JOURNAL OF RARE EARTHS, Vol. 31, No. 8, Aug. 2013, P. 741
Yellow upconversion luminescence in Ho3+/Yb3+ co-doped Gd2Mo3O9 phosphor SUN Jiayue (ᄭᆊ䎗)*, XUE Bing (㭯 ݉), SUN Guangchao (ᄭᑓ䍙), CUI Dianpeng (የ↓吣) (School of Science, Beijing Technology and Business University, Beijing 100048, China) Received 19 December 2012; revised 19 June 2013
Abstract: The strong yellow upconversion (UC) light emission was observed in Ho3+/Yb3+ co-doped Gd2Mo3O9 phosphor under the excitation of 980 nm diode laser. The phosphors were synthesized by the traditional solid-state reaction method. The structures of the samples were characterized by X-ray diffraction (XRD). Under 980 nm excitation, Ho3+/Yb3+ co-doped Gd2Mo3O9 exhibited strong yellow UC emission based on the green emission near 541 nm generated by 5F4,5S25I8 transition and the strong red emission around 660 nm generated by 5F55I8 transition, which assigned to the intra-4f transitions of Ho3+ ions. The doping concentrations of Ho3+ and Yb3+ were determined to be 0.01 mol Ho3+ and 0.2 mol Yb3+ for the strongest yellow emission. Then the dependence of UC emission intensity on excitation power density showed that the green and red UC emissions were involved in two-photon process. The possible UC mechanisms for the strong yellow emission were also investigated. The result indicated that this material was a promising candidate for the application in the yellow display field. Keywords: yellow upconversion; phosphor; Gd2Mo3O9:Ho,Yb; rare earths
Over the recent years, research on the UC materials, particularly the lanthanide doped phosphors, have attracted much attention due to their luminescent properties that can convert from infra-red radiation to visible or UV light[1–3]. More and more attention has been paid to the research on applications in compact solid state lasers, luminescence display panel, biological imaging and solar energy conversion[4–7]. Among the various lanthanide ions studied for UC luminescence, Ho3+ has attracted considerable attention owing to its dominant green and red UC emission[8,9]. Yb3+ ions is commonly an ideal sensitizer for the Ho3+ ions, due to its large absorption across section around 980 nm and can efficiently transfer the excitation energy to Ho3+ ions. As we known, the rare earth tungstate and molybdate matrix are highly chemically, photo-thermally, and photo-chemically stable and have a broad optical transparence from the visible to the NIR regions[11,12]. We also know that the solid state method used to prepare phosphor powders will result in the increase of particle sizes; this can protect the activator from being broken when the size of the particle was too small. So the phosphor powders prepared by solid state method had a relatively strong emission when excited. In this work, we synthesized Ho3+/Yb3+ co-doped Gd2Mo3O9 phosphors by the solid state method and investigated UC luminescence with various Ho3+ and Yb3+
ions in Gd2Mo3O9. The structure and UC luminescence properties were studied. Moreover, the optimum doping concentrations of Ho3+ and Yb3+ ions in Gd2Mo3O9 on UC emission intensity in accord with concentration quenching effect were shown and the UC mechanism of Ho3+/Yb3+ co-doped Gd2Mo3O9 phosphors was illustrated in detail.
1 Experimental 1.1 Preparations The xHo3+/0.1Yb3+ (x=0.001, 0.005, 0.010, 0.015, 0.020, 0.025) doped Gd1.9–xMo3O9 and yYb3+/0.01Ho3+ (y=0.05, 0.10, 0.15, 0.20, 0.25, 0.30) doped Gd1.99–yMo3O9 phosphors were synthesized by the traditional solid-state reaction method. The starting materials were Ho2O3 (99.99%), Gd2O3 (99.99%), Yb2O3 (99.99%) and MoO3 (A.R.). Na2CO3 with purity of 99.9% was used as flux to depress the sintering temperature, shorten the reaction time and improve the crystallization. The starting materials were weighed according to certain stoichiometric ratio and grinded thoroughly in an agate mortar. Then the mixtures were put into alumina crucibles and calcined in a muffle furnace at 800 ºC for 4 h. Finally, the corresponding phosphors were obtained when the furnace cooled naturally down to room temperature.
