Controlled Generation of Tm2+ Ions in Nanocrystalline BaFCl:Tm3+ by

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Article 2+

Controlled Generation of Tm Ions in Nanocrystalline BaFCl:Tm by X-Ray Irradiation 3+

Jun Zhang, and Hans Riesen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12035 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 16, 2017

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The Journal of Physical Chemistry

Controlled Generation of Tm2+ Ions in Nanocrystalline BaFCl:Tm3+ by X-ray Irradiation

Jun Zhang and Hans Riesen* School of Physical, Environmental and Mathematical Sciences The University of New South Wales Canberra, ACT 2600 Australia *Corresponding author: [email protected]

1 ACS Paragon Plus Environment


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Abstract: An investigation of the photoluminescence properties of divalent thulium generated by X-irradiation (=X-ray irradiation) of nanocrystalline BaFCl:Tm3+ (250 ± 50 ppm) is reported. The X-irradiated samples show typical Tm2+ f-f luminescence with an excited state lifetime τ = 0.98 ± 0.05 ms in the near infrared (~1140 nm) but no d-f luminescence is observed for the BaFCl host material within the visible range. The Tm2+ ions are relatively stable under dim room light but are photoionized rapidly by sunlight or by fluorescent tube lighting. To study the X-ray storage mechanism, photoluminescence intensities of both Tm3+ and Tm2+ transitions were measured and the reverse photoionization of Tm2+ was investigated as a function of the laser power density and the initial radiation dose. The photoionization data was then modelled by employing equations based on dispersive first order kinetics using a standard Γ distribution function for the separation between the reduced thulium ions and the hole traps. In accord with previous reports, the hole traps are most likely oxide impurities such as O2- and the average of the separation is found to be ~6-7 Å i.e. a few interionic spacings.


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Introduction The optical properties of rare earth (RE) ions in wide bandgap semiconductors have attracted a vast number of investigations due to their actual and potential applications including storage phosphors, up-conversion materials and solid-state lasers.1-5 It is well documented that RE ions are significantly more stable in the trivalent than divalent state6-8 and luminescent materials based on divalent RE ions require an appropriate host matrix to be stable9-11. For the stabilisation of divalent RE ions and for high quantum yields, the tetragonal BaFCl host material with the matlockite structure (P4/nmm) presents a promising opportunity because of its high chemical stability as well as the low phonon energy (~300 cm-1).12-17 BaFCl is an extraordinary host that can be readily prepared in the form of single macroscopic crystals or as a powder on the micrometer or nanometer scale. Many reports in the literature employed single crystals and micrometer-sized powders and hence the exploration of nanocrystalline samples is warranted for potential applications. Recently, we have successfully prepared nanocrystalline BaFCl doped with Eu2+ and Sm2+ ions by facile co-precipitation and ball-milling methods.18-20 Moreover, very efficient X-ray generation of stable Sm2+ ions was observed in co-precipitated nanocrystalline BaFCl:Sm3+, pointing to a promising potential for applications in dosimetry and medical imaging.21 Importantly, the sensitivity of the Sm3+ → Sm2+ reduction upon Xirradiation is over five orders of magnitude higher in the nanocrystalline sample compared to a microcrystalline sample prepared by high temperature sintering. This is most likely due to the higher defect density in the nanocrystalline sample and microstructural arrangement of oxide impurities. A range of investigations on preparation conditions, luminescent and hole-burning properties, and the storage mechanism has been reported for this material.22-24 These results indicate that other



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stable divalent RE ions may also be generated readily by X-irradiation in the BaFCl host lattice. Compared to Eu2+ and Sm2+ ions, Tm2+ is less stable as is reflected in its lower reduction potential; the standard reduction potentials for Eu3+/Eu2+, Sm3+/Sm2+ and Tm3+/Tm2+ are ~-0.3 V, ~-1.6 V and ~-2.3 V, respectively.6, 7 Thus it is not surprising that the photoluminescence of Tm2+ doped materials has been the subject of few reports only to date. Kiss, McClure and Loh first reported the spectroscopic properties and energy levels of Tm2+ in MX2 (M = Ca, Sr, Ba; X = F, Cl) crystals in the 1960s.2527

