Thin films of vapor-deposited Ni(II) and Co(II) complexes of tetraphenylporphyrin (NiTPP and CoTPP) were studied supported on gold and embedded in Al-Al(2)O(3)-MTPP-Pb tunnel diodes, where M = Ni or Co. Thin films deposited onto polycrystalline gold were analyzed by ultraviolet photoelectron spectroscopy (UPS) using He I radiation. Scanning tunneling microscopy (STM) and orbital-mediated tunneling spectroscopy (STM-OMTS) were performed on submonolayer films of CoTPP and NiTPP supported on Au(111). Inelastic electron tunneling spectroscopy (IETS) and OMTS were measured in conventional tunnel diode structures. The highest occupied pi molecular orbital of the porphyrin ring was seen in both STM-OMTS and UPS at about 6.4 eV below the vacuum level. The lowest unoccupied pi molecular orbital of the porphyrin ring was observed by STM-OMTS and by IETS-OMTS to be located near 3.4 eV below the vacuum level. The OMTS spectra of CoTPP had a band near 5.2 eV (below the vacuum level) that was attributed to transient oxidation of the central Co(II) ion. That is, it is due to electron OMT via the half-filled d(z)(2) orbital present in Co(II) of CoTPP. The NiTPP OMTS spectra show no such band, consistent with the known difficulty of oxidation of the Ni(II) ion. The STM-based OMTS allowed these two porphyrin complexes to be easily distinguished. The present work is the first report of the observation of STM-OMTS, tunnel junction OMTS, and UPS of the same compounds. Scanning tunneling microscope-based orbital-mediated tunneling provides more information than UPS or tunnel junction-based OMTS and does so with molecular-scale resolution.

Thin films of vapor deposited Cu(II), Ni(II), and Co(II) complexes of tetraphenylporphyrin (CuTPP, NiTPP, and CoTPP) supported on gold are studied by Reflectance Absorption Infrared Spectroscopy (RAIR) and X-ray photoelectron spectroscopy (XPS). In addition, scanning tunneling microscopy (STM) images of ultrathin (submonolayer) thermally deposited films of CoTPP and NiTPP on Au(111) are reported. The constant current STM images show a remarkable bias dependence that allows for the selective identification of different metallotetraphenylporphyrins (MTPPs). High resolution STM data are presented that clearly show the individual MTPP molecules and allow easy differentiation between NiTPP and CoTPP, even when mixed monolayer structures are studied. The strong tunneling enhancement through the Co(II) d z 2 orbital at intermediate bias values parallels what is observed in Co(II) phthalocyanines and is especially interesting since the cobalt ion in CoTPP is presumed to be located several Angstroms above the Au(111) plane relative to the cobalt ion in CoPc.

The production of a novel two-dimensional bimolecular surface structure using weak noncovalent interactions is demonstrated and observed by scanning tunneling microscopy. This work follows the three-dimensional (3D) ideas of crystal engineering and applies the concepts of supramolecular synthons to molecular systems constrained to 2D by physisorption on a conducting surface. We demonstrate a well-ordered planar structure that self-assembles through the influence of fluorine-phenyl interactions. This study provides a concrete example of the "bottom up" construction of nanostructures and of the systematic design of self-organized layers. To our knowledge, this is the first in a new class of fully 2D materials based both upon weak intermolecular interactions and upon image charges and weak interactions associated with adsorption on metal surfaces.

Binary thin films of cobalt(II) phthalocyanine (CoPc) and cobalt(II) tetraphenylporphyrin (CoTPP) were prepared at submonolayer coverage on Au(111)/mica substrates byvapor deposition. All sample preparation and analysis were done under an ultrahigh vacuum. Scanning tunneling microscopy (STM) constant-current images of CoPc/CoTPP mixtures showed two close-packed surface structures, with different compositional percentages and some disorder. CoPc was also observed exclusively in one-dimensional chains and as single, isolated molecules below 220 K. Occupied and unoccupied orbital energy levels were identified by STM and tunnel-diode-based orbital-mediated tunneling spectroscopy. Occupied energy levels were also confirmed by ultraviolet photoelectron spectroscopy. The transient oxidation of the Co d(z2) orbital is identified in STM dI/dV(V) curves just negative of the 0 V sample bias for both molecules. Nearly identical constant-current contours are observed over the central Co2+ ions of CoTPP and CoPc, indicating that the attenuation of the d(z)2 orbital-mediated tunneling current induced by the structure of TPP relative to Pc is at most a factor of about 10. The orbital-mediated tunneling spectra of CoTPP and CoPc are distinctly different and allow these structurally similar species to be differentially identified.

Thin films of vapor-deposited Ni(II) octaethylporphyrin (NiOEP) were studied supported on gold. Thin films thermally deposited onto flame-annealed Au (111) were analyzed by ultraviolet photoelectron spectroscopy (UPS) using He I radiation and by X-ray photoelectron spectroscopy (XPS) using Mg Kα radiation. Reflectance absorption infrared spectroscopy (RAIRS) was used to study NiOEP films on polycrystalline gold. Scanning tunneling microscopy (STM) and orbital-mediated tunneling spectroscopy (STM-OMTS) were performed on submonolayer films of NiOEP supported on Au(111). The highest occupied π molecular orbitals of the porphyrin ring were seen in both STM-OMTS and in UPS at about 6.4 and 6.8 eV below the vacuum level. The lowest unoccupied π* molecular orbital of the porphyrin ring also was observed by STM-OMTS to be located near 3.4 eV below the vacuum level.