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Mysteries of Electron-Electron Interaction in Graphene Revealed

 

It seems there’s a new development in graphene research daily. One day the ‘wonder material’ is made into a semiconductor, and the next it becomes an insulator

Despite all the research that is ongoing with graphene, no one has been quite sure what role electron-electron interaction played in giving graphene its unique properties, including its high electron mobility.

Now Michael Crommie, a physicist who holds joint appointments with Lawrence Berkeley National Laboratory’s (LBL) Materials Sciences Division and University of California Berkeley’s Physics Department, has peered into these interactions with scanning tunneling microscopy (STM) and solved the mystery

Crommie is among the researchers who as been pushing the boundaries of graphene’s capabilities since he discovered that graphene could be stretched, and in so doing, introduced the concept of ‘straintronics’ for graphene. One of the offshoots of that research has been the idea of using graphene for piezoelectric applications

In this latest research, which was published in the journal Nature Physics (“Mapping Dirac quasiparticles near a single Coulomb impurity on graphene”), Crommie and his colleagues looked at and recorded how electrons and holes react to a charged impurity (a single Coulomb potential) that had been located on a gated graphene device.

The researchers were already aware that the electrons traveled through graphene at relativistic speeds comparable to the speed of light. It is for this reason that physicists have referred to them as “Dirac quasiparticles,” which travel at the speed of light if they are massless excitations, but cannot reach that speed if the have mass. 

“In graphene, electrons behave as massless Dirac fermions,” Crommie says in the LBL press release . “As such, the response of these electrons to a Coulomb potential is predicted to differ significantly from how non-relativistic electrons behave in traditional atomic and impurity systems. However, until now, many key theoretical predictions for this ultra-relativistic system had not been tested.”

Crommie and his team took theoretical predictions of how Dirac quasiparticles would respond to a charged impurity in graphene and compared those with their physical measurements. They were able to take the physical measurements by using an STM tip to combine cobalt monomers into cobalt trimers that served as the charged impurities. Then, with the same STM, the team measured the response of the Dirac quasiparticles to the Coulomb potential created by the trimers.

“Theorists have predicted that compared with other materials, electrons in graphene are pulled into a positively-charged impurity either too weakly, the subcritical regime; or too strongly, the supercritical regime,” Crommie further noted in the LBL press release. “In our study, we verified the predictions for the subcritical regime and found the value for the dielectric to be small enough to indicate that electron–electron interactions contribute significantly to graphene properties. This information is fundamental to our understanding of how electrons move through graphene.”

By measuring how Dirac fermions react when they are near a charged impurity in the grapehen (a Coulomb potential), the researchers have been able to determine graphene's dielectric constant and in this way they have confirmed that electron-electron interaction is a factor in how electrons behave inside graphene.

Prospect of Ubiquitous Flexible Displays Just Got a Lot More Realistic

 

The ability to manufacture flexible lithium-ion (Li-ion) batteries is not new, as evidenced by some of the commercial efforts over the last couple of years to bring this technology to market. 

However, the quest to achieve flexibility for these Li-ion batteries has typically meant using materials that sacrifice performance to the degree that they have little hope of powering consumer gadgets, like a flexible display.

Keon Jae Lee, a professor in the Department of Materials Science and Engineering, Flexible and Nano-bio-energy Device Lab, at Korea Advanced Institute of Science and Technology (KAIST), and his colleagues have developed a new manufacturing technique that should allow for inorganic materials to be used that will produce better battery performance. (A demonstration video of the flexible Li-ion can be seen below.) 

You may remember back in May that Lee developed a method for producing nanogenerators more cheaply and easily than ever before.  It seems Lee has done the same with flexible Li-ion batteries.

The current research, which was published in the journal Nano Letters (“Bendable Inorganic Thin-Film Battery for Fully Flexible Electronic Systems”), demonstrated the use of high-density inorganic thin films employing a newly developed universal transfer approach. The process results in what the researchers describe as the “highest performance ever achieved for flexible LIBs [Li-ion batteries].”

The so-called “universal transfer approach” involves physical delamination of the battery’s mica substrate using sticky tapes. One of the key steps of the process comes right before the delamination when lithium cobalt oxide (LiCoO2) is exposed to high-temperature annealing to create the crystallinity of material that results in the high-performance solid-state LIB.

