Tm3+ co-doped KGd(WO4)2 powders

JOURNAL OF RARE EARTHS, Vol. 28, No. 5, Oct. 2010, p. 697

Upconversion luminescence of Yb3+/Ho3+/Er3+/Tm3+ co-doped KGd(WO4)2 powders DU Haiyan (ᴰ⍋➩)1, LAN Yujing (㪱䲼䴪)1, XIA Zhiguo (໣ᖫ೑)2, SUN Jiayue (ᄭᆊ䎗)1 (1. College of Chemistry and Environmental Engineering, Beijing Technology and Business University, Beijing 100048, China; 2. School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China) Received 8 November 2009; revised 7 April 2010

Abstract: Different lanthanide ions (Yb3+/Ho3+/Er3+/Tm3+) codoped KGd(WO4)2 phosphors were prepared by high-temperature solid-state reaction. The upconversion luminescence properties of two-ion and three-ion co-doped KGd(WO4)2 phosphors were investigated in detail. The concentration quenching effect of the two-ion co-doped KGd(WO4)2 phosphors was studied, and the optimum concentration of Ho3+, Er3+ and Tm3+ are 2 mol.%, 2 mol.% and 3 mol.%, respectively. The Yb3+/Ho3+/Tm3+ co-doped KGd(WO4)2 sample is the best white upconversion luminescence material among three-ion co-doped KGd(WO4)2 phosphors. The CIE coordinates for the samples were calculated, and chromaticity coordinates were close to white light regions. The upconversion luminescence mechanism of the samples was discussed. The upconversion luminescence of Ho3+ and Er3+ was both produced through two-photon process. The emissions of Ho3+ at 546 and 654 nm originated from the transition of Ho3+ (5S2ĺ5I8 and 5F5ĺ5I8), while those of Er3+ at 530 and 551 nm were attributed to the transition of Er3+ (2I11/2ĺ4I15/2 and 4S3/2ĺ 4I15/2). The upconversion luminescence mechanism of Tm3+ was a three-photon process. The blue emission of Tm3+ was due to the transition of Tm3+ (1G4ĺ3H6). Keywords: upconversion luminescence; KGd(WO4)2 powder; white light phosphor; rare earths

Inorganic nanocrystals doped with trivalent rare earth ions can easily upconvert the absorbed near-infrared (NIR) light into visible light by use of cost-effective and high-power NIR diode lasers, because rare earth ions usually have an energy level structure of equally spaced long-live excited states[1,2]. Recently, much attention has been paid to the research on inorganic upconversion phosphors due to their potential applications, such as volumetric displays[3], temperature sensors[4], photodynamic therapy[5] and biological analyses as boilable[6,7], and so on. In addition, upconversion phosphors have also been used in GaAs light-emitting diodes. A GaAs diode, which yields IR emission, can be covered with an upconversion phosphors layer and therefore allows the fabrication of LED with different colors[8]. There are growing interests focused on white upconversion luminescence via single-wavelength excitation in inorganic phosphors. Many white upconversion luminescence material have been reported, for example, YAlO3:Yb3+, Er3+,Tm3+[9], Y2O3:Yb3+,Er3+,Tm3+[10], Gd2O3:Yb3+,Er3+, Tm3+, Ho3+[11] and Lu3Ga5O12: Yb3+, Er3+,Tm3+[12]. Since the white upconversion luminescence material has been proved to be promising, more efforts should be offered to explore novel suitable phosphors for solid-state white light source based on diodes. KGd(WO4)2 belongs to the double molybdate and tungstate crystals with general formula MRE(XO4)2 (M=Li, Na, K; X=W, Mo), which have been reported as good host materials for solid-state lasers[13,14]. In this paper, the upconver-

sion luminescence properties of KGd(WO4)2 co-doped with different rare earth dopants were investigated. The work to obtain white light emitting upconversion luminescence phosphors was also reported. Moreover, the mechanism to produce upconversion emission was discussed. Yb3+ ion was chosen as sensitizing ion to increase the emission intensity, and the upconversion luminescence of the samples mainly originated from Ho3+, Er3+ and Tm3+.