Subsequently, Tm2+ was also doped into several other hosts such as SrB4O7,9

MZnCl4 (M = Ba, Sr),10 and CsCaX (X = Cl, Br, I).28 It is noted that the majority of these Tm2+ doped systems were studied in the form of single crystals. Very recently, it has been shown that Tm2+ based materials have a promising potential for applications in solar radiation conversion devices, such as luminescent solar concentrators, due to their outstanding broadband absorption and the large Stokes shift which minimizes self-absorption.29 In contrast to the limited literature on Tm2+, Tm3+ doped materials have received considerable attention as Tm3+ is an excellent activator for up-conversion materials.1, 3, 30

In this context, controlling the reduction of trivalent Tm3+ to divalent Tm2+

(especially when both Tm3+ and Tm2+ can be stabilized in the same host) by Xirradiation may render X-ray phosphors that use the change of up-conversion intensity as an indicator of X-ray dose. The present paper reports on the generation of Tm2+ ions in nanocrystalline BaFCl:Tm3+ by X-irradiation. Photoluminescence properties of the as-prepared samples were studied before and after exposure to X-rays. The reverse photobleaching mechanism was also studied by applying a range of power densities and for samples


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that were subject to a range of radiation doses. These results are significant for a better understanding of co-precipitated BaFCl:RE nanocrystals with possible applications such as X-ray storage phosphors, up-conversion materials and luminescent solar concentrators.

Experimental methods ACS reagent grade chemicals from Sigma-Aldrich were used without further purification. The nanocrystalline BaFCl:Tm3+ powders were prepared by a previously reported co-precipitation method.21 In brief, 0.6 mL of an aqueous thulium chloride solution (TmCl3, 1 mg·mL-1) was first added to 25 mL of a barium chloride solution (BaCl2, 0.4 M), followed by the addition of 25 mL of an ammonium fluoride solution (NH4F, 0.2 M). The mixture was briefly shaken and then kept at a constant temperature of 20 °C. After two hours, the obtained white nanocrystalline precipitate was separated from the solution by centrifugation (4000 rpm, 12 minutes) and then dried at 70 °C for 12 hours. The concentration of thulium ions in the as-prepared BaFCl:Tm3+ samples was determined to be 250 ± 50 ppm by ICP-OES (PerkinElmer Optima 8000). For this analysis 100 mg of the sample was dissolved in 50 mL 1 M HNO3. The powder X-ray diffraction (XRD) pattern was measured using a Rigaku MiniFlex-600 benchtop diffractometer operating at 40 kV and 15 mA with Cu Kα radiation. Scanning electron microscopy (SEM) images were recorded on a JEOL 7001 Field Emission Scanning Electron Microscope. The X-irradiation was undertaken by exposure in the powder X-ray diffractometer (Rigaku Miniflex-600, 40 kV, 15 mA, Cu Kα), and the effective radiation dose was cross-calibrated using a Sirona HELIODENT Plus dental X-ray source with known



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dose. The sample holder consisted of a black anodised aluminium disc with a milledout counterbore of 5 mm diameter and 0.5 mm depth. The nanocrystalline powder was manually pressed into the counterbore. Luminescence spectra were accumulated on a SPEX 500M monochromator equipped with Andor iDus cameras (Model DU490A-1.7 InGaAs CCD for the infrared range and Model DV401A-BV Si CCD for the visible range) and appropriate interference or colour glass filters were used to enhance the rejection of excitation light. For infrared luminescence measurements, if not specified otherwise, the light of a focused 462 nm blue laser diode (Thorlabs, L462P1400MM) served as the excitation source. The laser power was measured by an optical power meter (Thorlabs, PM120VA) and the size of the focused laser light was ~0.02 cm2. To optimise the signal, either a 470 nm LED (Thorlabs, M470L3) or a 365 nm LED was employed as the excitation source for the luminescence of Tm3+. A closed cycle cryostat (CTI-Cryogenics Cryodyne model 22) was used to cool the sample for low temperature experiments. A SPEX 1704 monochromator, equipped with a halogen lamp (24V, 150W) as the light source, was employed to scan the excitation spectrum and the infrared luminescence was monitored by a InGaAs amplified detector (Thorlabs, PDF10C/M). Results and Discussion The powder X-ray diffraction pattern and SEM images of the nanocrystalline BaFCl:Tm3+ samples as-prepared by co-precipitation are shown in Figure 1. All the prominent peaks in the XRD pattern match the standard BaFCl data (JCPDS No. 761368) well and can be indexed to the pure tetragonal matlockite structure (space group P4/nmm). From SEM images as shown in the insets of Figure 1, the average particle


