JOURNAL OF RARE EARTHS, Vol. 34, No. 1, Jan. 2016, P. 21

Foundation item: Project supported by Fundamental Research Grant Scheme of Malaysia (J130000.2526.03H97) and  the National Natural Science Foundation of China (51372142)

* Corresponding author: LIU Hong (E-mail: hongliu@sdu.edu.cn; Tel.: + 86-531-88362807)

DOI: 10.1016/S1002-0721(14)60573-1

Effects of Eu3+ and Dy3+ doping or co-doping on optical and structural properties of BaB2Si2O8 phosphor for white LED applications

LEOW Tingqiao (廖侹乔)1, LIU Hong (刘 宏)2,*, R. HUSSIN1, Z. IBRAHIM1, K. DERAMAN1, H.O. LINTANG3, W.N.W SHAMSURI1

(1. Phosphor Research Group, Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, Skudai 81310, Johor, Malaysia; 2. State Key Labora-tory of Crystal Materials, Shandong University, Jinan 250100, China; 3. Catalytic Science and Technology Research Group, Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, Skudai 81310, Johor, Malaysia)

Received 27 April 2015; revised 26 October 2015

Abstract: A series of Eu3+ and Dy3+ doped/co-doped as well as un-doped BaB2Si2O8 phosphors were synthesized via solid state reac-

tion method. The PL result showed typical blue and green emission from Dy3+ and red emission from Eu3+. The f-f transitions in-volving the lanthanide ions along with dopant site occupancy were discussed thoroughly. Phonon assisted energy transfer process was observed from Eu3+ to Dy3+, which enhanced the emissions of Dy3+. Combinations of the emissions from Eu3+

and Dy3+ showed a possible white to red tuneable emission on the CIE diagram. The white warmth emissions of the phosphor were revealed to be ad-justable through designing the dopant concentration and excitation wavelengths. An unusual energy transfer that originated from Eu3+ to Dy3+ was also discovered and the energy transfer mechanism was discussed. Proposed energy transfer mechanism was investigated using luminescence decay lifetime. All the phosphor exhibited efficient excitation in the UV range which matched well with the emis-sions from GaN-based LED chips. This presented the BaB2Si2O8 phosphor as a promising candidate for white LED applications. The ef-fects of doping on the structural properties and the optical band gap of BaB2Si2O8 phosphor were also discussed in this study.

Keywords: BaB2Si2O8:Eu3+/Dy3+; phosphor; energy transfer; rare earths

The development of white light emitting diode (LED) has drawn increasing interest in the past few years for its potential applications in solid state lighting, i.e., flat panel display and illuminations systems[1–4]. A consider-able amount of studies revolving LED were carried out majorly due to its high energy efficiency, long usable lifetime, compact size and environmental friendliness[5,6]. Nowadays, commercially available white LEDs were produced from InGaN-based LED chips coated with red/ green/blue (RGB) phosphor[7,8]. However, such white LED suffers a major drawback in low color rendering index due to the lack of red component[9]. Widely avail-able (Y,Gd)BO3:Eu3+ red phosphor has a chief disadvan-tage in low efficiency[10]. Therefore, the search for an in-organic red emitting phosphor is ongoing. Also, the distinct emission of Dy3+ in the blue and green regions can serve as a key component for white light produc-tion. With the introduction of AlGaN and InGaN LED chips, the band gap of the devices can be tuned from 3.2 to 6 eV by varying the content ratio of Al/In to GaN[3, 7, 8]. This in turn produces emissions from 200 to 400 nm wavelength that benefits lanthanide based phosphor for its ability to be efficiently excited in the UV regions.

BaB2Si2O8 which is also named as maleevite is an analogous species of danburite (CaB2Si2O8) and pekovite (SrB2Si2O8). BaB2Si2O8 is a good candidate for phosphor host materials owing to its colorless transmitted light, clarity, hardness (Mohs hardness=7) as well as chemi-cally and physically stable properties[11]. As far as cost concerned, the abundance of precursor material of B2O3, BaCO3 and SiO2 together with the low sintering tem-perature give researchers and manufacturers an edge on cost effectiveness. Lately, Wang et al. and Saradhi et al. reported that BaB2Si2O8 and SrB2Si2O8 as phosphor ma-terials showed promising results[12,13]. Nevertheless, based on extensive review in literature, the properties of Dy3+ doped and Eu3+/Dy3+ co-doped BaB2Si2O8 had never been investigated before.

With these motives in mind, the discovery of a novel and inexpensive BaB2Si2O8 phosphor is needed urgently. Therefore, the optical and structural properties of Eu3+ and Dy3+ singly doped and Eu3+/Dy3+ co-doped BaB2Si2O8 were studied in this article.

Additionally, the energy transfer mechanism of co- doped system had always been of high interest among phosphor researchers[14,15]. In many cases, the role of co-doping Dy3+ usually resulted in an enhanced lumi-

22 JOURNAL OF RARE EARTHS, Vol. 34, No. 1, Jan. 2016

nescence of Eu3+ [16–19]. However, in this study we dis-covered an out of norm energy transfer that originated from Eu3+ to Dy3+. This in turn demoted the lumines-cence of Eu3+ and enhanced Dy3+ emission instead. Therefore the energy transfer mechanisms were also in-vestigated in this study.

