From a carpet of nanorods to a thin film solar cell absorber within a few seconds

The transformation from a layer of closely packed nanorods
(top left) to a polycrystalline semiconductor thin film (top right)
can be observed in by in-situ X-ray diffraction in real time.
The intensities of the diffraction signals are color coded in the
image at the bottom. A detailed analysis of the signals reveals
that the transformation of the nanorods into kesterite crystals
takes only 9 to 18 seconds.
Picture: R. Mainz/A. Singh

Research teams at the HZB and at the University of Limerick, Ireland, have discovered a novel solid state reaction which lets kesterite grains grow within a few seconds and at relatively low temperatures. For this reaction they exploit a transition from a metastable wurtzite compound in the form of nanorods to the more stable kesterite compound. 

At the EDDI Beamline at BESSY II, the scientists could observe this process in real-time when heating the sample: in a few seconds Kesterite grains formed. The size of the grains was found to depend on the heating rate. With fast heating they succeeded in producing a Kesterite thin film with near micrometer-sized crystal grains, which could be used in thin film solar cells. These findings have now been published in the journal “Nature Communications”.

Grain formation during growth of kesterite solar cells observed in real-time

As starting material for the formation of the kesterite film serves a “carpet of nanorods”: With the help of solution-based chemical processing, the chemists around Ajay Singh and Kevin Ryan at the University of Limerick have fabricated films of highly ordered wurtzite nanorods, which have exactly the same composition as kesterite Cu₂ZnSnS₄. With the help of real-time X-ray diffraction at the EDDI beamline of BESSY II, HZB physicists around Roland Mainz and Thomas Unold could now observe how a phase transition from the metastable wurtzite phase to the stable kesterite phase leads to a rapid formation of a thin film with large kesterite grains. “It is interesting to see that the complete formation of the kesterite film is so fast”, says Mainz. And the faster the samples are heated up, the larger the grains grow. Mainz explains that at low heating rate, the transition from wurtzite to kesterite starts at lower temperature at which many small grains form – instead of a few larger grains. Additionally, more defects are formed at lower temperatures. During fast heating, the transition takes place at higher temperature at which grains with less defects form.

Moreover, the comparison of the time-resolved evolution of the phase transition during slow and during fast heating shows that not only the grain growth is triggered by the phase transition, but also the phase transition is additionally accelerated by the grain growth. The HZB physicists have developed a model which can explain these findings. By means of numerical model calculations, they demonstrated the accordance of the model with the measured data.

Novel synthesis pathway for thin film semiconductors with controlled morphology
The work points towards a new pathway for the fabrication of thin microcrystalline semiconductor films without the need of expensive vacuum technology. Cu₂ZnSnS₄-based kesterite semiconductors have gained increasing attention in the past, since they are a promising alternative for the Cu(In,Ga)Se₂chalcopyrite solar cells which already achieved efficiencies above 20%. Kesterite has similar physical properties as the chalcopyrite semiconductors, but consist only of elements which are abundantly present in the earth crust. The new procedure could also be interesting for the fabrication of micro- and nanostructured photoelectric devices as well as for semiconductor layers consisting of other materials, says Mainz. “But we continue to focus on kesterites, because this is a really exciting topic at the moment.”

Source: http://www.helmholtz-berlin.de/pubbin/news_seite?nid=13909;sprache=en;typoid=3228

Idaho scientists discover clue in the case of the missing silver

Some come to Idaho to travel the highways that lead to the Tetons, to Yellowstone, to small towns and big adventures. Idaho National Laboratory researcher Isabella van Rooyen came, all the way from South Africa, looking for a piece of silver 500,000 times smaller than a poppy seed. 

The silver was somewhere inside irradiated tristructural-isotopic (TRISO) fuel particles — a safer, more efficient, next-generation nuclear fuel — the "poppy seed" in question. Break a TRISO fuel particle open and it looks like a jaw breaker on the inside. An outer shell of carbon coats a layer of silicon carbide, which coats the uranium center where the energy-releasing fission happens. These layers are meant to contain the radioactive products of fission, which includes little bits of silver. Containment of the radioactive material is built right into the fuel itself.

