Formation of giant crystalline grain via delayed growth process driven by organic molecular anisotropy

Abstract

The growth of (001)-oriented pentacene (${\text{C}}_{22}{\text{H}}_{14}$, Pn) thin films on silicon surfaces has been extensively studied to elucidate the role of molecular anisotropy in nucleation and island evolution in organic film growth. In situ real-time low-energy electron microscopy studies of growth of Pn revealed a delayed, low-density nucleation that could be related to the difference in the orientation of this anisotropic molecule in its diffusing state and in the crystalline film. In contrast to the growth of Pn on self-assembled monolayers or ${\text{SiO}}_{2}$, we observed a delayed nucleation and formation of extraordinarily large grains (in submillimeter scale) on semiconducting $\ensuremath{\alpha}\sqrt{3}\text{-Bi-Si}(111)$ and on semimetallic Bi(0001)/Si(111) with a continuation in film growth after stopping Pn deposition. The delayed and very low-density nucleation and continuing growth after stopping deposition could be explained by a incorporation-limited growth processes resulted from a large energy barrier for Pn nucleation in standing-up orientation, as the molecule needs to reorient itself from a lying-down, diffusing state in order to build into the crystalline film.