Up-conversion luminescence of Er₂Mo₄O₁₅ under 980 and 1550 nm excitation

Abstract

Y_2−xEr_xMo₄O₁₅ (x = 0.04, 0.08, 0.16, 0.32, 0.64, 2.0) phosphors were synthesised at 700 °C through a solid-state reaction method. The samples were characterised by XRD and emission spectra analysis. Y_2−xEr_xMo₄O₁₅ samples showed a strong green emission (²H_11/2, ⁴S_3/2 → ⁴I_15/2) and a weak red emission (⁴F_9/2 → ⁴I_15/2) under 1550 and 980 nm excitation. The green emission intensity was enhanced increasing the Er³⁺ ion doping content, whereas the red emission was basically unchanged. Thus, Er₂Mo₄O₁₅ exhibited an abnormal green up-conversion luminescence without concentration quenching under 1550 and 980 nm excitation. The dominating green up-conversion luminescence was due to the larger adjacent Er³⁺⋯Er³⁺ distance caused by the special structure of Er₂Mo₄O₁₅, which limited the energy-transfer process and cross-relaxation responsible for the red up-conversion emission. Therefore, the up-conversion luminescence mechanism was excited-state absorption under 1550 nm excitation.

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1. Introduction

Rare-earth molybdate is a kind of typical composite oxide. Owing to the flexibility of coordination number and geometry for both R³⁺ and Mo⁶⁺ centres,^1,2 they can form a large family of solid oxides,³ for example, Ln₂MoO₆ (Ln = rare earth),⁴ Ln₂Mo₂O₉,⁵ Ln₂Mo₃O₁₂,⁶ Ln₂Mo₅O₁₈,⁷ Ln₂Mo₄O₁₅,⁸ Ln₄Mo₇O₂₇,⁹ Ln₆MoO₁₂,¹⁰ Ln₆Mo₁₀O₃₉,¹¹ Ln₆Mo₁₂O₄₅ (ref. 7) and ALnMo₂O₈ (A = alkali metal).^12,13 Rare-earth molybdate presents important applications in catalysis, ionic conductors, laser crystals and solid state lighting, such as Eu³⁺-doped Gd₂MoO₆,¹⁴ La₂Mo₂O₉,¹⁵ Gd₂(MoO₄)₃ (ref. 16) and NaYEu(MoO₄)₂.¹⁷ In recent years, rare-earth molybdate has attracted much attention in the field of up-conversion luminescence (UCL).^18–20

Among them, Ln₂Mo₄O₁₅ is a newly discovered rare-earth molybdate, so related research is scarce. Ln₂Mo₄O₁₅ is characterised by drastic changes in the structure and performance with different rare-earth ion substitutions. The structure of Ln₂Mo₄O₁₅ can be divided into three types: type I: Ln = La, monoclinic, space group P21/n;^21,22 type II: Ln = Ce–Tb, triclinic, space group, P1̄^23,24 and type III: Ln = Dy–Lu and Y, monoclinic, space group P21/c.^25,26 The structure of Er₂Mo₄O₁₅ is shown in Fig. 1, which is similar to that of Y₂Mo₄O₁₅, and the two compounds can form a continuous solid solution (Y_2−xEr_xMo₄O₁₅, x = 0–2).^1,27 Ln₂Mo₄O₁₅ can be applied to optical electronics devices^28–30 owing to its unique negative thermal expansion properties.^31–35 Several studies can been found about the down-shift in luminescence, such as the Eu³⁺-doped R₂Mo₄O₁₅ (R = Y, Gd, La)^36–39 and Y₂Mo₄O₁₅:xSm³⁺ (0.01 ≤ x ≤ 0.20),¹ which can be applied to white LED. However, the UCL of Ln₂Mo₄O₁₅ has not been reported yet.

Fig. 1 Crystal structure diagram of Er₂Mo₄O₁₅.

In this paper, Er₂Mo₄O₁₅ phosphors were synthesised using the conventional solid-state reaction method, and their UCL properties were characterised for the first time under 1550 and 980 nm excitation. The UCL mechanism was also analysed.

