Photoluminescence and Cathodoluminescence Properties of

Mar 16, 2012 - Nanocrystalline BaFCl:Sm3+ X-ray Storage Phosphor ... Unit, Mark Wainwright Analytical Centre, The University of New South Wales, Sydne...
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Photoluminescence and Cathodoluminescence Properties of Nanocrystalline BaFCl:Sm3+ X-ray Storage Phosphor Zhiqiang Liu,† Marion Stevens-Kalceff,‡ and Hans Riesen*,† †

School of Physical, Environmental and Mathematical Sciences, The University of New South Wales, Canberra, ACT 2600, Australia School of Physics and Electron Microscope Unit, Mark Wainwright Analytical Centre, The University of New South Wales, Sydney, NSW 2052, Australia

‡

ABSTRACT: Nanocrystalline BaFCl:Sm3+ is an efficient photoluminescent X-ray storage phosphor. To gain a better understanding of the storage mechanism, the photoluminescence and cathodoluminescence properties of nanocrystalline BaFCl:Sm3+ before and after X-irradiation were investigated. The results were compared with those obtained for microcrystalline BaFCl:Sm3+ prepared by high-temperature sintering. Significant differences in the local coordination environment of Sm3+ ions between the two samples were revealed by comparing the photoluminescence spectra at 2 and 293 K. From the cathodoluminescence microanalysis, it follows that the Sm3+ ions in the as-prepared nanocrystalline BaFCl:Sm3+ are mainly located on or close to the surface of the nanoparticles, whereas those in the microcrystalline sample are distributed homogeneously throughout the microcrystallites. For nanocrystalline BaFCl:Sm3+, the X-irradiation-induced reduction of Sm3+ to Sm2+ ions and the photoionization of Sm2+ to Sm3+ ions during photobleaching were investigated by monitoring the photoluminescence intensities of both Sm2+ and Sm3+ ions. The two processes can be modeled well assuming dispersive firstorder kinetics, where the rate constant of the electron transfer is given by an exponential function of the distance between the shallow electron traps and the Sm3+ centers.

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rium ions in LiBaB9O15,12 Ba3BP3O12,13 and Ba2SiO414 and identified different crystallographic sites of Sm2+ ions in the host lattice based on the temperature-dependent and nondegenerate 5D0−7F0 transition of Sm2+ ions. Nogami and Suzuki15 reported the increase of Sm2+ fluorescence intensity as well as the decrease of Sm3+ intensity by increasing the Xirradiation period in an Al2O3−SiO2 glass. Furthermore, the photobleaching of Sm2+ ions has also been the subject of continuous research interest. It has been well-accepted that the photobleaching of Sm2+ ions can be rationalized by the photoionization of Sm2+ to Sm3+ ions, as proposed by Winnacker et al. for the observation of photon-gated spectral hole burning of Sm2+ ions.4 Tanaka et al. reported the photobleaching and recovery of Sm2+ luminescence in BaFCl single crystals when irradiated with 325 nm He−Cd laser and 488 nm Ar+ ion laser light.16 According to Tanaka et al., the photoionization of Sm2+ to Sm3+ ions is followed by the trapping of released electrons at halogen vacancies in the BaFCl crystals, during the photobleaching process. However, in their work, no change of Sm3+ luminescence intensity was detected during the photobleaching to confirm the photoionization of Sm2+ to Sm3+ ions. Similar reversible photobleaching of Sm2+

INTRODUCTION Spectroscopic properties of samarium-activated inorganic materials have been investigated extensively due to their potential applications in solid-state lasers, luminescence materials, and optical data storage media.1−3 In particular, considerable attention has been directed to the Sm2+ ion-doped alkaline earth fluorohalides MeFX (Me = Ca, Sr, and Ba; X = Cl, Br, and I) after the first report of photon-gated spectral hole burning in BaFCl:Sm2+ by Winnacker et al.4 We have recently reported the application of nanocrystalline BaFCl:Sm3+ as a highly efficient photoluminescent storage phosphor for ionizing radiation.5 Upon X-ray, γ-ray, or β-irradiation, the Sm3+ ions in the BaFCl host are reduced to Sm2+ ions that can be read out efficiently by measuring the narrow 5DJ−7FJ f−f luminescence lines via excitation into the very intense, parity-allowed 4f6 → 4f55d transitions in the blue-violet region of the spectrum. The reduction of Sm3+ to Sm2+ ions by X-ray and γ-ray irradiation has been realized in a variety of crystalline and amorphous hosts such as alkaline earth fluorohalides,6 alkaline earth borophosphates,7,8 silicate glasses,9 and sodium aluminoborate glasses.10 Because the Sm3+ and Sm2+ ions display very different emission spectra, luminescence spectroscopy can be utilized for the characterization of the oxidation state, the crystallographic sites, and the local coordination environment of samarium ions in the host lattice.11 Huang et al. have demonstrated X-irradiation-induced valence changes of sama© 2012 American Chemical Society

