One-step surfactant-free controllable synthesis and tunable up-conversion/down-shifting white light emissions of Sr₂YF₇ crystals doped with Ln³⁺ ions

Abstract

In this work, uniform and monodisperse Sr₂YF₇ spheres were successfully synthesized via a facile one-step hydrothermal route without employing any surfactant. Particularly, a well-defined morphology of Sr₂YF₇ crystals ranging from spherical to octahedral shape was first achieved by adjusting the pH values. In addition, the spherical products doped with 6% Ln³⁺ (Ln = La, Ce, Pr, Sm, Gd, and Dy) show different sizes ranging from 300 to 600 nm. The Sr₂YF₇ crystals also exhibit excellent multicolor DS/UC luminescence properties after doping with different Ln³⁺ ions. Energy transfer from Tb³⁺ to Eu³⁺ was observed in Sr₂YF₇ crystals; the Sr₂YF₇:Dy³⁺ crystals showed nearly white light emission under UV excitation; Sr₂YF₇:Yb³⁺/Er³⁺/Tm³⁺ crystals could produce pure white light with a CIE color coordinate of (0.3123, 0.3005) by controlling the doping concentrations of Er³⁺ and Tm³⁺ ions under 980 nm excitation. The as-prepared phosphors may potentially serve as light, color displays and markers in biological imaging.

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1 Introduction

Nowadays, inorganic rare earth (RE) materials doped with trivalent lanthanide ions (Ln³⁺) are drawing attention greatly because of their wide application in various devices such as white light-emitting diodes,¹ solid-state lasers,² solar cells,³ and sensors.⁴ Among the many inorganic rare earth luminescent matrix materials,^5–7 rare earth fluoride is of particular interest due to its toxicity-free nature and high stability compared to some organic dyes and semiconductor quantum dots,^8,9 as well as its utmost efficiency for up-conversion emission.¹⁰ It is well known that fluoride materials have low lattice phonon energy to effectively reduce phonon-assisted nonradiative relaxation according to theoretical calculations.¹¹

Thus far, there have been several approaches to prepare RE-based fluorides with controllable phases, morphologies and chemical compositions.¹² The three most common methods are thermal decomposition, high-temperature coprecipitation and hydro(solvo)-thermal synthesis, respectively. However, the first two ways may also bring some inevitable problems like complex experimental conditions, pollution and safety concerns^13–15 despite high manufacturing yields. It seems that the hydrothermal method suits better to gain highly crystalline fluoride materials under relatively mild conditions (convenient, simple and environment-friendly). Generally, surfactants were employed to control the morphology and shapes during the preparation of materials by a wet chemical method. In fact, it is difficult to remove the residual surfactants entirely and they may even further affect the phosphors' luminous quenching centers and luminescence properties.¹⁶ Therefore, it is of significant importance to find a proper way to simplify the synthetic process of RE-based fluorides without any additives in the premise of desirable results.

Up to now, much effort has been made for the synthesis of RE-based fluorides (RE = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) with controllable crystalline phases, shapes and sizes.^17–20 In the family of Y³⁺-based fluorides, except for NaYF₄,²¹ YOF,²² LiYF₄ (ref. 23) and BaYF₅,²⁴ Sr₂YF₇, as an important matrix for Ln³⁺ ions to fabricate up-conversion (UC) or down-shifting (DS) phosphors, has aroused much research interest. Recently, Chen's group has reported Sr₂YF₇:Ln³⁺ nanoparticles for biodetection by a thermal decomposition method;²⁰ Xia has prepared Sr₂LnF₇:Yb³⁺, Er³⁺ (Ln = Y, Gd) nanocrystals by an EDTA-assisted method;²⁵ Ma reported ultrasmall face-centered cubic Sr₂YF₇:Yb³⁺, Er³⁺/Yb³⁺, Tm³⁺ nanocrystals under the condition of oleic acid;²⁶ Han's group has also made the hierarchical microspheres Sr₂YF₇:Eu³⁺ by a hydrothermal method.²⁷ Although several attempts have been made for the synthesis of Ln³⁺-doped Sr₂YF₇, only color emissions were produced, as previously reported in the literature. To date, a systematic survey on the optical properties, in particular UC and DS white light emission of Ln³⁺ in the Sr₂YF₇ host, is still lacking. To gain deep insights into the understanding of the electronic structure and optical properties (in particular, white light emission) of Ln³⁺ ions in the Sr₂YF₇ host is of vital importance for optimizing their optical performance for potential applications.

