Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 141–148
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Synthesis of Eu3+-activated BaMoO4 phosphors and their Judd–Ofelt analysis: Applications in lasers and white LEDs C. Shivakumara a,⇑, Rohit Saraf a, Sukanti Behera a, N. Dhananjaya b, H. Nagabhushana c a
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India Department of Physics, B.M.S. Institute of Technology, Bangalore 560 064, India c C.N.R. Rao Center for Advanced Materials, Tumkur University, Tumkur 572 103, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+ phosphors were synthesized using nitrate–citrate gel combustion method. Host BaMoO4 phosphor revealed white emission upon excitation at 370 nm. 3+ Eu -activated BaMoO4 exhibited red luminescence which was confirmed by CIE chromaticity diagram. Judd–Ofelt parameters and other radiative properties of Eu3+-activated BaMoO4 have been determined. These phosphors can be used in red lasers and optical display devices.
BaMoO4: Eu
a r t i c l e
i n f o
Article history: Received 13 November 2014 Received in revised form 12 June 2015 Accepted 16 June 2015 Available online 19 June 2015 Keywords: Nitrate–citrate gel combustion Eu3+ Phosphors Judd–Ofelt analysis Lasers
a b s t r a c t Eu3+-activated BaMoO4 phosphors were synthesized by the nitrate–citrate gel combustion method. The Rietveld refinement analysis confirmed that all the compounds were crystallized in the scheelite-type tetragonal structure with I41/a (No. 88) space group. Photoluminescence (PL) spectra of BaMoO4 phosphor reveals broad emission peaks at 465 and 605 nm, whereas the Eu3+-activated BaMoO4 phosphors show intense 615 nm (5D0 ? 7F2) emission peak. Judd–Ofelt theory was applied to evaluate the intensity parameters (X2, X4) of Eu3+-activated BaMoO4 phosphors. The transition probabilities (AT), radiative lifetime (srad), branching ratio (b), stimulated emission cross-section (re), gain bandwidth (re Dkeff) and optical gain (re srad) were investigated by using the intensity parameters. CIE color coordinates confirmed that the BaMoO4 and Eu3+-activated BaMoO4 phosphors exhibit white and red luminescence, respectively. The obtained results revealed that the present phosphors can be a potential candidate for red lasers and white LEDs applications. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Rare earth ions have been extensively employed as activators for various phosphors. Lanthanide ions (Ln3+) exhibit sharp emission peaks due to intra 4f–4f transitions which were quite characteristic [1]. Trivalent europium ions (Eu3+) activated phosphors ⇑ Corresponding author. Tel.: +91 80 2293 2951; fax: +91 80 2360 1310. E-mail address: [email protected] (C. Shivakumara). http://dx.doi.org/10.1016/j.saa.2015.06.045 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
were considered to obtain a red-emitting phosphor with proper CIE chromaticity coordinates. This was because the lowest excited level (5D0) of the 4f6 configuration in Eu3+ was situated below the 4f55d configuration. Eu3+ provides a particularly favorable situation for substitution in A2+ sites with favorable isostructural replacement. Trivalent europium ions exhibited narrow band emissions, long lifetimes and large Stokes shifts (emission of lower energy radiation upon excitation by higher energy radiation). The intensities and splitting of the spectral lines provide useful
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information concerning the local site symmetry, sizes of cations and properties of the chemical bonding [2]. Luminescent europium complexes, in particular, act as light conversion molecular devices by absorbing ultraviolet (UV) light and by emitting light in the red visible spectral region. Eu3+ ions exhibit intense 5D0 ? 7F2 emission in the red spectral region at 615 nm, when they occupy the lattice sites without centrosymmetry. This was highly favorable for improving the color purity of the phosphor. These phosphors were relatively stable and have strong absorption in the near-UV region; therefore, they were promising candidates as a red component for white LED emission devices [3]. Over the past few years, metal molybdates have received great research interest because of their potential applications in various fields, such as photoluminescence [4], optic fiber [5], humidity sensor [6], catalysts [7], microwave applications [8] and solid-state lasers [9]. Metal molybdates of relatively large bivalent cations (e.g., Ca, Ba, Pb and Sr; ionic radius > 0.99 Å) mainly exist in a scheelite-type tetragonal structure, whereas those with small cationic radii (e.g., Zn, Fe, Mn, Co and Ni) generally have a wolframite-type monoclinic structure. Among them, scheelite-type BaMoO4 is one of the most important inorganic materials which exhibit self luminescence in almost entire visible spectrum. Also, they serve as excellent host materials for trivalent lanthanide ions, which produce spectral emission lines due to f–f transitions [10]. Several synthetic routes have been employed for the formation of BaMoO4, such as solid state reaction [11], Czochralski technique [12], spontaneous crystallization [13], hydrothermal method [14] and complex polymerization [15]. All these synthesis methods exhibit several problems, such as presence of organic compounds, surfactants, expensive precursors, very high temperature and long processing time. For instance, Wu et al. [11] reported the synthesis of BaMoO4: Eu3+ phosphors by the solid state method at 1000 °C for 3 h. Similarly, in the Czochralski technique (1100–1200 °C, 30–40 h) [12] and spontaneous crystallization method (950 °C, 72 h) [13], high temperatures and long processing time were required for the preparation of barium molybdate. Zhang et al. [14] prepared the BaMoO4: Eu3+ microspheres via a hydrothermal route by using sodium dodecyl benzenesulfonate (SDBS) as surfactant. Nitrate–citrate gel combustion method overcome these problems and offer advantages such as low heating temperature, short reaction time, phase purity of products, less expensive, low energy requirements, better compositional control and relative simplicity of the process. As per our knowledge, the spectroscopic Judd–Ofelt investigation of Eu3+-activated BaMoO4 have not been reported so far and need careful examination. In the present work, Eu3+-activated BaMoO4 phosphors were synthesized by the nitrate–citrate gel combustion technique. The crystallographic structural parameters, surface morphology and functional groups were analyzed by powder X-ray diffraction (XRD), Field emission Scanning electron microscopy (FESEM), and Fourier transform infrared (FTIR) spectroscopy, respectively. Further, we have used the Judd–Ofelt theory to calculate the intensity parameters and various other radiative properties such as transition probability, radiative life time, branching ratio, stimulated emission cross-section, gain bandwidth and optical gain for possible optical applications. Experimental Synthesis BaMoO4: Eu3+ (0, 1, 3 and 5 mol%) phosphors were prepared by the nitrate–citrate gel combustion method. In the synthesis of Eu3+ doped BaMoO4, Ba(NO3)2, (NH4)6Mo7O244H2O, Eu2O3 and citric
acid were used as the starting materials. The typical preparation of BaMoO4: Eu3+ (1 mol%) compound was included in the following steps: Ba(NO3)2 (1.2937 g) and (NH4)6Mo7O244H2O (0.8828 g) were dissolved in 50 ml distilled water separately. Eu2O3 (0.0088 g) was dissolved in 5 ml of 6 N HNO3 and added into the barium nitrate solution followed by the addition of ammonium molybdate solution. Citric acid (1.3699 g) was dissolved in 50 ml of distilled water and added to the metal ions containing nitrate solution. The clear solution was heated on a hot plate around 80 °C to remove water and oxides of nitrogen to form a thick viscous gel. The resulting gel was further heated to get dry powder. Finally, the powder was grind in an agate mortar and pestle and calcined in a muffle furnace at 700 °C for 2 h in an ambient atmosphere. Characterization The phase purity of BaMoO4: Eu3+ phosphors was examined by powder X-ray diffraction measurements using PANalytical X’pert Pro diffractometer, Cu Ka radiation (k = 1.5418 Å) with a nickel filter. For Rietveld refinement analysis, data were collected at a scan rate of 1°/min with a 0.02° step size for 2h from 10° to 80°. The structural parameters were refined by Rietveld method using FullProf Suite-2000 programme. The surface morphology of these phosphors were investigated using FESEM (FEI Sirion XL 30) operated at an accelerating voltage of 10 kV. FTIR spectra were recorded using Perkin Elmer Spectrometer, Frontier using KBr as a reference. UV–Visible absorption spectra have been recorded for powders on Perkin Elmer Lambda 750 spectrophotometer. The PL studies have been carried out using a Horiba Flurolog-3 spectrofluorimeter (450 W Xenon lamp). All the measurements were performed at room temperature.
