Orbital Mediated Tunneling Spectroscopy Research Articles

Investigation of Metal Free Naphthalocyanine Vapor Deposited on Au(111)

Naphthalocyanines (Ncs) are promising candidates for future components in electronic devices and applications. To maximize the efficiency of Nc devices, it is critical to understand their structural and electronic properties and how these are impacted by deposition methods. The formation of a metal free naphthalocyanine (H2Nc) self-assembled monolayer on a Au(111) crystal was investigated by scanning tunneling microscopy under ultra-high-vacuum conditions at room temperature. A rigorous purification and processing procedure was developed to produce high purity, low defect, and well-ordered monolayers. High-resolution STM images reveal epitaxial growth of H2Nc on Au(111) with the observed structure having a molecular spacing of 1.6 ± 0.05 nm, with molecules orientated slightly off (roughly 2.5°) the low density packing direction of Au(111). A commensurate structure having 4 molecules per unit cell and unit cell parameters of A = 3.25 ± 0.05 nm, B = 3.17 ± 0.05 nm, and α = 87.5 ± 2° is proposed. Orbital-mediated tunneling spectroscopy was used to examine the electronic properties of individual molecules within the thin film. The first ionization potential and electron affinity of H2Nc adsorbed on Au(111) were measured to be −0.68 ± 0.03 and 1.12 ± 0.02 eV, relative to the Fermi energy.

Structural and Electronic Properties of Columnar Supramolecular Assemblies Formed from Ionic Metal-Free Phthalocyanine on Au(111)

Highly ordered assemblies prepared from tetra (4-sulfonatophenyl) phthalocyanine (TSPc) by employing very acidic aqueous solutions were deposited onto Au(111) substrates and studied in UHV using X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and orbital mediated tunneling spectroscopy (OMTS). XPS of the TSPc aggregates shows that the ratio of protonated to unprotonated nitrogens does not change with decreasing solution pH. STM images of TSPc deposited from pH <1 solutions reveal ordered branched web-like assemblies hundreds of nanometers in length, generally 2 nm tall and having variable widths. High-resolution UHV-STM images of TSPc aggregates on Au(111) reveal detailed coherent columnar architecture with the phthalocyanine macrocycles orientated parallel to the substrate surface. OMTS was used to identify high-energy occupied orbitals, the LUMO of the TSPc aggregates, and the results are contrasted with the same molecular states in unsubstituted metalated phthalocyanines (M...

A Self-Assembled Two-Dimensional Zwitterionic Structure: H6TSPP Studied on Graphite

Well-defined monolayer islands of protonated meso-tetrakis(4-sulfonatophenyl)porphine (H2[H4TSPP]) self-assembled on highly oriented pyrolytic graphite (HOPG) from 0.75 M HCl solutions are studied in ultrahigh vacuum using scanning tunneling microscopy (STM), orbital-mediated tunneling spectroscopy (OMTS), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). HOPG proved to be a superior substrate because it does not form a thin surface chloride in the presence of HCl, as does Au(111). Unlike meso-tetrakis(4-carboxyphenyl)porphine (Hx[HyTCPP]), the carboxylate analog, monolayers are stable on HOPG and can be studied at room temperature without the addition of a second stabilizing compound. Thus, the surface organizational energy for H2[H4TSPP] is significantly stronger than that of Hx[HyTCPP]. This increased interaction is also apparent in the fact that the H2[H4TSPP] monolayer has a much smaller unit cell. It is suggested that sulfonate groups may be more useful than carboxylates for creating self-assembled structures on HOPG. Protonation of the porphyrin nitrogens in the surface species is confirmed by XPS. High-resolution images of single molecule layers show a well-defined deformation of the porphyrin ring, as expected with complete protonation of the central nitrogen atoms. OMTS and UPS were used to identify the HOMO and LUMO of the H2[H4TSPP] monolayer species, and results are contrasted to those of nickel(II) tetraphenylporphyrin (NiTPP). H2[H4TSPP] previously has been shown to form well-defined nanorods on HOPG. I−V curves of single and stacked rods taken by STM are consistent with conduction in a band formed from the LUMO of H2[H4TSPP].

Differing HOMO and LUMO Mediated Conduction in a Porphyrin Nanorod

In this communication we provide the first UHV-STM images and STM-based current-voltage (I-V) and orbital mediated tunneling spectroscopy (OMTS) data on a self-assembled porphyrin nanostructure at the single structure level. We will show that transverse conductivity over distances less than 10 nm can occur by barrier type tunneling but that long distance conduction solely occurs through the LUMO band. These nanorods are very highly rectifying.

