1. Introduction

To overcome the resolution reduction caused by light scattering, some special scintillator screens have recently been developed and yielded excellent results in X-ray micro-imaging. Several modes of the screen have been proposed, including transparent [1,2], columnar-structured [3] and pixel-structured [4] scintillator films, and scintillator fibers [5]. Among them, the transparent scintillator film is a more effective route in X-ray imaging with a spatial resolution in the micrometer and even submicrometer scale [1,2]. As a consequence, the exploration of the transparent films with high scintillation performance will be crucial for the development of high-spatial-resolution X-ray imaging. A remarkable improvement of the detection efficiency can be expected by increasing the absorption and luminescent efficiency of scintillator materials. High-density materials are the most attractive candidates owing to their high X-ray absorption coefficient [6–8]. Lutetium orthotantalate (LuTaO₄) exhibits two types of structures, i.e., M-type with space group 12/a and M′-type with P2/a [9]. Rare-earth activated M′-type LuTaO₄ has been proved to be a superior X-ray luminescence material due to its outstandingly high density (9.75 g⋅cm⁻³), stable chemical property and strong irradiation hardness [7,10]. It could be a promising candidate of scintillator films. Up to now, however, all researches on rare-earth activated LuTaO₄ are limited to polycrystalline powders [7–10] because no corresponding single crystals or transparent thin films are available.

In various thin film deposition techniques, the sol-gel route has the advantages of low synthesis temperature, reduced time and simple operations. Simultaneously, the starting materials are mixed homogeneously on a molecular level and the doping concentration can be achieved from a few ppm to several percents [11]. Thus it is especially suitable for fabricating the luminescent films [11–13]. Here, we present the preparation and luminescence properties of the M′-type LuTaO₄:Ln³⁺ (Ln = Eu, Tb) scintillator films. The crystallization, morphology, photoluminescence (PL) and X-ray excited luminescence (XEL) of the thin films are discussed. The results demonstrate that this novel scintillator film is an excellent candidate for the applications in X-ray micro-imaging.

2. Experimental details

Lu_0.9Ln_0.1TaO₄ (Ln = Eu, Tb) scintillator films were prepared by Pechini sol-gel processing with spin-coating technique. Stoichiometric amounts of Lu(NO₃)₃·6H₂O (99.99%) and Eu₂O₃ (99.99%) were fully dissolved in 2-methoxyethanol (99%) and HCl (36 wt%), respectively, and Tb(NO₃)₃⋅6H₂O (99.99%) was dissolved in 2-methoxyethanol. The prescribed amount of TaCl₅ (99.99%) was dissolved into the mixed solution of HCl and 2-methoxyethanol by stirring at 80°C. These prepared solutions were mixed together at the molar ratio of Ta⁵⁺:Lu³⁺:Ln³⁺ (Ln = Eu, Tb) = 1:0.9:0.1. Successively, an appropriate amount of polyethylene glycol 400 (PEG400) (A.R.) was added. The mixture was refluxed at 130°C for 1 h and laid up for 48 h at room temperature to form a sol. The sol was spin-coated on a carefully cleaned quartz glass substrate. The as-formed film was pyrolyzed on a hot plate at 150°C for 5 min and then immediately transferred it into a 450°C muffle furnace for 1 h. Repetitive coatings and heat treatments were carried out so as to obtain a thicker film. After five repetitive coatings, it was finally heated up to 1200°C with a heating rate of 1°C/min and sintered for 2 h. Crack-free, dense and transparent thin film was obtained.

The structure was analyzed by an X-ray diffractometer (XRD, DX2700, Haoyuan Instrument Co., Ltd., Dandong, PR China) with Cu K_α radiation (λ = 0.15405 nm) operated at 40 kV and 30 mA. The morphology was observed with a Philips XL30 scanning electron microscope (SEM). The transmission measurement was performed on a JASCO V-570 ultraviolet-visible-near infrared spectrophotometer. The PL spectrum was recorded on a Hitachi F7000 fluorescence spectrophotometer. The decay time was determined by an FLS 920 fluorescence spectrometer (Edinburgh Instrument, UK) with a Xe lamp as the excitation source. XEL spectrum was measured using an X-ray excited spectrometer, where an F30-III X-ray tube (W anticathode target) was used as the source, and operated at 100 kV and 6 mA. Luminescence spectrum was recorded on Zolix SBP300 monochromator and Hamamatsu PMTH-CR131 photomultiplier with the data acquired by a computer. All measurements were carried out at room temperature.

3. Results and discussion

The XRD patterns of the LuTaO₄:Ln³⁺ (Ln = Eu, Tb) scintillator films deposited on quartz glass substrates and the standard data of LuTaO₄ (JCPDS 24-1263) are presented in Fig. 1. It indicates that the diffraction peaks of the samples coincide well with the standard data and no any peaks from the starting materials or impurities are observed, which demonstrates that the both samples belong to monoclinic phase M′-type LuTaO₄ with P2/a space group and the doping ions are in eight-coordination with C₂ symmetry. And the diffraction peaks with higher intensity and narrower FWHM (full width at half maximum) suggest that the both samples have a good crystallization.

