β-NaYF₄:Yb,Tm: upconversion properties by controlling the transition probabilities at the same energy level

Abstract

A series of Yb³⁺/Tm³⁺ co-doped β-NaYF₄ (β-NaYF₄:Yb,Tm) crystals have been successfully synthesized via the molten salt synthesis (MSS) method. A unique strong green emission of Tm³⁺ was observed, which cannot be obtained by any other method. The red, green and blue (RGB) light emissions have been obtained simultaneously in the presence of a single activator (Tm³⁺). The influencing factors of crystal growth have been studied in detail, in order to understand the change of the transition probabilities of Tm³⁺. Subsequent studies indicate that the RGB light emissions of Tm³⁺ can be effectively tuned by varying the concentration of sensitizers (Yb³⁺ and Mn²⁺) and the reaction time. Consequently, the effective multicolor upconversion (UC) luminescence was obtained, especially a strong white light, which was not obtained in the β-NaYF₄:Yb,Tm system earlier. To our knowledge, it is the first time that the UC properties in the β-NaYF₄:Yb,Tm system have been investigated by altering the transition probabilities at the same energy level of Tm³⁺.

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1. Introduction

Rare-earth element (REE) doped materials with upconversion (UC) luminescence have been the focus of materials research owing to their applications in solid-state blue-green lasers, three-dimensional color displays, solar cells, and fluorescent bio-labels.^1–4 In particular, RE³⁺-doped materials that are capable of white-light UC emission have attracted enormous attention.^5–11 Many efforts have been made to obtain white-light UC emission with a high color rendering index by mixing three monochromatic light sources red, green and blue (RGB).^12–15 NaYF₄ with a hexagonal structure is one of the most efficient host materials with UC luminescence due to the relatively low lattice phonon energy.^16–21 Meanwhile, in the RE³⁺-doped UC luminescent materials, Tm³⁺ is generally considered to be one of the most efficient RE³⁺ for obtaining laser emission, frequency UC luminescence, as well as to be used in optical amplifiers.²²

Recently, research has been focused on the development of different methods for the design and fabrication of materials with multicolor UC luminescence, especially white-light UC emission. One commonly used method of tuning UC luminescence is to control the type and concentration of the doping ions, or the range of pump power. For example, Zhao et al. significantly enhanced the UC luminescence of nanocrystals by increasing the activator (Tm³⁺) concentration, in combination with elevation of irradiance excitation in Yb³⁺/Tm³⁺ co-doped NaYF₄ nanocrystals.²³ Garry Glaspell et al. obtained RGB and white light emission by adjusting the concentrations of the RE³⁺ (Yb³⁺, Er³⁺, Ho³⁺, and Tm³⁺) in the Y₂O₃ nanocrystals.²⁴ Yang et al. adjusted the UC color by controlling the range of pump power.²⁵ However, there is almost no reference mentioned about the green light emission (¹D₂ → ³H₅, 550 nm) of Tm³⁺ and controlling of the transition probabilities of the ¹D₂ level.^25–34 Notably, in our past research,³⁵ the ¹D₂ level transitions of Tm³⁺ are changed greatly in the Yb³⁺/Tm³⁺ co-doped KMnF₃ system. Accordingly, the transition probability ratio of ¹D₂ → ³F₄ (450 nm, blue) to ¹D₂ → ³H₅ (550 nm, green) at the same ¹D₂ level exhibited a significant change. As a result, a strong green emission was firstly observed, which originated from the excitation of the activator (Tm³⁺). In the previous reports, UC luminescence has been widely studied and applied. The UC luminescence system of doping Yb³⁺/Tm³⁺ is mainly limited to the ternary doped systems, such as Yb³⁺/Er³⁺/Tm^3+ 36 or Yb³⁺/Ho³⁺/Tm^3+ 25 systems. No simultaneous generation of red, green and blue emissions was demonstrated with a single activator.^37–41 Few investigations have focused on single activator ions.