Results and discussion Powder X-ray diffraction Fig. S1 shows the powder XRD patterns of BaMoO4: Eu3+ (0, 1, 3 and 5 mol%) phosphors calcined at 700 °C for 2 h. All diffraction peaks could be indexed to the scheelite type tetragonal phase with I41/a (No. 88) space group which were in consistent with the JCPDS Card No. 72–0747. No traces of additional peaks were observed in the XRD patterns, which confirm the formation of single phase compounds. The structural parameters were obtained from the Rietveld refinement method using powder XRD data. The patterns were typically refined for lattice parameters, scale factor, backgrounds, pseudo-Voigt profile function (u, v and w), atomic coordinates and isothermal temperature factors (Biso). The refinement result reveals that all the compounds crystallized in the tetragonal scheelite-structure with space group I41/a (No. 88). The observed, calculated and the difference XRD patterns of BaMoO4: Eu3+ phosphors are shown in Fig. 1. There was a good agreement between the observed and calculated patterns. In Table 1, we have summarized refined structural parameters for all compounds. The crystal structure was modeled through VESTA program [16] using the Rietveld refined structural parameters. In scheelite BaMoO4 structure (Fig. S2), Ba2+ atoms were bonded to eight oxygen atoms which results in deltahedral [BaO8] clusters. In case of Eu3+-activated BaMoO4 compounds, Eu3+ atoms were also coordinated with eight oxygen atoms. On the other hand, the Mo6+ atoms were coordinated to four oxygen atoms which form [MoO4] clusters. These [MoO4] clusters were slightly distorted in the lattice and exhibit a particular characteristic related to differences in the OAMoAO bond angles (Fig. S2).
C. Shivakumara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 141–148
143
BaMoO4: Eu3+ (1 mol%)
BaMoO4
BaMoO4: Eu3+ (3 mol%)
BaMoO4: Eu3+ (5 mol%)
Fig. 1. Observed, calculated and the difference XRD patterns of BaMoO4: Eu3+ phosphors.
Table 1 Rietveld refined structural parameters for BaMoO4: Eu3+ phosphors. 3+
3+
Compounds
BaMoO4
BaMoO4: Eu (1 mol%)
BaMoO4: Eu (3 mol%)
Crystal system Space group
Tetragonal I41/a (No. 88)
Tetragonal I41/a (No. 88)
Tetragonal I41/a (No. 88)
Tetragonal I41/a (No. 88)
5.580(4) 12.829(9) 399.14(5)
5.579(2) 12.828(9) 399.33(4)
5.579(4) 12.825(9) 399.26(5)
0.0000 0.2500 0.6250
0.0000 0.2500 0.6250
0.0000 0.2500 0.6250
0.0000 0.2500 0.6250
0.0000 0.2500 0.1250
0.0000 0.2500 0.1250
0.0000 0.2500 0.1250
0.0000 0.2500 0.1250
0.2483(3) 0.1355(2) 0.0496(1)
0.2417(3) 0.1361(3) 0.0481(2)
0.2351(2) 0.1422(2) 0.0516(8)
0.2357(3) 0.1421(2) 0.0506(9)
6.63 8.53 13.53 0.39 4.07 5.47
7.57 9.95 12.22 0.48 7.01 8.43
7.73 10.6 12.44 0.73 6.87 6.93
6.90 9.40 13.31 0.49 5.64 6.75
Lattice parameters a (Å) 5.582(3) c (Å) 12.872(6) 3 Cell volume (Å ) 399.47(3) Atomic positions Ba/Eu (4b) x y z Mo (4a) x y z O (16f) x y z RFactors Rp Rwp Rexp
v2 RBragg RF
BaMoO4: Eu (5 mol%)
3+
The average crystallite size was estimated using Scherrer’s equation:
D¼
kk b cos h
ð1Þ
where k is the wavelength (1.5418 Å) of X-rays, b is the full width at half maximum (FWHM), h is the diffraction angle, k is the shape factor (0.9) and D is the average crystallite size. The average crystallite size for BaMoO4: Eu3+ phosphors were found to be 121–136 nm. Further, it was known that the FWHM can be expressed as a linear combination of the contribution from the lattice strain and crystallite size [17]. The effects of the strain and crystallite size on the FWHM were estimated by Williamson and Hall (W–H) plots using the relation:
b cos h ¼
kk þ 4e sin h D
ð2Þ
where k is the wavelength of X-rays (1.5418 Å), b is FWHM, h is the diffraction angle, k is the shape factor (0.9), D is the average crystallite size and e is the micro strain. For estimation of lattice strain, b cosh was plotted against 4sinh (Fig. S3). The slope of line gives the strain (e) and intercept (kk/D) on y-axis gives crystallite size (D). The estimated lattice strain values are shown in Table S1. The crystallite size calculated from the W–H plot was to some extent higher than those expected using Scherrer’s method. The discrepancy in the values was due to the fact that in Scherrer’s method, the strain component was assumed to be zero.
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FESEM analysis The surface morphology of BaMoO4 and 5 mol% Eu3+ doped BaMoO4 phosphors are shown in Fig. 2. The micrographs of BaMoO4 phosphor revealed the large quantity of spherical particles agglomerate and form submicrometer particles (Fig. 2a and b). In Fig. 2c and d, the 5 mol% Eu3+ doped BaMoO4 phosphor possess similar spherical particles aggregate with different particle size distribution. Therefore, we observed that the dopant ion concentration does not influence the morphology of the sample.
In the scheelite type tetragonal structure, there are 26 different vibration modes which are represented by the following equation:
C ¼ 3Ag þ 5Bg þ 3Bu þ 5Eg þ 5Eu
ð3Þ
All even vibrations Ag, Bg and Eg are Raman-active modes, while the odd modes 4Au and 4Eu are active only in infrared frequencies and remaining five vibrations (1Au and 1Eu are acoustic modes and 3Bu are silent modes) are IR inactive modes [18]. Fig. 3 shows the FTIR spectra of BaMoO4: Eu3+ phosphors measured in the wave number region of 350–2000 cm1. The molybdates with a scheelite-type tetragonal structure have 8 stretching and bending vibrational modes in FTIR spectra. In Fig. 3, only four modes (2Au and 2Eu) were observed. The characteristic strong and broad absorption bands with two modes around 825 cm1 in all samples can be assigned to m3(1Eu and 1Au) antisymmetric stretching vibrations originating from the Mo–O stretching vibration in MoO2 tetrahedron. Similarly, two other modes m4(1Au and 1Eu) 4 were ascribed to the Mo–O bending vibrations in [MoO4] clusters which were located in the same position around 376 cm1. The peaks observed around 1430 cm1 in all Eu3+-activated BaMoO4 were associated to the CO2 3 group.
3+ BaMoO4: Eu (3 mol%)
The optical energy band gap (Eg) of BaMoO4: Eu phosphors were estimated from UV–Vis absorption spectra using Wood and
1430
825
3+ BaMoO4: Eu (1 mol%)
1426
376 825
BaMoO4
376 825 4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumbers (cm ) Fig. 3. FTIR spectra of BaMoO4: Eu3+ phosphors.
Tauc relation [19]. The energy band gap was calculated with absorbance and photon energy by the following equation:
ahm ¼ A hm Eg 3+
376
376
UV–Vis absorption spectroscopy
1430
822
Transmittance (a.u.)
FTIR studies
3+ BaMoO4: Eu (5 mol%)
n
ð4Þ
where a = Absorption coefficient, hm = Photon energy, Eg = Band gap energy, A is a constant that is different for different types of
(a)
(b)
(c)
(d)
Fig. 2. FE-SEM micrographs of (a and b) BaMoO4 and (c and d) BaMoO4: Eu3+ (5 mol%) phosphors.
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Photoluminescence properties
370 nm
Ex Em
465 nm
PL Intensity (a.u.)
transitions, indicated by different values of n, where n = 1/2, 2, 3/2, or 3 for allowed direct, allowed indirect, forbidden direct, and forbidden indirect electronic transitions, respectively. Fig. 4 depicts the plots of (ahm)2 versus the photon energy (hm) for BaMoO4: Eu3+ phosphors. The Eg values were estimated by extrapolating the linear portion of the curve or tail to the x-axis and their values are shown in Table S1. It was observed that the Eg values increases from 4.03 to 4.19 eV with increase in Eu3+ concentration. This indicates the influence of [EuO8] clusters on the electronic structure of the host materials. These band gap energy values nearly matched with the literature [18].
