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White GRaphene 07-20-15_BORON-1-rnx250Pioneering new research by the Univ. of Exeter could pave the way for miniaturized optical circuits and increased internet speeds, by helping accelerate the ‘graphene revolution’.

Physicists from the Univ. of Exeter in collaboration with the ICFO Institute in Barcelona have used a ground-breaking new technique to trap light at the surface of the wonder material graphene using only pulses of laser light.

Crucially, the team of scientists have also been able to steer this trapped light across the surface of the graphene, without the need for any nanoscale devices. This dual breakthrough opens up a host of opportunities for advances in pivotal electronic products, such as sensors and miniaturized integrated circuits.

The new research features in the latest online edition of the respected scientific journal, Nature Physics.

Dr. Tom Constant, lead author on the paper and part of Exeter’s Physics and Astronomy Department said: ” This new research has the potential to give us invaluable insight into the wonder material and how it interacts with light. A more immediate commercial application could be a simple device that could easily scan a piece of graphene and tell you some key properties like conductivity, resistance and purity .”

Dr. Constant and his colleagues used pulses of light to be able to trap the light on the surface of commercially-available graphene. When trapped, the light converts into a quasi-particle called a ‘surface plasmon’, a mixture of both light and the graphene’s electrons.

Additionally, the team have demonstrated the first example of being able to steer the plasmons around the surface of the graphene, without the need to manufacture complicated nanoscale systems. The ability both to trap light at a surface, and direct it easily, opens up new opportunities for a number of electronic-based devices, as well as help to bridge the gap between the electronics and light.

Dr. Constant said: “Computers than can use light as part of their infrastructure have the potential to show significant improvement. Any advance that reveals more about light’s interaction with graphene-based electronics will surely benefit the computers or smartphones of the future.”

Source: Univ. of Exeter

jeffrey-grossman-mitWith the intensifying drought in California, the state has accelerated the construction of desalination plants. Yet due to high construction and operating costs, as well as environmental concerns, we’re not likely to see reclaimed seawater represent more than a small fraction of America’s clean water reserves for some time to come. Aside from other costs, the immense amounts of energy required to make clean water from seawater continues to make desalination a niche solution in most parts of the world.

When Jeffrey Grossman, a professor at MIT’s Department of Materials Science and Engineering (DMSE), began looking into whether new materials might reduce the cost of desalination, he was surprised to find how little research and development money was being applied to the problem.

“A billion people around the world lack regular access to clean , and that’s expected to more than double in the next 25 years,” Grossman says. “Desalinated water costs five to 10 times more than regular municipal water, yet we’re not investing nearly enough money into research. If we don’t have clean energy we’re in serious trouble, but if we don’t have water we die.”

At the Grossman Group, which explores the development of new materials to address clean energy and water problems, a possible solution may be at hand. Grossman’s lab has demonstrated strong results showing that new made from could greatly improve the energy efficiency of desalination plants while potentially reducing other costs as well.

Graphene, which results from slicing off an atom-thick layer of graphite, is increasingly emerging as something of a wonder material. The Grossman Group, for example, is also looking into using it as a cheaper alternative to silicon for making solar cells.

Graphene Nano Membrane 071615

“It’s never been a more exciting time to be a materials scientist,” says Grossman. “When you look at clean tech or water filtration, you find that the energy conversion bottleneck stems from the material. We can now design materials pretty much all the way down to the scale of the atom in almost any way we want, tailoring materials in ways that were previously impossible. There’s a convergence emerging in which we are facing enormously pressing problems that can only be solved by developing .”

Graphene filters: Up to 50 percent less energy

First isolated in 2003, graphene has different electrical, optical, and mechanical properties than graphite. “It’s stronger than steel, and it has unique sieving properties,” Grossman says. At only an atom thick, there’s far less friction loss when you push seawater through a perforated graphene filter compared with the polyamide plastic filters that have been used for the last 50 years, he says.

“We have shown that perforated graphene filters can handle the water pressures of desalination plants while offering hundreds of times better permeability,” Grossman explains. “The process of pumping seawater through filters represents about half the operating costs of a desalination plant. With graphene, we could use up to 50 percent less energy.”

Another advantage is that graphene filters don’t become fouled with bio-growth at nearly the rate that occurs with polyamide filters. Desalination plants often run at reduced efficiency due to the need to frequently clean the filters. In addition, the chlorine used to clean the filters reduces the structural integrity of the polyamide, requiring frequent replacement. By comparison, graphene is resistant to the damaging effects of chlorine.

