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Advances in Phosphors for Light-emitting Diodes Chun Che Lin and Ru-Shi Liu* Department of Chemistry, National Taiwan University, Taipei 106, Taiwan ABSTRACT: Light-emitting diodes (LEDs) are excellent candidates for general lighting because of their rapidly improving efficiency, durability, and reliability, their usability in products of various sizes, and their environmentally friendly constituents. Effective lighting devices can be realized by combining one or more phosphor materials with chips. Accordingly, it is very important that the architecture of phosphors be developed. Although numerous phosphors have been proposed in the past several years, the range of phosphors that are suitable for LEDs is limited. This work describes recent progress in our understanding of the prescription, morphology, structure, spectrum, and packaging of such phosphors. It suggests avenues for further development and the scientific challenges that must be overcome before phosphors can be practically applied in LEDs.
F
ollowing an increasing awareness of climate change and environmental issues, people are looking for alternatives to fossil fuels as energy sources that do not emit carbon dioxide.13 White light-emitting diodes (WLEDs) are extensively used and are very important as they significantly reduce global power requirements and the use of fossil fuels.4 They have attracted substantial attention owing to their extraordinary luminous efficiency, low power consumption, reliability, and environmental friendliness. The quest for phosphors for lighting is one of the most important and urgent challenges to be met by advanced science and high technology, and novel and acceptable fluorescent-material-based solid-state lighting (SSL) must be developed.5 As presented in Tables 1 and 2, the intensity, width, durability, and thermal quenching of commercial phosphors for lightemitting diodes (LEDs) suggest two exciting approaches of UV-LED chip (with the wavelength of 380420 nm) and blue-LED chip (with the wavelength of 450480 nm) for accelerating this development. One involves mixing the emissions from red, green, and blue (RGB) phosphors with a UVLED chip, as in a device with the schematic structure that is shown in the left inset in Figure 1A. The disadvantages of such a device are low efficiency of the red phosphors (due to the large Stokes shift) and the need for complex coating technology (e.g., problems of sedimentation and uniformity distribution of phosphors in silicon resin). Furthermore, mixing powders and finding high-efficiency compounds are more difficult for this type device. However, there are several benefits, including high color rendering index, high luminous efficiency, and stable light color that are almost independent of the changed current. According to the relevant literature,6,7 all such devices use inorganic phosphors that are mixed with quantum dots, which act as an alternative to a down-converter in WLEDs. Therefore, we propose a novel mixture of variously colored quantum dots (InP) and silicon resin as a color-converting material, which can be applied to a UV-LED chip, as displayed in the right inset in Figure 1A. In the r 2011 American Chemical Society
case of nontoxic InP QDs,8 the full color emission wavelengths can be easily adjusted by controlling the particle size (quantum confinement effect), and such QDs can be dispersed uniformly in silicon resin. This fact can perhaps be exploited to solve the problems of the efficiency and coating technology of UV-LED devices. The other method for lighting devices involves Ce3þ-doped yttrium aluminum garnet (YAG/Ce)-based blue-LED chips, which has been commercialized. The left inset in Figure 1B presents such a device. It can offer advantages such as easy fabrication and low cost. Nevertheless, the unstable light color results in the halo phenomenon under the different output current. The light from YAG/Ce-based WLEDs is colder and bluer than that from a traditional incandescent lamp. Its color rendering is poor owing to the red deficiency of the yellow phosphor. This problem has attracted the attention of many