- Email: [email protected]
The photoluminescent properties of Eu3+ in MgO–Ga2O3–SiO2 nanocrystalline glass-ceramic
Journal of Physics and Chemistry of Solids 71 (2010) 1656–1659
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
The photoluminescent properties of Eu3 + in MgO–Ga2O3–SiO2 nanocrystalline glass-ceramic Lixin Yu a,n, Hai Liu a, Wei Sun a, Masayuki Nogami b a b
Department of Materials Science and Engineering, Nanchang University, Nanchang 330031, PR China Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa Nagoya 466-8555, Japan
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 October 2009 Received in revised form 16 May 2010 Accepted 4 September 2010
The MgO–Ga2O3–SiO2 glass-ceramic (GC) containing MgGa2O4 nanocrystals and glasses doped with Eu3 + ions were prepared by the sol–gel method. The down-conversion and up-conversion luminescence (UCL) properties were studied. The results indicated that the relative intensity of f–f transitions of Eu3 + decreased in contrast with that of charge transfer (CT) absorption with the increase in heating temperature. Using a Xe lamp and 800 nm femtosecond (fs) laser excitation, strong red luminescence of Eu3 + in MgO–Ga2O3–SiO2 glasses and GC was observed. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials D. Luminescence
1. Introduction GC is a multiphase material that consists of a glassy phase and a nanocrystalline phase. The combination of easy fabrication and manipulation in air, large transparency and the ability to incorporate rare earth (RE) ions in nanocrystals makes this material unique in the field of optical material engineering. Transparent and luminescent nanocrystalline glass-ceramic doped with RE has been one of the research focuses in our time due to its many potential uses in telecommunication systems, such as up- conversion fibers, optical amplifiers, solid-state lasers and 3D displays [1–3]. Moreover, GC can combine the advantages of crystalline phase and amorphous glass. Previously, researches on this field concentrated on the fluoride GC due to low phonon energy [4–7]. Recently, oxide GC has attracted considerable attention due to many of its advantages, better ability to endure high temperature and high electron beam current over the fluoride, environmental friendly, etc [8–10]. But, the study on oxide GC was very less. Moreover, GC was generally obtained by the high temperature melting and quenching technique. In contrast with the traditional melting method, the sol–gel method has the advantage of good chemical homogeneity and accurate composition. Also, the gel-derived GC can be obtained at a temperature much lower than that necessary for the melting of an oxide mixture.
n Corresponding author. Department of Materials Science and Technology, Nanchang University, 999 Xuefu Road, Nanchang 330031, PR China. Tel.: + 86 791 3969553; fax: + 86 791 3969558. E-mail address: [email protected] (L. Yu).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.09.003
MGa2O4 (M ¼Zn, Mg) such as ZnGa2O4, MgGa2O4 phosphors with self-activated blue emission band have aroused considerable attention due to their potential applications in field emission display. Previously, the bulk powders have been intensively studied [11]. MgGa2O4 was an ideal nanocrystalline phase, which can be embedded into glasses [12]. As zinc oxide was easier to form zinc silicate in silicate glasses, we select MgGa2O4 as nanosized phase in silicate glasses. To our knowledge, no reports on luminescence nanocrystalline MgGa2O4 in glass-ceramics doped with RE have been reported. On the basis of the above consideration, in this paper, we prepared MgO–Ga2O3–SiO2 nanosized GC by the sol–gel method. The down-, up-conversion luminescence and UCL properties were investigated.
2. Experimental 5MgO–5Ga2O3–90SiO2 (MGS) glass and GC containing 1% Eu2O3 (mol ratio) were prepared by the sol–gel method. Tetraethoxysilane (TEOS), MgCl2 6H2O, GaCl3 and EuCl3 6H2O were used as starting materials. Firstly, the solution containing TEOS, ethanol and deionized water (containing 0.15 mol/l HCl) was vigorously stirred for 1 h at room temperature. MgCl2 6H2O, GaCl3 and EuCl3 6H2O dissolved in ethanol were added to the above solution and stirred for half an hour to obtain homogeneous solution. The above obtained solution was cast into plastic container and placed at room temperature for about 4 weeks to form a stiff gel of 1–3 mm thickness. Finally, the gel was sintered at 700, 800, 900 and 1000 1C for 3 h, and the corresponding samples were labeled as MGS7, MGS8, MGS9 and MGS10,
L. Yu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1656–1659
3. Results and discussion Fig. 1 shows the XRD patterns of MGS samples calcined at different temperatures. As seen, the broad band ranging from 201 to 251 was from typical amorphous SiO2. When the heating temperature was 700 and 800 1C, the MGS7 and MGS8 were of typical glass nature. When the temperature was increased to 900 and 1000 1C, several diffraction peaks (2y) at 35.491, 43.831 and 63.721 in MGS9 and MGS10 were observed. All the diffraction peaks of MGS9 and MGS10 can be assigned to those of MgGa2O4 spinal structures (JCPDS 38-1240). For MGS10, diffraction peaks became sharper than those of MGS9, indicating the increase in particle size . The particle size was determined by the strongest diffraction peak at 35.491 using the Scherrer equation d¼
kl cos yL
where k is the constant (0.89), l the wavelength of X-ray and L the half width of the diffraction peak. Thus the size of MgGa2O4 nanocrystals can be estimated to be about 4 nm in MGS9 and 8 nm in MGS10 GC. But it should be noted that MGS10 is not transparent. Here we studied only the luminescent properties of Eu3 + in MGS7, MGS8 and MGS9. Fig. 2 shows the excitation spectra of Eu3 + ions in MGS glass and GC monitoring 614 nm emissions. The broad band ranging from 220 to 350 nm centered at about 288 nm was associated with the CT transition absorption from the 2p orbital of O2 ions to the 4f vacant orbital of Eu3 + ions, while the sharp lines at 317, 360, 380, 396, 412 and 464 nm with the direct excitation of f–f shell transitions of Eu3 + can be contributed to 7F0–5H5, 7F0–5D4, 7 F0–5D4,3,2, 7F0–5L6, 7F0–5D3 and 7F0–5D2 transitions absorption, respectively. For MGS samples, the position and configuration of
the CT band hardly changed with increase in heating temperature. But it is obvious that the relative proportion of f–f transitions absorption in CT absorption gradually decreased from MGS7 to MGS9 samples. Fig. 3 shows the emission spectra of Eu3 + in MGS samples at 285 nm excitation pumped by the Xe lamp. It can be seen that from 550 to 720 nm five-group emission lines located at 576, 585–604, 604–638, 655 and 703 nm were observed, which were from 5D0–7FJ ( J¼0–4) transition emissions. The strongest emission at 618 nm was red emission from 5D0–7F2 transitions. An evident change can be seen that the relative intensity of 5 D0–7F0 transition increased with the increase in heating temperature. The integrated intensity ratios of 5D0–7F2/5D0–7F1 and 5D0–7F0/5D0–7F1 were determined and are listed in Table 1. It is obvious that the intensity ratio of 5D0–7F2/5D0–7F1 changed a little for different samples, indicating that the relative proportion of inversion centers to non-inversion centers hardly changed. Since the 5D0–7F1 line is due to the magnetic dipole transition, the intensity of this line is considered to be almost independent of the local field strength or hosts. Therefore, the increase of intensity ratio of 5D0–7F0/5D0–7F1 from MGS7 to MGS9 indicated that the 5 D0–7F0 transition probability increased. Fig. 4 shows the UCL spectra of Eu3 + in MGS samples at fs laser excitation (800 nm). Several strong emission lines ranging from 550 to 720 nm associated with 5D0–7FJ (J¼0–4) of Eu3 + ions in
2.0 MGS9 MGS8
1.6 Intensity (a.u.)
respectively. The final thickness of as prepared samples was about 1.5 mm. X-ray diffraction (XRD) was performed using a Philips X0 pert diffractometer with Cu target as the radiation source. The excitation and emission spectra were recorded using a monochromator (Jobin Yvon, HR 320) and a photomultiplier (Hamamatsu, R955). A 500 W xenon lamp, with the light passing through a monochromator (Jobin Yvon H20), was used as the excitation source. The optical absorption spectra were measured using a UV–vis spectrophotometer (JASCO, V-570). UCL spectra were measured by fs pulse using a Ti:sapphire regenerative amplifier laser system (Spectra Physics, Hurricane) operating at a wavelength of 800 nm with a 1 kHz repetition rate and approximately 130 fs pulse duration. The power density can be varied from 0.15 to 0.7 W/mm2.
MGS7
1.2 0.8 0.4 0.0 200
Intensity (a. u.)
Intensity (a.u.)
250
400 300 350 Wavelength (nm)
450
500
Fig. 2. Excitation spectra of Eu3 + in MGS samples monitoring 615 nm emission.
1.2 MGS10 MGS9 MGS8 MGS7
1657
MGS9 MGS8 MGS7
0.8
0.4 0 20
30
40
50 2θ (degree)
60
70
Fig. 1. XRD patterns of MGS samples at different heating temperatures.
450
500
550 600 650 Wavelength (nm)
700
Fig. 3. Emission spectra of Eu3 + in MGS samples at 285 nm excitation.
750
1658
L. Yu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1656–1659
5
Table 1 Intensity ratio of 5D0–7F2/5D0–7F0 and 5D0–7F2/5D0–7F1 transitions.
D0–7F0/5D0–7F1 5 D0–7F2/5D0–7F1
MGS9
MGS8
MGS9
3
MGS8
0.84 4.32
1.10 4.04
2.17 4.01
2
MGS7
1200000
Intensity (a.u.)
5
4
MGS7
1 0 8
MGS9 MGS8
6
intensity (a.u.)
MGS7
MGS9 MGS8 MGS7
800000
4 2 200
400000
400 600 Wavelength (nm)
800
Fig. 5. Absorption spectra of MGS samples doped with Eu3 + .
0 300
400
500 700 600 Wavelength (nm)
800
Fig. 4. UCL spectra of Eu3 + in MGS glass and GC at fs laser excitation.
three samples were observed. For MGS9, except for characteristic emissions of Eu3 + ions, a broad blue emission ranging from 380 to 520 nm centered at 430 nm occurred. But in MGS7 and MGS8 glasses, no blue emission band was observed. According to the results of XRD (Fig. 1), MgGa2O4 nanocrystals existed in MGS9 GC. In ZnGa2O4 phosphors and film, the same emission band was also observed [11]. Therefore the blue emission band was assigned to the MgGa2O4 nanocrystals. According to Ref. [11], when Ga–O octahedral sites became distorted, the emission of self-activation centers located at 360 nm, while octahedral sites were regular, showing emission at 430 nm. Thus we suggest that the origin of the blue emission band from self-activation luminescence was associated with the CT between Ga3 + ions at regular Oh sites and the surrounding O2. In addition, the emission corresponding to the CT excitation and fs excitation is different in comparison with Fig. 3. This result indicates that the emission mechanism of Eu3 + at fs excitation is different from the CT excitation. To better understand the mechanism of UCL of Eu3 + in MGS glasses and GC, the UCL integrated intensity of 5D0–7FJ transition emissions as a function of the excitation power was investigated. The UCL intensity can be expressed as Ippn where I is the integrated intensity of UCL, p the pump power of fs laser and n the number of photons absorbed per visible photon emitted for any UCL mechanism. According to the above equation, n is the slope of log–log plot between the UCL intensity and the pump power. The slope of the logarithmic lines determined by linear fitting was about 2.09 in MGS7, 1.87 in MGS8 and 1.78 in MGS9. This result indicated that a two-photon excitation predominated in MGS glasses and GC. Basically, UCL can take place by two different processes, i.e., radiatively by the excited state absorption and nonradiatively by the energy-transfer up-conversion between two excited ions [13]. In transparent glass host, two-photon UCL of Er3 + , Tm3 + and Ho3 + was the general phenomenon [14,15]. The mechanism includes the energy transfer from one ion A to another ion B, the two-step absorption from one ion, the two-photon simultaneous
absorption and so on [16]. To clarify the UCL processes, the absorption spectra (a) and the enlarged spectra (b) are shown in Fig. 5. The strong broad absorption at 200–350 nm was from CT. The sharp absorption line at 396 nm originated from 7F0–5L6 transitions. An evident change can be observed that the absorption edge of MGS9 GC redshifted in contrast with that in MGS7 and MGS8 glasses, which were from the absorption of MgGa2O4 nanocrystals. At fs laser excitation, the absorption of 7F0–5L6 transition (396 nm) does not perfectly correspond to two-photon excitation. Thus, In MGS samples, the electrons at ground state 7F0 level can be excited to 5L6 level with the assistance of phonon, then relaxed to 5D3, 5D2, 5D1, and resulting in the lowest excited state 5D0 level. As a consequence, 5D0–7FJ emissions occurred. It should be noted that the two-photon absorption (400 nm) was locating at the edge of absorption of MGS9 GC because of the redshift of absorption. Thus the intensity of Eu3 + in MGS9 GC is higher than that in MGS glasses.
4. Conclusions In conclusion, the MgO–Ga2O3–SiO2 glass and GC containing MgGa2O4 nanocrystals doped with Eu3 + ions were prepared by the sol–gel method. The down-conversion luminescence and UCL properties were studied. The results indicated that the relative intensity of f–f transitions of Eu3 + ions gradually decreased in contrast with that of the CT absorption from MGS7 glass to MGS9. Using a Xe lamp and 800 nm fs laser excitation, strong red luminescence of Eu3 + in MgO–Ga2O3–SiO2 glasses and GC was observed. The UCL mechanisms of Eu3 + in MGS glass and GC can be assumed to be the two-photon excitation.
Acknowledgement The authors are grateful to the Nation Natural Science Foundation of China (Grant no. 10504030) for the financial support. References [1] F. Lahoz, I.R. Martin, U.R. Rodriguez-Mendoza, I. Iparraguirre, J. Azkargorta, A. Mendioroz, R. Balda, J. Fernandez, V. Lavin, Opt. Mater. 27 (2005) 1762. [2] M. Goncalves, L. Santos, R. Almeida, C. R. Chim: 5 (2002) 845. [3] M. Mortier, A. Monteville, G. Patriarche, G. maze, F. Auzel, Opt. Mater. 16 (2001) 255.
L. Yu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1656–1659
[4] [5] [6] [7]
M. Mortier, Philos. Mag. B 82 (2002) 745. G. Jones, S. Houde-Walter, Opt. Lett. 30 (2005) 2122. X. Qiao, X. Fan, M. Wang, X. Zhang, Opt. Mater. 27 (2004) 597. W. Pisarski, T. Goryczka, J. Pisarska, W. Ryba-Romanowski, J. Phys. Chem. B 111 (2007) 2427. [8] X. Duan, D. Yuan, F. Yu, L. Wang, Appl. Phys. Lett. 89 (2006) 183119. [9] G. Beall, L. Pinckney, J. Am. Ceram. Soc. 82 (1999) 5. [10] K. Shioya, T. Komatsu, H. Kim, R. Sato, K. Matusita, J. Non-Cryst. Solids 189 (1995) 16.
