Shun-ichiro Watanabe

Electrostatic force microscope imaging analyzed by the surface charge method

The electrostatic force acting between the probe of the force microscope and the sample surface is theoretically investigated by using the surface charge method. The actual shapes of the probe (an etched tungsten wire) and the sample (gratinglike electrodes) which are used in the experiments are considered to calculate the electrostatic forces quantitatively. It is shown that the macroscopic probe has to be taken into account for the force measurement. However, only the top part of the probe is responsive for the measurement of the force gradient. The imaging properties through the detection of the electrostatic force are also investigated experimentally by using the dynamic (ac) mode force microscope. The potentiometric and topographic effects on the imaging are discussed on the basis of theoretical calculations and the experimental results.

Electron spin resonance observation of field-induced charge carriers in ultrathin-film transistors of regioregular poly(3-hexylthiophene) with controlled in-plane chain orientation

Ultrathin-film polymeric transistors with controlled in-plane chain orientation have been fabricated based on Langmuir–Blodgett technique by cospreading liquid crystal molecule with regioregular poly(3-hexylthiophene). The mobilities parallel to the chain direction reached 0.001–0.01 cm2/V s for 1 to 5 monolayer thick transistors. Mobility ratio was about 2 for the parallel and perpendicular directions. Electron spin resonance (ESR) signals of the field-induced polarons exhibited clear in-plane anisotropies due to unpaired π-electrons. The anisotropic ESR spectra together with the optical dichroism determine the detailed molecular orientations in the channel of such ultrathin transistors.

Direct observation of the charge carrier concentration in organic field-effect transistors by electron spin resonance

Charge carrier concentration in operating field-effect transistor (FET) of regioregular poly(3-hexylthiophene) has been directly determined by electron spin resonance (ESR). ESR signals of field-induced polarons are observed around g=2.003 under the application of negative gate-source voltage (Vgs). Upon applying drain-source voltage (Vds), ESR intensity decreases linearly in the low Vds region, reaching to about 50% of the initial intensity at the pinch-off point (Vds≅Vgs). For larger absolute values of Vds, it becomes nearly Vds independent. These behaviors are well explained by the change in the carrier concentration obtained by the FET theory using gradual channel approximation.

Electron Spin Resonance Observation of Gate-induced Charge Carriers in Organic Field-effect Devices Fabricated on Silicon Substrates

Electron spin resonance (ESR) measurements have been performed on metal–insulator–semiconductor (MIS) diode structures of regioregular poly(3-hexylthiophene), RR-P3HT, fabricated on silicon substrates with SiO2 layers as gate insulators. The conductivity of substrates was chosen so that it does not significantly lower the quality factor of the ESR cavity. Clear ESR signals due to field-induced polarons have been observed at g-values of around 2.002, consistent with those observed in the MIS devices of RR-P3HT fabricated on Al2O3 gate insulators. Carrier spins tend to saturate above the charge concentration of about 0.3%, suggesting the conversion of polarons with spin 1/2 to spinless bipolarons for higher carrier concentrations. The angular dependence of ESR signals exhibits distinct anisotropy, reflecting the fact that the surface of SiO2 is flatter than that of Al2O3. These results demonstrate that ESR can be performed on organic field-effect devices fabricated on silicon substrates.

Electron Spin Resonance Observation of Gate-Induced Ambipolar Charge Carriers in Organic Devices

Electron spin resonance (ESR) measurements are performed on gate-induced ambipolar charge carriers in composites of regioregular poly(3-hexylthiophne) (RR-P3HT) and C60 using metal–insulator–semiconductor (MIS) diode structures with the RR-P3HT–C60 composite as the active semiconductor and Al2O3 as the gate insulator. Clear ESR signals of g ≈2.002 and g ≈1.998 are successfully observed for negative and positive gate voltages (VG), respectively, and they monotonically increase as |VG| increases in each accumulation mode. We demonstrate microscopically that positive and negative charge carriers with the signals g ≈2.002 and g ≈1.998 are ascribable to RR-P3HT and C60, respectively.

Force microscope using a twin‐path interferometer

In this article, a force microscope using a twin‐path (common‐path) laser interferometer is presented. The interferometer was developed for the displacement detection of a small cantilever. Since the two interfering beams were very close to each other, the perturbation caused by the environmental disturbances were minimized. The optical path difference between the two beams was easily adjusted by the moire effect of the gratings. The vibrational amplitude of 10−3 nm was measured under the resonance conditions of the tungsten lever. The drift rate of the interference signal was less than 10−3 nm/s. An etched‐resist coated with thin gold film was imaged by using electrostatic forces under the conditions of ac mode operation.

Variable-Range Hopping Conduction in Ion-Gel-Gated Electrochemical Transistors of Regioregular Poly(3-hexylthiophene)

Ion-gel-gated transistors enable controlled carrier doping into conjugated polymers in identical devices measurable at low temperatures. Using this technique, the carrier transport of regioregular poly(3-hexylthiophene) was investigated in a wide temperature range down to 15 K. At room temperature, carrier mobility exhibited an electrochemical behavior with a minimum at a doping level of approximately 0.4% and a subsequent sharp increase up to 0.5 cm 2 V -1 s -1 . At doping levels above ∼3%, the ohmic conductivity of the transistor channel deviated from thermal activation and showed a quasi-one-dimensional variable-range hopping behavior. This result indicates the enhanced carrier transport through the overlap of carrier wavefunctions developing toward a doping level of ∼10% under electrochemical doping in a transistor channel.

Dynamic mode force microscopy for the detection of lateral and vertical electrostatic forces

In this letter, a variant of scanning force microscope for detecting attractive forces is reported. The force gradients of the attractive forces acting in two orthogonal directions were detected simultaneously from the resonant frequency shifts of a cantilever oscillating in two directions. Using the fine electrode sample, the distributions of the electrostatic forces acting in lateral and vertical directions were visualized separately.

Two‐directional dynamic mode force microscopy: Detection of directional force gradient

In this article, we report the theoretical and experimental investigations on a dynamic mode force microscopy for detecting the force gradients in two directions. The motion of the probe oscillated in two directions was analyzed on the basis of the mechanical dynamics theory to provide the ability of detecting the force gradient in vertical and lateral directions separately. When the spring constants of the cantilever probe in two orthogonal directions were different, the gradients of the attractive forces acting in two orthogonal directions were detected separately from the resonant frequency shifts. The theoretical consideration was confirmed by the experiment in which the gradients of electrostatic force acting on a fine electrode sample were measured.

A compact interferometer using moire effect for the phase compensation

A compact interferometer using a grating beamsplitter is reported. The phase of the interference signal was adjusted by using the moire effect of the gratings, so that a small optical path difference was compensated precisely by the magnified displacement of the grating. The optical system was analyzed theoretically on the basis of the Fourier optics. For the interferometric measurement in the microscopic region, a compact optical system was assembled from the microscopic objective and the 25‐μm gratings. The characteristics of the proposed interferometer were investigated in the measurements of film thickness, piezoelectric vibration, and photoacoustic effect.