Study of electron orbits in multilayer graphene finds unexpected energy gaps

 Stacking of graphene sheets creates regions where the moiré alignment is of type AA (all atoms have neighbors in the layer below), AB (only A atoms have neighbors) or BA (only B atoms have neighbors). In the figure, AA regions are blue-white, while AB and BA regions are red and yellow, respectively. Credit: Courtesy of Phillip First

Researchers have taken one more step toward understanding the unique and often unexpected properties of graphene, a two-dimensional carbon material that has attracted interest because of its potential applications in future generations of electronic devices.

In the Aug. 8 advance online edition of the journal Nature Physics, researchers from the Georgia Institute of Technology and the National Institute of Standards and Technology (NIST) describe for the first time how the orbits of electrons are distributed spatially by magnetic fields applied to layers of epitaxial graphene.

The research team also found that these electron orbits can interact with the substrate on which the graphene is grown, creating energy gaps that affect how electron waves move through the multilayer material. These energy gaps could have implications for the designers of certain graphene-based electronic devices.

"The regular pattern of energy gaps in the graphene surface creates regions where electron transport is not allowed," said Phillip N. First, a professor in the Georgia Tech School of Physics and one of the paper's co-authors. "Electron waves would have to go around these regions, requiring new patterns of electron wave interference. Understanding such interference will be important for bi-layer graphene devices that have been proposed, and may be important for other lattice-matched substrates used to support graphene and graphene devices."

In a magnetic field, an electron moves in a circular trajectory - known as a cyclotron orbit - whose radius depends on the size of the magnetic field and the energy of electron. For a constant magnetic field, that's a little like rolling a marble around in a large bowl, First said.

"At high energy, the marble orbits high in the bowl, while for lower energies, the orbit size is smaller and lower in the bowl," he explained. "The cyclotron orbits in graphene also depend on the electron energy and the local electron potential - corresponding to the bowl - but until now, the orbits hadn't been imaged directly."

Placed in a magnetic field, these orbits normally drift along lines of nearly constant electric potential. But when a graphene sample has small fluctuations in the potential, these "drift states" can become trapped at a hill or valley in the material that has closed constant potential contours. Such trapping of charge carriers is important for the quantum Hall effect, in which precisely quantized resistance results from charge conduction solely through the orbits that skip along the edges of the material.

The study focused on one particular electron orbit: a zero-energy orbit that is unique to graphene. Because electrons are matter waves, interference within a material affects how their energy relates to the velocity of the wave - and reflected waves added to an incoming wave can combine to produce a slower composite wave. Electrons moving through the unique "chicken-wire" arrangement of carbon-carbon bonds in the graphene interfere in a way that leaves the wave velocity the same for all energy levels.

In addition to finding that energy states follow contours of constant electric potential, the researchers discovered specific areas on the graphene surface where the orbital energy of the electrons changes from one atom to the next. That creates an energy gap within isolated patches on the surface.

"By examining their distribution over the surface for different magnetic fields, we determined that the energy gap is due to a subtle interaction with the substrate, which consists of multilayer graphene grown on a silicon carbide wafer," First explained.

In multilayer epitaxial graphene, each layer's symmetrical sublattice is rotated slightly with respect to the next. In prior studies, researchers found that the rotations served to decouple the electronic properties of each graphene layer.

"Our findings hold the first indications of a small position-dependent interaction between the layers," said David L. Miller, the paper's first author and a graduate student in First's laboratory. "This interaction occurs only when the size of a cyclotron orbit - which shrinks as the magnetic field is increased - becomes smaller than the size of the observed patches."

The origin of the position dependent interaction is believed to be the "moiré pattern" of atomic alignments between two adjacent layers of graphene. In some regions, atoms of one layer lie atop atoms of the layer below, while in other regions, none of the atoms align with the atoms in the layer below. In still other regions, half of the atoms have neighbors in the underlayer, an instance in which the symmetry of the carbon atoms is broken and the Landau level - discrete energy level of the electrons - splits into two different energies.

