Competition between Energy Transfer and Energy Migration

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C: Plasmonics, Optical Materials, and Hard Matter

Competition between Energy Transfer and Energy Migration Processes in Neat and Eu -Doped TbPO 3+

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Irene Carrasco, Fabio Piccinelli, Ivo Romet, Vitali Nagirnyi, and Marco Bettinelli J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01374 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 18, 2018

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The Journal of Physical Chemistry

Competition Between Energy Transfer and Energy Migration Processes in Neat and Eu3+-Doped TbPO4 Irene Carrasco1,2*, Fabio Piccinelli1,3, Ivo Romet4, Vitali Nagirnyi4, Marco Bettinelli1,3 1

Luminescent Materials Laboratory, Dept. of Biotechnology, University of Verona, Strada Le

Grazie 15, 37134 Verona, Italy. 2

Advanced Technology Institute, Department of Electrical and Electronic Engineering,

University of Surrey. Guildford GU2 7XH, United Kingdom. 3

INSTM, UdR Verona, Strada Le Grazie 15, 37134 Verona, Italy.

4

Institute of Physics, University of Tartu, W. Ostwald str. 1, 50411 Tartu, Estonia.

*Corresponding author. Email: [email protected]; Phone number: +4401483686083

1 ACS Paragon Plus Environment

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Abstract The luminescence spectroscopy of various TbPO4, EuPO4 and TbPO4:Eu3+ polycrystalline materials has been systematically investigated upon near and vacuum UV excitation. Evidence was found for an efficient overall Tb3+→Eu3+ energy transfer in TbPO4:Eu3+. Upon UV excitation, this process leads to a significant change in the resulting emission colour of the materials from green to red even for low europium doping levels. The efficiency of the transfer at room temperature was evaluated. Fast migration among Tb3+ ions in the Tb3+-based materials was also observed, which in the case of TbPO4 causes partial luminescence quenching due to energy transfer to killer centres.


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1. Introduction In concentrated lanthanide compounds the absorbed excitation energy can migrate over the Ln3+ sublattice via multistep energy transfer, leading either to luminescence concentration quenching or to light emission. Avoiding luminescence quenching is an important issue in most applications, whereas energy transfer control is the basis of many luminescence based technologies.1,

2

As a consequence, the knowledge of the

processes of energy migration and transfer to emission centres is of crucial importance for the development of novel light emitters. Tb3+ and Eu3+ ions are frequently used in luminescent materials due to the possibility of combining their green and red emission, respectively, in the same substance. It is well known that Tb3+ is a good sensitizer for Eu3+ in the visible and ultraviolet (UV) spectral ranges, while the Tb3+→Eu3+ energy transfer process has been widely documented and discussed.3-5 In the case of Tb3+-based materials containing Eu3+ as a dopant, the Tb3+→Eu3+ energy transfer processes depend on the relative efficiency of the Tb3+→Tb3+ energy migration and the donor-activator transfer.6, 7 The role of energy migration and direct transfer to impurities in the emission from the 5D4 level of Tb3+ in Tb3+-doped and Tb3+-based compounds has been already discussed in detail in garnets 8 and borates.9 Lanthanide orthophosphates are interesting hosts for luminescent applications thanks to their good optical transparency and high thermal and chemical stability.10-12 Tb3+→Eu3+ energy transfer has already been reported for Eu3+-doped TbPO4 nanowires (hydrated and anhydrous),13 glass-ceramics

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and photonic crystals with monoclinic and

hexagonal crystal phase.15, 16 However, to the best of our knowledge the luminescence properties of Eu3+ and the Tb3+→Eu3+ energy transfer in tetragonal TbPO4 powders have never been reported. In the present work, the Tb3+→Tb3+ and Tb3+→Eu3+ energy transfer processes were analysed by the systematic study of the luminescent properties



of various TbPO4, Tb1-xEuxPO4 and EuPO4 polycrystalline materials upon near-UV (NUV) and vacuum-UV (VUV) excitation. 2. Experimental Various samples of TbPO4, Tb0.99Eu0.01PO4, Tb0.975Eu0.025PO4 and EuPO4, were synthesized by a conventional solid state reaction starting from Tb4O7 (99.99%), (NH4)2HPO4 (>99%) and Eu2O3 (99.99%) powders. Appropriate amounts of precursors were carefully mixed in an agate mortar and pressed into pellets under a load of 10 tons. Pellets were heat treated in air atmosphere at 400 oC (3 hours) and at 1250 oC (4 hours), with intermediate grindings. The purity of the phases was confirmed by powder X-ray diffraction (PXRD). The PXRD measurements were carried out with a Thermo ARL X’TRA powder diffractometer, operating in the Bragg-Brentano geometry and equipped with a Cu-anode X-ray source (Kα, λ =1.5418 Å), using a Peltier Si (Li) cooled solid state detector. The patterns were collected with a scan rate of 0.03o/s in the 5-90o 2θ range. The data for the various phosphates are presented in Figure 1.

