1. Introduction

Lanthanide-doped fluorides materials could be used as multifunctional materials due to their unique optical, X-ray absorption and magnetic characteristics. At present, they have been successfully applied to light emitting diodes [1–3], solid-state lasers [4], three-dimensional display [5–7], solar cells [8,9], color displays [5–7], biomarkers [10,11] and photo activated researching [12,13]. Among them, control synthesis, UC efficiency, multicolor UC emission and multifunction are focus topics in developing UC crystal [14–17]. It is necessary to expedite research on multicolor UC emission due to their promising applications in the broad areas of biomedical imaging [18], display [19], complex data analysis [20] and encoding [21]. Very recently, Yin et al. reported that steady-state three-primary-color UC luminescence from core-shell structured NaYF₄ nanocrystals [22]. Deng et al. designed an unsteady UC full-color luminescence in NaYF₄ UC nanocrystals with a five-layer core–shell structure by adjusting the pulse width [7]. However, studies on the corresponding tunable multi-color luminescence mostly restricted on core-shell structured crystals, which undergoes complex chemical processing and increases the size of crystal. Hence, it is important to prepare single structure crystal with multicolor UC emission.

In the development of advanced multi-function devices, optical-magnetic dual-function materials have received widespread attention due to their excellent magnetic and optical properties [23–26]. The traditional optical-magnetic dual-function material is a kind of composite material that combines optical materials with magnetic materials. It is difficult to realize the interaction between magnetic-optical materials due to the separation of the magnetic-optical two phases. Rare earth ion doping materials are ideal for achieving these optical and magnetic dual functions due to their excellent magnetic properties, abundant 4f energy levels and excellent light stability. At present, most studies about optical and magnetic dual-function material was limited to Fe₃O₄ core-shell structure. However, the intensity of UC luminescence will be weaker in the presence of the Fe₃O₄-shielding, because both excitation and emission light are absorbed by the Fe₃O₄ shell [27]. Now, we introduce magnetic properties in NaErF₄ by doping Fe³⁺ ion. As we all know, Fe³⁺ ion is an ideal paramagnetic relaxation agent used in magnetic resonance imaging because of its large magnetic moment. We intend to synthesize a kind of crystal with high magnetic and luminescence intensities by doping Fe³⁺ ion in NaErF₄ microcrystals.

In this work, we report the effects of Fe³⁺ ion doping concentration and the molar ratio of NaF to Ln(NO₃)₃ (F:Ln; Ln includes all metallic elements Er, Yb and Fe) on the crystal structure and UC properties of NaErF₄: 20% Yb³⁺. Compared with the Fe³⁺-absent sample, UC luminescence intensities are significantly enhanced. The mechanisms of the UC luminescence in the green region showing stronger under 1560 nm excitation than that under 980 nm excitation are discussed in detail. Moreover, the magnetization of NaErF₄: Yb³⁺ microcrystals were increased by doping Fe³⁺ ion. It is expected that the as-prepared crystalline with double-color emissions and magnetic characters could be promising multifunctional materials.

2. Experimental

2.1 Materials

All chemicals were of analytical grade and used without further purification. Yb(NO₃)₃·6H₂O (99.999%) and Er(NO₃)₃·6H₂O (99.999%) were supplied by Yutai Qingda Chemical Technology Co, Ltd. China. Fe(NO₃)₃·6H₂O (AR), NaF (AR) and Ethylenediaminetetraacetic acid C₁₀H₁₆N₂O₈ (EDTA) were supplied by Beijing Fine Chemical Company.

2.2 Sample preparation

We synthesized 0.5 mmol NaErF₄ microcrystals doped with Yb³⁺, Fe³⁺ ions through a hydrothermal method. Firstly, all rare-earth nitrates and Fe³⁺ ion aqueous solutions in a certain ratio (n (Er³⁺): n (Yb³⁺): n (Fe³⁺) = (80-x): 20: x mol%) were added into the EDTA solution. Subsequently, 8 mL of NaF aqueous solution (the molar ratio of F:Ln equals 7:1) was added into the above mixture and then kept stirring for 30 min. The obtained solution was transferred into 25 ml Teflon-lined stainless autoclave, and reacted at 180 ℃ for 12 h in an oven. After the autoclave was cooled to room temperature, the products were washed several times with deionized water and dried in air at 80 ℃ overnight. Other Fe³⁺-undoped/doped NaErF₄: 20% Yb³⁺ with different crystal structures were prepared by a similar process as described above only by tuning the molar ratio of F:Ln (8 or 9) and Fe³⁺ ion concentration (0, 3, 5, 7 and 10 mol%).