Experimentally, the researchers examined a sample of epitaxial graphene grown at Georgia Tech in the laboratory of Professor Walt de Heer, using techniques developed by his research team over the past several years.

They used the tip of a custom-built scanning-tunneling microscope (STM) to probe the atomic-scale electronic structure of the graphene in a technique known as scanning tunneling spectroscopy. The tip was moved across the surface of a 100-square nanometer section of graphene, and spectroscopic data was acquired every 0.4 nanometers.

The measurements were done at 4.3 degrees Kelvin to take advantage of the fact that energy resolution is proportional to the temperature. The scanning-tunneling microscope, designed and built by Joseph Stroscio at NIST's Center for Nanoscale Science and Technology, used a superconducting magnet to provide the magnetic fields needed to study the orbits.

According to First, the study raises a number of questions for future research, including how the energy gaps will affect electron transport properties, how the observed effects may impact proposed bi-layer graphene coherent devices - and whether the new phenomenon can be controlled.

"This study is really a stepping stone in long path to understanding the subtleties of graphene's interesting properties," he said. "This material is different from anything we have worked with before in electronics."

Unprecedented look at oxide interfaces reveals unexpected structures on atomic scale

August 4, 2010

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A new scanning tunneling microscopy and low energy electron diffraction technique developed at Oak Ridge National Laboratory captured this 50 nm x 50 nm image of an oxide surface. Each bright dot is a single atom of material.

Thin layers of oxide materials and their interfaces have been observed in atomic resolution during growth for the first time by researchers at the Center for Nanophase Materials Sciences at the Department of Energy's Oak Ridge National Laboratory, providing new insight into the complicated link between their structure and properties.

"Imagine you suddenly had the ability to see in color, or in 3-D," said the CNMS's Sergei Kalinin. "That is how close we have been able to look at these very small interfaces."

The paper was published online in ACS Nano with ORNL's Junsoo Shin as lead author.

A component of magnetoelectronics and spintronics, oxide interfaces have the potential to replace silicon-based microelectronic devices and improve the power and memory retention of other electronic technologies.

However, oxide interfaces are difficult to analyze at the atomic scale because once the oxides are removed from their growth chamber they become contaminated. To circumvent this problem, ORNL researchers led by Art Baddorf built a unique system that allows scanning tunneling microscopy and low energy electron diffraction to capture images of the top layer of the oxide while in situ, or still in the vacuum chamber where the materials were grown by powerful laser pulses.

Many studies of similar oxide interfaces utilize a look from the side, typically achieved by aberration corrected scanning transmission electron microscopy (STEM). The ORNL team has used these cross-sectional images to map the oxide organization.

However, like a sandwich, oxide interfaces may be more than what they appear from the side. In order to observe the interactive layer of the top and bottom oxide, the group has used scanning tunneling microscopy to get an atomically resolved view of the surface of the oxide, and observed its evolution during the growth of a second oxide film on top.

"Instead of seeing a perfectly flat, square lattice that scientists thought these interfaces were before, we found a different and very complicated atomic ordering," said Baddorf. "We really need to reassess what we know about these materials."

Oxides can be used in different combinations to produce unique results. For instance, isolated, two oxides may be insulators but together the interface may become conductive. By viewing the atomic structure of one oxide, scientists can more effectively couple oxides to perform optimally in advanced technological applications such as transistors.

Kalinin says the correct application of these interface-based materials may open new pathways for development of computer processors and energy storage and conversion devices, as well as understanding basic physics controlling these materials.

"In the last 10 years, there has been only limited progress in developing beyond-silicon information technologies," Kalinin said. "Silicon has limitations that have been reached, and this has motivated people to explore other options."

Atomic resolution of interface structures during oxide growth will better enable scientists to identify defects of certain popular oxide combinations and could help narrow selections of oxides to spur new or more efficient commercial applications.

Provided by Oak Ridge National Laboratory (news : web)

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World record data density for ferroelectric recording

August 17, 2010 Scientists at Tohoku University in Japan have recorded data at a density of 4 trillion bits per square inch, which is a world record for the experimental "ferroelectric" data storage method. As described the journal Applied Physics Letters, which is published by the American Institute of Physics, this density is about eight times the density of today's most advanced magnetic hard-disk drives.

