Radio-, Thermo- and Photoluminescence Properties of Lu₂O₃:Eu and Lu₂O₃:Tb Nanopowder and Film Scintillators

Abstract

This work is dedicated to the preparation and characterization of the radio-, thermo-, and photoluminescent properties of Lu₂O₃:Eu and Lu₂O₃:Tb nanopowder (NPs) scintillators, prepared by means of hydrothermal processing, and their film analogues made of these NPs by the spin coating method. The luminescent properties of NPs and films were characterized by cathodoluminescence (CL), photoluminescence (PL), X-ray excited radioluminescence (RL), and thermoluminescence (TL) at low and high temperatures. In Lu₂O₃:Eu NPs and films, mostly the luminescence of Eu³⁺ ions occupying the C₂ site of the host, with the most intensive peaks at 611.6 nm and a decay time of 1.5 ms, was observed. On the contrary, two types of Tb³⁺ centers in the C₂ and C_3i sites with the main emission lines at 542.4 and 544.0 nm and the corresponding 4f→5d excitation bands at 270 and 305 nm and decay times of t_1/e = 2.17 and 3.96 ms were observed in the case of Lu₂O₃:Tb NPs and films. Indications were noted that Tb³⁺ in the C_3i symmetry position was most active in the CL spectra of Lu₂O₃:Tb NPs and a respective film. Thermoluminescent peaks at 110 °C and 170 °C for Lu₂O₃:Eu NPs and at 75 °C and 120 °C in Lu₂O₃:Tb NPs were observed corresponding to the hole and electron traps, respectively. Significantly different onsets of temperature quenching of Eu³⁺ and Tb³⁺ luminescence in Lu₂O₃:Eu and Lu₂O₃:Tb NPs were found at ~90 °C and ~320 °C, respectively.

1. Introduction

Lu₂O₃ (lutetia) is a structural analog of Y₂O₃ sesquioxide. Both are well-known hosts for rare-earth doped phosphors and scintillators. Eu³⁺ and Tb³⁺ doped Lu₂O₃ oxides are mostly researched as sintered ceramics and nanocrystalline powder (NP) X-ray phosphors [1,2,3]. Due to its extremely high density of 9.42 g/cm³ (one of the densest inorganic compounds) and high effective Z-number Z_eff = 67.3, Lu₂O₃-based phosphors attract attention as film scintillating screens for 2D/3D microimaging using X-ray or synchrotron radiation [4,5]. However, the properties of these materials in the various crystalline forms were found to be influenced by the differences in the methods and conditions of their preparation [6]. Understanding of these dependencies is important for deliberate management of phosphors properties according to the application needs.

Apart from the liquid phase epitaxy (LPE), which enables receiving Lu₂O₃:Eu single crystalline films from a few to tens of microns thick [5,6], other methods of film preparation, such as magnetron sputtering [7], pulse laser deposing [8,9], spin coating [10,11], etc., can be utilized for the fabrication of films of reasonable thicknesses. Most interesting are the methods of Lu₂O₃ film preparation, with thicknesses in the range of tens of nm to one micron, e.g., in the range where LPE cannot be used. This can be realized in the case of film preparation from the respective Lu₂O₃ based NPs using spin coating (SC) technology [10,11].

The aim of our work is the preparation and characterization of the luminescent properties of Lu₂O₃:Eu and Lu₂O₃:Tb NPs as well as their thin film counterparts with thicknesses in the submicron range, prepared by the SC method using the mentioned NPs. Such a study is especially important for producing scintillating screens for microimaging with high special resolution in the micron scale. For characterization of the luminescent properties of NP and films, data from complementary spectroscopic methods, such as cathodoluminescence, photoluminescence, radioluminescence, and thermoluminescence, were used.

