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Nanotubes: left or right?
RESEARCH NEWS
Nanotubes: left or right? NANOTECHNOLOGY The separation of different kinds of carbon nanotubes is important because their electrical, mechanical, and optical properties are closely related to their structure. Now a team of researchers in Japan has taken a significant step toward the preparation of a single type of carbon nanotube by separating nanotube optical isomers for the first time [Peng et al., Nat. Nanotechnol. (2007) doi:10.1038/ nnano.2007.142]. Chromatography can be used to separate nanotubes by size, but little attention has been paid to the fact that the resulting samples have equal distributions of left- or right-handed helices, says Naoki Komatsu from Shiga University of Medical Science, who worked with colleagues at Kyoto University and Osaka ElectroCommunication University. These mirror image forms, or optical isomers, may display different chemical or optical properties. To separate left- from right-handed nanotubes, the team used a chiral surfactant, meta-phenylene-bridged zinc(II) diporphyrins. Mirror image forms of these ‘chiral nanotweezers’ are first synthesized separately. One version is then introduced into a suspension of nanotubes in methanol, where the chiral surfactant forms a soluble complex with the nanotubes. After removing the insoluble component, the surfactant is removed, leaving a solution enriched with one chiral form. Circular dichroism experiments confirm that the resulting sample is optically active. Future work will concentrate on obtaining an ‘optically pure’ sample of nanotubes with only one chirality. Komatsu believes this will enable determination of reliable and precise physical data of the structure of carbon nanotubes.
Pauline Rigby
Superconductors find breaking up hard to do MAGNETIC BEHAVIOR
Despite many years of research, high-temperature superconductors remain an enigma. In conventional superconductors, electrons pair up at the superconducting transition temperature (Tc), creating an energy gap in the electronic density of states. However, in high-Tc superconductors, a partial gap exists above Tc. The question is, is this energy gap associated with pairing and, if so, at what temperature do pairs form? Recent research provides a more detailed picture of this behavior. A team from Princeton University and the Central Research Institute of Electric Power Industry in Japan have performed the first spatially resolved measurements of energy gap formation in the high-Tc superconductor, Bi2Sr2CaCu2O8+δ, as a function of temperature and doping [Gomes et al., Nature (2007) 447, 569]. “We have developed a unique ability to perform spectroscopic measurements at a specific atomic site as a function of temperature,” explains Ali Yazdani of Princeton University. The group used a specially designed, variable temperature, ultrahigh vacuum scanning tunneling microscope (STM) to probe the evolution of electron or Cooper pairs as a function of temperature (and doping) in real space. By varying the energy of the tunneling electrons, the STM can break apart electron pairs. “The key discovery is that pairs appear not to break up until temperatures well above the critical temperature and survive in small puddles (1-3 nm) up to very high temperatures,” says
Map of electron pairs (shown in red) as they form in Bi2Sr2CaCu2O8+δ. From top left, the same 30 nm2 area is shown with decreasing temperature. Even at 10°C above Tc, electron pairs still exist in small regions. (Courtesy of Ali Yazdani.)
Yazdani. The finding that Cooper pairs persist in small regions, even when the entire sample is too warm to exhibit superconductivity, is key to understanding superconductivity. “If we can figure out the details of what is happening at these local patches within the samples, it might be possible to construct a material that performs better overall,” says Yazdani. Cordelia Sealy
How to create tiny metal patterns NANOTECHNOLOGY Growth of metal nanoparticles without an organic shell is of importance in the fabrication of conductive nanowires that require intimate electrical contacts. Metal nanostructures can now be fabricated chemically on surfaces with lyophilic and lyophobic patterns by a technique called wetting driven self-assembly (WDSA), say researchers from The Weizmann Institute of Science in Israel [Chowdhury et al., Nano Lett. (2007) doi: 10.1021/nl070842x]. The new approach can be used to immobilize discrete particles of metal, 2.2 ± 0.5 nm high and 27 ± 6 nm wide on pre-defined surface sites. The metal features obtained are stable, suggesting that the route could be used to confine a wide range of metal species. In a process known as constructive nanolithography (CN), Jacob Sagiv and colleagues self-assemble silane monolayers onto Si substrates, then use a biased scanning tunneling microscope (STM) tip to oxidize -CH3 groups located at the surface of the silane electrochemically. Oxidation gives rise to narrow lines of -COOH groups, creating lyophilic patterns
on a lyophobic background. The modified substrate is then retracted rapidly through a thiol melt at temperatures well above the molecule’s melting point. Under these conditions, wettability drives selective self-assembly of the melt onto the lyophilic areas. No traces of melt are found on the lyophobic background. The melt solidifies on the sample upon exposure to ambient temperature. Self-assembly of melts is not limited to thiols. Any nonvolatile material that has an appropriate melting temperature for the technique and exhibits surface wetting properties could be used. Formation of a clean product facilitates further chemical processing, including oxidation or photoreaction reactions that may be required to produce immobilization sites for the metal. A solution of AgCH3COO is used as a source of metal ions. Once immobilized, the Ag+ ions are reduced to Ag(0), creating elemental nanoparticles. If required, the nanoparticles can be treated with a Ag enhancer to increase their height.
