Structural and optical characterization of RE (Eu²⁺, Ce³⁺) doped SrMg₂Al₆Si₉O₃₀ nanocrystalline phosphor

Abstract

This article present the reports on optical study of Eu²⁺ and Ce³⁺ doped SrMg₂Al₆Si₉O₃₀ phosphors, which has been synthesized by combustion method at 550 °C. Here SrMg₂Al₆Si₉O₃₀:Eu²⁺ emission band observed at 425 nm by keeping the excitation wavelength constant at 342 nm, whereas SrMg₂Al₆Si₉O₃₀:Ce³⁺ ions shows the broad emission band at 383 nm, under 321 nm excitation wavelength, both the emission bands are assigned due to 5d–4f transition respectively. Further, phase purity, morphology and crystallite size are confirmed by XRD, SEM and TEM analysis. However, the TGA analysis is carried out to know the amount of weight lost during the thermal processing. The CIE coordinates of SrMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor is observed at x = 0.160, y = 0.102 respectively, which may be used as a blue component for NUV-WLEDs. The critical distance of energy transfer between Ce³⁺ ions and host lattice is found to be 10.65 Å.

1 Introduction

Today, the whole world facing an problem associated with energy conservation and consumption reduction. Advanced materials is an emerging fields to solve the problem deals with energy by using a novel materials having the energy saving potential and storage capabilities. Thus, in case of applied as well as basic research fields, recently rare earth doped oxides based phosphor materials received an fast-growing interest due to their merits in high luminous efficiency, energy savings, potential, environmental friendliness, long persistence and reliability, high thermal and chemical stability [1, 2]. Among the different host used in luminescent materials aluminosilicates is an best candidates due to their physical, chemical properties and stability. There are very few work has been done on rare earth activated aluminosilicates host phosphors, because they have required high temperature for the phase formation due the combination of tetrahedral Al³⁺ and Si⁴⁺ bonding in their crystal structure. Also the mechanism of energy transfer between two different emission centres plays an important role in the development of luminescent display devices due to their theoretical and practical applications [3]. The luminescence efficiency of various rare earth ions can be enhanced by the energy transfer process from other co-doped rare earth ions; because most of the rare earth ions, have only narrow f–f transition lines from 300 to 500 nm locate at the range of near-UV, have low oscillator strength. Therefore rare earth ions have limitations in the application of UV–LED phosphors. Thus to avoid this drawback, RE ions with broad band emitting ions, such as Ce³⁺, Eu²⁺ can be co-doped as sensitizers and it help in improving the luminescence properties of the phosphors materials [4,5,6,7]. There are some well known reported aluminosilicates based phosphor such as, BaMg₂Al₆Si₉O₃₀:Eu²⁺ [8], NaAlSiO₄:Dy³⁺ [9], Sr₃Al₈SiO₁₇:Eu²⁺ [10]. Wei Lu et al. had reported SrMg₂Al₆Si₉O₃₀:Eu²⁺, Mn²⁺ phosphor synthesized by solid state reaction method [11]. In the present work we have reported optical properties of Eu²⁺ and Ce³⁺ doped SrMg₂Al₆Si₉O₃₀:phosphors, which has been synthesized here by combustion method successfully.