Foundation item: Project supported by National Natural Science Foundation of China (20976002), Beijing Municipal Natural Science Foundation (2122012), Key Projects for Science and Technology of Beijing Education Commission (KZ201310011013), Project of Transformation and Industrialization of College Scientific & Technological Achievements, and Projects of the Combination of Manufacture, Education & Research of Guangdong Province (2011B090400100) * Corresponding author: SUN Jiayue (E-mail: [email protected]
; Tel.: +86-10-68985467) DOI: 10.1016/S1002-0721(12)60351-2
JOURNAL OF RARE EARTHS, Vol. 31, No. 8, Aug. 2013
1.2 Characterization The X-ray diffraction (XRD) patterns were analyzed on a Shimadzu model XRD-6000 powder diffraction with Cu K radiation, 40 kV, 30 mA. The upconversion emission spectra among the wavelength range of 400– 700 nm were recorded by using a FluoroLog-3 spectrophotometer equipped with an external power-controllable 980 nm semiconductor laser (Beijing Viasho Technology Company, China).
2 Results and discussion X-ray powder diffraction is an important approach to identify the crystal structure and the phase purity of materials. Fig. 1 shows the XRD patterns at different Yb3+ and Ho3+ doping concentrations. From the figure we can observe that nearly all the diffraction peaks agree with the standard data of Gd2Mo3O9 (JCPDS No. 33- 0548), the final products mainly exist in Gd2Mo3O9 phase except a weak peak at 49.3 ºC. Based on the reports[13,14], it belongs to the Gd2(MoO4)3 phase (JCPDS card No. 710915). The obviously shift of the peaks from high angles to low indicates that Yb3+ with relatively larger size (R3+=0.0858 nm) effectively doped into the host lattice and replaced Gd3+ with small size (R3+=0.0038 nm) successfully. The UC luminescence spectra of Ho3+/Yb3+ co-doped Gd2Mo3O9 depending on Ho3+ concentrations with fixed Yb3+ at 0.1 mol is presented in Fig. 2. The inset of Fig. 2 shows that the intensities of green and red emissions dramatically increase with the Ho3+ concentration increasing up to 0.01 mol. Whereas the emission decreasing when the ratio of Ho3+/Yb3+ beyond 0.01/0.1. Similarly, the UC luminescence spectra depending on Yb3+ concentrations when fixed Ho3+ at 0.01 mol in the Gd2Mo3O9 matrix is presented in Fig. 3. From the inset of Fig. 3, we can get that the intensity of UC emission enhanced with Yb3+ concentration increasing up to 0.2 mol, when the ratio of Yb3+/Ho3+ is beyond 0.2/0.01, the
Fig. 1 XRD patterns of samples Gd2Mo3O9:xHo3+/0.1Yb3+ (x= 0.005, 0.015, 0.025) and Gd2Mo3O9:0.01Ho3+/yYb3+ (y= 0.1, 0.2, 0.3) prepared at 800 ºC for 4 h
Fig. 2 Photoluminescence spectra of Gd2Mo3O9 UC phosphors with various Ho3+ concentrations when Yb3+ concentration fixed at 0.1 mol (the inset shows the green and red emission intensity tendency when changing the Ho3+ concentration)
Fig. 3 Photo-luminescence spectra of Gd2Mo3O9 UC phosphors with various Yb3+ concentrations when Ho3+ concentration fixed at 0.01 mol (the inset shows the green and red emission intensity tendency when changing the Yb3+ concentration)
concentration quenching effect occurs. So the optimum Yb3+ concentration in the Gd2Mo3O9:yYb3+,0.01Ho3+ matrix is 0.2 mol. The concentration quenching may be produced by the enhancement of cross relaxation between the high amounts of dopants. The dependence of UC luminescence intensity on the pump power is essential for identification of the UC mechanism. For any UC process, the relationship between the UC emission intensity (I) and the pump power (P) is approximately expressed by the following equation: I v Pm (1) In the upconversion process, the upconversion emission intension I is measured as the function of the pump power P, and I is proportional to the exponent of P, m is the number of excitation photons absorbed by rare earth ions at ground state to transit to the emitting state. So the dependence of integrated emission intensity on the excitation power follows an exponential function, and the index m is an indicator for the number of needed infrared
