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Anomalous enhancement of radioluminescence in Lu2-xYxSiO5:Ce3+ and ZnxMg1-xWO4 mixed oxide nanocrystals
Optical Materials 98 (2019) 109455
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Anomalous enhancement of radioluminescence in Lu2-xYxSiO5:Ce3þ and ZnxMg1-xWO4 mixed oxide nanocrystals Yuri Malyukin *, Vladyslav Seminko, Pavel Maksimchuk, Iryna Bespalova, Anna Yakubovskaya, Irina Tupitsyna Institute for Scintillation Materials, National Academy of Sciences of Ukraine, 61001, 60 Nauky Ave., Kharkiv, Ukraine
A R T I C L E I N F O
A B S T R A C T
Keywords: Scintillator Cation mixing Nanocrystals Silicates Tungstates
We have observed an enhancement of radioluminescence intensity in mixed oxide nanocrystals with activator (Lu2-xYxSiO5:Ce3þ nanocrystals) and intrinsic luminescence (ZnxMg1-xWO4 nanocrystals) which value is higher as compared to the one observed for bulk scintillation crystals. This effect was explained as a result of both reduction of thermalization length of geminate electron-hole pairs and decrease of the content of electron traps formed by oxygen vacancies at cation mixing. More sufficient change of the content of oxygen vacancies at cation mixing for nanocrystals than for bulk crystals leads to stronger increase of radioluminescence intensity.
1. Introduction The crystals of various structures and chemical compositions which are able to emit photons after interaction with the high energy particles (γ quanta, α particles, etc.), so-called scintillators, are widely used in a great diversity of applications ranging from the cargo inspection on the borders to the high energy physics experiments [1]. The mechanism of the light production by scintillators is multistage including the following consecutive stages: the generation of electron-hole pairs by the high energy particle; the thermalization of the electron-hole pairs and their annihilation with photon emission. For effective electron-hole annihi lation and, hence, the high light output, the distances between the geminate electrons and holes after their thermalization should be less than Onsager radius RO ¼ e2 =4πε0 εkB T, which is typically in the order of 1–10 nm (at room temperature) [2]. The thermalization length is determined by the diffusion rates or the effective masses of the electrons and holes in the conduction and valence bands, respectively. Hence, the increase of the probability of radiative annihilation of geminate electron-hole pairs could be achieved through the modification of the conduction and/or valence bands to suppress the movement of electrons and holes. In the number of publication the light output of the mixed oxide scintillators was shown to increase at the replacement of the part of regular cations by other cations [3–6]. The maximum light output of scintillators increased up to two times with the replacement of approximately 50% of regular cations. A number of different ideas have been proposed to explain the observed effect including the modification
of electron-phonon interaction, local fluctuations of the bandgap edges of the conduction and valence bands, and formation of clusters enriched by different cations [5,6]. Here, the radioluminescence of Lu2-xYxSiO5:Ce3þ nanocrystals with activator luminescence and ZnxMg1-xWO4 nanocrystals with intrinsic luminescence were studied with variation in the ratio of different cat ions. The effect of the radioluminescence increase at cation mixing for the nanocrystals is much stronger in comparison with bulk scintillation crystals. The observed anomalous enhancement of radioluminescence in mixed oxide nanocrystals could be explained by combined effect of cation mixing and change of the content of electron traps for mixed nanocrystals. 2. Materials and methods Mixed Lu2-xYxSiO5:Ce3þ (C ¼ 1 at.%) nanocrystals were synthesized by the sol–gel technique. As starting reagents powders of lutetium oxide (Lu2O3, 99.99%), yttrium oxide (Y2O3, 99.99%), and cerium oxide (CeO2, 99.99%) were taken. At the first stage of the synthesis, solutions of rare-earth nitrates Re(NO3)3 (c ¼ 0.5 mol/l) were obtained from corresponding oxides by dissolving them in nitric acid followed by heating to t ¼ 60–80 � C. Aqueous solutions of metal nitrates, TEOS (Si (OC2H5)4, 98.0%) solution in anhydrous ethanol, and surfactant (poly oxyethylene) were mixed at room temperature in the calculated stoi chiometric ratios. An aqueous solution of ammonia NH4OH (10 wt%) was used to neutralize the resulting mixture to pH value of ~8 for
* Corresponding author. E-mail address: [email protected] (Y. Malyukin). https://doi.org/10.1016/j.optmat.2019.109455 Received 3 September 2019; Received in revised form 12 October 2019; Accepted 14 October 2019 Available online 24 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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Optical Materials 98 (2019) 109455
