Magnetic actuation enables nanoscale thermal analysis

 Polymer nano-films and nano-composites are used in a wide variety of applications from food packaging to sports equipment to automotive and aerospace applications. Thermal analysis is routinely used to analyze materials for these applications, but the growing trend to use nanostructured materials has made bulk techniques insufficient.

In recent years an atomic force microscope-based technique called nanoscale thermal analysis (nanoTA) has been employed to reveal the temperature-dependent properties of materials at the sub-100 nm scale. Typically, nanothermal analysis works best for soft polymers. Researchers at the University of Illinois at Urbana-Champaign and Anasys Instruments, Inc. have now shown that they can perform nanoscale thermal analysis on stiff materials like epoxies and filled composites.

"This new technique lets us measure temperature and frequency-dependent properties of materials rapidly over a wide bandwidth," noted William King, the College of Engineering Bliss Professor in the Department of Mechanical Science and Engineering at Illinois, who led the research. The technique works by flowing a current around the U-shaped arms of a self-heating atomic force microscope (AFM) cantilever and interacting that current with a magnetic field. The magnetic field allows the tip-sample force to be modulated right near the tip of the AFM.

"We are able to achieve nanometer-scale force control that is independent from the heating temperature," according to Byeonghee Lee, first author of the paper.

"Conventional nanothermal analysis has struggled with highly filled, highly crosslinked materials and sub-100 nm thin films. This new technique has allowed us to reliably measure and map glass transitions and melting transions on classes of materials that were previously very challenging," said Craig Prater, chief technology officer at Anasys Instruments and co-author on the paper.

The research was performed in King's Nanoengineering Laboratory and at Anasys Instruments. King is also affiliated with the Department of Materials Science and Engineering, the Department of Electrical and Computer Engineering, the Beckman Institute for Advanced Science and Technology, the Micro and Nanotechnology Laboratory, and the Materials Research Laboratory, all at the University of Illinois. The research was sponsored by the Air Force Office of Scientific Research and the National Science Foundation.

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The above story is reprinted from materials provided by University of Illinois College of Engineering.

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Journal Reference:

Byeonghee Lee, Craig B Prater, William P King. Lorentz force actuation of a heated atomic force microscope cantilever. Nanotechnology, 2012; 23 (5): 055709 DOI: 10.1088/0957-4484/23/5/055709

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New nanotech technique for lower-cost materials repair

In the super-small world of nanostructures, a team of polymer scientists and engineers at the University of Massachusetts Amherst have discovered how to make nano-scale repairs to a damaged surface equivalent to spot-filling a scratched car fender rather than re-surfacing the entire part. The work builds on a theoretical prediction by chemical engineer and co-author Anna Balazs at the University of Pittsburgh.

Their discovery is reported this week in the current issue of Nature Nanotechnology. The new technique has many practical implications, especially that repairing a damaged surface with this method would require significantly smaller amounts of material, avoiding the need to coat entire surfaces when only a tiny fraction is cracked, says team leader and UMass Amherst polymer scientist Todd Emrick.

"This is particularly important because even small fractures can then lead to structural failure but our technique provides a strong and effective repair. The need for rapid, efficient coating and repair mechanisms is pervasive today in everything from airplane wings to microelectronic materials to biological implant devices," he adds.

At nano-scale, damaged areas typically possess characteristics quite distinct from their undamaged surrounding surface, including different topography, wetting characteristics, roughness and even chemical functionality, Emrick explains. He adds, "Anna Balazs predicted, using computer simulation, that if nanoparticles were held in a certain type of microcapsule, they would probe a surface and release nanoparticles into certain specific regions of that surface," effectively allowing a spot-repair.

This vision of capsules probing and releasing their contents in a smart, triggered fashion, known as "repair-and-go," is characteristic of biological process, such as in white blood cells, Emrick adds.

He says the experimental work to support the concept required insight into the chemistry, physics and mechanical aspects of materials encapsulation and controlled release, and was achieved by collaboration among three polymer materials laboratories at UMass Amherst, led by Alfred Crosby, Thomas Russell and himself.

The researchers show how using a polymer surfactant stabilizes oil droplets in water (in emulsion droplets or capsules), encapsulating nanoparticles efficiently, but in a manner where they can be released when desired, since the capsule wall is very thin.

"We then found that the nanoparticle-containing capsules roll or glide over damaged substrates, and very selectively deposit their nanoparticle contents into the damaged (cracked) regions. Because the nanoparticles we use are fluorescent, their localization in the cracked regions is clearly evident, as is the selectivity of their localization."

Using rapid and selective deposition of sensor material in damaged regions, their innovative work also provides a precise method for detecting damaged substrates, he stresses. Finally, the new encapsulation techniques allow delivery of hydrophobic objects in a water-based system, further precluding the need for organic solvents in industrial processes that are dis-advantageous from an environmental standpoint.

