Origin of the bimodal island size distribution in ultrathin films of para-hexaphenyl on mica

Ultrathin films of para-hexaphenyl (6$P$) were prepared on freshly cleaved and sputter-amorphized mica(001) by physical vapor deposition. Ex situ atomic force microscopy (AFM) revealed a bimodal island size distribution for the films on both surfaces. On freshly cleaved mica long needlelike islands exist, which are surrounded by small crystallites. On the sputter-amorphized substrates, large dendritic islands exist which are again surrounded by small, compact islands. We could prove by thermal desorption spectroscopy that the small islands are the result of adsorbate-induced subsequent nucleation, when the films were exposed to air. In case of the freshly cleaved mica, islands grow on a wetting layer in vacuum. This layer dewets and forms the small islands upon venting, due to the adsorption of water. In the case of the amorphous mica substrate an equilibrium exists between the islands and a two-dimensional gas phase in the sub-monolayer regime. Again, the latter phase nucleates after venting. In a particular coverage range, islands due to nucleation during deposition and subsequent nucleation coexist on the substrate, leading to the bimodal island size distribution. Kinetic Monte Carlo (KMC) simulations were performed to model the nucleation process after venting on the sputter-modified mica substrate. The density of the subsequently nucleated islands just depends on the initial coverage and the critical island size. A critical cluster size of $i$ $=$ 7 molecules was determined for 6$P$ on amorphized mica, by comparing the KMC results with the AFM images in case of adsorbate-induced nucleation. Furthermore, the experimentally obtained island size distributions could be well reproduced by KMC simulations.

Figure 1

(a) AFM image ($4\times 4$ μm) for 6$P$ on a freshly cleaved mica(001) surface. Deposition temperature, 400 K; deposition rate, 0.06 ML/min; deposited amount, 0.62 ML (standing monolayer equivalents). (b) AFM image (8 $\times$ 8 μm) for 6$P$ on a sputtered mica(001) surface. Deposition temperature, 400 K; deposition rate, 0.07 ML/min; deposited amount, 0.18 ML. (c) Cross section along the line as indicated in (a), demonstrating different heights of the needles and clusters. (d) Cross section along the line as indicated in (b), demonstrating the same heights for the large and small islands, consisting of one layer of standing molecules.

Figure 2

(a) Thermal desorption spectra of 6$P$ (mass 61 amu) on freshly cleaved mica(001), deposited at room temperature. Curve a: deposited amount, 10 Hz (1.6 nm mean thickness); curve b: deposited amount, 2 Hz (0.32 nm mean thickness); curve c: deposited amount, 2 Hz, but afterwards exposed to air and evacuated again before desorption. (b) AFM image (1 $\times$ 1 μm) of 6$P$ on freshly cleaved mica. Deposition temperature, 400 K; deposition rate, 0.05 nm/min; deposited amount, 2 Hz; equivalent to 0.32 nm mean thickness.

Figure 3

Multiplexed thermal desorption spectra of water ($m$ $=$ 18 amu) after the deposition of 2-Hz equivalents of 6$P$ on the freshly cleaved mica, prior to venting the vacuum chamber (a) and after venting and reevacuation (b).

Figure 4

Large-scale AFM images (32 $\times$ 32 μm) for 6$P$ on sputter-modified mica(001) surfaces. Deposition temperature, 400 K; deposition rate, 0.08 ML/min; deposited amount: (a) 0.036 ML, (b) 0.04 ML, (c) 0.077 ML, (d) 0.147 ML. The small islands are hardly visible in this representation.

Figure 5

Small-scale AFM images for 6$P$ on sputter-modified mica (001) surfaces. Deposition temperature, 400 K; deposition rate, 0.08 ML/min; deposited amount: (a) 0.014 ML, (b) 0.03 ML, (c) 0.036 ML, (d) 0.04 ML.

Figure 6

AFM images (8 $\times$ 8 μm) for different coverages of 6P grown on sputter-modified mica(001) at 400 K, with a deposition rate of 0. 1 ML/min: (a) 0.18 ML, (b) 0.47 ML, (c) 0.94 ML.

