- Email: [email protected]
Fabrication and spectroscopic characterization of Er3+:Lu2O3 transparent ceramics
Materials Letters 94 (2013) 5–7
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication and spectroscopic characterization of Er3 þ :Lu2O3 transparent ceramics Neng-Li Wang *, Xi-Yan Zhang, Peng-He Wang School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, Jilin, 130022, China
a r t i c l e i n f o
abstract
Article history: Received 10 October 2012 Accepted 7 December 2012 Available online 20 December 2012
Er3 þ :Lu2O3 transparent ceramics were fabricated by a solid-state reaction method and vacuum sintering using high purity Lu2O3 and Er2O3 powders as starting materials and 0.5 wt% TEOS as sintering aid. The highest transmittance of the ceramic sample reaches 67% from Visible (Vis) to near infrared (NIR) region. The ceramic sample demonstrated a 1.5 mm NIR broad bandwidth emission under a 980 nm excitation, which makes Er3 þ :Lu2O3 ceramics a promising 1.5 mm tunable laser material. With a 980 nm laser diode (LD) excitation, the ceramic sample also exhibited a strong upconversion luminescence, which is centered at 565 nm and 660 nm. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ceramics Sintering Lutetium oxide Luminescence
1. Introduction Lutetium oxide (Lu2O3) is one of the attractive sesquioxides and crystallizes in a cubic bixbyite structure with space group Ia3. It possesses extremely high density (9.42 g/cm3), high thermal conductivity (12.5 W/mK), and a wide band gap ( 6.4 eV), which favor its applications not only as scintillators for radiation detection but also as a promising host material for laser gain media [1–4]. Growth of sesquioxide single crystals has been performed by various methods such as Czochralski (CZ) [5], Heat Exchanger Method (HEM) [6], Bridgman technique [7], and recently, the micro-pulling-down method (m–PD) was also demonstrated [8,9]. However, it is difficult to grow a high-quality Lu2O3 single crystal because of its high melting point (about 24901 C for Lu2O3). Compared with single crystals, polycrystalline transparent ceramics have many advantages such as larger doping concentrations with controllable distribution in the volume of material, increased compositional versatility, larger volumes, increased mechanical and thermal properties and so on [10]. In recent years, trivalent rare earth ion (RE3þ )-doped Lu2O3 transparent ceramics, such as Eu3 þ :Lu2O3 [11,12] and Nd3 þ :Lu2O3 [13] have been studied. The Er3 þ ion is ideally suited to convert infrared light (IR) to visible since the 4fn electronic levels provide intermediate levels with long lifetime, which are easily accessible with NIR radiation. For example, there are two levels in the NIR (4I9/2 and 4I11/2) around 800 and 980 nm, which can be conveniently populated with low-cost commercial LD [14]. In addition, NIR emission with the wavelength of about 1.5 mm in Er3 þ ion-doped materials also have been attracting researcher’s interests intensively. Owning to these unique configurations, Er3þ ion-doped materials are
n
Corresponding author. Tel.: þ86 431 8558 3016. E-mail address: [email protected] (N.-L. Wang).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.025
widely used in solar cells, high-speed optical fiber communications, eye-safe lasers, full color all-solid-state displays, and so on [15–18]. In the present work, we fabricated 2 at% Er3 þ :Lu2O3 transparent ceramics by solid-state reaction and vacuum sintering using high purity Lu2O3 and Er2O3 powders as raw materials with tetraethyl orthosilicate (TEOS) as sintering aid. The upconversion and infrared luminescence of the Er3 þ :Lu2O3 transparent ceramics are investigated. 2. Experimental Commercially available high purity powders of Lu2O3 and Er2O3 were mixed with the stoichiometric proportion of Lu1.96Er0.04O3 and ball-milled with zirconia balls for 24 h in anhydrous alcohol, with 0.5 wt% TEOS as sintering aid. After drying, the ball-milled powder mixture was calcined at 11001C for 2 h, and then dry-pressed at 30 MPa in a steel mold, and cold isostatically pressed at 200 MPa. Finally, Lu3 þ :Lu2O3 transparent ceramics were obtained by sintering the powder compacts at 18501C for 10–12 h in vacuum. Mirror-polished ceramic sample on both sides was used to measure the optical transmittance on a Perkin-Elmer Lambda 950 UV/Vis/NIR spectrophotometer. The excitation and emission spectra of the ceramics were measured using a Jobin-Yvon Fluorolog-3 spectrofluorometer. The up-conversion luminescence spectrum was obtained on a Shimadzu RF-5301PC spectrofluorometer upon excitation of a 980 nm LD. All the characterizations were performed at room temperature.
