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Quantum Dot Solar Cells Break Conversion Efficiency Record

Quantum dots have offered an attractive option for photovoltaics. Multijunction solar cells made from colloidal quantum dots (CQD) have been able to achieve around 7-percent conversion efficiency in the lab. While figures like this may not seem too impressive when compared to silicon solar cells, their promised theoretical conversion efficiency limit is an eye-popping 45 percent. This is possible because when a single photon is absorbed by a quantum dot, it produces more than one bound electron-hole pair, or exciton, thereby doubling normal conversion efficiency numbers seen in single-junction silicon cells.

Now researchers at the Massachusetts Institute of Technology (MIT) have raised the bar for quantum dot-based solar cells by producing one that changes light to electricity with 9-percent conversion efficiency. Furthermore, says the MIT team, it can be produced using an inexpensive production method that promises to keep manufacturing costs down.

The researchers, who published their findings in the journal Nature Materials, hit upon a way to produce quantum dot solar cells through a solution processing technique that doesn’t require high temperatures or a vacuum atmosphere to achieve stability for the solar cells when they are exposed to air. By using ligand treatments, which involve molecules or ions that bind to a central metal, the researchers were able to align the bands of the quantum dot layers, improving the performance of the films.

“Every part of the cell, except the electrodes for now, can be deposited at room temperature, in air, out of solution. It’s really unprecedented,” said graduate student Chia-Hao Chuang in a press release.

The processing technique for the quantum dot layers allows for the dots to do what they do well individually and also to work together in the transport of electrical charge to the edges of the film where it can then be collected to provide an electrical current.

Nine-percent efficiency may still seem low to casual observers, but the development of quantum dots for photovoltaics has been so rapid that researchers are impressed by the latest development.

“Silicon had six decades to get where it is today, and even silicon hasn’t reached the theoretical limit yet. You can’t hope to have an entirely new technology beat an incumbent in just four years of development,” said professor Vladimir Bulović in the release.

The researchers still need to determine why these films are so stable and there’s still a long way to go before they are commercially viable. But they now hold the National Renewable Energy Lab (NREL) record for quantum dot solar efficiency.

Graphene Plasmonic Circuits Take a Critical Step Foward

When one takes into account the rough and tumble world for electrons in electronic devices, one would think it might make more sense just to use photons instead. The problem has been that light takes up a lot of space. The components for such a photonic system can’t get any smaller than a wavelength of light, which would translate to devices many times larger than today’s.

However, the field of plasmonics, which takes advantage of the surface plasmons that are generated when photons hit a metal structure, has looked to be a way forward for confining electromagnetic energy to subwavelength scales and create smaller photonic logic circuits. Recent research has shown that graphene and other two-dimensional materials produce plasmons too. These so-called graphene plasmons have an advantage over surface plasmons because their confinement of electromagnetic energy at subwavelength scales can be tuned and controlled by a gate voltage.

Now Spanish researchers at CIC nanoGUNE outside of San Sebastian, the Institute of Photonic Sciences (ICFO) near Barcelona, and the company Graphenea located at the CIC nanoGUNE research center have demonstrated that an optical antenna made from graphene can capture infrared light and transform it into graphene plasmons.

The research, which was recently published in the journal Science, demonstrated that a metal rod on graphene can act as an antenna for infrared light and transform it into graphene plasmons in much the same way a radio antenna converts radio waves into electromagnetic waves in a metal cable.

“We introduce a versatile platform technology based on resonant optical antennas for launching and controlling of propagating graphene plasmons, which represents an essential step for the development of graphene plasmonic circuits”, said team leader Rainer Hillenbrand in a press release.

The researchers believe that the graphene-based optical antennas provide a number of advantages. “The excitation of graphene plasmons is purely optical, the device is compact and the phase and wavefronts of the graphene plasmons can be directly controlled by geometrically tailoring the antennas,” said Pablo Alonso-González, who performed the experiments at nanoGUNE, in a press release. “This is essential to develop applications based on focusing and guiding of light.”

Graphene Grown Directly on Insulator Substrate

Initially, laboratory-produced graphene was the result of what has been dubbed the “Scotch Tape” method, in which graphene is pulled off in single-layer flakes directly from graphite. The method does produce a high quality, single crystal graphene that possesses many attractive properties like high electron mobility, but it is decidedly not scalable.

Chemical Vapor Deposition (CVD) has been hotly pursued as a potentially scalable way to grow graphene on a metal substrate like copper or nickel. However, it has struggled with the step of peeling the graphene off of the metal substrate without ruining or contaminating it.

