Category Archives: energy

Carrying a solar cell on a pencil or glass slide?

Caption: Ultra-thin solar cells are flexible enough to bend around small objects, such as the 1mm-thick edge of a glass slide, as shown here. Credit: Juho Kim, et al/ APL

Caption: Ultra-thin solar cells are flexible enough to bend around small objects, such as the 1mm-thick edge of a glass slide, as shown here. Credit: Juho Kim, et al/ APL

Yes, this is another wearable electronics story and this time, it’s from South Korea. A June 20, 2016 news item on ScienceDaily announces remarkably thin and flexible photovoltaics,

Scientists in South Korea have made ultra-thin photovoltaics flexible enough to wrap around the average pencil. The bendy solar cells could power wearable electronics like fitness trackers and smart glasses. …

A June 20, 2016 American Institute of Physics news release on EurekAlert, which originated the news item, expands on the theme,

Thin materials flex more easily than thick ones – think a piece of paper versus a cardboard shipping box. The reason for the difference: The stress in a material while it’s being bent increases farther out from the central plane. Because thick sheets have more material farther out they are harder to bend.

“Our photovoltaic is about 1 micrometer thick,” said Jongho Lee, an engineer at the Gwangju Institute of Science and Technology in South Korea. One micrometer is much thinner than an average human hair. Standard photovoltaics are usually hundreds of times thicker, and even most other thin photovoltaics are 2 to 4 times thicker.

The researchers made the ultra-thin solar cells from the semiconductor gallium arsenide. They stamped the cells directly onto a flexible substrate without using an adhesive that would add to the material’s thickness. The cells were then “cold welded” to the electrode on the substrate by applying pressure at 170 degrees Celcius and melting a top layer of material called photoresist that acted as a temporary adhesive. The photoresist was later peeled away, leaving the direct metal to metal bond.

The metal bottom layer also served as a reflector to direct stray photons back to the solar cells. The researchers tested the efficiency of the device at converting sunlight to electricity and found that it was comparable to similar thicker photovoltaics. They performed bending tests and found the cells could wrap around a radius as small as 1.4 millimeters.

The team also performed numerical analysis of the cells, finding that they experience one-fourth the amount of strain of similar cells that are 3.5 micrometers thick.

“The thinner cells are less fragile under bending, but perform similarly or even slightly better,” Lee said.

A few other groups have reported solar cells with thicknesses of around 1 micrometer, but have produced the cells in different ways, for example by removing the whole substract by etching.

By transfer printing instead of etching, the new method developed by Lee and his colleagues may be used to make very flexible photovoltaics with a smaller amount of materials.

The thin cells can be integrated onto glasses frames or fabric and might power the next wave of wearable electronics, Lee said.

Here’s a link to and a citation for the paper,

Ultra-thin flexible GaAs photovoltaics in vertical forms printed on metal surfaces without interlayer adhesives by Juho Kim, Jeongwoo Hwang, Kwangsun Song, Namyun Kim, Jae Cheol Shin, and Jongho Lee. Appl. Phys. Lett. 108, 253101 (2016); http://dx.doi.org/10.1063/1.4954039

This paper is open access.

Turning gold into see-through rubber for an updated Rumpelstiltskin story

Rumpelstiltskin is a fairy tale whereby a young girl is trapped by her father’s lie that she can spin straw into gold. She is forced to spin gold by the King under pain of execution when an imp offers to help in exchange for various goods. As she succeeds each time, the King demands more until finally she has nothing left to trade for the imp’s help. Well, there is one last thing: her first-born child. She agrees to the bargain and she marries the King. On the birth of their first child, the imp reappears and under pressure of her pleas makes one last bargain. She must guess his name which she does, Rumplestiltskin. (The full story along with variants is here in this Wikipedia entry.)

With this latest research, we have a reverse Rumpelstiltskin story where gold is turned into something else according to a June 13, 2016 news item on Nanowerk (Note: A link has been removed),

Flexible solar panels that could be rolled up for easy transport and other devices would benefit from transparent metal electrodes that can conduct electricity, are stretchable, and resist damage following repeated stretching. Researchers found that topology and the adhesion between a metal nanomesh and the underlying substrate played key roles in creating such materials. The metal nanomesh can be stretched to three times its length while maintaining a transparency comparable to similar commercial materials used in solar cells and flat panel displays. Also, nanomeshes on pre-stretched slippery substrates led to electrodes that didn’t wear out, even after being stretched 50,000 times (Proceedings of the National Academy of Sciences, “Fatigue-free, superstretchable, transparent, and biocompatible metal electrodes”).

Tuning topology and adhesion of metal nanomeshes has led to super stretchable, transparent electrodes that don’t wear out. The scanning electron microscopy image shows the structure of a gold mesh created with a special lithographic technique that controlled the dimensions of the mesh structure. Optimizing this structure and its adhesion to the substrate was key to achieving super stretchability and long lifetimes in use—nanomeshes on pre-stretched slippery substrates did not show signs of wear even after repeated stretching, up to 50,000 cycles.