Received: February 9, 2012 Revised: March 15, 2012 Published: March 16, 2012 8322

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Figure 1. Photoluminescence spectra of nanocrystalline (red solid line) and microcrystalline (blue dashed line) BaFCl:Sm3+ at 293 K (a) before and (b) after X-irradiation. The nanocrystalline and microcrystalline samples were exposed to 560 and 3800 Gy Cu Kα radiation (40 kV, 25 mA), respectively. The luminescence was excited by 0.14 W cm−2 light of 405 nm blue-violet laser diode. The spectra are offset on the vertical scale for clarity. Prominent 4GJ−6HJ (Sm3+) and 5DJ−7FJ (Sm2+) transitions are denoted.

Figure 2. Photoluminescence spectra of the nanocrystalline (red solid line) and microcrystalline (blue dashed line) BaFCl:Sm3+ at 2 K (a) before and (b) after X-irradiation. The rest is as in Figure 1.

were investigated by monitoring the photoluminescence intensities of both Sm2+ and Sm3+ ions. To the best of our knowledge, this is the first report that presents direct evidence for the photoionization of Sm2+ to Sm3+ ions during the photobleaching process.

ions in BaFCl single crystal was also observed by Mikhail et al. in photoluminescence and cathodoluminescence experiments,17 in which a nonradiative decay mechanism was proposed based on the Sm3+ luminescence being unchanged within the experimental error. Other materials doped with Sm2+ ions such as Mg0.5Sr0.5FCl0.5Br0.5:Sm2+, BaFCl0.5Br0.5:Sm2+,18 Al2O3− SiO2:Sm2+ glass,19 and Li2O-BaO-B2O3:Sm2+ glass20 were also reported to exhibit photobleaching. The photobleaching effect of Sm2+ ions in these materials is generally ascribed to the charge transfer from Sm2+ ions to nearby defects in the host. In this article, we report photoluminescence and cathodoluminescence properties of Sm3+ and Sm2+ ions in nanocrystalline BaFCl:Sm3+. The results are compared with those for microcrystalline BaFCl:Sm3+ prepared by high-temperature sintering. To study the details of the X-ray storage mechanism of nanocrystalline BaFCl:Sm3+, the X-irradiation-induced reduction of Sm3+ ions and the photobleaching of Sm2+ ions

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EXPERIMENTAL METHODS

ACS reagent grade chemicals were used without further purification. The nanocrystalline BaFCl:Sm3+ was prepared by a coprecipitation method.21 The microcrystalline BaFCl:Sm3+ was obtained by sintering a mixture of BaCl2, BaF2, and SmCl3·6H2O for 1 h at 900 °C. The X-ray irradiation of the samples was realized by exposure in a powder X-ray diffractometer (40 kV, 25 mA, Cu Kα), and the radiation dose was cross-calibrated against a dental X-ray unit (Belmont Searcher model DX-068). 8323

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Figure 3. (a) Photoluminescence spectra of the X-irradiated nanocrystalline BaFCl:Sm3+ at various temperatures from 2 to 293 K. (b) Integrated photoluminescence intensity of Sm2+5DJ−7F0 transitions as a function of temperature. The sample was exposed to 560 Gy Cu Kα radiation (40 kV, 25 mA). The luminescence spectra were excited by a 0.14 W cm−2 of 405 nm blue-violet laser diode light.

