Yb3+ co-doped transparent glass-ceramics containing Ba2LaF7 nanocrystals under heat treatment

Optical Materials 36 (2014) 639–644

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Optical Materials journal homepage: www.elsevier.com/locate/optmat

Energy transfer and upconversion emission of Er3+/Tb3+/Yb3+ co-doped transparent glass-ceramics containing Ba2LaF7 nanocrystals under heat treatment Ho Kim Dan a,b, Dacheng Zhou a, Rongfei Wang a, Xue Yu a, Qing Jiao a, Zhengwen Yang a, Zhiguo Song a, Jianbei Qiu a,⇑ a b

School of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, PR China Department of Research Administration and International Relations, Tuyhoa Industrial College, Phuyen 620900, Viet Nam

a r t i c l e

i n f o

Article history: Received 1 September 2013 Received in revised form 31 October 2013 Accepted 31 October 2013 Available online 2 December 2013 Keywords: Upconversion Ba2LaF7 Glass-ceramics Energy transfer Er3+/Tb3+/Yb3+

a b s t r a c t Transparent glass-ceramics SiO2–AlF3–BaF2–TiO2–LaF3 (SABTL) containing Ba2LaF7 nanocrystals were successfully prepared by heat treatment process through conventional melting method. The crystal size in the glass-ceramics increased gradually under the changing of heat treatment temperatures and times, which was confirmed by the results of XRD, TEM measurements. The intensity of the blue, green and red upconversion luminescence around 490 nm, 525 nm, 546 nm, 657 nm which originate from the transitions 5D4 ? 7FJ ( J = 6 and 5) of Tb3+ ions and (2H11/2, 4S3/2, 4F9/2) ? 4I15/2 transitions of Er3+ ions, respectively, were strongly observed after heat treatment under 980 nm laser diode excitation. The intensity of upconversion luminescence was increased gradually with the increase of Yb3+ concentrations and reaches its maximum at 2.5 mol%. The upconversion luminescence and energy transfer process between Tb3+, Yb3+ and Er3+ ions in the glass-ceramics were discussed. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The frequency upconversion (UC) of infrared light to visible light has been extensively investigated in rare-earth (RE) doped crystals and glasses owing to the potential applications in visible UC lasers. The UC of RE doped glass materials have been extensively investigated due to their potential applications to many fields, such as three dimensional displays, short-wavelength laser, solid state lasers, optical data storage, sensor and energy solar cells [1–7]. In recent years, RE doped oxyfluoride glass-ceramics have attracted intensive research interest for their combined merits of low phonon energy for fluoride and desirable mechanical and chemical properties for oxide. The first successful synthesis of glass-ceramics with high UC efficiency can date back to (Pb, Cd)F2 system by Wang and Ohwaki [8]. Subsequently, oxyfluoride glass-ceramics doped with RE ions had been researched widely in the past few decades [9–14]. Among the RE ions, Er3+ ion is the most popular as well as one of most efficient ions because it has a favorable energy level structure with 4I15/2 ? 4I11/2 transition in the near-infrared spectral region which can be easily excited using

⇑ Corresponding author. Tel.: +86 871 518 8856; fax: +86 871 533 4185. E-mail address: [email protected] (J. Qiu). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.10.045

a 980 nm semiconductor laser as excitation source [15,16]. However, a problem of weak UC luminescence remains for the glasses doped with only Er3+ ions due to its small absorption cross-section. In order to improve the emission of Er3+ ions, the sensitization with Yb3+ ions may be a good choice because of the efficient energy transfer (ET) from Yb3+ to Er3+ ions. Several efforts, ET and choice of glass host with low phonon energy, etc., were put forward to enhance UC luminescence [17–20]. Recently, Hu et al. [21] have investigated characteristics and mechanism of UC luminescence in Er3+/Yb3+/Tb3+ co-doped oxyfluorogermante glasses, but not the transparent glass-ceramics materials. Based on the above considerations, in the present study, we developed a novel kind of Er3+/Tb3+/Yb3+ co-doped transparent glass-ceramics containing Ba2LaF7 nanocrystals. The reagents (SiO2, AlF3, TiO2, LaF3, BaF2, ErF3, TbF3 and YbF3) of this kind of glass-ceramics avoided toxic elements Pb and Cd [22,23]. In addition, as we well know, the crystallization of a fluoride phase is achieved by heat treatment slightly above the glass transition temperature. The base glass composition, the temperature and time of heat treatment will influence the crystallization process, the resulting phase composition, crystal size, volume concentration and distribution. Therefore, in this paper, the ET and UC emission of Er3+/Tb3+/Yb3+ co-doped transparent glass-ceramics SABTL containing Ba2LaF7 nanocrystals under heat treatment were investigated.

