Article pubs.acs.org/JPCA
Photoreduction of Sm3+ in Nanocrystalline BaFCl Nicolas Riesen,*,† Alexandre François,† Kate Badek,‡ Tanya M. Monro,§,† and Hans Riesen‡ †
Institute for Photonics & Advanced Sensing and School of Physical Sciences, North Terrace Campus, University of Adelaide, Adelaide, South Australia 5005, Australia ‡ School of Physical, Environmental and Mathematical Sciences, The University of New South Wales, Canberra, Australian Capital Territory 2600, Australia § University of South Australia, Research and Innovation, Hawke Building, 55 North Terrace, Adelaide, South Australia 5000, Australia ABSTRACT: We demonstrate that exposure of nanocrystalline BaFCl:Sm3+ X-ray storage phosphor to blue laser pulses with peak power densities on the order of 10 GW/cm2 results in conversion of Sm3+ to Sm2+. This photoreduction is found to be strongly powerdependent with an initial fast rate, followed by a slower rate. The photoreduction appears to be orders of magnitude more efficient than that for previously reported systems, and it is estimated that up to 50% of the samarium ions can be photoreduced to the divalent state. The main mechanism is most likely based on multiphoton electron−hole creation, followed by subsequent trapping of the electrons in the conduction band at the Sm3+ centers. Nanocrystalline BaFCl:Sm3+ is an efficient photoluminescent X-ray storage phosphor with possible applications as dosimetry probes, and the present study shows for the first time that the power levels of the blue light have to be kept relatively low to avoid the generation of Sm2+ in the readout process. A system comprising the BaFCl:Sm3+ nanocrystallites embedded into a glass is also envisioned for 3D memory applications. Because the deep electron traps, Sm2+, are relatively stable, it is possible to read out the storage center multiple times by photoexcited luminescence as long as the laser/light power density is low enough. Note also that the Sm2+ centers are stable indefinitely under ambient room light and up to temperatures of 200 °C.2 However, it is also possible to reverse the Sm2+ ions back to the trivalent oxidation state by a two-photon ionization process, and power densities on the order of 20 mW/cm2 are sufficient to bleach the Sm2+ center over a time scale of ∼1 h.2 Because of the small size of the crystallites on the order of magnitude of 100 nm and their high sensitivity to radiation, in particular, in the X-ray region, the nanocrystalline BaFCl:Sm3+ system renders itself as a potential nanoprobe for ionizing radiation, for example, as embedded into optical fibers, at the tip of a fiber or directly in biological cells, for instance, enabling in vivo dosimetry.6 To be able to undertake measurements of radiation exposure in real time, for example, with nanocrystals embedded at the tip of a fiber, it is advantageous to rapidly read out the Sm2+ emission by employing pulsed laser sources. The use of pulsed laser sources can also assist in avoiding unintentional changes in the valence state (i.e., bleaching) compared with using CW sources for the readout. However, if the peak powers of the pulsed laser sources are sufficiently high they too can potentially induce bleaching. To gain a better
1. INTRODUCTION Nanocrystalline BaFCl:Sm3+, prepared by coprecipitation, is an efficient photoluminescent X-ray storage phosphor. Exposure of the BaFCl host to ionizing radiation results in the creation of electron−hole pairs, and the Sm3+ ions act as deep traps for the electrons; in particular, the samarium ion is reduced to the divalent oxidation state in the storage mechanism.1,2 The luminescence spectrum of the resultant Sm2+, in the BaFCl nanocrystalline host, consists of very discrete and narrow f-f transitions from the 5DJ to the 7FJ multiplet in the red region of the visible spectrum, with the 5D0 → 7F0 line at ∼687 nm being dominant at room temperature.3 The system can be efficiently excited at ∼420 nm, where relatively intense interconfigurational 4f6 → 4f55d1 transitions are observed.4,5 The main peak of these transitions displays a molar extinction coefficient of ∼400 l/(mol cm).4,5 Figure 1 summarizes the behavior of nanocrystalline BaFCl:Sm3+ upon X-irradiation. The narrow 5DJ → 7FJ Sm2+ transitions in the red part of the visible spectrum can be selectively excited at 425 nm (Figure 1a), and the Sm3+ spectrum can be selectively excited at 401 nm (Figure 1b). The latter shows minimal change after 30 mGy of 60 kVp Xirradiation as only ∼0.2% of the Sm3+ ions are converted to Sm2+ with such a dose. The selective excitation spectra in Figure 1c show the 4f6 → 4f55d1 transitions of Sm2+ and the f-f transitions of Sm3+ when observed at 639 and 687 nm, respectively. The minute amount of Sm2+ that is present in the nonirradiated sample (Figure 1a) is due to background radiation. © 2015 American Chemical Society
Received: April 8, 2015 Revised: May 12, 2015 Published: May 18, 2015 6252
DOI: 10.1021/acs.jpca.5b03404 J. Phys. Chem. A 2015, 119, 6252−6256
Article
The Journal of Physical Chemistry A
Figure 4. Time-dependent increase in signal strength for the dominant 687 nm Sm2+ peak for excitation wavelengths of 430 and 480 nm, normalized to the readout power. Measurements were taken over 5 s at 7 s intervals. The data has been filtered with the MATLAB smooth function. The power densities (and hence signal counts) were lower compared with the experiment of Figure 3a because the spectrometer slit size was reduced to its minimum.
