**3. Conclusions**

In conclusion, "defect engineer" by implantation of ion beam is the main mechanism involved in interaction of CNTs and ion beam. The defect is used to modify CNTs by adding some functional group, functional material, and nanoparticle, and the CNTs should have some complex new properties. The atom and structure near the defect should be reconstructed which leads to the transformation of CNTs; some new carbon nanostructure can be formed such as carbon onion, amorphous carbon nanowire with different hollow structure, carbon nanoparticles, and carbon nanofiber. The reconstruction also can be used to fabricate the junctions of CNTs, form the carbon nanowire networks, and realize the welding of the CNTs.
