MIT: Material (molybdenum ditelluride) could bring optical communication onto silicon chips


Researchers have designed a light-emitter and detector that can be integrated into silicon CMOS chips. This illustration shows a molybdenum ditelluride light source for silicon photonics. Image: Sampson Wilcox

Ultrathin films of a semiconductor that emits and detects light can be stacked on top of silicon wafers.

The huge increase in computing performance in recent decades has been achieved by squeezing ever more transistors into a tighter space on microchips.

However, this downsizing has also meant packing the wiring within microprocessors ever more tightly together, leading to effects such as signal leakage between components, which can slow down communication between different parts of the chip. This delay, known as the “interconnect bottleneck,” is becoming an increasing problem in high-speed computing systems.

One way to tackle the interconnect bottleneck is to use light rather than wires to communicate between different parts of a microchip. This is no easy task, however, as silicon, the material used to build chips, does not emit light easily, according to Pablo Jarillo-Herrero, an associate professor of physics at MIT.

Now, in a paper published today in the journal Nature Nanotechnology, researchers describe a light emitter and detector that can be integrated into silicon CMOS chips. The paper’s first author is MIT postdoc Ya-Qing Bie, who is joined by Jarillo-Herrero and an interdisciplinary team including Dirk Englund, an associate professor of electrical engineering and computer science at MIT.

The device is built from a semiconductor material called molybdenum ditelluride. This ultrathin semiconductor belongs to an emerging group of materials known as two-dimensional transition-metal dichalcogenides.

Unlike conventional semiconductors, the material can be stacked on top of silicon wafers, Jarillo-Herrero says.

“Researchers have been trying to find materials that are compatible with silicon, in order to bring optoelectronics and optical communication on-chip, but so far this has proven very difficult,” Jarillo-Herrero says. “For example, gallium arsenide is very good for optics, but it cannot be grown on silicon very easily because the two semiconductors are incompatible.”

In contrast, the 2-D molybdenum ditelluride can be mechanically attached to any material, Jarillo-Herrero says.

Another difficulty with integrating other semiconductors with silicon is that the materials typically emit light in the visible range, but light at these wavelengths is simply absorbed by silicon.

Molybdenum ditelluride emits light in the infrared range, which is not absorbed by silicon, meaning it can be used for on-chip communication.

To use the material as a light emitter, the researchers first had to convert it into a P-N junction diode, a device in which one side, the P side, is positively charged, while the other, N side, is negatively charged.

In conventional semiconductors, this is typically done by introducing chemical impurities into the material. With the new class of 2-D materials, however, it can be done by simply applying a voltage across metallic gate electrodes placed side-by-side on top of the material.

“That is a significant breakthrough, because it means we do not need to introduce chemical impurities into the material [to create the diode]. We can do it electrically,” Jarillo-Herrero says.

Once the diode is produced, the researchers run a current through the device, causing it to emit light.

“So by using diodes made of molybdenum ditelluride, we are able to fabricate light-emitting diodes (LEDs) compatible with silicon chips,” Jarillo-Herrero says.

The device can also be switched to operate as a photodetector, by reversing the polarity of the voltage applied to the device. This causes it to stop conducting electricity until a light shines on it, when the current restarts.

In this way, the devices are able to both transmit and receive optical signals.

The device is a proof of concept, and a great deal of work still needs to be done before the technology can be developed into a commercial product, Jarillo-Herrero says.

This paper fills an important gap in integrated photonics, by realizing a high-performance silicon-CMOS-compatible light source, says Frank Koppens, a professor of quantum nano-optoelectronics at the Institute of Photonic Sciences in Barcelona, Spain, who was not involved in the research.

“This work shows that 2-D materials and Si-CMOS and silicon photonics are a natural match, and we will surely see many more applications coming out of this [area] in the years to come,” Koppens says.

The researchers are now investigating other materials that could be used for on-chip optical communication.

Most telecommunication systems, for example, operate using light with a wavelength of 1.3 or 1.5 micrometers, Jarillo-Herrero says.