SUN Jiayue et al., Yellow upconversion luminescence in Ho3+/Yb3+ co-doped Gd2Mo3O9 phosphor
photons of generating one visible photon[17,18]. The dependences of integrated UC emission intensities on LD working current is shown in Fig. 4. The values m for Gd2Mo3O9:0.01Ho3+,0.2Yb3+ matrix are 2.07 for the red emission and 2.04 for the green emission, indicating that the UC emissions are involved in two-photon process. According to the fitted slopes for green and red emission, the UC luminescence mechanisms of the Ho3+/Yb3+ co-doped Gd2Mo3O9 systems has been investigated, and the relative diagrams is shown in Fig. 5. Under 980 nm infrared excitation, there are two way contributing to the 5 I6 level[19,20]: (1) the ground state absorption (GSA) process (5I85I6) and (2) the energy transfer (ET) process from Yb3+ to Ho3+ ions. The process is described as follows. Yb3+ absorbs a photon and jumped from 2F7/2 to excited state 2F5/2, and then the energy from 2F5/2 level transmits to the 5I6 level by 2F5/2(Yb3+)+5I8(Ho3+) 2 F7/2(Yb3+)+5I6(Ho3+) process. For the red emission, the 5 F5 state of Ho3+ involved a two ET process: (1) 2 F5/2(Yb3+)+5I8(Ho3+)2F7/2(Yb3+)+5I6(Ho3+), (2) 2F5/2 (Yb3+)+5I7(Ho3+)2F7/2(Yb3+)+5F5(Ho3+) (The 5I7 level population comes from the non-radiative relaxes of 5I6 to the 5I7 level), and a non-radiative relax from the 5F4/5S2 level. Meanwhile, green emission is produced by the
F4/5S2 state to the 5I8 state, the two ET process contributes to 5F4/5S2 as follows: (1) 2F5/2(Yb3+) +5I8(Ho3+)2F7/2(Yb3+)+5I6(Ho3+) (2) 2F5/2(Yb3+)+5I6(Ho3+)2F7/2(Yb3+)+5F4/5S2(Ho3+) The color coordinates (x, y) of the upconversion emission in the samples of 0.01Ho3+/0.2Yb3+ are calculated under different excitation powers. The relative point is shown in Fig. 6. In the figure, we can obviously observe that the color coordinates (0.418, 0.564) at the pump power density of 800 mW/cm2 is very close to the color purity of yellow-light under 980 nm excitation. The color coordinates tendency toward to the pure yellow region when the pump power is enhanced from 200 to 800 mW/cm2. The relative reason can be the two-photon process of the green and red UC emissions resulting in the sensitivity to pump changing. At the Tm3+/Ho3+/ Yb3+-tri-doped system, the three-photon process blue emission is more sensitive than two-photon process green and red emission, which caused the blue region shift when the pump power is enhanced. While absent the blue emission it will be toward yellow region. Furthermore, the high enhancement of the red and green emissions causes the strong red light and green light, which combined to a color purity of yellow light.
Fig. 4 Power dependence of the upconversion emission intensity of 0.01Ho3+/0.2Yb3+ co-doped Gd2Mo3O9 at 541 nm and 660 nm
Fig. 6 CIE chromaticity diagram with the calculated color coordinates under the excitation of 980 nm LD with various pump powers a-200 mW/cm2; b-400 mW/cm2; c-600 mW/cm2; d-800 mW/cm2
Fig. 5 Energy level diagram of Ho3+ and Yb3+ in Gd2Mo3O9 and possible UC mechanisms under 980 nm excitation
Ho3+/Yb3+ co-doped Gd2Mo3O9 phosphor with various concentrations for yellow UC emission was successfully synthesized by the high temperature solid-state method. In this paper we introduced the green and red emission property. The Ho3+/Yb3+ co-doped Gd2Mo3O9 phosphors had strong green luminescent center at 541 nm and red
luminescent center at 660 nm. The optimum Yb3+ and Ho3+ concentration for the Gd2Mo3O9/Ho3+/Yb3+ system were 0.2 and 0.01 mol. The red emission could be ascribed to Ho3+:5F55I8 transitions and green emission ascribed to 5F4,5S25I8 transition. The relation between the intensity of emission and laser pumping power revealed that two photon processes were responsible for green and red UC luminescence. The CIE coordinate of the 0.01Ho3+, 0.2Yb3+ co-doped Gd2Mo3O9 under different pump power had been shown and the yellow-light emission tendency could be observed. The results indicated that Ho3+/Yb3+ co-doped Gd2Mo3O9 UC phosphor could be a candidate for efficient yellow luminescence.