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size of the sample is estimated to be ~200 nm and no noticeable change of the particle morphology is observed upon X-irradiation. The nanocrystalline BaFCl:Tm3+ samples were then measured by fluorescence spectroscopy to confirm that Tm3+ ions were successfully doped into the BaFCl host. As indicated in Figure 2, the three peaks at 450, 650 and 657 nm can be assigned to the Tm3+ f-f transitions 1D2 → 3F4, 1G4 → 3F4 and 1D2 → 3H4, respectively.

Figure 1. XRD pattern of nanocrystalline BaFCl:Tm3+ in comparison with the standard BaFCl powder diffraction file (JCPDS No. 76-1368). The inset shows typical SEM images of the nanocrystalline BaFCl:Tm3+ sample (a) before and (b) after Xirradiation (the markers indicate 1 µm).



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Figure 2. Photoluminescence spectra of nanocrystalline BaFCl:Tm3+ at room temperature : (a) excited by a 365 nm LED (Thorlabs FGUVS and FGL400 filters used), (b) excited by a 470 nm LED (Thorlabs FES550 and FEL600 interference filters used). In order to investigate the generation of Tm2+ ions by X-irradiation, the infrared luminescence spectra of nanocrystalline BaFCl:Tm3+ before and after X-irradiation at room temperature were investigated. As follows from Figure 3a, no emission peaks associated with Tm2+ ions were observed in the spectrum of the sample before Xirradiation. After X-irradiation, a relatively broad peak at ~1140 nm appears and this band can be assigned to the

2

F5/2 → 2F7/2 f-f transition of Tm2+ ions. The

photoluminescence intensity of Tm2+ ions at ~1140 nm increases with increasing Xray dose (shown for the range of 0 to 166 Gy), indicating a large dynamic dose range for the generation of Tm2+ ions by X-irradiation in nanocrystalline BaFCl:Tm3+. Figure 3b shows the excitation spectrum of the X-irradiated sample as measured by monitoring the 2F5/2 → 2F7/2 transition at ~1140 nm. The broad and intense 4f13 → 4f125d1 excitation band is located mostly in the blue-violet range. The stability of the Tm2+ ions in BaFCl:Tm3+ after X-irradiation under different environmental conditions


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was also studied as this is an important parameter for practical applications. To minimize the photobleaching effect from the excitation light in the readout period, a 425 nm LED with a relatively low power density of 0.05 mW·cm-2 was applied to excite the Tm2+ luminescence. As is displayed in Figure 3c, no oxidation of Tm2+ ions is observed within the experimental error when the irradiated sample is kept in the dark for ~100 h although there seems to be an initial slight build-up of signal due to charge equilibration. In comparison, nearly half of the Tm2+ ions got bleached after exposing the irradiated sample to room light (fluorescent lamp with a power density of ~0.70 mW·cm-2 for a couple of days or to natural sunlight for several minutes. Figure 3d shows the photoluminescence decay of Tm2+ in the 2F5/2 → 2F7/2 transition at ~1140 nm excited by a modulated 425 nm LED. The decay is well described by single exponential function and the lifetime was determined to be 0.98 ± 0.05 ms which is well within the reported range of 500 µs to 5 ms for other host materials.28, 31



