Pairing Quantum Dots With Fullerenes for Nanoscale Photovoltaics

"This is the first demonstration of a hybrid inorganic/organic, dimeric (two-particle) material that acts as an electron donor-bridge-acceptor system for converting light to electrical current," said Brookhaven physical chemist Mircea Cotlet, lead author of a paper describing the dimers and their assembly method inAngewandte Chemie.

By varying the length of the linker molecules and the size of the quantum dots, the scientists can control the rate and the magnitude of fluctuations in light-induced electron transfer at the level of the individual dimer."This control makes these dimers promising power-generating units for molecular electronics or more efficient photovoltaic solar cells," said Cotlet, who conducted this research with materials scientist Zhihua Xu at Brookhaven's Center for Functional Nanomaterials (CFN).

Scientists seeking to develop molecular electronics have been very interested in organic donor-bridge-acceptor systems because they have a wide range of charge transport mechanisms and because their charge-transfer properties can be controlled by varying their chemistry. Recently, quantum dots have been combined with electron-accepting materials such as dyes, fullerenes, and titanium oxide to produce dye-sensitized and hybrid solar cells in the hope that the light-absorbing and size-dependent emission properties of quantum dots would boost the efficiency of such devices. But so far, the power conversion rates of these systems have remained quite low.

"Efforts to understand the processes involved so as to engineer improved systems have generally looked at averaged behavior in blended or layer-by-layer structures rather than the response of individual, well-controlled hybrid donor-acceptor architectures," said Xu.

The precision fabrication method developed by the Brookhaven scientists allows them to carefully control particle size and interparticle distance so they can explore conditions for light-induced electron transfer between individual quantum dots and electron-accepting fullerenes at the single molecule level.

The entire assembly process takes place on a surface and in a stepwise fashion to limit the interactions of the components (particles), which could otherwise combine in a number of ways if assembled by solution-based methods. This surface-based assembly also achieves controlled, one-to-one nanoparticle pairing.

To identify the optimal architectural arrangement for the particles, the scientists strategically varied the size of the quantum dots -- which absorb and emit light at different frequencies according to their size -- and the length of the bridge molecules connecting the nanoparticles. For each arrangement, they measured the electron transfer rate using single molecule spectroscopy.

"This method removes ensemble averaging and reveals a system's heterogeneity -- for example fluctuating electron transfer rates -- which is something that conventional spectroscopic methods cannot always do," Cotlet said.

The scientists found that reducing quantum dot size and the length of the linker molecules led to enhancements in the electron transfer rate and suppression of electron transfer fluctuations.

"This suppression of electron transfer fluctuation in dimers with smaller quantum dot size leads to a stable charge generation rate, which can have a positive impact on the application of these dimers in molecular electronics, including potentially in miniature and large-area photovoltaics," Cotlet said.

"Studying the charge separation and recombination processes in these simplified and well-controlled dimer structures helps us to understand the more complicated photon-to-electron conversion processes in large-area solar cells, and eventually improve their photovoltaic efficiency," Xu added.

A U.S. patent application is pending on the method and the materials resulting from using the technique, and the technology is available for licensing. This work was funded by the DOE Office of Science.

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A Simple, Mildly Invasive Solution for Conserving Historic Buildings

Her European doctoral thesis, undertaken at the Tecnalia Construction Unit and presented at the University of the Basque Country (UPV/EHU), is entitledRehabilitation of masonry arches by a compatible and minimally invasive strengthening system.

The solution proposed is based on a composite material known as BTRM (Basalt Textile-Reinforced Mortar). This involves a series of tissues of basalt embedded in an inorganic matrix (cement-free mortar modified with polymers). With her research, Ms Garmendia aimed at contributing greater knowledge on the behaviour of masonry arches, as well as on the efficaciousness of the BTRM system applied to such arches.

