anomalous nir luminescence in mn2+doped fluoride ...download.xuebalib.com/xuebalib.com.36637.pdf ·...

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Anomalous NIR Luminescence in Mn 2+ -Doped Fluoride Perovskite Nanocrystals

Enhai Song , Sha Ding , Ming Wu , Shi Ye , * Fen Xiao , Shifeng Zhou , and Qinyuan Zhang *

E. H. Song, S. Ding, M. Wu, Prof. S. Ye, F. Xiao, Prof. S. F. Zhou, Prof. Q. Y. Zhang State Key Laboratory of Luminescent Materials and Devices Institute of Optical Communication Materials South China University of Technology Guangzhou 510640 , China E-mail: [email protected]; [email protected]

DOI: 10.1002/adom.201400066

visible emission at approximately 478 nm originating from the 1 G 4 → 3 H 6 transitions of Tm 3+ limits the emission-penetration depth in tissues due to the strong absorp-tion of short-wavelength light below 600 nm. [ 8,9 ] Therefore, there is still a need for exploring a new mechanism and structure that would produce a pure NIR UC emission.

In stark contrast to the rare earth ions that typically exhibit multipeak emission at a fi xed wavelength, transition metal ions, especially Mn 2+ , exhibit unique luminescence features, such as a tunable wavelength and singe-band emission. [ 10–12 ] According to the Tanabe–Sugano dia-gram of Mn 2+ , the emission of Mn 2+ can be easily tuned from green to deep red by changing the crystal fi eld strength. [ 13,14 ] For example, the tetrahedrally coordi-

nated Mn 2+ (weak fi eld strength) exhibits typically green emis-sion. However, the octahedrally coordinated Mn 2+ (strong fi eld strength) displays orange to red emission. [ 15,16 ] In addition, the excitation window of Mn 2+ can be extended from yellow light to the vacuum ultraviolet (VUV) region, which would enable signifi cant applications of Mn 2+ -doped luminescent mate-rials in white-LEDs, [ 16,17 ] plasma display panels (PDP) [ 18 ] and cathode-ray tubes (CRT). [ 19 ] More interestingly, the tunable single-band UC with high chromatic purity can also be realized in some Mn 2+ /Yb 3+ codoped systems, such as CsMnCl 3 :Yb 3+ (690 nm), [ 20 ] RbMnCl 3 :Yb 3+ (630 nm), [ 20 ] MnCl 2 :Yb 3+ (656 nm), [ 21 ] CsMnBr 3 :Yb 3+ (680 nm), [ 22 ] LaMgAl 11 O 19 :Yb 3+ ,Mn 2+ (514 nm) [ 23 ] and GaMgB 5 O 10 :Yb 3+ ,Mn 2+ (620 nm). [ 24 ] However, the synthesis of new Mn 2+ activated structures that produce single-band NIR UC has been a signifi cant fundamental challenge for a long time.

Herein, we report the realization of single-band NIR UC emission in a Mn 2+ -doped KZnF 3 nanostructure using a heavy doping strategy. In the newly developed system, upon excita-tion with a 396 nm light and/or a 976 nm laser, an anomalous NIR emission band with a central wavelength at ∼770 nm was observed. The Mn 2+ -concentration and temperature-dependent Stokes and UC luminescence properties have been investi-gated. The mechanism of the NIR UC emission has been investigated and discussed in detail. A comprehensive under-standing the nature of the Stokes and UC luminescence of Mn 2+ is of fundamental importance for advancing technolog-ical applications.

An anomalous near-infrared (NIR) upconversion (UC) emission band at approximately 770 nm is demonstrated in KZnF 3 :Yb 3+ ,Mn 2+ nanocrystals with heavy Mn 2+ doping. This band would enable advanced biological imaging with improved resolution and enhanced penetration depth. Careful studies based on structure analysis, excitation and emission spectra, and lumines-cence decay curves indicate that this unusual NIR emission (770 nm) origi-nates from the 6 A 1g (S) 4 T 1g (G)→ 6 A 1g (S) 6 A 1g (S) transitions of the Mn 2+ –Mn 2+ dimers. The infl uence of Mn 2+ concentration and temperature on the Stokes and UC luminescence properties are also investigated. The proposed mecha-nism for the observed NIR UC emission involves ground state absorption and excited state absorption processes. The present results not only provide a useful and effective approach to achieving pure NIR UC emission, and also new insights into the development of advanced photonic devices and technologies.