2. Sample Preparation and Experimental Techniques

The Lu₂O₃:3%Eu and Lu₂O₃:1%Tb NPs were sintered by means of hydrothermal processing [2]. For preparation of these NP phosphors, appropriate amounts of Lu₂O₃ and/or Eu₂O₃ and Tb₄O₇ oxides were dissolved in diluted HNO₃ at 80 °C. Then solid KOH was added until pH = 12. The immediately-formed suspension was cooled to room temperature (RT) and poured into an autoclave with a Teflon liner. The autoclave was heated at 230 °C for 10 h. After cooling, the precipitate was separated and washed several times with a water–ethanol mixture and dried. Finally, the powder was heat-treated at 900 °C for 5 h and then for 2 h at 950 °C in air.

The Lu₂O₃:Eu and Lu₂O₃:Tb thin NP films were prepared on quartz substrates using the SC method. Zinc acetate 2-hydrate (Zn(CH₃COO)₂*2H₂O, 99.5%) was dissolved at RT in isopropanol. Afterwards the ethanolamine (NH₂CH₂CH₂OH, MEA 98.0%) was added to the solution as a sol stabilizer. The molar ratio of ethanolamine to zinc acetate in solution was adjusted to 1:1 and the concentration of zinc acetate was 1.0 mol/L. The prepared mixtures were stirred at 333 K using a magnetic stirrer for about 1 h to obtain a clear homogeneous solution. Then 0.003 g of Lu₂O₃:Eu or Lu₂O₃:Tb NPs was added to the respective solutions. The thin NP film was deposited onto quartz substrates by the SC method with a speed rotation of 2000 r/min for 20 s. Each coated layer was dried at 353 K for 30 min to evaporate the solvent. The thickness of each dry layer was about 50 nm. The process was repeated to obtain a desired thickness of films in the 100–300 nm range. Finally, the thin NP films were heated in ambient atmosphere at the temperature of 823K for 1 h [12,13].

A D8 Advance diffractometer from Bruker (Billerica, MA, USA) with Cu K_α1 radiation of 1.54060 Å wavelength was used for X-ray diffractometry to determine the phase purity of samples. The morphology of the nanoparticles and films were studied using Hitachi S-3400N (Hitachi High-Technologies, Tokyo, Japan) and JEOL JSM 820 (JEOL, Tokio, Japan) scanning electron microscopes, respectively. Cathodoluminescence (CL) spectra of Lu₂O₃:Eu and Lu₂O₃:Tb NPs and films with 1.0 nm resolution were measured at RT using the JEOL JSM-820 electron microscope, additionally equipped with a high-sensitivity spectrometer Steller Net Silver Nova with a grating monochromator, respective software, and an electronically-cooled CCD detector (StellarNet, Tampa, USA) working in the 200–1200 nm range. The photoluminescence (PL) emission and excitation spectra as well as PL decay kinetics of Lu₂O₃:Eu and Lu₂O₃:Tb NPs at RT were investigated using a FLS 980 Edinburg Instruments spectrometer (Edinburgh Instruments Ltd., Livingston Scotland) Thermally stimulated luminescence (TSL) glow curves, TSL luminescence spectra, and radioluminescence (RL) spectra were recorded using a Lexsygresearch Fully Automated TL/OSL Reader from Freiberg Instruments GmbH, (Freiberg, Germany). The source of white X-rays was a Varian VF-50J RTG X-ray tube with a W-anode. The TSL glow curves were collected with a 9235QB-type photomultiplier from ET Enterprises (Uxbridge, UK). TSL emission spectra were recorded with an Andor DU420A-OE CCD camera and were not corrected for the system sensitivity. All modules were operated by means of LexStudio 2 software and resultant data were processed under LexEva 2 analytical software dedicated to the TL/OSL Reader and supplied by the manufacturer (Freiberg Instruments GmbH, Freiberg, Germany).

3. Structural and Morphological Properties of Lu₂O₃:Eu and Lu₂O₃:Tb NPs and Films

The structure and morphology of NPs and films were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Measured XRD patterns of the Lu₂O₃:Eu and Lu₂O₃:Tb powders are presented in Figure 1 together with a simulated pattern ICSD#40471 of Lu₂O₃ [3]. Analogous data were collected for films. As expected, they did not differ from the powders despite small background from the amorphous substrate. Therefore they are not separately presented here.

