Interaction of Graphene and Templated Cluster Arrays with CO, H2, and O2

Abstract

Interaction of graphene and templated cluster arrays with CO, H2 and O2 was studied by means of scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). The experimental data was complemented by ab initio density functional theory (DFT) calculations. As a prerequisite for studies of gas interaction, the binding mechanism of the clusters to graphene, as well as the growth and structure of Pt clusters, was investigated in detail. The formation of cluster lattices on graphene on Ir(111) has been explained by graphene rehybridization. This DFT picture was tested by comparing calculated core level spectra to XPS measurements. For pristine graphene, DFT and XPS agree on a 140 meV modulation of the C 1s core level shifts (CLS), which correlates with the height modulation of the graphene layer above the Ir surface. With Pt clusters adsorbed, measured Pt 4f CLS of the adsorbed clusters also support the calculations. The modulation of the C 1s spectrum is strengthened with clusters adsorbed, and C atoms both under and in the vicinity of the Pt clusters are experimentally distinguished as a broad shoulder of positive C 1s CLS. The calculations suggest that the sp2 to sp3 rehybridization of graphene displaces the involved C atoms closer to the Ir(111) surface, implying chemical bond formation. The signature of these bonds in the Ir 4f spectrum was identified. The growth of Pt clusters, their structure, and their interaction with the graphene layer was studied as a function of Pt coverage. STM measurements revealed that once Pt clusters are two-layered, their further growth is restricted to two dimensions. The threshold for sintering was found to be at 0.75ML Pt, so that the upper size limit for Pt clusters arranged in a lattice is 65 atoms. The cluster-graphene interaction, as well as the graphene-Ir surface interaction was investigated with spectra of the C 1s and the Ir 4f regions, respectively. It was found that the related components, namely the C 1s shoulder and the Ir interface component, agree regarding their relative intensities. Combining these results, schematic representations of the clusters and their binding geometry with the graphene layer was derived. Pt cluster arrays were tested for their stability with respect to CO gas exposure. Cluster stability and adsorption-induced processes were analyzed as a function of cluster size. Small clusters containing fewer than 10 atoms are unstable upon CO adsorption. They sinter through Smoluchowski ripening – cluster diffusion and coalescence – rather than the frequently reported Ostwald ripening mediated by metal-adsorbate complexes. Larger clusters remain immobile upon CO adsorption, but become more three-dimensional. An implication of the CO-induced cluster mobility is the sensitivity of Pt cluster array growth to the CO background pressure. In order to generalize the results, the study was extended to the adsorption of other gases (H2, O2) on Pt clusters, as well as to the adsorption of CO on other metal clusters (Ir, Au). The temperature, time and flake-size-dependent intercalation phases of oxygen underneath graphene on Ir(111) formed upon exposure to molecular oxygen were studied. Through the applied pressure of molecular oxygen, the atomic oxygen created on the bare Ir terraces is driven underneath the graphene flakes. The importance of substrate steps and of the unbinding of graphene flake edges from the substrate for the intercalation is identified. Using CO titration to selectively remove oxygen from the bare Ir terraces, the energetics of intercalation were uncovered. Cluster decoration techniques were used as an efficient tool to visualize intercalation processes in real space. To give an outlook, the study was extended to the intercalation of hydrogen. Intercalation of graphene often leads doping of graphene. Comparing XPS and angular resolved photoemission spectroscopy (ARPES) data for various systems, it was found that the Dirac point and the C 1s core level shift in the same direction. These shifts can be described in terms of a rigid band model and the C 1s core level shift may be used to estimate the level of doping. It was found that graphene/Ir(111)is p-doped by H and O intercalation, whereas it is n-doped after Eu intercalation. The graphene layer is also doped by Pt and Ir clusters on top of it. The smallest clusters n-dope the graphene by charge spill-out. The largest Pt coverage results in a slight p-doping of the graphene layer. When Pt clusters coalesce upon CO exposure, the level of doping is reduced