According to Grossman, you could easily replace polyamide filters with graphene filters in existing plants. Like polyamide filters, graphene filters can be mounted on robust polysulfone supports, which have larger holes that sieve out particulates.

“We have shown that perforated graphene filters can handle the water pressures of desalination plants while offering hundreds of times better permeability,” Grossman explains. “The process of pumping seawater through filters represents about half the operating costs of a desalination plant. With graphene, we could use up to 50 percent less energy.”

Another advantage is that graphene filters don’t become fouled with bio-growth at nearly the rate that occurs with polyamide filters. Desalination plants often run at reduced efficiency due to the need to frequently clean the filters. In addition, the chlorine used to clean the filters reduces the structural integrity of the polyamide, requiring frequent replacement. By comparison, graphene is resistant to the damaging effects of chlorine.

According to Grossman, you could easily replace polyamide filters with graphene filters in existing plants. Like polyamide filters, graphene filters can be mounted on robust polysulfone supports, which have larger holes that sieve out particulates.

Yet, significant challenges remain in bringing down costs. The Grossman Group has made good progress in creating high volumes of graphene at a reasonably low cost. A more serious challenge, however, is cost-effectively poking uniform holes in the graphene in a highly scalable manner.

“A typical plant has tens of thousands of membranes, configured in two-meter long tubes, each of which has 40 square meters of rolled up active membrane,” Grossman says. “We have to match that volume at the same cost, or it’s a nonstarter.”

Making graphene on the cheap

The traditional way to make graphene—since its first isolation in 2003, mind you—is to peel it off with adhesive. “You literally take a piece of Scotch Tape to graphite and you peel,” Grossman explains. “If you keep doing this, you eventually wind up with a single layer. The problem is it would take forever to peel off enough graphene for a desalination plant.”

Another approach is to “grow” graphene by applying super-hot gases to copper foil. “Growing graphene provides the best quality, which is why the semiconductor industry is interested in it,” Grossman says. The process, however, is very expensive and energy-intensive.

Instead, the Grossman Group is using a much more affordable chemical approach, which produces sufficient quality for creating desalination membranes. “Fortunately, our application doesn’t require the best quality,” says Grossman. “With the chemical technique, we put graphite in a solution, and apply low temperature chemistry to break apart the entire chunk of graphite into sheets. We can get lots of graphene very cheaply and quickly.”

Creating pores that block salt but let water molecules pass is a steeper challenge. The reason desalination is possible in the first place is that when diffused in water, salt ions bond with water molecules, thereby creating a larger entity. But the difference in size compared to a free water molecule is still frustratingly small.

“The challenge is to find the sweet spot of about 0.8 nanometers,” Grossman says. “If your pores are at 1.5 nm, then both the water and salt will pass through. If they’re half a nanometer, then nothing gets through.”

A 0.8 nm hole is “smaller than we’ve ever been able to make in a controllable way with any other material,” Grossman says. “And we need to do this over a very large area very consistently and cheaply.”

The Grossman Group is pursuing three techniques to make nanoporous graphene membranes, all of which use chemical and thermal energy rather than mechanical processes. “If you tried to use lithography, it would take years,” Grossman says. “Our first approach involves making the holes too big, and then carefully filling them in. Another tries to make them exactly the right size, and the third involves starting with a material without holes and then carefully ripping it apart.”

The chemical technique for making graphene actually produces graphene oxide, which is considered undesirable for semiconductors, but is fine for filters. As a result, the researchers were able to avoid the difficult step of removing the oxygen from the graphene oxide. In fact, they found a way to use the oxygen to their advantage.

“By controlling the way the oxygen is bonded to the graphene sheet, we can use chemical and thermal energy to drill the holes with the help of the oxygen,” Grossman says.

First target: Brackish water

As the Grossman Group continues to work on the challenge of manufacturing and perforating graphene sheets, Grossman is looking to leverage other benefits of graphene filters to help bring the technology to market.

Although graphene should improve efficiency with seawater and the even saltier, dirtier water used in hydraulic fracturing, it will likely debut in plants that clean brackish water, such as found in estuaries. “It turns out that higher permeability even by a factor of two or three would make a bigger difference with brackish water than with seawater,” Grossman says. “You lower the energy consumption in both cases, but more so for brackish water.”

Graphene filters could also enable the construction of smaller, cheaper plants. “With graphene you have more choices in how you operate the plant,” Grossman says. “You could apply the same pressures but get more water out, or you could operate it at lower pressures and get the same amount of water, but at a lower energy cost.”

