Thin film evolution: Dewetting dynamics of a bimodal molecular system

Deposition of submonolayer coverages of 3,4,9,10-perylenetetracarboxylic diimide onto NaCl(001) results in the bimodal growth of coexisting square and needle-shaped islands. The epitaxy of each island type was determined from molecular and atomic resolution noncontact atomic force microscopy images. Postdeposition, the needle islands grow while the square islands disappear completely within days. This is characterized as a dewetting transition, the dynamics of which has been described and empirically fit to the monomolecular growth model, a model typically applied to biological growth.

Figure 1

[(a) and (c)] Molecular and atomic resolution images of (a) a 1 ML square island of PTCDI on NaCl, $\Delta f=-15\text{Hz}$, and (b) a multilayer needle island of PTCDI on NaCl, $\Delta f=-24\text{Hz}$. Large and small inset regions of PTCDI and NaCl, respectively, have been flattened and adjusted for contrast to make the structures more visible using WSXM (Ref. 34). (c) $2\times 2$ brick wall square-island structure. (d) Needle-island structure (top layer of molecules only shown for clarity).

Figure 2

Postdeposition growth of needle islands and the disappearance of square islands over time. [(a)–(d)] $770\times 770{\text{nm}}^2$, $\Delta f=-2–-4\text{Hz}$, 0.3 ML PTCDI on NaCl images, and corresponding time since deposition, with both square and needle-shaped islands visible. (e) Differential concentration of 2D molecular gas $\left(\text{molecules}/{\text{nm}}^2\right)$, (f) area of exposed NaCl substrate, (g) differential population of 2D molecular gas (population less an unknown steady-state offset), and (h) population of square and needle islands plotted as functions of time, with monomolecular growth and decay fits.

Figure 3

(Color online) Island volume histograms of (a) square (metastable) islands and (b) needle (stable) islands for three different times. Solid lines in (b) are fits to a Gaussian with an exponential tail and are intended as a guide to the eye.