H.K. Dan et al. / Optical Materials 36 (2014) 639–644

3. Results and discussion The transparent glass-ceramics was prepared and the nanocrystals structures in the glass-ceramics were monitored by XRD. The XRD patterns of glass-ceramics after heat treatment at different temperatures and times are shown in Fig. 1. The crystallites size D for a given (h k l) plane was estimated from the XRD patterns following the Scherrer equation:

D¼

kK ; b  cos h

where K = 0.9, k is the wavelength of the incident XRD [Cu Ka (k = 0.154056 nm)], b is the full width of half maximum in radians and h is the diffraction angle for the (h k l) plane [24,25]. By using Scherrer equation, the average of Ba2LaF7 crystallites size can be calculated for SABTL-560, SABTL-580, SABTL-600, SABTL-620, SABTL-4 h, SABTL-6 h, SABTL-9 h and SABTL-12 h samples, respectively. The results calculation of Ba2LaF7 crystallites size and the relationship between crystal size with the heat-treated different temperatures and times in the SABTL glass-ceramics are shown in Fig. 2. Clearly in Fig. 2a, the increase for the heat treatment temperatures leads to the crystal size increased from about 4.5 to 11 nm. Similarly, the increase for the heat treatment times also leads to the crystal size increased (see in Fig. 2b).

80 SABTL-620

40 0

SABTL-600

40

SABTL-580

0 40

SABTL-560

0 40

SABTL-Glass

10

20

30

40

50

(422)

70

(333)

(400)

60

(331) (420)

JDCDS 00-048-0099: Ba2LaF7

(311) (222)

(200)

(111)

0

(220)

0 40

80

90

2θ (degree)

(b) 120

SA ATBL--12h

80 40 0 80

SATBL S L-9h

40 0 80

SATB S BL-6h

40 0 40

10

20

30

40

50

70

(333)

(422)

(400)

60

(331) ( ) (420)

JDC CDS 00-048 0 8-0099 9:Ba2LaF L 7 (311) (222)

(111)

0

(220)

SATB S BL-4h

(200)

All the raw materials (SiO2, AlF3, TiO2, LaF3, BaF2, ErF3, TbF3 and YbF3) are of analytical reagent grade. Glass with composition of 50SiO2–(8.5x)AlF3–30BaF2–5TiO2–5LaF3–xErF3–0.5TbF3–1YbF3 (SABTL-1) (in mol.%, x = 0.05, 0.1, 0.3, 0.5), 50SiO2–(8.8y)AlF3– 30BaF2–5TiO2–5LaF3–0.2ErF3–yTbF3–1YbF3 (SABTL-2) (in mol.%, y = 0.1, 0.3, 0.5, 1.0) and 50SiO2–(9.3z)AlF3–30BaF2–5TiO2– 5LaF3–0.2ErF3–0.5TbF3–zYbF3 (SABTL-3) (in mol.%, z = 1.0, 1.5, 2.0, 2.5, 3.0) were prepared by a conventional melting method. Reagents of SiO2 (99.99%), AlF3 (99.99%), TiO2 (99.99%), BaF2 (99.99%), LaF3 (99.99%), ErF3 (99.99%), TbF3 (99.99%) and YbF3 (99.99%) were used as raw materials. The mixtures (about 10 g), which compacted into a platinum crucible, were set in an electric furnace. After holding at 1450 °C for 45 min under air atmosphere in an electric furnace, the melts were quenched by putting it onto a polished plate of stainless steel. All the glasses were annealed at 540 °C for 4 h to remove thermal strains. To identify the crystallization phase, XRD analysis was carried out with a powder diffractometer (BRUKER AXS GMBH) using Cu Ka radiation. The sizes, shape, structure and component compositions of the asprepared nanocrystals were characterized by transmission electron microscopy (TEM, JEM-2100) at 200 kV. The optical absorption spectrum in the wavelength range of 350–1800 nm was measured on a HITACHI U-4100 spectrophotometer. The UC luminescence spectra of Er3+/Tb3+/Yb3+ co-doped samples under 980 nm laser diode (LD) excitation was measured by using a HITACHI F-7000 fluorescence spectrophotometer in the wavelength range of 400–700 nm. Polished samples glass 50SiO2–8.3AlF3–30BaF2–5TiO2–5LaF3– 0.2ErF3–0.5TbF3–1YbF3 was then heat treated at four different temperatures: 560, 580, 600 and 620 °C, which were selected to carry out heat treatment for 3 h to form transparent glass-ceramics and the fabricated samples were named as SABTL-560, SABTL-580, SABTL-600, SABTL-620. At the same time, the polished samples were selected to carry out heat treatment at 620 °C for different times 4, 6, 9 and 12 h to form transparent glass-ceramics and the fabricated samples were named as SABTL-4 h, SABTL-6 h, SABTL9 h, SABTL-12 h. The samples were cut into the size of 10 mm  10 mm  2 mm and polished for optical measurements. All measurements were performed at the ambient temperatures.

