Coverage-dependent crystalline domain structures of a tetracene thin film on H/Si(001)

Abstract

Atomic force microscopy, transverse shear microscopy, and friction force microscopy have been used to study coverage-dependent crystalline domain structures of a tetracene film on a hydrogen-passivated Si (001)-2 × 1 surface. Though submonolayer fractals present some nonepitaxial domains, the coalesced first monolayer, which possesses a partial commensurate registration with the substrate lattice, shows two lattice domains (major domains) orthogonally oriented with each other. The second-layer lattice exhibits 90° azimuthal rotation from the first-layer lattice, and the third and subsequent layers show a commensurate registration with their respective underlayers. The major-domain boundaries are not the preferable nucleation sites, indicating a potential energy barrier at the boundary. Meanwhile, the domain structure rigidity increases with the layer height until it saturates on the fourth layer, where the bulklike structure emerges with the formation of two (molecular tilting) subdomains in a major domain. The authors conclude that the structural-phase transition can take place locally wherever the fourth molecular layer emerges, and significant bulklike crystallization occurs at a nominal coverage of ∼3–4 ML.