Grossman notes that it can take years or even decades to site and permit a plant in heavily populated coastal areas. “A lot of effort goes into how you’re going to build the plant and where you’re going to find enough land,” Grossman says. “Having the option to build a smaller plant would be a big advantage.”

Explore further: Nanoporous graphene could outperform best commercial water desalination techniques

MIT-Graphene-Pores_0

Researchers created pores in a graphene sheet (in purple) and then placed it over a layer of silicon nitride (in blue) that had been punctured by an ion beam. This allows specific hydrated ions, which are surrounded by a shell of water molecules, to pass through.

Image: Jose-Luis Olivares/MIT

MIT News Office
October 5, 2015

The surface of a single cell contains hundreds of tiny pores, or ion channels, each of which is a portal for specific ions. Ion channels are typically about 1 nanometer wide; by maintaining the right balance of ions, they keep cells healthy and stable. Like biological channels, graphene pores are selective for certain types of ions.

Now researchers at MIT have created tiny pores in single sheets of graphene that have an array of preferences and characteristics similar to those of ion channels in living cells.

Each graphene pore is less than 2 nanometers wide, making them among the smallest pores through which scientists have ever studied ion flow. Each is also uniquely selective, preferring to transport certain ions over others through the graphene layer.

“What we see is that there is a lot of diversity in the transport properties of these pores, which means there is a lot of potential to tailor these pores to different applications or selectivities,” says Rohit Karnik, an associate professor of mechanical engineering at MIT.

Karnik says graphene nanopores could be useful as sensors — for instance, detecting ions of mercury, potassium, or fluoride in solution. Such ion-selective membranes may also be useful in mining: In the future, it may be possible to make graphene nanopores capable of sifting out trace amounts of gold ions from other metal ions, like silver and aluminum.

Karnik and former graduate student Tarun Jain, along with Benjamin Rasera, Ricardo Guerrero, Michael Boutilier, and Sean O’Hern from MIT and Juan-Carlos Idrobo from Oak Ridge National Laboratory, publish their results today in the journal Nature Nanotechnology.

Dynamic personality

In living cells, the diversity of ion channels may arise from the size and precise atomic arrangement of the channels, which are slightly smaller than the ions that flow through them.

“When nanopores get smaller than the hydrated size of the ion, then you start to see interesting behavior emerge,” Jain says.

In particular, hydrated ions, or ions in solution, are surrounded by a shell of water molecules that stick to the ion, depending on its electrical charge. Whether a hydrated ion can squeeze through a given ion channel depends on that channel’s size and configuration at the atomic scale.

Karnik reasoned that graphene would be a suitable material in which to create artificial ion channels: A sheet of graphene is an ultrathin lattice of carbon atoms that is one atom thick, so pores in graphene are defined at the atomic scale.

To create pores in graphene, the group used chemical vapor deposition, a process typically used to produce thin films. In graphene, the process naturally creates tiny defects. The researchers used the process to generate nanometer-sized pores in various sheets of graphene, which bore a resemblance to ultrathin Swiss cheese.

The researchers then isolated individual pores by placing each graphene sheet over a layer of silicon nitride that had been punctured by an ion beam, the diameter of which is slightly smaller than the spacing between graphene pores. The group reasoned that any ions flowing through the two-layer setup would have likely passed first through a single graphene pore, and then through the larger silicon nitride hole.

The group measured flows of five different salt ions through several graphene sheet setups by applying a voltage and measuring the current flowing through the pores. The current-voltage measurements varied widely from pore to pore, and from ion to ion, with some pores remaining stable, while others swung back and forth in conductance — an indication that the pores were diverse in their preferences for allowing certain ions through.

“The picture that emerges is that each pore is different and that the pores are dynamic,” Karnik says. “Each pore starts developing its own personality.”

New frontier

Karnik and Jain then developed a model to interpret the measurements, and used it to translate the experiment’s measurements into estimates of pore size. Based on the model, they found that the diameter of many of the pores was below 1 nanometer, which — given the single-atom thickness of graphene  — makes them among the smallest pores through which scientists have studied ion flow.

With the model, the group calculated the effect of various factors on pore behavior, and found that the observed pore behavior was captured by three main characteristics: a pore’s size, its electrical charge, and the position of that charge along a pore’s length.

Knowing this, researchers may one day be able to tailor pores at the nanoscale to create ion-specific membranes for applications such as environmental sensing and trace metal mining.

“It’s kind of a new frontier in membrane technologies, and in understanding transport through these really small pores in ultrathin materials,” Karnik says.

