Energy transfer of the quantum-cutter couple Pr3+–Mn2+ in CaF2:Pr3+, Mn2+ nanoparticles

Journal of Luminescence 179 (2016) 555–561

Contents lists available at ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Full Length Article

Energy transfer of the quantum-cutter couple Pr3 þ –Mn2 þ in CaF2:Pr3 þ , Mn2 þ nanoparticles Ana Kuzmanoski a, Vladimir Pankratov b,n, Claus Feldmann a,n a b

Karlsruhe Institute of Technology (KIT), Institute of Inorganic Chemistry, Engesserstraße 15, D-76131 Karlsruhe, Germany Research Center of Molecular Materials, University of Oulu, PO Box 3000, 90014 Oulu, Finland

art ic l e i nf o

a b s t r a c t

Article history: Received 28 January 2016 Received in revised form 4 July 2016 Accepted 23 July 2016

CaF2:Pr (1 mol%), CaF2:Mn (5 mol%) and CaF2:Pr,Mn (1 mol%, 5 mol%) nanoparticles are prepared via a microwave-mediated synthesis in ionic liquids. The nanoparticles are highly crystalline and exhibit particle diameters o 50 nm.In contrast to bulk-CaF2:Pr,Mn,energy transfer between Pr3 þ and Mn2 þ under 1 S0-1I6 relaxation on Pr3 þ and 4G(4T1g)-6S(A1g) emission of Mn2 þ is observed for the first time. Such energy transfer represents the essential first step of the quantum-cutting cascade via the Pr3 þ –Mn2 þ couple, which is most interesting as both expected photons – 3P0-3H4 emission of Pr3 þ and 4G(4T1g)-6S (A1g) emission of Mn2 þ – are emitted in the green spectral range. While bulk crystals were said not to show energy transfer due to prohibiting selection rules, vacuum ultraviolet (VUV) spectroscopy of CaF2: Pr, Mn nanoparticles firstly proves efficient Pr3 þ -Mn2 þ energy transfer, which can be ascribed to the reduced site symmetry and considerable spin–orbit interaction in the nanocrystals. & 2016 Elsevier B.V. All rights reserved.

Keywords: Calcium fluoride Nanoparticle Pr–Mn couple Quantum cutting

1. Introduction Quantum cutting by two-photon emission – viz. the absorption of high-energy photons followed by emission of two photons, preferentially, in the visible at similar wavelength – is a fascinating luminescence process [1–3]. Technically, two-photon emission can be highly relevant for high-energy excitation such as Xe plasma, lasers or X-rays. Quantum cutting was first shown for YF3:Pr3 þ and NaYF4: Pr3 þ [4,5]. Herein, specific f-levels on a single rare-earth metal ion allow establishing the two-photon emission cascade. Although twophoton emission was evidenced, the total quantum yield can be o100% nevertheless. Another drawback of single-ion-based quantum cutting such as on Pr3 þ or Tm3 þ is related to the fact that one of the emitted photons is outside the visible spectral range and therefore less useful in practice. A remarkable quantum-cutting system was discovered by Meijerink et al. with LiGdF4:Eu3 þ [6,7]. After excitation of Gd3 þ , energy transfer to Eu3 þ occurs twice under emission of two photons in the red spectral range. Thus, emission of two photons in the similar wavelength range was proven for the first time. Moreover, an exceptional quantum yield of 195% was shown. The drawback of the Gd3 þ –Eu3 þ quantum-cutting couple, however, is related to the narrow and weak-intensity absorption via f–f

n

Corresponding authors. E-mail addresses: [email protected], vladimirs.pankratovs@oulu.fi (V. Pankratov), [email protected] (C. Feldmann). http://dx.doi.org/10.1016/j.jlumin.2016.07.040 0022-2313/& 2016 Elsevier B.V. All rights reserved.

transitions of Gd3 þ . Despite of the high quantum yield, the emission intensity is nevertheless low therefore. Another interesting quantum-cutting system is related to the Pr3 þ –Mn2 þ couple with the underlying energy level scheme and luminescence processes shown in Fig. 1 [8,9]. Similar to Gd3 þ – Eu3 þ , again two photons – a first photon from Pr3 þ and a second photon from Mn2 þ – are emitted in the same spectral range which is green light, here. In contrast to the forbidden and therefore weak-intensity excitation via the f-levels on Gd3 þ in LiGdF4:Eu, here, the excitation starts with the fully allowed 3H4-5d transition on Pr3 þ (Fig. 1) [8]. After relaxation from the d-band to the 1S0 level on Pr3 þ , Pr3 þ -Mn2 þ energy transfer occurs under 1S0-1I6 relaxation on Pr3 þ and 6S(A1g)-4A1(4G) excitation on Mn2 þ . Subsequent to certain non-emissive relaxation, finally, two-photon emission occurs via 3PJ-3H4 transition on Pr3 þ and 4T1g(4G)-6S (A1g) transition on Mn2 þ (Fig. 1). In general, two-photon emission of the Pr3 þ –Mn2 þ couple has certain prerequisites [8]: (i) For cascade emission the 1S0 level on Pr3 þ needs to be located at lower energy than the 5d band; (ii) Spectral overlap of the 1S0-1I6 emission of Pr3 þ and the absorption of Mn2 þ is essential; (iii) The distance between Pr3 þ and Mn2 þ needs to be sufficient for optimal energy transfer. Spectroscopic studies involving host lattices like YF3, CaF2, LiBaF3, SrY2F8 or KMgF3 have shown that – after co-doping with Pr3 þ and Mn2 þ in various concentrations – the above prerequisites are fulfilled [8,10,11]. Fluoride host lattices are – with few oxides as exceptions [12–14] – prerequisite for Pr3 þ –Mn2 þ based quantum cutting since the 5d level is only at higher energy as the 1S0 level on Pr3 þ for highly ionic,

