1. Introduction

White upconversion (UC) luminescence materials have attracted increasing attention recently due to their superior photo-stability and chemical durability as well as their potential applications in optical devices, color displays and high-power continuous wave lasers [1–4]. Compared to the traditional white light obtained by UV or blue light excitation, UC white light is achieved from the excitation of a near-infrared (NIR) diode laser, which is more compact, powerful, inexpensive, and commercially available than other forms of excitation [5]. By doping with several f-f transitions of lanthanide (Ln³⁺) ions, such as Yb³⁺/Er³⁺/Tm³⁺ [6, 7], Yb³⁺/Ho³⁺/Tm³⁺ [8] or Yb³⁺/Er³⁺/Pr³⁺ [9], into a single host, the white UC luminescence consisting of blue, green and red (RGB) emissions can be obtained, which is the most popular strategy for producing white UC luminescence from near-infrared excitation. Usually, blue UC emission originates from Tm³⁺ or Pr³⁺, whereas green and red UC emissions come from Er³⁺ or Ho³⁺ ions [10, 11]. The Yb³⁺ ion acts as a sensitizer and conspicuously increases the optical pump efficiency, which can be ascribed to the high absorption cross section of the Yb³⁺ ion at approximately 980 nm and the efficient energy transfer from the Yb³⁺ ion to the others [11]. Nevertheless, owing to the narrow and fixed emission spectra of Ln³⁺ ions [11–14], there are obvious differences between the white UC luminescence based on Ln³⁺ ions and natural white light. Accordingly, these differences inevitably limit further practical applications [12, 15–17].

The transition metal ion Mn²⁺ compared with Ln³⁺ ions exhibits a broad emission band [18–20]. Moreover, because the emission of Mn²⁺ is ligand-field dependent, whereas the emission of Yb³⁺ is independent of the ligand field, tunable broadband UC luminescence can be achieved in some Yb³⁺/Mn²⁺ co-doped systems by near-infrared (NIR) laser excitation at approximately 980 nm laser [21]. However, the broad band UC emission of Mn²⁺ ion has been barely applied in designing white UC luminescent materials [22]. Most recently, it has been reported that the ⁴T_1g(G) excited state of Mn²⁺ ion can be used to promote the energy transfer from the ²H_9/2 and ⁴S_3/2 levels of Er³⁺ (¹D₂ and ¹G₄ levels of Tm³⁺), then back-energy transfer to the ⁴F_9/2 level of Er³⁺ (³F_2,3 level of Tm³⁺), thus realizing the tuning of pure red or NIR emission [23–25]. In addition, since the energy position of the lowest excited state ⁴T_1g(G) of Mn²⁺ ions is host-dependent, the green UC emission of Er³⁺ can also be selectively enhanced via doping Mn²⁺ ions in suitable host, for example, Yb₃Al₅O₁₂:Er³⁺,Mn²⁺ [26]. However, the broad band UC emission of Mn²⁺ ions has not been observed in these systems though part of the energy absorbed by Yb³⁺ or other rare earth ions has been transferred to the emitting state ⁴T_1g(G) of Mn²⁺ ions. To further investigate the energy transfer between Mn²⁺ and rare earth ions and get white UC luminescence, we investigate the UC emission of KZnF₃:Tm³⁺,Yb³⁺,Mn²⁺ perovskite nanocrystals. The perovskite host KZnF₃ is chosen owing to the fact that the Mn²⁺ UC emission (centered at 585 nm) can be achieved in KZnF₃:Yb³⁺,Mn²⁺ [27], which would benefit the discussion of the theme. Furthermore, due to the low phonon energy of fluoride host, the Tm³⁺/Yb³⁺codoped KZnF₃ would show efficient UC luminescence [28]. It is found that the tunable white light UC luminescence can be easily realized in this system by changing the Mn²⁺/Tm³⁺ ions content ratio. The influence of concentration, pump power and temperature on light emitting color and energy transfer between Mn²⁺ and Tm³⁺ have been investigated. The present work not only provides a new strategy to get white UC luminescence but also gives new insights on energy transfer between Mn²⁺ and Tm³⁺ ions.

