Luminescence and energy transfer in β-NaGdF₄:Eu³⁺,Er³⁺ nanocrystalline samples from a room temperature synthesis

Abstract

Phase pure hexagonal β-NaGdF₄:Eu³⁺,Er³⁺ samples of less than 10 nm in crystallite size were synthesized in anhydrous ethylene glycol within 24 hours at room temperature. The materials were characterized by powder X-ray diffraction, electron microscopy, and luminescence spectroscopy. After Gd³⁺ excitation at 273 nm Eu³⁺ luminescence from the ⁵D_J states dominates over Gd^{3+ 6}P_J emission and only weak Er³⁺ emissions could be observed due to strong Gd→Eu and weak Gd→Er energy transfer. Excitation at 377 nm results in Er^{3+ 2}H_9/2 and Eu^{3+ 5}D_J emissions. The concentration dependence of the Er³⁺ and Eu³⁺ luminescence intensities points to independent behavior of the dopant ions without Er→Eu or Eu→Er energy transfer. Eu³⁺ excitation at 394 nm results in Eu^{3+ 5}D_J emissions only and confirms the absence of Eu→Er energy transfer. Eu³⁺ luminescence from the ⁵D_J states prevails for all investigated excitations. The optimum Eu³⁺ doping level is close to 5%. Significant Er³⁺ luminescence was observed for 377 nm excitation only. The strong ²H_9/2 and weak ⁴S_3/2 Er³⁺ emissions indicate small losses by multiphonon relaxation and a good quality of the nanomaterial. Er co-doping hinders the energy migration on the Gd³⁺ and Eu³⁺ sublattices which results in higher Gd³⁺/Eu³⁺ and Eu^{3+ 5}D₁/⁵D₀ emission ratios.

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1. Introduction

Hexagonal β-NaREF₄ (RE³⁺ = rare earth) is an excellent host lattice for luminescent materials.^1–6 The rare earth ions occupy two sites with nine-fold coordination, one of them half occupied by Na⁺.⁷ The absence of an inversion centre on the rare earth sites and the microscopic disorder give rise to line broadening and an enhanced spectroscopic overlap. The high coordination number of nine results in long RE–F distances and a low effective phonon energy which decreases the multiphonon relaxation in this lattice.

Synthesis of phase pure β-NaREF₄ nanomaterials is challenging. At high temperatures, β-NaREF₄ crystallizes from solutions in a two-step process according to the Ostwald phase rule.⁸ First, metastable cubic α-NaREF₄ is formed which may transform to the thermodynamically stable β-NaREF₄ or persist as a metastable, kinetically stabilized phase. Luminescence from α-NaREF₄ is significantly weaker than from β-NaREF₄.⁹ The thermodynamic stability of β-NaREF₄ decreases for smaller RE³⁺ ionic radii.¹⁰ Thus, the synthesis of highly luminescent material requires an accurate optimization in order to obtain phase pure β-NaREF₄.

Several synthetic pathways are reported in the literature which show a high selectivity for the β-NaREF₄ phase and a good size control of the nanoparticles.^11–14 Most commonly, octadecene is used as a solvent with oleic acid or oleylamine as surfactants and the synthesis is performed at high temperatures. Recently, room temperature syntheses of fluoride nanoparticles were investigated.^15–19 They provide several advantages since they can be performed in conventional solvents, e.g. water, ethylene glycol, or diethylene glycol, and require no heating. Several room temperature syntheses for phase pure β-NaREF₄ nanoparticles were reported.^15,16,19 However, the obtained particle size distribution is often broad due to a weak size control. With the help of surfactants it is possible to improve the size control. Ionic liquids were successfully employed in the room temperature synthesis of β-NaGdF₄:Eu³⁺ nanoparticles with 2 nm crystallite size in aqueous solutions.¹⁶ A small crystallite size was recently achieved for the synthesis of β-NaGdF₄:Eu³⁺ and Er³⁺ nanocrystalline samples in ethylene glycol¹⁹ and KGdF₄:Ce³⁺,Ln³⁺ (Ln = Tb or Eu) nanoparticles in water and diethylene glycol solutions.¹⁸ The nanocrystallites obtained from both syntheses form aggregates smaller than 100 nm. The crystallite size is less than 10 nm without addition of further surfactants, e.g. ionic liquids. It was observed that the F⁻ concentration from NH₄F in the ethylene glycol synthesis affects the aggregate size.¹⁹ Here, we extend the room temperature synthesis in ethylene glycol,¹⁹ developed by us towards β-NaGdF₄:Eu³⁺,Er³⁺ co-doped nanoparticles. The absence of water during the synthesis constitutes a distinct advantage for the synthesis of luminescence materials. It minimizes OH⁻ impurities and the luminescence quenching resulting thereof. A synthesis under dry conditions is also beneficial for a good repeatability of the results, as suggested by comparison with the literature.¹⁹