556

A. Kuzmanoski et al. / Journal of Luminescence 179 (2016) 555–561

Fig. 1. Two-photon emission based on the Pr3 þ –Mn2 þ quantum-cutting couple (solid arrows indicate radiative emission; dotted arrows indicate non-radiative energy transfer; zig-zag lines indicate non-radiative relaxation) [8].

wide band-gap materials [8] . Until now, however, no energy transfer from Pr3 þ to Mn2 þ – in spite of favorable spectral overlap and a considerable number of nearest-neighbor pairs – was observed, independent of temperature of measurement and concentration of dopants [8]. Obviously, relaxation from the 1S0 state via energy levels on Pr3 þ only is favored in comparison to Pr3 þ -Mn2 þ energy transfer. This finding could be related to transitions conflicting with the total angular momentum selection rule [15,16], which is normally lifted in a crystalline lattice but might be dominating for the Pr3 þ –Mn2 þ couple. In this work, we report on a novel ionic-liquid-based synthesis of CaF2:Pr3 þ , CaF2:Mn2 þ and CaF2:Pr3 þ , Mn2 þ nanoparticles exhibiting diameters of 20–30 nm. Fluorescence spectroscopy was performed to study excitation and emission of all doped CaF2 nanoparticles at low temperature (10 K). Nanoparticles are intended to reduce the symmetry of the infinite, periodic lattice by size limitation and to trigger the aimed energy transfer under 1S0-1I6 relaxation on Pr3 þ and 6S (A1g)-4A1(4G) excitation on Mn2 þ . For the first time, we can indeed clearly show such Pr3 þ –Mn2 þ energy transfer that also points to the occurrence of Pr3 þ –Mn2 þ -based two-photon emission.

2. Material and methods 2.1. Starting materials All chemicals were used as received from the supplier. Calcium chloride (CaCl2, VWR, 99%), manganese(II) chloride (MnCl2  H2O, VWR, 99.9%), praseodymium chloride hydrate (PrCl3  H2O, Acros Organics, 99.99%) and 3-octyl-1-methyl-imidazolium tetrafluoroborate ([OMIm][BF4], IoLiTec, 99%) were used as starting materials. [OMIm][BF4] served as a fluoride precursor. The synthesis of [MeBu3N][N(SO2CF3)2] as the ionic liquid was performed based on the reaction of tributylmethylammonium chloride ([N(CH3)(C4H9)3] Cl) and lithium bis(trifluoromethanesulfonyl)imide (Li[N(SO2CF3)2]) following a procedure given elsewhere [17]. All synthesis and heating were performed under inert conditions (argon), especially, to exclude any oxidation of Pr3 þ and/or Mn2 þ . 2.2. CaF2:Pr3 þ , CaF2:Mn2 þ , CaF2:Pr3 þ , Mn2 þ nanoparticles To obtain highly nanocrystalline CaF2:Pr3 þ , CaF2:Mn2 þ and CaF2:Pr3 þ ,Mn2 þ nanoparticles, a microwave-assisted synthesis in

