White light-emitting, tunable color luminescence, energy transfer and paramagnetic properties of terbium and samarium doped BaGdF₅ multifunctional nanomaterials

Abstract

A series of BaGdF₅:x% Tb³⁺,y% Sm³⁺ phosphors are synthesized by a hydrothermal method, taking the form of irregular nanospheres with average sizes of about 20 nm. An energy transfer from Tb³⁺ to Sm³⁺ is observed in the BaGdF₅:Tb³⁺,Sm³⁺ system, which is justified by the luminescence spectra. Simultaneously, a resonance-type energy transfer from Tb³⁺ to Sm³⁺ is demonstrated to occur via the dipole–dipole interaction, for which the critical distance (R_Tb–Sm) is calculated to be 13.49 Å. Furthermore, based on the rare earth concentrations and excitation wavelengths, multiple (white, orange red, green and green yellow) emissions are obtained by Sm³⁺ ion co-activated BaGdF₅:Tb³⁺ phosphors, which could make them good candidates to be used as full-color phosphors for nUV-LED. More importantly, the obtained samples also exhibit paramagnetic properties at room temperature and low temperature.

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1. Introduction

Luminescent materials based on rare earth ions emission have been extensively investigated for their wide applications in modern society such as television tubes, mobile telephone screens, fluorescent lamps, white light emitting diodes^1–6 and active laser materials.^7–9 Therefore, an increasing number of researchers have paid attention to studying and developing luminescent nanomaterials, such as phosphates,^10–13 lanthanide-doped oxides^14,15 and fluorides.^16–18 Among these materials, rare-earth fluoride (such as BaGdF₅) nanocrystals are interesting and much-studied host materials for rare earth ion emissions, such as Tb³⁺, Sm³⁺, etc., for two reasons: (1) rare-earth fluoride (such as BaGdF₅) nanocrystals possess low phonon energy and toxicity, large effective Stokes shifts, high refractive indices, and multicolor emission as well as high resistance to photo-bleaching, blinking, and photochemical degradation.¹⁹ (2) For fluorides containing Gd³⁺ ions, the Gd³⁺ (4(f⁷)) transition has a strong absorption peak at ∼272 nm (S → I transition); thus, energy transfer is possible to the excited states of the activators (Ln³⁺).^20,21 Therefore, BaGdF₅ is considered to be an important luminescent host material.

Generally, the size, shape and independent luminescence properties of nanophosphors are beneficial for lighting applications; however, the implementation of tunable multicolor emissions makes the nanophosphors highly suitable for color display applications. The multicolor tuning of nanophosphors can be achieved through co-doping the host nanocrystals with Ln³⁺ ions. For example, Zhang²² et al. have successfully prepared GdOF:Ln³⁺ (Ln = Eu, Tb, Tm, Dy, Ho, Sm) microspheres by a facile hydrothermal method, and multicolored luminescence, including white emission, has been successfully achieved for co-doped GdOF:Ln³⁺ phosphors by changing the doped Ln³⁺ ions and adjusting their doping concentrations due to the simultaneous luminescence of Ln³⁺ in the GdOF host; thus, these materials have potential applications in field-emission display devices. Multicolor emitting GdF₃:Ce³⁺,Ln³⁺ (Ln³⁺ = Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺) nanocrystals were also successfully synthesized via a simple co-precipitation approach by Grzyb²³ et al.; these can be used in medical engineering applications, such as biological labeling and bioseparation. More importantly, Yang²⁴ et al. have successfully prepared multicolor emitting BaGdF₅:Ce³⁺/Ln³⁺ (Ln = Sm, Dy, Eu, Tb) nano/submicrocrystals and have also reported their size- and shape-controllable synthesis and luminescence properties. However, in these studies, the emission color could only be tuned by changing the doped Ln³⁺ ions or adjusting their doping concentrations. 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. Multicolor emitting Tb³⁺/Sm³⁺:Ca₂Gd₈Si₆O₂₆ phosphor powders at different excitation wavelengths were also synthesized via a solvothermal reaction method by Raju.²⁵ Tunable color nanomaterials of NaGdF₄:Tb³⁺/Sm³⁺ have also been reported by Guan¹⁸ et al. However, to the best of our knowledge, no reports have been published on the development of Tb³⁺,Sm³⁺ co-doped BaGdF₅ single-component phosphors and their excitation induced tunable multicolor emissions. In addition, since the Gd³⁺ (4f⁷,⁸S_7/2) ions with seven unpaired 4f electrons possess a large electron magnetic moment, rare earth ions-doped BaGdF₅ materials can be use as multifunctional materials which have potential applications in biomedicine.

