Mn2+ codoped Na(Sr,Ba)PO4 and Ba2Mg(BO3)2 phosphors

Mn2+ codoped Na(Sr,Ba)PO4 and Ba2Mg(BO3)2 phosphors

Journal Pre-proof 2+ 2+ Luminescent properties and energy transfer of Eu /Mn codoped Na(Sr,Ba)PO4 and Ba2Mg(BO3)2 phosphors Kevin Bertschinger, Hyemin...

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Journal Pre-proof 2+ 2+ Luminescent properties and energy transfer of Eu /Mn codoped Na(Sr,Ba)PO4 and Ba2Mg(BO3)2 phosphors Kevin Bertschinger, Hyemin Park, Ha Jun Kim, Yongseon Kim, Won Bin Im, Sungho Choi, Jae Yong Suh PII:

S0022-2313(19)32040-X

DOI:

https://doi.org/10.1016/j.jlumin.2019.116958

Reference:

LUMIN 116958

To appear in:

Journal of Luminescence

Received Date: 17 October 2019 Revised Date:

3 December 2019

Accepted Date: 8 December 2019

Please cite this article as: K. Bertschinger, H. Park, H.J. Kim, Y. Kim, W.B. Im, S. Choi, J.Y. Suh, 2+ 2+ Luminescent properties and energy transfer of Eu /Mn codoped Na(Sr,Ba)PO4 and Ba2Mg(BO3)2 phosphors, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2019.116958. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Luminescent Properties and Energy Transfer of Eu2+/Mn2+ Codoped Na(Sr,Ba)PO4 and Ba2Mg(BO3)2 Phosphors Kevin Bertschinger a,*, Hyemin Park b, Ha Jun Kim c, Yongseon Kim d, Won Bin Im c, Sungho Choi b, Jae Yong Suh a,* a

Department of Physics, Michigan Technology University, Houghton, MI 49931, United States

b

Advanced Battery Materials Research Group, Korea Research Institute of Chemical

Technology (KRICT), Daejeon 34114, Republic of Korea c

Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of

Korea d

Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of

Korea

ABSTRACT: Two series of the phosphors Na(Sr,Ba)PO4 and Ba2Mg(BO3)2 codoped with different concentrations of Eu2+ and Mn2+ were synthesized using a solid-state reaction method. The energy transfer between Eu2+ and Mn2+ in both host crystals was investigated by steady-state and time-resolved photoluminescence measurements. For Na(Sr,Ba)PO4, Eu2+ has an emission centered at 460 nm which overlaps with multiple excitation transition of Mn2+. In contrast, for Ba2Mg(BO3)2, Eu2+ emission is red-shifted to 612 nm, which causes a single spectral overlap between Eu2+ and Mn2+ energy levels. As a result, the energy transfer efficiency is improved between Eu2+ and Mn2+ in Ba2Mg(BO3)2 with a maximum energy transfer efficiency of 91% compared to that in Na(Sr,Ba)PO4 with that of 57%.

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1. INTRODUCTION There is a continuing interest in making cheap, efficient, and spectrally improved white light sources.1 Presently used commercial white light emitting devices (LEDs) are typically based on the blue emission of GaN, green emission of InGaN, and yellow emission of yttrium aluminum garnet (YAG) doped with the rare-earth ion Ce3+.2-5 Doping by transition-metal or rare-earth ions into inorganic compounds is known as most effective way to improve the luminescent properties of white light sources. The color rendering index, for instance, can easily be improved by incorporating a red component.6 The single component phosphors are regarded as an efficient and cost-saving white light source over its predecessors.7 In this regard, there are a great number of reports for single component white light sources codoped with Eu2+ and Mn2+ such as Sr3Y(PO4)3, Mg2Al4SiO18, and Ca3Mg3(PO4)4, to name a few.3, 6-8 One approach to making a single component white light source takes advantage of the energy transfer (ET) mechanism between two different kinds of ions in one inorganic phosphor host. To illustrate, the transition metal Mn2+ shows shifted-emission characteristics from green to red through crystal field interaction of the host material.9 However, Mn2+ generally has a poor absorption because of the spin forbidden transitions in the d orbitals.10 The Eu2+, on the other hand, is known to be an efficient sensitizer for Mn2+, and thus it can greatly improve the total luminescence through energy transfer process between the ions.11-13 Eu2+ can also be a dopant with blue emission from the 4f-5d transition in the ultraviolet (UV) absorption band.6 Therefore, the combination of Eu2+ and Mn2+ in an inorganic host material can create a single component UV pumped white light and tunable LED.3, 13-15 The energy transfer process in codoped phosphors is of technological importance but is also of scientific interest. Eu2+ has an allowed transition resulting in a dipole moment while Mn2+ has a

