No 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.

We investigated hydrogen absorption/desorption and structural properties in mechanically milled graphite under hydrogen pressures up to 6 MPa to clarify catalytic and hydrogen pressure effects in the milling. The results indicate that a small amount of iron contamination during milling plays a quite important role as a catalyst for hydrogen

Orientationally ordered structures of two-dimensional (2D) C(60) at low temperature have been investigated theoretically and experimentally. Using total energy optimization with a phenomenological potential, we find the ground state is a close packed hexagonal lattice in which all the molecules have the same orientation. Several local minima of the potential energy surface are found to be associated with other 1 x 1 lattices as well as 2 x 2 lattices. The energies of the orientational domain boundaries of the 1x1 lattices are also computed, and two kinds of which yield negative values. A majority of these theoretical findings are confirmed by our low-temperature scanning tunneling microscopy study of a 2D C(60) array supported on a self-assembled monolayer.