which indicates that the samarium ions are introduced into the nanocrystalline and microcrystalline hosts predominantly in the trivalent oxidation state. As is shown in Figure 1b, the 293 K photoluminescence spectra after X-irradiation of the two samples display similar narrow and characteristic Sm2+ luminescence lines at 629, 640, 663, 687, 702, and 727 nm originating from the 5D1−7F0,1,2 and 5D0−7F0,1,2 f−f transitions.22 However, for the microcrystalline sample, which was irradiated with a seven times higher X-ray dose than the nanocrystalline sample, the broad Sm3+ emission lines are still prominent in comparison with the sharp Sm2+ lines. We note here that the Sm2+ emission is much more efficiently excited than the Sm3+ luminescence since the Sm2+ and Sm3+ luminescence are excited by parity-allowed f−d and parityforbidden f−f transitions, respectively. Hence, the majority of samarium ions in the microcrystalline sample are still in the +3 oxidation state even after a very large X-ray dose administered for the spectrum shown in Figure 1b. Another variation in the photoluminescence spectra between the two samples after Xirradiation is that two emission lines at 686 and 687 nm are observed in the nanocrystalline sample for the 5D0−7F0 transition of Sm2+ ions, whereas only one line at 687 nm is detected in the microcrystalline sample. Because the 5D0 and 7 F0 are nondegenerate energy levels, each emission line associated with this transition must correspond to a different Sm2+ center. It thus follows that after X-irradiation, two different Sm2+ species are present in the nanocrystalline sample, whereas only one site is observed for the microcrystalline sample at 293 K. In the samarium-doped BaFX materials, the samarium ions enter the host lattice by replacing barium ions. Considering the differences in oxidation state and ionic radius between the Sm3+/2+ and the Ba2+ ions, multiple sites of Sm3+ and Sm2+ ions in BaFCl host can be expected, in particular when the samarium ions are near the surface. Figure 2 displays the 2 K photoluminescence spectra of nanocrystalline and microcrystalline BaFCl:Sm3+ before and after X-irradiation. The photoluminescence spectrum of the microcrystalline sample shows very similar broad 4GJ−6HJ

To measure the photoluminescence spectra, the samples were mounted on the coldfinger of a closed-cycle cryostat (CCR, Janis/Sumitomo SHI-4.5) and excited by the light of a focused 405 nm blue-violet laser diode. The emission light was collimated and then focused onto the entry slit of a Spex 1404 monochromator (1200 grooves/mm holographic grating) using 75 and 200 mm lenses. The light was modulated with a chopper and detected by a photomultiplier (Hamamatsu R928). The signal was processed by a current-to-voltage preamplifier (Femto DLPCA-200) and a lock-in amplifier (Stanford Research System SR810 DSP) before being collected on a PC. The cathodoluminescence spectra were collected by a JEOL 7001F Field Emission Scanning Electron Microscope equipped with a Gatan XiCLone cathodoluminescence system using either a Peltier-cooled Hamamatsu high-sensitivity photomultiplier R943-02 or a Peltier-cooled Princeton Instruments and UV-enhanced Pixis 100 CCD with gratings blazed at 500 nm.

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RESULTS AND DISCUSSION

Photoluminescence Spectroscopy. The photoluminescence spectra of nanocrystalline and microcrystalline BaFCl:Sm3+ samples before and after X-irradiation were recorded at temperatures between 2 and 293 K. The spectra of the two samples obtained at the two temperature limits are shown in Figures 1 and 2. As follows from Figure 1a, four broad emission lines at 559, 595, 639, and 701 nm are observed in the spectrum of the nanocrystalline sample at 293 K before X-irradiation. These lines, or groups of lines, can be assigned to the f−f transitions 4 G5/2−6H5/2, 4G5/2−6H7/2, 4G5/2−6H9/2, and 5G5/2−6H11/2 that are typical of Sm3+ ions.14 In comparison with the nanocrystalline sample, the 4GJ−6HJ emission lines of Sm3+ ions are redshifted by ∼3 nm in the microcrystalline sample at 293 K, implying different local structures of Sm3+ ions in the two samples. No emission lines associated with Sm2+ ions are observed in the spectra of the two samples before X-irradiation, 8324

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Figure 4. Secondary electron images of typical (a) nanocrystalline and (b) microcrystalline BaFCl:Sm3+. The dimension marker indicates 1.0 μm.