(a)

Intensity (a. u. )

2. Experimental

Intensity (a. u. )

640

80

90

2θ (degree) Fig. 1. XRD patterns of the SABTL glass and glass-ceramics after heat treatment: (a) at 560, 580, 600 and 620 °C for 3 h, (b) at 620 °C for 4, 6, 9 and 12 h.

The TEM micrograph of the SABTL glass-ceramics after heat treatment at 620 °C for 3 h is shown in Fig. 3. It demonstrates that the nanocrystals were distributed homogeneously among the glass matrix and the mean sizes of nanocrystals were about 11.05 nm, which was similar to those calculated by Scherrer equation. The absorption spectra of Er3+/Tb3+/Yb3+ co-doped SABTL glassceramics in the 350–1800 nm regions are shown in Fig. 4. The absorption spectra consisted of absorption band peaks at about 378, 406, 488, 521, 545, 651, 797, 975 and 1529 nm, each of which were corresponding to the transitions from the ground-state 4I15/2, to the excited states 4G11/2, 4F7/2, 2H9/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2 transitions of Er3+ ions, respectively. The band centered at 975 nm can be identified by transitions that originated from the Yb3+ ions ground multiple 2F7/2 to the excited multiple 2F5/2 level. The absorption spectra of Tb3+ ions can be assigned to the transitions from ground-state 7F6 to the excited states 5D3 (378 nm) and 5D4 (484 nm), respectively. The optical image of glass-ceramic’s sample SABTL-620 (inset of Fig. 4c) display good transparency in the visible wavelength range due to the far smaller size of Ba2LaF7 nanocrystals than the visible wavelength. The UC emission spectra of Er3+-doped, Tb3+/Yb3+ co-doped and Er3+/Tb3+/Yb3+ co-doped glass-ceramics samples, which were heattreated at 560 °C for 3 h under 980 nm LD excitation are shown in Fig. 5. As can be observed, in the Er3+-doped glass-ceramics there were three well-known emission bands centered at 525, 546 and 657 nm associated with the transition from 2H11/2, 4S3/2 and 4F9/2

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12

14

(a)

(b)

Crystal size (nm)

Crystal size (nm)

10

8

13

12 1

6

4

11 560

580

600

620

2

4

6

0

8

10

12

Time (h)

T Tem mperaturre ( C)

Fig. 2. Relationship between crystal size with temperature and time in the SABTL glass-ceramics under heat treatment.

3+ 5

3+ 4

3+ 5

Absorption intensity (a. u. )

Er : F7/2

Tb : D4

Tb3+ : 4 D3 Er : G11/2

(c)

SABTL-0.2Er0.5Tb1Yb

3+ 2

Yb : F5/2 3+ 4 Er : I11/2

2

H11/2 2 4

S3/2

F9/2

4

I13/2

4

I9/2

(b)

4

G11/2

(c)

SABTL-0.2Er0Yb

2

H11/2

4

F 4 2 H9/2 7/2 S3/2 F9/2

2

4

I9/2

4

I13/2

4

I11/2

3+ 5

Tb : D3

(a)

(b)

SABTL-0.5Tb0.5Yb

3+ 5

Tb : D4

(a)

3+ 2

Yb : F5/2

400

600

800

1000

1200

1400

1600

1800

Wavelength (nm) Fig. 3. HRTEM image after heat-treated at 620 °C for 3 h. Inset: the selected area electron pattern.

