ZnWO₄/ZnWO₄ : Eu³⁺ inverse opal photonic crystal scintillator: efficient phosphors in radiation detection

Abstract

Phosphors with photonic crystal (PC) structures may demonstrate modulated optical properties and have potential applications in various optical devices. In this study, we represent the fabrication of ZnWO₄ and ZnWO₄ : Eu³⁺ inverse opal photonic crystal (IOPC) scintillators based on polymethyl methacrylate (PMMA) templates. Typical modified photoluminescent properties in PCs were observed such as the suppression of emissions near the edge of photonic stop bands (PSBs) and prolonged decay time constants. Moreover, it is very interesting to observe that the luminescent quantum efficiency of the IOPC samples was remarkably enhanced compared to the corresponding ground powder references (REFs) and the quantum efficiency of ZnWO₄ IOPCs was as high as 70%, which was almost the optimum among various ZnWO₄-based phosphors reported before. Furthermore, the energy transfer (ET) efficiency from tungstate groups to Eu³⁺ was determined based on luminescent dynamics, which indicated that the efficiency in the IOPCs was considerably improved compared to that in the REFs owing to the suppression of the spontaneous emission rate for the tungstate groups. It was also demonstrated that ZnWO₄ : Eu³⁺ IOPCs could be used as white light phosphors.

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1. Introduction

Inorganic scintillation materials have been widely used in contemporary radiation detectors for medical imaging, industrial inspection, dosimetry, and high-energy physics due to their unique properties such as high scintillation light yield, large atomic number and density and high stopping power.¹ A scintillator is a fluorescent material that interacts with high-energy particles or rays. The working principle of a scintillation detector is that it converts a fraction of the energy of high-energy particles or high-energy rays into visible or ultraviolet light. Therefore, a fluorescent material (scintillator) is one of the scintillation detector cores. Improving the optical performance of the scintillator is the key to improve the detection capability of a scintillation detector.

It is known that tungstates are a very important family of inorganic scintillation materials. Among the tungstates, ZnWO₄ is a material that has been widely used in scintillation detectors,^2–6 anodes,⁷ laser hosts,^8,9 luminescent materials,¹⁰ gas-sensing,¹¹ up-conversion materials,^12,13 photo-catalysts^13,14 and ceramics,^15,16 on account of its high chemical stability, high light yield, high X-ray absorption coefficient, low afterglow to luminescence, non-hygroscopicity, nontoxicity and low cost compared to similar materials.¹⁷ Recently, there have been a number of reports concerning the improvement in the optical properties of ZnWO₄ achieved by introducing rare earth (RE) ions,^18,19 owing to the fact that the energy of the excited tungstate group can be effectively transferred to RE ions. However, this new scintillation detector still has some shortcomings. Firstly, preparation methods are difficult, having many disadvantages such as the requirement for strict synthesis media control, high synthesis costs and complexities in the synthesis reaction.^2–6 Secondly, a high average refractive index is not conducive to the collection of scintillated light. Lastly, the fluorescence efficiency is relatively low. Therefore, more investigations are directed towards solving these problems.

Since the pioneering works by Yablonovitch²⁰ and John²¹ in 1987, photonic crystals (PCs), as a type of materials with a periodically varying refractive index, have attracted considerable scientific and technological interest. PCs are composites of dielectric materials with a periodicity of the order of the wavelength of light.^20,21 Due to the periodic variation of the dielectric constant, the crystals possess photonic band structures, analogous to electronic band structures in solid state physics.²² The propagation of light in these structures gives rise to the concept of the photonic stop band (PSB), which is the essence of their functionality. The frequency regions of photonic band gap in which electromagnetic wave model of light is limited, or even a total absence. The opening of a band gap forbids the propagation of electromagnetic waves in these frequency ranges, whereas complementarily, the emission of light is also precluded or fundamentally modified.²³ The control of spontaneously emitted light through modification of photonic structure is a central issue throughout this research. To date, the modification of spontaneous emission by embedding luminescent species in three-dimensional PCs, including organic dyes, semi-conductors and RE ions, has been widely studied and various phenomena have been observed such as the modulation of the spontaneous emission rate and the ET process, the change in the effective refractive index and the improvement in the luminescent quantum yield in inverse opal photonic crystals (IOPCs) containing embedded RE ions.^24–29 Despite the fact that much research has been done, the effect of the modification of three-dimensional PCs on the spontaneous emission of RE ions as well as other species has not been clarified; this depends on the embedded luminescent species, the structure of PCs and the host matrices.

