A novel orange emitting BaS:xYb2+ phosphor for white light LEDs

A novel orange emitting BaS:xYb2+ phosphor for white light LEDs

Materials Research Bulletin 51 (2014) 202–204 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.c...

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Materials Research Bulletin 51 (2014) 202–204

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

A novel orange emitting BaS:xYb2+ phosphor for white light LEDs Yanmin Yang *, Xianyuan Su, Xiaodong Li, Fang Yu, Chao Mi, Gang Li Luminescence and Display Research Institute, College of Physics Science and Technology, Hebei University, Baoding 071002, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 October 2013 Received in revised form 26 November 2013 Accepted 15 December 2013 Available online 19 December 2013

A series of Yb2+-doped BaS orange emitting phosphors were synthesized by solid-state method. The crystal structure and optical properties were investigated. The results indicate that BaS:xYb2+ phosphors can be efficiently excited by both UV light and blue-LED chips to effectuate orange emitting which corresponding to the allowed 4f–5d transition of Yb2+ ions. The optimized Yb2+ concentration was 0.1 mol%. The preferable excitation spectrum profile and the intense emission show that BaS:xYb2+can be used as a potential orange emitting phosphor for UV, blue light exciting white light LEDs. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Optical materials D. Optical properties

1. Introduction Rare earth (RE) ions have unique optical, electric and magnetic properties due to their electronic configuration of 4fn5dm6s2 (n = 1–14, m = 0–1). In recent decades, RE ions doped luminescence materials have attracted great attention due to their practical and potential applications in many fields such as display devices, lighting and laser sources, etc. [1–6]. It is generally accepted that the RE ions are usually present in luminescence materials in their trivalent state. However, it is known that some RE ions, such as Sm, Eu and Yb, are divalent rather than trivalent in luminescence materials. It is based on the fact that the optical spectra of divalent RE ions, usually arising from 4f–5d electronic transitions, differ greatly from those of trivalent RE which are ascribed to 4f–4f transitions. The emission spectrum of the former is characterized by broad bands, whereas that of the latter consists of several sharp lines [7]. In recent years, the luminescence properties of Eu2+ ions have been extensively investigated in many luminescence materials and well understood. Compared with Eu2+ ions, the luminescence spectra of Yb2+ have been less reported in the literatures. The luminescence spectra of Yb2+ have been reported on alkaline earth halides [8,9], fluorides [10], sulfates [11], nitric oxide [12,13], phosphates [14], and so on. However, the luminescence spectra of Yb2+ doped in sulfide have not been reported yet. Yb2+ doped luminescence materials have greater potential value in application than Eu2+ doped luminescence materials, because Yb2O3 is much cheaper than Eu2O3.

* Corresponding author. Tel.: +86 15930284830; fax: +86 312 5079423. E-mail address: [email protected] (Y. Yang). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.12.016

In this paper, we report a series of novel orange emitting phosphors BaS:xYb2+. The structure and the optical properties of BaS:xYb2+ were investigated. 2. Experimental The BaS:xYb2+ (x = 0, 0.07, 0.1, 0.2, 0.5, 1 mol%) phosphors were synthesized by solid-state method. The starting components BaSO4 and Yb2O3 were adequately mixed and ground in an agate mortar. Then the mixtures were sintered at 1250 8C for 3 h in a reduction atmosphere (10% H2 + 90% N2). Then cool down to room temperature (RT) spontaneously, and pulverize for further measurements. The crystal structures of the samples were monitored by X-ray phase analysis on a Bruker D8 X-ray diffractometer. The detailed photoluminescence (PL) emission and excitation spectra were measured on FLS920 steady state and transient fluorescence spectrometer. The absorption spectrum was obtained by U-4100 Spectrophotometer (Solid). All the above were carried out at RT. 3. Results and discussion The XRD patterns of BaS samples with different Yb2+ doped concentration, as well as the standard pattern were shown in Fig. 1(a). No impurity phase has been observed in these samples, which clearly indicating that the obtained samples are single phase and the dopants Yb2+ do not significantly influence the structure of the host in our experimental range. Fig. 1(b) shows the schematic of the crystal. In the structure of BaS only one type of Ba2+ lattice site exists in this host, which is coordinated with six sulfur atoms. Therefore, it is expected that the Yb2+ ions doped in this host will

