Luminescent Properties of Sr₄Si₃O₈Cl₄:Eu²⁺, Bi³⁺ Phosphors for Near UV InGaN-Based Light-Emitting-Diodes

Abstract

Sr₄Si₃O₈Cl₄ co-doped with Eu²⁺, Bi³⁺ were prepared by the high temperature reaction. The structure and luminescent properties of Sr₄Si₃O₈Cl₄:Eu²⁺, Bi³⁺ were investigated. With the introduction of Bi³⁺, luminescent properties of these phosphors have been optimized. Compared with Sr_3.90Si₃O₈Cl₄:0.10Eu²⁺, the blue-green phosphor Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺ shows stronger blue-green emission with broader excitation in near-UV range. Bright blue-green light from the LED means this phosphor can be observed by the naked eye. Hence, it may have an application in near UV LED chips.

1. Introduction

White light-emitting-diodes (LEDs) have more advantages, such as energy saving, long lifetime, environmental friendly and high efficiency, compared with fluorescent lamps [1,2,3]. At present, white LEDs can be obtained by combining near UV LED chips with blue/green/red tricolor phosphors, in order to obtain a higher efficiency white LED with appropriate color temperature and higher color-rendering index. This type of white LED is expected to dominate the market in the near future [4,5]. So it is important to find new tricolor phosphors for near UV LED chips.

The phosphors for near UV LED chips should exhibit intense broad emission with broad excitation band in near UV range. Compared with the phosphors activated by some trivalent rare earth ions (such as Eu³⁺), Eu²⁺ ions in many phosphors have broader emission with broad excitation band in near UV range [6,7,8,9], which are assigned to the 4f⁶5d¹ → 4f⁷ transitions. In order to strengthen and broaden the absorption of phosphors in near UV range, one important approach is to introduce sensitizer to strengthen the excitation band in the phosphor. We consider that Bi³⁺ is probably an eligible co-activator. On one hand, the introduction of Bi³⁺ will influence their sub-lattice structure around the luminescent center ion. On the other hand, Bi³⁺ is a very good sensitizer of luminescence in many hosts [10,11,12]. It can efficiently absorb the UV-light and transfer the energy to the luminescent center, then the excitation band would be broadened and the emission intensity of the luminescent center would be strengthened.

Alkaline earth halo-silicates are well known good hosts for inorganic luminescent materials, due to their low synthesis temperature and high luminescence efficiency [9]. Alkaline earth chlorosilicate Sr₄Si₃O₈Cl₄ can be served as the host of phosphors, which has an orthorhombic crystal structure [13]. The luminescent properties of Sr₄Si₃O₈Cl₄ doped with Eu²⁺ were firstly reported by Burrus et al. [13]. The luminescent properties of Sr₄Si₃O₈Cl₄ doped with Eu²⁺ were optimized by co-doping with some divalent metal ions (such as Ca²⁺, Mg²⁺, Zn²⁺) [14,15,16].

In this paper, Sr₄Si₃O₈Cl₄ co-doped with Eu²⁺, Bi³⁺ were prepared by the high temperature reaction in the reduction atmosphere. The luminescent properties of Sr₄Si₃O₈Cl₄:Eu²⁺, Bi³⁺ were investigated. Finally, single blue-green LED was fabricated by combining the blue-green phosphor with ~395 nm emitting LED chips.

2. Results and Discussion

The XRD patterns of Sr_3.90Si₃O₈Cl₄:0.10Eu²⁺ and Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺ are shown in Figure 1. They are almost consistent with the JCPDS card 40-0074 [Sr₄Si₃O₈Cl₄]. This indicates that the phosphor Sr₄Si₃O₈Cl₄:Eu²⁺ almost shares the same phase as Sr₄Si₃O₈Cl₄, except for a few peaks of SrSiO₃ (marked by star shape). This result is in accordance with the references [15,16]. The impurity phase may be due to loss of chlorine content during the firing process. Eu²⁺ and Bi³⁺ maybe occupy the site of Sr²⁺ in an octahedral site, because the ion radii of Bi³⁺ (103 pm) and Eu²⁺ (117 pm) are close to that of Sr²⁺ (118 pm). Meanwhile, with the doping of Bi³⁺/Eu²⁺, a slight shift in the diffraction peaks toward higher angles can be found in Figure 1. This shift is due to the difference of ionic radius between Bi³⁺/Eu²⁺ and Sr²⁺.

Figure 2a is the excitation spectra of the phosphors Sr_4−4xSi₃O₈Cl₄:4_xEu²⁺ (x = 0.015, 0.020, 0.025, 0.030) by monitoring emission at 490 nm. These four curves are of similar shapes. The broad excitation band from 230 nm to 430 nm is due to the 4f–5d transitions of Eu²⁺. These broad and intense excitation bands in near UV range match well with near-UV LED chip. When the content of Eu²⁺ is 0.10, the excitation intensity is the strongest.

