Colloidal quantum dots: Performance boost next-generation solar cell technology

Researchers from the University of Toronto (U of T), the King Abdullah University of Science & Technology (KAUST) and Pennsylvania State University (Penn State) have created the most efficient solar cell ever made based on colloidal quantum dots (CQD).

The discovery is reported in the latest issue of Nature Materials.

Quantum dots are nanoscale semiconductors that capture light and convert it into an energy source. Because of their small scale, the dots can be sprayed on to flexible surfaces, including plastics. This enables the production of solar cells that are less expensive to produce and more durable than the more widely-known silicon-based version. In the work highlighted by the Nature Materials paper, the researchers demonstrate how the wrappers that encapsulate the quantum dots can be shrunk to a mere layer of atoms.

"We figured out how to shrink the passivating materials to the smallest imaginable size," states Professor Ted Sargent, corresponding author on the work and holder of the Canada Research Chair in Nanotechnology at U of T.

A crucial challenge for the field has been striking a balance between convenience and performance. The ideal design is one that tightly packs the quantum dots together. The greater the distance between quantum dots, the lower the efficiency.

However the quantum dots are usually capped with organic molecules that add a nanometer or two. When working on a nanoscale, that is bulky. Yet the organic molecules have been an important ingredient in creating a colloid, which is a substance that is dispersed in another substance. This allows the quantum dots to be painted on to other surfaces.

To solve the problem, the researchers have turned to inorganic ligands, which bind the quantum dots together while using less space. The result is the same colloid characteristics but without the bulky organic molecules.

"We wrapped a single layer of atoms around each particle. As a result, they packed the quantum dots into a very dense solid," explains Dr. Jiang Tang, the first author of the paper who conducted the research while a post-doctoral fellow in The Edward S. Rogers Department of Electrical & Computer Engineering at U of T.

The team showed the highest electrical currents, and the highest overall power conversion efficiency, ever seen in CQD solar cells. The performance results were certified by an external laboratory, Newport, that is accredited by the US National Renewable Energy Laboratory.

"The team proved that we were able to remove charge traps -- locations where electrons get stuck -- while still packing the quantum dots closely together," says Professor John Asbury of Penn State, a co-author of the work.

The combination of close packing and charge trap elimination enabled electrons to move rapidly and smoothly through the solar cells, thus providing record efficiency.

"This finding proves the power of inorganic ligands in building practical devices," states Professor Dmitri Talapin of The University of Chicago, who is a research leader in the field. "This new surface chemistry provides the path toward both efficient and stable quantum dot solar cells. It should also impact other electronic and optoelectronic devices that utilize colloidal nanocrystals. Advantages of the all-inorganic approach include vastly improved electronic transport and a path to long-term stability."

"At KAUST we were able to visualize, with incredible resolution on the sub-nanometer length scale, the structure and composition of this remarkable new class of materials," states Professor Aram Amassian of KAUST, a co-author on the work.

"We proved that the inorganic passivants were tightly correlated with the location of the quantum dots; and that it was this new approach to chemical passivation, rather than nanocrystal ordering, that led to this record-breaking colloidal quantum dot solar cell performance," he adds.

As a result of the potential of this research discovery, a technology licensing agreement has been signed by U of T and KAUST, brokered by MaRS Innovations (MI), which will will enable the global commercialization of this new technology.

"The world -- and the marketplace -- need solar innovations that break the existing compromise between performance and cost. Through U of T's, MI's, and KAUST's partnership, we are poised to translate exciting research into tangible innovations that can be commercialized," said Sargent.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University of Toronto, via EurekAlert!, a service of AAAS.

Journal Reference:

Jiang Tang, Kyle W. Kemp, Sjoerd Hoogland, Kwang S. Jeong, Huan Liu, Larissa Levina, Melissa Furukawa, Xihua Wang, Ratan Debnath, Dongkyu Cha, Kang Wei Chou, Armin Fischer, Aram Amassian, John B. Asbury, Edward H. Sargent. Colloidal-quantum-dot photovoltaics using atomic-ligand passivation. Nature Materials, 2011; DOI: 10.1038/nmat3118

Lasers could be used to detect roadside bombs

 A research team at Michigan State University has developed a laser that could detect roadside bombs -- the deadliest enemy weapon encountered in Iraq and Afghanistan.