researchers, who have sought to improve the color rendering property of phosphor-converted (pc) WLEDs. Coreshells or coremultishells of CdSe-based core QDs have been widely utilized to improve the color rendering index (CRI) of WLEDs.9,10 The right inset in Figure 1B shows a new combination of broad emission Lu3 Al5O12/Ce3þ (LuAG/Ce) yellowgreen phosphors and highly efficient red InP QDs. Moreover, Figure 1C exhibits the efficiency curves of different LED chips plotted as a function of correlated color temperature (CCT). To realize a low CCT of a blue-LED chip, a very thick phosphor layer must be formed to reduce the transparency of blue light. A very thick phosphor layer limits brightness because it prevents fluorescent light from traveling from the LED side to the front of the device. If a violet LED is used to yield low CCT, then the phosphor layer can be optimized to pass light Received: February 23, 2011 Accepted: May 6, 2011 Published: May 11, 2011 1268
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Table 1. Examples of WLEDs that Incorporate UV-LEDs Excitable Phosphors. (O: Good; 4: Medium; : Bad) emission characteristics LED
phosphor
violet LED
blue phosphor
green phosphor
red phosphor
chemical composition
intensity
width
durability
thermal quenching
(Sr,Ca,Ba,Mg)10(PO4)6Cl2/Eu
O
narrow
O
4
(Ba,Sr)MgAl10O17/Eu
O
middle
O
O
(Sr,Ba)3MgSi2O8/Eu
O
narrow
4
4
SrGa2S4/Eu
O
middle
β-sialon/Eu
O
middle
O
O
SrSi2O2N2/Eu
O
middle
O
O
Ba3Si6O12N2/Eu
O
middle
O
O
BaMgAl10O17/Eu,Mn SrAl2O4/Eu
O 4
narrow broad
O 4
O 4
(Sr,Ca)S/Eu
O
broad
(Ca,Sr)2Si5N8/Eu
4
broad
4
4
CaAlSiN3/Eu
O
broad
O
O
La2O2S/Eu
4
narrow
4
4
3.5MgO 3 0.5MgF2 3 GeO2/Mn
4
narrow
O
O
(Sr,Ca,Ba,Mg)10(PO4)6Cl2/Eu,Mn
4
broad
O
O
Ba3MgSi2O8/Eu,Mn
O
broad
4
4
Table 2. Examples of WLEDs that Incorporate Blue-LEDs Excitable Phosphors. (O: Good; 4: Medium; : Bad) emission characteristics LED
phosphor
blue LED
green phosphor
yellow phosphor
red phosphor
chemical composition
intensity
width
durability
thermal quenching
Y3(Al,Ga)5O12/Ce
4
broad
O
4
SrGa2S4/Eu
O
middle
(Ba,Sr)2SiO4/Eu Ca3Sc2Si3O12/Ce
O O
middle broad
4 O
4 O
CaSc2O4/Ce
O
broad
O
O
β-sialon/Eu
O
middle
O
O
(Sr,Ba)Si2O2N2/Eu
O
middle
4
O
Ba3Si6O12N2/Eu
O
middle
O
O
(Y,Gd)3Al5O12/Ce
O
broad
O
4
Tb3Al5O12/Ce
4
broad
O
4
CaGa2S4/Eu (Sr,Ca,Ba)2SiO4/Eu
O O
middle broad
O
4
Ca-R-sialon/Eu
O
middle
O
O
(Sr,Ca)S/Eu
O
broad
(Ca,Sr)2Si5N8/Eu
O
broad
4
4
CaAlSiN3/Eu
O
broad
O
O
(Sr,Ba)3SiO5/Eu
O
broad
O
K2SiF6/Mn
O
narrow
O
O
from the LED side to the front of the device. The low CCT can be obtained just by adjusting the mixing ratio of the RGB phosphors even without adjusting the thickness of the phosphor layer. As expected from the device scheme in Figure 1, semiconductor nanocrystals or QDs are a promising alternative to down-converting materials in WLEDs because of their attractive properties, including size-tunable optical characteristics, broad absorption spectrum, narrow emission band, high quantum yields, and low light scattering. 11,12 The criteria that must be applied and the approach that must be taken to enable inorganic phosphors to be used in LEDs are of concern.
On the basis of the above mentioned, rare-earth ions, like Eu2þ, Ce3þ, Tb3þ, and Sm3þ, which work as luminescence centers in most inorganic phosphors, tend to be very expensive. Moreover, a large number of productions were usually synthesized under high temperatures, high pressures, or reducing conditions. The development of available approaches for fluorescent materials without using rare-earth ions at relatively low temperature under air atmosphere is agreeable to apply in the future lighting. We will introduce some non-rare-earth-based phosphors in this Perspective. Notably, the blue-color-emitting GaZnON materials have not yet been reported in previous literature. This Perspective highlights some characteristics of 1269
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Figure 1. Structures for generating white light from fluorescent materials, based on (A) UV chip and (B) blue chip. (C) Efficiency of LED chips as a function of correlated color temperature.
phosphors and provides an overview of the subject area for WLEDs in the future.
