Kaname Kanai

Ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy of [6,6]-phenyl-C61-butyric acid methyl ester in gas and solid phases

The electronic structure of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was studied using ultraviolet photoelectron spectroscopy of vapor and thin film and inverse photoemission spectroscopy of thin film. The threshold ionization energy of PCBM was found to be 7.17±0.04 eV in gas phase and 5.96±0.02 eV in solid phase. The threshold electron affinity was 3.9±0.1 eV in solid phase. These values are 0.4−0.6 eV smaller than C60. The density functional theory calculations gave consistent results with these trends and suggested that the electron donation from the side chain to C60 backbone raised the C60-backbone-derived π orbitals of PCBM. The polarization energy of PCBM is 1.21 eV, which is almost the same as C60 but is about 0.5 eV smaller than the value of typical aromatic hydrocarbons.

Atmospheric effect of air, N2, O2, and water vapor on the ionization energy of titanyl phthalocyanine thin film studied by photoemission yield spectroscopy

The effect of atmospheric gases on the ionization energy (I) of titanyl phthalocyanine thin film was investigated by an apparatus of photoemission yield spectroscopy developed for the measurements of I for the same specimen both in vacuum and under gaseous atmosphere. It was found that the value of I is affected by the exposure to various ambient gases (i.e., air, nitrogen, and oxygen of 1 atm, and water vapor corresponding to 27% relative humidity at 300 K), and that the effect strongly depends on the gas. The ionization energies in vacuum could be determined as the onset I0 of the cube-root plot of the photoemission yield as a function of photon energy. When the sample was exposed to gases, the cube-root plot still gives an onset, but often a long tail at the low-energy side with another onset It was also observed. The first exposure to air did not affect both I0 and It much, while the following evacuation-exposure cycles caused mostly reversible decrease and increase of I0 and It by about 0.2 and 0.4 e...

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.

Doping Effect of Tetrathianaphthacene Molecule in Organic Semiconductors on Their Interfacial Electronic Structures Studied by UV Photoemission Spectroscopy

The n-type doping ability of the electron-donating organic molecule tetrathianaphthacene (TTN) was examined by ultraviolet photoemission spectroscopy (UPS) for two kinds of matrix organic semiconductors, hexadecafluorophthalocyaninatozinc (F16ZnPc) and tris(8-hydroxyquinoline)aluminum (Alq3). The observed dependence of the UPS spectra suggests that TTN acts as a good donor in F16ZnPc, while the effect was not so significant for Alq3. These results and the consideration of the energy parameters of these molecules indicate that a close match between the highest occupied molecular orbital of the dopant and the lowest unoccupied molecular orbital of the matrix is important for efficient n-type doping.

Halide-Substituted Electronic Properties of Organometal Halide Perovskite Films: Direct and Inverse Photoemission Studies

Solution-processed perovskite solar cells are attracting increasing interest due to their potential in next-generation hybrid photovoltaic devices. Despite the morphological control over the perovskite films, quantitative information on electronic structures and interface energetics is of paramount importance to the optimal photovoltaic performance. Here, direct and inverse photoemission spectroscopies are used to determine the electronic structures and chemical compositions of various methylammonium lead halide perovskite films (MAPbX3, X = Cl, Br, and I), revealing the strong influence of halide substitution on the electronic properties of perovskite films. Precise control over halide compositions in MAPbX3 films causes the manipulation of the electronic properties, with a qualitatively blue shift along the I → Br → Cl series and showing the increase in ionization potentials from 5.96 to 7.04 eV and the change of transport band gaps in the range from 1.70 to 3.09 eV. The resulting light absorption of MAPbX3 films can cover the entire visible region from 420 to 800 nm. The results presented here provide a quantitative guide for the analysis of perovskite-based solar cell performance and the selection of optimal carrier-extraction materials for photogenerated electrons and holes.

Anomalous electronic structure of ionic liquids determined by soft x-ray emission spectroscopy: Contributions from the cations and anions to the occupied electronic structure

Soft x-ray emission spectroscopy was used for elucidating the electronic structure of ionic liquids [C(4)mim](+)PF(6)(-) and [C(4)mim](+)OTf(-), where [C(4)mim](+) stands for methylbutylimidazolium cation and OTf(-) for the trifluoromethanesulfonate anion. Nonresonant spectra measured above N, O, and F 1s edges selectively probed the molecular orbitals (MOs) of the cation and anions. They give a clear evidence that the highest occupied molecular orbital of the [C(4)mim](+) cation contributes to the topmost occupied states of the ionic liquids [C(4)mim](+)PF(6)(-), while both cationic and anionic MOs contribute for the case of [C(4)mim](+)OTf(-). Resonant soft x-ray emission spectra at the N 1s edge of these ionic liquids revealed that the energy gap of [C(4)mim](+)PF(6)(-) is solely determined by the [C(4)mim](+) cation, in contrast to usual ionic crystals. The ionic liquids form a new class of the ionic materials from the viewpoint of the electronic structure.