2.3 Characterization

The structures of as-prepared NaErF₄: 20% Yb³⁺ microcrystals were analyzed by a Rigaku RU-200b X-ray powder diffractometer (XRD) using nickel-filtered Cu-Kα radiation (λ = 1.5406 Å). The sizes and morphologies of the samples were investigated by scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). Luminescence properties were analyzed by a Hitachi fluorescence spectrometer F-7000 equipped with a power-controllable 980 nm and 1560 nm CW diode laser (all excitation power density were fixed at 46 mw·mm^-2). The magnetization was measured by vibrating sample magnetometer (VSM, Lake shore 7404). Inductively coupled plasma optical emission spectrometer (ICP-OES) analysis was performed using a PerkinElmer ICP.

3. Results and discussion

3.1 Factors affecting the phase and the structure

Figure 1 and Fig. 2 present the XRD patterns and the corresponding SEM images of all as-prepared NaErF₄: 20% Yb³⁺ microcrystals. When the molar ratio of F:Ln equals 7:1 [Fig. 1(a)], without Fe³⁺ ion doping, all diffraction peaks of the microcrystals can be well matched with pure α-NaErF₄ (JCPDS No. 77-2041). Accordingly, the SEM image [Fig. 2(a)] showed a large number of small spheres. With increasing Fe³⁺ ion doping concentration from 10 mol% to 17 mol%, the samples included two distinct phases, α+β-NaErF₄ phases, as displayed in the XRD patterns. The peaks of α-NaErF₄ phase fall sharply, and the β-NaErF₄ phase dominates on a large scale, due to the fact that the crystal has a tendency to transform from the cubic to hexagonal phase. The related SEM image Fig. 2(b) shows that part of samples transformed from sphere to irregular sphere. A lot of spherical particles were attached on the large hexagonal prism surfaces [Fig. 2(c) and Fig. 2(d)]. When the Fe³⁺ ion concentration was further increased to 20 mol%, pure β-NaErF₄ (JCPDS No.27-0689) microcrystals were achieved. The associative SEM image [Fig. 2(e)] only emerged the hexagonal prisms. SEM result [Fig. 2(j)] also shows that pure hexagonal prisms were formed. To reveal the differences caused by Fe³⁺ ion doping, the main diffraction peak range from 16 to 18 degree was magnified, as shown in Fig. 1(b). The diffraction peaks gradually shift toward higher angle with increasing Fe³⁺ ion dopant concentrations. This may be due to the fact that the ionic radius of Fe³⁺ (0.64 Å) is much smaller than the ionic radius of Er³⁺ (1.004 Å), and the radius of the Fe³⁺ ion is small enough to replace the Er³⁺ ion. According to Bragg's law, 2dsinθ = nλ, where d is the interfacial distance, θ is the diffraction angle, and λ is the diffraction wavelength [28]. When an Er³⁺ ion is replaced by a smaller Fe³⁺ ion, the lattice constant decreases slightly, the corresponding unit volume becomes smaller, and the interfacial distance decreases, resulting in the diffraction peak shifting to a high angle [29]. Similar results were also observed in the system of NaYbF₄:Yb³⁺, Tm³⁺ codoped Fe³⁺ [30] and NaYF₄:Yb³⁺, Er³⁺ codoped Zn²⁺ [33].

 

Fig. 1. XRD patterns of (a) Ln:F = 1:7, NaErF₄: 20% Yb, xFe (x = 0, 10, 13, 17 and 20 mol%) samples; (b) The main diffraction peaks of the NaErF₄: 20% Yb³⁺ crystals with different concentrations of Fe³⁺ ion, Ln:F = 1:7; (c) Ln:F = 1:8, NaErF₄: 20% Yb, xFe (0, 3, 5, 7 and 10 mol%) samples; (d) Ln:F = 1:9, NaErF₄: 20%Yb, xFe (x = 0, 3, 5, 7 and 10 mol%) samples and standard α-NaErF₄ (JCPDS No: 77-2041) and β-NaErF₄ (JCPDS No: 27-0689).

