808 nm NIR light excited single-band red upconversion emission in lanthanide-doped KMnF3 nanocrystals

Journal of Alloys and Compounds 721 (2017) 531e537

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808 nm NIR light excited single-band red upconversion emission in lanthanide-doped KMnF3 nanocrystals Mingye Ding a, b, Min Xu a, Chunhua Lu b, *, Junhua Xi a, Zhenguo Ji a, Daqin Chen a, ** a b

College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, PR China Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing, 210009, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 March 2017 Received in revised form 3 June 2017 Accepted 5 June 2017

A new type of upconversion nanoparticles (UCNPs) whose excitation and emission peaks are both located in the biological window has been developed based on the combination of Nd3þ sensitization and efficient energy transfer between Mn2þ and Er3þ ions. In our design, Nd3þ ions act as sensitizers for efficient absorption of 808 nm NIR photons, while the usage of Yb3þ ions as energy migrators facilitates the efficient energy transfer from Nd3þ to Er3þ ions. More importantly, the introduction of Mn2þ ions plays a key role in the realization of single-band red UC emission of Er3þ ions via the efficient energy transfer process between Mn2þ and Er3þ. Consequently, upon 808 nm laser excitation, single-band red UC emission can be obtained in the as-synthesized Nd3þ/Yb3þ/Er3þ tri-doped KMnF3 nanocrystals, which could be considered as attractive alternative to conventionally used UCNPs for deep-tissue imaging with mitigation of the attenuation effect associated with the emission peaks outside biological window and the overheating constraints imposed by the conventional 980 nm laser excitation. © 2017 Elsevier B.V. All rights reserved.

Keywords: Upconversion Nd3þ-sensitization Single-band emission KMnF3 nanocrystals Bioimaging

1. Introduction Lanthanide-doped upconversion nanomaterials, which enable the conversion of two or more low-energy photons into one highenergy photon through multi-photon process, have attracted increasing attention over the past few years owing to their various potential applications ranging from solid-state lasers and threedimensional displays to solar cells and sensors [1e5]. Especially, as an attractive alternative to conventionally used semiconductor quantum dots and organic dyes, upconversion nanoparticles (UCNPs) have been regarded as a promising new class of biomarkers due to their unique optical features, including sharp emission peaks, large anti-Stokes shifts, long luminescent lifetimes, minimized background autofluorescence, high photostability and low toxicity [6e8]. Despite these advantages, there are still some challenges that may prevent their practical use in biological imaging. Generally, the widely used UCNPs are simultaneously doped with Yb3þ ions as sensitizers and Tm3þ, Ho3þ or Er3þ ions as activators. The input near-infrared (NIR) photons (~980 nm) are absorbed by sensitizers and then transferred to the activators,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C. Lu), [email protected] (D. Chen). http://dx.doi.org/10.1016/j.jallcom.2017.06.053 0925-8388/© 2017 Elsevier B.V. All rights reserved.

which promotes the activators to higher excited states, eventually leading to radiation of ultraviolet, blue, green, red and NIR photons. Unfortunately, the excitation wavelength of conventional Yb3þsensitized UCNPs is restricted to 980 nm, which largely overlaps the absorption band of water molecules dominant in biological tissues. As a consequence, continuous 980 nm laser irradiation to trigger UC process could be greatly attenuated as it passes through the biological tissues, resulting in a risk of local temperature rise and even overheating of tissues [9,10]. Therefore, to address the issue of NIR laser-induced overheating effect, it is highly desirable to shift the excitation wavelength from 980 nm to an appropriate range. Compared with 980 nm excitation source, 808 nm laser is considered as promising alternative to largely alleviate the overheating effect induced by 980 nm laser excitation due to the low absorption coefficient for water molecules in biological tissues. On the basis of large absorption cross section around 808 nm of Nd3þ ions and the efficient Nd3þ / Yb3þ energy transfer process, intense photon UC process under 808 nm laser excitation could be easily realized by selecting Nd3þ as new NIR absorber and sensitizer, lanthanide ions (such as Er3þ, Tm3þ and Ho3þ) as activators and Yb3þ as energy transfer bridging ion. As we know, the lanthanide ions (Ln3þ), possessing abundant metastable excited states, can emit photons covering from ultraviolet to NIR region when doped in the appropriate host lattices