Now researchers at the Massachusetts Institute of Technology (MIT) and the University of Michigan have devised a way in which graphene can be grown directly onto an insulator like glass or silicon, eliminating the need to peel the graphene off of a metal substrate.

In the research, which was published in Nature’s Scientific Reports, the team was able to grow the graphene at the interface of nickel and silicon dioxide. They followed that step by mechanical removal of the nickel from the substrate.

This process eliminates the need for the wet etching steps that researchers from the National University of Singapore (NUS) employed in a similar process where graphene was grown at the interface between copper and a silicon substrate. Some researchers believe that the Singaporean method would suffer from contamination and wouldn’t be able to produce large, uniform films.

In contrast to this NUS method, the new process grows graphene  on both sides of the nickel. In this way, it is possible to peel away the nickel and it will leave the graphene behind on top of the silicon substrate.

The researchers developed the method while working with a glass manufacturer so the aim was always to have a scalable production method.

“To meet their manufacturing needs, it must be very scalable,” says MIT's A. John Hart, in a press release. “We were inspired by the need to develop a scalable manufacturing process that could produce graphene directly on a glass substrate.”

While Hart cautions that the work is still at a preliminary stage due to a lack of consistent uniformity in the graphene films, it does open up some attractive potential applications.

Hart adds: “The ability to produce graphene directly on nonmetal substrates could be used for large-format displays and touch screens, and for ‘smart’ windows that have integrated devices like heaters and sensors.”

Smallest and Fastest Nanomotor Could Advance Drug Delivery

Engineers at the University of Texas Austin have developed what they claim is the smallest, fastest spinning and longest lasting nanomotor built to date. The engineers have demonstrated that their nanomotor is capable of rotating for 15 continuous hours at a speed of 18 000 rpm. These performance figures are on an entirely different scale to those of similar nanomotors that run anywhere from 14 rpm to 500 rpm and have only rotated for a few seconds or minutes.

The nanomotors, which are described in the journal Nature Communications, were built in a bottom-up manufacturing technique in which nanowires serve as rotors, patterned nanomagnets are the bearings, and quadrupole microelectrodes act as stators.

The entire structure of the nanomotor is smaller than 1 square micrometer, which means it is conceivable that they could fit inside some living cells. The nanomotors are also capable of rapidly mixing and pumping biochemicals as well as moving through liquids.

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What Is the Status of U.S. Nanomanufacturing?

The U.S. Government Accountability Office (GAO) made public yesterday a report on the state of nanotechnology in the United States compared to the rest of the world.

The GAO prepared the report, "Nanomanufacturing and U.S. Competitiveness: Challenges and Opportunities," at the request of the U.S. Congress, and GAO chief scientist Timothy M. Persons presented its findings in a testimony before the Subcommittee on Research and Technology of the House of Representatives.

I was interviewed extensively by two GAO economists for the accompanying report "Nanomanufacturing: Emergence and Implications for U.S. Competitiveness, the Environment, and Human Health," where I shared background information on research I helped compile and write on global government funding of nanotechnology.

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Nanotubes and Graphene Foam Make Hybrid Energy Storage Device

A paper in the journal Science earlier this year suggested that the problem of nomenclature for energy storage devices—specifically, defining the difference between what is a supercapacitor and what is a pseudocapacitor—is beginning to hold back development in the field.

To confuse matters further, researchers out of University of California Riverside have now developed an energy storage device that they define as a hybrid between a supercapacitor and a pseudocapacitor, but they prefer to term simply a supercapacitor.

The research, which is published in the journal Nature Scientific Reports, used hydrous ruthenium oxide (RuO2) nanoparticles that were modified by carbon nanotubes (CNT) and graphene foam as the electrode material for the supercapacitor. They then put the electrodes in an aqueous electrolyte. The combination not only operated safely, but also provided more energy and power density than what today’s commercially available supercapacitors can give.

“Besides high energy and power density, the designed graphene foam electrode system also demonstrates a facile and scalable binder-free technique for preparing high energy supercapacitor electrodes,” said graduate student Wei Wang in a press release. “These promising properties mean that this design could be ideal for future energy storage applications.”

The graphene-and-nanotube hybrid foam was  dipped into a slurry of the RuO2 resulting in a few-layer-thick graphene foam architecture covered with hybrid networks of RuO2 nanoparticles and anchored nanotubes. This design merges the supercapacitor's high  conductivity and pseudocapacitor's high specific capacitance, according to the researchers.

“The resulting hybrid device enables enough electrolyte access to the active materials (CNT-RuO2 network layer),” Wang told the Nanoclast in an e-mail. “And at the same time, the embedded CNTs in the CNT-RuO2 network layer work as a conductive framework.”