2. Experimental

The series of KZnF₃:1 mol%Yb³⁺,x%Tm³⁺,y%Mn²⁺ (x = 0-0.5 mol, y = 0-20 mol) samples were synthesized by a simple hydrothermal method using oleic acid as a stabilizing agent according to reference [27]. All of the chemical reagents were used as received without further purification. The reagents used in the experiment were Yb₂O₃ (99.998%), Tm₂O₃ (99.99%), C₄H₆MnO₄·4H₂O (AR), Zn(CH₃COO)₂·2H₂O (99.99%), KOH (AR), KF (AR) and oleic acid (AR). Yb₂O₃ and Tm₂O₃ were supplied by the Alfa Aesar Reagent Company, and the other reagents were supplied by the Aladdin Industrial Corporation. In a typical synthesis, 1.5 mmol of KOH, 2 mL of distilled water, 15 mL of ethanol and 5 mL of oleic acid were mixed together under magnetic stirring to form a homogeneous solution. Next, 5 mL of an aqueous solution containing stoichiometric amounts of Zn(CH₃COO)₂·2H₂O, C₄H₆MnO₄·4H₂O,Tm(NO₃)₃ and Yb(NO₃)₃ were added to the solution under vigorous stirring. After 5 minutes, 5 mL of an aqueous solution containing 12 mmol KF were added to the complex under vigorous stirring. The mixture was agitated for 30 min and then transferred into a 50 mL autoclave, sealed and hydrothermally treated at 180 °C for 8 hours. After the reaction, the system was naturally cooled to room temperature; the products were collected and centrifuged several times with distilled water and absolute ethanol to remove the residual remnant substances and were finally dried at 60 °C for several hours.

The crystal structure of the products was characterized by a Philips Model PW1830 X-ray powder diffractometer with Cu-K_α radiation (λ = 1.5406 Å) at a tube voltage of 40 kV and a tube current of 40 mA. The size and shape of the samples were measured by a transmission electron microscope (TEM, FEI Tecnai G2 F30). The UC emission spectra were measured on a TRIAX320 fluorescence spectrofluorometer (Jobin-Yvon Co., France) equipped with a R928 photomultiplier tube as the detector and a 976 nm laser diode (LD, Coherent Corp.) as excitation source. Before formally measuring the UC emission spectra, the spectrofluorometer has been corrected for the wavelength system response using a standard luminescent sample. The temperature-dependent UC emission spectra were measured by the same spectrofluorimeter equipped with a TAP-02 high-temperature fluorescence instrument (Tian Jin Orient–KOJI instrument Co., Ltd.). The excitation and emission spectra were measured by a FSL920 spectrophotometer (Edinburgh Instruments Ltd.).

3. Results and discussions

3.1 XRD characterization

Figure 1(a) shows the X-ray powder diffraction pattern of the KZnF₃:1%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺ sample. The diffraction peaks are all in agreement with the Joint Committee on Powder Diffraction Standards Card No 06-0439 (JCPDS No 06-0439), indicating that the obtained samples have an identical crystal structure to that of cubic perovskite KZnF₃. No extra phases or significant changes are detected, which clearly suggests that Yb³⁺, Tm³⁺ and Mn²⁺ ions have been successfully incorporated into the host lattice. As the radius of Mn²⁺ (r = 0.80 Å) is similar to that of Zn²⁺ (r = 0.74 Å) ion [29], the Mn²⁺ ions occupy Zn²⁺ sites in this host lattice. The Yb³⁺ and Tm³⁺ ions substituted at both the K⁺ and Zn²⁺ sites [30]. Figure 1(b) provides the TEM image of KZnF₃:1%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺. The nanocrystals have a quasi-cube shape with an average diameter of approximately 70 nm. The HR-TEM image shown in Fig. 1(c) reveals that the highly crystalline nature, structural uniformity and interplanar distance (d = 0.405 nm) of the nanocrystals match well with the (100) lattice planes of cubic pervoskite KZnF₃. The selected-area electron diffraction (SAED) patterns shown in Fig. 1(d) demonstrate the single crystalline nature of the nanocrystal, which can be readily indexed as cubic phase KZnF₃.

