Scientists have designed a new way to create a single-molecule diode that performs 50 times better than past models.
These single-molecule diodes are the first that could be used for real-world applications in nanoscale devices, Columbia University School of Engineering and Applied Sciencereported. The idea of creating a single-molecule diode was first proposed in the 1970s by Arieh Aviram and Mark Ratner, who theorized that a molecule could act as a “rectifier” to conduct one-way currents.
molecular electronics ever since its inception with Aviram and Ratner’s 1974 seminal paper, represents the ultimate in functional miniaturization that can be achieved for an electronic device,” said Latha Venkataraman, associate professor of applied physics at Columbia Engineering.
Since the 1974 paper, scientists have determined single-molecules attached themselves to metal electrodes, and act as a number of circuit elements such as switches, resistors, and diodes. A diode works as an “electricity valve,” and requires an asymmetrical structure in order to create different environments for electricity flowing in each direction.
“While such asymmetric molecules do indeed display some diode-like properties, they are not effective,” said Brian Capozzi, a PhD student working with Venkataraman and lead author of the paper. “A well-designed diode should only allow current to flow in one direction-the ‘on’ direction-and it should allow a lot of current to flow in that direction. Asymmetric molecular designs have typically suffered from very low current flow in both ‘on’ and ‘off’ directions, and the ratio of current flow in the two has typically been low. Ideally, the ratio of ‘on’ current to ‘off’ current, the rectification ratio, should be very high.”
To remedy this, the researchers worked to develop asymmetry in the environment around the molecular junction. They accomplished this by surrounding the active molecule with an ionic solution and employed the use of gold metal electrodes that differed in size to contact the molecule. The method led to rectification ratios as high as 250, which is 50 times higher than earlier designs.
“It’s amazing to be able to design a molecular circuit, using concepts from chemistry and physics, and have it do something functional,” Venkataraman said. “The length scale is so small that quantum mechanical effects are absolutely a crucial aspect of the device. So it is truly a triumph to be able to create something that you will never be able to physically see and that behaves as intended.”
The findings were published in a recent edition of the journal Nature Nanotechnology.
A new semiliquid battery developed by researchers at The University of Texas at Austin has exhibited encouraging early results, encompassing many of the features desired in a state-of-the-art energy-storage device. In particular, the new battery has a working voltage similar to that of a lithium-ion battery, a power density comparable to that of a supercapacitor, and it can maintain its good performance even when being charged and discharged at very high rates.
The researchers, led by Assistant Professor Guihua Yu, along with Yu Ding and Yu Zhao, at UT Austin, have published their paper on the new membrane-free, semiliquid battery in a recent issue of Nano Letters. The researchers explain that the battery is considered “semiliquid” because it uses a liquid ferrocene electrolyte, a liquid cathode, and a solid lithium anode.
“The greatest significance of our work is that we have designed a semiliquid battery based on a new chemistry,” Yu told Phys.org. “The battery shows excellent rate capability that can be fully charged or discharged almost within one minute while maintaining good energy efficiency and reasonable energy density, representing a promising prototype liquid redox battery with both high energy density and power density for energy storage.”
The battery is designed for applications in two of the biggest areas of battery technology: hybrid electric vehicles and energy storage for renewable energy resources.
As shown in the figure above, the battery’s high power density (1400 W/L) and good energy density (40 Wh/L) put it in the uniquely favorable position of combining a power density that is as high as that of current supercapacitors with an energy density on par with those of state-of-the-art redox flow batteries and lead-acid batteries, though slightly lower than that of lithium-ion batteries. This combination is especially attractive for electric vehicles, where the power density corresponds to top speed and the energy density to the vehicle’s range per charge.
The researchers also report in their paper that the new battery has a high capacity (137 mAh/g) and a high capacity retention of 80% for 500 cycles.
The researchers attribute the battery’s good performance in large part to its liquid electrode design that enables its high rate capability, which is basically a measure of how fast the battery operates. The ions can move through the liquid battery very rapidly compared to in a solid battery, and the redox reactions in which the electrons are transferred between electrodes also occur at very high rates in this particular battery. For comparison, the values used to measure these rates (the diffusion coefficient and the reaction constant) are orders of magnitude greater in the new battery than in most conventional flow batteries.