For mixed Lu2-xYxSiO5:Ce3þ nanocrystals increase of luminescence in tensity was observed as compared to Lu2SiO5:Ce3þ nanocrystals, which was accompanied by shift of luminescence maximum from 390 nm to 410 nm. At further increase of yttrium content decrease of luminescence intensity and reverse shift of luminescence maximum was observed. Analysis of luminescence spectra of Lu2SiO5:Ce3þ and Y2SiO5:Ce3þ nanocrystals described in our previous paper [7] have shown the com plex nature of Ce3þ luminescence band in these nanocrystals. Regular Ce3þ centers provide the wide luminescence band with maximum at 430 nm, while irregular Ce3þ centers form additional band at 390 nm. Excitation spectra of Ce3þ luminescence [7] taken at 380–400 nm had their maxima at 310–315 nm, while the ones taken at 500 nm peaked at 370 nm. Dependence of excitation spectra on the luminescence wave length confirmed the complex nature of the luminescence bands. Luminescence spectra obtained at selective excitation of regular (λexc ¼ 370 nm) and irregular (λexc ¼ 310 nm) Ce3þ luminescent centers are shown in Fig. 2a and b, respectively. Both spectra consist of single wide bands with maxima at 430 nm (λexc ¼ 370 nm) and 380 nm (λexc ¼ 310 nm). However, the dependences of luminescence intensities of these bands on the yttrium content were sufficiently different. For the luminescent band obtained at λexc ¼ 370 nm (regular Ce3þ centers) in crease of Y3þ content leads to increase of luminescence intensity up to 50% of Y3þ ions, for higher yttrium concentrations intensity decreases. At the same time, the intensity of the luminescence band obtained at selective excitation of irregular (λexc ¼ 310 nm) Ce3þ luminescent cen ters decreases with increase of yttrium content from 0 to 50%, while further increase of yttrium content did not led to sufficient change of 380 nm luminescence intensity. Overall, the magnitude of luminescence intensity change was 1.6 times for 430 nm band and 1.1 times for 380 nm band. At the same time the full concentration doped Ce3þ ions remained a constant in all studied samples. Hence, the spectral and in tensity variation presented in Figs. 1a and 2a are the result of selective excitation of non-equivalent Ce3þ centers. More sufficient increase of luminescence intensity was observed at Xray excitation (25 kV, 40 μA) of Lu2-xYxSiO5:Ce3þ nanocrystals. In Fig. 3 the X-ray luminescence spectra of Lu2-xYxSiO5:Ce3þ nanocrystals with different yttrium content are shown. The X-ray luminescence spectra of Lu2-xYxSiO5:Ce3þ nanocrystals consist of a single band with maximum at 470 nm, so there maxima are sufficiently red-shifted as compared to the maxima of luminescence bands observed in photoluminescence spectra at λexc ¼ 370 nm ascribed to regular Ce3þ centers. The possible explanation of these red-shift of Xray luminescence was proposed in our previous paper [7] and consist in the complex nature of 430 nm luminescence band formed by lumines cence of Ce3þ centers with different oxygen coordination. Luminescence of both these centers can be excited by 370 nm excitation, but only one of them takes part in X-ray luminescence due to peculiarities of relax ation of high-energy excitation [7]. The intensity of X-ray luminescence demonstrates the same behavior depending on the yttrium content, namely, it increases at increase of Y3þ content from 0 to 50%, after which it decreases again. However, the magnitude of this effect is higher than for 430 nm photoluminescence band, as the X-ray luminescence intensity increases in 2.5 times (see inset) compared with 1.6 times for photoluminescence. Increase of X-ray luminescence intensity was observed also at addi tional high-temperature treatment of Lu2-xYxSiO5:Ce3þ nanocrystals in air (700 � C, 1 h). In Fig. 3b the luminescence spectra of Lu2SiO5:Ce3þ nanocrystals before and after such treatment are shown. The X-ray luminescence intensity of Lu2SiO5:Ce3þ nanocrystals after treatment in air is ~1.5 times higher than before treatment.
complete precipitation of hydroxides. After that, the suspension was maintained for 6 h at a temperature of 70–80 � C for partial removal of water, alcohol and nitric acid. The resulting product was gradually heated to a temperature of 100–120 � C and held at this temperature for 1.0–1.5 h (the process of drying and partial dehydration), and then at 250 � C for 4 h (thermal dehydration). After treatment at 750 � C for 4 h and at 1000 � C for 2 h in argon atmosphere Lu2-xYxSiO5:Ce3þ nano powders were obtained. ZnxMg1-xWO4 nanocrystals were produced using the method of liquid-phase synthesis, followed by crystallization in molten LiNO3 (99.95%). Amorphous ZnxMg1-xWO4 samples were prepared by copre cipitation from aqueous solutions of Zn(NO3)2 (99.99%), Mg(NO3)2 (99.995%) and Na2WO4 (99.995%) at room temperature. Cleaned and dried precipitates were mixed with lithium nitrate in a weight ratio of 1:10 and melted at 300 � C followed by aging for 16 h. The reaction product was washed, filtered and dried at 80 � C in air. For high temperature treatment alundum crucibles were used. X-ray luminescence was excited by X-ray tube (25 kV, 40 μA) and registered using the SDL-1 grating monochromator (spectral resolution – 1 nm) with the Hamamatsu R9110 PMT in the photon counting mode. Luminescence and excitation spectra were taken using Lumina spec trofluorimeter (Thermo Scientific, USA) (spectral resolution – 0.5 nm). All spectra shown in the paper are corrected for the spectral sensitivity. 3. Results and discussion 3.1. Lu2-xYxSiO5:Ce3þ nanocrystals Lu2-xYxSiO5:Ce3þ nanocrystals were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM) methods. The size of synthesized nanocrystals was about 30–40 nm (Table S1) that exceeds typical Onsager radius (Fig. S1). Lu2SiO5:Ce3þ, Y2SiO5:Ce3þ and mixed Lu2-xYxSiO5:Ce3þ nanocrystals have the same monoclinic P21/c structure which is not typical for bulk Lu2SiO5:Ce3þ and Y2SiO5:Ce3þ crystals, which have C2/c crystal structure, and corresponds to Gd2SiO5type structure (XRD of Lu2SiO5:Ce3þ nanocrystals was discussed in de tails in our previous paper [7]). Previously, the same P21/c structure was observed for 30 nm Lu2SiO5:Ce3þ nanocrystals prepared using the methods of solution combustion synthesis (SCS) [8]. In Fig. 1a the luminescence spectra of Lu2-xYxSiO5:Ce3þ nanocrystals at 325 nm excitation are shown. For both Lu2SiO5:Ce3þ and Y2SiO5:Ce3þ nanocrystals spectra consist of a wide band with maximum at 390 nm.