1. Introduction

Functional light sources with broadband emission and tunable wavelengths have been extensively investigated in recent years due to their numerous applications in the fi elds of lighting, displays, continuous-wave lasers, anti-counterfeiting, bio-logical imaging, photoelectronics and photonic devices. [ 1–4 ] In particular, light sources that emit in the near-infrared (NIR) (700–1100 nm) region are attractive due to the cells and tis-sues exhibiting weak auto-fl uorescence and the low transmis-sion loss of the optical signal in this wavelength range. [ 5–7 ] Therefore, it is crucial to design nanocrystals with both emis-sion and excitation of luminescence in the NIR region for in vitro and in vivo imaging applications. [ 8,9 ] For this purpose, the Yb 3+ /Tm 3+ codoped NaYF 4 upconversion (UC) nanocrystals are of great interest as a potential candidate for their effi cient NIR UC emission near 800 nm upon excitation by a 976 nm diode laser. [ 7–9 ] However, the accompanying simultaneous

Adv. Optical Mater. 2014, 2, 670–678

http://doi.wiley.com/10.1002/adom.201400066

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2. Results and Discussion

2.1. Phase Characterization

Figure 1 (a) shows the XRD patterns of KZn 1- x Mn x F 3 ( x = 0.05–0.40). All of the diffraction peaks of the samples corre-spond to cubic perovskite KZnF 3 ((JCPDS 06–0439), and no impurity phases or extra peaks were detected in these sam-ples, which indicated that the Mn 2+ ions have been success-fully incorporated into the KZnF 3 host lattice. As the Mn 2+ concentration increases, the diffraction peaks of the samples gradually shift to lower angles (as shown on the right side of Figure 1 (a)) due to the partial replacement of Zn 2+ ions (Coordination Number (CN) = 6, r = 0.74Å) with Mn 2+ ions (CN = 6, r = 0.80Å). [ 25 ] Figure 1 (b) shows a typical transmis-sion electron microscope (TEM) image of KZn 0.80 Mn 0.20 F 3 . The obtained nanocrystals have a quasi-cubic morphology with an average diameter of ∼61 nm. Figure 1 (c) shows the high reso-lution TEM image of a single nanocrystal, indicating the per-fect crystallinity of the prepared nanocrystal. The space of the crystal facets were measured to be 0.409 and 0.293 nm, which

are consistent with the (100) and (110) crystal facets ( d 100 = 0.405 nm and d 110 = 0.287 nm) of cubic-phase KZnF 3 (JCPDS 06–0439). The slight increases in the d values are due to the par-tial substitution of Zn 2+ ions by Mn 2+ ions. The regular selected electron diffraction patterns shown in the inset of Figure 1 (c) reveal the single-crystal nature of the nanocrystal.

2.2. Stokes Emission Properties

Figure 2 (a) shows the emission spectra of KZn 1- x Mn x F 3 upon 396 nm light excitation. When the Mn 2+ concentration is less than x = 0.10, a single visible emission band centered at 585 nm appeared in the spectra, corresponding to the 4 T 1g (G)→ 6 A 1g (S) transitions of Mn 2+ . [ 26 ] When the concentration of Mn 2+ reaches x = 0.10, an anomalous NIR emission band located at ∼770 nm appears in the emission spectra, which has never been previ-ously reported for conventional Mn 2+ -doped phosphors. The full width at half maximum (FWHM) of the NIR luminescence is ∼105 nm, which is much larger than that of the visible emis-sion (60 nm). The concentration-dependent integrated emission


Figure 1. (a) XRD patterns of KZn 1- x Mn x F 3 ( x = 0.05, 0.10, 0.20, and 0.40). A standard diffraction pattern of KZnF 3 (JCPDS 06–0439) is included for comparison. (b) TEM image of KZn 0.80 Mn 0.20 F 3 nanocrystals and a typical nanocrystal (inset). (c) High-resolution TEM image of a nanocrystal with its selected electron diffraction pattern (inset).

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intensity of the two emission bands upon 396 nm excitation is shown in Figure 2 (b). The visible emission band exhibits a maximum intensity at x = 0.10, and the NIR emission band exhibits a maximum intensity at x = 0.30, then which sharply decreases due to the concentration quenching effect. These luminescence features demonstrate that the visible and NIR emission bands originated from two different emission centers. It is important to note that the pure NIR emission (770 nm) can also be obtained in KMnF 3 (the inset of Figure 2 (a)), indi-cating that the concentration quenching of the visible emission is more severely affected by the Mn 2+ centers than the active centers associated with NIR emission. In order to obtain more insight into the active centers, the luminescence dynamics were studied by determining the concentration-dependent lumines-cence decay curves (Figures 2 (c,d)). At a low concentration ( x = 0.01 and 0.05), the visible emission exhibits a single exponen-tial decay behavior, [ 27 ] and the decay lifetime is estimated by the following equation

( ) exp( / )0I t I A t τ= + − (1)

where I (t) and I 0 represent the emission intensity at time t and 0, respectively. A is a constant, and τ is the decay lifetime. The fi tted decay lifetimes are 65.270 and 59.767 ms for x = 0.01 and 0.05, respectively. For Mn 2+ -heavy doping ( x = 0.10–0.30), the visible emission exhibits a non-exponential decay behavior that

is probably associated with the prevailing nonradiative transition. Under this condition, the decay lifetime is expressed by the mean decay lifetime ( τ m ), [ 4 ] which can be estimated by