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Fig. 2. SEM images of NaErF₄: 20%Yb³⁺ microcrystals with different Ln to NaF ratios and different Fe³⁺ ion doping concentrations; (a) –(e) are NaErF₄: 20% Yb, xFe (x = 0, 10, 13, 17 and 20 mol%), respectively, Ln:F = 1:7; (F) –(g) are NaErF₄: 20%Yb, xFe (x = 0, 3, 5, 7 and 10 mol%), respectively, Ln:F = 1:8; (k) –(o) are NaErF₄: 20% Yb, xFe (x = 0, 3, 5, 7 and 10 mol%), Ln:F = 1:9, respectively; (p) EDS of the NaErF₄: 20% Yb³⁺, 20% Fe³⁺ microcrystals, Ln:F = 1:7.

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When the molar ratio of Ln:F equals 1:8, Fe³⁺ ion doping concentration increasing from 0 mol% to 7 mol%, the XRD patterns of samples can be indexed as a mixture of α+β-NaErF₄ phases [Fig. 1(c)]. The relevant SEM images [Figs. 2(f)–Figs. 2(i)] show two kinds of crystals evidently, which are small spheres and large hexagonal prisms, respectively. When doping 10 mol% Fe³⁺ ion, the sample eventually turned into a pure hexagonal phase.

When increasing NaF concentration to the molar ratio 1:9 (Ln:F), all samples were well matched with the standard β-NaErF₄XRD pattern [Fig. 1(d)]. With increasing Fe³⁺ ion doping concentration (0-10 mol%), the samples still remained hexagonal prisms and the size hardly changed [Figs. 2(k)–Figs. 2(o)].

The phase transformation of NaErF₄: 20% Yb³⁺ microcrystals can be partly attributed to the strong effect of doping Fe³⁺ ion on crystal growth. The effective ionic radius of Er³⁺ ion and Fe³⁺ ion are 1.144 Å and 0.64 Å, respectively. Fe³⁺ ion is small enough to enter any crystal site, substituting for the Er³⁺ ion. Consequently, substituting the Er³⁺ ion with smaller Fe³⁺ ion can cause the host lattice to shrink, which resulted in phase transformation [31,32]. Further, the XRD results [Fig. 1(b)] show that the peak shifts toward a higher 2θ angle, that is, the corresponding lattice constants slightly shrinking and the Er³⁺ ion is substituted by Fe³⁺ ion has occurred. In addition, previous reports have demonstrated that the size of doped ion plays an important role in the formation of a particular crystalline phase [33–35]. On the other hand, the molar ratio of NaF to Ln could also induce phase transformation [36]. Figure 2(a), Fig. 2(f) and Fig. 2(k) show that the cubic to hexagonal phase transformation is accompanied by the dissolution–recrystallization process in order to minimize their surface energy through the consumption of smaller particles as well as enhanced sizes [36,37]. Some particles transformed from the cubic to hexagonal phase because the cubic NaErF₄ crystals are unstable. The chemical potential of the crystal would fall under high fluorine sources [38]. Therefore, some spherical particles gradually dissolved to form a hexagonal plate, and some of the particles attached to the surface of hexagonal plate when the molar ratio of NaF to Ln is increased [39]. Further increasing the molar ratio of NaF to Ln, the spherical particles disappeared completely, which suggested that the dissolution–recrystallization process occurred. The corresponding elemental components of NaErF₄: 20% Yb³⁺, 20% Fe³⁺, Ln:F = 1:7 microcrystals were detected by the EDS. As shown in Fig. 2(p), the EDS result indicated that the main components were Na, Er, Yb, Fe and F elements. And ICP analyzed compositions of the Yb³⁺, Fe³⁺ doped NaErF₄ microcrystals (Ln:F = 1: 8), the results presented in Table 1, which accords with the initial designed ratios. Therefore, RE³⁺ and Fe³⁺ ions have been effectively incorporated into NaErF₄ crystals.

Table 1. Compositions of the Yb³⁺, Fe³⁺ doped NaErF₄ microcrystals (Ln:F = 1: 8) measured by ICP.

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3.2 UC photoluminescence and magnetic properties

Figure 3 shows the UC luminescence spectra of all as-prepared NaErF₄: 20% Yb³⁺, x% Fe³⁺ samples under 980 nm or 1560 nm excitation, respectively. As shown in UC luminescence spectra, there are three emission peaks located in 520 nm, 539 nm and 664 nm, which were assigned to ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 energy levels of Er³⁺ ion, respectively. The emission intensities of NaErF₄ samples were gradually enhanced with increasing Fe³⁺ ion doping concentration under 980 nm or 1560 nm excitation, respectively [Fig. 3(a) and Fig. 3(b), Fig. 3(d) and Fig. 3(e), Fig. 3(g) and Fig. 3(h)]. This may be caused by two reasons: one is that the increase of particle size, which reduces surface quenching centers; another is the distortion of the local symmetry around Er³⁺ ion after the doping of the Fe³⁺ ion, which increases the radiative transition probabilities of the Er³⁺ ion [40].