3.2. Zn1-хMgхWO4 nanocrystals XRD analysis has shown that all samples are monophasic and have monoclinic wolframite structure slightly distorted in comparison to zinc tungstate ZnWO4 (JCDPS No. 15-0774). Although all nanocrystals were
Fig. 1. Luminescence spectra of Lu2-xYxSiO5:Ce3þ nanocrystals with different yttrium content at λexc ¼ 325 nm. 2
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Fig. 2. Luminescence spectra of Lu2-xYxSiO5:Ce3þ nanocrystals with different yttrium content at a) λexc ¼ 370 nm, b) λexc ¼ 310 nm.
Fig. 3. X-ray luminescence spectra of a) Lu2-xYxSiO5:Ce3þ nanocrystals with different yttrium content; b) Lu2SiO5:Ce3þ nanocrystals before and after hightemperature treatment in air (700 � C, 1 h).
Fig. 4. Luminescence spectra of Zn1-хMgхWO4 nanocrystals with different magnesium content at a) λexc ¼ 270 nm, b) λexc ¼ 355 nm. 3
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obtained under identical conditions, samples of the starting compounds had smaller sizes as compared with the mixed nanocrystals. Zn1хMgхWO4 nanocrystals had grain-like shape with dimensions of 40–200 nm, the average sizes for nanoparticles of different compositions are shown in Table S1 [9,10]. The luminescence spectra of Zn1-хMgхWO4 nanocrystals at different excitation wavelengths are shown in Fig. 4. The luminescence spectra obtained at λexc ¼ 270 nm consist of the single wide band with maximum at 480 nm. Increase of magnesium content leads to increase of intensity of this band, while its maximum retains the same value until magnesium concentration reaches 100%, for MgWO4 the maximum is shifted to 460 nm. At λexc ¼ 355 nm the luminescence spectra are completely different consisting of the wide band with maximum at 650 nm. Contrary to 480 nm luminescence band, intensity of 650 nm luminescence band decreases with increase of magnesium content from 0 to 50%, after which an increase of luminescence intensity is observed. For MgWO4 the maximum of this band is red-shifted to 730 nm. The luminescence band of Zn1-хMgхWO4 nanocrystals with maximum at 480 nm was observed before for bulk ZnWO4 crystals and relates to radiative relaxation of excitation in WO66 oxyanion complex involving electron transfer from the 5d tungsten orbitals to the 2p ox ygen orbitals, described also as a relaxation of the self-trapped exciton [11–13]. More advanced studies have shown that this band is complex and the sub-bands related to transitions in undistorted and distorted WO66 oxyanion complexes were resolved [14,15]. The similar results were obtained for MgWO4 [16,17]. The luminescence band with maximum at 650 nm can be related to irregular luminescence centers such as WO66 centers distorted by oxygen vacancies [9]. The X-ray luminescence spectra of Zn1-хMgхWO4 nanocrystals consist of single band with maximum at 495 nm (Fig. 5). As well as the lumi nescence band obtained at 270 nm excitation, this band is also related to radiative relaxation in WO66 oxyanion complexes. Intensity of this band increases at increase of magnesium content reaching the maximum value at 50% of magnesium ions. The magnitude of this change is suf ficiently higher than the change of the band at 270 nm excitation (Fig. 4a), while at 270 nm excitation cation mixing leads to increase of 480 nm luminescence intensity in 1.5 times, at X-ray excitation intensity increases in 4.5 times at 50% of magnesium ions as compared to ZnWO4 nanocrystals.
