Gas Molecules Adsorption on Carbon Nanotubes
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Gas Molecules Adsorption on Carbon Nanotubes Jijun Zhao1, Alper Buldum1, Jie Han2, Jian Ping Lu1 1
Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255 2 NASA Ames Research Center, Mail Stop T27A-1, Moffett Field, CA 94035 Abstract We study electronic properties of single-walled carbon nanotubes (SWNTs) upon gas molecules adsorption with first principles methods. The adsorption energy, charge transfer, and electronic structures are studied for various gas molecules (NO2, O2, NH3, N2, CO2, CO, H2O, H2, He). Most molecules adsorb weakly on SWNTs and can be either charge donor or accepter to the nanotubes. The electronic properties of nanotubes are sensitive to the adsorption of certain molecules such as NO2 and O2. Charge transfer and gas-induced charge fluctuations are found to significantly affect the transport properties of SWNTs. Our results are consistent with recent experiments. The outstanding electronic properties of carbon nanotubes make them attractive for molecular electronic devices [1-7]. Pure SWNTs are ideal one-dimensional (1-D) quantum wires and can be either metallic or semiconducting depending on their diameter and chirality [1]. Recently, the dependence of electronic properties on chemical environment of SWNTs has attracted considerable attention [8-12]. Experimental studies include electrical resistance [8,9], NMR [10], and thermoelectricpower [11]. Upon exposure to O2, NO2, or NH3 gases, the conductance of semiconducting tubes is changed dramatically [8,9]. NMR experiments also reveal the effect of oxygen on density of state near the Fermi level [10]. The resistance of metallic nanotube bundle decreases as gas molecules are removed from the samples [11]. Theoretically, it has been shown that O2 adsorption has significant effect on the electronic properties of small semiconducting nanotubes [12]. Here we report first principles calculations on the electronic properties of SWNTs with adsorption of several gas molecules including O2, NO2, N2, and H2O. In addition to the charge transfer, we found that the adsorbed gas molecules induce charge fluctuations on nanotube, which has significant effects on transport properties. The density functional electronic structure calculations are performed with either localized basis (DMol) or plane-wave basis (CASTEP). The equilibrium geometries, adsorption energy and charge transfer are calculated using the DMol program [13], in which an effective core potential and a double numerical basis including d-polarization function are adopted. The density functional is treated by local density approximation with exchange-correlation potential parameterized by Wang and Perdew [14]. The band structures and the charge density are calculated using a plane-wave pseudopotential program (CASTEP) [15]. The ion-electron interaction is modeled by Troullier-Martin norm-conserving nonlocal pseudopotential [16]. The energy cutoff of plane wave basis is 760 eV. We studied both zigzag (10,0), (17,0) and armchair (5,5) tubes. 1-D p
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