Takafumi Miyazaki

Electronic structures of organic molecular materials for organic electroluminescent devices studied by ultraviolet photoemission spectroscopy

Electronic structures of evaporated films of five organic light-emitting and carrier-injecting materials for organic electroluminescent devices were studied by ultraviolet photoemission spectroscopy. The compounds examined were (i) light-emitting materials tris(8-hydroxyquinolino) aluminum (Alq3) and 1,2,3,4,5-pentaphenylcyclopentadiene, (ii) a hole-injecting material N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine, and (iii) electron-injecting materials N,N′-diphenyl-1,4,5,8-naphthyletracarboxyldiimide and 1,3,5-tris(5-phenyl-1,3,4-oxadiazol-2-yl)benzene. The spectral features corresponding to the top parts of the valence states, which dominate the electric properties of the materials, were assigned by the comparison with the simulated density of states obtained from PM3 molecular orbital calculations. Using these calculations, the evolution of the electronic structure of each molecule from those of constituent parts was discussed. The characters of the unoccupied states obtained by t...

Ultraviolet photoelectron spectroscopy of poly(pyridine-2,5-diyl), poly(2,2′-bipyridine-5,5′-diyl), and their K-doped states

Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π‐conjugated polymers, poly(pyridine‐2,5‐diyl) (PPy) and poly(2,2′‐bipyridine‐5,5′‐diyl) (PBPy) which exhibit n‐type electrically conducting properties. The two compounds show similar spectra and they were analyzed with MO calculations and the comparison with the data of related molecules. The ionization threshold energies of PPy and PBPy were found to be 6.3 and 6.35 eV, respectively. These values are higher than those of π‐conjugated conducting polymers capable of p doping. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap and the intensity of the state at 0.65 eV from EF grows as the doping proceeds. This finding and the change of optical absorption spectra upon doping indicate that bipolaron bands are formed in K‐doped PBPy. While K‐doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K‐doped PBPy. The difference of the dependence on dopant concentration between K‐doped PPy and K‐doped PBPy is discussed based on the conformational difference between these polymers.Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π‐conjugated polymers, poly(pyridine‐2,5‐diyl) (PPy) and poly(2,2′‐bipyridine‐5,5′‐diyl) (PBPy) which exhibit n‐type electrically conducting properties. The two compounds show similar spectra and they were analyzed with MO calculations and the comparison with the data of related molecules. The ionization threshold energies of PPy and PBPy were found to be 6.3 and 6.35 eV, respectively. These values are higher than those of π‐conjugated conducting polymers capable of p doping. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap and the intensity of the state at 0.65 eV from EF grows as the doping proceeds. This finding and the change of optical absorption spectra upon doping indicate that bipolaron bands are formed in K‐doped PBPy. While K‐doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K‐doped PBPy....

Angle-resolved ultraviolet photoelectron spectroscopy of thin films of bis(1,2,5-thiadiazolo)-p-quinobis (1,3-dithiole) on the MoS2 surface

In this paper we report on the angle-resolved ultraviolet photoelectron spectroscopy on ultrathin films of bis(1,2,5-thiadiazolo)-p-quinobis (1,3-dithiole) (BTQBT) deposited on a MoS2 surface with synchrotron radiation, and describe the quantitative analysis of the angular distribution of photoelectron from the highest occupied state. The observed angular distributions were better explained by those calculated with the single-scattering approximation combined with molecular orbital calculation considering intramolecularly scattered waves than the previously used independent-atomic-center approximation combined with molecular orbital calculation. Further, the low-energy-electron diffraction measurements were performed on the film. From the low-energy-electron diffraction, the two-dimensional lattice of the ultrathin films on the MoS2 was found to be MoS2(0001)−(13×13, R=±13.9°)-BTQBT, and from the analysis of the photoelectron angular distributions, it was found that at the lattice points the molecules lie...

Structure and catalysis of layered rock-salt type oxides for methane oxidation

A comparison of X-ray photoemission spectra of LiFeO 2 , LiCoO 2 , and Li x Ni 2-x O 2 for the range of 0 <x ≤ 1 have been measured. A main O Is XPS peak of LiNiO 2 and LiCoO 2 with a hexagonal type was observed at 531.9 eV, while that of LiInO 2 , LiFeO 2 with a tetragonal type and Li x Ni 2-x O 2 (x < 0.6) with a cubic type were about 531.4 eV. It was inferred from these results that the layered superstructure of LiNiO 2 and LiCOO 2 largely affected the formation of the active oxygen species for the high selective coupling reaction of methane.

Photoelectron Angular Distribution of Thin Films of Copper Phthalocyanine on MoS2 Surfaces: Quantitative Determination of Molecular Orientation

Abstract The angle-resolved ultraviolet photoelectron spectra of ultrathin films of copper phthalocyanine (CuPc) on an MoS2 substrate were measured using synchrotron radiation. The azimuthal angle dependence of the photoelectron intensity was analyzed using independent-atomic-center approximation combined with molecular orbital calculation. The results indicate that CuPc molecules lie flat on the substrate and that the film grows epitaxially with respect to the three equivalent directions of the MoS2 surface.