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forbidden transition indicating a quadrupole moment. The ET rate for dipole-quadrupole interaction is given by

with W as the ET rate, Qa as the area under the absorption band of Mn2+, fd as the oscillator strength of the dipole Eu2+, fq as the quadrupole oscillator strength of Mn2+, τs as the lifetime of sensitizer without the activator, and R as the distance between the sensitizer and the activator.16 The term

∫ [F ( E)F (E) / E s

a

4

]dE

is the overlap integral where Fs(E) is the emission distribution

of the sensitizer Eu2+, Fa(E) is the absorption distribution of Mn2+, and E is the energy in eV. The value of the integral is determined by the spectral overlap between the absorption of the activator and the emission of the sensitizer.17 As the overlap between the emission of the sensitizer Eu2+ and the absorption of the activator Mn2+ increases, so does the energy transfer rate between them. In this work, we studied the effect of the overlap integral on the ET rate by shifting the emission of Eu2+ substantially. For this reason, we have selected Na(Sr, Ba)PO4 for the blueish emission of Eu2+ and Ba2Mg(BO3)2 for the red emission of Eu2+ while the excitation of Mn2+ remains relatively the same. To fully analyze the ET process, a series of Na(Sr,Na)PO4 and Ba2Mg(BO3)2 based phosphors were synthesized with different concentrations of Eu2+ and Mn2+. X-ray diffraction (XRD), photoluminescence (PL), and time-resolved PL (TRPL) were used to characterize each synthesized phosphor. The XRD measurements were used to characterize the phases and crystal quality of the hosting materials. PL and TRPL measurements were used to study the ET process along with concentration quenching. In addition, we performed density functional theory (DFT) calculations to confirm the state stability of the two codoped phosphor crystals. It is revealed in our study that fluorescence quenching sets the upper limits of dopant

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concentrations for achieving an optimized PL enhancement in Ba2Mg(BO3)2. We also emphasize that the change of the spectral overlap of the sensitizer Eu2+ emission with the excitation of the activator Mn2+ increases the ET efficiency between co-doped luminescent ions. 2. EXPERIMENTAL DETAILS 2.1. Materials Synthesis Na(Sr,Ba)PO4 and Ba2Mg(BO3)2 codoped with Eu2+ and Mn2+ were prepared by using a conventional solid-state reaction. For the phosphate compound, Na2CO3 (99%, High Purity Chemicals), SrCO3 (99.9%, High Purity Chemicals), BaCO3 (99%, High Purity Chemicals), and (NH4)2HPO4 (99.99%, Sigma-Aldrich) were used as starting reagents. Similarly, BaCO3 (99.95%, High Purity Chemicals), Mg(CO3)4·Mg(OH)2·5H2O (99%, Aldrich), and H3BO3 (99.99%, High Purity Chemicals) were used for the preparation of alkali earth metal-based borate Ba2Mg(BO3)2. Eu2O3 (99.99%, Rare Earth Co.), and MnCO3 (99.9%, High Purity Chemicals) were used as the activators for both phosphor compositions. The given materials were weighed and thoroughly ground with acetone. The mixture powders were then calcined at 1150 ºC for 10 h in a reducing atmosphere of H2 (20%)/N2 (80%) mixture gas for phosphate, while the mixture was pre-heated at 600 ºC for 30 min in air followed by calcined at 950 ºC for 6 h in a reducing atmosphere of H2 (5%)/N2 (95%). More detailed synthetic conditions were presented in our previous work.18,19 2.2. Characterization Methods To study the luminescence properties, the PL measurements were performed using 405 nm pulsed laser of 10 MHz repetition rate to excite the Na(Sr, Ba)PO4 and Ba2Mg(BO3)2 based phosphors. The excitation laser pulses were incident on sample reflected off a dichroic mirror and through an objective lens (40×), and the emission was sent to a spectrometer (Ocean Optics,

