Key role of molecular kinetic energy in early stages of pentacene island growth

Abstract

Organic molecular beam deposition is studied systematically at thermal and hyperthermal regimes aiming at investigating the role of molecular kinetic energy on the growth mechanism of pentacene submonolayers on SiO_x/Si. We show that the kinetic energy of the impinging molecule (E_k) plays a crucial role in determining island structure and shape, distribution of island sizes, the crystalline quality of the first monolayer, and even the growth mode of subsequent layers. With increasing E_k, the island structure changes from fractal to nonfractal, the shape becomes more anisotropic and the island size more uniform, pointing to correlated island growth. Moreover, while 3D island growth is observed for thermal organic molecular beam deposition, supersonic molecular beam deposition gives rise to layer-by-layer growth, at least for the first two layers. When E_k≥5.0 eV, the first monolayer is composed of large single crystalline domains which can extend over up to 10 μm, inferred from comparing atomic force micrographs of height and net transverse shear force. In these growth conditions both the high surface diffusivity and energy redistribution play a major role. We propose a mechanism where the energy dissipation occurring during the molecule–surface collision leads to the reorientation of whole islands during island coalescence, resulting in the elimination of grain boundaries.