Different Roles of Ce³⁺ Optical Centers in Oxyorthosilicate Nanocrystals at X-ray and UV Excitation

Abstract

Luminescence properties of Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ nanocrystals were studied using photo- and X-ray luminescence techniques. The crystal structure of Re₂SiO₅ nanocrystals (P2₁/c space group) differs from the crystal structure of Re₂SiO₅ bulk crystals (C2/c space group) with 9- and 7-oxygen-coordinated cation positions instead of 6- and 7-coordinated ones observed for Re₂SiO₅ bulk crystals. Two optical centers (Ce1 and Ce2) were observed for Re₂SiO₅:Ce³⁺ nanocrystals originating from cerium ions substituting 9- and 7-oxygen-coordinated cation sites. Preferential substitution of larger cation sites by cerium ions leads to higher photoluminescence intensity of Ce1 centers, however, Ce2 centers are the main centers for electron-hole recombination, so only Ce2 band is observed in X-ray luminescence spectra. The features of oxygen coordination of Ce1 and Ce2 centers and high content of oxygen vacancies in Re₂SiO₅:Ce³⁺ nanocrystals can provide preferential trapping of electrons near Ce2 centers, and therefore, the dominant role of Ce2 band in X-ray luminescence spectra.

1. Introduction

Cerium-doped oxyorthosilicates (Re₂SiO₅) have gained a lot of attention in the last 30 years as highly-efficient scintillation materials [1,2,3]. Specifically, Lu₂SiO₅:Ce³⁺ crystals were widely studied by different groups due to their good scintillation properties including high scintillation yield (~25,000 photons/MeV) and short decay time (less than 40 ns) [4,5]. However, the wide-scale use of Lu₂SiO₅:Ce³⁺ crystals as scintillators is hampered by the long-lasting afterglow caused by the high number of shallow electron traps whose origin is still under discussion [6,7].

Lu₂SiO₅ and Y₂SiO₅ crystals have two positions for doped rare-earth ions which were earlier marked as Re1 and Re2. Already, the first studies of luminescence spectra of Lu₂SiO₅:Ce³⁺ crystals have revealed two luminescence centers (at 400 nm and 480 nm) which were attributed either to cerium ions in Lu1 and Lu2 position [8], or to one cerium ion in regular position and other in interstitial one [9]. The attribution of the main luminescence peak to 5d→4f transitions of Ce³⁺ ions coordinated by six oxygen anions, which was widely accepted in early papers on this subject [8], was later reconsidered [10], and now it is generally accepted that this band is formed by the luminescence of Ce³⁺ ions coordinated by seven oxygen ions, while that observed at 480 nm is ascribed to luminescence of six-coordinated Ce³⁺ ions. In the same way, two luminescence centers in Y₂SiO₅:Ce³⁺ bulk crystals and single crystalline films [11,12] have been ascribed to Ce³⁺ ions in 6- and 7-coordinated positions.

The studies of the structural and optical properties of Re₂SiO₅:Ce³⁺ nanocrystals (30 nm or less) have shown that their crystal structure is different from the structure of their bulk counterparts possessing P2₁/c space group instead of C2/c space group typical for Re₂SiO₅:Ce³⁺ bulk crystals, so their luminescent and scintillation characteristics are different as well [13]. Lu₂SiO₅:Ce³⁺ nanocrystals have sufficiently lower thermoluminescence [7,14] and afterglow levels [13], which may be attributed [8,13] to the higher content of oxygen vacancies (and, so, of F-centers that are possibly responsible for afterglow) in Lu₂SiO₅:Ce³⁺ bulk crystals as compared to Lu₂SiO₅:Ce³⁺ nanocrystals. However, the origin of this effect is still obscure, and requires more profound understanding.

In Re₂SiO₅ nanocrystals, two different positions for cerium ions are available—9-oxygen-coordinated one and 7-oxygen-coordinated one, but only one optical center was reported [13]. In this paper, more detailed study of Ce³⁺ X-ray and photoluminescence in Re₂SiO₅ nanocrystals is shown. In contrast to previous reports [8,13], two optical centers were observed in Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ nanocrystals ascribed to Ce³⁺ ions with different oxygen coordination, the first of which is dominant at photo- and second one at X-ray excitation.

