Second-Layer Induced Island Morphologies in Thin-Film Growth of Fullerenes

Deposition of fullerenes on the ${\mathrm{CaF}}_2(111)$ surface yields peculiar island morphologies with close similarities to previous findings for (100) surfaces of other ionic crystals. By means of noncontact atomic force microscopy we find a smooth transition from compact, triangular islands to branched hexagonal islands upon lowering the temperature. While triangular islands are two monolayers high, hexagonal islands have a base of one monolayer and exhibit a complicated structure with a second-layer outer rim and trenches oriented towards the interior. By developing a kinetic growth model we unravel the microscopic mechanisms of the structure formation.

Figure 1

(a)–(c) NC-AFM images of ${\mathrm{C}}_{60}$ islands on ${\mathrm{CaF}}_2(111)$ at three different growth temperatures. (d)–(f) Magnified images of single islands: a compact triangle (d), and hexagonal islands with morphologies I (e) and II (f). (g),(h) Height profiles along lines scans shown in (d),(e).

Figure 2

(a)–(c) Simulated configurations of the growth model at different temperatures. (d)–(f) Magnified single island structures from the simulations resemble the same morphologies as found in experiments [Figs. 1d, 1e, 1f].

Figure 3

Sketch of elementary upward jump processes facilitating the growth of a double-layer triangle. The processes at the $A$ step and the corner between the $A$ and $B$ step involve only two ${\mathrm{C}}_{60}$ and are, therefore, more likely than the process at the $B$ step involving three ${\mathrm{C}}_{60}$ molecules.

Figure 4

Hexagonal islands from the KMC simulations showing the evolution of the two morphologies I [upper row (a)–(d)] and II [lower row (e)–(h)].