Morphology evolution and pure red upconversion mechanism of β-NaLuF₄ crystals

Abstract

A series of β-NaLuF₄ crystals were synthesized via a hydrothermal method. Hexagonal phase microdisks, microprisms, and microtubes were achieved by simply changing the amount of citric acid in the initial reaction solution. Pure red upconversion (UC) luminescence can be observed in β-NaLuF₄:Yb³⁺, Tm³⁺, Er³⁺ and Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺. Based on the rate equations, we report the theoretical model about the pure red UC mechanism in Yb³⁺/Tm³⁺/Er³⁺ doped system. It is proposed that the pure red UC luminescence is mainly ascribed to the energy transfer UC from Tm³⁺:³F₄ → ³H₆ to Er³⁺:⁴I_11/2 → ⁴F_9/2 and the cross-relaxation (CR) effect [Er³⁺:⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2] rather than the long-accepted mechanism [CR process among Er³⁺:⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2]. In addition, compared to the Li⁺-free counterpart, the pure red UC luminescence in β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ with 15 mol% Li⁺ doping is enhanced by 13.7 times. This study provides a general and effective approach to obtain intense pure red UC luminescence, which can be applied to other synthetic strategies.

Introduction

Recently, lanthanide (Ln) doped upconversion (UC) materials have aroused extensive attention because of their potential applications in fields such as flat-panel displays, therapeutics, photovoltaics, and biological imaging^{1,2,3,4,5,6,7,8,9}. Their advantages include long luminescence lifetimes, low toxicity and high photochemical stability^{10,11,12,13,14}, which make them more desirable than conventional fluorescent materials. As an important fluoride, β-NaLuF₄ has excellent UC luminescence due to its high refractive index, low phonon energy, and high thermal stability, which has attracted significant interest^15,16,17,18. In contrast to green and blue light, red light (600–700 nm) can deeply penetrate biotissues owing to the lack of efficient endogenous absorbers^19,20,21. Consequently, a strategy to achieve high purity of red UC luminescence will be useful for UC applications, especially for biological imaging. As is known, UC materials usually display multipeak emissions due to Ln ions have more than one metastable excited state¹⁹. Thus, avoiding the blue and green emissions and boosting the red emission are needed to obtain high pure red UC luminescence. For instance, Tan et al. reported the pure red UC emission in NaYbF₄ nanocrystals doping with high Er³⁺ content²². Chan et al. revealed a UC mechanism in which energy transfer (ET) from Tm³⁺:³F₄ → ³H₆ to Er³⁺:⁴I_11/2 → ⁴F_9/2, leading to the population of red-emitting manifold (Er³⁺:⁴F_9/2)²³. Therefore, it can be concluded that high-content doping of Er³⁺ and low-content doping of Tm³⁺ can induce a tremendous increase in the red to green ratio (RGR) and ultimately pure red UC luminescence. There are three common cross-relaxation (CR) processes among Er³⁺ that account for the enhancement of RGR in Yb³⁺/Er³⁺ codoped system. The first CR effect is the long-accepted and most popular mechanism^22,24,25,26. According to Capobianco et al.’s report²⁶, the enhanced red UC emission was obtained owing to a CR process [Er³⁺:⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2], which directly populates the ⁴F_9/2 state. The second CR effect was proposed by Gao et al.²⁷, which was derived from Er³⁺:⁴S_3/2 + ⁴I_13/2 → ⁴F_9/2 + ⁴I_11/2, resulting in the promotion of red emission and quenching of green emission. The third CR effect was proposed by Salas et al.²⁸, they reported that the increased red UC luminescence mainly comes from the CR process [Er³⁺:⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2], and subsequently energy transfer UC (ETU) process [I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴F_9/2 (Er³⁺) + ²F_7/2 (Yb³⁺)]. However, there is no theoretical model about the pure red UC mechanism in Yb³⁺/Tm³⁺/Er³⁺ doped system. Besides, compared to traditional phosphors, the main drawback of UC materials is their low UC luminescence efficiency. It is still a challenge to obtain the intense pure red UC luminescence, and an effective method to enhance the pure red UC emission is urgently needed. It has been proved that the doping of Li⁺ can greatly increase the UC luminescence intensity^29,30,31. As is known, Li⁺ can be easily doped into the host lattice substitutionally or interstitially owing to its small ionic radius, which would reduce the symmetry of crystal field around Ln ions, inducing the enhancement of UC emission intensity. However, there is no report on the increase of pure red UC luminescence by introducing Li⁺ in β-NaLuF₄:Yb³⁺, Tm³⁺, Er³⁺.

