Tunable broadband white upconversion (UC) luminescence has been demonstrated in Yb3+/Tm3+/Mn2+ tri-doped KZnF3 nanocrystals from the excitation of a 976 nm laser diode (LD). The white light is composed of three sharp band peaks at 480, 650 and 700 nm, originating from the UC emissions of Tm3+ ions, and one broad band centered at 585 nm, originating from exchange-coupled Yb3+–Mn2+ dimers. The effects of the concentration, pump power and temperature on the UC luminescence properties of KZnF3:Yb3+,Tm3+,Mn2+ nanocrystals have been investigated. By changing the Mn2+/Tm3+ content ratio, various colors of the UC luminescence can be easily obtained in KZnF3:Yb3+,Tm3+,Mn2+ nanocrystals, which gives these nanocrystals potential applications in the fields of lighting, displays and lasers.
© 2014 Optical Society of America
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 . By doping with several f-f transitions of lanthanide (Ln3+) ions, such as Yb3+/Er3+/Tm3+ [6, 7], Yb3+/Ho3+/Tm3+  or Yb3+/Er3+/Pr3+ , 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 . 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 . However, the broad band UC emission of Mn2+ ion has been barely applied in designing white UC luminescent materials . Most recently, it has been reported that the 4T1g(G) excited state of Mn2+ ion can be used to promote the energy transfer from the 2H9/2 and 4S3/2 levels of Er3+ (1D2 and 1G4 levels of Tm3+), then back-energy transfer to the 4F9/2 level of Er3+ (3F2,3 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 4T1g(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+ . 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 4T1g(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+ , 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 . 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.
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 . 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-Kα 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 , the Mn2+ ions occupy Zn2+ sites in this host lattice. The Yb3+ and Tm3+ ions substituted at both the K+ and Zn2+ sites . 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.
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 |2F7/2,4T1g(G)>→|2F7/2,6A1g(S)> transitions of exchange-coupled Yb3+-Mn2+ dimers . 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 3F2,3→3H6 transitions of Tm3+ ion , 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+ , CaMoO4:Yb3+,Tm3+ , or NaYF4:Yb3+,Tm3+ , in which the blue emission located at 480 nm is much stronger than the red UC emissions.
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 . 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.
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).
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 (Iup) is related to the pump power (P) through the following formula :Fig. 5, the n values are calculated to be approximately 2.98, 1.61, 2.22 and 1.70 for 480, 585, 650 and 700 nm, respectively. These n values reveal that the emissions at 480 and 650 nm belong to three-photon UC processes, whereas the other emissions result from two-photon UC processes. Consequently, the different UC processes of the emission peaks lead to the pump-power tunable white light, as shown in Fig. 4.
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 6A1g(S) to the 4T1g(P), 4Eg(D), 4T2g(D), [4A1g(G),4Eg(G)], 4T2g(G) and 4T1g(G) energy levels , 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 3F2,3→3H6 transitions of Tm3+ ions , respectively. Owing to the excited state 4T1g(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 4T1g(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 . 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:
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.
For the Tm3+ ions, the 3F2,3 energy level is thermally coupled with 3H4; thus, the transition probability from 3H4 to 3F2,3 can be enhanced by increasing the temperature (thermal population processes), resulting in enhancing the UC emission of Tm3+ at 700 nm . 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 :Eq. (4), it is deduced that the higher temperature would lead to the higher nonradiative relaxation rates; thus, the UC emission intensity will decrease with increasing temperature. On the other hand, higher temperatures correspond to higher phonon densities, which promotes the phonon-assisted energy transfer; e.g., the energy transfer possibilities from the 1G4 state of Tm3+ to the |2F7/2,4T1g(G)> state of the Yb3+-Mn2+dimer and from the |2F7/2,4T1g(G)> state of the Yb3+-Mn2+ dimer to the 3F2,3 state of Tm3+ could be simultaneously enhanced. Consequently, the UC emission at 700 nm is increased remarkably when the temperature is increased from 300 to 473 K. As the temperature is increased further, the emission of Tm3+ at 700 nm decreases dramatically due to the serious thermal quenching effect. The temperature dependent UC emission properties of KZnF3:Yb3+,Tm3+ and KZnF3:Yb3+,Tm3+,Mn2+ firmly demonstrate that the UC emission of Tm3+ ions can be selectively enhanced by doping with Mn2+ ions.
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|2F7/2,4T1g(G)> state of the Yb3+-Mn2+ dimers and then to the 3F2,3 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.
This work is financially supported by NSFC (Grant Nos. 51125005, 21101065 and U0934001), Ministry of Education (Grant No. 20100172110012), and the Fundamental Research Funds for the Central Universities, SCUT.
