The mechanism of vacancy formation on the surface of Pt(111) upon C60 deposition

Abstract

It has been recently shown that the interaction of fullerenes with transition metal surfaces leads to the formation of one vacancy per C60 molecules in the metal [1]. We show by a combination of Scanning Tunneling Microscopy (STM), and real time-X-ray Photoelectrons Spectroscopy (XPS) that a charge transfer from the surface towards the molecule precedes the vacancy formation. As this charge is returned to the surface, the ejection of a surface atom takes place. At RT (Fig. 1-a), all the molecules were observed by STM to have an Apparent Height (AH) of 6.6 ± 0.3 Å (they will be called M molecules from now on), and to be oriented in different position [2]. However, at about 500 K (Fig. 1- c) something drastic happens to the sample: brighter molecules (named B, AH = 7.5 ± 0.3 Å) and dimmer molecules (called D, AH = 4.8 ± 0.5 Å) were observed, together with M molecules. B and D fullerenes are all sitting on a hexagon (as it can be seen by the molecular orbital resolution in Fig. 1-c) and all are oriented in the same way. Further annealing to 600 K allowed no more M molecules, and all the fullerene were either B or D. Moreover, above 675 K all the fullerenes were of type D. Therefore, an overall decrease in AH occurs (from M to D), which is compatible with the vacancy formation. However, this change is not direct, all the molecules pass through a B phase first. In order to explain this process, fast-real time XPS has been performed, to track the changes of the core level C1s peak at different temperatures on 0.6 ML of C60 on Pt(111) (Fig. 2). The intensity of the C1s peak is plotted in terms of the temperature (Y axis) and of the binding energy (bottom X axis). A shift towards lower Binding Energy (BE) of the main C1s peak of 0.16 eV occurs at 450 K, which is consistent with charge transfer to the molecule, the same which is observed by STM. Thus, charge accumulation on the molecules, starting at about 450 K, could be a precursor stage to the vacancy formation as observed in the STM as an increase of the apparent height of the molecule and by XPS as a shift in binding energy of the C1s peak.