Highly Enhanced Upconversion Luminescence Through Partially Isolate Yb3 + in Core-shell-shell Structured NaYF4 ∶Yb3 +,Ln3 +@NaYF4 ∶Yb3 +@NaYF4(Ln=Ho, Tm) Nanocrystals

LIAO Hua-zhen XIAO Ping YE Song WANG De-ping

(School of Materials Science and Engineering, Tongji University, Shanghai 201804, China)

Abstract： In this research,the Yb3+ /Ln3+(Ln=Ho3+ and Tm3+) doped β-NaYF4 core nanocrystals(NCs) with sub-10 nm size were synthesized with co-precipitation method,based on which the core-shell-shell structured NaYF4∶Yb3+,Ln3+@NaYF4∶Yb3+ @ NaYF4 NCs were constructed. The successfully growth of the middle active-shell and the outmost inert-shell were proved by XRD and TEM measurements. The spectral results indicated that partially isolate Yb3+dopant in active-core and active-shell and the growth of the outmost inert-shell can effectively improve the upconversion(UC) emission intensity of Ho3+ and Tm3+, which is resulted from the enhanced absorption of 980 nm excitation light and energy transfer efficiency ascribing to the increased Yb3+ concentration quenching threshold. Moreover,through adjusting Yb3+ doping concentration in the middle-shell, the tunable emission can be obtained. This research suggested a general route for the development of highly-efficient luminescent upconversion nanocrystals(UCNCs) in a broad color range.

Key words： upconversion； core-shell-shell structure； β-NaYF4

1 Introduction

Upconversion(UC) luminescence is an anti-Stokes process in which the near infrared(NIR)light can be converted into shorter wavelength light in ultraviolet or visible rangesviamulti-photon processes[1-3]. In recent years, the lanthanide-doped upconversion crystals(UCNCs) have provoked considerable interest due to their prominent luminescence properties of narrow emission bands, long excited-state lifetimes, tunable emissions, low background interference and deep tissue penetration[4-7],which thus have been prevalent in many applications like bioimaging, detection, photothermal therapy,and optical temperature sensors[8-11]. As we know,the accomplishment of strong UC emission intensity is of particular significance for UCNCs to determine their application depth and width. However, it still remains a challenge to obtain high-efficient UC luminescence due to the influences of nonradiative decays, small excitation cross sections of the rare earth ions and abundant surface quenching centers caused by the large surface-to-volume ratio of the UCNCs[12-14]. So far, many efforts have been made to overcome these negative factors, and one commonly used method is to choose low phonon energy fluorides like NaLnF4(Ln=Y, Gd and Lu), CaF2and YF3as doping host to reduce nonradiative transitions[15-17]. Besides, many other attempts have also been tried to improve UC emission intensity, such as increasing excitation power density[18-19], changing the crystal field symmetry through doping cations[8,20]and attaching noble metals nanoparticles on UCNCs to generate surface plasmon resonance(SPR) enhancement[21-22]. However, the abovementioned methods show limited enhancements and also involve complex preparation procedures or suffer from high cost.