It is clear that, to the level of the detection limit of the technique, no foreign phases were observed in any of the phosphors. In both cases the diffraction lines were broadened signifying that they were composed of nanosized crystallites which was proven by means of the high resolution SEM images presented below. Furthermore, no broad band underlying the regular XRD pattern was seen for any of the powders which implied that the amorphous fraction was not present or its amount was insignificant. This is advantageous for spectroscopic properties.

However, as shown in Figure 2, the morphology of the samples was noticeably different in the case of Lu₂O₃:Eu (Figure 2a) and Lu₂O₃:Tb (Figure 2b) powders despite the apparently identical conditions of preparation of the phosphors. Ball-like particles were formed in the case of Lu₂O₃:Eu whereas dendrite-like structures appeared in the case of Lu₂O₃:Tb (Figure 2a,b, respectively). The reasons for such a significant difference are not clear and will be further investigated. One can presume that the unavoidable presence of Tb⁴⁺ during the formation of the Lu₂O₃:Tb particles may play a crucial role here. The SEM images prove that both powders are nanocrystalline with crystallites smaller than 100 nm. Particles tended to agglomerate but no hard aggregates were exposed.

The morphology of Eu- and Tb-doped Lu₂O₃ films is shown in Figure 2c,d, respectively. The respective Lu₂O₃:Eu and Lu₂O₃:Tb NPs were embedded in the specific needle-like nets onto the quartz substrate surface. Thus, in the case of Lu₂O₃:Tb, more uniform phosphor was netted onto substrate surface with a small quantity of the large-dimensional conglomerates observed. For Lu₂O₃:Eu films, the quantity and size of such conglomerates were significantly larger. Apparently, differences in the morphology of Lu₂O₃:Eu and Lu₂O₃:Tb NPs (Figure 2a,b) led to the difference in the surface morphology of the respective films, formed by means of the SC method. In the future a significant effort should be made towards improvement of the film morphology.

The difference in the uniformity of the Lu₂O₃:Eu and Lu₂O₃:Tb NPs films was also confirmed by the respective absorption spectra of these films with thicknesses in the 100–300 nm range. Namely, the background level significantly increased with the thickness for Lu₂O₃:Eu films (Figure 3a) while in the case of Lu₂O₃:Tb films, only insignificant differences were seen (Figure 3b). It is thus expected that optimization of film fabrication (necessarily combined with optimization of the NPs preparation) may create high levels of film uniformity and consequently homogeneity of their spectroscopic properties.

4. Luminescent Properties of Lu₂O₃:Eu and Lu₂O₃:Tb NPs and Films

The Lu₂O₃ lattice provides two positions with different symmetry of Lu³⁺ cations and consequently also of the Eu³⁺ and Tb³⁺ dopants [3]. These are centrosymmetric C_3i (often termed S₆ in the spectroscopic literature which is not fully correct) and lower-symmetry non-centrosymmetric C₂ sites. The population of the latter is triple the population of the former. Generally speaking, for the C_3i sites the electric dipole 4f–4f transitions are forbidden and emission spectra of the Lu₂O₃:Eu and Lu₂O₃:Tb NPs are dominated by transitions of Eu³⁺ and Tb³⁺ ions occupying the C₂ sites [2,3] unless direct selective excitation into the dopant in the C_3i site is applied. Furthermore, at least for the Eu³⁺ activator, significant Eu(C_3i)→Eu(C₂) energy transfer occurs when the dopant content increases above ~1 mol% [2].

It was proven that the Eu³⁺ and Tb³⁺ luminescence can be different in the various Lu₂O₃ crystalline forms (single crystals, powders, films) [6]. This was attributed mainly to a different concentration of oxygen vacancies which interfere with the host–activator energy transfer under excitation above the Lu₂O₃ host band gap in the different crystalline Lu₂O₃ samples [6].

The luminescent properties of Lu₃O₃:Eu NPs and Lu₂O₃:Tb NPs were characterized by cathodoluminescence (CL), X-ray excited radioluminescence (RL), photoluminescence (PL), and thermoluminescence (TSL) measurements in a broad range of temperatures, both below and above room temperature. Due to the very low thicknesses of the Lu₂O₃:Eu NP films (100–300 nm) their RL and TSL properties could not be effectively investigated.