In this work, we focus our attention on BaGdF₅ as a host with Tb³⁺ and Sm³⁺ ions as activators to synthesize a series of BaGdF₅:Tb³⁺,Sm³⁺ nanophosphors through an L-arginine hydrothermal method. In addition, we report the phase, morphology, color tunable luminescence and paramagnetic properties of trivalent rare earth ions-activated BaGdF₅ powders. Furthermore, the luminescence and energy transfer properties of the Tb³⁺ or Sm³⁺ doped BaGdF₅ phosphors have been discussed in detail.

2. Experimental section

2.1 Chemicals

All chemicals were of analytical grade and were utilized as purchased without any further purification. The Gd(NO₃)₃, Tb(NO₃)₃, and Sm(NO₃)₃ stock solutions were prepared by dissolving corresponding appropriate amounts of Gd₂O₃ (99.99%), Tb₄O₇ (99.99%), and Sm₂O₃ (99.99%) in dilute HNO₃ (15 mol L⁻¹) under heating with agitation, followed by evaporating the excess solvent. Gd₂O₃, Tb₄O₇, and Sm₂O₃ were purchased from Jiangxi Ganzhou Rare-Earth Limited Corporation.

2.2 Preparation

A series of rare earth-doped BaGdF₅ phosphors were synthesized by a facile hydrothermal process without further sintering treatment. Firstly, 2 mmol of RE(NO₃)₃ (Gd(NO₃)₃, Tb(NO₃)₃, and/or Sm(NO₃)₃) were added to a 100 mL flask with 0.5226 g barium nitrate (Ba(NO₃)₃). Then, after vigorous stirring for 20 min, 0.6968 g L-arginine were slowly added to the above solution. Finally, after additional agitation for 20 min, 0.8398 g sodium fluoride (NaF) was poured into the above solution. The as-obtained 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 room temperature naturally, the precipitates were separated by centrifugation, washed several times with deionized water and ethanol in sequence and dried in air at 60 °C for 12 h.

2.3 Characterization methods

The crystalline nature and phase structure of BaGdF₅:Tb³⁺,Sm³⁺ 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 and 30 kV; the scanning speed, step length and diffraction range were 10° min⁻¹, 0.1° and 20° to 75°, respectively. The sizes and morphologies of the samples were inspected using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) micrographs were acquired 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 magnetic performance of the samples was measured by a vibrating sample magnetometer (VSM, MPMS SQUID XL).

3. Results and discussion

3.1 XRD analysis

The XRD patterns of the prepared BaGdF₅ samples doped with Tb³⁺ and/or Sm³⁺ by the hydrothermal method without heat treatment are shown in Fig. 1. It can be seen that all the diffraction peaks can be readily indexed to cubic BaGdF₅, agreeing well with the data reported in the JCPDS standard card (no. 24-0098); no impurity peaks are detected, implying that the prepared samples were pure phase BaGdF₅. However, compared with the reference data, a small shift to larger angles in the XRD peaks could be observed, which is in agreement with the literature.^26–28 This is because the ionic radii of the Tb³⁺ and Sm³⁺ ions are smaller than that of Gd³⁺ ion. Moreover, the sharp, strong peaks indicate that the products synthesized at low temperatures are still highly crystalline, which is beneficial to obtain stronger luminescence.

Fig. 1 XRD patterns of the BaGdF₅:Tb³⁺, BaGdF₅:Sm³⁺, and BaGdF₅:Tb³⁺,Sm³⁺ samples; the corresponding standard data of BaGdF₅ (JCPDS no. 24-0098) is given as a reference.

The SEM images shown in Fig. 2 present the morphologies of the materials prepared by the hydrothermal method. From Fig. 2(a)–(c), it can be seen that the doped samples (BaGdF₅ (a), BaGdF₅:2% Tb³⁺ (b) and BaGdF₅:2% Tb³⁺,2% Sm³⁺ (c)) exhibit an irregular nanosphere morphology with a diameter of about 20 nm, which demonstrates that the doping species and the doping quality of the rare earth ions do not alter the morphology. From the TEM images, it can be clearly seen that these nanospheres have an average diameter of about 20 nm (Fig. 2(d)). The corresponding HRTEM image clearly shows lattice fringes with an interplanar spacing of 3.3 Å, ascribed to the (111) plane of BaGdF₅ (Fig. 2(e)).²⁷

Fig. 2 SEM images of BaGdF₅ (a), BaGdF₅:2% Tb³⁺ (b) and BaGdF₅:2% Tb³⁺,2% Sm³⁺ (c) phosphors; TEM (d) and HRTEM images (e) of BaGdF₅:2% Tb³⁺,2% Sm³⁺.

