J. Phys. Chem. C 2008, 112, 15071–15074
15071
White Up-Conversion Luminescence in Rare-Earth-Ion-Doped YAlO3 Nanocrystals Wancong Lu¨,†,‡ Xinghua Ma,†,‡ Han Zhou,†,‡ Guitang Chen,†,‡ Jianfu Li,† Zhaojie Zhu,† Zhenyu You,† and Chaoyang Tu*,† Key Laboratory of Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002 Fujian, P. R. China, and Graduate School of Chinese Academy of Sciences, 100039 Beijing, P. R. China ReceiVed: June 13, 2008; ReVised Manuscript ReceiVed: July 24, 2008
Room-temperature bright white up-conversion (UC) luminescence in Yb3+/Er3+/Tm3+ ion-doped YAlO3 nanocrystals was achieved under diode laser excitation of 974.5 nm. The white light consists of the blue (1G4 f 3H6 of Tm3+), green (2H11/2/ 4S3/2 f 4I15/2 of Er3+), and red (1G4 f 3F4 of Tm3+ and 4F9/2 f 4I15/2 of Er3+) UC emissions. The UC mechanisms were proposed based on spectral, pump power dependence and kinetic analyses. The white light may be a possible candidate for applications in the field of lighting, displays, and photonics. Introduction Recently, rare-earth-ion-doped up-conversion (UC) nanocrystals are being investigated for a wide range of potential applications, such as photonic, displays, and luminescent marker systems due to their powerful capability to transform infrared (IR) light into visible radiation together with the availability of cost-effective and high-power IR diode lasers.1-7 Although the nanocrystals have some disadvantages compared with the bulk crystals in the UC emission, e.g., a more efficient nonradiative process and a lower efficiency,3 They have exhibited a number of potential applications such as photonics,3 displays,4-6 luminescence markers,7 UC lasers,6,8 etc. Furthermore, specific UC emissions in nanocrystals can be artificially devised, e.g., the single-band emitting of red, green, and blue UC emissions designs in Y2O3 nanocrystals.9 The demand of ongoing miniaturization in luminescence materials can also be well satisfied in nanocrystals.10 White UC luminescence via single-wavelength excitation in inorganic nanocrystals deserves special attention due to its many promising application. Combined with the investigations in refs 3 and 4, Yb3+, Er3+, and Tm3+ ions are directly incorporated in the Y2O33 and YF34 nanocrystal host lattice, which can generate white UC light. However, according to the investigation in ref 8, non white UC light can be achieved in LaF38 nanocrystals. Such a contradictive phenomenon indicates that white UC light only can be achieved in suitable host lattices when Yb3+, Er3+, and Tm3+ ions are directly incorporated into host lattices. In the paper, YAlO3 was chosen as the host material due to yttrium aluminates based on the system Y2O3-Al2O3, such as Y3Al5O12 (YAG), Y4Al2O9 (YAM), and YAlO3, which are wellknown as important materials for advanced optical technologies.11-13 We also obtained the YAlO3 nanocrystals with hexagonal structure. It should be mentioned here that it greatly challenges our ability to synthesize single-phase YAlO3 because it is always formed together with the YAM or YAG and it is important to strictly control the stoichiometric Y/Al ratio on an * To whom correspondence should be addressed. Tel.: +86-591-83711368. Fax: +86-591-8371-4946. E-mail: tcy@ fjirsm.ac.cn. † Fujian Institute of Research on the Structure of Matter. ‡ Graduate School of Chinese Academy of Sciences.
