Atoms in a nanocrystal cooperate, much like in biomolecules


atomsinanano
Nanocrystals of cadmium selenide, known for their brilliant luminescence, display intriguing chemical behavior resulting from positive cooperation between atoms, a behavior akin to that found in biomolecules. Credit: Prashant Jain        

(Phys.org) —Researchers have long thought that biological molecules and synthetic nanocrystals were similar only in size. Now, University of Illinois at Urbana-Champaign chemists have found that they can add reactivity to the list of shared traits. Atoms in a nanocrystal can cooperate with each other to facilitate binding or switching, a phenomenon widely found in biological molecules.

The finding could catalyze manufacturing of for smart sensors, solar cells, tiny transistors for optical computers, and medical imaging. Led by chemistry professor Prashant Jain, the team published its findings in the journal Nature Communications.

“In geological, industrial and domestic environments, the nanoscale grains of any material undergo chemical transitions when they are put under reactive conditions,” Jain said. “Iron rusting over time and diamond forming from carbon are examples of two commonly occurring transitions. Understanding how these transitions occur on the scale of the tiniest grains of the material is a major motivation of our work.”

Scientists can exploit such transitions to make nanocrystals that conform to a particular structure. They can make a nanocrystal of one material and transform it into another material, essentially using the original nanocrystal framework as a template for creating a nanocrystal of the new material with the same size and shape. This lets researchers create nanocrystals of new materials in shapes and structures they may not be able to otherwise.

In the new study, the researchers transformed tiny crystals of the material cadmium selenide to crystals of copper selenide. Copper selenide nanocrystals have a number of interesting properties that can be used for solar energy harvesting, optical computing and laser surgery. Transformation from cadmium selenide creates nanocrystals with a purity difficult to attain from other methods.

The researchers, including graduate student Sarah White, used advanced microscopy and spectroscopy techniques to determine the dynamics of the atoms within the crystals during the transformation and found that the transformation occurs not as a slow diffusion process, but as a rapid switching thanks to co-operativity.

The researchers saw that once the cadmium-selenide nanocrystal has taken up a few initial copper “seed” impurities, atoms in the rest of the lattice can cooperate to rapidly swap out the rest of the cadmium for copper. Jain compares the crystals to hemoglobin, the molecule in red blood cells that carries oxygen. Once one oxygen molecule has bound to hemoglobin, other binding sites within hemoglobin slightly change conformation to more easily pick up more oxygen. He posits that similarly, copper impurities might cause a structural change in the nanocrystal, making it easier for more copper ions to infiltrate the nanocrystal in a rapid cascade.

The researchers reproduced the experiment with silver, in addition to copper, and saw similar, though slightly less speedy, cooperative behavior.

Now, Jain’s team is using its advanced imaging to watch transitions happen in single nanocrystals, in real time.

“We have a sophisticated optical microscope in our lab, which has now allowed us to catch a single nanocrystal in the act of making a transition,” Jain said. “This is allowing us to learn hidden details about how the transition actually proceeds. We are also learning how one nanocrystal behaves differently from another.”

Next, the researchers plan to explore biomolecule-like cooperative phenomena in other solid-state materials and processes. For example, co-operativity in catalytic processes could have major implications for or manufacturing of expensive specialty chemicals.

“In the long term, we are interested in exploiting the co-operative behavior to design artificial smart materials that respond in a switch-like manner like hemoglobin in our body does,” Jain said.

Explore further:     Researchers extend galvanic replacement reactions to metal oxide nanocrystals

More information: “Co-operativity in a nanocrystalline solid-state transition.” Sarah L. White, Jeremy G. Smith, Mayank Behl, Prashant K. Jain. Nature Communications 4, Article number: 2933 DOI: 10.1038/ncomms3933

Read more at: http://phys.org/news/2013-12-atoms-nanocrystal-cooperate-biomolecules.html#jCp

Read more at: http://phys.org/news/2013-12-atoms-nanocrystal-cooperate-biomolecules.html#jCp

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Tetrapod Quantum Dots Light the Way to Stronger Polymers


Berkeley Lab Researchers Use Fluorescent Tetrapod Quantum Dots to Measure the Mechanical Strength of Polymer Fibers

qdot-images-3.jpgFluorescent tetrapod nanocrystals could light the way to the future design of stronger polymer nanocomposites. A team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed an advanced opto-mechanical sensing technique based on tetrapod quantum dots that allows precise measurement of the tensile  strength of polymer fibers with minimal impact on the fiber’s mechanical properties.

