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Graphical Abstract: The crystal structure, band structure, photoluminescence and cathodoluminescence properties of SrSiAl2O3N2: Eu2+ phosphor. 254x231mm (300 x 300 DPI)
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DOI: 10.1039/C5DT00800J
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Xicheng Wang,a Zhengyan Zhao,a Quansheng Wu,a Yanyan Li,a Chuang Wang,a Aijun Mao,a and Yuhua Wang.a,* 5
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Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A series of SrSiAl2O3N2: Eu2+ (0.005≤x≤0.05) phosphors were successfully synthesized through a pressureless, facile, and efficient solid state route. The crystal structure, band structure, and their photoluminescence or cathodoluminescence properties were investigated in detail. The phosphors exhibit rods shape morphology with a uniform Eu2+ distribution. Under n-UV excitation the emission spectra shift from 477 to 497nm with an increase of Eu2+ concentration. The concentration quenching mechanism of Eu2+ emission was dominated by the dipole-dipole interaction. The thermal stability is comparable to that of the commercial Ba2SiO4:Eu2+ phosphor. The phosphor also exhibits high current saturation, high resistance under low voltage electron bombardment. All the results indicate that the SrSiAl2O3N2: Eu2+ can be considered as candidates applied for both white LEDs and FEDs.
1Introduction
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Following an increasing awareness of climate changes and environmental issues, people are looking for new technologies to reduce energy consumption and provide eco-friendly illumination and display.1, 2 In recent years the solid-state lighting technology based on white light-emitting diodes (WLEDs), which is considered as energysaving and environmentally friendly lighting sources, has attracted considerable attention from both academic and industrial communities.3-5 The most widespread commercial strategy to achieve WLED is the combination of a InGaN LED chip and Ce3+-doped Y3Al5O12(YAG : Ce3+) yellow phosphor,6 however, this type of white light exhibits a poor color rending index (Ra< 75) because of the color deficiency in the red and blue-green spectral range.7 An alternative approach to generate white light is the combination of an near ultraviolet (n-UV) LED with red, green, and blue (RGB) phosphors.7 Accordingly, it is urgent to develop new phosphors that can be effectively excited in the n-UV range.8 As far as displays concerned, being the most promising next generation flat panel displays, field emission display (FED) has attracted much attention due to its potential to provide displays with thin panels, self-emission, wide viewing, quick response time, high brightness, high contrast ratio, light weight, and low power consumption.9-11 To realize full-color FEDs, it is necessary to develop good blue and green phosphors with high luminance, high efficiency, high saturation current densities, long time under low electron acceleration voltages (≤10 kV).12, 13Therefore, the Development of new high performance phosphor is an important issue for the FED as well. During the past decade, (oxo)nitridosilicates have demonstrated remarkable potential capacity as host materials for phosphors due This journal is © The Royal Society of Chemistry [year]
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to their outstanding thermal and chemical stability and excellent luminescence properties when activated with Eu2+ ,e.g. α/βSiAlONs,14-16M2Si5N8 (M=Ca,Sr,Ba),17, 18 CaAlSiN3,19, 20 MYSi4N7 (M = Sr, Ba),21, 22 MSiN2,23, 24 MSi2O2N2 (M = Ca, Sr, Ba)25, 26 and so on.27-29 These phosphors are with a broad emission range from green to red and some have been put into practical use, however, the Eu2+ doped (oxo)nitridosilicate phosphors emitting blue to green light are still few, which is also indispensable to full-color emission for both WLEDs and FED displays. The oxonitridosilicate phase SrSiAl2O3N2 has been studied for many years. The synthesis and crystal structure of the compound was first reported by Schnick et al.30 Xie et al later reported the photoluminescence properties of Eu2+ in this host.31 Recently, Liu’ s group investigated the energy transfer process of Ce3+/Eu2+ in the SrSiAl2O3N2 and its Ba-containing solid solutions in detail.32 But as for the synthesis method in above previous studies, either the raw material used such as Si(NH)2 or metal nitrides was labile, which need to be handled in a glove box, or the preparation must be under high pressure. So it is meaningful to find another simple and suitable synthesis method for mass production. And to the best of our knowledge, there is no report on the cathodoluminescence (CL) properties of the SrSiAl2O3N2. In consideration of the superior stability of oxonitridosilicate phosphors compared to sulfide and oxide phosphors, it is significant to study the CL properties of the blue-green emitting phosphor and explore its potential used in FEDs. In the present work, we have developed a pressureless, facile, and efficient route through the solid state reaction for the synthesis of the SrSiAl2O3N2 phosphor using the cheap and stable raw material (strontium carbonate and silicon nitride) with flux. The [journal], [year], [vol], 00–00 |1
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Synthesis, structure, and luminescence properties of SrSiAl2O3N2: Eu2+ phosphor for light-emitting devices and field emission displays
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structure, morphology, PL and CL properties were investigated in detail. The motivation of the study is not only to probe into the mechanism of structural and luminescence properties but also to explore its application in WLEDs and FED displays.
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Crystal structure. The Rietveld analysis for SrSiAl2O3N2 was carried out using the structure of synthesized by the silicon diimide as the starting model.30
2.Experimental Preparation. The SrSiAl2O3N2: Eu2+samples were prepared by solid state reaction. Briefly, the constituent raw materials SrCO3 (A.R.), Si3N4 (A.R.), AlN (A.R.), and Eu2O3 (99.999%) were weighed in stoichiometric proportions, 3wt% AlF3 was introduced as the flux. The raw materials were finely ground, and then sintered in boron nitride (BN) crucibles at 1400oC for 5h under a flowing gas of N2-NH3 in a tube furnace. The products were then cooled down to room temperature in the furnace, ground, and pulverized for further measurements. Characterization. The phase purity of samples was analyzed by X-ray diffraction (XRD) using a Bruker D2 PHASER X-ray Diffractometer with graphite monochromator using Cu Kα radiation (λ = 1.54056 Å), operating at 30 kV and 15 mA. Rietveld refinement33 was performed using the software GSAS.34,
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Figure 1.Rietveld refinement and crystal structure of SrSiAl2O3N2 host
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The calculations of the electronic structure for SrSiAl2O3N2were carried out with density functional theory (DFT) and performed with the Cambridge Serial Total Energy Package (CASTEP) code.36 The exchange-correlation functional generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) was chosen for the theoretical basis of density function.37, 38 There are two steps for the calculation. First, the crystallographic data refined from the XRD data were used to optimize its crystal structure. The second step was to calculate its band structure and density of states (DOS) for the optimized structure. The morphology of the sample was examined using scanning electron microscopy (SEM; Hitachi S-4800). The element composition and high-resolution transmission electron microscopy (HRTEM) were obtained on an FEI Tecnai F30transmission electron microscopy (TEM, FEI Tecnai F30, operated at 300 kV) equipped with an energy dispersive X-ray spectroscopy (EDX). Reflectance spectra were measured on PE lambda950 UV-vis spectrophotometer using the BaSiO4 white power as the reference. The photoluminescence (PL), photoluminescence excitation (PLE) spectra were recorded at room temperature using a FLS-920T fluorescence spectrophotometer (Edinburgh Instruments) equipped with a 450W Xe light source and double excitation monochromators. The PL decay curves were measured using an FLS-920T fluorescence spectrophotometer with an F900 nanosecond flash hydrogen lamp as the light source. High temperature luminescence intensity measurements were carried out using an aluminum plaque with cartridge heaters; the temperature was measured by thermocouples inside the plaque and controlled with a standard TAP-02 high temperature fluorescence controller (Orient KOJI instrument Co., Ltd.). The quantum efficiency (QE) was measured using a Fluorolog-3 spectrofluorometer equipped with a 450W xenon lamp (Horiba Jobin Yvon). The CL properties of the samples were obtained using a modified MpMicro-S instrument (Horiba Jobin Yvon).