In this study, we report the fabrication of ZnWO₄ and ZnWO₄ : Eu³⁺ IOPC scintillators by the PMMA template technique and their unique photoluminescent properties. We observed the effect of modulation of IOPCs on the spontaneous emission of the broad band luminescence of tungstate groups and the narrow band emission of Eu³⁺ ions. Furthermore, we determined that the quantum efficiency of ZnWO₄ : Eu³⁺ IOPCs reached 70%, which has been rarely reported before.^8,30,31 According to previous literature reports, the highest inner quantum efficiency at room temperature was 66%,⁸ which was determined by the luminescent lifetime in ZnWO₄ : Dy³⁺.

2. Experimental

2.1 Sample preparation

The starting materials for the synthesis of the IOPC, ZnWO₄ : Eu³⁺ were zinc nitrate Zn(NO₃)₂·6H₂O, europium nitrate Eu(NO₃)₃·6H₂O, ammonium metatungstate, methyl methacrylate (MMA), sodium hydroxide, potassium persulfate, ethanol and citric acid and all of these chemicals were of analytical grade and were used directly without any further purification.

Zn(NO₃)₂·6H₂O was obtained from Tianjin GuangFu Technology Development CO., LTD. Eu(NO₃)₃·6H₂O was bought from the National Engineering Research Centre of Rare Earth Metallurgy and Function Materials. Ammonium metatungstate, sodium hydroxide, MMA, ethanol and citric acid were purchased from Beijing Chemical Plant. Potassium persulfate was received from Tianjin FuChen Chemical Reagent Factory.

ZnWO₄ : Eu³⁺ IOPCs with different Eu³⁺ doping concentrations were prepared by the sol–gel method with a PMMA latex sphere as the template. Firstly, monodisperse PMMA latex spheres with controllable sizes were synthesized by polymerization, in which MMA, sodium hydroxide and potassium persulfate were the starting materials. Then, a colloidal suspension (5% solid content) of PMMA microspheres was dropped onto a glass substrate and placed in an oven at 32 °C for 24 h. The PMMA colloidal spheres slowly self-organized into highly ordered colloidal arrays on the glass substrate, driven by the surface tension of the liquid during the evaporating process. Following deposition, the opals were sintered for 40 min at 120 °C to enhance their physical strength. In the preparation of the ZnWO₄ : Eu³⁺ precursor sol, appropriate amounts of Zn(NO₃)₂·6H₂O, Eu(NO₃)₃·6H₂O and ammonium metatungstate were dissolved in a mixed solution of deionized water and ethanol with a volume ratio of 1 : 1. The mixed solution contained citric acid as the chelating agent. The mixture was stirred for 4 h to form a transparent solution. The prepared precursor solutions were used to infiltrate the voids of the opal template through capillary force. After infiltration, the resulting products were dried in air at room temperature. Annealing was carried out at a slowly elevating temperature (1 °C min⁻¹) up to 500 °C for 3 h. By controlling the diameters of the PMMA latex spheres during polymerization, PSB of IOPCs was finely tuned. PSBs of ZnWO₄ : Eu³⁺ IOPCs measured in the normal direction were located at 424, 506, 618 and 780 nm and the corresponding IOPC samples were denoted as PC1–PC4, respectively. The powder reference samples (REF) were prepared by grinding the corresponding IOPCs to destroy the regular 3D structure.

2.2 Measurements

The phase structure of the samples was characterized by X-ray diffractometry, using a monochromatized Cu target radiation resource (λ = 1.54 Å). The surface morphology of the samples was measured with a JEOL JSM-7500 field emission scanning electron microscope (FE-SEM) at an accelerating voltage of 15 kV. A Hitachi H-800 transmission electron microscope (TEM) was operated at an accelerating voltage of 200 kV. The hybrid was scraped from the glass substrate, dispersed in ethanol and then dropped onto a copper grid for TEM measurements. The reflectance spectra of the resulting samples were obtained with a UV-1800 UV-visible spectrometer in a range of 200–1100 nm. The excitation and emission spectra were obtained at room temperature using a fluoroSENS SENS-9000 spectrophotometer. The luminescent dynamics were investigated using a laser system consisting of a Nd:YAG pumping laser (1064 nm), a third-order Harmonic-Generator (355 nm) and a tunable optical parameter oscillator with a pulse duration of 10 ns, a repetition frequency of 10 Hz and a line width of 4–7 cm⁻¹. The luminescent quantum efficiency of the powder samples and IOPCs was measured with a fluoroSENS spectrophotometer equipped with a BaSO₄ integrated sphere. 280 nm light separated from an internal Xenon lamp was used as an excitation source.

