Tuesday, December 6, 2011

On the road to plasmonics with silver polyhedral nanocrystals: Researchers find simpler approach to making plasmonic materials

 The question of how many polyhedral nanocrystals of silver can be packed into millimeter-sized supercrystals may not be burning on many lips but the answer holds importance for one of today's hottest new high-tech fields -- plasmonics! Researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) may have opened the door to a simpler approach for the fabrication of plasmonic materials by inducing polyhedral-shaped silver nanocrystals to self-assemble into three-dimensional supercrystals of the highest possible density.

Plasmonics is the phenomenon by which a beam of light is confined in ultra-cramped spaces allowing it to be manipulated into doing things a beam of light in open space cannot. This phenomenon holds great promise for superfast computers, microscopes that can see nanoscale objects with visible light, and even the creation of invisibility carpets. A major challenge for developing plasmonic technology, however, is the difficulty of fabricating metamaterials with nano-sized interfaces between noble metals and dielectrics.

Peidong Yang, a chemist with Berkeley Lab's Materials Sciences Division, led a study in which silver nanocrystals of a variety of polyhedral shapes self-assembled into exotic millimeter-sized superstructures through a simple sedimentation technique based on gravity. This first ever demonstration of forming such large-scale silver supercrystals through sedimentation is described in a paper in the journal Nature Materials titled "Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices." Yang, who also holds appointments with the University of California Berkeley's Chemistry Department and Department of Materials Science and Engineering, is the corresponding author.

"We have shown through experiment and computer simulation that a range of highly uniform, nanoscale silver polyhedral crystals can self-assemble into structures that have been calculated to be the densest packings of these shapes," Yang says. "In addition, in the case of octahedra, we showed that controlling polymer concentration allows us to tune between a well-known lattice packing structure and a novel packing structure that featured complex helical motifs."

In the Nature Materials paper Yang and his co-authors describe a polyol synthesis technique that was used to generate silver nanocrystals in various shapes, including cubes, truncated cubes, cuboctahedra, truncated octahedra and octahedra over a range of sizes from 100 to 300 nanometers. These uniform polyhedral nanocrystals were then placed in solution where they assembled themselves into dense supercrystals some 25 square millimeters in size through gravitational sedimentation. While the assembly process could be carried out in bulk solution, having the assembly take place in the reservoirs of microarray channels provided Yang and his collaborators with precise control of the superlattice dimensions.

"In a typical experiment, a dilute solution of nanoparticles was loaded into a reservoir that was then tilted, causing the particles to gradually sediment and assemble at the bottom of the reservoir," Yang says. "More concentrated solutions or higher angles of tilt caused the assemblies to form more quickly."

The assemblies generated by this sedimentation procedure exhibited both translational and rotational order over exceptional length scales. In the cases of cubes, truncated octahedra and octahedra, the structures of the dense supercrystals corresponded precisely to their densest lattice packings. Although sedimentation-driven assembly is not new, Yang says this is the first time the technique has been used to make large-scale assemblies of highly uniform polyhedral particles.

"The key factor in our experiments is particle shape, a feature we have found easier to control," Yang says. "When compared with crystal structures of spherical particles, our dense packings of polyhedra are characterized by higher packing fractions, larger interfaces between particles, and different geometries of voids and gaps, which will determine the electrical and optical properties of these materials."

The silver nanocrystals used by Yang and his colleagues are excellent plasmonic materials for surface-enhanced applications, such as sensing, nanophotonics and photocatalysis. Packing the nanocrystals into three-dimensional supercrystals allows them to be used as metamaterials with the unique optical properties that make plasmonic technology so intriguing.

"Our self-assembly process for these silver polyhedral nanocrystals may give us access to a wide range of interesting, scalable nanostructured materials with dimensions that are comparable to those of bulk materials," Yang says.

Co-authoring the Nature Materials paper with Yang were Joel Henzie, Michael Grünwald, Asaph Widmer-Cooper and Phillip Geissler, who also holds joint appointments with Berkeley Lab and UC Berkeley.

This research was supported in part by the Defense Advanced Research Projects Agency and DOE's Office of Science.

