Temperature Dependent Optical Performance of the NaSr(PO₃)₃:Eu²⁺ Blue Phosphors

NaSr(PO₃)₃:Eu²⁺ 청색 형광체의 온도 의존적 형광 특성

Abstract

Eu²⁺ doped polyphosphate NaSr(PO₃)₃ blue-emitting phosphors were synthesized by the conventional solid state method in a reductive atmosphere. The phase formation of NaSr(PO₃)₃ phosphors were characterized by using the X-ray powder diffraction (XRD) measurement. The photoluminescence emission and excitation spectra of the NaSr(PO₃)₃:Eu²⁺ phosphor, and decay curves were measured. Under the near-UV excitation, the phosphor exhibits a band emission around 420 nm assigned to the 4f⁶5d→f7(8S^7/2) transition of Eu²⁺. The temperature dependent emission spectra and decay curves were measured to elevate the thermal properties of the Eu²⁺ doped phosphors. The as-prepared NaSr(PO₃)₃:Eu²⁺ phosphors show a strong temperature dependent performance, which can serve as a promising temperature sensor.

고상법을 이용하여 Eu 이온이 첨가된 다중인산염 NaSr(PO₃)₃형광체를 합성하고 탄소열환원법을 통하여 NaSr(PO₃)₃:Eu²⁺ 형광체를 완성하였다. X선 회절 측정(XRD)을 통하여 형광체의 결정상을 확인하였다. NaSr(PO₃)₃:Eu²⁺ 형광체의 Eu²⁺ 농도 변화에 따른 발광 방출 및 여기 스펙트럼과 수명시간을 측정하였다. 320 nm 근자외선 여기에서 NaSr(PO₃)₃:Eu²⁺ 형광체는 Eu²⁺의 4f⁶5d → f⁷(8S^7/2) 전이에 따른 420 nm 중심의 밴드를 방출하였다. Eu²⁺ 첨가 된 형광체의 열적 특성을 증가시키기 위해 온도 의존적 방출 스펙트럼과 형광 감쇠 곡선을 측정하고 형광체의 온도 의존성을 설명하였다. 본 연구로 희토류 이온이 첨가된 NaSr(PO₃)₃:Eu²⁺는 디스플레이, 검출기 등에 온도 센서 역할을 할 수 있는 우수한 온도 의존성을 보여준다.

Ⅰ. INTRODUCTION

The divalent Eu²⁺ ion with 4f⁷ electron configuration usually exhibits broad fluorescence emission due to f - d allowed transitions. The fluorescence of Eu²⁺ doped phosphors represent strong broad absorption bands in the ultraviolet (350 ~ 380 nm) or near - ultraviolet (380 ~ 420 nm) range and various emission wavelengths from ultraviolet to red^[1-3]. The allowed 5d - 4f electrical dipole transition originates from the lowest band of the 4f⁶5d¹ configuration to the 8S^7/2 grounded state of the 4f⁷ configuration, resulting in a tun able broadband emission, which is highly dependent on the crystal field of the host lattices^{[4, 5]}. Thus, the luminescence properties of Eu²⁺ activated phosphors have been extensively investigated^[6-8].

Polyphosphate, which are used as a representative series of host for rare earth activators, shows outstanding advantages such as environmental safety, high luminous efficiency and relatively low cost^{[9, 10]}. Therefore, these polyphosphate are suitable for the application of illumination and used as potential candidate as phosphor for light emitting diodes (LED) industry by structure engineering^{[11, 12]}. Particularly, the PO₄^3- tetrahedral in the polyphosphate host with rigid PO₄ structure is benefit for the formation and stabilization of the divalent Eu^{2+ [13, 14]} In addition, alkaline earth metal ions such as Sr²⁺, Ba²⁺ are ideal ions to be suitable for Eu²⁺ substitution due to the similarity of ionic radii^[15-19]. However, the luminescence efficiency of the Eu²⁺ doped polyphosphate is strongly influenced by the temperature and the bad thermal stability limits the application in white LED. Therefore, it is necessary to study the thermal stability of the Eu²⁺ doped polyphosphate and investigate the application of these kinds of phosphors in other field^[20-22].