Figure 1. Room temperature PXRD patterns of the TbPO4, Tb0.99Eu0.01PO4, Tb0.975Eu0.025PO4 and EuPO4 materials.


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The phase identification was performed with the PDF-4+2015 database provided by the International Centre for Diffraction Data (ICDD). All the diffraction peaks in the PXRD patterns are compatible with the ICDD data on TbPO4 (PDF card No. 01-076-1531) and EuPO4 (PDF card No. 01-084-0919). Tb1-xEuxPO4 (x = 0, 0.01, 0.025) powders belong to the zircon family with tetragonal crystal structure (space group I41/amd), whereas EuPO4 is of a monazite-type with monoclinic structure (space group P21/n). Room temperature luminescence upon NUV excitation was measured at the University of Verona with a Fluorolog 3 (Horiba-Jobin Yvon) spectrofluorometer, equipped with a Xe lamp, a double excitation monochromator, a single emission monochromator (mod.HR320) and a photomultiplier in photon counting mode for the detection of the emitted signal. Luminescence studies upon excitation in the VUV-NUV spectral range were carried out at the Institute of Physics, University of Tartu. For the excitation in the region 3-11 eV, a vacuum system coupled to a VUV light source (Heraeus deuterium discharge lamp, 200 W) through a Seya-Namioka McPherson 234/302 monochromator equipped with a 300g/mm grating (spectral resolution 0.4 nm, dispersion 16 nm/mm, optimization for 140 nm) was used. The samples were mounted on a sample holder of a closed cycle helium cryostat provided by ARS (Advanced Research System, Inc.) suitable for low temperature (LT) experiments. The excitation source, primary monochromator and cryostat were connected to a vacuum system and the high vacuum was created with a turbo-molecular pump. The emission energy in the range 1-6 eV was selected by means of an Andor SR 303i-B grating monochromator equipped with a CCD (Andor iDus 416) or with various photomultiplier tubes (H8259, H8259-01, H8259-02). Measurements were carried out at 5 and 300 K and controlled by using a LabView based software.



3. Results 3.1 Near-UV excited luminescence

Figure 2. Room temperature emission spectra of TbPO4, EuPO4 and Tb1-xEuxPO4 upon excitation at 377 nm, 394 nm and 377 nm, respectively. The spectrum of TbPO4 is normalized to Tb3+ 5D4→7F5 emission, whereas the spectra of Tb1-xEuxPO4 and EuPO4 are normalized to Eu3+ 5D0→7F4 emission. The inset shows in detail the Tb3+ emission bands for Tb1-xEuxPO4. The bands marked with * are due to Eu3+ impurities.

Figure 2 shows the room temperature emission spectra of TbPO4 and Tb1-xEuxPO4 (x = 0.01, 0.025) upon NUV excitation at 377 nm into the 5D3 level of Tb3+, and the spectrum of EuPO4 upon excitation into the 5L6 level of Eu3+ at 394 nm. The spectrum of TbPO4 presents strong bands originating from the transitions from the 5D4 level of Tb3+ but no significant emission from 5D3, as a result of the fast cross relaxation processes present in concentrated terbium materials.17 For this compound four main emission bands in the 450-630 nm range and three weaker bands in the 640-690 nm range can be observed, the most intense being the 5D4→7F5 green emission at 542 nm. The two weak bands around 700 nm are caused by unintentional and unavoidable Eu3+