3. Results and discussion
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Figure 1 illustrates the experimental and calculated XRD profiles as well as their differences for the Rietveld refinement of SrSiAl2O3N2 at room temperature. The results of Rietveld refinement indicate that no any impurity was observed and the synthesis method was effective. The SrSiAl2O3N2 crystallizes as an orthorhombic structure with the space group P2 12121. Lattice parameters were determined to be a = 4.9489(2) Å, b = 7.9698(3) Å, c = 11.3435(4) Å, α = β = γ = 90°, V = 447.40(3) Å3, Z = 4 and the refinement converged to satisfy the reflection condition well with the residual factors Rwp = 8.87% and Rp = 6.26% (see Table 1). The atomic coordinates, isotropic displacement parameters, and site occupancy factors for the SrSiAl2O3N2 host are listed in Table 2. Table 1. Crystallographic data for SrSiAl2O3N2
Formula Crystal system Space group Lattice parameters a(Å) b(Å) c(Å) α° β° γ° Cell volume (Å3) Z Calculated Density R-factors Rwp Rp
SrSiAl2O3N2 orthorhombic P212121 (No.19) 4.9489(2) b=7.9698(3) c=11.3435(4) 90 90 90 447.40(3) 4 3.64715 g/cm3 0.0887 0.0626
The crystal structure of SrSiAl2O3N2 along [100] direction are also shown in the Figure 1 inset. Three kinds of corner-sharing tetrahedral including AlO3N, AlO2N2 (aqua) and SiON3 (rose)
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Table 2. Atomic coordinates and isotropic displacement parameters for SrSiAl2O3N2 Atom Wyckoff S.O.F
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y
Sr
4a
1
-0.0070(8)
0.95890(24)
0.66501(22)
z
0.0314(11)
Uiso
Al1
4a
1
0.0322(27)
0.8385(8)
0.3397(8)
0.0131(23)
Al2
4a
1
0.0427(20)
0.5310(8)
0.5445(5)
0.0049(25)
Si1
4a
1
0.0075(24)
1.2010(8)
0.4190(6)
0.0178(25)
O1
4a
1
-0.045(5)
0.6429(18)
0.4232(13)
0.114(8)
O2
4a
1
0.1883(20)
0.6254(12)
0.6550(13)
0.004(5)
O3
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1
0.079(7)
1.2934(24)
0.2984(19)
0.219(13)
N1
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1
0.1715(24)
1.0376(17)
0.4255(11)
0.037(4)
N2
4a
1
0.169(4)
1.3296(23)
0.5122(18)
0.043(9)
Band structure. The band structure of SrSiAl2O3N2 was calculated using density function theory (DFT) methods based on the crystal structure refinement. As shown in Figure 2, this compound possesses a direct band gap of 3.83eV with the valence band (VB) maximum and the conduction band (CB) minimum at the G point of the Brillouin zone.
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Figure 3. Total and partial density of states of SrSiAl2O3N2 Figure 2. Band structure of SrSiAl2O3N2 15
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Figure 3 shows the partial density of states (DOS) for the Sr, Si, Al, N and O atoms, as well as total density of states for the host of SrSiAl2O3N2. The valence band is mainly composed by N2p and O 2p orbitals, The N 2p orbital contribute to the top of the valence band more than the O 2p orbital does, while the bottom of the conduction band is dominated by the Sr3d orbital. The calculation demonstrates that SrSiAl2O3N2 is a favourable host material. On the one hand it provides a suitable band gap for Eu2+ to act as the emission center; on the other hand it is propitious to luminescence since the transition probability of the direct band
gap is higher than that of the indirect band gap due to no phonons involved in the transition process.39
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form a highly condensed network. The Sr atoms (lime balls) are located in the channels of the network and they are coordinated by three nitrides and six oxides.
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Figure 4. Diffuse reflection spectra of undoped SrSiAl2O3N2 and Sr0.98Eu0.02SiAl2O3N2 samples. (Inset) Relationship between the absorption coefficient and the photon energy for SrSiAl2O3N2 5
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Figure 4 depicts the diffuse reflectance spectra (DRS) of the undopedSrSiAl2O3N2 and Eu2+doped SrSiAl2O3N2.Clearly, the undoped SrSiAl2O3N2 shows a remarkable drop in reflection around 270 nm, which corresponding to the band transition of the host lattice. The intense reflection in the visible spectral range agrees well with the observed white body color of the host sample. The Eu2+ doped SrSiAl2O3N2 shows a wide absorption range with the maximum at about 400nm, which indicates that the doped Eu2+ ions create localized energy levels within the band gap of the SrSiAl2O3N2 host.40 The band gap (Eg) is estimated according to the Equation as following: (αhν)n = A(hν - Eg) (1) Where hν is the incident photon energy, α is the absorption coefficient, and A is a constant. The value of n depends on the type of inter band transition: n = 2 for a direct transition and n = 1/2 for an indirect transition.41 The absorption coefficient α can be obtained via the Kubelka–Munk function:
=
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Figure 5. (a) SEM image, (b) TEM image, (c) HRTEM image and EDX spectrum of SrSiAl2O3N2: 0.02Eu2+
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show that well-crystallized SrSiAl2O3N2:Eu2+ powders have been obtained, which further confirms that the convenient synthesis method is also efficient.
( 1- R)2 2 R (2)
R is the observed reflectance in the DRS. The value of (αhν) 2 is plotted against the incident energy hν in the inset of Figure 4. By extrapolating the linear portion to the photon energy axis,42 the value of Eg was determined to be about 4.47 eV. This is consistent with the value of 3.83 eV obtained from DFT calculation in view of the generally accepted fact that the calculated band gap is somewhat underestimated.43 Morphology of the SrSiAl2O3N2: Eu2+ phosphor. Figure 5(a) shows the typical SEM imagine of the SrSiAl2O3N2: 0.02Eu2+ sample. The particles exhibit rods shape morphology with the size ranging from 5 to 10μm. This is also in line with the typical low-magnification as shown in Figure 5(b). The corresponding EDX spectrum analysis (Figure 5(c)) indicates that the product has a chemical composition of Sr, Si, Al, O, N, and no impurity element exists. The inter-planar spacing was measured to be 0.2315 nm (Figure 5(d)), which matches well with the (211) inter planar distances of the monoclinic SrSiAl2O3N2. These results
Figure 6. Excitation and emission spectra of SrSiAl2O3N2: xEu2+ with varying Eu2+ concentration 50
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PL properties. Figure 6 illustrates the excitation and emission spectra of SrSiAl2O3N2: xEu2+ with varying Eu2+ concentrations. The excitation spectrum exhibits a remarkable broad band between 250 and 450 nm, which is attributed to the 4f7→4f65d1 transition of the Eu2+ ions. It also consists of three bands peaking at about 294, 351 and 395nm, which correspond to the transitions of Eu2+ from the ground state to its field-splitting levels of the 5d state.44 The emission spectra of the SrSiAl2O3N2: xEu2+ (0.005≤x≤0.05) phosphors show broad, symmetric bands in the range of 400-600 nm, which corresponds to the allowed 4f65d1→4f7 electronic transitions of Eu2+. As the Eu2+ concentration increases from x =
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0.005 to x = 0.05, the full width at half-maximum (FWHM) of the SrSiAl2O3N2: xEu2+ emission spectra remain the same at about 88 nm, while the emission band shifts toward longer wavelength from 475 to 497 nm. This red-shift phenomenon could be explained in terms of the energy transfer of the Eu2+ ions from the higher 5d levels to those at the lower levels. This causes a decrease in the emission energy from 5d excited state to the 4f ground state and therefore the emission shifts to longer wavelength.45
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2+
Figure 7 Decay curves of Eu emission in SrSiAl2O3N2: xEu .
