Chemically Induced Conductance Switching in Carbon Nanotube Circuits

The chemical reactivity of carbon nanotubes in ${\mathrm{H}}_2{\mathrm{SO}}_4$ is investigated using individual, single-walled carbon nanotubes (SWNTs) incorporated into electronic devices. Exploiting the device conductance as a sensitive indicator of chemical reactions, discrete oxidation and reduction events can be clearly observed. During oxidation, a SWNT opens circuits to a nanometer-scale tunnel junction with residual conduction similar to Frenkel-Poole charge emission. When electrochemically reduced, a SWNT returns to its original conductance. This redox cycle can be repeated many times, suggesting a novel chemical method of reversibly switching SWNT conductivity.

Figure 1

(a) Atomic force microscopy image in which PMMA protects the entire device except for a short segment of exposed SWNT sidewall. Scale bar is $1\mu \mathrm{m}$. (b) Characterization and chemical modification proceeds with 5 independent electrodes: Pt counter and reference electrodes (CE, RE) to control the electrolyte, a backgate voltage $V_g$ to affect the SWNT carrier concentration, and two working electrodes (WE) connected at either end of the SWNT device. (c) $G(V)$ for a semiconducting SWNT FET, showing electrostatic modulation as a function of backgate voltage $V_g$ in air and $V_{\mathrm{WE}}$ in deionized ${\mathrm{H}}_2\mathrm{O}$. Note that conventional definitions of $V_g$ and $V_{\mathrm{WE}}$ require the top axis to be reversed.

Figure 2

(a) $G(V_{\mathrm{WE}})$ for voltage ramps staying within and exceeding past the SWNT oxidation threshold ($V_{\mathrm{WE}}>0.9\mathrm{V}$). An oxidation event results in a flat, zero conductance state stable until $V_{\mathrm{WE}}$ again exceeds the reduction threshold ($V_{\mathrm{WE}}<0.0\mathrm{V}$). Reduction returns $G(V_{\mathrm{WE}})$ to its original shape and magnitude (dashed curve). Measurements of $G(t)$ at constant $V_{\mathrm{WE}}$ reveal sharp steps under oxidizing (b) and reducing (c) electrochemical conditions; two complete redox cycles from the same SWNT device are depicted here.

Figure 3

Electrical characterization of an oxidized SWNT. (a) $I\mathrm{\text{−}}V$ characteristics are highly nonlinear, approximately exponential in voltage, and nearly temperature independent at or below room temperature. (b) Differential conductance exhibits a broad, deep conductance suppression at small bias, corresponding to the low conductance state shown in Fig. 2. (c) The temperature and bias dependence are well fit by a Frenkel-Poole mechanism in which the current is primarily regulated by thermal emission from localized states within the barrier (d).