As a typical solution-based approach, the hydrothermal method has been widely applied to synthesize inorganic materials with controllable structures and morphologies^32,33. During the hydrothermal treatment, a series of external parameters such as the pH value, citrate ions (Cit³⁻) content, NaF content, reaction time and temperature may have significant effects on the morphology evolution of particles^18,34,35,36. In particular, the addition of chelating agent has a great impact on the kinetics of crystal growth^37,38. Cit³⁻ as a shape modifier plays a critical role in the shape evolution of the final products due to its high thermal stability and ability to form complexes with other metal ions^39,40,41.

In this article, a series of β-NaLuF₄ crystals were prepared via a hydrothermal method using citric acid as a chelating agent, and their pure red UC luminescence were studied. Hexagonal phase microdisks, microprisms, and microtubes were achieved by simply changing the amount of citric acid in the initial reaction solution. Importantly, pure red UC luminescence can be observed in β-NaLuF₄:Yb³⁺, Tm³⁺, Er³⁺ and Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺. Based on the rate equations, the theoretical model about the pure red UC mechanism in Yb³⁺/Tm³⁺/Er³⁺ doped system is presented. The red UC emission of 660 nm in Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ is greatly increased compared to the Li⁺-free sample under 980 nm excitation at room temperature.

Results and Discussion

Morphology evolution of β-NaLuF₄ crystals

Citric acid has been regarded as one of the most effective chelating agents because of its ability to regulate the morphology and dimension of the samples in the hydrothermal process⁴². In the present system, citric acid also plays a critical role in the morphology evolution of β-NaLuF₄ crystals. Figure 1 shows the XRD patterns of the as-prepared β-NaLuF₄ samples with different citric acid contents from 2 to 8 mmol. As can be seen, all the diffraction peaks can be well indexed to pure β-NaLuF₄, which is consistent with the standard card (JCPDS 27-0726). No other impurity peaks are detected, indicating the high purity of β-NaLuF₄ samples. It is worth to note that the relative intensities of (100), (110), (101) and (201) peaks display some differences from each other, implying the existence of oriented growth under different Cit³⁻ contents. The above XRD results are supported by the corresponding SEM images, as exhibited in Fig. 2. When the adding citric acid is 2 mmol (Fig. 2a), regular hexagonal phase microdisks with an average size of 0.79 μm in height and 7.58 μm in diameter are obtained. As the citric acid content increases to 3 mmol (Fig. 2b), short hexagonal phase microprisms with uniformity and smooth surfaces are achieved. The mean height and diameter of the prisms are 2.12 μm and 8.51 μm, respectively. Further increasing the citric acid content to 8 mmol, hexagonal phase microtubes with hollow structure are presented in Fig. 2c. The tubes have an average height of 9.47 μm and an average diameter of 1.88 μm. The ratios of height to diameter (H/D ratios) are calculated to be about 0.10, 0.25, and 5.04 when the adding citric acid is 2, 3, and 8 mmol. From the above analysis, it can be concluded that the H/D ratio is increased as the citric acid content increases from 2 to 8 mmol. Based on the high anisotropic structure of β-NaLuF₄⁴³, when the adding citric acid increases from 2 to 8 mmol, Cit³⁻ absorbs onto the {0001} facets more strongly than the facets. Thus, the growth rate along [0001] direction is faster than that along direction, resulting in the morphology evolution from disks to tubes and the enhancement of H/D ratio. The hollow structure of the tubes is generated owing to the growth rate at the center is lower than that at the edges⁴⁴. The corresponding schematic diagrams of β-NaLuF₄ crystals under different citric acid contents are displayed in Fig. 2(d–f).