Here, we produced uniform and monodisperse Sr₂YF₇:Ln³⁺ microcrystals by a facile one-pot hydrothermal approach without any surfactant. The morphologies and sizes of the Sr₂YF₇ microcrystals could be easily adjusted by doping with different Ln³⁺ ions and changing the pH values. We performed a study of multi-doping Ln³⁺ ions in the Sr₂YF₇ host. The prepared Sr₂YF₇:Ln³⁺ (Ln = Dy, Tb, Eu, Yb, Er, Tm) phosphors showed tunable DS/UC luminescence properties. Specifically, we first found the energy transfer of Tb³⁺ → Eu³⁺ and obtained white light emission of tri-doped Yb³⁺–Er³⁺–Tm³⁺ in the Sr₂YF₇ host, respectively.

2 Experimental section

Chemicals

RE₂O₃ (RE = Yb, Er, Ho, Dy, Gd, Eu, Sm and La) (99.99%), Ce(NO₃)₃ (99.99%), Tb₄O₇ (99.99%) and Pr₆O₁₁ (99.99%) were acquired from Goring High-tech Material Corporation Limited (China). Strontium chloride (SrCl₂), sodium tetrafluoroborate (NaBF₄), sodium hydroxide (NaOH), and hydrochloric acid (HCl) were purchased from Aladdin Industrial Corporation (China) and used without further purification. RECl₃ (RE = Yb, Er, Ho, Dy, Tb, Gd, Eu, Sm, Pr and La) were prepared by dissolving the corresponding rare earth oxides in dilute HCl and then evaporating the excess HCl.

Synthesis

In a typical synthesis for the preparation of the Sr₂YF₇ host, 2 mmol SrCl₂, 1 mmol YCl₃ solution and 30 mL deionized water were added together to form a transparent solution by stirring in a 100 mL beaker for 15 minutes. Subsequently, 8 mmol NaBF₄ was added dropwise into the above solution under vigorous stirring. The pH was adjusted to the required value by adding 1 mol L⁻¹ NaOH solution dropwise. The volume of the system was finally maintained at 35 mL. After the process of enough stirring, the obtained solution was transferred into a Teflon bottle held in a stainless steel autoclave, sealed and kept at 220 °C for 24 h, and then cooled to room temperature naturally. The final products were centrifuged, washed several times with deionized water and ethanol, and then dried at 80 °C for 12 h. Other samples (Sr₂YF₇:Ln³⁺) were synthesized by a similar method except for using different stoichiometric ratios of rare earth ions.

Characterization

Powder X-ray diffraction (XRD) measurements were performed using a Purkinje Genera Instrument MSALXD3 under the condition of Cu Kα radiation (λ = 0.15406 nm) at a scanning rate of 8° min⁻¹ from 20° to 70° with 2θ, 20 mA and 36 kV. The morphology and energy-dispersive spectrometry of the samples were inspected using a field emission scanning electron microscope (FE-SEM, XL30, Philips) operating at an accelerating voltage of 10 kV. The DS PL measurements were conducted using a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The UC PL measurements were carried out using a 980 nm laser with MDL-N-980-8W as the excitation source. The luminescence decays were recorded using an FLSP920 fluorescence spectrophotometer and a Shimadzu R9287 photomultiplier (200–900 nm) accompanied by a liquid nitrogen-cooled InGaAs (800–1700 nm) diode as the detector. All the measurements were performed at room temperature.

3 Results and discussion

3.1. Phase identification and morphology of Sr₂YF₇ hosts

Sr₂YF₇ has a tetragonal crystal structure with a space group of I4/mcm (a = b = 11.416 Å, c = 13.291 Å and Z = 10). As shown in Fig. 1a, the well-defined diffraction peaks of the obtained products can fit well with the standard data of Sr₂YF₇ (JCPDS no. 53-0675). The crystallite size of the samples can be estimated from the Scherrer equation, D = 0.941λ/β cos θ, where D is the average grain size, the factor 0.941 is characteristic of spherical objects, λ is the X-ray wavelength (0.15405 nm), and θ and β are the diffraction angle and full-width at half-maximum (FWHM, in radian) of an observed peak, respectively. The strongest peaks (302) at 2θ = 26.976° were used to calculate the average crystallite size (D) of the Sr₂YF₇ host. The estimated average crystallite size is about 26 nm.

Fig. 1 XRD pattern (a) and SEM image (b) of the as-synthesized Sr₂YF₇ host under the conditions of Sr²⁺/Y³⁺/BF₄⁻ = 2 : 1 : 8 and pH = 3. Sr₂YF₇ has the stoichiometric ratio of Sr : Y : F = 2 : 1 : 7, so we use excess NaBF₄ to get pure-phase Sr₂YF₇ products.

