Triple-doped KMnF3:Yb3+/Er3+/Tm3+ nanocubes: four-color upconversion emissions with strong red and near-infrared bands

Triple-doped (Yb3+/Er3+/Tm3+) KMnF3 nanocubes with uniform sizes of 250 nm were synthesized by a facile hydrothermal route using the oleic acid as the capping agent. It was found that these nanocubes can simultaneously exhibited four-color (blue, green, red and NIR) upconversion emissions under a single 980 nm near-infrared (NIR) laser excitation, which should have potential multicolor in vivo imaging applications. Specifically, the red (660 nm) and NIR (800 nm) peaks, known as two “optical windows” for imaging biological tissues, were strong. The spectral and pump analyses indicated the two-photon processes were responsible for the both red and NIR emissions.

Scientific RepoRts | 5:17088 | DOI: 10.1038/srep17088 which are determined by the nature of light color itself are difficult, and thus it limits the practical applications of multicolor in vivo imaging.
Due to multicolor emission characteristics of UCNPs, simultaneous detection of multiple analytes or optical probes in a complex sample should be feasible if appropriate UCNPs were prepared and used. Ideally, in vivo multicolor tissue characterization 21 relies on: (1) the identification of multiple targets; (2) target-specific optical probes with distinct fluorescent properties; and (3) effective real-time multicolor optical cameras that permit accurate unmixing of different fluorescent probes with a single NIR excitation in vivo. The present rare-earth doped nanocrystals are generally not suitable for multiplexing biodetection, due to their limited number of colors. It is therefore necessary to develop UCNPs with multicolor fluorescence emissions under NIR excitation at the same wavelength. Along this line, several recent studies focused on multicolor UC emission with a more boarder spectrum of color output by using different host/activator combinations. Rantanen et al. 22 demonstrated simultaneous detection of two analytes using UC donors with multipeak emission characteristics. Nann et al. 23 reported the preparation of complex colloidal UCNPs systems and observation of the four-color (blue, green, red and NIR) emissions. They synthesized four different types of UCNPs by doping NaYbF 4 with different rare-earth ions, and thus obtain a four-color UC emission system for the potential multiplexing analysis by mixing these UCNPs. Jiang et al. 24 prepared core-shell structured nanoparticles with UCNP core and dye-doped silica shell to enable multi-color emissions for multiplex bioassays.
Herein, we developed a facile hydrothermal strategy to obtain multicolor emissions by preparing tri-doped KMnF 3 :Yb 3+ /Er 3+ /Tm 3+ . The as-prepared UCNPs exhibit four-color (blue, green, red and NIR) UC emissions upon a single excitation at 980 nm, which should have a potential use in multicolor in vivo imaging for simultaneously providing multi-color excitation lights with deep imaging depth. In addition, UCNPs are a promising candidate to harvest NIR sunlight and improve the power conversion efficiency of solar cells, i.e., dye sensitized solar cell (DSSC) 25 . As some DSSC are designed based on the simultaneous adsorption of different dyes which have different absorption bands, the developed UCNPs with multi-color emission bands matching the absorption bands of dyes may have a potential to improve overall absorption efficiency 12 . Figure 1a is a typical SEM image of as-prepared KMnF 3 :20%Yb 3+ /2%Er 3+ /2%Tm 3+ UCNPs and Fig. 1b shows the average size distribution of the samples corresponding to those in Fig. 1a. It can be seen that the UCNPs are well dispersed and exhibit uniform nanocube shape with an average size of 250 nm. The crystal structures and the phase purity of the as-prepared tri-doped KMnF 3 nanocubes were examined by the X-ray diffraction (XRD) analysis (see Fig. 1c). All peaks are sharp and match well with the standard JCPDS No.17-0116 of KMnF 3 , indicating high phase purity and crystallinity of obtained samples.
Secondly, UC emission intensity (I) was further measured as a function of laser power (P) (Fig. 3) to explore the UC mechanism of Yb 3+ , Tm 3+ , and Er 3+ ions in KMnF 3 matrix. Because I UC ∝ P n holds for the unsaturated UC process, where n is the number of pump photons absorbed per upconverted photons emitted 27 , the value of n can thus be determined to be the slope after linearly fitting the I-P data in a double logarithmic plot. For the tri-doped KMnF 3 :Yb 3+ /Er 3+ /Tm 3+ sample, the obtained n values are 2.94, 1.95, 1.92, and 1.99 respectively for the UC emission peaks at 476 nm (blue), 540 nm (green), 660 nm (red), and 800 nm (NIR). Therefore, it can be deduced that the three-photon process is responsible for blue UC emission, two-photon process is responsible for green red and 800 nm UC emissions.
At last, the overall UC emission mechanism and population process in rare-earth doped KMnF 3 is schematically illustrated in Fig. 4. Upon excitation at 980 nm, the red UC emission (660 nm) can be ascribed to nonradiative energy transfer from the 4 S 3/2 levels of Er 3+ to the 4 T 1 level of Mn 2+ , followed by the falling-back transition to the 4 F 9/2 level of Er 3+ and the 4 F 9/2 to 4 I 15/2 transition.
It would be interesting to have a closer look at the role of Mn 2+ played in the multi-photon excited mechanism, based on the literature findings, for both double-doped KMnF 3 :Yb/Er system and triple-doped KMnF 3 :Yb/Er/Tm system. For the simpler double-doped KMnF 3 :Yb/Er system, it is accepted that Mn 2+ ions play the important role in the single-band UC emission (the complete disappearance of 540 nm green emission and appearance of only 660 nm red emission). According to the literature, close proximity Scientific RepoRts | 5:17088 | DOI: 10.1038/srep17088 and excellent overlap of energy levels of the Mn 2+ and Er 3+ ions in the host lattices cause very efficient nonradiative energy transfer from the 2 H 11/2 and 4 S 3/2 levels of Er 3+ to the 4 T 1 level of Mn 2+ 16,28 . And this nonradiative energy transfer process is followed by the back-energy transfer to the 4 F 9/2 level of Er 3+ , thus leading to only 660 nm red emission. The mechanism is illustrated in the right part of Fig. 4 where only three Yb 3+ , Mn 2+ and Er 3+ ions are involved. On the other hand, for the more complex triple-doped KMnF 3 :Yb/Er/Tm system, as illustrated in Fig. 4 where all four Yb 3+ , Mn 2+ , Er 3+ and Tm 3+  ions are involved, reappearance of 540 nm green emission is due to the additional resonant cross relaxation process between Er 3+ and Tm 3+ ions: 3 F 4 (Tm 3+ ) + 4 F 9/2 (Er 3+ ) → 1 G 4 (Tm 3+ ) + 4 I 15/2 (Er 3+ ). This process causes the population of 1 G 4 state of Tm 3+ ions and depopulation of 4 F 9/2 state of Er 3+ ions. Because the energy level of 1 G 4 state (Tm 3+ ) equals to that of 4 F 7/2 , photons loose fraction of energy in 4 F 7/2 (Er 3+ ) and drop to 2 H 11/2 / 4 S 3/2 (Er 3+ ) state through the multiphonon assisted relaxations, and finally leading to 540 nm green emission.
For the blue (476 nm) and NIR (800 nm) emissions, the energy transfer from the first Yb 3+ → Tm 3+ excites the 3 H 6 → 3 H 5 transition, at the same time the redundant energy dissipated by phonons. Then, the Tm 3+ ion is firstly relaxes to the lower 3 F 4 state and further promoted to the 3 F 2 , 3 state through a continuous Yb 3+ → Tm 3+ energy transfer process. The 3 H 4 state can be populated by the efficient nonradiative relaxation from the 3 F 2,3 state. The strong NIR UC (800 nm) is due to the 3 H 4 → 3 H 6 transition. In addition, the blue emission (476 nm) corresponds to the of 1 G 4 → 3 H 6 transition, where the 1 G 4 level is populated by the efficient energy transfer from the 3 H 4 state. The unexpected green emission (540 nm) is attributed to the co-doping of Tm 3+ /Er 3+ ions in KMnF 3 matrix. The resonant cross relaxation process 3 F 4 (Tm 3+ ) + 4 F 9/2 (Er 3+ ) → 1 G 4 (Tm 3+ ) + 4 I 15/2 (Er 3+ ) between Er 3+ and Tm 3+ ions leads to the population of 1 G 4 state of Tm 3+ ions and depopulation of 4 F 9/2 state of Er 3+ ions, and then to 2 H 11/2 / 4 S 3/2 state through the multiphonon assisted relaxations 29 .

