A single-phase, color-tunable, broadband-excited white light-emitting phosphor Y2WO6: Sm3+

Journal of Luminescence 146 (2014) 33–36

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A single-phase, color-tunable, broadband-excited white light-emitting phosphor Y2WO6: Sm3 þ Zhongfei Mu a,b, Yihua Hu b,n, Li Chen b, Xiaojuan Wang b, Guifang Ju b, Zhongfu Yang b, Yahong Jin b a b

Experimental Teaching Department, Guangdong University of Technology, Guangzhou 510006, PR China School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 4 March 2013 Received in revised form 7 September 2013 Accepted 18 September 2013 Available online 25 September 2013

Un-doped and Sm3 þ doped Y2WO6 were synthesized with solid state reactions. X-ray diffraction measurements show that all the samples present single-phase Y2WO6. The combination of blue emission in a broad band from the host and several groups of emission lines in yellow, orange, and red spectral region results in a desired white light. The chromaticity coordinates and color temperature can be tuned by changing the doping concentration of Sm3 þ . The mechanisms of energy transfer were analyzed. The present results prove Sm3 þ doped Y2WO6 is a promising single-phase, color-tunable, broadband-excited white light-emitting phosphor based on AlGaN-based ultraviolet light-emitting diodes. & 2013 Elsevier B.V. All rights reserved.

Keywords: Y2WO6: Sm3 þ Energy transfer White light-emitting

1. Introduction Today's commercial phosphor-converted white light-emitting diodes (LEDs) normally use three combinations, i.e. a 450–470 nm blue LED chip and a yellowish phosphor Y3Al5O12: Ce3 þ (YAG: Ce), an ultraviolet (UV) LED chip and tri-color phosphors, an UV LED chip and a single-phase white light-emitting phosphor. The first approach suffers some pitfalls such as poor color rendering index and low stability of color temperature [1–5]. The characters of obtained white light are related to blue LED chip and phosphor. Thus deterioration of the chip or phosphor can cause significant color change. The second approach could conquer the pitfalls encountered by the first one because predominant components of white light do not come from LED chip but phosphors. However, the mixing of tri-color phosphors does not only bring many intractable technical problems but also increase manufacturing cost [6–8]. Therefore, more and more attentions have been paid to single-phase white light-emitting phosphors because of their simple fabrication process [9–18]. Three situations exist for the achievement of white light with a single-phase phosphor combined with a UV LED. Firstly, two or more activators are doped into a suitable host. White light is composed of different components from these activators. The most commonly used activator combinations are Eu2 þ –Mn2þ [9,10], Ce3 þ –Eu2 þ [11,12], Ce3 þ –Tb3 þ [13,14], Ce3 þ –Dy3 þ –Mn2 þ [15,16],

n

Corresponding author. Tel.: þ 86 20 39322262; fax: þ86 20 39322265. E-mail address: [email protected] (Y. Hu).

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.09.043

and Eu2 þ –Eu3 þ –Tb3 þ [17,18]. This situation generally involves energy transfer (ET) between different activators. Secondly, only one activator is used. White light is composed of multi-color emissions of this activator. Eu3 þ and Tb3 þ can play such a role [19,20]. Low doping concentration of activator and low phonon energy in the host are required in this situation. Otherwise nonradiative relaxation between neighboring energy levels would quench the emission from excited states with higher energy (5D3, 5D2, 5D1 for Eu3 þ and 5D3 for Tb3 þ ). The last situation which is seldom considered is the combination of the emissions from both a host and an activator. A typical example is Sr2CeO4: Eu3 þ , which can emit white light under UV excitation [21,22]. In this system, charge transfer (CT) emission from the host (Ce–O CT state) overlaps a large part of visible spectrum region and emissions from residual region are fulfilled by Eu3 þ . However, emissions from Eu3 þ are mainly composed of several sharp lines in red spectral region from 590 to 620 nm due to 5D0–7F2, 1 transitions [23]. It will be better if the activator can give out multi-color emissions, such as Sm3 þ with multi-color luminescence in yellow (562 nm, 4G5/2-6H5/2), orange (601 nm, 4G5/2-6H7/2), and red (644 nm, 4G5/2-6H9/2) regions [24–27]. Therefore, the present study focuses on Y2WO6: Sm3 þ . Crystal structure of Y2WO6 has been investigated for a long period since 1960s. However, it still has not been understood comprehensively until now. According to relevant literatures, yttrium tungstates with the composition of Y2WO6 have mainly fallen into several crystallographic forms: monoclinic P21/m, orthorhombic P212121, etc. Beaury et al. synthesized Y2WO6 at high temperature in 1978. The authors indicate that their Y2WO6 was orthorhombic with a space group of P212121 [28]. Bye et al.