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[11,12]. Especially, the UC emission in red and NIR region (biological window, 650e900 nm) permits high biological tissue penetration depth, making Ln3þ-doped UCNPs suitable as optical probes for biological imaging [13,14]. However, there are some emission peaks outside biological window, which cannot escape from the deep biological tissue and cause the interference to biological imaging signal, leading to the decrease of the resolution and sensitivity. Simultaneously, this kind of multi-band emission feather of Ln3þ activators leads to energy deconcentration and is adverse to obtaining high efficient UC emission for a specific emitting state of Ln3þ. Consequently, exploring applicable strategy to realize highefficient single-band emission of Ln3þ ions remains a big challenge, but is highly desirable. Till now, several efforts have been devoted to achieving single-band red UC emission (~660 nm) of UCNPs: (1) increasing the sensitizer's content [15]; (2) choosing an appropriate matrix (e. g. YOF, Bi2O3, KMgF3, Na3Zr(Hf)F7) [16e20]; (3) doping ions (e. g. Mn2þ, Ce3þ ions) with suitable energy level [21e24]; (4) Coating organic dyes with a certain absorption peak [25]. Among these methods, the strategy of co-doping with Mn2þ ions has been demonstrated to facilitate the red UC emission of Er3þ ions via efficient energy transfer between Mn2þ and Er3þ ions. Based on the Nd3þ-sensitized UC process in conjunction with the energy transfer between Er3þ and Mn2þ ions, we have developed a new type of UCNPs that exhibits single-band red UC emission under 808 nm laser excitation. In this work, Nd3þ/Yb3þ/Er3þ tri-doped KMnF3 nanocrystals were successfully synthesized by a facile solvothermal method. Moreover, the UC luminescent properties and the corresponding energy transfer mechanism have been systematically investigated and discussed in detail. To best of our knowledge, analogous study has been rarely reported.

2. Experimental section 2.1. Materials Ytterbium chloride hexahydrate (YbCl3.6H2O, 99.99%), erbium chloride hexahydrate (ErCl3$6H2O) and neodymium chloride hexahydrate (NdCl3.6H2O, 99.99%) were supplied by Beijing Founde Star Science & Technology Co., Ltd (China). Manganese chloride tetrahydrate (MnCl2$4H2O, 98%), potassium hydroxide (KOH), potassium fluoride (KF), oleic acid (OA), ethanol were purchased by Sinopharm Chemical Reagent Co., Ltd (China). All reagents were used without further purification.

2.2. Synthesis In a typical procedure, 36 mmol KOH was firstly dissolved in 5 mL deionized water, followed by addition of 30 mL oleic acid and 15 mL ethanol under magnetic stirring. Then 5 mL aqueous solution containing (0.012  y) mmol YbCl3.6H2O (y ¼ 5, 10, 20, 30, 40), 0.012 mmol ErCl3$6H2O, (1.188 e 0.012  y e 0.012  x) mmol MnCl2$4H2O and (0.012  x) mmol NdCl3.6H2O (x ¼ 0, 0.5, 1, 2, 5, 10) was added to the above solution. After vigorous stirring for 15 min, 5 mL aqueous solution containing 10.5 mmol kF was introduced into the mixed solution. The mixture was agitated for another 15 min and then transferred to a 100 mL Teflon-lined autoclave and heated to 220  C for 3 h. After cooling down to the room temperature, the products were separated by centrifugation and washed with deionized water and ethanol several times, and then dried under vacuum at 60  C overnight. The detailed chemical compositions of lanthanide ions in the as-synthesized KMnF3 NPs are provided in Table 1.

Table 1 The detailed chemical compositions of lanthanide ions in KMnF3 nanocrystals determined by ICP-OES analysis. Sample

Theoretical Value (mol %)

ICP-OES measurement (mol%)