2. Experimental

All chemicals were of analytical grade and were used without further purification. The raw materials used in this experiment included Y₂O₃ (99.99%, Guangzhou Rare Earth Co. Ltd.), Er₂O₃ (99.99%, Guangzhou Rare Earth Co. Ltd.), and (NH₄)₆Mo₇O₂₄·4H₂O (AR). After weighing stoichiometrically to the designed compositions of Y_2−xEr_xMo₄O₁₅ (x = 0.04, 0.08, 0.16, 0.32, 0.64, 2.0), for Er₂Mo₄O₁₅, 2.00 g of Er₂O₃ and 3.69 g of (NH₄)₆Mo₇O₂₄·4H₂O were mixed thoroughly by grinding for 30 minutes. Samples were obtained by annealing the mixture at 700 °C for 2 h, and then naturally cooling it to room temperature. The samples were ground and subsequently characterised.

X-ray powder diffraction (XRD) was performed at 40 kV and 40 mA, with a D/MAX-Ultima X-ray generator employing Cu Kα radiation (λ = 0.15406 nm) over the 2θ range of 10° to 70°. The morphology of samples was examined by using a JEOL JSM-7001F thermal field emission scanning electric microscope (SEM). The particle size was analysed using a Coulter LS100Q laser diffraction particle size analyser. Diffuse reflectance spectra were measured using a Shimadzu UV-3600 UV/VIS/NIR spectrophotometer. The Fourier transform infrared (FTIR) spectra were recorded in KBr pellets using a Nicolet 550 Magna-IR spectrophotometer. A micro-Raman spectroscope (Jobin Yvon HR800, excited by 633 nm He–Ne laser with a laser spot size of 1 μm², in line mapping mode) was adopted to measure the maximum phonon energy of the Er₂Mo₄O₁₅ host lattice. Photoluminescence (PL) spectra and UCL spectra (200–900 nm) were recorded using a Hitachi F-4500 fluorescence spectrophotometer (slit width: 1 nm) equipped with a 1550 (0–700 mW, 0–3.2 A, CNI Laser, MDL-III-1550 nm) and 980 nm (0–800 mW, 0–1.2 A, CNI Laser, MDL-III-980 nm) fibre laser diode (LD). The scanning speed was 1200 nm min⁻¹ and the power of the LD was measured with a laser power meter (LPE-type). The UCL brightness of the phosphors was recorded using a CS-100A luminance meter, and the LD was focused with a light beam area of about 1 mm² before UCL brightness measurement.

3. Results and discussion

Fig. 2 shows the XRD pattern for the Er₂Mo₄O₁₅ sample after heating at 700 °C for 2 h. The pattern exhibits the peak positions of crystalline monoclinic Er₂Mo₄O₁₅, which matched well with the standard card (PDF no. 52-1800, the XRD patterns of the Y_2−xEr_xMo₄O₁₅ samples, x = 0.04, 0.08, 0.16, 0.32, 0.64 are shown in Fig. S1† and the XRD patterns of the Y_1.96Er_0.04Mo₄O₁₅ sample sintered at 700, 800 and 900 °C for 2 h are shown in Fig. S2 in the ESI†). Compared with the standard card, the diffraction peaks of the Er₂Mo₄O₁₅ sample were slightly offset from the large angle direction, which may have resulted in lattice distortion due to the characteristics of the drastic changes in the structure of the Ln₂Mo₄O₁₅ and the preparation conditions.³ The SEM of the as-prepared Er₂Mo₄O₁₅ sample exhibits particles with round edges, smooth surfaces, and irregular blocks (Fig. 3). The mean particle size is 11.9 μm and the median particle size is 11.1 μm. The mean/median ratio is 1.08 (Fig. S3†). This result indicates that the particles are uniform in size distribution, which makes them very suitable for industrial applications, such as coating and printing.

Fig. 2 XRD patterns of Er₂Mo₄O₁₅ sample.

Fig. 3 FESEM micrograph of Er₂Mo₄O₁₅ sample.

Fig. 4 shows the FTIR and Raman spectra of the Er₂Mo₄O₁₅ sample. The absorption bands at 750–960 cm⁻¹ are attributed to the stretching vibration of tetrahedral [MoO₄]. The absorption peaks at 1630 and 3500 cm⁻¹ arise from the absorption of the O–H vibration (Fig. 4a). The Raman spectrum of rare-earth molybdates between 450–700 cm⁻¹ is a blank area, and the stretching vibration tetrahedral [MoO₄] is located between 700–950 cm⁻¹.⁴⁰ However, there are several peaks between 450–700 cm⁻¹ in the Raman spectrum of the Er₂Mo₄O₁₅ sample, which can be assigned to the characteristic peaks of the Er³⁺ ion (Fig. 4b).⁴¹ Therefore, the maximum phonon energy of the Er₂Mo₄O₁₅ host lattice is ħω = 955 cm⁻¹ (Fig. 4b).

