We show that the orientation of pentacene molecules is controlled by the electronic structure of the surface on which they are deposited. We suggest that the near-Fermi level density of states above the surface controls the interaction of the substrate with the pentacene pi orbitals. A reduction of this density as compared to noble metals, realized in semimetallic Bi(001) and Si(111)(5 x 2)Au surfaces, results in pentacene standing up. Interestingly, pentacene grown on Bi(001) is highly ordered, yielding the first vertically oriented epitaxial pentacene thin films observed to date.

Thin Bi(001) films grown by ultrahigh vacuum deposition on Si(111)-7×7 surfaces at room temperature, were annealed at ∼400K in order to improve their morphology by reducing the step density on the surface. Annealed, well-ordered Bi(001) films have been subsequently used as substrates for growth of pentacene (Pn). It has been determined using low-energy electron microscope that Pn nucleates on Bi(001) into a highly ordered, crystalline layer, with Pn molecules “standing up” on the Bi surface, and the (001) plane on the growth front. Moreover, the Pn layer is aligned with the Bi(001) surface having a “point-on-line” commensurate relationship with the substrate. The Pn∕Bi(001) film crystallizes in a bulk-like structure directly from the first Pn layer. Formation of the thin film phase reported for the Pn growth on SiO2 and other inert substrates was not observed in our experiments.

No abstract

Using scanning tunneling microscopy (STM) and first-principles local-spin-density-approximation calculations to study submonolayer films of Co (1-c)Ag (c)/Ru(0001) alloys, we have discovered a novel phase-separation mechanism. When the Ag concentration c exceeds 0.4, the surface phase separates between a dislocated, pure Ag phase and a pseudomorphically strained Co(0.6)Ag (0.4) surface alloy. We attribute the phase separation to the competition between two stress relief mechanisms: surface alloying and dislocation formation. The agreement between STM measurements and our calculated phase diagram supports this interpretation.