 

Fig. 3. (a)–(b) UC luminescence spectra of α-NaErF₄: 20% Yb³⁺ microcrystals doped with different Fe³⁺ ion concentrations, Ln:F = 1:7; (d)–(e) α+β-NaErF₄: 20%Yb³⁺ microcrystals, Ln:F = 1:8; (g)–(h) β-NaErF₄: 20% Yb³⁺ microcrystals, Ln:F = 1:9; (c), (f), (i) are red to green luminescence ratios of NaErF₄: Yb³⁺, Fe³⁺ as a function of Fe³⁺ ion concentration under 980 or 1560 nm light excitation (excitation power density = 46 mw·mm^-2).

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NaErF₄: 20% Yb³⁺ microcrystals doped with Fe³⁺ ion under 980 nm excitation presented dominant red emission and relatively weak green emission, and the red to green ratios increased slightly by increasing the Fe³⁺ ion doping concentration [Fig. 3(c), Fig. 3(f) and Fig. 3(i)]. However, NaErF₄: 20% Yb³⁺ microcrystals excited by 1560 nm light presented much stronger green emission than those excited by 980 nm light, and the ratios of red to green were decreasing. All excitation power density is 46 mw·mm^-2. The distinct emission colors displayed by NaErF₄: 20% Yb³⁺, doped with different Fe³⁺ ion concentration excited by 980 nm or 1560 nm light were due to the fact that the different interaction mechanisms between Yb³⁺ ion and Er³⁺ ion.

Figure 4 depicts possible UC mechanisms under 980 nm or 1560 nm excitation. As Yb³⁺ ion has a large absorption cross sectional area about 980 nm, thus Yb³⁺ ion from ²F_7/2 level is pumped to the ²F_5/2 excited state by absorbing 980 nm photons. And then the excited state electrons of Yb³⁺ ion in ²F_5/2 level transfer its energy to Er³⁺ ion in ⁴F_15/2 ground state to populate the ⁴I_11/2 level. Following, the excited electrons of Er³⁺ ion in ⁴I_11/2 level, one part absorb energy to the ⁴F_7/2 level, another part relax to the ⁴I_13/2 level. Subsequently, the nonradiative relaxations of (Er³⁺) ⁴F_7/2→²H_11/2, ⁴F_7/2→⁴S_3/2 and ⁴S_3/2→⁴F_9/2 fill the ²H_11/2, ⁴S_3/2 (the green emitting) and ⁴F_9/2 (the red emitting) energy levels, respectively [41]. NaErF₄: 20% Yb³⁺, x% Fe³⁺ excited by 980 nm light presented bright red light. This phenomenon is significantly different from the well-known UC matrix material NaYF₄ doped Yb³⁺/Er³⁺ with a small red to green ratio. UC luminescence has a large ratio of red to green, mainly due to the smaller distance between Er³⁺ ions in NaErF₄ matrix material compared with NaYF₄ doped Yb/Er nanocrystals. The decrease in the distance between Er³⁺ and Er³⁺ ions results in enhanced cross-relaxation effects (⁴F_7/2→⁴F_9/2, ⁴I_11/2→⁴F_9/2). As a result, the UC luminescence in NaErF₄ matrix is mainly red emission under 980 nm excitation [42,43].

 

Fig. 4. Possible UC luminescence processes of NaErF₄: 20% Yb³⁺ samples excited by 980 nm or 1560 nm laser.