Fig. 4 FTIR (a) and Raman (b) spectrum of Er₂Mo₄O₁₅ sample.

Fig. 5 shows the diffuse reflectance spectrum and the PL spectrum of Er₂Mo₄O₁₅ phosphor samples excited at 380 nm. The absorption peaks at 380, 407, 452, 490, 523, 546, 654, 799, 978 and 1505 nm are assigned to the transitions of ⁴I_15/2 to ⁴G_11/2, ²H_9/2, ⁴F_5/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 levels of Er³⁺ ions,⁴² respectively. Er₂Mo₄O₁₅ shows broad and intense absorption peaks at 1500–1540 nm in the NIR region. The emission peaks were observed at 409, 467, 532 and 555 nm under 380 nm excitation, which can be attributed to the transition of Er³⁺ ions ²H_9/2, ⁴F_5/2, ²H_11/2 and ⁴S_3/2 level to ⁴I_15/2 ground state, respectively. It means that Er³⁺ ions are excited to the ⁴G_11/2 level by absorption of 380 nm photons, and they decay to the ²H_9/2, ⁴F_5/2, and ²H_11/2/⁴S_3/2 levels rapidly by non-radiative relaxation processes (e.g. ⁴G_11/2 → ²H_9/2, ²H_9/2 → ⁴F_3/2 → ⁴F_5/2, ⁴F_5/2 → ⁴F_7/2 → ²H_11/2 in the energy level diagram of Fig. 5b) and then return to the ⁴I_15/2 ground state by producing 409, 467, 532, and 555 nm emissions, respectively. However, no red emission of Er³⁺ ions (⁴F_9/2 → ⁴I_15/2) between 640 and 680 nm was observed. It suggests that the non-radiative relaxation process ⁴S_3/2 → ⁴F_9/2 can hardly occur, and it is helpful for us to understand the UCL mechanism (Fig. 5b).

Fig. 5 (a) Diffuse reflectance spectrum and (b) PL spectrum of Er₂Mo₄O₁₅ (inset: energy level diagram of Er³⁺).

Through pumping with a 1550 nm laser, Y_2−xEr_xMo₄O₁₅ samples exhibited a bright green emission luminescence during day time as observed with the naked eye. Fig. 6a presents the UCL spectra of Y_2−xEr_xMo₄O₁₅ (x = 0.08, 0.16, 0.32, 0.64, 2.0) samples with 1550 nm pumping. The spectrum is mainly composed of three emission bands: (1) the NIR emission band between 780–830 nm (NIR) belongs to the ⁴I_9/2 → ⁴I_15/2 transition of Er³⁺ ions. (2) The red emission band between 640–700 nm (R) belongs to the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ ions. (3) The green emission band between 545–580 nm (G₂) belongs to the ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺ ions, and the green emission band located between 520–540 nm (G₁) belongs to the ²H_11/2 → ⁴I_15/2 transition of Er³⁺ ions. The peak of the blue emission (²H_9/2 → ⁴I_15/2) located at 409 nm is very weak. The red emission band appeared in the UCL spectra compared with the PL spectra in Fig. 5b. The green up-conversion emission (²H_11/2, ⁴S_3/2 → ⁴I_15/2) intensity was enhanced with the increase in Er³⁺ ion concentration under equivalent excitation conditions. Concentration quenching was not observed, even when the Y³⁺ ions were completely replaced by Er³⁺ ions (Fig. 6a). The relationship between the green up-conversion emission (⁴S_3/2 → ⁴I_15/2) intensity and the Er³⁺ doping concentration was linear (inset in Fig. 6a), indicating that the UCL mechanism of Y_2−xEr_xMo₄O₁₅ was mainly ESA under 1550 nm excitation, which was different from the results of Meijerink et al.,⁴³ who demonstrated that energy transfer (ETU) between Er³⁺ ions was responsible for the UCL in Gd₂O₂S:Er³⁺ under 1510 nm excitation even if the Er³⁺ ion concentration is only 0.1 mol%.

