How to separate out semiconducting carbon nanotubes

August 17, 2016

Artistic rendition of a metallic carbon nanotube being pulled into solution, in analogy to the work described by the Adronov group (credit: Alex Adronov, McMaster University)

Researchers at McMaster University in Canada have developed a radically improved way to purify single-wall carbon nanotubes (SWNTs) — flexible structures that are one nanometer in diameter and thousands of times longer, ­and that may revolutionize computers and electronics, replacing silicon.

To do that, we need to separate out semiconducting (sc-SWNTs) and metallic (m-SWNTs) nanotubes. That’s a challenging problem, because both are created simultaneously in the process* of producing carbon nanotubes.

“Once we have a reliable source of pure nanotubes that are not very expensive, a lot can happen very quickly,” says Alex Adronov, a professor of Chemistry at McMaster whose research team has developed a new and potentially cost-efficient way to purify carbon nanotubes.

Separating out semiconducting carbon nanotubes

Previous researchers have created polymers that could allow semiconducting carbon nanotubes to be dissolved and washed away, leaving metallic nanotubes behind, but there has not been such a process for doing the more-useful opposite: dispersing the metallic nanotubes and leaving behind the valuable semiconducting structures.

Single-wall carbon nanotube (credit: NASA)

Now, Adronov’s research group has reversed the electronic characteristics (from electron-rich to electron-poor) of a polymer known to disperse semiconducting nanotubes, while leaving the rest of the polymer’s structure intact. That is, they have reversed the purification process — leaving the semiconducting nanotubes behind while making it possible to disperse the metallic nanotubes.**

The next step, he explains, is for his team or other researchers to exploit the discovery by finding a way to develop even more efficient polymers and scale up the process for commercial production.

The unique properties of SWNTs — high ten­sile strength, the high aspect ratio, thermal and electrical conductivity, and extraordinary optical characteristics — could make carbon nanotubes potentially valuable as advanced materials in a variety of applications, including “field-effect transistors, photovoltaics, flexible electronics, sensors, touch screens, high-strength fibers, biotech­nological constructs, and various other devices,” the researchers note in the current cover story of Chemistry – A European Journal.

Financial support for this work was provided by the Discovery and Strategic Grant programs of the Natural Science and Engi­neering Research Council (NSERC) of Canada.

* These processes include high-pressure carbon monoxide disproportionation (HiPCO),carbon vapor deposition (CVD),arc discharge,laser ablation, and plasma torch growth.

** “We expect that relatively electron-poor con­jugated polymers should disperse m-SWNTs to a greater extent when compared to structurally similar electron-rich conjugated polymers. Here, we demonstrate this concept through the comparison of a poly(fluorene-co-pyridine) conjugated polymer before and after post-polymerization functionalization. By par­tially methylating the pyridine units, cationic charges are intro­duced onto the conjugated backbone, which convert the polymer from being electron-rich to electron-poor. This enables the comparison of two polymers that are identical in length and polydispersity, and differ primarily in their electronic character­istics. We show that the electron-poor conjugated polymer re­sults in dispersions that are enriched in m-SWNTs, while the electron-rich counterpart solely selects for sc-SWNTs, thus pro­viding evidence that the electronic structure of a conjugated polymer plays an important role in determining its selectivity for different SWNT types.” — Darryl Fong et al./Chemistry – A European Journal.

Abstract of Influence of Polymer Electronics on Selective Dispersion of Single-Walled Carbon Nanotubes

In the pursuit of next-generation polymers for the selective dispersion and purification of single-walled carbon nanotubes (SWNTs), understanding the key parameters dictating polymer selectivity is imperative. Simple modification of a poly(fluorene-co-pyridine) backbone, such that it is transformed from being electron-rich to -poor, has a significant impact on the electronic nature of the SWNTs dispersed. The unmodified copolymer bearing an electron-rich fluorene co-monomer preferentially forms stable colloids with sc-SWNTs, while the methylated copolymer bearing electron-withdrawing cationic charges produces dispersions that are more enriched with m-SWNTs.