Implantation of hydrogen and other ions into the superstructure of carbon nanotubes (CNT) :: SPE RESST

Implantation of hydrogen and other ions into the superstructure of carbon nanotubes (CNT)

Simulation of the processes of low-energy ion implantation (in particular, of hydrogen ions) into the superstructures of carbon nanotubes was carried out by the method of molecular dynamics. Also, a semianalytic theory on the kinetics of oriented interactions between ions and nanotubes, well-describing the results of mathematic simulation, was developed.

For the first time the effective ion-atom pair potentials have been introduced into the methods of restricted molecular dynamics for simulation of the interaction between ions and carbon nanotubes. The introduced potentials are compatible with the universal potential ZBL and the attractive potentials of Tersoff (C-C) and Brenner (H-C), being specific for carbon nanostructures, which take into consideration the ion-atom attraction (Figure 1).


a)	b)

Figure 1 – Radial dependences of ion-atom interaction potentials for hydrogen ions (a) and carbon ions (b). Solid curves – the universal potential ZBL.

For the first time in the world obtained were the ion-beam orientation dependences of implantation profiles for hydrogen and proper carbon ions of keV energies implanted into the superstructures (bundles) of carbon nanotubes (CNT) (Figure 2).


a)	b)

Figure 2 – Implantation profiles of carbon ions (a) and hydrogen ions (b)

The investigations have shown that the shapes and directional dependences of profiles conform to the ion dynamics in the potential reliefs of a transverse continuous potential, and essentially differ depending on the type of an ion-atom interaction potential. In the case of potentials with a strong ion-atom coupling (that are characteristic, first of all, for hydrogen) the ion attraction to the wall of carbon nanotubes exerts a cardinal influence on the range in the channeling mode, and also on the implantation profile shape.

The interrelation of implantation profiles with dynamics and kinetics of ion channeling in CNT has been established.

The reliefs of continuous Lindhard potentials in the superstructures of carbon nanotubes were calculated for ions of carbon and hydrogen. The reliefs for hydrogen ions are presented in Figure 3. The saturation of gray tone is proportional to the logarithm of potential value.


a)	b)	c)

Figure 3 – Relief of the continuous potentials in the CNT superstructure for hydrogen ions: a – universal ion-atomic repulsive potential ZBL; b, c – Brenner potential (weak and strong coupling)

For the first time, the simulation results showed that the literature data limiting the behaviour of critical angles ψ_c(E) of ion dechanneling from the nanotubes by the energy dependence ∞E^-½, can be applied only for high-energy ions, or for the angle of the low threshold of ion penetration through the CNT wall.

The critical angles ψ_c(E) of low-energy ion channeling in the nanotubes were introduced and verified by the simulation.

The angular width in the directional dependences of the CNT stopping power and ion implantation profiles agrees well with the results of ψ_c(E) calculations by these formulae for all the potentials.

Multipeak profiles of ion implantation into the CNT bundles (Figure 4), are, for the first time, theoretically described by means of a developed phenomenological model which enables to determine, using the simulation data, the principal kinetic parameters of the directional effect.


a)	b)

Figure 4 – Results from the theoretical description of the profiles of different-energy carbon ion implantation into the CNT bundle, calculated by the molecular dynamics method, versus the ions energy (a) and the angle of beam orientation to the nanotube axis (b).