Magnetic-downconversion luminescent bifunctional BaGdF₅:Dy³⁺,Eu³⁺ nanospheres: energy transfer, multicolor luminescence and paramagnetic properties

Abstract

A series of Dy³⁺ or/and Eu³⁺ doped cubic BaGdF₅ phosphors were synthesized for the first time by an L-arginine hydrothermal method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence spectroscopies (PL) and luminescence decay. The results indicate that the as-prepared samples are a pure cubic phase of BaGdF₅, taking on irregular nanoparticles with an average size of 20 nm. The as-prepared Dy³⁺ or Eu³⁺ single doped samples show strong blue and red emissions, originating from the ⁴F_9/2 → ⁶H_15/2 transition of the Dy³⁺ ions and the ⁵D₁ → ⁷F_J (J = 1, 2) and ⁵D₀ → ⁷F_J (J = 1, 2, 4) transition of the Eu³⁺ ions. Based on the rare earth concentrations and excitation wavelengths, multiple (white, red, blue and green yellow) emissions are obtained by Eu³⁺ ion co-activated BaGdF₅:Dy³⁺ phosphors. In addition, the energy migration from Dy³⁺ to Eu³⁺ has been reported in detail. Furthermore, the obtained samples also exhibit paramagnetic properties at room temperature and low temperature. It is obvious that Dy³⁺, Eu³⁺ co-doped BaGdF₅ nanomaterials with tunable multicolor emissions may have potential application in the field of full-color displays.

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1. Introduction

Luminescent materials based on rare earth ion emission have been extensively investigated for their wide applications in the field of full-color displays,^1,2 light emitting diodes (LED),^3,4 fluorescence imaging^5–7 as well as active laser materials^8–10 in modern society. Therefore, an increasing number of researchers have paid attention to studying and developing luminescent nanomaterials, such as phosphates,^11–14 lanthanide-doped oxides^15,16 and fluorides.^17–19 Among these materials, the rare-earth fluoride (such as BaGdF₅) nanocrystals are an interesting and highly-focused host material for rare earth ion emission, such as Dy³⁺, Eu³⁺, etc. for two reasons: (1) the rare-earth fluoride (such as BaGdF₅) nanocrystals possess a low phonon energy and toxicity, large effective Stokes shifts, high refractive index, multicolor emission as well as high resistance to photo-bleaching, blinking, and photochemical degradation.²⁰ (2) For the fluorides containing the Gd³⁺ ions, the Gd³⁺ (4f⁷) has a strong absorption peak at ∼272 nm (S → I transition) and thereby energy transfer is possible to the excited states of the activators (Ln³⁺);^21,22 therefore, BaGdF₅ is considered to be a prominent luminescent host material.

Nowadays, many red phosphors have been successfully prepared via the Eu³⁺ doped compounds based on the characteristic emissions resulting from the transitions of ⁵D₀ → ⁷F_J (J = 1, 2, 3, 4). Furthermore, the emitting color of Dy³⁺ of ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 belong to the blue and yellow regions, respectively. Thereby, it is feasible to produce Dy³⁺ and Eu³⁺ co-activated white light or color phosphors. Obviously, tunable photoluminescence properties of the Dy³⁺ and Eu³⁺ ions have been widely studied in many hosts, such as NaGdF₄,²³ GdVO₄,²⁴ SrY₂O₄ (ref. 25) and Y₂O₂S.²⁶ In view of the above studies and take into consideration the characteristic emissions of dysprosium and europium ions, it is expected that multicolor emissions also could be achieved in BaGdF₅ system by adjusting different excitation wavelengths and properly designed activator contents. More importantly, up to now, most reports about BaGdF₅ nanometer materials have mainly focused on up conversion luminescence in the literature.^27–29 However, there are scarcely ever reports about BaGdF₅ down conversion luminescence. Meanwhile, BaGdF₅ shows relatively excellent chemical and photophysical stability. Hence, it is highly valuable to obtain color-tunable emissions through adjusting the excitation wavelengths and doping Dy³⁺ along with Eu³⁺ into BaGdF₅ host.

It is known that some of the Ln³⁺ ions like Gd³⁺, having seven unpaired electrons, also can show magnetic properties, which could also be used in designing new functional materials. Recently, Ajithkumar reported that Gd₂O₂S:Yb³⁺/Er³⁺ phosphor exhibit strongly magnetic, thus making them suitable as an MRI agent.³⁰ Ju³¹ has successfully synthesized amine-functionalized lanthanide-doped KGdF₄ nanocrystals via a facile one-step solvothermal route by employing polyethylenimine as the surfactant and capping ligand, which have been demonstrated to be potential T1-MRI contrast agents. A simple and fast (7 min) procedure for synthesis of gadolinium phosphate (GdPO₄) nanocubes (edge = 75 nm) based on the microwave-assisted heating at 120 °C of gadolinium acetylacetonate and phosphoric acid solutions in buthylene glycol is reported by Ocaña,³² which are demonstrated potential applications as biolabels for in vitro optical imaging and as negative contrast agent for magnetic resonance imaging. Therefore, the BaGdF₅ nanomaterial not only has a very good luminescence property, but also has a magnetic property. So it is a multifunctional material, and it can be used in display devices and biological fields.

