Simple method for simultaneously achieving red and green up-conversion luminescence

Abstract

The simultaneous emission of red and green light with high brightness and color purity was obtained from Er³⁺-doped NaYbF₄-based up-conversion nanoparticles excited by 980 and 1550 nm excitation. The ²F_5/2 level of Yb³⁺ showed high absorption efficiency at 980 nm. The ⁴I_13/2 level of Er³⁺, an excellent UC intermediate with high energy and a long lifetime (milliseconds), absorbed more energy in the cross section (6.0 × 10⁻²⁰ cm²) than did the Yb^{3+ 2}F_5/2 level (1.2 × 10⁻²⁰ cm²) and was efficiently and directly pumped by light with a wavelength of ∼1500 nm. In contrast to particles resulting from other methods using complex coating for achieving multi-color emission in a single sample, the as-prepared luminescent NaYbF₄:25% Er³⁺ up-conversion nanoparticles were designed to be single-layered and spherical and showed excellent dispersibility and uniform sizes. Nanoparticles prepared by this method exhibit a great advantage based on the simple preparation process and small particle size. On this basis, we expect to easily achieve the emission of three primary colors by these reasonably designed core–shell particles.

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1. Introduction

Multi-color up-conversion luminescence (UCL) materials have major application prospects in biological imaging, multiple detection and treatment of disease, anti-counterfeiting and multi-color coding, and display areas.^1–4 The tunable range of the up-conversion (UC) emission color, however, is seriously limited due to the finite excitation wavelength, type of activator, and inappropriate emission levels, which also hinder the wide application of UCL materials.^5,6 Current methods for achieving multi-color emission can be divided into three types. (1) The first type of method involves achieving emissions from multiple samples by exciting them with a single wavelength.^7,8 Here, the emission color can be finely adjusted by changing the host,⁹ doping concentrations of rare earth ions,^10,11 particle size^12,13 and crystal structure,^14,15 or co-doping with other ions.^16,17 Furthermore, the UCL color can be tuned using different activator combinations and core–shell structures excited with the single wavelength.^18,19 (2) The second type of method involves achieving emissions from multiple samples by exciting them with multiple wavelengths.²⁰ These two methods usually face chromatic aberration problems resulting from different types of particles having inconsistent sedimentation rates and being unevenly mixed. (3) The third type of method involves achieving multi-color emission from a single sample under multi-wavelength excitation, and this method presents excellent application characteristics. Wen et al. prepared NaYF₄ up-conversion nanoparticles (UCNPs) with a five-layer core–shell structure to obtain blue-green two-color emission under excitation with light of wavelengths of 980 and 808 nm.²¹ Deng et al.²² designed an unsteady UC full-color luminescence in NaYF₄ UCNPs with a five-layer core–shell structure by adjusting the pulse width. These UCNPs realize multi-color emission but require a relatively complex preparation process and large particle size. Consequently, there is an urgent need to obtain a simple method for realizing multi-color emission.

Green and red double color UCL has been realized in micron-sized single Y₂O₂S:Yb³⁺, Er³⁺ UC materials under 980 and 1550 nm dual wavelength excitation.²³ Yb³⁺ and Er³⁺ ions form two sensitization systems excited at 980 and 1550 nm, respectively, and exhibit different UCL characteristics. In the current work, NaYbF₄:25% Er³⁺ UCNPs were shown to simultaneously emit red and green light when excited with light of wavelengths of 980 and 1550 nm.

2. Experimental

2.1 Materials

All materials purchased were used as described below without further purification. 1-Octadecene (OM; 90%), oleic acid (OA; 90%), Y(CH₃COO)₃·4H₂O, Yb(CH₃COO)₃, and Er(CH₃COO)₃ were purchased from Alfa Aesar. Ammonium fluoride (NH₄F), sodium hydroxide (NaOH), methanol, cyclohexane, and ethanol were purchased from Tianjinzhiyuan Chemical Reagent Co and used as received. For each experiment, Re(CH₃COO)₃ was dissolved in deionized water, and NaOH and NH₄F were dissolved in methanol solution in advance to reduce the experimental error.