2. Materials and Methods

Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ (C = 1 at. %) nanocrystals were synthesized by the sol–gel technique [15]. As starting reagents, powders of lutetium oxide (Lu₂O₃), yttrium oxide (Y₂O₃), and cerium oxide (CeO₂) were used. At the first stage of the synthesis, solutions of rare-earth nitrates Re(NO₃)₃ (c = 0.5 mol/L) were obtained from corresponding oxides by dissolving them in nitric acid followed by heating to t = 60–80 °C. Aqueous solutions of metal nitrates, TEOS (Si(OC₂H₅)₄) solution in anhydrous ethanol, and surfactant (polyoxyethylene) were mixed at room temperature in the calculated stoichiometric ratios. An aqueous solution of ammonia NH₄OH (10 wt. %) was used to neutralize the resulting mixture to pH value of ~8 for 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 Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ nanopowders were obtained. For a comparison of X-ray and photoluminescent properties between bulk and nanocrystals, Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ bulk crystals (C = 1 at. %) grown by Czochralski technique (purity—99.9 %) were taken. 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 spectrofluorimeter (Thermo Scientific, Waltham, MA, USA) (spectral resolution—0.5 nm). All spectra shown in the paper have been corrected for spectral sensitivity.

3. Results and Discussion

Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ nanocrystals obtained by the methods described in the previous section were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM) methods. The size of synthesized nanocrystals was about 30 nm (Supplementary Materials Figure S1). XRD of Lu₂SiO₅:Ce³⁺ nanocrystal is shown in Figure S2. Both Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ nanocrystals have the same monoclinic P2₁/c structure. This structure is not typical for bulk Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ crystals, which have C2/c crystal structure, and corresponds to Gd₂SiO₅-type structure. Previously, the same P2₁/c structure was observed for 30 nm Lu₂SiO₅:Ce³⁺ nanocrystals prepared using the methods of solution combustion synthesis (SCS) [13].

In Figure 1a photoluminescence and X-ray luminescence spectra of Lu₂SiO₅:Ce³⁺ bulk crystals are shown. The spectra obtained at X-ray excitation consist of an intensive luminescence band with maximum at 400 nm with less intensive sub-band at 420 nm (corresponding to 5d→²F_5/2 and 5d→²F_7/2 transitions of Ce³⁺ ion). The photoluminescence spectra at λ_exc = 325 nm consist of two bands with maxima at 400 nm and 470 nm. Excitation spectra taken at λ_reg = 400 nm consist of three bands with maxima at 270 nm, 295 nm and 355 nm (Figure 1b). Excitation spectra taken at λ_reg = 480 nm are sufficiently less intensive and poorly resolved with main peak observed at 325 nm and other peaks at 265 nm, 295 nm and 370 nm. Photoluminescence spectra obtained at selective excitation of different bands (λ_exc = 260 nm, 295 nm, and 360 nm) are shown in Figure 1a. Excitation at 260 nm, 295 nm, and 360 nm leads to luminescence with maximum at 400 nm and sub-band at 420 nm. This luminescence can be assigned to 5d→²F_5/2 and 5d→²F_7/2 transitions of 7-oxygen coordinated Ce³⁺ ion (Ce1 center). In the same way, the band at 470 nm can be ascribed to luminescence of 6-oxygen coordinated Ce³⁺ ion (Ce2 center). So, at 325 nm excitation the complex luminescence band with maximum is observed due to superposition of Ce2 (6-oxygen coordinated Ce³⁺ ion) and Ce1 luminescence.

The difference between the excitation spectra taken at different parts of the wide luminescence band clearly confirm its complex nature. In [8] the study of absorption spectra of Lu₂SiO₅:Ce³⁺ crystals allowed the authors to suppose the presence of two optical centers in Lu₂SiO₅:Ce³⁺:Ce1 (absorption bands at 3.5 eV (355 nm), 4.2 eV (295 nm), and 4.7 eV (264 nm)), and Ce2 (absorption band at 3.8 eV (325 nm)). They have attributed Ce1 center to 6-oxygen-coordinated Ce³⁺ ions, and Ce2 center to 7-oxygen-coordinated Ce³⁺ ions, but in the following papers [10] this attribution was reconsidered, and now Ce1 center is usually assigned to 7-oxygen-coordinated Ce³⁺ ions, and Ce2 center to 6-oxygen-coordinated Ce³⁺ ions. The excitation spectrum taken at 400 nm almost fully coincide with absorption spectrum of Ce1 center, while some additional low-intensive bands (at 265 nm, 295 nm and 370 nm) in the excitation spectrum taken at 480 nm can be ascribed either to excitation energy transfer between Ce1 and Ce2 centers, or to impact of luminescence of Ce1 centers into luminescence of Ce2 centers at 480 nm.