The data-recording device scans a tiny cantilever tip that rides in contact with the surface of a ferroelectric material. To write data, an electric pulse is sent through the tip, changing the electric polarization and nonlinear dielectric constant of a tiny circular spot in the substrate beneath. To read data, the same tip detects the variations in nonlinear dielectric constant in the altered regions.

"We expect this ferroelectric data storage system to be a candidate to succeed magnetic hard disk drives or flash memory, at least in applications for which extremely high data density and small physical volume is required," said Dr. Yasuo Cho.

In earlier experiments, the researchers had noticed one problem: When the data being written required that several consecutive marks be written next to each other, the written polarized regions expanded the normal diameter and coalesced to the point the bits were not distinct. Cho and Kenkou Tanaka then developed a method for anticipating strings of consecutive marks in the data and reducing the writing-pulse voltage by up to about 10 percent, which resulted in clear and distinct data marks.

While ferroelectric storage has the advantage of using only electric methods -- nothing magnetic or thermal -- to achieve its record-high density, Cho and Tanaka are well aware that many practical improvements would be needed for commercial viability. Such advances would include increasing the speed and accuracy of reading the data and developing a low-cost ferroelectric substrate.

Another risk is that existing data storage technologies continue to improve beyond the ferroelectric's capabilities. Disk drive maker Seagate, for example, has said it can envision achieving a density of 50 trillion bits per square inch.

Provided by American Institute of Physics

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Nanoparticles for cultural heritage conservation

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(PhysOrg.com) -- The conservation of Mayan wall paintings at the archaeological site of Calakmul (Mexico) will be one on the subjects touched upon by Piero Baglioni (based at the University of Florence) in his invited lecture at the 3rd European Chemistry Congress in Nurnberg in September.

In a special issue of Chemistry -- A European Journal, which contains papers by many of the speakers at this conference, he reports on the latest developments on the use of humble calcium and barium hydroxides nanoparticles as a versatile and highly efficient tool to combat the main degradation processes that affect wall paintings.

La Antigua Ciudad Maya de Calakmul is located in the Campeche state (Mexico) and is one of the most important cities of the Classic Maya period (AD 250-800). The excavation of this site (set up in 1993) involves, under the supervision of the archaeologist Ramon Carrasco, archaeologists, architects, engineers, conservators and epigraphists, besides other specialists. Since 2004, the Center for Colloid and Surface Science (CSGI) at the University of Florence (CSGI), and currently directed by Piero Baglioni, has been an active partner, being involved in the study of the painting technique and in the development of nanotechnology for the consolidation and protection of the wall paintings and limestone.

Over the last decades, polymers, mainly acrylic and vinyl resins, have been widely used to consolidate wall paintings and to confer protection and hydrorepellency to the painted layer. However, contrary to the expectations, polymers used for the protection of wall paintings have induced further degradation of the works of art and their chemical modifications, such as cross-linking, strongly hampers their removal. Hence, there has been a need to develop new methods of conservation.

In Florence, Piero Baglioni and his group have pioneered the use of calcium hydroxide nanoparticles to restore wall paintings, the degradation of which is basically due to the transformation of calcium carbonate into gypsum. Nanoparticles of calcium hydroxide efficiently interact with carbon dioxide to reform calcium carbonate and replace the degraded original ligand, leading to the re-cohesion of the paint layer. However, when large amounts of soluble sulfates (i.e., sodium or magnesium sulfates) are present in a wall painting, consolidation with calcium hydroxide nanoparticles might not produce durable results. In fact, sulfate ions can react with calcium hydroxide to give a double-exchange reaction, producing the slightly soluble gypsum (calcium sulfate dihydrate). Barium hydroxide nanoparticles represent a really useful alternative and a complementary tool to hinder this process. Hence, mixed formulations can be used for the pre-consolidation of surfaces largely contaminated by sulfates.