However, molybdenum ditelluride emits light at 1.1 micrometers. This makes it suitable for use in the silicon chips found in computers, but unsuitable for telecommunications systems.

“It would be highly desirable if we could develop a similar material, which could emit and detect light at 1.3 or 1.5 micrometers in wavelength, where telecommunication through optical fiber operates,” he says.

To this end, the researchers are exploring another ultrathin material called black phosphorus, which can be tuned to emit light at different wavelengths by altering the number of layers used. They hope to develop devices with the necessary number of layers to allow them to emit light at the two wavelengths while remaining compatible with silicon.

“The hope is that if we are able to communicate on-chip via optical signals instead of electronic signals, we will be able to do so more quickly, and while consuming less power,” Jarillo-Herrero says.

The research was supported by Center for Excitonics, an EFRC funded by the U.S. Department of Energy.


Rice U: Nano-Shells could deliver more chemo with fewer side effects

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Researchers from Rice University and Northwestern University loaded light-activated nano-shells (gold and light blue) with the anticancer drug lapatinib (yellow) by encasing the drug in an envelope of albumin (blue). Light from a near-infrared laser (center) was used to remotely trigger the release of the drug (right) after the nano-shells were taken up by cancer cells. Credit: A. Goodman/Rice University

Researchers investigating ways to deliver high doses of cancer-killing drugs inside tumors have shown they can use a laser and light-activated gold nanoparticles to remotely trigger the release of approved cancer drugs inside cancer cells in laboratory cultures.

The study by researchers at Rice University and Northwestern University Feinberg School of Medicine appears in this week’s online Early Edition of the Proceedings of the National Academy of Sciences. It employed gold nanoshells to deliver toxic doses of two drugs — lapatinib and docetaxel — inside breast cancer cells. The researchers showed they could use a laser to remotely trigger the particles to release the drugs after they entered the cells.

Though the tests were conducted with cell cultures in a lab, the research was designed to demonstrate clinical applicability: The nanoparticles are nontoxic, the drugs are widely used and the low-power, infrared laser can noninvasively shine through tissue and reach tumors several inches below the skin.

“In future studies, we plan to use a Trojan-horse strategy to get the drug-laden nanoshells inside tumors,” said Naomi Halas, an engineer, chemist and physicist at Rice University who invented gold nanoshells and has spent more than 15 years researching their anticancer potential. “Macrophages, a type of white blood cell that’s been shown to penetrate tumors, will carry the drug-particle complexes into tumors, and once there we use a laser to release the drugs.”

Co-author Susan Clare, a research associate professor of surgery at the Northwestern University Feinberg School of Medicine, said the PNAS study was designed to demonstrate the feasibility of the Trojan-horse approach. In addition to demonstrating that drugs could be released inside cancer cells, the study also showed that in macrophages, the drugs did not detach prior to triggering.

“Getting chemotherapeutic drugs to penetrate tumors is very challenging,” said Clare, also a Northwestern Medicine breast cancer surgeon. “Drugs tend to get pushed out of tumors rather than drawn in. To get an effective dose at the tumor, patients often have to take so much of the drug that nausea and other side effects become severe. Our hope is that the combination of macrophages and triggered drug-release will boost the effective dose of drugs within tumors so that patients can take less rather than more.”

If the approach works, Clare said, it could result in fewer side effects and potentially be used to treat many kinds of cancer. For example, one of the drugs in the study, lapatinib, is part of a broad class of chemotherapies called tyrosine kinase inhibitors that target specific proteins linked to different types of cancer. Other Federal Drug Administration-approved drugs in the class include imatinib (leukemia), gefitinib (breast, lung), erlotinib (lung, pancreatic), sunitinib (stomach, kidney) and sorafenib (liver, thyroid and kidney).

“All the tyrosine kinase inhibitors are notoriously insoluble in water,” said Amanda Goodman, a Rice alumna and lead author of the PNAS study. “As a drug class, they have poor bioavailability, which means that a relatively small proportion of the drug in each pill is actually killing cancer cells. If our method works for lapatinib and breast cancer, it may also work for the other drugs in the class.”

