Journal of Nanomaterials
Volume 2020 (2020), Article ID 8509380, 11 pages
https://doi.org/10.1155/2020/8509380

Research Article

Morphology Manipulation and Upconversion Luminescence Enhancement of β-NaLuF₄: Er³⁺ Hexagonal Microtubes via Sr²⁺ Doping

Han Lin, Zhonggen Wang, and Yan Hong

School of Electrical and Information Engineering, Anhui University of Science and Technology, Huainan, 232001 Anhui, China

Correspondence should be addressed to Han Lin; iyishuihan@126.com

Received 11 May 2020; Revised 15 July 2020; Accepted 21 July 2020; Published 28 August 2020

Academic Editor: Hiromasa Nishikiori

Copyright © 2020 Han Lin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A new and facile strategy to enhance the upconversion luminescence (UCL) emission of NaLuF₄: Er³⁺ microcrystals (MCs) using strontium (Sr) as a dopant has been reported. With the introduction of Sr²⁺, the products change from long NaLuF₄: Er³⁺ hexagonal microtubes to short hexagonal microtubes and finally to hexagonal microprisms. The growth mechanism is profoundly discussed according to the different reaction time-dependent morphologies. More importantly, the total fluorescence intensity is significantly reinforced by doping Sr²⁺ ions. When 18% Sr²⁺ is doped into NaLuF₄: Er³⁺ hexagonal microtubes, the maximum green and red luminescence intensities are about 5.8 and 4.4 times higher than those of Sr²⁺-free samples, respectively. The influences of Sr²⁺ ion doping content on the phase, the morphology, and the local crystal field symmetry of the as-synthesized NaLuF₄ crystals are investigated.

1. Introduction

Upconversion (UC) micro- and nanocrystals refer to a nonlinear optical process in which lower-energy photons are absorbed and higher-energy photons are emitted [1]. In the past few decades, countless scientific researches about trivalent lanthanide ion- (Ln³⁺-) doped UC materials have aroused widespread attention owing to their applications in many promising fields, such as photovoltaic cells, biological detection, temperature sensing, DNA detection, and photodynamic therapy (PDT) [2–10]. As a typical upconversion matrix material, NaYF₄ crystal has attracted much attention due to its low phonon energy (<350 cm⁻¹), long luminescence lifetime, and high thermal stability and has been widely studied [6, 10–14]. The crystal structure of NaLuF₄ is similar to NaYF₄, with a smaller ion radius ( Å,  Å), and it is also considered an ideal substitute for the ideal UC matrix [15–20]. Especially Lu³⁺ in NaLuF₄ could sensitize the activator and perform volume compensation, resulting in enhanced emission. Despite these advantages, there are still some insurmountable problems. In particular, the efficiency is low, which limits its application to a certain extent for NaLnF₄ compounds. Therefore, it is imperative to enhance the luminescent intensity of UC crystal.

It is generally believed that the UC emission of Ln³⁺-doped materials depends on the internal 4f transition probabilities, which is significantly impacted by the local crystal field symmetry of Ln³⁺ ions [21]. Doping impurity ions is an effective way to improve optical performance, which is dependent on local symmetrical tailoring, surface passivation, and surface-plasmon coupling [22, 23]. In particular, alkaline earth ion (Li⁺, K⁺, Mg²⁺, Ba²⁺, and Ca²⁺) or trivalent Ln³⁺ ion doping methods have already been proved to benefit from modifying the morphology and crystallographic phase of UC crystals. In previous studies, Wang and coworkers studied the effect of Li⁺ dopant on NaYF₄: Yb³⁺, Er³⁺ nanocrystals. They observed 34-time green and 101-time red emission enhancement and proposed that UC enhancement could be attributed to local crystal field distortion or disrupted local crystal field symmetry around rare-earth ions [24]. Zhou et al. claimed that by introducing Ca²⁺ ions, the β-NaLuF₄: Yb³⁺/Er³⁺, Er³⁺/Tm³⁺ microcrystals could be resized simultaneously and realize green and red emission enhancement by 7.5 and 7.9 times, respectively [25]. Shao and coworkers doped Ca²⁺ into NaYF₄: Nd³⁺ microcrystal, and the near-infrared luminescence intensity at 1.06 μm increased by about three times [26]. Furthermore, doping some special ions could not only improve the luminescence performance but also modify the morphology. Huang and coworkers reported that Sr²⁺ and Tb³⁺ codoping could regulate the size and morphology of NaCeF₄ nanorods and Sr²⁺ doping can tune the emission intensity of Tb³⁺-doped NaCeF₄ nanocrystals [27]. These investigations have proved that cationic doping can change electron distribution density and can control the phase transition, morphology, and luminescence intensity of microcrystals or nanocrystals. However, there have been few systematic studies focusing on the structure, morphology, and luminescence of NaLuF₄: Er³⁺ microcrystals by Sr²⁺ doping.

