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Yb3+ doped Na (Y1.5Na0.5) F6 single-crystal nanorods
Journal of Alloys and Compounds 493 (2010) 476–480
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and multicolor upconversion of Tm3+ /Er3+ /Yb3+ doped Na (Y1.5 Na0.5 ) F6 single-crystal nanorods Songjun Zeng, Guozhong Ren, Qibin Yang ∗ Institute of Modern Physics, Xiangtan University, Xiangtan 411105, China
a r t i c l e
i n f o
Article history: Received 30 September 2009 Received in revised form 18 December 2009 Accepted 19 December 2009 Available online 4 January 2010 Keywords: Nanorods Upconversion mechanisms Hydrothermal method Na (Y1.5 Na0.5 ) F6
a b s t r a c t Er3+ /Tm3+ /Yb3+ tri-doped Na (Y1.5 Na0.5 ) F6 single-crystal nanorods were synthesized by a facile hydrothermal treatment method. The tri-doped sample was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM). The sample showed a pure hexagonal phase structure with space group P63 /m and nanorod morphology. Under 980 nm excitation, intense red, green and blue upconversion emissions were observed. The red and green upconversion are consistent with 4 F9/2 → 4 I15/2 and 2 H11/2 , 4 S3/2 → 4 I15/2 transition of Er3+ , respectively, while the blue upconversion originates from 1 G4 → 3 H6 transition of Tm3+ . Moreover, the Er3+ /Tm3+ /Yb3+ tri-doped Na (Y1.5 Na0.5 ) F6 nanorods exhibit strong red and green emission which is ascribed to the sensitization of Tm3+ to Er3+ . The energy transfer upconversion mechanisms for the fluorescent intensity are also discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, one dimensional (1D) nanoscale materials, such as nanowires, nanotubes, nanobelts and nanorods, have attracted much interest due to their potential applications in nanoscale optoelectronics [1,2], electronics [3,4], lasers [1,5] and biological labels [6], etc. Many unique and fascinating properties for this kind of materials such as high luminescence efficiency and a lowered lasing threshold have already been reported in literatures [7,8]. Among all the unique properties, the upconversion (UC) properties have aroused great interest due to their potential for use in color displays [9], solar cells [10], biological detection [11,12], and so on. Among all of the commonly used UC host materials, including oxyfluorides [13,14], oxysulfides [15], fluorides [16] and others, NaYF4 is one of the most efficient UC host materials by means of doping with lanthanide ions owing to the low phonon energy of the crystal lattice [17]. The crystal structure of NaYF4 exhibits two polymorphic forms, namely, cubic and hexagonal phases, depending on the synthesis conditions and methods. It has been reported that hexagonal phase NaYF4 is a much better host lattice than the cubic phase NaYF4 for the upconversion luminescence [18]. Hereby, it is essential to synthesize pure hexagonal phase NaYF4 crystal with uniform size and high quality structure under simple processes without any other phases. Several reports have been devoted to synthesize hexagonal phase NaYF4 nano- and micro-
∗ Corresponding author. Tel.: +86 0732 8292113. E-mail addresses: [email protected], [email protected] (Q. Yang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.12.133
crystals doped with the lanthanide ions [19,20] and investigate the influence of crystal phase on the luminescence properties [21,22]. Recently, Ln3+ tri-doped NaYF4 nanoparticles [23] and Ln3+ doped NaYF4 /polymer composite fibers prepared by electrospinning [24] have been reported. However, there were rare reports on highly uniform rare earth ions doped hexagonal phase NaYF4 with 1D nanostructure such as nanorod shape. In the present work, the highly uniform Na (Y1.5 Na0.5 ) F6 singlecrystal nanorods doped with Er3+ /Tm3+ /Yb3+ were synthesized via a simple hydrothermal treatment. The as-prepared nanorods emit strong multicolor NIR-to-visible UC fluorescence via excitation by diode laser (LD) with wavelength of 980 nm. And the corresponding UC mechanism was investigated in detail. 2. Experimental details All the rare-earth oxides were 99.99% purity. Rare earth nitrate RE(NO3 )3 (RE = Y, Tm, Er, Yb) solution with 0.1 mol L−1 was formed by dissolving RE2 O3 (99.99%) in HNO3 and excess nitric acid was removed by evaporation under elevated temperature. Other chemicals were of analytical grade and used as received without further purification. The NaYF4 nanorods were synthesized using the method described in Ref. [25] with some modifications. First, 1.2 g NaOH, 2 mL distilled water, 9 mL ethanol, and 20 mL oleic acid were mixed together under agitation to form a homogeneous solution. Next, 1.77 mL Y(NO)3 (0.5 M), 0.2 mL Tm(NO)3 (0.05 M), 0.1 mL Er(NO)3 (0.05 M), 0.2 mL Yb(NO)3 (0.5 M), and 8 mL NaF (1.0 M) aqueous solution were added to the solution under vigorous stirring. The resulting mixture was vigorously stirred for another 30 min. The resulting solution was then transferred into a 50 mL stainless Teflon-lined autoclave, sealed, and heated at 190 ◦ C for 24 h. After that the system was then allowed to cool down to room temperature naturally and the products were deposited at the bottom of the vessel. The obtained nanorods were washed with ethanol and water to remove oleic acid and other remnants. Finally dried in air
S. Zeng et al. / Journal of Alloys and Compounds 493 (2010) 476–480
477
Fig. 1. XRD pattern of the as-prepared Na (Y1.5 Na0.5 ) F6 sample.