Halas invented nanoshells at Rice in the 1990s. About 20 times smaller than a red blood cell, they are made of a sphere of glass covered by a thin layer of gold. Nanoshells can be tuned to capture energy from specific wavelengths of light, including near-infrared (near-IR), a nonvisible wavelength that passes through most tissues in the body. Nanospectra Biosciences, a licensee of this technology, has performed several clinical trials over the past decade using nanoshells as photothermal agents that destroy tumors with infrared light.

Clare and Halas’ collaboration on nanoshell-based drug delivery began more than 10 years ago. In earlier work, they showed that a near-IR continuous-wave laser — the same kind that produces heat in the photothermal applications of nanoshells — could be used to trigger the release of drugs from nanoshells.

In the latest study, Goodman contrasted the use of continuous-wave laser triggering and triggering with a low-power pulse laser. Using each type of laser, she demonstrated the remotely triggered release of drugs from two types of nanoshell-drug conjugates. One type used a DNA linker and the drug docetaxel, and the other employed a coating of the blood protein albumin to trap and hold lapatinib. In each case, Goodman found she could trigger the release of the drug after the nanoshells were taken up inside cancer cells. She also found no measureable premature release of drugs in macrophages in either case.

Halas and Clare said they hope to begin animal tests of the technology soon and have an established mouse model that could be used for the testing.

“I’m particularly excited about the potential for lapatinib,” Clare said. “The first time I heard about Naomi’s work, I wondered if it might be the answer to delivering drugs into the anoxic (depleted of oxygen) interior of tumors where some of the most aggressive cancer cells lurk. As clinicians, we’re always looking for ways to keep cancer from coming back months or years later, and I am hopeful this can do that.”

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Materials provided by Rice UniversityNote: Content may be edited for style and length.

MIT and Lamborghini to develop graphene-enhanced supercar – Powered Only by Supercapacitors

 November 10, 2017

Lamborghini and MIT have announced a collaboration on a 3-year project to develop a graphene-enhanced supercapacitor electric vehicle. 

The Lamborghini-MIT partnership could, however, end up being extended as there is no target date for the car’s completion.

MIT and Lamborghini develop graphene-enhanced supercar image

The planned graphene-enhanced Terzo Millennio (“third millennium”) supercar may be a real gamechanger. 

This concept car is to be a fully electric, supercapacitor-powered automobile that can be charged in minutes – with no bulky battery. 

It will reportedly be “covered in a sheet of graphene”, but this description does not sound extremely accurate… We will have to wait for further information on this project.

According to reports, the bodywork of the car will utilize Lamborghini’s expertise in carbon fibre, which results in significant weight reduction. 

However, the joint plan is apparently for the carbon panels to also act as an accumulator for energy storage. 

But Lamborghini and MIT also want the car to self-heal. Cracks and minor damage will be automatically detected in the carbon structure and then repaired using “microchannels” in the bodywork filled with “healing chemistries”…. stay tuned … !

Source:  wired

U of Waterloo: Energy storage capacity of supercapacitors doubled by researchers

Researchers in Canada have developed a technique for improving the energy storage capacity of supercapacitors. These developments could allow for mobile phones to eventually charge in seconds.

A supercapacitor can store far more electrical energy than a standard capacitor. They are able to charge and discharge far more rapidly than batteries, making them a much-discussed alternative to traditional batteries.

The main drawback of supercapacitors as a replacement for batteries is their limited storage: while they can store 10 to 100 times more electrical energy than a standard capacitor, this is still not enough to be useful as a battery replacement in smartphones, laptops, electric vehicles and other machines.

At present, supercapacitors can store enough energy to power laptops and other small devices for approximately a tenth as long as rechargeable batteries do. 

Increases in the storage capacity of supercapacitors could allow for them to be made smaller and lighter, such that they can replace batteries in some devices that require fast charging and discharging.

A team of engineers at the University of Waterloo were able to create a new supercapacitor design which approximately doubles the amount of electrical energy that it can hold

They did this by coating graphene with an oily liquid salt in the electrodes of supercapacitors. By adding a mixture of detergent and water, the droplets of the liquid salt were reduced to nanoscale sizes.