In a study led by Paul Alivisatos, Berkeley Lab director and the Larry and Diane Bock Professor of Nanotechnology at the University of California (UC) Berkeley, the research team incorporated into polymer fibers a population of tetrapod quantum dots (tQDs) consisting of a cadmium-selenide (CdSe) core and four cadmium sulfide (CdS) arms. The tQDs were incorporated into the polymer fibers via electrospinning, among today’s leading techniques for processing polymers, in which a large electric field is applied to droplets of polymer solution to create micro- and nano-sized fibers. This is the first known application of electrospinning to tQDs.

 

Fluorescent tetrapod quantum dots or tQDs (brown) serve as stress probes that allow precise measurement of polymer fiber tensile strength with minimal impact on mechanical properties. Inserts show relaxed tQDs (upper) and stressed tQDs (lower).

“The electrospinning process allowed us to put an enormous amount of tQDs, up to 20-percent by weight, into the fibers with minimal effects on the polymer’s bulk mechanical properties,” Alivisatos says. “The tQDs are capable of fluorescently monitoring not only simple uniaxial stress, but stress relaxation and behavior under cyclic varying loads. Furthermore, the tQDs are elastic and recoverable, and undergo no permanent change in sensing ability even upon many cycles of loading to failure.”

Alivisatos is the corresponding author of a paper describing this research in the journal NANO Letters titledTetrapod Nanocrystals as Fluorescent Stress Probes of Electrospun Nanocomposites.” Coauthors were Shilpa Raja, Andrew Olson, Kari Thorkelsson, Andrew Luong, Lillian Hsueh, Guoqing Chang, Bernd Gludovatz, Liwei Lin, Ting Xu and Robert Ritchie.

From left, Andrew Olson, Shilpa Raja and Andrew Luong are members of Paul Alivisatos's research group who used electrospinning to incorporate tetrapod quantum dot stress probes into polymer fibers. (Photo by Roy Kaltschmidt)

From left, Andrew Olson, Shilpa Raja and Andrew Luong are members of Paul Alivisatos’s research group who used electrospinning to incorporate tetrapod quantum dot stress probes into polymer fibers. (Photo by Roy Kaltschmidt)

Polymer nanocomposites are polymers that contain fillers of nanoparticles dispersed throughout the polymer matrix. Exhibiting a wide range of enhanced mechanical properties, these materials have great potential for a broad range of biomedical and material applications. However, rational design has been hampered by a lack of detailed understanding of how they respond to stress at the micro- and nanoscale.

“Understanding the interface between the polymer and the nanofiller and how stresses are transferred across that barrier are critical in reproducibly synthesizing composites,” Alivisatos says. “All of the established techniques for providing this information have drawbacks, including altering the molecular-level composition and structure of the polymer and potentially weakening mechanical properties such as toughness. It has therefore been of considerable interest to develop optical luminescent stress-sensing nanoparticles and  find a way to embed them inside polymer fibers with minimal impact on the mechanical properties that are being sensed.”

The Berkeley Lab researchers met this challenge by combining semiconductor tQDs of CdSe/CdS, which were developed in an earlier study by Alivisatos and his research group, with electrospinning. The CdSe/CdS tQDs are exceptionally well-suited as nanoscale stress sensors because an applied stress will bend the arms of the tetrapods, causing a shift in the color of their fluorescence. The large electric field used in electrospinning results in a uniform dispersal of tQD aggregates throughout the polymer matrix, thereby minimizing the formation of stress concentrations that would act to degrade the mechanical properties of the polymer. Electrospinning also provided a much stronger bond between the polymer fibers and the tQDs than a previous diffusion-based technique for using tQDs as stress probes that was reported two years ago by Alivisatos and his group. Much higher concentrations of tQDs could also be a achieved with electrospinning rather than diffusion.

When stress was applied to the polymer nanocomposites, elastic and plastic regions of deformation were easily observed as a shift in the fluorescence of the tQDs even at low particle concentrations. As particle concentrations were increased, a  greater fluorescence shift per unit strain was observed. The tQDs acted as non-perturbing probes that tests proved were not adversely affecting the mechanical properties of the polymer fibers in any significant way.

“We performed mechanical tests using a traditional tensile testing machine with all of our types of polymer fibers,” says Shilpa Raja, a lead author of the Nano Letters paper along with Andrew Olson, both members of Alivisatos’ research group. “While the tQDs undoubtedly change the composition of the fiber – it is no longer pure polylactic acid but instead a composite – we found that the mechanical properties of the composite and crystallinity of the polymer phase show minimal change.”