3.2 Fluorescence performance

Fig. 3 presents the room temperature photoluminescence excitation and emission spectra for BaGdF₅:2% Tb³⁺ (a), BaGdF₅:2% Sm³⁺ (b), and BaGdF₅:2% Tb³⁺,2% Sm³⁺ (c). The excitation spectrum of BaGdF₅:2% Tb³⁺ was obtained by monitoring the characteristic emission of Tb³⁺ (543 nm, ⁵D₄ → ⁷F₅). It can be found that the excitation spectrum exhibits a sharp absorbance peak at 272 nm, attributed to the Gd³⁺ ions, and several weak absorbance peaks between 350 and 400 nm, ascribed to the f–f transitions of Tb³⁺; this manifests that the Gd³⁺ ions transfer excitation energy to the Tb³⁺ ions. The emission spectrum of BaGdF₅:2% Tb³⁺ is obtained by excitation at 272 nm. One can see that there are four main emission peaks in the range of 400 to 700 nm at 486, 543, 583, and 620 nm, which correspond to the ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃, transitions, respectively. Fig. 3(b) gives the excitation and emission spectra for the luminescence of Sm³⁺ in BaGdF₅:2% Sm³⁺. The excitation spectrum of the BaGdF₅:2% Sm³⁺ product monitored at 594 nm consists of an absorbance peak of Gd³⁺ ions at 272 nm and some absorbance peaks of Sm³⁺ in the longer wavelength region. The presence of the excitation bands of Gd³⁺ indicates the existence of energy transfer from Gd³⁺ to Sm³⁺. The emission spectrum of the BaGdF₅:2% Sm³⁺ product excited at 272 nm exhibits three main groups of emission lines at about 558, 594, and 647 nm, which are ascribed to the ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, and ⁴G_5/2 → ⁶H_9/2 transitions. In Fig. 3(c), we can clearly see that the excitation spectrum of the BaGdF₅:2% Tb³⁺,2% Sm³⁺ product monitored at 543 (Tb³⁺) and 594 (Sm³⁺) nm includes very similar Tb³⁺, Sm³⁺ and Gd³⁺ excitation peaks, with an intense excitation peak at 272 nm. The emission spectrum of BaGdF₅:2% Tb³⁺,2% Sm³⁺ shows peaks at 594 and 648 nm due to the ⁴G_5/2 → ⁶H_J/2 (J = 7, 9) transitions of Sm³⁺ and at 543, 581 and 619 nm, ascribed to the ⁵D₄ → ⁷F_J (J = 5, 4, 3) transitions of Tb³⁺. The above discussions strongly demonstrate that Gd³⁺ can transfer energy to Tb³⁺ and Sm³⁺, respectively.

Fig. 3 Photoluminescence excitation and emission spectra for BaGdF₅:2% Tb³⁺ (a), BaGdF₅:2% Sm³⁺ (b), and BaGdF₅:2% Tb³⁺,2% Sm³⁺ (c).

Fig. 4(a) gives the emission spectrum of BaGdF₅:x% Sm³⁺ in the wavelength range of 500 nm to 700 nm, excited at 272 nm. There are three main sharp emission peaks, corresponding to transitions from the excited state ⁴G_5/2 to the ground states ⁶H_J: ⁴G_5/2–⁶H_5/2 (558 nm), ⁶H_7/2 (594 nm), and ⁶H_9/2 (646 nm). The emission intensity at 594 nm is stronger than that at 558 and 646 nm; namely, the intensity of the yellow emission is greater than the other emissions. Therefore, the BaGdF₅:x% Sm³⁺ phosphors emit strong yellow light. As shown in Fig. 4(b), the emission intensity increases with increasing Sm³⁺ doping concentration; the most luminous sample was obtained at the Sm³⁺ concentration of 3%, then decreased as the concentration increased further as a result of the concentration quenching effect.

Fig. 4 Representative photoluminescence emission spectra of BaGdF₅:x% Sm³⁺ (a); the variation of the ⁴G_5/2 energies of Sm³⁺ in the BaGdF₅:x% Sm³⁺ system with different Sm³⁺ doping concentrations (b).