Figure 1: XRD patterns of β-NaLuF₄ crystals with different citric acid contents.

(a–c) refer to 2, 3, 8 mmol, respectively. The vertical red lines are the standard profiles of β-NaLuF₄ (JCPDS 27-0726).

Figure 2

SEM images (a–c) and the corresponding schematic diagrams (d–f) of β-NaLuF₄ crystals with different citric acid contents: (a,d) 2 mmol, (b,e) 3 mmol, and (c,f) 8 mmol. Scale bars = 5 μm.

Pure red UC mechanism of β-NaLuF₄:Yb³⁺, Tm³⁺, Er³⁺ crystals

A series of β-NaLuF₄:Yb³⁺, Tm³⁺, Er³⁺ crystals were synthesized by adding 3 mmol citric acid. Figure 3 shows the UC emission spectra (normalized to Er³⁺ 540 nm emission) of (a) β-NaLuF₄:20% Yb/xEr, (b) β-NaLuF₄:20% Yb/0.5% Tm/xEr, and (c) β-NaLuF₄:20% Yb/1% Tm/xEr (x = 0.5, 2, 5, 10, 20%) under 980 nm excitation at room temperature. Green emissions at around 520/540 nm correspond to the transitions of Er³⁺:²H_11/2/⁴S_3/2 → ⁴I_15/2. Red emissions at approximately 660 nm and 696 nm are attributed to the transition of Er³⁺:⁴F_9/2 → ⁴I_15/2 and the transition of Tm³⁺:³F₃ → ³H₆, respectively. As can be seen, compared with Tm³⁺-free group [Fig. 3(a)], the RGR is greatly increased in 0.5% Tm³⁺-group [Fig. 3(b)] and 1% Tm³⁺-group [Fig. 3(c)]. The maximum RGR is observed in 0.5% Tm³⁺ and 1% Tm³⁺-groups doped with 20 mol% Er³⁺. From the insets of Fig. 3(a–c) and Table 1, it can be clearly seen that the RGR is almost unchanged in Tm³⁺-free group while dramatically enhanced in 0.5% Tm³⁺ and 1% Tm³⁺-groups with the increase of Er³⁺ dopant content. The RGR in 0.5% Tm³⁺ (R/G = 43.7) and 1% Tm³⁺ (R/G = 49.3)-groups with 20% Er³⁺ doping are increased by 26 and 19 times compared to their 0.5% Er³⁺ doping (R/G = 1.66, 2.57). Consequently, low-content doping of Tm³⁺ and high-content doping of Er³⁺ induce great enhancement in the RGR. Figure 4 shows the pump power dependence of green and red UC emissions in 0.5% Tm³⁺ and 1% Tm³⁺-groups with 0.5%, 5% and 20% Er³⁺ doping under 980 nm excitation. According to the formula⁴⁵: I_uc ∝ Pⁿ, where I_uc is the output UC emission intensity, P is the infrared excitation power, n is the absorbed laser photon number when emitting an UC photon. As shown in Fig. 4(a,c), the slopes of green emission (Er³⁺:²H_11/2/⁴S_3/2 → ⁴I_15/2) are 1.23, 1.40, and 1.72 in β-NaLuF₄:20% Yb/0.5% Tm/xEr; 1.15, 1.53, and 1.61 in β-NaLuF₄:20% Yb/1% Tm/xEr (x = 0.5, 5, 20%). The slopes of red emission (Er³⁺:⁴F_9/2 → ⁴I_15/2) are 1.22, 1.10, and 0.64 in 0.5% Tm³⁺-group; 1.27, 1.16, and 1.08 in 1% Tm³⁺-group with 0.5%, 5% and 20% Er³⁺ doping. On the basis of the above analysis, it can be concluded that green emission varies from one-photon process to two-photon processes, and red emission (660 nm) keeps one-photon process in 0.5% Tm³⁺ and 1% Tm³⁺-groups.