Tuning topology and adhesion of metal nanomeshes has led to super stretchable, transparent electrodes that don’t wear out. The scanning electron microscopy image shows the structure of a gold mesh created with a special lithographic technique that controlled the dimensions of the mesh structure. Optimizing this structure and its adhesion to the substrate was key to achieving super stretchability and long lifetimes in use—nanomeshes on pre-stretched slippery substrates did not show signs of wear even after repeated stretching, up to 50,000 cycles.

A June 9, 2016 US Dept. of Energy news release,which originated the news item, provides more detail,

Next-generation flexible electronics require highly stretchable and transparent electrodes. Fatigue, structural damage due to repeated use, is deadly in metals as it leads to poor conductivity and it commonly occurs in metals with repeated stretching—even with short elongations. However, few electronic conductors are transparent and stretchable, even fewer can be cyclically stretched to a large strain without causing fatigue. Now researchers led by the University of Houston found that optimizing topology of a metal nanomesh and its adhesion to an underlying substrate improved stretchability and eliminated fatigue, while maintaining transparency. A special lithographic technique called “grain boundary lithography” controlled the dimensions of the mesh structure. The metal nanomesh remained transparent after being stretched to three times its length. Gold nanomeshes on prestretched slippery substrates impressively showed no wear when stretched 50,000 times. The slippery surface advantageously allowed the structure of the nanomesh to reorient to relax the stress. Such electrically conductive, flexible, and transparent electrodes could lead to next-generation flexible electronics such as advanced solar cells.  The nanomesh electrodes are also promising for implantable electronics because the nanomeshes are biocompatible.

Here’s a link to and a citation for the paper,

Fatigue-free, superstretchable, transparent, and biocompatible metal electrodes by Chuan Fei Guo, Qihan Liu, Guohui Wang, Yecheng Wang, Zhengzheng Shi, Zhigang Suo, Ching-Wu Chu, and Zhifeng Ren. Proceedings of the National Academy of Sciences, vol. 112 no. 40,  12332–12337, doi: 10.1073/pnas.1516873112

This paper appears to be open access.

Cleaning up nuclear waste gases with nanotechnology-enabled materials

Swiss and US scientists have developed a nanoporous crystal that could be used to clean up nuclear waste gases according to a June 13, 2016 news item on Nanowerk (Note: A link has been removed),

An international team of scientists at EPFL [École polytechnique fédérale de Lausanne in Switzerland] and the US have discovered a material that can clear out radioactive waste from nuclear plants more efficiently, cheaply, and safely than current methods.

Nuclear energy is one of the cheapest alternatives to carbon-based fossil fuels. But nuclear-fuel reprocessing plants generate waste gas that is currently too expensive and dangerous to deal with. Scanning hundreds of thousands of materials, scientists led by EPFL and their US colleagues have now discovered a material that can absorb nuclear waste gases much more efficiently, cheaply and safely. The work is published in Nature Communications (“Metal–organic framework with optimally selective xenon adsorption and separation”).

A June 14, 2016 EPFL press release (also on EurekAlert), which originated the news item, explains further,

Nuclear-fuel reprocessing plants generate volatile radionuclides such as xenon and krypton, which escape in the so-called “off-gas” of these facilities – the gases emitted as byproducts of the chemical process. Current ways of capturing and clearing out these gases involve distillation at very low temperatures, which is expensive in both terms of energy and capital costs, and poses a risk of explosion.

Scientists led by Berend Smit’s lab at EPFL (Sion) and colleagues in the US, have now identified a material that can be used as an efficient, cheaper, and safer alternative to separate xenon and krypton – and at room temperature. The material, abbreviated as SBMOF-1, is a nanoporous crystal and belongs a class of materials that are currently used to clear out CO2 emissions and other dangerous pollutants. These materials are also very versatile, and scientists can tweak them to self-assemble into ordered, pre-determined crystal structures. In this way, they can synthesize millions of tailor-made materials that can be optimized for gas storage separation, catalysis, chemical sensing and optics.

The scientists carried out high-throughput screening of large material databases of over 125,000 candidates. To do this, they used molecular simulations to find structures that can separate xenon and krypton, and under conditions that match those involved in reprocessing nuclear waste.

Because xenon has a much shorter half-life than krypton – a month versus a decade – the scientists had to find a material that would be selective for both but would capture them separately. As xenon is used in commercial lighting, propulsion, imaging, anesthesia and insulation, it can also be sold back into the chemical market to offset costs.

The scientists identified and confirmed that SBMOF-1 shows remarkable xenon capturing capacity and xenon/krypton selectivity under nuclear-plant conditions and at room temperature.

The US partners have also made an announcement with this June 13, 2016 Pacific Northwest National Laboratory (PNNL) news release (also on EurekAlert), Note: It is a little repetitive but there’s good additional information,

Researchers are investigating a new material that might help in nuclear fuel recycling and waste reduction by capturing certain gases released during reprocessing. Conventional technologies to remove these radioactive gases operate at extremely low, energy-intensive temperatures. By working at ambient temperature, the new material has the potential to save energy, make reprocessing cleaner and less expensive. The reclaimed materials can also be reused commercially.

Appearing in Nature Communications, the work is a collaboration between experimentalists and computer modelers exploring the characteristics of materials known as metal-organic frameworks.