The laser, which has comparable output to a simple presentation pointer, potentially has the sensitivity and selectivity to canvas large areas and detect improvised explosive devices -- weapons that account for around 60 percent of coalition soldiers' deaths. Marcos Dantus, chemistry professor and founder of BioPhotonic Solutions, led the team and has published the results in the current issue of Applied Physics Letters.

The detection of IEDs in the field is extremely important and challenging because the environment introduces a large number of chemical compounds that mask the select few molecules that one is trying to detect, Dantus said.

"Having molecular structure sensitivity is critical for identifying explosives and avoiding unnecessary evacuation of buildings and closing roads due to false alarms," he said.

Since IEDs can be found in populated areas, the methods to detect these weapons must be nondestructive. They also must be able to distinguish explosives from vast arrays of similar compounds that can be found in urban environments. Dantus' latest laser can make these distinctions even for quantities as small as a fraction of a billionth of a gram.

The laser beam combines short pulses that kick the molecules and make them vibrate, as well as long pulses that are used to "listen" and identify the different "chords." The chords include different vibrational frequencies that uniquely identify every molecule, much like a fingerprint. The high-sensitivity laser can work in tandem with cameras and allows users to scan questionable areas from a safe distance.

"The laser and the method we've developed were originally intended for microscopes, but we were able to adapt and broaden its use to demonstrate its effectiveness for standoff detection of explosives," said Dantus, who hopes to net additional funding to take this laser from the lab and into the field.

This research is funded in part by the Department of Homeland Security. BioPhotonic Solutions is a high-tech company Dantus launched in 2003 to commercialize technology invented in a spinoff from his research group at MSU.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Michigan State University.

Journal Reference:

Marshall T. Bremer, Paul J. Wrzesinski, Nathan Butcher, Vadim V. Lozovoy, Marcos Dantus. Highly selective standoff detection and imaging of trace chemicals in a complex background using single-beam coherent anti-Stokes Raman scattering. Applied Physics Letters, 2011; 99 (10): 101109 DOI: 10.1063/1.3636436

A guiding light for new directions in energy production: Optofluidics could help solve the energy challenge

The science of light and liquids has been intimately entwined since Léon Foucault discovered the speed of light in 1862, when he observed that light travels more slowly in water than in air. This physical harmony between the two materials is now being harnessed to collect and drive light to where it can be the most useful. October's issue of Nature Photonics focuses on optofluidics, the study of microfluidics -- the microscopic delivery of fluids through extremely small channels or tubes -- combined with optics. In a review written by Demetri Psaltis, Dean of EPFL's School of Engineering, he and his co-authors argue that optofluidics is poised to take on one of this century's most important challenges: energy.

"By directing the light and concentrating where it can be most efficiently used, we could greatly increase the efficiency of already existing energy producing systems, such as biofuel reactors and solar cells, as well as innovate entirely new forms of energy production" explains Psaltis. "EPFL is the world leader in optofluidics, our institution is in a position to develop truly efficient and disruptive energy sources."

Sunlight is already used for energy production besides conventional solar panels. For example, it is used to convert water and carbon dioxide into methane in large industrial biofuel plants. Prisms and mirrors are commonly employed to direct and concentrate sunlight to heat water on the roofs of homes and apartment buildings. These techniques already employ the same principles found in optofluidics -- control and manipulation of light and liquid transfer -- but often without the precision offered by nano and micro technology.

A futuristic example: Optofluidic solar lighting system

How can we better exploit the light that hits the outside of a building? Imagine sunlight channelled into the building An optofluidic solar lighting system could capture sunlight from a roof using a light concentrating system that follows the sun's path by changing the angle of the water's refraction, and then distribute the sunlight throughout the building through light pipes or fibre optic cables to the ceilings of office spaces, indoor solar panels, or even microfluidic air filters. Using sunlight to drive a microfluidic air filter or aliment an indoor solar panel -- which would be protected from the elements and last longer -- is a novel way to use solar energy to supplement non-renewable resources.

In such a system, it would be essential to deviate from the secondary devices such as air filtrage and solar panels to maintain a comfortable constant light source for ceiling lighting -- the flickering of the light source due to a cloud passing over would be intolerable. In order to modulate these different channels to maintain a constant light source, a system using electrowetting could deviate light from one channel into another both easily and inexpensively. A droplet of water sits on the outer surface of light tube. A small current excites the ions in the water, pushing them to the edge of the droplet and expanding it just enough for it to touch the surface of another tube. This expanded droplet then creates a light bridge between the two parallel light tubes, effectively moderating the amount of light streaming through either one.