 

Fig. 1 XRD pattern (a), TEM image (b), HR-TEM image (c) and Selected area electron diffraction (SAED) patterns (d) of KZnF₃:1%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺.

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3.2 Luminescence properties

Figure 2(a) shows the UC emission spectra of KZnF₃:1%Yb³⁺,2.5%Mn²⁺ under the excitation of a 976 nm LD. The UC emission spectrum of KZnF₃:1%Yb³⁺,2.5%Mn²⁺ consists of a broad emission band centered at 585 nm, which can be ascribed to the |²F_7/2,⁴T_1g(G)>→|²F_7/2,⁶A_1g(S)> transitions of exchange-coupled Yb³⁺-Mn²⁺ dimers [20]. The full width at the half maximum (FWHM) of the UC emission band is calculated to be approximately 58 nm. In contrast to the single broadband UC emission of KZnF₃:1%Yb³⁺,2.5%Mn²⁺, multiple sharp emissions characteristics have been observed in KZnF₃:1%Yb³⁺,0.1%Tm³⁺. As shown in Fig. 2(b), these typical sharp emission peaks at 480, 650 and 700 nm can be ascribed to the ¹G₄→³H₆, ¹G₄→³F₄, and ³F_2,3→³H₆ transitions of Tm³⁺ ion [11], respectively. Notably, in KZnF₃:1%Yb³⁺,0.1%Tm³⁺, the red UC emissions located at 650 and 700 nm are relative strong and only slightly weaker than the blue emission (480 nm), suggesting that this is a potentially interesting material for displays and luminescent devices where the strong red emissions are required. Additionally, this spectrum is quite different from other Tm³⁺/Yb³⁺co-dopedsystems reported in the literature, such as BiPO₄:Yb³⁺,Tm³⁺ [5], CaMoO₄:Yb³⁺,Tm³⁺ [31], or NaYF₄:Yb³⁺,Tm³⁺ [32], in which the blue emission located at 480 nm is much stronger than the red UC emissions.

 

Fig. 2 The UC emission spectra of KZnF₃:1%Yb³⁺,2.5%Mn²⁺ (a), and KZnF₃:1%Yb³⁺, 0.1%Tm³⁺ (b).