The X-ray and photoluminescence spectra of Y₂SiO₅:Ce³⁺ bulk crystals are shown in Figure 2a, and luminescence excitation spectra are shown in Figure 2b. They closely resemble the spectra obtained for Lu₂SiO₅:Ce³⁺ bulk crystals, and the same attribution of luminescence bands to cerium ions with different oxygen coordination seems reasonable.

In Figure 3a, photoluminescence and X-ray luminescence spectra of Lu₂SiO₅:Ce³⁺ nanocrystals are shown. Maximum of X-ray luminescence spectrum (470 nm) is red-shifted as compared to maxima of photoluminescence spectra. Excitation spectra (Figure 3b) taken at λ_reg = 400 nm consist of two intensive bands with maxima at 320 nm and 370 nm, and low-intensive band at 275–280 nm. At λ_reg = 500 nm, the excitation band at 320 nm is absent in the excitation spectra, and only 370 nm excitation band and low-intensive 275–280 nm can be observed. Photoluminescence spectra obtained at different excitation are shown in Figure 3a. At λ_exc = 320 nm the single luminescence band is observed with maximum at 380 nm, while at λ_exc = 370 nm the spectra consist of the luminescence band with maximum at 430 nm.

In Figure 4a,b photoluminescence, excitation and X-ray luminescence spectra of Y₂SiO₅:Ce³⁺ nanocrystals are shown. These spectra are similar to the spectra of Lu₂SiO₅:Ce³⁺ nanocrystals.

As Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ nanocrystals have Gd₂SiO₅-type structures, it seems reasonable to compare the obtained spectra with spectra of bulk Gd₂SiO₅:Ce³⁺ crystals which were discussed in the number of papers before [3,16]. The luminescence band with maximum at 430 nm was previously reported for Gd₂SiO₅:Ce³⁺ bulk crystals [17], and in [18,19], the complex character of this band was shown. The luminescence band with maximum at 380 nm was not observed before neither in the spectra of Lu₂SiO₅:Ce³⁺ (or Y₂SiO₅:Ce³⁺) bulk crystals, nor in the spectra of Gd₂SiO₅:Ce³⁺ bulk crystals, so it seems to be specific to nanocrystals, and may be related either to F-centers or surface-related defects.

Analysis of the luminescence band with maximum at 430 nm clearly confirms its complex nature (Figure 5). This band can be decomposed into two bands with maxima at 430 nm (2.9 eV) and 470 nm (2.65 eV). The sub-band at 430 nm is about 1.5 times more intensive than that at 470 nm. Previously for Lu₂SiO₅:Ce³⁺ bulk crystals, the tendency of Ce³⁺ ions to substitute preferably larger (7-oxygen coordinated) lutetium sites was shown. In Gd₂SiO₅ structure the average distance between rare-earth cation and surrounding ligands is ~2.49 Å for 9-oxygen coordinated site and ~2.39 Å for 7-oxygen coordinated site, so Ce³⁺ ions should substitute preferably 9-oxygen coordinated sites. Taking this fact into account, 430 nm band can be attributed to cerium ions at 9-oxygen coordinated sites (Ce1) and 470 nm band to cerium ions at 7-oxygen coordinated sites (Ce2). So, contrary to previous reports [7,13], which revealed only one optical center in Re₂SiO₅:Ce³⁺ (Re = Lu, Y, Gd) nanocrystals, we clearly observed two different optical centers similar to the centers observed for Gd₂SiO₅:Ce³⁺ bulk crystals. Most remarkably, the maximum of X-ray luminescence spectra coincides with the maximum of Ce2 luminescence. This fact does not correspond with situation in Gd₂SiO₅:Ce³⁺ bulk crystals where the maxima of UV-excited and high-energy excited luminescence coincide [13]. Moreover, the redshift of radioluminescence maximum for Gd₂SiO₅:Ce³⁺ nanocrystals, as compared to corresponding peak for Gd₂SiO₅:Ce³⁺ bulk crystals, was mentioned previously by [13], but the authors of that report were unable to give any explanation for this unexpected result. Coincidence of the maximum of X-ray luminescence spectra with the maximum of Ce2 photoluminescence makes it possible to argue that despite the fact that cerium ions preferentially occupy larger 9-oxygen coordinated positions, recombination of electron-hole pairs formed at high-energy excitation occurs almost exclusively at smaller 7-oxygen coordinated sites.

