Functionalizing Single-Walled Carbon Nanotube Networks: Effect on Electrical and Electrochemical Properties

Abstract

Chemical functionalization is an important aspect of single-walled carbon nanotube (SWNT) research, of interest to many proposed applications of SWNTs, including electrical and electrochemical sensing. In this study, the effects of two common in situ treatments on the electrochemical and solution conductance properties of SWNTs are assessed. The first is acid reflux, used for the purification of SWNTs and a common first step toward chemical functionalization of SWNTs. The second is an air plasma treatment, compatible with microfabrication processing. Rather than studying bulk quantities and using bulk analysis techniques, we investigate two-dimensional networks of individual SWNTs grown on an insulating substrate, enabling the effects of the treatments to be investigated at the level of individual SWNTs, as well as ensemble average behavior. The SWNTs are grown using catalyzed chemical vapor deposition, and electrical, electrochemical, atomic force microscopy, field emission scanning electron microscopy, and micro-Raman analysis are performed before and after applying the treatments. It is found that the major effect of the acid treatment is cutting of the SWNTs followed by gradual etching at the cut ends. Micro-Raman spectroscopy indicates preferential oxidative attack at the metallic SWNTs and minimal damage to the sidewalls. In contrast, plasma treatment does not affect the morphology of the SWNTs. Raman microscopy indicates a dramatic change in SWNT electronic structure, with a possible increase in sp3-hybridized carbon. Both treatments have a negligible effect on the voltammetric response of a simple outer-sphere electron-transfer redox process, Ru(NH3)63+/2+. However, both acid reflux and air plasma treatment enhance the electron-transfer kinetics for the oxidation of inner-sphere dopamine. In both cases this is likely due to the creation of defect sites. A key result of these studies is the strong correlation between increasing functionalization (with a view to increasing chemical sensitivity) and decreasing conductivity, which is an important consideration for electrical and electrochemical applications. It is clear that a balance must be struck between the two to enhance the performance of a SWNT device.