Effective Red Compensation of Sr 2 SiO 4 : Dy 3 + Phosphor by CodopingMn 2 + Ions and Its Energy Transfer

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 α′-Sr2SiO4. The codoping concentration range of Mn2+ was ≤ 4.0 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.


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][2][3][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][9][10][11].
As known, Mn 2+ 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 Mn 2+ /Dy 3+ or Eu 2+ /Dy 3+ [20][21][22].Therefore, there is reason to believe that the 3d 5 configured Mn 2+ ions may effectively compensate the red emission of Sr 2 SiO 4 : Dy 3+ phosphor as a red-emitting center with suitable doping concentration.
In this paper, Dy 3+ and Mn 2+ ions codoped Sr 2 SiO 4 powders were prepared by the solid-state reaction method using 5 wt% NH 4 Cl as the flux.The phase compositions and luminescence properties of prepared powders were systematically investigated as well as the energy transfer process from Dy 3+ to Mn 2+ .It was found that the broad emission band of Mn 2+ centered at 650 nm was observed from Sr 2 SiO 4 : Mn 2+ , Dy 3+ powders.The energy transfer from Dy 3+ to Mn 2+ dramatically changed the CIE chromaticity coordinates from (0.310, 0.340) to (0.332, 0.326).Sr 2 SiO 4 : Dy 3+ , Mn 2+ with high and almost pure white emission has great potential to act as single-matrix white phosphor for white LEDs.

Materials and Experimental Details
All powder samples were synthesized by the solid-state reaction method.High purity Dy 2 O 3 (>99.99%,the others are analytical grade), SrCO 3 , SiO 2 , MnCO 3 , Li 2 CO 3 (the charge compensation agent), and NH 4 Cl (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% H 2 + 95% N 2 ).Series of (Sr 0.99−x , Mn x ) 2 SiO 4 : Dy 0.02 powders and (Sr 0.98−x , Dy x ) 2 SiO 4 : Mn 0.04 (x = 0.0-8.0,mol%) were prepared.
The crystalline phases of the synthesized powders were determined by the X-ray diffraction (XRD, D/Max2500, Rigaku, Japan) using Cu Kα radiation (λ = 1.5406Å) 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.

