Motivation

Chemical functionalisation is an important aspect of SWNT research, of interest to many proposed applications of SWNTs, including electrical and electrochemical sensing. We investigate the effects of two common in situ chemical treatments on the electrochemical and solution conductance properties of SWNTs. The first is acid reflux, used for the purification of SWNTs and a common first step toward chemical functionalisation 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 SWNTs grown on insulating Si/SiO₂ using cCVD. Electrical, electrochemical, AFM, FE-SEM, and micro-Raman analysis are performed before and after applying the treatments.

Acid Reflux

The major effect of the acid treatment is cutting of the SWNTs followed by gradual etching at the cut ends, as shown by FE-SEM and AFM (Figure 1a). With the spectral resolution offered by micro-Raman spectroscopy, there is minimal evidence for significant sidewall functionalisation. In addition, the spectra indicate preferential etching of the metallic SWNTs (Figure 1b). Cutting of the SWNTs leads to breakdown of the SWNT network and, hence, breakdown in its electrical conductivity.

Figure 1: a) FE-SEM images of SWNT networks after growth and after reflux in 3 M HNO₃ for 2 hours, 6 hours and 14 hours. Scale bar is 10 μm. AFM image of a SWNT sample after 14 hours ₃ reflux. b) Raman spectra of a SWNT sample after growth (blue) and after 14 hours ₃ reflux (red).

Plasma Ashing

By contrast to acid reflux, the plasma treatment does not destroy the morphology of the SWNTs. However, an EFM-based technique suggests an increase in the intrinsic resistivity of the SWNTs post treatment (Figure 2), suggesting a significant change in the electronic structure of the SWNTs.

Figure 2: SCM phase before (left) and after (right) air plasma treatment. Scale bar is 2 μm.

Electrochemistry

Effect of functionalisation on the electrochemical response of SWNT networks

Both treatments have a negligible effect on the voltammetric response of a simple outer-sphere electron-transfer redox process, Ru(NH₃)₆^3+/2+. However, both acid reflux and air plasma treatment enhance the electron-transfer kinetics for the oxidation of inner-sphere dopamine (Figure 3). In both cases this is likely due to the creation of defect sites.

Figure 3: CV of dopamine at pristine (blue) and ashed (red) SWNT network electrodes.

A key result of these studies is the strong correlation between increasing functionalisation (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.