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Upconversion photoluminescence of ZrO2:Er3+ nanocrystals synthesized by using butadinol as high boiling point solvent
Optical Materials 28 (2006) 246–249 www.elsevier.com/locate/optmat
Upconversion photoluminescence of ZrO2:Er3+ nanocrystals synthesized by using butadinol as high boiling point solvent Ruokun Jia, Wensheng Yang, Yubai Bai *, Tiejin Li College of chemistry, Jilin University, Changchun 130023, P.R. China Received 23 July 2004; accepted 15 November 2004 Available online 8 February 2005
Abstract Erbium doped ZrO2 (ZrO2:Er3+) nanocrystals are prepared by a butadinol low thermal crystallization method. The crystallization temperature is reduced to 500 °C. X-ray diffraction (XRD) spectra show there exists monoclinic and cubic ZrO2 phases. Green, yellow and red lights are seen from the upconverting luminescence spectra under 980 nm excitation. The results confirm that upconverting emission is due to excited-state absorption (ESA) and energy transfer upconverting (ETU) process. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Upconverting character of rare-earth ion Er has received a lot of attentions due to its application potentials in fiber amplifier [1], short-wave laser [2] and biological labeling [3–7]. When excited by a near-infrared laser, the Er3+ ion shows emission at 550 and 660 nm [8–13]. Usually, Er must be doped into a lattice to emit luminescence upon excitation. Both Y2O3 and ZrO2 are considered to be suitable doping hosts for the rare-earth ions [14,15]. Compared to Y2O3, ZrO2 nanocrystal is chemically more stable and it does not decompose even at pH value of 3. Monochromatic green light is available at low dopant concentration and pure red light will be seen at high dopant concentration for the Er3+ doped ZrO2 nanocrystals. It is known that ZrO2 has three polymorphs: monoclinic, tetragonal, and cubic phases. Both the monoclinic and tetragonal phases exist at low dopant concentration, while cubic exists at high dopant concentration. Prasad and co-workers reported the synthesis of Er3+ doped ZrO2 nanocrystals by using a *
Corresponding author. Tel.: +86 431 8499192; fax: +86 431 8923907. E-mail address: [email protected] (Y. Bai). 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.11.034
sol-emulsion-gel method [16], in which the precursor of zirconia propoxide was soled to propanol and then dispersed into a mixed system of span80 and cyclohexane. Calcining temperature as high as 1000 °C was necessary to induce the gel particles separated by centrifugation into crystals with excellent upconverting. Such high calcining temperature will lead to the dramatic growth of the particle size. From the point of practical application, it is meaningful, beneficial to find an approach to produce smaller Er3+ doped ZrO2 nanocrystals with demanded upconversion emission. In this work, we use butadinol as the solvent for the sake of preventing hydrolyzation and increasing the temperature of the solvent to 200 °C. It is found that the ZrO2:Er3+ nanocrystals with required upconverting emission can be obtained at a calcining temperature as low as 500 °C.
2. Experimental ZrOCl2 Æ 8H2O (AR, Shanghai Chemical Reagent Corp.) and Er2O3 (GR) were used as the starting materials. Er2O3 was dissolved by nitric acid, and then precipitated by ammonia. Erbium acetate was obtained by dissolving the precipitate in acetic acid. To synthesize
R. Jia et al. / Optical Materials 28 (2006) 246–249
247
ZrO2:Er3+ nanoparticles, first, 7.5 g ZrOCl2 Æ 8H2O was added to the 80 ml 1,3-butadinol and stirred for 30 min at 200 °C, then a stoichiometric amount of solid erbium acetate was added into the solution and the system was further stirred for 3 h at 200 °C. The resulting sol was gelled by the controlled addition of 4 g NaOH. Then the gel particles were separated by centrifugation followed by washing with acetone. The product was dried at 60 °C in air for 12 h and then calcined at 500 °C for 2 h. The X-ray diffraction patterns were collected by using a Rigaku D/max-rA X-ray diffraction (XRD) instrument. A JY-UV-Raman argon ion laser with a wavelength of 488 nm and a semiconductor laser with a wavelength of 980 nm were employed to acquire the luminescence spectra. Morphology observations were conducted with a H8100 transmission electron microscopy (TEM).
perature of 500 °C, much lower than those reported in the literatures (up to 1100 °C for the monoclinic phase and above 2370 °C for the cubic phase [17]. When a base was added, Zr4+ ions tend to exit in the form of undefined ZrO2 Æ xH2O, whose growth is difficult to control. The employment of butadinol allows the reaction to go at temperature as high as 200 °C. At this time, most of the water is likely to be driven out the system and the growth of the particles are controlled. The diameter of Er3+ ion is larger than that of Zr4+; the introduction of such rare-earth ion will induce the change of ZrO2 lattice from monoclinic to cubic [18]. Therefore, the cubic phase becomes dominant with increased dopant concentrations. The crystallite sizes of the ZrO2:Er3+ nanocrystals are calculated to be about 27 nm by the Scherrers equation. TEM observations showed the average particle size of the nanocrystals is about 70 nm (Fig. 2), larger than that calculated from the XRD patterns due to the conglomeration of the nanocrystals.
3. Results and discussion
3.2. Upconverting properties
3.1. Structural investigations
Fig. 3 shows the upconverting spectra of the ZrO2:Er3+ nanocrystals with different dopant concentrations under 980 nm excitation. The emission centered at 550 nm corresponds to 2H11/2, 4S3/2 ! 4I15/2 transition, and the one centered at 660 nm corresponds to 4 F9/2 ! 4I15/2 transition. With increased dopant concentrations, the emission centered at 550 nm becomes weaker and the one centered at 660 nm becomes dominant. It is concluded that both the structures and upconverting properties of the are dependent on the dopant concentrations. It is seen that the monoclinic phase is related to the green emission and the cubic usually leads to red emission. When the monoclinic and cubic coexist in the nanocrystals, the powders show yellow or orange color under the 980 nm laser as a result of mixing of red and green emissions. Therefore, the upconverting emission can be mediated by the dopant concentrations.
Fig. 1 shows the powder X-ray diffraction patterns of the ZrO2:Er3+ nanocrystals. Without the dopant, the ZrO2 presents the monoclinic phase (see Fig. 1a). At high dopant concentration, that is, 20 mol%, the nanocrystals show cubic phase (see Fig. 1g). In the range of dopant concentrations from 0.1 to 12 mol%, the nanocrystals are the mixtures of both monoclinic and cubic phases (see Fig. 1b–f) and the cubic becomes dominant with increased dopant concentrations. It is seen that the ZrO2 nanocrystals can be obtained at calcining tem-
a b c d
e
f
g 30
40
50
60
2-theta Fig. 1. Powder XRD patterns of ZrO2:Er3+ nanocrystals with different dopant concentrations of Er. (a) 0 mol%, (b) 0.1 mol%, (c) 1 mol%, (d) 4 mol%, (e) 6 mol%, (f) 12 mol%, and (g) 20 mol%.
Fig. 2. TEM micrograph of the ZrO2:Er3+ nanocrystals.
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R. Jia et al. / Optical Materials 28 (2006) 246–249
a
4F 7/2 2 H11/2 4 S3/2 4
F 9/2
b
4
I 9/2
4
I 11/2 I13/2
c d e
f
500
550
600
650
980 nm
4
4
I 15/2
Er3+
Er3+
low dopant concentration
Er3+
high dopant concentration
Fig. 5. Schematic representation of upconverting process at low and high dopant concentrations.
700
Wavelength (nm)
I15/2
448 nm excitation 4
4
S3/2
2
4
H11/2
I15/2
I15/2
980 nm excitation 500
550
600
650
700
Wavelengh (nm)
Fig. 4. Comparison of the Stokes and anti-Stokes spectra of the ZrO2:Er3+ nanocrystal with a dopant concentration of 20% upon excitation at 488 and 980 nm.
Fig. 4 shows comparison of the upconverting and downconverting emission spectra in the range of 500– 700 nm under 980 and 488 nm excitation, respectively. It is noted that the green emission corresponding to 4 S3/2, 2H11/2 ! 4I15/2 transition is observed only upon excitation at 488 nm. Similar red emission corresponding to 4F9/2 ! 4I15/2 transition is observed under both 488 and 980 nm excitation.
EnergyX10 3 (cm-1) 25 4 F3/2 4 F5/2 4 F 2 7/2 H 11/2 4 S 3/2
20
8 6
4
F 9/2
15
4
I9/2
10
5
3.3. Upconverting mechanism 0
It is expected that the distance when the interactions between the doped Er3+ ions are weak at low dopant concentration, the emission will be mainly contributed
4
I11/2
4
I13/2
980 nm
4
14928cm-1
F9/2
14706cm-1
4
I15/2
15151cm-1
S3/2
15270cm-1
4
4
15373cm-1
,
11/2
980 nm
H
15492cm-1
2
by the excited-state absorbtion (ESA) process. Electrons in the ground state will be excited to the 4I11/2 level by the 980 nm excitation. Then they will reabsorb the excited light to arrive at the 4F7/2 level, and undergo a non-radiative decay to the 2H11/2 and 4S3/2 level. The green emission centered at about 550 nm will be observed as a result of radiative decay from the 4S3/2, 2 H11/2 level to the ground state. At high dopant concentration, the interactions between the doped Er3+ ions becomes stronger and the energy transfer upconverting (ETU) process will becomes dominant at this time. Therefore the green emission disappears and the red one is observed upon excitation at 980 nm. The enhanced interactions between the Er3+ ions induce increased relaxation of 4I11/2–4I13/2 at higher dopant concentrations (Fig. 5). It is known that the 4I13/2 level has a lifetime of longer than 5 ms [19], so the electrons populated in this level will reabsorb the excited light of 980 nm to arrive at 4F9/2 level, then go back to the ground state to emit the red light. It is seen that the energy of transition between the 4I13/2 and 4F9/2 levels does not match that of the excitation light of 980 nm exactly.
980 nm
Fig. 3. Upconverting spectra of the ZrO2:Er3+ nanocrystals with different dopant concentrations of Er under 980 nm excitation. (a) 0.1 mol%, (b) 1 mol%, (c) 4 mol%, (d) 6 mol%, (e) 12 mol%, (f) 20 mol%.
8 7 6
4
I15/2
Er3+
Er3+
Fig. 6. The energy levels of Er3+ with Stark splitting of 4F9/2 and 4I15/2 levels in cubic ZrO2:Er3+ nanocrystals.
R. Jia et al. / Optical Materials 28 (2006) 246–249
However, the transition still takes place due to the Stark splitting of the 4F9/2 energy level, which results in the broadened absorbed band corresponding to transition between the 4I13/2 and 4F9/2 levels (Fig. 6). It is noted that for the sample with highest dopant concentration (20 mol%), no green emission can be observed under 980 nm excitation. This means the electrons can not be excited to the 4F7/2 level. When the 488 nm excitation is employed, the electrons can be elevated to the 4F7/2 level directly, both green and red emissions are available at this time (see Fig. 4).
4. Conclusions In summary, Erbium doped ZrO2 nanocrystals are prepared by a butadinol low thermal crystallization method. The crystallization temperature is reduced to 500 °C. X-ray diffraction (XRD) spectra shows that there exists monoclinic and cubic ZrO2 phases. Green, yellow and red light are seen from the upconverting luminescence spectra under 980 nm excitation. The color of emission and crystalline phases were determined by Er3+ ions dopant concentration. The results confirm that upconverting emission is due to excited-state absorption (ESA) and energy transfer upconverting (ETU) process.
Acknowledgement The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (NNSFC), 863 and 973 projects.
249
References [1] C.G. Valley, Opt. Fiber Technol. 7 (2001) 21. [2] E. Downing, L. Hesselink, J. Ralson, R. MacFarlane, Science 272 (1996) 1185. [3] J. Hampl, M. Hall, N.A. Mufti, Y.-m.M. Yao, D.b. MacQueen, W.H. Wright, D.E. Cooper, Anal. Biochem. 288 (2001) 176. [4] R.S. Niedbala, H. Feindt, K. Kardos, T. Vail, J. Burton, B. ka, S. Li, D. Milunic, P. Bourdelle, R. Vallejo, Anal. Biochem. 293 (2001) 22. [5] P.L.A.M. Corstjens, M. Zuiderwijk, M. Nilsson, H. Feindt, R.S. Niedbala, H.J. Tanke, Anal. Biochem. 312 (2003) 191. [6] P. Corstjens, M. Zuiderwijk, A. Brink, S. Li, H. Feindt, R.S. Neidbala, H. Tanke, Clin. Chem. 47 (10) (2001) 1885. [7] F. Van de Rijke, H. Zijlmans, S. Li, T. Vail, A.K. Raap, R.S. Niedbala, H.J. Tanke, Nat. Biotechnol. 19 (3) (2001) 273. [8] J.A. Capobianco, F. Vetrone, J.C. Boyer, A. Speghini, M. Bettinelli, Opt. Mater. 19 (2002) 259. [9] J.A. Capobianco, F. Vetrone, T. DAlesio, G. Tessari, A. Speghini, M. Bettinelli, Phys. Chem. Chem. Phys 2 (2000) 3203. [10] J. Silver, M.I. Martinez-Rubio, T.G. Ireland, R. Withnall, J. Phys. Chem. B 105 (2001) 7200. [11] J.A. Capobianco, F. Vetrone, J.C. Boyer, J. Phys. Chem. B 106 (2002) 1181. [12] F. Vetrone, J.C. Boyer, J.A. Capobianco, J. Phys. Chem. B 106 (2002) 5622. [13] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, Chem. Mater. 15 (2003) 2737. [14] J. Silver, M.I. Martinez-Rubio, T.G. Ireland, G.R. Fern, R. Withnall, J. Phys. Chem. B 105 (2001) 948. [15] J. Silver, M.I. Martinez-Rubio, T.G. Ireland, G.R. Fern, R. Withnall, J. Phys. Chem. B 105 (2001) 9107. [16] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, J. Phys. Chem. B 106 (2002) 1909. [17] A.E. Bohe´, J. Andrade-Gamboa, D.M. Pasquevich, A.J. Tolley, J.L. Pelegrina, J. Am. Ceram. Soc 83 (4) (2000) 755. [18] E. Djurado, F. Boulch, G. Dezanneau, P. Bouvier, J. Phys. Chem. B 107 (2003) 8321. [19] S. Tanabe, H. Hayashi, T. Hanada, N. Onodera, Opt. Mater. 19 (2002) 343.
Upconversion photoluminescence of ZrO2:Er3+ nanocrystals synthesized by using butadinol as high boiling point solvent Ruokun Jia, Wensheng Yang, Yubai Bai *, Tiejin Li College of chemistry, Jilin University, Changchun 130023, P.R. China Received 23 July 2004; accepted 15 November 2004 Available online 8 February 2005
Abstract Erbium doped ZrO2 (ZrO2:Er3+) nanocrystals are prepared by a butadinol low thermal crystallization method. The crystallization temperature is reduced to 500 °C. X-ray diffraction (XRD) spectra show there exists monoclinic and cubic ZrO2 phases. Green, yellow and red lights are seen from the upconverting luminescence spectra under 980 nm excitation. The results confirm that upconverting emission is due to excited-state absorption (ESA) and energy transfer upconverting (ETU) process. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Upconverting character of rare-earth ion Er has received a lot of attentions due to its application potentials in fiber amplifier [1], short-wave laser [2] and biological labeling [3–7]. When excited by a near-infrared laser, the Er3+ ion shows emission at 550 and 660 nm [8–13]. Usually, Er must be doped into a lattice to emit luminescence upon excitation. Both Y2O3 and ZrO2 are considered to be suitable doping hosts for the rare-earth ions [14,15]. Compared to Y2O3, ZrO2 nanocrystal is chemically more stable and it does not decompose even at pH value of 3. Monochromatic green light is available at low dopant concentration and pure red light will be seen at high dopant concentration for the Er3+ doped ZrO2 nanocrystals. It is known that ZrO2 has three polymorphs: monoclinic, tetragonal, and cubic phases. Both the monoclinic and tetragonal phases exist at low dopant concentration, while cubic exists at high dopant concentration. Prasad and co-workers reported the synthesis of Er3+ doped ZrO2 nanocrystals by using a *
Corresponding author. Tel.: +86 431 8499192; fax: +86 431 8923907. E-mail address: [email protected] (Y. Bai). 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.11.034
sol-emulsion-gel method [16], in which the precursor of zirconia propoxide was soled to propanol and then dispersed into a mixed system of span80 and cyclohexane. Calcining temperature as high as 1000 °C was necessary to induce the gel particles separated by centrifugation into crystals with excellent upconverting. Such high calcining temperature will lead to the dramatic growth of the particle size. From the point of practical application, it is meaningful, beneficial to find an approach to produce smaller Er3+ doped ZrO2 nanocrystals with demanded upconversion emission. In this work, we use butadinol as the solvent for the sake of preventing hydrolyzation and increasing the temperature of the solvent to 200 °C. It is found that the ZrO2:Er3+ nanocrystals with required upconverting emission can be obtained at a calcining temperature as low as 500 °C.
2. Experimental ZrOCl2 Æ 8H2O (AR, Shanghai Chemical Reagent Corp.) and Er2O3 (GR) were used as the starting materials. Er2O3 was dissolved by nitric acid, and then precipitated by ammonia. Erbium acetate was obtained by dissolving the precipitate in acetic acid. To synthesize
R. Jia et al. / Optical Materials 28 (2006) 246–249
247
ZrO2:Er3+ nanoparticles, first, 7.5 g ZrOCl2 Æ 8H2O was added to the 80 ml 1,3-butadinol and stirred for 30 min at 200 °C, then a stoichiometric amount of solid erbium acetate was added into the solution and the system was further stirred for 3 h at 200 °C. The resulting sol was gelled by the controlled addition of 4 g NaOH. Then the gel particles were separated by centrifugation followed by washing with acetone. The product was dried at 60 °C in air for 12 h and then calcined at 500 °C for 2 h. The X-ray diffraction patterns were collected by using a Rigaku D/max-rA X-ray diffraction (XRD) instrument. A JY-UV-Raman argon ion laser with a wavelength of 488 nm and a semiconductor laser with a wavelength of 980 nm were employed to acquire the luminescence spectra. Morphology observations were conducted with a H8100 transmission electron microscopy (TEM).
perature of 500 °C, much lower than those reported in the literatures (up to 1100 °C for the monoclinic phase and above 2370 °C for the cubic phase [17]. When a base was added, Zr4+ ions tend to exit in the form of undefined ZrO2 Æ xH2O, whose growth is difficult to control. The employment of butadinol allows the reaction to go at temperature as high as 200 °C. At this time, most of the water is likely to be driven out the system and the growth of the particles are controlled. The diameter of Er3+ ion is larger than that of Zr4+; the introduction of such rare-earth ion will induce the change of ZrO2 lattice from monoclinic to cubic [18]. Therefore, the cubic phase becomes dominant with increased dopant concentrations. The crystallite sizes of the ZrO2:Er3+ nanocrystals are calculated to be about 27 nm by the Scherrers equation. TEM observations showed the average particle size of the nanocrystals is about 70 nm (Fig. 2), larger than that calculated from the XRD patterns due to the conglomeration of the nanocrystals.
3. Results and discussion
3.2. Upconverting properties
3.1. Structural investigations
Fig. 3 shows the upconverting spectra of the ZrO2:Er3+ nanocrystals with different dopant concentrations under 980 nm excitation. The emission centered at 550 nm corresponds to 2H11/2, 4S3/2 ! 4I15/2 transition, and the one centered at 660 nm corresponds to 4 F9/2 ! 4I15/2 transition. With increased dopant concentrations, the emission centered at 550 nm becomes weaker and the one centered at 660 nm becomes dominant. It is concluded that both the structures and upconverting properties of the are dependent on the dopant concentrations. It is seen that the monoclinic phase is related to the green emission and the cubic usually leads to red emission. When the monoclinic and cubic coexist in the nanocrystals, the powders show yellow or orange color under the 980 nm laser as a result of mixing of red and green emissions. Therefore, the upconverting emission can be mediated by the dopant concentrations.
Fig. 1 shows the powder X-ray diffraction patterns of the ZrO2:Er3+ nanocrystals. Without the dopant, the ZrO2 presents the monoclinic phase (see Fig. 1a). At high dopant concentration, that is, 20 mol%, the nanocrystals show cubic phase (see Fig. 1g). In the range of dopant concentrations from 0.1 to 12 mol%, the nanocrystals are the mixtures of both monoclinic and cubic phases (see Fig. 1b–f) and the cubic becomes dominant with increased dopant concentrations. It is seen that the ZrO2 nanocrystals can be obtained at calcining tem-
a b c d
e
f
g 30
40
50
60
2-theta Fig. 1. Powder XRD patterns of ZrO2:Er3+ nanocrystals with different dopant concentrations of Er. (a) 0 mol%, (b) 0.1 mol%, (c) 1 mol%, (d) 4 mol%, (e) 6 mol%, (f) 12 mol%, and (g) 20 mol%.
Fig. 2. TEM micrograph of the ZrO2:Er3+ nanocrystals.
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R. Jia et al. / Optical Materials 28 (2006) 246–249
a
4F 7/2 2 H11/2 4 S3/2 4
F 9/2
b
4
I 9/2
4
I 11/2 I13/2
c d e
f
500
550
600
650
980 nm
4
4
I 15/2
Er3+
Er3+
low dopant concentration
Er3+
high dopant concentration
Fig. 5. Schematic representation of upconverting process at low and high dopant concentrations.
700
Wavelength (nm)
I15/2
448 nm excitation 4
4
S3/2
2
4
H11/2
I15/2
I15/2
980 nm excitation 500
550
600
650
700
Wavelengh (nm)
Fig. 4. Comparison of the Stokes and anti-Stokes spectra of the ZrO2:Er3+ nanocrystal with a dopant concentration of 20% upon excitation at 488 and 980 nm.
Fig. 4 shows comparison of the upconverting and downconverting emission spectra in the range of 500– 700 nm under 980 and 488 nm excitation, respectively. It is noted that the green emission corresponding to 4 S3/2, 2H11/2 ! 4I15/2 transition is observed only upon excitation at 488 nm. Similar red emission corresponding to 4F9/2 ! 4I15/2 transition is observed under both 488 and 980 nm excitation.
EnergyX10 3 (cm-1) 25 4 F3/2 4 F5/2 4 F 2 7/2 H 11/2 4 S 3/2
20
8 6
4
F 9/2
15
4
I9/2
10
5
3.3. Upconverting mechanism 0
It is expected that the distance when the interactions between the doped Er3+ ions are weak at low dopant concentration, the emission will be mainly contributed
4
I11/2
4
I13/2
980 nm
4
14928cm-1
F9/2
14706cm-1
4
I15/2
15151cm-1
S3/2
15270cm-1
4
4
15373cm-1
,
11/2
980 nm
H
15492cm-1
2
by the excited-state absorbtion (ESA) process. Electrons in the ground state will be excited to the 4I11/2 level by the 980 nm excitation. Then they will reabsorb the excited light to arrive at the 4F7/2 level, and undergo a non-radiative decay to the 2H11/2 and 4S3/2 level. The green emission centered at about 550 nm will be observed as a result of radiative decay from the 4S3/2, 2 H11/2 level to the ground state. At high dopant concentration, the interactions between the doped Er3+ ions becomes stronger and the energy transfer upconverting (ETU) process will becomes dominant at this time. Therefore the green emission disappears and the red one is observed upon excitation at 980 nm. The enhanced interactions between the Er3+ ions induce increased relaxation of 4I11/2–4I13/2 at higher dopant concentrations (Fig. 5). It is known that the 4I13/2 level has a lifetime of longer than 5 ms [19], so the electrons populated in this level will reabsorb the excited light of 980 nm to arrive at 4F9/2 level, then go back to the ground state to emit the red light. It is seen that the energy of transition between the 4I13/2 and 4F9/2 levels does not match that of the excitation light of 980 nm exactly.
980 nm
Fig. 3. Upconverting spectra of the ZrO2:Er3+ nanocrystals with different dopant concentrations of Er under 980 nm excitation. (a) 0.1 mol%, (b) 1 mol%, (c) 4 mol%, (d) 6 mol%, (e) 12 mol%, (f) 20 mol%.
8 7 6
4
I15/2
Er3+
Er3+
Fig. 6. The energy levels of Er3+ with Stark splitting of 4F9/2 and 4I15/2 levels in cubic ZrO2:Er3+ nanocrystals.
R. Jia et al. / Optical Materials 28 (2006) 246–249
However, the transition still takes place due to the Stark splitting of the 4F9/2 energy level, which results in the broadened absorbed band corresponding to transition between the 4I13/2 and 4F9/2 levels (Fig. 6). It is noted that for the sample with highest dopant concentration (20 mol%), no green emission can be observed under 980 nm excitation. This means the electrons can not be excited to the 4F7/2 level. When the 488 nm excitation is employed, the electrons can be elevated to the 4F7/2 level directly, both green and red emissions are available at this time (see Fig. 4).
4. Conclusions In summary, Erbium doped ZrO2 nanocrystals are prepared by a butadinol low thermal crystallization method. The crystallization temperature is reduced to 500 °C. X-ray diffraction (XRD) spectra shows that there exists monoclinic and cubic ZrO2 phases. Green, yellow and red light are seen from the upconverting luminescence spectra under 980 nm excitation. The color of emission and crystalline phases were determined by Er3+ ions dopant concentration. The results confirm that upconverting emission is due to excited-state absorption (ESA) and energy transfer upconverting (ETU) process.
Acknowledgement The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (NNSFC), 863 and 973 projects.
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