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USB 2000) in a reflection geometry. For the TRPL measurements, two separate methods were implemented to measure fluorescence and phosphorescence, each of which has different time scales. For the fluorescence measurements, we used a home-built time-correlated single photon counting (TCSPC) measurement setup. The samples were excited using a pulsed laser (Aurea Technology, PIXEA-405) of 50 ps at a wavelength of 405 nm, and the emission was detected with single photon avalanche diode (SPAD, Excelitas Technologies, SPCM-AQRH-14). A dichroic mirror at 405 nm was used to redirect the laser light toward the sample while allowing the emission to pass through toward the SPAD detector. A monochromator (Newport, Cornerstone 260) was used to select the targeted PL wavelengths. An event counter (Picoquant, Hydraharp 400) was used to determine the luminescence center lifetime by using the synchronization of the pulse laser as a start time, and the detection of the emission as the stop time where multiple detection of fluorescence creates a series of histograms for different arrival times.20 The TCSPC method is inadequate for phosphorescence of lifetimes longer than 100 µs because of long and limited acquisition time originated from a low repetition of excitation. To measure phosphorescence lifetimes > 100 µs, an optical chopper was used to create a periodic excitation at 20 Hz from a 405 nm pulsed laser source at 10 MHz repetition rate (Aurea Technology, PIXEA-405). During the emission process, a multichannel scaler (Stanford Research Systems, SR430) with a SPAD was used to count the detected photons before the next excitation cycles.20 3. RESULTS AND DISCUSSION 3.1. Crystal Structures The XRD patterns of Eu2+ and/or Mn2+ doped Na(Sr,Ba)PO4 and Ba2Mg(BO3)2 are shown in Figure 1. Note that the optimal Eu2+/Mn2+ molar ratio is varied in each host compound, which

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means that the tailored red/blue emission band position and relative intensity lead to diverse decay behaviors. Herein, we show only the representative XRD patterns for each phosphor composition. For Na(Sr,Ba)PO4, no traceable amounts of impurity phases and noticeable diffraction angle change were detected, and thus, all the reflections could be well indexed to a hexagonal Olgite structure for Na(Sr,Ba)PO4 (Fig. 1a).19 The as-prepared Eu2+ and/or Mn2+ doped Na(Sr,Ba)PO4 samples are of single phase and the incorporated co-activators do not cause any significant change. For Ba2Mg(BO3)2, however, there exist some unindexed peaks with low intensity, which could have originated from the presence of Ba- or Mg-carbonates, but they were not taken into consideration for the luminescent behavior because these impurities could be detected all the given borate phosphors (Fig. 1b and Fig. S1).18

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Figure 1. X-ray diffraction patterns of Eu2+/Mn2+ doped (a) Na(Sr,Ba)PO4 and (b) Ba2Mg(BO3)2 phosphors with given activator concentrations.18, 19 Possible final states for the synthesis process of Ba2Mg(BO3)2 phosphor were investigated by DFT calculations to examine the stability of the crystal structures as well as the formation of impurity phases given the doping level of Eu and Mn in the host material (refer to Supporting Information for the calculation method). The ratio of cations was set to be (Ba+Eu+Mn):Mg:B = 2:1:2, which was the same condition as the experimental design of this study. Meanwhile, Ba, Eu, Mn, and Mg were assumed to be at monoxide states at the synthetic condition of 1150

in

H2/N2 atmosphere. The calculation results of relative reaction energies are summarized in Figure 2. The most stable product state is (c) where Eu is doped in a Ba site while Mn is not doped in the host crystal but exists as an impurity phase, followed by state (b) in which both Eu and Mn are doped in Ba sites. Both the states have lower energies than state (a), an undoped state of Eu and Mn, indicating that the occupation of Ba sites by dopants offers a higher stability than the undoped state. On the other hand, the states in which Eu is not doped (d) or Mn is doped in Mg site (e) are appeared to be unstable. Conclusively, the DFT calculations clearly predict that Eu and Mn would occupy the lattice site of Ba in the Ba2Mg(BO3)2 host. The calculation result for Na(Sr,Ba)PO4 was reported in our previous work,19 which also indicated that Eu and Mn preferred the same doping site (2d of P3m1), as they do in the Ba2Mg(BO3)2 host.