The photoluminescence excitation spectra of the BaGdF₅:Tb³⁺, BaGdF₅:Sm³⁺ and BaGdF₅:Tb³⁺,Sm³⁺ nanoparticles at different emission wavelengths are exhibited in Fig. 5. The photoluminescence excitation spectra of BaGdF₅:Tb³⁺ and BaGdF₅:Tb³⁺,Sm³⁺ when monitoring the Tb³⁺ emission wavelength at 543 nm have f–f transitions of ⁷F₆ → ⁵G₂, ⁷F₆ → ⁵D₂ and ⁷F₆ → ⁵D₃ of Tb³⁺ at 340, 349, and 374 nm, respectively. The photoluminescence excitation spectra of BaGdF₅:Sm³⁺ and BaGdF₅:Tb³⁺,Sm³⁺ when monitoring the Sm³⁺ emission wavelength at 594 nm show ⁶H_5/2 → ⁴F_9/2 (372 nm), ⁶H_5/2 → ⁴F_7/2 (399 nm), ⁶H_5/2 → ⁴F_5/2 (463 nm) and ⁶H_5/2 → ⁴I_11/2 (475 nm) transitions of Sm³⁺. Therefore, the excitation position of Tb^{3+ 7}F₆ → ⁵D₃ (374 nm) and the Sm^{3+ 6}H_5/2 → ⁴F_9/2 (372 nm) transitions are very close. In view of the synchronous emissions of these two ions, it is believed that a near-UV wavelength around 372 nm may be used to efficiently excite these co-doped nanoparticles, which properly fits the requirements for WLEDs.²⁵ Moreover, the excitation intensity of 372 nm in the BaGdF₅:2% Tb³⁺,2% Sm³⁺ phosphors monitored at 594 nm is stronger than the excitation peak of 372 nm in the BaGdF₅:2% Sm³⁺ phosphors when monitored at 594 nm. More importantly, the Tb³⁺ and Sm³⁺ co-doped BaGdF₅ phosphors show weak excitation peaks (⁷F₆ → ⁵D₃, 374 nm) compared with the Tb³⁺-only doped BaGdF₅ phosphors, which indicates that the energy transfer from Tb³⁺ to Sm³⁺ is expected. Meanwhile, the excitation peaks overlap between 372 and 374 nm in the BaGdF₅:2% Tb³⁺,2% Sm³⁺ phosphors when monitored at 594 nm. Therefore, the energy transfer from Tb³⁺ to Sm³⁺ is still possible.

Fig. 5 The excitation spectra of the BaGdF₅:2% Tb³⁺, BaGdF₅:2% Sm³⁺ and BaGdF₅:2% Tb³⁺,2% Sm³⁺ phosphors.

Generally speaking, one of the essential conditions commonly required for energy transfer is overlap between the emission spectrum of the sensitizer and the excitation spectrum of the activator. Fig. 6 gives the excitation spectra of BaGdF₅:2% Sm³⁺ and the emission spectra of BaGdF₅:2% Tb³⁺. From Fig. 6, one can see that the ⁵D₄ → ⁷F₆ (486 nm) emission transition of Tb³⁺ and the ⁶H_5/2 → ⁴I_9/2 (480 nm) excitation transition of Sm³⁺ obviously intersect. Thereby, one can speculate that Tb³⁺ ions can transfer energy to Sm³⁺ ions when they are co-doped in a BaGdF₅ host. This type of energy transfer is quite common and has been observed in several Tb³⁺ and Sm³⁺ co-activated phosphors, such as LaCaO₃,²⁹ BaCeF₅,³⁰ NaGd(WO₄)₂,³¹ and NaGdF₄.¹⁸

Fig. 6 Photoluminescence emission and excitation spectra of BaGdF₅:2% Tb³⁺ and BaGdF₅:2% Sm³⁺.

In order to further establish the existence of the energy transfer process from Tb³⁺ to Sm³⁺ in the BaGdF₅ host, the photoluminescence excitation spectra of BaGdF₅:2% Tb³⁺ and BaGdF₅:2% Tb³⁺,2% Sm³⁺ are given in Fig. 7. The spectrum of BaGdF₅:2% Tb³⁺,2% Sm³⁺ reveals the ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃, transitions of Tb³⁺ ions as well as the ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, and ⁴G_5/2 → ⁶H_9/2 transitions of Sm³⁺ ions. However, for the BaGdF₅:2% Tb³⁺ sample, only Tb³⁺ emission bands without Sm³⁺ emission at 594 nm and 647 nm, corresponding to the ⁴G_5/2 → ⁶H_7/2, and ⁴G_5/2 → ⁶H_9/2 transitions, are observed. This further proves that the Tb³⁺ ions transfer energy to the Sm³⁺ ions. Meanwhile, this phenomenon is beneficial to obtain polychromatic light.