“The potential weakness of this battery is the lithium anode in terms of long-term stability and safety,” Yu said. “More advanced lithium anode protection is required to fully suppress self-discharge. We suppose that other metals like zinc and magnesium may also function as the anode for such a battery as long as the electrolyte compatibility is resolved. We also expect that other organometallic compounds with multi-valence-state metal centers (redox centers) may also function as the anode, which eventually would make the battery fully liquid.”
In the future, the researchers plan to test the long-term durability of the battery, especially its lithium anode, under realistic operating conditions. In addition, the researchers want to find a way to increase the solubility of ferrocene in order to further increase the energy density to compete with current lithium-ion batteries while maintaining its very high power density.
More information: Yu Ding, et al. “A Membrane-Free Ferrocene-Based High-Rate Semiliquid Battery.” Nano Letters. DOI: 10.1021/acs.nanolett.5b01224
Scientists have moved graphene—the incredibly strong and conductive single-atom-thick sheet of carbon—a significant step along the path from lab bench novelty to commercially viable material for new electronic applications.
Researchers from the University of Manchester, together with BGT Materials Limited, a graphene manufacturer in the United Kingdom, have printed a radio frequency antenna using compressed graphene ink. The antenna performed well enough to make it practical for use in radio-frequency identification (RFID) tags and wireless sensors, the researchers said. Even better, the antenna is flexible, environmentally friendly and could be cheaply mass-produced. The researchers present their results in the journal Applied Physics Letters, from AIP Publishing.
The study demonstrates that printable graphene is now ready for commercial use in low-cost radio frequency applications, said Zhirun Hu, a researcher in the School of Electrical and Electronic Engineering at the University of Manchester.
“The point is that graphene is no longer just a scientific wonder. It will bring many new applications to our daily life very soon,” added Kostya S. Novoselov, from the School of Physics and Astronomy at the University of Manchester, who coordinated the project.
Graphene Gets Inked
Since graphene was first isolated and tested in 2004, researchers have striven to make practical use of its amazing electrical and mechanical properties. One of the first commercial products manufactured from graphene was conductive ink, which can be used to print circuits and other electronic components.
Graphene ink is generally low cost and mechanically flexible, advantages it has over other types of conductive ink, such as solutions made from metal nanoparticles.
To make the ink, graphene flakes are mixed with a solvent, and sometimes a binder like ethyl cellulose is added to help the ink stick. Graphene ink with binders usually conducts electricity better than binder-free ink, but only after the binder material, which is an insulator, is broken down in a high-heat process called annealing. Annealing, however, limits the surfaces onto which graphene ink can be printed because the high temperatures destroy materials like paper or plastic.
The University of Manchester research team, together with BGT Materials Limited, found a way to increase the conductivity of graphene ink without resorting to a binder. They accomplished this by first printing and drying the ink, and then compressing it with a roller, similar to the way new pavement is compressed with a road roller.
Compressing the ink increased its conductivity by more than 50 times, and the resulting “graphene laminate” was also almost two times more conductive than previous graphene ink made with a binder.
The high conductivity of the compressed ink, which enabled efficient radio frequency radiation, was one of the most exciting aspects of the experiment, Hu said.
Paving the Way to Antennas, Wireless Sensors, and More
The researchers tested their compressed graphene laminate by printing a graphene antenna onto a piece of paper. The antenna measured approximately 14 centimeters long, and 3.5 millimeter across and radiated radio frequency power effectively, said Xianjun Huang, who is the first author of the paper and a PhD candidate in the Microwave and Communcations Group in the School of Electrical and Electronic Engineering.
Printing electronics onto cheap, flexible materials like paper and plastic could mean that wireless technology, like RFID tags that currently transmit identifying info on everything from cattle to car parts, could become even more ubiquitous.
Most commercial RFID tags are made from metals like aluminium and copper, Huang said, expensive materials with complicated fabrication processes that increase the cost.
“Graphene based RFID tags can significantly reduce the cost thanks to a much simpler process and lower material cost,” Huang said. The University of Manchester and BGT Materials Limited team has plans to further develop graphene enabled RFID tags, as well as sensors and wearable electronics.
SOURCE: American Institute of Physics
Ever since single-layer graphene burst onto the science scene in 2004, the possibilities for the promising material have seemed nearly endless. With its high electrical conductivity, ability to store energy, and ultra-strong and lightweight structure, graphene has potential for many applications in electronics, energy, the environment, and even medicine.
Now a team of Northwestern University researchers has found a way to print three-dimensional structures with graphene nanoflakes. The fast and efficient method could open up new opportunities for using graphene printed scaffolds regenerative engineering and other electronic or medical applications.
