Single Walled Carbon Nanotubes (SWNTs) as a Gas Sensor
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Single Walled Carbon Nanotubes (SWNTs) as a Gas Sensor Bhabendra K. Pradhan, Gamini U. Sumanasekera, Clement K. W. Adu, Hugo Romero, Peter C. Eklund* Department of Physics, The Pennsylvania State University, 104 Davey Laboratory University Park, PA 16802-6300 ABSTRACT: A thermoelectric "nano-nose" has been built from tangled bundles of singlewalled carbon nanotubes (SWNT). The detector’s thermoelectric response ∆S is sensitive to the logarithmic energy derivative of the additional bundle resistivity (ρa) which is identified with the adsorbed molecules. The response is therefore specific to the details of the interaction of the adsorbed molecule with the nanotube wall; even gases such as He, N2 and H2 can be easily detected. Plots of ∆S vs. ρa are sensitive to whether oxidation or reduction of the tube wall is taking place, and to whether the gas molecule is physisorbed or chemisorbed. The utility of the sensor stems from the amphoteric nature of the SWNT, the quasi-one-dimensional character of the charge conduction and the high specific surface area of SWNTs. INTRODUCTION A single-wall carbon nanotube (SWNT) can be envisioned as a monolayer graphene sheet rolled up into a long seamless cylinder (1). First reported in the products of an electric arc in 1993 by research scientists in NEC-Tsukuba (2) and IBM-Almaden (3), and later produced in much more significant quantities and higher purity by pulsed laser vaporization at Rice University (4), these molecular filaments are now under active investigation for a variety of fundamental reasons, as well as potential new technologies (5). Both the electric arc and the laser produce well-ordered, tightly packed bundles of tubes containing tens to hundreds of SWNTs (6, 4). Within a bundle, the tubes are bound together by a weak van der Waals force. If all the allowed helicities are statistically populated, a random distribution of ~ 1/3 metallic tubes and ~ 2/3 semiconducting tubes would exist in a macroscopic sample (1). For the tube diameters found in our samples, previous scanning tunneling microscopy (7), optical absorption spectroscopy (8, 9) and theoretical calculations (10) report that the semiconducting gap Egap ~ 0.6 eV. Similar to graphite, SWNTs are amphoteric (11, 12). That is, they can be doped by electron donors or Figure 1 Schematic structure of a single wall carbon nanotube acceptors that attach to the tube wall, removing bundle showing the pore, groove, channel and surface sites or adding electrons to the carbon π electron available for gas adsorption. Dashed line indicates the nuclear skeleton of the nanotubes. Binding energies (EB) and specific conduction bands. surface area contributions (σ) for hydrogen adsorption on these The bundle structure of SWNTs sites are indicated (13). produces at least four distinct sites in which gas molecules can adsorb (see Fig. 1) : on the external bundle surface, in a groove formed at the contact between adjacent tubes on the outside of the bundle, within an interior pore of an *
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