Supplementary Data: Tungsten–Hydrogen Complexes on Graphene on Ir(111)

W and W–H complexes on graphene on Ir(111) were studied by means of low temperature scanning tunneling microscopy. H is found to coadsorb at the sample surface during W evaporation. Five different W-related structures on graphene on Ir(111) were found and characterized. They are identified as WHn, with n = 0 , 1 , 2 , 3 , 4 ?> , as shown by electric field induced dehydrogenation. They exhibit a number of peculiar effects, such as electric field induced shifts of spectroscopic features as well as reversible switching and charging effects. In addition, two different structures of H on Ir(111) were characterized.


I. H ON IR(111)
As explained in the main article, a line-like pattern could be observed after the adsorption of H at the Ir(111) surface at 35 K and at a low coverage (see Fig. 1a). A comparison to atomically resolved STM topographs of graphene on Ir(111) (not shown) yields that the line-pattern is oriented along the [110] directions of the Ir(111) surface. Tunneling at higher voltages (e.g., 1 V) or at large currents (e.g., when contacting the surface with the STM tip at low voltages) leads to changes of the pattern as illustrated in the inset of Fig. 1a. The low stability of the line-pattern under the STM tip is consistent with previous reports of a low diffusion barrier of H on Ir(111) 1,2 .
At higher H 2 exposure (e.g., 1 Langmuir) regions with a hexagonal pattern (see Fig. 1b) were present in addition to the line pattern mentioned above. Like the latter one, the hexagonal structure could easily be modified by the STM tip. Often an increase of the apparent height in the downwards pointing triangles of the hexagonal structure was observed (Figs. 1b and 1c). Occasionally such changes occured at voltage as low as 50 mV (current 100 pA). However, they became more frequent at higher bias. At low coverages the line-pattern is found only on narrow terraces. This observation can be explained as follows.
Step edges locally increase the reactivity of a metal surface towards the dissociative adsorption of hydrogen 4 . Thus, close to step edges the H density is increased and the critical H density of ≈ 25% of a monolayer 5 needed for the formation of the line-structure is first reached on narrow terraces. Confinement effects of delocalized H on narrow terraces may enhance the accumulation of H in these areas.
The hexagonal structure has the same orientation as the Ir(111) lattice. Its periodicity of ≈ 0.8 nm suggests a commensurate 3 × 3 superstructure. An atomistic model for the hexagonal structure is proposed in Figs. 1b and c. Since a higher hydrogen dose is needed to form this structure its H density is expected to be higher than in the line-pattern. If  every third row of Ir atoms is occupied as illustrated by the lines in Fig. 1b and c, a honeycomb lattice of unoccupied lattice sites remains. Motivated by a related observation from H on Cu 3 , we expect that the protrusions of the hexagonal pattern are due to pristine substrate atoms. Consequently, the Ir atoms in the upwards pointing triangles that are defined by the line-pattern have to be occupied by H atoms. Thus, in this model each protrusion corresponds to an unoccupied Ir site. The observed switching is consistent with the proposed structure. It corresponds to the removal of a single H atom from an occupied lattice position that is not in the line-structure. With the neighboring unoccupied atom the dehydrogenated site thus forms a downwards pointing triangle, as observed.
In the above model, the hexagonal structure corresponds to a hydrogen coverage of ≈ 89% of a monolayer. Further increase of the coverage should cause the hexagonal structure to disappear. Indeed, some areas appear flat in STM topographs at high hydrogen coverage.
Moreover, flat areas occasionally transformed to the hexagonal structure with a fraction of triangular protrusions during imaging as expected for a random removal of H atoms. * altenburg@physik.uni-kiel.de