Combinatorial chemistry is used to optimize novel phosphors for application in light-emitting diodes. YVO4/Bi 3þ,Eu3þ: Spectral Characteristics and Formula. Some intelligent algorithms, such as genetic algorithms, Monte Carlo techniques, simulated annealing, and artificial neural networks algorithms, can be used to improve the efficiency of new materials and to optimize the objective compounds.1316 Recently, Sohn et al.17 employed solid-state combinatorial chemistry to synthesize AE2Si5N8/Eu2þ (AE = alkaline earth elements) phosphors and analyze their composition. Liu et al.18 screened (YxLu1xy)3Al5O12/Ce3y greenyellow phosphors using the same method
(combi-chem). Figure 2A schematically depicts the developed drop-on-demand inkjet delivery system. A driving circuit and a computer that controls the motor drive the piezoelectric inkjet heads and the XY stage of a substrate. The emission spectra of the samples in the library were measured using an automatic system that was comprised of a Hg Lamp, a portable optical fiber spectrometer (Ocean Optics, Inc., model SD2000), and an XY stage. Figure 2B displays the composition map of the Y1stVO4/Bi3þs,Eu3þt combinatorial library and a luminescent photograph obtained under 365 nm excitation. The first column clearly reveals faint red emission from Eu3þ at various concentrations (t = 00.060) without Bi3þ codoping (s = 0). The first row clearly reveals that without Eu3þ codoping (t = 0), the strong green emission of Bi3þ increases with Bi3þ concentration from s = 0 to 0.050; the increase becomes negligible when the Bi3þ content exceeds s = 0.025. Furthermore, the emission colors in all other rows (t = 0.005, 0.015, 0.030, 0.045, and 0.060) remain almost constant with different Bi3þ concentrations, but the brightness increases with Bi3þ concentration from s = 0.005 to 0.040 and then declines as the Bi3þ concentration increases 1270
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Figure 2. (A) Drop-on-demand inkjet delivery system. (B) Composition map of the (Y1stBisEut)VO4 combinatorial library and luminescent photograph under 365 nm excitation. (C) Emission spectra of samples with s = 00.050 and t = 0.005 in the (Y1stBisEut)VO4 combinatorial library under 365 nm excitation. (D) Emission spectra of samples with s = 0.040 and t = 00.060 in the (Y1stBisEut)VO4 combinatorial library under 365 nm excitation. (E) Emission spectrum of a warm-white LED lamp. (Adapted from ref 16.)
further. These results suggest that Eu3þ cannot emit efficiently without Bi3þ codoping under 365 nm excitation but that excessive Eu3þ codoping severely reduces Bi3þ luminescence. Figure 2C shows the emission spectra of the samples at t = 0.005 and s = 00.050 in the combinatorial library under 365 nm excitation. The emission lines at 592, 618, 650, and 702 nm are attributed to the 5D0 f 7FJ (J = 1, 2, 3, 4) transitions of Eu3þ, and the broad emission band with a peak at 545 nm is assigned to the 3P1 f 1S0 transition of Bi3þ.16 The emission intensity of Eu3þ increases continuously with Bi3þ concentration from s = 0 to 0.050,
indicating the transfer of energy between Bi3þ and Eu3þ. Meanwhile, the emission intensity of Bi3þ increases with concentration. The inset in Figure 2C plots the variation of the relative height of the Bi3þ emission peaks at 545 nm and the Eu3þ emission peaks at 618 nm with Bi3þ concentration. The rates of increase of Eu3þ and Bi3þ emissions are similar to each other at s = 00.025, but the rate of increase of Eu3þ emission exceeds that of Bi3þ at s = 0.0250.050. The energy transfer from Bi3þ to Eu3þ does not influence Bi3þ emission if the concentration of Bi3þ does not exceed s = 0.040 and that of Eu3þ is less than 1271
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Figure 3. (A) PL spectra of (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes (excited at 370 nm). (B) Scanning electron micrographs of (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes. (C) Conductivity of (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes as a function of time. (D) PL spectra of as-received and hydrated (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with AlF3 and H3BO3 fluxes (excited at 370 nm). (Adapted from refs 21.)
t = 0.005. However, Bi3þ emission is clearly degraded by the shortening of the mean distance between Bi3þ and Eu3þ, and Eu3þ then efficiently captures energy from Bi3þ. The results in Figure 2D agree well with those in Figure 2C when the emission of Bi3þ declines gradually as the Eu3þ content increases from t = 0 to 0.060, indicating that the shortening of the distance between Bi3þ and Eu3þ results in the transfer of energy from Bi3þ to Eu3þ to an extent that increases gradually with Eu3þ concentration. The above results concerning the photoluminescence that is excited by UV light (365 nm) are consistent with the results of conventional-scale synthesis, revealing that the combinatorial chemistry technique is fast, reliable, and reproducible. Therefore, a novel prescription of phosphors can be found rapidly and exhaustively by using a combinatorial chemistry approach. A practical white LED lamp was fabricated by coating a NUV-LED chip with commercially available Sr3MgSi2O8/Eu2þ (3128) blue phosphor and self-optimized (Y0.956Bi0.040Eu0.004)VO4 phosphor. Figure 2E presents the emission spectrum of the prepared LED lamp, with a Ra of up to 90.3.
