Journal of Nanomaterials

Volume 2012 (2012), Article ID 848274, 7 pages

doi:10.1155/2012/848274

Research Article

Effective Red Compensation of  :  Phosphor by Codoping Ions and Its Energy Transfer

Le Zhang, Zhou Lu, Pengde Han, Lixi Wang, and Qitu Zhang

College of Materials Science and Engineering, Nanjing University of Technology, Jiangsu, Nanjing 210009, China

Received 15 August 2012; Accepted 28 September 2012

Academic Editor: Su Chen

Copyright © 2012 Le Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Mn²⁺ ions codoped Sr₂SiO₄ : Dy³⁺ phosphors were prepared by the solid-state reaction method using NH₄Cl as the flux. Their phase compositions, photoluminescence properties, and the energy transfer process were systematically investigated. All Mn/Dy codoped powders were ′-Sr₂SiO₄. The codoping concentration range of Mn²⁺ was  mol% to keep the structure undamaged. The broad red emission of Mn²⁺ centered at 647 nm in Sr₂SiO₄ : Mn, Dy powders, which effectively compensated the red emission of Sr₂SiO₄ : Dy³⁺ phosphor. The CIE chromaticity coordinates dramatically changed from (0.310, 0.340) to (0.332, 0.326) due to the red enhancement via the energy transfer from Dy³⁺ to Mn²⁺. This energy transfer is realized by the exchange interaction. But the luminescence quenching of Sr₂SiO₄ : Dy, Mn phosphor was mainly caused by the electric multipoles interaction. The concentration optimized (Sr0.96, Mn0.02, Dy0.02)₂SiO₄ phosphor with high and almost pure white emission has great potential to act as a single-matrix white phosphor for white LEDs.

1. Introduction

White light emitting diodes have attracted more attention due to their potential applications in extensive fields, such as device indicators, backlight, automobile headlights, and general illumination [1–4]. Current white LEDs are composed of blue-emitting GaN chip and yellow-emitting YAG : Ce phosphor. However, YAG : Ce has a deficient red emission, leading the white LEDs to have a low color rendering index (CRI < 80) [5]. To enrich the red emission, the phosphor blend of YAG : Ce and a red emitting phosphor is generally applied [6, 7]. Another technology is the combination of tricolor phosphors with an UV-LED chip. The above phosphor mixtures give fluorescence reabsorption and nonuniformity of luminescence properties, resulting in a loss of luminous efficiency and time-dependent shift of the color point. Therefore, a single-phase phosphor with direct white light emission is desirable [8–11].

At present, the most of this kind phosphors have at least two luminescent centers, such as Eu²⁺/Mn²⁺ [3, 12, 13], Ce³⁺/Eu²⁺ [8], or Eu²⁺ (in different lattice sites) [9]. As the natural white light emission ions, Dy³⁺ ions have two dominant emission bands of blue (485 nm, ⁴F9/2 → ⁶H15/2) and yellow (570 nm, ⁴F9/2 → ⁶H13/2) [14, 15]. Recently, Dy³⁺ doped alkaline earth orthosilicates (Sr₂SiO₄ : Dy³⁺) powders have attracted much attention due to its excellent emission characteristic, single luminescent center, and high absorption efficiency in the UV region [16–18]. This phosphor can emit white light with CIE chromaticity coordinates (0.310, 0.340). However, there is still a gap comparing with the pure white light (0.330, 0.330) due to the weak red emission at ~665 nm. Predictably, the compensation of red light will effectively reduce this gap and enhance the CRI and then obtain warm white light.

As known, Mn²⁺ ions can effectively absorb UV light and then emit different colors from green to deep red depending on the surrounding crystal field strength [19]. Red or white phosphors were successfully obtained by adjusting the concentration ratios of Mn²⁺/Dy³⁺ or Eu²⁺/Dy³⁺ [20–22]. Therefore, there is reason to believe that the 3 configured Mn²⁺ ions may effectively compensate the red emission of Sr₂SiO₄ : Dy³⁺ phosphor as a red-emitting center with suitable doping concentration.