Meni Wanunu, an assistant professor of physics at Northeastern University, says the group’s work with graphene membranes may significantly improve on commercial membranes used for water purification, which require large amounts of pressure to push water through.

“If these were replaced with graphene, since it is so thin, the pressure required to push water through would be among the lowest imaginable, if not the lowest,” says Wanunu, who was not involved in the research. “However, it is only through a fundamental understanding of ion transport that the overall anticipated behaviors of bulk graphene membranes can be drawn. The work here is fundamental, and will surely guide current and future graphene membrane design principles in years to come.”

This research was funded, in part, by the U.S. Department of Energy.

S Korea Graphene Sensors fibersensorx250Scientists in Korea have developed wearable, graphene-coated fabrics that can detect dangerous gases present in the air, alerting the wearer by turning on a light-emitting diode (LED) light.

The researchers, from the Electronics and Telecommunications Research Institute and Konkuk Univ. in the Republic of Korea, coated cotton and polyester yarn with a nanoglue called bovine serum albumin (BSA). The yarns were then wrapped in graphene oxide sheets.

Graphene is an incredibly strong one-atom-thick layer of carbon, and is known for its excellent conductive properties of heat and electricity. The graphene sheets stuck very well to the nanoglue—so much so that further testing showed the fabrics retained their electrical conducting properties after 1,000 consecutive cycles of bending and straightening and ten washing tests with various chemical detergents. Finally, the graphene oxide yarns were exposed to a chemical reduction process, which involves the gaining of electrons.

The reduced-graphene-oxide-coated materials were found to be particularly sensitive to detecting nitrogen dioxide, a pollutant gas commonly found in vehicle exhaust that also results from fossil fuel combustion. Prolonged exposure to nitrogen dioxide can be dangerous to human health, causing many respiratory-related illnesses. Exposure of these specially treated fabrics to nitrogen dioxide led to a change in the electrical resistance of the reduced graphene oxide.

The fabrics were so sensitive that 30 mins of exposure to 0.25 ppm of nitrogen dioxide (just under five times above the acceptable standard set by the U.S. Environmental Protection Agency) elicited a response. The fabrics were three times as sensitive to nitrogen dioxide in air compared to another reduced graphene oxide sensor previously prepared on a flat material.

The new technology, according to the researchers, can be immediately adopted in related industries because the coating process is a simple one, making it suitable for mass production. It would allow outdoor wearers to receive relevant information about air quality. The materials could also be incorporated with air-purifying filters to act as “smart filters” that can both detect and filter harmful gas from air.

“This sensor can bring a significant change to our daily life since it was developed with flexible and widely used fibers, unlike the gas sensors invariably developed with the existing solid substrates,” says Dr. Hyung-Kun Lee, who led this research initiative. The study was published online in Scientific Reports.

Source: Electronics and Telecommunications Research Institute

graphene-silicon-perovskite-solar-cell-id41503Silicon absorbers primarily convert the red portion of the solar spectrum very effectively into electrical energy, whereas the blue portions are partially lost as heat. To reduce this loss, the silicon cell can be combined with an additional solar cell that primarily converts the blue portions.

Teams at Helmholtz Zentrum Berlin (HZB) have already acquired extensive experience with these kinds of tandem cells. A particularly effective complement to conventional silicon is the hybrid material called perovskite. It has a band gap of 1.6 electron volts with organic as well as inorganic components. However, it is very difficult to provide the perovskite layer with a transparent front contact. While sputter deposition of indium tin oxide (ITO) is common practice for inorganic silicon solar cells, this technique destroys the organic components of a perovskite cell.

Graphene as transparent front contact
Now a group headed by Prof. Norbert Nickel has introduced a new solution. Dr. Marc Gluba and PhD student Felix Lang have developed a process to cover the perovskite layer evenly with graphene (“Perovskite Solar Cells with Large-Area CVD-Graphene for Tandem Solar Cells”). Graphene consists of carbon atoms that have arranged themselves into a two-dimensional honeycomb lattice forming an extremely thin film that is highly conductive and highly transparent.
silicon-perovskite tandem solar cell
The perovskite film (black, 200-300 nm) is covered by Spiro.OMeTAD, Graphene with gold contact at one edge, a glass substrate and an amorphous/crystalline silicon solar cell. (Image: F. Lang / HZB)
Fishing for graphene
As a first step, the scientists promote growth of the graphene onto copper foil from a methane atmosphere at about 1000 degrees Celsius. For the subsequent steps, they stabilise the fragile layer with a polymer that protects the graphene from cracking. In the following step, Felix Lang etches away the copper foil. This enables him to transfer the protected graphene film onto the perovskite.
“This is normally carried out in water. The graphene film floats on the surface and is fished out by the solar cell, so to speak. However, in this case this technique does not work, because the performance of the perovskite degrades with moisture. Therefore we had to find another liquid that does not attack perovskite, yet is as similar to water as possible”, explains Gluba.
Ideal front contact
Subsequent measurements showed that the graphene layer is an ideal front contact in several respects. Thanks to its high transparency, none of the sunlight’s energy is lost in this layer. But the main advantage is that there are no open-circuit voltage losses, that are commonly observed for sputtered ITO layers. This increases the overall conversion efficiency.
“This solution is comparatively simple and inexpensive to implement”, says Nickel. “For the first time, we have succeeded in implementing graphene in a perovskite solar cell. This enabled us to build a high-efficiency tandem device.”
Source: Helmholtz Zentrum Berlin

