Growth mode and electronic structure of the epitaxial ${\mathrm{C}}_{60}$(111)/GeS(001) interface

The heteroepitaxial growth of ${\mathrm{C}}_{60}$ on GeS(001) has been studied using low-energy electron diffraction, selected-area electron diffraction, high-resolution electron microscopy, x-ray diffraction, and x-ray and ultraviolet-photoelectron spectroscopy (UPS). The simultaneous observation of diffraction spots characteristic of the substrate and the ${\mathrm{C}}_{60}$(111) overlayer allows us to specify the geometry of the epitaxy. The shape of the intensity curves of the C 1s and Ge 3d photoemission lines strongly suggests a layer-by-layer-type growth, confirmed by the observation in the synchrotron x-ray diffraction spectrum of finite-size oscillations on the (111) Bragg reflection peak of a thin ${\mathrm{C}}_{60}$(111) film. From a theoretical simulation of the C 1s and Ge 3d line-intensity curves, the mean free path of a C 1s and Ge 3d photoelectron in solid ${\mathrm{C}}_{60}$ is estimated to about 15.4 and 17 Å, respectively. The plot of the film thickness versus deposition time shows evidence for a small difference in sticking coefficient between the first monolayer and the upper ones. A detailed analysis of the C 1s line shapes for normal and grazing emission suggests the existence of inequivalent carbon sites at the interface. The first valence-band feature of the substrate presents a downward band bending of about 200 meV with increasing ${\mathrm{C}}_{60}$ coverage. From the shift of the cutoff in the UPS spectra we deduce a work function increase of about 100 meV upon monolayer adsorption. The characteristic spectral features of ${\mathrm{C}}_{60}$ observed in the UPS spectra for bulk fullerite are slightly broadened and shifted to lower binding energies at submonolayer coverages and show no direct evidence for significant hybridization, indicating that the ${\mathrm{C}}_{60}$-substrate interaction is mainly dominated by van der Waals bonding. All these observations can be explained by a positive effective dipole of about 8×${10}^{\mathrm{-}31}$ C m induced on the ${\mathrm{C}}_{60}$ molecule upon adsorption onto the GeS substrate.