Fluxes considerably improve the stability of phosphors against moisture. BaMgAl10O17/Eu,Mn: Moisture Measurement and Morphology. The flux plays an important role in improving crystallinity, promoting grain growth. It is even involved in controlling the morphology and size of a phosphor.19 On the basis of previous investigations, the addition of a flux enhances the luminescence intensity and reduces the reaction temperature of fluorescent materials. The stability of the moisture content and the emission intensity of BaMgAl10O17/Eu,Mn phosphors vary with various fluxes, such as AlF3, BaF2, and H3BO3. Figure 3A shows the PL spectra of (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes. The spectra have similar shapes but various emission intensities. 1272
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Figure 4. (A) Calculated density of states (DOS) of (a) pure KSrPO4 and KSr1xPO4/Eux (x = 3.125%) in the (b) non-spin-polarized state, (c) spin-polarized state, and (d) spin-polarized state with on-site Coulomb interaction. (B) Majority spin orbital structure and possible mechanism of electronic transition in the KSrPO4/Eu system. (Adapted from ref 24.)
The fluxes significantly enhance the luminescence intensity and color saturation. Accordingly, the phosphor that is synthesized with AlF3 has a higher relative emission intensity (116%) and color purity than commercial BAM phosphor. The involvement of fluxes in the mechanism of BAM particle growth has not been elucidated in detail. However, fluxes are broadly agreed to be
compounds of alkali or alkaline earth metals with low melting points; when fluxes melt, the surface tension of the liquid helps particles coagulate and facilitates their sliding and rotation, providing more opportunities for particleparticle contact and promoting particle growth. Recent work has established that when fluorides are used as fluxes, BAM particle growth may 1273
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proceed by the following mechanism20 BaCO3 þ MgO þ 4=3AlF3 f BaMgF4 þ 2=3Al2 O3 þ CO2 BaMgF4 þ 17=3Al2 O3 f BaMgAl10 O17 þ 4=3AlF3 MgO þ 2BaF2 f BaMgF4 þ BaO BaMgF4 þ 5Al2 O3 þ 2BaO f BaMgAl10 O17 þ 2BaF2 As revealed by these chemical equations, the fluoride reacts with some of the raw material to produce BaMgF4 at approximately 1000 °C, which melts at a temperatures of over 1000 °C. BaMgAl10O17 will begin to be created in BaMgF4 solutions at about 1200 °C. At around 1600 °C, all of the BAM are created. Figure 3B displays the SEM morphology of (Ba0.85Eu0.15) (Mg0.7Mn0.3)Al10O17 with various fluxes. The widely dispersed BAM phosphor has a plate-shaped morphology because of its crystallographic characteristics. A comparison with a phosphor prepared without flux demonstrates that the growth of particles in the presence of flux is extensive; the flux improves their crystallinity, and the phosphor particles with a fluoride-based flux have a hexagonal surface morphology. A sample that is prepared by H3BO3 flux is semispherical and not smooth. The results also indicate that the morphology of phosphor particles affects their luminous properties and stability.21 Mishra et al.22 observed that moisture can easily interact with the intermediate planes of the aluminates in a β-alumina structure, degrading the BAM. Chemical stability is regarded as an important parameter in the application of phosphors in WLEDs, plasma display panels, and fluorescent lamps. Figure 3C plots the conductivity of (Ba0.85Eu0.15) (Mg0.7Mn0.3)Al10O17 as a function of time with various fluxes. The conductivity increases with time because some ions of the phosphor host are dissolved in water. Specifically, the conductivity varies only a little in the synthesis of BAM with AlF3 or BaF2 fluoride-based fluxes, enhancing the stability against moisture. This result is obtained because phosphor particles with a fluoride-based flux, which have a highly crystalline hexagonal surface morphology, cannot easily interact with the moisture. However, the sample with H3BO3 as the flux has the lowest stability owing to the absence of highly crystalline particles and the smoothness of the surface. Figure 3D shows the PL spectra of the as-received and hydrated (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with AlF3 and H3BO3 as fluxes. The emission spectra indicate a reduction in the emission intensity of both of the water-treated samples. The total integrated radiance of the hydrated samples with AlF3 and H3BO3 as fluxes is 9 and 22% less than that of fresh powder, respectively. Hence, synthesis with fluxes can enhance the stability against moisture; AlF3 outperforms BaF2 or H3BO3. Furthermore, the selection of suitable fluxes in the synthesis of any compounds can improve the stability against moisture. Chemical stability is an important requirement of phosphors for use in LEDs.