atomic scale.14,15 The hex-YAlO3 belongs to the P63/mmc-fcba space group with the following structural features: close-packed O layers in hc2 stacking; Y in octahedral, Al in trigonal bipyramidal voids. AlO5 trigonal bipyramids share vertices to form infinite slabs. Space lattice parameters are a ) 0.368, c ) 1.052 nm, c/a ) 2.859, V ) 0.1234 nm,3 and Z ) 2. Due to ionic size considerations in the hex-YAlO3 lattice, the doped rare-earth (Re3+) ions will predominantly enter the octahedral sites by replacing the Y3+ and possess a D3d symmetry.16,19 This hex-YAlO3 structure is stable at T < 1223 K.16 UC analysis of Er3+ in the YAlO3 crystal under the excitation at 1500 nm has been studied.21-23 It is very interesting to study the white UC luminescence using Yb3+-, Er3+-, and Tm3+-doped hex-YAlO3 nanocrystals, under diode laser excitation of 974.5 nm, and discuss the relevant mechanisms associate the UC processes. Experimental Section Chemicals. Yttrium oxide (Y2O3, 99.999%), erbium oxide (Er2O3, 99.99%), thulium oxide (Tm2O3, 99.9%) ytterbium oxide (Yb2O3, 99.99%), aluminum nitrate (Al(NO3)3 · 9H2O, 99.9%), hydrated citric acid (C6H8O7 · H2O, AR grade), and ammonia (NH4NO3, AR grade) were used as staring materials. Citric acid was used as both complexing agent for the xerogel process and fuel for the combustion. Stoichiometric Y3+ (including Re3+) and Al3+ (Y/Al )1 in mole ratio) were used to form the YAlO3 structure, and the molar ratio of citric acid to metal cations was 3:2. Sample Preparation. YAlO3 powder samples doped with (1) 0.5 atom % Er3+, (2) 0.5 atom % Er3+ and 5 atom % Yb3+, (3) 0.5 atom % Tm3+ and 5 atom % Yb3+, and (4) 0.5 atom % Er3+, 0.5 atom % Tm3+, and 5 atom % Yb3+ ions were prepared by using a procedure described briefly as follows.15 Y2O3, Er2O3, Yb2O3, and Tm2O3 were first dissolved in nitric acid to form lanthanum nitrate solution, respectively. Yttrium, aluminum, erbium, thulium, and ytterbium nitrates with corresponding mole ratio of cations were completely mixed to form a mixed solution. Then, citric acid was added into the mixed solution with a mole ratio of (Y + Re) to citric acid of 2:3. The pH of the solution was further adjusted to 3 by ammonium hydroxide, which is then dried at 90 °C for 24 h, and finally calcined at 900 °C for 10 min. It is important to note that prolonged heat treatment at
10.1021/jp805205v CCC: $40.75 2008 American Chemical Society Published on Web 08/30/2008
15072 J. Phys. Chem. C, Vol. 112, No. 38, 2008
Lu¨ et al.
Figure 1. XRD pattern of the Yb3+/Tm3+/Er3+-doped YAlO3 powder after the heat treatment at 900 °C for 10 min.
high temperature failed to generate pure-phase hex-YAlO3. The proposed stoichiometric equation for the synthesis reaction is given below:
Figure 2. TEM image of YAlO3 powder sample heated at 900 °C for 10 min.
(1 -x)Y(NO3)3 + xRe(NO3)3 + Al(NO3)3 + C6H8O7 f Y1 -xRexAlO3 + 6CO2 + 4H2O + 3NO2 + 3N2 (1) where in this case Re ) Er, Tm, and Yb and x is the corresponding mole ratio of each cation ion. All the nanocrystalline samples were kept in air without any further precaution. Characterization. X-ray powder diffraction (XRD) analysis was performed to identify the crystallization phase with a power diffractometer (Rigaku, DMAX2500PC) operated at 40 kV and 100 mA, using Cu KR as the radiation (λ ) 1.5405 Å). The 2θ scan range was 5-85° with a step size of 0.05°. The microstructures of the sample were analyzed by a transmission electron microscope (TEM, JEM-2010). The visible up-conversion fluorescence signals were detected with InP/InGaAs photomultiplier tubes (R928) excited by a power-controllable 974.5-nm diode laser (DPL-II, Module-HTL98M10) with the maximum power output of 9 W. All the measurements were carried out at room temperature. Results and Discussion Figure 1 shows a record of the XRD pattern of Yb3+/Tm3+/ Er3+-doped YAlO3 powder heated at 900 °C for 10 min. All the peaks are in good agreement with those of YAlO3 found in the JCPDS Card No. 74-1344; therefore, it is confirmed that the single phase of hexagonal YAlO3 is obtained. Figure 2 shows a TEM image of YAlO3 powder, which is uneven nanocrystals and encompassed several pores, and all of them maintain at the tens of nanometers level and tend to aggregate badly. Figure 3 shows a record of the UC luminescence of YAlO3 nanocrystals doped with various concentrations of rare-earth ions under diode laser excitation of 974.5 nm. As compared with Figure 3b and c, the UC bands centered at 484, 653, 523, 546, and 656 nm in Figure 3d can be easily assigned to the intra-4f electronic transitions 1G4f 3H6 (blue) and 1G4f 3F4 (red) of Tm3+ ions, 2H11/2/ 4S3/2f 4I15/2 (green) and 4F9/2f 4I15/2 (red) of Er3+ ions, respectively.3,4,6,9 It is noted that the UC band centered at 653 nm in Figure 3c arising from the transition 1G4f 3F of Tm3+ ions coincides with that of Er3+ ions in Figure 3b. 4 Here, the red UC band in Figure 3d can be attributed to
Figure 3. Up-converted emission spectra of YAlO3 nanocrystals under diode laser excitation at 974.5 nm doped with the following: (a) 0.5 atom % Er3+, (b) 0.5 atom % Er3+ and 5 atom % Yb3+, (c) 0.5 atom % Tm3+ and 5 atom % Yb3+, and (d) 0.5 atom % Er3+, 0.5 atom % Tm3+, and 5 atom % Yb3+ ions.
