Scanning tunneling microscopy study of the structure and orbital-mediated tunneling spectra of cobalt(II) phthalocyanine and cobalt(II) tetraphenylporphyrin on au(111): mixed composition films

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.     

Supramolecular structures of coronene and alkane acids at the Au(111)-solution interface: a scanning tunneling microscopy study

Scanning tunneling microscopy (STM) is utilized to study the solution-solid interface formed between Au(111) and solutions of coronene in hexanoic, heptanoic, and octanoic acids. In all three cases adsorbed coronene is observed and lays flat on the metal surface. Heptanoic and hexanoic acid solutions produce a hexagonal symmetry monolayer. For the heptanoic and hexanoic cases, dipole-image dipole interactions and H bonding stabilize a surface structure in which 12 acid molecules surround each coronene and produce a coronene spacing of 1.45 nm. In the case of octanoic acid as solvent, the incorporation of the solvent into the monolayer is not as strongly favored. The coronene spacing can range from close-packed (1.2 nm) with no solvent presumed present in the monolayer, to 1.50 nm with up to 12 solvent molecules surrounding each coronene. The close-packed regions have hexagonal symmetry, as do those with the largest (1.5 nm) spacing. Heptanoic acid solutions give the clearest STM images and are associated with the most stable two-component monolayer. The present paper demonstrates that non-covalent interactions at the solution-metal interface can lead to complex multicomponent monolayer structures.     

Single molecule imaging of oxygenation of cobalt octaethylporphyrin at the solution/solid interface: thermodynamics from microscopy

For the first time, the pressure and temperature dependence of a chemical reaction at the solid/solution interface is studied by scanning tunneling microscopy (STM), and thermodynamic data are derived. In particular, the STM is used to study the reversible binding of O(2) with cobalt(II) octaethylporphyrin (CoOEP) supported on highly oriented pyrolytic graphite (HOPG) at the phenyloctane/CoOEP/HOPG interface. The adsorption is shown to follow the Langmuir isotherm with P(1/2)(298K) = 3200 Torr. Over the temperature range of 10-40 °C, it was found that ΔH(P) = -68 ± 10 kJ/mol and ΔS(P) = -297 ± 30 J/(mol K). The enthalpy and entropy changes are slightly larger than expected based on solution-phase reactions, and possible origins of these differences are discussed. The big surprise here is the presence of any O(2) binding at room temperature, since CoOEP is not expected to bind O(2) in fluid solution. The stability of the bound oxygen is attributed to charge donation from the graphite substrate to the cobalt, thereby stabilizing the polarized Co-O(2) bonding. We report the surface unit cell for CoOEP on HOPG in phenyloctane at 25 °C to be A = (1.46 ± 0.1)n nm, B = (1.36 ± 0.1)m nm, and α = 54 ± 3°, where n and m are unknown nonzero non-negative integers.     

An iets study of alumina and magnesia supported —sihx and their use as substrates for inorganic vibrational spectroscopy

Tunneling spectra of Al₂O₃/—SiH_x, MgO/—SiH_x, Al₂O₃/—SiD_x, and Al₂O₃/—SiH_x + NCS^? are reported. Analysis of the vibrational spectra observed from isotopic substitution of the barriers obtained by deposition of a thin film of SiO onto alumina and magnesia indicate that the supported species is —SiH. The Al₂O₃/—SiH barrier can be used as a support for studying inorganic ions by IETS.     

Scanning tunneling microscopy, orbital-mediated tunneling spectroscopy, and ultraviolet photoelectron spectroscopy of metal(II) tetraphenylporphyrins deposited from vapor.

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.     

The interaction of ions and easily ionized species with oxide surfaces as studied by tunneling spectroscopy

The thin film properties of alumina and magnesia, the substrates most frequently used in tunneling spectroscopy, are reviewed. The composite barriers, Al₂O₃/SiHO_x and Al_xSO_y are also discussed. Tunneling studies provide information about absorbate-surface interactions; weak counterion adsorption effects through very strong direct chemical modifications can be studied. Tunneling studies of anionic transition metal complexes adsorbed on alumina provide significantly more information about the nature of the surface species than corresponding IR studies. The adsorption of TCNE on alumina from solution is used to exemplify how tunneling spectroscopy may be integrated into a manifold of conventional techniques (IR, ESR, and elution studies in this case).     

Fabrication of graphene with CuO islands by chemical vapor deposition

Graphene prepared on Cu foil by chemical vapor deposition was studied as a function of post growth cooling conditions. CuO islands embedded in the graphene film were discovered and studied by scanning electron microscopy, atomic force microscopy, and X-ray photoemission spectroscopy. It is shown that nanostructured holes can be formed within a graphene film by reduction using hydrogen cooling immediately after film growth. We also observe the formation of symmetrical oxide islands in these holes. This study provides an easy way to fabricate a graphene + CuO composite, and the method may be extended to other graphene based structures.     

Commensurate ordering of iron phthalocyanine on Ag(1 1 1) surface

Epitaxial growth of iron phthalocyanine on a silver (1 1 1) surface has been studied with scanning tunneling microscopy in ultrahigh vacuum. While several structures were observed, this report concentrates on what appeared to be the most stable phase. This phase has commensurate two-dimensional ordering with oblique unit vectors (b₁, b₂) where b₁ = 5a₁ + 2a₂ and b₂ = 5a₂, where a₁ and a₂ are unit vectors of Ag(1 1 1) surface. The rotation angle of iron phthalocyanine relative to the silver [1 0 ] direction was determined to be 16.63° from the high-resolution image analysis. The oblique structure is more commensurate than that of iron phthalocyanine on graphite and of copper phthalocyanine on the Ag(1 1 1) surface. The observed FePc on Ag(1 1 1) structure is similar to that of FePc on Au(1 1 1), but has significantly higher surface density. Bias dependence of the images was observed and is consistent with the expected small HOMO–LUMO gap.     

Synthesis of [n]- and [n.n]cyclophanes by using Suzuki-Miyaura coupling

Reaction of the bis-9-BBN adduct of several dienes with 1,3-dibromobenzene via Suzuki coupling leads to a series of [n]metacyclophanes ranging in size from 10 to 17 atom members. In each case, two carbon-carbon bonds are formed in one reaction vessel. However, when the bis-9-BBN adduct of 1,5-hexadiene is coupled with a variety of aryl dihalides, larger [n.n]cyclophanes were formed in preference to the [n]cyclophanes. Four carbon-carbon bonds are formed in this instance. Single-crystal X-ray analyses of these [n.n]cyclophanes reveal interestingly shaped molecules with large cavities.