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Based on the above results, a series of KZnF₃:1%Yb³⁺,x%Tm³⁺,y%Mn²⁺ (x = 0-0.5; y = 0-20) samples were synthesized. Figure 3(a) shows the UC emission spectra of KZnF₃:1%Yb³⁺, x%Tm³⁺,2.5%Mn²⁺ samples with x varying from 0 to 0.5. As the Tm³⁺ content increases, the emission intensities of the Tm³⁺ ions and the Yb³⁺-Mn²⁺ dimers decrease monotonically. When the concentration of Tm³⁺ rises to 0.5%, the UC emissions of the Tm³⁺ ions disappears, and only the emission of the Yb³⁺-Mn²⁺ dimers can be detected, implying the serious concentration quenching effect of Tm³⁺ in KZnF₃. The decrease in UC emission of the Yb³⁺-Mn²⁺ dimers may be ascribed to the energy transfer of Yb³⁺-Mn²⁺ dimer →Tm³⁺. Figure 3(b) gives the UC emission spectra of KZnF₃:1%Yb³⁺,0.1%Tm³⁺,y%Mn²⁺ samples with varying Mn²⁺ content. With the doping concentration of Mn²⁺ increases from 1 to 20%, the emission intensities of Tm³⁺ and the Yb³⁺-Mn²⁺dimers are significantly changed. Figure 3(c) shows the UC emission intensity of the emissions at approximately 480, 585, 650 and 700 nm as a function of the Mn²⁺ content (y).The emission intensities at 480 and 650 nm gradually decrease with the increasing Mn²⁺ content, whereas the emission intensities at 585 and 700 nm increase initially and reach their maximums at y = 2.5 and 5%, respectively. As the value of y further increases, the intensities of the emissions decrease sharply due to the serious nonradiative transition processes. When the doping concentration of Mn²⁺ reaches to 20%, the broad band Mn²⁺ UC emission fully is quenched. While the emissions of Tm³⁺ can be detected. The facts demonstrate the occurrence of energy transfer between Tm³⁺ ions and Yb³⁺-Mn²⁺ dimers. Additionally, the energy transfer from Yb³⁺-Mn²⁺ dimers or Mn²⁺ ions to Tm³⁺ ions is more efficient than that from Tm³⁺ ions to Yb³⁺-Mn²⁺ dimers or Mn²⁺ ions. These nonradiative processes in concentrated Mn²⁺ compounds and effect of rare earth doping in Mn compounds have been discussed by U. Kambli et al [33]. Due to the variation of the relative intensities of the 480, 585, 650 and 700 nm, the CIE chromaticity coordinates of the systems are greatly changed. Table 1 summarizes the CIE chromaticity coordinates of the KZnF₃: 1%Yb³⁺,1%Tm³⁺,y%Mn²⁺ (y = 0, 1, 2.5, 5 and 10) and KZnF₃:1%Yb³⁺,2.5%Mn²⁺ samples based on the emission spectra of the samples, as shown in Fig. 3(d). The chromaticity coordinates for KZnF₃:1%Yb³⁺,0.1%Tm³⁺ and KZnF₃:1%Yb³⁺,2.5%Mn²⁺ are calculated to be (0.198, 0.142) and (0.522,474), falling within the blue (point 1) and yellow (point 6) regions, respectively. For KZnF₃:1%Yb³⁺,1%Tm³⁺,y%Mn²⁺ samples, as the concentration of Mn²⁺ increases from 1 to 10%, the CIE chromatic coordinates of the samples are tuned from (0.285, 0.243) to (0.443, 0.388), corresponding to white and yellow hues, respectively. It is worth noting that a white UC luminescence has been obtained in KZnF₃:1%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺ (point 2) and KZnF₃:1%Yb³⁺,0.1%Tm³⁺,5%Mn²⁺ (point 3). These results indicate that the emission color of KZnF₃:1%Yb³⁺,x%Tm³⁺,y%Mn²⁺ phosphors can be easily tuned by changing the Tm³⁺/Mn²⁺content ratio depending on the demands.

 

Fig. 3 The UC emission spectra of KZnF₃:1%Yb³⁺,x%Tm³⁺,2.5%Mn²⁺(x = 0, 0.1, 0.2 and 0.5) (a), and KZnF₃:1%Yb³⁺,0.1%Tm³⁺,y%Mn²⁺ (y = 0, 1, 2.5, 5, 10 and 20) (b). UC emission intensity of KZnF₃:1%Yb³⁺,0.1%Tm³⁺,y%Mn²⁺ as function of Mn²⁺ content (c) and CIE chromaticity coordinates (d) of KZnF₃:1%Yb³⁺,x%Tm³⁺,y%Mn²⁺ (1-5: for x = 0.1, y = 0, 1, 5.0, 2.5 and 10, respectively; 6: for x = 0, y = 2.5).

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Table 1. CIE chromaticity coordinates for KZnF₃:1%Yb³⁺,x%Tm³⁺,y%Mn²⁺ under 976 nm LD excitation.

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Figure 4(a) shows the UC emission spectra and the corresponding CIE chromaticity coordinates of KZnF₃:1.0%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺ with varying pump-power density. It can be seen that all the emission spectra consist of three sharp emission bands (located at 480, 650 and 700 nm) and one broadband emission band (centered at 585 nm), corresponding to the Tm³⁺ ions and Yb³⁺-Mn²⁺ dimers, respectively. As the pump-power density gradually increases, the peak positions of all emission bands exhibit no changes, whereas the emission intensity has been greatly enhanced. Due to the blue emission increased faster than of the yellow and red emissions with increasing the pump-power density, and a color-point-tunable white light is obtained. Figure 4(b) displays the CIE chromaticity coordinates of KZnF₃:1%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺ with various pump-power densities. By changing the pump-power from 370 to 1500 mW (20.9 to 84.9 W/cm²), the CIE chromaticity coordinates can be tuned from (0.335, 0.304) to (0.285, 0.243), showing a fine-tuning emission property. Particularly, when the pump power is varied to 370 mW (20.9 W/cm²), the chromaticity coordinates (0.335, 0.304) are very close to those of the standard white point (0.33, 0.33).