3. Results and discussion Fig. 1 shows the transmittance spectrum and the photograph of the Er3 þ :Lu2O3 transparent ceramics, the highest transmittance of
6
N.-L. Wang et al. / Materials Letters 94 (2013) 5–7
the sample reaches 67% in the wavelength from 380 to 2200 nm. Absorption bands centered at 380, 522, 653, 808, 980 and 1536 nm are attributed to the transitions of Er3 þ ion from its ground state of 4 I15/2 to the excited states of 4G11/2, 2H11/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/ 2, respectively. With broad absorption bands in NIR region, Er3 þ :Lu2O3 transparent ceramics are advantageous for lower dependency on pump wavelength and temperature control of a diode laser and are hopeful to be a kind of laser media for miniaturization of laser devices. The excitation spectrum of the sample (Fig. 2(a)) with the monitoring wavelength of 1533 nm consists of three main peaks, which located at 798, 822 and 980 nm respectively, and the peak centered at 980 nm has the highest intensity. Fig. 2(b) shows the emission spectrum of the sample (lex ¼980 nm). The peak located at 1533 nm, which is corresponding to the 4I13/2-4I15/2 transition of Er3 þ ion, has the highest luminescence intensity. It can be seen that the emission bandwidth at 1533 nm is broad, with the fullwidth at half-maximum (FWHM) value of 16.3 nm. Broad emission bandwidths are beneficial to widely tunable laser output and make Er3 þ :Lu2O3 transparent ceramics a promising 1.5 mm tunable laser materials. Fig. 3 shows the upconversion emission spectrum of Er3 þ :Lu2O3 transparent ceramics under excitation of 980 nm LD. There are two emission bands in the spectrum, that is the green light emission centered at 565 nm and the red light emission centered at 660 nm. The upconversion mechanism of Er3 þ ions doped materials can be
70
attributed to either an excited state absorption (ESA) or energy transfer (ET) process, and the origin of green and red upconversion luminescence of Er3 þ :Lu2O3 ceramics is suggested as the following: the 980 nm LD excites the Er3 þ ion from ground state of 4I15/2 to the excited level of 4I11/2 through a ground state absorption (GSA) process, then multi-phonon relaxation (MPR) happens and populates 4I13/2 level. After the first-level excitation, the same laser pumps the excited atom from the 4I11/2 and 4I13/2 level to the 4F7/2 and 4F9/2 level, respectively. Subsequent MPR processes populate the 4S3/2/2H11/2 and 4F9/2 levels, and bright green (centered at 565 nm) and red (centered at 660 nm) emissions can be observed due to the radiative transitions of 4S3/2/2H11/2-4I15/2 and 4F9/ 4 2- I15/2, respectively. In addition, the mechanism of cooperative ET process between two neighboring Er3 þ ions cannot be ignored at the higher doping concentration. Two excited Er3 þ at 4I11/2 level interact with each other and one Er3 þ is de-excited to 4I15/2 level while the other is excited to 4F7/2 level [19,20].