605 nm
300
350
400
450
500
550
600
650
Wavelength (nm) The PL spectra of BaMoO4 phosphor show broad emission peaks around 465 and 605 nm, upon excitation at 370 nm (Fig. 5). The broad band suggests that the emission process was a typical multi phonon or multi level process (a solid system in which the relaxation occurs by several paths, involving the participation of numerous energy states within the band gap). The BaMoO4 emission spectrum consists of two broad peaks, first peak (465 nm) corresponds to blue and green emissions and another peak (605 nm) exhibits red luminescence. The combination of blue, green and red emission leads to the formation of white luminescence. The blue emission of BaMoO4 shows that slightly distorted tetrahedral symmetry leads to an absorption band which corresponds to A1 ? T1(2) transition [20]. Marques et al. [21] using the complex polymerization method reported that green PL emission was a result of structural disorderness in MoO4 cluster for the BaMoO4. In addition, the presence of red emission band was ascribed to the radiative transitions within defect centers, such as Frenkel defects (oxygen ions shifted to intersite positions with a simultaneous creation of a vacancy) on the surface. The presence of distorted tetrahedral [MoO4] clusters was due to different angles between OAMoAO (106.73° and 115.09°) (Fig. S2), crystallinity degree and surface defects [4]. Fig. 6 illustrates the excitation and emission spectra of BaMoO4: Eu3+ red phosphors. The excitation spectrum was monitored at 615 nm which consists of some sharp peaks between 350 and 500 nm (Fig. 6a). The sharp peaks within the 4f electron configuration of Eu3+ ions can be assigned to 7F0 ? 5D4 (362 nm), 7F0 ? 5G2 (382 nm), 7F0 ? 5L6 (394 nm), 7F0 ? 5D3 (416 nm) and 7F0 ? 5D2 (464 nm). Fig. 6b and c shows the emission spectra of BaMoO4: Eu3+ red phosphors which were excited at two resonant (394 and
BaMoO4, Eg = 4.03 eV 3+
BaMoO4: Eu (1 mol%), Eg = 4.09 eV 3+
BaMoO4: Eu (3 mol%), Eg = 4.17 eV 3+
2
(α αhν) (eV cm )
-1 2
BaMoO4: Eu (5 mol%), Eg = 4.19 eV
Fig. 5. PL excitation and emission spectra of BaMoO4 phosphor.
464 nm) wavelengths. Upon excitation, the Eu3+-activated BaMoO4 red phosphors show emission peaks at 5D1 ? 7F1 (534 nm), 5D1 ? 7F2 (555 nm), 5D0 ? 7F1 (592 nm), 5D0 ? 7F2 (615 nm), 5D0 ? 7F3 (654 nm) and 5D0 ? 7F4 (702 nm) due to intra-4f shell transitions of Eu3+ ions [22]. This result was the crucial evidence that the energy of photo excited carriers in BaMoO4 transfer to Eu3+ ions. The emission bands from higher excited states at 534 and 555 nm was due to incomplete multiphonon relaxation of transferred electrons to the higher excited states of Eu3+ then to its lowest state. The orange emission at 592 nm was magnetic-dipole allowed and independent of the host environment. The most intense red emission peak at 615 nm was an electric-dipole transition and hypersensitive to host structure and symmetry. The weak emission peaks at 654 and 702 nm were observed due to forbidden 5D0 ? 7F3 and allowed 5D0 ? 7F4 transitions respectively. Further, we did not see any appreciable change in the emission intensity for different excitation wavelengths. The PL emission peaks revealed that the relative intensity of 5D0 ? 7F2 transition increases as the dopant concentration of Eu3+ ions increase from 1, 3 to 5 mol% (Fig. 6b and c). This was due to the increase in the transfer of excitation energy from [MoO4]2 ionic clusters to the [EuO8] [22]. A universal quantitative model of color space was established by the Commission International de l’Eclairage (CIE) in 1931. The CIE chromaticity coordinates (x, y) of BaMoO4: Eu3+ phosphors were calculated from the PL spectra. Fig. 7a displays that the obtained CIE color coordinates for BaMoO4 phosphor (kex = 370 nm) was (0.305, 0.275), which lies in the white region. The CIE coordinates for Eu3+-activated BaMoO4 (1, 3 and 5 mol%) phosphors under kex = 394 were found to be (0.552, 0.388), (0.572, 0.360) and (0.592, 0.368), respectively (Fig. 7a). While upon kex = 464 nm, the calculated color coordinates were (0.588, 0.367), (0.603, 0.377) and (0.622, 0.356) for 1, 3 and 5 mol% Eu3+-activated BaMoO4 phosphors, respectively (Fig. 7b). The chromaticity coordinates of Eu3+-activated BaMoO4 phosphors were located in the red region of CIE chromaticity diagram. Hence, the present phosphors can be useful for the production of artificial white light to be similar to those of natural white light owing to its better spectral overlap and also as red component in white LEDs. Judd–Ofelt intensity parameters and radiative properties
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
hν (eV) Fig. 4. Plots of (ahm)2 versus the photon energy (hm) of BaMoO4: Eu3+ phosphors.
The radiative transitions of rare earth (RE) ions in different host matrices have been studied using Judd–Ofelt (J–O) theory [23,24]. The J–O intensity parameters (X2, X4) provide valuable information about the bonding nature of the RE ion with its surrounding ligands and the local structure around the RE ion site [25]. The J–O parameters were calculated from the PL emission spectra.
C. Shivakumara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 141–148
(a)
(a)
7
7
ex
= 394 nm
= 370 nm ex
BaMoO4: Eu
BaMoO4
BaMoO4: Eu
3+
(1 mol%)
3+
(3 mol%)
3+
(5 mol%)
7
5 F 0→ D3
5 F 0→ G2
BaMoO4: Eu
7
7
5 F0→ D4
PLE Intensity (a.u.)
λem = 615 nm
5 F 0→ D2
5 F 0→ L 6
146
340
360
380
400
420
440
460
480
500
5 7 D0→ F3
52 0 54 0 56 0
: Eu 3+
7 5 D0→ F2
700 720
680
4 : Eu 3+
(5 mol%)
)
)
Fig. 7. CIE chromaticity diagram of (a) BaMoO4 (kex = 370 nm), BaMoO4: Eu3+ (kex = 394 nm) phosphors and (b) BaMoO4: Eu3+ (kex = 464 nm) phosphors.
independent of host matrix whereas that of the 5D0 ? 7FJ (J = 2, 4) transitions depend solely on the XJ parameters. The integrated emission intensities of the spontaneous (radiative) emission of the transition between two manifolds 5D0 and 7FJ (J = 2, 4) were associated with the radiative emission rates and is as follows:
PL Intens ity (cps)
7 5 D0→ F4
5 7 D0→ F3
70 0 72 0
64 0 66 0 68 0
ol%
BaM oO
4
A0—2;4 I0—2;4 hm0—1 ¼ A0—1 I0—1 hm0—2;4
5.0x 6 10
4
60 0 62 0
(3 mol%)
3+
)
1.0x 7 10
BaM oO
56 0 58 0
ol%
ol%
1.5x 7 10
4
52 0 54 0
(5 m
(3 m
(1 m
BaM oO
Wa vel eng th ( nm )
(1 mol%)
3+
BaMoO4: Eu
2.0x1 7 0
5 7 D0→ F1
4
: Eu 3+
BaM oO
7 5 D1→ F1 5 7 D1→ F2
λ ex
6 =4
64 0 66 0
58 0 60 0 62 0
4
nm
3+
BaMoO4: Eu
5.0x 6 10
4
(c)
= 464 nm
BaMoO4: Eu
1.0x1 7 0
BaM oO
(nm )
ex
1.5x1 7 0
BaM oO
Wa vele ngt h
(b)
PL Intens ity (cps)
m
7 5 D0→ F4
4n
5 7 D1→ F1 5 7 D1→ F2
λ ex
9 =3
2.0x107
5 7 D0→ F1
)
(b
5 7 D0→ F2
Wavelength (nm)
: Eu 3+
: Eu 3+
: Eu 3+
(1 m
(3 m
ol%
(5 m
ol%
ol%
)
)
)
Fig. 6. (a) Excitation spectra of BaMoO4: Eu3+ (1 mol%) phosphor and (b and c) emission spectra of BaMoO4: Eu3+ red phosphors. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The J–O parameters were used to predict the radiative properties of excited states of Eu3+ ion such as transition probabilities (AT), radiative lifetime (srad), branching ratio (b), stimulated emission cross-section (re) and optical gain by using the expression reported in literature [26]. The intensity of the 5D0 ? 7F1 transition was
ð5Þ
where I0–2,4 is the integrated emission intensity and hm0–J is the energy corresponding to transition 5D0 ? 7FJ (J = 1, 2, 4). The transition 5D0 ? 7F0 is left out due to their small emission intensities. Due to the magnetic character of the 5D0 ? 7F1 transition and its weak dependence on crystal field effects, the value of the radiative emission rate A0–1 was estimated to be about 50 s1 [26]. The radiative emission rates A0–2,4 were related to forced electric dipole transitions and they may be written as a function of the J–O intensity parameters:
A0—2;4 ¼
64p4 ðm0—2;4 Þ3 e2 3hc
3
1 4pe0
v
X
J¼2;4
XJ
D
5
E2 D0 UðJÞ 7 FJ
ð6Þ
where v is the Lorentz local field correction factor given as function 2 nðn2 þ2Þ . The of the index of refraction n of the host v ¼ 9 D
5
E2 D0 UðJÞ 7 F2 are the square reduced matrix elements whose
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C. Shivakumara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 141–148 Table 2 Calculated Judd–Ofelt intensity parameters, radiative emission rates, lifetime, branching and asymmetry ratios for BaMoO4: Eu3+ phosphors. kex (nm)
Eu (mol%)
J–O intensity parameters (1020 cm2)
X2
X4
394
1
1.98
1.86
Transitions
A0–2,4 (s1)
A0–1 (s1)
AT (s1)
srad (ms)
b (%)
Asymmetry ratio
5
D0 ? 7 F 1 D0 ? 7 F 2 5 D0 ? 7 F 4 5 D0 ? 7 F 1 5 D0 ? 7 F 2 5 D0 ? 7 F 4 5 D0 ? 7 F 1 5 D0 ? 7 F 2 5 D0 ? 7 F 4
– 125.55 57.60 – 128.33 48.78 – 132.92 48.95
50 – – 50 – – 50 – –
233.15
4.29
2.41
227.11
4.40
231.87
4.31
21.44 53.85 24.71 22.01 56.51 21.48 21.56 57.33 21.11
5
– 157.26 42.58 – 159.23 44.12 – 178.58 45.10
50 – – 50 – – 50 – –
249.84
4.00
3.02
253.35
3.95
273.68
3.65
20.01 62.94 17.05 19.74 63.65 16.61 18.27 65.25 16.48
5
464
3
2.02
1.58
5
2.10
1.59
1
2.48
1.38
D0 ? 7 F 1 D0 ? 7 F 2 5 D0 ? 7 F 4 5 D0 ? 7 F 1 5 D0 ? 7 F 2 5 D0 ? 7 F 4 5 D0 ? 7 F 1 5 D0 ? 7 F 2 5 D0 ? 7 F 4 5
3
2.51
1.43
5
2.82
1.46
2.46
2.55
3.05
3.43
Table 3 Effective bandwidth of the emission transition (Dkeff), stimulated emission cross-section (re), gain bandwidth (re Dkeff) and optical gain (re srad) of BaMoO4: Eu3+ phosphors. kex (nm) 394
Eu (mol%)
Transitions
Dkeff (nm)
re (1022 cm2)
re Dkeff (1028 cm3)
re srad (1025 cm2 s1)
1
5
8.54 7.51 8.04 8.67 9.60 6.13 7.83 9.28 5.08 9.29 7.74 5.16 8.58 9.44 5.27 7.95 9.28 4.80
2.66 8.98 6.46 2.62 7.18 7.15 2.90 8.69 7.69 2.45 12.39 7.88 2.65 8.95 7.28 2.86 9.21 8.29
2.27 6.74 5.19 2.27 6.89 4.38 2.27 8.06 3.91 2.26 9.59 4.07 2.27 8.45 3.84 2.27 8.55 3.98
11.41 38.52 27.71 11.53 31.59 31.46 12.50 37.45 33.14 9.80 49.56 31.52 10.47 35.35 28.76 10.44 33.62 30.26
D0 ? F1 D0 ? 7F2 5 D0 ? 7F4 5 D0 ? 7F1 5 D0 ? 7F2 5 D0 ? 7F4 5 D0 ? 7F1 5 D0 ? 7F2 5 D0 ? 7F4 5 D0 ? 7F1 5 D0 ? 7F2 5 D0 ? 7F4 5 D0 ? 7F1 5 D0 ? 7F2 5 D0 ? 7F4 5 D0 ? 7F1 5 D0 ? 7F2 5 D0 ? 7F4 5
3
5
464
1
3
5
7
values were independent of the chemical environment of the Eu3+ ion [27]. Their values were known and are solely E2 E2 D D 5 D0 Uð2Þ 7 F2 = 0.0032 and 5 D0 Uð4Þ 7 F4 = 0.0023. Thus, using Eqs. (5) and (6) the values of X2, 4 were obtained. The total radiative transition probability (AT) can be calculated using the equation below:
AT ðwJ Þ ¼
X AJJ0
ð7Þ
J0
The radiative lifetime srad(wJ) of an emitting state was related to its total transition probabilities to all sublevels, is given by
srad ðwJ Þ ¼
1 AT ðwJ Þ
ð8Þ
The branching ratio b(wJ) corresponding to the emission from an excited level to its lower levels is defined as
bðwJ Þ ¼
AðwJ ; w0J0 Þ AT ðwJ Þ
ð9Þ
The stimulated emission cross-section (re) was an essential parameter to signifies the rate of energy extraction from the lasing material and is calculated as
"
# k4p re ðkp Þ ¼ AT ðwJ Þ 8pcn2 Dkeff
ð10Þ
where kp is the emission peak wavelength, c is the velocity of light and Dkeff is the effective bandwidth of the emission transition. The Judd–Ofelt intensity parameters, radiative emission rates, lifetime, branching and asymmetry ratios are presented in Table 2. The magnitude of X2 parameter depends on Eu3+–O2 covalence and also explains the symmetry of coordination environment around Eu3+ ion [28]. The trend observed in the J–O parameters (X2 > X4) for BaMoO4: Eu3+ confirmed the low symmetry at the Eu3+ site and high covalence between the Eu3+ ion and the surrounding O2 ion. The radiative transition probability (A) and branching ratio (b) were considerably higher for the 5D0 ? 7F2 transition and increase with increasing Eu3+ concentration. It was recognized that an emission level with b value near 50% becomes a potential laser emission transition [29]. The asymmetric ratios for BaMoO4: Eu3+ phosphors found to be in increasing order with increase in Eu3+ concentration. This revealed that the Eu3+ ion occupy sites with a low symmetry and without an inversion center [30]. Table 3 illustrates the effective bandwidth of the emission transition (Dkeff), stimulated emission cross-section (re), gain bandwidth (re Dkeff) and optical gain (re srad) for Eu3+-activated
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BaMoO4 phosphors. The stimulated emission cross-section value is an important factor to predict the laser performance of a material and also the rate of the energy extraction from the lasing material. The trend of re for the 5D0 emission transition were observed as 7 F2 > 7F4 > 7F1 for Eu3+-activated BaMoO4 phosphors. The stimulated emission cross-section values of the prepared phosphors for 5 D0 ? 7F2 transition were found to be better than those registered for Eu3+ doped ZnAlBiB [31], Eu3+ doped KLa(PO3)4 [29] and Eu3+ ions in lithium aluminium borophosphate glasses [32]. The higher stimulated emission cross section value was an attractive feature for low threshold and high gain laser applications which were used to claim good laser action [29]. The product of emission cross-section and the effective bandwidth of the emission transition is a significant parameter to predict the bandwidth of the optical amplifier [33]. The higher the product values were, the better was the amplifiers performance. It was noteworthy that the maximum value of gain bandwidth was 9.59 1028 cm3 corresponding to 5D0 ? 7F2 transition for BaMoO4: Eu3+ (1 mol%) under kex = 464 nm. Similarly, product of radiative lifetime and stimulated emission cross section is a vital aspect for high optical amplifier gain. The obtained gain bandwidth and optical gain values corresponding to 5D0 ? 7F2 transition were found to be larger than Eu3+ doped ZnAlBiB [31]. Therefore, we can conclude that the 5 D0 ? 7F2 transition (615 nm) provide favorable lasing action, high gain bandwidth and optical gain for amplifiers. The large values of branching ratio, stimulated emission cross-section, gain bandwidth and optical gain suggest that the Eu3+-activated BaMoO4 phosphors can be useful for red laser applications and also for the development of color display devices. Conclusions
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.06.045. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21]
BaMoO4: Eu3+ phosphors were prepared by the facile nitrate–citrate gel combustion process. All the compounds have a monophasic scheelite-type tetragonal structure. FE-SEM revealed that the spherical particles agglomerate and form submicrometer particles. Enhancement in PL emission intensity with increase in Eu3+ concentration was observed. Based on the CIE chromaticity diagram, BaMoO4 phosphor revealed white emission whereas Eu3+-activated BaMoO4 phosphors exhibited red luminescence. The Judd–Ofelt parameters (X2 > X4) for BaMoO4: Eu3+ confirmed the higher covalent character of Eu3+–O2 bond and lower symmetry at the Eu3+ site. The present Eu3+-activated BaMoO4 phosphors show the higher stimulated emission cross-section, branching ratio, gain bandwidth and optical gain parameters for 5D0 ? 7F2 transition in comparison with different host matrix such as glasses and oxides. The results point out that these phosphor materials can be a good candidate for the development of artificial white light and red component in white LEDs for display devices as well as for red laser applications.