 

Fig. 4 Pump-power dependence of UC emission spectra (a) and CIE chromaticity coordinates (b) of KZnF₃:1%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺.

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To clarify the UC mechanism and the dependence of the color coordinates on the pump power, the pump-power-dependent UC behavior of all emission bands are investigated. As is well known, in the UC processes, the UC luminescence intensity (I_up) is related to the pump power (P) through the following formula [34]:

 

Fig. 5 Double-logarithmic plots of the pump-power dependent UC emission intensity of the KZnF₃:1%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺ sample.

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Figure 6(a) shows the excitation spectrum of KZnF₃:1%Yb³⁺,2.5%Mn²⁺ monitored at 585 nm and the UC emission spectrum of KZnF₃:1%Yb³⁺,0.1%Tm³⁺ under a 976 nm laser beam excitation. It can be observed that the excitation spectrum is composed of six sub-bands located at 306, 333, 353, 396, 435 and 532 nm, corresponding to the transitions of Mn²⁺ from the ground state ⁶A_1g(S) to the ⁴T_1g(P), ⁴E_g(D), ⁴T_2g(D), [⁴A_1g(G),⁴E_g(G)], ⁴T_2g(G) and ⁴T_1g(G) energy levels [35], respectively. Meanwhile, the emission spectrum of KZnF₃: 1%Yb³⁺, 0.1%Tm³⁺ consists of three sharp emission bands centered at 480, 650 and 700 nm, originating from the ¹G₄→³H₆, ¹G₄→³F₄ and ³F_2,3→³H₆ transitions of Tm³⁺ ions [11], respectively. Owing to the excited state ⁴T_1g(G) of Mn²⁺ is intermediate between that of the ¹G₄ excited state and the ³F₂,₃ excited state of Tm³⁺, the energy transfer between Mn²⁺ and Tm³⁺ ions can be expected in Yb³⁺/Tm³⁺/Mn²⁺ tri-doped KZnF₃, in which the energy is transferred from the ¹G₄ state of Tm³⁺ to the ⁴T_1g(G) state of Mn²⁺ and then back-transfer to the ³F₂,₃ state of Tm³⁺. However, as the excitation spectrum and UC emission spectrum have no spectral overlap, the resonance bi-directional energy transfer mechanism between the Mn²⁺ and Tm³⁺ ions could be ruled out. The energy transfer mechanism between the Mn²⁺ and Tm³⁺ ions should be attributed to the phonon-assisted energy transfer mechanism [36]. In addition, since the Yb³⁺-Mn²⁺ dimer has a similar excitation spectrum to that of Mn²⁺ ion, it is deduced that the phonon-assisted bi-directional energy transfer between Tm³⁺ and exchange-coupled Yb³⁺-Mn²⁺ dimers also exist in this system, which has been demonstrated in Fig. 3. Accordingly, a possible energy transfer mechanism of the Yb³⁺/Tm³⁺/Mn²⁺ tri-doped system is proposed, as shown in Fig. 6(b). The possible energy transfer processes are shown as follows:

 

Fig. 6 Excitation spectrum of KZnF₃:1%Yb³⁺,2.5%Mn²⁺ (monitored at 585 nm) and UC emission spectrum (excited by 976 nm LD) of KZnF₃:1%Yb³⁺,0.1%Tm³⁺ (a), UC energy levels diagram of Yb³⁺/Tm³⁺/Mn²⁺ tri-doped KZnF₃ (b).

