Yewon Kim

Highly luminescent silica-coated CdS/CdSe/CdS nanoparticles with strong chemical robustness and excellent thermal stability

We present facile synthesis of bright CdS/CdSe/CdS@SiO2 nanoparticles with 72% of quantum yields (QYs) retaining ca 80% of the original QYs. The main innovative point is the utilization of the highly luminescent CdS/CdSe/CdS seed/spherical quantum well/shell (SQW) as silica coating seeds. The significance of inorganic semiconductor shell passivation and structure design of quantum dots (QDs) for obtaining bright QD@SiO2 is demonstrated by applying silica encapsulation via reverse microemulsion method to three kinds of QDs with different structure: CdSe core and 2 nm CdS shell (CdSe/CdS-thin); CdSe core and 6 nm CdS shell (CdSe/CdS-thick); and CdS core, CdSe intermediate shell and 5 nm CdS outer shell (CdS/CdSe/CdS-SQW). Silica encapsulation inevitably results in lower photoluminescence quantum yield (PL QY) than pristine QDs due to formation of surface defects. However, the retaining ratio of pristine QY is different in the three silica coated samples; for example, CdSe/CdS-thin/SiO2 shows the lowest retaining ratio (36%) while the retaining ratio of pristine PL QY in CdSe/CdS-thick/SiO2 and SQW/SiO2 is over 80% and SQW/SiO2 shows the highest resulting PL QY. Thick outermost CdS shell isolates the excitons from the defects at surface, making PL QY relatively insensitive to silica encapsulation. The bright SiO2-coated SQW sample shows robustness against harsh conditions, such as acid etching and thermal annealing. The high luminescence and long-term stability highlights the potential of using the SQW/SiO2 nanoparticles in bio-labeling or display applications.

PLASTIC SCINTILLATOR FOR RADIATION DOSIMETRY

Inorganic scintillators, composed of high-atomic-number materials such as the CsI(Tl) scintillator, are commonly used in commercially available a silicon diode and a scintillator embedded indirect-type electronic personal dosimeters because the light yield of the inorganic scintillator is higher than that of an organic scintillator. However, when it comes to tissue-equivalent dose measurements, a plastic scintillator such as polyvinyl toluene (PVT) is a more appropriate material than an inorganic scintillator because of the mass energy absorption coefficient. To verify the difference in the absorbed doses for each scintillator, absorbed doses from the energy spectrum and the calculated absorbed dose were compared. From the results, the absorbed dose of the plastic scintillator was almost the same as that of the tissue for the overall photon energy. However, in the case of CsI, it was similar to that of the tissue only for a photon energy from 500 to 4000 keV. Thus, the values and tendency of the mass energy absorption coefficient of the PVT are much more similar to those of human tissue than those of the CsI.

CALCULATION OF GAMMA SPECTRA IN A PLASTIC SCINTILLATOR FOR ENERGY CALIBRATIONAND DOSE COMPUTATION

Plastic scintillation detectors have practical advantages in the field of dosimetry. Energy calibration of measured gamma spectra is important for dose computation, but it is not simple in the plastic scintillators because of their different characteristics and a finite resolution. In this study, the gamma spectra in a polystyrene scintillator were calculated for the energy calibration and dose computation. Based on the relationship between the energy resolution and estimated energy broadening effect in the calculated spectra, the gamma spectra were simply calculated without many iterations. The calculated spectra were in agreement with the calculation by an existing method and measurements.

Study on the fast signal transfer for large-area X-ray image sensors

A large area X-ray CMOS image sensor (LXCIS) is a well-known imaging device for high speed and resolution. In design and fabrication process, we found several problems in making LXCIS, especially in signal transferring. A 3-transistor active pixel sensor (3T APS) in LXCIS has a long signal line about 16.896 cm as a worst case. This long signal line consists of metal and it has resistance and capacitance about 21.12 kΩ and 71.87 pF each. We have optimized 3T APSs transistors, applied boosting circuit, and designed a low parasitic resistance and capacitance. From our simulation result, we obtained a high speed operation, which ranges from 13.5 frame per second (FPS) to 18.6 FPS in 1536 × 3072 pixel arrays, and a high dynamic range by increasing maximum voltage of pixel output signal.

Investigation of Magnetic Characteristics for a FeCo/Si Multilayer with the Variation of the Bias Potential at Substrate

x-ray, polarized neutron scattering and SQUID are used to investigate the growth of the sputtered Fe89Co11-Si multilayers to determine the crystallization and it’s influence on the magnetic properties. These properties depend on the sputtering parameters. The substrate temperature during deposition is found to have the most important influence on the crystallinity. High energy on the substrate achieved by means of an Argon bombardment under a bias potential of –70 V produces a large crystallinity of the Fe89Co11 material. On the contrary, a low energy on the substrate under a bias potential of +100 V and floating, results in a low crystallinity. In the x-ray measurements of the monochromators deposited with the bias potential of –70 V, the (110) peak of Fe89Co11 is observed with nearly a hundredfold more intensity than the one with the bias potential of +100 V and floating. The minus bias potential leads to a high coercivity but reduced remanence compared with the floating. The +100 V bias potential gives rise to a distinct magnetic anisotropy and in the soft direction the remanence achieved is 94%.