In this paper, Dy³⁺ and Mn²⁺ ions codoped Sr₂SiO₄ powders were prepared by the solid-state reaction method using 5 wt% NH₄Cl as the flux. The phase compositions and luminescence properties of prepared powders were systematically investigated as well as the energy transfer process from Dy³⁺ to Mn²⁺. It was found that the broad emission band of Mn²⁺ centered at 650 nm was observed from Sr₂SiO₄ : Mn²⁺, Dy³⁺ powders. The energy transfer from Dy³⁺ to Mn²⁺ dramatically changed the CIE chromaticity coordinates from (0.310, 0.340) to (0.332, 0.326). Sr₂SiO₄ : Dy³⁺, Mn²⁺ with high and almost pure white emission has great potential to act as single-matrix white phosphor for white LEDs.

2. Materials and Experimental Details

All powder samples were synthesized by the solid-state reaction method. High purity Dy₂O₃ (>99.99%, the others are analytical grade), SrCO₃, SiO₂, MnCO₃, Li₂CO₃ (the charge compensation agent), and NH₄Cl (5 wt%, the flux) were used as the starting materials. Stoichiometrical amounts of starting materials were mixed with ethanol and then ball milled using planetary milling machine for 8 h. The synthesis was performed at 1000°C for 4 h under the reducing atmosphere (5% H₂ + 95% N₂). Series of (, )₂SiO₄ : Dy0.02 powders and (, )₂SiO₄ : Mn0.04 (–8.0, mol%) were prepared.

The crystalline phases of the synthesized powders were determined by the X-ray diffraction (XRD, D/Max2500, Rigaku, Japan) using Cu K  radiation ( Å) in the range of 5°~80° with a step size of 0.02°. The photoluminescence spectra of the phosphors were measured using a fluorescent spectrophotometer (FL3-221, HOROBA, Jobin Yvon, France) with a 450 W xenon lamp as the excitation source (the multiplication voltage: 700 V; the slit width: 2.5 nm). All measurements were carried out at room temperature.

3. Results and Discussion

3.1. Phase Compositions of (Sr, Mn, Dy)₂SiO₄ Powders

Figure 1 shows the XRD patterns of 1 mol% DyMn (~8.0 mol%) doped Sr₂SiO₄ powders. The overlap between the diffraction peaks of  -Sr₂SiO₄ (JCPDS no. 39-1256) and -Sr₂SiO₄ (no. 38-0271) was too much to index them precisely, especially at about 30–32°. Therefore, only the diffraction peaks with clear ownership were indexed. Obviously, the phase compositions of Dy/Mn codoped powders were almost pure  -Sr₂SiO₄ when  mol%. The ionic radiuses of Sr²⁺, Mn²⁺, and Dy³⁺ are 1.12, 0.80, and 0.91 Å, respectively. Therefore, the diffraction peaks of all samples slightly shifted to the higher angle with increasing the substitution concentration of Mn²⁺ (Figure 1 (right)). However, minor SrSiO₃ and MnO₂ were found when the Mn concentration was increased to 8.0 mol%. In general, the substitution of Mn²⁺ with smaller ionic radius cannot change the crystal structure of host. However, the difference of ionic radiuses between Sr²⁺ and Mn²⁺ is 40.0% ((1.12–0.80)/0.80, much bigger than 15%) and the electronegativity of Mn²⁺ (1.55) is much bigger than that of Sr²⁺ (0.95), which make the excess Mn exist in “MnO₂” form [23].

Figure 1: XRD patterns of (, )₂SiO₄ : Dy0.02 (, mol%) powders as a function of Mn²⁺ concentration.

When the Dy concentration was fixed at 1.0 mol%, the codoping concentration of Mn²⁺ for the maximum red emission was 2.0 mol% (see later part “3.2 Luminescence of powders”). Thus, the samples (, )₂SiO₄ : Mn0.04 (–8.0, mol%) as a function of Dy concentration were also synthesized to obtain the simultaneous maximum of the emission of Dy and Mn. Their XRD patterns are shown in Figure 2. All Mn/Dy codoped powders were also phase. All diffraction peaks slightly shifted to higher angle with increasing the substitution concentration of Dy³⁺. In fact, the phase compositions of all Mn/Dy codoped powders are consistent with the regularity suggested by our previous study [17]. Without any doping, the phase composition of host powders using 5.0 wt% NH₄Cl as the flux is almost pure phase [16]. But the smaller ions (Dy³⁺/Mn²⁺) doped or codoped powders are  -Sr₂SiO₄. The doping of smaller ions within certain concentration range is beneficial to the formation of    phase. Here, the structure is still stable even though the codoping concentration of Dy³⁺ reaches 8.0 mol%. However, the codoping concentration range of Mn²⁺ to keep the structure undamaged is ≤4.0 mol% in our experiments due to the big difference of ionic radius between Mn²⁺ and Sr²⁺. Therefore, the codoped Mn²⁺ ions are considered to be effective to occupy the Sr²⁺ site in the range of 0.0~4.0 mol%.