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To further understand the process of energy transfer, the decay curves for SrSiAl2O3N2: xEu2+ (x = 0.01, 0.02, 0.03,0.04 and 0.05) were measured and depicted in Figure 7. These decay curves are plotted as a semi-logarithmic plot and can be well fitted into a single exponential equation,46
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𝐼(𝑇) = 𝐴1 exp(−𝑡/𝜏)
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(3)
where I and A1 correspond to the luminescence intensity at time t, and τ is the lifetime. The lifetime of Eu2+ is shortened with increasing Eu2+ concentration. The values decreased from 0.578 to 0.548 μs with the Eu2+ concentration increased from x = 0.01 to x = 0.05. As the Eu2+ concentration increasing, the distance of
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Figure 8. CIE chromaticity diagram for SrSiAl2O3N2: xEu2+ phosphors with different Eu2+ dopant concentrations. The inset shows these phosphors under 365nm excitation in a UV box.
the luminescence centers (Eu-Eu) decrease, then the possibility of energy transfer increases and thus leads to a faster decay. This further indicates that the red-shift is mainly caused by the energy transfer processes, and the decay lifetimes are also suitable for solid-state lighting when used as WLED phosphors.25 The Stokes shifts of SrSiAl2O3N2: xEu2+ (0.005≤x≤0.05), roughly estimated from the maxima in the excitation and emission spectra, are in the range of 7276-8107 cm-1, as shown in Table 3. The CIE chromaticity coordinates of the SrSiAl2O3N2: xEu2+ phosphors with different Eu2+concentration are summarized in Table 3 and also shown in Fig. 8. It is clearly observed that the emission hue of the SrSiAl2O3N2: xEu2+ phosphors changed from cyan to blue-green as the x value increased. The chromaticity index varies from (0.25, 0.50) for the concentration with 0.5% to (0.42, 0.52) for the concentration with 5%.
Table 3.Excitation, Emission bands, stokes shifts, Normalized PL Intensity and the CIE Coordinates forSrSiAl2O3N2: xEu2+
Eu2+ concentration
FHWM of
λem
Stokes shifts
Normalized PL
emission (nm)
(nm)
(cm-1)
Intensity (%)
x = 0.005
85
475
8107
74.87
(0.1820,0.2638)
x = 0.01
86
477
7937
85.24
(0.1854,0.2879)
x = 0.02
88
484
7828
100
(0.1974,0.3387)
x = 0.03
88
489
7798
90.11
(0.2045,0.3661)
x = 0.04
88
493
7727
79.25
(0.2084,0.3795)
x = 0.05
89
497
7276
75.32
(0.2207,0.4154)
CIE (x,y)
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Figure 9. Emission intensity of SrSiAl2O3N2: xEu2+ phosphors as a function of Eu2+ concentration. Inset: log(I/x) dependence of log(x)
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Figure 9 shows the PL intensity of SrSiAl2O3N2: xEu2+ phosphors with various Eu2+ concentrations under 365nm excitation. The optimal doping concentration was observed at x = 0.02, the concentration quenching was observed when the content of Eu2+ exceeds 2mol%. The internal quantum efficiency of SrSiAl2O3N2: 0.02Eu2+ was measured to be 40.2%. The concentration quenching is due to the nonradiative energy transfer between Eu2+ ions, the possibility of which increases as the concentration of Eu2+ increases. Blasse47 suggested that the critical distance (Rc) of energy transfer can also be calculated by the critical concentration of the activator ion:
3V Rc=2 4xc Z
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Figure 10. (a) The emission spectra of theSrSiAl2O3N2:0.02Eu2+ sample under different excitation wavelengths and (b) TRPL spectra of the SrSiAl2O3N2:0.02Eu2+ sample.
(4)
Where xc is the critical concentration, Z is the number of formula units per unit cell, and V is the volume of the unit cell. For SrSiAl2O3N2 host, when Z = 4, xc = 0.02, and V = 447.40(3) Å3, the obtained Rc value is 22.02Å, which is similar to the Rc value for Eu2+centers in several oxide and apatite lattices.48 It is worth noting that this value is much further than the distance of the nearest neighbor Sr2+ ions (4.48 Å) according to the crystal structure results. So it is reasonable that the concentration quenching occurs at such a relative low Eu2+ concentration. Energy transfer is generally associated with multipolar interactions, radiation reabsorption or exchange. The radiation reabsorption requires a large overlap of the wave functions of the donor and the acceptor, and the critical distance for the exchange interaction is approximately 5Å. Based on the calculated Rc value and small overlap of the emission spectra and the excitation spectra, it seems that the electric multipolar interactions are the most relevant to process of energy transfer.49 This journal is © The Royal Society of Chemistry [year]
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The interaction mechanism between Eu2+ ions can be expressed by the following equation:50 𝐼
= 𝑥
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50
𝑘 𝜃
(5)
1+𝛽(𝑥) 3
Where k and β are constants for each type of interaction for a given host lattice and x is the activator concentration. According to results published by Van Uitert, θ = 6, 8 , 10 corresponds to dipole-dipole, dipole-quadrupole, quadrapole-quadrapole interactions, respectively.51 The inset of Figure 8 illustrates the(I/x) dependence on x on a logarithmic scale. The dependence of log (I/x)on log (x) was found to be relatively linear, and the slope was determined to be -1.016. The value of θ was found to be most close to 6, indicating that the concentration quenching mechanism of Eu2+ emission was dominated by the dipole-dipole
[journal], [year], [vol], 00–00 |2
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1 A exp( .
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Figure 11. Temperature-dependent luminescent spectra of theSrSiAl2O3N2:0.02Eu2+ from 25 to 250 oC
The thermal stability of phosphors is one of the important parameters for the application of LEDs. The thermal quenching properties of the SrSiAl2O3N2:0.02Eu2+ sample were also investigated. Figure 11 presents the temperature-dependent luminescent spectra of SrSiAl2O3N2:0.02Eu2+ with 365nm excitation. When the temperature increased from 25 oC to 250 oC, the PL intensities decrease while the peak positions almost keep the same. As displayed in top-right corner of the Figure 10, the SrSiAl2O3N2:0.02Eu2+ exhibits higher thermal stability than commercial Ba2SiO4:Eu2+ phosphor, especially when the temperature is above 150 oC. The intensity at 150 oC remains 56% of the initial intensity, while the one of the BaSiO4 decrease to the 49%. The activation energy for the thermal quenching was estimated by the equation as follows:54
I0
I (T )
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E ) K BT
(6)
Where I0 is the initial intensity, I(T) is the intensity at a given temperature T, A is a constant, ΔE is the activation energy for thermal quenching, and KB is the Boltzmann’s constant. As shown in the top-left corner of Figure 10, ΔE for SrSiAl2O3N2:0.02Eu2+ is 0.26 eV, which is similar to that of the (Ca, Sr)YSi4N7:Eu2+ phosphor in our previous study,55 indicating its good thermal stability.
Figure 12. (a) SEM images of theSrSiAl2O3N2:0.02Eu2+ sample and (b) monochromatic CL image at 484 nm of the same area.
