Ab initio study of curvature effects on the physical properties of CH4-doped nanotubes and nanoropes

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