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Spectroscopic Investigation of Up-Conversion Properties in Green Emitting BaMgF4:Yb3+,Tb3+ Phosphor Bhushan P. Kore,*,† Ashwini Kumar,*,† Anurag Pandey,† Robin E. Kroon,† Jacobus J. Terblans,† Sanjay J. Dhoble,‡ and Hendrik C. Swart† †

Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa Department of Physics, RTM Nagpur University, Nagpur 440033, India

‡

S Supporting Information *

ABSTRACT: In this work we have comprehensively studied the up-conversion (UC) properties of BaMgF4:Yb3+,Tb3+ phosphor for the first time. BaMgF4:Yb3+,Tb3+ phosphors were prepared by a simple and low cost precipitation method. To determine the influence of dopant concentration on luminescence properties, the corresponding UC luminescence spectra of BaMgF4:Yb3+,Tb3+ phosphors were studied under NIR excitation. Emission spectra under NIR excitation reveal the vital role of Tb3+ concentration in spectral tuning from the blue to green region. The UC decay curves were also studied to explore the possible energy transfer (ET) mechanisms between Yb3+ and Tb3+. The results reported here are expected to provide an approach for better understanding ET mechanisms in many Yb3+/Tb3+ codoped UC phosphors. This study will be helpful in applications where precisely defined optical transitions is an essential criterion.

1. INTRODUCTION Up-conversion (UC) is an antistokes emission, where the absorption of two or more, longer wavelength photons results in the emission of a single photon having a wavelength shorter than the absorbed photons.1−3 The rare earth (RE) doped UC materials show exciting optical features which are poised to be applied in a variety of applications such as displays,4 lasers,5,6 optical amplifiers,7 solar cells,8 lighting,9 biological labeling,7,10,11 imaging,7,10,12 etc. Most of the RE ions possess unique light emission at nearly fixed wavelengths owing to the 4f−4f transitions which are little affected by the crystal field as the electrons in the 4f orbitals are strongly shielded by electrons from the 5s and 5p orbitals. In most frequently studied examples Yb3+ transfers its energy to Er3+ resulting in UC emission. The two or more Yb3+ (sensitizer) ions excite the single Er3+ (activator) ion which after de-excitation emits light in the green and/or red region.13−15 Sometimes, two identical lanthanide ions can interact in such a way that the interaction results in UC emission.16 Over the last few decades extensive research has been done in the lanthanide doped UC phosphors, mostly on the fluoride based phosphors.1,14,15,17−19 The fluoride-based materials are © 2017 American Chemical Society

found to be more appropriate phosphors in UC due to their low phonon energy. The multicolor UC in many fluoride based host has been studied by various groups for their applications in display technology,4,9 laser,5,6 biomedicine,7,10−12 etc. There are several reports published on the UC properties of NaYF4 and other fluoride based phosphors, with different combinations of RE ions. The very well-known and best-studied examples of fluoride based systems are NaYF4:Yb3+,Er3+;13−15 NaYF4:Yb3+,Tm3+;13−15 NaYbF4:Tm3+;20 NaGdF4:Yb3+,Er3+;21 LiYF4:Yb3+,Tm3+;22 LiYbF4:Eu3+; and Tb3+.23 The vastly studied lanthanide pairs in UC are Er3+, Ho3+, and Tm3+ with Yb3+ as a sensitizer,14,15,20−24 whereas, very few UC studies have been carried out for Yb3+/Tb3+ and Yb3+/Eu3+ pairs.25−28 The reason behind these two categories of lanthanides is the way they transfer their energies. Er3+, Ho3+, and Tm3+ codoped with Yb3+ can be excited by ground state absorption (GSA), excited-state absorption (ESA), and energy transfer up-conversion (ETU) mechanisms whereas the energy transfer from Yb3+ to Tb3+/Eu3+ is due to co-operative energy Received: January 8, 2017 Published: April 13, 2017 4996

DOI: 10.1021/acs.inorgchem.7b00044 Inorg. Chem. 2017, 56, 4996−5005

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Inorganic Chemistry transfer (CET).16 Since, Tb3+ and Eu3+ do not have energy levels, which can absorb the NIR light directly, Yb3+ is the best choice in order to obtain the UC luminescence in Tb3+/ Eu3+ doped system. Yb3+ has a single electronic transition within the 4f subshell.1 The transition from the lower level through ground state absorption can be easily achieved by using NIR radiation, and resulting energy from two or more excited Yb3+ ions can be utilized in the excitation of a single Tb3+ or Eu3+ ion, for gaining improved UC emission. Furthermore, the Yb3+−Tb3+ pair is widely studied in glasses,29,30 whereas very few reports are published which show efficient green UC emission in polycrystalline phosphors. Recently, we have reported the efficient down-conversion emission in BaMgF4:Ce3+, Tb3+ phosphor.31 The obtained results showed efficient green emission from Tb3+ when codoped with different concentration of Ce3+. Therefore, we planned further to study the UC properties of this material. In this report, we comprehensively studied UC properties of BaMgF4:Yb3+,Tb3+ phosphor with different possible energy transfer mechanisms to elucidate the cross-relaxation in Tb3+, co-operative energy transfer from Yb3+ to Tb3+ and Tb3+ emission quenching due to phonon assisted energy transfer from Tb3+ to Yb3+. Color tunability of the prepared phosphor, as a function of dopant concentration and pump power have also been analyzed with the help of Commission Internationale de L′Eclairage (CIE) chromaticity.

change in the color of emitted light with respect to doping concentration and laser power.

4. RESULTS AND DISCUSSION 4.1. XRD Study. Figure 1 shows the XRD pattern of BaMgF4:Yb,Tb (Tb = 2 mol %, Yb = 2, 5, and 10 mol %). The

Figure 1. (a) XRD patterns of annealed BaMgF4:Yb3+,Tb3+ (Tb = 2 mol %, Yb = 2, 5, and 10 mol %) phosphor, (b) the magnified view of diffraction peak at 19.58 2θ angle.

2. EXPERIMENTAL SECTION

XRD pattern of the as prepared samples matched well with the XRD pattern reported in ICDD file no. 87-0201. After doping the sample with different concentrations of Yb3+ a shift in the diffraction peaks toward higher 2θ values was observed, as shown in Figure 1b. This suggests that Yb3+ ions are substituting Ba2+ ions in the BaMgF4 matrix. The substitution of Ba2+ (0.135 nm, 6-fold coordinated)32 ions by smaller Yb3+ ions (0.086 nm, 6-fold coordinated)32 causes a shift in the diffraction peaks toward a higher 2θ angle. The shift in diffraction peaks toward higher 2θ angle suggest shrinkage in the unit cell, which is in accordance with the results reported in our previous studies.31 4.2. SEM and EDS Analysis. Figure 2 shows the SEM images of the as-prepared undoped and doped BaMgF4 compounds. It is clearly seen that the morphology of the phosphor changes with incorporation of activators and annealing. The particles in the pure BaMgF4 sample are observed to be rod shape with average rod length in the range of 1 to 1.5 μm. The unannealed samples doped with Yb3+ and Tb3+ showed a flower like structure; these flowers actually consist of a number of two-dimensional sheets. The thickness of the sheets is approximately below 100 nm, whereas the length varies from 4−6 μm. After annealing the sample, these flower like structures were agglomerated to form spherical particles with an average diameter in the range 2−3 μm, as shown in Figure S1. The EDS data shows the elemental distribution showing the presence of F, Ba, and Mg elements in expected stoichiometric proportion in the BaMgF4 host. The elemental mapping further illustrates that all the elements are uniformly distributed, as shown in Figure S2. In some EDS spectra traces of carbon and chlorine were also observed. The traces of carbon are from the sample grid and Cl traces are due to chlorinated precursors. The presence of Cl in some samples suggests that Cl was not removed completely even after washing the samples with distilled water, a number of times.

The experimental procedure of preparation of BaMgF4:Ce3+,Tb3+ is given elsewhere.31 A similar procedure was adopted for the synthesis of BaMgF4:Yb3+,Tb3+ phosphors, but here Yb3+ was used as codopant instead of Ce3+. The starting materials used were BaCl2, MgCl2, NH4F, LiCl, ZnCl2, Yb(NO3)3, and Tb(NO3)3. All the reagents used were of high purity procured from Sigma-Aldrich (99.999%). Stoichiometric amounts of BaCl2 and MgCl2 were dissolved in double distilled water, in a separate beaker. The transparent solutions were then mixed in a container under vigorous magnetic stirring at room temperature for 10−20 min. In the next step stoichiometric amounts of Yb(NO3)3, and Tb(NO3)3, dissolved in distilled water were added dropwise to the solution mixture with continuous magnetic stirring at room temperature for 10 min. NH4F (the precipitating agent) was then added to the mixture under vigorous stirring. NH4F immediately reacted with the solution, and a slurry-like white precipitate was formed. The precipitated mixture was stirred again for 2 h at 80 °C. Finally, the precipitate was washed several times with distilled water and dried overnight in an oven at 60 °C. After cooling to room temperature, the obtained powder was annealed at 800 °C for 4 h in open atmosphere, which resulted in the final product.

3. CHARACTERIZATION TECHNIQUES X-ray diffraction (XRD) patterns of synthesized materials were recorded in a broad range of Bragg angles (10° ≤ 2θ ≤ 80°) using a Bruker D8 advance X-ray diffraction measuring instrument with Cu Kα target radiation (λ = 0.154 nm). The morphology of the prepared phosphors was observed by JSM7800F JEOL field emission scanning electron microscope (FESEM), operated at 30 kV. Energy dispersive X-ray spectroscopy (EDS) was used for composition analysis using an Oxford instrument (X-MaxN 80). UC emission and decay curves were measured upon excitation with a 980 nm laser using an Edinburgh FLS-980 fluorescence spectrometer. The color coordinates were calculated by Commission Internationale de L′Eclairage (CIE) software (GoCIE) to demonstrate the 4997


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Inorganic Chemistry

Figure 2. SEM images, elemental distribution and EDS spectra of (1) BaMgF4 annealed host sample showing uniformity of the particles, (2) BaMgF4:Yb3+,Tb3+ unannealed sample, and (3) BaMgF4:Yb3+,Tb3+ annealed sample.

We further found that the presence of Cl in final product does not affect the UC emission of BaMgF4:Yb,Tb phosphor. 4.3. Luminescence Properties. For studying the UC properties of this material, the BaMgF4 host was doped with different concentration of Yb3+ and Tb3+. Figure 3 shows the UC emission spectra of BaMgF4:Yb3+,Tb3+ annealed phosphors, under 980 nm laser excitation. The unannealed samples of BaMgF4:Yb3+,Tb3+ were first checked for UC, but the

samples showed very poor UC. From the XRD pattern of the unannealed samples we noticed that before annealing the material, the BaMgF4 phase was not formed as shown in Figure S3. Therefore, annealing is very essential in order to get a pure phase compound and good UC properties. In the first experiment, the concentration of Tb3+ (2 mol %) was fixed and concentration of Yb3+ varied from 1−10 mol %. Figure 3a shows the UC emission in sample when excited with 980 nm 4998


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Inorganic Chemistry

Figure 3. (a) PL emission spectra of (a) BaMgF4:xYb,Tb (Tb = 2 mol % fixed) phosphor, (b) BaMgF4:Yb,xTb (Yb = 3 mol % fixed) phosphor under 980 nm excitation wavelength, at 0.48 W fixed laser power. CIE chromaticity diagram for visualization of color tunability in (c) BaMgF4:xYb,Tb (Tb = 2 mol % fixed) phosphor on varying Yb3+ concentration and (d) BaMgF4:Yb,xTb (Yb = 3 mol % fixed) phosphor on varying Tb3+ concentration.

chromaticity diagram shown in Figure 3c,d illustrates the variation in color coordinates with change in concentration of the Yb3+ and Tb3+ ions, at a fixed laser power. The color coordinates corresponding to different concentration of Yb3+ and Tb3+ are given in Table S1 and S2, respectively. For lower concentration of Tb, the CIE coordinates were in the blue region whereas for higher concentration they shifted to the green region, as shown in Figure 3d. The decrease in blue luminescence from the 5D3 level, with corresponding change in color with increase in concentration of Tb3+, is due to the crossrelaxation among neighboring Tb3+ ions,35,36 and this part is elaborated more in the next section. 4.4. Energy Transfer Processes Involved in BaMgF4:Yb3+,Tb3+ Phosphor. 4.4.1. Cross-Relaxation (CR). The CR process in Tb3+ plays a very important role for getting green emission.34,36 At lower doping concentration of Tb3+ ions, the interaction between neighboring Tb3+ ions is very little, but after doping sufficiently higher concentrations the interaction between neighboring Tb3+ ions increases.34 As a result of this interaction the neighboring Tb3+ ions exchange their energies among themselves. The energy level scheme of Tb3+ consists of a number of low energy states 7FJ (J = 6, 5, 4, 3, 2, 1, and 0) and 5D2, 5D3, and 5D4 excited states. At lower concentration of Tb3+ (in the present case, 0.1 mol %), the transitions from 5D3 to 7FJ are significant, which contribute to the blue color of the emission. When the concentration of Tb3+ is sufficiently higher, the neighboring Tb3+ ions start interacting with each other. Cross-relaxation may then occur (i.e., a transition from 5D3 to 5D4 in one ion together with a transition from 7F6 to 7F0 an adjacent ion) which results in predominantly green emission due to 5D4 to 7FJ transitions,37 as shown in Figure 4a. It is difficult to get UV or blue emission from Tb3+ by CET from two Yb3+ ions, because two photons will not be

laser. The emission spectra shows characteristic emission of Tb3+. The UC spectra are composed of blue, green and red emission bands, which are assigned to 5D3 → 7F6 (379 nm), 5 D3 → 7F5 (414 nm), 5D3 → 7F4 (437 nm), 5D4 → 7F6 (489 nm), 5D4 → 7F5 (544 nm), 5D4 → 7F4 (583 nm), and 5D4 → 7 F3 (621 nm) transitions of Tb3+,33,34 respectively. The maximum emission intensity was observed for 3 mol % concentration of Yb3+ doping. In the second experiment, the concentration of Yb3+ (3 mol %) was kept constant and the concentration of Tb3+ was varied from 0.1−3 mol %. Figure 3b shows the UC emission spectra of annealed BaMgF4:Yb3+,Tb3+ phosphor. At lower concentration of Tb3+ (0.1 mol %, 0.2 mol %) the blue emission at 489 nm which corresponds to 5D4→ 7F6 transition of Tb3+ was found to be dominant, together with small but significant emissions at shorter wavelength originating from the 5D3 level. With further increase in Tb3+ concentration, the green emission from Tb3+ was dominant. The maximum UC emission was optimized for 2 mol % concentration of Tb3+. At this concentration, we have recorded the power dependent UC emission. After careful investigation of obtained UC emission, we observed that for a lower concentration of Tb3+ the emission peak was shifted slightly toward higher wavelength. This shifted peak is actually due to a co-operative upconversion luminescence of Yb3+-Yb3+ pairs. We believe that there is an emission from Yb3+ in this region (especially for lower concentration of Tb3+), together with the Tb3+ emission and this complicates the spectra. The blue emission from the Yb3+ can be achieved by simultaneous de-excitation of Yb3+ -Yb3+ ion pair. Using the 980 nm laser pump the Yb3+ ions can be excited from the 2F7/2 level to the 2 F5/2 level, the excited Yb3+−Yb3+ ion pair can emit in the blue region. It seems that this peak is only detectable for lower concentration of Tb where the Yb3+ is more dominating. The 4999


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in this Eu3+ and Tb3+ have their first excited state present at much higher energy than the ground state of Yb3+.40 In CET the combined energy from a pair of Yb3+ ions excited by NIR radiation is utilized by single Tb3+ ion to make transition to its excited state, as shown in Figure 4b. 4.4.3. Phonon Assisted Energy Transfer (PAET). Another possible energy transfer mechanism is the phonon assisted or accretive energy transfer mechanism, in which the Yb3+ quenches the Tb3+ emission. The PAET takes place when there is an energy mismatch between neighboring Tb3+ and Yb3+ RE ions, and this energy gap is further compensated by emission/absorption of one or more phonons through multiphonon relaxation,25,41−43 as shown in Figure 4c. The energy difference between the 2F5/2 to 2F7/2 transitions of Yb3+ and 5D4 to 7F0 transitions of Tb3+ is compensated by emission of one or more phonons. The transition from 5D4 to some intermediate level is radiative and the energy from Tb3+ is utilized in the excitation of Yb3+ together with emission of one or more phonons having energy from the intermediate level to 7 F0 level.29 The excess unutilized energy from the intermediate level of Tb3+ to the 7F0 level is dissipated in the lattice which gives rise to the formation of multiple phonons.29,41,44 4.5. UC Decay Analysis. In order to get deeper insight into the energy transfer processes that took place inside the BaMgF4:Yb3+,Tb3+ phosphor after 980 nm excitation, fluorescence decay measurements have been carried out. Figure 5 shows the time-resolved luminescence of the Tb3+ emission at 379 nm (5D3 → 7F6) and 544 nm (5D4 → 7F5), for different concentration of Yb3+ and Tb3+. The calculated values of lifetime illustrate a dependence of lifetime on the dopant concentrations. As presented in Table 1, the decay curve of the 5 D4 → 7F5 transition in BaMgF4:Yb;Tb with varying Tb3+ concentration shows exponential decay with a lifetime in the

Figure 4. Scheme for energy transfer between Yb3+ → Tb3+, Tb3+ → Tb3+, and Tb3+ → Yb3+ through (a) cross-relaxation, (b) co-operative energy transfer, and (c) phonon assisted energy transfer mechanisms.

able to populate the 5D3 level. The possible explanation could be that some Tb3+ ions which are already in the excited state (5D4) are further get excited and make transition to the higher energy state 5D1 by accepting the energy from Yb3+ ion.38 Tb3+ ions in the excited-state 5D1 relax to state 5D3 by nonradiative phonon assisted transitions. Now this Tb3+ ion in the 5D3 excited state can either relax radiatively (5D3 to 7FJ transitions) or relax nonradiatively to the 5D4 state via the cross-relaxation process already described, thereby also exciting an adjacent Tb3+ ion from the 7F6 to 7F0 state.36 As a result of this the blue emission which is dominant at lower concentration gets suppressed for higher concentrations, and the green emission dominates after CR energy transfer between neighboring Tb3+ ions. 4.4.2. Co-Operative Energy Transfer (CET). Usually the CET can be observed in which a large energy difference is present between the ground state of the donor ion and first excited state of the acceptor ion.16,39 The most commonly studied examples are Yb3+/Eu3+ and Yb3+/Tb3+ doped systems,

Figure 5. Luminescence decays of BaMgF4:Yb3+,Tb3+ phosphor under 980 nm excitation for Tb3+ emission at 379 nm (5D3 → 7F6) and 544 nm (5D4 → 7F5), (a, b) for different concentrations of Yb3+ at Tb3+ = 2 mol % fixed and (c, d) for different concentration of Tb with Yb = 3 mol % fixed. The decay curves for 5D3 and 5D4 level are fitted with single and double exponential equations, respectively. 5000


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4.6. Power Variation. To give an additional evidence for the above CET and PAET process, we measured the UC emission with laser power variation. The number of photons contributing in UC process can be estimated by plotting the UC luminescence intensity versus laser power at fixed excitation wavelength. The UC luminescence intensity (IUC) and incident pump power (IP) are related by the following equation,47

Table 1. Luminescence Decay Time Calculated for BaMgF4:Yb,xTb (Yb = 3 mol %) Phosphor BaMgF4:Yb,xTb

τ (ms) 544 nm

τ (ms) 379 nm

0.1 mol % 0.2 mol % 0.5 mol % 0.7 mol % 1 mol % 2 mol % 3 mol %

2.43 2.37 2.40 2.53 3.45 3.63 3.89

1.04 1.28 1.10 0.86 0.87 0.71 0.82

IUC = α(IP)n where α is a proportionality constant and n is a number indicating total number of photons taking part in UC process. The variation of the UC luminescence intensity with pumping laser power (at 980 nm) was fully examined to analyze the nature of the UC process. Figure 6a shows the linear trend between the UC emission intensity and laser power, Figure 6b,c reveals that points plotted on a log scale can be fitted with a linear function, the peaks arising from 5D3 level have slope around 1.44, 1.42, and 1.32 for 379, 414, and 435 nm peaks, respectively. The magnified view of transitions arising from 5D3 level is shown in Figure S4. On the other hand, the power dependence of 489, 544, 583, and 621 nm emission peaks arising from 5D4 level has slope of 1.25, 1.29, 1.33, and 1.34, respectively. However, the experimentally determined slopes are most of the times noninteger and retain smaller value due to cross-relaxation and nonradiative losses.48 Therefore, the obtained results indicates that two photons are involved in the energy transfer from the Yb3+ ions to Tb3+ ions {[2Yb3+](2F5/2 → 2F7/2) → [Tb3+](7F6 → 5D4)]}, this suggests the energy transfer is due to CET mechanism. On the other hand, the lower values of slope around 1 indicates there is also a phonon assisted energy transfer from Tb3+ to Yb3+.49

range of 2−4 ms, which is consistent with the radiative decay time previously reported in the literature.40,45,46 The decay time values for the 5D3 level initially increased with the increasing amount of Tb3+ ions then lowered with higher doping. Earlier we observed that the sample doped with 0.2 to 0.7 mol % or higher percentage of Tb3+ showed cross-relaxation and quenched the emission arising from 5D3 level with increase in Tb3+ concentration. The decay curves measured at 379 nm also show decrease in lifetime with increasing the Tb3+ concentration. This shorter decay time value might be due to the cross relaxation process, as observed in the UC photoluminescence spectra with varying Tb3+concentration. On the other hand an increasing concentration of Yb3+ ions in BaMgF4:xYb3+,Tb3+ did not change the shape of the decay curves; as a result of this very minute variation in lifetime values of Tb3+ emission was observed. In BaMgF4:Yb3+,Tb3+ phosphor the Tb3+ emission initially increased and then decreased after 3 mol % doping of Yb3+, this shows that energy transfer from Tb3+ to Yb3+ which is associated with phonons as large part of energy from Tb3+ is dissipated in the lattice.

Figure 6. (a) PL emission spectra of BaMgF4:Yb, Tb (Yb = 3 mol % fixed and Tb = 2 mol % fixed) phosphor under 980 nm excitation wavelength within a laser power range of 0.1 to 1.5 W. The logarithmic dependence of UC emission intensity as a function of logarithmic of pump power for BaMgF4:Yb, Tb (Yb = 3 mol % fixed and Tb = 2 mol % fixed) phosphor, (b) for 379, 414, and 435 nm emissions originating from the Tb 5D3 level and (c) 489, 544, 583, and 621 nm emissions originating from the Tb 5D4 level. (d) CIE chromaticity diagram of BaMgF4:Yb, Tb (Yb = 3 mol % fixed and Tb = 2 mol % fixed) phosphor, for different power range of 0.1−1.5 W. 5001


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Yb3+ ions is difficult to conclude from the obtained results. Energy transfer between RE pairs with a small energy mismatch is also possible. The energy mismatch can be compensated by emission of several phonons.44 In the plot of ln(IUC) versus ln(IP) we calculated a slope of 1.29, 1.34, 1.33, and 1.25 for 544, 621, 583, and 489 nm, respectively. This suggests energy transfer from 2F5/2 of Yb3+ to 5D4 is likely through a phononassisted process as shown by transition (v). However, transition (iv) could not be as dominant as transition (v) for the same reason as given for transition (iii). This also reduces the probability of multiphonon relaxation shown by transition (vi), but from experimental results it is difficult to predict the role of multiphonon relaxation in energy transfer process. Further, the excitation up to the 5D3 level is not possible by the absorption of only two photons. The possible explanation for this could be some electrons from the 5D4 level, which have longer lifetime than the 5D3 level, make transition to the 5D3 level by absorbing an extra IR photon, as shown by transition (vii). From all of the above discussion we finally conclude that the contribution of energy transfer from co-operative process is more dominant than corresponding to phonon assisted process. To further improve the UC properties of BaMgF4:Yb3+,Tb3+ phosphor we carried out some additional experiments. As in this phosphor, we have doped trivalent RE ion in place of divalent cation, which creates charge imbalance in the system. When single Yb3+ and Tb3+ ions substitutes for two Ba2+ site it will form two positively charged defects, which can distort the lattice and induce more nonradiative process. In order to overcome this charge imbalance we initially doped monovalent cation (Li) in the optimized BaMgF4:Yb3+,Tb3+ phosphor. The Li+ ion can compensate the excess charge on these Yb3+/Tb3+ ions. In principle, two lithium ions can compensate the two excess charges on Yb3+ and Tb3+ ions. However, this did not yield any improvement in the UC emission characteristics, the UC luminescence results of Li doped BaMgF4:Yb3+,Tb3+ phosphor are shown in Figure S5. Even after substituting the different amount of Li+ ions we do not see any increase in UC emission rather we observe a continuous decrease in UC emission with increase in Li+ ion concentration. We also tried the substitution of additional divalent cation (Zn). The single Zn2+ ion can compensate the two excess charges on Yb3+ and Tb3+ ions. The Zn substituted BaMgF4:Yb3+,Tb3+ phosphor was also showing a similar trend in UC emission, the UC luminescence results of Zn doped BaMgF4:Yb3+,Tb3+ phosphor are shown in Figure S6. From the obtained results it was finally concluded that in BaMgF4 host when the trivalent RE ions are substituted, the two RE ions are actually balances the charges of 3 divalent cations (Ba2+) in the host. Therefore, when we add monovalent or divalent cations as charge compensators, there was no further improvement in the UC luminescence properties. Hence, the addition of monovalent/divalent cations creates charge imbalance inside the BaMgF 4 :Yb 3+ ,Tb 3+ phosphor due to which there is further reduction in the UC luminescence. Therefore, in this system there is no need to add any extra monovalent or divalent cation in order to neutralize the excess charges. 4.7. Possible Application. The obtained results show the BaMgF4:Yb3+,Tb3+ phosphor as a promising material for upconverting part of the IR spectrum to visible photons. In most of the solar cell applications, the solar energy which falls in the IR spectrum remains unutilized due to the lack of absorption of IR spectrum by solar materials.7,50 About 50− 60% of sunlight is distributed within the NIR region of 800−

Therefore, the UC emission results suggest that the energy transfer from Yb3+ to Tb3+ involves multiple energy transfer process. We also observed that the slope calculated for the peak arising from the 5D3 level is around 1.5, which suggests two photons involved in the UC process, but it is impossible to get emission from the 5D3 level by two 980 nm photons. This suggests that in fact it is a 2-photon co-operative process to excite an electron to the 5D4 level followed by an independent ESA (excited state absorption) process from the long-lived 5D4 to the higher 5D3 level. Hence, it does not require 3 photons simultaneously and has the signature of a 2-photon process only. The color coordinates of BaMgF4:Yb3+,Tb3+ phosphor with variation in laser power are shown in Figure 6d. For lowest laser power the CIE coordinates lie in bluish green region whereas for rest of the other power variation the CIE coordinates have nearly same value with little variation. This also suggest the cross relaxation in Tb3+ can be easily tuned by changing the laser power to get rid of unwanted color emission or to get a pure green color emission. Above 0.9W laser power the CIE coordinates are almost same as given in Table. S3. Therefore, we have excluded these points from the CIE diagram. The above results suggest that initially there is crossrelaxation energy transfer among neighboring Tb3+ ions, which quenches the blue emission from Tb3+ as we go on increasing the Tb3+ concentration. As a results of this the green emission is more dominant for higher concentration of Tb3+ doping, the cross-relaxation energy transfer is shown in Figure 7 by

Figure 7. Energy level model illustrating various types of possible energy transfer mechanism between Yb3+−Tb3+ pair.

transition of type (i). The next is most dominant energy transfer by co-operative process, in this two Yb3+ ions simultaneously transfer their energies to single Tb3+ ion as shown by transition (ii),16,25,26 populating the 5D4 excited level and giving rise to emission from this level, generally dominated by the green 5D4-7F5 emission at 544 nm. The reverse of this that is energy transfer from one Tb3+ ion to two or more Yb3+ ions is not witnessed in the present phosphor, as shown in Figure 7 by transition (iii). For co-operative ET from the 5D4 level of Tb3+, the energy is concurrently transferred to two neighboring Yb3+ ions,16,25 resulting in emission of two IR photons. However, from the UC spectra it is difficult to get information about the CET from Tb3+ to Yb3+, as the range of Yb3+ emission is beyond the reach of the instrument used. Hence, the energy transfer from one Tb3+ ion to two or more 5002


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Inorganic Chemistry 1700 nm, and most of the solar materials are unable to absorb the NIR photons. One such application is highly trending perovskite based photovoltaic cells in which MAPbX3 and FAPbX3 (where, X = I, Br, Cl)51−54 material are used. The perovskite solar materials can be efficiently excited with green light with the help of BaMgF4:Yb3+,Tb3+ phosphors which will convert some part of the IR spectrum to green photons,55−57 as shown in Figure 8. This will not only increase the solar cells efficiency, but also the IR losses inside solar cells can be reduced to some extent.19

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Yb and Tb, and with power variation, and UCPL spectra of samples doped with Li and Zn. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bhushan P. Kore: 0000-0003-3921-6194 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation of South Africa (84415). The financial assistance from the University of the Free State is highly recognized. The Edinburgh Instruments FLS980 system used in this study was funded by the National Research Foundation of South Africa (Grant EQP14080486021).

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Figure 8. Comparison of solar spectrum and the emission spectrum of the BaMgF4:Yb3+,Tb3+ phosphor.

5. CONCLUSIONS To summarize, we have demonstrated that Tb3+ doped BaMgF4 can effectively up-convert the low energy photons to high energy visible photon using Yb3+ as a sensitizer. Investigation of the UC processes and mechanisms indicates that two IR photons each exciting Yb3+ can, through CET, excite an electron in a Tb3+ ion to the 5D4 level from which green luminescence was observed. Furthermore, it is interesting to observe that the emission color changed from blue to green without the transformation of the crystalline phase. The dependence of UC emission on Tb3+ contents show color tunability in this phosphor, whereas the color stability over the wide range of pumping power suggests good color stability. The possible energy transfer mechanisms between Yb3+ and Tb3+ are discussed in detail based on results obtained from fluorescence lifetime and pump power variation measurements. The energy differences between the Tb3+ and Yb3+ levels and UC luminescence versus laser power variation results force us to conclude that the co-operative energy transfer is the dominant mechanism responsible for the observed UC luminescence. We have discussed the possible application of the BaMgF4:Yb3+,Tb3+ phosphor as a spectral converter in photovoltaics. The obtained results also put forward the usefulness of this phosphor in other applications as well.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00044. SEM images, XRD patterns of annealed and unannealed samples, CIE coordinates for different doping of levels of 5003


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