Alternatively, one effective strategy for achieving enhanced UC emission is to increase the dopant concentration, which can simultaneously improve absorption ability and energy transfer efficiency[11,23]. However, the excessive doping of lanthanide ions in the same spatial structure will induce deleterious interaction between doped ions and thus leads to severely concentration quenching[24-25].Therefore, another protocol of fabricating core-shell structured nanoparticles was proposed to improve the UC luminescence properties, which can not only separate the luminescent centers from surface defects and ligands but also make it possible for higher doping concentration as well as tunable emission with diverse dopants[26-29]. For example, Zhanget al.grew the monolayer NaGdF4shell outside the NaYF4∶Yb,Er core NCs to passivate the surface defects of the NCs and thus enhance the overall UC emission intensity[30]. Apart from coating inert-shell, the active shell was also applied to construct core-shell structured nanoparticles. Chienet al. overcoated the LiYF4∶Yb3+/Er3+/Ho3+/Tm3+core nanoparticles with an active LiYF4∶Yb3+shell and thus obtained 3 times enhanced UC emission intensity by suppressing surface-related deactivations[31]. Moreover, in addition to improving UC efficiency, multifunctionality or multicolor emission was realized through the construction of core-shell-shell structured UCNCs[32-35].For instance, Penget al. prepared NaYF4∶Yb/Er@NaYF4∶Yb@NaNdF4∶Yb core-shell-shell nanoparticles that can be effectively excited by 808 nm or 980 nm laser[15]. Siefeet al. designed a unique β-NaYbF4@ NaY0.8-xErxGd0.2F4@ NaY0.8Gd0.2F4coreshell-shell architecture, in which sensitizers and activators were respectively partitioned into the core and the interior shell, and thus the core-shell-shell design was 2-fold brighter than the standard NaY0.58-Gd0.2Yb0.2Er0.02F4@NaY0.8Gd0.2F4core-shell design[36].By constructing NaGdF4∶ Eu/Tb@NaGdF4∶Yb,Tm@NaGdF4∶Yb,Nd core-shell-shell UCNCs, intense and multicolor emissions can be obtained with 808 nm excitation[33]. The magnetic-optical dual-modal imaging was also reported by fabricating NaGdF4∶Yb,Ho,Ce/NaYF4∶Nd,Yb/NaGdF4core/double-shell structured nanoparticles[37]. Therefore, the proper design of core-shell or core-shell-shell structured NCs can essentially improve the UC emission properties of lanthanides as it can greatly increase their doping level by isolating them into separated spatial structures.

In our research,the pure β-NaYF4∶Yb3+,Ln3+(Ln=Ho, Tm) core NCs were firstly prepared, and then through the construction of active-core@activeshell@ inert-shell structured β-NaYF4∶Yb3+,Ln3+@NaYF4∶x%Yb3+@NaYF4NCs,the greatly enhanced UC emission can be obtained. The Yb3+concentration dependent UC luminescence properties of the core-shell and core-shell-shell NCs were investigated, and the importance of partially isolate Yb3+dopant in the core and middle-shell, as well as the outmost inert shell growth on the enhancement of UC emission intensity were systematically discussed.

2 Experiments

2.1 Materials

YbCl3·6H2O, HoCl3·6H2O, TmCl3·6H2O and YCl3·6H2O with 99. 99% purity were purchased by Jiangxi Anrre Advanced Materials Co.,Ltd. Ammonium fluoride(NH4F,99.99%), sodium oleate(NaOA, ＞97%), oleic acid(OA,90%),methanol(MeOH,＞99.7%), 1-octadecene(ODE,90%) and cyclohexane( ＞97%) were purchased from Shanghai Aladdin Bio-chem Technology Co.,Ltd. Absolute ethanol( ＞99. 7%) was supplied from Sinopharm Chemical Reagent Co. Ltd.

2.2 Synthesis of β-NaYF4 ∶Yb3+,RE3+(RE =Ho and Tm) Core NCs

The β-NaYF4∶0.1Yb3+,0.02Ho3+and β-NaYF4∶0.1Yb3+,0.01Tm3+core NCs (named as CHoand CTm, respectively) were synthesized through co-precipitation method[26]. In the synthesis of CHoand CTmcore NCs, 0. 88 mmol YCl3·6H2O, 0. 10 mmol YbCl3·6H2O together with 0.02 mmol HoCl3·6H2O or 0. 01 mmol TmCl3·6H2O were firstly dropped in a three-necked flask with 15 mL ODE and 6 mL OA, and then the above mixed solution was heated to 150 ℃ and kept for 35 min under argon(Ar) atmosphere. After the above solution cooled down naturally,4 mmol NH4F and 2.5 mmol NaOA that dissolved in two sets of 10 mL MeOH were dropped, and then the mixed solution was incubated at 65 ℃and kept for 40 min. Finally, the resulting solution was heated to 280 ℃for 30 min under Ar atmosphere and then naturally cooled down. The resulted core NCs were firstly precipitated, collected and re-dispersed.