2.2 Synthesis of NaYbF₄:xEr³⁺ (x = 10, 25, 50, 90 mol%)

A volume of 3 mL of oleic acid and a volume of 7 mL of octadecene were added, respectively, to two vials by using a two-way valve, and a volume of 2 mL (0.4 mmol) of Re(CH₃COO)₃ (0.2 M) was added in proportion (Yb = 0.36, 0.3, 0.2, 0.04 mmol, as Er = 0.04, 0.1, 0.2, 0.36 mmol, respectively). A thermometer able to measure temperatures between 0 and 360 °C was inserted into each solution to precisely measure the temperature. The samples became pale yellow after being heating to 150 °C and kept at this temperature for 45 min, and a rare earth oleate precursor solution was obtained as the sample was cooled to room temperature. Methanol solutions of NaOH (1 mL, 1 M) and NH₄F (4 mL, 0.4 M) were added to the respective flasks, and kept at 50 °C for 40 min. The temperature was then increased to 100 °C in a vacuum environment in order to remove the methanol from the mixtures. Each solution was quickly heated (15 min) to 290 °C under an argon environment and then, when bubbles were no longer produced, kept warm at 290 °C for 1.5 h, and desired product was obtained as the solution was cooled to room temperature. A volume of 8 mL of an ethanol solution was added to each solution, and the resulting mixture was centrifuged at 9000 rad per min for 6 min. The resulting product in each case was dissolved in 4 mL of cyclohexane and 8 mL of ethanol, and the resulting solution was centrifuged at 9000 rad per min for 6 min. The previous step was then repeated with 4 mL of ethanol and 4 mL of methanol instead of 8 mL of ethanol. The resulting sample in each case was dissolved in 5 mL of cyclohexane and sealed and stored in a glass vial at 4 °C.

2.3 Characterizations

The size and morphology of each sample were determined by performing JEM-2100F high-resolution transmission electron microscopy (HRTEM). The UCL spectra were acquired by using an Edinburgh FS5 fluorescence spectrometer, and power-tunable 980 nm-wavelength (maximum power: 800 mW) and 1550 nm-wavelength (maximum power: 800 mW) laser diodes (LDs) were used as excitation sources.

3. Results and discussion

⁴I_11/2 of Er³⁺ was pumped directly by light with a wavelength of ∼980 nm due to the relatively large energy absorption section (1.7 × 10⁻²¹ cm²) and millisecond-range lifetime of the ⁴I_11/2 level.²³ In addition, the ⁴I_13/2 level of Er³⁺ was found in the current work to be an excellent UC intermediate energy level with a long lifetime, and with an energy absorption cross section (6.0 × 10⁻²⁰ cm²) much larger than that of the Yb^{3+ 2}F_5/2 level (1.2 × 10⁻²⁰ cm²) and that was efficiently pumped by light with a wavelength of ∼1500 nm. Therefore, a high UCL efficiency was achieved using Er³⁺ ions as the activator with 980 and 1550 nm dual-wavelength excitation.²⁴ An efficient single red UCL was easily obtained at 1500 nm-wavelength excitation relying on the non-radiative ⁴I_9/2 → ⁴I_11/2 relaxation, which is based on the relatively short lifetime (10 microseconds) of Er³⁺ ions and small energy gap between ⁴I_9/2 and ⁴I_11/2 levels (ΔE ≈ 1900 cm⁻¹).^25,26 Fig. 3a and b are the main green and red double-color UCL mechanism of Y₂O₂S:Yb³⁺, Er³⁺ UC materials under 980 and 1550 nm dual-wavelength excitation. However, the opposite is the case for the β-NaYF₄ host, as β-NaYF₄:Er³⁺ (ref. 27 and 28) and LiYF₄:Er³⁺ (ref. 29) show yellow-green UCL when excited at a wavelength of 1500 nm. This condition remains unchanged even upon adding Yb³⁺ ions (Fig. S1f†), and high-color-purity emission of red light is difficult to obtain from the β-NaYF₄ host. Therefore, in this work, spherical NaYbF₄:x% Er³⁺ UCNPs (Fig. 1), with an average particle size of 18.34 nm, were prepared through co-precipitation. The distance between adjacent lattice planes of this product was measured using the HRTEM images to be about 0.51 nm, which confirmed the product to be β-phase NaYbF₄ (Fig. 1b).

Fig. 1 (a), (b) HRTEM images and (c) corresponding particle size histograms of NaYbF₄:25% Er³⁺.

NaYbF₄:x% Er³⁺ showed different luminescence properties with different Er³⁺ doping concentrations and presented distinct emission colors under excitation with wavelengths of 980 nm and 1550 nm (Fig. S1†). Under excitation at 980 nm, NaYbF₄:10% Er³⁺ showed green UCL with a red-to-green emission ratio (I_r/I_g) of 0.59 (Fig. S1a†) and NaYbF₄:25% Er³⁺ showed red UCL, specifically at 654 nm (I_r/I_g = 3.29, Fig. 2), whereas NaYbF₄:50% Er³⁺ exhibited decreased red UCL color purity (I_r/I_g = 2.19, Fig. S1†) and exhibited luminescence quenching. However, Wei et al.³⁰ reported that NaYbF₄:50 mol% Er³⁺ exhibits the optimal red UCL performance when excited with light at a wavelength of 980 nm, this condition is mainly resulted from the large particle size (∼100 nm) and crystallinity (Fig. S2†). NaYbF₄:xEr³⁺ excited at 1550 nm showed a constant green UCL (Fig. S1b†) and exhibited the optimal emission characteristics when the doping concentration of Er³⁺ was 25 mol% (I_r/I_g = 0.45, Fig. 2). Therefore, emission of both red and green light with high brightness and color purity was achieved using NaYbF₄:25% Er³⁺ subjected to dual-wavelength excitation, at 980 and 1500 nm; the corresponding CIE chromaticity coordinates were determined to be (0.62, 0.31) and (0.29, 0.64), respectively. The double-color UCL samples prepared by this method were each shown to have a simple structure and small particle size, and the preparation process was simple and easily controlled. Multi-color emission was achieved by adjusting the relative intensities of the two lasers (Fig. 2b).

