Aluminum-lanthanum-silicate glasses with different Eu doping concentration have been synthesized by conventional melt-quenching method at 1680°C in reductive atmosphere. Under 395nm excitation, samples with low Eu doping concentration show mainly the cyan broad emission at 460nm due to 4f65d1-4f7 transition of Eu2+; and the samples with higher Eu doping concentration show mainly some narrow emissions with maximum at 616nm due to 5D0-7FJ (J=0, 1, 2, 3, 4) transitions of Eu3+. Cyan-white-red tunable luminescence under 395nm excitation has been obtained by changing the Eu doping concentration.
©2008 Optical Society of America
In recent years, solid state lighting is becoming an interesting field for many advantages, such as long lifetime, small bulk, toxicity-free and energy-saving, etc . Now, its luminescent efficiency has already exceeded that of the incandescent lamps and is expected to have the higher luminescent efficiency to replace the conventional fluorescent lamps [2, 3]. Undoubtedly, solid state lighting is a challenge for the conventional lighting.
In the field of solid state lighting, wavelength conversion phosphors play an important role as they did in fluorescent lamps [4-8]. However many presently-used rare earth doped phosphors are not suitable for the excitation of InGaN-based light emitting diode (LED) chips because they cannot be excited efficiently by presently-used LED chips (near ultraviolet to blue region), which results in low luminescence efficiency. Up to now, the luminous flux from one white LED is very small that many of white LEDs are necessary to obtain enough brightness for general lighting. Many studies have been carried out to solve this problem, in which useful method is to increase output power of LED chips. However, this also will increase the chip temperature, which may cause a deterioration of the resin, which is used to fix the powder phosphors onto the LED chip, and decrease the luminous efficiency and lifetime . Some novel durable phosphors without resin, such as Ce-doped YAG glass-ceramics are proposed . Furthermore, in the development of phosphors for solid state lighting, blue, green and yellow phosphors have been studied sufficiently [4-7]. Up to now, the major problem is lack of excellent red phosphors. According to colorimetric theory, red phosphor is highly required to realize excellent white lighting . So, economic red or orange phosphors are urgently needed for the development of efficient white-emitting LEDs . Bright red color emitting phosphors are, however, very difficult to obtain due to the quantum yield drop with increasing Stokes shift.
Europium ion is usually good choice for many luminescent materials : Eu2+ usually can show efficient blue or green emission under near-ultraviolet excitation due to its 4f-5d transition; and Eu3+ is excellent red activator in many classic phosphors, such as Y2O3:Eu3+ and (Y,Gd)BO3:Eu3+, etc . However, the red luminescence of Eu3+ usually can’t be efficiently excited by near-ultraviolet light because its excitation peaks in this region are usually due to parity-forbidden f-f transitions . To realize efficient red luminescence under near-ultraviolet excitation, suitable host materials or energy transfer processes is necessary.
Compared with crystals, glassy materials are good choice because of its wide, inhomogeneous line width, compositional variety, and easy mass production. In this paper, we present a suitable host from glasses in which Eu can show efficient luminescence under near-UV or violet excitation. In view of the thermal and mechanical stability of silicate glasses, Al2O3-La2O3-SiO2 glass was selected as host matrix. We present efficient red or white-color emission from Eu doped glasses under near-UV or violet excitation.
Glasses were prepared by conventional melt-quenching method. Analytic reagents of La2O3 (99.99%), Eu2O3 (99.99%) and SiO2 (A.R.) were used as raw materials and Al2O3 was introduced from alumina crucible (purity>99.5%) directly. In the synthesis process, 2wt% carbon powder (A.R.) was added to create a reductive atmosphere. After careful calculation and weighing (Ln2O3:SiO2=1:4 in mole ratio, Ln=La+Eu), the batches of about 12g were melted in a 20mL arc alumina crucible with an alumina lid at 1680°C for 2 to 4h. The melts were poured onto a cold brass plate and then pressed by another plate. We performed inductively coupled plasma atomic emission spectra (ICP-AES) measurement for all our studied glasses. ICP-AES results indicated that some alumina has been molten into the final products; and all our glass samples show similar composition of 27.5Al2O3-14.5Ln2O3-58SiO2 (short as ALSO). These glasses show similar composition though the melting duration and europium doping concentration are different.