“This is a great example of computer-inspired material discovery,” said materials scientist Praveen Thallapally of the Department of Energy’s Pacific Northwest National Laboratory. “Usually the experimental results are more realistic than computational ones. This time, the computer modeling showed us something the experiments weren’t telling us.”

Waste avoidance

Recycling nuclear fuel can reuse uranium and plutonium — the majority of the used fuel — that would otherwise be destined for waste. Researchers are exploring technologies that enable safe, efficient, and reliable recycling of nuclear fuel for use in the future.

A multi-institutional, international collaboration is studying materials to replace costly, inefficient recycling steps. One important step is collecting radioactive gases xenon and krypton, which arise during reprocessing. To capture xenon and krypton, conventional technologies use cryogenic methods in which entire gas streams are brought to a temperature far below where water freezes — such methods are energy intensive and expensive.

Thallapally, working with Maciej Haranczyk and Berend Smit of Lawrence Berkeley National Laboratory [LBNL] and others, has been studying materials called metal-organic frameworks, also known as MOFs, that could potentially trap xenon and krypton without having to use cryogenics.

These materials have tiny pores inside, so small that often only a single molecule can fit inside each pore. When one gas species has a higher affinity for the pore walls than other gas species, metal-organic frameworks can be used to separate gaseous mixtures by selectively adsorbing.

To find the best MOF for xenon and krypton separation, computational chemists led by Haranczyk and Smit screened 125,000 possible MOFs for their ability to trap the gases. Although these gases can come in radioactive varieties, they are part of a group of chemically inert elements called “noble gases.” The team used computing resources at NERSC, the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility at LBNL.

“Identifying the optimal material for a given process, out of thousands of possible structures, is a challenge due to the sheer number of materials. Given that the characterization of each material can take up to a few hours of simulations, the entire screening process may fill a supercomputer for weeks,” said Haranczyk. “Instead, we developed an approach to assess the performance of materials based on their easily computable characteristics. In this case, seven different characteristics were necessary for predicting how the materials behaved, and our team’s grad student Cory Simon’s application of machine learning techniques greatly sped up the material discovery process by eliminating those that didn’t meet the criteria.”

The team’s models identified the MOF that trapped xenon most selectively and had a pore size close to the size of a xenon atom — SBMOF-1, which they then tested in the lab at PNNL.

After optimizing the preparation of SBMOF-1, Thallapally and his team at PNNL tested the material by running a mixture of gases through it — including a non-radioactive form of xenon and krypton — and measuring what came out the other end. Oxygen, helium, nitrogen, krypton, and carbon dioxide all beat xenon out. This indicated that xenon becomes trapped within SBMOF-1’s pores until the gas saturates the material.

Other tests also showed that in the absence of xenon, SBMOF-1 captures krypton. During actual separations, then, operators would pass the gas streams through SBMOF-1 twice to capture both gases.

The team also tested SBMOF-1’s ability to hang onto xenon in conditions of high humidity. Humidity interferes with cryogenics, and gases must be dehydrated before putting them through the ultra-cold method, another time-consuming expense. SBMOF-1, however, performed quite admirably, retaining more than 85 percent of the amount of xenon in high humidity as it did in dry conditions.

The final step in collecting xenon or krypton gas would be to put the MOF material under a vacuum, which sucks the gas out of the molecular cages for safe storage. A last laboratory test examined how stable the material was by repeatedly filling it up with xenon gas and then vacuuming out the xenon. After 10 cycles of this, SBMOF-1 collected just as much xenon as the first cycle, indicating a high degree of stability for long-term use.

Thallapally attributes this stability to the manner in which SBMOF-1 interacts with xenon. Rather than chemical reactions between the molecular cages and the gases, the relationship is purely physical. The material can last a lot longer without constantly going through chemical reactions, he said.

A model finding

Although the researchers showed that SBMOF-1 is a good candidate for nuclear fuel reprocessing, getting these results wasn’t smooth sailing. In the lab, the researchers had followed a previously worked out protocol from Stony Brook University to prepare SBMOF-1. Part of that protocol requires them to “activate” SBMOF-1 by heating it up to 300 degrees Celsius, three times the temperature of boiling water.

Activation cleans out material left in the pores from MOF synthesis. Laboratory tests of the activated SBMOF-1, however, showed the material didn’t behave as well as it should, based on the computer modeling results.

The researchers at PNNL repeated the lab experiments. This time, however, they activated SBMOF-1 at a lower temperature, 100 degrees Celsius, or the actual temperature of boiling water. Subjecting the material to the same lab tests, the researchers found SBMOF-1 behaving as expected, and better than at the higher activation temperature.

But why? To figure out where the discrepancy came from, the researchers modeled what happened to SBMOF-1 at 300 degrees Celsius. Unexpectedly, the pores squeezed in on themselves.

“When we heated the crystal that high, atoms within the pore tilted and partially blocked the pores,” said Thallapally. “The xenon doesn’t fit.”

Armed with these new computational and experimental insights, the researchers can explore SBMOF-1 and other MOFs further for nuclear fuel recycling. These MOFs might also be able to capture other noble gases such as radon, a gas known to pool in some basements.