Cathodoluminescence. As is well known, CL has the advantages of a much higher lateral and depth resolution in comparison with PL, which makes CL as a unique tool for luminescence mapping, so CL monochromatic images were used to investigate the Eu2+ distribution in the structures.16 Figure 12 shows the SEM image of the SrSiAl2O3N2:0.02Eu2+ sample and the corresponding CLmapping image at the same area. According to as-mentioned result of PL and the CL discussed as following, the monochromatic CL mappings of SrSiAl2O3N2:0.02Eu2+ phosphor is collected at wavelength of 484nm. It can be observed that Eu 2+ ions distribute uniformly in the rod-shaped particles except mirror agglomerated locations. The strong luminescence obtained from the microstructures indicates that the Eu2+ ions have been effectively incorporated into samples with a uniform distribution. Figure 13(a) shows CL intensity of SrSiAl2O3N2:0.02Eu2+ phosphor measured under different accelerating voltages when the probe current fixed at 50mA. The inset shows the CL intensities as a function of accelerating voltage. The intensity gradually increases as the applied voltages changes from 2 to 8kV. Similarly, under a 5kV electron beam excitation, the CL intensity also increases with increasing probe current from 40 to 100mA (Figure13 (b)). The increase in CL brightness with the increase in electron energy and probe current are attributed to the deeper penetration of the electrons into the phosphor body and the larger electron-beam current density.11 To find the physical origin that the CL intensity increases with the electron energy, we make the following analysis. We calculate the electron penetration depth for SrSiAl2O3N2:0.02Eu2+ using the following empirical Eq : A
E n
1.2
L =( ρ ) ( ) ,n = 1−0.29 lg Z (7) √Z
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Where A is the atomic or molecular weight of the material, ρ is the density, Z is the atomic number or the number of the electrons per molecule in the compounds, and E is the excitation voltage in unite of kV.56 For SrSiAl2O3N2: Eu2+, A = 245.6803, Z =4 , ρ =
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interaction.52 Additionally, when varying the excitation wavelength from 300 to 410 nm, there is no significant change in the emission spectra except the emission intensity. The spectra profiles are nearly the same with the peaks wavelength located at 484 nm(see Figure 10(a)), indicating the presence of single emission center of the Eu2+ in the SrSiAl2O3N2 host lattice. The intensities changes little. This fully proves that the phosphor can be efficiently excited over the broad range from 300-410 nm. In order to confirm that the broad band emission originates from single luminescent center, a series of lifetime decay curves were recorded by monitoring the SrSiAl2O3N2:0.02Eu2+ sample in different wavelengths from 400 to 600 nm at 5 nm interval, and the Time-Resolved PL (TRPL) spectra shown in Figure 10(b) were obtained by slicing them. From 21.97 ns to 634.76 ns, the shape and position of the emission spectra almost keep still. This further proves that there is single luminescent centre.53 This result agrees well with the same emission spectra under different excitations. This also indicates that Eu2+ ions in SrSiAl2O3N2 lattice occupy only one kind of crystallographic site Sr2+ site, which consists with the crystal structure of SrSiAl2O3N2, where only one Sr2+ ion is available for the substitution by Eu2+ ions.
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DOI: 10.1039/C5DT00800J
Figure 14. CL intensity and chromaticity coordinate decay of SrSiAl2O3N2: 0.02Eu2+ phosphor with the electron beam bombardment time (min)
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Figure.13 The CL spectra of SrSiAl2O3N2:0.02Eu2+ phosphor as (a) a function of probe current under 5 kV electron-beam excitation and (b) a function of voltage under 60 mA filament current. (The insets show the dependence of emission intensity on filament current and voltage, respectively.)
3.64715 g/cm3, thus the estimated electron depths at 2kV, 4kV, 6kV and 8kV are 1.01 nm, 5.71 nm, 15.67 nm and 32.11 nm, respectively. Hence, the increase in emission intensity with increasing applied voltage is due to an increase in the number of excited luminescence centers resulting from an increase in electron penetration depth. As for the cathodoluminescence, the Eu2+ ions are excited by the plasmons produced by the incident electrons. The deeper the electron penetration depth, the more plasmons will be produced, which results in more Eu2+ ions being excited and therefore the CL intensity increases.57 The CL intensity also increases with the increasing current from 40mA to 100mA under fixed excitation voltage of 5kV (Figure13 (b)). The CL intensity continuously increases with increasing beam current. This indicates that the phosphor is resistant to the current saturation, which is of benefit to FEDs. Obviously, the emission peaks of the studied phosphor screen are unchanged with the increase of excitation voltage and current, indicating a good stability under different voltages electron beam bombardment.
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The degradation property for phosphors is very important for FED application. Figure 14 shows the decay behaviour of the CL intensity of representative SrSiAl2O3N2:Eu2+ phosphor under continuous Va = 5kV, I = 60mA electron bombardment. The CL intensity of the studied sample monotonously decreases with prolonging the electron bombardment time. After the continuous electron radiation for 90min, the CL intensities of the phosphor still remain 72% of the initial value. This degradation of CL intensity could be ascribed to the accumulation of carbon at the surface during electron bombardment. The accretion of graphitic carbon during electronbeam exposure at high current densities is a well-known effect. This carbon contamination will prevent low-energy electrons from reaching the phosphor grains and also exacerbate surface charging and thus lower the CL intensity.58 The CIE color coordinates of the sample under a continuous electron beam radiation with different radiation time (min) were also investigated, as presented in Figure 13(“+” for X and “×” for Y). The CIE values are nearly invariable under a continuous electron radiation for 90min. In summary, the short time experiment (90min) indicates that the stability of the CL intensity and CIE color coordinate of the as– prepared sample is good, which shows potential advantages applied in the FED.
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We have successfully synthesized a series of SrSiAl2O3N2:xEu2+ (0.005≤x≤0.05) phosphors through a facile and efficient solid state method. DFT calculations reveal the SrSiAl2O3N2 is with a direct band gap of 3.83 eV, which agrees well with the value obtained from DRS. The samples exhibit rods shape morphology with a uniform Eu2+ distribution. The SrSiAl2O3N2:xEu2+ can be efficiently excited in the n-UV range. With increasing the Eu2+ concentration, the emission hue was changed from cyan to bluegreen due to the energy transfer of the Eu2+ ions from the higher 5d levels to those at the lower levels. The concentration quenching mechanism of Eu2+ emission was dominated by the dipole-dipole interaction. The thermal stability is comparable to that of the commercial Ba2SiO4:Eu2+ phosphor. The CL spectra of SrSiAl2O3N2: 0.02Eu2+ phosphor shows intense cyan emission peaking at 484nm. Under continuous low voltage electron-beam
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This work is supported by Gansu Industry and Information Technology Committee, Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120211130003) and the National Natural Science Funds of China (Grant No. 51372105). a
Key Laboratory for Special Function Materials and Structural Design of the Ministry of the Education, School of Physical Science and Technology, Lanzhou University, Lanzhou, 730000, China Tel.: +86-931-8912772 (office); Fax: +86-931-8913554 (office); E-mail: [email protected] 1.
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Dalton Transactions Accepted Manuscript
excitation, SrSiAl2O3N2:xEu2+ phosphors exhibit excellent degradation resistance and good color stability. All the results indicate that the SrSiAl2O3N2 phosphor have potential applications in both WLEDs and FED devices.
