Paper EM+NC-WeA10
Phase Selection in TiOPc/Ag (111) Molecular Films: Combined STM:STS Investigations*

Wednesday, October 22, 2008, 4:40 pm, Room 210

Titanyl phthalocyanine (TiOPc) has emerged as an important molecular component in building organic electronic devices. Its broad optical absorption spans the near infrared - visible region and high photoconductivity are most promising features, yet a tendency for polymorphism may limit TiOPc applications. Understanding how to control the molecular architecture in thin TiOPc films, and relate structure to electronic properties, is thus an important scientific goal. We present STM/STS studies of growth and two-dimensional crystallization of TiOPc films prepared by vapor deposition on Ag (111). We show how three distinctive TiOPc monolayer phases can be fabricated via flux control. At lowest TiOPc fluxes, a honeycomb phase forms, in which molecules organize into interlocked pairs. This phase optimizes quadrupole attraction and minimizes lateral dipole repulsion between TiOPc neighbors. At medium fluxes, a higher-energy hexagonal phase, stabilized by partly overlapped Pc rings between neighboring molecules emerges. At yet higher fluxes, a triangular network of misfit dislocations, with a characteristic domain size of ca. 14 nm is produced. We present molecular models of these three distinctive phases and describe how the film architecture is controlled primarily by electrostatic TiOPc-TiOPc interactions. Comparative STS Studies of the honeycomb and hexagonal monolayers reveal a metallic-like conductivity for both films. Finally, we show how these different TiOPc phases, employed as substrates for sequential C₆₀ deposition, yield distinctive donor-acceptor heterostructures with unique electronic characteristics.

^*This work has been supported by the Department of Commerce through the Nanomanufacturing Center of NIST, the Nanotechnolgy Fund of NASA, and the National Science Foundation under Surface Analytical Chemistry grant CHE0750203.