Nd

Yb

Er

Nd

Yb

Er

1 2 3 4 5 6 7 8 9 10 11 12

0 0 1 0.5 1 2 5 10 1 1 1 1

0 20 0 20 20 20 20 20 5 10 30 40

1 1 1 1 1 1 1 1 1 1 1 1

0 0 0.95 0.53 1.07 1.89 5.03 9.86 0.98 1.09 0.91 0.93

0 19.63 0 20.50 19.79 19.68 18.93 19.92 4.79 9.85 28.89 39.91

1.06 0.91 0.97 1.12 0.99 1.07 0.95 1.03 0.91 0.99 1.09 1.01

2.3. Characterization Powder X-ray diffraction (XRD) patterns were recorded by a ARL X’TRA diffractometer at a scanning rate of 10 /min in the 2q range from 10 to 80 with Cu Ka radiation (l ¼ 0.15406 nm). Transmission electron microscopy (TEM) measurements were carried out on a JEM-200CX with a field emission gun operating at an acceleration voltage of 200 kV. The absorption spectrum was measured on a UV-3101 PC spectrophotometer. The chemical compositions of the as-synthesized samples were determined by inductively Coupled Plasma Optical Emission Spectrometry (ICPOES) technique using a Perkin-Elmer Optima-5300DV spectrometer. UC emission spectra and decay curves were measured with an Edinburgh Instruments FS5 spectrofluorimeter equipped with an adjustable 808 nm laser diode module as excitation source. According to the method reported by Prasad et al. [26], UC quantum efficiencies of the as-synthesized nanocrystals were determined on an Edinburgh Instruments FLS980 fluorescence spectrophotometer equipped with an integrated sphere. All the measurements were performed at room temperature.

3. Results and discussion To examine the crystal structure and phase purity of the assynthesized KMnF3:Nd3þ/Yb3þ/Er3þ samples, X-ray diffraction (XRD) analysis is performed. Fig. 1 shows XRD patterns of KMnF3:20%Yb3þ/1%Er3þ/1%Nd3þ, KMnF3:20%Yb3þ/1%Er3þ, KMnF3:1%Er3þ/1%Nd3þ, KMnF3:1%Er3þ and KMnF3 nanoparticles as well as the standard data of cubic KMnF3 for comparison. As shown, all the diffraction peaks can be well indexed by the standard pattern of cubic phase of KMnF3 (JCPDS No. 17-0116). The crystal structure is determined with lattice parameters of a ¼ 4.189 nm, space group Pm-3m (221) [27]. No other impurity peaks can be detected, indicating that pure cubic KMnF3 nanocrystals have been successfully synthesized via a facile solvothermal method. As presented in Fig. 2a and Fig. 2b, representative transmission electron microscopy (TEM) image reveals that the as-synthesized Ln3þ-doped KMnF3 nanocrystals are nearly cubic in shape with an average size of ~32 nm. The selected area electron diffraction (SAED) pattern (inset in Fig. 2a) displays the spotty polycrystalline diffraction rings, corresponding to the (100), (110), (200) and (211) planes of cubicphased KMnF3 nanocubes. Moreover, the high-resolution TEM (HRTEM) image of an individual nanocube (inset in Fig. 2a) exhibits obvious lattice fringes corresponding well with the d100 spacing (0.42 nm) of cubic KMnF3 phase (JCPDS No. 17-0116). In Fig. 2c, the energy disperse spectroscopy (EDS) spectrum demonstrates a characteristic intensity profile of potassium, manganese, fluorine,

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Fig. 1. XRD patterns of the as-synthesized KMnF3 (a), KMnF3:1%Er3þ (b), KMnF3:20% Yb3þ/1%Er3þ (c), KMnF3:1%Er3þ/1%Nd3þ (d), KMnF3:20%Yb3þ/1%Er3þ/1%Nd3þ (e) samples. The standard data for cubic KMnF3 crystal (JCPDS No. 17-0116) is shown as a reference.

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Fig. 3. The room-temperature absorption spectrum of KMnF3:20%Yb3þ, 1%Er3þ, 10% Nd3þ sample.

6 A1 / 4A1 transitions of Mn2þ ions, respectively [28,29]. It is noted that the absorption peaks in the visible region (400e700 nm) can be assigned to 4I15/2 / 4F7/2, 4I15/2 / 2H11/2 and 4I15/2 / 4S3/2 transitions of Er3þ ions [20,30]. Obviously, three absorption peaks at 745 nm, 808 nm and 865 nm exist in the biological window (650e900 nm), corresponding to the 4I9/2 / 4F7/2, 4I9/2 / 4F5/2 and 4 I9/2 / 4F3/2 transitions of Nd3þ ions, respectively [31]. The absorption band centered at about 980 nm in NIR region can be ascribed to 2F7/2 / 2F5/2 transition of Yb3þ ions. The above results also powerfully indicate that Nd3þ/Yb3þ/Er3þ ions are successfully doped into KMnF3 host lattice. Compared with the absorption of Yb3þ at 980 nm, Nd3þ has a larger absorption cross section around 808 nm, making possible the Nd3þ-sensitized UC process. To confirm the significant roles of the Nd3þ and Yb3þ in 808 nm NIR light excited UC system, the UC luminescent properties of KMnF3:1%Nd3þ/1%Er3þ, KMnF3:20%Yb3þ/1%Er3þ, KMnF3: 1%Er3þ and KMnF3:20%Yb3þ/1%Er3þ/1%Nd3þ nanocrystals are investigated and shown in Fig. 4. Under the excitation of 808 nm laser, all the UC