In the present work, Eu²⁺ doped NaSr(PO₃)₃ phosphors were prepared by conventional high-temperature solid state method. The phase and crystal structure are investigated in detail. Furthermore, the temperature- dependent luminescence spectra and decay curves measurement were measured and the activation energy was confirmed. Results show that NaSr(PO₃)₃:Eu²⁺ phosphor is a potential temperature sensor.℃

Ⅱ. MATERIAL AND METHOD

1. Materials Synthesis.

The compounds NaSr(1-x)(PO₃)₃:xEu²⁺ (x = 0.1% ~ 5%) were synthesized by the conventional solid-state reaction. Stoichiometric amounts of Na₂CO₃, SrCO₃, NH₄H₂PO₄ completely and Eu₂O₃ were mixed and ground in an agate mortar with a small quantity of ethanol. Firstly, the mixture was heated to 400℃ and kept at this temperature for 5 h. After regrinding, they were put into crucibles and heated at 650℃ for 10 h. The as-prepared Eu³⁺ doped materials were grounded heated at 650℃ with a reduction atmosphere for 24 h. Finally, the reduced Eu²⁺ doped samples were obtained after cooling down the furnace to room temperature naturally.

2. Materials Characterization.

A X-ray Rigaku D/Max diffractometer operating at 40 kV, 30 mA and equipped by Cu Kα radiation (λ= 1.5405 Å) was used to record the X-ray diffraction (XRD) data. A 450 W Xe lamp dispersed by 25 cm monochromator (Acton Research Corp. Pro-250) was used as a light source for excitation and emission spectra. The luminescence was dispersed by a 75 cm monochromator (Acton Research Corp. Pro-750) and observed with a photomultiplier tube (PMT) (Hamamatsu R928). The sample was cooled in a closed cycle helium cryostat and measurements were taken in the temperature range of 10 ~ 300 K. The luminescence decay curves were excited by the 355 nm-pulsed Nd-YAG laser (Spectron SL802G)and collected by means of Tektronix DPO 3054 oscilloscope. Laser Sys. SL802G)and collected by means of a 500 MHz Tektronix DPO 3054 oscilloscope.

Ⅲ. RESULT and DISCUSSION

1. Phase identification

Fig. 1(a) depicts the XRD patterns of the as-prepared NaSr(1-x)(PO₃)₃:xEu²⁺ (x = 0.1% ~ 5%) phosphors along with the standard PDF card of NaSr(PO₃)₃ (PDF# 25-0857). All the reflection peaks are indexed well with the standard NaSr(PO₃)₃ phase with a triclinic space group of P-1(2). No detectable diffraction peaks corresponding to impurity phases are observed. As shown in Fig. 1(b), the XRD peaks located at 25.2º, corresponding to the (200) plane, show slightly shift to the higher angle direction with the Eu²⁺ content increasing, indicating that the Eu²⁺ with smaller ion radio (r = 1.25 Å, CN = 8) completely substitutes the Sr²⁺ ions (r = 1.26 Å, CN = 8) in the lattice of the as-prepared phosphors.^{[23, 24]}

Fig. 1. (a) The XRD patterns of NaSr_(1-x)(PO₃)₃:xEu²⁺ (x = 0.1% ~ 5%) as a function of Eu²⁺ contents and compared with the standard XRD data of NaSr(PO₃)₃ (PDF#No.25-0857).(b) Magnified XRD patterns in the region of 25 ~ 25.5 degree for NaSr_(1-x)(PO₃)₃:xEu²⁺ as a function of Eu²⁺ contents^[25].

Fig. 2 shows the crystal structure of NaSr(PO₃)₃ in the super cell matrix with double cell X. the structure contains several (PO₃)nn^- chains with a period of tetrahedra PO₄ by corner shared, and forming long belts running zig-zags perpendicular to axis c, the Na⁺ and Sr²⁺ ions occupy the structural tunnels between the chains in the three-dimensional crystal network, coordinating to six and eight terminal oxygens, respectively.

