Takayuki Miyamae

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...

Anion Configuration at the Air/Liquid Interface of Ionic Liquid [bmim]OTf Studied by Sum-Frequency Generation Spectroscopy

The air/liquid interface of a room temperature ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([bmim]OTf), is investigated using infrared-visible sum frequency generation (SFG) spectroscopy. The SFG spectra clearly show low-frequency modes [CF3-symmetric stretching (ss) mode and SO3-symmetric stretching (ss) mode] of the OTf anion, demonstrating the existence of anions polar oriented at the interface. The amplitude of the CF3-ss peak of the OTf anion has the opposite sign with respect to that of the SO3-ss peak, indicating that OTf anions at the surface have polar ordering where the nonpolar CF3 group points away from the bulk into the air, whereas the SO3 group points toward the bulk liquid. The line width of the SFG peak from the submerged SO3 group is appreciably narrower than that from IR absorption, suggesting the environment of the surface OTf anions is much more homogeneous than that of the bulk. The vibrational calculations also suggest that the anions and the cations form a more specific aggregated configuration at the surface as compared to the bulk.

Angle-resolved UV photoelectron spectra (UPS) of thin films of perylene-3,4,9,10-tetracarboxylic dianhydride on MoS2

Angle-resolved UV photoelectron spectra (ARUPS) were measured for thin films of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) deposited on cleaved MoS(2) surfaces. The take-off angle (theta) dependence of the photoelectron intensity of the highest pi band showed a sharp maximum at theta = 32-34 degrees. A spectral feature of the binding energy at approximately 8.9 eV, which is believed to originate from a pi state, showed a remarkably different theta dependence from that of the pi band. A quantitative analysis of the observed theta dependencies clearly indicates that (a) the feature at approximately 8.9 eV originates from the oxygen 2p non-bonding states and (b) the molecules lie flat on the substrate surface.

Morphology and chemical structure of poly(methyl methacrylate) surfaces and interfaces: restructuring behavior induced by the deposition of SiO2

Abstract The surface morphology, molecular structure, and the surface restructuring behavior after the deposition of a SiO 2 layer on poly(methyl methacrylate) (PMMA) have been investigated by using atomic force microscopy (AFM) in contact mode and sum-frequency generation (SFG) vibrational spectroscopy. AFM images show that the surface of PMMA is quite flat and sporadically contains dendritic features, which are micro-crystalline structures of PMMA. From the SFG measurements, the ester methyl groups of PMMA are pointed away from the surface plane at least in the surface region. By depositing a SiO 2 film, the SFG signal intensity is significantly reduced, indicating that the CO and the ester methyl groups are aligned nearly parallel to the surface plane.

Why can organic liquids move easily on smooth alkyl-terminated surfaces?

The dynamic dewettability of a smooth alkyl-terminated sol-gel hybrid film surface against 17 probe liquids (polar and nonpolar, with high and low surface tensions) was systematically investigated using contact angle (CA) hysteresis and substrate tilt angle (TA) measurements, in terms of their physicochemical properties such as surface tension, molecular weight/volume, dielectric constant, density, and viscosity. We found that the dynamic dewettability of the hybrid film markedly depended not on the surface tensions but on the dielectric constants of the probe liquids, displaying lower resistance to liquid drop movement with decreasing dielectric constant (ε < 30). Interfacial analysis using the sum-frequency generation (SFG) technique confirmed that the conformation of surface-tethered alkyl chains was markedly altered before and after contact with the different types of probe liquids. When probe liquids with low dielectric constants were in contact with our surface, CH3 groups were preferentially exposed at the solid/liquid interface, leading to a reduction in surface energy. Because of such local changes in surface energy at the three-phase contact line of the probe liquid, the contact line can move continuously from low-surface-energy (solid/liquid) areas to surrounding high-surface-energy (solid/air) areas without pinning. Consequently, the organic probe liquids with low dielectric constants can move easily and roll off when tilted only slightly, independent of the magnitude of CAs, without relying on conventional surface roughening and perfluorination.

Probing buried organic layers in organic light-emitting diodes under operation by electric-field-induced doubly resonant sum-frequency generation spectroscopy

Electric-field-induced doubly resonant sum-frequency generation (EFI-DR-SFG) spectroscopy was used to study the electric field distribution in multilayer organic light-emitting diodes (OLEDs). Remarkable correlations between the DR-SFG signal enhancement and the applied bias voltage were observed. The SFG signals attributed to 4,4′-bis[N-(1-naphthyl-N-phenylamino)-biphenyl] were significantly enhanced by applying a forward voltage, whereas those from Alq3 were increased by applying a reverse voltage. The large enhancement in EFI-DR-SFG intensity enables us to nondestructively probe the local electric field distribution at the buried organic layer within the OLED.

Electronic structure and molecular orientation of well-ordered polyethylene oligomer (n-C44H90) on Cu(100) and Au(111) surfaces studied by UV photoemission and low energy electron diffraction

Abstract The electronic structure and molecular orientation of tetratetracontane (n-C44H90) films on Cu(100) and Au(111) surfaces were investigated by angle-resolved UV photoemission spectroscopy (ARUPS) and low energy electron diffraction (LEED). The observed ARUPS spectra showed the drastic take-off angle dependence due to intramolecular band dispersion. A 2×1-like LEED pattern was observed for both substrates. From these results and theoretical simulation of ARUPS spectra based on independent-atomic center (IAC) approximation, we found that the C–C–C plane of the adsorbed TTC molecule is parallel to the substrate surface and its molecular axis is along a [110] direction for both substrates. We also measured the work function change by adsorption of TTC. The observed values were c.a. −0.3eV and −0.7eV for Cu(100) and Au(111) systems, respectively. Such decrease of the work function indicates the existence of a dipole layer at the interfaces in contrast to the traditional picture of energy level alignment at organic/metal interface assuming a common vacuum level at the interface. The dipole formation in such physisorbed systems can be explained by the polarization of the TTC molecule due to an image force.

Surface structure of sulfuric acid solution relevant to sulfate aerosol: molecular dynamics simulation combined with sum frequency generation measurement

Surface structure of aqueous sulfuric acid solution at a typical atmospheric concentration (0.2x, x: mole fraction) is investigated by close collaboration of molecular dynamics (MD) simulation and sum frequency generation (SFG) measurement. The SFG spectra of both O-H and S-O stretching vibrations are provided with different sets of polarization combination. These sets of experimental spectra are consistently elucidated by the MD calculations. In modeling the surface structure, there exists a major uncertainty about local ion composition at the surface region. To address this uncertainty, we performed MD simulations with various assumptions on the local dissociation constants of sulfuric acid, and searched for the condition to be consistent with the experimental spectra. We have thereby concluded that the first acid dissociation of sulfuric acid is almost complete at the surface, while the second dissociation is more strongly suppressed than in the bulk liquid. The present MD simulation elucidates the ion distribution and molecular orientation at the sulfuric acid solution surface, and also the concentration dependence of the SFG spectrum.