Emrick says, "Having realized the concept experimentally, looking forward we now hope to demonstrate recovery of mechanical properties of coated objects by adjusting the composition of the nanoparticles being delivered."

The work was supported by the National Science Foundations' (NSF) Materials Research Science and Engineering Center on Polymers at UMass Amherst, an NSF Integrative Graduate Education and Research Traineeship (IGERT) award, the NSF Center for Hierarchical Manufacturing, the U.S. Department of Energy and its Office of Basic Energy Science.

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The above story is reprinted from materials provided by University of Massachusetts Amherst.

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Journal Reference:

Katrina Kratz, Amrit Narasimhan, Ravisubhash Tangirala, SungCheal Moon, Ravindra Revanur, Santanu Kundu, Hyun Suk Kim, Alfred J. Crosby, Thomas P. Russell, Todd Emrick, German Kolmakov, Anna C. Balazs. Probing and repairing damaged surfaces with nanoparticle-containing microcapsules. Nature Nanotechnology, 2012; DOI: 10.1038/nnano.2011.235

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Hydrogen advances graphene use

 Physicists at Linköping University have shown that a dose of hydrogen or helium can render the "super material" graphene even more useful.

Graphene has engendered high expectations whereof its extreme properties depend on the fact that it consists of a single sheet of carbon atoms. However the attraction forces between the atoms cause the sheets to be drawn to each other. One solution is to add atomic hydrogen between the layers.

Presented in the eminent journal Physical Review A, the researchers' calculations show that the hydrogen at a given concentration affects the atomic 'van der Waals forces' and becomes repulsive instead of attractive. The result is that graphene sheets repel each other and float freely just a few nanometres apart (an example of the so-called quantum levitation).

Professor Bo E. Sernelius, who conducted the study with his former doctoral student Mathias Bostrom, identifies several possible applications of the discovery:

Storage of hydrogen as vehicle fuelCreation of a single graphene sheet by peeling them from a pile that has grown on a substrate of silicon carbide; a method developed at Linköping UniversityRepulsive forces are ideal for the manufacture of friction-free components on a Nano scale, for example, robots and sensors for medical purposes

In the present study the researchers began with two 'undoped' sheets of graphene on a substrate of silicon dioxide (silica). The starting position is the van der Waals attractive forces and the sheets are compelled closer together. However once atomic hydrogen is added, repulsive forces arise. A similar effect was observed using other gases such as molecular hydrogen (H2) and helium.

Graphene is a two-dimensional material, which means that it retains a very special character. It is flexible, transparent, stronger than a diamond and has a superior ability to conduct electric current. In 2010 Andre Geim and Konstantin Novoselov received a Nobel Prize in Physics because for the first time ever they succeeded in producing stable flakes of material.

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Journal Reference:

Mathias Boström, Bo E. Sernelius. Repulsive van der Waals forces due to hydrogen exposure on bilayer graphene. Physical Review A, 2012; 85 (1) DOI: 10.1103/PhysRevA.85.012508

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Scientists solve mystery of colorful armchair nanotubes

 Rice University researchers have figured out what gives armchair nanotubes their unique bright colors: hydrogen-like objects called excitons.

Their findings appear in the online edition of the Journal of the American Chemical Society.

Armchair carbon nanotubes -- so named for the "U"-shaped configuration of the atoms at their uncapped tips -- are one-dimensional metals and have no band gap. This means electrons flow from one end to the other with little resistivity, the very property that may someday make armchair quantum wires possible.

The Rice researchers show armchair nanotubes absorb light like semiconductors. An electron is promoted from an immobile state to a conducting state by absorbing photons and leaving behind a positively charged "hole," said Rice physicist Junichiro Kono. The new electron-hole pair forms an exciton, which has a neutral charge.

"The excitons are created by the absorption of a particular wavelength of light," said graduate student and lead author Erik Hároz. "What your eye sees is the light that's left over; the nanotubes take a portion of the visible spectrum out." The diameter of the nanotube determines which parts of the visible spectrum are absorbed; this absorption accounts for the rainbow of colors seen among different batches of nanotubes.

Scientists have realized that gold and silver nanoparticles could be manipulated to reflect brilliant hues -- a property that let artisans who had no notions of "nano" create stained glass windows for medieval cathedrals. Depending on their size, the particles absorbed and emitted light of particular colors due to a phenomenon known as plasma resonance.

In more recent times, researchers noticed semiconducting nanoparticles, also known as quantum dots, show colors determined by their size-dependent band gaps.

But plasma resonance happens at wavelengths outside the visible spectrum in metallic carbon nanotubes. And armchair nanotubes don't have band gaps.