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impurities; these bands are always present in different samples synthesized with several Tb4O7 starting reagents coming from different sources. In the case of EuPO4, the spectrum is composed only by transitions originated from the 5D0 level of Eu3+, as expected for phosphate hosts since, in these compounds, the highest phonon energy (ℏ߱௠௔௫ =1300 cm-1 18) allows fast nonradiative relaxation from the higher excited levels (i.e. 5D1, 5D2 and 5D3) to 5D0. For Tb1-xEuxPO4 (x = 0.01, 0.025) samples, the emission spectrum is also dominated by strong Eu3+ emission bands corresponding to transitions from the 5D0 level, and only weak bands corresponding to the 5D4→7F6 and 5D4→7F5 transitions of Tb3+ can be observed at 487 and 542 nm. These bands are presented in detail in the inset of Figure 2, where spectra are normalized to the 5D0→7F4 Eu3+ band. It is possible to observe that the intensity of the Tb3+ bands is reduced when the Eu3+ content is increased, while no other significant differences can be noticed between the 1% and 2.5 % Eu3+-doped samples. The emission data show clearly the presence of fast and efficient Tb3+→Eu3+ energy transfer. This process changes the emission colour of the material from green to red even by addition of small amounts of europium ions, as it can be observed in the CIE diagram and in the picture of the phosphors illuminated with a hand UV lamp presented in Figure 3.



Figure 3. CIE coordinates diagram of EuPO4 upon excitation at 394 nm and TbPO4 and Tb1-xEuxPO4 upon excitation at 377 nm, and typical picture of the phosphors illuminated with a UV hand lamp at 365 nm.

It can be noticed that the 5D0→7F4 transition has the strongest intensity for all samples containing Eu3+. The abnormally high intensity of this transition and its dependency on the environment have been discussed in the past. Sá Ferreira et al. 19 ascribed this effect in Na9[EuW10O36].14H2O polyoxometalate to geometric distortions in highly polarizable environments. On the other hand, it has been demonstrated that in a series of garnets and orthophosphates the intensity of the 5D0→7F4 transition depends on the average electronegativity and ionic radii of the host cations, and increases when the electronegativity decreases.20 From Figure 2 it is also clear that the emission spectrum of Eu3+ is different for EuPO4 and Tb1-xEuxPO4, as a result of the different point symmetries of these compounds. In


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the monoclinic space group P21/n of EuPO4, the Eu3+ ions are located at the Wyckoff position 4e with point symmetry Cs,21 whereas in tetragonal space group I41/amd of TbPO4) these are located in the 4a position with point symmetry D2d. 22 For EuPO4 five distinct emission bands corresponding to the 5D0→7FJ (J= 0, 1, 2, 3, 4) transitions can be observed. The splitting of the bands (particularly high for the 5

D0→7F2, 3, 4 bands) is compatible with the expected crystal-field splitting for the sites of

C1, Cs, C2 and C2v symmetry, in good agreement with the local point symmetry assigned for this material by Mullica et al. single crystals

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21

and the spectroscopic results reported for EuPO4

and nanocrystals.24 In the case of Tb1-xEuxPO4, only four emission

bands can be observed (no 5D0→7F0 transition) and the number of crystal-field components is lower than for EuPO4. These results are compatible with those expected for the sites of D2, D2d, D3 and S4 symmetry, which is also in good agreement with the local point symmetry of the tetragonal phase of this compound.22 Notice that for both materials, the total number of crystal field components is difficult to identify in a univocal way as a consequence of the overlap caused by the large splitting of the bands. In any case, a crystal field analysis lies beyond the scope of the present paper.

Figure 4. Room temperature excitation spectra of TbPO4 for emission at 543 nm, EuPO4 for emission at 684 nm and Tb1-xEuxPO4 for the Eu3+ emission at 695 nm. 9 ACS Paragon Plus Environment


Figure 4 shows the excitation spectra of the various samples. The spectrum of TbPO4 shows overlapping bands ranging from 300 nm to almost 400 nm, and a band at 480 nm. Table 1 shows the corresponding electronic transitions of the excitation bands. The spectrum of EuPO4 is composed of various bands around 300 and 320 nm, and in the range from 360 nm to 400 nm, and of other weaker bands around 415 nm, 460 nm, 520 nm and 600 nm. The corresponding transitions for these bands are also provided in Table 1. In the case of the Eu3+-doped samples the excitation spectra of the emission at 695 nm (5D0→7F4 Eu3+ transition) are dominated by the Tb3+ bands located in the NUV, whereas only a few weak Eu3+ excitation bands are observed. These results confirm the presence of an efficient Tb3+→Eu3+ energy transfer process in this material. Table 1. Correspondence between the electronic transitions and the observed lines in the excitation spectra of TbPO4 and EuPO4.

identified transition TbPO4 7 7

identified transition EuPO4 7 F0→5D4

wavelength (nm)