To our knowledge, the research of Tm³⁺ with green UC emission, which is an important and necessary supplement to the NIR to visible light UC materials, is still not sufficient compared with the more frequently studied Er³⁺. In general, multicolor UC luminescence is observed in the tri-doped Yb³⁺, Tm³⁺, and Er³⁺ system, which consists of blue (¹D₂ → ³F₄, ¹G₄ → ³H₆ of Tm³⁺), green (²H_11/2/⁴S_3/2 → ⁴I_15/2 of Er³⁺), and red (⁴F_9/2 → ⁴I_15/2 of Er³⁺, ¹G₄ → ³F₄ of Tm³⁺) UC emissions. In fact, green light emission (¹D₂ → ³H₅) can also be produced by the energy level transition of Tm³⁺, despite that it is weak or not detectable due to the low transition probabilities. Assumpção et al. observed a faint green emission (¹D₂ → ³H₅ of Tm³⁺) after the figure of UC emission spectra was magnified 4200 times.⁴² Poirier et al. have presented that the transition probability ratio of blue emission (¹D₂ → ³F₄, 450 nm) to green emission (¹D₂ → ³H₅, 550) is 22 357.96 : 98.48 by theoretical calculations.²² Our group acquired a strong green emission (¹D₂ → ³H₅, 550 nm) of Tm³⁺ from the Yb³⁺/Tm³⁺ co-doped KMnF₃, which resulted in the RGB light emissions being obtained simultaneously in the presence of a single activator (Tm³⁺).³⁵ Consequently, it is possible to realize multicolor UC luminescence by controlling the intensities and ratios of the RGB light emissions output from a single activator (Tm³⁺) in the Yb³⁺/Tm³⁺ co-doped fluoride system.

In this paper, a series of Yb³⁺/Tm³⁺ co-doped β-NaYF₄ (β-NaYF₄:Yb,Tm) crystals have been synthesized by utilizing the MSS method. The reaction factors (the concentration of the doping ions, the reaction time, and the pump power) have been investigated in detail, in order to understand the impact of these factors on the UC luminescence properties in this system. The research results showed that the RGB light emissions were simultaneously obtained with a single activator (Tm³⁺) in our work. It not only simplifies the production process and reduces production costs, but also provides a new method for adjustable UC luminescence. It is noteworthy that the outer electron level of Tm³⁺ can be changed by the MSS system, which resulted in the transition probabilities at the same energy level that can be changed by controlling some reaction factors mentioned above, which makes it possible to obtain multicolor UC luminescence (including white UC luminescence). In other words, it is the first time that the UC properties in the β-NaYF₄:Yb,Tm system have been investigated by controlling the transition probabilities at the same energy level.

2. Materials and methods

2.1 Chemicals and materials

All reagents were purchased and directly used without further purification. NaCl (A.R., 99.5%), MnCl₂·4H₂O (A.R., 99.0%), and NH₄HF₂ (A.R., 98.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). Yb(NO₃)₃·5H₂O (A.R., 99.9%), Tm(NO₃)₃·6H₂O (A.R., 99.99%), and YN₃O₉·6H₂O (A.R., 99.5%) were purchased from Aladdin Industrial Corporation.

2.2 Preparation

The typical synthesis of Yb³⁺,Tm³⁺ co-doped β-NaYF₄via the MSS method can be described as follows: NaCl, NH₄HF₂, YN₃O₉·6H₂O, Yb(NO₃)₃·5H₂O, and Tm(NO₃)₃·6H₂O were added into a closed Teflon-lined stainless steel autoclave with 50.0 mL capacity with a molar ratio of 1 : 10 : x : y : 0.5% (x + y = 99.5%), and maintained at 180 °C for 24 h. After cooling down to room temperature naturally, the resulting products were washed with deionized water several times and dried at 60 °C for 12 h.

2.3 Characterization

All the samples were characterized by powder X-ray diffraction (XRD, Cu Kα radiation, Rigaku D/max 2550VB, Japan) with a scanning rate of 8° min⁻¹ within the 2θ range of 10° to 80°. The morphology and particle size of the samples were determined by scanning electron microscopy (SEM, JSM-6700F, JEOL, Japan). The upconversion emission spectra were recorded with an Edinburgh Instruments FLS920 spectrophotometer from 300 to 900 nm with an external 0–600 mW adjustable continuous-wave 980 nm laser as the excitation source. All the measurements were performed at room temperature.

3. Results and discussion

3.1 Phase and morphology

The crystalline structures of the as-synthesized products were identified by powder X-ray diffraction (XRD). Fig. S1† shows the XRD patterns of the as-synthesized NaYF₄ samples, as well as the standard XRD data of the pure hexagonal NaYF₄ phase as a reference. All the diffraction peaks can be well indexed in the hexagonal phase of NaYF₄ (JCPDS no. 16-0334), indicating the high purity of the as-synthesized products.

