STM) in ultra-high vacuum to construct, via atomic manipulation, artificial nuclei on the surface of a gated graphene device. The “nuclei” were actually clusters made up of pairs, or dimers, of calcium ions. With the STM, the researchers pushed calcium dimers together into a cluster, one by one, until the total charge in the cluster became supercritical. STM spectroscopy was then used to measure the spatial and energetic characteristics of the resulting atomic collapse electronic state around the supercritical impurity. “The positively charged calcium dimers at the surface of graphene in our artificial nuclei played the same role that protons play in regular atomic nuclei,” Crommie says. “By squeezing enough positive charge into a sufficiently small area, we were able to directly image how electrons behave around a nucleus as the nuclear charge is methodically increased from below the supercritical charge limit, where there is no atomic collapse, to above the supercritical charge limit, where atomic collapse occurs.

” Observing atomic collapse physics in a condensed matter system is very different from observing it in a particle collider, Crommie says. Whereas in a particle collider the “smoking gun” evidence of atomic collapse is the emission of a positron from the supercritical nucleus, in a condensed matter system the smoking gun is the onset of a signature electronic state in the region nearby the supercritical nucleus. Crommie and his group observed this signature electronic state with artificial nuclei of three or more calcium dimers. “The way in which we observe the atomic collapse state in condensed matter and think about it is quite different from how the nuclear and high-energy physicists think about it and how they have tried to observe it, but the heart of the physics is essentially the same,” says Crommie.

If the immense promise of graphene-based electronic devices is to be fully realized, scientists and engineers will need to achieve a better understanding of phenomena such as this that involve the interactions of electrons with each other and with impurities in the material. “Just as donor and acceptor states play a crucial role in understanding the behavior of conventional semiconductors, so too should atomic collapse states play a similar role in understanding the properties of defects and dopants in future graphene devices,” Crommie says. “Because atomic collapse states are the most highly localized electronic states possible in pristine graphene, they also present completely new opportunities for directly exploring and understanding electronic behavior in graphene.” Journal reference: Science Provided by Lawrence Berkeley National Laboratory