Tunable color and energy transfer of Tm³⁺ and Ho³⁺ co-doped NaGdF₄ nanoparticles

Abstract

NaGdF₄:Tm³⁺, Ho³⁺ nanoparticles with luminescence properties were synthesized by a hydrothermal method. The morphology, structure and properties of the obtained samples were investigated in detail. The results indicate that the samples are pure hexagonal NaGdF₄. The luminescence properties reveal that the samples can emit blue and green light under 359 nm UV light excitation. It is found that in the Tm³⁺, Ho³⁺ co-doped NaGdF₄ system, the efficient energy transfer from Tm³⁺ to Ho³⁺ is observed from the photoluminescence spectra and the fluorescence decay curves. Furthermore, the hues can be adjusted from blue through to light blue and ultimately to bluish green. It is clear that these β-NaGdF₄ nanoparticles could have potential applications in fields such as solid-state lighting and color displays.

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1 Introduction

In recent years, an increasing number of researchers have a keen interest in studying and developing luminescent nanomaterials such as lanthanide-doped oxides,^1–3 phosphates,^4–7 and fluorides,^8–10 by reason of their distinct emission properties deriving from the 4f electron configuration.¹¹ Among all kinds of host materials for lanthanide-doped nanocrystals, rare-earth fluoride (such as NaGdF₄) nanocrystals have garnered much more interest on account of their outstanding properties, for example, low phonon energy, high refractive index and chemical stability, and relatively low crystallization temperature.¹² Therefore, many different rare earth-doped NaGdF₄ nanoparticles have been obtained, such as NaGdF₄:Eu³⁺,¹³ NaGdF₄:Ce³⁺/Ln³⁺ (Ln = Tb, Eu, Dy),¹⁴ NaGdF₄:Tb³⁺, Sm³⁺,¹⁵ NaGdF₄:Yb³⁺, Er³⁺,¹⁶ NaGdF₄:Yb³⁺, Tm^3+ 17 and NaGdF₄:Yb³⁺, Ho³⁺.¹⁸

Nowadays, many blue phosphors have been successfully prepared, such as KSrPO₄:Eu²⁺,¹⁹ Ba₃LaNa(PO₄)₃F:Eu^2+ 20 and Na₂Ba₆(Si₂O₇)(SiO₄)₂:Ce³⁺.²¹ However, the emission from these phosphors is too blue to create a suitable white light. Tm³⁺ ions are usually used as efficient blue light emissive activators and have blue emissions which are mainly attributed to the ¹D₂ → ³F₄ transition, which overlaps well with the ⁵I₈ → ⁵G₆ transition of Ho³⁺, as reported by Tian²² et al. and Xu²³ et al. Whereas Ho³⁺ emits green light. Therefore, a change in hue from blue to green is expected in the Tm³⁺, Ho³⁺ co-doped system. For example, Silvestre²⁴ presented the crystal growth of monoclinic KGd(WO₄)₂ co-doped with Tm³⁺ and Ho³⁺ by a top-seeded solution growth and slow cooling method. New types of fluorophosphate glass with different Tm³⁺, Ho³⁺ co-doping concentrations were prepared by Tian.²² In those chemical compounds, the Tm³⁺ and Ho³⁺ ions are excited by infrared rays. Up to now, studies on the luminescence properties of inorganic host materials co-doped with Tm³⁺ and Ho³⁺ ions under the excitation of UV light are limited. Following that, in this work, we aim to focus our attention on the luminescence of NaGdF₄ co-doped Tm³⁺ and Ho³⁺ ions under the excitation of UV light.

Hence, in this work, the hexagonal phase NaGdF₄:Tm³⁺, Ho³⁺ nanoparticles with down conversion luminescence including energy transfer and color tunable emissions have been reported and studied in detail.

2 Experimental

2.1 Materials

All the chemicals were used directly as received without further purification. Gd(NO₃)₃, Tm(NO₃)₃ and Ho(NO₃)₃ were prepared by dissolving the corresponding Gd₂O₃, Tm₂O₃ and Ho₂O₃ in HNO₃ solution at an elevated temperature followed by evaporating superfluous HNO₃. The sodium dodecyl sulfate (SDS) and sodium fluoride (NaF) were of analytical grade.