[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis of Eu3+-activated BaMoO4 phosphors and their Judd–Ofelt analysis: Applications in lasers and white LEDs C. Shivakumara a,⇑, Rohit Saraf a, Sukanti Behera a, N. Dhananjaya b, H. Nagabhushana c a
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India Department of Physics, B.M.S. Institute of Technology, Bangalore 560 064, India c C.N.R. Rao Center for Advanced Materials, Tumkur University, Tumkur 572 103, India b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+ phosphors were synthesized using nitrate–citrate gel combustion method. Host BaMoO4 phosphor revealed white emission upon excitation at 370 nm. 3+ Eu -activated BaMoO4 exhibited red luminescence which was confirmed by CIE chromaticity diagram. Judd–Ofelt parameters and other radiative properties of Eu3+-activated BaMoO4 have been determined. These phosphors can be used in red lasers and optical display devices.
BaMoO4: Eu
a r t i c l e
i n f o
Article history: Received 13 November 2014 Received in revised form 12 June 2015 Accepted 16 June 2015 Available online 19 June 2015 Keywords: Nitrate–citrate gel combustion Eu3+ Phosphors Judd–Ofelt analysis Lasers
a b s t r a c t Eu3+-activated BaMoO4 phosphors were synthesized by the nitrate–citrate gel combustion method. The Rietveld refinement analysis confirmed that all the compounds were crystallized in the scheelite-type tetragonal structure with I41/a (No. 88) space group. Photoluminescence (PL) spectra of BaMoO4 phosphor reveals broad emission peaks at 465 and 605 nm, whereas the Eu3+-activated BaMoO4 phosphors show intense 615 nm (5D0 ? 7F2) emission peak. Judd–Ofelt theory was applied to evaluate the intensity parameters (X2, X4) of Eu3+-activated BaMoO4 phosphors. The transition probabilities (AT), radiative lifetime (srad), branching ratio (b), stimulated emission cross-section (re), gain bandwidth (re Dkeff) and optical gain (re srad) were investigated by using the intensity parameters. CIE color coordinates confirmed that the BaMoO4 and Eu3+-activated BaMoO4 phosphors exhibit white and red luminescence, respectively. The obtained results revealed that the present phosphors can be a potential candidate for red lasers and white LEDs applications. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Rare earth ions have been extensively employed as activators for various phosphors. Lanthanide ions (Ln3+) exhibit sharp emission peaks due to intra 4f–4f transitions which were quite characteristic [1]. Trivalent europium ions (Eu3+) activated phosphors ⇑ Corresponding author. Tel.: +91 80 2293 2951; fax: +91 80 2360 1310. E-mail address: [email protected] (C. Shivakumara). http://dx.doi.org/10.1016/j.saa.2015.06.045 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
were considered to obtain a red-emitting phosphor with proper CIE chromaticity coordinates. This was because the lowest excited level (5D0) of the 4f6 configuration in Eu3+ was situated below the 4f55d configuration. Eu3+ provides a particularly favorable situation for substitution in A2+ sites with favorable isostructural replacement. Trivalent europium ions exhibited narrow band emissions, long lifetimes and large Stokes shifts (emission of lower energy radiation upon excitation by higher energy radiation). The intensities and splitting of the spectral lines provide useful
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information concerning the local site symmetry, sizes of cations and properties of the chemical bonding [2]. Luminescent europium complexes, in particular, act as light conversion molecular devices by absorbing ultraviolet (UV) light and by emitting light in the red visible spectral region. Eu3+ ions exhibit intense 5D0 ? 7F2 emission in the red spectral region at 615 nm, when they occupy the lattice sites without centrosymmetry. This was highly favorable for improving the color purity of the phosphor. These phosphors were relatively stable and have strong absorption in the near-UV region; therefore, they were promising candidates as a red component for white LED emission devices [3]. Over the past few years, metal molybdates have received great research interest because of their potential applications in various fields, such as photoluminescence [4], optic fiber [5], humidity sensor [6], catalysts [7], microwave applications [8] and solid-state lasers [9]. Metal molybdates of relatively large bivalent cations (e.g., Ca, Ba, Pb and Sr; ionic radius > 0.99 Å) mainly exist in a scheelite-type tetragonal structure, whereas those with small cationic radii (e.g., Zn, Fe, Mn, Co and Ni) generally have a wolframite-type monoclinic structure. Among them, scheelite-type BaMoO4 is one of the most important inorganic materials which exhibit self luminescence in almost entire visible spectrum. Also, they serve as excellent host materials for trivalent lanthanide ions, which produce spectral emission lines due to f–f transitions [10]. Several synthetic routes have been employed for the formation of BaMoO4, such as solid state reaction [11], Czochralski technique [12], spontaneous crystallization [13], hydrothermal method [14] and complex polymerization [15]. All these synthesis methods exhibit several problems, such as presence of organic compounds, surfactants, expensive precursors, very high temperature and long processing time. For instance, Wu et al. [11] reported the synthesis of BaMoO4: Eu3+ phosphors by the solid state method at 1000 °C for 3 h. Similarly, in the Czochralski technique (1100–1200 °C, 30–40 h) [12] and spontaneous crystallization method (950 °C, 72 h) [13], high temperatures and long processing time were required for the preparation of barium molybdate. Zhang et al. [14] prepared the BaMoO4: Eu3+ microspheres via a hydrothermal route by using sodium dodecyl benzenesulfonate (SDBS) as surfactant. Nitrate–citrate gel combustion method overcome these problems and offer advantages such as low heating temperature, short reaction time, phase purity of products, less expensive, low energy requirements, better compositional control and relative simplicity of the process. As per our knowledge, the spectroscopic Judd–Ofelt investigation of Eu3+-activated BaMoO4 have not been reported so far and need careful examination. In the present work, Eu3+-activated BaMoO4 phosphors were synthesized by the nitrate–citrate gel combustion technique. The crystallographic structural parameters, surface morphology and functional groups were analyzed by powder X-ray diffraction (XRD), Field emission Scanning electron microscopy (FESEM), and Fourier transform infrared (FTIR) spectroscopy, respectively. Further, we have used the Judd–Ofelt theory to calculate the intensity parameters and various other radiative properties such as transition probability, radiative life time, branching ratio, stimulated emission cross-section, gain bandwidth and optical gain for possible optical applications. Experimental Synthesis BaMoO4: Eu3+ (0, 1, 3 and 5 mol%) phosphors were prepared by the nitrate–citrate gel combustion method. In the synthesis of Eu3+ doped BaMoO4, Ba(NO3)2, (NH4)6Mo7O244H2O, Eu2O3 and citric
acid were used as the starting materials. The typical preparation of BaMoO4: Eu3+ (1 mol%) compound was included in the following steps: Ba(NO3)2 (1.2937 g) and (NH4)6Mo7O244H2O (0.8828 g) were dissolved in 50 ml distilled water separately. Eu2O3 (0.0088 g) was dissolved in 5 ml of 6 N HNO3 and added into the barium nitrate solution followed by the addition of ammonium molybdate solution. Citric acid (1.3699 g) was dissolved in 50 ml of distilled water and added to the metal ions containing nitrate solution. The clear solution was heated on a hot plate around 80 °C to remove water and oxides of nitrogen to form a thick viscous gel. The resulting gel was further heated to get dry powder. Finally, the powder was grind in an agate mortar and pestle and calcined in a muffle furnace at 700 °C for 2 h in an ambient atmosphere. Characterization The phase purity of BaMoO4: Eu3+ phosphors was examined by powder X-ray diffraction measurements using PANalytical X’pert Pro diffractometer, Cu Ka radiation (k = 1.5418 Å) with a nickel filter. For Rietveld refinement analysis, data were collected at a scan rate of 1°/min with a 0.02° step size for 2h from 10° to 80°. The structural parameters were refined by Rietveld method using FullProf Suite-2000 programme. The surface morphology of these phosphors were investigated using FESEM (FEI Sirion XL 30) operated at an accelerating voltage of 10 kV. FTIR spectra were recorded using Perkin Elmer Spectrometer, Frontier using KBr as a reference. UV–Visible absorption spectra have been recorded for powders on Perkin Elmer Lambda 750 spectrophotometer. The PL studies have been carried out using a Horiba Flurolog-3 spectrofluorimeter (450 W Xenon lamp). All the measurements were performed at room temperature.