Researchers hailed from several other institutions as well as those listed earlier, including University of California, Berkeley, Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, Brookhaven National Laboratory, and IMDEA Materials Institute in Spain. This work was supported by the [US] Department of Energy Offices of Nuclear Energy and Science.

Here’s an image the researchers have provided to illustrate their work,

Caption: The crystal structure of SBMOF-1 (green = Ca, yellow = S, red = O, gray = C, white = H). The light blue surface is a visualization of the one-dimensional channel that SBMOF-1 creates for the gas molecules to move through. The darker blue surface illustrates where a Xe atom sits in the pores of SBMOF-1 when it adsorbs. Credit: Berend Smit/EPFL/University of California Berkley

Caption: The crystal structure of SBMOF-1 (green = Ca, yellow = S, red = O, gray = C, white = H). The light blue surface is a visualization of the one-dimensional channel that SBMOF-1 creates for the gas molecules to move through. The darker blue surface illustrates where a Xe atom sits in the pores of SBMOF-1 when it adsorbs. Credit: Berend Smit/EPFL/University of California Berkley

Here’s a link to and a citation for the paper,

Metal–organic framework with optimally selective xenon adsorption and separation by Debasis Banerjee, Cory M. Simon, Anna M. Plonka, Radha K. Motkuri, Jian Liu, Xianyin Chen, Berend Smit, John B. Parise, Maciej Haranczyk, & Praveen K. Thallapally. Nature Communications 7, Article number: ncomms11831  doi:10.1038/ncomms11831 Published 13 June 2016

This paper is open access.

Final comment, this is the second time in the last month I’ve stumbled across more positive approaches to nuclear energy. The first time was a talk (Why Nuclear Power is Necessary) held in Vancouver, Canada in May 2016 (details here). I’m not trying to suggest anything unduly sinister but it is interesting since most of my adult life nuclear power has been viewed with fear and suspicion.

A new, stable open-shell carbon molecule from Oregon

This discovery could one day make organic solar cells more efficient than silicon ones. Researchers at the University of Oregon announced their discovery in a June 9, 2016 news item on ScienceDaily,

University of Oregon chemists have synthesized a stable and long-lasting carbon-based molecule that, they say, potentially could be applicable in solar cells and electronic devices.

The molecule changes its bonding patterns to a magnetic biradical state when heated; it then returns to a fully bonded non-magnetic closed state at room temperature. That transition, they report, can be done repeatedly without decomposition. It remains stable in the presence of both heat and oxygen.

A June 9, 2016 University of Oregon news release on EurekAlert, which originated the news item, provides more detail,

 

Biradical refers to organic compounds, known as open-shell molecules, that have two free-flowing, non-bonding electrons. Producing them using techniques to control their electron spin, and thus provide semiconducting properties, in a heated state has been hampered by instability since the first synthetic biradical hydrocarbon was made in 1907.

“Potentially our approach could help to make organic solar cells more efficient than silicon solar cells, but that’s probably far in the future,” said UO doctoral student Gabriel E. Rudebusch, the paper’s lead author. “Our synthesis is rapid and efficient. We easily can make a gram of this compound, which is very stable when exposed to oxygen and heat. This stability has been almost unheard of in the literature about biradical compounds.”

The four-step synthesis of the compound — diindenoanthracene, or DIAn — and how it held up when tested in superconducting materials were detailed in a proof-of-principle paper published online May 23 by the journal Nature Chemistry. The UO team collaborated with experts in Japan, Spain and Sweden.

The molecular framework for the new molecule involves the hydrocarbon anthracene, which has three linearly fused hexagonal benzene rings, in combination with two five-membered pentagonal rings.

“The big difference between our new molecule and a lot of other biradical molecules that have been produced is those five-membered rings,” said co-author Michael M. Haley, who holds the UO’s Richard M. and Patricia H. Noyes Professorship in Chemistry. “They have the inherent ability to accept electrons or give up electrons. This means DIAn can move both negative and positive charges, which is an essential property for useful devices such as transistors and solar cells. Also, we can heat up our molecule to 150 degrees Celsius, bring it back to room temperature and heat it up again, repeatedly, and we see no decomposition in its reaction to oxygen. The unique features of DIAn are essential if these molecules are to have a use in the real world.”

Haley’s lab is now seeking to develop derivatives of the new molecule to help move the technology forward into potential applications.

Here’s a link to and a citation for the paper,

Diindeno-fusion of an anthracene as a design strategy for stable organic biradicals by Gabriel E. Rudebusch, José L. Zafra, Kjell Jorner, Kotaro Fukuda, Jonathan L. Marshall, Iratxe Arrechea-Marcos, Guzmán L. Espejo, Rocío Ponce Ortiz, Carlos J. Gómez-García, Lev N. Zakharov, Masayoshi Nakano, Henrik Ottosson, Juan Casado & Michael M. Haley. Nature Chemistry (2016)  doi:10.1038/nchem.2518 Published online 23 May 2016

This paper is behind a paywall.

There is another June 9, 2016 University of Oregon news release by Jim Barlow about this discovery. It covers much of the same material but focuses more closely on Rudebusch’s perspective.