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Figure 3. (a) Room temperature photoluminescence spectra of X-irradiated nanocrystalline BaFCl:Tm3+ with 40 kV X-ray doses of i) 0, j) 12, k) 45, l) 84, m) 125, and n) 166 Gy. The luminescence was excited by a 462 nm laser diode with a power density of 0.42 W·cm-2. (b) Excitation spectrum (λem = ~1040 nm) of an X-irradiated (~160 Gy) sample. The inset shows the measurement up to 800 nm with an extra 495 nm long-pass filter. (c) Stability of Tm2+ ions in X-irradiated (~85 Gy) samples under different environmental conditions. Two datasets are shown for the sample in the dark (red and blue markers). For fluorescent lamp light conditions, the sample was placed ~80 cm under fluorescent tubes with a power density of ~0.70 mW·cm-2. The luminescence was excited by ~0.05 mW·cm-2 of 425 nm LED light. (d) Excited state lifetime measurement of Tm2+ in an X-irradiated (~160 Gy) sample at ~1140 nm.


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The temperature dependence of the photoluminescence spectra of Tm2+ ions generated by X-irradiation in nanocrystalline BaFCl:Tm3+ is shown in Figure 4 between 39 K and 293 K. The spectra are shown on the same y-scale to allow a comparison of intensities at different temperatures. As seen in this figure, the relatively broad emission band at ~1140 nm gradually resolves into two narrower peaks with higher luminescence intensities as the temperature drops to 39 K. This is because the homogenous linewidth at room temperature obscures the two transitions. Several vibronic sidebands of the Tm2+ 2F5/2 → 2F7/2 transition can also be observed at low temperature. Similar observations have been reported previously in the literature; notably, the Tm2+ 2F5/2 → 2F7/2 transition displays relatively strong vibronic coupling in comparison with other f-f transition in rare earth ions.28, 31 It is noted here that emission from the 4f125d states of Tm2+ has been observed in several hosts such as CsCaX3 (X=Cl, Br, I)28 and MCl2 (M=Ca, Sr, Ba)31, 32. In the present case, however, no emission based on Tm2+ d-f transitions could be found at either room or low temperature. Similar results have also been reported by Kate et al recently.29 In this latter work it was concluded that efficient non-radiative inter-configuration relaxation from the lowest 5d state to 2F5/2 state occurs.



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Figure 4. Photoluminescence spectra of the 2F5/2 → 2F7/2 transition of Tm2+ in the Xirradiated nanocrystalline BaFCl:Tm3+ at four temperatures. Vibronic sidebands of the Tm2+ 2F5/2 → 2F7/2 transition are labelled as Vb. The sample was X-irradiated by 300 Gy Cu Kα radiation (40 kV, 15 mA). The luminescence was excited by the light of a 462 nm laser diode with a power density of 16 mW·cm-2.

Figure 5 illustrates the X-ray induced reduction of Tm3+ to Tm2+ ions in nanocrystalline BaFCl:Tm3+ by displaying the integrated luminescence intensities of Tm2+ at ~1140 nm (2F5/2 → 2F7/2) and Tm3+ at 450 nm (1D2 → 3F4) as a function of cumulative X-ray dose. The increase of the Tm2+ luminescence intensity is approximately linear for doses below 5 Gy. However, at higher X-ray doses the increase of Tm2 + and the decrease of Tm3+ can be described by double-exponential functions. With ~30 Gy of 40 kV X-ray exposure more than half of the Tm3+ ions are reduced in the nanocrystalline sample. Both the photoluminescence intensities of Tm2+ and Tm3+ ions remain at near-constant values for doses >250 Gy i.e. saturation 12 ACS Paragon Plus Environment

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is reached at this dose level. From the data in Figure 5 we conclude that no more than 70% of the Tm3+ ions can be reduced to Tm2+ by X-irradiation. This limit implies that another impurity must be involved in the stabilisation of the Tm2+ ion; otherwise all the Tm3+ ions could be reduced by X-irradiation. Also, the dose dependences of the Tm2+ and Tm3+ luminescence intensities are not exactly the same, indicating that some of the Tm3+ ions may not be luminescent before the X-ray induced reduction.

Figure 5. Dependence of the integrated luminescence intensities of (a) Tm2+ ions at ~1140 nm (2F5/2 → 2F7/2) and (b) Tm3+ ions at 450 nm (1D2 → 3F4) in nanocrystalline BaFCl:Tm3+ as a function of cumulative X-ray dose. The luminescence was excited with a 462 nm laser diode (16 mW·cm-2) and a 365 nm LED (~5 mW·cm-2), respectively. The solid lines are fits by double exponential functions.