Results and Discussion
3.1.Phase Compositions of (Sr, Mn, Dy) 2 SiO 4 Powders.Figure 1 shows the XRD patterns of 1 mol% Dy•xMn (x = 0.0∼8.0mol%) doped Sr 2 SiO 4 powders.The overlap between the diffraction peaks of α -Sr 2 SiO 4 (JCPDS no.39-1256) and β-Sr 2 SiO 4 (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 α -Sr 2 SiO 4 when x ≤ 4.0 mol%.The ionic radiuses of Sr 2+ , Mn 2+ , and Dy 3+ 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 Mn 2+ (Figure 1 (right)).However, minor SrSiO 3 and MnO 2 were found when the Mn concentration was increased to 8.0 mol%.In general, the substitution of Mn 2+ with smaller ionic radius cannot change the crystal structure of host.However, the difference of ionic radiuses between Sr 2+ and Mn 2+ is 40.0%((1.12-0.80)/0.80,much bigger than 15%) and the electronegativity of Mn 2+ (1.55) is much bigger than that of Sr 2+ (0.95), which make the excess Mn exist in "MnO 2 " form [23].When the Dy concentration was fixed at 1.0 mol%, the codoping concentration of Mn 2+ for the maximum red emission was 2.0 mol% (see later part "3.2 Luminescence of (Sr, Mn, Dy) 2 SiO 4 powders").Thus, the samples (Sr 0.98−x , Dy x ) 2 SiO 4 : Mn 0.04 (x = 0.0-8.0,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 α phase.All diffraction peaks slightly shifted to higher angle with increasing the substitution concentration of Dy 3+ .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% NH 4 Cl as the flux is almost pure β phase [16].But the smaller ions (Dy 3+ /Mn 2+ ) doped or codoped powders are α -Sr 2 SiO 4 .The doping of smaller ions within certain concentration range is beneficial to the formation of α phase.Here, the structure is still stable even though the codoping concentration of Dy 3+ reaches 8.0 mol%.However, the codoping concentration range of Mn 2+ to keep the structure undamaged is ≤4.0 mol% in our experiments due to the big difference of ionic radius between Mn 2+ and Sr 2+ .Therefore, the codoped Mn 2+ ions are considered to be effective to occupy the Sr 2+ site in the range of 0.0∼4.0mol%.
From Figure 5 (a, excitation spectrum of Mn 2+ ) and (b, emission spectrum of Dy 3+ ), it can be seen that the excitation of Mn 2+ was overlapped by the emission of Dy 3+ , which indicated that a potential energy transfer from Dy 3+ to Mn 2+ in Sr 2 SiO 4 lattice might exist.Figure 5(c) shows the excitation spectrum of Dy/Mn codoped (Sr 0.97 Mn 0.02 ) 2 SiO 4 : Dy 0.02 powders.The monitored wavelength was 647 nm for the emission of Mn 2+ .However, the strong absorption of Dy 3+ at 349 nm appeared which strongly indicated that the energy transfer of Dy 3+ → Mn 2+ happened.In addition, the characteristic emission of Mn 2+ at ∼650 nm was also clearly observed under the excitation of 349 nm (Figure 5(d)).Though the emission of Sr 2 SiO 4 : Dy also contained the peak at 665 nm, the luminescence enhancement of Mn 2+ at ∼650 nm further proved the energy transfer from Dy 3+ to Mn 2+ .The luminescence intensities of Mn 2+ at 647 nm and Dy 3+ 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 Dy 3+ and 635-700 nm for Mn 2+ of all concentration-pairs is shown in Figure 7.
When the concentration of Dy 3+ was fixed at 1.0 mol%, the emission intensity of Dy 3+ gradually decreased with the increase of Mn 2+ concentration (Figure 7(b)) and the emission intensity of Mn 2+ firstly increased and then dropped down.The optimum concentration of Mn 2+ for the maximum red-emission was 2.0 mol%.Therefore, the samples (Sr 0.98−x , Dy x ) 2 SiO 4 : Mn 0.04 as a function of Dy 3+ concentration were also synthesized to obtain the strong emissions of Dy 3+ and Mn 2+ simultaneously.The detailed emission change is shown in Figure 7(a).With the increase of the concentration of Dy 3+ , the yellow emission intensity reached the maximum when the corresponding concentration of Dy 3+ was 1.0 mol%.But when the red emission intensity of Mn 2+ reached the maximum, the codoped concentration of Dy 3+ was 2.0 mol%, and then the great decrease of the two emissions occurred due to the self-concentration quenching of Dy 3+ ions.That is, the optimum concentration ranges of Dy 3+ and Mn 2+ to obtain the highest yellow and red emissions are 1.0∼2.0mol%.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 Mn 2+ 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 Dy 3+ and Mn 2+ were 2.0 mol% and 2.0 mol%, respectively.That is, the red emission of Sr 2 SiO 4 : Dy phosphor was effectively compensated by the codoping of Mn 2+ ions, the concentration-optimizing of Dy/Mn, and the energy transfer from Dy 3+ to Mn 2+ .
In order to elucidate the type of the interaction involved in the energy transfer from the donor ions (Dy 3+ ) to the acceptor ions (Mn 2+ ), the luminescence intensities of Dy 3+ and Mn 2+ were integrated from the emission spectra   4 T 1 ( 4 P) 4 T 2 ( 4 G) 4 T 1 ( 4 G) 6 A 1 (6S) 4 E( 4 G) of Dy/Mn codoped powders according to the VanUitert's work [25][26][27].The relationship between the luminescence intensity of the donor ions (Dy 3+ ) and the concentration of the acceptor ions (Mn 2+ ) satisfies the following formula [28]: I 0 and I are the luminescence intensity of donor ions (Dy 3+ ) without and with the appearance of acceptor ions (Mn 2+ ), respectively.A is a constant for one host.When the θ value is 3, 6, 8, and 10, the interaction type is the exchange interaction, dipoles-dipoles (d-d), dipoles-quadrupoles (dq), and quadrupoles-quadrupoles (q-q), respectively.The relation curve between log(I/I 0 (Dy)) and log(x(Mn)) is plotted in Figure 10(a).According to the slope of the fitted line y 1 for the curve (a), θ/3 is 1.02 and θ is 3.06.As stated in VanUitert's theory [25], the energy transfer from Dy 3+ to Mn 2+ is realized by the exchange interaction.
In the meantime, in Huang's theory [28] for the interaction type among the donor ions (Dy 3+ ), the relationship between the luminescent intensity (I) and the concentration of activators (C) should be expressed as where a = CΓ(1 − d/s)[X 0 (1 + A)/γ] d/s , γ is the transition probability of the donor ions (Dy 3+ ), d is the sample dimension (d = 3), Γ is a concentration-independent function, and C is the concentration of the donor ions (Dy 3+ ).s is the electric multipoles index.When s 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, K is a concentration-independent constant.Thus, the value of s could be obtained by the relation curve of log(I/C Dy ) ∼ log(C Dy ) plotted in Figure 10(b).According to the slope of the fitted line y 2 for the curve (b), s is approximately equal to 4.80, which means that the d-d interaction (s = 6) as well as the exchange interaction (s = 3) are involved in the self-concentration quenching of the 4 F 9/2 → 6 H 13/2 transition of Dy 3+ (571 nm).On the basis of a simple linearalgebraic calculation for the s value (3m + 6(1 − m) = 4.80), where the m 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 Sr 2 SiO 4 : Dy 3+ , Mn 2+ phosphor.

Conclusions
In this paper, Mn 2+ ions codoped Sr 2 SiO 4 : Dy 3+ phosphors were successfully prepared by the solid-state reaction method using NH 4 Cl as the flux.All Mn/Dy codoped powders were α -Sr 2 SiO 4 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 Mn 2+ is ≤4.0 mol% to keep the structure undamaged.The broad red emission of Mn 2+ centered at ∼650 nm was observed from Sr 2 SiO 4 : Mn, Dy powders, which effectively compensated the red emission of Sr 2 SiO 4 : Dy 3+ 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 Dy 3+ to Mn 2+ .This energy transfer is realized through the exchange interaction.But the d-d interaction plays a major role in the luminescence quenching of Sr 2 SiO 4 : Dy 3+ , Mn 2+ phosphor.The concentration optimized (Sr 0.96 , Mn 0.02 , Dy 0.02 ) 2 SiO 4 phosphor with almost pure white emission has great potential to act as a singlematrix white phosphor for white LEDs.