Room Temperature Hole-Burning of X-ray Induced Sm2+

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Room Temperature Hole-Burning of X‑ray Induced Sm2+ in Nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+ Prepared by Mechanochemistry Xianglei Wang,† Hans Riesen,*,† Marion A. Stevens-Kalceff,‡ and Rajitha Papakutty Rajan† †

School of Physical, Environmental and Mathematical Sciences, The University of New South Wales, Canberra, Australian Capital Territory 2600, Australia ‡ School of Physics and Electron Microscope Unit, Mark Wainwright Analytical Centre, The University of New South Wales, Sydney, New South Wales 2052, Australia ABSTRACT: Alloyed nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5 doped with Sm3+ ions was prepared by a facile ball milling method at room temperature. Spectral hole-burning properties of Sm2+ ions from X-irradiated sample were investigated in the 7 F0−5D0 transition between 2.5 K and room temperature. The alloying allows a “chemical” broadening of the inhomogeneous width of the 7F0−5D0 f−f transition to 40 cm−1; spectral holes with a homogeneous width of 5 cm−1 can be burnt, yielding a figure-of-merit of Γinh/Γhom = 8. Mechanochemical preparation methods have a significant potential for the preparation of functional materials for applications in frequency domain optical data storage and as X-ray storage phosphors by allowing the preparation of tailored solid solutions. de Vries and Wiersma,14 respectively. In contrast, nonphotochemical spectral hole-burning is a ubiquitous phenomenon for systems with hydrogen bonds, but translational movements or rotations of other atoms or groups of atoms can also lead to this effect.3−5 A potential application of persistent spectral hole-burning is in the field of high-density frequency domain optical data storage by encoding binary data at the frequencies of persistent spectral holes, and this has resulted in significant research activity.15−20 In order to overcome bleaching and unwanted hole-burning during the readout process, photon-gated spectral hole-burning, especially employing rare-earth doped crystals, was explored. Winnacker et al. were the first to report photongated spectral hole-burning in Sm2+ ion doped BaFCl crystals at 2 K in 1985.21 Subsequently, photon-gated persistent spectral hole-burning has been reported for Sm2+ doped glasses22,23 and a color center in a halide crystal.24 In the application of spectral hole-burning to optical data storage, the figure-of-merit is expressed as the ratio of the inhomogeneous to the homogeneous line width Γinh/Γhom, where Γhom is half the width of the burnt hole in the ideal case.25 Importantly, Jaaniso and Bill reported26 the first observation of persistent spectral hole-burning at room temperature in Sm2+ doped SrFCl0.5Br0.5 crystals in 1991 with a figure of merit Γinh/Γhom = 20. Jaaniso and Bill also investigated high temperature spectral holeburning properties in Sm doped MyM1−yFXxX1−x (M = Ca, Sr,

1. INTRODUCTION Spectral hole-burning is a powerful technique in laser spectroscopy, and it is based on frequency selective bleaching of a subset of optical centers within an inhomogeneously broadened transition by a narrow band laser, resulting in a spectral hole (dip) in the absorption spectrum at the selected frequency.1,2 On the basis of the stability of the spectral hole, the hole-burning mechanism can be classified into transient spectral hole-burning (TSHB) and persistent spectral holeburning (PSHB); both mechanisms have been intensively studied.3−5 Spectral holes in transient spectral hole-burning are usually based on excited state population storage and hence decay on the microsecond to millisecond time scale, while persistent spectral holes can have infinite lifetimes.6 Transient spectral hole-burning was first reported by Szabo for the R1-line of ruby at 4.2 K;7 a very large number of TSHB reports were published in subsequent years, as is very selectively illustrated by refs 8−11. Persistent spectral hole-burning can be based on two mechanisms, namely, photochemical hole-burning and nonphotochemical hole-burning. Photochemical spectral holeburning occurs when a subset of chromophores within the inhomogeneously broadened transition undergoes a chemical reaction in the excited state upon selective excitation, while nonphotochemical or photophysical spectral hole-burning is based on slight structural rearrangements of the host−guest interactions in the local environment.3−6 Possible chemical reactions in photochemical hole-burning include photoionization, phototautomerism, and photodecomposition as reported, for example, by Macfarlane and Vial,12 Gorokhovskii et al.13 and © 2014 American Chemical Society

Received: July 23, 2014 Revised: September 2, 2014 Published: September 7, 2014 9445