3.3. Discussion The effect of improvement of scintillation characteristics (such as light output or X-ray luminescence intensity) at cation mixing was observed before for high number of bulk crystals [5,6]. In Ref. [18] the increase of X-ray luminescence intensity was reported for Zn1-хMgхWO4 bulk crystals with maximum value of 1.5 times as compared to ZnWO4 at cation ratio 1:1. At the same time, change of Lu3þ cations to Y3þ ones for Lu2-xYxSiO5:Ce3þ bulk crystals leads to linear increase of the light output according to Vegard’s law [19], and no maxima was observed at 1:1 cation ratio (unlike bulk Lu2-xGdxSiO5:Ce3þ crystals, for which the light output reaches 30000 photons/MeV as compared to 18000 photo ns/MeV for Lu2SiO5:Ce3þ at Lu:Gd ¼ 1:1 [20]). The increase of photoluminescence intensity of the luminescence bands related to regular optical centers (Ce3þ ions or WO66 oxyanion complex) in Lu2-xYxSiO5:Ce3þ and Zn1-хMgхWO4 nanocrystals is accompanied by decrease of intensity of irregular optical centers (lowcoordinated Ce3þ ions or distorted WO66 oxyanion complex). In this way, cation mixing leads to redistribution of the content of different optical centers leading to higher content of regular ones. The similar effect was observed for Lu2-xGdxSiO5:Ce3þ bulk crystals where cation mixing led to increase of the content of 7-oxygen coordinated Ce3þ centers as compared to 6-oxygen coordinated ones, and was explained by loosening of the lattice at incorporation of bigger Gd3þ ions instead of Lu3þ ones [20]. At the same time, the increase of X-ray luminescence intensity cannot be explained just by redistribution of optical centers as the magnitude of this effect is sufficiently higher than for photoluminescence. The strong X-ray luminescence intensity increase at cation mixing for both Lu23þ and Zn1-хMgхWO4 nanocrystals can be treated in the xYxSiO5:Ce framework of the concept of electrons scattering by the random poten tials [21] produced by the substitution of the regular cations and leading to formation of localized electronic states. Really, for both Lu2-xYxSiO5: Ce3þ and Zn1-хMgхWO4 nanocrystals excited states (and their energy) of Y and Mg ions considerably differ from those observed for Lu and Zn ions (Fig. S2), respectively. The doped ions (Y or Mg) are not able to modify considerably the conduction bands of the nanocrystals; however, they can form random short-range scattering potentials. According to Lifshitz theory, such potentials lead to formation of localized electron states (Lifshitz tails) in the band gap [22]. Such localized states can appear in conduction band as well [22]. The electrons which are produced by a high-energy particle in mixed scintillating crystals can be captured by such localized states and then move only by the hopping mechanism. This mechanism can work both in mixed bulk and nanosized scintillators reducing thermalization length of geminate electron-hole pairs and increasing efficiency of their recombination with photon emission. Cation mixing also can change the content of structural defects in the lattice (for instance, oxygen vacancies). Electron traps formed by oxygen vacancies are usually considered as a sufficient obstacle for achieving high light output in both Lu2SiO5:Ce3þ [8,20] and ZnWO4 bulk crystals [23]. So, decrease of the concentration of oxygen vacancies (for instance, at high-temperature treatment of nanocrystals in air, Fig. 3b) leads to increase of the light output. Effective elimination of electron traps related to oxygen vacancies at partial substitution of lutetium ions by yttrium or gadolinium ones in Lu2SiO5:Ce3þ bulk crystals was pre viously reported in Ref. [24]. Also our own results (Fig. 4b) show that cation mixing in Zn1-хMgхWO4 nanocrystals leads to decrease of the luminescence band related to WO66 centers distorted by oxygen va cancies (650 nm) that can be related as to redistribution of optical centers, so to decrease of the content of oxygen vacancies at cation mixing. It should be noted that this effect is more pronounced for nanocrystals than for their bulk counterparts [9] in agreement with recent studies which have shown that oxide nanocrystals are charac terized by considerable higher concentration of oxygen vacancies in comparison with their bulk analogues due to relatively low oxygen
Fig. 5. X-ray luminescence spectra of Zn1-хMgхWO4 nanocrystals with different magnesium content. 4
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binding energy [25,26]. So, the effect of decrease of the content of ox ygen vacancies at cation mixing which is more pronounced for nano crystals than for bulk crystals can be one of the causes of stronger enhancement of X-ray luminescence intensity for oxide nanocrystals.
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4. Conclusions Cation mixing in Lu2-xYxSiO5:Ce3þ and ZnxMg1-xWO4 nanocrystals leads to sufficient increase of radioluminescence intensity as compared to Lu2SiO5:Ce3þ and ZnWO4 ones (in 2.5 and 4.5 times, respectively). The increase of radioluminescence intensity in nanocrystals at cation mixing is sufficiently higher than the one observed for corresponding bulk crystals, and cannot be explained only by redistribution of regular and irregular optical centers for nanocrystals as compared to their bulk analogues. The observed increase of X-ray luminescence intensity can be interpreted as a combined effect of cation mixing leading to reduction of thermalization length of geminate electron-hole pairs and decrease of the content of electron traps formed by oxygen vacancies in mixed nanocrystals. The latter effect should be more pronounced for nano crystals than for their bulk analogues due to higher content of oxygen vacancies in nanocrystals as compared to bulk crystals that can at least partially explain the stronger enhancement of X-ray luminescence in tensity for mixed oxide nanocrystals. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109455. References [1] P. Lecoq, A. Gektin, M. Korzhik, Inorganic Scintillators for Detector Systems: Physical Principles and Crystal Engineering, Springer, New York, 2016. [2] S.A. Payne, W.W. Moses, S. Sheets, L. Ahle, N.J. Cherepy, B. Sturm, S. Dazeley, G. Bizarri, W.S. Choong, Nonproportionality of scintillator detectors: theory and experiment. II, IEEE Trans. Nucl. Sci. 58 (2011) 3392–3402. [3] K. Kamada, T. Endo, K. Tsutumi, T. Yanagida, Y. Fujimoto, A. Fukabori, A. Yoshikawa, J. Pejchal, M. Nikl, Composition engineering in cerium-doped (Lu, Gd)3(Ga, Al)5O12 single-crystal scintillators, Cryst. Growth Des. 11 (2011) 4484–4490. [4] E.D. Bourret-Courchesne, G.A. Bizarri, R. Borade, G. Gundiah, E.C. Samulon, Z. Yan, S.E. Derenzo, Crystal growth and characterization of alkali-earth halide scintillators, J. Cryst. Growth 352 (2012) 78–83.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Anomalous enhancement of radioluminescence in Lu2-xYxSiO5:Ce3þ and ZnxMg1-xWO4 mixed oxide nanocrystals Yuri Malyukin *, Vladyslav Seminko, Pavel Maksimchuk, Iryna Bespalova, Anna Yakubovskaya, Irina Tupitsyna Institute for Scintillation Materials, National Academy of Sciences of Ukraine, 61001, 60 Nauky Ave., Kharkiv, Ukraine
A R T I C L E I N F O
A B S T R A C T
Keywords: Scintillator Cation mixing Nanocrystals Silicates Tungstates
We have observed an enhancement of radioluminescence intensity in mixed oxide nanocrystals with activator (Lu2-xYxSiO5:Ce3þ nanocrystals) and intrinsic luminescence (ZnxMg1-xWO4 nanocrystals) which value is higher as compared to the one observed for bulk scintillation crystals. This effect was explained as a result of both reduction of thermalization length of geminate electron-hole pairs and decrease of the content of electron traps formed by oxygen vacancies at cation mixing. More sufficient change of the content of oxygen vacancies at cation mixing for nanocrystals than for bulk crystals leads to stronger increase of radioluminescence intensity.
