A novel approach to prepare optically active ion doped luminescent materials via electron beam evaporation into ionic liquids

Abstract

A novel approach to prepare luminescent materials via electron-beam evaporation into ionic liquids is presented which even allows doping of host lattices with ions that have a strong size mismatch. To prove this, MgF₂ nanoparticles doped with Eu³⁺ were fabricated. The obtained nanoparticles featured an unusually high luminescence lifetime and the obtained material showed a high potential for application.

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Metal ion doped fluorides find a wide range of applications in optical and photonic technologies such as storage phosphors, solid-state lasers, dosimeters, scintillators and thermoluminescent materials^1–8 as they are excellent host materials for optically active ion centers because of their large band gaps and their low phonon energies. In particular, cheap, non-rare earth based host matrices such as alkaline earth fluorides⁹ are of special interest since rare earths are considered critical materials. When doping with optical centers such as rare earth ions, materials for advanced applications such as scintillation,⁴ upconversion^10,11 and quantum cutting¹² can be realised. The (crystal) structure of the host lattice and distribution of the dopant ion in the surrounding lattice determine the optical properties particularly in case of nanoparticles. By using novel synthetic methods involving different techniques for dopant ion incorporation, it is possible to yield new materials with astounding properties.¹³

For this reason, we developed a novel method to prepare optically ion doped luminescent materials via electron beam evaporation into ionic liquids. Ionic liquids (ILs) are an innovative class of solvents and have received substantial attention in the recent years. They can exhibit unusual solvent properties such as a low vapour pressure, wide liquidus ranges, high thermal stabilities, and wide electrochemical windows which are advantageous for a large number of applications.¹⁴ In the last decade, ILs received considerable attention as reaction media for the preparation and stabilization of nanomaterials because they can act as structure-directing agents and stabilize particles by steric and electrostatic interactions.^15,16 Especially because of their negligible vapor pressure, ILs appear to be favourable media for vacuum experiments and applications.¹⁷

So far, physical vapor deposition and electron beam evaporation have been used for thin film preparation. Kawano et al. reported the preparation of luminescent rare earth ion-doped CaF₂ and MgF₂ films by simultaneous evaporation of lanthanide fluorides and alkaline earth fluorides.¹ Undoped MgF₂ nanoparticles have been prepared by sol–gel methods and¹⁸ microwave¹⁹ or ultrasound-assisted methods.²⁰ Conversely, the synthesis by chemical routes implies the use of fluorinated salts, fluorinated organic precursors^21–24 or the additional use of hydrofluoric acid.²⁵

Recently, we developed a synthesis procedure for metal, metal–oxide²⁶ alloy,²⁷ and rare earth fluoride nanoparticles²⁸ under high vacuum via thermal evaporation into ILs modifying the original SMAD (solvated metal atom dispersion) technique.^29–31 By replacing the conventional solvents and stabilizing agents, which typically exhibit high vapour pressures, with ILs with negligible vapour pressure, it is possible to evaporate materials at ambient temperatures into a liquid substrate. Here we describe an easy, environmentally friendly access to small, luminescent Eu³⁺ doped MgF₂ nanoparticles by simple electron beam co-evaporation of bulk EuF₃ and MgF₂ into an ionic liquid using a SMAD setup which was also used for physical vapour deposition recently.²⁶ The electron beam evaporation setup used herein principally consists of a source of electrons which are accelerated and focused by a magnetic field onto a crucible containing the evaporant. The electron beam bombardment heats the sample by transfer of kinetic energy. One advantage of the e-beam system is the fact that high-power densities up to 10 kW cm⁻² can be applied in order to achieve high evaporation temperatures and high evaporation rates. In detail, the e-beam is emitted from a tungsten filament, accelerated and focused by electric and magnetic fields. The focused beam melts the sample in a small spot. In order to prevent overheating of the evaporant, the crucible is cooled with water. The e-beam spot can be focused with a shield cage and the therein applied potential. The material is evaporated and condenses onto the ionic liquid film which is contained on the inner surface of a rotating flask surrounding the evaporation source. The principle of the e-beam evaporation and pictures of the setup are shown in Fig. 1.

