Lanthanide-doped nanocrystals are the new-fashioned genre of nanomaterials which exhibit diverting luminescent and magnetic properties.^[1–4] The distinct luminescent properties urged the conversion of low-energy photons to high-energy photons by multiphoton routes. The host materials NaGdF₄ already had a wide range of applications in biomedical fields among various forms of lanthanide-doped nanoparticles,^[5,6] due to their multicolour emission character by doping rare earth ions and multifunction for surface functionalization. For instance, they played a perfect role that enabled them to significantly improve the quality of luminescence biomedical imaging,^[7–9] multiplexed biological labeling and therapy.^[10–14] Meanwhile, due to the feature that the NIR part of the solar spectrum could be converted to be visible in solar cells,^[15,16] NaGdF₄ are also regarded as a potential candidate for improving solar energy utilization. As they mainly depended on electronic transition probabilities, upconversion luminescence intensities of rare earth ions enable them to be destroyed by the local crystal field of the rare-earth ions. So the effective strategies to increase the upconversion emissions appeared accordingly as follows, first of all, the probability of an extremely efficient resonant or near-resonant process should be increased,^[17,18] and meanwhile the crystal symmetry should be reduced via tailing the local crystal field of the rare-earth ions.^[19,20] Recently, Dong et al. successfully fabricated luminescent–magnetic bifunctional NaGdF₄:Dy³⁺, Eu³⁺ nanomaterials and achieved the tunable multicolor luminescence,^[21] while Parthiban Ramasamy et al. synthesized NaGdF₄:Yb³⁺, Er³⁺ nanocrystals by Fe³⁺ doping with the enhanced upconversion luminescence and discussed their application in bioimaging.^[22] At the same time, Lei et al. introduced Ca²⁺ dopants into the grain lattices by substituting Gd³⁺ ions, then the irregular NaGdF₄:Yb³⁺, Er³⁺ nanocrystals converted into highly uniform nanorods. Meanwhile, they acquired highly intensified upconversion luminescence due to an amendment of the crystal structure of NaGdF₄.^[23]

In our study, a new strategy to enhance upconversion emission has been realized based on β-NaGdF₄:Yb³⁺, Er³⁺ nanocrystals tridoping with Mg²⁺ ions which had a series of high concentrations for the first time. Notably, the results demonstrate that via Mg²⁺ doping in the system, F⁻ vacancies formed, which results in a conversion of the morphology of Mg²⁺-doped NaGdF₄:Yb³⁺, Er³⁺ NCs from shaped nanoparticles to hexagonal nanoplates. Moreover, it has turned out that with the concentration of Mg²⁺ increasing, not only the intensity of green UC emission but also the intensity of red UC emission increase at the same time. It is worth noting that the growth of red UC emission is more outstanding, which is conducive to the detection of biological utilizing visible red emission.

The XRD patterns of the products (Fig. 1) demonstrated the crystallinity and phase transformation of the synthesized nanoparticles with various Mg²⁺ dopant concentrations. All of the synthesized nanoparticles exhibited well-defined peaks, confirming that they were highly crystalline and had sharp diffraction peaks. Then all the diffraction peaks could be indexed to the hexagonal phase of NaGdF₄ (JCPDS No. 27-0699) even when the Mg²⁺ doping concentration is as high as 70 mol%. The result indicates the successful synthesis of pure β-NaGdF₄ nanocrystals via the amended solvothermal route, which was reflected by the phenomenon that no extra peaks or significant differences were observed. Maybe it is the primary proof that a phase change does not occur after Mg²⁺ ions doping. In addition, as a result of the substitution of Gd³⁺ by the Mg²⁺ ions, the lattice constant and unit-cell volume increased with the increasing Mg²⁺ ion concentration, which caused the diffraction peaks to shift. This shift may have been caused by the alterations in the crystal cell volume following the Mg doping. At the same time, the crystallinity of the host has an obvious improvement, which is advantageous for the luminescence of the doped activators.

Fig. 1.

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	Fig. 1. (color online) (a) XRD patterns of β-NaGdF4:20-mol% Yb3+, 2-mol% Er3+, x mol% Mg2+ (x = 0, 40, 50, 60, and 70) nanoparticles and (b) amplified XRD patterns of the main diffraction peaks.

