Adsorption of Noble Gases on Individual Suspended Single-Walled Carbon Nanotubes
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The focus of this work is a study of the adsorption of noble gases on individual, suspended, single-walled carbon nanotubes (SWNTs). The surface of a SWNT is a cylindrical cousin of graphite, but the binding energy is smaller and the cylindrical geometry and lack of grain boundaries could be expected to lead to substantially different behavior. The coverage (areal density) on a nanotube can be measured with high precision from the mechanical resonance frequency shifts, yielding isotherms as a function of gas pressure analogous to volumetric isotherms on bulk substrates. The electrical conductance of the nanotubes can also be measured at the same time and the effects of the adsorbates on the conductance thereby investigated. We found that adsorbed noble gases form monolayers on SWNTs, analogous to those on conventional 2-dimensional (2D) substrates, with a variety of 2D phases as a result of atom-atom interactions. Based on a combination of the coverage and conductance isotherms, the behavior of the adsorbates on SWNTs was established, including the 2D phases, 2D critical and triple-point temperatures, binding energies, isosteric heats, and latent heats. The majority of measurements were on Ar and Kr, fewer on 4He and Xe. The binding energy of the noble gases on a SWNT was found to be lower than on graphite, as anticipated. For example, the binding energy for Ar on one device was about 725 K ± 50 K, about 30% lower than graphite. The lower binding energy allows isotherm measurements at lower temperatures compared because the required pressure are higher. In all cases we found that only a single atomic layer is formed before reaching the saturated vapor pressure of the adsorbate. Remarkably, although the binding energies of Ar were consistent between multiple SWNTs, the adsorbates on different devices did not behave in the same way. The device can be classified into two classes. Those in Class I show sharp first-order transitions between 2D phases, very similar to those on graphite, and the maximum monolayer coverage on SWNTs is consistent with that on graphite taking into account the radius of nanotube. Large and sharp risers at constant pressures in the coverage or conductance isotherms indicate the 2D liquid-vapor or commensurate solid-vapor conversion. Small and smoother risers following the large ones in the isotherms indicate the 2D incommensurate solid-liquid phase transitions. In contrast, devices in Class II do not show clear phase transitions, have non-vertical isotherms in regions where first-order jumps would be expected, and the monolayers seem to be not complete when the saturated vapor pressure is reached. The difference in isosteric heat between devices of the two classes within the region where the first-order transition is expected, may be due to the 2D L-V latent heat. The existence of the two different classes of behavior remains puzzling. It appears that in class II nanotubes the effects of atom-atom attractive interactions are suppressed, due possibly either to geometrical effects or the different electrical properties of different nanotube species.
- Physics