The mechanism of electronic transition was confirmed by theoretical calculations. Chen et al.23 calculated the electronic structure of RMO4 (R = Y, Gd; M = P, V) and elucidated its physical properties and
Figure 5. (A) Temperature-dependent emission spectra of Y3Al5O12/ Ce3þ and CaAlSiN3/Eu2þ obtained under 460 nm excitation. (B) Configuration coordinate diagram. (Adapted from ref 3.)
interatomic interactions by employing density functional theory (DFT). Although various densities of states (DOSs) of undoped hosts have been reported, the DOS of doped phosphors had not been calculated before the KSrPO4/Eu system was developed.24 Figure 4 shows the DOS of, and a possible mechanism of electron transition in, the KSrPO4/Eu system. The results reveal that pure KSrPO4 has a direct band gap of approximately 5.09 eV at point Γ in (a) of Figure 4A. The band gap is shifted only slightly when the 800 eV cutoff energy for the plane wave basis and the geometrical optimization of the KSrPO4 are considered, thus suggesting that these parameters have only a weak effect on the electronic structure of KSrPO4 systems. The reference energies are set to the highest-energy electron-occupied state. The upper and lower rows in (c) and (d) of Figure 4A concern the majority spin (spinup) and minority spin (spin-down), respectively. The electronic structure of KSrPO4 after it is doped with dilute Eu is presented below; band gap values of around 1.3 (b) and 2.5 eV (c) are obtained for non-spin-polarized and spin-polarized electrons, respectively. With respect to the spin polarization and the on-site Coulomb interaction of Eu 4f electrons, the majority spin Eu 4f states are fully occupied, and the main peak of the Eu 5d states appears at about 3.0 eV above the Fermi level in (d) of Figure 4A. Remarkably, the separation of 3.0 eV between the Eu 4f5d main 1274
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The Journal of Physical Chemistry Letters peaks, corresponding to the wavelength of 414 nm, is very close to the measured emission wavelength of 424 nm. Therefore, the calculations herein suggest a possible mechanism of excitation from Eu 4f to the conduction band (CB) via Eu 5d, followed by the main emission from Eu 5d to Eu 4f. On the basis of the above discussion, Figure 4B presents a proposed mechanism of electron transition in the KSrPO4/Eu system. Initially, the Eu 4f f CB excitation pumps electrons to the delocalized CB via Eu 5d. Then, nonradiative relaxation brings the electron to the main peak of Eu 5d close to the lower band edge of the CB. Finally, photoemission may be associated with the on-site Eu 5d4f transition.
The thermal stability of phosphors can affect color coordinates of lighting for practical packaging. The thermal stability of phosphors that are used in WLEDs must be understood. Figure 5A plots the relationship between the emission intensity and the environmental temperature of Y3Al5O12/Ce3þ and CaAlSiN3/Eu2þ, measured under 460 nm excitation. The emission intensity of the two samples declined as the temperature increased because the nonradioactive transition
Figure 6. Developed scheme of phosphors for WLEDs.