Conclusions
In summary, we have developed a facile hydrothermal method for preparation of tri-doped KMnF 3 nanocubes with simultaneous four-color (blue, green, red and NIR) UC emissions. Of particular interests, the red and NIR bands, known as so-called "optical window" for imaging biological tissues, are strong. The  spectral and pump dependence analyses indicate that two-photon process is responsible for the red and NIR emissions. We believe that this proof-of-concept demonstration of a multicolor emission across a broader spectra (blue to NIR) using tri-doped single KMnF 3 host system may have potential applications for multiplexing analysis and/or multi-optical window imaging of biological tissues. MnCl  Preparation of tri-doped KMnF 3 nanocubes. The rare-earth tri-doped KMnF 3 nanocubes were hydrothermally prepared by using MnCl 2 and KF as precursors at 180 °C. Typically, 1.5 g (27 mmol) KOH, 2 mL H 2 O, 4 mL ethanol (48 mmol) and 9 mL of (24 mmol) OA (90 wt%) were well mixed at the room temperature for 10 min. A white viscous solution was obtained. The 10 mL (0.2 mol/L) MnCl 2 solution, 15.5 mg (0.4 mmol) YbCl 3 ·6H 2 O, 1.5 mg (0.04 mmol) ErCl 3 ·6H 2 O and 1.5 mg (0.04 mmol) TmCl 3 ·6H 2 O was subsequently added and vigorously stirred for 20 min. Then 8 mL (1.25 mol/L) KF was added into the above solution. After incubation for 1 h, the mixture was transferred to a 50 mL Teflon-lined autoclave, and then heated at 180 °C for 24 h. After cooling down, the products were removed by centrifugation then washed with ethanol, and dried under vacuum at room temperature for 24 h.

Materials.
Characterization. X-Ray powder diffraction (XRD) chracterization were carried out on a Rigaku D/max-γB diffractometer equipped with a rotating anode and a Cu Kα source (λ = 0.15418 nm). SEM micrographs were obtained using a field emission scanning electron microscope (FESEM, MX2600FE). Upconversion luminescence spectra were measured by a regeneratively amplified 980 nm diode laser (Hi-Tech Optoelectronics Co. Ltd., Beijing). The emitted UC fluorescence signal was collected by a lens-coupled monochromator (Zolix Instruments Co. Ltd., Beijing) at 3 nm spectral resolution with an attached photomultiplier tube (Hamamatsu CR131). All measurements were performed at room temperature.