Hiroki Kurata

Photoinduced Charge Carrier Dynamics of Zn−Porphyrin−TiO2 Electrodes: The Key Role of Charge Recombination for Solar Cell Performance

Time resolved absorption spectroscopy has been used to study photoinduced electron injection and charge recombination in Zn-porphyrin sensitized nanostructured TiO(2) electrodes. The electron transfer dynamics is correlated to the performance of dye sensitized solar cells based on the same electrodes. We find that the dye/semiconductor binding can be described with a heterogeneous geometry where the Zn-porphyrin molecules are attached to the TiO(2) surface with a distribution of tilt angles. The binding angle determines the porphyrin-semiconductor electron transfer distance and charge transfer occurs through space, rather than through the bridge connecting the porphyrin to the surface. For short sensitization times (1 h), there is a direct correlation between solar cell efficiency and amplitude of the kinetic component due to long-lived conduction band electrons, once variations in light harvesting (surface coverage) have been taken into account. Long sensitization time (12 h) results in decreased solar cell efficiency because of decreased efficiency of electron injection.

SCANNING TUNNELING MICROSCOPE CONTRAST OF PERYLENE-3,4,9,10-TETRACARBOXYLIC-DIANHYDRIDE ON GRAPHITE AND ITS APPLICATION TO THE STUDY OF EPITAXY

Epitaxial films of perylene‐3,4,9,10‐tetracarboxylic‐dianhydride (PTCDA) on graphite (0001) were investigated by scanning tunneling microscopy. Molecular image contrast of PTCDA was found to depend strongly upon the molecular orientation and the position on graphite. In particular, the periodic discrepancy between PTCDA and graphite lattice points results in a modulation of contrast, which can be used to determine the epitaxial relation of PTCDA relative to the substrate accurately. By analyzing this modulation of contrast, we determined two kinds of epitaxial orientation of PTCDA. These orientations have no exact commensurate relation with graphite, but every lattice point of PTCDA lies on a lattice line parallel to the a axis (or b axis) of graphite. This specific feature contributes to decreasing the interfacial energy. The contrast mechanism of adsorbed molecules is also discussed.

Atomic level observation of octahedral distortions at the perovskite oxide heterointerface

For perovskite oxides, ABO3, slight octahedral distortions have close links to functional properties. While perovskite oxide heterostructures offer a good platform for controlling functionalities, atomistic understanding of octahedral distortion at the interface has been a challenge as it requires precise measurements of the oxygen atomic positions. Here we demonstrate an approach to clarify distortions at an atomic level using annular bright-field imaging in aberration-corrected scanning transmission electron microscopy, which provides precise mappings of cation and oxygen atomic positions from distortion-minimized images. This technique revealed significant distortions of RuO6 and ScO6 octahedra at the heterointerface between a SrRuO3 film and a GdScO3 substrate. We also found that structural mismatch was relieved within only four unit cells near the interface by shifting the oxygen atomic positions to accommodate octahedral tilt angle mismatch. The present results underscore the critical role of the oxygen atom in the octahedral connectivity at the perovskite oxide heterointerface.

Tuning magnetic anisotropy by interfacially engineering the oxygen coordination environment in a transition metal oxide

Strong correlations between electrons, spins and lattices--stemming from strong hybridization between transition metal d and oxygen p orbitals--are responsible for the functional properties of transition metal oxides. Artificial oxide heterostructures with chemically abrupt interfaces provide a platform for engineering bonding geometries that lead to emergent phenomena. Here we demonstrate the control of the oxygen coordination environment of the perovskite, SrRuO3, by heterostructuring it with Ca0.5Sr0.5TiO3 (0-4 monolayers thick) grown on a GdScO3 substrate. We found that a Ru-O-Ti bond angle of the SrRuO3 /Ca0.5Sr0.5TiO3 interface can be engineered by layer-by-layer control of the Ca0.5Sr0.5TiO3 layer thickness, and that the engineered Ru-O-Ti bond angle not only stabilizes a Ru-O-Ru bond angle never seen in bulk SrRuO3, but also tunes the magnetic anisotropy in the entire SrRuO3 layer. The results demonstrate that interface engineering of the oxygen coordination environment allows one to control additional degrees of freedom in functional oxide heterostructures.

