Influence of O$_{2}$ and N$_{2}$ on the conductivity of carbon nanotube networks D. J. Mowbray$^{1}$, ∗ C. Morgan$^{2}$, and K. S. Thygesen 1 $^{1}$Department of Physics, Center for Atomic-scale Materials Design (CAMD), Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark $^{2}$Department of Physics, Molecular and Materials Physics Group, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom (Received 11 February 2009; Published 22 May 2009) 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 O$_{2}$ and 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 O$_{2}$ and 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 O$_{2}$ than for N$_{2}$, and for semiconducting rather than metallic SWNTs junctions, in agreement with experiment. PACS numbers: 73.63.–b, 68.43.–h, 73.50.Lw arXiv:1001.2498v1 [cond-mat.mes-hall] 14 Jan 2010 I. INTRODUCTION Using single-wall carbon nanotubes (SWNTs) as nanosen- sors, both individually and in SWNT networks, has been one of the most promising potential applications of SWNTs since their discovery. 1,2 Several experimental studies have demonstrated that the conductance of SWNT systems is rather sensitive to the presence of even single-molecule concentra- tions of physisorbed gas molecules such as O$_{2}$ and N$_{2}$. 3–9 Further, by measuring conductivity of individually charac- terized SWNTs, 10 as well as thick (metal-like) and thin (semiconductor-like) SWNT networks, 8,9,11 the response of SWNTs to contaminants has been shown to correlate with the intrinsic electronic properties of the material. For example, it has been found that the presence of low-O$_{2}$ concentrations, independent of temperature, introduces an increase in conduc- tance of approximately 20% on thin SWNT networks, while an increase in conductance of only about 1% is found for thick SWNT networks. 8 (7,7) metallic armchair, (12,0) semimetallic zigzag, and (13,0) semiconducting zigzag SWNT junctions, shown schemati- cally in Fig. 1. Comparing our theoretical results for SWNT junctions with experimental measurements for SWNT net- works suggests that the surprising sensitivity to O$_{2}$ and N$_{2}$ may be partially due to an increased tunneling probability through O$_{2}$ and N$_{2}$ physisorbed at SWNT junctions. In Sec. II we describe experimental measurements of the influence of both O$_{2}$ and N$_{2}$ on the conductivity of SWNT networks and the characterization of these networks using Ra- man spectroscopy. A description of the DFT and NEGF model used to describe the microscopic origin of this effect is then provided in Sec. III. In Sec. IV we compare our theoretical results for the SWNT junction transmission with the SWNT network experiments, followed by a concluding section. On the other hand, previous theoretical studies have found that SWNTs are rather inert, so that gases tend only to ph- ysisorb to the SWNT surface. 12–19 For this reason, it was suggested that O$_{2}$ should not effect conductance through SWNTs, but only influence conductance at either SWNT- SWNT junctions, at the SWNT-metal contacts, or at SWNT defect sites. 12,20 Although the conductivity of SWNTs with molecules physisorbed at defect sites has been extensively studied, 21–23 the conductivity of four-terminal SWNT-SWNT junctions has been previously studied only for small pristine metallic SWNTs. 24,25 The possible influence of physisorbed molecules on SWNT-SWNT junctions has not been investi- gated. In this paper we address the microscopic origin of the in- crease in conductance of SWNT networks when exposed to O$_{2}$ or N$_{2}$ gas. To this end, we have performed density- functional theory (DFT) calculations of the intratube trans- mission within a SWNT and the intertube transmission be- tween two SWNTs in the nonequilibrium Green’s function (NEGF) formalism for O$_{2}$ and N$_{2}$ molecules physisorbed in Below we give a brief discussion of our experiments on SWNT network conductivity. A more detailed description may be found in Refs. 8 and 9. Figure 2 shows experimen- tal measurements of the conductance sensitivity to O$_{2}$ and N$_{2}$ II. EXPERIMENTAL RESULTS FIG. 1: (Color online) Schematics of a (13,0) SWNT junction with (a) physisorbed O$_{2}$ and (b) physisorbed N$_{2}$. (a) Physisorbed O$_{2}$ (b) Physisorbed N$_{2}$