Figure 3

UC emission spectra (normalized to Er³⁺ 540 nm emission) of (a) β-NaLuF₄:20% Yb/xEr, (b) β-NaLuF₄:20% Yb/0.5% Tm/xEr, and (c) β-NaLuF₄:20% Yb/1% Tm/xEr (x = 0.5, 2, 5, 10, 20%) under 980 nm excitation. The insets of (a–c) show the corresponding R/G ratio as a function of Er³⁺ concentration.

Table 1 Summary of RGR in (a) β-NaLuF₄:20% Yb/xEr, (b) β-NaLuF₄:20% Yb/0.5% Tm/xEr, and (c) β-NaLuF₄:20% Yb/1% Tm/xEr (x = 0.5, 2, 5, 10, 20%).

Figure 4

Double logarithmic relationship of (a,c) green and (b,d) red UC luminescence intensities versus pump powers in β-NaLuF₄:20% Yb/0.5% Tm/xEr and β-NaLuF₄:20% Yb/1% Tm/xEr (x = 0.5, 5, 20%) under 980 nm excitation.

In this paper, we built a theoretical model to have a deep understanding of the ET process in Yb³⁺/Tm³⁺/Er³⁺ doped system. We supposed ⁴F_7/2, ²H_11/2, and ⁴S_3/2 energy levels as a same level. When the Yb³⁺/Tm³⁺/Er³⁺ system doped with low Er³⁺ content, CR effect between Er³⁺ can be neglected. The corresponding rate equations are as follows:

N₀, N₁, N₂, N_2′, N₃, N₄, M₀, M₁, N_Yb0 and N_Yb1 are the population densities of the Er^{3+ 4}I_15/2, ⁴I_13/2, ⁴I_11/2, ⁴I_9/2, ⁴F_9/2, ⁴S_3/2/²H_11/2/⁴F_7/2, Tm^{3+ 3}H₆, ³F₄, Yb^{3+ 2}F_7/2 and ²F_5/2 levels, respectively. ω₀, ω₁, ω₂, ω₃, and ω₄ correspond to the ET rates of ²F_5/2 + ⁴I_15/2 → ²F_7/2 + ⁴I_11/2 (ET1), ²F_5/2 + ⁴I_13/2 → ²F_7/2 + ⁴F_9/2 (ET2), ²F_5/2 + ⁴I_11/2 → ²F_7/2 + ⁴F_7/2 (ET3), ⁴S_3/2 + ³H₆ → ⁴I_9/2 + ³F₄ (ET4), and ³F₄ + ⁴I_11/2 → ³H₆ + ⁴F_9/2 (ET5), respectively. ω₂₁, ω_2′2, and ω₄₃ are multiphonon relaxation (MPR) rates from Er³⁺:⁴I_11/2 → ⁴I_13/2, ⁴I_9/2 → ⁴I_11/2, and ⁴F_7/2 → ⁴F_9/2, respectively. A₃ and A₄ are the spontaneous radiative probabilities of Er^{3+ 4}F_9/2 and ⁴S_3/2/²H_11/2 levels, respectively. As presented in Fig. 5, the Er^{3+ 4}F_7/2 level is populated through the ET1 + ET3 processes. Then green UC emission is generated by the MPR processes of Er³⁺:⁴F_7/2 → ⁴S_3/2/²H_11/2 levels. For red UC emission (660 nm), the Er^{3+ 4}F_9/2 level is populated in two ways: (a) the MPR from Er^{3+ 4}F_7/2 level, (b) the ET4+ET5 processes. In consideration of the high Yb³⁺ content, many radiative and nonradiative processes can be ignored, such as the back ET process from Er³⁺ to Yb³⁺ (⁴I_11/2 + ²F_7/2 → ⁴I_15/2 + ²F_5/2), MPR processes, N₁, N₂ radiative emissions and so on. Under steady-state condition, the rate equations can be acquired as follows:

Figure 5: Proposed ET mechanism of green and red UC emissions in β-NaLuF₄:Yb³⁺, Tm³⁺, Er³⁺ (with low Er³⁺ content).

