• 대한화학회지
• /
• 제44권4호
• /
• pp.350-355
• /
• 2000

알루미늄 보레이트 화합물, $CaAl_2(BO_3)_2O$와 $BaAl_2(BO_3)_2O$은 매우 효과적으로 $Eu^{2+}$ 이온의 형광성을 나타내는 발광격자를 가지고 있다. 발광피크는 $Eu^{2+}$:$CaAl_2(BO_3)_2O$화합물에 있어서 450㎚이고 $Eu^{2+}$:$SrAl_2(BO_3)_2O$화합물에 있어서411㎚이며 $Eu^{2+}$:$BaAl_2(BO_3)_2O$ 화합물에 있어서 375㎚이다. 그러므로 $Eu^{2+}$:$CaAl_2(BO_3)_2O$ 화합물은 파란색을 내는 좋은 형광체이며, 특별히, 진공자외선 제논 플라마스 램프에 있어서 우수한 형광물질로 기대된다. $Eu^{2+}$ 이온은 PDP 형광체로써 관심이 높으며, Stokes shift는 형광물질의 도핑격자 크기에 의해서 결정이 된다. Stokes shift는 발광 파장을 결정하는 하나의 중요한 요소이다. 만일, 도핑격자의 크기가작아짐에 따라 $Eu^{2+}$ 이온의 5d 에너지 준위가 낮아지면, 발광 파장은 길어지고 Stokes shift크기는 커진다. 그러므로 결정격자의 크기를 측정하면 $Eu^{2+}$ 이온의 Stokes shift와 5d 에너지 준위 등의 발광성질을 예측할 수 있다.

• Journal of the Optical Society of Korea
• /
• 제19권3호
• /
• pp.311-316
• /
• 2015

We investigate the temperature dependence of the phosphor conversion efficiency (PCE) of the phosphor material used in a white light-emitting diode (LED) consisting of a blue LED chip and yellow phosphor. The temperature dependence of the wall-plug efficiency (WPE) of the blue LED chip and the PCE of phosphor are separately determined by analyzing the measured spectrum of the white LED sample. As the ambient temperature increases from 20 to $80^{\circ}C$, WPE and PCE decrease by about 4.5% and 6%, respectively, which means that the contribution of the phosphor to the thermal characteristics of white LEDs can be more important than that of the blue LED chip. When PCE is decomposed into the Stokes-shift efficiency and the phosphor quantum efficiency (QE), it is found that the Stokes-shift efficiency is only weakly dependent on temperature, while the QE decreases rapidly with temperature. From 20 to $80^{\circ}C$ the phosphor QE decreases by about 7% while the Stokes-shift efficiency changes by less than 1%.

• Bulletin of the Korean Chemical Society
• /
• 제30권7호
• /
• pp.1452-1458
• /
• 2009

The change in the electronic absorption and emission energy of 7-aminocoumarin derivatives in binary solvent mixtures has been studied. The electronic transition energy along with the Stokes' shift is correlated with the orientation polarizability of the solvent as well as the empirical solvent polarity parameters $E_T$ (30). It is observed that the emission peak shift traces the change of $E_T$ (30) value very closely in binary solvent mixtures. The emission transition more strongly depends on the solvent polarity than the absorption, which indicates the dipole moment gets larger on excitation. From the dependence of the Stokes’ shift of 7-aminocoumarins with the solvent polarity parameters and the ground state dipole moment obtained by the semi-empirical calculations, the excited state dipole moment was estimated. The fluorescence lifetime change of 7-aminocoumarins in binary solvent mixtures was measured and the results are explained in terms of molecular conformation and solvent polarity. The study indicates the empirical solvent polarity $E_T$ (30) is a good measure of microscopic solvent polarity and it probes in general the non-specific solvent interactions.