‘Getting into’ cellulose walls at the University of Cambridge (UK) and University of Melbourne (Australia)

“Getting into” as used in the headline is slang for exploring a topic in more depth which is what an international team of researchers did when they ‘got into’ cellulose. From a June 9, 2016 news item on phys.org (Note: Links have been removed),

In the search for low emission plant-based fuels, new research may help avoid having to choose between growing crops for food or fuel.

Scientists have identified new steps in the way plants produce cellulose, the component of plant cell walls that provides strength, and forms insoluble fibre in the human diet.

The findings could lead to improved production of cellulose and guide plant breeding for specific uses such as wood products and ethanol fuel, which are sustainable alternatives to fossil fuel-based products.

Published in the journal Nature Communications today, the work was conducted by an international team of scientists, led by the University of Cambridge and the University of Melbourne.

A June 9, 2016 University of Cambridge press release, which originated the news item, provides more detail,

“Our research identified several proteins that are essential in the assembly of the protein machinery that makes cellulose”, said Melbourne’s Prof Staffan Persson.

“We found that these assembly factors control how much cellulose is made, and so plants without them can not produce cellulose very well and the defect substantially impairs plant biomass production. The ultimate aim of this research would be breed plants that have altered activity of these proteins so that cellulose production can be improved for the range of applications that use cellulose including paper, timber and ethanol fuels.”

The newly discovered proteins are located in an intracellular compartment called the Golgi where proteins are sorted and modified.

“If the function of this protein family is abolished the cellulose synthesizing complexes become stuck in the Golgi and have problems reaching the cell surface where they normally are active” said the lead authors of the study, Drs. Yi Zhang (Max-Planck Institute for Molecular Plant Physiology) and Nino Nikolovski (University of Cambridge).

“We therefore named the new proteins STELLO, which is Greek for to set in place, and deliver.”

“The findings are important to understand how plants produce their biomass”, said Professor Paul Dupree from the University of Cambridge’s Department of Biochemistry.

“Greenhouse-gas emissions from cellulosic ethanol, which is derived from the biomass of plants, are estimated to be roughly 85 percent less than from fossil fuel sources. Research to understand cellulose production in plants is therefore an important part of climate change mitigation.”

“In addition, by using cellulosic plant materials we get around the problem of food-versus-fuel scenario that is problematic when using corn as a basis for bioethanol.”

“It is therefore of great importance to find genes and mechanisms that can improve cellulose production in plants so that we can tailor cellulose production for various needs.”

Previous studies by Profs. Persson’s and Dupree’s research groups have, together with other scientists, identified many proteins that are important for cellulose synthesis and for other cell wall polymers.

With the newly presented research they substantially increase our understanding for how the bulk of a plant’s biomass is produced and is therefore of vast importance to industrial applications.

Here’s a link to and a citation for the paper,

Golgi-localized STELLO proteins regulate the assembly and trafficking of cellulose synthase complexes in Arabidopsis by Yi Zhang, Nino Nikolovski, Mathias Sorieul, Tamara Vellosillo, Heather E. McFarlane, Ray Dupree, Christopher Kesten, René Schneider, Carlos Driemeier, Rahul Lathe, Edwin Lampugnani, Xiaolan Yu, Alexander Ivakov, Monika S. Doblin, Jenny C. Mortimer, Steven P. Brown, Staffan Persson, & Paul Dupree. Nature Communications 7,
Article number: 11656 doi:10.1038/ncomms11656 Published  09 June 2016

This paper is open access.

Improving fossil-fueled cars’ efficiency with graphene-based ballistic rectifier

UK and Chinese researchers have a developed a technology to make fuel use more efficient in fossil-fueled cars (from a June 2, 2016 news item on phys.org),

A graphene-based electrical nano-device has been created which could substantially increase the energy efficiency of fossil fuel-powered cars.

The nano-device, known as a ‘ballistic rectifier’, is able to convert heat which would otherwise be wasted from the car exhaust and engine body into a useable electrical current.

Parts of car exhausts can reach temperatures of 600 degrees Celsius. The recovered energy can then be used to power additional automotive features such as air conditioning and power steering, or be stored in the car battery.

The nano-rectifier was built by a team at The University of Manchester led by Professor Aimin Song and Dr. Ernie Hill, with a team at Shandong University. The device utilises graphene’s phenomenally high electron mobility, a property which determines how fast an electron can travel in a material and how fast electronic devices can operate.

A June 1, 2016 University of Manchester press release, which originated the news item, provides more detail,

The resulting device is the most sensitive room-temperature rectifier ever made. Conventional devices with similar conversion efficiencies require cryogenically low temperatures.

Even today’s most efficient internal combustion engines can only convert about 70% of energy burned from fossil fuels into the energy required to power a car. The rest of the energy created is often wasted through exhaust heat or cooling systems.

Greg Auton, who performed most of the experiment said: “Graphene has exceptional properties; it possesses the longest carrier mean free path of any electronic material at room temperature.

“Despite this, even the most promising applications to commercialise graphene in the electronics industry do not take advantage of this property. Instead they often try to tackle the the problem that graphene has no band gap.”

Professor Song who invented the concept of the ballistic rectifier said: “The working principle of the ballistic rectifier means that it does not require any band gap. Meanwhile, it has a single-layered planar device structure which is perfect to take the advantage of the high electron-mobility to achieve an extremely high operating speed.