Phosphor materials with Gd³⁺, Eu³⁺, and Er³⁺ are interesting in several aspects. LiGdF₄:Eu^3+ 20 and LiGdF₄:Er^3+ 21 are quantum cutters. For Gd³⁺ or Er³⁺ excitation, one high energy photon in the VUV can be converted in two lower energy photons. Also, β-NaGdF₄:Eu³⁺ shows quantum cutting.^16,22,23 However, the excitation energy of 6.1 eV is too low for Xe (7.3 eV) and too high for Hg discharge lamps (4.9 eV) which limits potential applications.²⁴ For LiGdF₄:Er³⁺ the dense energy level diagram of Er³⁺, the efficient energy migration between Gd³⁺ ions, and the weak absorption of the Gd^{3+ 6}P_J→⁸S_7/2 emission by Er³⁺ ions reduces the quantum cutting efficiency.^21,25 Ternary systems (with respect to the rare earth ion) may enhance the energy transfer from Gd³⁺ ions. The best known nanomaterial is α-NaGdF₄:1.5% Er³⁺,0.3% Tb³⁺.²⁶ LiGdF₄:Er³⁺,Tb³⁺ might be feasible as phosphor for Xe discharge lamps.²⁴

The development of LEDs reduced the need for lamp phosphors.^27–29 UV-A LEDs show efficient emission around 365 nm, which can excite white emission from Er³⁺, Eu³⁺ co-doped materials.^30–32 Another pathway for white emission focuses on upconversion in Yb³⁺, Er³⁺, Eu³⁺ co-doped materials.^33–38 Between 360–380 nm, Er³⁺ has a strong ⁴I_15/2→⁴G_11/2 absorption at 377 nm and a weaker ⁴I_15/2→⁴G_9/2 and ²K_J absorption at 364 nm,³⁹ which may result in blue and green Er³⁺ emissions. In combination with red emission from the Eu^{3+ 5}D_J states, white light may be obtained. β-NaGdF₄:Eu³⁺,Er³⁺ co-doped nanoparticles can also be excited via Gd³⁺ at 273 nm. Here, we investigate the luminescence of β-NaGdF₄:Eu³⁺,Er³⁺ co-doped nanoparticles upon UV excitation of Gd³⁺, Er³⁺, and Eu³⁺ ions.

2. Experimental

A room temperature synthesis of hexagonal β-NaGdF₄ nanocrystallites in ethylene glycol was recently reported by the authors.¹⁹ This synthesis was modified and optimized for Eu³⁺ and Er³⁺ co-doped β-NaGdF₄ nanocrystallites in the present work. The fluoride nanomaterials were prepared from rare earth acetates, NaCl, and NH₄F in anhydrous ethylene glycol. Anhydrous rare earth acetates were synthesized from the respective rare earth oxides RE₂O₃ (Eu₂O₃, Gd₂O₃, Er₂O₃, Metall Rare Earth Limited, 5 N), see Experimental section with powder X-ray diffraction (XRD) patterns and thermogravimetric analyses in Fig. S1 and S2 of ESI,† respectively. The anhydrous acetates were stored in a dry nitrogen glovebox. Four different 1.1 M rare earth acetate mother solutions were prepared in anhydrous ethylene glycol (EG) (Sigma Aldrich, 99.8%) containing Gd³⁺, 1% or 5% of Eu³⁺ or Er³⁺, and NaCl (Merck, reagent grade), see Table S1 of ESI.† A 10.6 wt% solution of NH₄F (Alfa Aesar, 96%) in EG was used as fluoride source. Two series of materials were prepared with Gd : Eu + Er molar ratios of 99 : 1 or 95 : 5. The Eu : Er molar ratios were 1 : 0, 0.8 : 0.2, 0.6 : 0.4, 0.4 : 0.6, 0.2 : 0.8, and 0 : 1 for 1% doped samples or 5 : 0, 4 : 1, 3 : 2, 2 : 3, 1 : 4, and 0 : 5 for 5% doped samples. The Na : RE molar ratio of the reaction mixture was 2 : 1 for all samples. The reaction solutions were prepared from the mother solutions as indicated in Table S1 (ESI†). In a typical reaction, 0.5 mL of rare earth acetate solution was mixed with 2.5 mL of NH₄F solution under stirring for 24 hours in a nitrogen atmosphere. This corresponds to a 12-fold molar excess of fluoride and a 9 wt% NH₄F in the total 3 mL EG. The nanoparticles were separated by centrifugation at 3000 rpm for 5 minutes. The product was washed three times with technical grade methanol and ethanol. After each washing step the nanoparticles were collected by centrifugation. Finally, the product was dried under vacuum.

