Enhanced blue and green upconversion in hydrothermally synthesized hexagonal NaY1−xYbxF4:Ln3+ (Ln3+ = Er3+ or Tm3+)

Journal of Alloys and Compounds 368 (2004) 94–100

Enhanced blue and green upconversion in hydrothermally synthesized hexagonal NaY1−x Ybx F4 :Ln3+ (Ln3+ = Er3+ or Tm3+) Lifang Liang a,b , Hao Wu a , Haili Hu a , Mingmei Wu a,1 , Qiang Su a,∗ a

State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou, Guangdong 510275, PR China b Department of Chemistry, Guangxi Normal College, Nanning 530001, PR China Received 29 April 2003; received in revised form 21 July 2003; accepted 28 July 2003

Abstract Hexagonal NaY1−x Ybx F4 :Ln3+ (x = 0.05–1.00, Ln3+ = Er3+ or Tm3+ ) have been prepared through hydrothermal synthesis. Powder X-ray diffraction (XRD) patterns have been presented to characterize the synthesized samples. The concentration of doped rare earth and pumping power on the upconversion emissions have been extensively investigated under 980 nm excitation. NaY1−x Ybx F4 :Er3+ emit green (2 H11/2 , 4 S3/2 → 4 I15/2 ), red (4 F9/2 → 4 I15/2 ), weak violet (2 H9/2 → 4 I15/2 ) and weak UV (4 G11/2 → 4 I15/2 ) upconversion emission. NaY1−x Ybx F4 :Tm3+ exhibit intense blue (1 G4 → 3 H6 , 1 D2 → 3 F4 ), weak red (1 G4 → 3 F4 ) and UV (1 D2 → 3 H6 ) upconversion emission. A two-photon process account for the green and red emission of an Er3+ ion with the energy transfer from Yb3+ ion. One blue emission (1 G4 → 3 H6 ) of Tm3+ results from a three-photon excitation, and the other (1 D2 → 3 F4 ) from a four-photon process. The measured data about emission intensity suggest that hexagonal NaYF4 :Yb3+ /Er3+ is a more efficient upconverting phosphor than cubic one. Comparison of the measured properties in our work with values from the reported literature suggests that hydrothermally synthesized NaYF4 :Yb3+ /Er3+ (or Tm3+ ) samples emit more pure green or blue emission than solid-state synthesized samples with certain concentration of doping rare earth ions. © 2003 Elsevier B.V. All rights reserved. Keywords: Phosphor (A); Chemical synthesis (B); Luminescence (D)

1. Introduction Many infrared to visible upconverting rare earth fluoride-based phosphors [1–5], which can convert multiple photons of lower energy to one photon of higher energy, have been well described since first report by Auzel and Ovsyankin et al. in 1966 on the upconversion of Yb3+ /Er3+ , Yb3+ /Tm3+ and Yb3+ /Ho3+ co-doped oxides [6,7]. The interest in both upconversion materials and upconversion processes is fueled by the recent advances in solid-state lasers and semiconductor laser diodes, which have generated more efficient IR sources. The controlled fabrication of a host with aimed structure is important for enhancing upconversion efficiency, because the phonons (or lattice vibration) in material can provide nonradiative decay ways to ∗ Corresponding author. Tel.: +86-20-84111038; fax: +86-20-84111038. E-mail address: [email protected] (Q. Su). 1 Co-corresponding author.

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.07.010

suppress upconversion luminescence. Therefore, it is necessary to choose a lattice with much lower phonon energy to overcome the phonon decay problem. Among the rare earth fluorides, hexagonal NaYF4 showing a high refractive index and low phonon energy [3], has been regarded as an excellent host matrix for performing infrared to visible upconversion as activated by either Yb3+ /Er3+ or Yb3+ /Tm3+ ion pairs [1–4]. Hexagonal structure of NaYF4 has three cation sites, one for rare earth ions (1a), one for both rare earth and sodium ions (1f), and the third for sodium ions (2h). Sites 1a and 1f both have C3h symmetry, whereas site 2 h has Cs symmetry [8]. Recently, the spectroscopic analysis of NaEuF4 [9] and NaYF4 :Pr3+ [10] indicates that sodium ions 2 h site is accommodated by a small proportion of rare earth ions. Both Er3+ and Tm3+ ions illustrating favorable electronic energy level schemes with equally spaced and long-lived excited states and being accessible under near-infrared radiation, are ideally suitable to convert infrared to visible light. The addition of Yb3+ ions as sensitizer to a host lattice can remarkedly enhance the infrared to visible upconversion