1 Experimental

Phosphor samples of BaB2Si2O8 doped/co-doped by 1 mol% Eu3+ and 1 mol.% Dy3+ were synthesized via solid state reaction. Precursor material with high purity was comprised of BaCO3 (Sigma Aldrich, 98%), H3BO3 (Qrec, 99.8%), SiO2 (Sigma Aldrich, 99%), Eu2O3 (Sigma Aldrich, 99.9%) and Dy2O3 (Sigma Aldrich, 99.9%). The raw materials were weighed in stoichiomet-ric balance to achieve the doping concentration as per Table 1. To obtain a homogeneous mixture, the raw ma-terials were mixed in a mortar and ball-milled and then dry-pressed to promote intimate contact. The mixtures were calcinated at 350 ºC for 45 min then sintered at 950 ºC for 5 h in air atmosphere. Overall white samples were ground for various characterizations. Eu3+ and Dy3+ sin-gly doped phosphor were denoted as BBSE and BBSD in this article, while co-doped were BBSED and un-doped BaB2Si2O8 as BBS.

To determine the structural properties, the samples were investigated by X-ray diffraction (XRD) on a PANalytical X’Pert PRO MRD PW3040 operating at 40 kV tube voltage and 35 mA tube voltage current using Cu Kα (λ=0.15406 nm) radiation source. Molecular vi-brational studies using infrared (IR) absorptions were performed on a Perkin Elmer Frontier FTIR spectrometer occupying KBr method. Elemental studies using energy dispersive X-ray (EDX) were collected using a Hitachi TM3000 tabletop SEM. For optical properties determi-nation, the diffused reflectance spectra in Ultravio-let-visible (UV- VIS) region of the phosphor were col-lected from a Shimadzu 3101-PV UV-Vis spectroscope from 200 to 800 nm range using BaSO4 as white refer-ence. Photoluminescence (PL) emission and excitation spectra, as well as PL decay curve were performed by a JASCO FP-8500 Series fluorescence spectrometer with a 150 W xenon arc lamp source. All measurements were performed at room temperature.

2 Results and discussion

2.1 X-ray diffraction

Fig. 1 exhibits the XRD patterns of Eu3+ and Dy3+ doped/co-doped BaB2Si2O8 samples. Basically, all sam-ples were identified to possess phases of BaB2Si2O8 that matched well with Crystallographic Open Database (C.O.D.) card No. 96-900-4836. The BaB2Si2O8 carries an orthorhombic structure that is a member of Pnma (62) space ground. However, few peaks due to SiO2 interme-diate phases also appeared. Intensities of peaks that cor-respond to BaB2Si2O8 increased in doped/co-doped sam-ples. In addition, the intermediate phases also began to diminish, indicating that a more stable structure was formed. The crystallite sizes of the samples are listed in Table 1, which are calculated using Scherrer’s equa-tion: R=0.9λ/βcosθ (1) where R is the crystallite size, λ is the wavelength of the X-ray radiation, θ is the diffraction angle and β is the in-strument corrected full width half maximum (FWHM). The corrected FWHM was obtained by determining the instrument broadening effect using an NIST Si sample (SRM 640). The peak near 24.41º that corresponds to (2 1 1) plane was used to calculate the crystallite size owing to its intense peak and definition, which minimized peak overlapping. The results showed that the crystallite size was 34.56 nm for un-doped sample and increased to 62.90, 60.23, and 63.01 nm for Eu3+ and Dy3+ singly

Fig. 1 XRD patterns of Eu3+ and Dy3+ doped/co-doped

BaB2Si2O8 samples

Table 1 Crystallite size, band gap and chromaticity coordinate of doped, co-doped and un-doped samples

Chromaticity coordinate Lifetime, τ/ms

Ex=390 nm Ex=350 nm Denotation Phosphor composition Crystallite size,

R/nm

Band gap

Eg/eV x y x y

Em=613 nm Em=575 nm

BBS BaB2Si2O8 34.56 3.42 – – – – – –

BBSD Ba0.99B2Si2O8:0.01Dy3+ 62.90 3.51 – – 0.3467 0.4061 - 0.3873

BBSE Ba0.99B2Si2O8:0.01Eu3+ 60.23 3.49 0.6170 0.3495 – – 2.218 –

BBSED Ba0.98B2Si2O8:0.01Dy3+,0.01Eu3+ 63.01 3.6 0.5399 0.3701 0.4237 0.3882 1.554 0.4773

LEOW Tingqiao et al., Effects of Eu3+ and Dy3+ doping or co-doping on optical and structural properties of … 23

doped and co-doped samples, respectively. This was as-signed to a better crystallization of BaB2Si2O8 causing a larger crystallite size formation. Improvement in crystal-lization and structure stability could be considered to be the flux effect of Eu3+ and Dy3+ acting as the role of promoting efficient incorporation of BaO, B2O3, and SiO2 to form the host lattice[20].