The mechanism involves the ETU from Tm³⁺ to Er³⁺²³.

The population density of Yb^{3+ 2}F_5/2 can be described as follows⁴⁶:

where σ is the absorption cross-section of Yb^{3+ 2}F_5/2 level, ρ is the pump rate of near-infrared (NIR) laser. Under steady-state condition, we get

By solving the above equations, we have

As can be seen from Equations (1) and (2), both red-emitting manifold N₃ and green-emitting manifold N₄ have linear relationships with pump power at low Er³⁺ dose, which are in good agreement with the experimental results shown in Fig. 4.

When the Yb³⁺/Tm³⁺/Er³⁺ system doped with high Er³⁺ dose, pure red UC luminescence can be obtained. As is known, CR is dependent on the distance among activators⁴⁶. The average distance between Er³⁺ reduces with the increase of Er³⁺ dopant dose, which would result in the enhancement of CR effect. Thus, the CR process among Er³⁺ plays an important role in the achievement of pure red UC luminescence. There are generally three CR processes between Er³⁺ that account for the increase of RGR (has been described in the section of “Introduction”), as displayed in Fig. 6. Additionally, the ET4+ET5 processes between Tm³⁺ and Er³⁺ also make a significant contribution to the high RGR. In the following sections, the above three CR effects are discussed systematically. As the Yb³⁺/Tm³⁺/Er³⁺ system doped with high Er³⁺ content, green UC emission can be neglected, and red UC emission (660 nm) mainly comes from two ways: (a) the CR effect among Er³⁺; (b) the ETU from Tm³⁺ to Er³⁺.

Figure 6: Proposed ET mechanism of red UC emission in β-NaLuF₄:Yb³⁺, Tm³⁺, Er³⁺ (with high Er³⁺ content).

The mechanism involves the CR effects among Er^{3+26,27,28,46} and ETU from Tm³⁺ to Er³⁺²³.

Theoretical model for the first CR effect

The corresponding rate equations are as follows:

ω_C1 is the CR rate for [Er³⁺:⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2 (CR1)]. CR1 is the long-accepted and most popular mechanism to account for the enhancement of RGR in Yb³⁺/Er³⁺ codoped system^22,24,25,26. By solving the equations under steady-state excitation, we get

As can be seen from Equation (5), the green-emitting level N₄ does not have quasi-quadratic relationship with pump power, which is not corresponding to Fig. 4(a,c) where n are 1.72 and 1.61 in 0.5% Tm³⁺ and 1% Tm³⁺-groups at high content (20 mol%) of Er³⁺ doping, indicating CR1 is not suitable for explaining the experimental results. Consequently, CR1 has nothing to do with the increase of red emission (660 nm) at high Er³⁺ content in Yb³⁺/Tm³⁺/Er³⁺ doped system.

Theoretical model for the second CR effect

The corresponding rate equations are as follows:

ω_C2 is the CR rate for [Er³⁺:⁴S_3/2 + ⁴I_13/2 → ⁴F_9/2 + ⁴I_11/2 (CR2)]. By solving the equations under steady-state condition, we have

As can be seen from Equations (7) and (8), red-emitting manifold N₃ and green-emitting manifold N₄ have linear and quasi-quadratic relationships with pump power at high Er³⁺ content, which correspond to the relevant results in Fig. 4. The power dependence of RGR for β-NaLuF₄:20% Yb/1% Tm/20% Er is exhibited in Fig. 7(a). It can be clearly seen that the RGR is decreased with the increase of pump power. The ratio of N₃ to N₄ shows the inverse proportional relationship to pump power [Equation (9)], which is in accordance with the result in Fig. 7(a). From the above analysis, it can be deduced that CR2 maybe makes a contribution to the high RGR when the Yb³⁺/Tm³⁺/Er³⁺ system doped with high Er³⁺ dose.