In this work, we aim to focus our attentions on BaGdF₅ as a host, while Dy³⁺ and Eu³⁺ ions as activators to synthesize a series of BaGdF₅:Dy³⁺,Eu³⁺ nanophosphors through a simple hydrothermal method. In addition, we reported the phase, morphology, color tunable luminescence and paramagnetic properties behavior of trivalent rare-earth ions activated BaGdF₅ powders. Furthermore, the luminescence and energy transfer properties of Dy³⁺ and Eu³⁺ co-doped BaGdF₅ phosphors have been discussed in detail.

2. Experimental

2.1 Materials

All chemicals were of analytical grade and utilized as purchased without any further purification. The Gd(NO₃)₃, Dy(NO₃)₃, and Eu(NO₃)₃ stock solutions were prepared by dissolving corresponding appropriate amounts of Gd₂O₃ (99.99%), Dy₂O₃ (99.99%), and Eu₂O₃ (99.99%) in dilute HNO₃ (15 mol L⁻¹) under heating with agitation followed by evaporating the excess solvent. The Gd₂O₃, Dy₂O₃, and Eu₂O₃ were purchased from Jiangxi Ganzhou Rare-Earth Limited Corporation.

2.2 Preparation

A series of rate-earth doped BaGdF₅ phosphors were synthesized by a facile hydrothermal process without further sintering treatment. Firstly, 2 mmol of RE(NO₃)₃ (RE = Gd, Dy, Eu) were added into a 100 mL flask with 0.5226 g nitrate barium (Ba(NO₃)₂). Then after vigorous stirring for 20 min, 0.6968 g L-arginine were slowly added into the above solution. Finally, after additional agitation for 20 min, 0.8398 g sodium fluoride (NaF) were poured into the above solution. The as-obtained mixing solution was transferred into a 50 mL Teflon autoclave, which was tightly sealed and maintained at 180 °C for 25.5 h. As the autoclave was cooled to the room temperature naturally, the precipitates were separated by centrifugation washed with deionized water and ethanol in sequence each several times and dried in air at 60 °C for 12 h.

2.3 Characterizations

The crystalline nature and phase structure of BaGdF₅:Dy³⁺,Eu³⁺ were examined by X-ray powder diffraction (XRD) performed on a Rigaku D/max-RA X-ray diffractometer with Cu K_χ radiation, operating at 20 mA, 30 kv, scanning speed, step length and diffraction range were 10° min⁻¹, 0.1° and 20–75°, respectively. The size and morphology of the sample were inspected using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan). The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) micrographs were performed using a FEI Tecnai G2 S-Twin transmission electron microscope with a field emission gun operating at 200 kV. Photoluminescence (PL) excitation and emission spectra were recorded with a Jobin Yvon FluoroMax-4 equipped with a 150 W xenon lamp as the excitation source at room temperature. The luminescence decay curves of the phosphors were measured using a HITACHI F-7000 spectrometer with a 350 W xenon lamp as the excitation source.

3. Results and discussion

3.1 Crystallization behavior and structure

The XRD patterns of BaGdF₅ phosphors have been shown in Fig. 1. It can be seen that all the diffraction peaks of these samples are basically in agreement with pure cubic phase BaGdF₅ crystal in the Fm3m space group (lattice constants a = b = 6.023 Å, c = 6.023 Å, JCPDS card 24-0098) and no other phases or impurities can be detected, which implying that the prepared samples were pure phase BaGdF₅. Moreover, the sharp and strong peaks indicate that the products synthesized at low temperatures are still highly crystalline, which is very important for phosphors, since higher crystallinity always means fewer traps and stronger luminescence.^28,33 However, it is worth pointing out that compared with the corresponding standard card, the main peaks slightly shift towards to the higher degree by the introduction of Dy³⁺ ions, but for the co-doping of Dy³⁺ and Eu³⁺ ions, the main peaks shifting towards to the higher degree slightly decreases, which is in line with the literature.^24,28,29,34 The reason is that the ionic radii of the Gd³⁺ ions (0.938 Å) are larger than that of Dy³⁺ ions (0.912 Å) and smaller than that of Eu³⁺ ions (0.947 Å).²⁴

Fig. 1 XRD patterns of the as-prepared BaGdF₅:Dy³⁺, BaGdF₅:Eu³⁺ and BaGdF₅:Dy³⁺,Eu³⁺ samples, the corresponding standard data of BaGdF₅ (JCPDS no. 24-0098) is given as reference.