The SEM indicates that the morphology of products shows uniform and monodisperse spheres with a diameter of 500 nm, as shown in Fig. 1b. Moreover, the high-magnification SEM image (inset of Fig. 1b) clearly shows that the obtained product is polycrystalline,^28,29 as it further actually consists of many smaller grains with a size of 20–40 nm. The size is basically in agreement with the results estimated from the above-mentioned Scherrer equation (26 nm). Because smaller nanograins contribute more to the broadening of the diffraction peaks, the average nanocrystal size estimated from the Scherrer equation is smaller than that determined from the SEM topology in the case of Sr₂YF₇ spheres.³⁰ The barely low agglomeration of spherical particles are beneficial for practical applications like form coating and ceramic. Generally speaking, the phase and morphology of nano/micromaterials not only depend on the aimed compounds' intrinsic structures but also involve many experiment factors like types of raw materials, pH values, doping ions, temperatures and reaction times. Here, we used NaBF₄ as a F⁻ source and kept the other conditions unchanged (temperature, reaction time and free-surfactant). The effects of pH values and various doping rare earth ions on the phase/morphology of crystals are discussed in detail next.

3.1.1. Effect of pH on the phase and morphology. To study the role that pH plays in the system, we add sodium hydroxide (NaOH) to change the pH of the initial solution under the other same condition. Fig. 2 illustrates the typical XRD patterns of Sr₂YF₇ samples with different pH values. Compared with the standard peaks, no other diffraction peaks are detected and all the diffraction peaks of the as-prepared Sr₂YF₇ particles are barely shifted. The more significantly important thing is the high crystallinity, which usually suggests less defects and stronger luminescence.

Fig. 2 XRD patterns of the Sr₂YF₇ host with different pH values.

To precisely analyse the influence of pH values on the morphological evolution, the SEM images are provided in Fig. 3. First, the pH is 3 during the process of preparation. As shown in Fig. 3a, the original Sr₂YF₇ host consists of numerous uniform and monodisperse spheres whose surfaces are consisting of many smaller grains with a size of 20–40 nm (Fig. 1b). When the pH was adjusted to 4 (Fig. 3b), it was obvious that the morphology of crystals remained unchanged, which implied the coexistence of sphere-like and octahedron-like shapes. The shapes were assembled by many rhombus-like nanoparticles, showing a faceted morphology.³¹ Small particles have high surface energy and the nanoparticles quickly aggregate together to reduce the energy.^32,33 Upon further addition of NaOH to the solution (pH = 5), the morphologies of the obtained crystals all converted into octahedral shapes (Fig. 3c). At pH = 7, most of the octahedral shapes become larger and some even combination of polyhedral structures (Fig. 3d). The schematic of the morphological evolution can be seen in Fig. 3e.

Fig. 3 SEM images of Sr₂YF₇ crystals prepared at different pH values: (a) pH = 3, (b) pH = 4, (c) pH = 5, and (d) pH = 7. Schematic of the morphological evolution (e).

By continuously increasing the pH value, abnormal phase transition occurs and the resulting product consists of many smaller nanoparticles forming irregular aggregations. Thus, we can infer that various pH values are the key elements influencing the morphology and phase of the final products. During the synthetic process, F⁻ ions are released from the hydrolysis of NaBF₄:

BF₄⁻ + 3H₂O → 3HF + F⁻ + H₃BO₃

The high pH values increase the concentration of F⁻ ions in the solution, which affect the rate of crystal nucleation and crystal growth.³² By adding OH⁻ ions into the solution to adjust the pH, the nucleation rate could be changed owning to the competition of OH⁻ ions and F⁻ ions. With the increase in OH⁻ ions, the unavoidable chelation between OH⁻ anions and Y³⁺/Sr²⁺ cations takes place, thus inhibiting the integration between F⁻ ions and Y³⁺/Sr²⁺ ions to get larger crystals by kinetics.³⁴ Some researchers have also investigated that Na⁺ ions may be selectively absorbed on the different crystal facets of nanoparticles, leading to different shapes.³⁵ In summary, the pH values of the initial solution can effectively change the balance between the ion transport rates and chemical potential, and then affect the crystal orientation.³⁶ Based on these analysis results, the size- and morphology-controllable Sr₂YF₇ nanocrystals can be effectively synthesized by a facile one-pot hydrothermal approach without surfactants.