The scanning electron microscopy (SEM) image of the β-NaYF₄:10% Yb,0.5% Tm samples prepared at 180 °C for 24 h is exhibited in Fig. S2,† in which the resulting crystals are clearly hexagonal prisms.

3.2 UC luminescence properties

The UC emission spectrum of the β-NaYF₄:10% Yb,0.5% Tm crystals prepared at 180 °C for 24 h is shown in Fig. 1A. The spectrum was divided into three groups according to the initial Tm³⁺ levels: the ³F_2,3 and ³H₄ groups (two photon transition group), with 700 nm, 750 nm and 800 nm peaks, the ¹G₄ group (three photon transition group), with 475 nm and 650 nm peaks, and the ¹D₂ group (four photon transition group), with peaks at 360 nm, 450 nm and 550 nm. Notably, we observed a prominent band at 550 nm (¹D₂ → ³H₅) in Fig. 1A. The RGB light emissions can be obtained simultaneously by a single activator in the Yb³⁺/Tm³⁺ co-doped NaYF₄ system, which are the red light emission (¹G₄ → ³F₄, 650 nm), green light emission (¹D₂ → ³H₅, 550 nm) and blue light emission (¹D₂ → ³F₄, 450 nm, ¹G₄ → ³H₆, 475 nm). A similar phenomenon can be observed in the Yb³⁺/Tm³⁺ co-doped KMnF₃ system reported recently by our group, which has never been reported before.^43,44

Fig. 1 Room temperature UC emission spectra of (A) the β-NaYF₄:10% Yb³⁺,0.5% Tm³⁺ and (B) the β-NaYF₄:5% Yb³⁺,0.5% Tm³⁺. The samples were tested under laser excitation at 980 nm with the pump power of 250 mW.

Subsequently, various reaction factors (the concentration of the doping ions, the reaction time, and the pump power) were studied in detail, in order to understand the influence of these factors on the UC luminescence properties of the β-NaYF₄:Yb,Tm system. Here, we mainly studied the influence of the reaction factors on the transitions at the same energy level, which has been seldom studied earlier.

3.2.1 The effect of the sensitizer (Yb³⁺). The UC emission spectra of the β-NaYF₄:xYb,0.5% Tm crystals with different concentrations of Yb³⁺ (10%, 20%, 30%, 50%, 70%, and 99.5%) are similar in the peak intensity and peak positions (Fig. 1A and S3†), except the UC emission spectrum of the β-NaYF₄:5% Yb,0.5% Tm crystals (Fig. 1B).

The emission intensity diagrams of the β-NaYF₄:xYb,0.5% Tm samples with different concentrations of Yb³⁺ are shown in Fig. 2. A slight change of the overall integrated emission intensities (Fig. 2A) and the integrated intensities of two photon transition (Fig. 2B) with increasing the Yb³⁺ concentration was observed. The emission intensities of different transitions in the ¹G₄ group are shown in Fig. 2C. The emission intensity ratios between the ¹G₄ → ³H₆ and ¹G₄ → ³F₄ transitions remained the same while the concentration of Yb³⁺ increased from 10% to 70%. The blue emissions (¹G₄ → ³H₆, 475 nm) were significantly enhanced at the Yb³⁺ concentration of 5% and 99.5%, which indicated that the three photon transition group was enhanced. As shown in Fig. 1B and 2D, the green emission (¹D₂ → ³H₅, 550 nm) was extremely strong, whereas the ultraviolet emissions (¹D₂ → ³H₆, 350 nm) and blue emission (¹D₂ → ³F₄, 450 nm) were very weak when the concentration of Yb³⁺ was 5%, the intensity ratio of the blue emission (¹D₂ → ³F₄, 450 nm) and green emission (¹D₂ → ³H₅, 550 nm) is approximately 1 : 9. The intensity ratio of the emission peaks at 450 nm to 550 nm was close to 3 : 2 while the concentration of Yb³⁺ was in the range of 10% to 99.5%, despite that their intensities are both decreased significantly at the Yb³⁺ concentration of 99.5%, which could be due to the weakened four photon transition. The results of the experiments showed that the outer electron energy levels of Tm³⁺ must be affected by the MSS method. The blue emission (¹D₂ → ³F₄, 450 nm) was suppressed and replaced by the green emission (¹D₂ → ³H₅, 550 nm). This inhibition of the blue emission (¹D₂ → ³F₄, 450 nm) was weakened by the doped Yb³⁺. The inhibition could only be fully realized with the low concentration of Yb³⁺ (5%), and cannot be eliminated even in the NaYbF₄ (Yb³⁺ 99.5%):Tm sample.