“Unlike conventional rectifiers or diodes, the ballistic rectifier does not have any threshold voltage either, making it perfect for energy harvest as well as microwave and infrared detection”.

The Manchester-based group is now looking to scale up the research by using large wafer-sized graphene and perform high-frequency experiments. The resulting technology can also be applied to harvesting wasted heat energy in power plants.

Oily nanodiamonds

Nanodiamonds if successfully extracted from oil could be used for imaging and communications and the world’s leading program for extracting nanodiamonds (also known as diamondoids) is in California (US). From a May 12, 2016 news item on Nanowerk,

Stanford and SLAC National Accelerator Laboratory jointly run the world’s leading program for isolating and studying diamondoids — the tiniest possible specks of diamond. Found naturally in petroleum fluids, these interlocking carbon cages weigh less than a billionth of a billionth of a carat (a carat weighs about the same as 12 grains of rice); the smallest ones contain just 10 atoms.

Over the past decade, a team led by two Stanford-SLAC faculty members — Nick Melosh, an associate professor of materials science and engineering and of photon science, and Zhi-Xun Shen, a professor of photon science and of physics and applied physics – has found potential roles for diamondoids in improving electron microscope images, assembling materials and printing circuits on computer chips. The team’s work takes place within SIMES, the Stanford Institute for Materials and Energy Sciences, which is run jointly with SLAC.

Close-up of purified diamondoids on a lab bench. Too small to see with the naked eye, diamondoids are visible only when they clump together in fine, sugar-like crystals like these. Photo: Christopher Smith, SLAC National Accelerator Laboratory

Close-up of purified diamondoids on a lab bench. Too small to see with the naked eye, diamondoids are visible only when they clump together in fine, sugar-like crystals like these. Photo: Christopher Smith, SLAC National Accelerator Laboratory

A March 31, 2016 Stanford University news release by Glennda Chui, which originated the news item, describes the work in more detail,

Before they can do that [use nanodiamonds in imaging and other applications], though, just getting the diamondoids is a technical feat. It starts at the nearby Chevron refinery in Richmond, California, with a railroad tank car full of crude oil from the Gulf of Mexico. “We analyzed more than a thousand oils from around the world to see which had the highest concentrations of diamondoids,” says Jeremy Dahl, who developed key diamondoid isolation techniques with fellow Chevron researcher Robert Carlson before both came to Stanford — Dahl as a physical science research associate and Carlson as a visiting scientist.

The original isolation steps were carried out at the Chevron refinery, where the selected crudes were boiled in huge pots to concentrate the diamondoids. Some of the residue from that work came to a SLAC lab, where small batches are repeatedly boiled to evaporate and isolate molecules of specific weights. These fluids are then forced at high pressure through sophisticated filtration systems to separate out diamondoids of different sizes and shapes, each of which has different properties.

The diamondoids themselves are invisible to the eye; the only reason we can see them is that they clump together in fine, sugar-like crystals. “If you had a spoonful,” Dahl says, holding a few in his palm, “you could give 100 billion of them to every person on Earth and still have some left over.”

Recently, the team started using diamondoids to seed the growth of flawless, nano-sized diamonds in a lab at Stanford. By introducing other elements, such as silicon or nickel, during the growing process, they hope to make nanodiamonds with precisely tailored flaws that can produce single photons of light for next-generation optical communications and biological imaging.

Early results show that the quality of optical materials grown from diamondoid seeds is consistently high, says Stanford’s Jelena Vuckovic, a professor of electrical engineering who is leading this part of the research with Steven Chu, professor of physics and of molecular and cellular physiology.

“Developing a reliable way of growing the nanodiamonds is critical,” says Vuckovic, who is also a member of Stanford Bio-X. “And it’s really great to have that source and the grower right here at Stanford. Our collaborators grow the material, we characterize it and we give them feedback right away. They can change whatever we want them to change.”

Squeezing light into extremely thin layers

A May 4, 2016 Rice University (US) news release (also on EurekAlert) describes research on molybdenum disulfide and its light absorption properties,

Mechanics know molybdenum disulfide (MoS2) as a useful lubricant in aircraft and motorcycle engines and in the CV and universal joints of trucks and automobiles. Rice University engineering researcher Isabell Thomann knows it as a remarkably light-absorbent substance that holds promise for the development of energy-efficient optoelectronic and photocatalytic devices.

“Basically, we want to understand how much light can be confined in an atomically thin semiconductor monolayer of MoS2,” said Thomann, assistant professor of electrical and computer engineering and of materials science and nanoengineering and of chemistry. “By using simple strategies, we were able to absorb 35 to 37 percent of the incident light in the 400- to 700-nanometer wavelength range, in a layer that is only 0.7 nanometers thick.”

Thomann and Rice graduate students Shah Mohammad Bahauddin and Hossein Robatjazi have recounted their findings in a paper titled “Broadband Absorption Engineering To Enhance Light Absorption in Monolayer MoS2,” which was recently published in the American Chemical Society journal ACS Photonics. The research has many applications, including development of efficient and inexpensive photovoltaic solar panels.