2.3 Synthesis of β-NaYF4 ∶Yb3+,RE3+(RE =Ho and Tm)@NaYF4∶Yb3+ Core-shell NCs

The core-shell NCs with various Yb3+doping levels in shell layer, including β-NaYF4∶0.1Yb3+,0.02Ho3+@NaYF4∶xYb3+(x=0, 0. 025, 0. 05,0.1, 0. 2, 0. 3, 0. 5 and 0. 7, named as CHoS0,CHoS2.5, CHoS5, CHoS10, CHoS20, CHoS30, CHoS60and CHoS70, respectively) and β-NaYF4∶0.1Yb3+,0.01 Tm3+@NaYF4∶yYb3+(y=0,0.05,0.1,0.2,0.4,0.6, named as CTmS0, CTmS5, CTmS10, CTmS20,CTmS40and CTmS60, respectively) were synthesized with the as-prepared core NCs as seeds by using seed-mediated growth method. In the typical synthetic procedure,1 mmol YCl3·6H2O and YbCl3·6H2O were firstly added into a three-necked flask together with 15 mL ODE and 6 mL OA. And then,the above mixed solution was heated to 160 ℃and kept for 35 min under Ar atmosphere. After the solution was cooled down, the as-synthesized core NCs were added with a core to shell molar ratio of 1 ∶4 and the temperature was maintained at 85 ℃ for around 15 min to remove the cyclohexane. When the mixture was cooled down to room temperature, 4 mmol NH4F and 2.5 mmol NaOA were dissolved in two sets of 10 mL MeOH respectively under ultrasound and slowly dropped into the above solution.After that, the MeOH was completely removed by keeping the temperature at 60° C for 40 min. Finally, the solution was heated to 270 ℃and kept for 120 min under Ar atmosphere. After cooled down to room temperature, the obtained core-shell NCs were precipitated, collected and re-dispersed.

2.4 Synthesis of β-NaYF4 ∶Yb3+,RE3+(RE =Ho and Tm)@NaYF4∶Yb3+@NaYF4 Coreshell-shell NCs

By using seed-mediated growth method, the core-shell-shell NCs were synthesized with the assynthesized core-shell NCs as seeds. Firstly,1 mmol YCl3·6H2O was firstly dropped into a three-necked flask together with 6 mL OA and 15 mL ODE, and then this solution was heated to 150 ℃ and kept over 35 min under Ar atmosphere. After that, the as-synthesized core-shell NCs were dropped into the above solution with a core-shell to shell molar ratio of 1∶4 and then maintained at 85 ℃for around 15 min to remove the cyclohexane. When the mixture was cooled down, 2. 5 mmol NaOA and 4 mmol NH4F were dissolved in two sets of 10 mL MeOH respectively under ultrasound before slowly dropped into the above solution. After that, the MeOH can be completely removed by keeping the temperature at 60 ℃for 40 min. Subsequently, the solution was heated to 280 ℃for 150 min and then cooled down to room temperature. The obtained core-shell-shell NCs were precipitated, collected and re-dispersed.According to Yb3+doping condition, the Ho3+doped core-shell-shell NCs were named as CHoS2.5S,CHoS5S,CHoS10S,CHoS20S,CHoS30S, CHoS60S and CHoS70S, respectively, and the Tm3+doped core-shell-shell NCs were named as CTmS0S, CTmS5S, CTmS10S, CTmS20S,CTmS40S and CTmS60S,respectively.

2.5 Characterization

The structure and phase of the NCs were characterized by X-ray diffraction (XRD) measurement using RigakuD/maxrB diffractometer. The morphology and size of the NCs were determined by transmission electron microscope (TEM, JEM-2100F).The UC emission spectra were acquired with photomultiplier detector (Zolix DlnGaAs1700 InGaAs Detector and PMTH-S1-CR131 Photomultiplier Tube with Housing) together with a 980 nm laser as an external excitation source.

3 Results and Discussion

3.1 Crystal Structure, Phase and Morphology

The structure and phase purity of the as-synthesized core NCs of CHoand CTmwere firstly characterized with XRD measurements. As can be observed from Fig.1 that both of which are identified as pure β-NaYF4phase, and the broad diffraction peaks suggests very small particle sizes. The TEM images of these as-synthesized core NCs are shown in Fig.2,the spherical core NCs are well dispersed and uniform in size with statistical average diameter around 7.2 nm for CHoand 6.3 nm for CTm, respectively,by random measurements of 200 particles.