L. Liang et al. / Journal of Alloys and Compounds 368 (2004) 94–100

efficiency due to the energy transfer from Yb3+ to Er3+ (or Tm3+ ), which serves as the dominant mechanism. The purity of a NaYF4 -based phosphor and the good homogeneous distribution of Yb3+ and Er3+ or Tm3+ doping ions into the lattice have obvious effects on upconversion properties. NaYF4 was generally prepared via solid-state reaction from stoichiometric mixture of highly purified NaF and YF3 in a sealed tube and/or under HF atmosphere to assure complete fluorination [1–4]. The complex phase diagram [11] in NaF–YF3 system indicates that hexagonal phase is the lower temperature form while cubic phase is the higher temperature form, and it is difficult to obtain well-crystallized hexagonal NaYF4 materials. Therefore, environmentally benign and one-step route to hexagonal NaYF4 should be developed, especially with a simple apparatus and performed under mild condition. The increasing interest in hydrothermal synthesis derives from its advantages in terms of high reactivity of reactants, easy control of solution or interface reaction, formation of metastable and unique condensed phases favored, less air pollution, and low energy consumption. Compared with traditional solid-state synthesis, hydrothermal synthesis has been found to be an effective route for rational synthesis of complex fluorides. Demianets’s group [12] used high temperature (450–550 ◦ C) hydrothermal method to synthesize some single crystals in system AF–BF3 (A = alkaline metal, B = rare earth element). Feng and co-workers exploited a milder hydrothermal route (below 240 ◦ C) to synthesize complex fluorides [13–16]. In present work, both undoped complex fluorides NaLnF4 (Ln3+ = Y3+ , Er3+ –Yb3+ ) and doped NaY1−x Ybx F4 :Ln3+ (Ln3+ = Er 3+ or Tm3+ ) solid solutions with hexagonal phase are prepared under milder hydrothermal condition (below 220 ◦ C). The infrared to visible upconversion property under 980 nm excitation has been studied in NaY1−x Ybx F4 :Ln3+ (Ln3+ = Er3+ or Tm3+ ) samples, particularly, the effect of the hosts, the concentration of doped rare earth ions, and the supplied power to stimulate excitation on the upconversion property have been extensively and comparatively investigated. Some comparisons on upconversion properties between hydrothermally synthesized samples prepared by this work and solid-state synthesized samples reported by others are also provided.

95

under an autogeneous pressure for some time. After the autoclave was cooled and depressurized, the final powder product was washed with deionized water and dried in air at room temperature. 2.2. Characterization All products were characterized by powder X-ray diffraction (XRD) on a Shimadzu XD-3A diffractometer with Ni-filtered Cu K␣ radiation (λ = 1.5406 Å) at room temperature. Infrared to visible upconversion spectra of NaY1−x Ybx F4 :Ln3+ (x = 0.05–1.00, Ln3+ = Er 3+ or Tm3+ ) was obtained by pumping with a 980 nm diode laser and the luminescence emitted from the sample was collected and analyzed by a AB2 luminescence spectrophotometer (Spectronic Inc.).

3. Results and discussion 3.1. Structural investigation Powder X-ray diffraction patterns (Fig. 1) confirm that hexagonal NaLnF4 (Ln3+ = Y3+ , Er3+ –Yb3+ ) and NaY1−x Ybx F4 :Ln3+ (x = 0.05–1.00, Ln3+ = Er 3+ or Tm3+ ) are prepared using hydrothermal synthesis. The dependence of unit cell parameters on Yb3+ concentration in hexagonal NaY1−x Ybx F4 (x = 0.00–1.00) is shown in Fig. 2. The unit cell parameters a (Å), c (Å) and v (unit cell volume) (Å3 ) decrease linearly with the increasing of Yb3+ concentration, obeying Vegard’s rule [17]. The result reveals that Y3+ and Yb3+ ions can incorporate completely into hexagonal NaY1−x Ybx F4 lattice at any concentration by hydrothermal method. As mentioned above, Yb3+ /Ln3+ ions (Ln3+ = Er3+ or Tm3+ ) can be easily codoped into NaYF4 lattice, and the Er3+ or Tm3+ can be easily doped into NaYbF4 lattice, either, through the milder hydrothermal route.

