New principle for self-assembly of patterned nanoparticles


programmednaAnimal and plant cells are prominent examples of how nature constructs ever-larger units in a targeted, preprogrammed manner using molecules as building blocks. In nanotechnology, scientists mimic this ‘bottom-up‘ technique by using the ability of suitably structured nano materials to ‘self-assemble‘ into higher order architectures. Applying this concept, polymer scientists from Bayreuth, Aachen, Jena, Mainz, and Helsinki have recently published an article in the prestigious journal Nature that describes a new principle for the self-assembly of patterned nanoparticles. This principle may have important implications for the fundamental understanding of such processes as well as future technologies.

Animal and plant cells are prominent examples of how nature constructs ever-larger units in a targeted, preprogrammed manner using molecules as building blocks. In nanotechnology, scientists mimic this ‘bottom-up’ technique by using the ability of suitably structured nano materials to ‘self-assemble’ into higher order architectures. Applying this concept, polymer scientists from Bayreuth, Aachen, Jena, Mainz, and Helsinki have recently published an article in the prestigious journal Nature that describes a new principle for the self-assembly of patterned nanoparticles. This principle may have important implications for the fundamental understanding of such processes as well as future technologies.

However, the process of self-assembly does not end with the nanoparticles. If the nanoparticles formed by each type of macromolecule were left to their own, spherical superstructures would result on the one hand and linear superstructures on the other. Müller’s team has developed and implemented a different approach. The nanoparticles with one and two bonding sites are mixed so that they aggregate together into a completely new superstructure in a process of co-assembly. In the final superstructure, the nanoparticles originating from the A-B-C molecules and nanoparticles formed by the A-D-C molecules alternate in a precisely defined pattern.

When viewed under a transmission electron microscope, the new superstructure bears a strong resemblance to a caterpillar larva, because it also consists of a series of clearly separate, regularly ordered sections. Müller’s research team has thus coined the term “caterpillar micelles” for such co-assembled superstructures.

The research findings recently published in Nature represent a breakthrough in the field of hierarchical structuring and nano-engineering as it allows creating new materials by self-assemble preprogrammed particles. This could be a game changer, because so far only top-down procedures, i.e., extracting a microstructure from a larger complex, are widely accepted structuring processes. “The limitations of this technique will become all too apparent in the near future,” explained Müller. “Only rarely is it possible to generate complex structures in the nanometer range.”

However, a bottom-up principle of self-assembly based on that employed in nature could well represent the best way forward. One factor that makes this particularly attractive is the large number of macromolecules, which are readily available as building blocks. They can be used to incorporate specific properties in the resultant superstructures, such as sensitivity to environmental stimuli (e.g. temperature, light, electric and magnetic fields, etc.) or give them the ability to be switched on and off at will. Possible applications include nanolithography and the delivery of drugs in which the time and site of release of active substances can be preprogrammed. Here, the similarity to the structural principles of animal and plant cells becomes apparent again, where various properties are compartmentalized into areas of limited space.

The macromolecules carrying diverse functional segments can be hundreds of times smaller than a micrometer. The superstructures that such macromolecules produce have correspondingly high resolution. “Future technologies – such as tailor-made artificial cells, transistors, or components for micro/nano-robotics – may benefit significantly from this particularly delicate structuring,” explained Müller. “The research findings we published in Nature do not yet have any immediate real-world applications. Nevertheless, the better we understand bottom-up processes starting with molecules in the nanometer range and moving on to the higher hierarchical levels in the micrometer range, the more likely future technologies will be within our grasp.” The caterpillar micelles are in no way the only superstructures that can be produced with the self-assembling nanoparticles. “Such soft nanoparticles can be combined with inorganic or biological nano- and microparticles to create previously unknown materials with specific functions. The number of possible combinations is practically endless,” concluded Müller.

Read more at: http://phys.org/news/2013-11-principle-self-assembly-patterned-nanoparticles.html#jCp

 

Watching solar cells grow


201306047919620(Nanowerk News) For the first time, a team of  researchers at the Helmholtz Zentrum Berlin (HZB) led by Dr. Roland Mainz and  Dr. Christian Kaufmann has managed to observe growth of high-efficiency  chalcopyrite thin film solar cells in real time and to study the formation and  degradation of defects that compromise efficiency. To this end, the scientists  set up a novel measuring chamber at the Berlin electron storage ring BESSY II,  which allows them to combine several different kinds of measuring techniques.  Their results show during which process stages the growth can be accelerated and  when additional time is required to reduce defects. Their work has now been  published online in Advanced Energy Materials (“Formation of CuInSe2 and  CuGaSe2 Thin-Films Deposited by Three-Stage  Thermal Co-Evaporation: A Real-Time X-Ray Diffraction and Fluorescence  Study”).