ionic liquids was performed in argon atmosphere. In the first step, 140 mg (1.3 mmol) of CaCl2, 13 mg (0.068 mmol) and of PrCl3  H2O 3.6 mg (0.013 mmol) of MnCl2  4H2O were dissolved in 5 mL of ethanol. The dopant concentration was adjusted to 1 mol% of Pr3 þ and 5 mol% of Mn2 þ . This results in solution A. As solution B, 1 mL of the fluoride precursor [OMIm][BF4] (OMIm: octylmethylimidazolium) and 10 mL of the ionic liquid [Bu3MeN] [N(SO2CF3)2] (Bu3MeN: tributylmethylammonium) were mixed and heated in a microwave oven (MLS rotaprep) to 100 °C. At this temperature, solution A was injected into solution B under vigorous stirring. The instantaneous turbidity of the system indicated the nucleation of a solid compound. Thereafter, ethanol was evaporated so that the preformed, doped CaF2 nanoparticles were present in the pure ionic liquid. Herein, the nanoparticles were rapidly heated via microwave irradiation to 200 °C for 20 min. After cooling, the nanoparticle suspension was diluted with ethanol to reduce the viscosity of the ionic liquid. To remove the ionic liquid, the nanoparticles were repeatedly centrifuged and redispersed in ethanol. Finally, the doped CaF2 nanoparticles were collected as a white powder and dried at 60 °C in air. For analytical characterization, the nanoparticles were applied as suspension in ethanol or as powder samples. 2.3. Analytical methods 2.3.1. Dynamic light scattering (DLS) DLS was performed to obtain particle diameter and particle size distribution of the as-prepared CaF2 suspensions. Measurements were conducted with a Malvern Instruments Nanosizer ZS, equipped with a He–Ne laser (detection via non-invasive backscattering at an angle of 173° 256 detector channels). 2.3.2. Scanning electron microscopy (SEM) SEM was carried out with a Zeiss Supra 40 VP microscope. To this concern, diluted ethanol suspensions of doped CaF2 were deposited on silicon wafers and evaporated. The acceleration voltage was in the range of 5–10 kV and the working distance was 3 mm. Average particle diameters were calculated by statistical evaluation of at least 100 nanoparticles (Scandium 5.0 software package). 2.3.3. Energy-dispersive X-ray analysis (EDX) EDX was performed with an Ametek EDAX device mounted on the above described Zeiss SEM Supra 40 VP scanning electron microscope. For this purpose, the doped CaF2 nanoparticles were pressed to dense pellets to guarantee for a smooth surface and a quasi-infinite layer thickness. These pellets were fixed with conductive carbon pads on aluminum sample holders. EDX was only used to validate the presence and ratio of calcium and the rare earth metals. Lighter elements such as fluorine cannot be reliably quantified via the method. 2.3.4. X-ray powder diffraction (XRD) Phase identification and purity were validated by X-ray powder diffraction (XRD, Stoe STADI-P diffractometer) with a Gemonochromatized Cu-Kα1 radiation (40 kV, 40 mA). 2.3.5. Fourier-transform infrared spectroscopy (FT-IR) Infrared spectroscopy was performed on a Bruker Vertex 70 FTIR spectrometer. The doped CaF2 nanoparticles were pesteled and diluted with KBr (1 mg of sample per 300 mg of KBr) and pressed to pellets. 2.3.6. Luminescence spectroscopy CaF2 belongs to the class of wide band-gap materials, and its experimental band-gap was reported to be around 11.5 eV [18]. To

A. Kuzmanoski et al. / Journal of Luminescence 179 (2016) 555–561

get such excitation energy range, in this paper, we use the synchrotron radiation, which, due to its broad and continuous spectrum, is a very useful tool for the investigation of optical and luminescence properties of wideband-gap materials [19–21], where ultraviolet (UV) and vacuum ultraviolet (VUV) excitations are dominant. The method was also extended to studies of electronic excitations in semiconductor nanocrystals [22] and two dimensional systems [23]. In the present study, luminescence properties of doped CaF2 nanocrystals in the UV-VUV spectral range have been studied with synchrotron radiation from the MAX III storage ring of MAX IV synchrotron. The mobile end station designed for photoluminescence measurements of emission and excitation spectra was installed on the FinEst branch of I3 undulator line. The details of the FINEST branch and I3 beam line were reported elsewhere [24,25]. For emission measurements, the luminescence excitation energy was varied by using a 450 lines/mm Au grating of 6.65 m off-plane Eagle primary monochromator. Vertical polarization was used for excitation. The excitation spectra were recorded in the 225–100 nm (5.5– 12 eV) spectral range with a spectral resolution of 0.3 nm. Excitation spectra of sodium salicylate were normalized to equal synchrotron radiation intensities impinging onto the sample. A LiF filter was applied for excitation energies below 12 eV in order to avoid second order of excitation. An ARC SpectraPro 300i Czerny-Turner type spectrometer was used for the detection of luminescence in the UV– NIR spectral range. The grating 300 l/mm blazed at 500 nm was used. The emission signal was collected by a fiber optic system and delivered to the ARC monochromator. Suitable glass filters were used to suppress second order and stray light. The ARC spectrometer was equipped by the PMT detector (Hamamatsu H8259-01). The spectral resolution of the analyzing monochromator was typically 11 nm. Emission spectra were corrected for the spectral response of the detection system. Powders were slightly pressed into pellets and installed onto the sample holder of a flow-type liquid helium cryostat allowing a temperature range of 7 to 320 K.