Results and discussion Powder X-ray diffraction Fig. S1 shows the powder XRD patterns of BaMoO4: Eu3+ (0, 1, 3 and 5 mol%) phosphors calcined at 700 °C for 2 h. All diffraction peaks could be indexed to the scheelite type tetragonal phase with I41/a (No. 88) space group which were in consistent with the JCPDS Card No. 72–0747. No traces of additional peaks were observed in the XRD patterns, which confirm the formation of single phase compounds. The structural parameters were obtained from the Rietveld refinement method using powder XRD data. The patterns were typically refined for lattice parameters, scale factor, backgrounds, pseudo-Voigt profile function (u, v and w), atomic coordinates and isothermal temperature factors (Biso). The refinement result reveals that all the compounds crystallized in the tetragonal scheelite-structure with space group I41/a (No. 88). The observed, calculated and the difference XRD patterns of BaMoO4: Eu3+ phosphors are shown in Fig. 1. There was a good agreement between the observed and calculated patterns. In Table 1, we have summarized refined structural parameters for all compounds. The crystal structure was modeled through VESTA program [16] using the Rietveld refined structural parameters. In scheelite BaMoO4 structure (Fig. S2), Ba2+ atoms were bonded to eight oxygen atoms which results in deltahedral [BaO8] clusters. In case of Eu3+-activated BaMoO4 compounds, Eu3+ atoms were also coordinated with eight oxygen atoms. On the other hand, the Mo6+ atoms were coordinated to four oxygen atoms which form [MoO4] clusters. These [MoO4] clusters were slightly distorted in the lattice and exhibit a particular characteristic related to differences in the OAMoAO bond angles (Fig. S2).
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143
BaMoO4: Eu3+ (1 mol%)
BaMoO4
BaMoO4: Eu3+ (3 mol%)
BaMoO4: Eu3+ (5 mol%)
Fig. 1. Observed, calculated and the difference XRD patterns of BaMoO4: Eu3+ phosphors.
Table 1 Rietveld refined structural parameters for BaMoO4: Eu3+ phosphors. 3+
3+
Compounds
BaMoO4
BaMoO4: Eu (1 mol%)
BaMoO4: Eu (3 mol%)
Crystal system Space group
Tetragonal I41/a (No. 88)
Tetragonal I41/a (No. 88)
Tetragonal I41/a (No. 88)
Tetragonal I41/a (No. 88)
5.580(4) 12.829(9) 399.14(5)
5.579(2) 12.828(9) 399.33(4)
5.579(4) 12.825(9) 399.26(5)
0.0000 0.2500 0.6250
0.0000 0.2500 0.6250
0.0000 0.2500 0.6250
0.0000 0.2500 0.6250
0.0000 0.2500 0.1250
0.0000 0.2500 0.1250
0.0000 0.2500 0.1250
0.0000 0.2500 0.1250
0.2483(3) 0.1355(2) 0.0496(1)
0.2417(3) 0.1361(3) 0.0481(2)
0.2351(2) 0.1422(2) 0.0516(8)
0.2357(3) 0.1421(2) 0.0506(9)
6.63 8.53 13.53 0.39 4.07 5.47
7.57 9.95 12.22 0.48 7.01 8.43
7.73 10.6 12.44 0.73 6.87 6.93
6.90 9.40 13.31 0.49 5.64 6.75
Lattice parameters a (Å) 5.582(3) c (Å) 12.872(6) 3 Cell volume (Å ) 399.47(3) Atomic positions Ba/Eu (4b) x y z Mo (4a) x y z O (16f) x y z RFactors Rp Rwp Rexp
v2 RBragg RF
BaMoO4: Eu (5 mol%)
3+
The average crystallite size was estimated using Scherrer’s equation:
D¼
kk b cos h
ð1Þ
where k is the wavelength (1.5418 Å) of X-rays, b is the full width at half maximum (FWHM), h is the diffraction angle, k is the shape factor (0.9) and D is the average crystallite size. The average crystallite size for BaMoO4: Eu3+ phosphors were found to be 121–136 nm. Further, it was known that the FWHM can be expressed as a linear combination of the contribution from the lattice strain and crystallite size [17]. The effects of the strain and crystallite size on the FWHM were estimated by Williamson and Hall (W–H) plots using the relation:
b cos h ¼
kk þ 4e sin h D
ð2Þ
where k is the wavelength of X-rays (1.5418 Å), b is FWHM, h is the diffraction angle, k is the shape factor (0.9), D is the average crystallite size and e is the micro strain. For estimation of lattice strain, b cosh was plotted against 4sinh (Fig. S3). The slope of line gives the strain (e) and intercept (kk/D) on y-axis gives crystallite size (D). The estimated lattice strain values are shown in Table S1. The crystallite size calculated from the W–H plot was to some extent higher than those expected using Scherrer’s method. The discrepancy in the values was due to the fact that in Scherrer’s method, the strain component was assumed to be zero.
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FESEM analysis The surface morphology of BaMoO4 and 5 mol% Eu3+ doped BaMoO4 phosphors are shown in Fig. 2. The micrographs of BaMoO4 phosphor revealed the large quantity of spherical particles agglomerate and form submicrometer particles (Fig. 2a and b). In Fig. 2c and d, the 5 mol% Eu3+ doped BaMoO4 phosphor possess similar spherical particles aggregate with different particle size distribution. Therefore, we observed that the dopant ion concentration does not influence the morphology of the sample.
In the scheelite type tetragonal structure, there are 26 different vibration modes which are represented by the following equation:
C ¼ 3Ag þ 5Bg þ 3Bu þ 5Eg þ 5Eu
ð3Þ
All even vibrations Ag, Bg and Eg are Raman-active modes, while the odd modes 4Au and 4Eu are active only in infrared frequencies and remaining five vibrations (1Au and 1Eu are acoustic modes and 3Bu are silent modes) are IR inactive modes [18]. Fig. 3 shows the FTIR spectra of BaMoO4: Eu3+ phosphors measured in the wave number region of 350–2000 cm1. The molybdates with a scheelite-type tetragonal structure have 8 stretching and bending vibrational modes in FTIR spectra. In Fig. 3, only four modes (2Au and 2Eu) were observed. The characteristic strong and broad absorption bands with two modes around 825 cm1 in all samples can be assigned to m3(1Eu and 1Au) antisymmetric stretching vibrations originating from the Mo–O stretching vibration in MoO2 tetrahedron. Similarly, two other modes m4(1Au and 1Eu) 4 were ascribed to the Mo–O bending vibrations in [MoO4] clusters which were located in the same position around 376 cm1. The peaks observed around 1430 cm1 in all Eu3+-activated BaMoO4 were associated to the CO2 3 group.
3+ BaMoO4: Eu (3 mol%)
The optical energy band gap (Eg) of BaMoO4: Eu phosphors were estimated from UV–Vis absorption spectra using Wood and
1430
825
3+ BaMoO4: Eu (1 mol%)
1426
376 825
BaMoO4
376 825 4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumbers (cm ) Fig. 3. FTIR spectra of BaMoO4: Eu3+ phosphors.
Tauc relation [19]. The energy band gap was calculated with absorbance and photon energy by the following equation:
ahm ¼ A hm Eg 3+
376
376
UV–Vis absorption spectroscopy
1430
822
Transmittance (a.u.)
FTIR studies
3+ BaMoO4: Eu (5 mol%)
n
ð4Þ
where a = Absorption coefficient, hm = Photon energy, Eg = Band gap energy, A is a constant that is different for different types of
(a)
(b)
(c)
(d)
Fig. 2. FE-SEM micrographs of (a and b) BaMoO4 and (c and d) BaMoO4: Eu3+ (5 mol%) phosphors.
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Photoluminescence properties
370 nm
Ex Em
465 nm
PL Intensity (a.u.)
transitions, indicated by different values of n, where n = 1/2, 2, 3/2, or 3 for allowed direct, allowed indirect, forbidden direct, and forbidden indirect electronic transitions, respectively. Fig. 4 depicts the plots of (ahm)2 versus the photon energy (hm) for BaMoO4: Eu3+ phosphors. The Eg values were estimated by extrapolating the linear portion of the curve or tail to the x-axis and their values are shown in Table S1. It was observed that the Eg values increases from 4.03 to 4.19 eV with increase in Eu3+ concentration. This indicates the influence of [EuO8] clusters on the electronic structure of the host materials. These band gap energy values nearly matched with the literature [18].