∫τ =∞

( )/ Imax0

I t dtmt

(2)

where I (t) is the luminescence intensity at t and I max = I (0). The calculated values of τ m are 38.907, 16.886 and 0.455 ms for x = 0.10, 0.20, and 0.30, respectively. It is interesting to note that the NIR emission always exhibits a single-exponential decay behavior in the range of x = 0.10–0.40, and the decay lifetime is determined to be 0.425, 0.384, 0.252, and 0.205 ms for x = 0.10, 0.20, 0.30, and 0.40, respectively. Such important differences in the decay dynamics further confi rm that the visible and NIR emission bands originate from two different emission centers. The monotonously shortening decay lifetime of the two emission bands suggests that the nonradiative loss occurs among the Mn 2+ ions as the Mn 2+ concentration increases.

The excitation spectra were further studied by monitoring the emission wavelength at 585 and 770 nm. As shown in Figure 3 , these two emission bands display similar excita-tion features with several isolated excitation bands at 307, 333, 353, 396, 435, and 532 nm, which can be attributed to the electronic

transitions of Mn 2+ from ground state 6 A 1g (S) to excited states 4 T 1g (P), 4 E g (D), 4 T 2g (D), [ 4 A 1g (G), 4 E g (G)], 4 T 2g (G) and 4 T 1g (G), respectively. [ 26 ] This result fi rmly indicates that both the vis-ible and NIR emission originate from the Mn 2+ ions. It is well


Figure 2. (a) Emission spectra and (b) corresponding integrated emission intensity of KZn 1- x Mn x F 3 ( x = 0.01–0.40). (c) Luminescence decay curves at 585 nm and (d) 770 nm for KZn 1- x Mn x F 3 ( x = 0.01–0.40). The inset shows the emission spectrum of KMnF 3 .The emission spectra and luminescence decay curves are obtained upon excitation by a 396 nm light.

Figure 3. Excitation spectra (monitoring wavelengths are 585 and 770 nm) of KZn 0.80 Mn 0.20 F 3 .


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known that isolated Mn 2+ ions can only emit visible light in the range of 490–710 nm with a rather long lifetime (i.e., on the order of 10 ms). [ 28 ] However, it is interesting that the NIR emission exhibits a much short decay process that is less than 0.5 ms. In addition, the NIR emission can be exclusively observed in KMnF 3 where the doping concentration is much higher. The contribution from the crystal-fi eld change and different site substi-tutions can also be excluded because blue-shifted luminescence is expected due to the Mn 2+ ions would occupy only Zn 2+ sites in KZnF 3 . [ 29 ] Based on these facts, the NIR emission cannot be attributed to isolated Mn 2+ ions.

To explore the origin of the observed unusual NIR emission in KZn 1- x Mn x F 3 , the crystal structure of KZnF 3 and the activa-tion process of Mn 2+ were further analyzed. KZnF 3 possesses a typical cubic perovskite structure with a P m -3 m space group, and its lattice constants are a = 0.405 nm. In this structure, the Zn 2+ ions are located at the center of the ZnF 6 octahedra, and the K + ions are located at the tetradecahedron cavity sur-rounded by ZnF 6 octahedra (ICSD 63157). The ZnF 6 polyhedra are connected to each other through F − ions, and the distance between the two nearest Zn 2+ ions in KZnF 3 is approximately 0.405 nm (ICSD 63157). As mentioned above, the incorporated Mn 2+ ions will substitute for Zn 2+ , resulting in the for-mation of Mn 2+ (Zn 2+ )-Mn 2+ (Zn 2+ ) dimers, especially at a high doping concentration. The exchange interaction between Mn 2+ and Mn 2+ ions leads to split-ting of the ground state ( 6 A 1 , S = 5/2) and the fi rst excited state ( 4 T 1 , S = 3/2), which are primarily governed by the linear combi-nation of two spin states, S i and S j (i.e., ( S i + S j ),…, ( S i – S j )). [ 30 ] Therefore, the spin state is S = 5, 4, 3, 2, 1, and 0 for the ground state ( 6 A 1g (S) 6 A 1g (S)) and S = 4, 3, 2, and 1 for the fi rst excited state ( 6 A 1g (S) 4 T 1g (G)). The Mn 2+ -Mn 2+ dimers can also act as the chromophoric centers, and the observed NIR luminescence is most likely associated with the radiative transition from the excited state ( S = 1) to the ground state with various spin com-ponents. The changes in the spin components would cause a shift in the emission peak. The changes in the spin selection rule from spin forbidden ( S = 5/2→ S = 3/2) to spin allowed ( S = 1→ S = 1) may result in shortening of the decay lifetime. [ 31 ] Hence, it is reasonable to suggest that the NIR emission in KZn 1-x Mn x F 3 originates from the 6 A 1g (S) 4 T 1g (G)→ 6 A 1g (S) 6 A 1g (S) transitions of coupled Mn 2+ -Mn 2+ dimers.