But it doesn't always work perfectly. Occasionally, in just one or two out of 100 particles, silver escapes the center. It moves around the particle, and potentially gets out. Since the 1970s, scientists have been wondering exactly how this happens.

"I find it absolutely fascinating," said Van Rooyen. She has been studying the TRISO-silver problem since 2006. "I have a natural tendency to know what is going on [inside the fuel]."

And it does take a sixth science-minded sense: The silver seems to jump the silicon carbide layer as though by magic. There is no obvious point of exit, or forcible silver-shaped hole, to be found.  The transport mechanism that brings it from the inside out is a mystery that spans decades. It is a wrinkle in the plan to make TRISO the most efficient, and potentially the safest, fuel of the future.

In South Africa, Van Rooyen worked on a number of hypotheses for the TRISO problem. For example, did it piggyback out of the TRISO fuel particle attached to another element? Were there almost-too-tiny-to-see nanotubes forming in the silicon carbide layer?

One possibility seemed most probable to Van Rooyen. But to test it, to even begin to see if it was correct, she needed to be able to get a closer look. And she needed irradiated TRISO fuel.

Roads less traveled

There are roads in Idaho that will take you on long trips to lakes and mountains. But it was a different type of road that Van Rooyen came here to travel. Nanoroads describe the networks where each layer of the TRISO particle meets the next and where the grains that make up the layers themselves align with each other. These are the roads that Van Rooyen came to travel. 

The bright white triangle is where researchers spotted silver fission products congregating in a TRISO fuel particle.

Could the nanoroads be the silver precipitate's path out of the TRISO fuel particle? They do offer a path of lesser resistance, a point of potential weakness in the silicon carbide. The first step would be to see if silver could be found along these roads.

Van Rooyen's method of investigation was a Scanning Transmission Electron Microscope operated by Yaqio Wu, a Boise State University research associate professor and instrument lead of the Materials and Characterization Suite at the Center for Advanced Energy Studies.  

Somewhere along one of the nanoroad grain boundaries, Van Rooyen and Wu, along with materials engineer Tom Lillo, might be able to spot the silver precipitate.

"We were really like private investigators," Van Rooyen said. The silver's presence on the nanoroads — if that's where it was — would be a lynchpin clue in the mystery.

After a year of patience and administrative work, she finally got her hands on actual, irradiated samples.

Eureka moment

At a research briefing on the morning the team received the samples, they discussed the fact that they were looking for a needle in a haystack. For one, the bits of silver were so small. And not all TRISO particles emit silver. Would there even be silver in the specific sample they were looking at?

But what came that afternoon was one of the rare eureka moments — a discovery that seems to come into existence in an instant. 

This cross-section of a TRISO fuel pellet shows                  TRISO fuel particles at    the 10 mm scale.

After years of exploring and discarding various hypotheses about the location of the silver, Van Rooyen and her team placed the irradiated TRISO particle under the electron microscope. This would be the closest, most careful look at the nanoroads in irradiated TRISO ever.

On that very afternoon, microscope operator Wu zoomed in and they found the silver precipitate. It was wedged at the intersection of two layers of TRISO coating, at the nanoroads between grains.

It was "an absolute wow moment," said Van Rooyen. "We made such a commotion that people from other labs were coming to have a look."

The journey is far from over. Next, Van Rooyen and her team will observe the silver to see how far it moves through the silicon carbide and try to determine exactly how it is able to get out. Time and hard work will tell if the nanoroads hypothesis is correct.

For Van Rooyen, the search for the silver is just the beginning. This new section of the problem is the next adventure. "This is where the fun starts," she said.