Fig. 2 The emission intensity diagrams of β-NaYF₄:xYb,0.5% Tm prepared with different concentrations of Yb³⁺. (A) The intensities of the overall integrated emission (four photon transition group + three photon transition group + two photon transition group). (B) The integrated intensities of two photon transition. (C) The emission intensities of different transitions in the ¹G₄ group. (D) The emission intensities of different transitions in the ¹D₂ group. The samples were tested under laser excitation at 980 nm with the pump power of 250 mW.

All the UC emission spectra of the β-NaYF₄:xYb,0.5% Tm samples prepared with different concentrations of Yb³⁺ can be readily converted to the CIE-1931 chromaticity diagrams, as plotted in Fig. 3. The corresponding chromaticity coordinates of the samples were calculated based on the emission spectra, which were (0.249, 0.3313), (0.2745, 0.2659), (0.2738, 0.246) and (0.2369, 0.1625), respectively. As shown in the chromaticity diagrams, the color tone of the samples gradually shifted from green to blue whitish, and blue with the increasing concentration of Yb³⁺.

Fig. 3 CIE chromaticity diagram of the β-NaYF₄:xYb,0.5% Tm samples prepared with different concentrations of Yb³⁺: (A) 5% Yb³⁺, (B) 10% Yb³⁺, (C) 70% Yb³⁺, and (D) 99.5% Yb³⁺. The samples were tested under laser excitation at 980 nm with the pump power of 250 mW.

3.2.2 The effect of the sensitizer (Mn²⁺). Mn²⁺ is capable of transferring energy from the higher energy levels to the lower levels.⁴⁵ As shown in Fig. 5C, the exchange-energy transfer process occurred from the ¹D₂ level and ¹G₄ level of Tm³⁺ to the ⁴T₁ level of Mn²⁺, and then passed to the lower energy level. Therefore, the disappearance of the higher level emissions will add difficulty to investigate transition possibilities. Due to the substitution of Y³⁺ ions by smaller Mn²⁺ ions in the host lattice, which will change the crystal structure, the UC luminescence will be affected. However, the transfer function of Mn²⁺ was substantially decreased by extending the reaction time in our KMnF₃:Yb,Tm system, making the RGB light emissions originate from the excitation of Tm³⁺ which can be clearly observed.³⁵ Based on the research mentioned above, we doped different concentrations of Mn²⁺ into the β-NaYF₄:Yb,Tm system with a longer reaction time (24 h). The ICP (Inductive Coupled Plasma Emission Spectrometer) test was carried out on the samples doped with Mn²⁺ mole fraction of 1%, 5%, 10%, and 30%, and found that the mole fraction of the doped Mn²⁺ was 0.5%, 1.0%, 2.0%, and 8.0%. We then focused on investigating the change of the transition possibilities at the same energy level.

The emission intensity diagrams of the β-NaYF₄:10% Yb,xMn,0.5% Tm samples with different concentrations of Mn²⁺ are shown in Fig. 4. The emission intensities of the overall integrated transitions (Fig. 4A), the two photon transition (Fig. 4B), and the three photon transition in the ¹G₄ group (Fig. 4C) all underwent slight changes with increasing Mn²⁺ concentrations. Also, Mn²⁺ had a great influence on the four photon transition, as exhibited in Fig. 4D. In particular, the green emission (¹D₂ → ³H₅, 550 nm) was significantly enhanced when the concentration of Mn²⁺ is 0.5%, whereas the ultraviolet emissions (¹D₂ → ³H₆, 350 nm) and the blue emission (¹D₂ → ³F₄, 450 nm) seemed weak. The enhancement of green emission was also further confirmed by the UC emission spectrum (Fig. 5A), which was quite different from the spectrum of the β-NaYF₄:10% Yb,0.5% Tm samples (Fig. 1A). In general, the emission intensity ratio of the ¹D₂ → ³H₅ to ¹D₂ → ³F₄ transition gradually decreased with increasing Mn²⁺ concentrations. The spectrum (Fig. 5B) indicated that the effect of Mn²⁺ disappeared when the concentration of Mn²⁺ reached 8.0%, which was nearly the same with the spectrum without doping Mn²⁺ (Fig. 1A). The results of the experiments showed that a small amount of Mn²⁺ (0.5%) can be doped in the position of Y³⁺,⁴⁵ so as to promote the change of the outer electron energy levels of Tm³⁺. At the same concentration of 10% Yb³⁺, the blue emission (¹D₂ → ³F₄, 450 nm) was totally suppressed. With the increase of Mn²⁺ concentration, the doping of Mn²⁺ gradually destroyed the order in the crystal structure, and the fluorescence was basically not changed with or without 8.0% Mn²⁺ in the β-NaYF₄:10% Yb,0.5% Tm samples.