BTRM system

The researcher was able to show that, thanks to the physicochemical characteristics of BTRM components (resistance to high temperatures, permeability water vapour, flexibility, etc.), this composite material is compatible with the elements to be reinforced in arches. Moreover, it involves an easy-to-apply technology for buildings and especially for those with complex geometries like arches or vaulting. Also notable is its competitive cost compared to the more usual reinforcement methods employed to date.

The work carried out to arrive at these conclusions used a comprehensive, integral approach, with this reinforcement solution applied to stone buildings and, more specifically, to stonemasonry. First, mineralogical and mechanical characterisation tests were carried out on the materials making up the construction, both at the level of each constituent material and also for the overall structure; for the latter, 24 medium-scale, prismatic concrete test pieces were made, varying the material type (mortar and stone) and its bonding. Then the proposed reinforcement system put forward was looked at in more detail; carrying out physicochemical tests on the basalt tissue, the inorganic matrix and the tissue-matrix compound.

A third stage involved carrying out trials on twelve arches -- technical control of displacement to the point of reaching collapse. These arches were built and reinforced according to different criteria, both in terms of their typology (stonemasonry with or without mortar between the keystones/voussoirs) and in terms of reinforcement (without BTRM either with piers -- the lower surfaces of the arches -- , or with extrados -- the upper surfaces of the arches -- , or with both). Finally, various calculation methods were employed to mathematically evaluate the effect of the reinforcement solution proposed.

In conclusion, the experimental results reached with the PhD have shown the physicochemical compatibility between the BTRM system and the corresponding substrate of the stone construction to be reinforced, as well as validating its mechanical effectiveness in the reinforcement of arched structures. Thus, it was verified that this reinforcement solution could be the optimum alternative to traditional methods.

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Origami Not Just for Paper Anymore: DNA, Folded Into Complex Shapes, Could Have a Big Impact on Nanotechnology

Trying to build DNA structures on a large scale was once considered unthinkable. But about five years ago, Caltech computational bioengineer Paul Rothemund laid out a new design strategy called DNA origami: the construction of two-dimensional shapes from a DNA strand folded over on itself and secured by short"staple" strands. Several years later, William Shih's lab at Harvard Medical School translated this concept to three dimensions, allowing design of complex curved and bent structures that opened new avenues for synthetic biological design at the nanoscale.

A major hurdle to these increasingly complex designs has been automation of the design process. Now a team at MIT, led by biological engineer Mark Bathe, has developed software that makes it easier to predict the three-dimensional shape that will result from a given DNA template. While the software doesn't fully automate the design process, it makes it considerably easier for designers to create complex 3-D structures, controlling their flexibility and potentially their folding stability.

"We ultimately seek a design tool where you can start with a picture of the complex three-dimensional shape of interest, and the algorithm searches for optimal sequence combinations," says Bathe, the Samuel A. Goldblith Assistant Professor of Applied Biology."In order to make this technology for nanoassembly available to the broader community -- including biologists, chemists, and materials scientists without expertise in the DNA origami technique -- the computational tool needs to be fully automated, with a minimum of human input or intervention."

Bathe and his colleagues described their new software in the Feb. 25 issue ofNature Methods. In that paper, they also provide a primer on creating DNA origami with collaborator Hendrik Dietz at the Technische Universitaet Muenchen."One bottleneck for making the technology more broadly useful is that only a small group of specialized researchers are trained in scaffolded DNA origami design," Bathe says.

Programming DNA

DNA consists of a string of four nucleotide bases known as A, T, G and C, which make the molecule easy to program. According to nature's rules, A binds only with T, and G only with C."With DNA, at the small scale, you can program these sequences to self-assemble and fold into a very specific final structure, with separate strands brought together to make larger-scale objects," Bathe says.

Rothemund's origami design strategy is based on the idea of getting a long strand of DNA to fold in two dimensions, as if laid on a flat surface. In his first paper outlining the method, he used a viral genome consisting of approximately 8,000 nucleotides to create 2-D stars, triangles and smiley faces.