Fig. 6 (a) UCL spectra of Y_2−xEr_xMo₄O₁₅ (x = 0.08, 0.16, 0.32, 0.64, LD current 3 A; and X = 2.0, excitation current 2.6 A) under 1550 nm excitation (inset: the Er³⁺ concentration dependence of the green emission (⁴S_3/2 → ⁴I_15/2) intensity under 1550 nm excitation). (b) CIE chromaticity coordinates of Er₂Mo₄O₁₅ sample vs. pumping power of 1550 nm LD.

Fig. 7 shows the UCL spectra of the Er₂Mo₄O₁₅ sample under 1550 and 980 nm excitation. Compared with the UCL spectrum excited at 1550 nm, Er₂Mo₄O₁₅ phosphors showed similar emission peak positions and profiles under 980 nm excitation, whereas the components of red emission were enhanced significantly. Er₂Mo₄O₁₅ phosphors demonstrated almost the same UCL brightness under a relatively low powered 1550 and 980 nm excitation (inset in Fig. 7). The brightness of Er₂Mo₄O₁₅ under 1550 nm excitation was much greater than that under 980 nm excitation with increasing excitation power, suggesting that Er₂Mo₄O₁₅ phosphors present higher UCL efficiency under the 1550 nm excitation. However, the UCL brightness of Er₂Mo₄O₁₅ experienced a saturation effect under the higher excitation power of 1550 nm (>200 mW) (inset in Fig. 7).

Fig. 7 Normalized UCL spectra of Er₂Mo₄O₁₅ (1550 and 980 nm excitation) and contrast samples Y₂O₂S:Yb³⁺ and Er³⁺ (980 nm excitation) (inset: UCL brightness of Er₂Mo₄O₁₅ versus excitation power under 1550 and 980 nm).

As we can see from Fig. 8, the green emission intensity of Er₂Mo₄O₁₅ rose rapidly with the increment in the excitation current of 1550 nm LD (and 980 nm LD), whereas the red emission almost remained unchanged. The increasing rate of green emission intensity was much faster than that of red emission with the same increment in excitation current. The ratio of green and red emission intensities (I_G/I_R) (inset in Fig. 8) also confirmed this observation. The I_G/I_R increased with the increment in excitation current of 1550 and 980 nm LD. The samples presented green emission when I_G : I_R = 7.07 : 1 and 12.99 : 1, whereas at 0.5 and 1.1 A under 1550 nm excitation, the I_G : I_R were 3.12 : 1 and 7.87 : 1 under 980 nm excitation, respectively. The emission colour always focused on the green area with the increment in current, and the green emission was adjusted by changing the current. Such emission behaviour indicates that Er₂Mo₄O₁₅ phosphors have quite good colour stability (Fig. 6b).

Fig. 8 Green (I_G) and red (I_R) UCL intensity of Er₂Mo₄O₁₅ phosphors versus excitation current of 1550 and 980 nm LD (inset: relationship between I_G/I_R and 1550 and 980 nm LD excitation current).

The red emission component in the UCL spectra can be enhanced by increasing the doping concentration of Er³⁺ ions in conventional UCL materials under 980 nm excitation. For example, TiO₂:Er³⁺, BaTiO₃:Er³⁺ or Y₂O₃:Er³⁺ presented significant red emission, whereas dominating red emission was obtained by further increasing the concentration of Er³⁺ ions above 1 mol% due to the cross relaxation processes between Er³⁺ ions.⁴⁴ Because of the small energy gap between the ⁴I_9/2 and ⁴I_11/2 levels, the ⁴F_7/2 levels can be populated via a non-radioactive transition of ⁴I_9/2 → ⁴I_11/2 and a subsequent ⁴I_11/2 + hν_{1550 nm} → ⁴F_7/2 transition. Moreover, in the condition ⁴I_9/2 + hν_{1550 nm} → ²H_11/2, the ⁴S_3/2 transition can hardly occur due to the short lifetime of the ⁴I_9/2 level. Therefore, Er³⁺-doped UCL materials usually present high red emission under 1550 nm excitation. For example, M₂O₂S:Er (M = Y, Gd, La),⁴⁵ Yb³⁺ and Er³⁺ co-doped Y₂O₂S,⁴⁶ Y₂O₃,⁴⁷ LaOCl⁴⁸ and YOCl⁴⁹ presented high colour purity red emission. The as-prepared Y_2−xEr_xMo₄O₁₅ phosphor, even when Er₂Mo₄O₁₅ was present, showed a dominating green emission under 1550 or 980 nm excitation that was independent of the amount of doping Er³⁺ ions. This abnormal result is obviously different from the above conclusion. A similar phenomenon can also be found in our previous study for NaY(WO₄)₂. Er³⁺ and Yb³⁺/Er³⁺ co-doped samples presented high purity green-coloured emissions whether under 1550 or 980 nm excitation when the optimum doping concentration of Er³⁺ of 24 mol% was used.⁵⁰ Thus, this UCL mechanism is worthy of further research.