Considering the above points, we incorporate Sr²⁺ as a dopant ion into NaLuF₄: Er³⁺ for adjusting the local coordination symmetry of Ln³⁺ ions, thereby enhancing the emission efficiency of UC. Through XRD, TEM, and UC emission spectrum characterization analysis, the phase structure, morphology evolution, and UC luminescence property are studied. The results illustrate that, at low concentration of Sr²⁺, the morphology tended to be hexagonal microtubes with scrappy ends, while high concentration of Sr²⁺ induced the generation of a prism-shaped morphology. Growth process reveals that Sr²⁺-doped NaLuF₄: Er³⁺ hexagonal microtubes are obtained by oriented attachment and Ostwald ripening. Ostwald ripening refers to the growth of larger particles with the help of smaller particles. The main reason is that particles with larger size and smaller surface to volume ratio are better than smaller particles with less energy stability. In addition, it is found that doping Sr²⁺ ions into NaLuF₄: Er³⁺ induced effective reinforcement. When 18% Sr²⁺ is doped into NaLuF₄: Er³⁺, the green and red luminescence intensities increase by 5.8 and 4.4 times, respectively. A plausible mechanism for boosting UC luminescence of the synthesized microcrystals is discussed.

2. Experimental

2.1. Materials

Lutetium oxide (Lu₂O₃, 99.999%) and erbium oxide (Er₂O₃, 99.999%) were purchased from Beijing Lansu Co. China. Sodium fluoride (NaF, AR), Sr(NO₃)₂, polyvinylpyrrolidone (PVP-K30), and ethanol (AR) were provided by Sinopharm Chemical Reagent Co. Ltd. The metal oxides were dissolved in 10% hydrochloric solution at elevated temperature and then completely evaporated to prepare the corresponding rare-earth chlorides (LnCl₃, Ln: Lu/Er). All of the chemicals were used as received without further refinement. Deionized water was used throughout the experiment.

2.2. Experimental Section

2.2.1. Preparation of Different Sr²⁺-Doped β-NaLuF₄:6%Er³⁺ Micromaterials

Firstly, based on the stoichiometric ratio, 0.2234 g PVP-K30 was dissolved by magnetic agitation in 18 mL distilled water to form a solution of desired molar concentration and 10 mL of an aqueous solution containing certain stoichiometric amounts of LnCl₃ (Ln = Lu and Er), Sr(NO₃)₂, and NaF was added dropwise into the above mixture. The precursor solution was magnetically stirred for 10 min, then transferred to a 60 mL Teflon-lined autoclave and hydrothermally treated at a temperature at 195°C for 24 h. After hydrothermal treatment, the autoclave was naturally cooled to room temperature. The products were washed alternately with deionized water and ethanol to remove impurities and excessive surfactant, collected by centrifugation, and then dried at 80°C for 8 h for further characterization. Similarly, the same synthesis procedure was used for different Sr²⁺-doped NaLuF₄:6%Er³⁺ microtubes through adding the Sr(NO₃)₂ with different concentrations.

2.2.2. Preparation of Sr²⁺-Doped β-NaLuF₄:6%Er³⁺ Micromaterials at Different Durations

In the time-dependent experiment, the addition of Sr²⁺ ions was defined as 10%. The only difference was that the hydrothermal reaction times were performed for 0.5 h, 2 h, 5 h, and 12 h to achieve shape evolution.

2.3. Characterizations

Structures of the as-prepared samples were identified by an X-ray diffractometer (XRD, Ultima-III) using Cu-Kα radiation ( nm). The morphological behavior of final compounds was monitored by a Hitachi S-3400N field emission scanning electron microscope (FE-SEM) and a JEOL JEM-200CX transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectroscope (EDS). The photoluminescence spectra were recorded on a FLS920 spectrofluorimeter equipped with a power-controllable 980 nm diode laser. To compare the emission intensities between different samples, the emission spectra were measured under the same parameters.