2.2 IR absorption

IR absorption spectra of singly doped, co-doped and un-doped BaB2Si2O8 are shown in Fig. 2. All spectra ap-peared to be very similar. The overall IR absorption spectra can be sorted by separating into three regions. Firstly, the 1300–1800 cm–1 region that two narrow ab-sorption bands resided. Secondly, the occurrence of in-tense broad band coupled with few narrow bands in the region of 800 to 1300 cm–1. Lastly, the region between 400 to 800 cm–1 that contained multiple sharp weak bands. The strong narrow band near 1638 cm–1 was the result of vibrations from hydroxyl group, probably the moisture absorbed by KBr. Two strong broad bands around 1093 and 1150 cm–1 were ascribed to the asym-metric stretching vibrations of B–O and Si–O–Si linkage from their tetrahedral unit respectively[13,21]. Weak nar-row bands situated around 423, 516 and 1398 corre-sponded to the vibrations of metal cations, which was Ba2+ in this case[22,23]. Intense absorption band near 1017 cm–1 was assigned to the stretching modes of non- bridging oxygen in [SiO4]–1 and B–O bond in [BO4]–1 units. Bending vibrations of B–O bond in [BO4]–1 units were observed in the intense broad band around 868 and 963 cm–1 [13,21,24,25]. The weak narrow bands centered at 628 and 734 cm–1 corresponded to B–O–B bending vi-bration of [BO4]–1 units[13]. While the narrow band re-sided around 674 cm–1 was ascribed to the vibrations of Si–O–B linkage[13,24]. The relatively stronger narrow band near 590 cm–1 was assigned to vibrations from Si–O–Si link[26]. The narrow band located near 465 cm–1 was at-tributed to the overlapped bending vibrations of Si–O–Si,

Fig. 2 IR absorption spectra of Eu3+ and Dy3+ doped/co-doped

BaB2Si2O8 samples

O–Si–O and B–O–B from their tetrahedral units corre-spondingly[13,21,24]. Throughout the absorption spectra, many vibrational modes of [SiO4]–1 and [BO4]–1 tetrahe-dral group superimposed upon each other owing to their likeness in molecular structure. On the other hand, no absorptions were detected within 1200 to 1350 cm–1 range which usually resembles the vibrational modes of trigonal [BO3]–2 units[27]. This indicated that no trigonal borate units existed in the host structure, which was ex-pected for that the borate units in BaB2Si2O8 were in tet-rahedral form.

2.3 Energy dispersive X-ray

To verify the incorporation of Eu3+ and Dy3+ ions in the BaB2Si2O8 host, EDX was performed as is shown in Fig. 3. The dopings of Eu2+ and Dy3+ were confirmed from the peaks of their corresponding atoms, along with the Ba, Si, B and O atoms from the host lattice.

2.4 Diffused reflectance spectra

Normalized diffused reflectance spectra of Eu3+ and Dy3+ singly doped/co-doped BaB2Si2O8 phosphor col-lected in the UV-VIS range are exhibited in Fig. 4. All spectra showed a rapid reduction in reflectance near 360 nm and achieved minima around 240–250 nm. These

Fig. 3 EDX analysis of Eu3+ and Dy3+ singly doped/co-doped

BaB2Si2O8 samples

24 JOURNAL OF RARE EARTHS, Vol. 34, No. 1, Jan. 2016

Fig. 4 Normalized diffused reflectance spectra and Kubelka-

Munk functions of Eu3+ and Dy3+ singly doped/co- doped BaB2Si2O8 samples

reductions in reflectance were attributed to the absorption of the BaB2Si2O8 host lattice. In order to investigate the optical band gap of the prepared samples, the spectra were converted to an absorption-equivalent spectra using Kubelka-Munk function. The equation of the function is as follows[28]: F(R∞)=(1–R∞)2/2R∞ (2) where R∞ is the reflectance of an infinitely thick sample with respect to the probed wavelength. The Kubelka- Munk converted function is shown in the inset of Fig. 4.

Subsequently, Tauc’s method was employed to esti-mate the band gap using the relation[29] F(R∞)hν=A(hν–Eg)n (3) lnF(R∞)hν=lnA(hν–Eg)n (4) lnF(R∞)hν=nln(hν–Eg)+lnA (5) where A is a constant, Eg is the optical band gap energy and the value of n is 1/2 or 2 for direct allowed and indi-rect allowed transitions respectively. To determine the type of transition that took place, the logarithm of Eq. (3) was taken. Then, by plotting a linear function of lnF(R∞)hν=ln(hν), the value of n was determined from the gradient of the linear plot. The values of n for all spectra were determined to be 2, which corresponded to indirect allowed transitions. Finally, the band gaps were determined by plotting [F(R∞)hν]1/n=hν, found in the in-tersection point between a fitted baseline and the ex-trapolation from the linear part of the function. The men-tioned plots are displayed in Fig. 5.