2.2 Preparation

A series of rare earth (RE)-doped NaGdF₄ nanocrystals were synthesized by a facile hydrothermal process without further sintering treatment. In a typical procedure of preparing the representative NaGdF₄:3%Tm³⁺, 0.5%Ho³⁺: 19.3 mL Gd(NO₃)₃, 1.2 mL Tm(NO₃)₃ and 0.2 mL Ho(NO₃)₃ were poured into a 100 mL flask with 1.1535 g (1 : 2 molar ratio for RE³⁺/SDS) of SDS. After vigorous stirring for 30 min, 1.007 g of NaF (1 : 12 molar ratio for RE(NO₃)₃/NaF) was slowly added into the above solution. After an additional agitation for 40 min, the resultant milky colloidal suspension was transferred to a Teflon bottle held in a stainless steel autoclave, and then heated at 180 °C for 24 h. Finally, after the autoclave was naturally cooled to room-temperature, the precipitate was separated by centrifugation, then washed sequentially with deionized water and ethanol several times, and then air dried at 60 °C for 12 h. Other samples were prepared with a similar procedure, except for changing the value of the RE(NO₃)₃.

2.3 Characterization

The morphology, size and composition of the samples were observed by a field emission electron microscope (FESEM) equipped with an energy-dispersive X-ray spectrometer (EDS).

X-ray powder diffraction (XRD) data for the prepared samples (NaGdF₄:Tm³⁺, Ho³⁺) were taken using a Rigaku D/max-RA X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm) and a Ni filter, operating at 20 mA, 30 kV, and a scanning speed, step length and diffraction range of 10° min⁻¹, 0.1° and 10–60°, respectively. The measuring of the photoluminescence (PL), photoluminescence excitation (PLE) spectra and the luminescence decay curves was performed by a HITACHI F-7000 Fluorescence Spectrophotometer using a 150 W Xe lamp as the excitation source and scanning at 1200 nm min⁻¹. The excitation and emission slits were set to 2.5 and 5.0 nm. All of the measurements were performed at room temperature.

3 Results and discussion

3.1 Crystallization behaviour and morphology

Fig. 1 shows the representative XRD patterns of NaGdF₄:3%Tm³⁺, x%Ho³⁺ with different doping content. One can see that no impurity peaks are observed in all of the samples. Simultaneously, all of the diffraction peaks are found to be in good agreement with those reported in JCPDS file 27-0699, regardless of the doping content, suggesting that the obtained samples are single phase and the doped Tm³⁺ or co-doped Tm³⁺, Ho³⁺ ions do not cause any significant changes in the host structure. Moreover, sharp and strong peaks indicate that the products synthesized at low temperature still have high crystallinity. The crystal structure of hexagonal NaGdF₄ is shown in Fig. 1(b).²⁵ As shown in Fig. 1(b), there are two crystallographic positions of the cations in the unit cell: six-fold coordinated Na⁺ sites and nine-fold coordinated Gd³⁺ sites. According to the effective ionic radii of cations, the rare-earth ions are proposed to occupy the Gd³⁺ sites or Na⁺ sites. This is because the effective radii of the Tm³⁺ ions (1.052 Å) and Ho³⁺ ions (1.072 Å) are similar to that of the Gd³⁺ ions (1.107 Å). In view of the effective radii, Tm³⁺ and Ho³⁺ ions are expected to occupy the Gd³⁺ sites preferentially. Simultaneously, based on the valence state analysis, the rare-earth ions are also much more likely to occupy the Gd³⁺ sites.

Fig. 1 (a) XRD patterns of NaGdF₄:3.0%Tm³⁺, x%Ho³⁺ (x = 0, 1.0, 1.5) samples. The standard for NaGdF₄ (JCPDS card no. 27-0699) is shown as a reference. (b) Scheme of the hexagonal-phase NaGdF₄ structures.²⁵

In order to study the influence of doped rare earth ions and surfactant on the morphology of the samples, FESEM measurements were done. The FESEM images shown in Fig. 2 present the morphology of the different samples prepared by the hydrothermal method. From Fig. 2, it can be seen that both the NaGdF₄ (without SDS, (a)) and NaGdF₄:3%Tm³⁺, 3%Ho³⁺ (without SDS, (b)) samples exhibit an irregular particle morphology with a diameter of about 50 nm, indicating that the co-doped Tm³⁺ and Ho³⁺ ions do not change the NaGdF₄ morphology. The NaGdF₄:3%Tm³⁺, 3%Ho³⁺ (with SDS, (c)) sample exhibits a rod-like morphology with a diameter of about 50 nm and a length of about 250 nm. Furthermore, the nanorod is assembled by the nanoparticles with a diameter of about 50 nm. This indicates that SDS plays an important role in the formation of nanorods. SDS is a kind of anionic surfactant. It enables a reduction in the surface tension of the solution and hinders the formation of a new phase in the process of crystal growth. In the formation of the nanorods, it can act as structural guide reagent, therefore, the nanoparticles self-assemble and aggregate into a rod-like shape. The EDS result of NaGdF₄:3%Tm³⁺, 3.0% Ho³⁺ (Fig. 2(d)) reveals that the nanorods are mainly composed of Na, Gd, Tm, Ho and F. Note that the strong signals for Pt come from the platinum spraying process. All of these results further prove that the samples are NaGdF₄:Tm³⁺, Ho³⁺. In the following work, all the luminescence properties of Tm³⁺ and/or Ho³⁺ doped NaGdF₄ are measured using the samples prepared with SDS.

