Hirokazu Tada

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.

Scanning Tunneling Microscopy and Spectroscopy of Phthalocyanine Molecules on Metal Surfaces

We studied the electronic structure of cobalt-phthalocyanine (CoPc) molecules on Au(111) and Cu(100) surfaces by scanning tunneling microscopy and spectroscopy at 5 K. In the differential conductance (dI/dV) spectra, there were some peaks related to the highest occupied molecular orbital (HOMO), the lowest unoccupied MO (LUMO) and the d-orbitals of the Co atom. CoPc molecules on the Cu(100) surface had new electronic states between the peaks related to HOMO and LUMO, while those on the Au(111) surface did not show additional peaks. A dI/dV image indicated that the new states were generated by the hybridization between the LUMO of molecules and the electronic states of the Cu(100) surface.

Atomic force microscope anodization of Si(111) covered with alkyl monolayers

Alkyl monolayers on Si were prepared through the reaction between 1-alkenes and hydrogen-terminated Si by heat treatment. The monolayers were characterized by atomic force microscopy (AFM), force curve and water contact angle measurements. It was found that surface properties were modified by the formation of highly ordered closely packed monolayers. The monolayers were anodized with a contact-mode AFM by applying voltage between the conductive cantilever and surface under ambient conditions, which resulted in nanometer-scale oxidation of surfaces. After anodization, patterned areas were modified by removing the silicon oxide and terminating the surface of the grooves with hydrogen atoms by NH4F etching, and by covering the etched surface with 1-octadecene molecules. The monolayers themselves showed high resistance to NH4F etching and air oxidation. AFM lithography of monolayers anchored covalently on Si was found to be useful for nanofabrication of organic/inorganic interfaces based on Si–C covalent bonds.

Molecular conformation of n-alkyl monolayers covalently bonded to Si(111) probed by infrared-visible sum-frequency spectroscopy

Abstract Infrared–visible sum-frequency spectra of n -alkyl (C n H 2 n +1 ; n =10–13, 18) monolayers anchored on the Si(1xa01xa01) surface through a Si–C covalent bond were measured for the C–H stretch region. Spectral shapes exhibited azimuth-orientation dependence compatible with the C 3v symmetry of the substrate, indicating that the monolayers were epitaxially constructed on the substrate. Three methyl bands dominated each spectrum, and two-weak methylene bands were identified. Two distinct intensity patterns of the methyl bands were observed depending on n . The observed spectra were interpreted with reference to reported molecular models of the monolayer that consist of alkyl chains with an all- trans head part and a twisted stem.

Non-contact atomic force microscopy using silicon cantilevers covered with organic monolayers via silicon–carbon covalent bonds

Silicon cantilevers covered with dodecyl monolayers anchored via silicon–carbon covalent bonds were prepared by a wet process and used for non-contact atomic force microscopy (NC-AFM) of TiO2(110)-(1 × 1) surfaces. Clear images of atomic rows on atomically flat terraces were observed with the dodecyl-coated samples when they were biased around 2.0 V with respect to the cantilevers. The bias voltage required to give clear images for alkyl-coated cantilevers was higher than that for uncoated ones. Since the cantilevers are thermally and chemically stable, they are applicable to various force microscopy to distinguish chemical species on surfaces.

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.

Preparation of light-emitting organic field-effect transistors with asymmetric electrodes

Light-emitting organic field-effect transistors (LEOFETs) based on Poly [2-methoxy, 5-(2-ethyl-hexoxy)-1, 4-phenylenevinylene] (MEH-PPV), α-sexithiophene (α-6T) and N,N-Ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) were prepared on a SiO2 gate insulator. The LEOFETs based on MEH-PPV and α-6T showed a p-type semiconducting behavior whereas PTCDI-C13 operated in n-type FET. Asymmetric electrodes of Au-Al were prepared by twice of photolithography and lift-off techniques, and by electroplating of Au onto Al electrode to improved device performances. The emission efficiency of the devices with Au/Cr-Al was approximately 20 times higher than that of the device with Au/Al-Au/Al electrodes at the gate and drain voltages of -100 V. The emission region was observed with an optical microscope. The emission region was found to be very homogeneous along the drain electrode, and it did not shift when the gate and drain voltages changed. Although the carrier injection was improved by using asymmetric electrodes, the number of the carriers injected from the drain electrode was still lower than that from the source electrode in the unipolar devices.