The intercepting points gave the band gap energies of SBS, SBSD, SBSE and SBSED to be 3.42, 3.51, 3.49 and 3.60 eV respectively. The Eg values are also summa-rized in Table 1. Blue-shift in band gap was observed in lanthanide doped/co-doped BaB2Si2O8 compared to un-doped. The coordination numbers of Ba2+ in malee-vite were determined to be 7 and 10 in the report by Pautov et al.[11]. The ionic radii of seven coordinated Ba2+, Eu3+ and Dy3+ ions were 138, 101 and 97 pm respec-tively[30]. The lanthanide ions were predicted to replace

Fig. 5 Plots of [F(R∞)hv]1/2 vs hv of Eu3+ and Dy3+ singly

doped/co-doped BaB2Si2O8 samples Ba2+ ions but not B3+ and Si4+ ions. Two Eu3+/Dy3+ would replace three Ba2+ to compensate the charge dif-ference. The B3+ and Si4+ ionic radii (11 and 26 pm re-spectively for CN=4 in tetrahedral) differed from Ba2+ ionic radii to a further extent than the lanthanide ions, hence they had a very low tendency of substitutions. The increase in band gap was attributed to the Burstein-Moss effect, an increase in number of charge carriers occurs when Ba2+ ions were substituted by Eu3+ and Dy3+ ions[31,32]. The Fermi level was then elevated to higher energy due to excess of charge carriers. The substitutions of Ba2+ by Eu3+ and Dy3+ ions were common due to their similarity in ionic radius. Furthermore, the band gap of the Dy3+ doped phosphor appeared to be blue-shift rela-tive to Eu3+ doped, which was ascribed to the larger dopant ionic radius of Dy3+ [33]. In the co-doped samples, the combined effect of both dopants led to the widest band gap among prepared samples.

On the other hand, another possible explanation for the blue-shift in band gap can be related with the increase in crystallite size. As seen in Table 1, the band gap energy value increased when the crystallite sizes increased as well. The increment of crystallite sizes reportedly led to reduced stress, causing the widened band gap[34–36]. In general, the observation of increasing band gap led us to conclude that the doping or co-doping of 1 mol.% of Eu3+ or/and Dy3+ produces a considerable effect on the band structure of the BaB2Si2O8 host lattice.

2.5 Photoluminescence

2.5.1 BaB2Si2O8:Eu3+ Photoluminescence excitation and emission studies of

Eu3+ doped BaB2Si2O8 are shown in Fig. 6. When excited at 390 nm, sharp emission peaks were observed at around 580, 590, 613, 650, 691 and 702 nm which cor-responded to the de-excitation transitions between the first excited state of 5D0 to 4Fj multiplet states of Eu3+ ions, where j=0, 1, 2, 3, 4[37]. This reveals that the Eu ions incorporated into BaB2Si2O8 were in trivalent oxida-

LEOW Tingqiao et al., Effects of Eu3+ and Dy3+ doping or co-doping on optical and structural properties of … 25

Fig. 6 Photoluminescence excitation spectra (λem=613 nm),

emission spectra (λex=390 nm) and decay curve (λex=390 nm, λem=613 nm) (inset) of Eu3+ doped and Eu3+/Dy3+ co-doped BaB2Si2O8

tion state. No unusual reductions of Eu3+ to Eu2+ were detected as opposed to the works of Saradhi et al. who found a significant amount of Eu2+ existence, even in oxidizing sintering environment[13]. Emission peaks around 580, 650 and 702 nm corresponded to the mag-netic dipole transitions from 5D0 to 4F0, 5D0 to 4F3 and 5D0 to 4F5. While emission peak around 691 nm were as-signed to the electric dipole transition from 5D0 to 4F4. The emission peaks that came into interest were the two most prominent peaks at 590 and 613 nm. The former emission corresponded to 5D0 to 4F1 magnetic dipole transition, and the intensity was unaffected by host crystal environment because the transition was parity-allowed in Laporte selection rule[2]. Conversely, the latter emission around 613 nm corresponds to the 5D0 to 4F2 electric di-pole transition. The 5D0 to 4F2 transition with ∆J=2 was a forbidden transition. It was widely known that this tran-sition is only allowed in low symmetry site[38]. Therefore, the intensity of 613 nm is hypersensitive to Eu3+ site oc-cupancy. Its intensity increases along with low site sym-metry occupancy. Since the 5D0 to 4F1 transition is inde-pendent of host lattice, the intensity ratio, S of the two mentioned transition could work as a measurement tool for site symmetry following the expression below: SEu3+=∫(I613 nm)dν/∫(I500 nm)dν (6)

The peaks were firstly de-convoluted to avoid over-lapping. Here, the value SEu3+ was determined to be 3.25, indicating that the Eu3+ positioned in a site with very low inversion symmetry.

The de-convolution using Gaussian fit was performed to further study the emission peaks of Eu3+ in BaB2Si2O8, shown in Fig. 7. It can be clearly seen that the overlapped emission peaks could be sorted into eight distinct peaks which corresponded to the f-f transition of Eu3+. Closer inspection of the emission peak near 590 and 613 nm re-vealed that the peaks comprised of two weaker peaks

Fig. 7 Gaussian fit of Eu3+ doped and Eu3+/Dy3+ co-doped sam-

ples near red emission region very close apart, giving off an asymmetric peak shape. As mentioned before, Eu3+ ions substituted Ba2+ ions due to their similarity in ionic radius. Hence, the peaks split-ting was assigned to the different occupancy site of Ba2+, namely the 7-coordinated site and 10-coordinated site that the Ba2+ occupied in BaB2Si2O8 crystal structure[11]. The Commission International del’Eclairage (CIE) chromaticity coordinate was calculated from the emis-sion spectrum to be (0.6170, 0.3495). The coordinates are plotted in Fig. 10 and the actual image of the red lu-minescence from the phosphor under 390 nm excitations is shown in the inset.