4. Conclusion The Er3 þ :Lu2O3 transparent ceramics with the maximum optical transmittance of 67% from Vis to NIR were fabricated by a solid-state reaction method and vacuum sintering. The sample demonstrated a 1.5 mm NIR broad bandwidth emission under a 980 nm excitation, which makes Er3 þ :Lu2O3 ceramics a promising 1.5 mm tunable laser material. With a 980 nm LD excitation, the ceramics demonstrated a strong upconversion luminescence, the green and red upconversion luminescence can be attributed to 4S3/2/2H11/2-4I15/2 and 4F9/2-4I15/2 transitions of Er3 þ ions respectively.
I9/2
I11/2
4
4I 13/2
Luminescence intensity / a.u.
565
F9/2
40
4
4
50
30
G11/2
10
4
0
0
11/2
20
2H
Transmittance / %
60
500
1000
1500
2000
500
550
600
650
700
1533
λ em=1533 nm
822
900
Wavelength / nm
950
Fig.3. Upconversion spectrum of Er3 þ :Lu2O3 ceramics.
Luminescence intensity / a.u.
Luminescence intensity / a.u.
660
Wavelength / nm
980
850
539
450
Fig.1. Transmittance spectrum and photograph of Er3 þ :Lu2O3 ceramics (1 mm thick).
800
554
2500
Wavelength / nm
798
λ ex= 980 nm
1000
1300
λ ex= 980 nm
1400
1500
Wavelength / nm
Fig.2. Excitation (a) and emission (b) spectrum of Er3 þ :Lu2O3 ceramics.
1600
750
N.-L. Wang et al. / Materials Letters 94 (2013) 5–7
Acknowledgment Financial aid from Changchun University of Science and Technology (Contract No. 2009A0626) is gratefully appreciated. References [1] Capobianco JA, Vetrone F, Boyer JC, Speghini A, Bettinelli M. Opt Mater 2002;19:259–68. [2] Lu J, Takaichi K, Uematsu T, Shirakawa A, Musha M, Ueda K, et al. Appl Phys Lett 2002;81(23):4324–6. [3] Sanghera J, Frantz J, Kim W, Villalobos G, Baker C, Shaw B, et al. Opt Lett 2011;36(4):576–8. [4] Kalyvas N, Liaparinos P, Michail C, David S, Fountos G, Wo´jtowicz M, et al. Appl Phys A 2012;106:131–6. [5] Fornasiero L, Mix E, Peters V, Petermann K, Huber G. Ceram Int 2000;26(6):589–92.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
7
¨ Peters R, Krankel C, Petermann K, Huber G. J Cryst Growth 2008;310:1934–8. Petermann K, Fornasiero L, Mix E, Peters V. Opt Mater 2002;19:67–71. Fukabori A, Chani V, Kamada K, Yoshikawa A. J Cryst Growth 2012;352:124–8. Sangla D, Aubry N, Nehari A, Brenier A, Tillemetn O, Lebbou K, et al. J Cryst Growth 2009;312:125–30. Lupei V. J Alloys Compd 2008;451:52–5. Wang ZF, Zhang WP, Lin L, You BG, Fu YB, Yin M. Opt Mater 2008;30:1484–8. Shi Y, Chen QW, Shi JL. Opt Mater 2009;31:729–33. Zhou D, Shi Y, Xie JJ, Ren YY, Yun P. J Am Ceram Soc 2009;92(10):2182–7. Capobianco JA, Vetrone F, Boyer JC, Speghini A, Bettinelli M. J Phys Chem B 2002;106:1181–7. Lu SZ, Yang QH, Zhang B, Zhang HJ. Opt Mater 2011;33:746–9. Chandra S, Deepak FL, Gruber JB, Sardar DK. J Phys Chem C 2010;114:874–80. Xu CF, Yang LW, Han HL, Zhang YY, Chu PK. Opt Mater 2010;32:1188–92. Georgescu S, Toma O, Voiculescu AM, Matei C, Birjega R, Petrescu L. Physica B 2012;407:1124–7. Mao Y, Tran T, Guo X, Huang JY, Shih CK, Wang KL, et al. Adv Funct Mater 2009;19:748–54. Zhang J, Wang SW, An LQ, Liu M, Chen LD. J Lumin 2007;122–23 :8–10.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication and spectroscopic characterization of Er3 þ :Lu2O3 transparent ceramics Neng-Li Wang *, Xi-Yan Zhang, Peng-He Wang School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun, Jilin, 130022, China
a r t i c l e i n f o
abstract
Article history: Received 10 October 2012 Accepted 7 December 2012 Available online 20 December 2012
Er3 þ :Lu2O3 transparent ceramics were fabricated by a solid-state reaction method and vacuum sintering using high purity Lu2O3 and Er2O3 powders as starting materials and 0.5 wt% TEOS as sintering aid. The highest transmittance of the ceramic sample reaches 67% from Visible (Vis) to near infrared (NIR) region. The ceramic sample demonstrated a 1.5 mm NIR broad bandwidth emission under a 980 nm excitation, which makes Er3 þ :Lu2O3 ceramics a promising 1.5 mm tunable laser material. With a 980 nm laser diode (LD) excitation, the ceramic sample also exhibited a strong upconversion luminescence, which is centered at 565 nm and 660 nm. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ceramics Sintering Lutetium oxide Luminescence
1. Introduction Lutetium oxide (Lu2O3) is one of the attractive sesquioxides and crystallizes in a cubic bixbyite structure with space group Ia3. It possesses extremely high density (9.42 g/cm3), high thermal conductivity (12.5 W/mK), and a wide band gap ( 6.4 eV), which favor its applications not only as scintillators for radiation detection but also as a promising host material for laser gain media [1–4]. Growth of sesquioxide single crystals has been performed by various methods such as Czochralski (CZ) [5], Heat Exchanger Method (HEM) [6], Bridgman technique [7], and recently, the micro-pulling-down method (m–PD) was also demonstrated [8,9]. However, it is difficult to grow a high-quality Lu2O3 single crystal because of its high melting point (about 24901 C for Lu2O3). Compared with single crystals, polycrystalline transparent ceramics have many advantages such as larger doping concentrations with controllable distribution in the volume of material, increased compositional versatility, larger volumes, increased mechanical and thermal properties and so on [10]. In recent years, trivalent rare earth ion (RE3þ )-doped Lu2O3 transparent ceramics, such as Eu3 þ :Lu2O3 [11,12] and Nd3 þ :Lu2O3 [13] have been studied. The Er3 þ ion is ideally suited to convert infrared light (IR) to visible since the 4fn electronic levels provide intermediate levels with long lifetime, which are easily accessible with NIR radiation. For example, there are two levels in the NIR (4I9/2 and 4I11/2) around 800 and 980 nm, which can be conveniently populated with low-cost commercial LD [14]. In addition, NIR emission with the wavelength of about 1.5 mm in Er3 þ ion-doped materials also have been attracting researcher’s interests intensively. Owning to these unique configurations, Er3þ ion-doped materials are
n
Corresponding author. Tel.: þ86 431 8558 3016. E-mail address: [email protected] (N.-L. Wang).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.12.025
widely used in solar cells, high-speed optical fiber communications, eye-safe lasers, full color all-solid-state displays, and so on [15–18]. In the present work, we fabricated 2 at% Er3 þ :Lu2O3 transparent ceramics by solid-state reaction and vacuum sintering using high purity Lu2O3 and Er2O3 powders as raw materials with tetraethyl orthosilicate (TEOS) as sintering aid. The upconversion and infrared luminescence of the Er3 þ :Lu2O3 transparent ceramics are investigated. 2. Experimental Commercially available high purity powders of Lu2O3 and Er2O3 were mixed with the stoichiometric proportion of Lu1.96Er0.04O3 and ball-milled with zirconia balls for 24 h in anhydrous alcohol, with 0.5 wt% TEOS as sintering aid. After drying, the ball-milled powder mixture was calcined at 11001C for 2 h, and then dry-pressed at 30 MPa in a steel mold, and cold isostatically pressed at 200 MPa. Finally, Lu3 þ :Lu2O3 transparent ceramics were obtained by sintering the powder compacts at 18501C for 10–12 h in vacuum. Mirror-polished ceramic sample on both sides was used to measure the optical transmittance on a Perkin-Elmer Lambda 950 UV/Vis/NIR spectrophotometer. The excitation and emission spectra of the ceramics were measured using a Jobin-Yvon Fluorolog-3 spectrofluorometer. The up-conversion luminescence spectrum was obtained on a Shimadzu RF-5301PC spectrofluorometer upon excitation of a 980 nm LD. All the characterizations were performed at room temperature.
3. Results and discussion Fig. 1 shows the transmittance spectrum and the photograph of the Er3 þ :Lu2O3 transparent ceramics, the highest transmittance of
6
N.-L. Wang et al. / Materials Letters 94 (2013) 5–7
the sample reaches 67% in the wavelength from 380 to 2200 nm. Absorption bands centered at 380, 522, 653, 808, 980 and 1536 nm are attributed to the transitions of Er3 þ ion from its ground state of 4 I15/2 to the excited states of 4G11/2, 2H11/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/ 2, respectively. With broad absorption bands in NIR region, Er3 þ :Lu2O3 transparent ceramics are advantageous for lower dependency on pump wavelength and temperature control of a diode laser and are hopeful to be a kind of laser media for miniaturization of laser devices. The excitation spectrum of the sample (Fig. 2(a)) with the monitoring wavelength of 1533 nm consists of three main peaks, which located at 798, 822 and 980 nm respectively, and the peak centered at 980 nm has the highest intensity. Fig. 2(b) shows the emission spectrum of the sample (lex ¼980 nm). The peak located at 1533 nm, which is corresponding to the 4I13/2-4I15/2 transition of Er3 þ ion, has the highest luminescence intensity. It can be seen that the emission bandwidth at 1533 nm is broad, with the fullwidth at half-maximum (FWHM) value of 16.3 nm. Broad emission bandwidths are beneficial to widely tunable laser output and make Er3 þ :Lu2O3 transparent ceramics a promising 1.5 mm tunable laser materials. Fig. 3 shows the upconversion emission spectrum of Er3 þ :Lu2O3 transparent ceramics under excitation of 980 nm LD. There are two emission bands in the spectrum, that is the green light emission centered at 565 nm and the red light emission centered at 660 nm. The upconversion mechanism of Er3 þ ions doped materials can be
70
attributed to either an excited state absorption (ESA) or energy transfer (ET) process, and the origin of green and red upconversion luminescence of Er3 þ :Lu2O3 ceramics is suggested as the following: the 980 nm LD excites the Er3 þ ion from ground state of 4I15/2 to the excited level of 4I11/2 through a ground state absorption (GSA) process, then multi-phonon relaxation (MPR) happens and populates 4I13/2 level. After the first-level excitation, the same laser pumps the excited atom from the 4I11/2 and 4I13/2 level to the 4F7/2 and 4F9/2 level, respectively. Subsequent MPR processes populate the 4S3/2/2H11/2 and 4F9/2 levels, and bright green (centered at 565 nm) and red (centered at 660 nm) emissions can be observed due to the radiative transitions of 4S3/2/2H11/2-4I15/2 and 4F9/ 4 2- I15/2, respectively. In addition, the mechanism of cooperative ET process between two neighboring Er3 þ ions cannot be ignored at the higher doping concentration. Two excited Er3 þ at 4I11/2 level interact with each other and one Er3 þ is de-excited to 4I15/2 level while the other is excited to 4F7/2 level [19,20].
4. Conclusion The Er3 þ :Lu2O3 transparent ceramics with the maximum optical transmittance of 67% from Vis to NIR were fabricated by a solid-state reaction method and vacuum sintering. The sample demonstrated a 1.5 mm NIR broad bandwidth emission under a 980 nm excitation, which makes Er3 þ :Lu2O3 ceramics a promising 1.5 mm tunable laser material. With a 980 nm LD excitation, the ceramics demonstrated a strong upconversion luminescence, the green and red upconversion luminescence can be attributed to 4S3/2/2H11/2-4I15/2 and 4F9/2-4I15/2 transitions of Er3 þ ions respectively.