3. Results and discussion 3.1. Synthesis and characterization The synthesis of the CaF2:Pr, CaF2:Mn and CaF2:Pr,Mn nanoparticles was performed in ionic liquids (ILs) as liquid reaction media. ILs in recent years have attracted considerable attention for preparing nanoparticles due to their wide liquid range and noncoordinating properties as well as due to their thermal and chemical stability [26]. We have already used IL-based synthesis for preparing LaPO4:Ce,Tb and YVO4:Eu nanoparticles. Due to hightemperature treatment in the IL, highly crystalline nanoparticles were obtained and show high quantum yields like 90% for LaPO4: Ce,Tb or 45% for YVO4:Eu [27,28]. Because of the non-coordinating

557

properties of the ILs, however, microwave heating turned out as optimal for short-timed heating and crystallization at low degree of agglomeration. This strategy of a microwave-supported synthesis in ionic liquids has been here applied for preparing doped CaF2 nanoparticles as well (Fig. 2). In a first step of the synthesis, a solution of [OMIm][BF4] (OMIm: octylmethylimidazolium) as the fluoride precursor in [Bu3MeN][N(SO2CF3)2] (Bu3MeN: tributylmethylammonium) as the ionic liquid was heated to 100 °C. Thereafter, a solution of CaCl2, MnCl2  4H2O and PrCl3  H2O in ethanol was injected. The nucleation of doped CaF2 started immediately as indicated by the occurrence of a slight opalescence. It is to be noted that ethanol was used as a co-solvent to increase the solubility of the metal chloride precursors and to lower the viscosity of the ionic liquid. After nucleation at 100 °C, ethanol was distilled off and the weakly crystalline nanoparticles were crystallized in the pure ionic liquid by heating to 200 °C in the microwave oven (Fig. 2). Synthesis and heating were performed under argon, especially, to exclude any oxidation of Pr3 þ and/or Mn2 þ . Finally, the suspension was naturally cooled to room temperature and again diluted with ethanol as a co-solvent to reduce the viscosity of the IL. Subsequent to centrifugation and suited washing procedures, the colorless nanoparticles were collected and dried. Size and size distribution of the as-prepared doped CaF2 nanoparticles, first of all, were studied in suspension (ethanol) via dynamic light scattering (DLS) (Fig. 3). Here, CaF2:Pr and CaF2:Pr, Mn exhibit a significantly smaller mean diameter (20–50 nm) as compared to CaF2:Mn (50–70 nm). In addition to dynamic light scattering, particle diameter and morphology of the doped CaF2 nanoparticles were also validated by scanning electron microscopy (SEM) (Fig. 3). SEM images show uniform, spherical particles with a diameter of 20–30 nm for CaF2:Pr and CaF2:Pr,Mn. Again, a larger diameter is observed for CaF2:Mn with 30–50 nm (Fig. 3). Thus, the data from DLS and SEM are in good agreement. The systematically larger particle size of CaF2:Mn - despite of identical conditions for particle nucleation and growth – can be attributed to the specific doping. Since Mn2 þ is similarly charged as the regular lattice cation Ca2 þ , almost no charge stabilization occurred during particles growth. In contrast, the excess charge of Pr3 þ – in relation to the regular lattice cation Ca2 þ – leads to certain charge stabilization of the CaF2:Pr and CaF2:Pr,Mn nanoparticles. This effect results in a reduced particle growth, and thus, a smaller particle size. Similar findings are well known from colloid chemistry for the nucleation and growth of particles in water, but they are obviously also relevant in ionic liquids. Composition, crystallinity and purity of the as-prepared, doped CaF2 nanoparticles were demonstrated by X-ray powder diffraction (XRD) (Fig. 4). Accordingly, the nanoparticles adopt the cubic fluorite-type structure. All Bragg peaks match well with reference data of bulk-CaF2. The width of the diffraction peaks indicates the presence of nanoparticles (Fig. 4). A systematic shift of differently

Fig. 2. Scheme illustrating the ionic-liquid-based synthesis of CaF2:Pr3 þ ,Mn2 þ nanoparticles.

558

A. Kuzmanoski et al. / Journal of Luminescence 179 (2016) 555–561

Fig. 3. DLS and SEM images of the as-prepared CaF2:Mn, CaF2:Pr and CaF2:Pr,Mn nanoparticles.