Fig. 4. Absorption spectra of Er3+/Tb3+/Yb3+ co-doped in the SABTL glass-ceramics: (a) SABTL -0.5Tb0.5Yb, (b) SABTL -0.2Er, (c) SABTL -0.2Er0.5Tb1Yb.

levels to ground-state of Er3+ ions [26–28]. The UC emission bands centered at 415, 437, 490, 545, 585 and 622 nm was consistent with: 5D3 ? 7FJ (J = 5, 4) and 5D4 ? 7FJ (J = 6, 5, 4 and 3) transitions of Tb3+ ions, respectively [29]. While in the Er3+/Tb3+/Yb3+ codoped glass-ceramics, all the three fundamental colors red, green and blue were obtained simultaneously. And the UC mechanism is similar: the blue UC emission originating from 5D4 ? 7F6 transition of Tb3+ ions, green UC emission originating from 2H11/2 ? 4I15/ 4 4 3+ ions and 5D4 ? 7F5 transition of 2, S3/2 ? I15/2 transitions of Er Tb3+ ions, the red UC emission mainly related with 4F9/2 ? 4I15/2 transition of Er3+ ions. It is interesting to point out that, the blue and red UC emission of Tb3+ ions were diminished, while the green and red UC emission of Er3+ ions were enhanced [30]. The optical image of UC emission glass-ceramic’s sample SABTL-560 is shown in inset of Fig. 5. Fig. 6 shows the UC emission spectra of the SABTL-0.2Er0.5Tb1Yb glass and glass-ceramics, which were heat-treated at different temperatures for 3 h, under the excitation of 980 nm LD. In

contrast to the slight UC luminescence observed through the precursor glass, the strong UC luminescence of the transitions 5 D3 ? 7F6 (415 nm), 2H11/2 ? 4I15/2 (525 nm), 4S3/2 ? 4I15/2 (546 nm), 4D4 ? 7F4 (585 nm), 4D4 ? 7F3 (622 nm) and 4F9/ 4 3+ 3+ 3+ co-doped were observed in 2 ? I15/2 (657 nm) of Er /Tb /Yb the glass-ceramics. The inset of Fig. 6 shows ratios of the 4F9/ 4 4 4 2 ? I15/2 (red: 657 nm) to S3/2 ? I15/2 (green: 546 nm) UC emission intensities in the SABTL glass-ceramics. From this figure, it can be seen that the ratio (Ired/Igreen) varies from about 0.8 for the SABTL-560 to about 1.1 for the SABTL-620. A pronounced increase in the ratio implies that in the heat treatment temperatures from 560 to 620 °C. The reasons contributing to the increase of ratio (Ired/Igreen) after heat treatment: in the case of the specimens after heat treatment or laser irradiation, since it is known that RE ions practically dispersive into precipitated nanocrystal, RE ions are condensed in the glass-ceramics, so that the distances between RE ions become closer and consequently result in increasing of UC luminescence [26,31].

H.K. Dan et al. / Optical Materials 36 (2014) 639–644

3+ 4

70

4

Er : S3/2→ I15/2

(a) (b) (c) 3+ 5

3+ 4

Er : F9/2→ I15/2 (c)

(c)

(b)

7

7

(a)

(b)

0 450

500

550

600

650

700

Wavelength (nm)

SABTL-Glass SABTL-560 Er3+: 4S →4I 3/2 15/2 SABTL-580 1.1 SABTL-600 SABTL-620 Ratio I Red /IGreen

(a) (b) (c) (d) (e)

300

3+ 2

Er : H11/2→ I15/2

4

1.0

(e) 0.9 0.8 0.7

4

3+ 4

Er : F9/2→ I15/2

560 570 580 590 600 610 620

(d)

0

Temperature ( C)

200 (c) 7

Tb : D4→ F6

Intensity (a. u. )

400

(b)

3+ 5

100

0 450

500

(a)

550

600

4

3+ 4

4

Er : S3/2→ I15/2

(d) (c)

1.14 3+ 2

40

4

Er : H11/2→ I15/2

1.12 1.10 1.08 1.06

(b)

1.04 1.02

30

1.00 4

6

8

10

12

Time (h)

(a)

10

3+ 5

7

Tb : D4→ F6

0

450

500

550

600

650

700

Wavelength (nm)

Fig. 5. UC emission spectra of (a) SABTL-0.2Er, (b) SABTL-0.5Tb0.5Yb and (c) SABTL0.2Er0.5Tb1Yb, which were heat-treated at 560 °C for 3 h.