Samples were characterized by powder XRD and transmission electron microscopy (TEM) with respect to phase purity and size, respectively. Luminescence and excitation spectra were measured from the powder samples, see Experimental section of ESI.†

3. Results and discussion

Nanocrystalline hexagonal β-NaGdF₄:x% Eu³⁺,y% Er³⁺ samples were obtained in high phase purity at room temperature, as it is demonstrated by the XRD patterns in Fig. 1 for various Eu³⁺ and Er³⁺ concentrations. The calculated β-NaGdF₄ peak positions are reported at the bottom of Fig. 1 as blue ticks. The diffraction peaks are indexed by the hexagonal β-NaGdF₄ phase crystallizing with space group P6̄. The refined lattice parameters are a = 6.0304(9) Å and c = 3.6111(7).⁷ Two sharp impurity lines at diffraction angles of 38.8° and 56.1° are readily assigned to the (200) and (220) reflections of NaF⁴⁰ and marked by red asterisks in Fig. 1. The absence of the strongest (111) diffraction peak of the cubic α-NaGdF₄ phase at 28.1° confirms the absence of the cubic NaGdF₄ polymorph in all samples. The spectroscopic response is not altered by the NaF impurity, which only dilutes the samples. On the contrary, repeated washings or the use of water for a complete NaF removal can influence the sample purity and quench a large share of the luminescence.

Fig. 1 XRD patterns of β-NaGdF₄:x% Eu³⁺,y% Er³⁺ nanocrystalline samples with x = 0, 0.2, 0.4, 0.6, 0.8, 1, and y = 1 − x (A) and x = 0, 1, 2, 3, 4, 5, and y = 5 − x (B). Patterns were measured with CuK_α1 radiation at room temperature. The calculated peak positions for β-NaGdF₄ are shown as blue ticks at the bottom. Two sharp peaks from a NaF impurity are marked by red asterisks.

The broad width of the XRD peaks indicates the nanometer particle size of the materials. From the line width the crystallite size of the β-NaGdF₄:Eu³⁺,Er³⁺ samples was estimated by the Scherrer equation:

where K = 0.92, λ = 1.5406 Å is the wavelength of CuK_α1 radiation, FWHM is the full-width at half-maximum of the diffraction peak in radians, and 2θ is the diffraction angle. The calculated crystallite size ranges between 2 and 8 nm for all samples. This is consistent with a crystallite size below 10 nm as derived from TEM images, see Fig. 2. The crystallite size shrinks with increasing Er³⁺ doping, which is consistent with a decreasing phase stability of the hexagonal β-NaREF₄ phase towards smaller radii of the rare earth ions.¹⁰

Fig. 2 TEM micrographs of a β-NaGdF₄:5% Eu³⁺ nanocrystalline sample from room temperature synthesis. The scale bar at the bottom of the pictures indicates distances of 200 nm (A), 50 nm (B), and 20 nm (C), respectively.

TEM micrographs provide more details on the nanocrystal size and shape. Fig. 2 shows an overview on the particle size of β-NaGdF₄:5% Eu³⁺. Fig. S3 of ESI† depicts TEM micrographs of further representative samples. As proven by XRD, the crystallite size is below 10 nm, see Fig. 2C. The nanocrystals form larger aggregates upon drying. Their size ranges between 50 and 100 nm, see Fig. 2. The aggregate size increases with the NH₄F concentration from 3 to 9 wt% in the EG solutions.¹⁹ The optimum concentration of 9 wt% NH₄F for a pure hexagonal phase and improved luminescence¹⁹ was used in this work for all samples.

Individual nanocrystals of 2–3 nm size are shown in Fig. 2C. The dopant concentration has no detectable influence on the crystallite size. When up to 5% Eu³⁺ ions substitute for Gd³⁺, only a negligible influence on the stability of the hexagonal phase is expected as their ionic radii are very close (1.120 Å and 1.107 Å, respectively, for a nine-fold coordination).⁴¹ The smaller Er³⁺ ions (1.062 Å)⁴¹ can induce a variation in crystallite size and shape due to the decreasing stability of the hexagonal phase along the trivalent rare earth cation series. However, within error limits, no change in crystallite size was detectable from the TEM micrographs of the β-NaGdF₄:Eu³⁺,Er³⁺ nanocrystallites. Comparable aggregates of particles were observed in the room temperature synthesis of KGdF₄:Ce³⁺/Ln³⁺ in water with diethylene glycol as co-solvent.¹⁸

3.1. β-NaGdF₄:Eu³⁺ and β-NaGdF₄:Er³⁺ nanocrystallites

Emission spectra of β-NaGdF₄:Eu³⁺ and β-NaGdF₄:Er³⁺ nanocrystallites are shown in Fig. 3 together with the energy level diagrams of Eu³⁺, Gd³⁺, and Er³⁺. Excitation and emission transitions are indicated by arrows in the diagrams. All observed bands are assigned to 4f–4f intraconfigurational transitions of the respective rare earth ion. The spectra were collected for dopant concentrations of 1% and 5%.