Excitation peaks that appeared around 532, 466, 419, 389, 379, 358 and 321 nm were ascribed to the transition from the ground state 7F0 to 5H6, 5D4, 5L7, 5L6, 5D3, 5D2 and 5D1 respectively[39]. The wide excitation band arising from 291 nm and peaked at 226 nm was assigned to the charge transfer band of valence electron from O2– ligand in the host lattice to unfilled orbital of Eu3+ [40]. Collating the sharp reduce in reflectance in DRS and the charge transfer band in the PL excitation revealed strong inter-action between the host lattice and Eu3+ ions. The de-tailed transitions of Eu3+ are depicted in the energy level diagram in Fig. 9. 2.5.2 BaB2Si2O8:Dy3+

The PL excitation and emission of BaB2Si2O8 phos-phor activated by Dy3+ are presented in Fig. 8. Three well resolved peaks could be observed in the emission spec-trum, which were the 483, 575 and 661 nm emission peaks when excited at 350 nm. 483, 575 and 661 nm emissions arise from the radiative relaxation transition from 4F9/2 to multiplet ground states of 6Hj level, where j=15/2, 13/2 and 11/2[41,42]. The emission peaks that most interested us were the prominent 483 and 575 nm emis-sions. The emission near 575 nm was the electric dipole transition from 4F9/2 to the lower lying 6H13/2 level, which is only allowed for such condition that Dy3+ ions occupy a site without inversion center. On the other hand, the

26 JOURNAL OF RARE EARTHS, Vol. 34, No. 1, Jan. 2016

Fig. 8 Photoluminescence excitation spectra (λem=575 nm),

emission spectra (λex=350 nm) and decay curve (λex= 350 nm, λem=575 nm) (inset) of Dy3+ doped and Eu3+/ Dy3+ co-doped BaB2Si2O8

483 nm emission was associated with magnetic dipole transition from 4F9/2 to 6H15/2 level that was reported in-dependent of host media site symmetry. Similar to the case of Eu3+, the intensity ratio could therefore be used to determine the extent of site symmetry using the relation: SDy3+=∫(I575 nm)dν/∫(I404 nm)dν (7) Here, the SDy3+ was calculated to be 1.86, indicating that the activator occupied a site without inversion center. The chromaticity coordinates were determined to be (0.3467, 0.4061), also plotted in CIE diagram in Fig. 10. The emitting peaks combined to produce a pale yellow-ish white emission.

By observing the emission at 575 nm, multiple peaks could be identified from the excitation spectra. Namely, the peaks near 474, 450, 421, 388, 354, 322 and 293 nm that were associated with transitions from ground state 6H15/2 to 4F9/2, 4I15/2, 4G11/2, 4I13/2, 6P7/2, 6P3/2

and 4P5/2 re-

spectively[43]. Lastly, the broad excitation band extended from 251 nm to VUV region was ascribed to the charge transfer band of Dy3+ and O2– ligand. Similarly, the CTB compared to DRS showed strong interaction of Dy3+ and host matrix. 2.5.3 BaB2Si2O8:Eu3+,Dy3+

To study the effects of Eu3+ and Dy3+ co-doping on the photoluminescence properties, the co-doped phosphor were excited by different excitation wavelengths. The excitation and emission spectra of Eu3+ and Dy3+ co- doped BaB2Si2O8 are shown separately in Fig. 6 and with excitation wavelength of 390 and 350 nm, respectively. The emissions of Eu3+ in the co-doped phosphor ap-peared more dominant when excited at 390 nm, with an additional emission shoulder near 575 nm that corre-sponded to the emission from Dy3+. This was exhibited clearly in the de-convoluted emission spectrum of co-doped phosphor in Fig. 7. On the other hand, the Dy3+

emissions near 483 and 575 nm were more profound than Eu3+ emission when excited at 350 nm, with the ad-

ditional Eu3+ emission noticed near 613 nm. However, it is worthwhile to mention that the emission of Eu3+ de-creased significantly (~40%) in co-doped phosphor in comparison to singly doped phosphor, especially the 613 nm emission. On the contrary, the Dy3+ emissions in co-doped phosphor were revealed to be enhanced by a rough 75% in relative to singly doped phosphor. More-over, the excitation spectra of co-doped phosphor re-corded by observing 575 nm emissions showed traces of excitation peaks that resembled Eu3+ excitations transi-tions. The acromion near 466 nm and the peak near 532 nm (denoted as g and h in Fig. 8) corresponded to the transition from 7F0 to 5D1 and 5D2 respectively. The exci-tation spectra from observing 613 nm emissions showed no difference from the singly doped phosphor. Hence, we can conclude that the enhancement in Dy3+ emission and decrease in Eu3+ emissions were caused by an energy transfer progress between the two ions. We proposed that the energy absorbed from 7F0 to 5D1 and 5D2 of Eu3+ were transferred to the 4F9/2 level of Dy3+ through a pho-non-assisted transition. Subsequently, the usual de-exci-tation of 4F9/2 to 6Hj level (j=15/2, 13/2 and 11/2) took place, enhancing the Dy3+ emissions. The energy transfer mechanism is illustrated in the energy diagram in Fig. 9. We believe that the energy transfer transitions were pos-sible due to the adjacent of energy between 5D1 and 5D2 of Eu3+ ion and 4F9/2 of Dy3+ ions.

