Nucleation and Submonolayer Growth of ${\mathrm{C}}_{60}$ on KBr

Noncontact atomic force microscopy has been applied to the prototypical molecule-insulator system ${\mathrm{C}}_{60}$ on KBr to study nucleation and submonolayer growth. Overview images reveal an island growth mode with unusual branching structures. Simultaneous molecular and atomic resolution on the ${\mathrm{C}}_{60}$ and KBr surfaces, respectively, was obtained revealing a coincident $8\times 3$ superstructure. Also, a $21\pm 3\mathrm{p}\mathrm{m}$ apparent height difference was observed in atomic force microscopy topographies between some first layer molecules. One of the initial nucleation sites of the ${\mathrm{C}}_{60}$ islands was determined by observation of loosely bound molecules at kink sites in monatomic KBr steps, in conjunction with the observation that islands form preferentially at step edges.

Figure 1

Overview images, each $600\mathrm{n}\mathrm{m}\times 600\mathrm{n}\mathrm{m}$. (a) 0.05 ML ${\mathrm{C}}_{60}$, frequency shift, $\Delta f=-3\mathrm{H}\mathrm{z}$, oscillation amplitude, $A=7\mathrm{n}\mathrm{m}$, tip-sample bias, $V_{\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{s}}=0.0\mathrm{V}$; (b) 0.1 ML ${\mathrm{C}}_{60}$, $\Delta f=-5.6\mathrm{H}\mathrm{z}$, $A=7\mathrm{n}\mathrm{m}$, $V_{\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{s}}=0.8\mathrm{V}$; (c) 0.2 ML ${\mathrm{C}}_{60}$, $\Delta f=-2.2\mathrm{H}\mathrm{z}$, $A=7\mathrm{n}\mathrm{m}$, $V_{\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{s}}=-0.75\mathrm{V}$; (d) 0.2 ML ${\mathrm{C}}_{60}$, $\Delta f=-2.0\mathrm{H}\mathrm{z}$, $A=7\mathrm{n}\mathrm{m}$, $V_{\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{s}}=-0.75\mathrm{V}$. Region 1 is one ${\mathrm{C}}_{60}$ layer high; region 2 is two ${\mathrm{C}}_{60}$ layers high.

Figure 2

Atomic/molecular resolution image of ${\mathrm{C}}_{60}$ on KBr. $\Delta f=-11.3\mathrm{H}\mathrm{z}$, $A=7\mathrm{n}\mathrm{m}$, $V_{\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{s}}=0.25\mathrm{V}$, image size: $13\mathrm{n}\mathrm{m}\times 13\mathrm{n}\mathrm{m}$. $5\mathrm{n}\mathrm{m}\times 5\mathrm{n}\mathrm{m}$ insets are adjusted for contrast to show the corrugation on each surface more clearly. A schematic diagram of the $8\times 3$ coincident structure is shown in (b).

Figure 3

(a) Molecular resolution of single layer of ${\mathrm{C}}_{60}$ shows some molecules are dim, or effectively shorter. $\Delta f=-11.3\mathrm{H}\mathrm{z}$, $A=7\mathrm{n}\mathrm{m}$, $V_{\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{s}}=0.25\mathrm{V}$, image size: $20\mathrm{n}\mathrm{m}\times 20\mathrm{n}\mathrm{m}$. (b) Cross section of image above taken at the position marked by the black line, showing the observed height difference of 21 pm. This inset image shows two orientations of the ${\mathrm{C}}_{60}$ molecule.

Figure 4

Atomic resolution of KBr showing kink sites in $⟨100⟩$ steps. The blurred, or noisy areas, are most likely ${\mathrm{C}}_{60}$ molecules loosely bound at the kink sites. In (b) the molecule marked by the arrow has been removed in the imaging process, revealing the underlying KBr structure. (a) $\Delta f=-7.1\mathrm{H}\mathrm{z}$, $A=7\mathrm{n}\mathrm{m}$, $V_{\mathrm{b}\mathrm{i}\mathrm{a}\mathrm{s}}=0.25\mathrm{V}$; image size, $18\mathrm{n}\mathrm{m}\times 18\mathrm{n}\mathrm{m}$. (b) $\Delta f=-7.4\mathrm{H}\mathrm{z}$, $A=7\mathrm{n}\mathrm{m}$; image size, $18\mathrm{n}\mathrm{m}\times 18\mathrm{n}\mathrm{m}$.