3. Results and Discussion

3.1. XRD Analysis

The crystallinity and phase purity of the as-prepared NaLuF₄:6%Er³⁺ micromaterials by Sr²⁺ doping at different concentrations (0, 5, 10, 18, 25, and 30%) are presented in Figure 1(a). As Sr²⁺ doping concentration rises from 0 to 10%, all the distinct diffraction peaks in the patterns can be easily labelled as hexagonal phase NaLuF₄ (JCPDS 27-0726). No other impurity peak is detected, indicating the good crystallinity of these samples. Compared with the standard peaks, an enhanced intensity of peak at (110) about 30.3° can be observed. The enhancement of the peak indicates that the crystal has preferential growth in this direction. It should be noticed that the (101) diffraction peak about 31.2° became higher than the (110) diffraction peak as the concentration of Sr²⁺ increases up to 18%, which indicates the presence of preferred orientation growth under different Sr²⁺ doping concentrations. It is worth noting that when the Sr²⁺ concentration was further increased to 25%, an impurity, which is the strontium fluoride (SrF₂) phase (JCPDS No. 86-2418), appears.

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(b)

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Figure 1: (a) XRD patterns of the as-prepared NaLuF₄:6%Er³⁺, % Sr²⁺ (, 5, 10, 18, 25, 30) microcrystals. The vertical black lines are the standard profiles of β-NaLuF₄ (JCPDS 27-0726) and SrF₂ (JCPDS 86-2418). (b) FWHM of the 31.2° peak vs. content of Sr²⁺ ions. (c) The enlarged view at °. (d) Variation trend of the -axis length and cell volume of the β-NaLuF₄:6%Er³⁺ samples doped with different Sr²⁺ contents.

It can be seen from Figure 1(b) that as the concentration of Sr²⁺ increases to 18%, the full width at half maximum (FWHM) of the diffraction peak becomes narrower and then widens rapidly when the concentration of Sr²⁺ further increases to 30%. Narrow FWHM means good crystallinity; therefore, when Sr²⁺ doping concentration is less than 18%, the crystallinity increases; and when Sr²⁺ doping concentration is more than 18%, the crystallinity decreases. The magnified XRD patterns in the vicinity of ° are shown in Figure 1(c). The peak shifts toward a smaller angle with more and more Sr²⁺ doping, indicating that the spacing of crystal planes increases and the lattice expands. Based on the Bragg equation [15, 25],

where is a positive integer, is the interplanar spacing of the diffracting planes, is the incident wavelength, and is the intersection angle. Since the ionic radius of Sr²⁺ (1.18 Å) is larger than that of Lu³⁺ (0.86 Å) and Na⁺ (1.02 Å) ions, replacing Lu³⁺ and Na⁺ by Sr²⁺ would enlarge the lattice. The similar phenomenon is also observed in the Ca²⁺-doped NaLuF₄: Yb³⁺, Er³⁺, Er³⁺/Tm³⁺ microcrystals [25]. To probe and detect the variation of the crystal structure under different Sr²⁺ contents, MC samples prepared with different Sr²⁺ contents were characterized by XRD. Unit cell volume and structural parameters were visually depicted in Figure 1(d). It turns out as expected that the -axis length and unit cell volume increase gradually with the increase of Sr²⁺ doping concentration. Sr²⁺ doping can alter the lattice parameter and plays an essential role in the control of UC luminescence emission.

