Tm3+ codoped LiTaO3 polycrystals

Tm3+ codoped LiTaO3 polycrystals

Journal of Alloys and Compounds 591 (2014) 105–109 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

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Journal of Alloys and Compounds 591 (2014) 105–109

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

White upconversion emission in Er3+/Yb3+/Tm3+ codoped LiTaO3 polycrystals Liansheng Shi a,⇑, Chaofeng Li a, Qinyun Shen a, Zhaozhong Qiu b a b

School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150080, China Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China

a r t i c l e

i n f o

Article history: Received 6 August 2013 Received in revised form 26 December 2013 Accepted 26 December 2013 Available online 6 January 2014 Keywords: White upconversion emission Tm/Er/Yb:LiTaO3 CIE 1931 color coordinates

a b s t r a c t White upconversion emission was observed in Er3+/Yb3+/Tm3+:LiTaO3 polycrystals prepared by solidstate reaction method. The structural properties of Er3+/Yb3+/Tm3+:LiTaO3 polycrystals were confirmed by X-ray diffraction (XRD) patterns. The color coordinate of (0.33, 0.33) in 1 mol% Er3+/20 mol% Yb3+/1 mol% Tm3+:LiTaO3 polycrystal was found to be equal to the standard point of energy white-light. The blue emission was populated via the cooperative sensitization upconversion process, and efficient two-photon green upconversion and one-photon red upconversion emissions were observed. The increased green upconversion emission was attributed to the fast cross relaxation process of 3H4 (Tm) + 4I13/2 (Er) ? 3H6 (Tm) + 4S3/2 (Er). The gained excellent optical quality of white upconversion emission suggested that Er3+/Yb3+/Tm3+:LiTaO3 polycrystal will be a potential laser candidate material for the application of the three-dimensional backlighting. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Rare-earth ions doped upconversion materials (REUMs), exhibiting the promising applications in the field of bio-label, solid-state multicolor display, photonic applications and solar cell, have attracted considerable attention [1–5]. Especially, white upconversion (UC) emission could meet the practical application requirements for the three-dimensional backlighting [6]. REUMs have the powerful capability to convert the near infrared (NIR) light into visible UC emission under the cost-effective and highpower NIR-diode lasers [7,8]. Driven by the need for the white UC emission, many studies on the optical characteristics of rare earth (RE) ions have been developed. N.K. Giri has reported that the white UC emission is obtained in Tm3+/Ho3+/Yb3+ codoped tellurite glass under NIR excitations [9]. It has been reported by Y. Dwivedi that by tuning the concentration of RE ions and the incident power density could adjust the optical quality of white light in Pr/Er/Yb-codoped tellurite glass under 980 nm excitation [10]. H. Lin also obtained the white UC emission, which consists of the blue, green, and red UC radiations, in b-YF3:Nd3+/ Yb3+/Tm3+/Ho3+ nano-ceramic under 796 nm excitation [11]. Researches in this area based on RE ions doped nanosystems, glass-systems, polymer-systems, and other systems have been

⇑ Corresponding author. Tel.: +86 13936416655. E-mail address: [email protected] (L. Shi). 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.12.234

widely demonstrated [12–14]. Among these host materials, lithium tantalite (LiTaO3), exhibiting the piezoelectric, ferroelectric, acousto-optical, electro-optical and nonlinear optical properties [15–17], has attracted considerable attention. RE:LiTaO3 could provide more probability to combine the lasing characteristics of RE ion with the nonlinear optical property presented by LiTaO3 host material. Since the application of LiTaO3 polycrystal to laser materials is important, many studies on the luminescence of RE ions doped LiTaO3 single crystals have been expanded. J. Liao reported the red, yellow, green and blue–four-color light from a single, aperiodically poled LiTaO3 crystal [18]. It has been reported by W. Ryba-Romanowski that Ho3+-doped LiTaO3 crystal emits intense green luminescence under 647.1 nm excitation [19]. P.X. Zhang grown the Yb3+/Mg2+-codoped with LiTaO3 crystal by the Czochralski technique, and the laser properties of this crystal were discussed [20]. Therefore, it is essential to study the white optical characteristics of RE ions in LiTaO3 host material under 980 nm excitation. In this paper, Er3+/Tm3+/Yb3+-codoped LiTaO3 polycrystals, emitting the bright white UC emission, were synthesized by solid-state reaction method at 1200 °C. The structural properties of Er3+/Tm3+/Yb3+:LiTaO3 polycrystals were understood by the X-ray diffraction (XRD) spectra. The color coordinates of LiTaO3 polycrystals codoped with different concentration of Er3+, Yb3+ and Tm3+ ions were calculated based on the 1931 CIE standard. The power pump dependences and the UC mechanism were measured and discussed.

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2. Experimental

6000 3+

3+

Pure LiTaO3

3+

LiTaO3. polycrystals codoped with Er , Yb and Tm ions were prepared by solid-state reaction method at 1200 °C. The purities of Li2CO3, Ta2O5, Tm2O3, Er2O3 and Yb2O3 are 99.99%. The mixtures, which were fully ground in an agate mortar by hand at least for 4 h, were pressed into a disk under 20 MPa. The mixture of the chemical precursors was heated at 750 °C for 4 h to resolve the Li2CO3 into Li2O and CO2. The polycrystals were generated at 1200 °C for 22 h. The raw material compositions (mol%) of the prepared Er3+/Yb3+/Tm3+:LiTaO3 polycrytals are shown in Table 1. The crystallization phase was identified by X-ray diffraction spectra measured by an XRD-6000 diffractometer using a copper Ka radiation source. A SPEX1000 M spectrometer with a photomultiplier tube under 980 nm excitation was used to measure the UC emission spectra. The spot size of focused laser in the disk sample was measured to be about 1 mm in diameter. All these measures were performed at room temperature. Based on the 1931 CIE standard, the CIE chromaticity coordinates for the upconversion fluorescence of Er3+/Yb3+/Tm3+:LiTaO3 polycrystals were calculated and marked in the CIE standard chromaticity diagram.

5000

EYT-2/10/1 Intensity (a.u.)

4000

EYT-1/10/2 3000

EYT-1/6/3 2000

EYT-1/10/1 1000

EYT-1/20/1 0 10

20

30

40

50

60

70

80

Fig. 1. The X-ray diffraction patterns of LiTaO3 polycrystals tri-doped with Er3+, Yb3+ and Tm3+ ions.

The XRD spectra of the pure LiTaO3, EYT-1/10/1, EYT-1/20/1, EYT-1/10/2, EYT-2/10/1 and EYT-1/6/3 polycrystals are displayed in Fig. 1. It can be seen that all the diffraction peaks of these polycrystals are well indexed to the phase of pure LiTaO3 polycrystal, indicating that there is no new phase in EYT-1/10/1, EYT-1/20/1, EYT-1/10/2, EYT-2/10/1 and EYT-1/6/3 polycrystals. The Er3+, Yb3+ and Tm3+ ions have no effect on the structures, and EYT-1/10/1, EYT-1/20/1, EYT-1/10/2, EYT-2/10/1 and EYT-1/6/3 polycrystals are still trigonal system. Therefore, it is proposed that Er3+, Yb3+ and Tm3+ ions may occupy the normal Li-site or Nb-site rather than interstitial sites in LiTaO3 crystal lattice. It should be noted that there are two peaks around 30° in EYT-1/20/1 polycrystal and the peak intensities at 23° and 48° of EYT-1/20/1, EYT-1/10/2, EYT-2/10/1 and EYT-1/6/3 are lower than those of the pure LiTaO3. Two peaks around 30° in EYT-1/20/1 polycrystals may be attributed to some degradation of crystal quality for EYT-1/20/1. This is because the doped concentrations of Er3+, Yb3+ and Tm3+ ions are large in EYT-1/20/1 polycrystal. Fig. 2 shows the UC emission spectra of Er3+/Yb3+/Tm3+:LiTaO3 polycrystals irradiated by a focused power-controllable 980 nm diode laser with 250 mW. The green UC emissions centered at 525 nm and 550 nm correspond to the 2H11/2 ? 4I15/2 and 4 S3/2 ? 4I15/2 transitions of Er3+ ion, respectively [21]. The red UC emission at 660 nm is attributed to the 4F9/2 ? 4I15/2 transition of Er3+ ion [22]. The blue emission has a luminescence peak at 480 nm that corresponds to the 1G4 ? 3H6 of Tm3+ ions [23]. As illustrated in Fig. 2, the strongest blue UC emission is observed for YET-1/20/1, and YET-1/6/3 presents the maximum intensities of green and red UC emissions. In general, the intensity of UC emission depends strongly on the local environment, the dopant concentration and the distribution of active ions in a host material. In the case of the dopant concentration, the intensity of UC emission should be increased with the increasing dopant concentration. Compared EYT-1/10/2 with EYT-1/10/1 polycrystal, the intensity of the blue UC emission in EYT-1/10/2 should be higher than that in EYT-1/10/1 since the concentration of Tm3+ ions in EYT-1/10/2 is larger than that in EYT-1/10/1, which contradicts the experimental results shown in Fig. 2. Similarly, the intensities of the green and

EYT-1/10/1 EYT-1/20/1 EYT-2/10/1 EYT-1/6/3

Intensity (a.u.)

3. Results and discussion

EYT-1/10/2

450

500

550

600

650

700

Wavelength (nm) Fig. 2. UC emission spectra of LiTaO3 polycrystals tri-doped with Er3+, Yb3+ and Tm3+ ions under 980 nm excitation.

red UC emissions in EYT-2/10/1 is also expected to be stronger than those in EYT-1/10/1 polycrystal since the concentration of Er3+ ions in EYT-2/10/1 is larger than that in EYT-1/10/1, in disagreement with Fig. 2. It is proposed that the fluorescence intensities of these Er3+/Yb3+/Tm3+:LiTaO3 polycrystals are regardless of the RE ion concentration. Therefore, the local environment and the distribution of active ions play an important role in the optical characteristics of RE ions in Er3+/Yb3+/Tm3+:LiTaO3 polycrystals. This will be understood in the next section. In order to reflect the true color of luminescence, CIE 1931 color coordinates for UC emissions of Er3+/Yb3+/Tm3+:LiTaO3 polycrystals under 980 nm excitation with 250 mW are illustrated in Fig 3. CIE 1931 color coordinate can be calculated by the following formula [24]:





X ; XþY þZ Z



Y ; XþY þZ



Z XþY þZ

ð1Þ

700

PðkÞx0 ðkÞdk

ð2Þ

PðkÞy0 ðkÞdk

ð3Þ

PðkÞz0 ðkÞdk

ð4Þ

425

Table 1 Raw material compositions (mol%) of samples.



Z

700

425 3+

Samples

Er

EYT-1/10/1 EYT-1/20/1 EYT-1/10/2 EYT-2/10/1 EYT-1/6/3

1.0 1.0 1.0 2.0 1.0

3+

3+

Yb

Tm

10.0 20.0 10.0 10.0 6.0

1.0 1.0 2.0 1.0 3.0



Z

700

425

where k is the wavelength of the equivalent monochromatic light, P(k) is the tristimulus values for a color with a spectral power

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n EYT -1/10/1 = 1.99

nEYT-1/20/1 =1.92

nEYT-2/10/1 =2.34

nEYT-1/6/3 =2.32

nEYT-1/10/2 =2.01

Log (Intensity (a.u.))

Blue Emission n EYT -1/10/1 =1.87

nEYT-1/20/1 =1.84

nEYT-2/10/1 =1.92

nEYT-1/6/3 =1.94

nEYT-1/10/2 =2.02

Green Emission n EYT -1/10/1 =1.03

nEYT-1/20/1 =1.07

nEYT-2/10/1 =1.10

nEYT-1/6/3 =1.05

nEYT-1/10/2 =1.12

Red Emission

Fig. 3. Color coordinates of the multicolor upconversion emissions for Er/Yb/ Tm:LiTaO3 polycrystals. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2.20

2.25

2.30

2.35

2.40

2.45

2.50

Log (Pump Power (mW)) Fig. 4. Pump power dependences of Tm/Yb/Er:LiTaO3 polycrystals under 980 excitation.

considered here since Yb3+ ion has a large absorption cross section and can transfer efficiently its energy to Er3+ and Tm3+ ions [29]. The two-photon process for populating the green emissions is described as follows [30]: The Yb3+ ions at the 2F5/2 state excite the Er3+ ions at the ground 4I15/2 state to the upper 4F7/2 state via the two successive energy transfers (ET1 and ET2):

ET1 : 4 I15=2 ðErÞ þ 2 F5=2 ðYbÞ ! 4 I11=2 ðErÞ þ 2 F7=2 ðYbÞ ET2 : 4 I11=2 ðErÞ þ 2 F5=2 ðYbÞ ! 4 F7=2 ðErÞ þ 2 F7=2 ðYbÞ The 4S3/2/2H11/2 states of Er3+ ions, populated via the nonradiative relaxation process (NRP) of the 4F7/2 state, depopulate to the ground 4I15/2 state, producing the green UC emissions. According to Fig. 4, it is proposed that the blue emitting 1G4 state of Tm3+ ions is populated through the cooperative sensitization upconversion (CSU) process: 2 [2F5/2 (Yb)] + 3H6 (Tm) ? 1G4 (Tm) + 2F7/2 (Yb) since the slope values for the blue UC emission deviate from the expected n = 2. The linear dependence on the excitation power

4

2.0

F7/2 (3)

1.5

4

G4

H 4 11/2 S3/2

F2,3

F9/2

4

F9/2

3

H4

4

I11/2

4

I11/2

2

F5/2

(2)

3

H5

CR

4

I13/2

3

4

I13/2

0.5

2

3

4

(5)

1.0

F7/2

1

2

H 4 11/2 S3/2

Energy (104 cm-1)

distribution, x0 (k), y0 (k) and z0 (k) are three color-matching functions, respectively, and X, Y, and Z are the three tristimulus values, respectively. It can be seen from Fig. 3 that the color coordinates (x, y) of multicolor UC emissions in EYT-1/10/1, EYT-1/20/1, EYT-1/10/2, EYT-2/10/1 and EYT-1/6/3 polycrystals are (0.33, 0.34), (0.33, 0.33), (0.31, 0.36), (0.33, 0.34) and (0.33, 0.36), respectively. Obviously, the color coordinates of these polycrystals have a good match with the standard point of equal energy white-light (0.33, 0.33) under 980 nm excitation. Especially, the color coordinate of EYT-1/20/1 polycrystal (0.33, 0.33) is found to be the standard point of equal energy white-light, suggesting a best intensity and color purity. The obtained successfully white UC emissions indicate that Er3+/Yb3+/Tm3+:LiTaO3 polycrystal could be considered as a promising material in developing UC white-light illuminations for electrooptical devices. As for an ‘‘unsaturated’’ UC process, the relation: If / Pn [25] is used to obtain the required number (n) of photons to produce the fluorescence, where If is the intensity of UC emission, and P is the excitation power. Fig. 4 shows the pump power dependences of Er3+/Yb3+/Tm3+:LiTaO3 polycrystals for the blue, green and red UC emissions under 980 nm excitation. As for the blue UC emission, the n values are equal to 1.99, 1.92, 2.34, 2.32 and 2.01 respectively, for EYT-1/10/1, EYT-1/20/1, EYT-2/10/1, EYT-1/6/3 and EYT-1/10/2 polycrystals. The n values of 1.99, 1.92 suggest that the blue UC emission is a two-photon process in EYT-1/10/1 and EYT-1/20/1, and the n values of 2.34, 2.32 and 2.01 deviate from the expected n = 2, implying that a three-photon process is involved to populate the blue UC emission in EYT-2/10/1, EYT-1/ 6/3 and EYT-1/10/2 besides the two-photon process. As for the green UC emissions, the n values are found to be 1.87, 1.84, 1.92. 1.94 and 2.02, respectively, for EYT-1/10/1, EYT-1/20/1, EYT-2/10/ 1, EYT-1/6/3 and EYT-1/10/2 polycrystals, confirming that the green emitting 2H11/2/4S3/2 states are populated via a two-photon process. The linear power pump dependences for the red UC emission (n = 1.03, 1.07, 1.10, 1.05 and 1.12 for EYT-1/10/1, EYT-1/20/1, EYT-2/10/1, EYT-1/6/3 and EYT-1/10/2 may be due to the ‘‘saturation’’ of the UC process [26]. Energy level diagrams of Er3+, Yb3+ and Tm3+ ions, as well as the proposed UC mechanism [27,28] are illustrated in Fig. 5. Under 980 nm excitation, the laser excitation of Yb3+ ions is only

2.15

F4

(4) 4

0.0

2

I15/2

(1)

F7/2

3+

Er

4

3

3+

Yb

H6

3+

Tm

3+

Er

I15/2

Fig. 5. The energy level diagrams of Er3+, Yb3+ and Tm3+ ions, as well as the UC mechanism.

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(the slope value is about 1.0) for the red UC emission means only a ‘‘one-photon’’ process to populate the red emitting 4F9/2 state of Er3+ ions, which may be due to the ‘‘saturation’’ of the UC process. The ground 4I15/2 state of Er3+ ion, the intermediate 4I11/2 state and the green emitting 4S3/2/2H11/2 states are named as 1, 2 and 3, respectively, which are used to discuss the ‘‘saturation’’ of the UC process. Assumingly, the 4I11/2 state is populated by GSA process, and the state 3 is populated only by ESA or energy transfer upconversion (ETU). The sensitization effect caused by the sensitizer and the excited state bleaching are excluded here. Therefore, when state 3 is populated by the ESA process, we can obtain the rate equations under the steady-state excitation:

0 ¼ dN2 =dt ¼ qp r1 N1  qp r2 N2  A2 N2

ð5Þ

0 ¼ dN3 =dt ¼ qp r2 N2  A3 N3

ð6Þ

qp ¼

kp hcpW 2p

p

ð7Þ

where Ni is the population density, ri (i = 1, 2 and 3) is the absorption cross section, Ai (i = 1, 2 and 3) is the spontaneous emission constant, qp is the pump rate, kP, P and wP are the excited wavelength, the incident pump power and the pump radius, respectively, h is the Planck’s constant, and c is the rate of light. As for the state 2, there are two behaviors shown as following: (1) When the state 2 linearly decays primarily to the ground state 1, yielding the luminescence, the ESA term qpr2N2 in Eq. (5) can be neglected, and the conclusions N2 = qpr1N1/ A2 / qp / P and N3 = qpr2N2/A3 / qpN2 / q2p / P2. (2) As ESA process play an important role in the state 2, the linear decay term (A2N2) can be neglected in Eq. (5). The relations: N2 = qpr1N1/qpr2 / q0p / P0 and N3 = qpr2N2/ A3 / qpN2 / qp / P are obtained. The above conclusions indicate that the slope value n = 1 means the linear decay process for the depletion of the intermediate excited states (4I11/2) in at the high pump power. As for the ‘‘saturation’’ process for the red emitting 4F9/2 state of Er3+ ions, we assume firstly the intermediate 4I13/2 state and the red emitting 4F9/2 state are named as 4 and 5, respectively. The steadystate rate equations can thereby be expressed as follows:

dN2 =dt ¼ qp r1 N1  W 2 N2  A2 N 2

ð8Þ

dN4 =dt ¼ W 2 N2  qp r4 N4  A4 N 4

ð9Þ

dN5 =dt ¼ qp r4 N4  A5 N5

ð10Þ

where N4 and N5 are the population density, r4 is the absorption cross section, Ai (i = 2, 4 and 5) is the spontaneous emission constant, a nd W2 is the nonradiative relaxation rate of the state 2 (the 4I11/2 state of Er3+ ions). Under the steady-state excitation, Eq. (8)–(10) can be simplified:

W 2 N 2 ¼ q p r 1 N 1  A2 N 2

ð11Þ

qp r4 N4 ¼ W 2 N2  A4 N4

ð12Þ

A5 N5 ¼ qp r4 N4

ð13Þ

In Eq. (11), the term A2N2 is neglected since we pay attention to discuss the populations of the states 1, 4 and 5 (i.e. 4I15/2, 4I13/2 and 4 F9/2 of Er3+ ions) which are direct relate to the red UC emission. Then, we can obtain the following equation:

qp r1 N1 ¼ qp r4 N4 þ A4 N4

ð14Þ

Combined Eq. (13), (14), and (7), the term qpr4N4 is neglected as the state 4 linearly decays primarily to the ground state 1, and then we can obtain the relation N5 / P2; and when the ESA process play an important role in the state 4, the linear decay term A4N4 is not considered and the relation N4 / P is obtained. Therefore, under the ‘‘unsaturated’’ UC process (i.e. at low laser pump), the ESA process of state 4 is neglected, and the fitting slope value for the pump power dependence is equal to 2. Inversely, under the ‘‘saturated’’ UC process (i.e. at high laser pump), the linearly decays at the state 4 is ignored, and the fitting slope value would be 1. Studies on the previous works suggest that there is a cross relaxation (CR) process between Tm3+ and Er3+ ions: 3H4 (Tm) + 4I13/2 (Er) ? 3H6 (Tm) + 4S3/2 (Er), which leads to an increased intensity of the green UC emission in the Er3+/Yb3+/ Tm3+-codoped system. The maximum intensity of green UC emission observed in EYT-1/6/3 polycrystal could be attributed to the fast CR process. This is because the rate of CR process is inversely proportional to the distance between two neighboring ions. As the concentration of Tm3+ ions is up to 6 mol%, the distance between Tm3+ and Er3+ ions shortens. The shortening distance means an efficient CR process in EYT-1/6/3 polycrystal. 4. Conclusion Bright white UC emission is generated through frequency UC under 980 nm excitation at room temperature in Er/Yb/Tm:LiTaO3 polycrystals prepared by the solid-state reaction method. The standard color coordinate of (0.33, 0.33) is observed in 1 mol% Er3+/ 20 mol% Yb3+/1 mol% Tm3+:LiTaO3 polycrystal. Experimental results show that LiTaO3 polycrystals codoped with the different concentrations of Er3+, Yb3+ and Tm3+ ions also present the white UC emissions, and their color coordinates are very close to the standard equal energy white-light illuminate (0.33, 0.33). References [1] S. Heer, K. Kömpe, H.U. Güdel, M. Haase, Adv. Mater. 16 (2004) 2102. [2] K. Teshima, S.H. Lee, N. Shikine, T. Wakabayashi, K. Yubuta, T. Shishido, S. Oishi, Cryst. Growth Des. 11 (2011) 995. [3] Y.N. Qian, R. Wang, B.F. Zhang, B. Wang, Opt. Lett. 38 (2013) 3731. [4] G. Glaspell, J. Anderson, J.R. Wilkins, M.S. EI-Shall, J. Phys. Chem. C 112 (2008) 11527. [5] W.Q. Zou, C. Visser, J.A. Maduro, M.S. Pshenichnikov, J.C. Hummelen, Nat. Photon. 6 (2012) 560. [6] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Science 273 (1996) 1185. [7] E. Heumann, S. Bär, K. Rademaker, G. Huber, S. Butterworth, A. Dienng, W. Seelert, Appl. Phys. Lett. 88 (2006) 061108. [8] Y.N. Qian, B. Wang, R. Wang, L.L. Xing, Y.L. Xu, RSC Advances 3 (2013) 13507. [9] N.K. Giri, D.K. Rai, S.B. Rai, Appl. Phys. B 91 (2008) 437. [10] Y. Dwivedi, A. Rai, S.B. Rai, J. Appl. Phys. 104 (2008). 043509-1-4. [11] H. Lin, D.Q. Chen, Y.L. Yu, Z.F. Shan, P. Huang, Y.S. Wang, J.L. Yuan, J. Appl. Phys. 107 (2010) 103511. [12] W.C. Lü, X.H. Ma, H. Zhou, G.T. Chen, J.F. Li, Z.J. Zhu, Z.Y. You, C.Y. Tu, J. Phys. Chem. C 112 (2008) 15071. [13] N.Q. Wang, X. Zhao, C.M. Li, E.Y.B. Pun, H. Lin, J. Lumin. 130 (2010) 1044. [14] A. Rizzo, N. Solin, L. Lindgren, M.R. Andersson, O. Inganäs, Nano Lett. 10 (2010) 2225. [15] K.S. Abedin, T. Tsuritani, M. Sato, H. Ito, Appl. Phys. Lett. 70 (1997) 10. [16] S.N. Zhu, Y.Y. Zhu, Z.J. Yang, H.F. Wang, Z.Y. Zhang, J.F. Hong, C.Z. Ge, N.B. Ming, Appl. Phys. Lett. 67 (1995) 320. [17] T. Fujiwara, M. Takahashi, M. Ohama, A.J. Ikushima, Y. Furukawa, K. Kitamura, Electron. Lett. 35 (1999) 499. [18] J. Jiao, J.L. He, H. Liu, J. Du, F. Xu, H.T. Wang, S.N. Zhu, Y.Y. Zhu, N.B. Ming, Appl. Phys. B 78 (2004) 265. [19] W. Ryba-Romanowski, P.J. Deren´, S. Goła˛b, G. Dominiak-Dzik, J. Appl. Phys. 88 (2000) 6078. [20] P.X. Zhang, Y. Hang, J. Gong, C.C. Zhao, J.G. Yin, L.H. Zhang, J. Cryst. Growth 364 (2013) 57. [21] Y.N. Qian, R. Wang, B. Wang, C. Xu, W. Xu, L.L. Xing, Y.L. Xu, J. Quant. Spectrosc. Ra. 129 (2013) 60. [22] F. Liu, E. Ma, D.Q. Chen, Y.L. Yu, Y.S. Wang, J. Phys. Chem. B 110 (2006) 20843. [23] C.B. Zheng, Y.Q. Xia, F. Qin, Y. Yu, J.P. Miao, Z.G. Zhang, W.W. Cao, Chem. Phys. Lett. 509 (2011) 29. [24] H. Gong, D.L. Yang, X. Zhao, E.Y.B. Pun, H. Lin, Opt. Mater. 32 (2010) 554.

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