(e) (d)

2. Experimental

(c)

2.1. Hydrothermal synthesis

(b)

All complex fluorides, undoped NaLnF4 (Ln3+ = Er3+ –Yb3+ ) and doped NaY1−x Ybx F4 :Ln3+ (x = 0.05–1.00, Ln3+ = Er3+ or Tm3+ ) were prepared via hydrothermal synthesis. The reactant mixture with a molar ratio of 3.00 NaF:1.00 Ln(NO3 )3 :6.00 NH4 HF2 :700 H2 O was sealed in a 23 ml Teflon-lined stainless steel autoclave (Parr-type, 80% charged, pH 3.0–3.5) and heated at 220 ◦ C

(a)

Y3+ ,

20

30

40

50

60

70

2 theta (deg.)

Fig. 1. XRD patterns of hexagonal (a) NaY0.94 Yb0.05 Er0.01 F4 , (b) NaYF4 , (c) NaYbF4 , (d) NaErF4 and (e) NaTmF4 synthesized at 220 ◦ C for 2 days.

96

L. Liang et al. / Journal of Alloys and Compounds 368 (2004) 94–100

v

109

(e)

107 105

Intensity (a.u.)

(d)

c

3.51 3.49 3.47

(c) (b)

6.00

a

(a) 5.94 5.88 0

20

80

60

40

100

3+

450

550 Wavelength (nm)

650

Yb mol% Fig. 2. Unit cell parameters a (Å), c (Å) and v (unit cell volume) (Å3 ) vs. Yb3+ concentration in hexagonal NaY1−x Ybx F4 (x = 0.00–1.00).

3.2. Upconversion for NaY1−x Ybx F4 :Er3+

4

G11 / 2- I15 / 2

4

400

420 (nm)

4

F9 / 2 - H15 / 2

380

4

2

4 4

H9 / 2- I15 / 2

2

Intensity (a.u.)

4

H11 / 2 , S3 / 2- I15 / 2

The upconversion of 980 nm radiation on hexagonal NaY1−x Ybx F4 :Er3+ (x = 0.10–1.00) produces four emission bands of Er3+ ions from UV to visible range (Fig. 3). The strong green emissions at 520.6, 528.6, 539.0, 545.6 and 549.6 nm are typically assigned to the (2 H11/2 , 4 S3/2 ) → 4I 15/2 transitions and the weaker red emission at 652.2 nm corresponds to 4 F9/2 → 4 I15/2 transition. A much weaker violet emission at 408.4 nm attributed to 2 H9/2 → 4 I15/2 electronic transition is observed, especially in the amplified inserted figure. A trace of UV emission around 381 nm due to 4 G11/2 → 4 I15/2 transition is also obviously shown in the inserted Fig. 3. As illustrated in Fig. 4, the emission intensities in the upconversion spectra of hexagonal NaY0.94 Yb0.05 Er0.01 F4 are increased slightly with an increase of reaction time up to about 2 days. Further extendance of reaction rarely reduces the intensities.

(A) (B)

400

600 Wavelength (nm)

800

Fig. 3. Upconversion emission spectra of hexagonal (A) NaY0.79 Yb0.20 Er0.01 F4 and (B) NaYb0.99 Er0.01 F4 under 980 nm excitation.

Fig. 4. Upconversion emission intensity vs. reaction time in the synthesis of hexagonal NaY0.94 Yb0.05 Er0.01 F4 : (a) 6.0 h; (b) 12 h; (c) 48 h; (d) 5 days; (e) 15 days (under 980 nm excitation).