The research team believes their tQD probes should prove valuable for a variety of biological, imaging and materials engineering applications.

“A big advantage in the development of new polymer nanocomposites would be to use tQDs to monitor stress build-ups prior to material failure to see how the material was failing before it actually broke apart,” says co-lead author Olson. “The tQDs could also help in the development of new smart materials by providing insight into why a composite either never exhibited a desired nanoparticle property or stopped exhibiting it during deformation from normal usage.”

For biological applications, the tQD is responsive to forces on the nanoNewton scale, which is the amount of force exerted by living cells as they move around within the body. A prime example of this is metastasizing cancer cells that move through the surrounding extracellular matrix. Other cells that exert force include the fibroblasts that help repair wounds, and cardiomyocytes, the muscle cells in the heart that beat.

“All of these types of cells are known to exert nanoNewton forces, but it is very difficult to measure them,” Raja says.

“We’ve done preliminary studies in which we have shown that cardiomyocytes on top of a layer of tQDs can be induced to beat and the tQD layer will show fluorescent shifts in places where the cells are beating. This could be extended to a more biologically-relevant environment in order to study the effects of chemicals and drugs on the metastasis of cancer cells.”

Another exciting potential application is the use of tQDs to make smart polymer nanocomposites that can sense when they have cracks or are about to fracture and can strengthen themselves in response.

“With our technique we are combining two fields that are usually separate and have never been combined on the nanoscale, optical sensing and polymer nanocomposite mechanical tunability,” Raja says. “As the tetrapods are incredibly strong, orders of magnitude stronger than typical polymers, ultimately they can make for stronger interfaces that can self-report impending fracture.”

This research was primarily supported by the DOE Office of Science.

#  #  #

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit the Office of Science website at science.energy.gov/.

 

Additional Information

For more about the research of Paul Alivisatos go here

Continuous Flow Synthesis Method for Fluorescent Quantum Dots


NANOSPHERESQuantum dots have potential applications in fields as diverse as medicine, photovoltaics, and quantum computing. The Center for Applied Nanotechnology (CAN) in Hamburg are making great strides in making high quality quantum dots available for research and large-scale production.

Quantum dots are nano-scale particles of semiconductor material, which are so small that quantum effects start to directly affect the particles’ electrical, optical and magnetic properties. This has many interesting implications on a larger scale – for example, fluorescent quantum dots can be designed which emit different colors depending only on their size.

The properties of quantum dots have been investigated in labs to a fairly high degree, so current research is focusing on very challenging applications, or on bringing the technology into the commercial realm.

This rapid development puts greater and greater demands on the quality of the nanoparticles – a challenge which CAN is rising to with a continuous-flow production method for their CANdots® product range. Daniel Ness from CAN explains the benefits of this technique:

“Some of our nanoparticle products, including the new Series A nanoparticles with visible-range fluorescence, use our continuous-flow synthetic method. This replaces more conventional batch synthesis, and greatly improves the reproducibility of the product, as well as being much easier to scale to higher production volumes.

“The process is also less dependent on highly trained technicians, as the parameters are easier to control. We are now working on adapting this process for our other CANdots® products, and we have a patent pending on the process itself.”

 

CAN’s new product, Series A Plus, are fluorescent quantum dots made of CdSe, with applications in LEDs and solid state lighting, single particle spectroscopy and as markers for biological imaging. They were launched at the 2013 NSTI Nanotech Expo in Washington.

Daniel Ness, CAN

The CANdots® range covers many more types of quantum dots and nanocrystals, including Series C NIR/IR emitters based on PbS, and Series X rare-earth doped quantum dots with distinctive emission features ideal for tagging and security labelling.

CAN was founded in 2005 as a spin-out from the University of Hamburg, focused on the transfer of their expertise in the production of nanomaterials from research into industry. The center is working with an array of companies and universities to design and develop new nanotechnology products.

CAN are currently seeking industrial partners to work on scale-up of their continuous flow nanoparticle production process, particularly for applications in photovoltaics, LEDs, and life sciences.