34

Z. Mu et al. / Journal of Luminescence 146 (2014) 33–36

reported the luminescence properties of Eu3 þ in monoclinic Y2WO6 and found that there are three nonequivalent Y3 þ sites in the host [29]. Luminescence of Eu3 þ in different Y3 þ sites is site-selective. The energy can only be transferred from the WO66  complexes to Eu3 þ on the C2 sites. This process was performed by quadrupole–quadrupole interaction. Tian et al. prepared Eu3 þ doped and MoO66  substituted Y2WO6 with solid state reactions and investigated the luminescence properties of these phosphors [30]. Recently, Huang et al. reported a novel low temperature polymorph Y2WO6 with tetragonal structure [31]. Wang et al. prepared metastable Y2WO6: Ln3 þ (Ln¼Eu, Er, Sm and Dy) microspheres via hydrothermal method. In their reports, strong multicolor visible light emission under UV–visible light excitation of Ln3 þ was referred simply [32]. In this study, we synthesized pure and Sm3 þ doped Y2WO6 with solid state reactions and investigated the structure and luminescence properties of these phosphors. Our results show that prepared phosphors belong to monoclinic form and show intense white light emission due to ET from W–O complexes to doped Sm3 þ . Mechanism of the ET in Y2WO6: Sm3 þ was discussed in detail.

2. Experimental All the investigated phosphors were synthesized with solid state reactions. Raw materials WO3 (A.R.), Y2O3 (4 N) and Sm2O3 (4 N) were weighed according to the composition of Y2  xWO6: Sm3x þ (x ¼0, 0.01, 0.03, 0.05, 0.07, and 0.09). The weighed materials were ground for 1 h and sintered at 1350 1C in a tubal furnace for 6 h. To be clear, obtained samples are abbreviated to be YWO, YWO–Sm0.01, YWO–Sm0.03, YWO–Sm0.05, YWO–Sm0.07 and YWO– Sm0.09 according to the content of Sm3 þ . The phase identification of prepared phosphors was carried out by a Beijing MSAL-XD-2 X-ray powder diffractometer (XRD). The photoluminescence (PL) and PL excitation (PLE) spectra of the phosphors were measured with a Hitachi F-7000 fluorescence spectrophotometer. All the measurements were performed at room temperature (RT).

3. Results 3.1. Structure and morphology To check the phase purity of the samples, XRD measurements were performed. As examples, the XRD patterns of un-doped YWO and YWO–Sm0.09 with biggest doping concentration of Sm3 þ were plotted in Fig. 1. The ICSD cards #20955 was also exhibited in Fig. 1 for comparison [33]. It can be observed in Fig. 1 that XRD patterns

of two typical samples are consistent with the ICSD card #20955. This indicates that expected single phase YWO has been obtained in this work. Effective ionic radii of involved cation in our experiments are as 3þ 6þ follows: R(Y3þ 8 )¼ 0.1019 nm, R(Sm8 )¼0.1079 nm, and R(W6 )¼ 0.060 nm [34], the subscript here denotes coordination number (CN) of the cation. According to the difference of radii and charge numbers among these ions and the ratio of raw materials, it is assumed that most of Sm3þ come into polyhedron with CN¼ 8 to replace Y3 þ . Results of XRD measurements show that all the Sm3 þ doped YWO samples present single phase (Fig. 1). That is to say, though 4.5% Y3þ was replaced by Sm3 þ , these doped ions do not change the phase structure of the host. This observation indicates that most of Sm3þ come into Y3 þ sites according to our anticipation.

