Controllable multicolor output, white luminescence and cathodoluminescence properties of high quality NaCeF₄:Ln³⁺ (Ln³⁺ = Eu³⁺, Dy³⁺, Tb³⁺) nanorods

Abstract

Herein, a series of hexagonal phase lanthanide (Ln³⁺, Ln³⁺ = Eu³⁺, Dy³⁺, Tb³⁺) doped NaCeF₄ nanorods (NRs) with uniform morphology and monodispersity have been successfully synthesized via a typical hydrothermal method using oleic acid as the capping agent. The crystal phase and microstructure of the obtained NRs were analyzed by X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM). The down conversion (DC) luminescence properties and mechanisms of the as-prepared NaCeF₄:Ln³⁺ NRs have been discussed in detail. The as-prepared samples show the characteristic f–f transition of Ln³⁺ (Ln³⁺ = Eu³⁺, Dy³⁺, Tb³⁺). The decay time and quantum yield of these obtained NRs are also studied. Moreover, tunable multicolor, especially white emissions, can be successfully achieved via varying the doping ions and doping concentration. By increasing the content of Eu³⁺, the emission colors vary from light green to white and finally to light red under the excitation of 395 nm. The calculated CIE coordinates of the obtained white emissions are (0.33, 0.31), which are very close to the standard white light located at (0.33, 0.33). This is the first time to demonstrate that white light emission is achieved via only singly-doping Eu³⁺ into the NaCeF₄ system. In addition, the multicolor output changes from yellowish-green to yellow under the excitation at 261 nm, which was also obtained by only tuning the doped content of Dy³⁺ in the NaCeF₄ host. As for Tb³⁺, bright yellowish green emissions were obtained under excitation at 261 nm. Moreover, the cathodoluminescence (CL) spectra demonstrate that these NRs can emerge as ideal nanophosphors under electron beam excitation. Therefore, the as-prepared NaCeF₄:Ln³⁺ NRs with tunable multicolor output and bright white emissions might be applied in field-emission devices, multicolor displays and solid state lasers.

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1. Introduction

In recent years, considerable attentions have been focused on the shape and size-controlled synthesis of Ln³⁺ doped nanocrystals^1–18 because of their shape/size dependent properties and potential applications in optics, lasers, light emitting devices, and biological labeling/imaging. It is well known that the morphology and size of nanocrystals have great influence on their physical and chemical properties.¹⁹ Thus, the controlled fabrication of Ln³⁺-doped nanomaterials with desirable shapes and sizes is of great significance.

Ln³⁺-doped nanocrystals exhibit excellent photoluminescence (PL) capabilities owing to two different energy transfer mechanisms, namely, the up conversion^20–29 (UC) and DC^30–40 processes. DC generation is the one that converts higher-energy irradiation to lower-energy photon emissions,^1,2 while the UC process is the exact opposite. Among all the exploited Ln³⁺-doped nanocrystals, fluorides (NaLnF₄), are considered as the most efficient lattice hosts. However, Ln³⁺-doped nanocrystals using NaCeF₄ as the host matrix and high quality NaCeF₄ nanocrystals with monodispersity are still rarely reported. In contrast to the well-established NaYF₄ host, the NaCeF₄ system we selected here can serve not only as the perfect host material but also as an activator, which makes it simpler to obtain the multicolor output and intense white emissions by singly doping Ln³⁺ in a system. Apart from the advantages of its PL properties, the price of cerium oxide, which is used as a raw material in our experiments, is quite reasonable compared to other oxides.⁴¹ As previously reported, the CeF₃ and NaCeF₄ nanocrystals were synthesized through the liquid–solid-solution approach by Li et al.,⁴¹ in which the focus was on the nucleation and growth of the nanocrystals and it did not reveal the effect of Ln³⁺ doping on their PL property. In addition, NaCeF₄ nanoparticles with different shapes and sizes were obtained by using the solvothermal method via tuning the pH value and sodium concentration of the solution.³⁶ However, the as-prepared NaCeF₄ nanoparticles were irregular and not uniform. Recently, hexagonal phase NaCeF₄ NRs were successfully fabricated via the polyol-mediated solvothermal route by Qu et al.³⁷ The major work contributed to achieve the varied morphology and size of NaCeF₄ nanocrystals by changing the additive content of NH₄F and NaNO₃. Therefore, it is important to develop a proper method and experimental parameters to achieve hexagonal phase NaCeF₄ nanocrystals with a uniform shape and monodispersity. Moreover, the systematic study on the DC luminescence of the Ln³⁺-doped NaCeF₄ nanocrystals is of significance. Many recent literatures^30–33 have focused on phosphors, which were co-doped with Ce³⁺, Mn²⁺, Eu³⁺, Dy³⁺, and Tb³⁺ to obtain tunable multicolor emissions or even white light for applications in light emitting devices. The efficient and intense energy transfer from Ce³⁺ to Mn²⁺/Dy³⁺/Tb³⁺ in these phosphors has been systematically studied. However, white emissions obtained by singly doping Ln³⁺ into the hosts still remains a great challenge and there are few reports on Ln³⁺ doped NaCeF₄ nanocrystals.