Rice Pillard Graphene 0914_HYBRID-1-WEBx250Rice Univ. researchers discovered that putting nanotube pillars between sheets of graphene could create hybrid structures with a unique balance of strength, toughness and ductility throughout all three dimensions.

Carbon nanomaterials are common now as flat sheets, nanotubes and spheres, and they’re being eyed for use as building blocks in hybrid structures with unique properties for electronics, heat transport and strength. The Rice team is laying a theoretical foundation for such structures by analyzing how the blocks’ junctions influence the properties of the desired materials.

Rice materials scientist Rouzbeh Shahsavari and alumnus Navid Sakhavand calculated how various links, particularly between carbon nanotubes and graphene, would affect the final hybrid’s properties in all directions. They found that introducing junctions would add extra flexibility while maintaining almost the same strength when compared with materials made of layered graphene.

Their results appear in Carbon.

Carbon nanotubes are rolled-up arrays of perfect hexagons of atoms; graphene is a rolled-out sheet of the same. Both are super-strong and excel at transmitting electrons and heat. But when the two are joined, the way the atoms are arranged can influence all those properties.

Rice Pillard Graphene 0914_HYBRID-1-WEBx250

Carbon nanotube pillars between sheets of graphene may create hybrid structures with a unique balance of strength, toughness and ductility throughout all three dimensions, according to Rice Univ. scientists. Five, seven or eight-atom rings at the junctions can force the graphene to wrinkle. Image: Shuo Zhao and Lei Tao/Rice Univ.

“Some labs are actively trying to make these materials or measure properties like the strength of single nanotubes and graphene sheets,” Shahsavari said. “But we want to see what happens and quantitatively predict the properties of hybrid versions of graphene and nanotubes. These hybrid structures impart new properties and functionality that are absent in their parent structures—graphene and nanotubes.”

To that end, the lab assembled three-dimensional computer models of “pillared graphene nanostructures,” akin to the boron-nitride structures modeled in a previous study to analyze heat transfer between layers.

“This time we were interested in a comprehensive understanding of the elastic and inelastic properties of 3-D carbon materials to test their mechanical strength and deformation mechanisms,” Shahsavari said. “We compared our 3-D hybrid structures with the properties of 2-D stacked graphene sheets and 1-D carbon nanotubes.”

Layered sheets of graphene keep their properties in-plane, but exhibit little stiffness or thermal conductance from sheet to sheet, he said. But pillared graphene models showed far better strength and stiffness and a 42 percent improvement in out-of-plane ductility, the ability to deform under stress without breaking. The latter allows pillared graphene to exhibit remarkable toughness along out-of-plane directions, a feature that is not possible in 2-D stacked graphene sheets or 1-D carbon nanotubes, Shahsavari said.

The researchers calculated how the atoms’ inherent energies force hexagons to take on or lose atoms to neighboring rings, depending on how they join with their neighbors. By forcing five, seven or even eight-atom rings, they found they could gain a measure of control over the hybrid’s mechanical properties. Turning the nanotubes in a way that forced wrinkles in the graphene sheets added further flexibility and shear compliance, Shahsavari said.

When the material did fracture, the researchers found it far more likely for this to happen at the eight-member rings, where much of the strain gathers when stressed. That leads to the notion the hybrids can be tuned to fail under particular circumstances.

“This is the first time anyone has created such a comprehensive atomistic ‘lens’ to look at the junction-mediated properties of 3-D carbon nanomaterials,” Shahsavari said. “We believe the principles can be applied to other low-dimensional materials such as boron nitride and molybdenum/tungsten or the combinations thereof.”