4.1. Lu₂O₃:Eu NPs and Films

Cathodoluminescence: The CL spectra of both Lu₂O₃:Eu NPs and films showed quite similar shapes (Figure 3). The most intensive CL emission peak was observed at 612 nm, which corresponded to the ⁵D₀→⁷F₂ transitions of Eu³⁺ ions in the C₂ site. In both spectra some contribution from the ⁵D₀→⁷F₀ transitions as well as the low-intensity ⁵D₀→⁷F₁ transitions of Eu³⁺ ions was also observed. According to literature, in this range there might also be some light generated by the Eu³⁺ occupying the C_3i site [2]. It is noteworthy that some contribution from the ⁵D₁ level in the ~525–570 nm range of wavelengths was also evident in the CL spectra of Lu₂O₃:Eu NP (Figure 3 and Figure 4).

Photoluminescence: The PL emission spectra of Lu₂O₃:Eu NPs at RT were quite similar at different excitation energies and consisted of a group of strong lines caused by the ⁵D₀→⁷F_j (J = 0–4) transitions of Eu³⁺ ions (Figure 5a,b). The most intensive emission peak was observed at 611.6 nm (Figure 5a, curve 1) which corresponded to the ⁵D₀→⁷F₂ transition of Eu³⁺ ions in the C₂ site. The ⁵D₀→⁷F₀ transition of Eu³⁺ ions in the C₂ sites at 580.5 nm was clearly observed in the PL spectra of Lu₂O₃:Eu NPs (Figure 5b). That indirectly confirmed the very good quality of the phosphor [8]. No emission of Eu²⁺ ions was detected in the Lu₂O₃:Eu NPs. As is presented in Figure 5c, the main luminescent line showed basically exponential decay with the time constant τ = 1.5 ms. This was noticeably longer than in the case of coarse grains of the same composition, when the decay was about 1.0 ms [14]. The elongation of the decay time in nanopowders was described and explained first by Meltzer [15,16]. Good quality NPs show longer decay times than their micron-sized counterparts because the effective refractive index n felt by the emitting ion in nanopowders is significantly lower than in the coarse-grained materials [14,15,16,17,18]. In the latter, the refractive index n is defined by the host, while in the former, air (vacuum) contributes to the effective n value. The exponential, and much longer than in crystals, decay of Eu³⁺ luminescence in the Lu₂O₃:Eu NPs prove their high quality and low population of defects.

The excitation spectrum of the Eu³⁺ 611.6 nm luminescence (C₂ site) of Lu₂O₃:Eu NP (Figure 5a, curve 2) consisted of sharp lines in the 270–550 nm range, corresponding to the intrinsic 4f–4f transitions of Eu³⁺ ions. Narrow, well-resolved excitation lines again attested to the high structural quality of the fabricated Lu₂O₃:Eu NPs. A strong wide excitation band peaking at approximately 260 nm was related to the O²⁻→Eu³⁺ charge transfer transitions (CTT). One might be surprised by the high intensity of the 4f→4f excitation transitions of Eu³⁺ compared to its CTT band. This was already explained years ago [19] for Lu₂O₃:Eu and even earlier [20] for electronic and vibronic transitions of Pr³⁺ in other compositions as resulting from the saturation effect which effectively suppresses the measured intensity of the CCT. This occurs because the very high absorption co-efficient of the charge transfer transitions precludes excitation of the whole depth of the phosphor layer, while much lower absorptivity due to 4f→4f transitions allows the deeper penetration of the sample by radiation in the area of their occurrence.