A comparison between the emission spectra of hexagonal NaY0.78 Yb0.20 Er0.02 F4 sample synthesized by hydrothermal method in our present work and NaY0.8 Yb0.18 Er0.02 F4 sample synthesized by solid-state method in previously published work [4] indicates that the green emission is dominant in the former and the red emission is prominent in the latter. The different upconverting properties between the two samples synthesized by different methods are surely attributed to their purities. The solid-state synthesized sample often contains YOF:Yb3+ /Er3+ phosphor and/or other oxide impurities with higher phonon energy, which make multiphonon relaxation (4 S3/2 → 4 F9/2 ) much more favorable and results in the increase of the population on 4F 9/2 level. The disadvantages in solid-state synthesis for a complex fluoride indicate that the controlled and rational fabrication of hexagonal NaYF4 :Yb3+ /Er3+ at a much lower reaction temperature for a reasonable reaction time are quite significant. The mild hydrothermal synthesis used in our present work is proved to meet this aim. The effect of Er3+ concentration on the emission intensities of hexagonal NaY0.80−x Yb0.20 Erx F4 and NaYb1.00−x Erx F4 (x = 1, 2, 3, 4 and 6%) is shown in Fig. 5. The green emission intensity decreases with the increase of Er3+ concentration, due to the interaction between neighboring Er3+ ions. The optimum concentration of Er3+ in NaY0.80−x Yb0.20 Erx F4 for intense green emission is at 1.0–3.0 mol%. This is in good agreement with that reported previously to obtain the highest output of the green luminescence for its practical application [1–3]. The decreasing in green transition becomes more obvious than that in red emission at a higher Er3+ concentration as shown in Fig. 5I. The relative ratio of green and red emission intensity with integrated area (fg/r ) clearly decreases as Er3+ concentration increases (Fig. 5II). For example, the ratio fg/r is approximately 5.0:1.0 in NaY0.79 Yb0.20 Er0.01 F4 (1.0 mol% Er3+ ), while it is about 2.0:1.0 in NaY0.74 Yb0.20 Er0.06 F4 (6.0 mol% Er3+ ) (Fig. 5II(A)). Based on those results, it

L. Liang et al. / Journal of Alloys and Compounds 368 (2004) 94–100

(I) ( B)

( A)

97

( II )

5.0

(e) 4.0

Intensity (a.u.)

(d)

(A) 3.0

fg / r

(c) (b)

2.0

(a)

1.0

(B)

0.0 550

650

550

0

650

1

2

3 Er

Wavelength (nm)

3+

4

5

6

( mol %)

Fig. 5. (I) Upconversion emission intensity and (II) green to red emission intensity ratio (fg/r ) vs. Er3+ concentration in hexagonal (A) NaYF4 :Yb3+ /Er3+ (20 mol% Yb3+ ) and (B) NaYbF4 :Er3+ . (a) 1.0 mol% Er3+ ; (b) 2.0 mol% Er3+ ; (c) 3.0 mol% Er3+ ; (d) 4.0 mol% Er3+ ; (e) 6 mol% Er3+ (under 980 nm excitation).

can be confirmed that higher Er3+ concentrations will cause concentration quenching. In order to obtain sufficient absorption of infrared radiation, the addition of Yb3+ ions is necessary in the infrared to visible upconversion. The emission intensity of green transition is more prominent than that of red transition at lower Yb3+ concentration (Figs. 3 and 6). The dependence of emission intensity on Yb3+ concentration in the hexagonal NaY0.98−x Ybx Er3+ 0.02 F4 (x = 5, 10, 20, 30, 40 and 50%) is described in Fig. 6. With the addition of Yb3+ into the hexagonal host lattice the emission intensity of green transition becomes weaker while the intensity of red transition becomes stronger (Fig. 6I). For example, the relative intensity ratios (fg/r ) of green and red emission in hexagonal NaY0.93 Yb0.05 Er0.02 F4 (5% Yb3+ ) and NaY0.48 Yb0.50 Er0.02 F4 (50% Yb3+ ) are approximately 6.0:1.0 and 1.0:1.0, respectively (Fig. 6II). The presence of more Yb3+ ions in lattice makes the distance between Yb3+