emission lines of Sm3+ ions at both 2 and 293 K before Xirradiation (Figure 2a). In contrast, the 4GJ−6HJ emission lines of Sm3+ ions in the nanocrystalline sample become much narrower at 2 K in comparison with the 293 K spectrum. It clearly follows that the nanocrystalline spectra at 293 K are broadened by electron−phonon interactions that can be frozen out at 2 K, whereas the microcrystalline sample is subject to severe inhomogeneous broadening, which is temperature independent. The different temperature dependence of Sm3+ emission lines between the two samples confirms significant differences of the local environment of the Sm3+ ions in the two samples. This is most likely caused by the different preparation methods of the two samples. In the case of the microcrystalline sample, the Sm3+ ions are forced into the BaFCl host lattice during high-temperature sintering. In contrast, a core−shell structure, consisting of a BaFCl core and a BaFCl:Sm3+ shell, is formed in nanocrystalline BaFCl:Sm3+ system based on the coprecipitation method;21,23 that is, the Sm3+ ions are mainly distributed on or near the surface of the nanoparticles. Notwithstanding the fact that significantly different Sm3+ spectra are observed for the two samples before X-irradiation, similar Sm2+ emission lines were recorded at 2 K after Xirradiation as is shown in Figure 2b. Specifically, the Sm2+ emission lines at 570, 587, 612, and 642 nm, originating from the 5D2−7F1,2,3,4 transitions,22 dominate the spectra of the two samples at 2 K with the strongest emission line at 2 K from the 5 D2−7F1 transition at 570 nm instead of the 5D0−7F0 transition at 687 nm at 293 K. For the 5D0−7F0 transition at 2 K, five emission lines at 685, 686, 687, 688, and 689 nm are observed for the nanocrystalline sample, whereas two lines at 686 and 687 nm are present in the microcrystalline sample; this implies the presence of multiple Sm2+ sites in both samples at 2 K. We have investigated the temperature dependence of the photoluminescence spectra of Sm2+ ions generated by Xirradiation in nanocrystalline BaFCl:Sm3+ in detail, as is illustrated in Figure 3. At temperatures below 60 K, the spectra of the nanocrystalline sample are dominated by the emission lines at 561, 570, 587, 612, 680, and 723 nm due to the 5D2−7FJ f−f transitions of the Sm2+ ions. However, at temperatures above 100 K, the photoluminescence from the 5D2 excited-state multiplet is subject to thermal quenching, which is a characteristic behavior for BaFCl:Sm2+.22 In contrast, the photoluminescence intensity of the 5D0,1−7F0,1,2 transitions increases from 2 to 100 K. For temperatures above 100 K, a gradual drop in intensity of the 5D1−7F0,1,2 transitions and growth in the emission lines of 5D0−7F0,1,2 occur. A similar temperature dependence of the Sm2+ photoluminescence was also reported by Shen and Bray.24 This behavior is associated

with nonradiative and phonon-assisted relaxation processes between the 5D2 and the 5D1 and the 5D1 and the 5D0 multiplets. At temperatures below 60 K, these processes are frozen out and negligible; hence, the strongest emission lines originate from the 5D2 level. However, with increasing temperature, nonradiative relaxation is thermally induced and become more pronounced, resulting in luminescence quenching of the 5D2 and 5D1 levels. Cathodoluminescence Spectroscopy. Cathodoluminescence is the nonincandescent emission of light when electrons are used as the means of excitation. It is very valuable as a complementary technique to photoluminescence spectroscopy.25 In an electron microscope, the finely focused beam of high-energy electrons enables the collection of high sensitivity, high spatial resolution cathodoluminescence data (luminescence spectroscopy and images), and correlated topography images (i.e., secondary electron images). Figure 4 shows typical scanning electron microscope (SEM) secondary electron images from the as-prepared nanocrystalline and microcrystalline BaFCl:Sm3+. As is illustrated in Figure 4, the nanocrystalline particles are typically plateletlike particulates of approximately uniform thickness. In contrast, the microcrystalline particles are significantly larger, less uniform, and irregular in shape with smaller rounded fragmented grains. The differences between the samples in morphology are caused by the different preparation methods. Figure 5 shows the cathodoluminescence spectra of the asprepared nanocrystalline and microcrystalline BaFCl:Sm3+, collected using the high-sensitivity photomultiplier. Each spectrum was collected from a 1200 μm2 field of particles and is therefore the sum of the cathodoluminescence emission from the particles within the field. As shown in Figure 5, sharp emission lines associated with 5DJ−7FJ transitions of the Sm2+ ions dominate the cathodoluminescence spectrum of the nanocrystalline BaFCl:Sm3+, although the Sm3+ emission lines are still discernible. As compared with that of the nanocrystalline sample, the spectrum of the microcrystalline sample mainly presents the characteristic Sm3+ 4G5/2−6HJ (J = 5/2, 7/2, 9/2, 11/2) emission lines at 558, 597, 645, and 702 nm. From a detailed examination, it follows that the Sm3+ emission lines in the cathodoluminescence spectrum of the microcrystalline sample are shifted to the red by about 3 nm in comparison with the nanocrystalline sample, which is consistent with the results of photoluminescence spectroscopy. The coexistence of Sm3+ and Sm2+ emission lines in the cathodoluminescence spectra of the two samples indicates that the Sm3+ ions are partially reduced to Sm2+ ions during electron excitation. Furthermore, the Sm2+ emission lines in the nanocrystalline sample are much 8325

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Figure 5. Cathodoluminescence spectra of nanocrystalline (red solid line) and microcrystalline (blue dashed line) BaFCl:Sm3+. The spectra were collected at 15 keV and 3 nA at 295 K. The cathodoluminescence intensities were normalized at 370 nm. Prominent 4GJ−6HJ (Sm3+) and 5DJ−7FJ (Sm2+) transitions are denoted. The inset shows the spectra in the region of the 4G5/2−6H5/2 transition of Sm3+ to illustrate the 3 nm shift observed between the two samples.