Fig. 3 Emission spectra of β-NaGdF₄:1% and 5% Eu³⁺ nanocrystalline samples are shown on the left for 273 nm Gd³⁺ (black trace) and 395 nm Eu³⁺ excitation (red trace), respectively. Emission spectra of β-NaGdF₄:1% and 5% Er³⁺ nanocrystalline samples are shown on the right for 273 nm Gd³⁺ (black trace) and 377 nm Er³⁺ excitation (green trace), respectively. In the middle, the main transitions are indicated in energy level schemes for Eu³⁺ (red), Gd³⁺ (purple), and Er³⁺ (green) ions. Presentation of possible multiphonon relaxation and energy transfer pathways are omitted for clarity.

After Gd^{3+ 8}S_7/2→⁶I_J excitation at 36 630 cm⁻¹ (273 nm), β-NaGdF₄:1% Eu³⁺ shows the typical Eu³⁺ emission bands due to the ⁵D_J→⁷F_J transitions between 14 000 and 19 000 cm⁻¹. The intensity of the Gd^{3+ 6}P_7/2→⁸S_7/2 emission band at 32 260 cm⁻¹ (310 nm) is weak due to an efficient Gd³⁺→Eu³⁺ energy transfer.

For direct Eu^{3+ 7}F₀→⁵L₆ excitation at 25 380 cm⁻¹ (394 nm), β-NaGdF₄:5% Eu³⁺ shows identical Eu³⁺ emissions as for Gd³⁺ excitation, see the traces on the left hand side of Fig. 3. The Eu^{3+ 5}D₀→⁷F₂ emission at 16 260 cm⁻¹ (615 nm) has the strongest intensity. Emissions are detected from the ⁵D₀ and ⁵D₁ states. Most prominent are the ⁵D₀→⁷F_J transitions with J = 1 and 4 at 16 920 cm⁻¹ (591 nm) and 14 368 cm⁻¹ (696 nm), respectively. Significantly less intensity is observed for the ⁵D₁→⁷F_J transitions. They will be addressed in more detail in the following sections. The emission spectra are comparable to those reported in the literature for β-NaGdF₄:Eu³⁺ nanoparticles^{9,16,22,42,43} and bulk material.^6,23

For Gd³⁺ excitation at 273 nm, β-NaGdF₄:1% Er³⁺ shows predominantly Gd^{3+ 6}P_7/2→⁸S_7/2 emission at 32 260 cm⁻¹ (310 nm) and minor Er³⁺ emissions, see the right hand side of Fig. 3. The Gd³⁺→Er³⁺ energy transfer is less efficient than for Gd³⁺→Eu³⁺, in good agreement with reports in literature.^20,21 The Er³⁺ emissions range from 17 500 to 26 500 cm⁻¹. Er^{3+ 2}P_3/2→⁴I_13/2 and ²P_3/2→⁴I_11/2 emissions are observed at 25 063 cm⁻¹ (399 nm) and 21 368 cm⁻¹ (468 nm), respectively. The Er^{3+ 2}P_3/2→⁴I_15/2 transition is visible only as a shoulder of the Gd^{3+ 6}P_7/2→⁸S_7/2 band. The band at 24 510 cm⁻¹ (408 nm) is assigned to the ²H_9/2→⁴I_15/2 transition, while only minor emissions from the ²H_11/2 and ⁴S_3/2 states are recorded at 18 349 cm⁻¹ (545 nm) and 19 011 cm⁻¹ (526 nm), respectively. A direct Er³⁺ excitation of β-NaGdF₄:5% Er³⁺ into the ⁴G_11/2 state at 26 525 cm⁻¹ (377 nm) results in stronger Er³⁺ emissions, see the green trace on the right hand side of Fig. 3. The ²H_9/2→⁴I_15/2 and ²H_9/2→⁴I_13/2 transitions show the highest intensities at 24 510 cm⁻¹ (408 nm) and 17 986 cm⁻¹ (556 nm), respectively. Also the green emissions ²H_11/2 and ⁴S_3/2→⁴I_15/2 increase in intensity. For comparison, emission spectra of bulk β-NaGdF₄:5% Er³⁺ with micrometer particle size are shown in Fig. S4 of ESI† for Gd³⁺ and Er³⁺ excitation, as well as normalized luminescence spectra of bulk β-NaGdF₄ with 1% and 5% Er³⁺ in Fig. S5 (ESI†).