That single strand of DNA serves as a"scaffold" for the rest of the structure. Hundreds of shorter strands, each about 20 to 40 bases in length, combine with the scaffold to hold it in its final, folded shape.

"DNA is in many ways better suited to self-assembly than proteins, whose physical properties are both difficult to control and sensitive to their environment," Bathe says.

Bathe's new software program interfaces with a software program from Shih's lab called caDNAno, which allows users to manually create scaffolded DNA origami from a two-dimensional layout. The new program, dubbed CanDo, takes caDNAno's 2-D blueprint and predicts the ultimate 3-D shape of the design. This resulting shape is often unintuitive, Bathe says, because DNA is a flexible object that twists, bends and stretches as it folds to form a complex 3-D shape.

According to Rothemund, the CanDo program should allow DNA origami designers to more thoroughly test their DNA structures and tweak them to fold correctly."While we have been able to design the shape of things, we have had no tools to easily design and analyze the stresses and strains in those shapes or to design them for specific purposes," he says.

At the molecular-level, stress in the double helix of DNA decreases the folding stability of the structure and introduces local defects, both of which have hampered progress in the scaffolded DNA origami field.

Postdoctoral researcher Do-Nyun Kim and graduate student Matthew Adendorff, both of the Bathe lab, are now furthering CanDo's capabilities and optimizing the scaffolded DNA origami design process.

Building nanoscale tools

Once scientists have a reliable way to assemble DNA structures, the next question is what to do with them. One application scientists are excited about is a"DNA carrier" that can transport drugs to specific destinations in the body such as tumors, where the carrier would release the cargo based on a specific chemical signal from the target cancer cell.

Another possible application of scaffolded DNA origami could help reproduce part of the light-harvesting apparatus of photosynthetic plant cells. Researchers hope to recreate that complex series of about 20 protein subunits, but to do that, components must be held together in specific positions and orientations. That's where DNA origami could come in.

"DNA origami enables the nanoscale construction of very precise architectural arrangements. Researchers are exploiting this unique property to pursue a number of applications at the nanoscale, including a synthetic photocell," Bathe says."While applications such as this are still quite far off on the horizon, we believe that predictive engineering software tools are essential for progress in this direction."

Novel applications may also grow out of a new competition being held at Harvard this summer, called BIOMOD. Undergraduate teams from about a dozen schools, including MIT, Harvard and Caltech, will try to design nanoscale biomolecules for robotics, computing and other applications.

In the meantime, Bathe is focusing on further developing CanDo to enable automated DNA origami design."Once you have an automated computational tool that allows you to design complex shapes in a precise way, I think we're in a much better position to exploit this technology for interesting applications," he says.

For DNA origami to have a broad impact, it needs to become routine to simply order up DNA parts to build any configuration you can dream up, Bathe says. He notes:"Once non-specialists can design arbitrary 3-D nanostructures using DNA origami, their imaginations can run free."

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Functioning Synapse Created Using Carbon Nanotubes: Devices Might Be Used in Brain Prostheses or Synthetic Brains

The team, which was led by Professor Alice Parker and Professor Chongwu Zhou in the USC Viterbi School of Engineering Ming Hsieh Department of Electrical Engineering, used an interdisciplinary approach combining circuit design with nanotechnology to address the complex problem of capturing brain function.

In a paper published in the proceedings of the IEEE/NIH 2011 Life Science Systems and Applications Workshop in April 2011, the Viterbi team detailed how they were able to use carbon nanotubes to create a synapse.

Carbon nanotubes are molecular carbon structures that are extremely small, with a diameter a million times smaller than a pencil point. These nanotubes can be used in electronic circuits, acting as metallic conductors or semiconductors.

"This is a necessary first step in the process," said Parker, who began the looking at the possibility of developing a synthetic brain in 2006."We wanted to answer the question: Can you build a circuit that would act like a neuron? The next step is even more complex. How can we build structures out of these circuits that mimic the function of the brain, which has 100 billion neurons and 10,000 synapses per neuron?"

