Abstract Submitted to the    NANOTUBE'04 Conference:

It is well know that irradiation of graphite and carbon nanostructures results in the production of defects, especially vacancies and adatoms on the surface. Theoretical calculations show that these defects are magnetic [1], but the high adatom mobility suggests that recombinationand clustering is likely to significantly reduce the effect at room temperature. Recent experimental results [2] indicate that proton bombardment of graphite results in a strong residual magnetic signal, but He ion bombardment has a much smaller effect. To attempt to understand this, we have performed spin-polarized density functional theory calculations of the properties of H and He adsorbed on an ideal and defected graphene sheet.

We find that helium is very weakly interacting with surface, and although magnetic when adsorbed at a vacancy, this is a very energetically unfavourable site. For hydrogen, we find that it will adsorb strongly at the vacancy site, resulting in a magnetic moment double that of the naked vacancy - combined with its high mobility and recombination energy on graphite, hydrogen at a vacancy represents a strong candidate for explaining the experimental results. Further, we consider the properties of H adsorbed at vacancy sites on single-walled carbon nanotubes, and discuss how the magnetic properties depend on the tube physical and electronic structure.

[1] P. O. Lehtinen, A. S. Foster, A. Ayuela, A. Krasheninnikov, K. Nordlund and R. M. Nieminen Phys. Rev. Lett 91 (2003) 017202
[2] P. Esquinazi, D. Spemann, R. Höhne, A. Setzer, K.-H. Han, and T. Butz Phys. Rev. Lett. 91, 227201 (2003)