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Fig. 1. (a) XRD patterns of BaS:xYb2+ and (b) space group of BaS.

substitute for the Ba2+ ions and exhibit only one type of the lattice position. The PL emission and excitation spectra of BaS:0.1% Yb2+ were presented in Fig. 2. The excitation spectrum monitored at 617 nm covers from 200 to 500 nm, and consists of three bands around 277, 347 and 434 nm. This indicates that the phosphor can be excited by UV-LED and blue LED chips, which is necessary for improving the quality of W-LED. Upon 277, 347 and 434 nm light excitation respectively, the emission spectrum exhibits a broad band extending from 500 to 700 nm with a peak at 617 nm, which can be attributed to the typical 4f–5d transition of Yb2+. The inset was the digital photograph of the obtained phosphor BaS:0.1% Yb2+ under the excitation of 365 nm. The phosphor appears orange light when excited at 365 nm light. The emission international chromaticity coordinates (x, y) of the phosphor were (x = 0.586, y = 0.386). This phosphor can be used as near-UV, blue light exciting luminescent materials. A model of the excitation and emission mechanism is presented in Fig. 3 as followed. The Yb2+ in the 4f14 ground state is excited by the three, above mentioned excitation bands (see Fig. 2) in the 4f135d main-fold, which is partially located in the conduction band. After non-radiative relaxation within the 5d state, emission peak takes place at 617 nm, which agrees with the literature [15].

With doping concentration of Yb2+ increasing from 0.07 mol% to 1 mol%, the highest luminescence intensity has been achieved at 0.1 mol% Yb2+ as shown in Fig. 4. The concentration quenching occurs when Yb2+ concentration is larger than 0.1 mol%. The concentration quenching mechanism is generally associated with non-radiative energy transfer [16–18]. According to the Dexter theory, the emission intensity (I) per activator ion follows the equation [19] I u=3 1 ¼ K½ð1 þ bxÞ  x

(1)

where x is the activator concentration and I/x is the emission intensity (I) per activator concentration (x); K and b are constants for the same excitation condition for a given host. According to Eq. (1), u = 6, 8, and 10 for dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The inset of Fig. 4 shows I/x depending upon the x on a logarithmic scale. The dependence of log ðI=xYb2þ Þ vs. log ðxYb2þ Þ is quite linear and the slope (u/3) was 1.9713. Thus the value of u can be calculated as 5.91, which is approximately equal to 6. It indicates that the concentration quenching mechanism of Yb2+ emission was responsible for the electric dipole–dipole interaction which agrees with the literature [8].

300 λem = 617 nm

250

comduction band

λex = 434 nm

Intensity/a.u.

λex = 347 nm λex = 277 nm

200

5d

150 100

434 nm

347 nm 277 nm

617 nm

50 4f

0 200

Emission

Excition

300

400

500

600

Wavelength/nm Fig. 2. The PL emission and excitation spectra of BaS:0.1 mol%Yb2+.

700

Valence band Fig. 3. The luminescent mechanism for BaS:Yb2+.

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8 BaS:Yb

(αhν) 1/2/(a.u.)

6

4

2

3.0

3.2

3.4

3.6

3.8

4.0

hν (ev) Fig. 4. The emission spectra (lex = 434 nm) of BaS:xYb2+ phosphors for various Yb2+concentrations (x = 0.07, 0.1, 0.2, 0.5, 1 mol%). The inset shows the curve of log ðI=xYb2þ Þ on log ðxYb2þ Þ in the BaS:xYb2+ phosphors according to Eq. (1).

While discussing the mechanism of energy transfer in phosphor, it is possible to calculate the critical distance (Rc) by the critical concentration (xc) and the number of cation (N) present in the unit cell [20]. In this case, the xc is the optimized concentration of Yb2+ ion. The critical distance (Rc) obtained from the experimental data of concentration quenching is represented as  1=3 3V Rc  2 4pxc N

(2)

Taking the values of V (0.260 nm3), N (4), and xc (0.1), the Rc turned to be 1.1 nm. The absorption spectrum in the ultraviolet to the visible region of BaS:Yb2+ has been obtained, which was used to calculate the optical energy gap Eg of the semiconductor. For the absorption by indirect transitions, the energy gap is estimated according to Eq. (3) [21]. 2