Led by Ramille Shah, assistant professor of materials science and engineering at Northwestern’s McCormick School of Engineering and of surgery in the Feinberg School of Medicine, and her postdoctoral fellow Adam Jakus, the team developed a novel graphene-based ink that can be used to print large, robust 3-D structures.
“People have tried to print graphene before,” Shah said. “But it’s been a mostly polymer composite with graphene making up less than 20 percent of the volume.”
With a volume so meager, those inks are unable to maintain many of graphene’s celebrated properties. But adding higher volumes of graphene flakes to the mix in these ink systems typically results in printed structures too brittle and fragile to manipulate. Shah’s ink is the best of both worlds. At 60-70 percent graphene, it preserves the material’s unique properties, including its electrical conductivity. And it’s flexible and robust enough to print robust macroscopic structures. The ink’s secret lies in its formulation: the graphene flakes are mixed with a biocompatible elastomer and quickly evaporating solvents.
“It’s a liquid ink,” Shah explained. “After the ink is extruded, one of the solvents in the system evaporates right away, causing the structure to solidify nearly instantly. The presence of the other solvents and the interaction with the specific polymer binder chosen also has a significant contribution to its resulting flexibility and properties. Because it holds its shape, we are able to build larger, well-defined objects.”
Supported by a Google Gift and a McCormick Research Catalyst Award, the research is described in the paper “Three-dimensional printing of high-content graphene scaffolds for electronic and biomedical applications,” published in the April 2015 issue of ACS Nano. Jakus is the paper’s first author. Mark Hersam, the Bette and Neison Harris Chair in Teaching Excellence, professor of materials science and engineering at McCormick, served as coauthor.
An expert in biomaterials, Shah said 3-D printed graphene scaffolds could play a role in tissue engineering and regenerative medicine as well as in electronic devices. Her team populated one of the scaffolds with stem cells to surprising results. Not only did the cells survive, they divided, proliferated, and morphed into neuron-like cells.
“That’s without any additional growth factors or signaling that people usually have to use to induce differentiation into neuron-like cells,” Shah said. “If we could just use a material without needing to incorporate other more expensive or complex agents, that would be ideal.”
The printed graphene structure is also flexible and strong enough to be easily sutured to existing tissues, so it could be used for biodegradable sensors and medical implants. Shah said the biocompatible elastomer and graphene’s electrical conductivity most likely contributed to the scaffold’s biological success.
“Cells conduct electricity inherently — especially neurons,” Shah said. “So if they’re on a substrate that can help conduct that signal, they’re able to communicate over wider distances.”
The graphene-based ink directly follows work that Shah and her graduate student Alexandra Rutz completed earlier in the year to develop more cell-compatible, water-based, printable gels. As chronicled in a paper published in the January 2015 issue of Advanced Materials, Shah’s team developed 30 printable bioink formulations, all of which are compatible materials for tissues and organs. These inks can print 3-D structures that could potentially act as the starting point for more complex organs.
“There are many different tissue types, so we need many types of inks,” Shah said. “We’ve expanded that biomaterial tool box to be able to optimize more mimetic engineered tissue constructs using 3-D printing.”
- Adam E. Jakus, Ethan B. Secor, Alexandra L. Rutz, Sumanas W. Jordan, Mark C. Hersam, Ramille N. Shah. Three-Dimensional Printing of High-Content Graphene Scaffolds for Electronic and Biomedical Applications. ACS Nano, 2015; 9 (4): 4636 DOI: 10.1021/acsnano.5b01179
MIT researchers managed to use graphene, deposited on top of a similar 2D material called hexagonal boron nitride (hBN), to couple the properties of the different 2D materials to provide a high degree of control over light waves. They state this has the potential to lead to new kinds of light detection, thermal-management systems, and high-resolution imaging devices.
Both materials are structurally alike (in that they’re both composed of hexagonal arrays of atoms that form 2D sheets), but they react to light differently. These different reactions, though, were found by the researchers to be complementary, and assist in gaining control over the behavior of light. The hybrid material blocks light upon applying a particular voltage to the graphene, while allowing a special kind of emission and propagation, called “hyperbolicity,” when a different voltage is applied. This means that an extremely thin sheet of material can interact strongly with light, allowing beams to be guided, funneled, and controlled by voltages applied to the sheet. This poses a phenomenon previously unobserved in optical systems.