Star-shaped trimeric quaternary ammonium bromide surfactants: adsorption and aggregation properties.

Novel star-shaped trimeric surfactants consisting of three quaternary ammonium surfactants linked to a tris(2-aminoethyl)amine core were synthesized. Each ammonium had two methyls and a straight alkyl chain of 8, 10, 12, or 14 carbons. The adsorption and aggregation properties of these tris(N-alkyl-N,N-dimethyl-2-ammoniumethyl)amine bromides (3C(n)trisQ, in which n represents alkyl chain carbon number) were characterized by equilibrium and dynamic surface tension, rheology, small-angle neutron scattering (SANS), and cryogenic transmission electron microscopy (cryo-TEM) techniques. 3C(n)trisQ showed critical micelle concentrations (CMC) 1 order of magnitude lower than that of the corresponding gemini surfactants with an ethylene spacer and the corresponding monomeric surfactants. The logarithm of the CMC decreased linearly with increasing hydrocarbon chain length for 3C(n)trisQ. The slope of the line, which is well-known as Klevens equation, was larger than those of the monomeric and gemini surfactants; however, considering the total carbon number in the chains, the slope was shallower than the monomeric and was close to the gemini. Through the results such as surface tensions at the CMC (32-34 mN m(-1)) and the parameters of standard free energy, CMC/C(20) and pC(20), it was found that 3C(n)trisQ could adsorb densely at the air/water interface despite the strong electrostatic repulsion between multiple quaternary ammonium headgroups. Moreover, dynamic surface tension measurements showed that the kinetics of adsorption for 3C(n)trisQ to the air/water interface was slow because of their bulky structures. Furthermore, the results of rheology, SANS, and cryo-TEM determined that 3C(n)trisQ with n = 10 and 12 formed ellipsoidal micelles at low concentrations in solution and the structures transformed to threadlike micelles with very few branches for n = 12 as the concentration increased, but for n = 14 threadlike micelles formed at relatively low concentrations.

Formation of pseudomorphic nanocages from Cu2O nanocrystals through anion exchange reactions.

Creating semiconductor nanocages The surface area of nanomaterials can be increased by creating open cage structures. Now it seems that the shape of nanocrystals can be used as a tool to manipulate crystal structure in nanocrystals. Wu et al. show how single nanocrystals of copper oxide are converted through anion exchange reactions to multiply twinned open cages of a copper sulfide in a process that changes the crystal lattice symmetries. These structures were then converted into cadmium sulfide nanocages through cation exchange. Science, this issue p. 1306 Copper oxide nanocrystals can be converted in copper sulfide and cadmium sulfide nanocages via exchange reactions. The crystal structure of ionic nanocrystals (NCs) is usually controlled through reaction temperature, according to their phase diagram. We show that when ionic NCs with different shapes, but identical crystal structures, were subjected to anion exchange reactions under ambient conditions, pseudomorphic products with different crystal systems were obtained. The shape-dependent anionic framework (surface anion sublattice and stacking pattern) of Cu2O NCs determined the crystal system of anion-exchanged products of CuxS nanocages. This method enabled us to convert a body-centered cubic lattice into either a face-centered cubic or a hexagonally close-packed lattice to form crystallographically unusual, multiply twinned structures. Subsequent cation exchange reactions produced CdS nanocages while preserving the multiply-twinned structures. A high-temperature stable phase such as wurtzite ZnS was also obtained with this method at ambient conditions.

