Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
MgO:Eu3+ red nanophosphor: Low temperature synthesis and photoluminescence properties P.B. Devaraja a,b, D.N. Avadhani b, S.C. Prashantha c,⇑, H. Nagabhushana a, S.C. Sharma d, B.M. Nagabhushana e, H.P. Nagaswarupa c, H.B. Premkumar c a
Centre for Nano Research (CNR), Tumkur University, Tumkur 572 103, India Department of Physics, C.M.R.T.U, RV College of Engineering, Bangalore 560 091, India Department of Science, East West Institute of Technology, Bangalore 560 091, India d B.S.Narayan Center of Excellance for Advanced Materials, Department of Mechanical Engineering, B.M.S.Institute of Technology, Yelahanka, Bangalore 560 064, India e Department of Chemistry, M.S. Ramaiah Institute of Technology, Bangalore 560 054, India b c
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
Nanomaterials offer promising
opportunities for applications in various fields. Magnesium oxide (MgO) is an excellent wide band gap insulator. 3+ Structural and PL of MgO:Eu nanophosphor was studied. Highest emission was recorded at 3 mol% of Eu3+ doped MgO.
a r t i c l e
i n f o
Article history: Received 13 June 2013 Received in revised form 4 October 2013 Accepted 17 October 2013 Available online 25 October 2013 Keywords: MgO:Eu3+ Phosphor Photoluminescence Optical absorption
a b s t r a c t Nanoparticles of Eu3+ doped (0–9 mol%) MgO were prepared using low temperature (400 °C) solution combustion technique with metal nitrate as precursor and glycine as fuel. The powder X-ray diffraction (PXRD) patterns of the as-formed products show single cubic phase and no further calcination was required. The crystallite size was obtained using Scherer’s formula and was found to be 5–6 nm. The effect of Eu3+ ions on luminescence characteristics of MgO was studied and the results were discussed in detail. These phosphors exhibit bright red emission upon 395 nm excitation. The characteristic photoluminescence (PL) emission peaks at 580, 596, 616, 653, 696 and 706 nm (5D0 ? 7Fj = 0, 1, 2, 3, 4) were recorded due to Eu3+ ions. The electronic transition corresponding to 5D0 ? 7F2 of Eu3+ ions (616 nm) was stronger than the magnetic dipole transition corresponding to 5D0 ? 7F1 of Eu3+ ions (596 nm). The international commission on illumination (CIE) chromaticity co-ordinates were calculated from emission spectra, the values (x, y) were very close to national television system committee (NTSC) standard value of red emission. Therefore the present phosphor was highly useful for display applications. Ó 2013 Elsevier B.V. All rights reserved.
Introduction
⇑ Corresponding author. Tel.: +91 9945954010; fax: +91 9886021344. E-mail addresses: [email protected], [email protected] (S.C. Prashantha), [email protected] (H. Nagabhushana). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.060
Recently, much attention for red emitting long lasting luminescent phosphors have been focused to realize a full palette of persistent phosphor colors. This property makes long persistent phosphors, potential candidate to be applied in luminescent fields.
47
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
the sources of Mg and Eu, respectively. Glycine (NH2CH2COOH, 99.9%, Merck Ltd.) was used as a fuel for the synthesis of Eu3+ doped MgO powders. Calculated amount of Eu2O3 was dissolved in 1:1 HNO3 and excess nitric acid was evaporated on sand bath at 70 °C to obtain transparent europium nitrate solution. Aqueous solution containing stoichiometric amount of chemicals were taken in a Petri dish of 300 ml capacity. The stoichiometry of composition was calculated based on the total oxidizing and reducing valences of oxidizer and the fuel [24,25]. The excess water was allowed to evaporate by heating over a hot plate until a wet powder was left out. Then the petri dish wass introduced into a muffle furnace maintained at 400 ± 10 °C. The reaction mixture undergoes thermal dehydration and ignites at one spot with liberation of gaseous products such as oxides of nitrogen and carbon. The combustion propagates throughout the reaction mixture. The entire combustion process for producing MgO powder takes 5 min. Theoretical equation assuming complete combustion of the redox mixture used for the synthesis of cubic MgO may be written as 9MgðNO3 Þ2 þ 10NH2 CH2 COOH ! 9MgO þ 14N2 þ 25H2 O þ 20CO2
The crystalline nature of the powder sample was characterized by PXRD using Philips X-ray diffractometer (Shimadzu) using Cu Ka (1.541 Å) radiation with a nickel filter. FTIR studies of the samples were performed with a Perkin Elmer FTIR spectrophotometer (Spectrum-1000). The optical absorption studies of the sample were made in the range 200–800 nm using Elico SL-150 spectrophotometer. Photoluminescence studies were made using Horiba, (model fluorolog-3) spectrofluoremeter at RT. The surface morphology of the product was examined by Hitachi table top (SEM) (Model TM 3000). Results and discussion
(311) (222)
(220)
(111)
(200)
Fig. 1 shows the XRD pattern of undoped and Eu3+ doped MgO powder. The as-formed sample prepared by combustion synthesis show pure single cubic phases and no further calcination was required. All the X-ray diffraction peaks of the samples at (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) were indexed well and matched with JCPDS card No. 4–829 and belongs to a space group fm-3m (2 2 5) [26]. The lattice parameters and unit cell volume for cubic MgO
(d)
(c)
(b) (a)
Experimental Synthesis The raw materials, magnesium nitrate (Mg(NO3)26H2O: 99.9%, Merck Ltd.), and europium oxide (Eu2O3: 99.99%, Rolex Ltd.) were
ð1Þ
Characterization
Intensity (a.u)
Also, the demand for developing efficient luminescent materials such as rare earth activated powders attracted researchers because of their possible photonic applications, good luminescent characteristics, stability in high vacuum and absence of corrosive gas emission under electron bombardment when compared to currently used sulfide based phosphors [1,2]. Red emitting persistent phosphors were also desired in biomedical applications, e.g. in vivo imaging, where the phosphor emission should be situated between 600 and 1100 nm where biological tissue has the highest transparency. This was exactly the spectral region where the eye sensitivity was highest under very low illumination levels [3–8]. Currently, nanomaterials have become important with in the field of luminescence as they exhibit excellent optoelectronic properties. They have potential to be efficient phosphors in flat panel displays, solar energy converters, optical amplifiers, electroluminescent devices, photodiodes, bio-detectors, in lamp industries, radiation dosimetry, color display etc. [9–14]. In particular, oxide nanophosphors with the incorporation of rare earth activators have revealed major luminescence effects. In addition, various vacancies and defects of host materials results in wide variety of attractive luminescence features [15]. Among the rare earth ions, red-emitting trivalent europium (Eu3+) is recognized as an efficient red luminescent phosphors due to its 5D0 ? 7Fj (j = 0, 1, 2, 3, 4) transitions which were used in color television displays and mercury free lamps. Efforts have been made to enhance the luminescence nature of Eu3+ in host materials with low phonon energies. An appropriate selection of the host lattice and the suitable Eu3+ dopant concentration enhances the red emission [16–18]. The design of phosphors applied in the vacuum ultra violet (VUV) region compared to conventional mercury discharge sources. When, the phosphor host itself or doped activators should exhibit a strong absorption around these (147 or 172 nm) wavelengths and the proper hosts needed to hold the rare earth ions and their luminescence properties in the VUV range should be understood [17]. It has been known that reduction in the particle size in a crystalline system can result in remarkable modification of properties compared to those of bulk due to high surface-to-volume ratio. The modification of luminescence properties can be brought using different preparation techniques, different capping agents and use of suitable sensitizer, etc. [16]. The changes in the electrical and optical characteristics of very small particles were caused by the quantum size effect, which was generated by an increase in the band gap due to a decrease in the quantum allowed state that exists in small particles and the high surface-to-volume ratio, which improves the surface and interface effects. [19,20]. Over the past few years, synthesis of inorganic nanoscale materials with specific morphologies attracted the phosphor developers. Oxide phosphors were synthesized by a variety of routes such as solid-state reactions, sol–gel, hydrothermal, co-precipitation, microwave techniques etc. In the present studies, MgO:Eu3+ nanopowders were synthesized by low temperature combustion synthesis route. This process provides molecular level of mixing and high degree of homogeneity, short reaction time that leads to reduction in crystallization temperature and prevents from segregation during heating [21–23]. In this paper a novel and simple solution combustion synthesis of MgO nanocrystals with size of 5–6 nm and the effect of Eu3+ ions on the photoluminescence properties were studied systematically.