F6→3H6

304

F6→3H7 + 5D0

318

7

F0→ L7

380

F0→5L6

394

F0→5D3

413

7

F6→ D1

328

F6→5L8 + 5G3

341

7

F6→5G4+ 5L9 + 5D2

351

7

7

F6→ G5

359

F0→ D2

463

F6→5L10

370

7

F0→5D1

524

F6→5G6, 5D3

377

7

F1→5D1

530

488

7

F0→ D0

587

7

F1→5D0

593

7

5

375

5

7

7 7

5

360

F0→3G3

7

7 7

wavelength (nm)

5

F6→ D4

5

5


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Figure 5. (a) Room temperature decay curves of the 5D4 (Tb3+) emission at 543 nm upon excitation at 377 nm in TbPO4 and Tb1-xEuxPO4. (b) Room temperature decay curves of the 5D0 (Eu3+) emission at 695 nm upon excitation at 394 nm (EuPO4) and at 377 nm (Tb1-xEuxPO4).

The room temperature decay curves for the Tb3+ and Eu3+ emissions are presented in Figure 5. In all cases, the decay curves are non-exponential and therefore e-folding times τ1/e were evaluated to estimate the dynamics at short time after the excitation pulse. Table 2 presents the estimated values of the decay constant for the materials studied. The exponential decay constant for Y0.99Tb0.01PO4 10 is included to compare the behaviour of the Tb3+ 5D4 level in concentrated and diluted compounds. Figure 5(a) shows the decay curves for the Tb3+ emission from 5D4 in TbPO4 and Tb1-xEuxPO4. The 11 ACS Paragon Plus Environment


profile of the decay curves is not the same in the undoped and the Eu3+-doped samples. The initial part in Tb1-xEuxPO4 is faster than in TbPO4, whilst the characteristic times of the approximately exponential tail at long times are close and equal to about 1 ms in both systems. Figure 5(b) shows the decay curves of the Eu3+ emission at 695 nm in EuPO4 and Tb1-xEuxPO4. The curves are not perfectly exponential and for the two Tb3+-based samples the curves are almost identical, both presenting a small buildup at short times (see inset of Figure 5(b)) which indicates that the energy transfer from Tb3+ to Eu3+ is fast. For EuPO4 the decay is significantly shortened, probably as a result of energy migration to quenching impurities due to the high concentration of Eu3+ ions. Table 2. Decay data for the luminescent levels of Tb3+ and Eu3+ in Tb1-xEuxPO4 and YPO4 upon NUV excitation.

material

excitation wavelength (nm) 377

emitting level

τ1/e (ms)

5

D4 (Tb3+)

0.16

355 394

5

D4 (Tb3+)

τd =2.3 10

5

D0 (Eu3+)

0.9

Tb0.99Eu0.01PO4

377

5

D4 (Tb3+)

0.06

Tb0.99Eu0.01PO4

377

5

D0 (Eu3+)

2.8

Tb0.975Eu0.025PO4

377

5

D4 (Tb )

0.06

Tb0.975Eu0.025PO4

377

5

D0 (Eu3+)

2.7

TbPO4 Y0.99Tb0.01PO4 EuPO4

3+

3.2 VUV excited luminescence The room temperature emission spectra upon VUV excitation at 160 nm (Figure 6) present, for all the samples, various emission bands in the visible range and no significant differences can be observed if compared with the spectra obtained upon NUV excitation that have been presented and discussed in Section 3.1. The room


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temperature VUV-NUV excitation spectra in the 140-220 nm range are also shown in Figure 6.

Figure 6. Room temperature excitation (dotted lines) and emission (solid lines) spectra of: (a) Tb0.99Eu0.01PO4 and Tb0.975Eu0.025PO4, (b) EuPO4 and (c) TbPO4.

The excitation spectrum of the Tb3+ emission at 543 nm in TbPO4 is composed of various peaks in the 170-240 and 250-300 nm range that can be assigned to the various spin allowed and spin forbidden 4f8→4f75d transitions of Tb3+.25-27 The features around 140-160 nm can be attributed to the absorption of the PO43- anions, as reported for other phosphates.28,

29

The excitation spectra of the Eu3+ emission at 620 nm in

Tb0.99Eu0.01PO4 and Tb0.975Eu0.025PO4 are very similar to the one in TbPO4 and can be analyzed in the same way. In the case of EuPO4, the spectrum is different from those reported in literature for Eu3+ in phosphates.26,

30

The features at 140-160 nm are

probably associated to the PO43- anions as discussed previously, whereas the broad band centred at around 240 nm can be attributed to the O-Eu charge-transfer transitions.31, 32 The intense peak located at 265 nm and the contribution in the 160-200 nm range are very difficult to assign in the frame of this study. The former could be due to a defect



present in the sample under investigation, maybe charge transfer from an O belonging to a PO3 centre (oxygen vacancy). In any case, the origin of this peak is beyond the scope of the present work.