Supercapacitors operate in the space between batteries and traditional capacitors when it comes to the metrics of energy density (the amount of energy stored per unit mass) and power density (the maximum amount of power that can be supplied per unit mass). Batteries can store a lot more energy than supercapacitors and capacitors can deliver power far more quickly than supercapacitors. However, since supercapacitors can deliver power fast and charge up quickly in comparison to batteries, they are attractive for many applications, like electric vehicles, where they could be charged up in minutes rather than hours. But compared to batteries supercapacitors have low energy density, so the aim has been to increase how much energy they can store. If supercapacitor energy density could be increased, they could potentially replace chemical-based batteries.

While nanomaterials have been touted as good alternatives to traditional activated carbon for supercapacitor electrodes, it’s not been clear that they can provide enough surface area to make the resulting devices match the energy density of lithium-ion (Li-ion) batteries. On average those batteries have a specific energy density of 200 Watt-hour/kilogram,  whereas today’s supercapacitors can get around 28 Wh/kg. In figures released by the UC Riverside researchers, they claimed a full cell energy density of their device of 39.28 Wh/kg. Whether this will be enough to really be a breakthrough for supercapacitors in all-electric vehicles remains to be seen.

Bottom-Up Manufacturing of Nanowires on Silicon Expands Its Capabilities

With each passing day, the limitations of silicon chips become increasingly pressing. The inexorable march of Moore’s Law to smaller and smaller dimensions in order to double the number of transistors on integrated circuits every two years is beginning to sound like the footsteps of an army of giants. Despite brilliant engineering twists and tweaks, eventually silicon is not going to be able to avoid getting crushed by this onslaught, at least not without some help from other materials.

The problems with silicon are not just limited to an inability to meet the demands put on it by the shrinking dimensions of chips. It’s also not particularly good at a number of things that would be nice capabilities. For instance, you can’t really get silicon to emit light for photonic applications, and silicon circuits can’t withstand temperatures beyond 250 degrees Celsius. This last limitation requires all sorts of complicated engineering of the circuits to account for its temperature sensitivity.

But now, researchers at the University of California Davis have found a way to combine silicon with nanowires to create circuits that not only will make possible smaller dimensions, but also emit light for optoelectronic applications and withstand high temperatures found in hostile environments such as inside a jet engine. 

"Silicon can't do everything," said Saif Islam, professor of electrical and computer engineering at UC Davis, in a press release. "In the foreseeable future, society will be dependent on a variety of sensors and control systems that operate in extreme environments, such as motor vehicles, boats, airplanes, terrestrial oil and ore extraction, rockets, spacecraft, and bodily implants.”

While combining silicon with non-silicon materials has offered more robust performance, it has remained a challenge to grow non-silicon materials as layers over silicon because of incompatibilities in the crystal structures and differences in thermal properties.

To overcome this limitation, Islam has grown nanopillars from gallium arsenide, gallium nitride, or indium phosphide on top of the silicon to serve as foundations on top of which nanowires are used to bridge the gap between the nanopillars.

"We can't grow films of these other materials on silicon, but we can grow them as nanowires," Islam explained in the release. In the video below, Islam explains how nanowires can be grown on silicon and can improve its capabilities.

In research, which was published in the journal Advanced Materials (“3D-Transistor Array Based on Horizontally Suspended Silicon Nano-bridges Grown via a Bottom-Up Technique”), Islam and his colleagues developed a bottom-up technique to create these nanowire bridges. They note that it does not require the tedious nanowire alignment and contact formation processes that are typically required to connect these nanowires.

This bottom-up approach led to the researchers being able to get the nanowires to operate like transistors and form the basis of more complex circuits. The material also proved to be responsive to light.

Perhaps most attractive to the semiconductor industry is that the technology does not require any significant changes to the manufacturing of silicon integrated circuits.

Fiber-like Supercapacitors Could Be Woven Into Wearable Electronics

Initial hopes were that graphene and its cousin the carbon nanotube could serve as a replacements to activated carbon to push supercapacitors to the equivalent storage capacity of batteries. That hope soon waned when it became apparent that these carbon nanomaterials don’t even have the theoretical surface area—one of the key features for higher storage capacity in supercapacitors—of activated carbon.

As far back as four years ago, research started to move away from just trying to beat the energy density (the amount of energy stored per unit mass) of supercapacitors using activated carbon, but instead started looking at the interesting structures that could be built using graphene and carbon nanotubes as the electrode material for supercapacitors. This meant that new, smaller supercapacitors could be used to power microelectronic devices with unusual geometries.

Now an international team of researchers from Nanyang Technological University (NTU) in Singapore, Tsinghua University in China, and Case Western Reserve University in the United States has realized one of these potential new applications by developing a fiber-like supercapacitor made from both graphene and carbon nanotubes that could be woven right into clothing.