Figure 6. Cathodoluminescence spectra of the X-irradiated (red solid line) and nonirradiated (blue dashed line) nanocrystalline BaFCl:Sm3+. The spectra were collected at 15 keV and 3 nA at 295 K. The cathodoluminescence intensities were normalized at 370 nm. Prominent 5DJ−7FJ transitions of Sm2+ ions are denoted. It is noted here that the 5D0−7F0 emission at 686 nm due to a minority site (see Figure 1a) is absent in the cathodoluminescence spectra shown in Figures 5 and 6. This site bleaches rapidly in the electron beam and likewise undergoes much faster photobleaching than the 687 nm site.

more intense than those in the microcrystalline sample, confirming a much higher reduction efficiency of the nanocrystallite than that of the microcrystallite by high-energy electrons. Photoluminescence and cathodoluminescence have often been utilized to distinguish between Sm3+ and Sm2+ ions in a range of different hosts. Mikhail et al. have compared the cathodoluminescence and photoluminescence of Sm2+ and Sm3+ in oxide environments26 and proposed that the cathodoluminescence detects predominantly Sm3+ ions due to ionization of Sm2+ ions by the electron beam, whereas photoluminescence is sensitive for detecting both oxidation states of samarium ions. However, in the present study, the Sm2+ emission lines dominate the cathodoluminescence spectrum of the nanocrystalline sample as is illustrated in Figure 5. To further investigate the bleaching of Sm2+ ions during cathodoluminescence measurements, the cathodoluminescence spectra of the X-irradiated and nonirradiated nanocrystalline BaFCl:Sm3+ were measured as is illustrated in Figure 6. It follows from this figure that both the X-irradiated and the nonirradiated samples present mainly the 5DJ−7FJ luminescence lines of Sm2+ ions, in contrast to the results reported by Mikhail et al.,17 which were obtained from a polished single crystal of BaFCl doped with 0.1 mol % Sm. Also, the X-irradiated sample presents a higher cathodoluminescence intensity of the Sm2+ emission lines as compared to the nonirradiated sample. During the cathodoluminescence measurements, both the reduction of Sm3+ ions and the oxidation of Sm2+ ions can occur upon electron bombardment, and the spectra obtained reflect the equilibrium between the two processes and therefore the two oxidation states. In addition to the characteristic Sm2+ and Sm3+ emission lines seen in Figure 5, a broad and relatively intense emission at around 350 nm can be observed in the cathodoluminescence spectra of nanocrystalline and microcrystalline samples. This

peak can also be observed in the cathodoluminescence spectrum of the undoped BaFCl particles (not shown here) when collected under the same conditions. The broad cathodoluminescence emission at 350 nm is related to either pre-existing or electron irradiation-induced defect centers in the BaFCl host lattice, and a number of potential identifications and associations have been proposed. In BaFCl, absorption at 440 and 540 nm has been associated with F(F−) and F(Cl−) centers, respectively,27 with associated luminescence observed at wavelengths greater than 1000 nm. 28 The broad cathodoluminescence emission peaks at 350 nm and is therefore unlikely to be related to the F-centers. Previous vacuum UV and X-ray luminescence investigations have reported a broad emission at 360 nm and attributed it to the radiative relaxation of self-trapped excitons29,30 with a strongly off-center configuration to account for the large Stokes shift.31 Alternatively, electron spin resonance and luminescence investigations have assigned this emission to electron recombination with Vk (Cl2−) centers.32,33 The cathodoluminescence observed at around 350 nm (in Figure 5) may possibly be also attributed to an oxygen impurity defect. Oxygen is a common impurity in barium fluorohalides and typically forms two types of oxygen-vacancy centers. In BaFCl, the type I defect consists of oxygen substituting for a fluorine ion and a neighboring chlorine vacancy. The type II oxygenvacancy defect is located completely in the chlorine sublattice with the oxygen and vacancy occupying the chlorine ion pair sites in the Matlockite structure of BaFCl.34−36 However, luminescent emission observed at ∼500 and ∼540 nm has been attributed to type I and type II oxygen-vacancy defects, respectively,34,37 that is, at significantly longer wavelengths. Cathodoluminescence microanalysis provides the opportunity to investigate the local relative spatial distribution of 8326

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Figure 7. (a) Secondary electron image of a microcrystalline BaFCl:Sm3+ particle. The dimension marker indicates 100 nm. (b) Normalized cathodoluminescence intensities at 10 keV and 10 nA at 350 (host lattice defect), 595 (Sm3+), and 687 nm (Sm2+) plotted as a function of position across the microcrystalline BaFCl:Sm3+ particle.