In addition to her colleagues Lillo and Wu, Van Rooyen would like to acknowledge Jim Madden for focused ion beam sample preparation; Jason Harp for isotope calculations; Joanne Taylor (Idaho State University); and Kristi Moser-McIntire (ISU) from CAES for their organization and support in the licensing of the CAES facility, which enabled the team to bring irradiated samples to the microscope.

Source: https://inlportal.inl.gov/portal/server.pt/community/newsroom/257/feature_story_details/1269?featurestory=DA_615322

The Attractive Properties of Core-Shell Nanorods

Researchers at Rensselaer discovered a new method
to create “branched” nanorods, as seen in this
scanning electron microscope image.
Such nanorods could one day enable new
nanoscale thermoelectric devices for power
generation, as well as nanoscale heat pumps
for cooling hot spots in nanoelectronics devices.

Photo Credit: Rensselaer/Ramanath

Because of their attractive properties, core-shell nanorods are expected to one day enable the development of new nanoscale thermoelectric devices for power generation, as well as nanoscale heat pumps for cooling hot spots in nanoelectronics devices. 

“Our discovery enables the realization of two very important attributes for heat dissipation and power generation from heat,” Ramanath said. “First, the core-shell junctions in the nanorods are conducive for heat removal upon application of an electrical voltage, or generating electrical power from heat. Second, the branched structures open up the possibility of fabricating miniaturized conduits for heat removal alongside nanowire interconnects in future device architectures.”

The researchers discovered that synthesis at high temperatures or with low amounts of the biomolecular surfactant L-glutathonic acid (LGTA) yields branched nanorod structures in highly regulated patterns. In contrast, synthesis at low temperatures or with high levels of LGTA results in straight nanorods without any branching. It is interesting to note that at the point of branching, atoms in the branch resemble a mirror image of the parent crystal – a finding that reinforces Ramanath’s conclusion that LGTA is able to induce branching through atomic-level sculpture. 

“Since LGTA is similar to biological molecules, our discovery could be conceivably used as a starting point to explore the use of proteins and enzymes to atomically sculpt such nanorod architectures through biological processes,” said Ramanath

Results of the study, titled “Surfactant-Directed Synthesis of Branched Bismuth Telluride/Sulfide Core/Shell Nanorods,” were recently published online and will be featured in an upcoming issue of the journal Advanced Materials. 

The full study may be viewed at: http://dx.doi.org/10.1002/adma.200702572 

Along with Ramanath and Purkayastha, co-authors of the paper include: Theodorian Borca-Tasciuc, associate professor of mechanical, aerospace and nuclear engineering at Rensselaer; Rensselaer materials science and engineering postdoctoral researcher Huafang Li; Rensselaer graduate students Makala S. Raghuveer and Darshan D. Gandhi; as well as materials science and engineering professor Raju V. Ramanujan, assistant professor Qingyu Yan, and postdoctoral researcher Zhong W. Liu of Nanyang Technological University in Singapore. 

Source: http://www.rpi.edu/research/magazine/winter08/boiling-3.html

Nanostarfruits are pure gold for research

Gold nanoparticles created by the Rice University lab of
Eugene Zubarev take on the shape of starfruit in a chemical
bath with silver nitrate, ascorbic acid and gold chloride.
Photo courtesy Zubarev Lab/Rice University

Rice University lab develops starfruit-shaped nanorods for medical imaging, chemical sensing.

They look like fruit, and indeed the nanoscale stars of new research at Rice University have tasty implications for medical imaging and chemical sensing.

Starfruit-shaped gold nanorods synthesized by chemist Eugene Zubarev and Leonid Vigderman, a graduate student in his lab at Rice’s BioScience Research Collaborative, could nourish applications that rely on surface-enhanced Raman spectroscopy (SERS).

The research appeared online this month in the American Chemical Society journal Langmuir.

The researchers found their particles returned signals 25 times stronger than similar nanorods with smooth surfaces. That may ultimately make it possible to detect very small amounts of such organic molecules as DNA and biomarkers, found in bodily fluids, for particular diseases.