"We would like to emphasize that solid state thin-film battery has been developed for quite a long time and commercialized for various applications," says Lee in Nanowerk. "Our achievement is not the development of a thin-film battery but focused on the development of a flexible thin-film battery using a transfer protocol for the ultra-thin battery film itself – with less than 10µm thickness."

What is perhaps most intriguing about the manufacturing process is that it manages to create an “all-in-one” product in which both the LED display and energy source are produced all together in one flexible device.

Lee further notes in the Nanowerk article: "Our novel transfer approach can be expanded to various high-performance flexible applications, such as thin-film nanogenerators, thin-film transistors, and thermoelectric devices. As far as we know, this is the first prototype of a fully functional all-flexible electronic system.”

Researchers Unravel Twenty-Five-Year-Old Riddle of Buckyball Formation

When Richard Smalley, Robert Curl, James Heath, Sean O'Brien, and Harold Kroto prepared the first buckminsterfullerene (C60) (or buckyball), they kicked off the next 25 years of nanomaterial science.

What’s interesting about the buckyball—considering all the carbon nanoscience that has followed it—is that nobody quite understood the mechanism by which buckyballs formed—until now. Researchers at Florida State University and the National Science Foundation claim to have solved the quarter-century old mystery of how these buckyballs take shape

The research, which was published in the journal Nature Communications (“Closed network growth of fullerenes”), discovered that “fullerenes self-assemble through a closed network growth mechanism.” They are able to pull this off by incorporation of atomic carbon and diatomic carbon.

Making this discovery required some ingenious approaches and a bit of serendipity since fullerene formation happens in an instant. “We started with a paste of pre-existing fullerene molecules mixed with carbon and helium, shot it with a laser, and instead of destroying the fullerenes we were surprised to find they’d actually grown," the university press release quotes the researchers as saying.

The researchers further determined that fullerenes did not grow by splitting open but instead managed their growth trick by absorbing and incorporating carbon from the surrounding gas. They knew this because the fullerene cages contained heavy metal atoms in their centers. “If the cages grew by splitting open, we would have lost the metal atoms, but they always stayed locked inside,” Paul Dunk, a doctoral student in chemistry and biochemistry at Florida State and lead author of the work noted in a press release.

The clearest applications for solving this riddle will be in better understanding fullerene formation in extraterrestrial environments where C60 crystals have been observed orbiting stars, leading to the speculation that fullerenes are more abundant than originally thought.

Harry Kroto, a Nobel Prize winner for the discovery of C60 and co-author of the current study, and now a professor at Florida State further noted in the release: “The results of our study will surely be extremely valuable in deciphering fullerene formation in extraterrestrial environments.” 

Colloidal Quantum Dot Solar Cells Break Conversion Efficiency Record

Over the years, this blog has reported on the work of Edward H. Sargent, and his research team at the University of Toronto, in employing colloidal quantum dots (CQD) for photovoltaics (PVs).

Last year, Sargent’s work, which had been backed by funding from the King Abdullah University of Science and Technology (KAUST) in Thuwal, Saudi Arabia, managed to develop a CQD multijunction PV that had a “graded recombination layer” that served as an interface between the visible and infrared junction, passing electrons between the two layers.

While being able to harvest both visible and invisible light with PVs was impressive, the PVs had comparatively low conversion efficiencies of 4.2 percent, which was significantly lower than the 5 percent levels that were the state of the art at the time for CQD multijunction PVs. Even with Sargent’s team reaching 6 percent with their CQD multijunction PVs later in the year,  it seemed there was still room for improvement, especially if you consider theoretical levels for CQDs of 42 percent.

Now Sargent and his team—along with both financial and research support from his Saudi backers at KAUST—have pushed the conversion efficiencies of these devices up to what they claim is a record-breaking 7 percent efficiency. 

The research, which was published in the journal Nature Nanotechnology in a paper titled “Hybrid passivated colloidal quantum dot solids” (available without a subscription),  looked at the problem of the high number of electron traps in the surfaces of CQD films. They determined that a hybrid passivation scheme could dramatically improve the combination of surface passivation and film density and thereby reduce the trap densities.