Figure 8. (a) Secondary electron image of a typical nanocrystalline BaFCl:Sm3+ particle. The dimension marker indicates 100 nm. (b) Normalized cathodoluminescence intensities collected at 10 keV and 10 nA at 350 (host lattice defect), 595 (Sm3+), and 687 nm (Sm2+) plotted as a function of position across the nanocrystalline BaFCl:Sm3+ particle.

luminescent centers within a BaFCl:Sm3+ crystallite. In our study, a sequence of cathodoluminescence spectra excited by a stationary, finely focused 10 keV, 10 nA electron beam has been collected using the CCD camera at 100 nm intervals across individual nanocrystalline and microcrystalline BaFCl:Sm3+ particles. Figures 7 and 8 show the examples of individual microcrystalline and nanocrystalline BaFCl:Sm3+ particles of approximately similar size, respectively, and the normalized cathodoluminescence intensities at 350 nm from host lattice defects, 595 nm from Sm3+ ions, and 687 nm from Sm2+ ions as a function of position across the nanocrystalline and microcrystalline particles. Note that the results shown in Figures 7 and 8 are based on a number of scans for different nanocrystalline and microcrystalline BaFCl:Sm3+ particles and thus represent typical results for the two samples. As shown in Figure 7, the normalized intensity profile of the cathodoluminescence emissions from the microcrystalline particle is not uniform across the particle due to the differences

in thickness of the particle along its length. More importantly, very similar intensity profiles between the host lattice defects and the samarium ions are observed, indicating that the samarium impurities are distributed homogeneously through the microcrystalline BaFCl:Sm3+ particle. In contrast, significant differences in the distribution of samarium ions and host lattice defects are observed across the nanocrystalline particle. As shown in Figure 8, the normalized intensity profile of the cathodoluminescence emission at 350 nm from the nanocrystalline particle is approximately maximized across the central region of the crystallite as would be expected for an uniformly thick particle. In comparison, the cathodoluminescence emissions at 687 and 595 nm associated with Sm2+ ions and Sm3+ ions, respectively, are enhanced near the edges of the particle. The enhanced cathodoluminescence emission from the edges of the particle indicates that the concentration of samarium ions is greater in the near-surface region and is not uniformly distributed 8327

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by electron centers varies from site to site, depending on the distance between the Sm3+ ion and the electron defect center. In particular, for the Sm3+ ions that are stabilized by chargecompensating defect centers and well built into the BaFCl lattice, the reduction process is less likely to occur, leading to the observation of the near-constant level of both Sm3+ and Sm2+ photoluminescence intensities with increasing X-irradiation dose. The reduction of the Sm3+ photoluminescence upon Xirradiation can be described by dispersive first-order kinetics with the reduction rate modeled as an exponential function of the distance between the electron acceptor, that is, Sm3+ ions, and the electron centers created by X-irradiation. We assume that the trapped electrons either decay rapidly by combining with Sm3+ centers or by recombination with the created holes. We follow the key concepts from the Inokuti and Hirayama model39 for photoinduced electron transfer and assume that the dose-based rate constant for the reduction of Sm3+ ions k is given by eq 1; that is, the distance dependence of the rate constant k is given by an exponential function of the form

throughout the nanocrystalline particle. This result is in accordance with those obtained from photoluminescence spectroscopy and the preparation method for the material. X-irradiation Induced Reduction of Sm3+ to Sm2+ Ions. Figure 9 illustrates the reduction of Sm3+ to Sm2+ ions in

⎛ −R ⎞ ⎟ k(R ) = k 0 exp⎜ ⎝ a ⎠

(1)

where R is the distance between the Sm3+ ions and the X-ray induced electron-centers. k0 and a normalize the distance scale and the fall off of the reduction rate. Considering the random distribution of Sm3+ ions and the electron centers generated by X-irradiation in the BaFCl host lattice, the steady-state luminescence intensity of Sm3+ (which is proportional to the concentration of Sm3+) φ as a function of X-ray dose (d) is then approximated by eq 2.40

Figure 9. X-ray dose dependence of the integrated photoluminescence intensities of Sm3+ ions at 595 nm (4G5/2−6H7/2) and Sm2+ ions at 687 nm (5D0−7F0) in nanocrystalline BaFCl:Sm3+. The Sm3+ and Sm2+ curves were fitted with eqs 2 and 3, respectively.