Fig. 2 shows the morphology and composition of the materials prepared by a facile hydrothermal method. From Fig. 2(a)–(c), one can see that the BaGdF₅ (a) and BaGdF₅:3%Dy³⁺ (b) and BaGdF₅:3%Dy³⁺,4%Eu³⁺ (c) samples are composed of a large number of nanoparticles with the size of about 20 nm, which demonstrates that the doping species and the doping quality of rare earth ions do not alter the morphology. Fig. 2(d) gives the EDS spectrum of BaGdF₅ nanoparticles doped with 3%Dy³⁺ and 4%Eu³⁺. The EDS result reveals that the nanoparticles are mainly composed of Ba, Gd, Dy, Eu, F, which further proved the samples to be BaGdF₅:Dy³⁺,Eu³⁺. From the TEM image, it can be clearly seen that these nanospheres have an average diameter about 20 nm (Fig. 2(e)). The corresponding HRTEM image shows clearly lattice fringes with interplanar spacing of 3.4 Å ascribed to the (111) plane of BaGdF₅ (Fig. 2(f)).²⁹

Fig. 2 FE-SEM image of BaGdF₅ (a) and BaGdF₅:3%Dy³⁺ (b) and BaGdF₅:3%Dy³⁺,4%Eu³⁺ (c) samples and EDS spectrum of BaGdF₅:3%Dy³⁺,4%Eu³⁺ samples (d) and TEM (e) and HRTEM images (f) of BaGdF₅:3%Dy³⁺,4%Eu³⁺.

3.2 Fluorescent performance

Fig. 3 presents the room temperature photoluminescence excitation and emission spectra for BaGdF₅:3%Dy³⁺ (a), BaGdF₅:3%Eu³⁺ (b), and BaGdF₅:3%Dy³⁺,3%Eu³⁺ (c). As shown in Fig. 3(a), it can be seen that the photoluminescence excitation spectrum monitored with 570 nm emission (⁴F_9/2 → ⁶H_13/2) of Dy³⁺ consists of a sharp excitation band at 272 nm and several bands centered at 323, 348, 362, 386, 451 nm, and 474 nm, corresponding to the characteristic f–f transitions of Gd³⁺ (⁸S_7/2 → ⁶I_J) and Dy³⁺ (⁶H_15/2 → ⁶P_3/2, ⁶H_15/2 → ⁶P_7/2, ⁶H_15/2 → ⁶P_5/2, ⁶H_15/2 → ⁴F_7/2, ⁶H_15/2 → ⁴I^15/2, and ⁶H_15/2 → ⁴F_9/2). These excitation peaks indicate that the phosphor can strongly absorb ultraviolet and blue light to obtain Dy³⁺ emission. Upon excitation with 272 nm, the as-prepared BaGdF₅:3%Dy³⁺ samples exhibit two main emission bands, whose positions are at 477 and 570 nm, corresponding to the magnetic dipole (⁴F_9/2 → ⁶H_15/2) and electric dipole transitions (⁴F_9/2 → ⁶H_13/2) of Dy³⁺ ions.³⁵ In addition, a bright blue light can be observed in the photo of BaGdF₅:3%Dy³⁺ under excitation at 272 nm.

Fig. 3 Photoluminescence excitation and emission spectra for BaGdF₅:3%Dy³⁺ (a), BaGdF₅:3%Eu³⁺ (b), and BaGdF₅:3%Dy³⁺,3%Eu³⁺ (c); insets are the corresponding luminescent photographs under 272 nm illumination.

Fig. 3(b) gives excitation and emission spectra of the as-prepared BaGdF₅:3%Eu³⁺ samples. From the Fig. 3(b), it can be found that the excitation spectrum of BaGdF₅:3%Eu³⁺ shows some peaks at 316, 364, 384, and 392 nm corresponding to the transitions of Eu³⁺ ion from the ground level ⁷F₀ to ⁵H₃, ⁵D₄, ⁵G₄ and ⁵L₆ excited levels, respectively. Simultaneously, the sharp excitation band at 272 nm is assigned to the f–f transition of Gd³⁺. The emission spectrum of BaGdF₅:3%Eu³⁺ phosphor is obtained by exciting at 272 nm. One can see that there are a number of emission peaks in the range of 400–750 nm at 508, 534, 552, 591, 616, and 697 nm, which correspond to ⁵D₂ → ⁷F₃, ⁵D₁ → ⁷F₁, ⁵D₁ → ⁷F₂, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, and ⁵D₀ → ⁷F₄ transitions of Eu³⁺ ions, respectively.³⁶ Generally, the intensity ratio of the electric dipole to magnetic dipole transitions is used to determine the symmetry of the local environment. The strong emission ascribed to the ⁵D₀ → ⁷F₁ magnetic dipole transition (591 nm), is about 3 times stronger than that of the electric dipole transition ⁵D₀ → ⁷F₂ (616 nm), indication that the Eu³⁺ ions occupy the symmetry sites. Thereby, the orange red emission is often dominant in the emission spectrum. Moreover, it can be seen that the as-prepared BaGdF₅:3%Eu³⁺ show a bright orange red light under UV irradiation, as shown in the inset of Fig. 3(b). As depicted in Fig. 3(a) and (b), the comparison between the excitation spectra of BaGdF₅:3%Dy³⁺ and BaGdF₅:3%Eu³⁺ reveals that there is a sharp excitation band at 272 nm is assigned to the f–f transitions of Gd³⁺, which indicates that efficient energy transfer appears from Gd³⁺ to Dy³⁺ and Eu³⁺.