Source: Rice Univ.

Harvesting heat produced by a car’s engine which would otherwise be wasted and using it to recharge the car’s batteries or powering the air-conditioning system could be a significant feature in the next generation of hybrid cars.
Prof Ian Kinloch, Professor of Materials Science
Prof Ian Kinloch, Professor of Materials Science
The average car currently loses around 70% of energy generated through fuel consumption to heat. Utilising that lost energy requires a thermoelectric material which can generate an electrical current from the application of heat.

Thermoelectric materials convert heat to electricity or vice-versa, such as with refrigerators. The challenge with these devices is to use a material that is a good conductor of electricity but also dissipates heat well.

Currently, materials which exhibit these properties are often toxic and operate at very high temperatures – higher than that produced by car engines. By adding graphene, a new generation of composite materials could reduce carbon emissions globally from car use.

Scientists from The University of Manchester working with European Thermodynamics Ltd have increased the potential for low cost thermoelectric materials to be used more widely in the automotive industry.
The team, led by Prof Ian Kinloch, Prof Robert Freer and Yue Lin, added a small amount of graphene to strontium titanium oxide.
The resulting composite was able to convert heat which would otherwise be lost as waste into an electric current over a broad temperature range, going down to room temperature.
Prof Freer said: “Current oxide thermoelectric materials are limited by their operating temperatures which can be around 700 degrees Celsius. This has been a problem which has hampered efforts to improve efficiency by utilising heat energy waste for some time.
“Our findings show that by introducing a small amount of graphene to the base material can reduce the thermal operating window to room temperature which offers a huge range of potential for applications.
“The new material will convert 3-5% of the heat into electricity. That is not much but, given that the average vehicle loses roughly 70% of the energy supplied to it by its fuel to waste heat and friction, recovering even a small percentage of this with thermoelectric technology would be worthwhile.”
The findings were published in the journal ACS Applied Materials and Interfaces (“hermoelectric power generation from lanthanum strontium titanium oxide at room temperature through the addition of graphene”). Graphene’s range of superlative properties and small size causes the transfer of heat through the material to slow leading to the desired lower operating temperatures.
Improving fuel efficiency, whilst retaining performance, has long been a driving force for car manufacturers. Graphene could also aid fuel economy and safety when used as a composite material in the chassis or bodywork to reduce weight compared to traditional materials used.
Source: University of Manchester

Graphene Perovskite 081115 324x182 EPFL scientists have created the first perovskite nanowire-graphene hybrid phototransistors. Even at room temperature, the devices are highly sensitive to light, making them outstanding photodetectors.

 

The lead-containing perovskite materials can turn light into electricity with high efficiency, which is why they have revolutionized solar cell technologies. On the other hand, graphene is known for its super-strength as well as its excellent electrical conductivity. Combining the two materials, EPFL scientists have created the first ever class of hybrid transistors that turn light into electricity with high sensitivity and at room temperature. The work is published in Small.

The lab of László Forró at EPFL, where the chemical activity is led by Endre Horváth, used its expertise in microengineering to create nanowires of the perovskite methylammonium lead iodide. This highly non-trivial route for the synthesis of nanowires was developed by him in 2014 and called slip-coating method. The advantage of nanowires is their consistency, while their manufacturing can be controlled to modify their architecture and explore different designs.

Making a device by depositing the perovskite nanowires onto graphene has increased the efficiency in converting light to electrical current at room temperature. “Such a device shows almost 750,000 times higher photoresponse compared to detectors made only with perovskite nanowires,” added Massimo Spina who fabricated the miniature photodetectors. Because of this exceptionally high sensitivity, the graphene/perovskite nanowire hybrid device is considered to be a superb candidate for even a single-photon detection.

This work was founded by the Swiss National Science Foundation. The hybrid devices were fabricated in part at EPFL’s Center for Micro/Nanotechnology.

Reference

WEF Nano Energy Crisis RTR4XD9A-layout-comp-628x330This post is part of a series examining the connections between nanotechnology and the top 10 trends facing the world, as described in the Outlook on the Global Agenda 2015. All authors are members of the Global Agenda Council on Nanotechnology. Special to the World Economic Forum: By Tim Harper

The late Richard Smalley, often considered to be one of the fathers of nanotechnology following his Nobel Prize-winning work on fullerenes, had a keen interest in energy. In many presentations he would ask the audience to call out what they considered to be the most pressing issues facing humanity. The answers were often similar to those identified in the World Economic Forum’s Global Risks Report, including persistent worries such as disease, clean water, poverty, inequality and access to resources. Smalley would then rearrange the list to put energy at the top and proceed to explain how a happy, healthy world of 9 billion could be achieved if we could only fix the problem of providing cheap and abundant clean energy.