doped samples is not observed, and in fact, not to be expected in view of the broad peak width. Using Scherrer's equation, mean crystallite sizes of 18 and 25 nm were calculated for CaF2:Pr and CaF2:Pr,Mn when considering the most intense Bragg peaks. For CaF2:Mn, a larger mean crystallite size was obtained (49 nm), which is in agreement with the data obtained by DLS and SEM. The chemical composition and purity of the doped CaF2 nanoparticles as well as the average dopant concentration were quantified by energy-dispersive X-ray analysis (EDX). Here, a Ca:F ratio of 1:2 is observed and in accordance with the expected stoichiometry of CaF2. Moreover, the dopant concentration was analyzed and the data are summarized in Table 1. Accordingly, the dopant concentration aimed in the synthesis is well reflected by the analytical data. By comparing the IR spectra of the pure IL used as reaction medium with that of the doped CaF2, the surface conditioning of the nanoparticles can be verified (Fig. 5). Spectra recorded from 4000 to 400 cm  1 show strong vibrations at 3400 and 1600 cm  1 that indicate the presence of ethanol from the particle purification procedure (ν(OH) and δ(ROH)). In addition, only weak absorptions stemming from the ionic liquid [Bu3MeN][N(SO2CF3)2] were observed (ν(CH)¼ 2950–2850 cm  1, fingerprint area ¼ 1450–

500 cm  1). As a result, the ionic liquid as reaction medium was successfully removed by the purification process.

4. Photoluminescence properties To investigate whether energy transfer occurs between Pr3 þ and Mn2 þ , the luminescence properties of singly doped (Pr3 þ or Mn2 þ ) as well as of co-doped (Pr3 þ and Mn2 þ ) CaF2 nanoparticles were studied. CaF2 belongs to the class of wide band-gap materials, and its experimental band-gap is reported to be around 11.5 eV ( 107 nm) [18]. In order to study excitation in this energy range, synchrotron radiation was used, which is a very useful tool for the investigation of luminescence properties of wide band-gap materials [19–21] and nanocrystalline semiconductors [22,23]. Due to the broad and continuous spectrum of the synchrotron source, ultraviolet (UV) and vacuum ultraviolet (VUV) transitions of luminescent materials can be studied with excellent resolution. 4.1. CaF2:Mn2 þ The excitation spectrum of CaF2:Mn2 þ nanoparticles in the VUV spectral range reveals several bands at energies higher than

A. Kuzmanoski et al. / Journal of Luminescence 179 (2016) 555–561

559

Fig. 4. XRD patterns of the as-prepared CaF2:Mn2 þ , CaF2:Pr3 þ and CaF2:Pr3 þ , Mn2 þ nanoparticles (bulk-CaF2 as reference: ICDD-No. 035-0816). Table 1 Composition and dopant concentration of the as-prepared CaF2:Pr3 þ , CaF2:Mn2 þ and CaF2:Pr3 þ ,Mn2 þ nanoparticles according to EDX analysis. Nanoparticles CaF2:Pr3 þ CaF2:Mn

2þ

CaF2:Pr3 þ , Mn2 þ

Ca Pr Ca Mn Ca Pr Mn

Expected ratio

Measured ratio

99 1 95 5 94 1 5

98.7 1.3 94.7 5.3 94.2 1.6 4.2

Fig. 6. Excitation spectra of CaF2:Mn2 þ nanoparticles (λem ¼ 590 nm, top), CaF2: Pr3 þ nanoparticles (λem ¼ 400 nm, middle) and CaF2:Pr3 þ ,Mn2 þ nanoparticles (λem ¼ 590 nm: solid line, λem ¼ 400 nm: dashed line, bottom). The bottom scale in eV corresponds to the energies of the photons with a wavelength at the top axis. All spectra were obtained at 10 K.

Fig. 5. FT-IR spectra of CaF2:Pr3 þ ,Mn2 þ nanoparticles ([Bu3MeN][N(SO2CF3)2] as a reference).

7 eV (177 nm) (Fig. 6: top). The excitation bands leading to 590 nm emission of Mn2 þ are located at 157 nm (7.9 eV) and 145 nm (8.6 eV) and are split by the crystal field of the host lattice in two sublevels. These bands can be assigned to parity forbidden transitions of electrons from the ground state 6S to the high excited state corresponding to the 6D term of the Mn2 þ ion (transitions of the type 3d5-3d44s) [29]. The intense excitation observed at energies higher than 9 eV (139 nm), in fact, is part of a broad excitation band (9–11 eV) observed earlier for bulk-CaF2:Mn2 þ and can be attributed to the charge transfer (CT) transition F(2p6) Mn(3d5)-Mn(3d54s)F(2p5) [29]. In sum, the VUV excitation spectrum of Mn2 þ in nanocrystalline CaF2 is in excellent agreement with the data for bulk-CaF2:Mn2 þ . The absence of excitation bands at energies below 7 eV (177 nm) down to 5.5 eV (225 nm) is noticeable. Obviously, there is no direct excitation leading to Mn2 þ emission in this region.