500

3+ 4

Er : F9/2→ I15/2

20

3+ 5

20

Tb : D4→ F3

(a)

3+ 5

7

Tb : D4→ F6

40

Tb : D4→ F4

3+ 5

SABTL-4h SABTL-6h SABTL-9h SABTL-12h

50

3+ 2

60

(a) (b) (c) (d)

4

4

80

60

7

Tb : D4→ F5 Er : H11/2→ I15/2

Intensity (a. u. )

100

SABTL-0.2Er SABTL-0.5Tb0.5Yb SABTL-0.2Er0.5Tb1Yb

Intensity (a. u. )

120

Ratio I Red /IGreen

642

650

700

Wavelength (nm) Fig. 6. UC emission spectra of SABTL-0.2Er0.5Tb1Yb glass and glass-ceramics, which were heat-treated at different temperatures.

The UC emission spectra of the SABTL-0.2Er0.5Tb1Yb glass-ceramics, which were heat-treated for various times at 620 °C, under the excitation of 980 nm LD are shown in Fig. 7. The UC luminescence intensity of the blue (490 nm), green (525 and 546 nm) and red (657 nm) also increased with the increasing of the heat-treated times. In addition, in the inset of Fig. 7 shows ratios of the 4F9/2 ? 4I15/2 (red: 657 nm) to 4S3/2 ? 4I15/2 (green: 546 nm) UC emission intensities in the SABTL glass-ceramics. From this figure, it can be seen that the ratio varies from about 1.01 for the 4 h to about 1.13 for the 12 h. A pronounced increase in the ratio implies that in the heat treatment time from 4 h to 12 h. The UC mechanism in the Er3+/Tb3+/Yb3+ co-doped glassceramics are schematically depicted in Fig. 8. First of all, the Yb3+ ion was excited by 980 nm LD radiation that corresponds to the 2 F7/2 ? 2F5/2 transition. Then, the ET between Yb3+ and Er3+ ions occur with considerable high efficiency, as here come with the following pair of transitions Yb3+ ions: 2F5/2 ? 2F7/2; Er3+ ions: 4 I15/2 ? 4I11/2. The secondly, either the same Yb3+ ions that absorbs a second 980 nm photon or another nearby Yb3+ ions being still in the 2F5/2 state transfers its energy to the same Er3+ ions. After excited state absorption (ESA), Er3+ ions reaches the 4F7/2 level and then quickly relaxes to the 2H11/2 level with multi-phonon relaxing

Fig. 7. UC emission spectra of SABTL-0.2Er0.5Tb1Yb glass-ceramics, which were heat-treated for various times at 620 °C.