In this paper, high quality and monodispersed hexagonal phase NaCeF₄ NRs doped with different contents of Ln³⁺ (Ln³⁺ = Eu³⁺, Dy³⁺, and Tb³⁺) were successfully prepared via a hydrothermal method. The crystal phase and microstructure of the as-prepared Ln³⁺ doped NaCeF₄ NRs were characterized by XRD and TEM, respectively. The PL properties were recorded by the excitation and emission spectra and the corresponding energy transfer mechanisms are discussed in detail. Interestingly, the tunable multicolor and especially white emissions are obtained by only changing the content of single-doped ions in NaCeF₄ host.

2. Experimental

2.1 Chemicals and materials

All the rare earth oxides (Sigma-Aldrich) were 99.99% of purity and the other chemicals (Sinopharm Chemical Reagent Co., China) were of analytical grade and used as received without further purification. Ln(NO₃)₃ (Ln = Ce³⁺, Eu³⁺, Dy³⁺, and Tb³⁺) solutions of 0.5 M were prepared via dissolving the corresponding rare earth oxides into dilute nitric acid.

2.2 Synthesis of the high quality Ln³⁺-doped NaCeF₄ NRs

High quality and monodispersed NaCeF₄ NRs doped with different contents of Ln³⁺ were synthesized through a hydrothermal procedure by using oleic acid as the stabilizing agent.^41–44 In a typical synthesis, 1.2 g of NaOH was dissolved in 2 mL of deionized water and then 10 mL of ethanol and 20 mL of oleic acid were added under vigorous stirring to obtain a transparent homogeneous solution. After stirring for 30 min, total amount (1 mmol) of Ln(NO₃)₃ (Ln = Ce³⁺, Eu³⁺, Dy³⁺, and Tb³⁺) with designed compositions were added into the aforementioned solution under vigorous agitation. After that, 6 mL of NaF aqueous solution (1 M) was added, and the resulting mixture was stirred vigorously for another 20 min. Finally, the obtained mixture was sealed and maintained at 190 °C for 24 h by transferring it into a 50 mL stainless Teflon-lined autoclave. The system was naturally cooled to room temperature after the reaction. The products were deposited at the bottom of the vessel. The as-prepared samples were separated by centrifugation and washed several times with ethanol and deionized water to remove oleic acid and other remnants, and then dried at 60 °C in air for 24 h.

