Concentration dependences of the collective diffusion coefficient for carbon nanotubes with different diameters. The temperature is 300 K. In the simulation process, the density of the left atoms is fixed at 0.0001 mol/cm³, while the density of the right atoms can be varied in a range between 0 mol/cm³–0.0250 mol/cm³. The remaining parameters are the same as those in Fig. 3.