3.1.2. Effect of doping Ln³⁺ on the phase and morphology. It has been investigated that the size and shape of alkaline earth-rare earth fluorides can be modified by doping Ln³⁺ ions.^27,37 Here, the Sr₂YF₇:6% Ln³⁺ (Ln = La, Ce, Pr, Sm, Gd and Dy) crystals have already been prepared and Fig. 4 displays the XRD results of these samples. All diffraction peaks of Sr₂YF₇:6% Ln³⁺ (Ln = La, Ce, Pr, Sm, Gd and Dy) can be readily indexed to the pure tetragonal phase of Sr₂YF₇ (space group: I4/mcm) according to the JCPDS file no. 53-0675. No additional peaks of other phases have been found, indicating that the Ln³⁺ ions are effectively built into the Sr₂YF₇ host lattice. Moreover, the characteristic diffraction peak corresponding to the (302) crystal plane has a slight shift toward the low 2θ direction after doping the Ln³⁺ ions (seeing dot line in Fig. 4), which reveals that Ln³⁺ ions were successfully incorporated into the crystal lattice of Sr₂YF₇ crystals and occupied the position of the original Y³⁺ ions. The ionic radii of Y³⁺ and Dy³⁺–La³⁺ are 0.9 Å and 0.97–1.1 Å, respectively. Therefore, Ln³⁺ ion dopants (Ln = La, Ce, Pr, Sm, Gd and Dy) are expected to occupy Y³⁺ sites because of a similar ionic radius and valence.¹⁰ However, when the Y³⁺ ions are replaced with Dy³⁺–La³⁺, the unit cell volume becomes larger, which is visually expressed as the diffraction peak shifts to a lower 2θ degree from Dy³⁺ to La³⁺ ions.³⁸

Fig. 4 XRD patterns of Sr₂YF₇:6% Ln³⁺.

The doped Ln³⁺ ions indeed make some subtle differences in the host.³⁷ This is the reason that the obtained monodisperse spheres of Sr₂YF₇ in the presence of 6% doped Ln³⁺ ions become bigger from La³⁺ to Dy³⁺ (Fig. 5). Once the lanthanide dopants from La³⁺ to Dy³⁺ with a radius ranging from 1.1 Å to 0.97 Å were mixed into the crystal lattice of Sr₂YF₇, the size of the Sr₂YF₇ microspheres composed of nanoparticles was remarkably changed with an average diameter ranging from 300 nm to 600 nm (Fig. S1†). In fact, the substitution of Y³⁺ ions with Ln³⁺ ions with a larger radius could decelerate the anisotropic crystalline growth, and the electron charge density of surface relevantly increases, which can substantially slow the diffusion rate of F⁻ ions towards the surface of the rare-earth fluoride crystal nuclei as a result of charge repulsion.^28,39

Fig. 5 SEM images of Sr₂YF₇ doped with different Ln³⁺ ions: (a) 6% La³⁺, (b) 6% Ce³⁺, (c) 6% Pr³⁺, (d) 6% Sm³⁺, (e) 6% Gd³⁺ and (f) Dy³⁺, respectively.

3.2. Luminescence properties

After investigating the effect of pH values and doped Ln³⁺ ions on the morphology and phase of the Sr₂YF₇ host, it was reported that Sr₂YF₇ is an excellent host to contain various rare earth ions for rich luminescence. Herein, we are devoted to the DS/UC luminescence properties including Dy³⁺, Tb³⁺ and Eu³⁺ single-doped, Tb³⁺/Eu³⁺, Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ co-doped and Yb³⁺/Er³⁺/Tm³⁺ tri-doped in the Sr₂YF₇ host. We used solid samples for all the luminescence measurements at room temperature.

3.2.1. DS photoluminescence. The Eu³⁺ ion is one of the most excellent red-emitting activators in rare-earth ion-doped commercial phosphors due to its dominant transition in the red spectral area. The Sr₂YF₇:10% Eu³⁺ crystals are selected as a representative example to show its PL properties. As shown in Fig. 6a, the excitation spectrum (monitored at 595 nm) consists of several narrow peaks, which are the Eu³⁺ characteristic f–f transitions (318 nm, ⁷F₀ → ⁵H₆; 362 nm, ⁷F₀ → ⁵D₄; 382 nm, ⁷F₀ → ⁵G₂; 394 nm, ⁷F₀ → ⁵L₆, strongest one; 465 nm, ⁷F₀ → ⁵D₂) of its 4f⁶ configurations. In addition, the characteristic emissions (excited at 394 nm) result from the transitions of ⁵D_0,1 → ⁷F_J (J = 1, 2, and 3) including ⁵D₁ → ⁷F₃ (558 nm), ⁵D₀ → ⁷F₁ (595 nm) and ⁵D₀ → ⁷F₂ (620 nm). Moreover, it is quite obvious that the emission centered at 595 nm belonging to the ⁵D₀ → ⁷F₁ magnetic dipole transition is stronger than 620 nm belonging to the ⁵D₀ → ⁷F₂ electric dipole transition, indicating that the Eu³⁺ ions are encased in a high symmetric environment according to the Judd–Ofelt theory.⁴⁰ Therefore, Eu³⁺ ions are often applied to study the local site characteristics as a structure probe.⁴¹ From Fig. 6b, the emission intensities of Eu³⁺ rapidly increase with the increase in doping concentration and reaches a maximum at x = 10, and then slowly decreases due to concentration quenching. With the increase in concentration of Eu³⁺ ions, the resonance energy transfer is allowed because the distance of the luminescent centers becomes short enough to bring about the energy transfer from one luminescent center to another. Therefore, the concentration quenching of Eu³⁺ ions takes place when the concentration of Eu³⁺ ions increases to a high enough level.