The UCL population mechanism can be further studied by determining the relationship between the up-conversion emission intensity and pump power, i.e.:

I_up ∝ I_pⁿ (1)

where I_up is the intensity of the up-conversion emission, I_p is the excitation power and n = (1, 2, 3…n) is the number of pump-photons required to populate the emitting levels.

Fig. 9 shows the double-logarithmic fitting of red and green UCL intensities (I_up) to excitation power (I_p) of the Er₂Mo₄O₁₅ sample under 980 and 1550 nm excitation. The red (n = 1.36) and green (n = 1.80) emissions under the 980 nm excitation are the two-photon process. However, under 1550 nm excitation, the n value of the red and green emission changed with the increment in excitation power: the n value of green emission decreased from 2.07 to 1.72 with the increment in excitation power and the red emission also showed a similar trend due to the saturation effects. Similar results can also be observed in Y₂O₂S:Yb³⁺,Er³⁺.⁴⁶

Fig. 9 Dependence of UCL intensities of Er₂Mo₄O₁₅ phosphor on pump power under 980 and 1550 nm excitation.

The UCL mechanisms consist of ground state absorption (GSA), ESA, ETU and so on. For the green UCL mechanism of Er³⁺ ions doped systems under 980 nm excitation: GSA process 1, ⁴I_15/2 + hν_{980 nm} → ⁴I_11/2, ESA or ETU process 2, ⁴I_11/2 + hν_{980 nm} → ⁴F_7/2, non-radiation relaxation process 3, ⁴F_7/2 → ²H_11/2 → ⁴S_3/2 and radiation process 4, ²H_11/2, ⁴S_3/2 → ⁴I_15/2 + green emission. However, the red UCL mechanism of the Er³⁺ ions-doped systems was much more complex under the 980 nm excitation, and three possible mechanisms are responsible for red UCL: (1) non-radiation relaxation: ⁴S_3/2 → ⁴F_9/2, (2) non-radiation relaxation: ⁴I_11/2 → ⁴I_13/2 and subsequent ⁴I_11/2 + hν_{980 nm} → ⁴F_7/2 transition and (3) cross relaxation process between Er³⁺ ions. Few UCL mechanisms have been reported under the 1550 nm excitation.

According to the Miyakawa–Dexter theory, the rate of multiphonon relaxation is dependent upon the energy gap separating the upper and lower states. The rate of the multiphonon relaxation (ω_P) can be expressed as:⁵¹

where ω₀ and α are the constants related to the properties of the matrix materials, ΔE is the energy gap (cm⁻¹) and ħω indicates the maximum phonon energy of the host lattice.

The 4f shell of rare-earth ions is shielded by the outermost layer 5s and 5p, and the matrix material possesses a relatively small impact on 4f electron transitions. The difference between the energy level data of Er³⁺ ions in different matrix material was small. On the other hand, the shape and peaks of Er₂Mo₄O₁₅ UCL spectra were similar to those of Y₂O₂S:Yb³⁺,Er³⁺ (Fig. 8).⁵² Therefore, we inferred the energy level data of Er³⁺ ions in Er₂Mo₄O₁₅ from that in Y₂O₂S:Er³⁺.⁵³ The energy gap ΔE of Er³⁺ ions ⁴S_3/2–⁴F_9/2 and ⁴I_11/2–⁴I_13/2 was 2980 and 3400 cm⁻¹, respectively, and the ΔE/ħω was 3.1 and 3.6, respectively. Red emission (⁴F_9/2 → ⁴I_15/2) of Er³⁺ ions was not observed in the PL spectrum when the ⁴G_11/2 level was pumped directly (Fig. 5b), indicating that the probability of non-radiation relaxation of ⁴S_3/2 → ⁴F_9/2 was quite low. Therefore, the probability of non-radiation relaxation of ⁴I_11/2 → ⁴I_13/2 was smaller than that of ⁴S_3/2 → ⁴F_9/2, according to the above energy gap law data; and the ⁴I_13/2 + hν_{980 nm} → ⁴F_7/2 process was also not the major cause of red emission. Thus, cross relaxation between Er³⁺ ions was responsible for red UCL (⁴F_9/2 → ⁴I_15/2), which depended on the distance between Er³⁺ ions.