Fig. 2. TEM image (a), size distribution (b) and EDS spectrum of KMnF3:20%Yb3þ, 1% Er3þ, 10%Nd3þ nanoparticles. Insets in Fig. 2a are the corresponding SAED pattern and HRTEM image of KMnF3 NPs.

ytterbium, neodymium and erbium elements, also confirming the successful incorporation of lanthanide ions into KMnF3 host lattice. Based on the above results, it can be concluded that Nd3þ/Yb3þ/ Er3þ triply doped KMnF3 nanocubes have been successfully synthesized by a typical solvothermal method. Fig. 3 presents the absorption spectrum of the as-prepared KMnF3:20%Yb3þ, 1%Er3þ, 10%Nd3þ nanoparticles in the range from 200 nm to 1100 nm. As shown, there are two prominent absorption bands centered at 306 nm and 396 nm as well as a wide band in the range of 400e600 nm, which can be ascribed to 6A1 / 4T1 and

Fig. 4. The UC luminescent spectra, emission intensity and corresponding luminescent photographs of KMnF3:20%Yb3þ/1%Er3þ/1%Nd3þ, KMnF3:1%Er3þ, KMnF3:20%Yb3þ/1% Er3þ and KMnF3:1%Er3þ/1%Nd3þ NPs (The blue columns and red numbers represent the UC intensity ratios). All of the spectra were recorded under excitation by a 808 nm laser at a power of 1.5 W. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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luminescent spectra exhibit single-band emission peak centered at ~668 nm, which can be assigned to the 4F9/2 / 4I15/2 transition of Er3þ ions. Obviously, the green emissions attributed to 2H11/2/4S3/ 4 3þ completely disappear because of the 2 / I15/2 transitions of Er efficient exchange-energy transfer process between Mn2þ and Er3þ ions [20,22]. It is observed that the UC emission intensity of KMnF3:20%Yb3þ/1%Er3þ is as weak as that of KMnF3: 1%Er3þ, suggesting that UC process hardly happens upon 808 nm laser excitation owing to the narrow absorption section around 808 nm. Compared with KMnF3:20%Yb3þ/1%Er3þ or KMnF3: 1%Er3þ NPs, the UC emission intensity of KMnF3:1%Nd3þ/1%Er3þ seriously decreases owing to the deleterious quenching effect caused by back energy transfer (BET) from Er3þ to Nd3þ ions. However, the UC luminescent intensity of KMnF3:20%Yb3þ/1%Er3þ/1%Nd3þ is about 34 times stronger than that of KMnF3:1%Nd3þ/1%Er3þ, indicating that the introduction of Yb3þ benefits the significant improvement of the UC emission. The UC quantum efficiency of KMnF3:20%Yb3þ/ 1%Er3þ/1%Nd3þ nanocrystals is determined to be about 0.023% with the excitation power density of ~20 W/cm2. Under 808 nm NIR light excitation, intense red UC emission can be clearly observed in Nd3þ/ Yb3þ/Er3þ triply doped KMnF3 nanoparticles (inset in Fig. 4). Based on the above results, it can be concluded that Nd3þ and Yb3þ ions play an important role in 808 nm NIR light excited UC process. To obtain high efficient UC emission in Nd3þ/Yb3þ/Er3þ triply doped KMnF3, the doping concentration of Nd3þ and Yb3þ ions could be optimized. Fig. 5a shows the UC luminescent spectra of KMnF3:Yb3þ/Er3þ/Nd3þ nanocrystals doped with different Nd3þ dopant concentration (0e10 mol%) under the excitation of 808 nm laser diode. As shown, the single-band red emission peaked at ~668 nm (Er3þ: 4F9/2 / 4I15/2) can be clearly observed in these samples. The red emission of Er3þ ions is enhanced gradually with an increase of Nd3þ content from 0 to 1 mol%, and then monotonously decreases upon further increasing Nd3þ concentration (1e10 mol%). It can be found that 1 mol% of Nd3þ doped UCNPs is the optimal concentration and exhibits a 16.8-fold enhancement on the red emission of Er3þ as compared with the non-Nd3þ sample (Fig. 5b). Remarkably, the red emission decreases as the Nd3þ concentration rises from 1 to 10 mol%, indicating that high concentrations of doped Nd3þ ions may quench the UC process, even when the total absorption around 808 nm is raised. The phenomenon can be ascribed to the good overlaps of the Er3þ transitions (4I11/2 / 4I13/2, 4I13/2 / 4I15/2) with the Nd3þ absorptions (4I9/ 4 4 4 2 / I13/2, I9/2 / I15/2), which grants an efficient BET from the metastable excited states of Er3þ to the 4IJ levels of Nd3þ [31]. Therefore, the optimal doping concentration of Nd3þ in

