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/.

 

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UC Berkeley, Berkeley Lab announce Kavli Energy NanoSciences Institute


(Nanotubes imagesNanowerk News) The Kavli Energy NanoSciences Institute  (Kavli ENSI) announced today (Thursday, Oct. 3) will be supported by a $20  million endowment, with The Kavli Foundation providing $10 million and UC  Berkeley raising equivalent matching funds. The Kavli Foundation also will  provide additional start-up funds for the institute. The Kavli ENSI will explore  fundamental issues in energy science, using cutting-edge tools and techniques  developed to study and manipulate nanomaterials – stuff with dimensions 1,000  times smaller than the width of a human hair – to understand how solar, heat and  vibrational energy are captured and converted into useful work by plants and  animals or novel materials.
This new Kavli Institute has already received matching fund  gifts from the Heising-Simons Foundation, establishing a Heising-Simons Energy  Nanoscience Fellows program, and a donation from the Philomathia Foundation,  establishing the Philomathia Discovery Fund.
“The field of nanoscience is poised to change the very  foundations of how we should think about future energy conversion systems,” said  Kavli ENSI Director Paul Alivisatos, who is also director of Berkeley Lab and  the Samsung Distinguished Chair in Nanoscience and Nanotechnology in UC  Berkeley’s College of Chemistry. “UC Berkeley and Berkeley Lab stand out  worldwide for their strong efforts in nanoscience and their research activities  related to energy, so energy nanoscience is a particular strength for us.”
“I am delighted to welcome the Kavli ENSI into the community of  Kavli institutes,” said Fred Kavli, Founder and Chairman of The Kavli  Foundation. “By exploring the basic science of energy conversion in biological  systems, as well as building entirely new hybrid and perhaps even completely  artificial systems, the Kavli ENSI is positioned to revolutionize our thinking  about the science of energy, and is positioned to do the kind of basic research  that will ultimately make this a better world for all of us.”
“This new partnership with the Kavli Foundation and Berkeley Lab  is significant and exciting,” said UC Berkeley Chancellor Nicholas Dirks. “The  Kavli Institute will expand our portfolio of research endeavors focused on  alternative sources of energy, one of the planet’s most pressing and complicated  challenges. Progress in the realm of energy nanosciences will be contingent on  successful collaboration across conventional scientific boundaries – the very  approach that has made Berkeley a global leader in alternative energy research.”
“There is simply no better time, given the issues surrounding  energy worldwide, to announce an institute dedicated to the basic science of  energy. This new Kavli Institute will have superb leadership and a large number  of extraordinary faculty affiliated with it,” said Robert W. Conn, President of  The Kavli Foundation. “I’d like as well to thank both the Heising-Simons  Foundation and the Philomathia Foundation for their confidence in Berkeley and  in this new Kavli Institute. Their matching gifts will help the Kavli ENSI at  Berkeley get off to a very strong start.” He added, “There is also no more  important time than now to invest in basic scientific research. History has  shown that discoveries in basic science have a profound impact on the economy of  nations, on the health of people, and on the well-being of societies.”
The Kavli ENSI will be the fifth nanoscience institute worldwide  established by The Kavli Foundation, joining Kavli Institutes at the California  Institute of Technology, Cornell University, Delft University of Technology in  the Netherlands and Harvard University. The foundation funds an international  program that includes research institutes, professorships, symposia and other  initiatives in four fields – astrophysics, nanoscience, neuroscience and  theoretical physics. It is also a founder of the Kavli Prizes, which recognize  scientists for their seminal advances in astrophysics, nanoscience and  neuroscience.
With the announcement of the Kavli ENSI, The Kavli Foundation  has established 17 institutes worldwide – 11 in the United States, three in  Europe and three in Asia.
Scientists at the Kavli Energy NanoSciences Institute will look  beyond today’s energy conversion approaches to explore unusual avenues found in  biological systems and to build entirely new hybrid or completely artificial  systems. For example, Kavli ENSI scientists plan to explore how plant pigments  capture energy from the sun and transport it for chemical storage, and how the  body’s molecular motors convert chemical energy into motion inside a cell.  Meanwhile, other scientists and engineers plan to build nanodevices that mimic  and improve on nature’s tricks, using materials ranging from graphene and metal  oxide frameworks to nanowires and nanolasers.
UC Berkeley and Berkeley Lab boast a long history of nanoscience  innovation, starting with Alivisatos’ work in the science of nanocrystals,  ranging from studies of their physical properties to synthesis and applications  in biological imaging and renewable energy. Nearly 100 research labs are devoted  to aspects of nanoscience and nanoengineering.
“The new Kavli ENSI institute is intended to allow us to explore  the principles of energy systems on small scales and is not focused on any  particular area of application,” Alivisatos emphasized. “Fred Kavli’s vision is  to support curiosity-driven science. This institute will help to foster a  long-term perspective.”
“Of course, we have all learned that innovative solutions to  pressing problems can often start in the basic sciences,” said institute  co-director Omar Yaghi, the James and Neeltje Tretter Chair and professor of  chemistry at UC Berkeley and a Berkeley Lab researcher. Yaghi’s work on the  nanoscale properties of metal oxide frameworks – porous composites of iron and  organic molecules – proved to have wide application in natural gas and hydrogen  storage and carbon capture.
Alivisatos said that much of today’s energy research focuses on  improving well-known technologies, such as batteries, liquid fuels, solar cells  and wind generators. On the nanoscale, however, energy is captured, channeled  and stored in totally different ways dictated by the quantum mechanical nature  of small-scale interactions.
“We don’t fully understand some foundational issues about how  energy is converted to work on really short length scales,” he said.
Research by UC Berkeley and Berkeley Lab chemist Graham Fleming  has shown, for example, that when leaf pigments capture light in the form of  photons, electrons are excited and interact in a coherent way not seen at larger  scales. This quantum coherence could potentially be incorporated into nanoscale  artificial systems to produce energy on a commercial scale.
While studying nanoscale motors inside cells, UC Berkeley  physicist Carlos Bustamante and Berkeley Lab theorist Gavin Crooks discovered  that energy flow does not always follow the standard rules of macroscopic  systems. Nanomotors can sometimes move backward, for example, akin to a ball  rolling uphill. Such quantum weirdness might be replicated to create more  efficient nanomachines or self-regulating nanoscale energy circuits.
Other Kavli ENSI scientists plan to investigate how heat flows  in nanomaterials and whether the vibrational energy, or phonons, can be  channeled to make thermal rectifiers, diodes or transistors analogous to  electronic switches in use today; develop novel materials, ranging from polymers  to cage structures and nanowires, with unusual nanoscale properties; or design  materials that could sort, count and channel molecules along prescribed paths  and over diverse energy landscapes to carry out complex chemical conversions.
“I think that by bringing together people who make new forms of  matter, others who know how to manipulate matter on a fine scale, and those who  try to understand how electrons or light propagate through these materials, we  will get the kind of out-of-the-box thinking from which whole new areas of  research emerge,” Yaghi said.
The new institute’s co-director, Peidong Yang, who is the S.K.  and Angela Chan Distinguished Professor of Energy in the College of Chemistry,  said that Kavli ENSI’s multidisciplinary, intellectually stimulating environment  will be ideal for learning “how to program the assembly of nanoscopic building  blocks to create the necessary interfaces so that energy flow, molecular and  charge-charge transport can be controlled in a cooperative manner.”
While the institute will not have separate lab space, its  administrative offices will be housed in two new buildings expected to be  completed next year: Campbell Hall on the UC Berkeley campus and the Solar  Energy Research Center at Berkeley Lab.
Source: Kavli Foundation