Fig. 4 The emission intensity diagrams of β-NaYF₄:10% Yb,xMn,0.5% Tm prepared with different concentrations of Mn²⁺. (A) The intensities of the overall integrated emission (four photon transition group + three photon transition group + two photon transition group). (B) The integrated intensities of two photon transition. (C) The emission intensities of different transitions in the ¹G₄ group. (D) The emission intensities of different transitions in the ¹D₂ group. The samples were tested under laser excitation at 980 nm with the pump power of 250 mW.

Fig. 5 (A) Room temperature UC emission spectra of the β-NaYF₄:10% Yb³⁺,0.5% Mn²⁺,0.5% Tm³⁺. (B) Room temperature UC emission spectra of the β-NaYF₄:10% Yb³⁺,8.0% Mn²⁺,0.5% Tm³⁺. (C) Energy level diagrams of the Yb³⁺, Tm³⁺, and Mn²⁺, and the efficient transfer process from the ¹D₂ level of Tm³⁺. The samples were tested under laser excitation at 980 nm with the pump power of 250 mW.

The results indicated that a small amount of Mn²⁺ could promote the green light emission. It suggested that the order of the crystal was destroyed, which transformed to the regular arrangement with increasing Mn²⁺ concentration. Meanwhile, the blue and green light emissions were slightly adjusted by the remaining transfer function of Mn²⁺.

All the UC emission spectra of the β-NaYF₄:10% Yb,xMn,0.5% Tm samples prepared with different concentrations of Mn²⁺ can be readily converted to the CIE-1931 chromaticity diagrams, as shown in Fig. 6. The corresponding chromaticity coordinates of samples were calculated based on the emission spectra, which were (0.2748, 0.4187), (0.3032, 0.3181), (0.2902, 0.301) and (0.2532, 0.2396), respectively. As shown in the chromaticity diagrams, the color tone of the samples gradually shifted from green to white, and ultimately to blue with the increasing concentration of Mn²⁺.

Fig. 6 CIE chromaticity diagram of the β-NaYF₄:10% Yb,xMn,0.5% Tm samples prepared with different concentration of Mn²⁺: (A) 0.5% Mn²⁺, (B) 1.0% Mn²⁺, (C) 2.0% Mn²⁺, and (D) 8.0% Mn²⁺. The samples were tested under laser excitation at 980 nm with the pump power of 250 mW.

3.2.3 The effect of the reaction time. The UC emission spectra of the β-NaYF₄:10% Yb,0.5% Tm crystals prepared at 0.5 h and 12 h are shown in Fig. 7. When the reaction time was 0.5 h, the intensity of the blue emission (¹G₄ → ³H₆, 475 nm) was strong, whereas the intensities of the green emission (¹D₂ → ³H₅, 550 nm), the blue emission (¹D₂ → ³F₄, 450 nm) and the red emission (¹G₄ → ³F₄, 650 nm) were all very weak (Fig. 7A). When the reaction time extended to 12 h, the intensities of all UC emission peaks were enhanced, as illustrated in Fig. 7B.

Fig. 7 Room temperature UC emission spectra of the β-NaYF₄:10% Yb³⁺,0.5% Tm³⁺ with the reaction time of (A) 0.5 h and (B) 12 h. The samples were tested under laser excitation at 980 nm with the pump power of 250 mW.

The emission intensity diagrams of the β-NaYF₄:10% Yb,0.5% Tm samples prepared with different reaction times are shown in Fig. 8. The overall integrated emission intensity gradually increased with the extension of reaction time (Fig. 8A), which was due to the increase of the crystal size and the crystallinity. In particular, when the reaction time was 0.5 h, the transition possibility at the same level changed, and the transition probability ratio of ¹D₂ → ³H₅ (550 nm) to ¹D₂ → ³F₄ (450 nm) increased significantly, as illustrated in Fig. 8B and C. Although the overall integrated emission intensity was fairly low at 0.5 h, the transition intensities of ¹G₄ → ³H₆ (475 nm) and ¹D₂ → ³H₅ (550 nm) were specifically enhanced in the ¹G₄ and ¹D₂ groups respectively. The results of the experiments showed that a shorter reaction time, and a lower crystallization degree, is more conducive to changing the outer electron energy levels of Tm³⁺, thereby the blue emission (¹D₂ → ³F₄, 450 nm) was totally suppressed. However, when the reaction time is over 6 hours, the effect was lost with the larger crystal size and the higher crystallization degree. In other words, the small size and low degree of crystallization were more advantageous to generate the green light emission.