Fig. 2 FESEM images of NaGdF₄ (without SDS, (a)), NaGdF₄:3%Tm³⁺, 3%Ho³⁺ (without SDS, (b)), and NaGdF₄:3%Tm³⁺, 3%Ho³⁺ (with SDS, (c)). EDS spectrum of NaGdF₄:3%Tm³⁺, 3%Ho³⁺ (d).

3.2 Luminescence properties of NaGdF₄:Tm³⁺

Fig. 3 shows the PLE and PL spectra of NaGdF₄:3%Tm³⁺. Monitoring the emission at 451 nm, the excitation spectrum displays an absorption band which is attributed to the transition of ³H₆ → ¹D₂ at 358 nm. Upon excitation at 358 nm, the PL spectrum of NaGdF₄:3%Tm³⁺ has a sharp emission peak which is due to the transition of ¹D₂ → ³F₄ at 451 nm. The quenching concentration of NaGdF₄:Tm³⁺ is 3.0% as shown in the inset of Fig. 3. Moreover, a bright blue light can be observed in the photo of NaGdF₄:3%Tm³⁺ under excitation at 358 nm.

Fig. 3 PLE and PL spectra of NaGdF₄:3%Tm³⁺. Inset: the dependence of the emission intensity on Tm³⁺ concentration and a photo of NaGdF₄:3%Tm³⁺ under excitation at 358 nm.

3.3 Luminescence properties of NaGdF₄:Tm³⁺, Ho³⁺

The PLE spectra of NaGdF₄:Tm³⁺, NaGdF₄:Ho³⁺ and NaGdF₄:Tm³⁺, Ho³⁺ nanoparticles at different emission wavelengths are exhibited in Fig. 4. The PLE spectra of NaGdF₄:3%Tm³⁺ and NaGdF₄:3%Tm³⁺, 0.5% Ho³⁺ which were monitored at the Tm³⁺ emission wavelength of 451 nm have the f–f transition of ³H₆ → ¹D₂ of Tm³⁺ at 358 nm. The PLE spectra of NaGdF₄:3%Ho³⁺ and NaGdF₄:3%Tm³⁺, 0.5% Ho³⁺ which were monitored at the Ho³⁺ emission wavelength of 540 nm have ⁵I₈ → ³H₆ (359 nm), ⁵I₈ → ⁵G₅ (416 nm) and ⁵I₈ → ⁵G₆ (449 nm) transitions of Ho³⁺. Therefore, the excitation positions of the Tm^{3+ 3}H₆ → ¹D₂ (358 nm) and the Ho^{3+ 5}I₈ → ³H₆ (359 nm) transitions are very close. In view of the synchronous emissions of these two ions, it is believed that a near-UV wavelength around 359 nm might be used to efficiently excite these co-doped nanoparticles, which properly fits the requirements for WLEDs.²⁶ Moreover, in the NaGdF₄:3%Tm³⁺, 0.5%Ho³⁺ phosphors, the excitation intensity of 359 nm monitored at the Ho³⁺ emission wavelength of 540 nm is weaker than that of the excitation peak of 358 nm when monitored at the Tm³⁺ emission wavelength of 451 nm. More importantly, the Tm³⁺ and Ho³⁺ co-doped NaGdF₄ phosphors show a weak excitation peak (³H₆ → ¹D₂, 358 nm) when compared with the only Tm³⁺ doped NaGdF₄ phosphors, which indicates the expected energy transfer from Tm³⁺ to Ho³⁺. Meanwhile, the excitation peak overlap between 358 and 359 nm in the NaGdF₄:3%Tm³⁺, 0.5%Ho³⁺ phosphors was seen when monitored at 540 nm. Therefore the energy transfer from Tm³⁺ to Ho³⁺ is still possible.