at 60 ◦ C for 6 h. In addition, the final products can be well-dispersed in a nonpolar solvent such as cyclohexane and aggregated by adding polar solvent such as ethanol. The phase composition and phase purity of the obtained nanorods were examined by XRD on a D/max-␥A System X-ray diffractometer at 40 kV and 40 mA with Cu K␣ radiation. TEM and the high-resolution transmission electron microscopy (HRTEM) images were obtained using JEM-2100 microscope operated at the accelerating voltage of 200 kV. The samples were dispersed in nonpolar cyclohexane solvent and were then subjected to ultrasonication to achieve a well-dispersed suspension. One drop of the suspension was placed on copper grid covered with hollow carbon films for TEM assays. The upconversion emission spectra at room temperature were detected by a spectrophotometer (R500) under the excitation of 980 nm LD. All the above measurements were carried out at room temperature.
3. Results and discussion Fig. 1 shows a typical XRD pattern of the as-prepared Na (Y1.5 Na0.5 ) F6 sample. All of the strong and sharp diffraction peaks in Fig. 1 can be readily indexed to hexagonal phase of -Na (Y1.5 Na0.5 ) F6 (space group: P63 /m), which are consistent with the literature data (JCPDS 16-0334). No other impurity peak was detected which revealed that the pure hexagonal phase of Na (Y1.5 Na0.5 ) F6 had been fabricated. As can be seen from XRD pattern, the high crystallinity can be obtained at the low hydrothermal treatment temperature (190 ◦ C). High crystallinity is important for phosphors, because it generally means less traps and stronger luminescence [26]. A typical TEM image in Fig. 2a shows that the obtained -Na (Y1.5 Na0.5 ) F6 : Tm3+ /Er3+ /Yb3+ from the hydrothermal condition exhibits rod-like morphology with high quality and uniform size of the 1500 nm in length and 90 nm in diameter, as estimated from the TEM images. The aspect ratio is about 16. Fig. 2b shows a single assynthesized Na (Y1.5 Na0.5 ) F6 nanorod. The HRTEM image (Fig. 2c) taken from the single nanorod presented in Fig. 2b shows clearly resolved lattice fringes. The interplanar distance d = 0.257 nm and 0.295 nm match that of the (2 0 0) and (1 1 0) lattice planes of hexagonal phase Na (Y1.5 Na0.5 ) F6 respectively. In addition, the growth direction of the as-prepared nanorods along the [1 1 0] direction can be also confirmed by using HRTEM pattern. Its fast Fourier transform (FFT) of the HRTEM image shown in the inset of Fig. 2c also reveals single-crystalline nature of as-synthesized nanorods and can also be readily indexed as hexagonal phase Na (Y1.5 Na0.5 ) F6 , which is in good agreement with the XRD results presented in Fig. 1. Under 980 nm excitation, the strong multicolor fluorescence can be observed in the Tm3+ /Er3+ /Yb3+ doped Na (Y1.5 Na0.5 ) F6 single-crystal nanorods. The upconversion luminescence spectra of Na (Y1.5 Na0.5 ) F6 nanorods doped with Tm3+ /Yb3+ , Er3+ /Yb3+
Fig. 2. TEM and HRTEM images of the as-prepared Na (Y1.5 Na0.5 ) F6 nanorods: (a) overview image, (b) single nanorod image, and (c) the typical HRTEM image for single nanorod. The inset of Fig. 2c is corresponding Fourier transform of HRTEM image.
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S. Zeng et al. / Journal of Alloys and Compounds 493 (2010) 476–480
Fig. 3. Room temperature upconversion spectra of Na (Y1.5 Na0.5 ) F6 single-crystal nanorods under 980 nm excitation (a) doped with 0.5 mol%Tm3+ , 10 mol%Yb3+ ions, (b) doped with 0.5 mol%Er3+ , 10 mol%Yb3+ ions, and (c) doped with 0.5 mol%Tm3+ , 0.5 mol%Er3+ and 10 mol%Yb3+ ions.