Today’s chalcopyrite thin film cells based on copper indium  gallium selenide are already reaching efficiencies of more than 20 percent. For  the fabrication of the extremely thin polycrystalline layers, the process of  coevaporation has lead to the best results so far: During coevaporation, two  separate elements are evaporated simultaneously, first indium (or gallium) and  selenium, then copper and selenium, and, finally, indium (or gallium) and  selenium again. This way, a thin film of crystals forms, which exhibit only a  small number of defects. “Until recently, we did not fully understand what  exactly happens during this coevaporation process,” says Dr. Roland Mainz of the  HZB’s Institute of Technology. The team of physicists worked for three years  using on-site and real-time measurements to find an answer to this question.

id31067_1Polycrystalline film growth during coevaporation in real time using in situ  X-ray diffraction and fluorescence analysis. (Figure: R. Mainz/C.Kaufmann/HZB)

Novel experimental chamber constructed

For these measurements they constructed a new kind of  experimental chamber, which allows for an analysis of polycrystalline  chalcopyrite film formation during coevaporation when exposed to synchrotron  light at BESSY II. In addition to the evaporation sources for the elements, this  vacuum chamber contains heating and cooling elements to control the evaporative  process. According to Mainz, “one of the main challenges was adjusting the  chamber, which weighs around 250 kilograms, with an accuracy of 10 micrometer.”  Because of thermal expansion during evaporation, the height has to be  automatically re-adjusted every few seconds.

Combination of x-ray diffraction and fluorescence analysis  

With this setup, for the first time worldwide they were able to  observe polycrystalline film growth using in situ X-ray diffraction and  fluorescence analysis during coevaporation in real time. “We are now able to see  how crystalline phases form and transform and when defects form during the  different stages of evaporation. “But we’re also able to tell when these defects  disappear again.” This takes place in the second process stage, when copper and  selenium are evaporated. Excess copper, which deposits at the surface in the  form of copper selenide helps to remove defects. “This was already known before  from previous experiments. But now, using fluorescence signals and numeric model  calculations, we are able to show how copper selenide penetrates the copper  indium selenide layer,” Mainz explains. Here clear-cut differences between  copper indium selenide and copper gallium selenide layers became apparent: While  copper is able to penetrate the copper-indium-selenide layer, in the case of  copper-gallium-selenide, which is otherwise pretty similar, it remains at the  surface. This could be one possible reason for why the use of pure copper  gallium selenide does not yield high efficiency solar cells.

id31067 2

Concrete steps for optimization
“We now know that for further optimization of the process it is  important to concentrate on the transition point into the copper-rich phase. Up  to now the process was performed very slowly throughout all stages to give  defects enough time to disappear. Our findings suggest that the process can be  accelerated at some stages and that it is sufficient to slow it down only at  points where defects are efficiently eliminated,” explains Mainz. Mainz is  already looking forward to future project EMIL, which is currently being set up  at BESSY II. Here even more powerful tools will become available for the study  of complex processes during growth of new types of solar cells in situ and in  real time.
Source: Helmholtz Zentrum Berlin

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Shedding a Light on the Cause for Nanoparticle Size Distribution


QDOTS imagesCAKXSY1K 8(Nanowerk News) When buying shoes it does not matter  how good-looking the shoes might be if the size does not fit. This is similar  with nanoparticles, which are made by the so-called emulsionsolvent evaporation  process. This process allows for the production of nanoparticles with high  purity. Nevertheless they can still be improved: so far, their size distribution  cannot be fully controlled. However, a defined size is of prime importance for  future applications, whether it is for drug delivery or for intelligent  coatings.
An interdisciplinary and international research collaboration at  the Max Planck Institute for Polymer Research in Mainz was able to rule out  coalescence as reason for the borad nanoparticle size distribution. Coalescence  describes the tendency of colloidal droplets to melt together. For the first  time, Daniel Crespy, who is group leader in the department of Katharina  Landfester, was able to prove that the coalescence between droplets during the  process is not significantly responsible for the broad size distribution of the  particles.
“This study elucidates the mechanism of a common process used  for the preparation of nanoparticles,“ says Daniel Crespy about his research  results.
The chemist labeled the original materials prior to the  preparation of the nanoparticles. Some polymers were labeled with red and others  with blue dyes. During the synthesis, the polymers and a solventwere emulsified  in water. After the evaporation of the solvent, solid nanoparticles are  obtained. This is a common method to produce all types of nanoparticles.  Crespy’s trick: Upon adding both red- and blue-labeled polymers to the solvent,  nanoparticles with both colors were obtained. The so-called negative control  shows that if red and blue particles are mixed, no aggregation occurs because  species with both dyes were not detected.
What happens if a red emulsion from polymer and solvent is mixed  with a blue emulsion? Less than every twelfth particle –around 8 percent – were  labeled with both red and blue dyes, which means that coalescence does not play  a significant role in the process.
For the first time, the scientists were able to directly  quantify the occurrence of coalescence. Together with Kaloian Koynov, who is  physicist and expert for spectroscopic methods at the MPI-P, Crespy could  monitor the coalescence of nanometer sized droplets by fluorescence correlation  spectroscopy.
The experimental results were finally confirmed by simulations  based on Monte-Carlo algorithms performed by Davide Donadio, group leader of a  Max Planck Research Group. Thanks to this study (“Particle Formation in the Emulsion-Solvent Evaporation  Process”), the reason for the broad size distribution could be attributed to  the process itself.
Source: Max Planck Institute for Polymer Research

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