Narrow Results

We report the development of laboratory astrophysics program in Taiwan. We begin with outlining Taiwan's participation in the FLASH collaboration for measuring the fluorescence yield using SLAC 28.5 GeV electron beam. We then report the domestic effort of studying cosmic ray shower properties using NSRRC 1.5 GeV electron beam.

Hybrid hydrogels are a new class of composite materials. Polyacrylamide (PAAm) hydrogels are mainly produced by free radical crosslinking copolymerization (FCC) of AAm in the presence of N, N′-methylene bis (acrylamide) (BIS) as the crosslinker. Pyranine doped PAAm-poly (N-vinyl pyrrolidone) (PVP) composite were prepared with different amounts of PVP varying in the range between 0.0015 and 0.1 gr. It was observed that pyranine molecules as a fluoroprobe bind to AAm and PVP chains upon the initiation of the polymerization, causing the fluorescence spectra of the bonded pyranines shift to the shorter wavelengths. The sol–gel phase transition and its universality were monitored and tested as a function of PVP contents. Observations around the critical point show that the gel fraction exponent, β, agreed with the percolation result for below 0.025 gr PVP contents. However, classical result was observed above 0.0125 gr PVP content.

In this paper, combining phosphorescence and fluorescence to form white light was realized based on DCJTB:PMMA/ITO/NPB/TCTA/FIrpic:TCTA/TPBi/Ir(ppy)3:TPBi/TPBi/Cs₂CO₃/Al. The effects of red fluorescence on this white light device was studied by adjusting the concentration of DCJTB. The study shows that the device with a DCJTB concentration of 0.7% in the color conversion layer (CCL) generates a peak current efficiency and power efficiency of 23.4 cd ⋅ A${}^{-1}$ and 7.5 lm ⋅ W${}^{-1}$, respectively. And it is closest to the equal-energy white point of (0.33, 0.33) which shows a CIE (Commission Internationale de L’Eclairage) coordinate of (0.35, 0.43) and a color rendering index (CRI) of 70 at current density of 10 mA ⋅ cm${}^{-2}$. In order to improve the efficiency, we design and fabricate both high efficient and pure white organic light-emitting diode (WOLED) by replacing the single blue emission layer (EML) with double EMLs of FIrpic:TCTA and FIrpic:TPBi. The further study shows that, when the layers of EML is three and the concentration of DCJTB at 0.7%, the device exhibits good performance specifically, at current density of 10 mA ⋅ cm${}^{-2}$, the current efficiency of 28.2 cd ⋅ A${}^{-1}$ (power efficiency of 10.3 lm ⋅ W${}^{-1}$), and the CIE coordinate of (0.33, 0.31) (CRI of 80.38).

In this work, Mn${}^{2+}/$Eu${}^{3+}$ co-doped Zn₂GeO₄ (Zn₂GeO₄:Mn${}^{2+},{\mathrm{\text{Eu}}}^{3+})$ was prepared by high-temperature solid phase method. Compared with common fluorescent materials Zn₂GeO₄:Mn${}^{2+}$, Zn₂GeO₄:Mn${}^{2+},{\mathrm{\text{Eu}}}^{3+}$ could not only emit strong green fluorescence of 535 nm, but also maintain excellent persistent luminescence performance. Through Density Functional Theory calculation, we obtained the fine band structure of Zn₂GeO₄:Mn${}^{2+},{\mathrm{\text{Eu}}}^{3+}$. The results of the band structure were consistent with the experimental spectral data. On this basis, we proposed a new luminescence mechanism model of Zn₂GeO₄:Mn${}^{2+},{\mathrm{\text{Eu}}}^{3+}$ to explain the phenomena observed in experiment reasonably, though which was not completely consistent with previous works. When Zn₂GeO₄:Mn${}^{2+},{\mathrm{\text{Eu}}}^{3+}$ was excited, electron–hole separation occurred in the valence band (VB), and the electron transitioned to the conduction band (CB) directly. Through CB, the electron was trapped by trap levels (⁷F${}_0{\sim }^7$F₅ of Eu${}^{3+})$ and maintained metastable for a long time. Under the action of thermal stimulation, electron returned to CB from the trap level slowly. The electron was captured again by the ⁴T₂(D) level of Mn${}^{2+}$. Then the electron transitioned down toward VB and recombined with the previous hole and emitted a photon with 535 nm (afterglow). The samples were being irradiated, trap levels accommodated the excited electrons to saturation. More electrons excited into the CB could not be captured by the trap levels any more. They were captured directly by the ⁴T₂(D) and transitioned directly to VB, then emitted green fluorescence.

In this work, the mixed system composed of Zn₂GeO₄: Mn and MgGeO₃: Mn, Eu was synthesized by the high temperature solid phase method. Under the external excitation, visual color of samples was yellow. However, after the excitation was completed, visual color turned to be red. From luminescence spectrum, it was found that Zn₂GeO₄: Mn emitted green fluorescence of 534 nm under the excitation of 375 nm light. At the same time, MgGeO₃: Mn, Eu emitted both fluorescence and persistent luminescence (PersL) of 668 nm. Moreover, the properties of PersL present samples were superior to other red PersL materials. Fine band structures from density functional theory (DFT) indicated that there were different luminescent mechanisms of Zn₂GeO₄: Mn and MgGeO₃: Mn, Eu. When Zn₂GeO₄: Mn was excited, electron transitioned from VB to CB directly. Through CB, the electron was captured by the ⁴T₂(D) of Mn ion, then the electron jumped from ⁴T₂(D) to VB and recombined at once with the previous hole and emitted a 534 nm photon. When MgGeO₃: Mn, Eu was excited, electron transitioned from ⁶A₁(S) of Mn ion to CB and left a hole. Through CB, electron was captured by ⁷F₆ levels of Eu${}^{3+}$ and remained metastable for a long time, which slowed down the recombined rate between electron and hole. Under thermal stimulation, the captured electron returned to CB from ⁷F₆ levels and was recaptured by the ⁴T₂(D) of Mn. The electron transitioned down toward ⁶A₁(S) and recombined with the hole immediately, then emitted a photon with 668 nm.