A new study by researchers from Rice University and colleagues at Los Alamos National Laboratory in the US has shown how to produce highly aligned, wafer-scale films based on a straightforward filtration process, a breakthrough that could lead to the development of new electronic and photonic devices. The flexible, inch-wide films are of densely packed, chirality-enriched, single-walled carbon nanotubes (CNTs), cylinders of graphene with its atoms organized in hexagons. It is how these hexagons are turned that specifies the tube’s chirality, thus determining its electronic properties.

As presented in Nature Nanotechnology [He et al. Nat. Nanotechnol. (2016) DOI: 10.1038/nnano.2016.44], the process depends on the correct solution of CNTs, and under the right conditions. When this happen, millions of the tubes assemble themselves into long rows that are more effectively aligned than achieved previously. The approach works for nanotubes synthesized by various methods, and film thickness is controllable.

"They formed what is called a monodomain in liquid crystal technology, in which all the rigid molecules line up in the same direction"Wade Adams

The researchers hope to develop computer chips that are bendable as opposed to brittle silicon, although the monodomain films they have produced are “chirality-enriched” and not single-chirality. However, as CNTs grow in batches of random types, they separated the nanotubes by chirality using a simple process to produce enriched films with nanotubes of different types and diameters, before making terahertz polarizers and electronic transistors.

They had discovered the filtration technique by serendipitously adding too much water to a nanotube-surfactant suspension, and then feeding it through a filter helped by vacuum. On assessing the resulting film by scanning electron microscope, it was found that, rather than dropping randomly onto the paper, millions of the nanotubes clumped together in tight and aligned rows. This showed something unusual was happening, provoking them into spending another year and over a 100 films to refine their approach to produce nanotube wafers of up to an inch wide and of any thickness.

Each element is significant: the type of filter paper and the vacuum pressure, as well as the concentration of nanotubes and surfactant. To explore why the CNTs line up in this way, they are continuing to look at the mechanics of how the first few nanotubes on the paper combine. With Van der Waals force bringing them together, and they look for their lowest-energy state, that of alignment. As the CNTs vary in length, the overhangs could force other tubes to line up on joining the array. The films can be separated from the paper, and then washed and dried for use, with the final films able to be patterned using lithography.