Figure 7

Coverage dependence of the small and large island density for 6$P$ on sputter-modified mica. The lines are to guide the eye.

Figure 8

Thermal desorption spectra of 6$P$ (mass 61 amu) on sputter-modified mica(001), deposited at room temperature. (a) Deposited amount, 2 Hz (0.32 nm mean thickness); (b) deposited amount, 2 Hz, but afterwards exposed to air and evacuated again before desorption.

Figure 9

Series of TDS for very small 6$P$ coverage on sputter-modified mica. The deposited amount in monolayers is given in the inset.

Figure 10

(a) Size distribution of the large 6$P$ islands on sputter-modified mica (001) after deposition of 0.147 ML at 400 K, as measured with ex situ AFM at room temperature. The curve is a fit according to the function as proposed by Amar and Family[27] for ISD, with $p$ $=$ 2. A typical island morphology is shown in the inset. (b) Size distribution of the small 6$P$ islands on sputter-modified mica (001) after deposition of 0.03 ML at 400 K, as measured with ex situ AFM at room temperature. The curve is a fit according to the function as proposed by Amar and Family[27] for ISD, with $p$ $=$ 1. A typical island morphology is shown in the inset.

Figure 11

(a) Voronoi tesselation for the island distribution of Fig. 4d (large islands). (b) Capture zone distribution obtained by averaging over four AFM images of the film corresponding to Fig. 4d. The curves are the scaling functions of Eq. (2) with parameter $i$ $=$ 0 through 4. The best fit yields $i\approx 2$.

Figure 12

(a) Voronoi tesselation for the island distribution of Fig. 5b (small islands). (b) Capture zone distribution obtained by averaging over four AFM images of the film corresponding to Fig. 5b. The curves are the scaling functions of Eq. (2) with parameter $i$ $=$ 0 through 4. The best fit yields $i\approx 1$.

Figure 13

Comparison of KMC simulations for adsorbate-induced subsequent nucleation without edge hopping (a) and with edge hopping, where $E$ $=$ $Q=0.05$ eV (b). Lattice sites, 1000 $\times$ 1000; initial coverage, 0.03 ML.

Figure 14

Comparison of KMC simulations for island densities as a result of adsorbate-induced subsequent nucleation (initial coverage, 0.03 ML; 1000 $\times$ 1000 lattice sites) with an AFM image of 6$P$ on amorphous mica. (a) KMC, critical island size $i$ $=$ 6; (b) KMC, critical island size $i$ $=$ 7; (c) AFM image (1 $\times$ 1 μm); coverage, 0.03 ML.

Figure 15

Island density as a function of the critical island size, for different initial coverages (0.02–0.1 ML), obtained by KMC for adsorbate-induced subsequent nucleation. No edge diffusion was considered.

Figure 16

Logarithm of the island density, ln$N$, versus the logarithm of the coverage, lnΘ, for various critical island size ($i$ $=$ 3–8), obtained by KMC for adsorbate-induced subsequent nucleation. No edge diffusion was considered.

Figure 17

Time development of the monomer (dashed lines) and island density (solid lines) for adsorbate-induced subsequent nucleation as a function of the critical island size $i$. Initial coverage, 0.03 ML; attempt frequency, ν $=$ 10${}^{13}$ s${}^{-1}$; diffusion energy, $Q$ $=$ 0.05 eV.

Figure 18

Island size distribution for KMC-simulated island films resulting from adsorbate-induced subsequent nucleation, with $i$ $=$ 5 (a) and $i$ $=$ 7 (b); initial coverage, 0.03 ML. Fit curves according to the function by Amar and Family,[27] with fit parameters $p$ $=$ 1, 2. For comparison an ISD from KMC-simulated nucleation during deposition is shown (c), assuming $i$ $=$ 3. This demonstrates the very good agreement with the function of Amar and Family with fit parameter $p$ $=$ 3.