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The spectra were collected by single scans with 0.2 nm step and an integration time of 0.5 s. A blue band-pass (Schott, BG25) and a yellow long-pass (Schott, GG475) color glass filter were placed before and after the sample, respectively. The cathodoluminescence spectra of the sample were collected using a Princeton Instruments Pixis 100 UVoptimized CCD (1340 × 100 pixels) with 1200 grooves/mm grating blazed at 500 nm and excited by a 10 keV, 5.25 nA electron beam in a JEOL 7001F field emission scanning electron microscope. The secondary electron images of the samples were taken of the same area. For room temperature spectral hole-burning, a spectroscopy CCD camera (Andor iDus model DV401A-BV, 1024 × 127 pixels) was used to capture the photoluminescence spectra as dispersed by a Spex 1704 1 m monochromator. The sample was excited by a blue LED (Semileds 417 nm, 0.5 mW/cm2). The spectral hole-burning process was realized by a diode laser (Thorlabs HL6738MG with a LDC201ULN laser controller and a TEC 2000 temperature controller, 688.2 nm, 80 mW/ cm2). A green LED (530 nm, 12 mW/cm2) was applied for the photon-gated hole-burning. For low temperature spectral holeburning, a closed cycle cryostat (Janis/Sumitomo SHI-4.5) was employed to cool the sample to 2.5 K.

Ba; X = Cl, Br, I) single crystals of the PbFCl family up to 430 K and found similar inhomogeneous line widths of around 30 cm−1.27 Holliday et al.28 studied spectral hole-burning properties in Sm2+ doped crystalline powders of substitutionally disordered microcrystals at room temperature and concluded that the inhomogeneously broadened line widths are mainly determined by the halide anions rather than the cations. They reported inhomogeneous line widths of the studied MxM1−xFCl0.5Br0.5:Sm2+ system of around 40 cm−1. Moreover, they also noted that substitutional disorder does not affect the homogeneous line width.28 The ability to conduct hole-burning at room temperature is a key requirement for any practical application in frequency or time domain optical data storage. However, in most systems the homogeneous line width dominates over the inhomogeneous broadening at room temperature and disables the observation of hole-burning. In subsequent years, a number of other room-temperature experiments were reported for rare earth ion doped crystals and glasses.29−34 It is stressed here that in the above literature, samples were prepared by relatively complex preparation methods at elevated temperatures and under a reducing atmosphere. For example, Hagemann et al. reported systematic studies of the crystallographic and optical properties of the SryBa1−yFClxBr1−x (x, y from 0 to 1) system.35,36 We have recently reported a facile mechanochemical method for the preparation of BaFCl:Sm3+ nanocrystals by ball milling at room temperature.37 In the current paper, nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+ was prepared in the same way with subsequent conversion of a significant fraction of the Sm3+ ions to Sm2+ ions by irradiating the as-prepared sample with approximately 560 Gy of 40 kV X-ray radiation in a powder Xray diffractometer. The spectral hole-burning properties of the resulting Sm2+ ions were investigated in the 7F0−5D0 transition between 2.5 K and room temperature.

3. RESULTS AND DISCUSSION The powder X-ray diffraction pattern of nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+ is shown in Figure 1 in comparison with MFX (M = Ba, Sr; X = Cl, Br) standard data. The broadened peaks in the XRD patterns indicate that the asprepared sample is a solid solution with random occupation of the cation and anion sites. The secondary electron images exhibit the morphology which displays clumped particles with irregular shapes. The cathodoluminescence spectrum shown in Figure 2 was normalized to the 687 nm Sm2+ emission. Both Sm3+ from 4 GJ−6HJ transitions and Sm2+ from 5DJ−7FJ transitions can be observed in the spectrum. The inset 3D plot shows the intensity as a function of electron beam dose, which indicates an attenuation of both Sm3+ and Sm2+ emissions at the same rate with the increase of electron beam exposure time. It is also