From the Fig. 3(c), we can clearly see that a manifest ⁶H_15/2 → ⁶P_7/2 (348 nm) transition of Dy³⁺ is observed when monitoring by the ⁵D₀ → ⁷F₁ (591 nm) emission of Eu³⁺ ions for BaGdF₅:3%Dy³⁺,3%Eu³⁺, which imply that the energy migration from Dy³⁺ to Eu³⁺. The emission spectra of BaGdF₅:3%Dy³⁺,3%Eu³ at 272 nm exciting showed the emission of both Dy³⁺ and Eu³⁺ ions, which consists of blue and yellow bands corresponding to the ⁴F_9/2 → ⁶H_15/2 (477 nm) and ⁴F_9/2 → ⁶H_13/2 (570 nm) transitions of Dy³⁺ ions and orange red and red bands attributed to the ⁵D₀ → ⁷F₁ (591 nm), ⁵D₀ → ⁷F₂ (616 nm), and ⁵D₀ → ⁷F₄ (697 nm) transition of Eu³⁺ ions, respectively. Hence, it is highly valuable to obtain color-tunable emissions through doping Dy³⁺ along with Eu³⁺ into BaGdF₅ host. As shown in the inset of Fig. 3(c), the photo of BaGdF₅:3%Dy³⁺,3%Eu³⁺ exhibits orange red light.

In order to further study the luminescent characteristics, including tunable color and energy transfer in Dy³⁺, Eu³⁺ co-doped BaGdF₅ host, a series of BaGdF₅:3%Dy³⁺,x%Eu³⁺ (x = 0, 1, 2, 3, 5, 6, 7, 8, 10) samples have been prepared. From Fig. 4(a), one can see that the characteristic emissions for both Dy³⁺ and Eu³⁺ ions can be clearly observed. Simultaneously, as shown in Fig. 4(a) and (b), with increasing Eu³⁺ concentration, the ⁴F_9/2 → ⁶H_13/2 (477 nm) and ⁴F_9/2 → ⁶H_13/2 (570 nm) transition emission intensities of Dy³⁺ decrease and the ⁵D₀ → ⁷F₁ (591 nm), ⁵D₀ → ⁷F₂ (616 nm), and (⁵D₀ → ⁷F₄ 697 nm) emission intensities of Eu³⁺ increase, implying that the Eu³⁺ ions emission is sensitized by Dy³⁺ ions through energy transfer. However, it is interesting that upon increasing the Eu³⁺ concentration above 7%, the intensities of the Eu³⁺ emission begin to decrease as a result of a self-quenching effect due to the interactions between Eu³⁺ ions. The above results testify that tunable color can be realized by adjusting the concentration of Eu³⁺ and there is an energy transfer from Dy³⁺ to Eu³⁺ in BaGdF₅ host.

Fig. 4 (a) Photoluminescence emission spectra of the BaGdF₅:3%Dy³⁺,x%Eu³⁺ (x = 0, 1, 2, 3, 5, 6, 7, 8, 10) samples with different Eu³⁺ doped concentrations (λ_ex = 384 nm) (b) dependence of the emission intensity on the Eu³⁺ concentration.

To further confirm the conclusion derived from Fig. 4(a) and (b) that luminescent characteristics and energy transfer both of Dy³⁺ and Eu³⁺ in BaGdF₅ host, a series of BaGdF₅:y%Dy³⁺,3%Eu³⁺ (y = 2, 3, 4, 5, 6) samples have been synthesized. The emission spectra for the BaGdF₅:y%Dy³⁺,3%Eu³⁺ (y = 2, 3, 4, 5, 6) samples have also been showed in Fig. 5. It is clearly that the characteristics emissions of Dy³⁺ and Eu³⁺ ions were observed under the 384 nm. In addition, from Fig. 5(a) and (b), one can see that the Dy³⁺ ions emission increases firstly and then decreases when the concentration of Dy³⁺ is above 3% as a result of a self-quenching effect due to the interactions between Dy³⁺ ions. More importantly, although the Eu³⁺ concentration is fixed, the emission intensity of Eu³⁺ increases with the Dy³⁺ of concentration increasing. The above conclusions further support the results that the energy transfer is from the Dy³⁺ to Eu³⁺ in BaGdF₅ host as well as color-tunable emissions through doping Dy³⁺ along with Eu³⁺ into BaGdF₅ host can be obtain.

