1. Introduction

In the past decades, scintillating materials have been drawing spectacular attention on account of their potential applications in the field of high energy engineering, industrial and medical imaging [1–3]. Owing to the much concern of nuclear security recently [4], great interest has been sparked in alternative scintillating materials systems ranging from new mono-crystal compositions to novel composite systems such as phosphor-based polymer materials, transparent ceramics and scintillating glasses [4,5]. However, there are several restrictions for the applications of these scintillating materials.

As for mono-crystals, its growth via Czochralski or Bridgman techniques is extremely complex and time-consuming, and always leads to the restricted cylindrical geometries [6]. Moreover, the techniques are only applicative for the materials that work well in such techniques. As for phosphor-based polymer materials, at first, the easily agglomerated phosphor particles encapsulated in the polymer matrix causes strong light scattering, which limits spatial resolution [7,8]. At second, the aged polymer matrix and mismatch of refractive indices between phosphor particles and polymer matrix reduce transparency, which also limits spatial resolution and hinders luminescent efficiency. As for transparent ceramics, there are still lots of challenges such as complicated and time-consuming procedures, limited compositions, tendency to scatter light, segregation of doping agents, etc [9]. The intensively explored scintillating glasses doped with rare earth ions are promising alternatives to single crystal and ceramic scintillators for practical application in high-energy physics, industrial and medical imaging fields owing to the advantages of transparency, low-cost, large-volume production and easy shaping of elements [2,7]. However, they suffer from low light yield resulting from high phonon energy, hindering their performance for practical applications [10,11].

Transparent oxyfluoride glass ceramics (GC), one of the most promising luminescent materials, are characterized by the homogeneous precipitation of fluoride nanocrystals in the oxide glass matrix [12–14]. When rare earth (RE) ions are introduced to the precursor glasses, these ions are preferentially enrich into the crystallized nanocrystals with low phonon energy after thermal treatment [13]. The low phonon energy of these precipitated nanocrystals can reduce the multi-phonon non-radiative relaxation rate, and enormously enhance the emission efficiency of RE ions [15]. In this case, the excellent optical properties of RE ions in nanocrystals can be utilized in glass matrix, with the advantages of glass preserved, such as good forming ability, high chemical, photochemical and mechanical stability [16–18]. However, to our best knowledge, reports concerning on oxyfluoride GC as potential scintillating materials are rare [19,20]. Hence, it is of paramount importance to search for oxyfluoride GC used as potential alternative scintillating materials.

Such GC can be achieved through controlled ucleation and crystallization from precursor glasses by appropriate heat-treatment procedures [21]. If the nanocrystals are formed, even the active ions have been participated into formed nanocrystals before thermal treatment, it is more convenient and energy-saving to acquire GC. Recently, Chen et al reported the fabrication and tunable upconversion luminescence in self-crystallized Er³⁺:K(Y_1-xYb_x)₃F₁₀ [21] and Na(Lu_1-xYb_x)F₄ GC [13]. In such system, nanocrystals are already formed via self-crystallization during melt-quenching, which facilitates the incorporation of more RE ions into the precipitated nanocrystals through further heat-treatment at relatively low temperature and prevents overgrowth of nanocrystals. That is, enabling high light yield and ensuring high transparency of derived GC at the same time.

As we all know, the green emission of Tb³⁺ matches the spectral sensitivity of conventional Charge Coupled Device (CCD) detectors very well so as to be widely applied in scintillating materials. But it is usually difficult for Tb³⁺ ions to participate into formed nanocrystals in GC compared with other RE ions such as Er³⁺ and Ho³⁺, making the enhancement of emissions in Tb³⁺-doped GC neglectable [18,22]. To overcome this problem, herein, self-crystallized KTb₂F₇ oxy-fluoride GC were successfully fabricated and systematically investigated. KTb₂F₇ nanocrystals were already formed before heat-treatment. By taking Tb³⁺ as host in KTb₂F₇ GC, greatly enhanced emissions for both PL and XEL are observed, resulting from the crystallization of KTb₂F₇ nanocrystals with low phonon energy. Our results indicate that KTb₂F₇ GC present potential applications in X-ray imaging such as industrial radiography and X-ray intensifying screen considering the good transparency, enhanced light yield of Tb³⁺ in KTb₂F₇ glass ceramics.