1. Introduction The crystals of various structures and chemical compositions which are able to emit photons after interaction with the high energy particles (γ quanta, α particles, etc.), so-called scintillators, are widely used in a great diversity of applications ranging from the cargo inspection on the borders to the high energy physics experiments [1]. The mechanism of the light production by scintillators is multistage including the following consecutive stages: the generation of electron-hole pairs by the high energy particle; the thermalization of the electron-hole pairs and their annihilation with photon emission. For effective electron-hole annihi lation and, hence, the high light output, the distances between the geminate electrons and holes after their thermalization should be less than Onsager radius RO ¼ e2 =4πε0 εkB T, which is typically in the order of 1–10 nm (at room temperature) [2]. The thermalization length is determined by the diffusion rates or the effective masses of the electrons and holes in the conduction and valence bands, respectively. Hence, the increase of the probability of radiative annihilation of geminate electron-hole pairs could be achieved through the modification of the conduction and/or valence bands to suppress the movement of electrons and holes. In the number of publication the light output of the mixed oxide scintillators was shown to increase at the replacement of the part of regular cations by other cations [3–6]. The maximum light output of scintillators increased up to two times with the replacement of approximately 50% of regular cations. A number of different ideas have been proposed to explain the observed effect including the modification
of electron-phonon interaction, local fluctuations of the bandgap edges of the conduction and valence bands, and formation of clusters enriched by different cations [5,6]. Here, the radioluminescence of Lu2-xYxSiO5:Ce3þ nanocrystals with activator luminescence and ZnxMg1-xWO4 nanocrystals with intrinsic luminescence were studied with variation in the ratio of different cat ions. The effect of the radioluminescence increase at cation mixing for the nanocrystals is much stronger in comparison with bulk scintillation crystals. The observed anomalous enhancement of radioluminescence in mixed oxide nanocrystals could be explained by combined effect of cation mixing and change of the content of electron traps for mixed nanocrystals. 2. Materials and methods Mixed Lu2-xYxSiO5:Ce3þ (C ¼ 1 at.%) nanocrystals were synthesized by the sol–gel technique. As starting reagents powders of lutetium oxide (Lu2O3, 99.99%), yttrium oxide (Y2O3, 99.99%), and cerium oxide (CeO2, 99.99%) were taken. At the first stage of the synthesis, solutions of rare-earth nitrates Re(NO3)3 (c ¼ 0.5 mol/l) were obtained from corresponding oxides by dissolving them in nitric acid followed by heating to t ¼ 60–80 � C. Aqueous solutions of metal nitrates, TEOS (Si (OC2H5)4, 98.0%) solution in anhydrous ethanol, and surfactant (poly oxyethylene) were mixed at room temperature in the calculated stoi chiometric ratios. An aqueous solution of ammonia NH4OH (10 wt%) was used to neutralize the resulting mixture to pH value of ~8 for
* Corresponding author. E-mail address: [email protected] (Y. Malyukin). https://doi.org/10.1016/j.optmat.2019.109455 Received 3 September 2019; Received in revised form 12 October 2019; Accepted 14 October 2019 Available online 24 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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For mixed Lu2-xYxSiO5:Ce3þ nanocrystals increase of luminescence in tensity was observed as compared to Lu2SiO5:Ce3þ nanocrystals, which was accompanied by shift of luminescence maximum from 390 nm to 410 nm. At further increase of yttrium content decrease of luminescence intensity and reverse shift of luminescence maximum was observed. Analysis of luminescence spectra of Lu2SiO5:Ce3þ and Y2SiO5:Ce3þ nanocrystals described in our previous paper [7] have shown the com plex nature of Ce3þ luminescence band in these nanocrystals. Regular Ce3þ centers provide the wide luminescence band with maximum at 430 nm, while irregular Ce3þ centers form additional band at 390 nm. Excitation spectra of Ce3þ luminescence [7] taken at 380–400 nm had their maxima at 310–315 nm, while the ones taken at 500 nm peaked at 370 nm. Dependence of excitation spectra on the luminescence wave length confirmed the complex nature of the luminescence bands. Luminescence spectra obtained at selective excitation of regular (λexc ¼ 370 nm) and irregular (λexc ¼ 310 nm) Ce3þ luminescent centers are shown in Fig. 2a and b, respectively. Both spectra consist of single wide bands with maxima at 430 nm (λexc ¼ 370 nm) and 380 nm (λexc ¼ 310 nm). However, the dependences of luminescence intensities of these bands on the yttrium content were sufficiently different. For the luminescent band obtained at λexc ¼ 370 nm (regular Ce3þ centers) in crease of Y3þ content leads to increase