Comparison between nucleation of pentacene monolayer islands on polymeric and inorganic substrates

We present a comparative study of the nucleation and growth of pentacene monolayer islands in the submonolayer regime onto inorganic substrates of $\mathrm{Si}{\mathrm{O}}_2$ and sapphire $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ and organic substrates of poly(methyl-metacrylate) (PMMA). We have determined the scaling laws that govern the saturated nucleation density per unit area $N$ as a function of two essential deposition parameters: the deposition rate $\kappa$ and the substrate temperature $T_s$. For all substrates, we found $N\propto {\kappa }^{\delta }$, with $0.8⩽\delta ⩽1.2$, and the $T_s$ dependence of the nucleation density follows a typical activated Arrhenius law. Whereas a critical nucleus size of $i=2$ is obtained for all investigated substrates, the activation energy for nucleation depends significantly on the dielectric. The difference in activation energy of nucleation on PMMA and $\mathrm{Si}{\mathrm{O}}_2$ is due to different molecule-substrate interactions.

Figure 1

Effect of deposition rate on the surface morphology measured by AFM for three different types of substrate: $\mathrm{Si}{\mathrm{O}}_2$, ${\mathrm{Al}}_2{\mathrm{O}}_3$, and PMMA. For all the samples, the substrate temperature was fixed at $65°\mathrm{C}$ and the nominal thickness of the pentacene films was always $0.5\mathrm{nm}$. All the images are $10\times 10\mu {\mathrm{m}}^2$ in size.

Figure 2

Dependence of pentacene island density on deposition rate $\mathrm{Si}{\mathrm{O}}_2$, ${\mathrm{Al}}_2{\mathrm{O}}_3$, and PMMA substrates at a fixed substrate temperature, $T_s=65°\mathrm{C}$.

Figure 3

Effect of substrate temperature on the surface morphology measured by AFM for two types of substrate: $\mathrm{Si}{\mathrm{O}}_2$ and PMMA. For all samples the deposition rates are in the range $0.45–0.50\mathrm{nm}∕\min$ and the nominal thickness of the pentacene films was fixed to $0.5\mathrm{nm}$. All the images are $10\times 10\mu {\mathrm{m}}^2$ in size.

Figure 4

(a) Variation of apparent coverage $\theta$ with substrate temperature $T_s$ for a fixed nominal thickness of $0.5\mathrm{nm}$. (b) Dependence of pentacene island density on substrate temperature $\mathrm{Si}{\mathrm{O}}_2$ and PMMA substrates at fixed deposition rate $(0.4\mathrm{nm}∕\min -0.5\mathrm{nm}∕\min )$.