The photobleaching (photoionization) effect of Tm2+ ions generated by Xirradiation in nanocrystalline BaFCl:Tm3+ samples was studied by applying the light of a 462 nm laser diode as the excitation source with power densities of 0.004, 0.10, 0.25, 0.50, 1.00, 2.50 and 5.00 W·cm-2. The photoluminescence spectra of Tm2+ at ~1140 nm (2F5/2 → 2F7/2) were repetitively measured by the spectrograph. The luminescence intensity of the ~1140 nm line was then integrated and normalized. 13 ACS Paragon Plus Environment


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Figure 6. (a) 3D plot of the photoluminescence intensity as a function of time and wavelength for an 80 Gy X-irradiated sample. The luminescence was excited by 2.50 W·cm-2 focused 462 nm laser diode. (b) Dependence of the normalized and integrated photoluminescence intensity of the ~1140 nm (2F5/2 → 2F7/2) Tm2+ luminescence line as a function of photobleaching time. For each bleaching curve 7200 full photoluminescence spectra were measured and integrated. The luminescence was excited by the light of a focused 462 nm laser diode with power densities of h) 0.004, i) 0.10, j) 0.25, k) 0.50, l) 1.00, m) 2.50, and n) 5.00 W·cm-2. All samples were initially exposed to 80 Gy Cu Kα X-ray (40 kV, 15 mA). The black solid lines are the 14 ACS Paragon Plus Environment

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results of a global fit by using eq (2). (c) Dependence of the parameter k0 on the laser power density. The solid line shows a fit to a power law k0 = A × (power)x.

As is indicated in the context of the X-ray generation of Tm2+ above, the data in Figure 5 and Figure 6 imply that the ease at which Tm3+ ions get reduced upon Xirradiation or Tm2+ ions get photoionized is subject to a distribution that must involve other impurities. It is well established that BaFX materials are ubiquitously contaminated with oxygen impurities.33-35 This is corroborated by the fact that Tm2+ can be generated in nanocrystalline BaFCl:Tm3+ also by exposure to UV radiation at ~200 nm (not illustrated here) where oxide impurities render strong absorbance.36 This supports the idea that such impurities, e.g. O2- centers, act as the hole trap upon X-irradiation. Thus following mechanism appears to be plausible: upon exposure to ionising radiation electron-hole pairs are generated in the BaFCl host material, the holes are then trapped by oxide ions.36 The electrons may be initially trapped directly by the Tm3+ ions or by anion vacancies, νa+. The latter will form F-centres that may diffuse through the lattice, as time progresses, and eventually the electron may be transferred from these centres to Tm3+ as well. Given that oxygen impurities are the most likely hole traps, it is plausible that the conversion efficiency Tm3+ → Tm2+ is strongly correlated with the separation between the Tm3+ ion and the hole trap, leading to a dispersion of the reduction kinetics. Likewise the photobleaching (photoionization) as summarized in Figure 6 also shows strong dispersion and this can again be understood in terms of the separation between the hole trap and the Tm2+ centre (electron trap). As we have recently discussed for the photobleaching of UV generated Sm2+ centres in the same host material36, we assume that the separation between the oxide



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impurity and thulium ions can be described by a standard gamma distribution and the electron transfer rate is exponentially dependent on the separation between the Tm2+ centre and the oxygen-based hole trap. The effective electron transfer rate k in the specific excited state in which the electron transfer occurs is then given by

k = k0 exp(−R / a f )

(1)

and the concentration of the Tm2+ centres decreases according to eq (2) upon photoionization, (R − Rm )γ −1 exp [−(R − Rm )] × exp [−k0 exp(−R)t ]dR ∫ Γ(γ ) Rm ∞