As can be observed from Fig. 1, the minimum distance between Er³⁺ ions in Er₂Mo₄O₁₅ was 5.2680 Å, which can also be confirmed from the data for Y₂Mo₄O₁₅.³⁷ This minimum distance was much larger than the other matrix, for example, the shortest distances between the rare earth ions in β-NaYF₄, Y₂O₂S, Y₂O₃ were 3.2344, 3.5751 and 3.5130 Å, respectively. Therefore, the luminescence energy transfer or diffusions were limited by the long distances between Er³⁺ ions that are based on a special structure, and which suppressed the cross relaxation between Er³⁺ ions and caused a weak red emission in Er₂Mo₄O₁₅. The study by Fischer et al.⁵⁴ shows that the average distance between Er³⁺ ions is 6.63 Å for β-NaYF₄:0.25 mol Er³⁺ and 7.53 Å for Gd₂O₂S:0.1 mol Er³⁺. These data further support our results. In addition, Er₂Mo₄O₁₅ is a compound oxide, and the Er³⁺ ions account for only 9.5% of the total number of atoms, which is another possible reason for the absence of UCL quenching.

In this system, Er₂Mo₄O₁₅ showed a strong green emission under 1550 nm excitation, and its UCL mechanism can be speculated from the above analysis (Fig. 10). For the green emission process:

GSA: ⁴I_15/2 + hν_{1550 nm} → ⁴I_13/2 process (1) in Fig. 10

ESA: ⁴I_13/2 + hν_{1550 nm} → ⁴I_9/2 process (2) in Fig. 10

ESA: ⁴I_9/2 + hν_{1550 nm} → ²H_11/2, ⁴S_3/2 process (3) in Fig. 10

Fig. 10 Schematic diagram of the up-conversion luminescence process under 1550 nm excitation.

For the red emission process:

GSA: ⁴I_15/2 + hν_{1550 nm} → ⁴I_13/2 process (1) in Fig. 10

ESA: ⁴I_13/2 + hν_{1550 nm} → ⁴I_9/2 process (2) in Fig. 10

Non-radiation relaxation: ⁴I_9/2 → ⁴I_11/2

ESA: ⁴I_11/2 + hν_{1550 nm} → ⁴F_9/2 process (4) in Fig. 10

The NIR emission intensity of Er₂Mo₄O₁₅ was much stronger than that of Y₂O₂S:Yb³⁺,Er³⁺, as shown in Fig. 7, which was the probable reason for the weak red UCL in Er₂Mo₄O₁₅ under the 1550 nm excitation.

4. Conclusions

Er₂Mo₄O₁₅ phosphors were synthesised at 700 °C using a solid-state reaction method. The UCL properties and mechanism were studied under 980 and 1550 nm excitation. The shortest distance between neighbouring rare earth ions was 5.2680 Å in Er₂Mo₄O₁₅, which was more than 1.6 times longer than that in β-NaYF₄. The red UCL caused by the cross relaxation process was suppressed due to the larger adjacent Er³⁺⋯Er³⁺ distance. Therefore, the up-conversion luminescence mechanism was mainly due to the excited-state absorption under the 1550 nm excitation. Er₂Mo₄O₁₅ exhibited a bright green up-conversion luminescence, and concentration quenching was not observed under 980 and 1550 nm excitation.

Acknowledgements

The authors thank the Research Program of Application Foundation (Main Subject) of the Ministry of Transport of PR China (No. 2015329225090), the National Natural Science Foundation of China (No. 11504039 and 51502031), and Fundamental Research Funds for the Central Universities (Grant No. 3132016120, 3132016221, 3132016222, and 3132016349) for their financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21580g

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