Fig. 1 Electron beam evaporation set-up. Upper left corner: working principle of the e-beam gun as an evaporation source: (a) tungsten filament, (b) electron beam, (c) heating spot, (d) crucible with sample, (e) magnetic field lines, (f) cooling head, (g) shield cage for focusing the e-beam spot, (h) material vapour. Upper right corner: photograph of the crucible during evaporation. Below: schematic diagram of the SMAD set-up. For experimental details see the ESI.†

The IL 1-butyl-3-methylimidazolium tetrafluoroborate [C₁C₄im][BF₄] was used because of prior excellent results in the synthesis of nanoscale metal fluorides³³ and its superior stabilizing properties against metal nanoparticle precipitation.²⁶ In order to show the intriguing performance of the method presented herein, doping of MgF₂ with Eu³⁺ as the optical center was used. Eu³⁺ and Mg²⁺ have a large difference in their ionic radii which typically hampers doping. However, doping inexpensive, cheap and abundant fluoride host matrices such as alkali earth fluorides is strongly preferred over the use of precious rare earth fluorides since they are considered critical materials. In a typical experiment, 40 ml of [C₁C₄im][BF₄] was placed in the glass bulb of the reactor. A mixture of 1800 mg MgF₂ (ABCR, 99.99%) and 200 mg EuF₃ (ABCR 99.95%; REO) was ground under an inert atmosphere and placed in a Fabmate® crucible (Kurt-Lesker company, England) for electron-beam evaporation. After evaporation, the as-prepared particles were precipitated from the ionic liquid by adding a dichloromethane–methanol mixture.

The powder X-Ray diffraction pattern of the particles prepared by electron beam evaporation reveals tetragonal rutile-type MgF₂.³² No additional reflections belonging to a distinct phase of undoped EuF₃ or another impurity could be observed (Fig. 2). For comparison, MgF₂:Eu³⁺ particles synthesized in a microwave reaction as described are displayed.³³

Fig. 2 Powder X-ray diffraction pattern of MgF₂:Eu³⁺ synthesized by different methods (microwave vs. electron beam evaporation). For comparison, the database pattern of tetragonal MgF₂ is shown (ICSD 394, space group P4₂/mnm). The asterisk denotes a reflection originating from the instrument.

During synthesis, a dopant concentration of about 5% was envisioned (see ESI†). However, corresponding to ICP measurements an actual concentration of 2.77 wt% or 1.17 mol% was found for the Eu³⁺ ion in the sample prepared by e-beam evaporation. The different evaporation rate of neat MgF₂ and EuF₃ under electron beam irradiation leads to the diminished molar concentration. By proper adjustment of the ratio of starting materials, the obtained concentration can be controlled. Fig. 3 shows the transmission electron micrograph of the precipitated and washed Eu³⁺ doped MgF₂ particles. After removal of the stabilizing IL, the particles form larger aggregates on the TEM grid which makes an exact size distribution difficult. However, it is clear that these aggregates consist of smaller, unsymmetrical particles with a diameter between 8 nm and 14 nm.

Fig. 3 Transmission electron micrograph of MgF₂:Eu³⁺ particles synthesized via electron beam evaporation.

The photoluminescence excitation spectra of the synthesized powdered material (Fig. 4) show the typical Eu³⁺ f–f lines corresponding to ⁷F_0,1 → ⁵D₁, ⁵D₂, ⁵L₆, ⁵G_2–6, ⁵D₄, ⁵H₄ transitions. In neither case any oxygen impurities can be traced (as would become visible by a broad and significant Eu–O charge transfer band around 250 nm).

Fig. 4 Comparison of the room temperature excitation spectra (λ_em = 611 nm) of the synthesized MgF₂:Eu particles (microwave vs. electron beam evaporation) and bulk EuF₃ used as the starting material.

The particles show orange-red emission originating from the optical center Eu³⁺. The emission spectra are displayed in Fig. 5. The emission arises from the Eu^{3+ 5}D₀ → ⁷F_0–4 f–f transitions. The symmetry of the Eu³⁺ ion incorporated in the MgF₂ host material is different from the Eu³⁺ ion in the starting material EuF₃ as can be judged by the so-called asymmetry ratio of the ⁵D₀ → ⁷F₁ transition (592 nm) with respect to the ⁵D₀ → ⁷F₂ (611 nm) transition. Indeed, the emission is similar to previously obtained Eu³⁺ emission in other nanoscale alkali earth fluorides including CaF₂, SrF₂ and BaF₂ which further proves the incorporation of the Eu³⁺ ion into the lattice and that a (close to) octahedral site symmetry was achieved.³⁴ Noticeably, the nanoscale size of the particles leads to several slightly different Eu³⁺ coordination and thus symmetry environments yielding a much broader emission spectrum and a less resolved crystal field fine structure³⁵ in comparison to the bulky starting material EuF₃.