2.3 Characterization

The crystal phase compositions of the as-prepared NaCeF₄ NRs doped with different contents of Dy³⁺ were examined by XRD utilizing a D/max-γA system X-ray diffractometer at 40 kV and 250 mA with Cu Kα radiation (λ = 1.54056 Å). The microstructures of the as-prepared samples were characterized by TEM, scanning TEM (STEM), selected area electron diffraction (SAED) patterns and high-resolution TEM (HR-TEM) via a JEM-2100F TEM equipped with an energy-dispersive X-ray spectroscopy (EDS) system using an accelerating voltage at 200 kV. The obtained samples for the TEM assays were prepared as follows: 0.1 mg of the as-prepared samples was dispersed in 1 mL of cyclohexane solvent to form a homogeneous colloidal mixture, and then one drop of the suspension was added on the TEM copper grid covered with a carbon film. The DC luminescence excitation/emission spectra and decay time profile were recorded by a Zolix Analytical Instrument (fluoroSENS 9000A). The decay time curves of the samples were recorded using the Zolix Analytical Instrument by monitoring the corresponding strongest excitation and emission wavelengths (excitation/emission bandpass: 8 mm, integrate time: 200 μs). The quantum yields of the Ln-doped NaCeF₄ NRs were measured by the affiliated quantum yield measurement system of the spectroscope, in which an integrating sphere was used as the sample chamber. The digital photographs of the as-prepared Ln-doped NaCeF₄ samples were taken by a Canon digital camera under the strongest excitation at the corresponding wavelength. A cathodoluminescence spectrometer (Gantan MonoCL3+) equipped on the environmental scanning electron microscope (Quanta 400 FEG, FEI) was used to measure the CL spectra of the samples (accelerating voltage of electron beam: 3.15 kV for NaCeF₄:Eu³⁺, 4.75 kV for NaCeF₄:Dy³⁺/Tb³⁺).

3. Results and discussion

3.1 Phase and microstructure study

The crystal structure of ALnF₄ exhibits two polymorphic forms (cubic and hexagonal phases), depending on the selected synthesis conditions and methods. It has been illustrated that the hexagonal phase ALnF₄ is a much better host lattice than its cubic counterpart for UC/DC luminescence.⁴⁵ Usually, in order to obtain the pure hexagonal phase ALnF₄, sufficiently high temperature and longer time duration are needed for reaction. Fig. 1 shows the XRD patterns of the as-prepared Dy³⁺ doped NaCeF₄ NRs via the hydrothermal procedure at 190 °C for 24 h. It can be seen that all the diffraction peaks of the products can be indexed to the pure hexagonal phase NaCeF₄ structure (JCPDS, number 50-0154). The diffraction peaks of the products are very sharp and strong, indicating the formation of the NaCeF₄ samples with high crystallinity through this method. In addition, one can observe from the red dotted line that the diffraction peaks of the NaCeF₄:Dy³⁺ samples shifted toward high angles gradually with increasing the content of Dy³⁺, indicating the decrease of the unit-cell volume. This is mainly ascribed to the smaller ionic radius of Dy³⁺ (r = 1.167 Å) which replaces the relatively larger Ce³⁺ (r = 1.283 Å).⁴⁶ Moreover, the broadened diffraction peaks reveal that the average crystalline size decreases with increasing the content of Dy³⁺. It should be noted that the Eu³⁺ and Tb³⁺ doped samples present similar results (data not shown).

Fig. 1 Typical XRD patterns of the NaCeF₄ NRs doped with different Dy³⁺ contents: (a) 0.02, (b) 0.05, (c) 0.1, and (d) 0.15, (e) the standard hexagonal phase NaCeF₄ (JCPDS number: 50-0154). The diffraction peaks of the residual NaF were indicated by green triangles. The peaks shift towards higher angles (marked by a red dotted line).

To investigate the morphology and size of the NaCeF₄:mDy³⁺ NRs, further TEM and STEM characterizations are performed and the results are shown in Fig. 2. As demonstrated, all of the samples present highly monodispersed and uniform NRs, and can be self-assembled into a two-dimensional ordered array. The aspect ratios of the samples were measured to be 8.02, 9.41, and 13.59, with the increase of the Dy³⁺ concentration from 0.02 to 0.10. This evolution in size is due to the effect of the Dy³⁺ dopant ion on the crystal growth rate through surface charge modification.⁷ Liu's⁷ and our previous reports⁴² reveal that the crystal size can be readily tuned by doping Ln³⁺ with different ionic radius in the NaLnF₄ host. The smaller Ln³⁺ dopants may result in the formation of larger-sized nanocrystals. Therefore, in our case, the increased size is mainly attributed to the doped Ln³⁺ (Dy³⁺) with a larger ionic radius.