Figure 7

(a) Power dependence of R/G ratio for β-NaLuF₄:20% Yb/1% Tm/20% Er, and (b) Er³⁺ concentration dependence of NIR/G ratio for β-NaLuF₄:20% Yb/1% Tm/xEr under 980 nm excitation. NIR emission corresponds to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺.

Theoretical model for the third CR effect

The corresponding rate equations are as follows:

ω_C3 is the CR rate for [Er³⁺:⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2 (CR3)], and red UC emission (660 nm) is obtained by the subsequent ETU process [I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴F_9/2 (Er³⁺) + ²F_7/2 (Yb³⁺)]. By solving the equations under steady-state condition, we get

As can be seen from Equation (10), red-emitting level N₃ ~ aρ + bρ², which is corresponding to the relevant results in Fig. 4(b,d) when we suppose the parameter “b” is close to zero. Equations (11) and (12) show the green-emitting manifold N₄ and ratio of N₃ to N₄ have quasi-quadratic and inverse proportional relationships with pump power, which are in good agreement with the results shown in Figs 4(a,c) and 7(a), respectively. Thus, CR3 can be used to explain the experimental results.

On the basis of the above analysis, it can be concluded that both CR2 and CR3 maybe are the appropriate ET mechanisms for the achievement of pure red UC luminescence at high Er³⁺ content in Yb³⁺/Tm³⁺/Er³⁺ doped system. According to our experimental results, there are three reasons to prove that CR3 (Er³⁺:⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2) is the main CR effect for the population process of ⁴F_9/2 manifold. First, the ratio of NIR to green (NGR) is enhanced with the increase of Er³⁺ content in β-NaLuF₄:20% Yb/1% Tm/xEr (NIR emission corresponds to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺), as presented in Fig. 7(b). The increasing NGR indicates that the population of Er^{3+ 4}I_13/2 level becomes larger and larger compared to Er^{3+ 4}S_3/2/²H_11/2 levels. As is known, the probability of MPR from ⁴I_11/2 to ⁴I_13/2 is quite low due to the low phonon energy in our system. Thus, the increasing NGR is mainly ascribed to CR3. Second, the decay curves of the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ in β-NaLuF₄:20% Yb/1% Tm/0.5% Er and β-NaLuF₄:20% Yb/1% Tm/20% Er are shown in Fig. 8. The decay lifetime was calculated based on the function: , where I(t) is the emission intensity at time t, and I_P is the peak intensity in the decay curve. The calculation results show that τ_{0.5% Er} = 1.40 ms and τ_{20% Er} = 0.55 ms. The energy transfer efficiency (ETE) can be evaluated by the following expression: η_ETE = 1 − τ_{20% Er}/τ_{0.5% Er}, the calculation result shows that η_ETE = 60.71%. With the increase of Er³⁺ dose, the enhanced red UC emission (660 nm) mainly comes from the enhancement of CR effect between Er³⁺ in Yb³⁺/Tm³⁺/Er³⁺ doped system. Thus, it is reasonable to consider that the probability of CR3 is 60.71%. Third, the ET process of CR2 (Er³⁺:⁴S_3/2 + ⁴I_13/2 → ⁴F_9/2 + ⁴I_11/2) needs the population process of Er^{3+ 4}I_13/2 manifold, which is mainly dependent on the ET process of CR3 (Er³⁺:⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2). Therefore, the CR3 process is required before the CR2 process, resulting in the leading role of CR3 in the CR effects.

Figure 8

Decay curves of the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ in β-NaLuF₄:20% Yb/1% Tm/0.5% Er and β-NaLuF₄:20% Yb/1% Tm/20% Er.