30
40
50
60
70
80
2 θ (degree) Fig. 1. XRD patterns of (a) undoped (b) 1 mol% (c) 5 mol% and (d) 9 mol% Eu3+ doped MgO.
48
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
were estimated using the following relations and for (2 0 0) plane are found to be 4.3 Å and 79.507 1030 m3 respectively.
2dsinh ¼ nk
ð2Þ
a dhkl ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 h þk þl
ð3Þ
The average particle size (D) was estimated from the line broadening in X-ray powder using Scherrer’s formula [D = (Kk)/b cosh, where, ‘K’ constant, ‘k’ wavelength of X-rays, and ‘b’ FWHM] was found to be in the range 5–6 nm [27,28]. No shift in diffraction peaks and no secondary phase were detected indicating that Eu3+ ions are obviously homogeneously mixed and effectively doped in the host lattice in Mg2+ sites since, the ionic radii are close each 2þ other (Since, the ionic radii of R3þ Eu = 1.07 nm and RMg = 0.72 nm are very close to each other). From the analysis of XRD, it was revealed that the introduction of an activator (Eu3+) did not influence the crystal structure of the phosphor matrix. Further, strain present in the MgO nanoparticles prepared by combustion method was estimated using the W–H equation.
b cos h ¼
0:9k þ 4e sin h D
ð4Þ
where ‘e’ is the strain associated with the nanoparticles [29]. The above equation represents a straight line between ‘4 sinh’ (x-axis) and ‘b cosh’ (y-axis). The slope of the line gives the strain and intercept of this line on y-axis gives grain size (D). It was found that the strain present on the surface of the sample was 3.2 104. The room temperature Fourier transform infrared (FTIR) spectra of pure and Eu3+ doped MgO samples synthesized via combustion method was recorded in the range 300–4000 cm1 using KBr pellets was shown in Fig. 2. The strong bond at 415 cm1 was associated with the characteristic vibrational mode of symmetric MgO6 octahedral of MgO. The absorption at 3500 cm1 indicate the presence of hydroxyl groups (surface adsorbed), which was probably due to the fact that the spectra were not recorded in situ and some water re-adsorption from the ambient atmosphere as occurred. The absorption in the range of 1300–1800 cm1 related to hydroxyl group of molecular water at 1635 cm1. Further, the peaks at 550–850 cm1 (c1) higher frequency of MgO stretching and 410–450 cm1 (c2) was lower frequency stretching. The peak at 880 cm1 was attributed to Mg–O–Mg interactions [30–32]. The UV–Vis absorption spectra of MgO samples were shown in Fig. 3. It was well established that nanoscale materials have large
surface to volume ratio. This results in the formation of voids on the surface as well inside the agglomerated nanoparticles. Such voids can cause fundamental absorption bands in the UV region. In addition, surface of nanoparticles were well known to comprise of several defects normally dangling bonds, regions of disorder and absorption of impurity species that result in the absorption of nanocrystals [24]. Thus in the UV-absorption spectrum of pure MgO nanocrystals shows absorption bands at 305 nm (4 eV) and 395 nm (3.14 eV) were identified with the excitation of 4-fold and 3-fold co-ordinated O2 anions in edges and corners (also called F+, F centers.) respectively. Also, the absorption band at 305 nm may be attributed to the surface defects in nano MgO:Eu3+ phosphor, which corresponds to ligand-to-metal charge-transfer (LMCT) group. The weak absorption band from 320–475 nm was attributed to the surface state of MgO [33]. Further, the intra configurationally 4f 4f transitions from the ground 7Fo level which corresponds to the excitation spectra. In case of smaller size nanoparticles, it was found that increase of defect distribution on the surface exhibit broad absorption bands due to large surface to volume ratio [34,35]. The optical energy gap (Eg) of pure and doped MgO samples prepared through combustion technique were estimated using Tauc relation [36] and their values were in the range 5.40–5.55 eV (Fig. 3). It was observed from the spectra the Eg values were shifting with increase of Eu3+ concentrations, which was attributed to quantum confinement effect [37–39]. Fig. 4 shows the SEM pictures of undoped and doped MgO samples prepared via combustion technique. Combustion derived MgO was foamy product with large agglomerates. Further, SEM results reveal that the powder is porous and highly agglomerated. It was well known that, combustion synthesis reaction was highly influenced by metal–ligand complex formation. Depending upon the type of fuel and metal ions, the nature of combustion differs from flaming (gas phase) to nonflaming (smoldering and heterogeneous) type. Generally, flaming reactions involves liberation of large quantity of gas. The pores and voids can be attributed to the large amount of gases escaping out of the reaction mixture during combustion. SEM photographs of the samples confirm smooth and uniform nanoparticles were obtained in this synthesis method [40]. The undoped and doped phosphors were agglomerated from few microns to a few tens of microns, fluffy and porous in nature. SEM photographs of the samples also reveal smooth and uniform nanoparticles were obtained. It was found that the dopant concentration does not influence the morphology of the sample.
6x108
(d)
305 nm
(a)
Uv-Vis absorption (au)
-1 2
3x108
1. Undoped 2. Eu-1mol% 3. Eu-5mol% 4. Eu-9mol%
395 nm
1 2 3 4
2
(b)
4x108
(α hν) (eV cm )
Transmittace %
(c)
2
5x108
undoped Eu 1 mol% Eu 5 mol% Eu 9 mol%
MgO:Eu3+ (1,5,9 mol%)
2x108
300
400
500
600
700
800
Wavelength (nm) 8
1x10
0 4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 2. FT-IR spectra of (a) undoped (b) 1 mol% (c) 5 mol% and (d) 9 mol% Eu3+ doped MgO.
3
4
5
6
Energy (eV) Fig. 3. Uv–Vis absorption spectra of (a) undoped (b) 1 mol% (c) 5 mol% and (d) 9 mol% Eu3+ doped MgO.
49
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
Fig. 4. SEM pictures of (a) undoped (b) 1 mol% (c) 5 mol% and (d) 9 mol% Eu3+ doped MgO.
1, 2, 3, 4). Fig. 7 shows comparison of emissions excited with 305 and 395 nm light. In particular, the emission peak at 616 nm corresponds to 5D0 ? 7F2 and occurs through the forced electric dipole (FED), while the 5D0 ? 7F1 at 596 nm wass the magnetic dipole transitions. The peak around 616 nm and 596 nm denote the Eu3+ ions occupy the Mg2+ sites at octahedral sites. These two emissions are of particular interest because they represent actually the local environment of the Eu3+ ions. The 5D0 ? 7F1 was magnetic dipole allowed and its intensity shows variation with the crystal field strength surrounding the Eu3+ ions whereas, 5D0 ? 7F2 hypersensitive transition was forced electric dipole allowed and its intensity was sensitive to the local structure acting on the Eu3+ ion [28,41–43,26]. The Eu3+ ions enter into the host lattice and replace magnesium ion located on the surface of the nanocrystals because of the porosity of MgO. As dopant concentration of Eu3+ increases, 5 D0 ? 7F2 transition dominates and the emission intensity increases. This may be attributed to the increase distortion of the local field around the Eu3+ ions [44,45].
7
F2
The excitation spectra of Eu doped MgO monitored at 616 nm emission was shown in Fig. 5. The excitation spectra consist of a charge transfer band (CTB) of Eu3+ – O2 band in the short ultraviolet region (305 nm). The main factor affecting the intensity of the charge transfer band was the efficiency of the energy process from the CTB to the Eu3+ emitting level. It shows that with increasing heat treatment, the interaction between O2 and Eu3+ ions in the host becomes stronger which facilitates the electron transfer from O2 to Eu3+ [17]. In the present study, the weak excitation peak observed at 395 nm suggests that the interaction between O2 and Eu3+ was stronger; hence the efficiency of the energy transfer process from the charge transfer band to Eu3+ emitting levels increases. The f–f transitions within the Eu3+, 4f6 configuration in longer spectral region with 7F0 ? 5D3 (395 nm) was the most prominent group. Photoluminescence emission spectra of MgO:Eu3+ excited with 395 nm was shown in Fig. 6. A series of emission peaks at 580 nm, 596 nm, 616 nm, 653 nm, 696 nm and 706 nm may be attributed to the characteristic transitions of Eu3+ ions from 5D0 ? 7Fj (j = 0,
200
250
300
350
396
400
450
398
500
Wavelength (nm) Fig. 5. Excitation spectra of MgO (emission at 616 nm).