2 Materials and methods

A series RE (RE = Eu²⁺, Ce³⁺) doped SrMg₂Al₆Si₉O₃₀ were synthesized via combustion synthesis route at 550 °C. The precursors used for the synthesis of SrMg₂Al₆Si₉O₃₀:Eu²⁺ and SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphors were analytical grade SrNO₃, Mg(NO₃)₂, Al(NO₃)₃·9H₂O, SiO₂(A.R.), NH₂CONH₂, (NH₄)₂Ce(NO₃)₆, Eu₂O₃. Here, metal nitrates were used as oxidizers, and urea (NH₂CONH₂) was used as fuel for combustion. Using the concept of propellant chemistry, the weight of all the ingredients used above were calculated and ground together in mortal pestle to the form a pasty solution. While crushing, to make the solution more homogeneous, small amount of dilute nitric acid was added to the mixture. After mixing for about 15 min, pasty solution was transferred to the china dish and the dish was then inserted into a vertical muffle furnace preheated at 550 °C. Within 3–5 min, the exothermic reaction between metal nitrates and organic fuel complexes at low temperature takes place resulting into the formation of a flame and then highly fluffy and porous crystalline material is formed which after crushing becomes ready for the characterization. Initially the prepared phosphor is well characterized by using X-ray diffraction techniques to confirm the phase purity of the phosphor. This study was carried out at room temperature using a PAN-analytical diffractometer with Cu-K_α radiation (1.5405 Å) under operating voltage at 40 kV and current 30 mA respectively having the scan step time at 10.3377 s in a wide range of Bragg angle 10° < 2θ < 80°. The photoluminescence emission and excitation spectra were recorded using Shimadzu make RF5301PC spectrofluorometer using solid sample holder. The measurement were done by taking the same amount of sample at room temperature. The surface morphology of the sample is observed with the help of scanning electron microscopy (SEM), JEOL-6380 A under the sub micrometer resolution. The crystallites size of the phosphor particles has been studies with the help of Transmission electron microscopy (TEM) at an 200 keV accelerating voltage. Thermal analysis of the sample is studies by using TGA, to know the amount of weight loss during the thermal processing the sample using Hitachi STA7200 thermal analyser on accurately weighed SrMg₂Al₆Si₉O₃₀ sample (about 10.37 mg) in suitable air atmosphere by heating from 40 to 900 °C at a heating rate of 10 °C/min.

3 Results and discussion

The prepared phosphor was characterized by using the following techniques at standard measurement.

3.1 X-ray diffraction

Figure 1 shows, the XRD pattern of SrMg₂Al₆Si₉O₃₀ phosphor by combustion synthesis technique which has not been reported elsewhere. All of the diffraction peaks are in good agreement with the standard JPCDS file no. 83-0741 [12]. Besides that there are some unwanted impurity peaks seen in diffraction pattern which may observed due to addition of atmospheric impurity during synthesis process. But from the single most intense peak in the diffraction pattern, we can conclude that the synthesized phosphor compound has the single phase. The peaks are broadened which indicates the nanocrystallinity of the SrMg₂Al₆Si₉O₃₀ sample. SrMg₂Al₆Si₉O₃₀ crystallizes in hexagonal unit cell with space group P6/mcc and being reported lattice parameters as a = b = 10.1273 Å, c = 14.2274 Å, α = β = 90°, γ = 120°. The cell volume (V) = 1263.70 Å³ and the number of molecules in the unit cell (Z) = 2. Figure 2a gives schematic representation of crystal structure of SrMg₂Al₆Si₉O₃₀ phosphor. As shown in the Fig. 2b, c SrMg₂Al₆Si₉O₃₀ provide 12-fold coordinated Sr²⁺ site and sixfold coordinated Mg²⁺ cation site for appropriate.

Fig. 1

X-ray diffraction pattern of SrMg₂Al₆Si₉O₃₀ phosphor

Fig. 2

a Schematic representation of crystal structure of SrMg₂Al₆Si₉O₃₀ phosphor, however b and c represent the Sr²⁺ (12 coordinated) and Mg²⁺ (6-coordinated) cation sites in the host lattice

3.2 Surface morphology

The visual image of the nano particles of SrMg₂Al₆Si₉O₃₀:Ce³⁺were analyzed by SEM micrographs as shown in Fig. 3. SEM micrographs and particle size investigation of the phosphor shows that crystallinity, particle size, and surface roughness of the phosphor materials shows noticeable effect on the PL intensity. It depicts the thin plate with laminar structure. First micrograph shows a fibrous microstructure with filter paper like morphology [13, 14] with voids, pores and cracks. The particles sizes are revealing slight larger than that from the XRD data as SEM shows the lateral dimension of the particles while XRD gives the regularity in the atomic arrangement. Lateral dimension of the particles is of the order of a few micrometers, on the other hand the individual nanoparticle with regular atomic lattice varies up to a few nanometers. The smooth surface of phosphor can reduce the non-radiation and scattering, hence is beneficial to the luminescence efficiency in application. The dense packed arrangement of small particles can avert the phosphors from aging. The SEM micrographs in Fig. 3 also show some agglomeration of interlinked particles. The combustion reaction produces substantial moles of gas for each mole of solid produce. The rapid evolution of these gases break up large agglomerates and yield a porous nature phosphor powder that occupies the volume of the reaction crucible. The surface area to volume ratio of the combustion-synthesized powder is usually very large due to the large porosity between individual particles, which has appeared in the SEM micrographs [15].