KMnF3:Yb3þ/Er3þ/Nd3þ nanocrystals is about 1 mol%. For the UC luminescent materials, a long lifetime usually means a highlyefficient UC emission [32,33]. Therefore, the luminescent decay curves of KMnF3:20%Yb3þ, 1%Er3þ, x %Nd3þ (x ¼ 0.5, 1, 2, 5, 10) are measured at the excitation wavelength of 808 nm, as displayed in Fig. 6a. Owing to the non-single-exponential feature of these decay curves, the average decay lifetime can be evaluated based on the following equation: t ¼

R

IðtÞdt=IP , where IP represents the peak

intensity in the decay curve, I(t) denotes the time-dependent UC luminescent intensity [34]. As shown in Fig. 6b, a considerable increase in the lifetime of the Er3þ: 4F9/2 level is observed up to 1 mol % Nd3þ, followed by a fast decrease with the increment of Nd3þ concentration. Obviously, the trend of lifetime variation is consistent with that of UC emission intensity variation, significantly evidencing the above-mentioned conclusion. The UC luminescent properties of KMnF3:Yb3þ/Er3þ/Nd3þ NPs doped with different Yb3þ concentration (5e40 mol%) are systemically investigated and presented in Fig. 7. Under 808 nm laser diode excitation, single-band red UC emission centered at ~668 nm originates from 4F9/2 / 4I15/2 transition of Er3þ ions (Fig. 7a). Fig. 7b shows the dependence of the red emission on the doping concentration of Yb3þ ions. With the increase of Yb3þ content from 5 mol% to 40 mol%, the relative intensity of red emission increases first and then decreases after reaching the maximum at 20 mol% Yb3þ, which may be attributed to the back energy transfer processes (Mn2þ / Er3þ, Er3þ / Yb3þ) and/or the concentration quenching effect. As a result, the optimal concentration of Yb3þ ions is considered as 20 mol% in order to obtain efficient Nd3þ-sensitized UC emission. In addition, the lifetimes of the Er3þ: 4F9/2 level in KMnF3:y%Yb3þ, 1%Er3þ, 1%Nd3þ samples are recorded under 808 nm NIR laser excitation, as displayed in Fig. 8. The average lifetime of the 4F9/2 level increases from 46 ms to 102 ms upon increasing the Yb3þ concentration from 5 mol% to 20 mol%, due to the occurrence of BET processes (Mn2þ / Er3þ and/or Er3þ / Yb3þ). However, the lifetime of Er3þ: 4F9/2 level decreases gradually from 102 ms to 56 ms with increasing the doping content of Yb3þ from 20 mol% to 40 mol%, owing to the existence of concentration quenching effect [35,36]. Significantly, the above results agree well with the dependence of red UC emission intensity on Yb3þ dopant concentration. It is well known that the UC emission intensity (IUC) is related to the pumping power (P): IUC f P n , where n denotes the required number of the pumping photon to excite activator from the ground state to the emitting excited state [37]. The value of n is obtained

Fig. 5. (a) UC emission spectra and the luminescent photographs of KMnF3:20%Yb3þ, 1%Er3þ, x%Nd3þ (x ¼ 0, 0.5, 1, 2, 5, 10) NPs; (b) The corresponding integrated emission intensity versus Nd3þ dopant concentration for KMnF3:Yb3þ/Er3þ/Nd3þ NPs. All of the spectra were recorded under excitation by a 808 nm laser at a power of 1.5 W.