2.3. UC emission Properties

We further attempted to introduce rare-earth ions (e.g., Yb 3+ ) into the KZn 1- x Mn x F 3 nanostructure to explore the possibility of producing UC luminescence. Figure 4 (a) shows the UC

emission spectra of KZn 0.995-x Mn x Yb 0.005 F 3 upon excitation by a 976 nm LD. At a low concentration ( x = 0.05), a single vis-ible UC emission band centered at 585 nm is observed, corre-sponding to the 2 , ( )7/2

41F T Gg → 2 , ( )7/2

61F A Sg transition of the

Yb 3+ -Mn 2+ dimers. [ 26 ] Interestingly, when the doping concentra-tion of Mn 2+ is high enough ( x = 0.10–0.40), an extra anoma-lous NIR emission band centered at 770 nm emerges in the luminescence spectra. This result is very interesting because the observed NIR UC emission band is located at the “optical window” of the living cells and tissues, which offers the pos-sibility of achieving high-resolution or deep penetration during biological imaging. Figure 4 (b) displays the corresponding integrated UC emission intensity of the visible (585 nm) and NIR (770 nm) emission bands as a function of the Mn 2+ ion doping concentration. The emission intensity of visible UC luminescence (585 nm) fi rst increases as the concentration of Mn 2+ increases before reaching a maximum at x = 0.10. Then, the emission intensity of visible UC luminescence (585 nm) sharply decreases and completely disappears as the doping concentration of Mn 2+ reaches x = 0.40, which is due to non-radiative transition loss. In comparison, the NIR UC emis-sion band (770 nm) reaches a maximum intensity at x = 0.20. These results indicate that a high doping concentration favors NIR UC emission (770 nm). Remarkably, single-band UP NIR


Figure 4. (a) Upconversion emission spectra and (b) corresponding integrated emission inten-sity of KZn 0.995- x Mn x Yb 0.005 F 3 ( x = 0.05–0.40) upon excitation of a 976 nm laser diode. Log–log plots of upconversion emission intensities as a function of pump power, ranging from 550 to 1500 mW at indicated wavelengths for the KZn 0.795 Mn 0.20 Yb 0.005 F 3 (c) and KZn 0.595 Mn 0.40 Yb 0.005 F 3 (d) samples. Solid lines represent linear fi ts with the slopes indicated in the fi gure.


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emission can be easily obtained via controlling the concentra-tion of Mn 2+ .

The UC luminescence process can be further studied by investigating the pumping power P dependence of the UC luminescence intensit y I up using the following function: [ 32 ]

∝upI Pn

(3)

where I up is the integrated intensity of the UC luminescence, P is the average power of the pumping laser, and n is the photon number. Therefore, the number of pumping photons ( n ) can be determined from the slope of the UC luminescence intensity as a function of the laser excitation power in a Log–Log plot. According to Figures 4 (c,d), the n values for the 585 and 770 nm emission in KZn 0.795 Mn 0.20 Yb 0.005 F 3 are calculated to be approx-imately1.57 and 1.60, respectively, and n = 1.53 was calculated for the 770 nm emission band in KZn 0.595 Mn 0.40 Yb 0.005 F 3 , which indicated that a two-photon process contributes to the two UC emission bands.

2.4. UC Mechanism

To gain insight into the UC mechanism of the NIR emission, the photoluminescence spectra of the KZn 0.80 Mn 0.20 F 3 and KZn 0.795 Mn 0.20 Yb 0.005 F 3 samples upon excitation at 396 nm were measured and are shown in Figure 5 (a). Both spectra display two emission bands located at approximately 585 and 770 nm corresponding to the 4 T 1g (G) → 6 A 1g (S) transitions of isolated Mn 2+ ions and 6 A 1g (S) 4 T 1g (G) → 6 A 1g (S) 6 A 1g (S) tran-sitions of exchange coupled Mn 2+ –Mn 2+ dimers, respectively. For KZn 0.795 Mn 0.20 Yb 0.005 F 3 , an extra emission band cen-tered at 974 nm was observed, which can be attributed to the 2 F 5/2 → 2 F 7/2 transitions of Yb 3+ . In comparison to the single Mn 2+ -doped sample, the emission intensity of the Yb 3+ /Mn 2+ codoped sample substantially decreased due to energy transfer from the visible or NIR emissions to Yb 3+ . [ 33 ] Based on the result shown in Figure 5 (a), the visible emission intensity decreased by 67%, and the NIR emission intensity simultane-ously decreased by 43%, indicating that the energy transfer between isolated Mn 2+ and Yb 3+ is more effi cient than that between Mn 2+ –Mn 2+ dimers and Yb 3+ . The excitation spectra corresponding to the emissions at 585, 770, and 974 nm exhibit similar features (shown in Figure 5 (b)), which strongly support that the 974 nm emission originates from the Mn 2+ → Yb 3+ energy transfer. In addition, the single Mn 2+ -doped KZnF 3 sample does not exhibit UC emission in the visible or NIR range. On the other hand, for Yb 3+ doped perovskite KZnF 3 , as the radius of the Yb 3+ (CN = 6, r = 0.87 Å; CN = 12, r = 1.62 Å) ion is intermediate between that of the K + (CN = 12, r = 1.74 Å) ion and Zn 2+ (r = 0.74 Å), [ 25 ] Yb 3+ ions could occupy both K + and Zn 2+ sites according to previous work about perovskite luminescent materials. [ 26,34,35 ] Therefore, it is reasonable to sug-gest that the NIR UC emission originates from Mn 2+ via energy transfer between Yb 3+ and the Mn 2+ –Mn 2+ dimers.