and Tm3+ /Er3+ /Yb3+ under 980 nm excitation are shown in Fig. 3. As can be observed, in Tm3+ /Yb3+ double doped Na (Y1.5 Na0.5 ) F6 nanorods (Fig. 3a) two visible emissions 476 nm (blue), 650 nm and a weak peaks at 690 nm (red) were observed which can be ascribed to 1 G4 → 3 H6 , 1 G4 → 3 F4 and 3 F3 → 3 H6 transitions. And in Er3+ /Yb3+ co-doped Na (Y1.5 Na0.5 ) F6 nanorods (Fig. 3b), there were three well-known emission bands centered at 520, 545 and 660 nm which are associated with the transition from 2 H11/2 , 4 S3/2 and 4 F9/2 level to ground state (4 I15/2 ) of Er3+ . As compared with Fig. 3a and b, in Fig. 3c the blue emission near 476 nm comes from the 1 G4 → 3 H6 transition of Tm3+ ions, the sharp peaks in the green region around 520 and 545 nm are assigned to the transition emissions 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 of Er3+ ions, respectively, while the peak in the red region of 660 nm corresponding to the 4 3+ ions and the weak red emission band centered 4F 9/2 → I15/2 of Er at 690 nm is ascribed to 3 F3 → 3 H6 transition of Tm3+ ions. It is noted that the emission band at 650 nm in Fig. 3a arising from the transition 1 G4 → 3 H4 of Tm3+ ions coincides with that of Er3+ ions in Fig. 3b. However, the contribution of Tm3+ ions to the red emission band in Fig. 3c can be neglected due to the intensity of 650 nm band in Fig. 3a is much smaller than that of the blue band, which is also expected in Fig. 3c. As shown in Fig. 3, the introduction of Tm3+ ions into the Na (Y1.5 Na0.5 ) F6 nanorods significantly influences the emissions of Er3+ ions. The emission intensity of red and green is obviously enhanced. Furthermore, the blue upconversion of Tm3+ decreases in some extent when Er3+ , Tm3+ and Yb3+ ions are tri-doped. Fig. 4 shows the upconversion spectrum of Tm3+ , Er3+ , Yb3+ codoped Na (Y1.5 Na0.5 ) F6 nanorods with different concentrations of Tm3+ ion under 980 nm excitation. The blue, red and green emission intensity increases remarkably with Tm3+ doping content increasing from 0.2 to 0.5 mol%. However, it falls abruptly when Tm3+ content reaches 0.7 mol%, indicating the occurrence of the concentration quenching. The upconversion spectrum of as-prepared Na (Y1.5 Na0.5 ) F6 nanorods with varying Yb3+ ion concentrations excited by 980 nm is presented in Fig. 5. With the increase of Yb3+ concentration, the whole upconversion emission intensity increases gradually. However, when the Yb3+ doping concentration increased up to 15%, the upconversion emission intensity decreased due to the concentration quenching.
Fig. 4. Upconversioin spectra of x% Tm3+ , 0.5% Er3+ , 10% Yb3+ co-doped Na (Y1.5 Na0.5 ) F6 nanorods with different Tm3+ concentrations under 980 nm excitation.
To understand the mechanism, the dependence of the blue, green and red upconversion emissions of 0.5% Er3+ , 0.5% Tm3+ , and 10% Yb3+ ions co-doped Na (Y1.5 Na0.5 ) F6 nanorods on excitation power was analyzed. Fig. 6 shows the log–log plot of the UC emission intensity as a function of the pump power. As shown in Fig. 6, all the green and red emission bands centered at 520, 545, 660 and 690 nm exhibit a quadratic relationship (slope = 1.96, 1.78, 1.73 and 1.91 for 520, 545, 660 and 690 nm, respectively), which reveals that two photons steps are involved in these upconversion emissions. While, a slope of 2.59 for the blue emission indicates that three phonons are necessary for the blue upconversion emission centered at 476 nm. Fig. 7 shows simplified energy level diagrams of the Er3+ , Tm3+ , and Yb3+ ions as well as proposed upconversion mechanisms to produce the multicolor radiation. In Er3+ /Tm3+ /Yb3+ doped system, excitation of a single level 2 F5/2 of Yb3+ is only considered because Yb3+ ions have larger absorption cross-section and Tm3+
Fig. 5. Upconversioin spectra of 0.5% Tm3+ , 0.5% Er3+ , x% Yb3+ co-doped Na (Y1.5 Na0.5 ) F6 nanorods with different Yb3+ concentrations under 980 nm excitation.
