C60superstructure and carbide formation on the Al-terminatedAl9Co2(001)surface

Abstract

We report the formation of an ordered C 60 monolayer on the Al 9 Co 2 (001) surface using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), and ab initio calculations. Dosing fullerenes at 300 K results in a disordered overlayer. However, the adsorption of C 60 with the sample held between 573-673 K leads to a [4, −2|1,3] phase. The growth of C 60 proceeds with the formation of two domains which are mirror symmetric with respect to the [100] direction. Within each domain, the superstructure unit cell contains six molecules and this implies an area per fullerene equal to 91Å 2. The molecules exhibit two types of contrast (bright and dim) which are bias dependent. The adsorption energies and preferred molecular configuration at several possible adsorption sites have been determined theoretically. These calculations lead to a possible scheme describing the configuration of each C 60 in the observed superstructure. Several defects (vacancies, protrusions,. . .) and domain boundaries observed in the film are also discussed. If the sample temperature is higher than 693 K when dosing, impinging C 60 molecules dissociate at the surface, hence leading to the formation of a carbide film as observed by STM and LEED measurements. The formation of Al 4 C 3 domains and the molecular dissociation are confirmed by XPS/UPS measurements acquired at different stages of the experiment. The cluster substructure present at the Al 9 Co 2 (001) surface dictates the carbide domain orientations.

Quasi-ordered C60molecular films grown on the pseudo-ten-fold (1 0 0) surface of the Al13Co4quasicrystalline approximant

“…It corresponds to an hexagonal structure with lattice parameters a = 3.34Å and γ = 120 • . A similar structure was reported for C 60 films on related Al 9 Co 2 (001) surface [29] and is consistent with the formation of an aluminum carbide thin film (Al 4 C 3 , R3m, a = 3.33Å and b = 24.97Å, γ=20 • ) [48]. The diffraction ring is essentially diffuse although some weak spots can be distinguished.…”

“…The corresponding adsorption energy is −3.31 eV, which is similar to the value calculated for C 60 on 5-fold Al-Pd-Mn quasicrystalline surface (−3.20 eV) [27] and on Al 9 Co 2 compound (−3.18 eV) [29]. In the latter case, the most favorable adsorption site is also a 4-fold site, located just above a subsurface Co atom.…”

“…The order is mediated by preferred adsorption of the C 60 molecules with a pentagonal face on top of a local five-fold symmetric site [27,28]. For similar growth conditions, a novel C 60 superstructure has also been reported on the Al 9 Co 2 (001) surface, a related intermetallic [29]. The unit mesh contained six molecules and ab initio calculations showed that they adopt two main adsorption configurations on the intermetallic substrate.…”

“…The Al 2p core-levels are slightly shifted by 0.2 eV towards higher binding energies upon dosing and annealing at 693 K. Annealing the film at higher temperatures leads to a broadening of the Al 2p spectrum ascribed to the presence of an extra feature on the high binding energy side at about 73.65 eV. [29] In addition, a new component is also observed at 282 eV in the C 1s spectrum upon annealing above 693 K. The intensity ratio of this additional component with respect to the main peak at 284 eV is growing with increasing annealing temperature as well as with annealing time. These changes in XPS core-level spectra are interpreted by the formation of aluminum carbide [29].…”

Strain-Relief Patterns and Kagome Lattice in Self-Assembled C60 Thin Films Grown on Cd(0001)

“…In the fullerene family, C 60 was the first member to be created and most extensively discussed. C 60 molecules form various self-assembled structures when deposited on different types of substrate, such as Au [4][5][6][7][8][9], Ag [10][11][12][13][14], Cu [15,16], Pb [17], Pd [18], P [19], Al [20], graphene [21][22][23][24], WSe 2 [25], Si [26][27][28][29], and Ge [30][31][32]. Many results indicate that C 60 molecules are easy to nucleate at the terrace edge and may form a close-packed structure under appropriate conditions, regardless of the types of substrate, such as the 2 √ 3 × 2 √ 3 R30 • domain on Au(111) [33,34] and the 4 × 4 superstructure on graphene/Cu(111) [22].…”