The emission spectra of Sr_4−4xSi₃O₈Cl₄:4_xEu²⁺ (x = 0.015, 0.020, 0.025, 0.030) under 320 nm excitation are shown in Figure 2b. The blue-green emission band is due to the 4f⁶5d–4f⁷ transition of Eu²⁺. With the increase of the Eu²⁺ content, the intensity of the blue-green emission becomes higher. When the content of Eu²⁺ is 0.10, the emission intensity is the strongest.

Figure 1. X-ray powder diffraction (XRD) patterns of Sr_3.90Si₃O₈Cl₄:0.10Eu²⁺ and Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺. The Asterisk represents the peak of SrSiO₃.

Figure 1. X-ray powder diffraction (XRD) patterns of Sr_3.90Si₃O₈Cl₄:0.10Eu²⁺ and Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺. The Asterisk represents the peak of SrSiO₃.

Figure 2. (a) Excitation spectra and (b) emission spectra of Sr_4–4xSi₃O₈Cl₄:4_xEu²⁺ (x = 0.015, 0.020, 0.025, 0.030).

Figure 2. (a) Excitation spectra and (b) emission spectra of Sr_4–4xSi₃O₈Cl₄:4_xEu²⁺ (x = 0.015, 0.020, 0.025, 0.030).

Since the luminescent mechanism of Eu²⁺ is the 4f–5d allowed electric-dipole transition, the energy transfer process of Eu²⁺ in the Sr₄Si₃O₈Cl₄ phosphors would be attributed to an electric multipole-mutipole interaction. When the energy transfer occurs between the same sites of activators, the intensity of multipole interaction can be determined by the change of the emission intensity from the emitting level. According to the report of Van Uitert [17], the emission intensity (I) of per activator ion follows the equation [18,19]:

$$\frac{I}{\chi }=K{[1+\beta {\chi }^{\frac{Q}{3}}]}^{-1}$$

(1)

where I is the emission intensity, χ is the Eu²⁺ concentration, K and β are constants for the same excitation condition for a given host crystal. Q is a constant of multipole interaction equals to 3, 6, 8 or 10 for energy transfer among the nearest-neighbor ions, dipole-dipole (d–d), dipole-quadruple (d–q) or quadruple-quadruple (q–q) interaction, respectively. To get a Q value for the emission center, the plot of log (I/χ) as a function of log (χ) for the Sr_4−4xSi₃O₈Cl₄:4_xEu²⁺ phosphors is plotted, as shown in Figure 3. It can be seen that a linear relation between log (I/χ) and log (χ) is found and the slope is −0.5299. The Q value can be obtained as 1.5897. That means the quenching is directly proportional to the activator ion concentration, which indicates that the concentration quenching is caused by the energy transfer among the nearest-neighbor Eu²⁺ ions in the Sr_4−4xSi₃O₈Cl₄: 4_xEu²⁺ phosphors.

Figure 3. The plot of log (I/x) as a function of log (x) for the Sr_4−4xSi₃O₈Cl₄:4_xEu²⁺phosphors (λ_ex = 320 nm).

Figure 3. The plot of log (I/x) as a function of log (x) for the Sr_4−4xSi₃O₈Cl₄:4_xEu²⁺phosphors (λ_ex = 320 nm).

The excitation spectra of the phosphors Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ (y = 0.00, 0.05, 0.10, 0.15, 0.20) by monitoring emission 490 nm is shown in Figure 4. With the doping of Bi³⁺, the shoulder peak around 400 nm is broadened with a little red shift, and the excitation intensity is enhanced. When the content of Bi³⁺ is 0.10, the excitation intensity is the strongest.

Figure 4. Excitation spectra of Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ (y = 0.00, 0.05, 0.10, 0.15, 0.20) by monitoring emission at 490 nm.

Figure 4. Excitation spectra of Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ (y = 0.00, 0.05, 0.10, 0.15, 0.20) by monitoring emission at 490 nm.

Figure 5 is the emission spectra of Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ (y = 0.00, 0.05, 0.10, 0.15, 0.20) under 320 nm excitation. The broad emission from 400 nm to 600 nm is due to the 4f⁶5d₁ → 4f⁷ transition of Eu²⁺ ions. The strongest peak is located at 490 nm. The concentration dependence of the relative integral emission intensity of Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ is shown in the inserted figure of Figure 5. When the content of Bi³⁺ is at 0.10, its emission intensity is the strongest, and its intensity is about 1.3 times higher than that of Sr_3.90Si₃O₈Cl₄:0.10Eu²⁺ without Bi³⁺. This result is consistent with their excitation spectra. Bright blue-green light can be observed from Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺ under 365 nm light excitation (seeing the inset of Figure 5). The compositions Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ are not charge balanced and have a slight excess of positive charge because of Bi³⁺ substituting Sr²⁺. The excess charge could be compensated by a number of mechanisms, for example, cation vacancies, varying slightly O contents, etc. [20]. In this series of phosphors, this kind of substitution did not influence the emission spectra shapes of Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ (seeing Figure 5).