Story Source:

The above story is reprinted from materials provided by DOE/Lawrence Berkeley National Laboratory.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Joel Henzie, Michael Grünwald, Asaph Widmer-Cooper, Phillip L. Geissler, Peidong Yang. Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices. Nature Materials, 2011; DOI: 10.1038/nmat3178

Recent advance in detonation theory

Taking the complex multi-dimensional movement into account, Hu et al. at the Xian Modern Institute proposed the use of the entropy principle to specify the final point of detonation, and the use of the Hamilton principle to describe the complex movement of detonation product particles and determine the real path from explosive to products. A new detonation model named the least-action detonation model (LADM) has been proposed [1].

The least-action principle is one of the basic principles of nature, from which nearly all physical equations can be derived, including the equations of Newtonian mechanics, relativistic mechanics, , Maxwell's equations, Fermat's principle in optics, and Schrödinger equations in quantum mechanics. In analytical mechanics, the least-action principle is known as Hamilton's principle, which is equivalent to Newton's Law and applies to complex mechanical systems. Hu and Li et al. introduced the Hamilton principle into detonation science to bypass the difficulty of describing the complex movement and transport effects in detonation [2].

Because the LADM model takes complex movement and transport effects into account, the detonation configuration described by the LADM model differs from that described by the ZND model. The flow after the reaction zone in the ZND model is the changing Taylor rarefaction, whereas it is a stationary state in the LADM model. From the displacement of titanium foils embodied in the explosive charge, the movement state of detonation product particles can be judged. The X-ray photograph in Figure 1 shows that the titanium foil initially moves 1 mm because of the moving particles in the reaction zone, and then stops moving and is in a stationary state, which coincides with the prediction of the LADM model.

Much evidence that detonation particles are in a stationary state has already been given in the literature on detonation in the form of data, graphs and photographs. However, such evidence has not been addressed because of its contradiction with the ZND model; moreover, the stationary state has never been considered as the essence of detonation. Blasting models derived for a stationary state have long been used in blasting engineering, but the stationary state has been considered only an assumption because it contradicts the ZND model.

Because the LADM model solves many problems relating to detonation phenomena that cannot be explained by the ZND model, the use of the LADM model also proposes a series of research subjects in detonation science.

In recent years, detonation science has focused on the sub-macroscopic and sub-microscopic structures of detonation phenomena. By contrast, the LADM model emphasizes the general direction decided by the second law of thermodynamics, to grasp the essence of the detonation process.

According to the LADM model in which complex movement and transport effects are taken into account, the detonation path and final point of detonation should differ from those suggested by the ZND model.

Compared with the ZND model, the LADM model incorporates many partial differential equations corresponding to the multiformity of detonation. It is a challenge to solve these partial differential equations, which involve mechanics, chemistry and simultaneously.

Many equations of state have been proposed to calculate the moving Chapman–Jouguet state of detonation products. As theory and experiments show that detonation products are in a stationary state, the establishment of new equations of state becomes an urgent task in the field of detonation science.

"Standard candles" calculations for the supernovae SN Ia explosion have shown that the ZND model is not able to simulate precisely the complex process of detonation and a new model is needed. The LADM model incorporating many partial differential equations is one such candidate model.

More information: 1 Hu S M, Tian Q Z, Xiao C,et al. A new detontion model and its examination by experiment(in Chinese).Sci Sin Phys Mech Astro,2011,41:1230-1238, doi:10.1360/132011-252

2 Hu S M,Li C F,Ma Y H,et al. A detonation model of high/low velocity detonation. Propellants, Explosives, Pyrotechnics,2007,32(1):73-79, doi10.1002/prep,20070010.

DOI: 10.1007/s11434-011-9942-2

Provided by Science in China Press

Highly efficient method for creating flexible, transparent electrodes developed

 As the market for liquid crystal displays and other electronics continues to drive up the price of indium -- the material used to make the indium tin oxide (ITO) transparent electrodes in these devices -- scientists have been searching for a less costly and more dynamic alternative, particularly for use in future flexible electronics.

Besides its high price, ITO has several drawbacks. It's brittle, making it impractical for use in flexible displays and solar cells, and there is a lack of availability of indium, which is found primarily in Asia. Further, the production of ITO films is relatively inefficient.