Fig. 5 Room temperature emission spectra (λ_ex = 393 nm) of the synthesized MgF₂:Eu particles and the starting material EuF₃ for comparison.

The luminescent lifetimes of the Eu³⁺ emission were determined for the material prepared via electron beam evaporation, via microwave-assisted method and for the starting material EuF₃. Herein, the material prepared via electron beam evaporation shows the longest luminescence lifetime of the ⁵D₀ → ⁷F₂ transition excited at λ_ex = 393 nm and monitored at λ_em = 612 nm (Table 1).

Table 1 Average luminescent lifetimes of the Eu³⁺ emission (λ_ex = 393 nm, λ_em = 612 nm) of the MgF₂:Eu³⁺ particles synthesized by different methods in comparison with the analog luminescent lifetime [small tau, Greek, macron] of the EuF₃ starting material

Material/composition	[small tau, Greek, macron] (ms)
EuF3 (pure)	2.1
MgF2:Eu by electron beam evaporation	2.6
MgF2:Eu 5% by microwave	1.5

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In summary, crystalline Eu³⁺ doped MgF₂ nanoparticles with a size between 8 nm and 14 nm have been synthesized via simultaneous e-beam evaporation of neat MgF₂ and EuF₃ into the ionic liquid [C₁C₄im][BF₄]. The particles show a bright orange-red luminescence, and further no impurities from oxygen could be detected. The incorporation of the Eu³⁺ ion into the MgF₂ host lattice was confirmed by luminescence spectroscopy. The quantum yield of the particles prepared via e-beam evaporation has been determined to be 3.7% which is in good agreement with an optical material purely relying on the intraconfigurational f–f emission of Eu³⁺ in an insulating nanoscale material such as MgF₂.

The luminescent lifetime of the nanoparticles of 2.6 ms is the longest compared with particles obtained from other preparation methods. This also confirms the promising properties of the synthesis method presented herein.

This work was supported in part by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office.