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Figure 2. Comparison of stability of the synthetic products based on synthetic reaction energy of Eu- and Mn- added Ba2Mg(BO3)2 from DFT calculations: (a) the case that both Eu and Mn are not doped in Ba2Mg(BO3)2 host but form secondary phases, (b) Eu and Mn are doped in Ba sites, (c) Eu is doped in a Ba site while Mn is not doped, (d) Mn is doped in a Ba site while Eu is not doped, and (e) Eu is doped in a Ba site and Mn is doped in a Mg site.

The equilibrium proportion of the states in the product can be estimated by the comparison of the Boltzmann factor of states (a)-(e) (Fig. 2). The calculation result suggests that the phases of state (c), (b), and (a) are to be mixed in the ratio of about 96%, 2.2%, and 1.8%, respectively. The synthesis product is thus expected to consist of the phases of state (c), suggesting that Eu may readily be doped in Ba2Mg(BO3)2 while doping of Mn is not so favorable as that of Eu. A similar tendency of the dopability of Eu and Mn was also observed in the Na(Sr,Ba)PO4 host.19 Therefore, the increase of Mn in the reactants does not affect the dopability of Eu. This calculation results agree with the experimental observation, as discussed in the next section. The observed reduction in luminescence intensity with increasing Mn concentration is not due to a possible decrease of Eu in competition with Mn for occupying the doping sites; rather, it is MnMn interactions that cause the luminescence quenching by as a result of the excessive doping

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amount of Mn or Eu+Mn at Ba sites over the critical concentration. We also note that the most stable state (Fig. 2c) contains secondary phases of magnesium borates and manganese oxide. Thereby, the small in intensity and unmatched to Ba2Mg(BO3)2 peaks observed in the XRD pattern (Fig. 1b) may originate from these impurity phases. 3.2. Photoluminescence Properties Na(Sr,Ba)PO4:Eu2+,Mn2+ exhibits dual peak emissions from Eu2+ centered at 480 nm and sixfold coordinated Mn2+ centered at 590 nm (Fig. 3). The blue emission of Eu2+ corresponds to 4f5d transition,6 and the PL intensity decreases by 85% once the activator Mn2+ is added to the host Na(Sr,Ba)PO4, which implies a large initial ET from Eu2+ to Mn2+. Then, the luminescence of Eu2+ increases slightly with increasing concentration of Eu2+, which is expected since the amount of Eu has increased.9 Similarly, the PL intensity for Mn2+ increases up to 83% when sensitized by Eu2+ at a concentration of 0.01 as expected from the ET process, and it further increases monotonically with increasing concentration of Eu (Fig. 3a). With the laser excitation at 405 nm, when Mn2+ is added to the host, the four sharp peaks become distinctively present at 584 nm, 606 nm, 681 nm, and 697 nm. The emission peaks can be attributed to the presence of Eu3+ with the 5D0 → 7Fj transitions with j = 1,2,4,5 for each respective transition wavelengths of 585 nm, 606 nm, 681 nm, and 697 nm. The transition 5D0 → 7F2 at 606 nm is a strong transition that is present without Mn2+.21 In the Mn2+ doped Na(Sr,Ba)PO4, these extra small peaks are not observed with the incoherent the ultra-violet (UV) excitations by a Xe lamp. However, the UV excitation exhibits a relatively higher intensity from Eu2+ at around 450 nm for the same Eu/Mn doping ratio because the pumping at the higher energies above 405 nm is more efficient for the excitation of Eu2+ (Fig. 3b). Thus, the emission enhancement along with the presence of three additional peaks are most likely caused by ET between Mn2+ and Eu3+.22, 23 The ET between Eu3+

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and Mn2+ is further supported by the reduction of Mn2+ lifetime as discussed in section 3.3 for Na(Sr,Ba)PO4. Finally, once the concentration ratio of Eu0.05/Mn0.05 is reached quenching occurs for Eu3+ resulting in the reduction of Eu3+ intensity for each observed sharp peak.