Fig.1 XRD patterns for the core NCs of CHo and CTm

Fig.2 TEM images of the core NCs of CHo((a), (b)) and CTm((c), (d))

By using CHoand CTmas seeds, the core-shell NCs with increasing Yb3+doping concentrations in the shell were synthesized through seed-mediated growth method. The XRD patterns in Fig.3(a) and(c) revealed that the core-shell NCs are pure β-NaYF4phase as no other peaks from impurities were detected, even for the high-level Yb3+doped samples, which indicates a good solid solubility of NaYF4for Yb3+. Besides, the sharpened diffraction peak profile for those core-shell NCs in comparison with the core NPs verified the successful growth of the shell layer. Furthermore, the core-shell-shell structured NC with an inert outmost NaYF4layer epitaxial grown on the core-shell NCs was synthesized and the corresponding XRD patterns in Fig. 3(b)and (d) indicated the final products still keep pure β-NaYF4phase.

Fig.3 XRD patterns for the Ho3+ doped core-shell NCs(a) and core-shell-shell NCs(b), and the Tm3+ doped core-shell NCs(c) and core-shell-shell NCs(d), respectively.

The morphology as well as the size distribution of the core-shell and the core-shell-shell NCs were further studied with TEM measurements. As can be observed in the representative TEM images for CHoS70, CHoS70S, CTmS60and CTmS60S in Fig. 4,along with the size increasing by growing the middle and the outmost shells, the morphology of the resultant NCs changed from sphere to hexagon for the core-shell NCs and then to ellipse for the core-shellshell NCs, respectively. It is worth mentioning that the resultant core-shell NCs and core-shell-shell NCs were still highly monodisperse and uniform. The diameters for the core-shell structured CHoS70and CTmS60were estimated to be 12.6 nm and 12.3 nm, respectively. While for the ellipse core-shell-shell NCs, the statistical average sizes are 29.1 nm(length) ×18.2 nm(width) for CHoS70S,and 27.3 nm(length) ×17.5 nm (width) for CTmS60S,respectively.

3.2 UC Luminescence Properties of Core-shellshell Structured NaYF4∶Yb3+,Ho3+@NaYF4∶Yb3+@NaYF4 NCs

The UC luminescence spectra of those core NCs and core-shell NCs under the excitation of 980 nm are presented in Fig.5(a), the characteristic Ho3+emission peaked at 491,546,651 nm can be clearly observed for all the NCs, which can be assigned to Ho3+：5F3→5I8,5S2/5F4→5I8and5F5→5I8transitions, respectively, as shown in Fig. 6. It is noticed that the UC emission intensity of the core NCs CHois extremely weak caused by the strong surface quenching effect, while the growth of inert NaYF4shell can effectively enhance the UC emission intensities because of the spatial separation of Ho3+and Yb3+ions from surface quenching centers like defects and organic ligands[15,28,38]. Moreover, if Yb3+was also doped in shell, the emission intensity of Ho3+can be further improved, and the maximum intensity increase of 64 times can be obtained in CHoS2.5.As can be observed in Fig. 5(a) - (b), the UC emission intensity obviously enhanced with the increase of Yb3+concentration up to 2.5%, and then rapidly decreased when 5% or higher Yb3+was doped in the shell. The first enhancement of UC emission intensity with the increase of Yb3+concentration can be explained from two aspects： more effective absorption of 980 nm excitation light resulted from the increased absorption centers,and more efficient energy transfer to Ho3+resulted from the faster energy migration between Yb3+. While the major reason for the reduction in UC emission intensity with high Yb3+doping is resulted from the concentration quenching effect, which imposed a loss in transferring the excitation energy to Ho3+[39-40].

Fig.5 UC luminescence spectra for the core, core-shell(a) and core-shell-shell(d) NCs, integrated emission intensities for the core-shell(b) and core-shell-shell(e) NCs, and green to red emission intensity ratios for the core-shell(c) and coreshell-shell(f) NCs, respectively.