Light’s interaction with graphene produces particles called plasmons, while light interacting with hBN produces phonons. The scientists found that when the materials are combined in a certain way, the plasmons and phonons can couple, producing a strong resonance. Also, the properties of the graphene allow precise control over light, while hBN provides very strong confinement and guidance of the light. Combining the two makes it possible to create new “metamaterials” that marry the advantages of both. The combined materials create a system that can be adjusted to allow light only of certain specific wavelengths or directions to propagate, and to selectively pick which frequencies to let through and which to reject.
Just as alchemists always dreamed of turning common metal into gold, their 19th century physicist counterparts dreamed of efficiently turning heat into electricity, a field called thermoelectrics. Such scientists had long known that, in conducting materials, the flow of energy in the form of heat is accompanied by a flow of electrons. What they did not know at the time is that it takes nanometric-scale systems for the flow of charge and heat to reach a level of efficiency that cannot be achieved with larger scale systems. Now, in a paper published in EPJ B Barbara Szukiewicz and Karol Wysokiński from Marie Curie-Skłodowska University, in Lublin, Poland have demonstrated the importance of thermoelectric effects, which are not easily modelled, in nanostructures.
Since the 1990s, scientists have looked into developing efficient energy generation from nanostructures such as quantum dots. Their advantage: they display a greater energy conversion efficiency leading to the emergence of nanoscale thermoelectrics. The authors evaluate the thermoelectric performance of models made of two quantum dots—which are coupled electrostatically—connected to two electrodes kept at a different temperature and a single quantum dot with two levels. First, they using the theoretical approach based on approximations to calculate the so-called thermoelectric figure of merit, expected to be high for systems with high energy conversion efficiency. Then, they calculated the charge and heat fluxes as a means to define the efficiency of the system.
They found that the outcomes of the direct calculations giving the actual—as opposed to theoretical—performance of the system were less optimistic. For most parameters with an excellent performance, calculated predictions turned out to be surprisingly poor. These findings reveal that effects that are not easily formalized using equations are important at the nanoscale. This, in turn, calls for new ways to optimize the structures before they can be used for nanoscale energy harvesting.
|Colonies of microbes produce methane gas and other compounds in the lab
of Stanford Professor Alfred Spormann. The research goal is to create large
microbial factories that convert electricity and carbon dioxide into renewable biofuels
New findings by Stanford engineering Professor Alfred Spormann and colleagues could pave the way for microbial “factories” that produce renewable biofuels and chemicals.
Stanford University scientists have solved a long-standing mystery about methanogens, unique microorganisms that transform electricity and carbon dioxide into methane.
In a new study, the Stanford team demonstrates for the first time how methanogens obtain electrons from solid surfaces. The discovery could help scientists design electrodes for microbial “factories” that produce methane gas and other compounds sustainably.
“There are several hypotheses to explain how electrons get from an electrode into a methanogen cell,” said Stanford postdoctoral scholar Jörg Deutzmann, lead author of the study. “We are the first group to identify the actual mechanism.”
The study is published in the current issue of the journal mBio.
“The overall goal is to create large bioreactors where microbes convert atmospheric carbon dioxide and clean electricity from solar, wind or nuclear power into renewable fuels and other valuable chemicals,” said study co-author Alfred Spormann, a professor of chemical engineering and of civil and environmental engineering at Stanford. “Now that we understand how methanogens take up electricity, we can re-engineer conventional electrodes to deliver more electrons to more microbes at a faster rate.”
The study also provided new insights on microbially influenced corrosion, a biological process that threatens the long-term stability of structures made of iron and steel.
“Biocorrosion is a significant global problem,” Spormann said. “The yearly economic loss caused by this process is estimated to be in the $1 billion range.”
Methane from microbes
Methane is an important fuel for heating, transportation, cooking and generating electricity. Most methane comes from natural gas, an abundant fossil fuel extracted from wells. However, burning natural gas emits carbon dioxide, which accelerates global warming.
Methanogens offer a promising alternative. These single-celled organisms resemble bacteria but belong to a genetically distinct domain called Archaea.
Commonly found in sediments and sewage treatment plants, methanogens thrive on carbon dioxide gas and electrons. The byproduct of this primordial meal is pure methane gas, which the microbes excrete into the air.
Researchers are trying to develop large bioreactors where billions of methanogens crank out methane around the clock. These microbial colonies would be fed carbon dioxide from the atmosphere and clean electricity from electrodes.