the contribution of Er3+ and Tm3+ ions, since compared with blue band in Figure 3c, the intensity of the 653-nm band cannot be neglected, and the shape of the red band in Figure 3d is exactly the same as that of Figure 3b and c. Interestingly, combining with Figure 3b-d, there is a competition between red UC emission coming from Er3+ and Tm3+ ions. Obviously, the population of 1G4 state increases and 4F9/2 state decreases in the tridoped system (details below). Additionally, Figure 3a shows the UC luminescence curve of the YAlO3/Er3+ (0.5 atom % Er3+), which indicates that it is very difficult to achieve the UC process in the Er3+ ions without Yb3+ ions. In general, color is represented by color coordinates. The chromaticity coordinates have been calculated from the spectra by the method using the1931 CIE (Commission International de l’Eclairage France) system. The CIE tristimulus values expressed as integrals (or sums) are calculated.17 The standard data are built into the routine: that is, color-matching functions taken every 1 nm in the range from 720 to 425 nm under the excitation at 974.5 nm with various outputs. As shown in Figure 4, the color coordinates of the Yb3+/Tm3+/Er3+-tridoped YAlO3 nanocrystals are calculated to be about (0.30, 0.35), (0.29, 0.34), (0.27, 0.33), (0.26, 0.32), and (0.24, 0.30), corresponding to the laser pump powers of about 458, 529, 600, 707, and 1027 mW, respectively. These color coordinates exactly fall within the white region of the 1931 CIE diagram. Among them, the
White Up-Conversion Luminescence
Figure 4. Calculated CIE color coordinates for white luminescence under various pump powers: (black circle) 458 mW, (red circle) 529 mW, (green circle) 600 mW, (blue circle) 707 mW, and (cyan circle) 1027 mW.
Figure 5. Pump power dependence of the blue, green, and red upconverted emissions in YAlO3 nanocrystals doped with (a) 0.5 atom % Er3+ and 5 atom % Yb3+, (b) 0.5 atom % Tm3+ and 5 atom % Yb3+, and (c) 0.5 atom % Er3+, 0.5 atom % Tm3+, and 5 atom % Yb3+ ions.
color coordinates at the pump power of 458 mW are close to the standard color coordinates (0.33, 0.33). The variation of color coordinates in the white region indicates a possible way to tune the white color via the laser pump power. The tendency of color coordinates toward theblue region at high pump powers is due to the fact that the three-photon process is involved to produce the blue UC emission, which is higher than that of the green and red UC ones (see Figure 4).3 To better understand the UC mechanism(s), the pump power dependence of the UC emissions is measured and shown in Figure 5. It is well-known that this relation is expressed as I ∝ Pn, where I is the intensity of the fluorescence, P is the pump power, and n is the number of photons required to populate the emitting state.18 Unexpectedly, the obtained slopes (n) are smaller than the expected values. This can be attributed to the competition between linear decay and UC processes for the depletion of the intermediate excited states.18 Panels a and b in Figure 5 show that three- and two-photon processes are involved to produce the blue, green, and red UC emissions, which are similar to other Yb3+/Tm3+- and Yb3+/Er3+-doped systems, respectively.8,10 The n value maintains 1.96 for green UC emission in Figure 5a and c, indicating that green UC emission comes from Er3+ ions exclusively. The n value of red UC emission is 1.78 in Figure 5c between 1.51 (Figure 5a) and 2.47
J. Phys. Chem. C, Vol. 112, No. 38, 2008 15073
Figure 6. Schematic representation of the energy level diagram for the Er3+, Tm3+, and Yb3+ ions as well as the proposed UC mechanisms to produce the blue, green, and red up-converted emissions.