Fig. 2 TEM (left panel), STEM (middle panel), and HR-TEM (right panel) images of the NaCeF₄ NRs doped with different contents of Dy³⁺: (a) 0.02, (d) 0.05, (g) 0.1, and (j) 0.15. (b), (e), (h) and (k) are the corresponding STEM images and (c), (f), (i) and (l) denote the corresponding HR-TEM images. The right-up inset of each HR-TEM image shows the corresponding SAED pattern.

The HR-TEM and SAED results (right panels in Fig. 2) show that the as-prepared NRs present a high crystallinity and single crystal nature. The interplanar crystal spacing of the [002] and [200] lattice planes measured from the HR-TEM results (right panels in Fig. 2) decreased gradually when the Dy³⁺ doping content was varied from 0.02 to 0.15, which coincides well with the analyzed results of the XRD patterns. Moreover, the preferred growth direction is along the [001] direction. During the STEM analysis, the corresponding EDS and EDS mapping are obtained and the results for the NaCeF₄:0.05Dy³⁺ NRs are presented in Fig. 3. The EDS results (Fig. 3f) reveal that the as-prepared NaCeF₄:0.05Dy³⁺ NRs are mainly composed of Na⁺, Ce³⁺, F⁻, Dy³⁺ and no other impurity is detected. Further EDS mappings show that the elements (Na⁺, Ce³⁺, F⁻, Dy³⁺) are uniformly dispersed within the regions of the NRs. It is noted that the Eu³⁺ or Tb³⁺ doped samples also exhibit highly uniformed and monodispersed NRs (data not shown).

Fig. 3 (a) STEM of the NaCeF₄:0.05Dy NRs, (b–e) the corresponding EDS mapping of the as-prepared NRs shown in a, and (f) EDS of the NaCeF₄:0.05Dy³⁺ samples.

3.2 Tunable multicolor and DC properties

As a doping ion, the Eu³⁺ concentration in a host has great influence on the emission intensity and the shapes of the spectra.³⁸ Therefore, we selected a series of doping contents of Eu³⁺ (0.02, 0.05, 0.1, and 0.15) to investigate its spectral and luminescence nature in the hexagonal NaCeF₄ host lattice. Fig. 4 shows the excitation and emission spectra of the Eu³⁺ doped NaCeF₄ NRs. As presented in Fig. 4a, the excitation spectra contains the characteristic peaks of Eu³⁺ within the configuration ⁴F₆ between 250 and 450 nm, which is analogous to the absorption curve for Eu³⁺ in the YF₃ and NaYF₄ host.³⁹ The excitation lines can be attributed to the corresponding energy transfer: (317 nm, ⁷F₀ → ⁵H₆; 361 nm, ⁷F₀ → ⁵D₄; 384 nm, ⁷F₀ → ⁵G₂; 395 nm, ⁷F₀ → ⁵L₆, strongest; and 415 nm, ⁷F₀ → ⁵D₃). In addition, these weak lines (such as 253, 286, and 297 nm) have little contribution to the emission. The excitation spectra of the Eu³⁺ doped NaCeF₄ host have considerable difference from that in oxide hosts, where the charge-transfer band (200–300 nm) of Eu³⁺–O²⁻ has been detected constantly. In comparison with the Ce³⁺/Dy³⁺ and Ce³⁺/Tb³⁺ co-doped nanomaterials, the combination between Eu³⁺ and Ce³⁺ also exhibits distinctive excitation properties, which can be attributed to the transitions of the different crystal field splitting levels of the 5d state for Ce³⁺ ions in the NaCeF₄ host.

Fig. 4 (a) Excitation and (b) emission spectra of NaCeF₄:mEu³⁺ (m = 0.02, 0.05, 0.1, and 0.15) NRs. (c) Luminescence decay (λ_ex = 395 nm, λ_em = 615 nm) spectra of NaCeF₄:mEu³⁺ NRs.