                Date Added: May 30, 2013                                 | Updated: Aug 19, 2013

Tetrapod Quantum Dots Light the Way


QDOTS imagesCAKXSY1K 8Fluorescent tetrapod nanocrystals could light the way to the future design of stronger polymer nanocomposites. A team of researchers with the U.S. Dept. of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed an advanced optomechanical sensing technique based on tetrapod quantum dots that allows precise measurement of the tensile strength of polymer fibers with minimal impact on the fiber’s mechanical properties.

In a study led by Paul Alivisatos, Berkeley Lab Dir. and the Larry and Diane Bock Prof. of Nanotechnology at the Univ. of California (UC) Berkeley, the research team incorporated into polymer fibers a population of tetrapod quantum dots (tQDs) consisting of a cadmium-selenide (CdSe) core and four cadmium-sulfide (CdS) arms. The tQDs were incorporated into the polymer fibers via electrospinning, among today’s leading techniques for processing polymers, in which a large electric field is applied to droplets of polymer solution to create micro- and nano-sized fibers. This is the first known application of electrospinning to tQDs.

Alivisatostetrapodx500

 

 

 

 

 

 

 

 

 

Fluorescent tetrapod quantum dots or tQDs (brown) serve as stress probes that allow precise measurement of polymer fiber tensile strength with minimal impact on mechanical properties. Inserts show relaxed tQDs  (upper) and stressed (lower).

Image:  LBN Laboratory.

“The electrospinning process allowed us to put an enormous amount of tQDs, up to 20% by weight, into the fibers with minimal effects on the polymer’s bulk mechanical properties,” Alivisatos says. “The tQDs are capable of fluorescently monitoring not only simple uniaxial stress, but stress relaxation and behavior under cyclic varying loads. Furthermore, the tQDs are elastic and recoverable, and undergo no permanent change in sensing ability even upon many cycles of loading to failure.”

Alivisatos is the corresponding author of a paper describing this research in NANO Letters. Co-authors were Shilpa Raja, Andrew Olson, Kari Thorkelsson, Andrew Luong, Lillian Hsueh, Guoqing Chang, Bernd Gludovatz, Liwei Lin, Ting Xu and Robert Ritchie.

Polymer nanocomposites are polymers that contain fillers of nanoparticles dispersed throughout the polymer matrix. Exhibiting a wide range of enhanced mechanical properties, these materials have great potential for a broad range of biomedical and material applications. However, rational design has been hampered by a lack of detailed understanding of how they respond to stress at the micro- and nanoscale.

“Understanding the interface between the polymer and the nanofiller and how stresses are transferred across that barrier are critical in reproducibly synthesizing composites,” Alivisatos says. “All of the established techniques for providing this information have drawbacks, including altering the molecular-level composition and structure of the polymer and potentially weakening mechanical properties such as toughness. It has therefore been of considerable interest to develop optical luminescent stress-sensing nanoparticles and find a way to embed them inside polymer fibers with minimal impact on the mechanical properties that are being sensed.”

The Berkeley Lab researchers met this challenge by combining semiconductor tQDs of CdSe/CdS, which were developed in an earlier study by Alivisatos and his research group, with electrospinning. The CdSe/CdS tQDs are exceptionally well-suited as nanoscale stress sensors because an applied stress will bend the arms of the tetrapods, causing a shift in the color of their fluorescence. The large electric field used in electrospinning results in a uniform dispersal of tQD aggregates throughout the polymer matrix, thereby minimizing the formation of stress concentrations that would act to degrade the mechanical properties of the polymer. Electrospinning also provided a much stronger bond between the polymer fibers and the tQDs than a previous diffusion-based technique for using tQDs as stress probes that was reported two years ago by Alivisatos and his group. Much higher concentrations of tQDs could also be achieved with electrospinning rather than diffusion.

When stress was applied to the polymer nanocomposites, elastic and plastic regions of deformation were easily observed as a shift in the fluorescence of the tQDs even at low particle concentrations. As particle concentrations were increased, a greater fluorescence shift per unit strain was observed. The tQDs acted as non-perturbing probes that tests proved were not adversely affecting the mechanical properties of the polymer fibers in any significant way.

“We performed mechanical tests using a traditional tensile testing machine with all of our types of polymer fibers,” says Shilpa Raja, a lead author of the Nano Letters paper along with Andrew Olson, both members of Alivisatos’ research group. “While the tQDs undoubtedly change the composition of the fiber—it is no longer pure polylactic acid but instead a composite—we found that the mechanical properties of the composite and crystallinity of the polymer phase show minimal change.”

The research team believes their tQD probes should prove valuable for a variety of biological, imaging and materials engineering applications.

