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Fields drive nanotubes round the bend
RESEARCH NEWS
Grafting for nanotube solubility NANOTECHNOLOGY Chinese researchers at the University of Science and Technology of China and Hefei University have made multiwalled carbon nanotubes (MWNTs) soluble in water by grafting polymers onto their surface [Hong et al., Chem. Mater. (2005) 17 (9), 2247]. Water solubility could lead to nanotube-based sensors, probes for biological environments, and drug-delivery agents. Chun-Yan Hong and coworkers used a reversible addition and fragmentation chain transfer (RAFT) polymerization to graft poly(N-isopropylacrylamide), or (PNIPAAM), onto the surface of the MWNTs. The MWNT-g-PNIPAAM products exhibit good solubility in water, chloroform, and tetrahydrofuran. Transmission electron microscopy indicates that the original MWNT bundles separate into dispersed, individual nanotubes. PNIPAAM is a thermosensitive polymer. Below a critical solution temperature of 32ºC, PNIPAAM chains assume a coiled conformation, providing an expanded shell on the surface of the MWNTs. At 35ºC, the PNIPAAM collapses to form a barrierlike shell that is more hydrophobic, and the nanotubes gradually precipitate from aqueous solution. The temperature-dependent behavior can be used to switch the MWNT surface from being hydrophobic to hydrophilic and back. Potential applications could include ‘smart’ sensors and probes, reusable catalysts, drug-delivery agents, and scaffolds for different types of tissue. The researchers suggest that, besides providing surface layers that respond to temperature changes, surface layers that respond to changes in pH or ionic strength could also be grafted onto MWNTs using the RAFT technique. John K. Borchardt
Fields drive nanotubes round the bend NANOTECHNOLOGY
Creating carbon nanotubes with sharp bends by chemical vapor deposition (CVD) is difficult. Once growth is interrupted, the catalyst particles at the nanotube tips become encased with carbon, blocking future growth. Now, researchers at University of California, San Diego have developed a technique to prevent this, creating nanotubes that are bent in sharp, predetermined angles by manipulating the strong alignment of their growth with the direction of electric field lines [AuBuchon et al., J. Phys. Chem. B (2005) 109, 6044]. Nanotubes were grown by plasma-enhanced CVD on Si wafers seeded with Ni catalyst nanoparticles. After growing an aligned array of straight nanotubes, the electric field direction was changed 90° to make L-shaped tubes. Further changes yielded zigzags. “One use might be to improve performance of [probes in] atomic force microscopy,” says principal investigator Sungho Jin. Integrated circuits could be shrunk further by replacing current 70 nm-wide electrical connectors made of metal wires with nanotube connectors as thin as 1.2 nm that are theoretically capable of supplying sufficiently large electric currents. Interconnects between microcircuit devices are often made with metal alloy solder, which expands and contracts at a different rate, so heating and cooling cycles cause fatigue cracking.
An array of carbon nanotubes bent through 90°. (Courtesy of Sungo Jin, UCSD.)
“If these interconnections were made with electrically conducting nanotube zigzags, which also act as springs, not only would we need much less space to make these interconnections, but the thermal-expansion mismatch also wouldn’t matter because the interconnections are flexible,” says Jin. “We call it the compliant nanointerconnect.” By modifying the method, the researchers have also made arrays of parallel, branched T- and Y-shaped nanotubes. The tips could act as high-density three-dimensional scaffolds for Pt catalyst particles in developing more efficient fuel cells. Mark Telford
Nanowires painted onto glass for fast electronics NANOTECHNOLOGY A team at Harvard University has shown that semiconductor nanowires can be painted onto virtually any substrate to make fast, highperformance electronic circuits [Friedman et al., Nature (2005) 434, 1085]. “These advances could bring powerful electronics and computing to virtually all facets of life at low cost, and may open doors for radio frequency tags or high-refresh-rate ‘e-paper’ displays that are fully integrated on a single piece of plastic or glass,” says Charles M. Lieber. The researchers used low-temperature processes to integrate nanowire transistors into logical inverters and fast ring oscillators on glass surfaces. “We have exploited the very reproducible, highperformance properties and solution assembly characteristics of nanowires,” explains Lieber. The circuits operate at frequencies in excess of
10 MHz. In comparison, organic and amorphous Si oscillators are limited to frequencies of ~100 kHz because of their low mobility, while carbon nanotube devices have reached only ~100 Hz through a lack of reproducibility. “Nanowires, on the other hand, combine the best of both worlds – high speed and reproducibility – to allow us to reliably create fast electronics on glass and plastic,” says Lieber. The devices currently require a supply voltage of 35 V for stable oscillation, but this can be improved by using higher-k dielectrics, more advanced nanowires materials, and shorter channel lengths. “These results should be appealing to both scientists and average consumers alike in providing a new outlook in the race towards lightweight and portable electronics,” says Lieber. Jonathan Wood
