DOE: Mixing Nanoparticles to Make “Multifunctional” Materials


Posted: Oct 20, 2013

Mixing nanoparticles to make multifunctional materials

201306047919620(Nanowerk News) Scientists at the U.S. Department of Energy‘s Brookhaven National Laboratory have developed a general approach for combining different types of nanoparticles to produce large-scale composite materials.

The technique, described in a paper published online by Nature Nanotechnology on October 20, 2013 (“A general strategy for the DNA-mediated self-assembly of functional nanoparticles into heterogeneous systems”), opens many opportunities for mixing and matching particles with different magnetic, optical, or chemical properties to form new, multifunctional materials or materials with enhanced performance for a wide range of potential applications.

The approach takes advantage of the attractive pairing of complementary strands of synthetic DNA-based on the molecule that carries the genetic code in its sequence of matched bases known by the letters A, T, G, and C.

After coating the nanoparticles with a chemically standardized “construction platform” and adding extender molecules to which DNA can easily bind, the scientists attach complementary lab-designed DNA strands to the two different kinds of nanoparticles they want to link up. The natural pairing of the matching strands then “self-assembles” the particles into a three-dimensional array consisting of billions of particles. Varying the length of the DNA linkers, their surface density on particles, and other factors gives scientists the ability to control and optimize different types of newly formed materials and their properties.

DNA linkers allow different kinds of nanoparticles to self-assemble and form relatively large-scale nanocomposite arrays. This approach allows for mixing and matching components for the design of multifunctional materials.

“Our study demonstrates that DNA-driven assembly methods enable the by-design creation of large-scale ‘superlattice’ nanocomposites from a broad range of nanocomponents now available-including magnetic, catalytic, and fluorescent nanoparticles,” said Brookhaven physicist Oleg Gang, who led the research at the Lab’s Center for Functional Nanomaterials (CFN). “This advance builds on our previous work with simpler systems, where we demonstrated that pairing nanoparticles with different functions can affect the individual particles’ performance, and it offers routes for the fabrication of new materials with combined, enhanced, or even brand new functions.”

Future applications could include quantum dots whose glowing fluorescence can be controlled by an external magnetic field for new kinds of switches or sensors; gold nanoparticles that synergistically enhance the brightness of quantum dots’ fluorescent glow; or catalytic nanomaterials that absorb the “poisons” that normally degrade their performance, Gang said.

“Modern nano-synthesis methods provide scientists with diverse types of nanoparticles from a wide range of atomic elements,” said Yugang Zhang, first author of the paper. “With our approach, scientists can explore pairings of these particles in a rational way.”

Pairing up dissimilar particles presents many challenges the scientists investigated in the work leading to this paper. To understand the fundamental aspects of various newly formed materials they used a wide range of techniques, including x-ray scattering studies at Brookhaven’s National Synchrotron Light Source (NSLS) and spectroscopy and electron microcopy at the CFN.

For example, the scientists explored the effect of particle shape. “In principle, differently shaped particles don’t want to coexist in one lattice,” said Gang. “They either tend to separate into different phases like oil and water refusing to mix or form disordered structures.”

The scientists discovered that DNA not only helps the particles mix, but it can also improve order for such systems when a thicker DNA shell around the particles is used.

They also investigated how the DNA-pairing mechanism and other intrinsic physical forces, such as magnetic attraction among particles, might compete during the assembly process.

For example, magnetic particles tend to clump to form aggregates that can hinder the binding of DNA from another type of particle. “We show that shorter DNA strands are more effective at competing against magnetic attraction,” Gang said.

For the particular composite of gold and magnetic nanoparticles they created, the scientists discovered that applying an external magnetic field could “switch” the material’s phase and affect the ordering of the particles.

“This was just a demonstration that it can be done, but it could have an application-perhaps magnetic switches, or materials that might be able to change shape on demand,” said Zhang.

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DNA linkers allow different kinds of nanoparticles to self-assemble and form  relatively large-scale nanocomposite arrays. This approach allows for mixing and  matching components for the design of multifunctional materials.

The third fundamental factor the scientists explored was how the particles were ordered in the superlattice arrays: Does one type of particle always occupy the same position relative to the other type-like boys and girls sitting in alternating seats in a movie theater-or are they interspersed more randomly?

“This is what we call a compositional order, which is important for example for quantum dots because their optical properties-e.g., their ability to glow-depend on how many gold nanoparticles are in the surrounding environment,” said Gang. “If you have compositional disorder, the optical properties would be different.” In the experiments, increasing the thickness of the soft DNA shells around the particles increased compositional disorder.

These fundamental principles give scientists a framework for designing new materials. The specific conditions required for a particular application will be dependent on the particles being used, Zhang emphasized, but the general assembly approach would be the same.