3.2. Morphology Study

3.2.1. Morphology Evolution of β-NaLuF₄:6%Er³⁺ Microcrystals via Sr²⁺ Doping

The corresponding SEM images of Sr²⁺-doped β-NaLuF₄:6%Er³⁺ microcrystals at different concentrations (0, 5, 10, 18, 25, and 30%) are shown in Figures 2(a)–2(f). Figures 2(g)–2(l) are the high-magnification SEM images corresponding to the microstructures, and insets are TEM images. As can be seen from Figure 2(a), the Sr²⁺-free sample is composed of microtubes. Most of the microtubes have scrappy ends, thin-walled tube, and nanoparticles on the surfaces disorderly and unsystematic. When Sr²⁺ doping is 5%, the hexagonal structure can be observed on the upper and lower surfaces of microtubes with improved ends and smooth surfaces, as exhibited in Figure 2(b), which is in good agreement with the TEM image. When Sr²⁺ doping increases to 10%, the length decreases much. Homogeneous short hexagonal tubes are present in Figure 2(c). When the Sr²⁺ doping concentration varied from 10% to 18%, it can be seen from Figure 2(d) that the walls of hexagonal microtubes are getting thicker and thicker, and the morphology changes to hexagonal microprisms with uniformity and concave centers on the upper and lower surfaces. The internal structure of the microcrystals cannot be detected in the TEM images of Figures 2(i) and 2(j) due to its large thickness. Besides, when the Sr²⁺ content exceeds 25%, the surface of hexagonal prisms is becoming rougher and rougher. According to XRD results, it belongs to the SrF₂ crystal. If Sr²⁺ doping gets to 30%, the flocculent structure appears on the surface of hexagonal prisms (Figure 2(f).

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Figure 2: SEM images of β-NaLuF₄:6Er³⁺ MCs doped with Sr²⁺ at different concentrations. (a–f) Refer to 0, 5, 10, 18, 25, and 30%, correspondingly. (g–l) Are corresponding high-resolution SEM images of the microstructures, and insets are TEM images.

Upon careful checking, as the Sr²⁺ concentration increases from 0 to 5, 10, 18, 25, and 30%, the average length of the corresponding NaLuF₄:6%Er³⁺ MCs is 12.5, 7.33, 4.52, 2.01, 1.12, and 0.6 μm, and the average diameter is 1.56, 0.69, 0.65, 0.53, 0.3, and 0.6 μm, respectively. The size evolution (Figure 3) of NaLuF₄:6%Er³⁺ microcrystals may be attributed to the significant influence of Sr²⁺ dopant on the crystal growth rate. As the Sr²⁺ ion increases, the phases change from the pure β-NaLuF₄ to the mixture of β-NaLuF₄ and SrF₂ is achieved. The shape of NaLuF₄:6%Er³⁺ is modified from pure hexagonal microtubes to hexagonal prisms with irregular particles.

Figure 3: The size evolution of NaLuF₄:6%Er³⁺ MCs.

3.2.2. Growth Mechanism of NaLuF₄ Hexagonal Microtubes via Sr²⁺ Doping

To elucidate the growth process of NaLuF₄ hexagonal microtubes, intermediate products with different durations (0.5 h, 2 h, 5 h, and 12 h) are obtained, and the doping amount of Sr²⁺ is fixed at 10%. According to the morphologies, the growth process is divided into four stages: (1) nucleation, (2) oriented attachment, (3) orientation growth, and (4) Ostwald ripening. In the first stage, the precursor after 0.5 h reaction is ultrasmall nanoparticles with a cubic morphology, as shown in Figure 4(a). Accordingly, the EDS results in Figure 4(b) demonstrate the presence of Sr²⁺, Lu³⁺, and Na⁺, where Lu³⁺ content is the maximum and Na⁺ content is the minimum. Therefore, SrF₂ may first nucleate stably and then add Lu³⁺ and Na⁺ into the crystal lattice of SrF₂ nucleus. The product after 0.5 h reaction is SrF₂. From the high-resolution TEM images of some cubic nanocrystals in Figure 4(c), the lattice fringes of 0.286 nm, 0.289 nm, and 0.335 nm are detected, respectively, corresponding to (111) and (220) planes of SrF₂ (JCPDS 86-2418). In the second stage, when the reaction time reaches 2 h, the nanoparticles aggregate together to form aggregates at the microscopic level, which means oriented attachment. Most of the products are still SrF₂ nanocrystals, and some NaLuF₄ hexagonal microtubes can be observed from Figure 4(d), and the hexagonal microtubes may originate from the oriented attachment of ultrasmall cubic nanocrystals and have similar morphology with the final product. In the third stage, when the reaction time is prolonged to 5 h, the larger structure starts to grow into NaLuF₄ hexagonal microtubes, as shown in Figure 4(e). It is observed in the red rectangle of Figure 4(e) that many SrF₂ nanocrystals align together and intend to fabricate to a new hexagonal tube via orientation growth. When the hexagonal tubes grow to a certain number, the growth process enters the fourth stage. At this stage, at least after 12 h reaction, pure NaLuF₄ can be obtained as shown in Figure 4(f).

