Eiichi Fujiwara

Visible light emission from polymer-based field-effect transistors

Field-effect transistors (FETs) based on poly [2-methoxy, 5-(2′-ethyl-hexoxy)-1, 4-phenylenevinylene] (MEH-PPV) were prepared with bottom-contact type interdigital electrodes of Cr/Au and Al/Au on the SiO2/Si substrates. MEH-PPV exhibited a p-type semiconducting behavior and orange light emission was observed when the devices were operated in vacuum. It was found that the luminescence efficiency of the FETs with Al/Au electrodes was higher than that of Cr/Au electrodes. The simultaneous injection of holes and electrons into MEH-PPV occurred efficiently with the application of Al/Au heteroelectrodes.

Monolayer and bilayer formation of 17,19-dotetracontadiyne at a liquid/solid interface

Monolayer and bilayer of 17,19-dotetracontadiyne (DTDY) on a graphite substrate were studied by scanning tunneling microscopy at a liquid solid interface of phenyloctane solution. The orientation of the layers was examined with respect to the highly oriented pyrolitic graphite. The first layer grew very quickly with many small domains some tens of nm in diameter, and the alkyl chains of the molecule in each domain align epitaxially along the a g -axis of graphite. When the solution remains at room temperature, the second layer of DTDY grew epitaxially on the first layer and the domain size was much larger than that of the first layer.

A scanning tunneling microscopy study on the monolayer epitaxy of [cyano(ethoxycarbonyl) methylene]-2-ylidene-4,5-dimethyl-1,3-dithiole on graphite and its dynamic feature

The monolayer film of [cyano(ethoxycarbonyl)methylene]-2-ylidene-4,5-dimethyl-1,3-dithiole (CEMYDD) vacuum-deposited on a (0001) graphite surface was investigated by scanning tunneling microscopy (STM). High-resolution STM images showed that all of the molecules formed the same two-dimensional unit cell in each domain and that the unit cell was almost equivalent to that in the (011) plane of the second-harmonic-generation active β-form crystal. Therefore the CEMYDD molecules were arranged as polar domains with a size of several hundred nm. The crystallographic orientation of the unit cell in the film with respect to the graphite surface was found to have commensurate epitaxial coincidence. This commensurate structure was stable under STM measurements, except near the domain boundaries where the moire-like, long-range, periodic contrast modulation due to the commensurate coincidence often varied while scanning with the STM tip. Although the arrangement of molecular images themselves moved only slightly, such a collective dynamic feature at boundaries could be detected from changes in the moire-like contrast modulation.

Field-Effect Transistors Based on Single-Crystalline Wires of Bis-(1, 2, 5-Thiadiazolo)-p-Quinobis(1, 3-Dithiole)

We prepared single-crystalline wires of bis(1, 2, 5-thiadiazolo)-p-quinobis(1, 3-dithiole), whose ends were anchored to the drain and source electrodes of bottom-contact-type field-effect transistors. The temperature dependence of carrier mobility was investigated in the range from 5 K to 330 K. The tunnel transport was found to be dominant at T 200 K, indicating that phonon scattering governs carrier transport in single-crystalline wires.

Pseudo-Commensurate Epitaxy of A Non-Planar Molecule of Isoindoline Derivative At Liquid-Solid Interface

To understand epitaxial growth of organic molecules from a phenyloctane solution, mono-molecular layers of a non-planar isoindoline derivative formed on the graphite (0001) surface were investigated by STM. The observed images showed that the layer is composed of many small single-crystalline 2D domains, where the lattice orientation was analyzed with respect to the graphite surface lattice. As a result, the molecules adsorbed with its isoindoline frame to be parallel to the HOPG surface and the epitaxial orientations were slightly fluctuated around commensurate orientation from domain to domain. This is named pseudo-commensurate epitaxy, which is resulted from a size effect; that is, due to broadening of Laue functions appearing in an expression of lattice-lattice interaction energy at interface of deposited layer and substrate surface.

Effect of molecular symmetry and packing efficiency on the epitaxial growth modes of monomolecular film on graphite studied by STM: the case of 2,4,6-tris(4,5-dimethyl-1,3-dithiol-2-ylidene)-1,3,5-cyclohexanetrione

The monomolecular film of 2,4,6-tris(4,5-dimethyl-1,3-dithiol-2-ylidene)-1,3,5-cyclohexanetrione (HMTDTCHTO) vacuum-deposited on a (0 0 0 1) surface of graphite was observed by scanning tunneling microscopy, in order to reveal its growth mode and structure. The unit lattice of the HMTDTCHTO monomolecular layer is incommensurate to the substrate lattice. The flat lying molecules form an ordered close-packed arrangement in the film on the graphite. The results are compared with those of 2,4,6-tris(1,3-dithiol-2-ylidene)-1,3,5-cyclohexanetrione (TDTCHTO). The growth modes of some monomolecular films are examined from a viewpoint of two-dimensional packing efficiency and molecular symmetry.