Parker emphasized that the actual development of a synthetic brain, or even a functional brain area is decades away, and she said the next hurdle for the research centers on reproducing brain plasticity in the circuits.

The human brain continually produces new neurons, makes new connections and adapts throughout life, and creating this process through analog circuits will be a monumental task, according to Parker.

She believes the ongoing research of understanding the process of human intelligence could have long-term implications for everything from developing prosthetic nanotechnology that would heal traumatic brain injuries to developing intelligent, safe cars that would protect drivers in bold new ways.

For Jonathan Joshi, a USC Viterbi Ph.D. student who is a co-author of the paper, the interdisciplinary approach to the problem was key to the initial progress. Joshi said that working with Zhou and his group of nanotechnology researchers provided the ideal dynamic of circuit technology and nanotechnology.

"The interdisciplinary approach is the only approach that will lead to a solution. We need more than one type of engineer working on this solution," said Joshi."We should constantly be in search of new technologies to solve this problem."

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Fat Turns Into Soap in Sewers, Contributes to Overflows

"We found that FOG deposits in sewage collection systems are created by chemical reactions that turn the fatty acids from FOG into, basically, a huge lump of soap," says Dr. Joel Ducoste, a professor of civil, construction and environmental engineering at NC State and co-author of a paper describing the research. Collection systems are the pipes and pumping stations that carry wastewater from homes and businesses to sewage-treatment facilities.

These hardened FOG deposits reduce the flow of wastewater in the pipes, contributing to sewer overflows -- which can cause environmental and public-health problems and lead to costly fines and repairs.

The research team used a technique called Fourier Transform Infrared (FTIR) spectroscopy to determine what the FOG deposits were made of at the molecular level. FTIR spectroscopy shoots a sample material with infrared light at various wavelengths. Different molecular bonds vibrate in response to different wavelengths. By measuring which infrared wavelengths created vibrations in their FOG samples, researchers were able to determine each sample's molecular composition.

Using this technique, researchers confirmed that the hardened deposits were made of calcium-based fatty acid salts -- or soap.

"FOG itself cannot create these deposits," Ducoste says."The FOG must first be broken down into its constituent parts: glycerol and free fatty acids. These free fatty acids -- specifically, saturated fatty acids -- can react with calcium in the sewage collection system to form the hardened deposits.

"Until this point we did not know how these deposits were forming -- it was just a hypothesis," Ducoste says."Now we know what's going on with these really hard deposits."

The researchers are now focused on determining where the calcium in the collection system is coming from, and how quickly these deposits actually form. Once they've resolved those questions, Ducoste says, they will be able to create numerical models to predict where a sewage system may have"hot spots" that are particularly susceptible to these blockages.

Ultimately, Ducoste says,"if we know how -- and how quickly -- these deposits form, it may provide scientific data to support policy decisions related to preventing sewer overflows."

The research was funded by the Water Resources Research Institute and the U.S. Environmental Protection Agency.

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Pier Review: Comparing Ultra High-Resolution Photographs from the Past and the Present Could Hold the Key to Restoring Hastings' Fire-Damaged Pier

Prior to the fire, NPL, the UK's National Measurement Institute, had been surveying the pier to support redevelopment plans and to monitor long-term changes in the pier. The project was part of the development of a world leading low-cost technique to assess long-term degradation of structures.

The technique is called Digital Image Correlation. It has been used in the laboratory for some time but NPL have recently been pioneering its use for looking at civil engineering structures. It involves taking ultra high-resolution panoramic photos -- images up to 1.4 Giga Pixels in size -- at two different times to identify structural changes. Advanced mathematical programs then analyse the pair of images to identify changes in the structure pixel by pixel. Using this information, engineers can understand how large structures change over time.

Following the devastating fire, NPL scientists returned to Hastings to take their second set of photos. They were then required to develop more advanced analysis techniques, which could deal with the much larger than anticipated changes to the Pier, and produce meaningful information about the structure. This work is proving more valuable than expected as considerable change has now taken place. In addition, the large panoramic images provide a snapshot of the structure in time, which is useful for archival purposes.

