THESE SCROLLS OF GRAPHITE are much too tiny to be seen, but they are stronger than diamonds. CNTs can be shaped in a variety of ways and can act as metals or semiconductors. They offer great potential in nanoelectronics, medicine, sensing and lasers and as strengthening elements in composite materials.
Several obstacles must be overcome before CNTs live up to their expectations. Chief among these is the tendency of CNTs to clump together like strands of angel-hair pasta. Other challenges include achieving a better understanding of CNT structures and more effective ways of processing the tubes, sorting them, placing them on substrates and engineering their properties.
Lehigh, in collaboration with DuPont and MIT, recently received a four-year, $1.25-million grant from NSF to solve these problems by developing new ways of manipulating CNTs in solution.
Lehigh will work with MIT, Cornell and DuPont through NSF’s Nanoscale Interdisciplinary Research Team (NIRT) program. Much of the focus will be on the wrapping of single-walled CNTs with single-stranded DNA, which forms a helix around the nanotubes. The DNA-CNT hybrid has proven effective in CNT dispersion, and researchers hope it will also aid in sorting and placing the tubes.
Several years ago, a DuPont-led group found that DNA strands could be used to separate CNTs according to their electronic characteristics. The discovery was published in Science and cited by Forbes as one of the top five nanotechnology breakthroughs of 2003.
Investigators on the NIRT team include DuPont scientist Ming Zheng; Anand Jagota, formerly of DuPont and now director of Lehigh’s bioengineering and life sciences program; Slava Rotkin of Lehigh’s physics department; Chris Kiely of Lehigh’s Center for Advanced Materials and Nanotechnology; and Yet-Ming Chiang of MIT’s materials science and engineering department.
The two main goals of the NIRT team are to place CNTs on a substrate in specific locations and with specific densities and orientations and to sort a heterogeneous sample of CNTs into constituent types.
To accomplish this, says Jagota, the team will seek to predict the structure of the DNA-CNT hybrid, given the DNA sequence and the CNT type, and to design experiments to control the placement and separation of the CNTs.
To gain greater control over the placing of CNTs on substrates, the researchers will apply a recently discovered technique called quasi-2D liquid crystal formation at a liquidsolid interface.
“If we can do what we’re hoping to do,” says Jagota, “we will have achieved a major advancement in CNT research.”
Kiely, who directs Lehigh’s Nanocharacterization Laboratory, will supervise characterization for the NIRT project. Rotkin will oversee the theoretical work.
Rotkin is seeking to determine whether and to what degree the nanotube structure, specifically its bandgap structure, is altered when the CNT is wrapped by the DNA strand. He is using quantum-field analytical and numerical quantum-mechanical calculations to examine different types of CNTs with different types of DNA wraps.
“The answer to the question – does a CNT wrapped with DNA stay the same or undergo a change in properties? – depends on the symmetry and geometry of the wrap,” says Rotkin.
Rotkin and his students have succeeded in plotting bandgap structure and mapping the areas of varying DNA-induced electric charges, which show up in repeating patterns.
“We more or less understand the rules of nature in regards to whether or not the bandgap structure [of a DNA wrapped CNT] changes,” he says. “But there are a huge number of nanotube types and a huge number of ways of wrapping a CNT.”
