Identifying Molecular Orientation of IndividualC60on aSi(111)−(7×7)

Abstract

Low temperature scanning tunneling microscopy (STM) has been used to identify the molecular orientation of individual C 60 on Si͑111͒-͑7 3 7͒ surfaces. The STM images of individual C 60 reveal clear and rich intramolecular features that are site and bias dependent. Theoretical simulations, using the local density approximation method with cluster models, uniquely reproduce the observed STM images and hence allow the unambiguous identification of the binding configurations of the adsorbed fullerenes with respect to the Si substrate. PACS numbers: 82.65.My, 61.16.Ch, 61.48. + c, 71.15.Mb Chemically stable, cage-structured molecules such as fullerenes form an interesting new family of adsorbates on surfaces. They differ significantly from the elemental or simple molecular adsorbates because of their threedimensional character on the atomic scale. A unique fundamental property of this type of adsorbates is the molecular orientation with respect to the host substrate. When an isolated molecule ceases its rotational motion on a surface, it may, in general, adopt a number of binding configurations and hence a multitude of orientations. Interaction between the molecules could yield still other orientational arrangements. This new property could serve as the basis for designing new catalysts with functionalized cage molecules or for fabricating thin films of desirable orientational orders. Thus, to understand how a cage molecule orients itself on a substrate is obviously a worthy but challenging task both theoretically and experimentally.Owing to its elegant cage structure comprised of 12 pentagons and 20 hexagons, the C 60 molecule has attracted considerable interest since its discovery [1,2], and many attempts have been made to unveil its internal cage structure with scanning tunneling microscopy (STM) [3][4][5][6]. It was hoped that STM may directly image the individual C 60 molecules, so that the orientation of C 60 molecules on a specific adsorption site of the supporting substrate could be determined. So far, only a few authors reported STM images of C 60 that reveal some intramolecular features but could not be directly correlated with the cage structures. For example, STM images with four slightly curved bright stripes were typically obtained for C 60 adsorbed on the Si(111) and Si(100) surfaces [3,4,7], regardless of their different surface atom arrangements and binding configurations. Even on a Au(110) surface, C 60 also appears as 2-3 bright stripes in the STM image [5]. Threefold symmetric intramolecular features were first observed for a close-packed C 60 layer on a Cu(111) surface [6] and were shown to be caused by the charge transfer from Cu to C 60 . Curiously, a similar image was also obtained for C 60 observed on a Ge(111) surface [8]. Apart from showing that the adsorbed C 60 molecules have fixed orientation on the surfaces, these experiments have left the challenge to identify the specific molecular orientation largely unanswered.

Unveiling Metal-Cage Hybrid States in a Single Endohedral Metallofullerene

“…2(a)]. Such an intramolecular pattern has been observed in STM images of other fullerene molecules such as C 60 and C 70 adsorbed on different surfaces, and ascribed to the combined contributions of the fullerenes and substrates [16,17]. The large positive bias STM images [Figs.…”

Calculation of images of orientedC60molecules using molecular orbital theory

“…Also, as mentioned above, the exact symmetry of the local field is not as important as the orientation of the C 60 molecule. 3 The biggest effect of the field due to the substrate on the C 60 molecules is to lift degeneracies in the molecular orbitals. None of the site groups support irreps.…”

“…On a Si͑111͒ 7 ϫ 7 surface, it appears that C 60 is oriented such that either a single bond or an edge atom faces the surface. 3 Similarly, on a Si͑100͒ 2 ϫ 1 surface there is an indication that C 60 exhibits a twofold symmetry, 10 which would be indicative of either a single or a double bond facing the surface. 7 While intermolecular interactions between C 60 molecules in a monolayer will have some effect on the observed images, these will be neglected in this paper.…”

“…However, there is found to be only a weak dependence on the adsorption site. 3 We will therefore only consider the effect of different orientations here. Care must be taken when deducing the orientation of C 60 molecules from STM images as they show the electron density and not the position of the carbon atoms directly.…”

Spheroidal geometry approach to fullerene molecules

“…Particular attention was given to peculiar electronic states due to topological defects which were observed in different kinds of carbon nanoparticles by scanning tunneling microscopy (STM). For example, STM images with five-fold symmetry (due to pentagons in the hexagonal graphitic network) were obtained in the C 60 fullerene molecule [3]. The peculiar electronic properties at the ends of carbon nanotubes (which include several pentagons) were probed experimentally in [4,5].…”