Aromatic molecules, such as pentacene, sexiphenyl and naphtho[2,3-a]pyrene, adsorbed on Au(111) surfaces are studied with x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), temperature programmed desorption (TPD) and scanning tunneling microscopy (STM). All the molecules developed a strong interface dipole indicative of charge transfer from the molecule to the substrate. At monolayer coverages or less a variety of two-dimensional ordered structures were discovered. Naphtho[2,3-a]pyrene, a 2D chrial adsorbate, forms chiral domains rater than a 2D racemic unit cell.
The alignment of the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the organic luminescent semiconductor Gaq3 relative to the Fermi level of Au was determined by depositing a Gaq3 thin film in a multi-step growth procedure on an Au foil. Before growth and after each deposition step the sample was characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. Such measurements offer a direct way to determine the electronic structure of the interface. Our experiments demonstrate that this method allows distinguishing between band bending, charging and interface dipole related shifts in the UP-spectra. The additional XPS measurements allow the precise determination of the band bending occurring across the interface while comparison between XPS and UPS work function measurements allows one to pinpoint the organic film thickness dependent onset of charging phenomena. Our results show that the interface dipoles at Gaq3 Schottky contacts with Au, Pt and Ag amount to 0.6 - 0.7 eV. Our experiments also show that final state screening shifts can be dismissed as insignificant in such orbital line-up measurements. This was shown at the chloroindium phthalocyanine (ClInPc)/highly oriented pyrolytic graphite (HOPG) interface where no such shifts were observed.
Most scanning tunneling microscopy (STM) work dealing with ordered organic films have primarily relied on highly ordered pyrolytic graphite as a substrate (HOPG). A second substrate, molybdenum disulfide (MoS2), is appearing more often as a substrate. In this paper, we introduce a new substrate for the imaging of ordered systems. Tungsten diselenide (WSe2) is a layered material similar in structure to MoS2. The change in surface periodicity for HOPG to MoS2 caused significant changes in the ordering of liquid crystals. This is not completely unexpected given the two different types of surfaces. In this contribution two n-alkyl-cyanobiphenyls (mCBs; m equals 6,8) were imaged on WSe2. The differences between MoS2 and WSe2 are less drastic when compared to HOPG, yet the ordered structure that was found for 8CB was a new monolayer type structure while the 6CB structure is very similar to what has been found for this molecule on MoS2.
Neal Armstrong, Lai Chau, Greg Collins, S. Chen, Ken Nebesny, Valorie Williams, Paul Lee, C. Arbour, James Danziger, E. Osburn, D. O'Brien, B. Parkinson
We discuss here our most recent results with the characterization of epitaxial deposits of various phthalolcyanine dyes formed by vacuum deposition (O/I-MBE) or solution deposition on the surface of metal dichalcogenide semiconductors, such as SnS2. Surface electron diffraction techniques used during the vacuum deposition process help to verify the type and extent of long range ordering of these dyes. SnS2 semiconductor substrates allow for the photoelectrochemical characterization of the dye layers, starting with the deposition of submonolayer amounts of material. High quantum yields per absorbed photon are seen for ultrathin films of InPc-Cl, VOPc, and CuPc on SnS2, and the photocurrent spectra suggest similar ordering at the monolayer level, even though multilayer structures are quite different. Ordered Pc thin films are also obtained for a new class of liquid crystalline phthalocyanines (LC-Pc), where the hydrocarbon side chains are attached to the Pc ring by amide linkages. Deposition of ultrathin films of these materials produces photocurrent spectra which are quite similar to those obtained for low coverages of the vacuum deposited Pc. Photocurrent spectra on SnS2 show that the first monolayer of material may have a completely different surface structure than the bulk of the multilayer LC-Pc thin film. The nature of dye/dye' interfaces and their effect on exciton dissociation events has also been explored using vacuum deposited materials. Superlattices of Pcs were formed by vacuum deposition, where the active dye was sandwiched between various spacer molecules with thicknesses down to a few molecular layers. Transient photocurrent yield spectra from such assemblies suggests that exciton dissociation events in such materials can be confined to within a few molecular layers of the dye/dye' interface.
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