“Squeezing light into these extremely thin layers and extracting the generated charge carriers is an important problem in the field of two-dimensional materials,” she said. “That’s because monolayers of 2-D materials have different electronic and catalytic properties from their bulk or multilayer counterparts.”

Thomann and her team used a combination of numerical simulations, analytical models and experimental optical characterizations. Using three-dimensional electromagnetic simulations, they found that light absorption was enhanced 5.9 times compared with using MoS2 on a sapphire substrate.

“If light absorption in these materials was perfect, we’d be able to create all sorts of energy-efficient optoelectronic and photocatalytic devices. That’s the problem we’re trying to solve,” Thomann said.

She is pleased with her lab’s progress but concedes that much work remains to be done. “The goal, of course, is 100 percent absorption, and we’re not there yet.”

Here’s a link to and a citation for the paper,

Broadband Absorption Engineering to Enhance Light Absorption in Monolayer MoSby Shah Mohammad Bahauddin, Hossein Robatjazi, and Isabell Thomann. ACS Photonics, Article ASAP DOI: 10.1021/acsphotonics.6b00081
Publication Date (Web): April 27, 2016

Copyright © 2016 American Chemical Society

This paper is behind a paywall.

Want better energy storage materials? Add salt

An April 22, 2016 news item on Nanowerk reveals a secret to better energy storage materials,

The secret to making the best energy storage materials is growing them with as much surface area as possible. Like baking, it requires just the right mixture of ingredients prepared in a specific amount and order at just the right temperature to produce a thin sheet of material with the perfect chemical consistency to be useful for storing energy. A team of researchers from Drexel University, Huazhong University of Science and Technology (HUST) and Tsinghua University recently discovered a way to improve the recipe and make the resulting materials bigger and better and soaking up energy — the secret? Just add salt.

An April 22, 2016 Drexel University news release (also on EurekAlert), which originated the news item, provides more detail,

The team’s findings, which were recently published in the journal Nature Communications, show that using salt crystals as a template to grow thin sheets of conductive metal oxides make the materials turn out larger and more chemically pure — which makes them better suited for gathering ions and storing energy.

“The challenge of producing a metal oxide that reaches theoretical performance values is that the methods for making it inherently limit its size and often foul its chemical purity, which makes it fall short of predicted energy storage performance,” said Jun Zhou, a professor at HUST’s Wuhan National Laboratory for Optoelectronics and an author of the research. Our research reveals a way to grow stable oxide sheets with less fouling that are on the order of several hundreds of times larger than the ones that are currently being fabricated.”

In an energy storage device — a battery or a capacitor, for example — energy is contained in the chemical transfer of ions from an electrolyte solution to thin layers of conductive materials. As these devices evolve they’re becoming smaller and capable of holding an electric charge for longer periods of time without needing a recharge. The reason for their improvement is that researchers are fabricating materials that are better equipped, structurally and chemically, for collecting and disbursing ions.

In theory, the best materials for the job should be thin sheets of metal oxides, because their chemical structure and high surface area makes it easy for ions to attach — which is how energy storage occurs. But the metal oxide sheets that have been fabricated in labs thus far have fallen well short of their theoretical capabilities.

According to Zhou, Tang [?] and the team from HUST, the problem lies in the process of making the nanosheets — which involves either a deposition from gas or a chemical etching — often leaves trace chemical residues that contaminate the material and prevent ions from bonding to it. In addition, the materials made in this way are often just a few square micrometers in size.

Using salt crystals as a substrate for growing the crystals lets them spread out and form a larger sheet of oxide material. Think of it like making a waffle by dripping batter into a pan versus pouring it into a big waffle iron; the key to getting a big, sturdy product is getting the solution — be it batter, or chemical compound — to spread evenly over the template and stabilize in a uniform way.

“This method of synthesis, called ‘templating’ — where we use a sacrificial material as a substrate for growing a crystal — is used to create a certain shape or structure,” said Yury Gogotsi, PhD, University and Trustee Chair professor in Drexel’s College of Engineering and head of the A.J. Drexel Nanomaterials Institute, who was an author of the paper. “The trick in this work is that the crystal structure of salt must match the crystal structure of the oxide, otherwise it will form an amorphous film of oxide rather than a thing, strong and stable nanocrystal. This is the key finding of our research — it means that different salts must be used to produce different oxides.”

Researchers have used a variety of chemicals, compounds, polymers and objects as growth templates for nanomaterials. But this discovery shows the importance of matching a template to the structure of the material being grown. Salt crystals turn out to be the perfect substrate for growing oxide sheets of magnesium, molybdenum and tungsten.

The precursor solution coats the sides of the salt crystals as the oxides begin to form. After they’ve solidified, the salt is dissolved in a wash, leaving nanometer-thin two-dimensional sheets that formed on the sides of the salt crystal — and little trace of any contaminants that might hinder their energy storage performance. By making oxide nanosheets in this way, the only factors that limit their growth is the size of the salt crystal and the amount of precursor solution used.