Figure 7. Low temperature (5 K) excitation (dotted lines) and emission (solid lines) spectra of: (a) Tb0.99Eu0.01PO4 and Tb0.975Eu0.025PO4, (b) EuPO4 and (c) TbPO4.

Figure 7 presents the 5 K VUV emission and excitation spectra for the phosphors studied. No significant differences have been found between the emission spectra at room and low temperature except for the reduction of the emission intensity of EuPO4 at 5 K. The low temperature excitation spectra of TbPO4 and Tb1-xEuxPO4 are in good agreement with those measured at room temperature, and can be interpreted in the same way. On the contrary, for EuPO4 the low temperature VUV-NUV excitation spectrum is significantly different, demonstrating no peaks in the 120-200 nm range and a reduced intensity of the peak at 240 nm. These changes with temperature can be caused by charge trapping processes. It has been found for several materials that when temperature is lowered the intensity of luminescence may be largely decreased because of the competing charge trapping process. This effect is strong in materials where self-trapping is possible, particularly in those which demonstrate the self-trapping of both electrons 14 ACS Paragon Plus Environment

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and holes.33 Recently the evidence of such process has been shown for Eu3+-doped Lu and Y phosphates upon X-ray excitation.34 Charge carrier trapping is usually observed upon interband excitation, however, it can also follow charge-transfer transitions,

32

which explains the reduction of the 240 nm band at low temperatures in EuPO4. As indicated for the RT spectrum, the intense peak at 265 nm can be related to a lattice defect. The profiles of the room temperature decay curves are similar to those measured upon NUV excitation and can be interpreted in the same way. For the 5D4 emission of Tb3+ the room temperature decays are in good agreement with the NUV-excited results. No significant differences can be observed between the low and room temperature curves and the low temperature decay is only slightly longer, indicating that for these materials quenching processes are only weakly activated by raising temperature. On the contrary, the room temperature decay curve of EuPO4 upon VUV excitation is faster than upon NUV excitation. 4. Discussion The significant shortening observed in the decay curves of the 5D4 level of Tb3+ when comparing diluted and concentrated compounds indicates that Tb3+→Tb3+ migration processes play an important role in the luminescence of TbPO4. This has sense if we consider that TbPO4 has a rather small cell volume (292.26 Å3).22 In fact, the distance between two neighbouring Tb3+ ions is only 3.79 Å and Tb3+ ions occupy only one crystallographic site in this material. The minimum Tb3+-Tb3+ distance is much smaller than the critical distance for the dipole-dipole interaction among the 5D4 levels of Tb3+ ions, which is estimated to be 8.3 Å for TbPO4, confirming the expected relevance of migration in this compound. The critical distance was obtained using Equation 1, which



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is directly derived from Dexter’s theory of energy transfer through multipolar interaction,35 in the case of a dipole-dipole interaction: 36

ܴ௖଺ = 3 ∙ 10ଵଶ ݂ௗ ‫׬‬

௙ೞ ሺாሻிಲ ሺாሻ ாర

݀‫ܧ‬

(1)

In the equation, fs and FA represent the normalized absorption line shape of the activator and emission line shape of the sensitizer, respectively, as a function of the transition energy E, both obtained from the experimental spectral overlap, while fd is the oscillator strength of the involved absorption transition of the activator that was assumed to be 3·10-7 for the 7F6→5D4 transition.37 The critical distance is probably underestimated, since at such short minimum Tb3+-Tb3+ distance (3.79 Å, i.e. less than 4 Å) even exchange interaction could be operative.38 It must also be noted that TbPO4 shows a magnetic (and structural) phase transition at low temperatures and that the 5D4 decay has been found to be perturbed by 5D4 exciton-exciton annihilation at strong resonant excitation power density.39,