1659
[11] J. Kim, H. Kang, W. Kim, J. Kim, J. Choi, H. Park, G. Kim, T. Kim, Y. Hwang, S. Mho, Appl. Phys. Lett. 82 (2003) 2029. [12] X. Duan, D. Yuan, F. Yu, L. Wan, Appl. Phys. Lett. 89 (2006) 183119. [13] M. Wermuth, T. Riederer, H. Gudel, Phys. Rev. B 57 (1998) 4369. [14] G. Qin, W. Qin, S. Huang, C. Wu, D. Zhao, B. Chen, S. Lu, J. Appl. Phys. 92 (2002) 6936. [15] G. Qin, W. Qin, C. Wu, S. Huang, J. Zhang, S. Lu, D. Zhao, H. Liu, J. Appl. Phys. 93 (2003) 4328. [16] G. Blasse, C. Grabmaier, in: Luminescent Materials, Springer, Berlin, 1994 p. 196.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
The photoluminescent properties of Eu3 + in MgO–Ga2O3–SiO2 nanocrystalline glass-ceramic Lixin Yu a,n, Hai Liu a, Wei Sun a, Masayuki Nogami b a b
Department of Materials Science and Engineering, Nanchang University, Nanchang 330031, PR China Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa Nagoya 466-8555, Japan
a r t i c l e in f o
a b s t r a c t
Article history: Received 23 October 2009 Received in revised form 16 May 2010 Accepted 4 September 2010
The MgO–Ga2O3–SiO2 glass-ceramic (GC) containing MgGa2O4 nanocrystals and glasses doped with Eu3 + ions were prepared by the sol–gel method. The down-conversion and up-conversion luminescence (UCL) properties were studied. The results indicated that the relative intensity of f–f transitions of Eu3 + decreased in contrast with that of charge transfer (CT) absorption with the increase in heating temperature. Using a Xe lamp and 800 nm femtosecond (fs) laser excitation, strong red luminescence of Eu3 + in MgO–Ga2O3–SiO2 glasses and GC was observed. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Optical materials D. Luminescence
1. Introduction GC is a multiphase material that consists of a glassy phase and a nanocrystalline phase. The combination of easy fabrication and manipulation in air, large transparency and the ability to incorporate rare earth (RE) ions in nanocrystals makes this material unique in the field of optical material engineering. Transparent and luminescent nanocrystalline glass-ceramic doped with RE has been one of the research focuses in our time due to its many potential uses in telecommunication systems, such as up- conversion fibers, optical amplifiers, solid-state lasers and 3D displays [1–3]. Moreover, GC can combine the advantages of crystalline phase and amorphous glass. Previously, researches on this field concentrated on the fluoride GC due to low phonon energy [4–7]. Recently, oxide GC has attracted considerable attention due to many of its advantages, better ability to endure high temperature and high electron beam current over the fluoride, environmental friendly, etc [8–10]. But, the study on oxide GC was very less. Moreover, GC was generally obtained by the high temperature melting and quenching technique. In contrast with the traditional melting method, the sol–gel method has the advantage of good chemical homogeneity and accurate composition. Also, the gel-derived GC can be obtained at a temperature much lower than that necessary for the melting of an oxide mixture.
n Corresponding author. Department of Materials Science and Technology, Nanchang University, 999 Xuefu Road, Nanchang 330031, PR China. Tel.: + 86 791 3969553; fax: + 86 791 3969558. E-mail address: [email protected] (L. Yu).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.09.003
MGa2O4 (M ¼Zn, Mg) such as ZnGa2O4, MgGa2O4 phosphors with self-activated blue emission band have aroused considerable attention due to their potential applications in field emission display. Previously, the bulk powders have been intensively studied [11]. MgGa2O4 was an ideal nanocrystalline phase, which can be embedded into glasses [12]. As zinc oxide was easier to form zinc silicate in silicate glasses, we select MgGa2O4 as nanosized phase in silicate glasses. To our knowledge, no reports on luminescence nanocrystalline MgGa2O4 in glass-ceramics doped with RE have been reported. On the basis of the above consideration, in this paper, we prepared MgO–Ga2O3–SiO2 nanosized GC by the sol–gel method. The down-, up-conversion luminescence and UCL properties were investigated.
2. Experimental 5MgO–5Ga2O3–90SiO2 (MGS) glass and GC containing 1% Eu2O3 (mol ratio) were prepared by the sol–gel method. Tetraethoxysilane (TEOS), MgCl2 6H2O, GaCl3 and EuCl3 6H2O were used as starting materials. Firstly, the solution containing TEOS, ethanol and deionized water (containing 0.15 mol/l HCl) was vigorously stirred for 1 h at room temperature. MgCl2 6H2O, GaCl3 and EuCl3 6H2O dissolved in ethanol were added to the above solution and stirred for half an hour to obtain homogeneous solution. The above obtained solution was cast into plastic container and placed at room temperature for about 4 weeks to form a stiff gel of 1–3 mm thickness. Finally, the gel was sintered at 700, 800, 900 and 1000 1C for 3 h, and the corresponding samples were labeled as MGS7, MGS8, MGS9 and MGS10,
L. Yu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1656–1659
3. Results and discussion Fig. 1 shows the XRD patterns of MGS samples calcined at different temperatures. As seen, the broad band ranging from 201 to 251 was from typical amorphous SiO2. When the heating temperature was 700 and 800 1C, the MGS7 and MGS8 were of typical glass nature. When the temperature was increased to 900 and 1000 1C, several diffraction peaks (2y) at 35.491, 43.831 and 63.721 in MGS9 and MGS10 were observed. All the diffraction peaks of MGS9 and MGS10 can be assigned to those of MgGa2O4 spinal structures (JCPDS 38-1240). For MGS10, diffraction peaks became sharper than those of MGS9, indicating the increase in particle size . The particle size was determined by the strongest diffraction peak at 35.491 using the Scherrer equation d¼
kl cos yL
where k is the constant (0.89), l the wavelength of X-ray and L the half width of the diffraction peak. Thus the size of MgGa2O4 nanocrystals can be estimated to be about 4 nm in MGS9 and 8 nm in MGS10 GC. But it should be noted that MGS10 is not transparent. Here we studied only the luminescent properties of Eu3 + in MGS7, MGS8 and MGS9. Fig. 2 shows the excitation spectra of Eu3 + ions in MGS glass and GC monitoring 614 nm emissions. The broad band ranging from 220 to 350 nm centered at about 288 nm was associated with the CT transition absorption from the 2p orbital of O2 ions to the 4f vacant orbital of Eu3 + ions, while the sharp lines at 317, 360, 380, 396, 412 and 464 nm with the direct excitation of f–f shell transitions of Eu3 + can be contributed to 7F0–5H5, 7F0–5D4, 7 F0–5D4,3,2, 7F0–5L6, 7F0–5D3 and 7F0–5D2 transitions absorption, respectively. For MGS samples, the position and configuration of
the CT band hardly changed with increase in heating temperature. But it is obvious that the relative proportion of f–f transitions absorption in CT absorption gradually decreased from MGS7 to MGS9 samples. Fig. 3 shows the emission spectra of Eu3 + in MGS samples at 285 nm excitation pumped by the Xe lamp. It can be seen that from 550 to 720 nm five-group emission lines located at 576, 585–604, 604–638, 655 and 703 nm were observed, which were from 5D0–7FJ ( J¼0–4) transition emissions. The strongest emission at 618 nm was red emission from 5D0–7F2 transitions. An evident change can be seen that the relative intensity of 5 D0–7F0 transition increased with the increase in heating temperature. The integrated intensity ratios of 5D0–7F2/5D0–7F1 and 5D0–7F0/5D0–7F1 were determined and are listed in Table 1. It is obvious that the intensity ratio of 5D0–7F2/5D0–7F1 changed a little for different samples, indicating that the relative proportion of inversion centers to non-inversion centers hardly changed. Since the 5D0–7F1 line is due to the magnetic dipole transition, the intensity of this line is considered to be almost independent of the local field strength or hosts. Therefore, the increase of intensity ratio of 5D0–7F0/5D0–7F1 from MGS7 to MGS9 indicated that the 5 D0–7F0 transition probability increased. Fig. 4 shows the UCL spectra of Eu3 + in MGS samples at fs laser excitation (800 nm). Several strong emission lines ranging from 550 to 720 nm associated with 5D0–7FJ (J¼0–4) of Eu3 + ions in
2.0 MGS9 MGS8
1.6 Intensity (a.u.)
respectively. The final thickness of as prepared samples was about 1.5 mm. X-ray diffraction (XRD) was performed using a Philips X0 pert diffractometer with Cu target as the radiation source. The excitation and emission spectra were recorded using a monochromator (Jobin Yvon, HR 320) and a photomultiplier (Hamamatsu, R955). A 500 W xenon lamp, with the light passing through a monochromator (Jobin Yvon H20), was used as the excitation source. The optical absorption spectra were measured using a UV–vis spectrophotometer (JASCO, V-570). UCL spectra were measured by fs pulse using a Ti:sapphire regenerative amplifier laser system (Spectra Physics, Hurricane) operating at a wavelength of 800 nm with a 1 kHz repetition rate and approximately 130 fs pulse duration. The power density can be varied from 0.15 to 0.7 W/mm2.
MGS7
1.2 0.8 0.4 0.0 200
Intensity (a. u.)
Intensity (a.u.)
250
400 300 350 Wavelength (nm)
450
500
Fig. 2. Excitation spectra of Eu3 + in MGS samples monitoring 615 nm emission.
1.2 MGS10 MGS9 MGS8 MGS7
1657
MGS9 MGS8 MGS7
0.8
0.4 0 20
30
40
50 2θ (degree)
60
70
Fig. 1. XRD patterns of MGS samples at different heating temperatures.
450
500
550 600 650 Wavelength (nm)
700
Fig. 3. Emission spectra of Eu3 + in MGS samples at 285 nm excitation.
750
1658
L. Yu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1656–1659
5
Table 1 Intensity ratio of 5D0–7F2/5D0–7F0 and 5D0–7F2/5D0–7F1 transitions.
D0–7F0/5D0–7F1 5 D0–7F2/5D0–7F1
MGS9
MGS8
MGS9
3
MGS8
0.84 4.32
1.10 4.04
2.17 4.01
2
MGS7
1200000
Intensity (a.u.)
5
4
MGS7
1 0 8
MGS9 MGS8
6
intensity (a.u.)
MGS7
MGS9 MGS8 MGS7
800000
4 2 200
400000
400 600 Wavelength (nm)
800
Fig. 5. Absorption spectra of MGS samples doped with Eu3 + .
0 300
400
500 700 600 Wavelength (nm)
800
Fig. 4. UCL spectra of Eu3 + in MGS glass and GC at fs laser excitation.
three samples were observed. For MGS9, except for characteristic emissions of Eu3 + ions, a broad blue emission ranging from 380 to 520 nm centered at 430 nm occurred. But in MGS7 and MGS8 glasses, no blue emission band was observed. According to the results of XRD (Fig. 1), MgGa2O4 nanocrystals existed in MGS9 GC. In ZnGa2O4 phosphors and film, the same emission band was also observed [11]. Therefore the blue emission band was assigned to the MgGa2O4 nanocrystals. According to Ref. [11], when Ga–O octahedral sites became distorted, the emission of self-activation centers located at 360 nm, while octahedral sites were regular, showing emission at 430 nm. Thus we suggest that the origin of the blue emission band from self-activation luminescence was associated with the CT between Ga3 + ions at regular Oh sites and the surrounding O2. In addition, the emission corresponding to the CT excitation and fs excitation is different in comparison with Fig. 3. This result indicates that the emission mechanism of Eu3 + at fs excitation is different from the CT excitation. To better understand the mechanism of UCL of Eu3 + in MGS glasses and GC, the UCL integrated intensity of 5D0–7FJ transition emissions as a function of the excitation power was investigated. The UCL intensity can be expressed as Ippn where I is the integrated intensity of UCL, p the pump power of fs laser and n the number of photons absorbed per visible photon emitted for any UCL mechanism. According to the above equation, n is the slope of log–log plot between the UCL intensity and the pump power. The slope of the logarithmic lines determined by linear fitting was about 2.09 in MGS7, 1.87 in MGS8 and 1.78 in MGS9. This result indicated that a two-photon excitation predominated in MGS glasses and GC. Basically, UCL can take place by two different processes, i.e., radiatively by the excited state absorption and nonradiatively by the energy-transfer up-conversion between two excited ions [13]. In transparent glass host, two-photon UCL of Er3 + , Tm3 + and Ho3 + was the general phenomenon [14,15]. The mechanism includes the energy transfer from one ion A to another ion B, the two-step absorption from one ion, the two-photon simultaneous
absorption and so on [16]. To clarify the UCL processes, the absorption spectra (a) and the enlarged spectra (b) are shown in Fig. 5. The strong broad absorption at 200–350 nm was from CT. The sharp absorption line at 396 nm originated from 7F0–5L6 transitions. An evident change can be observed that the absorption edge of MGS9 GC redshifted in contrast with that in MGS7 and MGS8 glasses, which were from the absorption of MgGa2O4 nanocrystals. At fs laser excitation, the absorption of 7F0–5L6 transition (396 nm) does not perfectly correspond to two-photon excitation. Thus, In MGS samples, the electrons at ground state 7F0 level can be excited to 5L6 level with the assistance of phonon, then relaxed to 5D3, 5D2, 5D1, and resulting in the lowest excited state 5D0 level. As a consequence, 5D0–7FJ emissions occurred. It should be noted that the two-photon absorption (400 nm) was locating at the edge of absorption of MGS9 GC because of the redshift of absorption. Thus the intensity of Eu3 + in MGS9 GC is higher than that in MGS glasses.
