Naoki Shimosako

Carrier-density dependence of photoluminescence from localized states in InGaN/GaN quantum wells in nanocolumns and a thin film

We have measured and analyzed the carrier-density dependence of photoluminescence (PL) spectra and the PL efficiency of InGaN/GaN multiple quantum wells in nanocolumns and in a thin film over a wide excitation range. The localized states parameters, such as the tailing parameter, density and size of the localized states, and the mobility edge density are estimated. The spectral change and reduction of PL efficiency are explained by filling of the localized states and population into the extended states around the mobility edge density. We have also found that the nanocolumns have a narrower distribution of the localized states and a higher PL efficiency than those of the film sample although the In composition of the nanocolumns is higher than that of the film.

Decomposition of dioctyl phthalate (DOP) using titanium dioxide photocatalyst in a vacuum

We have focused on photocatalytic materials to solve contamination problem for spacecraft. We have fabricated TiO2 thin films and measured decomposition rates of methyl orange (MO) and dioctyl phthalate (DOP) in vacuum by TiO2 thin films as a photocatalyst. From XRD results, fabricated TiO2 thin films have anatase-type crystal structure, which is known to have stronger decomposition activities than rutile-type TiO2. The TiO2 thin films we made were shown to decompose methylene blue (MB) solution, which means that the TiO2 thin films have general photocatalystic activity in atmosphere. In decomposition of MO in atmosphere and vacuum, TiO2 shows photocatalytic activity even in vacuum although the decomposition rate in vacuum is slower than that in atmosphere. In decomposition of DOP in vacuum, DOP was effused from an effusion cell in vacuum chamber and was deposited on a TiO2 thin film using the in-situ measurement apparatus at Tsukuba Space Center, JAXA. Transmission spectra of DOP on TiO2 thin films after UV irradiation were measured to estimate decomposition rate from absorbance of DOP. The results show that TiO2 thin films can decompose DOP even in vacuum. Moreover, H2O can promote the decomposition of DOP. In order to use photocatalyst materials in vacuum for long time, the studies on the durability of photocatalystic activity of TiO2 in vacuum and the effects of O2 and H2O are necessary in the future.

Effects of vacuum on photocatalytic activity of TiO2

Contamination outgassed from spacecrafts materials can degrade optical devices in orbit. Therefore, solving the contamination problem is important because it in uences the success of spacecraft missions. In this study, methyl red (MR) and oleamide were decomposed in vacuum by TiO2 photocatalyst. Absorbance spectra, mass decrease and GC/MS were measured after and before decomposition. In vacuum, TiO2 could decompose MR to intermediate products, whereas it could not decompose the intermediate products to volatile substances because TiO2 cannot open benzene rings of the intermediate products. On the other hand, TiO2 could decompose oleamide to volatile substances even in vacuum. However, decomposition by TiO2 stopped after a certain period of time in vacuum. The decrease in mass for oleamide by photocatalytic reaction in vacuum was enough compared to standard molecular contamination levels.