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3.3 Temperature-dependent UC luminescence properties

For luminescent materials, the thermal quenching property is a crucial parameter for practical applications as well as for fundamental studies because this property has a large influence on the luminescence properties. Based on these, Fig. 7(a) gives the temperature-dependent UC emission spectra of KZnF₃:1%Yb³⁺,0.1%Tm³⁺ under the excitation of a 976 nm LD. As the temperature increases, the peak positions of all emission bands (480, 650 and 700 nm) exhibit no significant changes, whereas the UC emission intensities decrease monotonously. When the temperature reaches to 573 K, the emissions at 480 and 650 nm are almost fully quenched, whereas the 700 nm emission can be detected, indicating that the thermal quenching of three-photon UC processes is more serious than that of the two-photon process. Figure 7(b) shows the corresponding UC emission intensities of all emission bands (480, 650 and 700 nm) as a function of temperature. The emission bands at 480 and 650 nm show a similar quenched behavior, which is owing to the fact that both emissions originate from the ¹G₄ state of Tm³⁺. While, for the Yb³⁺/Tm³⁺/Mn²⁺ tri-doped KZnF₃ nanocrystals, the thermal quenching behavior of the emission bands are obviously changed. Figure 7(c) displays the UC emission spectra of KZnF₃:1%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺ at different temperatures from 300 to 573 K. It can be observed that the emissions at 480, 585 and 650 nm exhibit monotonically decreasing behaviors with increasing temperature, while the UC emission intensity at 700 nm increases first and reaches a maximum intensity at 473 K, as shown in Fig. 7(d).The facts indicate that the thermal quenching behavior of 700 nm from Tm³⁺ has been remarkably changed by codoping with Mn²⁺ ions.

 

Fig. 7 The UC emission spectra of KZnF₃:1%Yb³⁺,0.1%Tm³⁺ (a) and KZnF₃:1%Yb³⁺, 0.1%Tm³⁺,1%Mn²⁺ (c) at various temperatures, and UC emission intensity of the KZnF₃:1%Yb³⁺,0.1%Tm³⁺ (b) and KZnF₃:1%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺ samples of as a function of temperature (d).

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For the Tm³⁺ ions, the ³F_2,3 energy level is thermally coupled with ³H₄; thus, the transition probability from ³H₄ to ³F_2,3 can be enhanced by increasing the temperature (thermal population processes), resulting in enhancing the UC emission of Tm³⁺ at 700 nm [37]. However, according to Fig. 7(a), the emission peaked at 700 nm in KZnF₃:1%Yb³⁺,0.1%Tm³⁺ monotonously decreases with the increasing temperature, implying that the thermal population processes are much weaker than the nonradiative transition processes in this system. Therefore, the UC emission intensity enhancement of 700 nm in KZnF₃:1%Yb³⁺,0.1%Tm³⁺,1%Mn²⁺ should be attributed to the bi-directional energy transfer between the Tm³⁺ ions and Yb³⁺-Mn²⁺ dimers (or Mn²⁺ ions). As is well known, the luminescence thermal quenching can be ascribed to nonradiative relaxation processes, and the nonradiative relaxation rate is dominated by the following simplistic equation [38]:

4. Conclusions

In summary, KZnF₃:Yb³⁺,Tm³⁺,Mn²⁺ nanocrystals have been successfully synthesized by a facile solvothermal method. Under 976 nm laser excitation, concentration- and pump-power-tunable room-temperature white UC luminescence has been demonstrated due to the blue (480 nm) and red emissions (650 and 700 nm) of Tm³⁺ ions and the yellow emission (585 nm) of Yb³⁺-Mn²⁺ dimers. The white color point of KZnF₃:Yb³⁺,Tm³⁺,Mn²⁺ can be easily tuned by varying the pump power of the laser or the Mn²⁺/Tm³⁺ content ratio. The energy transfer phenomena from the ¹G₄ state of Tm³⁺ ions to the|²F_7/2,⁴T_1g(G)> state of the Yb³⁺-Mn²⁺ dimers and then to the ³F_2,3 state of Tm³⁺ ions have been investigated and discussed in detail via concentration- and temperature-dependent UC emission spectra. The white-luminescent KZnF₃:Yb³⁺,Tm³⁺,Mn²⁺ nanocrystals may have potential applications in the fields of lighting, displays, lasers and photonics.