Additionally, it should be mentioned here that ALSO: 0.02Eu means 2mol% lanthanum was substituted by europium; and same for ALSO: 0.04Eu, ALSO: 0.06Eu, and ALSO: 0.08Eu. In the fabrication process, melting time can greatly affect the glass performance and luminescent properties. So every piece of glass was molten with two different times: 2 and 4 hours. The 2h-samples were defined as a (ALSO: 0.02Eu), b (ALSO: 0.04Eu), c (ALSO: 0.06Eu) and d (ALSO: 0.08Eu); and the 4h-samples were defined as a1 (ALSO: 0.02Eu), b1 (ALSO: 0.04Eu), c1 (ALSO: 0.06Eu) and d1 (ALSO: 0.08Eu).
The obtained glasses were annealed at 750°C for 20 hours, then were cut to 10×10×2mm3 size and polished for photoluminescence and absorption spectra measurements. The photoluminescence spectra were obtained on Hitachi F-4500 with a 450w Xenon lamp as excitation source. Absorption spectra were recorded on Hitachi U-4100, using a dual-beam spectrophotometer. Their pictures were captured under same condition (excitation wavelength: 395 nm, white balance: daylight).
3. Results and discussion
Figure 1 shows the absorption spectra of samples ALSO: 0.02Eu (a), ALSO: 0.04Eu (b), ALSO: 0.06Eu (c) and ALSO: 0.08 Eu (d). In 375~700nm region, they show obvious absorption from 450 to shorter wavelength. ALSO: 0.02Eu (sample a) shows absorption band at about 410nm due to the existence of Eu2+. ALSO: 0.08Eu (sample d) has no obvious absorption band but edge from 450nm, which is stronger than that of ALSO: 0.02Eu. The absorption curves of samples b and c are between them of samples a and d. These absorption peaks at about 400nm result in their pale-yellow body color.
Figure 2 shows the photoluminescence spectra of ALSO: 0.02Eu (a). With 346nm excitation, the emission spectrum (solid line) shows only a broad band with maximum at 455nm, which is due to the typical 4f65d1→4f7 transition of Eu2+ . And when monitoring at 455nm, the excitation spectrum (dashed line) shows a broad band with maximum at 346nm, due to the 4f75d→4f65d1 transition of Eu2+.
Under 395nm excitation, sample a shows cyan luminescence for naked eye; and a new weak peak appears at 616nm in the emission curve, which could be due to the 5D0→7F2 transition of Eu3+. In the present host, the lattice is more suitable for the existence of Eu3+ than Eu2+ because of the similar ion radii of Eu3+ and La3+ , so some Eu3+ ions still exist even reductive atmosphere was employed. Monitoring at 616nm, some narrow excitation peaks can also be observed at 395, 466 and 534nm besides of the broad band mentioned above, which could be due to the f-f transitions of Eu3+ .
Figure 3 shows the excitation and emission spectra of ALSO: 0.08Eu glass. Under 395nm excitation, the emission spectrum of ALSO: 0.08Eu shows mainly some narrow peaks at 578, 592, 616, 654 and 701nm due to the 5D0→7FJ (J=0, 1, 2, 3, 4) transitions of Eu3+ ions. When monitoring at 616nm, the observed excitation spectrum of ALSO: 0.08Eu shows a weak broad band at 260nm and some strong narrow peaks at 363, 383, 395, 415, 466 and 532nm. The broad band should be due to the charge transfer band (CTB) of Eu3+-O2-; and the other narrow peaks are due to the f-f transitions of Eu3+, which are marked in fig.3. In comparison with many classic Eu-doped phosphors, such as Y2O3:Eu3+, the f-f excitation intensity of ALSO: 0.08Eu is obviously stronger than that of CTB, which is very useful for the application of solid state lighting.
From Fig. 2 and 3, it can be seen that lower-doping-concentration sample shows cyan emission and higher one shows red emission under 395nm excitation. According to colorimetry theory, cyan and red are complementary color for each other. So it is possible to realize white emission by employing suitable Eu doping concentration between samples a and d.
Figure 4 shows the emission spectra of samples a to d and a1 to d1 under 395nm excitation. Sample a shows mainly a broad band of Eu2+ with the maximum at 460nm, and very weak red emission could also be observed. From samples a to d, the intensity of the broad band emission decreases and red emission increases with increasing Eu doping concentration. Sample d shows efficient red emission under 395nm excitation. a-d of insert (A) are the photos of these samples under the irradiation of 395nm light, in which it can be obviously seen that their luminescence color changes from cyan (a) to white (b) and red (c, d). Insert (B) shows the color coordinates calculated from their emission spectra, the purple light of excitation source has not been considered in the calculation, which can directly confirm the color variety.