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Figure 4: SEM images and corresponding HRTEM images and EDS analysis of intermediate products with different reaction time: (a–c) 0.5 h, (d) 2 h, (e) 5 h, and (f) 12 h.

Based on the above results, we propose the possible modification mechanism of Sr²⁺ doping on the size and morphology of the NaLuF₄ MCs, as schematically shown in Figure 5. As the reaction continues, the Ostwald ripening process dominates the growing process. When the cubic nanocrystals with high surface energy are redissolved, Ostwald ripening begins. Hexagonal microtubes are synthesized following the precursor of Lu³⁺ and Sr²⁺ exhausted, and then Sr²⁺-doped NaLuF₄ hexagonal microtubes can be obtained ultimately. On the other hand, at a low concentration of Sr²⁺, the length of NaLuF₄ hexagonal microtubes decreases, while a high concentration of Sr²⁺ induces the generation of a prism-shaped morphology.

Figure 5: Sketch map of the crystal growing process: (a) as-obtained products reacted with different time; (b) doping with different contents of Sr²⁺ as the reaction time is 24 h.

3.3. Sr²⁺ Induced UCL Modulation and Mechanism Analysis

Figure 6(a) illustrates the UCL spectra of NaLuF₄:6%Er³⁺ microcrystals doped by different amounts of Sr²⁺ ions under excitation of a 980 nm continuous wave laser diode. Three distinct visible emission bands can be observed in the range of 500-700 nm. Two green emissions at 505-529 nm and 529-569 nm are assigned to the (Er³⁺) ²H11/2→⁴I15/2, (Er³⁺)⁴S3/2→⁴I15/2 transitions, respectively. The red emission from 628 to 692 nm is attributed to the (Er³⁺) ⁴F9/2→⁴I15/2 transition. Additionally, the infrared emission bands in the range of 700-900 nm can also be found. Sr²⁺ has no obvious energy transfer to RE³⁺ ions. Obviously, in the case of Sr²⁺ doping, both the peak positions and profiles of the emission bands are unaltered. The only difference is the UC emission intensity. It is interesting to observe that when the doping concentration of Sr²⁺ increases from 0 to 18%, the UC emission intensity is initially strengthened and then exhibits an abrupt decrease when the Sr²⁺ amount surpasses 18%. Noticeably, the maximum intensity of the 505-569 nm and 628-692 nm emissions of 18% Sr²⁺-doped microcrystals is reinforced by 5.8 and 4.4 times, as shown in Figure 6(b).

Figure 6: (a) The upconversion emission spectra; (b) the integrated intensity variation of green, red, infrared, and all emissions of 6%Er³⁺, % Sr²⁺ samples with , 5, 10, 18, 25, and 30 under 980 nm excitation (color figure online).

The enhancement of the upconversion emission efficiency caused by Sr²⁺ doping may be attributed to three aspects: phase, crystal field symmetry, and morphology. Firstly, it is commonly recognized that good crystallinity is beneficial to the luminescence of lanthanide activators. According to the aforesaid XRD data, doping with Sr²⁺ improves the crystallinity of β-NaLuF₄:6%Er³⁺ microcrystals. From Figure 1(b), which exhibits the variation of FWHM with Sr²⁺ amount, doping 18% Sr²⁺ ion is the optimum concentration and can improve the crystallinity very well. Significantly, the trend of improved crystallinity well agreed with the trend of enhanced UC emission intensity of Figure 6. Hence, we can draw a reasonable conclusion that Sr²⁺ doping increases the crystallinity. Secondly, the drift of the diffraction peaks (Figure 1(c)) and the change of the cell parameters (Figure 1(d)) indicate that the introduction of Sr²⁺ ions leads to the decrease of the local crystal field symmetry around Er³⁺ ions. This is also considered to be the reason for boosting the UC emission intensity. However, if excessive Sr²⁺ ions are incorporated into the host matrices, the symmetry around the Er³⁺ activator will gradually become better, resulting in a decrease in emission efficiency. Eventually, the UC emission intensity is reduced when the Sr²⁺ amount surpasses 18%. The reasons are twofold. On the one hand, high concentration of dopant may damage the unit cell structure of the β-NaLuF₄:6%Er³⁺ microcrystals and cause the formation of impurity phase SrF₂. On the other hand, high Sr²⁺ doping leads to excessive F⁻ vacancies, which acts as the quenching center in the β-NaLuF₄:6%Er³⁺ microcrystals. This kind of quenching effect was reported by Cong et al.’s group in the Zn²⁺-doped NaGdF₄:Yb³⁺, Er³⁺ nanoparticles [28]. Finally, from the morphological point of view, 18% Sr²⁺ concentration is an optimal concentration. As can be seen from Figure 2, the microtubes without any irregular particles are obtained at this time. Under the excitation of 980 nm, the UC emission intensities of hexagonal microtubes and hexagonal microprisms are different, for the following reasons: when infrared light is irradiated onto the microtubes, the electric vectors spread along the surface and inner wall at the same time. When infrared light is irradiated on the microprisms, the electric vectors only spread along the surface. Therefore, under the same power of 980 nm infrared laser, the UC emission intensity of hexagonal microtubes is stronger than that of microprisms.

