${\mathrm{C}}_{60}$ on alkali halides: Epitaxy and morphology studied by noncontact AFM

Submonolayer coverages of ${\mathrm{C}}_{60}$ deposited on KBr and NaCl are characterized by noncontact atomic force microscopy. Island shapes are described both qualitatively and quantitatively and their dependence on growth parameters and substrate is discussed. At room temperature, both compact and branched island morphologies are observed to coexist. An analysis of the island areas shows a distinct transition from compact to branched morphology with increasing island area. High-resolution imaging reveals differing coincident epitaxy at steps and on open terraces, despite consistent morphology for islands at both substrate positions. Molecular dewetting is proposed as a mechanism for the formation of branched islands.

Figure 1

Examples of the two island types observed: (a) compact multilayer islands with incomplete upper layers and (b) branched two-layer islands (2) with enclosed first layer regions (1) and disconnected regions (3).

Figure 2

(Color online) Island with two disconnected regions: (a) overview ($\Delta f=-1.9\mathrm{Hz}$, $440\times 440{\mathrm{nm}}^2$) and (b) high resolution($\Delta f=-13.0\mathrm{Hz}$, $20\times 20{\mathrm{nm}}^2$, high pass $\text{filter}+\text{original}$ image) showing alignment of molecular overlayer across the gap, dashed lines are parallel. Red and blue tinted regions in both images indicated the two disconnected regions, indicating that the top region of molecular contrast in (b) belongs to the lower right segment in (a) and the bottom region of molecular contrast is taken from an edge of the upper left segment in (a).

Figure 3

Overviews with different rates, coverages, substrates, showing similar, representative areas. All images $600\times 600{\mathrm{nm}}^2$.

Figure 4

The plot in (a) shows a typical dimensionality plot for a particular rate and coverage ($\sim 0.004\mathrm{ML}∕\mathrm{s}$, 0.1 ML). Images of the particular islands corresponding to points on the graph are shown for three key regions: the compact, $\text{slope}=2$ region, near the transition, and above the transition where branching is fully developed. The parameters in the upper left corner are defined as follows: $a$ and $c$ are the slopes for the compact and branched regions, respectively, and $P_{\mathit{trans}}$ and $A_{\mathit{trans}}$ are the perimeter and area, respectively, at which the transition occurs. The plot in (b) summarizes the area at which the transition occurs as a function of coverage and shows the difference in trends for KBr and NaCl, represented approximately by the gray areas.

Figure 5

Typical plot of the shape factor [see Eq. (2)] as a function of island area for the same sample shown in Fig. 4a. Up to the transition area, denoted by the vertical dashed line, the shape is roughly hexagonal. At areas greater than the transition area, the shape approaches $\mathrm{SF}=0$, or “linelike.” The solid line corresponds to the fit from the dimensionality plot in Fig. 4a. The two encircled points show islands with both similar areas and similar shapes one of which was positioned at a step and the other positioned on an open terrace.

Figure 6

High-resolution imaging of an island found on an open terrace are shown over the edge of the island with KBr resolution in (a) ($\Delta f=-13.0\mathrm{Hz}$, $20\times 20{\mathrm{nm}}^2$), and on a first layer region in (b)($\Delta f=-7.0\mathrm{Hz}$, $30\times 30{\mathrm{nm}}^2$) with the vectors $c_1$ and $c_2$ representing the periodicity of the moiré. The corrugation of the moiré is $\sim 10\mathrm{pm}$, with a ${\mathrm{C}}_{60}$ corrugation of $\sim 20\mathrm{pm}$. One of two possible coincident structures is shown in (c) based on high-resolution imaging. High-resolution imaging over the edge of an island found adjacent to a step is also shown in (d) ($\Delta f=-11.3\mathrm{Hz}$, $13\times 13{\mathrm{nm}}^2$) (Ref. 13) with the corresponding $n\times 3$ structure observed in (e).

Figure 7

(a) 0.5 ML ${\mathrm{C}}_{60}$ deposited at $0.0045\mathrm{ML}∕\mathrm{s}$ on KBr at room temperature $(Z_{\mathit{max}}=4.79\mathrm{nm})$, (b) 0.5 ML ${\mathrm{C}}_{60}$ deposited at $0.0045\mathrm{ML}∕\mathrm{s}$ on KBr at $175°\mathrm{C}$ $(Z_{\mathit{max}}=16.6\mathrm{nm})$, (c) same sample as shown in (a) after annealing at $175°\mathrm{C}$ for $16\mathrm{h}$ $(Z_{\mathit{max}}=5.63\mathrm{nm})$, and (d) 0.5 ML ${\mathrm{C}}_{60}$ deposited at $0.0045\mathrm{ML}∕\mathrm{s}$ without annealing, aged for nine days $(Z_{\mathit{max}}=3.39\mathrm{nm})$. Though annealing, and similarly aging, has some effect on the structure, the overall branched morphology remains, whereas depositing molecules onto a heated substrate results in a compact island morphology.

Figure 8

Sequence of nc-AFM images shortly after deposition showing dynamics of the island morphology. The first images was acquired $\sim 45\min$ after deposition. Three areas where a first layer edge is exposed are encircled. Subsequent images were taken $\sim 8\min$ apart and the time from the first image is indicated. The exposed first layer region receeds from the edge building up a second layer rim. Once enclosed, the remaining first layer regions are stable, as are the two layer branches. $\Delta f\simeq -2.8\mathrm{Hz}$, $600\times 600{\mathrm{nm}}^2$.