In order to further increase the UC emission intensity, an outmost NaYF4shell was grown on the as-synthesized NaYF4∶Yb3+,Ho3+@ NaYF4∶Yb3+core-shell NCs to reduce surface quenching. As we can observe from Fig.5(a) -(b) and (d) -(e),the core-shell-shell structured NCs exhibit over one magnitude enhanced UC emission intensity compared with the core-shell NCs with the same Yb3+doping concentration in the shell. More interestingly, the UC emission intensity for the core-shell-shell NCs exhibits different Yb3+concentration dependency from the core-shell NCs, which monotonously increases with increasing Yb3+concentration up to 30%, and then decreases when further increase Yb3+concentration. This phenomenon can be reasonably explained as following： the coating of an inert NaYF4layer can spatially separate Yb3+from surface defects, and therefore effectively minimized the concentration quenching of Yb3+, or in another word, the concentration quenching threshold for the Yb3+located in the middle-shell can be greatly enhanced with the growth of the outmost inert-shell. As a result, further enhanced UC emission was obtained in the core-shell-shell NCs caused by both the improved absorption efficiency of 980 nm excitation light and the increased energy transfer efficiency to Ho3+with high Yb3+doping in the middle-shell.However, the intrinsic quenching centers like defects cannot be modified through surface coating. As a result, when the concentration of Yb3+in the middle-shell is too high and the energy migration becomes very fast, the concentration quenching will become dominate and therefore a decreased UC emission was observed.

It was noticed that the green to red emission intensity ratio(RG/R) of Ho3+changes with the increase of Yb3+doping level in shell, and which shows different Yb3+concentration dependency for the core-shell and the core-shell-shell NCs, as shown in Fig. 5(c) and (f). For the core-shell NCs,RG/Rfirstly increases with the increase of Yb3+doping up to 30% and then decreases when continue increasing Yb3+concentration. This is because the energy transfer efficiency from Yb3+to Ho3+increases with increasing Yb3+concentration, which effectively promote the population of Ho3+：5S2/5F4energy level, and thereforeRG/Rincreases accordingly[41-42]. Nevertheless, with increasing Yb3+concentration, the population of5S2/5F4level is gradually restrained caused by the energy back transfer from Ho3+to Yb3+[42], while the population of5F5level is promoted due to the saturation of the5I7level,which thus leads to decreasedRG/Rwhen Yb3+concentration is higher than 30%. While for the coreshell-shell NCs,RG/Rshows monotonous decrease with increasing Yb3+doping concentration in middle active-shell. This is because the coating of an inert NaYF4shell can effectively separate Yb3+from surface quenching centers and thus persistently promote the energy transfer from Yb3+to Ho3+, which resulted in the saturation of Ho3+：5I7, and the following2F5/2(Yb3+) +5I7(Ho3+)→2F7/2(Yb3+) +5F5(Ho3+) cross-relaxation process effectively populated the red emission level of Ho3+∶5F5,as shown in Fig.6.

Fig.6 Schematic energy levels of Yb3+ and Ho3+ showing the UC emission process

The luminescence photographs of the core-shell and core-shell-shell NCs under 980 nm excitation were shown in Fig.7(a) and (b), from which the drastically enhanced UC emission intensity after the optimizing of Yb3+concentration and the coating of inert NaYF4shell can be clearly observed. Fig.7(c) and (d) presented the shifting of chromaticity coordinates for the core-shell and core-shell-shell NCs with increasing Yb3+concentration. It can be seen that the CIE chromaticity coordinate of the core NCs is (0.337,0.403), corresponding to a yellowish green. With increasing Yb3+concentration in the active-shell, the chromaticity coordinates shifted from (0.355, 0.527) to (0.342, 0.408) for the core-shell NCs and from (0.302,0.624) to (0.361,0.549) for the core-shell-shell NCs. Therefore,tunable color emission can be achieved simply by tuning Yb3+concentration in the middle-shell,suggesting potential applications in display technology, multicolor biolabeling and multiplexed detection.