“There’s a great deal of interest in sensing applications,” said Zubarev, an associate professor of chemistry. “SERS takes advantage of the ability of gold to enhance electromagnetic fields locally. Fields will concentrate at specific defects, like the sharp edges of our nanostarfruits, and that could help detect the presence of organic molecules at very low concentration.”

SERS can detect organic molecules by themselves, but the presence of a gold surface greatly enhances the effect, Zubarev said. “If we take the spectrum of organic molecules in solution and compare it to when they are adsorbed on a gold particle, the difference can be millions of times,” he said. The potential to further boost that stronger signal by a factor of 25 is significant, he said.

Seen from the side, the nanostarfruit produced at Rice University take
on the appearance of carambola, or starfruit. The particles are about
55 nanometers wide and 550 nanometers long.
Photo courtesy Zubarev Lab/Rice University

Zubarev and Vigderman grew batches of the star-shaped rods in a chemical bath. They started with seed particles of highly purified gold nanorods with pentagonal cross-sections developed by Zubarev’s lab in 2008 and added them to a mixture of silver nitrate, ascorbic acid and gold chloride.

Over 24 hours, the particles plumped up to 550 nanometers long and 55 nanometers wide, many with pointy ends. The particles take on ridges along their lengths; photographed tip-down with an electron microscope, they look like stacks of star-shaped pillows.

Why the pentagons turn into stars is still a bit of a mystery, Zubarev said, but he was willing to speculate. “For a long time, our group has been interested in size amplification of particles,” he said. “Just add gold chloride and a reducing agent to gold nanoparticles, and they become large enough to be seen with an optical microscope. But in the presence of silver nitrate and bromide ions, things happen differently.”

When Zubarev and Vigderman added a common surfactant, cetyltrimethylammonium bromide (aka CTAB), to the mix, the bromide combined with the silver ions to produce an insoluble salt. “We believe a thin film of silver bromide forms on the side faces of rods and partially blocks them,” Zubarev said.

This in turn slowed down the deposition of gold on those flat surfaces and allowed the nanorods to gather more gold at the pentagon’s points, where they grew into the ridges that gave the rods their star-like cross-section. “Silver bromide is likely to block flat surfaces more efficiently than sharp edges between them,” he said.

The researchers tried replacing silver with other metal ions such as copper, mercury, iron and nickel. All produced relatively smooth nanorods. “Unlike silver, none of these four metals form insoluble bromides, and that may explain why the amplification is highly uniform and leads to particles with smooth surfaces,” he said.

Nanostarfruits begin as gold nanowires with pentagonal cross-sections.
Rice chemist Eugene Zubarev believes silver ions and bromide combine to
form an insoluble salt that retards particle growth along the pentagons’
flat surfaces. Photo courtesy Zubarev Lab/Rice University

The researchers also grew longer nanowires that, along with their optical advantages, may have unique electronic properties. Ongoing experiments with Stephan Link, an assistant professor of chemistry and chemical and biomolecular engineering, will help characterize the starfruit nanowires’ ability to transmit a plasmonic signal. That could be useful for waveguides and other optoelectronic devices.

But the primary area of interest in Zubarev’s lab is biological. “If we can modify the surface roughness such that biological molecules of interest will adsorb selectively on the surface of our rugged nanorods, then we can start looking at very low concentrations of DNA or cancer biomarkers. There are many cancers where the diagnostics depend on the lowest concentration of the biomarker that can be detected.”

The National Science Foundation and Welch Foundation supported the research.

Read the abstract at http://pubs.acs.org/doi/abs/10.1021/la300218z

Source: http://news.rice.edu/2012/03/26/nanostarfruits-are-pure-gold-for-research/

Traces of DNA exposed by twisted light

Structures that put a spin on light reveal tiny amounts of DNA with 50 times better sensitivity than the best current methods, a collaboration between the University of Michigan and Jiangnan University in China has shown.

Highly sensitive detection of DNA can help with diagnosing patients, solving crimes and identifying the origins of biological contaminants such as a pathogen in a water supply.