Said Gang, “We can vary the lengths of the DNA strands to change the distance between particles from about 10 nanometers to under 100 nanometers-which is important for applications because many optical, magnetic, and other properties of nanoparticles depend on the positioning at this scale. We are excited by the avenues this research opens up in terms of future directions for engineering novel classes of materials that exploit collective effects and multifunctionality.”

Source: Brookhaven National Laboratory

 

Read more: http://www.nanowerk.com/news2/newsid=32807.php#ixzz2iImpDZ6p

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Polymer Structures Serve as ‘Nanoreactors’ for Nanocrystals


QDOTS imagesCAKXSY1K 8Using star-shaped block co-polymer structures as tiny reaction vessels, researchers have developed an improved technique for producing nanocrystals with consistent sizes, compositions and architectures – including metallic, ferroelectric, magnetic, semiconductor and luminescent nanocrystals. The technique relies on the length of polymer molecules and the ratio of two solvents to control the size and uniformity of colloidal nanocrystals.

 

The technique could facilitate the use of nanoparticles for optical, electrical, optoelectronic, magnetic, catalysis and other applications in which tight control over size and structure is essential to obtaining desirable properties. The technique produces plain, core-shell and hollow nanoparticles that can be made soluble either in water or in organic solvents.

“We have developed a general strategy for making a large variety of nanoparticles in different size ranges, compositions and architectures,” said Zhiqun Lin, an associate professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “This very robust technique allows us to craft a wide range of nanoparticles that cannot be easily produced with any other approaches.”

The technique was described in the June issue of the journal Nature Nanotechnology. The research was supported by the Air Force Office of Scientific Research.

Georgia Tech professor Zhiqun Lin examines a gold nanoparticle toluene solution. The work is part of research on using star-shaped block co-polymers to create nanocrystals of uniform size and shape.

(Photo Credit:  Georgia Tech Photo: Gary Meek)

The star-shaped block co-polymer structures consist of a central beta-cyclodextrin core to which multiple “arms” – as many as 21 linear block co-polymers – are covalently bonded. The star-shaped block co-polymers form the unimolecular micelles that serve as a reaction vessel and template for the formation of the nanocrystals.

The inner blocks of unimolecular micelles are poly(acrylic) acid (PAA), which is hydrophilic, which allows metal ions to enter them. Once inside the tiny reaction vessels made of PAA, the ions react with the PAA to form nanocrystals, which range in size from a few nanometers up to a few tens of nanometers. The size of the nanoparticles is determined by the length of the PAA chain.

The block co-polymer structures can be made with hydrophilic inner blocks and hydrophobic outer blocks – amphiphilic block co-polymers, with which the resulting nanoparticles can be dissolved in organic solvents. However, if both inner and outer blocks are hydrophilic – all hydrophilic block co-polymers – the resulting nanoparticles will be water-soluble, making them suitable for biomedical applications.

Lin and collaborators Xinchang Pang, Lei Zhao, Wei Han and Xukai Xin found that they could control the uniformity of the nanoparticles by varying the volume ratio of two solvents – dimethlformamide and benzyl alcohol – in which the nanoparticles are formed. For ferroelectric lead titanate (PbTiO3) nanoparticles, for instance, a 9-to-1 solvent ratio produces the most uniform nanoparticles.

The researchers have also made iron oxide, zinc oxide, titanium oxide, cuprous oxide, cadmium selenide, barium titanate, gold, platinum and silver nanocrystals. The technique could be applicable to nearly all transition or main-group metal ions and organometallic ions, Lin said.

“The crystallinity of the nanoparticles we are able to create is the key to a lot of applications,” he added. “We need to make them with good crystalline structures so they will exhibit good physical properties.”

Earlier techniques for producing polymeric micelles with linear block co-polymers have been limited by the stability of the structures and by the consistency of the nanocrystals they produce, Lin said. Current fabrication techniques include organic solution-phase synthesis, thermolysis of organometallic precursors, sol-gel processes, hydrothermal reactions and biomimetic or dendrimer templating. These existing techniques often require stringent conditions, are difficult to generalize, include a complex series of steps, and can’t withstand changes in the environment around them.

Georgia Tech professor Zhiqun Lin (standing) watches research scientist Xinchang Pang tuning the experimental condition in the nanocrystal synthesis.

(Photo Credit:  Georgia Tech Photo: Gary Meek)

By contrast, nanoparticle production technique developed by the Georgia Tech researchers is general and robust. The nanoparticles remain stable and homogeneous for long periods of time – as much as two years so far – with no precipitation. Such flexibility and stability could allow a range of practical applications, Lin said.

“Our star-like block co-polymers can overcome the thermodynamic instabilities of conventional linear block co-polymers,” he said. “The chain length of the inner PAA blocks dictates the size of the nanoparticles, and the uniformity of the nanoparticles is influenced by the solvents used in the system.”