The luminescent intensities of the NaCeF₄:Eu³⁺ NRs were remarkably enhanced via increasing the doping content of Eu³⁺ from 0.02 to 0.15 (Fig. 4b). As demonstrated in Fig. 4b, the emission spectra of the NaCeF₄:mEu³⁺ NRs were obtained by the strongest excitation wavelength of 395 nm (⁷F₀ → ⁵L₆ transition of Eu³⁺). According to the proposed energy level diagram (Fig. 5), all the emission lines are composed of ⁵D_0,2 → ⁷F_J transitions of Eu³⁺, namely, 468 nm, ⁵D₂ → ⁷F₀; 485 nm, ⁵D₂ → ⁷F₂; 574 nm, ⁵D₀ → ⁷F₀; 591 nm, ⁵D₀ → ⁷F₁; 615 nm, ⁵D₀ → ⁷F₂; 649 nm, ⁵D₀ → ⁷F₃; and 690/695 nm, ⁵D₀ → ⁷F₄.³⁶ In comparison to the PL properties of the Dy³⁺ and Tb³⁺ doped NaCeF₄ system, the Eu³⁺ doped NaCeF₄ NRs present a very weak excitation peak centered at 261 nm of Ce³⁺, indicating that the intense emissions of the as-prepared NaCeF₄:Eu³⁺ NRs come from the intrinsic emissions of the Eu³⁺ rather than the energy transfer between the Ce³⁺ and Eu³⁺. Moreover, with increasing the Eu³⁺ content, the excitation peak centered at 261 nm of Ce³⁺ gradually disappears, indicating the hardness of the direct sensitization of Eu³⁺ by Ce³⁺, which is similar with a previous report.⁴⁷ This is mainly ascribed to the electron transfer quenching effect between Ce³⁺ and Eu³⁺, resulting in the poor energy transfer from Ce³⁺ to Eu³⁺.^47–49 The energy transfer probability depends on the critical distance of the energy transfer.^50,51 As demonstrated in Fig. 4b, the relative emission intensity ratio of the transitions (⁵D₂ → ⁷F_J)/(⁵D₀ → ⁷F_J) decreased when the doping contents of Eu³⁺ increased. Moreover, the strongest emission peak of Eu³⁺ is not constant, which is centered at 485 nm when the doping content of Eu³⁺ in NaCeF₄ host is low (0.02 and 0.05) and centered at 615 nm while increasing the Eu³⁺ composition to 0.1 and 0.15. In general, the differences in the relative emission intensity are mainly attributed to the doping content of Eu³⁺ and their predominant vibration frequencies available in the host.⁴⁰ The emission peaks centered at 591 nm (⁵D₀ → ⁷F₁) and 615 nm (⁵D₀ → ⁷F₂) are ascribed to the magnetic and electric dipole transitions of Eu³⁺, respectively, and the asymmetric environment of the Eu³⁺ ion in the host can be calculated by the integrated intensity ratio (denoted as A₂₁) of the electric to magnetic dipole transitions.^52–54 The A₂₁ values are calculated to be ∼1.47, 1.56, 1.58, and 1.80, when increasing the dopant content of Eu³⁺. The ⁵D₀ → ⁷F₂ transition is more sensitive than that of the ⁵D₀ → ⁷F₁ transition and the peak at 615 nm is always dominant compared with the one at 591 nm. The higher A₂₁ value indicates that a higher local disorder and distortion appears in the Eu³⁺ doped NaCeF₄ system, which can enhance the emission intensity enormously. The CIE coordinates (labeled from 1 to 4 in Fig. 8) are calculated to be (0.31, 0.36) for NaCeF₄:0.02Eu³⁺, (0.33, 0.31) for NaCeF₄:0.05Eu³⁺, (0.42, 0.32) for NaCeF₄:0.1Eu³⁺, and (0.43, 0.33) for NaCeF₄:0.15Eu³⁺ NRs, respectively. From the corresponding digital photographs of the powders (the insets in Fig. 8), the tunable multicolor outputs from green to bright white and finally to red were readily obtained by singly doping Eu³⁺ in the NaCeF₄ host. Moreover, the calculated CIE coordinates (0.33, 0.31) of the NaCeF₄:0.05Eu³⁺ NRs are very close to the standard white light emission (0.33, 0.33). These findings provide a new route for achieving white light emissions by only singly doping Ln³⁺, which is different with previously reported Ce³⁺/Mn²⁺ co-doped phosphors.⁵⁵ The PL decay profile of the ⁵D₀ level at 615 nm of Eu³⁺ for these NaCeF₄:Eu³⁺ NRs under the excitation of 395 nm is shown in Fig. 4c. The experimental data were fitted by a mono-exponential function and the lifetimes were calculated to be 4.79, 6.13, 7.61, and 8.15 ms, respectively. In addition, the quantum yields of the NaCeF₄:Eu³⁺ NRs were measured to be 33%, 40%, 49%, and 57%, respectively.