Scanning Tunneling Microscopy and Orbital-Mediated Tunneling Spectroscopy of N,N′-Dioctyl-1,8:4,5-naphthalenediimide Adsorbed on Highly Ordered Pyrolytic Graphite from Various Solvents and in Different Environments

Scanning tunneling microscopy (STM) and orbital-mediated tunneling spectroscopy (OMTS) are reported for N,N′-dioctyl-1,8:4,5-naphthalenediimide (diimide) adsorbed on highly ordered pyrolytic graphite (HOPG). The diimide forms well ordered monolayers either at the interface between HOPG and several phenylalkanes, or at the HOPG−air or HOPG−vacuum interface when adsorbed from toluene. Planar adsorption of the diimide ring on HOPG is observed. Hydrogen bonding, O and N interaction with HOPG, and π−π interactions appear to be the primary drivers for determining the monolayer structure which is stable and independent of the adsorption method. This is an unusual example since most alkane-substituted systems studied to date rely on alkane chain interactions (with HOPG and interdigitation) to drive the adsorbate structure on graphite. The observed unit cell has a = 2.0 ± 0.2 nm, b = 1.95 ± 0.2 nm, α = 67 ± 2°. The STM imaging is highly bias dependent and appears to be controlled (in the ±2 V bias region) by an un...

Scanning Tunneling Microscopy and Orbital-Mediated Tunneling Spectroscopy Study of 1,5-Di(octyloxy)anthracene Adsorbed on Highly Ordered Pyrolytic Graphite from Various Solvents and in Different Environments

Scanning tunneling microscopy (STM) and orbital mediated tunneling spectroscopy (OMTS) are reported for 1,5-di(octyloxy)anthracene (15DA) adsorbed on highly ordered pyrolytic graphite (HOPG). 15DA forms well-ordered monolayers either at the interface between HOPG and 1-phenyloctane or at the HOPG−air or HOPG−vacuum interface when spin doped from octane or dichloromethane. Octyl chain interdigitation and planar adsorption of the anthracene ring combine to produce structures which are stable independent of the adsorption method. The observed unit cell has a = 1.76 ± 0.06 nm, b = 1.07 ± 0.06 nm, and α = 77 ± 3°. Molecular orbital calculations are combined with bias-dependent imaging and OMTS results to show that the highest-occupied molecular orbital (HOMO) dominates the tunneling process over the voltage range from −2.5 to +1.0 V. The HOMO of 15DA on HOPG is found to occur at −7.0 eV below the vacuum level, in good agreement with previous UPS studies and with our calculations for the free molecule. We also ...

Electron Tunneling, a Quantum Probe for the Quantum World of Nanotechnology

Key events and concepts in the development of tunneling spectroscopy, especially in the context of the scanning tunneling microscope, are reviewed. Focus is placed on the single-molecule spectroscopy of adsorbates on surfaces. Examples of molecular images with sub-molecular resolution are provided. Molecular electronic spectra obtained in the STM are displayed and contrasted with results from techniques requiring large (>109) numbers of molecules. Correlation between the nanoscale method of orbital-mediated tunneling spectroscopy (OMTS) and the microscale methods of ultraviolet photoelectron spectra (UPS) and electrochemical oxidation and reduction potentials will be made. Mapping of electron-transport paths through a molecule as a function of electron energy will be demonstrated. Specific examples of STM based OMTS will be drawn from porphyrins and phthalocyanines on Au(111).

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.

Scanning Tunneling Microscopy, Orbital-Mediated Tunneling Spectroscopy, and Ultraviolet Photoelectron Spectroscopy of Nickel(II) Octaethylporphyrin Deposited from Vapor

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.

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.

Orbital Mediated Tunneling in Vanadyl Phthalocyanine Observed in both Tunnel Diode and STM Environments

For the first time orbital mediated tunneling spectroscopy (OMTS) is reported for the same species, vanadyl phthalocyanine (VOPc), in both the tunnel diode and STM environments. IETS and OMTS for a junction of the form [Al−Al2O3−VOPc−Pb] show both the expected bias symmetry of vibrational and electronic inelastic electron tunneling spectroscopy (IETS) bands and the highly bias asymmetric OMTS band associated with tunneling via direct occupation of the Pc ring π* orbital. STM studies of submonolayer coverage of VOPc on Au(111) produce well-resolved submolecular images of the molecule. Current imaging tunneling spectroscopy (CITS) allows one to easily identify VOPc islands, and the resulting I−V curves can be used to generate dI/dV spectra for VOPc and for bare gold regions. An intense band seen near +1.4 V bias (Au+) is identified as the same OMTS band as seen in the tunnel diode: resonance-like tunneling through the Pc ring π* orbital.

Orbital-Mediated Tunneling, Inelastic Electron Tunneling, and Electrochemical Potentials for Metal Phthalocyanine Thin Films

Orbital-mediated electron tunneling through both occupied and unoccupied orbitals of metal phthalocyanines imbedded in metal−insulator−metal tunnel junctions is reported and discussed in terms of transient oxidation and reduction of the molecular species. Electrochemical oxidation and reduction potentials for the solution phase molecular systems are compared to the orbital-mediated tunneling spectroscopy (OMTS) data and a strong correlation is observed. These results are consistent with a simple model. It is demonstrated that different redox sites and states within a given molecule within a thin film will shift in unique ways relative to the equivalent processes in solution. Inelastic scattering from phthalocyanine π−π* transitions and metal centered d−d transitions are also observed.