Figure 3. Photoluminescence of Na(Sr,Ba)PO4:Eu2+,Mn2+ excited with (a) laser source at 405 nm, and (b) ultra-violet light source. Emission peak centered at 460 nm is from Eu2+ while emission peak centered at 589 nm is from Mn2+. For a concentration ratio of Eu0.01/Mn0.05, the intensity of Mn increases substantially before it increases linearly with increasing concentration of Eu. UV excitation shows a relatively higher intensity from Eu2+ compared to laser excitation.

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Ba2Mg(BO3)2:Eu2+,Mn2+, in contrast, shows a single emission peak centered around 625 nm even though the phosphor is codoped with Eu2+ and Mn2+ (Fig. 4a). The broad peak centered 612 nm is PL of Eu2+ because the PL profile is present when Ba2Mg(BO3)2 is doped with only Eu. The peak at 560 nm is neither observed with the UV excitation nor identified by the nominal transitions of Eu2+ and Mn2+ (Fig.4b). This peak is thus a spectral artifact caused by the 405 nm dichroic mirror. The emission from Eu2+ is red-shifted owing to the energy splitting from the crystal field effect from the host Ba2Mg(BO3)2.24 When Mn is added to Ba2Mg(BO3)2, the emission peak shifts to 625 nm, representing the emission from the transition of 4T1(4G) → 6

A1(6S) in Mn2+.7 For the Mn2+ concentration of 0.1 in Ba2Mg(BO3)2, the PL enhances up to 97 %

in relative intensity compared to the maximum intensity at a concentration ratio of Eu0.2/Mn0.1. Importantly, as the concentration of Mn2+ increases from 0.1 to 0.3, the luminescent quenching occurs. The quenching is attributed to the Mn-Mn interactions with excessive Mn ions, and it is supported by a reduction in the lifetime of Mn, as further discussed in the next section.6 Since the emission quenches only with increasing concentration, its origin is likely due to the donor-killer mechanism as opposed to the cross relaxation mechanism which would result in an enhanced emission at lower energy levels.25

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Figure 4. Photoluminescence of Ba2Mg(BO3)2:Eu2+,Mn2+ excited with (a) laser source at 405 nm, and (b) ultra-violet light source. Inset shows photoluminescence of maximized intensity for concentration ratios Eu0.2/Mn0.0, Eu0.2/Mn0.2, and Eu0.2/Mn0.3. Emission of Ba2Mg(BO3)2 doped only with Eu2+ shows an emission centered at 612 nm.

3.3. Radiative Lifetimes and Energy Transfer TRPL is a method that can determine the decay dynamics of excited ions in phosphors. ET and its efficiency can be deduced by the reduction of lifetime of luminescent ions. The spectral overlap between the absorption of activator ions and the emission of sensitizer ions is required

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for ET to occur.26 The mean radiative lifetimes can be found by fitting a single exponential function of the form

I (t) = Ae



t

τ

where A is luminescence intensity and τ is mean radiative lifetime. We can calculate ET efficiency by using the equation proposed by Paulose27

η = 1−

τx τ0

with η as the ET efficiency, τ0 as the lifetime without the activator, and τx as the lifetime with activator.6 For Na(Sr,Ba)PO4, the ET between Eu2+ and Mn2+ is expected to occur since the broad emission at 460 nm overlaps with the excitation transitions 1E(4D), 4T2(4D) and 4E-6A1(4G) of Mn2+.13 The lifetimes of Eu2+ and Mn2+ were monitored at 460 nm and 580 nm, respectively, by using the monochromator as the emission peaks were separable. Note that the lifetimes of Mn2+ at different doping concentrations are much longer than those of Eu2+. The fluorescence lifetimes of Eu2+ are displayed in Figure 5a while the phosphorescence lifetimes of Mn2+ are displayed in Figure 5b. As the concentration of Eu2+ is increased in Na(Sr,Ba)PO4, the lifetime of Eu2+ decreases monotonically until a concentration of 0.2 is reached. After reaching a concentration of 0.2, the lifetime does not change (Fig. 5a). The lifetime of Mn2+ is 105 ms in the absence of Eu2+ in the Na(Sr,Ba)PO4 host, and it reduces to 47 ms with the Eu2+ presence. Since the only change in the Na(Sr,Ba)PO4 occurs in the presence of Eu, the only source of the Mn lifetime reduction must be the nonradiative interactions between Eu and Mn ions. Clearly, Eu3+ is present in Na(Sr,Ba)PO4, and ET occurs between Eu3+ and Mn2+ as indicated by the presence of emission peaks at 585 nm, 606 nm, 681 nm, and 697 nm once Mn2+ is added to the host. The presence of