To illustrate the number of incident photons responsible for the upconversion process, the relation of UCL intensities versus excitation power density was drawn. For the unsaturated upconversion process, the number () of photons needed to emit a green or red photon can be obtained using the following equation [29, 30]:

where is the number of pump photons absorbed, is the excitation power intensity, and is the emission intensity. From the double logarithmic plots of the emission intensity as a function of the excitation power (Figure 7), we can see that the 544 nm green emission and 655 nm red emission are ascribed to two-photon upconversion processes. The addition of Sr²⁺ ion cannot absorb 980 nm photons or transfer energy to Er³⁺. But it is worth noting that the value of changes slightly with the change of the concentration of Sr²⁺ ions and is smaller than that of Sr²⁺-free samples. It may be due to the fact that the upconversion process is relatively easy to saturate in NaLuF₄ MCs doped with Sr²⁺ ions [31]. Another subtle difference is the number of photons needed for the 544 nm upconversion and that of 655 nm. The main reason is that electrons require steps to reach the ⁴F9/2 state than the ⁴S3/2 and ²H11/2 states, and this will deplete more photons. So the 655 nm upconversion emission process will need more photons. According to the theoretical analysis, the possible upconversion routes are displayed in Figure 8, which lists the relevant energy levels of Er³⁺ as well as the emission processes. The 980 nm laser can excite an Er³⁺ ion to ⁴F7/2. Through nonradiative transitions, the electrons of Er³⁺ at ⁴F7/2 relax to ²H11/2, ⁴S3/2, or ⁴F9/2 and then transfer back to the ground state ⁴I15/2 and in the meanwhile generate luminescence at 526 nm, 544 nm, and 655 nm.

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Figure 7: Log-log of the UC emission intensity against laser excitation power for NaLuF₄:6%Er³⁺, % Sr²⁺ samples with , 5, 10, 18, 25, 30 (color figure online).

Figure 8: Proposed upconversion mechanism under 980 nm laser excitation.

4. Conclusions

In summary, NaLuF₄: Er³⁺ hexagonal microtubes doped with different concentrations of Sr²⁺ ions have been synthesized via a simple hydrothermal method. We systematically investigate the crystal structure, morphology, and UCL enhancement of hexagonal NaLuF₄: Er³⁺. With the increase of Sr²⁺ dosage, the length of NaLuF₄ hexagonal microtubes decreases and the morphologies change from hexagonal microtubes to hexagonal prisms. Extending the reaction time, NaLuF₄ hexagonal microtubes have undergone a typical Ostwald’s ripening. At the optimized concentration (18%) of Sr²⁺ ions, the green and red upconversion emissions are strengthened by 5.8 times and 4.4 times compared to that of Sr²⁺-free samples. A possible mechanism of upconversion emission enhancement has been identified. All the above results suggest that NaLuF₄: Er/Sr microphosphors have broad application prospects in the design and fabrication of high-brightness displays.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by Provincial Natural Foundation of Anhui Province (No. 1808085QF197, No. 1808085MF169), Natural Science Research Project of Anhui Universities of Science and Technology (No. QN201501), and Ph.D. Research Fund of Anhui Universities of Science and Technology.

References

J. Zhou, Q. Liu, W. Feng, Y. Sun, and F. Li, “Upconversion luminescent materials: advances and applications,” Chemical Reviews, vol. 115, no. 1, pp. 395–465, 2014.

