Carla Puglia

Physisorbed, chemisorbed and dissociated O2 on Pt(111) studied by different core level spectroscopy methods

Abstract For O 2 Pt (111) we have found four different adsorption phases which are formed at different substrate temperatures. At about 25 K the oxygen molecules physisorb on the surface. Two chemisorbed phases are observed at 90 and 135 K, respectively. An atomic phase, characterized by a sharp (2 × 2) LEED pattern, exists at a temperature above 150 K. Different spectroscopic techniques have been used to characterize the four different adsorption states: XPS studies of adsorbate and surface core level shifts, UPS, NEXAFS, autoionization and Auger spectroscopy. We conclude that oxygen adsorbs in two different molecular chemisorbed states which can be considered to be precursors for the thermally activated atomization process. The first of these molecular states is weakly chemisorbed at 90 K. It is adsorbed in a hollow site with a saturation coverage of 0.23 (molecules per Pt surface atom). We have identified this phase as a superoxo-like configuration. The second phase is more strongly bonded to the Pt substrate. It is characterized by a longer and weaker molecular σ bonding due to more charge transfer from the metallic substrate to the antibonding molecular 1 π g orbitals than for the first chemisorbed phase. With a coverage of 0.15 the oxygen molecules seem to be adsorbed in hollow or hollow-bridge sites. We have characterized this phase as a peroxo-like configuration of the oxygen molecule. For atomic oxygen on platinum we have found a coverage of 0.25 (oxygen atoms per Pt surface atom) and a threefold adsorption site, in agreement with previous studies. We discuss the XAS results according to a model for the density of states induced by the hybridization of the 2p atomic orbitals with the 6sp states and 5d band of the metal.

Overlayer structure from adsorbate and substrate core level binding energy shifts: CO, CCH3 and O on Pt(111)

Abstract By combining high resolution photoemission measurements of adsorption induced binding energy shifts of both adsorbate and substrate core levels the nature and distribution of adsorption sites in the CO/Pt(111) system can be determined. The existence of different surface shifted components demonstrates the local character of the surface core level shift. This is used to study the O/Pt(111) (2 × 2) and CCH3/Pt(111) “(2 × 2)” phases. The intensity relations of the different surface peaks suggest O/Pt(111) to be a true (2 × 2) phase, while the CCH3/Pt(111) “(2 × 2)” phase is proposed to consist of three equivalent (2 × 1) domains.

The electronic structure of iron phthalocyanine probed by photoelectron and x-ray absorption spectroscopies and density functional theory calculations

A joint experimental and theoretical work to explain the electronic and geometrical structure of an in situ prepared film of iron phthalocyanine (FePc) on silicon (100) is presented. FePc molecular films have been characterized by core and valence photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS), and the results have been interpreted and simulated by density functional theory (DFT) calculations. C1s and N1s PE spectra have been analyzed by taking into account all chemically nonequivalent C and N atoms in the molecule. In the Fe2p(32) spectra it has been possible to resolve two components that can be related to the open shell structure of the molecule. By valence PES and N1s XAS data, the geometrical orientation of the FePc molecules in the film could be determined. Our results indicate that for the FePc on Si(100), the molecules within the film are mainly standing on the surface. The experimental N1s XAS spectra are very well reproduced by the theoretical calculations, which are both angle and atomic resolved, giving a detailed description of the electronic and geometric structure of the FePc film. Furthermore, the asymmetry and the intensity angle variation of the first N1s XAS threshold feature could be explained by the presented DFT calculations as due to the chemical nonequivalence of the N atoms and the symmetry character of the lowest unoccupied molecular orbital.

Electronic structure of a vapor-deposited metal-free phthalocyanine thin film

The electronic structure of a vapor-sublimated thin film of metal-free phthalocyanine (H2Pc) is studied experimentally and theoretically. An atom-specific picture of the occupied and unoccupied electronic states is obtained using x-ray-absorption spectroscopy (XAS), core- and valence-level x-ray photoelectron spectroscopy (XPS), and density-functional theory (DFT) calculations. The DFT calculations allow for an identification of the contributions from individual nitrogen atoms to the experimental N1s XAS and valence XPS spectra. This comprehensive study of metal-free phthalocyanine is relevant for the application of such molecules in molecular electronics and provides a solid foundation for identifying modifications in the electronic structure induced by various substituent groups.

Scanning tunneling microscopy study of metal-free phthalocyanine monolayer structures on graphite

Low temperature scanning tunneling microscopy (STM) studies of metal-free phthalocyanine (H2Pc) adsorbed on highly oriented pyrolytic graphite (HOPG) have shown ordered arrangement of molecules for low coverages up to 1 ML. Evaporation of H2Pc onto HOPG and annealing of the sample to 670 K result in a densely packed structure of the molecules. Arrangements of submonolayer, monolayer, and monolayer with additional adsorbed molecules have been investigated. The high resolution of our investigations has permitted us to image single molecule orientation. The molecular plane is found to be oriented parallel to the substrate surface and a square adsorption unit cell of the molecules is reported. In addition, depending on the bias voltage, different electronic states of the molecules have been probed. The characterized molecular states are in excellent agreement with density functional theory ground state simulations of a single molecule. Additional molecules adsorbed on the monolayer structures have been observed, and it is found that the second layer molecules adsorb flat and on top of the molecules in the first layer. All STM measurements presented here have been performed at a sample temperature of 70 K.