of luminescence intensity up to 50% of Y3þ ions, for higher yttrium concentrations intensity decreases. At the same time, the intensity of the luminescence band obtained at selective excitation of irregular (λexc ¼ 310 nm) Ce3þ luminescent cen ters decreases with increase of yttrium content from 0 to 50%, while further increase of yttrium content did not led to sufficient change of 380 nm luminescence intensity. Overall, the magnitude of luminescence intensity change was 1.6 times for 430 nm band and 1.1 times for 380 nm band. At the same time the full concentration doped Ce3þ ions remained a constant in all studied samples. Hence, the spectral and in tensity variation presented in Figs. 1a and 2a are the result of selective excitation of non-equivalent Ce3þ centers. More sufficient increase of luminescence intensity was observed at Xray excitation (25 kV, 40 μA) of Lu2-xYxSiO5:Ce3þ nanocrystals. In Fig. 3 the X-ray luminescence spectra of Lu2-xYxSiO5:Ce3þ nanocrystals with different yttrium content are shown. The X-ray luminescence spectra of Lu2-xYxSiO5:Ce3þ nanocrystals consist of a single band with maximum at 470 nm, so there maxima are sufficiently red-shifted as compared to the maxima of luminescence bands observed in photoluminescence spectra at λexc ¼ 370 nm ascribed to regular Ce3þ centers. The possible explanation of these red-shift of Xray luminescence was proposed in our previous paper [7] and consist in the complex nature of 430 nm luminescence band formed by lumines cence of Ce3þ centers with different oxygen coordination. Luminescence of both these centers can be excited by 370 nm excitation, but only one of them takes part in X-ray luminescence due to peculiarities of relax ation of high-energy excitation [7]. The intensity of X-ray luminescence demonstrates the same behavior depending on the yttrium content, namely, it increases at increase of Y3þ content from 0 to 50%, after which it decreases again. However, the magnitude of this effect is higher than for 430 nm photoluminescence band, as the X-ray luminescence intensity increases in 2.5 times (see inset) compared with 1.6 times for photoluminescence. Increase of X-ray luminescence intensity was observed also at addi tional high-temperature treatment of Lu2-xYxSiO5:Ce3þ nanocrystals in air (700 � C, 1 h). In Fig. 3b the luminescence spectra of Lu2SiO5:Ce3þ nanocrystals before and after such treatment are shown. The X-ray luminescence intensity of Lu2SiO5:Ce3þ nanocrystals after treatment in air is ~1.5 times higher than before treatment.
complete precipitation of hydroxides. After that, the suspension was maintained for 6 h at a temperature of 70–80 � C for partial removal of water, alcohol and nitric acid. The resulting product was gradually heated to a temperature of 100–120 � C and held at this temperature for 1.0–1.5 h (the process of drying and partial dehydration), and then at 250 � C for 4 h (thermal dehydration). After treatment at 750 � C for 4 h and at 1000 � C for 2 h in argon atmosphere Lu2-xYxSiO5:Ce3þ nano powders were obtained. ZnxMg1-xWO4 nanocrystals were produced using the method of liquid-phase synthesis, followed by crystallization in molten LiNO3 (99.95%). Amorphous ZnxMg1-xWO4 samples were prepared by copre cipitation from aqueous solutions of Zn(NO3)2 (99.99%), Mg(NO3)2 (99.995%) and Na2WO4 (99.995%) at room temperature. Cleaned and dried precipitates were mixed with lithium nitrate in a weight ratio of 1:10 and melted at 300 � C followed by aging for 16 h. The reaction product was washed, filtered and dried at 80 � C in air. For high temperature treatment alundum crucibles were used. X-ray luminescence was excited by X-ray tube (25 kV, 40 μA) and registered using the SDL-1 grating monochromator (spectral resolution – 1 nm) with the Hamamatsu R9110 PMT in the photon counting mode. Luminescence and excitation spectra were taken using Lumina spec trofluorimeter (Thermo Scientific, USA) (spectral resolution – 0.5 nm). All spectra shown in the paper are corrected for the spectral sensitivity. 3. Results and discussion 3.1. Lu2-xYxSiO5:Ce3þ nanocrystals Lu2-xYxSiO5:Ce3þ nanocrystals were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM) methods. The size of synthesized nanocrystals was about 30–40 nm (Table S1) that exceeds typical Onsager radius (Fig. S1). Lu2SiO5:Ce3þ, Y2SiO5:Ce3þ and mixed Lu2-xYxSiO5:Ce3þ nanocrystals have the same monoclinic P21/c structure which is not typical for bulk Lu2SiO5:Ce3þ and Y2SiO5:Ce3þ crystals, which have C2/c crystal structure, and corresponds to Gd2SiO5type structure (XRD of Lu2SiO5:Ce3þ nanocrystals was discussed in de tails in our previous paper [7]). Previously, the same P21/c structure was observed for 30 nm Lu2SiO5:Ce3þ nanocrystals prepared using the methods of solution combustion synthesis (SCS) [8]. In Fig. 1a the luminescence spectra of Lu2-xYxSiO5:Ce3þ nanocrystals at 325 nm excitation are shown. For both Lu2SiO5:Ce3þ and Y2SiO5:Ce3þ nanocrystals spectra consist of a wide band with maximum at 390 nm.