1 Experimental The powder samples of KGd(WO4)2 doping different lanthanide ions (Yb3+/Ho3+/Er3+/Tm3+) were prepared by the high-temperature solid-state reaction. The starting materials were K2CO3 (A.R., WO3 (A.R.), Gd2O3 (99.99%), Yb2O3 (99.99%), Ho2O3 (99.99%), Er2O3 (99.99%) and Tm2O3 (99.99%). The mixture ratio of the reactants was as follows: (0.8–x–y–z) Gd2O3+K2CO3+4WO3+0.2Yb2O3+xHo2O3+yEr2O3+ zTm2O3 (x=0, 0.01, 0.02, 0.03 and 0.04; y=0, 0.01, 0.02, 0.03 and 0.04; z=0, 0.01, 0.02, 0.03 and 0.04). The weighed raw materials were thoroughly mixed in an agate mortar and then placed in a corundum crucible with a lid. Then, the samples were sintered at 900 qɋ for 4 h. The crystal structure and phase of the as-synthesized samples were characterized by using a SHIMADZU model XRD-6000 X-ray powder diffractometer (Cu KĮ radiation, 40 kV, 30 mA and a scanning speed (2ș) 2.0(q /min). The

Foundation item: Project supported by the National Natural Science Foundation of China (20876002, 20976002), the Beijing Natural Science Foundation (2091002), and Funding Project for Academic Human Resources Development in Institutions of Higher Learning under the Jurisdiction of Beijing Municipality Corresponding author: SUN Jiayue (E-mail: [email protected]; Tel.: +86-10-68985467) DOI: 10.1016/S1002-0721(09)60182-4

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upconversion luminescence spectra were recorded on a Hitachi F-4500 spectrophotometer equipped with an external power-controllable 980 nm semiconductor laser (Beijing Viasho Technology Company, China) as the excitation source, which is connected with an optic fiber accessory.

2 Results and discussion Fig. 1 shows the XRD patterns of the KGd(WO4)2 obtained by sintering the sample at 900 qɋ and the data of JCPDS Card (No. 45-0555) of KGd(WO4)2. The diffraction peaks of the phosphor match well with the value from the JCPDS files of KGd(WO4)2, indicating that the crystal lattice and basic structure of the KGd(WO4)2 host can be obtained by the high-temperature solid-state method. The upconversion luminescence properties of the KGd(WO4)2 phosphors doped with different rare earth ions were investigated in detail when the powders were excited by a 980 nm semiconductor laser. We divided all the samples into two groups. One group were the KGd(WO4)2 powders with two rare earth dopants, while the other group were those with three rare earth dopants. Fig. 2 shows the upconversion luminescence spectra of the two rare earth ions co-doped KGd(WO4)2 phosphors. As seen from Fig. 2(a), the peak at 546 nm of green emission and the peak at 654 nm of weaker red emission attributed to the transition emission of Ho3+ (5S2ĺ5I8 and 5F5ĺ5I8 ) are observed. It can also be seen from the graph that the concentration quenching effect of the KGd(WO4)2 powders with invariable Yb3+ concentration (20 mol.%) and different Ho3+ concentrations are obvious. With increasing Ho3+ concentration, both of the green and red emission intensities increase, maximize at 2 mol.%, and then decrease. Fig. 2(b) and (c) give the upconversion luminescence spectra of the Yb3+/Er3+ and the Yb3+/ Tm3+ co-doped KGd(WO4)2 materials, respectively. The peaks at 530 and 550 nm of the green emissions of KGd(WO4)2:Yb3+/Er3+ are corresponding to the transitions of Er3+ (2I11/2ĺ4I15/2 and 4S3/2ĺ4I15/2), and the peak at 478 nm of the blue emission of KGd(WO4)2: Yb3+, Tm3+ is due to the transition of Tm3+(1G4ĺ3H6). Similarly, the quenching effect

Fig. 1 XRD patterns of KGd(WO4)2 obtained from sintering the sample at 900 qɋ (1) and the data of JCPDS Card (No. 45-0555) of KGd(WO4)2 (2)

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of the Yb3+/Er3+ and the Yb3+/Tm3+ co-doped KGd(WO4)2 phosphors can also be found in the graphs. The green and blue emission intensities are at maximum when the Er3+ and Tm3+ concentrations are 2 mol.% and 3 mol.%, respectively. It can be concluded that the optimal concentration of Ho3+, Er3+ and Tm3+ for the KGd(WO4)2 with two rare earth dopants should be 2 mol.%, 2 mol.% and 3 mol.%, respectively. Fig. 3 gives the upconversion luminescence spectra of the three rare earth ions co-doped KGd(WO4)2 phosphors. As shown in Fig. 3 (a), the peaks at 478, 546 and 654 nm which correspond to the transitions of Tm3+ (1G4ĺ3H6), Ho3+ (5S2ĺ5I8) and Ho3+ (5F5ĺ5I8) are observed. When increasing the concentration of Tm3+ with invariable concentration of Ho3+ and Yb3+, the blue emission of the sample is getting stronger. The intensity of the green and red emission is en-

