Researchers at the University of Groningen (Netherlands), the University of Wuppertal (Germany) and IBM Zurich have developed a method for selecting semiconducting nanotubes from a solution and having them self-assemble on a circuit of gold electrodes, according to a report on the Groningen website.
Carrier with nanotube transistors. (Sylvia Germes)
The self-assembled transistors, which have nearly 100 percent purity and high electron mobility according to the report, are the result of a decade’s worth of research into how polymers connect to carbon nanotubes (rolled sheets of graphene).
The researchers worked with thiol side chains with the idea of having the polymer-wrapped nanotubes directed towards gold electrodes by sulfur binding to the metal. This process was patented by IBM, but Groningen researchers saw that it also connected metallic nanotubes to the transistors, which made them unusable.
Now, the scientists are using polymers with low concentrations of thiols to selectively bring semiconducting nanotubes onto the circuit. The article added, “The sulphur-gold bond is strong, so the nanotubes are firmly fixed: enough even to stay there after sonication of the transistor in organic solvents.”
The article continued, “The production process is simple: metallic patterns are deposited on a carrier, which is then dipped into a solution of carbon nanotubes. The electrodes are spaced to achieve proper alignment.”
Currently, transistor density is limited because nanotubes are spaced 500 nanometer apart, but the scientists are working on engineering different designs and developing possible applications for this technology.
The research was recently published in Advanced Materials. The abstract from the report said:
“In this paper, the fabrication of carbon nanotubes field effect transistors by chemical self-assembly of semiconducting single walled carbon nanotubes (s-SWNTs) on prepatterned substrates is demonstrated. Polyfluorenes derivatives have been demonstrated to be effective in selecting s-SWNTs from raw mixtures. In this work the authors functionalized the polymer with side chains containing thiols, to obtain chemical self-assembly of the selected s-SWNTs on substrates with prepatterned gold electrodes.
“The authors show that the full side functionalization of the conjugated polymer with thiol groups partially disrupts the s-SWNTs selection, with the presence of metallic tubes in the dispersion. However, the authors determine that the selectivity can be recovered either by tuning the number of thiol groups in the polymer, or by modulating the polymer/SWNTs proportions.
“As demonstrated by optical and electrical measurements, the polymer containing 2.5% of thiol groups gives the best s-SWNT purity. Field-effect transistors with various channel lengths, using networks of SWNTs and individual tubes, are fabricated by direct chemical self-assembly of the SWNTs/thiolated-polyfluorenes on substrates with lithographically defined electrodes. The network devices show superior performance (mobility up to 24 cm2 V−1 s−1), while SWNTs devices based on individual tubes show an unprecedented (100%) yield for working devices.
“Importantly, the SWNTs assembled by mean of the thiol groups are stably anchored to the substrate and are resistant to external perturbation as sonication in organic solvents.”