To verify this, the luminescence lifetimes of all singly doped and co-doped samples were recorded at 575 nm (excited at 250 nm) and 613 nm (excited at 390 nm) emissions and are shown in the insets of Fig. 6 corre-spondingly. The lifetime can be fitted with the single exponential decay equation[44]: I=Ioe–l/τ (8) where I is the intensity, Io is the initial intensity, t is time and τ is the luminescence lifetime. The luminescence lifetimes are listed in Table 1. The lifetime observed at 613 nm emission decreased from 2.21 ms in Eu3+ singly doped phosphor to 1.55 ms in co-doped phosphor. On the contrary, the emissions at 575 nm increased from 0.387 to 0.477 ms when traversing from Dy3+ singly doped to co-doped phosphor. This result revealed that the energy transfer from Eu3+ to Dy3+ did commence. Usually the energy transfer between Eu3+ and Dy3+ occurs in the op-posite direction, which is from Dy3+ to Eu3+

. In the past, energy transfer within Eu3+ and Dy3+ co-doped system usually occurred from Dy3+ to Eu3+, subsequently en-hancing the Eu3+ emissions. This was reported by several researchers in Eu3+ and Dy3+ co-doped compounds such as Sr3AlO4F, Ca2La8(GeO4)6O2 and CaO-Al2O3-B2O3 glass system[17–19]. We find this to be unusual as the en-ergy transfer originated from Eu3+ to Dy3+ in this study. In our account, the Eu3+ acted as a sensitizer, enhancing the Dy3+ emissions. The energy transfer efficiency, ηT of Eu3+ can be calculated by the equation[5]:

LEOW Tingqiao et al., Effects of Eu3+ and Dy3+ doping or co-doping on optical and structural properties of … 27

Fig. 9 Depicted transitions of Dy3+ and Eu3+ in BaB2Si2O8 phosphor

ηT=1Is/Is0 (9) where Is and Iso are the peak intensity of the sensitizer (Eu3+) in the presence and absence of acceptor (Dy3+). The value of ηT was calculated to be near 60% which in-dicated an efficient energy transfer.

The CIE coordinates of Eu3+ and Dy3+ co-doped

BaB2Si2O8 were calculated to be (0.5399, 0.3701) for 390 nm excitation and (0.4237, 0.3882) when excited at 350 nm and the coordinates are plotted in Fig. 10. As one can see, the chromaticity coordinates of Eu3+ or Dy3+ singly doped and Eu3+/Dy3+ co-doped samples could be connected on a line. The emission color shifted to the red region when Eu3+ was incorporated in the Dy3+ doped

Fig. 10 CIE chromaticity diagram plots of Eu3+ and Dy3+ singly

doped/co-doped BaB2Si2O8 samples

phosphor. This revealed that with the combinations from the blue and green emissions of Dy3+ together with the red emissions from Eu3+, a white light with tuneable white warmth can be produced by properly engineering the dopant concentrations and varying the excitation wavelengths. In addition, the red emissions from Eu3+ singly doped BaB2Si2O8 will serve as a promising can-didate as the red component in white LED applications. Overall, all the phosphor exhibited that they can be effi-ciently excited in the UV range, which is compatible with GaN-based LED chips.

3 Conclusions

In conclusion, a series of Eu3+/Dy3+ doped, Eu3+ and Dy3+ co-doped as well as un-doped BaB2Si2O8 phosphor were prepared via solid state reaction method. The dopants were found to stabilize the host crystal as well as improve crystallinity. Optical band gaps of the phosphor were widened gradually when dopants were incorporated, which was ascribed to the Burstein-Moss effect and in-creased crystallite size. Photoluminescence of the phos-phor showed a possible tuneable white to red emitting phosphor from the combination emission from Eu3+ and Dy3+ through careful engineering of the dopant concen-trations. The energy transfer from Eu3+ to Dy3+ were shown in the process which caused the enhanced inten-sity of Dy3+ emissions in co-doped samples.

Acknowledgements: The authors would like to express appre-ciation to the Phosphor Research Group, Physics Department, UTM for the preparation of equipment as well as The Ministry

28 JOURNAL OF RARE EARTHS, Vol. 34, No. 1, Jan. 2016

of Higher Education (MOHE) and Universiti Teknologi Malay-sia for their financial support.

References:

[1] Ma Q, Lu M K, Yuan Z M, Zhu Y N, Yang P. Synthesis and photoluminescence of highly enhanced green-emitting LaNbTiO6:xEr3+,yBi3+ micro/nanophosphors. Mater. Res. Bull., 2012, 47: 4126.

[2] Tian L H, Yang P, Wu H, Li F. Luminescence properties of Y2WO6:Eu3+ incorporated with Mo6+ or Bi3+ ions as red phosphors for light-emitting diode applications. J. Lumin., 2010, 130: 717.

[3] Horng R H, Wang W K, Huang S C, Huang S Y, Lin S H, Lin C F, Wuu D S. Growth and characterization of 380 nm InGaN/AlGaN LEDs grown on patterned sapphire sub-strates. J. Crys. Growth, 2007, 298: 219.

[4] Xie A, Yuan X M, Wang F X, Shi Y, Li J, Liu L, Mu Z F. Synthesis and luminescent properties of Eu3+-activated molybdate-based novel red-emitting phosphors for white LEDs. J. Alloys Compd., 2010, 501: 124.