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Dalton Transactions Accepted Manuscript
M. Sato, European Journal of Inorganic Chemistry, 2008, 2008, 5471-5475.
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Graphical Abstract: The crystal structure, band structure, photoluminescence and cathodoluminescence properties of SrSiAl2O3N2: Eu2+ phosphor. 254x231mm (300 x 300 DPI)
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DOI: 10.1039/C5DT00800J
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Xicheng Wang,a Zhengyan Zhao,a Quansheng Wu,a Yanyan Li,a Chuang Wang,a Aijun Mao,a and Yuhua Wang.a,* 5
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Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A series of SrSiAl2O3N2: Eu2+ (0.005≤x≤0.05) phosphors were successfully synthesized through a pressureless, facile, and efficient solid state route. The crystal structure, band structure, and their photoluminescence or cathodoluminescence properties were investigated in detail. The phosphors exhibit rods shape morphology with a uniform Eu2+ distribution. Under n-UV excitation the emission spectra shift from 477 to 497nm with an increase of Eu2+ concentration. The concentration quenching mechanism of Eu2+ emission was dominated by the dipole-dipole interaction. The thermal stability is comparable to that of the commercial Ba2SiO4:Eu2+ phosphor. The phosphor also exhibits high current saturation, high resistance under low voltage electron bombardment. All the results indicate that the SrSiAl2O3N2: Eu2+ can be considered as candidates applied for both white LEDs and FEDs.
1Introduction
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Following an increasing awareness of climate changes and environmental issues, people are looking for new technologies to reduce energy consumption and provide eco-friendly illumination and display.1, 2 In recent years the solid-state lighting technology based on white light-emitting diodes (WLEDs), which is considered as energysaving and environmentally friendly lighting sources, has attracted considerable attention from both academic and industrial communities.3-5 The most widespread commercial strategy to achieve WLED is the combination of a InGaN LED chip and Ce3+-doped Y3Al5O12(YAG : Ce3+) yellow phosphor,6 however, this type of white light exhibits a poor color rending index (Ra< 75) because of the color deficiency in the red and blue-green spectral range.7 An alternative approach to generate white light is the combination of an near ultraviolet (n-UV) LED with red, green, and blue (RGB) phosphors.7 Accordingly, it is urgent to develop new phosphors that can be effectively excited in the n-UV range.8 As far as displays concerned, being the most promising next generation flat panel displays, field emission display (FED) has attracted much attention due to its potential to provide displays with thin panels, self-emission, wide viewing, quick response time, high brightness, high contrast ratio, light weight, and low power consumption.9-11 To realize full-color FEDs, it is necessary to develop good blue and green phosphors with high luminance, high efficiency, high saturation current densities, long time under low electron acceleration voltages (≤10 kV).12, 13Therefore, the Development of new high performance phosphor is an important issue for the FED as well. During the past decade, (oxo)nitridosilicates have demonstrated remarkable potential capacity as host materials for phosphors due This journal is © The Royal Society of Chemistry [year]
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to their outstanding thermal and chemical stability and excellent luminescence properties when activated with Eu2+ ,e.g. α/βSiAlONs,14-16M2Si5N8 (M=Ca,Sr,Ba),17, 18 CaAlSiN3,19, 20 MYSi4N7 (M = Sr, Ba),21, 22 MSiN2,23, 24 MSi2O2N2 (M = Ca, Sr, Ba)25, 26 and so on.27-29 These phosphors are with a broad emission range from green to red and some have been put into practical use, however, the Eu2+ doped (oxo)nitridosilicate phosphors emitting blue to green light are still few, which is also indispensable to full-color emission for both WLEDs and FED displays. The oxonitridosilicate phase SrSiAl2O3N2 has been studied for many years. The synthesis and crystal structure of the compound was first reported by Schnick et al.30 Xie et al later reported the photoluminescence properties of Eu2+ in this host.31 Recently, Liu’ s group investigated the energy transfer process of Ce3+/Eu2+ in the SrSiAl2O3N2 and its Ba-containing solid solutions in detail.32 But as for the synthesis method in above previous studies, either the raw material used such as Si(NH)2 or metal nitrides was labile, which need to be handled in a glove box, or the preparation must be under high pressure. So it is meaningful to find another simple and suitable synthesis method for mass production. And to the best of our knowledge, there is no report on the cathodoluminescence (CL) properties of the SrSiAl2O3N2. In consideration of the superior stability of oxonitridosilicate phosphors compared to sulfide and oxide phosphors, it is significant to study the CL properties of the blue-green emitting phosphor and explore its potential used in FEDs. In the present work, we have developed a pressureless, facile, and efficient route through the solid state reaction for the synthesis of the SrSiAl2O3N2 phosphor using the cheap and stable raw material (strontium carbonate and silicon nitride) with flux. The [journal], [year], [vol], 00–00 |1
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Synthesis, structure, and luminescence properties of SrSiAl2O3N2: Eu2+ phosphor for light-emitting devices and field emission displays
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structure, morphology, PL and CL properties were investigated in detail. The motivation of the study is not only to probe into the mechanism of structural and luminescence properties but also to explore its application in WLEDs and FED displays.
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Crystal structure. The Rietveld analysis for SrSiAl2O3N2 was carried out using the structure of synthesized by the silicon diimide as the starting model.30
2.Experimental Preparation. The SrSiAl2O3N2: Eu2+samples were prepared by solid state reaction. Briefly, the constituent raw materials SrCO3 (A.R.), Si3N4 (A.R.), AlN (A.R.), and Eu2O3 (99.999%) were weighed in stoichiometric proportions, 3wt% AlF3 was introduced as the flux. The raw materials were finely ground, and then sintered in boron nitride (BN) crucibles at 1400oC for 5h under a flowing gas of N2-NH3 in a tube furnace. The products were then cooled down to room temperature in the furnace, ground, and pulverized for further measurements. Characterization. The phase purity of samples was analyzed by X-ray diffraction (XRD) using a Bruker D2 PHASER X-ray Diffractometer with graphite monochromator using Cu Kα radiation (λ = 1.54056 Å), operating at 30 kV and 15 mA. Rietveld refinement33 was performed using the software GSAS.34,
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Figure 1.Rietveld refinement and crystal structure of SrSiAl2O3N2 host
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The calculations of the electronic structure for SrSiAl2O3N2were carried out with density functional theory (DFT) and performed with the Cambridge Serial Total Energy Package (CASTEP) code.36 The exchange-correlation functional generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) was chosen for the theoretical basis of density function.37, 38 There are two steps for the calculation. First, the crystallographic data refined from the XRD data were used to optimize its crystal structure. The second step was to calculate its band structure and density of states (DOS) for the optimized structure. The morphology of the sample was examined using scanning electron microscopy (SEM; Hitachi S-4800). The element composition and high-resolution transmission electron microscopy (HRTEM) were obtained on an FEI Tecnai F30transmission electron microscopy (TEM, FEI Tecnai F30, operated at 300 kV) equipped with an energy dispersive X-ray spectroscopy (EDX). Reflectance spectra were measured on PE lambda950 UV-vis spectrophotometer using the BaSiO4 white power as the reference. The photoluminescence (PL), photoluminescence excitation (PLE) spectra were recorded at room temperature using a FLS-920T fluorescence spectrophotometer (Edinburgh Instruments) equipped with a 450W Xe light source and double excitation monochromators. The PL decay curves were measured using an FLS-920T fluorescence spectrophotometer with an F900 nanosecond flash hydrogen lamp as the light source. High temperature luminescence intensity measurements were carried out using an aluminum plaque with cartridge heaters; the temperature was measured by thermocouples inside the plaque and controlled with a standard TAP-02 high temperature fluorescence controller (Orient KOJI instrument Co., Ltd.). The quantum efficiency (QE) was measured using a Fluorolog-3 spectrofluorometer equipped with a 450W xenon lamp (Horiba Jobin Yvon). The CL properties of the samples were obtained using a modified MpMicro-S instrument (Horiba Jobin Yvon).
