Scanning tunneling microscopy of liquid crystals, perylene-tetracarboxylic-dianhydride, and phthalocyanine

Abstract

Scanning tunneling microscopy measurements conducted on pure liquid crystals and liquid crystals in combination with the dyes perylene-tetracarboxylic-dianhydride (PTCDA) and phthalocyanine (Pc), all adsorbed on highly oriented pyrolytic graphite (HOPG), are being reported on. The liquid crystal octylcyanobiphenyl (8CB) was used as a solvent for the dyes, allowing the preparation of ordered dye layers in a very simple way. A combination of vapor deposition of PTCDA and liquid deposition of 8CB provided films, where 8CB and PTCDA are embedded in each other on a small length scale. The samples were prepared in two different ways yielding either 8CB islands within a PTCDA monolayer or PTCDA islands surrounded by 8CB. The Pc films on HOPG exhibit a hexagonal molecular lattice that could be related to the lattice of the graphite substrate and for which a structural model is proposed. For dodekylcyanobiphenyl (12CB) on HOPG the ordering in unit cells containing 10 rather than 8 molecules is reported for the first time.

Metal d-Orbital Occupation-Dependent Images in the Scanning Tunneling Microscopy of Metal Phthalocyanines

“…STM images of individually distinguishable copper phthalocyanine (CuPc) molecules have been presented by Gimzewski and co-workers, Moeller, Lippel et al., Ludwig and associates, Fritz and co-workers, Kanai et al., and Petracek . In addition, images of the free acid (H 2 Pc) coadsorbed on graphite with a liquid-crystalline carrier have also been published by Freund and associates . While there have been reports of PbPc studied by STM, no molecular images have been observed. , To our knowledge, no other phthalocyanine systems have been studied by STM.…”

“…In all of these cases, the predominant features of the molecular image could be understood based on the organic material alone. Theoretical calculations of the STM image of CuPc predict that there should be an apparent hole in the center of the molecule, , and this is what is normally observed. , Even for H 2 Pc, there is an apparent hole in the center of the molecule . Since STM measures the local density of states, the “height” in constant-current mode is a complex function of the density of electrons with energy near the Fermi energyit may deviate significantly (in either a positive or negative direction) from the overall electron density.…”

Metal-free phthalocyanine (H2Pc) molecule adsorbed on the Au(111) surface: formation of a wide domain along a single lattice direction

“…The diameter of the molecule can be estimated as the nearest-neighbor distance (nnd) of the close-packed molecules. The nnd of H 2 Pc on the graphite surface was reported as ∼1.3 nm [24,25]. The fcc domain is wide enough to accommodate one molecule, but not two molecules.…”

Scanning Tunneling Microscopy of Metal Phthalocyanines: d6 and d8 Cases

“…While there have been reports of PbPc studied by STM, no molecular images have been observed. , In all of these cases, the predominant features of the molecular image could be understood on the basis of the organic material alone. Theoretical calculations of the STM image of CuPc predict that there should be an apparent hole in the center of the molecule, − and this is what is normally observed. , Even for H 2 Pc, there is an apparent hole in the center of the molecule . The explanation for these “holes”, it seemed to us, was that both the occupied and unoccupied orbitals localized on Cu lay more than 1 eV from the Fermi energy, while the MPc ligand LUMO lay close to the Fermi energy. , We reasoned that as the metal in a MPc system is varied, the corresponding variation in metal d orbital participation near the Fermi surface should produce profound changes in the STM images.…”