Fig. 6 PL excitation (left) and emission (right) spectra of Sr₂YF₇:10% Eu³⁺ (a) and PL emission intensity at 595 nm with different Eu³⁺ concentrations (b).

To find the optimum doping concentration (doping concentration when the sample shows the highest emission) in Sr₂YF₇:Tb³⁺ and Sr₂YF₇:Dy³⁺, different results are given in Fig. S2.†Fig. 7A displays the PL spectra of the acquired Sr₂YF₇:18% Tb³⁺ phosphors. When monitored with the 547 nm emission of Tb³⁺ (⁵D₄ → ⁷F₅), the excitation spectrum consists of a strong band with maximum at 217 nm in the range of 200–300 nm and several weak peaks in the range of 300–400 nm, corresponding to the spin-allowed f → d transition and f → f transitions of Tb³⁺ ions, respectively. Under 217 nm UV excitation, the emission spectrum exhibits the ⁵D₄ → ⁷F_j (j = 3, 4, 5, 6) transitions of Tb³⁺ ions. Four narrow peaks centered at 490 nm, 547 nm, 587 nm and 624 nm, originating from ⁵D₄ → ⁷F₆ (490 nm) in the blue region, ⁵D₄ → ⁷F₅ (547 nm) in the green region, and ⁵D₄ → ⁷F₄ (587 nm) and ⁵D₄ → ⁷F₃ (624 nm) in the red region. Apparently, the ⁵D₄ → ⁷F₅ transition at 547 nm is the dominant peak. For the Sr₂YF₇:2% Dy³⁺ sample (Fig. 7B), it can be easily seen that there are several sharp excitation bands at 325 nm, 350 nm, 364 nm and 388 nm, corresponding to the transitions of Dy³⁺ ions (⁶H_15/2 → ⁶P_3/2, ⁶P_7/2, ⁶P_5/2, ⁴F_7/2). Upon excitation at 350 nm, the as-prepared Sr₂YF₇:2% Dy³⁺ phosphors show nearly white emission when viewed with naked eyes (illustration in Fig. 7B). The emission spectrum consists of two main peaks at 482 nm and 575 nm, which are ascribed to the magnetic dipole (⁴F_9/2 → ⁶H_15/2) and electric dipole transitions (⁴F_9/2 → ⁶H_13/2) of Dy³⁺ ions, respectively.⁴² Just like Eu^{3+ 5}D₀–⁷F₂, the Dy^{3+ 4}F_9/2–⁶H_13/2 emission belongs to hypersensitive transitions with ΔJ = 2, which is strongly influenced by the surrounding environment. When Dy³⁺ is located at a lower-symmetry local site, this emission transition will be stronger in its emission spectra.^43–45 Here, the former (⁴F_9/2 → ⁶H_15/2) is stronger than the latter (⁴F_9/2 → ⁶H_13/2), indicating that Dy³⁺ ions are located in a high symmetrical chemical environment just like Eu³⁺ ions in the host.^43–45

Fig. 7 PL excitation (left) and emission (right) spectra of Sr₂YF₇:18% Tb³⁺ (A) and Sr₂YF₇:2% Dy³⁺ (B), respectively. In addition, the luminescence photo of the Sr₂YF₇:2% Dy³⁺ crystals under excitation at 350 nm UV light (illustration in B).