Reference:  Zhang J, Wang S W, Rong T J, Chen L D. Upconversion luminescence in Er3+ doped and Yb3+/Er3+ co-doped yttria nanocrystalline powders. J. Am. Ceram. Soc., 2004, 87(6): 1072.  Anh T K, Benalloul P, Barthou C, Giang L, Vu N, Minh L. Luminescence, energy transfer, and upconversion mechanisms of Y2O3 nanomaterials doped with Eu3+, Tb3+, Tm3+, Er3+, and Yb3+ ions. J. Nanomater., 2007, 2007: 48247.  Chung J H, Ryu J Ho, Lee S Y, Lee J H, Choi B G, Shim K B. Green upconversion luminescence from poly-crystalline Yb3+, Er3+ co-doped CaMoO4. J. Alloys Compd., 2012, 522: 30.  Auzel F. Upconversion and anti-stokes processes with f and d ions in solids. Chem. Rev., 2004, 104(1): 139.  Downing E, Hesselink L, Ralston J, Macfarlane R A threecolor, solid-state, three-dimensional display. Science, 1996, 273: 1185.  Wang X X, Wang J, Shi J X, Su Q, Gong M L. Intense red emitting phosphors for LED solid-state lighting. Mater. Res. Bull., 2007, 42:1669.  Wild J, Meijerink A, Rath J K, van Sark W G J H M, Schropp R E I. Upconverter solar cells: materials and applications. Energy Environ. Sci., 2011, 4: 4835.  Jiang Y F, Shen R S, Li X P, Zhang J S, Zhong H, Tian Y, Sun J S, Cheng L H, Zhong H Y, Chen B J. Concentration effects on the upconversion luminescence in Ho3+/Yb3+ co-doped NaGdTiO4 phosphor. Ceram. Int., 2012, 38(6): 5045.  Zhang X X, Hong P, Bass M, Chai B H T. Blue upconver-
JOURNAL OF RARE EARTHS, Vol. 31, No. 8, Aug. 2013 sion with excitation into Tm ions at 780 nm in Yb- and Tm-codoped fluoride crystals . Phys. Rev. B, 1995, 51(14): 9298.  Wei X T, Li Y, Cheng X R, Chen Y H, Yin M. Strong dependence of upconversion luminescence on doping concentration in holmium and ytterbium co-doped Y2O3 phosphor. J. Rare Earths, 2011, 29(6): 536.  Xia Z G, Zhou W, Du H Y, Sun J Y. Synthesis and spectral analysis of Yb3+/Tm3+/Ho3+-doped Na0.5Gd0.5WO4 phosphor to achieve white upconversion luminescence. Mater. Res. Bull., 2010, 45: 1199.  Wang J G, Jing X P, Yan C H, Lin J H, Liao F H. Influence of fluoride on f-f transitions of Eu3+ in LiEuM2O8 (M=Mo, W). J. Lumin., 2006, 121 (1): 57.  Zhao X X, Wang X J, Chen B J, Meng Q Y, Di W H, Ren G Z, Yang Y M. Novel Eu3+-doped red-emitting phosphor Gd2Mo3O9 for white-light-emitting-diodes (WLEDs) application. J. Alloys Compd., 2007, 433(1-2): 352.  Zhang L H, Zhong H Y, Li X P, Cheng L H, Yao L, Sun J S, Zhang J S, Hua R N, Chen B J. Solid state reaction synthesis and luminescence properties of Dy3+-doped Gd2Mo3O9 phosphor. Physica B, 2012, 407(1): 68.  Yang D M, Li C X, Li G G, Shang M M, Kang X J, Lin J. Colloidal synthesis and remarkable enhancement of the upconversion luminescence of BaGdF5:Yb3+/Er3+ nanoparticles by active-shell modification. J. Mater. Chem., 2011, 21: 5923.  Pollnau M, Gamelin D R, Luthi S R, Hehlen M P, Gudel H U. Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems. Phys. Rev. B, 2000, 61(5): 3337.  Tian Y, Hua R N, Yu J C, Sun J S, B. Chen J. The effect of excitation power density on frequency upconversion in Yb3+/Er3+ codoped Gd6WO12 nanoparticles. Mater. Chem. Phys., 2012, 133(2-3): 617.  Vetrone F, Boyer J C, Capobianco J A, Speghini A, Bettinelli M. Significance of Yb3+ concentration on the upconversion mechanisms in codoped Y2O3:Er3+, Yb3+ nanocrystals. J. Appl. Phys., 2004, 96(1): 661.  Xing L L, Wang R, Xu W, Qian Y N, Xu Y L, Yang C H, Liu X R. Upconversion white-light emission in Ho3+/ Yb3+/Tm3+ codoped LiNbO3 polycrystals. J. Lumin., 2012, 132(6): 1568.  Guo L N, Wang Y H, Zhang J, Dong P Y. Bright white up-conversion emission from Ho3+/Yb3+/Tm3+ tri-doped Y2SiO5 phosphors. J. Electrochem. Soc., 2011, 158(7): 225.