500
D0
5
D0
5
7
F0
D0
7
F1 394
F4
7
F3
D0
5
D0
7 5
392
1 mol% Eu 3 mol% Eu 5 mol% Eu 7 mol% Eu 9 mol% Eu
5
395nm
Photoluminescence Intensity (a.u)
PL Intensity (a.u)
305nm
550
600
650
700
Wavelength (nm) Fig. 6. Emission spectra of MgO: Eu3+ (1–9 mol%) (excited at 395 nm).
50
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
Photoluminescence Intensity (a.u)
613 nm
550
λ exc - 395 nm λ exc - 305 nm
596 nm 578 nm
600
650 nm
707 nm 694 nm
650
700
750
Wavelength (nm) Fig. 7. Comparison of emission spectra excited with 305 nm and 395 nm. Fig. 8. The energy level diagram of Eu3+ ion showing the states involved in the luminescence process and the transition probabilities.
MgO has a rock salt structure (fcc) with magnesium ions occupying octahedral sites within the anion closed packed structure. Its ionic constituents comprise a relatively small number of electrons, its nanostructures were expected to have novel properties superior to their bulk counterparts due to the quantum confinement effect of nanocrystals [46]. When RE3+ ions doped into the host, they could probably occupy these sites. On the other hand, as indicated in the emission spectrum of the undoped MgO nanocrystals, the strong emission band centered at 390 nm originating from the contribution of oxygen vacancies formed during the combustion process, which was justly favored for the 7F0–5L6 transition of Eu3+ ions [47]. Therefore, the enhanced emission comes mainly from radiative recombination of the large amount of trapped carriers excited from MgO. It can be assumed that the oxygen vacancies formed during the combustion process might act as sensitizer for the energy transfer to the rare-earth ions at the surface, due to the strong mixing of charge transfer states resulting in the highly enhanced luminescence. Based on this, Eu3+ was chosen to study luminescent properties of the material. The Eu3+ energy level diagram corresponding to the 5Do ? 7Fj (j = 0, 1, 2, 3, 4) emission peaks was shown in Fig. 8. When the phosphor was excited by 305 nm or 395 nm wavelength, Eu3+ ion was raised to 5L6 level from the ground state. Since, the separation between 5D0 ? 7Fj (j = 0, 1, 2, 3, 4) was large, the stepwise decay process stops here and returns to ground state by giving emission in the orange and red regions. If the Eu3+ impurity does not occupy centre of symmetry of the crystal lattice, then it will give both magnetic and electric dipole transitions. When the rare earth impurity ion was located at the centre of symmetry in the relevant crystal lattice only magnetic dipole transition were allowed. The energy-level diagram indicates the states involved in the luminescence processes and the transition probabilities for Eu3+ ions. According to the model, the system is first excited from the ground state (5D3 configuration) to the singlet state of the 5D3, 2, 1, 0 configurations. Then the electrons pass to the triplet state, mainly to level 4 because of symmetry reasons. The last transition 5Do was so much faster than any other step of the luminescence process. It may be considered at once that the singlet state does not affect the luminescent process. Non-radiative transitions occur between the third energy levels of the triplet state, named 5D3, 5D2, 5D1 and 5Do with probabilities from level 3 to level 2, level 2 to level 1, and level 1 to level 0 i.e., 5Do. Level 5Do to level 7F0, 1, 2, 3, 4, occurs radiative transitions to the ground state i.e., 5Do ? 7Fj states respectively. The increase in PL intensity observed might be due to the decrease of cross relaxation between Eu3+ ions in this process [48,49].
The variation of PL intensity and asymmetric ratio at different europium mol concentration was shown in Fig. 9. Eu3+ ion in MgO prepared by combustion synthesis was situated at the low symmetry sites. Eu3+ ions enter into Mg2+ lattice site. The ionic radii of Eu3+ and Mg2+ were 1.07 and 0.72 nm respectively. Since ionic radius of Mg2+ was smaller than Eu3+, host could accommodate only small percentage of impurity ions or it was suggested that Eu3+ ions will likely reside on the surface or on the grain boundaries of the nanocrystals to yield optimum strain relief. Moreover, there was charge imbalance in the host lattice due to doping of trivalent Eu3+ cations. This may absorb emitted light, resulting into decrease of intensity. It was shown that relative intensity of the emission lines of Eu3+ depends on the doping concentration of Eu3+ in the host phosphor. By increasing the Eu3+ mol concentration the transition 5D0 ? 7F2 (616 nm) has shown an enhanced emission and reaches saturation at 5 mol% concentration and PL intensity decreases thereafter it decreases due to the concentration quenching. The concentration quenching occurs because of the energy transfer from one activator to another. The critical distance for energy transfer (Rc) in MgO:Eu3+ can be calculated from the structural parameters with unit cell volume (V) and the number of total Eu3+ sites per unit cell (N), together with the critical concentration (Xc) [50].
Rc 2
3V 4p X c N
1=3 ð5Þ 0
For the MgO:Eu3+ system, N = 4, V = 79.507 (Å A)3 and Xc = 0.05. The 3+ 3+ critical transfer distance of Eu in MgO:Eu is determined to be 0 9.124 Å A. The critical0 energy distance between Eu3+ ions in MgO was greater than 5 A Å as a result overlapping between excitation and emission spectra decreases. Another important parameter, which was sensitive to the nature of the Eu3+ ions environment in the host lattice, was the asymmetric ratio (A21) [51,52]. Which, gives a measure of the degree of distortion from inversion symmetry of the local environment surrounding the Eu3+ ions in the host matrix.
H 5 I2 ð D0 !7 F 2 Þdk A21 ¼ H 5 I1 ð D0 !7 F 1 Þdk
ð6Þ
where I2: intensity of electric dipole transition at 616 nm (5D0 ? 7F2) and I1: intensity of the magnetic dipole transition at
51
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
Photoluminescence Intensity (a.u)
1.3440
I 616 nm / I 596 nm
578 nm 596 nm 616 nm
1.3424
5 mol %
1.3408
1
3
5
7
1
9
3
5
7
9
Eu3+ concentration (mol%)
Eu3+ Concentration (mol %)
Fig. 9. The effect of Eu3+ on the 596 nm and 616 nm emission peaks and variation of asymmetric ratio with Eu3+ concentration in MgO nanophosphors.
9.4
0.9 MgO:Eu 3+
Slope: -0.919 Q : 7.45
9.2
0.6
Y
log (I/x)
9.0
8.8
0.3
8.6 0.0 8.4
0.0 -2.0
-1.8
-1.6
-1.4
-1.2
between
log (x)
and
log (I/x)
-1.0
0.6
0.8
Fig. 11. CIE diagram of MgO:Eu3+ (5 mol%) nanophosphor. in
MgO:Eu3+
(1–9 mol%)
596 nm (5D0 ? 7F2). The values of A21 decrease with increase of Eu3+ ions. However, it is reasonable to believe that the doping of Eu3+ will introduce lattice defects, which will undoubtedly reduce the symmetry strength of the local environment of Mg2+ sites. Consequently, the symmetry ratio of MgO:Eu3+ decreases with the increase of doped Eu3+ concentration [53]. Van Uitert’s [54] early study has developed a theoretical description for the relationship between the luminescent intensity and the doping concentration. Van Uitert fingered out that if the energy transfer occurs among the same sorts of activators due to electric multipolar interaction, the relationship between the luminescent intensity and the doping concentration can be expressed by the relation 1 I ¼ k½1 þ bðXÞQ =3 X
0.4
X
log (x) Fig. 10. Relation nanophosphor.