Prediction of the Epitaxial Orientation of Ultrathin Organic Films on Graphite

A method of predicting the epitaxial orientation of organic monolayer on graphite substrate was proposed together with a misfit defined appropriately for the prediction. This method is based on a lattice matching mode, point-on-line coincidence, that has been found through our scanning tunneling microscopy (STM) investigations of organic epitaxy on graphite. In order to examine efficiency of the method, epitaxial orientations of bis-1,2,5-thiadiazolo-tetracyano-quinodimethane (BTDA-TCNQ) and bis-1,2,5-selenadiazolo-tetracyano-quinodimethane (BSDA-TCNQ) were predicted and the predictions were compared with the orientations observed by STM. In both cases, the orientation observed was found to be exactly one of the predicted orientations, which suggests not only the efficiency of the prediction method but also the importance of the point-on-line coincidence in organic epitaxy.

Thermal-sensitive viscosity transition of elongated micelles induced by breaking intermolecular hydrogen bonding of amide groups.

A heat-induced viscosity transition of novel worm-like micelles of a long alkyl-chain amidoamine derivative (C18AA) bearing intermolecular hydrogen-bonding group was investigated by cryo-TEM, FT-IR, and rheological measurements. At lower temperature, C18AA forms straight elongated micelles with a length on the order of micrometers due to strong intermolecular hydrogen-bonded packing of the amide groups, although the micelles rarely entangle and have low value of zero-shear viscosity. The straight elongated micelles likely became flexible and underwent a morphological transition from straight structure to worm-like structure at a certain temperature, which caused a drastic increase in viscosity due to entanglement of the micelles. This morphological transition was caused by a defect of intermolecular hydrogen bonding between the amide groups on heating. Furthermore, addition of LiCl, which acts as hydrogen-bond breaker, also promoted the viscosity transition, leading to a lowering of the transition temperature.

Effects of electron channeling in HAADF-STEM intensity in La2CuSnO6

Atomic resolution imaging using the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) can be applied to analyze the atomic structures of materials directly. This technique provides incoherent Z-contrast with the atomic number of the constituent elements. In the present work, unique contrasts that make intuitively interpreting the HAADF-STEM image in double perovskite oxide La(2)CuSnO(6) difficult were observed. Multislice simulation confirmed that this occurred as an effect of the channeling process of electrons in combination with the effect of Debye-Waller factors. This was confirmed because in the La(2)CuSnO(6) crystal, two independent Sn atoms and four independent La atoms in the unit cell had different Debye-Waller factors, and the La columns consisted of pairs of columns with a small separation, whereas the Sn atoms were arranged straight. Furthermore, the image contrast was examined by mutislice simulation, and two atomic La columns were separated in a projected plane and appeared as one column contrast using multislice simulation. As a result, the HAADF intensity did not decrease constantly with the increase in column separation, with the exception of a very thin sample, which could be interpreted by the specific change in the electron-channeling process.

Gelation Behavior by the Lanthanoid Adsorption of the Cyanobacterial Extracellular Polysaccharide

The self-organization behavior of an extracellular polysaccharide (sacran) extracted from the cyanobacterium Aphanothece sacrum in response to lanthanoid ion adsorption was investigated. Consequently, cryogenic TEM images revealed that sacran could be cross-linked by Nd(3+) trivalent ions and formed a fibrous nanostructural network containing water. Furthermore, sacran adsorbed trivalent metal ions at a 3:1 ratio, which was the theoretical ionic adsorption and showed more efficient adsorption than alginate based on electric conductivity titration. The critical gelation concentrations, Cg, where sacran formed tough gels upon metal ion binding were estimated. The Cg for trivalent metal ions was lower than that for divalent ions, and the Cg for lanthanoid ions was particularly low at 10(-3) to 10(-4) M, changing every four elemental numbers. The extracellular matrix of Aphanothece sacrum, sacran, may adsorb metal ions to create fibrous nanostructures that reinforce the jelly matrix.