Up to 45 images were stitched together to produce an ultra high-resolution final image 80,000 pixels wide -- 300-400 times more detailed than a typical camera-phone photograph. Processing a pair of these images, one before the fire and one after, can help highlight where the structure has apparently changed because of the fire.

Results have been very positive. Whilst the super-structure has been severely damaged and there are large visual changes, the cast iron framework -- or sub-structure -- seems much less affected. The sub-structure on the west side of the Pier appears to be remarkably similar pre and post fire. On the East side there are small areas where there are some changes, and one localised area of the sub-structure about half way along showing significant distortion. But the vast majority of the sub-structure seems largely unchanged. The area showing the most distortion -- presumably caused by the extreme heat -- was at a downwind point where anecdotally the fire was seen to be fiercest.

Digital Image Correlation allows the computer to effectively carry out the laborious checking of the whole structure. This means quicker and cheaper identification of areas which have been deformed or damaged, and hence may need closer inspection. This is important on large structures such as piers as it allows civil engineers to focus their efforts on the parts that most need attention, dramatically speeding the inspection process and reducing the cost of repair.

The project has also helped prove the concept of Digital Image Correlation for the measurement of changes in large structures, by providing NPL with a real-life case study enabling development of key analysis software.

Nick McCormick, Principle Research Scientist at NPL, said:"It was fortunate that we began the project before the fire, as the results will be invaluable in regenerating the pier when restoration funding is secured. From a scientific point of view, the scale of the changes actually proved very interesting, although challenging, and required us to develop far more advanced analysis techniques than originally intended. These will be hugely important in our work to develop low cost monitoring solutions for other structures. Obviously we hope the next one won't be so badly damaged part way through our study. For most applications we work on we would expect to monitor much less significant changes over time -- for example small cracks appearing in bridges or building subsidence -- so that problems can be remedied before they escalate to cause such serious damage."

Digital Image Correlation is one of a number of techniques that NPL is developing for low-cost examination of large civil engineering structures such as bridges, buildings, tunnels and piers.

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Carbon Fiber Used to Reinforce Buildings; Protect from Explosion

Sarah Orton, assistant professor of civil engineering in the MU College of Engineering, has focused on using carbon fiber reinforced polymer (CFRP), a fabric that can carry 143,000 pounds of force per square inch and has various applications to strengthen reinforced concrete buildings. CFRP has been used previously to strengthen buildings for earthquakes.

"CFRP has been used in places like California since the 1980s to protect buildings from earthquakes, but it has so many applications," Orton said."Now, we have to worry about damage caused by attacks. This fabric can be a great tool to protect people in threatened buildings."

To protect a building from an extreme event, CFRP can be used to increase the bending capacity of walls or columns. Previously, Orton invented an anchor that can be embedded in the column or joint to make CFRP more effective. In that work, Orton found that the anchors allow the CFRP to reach its full tension strength rather than separating from the concrete at only about half its strength.

CFRP can be used to protect an entire wall from an explosion. To study the effectiveness of different ways of applying CFRP, Orton worked with the U.S. Army Engineer Research and Development Center (ERDC) to detonate explosives near CFRP-reinforced concrete slabs. She found that CFRP, when layered and anchored, provided a significant amount of protection. However, she said that applying additional protection to the front of the concrete slab, such as a steel plate, would enhance the slab's performance.

Orton says the high costs of approximately$30 per square foot have kept CFRP from being widely implemented in non-earthquake prone areas.

"This is a really useful material," Orton said."I continue to be fascinated by the material's strength and applications. Retrofitting buildings with CFRP will help protect people from attacks and potentially collapse of the building."

The study,"Use of Carbon Fiber Anchors to Improve Performance of CFRP Strengthened Concrete Structures Subjected to Blast and Impact Loads," will be published in a special publication of the American Concrete Institute.

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