Ultraviolet Photoelectron Spectroscopy of n-Type Conducting Polymers

Abstract Ultraviolet photoelectron spectra were measured using synchrotron radiation for four kinds of π -conjugated polymers, poly(pyridine-2,5-diyl)[Ppy], poly(2,2′-bipyridine-5,5′-diyl)[Pbpy], poly(2-methylantraquinone-1,4-diyl)[P(2Me-1,4-AQ)], and poly(antraquinone-1,5-diyl)[P(1,5-AQ)], which exhibit n-type electrically conducting properties. Upon K-doping of Pbpy, two new states appeared in the originally empty energy gap. This finding indicates that bipolaron bands are formed in K-doped Pbpy. While K-doped Ppy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K-doped Pbpy. The UPS spectra of K-doped P(2Me-1,4-AQ) also show similar gap states, while it is almost absent in K-doped P(1,5-AQ). The changes in electronic structure of these polymers are discussed based on the conformational difference between these polymers.

Structure and catalysis of LixNi2−xO2 oxide systems for oxidative coupling of methane

Publisher Summary This chapter explains a study of the Li x Ni 2−x O 2 system catalysts with rock-salt structure. Catalysts have been characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) and are tested in the oxidative coupling of methane to elucidate different structure and surface species. The solid solution series Li x Ni 2−x O 2 (07 0.3 Ni 1.7 O 2 , and Li 0.6 Ni 1.4 O 2 have a typical cubic type structure analogous to the structure of NiO. NiO has a rock-salt type structure in which Ni cations are co-ordinated in the six co-ordinate location formed in the cubic closest packing (ccp) of oxygen ions. As for Li x Ni 2−x O 2 solid solution in which Li ions are dissolved into NiO, the solid solution, with the range of x x Ni 2−x O 2 catalysts with different lithium content for the range of 0 x Ni 2−x O 2 , with x>0.6, having the hexagonal structure, become a catalyst that has high selectivity to the oxidative coupling of methane (OCM) reaction. The results of XPS measurement for the solid solution series Li x Ni 2−x O 2 are discussed in the chapter. The formation of active O − species occurs in the lattice oxygen and on the catalyst surface. It is proposed that an active O − species of the catalyst surface extracts a hydrogen atom from methane, and accordingly the coupling of methyl radicals take place. From the relationship between these structures and the catalytic activity, it was inferred that the anisotropic layered superstructure of LiNiO 2 largely affected the formation of the active oxygen species for the oxidative coupling reaction.

DV-Xα Calculation and Ultraviolet Photoelectron Spectra of Gold Trichloride-Graphite Intercalation Compound (AuCl3-GIC)

We investigate the electronic structure of AuCl 3 graphite intercalation compounds by means of discrete variational (DV)-Xα molecular orbital calculation and ultraviolet photoelectron spectra (UPS). DV-Xα calculation of the Au 2 Cl 6 intercalate reveals the importance of relativistic effect due to the presence of heavy Au atoms. The terminal Cl 3 p orbital contributes mainly to the lowest unoccupied molecular orbital (LUMO) of Au 2 Cl 6 molecule in addition to the minor contribution of the Au 5 d component. The calculation of (Au 2 Cl 6 ) -0.5 , which is considered to be intercalate species realized in graphitic galleries through charge transfer from graphite, suggests that the Au 2 Cl 6 intercalate band is located at around the Fermi level E F as well as the graphitic π-band. As a consequence, the density of states at E F is 2-3 times larger than that expected only by the graphitic π-band, which is consistent with optical reflectance, magnetic susceptibility, and the electronic specific heat. The small h...

Galvanomagnetic, Optical Properties and Ultraviolet Photoelectron Spectra of Potassium-Oxygen-Graphite Intercalation Compounds

We investigated the Hall effect, the transverse magnetoresistance, the optical reflectivity and the ultraviolet photoelectron spectra for 1st and 2nd stage KO x -GICs (C 4 n KO x , stage number n =1, 2; x ∼2) in order to clarify the transport properties and the electronic structures. The Hall effect and the magnetoresistance indicate that the electrical conduction in KO x -GICs is governed by the majority electron carriers with low mobility in graphitic π bands and the minority hole carriers with high mobility in the intercalate bands, the latter of which originates mainly from the oxygen bands as suggested by the incident photon energy dependence of the ultraviolet photoelectron spectra. The optical reflectance spectra give the estimates of the Fermi energies, 1.9 eV and 1.35 eV for 1st and 2nd stage compounds, respectively, on the basis of the Drude-Lorentz model. The contribution of the intercalate hole carriers to the plasma frequency is small in good agreement with the Hall effect indicating the pres...

Separate production of hydrogen and carbon monoxide by carbon dioxide reforming reaction of methane

Abstract First, modeling of the carbon dioxide reforming reaction (CO 2 RR) mechanism was carried out by the chemical affinity values of the individual elementary reactions which would be involved in the overall reaction. The chemical affinity values were obtained from a composition of an outlet stream passed through a catalyst bed. The catalysts used were Ni, Ru, Rh, Pd and Pt supported on MgO. As a result, the decomposition of CH 4 to give carbonaceous deposits and H 2 was the slowest step in CO 2 RR. Then, a separate production of H 2 and CO over Ni catalysts from an alternate stream of CH 4 and CO 2 was proved by the use of a flow system equipped with two reactors in parallel, two selector valves, and two gas reservoirs. As a result, such mixtures were obtained as H 2 (74%)+CH 4 (18%) and CO(71%)+CO 2 (17%) when Ni/αAl 2 O 3 was used as a catalyst.