Aðhn  Eg Þ hn

(3)

where a is the absorption coefficient, A is a constant, Eg is the optical energy gap and hn is the photon energy of the incident radiation. The values of (ahn)1/2 are plotted as a function of the incident photon energy (hn) as illustrated in Fig. 5. According to the liner extrapolation of (ahn)1/2 = 0, the Eg was estimated to be about 3.57 eV. It is similar to that of BaS:Bi (4.25–4.28 eV) [22]. 4. Conclusions In summary, we have successfully synthesized the BaS:xYb2+ phosphors, and their luminescence performance were investigated. According to the absorption spectra, the energy gap (Eg) was estimated to be about 3.57 eV. The broad excitation band and intense emission upon near-UV and blue light excitation indicate

Fig. 5. Plot for (ahn)1/2 as a function of the incident photon energy for the BaS:Yb2+.

that BaS:xYb2+ can be a good orange emitting phosphor candidate for creating white light in white LEDs. Acknowledgements This work was supported by National Science Foundation of China (No. 50902042), China Postdoctoral Science Foundation (No. 20100480840) and Natural Science Foundation of Hebei Province (No. E2010000283). References [1] H.M. Zhu, R.F. Li, W.Q. Luo, X.Y. Chen, Phys. Chem. Chem. Phys. 13 (2011) 4411–4419. [2] C.H. Huang, T.M. Chen, J. Phys. Chem. C 115 (2011) 2349–2355. [3] C.M. Zhang, S.S. Huang, D.M. Yang, X.J. Kang, M.M. Shang, C. Peng, J. Mater. Chem. 20 (2010) 6674–6680. [4] X.N. Fang, H.W. Song, L.P. Xie, Q. Liu, H. Zhang, X. Bai, J. Chem. Phys. 131 (2009) 054506. [5] D.S. Kang, H.S. Yoo, S.H. Jung, H.K. Kim, D.Y. Jeon, J. Phys. Chem. C 115 (2011) 24334–24340. [6] W.R. Liu, C.H. Huang, C.W. Yeh, J.C. Tsai, Y.C. Chiu, Y.T. Yeh, Inorg. Chem. 51 (2012) 9636–9641. [7] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. [8] F.C. Palilla, B.E. O’Reilly, V.J. Abbruscato, J. Electrochem. Soc. 117 (1970) 87–91. [9] H.S. Yoo, S. Vaidyanathan, S.W. Kim, D.Y. Jeon, Opt. Mater. 31 (2009) 1555–1558. [10] S. Lizzo, A. Meijerink, G.J. Dirksen, G. Blase´, J. Lumin. 63 (1995) 223–234. [11] S. Lizzo, A. Meijerink, G. Blase´, J. Lumin. 59 (1994) 185–194. [12] A. Kirakosyan, A. Mnoyan, S.H. Cheong, G.Y. Lee, D.Y.J. Jeon, Solid State Sci. Technol. 2 (2013) R5–R8. [13] R.J. Xie, N. Hirosaki, M. Mitomo, J. Phys. Chem. B 109 (2005) 9490–9494. [14] Y.L. Huang, P.J. Wei, S.Y. Zhang, H.J. Seo, J. Electron. Soc. 158 (2011) H465. [15] M. Henke, J. Perbon, S. Ku¨ck, J. Lumin. 87–98 (2000) 1049–1051. [16] Y. Tian, B.J. Chen, B.N. Tian, R.N. Hua, J. Alloys Compd. 509 (2011) 6096–6101. [17] Y. Tian, B.J. Chen, R.N. Hua, N.S. Yu, CrystEngComm 14 (2012) 1760–1769. [18] B.N. Tian, B.J. Chen, Y. Tian, X.P. Li, J. Mater. Chem. C 1 (2013) 2338–2344. [19] D.L. Dexter, J.H. Schulman, Chem. Phys. 22 (1952) 63–70. [20] A. Meijerink, J. Nuyten, G. Blasse, J. Lumin. 44 (1989) 19–31. [21] Y.L. Wang, S.X. Dai, F.F. Chen, T.F. Xu, Q.H. Nie, Mater. Chem. Phys. 113 (2009) 407–411. [22] S. Surender, K. Ravi, S. Nafa, J. Alloys Compd. 509 (2011) L81–L84.