While the visible UC emission of Mn 2+ has been observed in some Yb 3+ /Mn 2+ codoped systems, the NIR UC emis-sion of Mn 2+ has not been previously documented. In Yb 3+ /Mn 2+ codoped KZnF 3 , both visible and NIR UC emission are

observed, and the ratio of NIR to visible UC emission can be easily tuned by changing the Mn 2+ doping concentration. The visible UC emission originated from the Yb 3+ –Mn 2+ dimers, and the corresponding ground state absorption/excited state absorption (GSA/ESA) UC mechanism is shown in Figure 6 (a). A schematic representation depicting the NIR UC emis-sion process is shown in Figure 6 (b). one Mn 2+ –Mn 2+ dimer and one Yb 3+ ion is proposed to form a trimer through an exchange interaction, and this trimer can act as a UC chromo-phoric unit similar to the Ti 2+ –Mn 2+ –Ti 2+ trimer [ 36 ] and trim-eric Cu(II) clusters. [ 37 ] In the current NIR UC emission mode, the ground, intermediate and emitting state of the Yb 3+ –Mn 2+ –Mn 2+ trimer are represented by , ( ) ( )2

7/26

16

1F A S A Sg g , , ( ) ( )2

5/26

16

1F A S A Sg g and , ( ) ( )27/2

61

41F A S T Gg g , respectively.

The GSA and ESA processes cooperatively contribute to photon pumping followed by the radiative transition from

, ( ) ( )27/2

61

41F A S T Gg g to , ( ) ( )2

7/26

16

1F A S A Sg g state, which results in NIR emission at 770 nm. The detailed UC processes are summarized as follows:


Figure 5. (a) Emission spectra of KZn 0.80 Mn 0.20 F 3 (short dash line) and KZn 0.795 Mn 0.20 Yb 0.005 F 3 (solid line). Inset shows the emission spectra of KZn 0.795 Mn 0.20 Yb 0.005 F 3 in the range of 850 to 1250 nm. (b) Excita-tion spectra (monitoring wavelengthsare 585, 770 and 974 nm) of KZn 0.795 Mn 0.20 Yb 0.005 F 3 .


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→, ( ) ( ) + (976 nm) , ( ) ( ) (GSA)27/2

61

61

25/2

61

61F A S A S hv F A S A Sg g g g

→, ( ) ( ) + (976 nm) , ( ) ( ) (ESA)25/2

61

61

27/2

61

41F A S A S hv F A S T Gg g g g

→, ( ) ( ) , ( ) ( ) + (770 nm)27/2

61

41

27/2

61

61F A S T G F A S A S hvg g g g

2.5. Temperature-Dependent Stokes and UC Emission Properties

For practical applications, the thermal stability is an important factor because the working temperature of most luminescent devices exceeds 300 K. In addition, the higher temperature typically results in lower luminescence effi ciency due to serious nonradiative loss and the different emission centers usually show different temperature-dependent emission behaviors. [ 34,38 ] Therefore, it is necessary to investigate the temperature-dependent luminescence properties of the two emission bands at temperatures higher than 300 K. To compare the differences in the thermal quenching behavior between the visible (585 nm) and NIR (770 nm) emissions, the temperature-dependent emis-sion properties of KZn 0.795 Mn 0.20 Yb 0.005 F 3 and KZn 0.8 Mn 0.2 F 3 have been studied.