Parthiban Ramasamy indicated that in the NaGdF₄ host lattice, the Fe³⁺ ions appeared via substituting the Gd³⁺ site according to the same charge valence (+3). Then Lei^[23] pointed out the results, confirmed the incorporation of Ca²⁺ into NaGdF₄ by substituting Gd³⁺. The charge balance in NaGdF₄ is perturbed after Mg²⁺ displacing Gd³⁺, which should be noted obviously. As shown in previous research work, F⁻ vacancies were formed in order to maintain the charge balance. A quantitative ICP–MS analysis of the synthesized nanoparticles has further confirmed the formed F⁻ vacancies (see Table 1). As clearly shown in Table 1, with the increasing of Mg²⁺ doping content, the content of Gd³⁺ decreases correspondingly, and the value of Gd/Mg ratio is the same as the nominal one, whereas the Na content remains unchanged. These situations confirm the incorporation of Mg²⁺ into NaGdF₄ by substituting Gd³⁺. Figure 2 plots SEM images and elements analysis spectra (EDS) of β-NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ NCs which codoped with 0 mol% to 7-mol% Mg²⁺. It is notably presented in each element such as Na, Gd, Mg, F, and so on.^[24] The result suggested that the peak intensity of Gd³⁺ was reduced while the intensity of Mg²⁺ was increased. All the above results indicate a substitution of Gd³⁺ with Mg²⁺ in NaGdF₄ lattice.

Fig. 2.

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	Fig. 2. (color online) SEM images and the EDS spectra of β-NaGdF4:20-mol% Yb3+, 2-mol% Er3+ nanoparticles with different Mg2+ concentrations (a) and (b): 0 mol%, (c) and (d): 40 mol%, (e) and (f): 60 mol%.

Table 1.

Table 1.

Table 1. ICP-MS analysis of the synthesized nanoparticles with different concentrations of Mg2+ ions codoping. Sample A: 0 mol% Mg2+; sample B: 40-mol% Mg2+; sample C: 50-mol% Mg2+; sample D: 60-mol% Mg2+; sample E: 70-mol% Mg2+. . Sample Gd/(Gd+Mg) Mg/(Gd+Mg) Nominal Measured Nominal Measured A 0.78 0.78 0 0 B 0.38 0.36 0.40 0.42 C 0.28 0.24 0.50 0.54 D 0.18 0.20 0.60 0.58 E 0.08 0.10 0.70 0.68

Table 1. ICP-MS analysis of the synthesized nanoparticles with different concentrations of Mg2+ ions codoping. Sample A: 0 mol% Mg2+; sample B: 40-mol% Mg2+; sample C: 50-mol% Mg2+; sample D: 60-mol% Mg2+; sample E: 70-mol% Mg2+. .

The detailed structures of the β-NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ nanoparticles were obtained by using TEM and HR-TEM. Figures 3(a)–3(e) show the low-magnification TEM images, which reveals the synthesized hexagonal phase NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ with the concentrations of 0-mol% to 70-mol% Mg²⁺. It should be noted that the morphologies of the prepared nanoparticles change obviously. At the same time, figure 3(f) shows the high resolution TEM (HR-TEM) pictures and the SAED pattern of the 60-mol% Mg²⁺-doped NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ NCs. The HR-TEM image shows the high crystalline property of the nanoparticles and the calculated inter-planar distance between the two adjacent lattice planes is 0.52 nm, corresponding to the (100) lattice plane of the hexagonal phase NaGdF⁴ (JCPDS card No. 27-0699). A perfect hexagonal crystal structure is demonstrated in the corresponding fast Fourier transform (FFT) of the HR-TEM image (inset in panel (f)). The above structure is in good agreement with the XRD result presented in Fig. 1.

Fig. 3.

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	Fig. 3. The TEM images of β-NaGdF4:20-mol% Yb3+, 2-mol% Er3+ NCs codoped with (a) 0-mol%, (b) 40-mol%, (c) 50-mol%, (d) 60-mol%, and (e) 70-mol% Mg2+ ions; (f) HR-TEM image of a single nanoplate of 60-mol% Mg2+ codoped NaGdF4:20-mol% Yb3+, 2-mol% Er3+ NCs. Inset: fast Fourier transform (FFT) of the HR-TEM image exhibits a hexagonal symmetry.

