Strong Bonding of single C60 molecules to (1x2)-Pt (110): an STM/DFT Investigation
(Result of the month 07/2007)

The interaction of single C₆₀ molecules with the (1x2)-Pt (110) surface has been studied by scanning tunneling microscopy and density functional calculations on slab models. Molecules are observed to be frozen at room temperature (RT). The majority of them are adsorbed symmetrically over the troughs of the missing-row-reconstructed substrate, and are coordinated to a bridging Pt-Pt site in the substrate second atomic layer via a 5-6 C-C bond perpendicular to the trough. Extensive density functional theory (DFT) calculations show that this configuration is the global energy minimum, suggesting that adsorbed molecules have enough rototranslational freedom to escape from the numerous local minima. The adsorption energy (3.81 eV) is the strongest ever found for C₆₀, and it is roughly proportional to the number of the Pt and C atoms at contact distance. Analysis of DFT results shows that the surface-adsorbate interaction is covalent in nature. Several molecules are trapped in the second most stable energy minimum revealed by DFT calculations, which corresponds to the C₆₀ unit facing a fourfold hollow site in the substrate second atomic layer via a hexagonal ring. A minority fraction of C₆₀ molecules appears to be adsorbed on surface defects. A careful investigation of their registry and height with respect to the regularly adsorbed units leads to an indirect structural characterisation of the nano-pits which act as their adsorption sites.

(A) Constant current high resolution STM image of a cluster of C60 molecules deposited on the Pt (110) surface at RT (sample bias = 2.1 V; tunneling current = 0.42 nA; 3.1 nm x 3.1 nm). (B) the same as in (A), convoluted with the current image. (C) Model showing the molecular orientation within the cluster. Black lines with arrows highlight the LUMO nodes with respect to the molecular cage.
The three brighter molecules to the left reside in two adjacent troughs of the (1x2) missing row reconstruction of the substrate, running along the [1 ‾1 0] direction (see Figure 2). The darker molecule in the upper right corner, rotated by 90° with respect to the others, resides in a surface defect (a nano-pit) of the substrate.

First row: High-resolution constant current STM images of C60 molecules adsorbed on (1x2)-Pt (110) in (A) M1 configuration, (sample bias=+2.0 V, tunneling current=1.0 nA); (B) M1 configuration, (sample bias=-2.0 V, tunneling current=1.2 nA); (C) M2 configuration, (sample bias=+2.0 V, tunneling current=1.0 nA). All three images are 1.8 nm x 1.8 nm in size. Second row: the same as in the top row, convoluted with the respective current images. Third row: DFT simulations. The DFT images were obtained by mapping the vertical coordinate of an isodensity surface. Lighter colours correspond to regions at larger distance from the substrate. Fourth row: Top views of the DFT relaxed structures in the M1 and M2 chemisorption configurations, showing only the bottom part of C60. Large and small circles represent Pt and C atoms, respectively. Darker Pt atoms are deeper into the surface. White (black) circles represent C atoms which are bonded (not bonded) to the surface.

STM image of the (1x2)-Pt (110) reconstructed surface with ~0.02 monolayers of C60 deposited at RT (sample bias = 0.21 V; tunneling current = 1.46 nA; 62 nm x 62 nm). White circles highlight apparent dimers; white arrows point to molecules adsorbed on surface defects. The upper left inset (16.5 nm x 19 nm) highlights an apparent fullerene trimer.