Fig. 8 The sample of β-NaYF₄:Yb,Tm prepared with different reaction times. (A) The intensities of the overall integrated emission (four photon transition group + three photon transition group + two photon transition group. The inset is the sample of 0.5 h that is amplified). (B) The emission intensities of different transitions in the ¹G₄ group (the inset is the sample of 0.5 h that is amplified). (C) The emission intensities of different transitions in the ¹D₂ group (the inset is the sample of 0.5 h that is amplified). The samples were tested under laser excitation at 980 nm with the pump power of 250 mW.

All the UC emission spectra of the β-NaYF₄:10% Yb,0.5% Tm samples prepared with different reaction times can be readily converted to the CIE-1931 chromaticity diagrams, as shown in Fig. 9. The corresponding chromaticity coordinates of samples were calculated based on the emission spectra, which were (0.2021, 0.2184), (0.2561, 0.2708) and (0.2745, 0.2659), respectively. As shown in the chromaticity diagrams, the color tone of the samples gradually shifted from blue to blue whitish with the increasing of reaction time.

Fig. 9 CIE chromaticity diagram of the β-NaYF₄:10% Yb³⁺,0.5% Tm³⁺ samples prepared with different reaction times: (A) 0.5 h, (B) 12 h, and (C) 24 h. The samples were tested under laser excitation at 980 nm with the pump power of 250 mW.

3.2.4 The effect of the activator (Tm³⁺). UC luminescence intensity can be significantly enhanced by using high concentrations of the activator (Tm³⁺) under relatively strong excitation, which was reported in β-NaYF₄:Yb³⁺,Tm³⁺ nanocrystals by Jin's group.²³ Notably, our research group has synthesized two types of UC luminescent materials in the Yb³⁺/Tm³⁺ co-doped systems.³⁵ Both of them had strong green emission from Tm³⁺, which was never reported by the other research groups. The RGB colors may be tuned by controlling the concentration of Tm³⁺ to realize white-light UC emission. However, further research proved that the change of the Tm³⁺ concentration is not helpful to tune RGB colors. As shown in Fig. 10, the transition intensity ratios at the same energy level changed slightly with increasing Tm³⁺ concentration, which indicated that changing the concentration of Tm³⁺ had no effect on the luminescence of Tm³⁺. The results showed that the change of the Tm³⁺ concentration is not helpful to the outer electron level of Tm³⁺ and was changed by the MSS system.

Fig. 10 The emission intensities diagrams of β-NaYF₄:10% Yb,xTm prepared with different concentrations of Tm³⁺. (A) The intensities of the overall integrated emission (four photon transition group + three photon transition group + two photon transition group). (B) The emission intensities of different transitions in the ¹G₄ group. (C) The emission intensities of different transitions in the ¹D₂ group. The samples were tested under laser excitation at 980 nm with the pump power of 250 mW.

3.2.5 The effect of the pump power. Finally, we studied the influence of the pump power on the transition probability at the same energy level. The pump power was modulated from 150 mW to 300 mW in the β-NaYF₄:10% Yb³⁺,1.0% Mn²⁺,0.5% Tm³⁺ system. In addition to the intensity reduction of the ¹D₂ group and the corresponding intensity increase of the ¹G₄ group with the power of 150 mW, the UC emission spectra of the samples changed slightly with the increasing of pump power, as presented in Fig. 11. The results indicated that the special fluorescence phenomenon caused by the change of the outer electron energy levels of Tm³⁺ with the crystal structure, had nothing to do with the pump power.

Fig. 11 The emission intensity diagrams of β-NaYF₄:10% Yb,1.0% Mn,0.5% Tm prepared with different pump powers under laser excitation at 980 nm. (A) The intensities of the overall integrated emission (four photon transition group + three photon transition group + two photon transition group). (B) The integrated intensities of two photon transition. (C) The emission intensities of different transitions in the ¹G₄ group. (D) The emission intensities of different transitions in the ¹D₂ group.