2. EXPERIMENTAL SECTION ACS reagent grade chemicals obtained from Sigma-Aldrich Pty Ltd. were used without further purification. Strontium chloride hexahydrate was obtained from May & Baker Ltd. Strontium fluoride was prepared by coprecipitation of strontium chloride heaxhydrate (SrCl2·6H2O) and ammonium fluoride (NH4F) and dried in the oven. The purity of this laboratory-made SrF2 was characterized by XRD. First, 0.104 g of barium chloride (BaCl 2 , 99.999%) and 0.133 g of strontium chloride hexahydrate (SrCl2·6H2O, 98%) were mixed and ground. The mixture was dried in an oven at 400 °C for 3 h and then ballmilled in a 10 mL of zirconia jar with two 12 mm zirconia balls on a Retsch mixer mill 200 at a frequency of 20 Hz for 1 h. Second, the same procedure was applied to a mixture of 0.088 g of barium fluoride (BaF2, 99.99%) and 0.063 g of strontium fluoride (SrF2). Third, the two ball-milled mixtures were mixed with 0.333 g of barium bromide dihydrate (BaBr2·2H2O, 99%), 0.126 g of strontium fluoride (SrF2), and 0.005 g of samarium chloride hexahydrate (SmCl3·6H2O, 99%). The final mixture was dried in the oven at 400 °C for another 3 h and ball-milled for 2 h to yield nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+. The as-prepared sample was irradiated at room temperature in a Siemens powder X-ray diffractometer (40 kV, 25 mA, Cu Kα) with 2θ of 60° for 2 h with 560 Gy of radiation, yielding a significant conversion of Sm3+ to Sm2+ ions. The photoluminescence spectra of the sample were measured at room temperature on a Horiba Jobin-Yvon Spex Fluoromax-3 fluorometer equipped with Datamax 2.2 software.

Figure 1. Powder X-ray diffraction pattern of nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+ compared with standard BaFCl, BaFBr, SrFCl, and SrFBr data from ICDD cards. The insets show secondary electron images of particles. The white dimension markers denote 100 nm. 9446

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∼560 Gy, about 30−40% of Sm3+ ions are converted to Sm2+ ions. The full width at half-maximum (fwhm) of the 5D0−7F0 Sm2+ transition at around 687 nm is 2 nm (∼42 cm−1), which is in accord with the results reported by Jaaniso and Bill27 and Holliday et al.28 Room temperature spectral hole-burning of Sm2+ ions generated by X-irradiation was observed by measuring the characteristic 5D0−7F0 transition line of Sm2+ ions at around 687 nm in luminescence with an excitation wavelength of 417 nm (blue LED). The hole-burning kinetics is shown in Figure 4. From Figure 4 we determine an inhomogeneous width of Γinh = 42 cm−1 and the shallowest hole burnt for 1 min displays a width of 10 cm−1 which yields a homogeneous width of Γhom = 5 cm−1, resulting in a figure of merit of about 8. Compared to previously reported values,26 the hole width is significantly larger, most likely because of the nanoscale of the crystals. The burning kinetics shown in the right-hand panel of Figure 4 can be fitted with a double-exponential function, indicating dispersive first order kinetics.40 The efficiency of photon-gated hole-burning and non-gated hole-burning was compared by irradiating the as-prepared sample in the powder diffractometer by X-ray radiation for the same duration and burning under the same conditions for the same period of time with and without green light (530 nm, 12 mW/cm2) at 2.5 K. It follows from Figure 5 that the hole in photon-gated burning is deeper than that for the non-gated burning, confirming gating with green light. A hole with depth of 15% was burnt in 4500 s (75 min) with the green gating light, while a hole with depth of 8% was burnt without gating in the same period, yielding a gating ratio of 2. In comparison, Winnacker et al.21 burnt a 2% deep hole in 2000 s at 687.9 nm (2 W/cm2) and a 20% deep hole in 3 s with an Ar laser as the gating light (514.5 nm, 20 W/cm2), yielding a gating ratio of 104. After normalization to a gating power of 1 W/cm2, the present gating ratio is 170 compared to the one of 500 from Winnacker et al.21 Again, this is likely due to the nanoscale size of the crystallites and, in particular, the higher self-gated burning rate. According to Winnacker et al.,21 the photon-gated spectral hole-burning of Sm2+ ions is based on a two-step

Figure 2. Cathodoluminescence spectrum of nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+. Both Sm3+ (4GJ → 6HJ) and Sm2+ (5DJ → 7 FJ) emissions can be observed. The inset shows a 3D plot of spectra collected as a function of electron beam dose.

notable in the spectrum that a group of narrow and red-shifted Sm3+ emissions can be observed in addition to the broad Sm3+ emission lines which are commonly observed in the photoluminescence spectrum. According to Liu et al.,38 these two groups of emission lines are attributed to different types of Sm3+ sites with different local coordination environments. Interestingly, the red-shifted sites appear to be created by the exposure to the electron beam as follows from a comparison with the photoluminescence spectra below. The photoluminescence spectra of the as-prepared sample observed with 401 and 415 nm excitation wavelengths before and after X-irradiation are shown in Figure 3. The emission peaks around 559, 594, 640, and 700 nm can be attributed to the characteristic 4GJ−6HJ transitions of Sm3+ ions, and the emission peaks around 629, 641, 687, 701, and 727 nm are the characteristic 5DJ → 7FJ transitions of the Sm2+ ions.39 The intensities of Sm3+ and Sm2+ emissions are decreasing and increasing, respectively, with increasing X-ray exposure, indicating a radiation induced conversion between the two ions. After 2 h of X-irradiation, which corresponds to a dose of

