Yasuyuki Kawakami

EXCIMER EMISSION OF ANTHRACENE, PERYLENE, CORONENE AND PYRENE MICROCRYSTALS DISPERSED IN WATER

Abstract Microcrystals of aromatic hydrocarbons such as perylene, pyrene, coronene and anthracene were prepared as an aqueous suspended solution by a stratificational sublimation with water onto a substrate cooled by liquid nitrogen. Aqueous solutions of microcrystals showed anomalous fluorescence spectra different from those of bulk crystals; coronene exhibited monomer and broad excimer emissions, while pyrene showed a red-shifted excimer band, and anthracene or perylene showed both excimer and monomer bands. The relative intensities of two bands were varied systematically with the size of the microcrystals. The dual emissions are explained by assuming two emission centers, one is in the mid and another is on the periphery of microcrystals. The diffusion lengths of exciton were estimated for anthracene and perylene microcrystals from the intensity ratio of two emission bands.

Perfect blackbody radiation from a graphene nanostructure with application to high-temperature spectral emissivity measurements

We report the successful fabrication of a novel type of blackbody material based on a graphene nanostructure. We demonstrate that the graphene nanostructure not only shows a low reflectance comparable to that of a carbon nanotube array but also shows an extremely high heat resistance at temperatures greater than 2500 K. The graphene nanostructure, which has an emissivity higher than 0.99 over a wide range of wavelengths, behaves as a standard blackbody material; therefore, the radiation spectrum and the temperature can be precisely measured in a simple manner. Here, the spectral emissivities of tungsten and tantalum are experimentally obtained using this ideal blackbody material and are compared to previously reported spectra. We clearly demonstrate the existence of a temperature-independent fixed point of emissivity at a certain wavelength. Both the spectral emissivity as a function of temperature and the cross-over point in the emissivity spectrum are well described by the complex dielectric function based on the Lorentz-Drude model with the phonon-scattering effect.

Fluorescence Lifetime of Organic Thin Films Alternately Deposited with Diamine Derivative and Aluminum Quinoline

Fluorescence lifetimes of organic thin films alternately deposited with diamine derivative (TPD) and aluminium quinoline (Alq3) were measured. The alternating deposition structure was shown to enhance the emission from Alq3 in spite of imperfect film structures. Energy transfer from TPD to Alq3 was evidenced by the correlation between lifetime and period number. It was found to be a competitive process with the TPD radiation process, and its rate of TPD radiation was estimated to be 1.2×1010 s-1. The authors suggested that the presence of the region mixed with TPD and Alq3 at the TPD/Alq3 interface plays an important role in the electroluminescence (EL) process. The Alq3 exciton confinement was clarified experimentally since the component of the faster fluorescence lifetime of Alq3 increased in the short-wavelength region. In addition, the authors proposed a possible EL model based on the suppression of the thermal inactivation of Alq3 excitons during their diffusion.

Displacement current measurements of the dynamic charge transfer of photosynthetic reaction centers in monolayer LB films

Abstract The photosynthetic reaction center (RC) from the photosynthetic bacterium Rhodopsuedomonas viridis is a unique protein in which lightinduced charge separation occurs. The charge-transfer process of the RCs was analyzed by a displacement current method in which two electrodes sandwiching the LB film without contact. A displacement current was detected for even a monolayer RC-LB film when it was irradiated with light. The action spectrum of the displacement current was in agreement with the absorption spectrum of solublized RCs, indicating that the current originates from the charge separation within the RCs. Furthermore, displacement currents for two types of LB films, in whichthe orientations of the RCs were opposite, yielded polarities that were opposite to each other. Thus, this method can be used to evaluate the unidirectional electron transfer in monolayers of RCs.

Handheld deep ultraviolet emission device based on aluminum nitride quantum wells and graphene nanoneedle field emitters

We report the successful fabrication of a compact deep ultraviolet emission device via a marriage of AlGaN quantum wells and graphene nanoneedle field electron emitters. The device demonstrated a 20-mW deep ultraviolet output power and an approximately 4% power efficiency. The performance of this device may lead toward the realization of an environmentally friendly, convenient and practical deep ultraviolet light source.

Improvement of the external extraction efficiency of OLED by using a pyramid array

We focused on the light out coupling efficiency to be improved. To improve out coupling efficiency, we considered air/substrate interface is the key place to be modified. We simulated various shaped array at that interface for high out coupling efficiency. As a result of simulation, we found that the pyramid array was best. We successfully fabricated a pyramid array film and attached to a white emission OLED panel for evaluation. The white emission OLED was optimized to be suitable for the pyramid array. The luminance and flux of OLED with pyramid array were increased by a factor of 2.0, 1.7 relatively. For further improvement in luminance, we found that additional use of Brightness Enhancement Film (BEF) with optimized OLED device is able to achieve much higher luminous efficiency of 3.0 times comparing to the normal OLED.