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Researchers at Northwestern University and the University of Oxford say that relatively simple methods of explaining chemical bond mechanisms typically taught at undergraduate levels turns out to be an accurate way to understand the arrangement of atoms on a oxide’s surface.

“For a long time we have not understood oxide surfaces,” said Laurence Marks, professor of materials science and engineering in the McCormick School of Engineering and Applied Science at Northwestern. “We only have had relatively simple models constructed from crystal planes of the bulk structure, and these have not enabled us to predict where the atoms should be on a surface.

“Now we have something that seems to work,” Marks said. “It’s the bond-valence-sum method, which has been used for many years to understand bulk materials. The way to understand oxide surfaces turns out to be to look at the bonding patterns and how the atoms are arranged and then to follow this method.”

These findings are published in Nature Materials.

According to a news release, NU graduate student James Enterkin matched the electron diffraction patterns from a strontium titanate surface with the patterns with scanning-tunneling microscopy images obtained by Bruce Russell at Oxford. Enterkin then combined these with density functional calculations and bond-valence sums, showing that those that had bonding similar to that found in bulk oxides were those with the lowest energy.

Ulrike Diebold, an expert in the investigation of metal oxide surfaces at the Institute of Applied Physics in Vienna, Austria, commented on the significance of this research in a separate article in Nature Materials. She writes, “This simple and intuitive, yet powerful concept [the bond-valence-sum method] is widely used to analyze and predict structures in inorganic chemistry. Its successful description of the surface reconstruction of SrTiO3 (110) shows that this approach could be relevant for similar phenomena in other materials.”

NU provides a fun 3D model of the surface of SrTiO3 (110) here. (Be sure to try to manipulated the model with your mouse.)

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A network with crystalline bundles of filaments. Credit: Yuri S. Velichko.

A team from Northwestern University reports in the new issue of Science about the role that X-rays can play in crystal formation. The researchers say they accidentally discovered that X-rays can trigger the formation of a new type of crystal that is composed of charged cylindrical filaments. These crystals are ordered like a bundle of pencils experiencing repulsive forces.

They hope their work will expand the use of X-rays from not just an analytical tool but also a method to control the structure of materials.

In a NU press release, Samuel Stupp, one of the paper’s authors, describes what the group thinks is going on with the X-rays. “The filaments are charged so one would expect them to repel each other, not to organize into a crystal. Even though they are repelling each other, we believe the hundreds of thousands of filaments in the bundles are trapped within a network and form a crystal to become more stable,” says Stupp, who is a professor of chemistry, materials science and engineering and medicine.

The discovery happened when members of Stupp’s research team, working on a separate organic project, zapped a solution of peptide nanofibers with synchrotron X-ray radiation. Unexpectedly, the solution turned opaque. “There was a dramatic change in the way filaments scattered the radiation,” says coauthor Honggang Cui. “The X-rays turned a disordered structure into something ordered - a crystal.”

The group theorizes that the X-rays increase the charge of the material and causes a hexagonal stacking of filaments. They say that because of repulsive forces, the filaments are positioned far apart from each other, with as much as 320 angstroms separating the filaments.

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The Daily Northwestern reported that Northwestern University received $2.4 million in government funding to develop flash-memory devices with enhanced capacity for U.S. military and intelligence use.

Allocated to NU’s Center for Integrated Nanosystems and International Institute for Nanotechnology, the money represents “substantial and welcome funding” for the field of nanoelectronics, says Fraser Stoddart, CINS director and NU Board of Trustees professor of chemistry.

The funding is part of $45.4 million for Illinois-based projects approved by Congress on Dec. 19 in a 2010 defense spending bill, according to the a news release on Sen. Dick Durbin’s (D-Il) web site.

Developing memory chips will involve building and mounting mechanical switches into infinitely stretching three-dimensional scaffolds on the molecular level, he said.

“Over 10 years ago, we developed two-dimensional switches, and this piece of research will put what we did with two into three (dimensions),” Stoddart continues. “If we managed to do this, it would create very dense flash memory.”

Although funded by the Defense Department, the technology will not be limited to surveillance and battlefield operations but could be used to increase the capacity of any flash memory device, he said.

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Dean Ho’s nanodiamond team at Northwestern always seems to be coming up with something new. This time, Ho and a team led by NWU cancer researcher Thomas J. Meade say they have figured out a way to couple gadolinium with nanodiamonds to make a MRI contrast agent that delivers greatly improved images.

“The results are a leap and not a small one,” says Meade in a NWU news release “it is a game-changing event for sensitivity. This is an imaging agent on steroids. The complex is far more sensitive than anything else I’ve seen.”

In the past, Ho has shown, at least with in vitro studies, that nanodiamonds seem to have excellent biocompatibility and can be used for drug, protein and DNA delivery. But researchers in that area are looking not only for a system to deliver drugs but also has a second function: tracking. (The ideal drug delivery system adds one more function that allows the material to be targeted to a particular tissue or site.)

In a paper study published online by the journal Nano Letters, the team says they have developed a gadolinium(III)-nanodiamond complex that demonstrated a greater than 10-fold increase in “relaxivity.” Relaxivity refers to ability of magnetic compounds to increase the relaxation rates of the surrounding water proton spins. Relaxivity is used to improve the contrast of the image.

“Nanodiamonds have been shown to be effective in attracting water molecules to their surface, which can enhance the relaxivity properties of the Gd(III)-nanodiamond complex,” said Ho. “This might explain why these complexes are so bright and such good contrast agents.”

“The nanodiamonds are utterly unique among nanoparticles,” Meade said. “A nanodiamond is like a cargo ship - it gives us a nontoxic platform upon which to put different types of drugs and imaging agents.”

The team also studied the toxicity of the Gd(III)-nanodiamond complex using fibroblasts and HeLa cells as biological testbeds, and found that that the material didn’t negatively affect th hybrid complex on cellular viability.

Now the focus is on moving from in vitro to in vivo. The researchers hope to be moving into preclinical application of the new contrast agent in various animal models.

They also think they can fine tune and improve the agent by nailing down how the structure of the Gd(III)-nanodiamond complex governs increased relaxivity.

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Micrographs of two single-phase FeTiO5 laminates with textured and untextured layers. Image (a), top, shows crack bifurcation within the textured layers. Image (b) shows tunnel cracks in the textured layer. Source: International Journal of Applied Ceramic Technology

The current issue of the International Journal of Applied Ceramic Technology reports on a Northwestern University group’s work related to using improved gelcasting techniques that allow new possibilities for manufacturing of certain laminates.

Gelcasting, a technique perfected at the Oak Ridge National Lab, is a method used to create large, near-net-shape ceramic and metal components with complex shapes from low-viscosity slurries composed of powders suspended in a liquid binder system. The components begin to be solidified when a chemical initiator is added to the slurry. This starts the formation of a polymer gel network. The slurry is quickly poured into a molds and allowed to dry. The additives and binders are burned out before sintering.

The team of Noah Shanti, David Hovis, Michelle Seitz, John Montgomery, Donald Baskin and Katherine Faber describe the use of more flexible gelcasting system – thermoreversible gelcasting – that allows more opportunities to manipulate the materials during the molding stage, something they found useful, for example, in toughening laminates.

The advantage of TRG over traditional gelcasting is that it is not time constrained (as long as the slurry temperature is kept above the transition temperature). Lamination is possible during gelcasting by adding successive layers of slurries selected because the properties and interfaces being sought. The teams describes concepts of tailoring the porosity and texture of the layers to, for example, strengthen the final laminate material by crack deflection, crack bifurcation and taking advantage of residual compressive stresses.

In one case, they describe the using the enhanced manipulation time to introduce a magnetic field to the materials during casting. The magnetic field aligns ceramic particles and allows the development of highly textured microstructures. The would not be possible using traditional gelcasting techniques because of the relatively brief window of opportunity before solidification begins.

The groups notes that while the use of TRG requires a good understanding of polymer chemistry, physics, and slurry rheology, not to mention drying and sintering kinetics, they predict the technique will find much use in applications ranging from strong bioceramic materials to solid oxide fuel cells.

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