0.2
ð7Þ
where X: Eu3+ ion concentration, k and b: constants, Q = 6, 8 and 10 for dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions. The value of Q can be determined by plotting log (X) vs log (I/X) (Fig. 10) which gives a linear graph having a intercept,
Table 1 CIE co-ordinates values of different Eu mol% concentration in MgO host matrix. Eu3+ concentration (mol%)
X
Y
1 3 5 7 9
0.5389 0.5577 0.645 0.5625 0.5592
0.3601 0.3699 0.353 0.3992 0.3791
slope = 0.919 and Q = 7.45, which was close to 8. The Q value indicates that the concentration quenching in MgO was due to dipole– dipole interaction [55]. The luminous color was depicted by studying color co-ordinates and color ratios of the phosphor. The values of chromaticity coordinates of the MgO:Eu3+ (1–9 mol%) phosphor have been estimated from 1931 CIE (International Commission on Illumination) system and was shown in Fig. 11. and their corresponding values were given in Table 1. It was observed that the CIE co-ordinates of 5 mol% Eu3+ activated MgO:Eu3+ phosphor was measured as
52
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
(x = 0.645, y = 0.353) which was very close to the NTSC (National Television System Committee) standard values (x = 0.67, y = 0.33) [56]. Their corresponding location has been marked in Fig. 11 with star in red region. Conclusions Phase pure Eu3+ doped MgO nanophosphors were prepared by simple and low cost solution combustion method. The powder Xray diffraction (PXRD) pattern showed the single phase, cubic structure and the particle size was observed in the nanoscale. The UV–Vis absorption of un-doped and doped phosphor show an intense absorption band in the region 300–400 nm corresponding to ligand-to-metal charge-transfer (O2 to Eu3+) band. The phosphor exhibits different emission (in the range 540 nm to 660 nm) due to Eu3+ corresponding to 5D0 ? 7Fj (j = 0, 1, 2, 3, 4) transitions. The transition centered at 616 nm was found to be hypersensitive in nature resulting in a strong and red emission. Enhancement in photoluminescence (PL) intensity of Eu3+ was observed due to the formation of different lattice sites in the host phosphor. In the present work, MgO doped with Eu3+ were investigated as red phosphor with good emission color purity due to the non-centrosymmetric site for the europium ion. 5 mol% Eu doped sample showed enhanced PL intensity and thereafter, it decreases due to concentration quenching. It was observed that the emission spectrum excited at 395 nm showed prominent spectral lines compared to 305 nm excitation. Further, 395 nm is close to visible region (NUV) which may be useful for LED applications. The excellent red emission properties and the estimated CIE chromaticity co-ordinates (x, y) were very close to NTSC standard value of red emission of this phosphor suggest that it may be used for display and white light emitting applications. Acknowledgements One of the author HN thanks to DST nano mission for funding of research project to Center for Nano Research, Tumkur University, Tumkur for extended to carry out this research work. The authors PBD, SCP and HPN thanks to VGST, Govt. of Karnataka, India, (VGST/CISEE-GRD 190) for extended to carry out this research work. References [1] H.F. Brito, J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, L.C.V. Rodrigues, Opt. Mater. Expr. 2 (2012) 371–381. [2] T. Maldiney, B. Viana, D. Gourier, C. Richard, D. Scherman, M. Bessodes, K. Van den Eeckhout, D. Poelman, P.F. Smet, Opt. Mater. Expr. 2 (2012) 261–268. [3] Danuta. Dutczak, Cees. Ronda, Andries. Meijerink, Thomas. Jüstel, J. Lumin. 141 (2013) 150–154. [4] P. Zhang, M. Xu, Z. Zheng, B. Sun, Y. Zhang, Trans. Nonferr. Mat. Soc. China. 16 (2006) 423–425. [5] X.X. Li, Y.H. Wang, Z. Chen, J. Alloy. Comp. 453 (2008) 392–394. [6] K.Y. Jung, J.H. Kim, J. Lumin. 128 (2008) 2004–2007. [7] H.S. Yoo, W.B. Im, J.H. Kang, D.Y. Jeon, Opt. Mater. 31 (2008) 131–135. [8] L. Zhou, W.C.H. Choy, J. Shi, M. Gong, H. Liang, J. Alloy Comp. 463 (2008) 302– 305. [9] Z. Zhang, Y. Wang, J. Electr. Soc. 154 (2007) J62–J64. [10] L.Y. Zhou, F.Z. Gong, J.X. Shi, M.L. Gong, H.B. Liang, Mater. Res. Bull. 43 (2008) 2295–2299. [11] T.S. Chan, R.S. Liu, I. Baginskiy, N. Bagkar, B.M. Cheng, J. Electr. Soc. 155 (2008) J284–J286. [12] Z.J. Zhang, S. Chen, J. Wang, X.X. Yang, J.T. Zhao, Y. Tao, H.H. Chen, Y. Huang, Opt. Mater. 32 (2009) 99–103.
[13] P. Zhang, L.X. Li, M.X. Xu, L. Liu, J. Alloy. Compd. 456 (2008) 216–219. [14] P. Zhang, M.X. Xu, Z.T. Zheng, B. Sun, Y.H. Zhang, Mater. Sci. Eng. B. 136 (2007) 159–164. [15] N. Dhananjaya, H. Nagabhushanan, B.M. Nagabhushana, B. Rudraswamy, C. Shivakumara, K.P. Ramesh, R.P.S. Chakradhar, Physica B. 406 (2011) 1645– 1652. [16] Ganngam. Phaomei, W. Rameshwor Singh, R.S. Ningthoujam, J. Lumin. 131 (2011) 1164–1171. [17] Feng Zhang, Yuhua Wang, Yan Wen, Dan Wang, Ye Tao, Opt. Mater. 33 (2011) 475–479. [18] H. Shabir, B. Lal, M. Rafat, J. Sol–Gel Sci. Tech. 53 (2010) 399–404. [19] S. Cho, R. Lee, H. Lee, J. Kim, C. Moon, S. Nam, J. Park, J. Sol–Gel Sci. Tech. 53 (2010) 171–175. [20] Juan Wang, Xu Yunhua, Mirrabos Hojamberdiev, Jianhong Peng, Gangqiang Zhu, J. Noncryst. Solid. 355 (2009) 903–907. [21] Yonghu Chen, Xuerui Cheng, Miao Liu, Zeming Qi, Chaoshu Shi, J. Lumin. 129 (2009) 531–535. [22] S. Ekambaram, K.C. Patil, J. Mater. Chem. 5 (1995) 905–908. [23] H.B. Premkumar, D.V. Sunitha, H. Nagabhushana, S.C. Sharma, B.M. Nagabhushana, C. Shivakumara, J.L. Rao, R.P.S. Chakradhar, J. Lumin. 135 (2013) 105–112. [24] S.C. Prashantha, B.N. Lakshminarasappa, B.M. Nagabhushana, J. Alloy Comp. 509 (2011) 10185–10189. [25] K.C. Patil, M.S. Hegde, Tanu Rattan, S.T. Aruna, Chemistry of Nanocrystalline Oxide Materials, Combustion Synthesis, Properties and Applications, World Scientific Publishing Co., Pt., Ltd., UK, 2008. [26] Gu Feng, Chun Zhong Li, Hai Bo Jiang, J. Cryst. Grow. 289 (2006) 400–404. [27] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435–445. [28] S.C. Prashantha, B.N. Lakshminarasappa, Fouran Singh, J. Lumin. 