In Calakmul, Mayan paintings have been successfully treated by using a mixture of calcium and barium hydroxide nanoparticles as a dispersion in 1-propanol. The consolidation effect was significant already after one week. The result of the application is that the paintings are now stable and do not show ongoing degradation processes. Thus, nanoscience has opened up enormous potential for Cultural Heritage conservation, due to the unique properties that the reduction in particle size confers to nanomaterials compared to their micrometric counterparts.

Study predicts nanoscience will greatly increase efficiency of next-generation solar cells

As the fastest growing energy technology in the world, solar energy continues to account for more and more of the world?s energy supply. Currently, most commercial photovoltaic power comes from bulk semiconductor materials. But in the past few years, scientists have been investigating how semiconductor nanostructures can increase the efficiency of solar cells and the newer field of solar fuels.

Although there has been some controversy about just how much nanoscience can improve solar cells, a recent overview of this research by Arthur Nozik, a researcher at the National Renewable Energy Laboratory (NREL) and professor at the University of Colorado, shows that semiconductor nanostructures have significant potential for converting solar energy into electricity.

In his overview, which is published in a recent issue of Nano Letters, Nozik has summarized the current status of several approaches to improving photovoltaics with nanoscience. As he explains, the advantages of semiconductor nanostructures arise from the quantum confinement of negative electrons and positive holes into very small regions of space in the nanocrystals. Quantum confinement can occur in one, two or three dimensions; in three dimensions, the semiconductors are called quantum dots. In any regime, the quantum confinement produces quantization effects, resulting in unique optical and electronic properties.

?There are two main theoretical advantages of incorporating quantum dots into solar cells and photovoltaics: higher efficiency and lower cost,? Nozik told PhysOrg.com. ?There is a theoretical possibility based on thermodynamic calculations of increasing the efficiency of present day solar cells by a very significant amount of 50-100%. In addition, quantum dots could lower the capital cost of solar cell production in terms of cost per unit area. The combination of lower cost per unit area and higher conversion efficiency would lower the cost of photovoltaic power expressed as cost per peak watt. Present silicon cells are expensive (about three times the cost of conventional electricity), but quantum dots are based on less expensive low-temperature solution chemistry methods, plus they could produce higher conversion efficiencies. However, there is still a lot of work to be done before quantum dots are commercially available.?

The basic principle of photovoltaic solar cells is to absorb photons from incident solar radiation with energies above the semiconductor band gap, and use the photons to create free electrons and holes (called charge carriers). In order to increase the efficiency of the system, it is important to form as many charge carriers as possible from the absorbed photons. This is where the quantum confinement effects become very useful, as the effects couple photogenerated electrons and holes into bound electron-hole pairs called excitons, and encourage the efficient formation of more than one exciton from a single absorbed photon. In quantum dots. the process is called multiple exciton generation (MEG). Among its advantages, MEG is more efficient and can occur with lower-energy photons in the visible region of the solar spectrum compared to a multiplication process of charge carriers in bulk semiconductors (a process called impact ionization, which is generally restricted to the ultraviolet region where solar photons are absent or scarce).

To generate multiple excitons, the MEG process must compete with the rapid cooling of initial photogenerated high-energy excitons (called ?hot excitons?). The hot excitons are created by the absorption of energetic blue or near-ultraviolet photons. In bulk semiconductors at room temperature and above, the photogenerated electrons and holes are uncoupled and exist as free charge carriers (called ?hot carriers?). The excess energy of hot excitons or hot carriers can quickly lose their excess kinetic energy through electron-phonon interactions and convert it into heat, which accounts for significant loss of conversion efficiency. However, Nozik notes that, despite some controversy, recent studies have shown that the rate of MEG can be much faster than the hot exciton cooling rate, resulting in an overall higher efficiency of electron-hole pair multiplication. But despite early initial reports of quantum yields of 200% in quantum dot photoelectrochemical solar cells, no quantum dot-based photovoltaic device to date has shown an actual enhanced power conversion efficiency due to MEG.

?Generally, the goal is to produce systems that have efficiencies close to the theoretical limit,? Nozik said. ?The theoretical efficiency is about 45%, while the lab efficiency of present quantum dot solar cells is about 3-5%. That?s a big gap; we need to understand what limits the efficiency in these new approaches.?

