Temperature-dependent self-assemblies of C₆₀ on (1x2)-Pt (110): an STM/DFT Investigation
(Result of the month 03/2008)

A) High resolution STM image of the equilibrium oblique C₆₀ chemisorption phase on (1x2)-Pt (110), obtained after annealing at 850 K; bias=2.1 V; I_T=1.28 nA; 11 nm x 11 nm. The inset shows the linear section through the vertical white line, indicating the apparent height difference between the bright (B) and dim (D) C₆₀ molecules in a row aligned along [5(overbar)58]. The black line evidences the peculiar zig-zag pattern of the B – D contrasted phase at the chosen positive bias.

B) Close-up of the same phase (6 nm x 6 nm), with the indication of the STM-derived superstructure unit cell.

C) the same as in B), convoluted with the current image. Two different orientations for both B and D molecules (B₁, B₂ and D₁, D₂, respectively.) are detected. B₁ (D₁) is related to B₂ (D₂) by an azimuthal rotation (i.e. a rotation with respect to an axis perpendicular to the surface and passing through the molecular centre of mass) of approximately 180°.

D) Possible models for the C₆₀/Pt (110) interface in the oblique phase, as derived from STM and LEED measurements: molecules adsorbed on a network of bi- and triatomic nanopits on the unreconstructed substrate surface (left); molecules adsorbed on the unreconstructed Pt (110) surface (right). B (D) molecules are represented with light (dark) grey C atoms with dark (light) topmost pentagon-hexagon rings. Pt atoms are the larger grey spheres. Darker Pt atoms are deeper into the surface.

Centre: experimental STM images; (bias=+2.0 V; IT=1.28 nA;1.8 nm x 1.8 nm) of a B (top) and of a D (bottom) molecule within the oblique chemisorption phase obtained after annealing at 850 K. The white circle highlights the single molecule boundary. The nodal feature separating the three main lobes of the charge density distribution is highlighted by three white segments.

Left: Chemisorption configurations and simulations of the STM images within the Tersoff-Hamann approximation for C₆₀ on a triatomic (V1) and a biatomic (V2) nanopit on (1x1)-Pt (110), as deduced by DFT calculations. Molecules bond to the surface approximately via a 5-6 C-C bond (b56). The polar tilt q is indicated in the top right corner. It represents the molecular tilt with respect to an ideal b56 bonding configuration, where the plane containing the topmost and the bottommost 5-6 C-C bonds would be exactly perpendicular to the surface. The direction of polar tilt is highlighted by means of red arrows.

Right: The same for the N1 (top) and N2 (bottom) chemisorption configurations for C₆₀ on unreconstructed (1x1)-Pt (110), as deduced by DFT calculations. Dh is the apparent height difference between the two bonding configurations at the given bias value deduced from the experimental/simulated STM images. Both Dh and the relative intensity and orientation of the intramolecular fine structure are better reproduced by the N1-N2 choice, where molecules tilt in opposite directions along the [1(overbar)10] direction.

A) (top left): STM image of a fullerene island obtained after dosing 0.2 ML of C₆₀ on the (1x2)-Pt (110) surface and annealing at 700 K; bias=0.76 V; IT=0.66 nA; 15 nm x 15 nm. Quasi-hexagonal unit cells are highlighted in black. The unit vector length is 9.6 Å, corresponding to a 4.5 % contraction of the intermolecular nearest neighbour (NN) distance with respect to bulklike fullerite (10.05 Å), and resulting in the densest 2D fullerene phase reported to date in the literature. Red dashed lines connect sites belonging to different registry domains at a mutual separation of 9.2 Å. The two domains (Regular, RDo, and Shifted, ShDo) are related by an adsorbate-induced rigid shift of several Pt close-packed rows by one Pt lattice parameter along the [001] direction. Molecules at the domain boundary are too close for VdW interactions. However the interdomain NN distance is in agreement with the equilibrium distance in C₁₂₀ dumbbell dimers (see the molecular model in the centre)

B) (top center): Solid sphere model of the phase boundary. C₆₀ molecules are not shown for clarity. C₆₀ chemisorption sites are marked in dark grey. Topmost Pt atoms are white.

C) (right) Close-up of the boundary where the fullerene dumbbell dimers formation occurs. RDo is marked by dark grey chemisorption sites in the lower part of the model. Occasionally, a molecule diffusing on ShDo approaches the domain boundary at a distance of 9.2 Å (site Sh1) from the NN molecule (site R1) belonging to RDo and together they form a dumbbell dimer. Accessible sites for a C₆₀ molecule approaching the already formed R1-Sh1 dimer are in green and marked Sh2a and Sh2b, while unaccessible sites are in red and marked by crosses. If close packing has to be maintained, the upcoming molecule is forced to adsorb on Sh2a, thereby forming a second dimer with the C₆₀ molecule residing in R2. By recursion of this "molecular zip” packing scheme based on a site-blocking mechanism, the dimerized linear domain-boundary can be explained. The relevant inter-site distances are indicated.

D) (bottom left) STM image of another fullerene island obtained after dosing 0.2 ML of C₆₀ on the (1x2)-Pt (110) surface and annealing at 700 K; bias=0.26 V; IT=1.13 nA; 18 nm x 18 nm. The black rectangle highlights a nanowire of 10 C₁₂₀ dumbbell dimers embedded in the quasi-hexagonal phase.