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ET between Eu3+ and Mn2+ would have caused the lifetime of Mn2+ to decrease. It is understood that the lifetime of Mn2+ does not reduce further because the concentration of Eu at 0.01 is large enough to cause maximum ET to occur with the additional Eu not having an effect, which is consistent with the saturated PL enhancement (Fig. 3)

Figure 5. Lifetime measurements for Eu2+ (a) and Mn2+ (b) in Na(Sr,Ba)PO4. The lifetime of Eu2+ reduces from 284 ns to 123 ns with increasing concentration of Eu (a). The Mn2+ lifetime without the sensitizer Eu2+ is 105 ms. Once Eu2+ is introduced in the host Na(Sr,Ba)PO4, the lifetime reduces to 46 ms and remains relatively constant for different concentrations of Eu2+. For Ba2Mg(BO3)2, ET can also be expected between Eu2+ and Mn2+ since there is a relatively big overlap between the Eu2+ emission at 612 nm and the direct excitation of Mn2+ at 625 nm from the 4T1(4G) → 6A1(6S) transition. Because the emissions from Eu2+ and Mn2+ are

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convoluted, the lifetimes cannot be measured separately through a spectral filtering. The fluorescence lifetime originates from Eu2+ since the lifetime is measured with Ba2Mg(BO3)2 doped with only Eu2+. Also, because PL of Eu2+ is an allowed transition compared to the Mn2+ dd forbidden transition, the lifetime of Eu2+ is expected to be much lower.6, 25 As the concentration of Mn increases, the lifetime of Eu2+ reduces exponentially until an ET efficiency of 91 % is reached (Fig. 6a). As it was previously mentioned, the PL increases substantially for a Mn concentration of 0.1, and, then, it diminishes down to 5% for higher concentrations. Thus, the luminescent quenching causes the large reduction in output intensity. However, the lifetime of Mn2+ independently decreases with increasing concentration, which indicates the Mn-Mn interactions (Fig. 6b).6

Figure 6. Lifetime measurements of Eu2+ (a) and Mn2+ (b) in Ba2Mg(BO3)2 host. The lifetime of Eu2+ decreases from 3.2 µs to 291 ns for increasing concentration of Mn in the host material. The

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lifetime of Mn2+ decreases from 60 ms to 20 ms with increasing Mn concentration, suggesting the concentration quenching with Mn-Mn interactions.

3.4. Discussion The ET process occurs between Eu2+ and Mn2+ in both phosphors, but the overall luminescent behavior is quite different. For Na(Sr,Ba)PO4, the calculated ET efficiency increases to 30% with an Eu concentration of 0.01, and it saturates at 57% for a concentration of 0.02 and higher (Fig. 7a). For Ba2Mg(BO3)2, in contrast, the ET efficiency rapidly increases above 90% with concentration increasing (Fig. 7b). This result indicates a higher ET efficiency for Ba2Mg(BO3)2 compared to Na(Sr,Ba)PO4 for the fixed dopant concentrations, as expected from the greater overlap of the absorption of Mn2+ and the emission of Eu2+ in Ba2Mg(BO3)2. The PL intensity with varied concentration ratio for each phosphor was measured and normalized by the maximum PL for each discernible luminescent peak. For Na(Sr,Ba)PO4, the luminescence decreases for Eu2+ less than 20 %, while the emission of Mn2+ increases to about 80 %, showing the similar trend with the ET enhancement. As the Eu concentration increases over 0.02, the emission of Eu2+ and Mn2+ gradually increase, while the ET efficiency saturates (Fig. 7a). The linear luminescence enhancement seen in both Eu2+ and Mn2+ occurs due to the addition of Eu2+ even though the ET efficiency remains the same. In comparison, Ba2Mg(BO3)2 exhibits a different PL behavior with Mn concentration increasing. Since the PL of Eu2+ and Mn2+ are convoluted in Ba2Mg(BO3)2, only the convoluted emission intensity can be analyzed. PL of Ba2Mg(BO3)2 shows the significant emission enhancement of 91% followed by luminescence quenching across the Mn2+ concentration of 0.1. The quenching observed in Ba2Mg(BO3)2 is further supported by the reduction in the measured lifetime of Mn2+ (Figure 5b).

