In situphotoelectron spectroscopy investigation of silicon cluster growth on fullerene surfaces

The growth of Si clusters on the C₆₀ surface was observed in situ using photoelectron spectroscopy in the ultraviolet (UPS) and x-ray regimes. The highly corrugated C₆₀ surface provides a wide range of bonding sites and might function as a template for the formation of cluster superstructures. The Si accumulates first in the interstitial bonding sites, which present a bonding environment defined by the fullerene molecules and dominated by carbon atoms. This arrangement is comparable to the formation of a surface fulleride and the interstitial sites apparently act as primary nucleation sites. The clusters then continue to grow and the SiSi bonding environment gains in importance. A numerical fit of the core level spectra (C1s and Si2p) reveals the relative contributions of the different bonding environments as the cluster growth proceeds. The absence of a SiC contribution excludes a substitutional doping of the fullerene cage with Si. The impact of initial and final state effects due to changes in the photoemission process as the cluster size is varied, can be identified and separated from the influence of the chemical shift (variation in bonding environment) on the core level position and shape. The UPS valence band spectra confirm the absence of a charge transfer or directed covalent bonding between the Si adsorbate and the fullerene surface. The valence band offset between the amorphous Si and the C₆₀ amounts to 1.20.3em0exeV, and the position of the C1s core level is likely pinned by the a-Si dangling bonds. The C₆₀ can function as a template for the formation of Si cluster arrays (superstructures) with the interstitial sites as primary nucleation sites and an expansion to three-dimensional structures is possible.