On the basis of doping Tb³⁺ and Eu³⁺ ions separately in the Sr₂YF₇ host, next we attempt to dope Tb³⁺ and Eu³⁺ ions simultaneously in the sample (Fig. 8). The PL spectra of Sr₂YF₇:12% Tb³⁺ (Fig. 8a) and Sr₂YF₇:6% Eu³⁺ (Fig. 8b) both contain the characteristics of Tb³⁺ and Eu³⁺ ion peaks, respectively. However, when doping 6% Eu³⁺ ions in Sr₂YF₇:12% Tb³⁺, the spectrum of Sr₂YF₇:12% Tb³⁺/6% Eu³⁺ sample presents significant difference, as shown in Fig. 8c. Under monitoring at 594 nm of Eu³⁺ ions in Fig. 8c, the main excitation peaks are just like that in Fig. 7a except from a few weak transition lines of Eu³⁺ ions at 318 nm (⁷F₀ → ⁵H₆), 362 nm (⁷F₀ → ⁵D₄), 382 nm (⁷F₀ → ⁵G₂) and 394 nm (⁷F₀ → ⁵L₆), respectively. It is worthwhile to notice that the co-doped emission spectrum (λ_ex = 217 nm) consists of not only the narrow peaks of Tb³⁺ ions at 490 nm (⁵D₄ → ⁷F₆) and 547 nm (⁵D₄ → ⁷F₅) but also the narrow peaks of Eu³⁺ ions at 594 nm (⁵D₀ → ⁷F₁) and 620 nm (⁵D₀ → ⁷F₂). Analyzing the results shown in Fig. 8c, the presence of the excitation bands and lines of Tb³⁺ (217 nm) in the excitation spectrum monitored with Eu³⁺ emission (594 nm) clearly indicates that an energy transfer has occurred from Tb³⁺ to Eu³⁺ in the Sr₂YF₇:12% Tb³⁺/6% Eu³⁺ sample.^46–51 Since excited at 217 nm of Tb³⁺, the emission spectrum also exists the peaks of Eu³⁺ ions centered at 594 and 620 nm besides the peaks of Tb³⁺ ions, also suggesting clearly that the energy transfer from Tb³⁺ to Eu³⁺ has occurred in the Sr₂YF₇:12% Tb³⁺/6% Eu³⁺ sample.^46–51

Fig. 8 PLE and PL spectra of Sr₂YF₇:12% Tb³⁺ (a), Sr₂YF₇:6% Eu³⁺ (b) and Sr₂YF₇:12% Tb³⁺/6% Eu³⁺ (c) crystals.

As we all know, the acceptor has no influence on the decay time of a donor when the radiative energy transfer processes are dominating in certain materials. However, the acceptor has a great influence on the decay time of a donor when the nonradiative energy transfer processes are dominating. With the increase in the concentration of acceptors, the decay time of the donor decreases gradually. Similar to the double exponential luminescence decay of Tb³⁺ in the Sr₂YF₇:12% Tb³⁺ sample, the luminescence decay curve of Tb³⁺ in the Sr₂YF₇:12% Tb³⁺/6% Eu³⁺ sample can also be well fitted into a double exponential function (Fig. 9). In addition, the life time τ for the ⁵D₄ state of Tb³⁺ is determined to be 0.228 ms in the Sr₂YF₇:12% Tb³⁺/6% Eu³⁺ sample, which is much shorter than that in the Sr₂YF₇:12% Tb³⁺ (0.559 ms) sample. Obviously, the shortening of the life time in Sr₂YF₇:12% Tb³⁺/6% Eu³⁺ with respect to Sr₂YF₇:12% Tb³⁺ is due to the occurrence of the energy transfer from Tb³⁺ to Eu³⁺ in the former sample.^46–51

Fig. 9 Decay curves for the luminescence of Tb³⁺ in Sr₂YF₇:12% Tb³⁺ and T Sr₂YF₇:12% Tb³⁺/6% Eu³⁺, respectively.

3.2.2. UC photoluminescence. To achieve multicolor emission as much as possible, the UC photoluminescence of Sr₂YF₇ crystals doped with the Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺ and Yb³⁺/Er³⁺/Tm³⁺ ions were investigated under 980 nm excitation, whereas the result of Sr₂YF₇:1% Er³⁺ shows the strongest green light by adjusting the single-doped concentration of Er³⁺ ions; we subsequently fixed the 1 mol% Er³⁺ ion concentrations to vary the concentrations of Yb³⁺ from 5 mol% to 20 mol%, as shown in Fig. 10A. It was found that the optimum concentration of Yb³⁺ ions is 15 mol%. The emission spectrum of Sr₂YF₇:15% Yb³⁺/1% Er³⁺ consists of four emission peaks. The weak violet emission centered at 408 nm is assigned to the ²H_9/2–⁴I_15/2 of Er³⁺.^52,53 The dominant emissions centered at 522/540 nm in the green region and a red emission centered at 652 nm, which are assigned to the ²H_11/2 → ⁴I_5/2, ⁴S_3/2 → ⁴I_5/2 and ⁴F_9/2 → ⁴I_5/2 transitions of Er³⁺ ions, respectively.^26,54 The green emissions of ²H_11/2 → ⁴I_5/2 and ⁴S_3/2 → ⁴I_5/2 are much stronger than the red emissions of ⁴F_9/2 → ⁴I_5/2. In addition, the sample presented vivid green slightly yellow when viewed with naked eyes. As the Sr₂YF₇:15% Yb³⁺/y% Tm³⁺ phosphors indicated in Fig. 10B, the proper concentration of Tm³⁺ ions is 0.5 mol% under 980 nm excitation. Additionally, the Sr₂YF₇:15% Yb³⁺/0.5% Tm³⁺ samples display blue emission at 465 nm (¹D₂ → ³F₄) and 476 nm (¹G₄ → ³H₆) and the weaker red emission at 646 nm (¹G₄ → ³F₄).