• Journal of Photoscience
• /
• 제12권2호
• /
• pp.57-61
• /
• 2005

The ground and excited state dipole moments of Coumarin 450 (C 450) laser dye were measured at room temperature in several solvents of varying dipole moments. The ground state dipole moment (${\mu}_g$) is estimated by using the modified Onsagar model and the excited state dipole moments (${\mu}_e$) were estimated by the method of solvatochromism as well as by utilizing the microscopic solvent polarity parameter ($E^N_T$). Further, the deviation of some of the points from the linearity of the $E^N_T$ versus Stokes shift indicates the existence of specific type of solute-solvent interaction. The excited state dipole moment of C 450 were found to be higher than those of the ground state and is interpreted in terms of the resonance structure of the molecule. A reasonable agreement has been observed between the values obtained by the method of solvatochromism and modified Onsagar model. It is observed that, corresponding to cyclohexane solution, the fluorescence maxima shift towards the red region with increasing the polarity of the solvents, hence the transition involved are of ${\pi}-{\pi}^*$ type.

• Bulletin of the Korean Chemical Society
• /
• 제13권6호
• /
• pp.613-620
• /
• 1992

The solvent-dependent photophysical properties of 2-biphenylcarboxylic acid (2BPCA) and 4-biphenylcarboxylic acid(4BPCA), which have a pre-twisted conformation in the ground state, have been investigated. The fluorescence spectra of 4BPCA show vibrational structure with a non-mirror image to the absorption spectra in nonpolar solvent while those of 2BPCA show no structure even in nonpolar solvents. As the solvent polarity increases, the fluorescence spectra become diffuse and broad with a strong red shift resulting in the large Stokes shift. The large fluorescence Stokes shift of BPCA's in polar solvent is also partially due to an intramolecular charge transfer (ICT) interaction in the excited state, as demonstrated by the large dipole moment in the excited state (7.6-10.6 D). The fluorescence decay behaviors of BPCA's (decay-times and their pre-exponential factors) also depend on solvent polarity in agreement with the solvent-dependent properties of the steady-state fluorecence. The data have been discussed in terms of change in the excited-state potential energy surface with respect to change of the dihedral angle of biphenyl moiety.

• Current Photovoltaic Research
• /
• 제7권2호
• /
• pp.29-32
• /
• 2019

Large band gap attenuation of CdS nanoclusters in hybrid sol gel matrix comprised of 3-(trimethoxysilyl)propyl methacrylate (TMSPM), 15 wt. % zirconium, and various amounts of cadmium acetate was observed after $H_2S$ exposure. Hybrid sol gel matrixes were prepared by hydrolysis and condensation reactions. The sol gels contained with various amount of cadmium acetate were spin coated to glass substrates and exposed to $H_2S$ gas. The UV-visible absorption peaks were shifted toward blue with increasing the amount of CdS nanoclusters and were shifted to the red after thermal process. Significant amount of -OH absorption peaks were reduced after thermal process. Strong room temperature photoluminescence (PL) of CdS nanoclusters was observed after exposing to $H_2S$ gas. The PL intensity increased for several minutes and slowly decreased thereafter. The luminescence peaks were continuously shifted toward blue as the time passed. Extraordinary Stokes shift (approximately 160 nm) was observed.

• 한국전기전자재료학회:학술대회논문집
• /
• 한국전기전자재료학회 2005년도 춘계학술대회 논문집 디스플레이 광소자 분야
• /
• pp.51-53
• /
• 2005

$Zn_{1-x}Cd_xO$ thin films were grown on (0001) sapphire substrates by pulsed laser deposition. The energy bandgap of $Zn_{1-x}Cd_xO$ films decreases withincreasing Cd content. An increase of Cd content also leads to the emission broadening and degraded crystallinity. The absorption edge and ultraviolet emission peak shift to lower energy from 3.357 eV to 3.295 eV and 3.338 eV to 3.157 eV, respectively, with increasing Cd content from 0.3% to 3%. The Stokes' shift between the absorption and emission indicates the increase of localization of exciton with Cd content.

