Ferromagnetism in Semihydrogenated Graphene Sheet

J. Zhou, Q. Wang, Q. Sun, X. S. Chen, Y. Kawazoe, and P. Jena

Single layer of graphite (graphene) was predicted and later experimentally confirmed to undergo metal-semiconductor transition when fully hydrogenated (graphane). Using density functional theory we show that when half of the hydrogen in this graphane sheet is removed, the resulting semihydrogenated graphene (which we refer to as graphone) becomes a ferromagnetic semiconductor with a small indirect gap. Half-hydrogenation breaks the delocalized π bonding network of graphene, leaving the electrons in the unhydrogenated carbon atoms localized and unpaired. The magnetic moments at these sites couple ferromagnetically with an estimated Curie temperature between 278 and 417 K, giving rise to an infinite magnetic sheet with structural integrity and magnetic homogeneity. This is very different from the widely studied finite graphene nanostrucures such as one-dimensional nanoribbons and two-dimensional nanoholes, where zigzag edges are necessary for magnetism. From graphene to graphane and to graphone, the system evolves from metallic to semiconducting and from nonmagnetic to magnetic. Hydrogenation provides a novel way to tune the properties with unprecedented potentials for applications.

Analysis
1. Published in June 2009 this is a DFT based theoretical study that talks about the effect of hydrogenation of graphene film.

2. Fully hydrogenated graphene, also known as “graphane” was experimentally prepared by K. S. Novoselov et al. and the results were published in Science (30 January 2009, Vol. 323). It was demonstrate that hydrogenation of graphene is reversible.

3. This paper discusses how the properties of graphene film will change if half hydrogenation can be achieved.

4. It has been demonstrated that hydrogen has potential to alter the properties of non-magnetic as well as magnetic materials.

5. Both graphene and graphane are non-magnetic in nature.

6. Using DFT calculation, the author of this paper shows that half hydrogenated graphene (authors call it graphone) is ferromagnetic in nature.

7. Partial saturation of carbon atoms in graphene breaks the pi (π) bonds and the p-electrons associated with unhydrogenated bond are localized and unpaired.

8. Along with magnetic properties, electronic properties of graphene also changes due to partial (half) hydrogenation of graphene. For example graphone becomes an indirect band gap semiconductor with a very small band gap as opposed to graphene (zero band gap conductors) or graphane (very large band gap insulator).

9. Magnetism also arises in 0D graphene nanodots, 1D graphene nanoribbons and 2D graphen nanoholes due to the presence of zigzag edges. However, Okada showed that arm chair nanoribbons are more favorable than zigzag ones. Thus the challenge is to produce the carbon material with zigzag edges still remains.

10. This theoretical study shows that magnetic graphene material can be achieved by selective hydrogenation of graphene.

11. When half the carbon atoms are hydrogenated, strong σ bonds are formed between C-H atoms and pi (π) bonds are broken, thus leaving electrons in unhydrogenated C atoms unpaired and localized.

12. Authors have found that unhyddrogenated C atoms have magnetic moment of 1 µB and are main cause of magnetization due to unpaired 2p electrons.

13. Authors have found that controlling the hydrogenation, i.e. after removing the hydrogen from the hydrogenated graphene (graphone) revert the magnetism and thus makes graphene non magnetic. The reason lies in that fact that two unsaturated carbon atoms form nearest neighbor where pz orbital forms pi (π) bonds, which quenches the magnetization.

14. Based on the above claims, authors infer that controlling the amount of hydrogenation can control the magnetic properties of graphene. Thus controlling the hydrogenation and the geometry is key to achieve predicted ferromagnetism.

Conclusion

15. In conclusion, it is shown theoretically that it is possible introduce ferromagnetism into graphene by surface modification with hydrogenation.

16. This method has the following advantages over the existing ones:
*.It is not necessary to substitutionally dope C atoms by foreign atoms such as transition metal atoms or B and N.
*. It is not necessary to cut the two-dimensional graphene into finite systems with zigzag edges like one-dimensional nanoribbon or zero-dimensional quantum dots.
*. It is not necessary to introduce carbon vacancies like nanoholes in graphene, where magnetism appears at the edge of the vacancy.

17. The disavantages of the above processes are the following:
*. Integrity of graphene structure is destroyed by vacancies or by substitution with foreign atoms or cutting into nanosize, where magnetism is inhomogeneously distributed.
*. In practical applications, nanoribbons need to be assembled, and the magnetic moments in the zigzag edges can be easily quenched in the assembly.
*. Magnetism can also be introduced by transition metal adsorption, but due to the strong d-d interactions, it is easy for transition metal atoms to form clusters.

18. All these methods are difficult to control and they are not reversible. Therefore, reversibility, controllability, integrity of structure, and homogeneity of magnetism make the graphone sheet very appealing for further experimental study.