The morphology conversion induced by Mg²⁺ doping in NaGdF₄:Yb³⁺, Er³⁺ NCs is shown in Fig. 4(a). As the capping function of excess F⁻ ions is on the crystal planes, the ratio of F⁻/RE³⁺ ions in the reaction solution has a key impact on the morphology of the finally formed NaREF₄ NCs. These results have already been confirmed by several research groups.^[25–27] The result which had been verified by Wang’s group indicated that when the F⁻/ ( ) ratio is set as 4:1 in the present system, no excess F⁻ is provided. Consistent with the above discussions, in this research when Mg²⁺ is incorporated into the NaGdF₄ lattices by substituting Gd³⁺, F⁻ vacancies are formed for the sake of maintaining the charge balance. The formation of F⁻ vacancies results in a conversion of the morphology of Mg²⁺-doped NaGdF₄:Yb³⁺, Er³⁺ NCs from shaped nanoparticles to hexagonal nanoplates. In order to acquire further proof of the proposed mechanism, two other specimens were synthesized in the same terms and conditions, the morphologies of which were shown in Figs. 4(b)–4(e). 60-mol% Mg²⁺-doped NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ NCs with F⁻ to ratio of 3.2:1 and NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ NCs with F⁻ to ratio of 4.8:1 were prepared respectively. The morphology is consistent with the result demonstrated from the TEM image of NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ NCs, and 60-mol% Mg²⁺-doped NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ NCs prepared with F⁻ to ratio of 4.0:1. It was concluded that the Mg²⁺ doping induced shape evolution in the system originated from excess F⁻ ions in the solution after the substitution of Gd³⁺ by Mg²⁺.

Fig. 4.

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Fig. 4. (a): Schematic illustration of the proposed mechanism for the morphology conversion induced by Mg2+ doping in β-NaGdF4: 20-mol% Yb3+, 2-mol% Er3+NCs. (b)–(e): TEM images of (b) NaGdF4: 20-mol% Yb3+, 2-mol% Er3+ NCs and (c) 60-mol% Mg2+ doped NaGdF4:20-mol% Yb3+, 2 mol% Er3+ NCs prepared with F to ratio of 4.0:1; (d) 60-mol% Mg2+-doped NaGdF4:20-mol% Yb3+, 2-mol% Er3+ NCs with F− to ratio of 3.2:1; (e) NaGdF4:20-mol% Yb3+, 2-mol% Er3+ NCs prepared with F− to ratio of 4.8:1 respectively.

Figure 5(a) exhibits the UC emission spectra of β-NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺, x-mol% Mg²⁺ (x = 0, 40, 50, 60, and 70) samples which are dispersed in cyclohexane and excited by a 980-nm cw laser at a power density of 500 mW cm⁻². The intensity of the UC emission increases gradually with the concentration of codoped Mg²⁺ from 0 mol% to 60 mol%. Then the UC intensity reduces when the concentration of Mg²⁺ is as high as 70 mol% compared to that of the sample of 60 mol%. Obviously, the UC emission intensity of the NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺:NCs doped with 60-mol% Mg²⁺ reached a maximum in relative terms, which is even higher than the intensity of non-codoped Mg²⁺ in NaGdF₄:20-mol% Yb³⁺ 2-mol% Er³⁺ nanoparticles. It is worth mentioned that with the concentration of Mg²⁺ increasing, not only the intensity of green UC emission but also the intensity of red UC emission increase correspondingly. The same trend for the dependence of the red ( ) and green ( , ) emission intensities due to the Mg²⁺ doping (see Fig. 5(b)), which indicates they follow the same UC pathways.

Fig. 5.

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	Fig. 5. (color online) (a) UC luminescence spectra of the samples after doping various concentrations of Mg2+ ions into the NaGdF4:20-mol% Yb3+, 2-mol% Er3+ matrix: x-mol% Mg2+ ions under diode laser excitation at 980 nm (x = 0, 40, 50, 60, 70). (b) Red, green, and total UC emission intensities versus Mg2+ doping content.