Fig. 10 UC emission spectra of Sr₂YF₇:x% Yb³⁺/1% Er³⁺ (A) and Sr₂YF₇:15% Yb³⁺/y% Tm³⁺ (B) under 980 nm excitation, respectively.

After a range of experiments about Sr₂YF₇:Yb³⁺/Er³⁺ and Sr₂YF₇:Yb³⁺/Tm³⁺ products, we observed that Sr₂YF₇:15% Yb³⁺/0.5% Er³⁺ phosphors showed yellow light with a CIE color coordinate of (0.3705, 0.3835) and Sr₂YF₇:15% Yb³⁺/0.5% Tm³⁺ showed blue light with a CIE color coordinate of (0.1612, 0.0961) in Fig. 11A-a and b, respectively. According to the color superposition principle, it is very possible to obtain UC white light by adjusting the appropriate doping content of Yb³⁺/Er³⁺/Tm³⁺ ions in the Sr₂YF₇ host. To prove our conjecture, a series of attempts were made next. First, we have synthesized Sr₂YF₇:15% Yb³⁺/0.5% Er³⁺/0.5% Tm³⁺ nanocrystals based on the previous reports.⁵⁵ It can be noted that the emission spectrum of the Sr₂YF₇:15% Yb³⁺/0.5% Er³⁺/0.5% Tm³⁺ sample (Fig. 11A-c) is composed of peaks at 465 nm and 476 nm due to the ¹D₂ → ³F₄ and ¹G₄ → ³H₆ transitions of Tm³⁺ ions in the blue region, green emissions at 522 nm and 540 nm from the ²H_11/2 → ⁴I_5/2 and ⁴S_3/2 → ⁴I_5/2 transitions as well as red emission at 652 nm ascribed to ⁴F_9/2 → ⁴I_5/2 of Er³⁺ ions, respectively. The red emission also involved the ¹G₄ → ³F₄ transition of Tm³⁺ ions at 646 nm despite its lower intensity, whose contribution in the red region can be neglected.⁵⁶ In addition, the sample of Sr₂YF₇:15% Yb³⁺/0.5% Er³⁺/0.5% Tm³⁺ shows white light with pale blue when viewed with naked eyes (Fig. 11A-c).

Fig. 11 (A) UC PL spectra of Sr₂YF₇:Yb³⁺/Er³⁺/Tm³⁺ and their corresponding luminescence photos under 980 nm excitation; (B) intensity ratios of RGB for the group c/d/e A; and (C) CIE chromaticity coordinates of group a/b/e in A.

It is well known that Yb³⁺ ions play an important role in sensitizing Er³⁺/Tm³⁺ ions because of their larger absorption cross section at around 980 nm and longer excited state lifetime. The energy transfer generates as a result of the spectral overlap between the emission ²F_5/2 → ²F_7/2 of Yb³⁺ ions and the absorption ⁴I_11/2 → ⁴I_15/2 of Er³⁺ ions as well as the absorption ³H₅ → ³H₆ of Tm³⁺ ions. As for Sr₂YF₇:Yb³⁺/Er³⁺, first of all, the photons are mostly absorbed by Yb³⁺ ions under excitation at 980 nm. It further transfers the energy to Er³⁺ ions to the ⁴F_7/2 level, which then relax to ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels by nonradiative relaxation. At last, the energy in these levels return to the ground state, resulting in green emissions at 522 nm and 540 nm (²H_11/2 → ⁴I_5/2, ⁴S_3/2 → ⁴I_5/2) and red emission at 652 nm (⁴F_9/2 → ⁴I_5/2). With respect to Sr₂YF₇:Yb³⁺/Tm³⁺, it has been proved that the energy transfer process between Yb³⁺ ions and Tm³⁺ ions is the three-photon process. First, the photons can be absorbed by the ²F_5/2 level of Yb³⁺. Then, the nonresonant ET1 takes place to excite Tm³⁺ ions into the excited states ³H₅, relaxing subsequently to the ³F₄ state. The ³F₄ level of Tm³⁺ is excited into ³F₂ and ³F₃ levels by nonresonant ET2 process, and then Tm³⁺ ions return to the ³H₆ state. Finally, the nonresonant ET3 populates the ¹G₄ level, which emits 474 nm (¹G₄ → ³H₆) and 650 nm (¹G₄ → ³F₄). The detailed energy level diagrams about Sr₂YF₇:Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺ and Yb³⁺/Er³⁺/Tm³⁺ are illustrated in Fig. 12.