and Er3+ ions to be shorter, resulting in energy-back-transfer from Er3+ to Yb3+ ions. The cross-relaxation diminishes the population in (2 H11/2 , 4 S3/2 ) levels and enhances the population in 4 F9/2 level, and then results in the decrease of green emission and the increase of red emission. To better understand the procedure of upconversion by which the (2 H11/2 , 4 S3/2 ), 4 F9/2 , 2 H9/2 and 4 G11/2 excited states are populated following 980 nm irradiation, a power dependence study of the emission intensity (Iem ) versus pump power (Ip ) is performed. Relation Iem ∝ Ipn exists in upconversion processes, in which n is the number of pumping photons required to populate the exciting state of a rare earth ion. The result from a fit of the curve log(Ip ) versus log(Iem ) is shown in Fig. 7. For hexagonal NaY0.79 Yb0.20 Er0.01 F4 , the slopes of green, red, violet and UV emission are 2.20, 1.96, 3.10 and 3.30, respectively. Based on experimental data, it is confirmed that both green and red emission emit from a two-photon excitation of Er3+

(I)

6

(f)

5

(e)

Intensity (a.u.)

( II )

4

(d)

fg / r

3

(c)

2

(b) 1

(a) 450

550 Wavelength (nm)

650

0 0

10

20

30

40

50

3+

Yb m o l %

Fig. 6. (I) Upconversion emission intensity and (II) green to red emission intensity ratio (fg/r ) vs.Yb3+ concentration in hexagonal NaYF4 :Yb3+ /Er3+ (2% Er3+ ). (a) 5 mol% Yb3+ ; (b) 10 mol% Yb3+ ; (c) 20 mol% Yb3+ ; (d) 30 mol% Yb3+ ; (e) 40 mol% Yb3+ ; (f) 50 mol% Yb3+ (under 980 nm excitation).

L. Liang et al. / Journal of Alloys and Compounds 368 (2004) 94–100

G4- H6

4.0

3

G4- F4

1

1

1

Intensity (a.u.)

violet n=3.1

Log (Iem) (a.u.)

1

3

D2- H6

green n=2.2

3

3

5.0

D2- F4

98

x10 (B)

UV n=3.3

red n=1.96

(A)

3.0 3.3

2.9 3.1 Log (Ip) (mW)

Fig. 7. Dependences of upconversion emission intensities (Iem ) on pumping power (Ip ) for hexagonal NaY0.79 Yb0.2 Er0.01 F4 sample excited at 980 nm.

ion, and violet and UV emission from a three-photon process. It is undoubtedly that the upconversion mechanism in NaY1−x Ybx F4 :Er3+ (x = 0.10–1.00) is mainly an energy transfer between Yb3+ and Er3+ ions under the excitation of 980 nm diode laser. The upconversion procedure is schematically illustrated in Fig. 8. 3.3. Upconversion for NaY1−x Ybx F4 :Tm3+ The upconversion emission spectra of the hexagonal NaY0.796 Yb0.200 Tm0.004 F4 and NaYb0.996 Tm0.004 F4 under 980 nm excitation (Fig. 9) are similar to those reported previously for Yb3+ /Tm3+ codoped fluoride lattices [5]. In addition to two obvious blue emissions centered at 450.4 nm (1 D2 → 3 F4 ) and 475.6 nm (1 G4 → 3 H6 ), two UV emission peaks at 346.4 and 363.0 nm (1 D2 → 3 H6 ) and a much weaker red Tm3+ emission at 650 nm (1 G4 → 3 F4 ) are also observed. The intensity of emissions depends on the population on 1 G4 and 1 D2 levels. For a Tm3+ ion, a three-step energy transfer process to populate 1 G4 and a four-step

700

500 Wavelength (nm)

300

2.7

Fig. 9. Upconversion emission spectra of hexagonal (A) NaY0.796 Yb0.20 Tm0.004 F4 and (B) NaYb0.996 Tm0.004 F4 under 980 nm excitation.