In keeping with the new fiber-like geometry of the supercapacitors, the researchers have released figures on the energy density of the novel supercapacitors by volume rather than by mass. They claim that the volumetric energy density is the highest yet reported for carbon-based microscale supercapacitors: 6.3 microwatt-hours per cubic millimeter, which is comparable to a 4-volt-500-microampere-hour thin-film lithium ion battery that can be used to power smart cards and RFID tags.

This record-breaking figure for volumetric energy density addresses one of the weaknesses of typical, activated carbon-based supercapacitors. Using activated carbon on the electrodes of supercapacitors could approximate the energy density of batteries by mass, but when it came to volume they were woefully deficient because they require large amounts of accessible surface area to store energy.

In research reported in journal Nature Nanotechnology, the team demonstrated that their hybrid fiber could store energy along its entire length, providing huge amounts of accessible surface area—396 square meters per gram of hybrid fiber.

The researchers produced the fiber-like supercapacitor by heating a solution of graphene and carbon nanotubes. The graphene and carbon nanotubes self assemble into an interconnected, porous network that runs the entire length of the fiber. The researchers have made flexible fibers as long as 50 meters and having a charge capacity of 300 Farad per cubic centimeter.

"We have tested the fiber device for 10 000 charge/discharge cycles, and the device retains about 93 percent of its original performance, while conventional rechargeable batteries have a lifetime of less than 1000 cycles," said Yuan Chen, a professor of chemical engineering at NTU, in a press release. "The fiber supercapacitor continues to work without performance loss, even after bending hundreds of times.”

The researchers envision the supercapcitor fibers being woven into clothing that could power biomedical monitoring devices a patient wears at home.

The next steps for the researchers will be to scale up the production method to bring down its costs to make it more attractive for commercialization. Meanwhile they will also be looking into applying the supercapacitor fibers  solar cells, biofuel production, wearable optoelectronics, and other systems.

Graphene Oxide Could Lead to Easy-to-Make Integrated Photonics

In order for us to enjoy our high-speed Internet connections, we depend on optical fiber carrying multiple beams of laser light at different wavelengths. A key property that makes optical networks function is something called optical nonlinearity, which is the ability of a medium to have its optical properties (transmission, refraction, etc.) manipulated by changing the intensity of the light traveling through it. Optical nonlinearity provides us the possibility to use light to control light so we can operate fiber optic networks.

Now, researchers at Swinburne University of Technology in Melbourne, Australia, have found that graphene oxide (GO) possesses a record-breaking optical nonlinearity that makes it suitable for use in high-performance integrated photonic devices for all-optical communications, biomedicine, and photonic computing. Associate Professor Baohua Jia at Swinburne told Nanoclast that the nonlinearity of the GO film they developed is 1000 times as large as previous results. 

In research which was published in the journal Advanced Materials (“In Situ Third-Order Non-linear Responses During Laser Reduction of Graphene Oxide Thin Films Towards On-Chip Non-linear Photonic Devices”), the Swinburne team spin coated a GO film onto a glass surface. The researchers then used a laser to create microstructures on the surface of GO film to tune the nonlinearity of the material.

The Swinburne researchers believe that their approach to laser writing structures on a GO film can serve as a method for tuning the nonlinearity of every optical component of integrated photonic devices.

This stands in contrast to today’s integrated photonic devices, in which multiple photonic functions are integrated in one device by building each component separately and then putting them together.

“Now we can provide a film, on which everything can be fabricated with laser and then it is automatically integratable,” said PhD student Xiaorui Zheng in a press release.

This should make the manufacturing process for integrated photonic devices, which still require clean rooms to be fabricated, dramatically easier. “Using this new method, we have demonstrated the possibility of manufacturing a scalable and cheap material,” Professor Jia said in the press release.

Over the past year, Swinburne has taken on graphene oxide as a research target for optics. Last October, a Swinburne team discovered that GO’s giant refractive index could be exploited for merging data storage with holography for improved security coding.

In this most recent research, the aim now is to fabricate a functional device.

Cyborg Beetles Detect Nerve Gas

The rival rock stars of nanotech—carbon nanotubes and graphene—have joined forces to become a super group of late. They are now being combined to make supercapacitors or just to make the manufacturing process for one of them less arduous.

Now researchers in South Korea have joined them together to create one monolithically integrated flexible electronic device that can be synthesized in a single step and be attached to, among other things, live stag beetles that can be set loose to detect a range of environmental conditions or nerve gas agents.

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

Dexter Johnson
Madrid, Spain
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