Fig. 12 Energy level diagrams of the Yb³⁺, Er³⁺ and Tm³⁺ ions as well as the proposed UC mechanisms.

The key to acquire the pure white light is properly enhancing the relative intensity of red emission.¹² The reduction of the green to red (G/R) ratio mainly concerns two energy transfer processes. One is the energy transfer from Er³⁺ ions to Yb³⁺ ions, resulting in the decrease of green (²H_11/2, ⁴S_3/2 → ⁴I_5/2) emissions. Another is the transfer energy from excited Yb³⁺ ions to Er³⁺ ions, which can directly populate the ⁴F_9/2 level originating from ²F_5/2(Yb³⁺) + ⁴I_13/2(Er³⁺) → ²F_7/2(Yb³⁺) + ⁴F_9/2(Er³⁺). However, some energy transfer processes between Er³⁺ ions and Tm³⁺ ions might also work in the complex system as follows: ³H₄(Tm³⁺) + ⁴I_13/2(Er³⁺) → ³H₆(Tm³⁺) + ⁴S_3/2(Er³⁺); ¹G₄(Tm³⁺) + ⁴I_15/2(Er³⁺) → ³F₄(Tm³⁺) + ⁴F_9/2(Er³⁺). The ultraviolet emission is hardly detected in a tri-doped Sr₂YF₇ system according to the available conclusion.¹⁰ In addition, the intensity ratios of the red to the green (R/G) emission increases gradually upon decreasing Tm³⁺ ions. Therefore, we conceived the idea of averaging the dopant concentrations of Er³⁺ ions and Tm³⁺ ions to obtain white light. The up-conversion emission spectra of Sr₂YF₇:Yb³⁺/Er³⁺/Tm³⁺ crystals with various concentrations of Er³⁺ and Tm³⁺ ions are shown in Fig. 11A-c–e. The relative intensity ratios of RGB were calculated by integral area of red band, green band and blue band from spectra.⁵⁷ From the corresponding information illustrated in Fig. 11B, we can directly observe the ratios of red band, green band and blue band of group c, d and e. The R/G ratio increases from 1 to 1.35 (inset of Fig. 11B). Ultimately, the target products Sr₂YF₇:15% Yb³⁺/0.4% Tm³⁺/0.3% Er³⁺ generate ideal UC white light, whose CIE color coordinate was calculated to be (0.3123, 0.3005) under a 980 nm pump with a power density of about 2 W cm⁻², as displayed in Fig. 11C (point e).

Conclusion

In summary, uniform and monodisperse Sr₂YF₇ spherical crystals have been successfully synthesized via a facile one-step hydrothermal route without any surfactant. We found that the pH values can change the morphology from spherical to octahedral shape and the various doped Ln³⁺ ions are beneficial to form different sizes of crystals ranging from 300 to 600 nm. The obtained Sr₂YF₇:Ln³⁺ (Ln = Dy, Tb, Eu, Tb/Eu, Yb/Er, Yb/Tm and Yb/Er/Tm) crystals show multicolor DS/UC luminescence properties. The energy transfer from Tb³⁺ ions to Eu³⁺ ions was observed in the Sr₂YF₇ host; the Sr₂YF₇:0.5% Dy³⁺ crystals show the nearly white emission under UV excitation at 217 nm; the ideal UC white light-emitting tri-doped Sr₂YF₇:15% Yb³⁺/0.4% Er³⁺/0.3% Tm³⁺ with a CIE color coordinate of (0.3123, 0.3005) was prepared under 980 nm excitation. In view of their simple production route and good properties, the Sr₂YF₇:Ln³⁺ materials may have potential applications in lighting, display devices and biological imaging.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This project is financially supported by the National Undergraduate Training Program for Innovation and Entrepreneurship of Southwest University, China (201710635040).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ce00280a

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