Jinhyun Choi

Warm-white-light emitting diode utilizing a single-phase full-color Ba3MgSi2O8:Eu2+, Mn2+ phosphor

The Ba3MgSi2O8:Eu2+, Mn2+ shows three emission colors: 442, 505, and 620 nm. The 442 and 505 nm emission originate from Eu2+ ions, while the 620 nm emission originates from Mn2+ ions. The excitation bands of three emission colors are positioned around 375 nm. Electron paramagnetic resonance measurement demonstrates that Eu2+ ions are occupied with three different Ba2+ sites. The red emission of Mn2+ ions has a long decay time of 750 ms due to persistent energy transfer from oxygen vacancies to Mn2+ ions, while the blue and green bands of Eu2+ ions have decay times of 0.32 and 0.64 μs, respectively. The fabricated white-light emitting diode using a 400-nm-emissive chip with a white-light emitting Ba3MgSi2O8:Eu2+, Mn2+ phosphor shows warm white light and higher color stability against input power in comparison with a commercial GaN-pumped (Y1−xGdx)3(Al1−yGay)5O12:Ce3+ phosphor.

Color variation of ZnGa2O4 phosphor by reduction-oxidation processes

The color of the emission of zinc gallate (ZnGa2O4) oscillates between ultraviolet and blue by hydrogen ambient reduction and air ambient oxidation heat treatments. The photoluminescence spectra and electron paramagnetic resonance signals show that ultraviolet emission of reduced ZnGa2O4 always accompanies 680 nm emission originating from single oxygen vacancies (VO*). The increasing difference in binding energy between Ga3+ and O2− in reduced ZnGa2O4 indicates that the configuration of octahedral sites is distorted due to VO* generation and it becomes more ionic which shifts the emission band from 430 to 360 nm. The x-ray diffraction patterns and Raman scattering spectra show that β-Ga2O3 from ZnGa2O4 is formed in both reduction and oxidation processes which suggests the vaporization of Zn ions. We propose a model in which the origin of 360 nm emission is the Ga–O transition at distorted octahedral sites with VO* in ZnGa2O4, whereas 430 nm emission originates from the Ga–O transition of regular octahedra...

Tunable color emission and solid solubility limit in Ba1−xCaxAl2O4:Eu0.0012+ phosphors through the mixed states of CaAl2O4 and BaAl2O4

The tunable color emission and solid solubility limit in Ba1−xCaxAl2O4:Eu0.0012+ phosphors was investigated by using cathodoluminescence (CL) and X-ray diffraction (XRD) measurements. The CL spectra showed that the tuning range of the color emission was between 500 and 430 nm. The XRD curves showed that the lattice constant of the (102) plane in the Ba0.6Ca0.4Al2O4:Eu0.0012+ phosphor had a minimum value, and that the single phase limit in the Ba1−xCaxAl2O4:Eu0.0012+ phosphors is below x value of 0.4.

Enhancement of the activation energy in coupled CdTe/ZnTe quantum dots and quantum-well structures with a ZnTe thin separation barrier

Temperature-dependent photoluminescence measurements showed that the activation energy of CdTe/ZnTe quantum dots (QDs) coupled with a quantum well is much larger than that of the QDs alone, This behavior is attributed to the tunneling of carriers via a thin separation layer from the quantum well to the QDs. The present observations can help improve understanding of the enhancement of the activation energy in coupled CdTe/ZnTe nanostructures.

Determination of the solid solubility of SrAl2o4 in CaAl2o4 through crystal field-dependent Eu2+ signatures

Abstract The solid solubility of the SrAl 2 O 4 –CaAl 2 O 4 system was investigated. Both XRD patterns and cathodoluminescence from Eu 2+ in the host lattices were utilized to investigate the solid solubility of SrAl 2 O 4 in CaAl 2 O 4 . A single phase exists in the range x = 1 to x = 0.5 in Sr 1−x Ca x Al 2 O 4 composition (i.e., the [CaAl 2 O 4 ] phase exists as a single phase from CaAl 2 O 4 to Sr 0.5 Ca 0.5 Al 2 O 4 ). The luminescence band maximum of Eu 2+ in [CaAl 2 O 4 ] phase shifts from 433 nm (CaAl 2 O 4 :Eu 2+ ) to 404 nm (Sr 0.9 Ca 0.1 Al 2 O 4 :Eu 2+ ). The blue-shift behavior of Eu 2+ emission can be explained in terms of Eu 2+ experiencing the weakening of crystal field strength with the decreasing x value of Sr 1−x Ca x Al 2 O 4 :Eu 2+ (0.01%). This phenomenon confirms that Blasse’s prediction regarding the Eu 2+ crystal field-dependent 4 f 6 5 d 1 energy level is applicable in oxide hosts.