The emission spectrum of the CaF2:Mn2 þ nanoparticles under VUV excitation contains a broad band peaking at 590 nm (Fig. 7: top). This Mn2 þ emission in CaF2 results from radiative transitions between the levels 4G(4T1g)-6S(A1g) on the Mn2 þ ion located on regular cation sites of the fluorite lattice [29,30]. In comparison to bulk-CaF2, the Mn2 þ emission band is slightly shifted towards longer wavelengths where the position of the Mn2 þ band is known to be at about 500 nm [29,30]. This shift could be due to slightly different crystal field strength of the Mn2 þ ions in nanocrystalline CaF2, which is perturbed by the nanoparticles’ surface. 4.2. CaF2:Pr3 þ While observing the Pr3 þ emission at 400 nm, the excitation spectrum of the CaF2:Pr3 þ nanoparticles shows a single excitation band (Fig. 6: middle). This peak at about 177–190 nm (7.0–6.5 eV) is attributed to the 4f-5d transition on Pr3 þ and exhibits an energy higher than the expected position of the 1S0 level (212 nm or 5.85 eV) in bulk-CaF2:Pr3 þ [29]. Based on such distance between the 4f-5d bands and the 1S0 level, one more condition for quantum cutting is fulfilled [8,29]. The separate 1S0 excitation band is not observed (Fig. 6: middle) because the 1I6-1S0 transition is forbidden according to the total angular momentum selection rule. As a consequence, the underlying excitation band is too weak in intensity. The emission spectrum of the CaF2:Pr3 þ nanoparticles upon excitation at 180 nm (6.89 eV) shows a set of Pr3 þ -lines that are clearly resolved in the spectrum (Fig. 7: middle). The most intense

560

A. Kuzmanoski et al. / Journal of Luminescence 179 (2016) 555–561

Fig. 7. Emission spectra of CaF2:Mn2 þ nanoparticles (λexc ¼145 nm, top), CaF2:Pr3 þ nanoparticles (λexc ¼ 180 nm, middle) and CaF2:Pr3 þ ,Mn2 þ nanoparticles (λexc ¼ 180 nm, bottom). The top scale in eV corresponds to the energies of the photons with a wavelength at the bottom axis. All spectra were obtained at 10 K.

co-doped CaF2:Pr3 þ ,Mn2 þ shows an additional strong excitation band from 177 nm (7 eV) to 225 nm (5.5 eV) compared to the excitation spectrum of the singly doped CaF2:Mn2 þ discussed above (Fig. 6: top and bottom). The position of this strong additional excitation band is more-or-less similar to the 4f-5d excitation band of Pr3 þ (Fig. 6: middle and bottom). Several aspects of the CaF2:Pr3 þ ,Mn2 þ system are promising in view of two-photon emission via the Pr3 þ –Mn2 þ photon cascade (Fig. 1): (i) The 1S0 level on Pr3 þ is located at lower energy as the 5d band; (ii) Energy transfer under 1S0-1I6 relaxation on Pr3 þ and 4 G(4T1g)-6S(A1g) excitation on Mn2 þ is observed. While excitation and emission spectra clearly evidence the Pr3 þ -Mn2 þ energy transfer, proving the two-photon emission is more difficult. Obviously, the intensity of the 1S0-1I6 emission on Pr3 þ is significantly reduced for CaF2:Pr3 þ ,Mn2 þ in comparison to CaF2:Pr3 þ (Fig. 6: middle and bottom). This decrease of the 1S0-1I6 intensity of Pr3 þ is in favor of intense 4G(4T1g)-6S(A1g) emission of Mn2 þ . Taking the energy-level scheme of the Pr3 þ -Mn2 þ couple into account (Fig. 1), this is an evitable first step of the quantumcutting cascade. The fact that 1S0-1I6 emission is still visible at low intensity for CaF2:Pr3 þ ,Mn2 þ can be ascribed to a partly incomplete energy transfer that could be improved by precise adjustment of the Pr3 þ and Mn2 þ ratio and/or concentration levels. Beside this first step of the quantum-cutting cascade, 3 P0-3H4 emission of Pr3 þ is expected (Fig. 1). 3P0-3H4 emission is indeed observed for CaF2:Pr3 þ as well as for CaF2:Pr3 þ ,Mn2 þ peaking at 485 nm (Fig. 6: middle and bottom, indicated by *). Both spectra show the emission at similar but low intensity. This indicates that the quantum-cutting cascade is indeed active. The low intensity of the 3P0-3H4 transition can be ascribed, on the one hand, to the conflicting total angular momentum selection rule, and on the other hand, to non-emissive relaxation because of the defective nanoparticles.

