Yuki Ohshima

Nanospot welding of carbon nanotubes

Single wall carbon nanotube (SWNT) bundles protruding from the SWNT layers on self-aligned Sn apexes were brought to a distance of 30 nm by a scanning tunneling microscope inside a transmission electron microscope. A straight bundle on the tip could be observed in situ in contact electrostatically with a looped bundle on the sample by applying tip bias voltages above 2.0 V. The bundles were welded at the nanometer size contact area by local Joule heating.

Direct observation of trapped carriers in polydiacetylene films by optical second harmonic generation

Trapped carriers in polydiacetylene (PDA) films were directly observed by the electric field induced second harmonic generation (EFISHG) using field effect transistor (FET) structure. Response of EFISHG signal from PDA-FET with applying voltage depended strongly on the polarity of gate voltage. For negative bias, which promotes hole injection from source electrode, EFISHG signal was not observed during bias application, whereas it was enhanced after turning off the bias. Electric field formed by trapped holes in PDA activated the EFISHG signal for the negative bias condition.

Growth of a single-wall carbon nanotube in the gap of scanning tunneling microscope

Single-wall carbon nanotubes (SWNTs) were grown in the tunneling gap of a scanning tunneling microscope (STM). We could observe their growth processes in situ by operating the STM in a transmission electron microscope. The STM tip and sample were covered by graphite layers. The tip was lightly touched to the sample and subsequently retracted. Occasionally, a carbon nanobridge was generated between the tip and the sample. The bridge had the shape of SWNT at the tip side.

Observation of electron behavior in ambipolar polymer-based light-emitting transistor by optical second harmonic generation

By using the optical second harmonic generation (SHG) measurement, we directly visualized the carrier behavior leading to carrier recombination and electroluminescence (EL) in ambipolar polymer-based organic light-emitting transistor (OLET) with an active layer of poly 9,9-di-n-octylfluorene-alt-benzothiadiszole (F8BT). Eliminating photoluminescence generated at 560 nm by a two-photon absorption process, the dynamical carrier motion in the F8BT-OLET was visualized by the electric field induced SHG induced at 420 nm. Diffusion-like electron transport that starts from the drain electrode was directly caught as the transits of the SHG images. Accordingly, EL was obtained at the edge of the source electrode. The electron mobility was estimated from the visualized carrier motion as 9.2×10-4cm2/Vs, which was larger than that obtained from the transfer curve of the OLET.

Direct observation of space charge field in tetracene field-effect transistor using time-resolved microscopic optical second harmonic generation

Using electric field-induced second harmonic generation (EFISHG) measurements, we directly probed a time-varying space charge field responsible for carrier injection followed by radiative recombination (electroluminescence; EL). Experiments using tetracene as the active layer of a top-contact field-effect transistor with application of a pulsed voltage showed that a space charge field remained around the source electrode—caused by injected holes, but not by electrons. Observing the time-varying EFISHG intensity at the electrodes clearly showed that the space charge field formed in the vicinity of the electrodes made a dominant contribution to the opposite-polarity carrier injection, directly leading to EL around the electrodes by carrier recombination. We concluded that EFISHG measurement is useful for probing carrier behavior in luminescent materials, by eliminating two-photon excited photoluminescence from the material induced using an incident laser.

Observation of Electron Injection into Organic Field-Effect Transistor with Au Electrodes using Electroluminescence under AC Electric Field

The electroluminescence (EL) from a tetracene field-effect transistor was examined by applying an AC voltage to elucidate electron injection from the source and drain Au electrodes. The EL intensity was proportional to the AC frequency up to 500 kHz, but it decayed rapidly with elapsed time when applying the AC voltage. The EL spectrum corresponded well with the photoluminescence spectrum. These results indicate that holes and electrons are injected into the tetracene alternately in accordance with the AC applied voltage, and that the EL is radiated due to the recombination of the injected holes and electrons, while the space charge formation leading to the gradual decay of the EL proceeds owing to the accumulation and trapping of excessive injected carriers.

Observation of Carrier Behavior in Organic Field-Effect Transistors with Electroluminescence under AC Electric Field

The carrier behavior in a tetracene thin film under AC electric field was examined by electroluminescence (EL) using the structure of organic field-effect transistors (OFETs). A square wave voltage was applied to the source electrode, whereas the drain and gate electrodes were connected to the ground. Electrons and holes were injected from the Au source electrode alternately, and they recombined at the interface between the tetracene thin film and the source electrode. A series of EL pulses was observed in accordance with the applied AC voltage, i.e., a transient EL was observed in a half cycle of AC. This result indicates that alternating carrier injection causes EL. Furthermore, EL intensity increased with the frequency and amplitude of AC voltage. It was suggested that the amount of electrons and holes injected from the source electrode in a unit time is controlled by the frequency and amplitude of AC voltage.

Induced Optical Chirality of Poly(diacetylene) Film by Circularly Polarized Light and Its Control by Changing Substrate Temperature

The strength of chirality of a blue-phase poly(diacetylene) (PDA) film polymerized from achiral monomers using circularly polarized light (CPL) was effectively controlled by changing substrate temperature (313 and 273 K). The obtained absorption spectra did not show a difference between PDA films polymerized using left- and right-CPL, whereas the chiral PDA films with opposite chirality were selectively produced by left- and right-CPL irradiations. The PDA film prepared at 313 K showed stronger chirality than that prepared at 273 K. Microscopy observation clearly indicates that the strength of chirality depends on the domain size.

Electroluminescence Enhanced from Electrode Interface in ITO/Tetracene/Al Diodes

We investigated emission spectra of ITO/tetracene/Al diodes for fully understanding carrier behavior in the device. By applying an A.C. square voltage, we showed that two distinct peaks were enhanced at around 545 nm and 615 nm. The peak at 545 nm is ascribed to intrinsic carrier process leading to bulk-originated EL, whereas the peak at 615 nm is due to interfacial carrier process via carrier traps nearby the electrodes, resulting in OLED degradation. To discriminate interface- and bulk-originated EL, A.C. frequency dependence of the EL spectrum was analyzed with taking into account carrier transit and carrier injection.

Direct Observation of Carrier Behavior Leading to Electroluminescence in Tetracene Field-Effect Transistor

By using electric-field-induced second-harmonic generation (EFISHG) measurements, we directly probed carrier motion in tetracene field-effect transistors (FETs), followed by electroluminescence (EL). Under a pulse voltage application, positive charges were injected from the source electrode and accumulated on the gate insulator. After the voltage was removed, positive charges remained and formed a space charge field. As a result, electrons were pulled into the FET to recombine with the remaining holes for radiating EL. The EFISHG measurement provides a way to directly probe the electric field transient that contributes to the EL mechanism of light-emitting transistors.