The researchers have used a variety of star-like di-block and tri-block co-polymers as nanoreactors. Among them are poly(acrylic acid)-block-polystyrene (PAA-b-PS) and poly(acrylic acid)-blockpoly(ethylene oxide) (PAA-b-PEO) diblock co-polymers, and poly(4-vinylpyridine)-block-poly(tert-butyl acrylate)-block-polystyrene (P4VP-b-PtBA-b-PS), poly(4-vinylpyridine)-block-poly (tert-butyl acrylate)-block-poly(ethylene oxide) (P4VP-b-PtBA-b-PEO), polystyrene-block-poly(acrylic acid)-block-polystyrene (PS-b-PAA-b-PS) and polystyrene-block-poly(acrylic acid)-block-poly(ethylene oxide) (PS-b-PAA-b-PEO) tri-block co-polymers.

For the future, Lin envisions more complex nanocrystals with multifunctional shells and additional shapes, including nanorods and so-called “Janus” nanoparticles that are composed of biphasic geometry of two dissimilar materials.

Georgia Tech professor Zhiqun Lin (standing) and research scientist Xinchang Pang compare two cadmium selenide (CdSe) nanocrystals made by Pang. The researchers are examining the absorption spectra of the nanocrystals in front of the computer.

(Photo Credit:  Georgia Tech Photo: Gary Meek)

Quantum dots for superior solar cells


QDOTS imagesCAKXSY1K 8

Researchers at the University of Toronto in Canada and KAUST in Saudi Arabia have made a solar cell out of colloidal quantum dot (CQD) films that has a record-breaking efficiency of 7%. This is almost 40% more efficient than the best previous devices based on CQDs.

CQDs are semiconductor particles only a few nanometres in size. They can be synthesized in solution, which means that films of the particles can be deposited quickly and without fuss on a wide range of flexible or rigid substrates – just like paint or ink can.

QD Solar Chip

CQD photovoltaics

CQDs could be used as the light-absorbing component in cheap, highly efficient inorganic solar cells. In a solar cell, high-energy photons hitting the photovoltaic material can produce excited electrons and holes (charge carriers) that have energies at least equal to or greater than the band gaps of the material. Another advantage of using CQDs as the photovoltaic material is that they absorb light over a wide spectrum of wavelengths thanks to the fact that the band gap can be tuned over a large energy range by simply changing the size of the nanoparticles.

There is a snag, however; the high surface area to volume ratio of nanoparticles results in bare surfaces that can became “traps” in which electrons invariably get stuck. This means that electrons and holes have time to recombine instead of producing useful current, something that inevitably reduces the efficiency of devices made from CQD films.

Surface passivation

A team led by Edward Sargent at Toronto may now have come up with a solution to this annoying problem. The researchers have succeeded in passivating the surface of CQD films by completely covering all exposed surfaces using a chlorine solution that they added to the quantum-dot solution immediately after it was synthesized. “We employed chlorine atoms because they are small enough to penetrate all of the nooks and crannies previously responsible for the poor surface quality of the CQD films,” explained Sargent.

QD Solar Chip 2

In the lab

The team then spin cast the CQD solution onto a glass substrate that was covered with a transparent conductor. Next, an organic linker was used to bind the quantum dots together. This final step in the process results in a very dense film of nanoparticles that absorbs a much larger amount of sunlight.

Reducing traps

“Our hybrid passivation scheme employs chlorine atoms to reduce the number of traps for electrons associated with poor CQD film surface quality while simultaneously ensuring that the films are dense and highly absorbing thanks to the organic linkers,” Sargent told nanotechweb.org.

Electronic spectroscopy measurements confirmed that the films hardly contained any electron traps at all, he adds. Synchrotron X-ray scattering measurements at sub-nanometre resolution performed by the scientists at KAUST corroborated the fact that the films were highly dense and contained very closely packed nanoparticles.

Low-cost photovoltaics on the horizon

“Most solar cells on the market today are made out of heavy crystalline materials,” explained Sargent, “but our work shows that light and versatile materials like CQDs could potentially become cost-competitive with these traditional technologies. Our results also pave the way for low-cost photovoltaics that could be fabricated on flexible substrates, for example, using roll-to-roll manufacturing (in the same way that newspapers are printed in mass quantities).”

The team is now looking at further reducing electron traps in CQD films for even higher efficiency. “It turns out that there are many organic and inorganic materials out there that might well be used in such hybrid passivation schemes,” added Sargent, “ so finding out how to reduce electron traps to a minimum would be good.”

The researchers say that they are also interested in using layers of different-sized quantum dots to make a multi-junction solar cell that could absorb over an even broader range of light wavelengths.

The current work is detailed in Nature Nanotechnology.