process. From the 2H11/2 level, the Er3+ ions decay radiative to the 4 I15/2 ground-state generating the intense green emission around 525 nm. The major contribution to the red (657 nm) emission is attributed to the 4F9/2 ? 4I15/2 transition. At the same time, one Tb3+ ion in the ground 7F6 state can be exited to 5D4 excited state through cooperative energy transfer (CET) from two Yb3+ ions in 2 F5/2 state [29,32]. Subsequently, the Tb3+ ions in 5D4 level radiatively relax to 7FJ (J = 3, 4 and 5) levels to produce red and green UC emission around 622, 585 and 545 nm, respectively [33]. The CET process can be described as follows: 2F5/2 (Yb3+) + 2F5/2 (Yb3+) + 7F6 (Tb3+) ? 2F7/2 (Yb3+) + 2F7/2 (Yb3+) + 5D4 (Tb3+) (CET). Furthermore, the ET process between Tb3+ and Er3+ ions were also discussed. Fig. 9 shows the UC emission spectra of the SABTL-1 glass-ceramics under the excitation of 980 nm LD. Clearly, in Fig. 9, the UC emission centered at 546 nm (Er3+: 4S3/2 ? 4I15/2) increases significantly with the increasing concentration of Er3+ ions in the SABTL-1 glass-ceramics. In contrast, the UC emission bands around 490 nm (Tb3+: 5D4 ? 7F6), 585 nm (Tb3+: 5 D4 ? 7F4) and 622 nm (Tb3+: 5D4 ? 7F3) originated from Tb3+ ions decreased with increasing molar concentration of Er3+ ions. This phenomenon can be explained by two reasons: First of all, as the molarity of Er3+ ions increased, the increased luminescent centers lead the emission intensity at 546 nm (Er3+: 4S3/2 ? 4I15/2) increased. Secondly, the possible ET from Tb3+ to Er3+ ions, contribute to the emission intensity at 546 nm improved while at 490, 585 and 622 nm decreased. The mechanism of ET from Tb3+ to Er3+ ions was proposed as follows: 5D4 (Tb3+) + 4S3/2 (Er3+) ? 7F6 (Tb3+) + 4I15/2 (Er3+). (ET). A variation of the molar concentration of Tb3+ ions while keeping the concentration of Er3+ ions in the glass-ceramics composition was also given for comparison in the second component of the SABTL-2 glass-ceramics. Fig. 10 shows the UC emission spectra of the SABTL-2 glass-ceramics, which were heat-treated at 620 °C, under the excitation of 980 nm LD. As shown in Fig. 10, the UC emission at 490 nm (Tb3+: 5D4 ? 7F6) was enhanced dramatically with the increasing concentration of Tb3+ ions in the SABTL-2 glass-ceramics. At the same time, the UC emission bands around 546 and 657 nm originated from Er3+ ions also increased (see inset of Fig. 10). The intense and broad green up converted emission bands around 546 nm consists in two contributions: (i) the 4S3/ 4 3+ ions and (ii) the 5D4 ? 7F5 transi2 ? I15/2 transition of the Er tion of the Tb3+ ions. These results implied that ET from Tb3+ to Er3+ ions might be occurred. For the result of Fig. 10, there are

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Fig. 8. Mechanism for UC and ET processes of Er3+/Tb3+/Yb3+ co-doped in SABTL glass-ceramics under 980 nm LD excitation.

7

(d)

(c)

0.8

(d) 0.4

(c) 600

610

620

(b)

7

Wavelength (nm)

(a)

0 500

550

600

650

700

Wavelength (nm) Fig. 9. UC emission spectra of SABTL-1 glass-ceramics.

cross-relaxation (CR) may be occur between two neighboring Er3+ ions [4F9/2–4I13/2]; [4I11/2–4I15/2], the efficiency of ET strongly depends on the distance of two Er3+ ions, the distance becomes contract with Er3+ ions addition and consequently the UC emission intensity of red light at 657 nm (3H4 ? 3H6) became stronger [7,34,35]. The mechanism of CR from Er3+ to Er3+ ions was suggested as follows: 4I11/2 (Er3+) + 4I13/2 (Er3+) ? 4I15/2 (Er3+) + 4F9/2 (Er3+). (CR). The possible ET processes in this system were shown in Fig. 8. Moreover, the role of Yb3+ ions in UC emission of Er3+/Tb3+/Yb3+ co-doped glass-ceramics were also discussed. The UC emission spectra of the SABTL-3 glass-ceramics, which were heat-treated

4

4

80

3+ 2

(b)

3+ 4

Er : S3/2→ I15/2

100

3+ 5

1.2

590

SABTL-0.2Er0.1Tb1Yb SABTL-0.2Er0.3Tb1Yb SABTL-0.2Er0.5Tb1Yb SABTL-0.2Er1.0Tb1Yb

Intensity (a. u.)

