Pattern Formation on Carbon Nanotube Surfaces

Abstract

Calculations of fluorine binding and migration on carbon nanotube surfaces show that fluorine forms varying surface superlattices at increasing temperatures. The ordering transition is controlled by the surface migration barrier for fluorine atoms to pass through next neighbor sites on the nanotube, explaining the transition from semi-ionic low coverage to covalent high coverage fluorination observed experimentally for gas phase fluorination between 200 and 250 C. The effect of solvents on fluorine binding and surface diffusion is explored.

Spectroscopy and Defect Identification for Fluorinated Carbon Nanotubes

“…Upon increasing the treatment time, a weak shoulder appears at low binding energies, indicating that different patterns can co-exist. [8,26] Because valence electrons are involved in bond formation, subtle differences in surface chemistry may be obtained by studying the valence band region. The influence of the plasma treatment on the valence electronic states of the CNT is revealed by Figure 3, which summarizes the UPS valence band spectra obtained from five different samples: pristine MWCNTs and MWCNTs exposed to CF 4 plasma for 2, 30, 60 and 300 s.…”

“…This was associated to the transition from "semi-ionic" low-coverage to covalent high-coverage fluorination. [8,9] Surface fluorine atoms were found to migrate rapidly at low temperatures before pairing up at third-neighbour spacing. We note that the term "semiionic" sometimes applied to this phase is somewhat misleading, because although the CÀF bonds are dilated, they remain covalent in this arrangement.…”

“…[3,4] The electrical conductivity of CNTs was reported to decrease after gas-phase fluorine functionalization; while pristine CNTs were good conductors, tubes fluorinated at temperatures of 250 8C and above were insulators. [5][6][7] By using density functional calculations, Ewels et al [8] showed that this transition temperature arises from the surface migration barrier for fluorine atoms on tube surfaces, to pass through next-neighbour sites which blocks dense packing and fluorine banding at low temperatures. This was associated to the transition from "semi-ionic" low-coverage to covalent high-coverage fluorination.…”

“…We note that the term "semiionic" sometimes applied to this phase is somewhat misleading, because although the CÀF bonds are dilated, they remain covalent in this arrangement. [8] For this reason, rather than using the terms "semi-ionic" and covalent for these different bonding arrangements, we prefer low-and high-density packing respectively. Fluorine coverage at the third neighbour is limited to a maximum F/C surface ratio of 0.25.…”

“…As the temperature reaches~200-250 8C, fluorine can overcome the activation barrier associated with adopting the energetically less favourable second-neighbour configuration, enabling a phase with close packing at the first neighbour, until finally arriving to a F/C ratio of 0.5 under further fluorine addition. [8] Fluorination in fluorine gas generally results in a high reaction ratio and a deep penetration into carbon-related materials which prevents a good control of sidewall fluorination. [10] In this context, plasma fluorination has the potential to limit the fluorination area to the external surface of the nanotubes if performed at room temperature; in addition it requires a reaction time orders of magnitude shorter than other fluorination methods.…”