5. Conclusions emission band at 400 nm relates to 1S0-1I6 emission. The occurrence of this transition is prerequisite for successful energy transfer to Mn2 þ [8]. Other weak emission bands are observed at about 280 nm (1S0-1G4 transition), 335 nm (1S0-1D2 transition), 480 nm (3P0-3H4 transition), 530 nm (3P0-3H5 transition) and 590 nm (3P0-3H6 transition). All these emission lines are well known for bulk-CaF2:Pr3 þ as well [31]. 4.3. CaF2:Pr3 þ ,Mn2 þ The excitation and emission spectra of the CaF2:Pr3 þ ,Mn2 þ nanoparticles, in general, are very comparable to the respective spectra of CaF2:Mn2 þ and CaF2:Pr3 þ (Figs. 6 and 7: bottom). Thus, the emission spectrum of CaF2:Pr3 þ ,Mn2 þ under excitation at 180 nm (6.89 eV), which corresponds to the 4f-5d excitation band of Pr3 þ clearly shows two main emission bands (Fig. 7: bottom): a comparably weak band at 400 nm and a much stronger broad band at 590 nm. Comparing this emission spectrum with spectra of the CaF2:Mn2 þ and CaF2:Pr3 þ nanoparticles (Fig. 7: top and middle), we can conclude that emission from both ions Pr3 þ and Mn2 þ occurs. Thus, after exciting of Pr3 þ , a significant part of the energy is transferred to Mn2 þ resulting in the 590 nm emission. This finding clearly validates successful energy transfer from Pr3 þ to Mn2 þ . This finding is further confirmed by the excitation spectrum of the CaF2:Pr3 þ ,Mn2 þ nanoparticles (Fig. 6: bottom). Indeed, the excitation spectrum for Pr3 þ emission (1S0-1I6) in the co-doped CaF2:Pr3 þ ,Mn2 þ is identical to the singly doped CaF2: Pr3 þ (Fig. 6: middle and bottom). On the other hand, the excitation spectrum for Mn2 þ emission (4G(4T1g)-6S(A1g) transition) in the

CaF2:Pr, CaF2:Mn and CaF2:Pr,Mn nanoparticles with a particle diameter of 20–30 nm (CaF2:Pr, CaF2:Pr,Mn) and 30–50 nm (CaF2: Mn) were prepared via microwave-mediated synthesis in ionic liquids. The nanoparticles are highly crystalline and contain the dopants in concentrations of 1 mol-% of Pr3 þ and 5 mol% of Mn2 þ only or of both ions together. Excitation and emission spectra prove sufficient energy transfer between Pr3 þ and Mn2 þ under 1 S0-1I6 relaxation on Pr3 þ and 4G(4T1g)-6S(A1g) excitation on Mn2 þ . In fact, this energy transfer relates to the essential first process of the quantum-cutting cascade via the Pr3 þ –Mn2 þ couple. Such energy transfer is here observed for the first time. Moreover, 4G(4T1g)-6S(A1g) emission of Mn2 þ as well as 3P0-3H4 emission of Pr3 þ are observed and point to two-photon emission. Till now Pr3 þ -Mn2 þ energy transfer as the first step of the quantum-cutting cascade could not be observed independent of the applied dopant concentration, temperature, and type of bulklattice material. Since sufficient spectral overlap of the underlying emission of Pr3 þ and excitation of Mn2 þ was proven, the lacking energy transfer is surprising and was ascribed to selection rules that prohibit an exchange-induced energy transfer. Energy transfer between Pr3 þ and Mn2 þ can involve a change of spin for both ions, what is expected to be a limiting factor. In addition, there might not be a Mn2 þ site in the bulk material having the appropriate symmetry to mediate the exchange interaction. Most selection rules, however, can be lifted partially if states of the appropriate symmetry are mixed in, in order to introduce odd parity. In view of the here addressed Pr3 þ –Mn2 þ couple, Meijerink et al. have previously studied the energy-transfer in depth in view

A. Kuzmanoski et al. / Journal of Luminescence 179 (2016) 555–561

of different fluoride lattices, different concentrations of Pr3 þ and Mn2 þ and different temperatures [8,10,11]. Although there is almost ideal spectral overlap of Pr3 þ and Mn2 þ not any Pr3 þ Mn2 þ energy transfer was observed. If the energy transfer would have been limited by the spin selection rule or the parity selection rule, at least some energy transfer would have been observed in the one or other lattice and/or concentration. On the other hand, it is well known that spin–orbit interactions become strong for rareearth ions [3,19]. For nanoparticles, moreover, it has been frequently reported that spin–orbit interactions can become even stronger than for the bulk materials [32–34]. As a consequence, spin–orbit interactions can be the background for the observation of Pr3 þ -Mn2 þ energy transfer in CaF2:Pr3 þ ,Mn2 þ nanoparticles. Moreover, in nanoparticles more pair formation can expected, which might be favorable because the volume is small and the surface states will have a different, probably higher, energy. Thus, instead of an even distribution, pair formation in the small particles may occur. This could also improve energy transfer, either by very close distances, or by cooperative two-ion processes.

[3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

Acknowledgments

[23] [24]

A.K. and C.F. thank the Deutsche Forschungsgemeinschaft for funding in the Priority Program SPP1708 “Synthesis near room temperature” for funding. V.P. acknowledges the financial support from University of Oulu Strategic Funding and Research Council of Natural Sciences of the Academy of Finland. The research leading to these results received funding from the European Community's Seventh Framework Program (FP7/2007–2013) CALIPSO under Grant agreement no. 312284. The authors thank also the crew of the MAX-IV laboratory for their support during the beam time operation.