 

Nano-rod solar cell generates hydrogen


QDOTS imagesCAKXSY1K 8A new type of solar collector that uses gold nano-rods could convert sunlight into energy without many of the problems associated with traditional photovoltaic solar cells.

24 February 2013 Will Parker

 

The developers of the new technique, from the University of California – Santa Barbara, say it is “the first radically new and potentially workable alternative to semiconductor-based photovoltaic devices to be developed in the past 70 years.” They provide details of the new solar hydrogen generator in the journal Nature Nanotechnology.

In conventional photovoltaic cells, sunlight hits the surface of semiconductor material, one side of which is electron-rich, while the other side is not. The photon excites the electrons, causing them to leave their positions, and create positively-charged “holes.” The result is a current of charged particles – electricity.

In the new technique, it is not semiconductor materials that provide the electrons and venue for the conversion of solar energy, but a “forest” of gold nano-rods operating in water. Specifically, gold nano-rods capped with a layer of crystalline titanium dioxide and platinum, and a cobalt-based oxidation catalyst deposited on the lower portion of the array.

“When nanostructures, such as nano-rods, of certain metals are exposed to visible light, the conduction electrons of the metal can be caused to oscillate collectively, absorbing a great deal of the light,” explained Martin Moskovits (pictured front center), a professor of chemistry at UCSB. “This excitation is called a surface plasmon.”

As the “hot” electrons in these plasmonic waves are excited by light particles, some travel up the nano-rod, through a filter layer of crystalline titanium dioxide, and are captured by platinum particles. This causes the reaction that splits hydrogen ions from the bond that forms water. Meanwhile, the holes left behind by the excited electrons head toward the cobalt-based catalyst on the lower part of the rod to form oxygen.

The researchers say that hydrogen production was clearly observable after about two hours. Importantly, the nano-rods were not subject to the photo-corrosion that often causes traditional semiconductor materials to fail and Moskovits says the device operated with no hint of failure for “many weeks.”

Though still in its infancy, the research promises a more robust method of converting sunlight into energy. “Despite the recentness of the discovery, we have already attained ‘respectable’ efficiencies. More importantly, we can imagine achievable strategies for improving the efficiencies radically,” Moskovits said.

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Related: Discuss this article in our forum Stanford announces peel-and-stick solar panels Solar steam generator outshines photovoltaic solar cells Solar power’s dirty secret: skyrocketing lead pollution Much simpler catalyst could fast-track hydrogen economy

See Summary of article here: http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2013.18.html

Source: University of California – Santa Barbara

‘Quantum dot’ solar cells offer bright future with reliable, low cost energy


London, July 30 (ANI): Researchers have made a breakthrough in the development of colloidal quantum dot (CQD) films, leading to the most efficient CQD solar cell ever.

Researchers from the University of Toronto (U of T) and King Abdullah University of Science and Technology (KAUST) created a solar cell out of inexpensive materials that was certified at a world-record 7.0 percent efficiency.

“Previously, quantum dot solar cells have been limited by the large internal surface areas of the nanoparticles in the film, which made extracting electricity difficult,” said Dr. Susanna Thon, a lead co-author of the paper.

“Our breakthrough was to use a combination of organic and inorganic chemistry to completely cover all of the exposed surfaces,” Dr. Thon stated.

Quantum dots are semiconductors only a few nanometres in size and can be used to harvest electricity from the entire solarspectrum – including both visible and invisible wavelengths. Unlike current slow and expensive semiconductor growth techniques, CQD films can be created quickly and at low cost, similar to paint or ink.

The researchers, led by U of T Engineering Professor Ted Sargent, paves the way for solar cells that can be fabricated on flexible substrates in the same way newspapers are rapidly printed in mass quantities.

The U of T cell represents a 37 percent increase in efficiency over the previous certified record. In order to improve efficiency, the researchers needed a way to both reduce the number of “traps” for electrons associated with poor surface quality while simultaneously ensuring their films were very dense to absorb as much light as possible. The solution was a so-called “hybrid passivation” scheme.

“By introducing small chlorine atoms immediately after synthesizing the dots, we’re able to patch the previously unreachable nooks and crannies that lead to electron traps. We follow that by using short organic linkers to bind quantum dots in the film closer together,” explained doctoral student and lead co-author Alex Ip.

Work led by Professor Aram Amassian of KAUST showed that the organic ligand exchange was necessary to achieve the densest film.

“The KAUST group used state-of-the-art synchrotron methods with sub-nanometer resolution to discern the structure of the films and prove that the hybrid passivation method led to the densest films with the closest-packed nanoparticles,” stated Professor Amassian.

The advance opens up many avenues for further research and improvement of device efficiencies, which could contribute to a bright future with reliable, low cost solar energy.

Their work featured in a letter published in Nature Nanotechnology. (ANI)