S. Zeng et al. / Journal of Alloys and Compounds 493 (2010) 476–480
FIg. 6. Dependence of upconversion emission intensity at 496, 520, 545, 660, 690 nm on excitation power under 980 nm in 0.5% Tm3+ , 0.5% Er3+ , 10% Yb3+ co-doped Na (Y1.5 Na0.5 ) F6 nanorods.
ions have no corresponding energy level. It is well know that the Yb3+ can efficiently sensitize Er3+ and Tm3+ ions. Under 980 nm excitation, the Yb3+ ions are excited from the 2 F7/2 level to the 2F 3+ and 5/2 level, then transfer their energies to the nearby Tm 3+ 3+ 4 4 Er ions. Thus Er ions are raised from I15/2 to I11/2 , and Tm3+ ions are raised from 3 H6 to 3 H5 excited states. The Tm3+ ions in 3 H level and Er3+ in 4 I 3 5 11/2 level will non-radiative transmit to F4 and 4 I13/2 levels respectively. Secondly, the Tm3+ in 3 F4 level are promoted to 3 F2 level and Er3+ ions in 4 I11/2 /4 I13/2 to 4 F7/2 /4 F9/2 levels by the same energy transfer. The Tm3+ ions in 3 F2 level and Er3+ ions in 4 F7/2 level relax non-radiatively to 3 H4 level and 4 2H 11/2 / S3/2 levels respectively. The third energy transfer processes, the Tm3+ in 3 H4 level is raised to 1 G4 level. Finally, the blue emission at 476 nm is generated by transition of Tm3+ ions 1 G4 → 3 H6 transition; the green emission at 520 and 545 nm are assigned to the Er3+ ions 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 transitions; the red emission are due to the Er3+ ions 4 F9/2 → 4 I15/2 (660 nm) the Tm3+ ions 3 F3 → 3 H6 transitions (690 nm). However, some other mechanism must exist, since the intensity of the red, green and blue emissions were changed in our triply doped sample as mentioned
479
above. For the Er3+ , Tm3+ and Yb3+ doped samples, Tm3+ behaved as a sensitizer. The intensification of red luminescence is possibly due to the efficient energy transfer processes between Tm3+ and Er3+ . (1) 4 I11/2 (Er3+ ) + 3 F4 (Tm3+ ) → 4 F9/2 (Er3+ ) + 3 H6 (Tm3+ ); (2) 4F 3+ 3 3+ 4 3+ 3 3+ 7/2 (Er ) + H6 (Tm ) → F9/2 (Er ) + F4 (Tm ). The red emission is weak in the Er3+ /Yb3+ doped sample, and it is significantly enhanced when Tm3+ is introduced to the system, indicating that the energy transfer processes play a dominant role in populating the 4 F9/2 excite state of Er3+ [27]. And Tm3+ behaves as the sensitizer to Er3+ for the green upconversion luminescence through the energy transfer process [28]: 3 H4 (Tm3+ ) + 4 I15/2 (Er3+ ) → 4 I9/2 (Er3+ ) + 3 H6 (Tm3+ ). On the other hand, the sensitization of Tm3+ to Er3+ depopulates 3 H4 and 3 F4 energy level and weakens the 476 nm emission. Blue luminescence can be quenched at a relative lower concentration of Tm3+ ions in Er3+ , Yb3+ and Tm3+ tri-doped glasses than in Yb3+ /Tm3+ co-doped sample by the following process: 1 G4 (Tm3+ ) + 3 H6 (Tm3+ ) → 3 H4 (Tm3+ ) + 3 H5 (Tm3+ ). Therefore, the blue emission of the tri-doped sample decreases compared with Tm3+ and Yb3+ ions co-doped sample. Based on the analyses above, by properly adjusting the concentrations of the Tm3+ , Er3+ and Yb3+ ions, the intensity of the blue, green and red upconversion emissions can be changed in the Na (Y1.5 Na0.5 ) F6 nanorods. 4. Conclusions In summary, Tm3+ /Er3+ /Yb3+ doped Na (Y1.5 Na0.5 ) F6 singlecrystal nanorods were synthesized via a simple hydrothermal method. TEM and XRD results show that these nanorods present uniform morphology and high quality. Under 980 nm excitation, intense green, red and blue upconversion was obtained. In addition, the upconversion mechanism was fully discussed in detail. For the Tm3+ /Er3+ /Yb3+ /tri-doped Na (Y1.5 Na0.5 ) F6 nanorods, the blue, green, and red upconversion emission can be attributed to the transitions 1 G4 → 3 H6 of Tm3+ , 2 H11/2 /4 S3/2 → 4 I15/2 , and 4 F9/2 → 4 I15/2 of Er3+ ions, respectively. In addition, the efficient energy transfer processes between Er3+ and Tm3+ occurred. Tm3+ behaves as the sensitizer to Er3+ for red and green luminescence of Er3+ and Er3+ acts as quenching center for the blue upconversion luminescence of Tm3+ ions. This hexagonal phase Na (Y1.5 Na0.5 ) F6 nanorods with multicolor upconversion emission might promise further fundamental research and be found application in nanoscale optoelectronics and solid-state lasers. Acknowledgments This work is supported by the National Natural Scientific Foundation of China (No. 10874144) and the Scientific Foundation of Education Department of Hunan Province (No. 08C885). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
3+
3+
Fig. 7. Simplified energy level diagram of Yb , Tm mechanism for the upconversion luminescence.