Fig. 7 Photoluminescence emission spectra of BaGdF₅:2% Tb³⁺ and BaGdF₅:2% Tb³⁺,2% Sm³⁺.

Taking into consideration the characteristic emissions of Tb³⁺ and Sm³⁺ ions as well as the existence energy transfer between Tb³⁺ and Sm³⁺ ions, a series of Tb³⁺ and Sm³⁺ co-doped samples with fixed Tb³⁺ ion content at 2% and varied Sm³⁺ ion concentrations from 0% to 9% were prepared. From Fig. 8(a), the characteristic emissions for both Tb³⁺ and Sm³⁺ ions can be clearly observed. Simultaneously, as shown in Fig. 8(a) and (b), the corresponding emission intensity of the Sm³⁺ activator (or energy acceptor) at a certain range was observed to increase, whereas that of the Tb³⁺ sensitizer (or energy donor) was simultaneously found to decrease monotonically with increasing Sm³⁺ dopant content. The above phenomena indicate that the emission intensity of Sm³⁺ is enforced by the introduction of Tb³⁺; this is attributed to the energy transfer phenomenon from Tb³⁺ ion to Sm³⁺ ion in the BaGdF₅ host material. Moreover, we also prepared a series of BaGdF₅:y% Tb³⁺,2% Sm³⁺ (y = 2, 3, 4, 5, 6) phosphors. From Fig. 8(c), the characteristic emissions for both Tb³⁺ and Sm³⁺ ions can also be clearly observed. Meanwhile, as exhibited in Fig. 8(c) and (d), although the Sm³⁺ concentration was fixed, the emission intensity of Sm³⁺ ions demonstrably increased. That phenomenon strongly demonstrates the occurrence of energy transfer from the Tb³⁺ to the Sm³⁺ ions. More importantly, the relative luminescence intensities of the three emission peaks of Tb³⁺ and the two emission peaks of Sm³⁺ vary with alternating doping concentrations of Sm³⁺. Therefore, the emitting color of the products can be tuned easily.

Fig. 8 Photoluminescence emission spectra of the BaGdF₅:2% Tb³⁺,x% Sm³⁺ (x = 0, 2, 3, 4, 5, 6, 7, 9) (a) and BaGdF₅:y% Tb³⁺,2% Sm³⁺ (y = 2, 3, 4, 5, 6) (c) samples with different Tb³⁺ or Sm³⁺ doped concentrations (λ_ex = 372 nm); the emission intensities of Tb³⁺ and Sm³⁺ as a function of Tb³⁺ or Sm³⁺ concentration (b and d).

In order to further demonstrate the existence of the energy transfer process from Gd³⁺ to Tb³⁺ and Sm³⁺ in the BaGdF₅ host as well as to take into consideration the characteristic emissions of terbium (Tb³⁺) and samarium (Sm³⁺) ions, it was expected that different colors of green emissions could be achieved in the BaGdF₅ system with properly designed activator contents. Therefore, Fig. 9(a) and (b) gives the emission spectra of BaGdF₅:2% Tb³⁺,x% Sm³⁺ (x = 0, 1, 2, 3, 4, 5, 6, 7, 9) (a) and BaGdF₅:y% Tb³⁺,2% Sm³⁺ (y = 1, 2, 3, 4, 5) (b) samples excited at 272 nm. From Fig. 9(a) and (b), we can clearly see that the emission spectrum of the BaGdF₅:2% Tb³⁺,x% Sm³⁺ (x = 0, 1, 2, 3, 4, 5, 6, 7, 9) (a) and BaGdF₅:y% Tb³⁺,2% Sm³⁺ (y = 1, 2, 3, 4, 5) (b) products excited at 272 nm contain very similar Tb³⁺ and Sm³⁺ emission peaks, which strongly proved the existence of energy transfer from Gd³⁺ to Tb³⁺ and Sm³⁺ ions. It is well known that Tb³⁺ ions emit green light and Sm³⁺ ions emit yellow light. As shown in Fig. 9(a) and (b), the emission intensities of the Tb³⁺ ions are much greater than those of the Sm³⁺ ions. Therefore, different colors of green emissions could be achieved in the BaGdF₅ system by properly adjusting the Sm³⁺ ion content.