Back in the early 2000s, most of the imagined solutions to the energy challenge involved novel materials such as carbon nanotubes for lossless electricity transmission, or hydrogen storage to enable fuel-cell vehicles. While novel materials like nanotubes never quite lived up to their promise, 15 years later many nanotechnologies, including the latest carbon-based material graphene, are now promising to deliver huge leaps in the way that we generate, store and use energy.

But these advances are not enabled by nanotechnologies in isolation. Many of the technologies identified in the Forum’s top 10 emerging technologies list for the past three years, from gene editing to additive manufacturing, also play a role, supporting our ability to understand the nanoscale processes in nature, generating new insights into how to move beyond conventional solar cells and copy some of nature’s tricks, such as photosynthesis.

Solar solutions

The problem is that conventional silicon-based solar cells, while effective, have many drawbacks. They are brittle, which means that they need to be fixed to a rigid support, and they only harvest a small amount of the spectrum of light generated by the sun. For instance, silicon is transparent to infrared light, which means a lot of potential energy available is not harvested.

Researchers at the University of California, Riverside, are helping to solve this by working with hybrid material combining inorganic semiconductor nanoparticles with organic compounds. These first capture two infrared photons that would normally pass right through a solar cell without being converted to electricity, then add their energies together to make one higher energy photon.

An alternative approach is the use of quantum dots. These are nanoscale particles where the response to different wavelengths can be tuned by altering their sizes. Because of their unique optical properties, they are finding increasing uses in lighting and televisions, but these properties are also useful in solar cells. While the efficiency of quantum-dot solar cells reported in recent studies is increasing to as high as 9%, the real breakthrough is that the new devices can be produced at room temperature and in an atmosphere, rather than an expensive and hard-to-maintain vacuum. Perhaps the most exciting aspect of quantum-dot solar cells, though, is that the quantum dots can be dispersed in other materials, leading to “spray on” low-cost and large-area solar cells that can be applied to buildings or vehicles.

A leaf out of nature’s book

But the big prize in advanced photovoltaics will come with achieving artificial photosynthesis. The aim is to enable the production of useful chemicals and fuels directly from sunlight and carbon dioxide, just as plants do. By combining nanotechnology and biology, researchers are mimicking the processes that occur in the leaf of a plant to produce fuels such as butanol and biodegradable plastics. Once combined with synthetic biology to precisely engineer the bacteria, the possibilities are endless.

Generating energy is only half the solution, though. It also has to be stored for later use. This is an addressable issue for energy utilities, who balance peaks and troughs in demand by using techniques such as pumping water uphill into hydro-electric dams. But such large-scale engineering solutions are not an option for off-grid communities in much of the developing world. Local energy use requires a cheap and efficient way of storing energy, as do electric vehicles and smartphones.

Nanomaterials, and graphene in particular, have been attracting significant interest as potential game-changers for energy storage. One driver for this is the high surface area of many nanomaterials, which increases the ability to store charge within a given volume. Graphene – which is formed from layers of carbon a single atom thick – has a tremendous surface area for a given amount of material, and has created a lot of excitement about graphene-based supercapacitors and anodes for lithium ion batteries.

One of the biggest problems with the lithium ion batteries is the amount of charge that can be stored in the conventional graphite-based anodes they use. Lithium is added to the graphite when the battery is charging and removed as it discharges, but the low capacity of graphite means that the anode is limited in the amount of energy it can store. Researchers have been looking at silicon anodes that promise 10 times better capacity for the best part of decade, but the constant stresses on the material results in a short lifetime. One way of addressing this issue has been to place the silicon in cage of fullerenes, nanotubes or nanowires. Companies such as XG Sciences and California Lithium Battery are developing graphene-coated silicon, or “silicon-graphene nano-composite anode material”.Nanotech World stock-photo-background-concept-wordcloud-illustration-of-nanotechnology-glowing-light-76352191

Fast-charging batteries

Taking a more bio-inspired approach, the Israeli company StoreDot is combining nanotechnology and biology to create nanoscale peptide crystals to produce a battery that will charge in less than a minute, while researchers in Singapore have recently developed a nanotube-based battery that could last more than 10 times as long as normal ion batteries and can also charge in minutes.

In the meantime, while we wait for current nanotechnology research to bear fruit, the biggest contribution that nanotechnology can make today is simply to reduce the amount of energy required to perform common tasks, such as heating and cooling.