3+
and Er , as well the possible
[12] [13] [14] [15]
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and multicolor upconversion of Tm3+ /Er3+ /Yb3+ doped Na (Y1.5 Na0.5 ) F6 single-crystal nanorods Songjun Zeng, Guozhong Ren, Qibin Yang ∗ Institute of Modern Physics, Xiangtan University, Xiangtan 411105, China
a r t i c l e
i n f o
Article history: Received 30 September 2009 Received in revised form 18 December 2009 Accepted 19 December 2009 Available online 4 January 2010 Keywords: Nanorods Upconversion mechanisms Hydrothermal method Na (Y1.5 Na0.5 ) F6
a b s t r a c t Er3+ /Tm3+ /Yb3+ tri-doped Na (Y1.5 Na0.5 ) F6 single-crystal nanorods were synthesized by a facile hydrothermal treatment method. The tri-doped sample was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM). The sample showed a pure hexagonal phase structure with space group P63 /m and nanorod morphology. Under 980 nm excitation, intense red, green and blue upconversion emissions were observed. The red and green upconversion are consistent with 4 F9/2 → 4 I15/2 and 2 H11/2 , 4 S3/2 → 4 I15/2 transition of Er3+ , respectively, while the blue upconversion originates from 1 G4 → 3 H6 transition of Tm3+ . Moreover, the Er3+ /Tm3+ /Yb3+ tri-doped Na (Y1.5 Na0.5 ) F6 nanorods exhibit strong red and green emission which is ascribed to the sensitization of Tm3+ to Er3+ . The energy transfer upconversion mechanisms for the fluorescent intensity are also discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, one dimensional (1D) nanoscale materials, such as nanowires, nanotubes, nanobelts and nanorods, have attracted much interest due to their potential applications in nanoscale optoelectronics [1,2], electronics [3,4], lasers [1,5] and biological labels [6], etc. Many unique and fascinating properties for this kind of materials such as high luminescence efficiency and a lowered lasing threshold have already been reported in literatures [7,8]. Among all the unique properties, the upconversion (UC) properties have aroused great interest due to their potential for use in color displays [9], solar cells [10], biological detection [11,12], and so on. Among all of the commonly used UC host materials, including oxyfluorides [13,14], oxysulfides [15], fluorides [16] and others, NaYF4 is one of the most efficient UC host materials by means of doping with lanthanide ions owing to the low phonon energy of the crystal lattice [17]. The crystal structure of NaYF4 exhibits two polymorphic forms, namely, cubic and hexagonal phases, depending on the synthesis conditions and methods. It has been reported that hexagonal phase NaYF4 is a much better host lattice than the cubic phase NaYF4 for the upconversion luminescence [18]. Hereby, it is essential to synthesize pure hexagonal phase NaYF4 crystal with uniform size and high quality structure under simple processes without any other phases. Several reports have been devoted to synthesize hexagonal phase NaYF4 nano- and micro-
∗ Corresponding author. Tel.: +86 0732 8292113. E-mail addresses: [email protected], [email protected] (Q. Yang). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.12.133
crystals doped with the lanthanide ions [19,20] and investigate the influence of crystal phase on the luminescence properties [21,22]. Recently, Ln3+ tri-doped NaYF4 nanoparticles [23] and Ln3+ doped NaYF4 /polymer composite fibers prepared by electrospinning [24] have been reported. However, there were rare reports on highly uniform rare earth ions doped hexagonal phase NaYF4 with 1D nanostructure such as nanorod shape. In the present work, the highly uniform Na (Y1.5 Na0.5 ) F6 singlecrystal nanorods doped with Er3+ /Tm3+ /Yb3+ were synthesized via a simple hydrothermal treatment. The as-prepared nanorods emit strong multicolor NIR-to-visible UC fluorescence via excitation by diode laser (LD) with wavelength of 980 nm. And the corresponding UC mechanism was investigated in detail. 2. Experimental details All the rare-earth oxides were 99.99% purity. Rare earth nitrate RE(NO3 )3 (RE = Y, Tm, Er, Yb) solution with 0.1 mol L−1 was formed by dissolving RE2 O3 (99.99%) in HNO3 and excess nitric acid was removed by evaporation under elevated temperature. Other chemicals were of analytical grade and used as received without further purification. The NaYF4 nanorods were synthesized using the method described in Ref. [25] with some modifications. First, 1.2 g NaOH, 2 mL distilled water, 9 mL ethanol, and 20 mL oleic acid were mixed together under agitation to form a homogeneous solution. Next, 1.77 mL Y(NO)3 (0.5 M), 0.2 mL Tm(NO)3 (0.05 M), 0.1 mL Er(NO)3 (0.05 M), 0.2 mL Yb(NO)3 (0.5 M), and 8 mL NaF (1.0 M) aqueous solution were added to the solution under vigorous stirring. The resulting mixture was vigorously stirred for another 30 min. The resulting solution was then transferred into a 50 mL stainless Teflon-lined autoclave, sealed, and heated at 190 ◦ C for 24 h. After that the system was then allowed to cool down to room temperature naturally and the products were deposited at the bottom of the vessel. The obtained nanorods were washed with ethanol and water to remove oleic acid and other remnants. Finally dried in air
S. Zeng et al. / Journal of Alloys and Compounds 493 (2010) 476–480
477
Fig. 1. XRD pattern of the as-prepared Na (Y1.5 Na0.5 ) F6 sample.