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Fig. 6. (a) Er3þ decay curves recorded for KMnF3:20%Yb3þ, 1%Er3þ, x%Nd3þ (x ¼ 0.5, 1, 2, 5, 10) NPs; (b) The corresponding emission lifetimes of Er3þ at its 4F9/2 excited state.

Fig. 7. (a) UC emission spectra of KMnF3:y%Yb3þ, 1%Er3þ, 1%Nd3þ (y ¼ 5, 10, 20, 30, 40) NPs; (b) The corresponding integrated emission intensity versus Yd3þ dopant concentration for KMnF3:Yb3þ/Er3þ/Nd3þ NPs. All of the spectra were recorded under excitation by a 808 nm laser at a power of 1.5 W.

Fig. 8. (a) Er3þ decay curves recorded for KMnF3:y%Yb3þ, 1%Er3þ, 1%Nd3þ (x ¼ 5, 10, 20, 30, 40) NPs; (b) The corresponding emission lifetimes of Er3þ at its 4F9/2 excited state.

from the slope of the fitted line in the double-logarithmic plot (logIUC versus logP). To deeply investigate the number of pumping photons involved in the Nd3þ-sensitized UC process, the excitation power-dependent UC emissions of red (~668 nm) for KMnF3:20% Yb3þ/1%Nd3þ/1%Er3þ nanoparticles are calculated and provided in Fig. 9. As shown, the slope of the linear fitting for the Er3þ: 4F9/

/ 4I15/2 transition is close to 2, indicating that only two-photon process is responsible for the red UC emission under 808 nm NIR light excitation [38]. To explain the realization of Nd3þ-sensitized single-red UC emission, the possible UC process in Yb3þ/Er3þ/Nd3þ tri-doped KMnF3 NPs is proposed and shown in Scheme 1 [22,31]. Nd3þ 2

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Scheme 2. The possible back energy transfer process in the Nd3þ/Yb3þ/Er3þ co-doped KMnF3 nanoparticles. Fig. 9. The double-logarithmic plot of UC luminescent intensity versus NIR pumping power for Er3þ: 4F9/2 / 4I15/2 transition in KMnF3:20%Yb3þ/1%Nd3þ/1%Er3þ samples under 808 nm laser excitation.

Scheme 1. Energy level diagrams of Nd3þ, Yb3þ, Er3þ and Mn2þ as well as the proposed energy transfer mechanism that facilitates single-band red emission of Er3þ ions under 808 NIR light excitation in KMnF3:Yb3þ/Er3þ/Nd3þ NPs.

ions act as sensitizers for the absorption of 808 nm NIR photons, while Yb3þ ions play the role of a bridge in the energy transfer process from sensitizers (Nd3þ ions) to the activators (Er3þ ions). Upon 808 nm NIR light excitation, the electrons of Nd3þ ions are excited from the ground state 4I9/2 to excited state 4F5/2/2H9/2, followed with non-radiative relaxation to the 4F3/2 state. Then the energy of these excited electrons could be transferred to adjacent Yb3þ ions, resulting in the population of the Yb3þ: 2F5/2 level. Through the energy migration between Yb3þ ions, the energy is finally transferred to Er3þ ions and populates its 4I11/2 level. The Nd3þ ions continuously absorb NIR photons and transfer the energy to Er3þ ions via the bridge ions (Yb3þ) in order to populate highenergy states of Er3þ: 4F9/2, 4F7/2 and 2H9/2 levels. The excited electrons at 4F7/2 and 2H9/2 states decay nonradiatively to 2H11/2, 4 S3/2 or 4F9/2 level of Er3þ ions. Owing to the existence of efficient exchange-energy transfer between Mn2þ and Er3þ ions, the excited electrons at 2H9/2 and 4S3/2 levels of Er3þ transfer their energy to the 4 T1 state of Mn2þ ions, followed by back energy transfer to the Er3þ:

4

F9/2 level, leading to the quenching of the 2H11/2, 4S3/2, 2H9/2 / 4I15/ 3þ ions. As a consequence, only red emission 2 transitions of Er originating from the Er3þ: 4F9/2 / 4I15/2 transition can be obtained. Remarkably, in the Nd3þ/Yb3þ/Er3þ co-doped KMnF3 system, there exist three types of BET processes (Scheme 2), which have significant impact on the single-band UC emission. The BET processes have been proposed and described as follows: (1) Er3þ / Nd3þ process: 4I11/2 (Er3þ) þ 4I9/2 (Nd3þ) / 4I13/2 (Er3þ) þ 4I13/2 (Nd3þ); 4I13/2 (Er3þ) þ 4I9/2 (Nd3þ) / 4I15/2 (Er3þ) þ 4I15/2 (Nd3þ). Owing to the good overlaps of the Er3þ emissions (4I11/2 / 4I13/2, 4I13/2 / 4I15/2) with the Nd3þ absorptions (4I9/2 / 4I13/2, 4I9/2 / 4I15/2), the efficient BET from the metastable excited states of Er3þ ions to the 4IJ levels of Nd3þ ions happens [39]. Therefore, to greatly suppress the cross-relaxation between Er3þ and Nd3þ, the dopant concentration of sensitizer (Nd3þ) should be restricted to a relative low level (~1 mol%) in our system. However, the low dopant concentration of Nd3þ ions results in a limited harvest of 808 nm photons. (2) Er3þ / Yb3þ process: 4S3/2 (Er3þ) þ 2F7/2 (Yb3þ) / 4I13/2 (Er3þ) þ 2F5/2 (Yb3þ). The increase of Yb3þ dopant concentration in the KMnF3 host lattice would decrease YbeEr interatomic distance and thus facilitate BET process from Er3þ to Yb3þ. Subsequently, this BET could suppress the population in excited states of 2H9/2, 2H11/2 and 4S3/2, which is conducive to enhancing red UC emission [40]. (3) Mn2þ / Er3þ process: 4I15/2 (Er3þ) þ 4T1 (Mn2þ) / 4F9/2 (Er3þ) þ 6A1 (Mn2þ). The resonances between the metastable states of Er3þ ions and the absorption bands of Mn2þ ions grant nonradiative energy transfer from Er3þ: 2H9/2, 4S3/2 levels to Mn2þ: 4T1 state, followed by BET to Er3þ: 4F9/2 level, resulting in the single-band red UC emission [23]. 4. Conclusions In conclusion, 808 nm NIR light excited single-red UC emission has been successfully realized in Nd3þ/Yb3þ/Er3þ tri-doped KMnF3 nanocrystals by using Nd3þ as sensitizer, Yb3þ as bridging center, Er3þ as activator and Mn2þ as an exchange-energy transfer medium. In order to obtain high efficient UC emission, the doping concentration of Nd3þ and Yb3þ ions are precisely optimized and could be set as 1 mol% and 20 mol%, respectively. In the tri-doped UC system, the doping of Nd3þ ions (sensitizer) plays a role in the absorption of NIR light (~808 nm), while the Yb3þ ions act as efficient energy migrators for the energy transfer from Nd3þ to Er3þ ions. Importantly, the introduction of host Mn2þ ions significantly benefits the realization of single-band red UC emission of Er3þ ions

M. Ding et al. / Journal of Alloys and Compounds 721 (2017) 531e537

through the efficient energy transfer process between Mn2þ and Er3þ, that is, 4S3/2 (Er3þ) þ 6A1 (Mn2þ) / 4I15/2 (Er3þ) þ 4T1 (Mn2þ), 2 H9/2 (Er3þ) þ 6A1 (Mn2þ) / 4I13/2 (Er3þ) þ 4T1 (Mn2þ) and 4I15/2 (Er3þ) þ 4T1 (Mn2þ) / 4F9/2 (Er3þ) þ 6A1 (Mn2þ). The assynthesized UCNPs can serve as ideal luminescent biomarkers for deep-tissue imaging without the concern of overheating effect and the constraints associated with conventional multi-peak emissions.

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Acknowledgments [20]

This work has been finally supported by Zhejiang Provincial Natural Science Foundation of China (LR15E020001 and LQ15E020004), the Natural Science Foundation of Jiangsu Province (No. BK20141459), the National Natural Science Foundation of China (51572065 and 61372025) and the 151 talent's projects in the second level of Zhejiang province.

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Appendix A. Supplementary data

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Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.06.053.

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