The "hybrid passivation scheme” consisted of introducing chlorine atoms to the quantum dots immediately after synthesizing them. This made it possible to coat areas on CQD films that had been unreachable before.

“Previously, quantum dot solar cells have been limited by the large internal surface areas of the nanoparticles in the film, which made extracting electricity difficult,” said lead co-author Susanna Thon, in a University of Toronto press release. “Our breakthrough was to use a combination of organic and inorganic chemistry to completely cover all of the exposed surfaces.” Thon is a post-doctoral fellow at the university.

The researchers believe that this latest 37-percent improvement in conversion efficiency for the CQD PVs represents a sign of things to come.

As Sargent further notes in the press release: “This work shows that the abundant materials interfaces inside colloidal quantum dots can be mastered in a robust manner, proving that low cost and steadily-improving efficiencies can be combined."

Graphene-based Tunnel Barriers Promise to Change both Electronics and Spintronics

Tunnel barriers have typically been made from metal oxides because they excel at separating conductors, such as graphene. Now researchers at the Naval Research Laboratory (NRL) have turned that all upside down by making graphene serve as a tunnel barrier for the first time

This finding by the NRL scientists is almost counterintuitive. So much work is currently done in finding ways of imbuing the conductor graphene with a band gap by using tunnel barriers. With the flow of research currently going in the direction of trying to make graphene into a semiconductor, it's unusual to have a group looking to make it serve as a tunnel barrier.

Nonetheless, in the NRL research, which was published in the ACS journal Nano Letters (“Graphene As a Tunnel Barrier: Graphene-Based Magnetic Tunnel Junctions”), the graphene serves as the electrically insulating barrier between two conducting materials. The NRL researchers were able to construct magnetic tunnel junctions, which form the backbone of read heads in the giant magnetoresistance (GMR) hard disk drives of today’s computers and magnetoresistive random access memory (MRAM), in a fully scalable lithographic process.

While GMR is a well-established technology, the area of non-volatile MRAM has been hindered by limitations resulting from the materials used. Among the big problems for MRAM has been the metal oxides used as tunnel barriers in these devices. They suffer from inconsistent thicknesses and other defects such as high resistance-area (RA) that result in high power consumption and localized heating. Graphene offers a solution to many of these problems. Because graphene is one atom thick it has very low RA, which in turn means it consumes little power but has fast switching speeds.

The NRL researchers believe that graphene-based magnetic tunnel junctions will exceed the performance of the metal oxide variety. This work represents a "paradigm shift in tunnel barrier technology for magnetic tunnel junctions (MTJs) used for advanced sensors, memory and logic," says Dr. Berend Jonker in the NRL press release covering the research.

By going against the flow of current research, the NRL researchers may have developed an alternative material that the 2011 International Technology Roadmap for Semiconductors (ITRS) believed may hold the key to creating "...electrically accessible non-volatile memory with high speed and high density [that] would initiate a revolution in computer architecture."

Drug Delivery Research Gets a New Nanotech Tool in its Arsenal

 

Earlier this year, IBM Zurich demonstrated, for the first time, the ability to image the charge distribution of a molecule. Now researchers at the University of Zurich, led by Prof. Madhavi Krishnan, have developed a method that makes possible the measurement of the electrostatic charge of nanoparticles for the first time. 

The research, which was published in the journal Nature Nanotechnology ("Measuring the size and charge of single nanoscale objects in solution using an electrostatic fluidic trap") developed a method by which “single nanoscale objects can be directly measured with high throughput by analyzing their thermal motion in an array of electrostatic traps.”

Krishnan and her colleagues set up the “electrostatic traps” by using glass plates the size of computer chips and creating energy holes between the two plates of glass. Each hole contains a weak electrostatic charge, so when a solution is dropped on the glass plates, particles get trapped there. Because molecules from the solution continue to bounce off the trapped particles, the particles are forced into a circular motion within the traps. It is this motion that enables the measurement of the charge of each particle.

