Polycrystalline domain structure of pentacene thin films epitaxially grown on a hydrogen-terminated Si(111) surface

摘要

Single-monolayer high pentacene (Pn) dendrites grown on a hydrogen-terminated Si(111) surface [H-Si(111)] under ultrahigh vacuum were observed by low-energy electron microscopy and microbeam low-energy electron, diffraction analyses. We determined the epitaxial structure (type I) inside a unique polycrystalline domain structure of such dendrites, each of which has six equivalent epitaxial orientations of Pn two-dimensional (2D) unit cells. There are three sets of these cells, which are rotated ±120° relative to each other. Domain boundaries inside each dendrite were successfully observed by scanning tunneling microscopy. In addition, we found another epitaxial relation (type Ⅱ): the polycrystalline domain structure and lattice parameters are similar to those of the type-Ⅰ dendrite; however, the 2D unit cells of the type-Ⅱ dendrite are rotated approximately 90° relative to those of the type-I dendrite. These results suggest that the crystal structure of the dendrites on H-Si(111) is determined mainly by the interaction between Pn molecules. Each dendrite is composed of domains that are exclusively of type Ⅰ or Ⅱ. The so-called point-on-line coincidences are found between the Pn 2D lattices of types Ⅰ and Ⅱ, and H-Si(111). The higher commensurability of the type-Ⅰ dendrites than the type-Ⅱ dendrites results in a higher probability of type-Ⅰ dendrite formation. Moreover, for both the type-Ⅰ and type-Ⅱ dendrites, we found supercell structures. We estimated the minimum interface energy between the dendrite and H-Si(111) from an island's free energy, which is necessary to reproduce the growth of a single-monolayer high dendrite.