The UK company Xefro, for instance, is making use of graphene to create a smart home-heating system which promises savings of up to 70%. The heaters make use of the high surface area of what is effectively a two-dimensional material to create an efficient heating material which is then applied as an ink. The ink can be printed on a variety of materials and in just about any shape, including water heaters. In a two-dimensional material, energy isn’t wasted in heating up the heater, so the heat can be turned on and off quickly. This both reduces energy use and makes the system ideal for use with smart thermostats.

Cool fractals

Meanwhile, another UK start-up called Inclusive Designs is addressing the problem of keeping things cool by combining nanomaterials and fractals with 3D printing. The company prints 3D fractal structures designed to absorb infrared (heat) and then removes the heat by making use of the high thermal conductivity of graphene, creating a cooling system with no liquids or moving parts.

Since Richard Smalley’s untimely death in 2005, the energy situation has improved, with an increasing number of countries now generating the majority of their power from renewable sources; electric vehicles are now a common sight. But cheap, efficient renewable-energy production – together with its storage and transmission – remains a challenge. The combination of nanotechnology, with a wide range of other emerging and transformative technologies, promises to make Smalley’s dream of a world of abundant, cheap, clean energy a reality over the coming decade.

GNT Thumbnail Alt 3 2015-page-001

Also Read: Genesis Nanotech Home Page

“Great Things from Small Things”

Also Read Our Online “Nano-News and Updates” at: GNT OnLine: “Great Things from Small Things”

More reading:
What does nanotech mean for geopolitics?
How new nanomaterials can boost renewables
Why energy poverty is the real energy crisis

Author: Tim Harper, CEO G2O Water International and co-founder of Xefro

Image: Solar panels are seen in the Palm Springs area, California April 13, 2015. REUTERS/Lucy Nicholson

White GRaphene 07-20-15_BORON-1-rnx250Three-dimensional structures of boron nitride might be the right stuff to keep small electronics cool, according to scientists at Rice Univ.

Rice researchers Rouzbeh Shahsavari and Navid Sakhavand have completed the first theoretical analysis of how 3-D boron nitride might be used as a tunable material to control heat flow in such devices.

Their work appears in Applied Materials and Interfaces.

In its 2-D form, hexagonal boron nitride (h-BN), aka white graphene, looks just like the atom-thick form of carbon known as graphene. One well-studied difference is that h-BN is a natural insulator, where perfect graphene presents no barrier to electricity.

But like graphene, h-BN is a good conductor of heat, which can be quantified in the form of phonons. (Technically, a phonon is one part—a “quasiparticle”—in a collective excitation of atoms.) Using boron nitride to control heat flow seemed worthy of a closer look, Shahsavari said.

“Typically in all electronics, it is highly desired to get heat out of the system as quickly and efficiently as possible,” he said. “One of the drawbacks in electronics, especially when you have layered materials on a substrate, is that heat moves very quickly in one direction, along a conductive plane, but not so good from layer to layer. Multiple stacked graphene layers is a good example of this.”

Heat moves ballistically across flat planes of boron nitride, too, but the Rice simulations showed that 3-D structures of h-BN planes connected by boron nitride nanotubes would be able to move phonons in all directions, whether in-plane or across planes, Shahsavari said.

White GRaphene 07-20-15_BORON-1-rnx250

A 3-D structure of hexagonal boron nitride sheets and boron nitride nanotubes could be a tunable material to control heat in electronics, according to researchers at Rice Univ. Image: The Shahsavari Group

The researchers calculated how phonons would flow across four such structures with nanotubes of various lengths and densities. They found the junctions of pillars and planes acted like yellow traffic lights, not stopping but significantly slowing the flow of phonons from layer to layer, Shahsavari said. Both the length and density of the pillars had an effect on the heat flow: more and/or shorter pillars slowed conduction, while longer pillars presented fewer barriers and thus sped things along.

While researchers have already made graphene/carbon nanotube junctions, Shahsavari believed such junctions for boron nitride materials could be just as promising. “Given the insulating properties of boron nitride, they can enable and complement the creation of 3-D, graphene-based nanoelectronics.

“This type of 3-D thermal-management system can open up opportunities for thermal switches, or thermal rectifiers, where the heat flowing in one direction can be different than the reverse direction,” Shahsavari said. “This can be done by changing the shape of the material, or changing its mass—say one side is heavier than the other—to create a switch. The heat would always prefer to go one way, but in the reverse direction it would be slower.”

Source: Rice Univ.


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