Growth mode and electronic structure of the epitaxialC60(111)/GeS(001) interface

Abstract

The heteroepitaxial growth of C60 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 C60(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-bylayer-type growth, confirmed by the observation in the synchrotron x-ray diffraction spectrum of finitesize oscillations on the (111)Bragg reflection peak of a thin C60(111)film. From a theoretical simulation of the C ls and Ge 3d line-intensity curves, the mean free path of a C 1s and Ge 3d photoelectron in solid C60 is estimated to about 15. 4 and 17 A. , respectively. The plot of the film thickness versus deposition time shows evidence for a small difference in sticking coefBcient 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 C60 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 C60 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 C60-substrate interaction is mainly dominated by van der Waals bonding. All these observations can be explained by a positive effective dipole of about 8 X 10 ' C m induced on the C60 molecule upon adsorption onto the GeS substrate.

Structural defects and epitaxial rotation of C60 and C70(111) films on GeS(001)

“…31 This work presents a high resolution transmission electron microscopy ͑HREM͒ study of fullerene films on GeS͑001͒ and investigates the relationship between the lattice mismatch and the observed epitaxial rotations. Although epitaxial C 60 ͑111͒/GeS͑001͒ have been reported to be single crystalline, 21 the present HREM experiments show the occasional presence of epitaxial rotations. Similar epitaxial rotations are more frequently observed in C 70 epitaxial films due to the larger lattice mismatch ͑about 6.4%͒.…”

“…An outstanding crystallographic quality was observed by LEED for the clean substrate and for the thin and thick C 60 films; the LEED patterns were diffuse for the C 70 films. 21,37 Preparation of the samples for TEM was done a͒ by polishing and ion milling the substrate side of the sample until a small hole was formed. The resulting wedge-shaped crystal areas contain both the fullerene film and substrate.…”

“…Heating the substrate resulted in well-ordered films with domain sizes exceeding 100 nm. 15,17,19,20 The best crystalline quality was obtained for films grown on lamellar substrates like GeS͑001͒, 18,21 GaSe͑0001͒, 22,23 MoS 2 ͑0001͒, 22,[24][25][26] and mica͑001͒. [27][28][29][30][31] The low surface energy of these materials and the absence of unsaturated chemical bonds at their surface relax the lattice matching conditions required for epitaxial growth and favor surface diffusion of the deposited molecules.…”

Optical Properties of Nanostructured Silicon

“…Although this technique usually suffers from extremely low count rates, the capability to discriminate more strictly against (the approximately constant background) fluorescence from other elements in a composite sample make it in certain cases useful. The signal can be detected by choosing a suitable energy window in the spectrometer and integrating the count rate for each incoming photon energy step [75]. Figure 17 is a typical XAS taken at Si L 2,3 region from porous silicon.…”

Ions colliding with clusters of fullerenes—Decay pathways and covalent bond formations

“…19 For high projectile charge states, the critical distances for electron transfer to the ion are large. In the fullerite bulk limit and for large clusters of fullerenes, the work function of 4.7 eV 20 13 . Here, the total ionization cross sections include single-and multipleionization and related cluster fragmentation processes.…”