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Figure 7. Energy transfer efficiency and relative intensity for different doping concentrations for both (a) Na(Sr,Ba)PO4 (b) and Ba2Mg(BO3)2. Relative intensity is found by normalizing the peak intensity value with the maximum value found for a given concentration of each ion. For Ba2Mg(BO3)2, luminescent quenching is observed across Mn2+ concentration of 0.1.

There have been few reports on ET between Mn2+ and Eu3+ that are similar to the ET process observed in Ba2Mg(BO3)2 for Eu2+ and Mn2+, and, so, it is worth discussing the presence of Eu3+ in Ba2Mg(BO3)2.22,

23, 28-30

Through X-ray photoelectron spectroscopy (XPS), the presence of

Eu3+ was confirmed (Fig. S2). However, the PL of Ba2Mg(BO3)2 does not support the presence of Eu3+, since Eu3+ would have three distinct peaks at 594 nm, 612 nm, and 624 nm corresponding to 5D0 → 7Fj for j = 0,1,2 transitions from the electric dipole.31 The broad emission of Eu2+ in Ba2Mg(BO3)2 is consistent with the work of Kim and Diaz which is identified as Eu2+.18, 24 The fluorescence lifetimes observed for Ba2Mg(BO3)2 would support the presence of Eu2+ as opposed to Eu3+, which has a phosphorescence forbidden transition.31 Also, since XPS is a surface analysis tool, the measurement may support only the presence of Eu3+ on the surface. Therefore, the phosphor Ba2Mg(BO3)2 has most likely coexisting ions Eu3+/Eu2+, but the steady state PL and lifetime measurements are more consistent for Eu2+ and Mn2+, as

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opposed to Eu3+ and Mn2+. For this reason, the ET is believed to occur between Eu2+ and Mn2+ in Ba2Mg(BO3)2 only with no effect from Eu3+ ions. In conjunction, the ET process also depends on the distance between the sensitizer and activator ions.26 The critical distance Rc can be found between the sensitizer and activator ions for ET to occur by using the concentration quenching method. The average distance can be found with Blasse equation32 1/3

 3V  Rc = 2    4π xc N 

where N is the number of Z ion in the unit cell, V is the volume of the unit cell, and xc is the critical concentration of Eu2+ and Mn2+. The critical concentration xc is defined when the luminescence of the sensitizer Eu2+ is reduced to 50% luminescence without the activator Mn2+.6, 32

The luminescence intensity of Eu2+ in Na(Sr,Ba)PO4 drops below 50% for the concentration of

xc = 0.01 + 0.05 = 0.06 (Figure 7a). From the XRD data, we find that N = 2 and V = 187 Å which gives the value of the critical distance of Rc = 14.39 Å for Na(Sr,Ba)PO4. In comparison, for Ba2Mg(BO3)2, the PL for both Mn2+ and Eu2+ is convoluted, and thus we only estimate that the critical concentration is reached at x = 0.2 + 0.1 = 0.3 since the ET efficiency is at 50% for this concentration ratio. The unit cell volume and Z value are found from the XRD data which gives N = 3, V = 408.42 Å, and the critical distance of Rc = 9.53 Å for Ba2Mg(BO3)2. In comparison, the critical distance Rc is found to be much smaller for Ba2Mg(BO3)2 than that for Na(Sr,Ba)PO4. Given the TRPL data obtained for Na(Sr,Ba)PO4 and Ba2Mg(BO3)2, we observe a higher ET efficiency for Ba2Mg(BO3)2, compared to Na(Sr,Ba)PO4 as expected from the overlap integral. Thereby, the result can be better understood by considering the difference in the energy level diagrams of these two hosts (Fig. 8). For Na(Sr,Ba)PO4, the emission of Eu2+ overlaps with

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several higher energy transition in Mn2+ before the energy is emitted from the 4T1(4G) → 6A1(6S) transition of Mn2+ (Fig. 8a). Importantly, the enhanced ET found in Ba2Mg(BO3)2 is mostly caused by the resonance wavelength overlap of the emission of Eu2+ and the excitation of Mn2+ by the 4T1(4G) → 6A1(6S) transition (Fig. 8b).

