Tuning bimetallic catalysts for a selective growth of SWCNTs

TL;DR

This paper investigates how the chemical composition of bimetallic catalysts influences the growth and properties of single-walled carbon nanotubes (SWCNTs), proposing a model to explain the observed effects.

Contribution

It compares three bimetallic catalysts and develops a phenomenological model linking catalyst composition to SWCNT diameter and electronic properties.

Findings

Ruthenium's limited carbon solubility leads to tangential growth mode.

Catalyst composition and temperature can tune SWCNT diameter and electronic type.

A model explains how alloying elements affect carbon solubility and nanotube characteristics.

Abstract

Recent advances in structural control during the synthesis of SWCNTs have in common the use of bimetallic nanoparticles as catalysts, despite the fact that their exact role is not fully understood. We therefore analyze the effect of the catalyst' s chemical composition on the structure of the resulting SWCNTs by comparing three bimetallic catalysts (FeRu, CoRu and NiRu). A specific synthesis protocol is designed to impede the catalyst nanoparticle coalescence mechanisms and stabilize their diameter distributions throughout the growth. Owing to the ruthenium component which has a limited carbon solubility, tubes grow in tangential mode and their diameter is close to that of their seeding nanoparticle. By using as-synthesized SWCNTs as a channel material in field effect transistors, we show how the chemical composition of the catalysts and temperature can be used as parameters to tune the diameter distribution and semiconducting-to-metallic ratio of SWCNT samples. Finally, a phenomenological model, based on the dependence of the carbon solubility as a function of catalyst nanoparticle size and nature of the alloying elements, is proposed to interpret the results.