This salt acts as an electrolyte (which is required for storage of electrical charge), as well as preventing the atom-thick graphene sheets sticking together, hugely increasing their exposed surface area and optimising energy storage capacity.

“We’re showing record numbers for the energy-storage capacity of supercapacitors,” said Professor Michael Pope, a chemical engineer at the University of Waterloo. “And the more energy-dense we can make them, the more batteries we can start displacing.”

According to Professor Pope, supercapacitors could be a green replacement for lead-acid batteries in vehicles, capturing the energy otherwise wasted by buses and high-speed trains during braking. In the longer term, they could be used to power mobile phones and other consumer technology, as well as devices in remote locations, such as in orbit around Earth.

“If they are marketed in the correct ways for the right applications, we’ll start seeing more and more of them in our everyday lives,” said Professor Pope.


New Efficient, Low-Temperature Catalyst for Converting Water and CO to Hydrogen Gas and CO2

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Brookhaven Lab chemists Ping Liu and José Rodriguez helped to characterize structural and mechanistic details of a new low-temperature catalyst for producing high-purity hydrogen gas from water and carbon monoxide.

Low-temperature “water gas shift” reaction produces high levels of pure hydrogen for potential applications, including fuel cells

UPTON, NY—Scientists have developed a new low-temperature catalyst for producing high-purity hydrogen gas while simultaneously using up carbon monoxide (CO). The discovery—described in a paper set to publish online in the Journal Science — could improve the performance of fuel cells that run on hydrogen fuel but can be poisoned by CO.

“This catalyst produces a purer form of hydrogen to feed into the fuel cell,” said José Rodriguez, a chemist at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory. Rodriguez and colleagues in Brookhaven’s Chemistry Division—Ping Liu and Wenqian Xu—were among the team of scientists who helped to characterize the structural and mechanistic details of the catalyst, which was synthesized and tested by collaborators at Peking University in an effort led by Chemistry Professor Ding Ma.

“This catalyst produces a purer form of hydrogen to feed into fuel cells.”

— José Rodriguez

Because the catalyst operates at low temperature and low pressure to convert water (H2O) and carbon monoxide (CO) to hydrogen gas (H2) and carbon dioxide (CO2), it could also lower the cost of running this so-called “water gas shift” reaction.

“With low temperature and pressure, the energy consumption will be lower and the experimental setup will be less expensive and easier to use in small settings, like fuel cells for cars,” Rodriguez said.

The gold-carbide connection

The catalyst consists of clusters of gold nanoparticles layered on a molybdenum-carbide substrate. This chemical combination is quite different from the oxide-based catalysts used to power the water gas shift reaction in large-scale industrial hydrogen production facilities.

“Carbides are more chemically reactive than oxides,” said Rodriguez, “and the gold-carbide interface has good properties for the water gas shift reaction; it interacts better with water than pure metals.”

operando x-ray diffraction studies of the gold-molybdenum-carbide catalyst over a range of temperatuClick on the image to download a high-resolution version.Wenqian Xu and José Rodriguez of Brookhaven Lab and Siyu Yao, then a student at Peking University but now a postdoctoral research fellow at Brookhaven, conducted operando x-ray diffraction studies of the gold-molybdenum-carbide catalyst over a range of temperatures (423 Kelvin to 623K) at the National Synchrotron Light Source (NSLS) at Brookhaven Lab. The study revealed that at temperatures above 500K, molybdenum-carbide transforms to molybdenum oxide, with a reduction in catalytic activity.


“The group at Peking University discovered a new synthetic method, and that was a real breakthrough,” Rodriguez said. “They found a way to get a specific phase—or configuration of the atoms—that is highly active for this reaction.”

Brookhaven scientists played a key role in deciphering the reasons for the high catalytic activity of this configuration. Rodriguez, Wenqian Xu, and Siyu Yao (then a student at Peking University but now a postdoctoral research fellow at Brookhaven) conducted structural studies using x-ray diffraction at the National Synchrotron Light Source (NSLS) while the catalyst was operating under industrial or technical conditions. These operandoexperiments revealed crucial details about how the structure changed under different operating conditions, including at different temperatures.

