Viability of sub-0.4-nm diameter carbon nanotubes

Hartree-Fock ~HF! and density functional theory calculations indicate that a ~4,0! carbon nanotube ~CNT! of sub-0.4-nm diameter is stable and its heat of formation is close to that of the stable C36 (D6h). Semiempirical molecular-orbital calculation shows that such narrow tubular structure is more stable than the corresponding opened fragment in the innermost zone of a large CNT. Simulated TEM images show that an extra-narrow CNT does not show the typical CNT image regardless of the microscopic resolution. In order to aid experimental identification, the Raman spectrum of the ~4,0! CNT obtained from HF calculation shows A1g breathing modes in 489–725 cm.