N (t) =

(2)

where we use an excluded volume with radius Rm as the oxide impurity cannot be on nearest-neighbour F and Cl sites; the latter situation would stabilize the trivalent ions. In eq (2), k0 determines the timescale of the bleaching curve and γ determines its deviation from single exponential behavior, i.e., it measures its dispersion. It is noted here that k0 is an effective rate constant that is a measure of the photoionization rate for a particular excitation power and its power dependence indicates whether the photoionization is a one or multiphoton process. The data in panel (b) of Figure 6 was used in a global fit by eq (2) where only the rate constant k0 was allowed to be independent. A near-perfect fit is obtained with the global parameters af = 0.45 Å, γ = 2.1 ± 0.5, a residual Tm2+ concentration of 2.5% (of the initially generated number of centres) and a radius for the excluded volume of Rm = 5 Å. The γ-parameter implies that the separation between the Tm2+ centres and the electron acceptors is on average ~6-7 Å i.e. a few interionic spacings. It is noted here that the absolute values of k0 and γ should not be overinterpreted as they are not fully independent. However, the relative trend of k0 as a function of laser power density clearly shows that the photobleaching process is based on a two-photon ionization mechanism, as follows 16 ACS Paragon Plus Environment

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from the non-linear behaviour that is illustrated in Figure 6c and the data is well described by a power law with an exponent of 1.46 ± 0.03.

Figure 7. (a) Photobleaching data for different initial X-ray doses of 10, 20, 40 and 80 Gy. The luminescence was excited by the light of a focused 462 nm laser diode with a power density of 2.50 W·cm-2. Experimental and fitted data are identified as solid and dotted lines, respectively. (b) Dependence of the parameter γ on the X-ray dose.

The dose dependence of the photobleaching is illustrated in Figure 7a. It shows that there are slight variations for different doses and the reduction is somewhat slower for higher doses. This is again due to the dispersive nature of the mechanism where higher dose of X-irradiation creates Tm2+ and hole trap pairs with larger separations. Again a global fit was conducted with the parameters af = 0.45 Å, k0 = 43961 s-1, a residual, relative to the initially created, Tm2+ concentration of 2.5% and a radius for the excluded volume of Rm = 5 Å. The variation of γ with dose is in line with this interpretation as it increases slightly with increasing dose (see Figure 7b).



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Conclusions Divalent thulium has been successfully generated in nanocrystalline BaFCl:Tm3+ by X-irradiation. The photoluminescence properties of nanocrystalline BaFCl:Tm3+ before and after X-irradiation were investigated. Typical Tm2+ f-f luminescence (~1140 nm) has been observed at both room and low temperature, but no d-f emission within the visible range could be detected. The lifetime of the excited Tm2+ in Xirradiated samples is 0.98 ± 0.05 ms that is comparable to values reported in the literature for similar hosts. The generated Tm2+ ions in nanocrystalline BaFCl are shown to be stable in the dark or under dimmed room light but bleach relatively rapidly under fluorescent tube lighting and in particular under sunlight. To study the X-ray storage mechanism, photoluminescence intensities of both Tm3+ and Tm2+ were monitored to investigate the valence state witching between Tm3+ and Tm2+ under Xirradiation or photoionization. Both the reduction upon X-irradiation and the photobleaching display dispersive first order kinetics that can be rationalised on the basis of a distribution of the separation between Tm and hole trap. It appears that a standard Γ-distribution adequately describes the dispersion in the photobleaching and the average separation between Tm and hole trap is ~6-7 Å. The present work supports ideas about the storage mechanism of RE doped alkaline earth fluorohalides X-ray phosphors as deduced previously for the BaFCl:Sm3+ system.23 In particular, it appears that oxygen-impurities play a major role in the mechanism and kinetics of the photoreduction and photoionization. The successful doping of Tm3+ ions into BaFCl host points to the possibility of creating X-ray phosphors that are based on the change of up-conversion intensity as an indicator of X-irradiation. Moreover, the controlled production of Tm2+ based luminescent materials could also lead to materials for luminescent solar concentrators.


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Acknowledgment We thank The University of New South Wales (UNSW) for supporting this work. J.Z. acknowledges UNSW for a University International Postgraduate Award. We are grateful to Kate Badek for her kind help with the ICP analysis. The Mark Wainwright Analytical Centre at UNSW is acknowledged for providing the SEM images.

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TOC Graphic


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Controlled Generation of Tm2+ Ions in Nanocrystalline BaFCl:Tm3+ by

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