A team of U.S. scientists has discovered that by tightly packing molecules, they could obtain nanothin sheets of materials showing surprising strength. As one enthusiast researcher says, "It's an amazing little marvel." The team adds that "even when suspended over a tiny hole and poked with an ultrafine tip, the membrane boasts the equivalent strength of an ultrathin sheet of plexiglass that maintains its structural integrity at relatively high temperatures." This could lead to highly sensitive pressure sensors in precision technological applications.
You can see on the left three atomic force microscope (AFM) images of a 2-μm-diameter membrane at different temperatures. "a, At 298 K; after force–displacement measurements were taken. b, At 373 K; the array did not melt nor tear even though a weak spot, and possibly a small hole, appeared where the force measurements stressed the array. c, At 413 K; the array has failed by ripping. Nevertheless, large portions of it still appear intact and stretched flat across the hole." (Credit: Klara Mueggenburg and Heinrich Jaeger, University of Chicago) Here is a link to a larger version of these pictures.
These ultrathin but extrastrong materials have been created by Heinrich Jaeger, a professor of physics at the University of Chicago, and his research group. His team also worked with Xiao-Min Lin, a physicist at Argonne National Laboratory’s Center for Nanoscale Materials.
Now, let's discover how the scientists did their experiments. "The experimental material consisted of gold particles separated by organic 'bumpers' to keep them from coming into direct contact. The research team suspended this array of nanoparticles in a solution, then spread the solution across a small chip of silicon, a popular semiconductor material. When the solution dried, it left behind a blanket of nanoparticles that drape themselves over holes in the chip, each hole measuring hundreds of nanoparticles in diameter. Then the researchers probed the strength of the freely suspended nanoparticle layer by poking it with the tip of an atomic force microscope."
It's also interesting to note how this new material compares with plexiglass. "Plexiglass draws its strength from the nature of its polymers, long chains of molecules that become entangled with one another. But the short-chain polymers the research group used to link the nanoparticles were scarcely long enough to qualify as polymers at all. 'They probably do not have the chance to entangle like a 'card-carrying' polymer would do,' Jaeger said. 'The molecules are anchored to the gold particles, but only on one end. The strength comes from compressing them between the gold particles.'"
This research work has been published by Nature Materials as an advance online publication under the name "Elastic membranes of close-packed nanoparticle arrays" (July 22, 2007). Here are some excerpts of the -- long -- abstract. "Nanoparticle superlattices are hybrid materials composed of close-packed inorganic particles separated by short organic spacers. Most work so far has concentrated on the unique electronic, optical and magnetic behaviour of these systems. Here, we demonstrate that they also possess remarkable mechanical properties. [... Their] remarkable strength is coupled with high flexibility, enabling the membranes to bend easily while draping over edges. The arrays remain intact and able to withstand tensile stresses up to temperatures around 370 K. The purely elastic response of these ultrathin membranes, coupled with exceptional robustness and resilience at high temperatures should make them excellent candidates for a wide range of sensor applications."
Besides sensors, the scientists think these nanoparticles might also serve as building blocks in assembling specially designed nanostructures. This sounds pretty vague to me.
Sources: University of Chicago news release, July 23, 2007; and various websites
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