3.2. Luminescence and ET 3.2.1. YWO Fig. 2 shows the PLE and PL spectra of un-doped YWO. Exciting sample with 280 or 308 nm, both obtained PL spectra are broad bands from 350 to 600 nm with a maximum at 450 nm. A broad band with an obvious saddle peaking at 280 and 308 nm can be observed in the PLE spectrum when emission at 450 nm is monitored. Generally, broad bands in the excitation spectra of un-doped tungstates such as CaWO4 can be ascribed to W–O CT states [35,36]. However, some researchers think that the electronic states will be dispersed into the energy bands with the width of a few eV. As a rule 2p electronic states of oxygen are dispersed into the valence band with some admixture of 5d electronic states of tungsten. 5d electronic states of tungsten mainly form the conduction band of the crystal. To be precise, these broad bands can be attributed to host absorption or the excitation of W–O complexes. Fig. 3 shows the PLE spectra of samples YWO–Smx (x ¼0, 0.01, 0.03, 0.05, 0.07, and 0.09) monitoring the emission at 613 nm, which is the typical emission of Sm3 þ transition from 4G5/2 to 6H7/ 2 [24–27]. All the spectra were composed of two parts, broad bands from 200 to 350 nm and some emission peaks from 350 to 550 nm. The latter can be attributed to f–f transition excitation of Sm3 þ . The broad bands from 200 to 350 nm can be attributed to host absorption for its similarity with PLE spectrum of YWO (Fig. 2). According to the difference of ion radii and charge number, Sm3 þ replaces Y3 þ in the lattice. We do not exclude the existence of CT excitation of Sm–O complexes. But it may be so weak that it cannot be found in the PLE spectra. The appearance of host absorption in the PLE spectra of Sm3 þ indicates the ET from the host to Sm3 þ . Fig. 4 shows the PL spectra of samples YWO–Smx (x¼ 0, 0.01, 0.03, 0.05, 0.07, and 0.09) excited by 308 nm. These spectra are λem=450 nm

intensity(a.u.)

intensity(a.u.)

YWO-Sm0.09

YWO

λex=308 nm

(a)

(b)

ICSD cards #20955

10

20

30

40

50

60

70

2θ (°) Fig. 1. Comparison of XRD patterns of un-doped YWO and YWO–Sm0.09 with ICSD card #20955.

200

300

400

500

600

wavelength(nm) Fig. 2. PLE spectrum monitoring the emission at 450 nm (a) and PL spectrum excited with 308 nm (b) of un-doped sample YWO.

Z. Mu et al. / Journal of Luminescence 146 (2014) 33–36

Table 1 The CIE chromaticity coordinates and color temperature of phosphors.

x=0.03

intensity(a.u.)

Phosphor x=0.01

Excitation wavelength CIE chromaticity (nm) coordinates

x=0.05 x=0.07

YWO–Sm0.01 YWO–Sm0.03 YWO–Sm0.05 YWO–Sm0.07 YWO–Sm0.09

x=0.09

250

300

350

35

400

450

500

308 308 308 308 308

x

y

0.226 0.253 0.277 0.304 0.318

0.190 0.203 0.227 0.251 0.267

Color temperature (K)

8979 6687

550

wavelength(nm) Fig. 3. PLE spectra monitoring the emission at 613 nm of Sm3 þ in samples YWO– Smx (x ¼0.01, 0.03, 0.05, 0.07, and 0.09).

x=0.01

intensity(a.u.)

x=0.03 x=0.05 x=0.07 x=0.09

350

400

450

500

550

600

650

Fig. 5. Color points corresponding to phosphors with different doping concentration of Sm3 þ in CIE 1931 diagram.

wavelength(nm) Fig. 4. PL spectra of samples YWO–Smx (x ¼0.01, 0.03, 0.05, 0.07, and 0.09) excited with UV light with a wavelength of 308 nm.

composed of two parts, i.e. broad band from 350 to 550 nm with a maximum at 450 nm and three groups of emission peaks from 550 to 680 nm. Origination of both parts is indubitable, i.e. the former from the emission of W–O complexes and the latter from doped Sm3 þ . The appearance of Sm3 þ emission peaks in the PL spectra of samples excited by 280 nm proves that there exactly exists ET from W–O complexes to Sm3 þ , since light with this wavelength cannot directly excite Sm3 þ but W–O complexes.