“A big advantage in the development of new polymer nanocomposites would be to use tQDs to monitor stress build-ups prior to material failure to see how the material was failing before it actually broke apart,” says co-lead author Olson. “The tQDs could also help in the development of new smart materials by providing insight into why a composite either never exhibited a desired nanoparticle property or stopped exhibiting it during deformation from normal usage.”

For biological applications, the tQD is responsive to forces on the nanoNewton scale, which is the amount of force exerted by living cells as they move around within the body. A prime example of this is metastasizing cancer cells that move through the surrounding extracellular matrix. Other cells that exert force include the fibroblasts that help repair wounds, and cardiomyocytes, the muscle cells in the heart that beat.

“All of these types of cells are known to exert nanoNewton forces, but it is very difficult to measure them,” Raja says.

“We’ve done preliminary studies in which we have shown that cardiomyocytes on top of a layer of tQDs can be induced to beat and the tQD layer will show fluorescent shifts in places where the cells are beating. This could be extended to a more biologically-relevant environment in order to study the effects of chemicals and drugs on the metastasis of cancer cells.”

Another exciting potential application is the use of tQDs to make smart polymer nanocomposites that can sense when they have cracks or are about to fracture and can strengthen themselves in response.

“With our technique we are combining two fields that are usually separate and have never been combined on the nanoscale, optical sensing and polymer nanocomposite mechanical tunability,” Raja says. “As the tetrapods are incredibly strong, orders of magnitude stronger than typical polymers, ultimately they can make for stronger interfaces that can self-report impending fracture.”

Source: Lawrence Berkeley National Laboratory

New flexible solar cell technology in development


QDOTS imagesCAKXSY1K 8US-based Natcore Technology with research partner Rice University has developed what it describes as an inorganic flexible thin film solar cell by solution processes.

The production process for the cells has the potential to move to a roll-to-roll manufacturing lineThe device was made using Natcore’s liquid phase deposition (LPD) process. A cadmium/selenium (CdSe) absorber layer was grown onto a back contact substrate based on single-walled carbon nanotubes (SWNT). LPD was also used to grow a copper/selenium (CuSe) window layer onto which silver contacts were deposited. The resulting solar device shows potential for this process to make a flexible solar cell, free of high temperature semiconductor processing.

With further work the process has potential for roll-to-roll (R2R) production. The company’s R&D centre is situated near a former Kodak R2R photo film plant in Rochester, New York state.

Black silicon

A few years ago Natcore Technologies began attracting interest for its LPD technology in an application for improving the light absorption properties of multi-crystalline silicon cells, known as black silicon solar cells. LPD, developed at Rice University, makes it possible to grow a wide range of inorganic materials on a range of substrates using a room-temperature, environmentally friendly chemical bath.

In the flexible solar cell work nanotubes were used for a back contact embedded into the absorber layer, reducing the diffusion length to the back contact, to potentially lead to higher efficiency, because of a lower percentage of hole electron recombination.

There is potential to make the development compatible with the company’s multi-junction tandem solar cell technology to enable high efficiency extremely thin and flexible solar cells.

Other companies bringing to market high efficiency flexible thin film solar cells include Alta Devices, which has developed a process of growing very thin layers of solar cell materials on gallium arsenide (GaAs) wafers. The California-based company has been working on a GaAs solar cell technology for military and other applications, targeting the commercial unmanned aerial vehicle (UAE) market where very lightweight and efficient solar cells on the wings of craft can extend flight times without adding extra weight.

Tetrapod Nanocrystals as Fluorescent Stress Probes of Electrospun Nanocomposites


Abstract Image

 

201306047919620

A nanoscale, visible-light, self-sensing stress probe would be highly desirable in a variety of biological, imaging, and materials engineering applications, especially a device that does not alter the mechanical properties of the material it seeks to probe. Here we present the CdSe–CdS tetrapod quantum dot, incorporated into polymer matrices via electrospinning, as an in situ luminescent stress probe for the mechanical properties of polymer fibers. The mechanooptical sensing performance is enhanced with increasing nanocrystal concentration while causing minimal change in the mechanical properties even up to 20 wt % incorporation. The tetrapod nanoprobe is elastic and recoverable and undergoes no permanent change in sensing ability even upon many cycles of loading to failure. Direct comparisons to side-by-side traditional mechanical tests further validate the tetrapod as a luminescent stress probe. The tetrapod fluorescence stress–strain curve shape matches well with uniaxial stress–strain curves measured mechanically at all filler concentrations reported.