• JSTS:Journal of Semiconductor Technology and Science
• /
• 제6권2호
• /
• pp.90-94
• /
• 2006

We report on the growth and characterization of InGaN/GaN MQWs on two different types of sapphire substrates and GaN substrates. The InGaN/GaN MQWs are grown by using metalorganic chemical vapor deposition. Our analysis of the satellite peaks in the HRXRD patterns shows, GaN substrates InGaN/GaN MQW compared to sapphire substrates InGaN/GaN MQW, more compressive strain on GaN substrates than on sapphire substrates. However, results of optical investigation of InGaN/GaN MQWs grown on GaN substrates and on sapphire substrates, which have lower Stokes-like shift of PL to GaN substrates compared to sapphire substrates, are shown to the potential fluctuation and the quantum-confined Stark effect induced by the built-in internal field due to spontaneous and straininduced piezoelectric polarizations. The InGaN/GaN MQWs are shown to quantify the Stokes-like shift as a function of x.

• Journal of Mechanical Science and Technology
• /
• 제16권10호
• /
• pp.1303-1313
• /
• 2002

Effects of the reduced frequency of upstream wake on downstream unsteady boundary layer flow were simulated by using a Wavier-Stokes code. The Wavier-Stokes code is based on an unstructured finite volume method and uses a low Reynolds number turbulence model to close the momentum equations. The geometry used in this paper is the MIT flapping foil experimental set-up and the reduced frequency of the upstream wake is varied in the range of 0.91 to 10.86 to study its effect on the unsteady boundary layer flow. Numerical solutions show that they can be divided into two categories. One is so called the low frequency solution, and behaves quite similar to a Stokes layer. Its characteristics is found to be quite similar to those due to either a temporal or spatial wave. The low frequency solutions are observed clearly when the reduced frequency is smaller than 3.26. The other one is the high frequency solution. It is observed for the reduced frequency larger than 7.24. It shows a sudden shift of the phase angle of the unsteady velocity around the edge of the boundary layer. The shift of phase angle is about 180 degree, and leads to separation of the boundary layer flow from corresponding outer flow. The high frequency solution shows the characteristics of a temporal wave whose wave length is half of the upstream frequency. This characteristics of the high frequency solution is found to be caused by the strong interaction between unsteady vortices. This strong interaction also leads to destroy of the upstream wake strips inside the viscous sublayer as well as the buffer layer.

• 한국전기전자재료학회논문지
• /
• 제15권5호
• /
• pp.435-440
• /
• 2002

In this study, we have investigated photo-induced changes of optical energy gap( $E_{OP)}$ and photoluminescence (PL) in amorphous ($\alpha$-) S $e_{100-x}$G $e_{x}$ (x=5, 25 and 33) thin films prepared by conventional thermal evaporation method. In the $\alpha$-S $e_{100-x}$G $e_{x}$ thin film, the $E_{OP}$ is obtained by a linear extrapolation of the ($\alpha$hν)$^{\frac{1}{2}}$ versus hν plot to the energy axis using the optical absorption coefficient ($\alpha$) calculated from the extinction coefficient k measured in the wavelength range of 290~900nm. Although the values of $\Delta$ $E_{OP}$ are very different, all films exhibit photo-induced photo-darkening (PD) effect that is a red shift of $E_{OP}$ . In particular, $\Delta$ $E_{OP}$ in $\alpha$-S $e_{75}$ G $e_{25}$ thin film exhibits the largest value (i, e., $\Delta$ $E^{OP}$ ~40meV for $\alpha$-S $e_{95}$ G $e_{5}$ , $\Delta$ $E_{OP}$ ~200meV for $\alpha$-S $e_{75}$ G $e_{25}$ , $\Delta$ $E_{OP}$ ~130meV for $\alpha$-S $e_{67}$ G $e_{33}$ ). PL spectra in $\alpha$-SeGe by hν$_{HeCd}$ have no-Stokes shift (SS) and show a tendency dependent on both composition and illumination time. We explain the energy-induced phenomena such as the PD and thermal bleaching, the native charged-defect generation and the no-SS PL, etc..the PD and thermal bleaching, the native charged-defect generation and the no-SS PL, etc..tc..