Up-scaling for industrial use

"The main challenge optofluidics faces in the energy field is to maintain the precision of nano and micro light and fluid manipulation while creating industrial sized installations large enough to satisfy the population's energy demand," explains David Erickson, professor at Cornell University and visiting professor at EPFL. "Much like a super computer is built out of small elements, up-scaling optofluidic technology would follow a similar model -- the integration of many liquid chips to create a super-reactor."

Since most reactions in liquid channels happen at the point of contact between the liquid and the catalyst-lined tubes, the efficiency of a system depends on how much surface area is available for reactions to take place. Scaling down the size of the channels to the micro and nano level allows for thousands more channels in the same available space, greatly increasing the overall surface area and leading to a radical reduction of the size needed (and ultimately the cost) for catalytic and other chemical reactions. Adding a light source as a catalyst to the directed flow of individual molecules in nanotubes allows for extreme control and high efficiency.

Their review in Nature Phontonics lays out several possibilities for up-scaling optofluidics, such as using optical fibers to transport sunlight into large indoor biofuel reactors with mass-produced nanotubes. They point out that the use of smaller spaces could increase power density and reduce operating costs; optofluidics offers flexibility when concentrating and directing sunlight for solar collection and photovoltaic panels; and by increasing surface area, the domain promises to reduce the use of surface catalysts -- the most expensive element in many reactors.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Ecole Polytechnique Fédérale de Lausanne, via EurekAlert!, a service of AAAS.

Journal Reference:

David Erickson, David Sinton, Demetri Psaltis. Optofluidics for energy applications. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.209

New method to grow synthetic collagen unveiled: New material may find use in reconstructive surgery, cosmetics, tissue engineering

In a significant advance for cosmetic and reconstructive medicine, scientists at Rice University have unveiled a new method for making synthetic collagen. The new material, which forms from a liquid in as little as an hour, has many of the properties of natural collagen and may prove useful as a scaffold for regenerating new tissues and organs from stem cells.

"Our work is significant in two ways," said Rice's Jeffrey Hartgerink, the lead author of a new paper about the research in Nature Chemistry. "Our final product more closely resembles native collagen than anything that's previously been made, and we make that material using a self-assembly process that is remarkably similar to processes found in nature."

Collagen, the most abundant protein in the body, is a key component of many tissues, including skin, tendons, ligaments, cartilage and blood vessels. Biomedical researchers in the burgeoning field of regenerative medicine, or tissue engineering, often use a combination of stem cells and collagen-like materials in their attempts to create laboratory-grown tissues that can be transplanted into patients without risk of immunological rejection.

Animal-derived collagen, which has some inherent immunological risks, is the form of collagen most commonly used in reconstructive and cosmetic surgery today. Animal-derived collagen is also used in many cosmetics.

Despite the abundance of collagen in the body, deciphering or recreating it has not been easy for scientists. One reason for this is the complexity collagen exhibits at different scales. For example, just as a rope is made of many interwoven threads, collagen fibers are made of millions of proteins called peptides. Like a rope net that can trap and hold items, collagen fibers can form three-dimensional structures called hydrogels that trap and hold water.

"Our supramolecules, fibers and hydrogels form in a similar way to native collagen, but we start with shorter peptides," said Hartgerink, associate professor of chemistry and of bioengineering.

With an eye toward mimicking collagen's self-assembly process as closely as possible, Hartgerink's team spent several years perfecting its design for the peptides.

Hartgerink said it's too early to say whether the synthetic collagen can be substituted medically for human or animal-derived collagen, but it did clear the first hurdle on that path; the enzyme that the body uses to break down native collagen also breaks down the new material at a similar speed.

A faculty investigator at Rice's BioScience Research Collaborative, Hartgerink said scientists must next determine whether cells can live and grow in the new material and whether it performs the same way in the body that native collagen does. He estimated that clinical trials, if they prove warranted, are at least five years away.

The paper's co-authors include Rice graduate students Lesley O'Leary, Jorge Fallas, Erica Bakota and Marci Kang. The research was funded by the National Science Foundation, the Robert A. Welch Foundation and the Norman Hackerman Advanced Research Program of Texas.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Rice University.