7 3+ 5

(a)

3+ 5

50

1.6

0.0 580

Tb : D4→ F6

100

120

(a) (b) (c) (d)

Er : H11/2→ I15/2

3+ 2

4

150

4

Intensity (a. u. )

Er : H11/2→ I15/2

200

Inetnsity (a. u. )

4

3+ 4

Er : F9/2→ I15/2 Tb : D4→ F3

3+ 4

Er : S3/2→ I15/2 Intensity (a. u. )

250

SABTL-0.05Er0.2Tb1Yb SABTL-0.1Er0.2Tb1Yb SABTL-0.3Er0.2Tb1Yb SABTL-0.5Er0.2Tb1Yb

Tb : D4→ F4

140 (a) (b) (c) (d)

60

3+ 4

4

Er : F9/2→ I15/2 120

Blue (I490nm) Green (I546nm)

100

Red (I657nm)

80 60

(d)

40 20 0 0.0

(c) 0.2

0.4

0.6

0.8

1.0

3+

(b)

Concentration Tb (mol %)

40

(a) 3+ 5

7

Tb : D4→ F6

20 0 450

500

550

600

650

700

Wavelength (nm) Fig. 10. UC emission spectra of SABTL-2 glass-ceramics.

at 620 °C, under the excitation of 980 nm LD are shown in Fig. 11. In the UC process, Yb3+ ions act as an efficient sensitizer. With the increase of Yb3+ concentration, the electron population of a Tb3+ ions: 5D4 energy level greatly increases due to more efficient ET from two adjacent Yb3+ ions to one Tb3+ ion [36]. A certain concentration of Yb3+ ion is necessary to ensure this cooperative UC process. However, too high a doping concentration may result in the self-quenching of Yb3+ ions [37,38]. Therefore, the blue, green and red UC emission intensity were increased gradually with the increases of Yb3+ concentration and reaches its maximum at 2.5 Yb3+ concentration, then decreases due to the self-quenching effect, as shown in inset of Fig. 11.

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H.K. Dan et al. / Optical Materials 36 (2014) 639–644

240

(a) (b) (c) (d) (e)

210 3+ 4

3+ 4

180 150

150

Intensity (a. u. )

Intensity (a. u. )

4

Er : S3/2→ I15/2

180

120 3+ 2

References

SABTL-0.2Er0.5Tb1.0Yb SABTL-0.2Er0.5Tb1.5Yb SABTL-0.2Er0.5Tb2.0Yb SABTL-0.2Er0.5Tb2.5Yb SABTL-0.2Er0.5Tb3.0Yb

4

Er : H11/2→ I15/2

90

4

Er : F9/2→ I15/2 I490 nm I546 nm I657 nm

(d)

120

(e)

90 60

(c)

30

(b)

0 1.0

1.5

2.0

2.5

3.0

(a)

3+

Yb concentration (mol %)

60 30 Tb3+: 5D →7F 4 6 0 500

550

600

650

700

Wavelength (nm) Fig. 11. UC emission spectra of SABTL-3 glass-ceramics.

4. Conclusions In this article, the ET and UC emission of Er3+/Tb3+/Yb3+ codoped in the SABTL glass-ceramics containing Ba2LaF7 nanocrystals were successfully investigated by using conventional melting method. The UC luminescence of Er3+/Tb3+/Yb3+ co-doped in the SABTL glass-ceramics has significantly enhanced in comparison with precursor glass before heat treatment. We deem that there was possibly an energy transition process from bands around 490 nm (5D4 ? 7F6), 545 nm (5D4 ? 7F5), 585 nm (5D4 ? 7F4) and 622 nm (5D4 ? 7F3) of Tb3+ ions to bands around 525 nm (2H11/ 4 4 4 4 4 2 ? I15/2), 546 nm ( S3/2 ? I15/2) and 657 nm ( F9/2 ? I15/2) of 3+ Er ions. The intensity of UC emission increases gradually with the increase of Yb3+ concentrations and reaches its maximum at 2.5 mol% Yb3+ concentration, then decreases due to the selfquenching effect. The data presented for this study might provide useful information for further development of the UC glassceramics associated with the ET from Yb3+, Tb3+ to Er3+ ions. Acknowledgments This work was supported by a Grant from the National Natural Science Foundation of China (No. 51272097, No. 61265004 and No. 61307111), the Nature and Science Fund from Yunnan Province Ministry of Education (No. 2011C13211708), Natural Science Foundation of Yunnan Province (2010ZC038), Postdoctoral Science Foundation of China (20110491759), Education Department Foundation of Yunnan Province (2011Y348) and Foundation of Yunnan province (2012FD009).

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