Figure 2: XRD patterns of (, )₂SiO₄ : Mn0.04 (, mol%) powders as a function of Dy³⁺ concentration.

3.2. Luminescence of (Sr, Mn, Dy)₂SiO₄ Powders

Figure 3(a) shows the excitation spectrum of Sr₂SiO₄ : Dy³⁺ powders ( nm). The excitation peaks included 349, 364, 386, 452, and 466 nm corresponding to the transitions of ⁶H15/2 → ⁴D7/2, ⁶P7/2, ⁴F21/2, ⁴I15/2 and ⁴F9/2, respectively. The strongest excitation peak was at 349 nm. Its corresponding emission spectrum (Figure 3(b)) exhibited the typical characteristics of Dy³⁺ ions, including two strong peaks centered at 477 nm and 571 nm and a weak emission at 665 nm. The integrated intensities of blue (477 nm) and yellow (571 nm) colors were almost equal. Figure 4(a) shows the excitation spectrum of Sr₂SiO₄ : Mn²⁺ powders ( nm). The main excitation peaks centered at 371, 416, and 543 nm, corresponding to the transitions from ⁶A₁(⁶S) ground state of Mn²⁺ to its ⁴T₂(⁴G), 4E(⁴G), and ⁴T₁(⁴G) excited state, respectively. Under the excitation of 416 nm, a broad emission centered at 647 nm was observed (Figure 4(b)). This emission was caused by the transition of ⁴T₁(⁴G)-⁶A₁(⁶S) and its Gauss fitting curves (dotted line) included two peaks at 645 nm and 719 nm, which maybe resulting from the two kinds of lattice environment of Sr²⁺ site (their coordination numbers were 9 and 10, resp.).

(a)

(b)

Figure 3: Excitation spectrum (a), ( nm) and emission spectrum (b), ( nm) of Dy³⁺ in (Sr0.99, Dy0.01)₂SiO₄.

(a)

(b)

Figure 4: Excitation spectrum (a), ( nm) and emission spectrum (b), ( nm) of Mn²⁺ in (Sr0.98, Mn0.02)₂SiO₄.

From Figure 5 (a, excitation spectrum of Mn²⁺) and (b, emission spectrum of Dy³⁺), it can be seen that the excitation of Mn²⁺ was overlapped by the emission of Dy³⁺, which indicated that a potential energy transfer from Dy³⁺ to Mn²⁺ in Sr₂SiO₄ lattice might exist. Figure 5(c) shows the excitation spectrum of Dy/Mn codoped (Sr0.97Mn0.02)₂SiO₄ : Dy0.02 powders. The monitored wavelength was 647 nm for the emission of Mn²⁺. However, the strong absorption of Dy³⁺ at 349 nm appeared which strongly indicated that the energy transfer of Dy³⁺ → Mn²⁺ happened. In addition, the characteristic emission of Mn²⁺ at ~650 nm was also clearly observed under the excitation of 349 nm (Figure 5(d)). Though the emission of Sr₂SiO₄ : Dy also contained the peak at 665 nm, the luminescence enhancement of Mn²⁺ at ~650 nm further proved the energy transfer from Dy³⁺ to Mn²⁺. The luminescence intensities of Mn²⁺ at 647 nm and Dy³⁺ at 571 nm asynchronously changed with their concentration variations shown in Figure 6 (only four concentration pairs with dramatic change are shown). The detailed change of the integrated intensities of 550–600 nm for Dy³⁺ and 635–700 nm for Mn²⁺ of all concentration-pairs is shown in Figure 7.