Fig. 2. Crystal structure of NaSr(PO₃)₃

2. Optical properties of NaSr(PO₃)3:x%Eu²⁺phosphors

Fig. 3 shows the typical excitation and emission spectra of NaSr(PO₃)₃:3%Eu²⁺ phosphor at 10 K. The excitation spectrum monitored at 421 nm consists of a structureless broad band with several peak features in the wavelength region 200 ~ 400 nm with a maximum at 325 nm, which consisted of well resolved bands of the 4f⁶5d¹ multiplets of the Eu²⁺ excited states at low temperature. Therefore, the excitation band could be assigned to the d - f transition of Eu²⁺. Obviously, the excitation spectrum covers UV to blue region indicating that NaSr(PO₃)₃:3%Eu²⁺ phosphors can be used as a near UV chips excited blue-emitting phosphor. The emission spectrum under the excitation of 320 nm consist of a broad asymmetric band peaked at 425 nm in the range of 400 ~ 480 nm, which corresponds to the Eu²⁺ allowed 4f⁶5d^{1 -} 4f⁷ electronic transitions.

Fig. 3. The normalized PL (λ_ex = 320 nm) and PLE (λ_em = 421 nm) spectra of NaSr(PO₃)₃:3%Eu²⁺ at 10 K.

Fig. 4(a) shows the PL intensity of NaSr(PO₃)₃:Eu²⁺ as a function of doped Eu²⁺ concentration. As shown in Fig. 4(b), the luminescence intensity increases with increasing Eu²⁺ concentration and reaches a maximum intensity at 3 mol%. However, when the doping concentration is higher than 3 mol%, the luminescence intensity dramatically decreases due to the concentration quenching, mainly caused by the nonradiative energy transition among the identical activator Eu²⁺ ions^[18]. Generally, nonradiative transition was attributed to radiation reabsorption, exchange interaction or electric multipolar interaction^[26]. Blasse pointed out that the critical distance (R_c) between the nearby Eu²⁺ ions can be estimated by the following Eq. (1):

Fig. 4. (a) PL spectra of NaSr_(1-x)(PO₃)₃:xEu²⁺ (x = 0.1% ~ 5%) under the excitation of 355 nm; (b) The corresponding PL intensities as a function of Eu²⁺ contents. (c) Concentration dependence of lg(I/xEu²⁺) on lg(xEu²⁺)for NaSr_(1-x)(PO₃)₃:xEu²⁺ (x = 0.1% ~ 5%) phosphor^[25].

\(R_{c}=2\left(\frac{3 V}{4 \pi x_{c} N}\right)^{1 / 3}\)       (1)

Where x_c is the critical concentration, V is the volume of unit cell and Z represents the number of formula units per unit cell. Here, N = 1.5, x_c = 0.03, and V = 365.19 ų. Thus, the critical transfer distance R_c is calculated to be 24.93 Å. Generally, the exchange interaction commonly occurs in the forbidden transition, that is to say, the nonradiative transition among the Eu²⁺ ions in NaSr(PO₃)₃:3%Eu²⁺ phosphors should be controlled by electric multipolar - multipolar interaction. The interaction type between the Eu²⁺ sensitizers can be calculated by the following Eq. (2):

\(\frac{1}{x}=\frac{k}{1+\beta(x)^{\theta / 3}}\)       (2)

where I represents the emission intensity, x represents the activator concentration, K and β are constants for each type of interaction under the same measurement conditions. The value of θ is a function of multipole - multipole interaction, where θ = 6, 8, and 10 represent electric dipole - dipole, dipole - quadrupole, and quadrupole - quadrupole interactions, respectively and θ = 3 stands for the energy transfer between nearest Eu²⁺ ions^[27]. The relationships of logI/x vs logx are shown in the inset of Fig. 4(c) and the slope of the fitting line is -0.8833. Hence, the value of θ is calculated to be to 2.41 and approximately equal to 3, which means that the dominant concentration quenching mechanisms between Eu²⁺ ions occur via energy transfer between nearest Eu²⁺ ions^{[25, 28]}.

In order to investigate the thermal stability of NaSr(PO₃)₃:3%Eu²⁺, PL spectra NaSr(PO₃)₃:3%Eu²⁺ as a function of temperature are measured under the excitation of the 320 nm Xe lamp. As shown in Fig. 5(a), the integrated PL intensity maintains a stable magnitudein the range of 10 ~ 90 K as shown in Fig. 5(b). With the temperature further elevated, the PL intensity decreases sharply due to thermal quenching. The quenching temperature T50 is usually defined as the temperature at which the intensity is half of the maximum intensity.