Journal Reference:

Lesley E. R. O'Leary, Jorge A. Fallas, Erica L. Bakota, Marci K. Kang, Jeffrey D. Hartgerink. Multi-hierarchical self-assembly of a collagen mimetic peptide from triple helix to nanofibre and hydrogel. Nature Chemistry, 2011; DOI: 10.1038/nchem.1123

Note: If no author is given, the source is cited instead.

Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment. Views expressed here do not necessarily reflect those of ScienceDaily or its staff.

New material synthesized: Graphene nanoribbons inside of carbon nanotubes

Physicists from Umea University (Sweden) and Finland have found an efficient way to synthesize graphene nanoribbons directly inside of single-walled carbon nanotubes.

The result was recently published in the journal Nano Letters.

Graphene, a one atom thin flake of plain carbon, has a wide range of unusual and highly interesting properties. As a conductor of electricity it performs as well as copper. As a conductor of heat it outperforms all other known materials. There are possibilities to achieve strong variations of the graphene properties by making graphene in the form of belts with various widths, so called nanoribbons. These nanoribbons are now the real focus of attention in physics and an extremely promising material for electronics, solar cells and many other things. However, it is has not been easy to make such ribbons.

Associate professor Alexandr Talyzin and his research group at the Department of Physics, Umea University, have together with colleagues from Professor Esko Kauppinen´s group, Aalto University in Finland, discovered a way to use the hollow space inside carbon nanotubes as a one-dimensional chemical reactor to make encapsulated graphene. An intriguing property of this space is that chemical reactions occur differently here compared to under bulk three-dimensional conditions.

"We used coronene and perylene, which are large organic molecules, as building blocks to produce long and narrow graphene nanoribbons inside the tubes. The idea of using these molecules as building blocks for graphene synthesis was based on our previous study," says Talyzin.

This study revealed that coronene molecules can react with each other at certain conditions to form dimers, trimers and longer molecules in a bulk powder form. The result suggested that coronene molecules can possibly be used for synthesis of graphene but need to be somehow aligned in one plane for the required reaction. The inner space of single-walled carbon nanotubes seemed to be an ideal place to force molecules into the edge-to-edge geometry required for the polymerization reaction.

In the new study, the researchers show that this is possible. When the first samples were observed by electron microscopy by Ilya Anoshkin at Aalto University, exciting results were revealed: all nanotubes were filled inside with graphene nanoribbons.

"The success of the experiments also depended a lot on the choice of nanotubes. Nanotubes of suitable diameter and in high quality were provided by our co-authors from Aalto University," says Talyzin.

Later the researchers found that the shape of encapsulated graphene nanoribbons can be modified by using different kinds of aromatic hydrocarbons. The properties of nanoribbons are very different depending on their shape and width. For example, nanoribbons can be either metallic or semiconducting depending on their width and type. Interestingly, carbon nanotubes can also be metallic, semiconducting (depending on their diameter) or insulating when chemically modified.

"This creates an enormous potential for a wide range of applications. We can prepare hybrids that combine graphene and nanotubes in all possible combinations in the future," says Talyzin.

For example, metallic nanoribbons inside insulating nanotubes are very thin insulated wires. They might be used directly inside carbon nanotubes to produce light thus making nano-lamps. Semiconducting nanoribbons can possibly be used for transistors or solar cell applications and metallic-metallic combination is in fact a new kind of coaxial nano-cable, macroscopic cables of this kind are used e.g. for transmitting radio signals.

The new method of hybrid synthesis is very simple, easily scalable and allows obtaining almost 100 percent filling of tubes with nanoribbons. The theoretical simulations, performed by Arkady Krasheninnikov in Finland, also show that the graphene nanoribbons keep their unique properties inside the nanotubes while protected from the environment by encapsulation and aligned within bundles of single-walled nanotubes.

"The new material seems very promising, but we have a lot of inter-disciplinary work ahead of us in the field of physics and chemistry. To synthesize the material is just a beginning. Now we want to learn its electric, magnetic and chemical properties and how to use the hybrids for practical applications," says Talyzin.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Umea University.

Journal Reference:

Alexandr V. Talyzin, Ilya V. Anoshkin, Arkady V. Krasheninnikov, Risto M. Nieminen, Albert G. Nasibulin, Hua Jiang, Esko I. Kauppinen. Synthesis of Graphene Nanoribbons Encapsulated in Single-Walled Carbon Nanotubes. Nano Letters, 2011; 110902093500003 DOI: 10.1021/nl2024678