Yutaka Wakayama

Copper-phthalocyanine based metal-organic interfaces: The effect of fluorination, the substrate, and its symmetry

Metal-organic interfaces based on copper-phthalocyanine monolayers are studied in dependence of the metal substrate (Au versus Cu), of its symmetry [hexagonal (111) surfaces versus fourfold (100) surfaces], as well as of the donor or acceptor semiconducting character associated with the nonfluorinated or perfluorinated molecules, respectively. Comparison of the properties of these systematically varied metal-organic interfaces provides new insight into the effect of each of the previously mentioned parameters on the molecule-substrate interactions.

Understanding energy-level alignment in donor-acceptor/metal interfaces from core-level shifts

The molecule/metal interface is the key element in charge injection devices. It can be generally defined by a monolayer-thick blend of donor and/or acceptor molecules in contact with a metal surface. Energy barriers for electron and hole injection are determined by the offset from HOMO (highest occupied) and LUMO (lowest unoccupied) molecular levels of this contact layer with respect to the Fermi level of the metal electrode. However, the HOMO and LUMO alignment is not easy to elucidate in complex multicomponent, molecule/metal systems. We demonstrate that core-level photoemission from donor-acceptor/metal interfaces can be used to straightforwardly and transparently assess molecular-level alignment. Systematic experiments in a variety of systems show characteristic binding energy shifts in core levels as a function of molecular donor/acceptor ratio, irrespective of the molecule or the metal. Such shifts reveal how the level alignment at the molecule/metal interface varies as a function of the donor-acceptor stoichiometry in the contact blend.

Optically and Electrically Driven Organic Thin Film Transistors with Diarylethene Photochromic Channel Layers

We achieved drain-current switching of diarylethene-channel field-effect transistors with light- and electric-field effects. The drain current was reversibly changed by alternating ultraviolet and visible light irradiation. Stress is placed on the fact that the on/off ratio realized by light irradiation was 1 × 10(2) (1 × 10(4)%) and this value is much larger than those in other photochromism-based transistors. These results indicate that the drain current was effectively controlled by light irradiation. Furthermore, the on and off states modulated by light were maintained without light irradiation even after 1 week, exhibiting that our transistor works as an optical memory. We clarified that the light-driven modulation can be attributed to the transformation in the π-conjugation system accompanied by photoisomerization. These findings have the potential to attain high-performance optoelectrical organic devices including optical sensors, optical memory, and photoswitching transistors.

Recent progress in photoactive organic field-effect transistors

Abstract Recent progress in photoactive organic field-effect transistors (OFETs) is reviewed. Photoactive OFETs are divided into light-emitting (LE) and light-receiving (LR) OFETs. In the first part, LE-OFETs are reviewed from the viewpoint of the evolution of device structures. Device performances have improved in the last decade with the evolution of device structures from single-layer unipolar to multi-layer ambipolar transistors. In the second part, various kinds of LR-OFETs are featured. These are categorized according to their functionalities: phototransistors, non-volatile optical memories, and photochromism-based transistors. For both, various device configurations are introduced: thin-film based transistors for practical applications, single-crystalline transistors to investigate fundamental physics, nanowires, multi-layers, and vertical transistors based on new concepts.

Light propagation within colloidal crystal wire fabricated by a dewetting process.

We present a colloidal crystal wire composed of thousands of connected microspheres that is fabricated by a simple dewetting process utilizing a drain phenomenon, and we directly observe the light propagation within the wire by near-field scanning optical microscopy. The optical properties of propagation light suggest that the propagation mechanism was attributed mainly to nanojet-induced mode coupling for the straight propagation component and partly to whispering-gallery mode coupling within the colloidal crystal wire.

How molecules accommodate a 2D crystal lattice mismatch: an unusual ‘mixed’ conformation of tetraphenylporphyrin

Tetraphenylporphyrin molecules adopt unusual unsymmetrical conformations in order to achieve perfect order at the region between differing 2-D crystal phases.

Substrate-Independent Growth of Atomically Precise Chiral Graphene Nanoribbons

Contributing to the need for new graphene nanoribbon (GNR) structures that can be synthesized with atomic precision, we have designed a reactant that renders chiral (3,1)-GNRs after a multistep reaction including Ullmann coupling and cyclodehydrogenation. The nanoribbon synthesis has been successfully proven on different coinage metals, and the formation process, together with the fingerprints associated with each reaction step, has been studied by combining scanning tunneling microscopy, core-level spectroscopy, and density functional calculations. In addition to the GNR’s chiral edge structure, the substantial GNR lengths achieved and the low processing temperature required to complete the reaction grant this reactant extremely interesting properties for potential applications.

Three-dimensional islands of Si and Ge formed on SiO2 through crystallization and agglomeration from amorphous thin films

The crystallization process was examined for amorphous thin films of silicon (a-Si) and germanium (a-Ge) on quartz glass (SiO2) substrate. Three-dimensional crystalline islands were formed through crystallization and agglomeration. These islands indicated a bimodal size distribution. The mechanism of crystalline island (c-Si, c-Ge) formation was discussed on the basis of thermodynamics. In studying the crystallization of the thin films, the influence of the film-substrate interfacial energy should be taken into consideration. It was found that the thickness of the as-deposited amorphous films is an essential factor in determining the crystallization behavior and in controlling island size. Above all, a high size uniformity of crystalline islands could be obtained under moderate thermal annealing conditions. q 1999 Elsevier Science S.A. All rights reserved.

Observation of light propagation across a 90° corner in chains of microspheres on a patterned substrate

To demonstrate light-path manipulation in arbitrary shapes we fabricated coupled-resonator optical waveguides (CROWs) having a 90°-corner structure on a lithographically patterned substrate. The spectra of propagation light within the CROWs were directly measured by guide-collection-mode near-field scanning optical microscopy. The spectra revealed that the propagation light through the CROWs has a larger transverse-magnetic polarization mode than a transverse-electric (TE) one. The most plausible cause of the lower intensity in the TE mode is that light leaks out to the Si substrate.

Solid-State Reactions in Binary Molecular Assemblies of F16CuPc and Pentacene

Various phases of binary molecular assemblies of perfluorinated Cu-phthalocyanine (F₁₆CuPc) and pentacene were examined using scanning tunneling microscopy (STM). Alloying, solid solutions, phase separation, and segregation were observed in assemblies on monolayers according to the mixture ratios. The main driving force behind such molecular blending is CH-F hydrogen bonds. Lattice matching and molecular symmetry are other factors that determine the assembly configuration. A detailed understanding of such solid-state reactions provides a guideline to the construction of multilayered binary assemblies, where intermixing between molecules takes place when multiple layers are stacked.