Fig. 5 (a) Photoluminescence emission spectra of the BaGdF₅:y%Dy³⁺,3%Eu³⁺ (y = 2, 3, 4, 5, 6) samples with different Dy³⁺ doped concentrations (λ_ex = 384 nm); (b) dependence of the emission intensity on the Dy³⁺ concentration.

In view of the above studies and take into consideration the characteristic emissions of dysprosium (Dy³⁺) and terbium (Eu³⁺) ions, it is expected that the multicolor tunable emissions could be achieved in BaGdF₅ system by properly designed activator contents. Therefore, a series of BaGdF₅:3%Dy³⁺,x%Eu³⁺ (x = 0, 1, 2, 3, 5, 6, 7) and BaGdF₅:y%Dy³⁺,3%Eu³⁺ (y = 2, 3, 4, 5, 6) samples are prepared. From Fig. 6(a) and (c), we can clearly see that the emission spectrum of the BaGdF₅:3%Dy³⁺,x%Eu³⁺ (x = 0, 1, 2, 3, 5, 6, 7) and BaGdF₅:y%Dy³⁺,3%Eu³⁺ (y = 2, 3, 4, 5, 6) products excited at 272 nm includes very similar Dy³⁺ and Eu³⁺ emission peaks. As shown in Fig. 6(b), the emission intensities of Eu³⁺ increase with the increasing of Eu³⁺ on concentrations. However, the emission intensities of Dy³⁺ ions decrease monotonously with an increase of Eu³⁺ ions concentration. From Fig. 6(d), one can see that the intensities of Dy³⁺ ions gradually increase until the content of Dy³⁺ ions is above 3%, which is ascribed to the cross relaxation between neighboring Dy³⁺ ions: Dy³⁺(⁴F_9/2) + Dy³⁺(⁶H_15/2) → Dy³⁺(⁶F_3/2) + Dy³⁺(⁶F_11/2). More importantly, although the Eu³⁺ concentration is fixed, the emission intensities of Eu³⁺ increase with the Dy³⁺ of concentration increasing at a certain range. All in all, the emission intensity of Dy³⁺ and Eu³⁺ varies with changing the doped concentration of Dy³⁺ and Eu³⁺ in BaGdF₅ phosphors, which is benefit to obtain the multicolor tunable emission lights.

Fig. 6 (a) Photoluminescence emission spectra of the BaGdF₅:3%Dy³⁺,x%Eu³⁺ (x = 0, 1, 2, 3, 5, 6, 7) samples with different Eu³⁺ doped concentrations (λ_ex = 272 nm); (b) dependence of the emission intensity on the Eu³⁺ concentration; (c) photoluminescence emission spectra of the BaGdF₅:y%Dy³⁺,3%Eu³⁺ (y = 2, 3, 4, 5, 6) samples with different Dy³⁺ doped concentrations (λ_ex = 272 nm); (d) dependence of the emission intensity on the Dy³⁺ concentration.

In order to search for a new and economical as well as highly efficient phosphor, many researchers have focused their interests on changing the excitation wavelength. Fig. 7 gives emission spectra of BaGdF₅:3%Dy³⁺,3%Eu³⁺ sample under different wavelengths excitation. One can see that the sample exhibit the typical emissions of Dy³⁺ and Eu³⁺ under 309, 323, 363, 374, 384 and 392 nm excitation. However, the emission intensities of Dy³⁺ and Eu³⁺ ions obviously vary. Upon excitation 323 and 363 nm, only the Dy³⁺ ions characteristic emissions are observed. Nevertheless, when excited at 309 and 392 nm, the emission intensities of Eu³⁺ ions are distinctly stronger than the emission intensities of Dy³⁺ ions. The above result easily proved that the multicolor luminescence can be achieved in BaGdF₅:Dy³⁺,Eu³⁺ system by adopting the appropriate excitation wavelength.

Fig. 7 Photoluminescence emission spectra of BaGdF₅:3%Dy,3%Eu³⁺ samples under different wavelengths excitation.