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from the excited states to the ground state increased by the crossing point (F), as shown in the configurationally coordinate diagram in Figure 5B; this effect is called thermal quenching. As a result, the emission peaks of YAG/Ce3þ were red-shifted from 560 to 570 nm, which is explained with reference to the Varshini equation for the temperature dependence of energy25 EðTÞ ¼ E0
aT 2 Tþb
where E(T) is the energy difference between the excited states and the ground states at temperature T; E0 is the corresponding energy difference at 0 K, and a and b are fitting parameters. Increasing the temperature reduces the transition energy and red shifts the emission peak. Additionally, the configurationally coordinate (red line) is slightly right-shifted from that of the original coordinate (black line), reducing the activation energy (ΔE = F00 to G). Sohn et al.26 found that a lower energy emission of CaAlSiN3/Eu2þ, which was attributed to an Al-rich local environment, made the polyhedron around the Eu2þ activator smaller, while a Si-rich local environment resulted in a larger polyhedron and a higher energy emission. Consequently, the emission peaks of CaAlSiN3/Eu2þ in Figure 5A were blue-shifted in a manner determined by the configuration coordinate (blue line), increasing the activation energy (ΔE = F0 to G). Prospects for Phosphors in WLEDs. As described above, in the search for suitable phosphors, significant effort has been made to develop highly efficient LED devices. Figure 6 presents the developed scheme of phosphors for WLEDs. In recent years, several highly luminous YAG/Ce or silicate-based blue-LED chips have been fabricated, but they have a low color-rendering index and low thermal stability. Although nitride and phosphate phosphors have high thermal stability, they have very low efficiency. The optimal mixture of Sr3MgSi2O8 (blue), LuAG (yellowgreen), and CaAlSiN3 (red) was recently proposed for use in a high-efficiency device. Additionally, considering the cost of precursors and synthetic processes, non-rare-earth-based phosphors maybe are excellent candidates for replacing doped materials in the preparation of LEDs. Okuyama et al.27 have reported full-color-emitting BCNO phosphors, which are produced by a one-step liquid process at relatively low temperature (800 °C). The peak positions of emission spectra shift from the UV (387 nm) to visible (571 nm) under different conditions,
Figure 7. (A) Diffuse reflection spectra for Ga2O3, ZnO, and GaZnON. (B) Excitation (λem = 450 nm) and emission (λex = 390 nm) spectra for GaZnON. 1275
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The Journal of Physical Chemistry Letters including the polyethylene glycol/boron ratio, reaction temperature, and heating time. Nanda et al.28 have successfully developed nanorod Ga2O3 that been used as a multilight emitter for WLEDs. They also explained the possible growth mechanism and luminescence mechanism for this compound. The bluish-green emission was ascribed to the oxygen vacancy, while the nitrogen formed the red emission. Especially, the solid-solution GaZnON was fabricated without doping rare-earth ions at considerably low temperatures (below 500 °C) under ambient atmospheric pressure. Figure 7A presents the reflection spectra of the precursors Ga2O3 and ZnO and the production of GaZnON. It clearly indicates that the optical band gap was estimated to be 4.9 and 3.3 eV for Ga2O3 and ZnO, respectively. The band gap of GaZnON, which has stronger absorption (380450 nm) than others, was measured to be 3.7 eV. Figure 7B shows the excitation and emission spectra of the GaZnON phosphor. The excitation wavelength, which is consistent with the reflection spectra, is suitable for the UV-chip- and blue-chip-based LEDs. The PL spectrum exhibits a pure blue emission band from 400 to 550 nm centered at 450 nm ascribed to the oxygen vacancy and urea. The chromaticity coordinate of this compound was found to be (0.1614, 0.1439), in the higher blue color purity region. We will perform more studies to verify the luminescence mechanism and the distribution of different elements in the near future. Briefly, considerable effort will be devoted to the development of the non-rare-earth-based phosphors for exercise in WLEDs. This Perspective highlights the development and tendency of phosphors and provides some methods that can readily be scaled up for industrial applications.
’ AUTHOR INFORMATION Corresponding Author
*Tel: þ886-2-33661169. Fax: þ886-2-23636359. E-mail: rsliu@ ntu.edu.tw.
’ BIOGRAPHIES Chun Che Lin received his B.S. degree in chemistry from Chung Yuan Christian University in 2005. He received his M.S. degree in chemistry from National Taiwan University in 2007. He is currently working on his Ph.D. in inorganic chemistry, focusing on the synthesis of fluorescent materials for particular applications. His current research interests include synthesis of phosphors and quantum dots for LEDs and bioapplications. Ru-Shi Liu is a professor at the Department of Chemistry, National Taiwan University. He obtained two Ph.D. degrees in chemistry, one from National Tsing Hua University in 1990 and another from the University of Cambridge in 1992. He worked at the Industrial Technology Research Institute from 1983 to 1985. ’ ACKNOWLEDGMENT The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contracts NSC 97-2113-M-002-012-MY3, and NSC 97-3114-M-002-002. ’ REFERENCES (1) Intergovernmental Panel on Climate Change (IPCC) Human and Natural Drivers of Climate Change 2007 Report, Summary for Policymakers; 2007; pp 118.
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