Fig. 5 The proposed energy level diagram and luminescence mechanisms of the NaCeF₄:Ln³⁺ NRs.

The NaCeF₄:mDy³⁺ (m = 0.02, 0.05, 0.1, and 0.15) NRs also exhibit unique DC properties and the excitation/emission spectra are illustrated in Fig. 6a and b, respectively. The excitation spectra (Fig. 6a) are composed of the intense absorption band centered at 261 nm of Ce³⁺ and the relative weak characteristic f–f transition lines of Dy³⁺, which are assigned to the corresponding energy transfer: 324 nm, ⁶H_15/2 → ⁶P_3/2; 350 nm, ⁶H_15/2 → ⁶P_7/2; 364/368 nm, ⁶H_15/2 → ⁶P_5/2; and 388/395 nm, ⁶H_15/2 → ⁴M_21/2, respectively. On the other hand, the emission spectra (Fig. 6b) excited via the intense Ce³⁺ absorption peak at 261 nm were dominated by two groups of emissions (477/482 nm, ⁴F_9/2 → ⁶H_15/2; and 572 nm, ⁴F_9/2 → ⁶H_13/2). There is still some weak emission transition such as the ⁴F_9/2 → ⁶H_11/2 transition of Dy³⁺ at 658 nm. The energy transfer mechanism of Dy³⁺ in the NaCeF₄ matrix is shown in Fig. 5. Under UV excitation, the energy transfer took place between the Ce³⁺ ions at first, and then transferred from Ce³⁺ (5d) to Dy³⁺. Finally, the excited levels of Dy³⁺ radiatively transferred to the lower energy levels. The multicolor emissions, such as yellowish-green for NaCeF₄:0.02/0.05/0.1Dy³⁺ and yellow for NaCeF₄:0.15Dy³⁺, are obtained under the excitation of 261 nm (insets in Fig. 8). The calculated CIE coordinates followed by the emission spectra are measured to be (0.30, 0.37) for NaCeF₄:0.02Dy³⁺, (0.31, 0.37) for NaCeF₄:0.05Dy³⁺, (0.32, 0.38) for NaCeF₄:0.1Dy³⁺, and (0.36, 0.41) for NaCeF₄:0.15Dy³⁺ as marked by 5 to 8 in Fig. 8. The PL decay profile (shown in Fig. 6c) of the NaCeF₄:mDy³⁺ NRs also matched well with the mono-exponential curve fit and the calculated decay times are 5.31, 4.48, 4.02, and 3.10 ms, respectively. The quantum yields of the NaCeF₄:mDy³⁺ NRs were tested to be 41%, 27%, 24%, and 13%, respectively.

Fig. 6 (a) Excitation and (b) emission spectra of NaCeF₄:mDy³⁺ (m = 0.02, 0.05, 0.1, and 0.15) NRs. (c) Luminescence decay (λ_ex = 261 nm, λ_em = 572 nm) spectra of NaCeF₄:mDy³⁺ NRs.