2. Experimental

Glass samples with nominal composition of 53SiO₂-10K₂CO₃-15Al₂O₃-18KF-4TbF₃ (in mol %) were prepared by melt-quenching method in air atmosphere. SiO₂, Al₂O₃, K₂CO₃, KF (A.R., all from Sinopharm Chemical Reagent Co., Ltd., China) and high purity EuF₃, TbF₃ (99.99%, from AnSheng Inorganic Materials Co., Ltd., China) were used as starting materials. The well ground stoichiometric chemicals were put into a covered alumina crucible and melted at 1550 °C for 1 h in air atmosphere. The melt was poured onto a stainless-steel plate and then pressed by another plate to form precursor glasses (labeled as PG). Subsequently, PG glasses were heat-treated for 2 h at 660 and 680 °C, which were labeled as GC660 and GC680, respectively. All samples were polished optically for further characterization.

To confirm the crystallization of all samples, X-ray diffraction (XRD) patterns were performed on a Philips X’Pert PRO SUPER X-ray diffraction apparatus with CuKa radiation (λ = 0.154056 nm) over the angular range 10°≤ 2θ ≤80° in a step size of 0.0167°. Transmittance spectra were measured on a U-3900 ultraviolet-visible (UV-VIS) spectrophotometer (Hitachi). The microstructure of GC was analyzed by a JEM-2010 transmission electron microscopy (TEM) (JOEL Ltd., Tokyo, Japan). Excitation, emission spectra and lifetime measurement were recorded on an Edinburgh FLS920 spectrofluorometer equipped with a continuous wave 450 W Xe lamp and a microsecond flashlamp (μF900) as excitation sources. X-ray tube operated under 50 kV and 3 mA was used as the source for the measurement of XEL spectra on FLS920. All the measurements were performed at room temperature.

3. Results and discussions

Figure 1(a) presents the XRD patterns of PG and GC samples. The distinct diffraction peaks in the XRD pattern of PG directly confirm the formation of nanocrystals before heat-treatment. With further heat-treatment, the characteristic diffraction peaks corresponding to monoclinic KTb₂F₇ nanocrystals (JCPDS No. 32-0849) become increasingly obvious, which indicates the growth of KTb₂F₇ nanocrystals after heat-treatment. The mean crystalline size D of KTb₂F₇ nanocrystals can be evaluated by Scherer’s equation [16],

 

Fig. 1 (a) XRD patterns of PG, GC660 and GC680, and the reference data of JCPDS card No. 32-0849 for KTb₂F₇; (b) Transmittance spectra; (c) TEM image of PG, the inset is corresponding SAED patterns; (d) HRTEM image of PG. (e) TEM image of GC680, the inset is corresponding SAED patterns; (f) HRTEM image of GC680.

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As described in the UV-VIS transmittance spectra of PG and GC given in Fig. 1(b), all samples maintain a fair transmittance up to 70% in VIS region owing to the much smaller size of precipitated KTb₂F₇ nanocrystals compared to the wavelength of VIS light, which will be favorable for their practical applications [15]. The absorption bands at 317, 341, 351, 368, 376 and 484 nm are attributed to the transitions of Tb³⁺ from ⁷F₆ ground state to ⁵H₇, ⁵L₇, ⁵L₉, ⁵D₂, ⁵D₃ and ⁵D₄ excited states, respectively [20].

To probe the microstructures of PG and GC samples, TEM and high-resolution TEM (HRTEM) images of PG and GC680 sample were performed (displayed in Fig. 1(c)-1(f)). TEM bright-field micrographs for both PG and GC680 clearly show that uniform spherical nanocrystals are lying on the gray background corresponding to the glass matrix. Both the size of nanocrystals and degree of crystallinity for GC increased compared to that of PG sample after proper heat-treatment, indicating the growth of KTb₂F₇ nanocrystals in GC. The mean size of KTb₂F₇ nanocrystals is about 30 and 36 nm in diameter for PG and GC680, respectively, which are similiar to the size estimated by Scherer’s equation. The polycrystalline diffraction feature of the precipitated nanocrystals is confirmed by the corresponding selected area electron diffraction (SAED) patterns (inset of Fig. 1(c) and Fig. 1(e)). The HRTEM images in Fig. 1(d) and 1(f) for PG and GC680 clearly reveal well-defined lattice structure and the relevant interplanar space d value measured by two separated crystal planes is about 0.330 and 0.586 nm for PG and GC680, which could be attributed to (111) and (101) crystal plane of KTb₂F₇ (d₍₁₁₁₎ = 0.328 nm and d₍₁₀₁₎ = 0.590 nm), respectively.