process to populate 1 D2 are probably the most important mechanism. The upconversion processes are depicted as the right scheme in Fig. 8. According to Fig. 8 and the relation Iem ∝ Ipn , slopes of n = 3 and 4 are expected for the 1 G4 and 1 D2 emission transitions. The experimental data are provided in Fig. 10 and further confirm the right scheme showing the upconversion process. For hexagonal NaY0.796 Yb0.20 Tm0.004 F4 , the slopes of blue emission (1 D2 → 3 F4 ) and UV emission (1 D2 → 3 H6 ) are 3.8 and 4.1, in agreement with a four-photon procedure. However, the slope of 2.3 of blue emission (1 G4 → 3 H6 ) has a deviation from the expected three-photon model. Similar deviation has also been observed in other Yb3+ /Tm3+ system as reported in literature [5,18,19]. When the Yb3+ concentration is fixed at 20 mol%, the emission intensity of one blue transition (1 G4 → 3 H6 ) is strong and that of the other blue transition (1 D2 → 3 F4 ) is weak (Fig. 11I(A)). By increasing the Tm3+ content up

5.2 1

2

4

H9/2

4

1 2

4

S3/2 4 F9/2 4 I9/2 4 I11/2

4

H11/2

3

2 3

2

I13/2

1

F2

3

2

H4

3

3+

Yb

F7/2

3 3+

3

D 2 - H 6 n=4.1

4.6

1

4.0

3

G 4- H 6 n=2.3

1

3

D 2- F 4 n=3.8

H5

F4

1

I15/2 3+

F5/2

3

F3

G4

3

2

Er

D2

4

3

F7/2

4

1

G11/2

Log (Iem) (a.u.)

G7/2,9/2

980 nm

2

H6

Tm

Fig. 8. Simplified energy level diagram for hexagonal NaY1−x Ybx F4 :Ln3+ (x = 0.05–1.00, Ln3+ = Er 3+ or Tm3+ ) system. Upward and downward solid arrows indicate photon absorption and emission, respectively.

3.4 2.70

2.90

3.10

3.30

Log (I p) (mW) Fig. 10. Dependences of upconversion emission intensities (Iem ) on pumping power (Ip ) for hexagonal NaY0.796 Yb0.20 Tm0.004 F4 sample excited at 980 nm.

L. Liang et al. / Journal of Alloys and Compounds 368 (2004) 94–100

(I) (f)

Intensity (a.u.)

(e) (d) (c) (b) (a) 450

550 450 Wavelength (nm)

( II )

Integrated area of blue emission (a.u.)

( B)

( A)

(B)

(A)

0.4

0.0

550

99

0.8

3+

Tm mol%

Fig. 11. Upconversion emission intensity (I) and integrated area of blue emission (II) vs. Tm3+ concentration in hexagonal (A) NaYF4 :Yb3+ /Tm3+ (20 mol% Yb3+ ) and (B) NaYbF4 :Tm3+ . (a) 0.05 mol% Tm3+ ; (b) 0.1 mol%Tm3+ ; (c) 0.2 mol% Tm3+ ; (d) 0.4 mol% Tm3+ ; (e) 0.6 mol% Tm3+ ; (f) 0.8 mol% Tm3+ (under 980 nm excitation).

(I)

Intensity (a.u.)

(e) ( d) (c) ( b) (a) 400

500 600 Wavelength (nm)

→ 3 F4 ) is enhanced considerably as increasing content from 10 to 30 mol%, the further increase of Yb3+ concentration from 30 to 50 mol% yields a trivial decrease of blue intensity (Fig. 12II). Comparing the upconversion spectra of hexagonal NaY0.796 Yb0.200 Tm0.004 F4 and NaYb0.996 Tm0.004 F4 (Fig. 9), we find the blue emission (1 D2 → 3 F4 ) and the UV emission (1 D2 → 3 H6 ) become relatively obvious in hexagonal NaYb0.996 Tm0.004 F4 . We agree with the view [18,19] that the excitation is mobile within the NaYbF4 lattice, it could thus readily reach the Tm3+ traps, which cause the high relative intensity of the two blue emissions (1 G4 → 3 H6 , 1 D2 → 3 F4 ) and the UV emission. All described above suggests that the Yb3+ concentration has a great effect on the intensity of blue and UV transitions, but a small effect on the red emission. When comparing the upconversion spectra of hexagonal NaY0.799 Yb0.20 Tm0.001 F4 synthesized via hydrothermal method with that of NaY0.729 Yb0.27 Tm0.001 F4 synthesized via solid-state method from literature [4], it is found that the emission intensity of red transition is very weak in 3H

6, Yb3+

Integrated area of blue emission (a.u.)

to 0.4 mol%, the intensity of both blue transitions (1 G4 → 3 H , 1 D → 3 F ) are enhanced. Further increase of Tm3+ 2 4 6 content results in a slight decrease of both the blue emission intensities (Fig. 11II(A)). In hexagonal NaYbF4 :Tm3+ (Fig. 11B), 1 D2 → 3 F4 transition becomes intense with the increase of Tm3+ concentration. The variation of blue emission intensity with increase of Tm3+ content is similar to that of NaY0.80−x Yb0.20 Tmx F4 (x = 0.05, 0.1, 0.2, 0.4, 0.6 and 0.8%). These phenomena can be attributed to concentration quenching of Tm3+ . The excess of Tm3+ ions causes the self-interaction between neighboring Tm3+ ions and results in a smaller population on 1 G4 and 1 D2 levels. Eventually, emission intensities of 1 G4 → 3 H6 and 1 D2 → 3 F4 transitions are diminished. In the same way, at Tm3+ concentration of 0.2 mol%, the emission intensity of 1 D2 → 3 F4 transition increase with the Yb3+ concentration from 10 to 50 mol%, but the emission intensity of 1 G4 → 3 H6 transition has an optimum at 30 mol% of Yb3+ content (Fig. 12I). On the other hand, the integrated area of emission intensity of blue transitions (1 G4 →

700

1D

2

( II )

10

20

30

40

50

3+

Yb mol%

Fig. 12. Upconversion emission intensity (I) and integrated area of blue emission (II) with various Yb3+ concentrations in hexagonal NaYF4 :Yb3+ /Tm3+ (0.2 mol% Tm3+ ). (a) 10 mol% Yb3+ ; (b) 20 mol% Yb3+ ; (c) 30 mol% Yb3+ ; (d) 40 mol% Yb3+ ; (e) 50 mol% Yb3+ (under 980 nm excitation).

100

L. Liang et al. / Journal of Alloys and Compounds 368 (2004) 94–100

the former, but the emission intensity of red transition is dominant in the latter. The reason may be the existence of impurity, especially oxides, in the product synthesized by solid-state method. Since the energy transfer involves many non-resonant steps between Yb3+ and Tm3+ ions, the phonons of host materials may play an important role in the blue and red emissions. Based on the analysis stated above, we find that the concentration quenching of Tm3+ is obvious, but the concentration quenching of Yb3+ cannot be observed obviously. The red emission is quite weak in all doping rare earth concentration range. When the concentration of Yb3+ ions is equal to or greater than 20 mol% and that of Tm3+ ions is between 0.2 and 0.6 mol%, the blue emissions are strong and the UV emission enhances with increasing Yb3+ concentrations. When controlling Yb3+ concentration between 20 and 30 mol%, the NaY1−x Ybx F4 :Tm3+ samples reveal more pure blue emissions. Therefore, the hexagonal NaY1−x Ybx F4 :Tm3+ samples synthesized by hydrothermal method are especially attractive for the simple preparation and their strong and nearly pure blue emission with certain concentration of doping rare earth ions.