As for Tb³⁺, it contains a low energy state (⁷F_J, J = 0, …, 6) and excited states (⁵D₃ and ⁵D₄), which results in blue or green emissions with a low or high doping content of Tb³⁺, respectively.⁵⁶ Similar to the Dy³⁺ doped NaCeF₄ samples, the excitation spectra of NaCeF₄:mTb³⁺ (m = 0.02 and 0.05) consist of a predominant Ce³⁺ absorption line, centered at 261 nm and relative weak f–f transitions of Tb³⁺ (351 nm, ⁷F₆ → ⁵D₂; 375 nm, ⁷F₆ → ⁵G₆), as observed in Fig. 7a. Under the excitation of a UV light at 261 nm, the emission spectra of the NaCeF₄:mTb³⁺ NRs consist of the transitions of Tb³⁺: 489 nm, ⁵D₄ → ⁷F₆; 543/546 nm, ⁵D₄ → ⁷F₅, strongest; 584 nm, ⁵D₄ → ⁷F₄; and 620 nm, ⁵D₄ → ⁷F₃, respectively. The bright green emissions of the NaCeF₄:Tb³⁺ NRs are presented (insets in Fig. 8) and the CIE coordinates are calculated to be (0.31, 0.51) for NaCeF₄:0.02Tb³⁺ and (0.33, 0.54) for NaCeF₄:0.05Tb³⁺ NRs. The mono-exponential fitting of the NaCeF₄:mTb³⁺ NRs shows the lifetime of the transition (⁵D₄ → ⁷F₅) and the decay times are measured to be 7.71 and 8.92 ms for m = 0.05 and 0.1, respectively (shown in Fig. 7c). The quantum yields of the NaCeF₄:mTb³⁺ NRs are recorded to be 25% and 30%, respectively.

Fig. 7 (a) Excitation and (b) emission spectra of NaCeF₄:mTb³⁺ (m = 0.02 and 0.05) NRs. (c) Luminescence decay (λ_ex = 261 nm, λ_em = 543 nm) spectra of NaCeF₄:mTb³⁺ NRs.

Fig. 8 The CIE chromaticity diagram of the as-prepared NaCeF₄:Eu³⁺/Dy³⁺/Tb³⁺ NRs. The upright insets present the digital photographs of the as-prepared powders under the excitation of a Xe lamp at the corresponding strongest excitation peak.

3.3 CL properties

The low-voltage CL properties of materials are significant for field-emission displays. To further reveal their potential applications in field-emission devices, the CL properties were investigated. As shown in Fig. 9, their CL spectra were similar to the corresponding PL spectra. Under the low-voltage electron beam excitation (accelerating voltage: 3.15 kV), the NaCeF₄:0.15Eu³⁺ NRs give bright yellowish-red light with emission peaks centered at 593, 618, and 697 nm owing to the transitions ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, and ⁵D₀ → ⁷F₄ of Eu³⁺, respectively. For the NaCeF₄:0.02Dy³⁺ NRs, the CL spectrum (excitation voltage: 4.75 kV) shows two intense emission peaks at 480 and 572 nm due to the transitions ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 of Dy³⁺, respectively. These NRs show greenish-yellow light under the electron beam excitation. As for the NaCeF₄:0.05Tb³⁺ NRs, the CL spectrum (excitation voltage: 4.75 kV) is composed of emission peaks at 491, 545, 587, and 622 nm, which result from ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃ transitions of Tb³⁺, respectively.

Fig. 9 CL spectra of (a) NaCeF₄:0.15Eu³⁺, (b) NaCeF₄:0.02Dy³⁺, and (c) NaCeF₄:0.05Tb³⁺ NRs. Accelerating voltages were 3.15, 4.75, and 4.75 kV for a–c, respectively.

4. Conclusion

In summary, the pure hexagonal phase NaCeF₄:Ln³⁺ (Ln³⁺ = Eu³⁺, Dy³⁺, Tb³⁺) NRs have been successfully fabricated via a hydrothermal method. The microstructural studies exhibit the formation of high quality NaCeF₄:Ln³⁺ NRs with monodispersity, uniform shape and single crystal nature. Furthermore, the tunable multicolor, especially white emissions can be obtained by adjusting the doping ions and the doping content. It is important that the CIE coordinates of NaCeF₄:0.05Eu³⁺ NRs are located in the white region and are calculated to be (0.33, 0.31), which is close to the standard white light emission (0.33, 0.33). These findings provide a simple way for white light emission by only singly doping Eu³⁺, which is different with previously demonstrated Ce³⁺/Mn²⁺ co-doped phosphors. The excellent DC emissions, tunable multicolor output and bright white light luminescence of the NaCeF₄:Ln³⁺ NRs may find applications in field-emission devices and solid state lighting.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (nos. 51102202 and 31370736), the Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20114301120006), the Hunan Provincial Natural Science Foundation of China (no. 12JJ4056), and the Scientific Research Fund of Hunan Provincial Education Department (13B062).

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