JOURNAL OF RARE EARTHS, Vol. 32, No. 9, Sep. 2014, P. 802

Foundation item: Project supported by Fund for Fostering Talents in Basic Science of the National Natural Science Foundation of China (J1103207), the National Natural Science Foundation of China (11274288, 21002097), the National Basic Research Program of China (2011CB932801, 2012CB933702) and Ministry of Education of China (20123402110034)

* Corresponding authors: ZHANG Zengming, WANG Zhongping (E-mail: zzm@ustc.edu.cn, zpwang@ustc.edu.cn; Tel.: +86-551-63607671, +86-551-63601850)

DOI: 10.1016/S1002-0721(14)60144-7

Upconversion luminescence of NaYF4:Yb,Er nanocrystals with high uniformity

YE Yangsen (叶杨森)1, JIANG Zhihao (江志浩)1, WANG Qizheng (王棋正)1, ZHU Zishu (朱子疏)1, WANG Xiao (王 潇)1, SUI Zhilei (随志磊)1, DAI Rucheng (代如成)2, WANG Zhongping (王中平)2,*, ZHANG Zengming (张增明)2,*, DING Zejun (丁泽军)1,3 (1. Department of Physics, University of Science and Technology of China, Hefei 230026, China; 2. The Centre for Physical Experiments, University of Science and Technology of China, Hefei 230026, China; 3. Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China)

Received 25 September 2013; revised 24 April 2014

Abstract: The NaYF4:Yb,Er nanocrystals were synthesized via the thermal decomposition of metal oleate precursors. The nanocrys-tals in hexagonal structure were highly uniform and in size of 25 nm. The bright upconversion luminescence was observed under the excitation of 980 nm laser and the upconversion emission spectra were investigated at different pump powers. The emission intensity ratio of red light to green light linearly increased with pump power increasing. This result indicated that there existed a large threshold power of saturation pump for the first excitation state in NaYF4:Yb,Er nanocrystals comparing to that in bulk material.

Keywords: upconversion luminescence; nanocrystals; NaYF4:Yb,Er; rare earths

Recently, new upconversion nanocrystals have drawn much attention due to a multitude of possible applica-tions in solar cells[1–3], laser materials[4], bio-probe[5], bio-labeling[6-8], lighting and display technologies[9,10]. These materials can be excited by near-infrared radiation (NIR) in the window of transparency of biological tissue[6] and emit bright, multi-color visible fluorescence. This NIR excitation can avoid autofluorescence background which has been a major challenge in traditional fluorescent la-beling under ultraviolet or visible excitation[11,12]. Among upconversion nanomaterials, the NaYF4 nanocrystal is one of the most efficient upconversion host materials to date[13]. Because of the high luminescence efficiency, that the upconversion luminescence from single nanocrystal in size of 27 nm in a confocal microscope has been achieved[8].

Most of current researches on upconversion are fo-cused on crystalline particles with the size larger than 100 nm. But with decreasing size, the possibility of sur-face quenching increases[14]. For nano-scale upconver-sion particles, the surface to volume ratio is large and the space distribution of sensitizers and active centers are easily controlled. However, most of the previous works were done for the upconversion luminescence of nanocrystals with different morphologies. Because of its difficulty in fabrication, quite few works were carried out for nanocrystals in uniform morphology.

This work presented an approach to synthesizing NaYF4:Yb,Er upconversion nanocrystals with the uni-form size of 25 nm. The upconversion luminescence of the nanocrystals was investigated. The intensity of up-conversion emissions and the intensity ratio of red light to green light were found to increase with pump power increasing for NaYF4:Yb,Er nanocrystals.

1 Experimental

1.1 Synthesis

All used reagents are analytical and without further purification. The NaYF4:Yb,Er nanocrystals were syn-thesized via the thermal decomposition of metal oleate precursors[14,15]. In a 100 mL beaker, Y2O3 (0.4508 g), Yb2O3 (0.1773 g) and Er2O3 (0.0199 g) were vigorously stirred with HCl (40 mL) until the solution became clear. Yttrium chloride hexahydrate (YCl3·6H2O), ytterbium chloride hexahydrate (YbCl3·6H2O) and erbium chloride hexahydrate (ErCl3·6H2O) were synthesized after the so-lution was evaporated to dryness. Deionized water was added into the beaker to dissolve the chloride crystal. The solution was mixed with OA (30 mL) and ODE (75 mL) magnetically in a 250 mL three-neck round-bottom flask for 20 min. The mixture was then heated to 150 ºC in N2 flow for 30 min to form a clear yellow solution. After the solution cooled down to room temperature, an-

YE Yangsen et al., Upconversion luminescence of NaYF4:Yb,Er nanocrystals with high uniformity 803

other mixture of 50 mL methanol, NH4F (0.7522 g) and NaOH (0.5116 g) was added into the three-neck round- bottom flask. After stirring for 30 min, the solution was heated to 110 ºC and maintained at this temperature in the N2 flow for 30 min to completely remove methanol. Then the solution was heated again to 280 ºC and aged for 45 min under the protection of N2 flow. After the so-lution cooled, the NaYF4:Yb,Er nanocrystals were pre-cipitated from the solution with acetone and washed with ethanol for two times and with ethanol/water (1:1 v/v) for two times. The nanocrystals were redispersed in 15 mL ethanol and then the final product was obtained after the ethanol was completely evaporated.

1.2 Characterization

X-ray diffraction (XRD) was carried out on an X-ray diffraction apparatus (MXP18AHF, MAC Science Co., Ltd.) with Cu Kα radiation. Size and morphology of specimen were characterized by a field emission scan-ning electron microscope (SEM) (Hitachi S-4800) and a transmission electron microscope (TEM) (JEOL-2010). The absorption spectrum was measured at incidence from 400 to 1150 nm by a double-beam spectropho-tometer (Shimazu Co., Inc. Solid Spec-3700). The up-conversion emission spectra were recorded by Acton SP2750 (Princeton Instruments). The excitation source of UC luminescence was 980 nm semiconductor laser and the spot size of laser was about 0.03 mm2.