4. Conclusion In summary, we have reported on the investigation of the near-infrared excited upconversion emission in hydrothermally synthesized NaY1−x Ybx F4 :Ln3+ (x = 0.05–1.00, Ln3+ = Er 3+ or Tm3+ ) samples. Green, red, violet and UV emissions in NaY1−x Ybx F4 :Er3+ (x = 0.05–1.00) have been observed. In the hexagonal NaYF4 :Yb3+ /Er3+ , the green emission is considerably strong and the other emissions are weak at a Yb3+ concentration in the range from 5.0 to 20 mol% and Er3+ concentration from 1.0 to 3.0 mol%. The green emission is obviously diminished in both NaYbF4 :Er3+ and NaYF4 :Yb3+ /Er3+ with a much higher Yb3+ content. Power-dependent study reveals that both green and red emissions result from a two-photon process of Er3+ . The strong blue emission and weak red emission are observed in both hexagonal NaYF4 :Yb3+ /Tm3+ and NaYbF4 :Tm3+ samples (Yb3+
20 mol%, Tm3+ = 0.2–0.6 mol%). For NaY1−x Ybx F4 :Tm3+ (x = 0.10–1.00), one blue emission (1 G4 → 3 H6 ) arises from a three-photon process and the other (1 D2 → 3 F4 ) arises from a four-photon excitation. Comparison with the solid-state synthesized NaYF4 :Yb3+ /Ln3+ (Ln3+ = Er 3+ or Tm3+ ) from literature, which display significant red emission because of

the existence of some oxide impurity, it is believed that hydrothermal synthesis can provide us with a benign and one-step route to obtain hexagonal form which demonstrates strong and much more pure green or blue emission with certain concentration of doping rare earth ions. As NaYbF4 :Ln3+ reveals similar upconversion properties to NaYF4 :Yb3+ /Ln3+ (Ln3+ = Er3+ or Tm3+ ), we believe that NaYbF4 will be as good a host as NaYF4 for upconversion materials. Acknowledgements The authors acknowledge Provincial Natural Science Foundation of Guangdong Province in China for financial support (No. 21423452), and gratefully thank the Yangjiang Rare Earth Factory for providing rare earth oxides. References [1] T. Kano, H. Yamamoto, Y. Otomo, J. Electrochem. Soc. 119 (11) (1972) 1561–1564. [2] N. Menyuk, K. Dwight, J.W. Pierce, Appl. Phys. Lett. 21 (1972) 159–161. [3] A. Bril, J.L. Sommerdijk, A.W. De Jager, J. Electrochem. Soc. 122 (5) (1975) 660–663. [4] R.H. Page, K.I. Schaffers, P.A. Waide, J.B. Tassano, S.A. Payne, W.F. Krupke, J. Opt. Soc. Am. B 15 (3) (1998) 996–1008. [5] L.F. Johnson, H.J. Guggenheim, T.C. Rich, F.W. Ostermayer, J. Appl. Phys. 43 (1972) 1125–1137, 2357–2365. [6] V.V. Ovsyankin, P.P. Feofilov, Sov. Phys. JETP Lett. 4 (1966) 317. [7] F. Auzel, C.R. Acad. Sci. 262 (1966) 1016; F. Auzel, C.R. Acad. Sci. 263B (1966) 819. [8] J.H. Burns, Inorg. Chem. 4 (6) (1965) 881–886. [9] D. Zakaria, R. Mahiou, D. Avignant, M. Zahir, J. Alloys Compd. 257 (1997) 65–68. [10] N. Martin, P. Boutinaud, M. Malinowski, R. Mahiou, J.C. Cousseins, J. Alloys Compd. 275–277 (1998) 304–306. [11] R.E. Homa, G.M. Hebert, H. Insley, C.F. Weaver, Inorg. Chem. 2 (5) (1963) 1005–1012. [12] L.N. Demianets, Prog. Cryst. Growth Charact. 21 (1990) 299. [13] C.-Y. Zhao, S.-H. Feng, R.-R. Xu, C.-S. Shi, J.Z. Ni, Chem. Commun. 10 (1997) 945–946. [14] X.M. Xun, S.H. Feng, J.Z. Wang, R.R. Xu, Chem. Mater. 9 (1997) 2966. [15] X.M. Xun, S.H. Feng, J.Z. Wang, R.R. Xu, Mater. Res. Bull. 33 (1998) 369–375. [16] S.H. Feng, R.R. Xu, Acc. Chem. Res. 34 (2001) 239–247. [17] L. Vegard, Z. Phys. 5 (1921) 17. [18] T. Riedener, H.U. Gudel, G.C. Vally, R.A. McFarlane, J. Lumin. 63 (1995) 327. [19] M. Dulick, G.E. Faulkner, N.J. Cockroft, D.C. Nguyen, J. Lumin. 48–49 (1991) 517.