3. Results and discussions

3.1 Structure and morphology of ZnWO₄ : Eu³⁺ IOPCs

The IOPC samples on the glass substrates (2.0 cm × 1.5 cm) display different colors when the direction of observation is changed. The color gradually changes, implying the formation of well-ordered IOPCs. The SEM and TEM images of the IOPCs were analyzed to determine the local structure and quality of the samples (Fig. 1). Fig. 1(a)–(d) shows the SEM images of the PMMA opal template, the magnified image of IOPC (PC4), the side-view image of the PC4 sample and the TEM image of PC4, respectively. Fig. 1(a) shows that long range PMMA opal is formed and the average diameter of the PMMA spheres is 570 nm; Fig. 1(b) displays the SEM image of the PC4 IOPC sample. It can be seen that the PC4 sample yields a long-range ordered hexagonal arrangement of inverse opal and the center-to-center distance of IOPC is 364 nm, which is about 36% smaller than the original size of the PMMA template due to the shrinkage of the spheres during calcination. The thickness of the sample is 13.5 μm observed from the side view of Fig. 1(c), indicating a multilayered structure. The TEM image in Fig. 1(d) shows that the wall of the IOPC sample consists of a large number of small nanoparticles with clear boundaries and the wall thickness of the IOPC sample is estimated to be about 75 nm.

Fig. 1 (a) The SEM image of the opal template. (b) The SEM image of the inverse opal PC sample (PC4). (c) The side-view image of the inverse opal PC sample (PC4). (d) The TEM image of the inverse opal sample (PC4). (e) The XRD patterns of pure ZnWO₄ PC and the corresponding REF and ZnWO₄ : Eu³⁺ IOPCs with different Eu³⁺ doped concentrations. Inset: the magnified XRD patterns of the (100) plane. (f) The EDX spectra of ZnWO₄ : 0.1%Eu³⁺ IOPC.

There exist two types of structure in tungstate: wolframite and scheelite. Zinc tungstate has the wolframite structure, which is monoclinic with space group P2/c. Its lattice parameters are a = 0.469263(5) nm, b = 0.572129(7) nm, c = 0.492805(5) nm, and β = 90.6321(9)°.^8,9,32 The RE ions replace zinc ions when they are doped into ZnWO₄ lattices. The crystalline structure of the doped ZnWO₄ samples was characterized by powder X-ray diffraction (XRD). Fig. 1(e) shows the XRD patterns of the pure ZnWO₄ sample, the corresponding REF sample and the ZnWO₄ samples with different Eu³⁺ doping concentrations in contrast to the standard card. It can be seen that the results for all of the samples are exactly in agreement with the standard cards (JCPDS 15-0774). No impurity peak appears, implying that all of the samples demonstrate the pure wolframite structure in monoclinic phase. Fig. 1(f) shows the EDX spectrum of the ZnWO₄ : 0.1%Eu³⁺ IOPCs, which shows that W, O, Zn and Eu all exist in the sample and this is the direct experimental evidence to prove the existence of Eu³⁺ in the ZnWO₄ matrix. The inset of Fig. 1(e) shows a little difference among all the XRD patterns; the diffraction peaks shift a little to a lower degree as the Eu³⁺ concentration is increased, owing to the fact that the ionic radius of the replaced Zn²⁺ (0.74 Å) is smaller than the ionic radius of Eu³⁺ (0.947 Å).⁴² From the analysis of the XRD patterns and EDX spectra, it can also be concluded that the Eu³⁺ ions have been effectively doped into the ZnWO₄ host lattices.

3.2 Reflectance/photoluminescence spectra of ZnWO₄/ZnWO₄ : Eu³⁺IOPCs

As is well known, the periodic electromagnetic modulation created by the PCs can yield PSB; higher in the frequency domain, a stop band typically opens in a certain region of the reciprocal space. Hence transmission at frequencies lying in this region is strongly inhibited, giving rise to a dip in transmittance³³ and a peak in reflectance.³⁴ Fig. 2(a) shows the optical reflectance spectra of ZnWO₄ : Eu³⁺ IOPC samples when the incidence is vertical to the (111) plane. From the reflectance spectra, it can be seen that all the IOPC samples from PC1 to PC4 display distinctive PSBs, centered at 424, 506, 618 and 780 nm, respectively, which corresponds to the colors purple, light green, orange and red, as shown in the inset, which shows that the IOPC samples present beautiful structural colors under normal indoor illumination. Every sample exhibits an expansion width of ∼120 nm. Note that as measured from the normal direction, the 315, 364, 457 and 570 nm PMMA templates display PSBs centered at 675, 780, 980 and 1240 nm, respectively. Due to a lower average refractive index caused by air spheres instead of polymer spheres and shrinkage of spheres during calcination, the PSBs of the IOPCs shift to the short wavelength compared with the PSBs of bare opals. Theoretically, the normal positions of PSBs in face centered cubic (FCC) PCs can be estimated by Snell's law, as follows:³⁵

n_eff = xn_ZnWO4 + (1 − x)n_air (2)

where λ is the central wavelength of PSB, m is the order of the Bragg diffraction, d_hkl is the plane distance, n_eff is the average refractive index, n_ZnWO4 is the refractive index of ZnWO₄ (∼2.2),^2,8 n_air is the refractive index of air, x is the filling factor, D is the center-to-center distance of the neighboring hollow spheres and θ is the angle from the incident light to the normal of the substrate surface (θ = 0°). It is easy to deduce that in the IOPC sample, n_eff = 1.33.

Fig. 2 (a) The reflectance spectra of the ZnWO₄ : Eu³⁺ IOPC sample measured at the normal (θ = 0°) direction. Inset: a digital image of PCs with different stop band positions. (b) Normalized excitation (λ_em = 470 nm) and emission (λ_ex = 280 nm) spectra of pure ZnWO₄ IOPC and corresponding REF and (c) normalized excitation (λ_em = 470 nm and λ_em = 618 nm) and emission (λ_ex = 280 nm) spectra of ZnWO₄ : Eu³⁺ IOPC samples and corresponding REF sample. (d) The quantum efficiencies of pure ZnWO₄ and ZnWO₄ : 0.5%Eu³⁺ IOPC samples and the corresponding REF samples.

The photoluminescent properties of pure ZnWO₄ IOPCs, ZnWO₄ : Eu³⁺ IOPCs and corresponding REF sample were characterized. Fig. 2(b) shows the excitation spectra (left) and emission spectra (right) of pure ZnWO₄ IOPCs and the REF sample. It can be seen that the excitation bands of ZnWO₄ IOPCs range from 240 to 320 nm (λ_em = 470 nm) and have a peak location of 280 nm. The tungstate emissions consist of a broad band with a maximum around 470 nm (λ_ex = 280 nm), originating from tungstate groups with a wolframite structure. The WO₆²⁻ octahedral complex and a slight deviation from a perfect crystal structure play roles in the emission bands, which are mainly determined by the charge-transfer transitions between the 2p orbitals of O²⁻ and the empty orbitals of the central W⁶⁺ ions in the WO₆²⁻ complex.^18,19 Fig. 2(c) displays the excitation and emission spectra of the ZnWO₄ : Eu³⁺ IOPCs and REF samples. In the emission spectra, the bands coming from tungstate and the sharp lines of the ⁵D₀–⁷F_J (J = 0–4) transitions for Eu³⁺ can be all identified under an excitation of 280 nm. Among them, the electronic dipole transition of ⁵D₀–⁷F₂ at 618 nm is dominant. The excitation spectra in Fig. 2(c) show that while monitoring the emissions of Eu³⁺ (λ_em = 618 nm) and tungstate group (λ_em = 470 nm), the excitation bands are the same, indicating efficient ET from the tungstate group to Eu³⁺.^18,19,36 From Fig. 2(b) and (c), it can be also seen that the excitation and emission spectra for IOPCs are similar to the REF samples, which match well with photomultiplier tubes or silicon photo diodes. The inner quantum efficiency for different samples was measured using the space integrated spectra in the integrated sphere. A comparison of the inner quantum efficiency between the IOPC and REF samples is shown in Fig. 2(d). The quantum efficiencies of pure ZnWO₄ IOPCs and ZnWO₄ : 0.5%Eu³⁺ IOPC samples are 70% and 51.9%, while the corresponding values for the REF samples are 24% and 9.6%, respectively. It is interesting to observe that the quantum efficiencies of pure ZnWO₄ IOPCs and ZnWO₄ : 0.5%Eu³⁺ IOPC samples are much higher than the corresponding REF samples. To the best of our knowledge, the inner quantum efficiency of 70% is almost the most efficient observed among the ZnWO₄-based phosphors, indicating that ZnWO₄ IOPCs are superior to the scintillator material. Note that an improvement in the inner quantum yield for IOPC samples relative to REF samples was also observed in YVO₄ : Dy³⁺.²⁹

3.3 Modification of the PSB on emission spectra

The space-dependent optical properties of the PSB and their modification on spontaneous emission is an important subject with respect to PC effects. Based on the results mentioned above, the PSB of PC4 (780 nm) is far away from the broad band emission of tungsten and the narrow emission of the Eu³⁺ ion, thus PC4 is selected as the reference sample. Fig. 3(a) and (b) present the emission spectra of ZnWO₄ : Eu³⁺ (0.5% molar ratio) for PC2 and PC3 in contrast to the reference sample (PC4) under 280 nm excitation. A fraction of the broad band emission of tungsten groups falls within the PSB of PC2 (PSB ∼ 506 nm), while the PSB of PC3 overlaps with the emission lines of ⁵D₀–⁷F₂ at the 618 nm transition. As shown in Fig. 3(a), the broad band emission is suppressed in contrast to PC4 in the range of 450–550 nm. In Fig. 3(b), significant inhibition of the ⁵D₀–⁷F₂ luminescent intensity for PC3 can be observed in contrast to PC4. The inhibition of light emission is a universal phenomenon for luminescent species embedded in PCs,^28,29 which can be understood as being due to the reduction in the number of optical modes available for photon propagation at frequencies within the PSB.³⁷

Fig. 3 Photonic crystal effect: steady-state emission spectra (λ_ex = 280 nm) of PC2 (a) and PC3 (b), in contrast with that of PC4 (the red line in (a) and (b)). (c) PSB position as a function of θ for PC2 and PC3 samples. Emission spectra obtained at different angles of incidence from PC2 (d) and PC3 (e).

Based on theory, the PSB of PCs varies largely with the incident angle, which will induce modulation of photoluminescence.³⁸ PC2 and PC3 samples were selected to study the angle-dependent photoluminescent properties. Fig. 3(c) summarizes the angle-dependence of PSB for the PC2 and PC3 samples, which shows that the central position of the PSB gradually shifts to short wavelengths with an increasing incident angle. Fig. 3(d) and (e) show the emission spectra obtained from PC2 and PC3 at different incidence angles with respect to the normal direction of the (111) plane. In Fig. 3(d), the expected angle-dependent spectral change is not obvious for the band emissions of tungsten groups. This could be mainly attributed to the fact that as the angle is varied, the shift of PSB remains within the emission bands, thus the total density of states (DOS) for the band emissions rarely changes. From Fig. 3(e), it can be observed that the luminescent intensity of ⁵D₀–⁷F₂ at 618 nm for the Eu³⁺ ion increases with an increasing incident angle. This is due to the suppression being the most remarkable at normal incidence (θ = 0°). When the incident angle increases from 0° to 40°, the central position of the PSB gradually deviates from the emission peak of the Eu³⁺ ion, leading to a weakened inhibition of light emission.

3.4 Luminescent dynamics of as-fabricated samples

As shown in Fig. 4(a) and (b), the luminescent decay dynamics of IOPCs and the corresponding REF samples are compared. It is obvious that in IOPC samples, the luminescent decay becomes slow in contrast to the REF samples, which agrees well with previous literature results.^29,39 All the decay curves can be well fitted by a double-exponential function:

I(t) = I₁(0)e^−t/τ₁ + I₂(0)e^−t/τ₂ (3)

where τ₁ is slower and τ₂ is faster decay time constant measured at a fixed wavelength and I₁(0) and I₂(0) represent the relative contributions of the slower decay and the faster decay components, respectively. In Fig. 4(a), the dynamics of undoped ZnWO₄, the slower decay components come from the charge transfer transition from excited 2p orbitals of O²⁻ to the empty orbitals of the central W⁶⁺ ions, while the faster decay components may originate from the emission from surface defect states of nano-particles.^40,41 The decay time constants of the slower and the faster decay components (longer and shorter constants) with respect to tungstate groups in IOPC and REF samples are listed in Table 1. It can be seen that the decay time constants of the slower decay components of the IOPC samples are longer than those of the REF samples on account of the modulation of PCs on spontaneous emission. The decay time constants of faster decay components do not change significantly for IOPC and REF samples, which indicates that a shorter constant is not subjected to the effect of the PC structure. Therefore, decay time constants of tungstate groups herein derive from slower decay components. In Fig. 4(b), the slower decay components originate from the ⁵D₀–⁷F₂ transitions of Eu³⁺ ions, whereas the faster decay components should come from the tails for the band emissions of tungstate groups. For IOPCs and REF, the lifetime constants of tungstate groups are deduced to be 39 μs and 28 μs, respectively. The lifetime of tungstate groups in IOPCs is prolonged by 40% in contrast to the REF sample. Similarly, the decay time constant for the ⁵D₀–⁷F₂ transition of Eu³⁺ is prolonged from 571 μs (REF) to 786 μs (IOPCs). This implies that the spontaneous emission rates in IOPCs are suppressed. According to the virtual cavity model, the radiation lifetime can be approximatively expressed as,where f(ED) is the electric dipole strength, λ₀² is the wavelength in vacuum and n_eff is the average refractive index (formula (2)). Based on eqn (4), it can be easily deduced that in the PCs, the radiation lifetime should greatly increase compared to the REF samples. The practical variation in IOPCs relative to REF samples is much smaller than the theoretical prediction according to eqn (1), (2) and (4), which could be attributed to the complex structure of the wolframite matrix itself, which makes the filling factor x of the lattice shrinkage of IOPCs deviate from the ideal value. In addition, the non-radiative relaxation also contributes to the depopulation of emission levels. Fig. 4(c) displays the decay time constant of tungstate groups and Eu³⁺ ion as a function of the location of PSB. As the location of the PSB varies, the lifetimes of tungstate groups and Eu³⁺ ions in the PCs do not fluctuate significantly with different PSBs, which indicate that the decay time constants are nearly independent of the location of PSB. This result is consistent with previous conclusions.^27,29,38,39 This further indicates that the change in the suppression of the spontaneous emission rate between IOPCs and REF originates from the variation in the effective refractive index rather than the DOS. Fig. 4(d) shows the dependence of decay time constants on the angle of the incident light for tungstate groups and Eu³⁺ ions in the IOPC sample. It can be concluded that the decay time constants of the two PCs are nearly independent of the incident angle of light, which further indicates that the spontaneous emission rates do not vary with the location of PSB, wherein the location of PSB shifts significantly as the incident angle varies.

Fig. 4 Luminescent decay curves of the PC and REF samples under 280 nm excitation. (a) Decay time curves of tungstate groups (a) and the ⁵D₀–⁷F₂ transition of Eu³⁺ ions (b) embedded in PCs and the corresponding REF sample. The dots are experimental data and the solid curves are fitting functions. (c) Dependence of decay time constants on PSB positions. (d) Dependence of decay time constants on angle of incident light with respect to surface normal in ZnWO₄ : Eu³⁺ IOPCs.

Table 1 Variation of longer and shorter decay time constants of tungstate groups in IOPC and REF samples

Constants/samples	PC1	PC2	PC3	PC4	REF
Longer constants (μs)	37.8	36.1	38.7	36.8	28.1
Shorter constants (μs)	7.50	7.45	7.57	7.47	7.40

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3.5 ET from tungstate groups to Eu³⁺ ions in ZnWO₄ : Eu³⁺ IOPC

The emission spectra for Eu³⁺-doped ZnWO₄ in various REF samples and IOPCs are shown in Fig. 5(a) and (b), respectively. It can be seen that the luminescent intensity of tungstate groups decreases and the luminescent intensity of Eu³⁺ ions increases as the amount of doped Eu³⁺ increases, which can be understood by considering that the tungstate groups absorb energy from UV excitation and transfer the energy to Eu³⁺ ions. Furthermore, the change in luminescent intensity in PC samples (Fig. 5(b)) is more obvious than in REF samples, implying that ET is more effective in PCs, which can be mainly attributed to the suppression of non-radiative transitions in the network of IOPCs. Moreover, the integral intensity of as-fabricated IOPC and REF samples, which decreases as the Eu³⁺ concentration increases in both cases, is demonstrated in Fig. 5(c). In order to confirm the abovementioned results, the dependence of the luminescent dynamics of Eu³⁺ ions and tungstate groups on the concentration of Eu³⁺ was further studied. The decay time constants of Eu³⁺ ions and tungstate groups versus the concentration of Eu³⁺ ions in IOPC and REF samples were studied, as shown in Fig. 5(d) and (e). Fig. 5(d) shows that the lifetime for the ⁵D₀–⁷F₂ transition of Eu³⁺ varies little as the concentration of Eu³⁺ ions is increased, while in Fig. 5(e), it can be seen that the decay time constants of tungstate groups decrease gradually as the doping concentration of Eu³⁺ ions is increased, further indicating ET from tungstate groups to Eu³⁺ ions. In order to further reveal the ET processes, the inverses of the decay time constants of tungstate groups were calculated, as shown in Fig. 5(f). As the Eu³⁺ concentration varies, the spontaneous decay rate of tungstate groups in IOPCs increases linearly and more rapidly than that in REF samples, indicating an improvement in the ET rate. The ET rate can be deduced as 7.6 ms⁻¹ mol⁻¹ in the IOPC samples and 5.9 ms⁻¹ mol⁻¹ in the REFs. Based on the luminescent dynamics of tungstate groups, the ET efficiency from tungstate groups to Eu³⁺ can be roughly estimated by,where η is the ET efficiency, τ₁ is the lifetime constant of tungstate groups with Eu³⁺ doping, and τ₂ is the lifetime constant of tungstate groups without Eu³⁺ doping. Fig. 5(g) shows the deduced dependence of ET efficiency from tungstate groups to Eu³⁺ ions as a function of Eu³⁺ concentration. It is interesting to observe that the ET efficiency gradually increases as the Eu³⁺ concentration increases and the ET efficiency from tungstate groups to Eu³⁺ in IOPCs is always higher than that in the REF, which could be attributed to the suppression of nonradiative relaxation in IOPCs. Fig. 5(h) displays the inner quantum efficiency of ZnWO₄ : Eu³⁺ IOPCs as a function of Eu³⁺ concentration, which was obtained from the space integrated spectra in the integrated sphere.²⁹ It is obvious that the quantum efficiency of the IOPC samples is higher than that of the REF samples, resulting from the special network structure of IOPC, which consists of a thin wall of ZnWO₄ layers (∼75 nm) and a large periodic air cavity (hundreds of nanometers). The improvement in quantum efficiency in IOPC samples is mainly attributed to the following two behaviors of IOPC special structure. First, according to Snell's law, light can only radiate into the ambient medium when the incident angle θ is smaller than the critical angle θ_c = a sin (n_amb/n_crystal), where n_amb and n_crystal are the refractive indices of the ambient environment and the scintillation crystal, respectively. PC structures exhibit a periodically modulated refractive index with characteristic dimensions in the range of the wavelength of the incident light, thus acting like a diffraction grating that scatters impinging photons into various diffraction orders, which reduces light trapping in the crystal and increases light leakage into the ambient environment⁴³ as a consequence further improving the collection efficiency of light. Therefore, it can be seen that the integral intensity in Fig. 5(c) and the luminescent quantum efficiency in Fig. 5(h) of ZnWO₄ IOPCs is higher than that of REF samples, which is consistent with previous literature.^44,45 Second, it is known that in the traditional phosphors, due to the random distribution of defect states in the host lattices, the excited energy will probably be captured by some defect states near the activator in the process of energy migration.⁴⁶ The long-term ET from tungstate groups to defect states is very effective. Inevitably, the quenching of luminescence will occur due to the ET from luminescent centers to defect states, which are randomly distributed in the lattices of the phosphors.⁴⁷ In IOPCs, the long-term ET should be suppressed largely on account of the thin wall thickness of each ZnWO₄ : Eu³⁺ layer, which is only about 75 nm, as shown in Fig. 1(d), and the existence of long periodic and connected air cavities between the two layers. In this case, the ET among tungstate groups and from tungstate groups to defect states can occur only within the thin wall of the ZnWO₄ : Eu³⁺ layer and then the emitted photons are scattered into the air cavity rather than being largely captured by the defect states, losing energy in the form of non-radiative transition through further long-range ET. As a consequence, the IOPC structure can effectively restrain long-range ET among tungstate groups, ET from tungstate groups to defect states and non-radiative relaxation of Eu³⁺ ions, improving the luminescent quantum efficiency. This result shows that three-dimensional (3D) IOPC is a significant structure with respect to improving the quantum efficiency of traditional RE oxide phosphors.²⁹ The quantum efficiency in ZnWO₄ IOPCs decreases as Eu³⁺ concentration increases. This may be attributed to the formation of lattice defects resulting from the replacement of Zn²⁺ by Eu³⁺ in the ZnWO₄ crystal lattice, which leads to the quenching of photoluminescence.

Fig. 5 Emission spectra of ZnWO₄ : Eu³⁺ with different Eu³⁺ doped concentrations in the REF samples (a) and IOPC samples (b) under excitation of 280 nm light. (c) Dependence of the integral emission intensity of ZnWO₄ : Eu³⁺ on the doping concentration of Eu³⁺ in IOPC samples and REF samples. The decay time constant of Eu³⁺ (d) and tungstate groups (e) versus the concentration of Eu³⁺ ions in IOPC samples and REF samples. (f) Inverse of tungstate groups lifetime versus the concentration of Eu³⁺ ions in IOPC and REF samples. The solid curves (red line) are fitting functions and the dots (square and triangle) are experimental data. ET efficiency of tungstate groups to Eu³⁺ ions (g) and quantum efficiency (h) versus the concentration of Eu³⁺ ions in PC and REF samples.

3.6 ZnWO₄ : Eu³⁺ IOPCs white light phosphors

As is well known, white light is traditionally generated by mixing blue, green, and red light together. Emissions from tungstate groups covers blue and green light but lacks the red component. In the ZnWO₄ host, the red ⁵D₀–⁷F₂ transition is dominant for Eu³⁺, which can supply the red component of tungstate groups. Because of the high quantum yield and efficient energy transfer among tungstate groups and Eu³⁺ ions, ZnWO₄ : Eu³⁺ IOPCs will potentially become efficient white light phosphors. Fig. 6 shows the CIE chromaticity coordinates diagram of ZnWO₄ : Eu³⁺ IOPCs with different Eu³⁺ doping concentrations (0, 0.3, 0.5, 1, 2 and 3% in molar ratios) under the excitation of 280 nm light. It can be seen that the CIE chromaticity coordinates of pure ZnWO₄ IOPCs is situated in the blue area. As the concentration of Eu³⁺ ions increases, the CIE chromaticity coordinates of ZnWO₄ : Eu³⁺ IOPC shifts to the white light area. When the concentration of Eu³⁺ is up to 3%, white light emission is obtained.

Fig. 6 The CIE chromaticity coordinates of ZnWO₄ : Eu³⁺ IOPCs with different Eu³⁺ doping concentrations under 280 nm excitation.

4. Conclusions

In this study, ZnWO₄ and ZnWO₄ : Eu³⁺ IOPCs were prepared by the PMMA template technique and their optical properties were systemically studied. Several new contributions in this study should be highlighted. Firstly, it is significant to observe that the luminescent quantum efficiency of the IOPC samples is remarkably enhanced compared to that of the corresponding REFs, which can be attributed to the decreased nonradiative transition in IOPC. The quantum efficiency of ZnWO₄ IOPC is as high as 70%, which is almost the optimum obtained in ZnWO₄-based phosphors and in various IOPC materials. Secondly, the modulation of the PC effect on the broad band emissions of tungstate groups and sharp line emissions of Eu³⁺ ions were observed simultaneously. The decay time constants for the emissions of tungstate groups and the ⁵D₀–⁷F₂ transitions of Eu³⁺ ions in the IOPC samples were observed to be prolonged by about 40%. Thirdly, it was observed that in the IOPCs, the ET efficiency from tungstate groups to Eu³⁺ ions was considerably improved compared to that of the REF samples. Finally, ZnWO₄ : Eu³⁺ IOPCs were used as white light phosphors.

Acknowledgements

This study was supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant no.61204071), the Major State Basic Research Development Program of China (973 Program) (no.2014CB643506) and the National Natural Science Foundation of China (Grant no. 11374127, 11304118, 61204015, 81201738, 81301289, 61177042 and 11174111).

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