References and links
1. N. Niu, P. P. Yang, F. He, X. Zhang, S. L. Gai, C. X. Li, and J. Lin, “Tunable multicolor and bright white emission of one-dimensional NaLuF4:Yb3+,Ln3+ (Ln = Er, Tm, Ho, Er/Tm, Tm/Ho) microstructures,” J. Mater. Chem. 22(21), 10889–10899 (2012). [CrossRef]
2. J. Yang, C. M. Zhang, C. Peng, C. X. Li, L. L. Wang, R. T. Chai, and J. Lin, “Controllable Red, Green, Blue (RGB) and bright white upconversion luminescence of Lu2O3:Yb3+/Er3+/Tm3+ nanocrystals through single laser excitation at 980 nm,” Chem. Eur. J. 15(18), 4649–4655 (2009). [CrossRef] [PubMed]
3. V. Mahalingam, F. Mangiarini, F. Vetrone, V. Venkatramu, M. Bettinelli, A. Speghini, and J. A. Capobianco, “Bright white upconversion emission from Tm3+/Yb3+/Er3+-doped Lu3Ga5O12 nanocrystals,” J. Phys. Chem. C 112(46), 17745–17749 (2008). [CrossRef]
4. G. Y. Chen, Y. Liu, Y. G. Zhang, G. Somesfalean, Z. G. Zhang, Q. Sun, and F. P. Wang, “Bright white upconversion luminescence in rare-earth-ion-doped Y2O3 nanocrystals,” Appl. Phys. Lett. 91(13), 133103 (2007). [CrossRef]
5. Z. Wang, J. Feng, M. Pang, S. H. Pan, and H. J. Zhang, “Multicolor and bright white upconversion luminescence from rice-shaped lanthanide doped BiPO4 submicron particles,” Dalton Trans. 42(34), 12101–12108 (2013). [CrossRef] [PubMed]
6. D. Q. Chen, Y. S. Wang, K. L. Zheng, T. L. Guo, Y. L. Yu, and P. Huang, “Bright upconversion white light emission in transparent glass ceramic embedding Tm3+/Er3+/Yb3+:β-YF3 nanocrystals,” Appl. Phys. Lett. 91(25), 251903 (2007). [CrossRef]
7. I. Etchart, M. Bérard, M. Laroche, A. Huignard, I. Hernández, W. P. Gillin, R. J. Curry, and A. K. Cheetham, “Efficient white light emission by upconversion in Yb3+-, Er3+- and Tm3+-doped Y2BaZnO5.,” Chem. Commun. (Camb.) 47(22), 6263–6265 (2011). [CrossRef] [PubMed]
8. L. W. Yang, H. L. Han, Y. Y. Zhang, and J. X. Zhong, “White emission by frequency up-conversion in Yb3+-Ho3+-Tm3+ triply doped hexagonal NaYF4 nanorods,” J. Phys. Chem. C 113(44), 18995–18999 (2009). [CrossRef]
9. Y. Dwivedi, A. Rai, and S. B. Rai, “Intense white upconversion emission in Pr/Er/Yb codoped tellurite glass,” J. Appl. Phys. 104(4), 043509 (2008). [CrossRef]
10. F. Wang, D. Banerjee, Y. S. Liu, X. Y. Chen, and X. G. Liu, “Upconversion nanoparticles in biological labeling, imaging, and therapy,” Analyst (Lond.) 135(8), 1839–1854 (2010). [CrossRef] [PubMed]
12. J. F. Suyver, A. Aebischer, D. Biner, P. Gerner, J. Grimm, S. Heer, K. W. Kra Mer, C. Reinhard, and H. U. Güdel, “Novel materials doped with trivalent lanthanides and transition metal ions showing near-infrared to visible photon upconversion,” Opt. Mater. 27(6), 1111–1130 (2005). [CrossRef]
14. J. Wang, F. Wang, C. Wang, Z. Liu, X. G. Liu, and Angew, “Single‐band upconversion emission in lanthanide‐doped KMnF3 nanocrystals,” Angew. Chem. Int. Ed. 50(44), 10369–10372 (2011). [CrossRef]
15. R. Chen, V. D. Ta, F. Xiao, Q. Y. Zhang, and H. D. Sun, “Multicolor hybrid upconversion nanoparticles and their improved performance as luminescence temperature sensors due to energy transfer,” Small 9(7), 1052–1057 (2013). [CrossRef] [PubMed]
18. S. F. Zhou, N. Jiang, B. Zhu, H. C. Yang, S. Ye, G. Lakshminarayana, J. H. Hao, and J. R. Qiu, “Multifunctional bismuth‐doped nanoporous silica glass: from blue‐green, orange, red, and white light sources to ultra‐broadband infrared amplifiers,” Adv. Funct. Mater. 18(9), 1407–1413 (2008). [CrossRef]
19. R. Martín-Rodríguez, R. Valiente, F. Rodríguez, F. Piccinelli, A. Speghini, and M. Bettinelli, “Temperature dependence and temporal dynamics of Mn2+ upconversion luminescence sensitized by Yb3+ in codoped LaMgAl11O19,” Phys. Rev. B 82(7), 075117 (2010). [CrossRef]
20. R. Valiente, O. S. Wenger, and H. U. Güdel, “Near-infrared-to-visible photon upconversion process induced by exchange interactions in Yb3+-doped RbMnCl3,” Phys. Rev. B 63(16), 165102 (2001). [CrossRef]
