Ab initio study of curvature effects on the physical properties ofCH4-doped nanotubes and nanoropes

Abstract

We have performed an ab initio study of the energetics, structural, electronic, and optical propertiesof the CH4-doped ultrathin 4 Å and large diameter carbon nanotubes (CNTs). No adsorption of theCH4 molecule has been seen either on the groove or on the interstitial sites of the achiral4 Å nanoropes. In the case of weakly bonded systems such as the adsorption of theCH4 molecules on the nanotubes, a prominent role of the dispersion forces in the binding isobserved. The negative contribution of the zero point vibrational energy to thebinding energy is seen to be appreciable. The effects of the tube diameter and thedifferent chiralities of the carbon nanotubes on the adsorbate-induced physicalproperties have been investigated. In the large diameter tubes, the binding of theCH4 moleculein the endohedral adsorption is much stronger than that in the exohedral adsorption. The binding ofthe CH4 molecules depends upon the chirality of the nanotube and we find no adsorption on thechiral (4, 2) tube. We find that the local density approximation (LDA) over-binds theCH4 molecule and the generalized gradient approximation (GGA) under-binds it, andfor a reliable theoretical estimate one should take some weighted averageof the binding energies (BEs) determined in the LDA and the GGA. Thecurrently calculated BE and the adsorbate concentration are in reasonableagreement with the measured data available for the (10, 10) nanotubes. Theelectronic structure of the pristine tube is quite altered by the adsorption of theCH4 molecule on the surface of the tube because of the breaking of the symmetry of the hostlattice except the chiral (4, 2) tube, which has practically no symmetry. The adsorptionincurs splitting in the states in the whole energy range, especially in the large curvature4 Å tubes. The bandgap of the semiconducting achiral zigzag nanotube is reduced,whereas that of a chiral semiconducting tube is enhanced, by the adsorption of theCH4 molecules. Theadsorption of CH4 molecules does not alter significantly the peak structure in the optical absorption of thepristine tube, except for some changes in the energy locations and the relative intensities inthe achiral tubes. Most of the calculated peaks in the optical absorption of the pristinelarge diameter (10, 0) and (10, 10) nanotubes have been observed in the experimentalmeasurements.