Additionally, we can see from insert (A) that samples a and b contain some bubbles. They may be due the mixture of CO and CO2, generated from the oxidation of carbon powder. Since they are usually disadvantages for the good mechanical or thermal properties of glass and should be removed as completely as possible. We used much longer time to remove the bubbles. a1-d1 show samples melted for 4h, they contain only few of bubbles and show obvious red-shift i.e.; compared to sample a, sample a1 shows obvious red peaks because major CO gas generated by carbon powder has been eliminated after a long melting time; then some Eu2+ ions are oxidized into trivalent Eu3+ again. Their color coordinates present that a1-d1 show obvious red shift: a1 is white-close and b1-d1 is pale-red to red.
In Al2O3-La2O3-SiO2 glass system, Eu shows two kinds of luminescence by employing reductive environment using carbon powder. Under near ultraviolet excitation, ALSO: 0.02Eu mainly shows cyan luminescence of Eu2+; and ALSO: 0.08Eu shows mainly red luminescence of Eu3+. By increasing Eu doping concentration or melting duration, tunable luminescence from cyan to red can be obtained under 395nm excitation. These results show that these glasses are promising materials for solid state lighting.
This work was financially supported by National Natural Science Foundation of China (Grant No.50672087 and No. 60778039), National Basic Research Program of China (2006CB806007), National High-Technology Research and Development Program of China (2006AA03Z304) and Program for Changjiang Scholars and Innovative Research Team in University (IRT0651).
References and links
1. S. Nakamura, T. Mukai, and M. Senoh, “Candela-class high-brightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes,” Appl. Phys. Lett. 64, 1687–1689(1994). [CrossRef]
2. L. S. Rohwer and A. M. Srivastava, “Development of Phosphors for LEDS,” Electrochem. Soc. Interface 36, 37–39 (2003).
3. J. Y. Taso, Light Emitting Diodes (LEDs) for General Illumination Update 2002 (Optoelectronics Industry Development Association, Washington DC, 2002).
4. P. Schlotter, J. Baur, Ch. Hielscher, M. Kunzer, H. Obloh, R. Schmidt, and J. Schneider, “Fabrication and characterization of GaN:InGaN:AlGaN double heterostructure LEDs and their application in luminescence conversion LEDs,” Mater. Sci. Eng. B 59, 390–394 (1999). [CrossRef]
5. S. Nakamura, S. Pearton, and G. Fasol, The Blue Laser Diode (Springer, Berlin, 2000).
6. R. Mueller-Mach, G. O. Mueller, M. R. Krames, and T. Trottier, “High-phower phosphor-converted light-emitting diodes based on III-Nitrides,” IEEE J. Sel. Top. Quantum Electron. 8, 339–345 (2002). [CrossRef]
7. K. Kobashi and T. Taguchi, “Imaging camera system of OYGBR-phosphor-based white LED lighting,” Proc. SPIE 5739, 25–32 (2005). [CrossRef]
8. U. Kaufmann, M. Kunzer, K. Köhler, H. Obloh, W. Pletschen, P. Schlotter, J. Wagner, A. Ellens, W. Rossner, and M. Kobusch, “Single Chip White LEDs,” Phys. Status Solidi A 192, 246–253 (2002). [CrossRef]
9. Y. Shimizu and K. Bandou, “Development of White LED light source,” Rare Earth , 40, 150–151 (2002).
10. S. Fujita, S. Yoshihara, A. Sakamoto, S. Yamamoto, and S. Tanabe, “YAG glass-ceramic phosphor for white LED (I): background and development,” Proc. SPIE5941, 594111(1–7) (2005). [CrossRef]
11. I. Abramov, “Retinal mechanism of color vision,” In Handbook of Sensory Physiology, M.G.F. Fuortes (Ed), (Springer-Verlag, Berlin, Heidelberg, and New York, 7, 1972), pp.567–607.
12. R. Le Toquin and A. K. Cheetham, “Red-emitting cerium-based phosphor materials for solid-state lighting applications,” Chem. Phys. Lett. 423, 352–356 (2006). [CrossRef]
13. G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer-Verlag, Berlin, 1994). [CrossRef]
14. R. D. Shannon, “Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallogr. A 32751 (1976). [CrossRef]
15. C. A. Morrison and R. P. Leavitt, “Spectroscopic properties of triply ionized lanthanides in transparent host crystals,” in Handbook on the Physics and Chemistry of Rare Earths, K.A. Gschneidner Jr. and L. Eyring, Eds., (North-Holland, Amsterdam, 5, 1982) pp. 491–687.