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To the beat of tiny drums
RESEARCH NEWS
To the beat of tiny drums
Nanorods change their stripes
NANOTECHNOLOGY Freestanding, one-nanoparticle-thick membranes have been made by researchers at The University of Chicago and Argonne National Laboratory. The nanoparticle superlattices are made of a standard mix of inorganic nanoparticles with organic spacers, but in contrast to much prior work, a high density of nanoparticles is separated by only short spacers [Mueggenburg et al., Nat. Mater. (2007) doi: 10.1038/ nmat1965]. The team investigated the mechanical stability of the layers stretched across holes. Surprisingly, the self-assembled membranes not only survived the stresses, but stood up to deformation even at temperatures near 100°C. The group found that interactions between the organic ligands provide the tensile strength needed for such thin, freestanding membranes. Heinrich M. Jaeger of The University of Chicago, who led the study, likens them more to ‘glue’ than ‘Velcro’. Not only do the membranes form easily, they’re also robust, as atomic force microscope (AFM) tests show. The membranes have an elastic response up to a deformation equal to the membrane thickness, withstand 1.5% strain, and suffer no
NANOTECHNOLOGY
False-color transmission electron micrograph of nanoparticle membranes on a Si substrate. Light green represents the hole area. (Courtesy of Heinrich Jaeger.) damage, even after repeated tests. Among 19 membranes, the average Young’s modulus was 6 GPa. “Nanoparticles offer the opportunity to assemble new types of solids, where the particles take on the role of ‘artificial atoms’,” says Jaeger. Like a nano-sized drum, the membranes could serve as sensitive pressure or acceleration sensors.
D. Jason Palmer
Carbon nanotubes show no sign of fatigue CARBON
This behavior is similar to It is readily agreed that that observed in muscle carbon nanotubes (CNTs) and soft tissue, which are tough customers, but a undergo large deformations team of researchers from of up to 10% strain levels the US and China has now while still retaining their carried out the first major original structural integrity study to quantify the over their life time. In order fatigue resistance of CNT to test the fatigue lifetime structures [Suhr et al., Nat. (Left) A 2 mm square block of vertically aligned, of the CNT block, it was Nanotechnol. (2007) 2, multiwalled nanotubes. (Right) The same block after subjected to half a million 417]. The results show that cycles of mechanical the ability of CNTs to resist being compressed and released more than 500 000 times. (Credit: Rensselaer/ Victor Pushparaj.) compression and release. wear and tear is similar to The structural integrity and properties of the block the behavior of muscles, stomach lining, and other soft remained unaltered, except for a minimal change in tissues. This ability, coupled with the strong electrical the structure of the original sample. conductivity of CNTs, suggests that they could be used By combining these types of nanotube blocks to create artificial muscles. with electroactive polymers, the researchers The team created a free-standing ~3 mm square block hope to develop, for example, robotic arms or made up of millions of vertically aligned, 2.5 mm tendons where there is a requirement to lift large tall, multiwalled nanotubes. The block was placed weights using simple machines. “Apart from between two steel plates and subjected to mechanical biosynthetic material applications, they also find compression and release. The researchers found usefulness in electromechanical applications like that the block behaves like a sponge or spring when pressure sensors based on combining both electrical compressed, reducing its size. On releasing the load and mechanical properties,” says Victor Pushparaj, or strain, the block returns to its original shape. This joint lead researcher from Rensselaer Polytechnic shows that the nanotubes are supercompressible and Institute. exhibit a cushioning effect. The nanotube block can Catherine Reinhold withstand very high strains of over 70%.
10
SEPTEMBER 2007 | VOLUME 10 | NUMBER 9
While regularly patterned nanorods or quantum dot arrays have been made before, typical approaches involve laborious layer-by-layer methods such as epitaxy. In contrast, a spontaneous reaction to create periodic CdS nanorod/Ag2S quantum dot structures by partial cation exchange has been reported by researchers at the University of California, Berkeley and Lawrence Berkeley National Laboratory [Robinson et al., Science (2007) 317, 355]. The group has previously shown complete and reversible exchange of this kind in a variety of nanocrystals, in which the resulting fully exchanged nanocrystals maintain their starting morphology. The rest of the trick is in strain engineering to favor a particular patterning result. CdS nanorods are added to a solution of toluene, methanol, and AgNO3. In a progressive cation-by-cation exchange, small and randomly spaced regions of Ag2S form at the surface of the nanorods. In an excess of Ag ions, the exchange continues to give nanorods of pure Ag2S. But with enough Ag ions to yield just 36% exchange, the result is a superlattice of Ag2S quantum dots along the length of the CdS rods – a ‘striped’ superlattice. Ab initio calculations show that beyond the initial, random exchange, growth of larger Ag2S regions is favored over smaller ones. The lattice mismatch of the adjoining Ag2S and CdS regions results in a repulsive, elastic force between the successive Ag2S sections, so that they remain regularly spaced. The solution-phase reaction is a cheap and easily scalable alternative to epitaxial approaches. However, a lot of work remains to be done to match the capabilities of epitaxial methods.