132 (2012) 3093–3097. [29] G.K. Williamson, W.H. Hall, Acta. Metal. 1 (1953) 22–31. [30] Latha Kumari, W.Z. Li, Charles H. Vannoy, Roger M. Leblanc, D.Z. Wang, Ceram. Intern. 35 (2009) 3355–3364. [31] T. Selvamani, T. Yagyu, S. Kawasaki, I. Mukhopadhyay, Catal Commun. 11 (2010) 537–541. [32] S.M. Borghei, S. Kamali, M.H. Shakib, A. Bazrafshan, M.J. Ghoranneviss, J. Fusi. Enery 30 (2011) 433–436. [33] K. Kviyarasu, P.A. Devarajan, Adv. Appl. Sci. Res. 2 (2011) 131–138. [34] G.A. Kumar, C.W. Chen, J. Ballato, R.E. Riman, Chem. Mater. 19 (2007) 1523– 1528. [35] S. Atalay, H.I. Adiguzel, F. Atalay, Mater. Sci. Eng. A 304 (2001) 796–799. [36] J. Tauc, R. Grigorovici, A. Vancu, Phys. Stat. Sol. 15 (1966) 627–638. [37] Lingling Peng, Yuhua Wang, Zhaofeng Wang, Qizheng Dong, Appl. Phys. A 102 (2011) 387–392. [38] M. Sterrer, T. Berger, O. Diwald, E. Knozinger, J. Am. Chem. Soc. 125 (2003) 195–199. [39] E. Garrone, A. Zecchina, F.S. Stone, Philos. Mag. Part B. 42 (1980) 683–703. [40] D.V. Sunitha, C. Manjunatha, C.J. Shilpa, H. Nagabhushana, S.C. Sharma, B.M. Nagabhushana, N. Dhananjaya, C. Shivakumara, R.P.S. Chakradhar, Spectrosc. Acta Part A: Mole. Biomole. Spectrosc. 99 (2012) 279–287. [41] Atsushi Ishizumi, Satoshi Fujita, Hisao Yanagi, Opt. Mater. 33 (2011) 1116– 1119. [42] A. Bahadur, Y. Dwivedi, S.B. Rai, Spectrosc. Acta Part A: Mole. Biomole. Spectrosc. 91 (2012) 217–221. [43] Hong-Sheng Huang, Zhi-Hong Liu, J. Lumin. 140 (2013) 114–119. [44] Vu Nguyen, Tran kim Anh, Gyu-chul Yi, W. Trek, J. Lumin. 122 (2007) 776–779. [45] Oleg Viagin, Andrey Masalov, Irina Ganina, Yuriy Malyukin, Opt. Mater. 31 (2009) 1808–1810. [46] S.Y. Yang, S.G. Kim, Powd. Tech. 146 (2004) 185–192. [47] Changhyun Jin, Hyunsu Kim, Sunghoon Park, Chongmu Lee, J. Alloy. Comp. 541 (2012) 163–167. [48] I.M. Nagpure, Subhajit Saha, S.J. Dhoble, J. Lumin. 129 (2009) 898–905. [49] W. Stambouli, H. Elhouichet, B. Gelloz, M. Férid, J. Lumin. 138 (2013) 201–208. [50] G. Blasse, J. Sol. Sta. Chem. 62 (1986) 207–211. [51] M. Catti, G. Gazzoni, G. Ivaldi, Acta Crystall. C 39 (1983) 29–34. [52] M. Chandrasekhar, H. Nagabhushana, S.C. Sharma, K.H. Sudheer kumar, N. Dhananjaya, D.V. Sunitha, C. Shivakumara, B.M. Nagabhushana, J. Alloy. Comp. doi.org/10.1016/j.jallcom.2013.08.149. [53] N. Dhananjaya, H. Nagabhushana, B.M. Nagabhushana, B. Rudraswamy, C. Shivakumara, R.P.S. Chakradhar, J. Alloy. Comp. 509 (2011) 2368–2374. [54] L.G. Van Uitert, J. Electr. Soc. 114 (1967) 1048–1053. [55] Fu Yang, Zhiping Yang, Yu Quanmao, Xu Yufeng Liu, Fachun Lu Li, Spectrosc. Acta Part A: Mole. Biomole. Spectrosc. 105 (2013) 626–631. [56] Devayani Chikte, S.K. OmanwarOmanwar, S.V. Moharil, J. Lumin. 142 (2013) 180–183.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
MgO:Eu3+ red nanophosphor: Low temperature synthesis and photoluminescence properties P.B. Devaraja a,b, D.N. Avadhani b, S.C. Prashantha c,⇑, H. Nagabhushana a, S.C. Sharma d, B.M. Nagabhushana e, H.P. Nagaswarupa c, H.B. Premkumar c a
Centre for Nano Research (CNR), Tumkur University, Tumkur 572 103, India Department of Physics, C.M.R.T.U, RV College of Engineering, Bangalore 560 091, India Department of Science, East West Institute of Technology, Bangalore 560 091, India d B.S.Narayan Center of Excellance for Advanced Materials, Department of Mechanical Engineering, B.M.S.Institute of Technology, Yelahanka, Bangalore 560 064, India e Department of Chemistry, M.S. Ramaiah Institute of Technology, Bangalore 560 054, India b c
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
Nanomaterials offer promising
opportunities for applications in various fields. Magnesium oxide (MgO) is an excellent wide band gap insulator. 3+ Structural and PL of MgO:Eu nanophosphor was studied. Highest emission was recorded at 3 mol% of Eu3+ doped MgO.
a r t i c l e
i n f o
Article history: Received 13 June 2013 Received in revised form 4 October 2013 Accepted 17 October 2013 Available online 25 October 2013 Keywords: MgO:Eu3+ Phosphor Photoluminescence Optical absorption
a b s t r a c t Nanoparticles of Eu3+ doped (0–9 mol%) MgO were prepared using low temperature (400 °C) solution combustion technique with metal nitrate as precursor and glycine as fuel. The powder X-ray diffraction (PXRD) patterns of the as-formed products show single cubic phase and no further calcination was required. The crystallite size was obtained using Scherer’s formula and was found to be 5–6 nm. The effect of Eu3+ ions on luminescence characteristics of MgO was studied and the results were discussed in detail. These phosphors exhibit bright red emission upon 395 nm excitation. The characteristic photoluminescence (PL) emission peaks at 580, 596, 616, 653, 696 and 706 nm (5D0 ? 7Fj = 0, 1, 2, 3, 4) were recorded due to Eu3+ ions. The electronic transition corresponding to 5D0 ? 7F2 of Eu3+ ions (616 nm) was stronger than the magnetic dipole transition corresponding to 5D0 ? 7F1 of Eu3+ ions (596 nm). The international commission on illumination (CIE) chromaticity co-ordinates were calculated from emission spectra, the values (x, y) were very close to national television system committee (NTSC) standard value of red emission. Therefore the present phosphor was highly useful for display applications. Ó 2013 Elsevier B.V. All rights reserved.
Introduction
⇑ Corresponding author. Tel.: +91 9945954010; fax: +91 9886021344. E-mail addresses: [email protected], [email protected] (S.C. Prashantha), [email protected] (H. Nagabhushana). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.10.060
Recently, much attention for red emitting long lasting luminescent phosphors have been focused to realize a full palette of persistent phosphor colors. This property makes long persistent phosphors, potential candidate to be applied in luminescent fields.
