Confirming the Dual Role of Etchants during the Enrichment of Semiconducting Single Wall Carbon Nanotubes by Chemical Vapor Deposition

The search for ways to synthesize single wall carbon nanotubes (SWCNT) of a given electronic type in a controlled manner persists despite great challenges because the potential rewards are huge, in particular as a material beyond silicon. In this work we take a systematic look at three primary aspects of semiconducting enriched SWCNT grown by chemical vapor deposition. The role of catalyst choice, substrate, and feedstock mixture are investigated. In terms of semiconducting yield enhancement, little influence is found from either the binary catalyst or substrate choice. However, a very clear enrichment is found as one adds nominal amounts of methanol to an ethanol feedstock. Yields of up to 97% semiconducting SWCNT are obtained. These changes are attributed to two known etchant processes. In the first, metal SWCNT are preferentially etched. In the second, we reveal etchants also preferentially etch small diameter tubes because they are more reactive. The etchants are confirmed to have a dual role, to preferentially etch metallic tubes and narrow diameter tubes (both metallic and semiconducting) which results in a narrowing of the SWCNT diameter distribution.