Figure 7 (a) shows the emission spectra of KZn 0.8 Mn 0.2 F 3 at various temperatures from 300 to 523 K upon 396 nm light excitation. Both the visible (585 nm) and NIR (770 nm) emis-sion bands exhibit substantial changes as the temperature increases. As shown in Figure 7 (c), at less than 400 K, the increase in temperature leads to enhancement of the lumines-cence intensity until saturation occurs. A further increase in temperature boosts the thermal quenching process due to an obvious decrease in emission intensity. At 473 K, the emission intensities of the visible (585 nm) and NIR emission (770 nm)

decreased to 87% and 62%, respectively, of the values meas-ured at 300 K. When the temperature increased to 523 K, the NIR emission band was thoroughly quenched. Therefore, the luminescence thermal quenching of the Mn 2+ -Mn 2+ dimers is more intense than that of isolated Mn 2+ ions. It is important to note that the luminescence can be renewed in this system. As shown in Figures 7 (b,d), when the samples were gradually cooled to room temperature, the emission intensities of the two emission bands return to their original values. A similar temperature-dependent luminescence characteristic has also been observed in the UC luminescence mode. Figure 8 shows the UC luminescence spectra of KZn 0.795 Mn 0.20 Yb 0.005 F 3 at various temperatures from 300 to 523 K upon excitation with a 976 nm LD. The thermal quenching effect of the NIR UC emission is also more intense than that of visible UC emis-sion. It is noted that both of the UC emission bands exhibit a decreasing trend when the temperature is more than 373 K. As the temperature reaches 523 K, the NIR UC emission is also completely quenched, and only the visible UC emission band can be observed. When the sample is gradually cooled to room temperature, the UC emission of the two emission bands returns to their original values. The observed phenomenon can be explained based on the confi gurational diagram. When the confi gurational coordinate curves of the excited and ground states intersect with each other, an electron in the excited state can cross the intersection assisted by thermal energy and non-radiatively decays to the ground state. [ 39 ] This process can be described by a transition probability per unit time given by [ 39 ]

=

−Δ⎛⎝⎜

⎞⎠⎟

expw sE

k TB (4)

where s is a frequency constant, ΔE is the activation energy, k B is Boltzmann constant, and T is the temperature. With


Figure 6. Schematic energy level diagrams for the Yb 3+ –Mn 2+ dimers and Yb 3+ –Mn 2+ –Mn 2+ trimers and the proposed upconversion mechanisms.


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increasing temperature, the nonradiative transition prob-ability increases resulting in a decrease in the emission inten-sity. For the visible and NIR emission bands, the decrease in UC emission intensity is primarily owing to the nonradiative loss processes involving 2 , ( )7/2

41F T Gg → 2 , ( )5/2

61F A Sg and

, ( ) ( )27/2

61

41F A S T Gg g → , ( ) ( )2

5/26

16

1F A S A Sg g . According to the UC modes of the Yb 3+ –Mn 2+ dimer and Yb 3+ –Mn 2+ –Mn 2+ trimer shown in Figure 6 , the energy gap between 2 , ( )7/2

41F T Gg and

2 , ( )5/26

1F A Sg is larger than that between , ( ) ( )27/2

61

41F A S T Gg g

and , ( ) ( )25/2

61

61F A S A S . Therefore, the nonradiative transition

possibility of 2 , ( )7/24

1F T Gg → 2 , ( )5/26

1F A Sg will be lower than that of , ( ) ( )2

7/26

14

1F A S T Gg g → , ( ) ( )25/2

61

61F A S A S , leading to

a faster decrease in the NIR UC emission as the temperature increases. These excellent temperature-tunable Stokes (UC) emission properties of KZn 0.80 Mn 0.20 F 3 (KZn 0.795 Mn 0.20 Yb 0.005 F 3 ) indicate that the obtained nanocrystals may have potential appli-cations in high-resolution fl uorescence imaging.

3. Conclusions

In addition to the visible emission centered at 585 nm, an anomalous NIR emission at approximately 770 nm has

been revealed in KZnF 3 :Yb 3+ ,Mn 2+ nanocrystals upon exci-tation with 396 nm light or 976 nm LD when the doping concentration of Mn 2+ is suffi cient. The pure NIR UC emis-sion at approximately 770 nm has been demonstrated in KZn 0.595 F 3 :0.40Mn 2+ ,0.005Yb 3+ for the fi rst time. The struc-ture analysis, excitation and emission spectra and lumines-cence decay curves clearly demonstrate that the NIR emission originates from the 6 A 1g (S) 4 T 1g (G)→ 6 A 1g (S) 6 A 1g (S) transi-tion of the coupled Mn 2+ –Mn 2+ dimers. A new GSA/ESA UC mechanism based on the Yb 3+ –Mn 2+ –Mn 2+ trimer has been proposed to explain the NIR UC emission. The present results allow for tuning of the luminescence of UC nanocrystals via rational heavy doping and may provide a useful approach for bio-imaging applications with an improved resolution and enhanced penetration depth.