Despite the controversy about MEG, Nozik concludes that the possibilities for quantum dot solar cells and other nanostructures that use quantum confinement look promising, although much more work still needs to be done. One issue that may help MEG to reach its full potential is to ensure that the additional excitons are being quickly collected, since they decay within about 20-100 picoseconds after formation. Most importantly, Nozik emphasizes that researchers should strive toward reaching the maximum theoretical efficiency of solar cells.

?There?s a certain degree of controversy about these third generation approaches because they?re new and not completely understood,? Nozik said. ?In the past, some results could not be reproduced in different labs. But now more and more people in recent years are reproducing positive results. Los Alamos and NREL are measuring these effects in a new U.S. DOE Energy Frontier Research Center with different techniques, and getting the same answer. So it is a real effect, a positive effect. However, some people are still skeptical and think that we?re never going to reach those values [of theoretical efficiency]. But there is no fundamental reason why we can?t reach those values. It just takes more research, more effort, and more understanding.?

New architectures for nano brushes: Bitty structures can be tailored in many shapes

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An atomic force microscopy topographic image of the nano-brushes. The relative heights of the brushes can be tailored by changing the substrate and initiators.

(PhysOrg.com) -- Just as cilia lining the lungs help keep passages clear by moving particles along the tips of the tiny hair-structures, man-made miniscule bristles known as nano-brushes can help reduce friction along surfaces at the molecular level, among other things.

In their latest series of experiments, Duke University engineers have developed a novel approach to synthesize these nano-brushes, which could improve their versatility in the future. These polymer brushes are currently being used in biologic sensors and microscopic devices, such as microcantilevers, and they will play an important role in the future drive to miniaturization, the researchers said.

Nano-brushes are typically made of polymer molecules grown on flat surfaces with strands of the molecules growing up and out from a surface, much like hairs on a brush. Polymers are large man-made molecules ubiquitous in the manufacture of everyday products.

Like microscopic orchard keepers, the Duke scientists have grafted bundles of polymer ?limbs? on flat surfaces known as substrates, already covered with brush bristles. In their approach, two dissimilar brushes can be joined and patterned on the micro-scale. Because the ?limbs? can be made out of a different substance than the substrate, the scientists believe these nano-structures are able to significantly modify the properties of a given surface.

To make such a nano-brush, scientists add a chemical known as an initiator to the flat surface, which spurs the growth of the strands.

?One of the common ways of growing brushes is much like a dot matrix printer, with an initiator being the ink ?printed? onto an inorganic substrate, such as a silicon wafer or a gold surface, which then causes the brush bristles to grow in specified patterns,? said Stefan Zauscher, Alfred M. Hunt Faculty Scholar and associate professor of mechanical engineering and materials science at Duke?s Pratt School of Engineering.

?In our patterning approach we are now also able to initiate polymer brush growth on existing brush substrates and thus obtain patterned block copolymer brushes, just like grafts, on polymeric substrates,? Zauscher said. ?The ability to create more intricate brush structures provides the potential for using them in biomedical applications as sensors for the detection of proteins or glucose.?

The results of his team?s experiments were published online in the journal Small. The research is supported by the National Science Foundation.

Zauscher said this new approach could be readily expanded to many other types of polymers, and to make either single or double layers of brushes. These nano-brushes, he said, would have many potential uses, and would open up the possibilities for building more complicated polymer architectures, which are much in demand for current and future technologies.

In recent research, published earlier in the journal Advanced Materials, Zauscher showed that stimulus-responsive nano-brushes resemble and act like sea anemones, which have a multitude of arms reaching up from an attached base. In the same fashion as these sea animals, nano-brushes can be used to capture and release micro-particles as they move across a surface.

Caltech researchers design a new nanomesh material

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Top: A scanning electron microscope image shows the grid of tiny holes in the nanomesh material. Bottom: In this drawing, each sphere represents a silicon atom in the nanomesh. The colorful bands show the temperature differences on the material, with red being hotter and blue being cooler.