Fig. 4 shows excitation spectra of nanocrystalline β-NaGdF₄:Eu³⁺ and Er³⁺ samples with 1% and 5% doping monitoring the Eu³⁺ and Er³⁺ emissions, respectively. Spectra are normalized to the Gd^{3+ 8}S_7/2→⁶I_J transition at 36 630 cm⁻¹ (273 nm). Absorption and emission via Eu³⁺ and Er³⁺ dopant bands increase with the doping level and allow a clear distinction from Gd³⁺ bands. In Fig. 4A the Eu^{3+ 5}D₀→⁷F₂ emission at 16 260 cm⁻¹ (615 nm) is monitored. Bands are assigned to direct Eu³⁺ as well as Gd³⁺ excitations followed by Gd³⁺→Eu³⁺ energy transfer. Eu³⁺ excitation bands range from 24 500 to 28 000 cm⁻¹ and 30 000 to 35 500 cm⁻¹. The intensity of the Eu³⁺ emission roughly doubles when the doping is increased from 1% to 5% Eu³⁺, in good agreement with an enhanced absorption according to the Lambert–Beer law. The prominent Eu^{3+ 7}F₀→⁵L₆ excitation is observed at 25 380 cm⁻¹ (394 nm). The strongest excitation band at 36 630 cm⁻¹ (273 nm) is attributed to the Gd^{3+ 8}S_7/2→⁶I_J transition. Weaker Gd³⁺ excitations are due to ⁸S_7/2→⁶P_J transitions at 32 468 cm⁻¹ (308 nm) and 32 787 cm⁻¹ (305 nm), reflecting the lower absorption coefficients of ⁶P_J compared to ⁶I_J states.³⁹

Fig. 4 (A) Normalized excitation spectra of the Eu^{3+ 5}D₀→⁷F₂ emission at 615 nm (16 260 cm⁻¹) for β-NaGdF₄:1% (black trace) and 5% Eu³⁺ (red trace) nanocrystalline samples. (B) Normalized excitation spectra of the Er^{3+ 4}S_3/2→⁴I_15/2 emission at 545 nm (18 349 cm⁻¹) for β-NaGdF₄:1% (black trace) and 5% Er³⁺ (green trace) nanocrystalline samples. Spectra are normalized to the Gd^{3+ 8}S_7/2→⁶I_J transition at 36 630 cm⁻¹ (273 nm), marked by an asterisk.

Normalized excitation spectra of the Er^{3+ 4}S_3/2→⁴I_15/2 emission at 18 349 cm⁻¹ (545 nm) in β-NaGdF₄ doped with 1 and 5% Er³⁺ are depicted in Fig. 4B. The low intensity of the Er³⁺ emission required a wider slit width which reduces the resolution of these spectra and results in broader peak width compared to Fig. 4A. The Er³⁺ excitation bands range from 26 000 to 28 000 cm⁻¹. Further bands at higher energy were too weak to be detected. The two Er³⁺ excitations at 26 525 cm⁻¹ (377 nm) and 27 473 cm⁻¹ (364 nm) originate from ⁴I_15/2→⁴G_J and ²K_J transitions with ⁴I_15/2→⁴G_11/2 being strongest. Increasing the doping from 1% to 5% Er³⁺ results in a distinct enhancement of the direct Er³⁺ excitations. The weaker emission from Er³⁺ compared to Eu³⁺ doped samples indicates smaller absorption coefficients, an increased non-radiative relaxation due to smaller energy gaps between the excited states of Er³⁺, and a less efficient energy transfer from Gd³⁺.

3.2. β-NaGdF₄:Eu³⁺,Er³⁺ co-doped nanoparticles

3.2.1. Gd³⁺ excitation at 273 nm. Emission spectra of β-NaGdF₄:Eu³⁺,Er³⁺ co-doped nanocrystallites are shown in Fig. 5 for Gd³⁺ excitation at 36 630 cm⁻¹ (273 nm). The spectra are normalized to the strongest Eu³⁺ emission ⁵D₀→⁷F₂ at 16’260 cm⁻¹ (615 nm). The Eu³⁺, Gd³⁺, and Er³⁺ emissions were already addressed in Section 3.1.

Fig. 5 Normalized luminescence spectra of β-NaGdF₄:x% Eu³⁺,y% Er³⁺ nanocrystalline samples for Gd³⁺ excitation at 273 nm (36 630 cm⁻¹). Spectra are normalized to the Eu^{3+ 5}D₀→⁷F₂ emission at 615 nm (16 260 cm⁻¹), marked by an asterisk.

The intensity distribution among the Eu³⁺ emissions is close to identical for all the spectra shown in Fig. 5. Small variations of the ⁵D₁vs.⁵D₀ emission intensities are discussed in more detail with Fig. 6. Most obvious in Fig. 5 is a strong decrease of the Gd³⁺ emission intensity with increasing Eu³⁺ doping. The changes of the luminescence intensities with doping are resolved in Fig. 7.