“Lateral growth of the 2D oxides was guided by salt crystal geometry and promoted by lattice matching and the thickness was restrained by the raw material supply. The dimensions of the salt crystals are tens of micrometers and guide the growth of the 2D oxide to a similar size,” the researchers write in the paper. “On the basis of the naturally non-layered crystal structures of these oxides, the suitability of salt-assisted templating as a general method for synthesis of 2D oxides has been convincingly demonstrated.”

As predicted, the larger size of the oxide sheets also equated to a greater ability to collect and disburse ions from an electrolyte solution — the ultimate test for its potential to be used in energy storage devices. Results reported in the paper suggest that use of these materials may help in creating an aluminum-ion battery that could store more charge than the best lithium-ion batteries found in laptops and mobile devices today.

Gogotsi, along with his students in the Department of Materials Science and Engineering, has been collaborating with Huazhong University of Science and Technology since 2012 to explore a wide variety of materials for energy storage application. The lead author of the Nature Communications article, Xu Xiao, and co-author Tiangi Li, both Zhou’s doctoral students, came to Drexel as exchange students to learn about the University’s supercapacitor research. Those visits started a collaboration, which was supported by Gogotsi’s annual trips to HUST. While the partnership has already yielded five joint publications, Gogotsi speculates that this work is only beginning.

“The most significant result of this work thus far is that we’ve demonstrated the ability to generate high-quality 2D oxides with various compositions,” Gogotsi said. “I can certainly see expanding this approach to other oxides that may offer attractive properties for electrical energy storage, water desalination membranes, photocatalysis and other applications.”

Here’s a link to and a citation for the paper,

Scalable salt-templated synthesis of two-dimensional transition metal oxides by Xu Xiao, Huaibing Song, Shizhe Lin, Ying Zhou, Xiaojun Zhan, Zhimi Hu, Qi Zhang, Jiyu Sun, Bo Yang, Tianqi Li, Liying Jiao, Jun Zhou, Jiang Tang, & Yury Gogotsi. Nature Communications 7, Article number:  11296 doi:10.1038/ncomms11296 Published 22 April 2016

This is an open access paper.

Transparent wood instead of glass for window panes?

The transparent wood is made by removing the lignin in the wood veneer. (Photo: Peter Larsson

The transparent wood is made by removing the lignin in the wood veneer. (Photo: Peter Larsson

Not quite ready as a replacement for some types of glass window panes, nonetheless, transparent (more like translucent) wood is an impressive achievement. According to a March 30, 2016 news item on ScienceDaily size is what makes this piece of transparent wood newsworthy,

Windows and solar panels in the future could be made from one of the best — and cheapest — construction materials known: wood. Researchers at Stockholm’s KTH Royal Institute of Technology [Sweden] have developed a new transparent wood material that’s suitable for mass production.

Lars Berglund, a professor at Wallenberg Wood Science Center at KTH, says that while optically transparent wood has been developed for microscopic samples in the study of wood anatomy, the KTH project introduces a way to use the material on a large scale. …

A March 31 (?), 2016 KTH Institute of Technology press release, which originated the news item, provides more detail,

“Transparent wood is a good material for solar cells, since it’s a low-cost, readily available and renewable resource,” Berglund says. “This becomes particularly important in covering large surfaces with solar cells.”

Berglund says transparent wood panels can also be used for windows, and semitransparent facades, when the idea is to let light in but maintain privacy.

The optically transparent wood is a type of wood veneer in which the lignin, a component of the cell walls, is removed chemically.

“When the lignin is removed, the wood becomes beautifully white. But because wood isn’t not naturally transparent, we achieve that effect with some nanoscale tailoring,” he says.

The white porous veneer substrate is impregnated with a transparent polymer and the optical properties of the two are then matched, he says.

“No one has previously considered the possibility of creating larger transparent structures for use as solar cells and in buildings,” he says

Among the work to be done next is enhancing the transparency of the material and scaling up the manufacturing process, Berglund says.

“We also intend to work further with different types of wood,” he adds.

“Wood is by far the most used bio-based material in buildings. It’s attractive that the material comes from renewable sources. It also offers excellent mechanical properties, including strength, toughness, low density and low thermal conductivity.”

The American Chemical Society has a March 30, 2016 news release about the KTH achievement on EurekAlert  highlighting another potential use for transparent wood,

When it comes to indoor lighting, nothing beats the sun’s rays streaming in through windows. Soon, that natural light could be shining through walls, too. Scientists have developed transparent wood that could be used in building materials and could help home and building owners save money on their artificial lighting costs. …

Homeowners often search for ways to brighten up their living space. They opt for light-colored paints, mirrors and lots of lamps and ceiling lights. But if the walls themselves were transparent, this would reduce the need for artificial lighting — and the associated energy costs. Recent work on making transparent paper from wood has led to the potential for making similar but stronger materials. Lars Berglund and colleagues wanted to pursue this possibility.

Here’s a link to and a citation for the paper,

Optically Transparent Wood from a Nanoporous Cellulosic Template: Combining Functional and Structural Performance by Yuanyuan Li, Qiliang Fu, Shun Yu, Min Yan, and Lars Berglund. Biomacromolecules, Article ASAP DOI: 10.1021/acs.biomac.6b00145 Publication Date (Web): March 4, 2016

Copyright © 2016 American Chemical Society

This paper appears to be open access.