3. Results and discussion
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Figure 1 illustrates the experimental and calculated XRD profiles as well as their differences for the Rietveld refinement of SrSiAl2O3N2 at room temperature. The results of Rietveld refinement indicate that no any impurity was observed and the synthesis method was effective. The SrSiAl2O3N2 crystallizes as an orthorhombic structure with the space group P2 12121. Lattice parameters were determined to be a = 4.9489(2) Å, b = 7.9698(3) Å, c = 11.3435(4) Å, α = β = γ = 90°, V = 447.40(3) Å3, Z = 4 and the refinement converged to satisfy the reflection condition well with the residual factors Rwp = 8.87% and Rp = 6.26% (see Table 1). The atomic coordinates, isotropic displacement parameters, and site occupancy factors for the SrSiAl2O3N2 host are listed in Table 2. Table 1. Crystallographic data for SrSiAl2O3N2
Formula Crystal system Space group Lattice parameters a(Å) b(Å) c(Å) α° β° γ° Cell volume (Å3) Z Calculated Density R-factors Rwp Rp
SrSiAl2O3N2 orthorhombic P212121 (No.19) 4.9489(2) b=7.9698(3) c=11.3435(4) 90 90 90 447.40(3) 4 3.64715 g/cm3 0.0887 0.0626
The crystal structure of SrSiAl2O3N2 along [100] direction are also shown in the Figure 1 inset. Three kinds of corner-sharing tetrahedral including AlO3N, AlO2N2 (aqua) and SiON3 (rose)
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Table 2. Atomic coordinates and isotropic displacement parameters for SrSiAl2O3N2 Atom Wyckoff S.O.F
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x
y
Sr
4a
1
-0.0070(8)
0.95890(24)
0.66501(22)
z
0.0314(11)
Uiso
Al1
4a
1
0.0322(27)
0.8385(8)
0.3397(8)
0.0131(23)
Al2
4a
1
0.0427(20)
0.5310(8)
0.5445(5)
0.0049(25)
Si1
4a
1
0.0075(24)
1.2010(8)
0.4190(6)
0.0178(25)
O1
4a
1
-0.045(5)
0.6429(18)
0.4232(13)
0.114(8)
O2
4a
1
0.1883(20)
0.6254(12)
0.6550(13)
0.004(5)
O3
4a
1
0.079(7)
1.2934(24)
0.2984(19)
0.219(13)
N1
4a
1
0.1715(24)
1.0376(17)
0.4255(11)
0.037(4)
N2
4a
1
0.169(4)
1.3296(23)
0.5122(18)
0.043(9)
Band structure. The band structure of SrSiAl2O3N2 was calculated using density function theory (DFT) methods based on the crystal structure refinement. As shown in Figure 2, this compound possesses a direct band gap of 3.83eV with the valence band (VB) maximum and the conduction band (CB) minimum at the G point of the Brillouin zone.
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Figure 3. Total and partial density of states of SrSiAl2O3N2 Figure 2. Band structure of SrSiAl2O3N2 15
20
Figure 3 shows the partial density of states (DOS) for the Sr, Si, Al, N and O atoms, as well as total density of states for the host of SrSiAl2O3N2. The valence band is mainly composed by N2p and O 2p orbitals, The N 2p orbital contribute to the top of the valence band more than the O 2p orbital does, while the bottom of the conduction band is dominated by the Sr3d orbital. The calculation demonstrates that SrSiAl2O3N2 is a favourable host material. On the one hand it provides a suitable band gap for Eu2+ to act as the emission center; on the other hand it is propitious to luminescence since the transition probability of the direct band
gap is higher than that of the indirect band gap due to no phonons involved in the transition process.39
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form a highly condensed network. The Sr atoms (lime balls) are located in the channels of the network and they are coordinated by three nitrides and six oxides.
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Figure 4. Diffuse reflection spectra of undoped SrSiAl2O3N2 and Sr0.98Eu0.02SiAl2O3N2 samples. (Inset) Relationship between the absorption coefficient and the photon energy for SrSiAl2O3N2 5
10
15
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Figure 4 depicts the diffuse reflectance spectra (DRS) of the undopedSrSiAl2O3N2 and Eu2+doped SrSiAl2O3N2.Clearly, the undoped SrSiAl2O3N2 shows a remarkable drop in reflection around 270 nm, which corresponding to the band transition of the host lattice. The intense reflection in the visible spectral range agrees well with the observed white body color of the host sample. The Eu2+ doped SrSiAl2O3N2 shows a wide absorption range with the maximum at about 400nm, which indicates that the doped Eu2+ ions create localized energy levels within the band gap of the SrSiAl2O3N2 host.40 The band gap (Eg) is estimated according to the Equation as following: (αhν)n = A(hν - Eg) (1) Where hν is the incident photon energy, α is the absorption coefficient, and A is a constant. The value of n depends on the type of inter band transition: n = 2 for a direct transition and n = 1/2 for an indirect transition.41 The absorption coefficient α can be obtained via the Kubelka–Munk function:
=
25
30
35
40
Figure 5. (a) SEM image, (b) TEM image, (c) HRTEM image and EDX spectrum of SrSiAl2O3N2: 0.02Eu2+
45
show that well-crystallized SrSiAl2O3N2:Eu2+ powders have been obtained, which further confirms that the convenient synthesis method is also efficient.
( 1- R)2 2 R (2)
R is the observed reflectance in the DRS. The value of (αhν) 2 is plotted against the incident energy hν in the inset of Figure 4. By extrapolating the linear portion to the photon energy axis,42 the value of Eg was determined to be about 4.47 eV. This is consistent with the value of 3.83 eV obtained from DFT calculation in view of the generally accepted fact that the calculated band gap is somewhat underestimated.43 Morphology of the SrSiAl2O3N2: Eu2+ phosphor. Figure 5(a) shows the typical SEM imagine of the SrSiAl2O3N2: 0.02Eu2+ sample. The particles exhibit rods shape morphology with the size ranging from 5 to 10μm. This is also in line with the typical low-magnification as shown in Figure 5(b). The corresponding EDX spectrum analysis (Figure 5(c)) indicates that the product has a chemical composition of Sr, Si, Al, O, N, and no impurity element exists. The inter-planar spacing was measured to be 0.2315 nm (Figure 5(d)), which matches well with the (211) inter planar distances of the monoclinic SrSiAl2O3N2. These results
Figure 6. Excitation and emission spectra of SrSiAl2O3N2: xEu2+ with varying Eu2+ concentration 50
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PL properties. Figure 6 illustrates the excitation and emission spectra of SrSiAl2O3N2: xEu2+ with varying Eu2+ concentrations. The excitation spectrum exhibits a remarkable broad band between 250 and 450 nm, which is attributed to the 4f7→4f65d1 transition of the Eu2+ ions. It also consists of three bands peaking at about 294, 351 and 395nm, which correspond to the transitions of Eu2+ from the ground state to its field-splitting levels of the 5d state.44 The emission spectra of the SrSiAl2O3N2: xEu2+ (0.005≤x≤0.05) phosphors show broad, symmetric bands in the range of 400-600 nm, which corresponds to the allowed 4f65d1→4f7 electronic transitions of Eu2+. As the Eu2+ concentration increases from x =
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0.005 to x = 0.05, the full width at half-maximum (FWHM) of the SrSiAl2O3N2: xEu2+ emission spectra remain the same at about 88 nm, while the emission band shifts toward longer wavelength from 475 to 497 nm. This red-shift phenomenon could be explained in terms of the energy transfer of the Eu2+ ions from the higher 5d levels to those at the lower levels. This causes a decrease in the emission energy from 5d excited state to the 4f ground state and therefore the emission shifts to longer wavelength.45
25
10 2+
2+
Figure 7 Decay curves of Eu emission in SrSiAl2O3N2: xEu .