47
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
the sources of Mg and Eu, respectively. Glycine (NH2CH2COOH, 99.9%, Merck Ltd.) was used as a fuel for the synthesis of Eu3+ doped MgO powders. Calculated amount of Eu2O3 was dissolved in 1:1 HNO3 and excess nitric acid was evaporated on sand bath at 70 °C to obtain transparent europium nitrate solution. Aqueous solution containing stoichiometric amount of chemicals were taken in a Petri dish of 300 ml capacity. The stoichiometry of composition was calculated based on the total oxidizing and reducing valences of oxidizer and the fuel [24,25]. The excess water was allowed to evaporate by heating over a hot plate until a wet powder was left out. Then the petri dish wass introduced into a muffle furnace maintained at 400 ± 10 °C. The reaction mixture undergoes thermal dehydration and ignites at one spot with liberation of gaseous products such as oxides of nitrogen and carbon. The combustion propagates throughout the reaction mixture. The entire combustion process for producing MgO powder takes 5 min. Theoretical equation assuming complete combustion of the redox mixture used for the synthesis of cubic MgO may be written as 9MgðNO3 Þ2 þ 10NH2 CH2 COOH ! 9MgO þ 14N2 þ 25H2 O þ 20CO2
The crystalline nature of the powder sample was characterized by PXRD using Philips X-ray diffractometer (Shimadzu) using Cu Ka (1.541 Å) radiation with a nickel filter. FTIR studies of the samples were performed with a Perkin Elmer FTIR spectrophotometer (Spectrum-1000). The optical absorption studies of the sample were made in the range 200–800 nm using Elico SL-150 spectrophotometer. Photoluminescence studies were made using Horiba, (model fluorolog-3) spectrofluoremeter at RT. The surface morphology of the product was examined by Hitachi table top (SEM) (Model TM 3000). Results and discussion
(311) (222)
(220)
(111)
(200)
Fig. 1 shows the XRD pattern of undoped and Eu3+ doped MgO powder. The as-formed sample prepared by combustion synthesis show pure single cubic phases and no further calcination was required. All the X-ray diffraction peaks of the samples at (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) were indexed well and matched with JCPDS card No. 4–829 and belongs to a space group fm-3m (2 2 5) [26]. The lattice parameters and unit cell volume for cubic MgO
(d)
(c)
(b) (a)
Experimental Synthesis The raw materials, magnesium nitrate (Mg(NO3)26H2O: 99.9%, Merck Ltd.), and europium oxide (Eu2O3: 99.99%, Rolex Ltd.) were
ð1Þ
Characterization
Intensity (a.u)
Also, the demand for developing efficient luminescent materials such as rare earth activated powders attracted researchers because of their possible photonic applications, good luminescent characteristics, stability in high vacuum and absence of corrosive gas emission under electron bombardment when compared to currently used sulfide based phosphors [1,2]. Red emitting persistent phosphors were also desired in biomedical applications, e.g. in vivo imaging, where the phosphor emission should be situated between 600 and 1100 nm where biological tissue has the highest transparency. This was exactly the spectral region where the eye sensitivity was highest under very low illumination levels [3–8]. Currently, nanomaterials have become important with in the field of luminescence as they exhibit excellent optoelectronic properties. They have potential to be efficient phosphors in flat panel displays, solar energy converters, optical amplifiers, electroluminescent devices, photodiodes, bio-detectors, in lamp industries, radiation dosimetry, color display etc. [9–14]. In particular, oxide nanophosphors with the incorporation of rare earth activators have revealed major luminescence effects. In addition, various vacancies and defects of host materials results in wide variety of attractive luminescence features [15]. Among the rare earth ions, red-emitting trivalent europium (Eu3+) is recognized as an efficient red luminescent phosphors due to its 5D0 ? 7Fj (j = 0, 1, 2, 3, 4) transitions which were used in color television displays and mercury free lamps. Efforts have been made to enhance the luminescence nature of Eu3+ in host materials with low phonon energies. An appropriate selection of the host lattice and the suitable Eu3+ dopant concentration enhances the red emission [16–18]. The design of phosphors applied in the vacuum ultra violet (VUV) region compared to conventional mercury discharge sources. When, the phosphor host itself or doped activators should exhibit a strong absorption around these (147 or 172 nm) wavelengths and the proper hosts needed to hold the rare earth ions and their luminescence properties in the VUV range should be understood [17]. It has been known that reduction in the particle size in a crystalline system can result in remarkable modification of properties compared to those of bulk due to high surface-to-volume ratio. The modification of luminescence properties can be brought using different preparation techniques, different capping agents and use of suitable sensitizer, etc. [16]. The changes in the electrical and optical characteristics of very small particles were caused by the quantum size effect, which was generated by an increase in the band gap due to a decrease in the quantum allowed state that exists in small particles and the high surface-to-volume ratio, which improves the surface and interface effects. [19,20]. Over the past few years, synthesis of inorganic nanoscale materials with specific morphologies attracted the phosphor developers. Oxide phosphors were synthesized by a variety of routes such as solid-state reactions, sol–gel, hydrothermal, co-precipitation, microwave techniques etc. In the present studies, MgO:Eu3+ nanopowders were synthesized by low temperature combustion synthesis route. This process provides molecular level of mixing and high degree of homogeneity, short reaction time that leads to reduction in crystallization temperature and prevents from segregation during heating [21–23]. In this paper a novel and simple solution combustion synthesis of MgO nanocrystals with size of 5–6 nm and the effect of Eu3+ ions on the photoluminescence properties were studied systematically.
30
40
50
60
70
80
2 θ (degree) Fig. 1. XRD patterns of (a) undoped (b) 1 mol% (c) 5 mol% and (d) 9 mol% Eu3+ doped MgO.
48
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
were estimated using the following relations and for (2 0 0) plane are found to be 4.3 Å and 79.507 1030 m3 respectively.
2dsinh ¼ nk
ð2Þ
a dhkl ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 h þk þl
ð3Þ
The average particle size (D) was estimated from the line broadening in X-ray powder using Scherrer’s formula [D = (Kk)/b cosh, where, ‘K’ constant, ‘k’ wavelength of X-rays, and ‘b’ FWHM] was found to be in the range 5–6 nm [27,28]. No shift in diffraction peaks and no secondary phase were detected indicating that Eu3+ ions are obviously homogeneously mixed and effectively doped in the host lattice in Mg2+ sites since, the ionic radii are close each 2þ other (Since, the ionic radii of R3þ Eu = 1.07 nm and RMg = 0.72 nm are very close to each other). From the analysis of XRD, it was revealed that the introduction of an activator (Eu3+) did not influence the crystal structure of the phosphor matrix. Further, strain present in the MgO nanoparticles prepared by combustion method was estimated using the W–H equation.
b cos h ¼
0:9k þ 4e sin h D
ð4Þ
where ‘e’ is the strain associated with the nanoparticles [29]. The above equation represents a straight line between ‘4 sinh’ (x-axis) and ‘b cosh’ (y-axis). The slope of the line gives the strain and intercept of this line on y-axis gives grain size (D). It was found that the strain present on the surface of the sample was 3.2 104. The room temperature Fourier transform infrared (FTIR) spectra of pure and Eu3+ doped MgO samples synthesized via combustion method was recorded in the range 300–4000 cm1 using KBr pellets was shown in Fig. 2. The strong bond at 415 cm1 was associated with the characteristic vibrational mode of symmetric MgO6 octahedral of MgO. The absorption at 3500 cm1 indicate the presence of hydroxyl groups (surface adsorbed), which was probably due to the fact that the spectra were not recorded in situ and some water re-adsorption from the ambient atmosphere as occurred. The absorption in the range of 1300–1800 cm1 related to hydroxyl group of molecular water at 1635 cm1. Further, the peaks at 550–850 cm1 (c1) higher frequency of MgO stretching and 410–450 cm1 (c2) was lower frequency stretching. The peak at 880 cm1 was attributed to Mg–O–Mg interactions [30–32]. The UV–Vis absorption spectra of MgO samples were shown in Fig. 3. It was well established that nanoscale materials have large
surface to volume ratio. This results in the formation of voids on the surface as well inside the agglomerated nanoparticles. Such voids can cause fundamental absorption bands in the UV region. In addition, surface of nanoparticles were well known to comprise of several defects normally dangling bonds, regions of disorder and absorption of impurity species that result in the absorption of nanocrystals [24]. Thus in the UV-absorption spectrum of pure MgO nanocrystals shows absorption bands at 305 nm (4 eV) and 395 nm (3.14 eV) were identified with the excitation of 4-fold and 3-fold co-ordinated O2 anions in edges and corners (also called F+, F centers.) respectively. Also, the absorption band at 305 nm may be attributed to the surface defects in nano MgO:Eu3+ phosphor, which corresponds to ligand-to-metal charge-transfer (LMCT) group. The weak absorption band from 320–475 nm was attributed to the surface state of MgO [33]. Further, the intra configurationally 4f 4f transitions from the ground 7Fo level which corresponds to the excitation spectra. In case of smaller size nanoparticles, it was found that increase of defect distribution on the surface exhibit broad absorption bands due to large surface to volume ratio [34,35]. The optical energy gap (Eg) of pure and doped MgO samples prepared through combustion technique were estimated using Tauc relation [36] and their values were in the range 5.40–5.55 eV (Fig. 3). It was observed from the spectra the Eg values were shifting with increase of Eu3+ concentrations, which was attributed to quantum confinement effect [37–39]. Fig. 4 shows the SEM pictures of undoped and doped MgO samples prepared via combustion technique. Combustion derived MgO was foamy product with large agglomerates. Further, SEM results reveal that the powder is porous and highly agglomerated. It was well known that, combustion synthesis reaction was highly influenced by metal–ligand complex formation. Depending upon the type of fuel and metal ions, the nature of combustion differs from flaming (gas phase) to nonflaming (smoldering and heterogeneous) type. Generally, flaming reactions involves liberation of large quantity of gas. The pores and voids can be attributed to the large amount of gases escaping out of the reaction mixture during combustion. SEM photographs of the samples confirm smooth and uniform nanoparticles were obtained in this synthesis method [40]. The undoped and doped phosphors were agglomerated from few microns to a few tens of microns, fluffy and porous in nature. SEM photographs of the samples also reveal smooth and uniform nanoparticles were obtained. It was found that the dopant concentration does not influence the morphology of the sample.
