Ab initio study of 4 Å armchair carbon nanoropes:: Orientation-dependent properties -: art. no. 075403

A comprehensive ab initio investigation of the effects of the relative orientation (r.o.) between the adjacent tubes in a rope on the stability, structural, electronic, optical and Raman-active properties has been performed for the nanoropes of the ultrathin 4 A diameter armchair (3,3) carbon nanotubes. A number of features associated with the large curvature effects in these tubes not discussed earlier are observed in the present study. Only the nanoropes possessing a maximum value of r.o. equal to 45degrees are stable. The symmetric rope with zero value of r.o. is metallic along the rope axis but is semiconductive normal to the axis whereas the asymmetric ropes characterized by nonzero r.o. are semimetallic along the tube axis as well as normal to the axis. The band gap for k(x)=k(y)=0 and the dispersion along the tube axis increases with the r.o. up to an angle of 30degrees for which it is 0.21 eV. A rope possessing an orientational disorder is also seen to be semimetallic along the tube axis as well as normal to the axis. The large curvature effects in the (3,3) nanoropes give rise to a number of results opposite to those seen in the large diameter tubes and ropes. The large curvature incurs a decrease in DOS at E-F by a factor of 2 in the asymmetric rope (r.o.=30degrees) over the symmetric rope, a result which is in contrast to the behavior seen for comparatively small curvature (10,10) ropes. In the (3,3) rope which contains large curvature, one observes a strong directional intertube covalent binding in contrast to the significant van der Waals interaction present in the large diameter tubes. The optical absorption in each rope reveals a strong peak in the energy range of 2.9-3.0 eV. In addition, each nanorope characterized by nonzero r.o. reveals very specific strong peak lying below 1.0 eV which may be used for identifying the r.o. of a rope. The present results for the isolated (3,3) nanotube are in good agreement with the available measured data for absorption and Raman scattering. For the disordered rope which contains four ropes possessing four different r.o.'s, quite strong absorption along with four peaks is seen throughout the energy up to 5.0 eV. The RBM frequencies for the ropes are seen to be somewhat higher by about 2% than that of the isolated nanotube.