Fig. 6 Expanded Eu³⁺ luminescence spectra from Fig. 5. Spectra are normalized to the Eu^{3+ 5}D₀→⁷F₂ emission at 615 nm (16 260 cm⁻¹), marked by an asterisk.

Fig. 7 Integrated relative intensity of the Eu^{3+ 5}D₀→⁷F₂ (red square), Gd^{3+ 6}P_J→⁸S_7/2 (purple circle), and Er^{3+ 2}P_3/2→⁴I_13/2 (green triangle) luminescence of the β-NaGdF₄:x% Eu³⁺,y% Er³⁺ nanocrystalline samples shown in Fig. 5. The empty (filled) symbols refer to 1% (5%) total doping with x% Eu³⁺ and y = 1 − x (y = 5 − x)% Er³⁺. Gd³⁺ was excited at 273 nm.

The Eu³⁺ emissions from the ⁵D₀ and ⁵D₁ to the ⁷F_J states are labelled in Fig. 6. The ⁵D₁→⁷F_J transitions with J = 0, 1, 2, and 3 are also clearly assigned. They correspond to the bands at 19 011 cm⁻¹ (526 nm), 18 657 cm⁻¹ (536 nm), 18 018 cm⁻¹ (555 nm), and 17 123 cm⁻¹ (584 nm). Further weaker bands are the ⁵D₀→⁷F₀ and ⁵D₀→⁷F₃ transitions at 17 270 cm⁻¹ (579 nm) and 15 385 cm⁻¹ (650 nm), respectively, and the ⁵D₁→⁷F₄ transition at 15 950 cm⁻¹ (627 nm). Much lower intensities show the ⁵D₂→⁷F_J transitions with J = 0, 2, and 3 which correspond to bands at 21 505 cm⁻¹ (465 nm), 20 492 cm⁻¹ (488 nm), and 19 608 cm⁻¹ (510 nm). The energy gap of 1 740 cm⁻¹ between the ⁵D₁ and ⁵D₀ states corresponds to 5 times the effective phonon energy of the β-NaGdF₄ host lattice. This gives rise to a competition between radiative and non-radiative relaxation from the ⁵D₁ state.

Interestingly, the relative emission from the ⁵D₁vs. the ⁵D₀ state decreases with increasing Eu³⁺ doping, as previously observed for β-NaGdF₄:Eu³⁺ samples.^6,9,19 Higher Eu³⁺ concentrations enhance the energy migration between Eu³⁺ ions. Cross relaxation rates may increase between ⁵D_J and ⁷F_J states and reduce the population of higher ⁵D_J states in favour of ⁵D₀. On the contrary, the co-doping with Er³⁺ ions increases the relative ⁵D₁ emission. The Er³⁺ ions hinder the energy migration on the Eu³⁺ sublattice and retain a higher ⁵D₁ population.

Normalized integrated intensities are reported in Fig. 7 for the Gd^{3+ 6}P_J→⁸S_7/2, Eu^{3+ 5}D₀→⁷F₂, and Er^{3+ 2}P_3/2→⁴I_13/2 emissions upon Gd³⁺ excitation at 273 nm. These transitions were chosen avoiding any overlap of emissions from different ions. The Gd³⁺ and Eu³⁺ intensities are displayed vs. the Eu³⁺ concentration on the bottom axis. Since the Er³⁺ emission is much weaker than the others, its intensity is displayed ten-fold expanded on the right hand axis vs. the Er³⁺ concentration on the top axis. The normalized integrated intensities in Fig. 7 were obtained from the luminescence spectra in Fig. S6 of ESI† which correspond to the normalized emission spectra in Fig. 5. The integrated intensities are displayed on a logarithmic scale in Fig. S7 of ESI.† According to the Lambert–Beer law, the absorbance A of a sample is proportional to the absorption coefficient ε, the concentration c, and the sample thickness d. In our standard luminescence setup, the excitation energy and d are fixed. The absorption coefficients ε of the rare earth ions are determined by the host lattice β-NaREF₄. Therefore, the logarithm of the absorbed number of photons is proportional to the concentration of the respective ion. Ideally, the absorption results in the emission of a photon from the same ion. Any deviation indicates additional decay pathways such as energy transfer or multiphonon relaxation.