nanocrystalline BaFCl:Sm3+ by displaying the integrated photoluminescence intensities of Sm3+ ions at 595 nm (4G5/2−6H7/2) and Sm2+ ions at 687 nm (5D0−7F0) as a function of the accumulated X-ray dose. No Sm2+ photoluminescence is observed before X-irradiation, and with increasing X-ray dose, the photoluminescence intensity of Sm3+ at 595 nm decreases, whereas that of Sm2+ at 687 nm increases, indicating the reduction of Sm3+ ions to Sm2+ ions by X-rays. After the nanocrystalline sample is irradiated with ∼20 Gy of 40 kV X-ray radiation, more than 50% of Sm3+ ions are reduced. With the irradiation dose increased up to ∼150 Gy, both the photoluminescence intensities of Sm3+ and Sm2+ ions become saturated and remain at near-constant values. Similar luminescence changes of the Sm2+ ions as well as that of the Sm3+ ions upon X-irradiation were also reported in samariumdoped Ba(Na1.3Sr0.7)Na(PO4)238 and Al2O3−SiO2 glass.15 However, no X-irradiation doses associated with the reduction process were given for these materials. It appears that the current work is the first report that quantifies the reduction of Sm3+ to Sm2+ ions as a function of X-ray dose. The reduction mechanism of Sm3+ to Sm2+ ions by Xirradiation in nanocrystalline BaFCl:Sm3+ can be interpreted based on the random distribution of distances between Sm3+ ions and electrons created by X-irradiation that are localized in shallow traps. Upon X-irradiation, free electrons and holes are generated in nanocrystalline BaFCl:Sm3+, and they are most likely trapped out on a very short time scale. In particular, the electrons are likely to be first trapped out at relatively shallow defect centers (such as oxygen defects, F-centers, etc.) in the host lattice. The defects then combine with Sm3+ ions in close proximity, resulting in the reduction of Sm3+ ions and the formation of Sm2+ ions. However, the reduction of Sm3+ ions

⎛ ⎞ ∞ 2 φ(d) = A exp⎜ −4πC [1 − exp( −k(R )d)]R dR ⎟ Rm ⎝ ⎠

∫

(2)

where C is a relative concentration of electron-centers created by X-irradiation (typically on the order of 10−4 −10−3 M41) and A is a scaling parameter. Rm is the radius of the excluded volume (closest distance between the Sm3+ ions and the trapped electrons39). Accordingly, the averaged luminescence increase of Sm2+ ions upon X-irradiation can be approximated by eq 3. ⎧ ⎛ ∞ θ(d) = A ⎨1 − exp⎜ −4πC [1 − exp( −k(R )d)] Rm ⎝ ⎩ ⎞⎫ R2dR ⎟⎬ ⎠⎭ ⎪

∫

⎪

⎪

⎪

(3)

Figure 9 shows fits of eqs 2 and 3 to the reduction of Sm3+ ions and creation of Sm2+ upon X-ray irradiation, respectively, with fitting parameters k0 = 0.3 Gy−1 and a = 2 Å. The dose-based reduction rate constant of Sm3+ ions is thus about ∼0.03 Gy−1 at the distance of 5 Å between Sm3+ ions and nearby electron centers. Photobleaching of Sm2+ Ions Generated by XIrradiation. To study the photobleaching effect of X-ray generated Sm2+ ions in nanocrystalline BaFCl:Sm3+, four samples irradiated with X-ray doses of 3, 5, 24, and 78 Gy were photobleached by 21 mW cm−2 light from a 405 nm blue LED. The photoluminescence intensities of Sm2+ ions at 687 nm (5D0−7F0) and Sm3+ ions at 595 nm (4G5/2−6H7/2) were 8328

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Figure 10. Photoluminescence intensity changes of Sm2+ and Sm3+ ions in nanocrystalline BaFCl:Sm3+ upon photobleaching by a 405 nm blue LED with a power density of 21 mW cm−2. The samples were exposed to Cu Kα radiation (40 kV, 25 mA) with X-ray doses of (a) 3, (b) 5, (c) 24, and (d) 78 Gy. The photoluminescence intensities of Sm3+ at 595 nm (blue lines) and Sm2+ at 687 nm (red lines) were measured using 405 nm blueviolet laser diode excitation. The curves were fitted with dispersive first-order kinetics as described in Figure 9 and the text, with the exception that a term related to the initial photoluminescence intensity of Sm3+ ions was added to eq 3.