4. Discussion 4.1. Generation of white light PL spectra of Sm3 þ doped samples cover most of visible light region from 380 to 680 nm(Fig. 4). It makes samples emit white light with different color coordination. The color coordination and relevant color temperature of different samples are calculated with PL spectrum data and listed in Table 1. The corresponding color points are plotted in CIE 1931 diagram (Fig. 5). It is observed that color points gradually come into white light region with the increment of Sm3 þ content. This indicates that white light can be obtained by combining blue emission from W–O complexes and yellow, orange, red emissions from Sm3 þ . The relative color temperature of YWO–Sm0.09 is calculated to be 6687 K. In recent years, blue emitting GaN-based LEDs (450–470 nm) and near ultraviolet (UV) emitting InGaN-based LEDs (370– 410 nm) have obtained extensively applications in lighting and display. However, the past twenty years have also witnessed the development of AlGaN-based UV LEDs with their emitting

wavelength shorter than 365 nm (band gap of GaN). Han and co-workers manufactured the first UV LED with an emitting wavelength 353.6 nm in 1998 [37]. In 2002, the first UV LED with wavelength 285 nm was made out by Adivarahan et al. [38]. Subsequently, UV LEDs with emitting wavelengths 280, 269 and 265 nm were accomplished one by one [39–41]. In 2011, Pernot et al. reported on the fabrication and characterization of high efficiency UV LEDs with emission wavelength ranging from 255 to 355 nm. External quantum efficiencies (EQEs) over 3% were obtained for all the investigated wavelengths with maximum value reaching 5.1% for 280 nm LED. By using enhanced light extraction technologies, such as, moth-eye structure on the back side of the sapphire substrate, we expect to improve these values by up to 50% [42]. These UV LEDs cannot be commercialized now for their lower quantum and luminous efficiency. However, their appearance makes it possible that phosphors with their excitation bands locating in UV region shorter than 365 nm are combined with UV LEDs to accomplish a desired visible light. Sm3 þ doped YWO can be efficiently excited by UV light with a wavelength of 308 nm and emit desired white light. Thus, it will be a promising single-phase, color-tunable, broadband-excited white lightemitting phosphor for AlGaN-based UV LEDs.

4.2. Mechanism of ET from CTS to Sm3 þ There are many excitation peaks from 350 to 500 nm in the PLE spectrum of Sm3 þ doped samples monitoring the emission at 613 nm (Fig. 3). These excitation peaks are mostly covered by the PL spectrum of W–O complexes under the 308 nm excitation (Fig. 2). The mostly covering of the PL spectrum of W–O complexes to the PLE spectrum of Sm3 þ provides advantageous condition to the resonance ET from W–O CT complexes to Sm3 þ .

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Z. Mu et al. / Journal of Luminescence 146 (2014) 33–36

Resonance ET can be performed by exchange interaction or multi-pole interaction. The former has two conditions. One is that the PL spectrum of donor overlaps the PLE spectrum of acceptor. The other is that the critical distance of the two (R0) is enough small (generally smaller than 0.7 nm [43–45]). In YWO, average length of Y–O and W–O bonds is 0.1876 and 0.2326 nm. From the ratio of raw materials and XRD measurement results, Sm3 þ replaces Y3 þ . If we suppose each Sm3 þ just obtain energy from W–O complexes which is nearest to itself, the biggest distance from donor (W–O complexes) to acceptor (Sm3 þ ) is 0.4202 nm. This value is smaller than 0.7 nm. So ET from W–O complexes to Sm3 þ mainly occurs by exchange interaction. 5. Conclusions A series of un-doped and Sm3 þ doped YWO were synthesized with solid state reactions. Luminescence properties of prepared phosphors were investigated in detail. XRD measurements show that all the samples present single phase YWO. The doping of Sm3 þ with a small content does not change the phase structure of host YWO. Un-doped YWO emits blue light in a broad band peaking at 450 nm excited with 280 or 308 nm. ET from the host to Sm3 þ was observed in Sm3 þ doping samples. The combination of blue emission in a broad band from the host and the several groups of emission lines in yellow, orange and red spectral region results in a desired white light. The color temperature can be tuned by the doping concentration of Sm3 þ . ET from the host to Sm3 þ is mainly performed by exchange interaction. The present results prove Sm3 þ doped YWO is a promising single-phase, colortunable, broadband-excited white light-emitting phosphor for AlGaN-based UV LEDs. Acknowledgements This research was supported by the National Natural Science Foundations of China (Grant Nos. 21271049). References [1] C.C. Lin, Y.S. Zheng, H.Y. Chen, et al., J. Electrochem. Soc. 157 (2010) II900. [2] K. Li, C.Y. Shen, Optik 123 (2012) 621.

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