The excitation and emission spectra of PG, GC660 and GC680 are depicted in Fig. 2(a) and 2(b) respectively. By monitoring the ⁵D₄ → ⁷F₅ transition of Tb³⁺ emission at 543 nm, a series of characteristic excitation peaks centered at 283, 302, 317, 340, 351, 368, 376 and 484 nm are observed and assigned to ⁷F₆ → ⁵I₈, ⁵H₆, ⁵H₇, ⁵L₇, ⁵L₉, ⁵D₂, ⁵D₃ and ⁵D₄ transitions of Tb³⁺, respectively [14]. With increasing the temperature of thermal treatment, remarkable enhancement of the excitation intensity occurred for GC660 and GC680 compared to that of PG.

 

Fig. 2 (a) Excitation spectra (λ_em = 543 nm) and (b) emission spectra (λ_ex = 376 nm) of Tb³⁺-doped PG, GC660 and GC680 samples. (c) Decay curves for ⁵D₄ → ⁷F₅ transition (543 nm) of Tb³⁺ in PG, GC660 and GC680 (λ_ex = 376 nm). (d) XEL spectra of KTb₂F₇ PG,GC660 and GC680 samples under the excitation of X-ray (50 kV, 3 mA), the inset is corresponding enhanced factor of PL spectra and XEL spectra of GC660 and GC680 samples compared with PG sample.

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As displayed in Fig. 2(b), under the excitation of 376 nm (⁷F₆ → ⁵D₃), four characteristic emission bands of Tb³⁺ are observed in PG, GC660 and GC680. The emission bands centered at 488, 542, 583 and 621 nm are assigned to ⁵D₄ → ⁷F_J (J = 6, 5, 4 and 3) of Tb³⁺, respectively [12]. A great enhancement of the emission intensity is also observed in emission spectra after heat-treatment and the integrated emission intensity is enhanced by 1.44 and 2.75 times for GC660 and GC680, respectively. More KTb₂F₇ nanocrystals are formed after further heat-treatment, leading to further decrease of phonon energy and reduction of structure defects [11]. As a result, the non-radiative transition rate is further reduced and the emission intensity is enhanced [23].

Figure 2(c) exhibits the decay curves of 543 nm emission (⁵D₄ → ⁷F₅) of Tb³⁺ in PG, GC660 and GC680 excited by 376 nm light. The average fluorescence lifetime $\tau$ can be evaluated by the following equation [16],

In order to investigate the potential application of such KTb₂F₇ GC in X-ray scintillator, the XEL spectra of PG and GC samples excited by X-ray (50 kV, 3 mA) were performed, as shown in Fig. 2(d). Under the excitation of X-ray, four intense emission bands centered at 489, 543, 587, and 620 nm are observed and they can be ascribed to ⁵D₄ → ⁷F_J (J = 6, 5, 4 and 3) of Tb³⁺, respectively [14]. Compared to PG, the integrated XEL emission intensity is enhanced by 1.36 and 2.15 times for GC660 and GC680, respectively. Similarly, the astonishing enhancement in the XEL spectra is also caused by the crystallization of KTb₂F₇ nanocrystals with low phonon energy, which reduces the probability of non-radiative transition [19].

As exhibited in the inset of Fig. 2(d), the enhanced factor for XEL and PL are different. This is because the interaction mechanism of ultraviolet light excitation differs remarkably from that of X-ray excitation [11]. Under ultraviolet excitation, energy is deposited by direct excitation into the Tb³⁺ ions [10]. However, scintillating process is more complicated. Usually, three phases are involved in one scintillating process: (1) Energy conversion: initial energy release with formation of “hot” electrons and holes. (2) Transfer to luminescent centers: formation of excitonic states and groups of excited luminescent centers. (3) Light emission: relaxation of excited luminescent centers and emission of scintillating light [7,24].

4. Conclusions

Transparent self-crystallized KTb₂F₇ GC were successfully manufactured. The average size of KTb₂F₇ nanocrystals was estimated to be around 36 nm. By taking Tb³⁺ as host, both PL and XEL of KTb₂F₇ GC are highly enhanced after further heat-treatment due to the formation of KTb₂F₇ nanocrystals with low phonon energy, which suggests that such KTb₂F₇ GC present potential application in X-ray scintillator for X-ray imaging and our strategy that takes active ions as host may contributes to designing other oxy-fluoride GC using active ions as host.