4. Conclusions In conclusion, the MgO–Ga2O3–SiO2 glass and GC containing MgGa2O4 nanocrystals doped with Eu3 + ions were prepared by the sol–gel method. The down-conversion luminescence and UCL properties were studied. The results indicated that the relative intensity of f–f transitions of Eu3 + ions gradually decreased in contrast with that of the CT absorption from MGS7 glass to MGS9. Using a Xe lamp and 800 nm fs laser excitation, strong red luminescence of Eu3 + in MgO–Ga2O3–SiO2 glasses and GC was observed. The UCL mechanisms of Eu3 + in MGS glass and GC can be assumed to be the two-photon excitation.
Acknowledgement The authors are grateful to the Nation Natural Science Foundation of China (Grant no. 10504030) for the financial support. References [1] F. Lahoz, I.R. Martin, U.R. Rodriguez-Mendoza, I. Iparraguirre, J. Azkargorta, A. Mendioroz, R. Balda, J. Fernandez, V. Lavin, Opt. Mater. 27 (2005) 1762. [2] M. Goncalves, L. Santos, R. Almeida, C. R. Chim: 5 (2002) 845. [3] M. Mortier, A. Monteville, G. Patriarche, G. maze, F. Auzel, Opt. Mater. 16 (2001) 255.
L. Yu et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1656–1659
[4] [5] [6] [7]
M. Mortier, Philos. Mag. B 82 (2002) 745. G. Jones, S. Houde-Walter, Opt. Lett. 30 (2005) 2122. X. Qiao, X. Fan, M. Wang, X. Zhang, Opt. Mater. 27 (2004) 597. W. Pisarski, T. Goryczka, J. Pisarska, W. Ryba-Romanowski, J. Phys. Chem. B 111 (2007) 2427. [8] X. Duan, D. Yuan, F. Yu, L. Wang, Appl. Phys. Lett. 89 (2006) 183119. [9] G. Beall, L. Pinckney, J. Am. Ceram. Soc. 82 (1999) 5. [10] K. Shioya, T. Komatsu, H. Kim, R. Sato, K. Matusita, J. Non-Cryst. Solids 189 (1995) 16.
1659
[11] J. Kim, H. Kang, W. Kim, J. Kim, J. Choi, H. Park, G. Kim, T. Kim, Y. Hwang, S. Mho, Appl. Phys. Lett. 82 (2003) 2029. [12] X. Duan, D. Yuan, F. Yu, L. Wan, Appl. Phys. Lett. 89 (2006) 183119. [13] M. Wermuth, T. Riederer, H. Gudel, Phys. Rev. B 57 (1998) 4369. [14] G. Qin, W. Qin, S. Huang, C. Wu, D. Zhao, B. Chen, S. Lu, J. Appl. Phys. 92 (2002) 6936. [15] G. Qin, W. Qin, C. Wu, S. Huang, J. Zhang, S. Lu, D. Zhao, H. Liu, J. Appl. Phys. 93 (2003) 4328. [16] G. Blasse, C. Grabmaier, in: Luminescent Materials, Springer, Berlin, 1994 p. 196.