Formation mechanism of CdTe self-assembled quantum dots embedded into ZnTe barriers

Photoluminescence spectra showed that the formation mechanism for CdTe layers grown on ZnTe thin films changed from a two-dimensional mode to a three-dimensional mode with increasing submonolayer CdTe layer thickness, and the temperature-dependent PL spectra indicated that the activation energy of CdTe quantum dots is larger than that of CdTe single quantum wells. The formation mechanism for the CdTe QDs is in reasonable agreement with a Stranski–Krastanov growth mode.

Structural and optical properties of Mn2+ and Li+ codoped ZnGa2O3.95M0.05 (M = S, Se, and Te) phosphors

Abstract Structural and optical properties on various kinds of Mn2+ and Li+ codoped ZnGa2O3.95M0.05 (M=S, Se, and Te) were investigated by using X-ray diffraction (XRD), cathodoluminescence (CL), and photoluminescence excitation (PLE) measurements. The results of the XRD patterns showed that the lattice constants of the ZnGa2O3.95M0.05:Mn2+, Li+ decrease with the substitution of S, Se, and Te for the oxygen in the ZnGa2O4. The CL and PLE spectra exhibited that the luminescence intensity of the ZnGa2O3.95Se0.05:Mn2+, Li+ was the strongest among various samples studied in this work. The enhancement of the luminescence intensity in the ZnGa2O3.95Se0.05:Mn2+, Li+ originated from the increase of the conductivity due to Li+ doping. These results can help improve understanding Mn2+ and Li+ doped ZnGa2O3.95Se0.05 phosphors for the application in field-emission display devices with high brightness.

Type-II interband transition of ZnS0.78Te0.22/ZnTe single quantum wells

Optical properties of ZnS0.78Te0.22/ZnTe single quantum wells grown on GaAs (100) substrates by hot wall epitaxy technique with varying the ZnS0.78Te0.22 well width from 0.3 to 1.8 nm were investigated by photoluminescence (PL) measurements at low temperature and by temperature-dependent PL measurements. PL results show the evidence of type-II transition and their peak energy shifts to higher energies as the ZnS0.78Te0.22 well width decreases. In addition, temperature-dependent PL measurements show the increase of the activation energy as the well thickness decreases, indicating the increase of confinement effect. This study makes it possible to introduce proper band diagram for this structure, and can give very useful information on their device applications.

Treatment outcomes of helical intensity-modulated radiotherapy for unresectable hepatocellular carcinoma.

Background/Aims This study reports treatment outcomes after helical intensity-modulated radiotherapy (IMRT) in unresectable hepatocellular carcinoma (HCC) patients for whom transarterial chemoembolization (TACE) was considered ineffective or unsuitable. Methods From January 2008 to December 2011, 22 unresectable HCC patients received helical IMRT. A daily dose of 1.8 to 4 Gy was delivered at five fractions per week to deliver a total dose of 30 to 60 Gy. The most-prescribed dose fractionation was a total dose of 50 to 57.5 Gy, with a daily dose of 2.3 to 2.5 Gy. Results In the entire group, the objective response rate of the primary tumor was 72.7%. In the eight patients with portal vein thrombosis (PVT), the objective response rate of PVT was 50.0%. Median disease progression-free survival was 11.8 months, and the 1-year disease progression-free survival rate was 40.2%. The median overall survival was 14.4 months, and the 1- and 2-year overall survival rates were 86.4% and 69.1%, respectively. PVT and Child-Pugh classifications were significant prognostic factors for overall survival in multivariate analyses. Conclusions Helical IMRT in patients with unresectable HCC resulted in high treatment response and survival rates. This study suggests helical IMRT is a practical treatment option for HCC patients in whom TACE is unsuitable or ineffective.

Predictive factors for radiation pneumonitis in lung cancer treated with helical tomotherapy.

Purpose Predictive factors for radiation pneumonitis (RP) after helical tomotherapy (HT) may differ from those after linac-based radiotherapy. In this study, we identified predictive factors for RP in patients with lung cancer treated with HT. Materials and Methods We retrospectively analyzed clinical, treatment-related and dosimetric factors from 31 patients with lung cancer treated with HT. RP was graded according to Common Terminology Criteria for Adverse Events version 4.0 and grade ≥2 RP was defined as a RP event. We used Kaplan-Meier methods to compute the actuarial incidence of RP. For univariate and multivariate analysis, the log-rank test and the Cox proportional regression hazard model were used. We generated receiver-operating characteristics (ROC) curves to define the cutoff values for significant parameters. Results The median follow-up duration was 6.6 months (range, 1.6 to 38.5 months). The 2-, 4-, and 6-month actuarial RP event rates were 13.2%, 58.5%, and 67.0%, respectively. There was no grade 4 or more RP. Ipsilateral V5, V10, V15, and contralateral V5 were related with RP event on univariate analysis. By multivariate analysis, ipsilateral V10 was factor most strongly associated with RP event. On the ROC curve, the cutoff values of ipsilateral V5, V10, V15, and contralateral V5 were 67.5%, 58.5%, 50.0%, and 55.5%, respectively. Conclusion In our study, ipsilateral V5, V10, V15, and contralateral V5 were significant predictive factors for RP after HT.