4. Conclusions

In conclusion, we have successfully synthesized the Yb³⁺/Tm³⁺ co-doped β-NaYF₄ crystals with the MSS method. The ¹D₂ level transition of Tm³⁺ underwent a special change in this system, which resulted in a strong green emission from Tm³⁺. The influence of reaction factors on the transition probability at the same energy level was studied in detail. Subsequent studies indicated that a small amount of doped ions (Yb³⁺ and Mn²⁺) will promote the green light emission of Tm³⁺, and lose efficacy with higher concentrations. The green light emission of Tm³⁺ will be more significant by a low degree of crystallization, which has nothing to do with the doping concentration of Tm³⁺ itself as well as the pump power. In conclusion, the special changes of the ¹D₂ level transition probability of Tm³⁺ were investigated and described in detail in the Yb/Tm co-doped fluoride system, following the corresponding characteristics: the RGB light emissions from a single activator (Tm³⁺), a variety of multicolor adjustment methods, and the absence of the concentration quenching effect. This study not only provided a novel way for the generation of multicolor UC luminescence, but also raised the possibility of constructing high-quality UC luminescent crystals with desirable optical properties beyond the limitation of the nanosize materials.

Acknowledgements

This work was supported by the National Sciences Foundation of China (no. 21271082 and 21301066).