With increasing Eu³⁺ doping, the Gd³⁺ emission intensity (purple circle) is strongly reduced and the Eu³⁺ emission (red square) increases due to Gd³⁺→Eu³⁺ energy transfer, see Fig. 7. The Eu³⁺ emission increases up to 5% doping where it almost reaches a plateau. The optimum Eu³⁺ doping is expected in the vicinity of 5%. For 10% Eu³⁺ doping the emission intensity is already lower than for 5% due to concentration quenching.¹⁹ The total Gd³⁺ and Eu³⁺ emission is approximately constant, as shown in Fig. S7 (ESI†). Sample to sample variations of the total luminescence intensity due to experimental errors are strongly reduced in the normalized data of Fig. 7, which yields clear trends along the series. The Er³⁺ emission (green triangle) increases with Er³⁺ concentration, but it is much weaker than the Gd³⁺ and Eu³⁺ emissions. Both the Gd³⁺→Er³⁺ and Eu³⁺→Er³⁺ energy transfers are inefficient for 273 nm excitation. However, Er³⁺ co-doping has another important effect: the relative Gd³⁺vs. Eu³⁺ emission is higher in the presence of Er³⁺ than without. A comparison of the spectra in Fig. 5 reveals a much stronger relative Gd³⁺ emission for β-NaGdF₄:1% Eu³⁺,4% Er³⁺ (Fig. 5B, green trace) than for β-NaGdF₄:1% Eu³⁺ (Fig. 5A, black trace). In Fig. 7 the Gd³⁺/Eu³⁺ emission intensity ratios of these samples are about 45/55 and 10/90, respectively. Er³⁺ acts as a neutral scatterer, as discussed above for the Eu^{3+ 5}D₁vs.⁵D₀ emission ratio. Co-doping reduces the energy migration on the Gd³⁺ and Eu³⁺ sublattices, decreases the Gd³⁺→Eu³⁺ energy transfer, and therefore retains the excitation energy on Gd³⁺. An optically inactive ion, such as Y³⁺, is likely to yield comparable results.

3.2.2. Er³⁺ excitation at 377 nm. Fig. 8 shows the normalized emission spectra of β-NaGdF₄:Eu³⁺,Er³⁺ nanocrystalline samples for Er^{3+ 4}I_15/2→⁴G_11/2 excitation at 26 525 cm⁻¹ (377 nm). The spectra are normalized to the Eu^{3+ 5}D₀→⁷F₂ emission, marked by an asterisk. The corresponding luminescence spectra are shown in Fig. S8 of ESI.† In Fig. S9 (ESI†), the low energy part of Fig. 8 is expanded for a better separation of the Eu³⁺ and Er³⁺ emissions. Fig. 9 shows the integrated intensities of the Eu^{3+ 5}D₀→⁷F₂ and Er^{3+ 2}H_9/2→⁴I_15/2 emissions on a logarithmic scale. These intensities were obtained from Fig. S8 (ESI†).

Fig. 8 Normalized luminescence spectra of β-NaGdF₄:x% Eu³⁺,y% Er³⁺ nanocrystalline samples for Er³⁺ excitation at 377 nm (26 525 cm⁻¹). Spectra are normalized to the Eu^{3+ 5}D₀→⁷F₂ emission at 615 nm (16 260 cm⁻¹), marked by an asterisk.

Fig. 9 Integrated intensity of the Eu^{3+ 5}D₀→⁷F₂ (red square) and Er^{3+ 2}H_9/2→⁴I_15/2 (green triangle) luminescence of β-NaGdF₄:x% Eu³⁺,y% Er³⁺ nanocrystalline samples shown in Fig. S8 (ESI†). The empty (filled) symbols refer to a 1% (5%) total doping for x% Eu³⁺ and y = 1 − x (y = 5 − x)% Er³⁺. Er³⁺ was excited at 377 nm (26 525 cm⁻¹). The integrated intensity is reported on a logarithmic scale.

Er³⁺ excitation at 377 nm results in strong Er³⁺ emission from the ²H_9/2, ²H_11/2, and ⁴S_3/2 states, see Fig. 8. A minor emission at 19 881 cm⁻¹ (503 nm) is ascribed to the Er^{3+ 4}G_11/2→⁴I_13/2 transition. At 377 nm Eu³⁺ is also directly excited, see Fig. 3. The resulting Eu^{3+ 5}D_J→⁷F_J emissions are shown on an expanded scale in Fig. S9 (ESI†). A comparison with Fig. 6 reveals the additional overlapping Er³⁺ emissions labelled a–c in Fig. S9 (ESI†).

Since the luminescence spectra in Fig. 8 are normalized to the Eu^{3+ 5}D₀→⁷F₂ emission, the strongly varying intensity of the Er³⁺ emissions allow a conclusive assignment of the Er³⁺ states. Their energies are reported in Table 1. The Eu³⁺ energies for β-NaGdF₄:Eu³⁺ nanocrystallites were reported elsewhere.¹⁸ Both Er³⁺ and Eu³⁺ emission intensities increase with doping. On the logarithmic scale of Fig. 9, they show a close to linear dependence on the concentration. This indicates the absence of significant energy transfer between Er³⁺ and Eu³⁺ ions. Both dopant ions behave rather independently and their luminescence intensity correlates with the respective absorption. For the same doping level, the Eu^{3+ 5}D₀→⁷F₂ emission shows an approximately five-times higher intensity than the Er^{3+ 2}H_9/2→⁴I_15/2 emission upon excitation at 377 nm.