Fig. 2 (a) CIE chromaticity coordinates, UCL spectra, and digital camera images of NaYbF₄:25% Er³⁺ under 980 and 1500 nm wavelength excitation. (b) Multi-color emission images of NaYbF₄:25% Er³⁺ under excitation with wavelengths of 980 nm (excitation power density = 0, 28.41, 32.13, 37.06, 44.52 mW mm⁻²) and 1550 nm (excitation power density = 38.03 mW mm⁻²) (Canon EOS 5D Mark III, Tv = 1 : 1250, Len: EF24-70 mm f/2.8 L II USM, Av = 2.8, ISO = 12800).

The red and green UCL displayed by NaYbF₄:25% Er³⁺ under 980 and 1550 nm-wavelength excitation was attributed to different interaction mechanisms between Yb³⁺ and Er³⁺ excited at these two excitation wavelengths. Wei et al.³⁰ attributed the enhancement of red UCL in highly doped Er³⁺ samples to the cross relaxation (CR, ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2, +⁴F_9/2) process between Er³⁺ ions. The CR process was enhanced as the Er³⁺ concentration was increased, and with increasing I_r/I_g (Fig. S3†). However, the I_r/I_g of Er³⁺-doped NaYbF₄:xEr³⁺ samples failed to increase as the Er³⁺ concentration was increased, indicating that the enhancement of red UCL was also related to Yb³⁺ co-doping.³¹ Therefore, the enhancement of red UCL in NaYbF₄:25% Er³⁺ excited at 980 nm was due to the combined effect of the CR process and reverse energy transfer from the Er^{3+ 4}G_11/2 level to Yb³⁺ (Fig. 3c). The green emission displayed by NaYbF₄:25% Er³⁺ excited with light of a wavelength of 1550 nm was mainly due to the radiation relaxation ⁴I_9/2 → ⁴I_11/2 occurring in other host being less likely to take place in NaYbF₄ because of the low phonon energy, thus making the energy transfer process Er³⁺ → Yb³⁺ → Er³⁺ difficult to transpire.³² Moreover, the lack of occurrence of the above CR process and the occurrence of weak red emission (Fig. 3d) were due to the difficulty in the ⁴F_7/2 level being pumped by light with a wavelength of 1550 nm (Fig. 4).

Fig. 3 (a, b) Double-color UCL mechanism of Er³⁺ under (a) 980 nm-wavelength and (b) 1500 nm-wavelength excitation. (c, d) UCL mechanism of NaYbF₄:25% Er³⁺ under (c) 980 nm-wavelength and (d) 1500 nm-wavelength excitation.

Fig. 4 Decay curves of the Er^{3+ 4}S_3/2 (550 nm), ⁴F_9/2 (654 nm), ⁴I_9/2 (800 nm), ⁴I_11/2 (1000 nm), ⁴I_13/2 (1535 nm) levels in an NaYbF₄:Er³⁺ sample under (a) 980 nm-wavelength and (b) 1550 nm-wavelength pulse LD excitation.

4. Conclusions

Single-layer spherical NaYbF₄:25% Er³⁺ UCNPs, with an average particle size of 18 nm, showed double-color emission, of red and green light, under dual-wavelength excitation, of 980 nm and 1550 nm wavelengths. In contrast to other methods for achieving multi-color emission from a single sample with complex coating processes, the as-prepared Er³⁺-doped NaYbF₄-based system is a single-layer structure. And compared with the existing research on multi-color luminescence, the method we have now described involves a simple preparation process, suggesting the potential of achieving mass production. And on this basis, the three primary colors may be expected to be easily achieved by a reasonable designed core–shell.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the Research Program of the National Natural Science Foundation of China (Grant No. 11504039 and 51502031), and the Application Foundation (Main subject) of the Ministry of Transport of PR China (No. 2015329225090). The Fundamental Research Funds for the Central Universities (Grant No. 3132016349 and 3132017061), Program for Liaoning Excellent Talents in University (LR2013020), and Foundation of Liaoning Educational Committee (L2013201, L2014212, L2014208) are also acknowledged for their financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra09330f

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