4. Experimental Section

Material Synthesis : The series of KZn 1- x Mn x F 3 (x = 0.01–1.0) and KZn 0.995- x Mn x Yb 0.005 F 3 ( x = 0.05–0.40) samples were synthesized with a simple hydrothermal method using oleic acid as a stabilizing agent

Figure 7. Temperature-dependent emission spectra of KZn 0.80 Mn 0.20 F 3 during heating (a) and cooling (b). (c) and (d) represent the corresponding integrated emission intensity of KZn 0.80 Mn 0.20 F 3 at various temperatures during heating and cooling, respectively. The emission spectra are obtained upon excitation at 396 nm.


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according to reference. [ 26 ] All of the chemical reagents were used as received without further purifi cation. The reagents used in the experiment are Yb 2 O 3 (99.998%), C 4 H 6 MnO 4 ·4H 2 O(AR), Zn(CH 3 COO) 2 ·2H 2 O (99.99%), KOH (AR) and KF (AR). The Yb 2 O 3 was supplied by Alfa Aesar Reagent Company, and the other reagents were supplied by Aladdin Industrial Corporation. In a typical synthesis, 3 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. Then, 5 mL of an aqueous solution containing stoichiometric amounts of Zn(CH 3 COO) 2 ·2H 2 O, C 4 H 6 MnO 4 ·4H 2 O and Yb(NO 3 ) 3 were added to the solution under vigorous stirring. Finally, 5 mL of an aqueous solution containing 8 mmol of KF was added to the complex under vigorous stirring. The mixture was agitated for 30 min and then transferred to a 50 mL autoclave, which was sealed and hydrothermally treated at 220 °C for 15 hours. After the reaction, the system was cooled to room temperature, and the products were collected and centrifuged several times with the distilled water and absolute ethanol to remove remnant substances, and fi nally, the products were dried at 60°C for several hours.

Material Characterization : 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 40 kV tube voltage and a 40 mA tube current. The size and shape of the samples were measured by a transmission electron microscope (TEM, JEM-2010). The UC emission spectra were measured on a TRIAX320 fl uorescence spectrofl uorometer (Jobin-Yvon Co., France) equipped with a continuous wave 976 nm laser diode (LD) (Coherent corp., USA). The UC luminescence decay curves

were collected by a Tektronix TDS 3012C Digital phosphor oscilloscope. The temperature-dependent Stokes and anti-Stokes emission spectra were measured using the same spectrofl uorimeter equipped with a TAP-02 high-temperature fl uorescence instrument (Tian Jin Orient–KOJI instrument Co., Ltd.). Photoluminescence and photoluminescence excitation spectra as well as fl uorescence decay curves were determined on a FLS920 combined with a time-resolved and steady-state fl uorescence spectrophotometer (Edinburgh Instruments Ltd.).

Acknowledgements This work is fi nancially supported by NSFC (Grant No. 51125005, 21101065 and U0934001), the Ministry of Education (Grant No. 20100172110012), and the Fundamental Research Funds for the Central Universities, SCUT.

[1] T. Kuykendall , P. Ulrich , S. Aloni , P. Yang , Nat. Mater. 2007 , 6 , 951 . [2] A. H. Mueller , M. A. Petruska , M. Achermann , D. J. Werder ,

E. A. Akhadov , D. D. Koleske , M. A. Hoffbauer , V. I. Klimov , Nano Lett. 2005 , 5 , 1039 .

Figure 8. Temperature-dependent upconversion emission spectra of KZn 0.795 Mn 0.20 Yb 0.005 F 3 during heating (a) and cooling (b). (c,d) These represent the corresponding integrated emission intensity of KZn 0.795 Mn 0.20 Yb 0.005 F 3 at various temperatures during heating and cooling, respectively. The upcon-version emission spectra are obtained upon excitation of a 976 nm laser diode.

Received: February 10, 2014 Revised: March 2, 2014

Published online: March 24, 2014


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[3] S. F. Zhou , C. Y. Li , G. Yang , G. Bi , B. Xu , Z. L. Hong , K. Miura , K. Hirao , J. R. Qiu , Adv. Funct. Mater. 2013 , 43 , 5436 .

[4] S. F. Zhou , N. Jiang , B. T. Wu , J. H. Hao , J. R. Qiu , Adv. Funct. Mater. 2009 , 19 , 2081 .

[5] N. M. Idris , M. K. Gnanasammandhan , J. Zhang , P. C. Ho , R. Mahendran , Y. Zhang , Nat. Med. 2012 , 18 , 1580 .

[6] M. Y. Xie , X. N. Peng , X. F. Fu , J. J. Zhang , G. L. Li , X. F. Yu , Scripta Mater. 2009 , 60 , 190 .

[7] M. Nyk , R. Kumar , T. Y. Ohulchanskyy , E. J. Bergey , P. N. Prasad , Nano Lett. 2008 , 8 , 3834 .