Table 1 Energies of the Er³⁺ states in β-NaGdF₄:Er³⁺

State	Energy [cm^−1]
^2P3/2	31 587
^4G9/2, ^2KJ	27 473
^4G11/2	26 525
^2H9/2	24 510
^2H11/2	19 011
^4S3/2	18 349
^4I11/2	10 219
^4I13/2	6524
^4I15/2	0

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In contrast to our observations in β-NaGdF₄:Eu³⁺,Er³⁺, efficient Er³⁺→Eu³⁺ energy transfer was reported in NaLa(WO₄)₂:x% Eu³⁺,3% Er³⁺ phosphors upon excitation at 378 nm with variable Eu³⁺ doping.³¹ Increasing the Eu³⁺ concentration enhanced the Eu³⁺ and quenched the Er³⁺ luminescence.

3.2.3. Eu³⁺ excitation at 394 nm. The normalized emission and luminescence spectra of β-NaGdF₄:Eu³⁺,Eu³⁺ nanocrystalline samples after Eu^{3+ 7}F₀→⁵L₆ excitation at 25 380 cm⁻¹ (394 nm) are shown in Fig. S10 and S11 of ESI,† respectively. The integrated intensities of the Eu^{3+ 5}D₀→⁷F₂ emission at 16 260 cm⁻¹ are reported in Fig. S12 of ESI.† On a logarithmic scale, the integrated intensity shows a linear dependence on the Eu³⁺ doping, as expected from the Lambert–Beer law in the absence of energy transfer. The spectra in Fig. S10 (ESI†) consist of Eu³⁺ emissions from the ⁵D₀ and ⁵D₁ states, similar to Fig. 6. With increasing Eu³⁺ doping, the emission from ⁵D₁ decreases relatively to that of ⁵D₀. Co-Doping with Er³⁺ counteracts that trend because the energy migration on the Eu³⁺ sublattice is reduced, as it was discussed in Section 3.2.1. A comparison to Fig. S9 (ESI†) reveals a very weak Er^{3+ 4}S_3/2→⁴I_15/2 emission at 545 nm (18 350 cm⁻¹) for β-NaGdF₄:1% Eu³⁺,4% Er³⁺ in Fig. S11B (ESI†). The virtual absence of Er³⁺ emissions upon Eu³⁺ excitation proves a negligible Eu³⁺→Er³⁺ energy transfer for 394 nm excitation.

4. Conclusions

A room temperature synthesis for phase pure hexagonal β-NaGdF₄ nanocrystalline samples was established. A crystallite size below 10 nm was determined by X-ray diffraction and electron microscopy. The energy transfer between Gd³⁺, Eu³⁺, and Er³⁺ ions was investigated depending on the excitation wavelength. For Gd³⁺ excitation at 273 nm, the Gd³⁺→Eu³⁺ energy transfer is very efficient and Eu³⁺ luminescence dominates the spectra. At 377 nm, both Er³⁺ and Eu³⁺ are excited. For identical dopant concentration, the Eu³⁺ emission is about five-times more intense than the Er³⁺ emission. The concentration dependence of the luminescence indicates an independent optical behaviour of both ions without any significant Er³⁺→Eu³⁺ or Eu³⁺→Er³⁺ energy transfer. Eu³⁺ excitation at 394 nm confirms the absence of Eu³⁺→Er³⁺ energy transfer and only Eu³⁺ luminescence is observed. Er³⁺ co-doping hinders the energy migration on the Gd³⁺ and Eu³⁺ sublattices which results in higher Gd³⁺/Eu³⁺ and Eu^{3+ 5}D₁/⁵D₀ emission ratios. Time-resolved spectroscopy may yield a deeper characterization of the energy transfer processes. This was beyond the scope of this work, which focused on the synthesis of the material and its dopant dependent luminescence. A strong Er^{3+ 2}H_9/2 emission indicates small multiphonon relaxation losses to lower states and proves a good quality of the nanomaterial. The synthesis under strictly anhydrous conditions minimizes OH⁻ impurities and the non-radiative losses resulting thereof.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial support by the EU FP7 ITN LUMINET (Grant agreement No. 316906) is gratefully acknowledged by all authors. The authors thank the Microscopy Imaging Centre (MIC) at University of Bern for the access to TEM instruments.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, additional TEM micrographs, XRD patterns, thermogravimetric analysis, luminescence spectra, and integrated emissions. See DOI: 10.1039/c7nj03242k

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