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exposure for thick and thin SWNT networks. These samples were initially placed under vacuum ( ∼ 1 × 10 − 6 mbar) and ir- radiated by a UV light-emitting diode (LED) ( λ ∼ 400 nm) at low intensity ( ∼ 0.03 mW/cm$^{2}$) for approximately 12 h to des- orb surface and interbundle adsorbates (surface dopants) from the SWNTs. Once the SWNT network’s conductance stabi- lized, the samples were exposed to either O$_{2}$ (99.5% pure) or N$_{2}$ (99.998% pure) at 1 atm. The conductance of the sam- ples was then monitored by periodically sampling ( Δ t ≈ 1 s) the current while applying a fixed bias of 1 mV to the thick (metal-like) SWNT network ( R ≈ 1 k Ω ) and 10 mV to the thin (semiconductor-like) SWNT network ( R ≈ 1000 k Ω ), as shown in Fig. 2. 

After 5 min of exposure to O$_{2}$, the thin network shows an increase in conductance of about 13% while the thick net- work’s conductance changes by about 7%. For the same ex- posure to N$_{2}$, both networks show substantially smaller con- ductance changes of 2%–3%. However, at exposure times of more than 2 h, the thin SWNT network response to N$_{2}$ is sim- ilar to that of the thick SWNT network to O$_{2}$. This might be caused by a weaker physisorption of N$_{2}$ to the SWNT net- works than O$_{2}$. The inset of Fig. 2 also shows that at very long exposure times the fractional change in conductance, Δ G = G/G ( 0 ) − 1, becomes saturated after 24 h. Further, the response to O$_{2}$ depicted in Fig. 2 shows that the conductance change for a thin SWNT network is about two to three times that of the thick SWNT network at all times. This suggests that the conductance change under O$_{2}$ exposure is an intrinsic property of the SWNT networks, present even at very low O$_{2}$ concentrations. Herein we shall focus on the microscopic ori- gin of the network sensitivity to O$_{2}$ and N$_{2}$, with the temporal behavior of the networks discussed elsewhere. 8,9 

We have performed the Raman spectroscopy to character- ize our SWNT network samples, which were produced via 

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the high-pressure carbon monoxide (HiPco) method. Fig- ure 3 shows the radial breating mode (RBM) Raman sig- nals of HiPco samples at excitation wavelengths λ$_{exc}$ ≈ 532 nm, λ$_{exc}$ ≈ 632.5 nm and λ$_{exc}$ ≈ 785 nm. The van Hove singularity energy separation was calculated using the tight- binding approximation with the carbon-carbon interaction en- ergy γ$_{0}$ ≈ 2.9 eV and carbon-carbon bond length a$_{C}$$_{−}$$_{C}$ ≈ 1.44 ˚ A. The SWNT diameter d dependence of the RBM frequency ν$_{RBM}$ for isolated SWNTs on SiO$_{2}$ has been shown 26 to be- have as ν$_{RBM}$ ≈ 248 /d$_{t}$ . The DFT calculated diameters for (7,7), (12,0), and (13,0) SWNTs of d ≈ 9.76, 9.79, and 10.66 ˚ A, respectively, are found to correlate well with the HiPco Ra- man shift, as shown in Fig. 3. This should ensure a good de- 