Fig. 4 The excitation spectra of NaGdF₄:Tm³⁺, NaGdF₄:Ho³⁺ and NaGdF₄:Tm³⁺, Ho³⁺ phosphors.

Fig. 5 displays the PL spectra of NaGdF₄:3%Ho³⁺ (dashed line) and NaGdF₄:3%Tm³⁺, 3%Ho³⁺ (solid line). From Fig. 5, the emissions at 485, 540 and 648 nm belong to the ⁵F₃ → ⁵I₈, ⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺, respectively. The emission at 451 nm belongs to the ¹D₂ → ³F₄ transition of Tm³⁺. Moreover, the strong emission ascribed to the ⁵S₂ → ⁵I₈ (540 nm) transition of Ho³⁺ in NaGdF₄:3%Tm³⁺, 3%Ho³⁺ (solid line), is about 2 times stronger than that of Ho³⁺ in NaGdF₄:3%Ho³⁺ (dashed line), which suggests that the energy transfer occurred from Tm³⁺ to Ho³⁺.

Fig. 5 The PL spectra of NaGdF₄:3%Ho³⁺ (dashed line) and NaGdF₄:3%Tm³⁺, 3%Ho³⁺ (solid line) under UV excitation (λ_ex = 359 nm).

Fig. 6 shows the PL spectra of the NaGdF₄:3%Tm³⁺, x%Ho³⁺ (x = 0, 0.5, 1.0, 1.5, 3.0) samples with different Ho³⁺ concentrations under excitation at 359 nm. From Fig. 6, one can see that all of the samples show a emission band in the blue region related to the electronic transitions of ¹D₂ → ³F₄ (451 nm) of Tm³⁺.¹⁷ The emission bands in the green region are due to the ⁵F₃ → ⁵I₈ (485 nm) and ⁵S₂ → ⁵I₈ (540 nm) transitions of Ho³⁺.^22,27 From the inset of Fig. 6, it is clear and reasonable to imply that the PL intensity of the Ho³⁺ activator (or energy acceptor) increases, whereas the PL intensity of the Tm³⁺ sensitizer (or energy donor) is simultaneously found to decrease monotonically, with an increasing concentration of Ho³⁺ ions, which indicates that there is an energy transfer from Tm³⁺ to Ho³⁺.

Fig. 6 A series of PL spectra for NaGdF₄:3%Tm³⁺, x%Ho³⁺(x = 0, 0.5, 1.0, 1.5, 3.0) under UV excitation (λ_ex = 359 nm). Inset: the emission intensity of Tm³⁺ and Ho³⁺ with different Ho³⁺ doping content.

3.4 Energy transfer mechanism study

So as to make clear the possibility of the Tm³⁺ → Ho³⁺ energy transfer, the optical properties of single Tm³⁺, Ho³⁺-doped samples were studied. As described in Fig. 7, the comparison of the PL spectrum of NaGdF₄:Tm³⁺ and the PLE spectrum of NaGdF₄:Ho³⁺ discloses an obvious spectral overlap between the emission band of Tm³⁺ centered at 451 nm (¹D₂ → ³F₄) and the excitation transaction of Ho³⁺ (⁵I₈ → ⁵G₆, 449 nm). Hence, an effective resonance-type energy transfer from Tm³⁺ to Ho³⁺ is expected.

Fig. 7 Spectral overlap between PL spectrum of NaGdF₄:3%Tm³⁺ (dashed line) and PLE spectrum of NaGdF₄:3%Ho³⁺ (solid line).

The fluorescence decay process of the Tm³⁺ ions in NaGdF₄:3%Tm³⁺, x%Ho³⁺ phosphors was investigated to further study the energy transfer between Tm³⁺ and Ho³⁺ by monitoring at 451 nm with an irradiation wavelength of 359 nm. From Fig. 8(a), one can see that all of the luminescence decay curves of Tm³⁺ in NaGdF₄:Tm³⁺, Ho³⁺ can be fitted well with a single exponential function as

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 Fig. 8(a), the corresponding luminescence decay times are determined to be 2.850, 2.447, 2.385 and 1.791 ms, respectively. Fig. 8(b) clearly shows that the lifetimes for Tm³⁺ ions are found to drastically decrease with increasing Ho³⁺ concentration. This is because the energy absorbed by Tm³⁺ transfers to Ho³⁺, which is strong evidence for the energy transfer from Tm³⁺ to Ho³⁺, as reported by Shang.²⁸

Fig. 8 (a) Decay curves for the luminescence of Tm³⁺ ions in NaGdF₄:3%Tm³⁺, x%Ho³⁺ phosphors with different doped Ho³⁺ concentrations. (b and c) Dependence of the fluorescence lifetime of Tm³⁺ and the energy transfer efficiency η_T versus doped Ho³⁺ concentrations (x) in NaGdF₄:3.0%Tm³⁺, x%Ho³⁺ phosphors.