6x108
(d)
305 nm
(a)
Uv-Vis absorption (au)
-1 2
3x108
1. Undoped 2. Eu-1mol% 3. Eu-5mol% 4. Eu-9mol%
395 nm
1 2 3 4
2
(b)
4x108
(α hν) (eV cm )
Transmittace %
(c)
2
5x108
undoped Eu 1 mol% Eu 5 mol% Eu 9 mol%
MgO:Eu3+ (1,5,9 mol%)
2x108
300
400
500
600
700
800
Wavelength (nm) 8
1x10
0 4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 2. FT-IR spectra of (a) undoped (b) 1 mol% (c) 5 mol% and (d) 9 mol% Eu3+ doped MgO.
3
4
5
6
Energy (eV) Fig. 3. Uv–Vis absorption spectra of (a) undoped (b) 1 mol% (c) 5 mol% and (d) 9 mol% Eu3+ doped MgO.
49
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
Fig. 4. SEM pictures of (a) undoped (b) 1 mol% (c) 5 mol% and (d) 9 mol% Eu3+ doped MgO.
1, 2, 3, 4). Fig. 7 shows comparison of emissions excited with 305 and 395 nm light. In particular, the emission peak at 616 nm corresponds to 5D0 ? 7F2 and occurs through the forced electric dipole (FED), while the 5D0 ? 7F1 at 596 nm wass the magnetic dipole transitions. The peak around 616 nm and 596 nm denote the Eu3+ ions occupy the Mg2+ sites at octahedral sites. These two emissions are of particular interest because they represent actually the local environment of the Eu3+ ions. The 5D0 ? 7F1 was magnetic dipole allowed and its intensity shows variation with the crystal field strength surrounding the Eu3+ ions whereas, 5D0 ? 7F2 hypersensitive transition was forced electric dipole allowed and its intensity was sensitive to the local structure acting on the Eu3+ ion [28,41–43,26]. The Eu3+ ions enter into the host lattice and replace magnesium ion located on the surface of the nanocrystals because of the porosity of MgO. As dopant concentration of Eu3+ increases, 5 D0 ? 7F2 transition dominates and the emission intensity increases. This may be attributed to the increase distortion of the local field around the Eu3+ ions [44,45].
7
F2
The excitation spectra of Eu doped MgO monitored at 616 nm emission was shown in Fig. 5. The excitation spectra consist of a charge transfer band (CTB) of Eu3+ – O2 band in the short ultraviolet region (305 nm). The main factor affecting the intensity of the charge transfer band was the efficiency of the energy process from the CTB to the Eu3+ emitting level. It shows that with increasing heat treatment, the interaction between O2 and Eu3+ ions in the host becomes stronger which facilitates the electron transfer from O2 to Eu3+ [17]. In the present study, the weak excitation peak observed at 395 nm suggests that the interaction between O2 and Eu3+ was stronger; hence the efficiency of the energy transfer process from the charge transfer band to Eu3+ emitting levels increases. The f–f transitions within the Eu3+, 4f6 configuration in longer spectral region with 7F0 ? 5D3 (395 nm) was the most prominent group. Photoluminescence emission spectra of MgO:Eu3+ excited with 395 nm was shown in Fig. 6. A series of emission peaks at 580 nm, 596 nm, 616 nm, 653 nm, 696 nm and 706 nm may be attributed to the characteristic transitions of Eu3+ ions from 5D0 ? 7Fj (j = 0,
200
250
300
350
396
400
450
398
500
Wavelength (nm) Fig. 5. Excitation spectra of MgO (emission at 616 nm).
500
D0
5
D0
5
7
F0
D0
7
F1 394
F4
7
F3
D0
5
D0
7 5
392
1 mol% Eu 3 mol% Eu 5 mol% Eu 7 mol% Eu 9 mol% Eu
5
395nm
Photoluminescence Intensity (a.u)
PL Intensity (a.u)
305nm
550
600
650
700
Wavelength (nm) Fig. 6. Emission spectra of MgO: Eu3+ (1–9 mol%) (excited at 395 nm).
50
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
Photoluminescence Intensity (a.u)
613 nm
550
λ exc - 395 nm λ exc - 305 nm
596 nm 578 nm
600
650 nm
707 nm 694 nm
650
700
750
Wavelength (nm) Fig. 7. Comparison of emission spectra excited with 305 nm and 395 nm. Fig. 8. The energy level diagram of Eu3+ ion showing the states involved in the luminescence process and the transition probabilities.
MgO has a rock salt structure (fcc) with magnesium ions occupying octahedral sites within the anion closed packed structure. Its ionic constituents comprise a relatively small number of electrons, its nanostructures were expected to have novel properties superior to their bulk counterparts due to the quantum confinement effect of nanocrystals [46]. When RE3+ ions doped into the host, they could probably occupy these sites. On the other hand, as indicated in the emission spectrum of the undoped MgO nanocrystals, the strong emission band centered at 390 nm originating from the contribution of oxygen vacancies formed during the combustion process, which was justly favored for the 7F0–5L6 transition of Eu3+ ions [47]. Therefore, the enhanced emission comes mainly from radiative recombination of the large amount of trapped carriers excited from MgO. It can be assumed that the oxygen vacancies formed during the combustion process might act as sensitizer for the energy transfer to the rare-earth ions at the surface, due to the strong mixing of charge transfer states resulting in the highly enhanced luminescence. Based on this, Eu3+ was chosen to study luminescent properties of the material. The Eu3+ energy level diagram corresponding to the 5Do ? 7Fj (j = 0, 1, 2, 3, 4) emission peaks was shown in Fig. 8. When the phosphor was excited by 305 nm or 395 nm wavelength, Eu3+ ion was raised to 5L6 level from the ground state. Since, the separation between 5D0 ? 7Fj (j = 0, 1, 2, 3, 4) was large, the stepwise decay process stops here and returns to ground state by giving emission in the orange and red regions. If the Eu3+ impurity does not occupy centre of symmetry of the crystal lattice, then it will give both magnetic and electric dipole transitions. When the rare earth impurity ion was located at the centre of symmetry in the relevant crystal lattice only magnetic dipole transition were allowed. The energy-level diagram indicates the states involved in the luminescence processes and the transition probabilities for Eu3+ ions. According to the model, the system is first excited from the ground state (5D3 configuration) to the singlet state of the 5D3, 2, 1, 0 configurations. Then the electrons pass to the triplet state, mainly to level 4 because of symmetry reasons. The last transition 5Do was so much faster than any other step of the luminescence process. It may be considered at once that the singlet state does not affect the luminescent process. Non-radiative transitions occur between the third energy levels of the triplet state, named 5D3, 5D2, 5D1 and 5Do with probabilities from level 3 to level 2, level 2 to level 1, and level 1 to level 0 i.e., 5Do. Level 5Do to level 7F0, 1, 2, 3, 4, occurs radiative transitions to the ground state i.e., 5Do ? 7Fj states respectively. The increase in PL intensity observed might be due to the decrease of cross relaxation between Eu3+ ions in this process [48,49].