It can be concluded that the pure red UC luminescence is mainly ascribed to the ETU from Tm³⁺:³F₄ → ³H₆ to Er³⁺:⁴I_11/2 → ⁴F_9/2 and the CR effect [Er³⁺:⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2 (CR3)] rather than the long-accepted and most popular mechanism (CR1 process among Er³⁺:⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2).

Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ crystals

A series of Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ crystals were synthesized by adding 3 mmol citric acid. Figure 9 presents the XRD patterns (a) and the main diffraction peak (b) of different Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ crystals. As shown in Fig. 9(a), all the diffraction peaks of the products can be indexed as pure β-NaLuF₄ (JCPDS 27-0726) even the Li⁺ concentration increases up to 20 mol%, indicating that Li⁺ doping has no influence on the crystal structure of the products. The corresponding UC emission spectra of the products under 980 nm excitation are shown in Fig. 10. As can be seen, the pure red UC luminescence is greatly enhanced after Li⁺ doping. Compared to the Li⁺-free sample, the pure red UC luminescence in β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ with 15 mol% Li⁺ doping is increased by 13.7 times. This phenomenon is mainly caused by the asymmetric surrounding environment around Ln ions after Li⁺ doping. Figure 9(b) exhibits that the main diffraction peak moves to the larger angles when the Li⁺ concentration is from 0 to 15 mol%, whereas shifts in reverse as the Li⁺ concentration increases up to 20 mol%. According to Bragg’s law 2dsinθ = nλ, where d represents the interplanar distance, θ represents the diffraction angle, and λ represents the diffraction wavelength. When d decreases, θ increases; when d increases, θ decreases. As is displayed in Fig. 11(a), Na⁺ and Ln³⁺ occupy the same lattice site in β-NaLuF₄ lattice. When the Li⁺ is introduced into the host lattice, it can replace Na⁺ (d decreases, θ increases, 0 < Li⁺ concentration ≤15 mol%) [Fig. 11(b)] or occupy the interstitial site (d increases, θ decreases, 15 < Li⁺ concentration ≤20 mol%) [Fig. 11(c)] due to its small ionic radius, leading to the contraction or expansion of unit cell. Both the contraction and expansion of unit cell would reduce the symmetry of crystal field around Ln ions, inducing the sharp increase of pure red UC luminescence intensity^29,30,31. The strongest UC luminescence intensity is acquired in β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ with 15 mol% Li⁺ doping, which is attributed to the most asymmetric surrounding environment around Ln ions, as shown in Fig. 9(b).

Figure 9

XRD patterns (a) and the main diffraction peak (b) of different Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ crystals. The vertical red lines are the standard profiles of β-NaLuF₄ (JCPDS 27-0726).

Figure 10

UC emission spectra of β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ crystals doped with different Li⁺ contents under 980 nm excitation.

Figure 11

Crystal structure of β-NaLuF₄ (a); possible changes in β-NaLuF₄ crystal lattice after Li⁺ doping: substitution by Li⁺ (b), and interstitial occupation by Li⁺ (c).

Conclusion

In summary, hexagonal phase microdisks, microprisms, and microtubes were achieved by simply changing the amount of citric acid in the initial reaction solution. Pure red UC luminescence can be observed in β-NaLuF₄:Yb³⁺, Tm³⁺, Er³⁺ and Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺. We prove that the low-content doping of Tm³⁺ and high-content doping of Er³⁺ induce great enhancement in the RGR. The RGR in 0.5% Tm³⁺ (R/G = 43.7) and 1% Tm³⁺ (R/G = 49.3)-groups with 20% Er³⁺ doping are increased by 26 and 19 times compared to their 0.5% Er³⁺ doping (R/G = 1.66, 2.57). Green emission varies from one-photon process to two-photon processes, and red emission (660 nm) keeps one-photon process in 0.5% Tm³⁺ and 1% Tm³⁺-groups. Based on the rate equations, we report the theoretical model about the pure red UC mechanism in Yb³⁺/Tm³⁺/Er³⁺ doped system. It is proposed that the pure red UC luminescence is mainly ascribed to the ETU from Tm³⁺:³F₄ → ³H₆ to Er³⁺:⁴I_11/2 → ⁴F_9/2 and the CR effect [Er³⁺:⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2 (CR3)] rather than the long-accepted and most popular mechanism (CR1 process among Er³⁺:⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2). Additionally, compared to the Li⁺-free sample, the pure red UC luminescence in β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ with 15 mol% Li⁺ doping is enhanced by 13.7 times. The results suggest that the enhanced pure red UC luminescence in Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ may have potential applications in flat-panel displays, solid-state lasers and light-emitting diodes. Besides, this study provides a general and effective approach to obtain intense pure red UC luminescence, which can be applied to other synthetic strategies to prepare many types of nanocrystals with high purity of red UC luminescence, making it suitable for the future bioapplications.