The energy transfer efficiency (η_T) from the sensitizer Tm³⁺ ions to the activator Ho³⁺ ions can be calculated using the following formula²⁹

η_T = 1 − I/I₀ (2)

where η_T is the energy transfer efficiency and I₀ and I are the peak intensities of the Tm³⁺ ions in the absence and presence of Ho³⁺ ions, respectively. Thus, the relationship between the energy-transfer efficiency and activator concentration of Ho³⁺ ion can be obtained (Fig. 8(c)). As shown in Fig. 8(c), the energy transfer efficiency monotonically increases with an increase in Ho³⁺ concentration and can even reach about 90% when x = 3.0. The high energy transfer efficiency implies that the energy transfer from Tm³⁺ to Ho³⁺ ions is especially efficient. Simultaneously, the main reason for the increase in the Ho³⁺ emission intensity is owing to the Tm³⁺ → Ho³⁺ energy transfer, rather than the energy absorbed by the Ho³⁺ ions themselves.⁷

3.5 Tunable color study

Aiming to research the effect of the Ho³⁺ ion doping content on the hue of NaGdF₄:3%Tm³⁺, we synthesized a series of samples with the chemical compositions of NaGdF₄:3%Tm³⁺, x%Ho³⁺ (x = 0, 0.5, 1.0, 1.5, 3.0). The Commission International de L’Eclairage (CIE) chromaticity coordinates of NaGdF₄:NaGdF₄:3%Tm³⁺, x%Ho³⁺ are presented in Fig. 9 and Table 1. The CIE chromaticity coordinate of pure Tm³⁺ activated NaGdF₄ is (0.179, 0.097), corresponding to a purplish blue color. From the CIE diagram of NaGdF₄:Tm³⁺, Ho³⁺ nanoparticles, it can be seen that with increasing concentration of Ho³⁺ ions, the chromaticity coordinates (x, y) can vary from (0.202, 0.194) to (0.219, 0.293), corresponding to the hue of these powders changing gradually from blue through to light blue and ultimately to bluish green. The digital photographs are also shown in the picture. The hues of the obtained samples change from purplish blue to light blue and eventually to bluish green as indicated in the inset of Fig. 9, which also demonstrates the role of the Ho³⁺ ions.

Fig. 9 CIE chromaticity diagram for NaGdF₄:3%Tm³⁺, x%Ho³⁺ (x = 0, 0.5, 1.0, 1.5, 3.0) excited at 359 nm.

Table 1 The corresponding chromaticity coordinates of selected NaGdF₄:Tm³⁺, Ho³⁺ nanoparticles

Samples	CIE (x, y)	Color
NaGdF4:0.03Tm^3+	(0.179, 0.097)	Purplish blue
NaGdF4:0.03Tm^3+, 0.005Ho^3+	(0.202, 0.194)	Blue
NaGdF4:0.03Tm^3+, 0.10Ho^3+	(0.211, 0.235)	Light blue
NaGdF4:0.03Tm^3+, 0.15Ho^3+	(0.212, 0.248)	Light blue
NaGdF4:0.03Tm^3+, 0.30EHo^3+	(0.219, 0.293)	Bluish green

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4 Conclusions

In summary, a series of Tm³⁺, Ho³⁺ co-doped NaGdF₄ nanoparticles were synthesized. The XRD patterns confirmed their hexagonal structure. In the NaGdF₄:Tm³⁺, Ho³⁺ systems, the energy transfer efficiency increases with an increase in Ho³⁺ concentration. For the NaGdF₄:3%Tm³⁺, x%Ho³⁺ (x = 0, 0.5, 1.0, 1.5, 3.0) nanoparticles, the emission could be tuned from purplish blue through to blue and ultimately to bluish green. Therefore, the β-NaGdF₄ nanoparticles could have potential applications in fields such as color displays and solid-state lighting.

Acknowledgements

This work was supported by the National Science Foundation of P. R. China (NSFC) (Grant no. 51072026, 50972020) and the Development of Science and Technology plan projects of Jilin province (Grant no. 20130206002 GX).

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