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Enhancement of upconversion emission in Y3Al5O12:Er3+ induced by Li+ doping at interstitial sites
Chemical Physics Letters 492 (2010) 40–43
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Enhancement of upconversion emission in Y3Al5O12:Er3+ induced by Li+ doping at interstitial sites Mingzhu Yang a, Yu Sui a,b,*, Shipeng Wang a, Xianjie Wang a, Yanqiu Sheng a, Zhiguo Zhang a, Tianquan Lü a, Wanfa Liu c a b c
Center for Condensed Matter Science and Technology (CCMST), Department of Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China International Center for Materials Physics, Academia Sinica, Shenyang 110015, People’s Republic of China Dalian Institute of Chemical Physics, Dalian 116023, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 10 January 2010 In final form 8 April 2010 Available online 11 April 2010
a b s t r a c t The influence of site occupancy of Li+, including substitutional and interstitial sites, on the upconversion emissions of Er3+ doped Y3Al5O12 powders is reported. The intensity of green emission increases slightly when the Li+ occupies substitutional site, but when the Li+ enters into interstitial site it enhances drastically accompanied with a change of emission spectra. These phenomena originate from the increase, induced by interstitial Li+, in the lifetime of 4 I11=2 level, the ratio of radiation rate in green emission and the absorptivity at 980 nm. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth ions doped upconversion (UC) materials have attracted considerable attentions recently for their applications in solid state visible lasers, color displays and biomedical imaging [1–4]. However, the emissions of most UC materials do not have enough intensity. Although adopting appropriate sensitizer and low photon energy host could increase the UC luminescence intensity, it is far from enough [5,6]. Therefore, how to increase the luminescence intensity of UC materials is still a formidable challenge. It is well known that the intra-4f electronic transitions of rareearth ions are parity forbidden according to the quantum mechanical selection rules. But, the forbiddance can be partially broken when the rare-earth ion situates at low symmetry sites [7–10]. Hence, the modification of rare-earth ions’ local environment can be a promising route to enhance their luminescence intensity. It has recently been reported that substitutional site Li+ could result in the enhancement of UC emission in Y2O3:Er3+ and Gd2O3:Eu3+ [7,8]. It indicates that the improved UC intensity may originate from the break of local crystal field symmetry around rare-earth ion by the Li+ doping. Moreover, O.A. Lopez reported that 1.0 mol% Li+ doping made the blue emission of YAG:Tm3+ increase by 87% [11]. However, the role of Li+ in the increasing emission intensity has not been interpreted in detail. Because Li+ ion is very small it can easily enter into the host lattice, occupying not only
* Corresponding author at: Center for Condensed Matter Science and Technology (CCMST), Department of Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China. E-mail address: [email protected] (Y. Sui). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.04.023
the substitutional sites, but also the interstitial sites [12,13]. Both the substitution of Y3+ ions and the occupation of interstitial sites can break the symmetry of crystal field around rare-earth ions and the break can enhance the UC intensity. However, there are few reports on comparing the effect of the two different Li+ site occupancies on the enhancement of emission intensity [12,13]. In this Letter, we report an investigation on the influence of Li+ site occupancy on the UC emission of Er3+ doped YAG powders. YAG is selected as the host lattice due to its high melting point, high chemical stability and relatively low phonon energy. Moreover, Er3+ doped YAG has been established as one of the model systems for generating efficient UC radiation under 980 nm laser excitation, since the abundant energy levels of Er3+ favor a lot of UC processes and the long lifetime 4 I11=2 level of Er3+ can be easily populated by 980 nm pump laser [14,15]. 2. Experimental YAG powders doped with 1 mol% Er3+ (YAG:Er3+) and different concentrations of Li+ were prepared by sol–gel combustion method. High purity Y2O3, Er2O3, Li2CO3, Al(NO3)39H2O and C6H8O7H2O were used as starting materials. The molar ratio of nitrate to citric acid was 1:2. The gel was rapidly heated to 200 °C and an autocombustion process took place. The precursor was then calcined at 900 °C in air. The XRD patterns of powders were recorded by a Rigaku D/max-cB diffractrometer using Cu Ka radiation (k = 0.15418 nm). The UC emission spectra were measured by a power controllable 980 nm diode laser and detected with a lenscoupled monochromator with an attached photomultiplier. The used maximum excitation power was 200 mW. Decay profiles were measured by square-wave modulation of the electric current
M. Yang et al. / Chemical Physics Letters 492 (2010) 40–43
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input to the 980 nm diode laser, and by recording the signals via a Tektronix TDS 5052 digital oscilloscope. The absorption spectra were measured by a Lambda 950 UV/Vis/NIR Spectrophotometer.
3. Results and discussion Fig. 1 displays XRD patterns of the YAG:Er3+ powders doped with 0, 7 and 13 mol% Li+. The main features of diffraction peaks are consistent with the standard pattern of JCPDS 33-0040 [Y3Al5O12], and no other phase can be detected. However, the position of diffraction peaks shifts slightly with different doping concentrations. The inset of Fig. 1 shows the main diffraction peak of the YAG:Er3+ powders doped with different contents of Li+. It is clear that the main diffraction peak shifts toward larger angles as the concentrations of Li+ increasing up to 7 mol%, then gradually reverses when the content of Li+ is over 9 mol%. These results indicate that the host lattice dimension shrinks with the concentration of Li+ from 0 to 7 mol%, and then begins to expand when the concentration of Li+ is over 9 mol%. The effective ionic radius of Y3+ ion and Li+ ion are 90.0 and 76.0 pm, respectively [16,17], hence, substituting the Y3+ ion with the smaller Li+ ion can induce the shrinking of the host lattice, whereas Li+ ions occupying interstitial sites leads to the expansion of the host lattice. Consequently, when the concentration of Li+ ions is below 7 mol%, Li+ ions occupy substitutional sites, but with higher concentrations, Li+ ions begin to take interstitial sites. Because both types of Li+ occupancies would break the local crystal field symmetry around the Er3+ ions, Li+ doping could help to break the forbidden transitions, change the lifetime of energy levels and consequently enhance the UC intensity [7,12]. Fig. 2 shows the UC luminescence in YAG:Er3+ with different concentrations of Li+ under 980 nm diode laser excitation, and the excitation power is 200 mW. The green emission ranging from 515 to 580 nm is associated with transitions of 2 H11=2 , 4 S3=2 ! 4 I15=2 and the red emission from 640 to 690 nm is attributed to the 4 F9=2 ! 4 I15=2 transition. As shown in Fig. 2, the green emission increases gently with concentrations of Li+ increasing to 7 mol%, but when the concentration of Li+ reaches 9 mol% it enhances dramatically accompanied with the change of emission spectra. The number of emission peak splits from four to six with the content of Li+ varying from 7 to 9 mol%. From the inset of Fig. 2, the maximum
Fig. 1. XRD patterns of YAG:Er3+ powders codoped with Li+ (a) 0 mol%, (b) 7 mol% and (c) 13 mol%. The inset is the main diffraction peaks of the YAG:Er3+ powders with different concentrations of Li+.
Fig. 2. UC luminescence spectra of YAG:Er3+ powders with different concentrations of Li+ under 980 nm excitation and a excitation power of 200 mW. The inset is the dependence of the intensity of the green and red UC emission as well as the ratio of green/red on the concentration of Li+.
green emission enhancement is measured to be about 36 times greater corresponding to the sample with the Li+ concentration of 13 mol%. The red emission also enhances with the increase of Li+ content, but the enhancement of red emission lags behind green emission when the Li+ content is below 7 mol%. As the concentration of Li+ is above 9 mol% the increases of green emission and red emission almost keep synchronous. To understand the UC processes in Er3+, Li+ codoped YAG powders, we measured the excitation power dependence of the green emission and red emission of YAG:Er3+ powder doped with 13 mol% Li+ under the excitation of 980 nm. As shown in Fig. 3, the values of slopes for the green emission and the red emission are 1.90 and 1.65, respectively. It indicates that the UC emission is a two-photon process. So the UC processes of the green and red emissions are described as Fig. 4. The Er3+ ion can be promoted to the 4 I11=2 state through ground state absorption (GSA) process, and then nonradiative relaxation occurred and populated the 4 I13=2 state. Subsequently, 4 I11=2 and 4 I13=2 states are further excited to the 4 F7=2 and 4 F9=2 states, respectively, via excited state
Fig. 3. The pump power dependence of the green and red UC emissions in YAG:Er3+ powders codoped with 13 mol% Li+.
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M. Yang et al. / Chemical Physics Letters 492 (2010) 40–43
Fig. 4. Energy level diagram of Er3+ ion as well as the UC processes of the green and red emissions.
absorption (ESA) or energy transfer UC (ETU) processes. After that nonradiative relaxation processes populate the 2 H11=2 /4 S3=2 and 4 F9=2 states. So the green and red emissions are observed by the transitions of 2 H11=2 , 4 S3=2 ! 4 I15=2 and 4 F9=2 ! 4 I15=2 . When the concentration of Er3+ ion increases, the ETU processes between neighboring Er3+ ions is effective [18]. As the concentration of Er3+ ion in our experiment is 1 mol%, the ETU processes, rather than the ESA processes, contribute to the green emission [12]. Li+ ions can dissociate the Er3+ clusters and the nonradiative relaxation rate is reduced. As indicated in Fig. 4, the decrease of nonradiative relaxation rate would increase the population ratio of the 4 I11=2 to 4 I13=2 level, so as to enhance the green emission and suppress the red emission. The interstitial Li+ ions are favor to separate the Er3+ clusters. With Li+ content changing from 7 to 9 mol%, the nonradiative process decreases obviously which enlarges the ratio of G/R [7]. Here, we mainly study the change of green emission for it is the dominant emission. When the concentration of Li+ ions is below 7 mol%, substituting Y3+ ions with Li+ ions can induce oxygen vacancy due to charge compensation, so substitutional site Li+ ions and oxygen vacancies around Er3+ ions can cause the crystal field distortion. However, for higher concentrations of Li+, the substitutional site and the interstitial site Li+ ions coexist in the host. These interstitial Li+ ions would induce the yttrium vacancies so there are four different types of defects around the Er3+ ions, which cause much severe crystal field distortion. When the content of Li+ is above 9 mol% the increase of emission peaks means that the Stark levels have been split, which proves that with the appearance of interstitial Li+ the crystal field symmetry around Er3+ have been seriously broken. The distortion of local symmetry of crystal field around Er3+ and the breaking up of Er3+ clusters would change the lifetime of energy levels, which can affect the UC intensity. The green emission intensity can be expressed as follows [12]:
Igreen bgreen hmgreen C s22 R21 N2
ð1Þ
where h is Planck constant, C is the coefficient of energy transfer process, R1 is the rate of ground state absorption process, N is the population of ground state level, bgreen is the ratio of the radiation rate to the decay rate in the 2 H11=2 =4 S3=2 state and s2 is the lifetime of 4 I11=2 level. According to Eq. (1), the increase in the parameters s2 and bgreen can lead to the increase of the green UC radiation. As shown in Fig. 5 the lifetime of 4 I11=2 level for the sample doped with
Fig. 5. Decay profiles of 4 I11=2 ! 4 I15=2 transition in YAG:Er3+ powders codoped with 7 and 9 mol% of Li+ under the excitation of 980 nm. The inset is the absorption spectra of YAG:Er3+ powders codoped with 7 and 9 mol% of Li+.
7 and 9 mol% Li+ is 0.853 ms and 0.982 ms, respectively. That is to say, the lifetime of intermediate 4 I11=2 state increases with the appearance of interstitial Li+. It is well understood that the reverse of the lifetime equals to the sum of the radiative transition and nonradiative relaxation rates [19]. As mentioned above, with Li+ content changing form 7 to 9 mol%, the radiative transition rate increases but the nonradiative relaxation rate decreases obviously. So the separation of the Er3+ clusters which decrease the nonradiative transition leads to the increase of lifetime of 4 I11=2 level. As evaluated from the ratio of green to red in the inset of Fig. 2, the parameter bgreen increases about 1.2 times when the content of Li+ changes from 7 to 9 mol%. Quantitatively, the increase of s2 and bgreen can lead to about 1.6 times fluorescence increase according to Eq. (1), while the increase is about 4.8 times from the experiment. So, the increase in the lifetime of 4 I11=2 level and bgreen is part of the reasons that cause the abrupt enhancement of emission intensity when interstitial Li+ ions appear. There must be other factors in the emission enhancement. As previously reported, the distortion of the local symmetry around Er3+ favors the spontaneous radiation [7,9]. The relationship between spontaneous radiation and stimulated absorption is expressed as Einstein formula:
Aij 8phm3 ¼ Bji c3
ð2Þ
where Aij is the spontaneous radiation coefficient from i to j level, Bji is the stimulated absorption coefficient from j to i level, h is Planck constant, c is velocity of light and m is the frequency between i and j levels. It is clear that the right side of Eq. (2) is a constant for certain energy levels i and j. Moreover, the absorptivity from j to i level is equal to Bji qðmÞ, where qðmÞ is the photon density of the excitation light. Since qðmÞ is a constant in our experiment, the Bji qðmÞ varies directly as Aij . As mentioned above, the interstitial Li+ ions induce seriously distortion of the local crystal field which favors the spontaneous radiation probability from 4 I11=2 to 4 I15=2 level. So the absorptivity would increase with the spontaneous radiation probability. As shown in the inset of Fig. 3, the absorptivity at 980 nm increases with the concentration of Li+ is from 7 to 9 mol%. This means that the absorptivity from 4 I15=2 to 4 I11=2 level increase with the appearance of interstitial Li+ ions, and so more energy is absorbed by Er3+ so the emission intensity is enhanced. So the increase in the lifetime of 4 I11=2 level, bgreen and the absorptivity at 980 nm
M. Yang et al. / Chemical Physics Letters 492 (2010) 40–43
lead to the drastic increase in the emission intensity when interstitial Li+ ions appear. 4. Conclusions In summary, we have found experimentally that the green radiation enhances drastically with an emission spectra change when interstitial Li+ ions appear. The interstitial Li+ ions lead to the severe break of the symmetry of crystal field around Er3+ and the separation of the Er3+ clusters. The abrupt enhancement of luminescence is therefore attributed to the increase in the lifetime of 4 I11=2 level, bgreen and the absorptivity at 980 nm. The significant enhancement of UC intensity by doping Li+ ions in YAG powders could also be used in transparent ceramics which is a perfect substitute of single crystal in solid state laser. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 50672019 and 10804024) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. Supported by ‘‘the Fundamental Research Funds for the Central Universities” (Grant No. HIT.NSRIF. 2009056).
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References [1] A.J. Silversmith, W. Lenth, R.M. Macfarlane, Appl. Phys. Lett. 51 (1987) 1977. [2] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Science 273 (1996) 1185. [3] F. Liu, E. Ma, D.Q. Chen, Y.L. Yu, Y.S. Wang, J. Phys. Chem. B 110 (2006) 20843. [4] S.F. Lim, R. Riehn, W.S. Ryu, N. Khanarian, C.K. Tung, D. Tank, R.H. Austin, Nano Lett. 6 (2006) 169. [5] P.V. dos Santos, E.A. Gouveia, M.T. de Araujo, A.S. Gouveia-Neto, A.S.B. Sombra, J.A. Medeiros Neto, Appl. Phys. Lett. 74 (1999) 3607. [6] E. Heumann, S. Bär, K. Rademaker, G. Huber, Appl. Phys. Lett. 88 (2006) 061108. [7] Y.F. Bai, K. Yang, Y.X. Wang, X.R. Zhang, Y.L. Song, Opt. Commun. 281 (2008) 2930. [8] S.H. Shin, J.H. Kang, D.Y. Jeon, D.S. Zang, J. Lumin. 114 (2005) 275. [9] L.D. Sun, C. Qian, C.S. Lia, X.L. Wang, C.H. Yan, Solid State Commun. 119 (2001) 393. [10] Y.F. Bai et al., J. Phys. Chem. C 112 (2008) 12259. [11] O.A. Lopez, J. Mckittrick, L.E. Shea, J. Lumin. 71 (1997) 1. [12] G.Y. Chen et al., Appl. Phys. Lett. 92 (2008) 113114. [13] G.Y. Chen, H.C. Liu, H.J. Liang, G. Somesfalean, Z.G. Zhang, J. Phys. Chem. C 112 (2008) 12030. [14] M. Liu, S.W. Wang, J. Zhang, L.Q. An, L.D. Chen, Opt. Mater. 29 (2007) 1352. [15] M. Kaczkan, M. Borowska, K. Kołodziejak, T. Łukasiewicz, M. Malinowski, Opt. Mater. 30 (2008) 703. [16] C.G. Dou, Q.H. Yang, X.M. Hu, J. Xu, Opt. Commun. 281 (2008) 692. [17] K.A. Hyeon, S.H. Byeon, J.C. Park, D.K. Kim, K.S. Suh, Solid State Commun. 115 (2000) 99. [18] R.R. Gonçalves et al., J. Non-Cryst. Solids 322 (2003) 306. [19] H.W. Song, J.W. Wang, B.J. Chen, H.S. Peng, S.Z. Lu, Chem. Phys. Lett. 376 (2003) 1.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Enhancement of upconversion emission in Y3Al5O12:Er3+ induced by Li+ doping at interstitial sites Mingzhu Yang a, Yu Sui a,b,*, Shipeng Wang a, Xianjie Wang a, Yanqiu Sheng a, Zhiguo Zhang a, Tianquan Lü a, Wanfa Liu c a b c
Center for Condensed Matter Science and Technology (CCMST), Department of Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China International Center for Materials Physics, Academia Sinica, Shenyang 110015, People’s Republic of China Dalian Institute of Chemical Physics, Dalian 116023, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 10 January 2010 In final form 8 April 2010 Available online 11 April 2010
a b s t r a c t The influence of site occupancy of Li+, including substitutional and interstitial sites, on the upconversion emissions of Er3+ doped Y3Al5O12 powders is reported. The intensity of green emission increases slightly when the Li+ occupies substitutional site, but when the Li+ enters into interstitial site it enhances drastically accompanied with a change of emission spectra. These phenomena originate from the increase, induced by interstitial Li+, in the lifetime of 4 I11=2 level, the ratio of radiation rate in green emission and the absorptivity at 980 nm. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth ions doped upconversion (UC) materials have attracted considerable attentions recently for their applications in solid state visible lasers, color displays and biomedical imaging [1–4]. However, the emissions of most UC materials do not have enough intensity. Although adopting appropriate sensitizer and low photon energy host could increase the UC luminescence intensity, it is far from enough [5,6]. Therefore, how to increase the luminescence intensity of UC materials is still a formidable challenge. It is well known that the intra-4f electronic transitions of rareearth ions are parity forbidden according to the quantum mechanical selection rules. But, the forbiddance can be partially broken when the rare-earth ion situates at low symmetry sites [7–10]. Hence, the modification of rare-earth ions’ local environment can be a promising route to enhance their luminescence intensity. It has recently been reported that substitutional site Li+ could result in the enhancement of UC emission in Y2O3:Er3+ and Gd2O3:Eu3+ [7,8]. It indicates that the improved UC intensity may originate from the break of local crystal field symmetry around rare-earth ion by the Li+ doping. Moreover, O.A. Lopez reported that 1.0 mol% Li+ doping made the blue emission of YAG:Tm3+ increase by 87% [11]. However, the role of Li+ in the increasing emission intensity has not been interpreted in detail. Because Li+ ion is very small it can easily enter into the host lattice, occupying not only
* Corresponding author at: Center for Condensed Matter Science and Technology (CCMST), Department of Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China. E-mail address: [email protected] (Y. Sui). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.04.023
the substitutional sites, but also the interstitial sites [12,13]. Both the substitution of Y3+ ions and the occupation of interstitial sites can break the symmetry of crystal field around rare-earth ions and the break can enhance the UC intensity. However, there are few reports on comparing the effect of the two different Li+ site occupancies on the enhancement of emission intensity [12,13]. In this Letter, we report an investigation on the influence of Li+ site occupancy on the UC emission of Er3+ doped YAG powders. YAG is selected as the host lattice due to its high melting point, high chemical stability and relatively low phonon energy. Moreover, Er3+ doped YAG has been established as one of the model systems for generating efficient UC radiation under 980 nm laser excitation, since the abundant energy levels of Er3+ favor a lot of UC processes and the long lifetime 4 I11=2 level of Er3+ can be easily populated by 980 nm pump laser [14,15]. 2. Experimental YAG powders doped with 1 mol% Er3+ (YAG:Er3+) and different concentrations of Li+ were prepared by sol–gel combustion method. High purity Y2O3, Er2O3, Li2CO3, Al(NO3)39H2O and C6H8O7H2O were used as starting materials. The molar ratio of nitrate to citric acid was 1:2. The gel was rapidly heated to 200 °C and an autocombustion process took place. The precursor was then calcined at 900 °C in air. The XRD patterns of powders were recorded by a Rigaku D/max-cB diffractrometer using Cu Ka radiation (k = 0.15418 nm). The UC emission spectra were measured by a power controllable 980 nm diode laser and detected with a lenscoupled monochromator with an attached photomultiplier. The used maximum excitation power was 200 mW. Decay profiles were measured by square-wave modulation of the electric current
M. Yang et al. / Chemical Physics Letters 492 (2010) 40–43
41
input to the 980 nm diode laser, and by recording the signals via a Tektronix TDS 5052 digital oscilloscope. The absorption spectra were measured by a Lambda 950 UV/Vis/NIR Spectrophotometer.
3. Results and discussion Fig. 1 displays XRD patterns of the YAG:Er3+ powders doped with 0, 7 and 13 mol% Li+. The main features of diffraction peaks are consistent with the standard pattern of JCPDS 33-0040 [Y3Al5O12], and no other phase can be detected. However, the position of diffraction peaks shifts slightly with different doping concentrations. The inset of Fig. 1 shows the main diffraction peak of the YAG:Er3+ powders doped with different contents of Li+. It is clear that the main diffraction peak shifts toward larger angles as the concentrations of Li+ increasing up to 7 mol%, then gradually reverses when the content of Li+ is over 9 mol%. These results indicate that the host lattice dimension shrinks with the concentration of Li+ from 0 to 7 mol%, and then begins to expand when the concentration of Li+ is over 9 mol%. The effective ionic radius of Y3+ ion and Li+ ion are 90.0 and 76.0 pm, respectively [16,17], hence, substituting the Y3+ ion with the smaller Li+ ion can induce the shrinking of the host lattice, whereas Li+ ions occupying interstitial sites leads to the expansion of the host lattice. Consequently, when the concentration of Li+ ions is below 7 mol%, Li+ ions occupy substitutional sites, but with higher concentrations, Li+ ions begin to take interstitial sites. Because both types of Li+ occupancies would break the local crystal field symmetry around the Er3+ ions, Li+ doping could help to break the forbidden transitions, change the lifetime of energy levels and consequently enhance the UC intensity [7,12]. Fig. 2 shows the UC luminescence in YAG:Er3+ with different concentrations of Li+ under 980 nm diode laser excitation, and the excitation power is 200 mW. The green emission ranging from 515 to 580 nm is associated with transitions of 2 H11=2 , 4 S3=2 ! 4 I15=2 and the red emission from 640 to 690 nm is attributed to the 4 F9=2 ! 4 I15=2 transition. As shown in Fig. 2, the green emission increases gently with concentrations of Li+ increasing to 7 mol%, but when the concentration of Li+ reaches 9 mol% it enhances dramatically accompanied with the change of emission spectra. The number of emission peak splits from four to six with the content of Li+ varying from 7 to 9 mol%. From the inset of Fig. 2, the maximum
Fig. 1. XRD patterns of YAG:Er3+ powders codoped with Li+ (a) 0 mol%, (b) 7 mol% and (c) 13 mol%. The inset is the main diffraction peaks of the YAG:Er3+ powders with different concentrations of Li+.
Fig. 2. UC luminescence spectra of YAG:Er3+ powders with different concentrations of Li+ under 980 nm excitation and a excitation power of 200 mW. The inset is the dependence of the intensity of the green and red UC emission as well as the ratio of green/red on the concentration of Li+.
green emission enhancement is measured to be about 36 times greater corresponding to the sample with the Li+ concentration of 13 mol%. The red emission also enhances with the increase of Li+ content, but the enhancement of red emission lags behind green emission when the Li+ content is below 7 mol%. As the concentration of Li+ is above 9 mol% the increases of green emission and red emission almost keep synchronous. To understand the UC processes in Er3+, Li+ codoped YAG powders, we measured the excitation power dependence of the green emission and red emission of YAG:Er3+ powder doped with 13 mol% Li+ under the excitation of 980 nm. As shown in Fig. 3, the values of slopes for the green emission and the red emission are 1.90 and 1.65, respectively. It indicates that the UC emission is a two-photon process. So the UC processes of the green and red emissions are described as Fig. 4. The Er3+ ion can be promoted to the 4 I11=2 state through ground state absorption (GSA) process, and then nonradiative relaxation occurred and populated the 4 I13=2 state. Subsequently, 4 I11=2 and 4 I13=2 states are further excited to the 4 F7=2 and 4 F9=2 states, respectively, via excited state
Fig. 3. The pump power dependence of the green and red UC emissions in YAG:Er3+ powders codoped with 13 mol% Li+.
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M. Yang et al. / Chemical Physics Letters 492 (2010) 40–43
Fig. 4. Energy level diagram of Er3+ ion as well as the UC processes of the green and red emissions.
absorption (ESA) or energy transfer UC (ETU) processes. After that nonradiative relaxation processes populate the 2 H11=2 /4 S3=2 and 4 F9=2 states. So the green and red emissions are observed by the transitions of 2 H11=2 , 4 S3=2 ! 4 I15=2 and 4 F9=2 ! 4 I15=2 . When the concentration of Er3+ ion increases, the ETU processes between neighboring Er3+ ions is effective [18]. As the concentration of Er3+ ion in our experiment is 1 mol%, the ETU processes, rather than the ESA processes, contribute to the green emission [12]. Li+ ions can dissociate the Er3+ clusters and the nonradiative relaxation rate is reduced. As indicated in Fig. 4, the decrease of nonradiative relaxation rate would increase the population ratio of the 4 I11=2 to 4 I13=2 level, so as to enhance the green emission and suppress the red emission. The interstitial Li+ ions are favor to separate the Er3+ clusters. With Li+ content changing from 7 to 9 mol%, the nonradiative process decreases obviously which enlarges the ratio of G/R [7]. Here, we mainly study the change of green emission for it is the dominant emission. When the concentration of Li+ ions is below 7 mol%, substituting Y3+ ions with Li+ ions can induce oxygen vacancy due to charge compensation, so substitutional site Li+ ions and oxygen vacancies around Er3+ ions can cause the crystal field distortion. However, for higher concentrations of Li+, the substitutional site and the interstitial site Li+ ions coexist in the host. These interstitial Li+ ions would induce the yttrium vacancies so there are four different types of defects around the Er3+ ions, which cause much severe crystal field distortion. When the content of Li+ is above 9 mol% the increase of emission peaks means that the Stark levels have been split, which proves that with the appearance of interstitial Li+ the crystal field symmetry around Er3+ have been seriously broken. The distortion of local symmetry of crystal field around Er3+ and the breaking up of Er3+ clusters would change the lifetime of energy levels, which can affect the UC intensity. The green emission intensity can be expressed as follows [12]:
Igreen bgreen hmgreen C s22 R21 N2
ð1Þ
where h is Planck constant, C is the coefficient of energy transfer process, R1 is the rate of ground state absorption process, N is the population of ground state level, bgreen is the ratio of the radiation rate to the decay rate in the 2 H11=2 =4 S3=2 state and s2 is the lifetime of 4 I11=2 level. According to Eq. (1), the increase in the parameters s2 and bgreen can lead to the increase of the green UC radiation. As shown in Fig. 5 the lifetime of 4 I11=2 level for the sample doped with
Fig. 5. Decay profiles of 4 I11=2 ! 4 I15=2 transition in YAG:Er3+ powders codoped with 7 and 9 mol% of Li+ under the excitation of 980 nm. The inset is the absorption spectra of YAG:Er3+ powders codoped with 7 and 9 mol% of Li+.
7 and 9 mol% Li+ is 0.853 ms and 0.982 ms, respectively. That is to say, the lifetime of intermediate 4 I11=2 state increases with the appearance of interstitial Li+. It is well understood that the reverse of the lifetime equals to the sum of the radiative transition and nonradiative relaxation rates [19]. As mentioned above, with Li+ content changing form 7 to 9 mol%, the radiative transition rate increases but the nonradiative relaxation rate decreases obviously. So the separation of the Er3+ clusters which decrease the nonradiative transition leads to the increase of lifetime of 4 I11=2 level. As evaluated from the ratio of green to red in the inset of Fig. 2, the parameter bgreen increases about 1.2 times when the content of Li+ changes from 7 to 9 mol%. Quantitatively, the increase of s2 and bgreen can lead to about 1.6 times fluorescence increase according to Eq. (1), while the increase is about 4.8 times from the experiment. So, the increase in the lifetime of 4 I11=2 level and bgreen is part of the reasons that cause the abrupt enhancement of emission intensity when interstitial Li+ ions appear. There must be other factors in the emission enhancement. As previously reported, the distortion of the local symmetry around Er3+ favors the spontaneous radiation [7,9]. The relationship between spontaneous radiation and stimulated absorption is expressed as Einstein formula:
Aij 8phm3 ¼ Bji c3
ð2Þ
where Aij is the spontaneous radiation coefficient from i to j level, Bji is the stimulated absorption coefficient from j to i level, h is Planck constant, c is velocity of light and m is the frequency between i and j levels. It is clear that the right side of Eq. (2) is a constant for certain energy levels i and j. Moreover, the absorptivity from j to i level is equal to Bji qðmÞ, where qðmÞ is the photon density of the excitation light. Since qðmÞ is a constant in our experiment, the Bji qðmÞ varies directly as Aij . As mentioned above, the interstitial Li+ ions induce seriously distortion of the local crystal field which favors the spontaneous radiation probability from 4 I11=2 to 4 I15=2 level. So the absorptivity would increase with the spontaneous radiation probability. As shown in the inset of Fig. 3, the absorptivity at 980 nm increases with the concentration of Li+ is from 7 to 9 mol%. This means that the absorptivity from 4 I15=2 to 4 I11=2 level increase with the appearance of interstitial Li+ ions, and so more energy is absorbed by Er3+ so the emission intensity is enhanced. So the increase in the lifetime of 4 I11=2 level, bgreen and the absorptivity at 980 nm
M. Yang et al. / Chemical Physics Letters 492 (2010) 40–43
lead to the drastic increase in the emission intensity when interstitial Li+ ions appear. 4. Conclusions In summary, we have found experimentally that the green radiation enhances drastically with an emission spectra change when interstitial Li+ ions appear. The interstitial Li+ ions lead to the severe break of the symmetry of crystal field around Er3+ and the separation of the Er3+ clusters. The abrupt enhancement of luminescence is therefore attributed to the increase in the lifetime of 4 I11=2 level, bgreen and the absorptivity at 980 nm. The significant enhancement of UC intensity by doping Li+ ions in YAG powders could also be used in transparent ceramics which is a perfect substitute of single crystal in solid state laser. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 50672019 and 10804024) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. Supported by ‘‘the Fundamental Research Funds for the Central Universities” (Grant No. HIT.NSRIF. 2009056).
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