Fig.7 Luminescence photographs and CIE chromaticity of the core-shell((a), (c)) and core-shell-shell NCs((b), (d))

3.3 UC Luminescence Properties of The Coreshell-shell Structured NaYF4 ∶Yb3+,Tm3+@NaYF4∶Yb3+@NaYF4 NCs

According to the above experimental results,the construction of β-NaYF4∶Yb3+,Ho3+@NaYF4∶Yb3+@NaYF4structure is an effective method to improve the UC emission intensity of Ho3+. The doping of sensitizer ions of Yb3+in the core and middle-shell can effectively increase the absorption efficiency of 980 nm excitation light and meanwhile minimize the concentration quenching, since the Yb3+ions were separated in different spatial structure. On the other hand, the growth of outmost inert NaYF4shell can effectively increase the concentration quenching threshold of Yb3+in the middleshell, and therefore further increased absorption efficiency and energy transfer efficiency can be obtained in higher Yb3+doped NCs. The improvement of UC emission intensity through the construction of activecore@ active-shell@ inert-shell structure is also applicable for other Yb3+sensitized UC systems, such as the Yb3+/Tm3+couple. Fig.8(a) -(b) present the UC emission spectra of Yb3+/Tm3+codoped core, core-shell and core-shell-shell NCs under 980 nm laser excitation, from which the blue and red emissions due to Tm3+：1D2→3F4(455 nm),1G4→3H6(478 nm), and1G4→3F4(650 nm) transitions can be clearly observed, and the UC conversion process was described in Fig. 9. We can also observe that the core NCs exhibit drastically increased emission intensity after the coating of an inert NaYF4shell, while when an active-shell containing Yb3+ion was grown, the emission intensity gradually decreased in comparing with CTmS0when increasing Yb3+concentration. This phenomenon can be explained by the fast energy migration between Yb3+,which led to the quenching of excitation energy through energy transfer to the surface quenching centers. Fortunately, the surface quenching can be suppressed after the growth of an outmost inert NaYF4shell, and therefore greatly improved emission intensity can be obtained, as shown in Fig.8(b). The greatly increased UC emission after the growth of inert NaYF4shell can be directly seen from the luminescence photographs from Fig.8(c) by our naked eyes. According to the integrated emission intensity ratio of the core-shell-shell NCs over the coreshell NCs in Fig.8(d),the higher the Yb3+concentration in the middle active-shell,the higher magnitude the UC luminescence enhanced. This is in accordance with our previous results that the surface modification is of great importance for the improvement of UC emission intensity when the energy migration between Yb3+became fast. Besides, it is worthy of mentioning that the different behavior of Yb3+/Ho3+and Yb3+/Tm3+codoped couples on the growth of Yb3+contained middle active-shell and the after growth of outmost NaYF4inert-shell may due to the different Yb3+concentration dependent energy transfer efficiency from Yb3+to Ho3+and Tm3+.

Fig.8 UC luminescence spectra for the core, core-shell NCs(a) and core-shell-shell NCs(b). (c)Luminescence photographs for the core-shell NCs of CTmS0,CTmS5,CTmS10,CTmS20,CTmS40,CTmS60(above,from left to right),and the core-shellshell NCs of CTmS5S,CTmS10S,CTmS20S,CTmS40S,CTmS60S(bottom,from left to right),respectively. (d)Integrated UC emission intensity ratio of core-shell-shell NCs over core-shell NCs.

Fig.9 Schematic energy levels for Yb3+ and Tm3+ showing the UC emission process

4 Conclusion

The Yb3+/Ln3+(Ln= Ho3+and Tm3+) codoped pure β-NaYF4core NCs with sub-10 nm size were synthesized by co-precipitation method, based on which the Yb3+doped active-shell and the inertshell were grown in sequence, and the successfully growth of the middle active-shell and the outmost inert-shell were proved by XRD and TEM measurements. The spectral results indicated that partially isolate Yb3+dopant in the core-shell-shell NCs is a desirable method to improve UC emission intensity,as it can effectively increase the absorption efficiency of 980 nm excitation light and meanwhile minimize Yb3+concentration quenching. Besides, the growth of outmost inert NaYF4shell can effectively increase the concentration quenching threshold of Yb3+in the middle-shell, and therefore further increased absorption efficiency and energy transfer efficiency can be obtained. Moreover, through adjusting Yb3+doping concentration in the middle-shell, the tunable emission can be obtained due to the change of the relative emission intensities of different emission colors.It was expected that these core-shell-shell structured NCs with bright UC emission may have potential applications in three-dimensional display and light emitting diode.