Those of you familiar with the work of the 1923 Nobel Prize winner in physics—Robert A. Millikan—might be thinking that this sounds remarkably similar to the traps he created to measure the velocity of oil drops. “But he examined the drops in a vacuum,” Prof. Krishnan explains in a press release. “We on the other hand are examining nano particles in a solution which itself influences the properties of the particles.”

The ability to measure charge in solution is critical. It is in fact the electrical charge of the particles within the solution that determines the consistency of various solutions ranging from blood to pharmaceuticals. “With our new method, we get a picture of the entire suspension along with all of the particles contained in it,” Krishnan says. “The charge of the particles plays a major role in this.”

The scientists believe that the ability to make these measurements of a single nanoparticle in real time will alter the way research is conducted for nanoparticles used in drug delivery. Any change in charge to a nanoparticle due to its reactions to various proteins and other large molecules can dramatically affect how the nanoparticle interacts in the body when carrying out a function like delivering a drug.

DNA Scaffold Delivers Payload of Synthetic Vaccines Safely and Effectively

About 18 months ago, the nanotech trade press was buzzing with the work of Hongbin Yu and Hao Yan, both from Arizona State University (ASU), when they developed a method that used DNA origami as a scaffold. When the DNA scaffolding was combined with “nano islands” made from gold, it enabled the manufacturing of smaller electronic memory devices. 

Now Yan has joined with Yung Chang, a biodesign immunologist also from ASU, to use three-dimensional DNA structures as a scaffold on which they piggybacked synthetic vaccine complexes to make the delivery of the vaccines safer and more effective. 

“When Hao treated DNA not as a genetic material, but as a scaffolding material, that made me think of possible applications in immunology,” said Chang, an associate professor in the School of Life Sciences and a researcher in the Biodesign Institute’s Center for Infectious Diseases and Vaccinology in a university press release. “This provided a great opportunity to try to use these DNA scaffolds to make a synthetic vaccine.”

The research, which was published in the journal Nano Letters ("A DNA Nanostructure Platform for Directed Assembly of Synthetic Vaccines"),  made its first test with the DNA scaffold by placing an immune stimulating protein called streptavidin (STV) and an immune response boosting compound called an adjuvant (CpG oligo-deoxynucletides) to different branches of the DNA structure.

After determining that cells would absorb the DNA structure with its synthetic vaccine payload, the researchers waited to see if an immune cascade response would follow. It did and was really beyond the researchers expectations.

The results showed that the mice that were given the full vaccine complex consisting of the DNA scaffold and the STV and GpG displayed an immune response nine times higher than those that had been injected solely with the STV and GpG.

"We were very pleased," said Chang in the press release. "It was so nice to see the results as we predicted. Many times in biology we don't see that."

This is really just a leaping off point, according to the researchers. They believe that this proof of concept indicates that an unlimited range of antigens could be used in this way for fighting a host of diseases.

Nanosensor Could Detect Prostate Cancer in its Early Stages

Sensors for detecting chemical biomarkers that indicate a disease have already been applied to some maladies, but they have not proven very effective at discovering low concentrations of those biomarkers, such as when a disease is in its early stages. This is a problem, because often the key to successful treatment is early detection. 

Now researchers at the London Centre for Nanotechnology at Imperial College London and the University of Vigo have developed plasmonic nanosensors that could enable early disease detection by picking up biomarker signals at very low concentrations

The research, which was published in the journal Nature Materials (“Plasmonic nanosensors with inverse sensitivity by means of enzyme-guided crystal growth”), demonstrated a signal-generation mechanism for nanoparticle sensors capable of creating “a signal that is larger when the target molecule is less concentrated.” Earlier this year we saw researchers at Brown University experiment with plasmonics for biosensors to measure glucose levels via saliva rather than blood.  

In the initial testing with the new London nanoparticle sensor the researchers looked for the biomarker associated with prostate cancer, called prostate specific antigen (PSA). The nanosensor was capable of detecting PSA in concentrations nine orders of magnitude smaller than today's enzyme-linked immunosorbent assay (ELISA) tests.