at 60 ◦ C for 6 h. In addition, the final products can be well-dispersed in a nonpolar solvent such as cyclohexane and aggregated by adding polar solvent such as ethanol. The phase composition and phase purity of the obtained nanorods were examined by XRD on a D/max-␥A System X-ray diffractometer at 40 kV and 40 mA with Cu K␣ radiation. TEM and the high-resolution transmission electron microscopy (HRTEM) images were obtained using JEM-2100 microscope operated at the accelerating voltage of 200 kV. The samples were dispersed in nonpolar cyclohexane solvent and were then subjected to ultrasonication to achieve a well-dispersed suspension. One drop of the suspension was placed on copper grid covered with hollow carbon films for TEM assays. The upconversion emission spectra at room temperature were detected by a spectrophotometer (R500) under the excitation of 980 nm LD. All the above measurements were carried out at room temperature.
3. Results and discussion Fig. 1 shows a typical XRD pattern of the as-prepared Na (Y1.5 Na0.5 ) F6 sample. All of the strong and sharp diffraction peaks in Fig. 1 can be readily indexed to hexagonal phase of -Na (Y1.5 Na0.5 ) F6 (space group: P63 /m), which are consistent with the literature data (JCPDS 16-0334). No other impurity peak was detected which revealed that the pure hexagonal phase of Na (Y1.5 Na0.5 ) F6 had been fabricated. As can be seen from XRD pattern, the high crystallinity can be obtained at the low hydrothermal treatment temperature (190 ◦ C). High crystallinity is important for phosphors, because it generally means less traps and stronger luminescence [26]. A typical TEM image in Fig. 2a shows that the obtained -Na (Y1.5 Na0.5 ) F6 : Tm3+ /Er3+ /Yb3+ from the hydrothermal condition exhibits rod-like morphology with high quality and uniform size of the 1500 nm in length and 90 nm in diameter, as estimated from the TEM images. The aspect ratio is about 16. Fig. 2b shows a single assynthesized Na (Y1.5 Na0.5 ) F6 nanorod. The HRTEM image (Fig. 2c) taken from the single nanorod presented in Fig. 2b shows clearly resolved lattice fringes. The interplanar distance d = 0.257 nm and 0.295 nm match that of the (2 0 0) and (1 1 0) lattice planes of hexagonal phase Na (Y1.5 Na0.5 ) F6 respectively. In addition, the growth direction of the as-prepared nanorods along the [1 1 0] direction can be also confirmed by using HRTEM pattern. Its fast Fourier transform (FFT) of the HRTEM image shown in the inset of Fig. 2c also reveals single-crystalline nature of as-synthesized nanorods and can also be readily indexed as hexagonal phase Na (Y1.5 Na0.5 ) F6 , which is in good agreement with the XRD results presented in Fig. 1. Under 980 nm excitation, the strong multicolor fluorescence can be observed in the Tm3+ /Er3+ /Yb3+ doped Na (Y1.5 Na0.5 ) F6 single-crystal nanorods. The upconversion luminescence spectra of Na (Y1.5 Na0.5 ) F6 nanorods doped with Tm3+ /Yb3+ , Er3+ /Yb3+
Fig. 2. TEM and HRTEM images of the as-prepared Na (Y1.5 Na0.5 ) F6 nanorods: (a) overview image, (b) single nanorod image, and (c) the typical HRTEM image for single nanorod. The inset of Fig. 2c is corresponding Fourier transform of HRTEM image.