[5] Li G G, Geng D L, Shang M M, Peng C, Cheng Z Y, Lin J. Tunable luminescence of Ce3+/Mn2+-coactivated Ca2Gd8(SiO4)6O2 through energy transfer and modulation of excitation: potential single-phase white/yellow-emitting phosphors. J. Mater. Chem., 2011, 21: 13334.

[6] Wang C, Zhao Z Y, Wu Q S, Xin S Y, Wang Y H. The pure-phase Ba3−xCaxSi6O12N2:Eu2+ green phosphor: syn-thesis, photoluminescence and thermal properties. Crys-tEngComm, 2014, 16: 9651.

[7] Yang C C, Lin C M, Chen Y J, Wu Y T, Chuang S R, Liu R S, Hu S F. Highly stable three-band white light from an InGaN-based blue light-emitting diode chip precoated with (oxy)nitride green/red phosphors. Appl. Phys. Lett., 2007, 90: 123503.

[8] Sharma T K, Towe E. Application-oriented nitride sub-strates: The key to long-wavelength nitride lasers beyond 500 nm. J. Appl. Phys., 2010, 107: 024516

[9] Que M, Ci Z, Wang Y, Zhu G, Xin S, Shi Y, Wang Q. Crystal structure and luminescence properties of a cyan emitting Ca10(SiO4)3(SO4)3F2:Eu2+ phosphor. CrystEng-Comm, 2013, 15: 6389.

[10] Li X X, Zhang Z H, Chen Z, Wang Y H, Huang X Y, Yu Q M, Lü X D. Photoluminescence of BaGdB9O16:Eu3+ co-doped Al3+ or Sc3+ under UV-VUV excitation. J. Rare Earths, 2009, 27: 38.

[11] Pautov L A, Agakhanov A A, Sokolova E, Hawthrone F C. Maleevite, BaB2Si2O8, and pekovite, SrB2Si2O8, new min-eral species from the dara-i-pioz Alkaline massif, Northern Tajikistan: description and crystal structure. Can. Miner., 2004, 42: 107.

[12] Wang Y H, Zhang Z Y, Zhang J C, Lu Y H. Electronic properties and rare-earth ions photoluminescence behav-iors in borosilicate: SrB2Si2O8. J. Solid State Chem., 2009, 182: 813.

[13] Saradhi M P, Boudin S, Varadaraju U V, Raveau B. A new BaB2Si2O8:Eu2+/Eu3+,Tb3+ phosphor – Synthesis and pho-toluminescence properties. J. Solid State Chem., 2010, 183: 2496.

[14] Wan J, Cheng L H, Sun J S, Zhong H Y, Li X P, Lu W L, Tian Y, Lin H, Chen B J. Energy transfer and colorimetric properties of Eu3+/Dy3+ co-doped Gd2(MoO4)3 phosphors. J. Alloys Compd., 2010, 496: 331.

[15] Wu L, Zhang Y, Gui M Y, Lu P Z, Zhao L X, Tian S, Kong Y F, Xu J J. Luminescence and energy transfer of a color tunable phosphor: Dy3+-, Tm3+-, and Eu3+-coacti-vated KSr4(BO3)3 for warm white UV LEDs. J. Mater. Chem., 2012, 22: 6463.

[16] Guan H X, Liu G X, Wang J X, Dong X T, Yu W S. Tun-able luminescence and energy transfer properties of NaGdF4:Dy3+,Eu3+ nanophosphors. New J. Chem., 2014, 38: 4901.

[17] Cai J L, Li R Y, Zhao C J, Tie S L, Wan X, Shen J Y. White light emission and energy transfer in Dy3+/Eu3+ co-doped aluminoborate glass. Opt. Mater., 2012, 34: 1112.

[18] Jeon Y I, Bharat L K, Yu J S. Synthesis and luminescence properties of Eu3+/Dy3+ ions co-doped Ca2La8(GeO4)6O2 phosphors for white-light applications. J. Alloys Compd., 2015, 620: 263.

[19] Bandi V R, Grandhe B K, Woo H J, Jang K, Shin D S, Yi S S, Jeong J H. Luminescence and energy transfer of Eu3+ or/and Dy3+ co-doped in Sr3AlO4F phosphors with NUV excitation for WLEDs. J. Alloys Compd., 2012, 538: 85.

[20] Liu B J, Gu M, Liu X L, Ni C, Wang D, Xiao L H, Zhang R. Effect of Zn2+ and Li+ codoping ions on nanosized Gd2O3:Eu3+ phosphor. J. Alloys Compd., 2007, 440: 341.

[21] Marzouk S Y, Seoudi R, Said D A, Mabrouk M S. Linear and non-linear optics and FTIR characteristics of borosili-cate glasses doped with gadolinium ions. Opt. Mater., 2013, 35: 2077.

[22] Gautam C, Yadav A K, Singh A K. A review on infrared spectroscopy of borate glasses with effects of different ad-ditives. International Scholarly Research Network Ce-ramics, 2012. 2012.

[23] Ryu H K, Heo J S, Cho S I, Chung C, Moona S H. Ther-mal decomposition mechanism of Sr(DPM)2. J. Electro-chem. Soc., 2000, 147: 1130.