D. Jason Palmer
To the beat of tiny drums
Nanorods change their stripes
NANOTECHNOLOGY Freestanding, one-nanoparticle-thick membranes have been made by researchers at The University of Chicago and Argonne National Laboratory. The nanoparticle superlattices are made of a standard mix of inorganic nanoparticles with organic spacers, but in contrast to much prior work, a high density of nanoparticles is separated by only short spacers [Mueggenburg et al., Nat. Mater. (2007) doi: 10.1038/ nmat1965]. The team investigated the mechanical stability of the layers stretched across holes. Surprisingly, the self-assembled membranes not only survived the stresses, but stood up to deformation even at temperatures near 100°C. The group found that interactions between the organic ligands provide the tensile strength needed for such thin, freestanding membranes. Heinrich M. Jaeger of The University of Chicago, who led the study, likens them more to ‘glue’ than ‘Velcro’. Not only do the membranes form easily, they’re also robust, as atomic force microscope (AFM) tests show. The membranes have an elastic response up to a deformation equal to the membrane thickness, withstand 1.5% strain, and suffer no
NANOTECHNOLOGY
False-color transmission electron micrograph of nanoparticle membranes on a Si substrate. Light green represents the hole area. (Courtesy of Heinrich Jaeger.) damage, even after repeated tests. Among 19 membranes, the average Young’s modulus was 6 GPa. “Nanoparticles offer the opportunity to assemble new types of solids, where the particles take on the role of ‘artificial atoms’,” says Jaeger. Like a nano-sized drum, the membranes could serve as sensitive pressure or acceleration sensors.
D. Jason Palmer
Carbon nanotubes show no sign of fatigue CARBON
This behavior is similar to It is readily agreed that that observed in muscle carbon nanotubes (CNTs) and soft tissue, which are tough customers, but a undergo large deformations team of researchers from of up to 10% strain levels the US and China has now while still retaining their carried out the first major original structural integrity study to quantify the over their life time. In order fatigue resistance of CNT to test the fatigue lifetime structures [Suhr et al., Nat. (Left) A 2 mm square block of vertically aligned, of the CNT block, it was Nanotechnol. (2007) 2, multiwalled nanotubes. (Right) The same block after subjected to half a million 417]. The results show that cycles of mechanical the ability of CNTs to resist being compressed and released more than 500 000 times. (Credit: Rensselaer/ Victor Pushparaj.) compression and release. wear and tear is similar to The structural integrity and properties of the block the behavior of muscles, stomach lining, and other soft remained unaltered, except for a minimal change in tissues. This ability, coupled with the strong electrical the structure of the original sample. conductivity of CNTs, suggests that they could be used By combining these types of nanotube blocks to create artificial muscles. with electroactive polymers, the researchers The team created a free-standing ~3 mm square block hope to develop, for example, robotic arms or made up of millions of vertically aligned, 2.5 mm tendons where there is a requirement to lift large tall, multiwalled nanotubes. The block was placed weights using simple machines. “Apart from between two steel plates and subjected to mechanical biosynthetic material applications, they also find compression and release. The researchers found usefulness in electromechanical applications like that the block behaves like a sponge or spring when pressure sensors based on combining both electrical compressed, reducing its size. On releasing the load and mechanical properties,” says Victor Pushparaj, or strain, the block returns to its original shape. This joint lead researcher from Rensselaer Polytechnic shows that the nanotubes are supercompressible and Institute. exhibit a cushioning effect. The nanotube block can Catherine Reinhold withstand very high strains of over 70%.
10
SEPTEMBER 2007 | VOLUME 10 | NUMBER 9
While regularly patterned nanorods or quantum dot arrays have been made before, typical approaches involve laborious layer-by-layer methods such as epitaxy. In contrast, a spontaneous reaction to create periodic CdS nanorod/Ag2S quantum dot structures by partial cation exchange has been reported by researchers at the University of California, Berkeley and Lawrence Berkeley National Laboratory [Robinson et al., Science (2007) 317, 355]. The group has previously shown complete and reversible exchange of this kind in a variety of nanocrystals, in which the resulting fully exchanged nanocrystals maintain their starting morphology. The rest of the trick is in strain engineering to favor a particular patterning result. CdS nanorods are added to a solution of toluene, methanol, and AgNO3. In a progressive cation-by-cation exchange, small and randomly spaced regions of Ag2S form at the surface of the nanorods. In an excess of Ag ions, the exchange continues to give nanorods of pure Ag2S. But with enough Ag ions to yield just 36% exchange, the result is a superlattice of Ag2S quantum dots along the length of the CdS rods – a ‘striped’ superlattice. Ab initio calculations show that beyond the initial, random exchange, growth of larger Ag2S regions is favored over smaller ones. The lattice mismatch of the adjoining Ag2S and CdS regions results in a repulsive, elastic force between the successive Ag2S sections, so that they remain regularly spaced. The solution-phase reaction is a cheap and easily scalable alternative to epitaxial approaches. However, a lot of work remains to be done to match the capabilities of epitaxial methods.
D. Jason Palmer