Notes and references

1. B.-C. Hong and K. Kawano, J. Alloys Compd., 2006, 408–412, 838–841.
2. P. F. Moulton, A. Mooradian and T. B. Reed, Opt. Lett., 1978, 3, 164–166.
3. L. G. Jacobsohn, A. L. Roy, C. L. McPherson, C. J. Kucera, L. C. Oliveira, E. G. Yukihara and J. Ballato, Opt. Mater., 2013, 35, 2461–2464.
4. W. Chen, S. L. Westcott, S. Wang and Y. Liu, J. Appl. Phys., 2008, 103, 113103.
5. M. Secu, S. Jipa, C. E. Secu, T. Zaharescu, R. Georgescu and M. Cutrubinis, Phys. Status Solidi B, 2008, 245(1), 159–162.
6. J. W. Stouwdam and F. C. J. M. van Veggel, Nano Lett., 2002, 2, 733; R. Yan and Y. Li, Adv. Funct. Mater., 2005, 15, 763.
7. M. M. Lezhnina, T. Jüstel, H. Kätker, D. U. Wiechert and U. H. Kynast, Adv. Funct. Mater., 2006, 16, 935; R. Chai, H. Lian, C. Li, Z. Cheng, Z. Hou, S. Huang and J. Lin, J. Phys. Chem. C, 2009, 113, 8070; S. Sivakumar, J. C. Boyer, E. Bovero, F. van Veggel and C. J. M. Frank, J. Mater. Chem., 2009, 19, 2392; X. Zhou, S. Li, X. Chen, Q. Zhu, Z. Wang and J. Zhang, J. Nanosci. Nanotechnol., 2008, 8, 1392.
8. G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer–Verlag, Berlin, Heidelberg, 1994.
9. C. Lorbeer, J. Cybinska, E. Zych and A.-V. Mudring, Opt. Mater., 2011, 34, 336.
10. S. Fischer, J. C. Goldschmidt, P. Löper, G. H. Bauer, R. Brüggemann, K. Krämer, D. Biner, M. Hermle and S. W. Glunzl, J. Appl. Phys., 2010, 108, 044912.
11. K. W. Krämer, D. Biner, G. Frei, H. U. Güdel, M. P. Hehlen and S. R. Lüthi, Chem. Mater., 2004, 16, 1244–1251.
12. C. Lorbeer, J. Cybinska and A.-V. Mudring, J. Mater. Chem. C, 2014, 2, 1862.
13. P. L. Archer, S. A. Santangelo and D. R. Gamelin, Nano Lett., 2007, 7, 1037.
14. N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123.
15. M. Antonietti, D. Kuang, B. Smarsly and Y. Zhou, Angew. Chem., Int. Ed., 2004, 43, 4988; A. Taubert, A. Uhlmann, A. Heddrich and K. Kirchhoff, Inorg. Chem., 2008, 47, 10758; K.-S. Kim, S. Choi, J.-H. Cha, S.-H. Yeon and H. Lee, J. Mater. Chem., 2006, 16, 1315; N. von Prondzinski, A. Babai, A.-V. Mudring and K. Merz, Z. Anorg. Allg. Chem., 2007, 633, 1490.
16. J. Dupont and J. D. Scholten, Chem. Soc. Rev., 2010, 39, 1780.
17. S. Kuwabata, T. Torimoto, A. Imanishi and T. Tsuda, Electrochemistry, 2012, 80, 498503; S. Kuwabata, T. Tsuda and T. Torimoto, J. Phys. Chem. Lett., 2010, 1, 3177.
18. S. Bas, U. Chatterjee and M. D. Soucek, J. Appl. Polym. Sci., 2012, 126, 998–1007.
19. Z. Quan, P. Yang, C. Li, J. Yang, D. Yang, Y. Jin, H. Lian, H. Li and J. Lin, J. Phys. Chem. C, 2009, 113, 4018–4025.
20. J. Lellouche, A. Friedman, J.-P. Lellouche, A. Gedanken and E. Banin, Nanomedicine, 2012, 8, 702; M. Eshed, J. Lellouche, E. Banin and A. Gedanken, J. Mater. Chem. B, 2013, 1, 3985.
21. S. Fujihara, M. Tada and T. Kimura, J. Sol–Gel Sci. Technol., 2000, 19, 311.
22. M. Tada, S. Fujihara and T. Kimura, J. Mater. Res., 1999, 14, 1610.
23. S. Fujihara, Y. Kadota and T. Kimura, J. Sol–Gel Sci. Technol., 2002, 24, 147.
24. S. Fujihara, H. Naito and T. Kimura, Thin Solid Films, 2001, 389, 227.
25. I. M. Thomas, Appl. Opt., 1988, 27, 3356.
26. K. Richter, A. Birkner and A.-V. Mudring, Angew. Chem., Int. Ed., 2010, 49, 2431–2435; K. Richter, P. S. Campbell, T. Baecker, A. Schimitzek, D. Yaprak and A.-V. Mudring, Phys. Status Solidi B, 2013, 250, 1152.
27. D. König, K. Richter, A. Siegel, A.-V. Mudring and A. Ludwig, Adv. Funct. Mater., 2014, 24, 2049–2056.
28. N. von Prondzinski, J. Cybinska and A.-V. Mudring, Chem. Commun., 2010, 46, 4393.
29. S. B. Kalidindi and B. R. Jagirdar, J. Phys. Chem. C, 2008, 112, 4042; S. Cingarapu, Z. Yang, C. M. Sorensen and K. J. Klabunde, Chem. Mater., 2009, 21, 1248.
30. P. L. Timms, Angew. Chem., 1975, 87, 295; U. Zenneck, Angew. Chem., 1990, 102, 171; P. D. Hooker and K. J. Klabunde, Chem. Mater., 1993, 5, 1089; K. J. Klabunde, P. L. Timms, P. S. Skell and S. Ittel, Inorg. Synth., 1979, 19, 59; P. L. Arnold, F. G. N. Cloke, P. B. Hitchcock and J. F. Nixon, J. Am. Chem. Soc., 1996, 118, 7630.
31. P. D. Hooker and P. L. Timms, J. Chem. Soc., 1988, 3, 158–159; P. Hooker, B. J. Tan, K. J. Klabunde and S. Suib, Chem. Mater., 1991, 3, 947.
32. H. W. Baur, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1976, 32, 2200–2204.
33. C. Lorbeer, J. Cybinska and A.-V. Mudring, Chem. Commun., 2010, 46, 571.
34. C. Lorbeer, F. Behrends, J. Cybinska, H. Eckert and A.-V. Mudring, J. Mater. Chem. C, 2014 10.1039/c4tc01214c.
35. V. Sudarsan, F. C. J. M. van Veggel, R. A. Herring and M. Raudsepp, J. Mater. Chem., 2005, 15, 1332.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental and instrumental details and synthesis of ionic liquids used. See DOI: 10.1039/c4cc05817h

‡ These authors contributed equally.

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