Fig. 3

SEM images of SrMg₂Al₆Si₉O₃₀ phosphor

3.3 Crystallites size

Transmission electron microscopy (TEM) is a useful technique in determining crystal morphology and particle size of materials. It is difficult to measure the actual grain size distribution due to agglomeration and low magnification of present SEM micrographs hence the TEM analysis of the phosphor has been carried out and the obtained results are presented below. The transmission electron micrograph (TEM) of SrMg₂Al₆Si₉O₃₀ were shown in Fig. 4. From the micrographs, the average crystallite size was observed to be 20 nm. The crystallites are randomly distributed and looks like core structure as those reported in oxides based nanomaterials.

Fig. 4

TEM images of SrMg₂Al₆Si₉O₃₀ phosphor

Here we have also estimated the crystallite from observed diffraction data by Debye Scherer’s equation, which is comes out to be approximately 28 nm.

3.4 TGA

Thermal analysis is mostly carried out to investigate phase changes, decomposition, and loss of water or oxygen and to construct related phase diagrams. In thermogravimetry (TG), loss of crystallization water or volatiles such as oxygen and CO₂ is given away by a weight loss. In differential thermal analysis (DTA) a phase change is generally related with either absorption or evolution of heat. Usually in DTA, exothermic reactions give an increase in temperature, and endothermic reaction leads to a decrease in temperature [16]. The results from thermo gravimetric and differential thermal analyses are generally reported in the form of curves describing the mass lost and temperature difference respectively from the sample material against temperature. The temperatures at which the thermal processes start on and ended are graphically demonstrated as below.

The typical TG/DTA curves (green line/blue line) recorded for SrMg₂Al₆Si₉O₃₀ matrix in air environment have been depicted in the Fig. 5. Thermogravimetry (TG) and differential thermal analysis (DTA) curves of the SrMg₂Al₆Si₉O₃₀ phosphor were recorded in the temperature range 40–900 °C. It shows basically four mass loss steps in the temperature region of 40–900 °C about at 49.8, 389.7, 587.2 and 877.2 °C. The thermal decomposition temperature T_dec (The temperature when the sample loses 10% of its initial weight) is measured to be 587.2 °C The continuous nature of TG curve shows that there is no phase change during the rise of temperature up to 900 °C [17]. The TG curves show a sharp drop down up to 500 °C because of the weight loss of the sample due to elimination of water and the auto combustion reaction. This fall becomes lower between 500 and 600 °C and after 600 °C it remains constant with no significant weight loss. Due to the evaporation of water, there is weight loss below 500 °C. The DTA curve shows that the nanoparticles has curve below zero level meaning endothermic decrease Endothermic peaks at 90°, 550° are seen in the DTA curves which corresponds to the mass loss due to the release of H₂O in the TG curve while exothermic peak at 600 °C is attributable to the phase formation or crystallization of SrMg₂Al₆Si₉O₃₀ phases.