Valence-band electronic structure of iron phthalocyanine : An experimental and theoretical photoelectron spectroscopy study

The electronic structure of iron phthalocyanine (FePc) in the valence region was examined within a joint theoretical-experimental collaboration. Particular emphasis was placed on the determination of the energy position of the Fe 3d levels in proximity of the highest occupied molecular orbital (HOMO). Photoelectron spectroscopy (PES) measurements were performed on FePc in gas phase at several photon energies in the interval between 21 and 150 eV. Significant variations of the relative intensities were observed, indicating a different elemental and atomic orbital composition of the highest lying spectral features. The electronic structure of a single FePc molecule was first computed by quantum chemical calculations by means of density functional theory (DFT). The hybrid Becke 3-parameter, Lee, Yang and Parr (B3LYP) functional and the semilocal 1996 functional of Perdew, Burke and Ernzerhof (PBE) of the generalized gradient approximation (GGA-)type, exchange-correlation functionals were used. The DFT/B3LYP calculations find that the HOMO is a doubly occupied π-type orbital formed by the carbon 2p electrons, and the HOMO-1 is a mixing of carbon 2p and iron 3d electrons. In contrast, the DFT/PBE calculations find an iron 3d contribution in the HOMO. The experimental photoelectron spectra of the valence band taken at different energies were simulated by means of the Gelius model, taking into account the atomic subshell photoionization cross sections. Moreover, calculations of the electronic structure of FePc using the GGA+U method were performed, where the strong correlations of the Fe 3d electronic states were incorporated through the Hubbard model. Through a comparison with our quantum chemical calculations we find that the best agreement with the experimental results is obtained for a U(eff) value of 5 eV.

Elucidating the 3d Electronic Configuration in Manganese Phthalocyanine

To shed light on the metal 3d electronic structure of manganese phthalocyanine, so far controversial, we performed photoelectron measurements both in the gas phase and as thin film. With the purpose of explaining the experimental results,three different electronic configurations close in energy to one another were studied by means of density functional theory. The comparison between the calculated valence band density of states and the measured spectra revealed that in the gas phase the molecules exhibit a mixed electronic configuration, while in the thin film, manganese phthalocyanine finds itself in the theoretically computed ground state, namely, the b1(2g)e3(g)a1(1g)b0(1g) electronic configuration.

Electron spectroscopy study of the initial stages of iron phthalocyanine growth on highly oriented pyrolitic graphite.

The nature of the intermolecular and substrate bonds of iron phthalocyanine adsorbed on highly oriented pyrolitic graphite has been investigated by x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. We find that the molecules grow in a highly ordered fashion with the molecules essentially plane-parallel to the surface in both the mono- and multilayers. The spectra obtained on both types of film are virtually identical, which shows that the bonds both between the adsorbate and substrate and between the molecular layers have a pure van der Waals nature. Supporting density functional theory results indicate that the layers are stabilized by weak hydrogen bonds within the molecular layers.

Molecular ordering in isonicotinic acid on rutile TiO2(110) investigated with valence band photoemission

The adsorption of isonicotinic acid on rutile TiO(2)(110) has been investigated using synchrotron-based valence band photoemission. Structural ordering in multilayer films of the molecules is found to give rise to a strong angular dependence in the valence band intensities when measured using linearly polarized radiation. Molecular ordering in this case is proposed to be induced by intermolecular hydrogen bonding which is found to be highly dependent upon the deposition rate of the isonicotinic acid. Through comparison of the experimental data with density functional calculated valence band spectra of hydrogen-bonded isonicotinic acid molecules, we can account for the angular dependence in terms of the spatial distribution of the molecular orbitals.

Experimental and theoretical study of electronic structure of lutetium bi-phthalocyanine.

Using Near Edge X-Ray Absorption Fine Structure (NEXAFS) Spectroscopy, the thickness dependent formation of Lutetium Phthalocyanine (LuPc2) films on a stepped passivated Si(100)2×1 reconstructed surface was studied. Density functional theory (DFT) calculations were employed to gain detailed insights into the electronic structure. Photoelectron spectroscopy measurements have not revealed any noticeable interaction of LuPc2 with the H-passivated Si surface. The presented study can be considered to give a comprehensive description of the LuPc2 molecular electronic structure. The DFT calculations reveal the interaction of the two molecular rings with each other and with the metallic center forming new kinds of orbitals in between the phthalocyanine rings, which allows to better understand the experimentally obtained NEXAFS results.