Análisis de Resultados del AG

Figure 3.38a displays an STM image taken after 45 min deposition of gold at room temperature on the surface partially covered with graphene. The image displays six metal terraces partially covered by graphene areas recognizable by the characteristic moiré periodicity. The light-grey features with irregular shapes are gold islands. The coverage of gold on the bare ruthenium areas is about 0.5 ML, while on the graphene areas it is 0.08 ML. Gold obviously nucleates preferably on the ruthenium areas where the bonding to the substrate is stronger than on graphene and the total energy is minimized. Furthermore, there is a high concentration of gold islands on the bare ruthenium at the edges of the graphene layers. The graphene edges can act as nucleation centres for the gold islands in a similar way as the dislocation sites or the lower step edges of bare ruthenium [111]: gold particles deposited on the surface diffuse and get trapped at the edges of the graphene layer where the islands formation then starts. It can, however, not be ruled out that this phenomenon is an effect of the scanning tip of the STM. By scanning, the weakly bounded islands on graphene could be moved by the tip to the bare ruthenium areas where they bind more strongly and are not affected by the microscope tip any more. It would be desirable to systematically investigate this effect performing STM measurements at various tip-sample distances.

Another interesting result is the shape of the gold islands: on graphene they exhibit a more compact shape than on the metal areas, and the fractal growth is not observed (figure 3.38b). This indicates that the diffusion of gold atoms at the edges of the islands grown on graphene is not limited as on the ruthenium areas so that the islands can assume compact shapes even at room temperature.

Figure 3.38b shows a gold island grown on graphene in detail. Interestingly it displays the same moiré modulation as the graphene layer with a modulation amplitude of about 0.5 Å. The apparent height of the island above the graphene layer thus oscillates between 3.2 and 3.7 Å

Using the height difference between gold islands grown on the bare Ru and islands grown on graphene one can estimate the height of the graphene layer above the metal. This estimate is based on the assumption that the local density of states of the Au islands is the same independently on the substrate on which they have grown. A height oscillating between 1.8 and 2.3 Å is obtained which is in agreement with the results of paragraph 3.2.3 I.

Unfortunately, gold deposited at room temperature on graphene does not show the formation of ordered arrays of clusters with one cluster per moiré unit cell, but shows, on the contrary, the tendency to form large compact islands.

a)

b)

Figure 3.38: STM images recorded after 45 min of Au deposition on the surface partially covered

with graphene. a) The image shows six metal terraces partially covered with graphene. The gold coverage on the metal areas is much higher than on graphene and several islands appear at the edges of the graphene layer. It = 1 nA, U= -0.7 V, 200 nm x 200 nm. b) Au island grown on

graphene. The islanddisplays the same height modulation as the graphene moiré structure. The compact shape is different from the dendritic shape of the islands grown on the bare metal. It = 1

Chapter 3 Graphene on Ru(0001)

Goriachko and co-workers deposited gold on the (12x12) structure formed by hexagonal boron nitride deposited on the Ru(0001) surface [71]. After annealing the sample to 900 K they observed the formation of well defined round nanoparticles of gold randomly distributed over the surface. This behaviour is completely different from the behaviour of iridium deposited on the (11x11) structure formed by graphene on the Ir(111) surface [68]. In this work the formation of regular arrays of Ir clusters was observed with tunable sizes that were stable up to 500 K. However, in the case of Ir, the surface could not be completely covered with the graphene layer and the regular arrays of Ir clusters were then only observed on the graphene islands.

These preliminary results show that Au can be deposited with the newly built source in a clean and controlled way. However, the islands formed on the graphene are too large which is most likely caused by a too high mobility of the Au atoms on the graphene at room temperature. In the future the temperature will therefore be lowered and the deposition of other metals will be tried. From its homogeneity the Ru(0001)/graphene system has great potential for engineering 2D arrays of nanoparticles.

Chapter 4

CO poisoning of O/Rh(111)

One of the key issues in the study of oxygen adsorption on transition metal surfaces is the saturation coverage. The saturation coverage is the main calibration point for surface analytical techniques, and it enters kinetic models of catalytic reactions. It is further a measure of the reactivity of a given metal.

Recently, there has been a debate about the oxygen saturation coverage on Rh(111), with several groups suggesting 0.5 ML and a STM group measuring only a (2x2) structure compatible with a maximum coverage of 0.25 ML. It will be shown here that the source of these discrepancies is the presence of CO molecules poisoning the rhodium surface and modifying its reactivity.