New publications, experiments, and breakthroughs – and what they mean.
Carbon nanotubes stretch out
Context: Little more than a nanometer wide, carbon nanotubes have become superstars of the nano world: unusually strong, electrically conductive, and stable at high temperatures. Fibers composed of nanotubes should outperform those made from any existing material. However, the length of the tubes – most are only tenths of a millimeter long – requires that they be lined up for peak performance. Now, researchers at Los Alamos National Laboratory and Duke University have created nanotubes that are centimeters long, and whose length is checked only by the size of the chamber used to create them.
Methods and Results: The Los Alamos team synthesized the nanotubes by flowing ethanol vapors at 900 degreesC over an iron catalyst spotted onto a silicon wafer. Tubes grew from these catalyst spots; the catalyst was pushed along the wafer surface in the direction of the gas flow. The longest tubes grew to four centimeters as straight lines across the length of the silicon wafer, terminating only at the wafer’s edge.
Why it matters: Bundles of carbon nanotubes, spun as fi bers, have been promoted for applications where high strength and low weight are critical, from sporting equipment like golf clubs or tennis rackets to science fiction dreams of “elevators” extending into outer space. Although the shorter tubes have many promising applications in their own right, bundles of them have failed to perform up to their potential because of weak links between the tubes. Lengthening the tubes reduces these problems, bringing researchers closer to exploiting the remarkable strength and conductivity of nanotube bundles. But the Los Alamos and Duke researchers have done more than advance a technology; they have done the unthinkable, building individual molecules as long as a paper clip.
Source: Zheng, L. X. et al. (2004) Ultralong single-wall carbon nanotubes. Nature Materials 3:673-6.
A friendlier route to zeolites
Context: Minerals called zeolites are essential to industrial chemistry because they help convert crude petroleum into useful chemicals, including the materials used in plastics. By dramatically reducing the cost of petrochemicals, zeolites make everything from pills to pocket protectors more aff ordable. Now researchers at the University of St. Andrews in Scotland have discovered a way to make these nanostructured minerals that is not only cheaper but also faster, safer, and less toxic.
Methods and Results: Zeolites are typically made in hot water at dangerously high pressures. The minerals are riddled with nanometer-wide pores; molecules tucked inside these pores react quickly and cleanly. Chemists create the zeolites through a “condensation reaction,” during which mineral precursors encapsulate molecules added as templates, forming a porous solid. Instead of making zeolites in water, Emily Cooper, a chemistry postdoc at St. Andrews, and her colleagues used liquid salts at a relatively low temperature. These liquids are made of charged molecules, or ions, so mineral precursors condense around them directly, eliminating the need for templates. Afterward, the salt ions are removed, leaving a structure with nanometer-sized holes. The recipe yielded five new nanoporous materials; two represented classes that had never been seen before.
Why it matters: The standard process for making zeolites is expensive and dangerous, and it requires specialized equipment. With the new technique, even a high-school laboratory should be able to make them. The millions of possible salt compositions produced through this process could result in the creation of families of zeolites with entirely new functions, leading to better and cheaper everyday products.
Source: Cooper, E. R. et al. (2004) Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues. Nature 430:1012-6.
Gotta Look Sharp
Atomic force microscopy makes electrical measurements
Context: The rate of corrosion in devices like batteries and semiconductors is often dictated by nanometer-sized imperfections. Conducting atomic force microscopes (AFMs) can image these nanoflaws, but accurately measuring their electrical properties requires knowing how much of the microscope’s sharp conductive tip comes in contact with the active surface. Using a mathematical model, Ryan O’Hayre, an assistant professor in the Stanford University Department of Mechanical Engineering, and his colleagues have found a way to indirectly measure this contact area, overcoming a limit of conductive tip microscopy and improving quality control.
Methods and Results: Researchers used a platinum-coated AFM tip to monitor the reaction between hydrogen and oxygen at the surface of a polymer fuel cell membrane; the fuel cell was chosen to show that nanoscale measurements can correlate with macroscale results. The rate of the reaction depends on how much force the tip applies to the membrane: the force pushes the materials together, causing them to deform slightly, and thus increases the area of interaction between the two. Crucially, the researchers showed that the area of interaction can be estimated by determining the hardness of the membrane, accompanied by a few assumptions and mathematical tricks. The researchers experimented across three orders of magnitude of force between tip and sample, and their results were all consistent with conventional experiments, making them more credible.
Why it matters: Conducting AFM can give nanoscale resolution to electrical measurements of semiconductors, fuel cells, batteries, and other devices. But while it was possible to measure relative changes in properties like conductivity, capacitance, and impedance across the surface of a single sample of material, comparing such measurements between materials had been impossible. Conducting AFM, while capable of fi nding flaws, could not measure their absolute severity, since different materials interacted with the AFM tip in diff erent ways. This refinement may convert conductive AFM from a research instrument into a useful tool in a number of industries.
Source: O’Hayre, R. et al. (2004) Quantitative impedance measurement using atomic force microscopy. Journal of Applied Physics 96:3540-9.