Fig. 9 Photoluminescence emission spectra of the BaGdF₅:2% Tb³⁺,x% Sm³⁺ (x = 0, 1, 2, 3, 4, 5, 6, 7, 9) (a) and BaGdF₅:y% Tb³⁺,2% Sm³⁺ (y = 1, 2, 3, 4, 5) (b) samples with different Tb³⁺ or Sm³⁺ doped concentrations (λ_ex = 272 nm); the inset shows the emission intensities of Tb³⁺ and Sm³⁺ as a function of Tb³⁺ or Sm³⁺ 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.³² Therefore, we researched the emission spectra of the different excitation wavelengths for the BaGdF₅:2% Tb³⁺,2% Sm³⁺ sample. From Fig. 10, one can see that when excited at 358 and 342 nm, the emission intensities of Sm³⁺ ions are obviously stronger than those of Tb³⁺ ions. However, upon excitation at 272 nm, the emission intensities of the Tb³⁺ ions are clearly greater than those of the Sm³⁺ ions. More importantly, we only can see the characteristic emissions of Sm³⁺ ions under excitation at 399, 463 and 475 nm. The above discussions powerfully imply that multicolor luminescence can be obtained in the BaGdF₅ host by adjusting the excitation wavelength.

Fig. 10 Photoluminescence emission spectra of BaGdF₅:2% Tb³⁺,2% Sm³⁺ samples excited at different wavelengths.

To understand the energy transfer and concentration quenching, the critical distance R_c between Tb³⁺ and Sm³⁺ can be calculated using the relationship given by Blasse:^33,34

R_Tb–Sm = 2 × [3V/(4πx_cZ)]^1/3

where V stands for the unit cell volume, x_c is the critical concentration at which the luminescence intensity of Tb³⁺ is half of that in the absence of Sm³⁺, and Z represents the number of sites that the activator ion can occupy in the host. For the BaGdF₅ host, V = 218.49 Å, Z = 2, and x_c = 0.02 + 0.065 = 0.085. The critical distance R_c is calculated to be 13.49 Å. According to Dexter's reports, the resonant energy transfer mechanism consists of two main aspects: (1) exchange interaction; (2) multipole–multipole interaction, which depends on the critical distance between the donor and the acceptor ions. Exchange interaction will take place when the value of R_c is less than 5 Å; otherwise, the multipolar interactions may dominate. Obviously, the energy transfer from Tb³⁺ to Sm³⁺ ions in the BaGdF₅ host can be attributed to multipolar interactions.

As a more detailed analysis method for the energy transfer mechanism, according to Dexter's energy transfer expressions of multipolar interaction, the following relationship can be obtained:^35,36

I_S0/I_S ∝ C^n/3

where I_S0 is the intrinsic luminescence intensity of Tb³⁺, and I_S is the luminescence intensity of Tb³⁺ in the presence of Sm³⁺. C is the sum of the concentration of Dy³⁺ and Eu³⁺; n = 6, 8 and 10, corresponding to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The plots of I_S0/I_S of Tb³⁺ with n = 6, 8, 10 are exhibited in Fig. 11. As shown in Fig. 10, a linear relationship is well-fitted at n = 6, indicating that the energy transfer mechanism occurs via a dipole–dipole interaction between the Tb³⁺ and Sm³⁺ ions.

Fig. 11 Dependence of I₀/I of Tb³⁺ on (a) C^6/3, (b) C^8/3, and (c) C^10/3.

A schematic proposed for the probable processes of energy transfer in the BaGdF₅:Tb³⁺,Sm³⁺ phosphors is exhibited in Fig. 12. During the whole excitation process, the electrons located at the ground state of the Gd³⁺ ions first absorb the photon energies of UV/n-UV light and then jump to the ⁶P_7/2 energy level. Instantaneously, the Gd³⁺ ion transfers the energy to the Tb³⁺ and Sm³⁺ ions. In the BaGdF₅:Tb³⁺,Sm³⁺ system, the Tb³⁺ ions, on the one hand, emit green light due to the ⁵D₄ → ⁷F₅ transition (543 nm). On the other hand, the Tb³⁺ ions transfer energy to the Sm³⁺ ions, which emit yellow light based on the transitions of ⁴G_5/2 → ⁶H_5/2 (565 nm) and ⁴G_5/2 → ⁶H_7/2 (594 nm).

Fig. 12 The proposed scheme of energetic processes occurring in the BaGdF₅:Tb³⁺,Sm³⁺ sample.

