engleski

He atoms in carbon nanotubes and nanotube bundles

We report results for the energetics and single-particle spectra of $^4$He and $^3$He atom confined by one, two, and three carbon nanotubes, obtained using both a smooth--tube model for the interaction between helium atoms and tubes, and a realistic model that takes the atomic structure of the tubes into account. The particular choice of geometry allows us to examine four possible physisorption sites of helium atoms in nanotube bundles: sites inside and outside the tube, in the grooves between tubes, and in the interstitial channels. We show how the energies and the wave functions of the inside and groove states change with the addition of the tubes. The effects of corrugation on the energy spectra and wave functions are examined in detail. In particular, effective masses and band gaps of $^4$He adsorbed in grooves are determined for various relative orientations of the nanotubes. The calculated single-particle spectra are then used to obtain the thermodynamic properties of adsorbed helium atoms in bundles of closed nanotubes. As a consequence of corrugation, a departure from the one-dimensional ideal gas behavior is observed ; this deviation depends on the temperature, density and bundle model. Finally, we discuss the formation of small clusters in the four possible adsorption sites. The cylindrical geometry provided by the carbon nanotubes leads to dramatic enhancement of the dimer binding. Comparison with the results obtained in the model potential which confines atoms to move on a cylindrical surface, shows that in order to provide a realistic estimate of the dimer binding energy, it is necessary to solve a full three-dimensional case.

helium; nanotubes; adsorption; corrugation; clusters

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