40

This behaviour is compatible with sizeable Tb3+-Tb3+

couplings, presumably also exchange interaction, and therefore very effective migration is undoubtedly present in the 5D4 subset. On the other hand, the short interionic distances and the relatively simple crystal structure of TbPO4 make possible the presence of processes that can compete with migration. The fast decay at short times after the pulse is related to the migrationassisted energy transfer from Tb3+ ions to non-emitting impurities, or Eu3+ ions in the case of the doped samples. The values of the e-folding times τ1/e are given in Table 2. The slow near-exponential decay at longer times is assigned to defect related emission, possibly Tb3+ ions close to defects. The fast component of the curves is shorter in the case of the Eu3+-doped samples, which is related to the migration-assisted energy transfer from Tb3+ ions to nearby Eu3+ ions. By taking into account the short time part of the decay curves, and the decay constants reported in Table 2, the efficiency of the 16 ACS Paragon Plus Environment

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Tb3+→Eu3+ energy transfer in the Eu3+-doped materials can be estimated using the equation:

ߟ் = 1 −

ఛ೅್షಶೠ ఛ೅್

(2)

For both materials, ηT is found to be slightly above 0.6, but a precise quantitative estimation is made difficult by the presence of quenching due to migration-assisted transfer to killer impurities that competes with the Tb3+→Eu3+ transfer and makes a reliable analysis of the decay curves complicated. 5. Conclusions In this work, the luminescence spectroscopy of various TbPO4, EuPO4 and TbPO4:Eu3+ polycrystalline materials has been systematically investigated upon NUV excitation. As a complementary tool, the VUV-excited luminescence at RT and LT has been also studied in detail. The resulting emission colour of the Eu3+-doped materials is significantly shifted from green to red even for low europium doping levels, indicating that in this system the overall Tb3+→Eu3+ energy transfer process is efficient. No remarkable differences have been found in the emission properties when varying the excitation energy from NUV to VUV. Evidence has been found for energy migration among Tb3+ ions in the Tb3+-based materials. Non-exponential decays from the 5D4 level of Tb3+ and the 5D0 level of Eu3+ were observed upon excitation at both 230 nm and 377 nm. In general, it is possible to conclude that TbPO4 is an interesting host for case studies of energy transfer processes. In the materials under investigation, the Tb3+→Eu3+ energy transfer depends on migration and interionic transfer, and leads to UV (deep and near) sensitization of the red 5D0 luminescence. Overall, energy migration in TbPO4 is very fast, whilst it can be much slower in other hosts (e.g., whitlockites 7) depending on the 17 ACS Paragon Plus Environment


crystal structure (in particular on interionic distances and the presence of multisites). In the TbPO4 host, the very fast migration leading to quenching due to killer impurities is a clear drawback for possible applications in frequency conversion devices. Acknowledgments This research was conducted thanks to the funding of the European Commission through the Marie Curie Initial Training network LUMINET, grant agreement No. 316906, and the Estonian Research Council (project PUT PRG111). Tartu group also acknowledges financial support from the ERDF funding in Estonia granted to the Centre of Excellence TK141 (Project No. 2014-2020.4.01.15-0011). Expert technical assistance by Erica Viviani (University of Verona) is gratefully acknowledged. References [1] Zhou, B.; Shi, B.; Jin, D.; Liu, X. Controlling Upconversion Nanocrystals for Emerging Applications. Nat. Nanotechnol. 2015, 10, 924-936. [2] Bettinelli, M.; Carlos, L.; Liu, X. Lanthanide-Doped Upconversion Nanoparticles. Physics Today 2015, 68, 38-44. [3] Holloway, W. W.; Kestigian, J. M.; Newman, R. Direct Evidence for Energy Transfer Between Rare Earth Ions in Terbium-Europium Tungstates. Phys. Rev. Lett. 1963, 11, 458-461. [4] Laulicht, I.; Meirman, S.; Ehrenberg, B. Fluorescent Linewidths and Excitation Transfer in Eu0.33Tb0.66P5O14 Crystals. J. Lumin. 1984, 31-32, 814-816. [5] Carrasco, I.; Bartosiewicz, K.; Nikl, M.; Piccinelli, F.; Bettinelli, M. Energy Transfer Processes in Ca3Tb2− xEuxSi3O12 (x= 0-2). Opt. Mater. 2015, 48, 252-257. [6] Carrasco, I.; Piccinelli, F.; Bettinelli, M. Luminescence of Tb-Based Materials Doped With Eu3+: Case Studies for Energy Transfer Processes. J. Lumin. 2017, 189, 71-77. 18 ACS Paragon Plus Environment

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[7]

Carrasco,

I.;

Piccinelli,

F.;

Bettinelli,

M.

Optical

Spectroscopy

of

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Competition between Energy Transfer and Energy Migration

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