Figure 8. Energy diagram for Na(Sr,Ba)PO4 (a) and Ba2Mg(BO3)2 (b) for both Eu2+ and Mn2+. The red arrow indicates nonradiative decay, while the blue arrow indicates radiative decay. For Na(Sr,Ba)PO4, the transition of Eu2+ overlaps with several excitation peaks in Mn2+ before emitting light at 589 nm from 4T1(4G) → 6A(6S) transition while, in Ba2Mg(BO3)2, the energy transition of Eu2+ overlaps with the 4T(4G) → 6A1(6S) transition only.

4. CONCLUSION

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The series of Ba2Mg(BO3)2:Eu2+, Mn2+ and Na(Sr,Ba)PO4:Eu2+,Mn2+ were synthesized for the PL, TRPL and ET measurements and detailed analysis of DFT calculations. We observed a substantial enhanced red emission from Ba2Mg(BO3)2:Eu2+,Mn2+ by adding Mn2+, followed by concentration quenching. The PL enhancement can be explained by the resonant ET between Eu2+ and Mn2+, and the quenching is attributed to Mn-Mn interactions. Likewise, for Na(Sr,Ba)PO4, the PL of Mn2+ was increased by the ET from Eu2+, and the ET efficiency reaches a steady state at a Eu2+ concentration of 0.02, showing only minor enhancement. It is concluded that the higher ET enhancement found in Ba2Mg(BO3)2 is due to the spectral overlap in the excitation of Mn+2 at 625 nm from the 4T1(4G) → 6A1(6S) transition and the emission of Eu2+ at 612 nm. In comparison, the overlap in Na(Sr,Ba)PO4 occurs between the emission of Eu2+ at 460 nm and the excitation transitions at 1E(4D), 4T2(4D) and 4E-6A1(4G) of Mn2+. These results clearly suggest that the doping ratio of luminescent ions into hosting materials need to be chosen such that the concentration quenching is avoided. Moreover, the spectral outputs can be tailored by controlling the emission wavelengths of the sensitizer in relation to the excitation wavelength of the activator in the host crystals.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. Density functional theory calculation, X-ray diffraction for Ba2Mg(BO3)2:0.2Eu2+,xMn2+ with varying concentration x of Mn2+, and x-ray photoelectron spectroscopy for Ba2Mg(BO3)2 (PDF) AUTHOR INFORMATION Corresponding Author

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*Email : [email protected] *Email: [email protected].edu ORCID Jae Yong Suh: 0000-0001-9357-5935

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Research Enhancement Fund-Research Seed grant from the Vice President of Research Office at Michigan Technological University (REF-RS Proposal No. 1805012). ABBREVIATIONS ET Energy Transfer, PL photoluminescence, TRPL Time resolved photoluminescence, TCSPC Time correlated single photon counting REFERENCES 1.

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Highlights • • • •

Energy transfer between Eu2+ and Mn2+ is improved as a result of the substantial red-shift of Eu2+ emission. Lifetimes of Eu2+ and Mn2+ are reduced as a result of energy transfer. A competition between photoluminescence enhancement and concentration quenching exists for varying dopant ratios Optimized doping level can be found for enhancing photoluminescence.

Author Statement Kevin Bertschinger: Methodology, Software, Investigation, Formal Analysis, Writing -Original Draft. Hyemin Park: Investigation, Resources. Ha Jun Kim: Investigation, Resources. Yongseon Kim: Formal Analysis, Software. Won Bin Im: Investigation, Resources. Sungho Choi: Investigation, Methodology, Resources. Jae Yong Suh: Conceptualization, Methodology, Validation, Supervision, Writing -Review & Editing.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: