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Yb3+: Cs2NaGdCl6 powders under 785 nm laser excitation
JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008, p. 880
Green, orange, and red upconversion luminescence in Nd3+/Yb3+: Cs2NaGdCl6 powders under 785 nm laser excitation WANG Dianyuan ()1, GUO Yanyan ()2, WU Xinghua ( )1, Zhang Yuxia ( )1 (1. College of Science, Jiujiang University, Jiujiang 332005, China2. College of Mechanical and Materials Engineering, Jiujiang University, Jiujiang 332005, China) Received 10 June 2008; revised 23 August 2008
Abstract: Nd3+:Cs2NaGdCl6 and Nd3+, Yb3+:Cs2NaGdCl6 polycrystalline powder samples were prepared by Morss method E. Under 785 nm semiconductor laser pumping, the upconversion luminescence of Nd3+ ions in Cs2NaGdCl6 was investigated at room temperature, and three upconversion emissions near 538 nm (Green), 603 nm (Orange), and 675 nm (Red) were observed and assigned to 4G7/2→4I9/2, (4G7/2→4I11/2; 4 G5/2→4I9/2), and (4G7/2→4I13/2; 4G5/2→4I11/2), respectively. The dependences of these upconverted emissions on laser power and Nd3+ ion concentration were investigated, to explore the upconversion mechanism. The effect of doping Yb3+ ions on the upconversion luminescence of Nd3+ in Cs2NaGdCl6 was also studied under 785 nm laser excitation. The energy transfer processes were discussed as the possible mechanism for the above upconversion emissions. Keywords: upconversion luminescence; energy transfer; excited state absorption; rare earths
In recent years, frequency upconversion of infrared light to visible light has been extensively investigated in trivalent rare-earth (RE) -ion-doped materials for a wide range of applications, such as, three-dimension volumetric display, all-solid-state compact laser devices operating in the blue-green region, optical data storage, infrared quantum counter detectors, and fluorescent labels, for sensitive detection of biomolecules [1]. Among trivalent RE ions, Pr3+, Nd3+, Er3+, Tm3+, and Ho3+ ions have been investigated most widely in the last two decades. Cs2NaGdCl6 crystallizes in the space group Fm3m and the Gd3+ ion in the GdCl63- moiety has Oh site symmetry. The Raman spectra and vibrational data show that this system has low phonon energy (phonon cutoff ≈300 cm–1) [2]. This character would reduce the probabilities of the phonon-induced, nonradiative transitions, increase the population of the 4F3/2 metastable level of Nd3+ ions, and in turn result in a high quantum yield of upconversion emissions. Efficient upconversion luminescence was observed in Cs2NaGdCl6:Nd3+,Ho3+ (λexc = 514.5 nm)[3] Cs2NaGdCl6: Tm3+ (λexc = 650 nm)[4] and Tm3+/Ho3+:Cs2NaGdCl6 (λexc = 785 nm)[5] crystals at room temperature. In this work, Nd3+ single-doped and Nd3+, Yb3+ co-doped Cs2NaGdCl6 polycrystalline powder samples were prepared using the Morss method E. Under 785 nm laser excitation, the visible upconversion emissions from Nd3+ ions were recorded. The mechanisms were studied through power de-
pendence and concentration dependence of upconverted emissions. The effect of Yb3+ ions on the upconversion luminescence of Nd3+ was also studied.
1 Experimental Polycrystalline powder samples Cs2NaGd1–xNdxCl6 (x= 0.02, 0.10, 0.25, 0.50, 0.75, 1.0), and Cs2NaNd0.02Gd0.98–y YbyCl6 (y=0.05, 0.10, 0.25, 0.50) were prepared using the Morss method E[6]. CsCl (99.9%), NaCl (AR), Gd2O3 (99.99%), Nd2O3 (99.99%), Yb2O3 (99.99%), and hydrochloric acid (AR) were used as the starting raw materials. GdCl3, NdCl3, and YbCl3 stock solutions were prepared by dissolving Gd2O3, Nd2O3, and Yb2O3 in hydrochloric acid and diluting with deionized water. Stoichiometric CsCl, NaCl, GdCl3, NdCl3, and YbCl3 solutions were mixed in an evaporating dish. The mixture was dissolved by using ultrasonic agitation until a uniform solution was achieved. Then the solution was concentrated by heating on a furnace until the excess free water evaporated. All samples were stored in sealed quartz tubes to reduce their exposure to atmospheric water. X-ray diffraction (XRD) was carried out on an MAC Science Co., Ltd. (Japan) MXP18AHF X-ray diffractometer with Cu Kα radiation. The fluorescence spectra were measured and corrected by using a Jobin-Yvon LABRAM-HR confocal laser microraman spectrometer system, which could work with a 785 nm laser (with a maximum power
Foundation item: Project supported by Scientific Project of Jiangxi Education Departments of China (2007330) and Science Foundation of Jiujiang University (05KJ01) Corresponding author: WANG Dianyuan (E-mail: [email protected]; Tel.: +86-792-8337198)
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WANG D Y et al., Green, orange, and red upconversion luminescence in Nd3+/Yb3+:Cs2NaGdCl6 powders under 785 nm
density of 1.45 mW/mm2). All experiments were carried out at room temperature.
2 Results and discussion The XRD patterns show that all resultant samples are crystallized in cubic Cs2NaGdCl6 with space group Fm3m (JCPDS card No.79-0722)[5]. Visible upconversion has been observed in Nd3+: Cs2NaGdCl6 powder under 785 nm laser. Under excitation, in resonance with the 4I9/2→4F5/2, 2H9/2 transitions, the spectrum shows three main bands near 538 nm (Green), 603 nm (Orange), and 675 nm (Red). As an example, Fig.1 shows the emission spectrum for the sample doped with 2at.% of Nd3+, obtained by exciting at 785 nm. An analysis of the energy level diagram and the upconverted emission spectrum suggests that these bands can be attributed to radiative transitions 4G7/2→4I9/2 (538 nm), (4G7/2→4I11/2; 4G5/2→4I9/2) (603 nm), and (4G7/2→4I13/2; 4G5/2→4I11/2) (675 nm). The concentration dependence of these three upconverted emission bands in Nd3+ single-doped samples has also been obtained and shown in Fig.2. From this figure, it can be seen that the concentration dependence of 603 and 675 nm emissions is very similar to each other, but very different from that of 538 nm. Therefore, it confirms that 603 and 675 nm emissions contain the fluorescence emission component from the 4 G5/2 excited state, that is, 4G5/2→4I9/2 and 4G5/2→4I11/2 transitions, respectively. The assignment of these three emissions is very different from Ref.[7]. To disclose the upconversion mechanisms, the upconversion emission intensity I is measured as a function of the pumped power P. For the upconversion process, I is proportional to the nth power of P, that is, I∝Pn, where n is the number of pump photons absorbed per upconverted photon emitted[8]. A plot of lgI versus lgP yields a straight line with slope n. The values of the slope n for 538, 603, and
Fig.1 Upconversion emission spectrum for the sample doped with 2at.% Nd3+ obtained by 785 nm laser excitation at room temperature
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675 nm upconverted emissions for 2at.%Nd3+ single-doped sample are 1.690, 1.930 and 1.926, respectively (as shown in Fig.3). This result indicates a two-photon upconversion process. The excited states for upconversion can be populated by several well-known mechanisms: (1) excited state absorption (ESA), (2) energy transfer (ET), and (3) photon avalanche. The photon avalanche upconversion mechanism has been ruled out because no inflection point was observed in the power study. Together with the energy level diagram of the Nd3+ ion in Cs2NaNdCl6, Fig.4 illustrates the possible mechanisms of population losses from 4F3/2 leading to upconversion processes: excited state absorption (ESA). The excited 4G7/2 and 4G5/2 states can be populated by ESA and ESA processes in Fig.4. As the concentration of Nd3+ increases, the cross-relaxation energy transfer process ET 4 F3/2 (Nd1)+4I9/2 (Nd2)→4I15/2 (Nd1)+4I15/2 (Nd2) becomes efficient and enhances the upconversion emission intensity. However, when the Nd3+ concentration exceeds 10at.%, the concentration quenching of 538 nm emission is remarkable by 4G7/2+4I9/2→4F5/2 + 4I15/2. The effect of the Yb3+ ion on the upconversion emission
Fig.2 Upconversion emission integrated intensity as a function of Nd3+ ion concentration
Fig.3 Power dependence of the upconverted emission intensity in 2at.%Nd3+:Cs2NaGdCl6 under 785nm laser excitation
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intensity of Nd3+ in Cs2NaGdCl6 was also presented. Fig.5 shows the concentration dependence of the upconversion emission intensity on Yb3+ ion concentration. It can be found that the doped Yb3+ ions enhance the upconverted emissions of Nd3+ and for the 2at.%Nd3+ doped sample, the optimal Yb3+ ion concentration is 25at.%. Fig.4 also shows the possible mechanism of this enhancement. First, by absorbing 785 nm laser photons, Nd3+ ions are excited from ground state to 2H9/2 and 4F5/2, and then relax to the 4F3/2 level through nonradiative relaxation. Second, energy transfer from Nd3+(4F3/2) to Yb3+(2F5/2) can occur, and this process will be more efficient as the Yb3+ ion concentration increases. Third, the energy transferred to Yb3+ will migrate over the Yb-sublattice until it is transferred to an Nd3+ ion by an energy transfer upconversion (ETU) process ET:
JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008 2
F5/2(Yb3+)+4F3/2(Nd3+)→2F7/2(Yb3+)+4G11/2(Nd3+). Finally, the 4G7/2 and 4G5/2 states of the Nd3+ ions are populated by nonradiative relaxation, so the intensity of the 538, 603, and 675 nm upconverted emissions become strong.
3 Conclusion Nd3+/Yb3+:Cs2NaGdCl6 polycrystalline powder samples were prepared, and the green, orange, and red upconversion fluorescence of Nd3+ doped and Nd3+, Yb3+ co-doped Cs2NaGdCl6 was measured and investigated under 785 nm laser excitation, at room temperature. With the help of the power and concentration dependences of upconverted emission intensities, all the possible ESA and ET processes responsible for these visible upconversion emissions were presented. The analysis of the plot of the upconversion emissions intensity of Nd3+ as a function of Yb3+ molar concentration indicated that the Yb3+ ion could enhance the upconverted emissions of Nd3+.
References:
Fig.4 Upconversion mechanisms and assignments of the main upconverted emission bands observed with 785 nm laser excitation in Nd3+/Yb3+:Cs2NaGdCl6
Fig.5 Upconversion emission intensity of Nd3+ as a function of Yb3+ ion concentration in Cs2NaGd0.98-yNd0.02YbyCl6
[1] Auzel F. Upconversion and anti-stokes processes with f and d ions in solids. Chem. Rev., 2004, 104: 139. [2] Tanner P A, Mak C S K, Kwok W M, Philips D L, Faucher M D. Ultraviolet f→f emission and crystal field analysis for Er3+ in Cs2NaErCl6. Phys. Rev. B, 2002, 66: 165203. [3] Wang D Y, Luo J L, Yu J M, Liu J Q, Wang Q K, Guo Y Y, Yin M, Zhang W P, Xia S D. Upconversion luminescence and energy transfer in Nd3+, Ho3+: Cs2NaGdCl6. J. Alloys Compd., 2005, 399: 57. [4] Joubert M F, Guy S, Cuerq S, Tanner P A. Room temperature blue upconverted luminescence via photon avalanche pumping in Cs2NaGdCl6:Tm3+. J. Lumin., 1997, 75: 287. [5] Wang D Y, Guo Y Y, Sun G H, Li J, Zhao L, Xu G P. Blue, green, yellow and red upconversion fluorescence in Tm3+/Ho3+:Cs2NaGdCl6 crystals under 785 nm laser excitation. J. Alloys Compd., 2007, 451: 122. [6] Morss L R, Siegal M, Stenger L, Edelstein N, Preparation of cubic chloro complex compounds of trivalent metals: Cs2NaMCl6. Inorg. Chem., 1970, 9: 1771. [7] Fernandez J, Balda R, Sanz M, Lacha L M, Oleaga A, Adam J L. Upconversion losses in Nd-doped fluoroarsenate glasses. J. Lumin., 2001, 94-95: 325. [8] Pollnau M, Gamelin D R, Luthi S R, Gudel H U. Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems. Phys. Rev. B, 2000, 61: 3337.
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Green, orange, and red upconversion luminescence in Nd3+/Yb3+: Cs2NaGdCl6 powders under 785 nm laser excitation WANG Dianyuan ()1, GUO Yanyan ()2, WU Xinghua ( )1, Zhang Yuxia ( )1 (1. College of Science, Jiujiang University, Jiujiang 332005, China2. College of Mechanical and Materials Engineering, Jiujiang University, Jiujiang 332005, China) Received 10 June 2008; revised 23 August 2008
Abstract: Nd3+:Cs2NaGdCl6 and Nd3+, Yb3+:Cs2NaGdCl6 polycrystalline powder samples were prepared by Morss method E. Under 785 nm semiconductor laser pumping, the upconversion luminescence of Nd3+ ions in Cs2NaGdCl6 was investigated at room temperature, and three upconversion emissions near 538 nm (Green), 603 nm (Orange), and 675 nm (Red) were observed and assigned to 4G7/2→4I9/2, (4G7/2→4I11/2; 4 G5/2→4I9/2), and (4G7/2→4I13/2; 4G5/2→4I11/2), respectively. The dependences of these upconverted emissions on laser power and Nd3+ ion concentration were investigated, to explore the upconversion mechanism. The effect of doping Yb3+ ions on the upconversion luminescence of Nd3+ in Cs2NaGdCl6 was also studied under 785 nm laser excitation. The energy transfer processes were discussed as the possible mechanism for the above upconversion emissions. Keywords: upconversion luminescence; energy transfer; excited state absorption; rare earths
In recent years, frequency upconversion of infrared light to visible light has been extensively investigated in trivalent rare-earth (RE) -ion-doped materials for a wide range of applications, such as, three-dimension volumetric display, all-solid-state compact laser devices operating in the blue-green region, optical data storage, infrared quantum counter detectors, and fluorescent labels, for sensitive detection of biomolecules [1]. Among trivalent RE ions, Pr3+, Nd3+, Er3+, Tm3+, and Ho3+ ions have been investigated most widely in the last two decades. Cs2NaGdCl6 crystallizes in the space group Fm3m and the Gd3+ ion in the GdCl63- moiety has Oh site symmetry. The Raman spectra and vibrational data show that this system has low phonon energy (phonon cutoff ≈300 cm–1) [2]. This character would reduce the probabilities of the phonon-induced, nonradiative transitions, increase the population of the 4F3/2 metastable level of Nd3+ ions, and in turn result in a high quantum yield of upconversion emissions. Efficient upconversion luminescence was observed in Cs2NaGdCl6:Nd3+,Ho3+ (λexc = 514.5 nm)[3] Cs2NaGdCl6: Tm3+ (λexc = 650 nm)[4] and Tm3+/Ho3+:Cs2NaGdCl6 (λexc = 785 nm)[5] crystals at room temperature. In this work, Nd3+ single-doped and Nd3+, Yb3+ co-doped Cs2NaGdCl6 polycrystalline powder samples were prepared using the Morss method E. Under 785 nm laser excitation, the visible upconversion emissions from Nd3+ ions were recorded. The mechanisms were studied through power de-
pendence and concentration dependence of upconverted emissions. The effect of Yb3+ ions on the upconversion luminescence of Nd3+ was also studied.
1 Experimental Polycrystalline powder samples Cs2NaGd1–xNdxCl6 (x= 0.02, 0.10, 0.25, 0.50, 0.75, 1.0), and Cs2NaNd0.02Gd0.98–y YbyCl6 (y=0.05, 0.10, 0.25, 0.50) were prepared using the Morss method E[6]. CsCl (99.9%), NaCl (AR), Gd2O3 (99.99%), Nd2O3 (99.99%), Yb2O3 (99.99%), and hydrochloric acid (AR) were used as the starting raw materials. GdCl3, NdCl3, and YbCl3 stock solutions were prepared by dissolving Gd2O3, Nd2O3, and Yb2O3 in hydrochloric acid and diluting with deionized water. Stoichiometric CsCl, NaCl, GdCl3, NdCl3, and YbCl3 solutions were mixed in an evaporating dish. The mixture was dissolved by using ultrasonic agitation until a uniform solution was achieved. Then the solution was concentrated by heating on a furnace until the excess free water evaporated. All samples were stored in sealed quartz tubes to reduce their exposure to atmospheric water. X-ray diffraction (XRD) was carried out on an MAC Science Co., Ltd. (Japan) MXP18AHF X-ray diffractometer with Cu Kα radiation. The fluorescence spectra were measured and corrected by using a Jobin-Yvon LABRAM-HR confocal laser microraman spectrometer system, which could work with a 785 nm laser (with a maximum power
Foundation item: Project supported by Scientific Project of Jiangxi Education Departments of China (2007330) and Science Foundation of Jiujiang University (05KJ01) Corresponding author: WANG Dianyuan (E-mail: [email protected]; Tel.: +86-792-8337198)
3') SGI)DFWRU\3UR ZZZILQHSULQWFQ
WANG D Y et al., Green, orange, and red upconversion luminescence in Nd3+/Yb3+:Cs2NaGdCl6 powders under 785 nm
density of 1.45 mW/mm2). All experiments were carried out at room temperature.
2 Results and discussion The XRD patterns show that all resultant samples are crystallized in cubic Cs2NaGdCl6 with space group Fm3m (JCPDS card No.79-0722)[5]. Visible upconversion has been observed in Nd3+: Cs2NaGdCl6 powder under 785 nm laser. Under excitation, in resonance with the 4I9/2→4F5/2, 2H9/2 transitions, the spectrum shows three main bands near 538 nm (Green), 603 nm (Orange), and 675 nm (Red). As an example, Fig.1 shows the emission spectrum for the sample doped with 2at.% of Nd3+, obtained by exciting at 785 nm. An analysis of the energy level diagram and the upconverted emission spectrum suggests that these bands can be attributed to radiative transitions 4G7/2→4I9/2 (538 nm), (4G7/2→4I11/2; 4G5/2→4I9/2) (603 nm), and (4G7/2→4I13/2; 4G5/2→4I11/2) (675 nm). The concentration dependence of these three upconverted emission bands in Nd3+ single-doped samples has also been obtained and shown in Fig.2. From this figure, it can be seen that the concentration dependence of 603 and 675 nm emissions is very similar to each other, but very different from that of 538 nm. Therefore, it confirms that 603 and 675 nm emissions contain the fluorescence emission component from the 4 G5/2 excited state, that is, 4G5/2→4I9/2 and 4G5/2→4I11/2 transitions, respectively. The assignment of these three emissions is very different from Ref.[7]. To disclose the upconversion mechanisms, the upconversion emission intensity I is measured as a function of the pumped power P. For the upconversion process, I is proportional to the nth power of P, that is, I∝Pn, where n is the number of pump photons absorbed per upconverted photon emitted[8]. A plot of lgI versus lgP yields a straight line with slope n. The values of the slope n for 538, 603, and
Fig.1 Upconversion emission spectrum for the sample doped with 2at.% Nd3+ obtained by 785 nm laser excitation at room temperature
881
675 nm upconverted emissions for 2at.%Nd3+ single-doped sample are 1.690, 1.930 and 1.926, respectively (as shown in Fig.3). This result indicates a two-photon upconversion process. The excited states for upconversion can be populated by several well-known mechanisms: (1) excited state absorption (ESA), (2) energy transfer (ET), and (3) photon avalanche. The photon avalanche upconversion mechanism has been ruled out because no inflection point was observed in the power study. Together with the energy level diagram of the Nd3+ ion in Cs2NaNdCl6, Fig.4 illustrates the possible mechanisms of population losses from 4F3/2 leading to upconversion processes: excited state absorption (ESA). The excited 4G7/2 and 4G5/2 states can be populated by ESA and ESA processes in Fig.4. As the concentration of Nd3+ increases, the cross-relaxation energy transfer process ET 4 F3/2 (Nd1)+4I9/2 (Nd2)→4I15/2 (Nd1)+4I15/2 (Nd2) becomes efficient and enhances the upconversion emission intensity. However, when the Nd3+ concentration exceeds 10at.%, the concentration quenching of 538 nm emission is remarkable by 4G7/2+4I9/2→4F5/2 + 4I15/2. The effect of the Yb3+ ion on the upconversion emission
Fig.2 Upconversion emission integrated intensity as a function of Nd3+ ion concentration
Fig.3 Power dependence of the upconverted emission intensity in 2at.%Nd3+:Cs2NaGdCl6 under 785nm laser excitation
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intensity of Nd3+ in Cs2NaGdCl6 was also presented. Fig.5 shows the concentration dependence of the upconversion emission intensity on Yb3+ ion concentration. It can be found that the doped Yb3+ ions enhance the upconverted emissions of Nd3+ and for the 2at.%Nd3+ doped sample, the optimal Yb3+ ion concentration is 25at.%. Fig.4 also shows the possible mechanism of this enhancement. First, by absorbing 785 nm laser photons, Nd3+ ions are excited from ground state to 2H9/2 and 4F5/2, and then relax to the 4F3/2 level through nonradiative relaxation. Second, energy transfer from Nd3+(4F3/2) to Yb3+(2F5/2) can occur, and this process will be more efficient as the Yb3+ ion concentration increases. Third, the energy transferred to Yb3+ will migrate over the Yb-sublattice until it is transferred to an Nd3+ ion by an energy transfer upconversion (ETU) process ET:
JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008 2
F5/2(Yb3+)+4F3/2(Nd3+)→2F7/2(Yb3+)+4G11/2(Nd3+). Finally, the 4G7/2 and 4G5/2 states of the Nd3+ ions are populated by nonradiative relaxation, so the intensity of the 538, 603, and 675 nm upconverted emissions become strong.
3 Conclusion Nd3+/Yb3+:Cs2NaGdCl6 polycrystalline powder samples were prepared, and the green, orange, and red upconversion fluorescence of Nd3+ doped and Nd3+, Yb3+ co-doped Cs2NaGdCl6 was measured and investigated under 785 nm laser excitation, at room temperature. With the help of the power and concentration dependences of upconverted emission intensities, all the possible ESA and ET processes responsible for these visible upconversion emissions were presented. The analysis of the plot of the upconversion emissions intensity of Nd3+ as a function of Yb3+ molar concentration indicated that the Yb3+ ion could enhance the upconverted emissions of Nd3+.
References:
Fig.4 Upconversion mechanisms and assignments of the main upconverted emission bands observed with 785 nm laser excitation in Nd3+/Yb3+:Cs2NaGdCl6
Fig.5 Upconversion emission intensity of Nd3+ as a function of Yb3+ ion concentration in Cs2NaGd0.98-yNd0.02YbyCl6
[1] Auzel F. Upconversion and anti-stokes processes with f and d ions in solids. Chem. Rev., 2004, 104: 139. [2] Tanner P A, Mak C S K, Kwok W M, Philips D L, Faucher M D. Ultraviolet f→f emission and crystal field analysis for Er3+ in Cs2NaErCl6. Phys. Rev. B, 2002, 66: 165203. [3] Wang D Y, Luo J L, Yu J M, Liu J Q, Wang Q K, Guo Y Y, Yin M, Zhang W P, Xia S D. Upconversion luminescence and energy transfer in Nd3+, Ho3+: Cs2NaGdCl6. J. Alloys Compd., 2005, 399: 57. [4] Joubert M F, Guy S, Cuerq S, Tanner P A. Room temperature blue upconverted luminescence via photon avalanche pumping in Cs2NaGdCl6:Tm3+. J. Lumin., 1997, 75: 287. [5] Wang D Y, Guo Y Y, Sun G H, Li J, Zhao L, Xu G P. Blue, green, yellow and red upconversion fluorescence in Tm3+/Ho3+:Cs2NaGdCl6 crystals under 785 nm laser excitation. J. Alloys Compd., 2007, 451: 122. [6] Morss L R, Siegal M, Stenger L, Edelstein N, Preparation of cubic chloro complex compounds of trivalent metals: Cs2NaMCl6. Inorg. Chem., 1970, 9: 1771. [7] Fernandez J, Balda R, Sanz M, Lacha L M, Oleaga A, Adam J L. Upconversion losses in Nd-doped fluoroarsenate glasses. J. Lumin., 2001, 94-95: 325. [8] Pollnau M, Gamelin D R, Luthi S R, Gudel H U. Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems. Phys. Rev. B, 2000, 61: 3337.
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