605 nm
300
350
400
450
500
550
600
650
Wavelength (nm) The PL spectra of BaMoO4 phosphor show broad emission peaks around 465 and 605 nm, upon excitation at 370 nm (Fig. 5). The broad band suggests that the emission process was a typical multi phonon or multi level process (a solid system in which the relaxation occurs by several paths, involving the participation of numerous energy states within the band gap). The BaMoO4 emission spectrum consists of two broad peaks, first peak (465 nm) corresponds to blue and green emissions and another peak (605 nm) exhibits red luminescence. The combination of blue, green and red emission leads to the formation of white luminescence. The blue emission of BaMoO4 shows that slightly distorted tetrahedral symmetry leads to an absorption band which corresponds to A1 ? T1(2) transition [20]. Marques et al. [21] using the complex polymerization method reported that green PL emission was a result of structural disorderness in MoO4 cluster for the BaMoO4. In addition, the presence of red emission band was ascribed to the radiative transitions within defect centers, such as Frenkel defects (oxygen ions shifted to intersite positions with a simultaneous creation of a vacancy) on the surface. The presence of distorted tetrahedral [MoO4] clusters was due to different angles between OAMoAO (106.73° and 115.09°) (Fig. S2), crystallinity degree and surface defects [4]. Fig. 6 illustrates the excitation and emission spectra of BaMoO4: Eu3+ red phosphors. The excitation spectrum was monitored at 615 nm which consists of some sharp peaks between 350 and 500 nm (Fig. 6a). The sharp peaks within the 4f electron configuration of Eu3+ ions can be assigned to 7F0 ? 5D4 (362 nm), 7F0 ? 5G2 (382 nm), 7F0 ? 5L6 (394 nm), 7F0 ? 5D3 (416 nm) and 7F0 ? 5D2 (464 nm). Fig. 6b and c shows the emission spectra of BaMoO4: Eu3+ red phosphors which were excited at two resonant (394 and
BaMoO4, Eg = 4.03 eV 3+
BaMoO4: Eu (1 mol%), Eg = 4.09 eV 3+
BaMoO4: Eu (3 mol%), Eg = 4.17 eV 3+
2
(α αhν) (eV cm )
-1 2
BaMoO4: Eu (5 mol%), Eg = 4.19 eV
Fig. 5. PL excitation and emission spectra of BaMoO4 phosphor.
464 nm) wavelengths. Upon excitation, the Eu3+-activated BaMoO4 red phosphors show emission peaks at 5D1 ? 7F1 (534 nm), 5D1 ? 7F2 (555 nm), 5D0 ? 7F1 (592 nm), 5D0 ? 7F2 (615 nm), 5D0 ? 7F3 (654 nm) and 5D0 ? 7F4 (702 nm) due to intra-4f shell transitions of Eu3+ ions [22]. This result was the crucial evidence that the energy of photo excited carriers in BaMoO4 transfer to Eu3+ ions. The emission bands from higher excited states at 534 and 555 nm was due to incomplete multiphonon relaxation of transferred electrons to the higher excited states of Eu3+ then to its lowest state. The orange emission at 592 nm was magnetic-dipole allowed and independent of the host environment. The most intense red emission peak at 615 nm was an electric-dipole transition and hypersensitive to host structure and symmetry. The weak emission peaks at 654 and 702 nm were observed due to forbidden 5D0 ? 7F3 and allowed 5D0 ? 7F4 transitions respectively. Further, we did not see any appreciable change in the emission intensity for different excitation wavelengths. The PL emission peaks revealed that the relative intensity of 5D0 ? 7F2 transition increases as the dopant concentration of Eu3+ ions increase from 1, 3 to 5 mol% (Fig. 6b and c). This was due to the increase in the transfer of excitation energy from [MoO4]2 ionic clusters to the [EuO8] [22]. A universal quantitative model of color space was established by the Commission International de l’Eclairage (CIE) in 1931. The CIE chromaticity coordinates (x, y) of BaMoO4: Eu3+ phosphors were calculated from the PL spectra. Fig. 7a displays that the obtained CIE color coordinates for BaMoO4 phosphor (kex = 370 nm) was (0.305, 0.275), which lies in the white region. The CIE coordinates for Eu3+-activated BaMoO4 (1, 3 and 5 mol%) phosphors under kex = 394 were found to be (0.552, 0.388), (0.572, 0.360) and (0.592, 0.368), respectively (Fig. 7a). While upon kex = 464 nm, the calculated color coordinates were (0.588, 0.367), (0.603, 0.377) and (0.622, 0.356) for 1, 3 and 5 mol% Eu3+-activated BaMoO4 phosphors, respectively (Fig. 7b). The chromaticity coordinates of Eu3+-activated BaMoO4 phosphors were located in the red region of CIE chromaticity diagram. Hence, the present phosphors can be useful for the production of artificial white light to be similar to those of natural white light owing to its better spectral overlap and also as red component in white LEDs. Judd–Ofelt intensity parameters and radiative properties
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
hν (eV) Fig. 4. Plots of (ahm)2 versus the photon energy (hm) of BaMoO4: Eu3+ phosphors.
The radiative transitions of rare earth (RE) ions in different host matrices have been studied using Judd–Ofelt (J–O) theory [23,24]. The J–O intensity parameters (X2, X4) provide valuable information about the bonding nature of the RE ion with its surrounding ligands and the local structure around the RE ion site [25]. The J–O parameters were calculated from the PL emission spectra.
C. Shivakumara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 141–148
(a)
(a)
7
7
ex
= 394 nm
= 370 nm ex
BaMoO4: Eu
BaMoO4
BaMoO4: Eu
3+
(1 mol%)
3+
(3 mol%)
3+
(5 mol%)
7
5 F 0→ D3
5 F 0→ G2
BaMoO4: Eu
7
7
5 F0→ D4
PLE Intensity (a.u.)
λem = 615 nm
5 F 0→ D2
5 F 0→ L 6
146
340
360
380
400
420
440
460
480
500
5 7 D0→ F3
52 0 54 0 56 0
: Eu 3+
7 5 D0→ F2
700 720
680
4 : Eu 3+
(5 mol%)
)
)
Fig. 7. CIE chromaticity diagram of (a) BaMoO4 (kex = 370 nm), BaMoO4: Eu3+ (kex = 394 nm) phosphors and (b) BaMoO4: Eu3+ (kex = 464 nm) phosphors.
independent of host matrix whereas that of the 5D0 ? 7FJ (J = 2, 4) transitions depend solely on the XJ parameters. The integrated emission intensities of the spontaneous (radiative) emission of the transition between two manifolds 5D0 and 7FJ (J = 2, 4) were associated with the radiative emission rates and is as follows:
PL Intens ity (cps)
7 5 D0→ F4
5 7 D0→ F3
70 0 72 0
64 0 66 0 68 0
ol%
BaM oO
4
A0—2;4 I0—2;4 hm0—1 ¼ A0—1 I0—1 hm0—2;4
5.0x 6 10
4
60 0 62 0
(3 mol%)
3+
)
1.0x 7 10
BaM oO
56 0 58 0
ol%
ol%
1.5x 7 10
4
52 0 54 0
(5 m
(3 m
(1 m
BaM oO
Wa vel eng th ( nm )
(1 mol%)
3+
BaMoO4: Eu
2.0x1 7 0
5 7 D0→ F1
4
: Eu 3+
BaM oO
7 5 D1→ F1 5 7 D1→ F2
λ ex
6 =4
64 0 66 0
58 0 60 0 62 0
4
nm
3+
BaMoO4: Eu
5.0x 6 10
4
(c)
= 464 nm
BaMoO4: Eu
1.0x1 7 0
BaM oO
(nm )
ex
1.5x1 7 0
BaM oO
Wa vele ngt h
(b)
PL Intens ity (cps)
m
7 5 D0→ F4
4n
5 7 D1→ F1 5 7 D1→ F2
λ ex
9 =3
2.0x107
5 7 D0→ F1
)
(b
5 7 D0→ F2
Wavelength (nm)
: Eu 3+
: Eu 3+
: Eu 3+
(1 m
(3 m
ol%
(5 m
ol%
ol%
)
)
)
Fig. 6. (a) Excitation spectra of BaMoO4: Eu3+ (1 mol%) phosphor and (b and c) emission spectra of BaMoO4: Eu3+ red phosphors. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The J–O parameters were used to predict the radiative properties of excited states of Eu3+ ion such as transition probabilities (AT), radiative lifetime (srad), branching ratio (b), stimulated emission cross-section (re) and optical gain by using the expression reported in literature [26]. The intensity of the 5D0 ? 7F1 transition was
ð5Þ
where I0–2,4 is the integrated emission intensity and hm0–J is the energy corresponding to transition 5D0 ? 7FJ (J = 1, 2, 4). The transition 5D0 ? 7F0 is left out due to their small emission intensities. Due to the magnetic character of the 5D0 ? 7F1 transition and its weak dependence on crystal field effects, the value of the radiative emission rate A0–1 was estimated to be about 50 s1 [26]. The radiative emission rates A0–2,4 were related to forced electric dipole transitions and they may be written as a function of the J–O intensity parameters:
A0—2;4 ¼
64p4 ðm0—2;4 Þ3 e2 3hc
3
1 4pe0
v
X
J¼2;4
XJ
D
5
E2 D0 UðJÞ 7 FJ
ð6Þ
where v is the Lorentz local field correction factor given as function 2 nðn2 þ2Þ . The of the index of refraction n of the host v ¼ 9 D
5
E2 D0 UðJÞ 7 F2 are the square reduced matrix elements whose
147
C. Shivakumara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 141–148 Table 2 Calculated Judd–Ofelt intensity parameters, radiative emission rates, lifetime, branching and asymmetry ratios for BaMoO4: Eu3+ phosphors. kex (nm)
Eu (mol%)
J–O intensity parameters (1020 cm2)
X2
X4
394
1
1.98
1.86
Transitions
A0–2,4 (s1)
A0–1 (s1)
AT (s1)
srad (ms)
b (%)
Asymmetry ratio
5
D0 ? 7 F 1 D0 ? 7 F 2 5 D0 ? 7 F 4 5 D0 ? 7 F 1 5 D0 ? 7 F 2 5 D0 ? 7 F 4 5 D0 ? 7 F 1 5 D0 ? 7 F 2 5 D0 ? 7 F 4
– 125.55 57.60 – 128.33 48.78 – 132.92 48.95
50 – – 50 – – 50 – –
233.15
4.29
2.41
227.11
4.40
231.87
4.31
21.44 53.85 24.71 22.01 56.51 21.48 21.56 57.33 21.11
5
– 157.26 42.58 – 159.23 44.12 – 178.58 45.10
50 – – 50 – – 50 – –
249.84
4.00
3.02
253.35
3.95
273.68
3.65
20.01 62.94 17.05 19.74 63.65 16.61 18.27 65.25 16.48
5
464
3
2.02
1.58
5
2.10
1.59
1
2.48
1.38
D0 ? 7 F 1 D0 ? 7 F 2 5 D0 ? 7 F 4 5 D0 ? 7 F 1 5 D0 ? 7 F 2 5 D0 ? 7 F 4 5 D0 ? 7 F 1 5 D0 ? 7 F 2 5 D0 ? 7 F 4 5
3
2.51
1.43
5
2.82
1.46
2.46
2.55
3.05
3.43
Table 3 Effective bandwidth of the emission transition (Dkeff), stimulated emission cross-section (re), gain bandwidth (re Dkeff) and optical gain (re srad) of BaMoO4: Eu3+ phosphors. kex (nm) 394
Eu (mol%)
Transitions
Dkeff (nm)
re (1022 cm2)
re Dkeff (1028 cm3)
re srad (1025 cm2 s1)
1
5
8.54 7.51 8.04 8.67 9.60 6.13 7.83 9.28 5.08 9.29 7.74 5.16 8.58 9.44 5.27 7.95 9.28 4.80
2.66 8.98 6.46 2.62 7.18 7.15 2.90 8.69 7.69 2.45 12.39 7.88 2.65 8.95 7.28 2.86 9.21 8.29
2.27 6.74 5.19 2.27 6.89 4.38 2.27 8.06 3.91 2.26 9.59 4.07 2.27 8.45 3.84 2.27 8.55 3.98
11.41 38.52 27.71 11.53 31.59 31.46 12.50 37.45 33.14 9.80 49.56 31.52 10.47 35.35 28.76 10.44 33.62 30.26
D0 ? F1 D0 ? 7F2 5 D0 ? 7F4 5 D0 ? 7F1 5 D0 ? 7F2 5 D0 ? 7F4 5 D0 ? 7F1 5 D0 ? 7F2 5 D0 ? 7F4 5 D0 ? 7F1 5 D0 ? 7F2 5 D0 ? 7F4 5 D0 ? 7F1 5 D0 ? 7F2 5 D0 ? 7F4 5 D0 ? 7F1 5 D0 ? 7F2 5 D0 ? 7F4 5
3
5
464
1
3
5
7
values were independent of the chemical environment of the Eu3+ ion [27]. Their values were known and are solely E2 E2 D D 5 D0 Uð2Þ 7 F2 = 0.0032 and 5 D0 Uð4Þ 7 F4 = 0.0023. Thus, using Eqs. (5) and (6) the values of X2, 4 were obtained. The total radiative transition probability (AT) can be calculated using the equation below:
AT ðwJ Þ ¼
X AJJ0
ð7Þ
J0
The radiative lifetime srad(wJ) of an emitting state was related to its total transition probabilities to all sublevels, is given by
srad ðwJ Þ ¼
1 AT ðwJ Þ
ð8Þ
The branching ratio b(wJ) corresponding to the emission from an excited level to its lower levels is defined as
bðwJ Þ ¼
AðwJ ; w0J0 Þ AT ðwJ Þ
ð9Þ
The stimulated emission cross-section (re) was an essential parameter to signifies the rate of energy extraction from the lasing material and is calculated as
"
# k4p re ðkp Þ ¼ AT ðwJ Þ 8pcn2 Dkeff
ð10Þ
where kp is the emission peak wavelength, c is the velocity of light and Dkeff is the effective bandwidth of the emission transition. The Judd–Ofelt intensity parameters, radiative emission rates, lifetime, branching and asymmetry ratios are presented in Table 2. The magnitude of X2 parameter depends on Eu3+–O2 covalence and also explains the symmetry of coordination environment around Eu3+ ion [28]. The trend observed in the J–O parameters (X2 > X4) for BaMoO4: Eu3+ confirmed the low symmetry at the Eu3+ site and high covalence between the Eu3+ ion and the surrounding O2 ion. The radiative transition probability (A) and branching ratio (b) were considerably higher for the 5D0 ? 7F2 transition and increase with increasing Eu3+ concentration. It was recognized that an emission level with b value near 50% becomes a potential laser emission transition [29]. The asymmetric ratios for BaMoO4: Eu3+ phosphors found to be in increasing order with increase in Eu3+ concentration. This revealed that the Eu3+ ion occupy sites with a low symmetry and without an inversion center [30]. Table 3 illustrates the effective bandwidth of the emission transition (Dkeff), stimulated emission cross-section (re), gain bandwidth (re Dkeff) and optical gain (re srad) for Eu3+-activated
148
C. Shivakumara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 151 (2015) 141–148
BaMoO4 phosphors. The stimulated emission cross-section value is an important factor to predict the laser performance of a material and also the rate of the energy extraction from the lasing material. The trend of re for the 5D0 emission transition were observed as 7 F2 > 7F4 > 7F1 for Eu3+-activated BaMoO4 phosphors. The stimulated emission cross-section values of the prepared phosphors for 5 D0 ? 7F2 transition were found to be better than those registered for Eu3+ doped ZnAlBiB [31], Eu3+ doped KLa(PO3)4 [29] and Eu3+ ions in lithium aluminium borophosphate glasses [32]. The higher stimulated emission cross section value was an attractive feature for low threshold and high gain laser applications which were used to claim good laser action [29]. The product of emission cross-section and the effective bandwidth of the emission transition is a significant parameter to predict the bandwidth of the optical amplifier [33]. The higher the product values were, the better was the amplifiers performance. It was noteworthy that the maximum value of gain bandwidth was 9.59 1028 cm3 corresponding to 5D0 ? 7F2 transition for BaMoO4: Eu3+ (1 mol%) under kex = 464 nm. Similarly, product of radiative lifetime and stimulated emission cross section is a vital aspect for high optical amplifier gain. The obtained gain bandwidth and optical gain values corresponding to 5D0 ? 7F2 transition were found to be larger than Eu3+ doped ZnAlBiB [31]. Therefore, we can conclude that the 5 D0 ? 7F2 transition (615 nm) provide favorable lasing action, high gain bandwidth and optical gain for amplifiers. The large values of branching ratio, stimulated emission cross-section, gain bandwidth and optical gain suggest that the Eu3+-activated BaMoO4 phosphors can be useful for red laser applications and also for the development of color display devices. Conclusions
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2015.06.045. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21]
BaMoO4: Eu3+ phosphors were prepared by the facile nitrate–citrate gel combustion process. All the compounds have a monophasic scheelite-type tetragonal structure. FE-SEM revealed that the spherical particles agglomerate and form submicrometer particles. Enhancement in PL emission intensity with increase in Eu3+ concentration was observed. Based on the CIE chromaticity diagram, BaMoO4 phosphor revealed white emission whereas Eu3+-activated BaMoO4 phosphors exhibited red luminescence. The Judd–Ofelt parameters (X2 > X4) for BaMoO4: Eu3+ confirmed the higher covalent character of Eu3+–O2 bond and lower symmetry at the Eu3+ site. The present Eu3+-activated BaMoO4 phosphors show the higher stimulated emission cross-section, branching ratio, gain bandwidth and optical gain parameters for 5D0 ? 7F2 transition in comparison with different host matrix such as glasses and oxides. The results point out that these phosphor materials can be a good candidate for the development of artificial white light and red component in white LEDs for display devices as well as for red laser applications.
[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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![Synthesis of Sr2Si5N8:Ce3+ phosphors for white LEDs via an efficient chemical vapor deposition [corrected].](/assets/img/hold.png)