Kono's lab ultimately determined that excitons are the source of color in batches of pure armchair nanotubes suspended in solution.

The results seem counterintuitive, Kono said, because excitons are characteristic of semiconductors, not metals. Kono is a professor of electrical and computer engineering and of physics and astronomy.

While armchair nanotubes don't have band gaps, they do have a unique electronic structure that favors particular wavelengths for light absorption, he said.

"In armchair nanotubes, the conduction and valence bands touch each other," Kono said. "The one-dimensionality, combined with its unique energy dispersion, makes it a metal. But the bands develop what's called a van Hove singularity," which appears as a peak in the density of states in a one-dimensional solid. "So there are lots of electronic states concentrated around this singularity."

Exciton resonance tends to occur around these singularities when hit with light, and the stronger the resonance, the more distinguished the color. "It's an unusual quality of these particular one-dimensional materials that these excitons can actually exist," Hároz said. "In most metals, that's not possible; there's not enough Coulomb interaction between the electron and the hole for an exciton to be stable."

The new paper follows on the heels of work by Kono and his team to create batches of pure single-walled carbon nanotubes through ultracentrifugation. In that process, nanotubes were spun in a mix of solutions with different densities up to 250,000 times the force of gravity. The tubes naturally gravitated toward separated solutions that matched their own densities to create a colorful "nano parfait."

As a byproduct of their current work, the researchers proved their ability to produce purified armchair nanotubes from a variety of synthesis techniques. They now hope to extend their investigation of the optical properties of armchairs beyond visible light. "Ultimately, we'd like to make one collective spectrum that includes frequency ranges all the way from ultraviolet to terahertz," Hároz said. "From that, we can know, optically, almost everything about these nanotubes."

Co-authors of the paper include Robert Hauge, a distinguished faculty fellow in chemistry at Rice; Rice alumnus Benjamin Lu; and professors Pavel Nikolaev and Sivaram Arepalli of Sungkyunkwan University, Suwon, Korea.

The research was supported by the Department of Energy, the Robert A. Welch Foundation, the Air Force Research Laboratory and the World Class University Program at Sungkyunkwan University.

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The above story is reprinted from materials provided by Rice University.

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Journal Reference:

Erik H. Hároz, Benjamin Y. Lu, Pavel Nikolaev, Sivaram Arepalli, Robert H. Hauge, Junichiro Kono. Unique Origin of Colors of Armchair Carbon Nanotubes. Journal of the American Chemical Society, 2012; : 120103131856006 DOI: 10.1021/ja209333m

Atomic Layer Deposition (ALD) to enable novel, high efficiency silicon nanorod solar cells

 Picosun Oy reports successful final results of the European Union 7th Framework Programme funded research project ROD-SOL. The goal of this multinational, inter-European, three years (2009-2011) project combining the efforts of both scientific and industrial partners has been to dramatically increase the efficiency of solar cells and reduce the costs of their manufacturing. This has been achieved with novel, innovative, silicon nanorod based concept. The amount of active photovoltaic material (Si) can be significantly reduced by growing the light-trapping nanorod “forests” (thickness from < 1µm to a few µm at most) on cheaper substrates such as glass or flexible foils. This has led to already promising over 9% energy conversion efficiencies with very good long-term stabilities of cells. Due to their effectively 3D geometry, the nanorod forests have high active surface area which enables efficient light absorption – much more efficient than in convenient 2D thin film solar cells. Also, the location of the p-n junction much closer to the surface than in normal solar cells radically improves the minority carrier charge transport and thus the amount of electricity that can be extracted from the cell.

Due to the micrometer/sub-micrometer dimensions of the nanorod forests (dense packing, rod diameters typically few hundreds of nm and lengths < 1 µm) ALD has proven to be ideal technique for manufacturing some of the most crucial cell components. To prevent recombination losses in the active photovoltaic layer and thus cell efficiency decrease, a recombination barrier i.e. passivation layer needs to be coated on the rods’ surface. An ultrathin ALD-deposited Al2O3 film serves ideally this purpose, and the gas-phase, surface-controlled and self-limiting nature of the ALD process ensures that even the deepest and narrowest between-the-rods nooks and crannies will be reliably covered with 100 % uniform, conformal and pinhole- and defect-free passivation film. Another central cell component where ALD has shown its indispensability is the transparent conductive oxide (TCO) layer that works as the current collector on the top of the cell. Different TCO deposition methods were investigated in the course of the project, and ALD turned out to be the ideal method regarding both the TCO film quality and the scalability of the technique, due to Picosun’s fast, efficient and easy-to-use HVM (High Volume Manufacturing) batch ALD system, which was developed specifically during the project ROD-SOL.