For better understanding the mechanism of energy transfer phenomena, the fluorescent decay curves of Dy³⁺ ions emission in the BaGdF₅:3%Dy³⁺,x%Eu³⁺ samples under monitored at 477 nm with irradiation of 384 nm were measured. From Fig. 8, one can see that all the luminescent decay times of Dy³⁺ in BaGdF₅:Dy³⁺,Eu³⁺ can be fitted well with a single exponential function as²⁴

I = I₀ + Ae^−t/τ (1)

where I and I₀ are the luminescence intensities at times t and 0, respectively, and τ is the luminescence life time. As shown in the Fig. 8(a), the corresponding luminescence decay times are determined to be 1.689, 1.631, 1.609, 1.570, 1.549 and 1.484 ms, respectively. From Fig. 8(b) clearly showed that the lifetimes for Dy³⁺ ions were found to drastically decrease with increasing the Eu³⁺ concentration. The decrease in the fluorescent decay cure further demonstrated that the existing energy transfer from Dy³⁺ to Eu³⁺.

Fig. 8 Decay curves for the luminescence of Dy³⁺ ions in BaGdF₅:3%Dy³⁺,x%Eu³⁺ samples (excited at 384 nm, monitored at 477 nm) (a). Dependence of energy transfer efficiency η_T on Eu³⁺ concentration (x) in BaGdF₅:3%Dy³⁺,x%Eu³⁺ (x = 4, 5, 6, 7, 8) samples (b).

To further understand the energy transfer process in depth, the energy transfer efficiencies (η_T) from Dy³⁺ to Eu³⁺ are calculated using the following formula:³⁷

η_T = 1 − τ/τ₀ (2)

where η_T is the energy transfer efficiency and τ₀ and τ are the lifetimes of Dy³⁺ ions in the absence and presence of Eu³⁺ ions, respectively. Thus, the relationship between the energy-transfer efficiency and activator concentration of Eu³⁺ ion can be obtained (Fig. 8(b)). As shown in Fig. 8(b), the energy transfer efficiency monotonically increases with an increase in Eu³⁺ concentration. The value of η_T reaches the maximum of 12% when x = 8.

According to Dexter and Schulman, concentration quenching is in many cases due to energy sink in the lattice is reached. As suggested by Blasse, the average separation R_Dy−Eu can be expressed by³⁸

R_Dy−Eu = 2 × [3V/(4πxZ)]^1/3 (3)

where V stands for the unit cell volume, x is the total concentration of Dy³⁺ and Eu³⁺, and Z represents the number of sites that activator ion can occupy in the host. For BaGdF₅ host, V = 218.49 Å, Z = 2. Thus, R_Dy−Eu is determined to be 17.35, 16.10, 15.15, 13.77, 13.24, 12.78, 12.38 and 11.71 Å for x = 0.04, 0.05, 0.06, 0.08, 0.09, 0.10, 0.11 and 0.13, respectively. Generally, the resonant energy transfer the mechanism consists of two main aspects: (1) exchange interaction; (2) multipole–multipole interaction. If critical distance (R_c) is less than 5 Å, the exchange interaction dominates and if the R_c is more than 5 Å, the multipole–multipole interaction will be the primary energy transfer mechanism. The critical concentration (x_c), at which the luminescence intensity of Dy³⁺ is half of that in the absence of Eu³⁺, is 0.1045. The critical distance R_Dy−Eu is calculated to be about 12.59 Å, which is much longer than 5 Å, implying that the multipole–multipole interaction is responsible to the energy transfer from Dy³⁺ to Eu³⁺ in the BaGdF₅ host.

As a more detailed analysis method for the energy transfer mechanism, according to Dexter's energy transfer expressions of multipolar interaction, the following relation can be obtained:^39,40

I_S0/I_S ∝ C^n/3 (4)

where I_S0 is the intrinsic luminescence intensity of Dy³⁺, and I_S is the luminescence intensity of Dy³⁺ in the presence of the Eu³⁺. C is the sum of the concentration of Dy³⁺ and Eu³⁺; n = 6, 8 and 10 is corresponding to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The plots of I_S0/I_S of Dy³⁺ on n = 6, 8, 10 is exhibited in Fig. 9. As shown in Fig. 9, a line relation is well-fitted at n = 6, indicating that the energy transfer mechanism via a dipole–dipole interaction between the Dy³⁺ and Eu³⁺ ions.

Fig. 9 Dependence of I₀/I of Eu³⁺ on (a) C^6/3, (b) C^8/3, and (c) C^10/3.