I9/2
I11/2
4
4I 13/2
Luminescence intensity / a.u.
565
F9/2
40
4
4
50
30
G11/2
10
4
0
0
11/2
20
2H
Transmittance / %
60
500
1000
1500
2000
500
550
600
650
700
1533
λ em=1533 nm
822
900
Wavelength / nm
950
Fig.3. Upconversion spectrum of Er3 þ :Lu2O3 ceramics.
Luminescence intensity / a.u.
Luminescence intensity / a.u.
660
Wavelength / nm
980
850
539
450
Fig.1. Transmittance spectrum and photograph of Er3 þ :Lu2O3 ceramics (1 mm thick).
800
554
2500
Wavelength / nm
798
λ ex= 980 nm
1000
1300
λ ex= 980 nm
1400
1500
Wavelength / nm
Fig.2. Excitation (a) and emission (b) spectrum of Er3 þ :Lu2O3 ceramics.
1600
750
N.-L. Wang et al. / Materials Letters 94 (2013) 5–7
Acknowledgment Financial aid from Changchun University of Science and Technology (Contract No. 2009A0626) is gratefully appreciated. References [1] Capobianco JA, Vetrone F, Boyer JC, Speghini A, Bettinelli M. Opt Mater 2002;19:259–68. [2] Lu J, Takaichi K, Uematsu T, Shirakawa A, Musha M, Ueda K, et al. Appl Phys Lett 2002;81(23):4324–6. [3] Sanghera J, Frantz J, Kim W, Villalobos G, Baker C, Shaw B, et al. Opt Lett 2011;36(4):576–8. [4] Kalyvas N, Liaparinos P, Michail C, David S, Fountos G, Wo´jtowicz M, et al. Appl Phys A 2012;106:131–6. [5] Fornasiero L, Mix E, Peters V, Petermann K, Huber G. Ceram Int 2000;26(6):589–92.
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
7
¨ Peters R, Krankel C, Petermann K, Huber G. J Cryst Growth 2008;310:1934–8. Petermann K, Fornasiero L, Mix E, Peters V. Opt Mater 2002;19:67–71. Fukabori A, Chani V, Kamada K, Yoshikawa A. J Cryst Growth 2012;352:124–8. Sangla D, Aubry N, Nehari A, Brenier A, Tillemetn O, Lebbou K, et al. J Cryst Growth 2009;312:125–30. Lupei V. J Alloys Compd 2008;451:52–5. Wang ZF, Zhang WP, Lin L, You BG, Fu YB, Yin M. Opt Mater 2008;30:1484–8. Shi Y, Chen QW, Shi JL. Opt Mater 2009;31:729–33. Zhou D, Shi Y, Xie JJ, Ren YY, Yun P. J Am Ceram Soc 2009;92(10):2182–7. Capobianco JA, Vetrone F, Boyer JC, Speghini A, Bettinelli M. J Phys Chem B 2002;106:1181–7. Lu SZ, Yang QH, Zhang B, Zhang HJ. Opt Mater 2011;33:746–9. Chandra S, Deepak FL, Gruber JB, Sardar DK. J Phys Chem C 2010;114:874–80. Xu CF, Yang LW, Han HL, Zhang YY, Chu PK. Opt Mater 2010;32:1188–92. Georgescu S, Toma O, Voiculescu AM, Matei C, Birjega R, Petrescu L. Physica B 2012;407:1124–7. Mao Y, Tran T, Guo X, Huang JY, Shih CK, Wang KL, et al. Adv Funct Mater 2009;19:748–54. Zhang J, Wang SW, An LQ, Liu M, Chen LD. J Lumin 2007;122–23 :8–10.