G. B. Shan and G. P. Demopoulos, “Near-infrared sunlight harvesting in dye-sensitized solar cells via the insertion of an upconverter-TiO₂ nanocomposite layer,” Advanced Materials, vol. 22, no. 39, pp. 4373–4377, 2010.

Y. Y. Cheng, B. Fückel, R. W. MacQueen et al., “Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion,” Energy & Environmental Science, vol. 5, no. 5, pp. 6953–6959, 2012.

F. Wang, D. Banerjee, Y. Liu, X. Chen, and X. Liu, “Upconversion nanoparticles in biological labeling, imaging, and therapy,” Analyst, vol. 135, no. 8, pp. 1839–1854, 2010.

P. Zhang, W. Steelant, M. Kumar, and M. Scholfield, “Versatile photosensitizers for photodynamic therapy at infrared excitation,” Journal of the American Chemical Society, vol. 129, no. 15, pp. 4526-4527, 2007.

S. Zhou, K. Deng, X. Wei et al., “Upconversion luminescence of NaYF₄: Yb³⁺, Er³⁺ for temperature sensing,” Optics Communications, vol. 291, pp. 138–142, 2013.

L. Lei, D. Chen, Y. Han et al., “Modifying the size and uniformity of upconversion Yb/Er: NaGdF₄ nanocrystals through alkaline-earth doping,” Nanoscale, vol. 5, no. 22, pp. 11298–11305, 2013.

J. Wang, T. Wei, X. Li et al., “Near-infrared-light-mediated imaging of latent fingerprints based on molecular recognition,” Angewandte Chemie, International Edition, vol. 53, no. 6, pp. 1616–1620, 2014.

C. Wang, H. Tao, L. Cheng, and Z. Liu, “Near-infrared light induced in vivo photodynamic therapy of cancer based on upconversion nanoparticles,” Biomaterials, vol. 32, no. 26, pp. 6145–6154, 2011.

X. Liu, M. Zheng, X. Kong et al., “Separately doped upconversion-C60 nanoplatform for NIR imaging-guided photodynamic therapy of cancer cells,” Chemical Communications, vol. 49, no. 31, pp. 3224–3226, 2013.

K. M. L. Taylor, A. Jin, and W. Lin, “Surfactant-assisted synthesis of nanoscale gadolinium metal-organic frameworks for potential multimodal imaging,” Angewandte Chemie, International Edition, vol. 47, no. 40, pp. 7722–7725, 2008.

Z. Chen, H. Chen, H. Hu et al., “Versatile synthesis strategy for carboxylic acid−functionalized upconverting nanophosphors as biological labels,” Journal of the American Chemical Society, vol. 130, no. 10, pp. 3023–3029, 2008.

F. Wang and X. Liu, “Recent advances in the chemistry of lanthanide-doped upconversion nanocrystals,” Chemical Society Reviews, vol. 38, no. 4, pp. 976–989, 2009.

Y. Han, C. Gao, Y. Wang et al., “Spatially confined luminescence process in tip-modified heterogeneous-structured microrods for high-level anti-counterfeiting,” Physical Chemistry Chemical Physics, vol. 20, no. 14, pp. 9516–9522, 2018.

Y. Han, C. Gao, S. Li, and Z. Jiang, “Manipulating emissions of an intensive red-emitting NaLuF₄:Yb/Er microrod by controlling its temporal behaviors,” Journal of Alloys and Compounds, vol. 838, article 155610, 2020.

F. Shi, J. Wang, X. Zhai, D. Zhao, and W. Qin, “Facile synthesis of β- NaLuF₄ : Yb/Tm hexagonal nanoplates with intense ultraviolet upconversion luminescence,” CrystEngComm, vol. 13, no. 11, pp. 3782–3787, 2011.

Y. Li, Y. Dong, T. A. Tuerxun·Aidilibike, X. Liu, J. Guo, and W. Qin, “Growth phase diagram and upconversion luminescence properties of NaLuF₄: Yb³⁺/Tm³⁺/Gd³⁺ nanocrystals,” RSC Advances, vol. 7, no. 70, pp. 44531–44536, 2017.

J. Yang, L. Song, X. Wang et al., “A facile route to the controlled synthesis of β-NaLuF₄: Ln³⁺ (Ln = Eu, Tb, Dy, Sm, Tm, Ho) phosphors and their tunable luminescence properties,” CrystEngComm, vol. 20, no. 33, pp. 4763–4770, 2018.

