Influence of ${\text{O}}_2$ and ${\text{N}}_2$ on the conductivity of carbon nanotube networks

We have performed experiments on single-wall carbon nanotube (SWNT) networks and compared with density-functional theory (DFT) calculations to identify the microscopic origin of the observed sensitivity of the network conductivity to physisorbed ${\text{O}}_2$ and ${\text{N}}_2$. Previous DFT calculations of the transmission function for isolated pristine SWNTs have found physisorbed molecules have little influence on their conductivity. However, by calculating the four-terminal transmission function of crossed SWNT junctions, we show that physisorbed ${\text{O}}_2$ and ${\text{N}}_2$ do affect the junction’s conductance. This may be understood as an increase in tunneling probability due to hopping via molecular orbitals. We find the effect is substantially larger for ${\text{O}}_2$ than for ${\text{N}}_2$, and for semiconducting rather than metallic SWNTs junctions, in agreement with experiment.

Figure 1

(Color online) Schematics of a (13,0) SWNT junction with (a) physisorbed ${\text{O}}_2$ and (b) physisorbed ${\text{N}}_2$.

Figure 2

(Color online) Fractional change in conductance $\Delta G=G/G\left(0\right)-1$ versus time $t$ in seconds and hours (inset) following exposure to ${\text{O}}_2$ and ${\text{N}}_2$ for thin $\left(◻,⊠\right)$ and thick $\left(○,\otimes \right)$ SWNT networks, respectively, on log-log and linear (inset) scales (Refs. 8, 9).

Figure 3

(Color online) Raman spectra and approximate diameter distribution of HiPco SWNT sample for an excitation wavelength (top) ${\lambda }_{\text{exc}}\approx 532\text{nm}$ (lower black curve), ${\lambda }_{\text{exc}}\approx 785\text{nm}$ (upper red curve), and (bottom) ${\lambda }_{\text{exc}}\approx 632.5\text{nm}$. The DFT calculated diameters of $d\approx 9.76\text{Å}$, $9.79\text{Å}$, and $10.66\text{Å}$ for (7,7), (12,0), and (13,0) SWNTs, respectively, are provided for comparison (dashed lines).

Figure 4

(Color online) Schematic of (13,0) SWNT junction consisting of four SWNT leads (light gray) coupled to the central scattering region via four SWNT primitive unit cells or layers (gray) “frozen” at their relative positions in the isolated SWNT. The atomic positions of the remaining eight SWNT layers (dark gray) and the physisorbed ${\text{O}}_2$ molecule have been relaxed. A SWNT separation consistent with experiments of approximately $3.4\text{Å}$ has been used. The intratube transport through the SWNT and the intertube transport between the SWNTs has been shown schematically.

Figure 5

(Color online) (a)–(c) Intratube transmission and (d)–(f) intertube transmission vs energy in eV relative to the Fermi energy for a pristine (black solid lines) SWNT junction, and with ${\text{N}}_2$ (blue dashed lines), and ${\text{O}}_2$ (red dash-dotted lines) physisorbed consisting of [(a) and (d)] metallic (7,7) SWNTs, [(b) and (e)] semimetallic (12,0) SWNTs, and [(c) and (f)] semiconducting (13,0) SWNTs. Schematics and isosurfaces of $\pm 0.02e/{\text{A}}^3$ for molecular orbitals on the physisorbed molecules are shown above the corresponding eigenenergies (dotted lines).