Fig. 2 Upconversion luminescence spectra of KGd0.8–x(WO4)2: 0.2Yb3+, xHo3+ (x=0.01, 0.02, 0.03 and 0.04) (a), KGd0.8–y(WO4)2:0.2Yb3+, yEr3+ (y=0.01, 0.02, 0.03 and 0.04) (b) and KGd0.8–z(WO4)2: 0.2Yb3+, zTm3+ (z=0.01, 0.02, 0.03 and 0.04) (c)

Fig. 3 Upconversion luminescence spectra of KGd0.8–x–z(WO4)2: 0.2Yb3+, xHo3+, zTm3+ (x=0.01, 0.02 and 0.03; z=0.01, 0.02 and 0.03) (a), KGd0.8–y–z(WO4)2:0.2Yb3+, yEr3+, zTm3+ (y= 0.01, 0.02 and 0.03; z=0.01, 0.02 and 0.03) (b) and KGd0.8–y–x(WO4)2:0.2Yb3+, yEr3+, xHo3+ (y=0.01, 0.02 and 0.03; x=0.01, 0.02 and 0.03) (c)

DU Haiyan et al., Upconversion luminescence of Yb3+/Ho3+/Er3+/Tm3+ co-doped KGd(WO4)2 powders

hanced when the concentration of Ho3+ is increasing from 1 mol.% to 3 mol.% with the changeless amount of Tm3+ and Yb3+. Similarly, Fig. 3(b) and (c) also show that the emissions of upconversion luminescence are affected by the different amount of the rare earth ion for KGd(WO4)2: Yb3+/Er3+/Tm3+ and KGd(WO4)2:Yb3+/Er3+/Ho3+, respectively. It is found that upconversion luminescence of different color can be obtained by adjusting the concentration of the rare earth dopants. So the three rare earth ions co-doped KGd(WO4)2 phosphors can be used as light source of diverse color. The Yb3+/Ho3+/Tm3+ co-doped KGd(WO4)2 samples are most suitable for producing white light by emitting blue, green and red light simultaneously. To measure the color of the visible emission that the human eye perceives, the Commission internationale de lƍeƍ clairage (CIE) coordinates were calculated. The calculated color coordinates for the KGd(WO4)2:Yb3+/Ho3+/Tm3+ samples are marked as sites (a~e) in CIE chromaticity diagram (Fig. 4), which are (0.25, 0.42), (0.24, 0.38), (0.21, 0.30), (0.19, 0.25) and (0.26, 0.41), respectively. The CIE chromaticity coordinates are close to white light regions. Further efforts should be offered to adjust the color components and enhance the upconversion efficiency for practical application. Among all the rare earth dopants, Yb3+ ion was chosen as the sensitizing ion to increase the upconversion luminescence intensities of the samples, due to the energy matching of the gap (between 2F7/2 and 2F5/2) of Yb3+. So the emissions of the phosphors doped with Yb3+ ion can be obviously observed under the 980 nm semiconductor laser excitation. In order to further understand the involved mechanism, the dependence of upconversion luminescence intensity upon the pump power of laser diode for different rare earth dopants except the sensitizing ion Yb3+ is explored, as shown in Fig. 5. The relationship between the intensity of the upconversion luminescence and the infrared excitation can be approximately expressed as follows[15]: Iucĝ(Iexc)n, where n is the number of infrared photons absorbed for emitting a visible

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photon. In the double-log coordinate graph shown in Fig. 5, the slope of Iuc–Iexc indicates the value of n, which is used for the deduction of the possible upconversion luminescence process. As shown in Fig. 5(a) and (b), the values of n for the emissions of Ho3+ at 546 and 654 nm are 1.73 and 1.66, and those of Er3+ at 530 and 551 nm are 1.85 and 1.84. It indicates that the emissions of Ho3+ and Er3+ originate from twophoton process. Fig. 5(c) shows that the value of n for the emissions of Tm3+ at 478 nm is 2.26, indicating that the blue emission of Tm3+ is produced through a three-photon process. Fig. 6 gives the schematic representation of the energy levels diagram for the Ho3+, Er3+, and Tm3+ and the sensitizing ion Yb3+ as well as the proposed upconversion luminescence mechanisms to produce the blue, green and red (white) upconverted emissions. A Yb3+ ion is excited by a 980 nm photon from ground state (4F7/2) to 4F5/2 through GSA process, and then promotes a Ho3+ ion from 5I8 to 5I6 by transferring the energy to it. Then the excited Er3+ ion transits to a higher level at 5I6 when another Yb3+ ion at 2F5/2 level continuously transfers the energy to it. The populated level 5I6 decays non-radiatively to 5I8, and then produces green emission at 546 nm with a radiative transition to ground state 5I8. The Ho3+ at 5I6 level transits to 5F5 with a non-radiative relaxation