Fig. 10exhibits the proposed energy transfer mechanism between Dy³⁺ and Eu³⁺, which is speculated according to literature.^31,41,42 Although the energy level ⁴F_9/2 of Dy³⁺ (21.14 × 10³ cm⁻¹) is slightly higher than the energy level ⁵D₀ of Eu³⁺ (17.29 × 10³ cm⁻¹), the energy transfer (ET) is possible due to the phonon-aided non-radiative relaxation from ⁴F_9/2 level of Dy³⁺ to the ⁵D₀ level of Eu³⁺.^43,44 When the samples were excited under UV irradiation, the Dy³⁺ firstly absorbed the energy, and then emitted blue and yellow lights due to the transition of the ⁴F_9/2 → ⁶H_15/2 (477 nm) and ⁴F_9/2 → ⁶H_13/2 (570 nm). Simultaneously, it delivered some of the energy to the Eu³⁺ ions, which emitted the red light based on the transition of the ⁵D₀ → ⁷F₁ (590 nm), ⁵D₀ → ⁷F₂ (616 nm) and ⁵D₀ → ⁷F₄ (697 nm).

Fig. 10 The proposed scheme of energetic process occurring in the BaGdF₅:Dy³⁺,Eu³⁺ host.

As Fig. 11(A) depicted, BaGdF₅:Dy³⁺ samples emitted blue light and the CIE chromaticity coordinates was calculated to be (0.238, 0.275) as result from the blue emission stronger than yellow emission in Dy³⁺ singly activated BaGdF₅ phosphors. The pure Eu³⁺-activated BaGdF₅ emitted orange-red light. From the CIE diagram of BaGdF₅:3%Dy³⁺,x%Eu³⁺ (x = 0, 1, 2, 3, 4, 5, 7, 8, 10) phosphors, it can be seen that when excited at 272 and 384 nm, the trend of their color tones changes from blue to orange red light by adjusting the doping concentration of Eu³⁺, the corresponding selected images are presented in Fig. 11(A) (point 1–6). In addition, for the BaGdF₅:y%Dy³⁺,3%Eu³⁺ (y = 0, 2, 3, 4, 5, 6) phosphors, one can see that upon excited 272 and 384 nm, the trend of their color tones changes from red to white light by adjusting the doping concentration of Dy³⁺, the corresponding selected images are presented in Fig. 11(B) (point 15–23). More importantly, BaGdF₅:3%Dy³⁺,3.0%Eu³⁺ excited at 309, 323, 363, 374, 384 and 392 nm, the corresponding CIE coordinates for all the excitation wavelengths are determined to be (24, 0.388, 0.189), (25, 0.262, 0.276), (26, 0.270, 0.287), (27, 0.344, 0.287), (28, 0.325, 0.302) and (29, 0.451, 0.365), which are located in light red, blue green, white and red regions, respectively. The corresponding selected images under corresponding excitation wavelengths are obtained by taking pictures when the Jobin Yvon FluoroMax-4 Fluorescence Spectrophotometer is exciting the products. So the surrounding environment and light source will influence the results of the picture. All of the results indicate that multicolor luminescence can be achieved through adopting different excitation wavelengths or adjusting appropriate concentration of Dy³⁺ and Eu³⁺ in BaGdF₅. Finally, the specific CIE chromaticity coordinates of BaGdF₅:Dy³⁺,Eu³⁺ samples are given in the Table 1.

Fig. 11 (A and B) CIE chromaticity diagram of the selected BaGdF₅:Dy³⁺, BaGdF₅:Eu³⁺, and BaGdF₅:Dy³⁺,Eu³⁺ samples under 272 and 384 nm excitation; (C) CIE chromaticity diagram of BaGdF₅:3%Dy³⁺,3%Eu³⁺ samples under 309, 323, 363, 374, 384 and 392 nm excitation. The selected luminescence photographs of the corresponding samples.