Fig. 5. (a) Temperature-dependent PL spectra of NaSr(PO₃)₃:3%Eu²⁺under 320 nm excitation. (b) Integrated emission intensity as a function of temperature in the range of 10 ~ 300 K, the red curve represents the fitting result according to the Arrhenius-type Mott equation.

In this case, the T50 of NaSr(PO₃)₃:3%Eu²⁺ is evaluated at 165 K. In trivalent rare 4f⁷ system, the electrons in 4f orbital is shield and barely influenced by the local environment, and the dominant nonradiative relaxation path is the multiphonon relaxation.

On the other hand, the 5d electrons are unshielded in the divalent Eu²⁺ of 4f⁵d electronic shell and the optical properties of Eu²⁺ are very sensitive to the environment, in particular influenced by the temperature.

The thermal quenching mechanism can be explained by the configuration coordinate diagram as shown in Fig. 6. The red parabola and the black parabola represent the excited state and ground state, respectively. The parabola of the excited state deviates from the equilibrium position and have a crossover point C with the parabola of the ground state. Thus, the electron in the excited state can reach the ground state nonradiatively by the red vertical broken arrows as shown in Fig. 6.

Fig. 6. The schematic configuration coordinate diagram for the excited state 5d and ground state 4f. The red dash curve shows the nonradiative relaxation route.

The energy difference between lowest excited state B and the cross point C is name as activation energy ∆, and it can be obtained from the following Eq. (3):

\(I_{T}=\frac{I_{0}}{1+\mathrm{C}_{1} \exp \left(-\frac{\Delta E}{\mathrm{kT}}\right)}\)       (3)

Similarly, the temperature dependent lifetime also have the following Eq. (4):

\(\tau_{T}=\frac{\tau_{0}}{1+\mathrm{C}_{2} \exp \left(-\frac{\Delta E}{\mathrm{kT}}\right)}\)       (4)

As shown in Fig. 7(a), the decay times cannot be fitted by single-exponential function and as shown in Fig. 7(b) Calculated average lifetimes of NaSr(PO₃)₃:3%Eu²⁺ as a function of temperature in the range of 10 ~ 300K, therefore, the average decay time can be employed to calculate the lifetime by the following Eq. (5):

Fig. 7. (a) Decay curves of NaSr(PO₃)₃:3%Eu²⁺ as a function of temperature. (b) Calculated average lifetimes of NaSr(PO₃)₃:3%Eu²⁺ as a function of temperature.

\(\tau=\frac{\int_{0}^{\infty} t I(t) d t}{\int_{0}^{\infty} I(t) d t}\)       (5)

According to the above equations, the activation energies are calculated to be 806 and 763 cm^-1 for NaSr(PO³)³:3%Eu²⁺ calculated through temperature dependent luminescence intensities and decay times, respectively. Compared with other published oxides blue phosphors such as Ca₃SiO₄Cl₂:Eu²⁺ (1281 cm^-1)^[6], Ba³BP³O₁₂:0.01Eu²⁺ (3465 cm^-1)^[5], Sr₉Mg_1.5(PO₄)₇:0.06Eu²⁺ (2417 cm^-1)^[20], the obtained borate blue phosphors NaSr(PO₃)₃:3%Eu²⁺ depicts sensitive temperature dependent characters. Due to the decay measurement is barely influenced by the instrumental conditions, it is suggests that Eu²⁺-doped NaSr(PO₃)₃ is an acceptable temperature senor for thermometry by calculating the temperature dependent lifetime.

Ⅳ. CONCLUSION

Polyphosphate NaSr(PO₃)₃:Eu²⁺ doped samples were prepared via conventional high-temperaure solid state reaction. The blue-emitting NaSr(PO₃)₃:Eu²⁺ phosphors can be efficiently excited by the UV to blue light and show a narrow emission band in the range of 400 ~ 480 nm at 10 K, assigned to the allowed Eu²⁺ d - f transition. The thermal quenching properties of NaSr(PO₃)₃:Eu²⁺ phosphors were evaluated according to the temperature dependent emission spectra and decay curves. The fitting results illustrate that NaSr(PO₃)₃:Eu²⁺ phosphors depict very low thermal stability (∆ 800 cm^-1), when compared with other reported oxides blue phosphors. The sensitive temperature dependent features demonstrate that NaSr(PO₃)₃:Eu²⁺ phosphors have potential to serve as temperature sensor candidates.