Katerina Busuttil
JULY-AUGUST 2007 | VOLUME 10 | NUMBER 7-8
13
Nanotubes: left or right? NANOTECHNOLOGY The separation of different kinds of carbon nanotubes is important because their electrical, mechanical, and optical properties are closely related to their structure. Now a team of researchers in Japan has taken a significant step toward the preparation of a single type of carbon nanotube by separating nanotube optical isomers for the first time [Peng et al., Nat. Nanotechnol. (2007) doi:10.1038/ nnano.2007.142]. Chromatography can be used to separate nanotubes by size, but little attention has been paid to the fact that the resulting samples have equal distributions of left- or right-handed helices, says Naoki Komatsu from Shiga University of Medical Science, who worked with colleagues at Kyoto University and Osaka ElectroCommunication University. These mirror image forms, or optical isomers, may display different chemical or optical properties. To separate left- from right-handed nanotubes, the team used a chiral surfactant, meta-phenylene-bridged zinc(II) diporphyrins. Mirror image forms of these ‘chiral nanotweezers’ are first synthesized separately. One version is then introduced into a suspension of nanotubes in methanol, where the chiral surfactant forms a soluble complex with the nanotubes. After removing the insoluble component, the surfactant is removed, leaving a solution enriched with one chiral form. Circular dichroism experiments confirm that the resulting sample is optically active. Future work will concentrate on obtaining an ‘optically pure’ sample of nanotubes with only one chirality. Komatsu believes this will enable determination of reliable and precise physical data of the structure of carbon nanotubes.
Pauline Rigby
Superconductors find breaking up hard to do MAGNETIC BEHAVIOR
Despite many years of research, high-temperature superconductors remain an enigma. In conventional superconductors, electrons pair up at the superconducting transition temperature (Tc), creating an energy gap in the electronic density of states. However, in high-Tc superconductors, a partial gap exists above Tc. The question is, is this energy gap associated with pairing and, if so, at what temperature do pairs form? Recent research provides a more detailed picture of this behavior. A team from Princeton University and the Central Research Institute of Electric Power Industry in Japan have performed the first spatially resolved measurements of energy gap formation in the high-Tc superconductor, Bi2Sr2CaCu2O8+δ, as a function of temperature and doping [Gomes et al., Nature (2007) 447, 569]. “We have developed a unique ability to perform spectroscopic measurements at a specific atomic site as a function of temperature,” explains Ali Yazdani of Princeton University. The group used a specially designed, variable temperature, ultrahigh vacuum scanning tunneling microscope (STM) to probe the evolution of electron or Cooper pairs as a function of temperature (and doping) in real space. By varying the energy of the tunneling electrons, the STM can break apart electron pairs. “The key discovery is that pairs appear not to break up until temperatures well above the critical temperature and survive in small puddles (1-3 nm) up to very high temperatures,” says
Map of electron pairs (shown in red) as they form in Bi2Sr2CaCu2O8+δ. From top left, the same 30 nm2 area is shown with decreasing temperature. Even at 10°C above Tc, electron pairs still exist in small regions. (Courtesy of Ali Yazdani.)
Yazdani. The finding that Cooper pairs persist in small regions, even when the entire sample is too warm to exhibit superconductivity, is key to understanding superconductivity. “If we can figure out the details of what is happening at these local patches within the samples, it might be possible to construct a material that performs better overall,” says Yazdani. Cordelia Sealy
How to create tiny metal patterns NANOTECHNOLOGY Growth of metal nanoparticles without an organic shell is of importance in the fabrication of conductive nanowires that require intimate electrical contacts. Metal nanostructures can now be fabricated chemically on surfaces with lyophilic and lyophobic patterns by a technique called wetting driven self-assembly (WDSA), say researchers from The Weizmann Institute of Science in Israel [Chowdhury et al., Nano Lett. (2007) doi: 10.1021/nl070842x]. The new approach can be used to immobilize discrete particles of metal, 2.2 ± 0.5 nm high and 27 ± 6 nm wide on pre-defined surface sites. The metal features obtained are stable, suggesting that the route could be used to confine a wide range of metal species. In a process known as constructive nanolithography (CN), Jacob Sagiv and colleagues self-assemble silane monolayers onto Si substrates, then use a biased scanning tunneling microscope (STM) tip to oxidize -CH3 groups located at the surface of the silane electrochemically. Oxidation gives rise to narrow lines of -COOH groups, creating lyophilic patterns
on a lyophobic background. The modified substrate is then retracted rapidly through a thiol melt at temperatures well above the molecule’s melting point. Under these conditions, wettability drives selective self-assembly of the melt onto the lyophilic areas. No traces of melt are found on the lyophobic background. The melt solidifies on the sample upon exposure to ambient temperature. Self-assembly of melts is not limited to thiols. Any nonvolatile material that has an appropriate melting temperature for the technique and exhibits surface wetting properties could be used. Formation of a clean product facilitates further chemical processing, including oxidation or photoreaction reactions that may be required to produce immobilization sites for the metal. A solution of AgCH3COO is used as a source of metal ions. Once immobilized, the Ag+ ions are reduced to Ag(0), creating elemental nanoparticles. If required, the nanoparticles can be treated with a Ag enhancer to increase their height.
Katerina Busuttil
JULY-AUGUST 2007 | VOLUME 10 | NUMBER 7-8
13