Dots, rods and tetrapods: CdSe gets in shape


Jan 31, 2011

QDOTS imagesCAKXSY1K 8Researchers from the South China University of Technology have presented a surfactant-free recipe for fabricating high-quality CdSe nanocrystals (NCs). The morphology, which includes irregular dots, rods, tetrapods and sphere-shapes, can be controlled easily by varying the experimental conditions. More importantly, the preparation techniques involved are simple, low-cost and can be used to fabricate other II-VI group semiconductor NCs.

CdSe Nanocrystals

CdSe nanocrystals: dots, nanorods and tetrapods

The CdSe NCs were produced with a fixed Cd/Se molar ratio of 2:1 and using 2.32 g of trioctylphosphine oxide (TOPO); at the same time, all the trioctylphosphine selenide (TOPSe) injections were kept at 1 ml, but with different concentrations. No other ligands were used in the case study.

Homogeneous CdSe NCs with different morphology were obtained under such experimental conditions. The sample quality (size distribution, optical properties, tetrapod selectivity) is as good as that of the best CdSe NCs synthesized by using extra ligands. As for the growth mechanism, we believed that the decomposition of TOPSe and cadmium myristate at a temperature of 240 or 300 °C would also supply in situ-generated TOP and myristic acid in the reaction mixture, which affected the anisotropic growth of CdSe NCs.

To further investigate the application of this surfactant-free recipe, the group is now optimizing the experimental conditions and has found that well controlled morphology of CdTe and/or CdSexTe1–x NCs can also be successfully fabricated.

Thanks to the easily controllable NC-growth kinetics, such a synthesis route is very promising for low cost, large-scale preparation of CdSe and CdTe NCs for application in solution-processed thin-film solar cells.

More information can be found in the journal Nanotechnology.

About the author

The study was funded by the National Natural Science Foundation of China (nos. 50703012, 50773023 and 50990065), the National Basic Research Program of China (973 program no. 2009CB623600) and SCUT grant (no. 2009ZZ0003). The experiments were performed at the Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Special Functional Materials group. Hongmei Liu is a PhD student in materials science and holds a bachelors degree in chemistry. Currently she is exploring the fabrication of high-quality semiconductor nanostructures, together with the measurement and application of the resulting nanostructures in the field of solution processed thin-film solar cell systems and other nano-electronic devices.

Nanorods could improve LED displays: Cornell


Oct. 24, 2012

Well-ordered nanorods could improve LED displays

Scientists have utilized the imaging capabilities of the Cornell High Energy Synchrotron Source (CHESS) to help develop enhanced light-emitting diode displays using bottom-up engineering methods.

Collaborative work between researchers from the University of Florida and CHESS has resulted in a novel way to make colloidal “superparticles” from oriented nanorods of semiconducting materials. The work was published in the journal Science, Oct. 19.

The team synthesized nanorods with a cadmium selenide and cadmium sulfide shell. Taking advantage of the compounds’ lattice mismatch interfaces, they assembled these rods into larger periodic colloidal structures, called superparticles.

The superparticles exhibit enhanced light emission and polarization, features that are important for fabrication of LED televisions and computer screens. The nucleated superparticles can further be cast into macroscopic polarized films. The films could increase efficiency in polarized LED television and computer screen by as much as 50 percent, the researchers say.

The team, which included CHESS scientist Zhongwu Wang, made use of the CHESS facility to collect small angle X-ray scattering data from specimens inside tiny diamond-anvil cells. They used this technique, in combination with high-resolution transmission electron microscopy, to analyze how nanorods with attached organic components could be formed into well-ordered structures.

The nanorods first align within a layer as hexagonally ordered arrays. Then the highly ordered nanorod arrays behave like a series of layered units, self-assembling into structures that exhibit long-range order as they grow into large superparticles. The elongated superparticles can be aligned in a polymer matrix into macroscopic films.

The project demonstrates how scientists are learning to recognize and exploit anisotropic interactions between nanorods, which can be adjusted during the synthesis process, to create single-domain, needle-like particles. The authors hope their work can lead to new processes of self-assembly to create nano-objects with other anisotropic shapes, perhaps even joining two or more types of objects to form well-defined mesoscopic and macroscopic architectures with greater and greater complexity.

The team was led by Charles Cao, professor of chemistry at the University of Florida. The lead author of the paper was Tie Wang of Cao’s group.