With those structural details in hand, Zhijun Zuo, a visiting professor at Brookhaven from Taiyuan University of Technology, China, and Brookhaven chemist Ping Liu helped to develop models and a theoretical framework to explain why the catalyst works the way it does, using computational resources at Brookhaven’s Center for Functional Nanomaterials (CFN).

“We modeled different interfaces of gold and molybdenum carbide and studied the reaction mechanism to identify exactly where the reactions take place—the active sites where atoms are binding, and how bonds are breaking and reforming,” she said.

Additional studies at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, and two synchrotron research facilities in China added to the scientists’ understanding.

“This is a multipart complex reaction,” said Liu, but she noted one essential factor: “The interaction between the gold and the carbide substrate is very important. Gold usually bonds things very weakly. With this synthesis method we get stronger adherence of gold to molybdenum carbide in a controlled way.”

That configuration stabilizes the key intermediate that forms as the reaction proceeds, and the stability of that intermediate determines the rate of hydrogen production, she said.

The Brookhaven team will continue to study this and other carbide catalysts with new capabilities at the National Synchrotron Light Source II (NSLS-II), a new facility that opened at Brookhaven Lab in 2014, replacing NSLS and producing x-rays that are 10,000 times brighter. With these brighter x-rays, the scientists hope to capture more details of the chemistry in action, including details of the intermediates that form throughout the reaction process to validate the theoretical predictions made in this study.

The work at Brookhaven Lab was funded by the U.S. DOE Office of Science.

Additional funders for the overall research project include: the National Basic Research Program of China, the Chinese Academy of Sciences, National Natural Science Foundation of China, Fundamental Research Funds for the Central Universities of China, and the U.S. National Science Foundation.

NSLS, NSLS-II, CFN, CNMS, and ALS are all DOE Office of Science User Facilities.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy.  The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.  For more information, please visit

NREL, University of Washington Scientists Elevate Quantum Dot Solar Cell World Record to 13.4 Percent

Researchers at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) established a new world efficiency record for quantum dot solar cells, at 13.4 percent.

Colloidal quantum dots are electronic materials and because of their astonishingly small size (typically 3-20 nanometers in dimension) they possess fascinating optical properties. 

Quantum dot solar cells emerged in 2010 as the newest technology on an NREL chart that tracks research efforts to convert sunlight to electricity with increasing efficiency. 

The initial lead sulfide quantum dot solar cells had an efficiency of 2.9 percent. Since then, improvements have pushed that number into double digits for lead sulfide reaching a record of 12 percent set last year by the University of Toronto. 

The improvement from the initial efficiency to the previous record came from better understanding of the connectivity between individual quantum dots, better overall device structures and reducing defects in quantum dots.

 NREL scientists Joey Luther and Erin Sanehira are part of a team that has helped NREL set an efficiency record of 13.4% for a quantum dot solar cell.

The latest development in quantum dot solar cells comes from a completely different quantum dot material. The new quantum dot leader is cesium lead triiodide (CsPbI3), and is within the recently emerging family of halide perovskite materials. 

In quantum dot form, CsPbI3 produces an exceptionally large voltage (about 1.2 volts) at open circuit.

“This voltage, coupled with the material’s bandgap, makes them an ideal candidate for the top layer in a multijunction solar cell,” said Joseph Luther, a senior scientist and project leader in the Chemical Materials and Nanoscience team at NREL. 

The top cell must be highly efficient but transparent at longer wavelengths to allow that portion of sunlight to reach lower layers. 
Tandem cells can deliver a higher efficiency than conventional silicon solar panels that dominate today’s solar market.

This latest advance, titled “Enhanced mobility CsPbI3 quantum dot arrays for record-efficiency, high-voltage photovoltaic cells,” is published in Science Advances. The paper was co-authored by Erin Sanehira, Ashley Marshall, Jeffrey Christians, Steven Harvey, Peter Ciesielski, Lance Wheeler, Philip Schulz, and Matthew Beard, all from NREL; and Lih Lin from the University of Washington.

