Single-walled carbon nanotubes (SWCNTs) used to be the darling of those who were looking for an alternative to silicon in digital electronics. The first SWCNT-based transistors were fashioned almost twenty years ago, but scaling up the use of SWCNTs since then to very large scale integration (VLSI) processes has remained elusive.
There were two persistent problems with SWCNTs that led to much of the research community pursuing graphene instead of SWCNTs as the next great post-silicon hope: an inconsistency between semiconducting and metallic nanotubes and the frustration of trying to get all of the nanotubes to align on a wafer.
Now researchers at Rice University claim that they have struck upon a method that produces a uniform and wafer-scale film of highly aligned and densely packed SWCNTs that may finally deliver on the long-promised potential of SWCNTs.
In research published in the journal Nature Nanotechnology, the Rice researchers’ method starts by preparing a well-dispersed CNT suspension, which requires getting just the right concentration of CNT powder with a surfactant in water. The next step involves a vacuum filtration method that has long been the established technique for creating wafer-scale films of CNTs with controllable thickness. The CNT suspension is poured into a filtration funnel with small pores. Pressure pushes the suspension through those pores so that CNTs are left behind on the filter membrane.
The SWCNTs spontaneously align as long as both the surfactant level in the dispersion and the CNT concentration are just right and the filtration process is done slowly and carefully. When these criteria are met, a wafer-scale, uniform and aligned SWCNT film forms on the filter membrane.
The film can be easily transferred onto a substrate by dissolving the filter membrane on the substrate, which leaves perfectly aligned SWCNTs in place. In addition to the problem of alignment, many methods that have been used for aligning SWCNTs result in low density. However, in this method the density is quite high with 1×106 CNTs found in a cross-sectional area of 1 square micrometer. Finally, the film can be patterned by standard photolithography methods.
The researchers have put the resulting material to the test by producing terahertz/infrared polarizers using a mix of metallic and semiconductor CNTs; and they fabricated thin-film transistors, polarized light-emission devices and polarization-sensitive photodetectors using only semiconducting CNTs.
The Rice team believes that this method should create not only new avenues for fundamental research in physics, chemistry and materials science, but will also enable the use of SWCNTs in electronics, optoelectronics, sensing, imaging and medicine.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.