Methods

Chemicals

All of the chemicals are of analytical grade and used as received without further purification. 1 M of Lu(NO₃)₃, 0.5 M of Yb(NO₃)₃, 0.1 M of Er(NO₃)₃, and 0.1 M of Tm(NO₃)₃ stock solutions were prepared by dissolving the corresponding rare earth oxide (99.99%) in dilute nitric acid (30%) at elevated temperature.

Preparation

A series of β-NaLuF₄ crystals with different morphologies were synthesized via a hydrothermal method using citric acid as a chelating agent. In a typical procedure, (2 mmol/3 mmol/8 mmol) of citric acid (2 M, 1 mL/1.5 mL/4 mL), 5 mmol of NaOH (4 M, 1.25 mL) and 10 mL of deionized water were mixed and stirred for 10 min. Then 1 mmol of Ln(NO₃)₃ [1 mmol of Lu(NO₃)₃ (1 M, 1 mL)] was added to the above mixture and then stirred for 30 min to form the RE-Cit³⁻ complex. Subsequently, an aqueous solution containing 8 mmol of NaF (1 M, 8 mL) and (9 mL/8.5 mL/6 mL) of deionized water was added into the chelated RE-Cit^3- complex to form a colloidal suspension and kept stirring for another 30 min. Finally, the suspension was transferred into a 50 mL-Teflon vessel, sealed in an autoclave and maintained at 200 °C for 10 h before cooling down naturally. The final products were separated by centrifugation, washed several times with ethanol and deionized water, then dried in air at 60 °C for 12 h. β-NaLuF₄: 20% Yb³⁺, (0%/0.5%/1%)Tm³⁺, (0.5%/2%/5%/10%/20%)Er³⁺ crystals and (8%/15%/20%)Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ crystals were prepared by a similar process (Cit³⁻ = 3 mmol) under the same experimental conditions. In particular, as for (8%/15%/20%)Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ crystals, after the formation of RE-Cit³⁻ complex, a mixture containing (0.64 mmol/1.2 mmol/1.6 mmol) of LiNO₃ (1 M, 0.64 mL/1.2 mL/1.6 mL), (7.36 mmol/6.8 mmol/6.4 mmol) of NaF (1 M, 7.36 mL/6.8 mL/6.4 mL), (0.32 mmol/0.6 mmol/0.8 mmol) of NH₄HF₂ (1 M, 0.32 mL/0.6 mL/0.8 mL) and (6 mL/5.5 mL/5.5 mL) of deionized water were added into the chelated RE-Cit³⁻ complex to form the colloidal suspension.

Characterization

The phase and structure of the as-prepared products were confirmed by powder X-ray diffraction (XRD) patterns using the D-Max 2200VPC XRD from Rigaku Company. Morphologies and grain sizes were verified by using an Oxford Quanta 400F Thermal Field Emission environmental Scanning Electronic Microscope (SEM). UC photoluminescence spectra were acquired on the Edinburgh Instrument FLSP920 steady-state fluorescence spectrometer equipped with a 2 W 980 nm laser diode. The spot size of the 980 nm laser on the samples is about 0.05 cm².