The multijunction approach is often used for space applications where high efficiency is more critical than the cost to make a solar module. 
The quantum dot perovskite materials developed by Luther and the NREL/University of Washington team could be paired with cheap thin-film perovskite materials to achieve similar high efficiency as demonstrated for space solar cells, but built at even lower costs than silicon technology–making them an ideal technology for both terrestrial and space applications.

“Often, the materials used in space and rooftop applications are totally different. It is exciting to see possible configurations that could be used for both situations,” said Erin Sanehira a doctoral student at the University of Washington who conducted research at NREL.

The NREL research was funded by DOE’s Office of Science, while Sanehira and Lin acknowledge a NASA space technology fellowship.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.

Connecting the Future of Electric Vehicles with Our Exploration of Space – “Back to the Future”

Special Contribution by Jason Torchinsky 

Yesterday, we reported on an alarming development for the future of electric cars: we may not have enough of the crucial minerals needed for their batteries to meet the expected demand. Supplies of nickel and cobalt are going to be needed in far larger quantities than ever before, and it’s looking like we may not have the necessary resources. 

Though, it’s worth mentioning that this is only a problem if you have what the intergalactic call a “planetary mindset.” There’s plenty of what we need just outside our door, in asteroids.

Asteroid mining has been discussed and planned and speculated about for decades, but so far there’s never really been a compelling economic reason to take the risks inherent in starting an entirely new, space-based industry.

Electric car demand may be that crucial factor that changes everything, though. Nickel and cobalt of sufficient quality and quantity may be becoming scarce on Earth, but there’s literally tons and tons and tons of the stuff pirouetting around in the inky black of space.

There’s incredibly, astoundingly valuable asteroids out there, and many we’ve already identified, like 241 Germania, which has as much mineral value in it as the entire Earth’s yearly GDP. Nickel and cobalt are abundant elements in these asteroids, and researchers have even already picked a dozen small asteroids close enough to Earth that they could be mined with just the technology that we have right now.

Those 12 asteroids are close enough to the L1 or L2 Lagrangian Points–stable areas where the gravity between two bodies, like the Earth and moon, cancel one another out–that getting them to these stable, accessible orbits is easy enough that researchers call them EROs, for Easily Retrievable Objects.

Companies like Planetary Resources have been working on asteroid mining for years, but have mostly been focused on the in-space uses of those resources, as opposed to bringing those resources back to Earth. This animation gives a sense of the way they’ve been thinking so far:

While in-space use of asteroid mineral resources is absolutely important, the recently seen expected demand for electric cars–most obviously seen in the amount of interest and pre-orders Tesla got for its upcoming Model 3–changes things dramatically. Electric car demand could easily be the backbone of the justification for asteroid mining that returns resources to Earth.

Where it was once thought that it didn’t make economic sense to mine asteroids for terrestrial use, that thinking is changing. In fact, a recent study by Noah Poponak of Goldman Sachs says the opposite:

“While the psychological barrier to mining asteroids is high, the actual financial and technological barriers are far lower. Prospecting probes can likely be built for tens of millions of dollars each and Caltech has suggested an asteroid-grabbing spacecraft could cost $2.6 billion.”

For comparison, $2.6 billion is how much money Lyft has raised. Lyft! What have they produced? Fuzzy pink car-moustaches and an app, neither of which can grab asteroid one.

Legally, things are looking good, too. An Obama-era law, the U.S. Commercial Space Launch Competitiveness Act, was passed that acknowledges that while legally no one can own the moon or an asteroid, private companies can own any materials taken from those celestial objects, which means asteroid mining for profit is legal.

If electric cars provide the economic push needed to get us to send grizzled robot space prospectors out to get that sweet, sweet space-cobalt, it’s hard not to see a possible significant competitive advantage for one of the key players, Tesla.