[24] Bootjomchai C, Laopaiboon J, Nontachat S, Tipparach U, Laopaiboon R. Structural investigation of borosilicate re-cycled-barium-bismuth glasses under the influence of gamma-irradiation through ultrasonic and FTIR studies. Nucl. Eng. Des., 2012, 248: 28.

[25] Pascuta P, Lungu R, Ardelean I. FTIR and Raman spec-troscopic investigation of some strontium-borate glasses doped with iron ions. J. Mater. Sci.: Mater. Electron., 2010, 21: 548.

[26] Rao T G V M, Kumar A R, Veeraiah N, Reddy M R. Op-tical and structural investigation of Sm3+-Nd3+ co-doped in magnesium lead borosilicate glasses. J. Phys. Chem. Sol., 2013, 74: 410.

[27] Leow T Q, Leong P M, Eeu T Y, Ibrahim Z, Hussin R. Study of structural and luminescence properties of lead lithium borophosphate glass system doped with Ti ions. Sains Malaysiana, 2014, 43: 929.

[28] Zhu G, Shi Y, Mikami M, Shimomura Y, Wang Y. Elec-tronic structure and photo/cathodoluminescence properties investigation of green emission phosphor NaBaScSi2O7:

LEOW Tingqiao et al., Effects of Eu3+ and Dy3+ doping or co-doping on optical and structural properties of … 29

Eu2+ with high thermal stability. CrystEngComm, 2014, 16: 6089.

[29] Som S, Mitra P, Kumar V, Kumar V, Terblans J J, Swarta H C, Sharma S K. The energy transfer phenomena and colour tunability in Y2O2S:Eu3+/Dy3+ micro-fibers for white emission in solid state lighting applications. Dalton Transactions, 2014, 43: 9860.

[30] Ahrens L H. The use of ionization potentials Part 1. Ionic radii of the elements. Geochimica et Cosmochimica Acta, 1952, 2: 155.

[31] Minami T, Yamamoto T, Miyata T. Highly transparent and conductive rare earth-doped ZnO thin films prepared by magnetron sputtering. Thin Solid Films, 2000, 366: 63.

[32] Han S C, Wang Y H, Zeng W, Chen W B, Li G. Persistent luminescence property of rare earth doped BaMg2Al6Si9O30 phosphor. J. Lumin., 2014, 152: 66.

[33] Yang Y, Jin S, Medvedeva J E, Ireland J R, Metz A W, Ni J, Hersam M C, Freeman A J, Marks T J. CdO as the ar-chetypical transparent conducting oxide. Systematics of dopant ionic radius and electronic structure effects on charge transport and band structure. J. Am. Chem. Soc., 2005, 127: 8796.

[34] Benramache S, Belahssen O, Guettaf A, Arif A. Correla-tion between crystallite size-optical gap energy and pre-cursor molarities of ZnO thin films. J. Semicond., 2014, 35: 0420011.

[35] Sharma R, Shishodia P K, Wakahara A, Mehra R M. In-vestigations of highly conducting and transparent Sc doped ZnO films grown by the sol-gel process. Materials Sci-ence-Poland, 2009, 27.

[36] Ashokkumar M, Muthukumaran S. Size dependent struc-

tural and optical properties of Cd0.9Zn0.1S thin films. Phys-ics Procedia, 2013, 49: 137.

[37] Hao Z M, Yang G C, Song X Z, Zhu M, Meng X, Zhao S N, Song S Y, Zhang H J. A europium(III) based metal- organic framework: bifunctional properties related to sensing and electronic conductivity. J. Mater. Chem. A, 2014, 2: 237.

[38] Sun W Z, Pang R, Li H F, Jia Y L, Zhang S, Jiang L H, Li C Y. Synthesis and photoluminescence properties of novel red-emitting Ca14Mg2(SiO4)8:Eu3+/Sm3+ phosphors. J. Rare Earths, 2015, 33: 814.

[39] Dai W B. Mechanism of the reduction and energy transfer between Eu2+ and Eu3+ in Eu-doped CaAl2Si2O8 materials prepared in air. J. Mater. Chem. C, 2014, 2.

[40] Li L, Zhang S. Dependence of charge transfer energy on crystal structure and composition in Eu3+-doped com-pounds. J. Phys. Chem. B, 2006, 110: 21438.

[41] Ma Q, Lu M, Yang P, Zhang A, Cao Y. Preparation and photoluminescence properties of euxenite-type YNbTiO6 and YNbTiO6:xRE3+,yMn2+ (RE=Er, Dy) micro/ nanophosphors. Mater. Res. Bull., 2013, 48: 3677.

[42] Zhai Y Q, Li X, Liu J, Jiang M. A novel white-emitting phosphor ZnWO4:Dy3+. J. Rare Earths, 2015, 33: 350.

[43] Geng W Y, Zhu G, Shi Y R, Wang Y H. Luminescent characteristics of Dy3+ doped calcium zirconium phos-phate CaZr4(PO4)6 (CZP) phosphor for warm-white LEDs. J. Lumin., 2014, 155: 205.

[44] Wang J, Cheng Y, Huang Y L, Cai P Q, Kim S I, Seo H J. Structural and luminescent properties of red emitting Eu3+-doped ternary rare earth antimonates R3SbO7 (R ¼ La, Gd, Y). J. Mater. Chem. C, 2014, 2: 5559.