From the emission spectra of Tb³⁺ and Sm³⁺ co-doped in BaGdF₅ (Fig. 8(a) and 9(a)), we can see that the emission intensities of Tb³⁺ ions and Sm³⁺ ions varied with increasing Sm³⁺ ion concentration although the concentration of Tb³⁺ ions is fixed; this is a feasible route to realize color-tunable emission under UV excitation. Therefore, the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of the BaGdF₅:2% Tb³⁺,x% Sm³⁺ (x = 0, 1, 2, 3, 4, 5, 6, 7, 9) phosphors excited at 272 nm with different Sm³⁺ doping concentrations are given in Fig. 13(A, 1–7). As shown in Fig. 13(A, 1–7), the pure Tb³⁺-activated BaGdF₅ emits strong green light. Then, when the Sm³⁺ ions are introduced into the BaGdF₅:2% Tb³⁺ phosphors, the emitted color moves to the yellowish green region. From Fig. 13(B, 15–18), one can see that the emitted color moves from yellowish green to green for the BaGdF₅:y% Tb³⁺,2% Sm³⁺ (y = 1, 2, 3, 4, 5) samples excited at 272 nm. From Fig. 13(A, 8), one can see that the pure Tb³⁺-activated BaGdF₅ excited at 372 nm emits strong blue light. For BaGdF₅:2% Tb³⁺,x% Sm³⁺ (x = 2, 4, 5, 6, 7, 9) (A, 9–14) and BaGdF₅:y% Tb³⁺,2% Sm³⁺ (y = 1, 2, 3, 4, 6) (B, 19–23) phosphors excited at 372 nm, the color is mainly concentrated in the white light. Moreover, the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of the BaGdF₅:2% Tb³⁺,2% Sm³⁺ phosphors excited at 272, 342, 358, 399, 463 and 475 nm are also given in Fig. 13(C). From Fig. 13(C), one can see that for the BaGdF₅:2% Tb³⁺,2% Sm³⁺ phosphors excited at 272, 342, 358, 399, 463, and 475 nm, the corresponding CIE coordinates for all the excitation wavelengths are determined to be (16, 0.344,0.472), (23, 0.319,0.310), (24, 0.354,0.328), (25, 0.491,0.404), (26, 0.426,0.465) and (27, 0.482,0.49), which are located in the yellowish green, white, white, orange red, greenish yellow and yellow regions, respectively. All of the results indicate that multicolor luminescence can be achieved by adopting different excitation wavelengths or adjusting the appropriate concentration of Sm³⁺ and Tb³⁺. Finally, the specific CIE chromaticity coordinates of the BaGdF₅:Tb³⁺,Sm³⁺ samples are given in Table 1.

Fig. 13 CIE chromaticity diagram (A–C) of BaGdF₅:Tb³⁺,Sm³⁺. Inset: the corresponding images under corresponding excitation wavelengths.

Table 1 The CIE chromaticity coordinates for the BaGdF₅:Tb³⁺,Sm³⁺ samples

Label	Sample	Excitation (nm)	CIE (x,y)	Color
1	BaGdF5:0.02Tb^3+	272	(0.279,0.480)	Green
8	BaGdF5:0.02Tb^3+	372	(0.223,0.275)	Blue
2	BaGdF5:0.02Tb^3+,0.01Sm^3+	272	(0.318,0.474)	Green
9	BaGdF5:0.02Tb^3+,0.02Sm^3+	372	(0.346,0.334)	White
3	BaGdF5:0.02Tb^3+,0.03Sm^3+	272	(0.337,0.454)	Green
10	BaGdF5:0.02Tb^3+,0.04Sm^3+	372	(0.351,0.327)	White
4	BaGdF5:0.02Tb^3+,0.05Sm^3+	272	(0.338,0.443)	Green
11	BaGdF5:0.02Tb^3+,0.05Sm^3+	372	(0.329,0.295)	White
5	BaGdF5:0.02Tb^3+,0.06Sm^3+	272	(0.327,0.423)	Green
12	BaGdF5:0.02Tb^3+,0.06Sm^3+	372	(0.340,0.312)	White
6	BaGdF5:0.02Tb^3+,0.07Sm^3+	272	(0.304,0.379)	Bluish green
13	BaGdF5:0.02Tb^3+,0.07Sm^3+	372	(0.336,0.305)	White
7	BaGdF5:0.02Tb^3+,0.09Sm^3+	272	(0.304,0.379)	Green
14	BaGdF5:0.02Tb^3+,0.09Sm^3+	372	(0.298,0.268)	White
15	BaGdF5:0.01Tb^3+,0.02Sm^3+	272	(0.344,0.436)	Yellowish green
19	BaGdF5:0.01Tb^3+,0.02Sm^3+	372	(0.335,0.301)	White
16	BaGdF5:0.02Tb^3+,0.02Sm^3+	272	(0.334,0.472)	Yellowish green
17	BaGdF5:0.03Tb^3+,0.02Sm^3+	272	(0.333,0.491)	Yellowish green
20	BaGdF5:0.03Tb^3+,0.02Sm^3+	372	(0.333,0.349)	White
21	BaGdF5:0.04Tb^3+,0.02Sm^3+	372	(0.357,0.349)	White
18	BaGdF5:0.05Tb^3+,0.02Sm^3+	272	(0.331,0.501)	Yellowish green
22	BaGdF5:0.06Tb^3+,0.02Sm^3+	372	(0.340,0.395)	White
23	BaGdF5:0.02Tb^3+,0.02Sm^3+	342	(0.319,0.310)	White
24	BaGdF5:0.02Tb^3+,0.02Sm^3+	358	(0.354,0.328)	White
25	BaGdF5:0.02Tb^3+,0.02Sm^3+	399	(0.491,0.404)	Orange red
26	BaGdF5:0.02Tb^3+,0.02Sm^3+	463	(0.426,0.465)	Greenish yellow
27	BaGdF5:0.02Tb^3+,0.02Sm^3+	475	(0.482,0.495)	Yellow