The entire process would be carbon neutral, Spormann explained. “When microbial methane is burnt as fuel, carbon dioxide gets recycled back into the atmosphere where it originated,” he said. “Natural gas combustion, on the other hand, frees carbon that has been trapped underground for millions of years.”
Producing microbial methane on an industrial scale will require major improvements in efficiency, Deutzmann said.
“Right now the main bottleneck in this process is figuring out how to get more electrons from the electrode into the microbial cell,” he said. “To do that, you first have to know how electron uptake works in methanogens. Then you can engineer and enhance the electron-transfer rate and increase methane production.”
In nature, methanogens acquire electrons from hydrogen and other molecules that form during the breakdown of organic material or bacterial fermentation.
“These small molecules are food for the microbes,” Deutzmann said. “They provide methanogens with electrons to metabolize carbon dioxide and produce methane.”
In the Spormann lab, methanogens don’t have to worry about food. Electrons are continuously supplied by a low-voltage current via an electrode. How those electrons get into the methanogen cell has been the subject of scientific debate.
“The leading hypothesis is that many microbes, including methanogens, take up electrons directly from the electrode,” Deutzmann said. “But in a previous study, we found evidence that microbial enzymes and other molecules could also play a role. From an engineering perspective, it makes a difference if you have to design an electrode to accommodate large microbial cells versus enzymes. You can attach a lot more enzymes to the electrode, because enzymes are a lot smaller.”
Experiments with enzymes
For the experiment, the Stanford team used a species of methanogen called Methanococcus maripaludis. Cultures of M. maripaludis were grown in flasks equipped with a graphite electrode, which provided a steady supply of electrons. The microbes were also fed carbon dioxide gas.
As expected, methane gas formed inside the flasks, a clear indication that the methanogens were taking up electrons and metabolizing carbon dioxide. But researchers also detected a build-up of hydrogen gas. Were these molecules of hydrogen shuttling electrons to the methanogens, as occurs in nature?
To find out, the Stanford team repeated the experiment using a genetically engineered strain of M. maripaludis. These mutant methanogens had six genes deleted from their DNA so they could no longer produce the enzyme hydrogenase, which microbes need to make hydrogen. Although the mutants were grown in the same conditions as normal methanogens, their methane output was significantly lower.
“When hydrogenase was absent from the culture, methane production plummeted 10-fold,” Spormann said. “This was a strong indication that hydrogen-producing enzymes are significantly involved in electron uptake.”
Further tests without methanogen cells confirmed that hydrogenase and other enzymes take up electrons directly from the electrode surface. The microbial cell itself is not involved in the transfer, as was widely assumed.
“It turns out that all kinds of enzymes are just floating around in the culture medium,” Deutzmann said. “These enzymes can attach to the electrode surface and produce small molecules, like hydrogen, which then feed the electrons to the microbes.”
Normal methanogen cells produce a variety of enzymes. Stirring, starvation and other biological factors can cause the cells to break open, releasing enzymes into the culture medium, Deutzmann said.
“Now that we know that certain enzymes take up electrons, we can engineer them to work better and search for other enzymes that do it even faster,” he added. “Another benefit is that we no longer have to design large, porous electrodes to accommodate the entire methanogen cell.”
The Stanford team also discovered that methanogen enzymes play a similar role in biocorrosion. The researchers found that granules of iron transfer electrons directly to hydrogenase. The enzyme uses these electrons to make hydrogen molecules, which, in turn, are consumed by methanogens. Eliminating hydrogenase from the environment could slow down the rate of corrosion, according to the scientists.
“At first we were surprised by these results, because enzymes were thought to degrade very quickly once they were outside the cell,” Spormann said. “But our study showed that free enzymes attached to an electrode surface can remain active for a month or two. Understanding why they are stable for so long could lead to new insights on reducing corrosion and on scaling up the production of microbial methane and other sustainable chemicals.”
The mBio paper was also co-authored by Stanford researcher Merve Sahin. The study was supported by the Global Climate and Energy Project at Stanford.
The latest version of a microfluidic device for capturing rare circulating tumor cells (CTCs) is the first designed specifically to capture clusters of two or more cells, rather than single cells. The new device, called the Cluster-Chip, was developed by the same Massachusetts General Hospital (MGH) research team that created previous microchip-based devices. Recent studies by MGH investigators and others have suggested that CTC clusters are significantly more likely to cause metastases than single circulating tumor cells.