"It really does not matter where the target DNA is from," said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Chemical Engineering at U-M. "In order to detect a specific DNA, we just need to know a small portion of its sequence."

Current DNA analysis methods rely on copying segments of a strand of DNA. The process unzips the double helix and then short, lab-made 'primer' DNA strands attach to each half of the original DNA. These primers kick-start the copying process, using the unzipped DNA as a template. Targeted segments of DNA can be replicated in this way, doubling every cycle. If enough DNA is produced before copying errors become a major problem, then further analysis can show whether the sample matches a suspect, for example.

But if the primers were very selective for the suspected DNA sequence, then a match could be determined by simply detecting whether the DNA had copied or not. Studies revealed that small amounts of DNA could be observed when spherical gold nanoparticles were attached to the primers. If the DNA matched suspicions, strings of particles bound together with DNA would form in the replication process. The nanoparticle solution would change color from red to blue, due to the way the strings of particles interact with light.

"Impressive detection limits were attained for short DNAs with nanoparticles; however, not for long DNA," Kotov said.

The problem, he explained, is that if the particles are further apart than a few nanometers, or millionths of a millimeter, "they do not interact strongly and the blue color does not happen." Longer strands are needed to differentiate between species and individuals with greater accuracy.

"If the strands are too short, you could mix up the DNA of a killer with that of the friend's dog—or a signature of malignant stomach cancer with the piece of a chicken burrito," Kotov said.

He and his partner Chuanlai Xu, a professor of food science and technology at Jiangnan University in China, led an effort to see whether a more subtle optical change would hold up to longer distances.

Rather than using spherical nanoparticles, the team started with nanorods, shaped like tiny Mike and Ike candies, about 62 nanometers long and 22 nanometers in diameter. They attached the primer DNA to the sides of these.

When nanorods line up, they tend to misalign by about 10 degrees. After a few rounds of copying, the gold and DNA structures resembled twisted rope ladders. Light passing through the spiral of golden spokes reacted by rotating.

"The light can be rotated even when the nanorods are far away from each other," Kotov said. "This gives our methods a tremendous advantage in sensitivity for long DNA strands."

The rotation happens because light is composed of electric and magnetic waves moving in tandem, and electric and magnetic fields exert forces on charged particles that have freedom to move, such as electrons in metals. The electrons in gold respond very well to the frequency of visible light waves, so they begin to move back and forth in the gold, synced with the light. This effect is a two-way street: the moving electrons in the gold can also affect the light waves.

Kotov compares the light to a rope with ripples running through it.

"Now imagine that the air around the rope can move more easily along certain directions," Kotov said.

For light passing through the gold nanorods, it's easiest if the electric wave moves up and down the length of the nanorods, so the light rotates as it moves from nanorod to nanorod and continues twisting after it leaves the structure. And depending on whether the light starts out rotating clockwise or counterclockwise, it feels the twist from the nanorods most at different wavelengths.

"For analytical purposes, this is a gift," Kotov said.

The two peaks in the amount of twisting for clockwise and counterclockwise light can be added together, which makes for a stronger signal and allows the method to identify a match with smaller amounts of DNA.

"The strength of the rotation reaches maximum when the gap between nanorods is 20 nanometers, which is exactly what we need for the detection of long, selective and species-specific DNA strands," Kotov said. "The calculations presented show that we can potentially increase the sensitivity even more in the future and to even longer DNAs."

The paper, "Attomolar DNA detection with Chiral Nanorod Assemblies," will be published in Nature Communications on Oct. 28. Kotov is a professor of chemical engineering, biomedical engineering, materials science and engineering, and macromolecular science and engineering.

This work was funded by the U.S. Department of Energy and National Science Foundation, National Natural Science Foundation of China, China Ministry of Science and Technology, and grants from the Ministries of Finance and Education in Jiangsu Province, China.

Source: http://www.ns.umich.edu/new/releases/21767-traces-of-dna-exposed-by-twisted-light