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Fig. 3. Room temperature upconversion spectra of Na (Y1.5 Na0.5 ) F6 single-crystal nanorods under 980 nm excitation (a) doped with 0.5 mol%Tm3+ , 10 mol%Yb3+ ions, (b) doped with 0.5 mol%Er3+ , 10 mol%Yb3+ ions, and (c) doped with 0.5 mol%Tm3+ , 0.5 mol%Er3+ and 10 mol%Yb3+ ions.
and Tm3+ /Er3+ /Yb3+ under 980 nm excitation are shown in Fig. 3. As can be observed, in Tm3+ /Yb3+ double doped Na (Y1.5 Na0.5 ) F6 nanorods (Fig. 3a) two visible emissions 476 nm (blue), 650 nm and a weak peaks at 690 nm (red) were observed which can be ascribed to 1 G4 → 3 H6 , 1 G4 → 3 F4 and 3 F3 → 3 H6 transitions. And in Er3+ /Yb3+ co-doped Na (Y1.5 Na0.5 ) F6 nanorods (Fig. 3b), there were three well-known emission bands centered at 520, 545 and 660 nm which are associated with the transition from 2 H11/2 , 4 S3/2 and 4 F9/2 level to ground state (4 I15/2 ) of Er3+ . As compared with Fig. 3a and b, in Fig. 3c the blue emission near 476 nm comes from the 1 G4 → 3 H6 transition of Tm3+ ions, the sharp peaks in the green region around 520 and 545 nm are assigned to the transition emissions 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 of Er3+ ions, respectively, while the peak in the red region of 660 nm corresponding to the 4 3+ ions and the weak red emission band centered 4F 9/2 → I15/2 of Er at 690 nm is ascribed to 3 F3 → 3 H6 transition of Tm3+ ions. It is noted that the emission band at 650 nm in Fig. 3a arising from the transition 1 G4 → 3 H4 of Tm3+ ions coincides with that of Er3+ ions in Fig. 3b. However, the contribution of Tm3+ ions to the red emission band in Fig. 3c can be neglected due to the intensity of 650 nm band in Fig. 3a is much smaller than that of the blue band, which is also expected in Fig. 3c. As shown in Fig. 3, the introduction of Tm3+ ions into the Na (Y1.5 Na0.5 ) F6 nanorods significantly influences the emissions of Er3+ ions. The emission intensity of red and green is obviously enhanced. Furthermore, the blue upconversion of Tm3+ decreases in some extent when Er3+ , Tm3+ and Yb3+ ions are tri-doped. Fig. 4 shows the upconversion spectrum of Tm3+ , Er3+ , Yb3+ codoped Na (Y1.5 Na0.5 ) F6 nanorods with different concentrations of Tm3+ ion under 980 nm excitation. The blue, red and green emission intensity increases remarkably with Tm3+ doping content increasing from 0.2 to 0.5 mol%. However, it falls abruptly when Tm3+ content reaches 0.7 mol%, indicating the occurrence of the concentration quenching. The upconversion spectrum of as-prepared Na (Y1.5 Na0.5 ) F6 nanorods with varying Yb3+ ion concentrations excited by 980 nm is presented in Fig. 5. With the increase of Yb3+ concentration, the whole upconversion emission intensity increases gradually. However, when the Yb3+ doping concentration increased up to 15%, the upconversion emission intensity decreased due to the concentration quenching.
Fig. 4. Upconversioin spectra of x% Tm3+ , 0.5% Er3+ , 10% Yb3+ co-doped Na (Y1.5 Na0.5 ) F6 nanorods with different Tm3+ concentrations under 980 nm excitation.
To understand the mechanism, the dependence of the blue, green and red upconversion emissions of 0.5% Er3+ , 0.5% Tm3+ , and 10% Yb3+ ions co-doped Na (Y1.5 Na0.5 ) F6 nanorods on excitation power was analyzed. Fig. 6 shows the log–log plot of the UC emission intensity as a function of the pump power. As shown in Fig. 6, all the green and red emission bands centered at 520, 545, 660 and 690 nm exhibit a quadratic relationship (slope = 1.96, 1.78, 1.73 and 1.91 for 520, 545, 660 and 690 nm, respectively), which reveals that two photons steps are involved in these upconversion emissions. While, a slope of 2.59 for the blue emission indicates that three phonons are necessary for the blue upconversion emission centered at 476 nm. Fig. 7 shows simplified energy level diagrams of the Er3+ , Tm3+ , and Yb3+ ions as well as proposed upconversion mechanisms to produce the multicolor radiation. In Er3+ /Tm3+ /Yb3+ doped system, excitation of a single level 2 F5/2 of Yb3+ is only considered because Yb3+ ions have larger absorption cross-section and Tm3+
Fig. 5. Upconversioin spectra of 0.5% Tm3+ , 0.5% Er3+ , x% Yb3+ co-doped Na (Y1.5 Na0.5 ) F6 nanorods with different Yb3+ concentrations under 980 nm excitation.