the 2.6× higher power for the 480 nm light. For the settings used, the initial rate of increase in signal is well over 1000 cpm for 480 nm. It is important to note that much greater rates of conversion could be achieved by using higher power, tighter focusing of the light, and a greater duty cycle. The amount of conversion between Sm3+ and Sm2+ is on the order of 1 to 2% over the ∼1/2 h time-frames considered. Over a time frame of ∼1 day, conversion of ∼20% was observed. These conversion levels were determined by comparing the intensity of the luminescence signal under the same experimental condition with that of a sample of known Sm2+ percentage. The latter was prepared by X-irradiation of BaFCl:Sm3+ with an X-ray dose that yields a conversion of ∼90% of the Sm3+ ions to the divalent state.2 Interestingly, when the X-irradiated sample is readout under comparable experimental conditions, the signal drops by ∼50% and reaches an equilibrium after ∼2 h. This indicates that the photoreduction and photoexcitation processes occur simultaneously under these specific conditions. The conversion of samarium is evidenced by an increase in not only the main 687 nm peak (5D0 → 7F0) but also lesser peaks at 629 (5D1 → 7F0), 641 (5D1 → 7F1), 702 (5D0 → 7F1), and 728 nm (5D0 → 7F2), as is clearly shown in Figure 5. The characteristic increase in signal with time is the same among all of these emission wavelengths for a fixed excitation wavelength. Because both the 430 and 480 nm excited spectra show comparable weak photoluminescence of the Sm3+ ion, that is, the weak absorption by f-f transitions is similar for the two wavelengths (see also Figure 1c), a resonant and nonresonant mechanism are likely responsible for the Sm3+ reduction. We note here that a wavelength of 401 nm would be more ideal to probe the resonant process because Sm3+ displays a relatively strong f−f transition (6H5/2 → 4K11/2) at this wavelength (see Figure 1c), with a molar extinction coefficient of about 3 l/(mol cm). Note also that the absorbance by Sm3+ is very weak at both 430 and 480 nm. Also, the 641 and 702 nm Sm2+ transitions have 4G5/2 → 6H9/2 and 4G5/2 → 6H11/2 emissions, respectively, from Sm3+ as a background. These background signals were subtracted from the signals in Figure 5. The conversion of samarium was also observed in smaller 30 nm crystals that were prepared by ball-milling a mixture of BaCl2 and BaF2 with a small percentage of SmCl3·6H2O.20 The conversion efficiency for these crystals was, however, found to
Figure 5. Time-dependent increase in signal strength of all the Sm2+ emission peaks when exciting samples at (a) 430 and (b) 480 nm. The signal counts were normalized with respect to their maximum values, F. Measurements were taken for 5 s at 1.3 min intervals. The spectrometer slit size was 0.1 mm. The average powers used were 0.65 ± 0.05 and 1.7 ± 0.2 mW for 430 and 480 nm, respectively.
decrease by several orders of magnitude for the same exposure time and power settings. This indicates that multiphoton electron−hole creation by exposure to laser light of high power density is likely the main mechanism for the reduction of the Sm3+ centers that act as the electron traps. For example, upon exposure to the intense light a chloride ion can be subject to electron loss and the electron may be excited into the conduction band with subsequent trapping at the Sm3+ centers. In the 30 nm crystals the electron−hole recombination length is expected to be much shorter and hence the electrons cannot reach the Sm3+ traps. The faster rate is possibly due to twophoton excitation into the Cl− → Sm3+ charge-transfer (CT) transition. It is also possible that O2−(Cl−) centers play some role in the photoreduction. Results obtained using electron paramagnetic resonance (EPR) spectroscopy on single crystals of BaFCl doped with Sm3+ indicate that such centers are likely responsible for the charge compensation.21 The excitation spectrum of nanocrystalline BaFCl:Sm3+ exhibits a broad transition at around 230 nm (not illustrated in Figure 1c) that is most likely due to a CT Sm3+ transition. We note here that a range of possible defect centers were previously investigated in the context of light-induced valence state switching in single crystals of BaFCl:La, where it was concluded that the photoelectrons from La2+ ions were most likely trapped at F centers.22
4. CONCLUSIONS Pulsed blue laser light with peak powers on the order of magnitude of 10 GW/cm2 is found to convert Sm3+ to Sm2+ in nanocrystalline BaFCl. The necessary power density is several orders of magnitude lower than for previously reported systems.8 This may be related to the high defect density and charge mobility provided by the nanocrystalline BaFCl host. 6255
DOI: 10.1021/acs.jpca.5b03404 J. Phys. Chem. A 2015, 119, 6252−6256
Article
The Journal of Physical Chemistry A
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There appear to be two mechanisms that lead to the photoreduction, with the slower process being significantly power- and wavelength-dependent. The saturation level of the first process appears to be strongly power-dependent, indicating dispersive first-order kinetics. The strong power dependence of the photoreduction could be used for the preparation of holeburning materials. After an initial preparation step with a power density of GW/cm2 of the divalent state, holes could be burnt by two-step photoionization at significantly lower power levels. This is in addition to the potential use of the nanocrystalline BaFCl:Sm3+ system in 3D memory applications17−19 if the crystallites can be embedded into a glass with similar refractive index (i.e., n ≈ 1.6) to minimize scattering. The relatively low power threshold and very high power dependence for the photoreduction would facilitate such an application. Hole burning may add a fourth dimension with the spatially resolved preparation of Sm2+ in 3D being followed by the frequencyselective hole-burning process; that is, literally a 4D memory may be possible.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +61 (08) 8313 0871. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS N.R., A.F., and T.M.M. acknowledge the support by an Australian Research Council (ARC) Georgina Sweet Laureate Fellowship FL130100044. H.R. acknowledges the ARC for financial support via an ARC Linkage project LP110100451.
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REFERENCES
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DOI: 10.1021/acs.jpca.5b03404 J. Phys. Chem. A 2015, 119, 6252−6256