(Figure 5b), indicating the fact that both the Er3+ and Tm3+ ions are responsible for the red UC band. It should be noted that the blue UC emission comes from Tm3+ ions exclusively, because only Tm3+ ions contribute to the blue UC emission. Remarkably, the decreased n value from 2.71 (Figure 5b) to 2.49 (Figure 5c) indicates that another mechanism happens involving a two-photon process to populate the 1G4 (Tm) state. Figure 6 shows the schematic representation of the energy levels diagram for the Er3+, Tm3+, and Yb3+ ions as well as the proposed UC mechanisms to produce the blue, green, and red (white) up-converted emissions.3,4,9 In the complex tridoped system, laser excitation of Yb3+ ions is only considered because the Tm3+ ions have no corresponding energy level and Er3+ ions suffer from a low efficiency as mentioned above. It is wellknown that the Yb3+ ions can efficiently sensitize Er3+ and Tm3+ ions,7 respectively. The blue UC emission comes from the Yb3+/ Tm3+ pairs via a well-known three-photon process, and the green band from Yb3+/Er3+ pairs is two-photon process.3,4,6,9 Here, the red band comes from Yb3+/Tm3+ and Yb3+/Er3+ pairs involves a mixed three/two-photon processes. Furthermore, some other mechanisms must be considered, since a still not elucidated two-photon process is observed here to promote the 1G4 (Tm) state (see Figure 6). We consider that the nearly resonant crossrelaxation process 3F4 (Tm) + 4F9/2 (Er) f 1G4 (Tm) + 4I15/2 (Er) is responsible for the promotion of 1G4 (Tm) state (energy mismatch of 383 cm-1). It is reasonable to believe the occurrence of the cross-relaxation process: First, the energy mismatch of 383 cm-1 can easily be dissipated by the phonons of YAlO3 lattice (maximum phonon energy of 686 cm-1)19,20 and allow this process to efficiently occur. Second, both the 3F (Tm) and 4F 4 9/2 (Er) are metastable states, which have enough time to allow this process to occur. Third, the fact that the population of the 1G4 state increases and the 4F9/2 state decreases as shown in Figure 3. Lastly, the rise time for the 1G4 (Tm) is reduced as compared to that YAlO3:Yb3+/Tm3+ nanocrystals, since the lifetime of the 3F4 (Tm) state is longer than that of the 4F9/2 (Er) state.12 All facts have been identified by spectral and kinetic investigations, suggesting the occurrence of the proposed cross-relaxation process. This new cross-relaxation process results in the enhancement of blue and part red (Tm) UC emissions. In fact, similar energy transfer from Er3+ to Tm3+ ions has been demonstrated in tellurite glasses.24 It should be mentioned here that this new cross-relaxation process is different from the cross-elaxation process: 3H4 (Tm) + 4I13/2 (Er) f 3H6 (Tm) + 4S3/2 (Er), which was demonstrated in ref 3.
15074 J. Phys. Chem. C, Vol. 112, No. 38, 2008 Conclusions Room-temperature white UC luminescence was for the first time achieved in Yb3+/Er3+/Tm3+-tridoped hex-YAlO3 nanocrystals under diode laser excitation at 974.5 nm. The calculated color coordinates fall exactly within the white region of the 1931 CIE diagram in a wide range of pump power. Energy transfer from Yb3+ to Tm3+ and Er3+ ions took place simultaneously in the hex-YAlO3 nanocrystals and produced blue, green, and red UC emissions via three-, two-, and mixed three/two-photon processes, respectively, which have been confirmed by pump power dependence. The two-photon cross-relaxation process 3F4 (Tm) + 4F9/2 (Er) f 1G4 (Tm) + 4I15/2 (Er) is first proposed for the promotion of the blue and part red (Tm) UC emissions. Energy also transfers from Er3+ to Tm3+ ions. The bright white luminescent nanocrystals may have potential application in the field of lighting, displays, and photonics. Acknowledgment. This work was supported by the National Defense Science Innovative Foundation Project of Chinese Academy of Sciences (Grant CXJJ-182), the Science & Technology Plan Project of Fujian Province of China (Grant 2005HZ1026 and 2007H0037) and the great project of FJIRSM (SZD08001-2). References and Notes (1) McKittrick, J.; Shea, E. L.; Bacalski, C. F.; Bosze, E. J. Displays 1999, 19, 169. (2) Shionoya, S.; Yen, W. M. Phosphor Handbook [M]; CRC Press: New York, 1999; p 197. (3) Chen, G. Y.; Liu, Y.; Zhang, Y. G.; Somesfalean, G.; Zhang, Z. G.; Sun, Q.; Wang, F. P. Appl. Phys. Lett. 2007, 91, 133103. (4) Chen, D. Q.; Wang, Y. S.; Zheng, K. L.; Guo, T. L.; Yu, Y. L.; Huang, P. Appl. Phys. Lett. 2007, 91, 1903.
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