Scanning tunneling microscopy investigation of atomic-scale carbon nanotube defects produced by Ar+ ion irradiation

Multi-wall carbon nanotubes (MWCNTs) dispersed on graphite (HOPG) substrate were irradiated with Ar+ ions of 30 keV, using a dose of D = 5 × 1011 ions/cm2. The irradiated nanotubes were investigated by scanning tunneling microscopy (STM) and spectroscopy (STS) under ambient conditions. Atomic resolution STM images revealed individual nanotube defects, which appeared as “hillocks” of 0.1–0.2 nm in height, due to the locally changed electronic structure. The results are in agreement with previous theoretical predictions. Electron density patterns (superstructures) were observed near the defect sites, which originated from the interference of incident waves and waves scattered by defects. The period of these superstructures is larger than the period determined by the atomic structure. After annealing at 450 °C in nitrogen atmosphere, the irradiated MWCNTs were investigated again. The effect of heat treatment on the irradiation-induced nanotube-defects was observed both on the STM images and on the recorded STS spectra.