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FIg. 6. Dependence of upconversion emission intensity at 496, 520, 545, 660, 690 nm on excitation power under 980 nm in 0.5% Tm3+ , 0.5% Er3+ , 10% Yb3+ co-doped Na (Y1.5 Na0.5 ) F6 nanorods.
ions have no corresponding energy level. It is well know that the Yb3+ can efficiently sensitize Er3+ and Tm3+ ions. Under 980 nm excitation, the Yb3+ ions are excited from the 2 F7/2 level to the 2F 3+ and 5/2 level, then transfer their energies to the nearby Tm 3+ 3+ 4 4 Er ions. Thus Er ions are raised from I15/2 to I11/2 , and Tm3+ ions are raised from 3 H6 to 3 H5 excited states. The Tm3+ ions in 3 H level and Er3+ in 4 I 3 5 11/2 level will non-radiative transmit to F4 and 4 I13/2 levels respectively. Secondly, the Tm3+ in 3 F4 level are promoted to 3 F2 level and Er3+ ions in 4 I11/2 /4 I13/2 to 4 F7/2 /4 F9/2 levels by the same energy transfer. The Tm3+ ions in 3 F2 level and Er3+ ions in 4 F7/2 level relax non-radiatively to 3 H4 level and 4 2H 11/2 / S3/2 levels respectively. The third energy transfer processes, the Tm3+ in 3 H4 level is raised to 1 G4 level. Finally, the blue emission at 476 nm is generated by transition of Tm3+ ions 1 G4 → 3 H6 transition; the green emission at 520 and 545 nm are assigned to the Er3+ ions 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 transitions; the red emission are due to the Er3+ ions 4 F9/2 → 4 I15/2 (660 nm) the Tm3+ ions 3 F3 → 3 H6 transitions (690 nm). However, some other mechanism must exist, since the intensity of the red, green and blue emissions were changed in our triply doped sample as mentioned
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above. For the Er3+ , Tm3+ and Yb3+ doped samples, Tm3+ behaved as a sensitizer. The intensification of red luminescence is possibly due to the efficient energy transfer processes between Tm3+ and Er3+ . (1) 4 I11/2 (Er3+ ) + 3 F4 (Tm3+ ) → 4 F9/2 (Er3+ ) + 3 H6 (Tm3+ ); (2) 4F 3+ 3 3+ 4 3+ 3 3+ 7/2 (Er ) + H6 (Tm ) → F9/2 (Er ) + F4 (Tm ). The red emission is weak in the Er3+ /Yb3+ doped sample, and it is significantly enhanced when Tm3+ is introduced to the system, indicating that the energy transfer processes play a dominant role in populating the 4 F9/2 excite state of Er3+ [27]. And Tm3+ behaves as the sensitizer to Er3+ for the green upconversion luminescence through the energy transfer process [28]: 3 H4 (Tm3+ ) + 4 I15/2 (Er3+ ) → 4 I9/2 (Er3+ ) + 3 H6 (Tm3+ ). On the other hand, the sensitization of Tm3+ to Er3+ depopulates 3 H4 and 3 F4 energy level and weakens the 476 nm emission. Blue luminescence can be quenched at a relative lower concentration of Tm3+ ions in Er3+ , Yb3+ and Tm3+ tri-doped glasses than in Yb3+ /Tm3+ co-doped sample by the following process: 1 G4 (Tm3+ ) + 3 H6 (Tm3+ ) → 3 H4 (Tm3+ ) + 3 H5 (Tm3+ ). Therefore, the blue emission of the tri-doped sample decreases compared with Tm3+ and Yb3+ ions co-doped sample. Based on the analyses above, by properly adjusting the concentrations of the Tm3+ , Er3+ and Yb3+ ions, the intensity of the blue, green and red upconversion emissions can be changed in the Na (Y1.5 Na0.5 ) F6 nanorods. 4. Conclusions In summary, Tm3+ /Er3+ /Yb3+ doped Na (Y1.5 Na0.5 ) F6 singlecrystal nanorods were synthesized via a simple hydrothermal method. TEM and XRD results show that these nanorods present uniform morphology and high quality. Under 980 nm excitation, intense green, red and blue upconversion was obtained. In addition, the upconversion mechanism was fully discussed in detail. For the Tm3+ /Er3+ /Yb3+ /tri-doped Na (Y1.5 Na0.5 ) F6 nanorods, the blue, green, and red upconversion emission can be attributed to the transitions 1 G4 → 3 H6 of Tm3+ , 2 H11/2 /4 S3/2 → 4 I15/2 , and 4 F9/2 → 4 I15/2 of Er3+ ions, respectively. In addition, the efficient energy transfer processes between Er3+ and Tm3+ occurred. Tm3+ behaves as the sensitizer to Er3+ for red and green luminescence of Er3+ and Er3+ acts as quenching center for the blue upconversion luminescence of Tm3+ ions. This hexagonal phase Na (Y1.5 Na0.5 ) F6 nanorods with multicolor upconversion emission might promise further fundamental research and be found application in nanoscale optoelectronics and solid-state lasers. Acknowledgments This work is supported by the National Natural Scientific Foundation of China (No. 10874144) and the Scientific Foundation of Education Department of Hunan Province (No. 08C885). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
3+
3+
Fig. 7. Simplified energy level diagram of Yb , Tm mechanism for the upconversion luminescence.
3+
and Er , as well the possible
[12] [13] [14] [15]
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