To understand the intensity of UC emission for Mg²⁺ codoped β-NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ nanoparticles in more detail, the main factors affecting UC luminescence of a definite nanophase are considered generally. On the one hand, in view of the crystal structure, β-NaGdF₄ is a disordered crystal, the randomly distributed sites of Na⁺/Gd³⁺ cations which at the same lattice contribute to multiple active sites for the doping Ln³⁺ ions. With the concentration of Mg²⁺ increasing in the NaGdF₄ host, F⁻ vacancies are generated more and more in order to maintain a charge balance. The amendment of the host crystal structure above all accelerates and diversifies the active sites for the Ln³⁺ ions. Then the number of possible energy-transfer processes increases, attributed to the presence of multiple independent sites for both Yb³⁺ and Er³⁺ ions in the host, which observably increases the probability of an extremely efficient resonant or near-resonant process,^[28–30] thus a large enhancement of the UC luminescence for Mg²⁺-doped samples occurred. On the other hand, depending on the intra 4f transition probability, the UC emissions of Ln³⁺-doped materials significantly affected by the local crystal field environment of Ln³⁺. The forbidden 4f–4f transitions of Ln³⁺ is broken efficiently by the intermixing of the f states of Ln³⁺ ions with higher electronic configurations. According to the theory of Judd–Ofelt (J–O),^[31] the forbidden transition probability is closely related to the crystal symmetry, and the probability of electric dipole transitions generally can be expressed as where is energy distance, is the matrix element of transition, (t=2, 4, 6) are J–O intensities parameters, which depend on the local environment and symmetry around the Ln³⁺ ions. As to the Ln³⁺-codoped materials, since the affection from the host property for the forbidden transition probability normally can be neglected, the forbidden transition probability is mainly determined by the local environment and symmetry of the Ln³⁺ ions. Gao et al. have used the tervalence Eu³⁺ ions as the probe, which are extremely sensitive to the environment, to study the local environment symmetry of the luminescence center within the NaYF⁴ and LiYF₄ micro-crystals. The result confirms a decrease in the symmetry of the Eu³⁺ ligand field, which is induced by doped ions. The underlying reason for the decrease is that the magnetic transition does not depend on the the Eu³⁺ ligand field, while the electric dipolar intensifies with decreasing environmental symmetry. So η can clearly reflect the symmetry of the Ln³⁺ ions, which stated on the local environment. The smaller value of η, the lower the symmetry, and the bigger the transition probability. So the lower crystal symmetry is always beneficial to higher UC intensity. Therefore, F⁻ vacancies formed (via the substitution of Gd³⁺ by Mg²⁺ ions) in the host should reduce the local crystal field symmetry around Yb³⁺ and Er³⁺ ions. Therefore, the lower symmetry favors the breaking of the forbidden transition of Ln³⁺ and meanwhile it is beneficial to enhancing the UC luminescence. As a result, the intensity of UC emission increases since Mg²⁺ tridoping into the host NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺.

Moreover, as is well known, a good crystallinity of the host is beneficial for the UC emission of the doped activators.^[32] As mentioned above, when the introduced contents of Mg²⁺ ions were increased, the amounts of excess F⁻ ions in the system were increased significantly, which changes the reaction environment and subsequently the growth behavior of the NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ NCs, and results in the modification of their crystallinity.^[33,34] With the content of Mg²⁺ ions increasing, the crystallinity of the host has an obvious improvement, corresponding to the XRD patterns (Fig. 1), which is advantageous for the luminescence of the doped activators.

As a function of the pump power for the two samples shown in Figs. 6(a) and 6(b), the luminescence intensities of UC emissions were measured for the sake of measuring the number of photons responsible for the UC mechanism. The dependence of the green and red emissions on the excitation power was calculated according to Auzel’s method, , where P is the pumping laser power, and n is the number of laser photons required in populating the upper emitting state.^[35] The slope n, which is nearly 2, indicates the following results: 1) both the red and green UC emissions are two photon absorption routes;^[36] 2) the Mg²⁺ doping does not affect the UC mechanism.

Fig. 6.

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	Fig. 6. (color online) (a) and (b): Power dependence of UC emissions of NaGdF4:20-mol% Yb3+, 2-mol% Er3+ of (a) 0-mol% and (b) 6-mol% Mg2+ dispersions in cyclohexane with a 980-nm semiconductor laser. (c): Energy level diagrams of Er3+ and Yb3+ ions, showing possible energy transfer mechanisms for red and green UC emissions of Er3+ activators in the hosts.

The diagram of the UC energy-transfer processes demonstrated in Fig. 6(c) explains the emissions. Obviously, they have two successive routes from Yb³⁺, which could excite Er³⁺ ions to the level, then the Er³⁺ ions relax to the and levels by the non-radiative relaxation processes, followed with the green emission. For the red UC luminescence process, the Er³⁺ ions relax non-radiatively from to the level, and then the level of Er³⁺ ions excited to the level by the Yb³⁺ ion energy transfer. The transition from the level to the ground state yields the red UC emission.

In summary, β-NaGdF₄ codoped with 20-mol% Yb³⁺, 2-mol% Er³⁺ and 0-, 40-, 50-, 60-, and 7-mol% Mg²⁺ were synthesized. We demonstrated that with the concentration of Mg²⁺ increasing, the morphology of the nanoparticles turns to change gradually and the UC emission is increased gradually as well. When doped with 60-mol% Mg²⁺, the UC emission intensity of nanoparticles reached the maximum level. The effect of Mg²⁺: 1) lowering the local crystal field symmetry around the Ln³⁺ activators, 2) improving the crystallinity of the NaGdF₄ NCs. With increasing Mg²⁺ doping content, the above two factors all induce a gradual intensification in the UC luminescence. As a consequence, the 60-mol% Mg²⁺-doped samples exhibit the best UC performance. This work significantly codoped Mg²⁺ into β-NaGdF₄:20-mol% Yb³⁺, 2-mol% Er³⁺ NCs, which has a great potential in some fields such as photocatalysis, biomedicine, and solar cells.