Now, researchers at UCLA report in the journal ACS Nano that they have developed a unique method for producing transparent electrodes that uses silver nanowires in combination with other nanomaterials. The new electrodes are flexible and highly conductive and overcome the limitations associated with ITO.

For some time, silver nanowire (AgNW) networks have been seen as promising candidates to replace ITO because they are flexible and each wire is highly conductive. But complicated treatments have often been required to fuse crossed AgNWs to achieve low resistance and good substrate adhesion. To address this, the UCLA researchers demonstrated that by fusing AgNWs with metal-oxide nanoparticles and organic polymers, they could efficiently produce highly transparent conductors.

The team of researchers represents a collaboration between the department of materials science and engineering at the UCLA Henry Samueli School of Engineering and Applied Science; the department of chemistry and biochemistry in the UCLA College of Letters and Science; and the California NanoSystems Institute (CNSI) at UCLA.

The team was led by Yang Yang, a professor of materials science and engineering, and Paul Weiss, director of the CNSI and a professor of materials science and engineering and of chemistry and biochemistry.

"In this work, we demonstrate a simple and effective solution method to achieve highly conductive AgNW composite films with excellent optical transparency and mechanical properties," said Yang who also directs the Nano Renewable Energy Center at the CNSI. "This is by far the best solution: a processed, transparent electrode that is compatible with a wide variety of substrate choices."

Scientists can easily spray a surface with the nanowires to make a transparent mat, but the challenge is to make the silver nanowires adhere to the surface more securely without the use of extreme temperatures (200° C) or high pressures, steps that make the nanomaterials less compatible with the sensitive organic materials typically used to make flexible electronics.

To meet this challenge, Rui Zhu, the paper's first author, developed a low-temperature method to make high-performance transparent electrodes from silver nanowires using spray coating of a unique combination of nanomaterials.

First, researchers sprayed a solution of commercially available silver nanowires onto a surface. They then treated the nanowires with a solution of titanium dioxide nanoparticles to create a hybrid film. As the film dries, capillary forces pull the nanowires together, improving the film's conductivity. The scientists then coated the film with a layer of conductive polymer to increase the wires' adhesion to the surface.

The AgNW composite meshes are highly conductive, with excellent optical transparency and mechanical properties. The research team also built solar cells using the new electrodes and found that their performance was comparable to that of solar cells made with indium tin oxide.

The research received support from the Office of Naval Research and the Kavli Foundation.

Story Source:

The above story is reprinted from materials provided by University of California - Los Angeles. The original article was written by Jennifer Marcus.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Rui Zhu, Choong-Heui Chung, Kitty C. Cha, Wenbing Yang, Yue Bing Zheng, Huanping Zhou, Tze-Bin Song, Chun-Chao Chen, Paul S. Weiss, Gang Li, Yang Yang. Fused Silver Nanowires with Metal Oxide Nanoparticles and Organic Polymers for Highly Transparent Conductors. ACS Nano, 2011; : 111104125342002 DOI: 10.1021/nn203576v

Saving Da Vinci's Last Supper from air pollution

In late 2009, the refectory of Santa Maria Delle Grazie Church, where the painting is located, installed a sophisticated heating, ventilation, and air conditioning system to protect the painting from the of Milan.

To test the effectiveness of their pollution countermeasures, Italian officials called on Constantinos Sioutas, Fred Champion professor of civil and environmental engineering at the USC Viterbi School of Engineering. For his ongoing research, Sioutas has designed unobtrusive air samplers that are compact and quiet.

"These sampling technologies are ideally suited for use in sensitive facilities such as art galleries and museums. They do not disrupt the day-to-day operations of the facility," Sioutas said.

A multi-national team that includes USC scientists used the monitors to determine that indoor pollution has been drastically reduced at the church, though visitors enjoying the painting remain a potential source of soiling. The team's findings will be presented in December in Milan.

The team deployed two sets of air quality monitors for one year at the church, and found that – for the most part – the Italian authority responsible for the facility housing the famous painting (Soprintendenza per i Beni Architettonici e per il Paesaggio di Milano) is winning the war with outdoor air pollution. Fine and coarse particulate matter concentrations were reduced around the painting by 88 and 94 percent, respectively from their corresponding outdoor levels.