21. P. Gerner, O. S. Wenger, R. Valiente, and H. U. Güdel, “Green and Red Light Emission by Upconversion from the near-IR in Yb3+ Doped CsMnBr3.,” Inorg. Chem. 40(18), 4534–4542 (2001). [CrossRef] [PubMed]
22. S. Ye, Y. J. Li, D. C. Yu, G. P. Dong, and Q. Y. Zhang, “Room-temperature upconverted white light from GdMgB5O10:Yb3+,Mn2+,” J. Mater. Chem. 21(11), 3735–3739 (2011). [CrossRef]
23. G. Tian, Z. J. Gu, L. J. Zhou, W. Y. Yin, X. X. Liu, L. Yan, S. Jin, W. L. Ren, G. M. Xing, S. J. Li, and Y. L. Zhao, “Mn2+ dopant-controlled synthesis of NaYF4:Yb/Er upconversion nanoparticles for in vivo imaging and drug delivery,” Adv. Mater. 24(9), 1226–1231 (2012). [CrossRef] [PubMed]
24. Z. N. Wu, M. Lin, S. Liang, Y. Liu, H. Zhang, and B. Yang, “Hot-injection synthesis of manganese-ion-doped NaYF4: Yb,Er nanocrystals with red up-convertingemission and tunable Diameter,” Part. Part. Syst. Charact. 30(4), 311–315 (2013). [CrossRef]
25. Y. Zhang, J. D. Lin, V. Vijayaragavan, K. K. Bhakoo, and T. T. Y. Tan, “Tuning sub-10 nm single-phase NaMnF3 nanocrystals as ultrasensitive hosts for pure intense fluorescence and excellent T1 magnetic resonance imaging,” Chem. Commun. (Camb.) 48(83), 10322–10324 (2012). [CrossRef] [PubMed]
26. Z. P. Li, B. Dong, Y. Y. He, B. S. Cao, and Z. Q. Feng, “Selective enhancement of green upconversion emissions of Er3+:Yb3Al5O12 nanocrystals by high excited state energy transfer with Yb3+-Mn2+ dimer sensitizing,” J. Lumin. 132(7), 1646–1648 (2012). [CrossRef]
27. E. H. Song, S. Ding, M. Wu, S. Ye, F. Xiao, G. P. Dong, and Q. Y. Zhang, “Temperature-tunable upconversion luminescence of perovskite nanocrystals KZnF3:Yb3+,Mn2+,” J. Mater. Chem. C 1(27), 4209–4215 (2013). [CrossRef]
28. F. Wang, Y. Han, C. S. Lim, Y. H. Lu, J. Wang, J. Xu, H. Y. Chen, C. Zhang, M. H. Hong, and X. G. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature 463(7284), 1061–1065 (2010). [CrossRef] [PubMed]
29. R. D. Shannon, “Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides,”Acta Cryst. A 32(5), 751–767 (1976). [CrossRef]
30. A. A. Antipin, A. V. Vinokurov, M. P. Davydova, S. L. Korableva, A. L. Stolov, and A. A. Fedii, “Optical spectra, EPR, and spin–lattice relaxation of Yb3+ ions in crystals having perovskite‐type structure,” Phys. Status Solidi B 81(1), 287–293 (1977). [CrossRef]
31. J. H. Chung, J. H. Ryu, S. W. Mhin, K. M. Kim, and K. B. Shim, “Controllable white upconversion luminescence in Ho3+/Tm3+/Yb3+ co-doped CaMoO4,” J. Mater. Chem. 22(9), 3997–4002 (2012). [CrossRef]
32. F. Wang and X. G. Liu, “Upconversion multicolor fine-tuning: visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles,” J. Am. Chem. Soc. 130(17), 5642–5643 (2008). [CrossRef] [PubMed]
33. U. Kambli and H. U. Güdel, “Transfer of electronic excitation energy in the antiferromagets RbMnCl3, CsMnCl3, CsMnBr3, and RbMnCl4,” Inorg. Chem. 23(22), 3479–3486 (1984). [CrossRef]
34. M. Pollnau, D. R. Gamelin, S. R. Lüthi, H. U. Güdel, and M. P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61(5), 3337–3346 (2000). [CrossRef]
35. F. Rodríguez and M. Moreno, “Thermal expansion around an impurity: study of KZnF3:Mn2+,” J. Phys. C Solid State Phys. 19(23), L513–L517 (1986). [CrossRef]
36. S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma, and Q. Y. Zhang, “Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties,” Mater. Sci. Eng. Rep. 71(1), 1–34 (2010). [CrossRef]
37. S. A. Wade, S. F. Collins, and G. W. Baxter, “Fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys. 94(8), 4743–4756 (2003). [CrossRef]
38. K. Z. Zheng, D. Zhao, D. Zhang, N. Liu, and W. P. Qin, “Temperature-dependent six-photon upconversion fluorescence of Er3+,” J. Fluor. Chem. 132(1), 5–8 (2011). [CrossRef]