3.2. Zn1-хMgхWO4 nanocrystals XRD analysis has shown that all samples are monophasic and have monoclinic wolframite structure slightly distorted in comparison to zinc tungstate ZnWO4 (JCDPS No. 15-0774). Although all nanocrystals were
Fig. 1. Luminescence spectra of Lu2-xYxSiO5:Ce3þ nanocrystals with different yttrium content at λexc ¼ 325 nm. 2
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Fig. 2. Luminescence spectra of Lu2-xYxSiO5:Ce3þ nanocrystals with different yttrium content at a) λexc ¼ 370 nm, b) λexc ¼ 310 nm.
Fig. 3. X-ray luminescence spectra of a) Lu2-xYxSiO5:Ce3þ nanocrystals with different yttrium content; b) Lu2SiO5:Ce3þ nanocrystals before and after hightemperature treatment in air (700 � C, 1 h).
Fig. 4. Luminescence spectra of Zn1-хMgхWO4 nanocrystals with different magnesium content at a) λexc ¼ 270 nm, b) λexc ¼ 355 nm. 3
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obtained under identical conditions, samples of the starting compounds had smaller sizes as compared with the mixed nanocrystals. Zn1хMgхWO4 nanocrystals had grain-like shape with dimensions of 40–200 nm, the average sizes for nanoparticles of different compositions are shown in Table S1 [9,10]. The luminescence spectra of Zn1-хMgхWO4 nanocrystals at different excitation wavelengths are shown in Fig. 4. The luminescence spectra obtained at λexc ¼ 270 nm consist of the single wide band with maximum at 480 nm. Increase of magnesium content leads to increase of intensity of this band, while its maximum retains the same value until magnesium concentration reaches 100%, for MgWO4 the maximum is shifted to 460 nm. At λexc ¼ 355 nm the luminescence spectra are completely different consisting of the wide band with maximum at 650 nm. Contrary to 480 nm luminescence band, intensity of 650 nm luminescence band decreases with increase of magnesium content from 0 to 50%, after which an increase of luminescence intensity is observed. For MgWO4 the maximum of this band is red-shifted to 730 nm. The luminescence band of Zn1-хMgхWO4 nanocrystals with maximum at 480 nm was observed before for bulk ZnWO4 crystals and relates to radiative relaxation of excitation in WO66 oxyanion complex involving electron transfer from the 5d tungsten orbitals to the 2p ox ygen orbitals, described also as a relaxation of the self-trapped exciton [11–13]. More advanced studies have shown that this band is complex and the sub-bands related to transitions in undistorted and distorted WO66 oxyanion complexes were resolved [14,15]. The similar results were obtained for MgWO4 [16,17]. The luminescence band with maximum at 650 nm can be related to irregular luminescence centers such as WO66 centers distorted by oxygen vacancies [9]. The X-ray luminescence spectra of Zn1-хMgхWO4 nanocrystals consist of single band with maximum at 495 nm (Fig. 5). As well as the lumi nescence band obtained at 270 nm excitation, this band is also related to radiative relaxation in WO66 oxyanion complexes. Intensity of this band increases at increase of magnesium content reaching the maximum value at 50% of magnesium ions. The magnitude of this change is suf ficiently higher than the change of the band at 270 nm excitation (Fig. 4a), while at 270 nm excitation cation mixing leads to increase of 480 nm luminescence intensity in 1.5 times, at X-ray excitation intensity increases in 4.5 times at 50% of magnesium ions as compared to ZnWO4 nanocrystals.