Figure 3. Room temperature photoluminescence spectra of nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+ before (dashed line) and after (solid line) 2 h of X-irradiation, excited at 401 nm (a) and 415 nm (b), respectively. The spectra are offset on the vertical scale for clarity. Prominent 4GJ−6HJ (Sm3+) and 5DJ−7FJ (Sm2+) transitions are denoted in (a) and (b), respectively. Approximately 40% of Sm3+ ions are converted to Sm2+ ions after 2 h of X-irradiation. 9447

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Figure 4. Room temperature (298 K) spectral hole-burning spectra of X-irradiated nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+ with different burning time observed for the 5D0−7F0 transition in photoluminescence is shown on the left; the hole depth as a function of burning time is shown on the right. The data have been fitted with a double-exponential function. The inset shows the hole-burning (difference) spectra.

quadratic temperature dependence, similar to other rare-earth ion doped disordered systems reported previously.28,29,41

4. CONCLUSIONS A facile mechanochemical method for the synthesis of alloyed alkaline earth fluorohalide crystals doped with rare earth metal ions at room temperature is reported. The Sm3+ ions in the asprepared nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+ can be converted to Sm2+ ions upon X-irradiation. The X-irradiated sample exhibits spectral hole-burning at room temperature with a figure of merit of about 8. The temperature contribution to the hole-width follows a T2.2 dependence. At room temperature, the fluence dependence of the depth of the burnt hole can be described by a double-exponential function, indicating dispersive first order kinetics. At low temperature, the photon gating ratio by green light is about ∼170 per W/cm2. The holeburning performance of the sample, especially at room temperature, points to the significant potential of mechano-

Figure 5. Non-gated hole-burning (a) and photon-gated (12 mW/cm2 green light) hole-burning (b) spectra of nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+ after burning for 75 min at 2.5 K. The laser was scanned during the burn period over ∼15 GHz in order to render a hole width that is commensurate with the resolution of the readout instrumentation.

photoionization of Sm2+ to Sm3+ ions with the following net reaction, Sm 2 + + (trap) → Sm 3 + + (trap)−

(1)

It was concluded that there are two different traps; one was attributed to Sm3+ ions, while the other trap could be due to Fcenters or oxygen defects. The temperature dependence of the hole width was investigated by burning a hole at 2.5 K and then subsequently measuring the luminescence spectrum at temperatures up to 295 K. Figure 6 illustrates that the hole width has a near

Figure 6. Temperature dependence of a spectral hole burnt at 2.5 K in the 5D0−7F0 transition of nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5:Sm3+. The data show a T2.2 dependence. The hole was initially burnt by a scanning laser (∼15 GHz) with gating for 3 h at 2.5 K. 9448