Fig. 5

TG/DTA curves of the SrMg₂Al₆Si₉O₃₀ phosphors

3.5 Luminescence

3.5.1 SMASO:Eu²⁺phosphor

Figure 6 shows, the excitation spectra of the SrMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor recorded at λ_em = 425 nm. It shows a broad band from 220 to 400 nm. The band ranging from 220 to 300 nm is weaker as compared to that in the region from 300 to 400 nm. Application point of view this character of the phosphor is very useful. The excitation band centred at 342 nm is due to the 4f⁷–4f⁶5d¹ transition of Eu²⁺ ions. Monitored under the λ_ex = 342 nm, the emission spectra of the SrMg₂Al₆Si₉O₃₀:Eu²⁺ is as shown in Fig. 7. It exhibits a broad emission band ranging from 380 to 550 nm with a shoulder around 390 nm which is the emission of the xenon lamp itself and an emission peak around 425 nm demonstrating blue luminescence, corresponding to the characteristic 4f⁶5d¹–4f⁷ transition of the Eu²⁺ ions. The particular emission of Eu³⁺ in the spectra is not visible demonstrating that the Eu³⁺ has been completely reduced to Eu²⁺. It is seen that, the luminescence intensity increases gradually with the increase in the concentration of Eu²⁺ ions and becomes maximum at 1 mol%. The emission band appears to be almost symmetric indicating that Eu²⁺ ions occupy only one type (either of 12-fold coordinated Sr²⁺ site or sixfold coordinated Mg²⁺ site) of site in the SrMg₂Al₆Si₉O₃₀ matrix. The ionic radii of Eu²⁺ is 1.17 Å for CN = 6 and 1.26 Å for CN = 12, Mg²⁺ is 0.72 Å for CN = 6 and that of Sr²⁺ is 1.44 Å for CN = 12. Thus, it is closer to Sr²⁺ for 12 CN, hence Eu²⁺ will occupy Sr²⁺ site [11]. The chromaticity diagram is a tool to specify how the human eye will experience light with a given spectrum, which could naturally illumine the changes of the emission colour of the phosphor. Figure 8 shows the 1931 CIE chromaticity diagram for SrMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor at λ_ex = 342 nm for 1 mol% of Eu²⁺ concentration. The colour coordinates x = 0.160, y = 0.102 for the blue emitting SrMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor which may be a promising blue emitting phosphor for w-LEDs application.

Fig. 6

Excitation curve of SrMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor (λ_em = 425 nm)

Fig. 7

Emission curve of SrMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor (λ_ex = 342 nm), where Eu²⁺ varies from 0.05 to 1 mol%

Fig. 8

CIE chromaticity diagram of SrMg₂Al₆Si₉O₃₀:Eu²⁺phosphor

3.5.2 SMASO:Ce³⁺ phosphor

The excitation spectrum of SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor monitored at λ_em = 383 nm is as shown in the Fig. 9 due to parity allowed 4f–5d transitions in Ce³⁺ ions. The energy of the electron in the 5d excited state is strongly influenced by the crystal field. Hence the 5d energy level splits into several energy sub-levels as shown in Fig. 10. The excitation spectrum is fitted well with five Gaussian broad bands peaking at 236, 249, 304, 318 and 324 nm. The Fig. 11, shows the emission spectrum of the SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor under the UV excitation of 321 nm from the xenon lamp. It shows a broad band with a peak at 383 nm. For SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor, Stokes shift of is found to be approximately equal to 5043 cm⁻¹ which is in agreement to the values stated in the literature. As observed from Fig. 12, the emission spectrum observed in isolated broad band, hence it is well fitted with two emission centres peaking at 372 and 406 nm, respectively. The intensity of the band at 372 nm is remarkably higher than the one at 406 nm. Ce³⁺ has only one electron in the 4f shell. The ground state of Ce³⁺ is split into ²F_7/2 and ²F_5/2 (as shown in Fig.) with an energy difference of approximately 2200 cm⁻¹ [18]. As shown in Fig. 13, the energy difference between ²F_7/2 and ²F_5/2 is found to be approximately 2251 cm⁻¹, which is in good agreement with the literature data. The asymmetric nature of the emission spectrum is consistent with prior research on the luminescence of Ce³⁺ in other hosts, such as Y₃Al₅O₁₂ (YAG) [19]. Figure 13 exhibits the emission spectra of SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor samples with different doping concentration. With the increase of Ce³⁺ ions concentration, the intensity of Ce³⁺ emission is increased initially till it reaches the maximum at 1 mol%, then gradually decreases due to the internal concentration quenching mechanism as shown in Fig. 14. In general, the concentration quenching mechanism is associated to energy transfer from one activator to another. The distance between the Ce³⁺ luminescent centers decreases with increase in the concentration of Ce³⁺ ions which marks more chance of nonradiative energy transfer between Ce³⁺ ions. The critical distance (R_C) for energy transfer between the Ce³⁺ ions [20], can be given by the equation:

Fig. 9

PL excitation spectra of SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor (λ_em = 383 nm)

Fig. 10

Gaussian fit PLE spectra of SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor (λ_em = 383 nm)

Fig. 11

PL emission spectra of SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor (λ_ex = 321 nm)

Fig. 12

Gaussian fit PL emission band of SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor

Fig. 13

Concentration quenching of SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor

Fig. 14

The plot of log(I/x) versus log(x) SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphors

$${R_C} \approx 2{\left( {\frac{{3{\text{V}}}}{{4\pi {{\text{x}}_{\text{c}}}N}}} \right)^{\frac{1}{3}}}$$

(1)

where V represents the volume of the unit cell, x_c represents the critical concentration of the activator ions and N represents the number of cations in the unit cell. For SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor, parameters are V = 1263.70 Å³, N = 2 and x_c = 1 Therefore, the R_C of SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor is estimated to be about 10.65 Å. As per Van Uitert [21] the energy transfer involves either multipolar interaction, radiation reabsorption or exchange interaction. As per Dexter [22] exchange interaction will come into play if the critical distance of energy transfer is < 5 Å. In SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor the R_c is found to be 10.65 Å > 5 Å, hence energy transfer by exchange interaction will not take place. If there exists considerable overlap between the luminescent spectra of the sensitizer and activator, radiation re absorption will come as outcome. There is no overlap between the excitation and emission spectra of SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor, hence no chance of energy transfer via radiation reabsorption. Thus, in SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor multipolar interaction is accountable for energy transfer. According to Van Uitert’s [23, 24] the correlation between the luminescent intensity and their corresponding doping concentrations of the luminescent centre can be mathematically given by the following equation,

$$\frac{1}{x}={\text{K}}{\left[ {1+\beta ({\text{x}})\frac{\theta }{3}} \right]^{ - 1}}$$

(2)

where I is the emission intensity, x is critical concentration, K and b are constants, θ is a constant of the multipolar interaction between rare earth ions and equals to 6, 8 or 10 for electric dipole–dipole (d–d), electric dipole–quadrupole (d–q) or electric quadrupole – quadrupole (q–q) interactions, respectively. In order to calculate θ, log(I/x) versus log(x) is plotted, which results in a straight line with a slope equal to −θ/3. As shown in Fig. 11, the slope is −1.41 there by θ can be calculated as 4.23, which is close to 6. Thus, the quenching in SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphor results from dipole–dipole interaction.

4 Summary

In the present work, Combustion synthesis technique has found to be successful methodology to synthesize a series of SrMg₂Al₆Si₉O₃₀:Ce³⁺ and SrMg₂Al₆Si₉O₃₀:Eu²⁺ nanophosphors at 600 °C. XRD pattern confirms the formation of pure phase at low temperature. Micro and nanoscale structure of phosphor is confirmed by SEM and TEM analysis the average crystallite size is found to be about 20 nm. TGA investigation shows the crystallization of SrMg₂Al₆Si₉O₃₀ phases at 600 °C. Optical properties of SrMg₂Al₆Si₉O₃₀:Ce³⁺ phosphors show a broad emission band from 350 to 500 nm, centred at 383 nm under 321 nm excitation. The optimal concentration has found to be 1 mol%. The critical distance of energy transfer between Ce³⁺ ions has calculated as 10.65 Å. The concentration quenching mechanism has been explained by the electric dipole interaction of the Ce³⁺ ions. Whereas Eu²⁺ emission in SrMg₂Al₆Si₉O₃₀ phosphor is observed at 425 nm under 342 nm excitation. The optimal concentration has found to be 1 mol%. Thus blue is an basic and most important colour component for the production of white light and there are only few rare earth available in periodic table that may gives an strong blue emission. Further, the SrMg₂Al₆Si₉O₃₀:Eu²⁺ phosphor showed excellent CIE chromaticity co-ordinates (x = 0.160, y = 0.102) in the blue region, as a result it would be quite useful as a blue phosphor for SSL technology.