Single-Walled Carbon Nanotubes: Density Functional Theory Study on Field Emission Properties
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Ongoing developments in the production and characterization of carbon nanotubes, as previously summarized, have opened up new fields in science and technology in the last few years. These materials are shown to be versatile building blocks because of their unique electronic, optical, and mechanical properties; for example, most recently, carbon nanotubes were found to act as ballistic field-effect transistors (FET), were fabricated as FETs with multiple, individually addressable gate segments, were applied in composites, and were shown to be photoconductive.
Furthermore, because carbon nanotubes emit high currents (up to 1 A/cm2) at low field (∼ 5 V/µm), and significant advances have been attained in the production of well-aligned tubes, the applicability for field emission previously recognized by de Heer et al. has now been widely acknowledged, and a prototype of a flat panel with color display was achieved. Most recently, different production procedures and treatments of nanotubes were shown to result in varying degrees of field emission.
Despite these technological developments, there are still unanswered questions regarding the field emission mechanism, as it was observed that the I–V characteristics of single-wall carbon nanotubes (SWCNTs) could deviate from Fowler–Nordheim's (FN) theory, which assumes the emission to occur by a tunneling mechanism. A range of experimental and theoretical studies were reported to explain this behavior, e.g., by Saito et al., suggesting that the deviation may occur because of the nonmetallic behavior of SWCNTs at the tip, while Bonard et al. attributed the nonlinearity to tube interactions, and others observed that the exposure of SWCNT samples to gas adsorption alters their emission properties. Although the effects of gas adsorption, in particular O2, were previously reported experimentally, the detailed chemistry at the adsorption sites under an external field and the resulting field emission properties are still not fully understood. Theoretically, much attention has been devoted thus far to the study of O2 adsorption at SWCNT tips, and their effects on field emission, including our recent systematic examination of the adsorption at capped and uncapped tips, performed under a local field to mimic the emission environment.
In this review, we outline some of the findings of our previous work, and in addition, report preliminary results on the effects of ozonation on SWCNTs. Indeed, although ozonation of carbon nanotubes enhances the propensity for functionalization, and alters the field emission properties, e.g., an enhancement in the first 3 min upon exposure to O3 has been observed, Thus far the effects of O3 have not been extensively explored theoretically. While a mixed quantum mechanics/molecular mechanics (ONIOM) study of O3 adsorption at the sidewall of a SWCNT, assuming only a 16-atom model at a high level of accuracy, has been recently reported, we carried out all-electron linear combination of atomic orbitals (LCAO) density functional theory (DFT) calculations in our preliminary investigation, previously also applied to extended SWCNT systems. We identify possible reaction sites for ozonation, which could play a role in the experimentally observed field emission enhancement. Such comprehensive studies, reiterated in this review for O2 adsorption, and including our new results for the ozonation at the tips, provide an outline for our ongoing interest in gaining insight into the effects of adsorbates on field emission characteristics in these materials, e.g., enhancement by Cs intercalation or deposition.