References [1] Q.Y. Zhang, X.Y. Huang, Prog. Mater. Sci. 55 (2010) 353 (Review). [2] C. Feldmann, T. Jüstel, C.R. Ronda, P.J. Schmidt, Adv. Funct. Mater. 13 (2003) 511 (Review).

[25]

[26] [27] [28] [29] [30] [31] [32]

[33] [34]

561

C. Ronda, J. Lumin. 100 (2002) 301 (Review). J.L. Sommerdijk, A. Bril, A.W. de Jager, J. Lumin. 8 (1974) 341. W.W. Piper, J.A. DeLuca, F.S. Ham, J. Lumin. 8 (1974) 344. R.T. Wegh, H. Donker, K.D. Oskam, A. Meijerink, Science 283 (1999) 663. R.T. Wegh, H. Donker, E.V.D. van Loef, K.D. Oskam, A. Meijerink, J. Lumin. 87– 89 (2000) 1017. A. Meijerink, R.T. Wegh, P. Vergeer, T. Vlugt, Opt. Mater. 28 (2006) 575. K.D. Oskam, R.T. Wegh, H. Donker, E.V.D. van Loef, A. Meijerink, J. Alloy. Compd. 300–301 (2000) 421. S. Kueck, I. Sokólska, J. Phys. Cond. Matter 18 (2006) 5447. P.S. Peijzel, A. Meijerink, R.T. Wegh, M.F. Reid, G.W. Burdick, J. Solid State Chem. 178 (2005) 448. C. de Mello, A. Meijerink, G. Blasse, J. Phys. Chem. Sol. 56 (1995) 673. A.M. Srivastava, D.A. Doughty, W.W. Beers, J. Electrochem. Soc. 143 (1996) 4113. A.M. Srivastava, W.W. Beers, J. Lumin. 71 (1997) 285. K.D. Oskam, A.J. Houtepen, A. Meijerink, J. Lumin. 97 (1994) 107. Q.Y. Zhang, X.Y. Huang, Prog. Mater. Sci. 55 (2010) 353. T. Welton, Chem. Rev. 99 (1999) 2071. G.W. Rubloff, Phys. Rev. B 5 (1972) 662. G. Blasse, J. Alloy. Comp. 225 (1995) 529. R.T. Wegh, A. Meijerink, R.-J. Lamminmaki, J. Holsa, J. Lumin. 87-89 (2000) 1002. V. Pankratov, A.I. Popov, L. Shirmane, A. Kotlov, C. Feldmann, J. Appl. Phys. 110 (2011) 053522/1-053522/7. V. Pankratov, V. Osinniy, A. Kotlov, A. Nylandsted Larsen, B. Bech Nielsen, Phys. Rev. B 83 (2011) 045308/1-045308/5. V. Pankratov, J. Hoszowska, J.-C. Dousse, J. Phys. : Condens. Matter 28 (2016) 01530. S. Urpelainen, M. Huttula, T. Balasubramanian, R. Sankari, P. Kovala, E. Kukk, E. Nõmmiste, S. Aksela, R. Nyholm, H. Aksela, in: Proceedings of the AIP Conference Proceedings, 1234, 2010, pp. 411–414. T. Balasubramanian, B.N. Jensen, S. Urpelainen, B. Sommarin, U. Johansson, M. Huttula, R. Sankari, E. Nõmmiste, S. Aksela, H. Aksela, R. Nyholm, in: Proceedings of the AIP Conference Proceedings, 1223, 2010, pp. 661–664. D. Freudenmann, S. Wolf, M. Wolff, C. Feldmann, Angew. Chem. Int. Ed. 50 (2011) 11050. G. Bühler, C. Feldmann, Angew. Chem. Int. Ed. 45 (2006) 4864. A. Zharkouskaya, H. Lünsdorf, C. Feldmann, J. Mater. Sci. 44 (2009) 3936. V.P. Denks, M.P. Kerikmyaé, A.L. Lust, T.I. Savikhina, Phys. Solid State 42 (2000) 261. P.J. Alonso, R. Alcala, J. Lumin. 22 (1981) 321. R. Pappalardo, J. Lumin. 14 (1976) 159. G. Akhgar, O. Klochan, L.H. Willems van Beveren, M.T. Edmonds, F. Maier, B. J. Spencer, J.C. McCallum, L. Ley, A.R. Hamilton, C.I. Pakes, Nano Lett. 16 (2016) 3768. J. Huang, Y. Hou, C. Liu, L. Jing, T. Ma, X. Sun, M. Gao, Chem. Mater. 27 (2015) 7918. S.R. Naik, A.V. Salker, J. Mater. Chem. 22 (2012) 2740.