Notes and references

1. F. Auzel, Chem. Rev., 2004, 104, 139–173.
2. E. Downing, L. Hesselink, J. Ralston and R. M. Macfarlance, Science, 1996, 273, 1185–1189.
3. E. D. Rosa, P. Salas, H. Desirenca, C. Angeles and R. A. Rodríguez, Appl. Phys. Lett., 2005, 87, 241912.
4. F. Van de Rijke, H. Zijlmans, S. Li, T. Vail, A. K. Raap, R. S. Niedbala and H. J. Tanke, Nat. Biotechnol., 2001, 19, 273–276.
5. S. Sivakumar, J.-C. Boyer, E. Bovero and F. C. J. M. van Veggel, J. Mater. Chem., 2009, 19, 2392–2399.
6. G. Li, D. Geng, M. Shang, Y. Zhang, C. Peng, Z. Cheng and J. Lin, J. Phys. Chem. C, 2011, 115, 21882–21892.
7. M.-S. Wang, S.-P. Guo, Y. Li, L.-Z. Cai, J.-P. Zou, G. Xu, W.-W. Zhou, F.-K. Zheng and G.-C. Guo, J. Am. Chem. Soc., 2009, 131, 13572–13573.
8. N. Guo, H. You, Y. Song, M. Yang, K. Liu, Y. Huang and H. Zhang, J. Mater. Chem., 2010, 20, 9061–9067.
9. K. Zheng, D. Zhang, D. Zhao, N. Liu, F. Shi and W. Qin, Phys. Chem. Chem. Phys., 2010, 12, 7620–7625.
10. T. Passuello, F. Piccinelli, M. Pedroni, M. Brttinelli, F. Mangiarini, R. Naccache, F. Vetrone, J. A. Capobianco and A. Speghini, Opt. Mater., 2011, 33, 643–646.
11. C. Zhang, P. Ma, C. Li, G. Li, S. Huang, D. Yang, M. Shang, X. Kang and J. Lin, J. Mater. Chem., 2011, 21, 717–723.
12. C. Zhang, S. Huang, D. Yang, X. Kang, M. Shang, C. Peng and J. Lin, J. Mater. Chem., 2010, 20, 6674–6680.
13. J. Yang, C. Zhang, C. Peng, C. Li, L. Wang, R. Chai and J. Lin, Chem. – Eur. J., 2009, 15, 4649–4655.
14. X. Liu, C. Lin and J. Lin, Appl. Phys. Lett., 2007, 90, 081904.
15. J. R. DiMaio, B. Kokuoz and J. Ballato, Opt. Express, 2006, 14, 11412–11417.
16. N. Niu, P. Yang, F. He, X. Zhang, S. Gai, C. Li and J. Lin, J. Mater. Chem., 2012, 22, 10889–10899.
17. J.-C. Boyer, F. Vetrone, L. A. Cuccia and J. A. Capobianco, J. Am. Chem. Soc., 2006, 128, 7444–7445.
18. D. Tu, L. Liu, Q. Ju, Y. Liu, H. Zhu, R. Li and X. Chen, Angew. Chem., Int. Ed., 2011, 50, 6306–6310.
19. A. Yin, Y. Zhang, L. Sun and C. Yan, Nanoscale, 2010, 2, 953–959.
20. Y. Wang, Y. Liu, Q. Xiao, H. Zhu, R. Li and X. Chen, Nanoscale, 2011, 3, 3164–3169.
21. F. Wang and X. Liu, Chem. Soc. Rev., 2009, 38, 976–989.
22. G. Poirier, V. A. Jerez, C. B. de Araújo, Y. Messaddeq, S. J. L. Ribeiro and M. Poulain, J. Appl. Phys., 2003, 93, 1493–1497.
23. J. Zhao, D. Jin, E. P. Schartner, Y. Lu, Y. Liu, A. V. Zvyagin, L. Zhang, J. M. Dawes, P. Xi, J. A. Piper, E. M. Goldys and T. M. Monro, Nat. Nanotechnol., 2013, 8, 729–734.
24. G. Glaspell, J. Anderson, J. R. Wilkins and M. S. El-Shall, J. Phys. Chem. C, 2008, 112, 11527–11531.
25. L. W. Yang, H. L. Han, Y. Y. Zhang and J. X. Zhong, J. Phys. Chem. C, 2009, 113, 18995–18999.
26. C.-H. Huang, W.-R. Liu and T.-M. Chen, J. Phys. Chem. C, 2010, 114, 18698–18701.
27. J. Capobianco, J. C. Boyer, F. Vetrone, A. Speghini and M. Bettinelli, Chem. Mater., 2002, 14, 2915–2921.
28. A. Patra, P. Ghosh, P. S. Chowdhury, M. A. R. Alencar, W. Lozano, N. Rakov and G. S. Maciel, J. Phys. Chem. B, 2005, 109, 10142–10146.
29. G.-S. Yi and G.-M. Chow, J. Mater. Chem., 2005, 15, 4460–4464.
30. H. Zhang, Y. Li, Y. Lin, Y. Huang and X. Duan, Nanoscale, 2011, 3, 963–966.
31. K. W. Krämer, D. Biner, G. Frei, H. U. Güdel, M. P. Hehlen and S. R. Lüthi, Chem. Mater., 2004, 16, 1244–1251.
32. F. Wang and X. Liu, J. Am. Chem. Soc., 2008, 130, 5642–5643.
33. F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen and X. Liu, Nat. Mater., 2011, 10, 968.
34. F. Vetrone, V. Mahalingam and J. A. Capobianco, Chem. Mater., 2009, 21, 1847–1851.
35. B. Ma, X. Liu, L. Liu, Y. Meng, B. Li, X. Zhao, P. Pu and X. Wang, RSC Adv., 2014, 4, 63100–63104.
36. Y. Li, J. Zhang, Y. Luo, X. Zhang, Z. Hao and X. Wang, J. Mater. Chem., 2011, 21, 2895.
37. B. Shao, Y. Feng, Y. Song, M. Jiao, W. Lü and H. You, Inorg. Chem., 2016, 55, 1912–1919.
38. D. Li, W. Qin, S. Liu, W. Pei, Z. Wang, P. Zhang, L. Wang and L. Huang, J. Alloys Compd., 2015, 653, 304–309.
39. A. A. Ansari, R. Yadav and S. B. Rai, RSC Adv., 2016, 6, 22074–22082.
40. L.-L. Li, R. Zhang, L. Yin, K. Zheng, W. Qin, P. R. Selvin and Y. Lu, Angew. Chem., Int. Ed., 2012, 124, 6225–6229.
41. Q. Chen, C. Wang, L. Cheng, W. He, Z. Cheng and Z. Liu, Biomaterials, 2014, 35, 2915–2923.
42. T. A. A. Assumpção, D. M. da Silva, L. R. P. Kassab and C. B. de Araújo, J. Appl. Phys., 2009, 106, 063522.
43. J.-C. Zhou, Z.-L. Yang, W. Dong, R.-J. Tang, L.-D. Sun and C.-H. Yan, Biomaterials, 2011, 32, 9059–9067.
44. Y. Wei, F. Lu, X. Zhang and D. Chen, J. Alloys Compd., 2007, 427, 333–340.
45. G. Tian, Z. Gu, L. Zhou, W. Yin, X. Liu, L. Yan, S. Jin, W. Ren, G. Xing, S. Li and Y. Zhao, Adv. Mater., 2012, 24, 1226–1231.

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Footnote

† Electronic supplementary information (ESI) available: XRD, SEM, UC emission spectra and CIE. See DOI: 10.1039/c6qi00121a

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