Table 1 The CIE chromaticity coordinates for BaGdF₅:Dy³⁺,Eu³⁺ samples

Labels	Samples	Excitation (nm)	CIE (x, y)
1	BaGdF5:0.03Dy^3+	272	(0.264, 0.303)
7	BaGdF5:0.03Dy^3+	384	(0.253, 0.267)
2	BaGdF5:0.03Dy^3+,0.01Eu^3+	272	(0.358, 0.345)
8	BaGdF5:0.03Dy^3+,0.01Eu^3+	384	(0.285, 0.288)
3	BaGdF5:0.03Dy^3+,0.02Eu^3+	272	(0.401, 0.362)
9	BaGdF5:0.03Dy^3+,0.02Eu^3+	384	(0.308, 0.299)
4	BaGdF5:0.03Dy^3+,0.03Eu^3+	272	(0.438, 0.376)
10	BaGdF5:0.03Dy^3+,0.03Eu^3+	384	(0.325, 0.302)
5	BaGdF5:0.03Dy^3+,0.04Eu^3+	272	(0.451, 0.379)
11	BaGdF5:0.03Dy^3+,0.05Eu^3+	384	(0.351, 0.310)
6	BaGdF5:0.03Dy^3+,0.07Eu^3+	272	(0.465, 0.365)
12	BaGdF5:0.03Dy^3+,0.07Eu^3+	384	(0.377, 0.319)
13	BaGdF5:0.03Dy^3+,0.08Eu^3+	384	(0.395, 0.329)
14	BaGdF5:0.03Dy^3+,0.10Eu^3+	384	(0.420, 0.338)
15	BaGdF5:0.03Eu^3+	272	(0.492, 0.400)
16	BaGdF5:0.03Eu^3+,0.02Dy^3+	272	(0.424, 0.368)
20	BaGdF5:0.03Eu^3+,0.02Dy^3+	384	(0.320, 0.297)
17	BaGdF5:0.03Eu^3+,0.04Dy^3+	272	(0.417, 0.367)
21	BaGdF5:0.03Eu^3+,0.04Dy^3+	384	(0.326, 0.308)
18	BaGdF5:0.03Eu^3+,0.05Dy^3+	272	(0.392, 0.348)
22	BaGdF5:0.03Eu^3+,0.05Dy^3+	384	(0.297, 0.280)
19	BaGdF5:0.03Eu^3+,0.06Dy^3+	272	(0.386, 0.344)
23	BaGdF5:0.03Eu^3+,0.06Dy^3+	384	(0.305, 0.289)
24	BaGdF5:0.03Dy^3+,0.03Eu^3+	309	(0.388, 0.335)
25	BaGdF5:0.03Dy^3+,0.03Eu^3+	323	(0.262, 0.276)
26	BaGdF5:0.03Dy^3+,0.03Eu^3+	363	(0.270, 0.287)
27	BaGdF5:0.03Dy^3+,0.03Eu^3+	374	(0.344, 0.287)
28	BaGdF5:0.03Dy^3+,0.03Eu^3+	392	(0.451, 0.365)

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3.3 Magnetic property

The inorganic compounds which include Gd atom exhibit paramagnetic properties. The seven unpaired inner 4f electrons of Gd³⁺ ions are closely bound to the nucleus and can be effectively shield by the outer closed-shell 5s² 5p⁶ electrons from the crystal field, which gives rise to the magnetic properties of Gd³⁺ ions. The magnetic moments related to Gd³⁺ ions are all localized and non-interacting, which led to paramagnetism of Gd³⁺ ions.⁴⁵ Hence, the as-prepared BaGdF₅:Dy³⁺,Eu³⁺ samples not only exhibit excellent fluorescent properties but also magnetic properties. The magnetic properties of the as-prepared samples were analyzed by a VSM. Fig. 12(A) and (B) show the hysteresis loops of BaGdF₅:3%Dy³⁺,3%Eu³⁺ nanospheres at room temperature (300 K) and low temperature (2 K), respectively. From Fig. 12(A) and (B), one can see that the BaGdF₅:3%Dy³⁺,3%Eu³⁺ exhibit a good paramagnetism in the magnetic range of −30 to 30 kOe at 300 K due to no coercivity or remanence and the magnetization was 2.02 emu g⁻¹ (Fig. 12(A)), which approaches the value reported for nanoparticles used for common bio-separation.^46,47 Whereas BaGdF₅:3%Dy³⁺,3%Eu³⁺ at low temperature (2 K) show superparamagnetism with a saturation magnetization value of 83.39 emu g⁻¹ (Fig. 12(B)). The increase in the magnetic susceptibility at low temperature might be due to the reduction in thermal fluctuation, which is typical behaviour in paramagnetic materials described by Curie's law.⁴⁸

Fig. 12 Magnetization–applied magnetic field curves of BaGdF₅:3%Dy³⁺,3%Eu³⁺ nanospheres at 300 K (A) and 2 K (B), respectively.

4. Conclusions

In summary, Dy³⁺ and Eu³⁺ co-doped nanophosphors have been synthesized through a hydrothermal method and the as-synthesized samples are in particle shape with the diameter of about 20 nm. Upon UV light excitation, the Dy³⁺ or Eu³⁺ ions singly activated BaGdF₅ phosphors exhibit excellent emission properties in their respective regions. In addition, the energy transfer process of Dy³⁺ → Eu³⁺ has been systematically investigated in the BaGdF₅ host. The energy migration from Dy³⁺ to Eu³⁺ has been verified to be an electric dipole–dipole mechanism, of which the critical distance (R_Dy−Eu) is estimated to be 12.59 Å. By adopting different excitation wavelengths or adjusting appropriate concentration of Dy³⁺ and Eu³⁺ ions, tunable photoluminescence are realized in Dy³⁺ and Eu³⁺ co-doped BaGdF₅ system. These single-component phosphors exhibit color-tunable emissions, which may have application in full-color display. In addition, the obtained samples also exhibit paramagnetic properties at room temperature and low temperature.

Acknowledgements

This present work was financially supported by the National Natural Science Foundation of China (Grant No. 21171066), and the Opening Research Funds Projects of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2010–05).

Notes and references

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