[8] H. Wong , H. L. Chan , J. H. Hao , Opt. Express 2010 , 18 , 6123 . [9] G. Y. Chen , T. Y. Ohulchanskyy , R. Kumar , H. Ågren , P. N. Prasad ,

ACS Nano 2010 , 4 , 3163 . [10] Y. Zhang , J. D. Lin , V. Vijayaragavan , K. K. Bhakoo , T. T. Y. Tan ,

Chem. Commun. 2012 , 48 , 10322 . [11] F. Auzel , Chem. Rev. 2004 , 104 , 139 . [12] J. F. Suyver , A. Aebischer , D. Biner , P. Gerner , J. Grimm , S. Heer ,

K. W. Kra Mer , C. Reinhard , H. U. Güdel , Opt. Mater. 2005 , 27 , 1111 . [13] J. Lü , F. Du , R. Zhu , Y. Huang , H. J. Seo , J. Mater. Chem. 2011 , 21 ,

16398 . [14] S. Sugano , Y. Tanabe , H. Kamimura , Multiplets of Transition-Metal

Ions in Crystals , Vol. 33 , Academic Press , New York 1970 . [15] L. Shi , Y. L. Huang , H. J. Seo , J. Phys. Chem. A 2010 , 114 , 6927 . [16] S. Ye , F. Xiao , Y. X. Pan , Y. Y. Ma , Q. Y. Zhang , Mater. Sci. Eng. R.

2010 , 71 , 1 . [17] D. Haranath , S. Mishra , S. Yadav , R. K. Sharma , L. M. Kandpal ,

N. Vijayan , M. K. Dalai , G. Sehgal , V. Shanker , Appl. Phys. Lett. 2012 , 101 , 221905 .

[18] A. Morell , N. El Khiati , J. Electrochem. Soc. 1993 , 140 , 2019 . [19] C. Bertail , S. Maron , V. Buissette , T. Le Mercier , T. Gacoin , J. Boilot ,

Chem. Mater. 2011 , 23 , 2961 . [20] R. Valiente , O. Wenger , H. U. Güdel , Chem. Phys. Lett. 2000 , 320 ,

639 .

[21] P. Gerner , C. Reinhard , H. U. Güdel , Chem-Eur. J. 2004 , 10 , 4735 . [22] C. Reinhard , P. Gerner , R. Valiente , O. S. Wenger , H. U. Güdel , J.

Lumin. 2001 , 94 , 331 . [23] R. Martín-Rodríguez , R. Valiente , F. Rodríguez , F. Piccinelli ,

A. Speghini , M. Bettinelli , Phys. Rev. B 2010 , 82 , 75111 . [24] S. Ye , Y. J. Li , D. C. Yu , G. P. Dong , Q. Y. Zhang , J. Mater. Chem.

2011 , 21 , 3735 . [25] R. D. Shannon , Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor.

Gen. Crystallogr. 1976 , 32 , 751 . [26] E. H. Song , S. Ding , M. Wu , S. Ye , F. Xiao , G. P. Dong , Q. Y. Zhang ,

J. Mater. Chem. C 2013 , 1 , 4209 . [27] Z. G. Xia , R. S. Liu , K. W. Huang , V. Drozd , J. Mater. Chem. 2012 ,

22 , 15183 . [28] J. Lü , F. P. Du , R. Zhu , Y. L. Huang , H. J. Seo , J. Mater. Chem. 2011 ,

21 , 16398 . [29] S. Ye , Z. S. Liu , X. T. Wang , J. G. Wang , L. X. Wang , X. P. Jing , J.

Lumin. 2009 , 129 , 50 . [30] C. R. Ronda , T. Amrein , J. Lumin. 1996 , 69 , 245 . [31] A. P. Vink , M. A. De Bruin , S. Roke , P. S. Peijzel , A. Meijerink , J.

Electrochem. Soc. 2001 , 148 , E313 . [32] M. Pollnau , D. R. Gamelin , S. R. Lüthi , H. U. Güdel , M. P. Hehlen ,

Phys. Rev. B 2000 , 61 , 3337 . [33] G. J. Gao , L. Wondraczek , J. Mater. Chem.C 2013 , 1 , 1952 . [34] Y. Zhang , J. H. Hao , C. L. Mak , X. H. Wei , Opt. Express 2011 , 19 ,

1824 . [35] J. H. Hao , Y. Zhang , X. H. Wei , Angew. Chem., Int. Ed. 2011 , 50 ,

6876 . [36] M. Herren , S. M. Jacobsen , H. U. Güdel , Inorg. Chem. 1989 , 28 ,

504 . [37] J. Yoon , E. I. Solomon , Coordin. Chem. Rev. 2007 , 251 , 379 . [38] V. Bachmann , C. Ronda , A. Meijerink , Chem. Mater. 2009 , 21 , 2077 . [39] J. Orive , J. L. Mesa , R. Balda , J. Fernández , J. R. Fernández , T. Rojo ,

M. I. Arriortua , Inorg. Chem. 2011 , 50 , 12463 .

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