Read more: http://www.nanowerk.com/news2/newsid=32595.php#ixzz2gjo9EJpz

Alivisatos (UC Berkley) Appointed Samsung Distinguished Chair in Nanoscience


 

By Public Affairs, UC Berkeley | August 22, 2013

BERKELEY —201306047919620Chemist Paul Alivisatos, one of the pioneers of nanoscience, has been appointed to the Samsung Distinguished Chair in Nanoscience and Nanotechnology at UC  Berkeley in recognition of his many scientific achievements.

The endowed chair, established through the support of Samsung Electronics Co., will help cement the campus’s leadership in research and innovation in an area that has great implications for many fields ranging from biology to energy, the Office of the Vice-Chancellor for Research announced Friday (Aug. 23). Alivisatos, director of the Lawrence Berkeley National Laboratory and a UC Berkeley professor of chemistry, is known for his research into quantum dot semiconductor nanocrystals, clusters of hundreds to thousands of atoms with novel properties that can be applied to electronic devices and solar cells as well as light-emitting diodes (LEDs).

Paul Alivisatos

Paul Alivisatos, the newly named Samsung Distinguished Chair in Nanoscience and Nanotechnology, in conversation with Dr. Young Hwan Kim of the Samsung Advanced Institute of Technology, Korea, at Alivisatos’s lab on the UC Berkeley campus. A delegation from SAIT visited UC Berkeley Thursday, Aug. 22.  (Photo by Roy Kaltschmidt, Berkeley Lab)

Dr. Youngjoon Gil, executive vice president of the Samsung Advanced Institute of Technology, welcomed the appointment.

“Historically, the invention of a new material can initiate a quantum leap in the development of industry,” said Dr. Gil. “Nanomaterials offer such opportunities for the electronics as well as the biosciences industry, where precise control and manipulation of energy is required. Quantum dot, pioneered by Professor Alivisatos, has established its commercial value by reproducing more realistic colors on displays. Through the establishment of the endowed chair, Samsung anticipates a closer partnership with UC Berkeley, the world’s leader in nanoscience, in exploring the commercial value of nanotechnology.”

Over the past two decades, UC Berkeley has become a brain trust in nanoscience and nanotechnology, with nearly a hundred nanoscience and nanotech researchers in the fields of biology, chemistry, physics and materials science. These researchers have made major advances in understanding the nano-scale molecular motors that move materials around inside cells or manipulate DNA; creating tiny motors, lasers and photonic devices for smaller electronic circuits; creating flexible and inexpensive solar cells from nanorods; and understanding the properties of new materials such as graphene and high-temperature superconductors.

Graham Fleming, UC Berkeley’s vice chancellor for research, lauded Samsung for its initiative in establishing this chair.

“The new chair helps build on our strengths in the conversation and utilization of energy on the nano scale,” said Fleming. “It is a fitting recognition of Paul’s achievements and his world-wide influence on the field of nanoscience. We look forward to continue expanding our relationship with Samsung in this area.”

Alivisatos is widely recognized for his contributions to the study of nanocrystals, ranging from control of their synthesis and fabrication to studies of their optical, electrical, structural, and thermodynamic properties. He demonstrated that semiconductor nanocrystals can be grown into rods as opposed to spheres. This achievement paved the way for a slew of new synthetic advances, developing methods for controlling the shape, connectivity and topology of nanocrystals.

Nanocrystals are typically a few nanometers in diameter — larger than molecules but smaller than bulk solids — and frequently exhibit physical and chemical properties somewhere in between. Given that a nanocrystal is virtually all surface and no interior, its properties can vary considerably as the crystal grows.

Alivisatos’s research has opened the door to a number of potential new applications for nanocrystals. These include their use as fluorescent probes for the study of biological materials and LEDs, and the fabrication of hybrid solar cells that combine nanotechnology with plastic electronics.

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