According to previously obtained results, the electron traps in Re₂SiO₅ structures are formed mostly by oxygen vacancies, while Ce³⁺ ions play the role of hole traps [20]. The formation of oxygen vacancies occurs mainly on the sites of non-silicon-bonded oxygen, which has sufficiently less binding energy compared to other oxygen ions, and therefore, can easily leave the lattice. The same should be hold for Lu₂SiO₅, and for Gd₂SiO₅-type structures, as observed for Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ nanocrystals. However, while for bulk Lu₂SiO₅ and Y₂SiO₅ the distance between cation site and non-silicon-bonded oxygen ions is almost the same for Re1 and Re2 sites (2.16 Å and 2.166 Å, respectively), for bulk Gd₂SiO₅ (and so, for Lu₂SiO₅ and Y₂SiO₅ nanocrystals), the surrounding of Re1 site includes only one non-silicon-bonded oxygen, and the surrounding of Re2 site includes three non-silicon-bonded oxygen ions [21]. So, the oxygen vacancies in cerium-doped crystals and nanocrystals Gd₂SiO₅-type structure are located preferentially near Ce2 ions. The high content of oxygen vacancies in Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ nanocrystals provided by oxygen-deficient conditions of sol-gel method used in this research and also by the decrease of vacancy formation energy in nanocrystals [22,23] can lead to the main role of relative location of oxygen vacancies and cerium ions. Electrons formed at X-ray excitation can move far away from their point of origin, but finally they are trapped on the oxygen vacancies near Ce2 centers and recombine with holes trapped on the same Ce2 centers. The supposition about the role of preferential location of oxygen vacancies close to Ce2 centers in the dynamics of relaxation of high-energy excitation is supported by the change of X-ray luminescence spectra after additional high-temperature treatment of Re₂SiO₅ nanocrystals in air for 1 h at 750 °C (in Figure 6 the results for Y₂SiO₅:Ce³⁺ nanocrystals are shown). The shift of the maximum after treatment in air corresponds to an increase of the relative impact of Ce1 centers and a decrease of the impact of Ce2 centers. As the initial synthesis did not include the stage of high-temperature treatment in air (only in argon atmosphere), additional treatment in air at 750 °C led to partial filling of oxygen vacancies by oxygen, so they could no longer take part in the processes of relaxation of high-energy excitation. So, while for samples with high content of oxygen vacancies (treated in argon), the electron-hole pairs should recombine preferentially on the Ce2 sites, for samples with lower content of oxygen vacancies (treated in air), the recombination on the Ce1 sites is probable as well. In this way, manipulation by the system of oxygen vacancies in oxyorthosilicate nanocrystals opens the way to changing the pathways of high-energy excitation recombination, and can be used for improving the scintillation properties of this material.

4. Conclusions

Lu₂SiO₅:Ce³⁺ and Y₂SiO₅:Ce³⁺ nanocrystals with Gd₂SiO₅-type structure were obtained by sol-gel method. Both nanocrystals have two optical centers (Ce1 and Ce2) formed by cerium ions substituting cations with different oxygen coordination. Photoluminescence spectra consist of the bands related to both centers, while X-ray luminescence spectra are formed by emission of Ce2 centers only. This fact was explained by inherent to Gd₂SiO₅-type structure preferential localization of oxygen vacancies near Ce2 sites which leads to higher recombination probability of electrons trapped by these oxygen vacancies with holes trapped at Ce2 sites, than with those trapped at Ce1 sites.