Fig. 5 Dependence of upconversion luminescence intensity upon the pump power of laser diode for KGd(WO4)2:Yb3+, Ho3+ (a), KGd(WO4)2:Yb3+, Er3+ (b) and KGd(WO4)2:Yb3+, Tm3+ (c)

Fig. 4 CIE chromaticity diagram for KGd0.77(WO4)2:0.2Yb3+, 0.02Ho3+, 0.01Tm3+ (a), KGd0.76(WO4)2:0.2Yb3+, 0.02Ho3+, 0.02Tm3+ (b), KGd0.75(WO4)2:0.2Yb3+, 0.02Ho3+, 0.03Tm3+ (c), KGd0.77(WO4)2:0.2Yb3+, 0.01Ho3+, 0.02Tm3+ (d) and KGd0.75(WO4)2:0.2Yb3+, 0.03Ho3+, 0.02Tm3+ (e)

Fig. 6 Schematic representation of the energy levels diagram for the Ho3+, Er3+, and Tm3+ and the sensitizing ion Yb3+ as well as the proposed upconversion luminescence mechanisms to produce the blue, green and red (white) upconverted emissions

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finally produces red emission at 654 nm with a radiative transition to ground state 5I8. It can be concluded that the occurrence of upconversion luminescence of Ho3+ ion can be attributed to a two-photon process. The upconversion mechanism for Er3+ is also a two-photon process. The Er3+ at ground state 4I15/2 is excited to 4I11/2 by a Yb3+ ion at 2F5/2 level, and then transits to a higher level at 4I7/2 when excited by another Yb3+ ion at 2F5/2 level. The populated level 4F7/2 decays non-radiatively to 2H11/2 and 4S3/2, finally produces emissions at 529 and 550 nm with a radiative transition to ground state 4I15/2, respectively. The upconversion for Tm3+ ion occurs through a three-photon process. A Tm3+ at 3H6 is firstly excited to 3H5 by an excited Yb3+ ion, and then relaxes non-radiatively to 3F4. When excited by another Yb3+ ion, the Tm3+ at 3F4 transits to 3F3, then decays to 3H4. After the third Yb3+ ion transferring the energy to it, the Tm3+ at 3H4 is excited to 1G4, and then finally transits to 3H6 radiatively, producing blue emission at 478 nm. For the three rare earth ions co-doped KGd(WO4)2 phosphors, cross relaxation also occurs when the upconversion luminescence is produced through the sensitization of Yb3+. As shown in Fig. 7, the cross relaxation process Ho3+ (5I6)+ Tm3+ (3H4)ĺHo3+ (5F4)+Tm3+ (3H6) and Er3+ (4I11/2)+Tm3+ (3H4)ĺEr3+ (4F7/2)+Tm3+ (3H6) are responsible for the promotions of Ho3+ (5F4) and Er3+ (4F7/2), respectively. The cross relaxation between Ho3+ and Er3+ was not observed, because the excited state energy level of Ho3+ is close to that of Er3+.

Fig. 7 Cross relaxation (CR) of the Ho3+/Tm3+ and Er3+/Tm3+ codoped KGd(WO4)2

3 Conclusions The powder samples of KGd(WO4)2 doped with different lanthanide ions (Yb3+/Ho3+/Er3+/Tm3+) were prepared by the high-temperature solid-state reaction. For the two-ion co-doped KGd(WO4)2 phosphors, the concentration quenching effect was studied, and the optimum concentrations of Ho3+, Er3+ and Tm3+ were 2 mol.%, 2 mol.% and 3 mol.%, respectively. The Yb3+/Ho3+/Tm3+ co-doped KGd(WO4)2 sample is the best white upconversion luminescence material among all the three-ion co-doped KGd(WO4)2 phosphors. The CIE coordinates for the KGd(WO4)2:Yb3+/Ho3+/Tm3+ samples were calculated, and chromaticity coordinates were

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close to white light regions. The upconversion luminescence mechanism for Ho3+, Er3+ and Tm3+ and the sensitizing ion Yb3+ was investigated. The upconversion luminescence of Ho3+ and Er3+ was both produced through two-photon process. The emissions of Ho3+ at 546 nm and 654 nm originated from the transition of Ho3+ (5S2ĺ5I8 and 5F5ĺ5I8), while those of Er3+ at 530 nm and 551 nm were attributed to the transition of Er3+ (2I11/2ĺ4I15/2 and 4S3/2ĺ 4I15/2). The upconversion luminescence mechanism of Tm3+ was a three-photon process. The blue emission of Tm3+ was due to the transition of Tm3+ (1G4ĺ3H6).

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