 

Fig. 1 XRD patterns of the M'-type LuTaO₄:Ln³⁺ (Ln = Eu, Tb) thin films.

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For the activated ions, the nominal doping amount (Ln³⁺/Ta⁵⁺ in mol ratio, Ln = Eu, Tb) is 0.1. In order to acquire the actual doping amount, Vegard's law [14], a_x = (1-x) a₁ + xa₂, is applied, where x denotes the actual doping amount of Eu³⁺ or Tb³⁺, a_x and a₁ and a₂ are the lattice constants of Lu_1-xLn_xTaO₄ (Ln = Eu, Tb), LuTaO₄ (5.239 Å [9]) and LnTaO₄ (5.368 Å for EuTaO₄ and 5.338 Å for TbTaO₄ [9]), respectively. The lattice constant a_x obtained by JADE 9.0 software package for Eu³⁺ or Tb³⁺ activated M′-type LuTaO₄ is 5.249 Å or 5.247 Å, respectively. According to Vegard's law, the actual doping amount of Eu³⁺ or Tb³⁺ in LuTaO₄ is about 0.078 or 0.081, respectively, which is lower than the nominal value of 0.1.

The SEM images of the samples are given in Fig. 2. Homogeneous, crack-free, smooth and dense scintillator films have been successfully prepared on quartz glass substrates by Pechini sol-gel technique. It can be observed that these samples have fine nanostructure with nanosized grains uniformly spreading over the entire substrate surfaces. The average size of the crystallites is estimated to be about 200 nm in diameter. The thicknesses of the thin films are about 339 and 330 nm, respectively, which are determined by the cross-sectional views shown in Figs. 2(c) and 2(d).

 

Fig. 2 The surface morphologies and cross-sectional views of the M′-type LuTaO₄:Ln³⁺ thin films, (a) and (c) Ln = Eu, (b) and (d) Ln = Tb.

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The optical transparency directly affects the light yield of the scintillator films. Figure 3 displays the transmission spectra of the samples deposited on the quartz glass substrates. The thin films are fairly transparent as presented in the insets of Fig. 3. A average transmittance of about 75% in the visible region is achieved for the both samples. Furthermore, the absorption coefficient α can be calculated by α = -lnT/d, where T and d denote the transmittance and the thickness of the thin film, respectively. On the basis of the relation of α² ∝ (hν - E_opt), here hν and E_opt indicate photon energy and optical band gap, respectively, we can estimate E_opt by plotting (lnT)² as a function of hν and drawing a tangential line near the absorption edge. The results are also given in the insets of Fig. 3. It indicates that the absorption edges are independently of the doping ions, which suggests the optical band gap of the M′-type LuTaO₄ thin film is about 5.5 eV. This result is experimentally determined for the first time.

 

Fig. 3 Transmission spectra of the M′-type LuTaO₄:Ln³⁺ thin films, (a) Ln = Eu, (b) Ln = Tb, the insets present their photographs and optical band gaps.

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The PL spectra of the samples are illustrated in Fig. 4.In the excitation spectrum of the LuTaO₄:Eu³⁺ thin film shown in Fig. 4(a), a broad band peaking at 209 nm is attributed to the charge transfer (CT) transition in which an electron is transferred from ${\text{TaO}}_{\text{4}}^{\text{3-}}$ group to Lu³⁺ site [7,9], and the excitation energy is transferred to Eu³⁺. A shoulder at 245 nm is originated from Eu³⁺ CT transition, which occurs due to electron delocalization from the filled 2p shell of the O²⁻ to the partially filled 4f shell of Eu³⁺ [15].

 

Fig. 4 Excitation and emission spectra of the M'-type LuTaO₄:Ln³⁺ thin films, (a) Ln = Eu (λ_ex = 209 nm, λ_em = 613 nm), (b) Ln = Tb (λ_ex = 209 nm, λ_em = 548 nm).

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In the emission spectrum of the Eu³⁺ activated thin-film sample also displayed in Fig. 4(a), the ⁵D₀→⁷F_J (J = 1, 2, 3, 4) transitions are observed in the range of 570-720 nm. The dominant emission peak located at 613 nm is caused by the electric dipole transition, indicating that the Eu³⁺ ions occupied the sites with non-inversion symmetry (C₂) in M′-type LuTaO₄ host lattice [16]. The ⁵D₀→⁷F_J emissions can be split by the symmetry circumstance around Eu³⁺. If the symmetry of the sites occupied by Eu³⁺ ions is monoclinic (C₂, C_s), three and five components for the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions could be observed [16], respectively. Eu³⁺ ions occupy C₂ sites in the M′-type LuTaO₄ host lattice, but only two and four components for the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions were respectively detected in our measurements. This is due to the limitation of the spectrometer resolution. The ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄ emissions are also split into three and four components, respectively. The emissions located in the range of 400-570 nm (intensity multiplied by 15) are very weak in comparison with the ⁵D₀→⁷F₂ transition. They can be ascribed to the ⁵D_J→⁷F_J' (J = 1, 2, 3; J' = 0, 1, 2, 3) transitions of Eu³⁺. Furthermore, the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions are of particular interest because they actually represent the local environment of the Eu³⁺ [17]. The ⁵D₀→⁷F₁ transition is magnetic-dipole allowed and its intensity exhibits very little variation with the crystal field strength surrounding the Eu³⁺ ions, whereas the intensity of the ⁵D₀→⁷F₂ electric-dipole transition is sensitive to the local structure acting on the Eu³⁺ ions. Therefore, the integral intensity ratio of electric-dipole to magnetic-dipole transition, known as the asymmetry ratio, is widely used for probing the local symmetry of Eu³⁺ in host lattice. The asymmetry ratio is 2.24, which is larger than that of the powder phosphor [8], implying a lower local symmetry of Eu³⁺ and leading to a more effective ⁵D₀→⁷F₂ emission in the thin film samples.

