Effect of uniaxial strain on the band gap of zigzag carbon nanotubes

Both zigzag (n,0) and armchair (n,n) CNTs are two specific CNTs and have attracted much attention due to their higher rotation symmetry, but the 2D graphene sheet model predicted that all armchair (n,n) CNTs are metallic, while zigzag (n  ,0) CNTs are either metallic if n=3qn=3q, or semiconducting in other cases. Our calculations for unstrained zigzag (n  ,0) CNTs (n=7–15)(n=7–15) show that the band gap Eg exhibits sawtoothlike periodic character with (3q  ,0) tubes having the lower nonzero values as compared with their neighbors (3q−1,0)(3q−1,0) and (3q+1,0)(3q+1,0) tubes. The strain and stress values corresponding to zero band gap or SMT show that, for a certain integer q  , the largest tensile strain and stress, the smallest tensile strain and stress, and the moderate compressive strain and stress are needed to produce the SMT for (3q−1,0)(3q−1,0), (3q,0)(3q,0) and (3q+1,0)(3q+1,0) CNTs, respectively. Furthermore, in each group the strain and stress needed to produce the SMT decrease with increasing the index n, except for (7,0) CNT with large curvature and thus large interaction between the σ* and π* electrons. These results have been explained satisfactory from the movement of the KF point due to both the curvature of the nanotube and uniaxial strain.