June 2005
13
Grafting for nanotube solubility NANOTECHNOLOGY Chinese researchers at the University of Science and Technology of China and Hefei University have made multiwalled carbon nanotubes (MWNTs) soluble in water by grafting polymers onto their surface [Hong et al., Chem. Mater. (2005) 17 (9), 2247]. Water solubility could lead to nanotube-based sensors, probes for biological environments, and drug-delivery agents. Chun-Yan Hong and coworkers used a reversible addition and fragmentation chain transfer (RAFT) polymerization to graft poly(N-isopropylacrylamide), or (PNIPAAM), onto the surface of the MWNTs. The MWNT-g-PNIPAAM products exhibit good solubility in water, chloroform, and tetrahydrofuran. Transmission electron microscopy indicates that the original MWNT bundles separate into dispersed, individual nanotubes. PNIPAAM is a thermosensitive polymer. Below a critical solution temperature of 32ºC, PNIPAAM chains assume a coiled conformation, providing an expanded shell on the surface of the MWNTs. At 35ºC, the PNIPAAM collapses to form a barrierlike shell that is more hydrophobic, and the nanotubes gradually precipitate from aqueous solution. The temperature-dependent behavior can be used to switch the MWNT surface from being hydrophobic to hydrophilic and back. Potential applications could include ‘smart’ sensors and probes, reusable catalysts, drug-delivery agents, and scaffolds for different types of tissue. The researchers suggest that, besides providing surface layers that respond to temperature changes, surface layers that respond to changes in pH or ionic strength could also be grafted onto MWNTs using the RAFT technique. John K. Borchardt
Fields drive nanotubes round the bend NANOTECHNOLOGY
Creating carbon nanotubes with sharp bends by chemical vapor deposition (CVD) is difficult. Once growth is interrupted, the catalyst particles at the nanotube tips become encased with carbon, blocking future growth. Now, researchers at University of California, San Diego have developed a technique to prevent this, creating nanotubes that are bent in sharp, predetermined angles by manipulating the strong alignment of their growth with the direction of electric field lines [AuBuchon et al., J. Phys. Chem. B (2005) 109, 6044]. Nanotubes were grown by plasma-enhanced CVD on Si wafers seeded with Ni catalyst nanoparticles. After growing an aligned array of straight nanotubes, the electric field direction was changed 90° to make L-shaped tubes. Further changes yielded zigzags. “One use might be to improve performance of [probes in] atomic force microscopy,” says principal investigator Sungho Jin. Integrated circuits could be shrunk further by replacing current 70 nm-wide electrical connectors made of metal wires with nanotube connectors as thin as 1.2 nm that are theoretically capable of supplying sufficiently large electric currents. Interconnects between microcircuit devices are often made with metal alloy solder, which expands and contracts at a different rate, so heating and cooling cycles cause fatigue cracking.
An array of carbon nanotubes bent through 90°. (Courtesy of Sungo Jin, UCSD.)
“If these interconnections were made with electrically conducting nanotube zigzags, which also act as springs, not only would we need much less space to make these interconnections, but the thermal-expansion mismatch also wouldn’t matter because the interconnections are flexible,” says Jin. “We call it the compliant nanointerconnect.” By modifying the method, the researchers have also made arrays of parallel, branched T- and Y-shaped nanotubes. The tips could act as high-density three-dimensional scaffolds for Pt catalyst particles in developing more efficient fuel cells. Mark Telford
Nanowires painted onto glass for fast electronics NANOTECHNOLOGY A team at Harvard University has shown that semiconductor nanowires can be painted onto virtually any substrate to make fast, highperformance electronic circuits [Friedman et al., Nature (2005) 434, 1085]. “These advances could bring powerful electronics and computing to virtually all facets of life at low cost, and may open doors for radio frequency tags or high-refresh-rate ‘e-paper’ displays that are fully integrated on a single piece of plastic or glass,” says Charles M. Lieber. The researchers used low-temperature processes to integrate nanowire transistors into logical inverters and fast ring oscillators on glass surfaces. “We have exploited the very reproducible, highperformance properties and solution assembly characteristics of nanowires,” explains Lieber. The circuits operate at frequencies in excess of
10 MHz. In comparison, organic and amorphous Si oscillators are limited to frequencies of ~100 kHz because of their low mobility, while carbon nanotube devices have reached only ~100 Hz through a lack of reproducibility. “Nanowires, on the other hand, combine the best of both worlds – high speed and reproducibility – to allow us to reliably create fast electronics on glass and plastic,” says Lieber. The devices currently require a supply voltage of 35 V for stable oscillation, but this can be improved by using higher-k dielectrics, more advanced nanowires materials, and shorter channel lengths. “These results should be appealing to both scientists and average consumers alike in providing a new outlook in the race towards lightweight and portable electronics,” says Lieber. Jonathan Wood
June 2005
13