Figure 5: Excitation spectrum (a), ( nm) of Mn²⁺ in (Sr0.98, Mn0.02)₂SiO₄ and emission spectrum (b), ( nm) of Dy³⁺ in (Sr0.99, Dy0.01)₂SiO₄. Excitation (c), ( nm) and emission (d), ( nm) spectra of Mn²⁺ in (Sr0.97, Mn0.02, Dy0.01)₂SiO₄.

Figure 6: Emission spectra of (Sr, Dy, Mn)₂SiO₄ with different concentration pairs of Dy and Mn ( nm).

(a)

(b)

Figure 7: Variations of integrated intensity of 550–600 nm for Dy³⁺ and 635–700 nm for Mn²⁺ in (Sr, Dy, Mn)₂SiO₄ ( nm).

When the concentration of Dy³⁺ was fixed at 1.0 mol%, the emission intensity of Dy³⁺ gradually decreased with the increase of Mn²⁺ concentration (Figure 7(b)) and the emission intensity of Mn²⁺ firstly increased and then dropped down. The optimum concentration of Mn²⁺ for the maximum red-emission was 2.0 mol%. Therefore, the samples (, )₂SiO₄ : Mn0.04 as a function of Dy³⁺ concentration were also synthesized to obtain the strong emissions of Dy³⁺ and Mn²⁺ simultaneously. The detailed emission change is shown in Figure 7(a). With the increase of the concentration of Dy³⁺, the yellow emission intensity reached the maximum when the corresponding concentration of Dy³⁺ was 1.0 mol%. But when the red emission intensity of Mn²⁺ reached the maximum, the codoped concentration of Dy³⁺ was 2.0 mol%, and then the great decrease of the two emissions occurred due to the self-concentration quenching of Dy³⁺ ions. That is, the optimum concentration ranges of Dy³⁺ and Mn²⁺ to obtain the highest yellow and red emissions are 1.0~2.0 mol%. From the typical emission spectra of four concentration-pairs shown in Figure 6, it was noted that the peak location of red emission changed with the variation of the Dy/Mn concentrations. A red shift occurred from 651 nm to 659 nm with the increase of Mn concentration (b~d) and this may be due to the exchange interactions between Mn²⁺ ions [24].

The CIE chromaticity coordinates of the four concentration-pair powders are shown in Figure 8. They were all in white light region. However, comparing with the point (a), points (b, c, and d) had the purer white color and their CIE coordinates were closer with the pure white point (0.330, 0.330; five stars). The optimized coordinates were (0.332, 0.326) with higher emission intensity when the codoping concentrations of Dy³⁺ and Mn²⁺ were 2.0 mol% and 2.0 mol%, respectively. That is, the red emission of Sr₂SiO₄ : Dy phosphor was effectively compensated by the codoping of Mn²⁺ ions, the concentration-optimizing of Dy/Mn, and the energy transfer from Dy³⁺ to Mn²⁺.

Figure 8: CIE chromaticity coordinates of (Sr, Dy, Mn)₂SiO₄ with different concentration pairs ( nm).

3.3. Energy Transfer from Dy³⁺ to Mn²⁺

Figure 9 shows the schematic diagram of energy transfer in Sr₂SiO₄ : Dy, Mn powders. The excitation peaks of Mn²⁺ at 416 and 543 nm (⁶A₁(6S) → ⁴T₂(⁴G)/⁴T₁(⁴G)) were covered by the emission peaks of Dy³⁺ at 477 nm and 571 nm (⁴F9/2 → ⁴H15/2/⁶H13/2). This overlap indicates that the energy transfer mechanism is likely to be the radiative reabsorption or nonradiative cross relaxation. However, it could not be the radiative transfer because the oscillator strengths of ⁶A₁(6S) → ⁴T₂(⁴D)/⁴T₁(⁴G) are very weak and the resulted energy transfer will be much weaker. Therefore, the energy transfer is likely to be the non-radiative cross-relaxation (CR) between the ⁴F9/2 → ⁴H15/2, ⁶H13/2 transitions of Dy³⁺, and the ⁶A₁(6S) → ⁴T₂(⁴G), ⁴T₁(⁴G) transitions of Mn²⁺ as shown in Figure 9.

