Atomic Hydrogen Adsorbate Structures on Graphene

Richard Balog, Bjarke Jørgensen, Justin Wells, Erik Lægsgaard, Philip Hofmann, Flemming Besenbacher, and Liv Hornekær

Analysis
1. This paper is published in |J|A|C|S| Communications in May, 2009 and presents Scanning Tunneling Microscopy study of hydrogen adsorption on graphene grown by thermal annealing of silicon carbide (SiC).

2. Chemical reaction of hydrogen atom with graphene is a potentially viable method to introduce bad gap in graphene (semi metal) to use it for electronic devices.

3. Authors present Scanning Tunneling Microscopy (STM) study of adsorbed hydrogen atoms at the basel plane of graphene grown on SiC.

4. At low coverage, hydrogen dimer structure and at high coverage hydrogen clusters are observed by STM.

5. Hydrogen dimer formation occur at the high tunneling probability area of graphene that is altered by 6×6 reconstruction of SiC(0001) (1×1) surface.

6. Observed hydrogenation of graphene on SiC was reversible by thermal annealing.
7. For hydrogenation graphene was exposed to 1600 K D-atom beam for 5 s at a flux of 1012-1013 atoms/cm2 s.

8. This article talks about the papers that have demonstrated that ortho and para positions are most favorable for hydrogen attachment i.e. energetically ortho and para dimmers are most stable configuration at the basel plane of graphene and graphite (1-3).

9. Authors did not observe significant difference in dimer formation on graphite or graphene. In their study, hydrogen monomers were also observed on graphene surface as opposed to fact that there is no reported study of hydrogen monomer on graphite.

10. The observation of monomer suggests that atomic hydrogen is more strongly bind to graphene than graphite.

11. Authors have fond that at low coverage the majority of hydrogen adsorbate follows 6x6 modulation of the SiC surface.

12. Both theory and experiment have suggested that deformation plays a key role in the reactivity of graphene.

13. Barrier for sticking decreases for higher curvature surface and binding energy increases. The decrease in barrier for sticking can be explained by hydrogen atom (H) chemisorptions causes change of sp2 hybridization to sp3, thus relaxation of carbon atoms towards hydrogen adsorbate.

14. During STM study and hydrogen deposition the sample was kept at room temperature. In STM adsorbed hydrogen should be visible as bright protrusion.

15. As the coverage is increased hydrogen tends to make large clusters similar to hydrogen on graphite.

16. It has been predicted theoretically that one sided hydrogenation is thermodynamically unstable (4,5).

17. Authors have observed that tip induced tunneling can desorb the hydrogen from the surface that implies weak binding of hydrogen with the graphene surface. They also observe that graphene can be recovered after annealing the hydrogen adsorbed surface at 800 0C.

Conclusion
18. Authors have studied graphene grown by thermal annealing on SiC (0001) substrate and found presence of different dimer structure at low hydrogen coverage and hydrogen clusters at high hydrogen coverage (high coverage of hydrogen is similar to the hydrogen on graphite)

19. Authors have observed tip induced desorption of hydrogen from graphene surface and also find the graphene can be recovered after annealing the hydrogenated graphene at 800 0C.

References
(1) Casolo, S.; Lovvik, O. M.; Martinazzo, R.; Tantardini, G. F. J. Chem. Phys.2009, 130, 054704.
(2) Hornekaer, L.; Sljivancanin, Z.; Xu, W.; Otero, R.; Rauls, E.; Stensgaard, I.; Laegsgaard, E.; Hammer, B.; Besenbacher, F. Phys. ReV. Lett. 2006, 96, 156104.
(3) Ferro, Y.; Teillet-Billy, D.; Rougeau, N.; Sidis, V.; Morisset, S.; Allouche, A. Phys. ReV. B 2008, 78, 8.
(4) Sofo, J. O.; Chaudhari, A. S.; Barber, G. D. Phys. ReV. B 2007, 75, p153401.
(5) Boukhvalov, D. W.; Katsnelson, M. I.; Lichtenstein, A. I. Phys. ReV. B 2008, 77, 035427