Thermally stimulated luminescence: The TSL glow curve and evolution of TSL emission spectra of Lu₂O₃:Eu NPs with temperature are shown in Figure 6a,b, respectively. The positions of the main TSL peaks are located at ~110°C and 170°C (Figure 6a). Already at room temperature some afterglow of Lu₂O₃:Eu NP was observed. The TSL emission spectra of Lu₂O₃:Eu NP were composed of Eu³⁺ luminescence exclusively (Figure 6b). According to the model of TSL processes [21,22,23], Eu³⁺ ions serve as electron traps, while the hole trap is expected to be connected with an intrinsic defect of the host. The relative intensity of the orange features of Eu³⁺ luminescence (~585–600 nm) in the TSL spectra (Figure 6b) was notably higher than in CL or PL spectra, presented in Figure 4 and Figure 5. This indicated that Eu³⁺ ions in the C_3i site were more susceptible for trapping electrons than those in the C₂ site and consequently more profoundly contribute to TSL. Such an observation was already reported in Lu₂O₃:Eu sintered ceramics [24]. Yet, in the ceramics it was observed only for lower concentrations of Eu, up to 1%, while here it was still seen for the Eu content of 3%. This size-effect should be a subject of more comprehensive and focused investigation in the future. Note that two spikes at ~650 nm and ~730 nm around 260–270 °C seen in Figure 6b were artifacts resulting from an electronic noise of CCD used in this experiment.

Radioluminescence: X-ray excited RL spectra of Lu₂O₃:Eu NPs were measured in the temperature range of 300–550 °C and the data are presented in Figure 7. Note how much less intense the relative intensity of the orange part (~585–600 nm) was in RL than it was in TSL (Figure 6b). Surprisingly, the maximum of RL intensity Lu₂O₃:Eu NP was observed around 320–330 °C. Hence, the overall efficiency of the RL was higher when temperature was well above the main TSL peaks (Figure 6). Therefore, this phenomenon had to be caused by a higher efficiency of energy transfer from the excited host to the dopant at such high temperatures. This, in turn, might imply that the light yield under X-ray reported in the past [1] might not reflect the real potential of this X-ray phosphor. This is a truly intriguing conclusion and this effect is worth much deeper investigation, and it is especially interesting that such results can be compared for different Eu concentrations.

4.2. Lu₂O₃:Tb NPs and films

Cathodoluminescence: The CL spectra of Lu₂O₃:Tb NPs and films were very similar (Figure 8). The most intensive emission peaks were observed at 543.5 and 552.2 nm, which upon site selective PL spectroscopy could be attributed to the ⁵D₄→⁷F_j transitions of Tb³⁺ ions in the Lu₂O₃ host (Figure 9). Taking into account the spectral position of these bands in the PL spectra (Figure 9), the contribution of Tb³⁺ luminescence in C_3i centers may appear significant compared to C₂ centers in the CL spectra of Lu₂O₃:Tb NPs and film. However, at present this is merely a supposition needing precise verification. Determination of a dominant type of emission center and estimation of the relative quantity of Tb³⁺(C₂) and Tb³⁺(C_3i) centers requires CL spectra recorded with a resolution much higher than 1 nm. This observation will be further verified and investigated in other crystalline forms of Lu₂O₃:Tb samples.

Photoluminescence: Contrary to the CL spectra, emissions from the two sites were clearly observed in the PL spectra of Lu₂O₃:Tb NP. Namely, the PL spectra of Tb³⁺ ions in the C₂ and C_3i positions possessed the main peaks at 542.4 and 544.0 nm, respectively (Figure 9a,b). Their excitation spectra also showed some differences, proving the presence of the two mentioned Tb³⁺ sites (Figure 9c). Namely, the main excitation 4f→5d bands peaked at 270 nm for the C₂ site and at 306 nm for the C_3i site of Tb³⁺ in the Lu₂O₃ host. A low-intensity excitation band around 365 nm was also observed in the PL excitation spectra and was proven to result from the spin-forbidden (sf) 4f→5d transitions of Tb³⁺ ions both in C_3i sites (Figure 9c). Therefore, excitation at 305 nm and 365 nm results in very similar PL spectra of Lu₂O₃:Tb NPs due to transitions within the Tb³⁺ (C_3i) (Figure 9a,b).

As expected, the decay kinetics of Tb³⁺ luminescence deviate from the exponential course, especially for the C_3i site. The latter was much longer than that of the C₂ sites (Figure 9d). For this reason, for decay curve characterization we have used the two decay times of τ_1/e = 2.17 ms and τ_1/20 = 3.96 ms for Tb in C₂ and τ_1/e = 7.32 ms and τ_1/20 = 19.36 ms for Tb in the C_3i site of the Lu₂O₃ host. As usual, there was a longer emission of ions in the site with inversion symmetry.