The LCN nanosensors are made up of gold nanoparticles that are floating in proteins derived from blood serum. On the surface of the gold nanoparticles are differnet antibodies. One antibody latches onto the PSA when it detects it while the other antibody creates a silver crystal coating that floats on the surface of the nanoparticle when it comes in the presence of the PSA. The silver crystal coating is detected by optical microscopes. The improved “signal-generation mechanism” is that this silver crystal coating is more apparent when the concentration of the PSA is low.

Professor Molly Stevens, senior author of the study from the Departments of Materials and Bioengineering at Imperial College London, notes in a press release: “It is vital to detect diseases at an early stage if we want people to have the best possible outcomes – diseases are usually easier to treat at this stage, and early diagnosis can give us the chance to halt a disease before symptoms worsen. However, for many diseases, using current technology to look for early signs of disease can be like finding the proverbial needle in a haystack. Our new test can actually find that needle. We only looked at the biomarker for one disease in this study, but we’re confident that the test can be adapted to identify many other diseases at an early stage.”

The First Printable Giant Magnetoresistive Devices Emerge

Last month, researchers at the University of Utah developed a “plastic paint” magnetic field sensor based on spintronics that looks as though it could ultimately find its way into consumer electronics. 

Along these lines, researchers in Germany are now reporting they have developed the first printable magnetic sensor that relies on giant magnetoresistance (GMR) effect. The research, initially published in the online edition of Advanced Materials ("Printable Giant Magnetoresistive Devices"), was performed by a team at IFW Dresden.

The researchers claim that the magneto-sensitive ink can be painted on just about any substrate and maintains a GMR ratio of up to 8 percent at ambient conditions. With a typical spin valve GMR ratio reaching just 5 percent for read heads inside your computer, this GMR ratio alone is pretty impressive. However, it is what the work augurs for the field of printable electronics that may have the most lasting impact.

"Our demonstrator with a magnetic switch printed on a postcard suggests that the vision of interactive fully printable electronics can become reality," Dr. Denys Makarov, leader of the group "Magnetic Nanomembranes" at the IFW Dresden, tells Nanowerk in the story covering the research.

The demonstrator Makarov refers to involved integrating the printable GMR sensor into a hybrid electronic circuit. The circuit consisted of an amplifier with a light emitting diode (LED) that had been printed on a postcard. A permanent magnet modifies the resistance of the printable magnetic sensor to switch the LED between its on/off states.

While it appears that the GMR ink can be applied by a variety of methods, such as roll-to-roll or flexography, each method demands different sized GMR flakes in the ink as well as different ink viscosity.

Makarov further notes in the Nanowerk story: “More investigations are required to understand the influence of the size of GMR flakes on the resulting GMR response of the magneto-sensitive ink. Furthermore, different binder solutions have to be tested to adjust the viscosity and conductivity of the ink."

New Method Developed for Making a Transistor from Graphene

 

Researchers from Germany and Sweden have developed a new method for creating a transistor from graphene, according to an artlcle at Phys.org.

The headline, "Researchers devise a way to create a graphene transistor," is a bit misleading. Researchers have been making transistors out of graphene for some time now,  and have been using a process based on silicon carbide, like the German-Swedish research. 

The real breakthrough for this latest line of research, which was published 17 July in the journal Nature Communications (“Tailoring the graphene/silicon carbide interface for monolithic wafer-scale electronics”),  appears to be how they engineered all the constituent parts of the transistor.

To be honest, I am not entirely clear on how even their process diverges drastically from the IBM research. After a year of trying to figure out how to connect all the parts of the graphene-based transistor without damaging it, the IBM team used electron beam lithography and a resist that was sensitive to electrons.The German-Swedish researchers used electron beam lithography too.

Maybe the difference between the two is the use of oxygen plasma etching, which converted the middle channel on the transistor from a contact into a gate. This could be the “tailoring the graphene/silicon carbide interface” of which the paper title speaks.

An important caveat to the research is that because the researchers had to scale up dramatically their transistor they don’t really know how much faster the transistor might be than the current variety. Furthermore, they’re not even sure how fast it might be when they scale the transistor down.

I am sure this work is helpful and evolutionary research in the development of graphene transistors, but I think maybe the Phys.org article has perhaps overstated its case when it says that this research “is the breakthrough computer engineers have been waiting for.”

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
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