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(18) Masumoto, Y.; Zimin, L. G.; Naoe, K.; Okamoto, S.; Arai, T. Persistent Spectral Hole-Burning in Semiconductor Microcrystals. Mater. Sci. Eng. B 1994, 27, L5−L9. (19) Harley, R. T.; Henderson, M. J.; Macfarlane, R. M. Persistent Spectral Hole-burning of Colour Centres in Diamond. J. Phys. C: Solid State Phys. 1984, 17, L233−L236. (20) Ambrose, W. P.; Sethna, J. P.; Sievers, A. J. Persistent Infrared Spectral Hole-Burning of NO2− Ions in Potassium Halide Crystals. I. Principle and Satellite Hole Generation. J. Chem. Phys. 1991, 95, 8816−8842. (21) Winnacker, A.; Shelby, R. M.; Macfarlane, R. M. Photon-Gated Hole-Burning: A New Mechanism Using Two-Step Photoionization. Opt. Lett. 1985, 10, 350−352. (22) Hirao, K.; Todoroki, S.; Cho, D. H.; Soga, N. RoomTemperature Persistent Hole-Burning of Sm2+ in Oxide Glasses. Opt. Lett. 1993, 18, 1586−1587. (23) Cho, D. H.; Hirao, K.; Soga, N. Persistent Spectral HoleBurning of Sm2+ in Borate Glasses. J. Non-Cryst. Solids. 1995, 189, 181−190. (24) Fedorov, V. V.; Mirov, S. B.; Ashenafi, M.; Xie, L. Spectroscopic Analysis and Persistent Photon-Gated Spectral Hole-Burning in LiF:F2− Color Center Crystal. Appl. Phys. Lett. 2001, 79, 2318−2320. (25) Moerner, W. E., Ed. Persistent Spectral Hole-Burning: Science and Applications; Springer: Berlin, 1988. (26) Jaaniso, R.; Bill, H. Room Temperature Persistent Spectral Hole-Burning in Sm-Doped SrFCl1/2Br1/2 Mixed Crystals. Europhys. Lett. 1991, 16, 569−574. (27) Jaaniso, R.; Bill, H. High-Temperature Spectral Hole-Burning on Sm-Doped Single Crystal Materials of PbFCl Family. J. Lumin. 1995, 64, 173−179. (28) Holliday, K.; Wei, C.; Croci, M.; Wild, U. P. Spectral HoleBurning Measurements of Optical Dephasing between 2−300 K in Sm2+ Doped Substitutionally Disordered Microcrystals. J. Lumin. 1992, 53, 227−230. (29) Wei, C.; Holliday, K.; Meixner, A. J.; Croci, M.; Wild, U. P. A Spectral Hole-Burning Study of BaFCl0.5Br0.5:Sm2+. J. Lumin. 1991, 50, 89−100. (30) Schnieper, M.; Trotta, F.; Bersier, S.; Bill, H. RoomTemperature Persistent Spectral Hole-Burning in SrFCl:Sm2+ Films: Temporal and Spatial Response. Appl. Phys. Lett. 1999, 75, 40−42. (31) Nogami, M.; Ishikawa, T. Room-Temperature Persistent Spectral Hole-Burning of Eu3+ Coupling with Al3+ in Glass. Phys. Rev. B 2001, 63, 104205−1∼6. (32) Nogami, M.; Suzuki, K. Formation of Sm2+ Ions and Spectral Hole-Burning in X-ray Irradiated Glasses. J. Phys. Chem. B 2002, 106, 5395−5399. (33) Kurita, A.; Kushida, T.; Izumitani, T.; Matsukawa, M. RoomTemperature Persistent Spectral Hole-Burning in Sm2+-Doped Fluoride Glasses. Opt. Lett. 1994, 19, 314−316. (34) Qiu, J.; Nouchi, K.; Miura, K.; Mitsuyu, T.; Hirao, K. RoomTemperature Persistent Spectral Hole-Burning of X-ray-Irradiated Sm3+-Doped Glass. J. Phys.: Condens. Matter. 2000, 12, 5061−5063. (35) Hagemann, H.; Kubel, F.; Bill, H. Crystallochemical Study of Mixed Strontium−Barium Fluorohalides. Mater. Res. Bull. 1993, 28, 353−362. (36) Hagemann, H.; Kubel, F.; Bill, H. Crystallochemical and Optical Study of Mixed Alkaline Earth-Samarium Fluorohalides of the Lead Fluoride Chloride Type. Mater. Res. Bull. 1995, 30, 405−412. (37) Liu, Z.; Stevens-Kalceff, M. A.; Wang, X.; Riesen, H. Mechanochemical Synthesis of Nanocrystalline BaFCl:Sm3+ Storage Phosphor by Ball Milling. Chem. Phys. Lett. 2013, 588, 193−197. (38) Liu, Z.; Stevens-Kalceff, M. A.; Riesen, H. Effects of Postannealing on the Photoluminescence Properties of Coprecipitated Nanocrystalline BaFCl:Sm3+. J. Phys. Chem. A 2013, 117, 1930−1934. (39) Liu, Z.; Stevens-Kalceff, M. A.; Riesen, H. Photoluminescence and Cathodoluminescence Properties of Nanocrystalline BaFCl:Sm3+ X-ray Storage Phosphor. J. Phys. Chem. C 2012, 116, 8322−8331.

chemical methods for the preparation of optical data storage material and also X-ray storage phosphors.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Australian Research Council (ARC Linkage Project LP110100451) is acknowledged for financial support of this work. X.W. thanks UNSW Canberra for a completion scholarship. Support from the Australian Microscopy & Microanalysis Research Facility at UNSW is acknowledged.

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REFERENCES

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