**1. Introduction**

Ion beam technology has some advantages for controlled construction and modification of nanostructures; i.e., (1) nonequilibrium phase transition of nanostructures can also be realized using ion beam technology, and the nucleation in phase transition process will not be confined due to thermodynamics of growth of material. (2) The lattice orientation of nanostructures can be controlled using a channeling effect of ion beam. In fact, when a beam of energetic ions interacts with crystal, it can destroy the nanoparticles while keeping maintained the crystal nuclei and nanoparticles with orientation consistent with channel direction of ion beam. (3) Ion implantation is an effective means for nucleation, nanophase formation, nanocrystal orientation, and precise doping. (4) Ion beam deposition technology is also an effective way to achieve high-quality thin films.

In the past three decades, ion beam technology has played an important role in the formation of nanostructures, such as alloying, amorphization, and phase transformation; nanocrystalline phase formation by ion implantation; nucleation induced by ion implantation; oriented nanocrystals in solid-state network; and nanocrystal size control [1–4].

CNTs can be employed as conducting wires and building blocks of many electronic and optoelectronic nanodevices due to their excellent mechanical and electronic properties [5–8].

In recent years, some important research topics in the interaction between the ion beams and CNTs have attracted widespread attention. The energy, doses, and the substrate temperature of ion beams should affect the interaction results.

In general, if the energy is very low (such as 100 eV), the cascade collision effect does not occur. The interaction between ion beam and CNTs is producing the defect in the graphite lattice, and the structure of CNTs is still graphite shell. In this range of ion beam energy, the number of defect on the CNT surface can be controlled precisely by adjusting the ion energy and ion doses, and the corresponding properties of CNTs can be tuned. These defects can be used as the source to add some new functional group, functional material, and nanoparticles. Even, the C atoms around the defects can be transferred into the carbon onion structure or diamond by the H ion beam. When the energy is high (such as 30 keV), the cascade collision effect occurs, and it can produce a large number of defects by the implantation of ion beam. In this ion beam energy range, the graphite layer structure of CNTs should be damaged under the irradiation of ion beam. The rearrangement of carbon atoms should happen. The amorphous carbon nanostructure, carbon onion structure, or diamond structure can be formed. The defects can also be used to link the CNTs, and the welding of CNTs can be realized. Therefore, the modification of CNTs, transformation of CNTs, welding of CNTs, fabrication of carbon nanowire networks, etc. have been interesting under the interaction between the ion beam and CNTs.