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The UC mechanism of 1560 nm excitation is distinguished from that by 980 nm light exciting. First, Er³⁺ ion has a strong absorption around 1560 nm, which matches well with the ⁴I_15/2→⁴I_13/2 level transition via ground state absorption. Following, the excited state electrons of Er³⁺ ion from ⁴I_13/2 level via excited-state absorption are pumped to the ⁴I_9/2 level. Then some of them decay through a nonradiative process from the ⁴I_9/2 level into the ⁴I_11/2 metastable level, some are populated from ⁴I_9/2 level to ²H_11/2 level, leading to green emission. Subsequently, one part electrons of the ⁴I_11/2 metastable level are pumped to the ⁴F_9/2 level, leading to red ⁴F_9/2→⁴I_15/2 emission, another part electrons occur the energy transfer from ⁴I_11/2 (Er³⁺) to ²F_5/2 (Yb³⁺) because the nonradiative decay ⁴I_9/2→⁴I_11/2 easily occurs [44]. The energy transfer from ⁴I_11/2 (Er³⁺) to ²F_5/2 (Yb³⁺) easily achieve due to their appropriate energy matching, which makes the energy transfer process of ²F_5/2 (Yb³⁺) to ⁴F_7/2 (Er³⁺) more effective, leading to much stronger green ²H_11/2, ⁴S_3/2→⁴I_15/2 emissions [7,45].

The CIE (Commission Internationale de I’Eclairage) chromaticity coordinates of the UC emission spectra of NaErF₄: 20% Yb³⁺, x Fe³⁺ (x = 0 and 10 mol%), Ln:F = 1:8 samples excited by 980 nm or 1560 nm light, as given in Fig. 5. Under 980 nm light excitation, NaErF₄: 20% Yb³⁺ showed red UC luminescence and NaErF₄: 20% Yb³⁺, 10% Fe³⁺ showed red-yellow UC luminescence. However, under 1560 nm light excitation, NaErF₄: 20% Yb³⁺, x% Fe³⁺ showed a constant green UC luminescence and exhibited green to green-yellow UC luminescence when the doping concentration of Fe³⁺ ion is 0% and 10%. The corresponding CIE chromaticity coordinates were determined to be (0.679, 0.3189) for 0% Fe³⁺ (λ_ex= 980 nm), (0.729, 0.3189) for 10% Fe³⁺ (λ_ex= 980 nm), (0.3883, 0.5964) for 0% Fe³⁺ (λ_ex= 1560 nm) and (0.3956, 0.5892) for 10% Fe³⁺ (λ_ex= 1560 nm), respectively. Therefore, the red and green UC luminescence samples were achieved by combining the 980 nm and 1560 nm laser exciting, respectively.

 

Fig. 5. The CIE chromaticity diagram of the NaErF₄: 20% Yb³⁺, x Fe³⁺(x = 0 and 10 mol%), Ln:F = 1:8, pumping power density of 980 nm or 1560 nm laser is 46 mw·mm^-2. (inset: corresponding chromaticity coordinates values).

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3.3 Magnetic properties

The magnetic hysteresis curves of samples NaErF₄: 20% Yb³⁺, x Fe³⁺ (0, 3, 5, 7 and 10 mol%), Ln:F = 1:8 measured at room temperature are shown in Fig. 6. The magnetization of the NaErF₄: 20% Yb³⁺ microcrystals can be modified from 1.00887 emu·g^-1 to 18.01561 emu·g^-1 at 10 kOe by increasing the Fe³⁺ ion doping concentration from 0 mol% to 10 mol%. This paramagnetic behavior could be mainly attributed to the intrinsic magnetic moment of Fe³⁺ ion [46]. When the doping concentration of Fe³⁺ ion is at a low concentration, the magnetic intensity increases with the increase of the doping concentration of Fe³⁺ ion, which agrees with the results of literatures [47–50]. Hopefully, these Fe³⁺-doped NaErF₄ microcrystals may be suitable for applications in magnetic resonance imaging such as MRI, CT and bio-separation.

 

Fig. 6. Magnetization vs. magnetic field of Yb³⁺ ion doped NaErF₄ microcrystals with x Fe³⁺ (x = 0, 3, 5, 7 and 10 mol%) ion, Ln:F = 1:8 measured at a room temperature.

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4. Conclusions

In summary, multifunctional Yb³⁺ ion doped NaErF₄ microcrystals with tuned crystal phase, size, optical and magnetic properties were realized by doping Fe³⁺ ion. The results show that the high molar ratio of F:Ln favors the pure hexagonal phase and more Fe³⁺ ion doping concentration promotes the phase transformation from cubic to hexagonal phase. Besides, the tunable two-color luminescence has been achieved by utilizing two kinds of lasers. Simultaneously, the doping of Fe³⁺ ion endows NaErF₄ with magnetic characteristics. These findings provide a new route of obtaining multifunctional materials with double-color control and optical-magnetic characteristics, and may open a new door for magnetic field detection and complex anticounterfeiting applications.