"It's a spectacular reduction," Sioutas said. "It is, frankly, very impressive."

Indoor sources of pollution, however, may still pose a threat of soiling on the . Nancy Daher, USC graduate student and lead author of a journal article on the team's findings, said that fatty lipids from the skin of visitors to the church still appeared in significant quantities around the painting – even with visitor access to the painting strictly regulated. Her article appears this month in Environmental Science and Technology.

Only a handful of patrons are allowed into the church via an airlock-style chamber at any given time, and are only allowed to stay for 15 minutes at a stretch.

Airborne lipids from visitors' skin can combine with dust in the air and, if they come in contact with the painting, soil it, Daher said.

"Even the painting itself is emitting," she said. Tiny particles of wax used in early repair efforts on the painting also can get into the air, soiling the in the same manner.

In addition to aiding in the conservation of the Last Supper, the team's research can be used as a benchmark for future studies aimed at protecting indoor artworks and antiquities.

Provided by University of Southern California (news : web)

Scientists develop new class of small molecules through innovative chemistry

Combining the power of with some advanced , the new approach could eventually expand by millions the number of provocative synthetic compounds available to explore as potential . This approach overcomes substantial molecular limitations associated with state-of-the-art approaches in small molecule synthesis and screening, which often serve as the foundation of current drug discovery efforts.

The study, led by Scripps Research Associate Professor Glenn Micalizio, was published Nov. 20, 2011, in an advanced online edition of the journal Nature Chemistry.

To frame the significance of this advance, Micalizio explains that high-throughput screening is an important component of modern drug discovery. In high-throughput screening, diverse collections of molecules are evaluated en masse for potential function in a biological area of interest. In this process, success is critically dependent on the composition of the molecular collections under evaluation. Modern screening centers maintain a relatively static collection of molecules, the majority of which are commercially available materials that have structures unrelated to natural products -- molecules that are appreciated as validated leads for drug development.

"This divergence in structure between natural products and commercially available synthetics lies at the heart of our inquiry," said Micalizio. "Why should we limit discovery of therapeutic leads to compound collections that are influenced by concerns relating to commercial availability and compatibility with an artificial set of constraints associated with the structure of modern screening centers?"

To expand the compounds available for investigation, the scientists embraced an approach to structural diversity that mimics nature's engine for the discovery of molecules with biological function. This process, termed "oligomerization," is a modular means of assembling structures (akin to the way that letters are used in a sequence to provide words with meaning) where a small collection of monomeric units can deliver a vast collection of oligomeric products of varying length, structure, and function (like the diversity of words presented in a dictionary).

Coupling this technique with a synthetic design aimed at generating molecules that boast molecular features inspired by the structures of bioactive natural products (specifically, polyketide-derived , which include erythromycin, FK-506, and epothilone), the scientists established a new chemical platform for the discovery of potential therapeutics.

Micalizio points out: "The importance of oligomerization to drive discovery is well appreciated in chemistry and biology, yet a means to realize this process as an entry to small molecule natural product-inspired structures has remained elusive. The crux of the problem is related to challenges associated with the control of shape for each member of a complex oligomer collection -- the central molecular feature that defines biological function."

"It is the stability associated with the shape of these new compounds that lies at the heart of the practical advance," he continued. "The unique features of this science now make possible the ability to synthesize large collections of diverse natural product-inspired structures that have predictable and stable three-dimensional shapes."

Micalizio said that the science described represents a first step toward revolutionizing discovery at the interface of chemistry, biology, and medicine by embracing nature's strategy for molecular discovery. Coupling this type of advance with modern screening technology that can handle the evaluation of large compound collections at low cost (such as work by Scripps Florida Professor Thomas Kodadek, a co-author of the new study), can dramatically enhance the future of pharmaceutically relevant science.

The potential of this vision was highlighted in the new study, in which a 160,000-member compound collection was employed to discover the first non-covalent small molecule ligand to the DNA binding domain of p53 -- an important transcription factor that regulates a variety of genes involved in cell cycle control and cell death.

More information: "A Biomimetic Polyketide-Inspired Approach to Small-Molecule Ligand Discovery," Nature Chemistry.

Provided by The Scripps Research Institute (news : web)