Figure 6 shows the electroluminescence (EL) spectra of the intense blue-green LED fabricated with Sr_3.90Si₃O₈Cl₄:0.10Eu²⁺ and Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺ under 20 mA current excitation. The emission band at ~400 nm is attributed to the emission of LED chip and the broad band from 430 nm to 600 nm is due to the emission of the phosphor. Comparing with curve a, the ratio of LED chip emission to phosphor emission in curve b is smaller. The result indicates phosphor Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺ can more efficiently absorb the emission of LED chip, and exhibit stronger blue-green emission, compared with Sr_3.90Si₃O₈Cl₄:0.10Eu²⁺. Bright blue-green light is observed from these two LEDs by the naked eye. This result is in accordance with their emission spectra. Their CIE chromaticity coordinates are calculated to be x = 0.169, y = 0.356, and x = 0.170, y = 0.362, respectively.

Figure 5. Emission spectra of Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ (y = 0.00, 0.05, 0.10, 0.15, 0.20) under 320 nm excitation, the inserted figures are the concentration dependence of the relative integral emission intensity of Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ and the image of Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺ under 365 nm excitation.

Figure 5. Emission spectra of Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ (y = 0.00, 0.05, 0.10, 0.15, 0.20) under 320 nm excitation, the inserted figures are the concentration dependence of the relative integral emission intensity of Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺ and the image of Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺ under 365 nm excitation.

Figure 6. Electroluminescence (EL) spectra of the blue-green light-emitting-diodes (LEDs) based on (a) Sr_3.90Si₃O₈Cl₄:0.10Eu²⁺ and (b) Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺ under 20 mA current excitation; the inserted figures are the images of these two LEDs.

Figure 6. Electroluminescence (EL) spectra of the blue-green light-emitting-diodes (LEDs) based on (a) Sr_3.90Si₃O₈Cl₄:0.10Eu²⁺ and (b) Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺ under 20 mA current excitation; the inserted figures are the images of these two LEDs.

3. Experimental Section

The phosphors Sr₄Si₃O₈Cl₄ doped with Eu²⁺ and Bi³⁺ were synthesized by a solid-state reaction. The raw materials SrCO₃ (A.R. grade, Aladdin, Shanghai, China), SiO₂ (A.R. grade, Aladdin, Shanghai, China), SrCl₂·6H₂O (A.R. grade, Aladdin, Shanghai, China), Bi₂O₃ (A.R. grade, Aladdin, Shanghai, China) and Eu₂O₃ (99.99% purity, Aladdin, Shanghai, China) were thoroughly mixed and ground together according to the given stoichiometric ratio. The mixture was first pre-fired at 400 °C for 2 h, and heated at 950 °C for 4 h. The process reduced CO atmosphere. The blue-green LED was fabricated by combing an LED chip (~395 nm) with the mixture of blue-green phosphor and epoxy resin (the ratio of mass is 1:1).

The structure of these phosphors was recorded by X-ray powder diffraction (XRD) using Cu K_α radiation on a RIGAKU D/max 2200 vpc X-ray Diffractometer (Rigaku Corporation, Osaka, Japan). Their photoluminescent spectra were recorded on a Cary Eclipse FL1011M003 (Varian Palo Alto, CA, USA) spectrofluorometer and the xenon lamp was used as excitation source. The electro-luminescence of blue-green LEDs was recorded on a high-accuracy array spectrometer (HSP6000, HongPu Optoelectronics Technology Co. Ltd., Hangzhou, China). All the measurements were performed at room temperature.

4. Conclusions

A series of phosphors, Sr_4−4xSi₃O₈Cl₄:4xEu²⁺, Sr_3.90−4ySi₃O₈Cl₄:0.10Eu²⁺, 4yBi³⁺, were prepared by solid state reaction at high temperature. Their structure and photo-luminescent properties were investigated. The doping of Bi³⁺ enhanced the excitation and emission intensity of these phosphors. The blue-green phosphor Sr_3.50Si₃O₈Cl₄:0.10Eu²⁺, 0.40Bi³⁺ exhibited intense blue-green emission with broader excitation in near-UV range. The LED based on this phosphor can emit intense blue-green light, which can be observed by the naked eye. Hence, it is considered to be a good candidate for the blue-green component of a white LED.