15
To further understand the process of energy transfer, the decay curves for SrSiAl2O3N2: xEu2+ (x = 0.01, 0.02, 0.03,0.04 and 0.05) were measured and depicted in Figure 7. These decay curves are plotted as a semi-logarithmic plot and can be well fitted into a single exponential equation,46
35
𝐼(𝑇) = 𝐴1 exp(−𝑡/𝜏)
20
30
(3)
where I and A1 correspond to the luminescence intensity at time t, and τ is the lifetime. The lifetime of Eu2+ is shortened with increasing Eu2+ concentration. The values decreased from 0.578 to 0.548 μs with the Eu2+ concentration increased from x = 0.01 to x = 0.05. As the Eu2+ concentration increasing, the distance of
40
Figure 8. CIE chromaticity diagram for SrSiAl2O3N2: xEu2+ phosphors with different Eu2+ dopant concentrations. The inset shows these phosphors under 365nm excitation in a UV box.
the luminescence centers (Eu-Eu) decrease, then the possibility of energy transfer increases and thus leads to a faster decay. This further indicates that the red-shift is mainly caused by the energy transfer processes, and the decay lifetimes are also suitable for solid-state lighting when used as WLED phosphors.25 The Stokes shifts of SrSiAl2O3N2: xEu2+ (0.005≤x≤0.05), roughly estimated from the maxima in the excitation and emission spectra, are in the range of 7276-8107 cm-1, as shown in Table 3. The CIE chromaticity coordinates of the SrSiAl2O3N2: xEu2+ phosphors with different Eu2+concentration are summarized in Table 3 and also shown in Fig. 8. It is clearly observed that the emission hue of the SrSiAl2O3N2: xEu2+ phosphors changed from cyan to blue-green as the x value increased. The chromaticity index varies from (0.25, 0.50) for the concentration with 0.5% to (0.42, 0.52) for the concentration with 5%.
Table 3.Excitation, Emission bands, stokes shifts, Normalized PL Intensity and the CIE Coordinates forSrSiAl2O3N2: xEu2+
Eu2+ concentration
FHWM of
λem
Stokes shifts
Normalized PL
emission (nm)
(nm)
(cm-1)
Intensity (%)
x = 0.005
85
475
8107
74.87
(0.1820,0.2638)
x = 0.01
86
477
7937
85.24
(0.1854,0.2879)
x = 0.02
88
484
7828
100
(0.1974,0.3387)
x = 0.03
88
489
7798
90.11
(0.2045,0.3661)
x = 0.04
88
493
7727
79.25
(0.2084,0.3795)
x = 0.05
89
497
7276
75.32
(0.2207,0.4154)
CIE (x,y)
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Figure 9. Emission intensity of SrSiAl2O3N2: xEu2+ phosphors as a function of Eu2+ concentration. Inset: log(I/x) dependence of log(x)
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Figure 9 shows the PL intensity of SrSiAl2O3N2: xEu2+ phosphors with various Eu2+ concentrations under 365nm excitation. The optimal doping concentration was observed at x = 0.02, the concentration quenching was observed when the content of Eu2+ exceeds 2mol%. The internal quantum efficiency of SrSiAl2O3N2: 0.02Eu2+ was measured to be 40.2%. The concentration quenching is due to the nonradiative energy transfer between Eu2+ ions, the possibility of which increases as the concentration of Eu2+ increases. Blasse47 suggested that the critical distance (Rc) of energy transfer can also be calculated by the critical concentration of the activator ion:
3V Rc=2 4xc Z
20
25
30
1/ 3
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Figure 10. (a) The emission spectra of theSrSiAl2O3N2:0.02Eu2+ sample under different excitation wavelengths and (b) TRPL spectra of the SrSiAl2O3N2:0.02Eu2+ sample.
(4)
Where xc is the critical concentration, Z is the number of formula units per unit cell, and V is the volume of the unit cell. For SrSiAl2O3N2 host, when Z = 4, xc = 0.02, and V = 447.40(3) Å3, the obtained Rc value is 22.02Å, which is similar to the Rc value for Eu2+centers in several oxide and apatite lattices.48 It is worth noting that this value is much further than the distance of the nearest neighbor Sr2+ ions (4.48 Å) according to the crystal structure results. So it is reasonable that the concentration quenching occurs at such a relative low Eu2+ concentration. Energy transfer is generally associated with multipolar interactions, radiation reabsorption or exchange. The radiation reabsorption requires a large overlap of the wave functions of the donor and the acceptor, and the critical distance for the exchange interaction is approximately 5Å. Based on the calculated Rc value and small overlap of the emission spectra and the excitation spectra, it seems that the electric multipolar interactions are the most relevant to process of energy transfer.49 This journal is © The Royal Society of Chemistry [year]
40
The interaction mechanism between Eu2+ ions can be expressed by the following equation:50 𝐼
= 𝑥
45
50
𝑘 𝜃
(5)
1+𝛽(𝑥) 3
Where k and β are constants for each type of interaction for a given host lattice and x is the activator concentration. According to results published by Van Uitert, θ = 6, 8 , 10 corresponds to dipole-dipole, dipole-quadrupole, quadrapole-quadrapole interactions, respectively.51 The inset of Figure 8 illustrates the(I/x) dependence on x on a logarithmic scale. The dependence of log (I/x)on log (x) was found to be relatively linear, and the slope was determined to be -1.016. The value of θ was found to be most close to 6, indicating that the concentration quenching mechanism of Eu2+ emission was dominated by the dipole-dipole
[journal], [year], [vol], 00–00 |2
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1 A exp( .
45
50
55
60
65
25
30
35
40
Figure 11. Temperature-dependent luminescent spectra of theSrSiAl2O3N2:0.02Eu2+ from 25 to 250 oC
The thermal stability of phosphors is one of the important parameters for the application of LEDs. The thermal quenching properties of the SrSiAl2O3N2:0.02Eu2+ sample were also investigated. Figure 11 presents the temperature-dependent luminescent spectra of SrSiAl2O3N2:0.02Eu2+ with 365nm excitation. When the temperature increased from 25 oC to 250 oC, the PL intensities decrease while the peak positions almost keep the same. As displayed in top-right corner of the Figure 10, the SrSiAl2O3N2:0.02Eu2+ exhibits higher thermal stability than commercial Ba2SiO4:Eu2+ phosphor, especially when the temperature is above 150 oC. The intensity at 150 oC remains 56% of the initial intensity, while the one of the BaSiO4 decrease to the 49%. The activation energy for the thermal quenching was estimated by the equation as follows:54
I0
I (T )
70
75
80
E ) K BT
(6)
Where I0 is the initial intensity, I(T) is the intensity at a given temperature T, A is a constant, ΔE is the activation energy for thermal quenching, and KB is the Boltzmann’s constant. As shown in the top-left corner of Figure 10, ΔE for SrSiAl2O3N2:0.02Eu2+ is 0.26 eV, which is similar to that of the (Ca, Sr)YSi4N7:Eu2+ phosphor in our previous study,55 indicating its good thermal stability.