Thermally stimulated luminescence: An easily recordable but not very intense afterglow was observed in Lu₂O₃:Tb NPs at RT (Figure 10). The TSL glow curve was composed of two superimposed peaks with maxima around 75 °C and 120 °C. The TL emission spectrum was exclusively generated by Tb³⁺ transitions. Due to the fact that Tb³⁺ ions typically serve as hole traps, and because it was Tb³⁺ which finally produced the TSL photons, the observed TSL peaks in Lu₂O₃:Tb NPs had to correspond to intrinsic electron trapping centers. These may result from oxygen vacancies. Similarly positioned TL peaks were reported for Lu₂O₃:Tb, Ca ceramics sintered at 1700 °C in a reducing atmosphere, where the oxygen vacancies were responsible for a very intense TL [25,26].

Radioluminescence: The temperature dependence of the RL spectra of Lu₂O₃:Tb NPs was measured up to 500 °C (Figure 11). As can be seen from this figure, the Tb³⁺ luminescence disappeared at about 400 °C. This resulted from a thermal quenching which was reported for Lu₂O₃:Tb sintered ceramics to start right at room temperature [27]. Note that in this experiment, the dependence of RL intensity on temperature was to some extent distorted by the appearance of TL photons which contributed quite significantly in the range of ~50–150 K (see Figure 10a).

5. Conclusions

The Lu₂O₃:Eu and Lu₂O₃:Tb NPs were fabricated by means of hydrothermal processing. From the respective NPs the Lu₂O₃:Eu and Lu₂O₃:Tb thin films (from 100 up to 300 nm thick) were prepared using a spin-coating method. Despite undergoing essentially the same method of preparation, different morphology of Lu₂O₃:Eu and Lu₂O₃:Tb NPs was observed: ball-like particles were formed in the case of Lu₂O₃:Eu and dendrite-like structures in the case of Lu₂O₃:Tb. The morphological differences of powders resulted in differences of the morphology and structural quality of the respective films. Accordingly, the Lu₂O₃:Tb films possessed a more uniform surface than Lu₂O₃:Eu counterparts.

It was found that the CL and PL emission spectra as well as PL decay kinetic of Lu₂O₃:Eu NP and films showed the luminescence of Eu³⁺ ions almost exclusively from the C₂ site of Lu₂O₃ host with the most intensive peak at 611.6 nm and the decay time of about 1.5 ms, about 50% longer than in coarse grained ceramics.

Contrary to Lu₂O₃:Eu, the PL emission and excitation spectra as well as PL decay kinetic of Lu₂O₃:Tb NPs and films at 300 K showed two types of Tb³⁺ luminescence centers in the C₂ and C_3i sites of the Lu₂O₃ host with the main emission peaks at 542.4 and 544.0 nm and corresponding maxima of excitation bands located at 270 and 305 nm due to the 4f→5d transition. The luminescence decay traces of Tb³⁺ ions in C₂ and C_3i sites were non-exponential, especially for Tb³⁺ in the C_3i position, and the decay times of the Tb³⁺(C₂) and Tb³⁺(C_3i) centers luminescence were equal to t_1/e = 2.17 and t_1/20 = 3.96 ms and t_1/20 = 7.32 and t_1/e = 19.36 ms, respectively.

The TL peaks at 110 °C and 170 °C and at 75 °C and 120 °C were observed in Lu₂O₃:Eu and Lu₂O₃:Tb NPs, respectively. They corresponded to the formation of the hole and electron trapping centers in the Lu₂O₃ host, respectively. Significantly different temperature dependences of X-ray excited luminescence in Lu₂O₃:Eu and Lu₂O₃:Tb NPs was also evident. Namely, the start of the temperature quenching of Eu³⁺ and Tb³⁺ luminescence in Lu₂O₃:Eu and Lu₂O₃:Tb NPs was observed at 90°C and 320°C, respectively.