AFM observation of surface reconstruction of C 60 single crystals

Abstract

We report an atomic force microscopic (AFM) examination of the photo-induced surface reconstruction of C 60 single crystals. The (111) and the (001) faces of C 60 single crystals were illuminated with photons with energies below the band gap (generation of Frenkel-type excitons) or above the band gap. In the case of sub-band-gap illumination, reconstructed surfaces formed on the crystals. The reconstruction was represented by fringes running in the [112] and [010] directions on the (111) and (001) surfaces, respectively. As a result of supra-band-gap illumination, however, there appeared only cracks, without any fringes, which were oriented in directions characteristic of each face. The results suggest that the photo-induced surface reconstruction of C 60 single crystals is induced via the recombination of Frenkeltype excitons.Since the discovery of C 60 [1], there have been numerous investigations on its structural and optical properties in the solid state [2]. Molecules of C 60 condense at room temperature into a face-centered cubic (fcc) structure held by rather weak intermolecular van der Waals forces [3]. A great number of studies of the surface morphology of epitaxial fullerene films on various types of substrate [4,5] have been conducted with atomic force microscopy (AFM) and/or scanning tunneling microscopy (STM). We recently also carried out AFM studies of the morphology and surface dislocations of fullerene single crystals and epitaxial films [6][7][8][9][10][11]. The reconstructed (0001) face of C 70 single crystals exhibited a long-range periodic array of partial dislocation fringes [8]. In a related fashion, AFM images of the reconstructed surface of C 60 epitaxial films on KBr(001) substrates showed many fringes in the equivalent [110] directions on the fcc(111) plane [11]. In the latter case, molecular-resolution images revealed that the reconstructed surface is contracted by ca. 3% in the [112] direction. Both of these observations suggest a stacking-faultdomain model involving periodic transitions between fcc and hexagonal-closed packed (hcp) structures in the surface layer.Solid C 60 is a molecular semiconductor with a band gap of 2.3 eV [12], and weak optical absorption in the range of 1.6 to 2.2 eV, which corresponds to Frenkel-type excitons [12,13]. Previous to the present work, several reports have appeared concerning photo-transformations of solid C 60 by optical illumination. First, the irradiation of oxygen-free C 60 films with an Ar-ion laser at 514.5 nm (2.41 eV) and 488.0 nm (2.54 eV) was focused to initiate the irreversible polymerization of C 60 [14][15][16]. Second, in the presence of oxygen, C 60 undergoes oxidation upon uv light irradiation [17,18]. Both the photo-induced polymerization and the photoinduced oxidation processes are initiated by supra-band-gap illumination. In this paper, we report an AFM investigation of photo-induced structural transformations of C 60 singlecrystal surfaces using both sub-band-gap and supra-band-gap illumination.

Photoinduced Dislocation Lines on the (111) Face of C60 Single Crystals

“…To confirm the wavelength threshold for photoinduced surface reconstruction, we also examined the effect of illumination of the surface of the C 60 single crystals with various photon energies, e.g., 3.81, 2.78, and 2.41 eV. 54 Cracked surfaces were created when the light energy was greater than the band gap (>2.3 eV). Figure 7 shows a cracked (111) surface for a C 60 single crystal illuminated at a photon energy of 2.41 eV.…”