Figure 9: Energy transfer diagram of Dy³⁺ → Mn²⁺.

In order to elucidate the type of the interaction involved in the energy transfer from the donor ions (Dy³⁺) to the acceptor ions (Mn²⁺), the luminescence intensities of Dy³⁺ and Mn²⁺ were integrated from the emission spectra of Dy/Mn codoped powders according to the VanUitert’s work [25–27]. The relationship between the luminescence intensity of the donor ions (Dy³⁺) and the concentration of the acceptor ions (Mn²⁺) satisfies the following formula [28]:

and are the luminescence intensity of donor ions (Dy³⁺) without and with the appearance of acceptor ions (Mn²⁺), respectively. is a constant for one host. When the    value is 3, 6, 8, and 10, the interaction type is the exchange interaction, dipoles-dipoles (d-d), dipoles-quadrupoles (d-q), and quadrupoles-quadrupoles (q-q), respectively. The relation curve between log((Dy)) and log((Mn)) is plotted in Figure 10(a). According to the slope of the fitted line for the curve (a), is 1.02 and is 3.06. As stated in VanUitert’s theory [25], the energy transfer from Dy³⁺ to Mn²⁺ is realized by the exchange interaction.

Figure 10: (a) log() of Dy³⁺ emission versus Mn²⁺ concentration in (, )₂SiO₄ : Dy0.02; (b) log() of Dy³⁺ emission versus Dy³⁺ concentration in (, )₂SiO₄ : Mn0.04.

In the meantime, in Huang’s theory [28] for the interaction type among the donor ions (Dy³⁺), the relationship between the luminescent intensity () and the concentration of activators () should be expressed as

where ,   is the transition probability of the donor ions (Dy³⁺), is the sample dimension (), is a concentration-independent function, and is the concentration of the donor ions (Dy³⁺). is the electric multipoles index. When value is 3, 6, 8 and 10, the interaction type is also exchange interaction, d-d, d-q, and q-q, respectively. After a mathematical operating,

is a concentration-independent constant. Thus, the value of could be obtained by the relation curve of plotted in Figure 10(b). According to the slope of the fitted line for the curve (b),    is approximately equal to 4.80, which means that the d-d interaction () as well as the exchange interaction () are involved in the self-concentration quenching of the ⁴F9/2 → ⁶H13/2 transition of Dy³⁺ (571 nm). On the basis of a simple linear-algebraic calculation for the value (), where the value is the fraction of the exchange interaction, about 40% of the non-radiative energy transfer occurs via the exchange interaction. Therefore, the d-d interaction is involved in a major way in the luminescence quenching of Sr₂SiO₄ : Dy³⁺, Mn²⁺ phosphor.

4. Conclusions

In this paper, Mn²⁺ ions codoped Sr₂SiO₄ : Dy³⁺ phosphors were successfully prepared by the solid-state reaction method using NH₄Cl as the flux. All Mn/Dy codoped powders were -Sr₂SiO₄ with the slight shift of the diffraction peaks to higher angle with the increase of the substitution concentrations of Mn/Dy. The codoping concentration range of Mn²⁺ is ≤4.0 mol% to keep the structure undamaged. The broad red emission of Mn²⁺ centered at ~650 nm was observed from Sr₂SiO₄ : Mn, Dy powders, which effectively compensated the red emission of Sr₂SiO₄ : Dy³⁺ phosphor. The CIE chromaticity coordinates dramatically changed from (0.310, 0.340) to (0.332, 0.326) due to the red enhancement via the energy transfer from Dy³⁺ to Mn²⁺. This energy transfer is realized through the exchange interaction. But the d-d interaction plays a major role in the luminescence quenching of Sr₂SiO₄ : Dy³⁺, Mn²⁺ phosphor. The concentration optimized (Sr0.96, Mn0.02, Dy0.02)₂SiO₄ phosphor with almost pure white emission has great potential to act as a single-matrix white phosphor for white LEDs.

Acknowledgments

The authors acknowledge the generous financial supports from the Natural Science Foundation of Jiangsu Province (BK2007724), National Natural Science Foundation of China (51202111), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Research and Innovation Program for College Graduates of Jiangsu Province (CXZZ12_0410).

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