Figure 4(b) shows the PL spectra of LuTaO₄:Tb³⁺ thin film. The band located at 209 nm in excitation spectrum is also the CT transition of ${\text{TaO}}_{\text{4}}^{\text{3-}}$, implying the excitation energy transfer from host lattice to Tb³⁺. The band peaking at 260 nm is attributed to the 4f⁸→4f⁷5d¹ transition of Tb³⁺, which is the dipolar electric parity allowed transition [15]. The emission spectrum consists of the typical emissions of Tb³⁺ owing to ⁵D₄→⁷F_J (J = 6, 5, 4, 3) transitions. Among them, the green emission at 548 nm corresponding to the ⁵D₄→⁷F₅ transition is the most intensive, which can be explained by the large values of the reduced matrix elements at J = 5 and the Judd–Ofelt theory [18,19]. The blue emissions at wavelengths below 475 nm originated from the ⁵D₃→⁷F_J (J = 6, 5, 4, 3) transitions of Tb³⁺ ions have not been observed. This is resulted from the cross-relaxation in the form of Tb³⁺(⁵D₃) + Tb³⁺(⁷F₆) →Tb³⁺(⁵D₄) + Tb³⁺(⁷F₀) at a higher Tb³⁺ doping amount [16].

The decay curves of the ⁵D₀→⁷F₂ and ⁵D₄→⁷F₅ for the LuTaO₄:Ln³⁺ (Ln = Eu, Tb) thin films are displayed in Fig. 5.They can be fitted by a single exponential function: I = I₀exp(t/τ), where I₀ denotes the initial intensity at t = 0, and τ is the 1/e lifetime. The decay times of the both samples are 1.08 and 1.10 ms, respectively.

 

Fig. 5 Decay curves of the M′-type LuTaO₄:Ln³⁺ thin films, (a) Ln = Eu (λ_ex = 209 nm, λ_em = 613 nm), (b) Ln = Tb (λ_ex = 209 nm, λ_em = 548 nm).

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It has clearly demonstrated in the excitation spectra given in Fig. 4 that the${\text{TaO}}_{\text{4}}^{\text{3-}}$ CT band is provided with the most intensive. That is to say that the indirect excitation through energy transfer from host lattice to Eu³⁺ or Tb³⁺ is more efficiently than the direct excitation of Eu³⁺ or Tb³⁺. Hence, high luminescence efficiency could be expected under ionizing radiation excitation. The scintillation spectra obtained under X-ray excitation are illustrated in Fig. 6.It reveals typical emissions of Eu³⁺ and Tb³⁺, and a higher signal-to-noise ratio means that the sol-gel derived M′-type LuTaO₄:Ln³⁺ (Ln = Eu, Tb) scintillator films have a superior XEL performance. Certainly, the emission intensity could be further improved by increasing the doping concentration of activated ion and the thin-film thickness.

 

Fig. 6 XEL spectra of the M'-type LuTaO₄:Ln³⁺ thin films, (a) Ln = Eu, (b) Ln = Tb.

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4. Conclusions

In conclusion, a new type of M′-type LuTaO₄:Ln³⁺ (Ln = Eu, Tb) scintillator films was successfully fabricated on quartz glass substrates via Pechini sol-gel technique, and characterized by XRD, SEM, transmission, PL and XEL. The thin films with the transmittance of 75% in the visible region are homogeneous, crack-free and dense. The optical band gap of the LuTaO₄ thin film with the value of 5.5 eV is experimentally determined for the first time. Under ultraviolet excitation, Eu³⁺ or Tb³⁺ activated M′-type LuTaO₄ thin film exhibits a dominated red or green emission, respectively, and the decay times of the ⁵D₀→⁷F₂ and ⁵D₄→⁷F₅ transitions are 1.08 ms or 1.10 ms, respectively. The samples display an excellent XEL performance. All results demonstrate that this novel scitillator film is expected to be a promising candidate for applications in high-spatial-resolution X-ray imaging. Further efforts will be focused on improving the thin-film thickness.