Segregating nanotubes

Single-walled carbon nanotubes (SWNTs) are contending to augment or replace silicon as the semiconducting layer of the ever-shrinking field-effect transistor. However, SWNT synthesis produces a mixture of semiconducting and metallic nanotubes; the metallic ones cause transistors to short. Separation techniques, such as burning off or electrically shocking the metallic encroachers, are often multi-step, unscalable processes. In a new technique, Zhenan Bao’s group at Stanford University in collaboration with researchers at the Samsung Advanced Institute of Technology in South Korea, attach amine-terminated silane molecules to a silicon wafer’s silicon dioxide surface layer. Then thin-film transistors are made in a single step by spin-coating an SWNT solution onto the prepared wafer: Semiconducting nanotubes bind to the amine groups (left image), and metallic ones spin off. In a separate experiment, metallic nanotubes were shown to bind selectively with phenyl-terminated silane molecules on the SiO₂ dielectric surface (right image). Measurements between the source and drain of amine-modified SWNT transistors revealed high current flow when on and a leakage current in the pico-amp range. Atomic force microscopy also confirmed that spin-coating conditions can be tuned to control film density and nanotube alignment. The next move for the researchers is to pattern a complete microcircuit by positioning amine and phenyl groups on flexible or rigid substrates to form semiconducting transistor networks connected by metallic nanotube wires. (M. C. LeMieux et al., Science 321 , 101, 2008 http://dx.doi.org/10.1126/science.1156588 .)