${\mathrm{C}}_{60}$ on strain-relief patterns of $\mathrm{Ag}∕\mathrm{Pt}\left(111\right)$: Film orientation governed by template superstructure

The adsorption, growth and overlayer formation of ${\mathrm{C}}_{60}$ fullerene molecules on strain-relief patterns induced by 2 monolayers of Ag on Pt(111) (Ag(2ML)/Pt(111)) is investigated by means of scanning tunneling microscopy. The Ag(2ML)/Pt(111) template surface consists of periodic crossing dislocations separating face-centered cubic (fcc) and hexagonal close-packed (hcp) stacking domains. At room temperature, ${\mathrm{C}}_{60}$ molecules are found to be sufficiently mobile on the template surface to cross the dislocations and to self-assemble into large hexagonally close-packed two-dimensional islands. The nucleation and growth of the ${\mathrm{C}}_{60}$ islands takes place not only at the step-edges, but also in the middle of terraces. The islands cover as well fcc as hcp domains of the template, in a near commensurate fashion with respect to the Ag(2ML)/Pt(111) superstructure. This commensurability manifests itself by a transfer of the strain relief superstructure into the molecular film via a “lock to register” mechanism. The Ag(2ML)/Pt(111) superstructure acts on the molecular film by orientating the ${\mathrm{C}}_{60}$ close-packed rows along the $\mathrm{Ag}⟨1\overline{1}0⟩$ direction. This is different from the usual orientation of a ${\mathrm{C}}_{60}$ film on a standard and unreconstructed Ag(111) substrate, namely it is rotated by 30°. The ${\mathrm{C}}_{60}$ “lock to register” phenomenon can find interpretation in a strong substrate-molecule interaction, which can result in a significant compression of the ${\mathrm{C}}_{60}$ lattice. Here, a special role is played by Ag(2ML)/Pt(111) small hcp stacking domains enclosed by strong discommensuration lines, foremost responsible for the “lock to register” mechanism.

Figure 1

(Color online) Topographic LT-STM images ($2\mathrm{nA}$, $-2\mathrm{V}$) (a) $130\mathrm{nm}\times 130\mathrm{nm}$ and (b) $40\mathrm{nm}\times 40\mathrm{nm}$ showing an Ag(2ML)/Pt(111) strain-relief pattern [(b) is a detail of (a)]. (c) A schematized top view of some unit cells of the Ag(2ML)/Pt(111) dislocation network superstructure shown in (a,b). The circles represent Ag atoms (model after Refs. 6, 9, see text). It is not clear, however, if the dislocations are present in the topmost Ag layer or are buried in the first Ag layer on the Pt(111) substrate. (d) FFT spectrum of image (b) illustrating the hexagonal symmetry of the Ag(2ML)/Pt(111) superstructure.

Figure 2

Topographic LT-STM images ($0.01\mathrm{nA}$, $-2\mathrm{V}$) illustrating the adsorption at RT of 0.4 ML ${\mathrm{C}}_{60}$ on the Ag(2ML)/Pt(111) strain-relief pattern: (a) $500\mathrm{nm}\times 500\mathrm{nm}$, (b) $28\mathrm{nm}\times 28\mathrm{nm}$. (b) displays a ${\mathrm{C}}_{60}$ 2D island at a step edge and two isolated ${\mathrm{C}}_{60}$ molecules in touch with Ag(2ML)/Pt(111) deep dislocations. In (b), a dual tone gray scale has been chosen to allow the visualization of the molecules and the substrate dislocation network.

Figure 3

LT-STM images ($0.01\mathrm{nA}$, $-2\mathrm{V}$) (a) $120\mathrm{nm}\times 120\mathrm{nm}$ and (b) $50\mathrm{nm}\times 50\mathrm{nm}$ showing ${\mathrm{C}}_{60}$ 2D islands grown in the middle of an Ag(2ML)/Pt(111) terrace. ((a) is a detail measured on the upper part of Fig. 2a.) The Ag(2ML)/Pt(111) superstructure is clearly imprinted in the ${\mathrm{C}}_{60}$ 2D islands. In both images (a) and (b), a dual tone gray scale has been chosen to allow a better visualisation of the ${\mathrm{C}}_{60}$ triangular features (or/and the ${\mathrm{C}}_{60}$ lattice) and the substrate dislocation network superstructure.

Figure 4

(Color online) (a) High resolution LT-STM image ($0.01\mathrm{nA}$, $-2\mathrm{V}$) $20\mathrm{nm}\times 20\mathrm{nm}$ measured on the ${\mathrm{C}}_{60}$ 2D island displayed on the image 3.(b). Highlighted triangular features (containing 10 ${\mathrm{C}}_{60}$ molecules) fingerprint the transfer of the Ag(2ML)/Pt(111) superstructure to the ${\mathrm{C}}_{60}$ film. (b) FFT spectrum of image (a) showing two spot hexagons: the big one corresponds to the ${\mathrm{C}}_{60}$ hcp stacking and the small one is related to the substrate superstructure transferred to the molecular film, as attested by the triangular features. The coincidence in the orientation of the two spot hexagons indicated by a dotted line in (b) shows that the ${\mathrm{C}}_{60}$ close-packed rows are aligned along the $\mathrm{Ag}⟨1\overline{1}0⟩$ direction.

Figure 5

(a) Schematic representation of ${\mathrm{C}}_{60}$ film orientation on Ag(2ML)/Pt(111) governed by the template superstructure, i.e., with ${\mathrm{C}}_{60}\text{−}{\mathrm{C}}_{60}$ close-packed rows along $\mathrm{Ag}⟨1\overline{1}0⟩$. Comparison with (b) ${\mathrm{C}}_{60}$ monolayer orientation on a standard and unreconstructed Ag(111) substrate with ${\mathrm{C}}_{60}\text{−}{\mathrm{C}}_{60}$ close-packed rows aligned along $\mathrm{Ag}⟨11\overline{2}⟩$ and which gives rise to a $(2\surd 3\times 2\surd 3)\mathrm{R}30°$ structure.[13, 14, 19] The small and big circles represent Ag atoms and ${\mathrm{C}}_{60}$ molecules, respectively, in both models (a) and (b).