Figure 12. (a) SEM images of theSrSiAl2O3N2:0.02Eu2+ sample and (b) monochromatic CL image at 484 nm of the same area.
Cathodoluminescence. As is well known, CL has the advantages of a much higher lateral and depth resolution in comparison with PL, which makes CL as a unique tool for luminescence mapping, so CL monochromatic images were used to investigate the Eu2+ distribution in the structures.16 Figure 12 shows the SEM image of the SrSiAl2O3N2:0.02Eu2+ sample and the corresponding CLmapping image at the same area. According to as-mentioned result of PL and the CL discussed as following, the monochromatic CL mappings of SrSiAl2O3N2:0.02Eu2+ phosphor is collected at wavelength of 484nm. It can be observed that Eu 2+ ions distribute uniformly in the rod-shaped particles except mirror agglomerated locations. The strong luminescence obtained from the microstructures indicates that the Eu2+ ions have been effectively incorporated into samples with a uniform distribution. Figure 13(a) shows CL intensity of SrSiAl2O3N2:0.02Eu2+ phosphor measured under different accelerating voltages when the probe current fixed at 50mA. The inset shows the CL intensities as a function of accelerating voltage. The intensity gradually increases as the applied voltages changes from 2 to 8kV. Similarly, under a 5kV electron beam excitation, the CL intensity also increases with increasing probe current from 40 to 100mA (Figure13 (b)). The increase in CL brightness with the increase in electron energy and probe current are attributed to the deeper penetration of the electrons into the phosphor body and the larger electron-beam current density.11 To find the physical origin that the CL intensity increases with the electron energy, we make the following analysis. We calculate the electron penetration depth for SrSiAl2O3N2:0.02Eu2+ using the following empirical Eq : A
E n
1.2
L =( ρ ) ( ) ,n = 1−0.29 lg Z (7) √Z
85
Where A is the atomic or molecular weight of the material, ρ is the density, Z is the atomic number or the number of the electrons per molecule in the compounds, and E is the excitation voltage in unite of kV.56 For SrSiAl2O3N2: Eu2+, A = 245.6803, Z =4 , ρ =
Dalton Transactions Accepted Manuscript
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interaction.52 Additionally, when varying the excitation wavelength from 300 to 410 nm, there is no significant change in the emission spectra except the emission intensity. The spectra profiles are nearly the same with the peaks wavelength located at 484 nm(see Figure 10(a)), indicating the presence of single emission center of the Eu2+ in the SrSiAl2O3N2 host lattice. The intensities changes little. This fully proves that the phosphor can be efficiently excited over the broad range from 300-410 nm. In order to confirm that the broad band emission originates from single luminescent center, a series of lifetime decay curves were recorded by monitoring the SrSiAl2O3N2:0.02Eu2+ sample in different wavelengths from 400 to 600 nm at 5 nm interval, and the Time-Resolved PL (TRPL) spectra shown in Figure 10(b) were obtained by slicing them. From 21.97 ns to 634.76 ns, the shape and position of the emission spectra almost keep still. This further proves that there is single luminescent centre.53 This result agrees well with the same emission spectra under different excitations. This also indicates that Eu2+ ions in SrSiAl2O3N2 lattice occupy only one kind of crystallographic site Sr2+ site, which consists with the crystal structure of SrSiAl2O3N2, where only one Sr2+ ion is available for the substitution by Eu2+ ions.
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Figure 14. CL intensity and chromaticity coordinate decay of SrSiAl2O3N2: 0.02Eu2+ phosphor with the electron beam bombardment time (min)
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Figure.13 The CL spectra of SrSiAl2O3N2:0.02Eu2+ phosphor as (a) a function of probe current under 5 kV electron-beam excitation and (b) a function of voltage under 60 mA filament current. (The insets show the dependence of emission intensity on filament current and voltage, respectively.)
3.64715 g/cm3, thus the estimated electron depths at 2kV, 4kV, 6kV and 8kV are 1.01 nm, 5.71 nm, 15.67 nm and 32.11 nm, respectively. Hence, the increase in emission intensity with increasing applied voltage is due to an increase in the number of excited luminescence centers resulting from an increase in electron penetration depth. As for the cathodoluminescence, the Eu2+ ions are excited by the plasmons produced by the incident electrons. The deeper the electron penetration depth, the more plasmons will be produced, which results in more Eu2+ ions being excited and therefore the CL intensity increases.57 The CL intensity also increases with the increasing current from 40mA to 100mA under fixed excitation voltage of 5kV (Figure13 (b)). The CL intensity continuously increases with increasing beam current. This indicates that the phosphor is resistant to the current saturation, which is of benefit to FEDs. Obviously, the emission peaks of the studied phosphor screen are unchanged with the increase of excitation voltage and current, indicating a good stability under different voltages electron beam bombardment.
45
50
The degradation property for phosphors is very important for FED application. Figure 14 shows the decay behaviour of the CL intensity of representative SrSiAl2O3N2:Eu2+ phosphor under continuous Va = 5kV, I = 60mA electron bombardment. The CL intensity of the studied sample monotonously decreases with prolonging the electron bombardment time. After the continuous electron radiation for 90min, the CL intensities of the phosphor still remain 72% of the initial value. This degradation of CL intensity could be ascribed to the accumulation of carbon at the surface during electron bombardment. The accretion of graphitic carbon during electronbeam exposure at high current densities is a well-known effect. This carbon contamination will prevent low-energy electrons from reaching the phosphor grains and also exacerbate surface charging and thus lower the CL intensity.58 The CIE color coordinates of the sample under a continuous electron beam radiation with different radiation time (min) were also investigated, as presented in Figure 13(“+” for X and “×” for Y). The CIE values are nearly invariable under a continuous electron radiation for 90min. In summary, the short time experiment (90min) indicates that the stability of the CL intensity and CIE color coordinate of the as– prepared sample is good, which shows potential advantages applied in the FED.
4 Conclusions 55
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We have successfully synthesized a series of SrSiAl2O3N2:xEu2+ (0.005≤x≤0.05) phosphors through a facile and efficient solid state method. DFT calculations reveal the SrSiAl2O3N2 is with a direct band gap of 3.83 eV, which agrees well with the value obtained from DRS. The samples exhibit rods shape morphology with a uniform Eu2+ distribution. The SrSiAl2O3N2:xEu2+ can be efficiently excited in the n-UV range. With increasing the Eu2+ concentration, the emission hue was changed from cyan to bluegreen due to the energy transfer of the Eu2+ ions from the higher 5d levels to those at the lower levels. The concentration quenching mechanism of Eu2+ emission was dominated by the dipole-dipole interaction. The thermal stability is comparable to that of the commercial Ba2SiO4:Eu2+ phosphor. The CL spectra of SrSiAl2O3N2: 0.02Eu2+ phosphor shows intense cyan emission peaking at 484nm. Under continuous low voltage electron-beam
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This work is supported by Gansu Industry and Information Technology Committee, Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120211130003) and the National Natural Science Funds of China (Grant No. 51372105). a
Key Laboratory for Special Function Materials and Structural Design of the Ministry of the Education, School of Physical Science and Technology, Lanzhou University, Lanzhou, 730000, China Tel.: +86-931-8912772 (office); Fax: +86-931-8913554 (office); E-mail: [email protected] 1.
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excitation, SrSiAl2O3N2:xEu2+ phosphors exhibit excellent degradation resistance and good color stability. All the results indicate that the SrSiAl2O3N2 phosphor have potential applications in both WLEDs and FED devices.
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