That’s because as we all know, Elon Musk is behind not just Tesla but SpaceX, likely the most successful private space-launch company around. SpaceX has capable launch vehicles and likely the expertise to design and build robotic mining spacecraft, which could give Tesla total control of their entire vertical from mining the resources in space, transporting them back to Earth (humans have been sending material from space to Earth since the start of the space program, remember), manufacturing those resources into batteries, and from there into electric cars.

Has this been Elon’s plan all along? Has all the Mars colonization hype just been a red-planet herring to distract us from his real preparations for large-scale asteroid mining?

Probably not, but it’s fun to think about. There’s also an environmental argument in favor of asteroid mining for electric car batteries. Where electric cars are far cleaner at the car level, they still take an environmental toll to build, since mining isn’t exactly the most eco-friendly endeavor. Moving that part of the equation off-planet would made the overall life cycle of an electric car vastly better for the Earth, for the simple reason it’s just not happening there.

NREL Research Yields Significant Thermoelectric Performance

Addition of thin films to fabrics could power portable electronics, sensorsScientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) reported significant advances in the thermoelectric performance of organic semiconductors based on carbon nanotube thin films that could be integrated into fabrics to convert waste heat into electricity or serve as a small power source.

The research demonstrates significant potential for semiconducting single-walled carbon nanotubes (SWCNTs) as the primary material for efficient thermoelectric generators, rather than being used as a component in a “composite” thermoelectric material containing, for example, carbon nanotubes and a polymer.

The discovery is outlined in the new Energy & Environmental Science paper, Large n- and p-type thermoelectric power factors from doped semiconducting single-walled carbon nanotube thin films.

NREL scientists Andrew Ferguson, left, and Jeffrey Blackburn stand in front of a screen displaying single-walled carbon nanotubes. (Photo by Dennis Schroeder/NREL)

“There are some inherent advantages to doing things this way,” said Jeffrey Blackburn, a senior scientist in NREL’s Chemical and Materials Science and Technology center and co-lead author of the paper with Andrew Ferguson.

These advantages include the promise of solution-processed semiconductors that are lightweight and flexible and inexpensive to manufacture. Other NREL authors are Bradley MacLeod, Rachelle Ihly, Zbyslaw Owczarczyk, and Katherine Hurst.

The NREL authors also teamed with collaborators from the University of Denver and partners at International Thermodyne, Inc., based in Charlotte, N.C.

Ferguson, also a senior scientist in the Chemical and Materials Science and Technology center, said the introduction of SWCNT into fabrics could serve an important function for “wearable” personal electronics.

By capturing body heat and converting it into electricity, the semiconductor could power portable electronics or sensors embedded in clothing.

Blackburn and Ferguson published two papers last year on SWCNTs, and the new research builds on their earlier work. The first paper, in Nature Energy, showed the potential that SWCNTs have for thermoelectric applications, but the films prepared in this study retained a large amount of insulating polymer.

The second paper, in ACS Energy Letters, demonstrated that removing this “sorting” polymer from an exemplary SWNCT thin film improved thermoelectric properties.

The newest paper revealed that removing polymers from all SWCNT starting materials served to boost the thermoelectric performance and lead to improvements in how charge carriers move through the semiconductor.

The paper also demonstrated that the same SWCNT thin film achieved identical performance when doped with either positive or negative charge carriers. These two types of material–called the p-type and the n-type legs, respectively–are needed to generate sufficient power in a thermoelectric device.

Semiconducting polymers, another heavily studied organic thermoelectric material, typically produce n-type materials that perform much worse than their p-type counterparts. The fact that SWCNT thin films can make p-type and n-type legs out of the same material with identical performance means that the electrical current in each leg is inherently balanced, which should simplify the fabrication of a device.

The highest performing materials had performance metrics that exceed current state-of-the-art solution-processed semiconducting polymer organic thermoelectrics materials.

“We could actually fabricate the device from a single material,” Ferguson said. “In traditional thermoelectric materials you have to take one piece that’s p-type and one piece that’s n-type and then assemble those into a device.”

The research was funded by a cooperative research and development agreement (CRADA) with partner International Thermodyne. The fundamental research in SWCNT separation and optical/electrical characterization is supported by the U.S. Department of Energy’s Office of Science.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.