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Quantum efficiency is an important parameter for phosphors. The quantum efficiencies for the BaGdF₅:2% Sm³⁺ and BaGdF₅:2% Tb³⁺,x% Sm³⁺ (x = 2, 3, 4, 5) phosphors are 4.42%, 6.01%, 10.51%, 8.36% and 5.07%, respectively. With additional research, we are studying methods of improving the quantum efficiency by optimizing the preparation conditions and the composition of the products.

3.3 Magnetic properties

In addition to the down-conversion multicolor emissions, the magnetic properties of the BaGdF₅ and BaGdF₅:2% Tb³⁺,2% Sm²⁺ samples at room temperature (300 K) and low temperature (2 K) were also investigated. From Fig. 13(A) and (B), one can see that the BaGdF₅ and BaGdF₅:2% Tb³⁺,2% Sm³⁺ samples exhibit good paramagnetism in the magnetic range of −30 000 to 300 000 Oe at 300 K due to the lack of coercivity or remanence, and the magnetizations were 2.09 and 2.01 emu g⁻¹, respectively (Fig. 14(A)), which approaches the value reported for nanoparticles used for common bioseparation.^37,38 Meanwhile, BaGdF₅ and BaGdF₅:2% Tb³⁺,2% Sm³⁺ show superparamagnetism at low temperature (2 K) with saturation magnetization values of 87.49 and 84.60 emu g⁻¹ (Fig. 14(B)). The increase in the magnetic susceptibility at low temperature may be due to the reduction in thermal fluctuation, which is typical behaviour in paramagnetic materials, as described by Curie's law.³⁹ Furthermore, the magnetism of the pure BaGdF₅:2% Tb³⁺,2% Sm³⁺ sample is slightly smaller than that of BaGdF₅. The reason is that introducing 2% Tb³⁺ and 2% Sm³⁺ ions to substitute for Gd³⁺ can decrease the molar concentration of Gd³⁺ in the BaGdF₅ nanocrystals.

Fig. 14 Magnetization–applied magnetic field curves of BaGdF₅ and BaGdF₅:2% Tb³⁺,2% Sm³⁺ nanospheres at 300 K (A) and 2 K (B), respectively.

4. Conclusions

To summarize, a simple hydrothermal method has been used to prepare BaGdF₅:Tb³⁺/Sm³⁺ nanocrystals. XRD and FE-SEM analysis indicated that the samples crystallized in the cubic structure with nanocrystalline morphology and an average diameter of 20 nm. The photoluminescence spectra of BaGdF₅:Sm³⁺ nanocrystals demonstrated that the Sm³⁺ in BaGdF₅ host emits yellow light, and energy is transferred from Gd³⁺ to Sm³⁺. By co-doping Tb³⁺ and Sm³⁺ ions into the host, due to the different excitation wavelengths, the colour tone of the phosphors shifts gradually from yellowish green (0.334,0.472) to white (0.319,0.310) and then to yellow (0.482,0.495). The energy migration from Tb³⁺ to Sm³⁺ has been verified to be an electric dipole–dipole interaction mechanism, of which the critical distance (R_Tb–Sm) is estimated to be 13.49 Å. Our results indicate that the developed phosphor has significant potential to be used as a single-component full-color phosphor for fabricating full-color devices. Furthermore, 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. 51272085) and the Opening Research Funds Projects of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2016-01).

Notes and references

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