H. Lin, X. Yan, J. Zheng, C. Dai, and Y. Chen, “Upconversion luminescence and Visible-Infrared Properties of β-NaLuF4:Er3+ Microcrystals Synthesized by the Surfactant-Assisted Hydrothermal Method,” Journal of Nanomaterials, vol. 2014, Article ID 903208, 11 pages, 2014.

J. Sun, H. Hao, W. Xu, Y. Song, Y. Wang, and X. Zhang, “Manipulation of microstructures and the stability of white emissions in NaLuF₄: Yb³⁺, Ho³⁺, Tm³⁺ upconversion crystals,” Optical Materials Express, vol. 8, no. 4, pp. 1043–1057, 2018.

Q. Qiang, S. du, X. Ma, W. Chen, G. Zhang, and Y. Wang, “A temperature sensor based on the enhanced upconversion luminescence of Li⁺ doped NaLuF₄:Yb³⁺,Tm³⁺/Er³⁺ nano/microcrystals,” Dalton Transactions, vol. 47, no. 26, pp. 8656–8662, 2018.

S. Sinha, A. Mondal, K. Kumar, and H. C. Swart, “Enhancement of upconversion emission and temperature sensing of paramagnetic Gd₂Mo₃O₉: Er³⁺/Yb³⁺ phosphor via Li⁺/Mg²⁺ co-doping,” Journal of Alloys and Compounds, vol. 747, pp. 455–464, 2018.

L. Lei, D. Chen, J. Xu, R. Zhang, and Y. Wang, “Highly intensified upconversion luminescence of Ca²⁺-doped Yb/Er:NaGdF₄ Nanocrystals prepared by a solvothermal route,” Chemistry, an Asian Journal, vol. 9, no. 3, pp. 728–733, 2014.

X. Wang, X. Zhang, Y. Wang et al., “Comprehensive studies of the Li⁺ effect on NaYF₄: Yb/Er nanocrystals: morphology, structure, and upconversion luminescence,” Dalton Transactions, vol. 46, no. 28, pp. 8968–8974, 2017.

A. Zhou, F. Song, Y. Han, F. Song, D. Ju, and X. Wang, “Simultaneous size adjustment and upconversion luminescence enhancement of β-NaLuF₄: Yb³⁺/Er³⁺, Er³⁺/Tm³⁺ microcrystals by introducing Ca²⁺ for temperature sensing,” CrystEngComm, vol. 20, no. 14, pp. 2029–2035, 2018.

Q. Shao, H. Zhang, J. Dai et al., “Preparation, growth mechanism, size manipulation and near-infrared luminescence enhancement of β-NaYF4:Nd³⁺ microcrystals via Ca²⁺ doping,” CrystEngComm, vol. 21, no. 4, pp. 741–748, 2019.

Z. Huang, Y. Yang, L. Gong, M. Ma, and C. Xu, “Sr²⁺ and Tb³⁺ doping tuning the size, morphology, and photoluminescence of NaCeF₄ nanorods via solvothermal method,” Chemical Engineering Journal, vol. 286, pp. 602–609, 2016.

T. Cong, Y. Ding, J. Liu, H. Zhao, and X. Hong, “Synthesis and optical properties of Zn²⁺ doped NaYF₄: Yb³⁺, Er³⁺ upconversion nanoparticles,” Materials Letters, vol. 165, pp. 59–62, 2016.

M. Pollnau, D. R. Gamelin, S. R. Lüthi, H. U. Güdel, and M. P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Physical Review B, vol. 61, no. 5, pp. 3337–3346, 2000.

K. Du, X. Xu, S. Yao et al., “Enhanced upconversion luminescence and controllable phase/shape of NaYF4:Yb/Er crystals through Cu²⁺ ion doping,” CrystEngComm, vol. 20, no. 14, pp. 1945–1953, 2018.

X. Deng, M. Yu, X. Zhou, Z. Xia, X. Chen, and S. Huang, “Highly bright and sensitive thermometric LiYF₄:Yb, Er upconversion nanocrystals through Mg²⁺ tridoping,” Journal of Materials Science: Materials in Electronics, vol. 31, no. 4, pp. 3415–3425, 2020.