Atmospheric Effect on the Ionization Energy of Titanyl Phthalocyanine Thin Film as Studied by Photoemission Yield Spectroscopy

The atmospheric effect on the ionization energy (I) of titanyl phthalocyanine (TiOPc) thin film was investigated by photoemission yield spectroscopy developed for the measurement of I for the same specimen both in vacuum and under gaseous atmosphere. The variation of I of TiOPc thin film induced by exposure to air was observed.

Development of Experimental Methods for Determining the Electronic Structure of Organic Materials

The principles and developments of experimental methods for determining the energy parameters governing the electronic structure of organic molecules and solids are reviewed with some historical descriptions. The energy parameters discussed are ionization energy of a molecule (Ig) and solid (Is), electron affinity of a molecule (Ag) and solid (As), and the energies of the vacuum level (VL) and the Fermi level (EF). The work function (Φ) can be also derived as the difference between the last two. Various types of spectroscopic and electrical measurements are discussed with typical examples, including the range of applicability and the merits and the demerits. Recent microscopic techniques for obtaining local information are also described.

Highly Ordered and Stable Layered “Polymer Nanosheets” Constructed with Amorphous Side Chains and π−π Stacking of Functional Groups in Ternary Comb Copolymers

We have developed a highly stable, layered structure for ternary copolymers in Langmuir-Blodgett (LB) films at a nanometer scale, with substantial durability over the long-term. In these ternary copolymer LB films, amorphous side chains support the layered structure, and the distance between the layers is controlled at the nanometer scale by the composition of hydrogenated and fluorinated side chains. In the present study, the fine structures of newly synthesized ternary comb copolymers with a carbazole ring in the solid state and molecular orientations in the LB films were investigated using wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), surface pressure-area (pi-A) isotherms, in-plane and out-of-plane X-ray diffraction (XRD), and atomic force microscopy (AFM). The WAXD results identified two short-spacing peaks related to the formation of the subcells for both the fluorinated and hydrogenated side chains. Further, SAXS measurements indicated that these ternary copolymers formed a highly ordered layer structure. In addition, monolayers on the water surface of these ternary copolymers were highly condensed. From the results of in-plane XRD and AFM, it was determined that the side chains and side-chain crystals could not form phase-separated structures in two-dimensional films. These structural features may result from enhancement of pi-pi interactions between the arranged carbazole rings. The side chains of the copolymers in the two-dimensional films are apparently in a miscible state, and monolayers form a homogeneous amorphous surface because of cancellation of differences in van der Waals forces between the two types of side chains. As a result, formation of a highly ordered layer structure in copolymer films having substantial durability over the long term is realized because amorphous side chains support the layer structure in the LB multilayers. Further, control of long spacing at a subnanometer level becomes possible due to changes in the tilt angle of the side chains, depending on their fluorocarbon content.

Control of Arrangement for s-Triazine Group in Comb Copolymers by the Langmuir−Blodgett Method and Its Structural Estimation by NEXAFS Spectroscopy

We investigated the molecular orientation of organized molecular films with regard to solid-state structures for newly synthesized comb copolymers with 2-vinyl-4,6-diamino-1,3,5-triazine (VDAT) by surface pressure-area (pi-A) isotherms, in-plane and out-of-plane X-ray diffraction (XRD), atomic force microscopy (AFM), and polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Since VDAT has adsorption ability to an adenine-thymine base pair of a DNA molecule, control of orientation for VDAT units in monolayers is possible to form surface patterning of biomolecules and construct candidates of new biochip materials. In the bulk state, hydrogenated and fluorinated comb copolymers containing VDAT form side-chain crystals for a two-dimensional lattice spacing of 4.2 and 5.0 A, respectively. From the results of the differential scanning calorimetric (DSC) measurements, sharp-shaped melting peaks appear on the relatively lower temperature side of the thermograms. This result supports the formation of side-chain crystals in the synthesized comb copolymers. These monolayers of copolymers on the water surface were extremely condensed, except for the VDAT:OA = 5:1 copolymer. From the in-plane XRD measurement of multilayers on solids, changes in the two-dimensional lattice structure of fluorinated comb copolymer films containing VDAT units, as opposed to their bulk state, were confirmed. It seems that these structural changes are caused by the stronger pi-pi interaction between the s-triazine rings rather than the van der Waals interaction between fluorocarbons. Polarized NEXAFS spectroscopy showed highly ordered orientation of s-triazine groups in the films based on the incident angle dependency of C and N1s-pi*(CN) transitions with synchrotron radiation. These experimental findings relate to well-ordered arrangement of functional groups supporting the side-chain rearrangement caused by the pi-pi interaction between the s-triazine rings.