3.3. Discussion The effect of improvement of scintillation characteristics (such as light output or X-ray luminescence intensity) at cation mixing was observed before for high number of bulk crystals [5,6]. In Ref. [18] the increase of X-ray luminescence intensity was reported for Zn1-хMgхWO4 bulk crystals with maximum value of 1.5 times as compared to ZnWO4 at cation ratio 1:1. At the same time, change of Lu3þ cations to Y3þ ones for Lu2-xYxSiO5:Ce3þ bulk crystals leads to linear increase of the light output according to Vegard’s law [19], and no maxima was observed at 1:1 cation ratio (unlike bulk Lu2-xGdxSiO5:Ce3þ crystals, for which the light output reaches 30000 photons/MeV as compared to 18000 photo ns/MeV for Lu2SiO5:Ce3þ at Lu:Gd ¼ 1:1 [20]). The increase of photoluminescence intensity of the luminescence bands related to regular optical centers (Ce3þ ions or WO66 oxyanion complex) in Lu2-xYxSiO5:Ce3þ and Zn1-хMgхWO4 nanocrystals is accompanied by decrease of intensity of irregular optical centers (lowcoordinated Ce3þ ions or distorted WO66 oxyanion complex). In this way, cation mixing leads to redistribution of the content of different optical centers leading to higher content of regular ones. The similar effect was observed for Lu2-xGdxSiO5:Ce3þ bulk crystals where cation mixing led to increase of the content of 7-oxygen coordinated Ce3þ centers as compared to 6-oxygen coordinated ones, and was explained by loosening of the lattice at incorporation of bigger Gd3þ ions instead of Lu3þ ones [20]. At the same time, the increase of X-ray luminescence intensity cannot be explained just by redistribution of optical centers as the magnitude of this effect is sufficiently higher than for photoluminescence. The strong X-ray luminescence intensity increase at cation mixing for both Lu23þ and Zn1-хMgхWO4 nanocrystals can be treated in the xYxSiO5:Ce framework of the concept of electrons scattering by the random poten tials [21] produced by the substitution of the regular cations and leading to formation of localized electronic states. Really, for both Lu2-xYxSiO5: Ce3þ and Zn1-хMgхWO4 nanocrystals excited states (and their energy) of Y and Mg ions considerably differ from those observed for Lu and Zn ions (Fig. S2), respectively. The doped ions (Y or Mg) are not able to modify considerably the conduction bands of the nanocrystals; however, they can form random short-range scattering potentials. According to Lifshitz theory, such potentials lead to formation of localized electron states (Lifshitz tails) in the band gap [22]. Such localized states can appear in conduction band as well [22]. The electrons which are produced by a high-energy particle in mixed scintillating crystals can be captured by such localized states and then move only by the hopping mechanism. This mechanism can work both in mixed bulk and nanosized scintillators reducing thermalization length of geminate electron-hole pairs and increasing efficiency of their recombination with photon emission. Cation mixing also can change the content of structural defects in the lattice (for instance, oxygen vacancies). Electron traps formed by oxygen vacancies are usually considered as a sufficient obstacle for achieving high light output in both Lu2SiO5:Ce3þ [8,20] and ZnWO4 bulk crystals [23]. So, decrease of the concentration of oxygen vacancies (for instance, at high-temperature treatment of nanocrystals in air, Fig. 3b) leads to increase of the light output. Effective elimination of electron traps related to oxygen vacancies at partial substitution of lutetium ions by yttrium or gadolinium ones in Lu2SiO5:Ce3þ bulk crystals was pre viously reported in Ref. [24]. Also our own results (Fig. 4b) show that cation mixing in Zn1-хMgхWO4 nanocrystals leads to decrease of the luminescence band related to WO66 centers distorted by oxygen va cancies (650 nm) that can be related as to redistribution of optical centers, so to decrease of the content of oxygen vacancies at cation mixing. It should be noted that this effect is more pronounced for nanocrystals than for their bulk counterparts [9] in agreement with recent studies which have shown that oxide nanocrystals are charac terized by considerable higher concentration of oxygen vacancies in comparison with their bulk analogues due to relatively low oxygen
Fig. 5. X-ray luminescence spectra of Zn1-хMgхWO4 nanocrystals with different magnesium content. 4
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binding energy [25,26]. So, the effect of decrease of the content of ox ygen vacancies at cation mixing which is more pronounced for nano crystals than for bulk crystals can be one of the causes of stronger enhancement of X-ray luminescence intensity for oxide nanocrystals.
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4. Conclusions Cation mixing in Lu2-xYxSiO5:Ce3þ and ZnxMg1-xWO4 nanocrystals leads to sufficient increase of radioluminescence intensity as compared to Lu2SiO5:Ce3þ and ZnWO4 ones (in 2.5 and 4.5 times, respectively). The increase of radioluminescence intensity in nanocrystals at cation mixing is sufficiently higher than the one observed for corresponding bulk crystals, and cannot be explained only by redistribution of regular and irregular optical centers for nanocrystals as compared to their bulk analogues. The observed increase of X-ray luminescence intensity can be interpreted as a combined effect of cation mixing leading to reduction of thermalization length of geminate electron-hole pairs and decrease of the content of electron traps formed by oxygen vacancies in mixed nanocrystals. The latter effect should be more pronounced for nano crystals than for their bulk analogues due to higher content of oxygen vacancies in nanocrystals as compared to bulk crystals that can at least partially explain the stronger enhancement of X-ray luminescence in tensity for mixed oxide nanocrystals. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.optmat.2019.109455. References [1] P. Lecoq, A. Gektin, M. Korzhik, Inorganic Scintillators for Detector Systems: Physical Principles and Crystal Engineering, Springer, New York, 2016. [2] S.A. Payne, W.W. Moses, S. Sheets, L. Ahle, N.J. Cherepy, B. Sturm, S. Dazeley, G. Bizarri, W.S. Choong, Nonproportionality of scintillator detectors: theory and experiment. II, IEEE Trans. Nucl. Sci. 58 (2011) 3392–3402. [3] K. Kamada, T. Endo, K. Tsutumi, T. Yanagida, Y. Fujimoto, A. Fukabori, A. Yoshikawa, J. Pejchal, M. Nikl, Composition engineering in cerium-doped (Lu, Gd)3(Ga, Al)5O12 single-crystal scintillators, Cryst. Growth Des. 11 (2011) 4484–4490. [4] E.D. Bourret-Courchesne, G.A. Bizarri, R. Borade, G. Gundiah, E.C. Samulon, Z. Yan, S.E. Derenzo, Crystal growth and characterization of alkali-earth halide scintillators, J. Cryst. Growth 352 (2012) 78–83.
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