(PhysOrg.com) -- Computers, light bulbs, and even people generate heat?energy that ends up being wasted. With a thermoelectric device, which converts heat to electricity and vice versa, you can harness that otherwise wasted energy. Thermoelectric devices are touted for use in new and efficient refrigerators, and other cooling or heating machines. But present-day designs are not efficient enough for widespread commercial use or are made from rare materials that are expensive and harmful to the environment.

Researchers at the California Institute of Technology (Caltech) have developed a new type of material?made out of silicon, the second most abundant element in Earth's crust?that could lead to more efficient thermoelectric devices. The material?a type of nanomesh?is composed of a thin film with a grid-like arrangement of tiny holes. This unique design makes it difficult for heat to travel through the material, lowering its thermal conductivity to near silicon's theoretical limit. At the same time, the design allows electricity to flow as well as it does in unmodified silicon.

"In terms of controlling thermal conductivity, these are pretty sophisticated devices," says James Heath, the Elizabeth W. Gilloon Professor and professor of chemistry at Caltech, who led the work. A paper about the research will be published in the October issue of the journal Nature Nanotechnology.

A major strategy for making thermoelectric materials energy efficient is to lower the thermal conductivity without affecting the electrical conductivity, which is how well electricity can travel through the substance. Heath and his colleagues had previously accomplished this using silicon nanowires?wires of silicon that are 10 to 100 times narrower than those currently used in computer microchips. The nanowires work by impeding heat while allowing electrons to flow freely.

In any material, heat travels via phonons?quantized packets of vibration that are akin to photons, which are themselves quantized packets of light waves. As phonons zip along the material, they deliver heat from one point to another. Nanowires, because of their tiny sizes, have a lot of surface area relative to their volume. And since phonons scatter off surfaces and interfaces, it is harder for them to make it through a nanowire without bouncing astray. As a result, a nanowire resists heat flow but remains electrically conductive.

But creating narrower and narrower nanowires is effective only up to a point. If the nanowire is too small, it will have so much relative surface area that even electrons will scatter, causing the electrical conductivity to plummet and negating the thermoelectric benefits of phonon scattering.

To get around this problem, the Caltech team built a nanomesh material from a 22-nanometer-thick sheet of silicon. (One nanometer is a billionth of a meter.) The silicon sheet is converted into a mesh?similar to a tiny window screen?with a highly regular array of 11- or 16-nanometer-wide holes that are spaced just 34 nanometers apart.

Instead of scattering the phonons traveling through it, the nanomesh changes the way those phonons behave, essentially slowing them down. The properties of a particular material determine how fast phonons can go, and it turns out that?in silicon at least?the mesh structure lowers this speed limit. As far as the phonons are concerned, the nanomesh is no longer silicon at all. "The nanomesh no longer behaves in ways typical of silicon," says Slobodan Mitrovic, a postdoctoral scholar in chemistry at Caltech. Mitrovic and Caltech graduate student Jen-Kan Yu are the first authors on the Nature Nanotechnology paper.

When the researchers compared the nanomesh to the nanowires, they found that?despite having a much higher surface-area-to-volume ratio?the nanowires were still twice as thermally conductive as the nanomesh. The researchers suggest that the decrease in thermal conductivity seen in the nanomesh is indeed caused by the slowing down of phonons, and not by phonons scattering off the mesh's surface. The team also compared the nanomesh to a thin film and to a grid-like sheet of silicon with features roughly 100 times larger than the nanomesh; both the film and the grid had thermal conductivities about 10 times higher than that of the nanomesh.

Although the electrical conductivity of the nanomesh remained comparable to regular, bulk silicon, its thermal conductivity was reduced to near the theoretical lower limit for silicon. And the researchers say they can lower it even further. "Now that we've showed that we can slow the phonons down," Heath says, "who's to say we can't slow them down a lot more?"

The researchers are now experimenting with different materials and arrangements of holes in order to optimize their design. "One day, we might be able to engineer a material where you not only can slow the phonons down, but you can exclude the phonons that carry heat altogether," Mitrovic says. "That would be the ultimate goal."