It is well-accepted that the photobleaching of Sm2+ ions is facilitated by the same mechanism as the photon-gated spectral hole burning of BaFCl:Sm2+ reported by Winnacker et al. in 1985.4 These researchers described the photon-gated spectral hole burning as a two-step photoionization process in which two different kinds of electron traps, that is, Sm3+ (type I) or other unknown centers (type II), are involved. On the basis of the room temperature hole-burning studies of polycrystalline SrFCl0.5Br0.5:Sm2+ thin films, Jaaniso et al.42 proposed that the photoionization of Sm2+ ions occurs in local Sm environments, and the photoproducts can be partially converted back to Sm2+ ions. In particular, according to Jaaniso et al., a photochromic center, where two electrons are shared by a Sm3+ ion and a vacancy, is possibly formed in the hole-burning process. Similarly, the hole-burning process of Sm2+ ions generated in nanocrystalline BaFCl:Sm3+ by X-rays is also strongly associated with the local coordination environment of Sm

monitored during the photobleaching process by measuring the luminescence spectrum as excited by a 405 nm blue-violet laser diode. The results are summarized in Figure 10. As follows from Figure 10, the photoluminescence intensity of Sm2+ ions at 687 nm decreases, whereas the 595 nm line associated with the Sm3+ ions increases with progressing 405 nm LED illumination in all four samples. In 15 min, the photoluminescence intensity of Sm2+ ions decreases by about 66, 62, 48, and 35% of the initial values for the samples irradiated with 3, 5, 24, and 78 Gy X-irradiation, respectively. After 2 h of bleaching, the photoluminescence intensities of both Sm3+ ions and Sm2+ ions approach almost constant values. It is noted here again that the photoluminescence intensities of the Sm3+ and Sm2+ ions cannot be used to compare the concentrations of the two oxidation states unless corrections are undertaken for the excitation efficiencies. 8329

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The Journal of Physical Chemistry C

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ACKNOWLEDGMENTS The Australian Research Council (ARC Discovery Project DP0772426; ARC Linkage Project LP110100451) is acknowledged for financial support of this work. Z.L. thanks The University of New South Wales Canberra for a Research Training Scholarship.

ions. As far as the electron traps involved in the hole-burning process are concerned, our recent investigation43 showed that the non-Sm3+ electron traps probably play an important part in the photoionization process. Accordingly, the photobleaching effect of Sm2+ ions observed in nanocrystalline BaFCl:Sm3+ can be rationalized as the photoionization of Sm2+ to Sm3+ ions and a subsequent trapping of the released electrons at nearby defects. To the best of our knowledge, this is the first report that demonstrates clear evidence of the recovery of Sm3+ photoluminescence and thus the photoionization of Sm2+ to Sm3+ ions during the photobleaching process. As is the case for the reduction of Sm3+ ions upon Xirradiation, the photoionization of Sm2+ ions during photobleaching under 405 nm LED is also subject to randomly distributed distances between Sm2+ centers and electronaccepting defects, that is, hole centers, in the BaFCl host lattice. The photobleaching of Sm2+ ions can be well-fitted with the dispersive first-order kinetics given for the reduction of Sm3+ to Sm2+ ions upon X-irradiation in eqs 2 and 3, except that a term related to the initial photoluminescence intensity of Sm3+ ions before photobleaching was added to eq 3. Also, the dose d in these equations is replaced by the fluence (energy/ area) of the bleaching light. Figure 10 displays the fitted curves of the photobleaching of Sm2+ ions and recovery of Sm3+ ions, respectively. Assuming a concentration of the defect centers in the host lattice of 10−3 M,41 the fit yields the parameters k0′ = 0.9 cm2 J−1 and a′ = 3 Å.

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CONCLUSIONS The photoluminescence and cathodoluminescence properties of nanocrystalline and microcrystalline BaFCl:Sm3+ have been investigated. Significant differences between the local structure of the nanocrystalline and microcrystalline BaFCl:Sm3+ samples were revealed and are found to be due to the different methods of sample preparation: coprecipitation or high-temperature sintering, respectively. The Sm3+ ions in the nanocrystalline BaFCl:Sm3+ are mainly located near the surface of the nanoparticles, whereas those in the microcrystalline sample are distributed homogeneously throughout the crystallites. Nanocrystalline BaFCl:Sm3+ is an efficient photoluminescent X-ray storage phosphor. The luminescence properties of nanocrystalline BaFCl:Sm3+ before and after X-irradiation have therefore been investigated in detail. The storage mechanism and efficiency of the nanocrystalline BaFCl:Sm3+ were investigated by monitoring the photoluminescence intensities of both Sm3+ and Sm2+ ions during X-irradiation and photobleaching processes. Upon X-irradiation, the Sm3+ centers combine with electrons trapped in shallow defects and are reduced to Sm2+ ions. During the reverse photobleaching process, the Sm2+ ions are photoionized to Sm3+ ions. Both the reduction and the photobleaching processes can be modeled by dispersive first-order kinetics where the rate constant is assumed to be given by an exponential function of the distance between the defect centers and the samarium ions.

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The authors declare no competing financial interest. 8330

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