2 Results and discussion

2.1 Morphology and crystalline structure

Fig. 1 shows XRD pattern of NaYF4:Yb,Er nanocrys-tals. All the diffraction peaks can be indexed to a pure hexagonal phase (β-phase) of NaYF4 according to the standard data of JCPDS 28-1192. There is not any extra diffraction peak from impurities in XRD pattern. This

Fig. 1 XRD pattern of NaYF4:Yb,Er nanocrystals

indicates that the Yb3+ and Er3+ ions are effectively doped in the host lattice.

SEM image in Fig. 2 and TEM image in Fig. 3 for the hexagonal NaYF4:Yb,Er nanocrystals illustrate that the synthesized nanocrystals are quite uniform and in size of 25 nm. TEM image shows that the morphology of NaYF4:Yb,Er nanocrystals is the uniform hexagonal tab-let. This tablet morphology is of large ratio of surface to volume and in favor of the absorbing pump laser.

2.2 Absorption and emission spectra

Fig. 4 displays the absorption spectrum of NaYF4:Yb,Er nanocrystals at room temperature. There exists a sharp absorption band at about 970 nm from the sensitizer Yb3+. The upconversion luminescence spectrum is displayed in Fig. 5, in which NaYF4:Yb,Er nanocrystals are under the excitation of 980 nm laser. The schematic of energy lev-els for Yb3+ and Er3+ ions is given in Fig. 6. As sensitiz-ers, the Yb3+ ion is easily excited from ground state 2F7/2 to excitation state 2F5/2 due to a large absorption cross section for 980 nm pump laser. The Er3+ ion is excited from ground state 4I15/2 to excitation state 4I11/2 by energy transfer (ET) from the excited Yb3+ ion because of en-ergy level match between them. The populations at 4I11/2 can be excited to 4F7/2 and by excitation state absorption (ESA) processes. Subsequent nonradiative processes

Fig. 2 SEM image of the hexagonal NaYF4:Yb,Er nanocrystals

Fig. 3 TEM image of the hexagonal NaYF4:Yb,Er nanocrystals

804 JOURNAL OF RARE EARTHS, Vol. 32, No. 9, Sep. 2014

Fig. 4 Absorption spectrum of NaYF4:Yb,Er nanocrystals dis-

persed in ethanol (0.025 g/mL)

Fig. 5 Upconversion emission spectrum of NaYF4:Yb,Er nano-

crystals powder under the excitation of 980 nm laser

Fig. 6 Schematic of the upconversion processes of Er3+ in β-

NaYF4:Yb3+,Er3+ (CR means cross relaxation) within the Er3+ ions populate their radiation states of 2H11/2 and 4S3/2. Another mechanism is the cross relaxa-tion (CR) between Er3+ ions. CR process is to excite the population at 4I11/2 to 4F9/2 by the transition from 4F7/2 to 4F9/2 in nearby Er3+ ions. The characteristic emission

peaks in green band are attributed to the 2H11/2→ 4I15/2 and

4S3/2→4I15/2 (522 nm and 541 nm, respectively). The

emission peak in red band arises from 4F9/2→4I15/2 (650

nm). The emission peak in blue band (430 nm) is rela-tively weak and omitted here.

2.3 Upconversion luminescence

Fig. 7 shows the upconversion emission spectra of NaYF4:Yb,Er nanocrystals under excitation of 980 nm laser with different pump powers. With increasing pump power, the upconversion luminescence intensities in-creased greatly. Fig. 8 plots the double logarithmic curve of emission intensity versus pump power. The slopes are 2.19 and 1.81 for the green light and 2.10 for red light. All these slopes are close to 2, indicating that the two- photon processes are responsible for both red and green

Fig. 7 Upconversion luminescence of NaYF4:Yb,Er nanocrys-

tals powder at different pump powers under the excita-tion of 980 nm laser

Fig. 8 Double logarithmic plots of emission intensity of NaYF4:

Yb,Er nanocrystals powder versus pump power

YE Yangsen et al., Upconversion luminescence of NaYF4:Yb,Er nanocrystals with high uniformity 805

upconversion emissions in the NaYF4:Yb,Er nanocrys-tals.

Fig. 9 displays the dependence of the intensity ratio of red light to green light for NaYF4:Yb,Er nanocrystals on pump power. Different from bulk materials[16], this ratio for nanocrystals increases almost linearly with the excit-ing power. In general, the populations at ground state are completely excited under strong pump. With further in-creasing pump power, the ESA process dominates the populations to higher energy levels. This results in the competition between red and green emissions of the up-conversion luminescence process. Thus, it is possible to modulate the color from single irradiation source by changing the excitation power only[16]. But the present result indicates that the threshold power of saturation pump for the first excitation state in NaYF4:Yb,Er nanocrystals is much larger than that for bulk materials.

Fig. 9 Pump power dependence of the intensity ratio of red light

to green light of NaYF4:Yb,Er nanocrystals powder

3 Conclusions

The NaYF4:Yb,Er nanocrystals in size of 25 nm and with high uniformity were synthesized via the thermal decomposition of metal oleate precursors. The upconver-sion emission spectra were investigated under different pump powers. The nanocrystals displayed the bright up-conversion luminescence by laser excitation of 980 nm. With increasing pump power, the emission intensity ratio of red light to green light increased linearly. This result indicated that there was a larger threshold power of satu-ration pump for the first excitation state in NaYF4:Yb,Er nanocrystals comparing to that in bulk materials.

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