The variation of PL intensity and asymmetric ratio at different europium mol concentration was shown in Fig. 9. Eu3+ ion in MgO prepared by combustion synthesis was situated at the low symmetry sites. Eu3+ ions enter into Mg2+ lattice site. The ionic radii of Eu3+ and Mg2+ were 1.07 and 0.72 nm respectively. Since ionic radius of Mg2+ was smaller than Eu3+, host could accommodate only small percentage of impurity ions or it was suggested that Eu3+ ions will likely reside on the surface or on the grain boundaries of the nanocrystals to yield optimum strain relief. Moreover, there was charge imbalance in the host lattice due to doping of trivalent Eu3+ cations. This may absorb emitted light, resulting into decrease of intensity. It was shown that relative intensity of the emission lines of Eu3+ depends on the doping concentration of Eu3+ in the host phosphor. By increasing the Eu3+ mol concentration the transition 5D0 ? 7F2 (616 nm) has shown an enhanced emission and reaches saturation at 5 mol% concentration and PL intensity decreases thereafter it decreases due to the concentration quenching. The concentration quenching occurs because of the energy transfer from one activator to another. The critical distance for energy transfer (Rc) in MgO:Eu3+ can be calculated from the structural parameters with unit cell volume (V) and the number of total Eu3+ sites per unit cell (N), together with the critical concentration (Xc) [50].
Rc 2
3V 4p X c N
1=3 ð5Þ 0
For the MgO:Eu3+ system, N = 4, V = 79.507 (Å A)3 and Xc = 0.05. The 3+ 3+ critical transfer distance of Eu in MgO:Eu is determined to be 0 9.124 Å A. The critical0 energy distance between Eu3+ ions in MgO was greater than 5 A Å as a result overlapping between excitation and emission spectra decreases. Another important parameter, which was sensitive to the nature of the Eu3+ ions environment in the host lattice, was the asymmetric ratio (A21) [51,52]. Which, gives a measure of the degree of distortion from inversion symmetry of the local environment surrounding the Eu3+ ions in the host matrix.
H 5 I2 ð D0 !7 F 2 Þdk A21 ¼ H 5 I1 ð D0 !7 F 1 Þdk
ð6Þ
where I2: intensity of electric dipole transition at 616 nm (5D0 ? 7F2) and I1: intensity of the magnetic dipole transition at
51
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
Photoluminescence Intensity (a.u)
1.3440
I 616 nm / I 596 nm
578 nm 596 nm 616 nm
1.3424
5 mol %
1.3408
1
3
5
7
1
9
3
5
7
9
Eu3+ concentration (mol%)
Eu3+ Concentration (mol %)
Fig. 9. The effect of Eu3+ on the 596 nm and 616 nm emission peaks and variation of asymmetric ratio with Eu3+ concentration in MgO nanophosphors.
9.4
0.9 MgO:Eu 3+
Slope: -0.919 Q : 7.45
9.2
0.6
Y
log (I/x)
9.0
8.8
0.3
8.6 0.0 8.4
0.0 -2.0
-1.8
-1.6
-1.4
-1.2
between
log (x)
and
log (I/x)
-1.0
0.6
0.8
Fig. 11. CIE diagram of MgO:Eu3+ (5 mol%) nanophosphor. in
MgO:Eu3+
(1–9 mol%)
596 nm (5D0 ? 7F2). The values of A21 decrease with increase of Eu3+ ions. However, it is reasonable to believe that the doping of Eu3+ will introduce lattice defects, which will undoubtedly reduce the symmetry strength of the local environment of Mg2+ sites. Consequently, the symmetry ratio of MgO:Eu3+ decreases with the increase of doped Eu3+ concentration [53]. Van Uitert’s [54] early study has developed a theoretical description for the relationship between the luminescent intensity and the doping concentration. Van Uitert fingered out that if the energy transfer occurs among the same sorts of activators due to electric multipolar interaction, the relationship between the luminescent intensity and the doping concentration can be expressed by the relation 1 I ¼ k½1 þ bðXÞQ =3 X
0.4
X
log (x) Fig. 10. Relation nanophosphor.
0.2
ð7Þ
where X: Eu3+ ion concentration, k and b: constants, Q = 6, 8 and 10 for dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions. The value of Q can be determined by plotting log (X) vs log (I/X) (Fig. 10) which gives a linear graph having a intercept,
Table 1 CIE co-ordinates values of different Eu mol% concentration in MgO host matrix. Eu3+ concentration (mol%)
X
Y
1 3 5 7 9
0.5389 0.5577 0.645 0.5625 0.5592
0.3601 0.3699 0.353 0.3992 0.3791
slope = 0.919 and Q = 7.45, which was close to 8. The Q value indicates that the concentration quenching in MgO was due to dipole– dipole interaction [55]. The luminous color was depicted by studying color co-ordinates and color ratios of the phosphor. The values of chromaticity coordinates of the MgO:Eu3+ (1–9 mol%) phosphor have been estimated from 1931 CIE (International Commission on Illumination) system and was shown in Fig. 11. and their corresponding values were given in Table 1. It was observed that the CIE co-ordinates of 5 mol% Eu3+ activated MgO:Eu3+ phosphor was measured as
52
P.B. Devaraja et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 46–52
(x = 0.645, y = 0.353) which was very close to the NTSC (National Television System Committee) standard values (x = 0.67, y = 0.33) [56]. Their corresponding location has been marked in Fig. 11 with star in red region. Conclusions Phase pure Eu3+ doped MgO nanophosphors were prepared by simple and low cost solution combustion method. The powder Xray diffraction (PXRD) pattern showed the single phase, cubic structure and the particle size was observed in the nanoscale. The UV–Vis absorption of un-doped and doped phosphor show an intense absorption band in the region 300–400 nm corresponding to ligand-to-metal charge-transfer (O2 to Eu3+) band. The phosphor exhibits different emission (in the range 540 nm to 660 nm) due to Eu3+ corresponding to 5D0 ? 7Fj (j = 0, 1, 2, 3, 4) transitions. The transition centered at 616 nm was found to be hypersensitive in nature resulting in a strong and red emission. Enhancement in photoluminescence (PL) intensity of Eu3+ was observed due to the formation of different lattice sites in the host phosphor. In the present work, MgO doped with Eu3+ were investigated as red phosphor with good emission color purity due to the non-centrosymmetric site for the europium ion. 5 mol% Eu doped sample showed enhanced PL intensity and thereafter, it decreases due to concentration quenching. It was observed that the emission spectrum excited at 395 nm showed prominent spectral lines compared to 305 nm excitation. Further, 395 nm is close to visible region (NUV) which may be useful for LED applications. The excellent red emission properties and the estimated CIE chromaticity co-ordinates (x, y) were very close to NTSC standard value of red emission of this phosphor suggest that it may be used for display and white light emitting applications. Acknowledgements One of the author HN thanks to DST nano mission for funding of research project to Center for Nano Research, Tumkur University, Tumkur for extended to carry out this research work. The authors PBD, SCP and HPN thanks to VGST, Govt. of Karnataka, India, (VGST/CISEE-GRD 190) for extended to carry out this research work. References [1] H.F. Brito, J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, L.C.V. Rodrigues, Opt. Mater. Expr. 2 (2012) 371–381. [2] T. Maldiney, B. Viana, D. Gourier, C. Richard, D. Scherman, M. Bessodes, K. Van den Eeckhout, D. Poelman, P.F. Smet, Opt. Mater. Expr. 2 (2012) 261–268. [3] Danuta. Dutczak, Cees. Ronda, Andries. Meijerink, Thomas. Jüstel, J. Lumin. 141 (2013) 150–154. [4] P. Zhang, M. Xu, Z. Zheng, B. Sun, Y. Zhang, Trans. Nonferr. Mat. Soc. China. 16 (2006) 423–425. [5] X.X. Li, Y.H. Wang, Z. Chen, J. Alloy. Comp. 453 (2008) 392–394. [6] K.Y. Jung, J.H. Kim, J. Lumin. 128 (2008) 2004–2007. [7] H.S. Yoo, W.B. Im, J.H. Kang, D.Y. Jeon, Opt. Mater. 31 (2008) 131–135. [8] L. Zhou, W.C.H. Choy, J. Shi, M. Gong, H. Liang, J. Alloy Comp. 463 (2008) 302– 305. [9] Z. Zhang, Y. Wang, J. Electr. Soc. 154 (2007) J62–J64. [10] L.Y. Zhou, F.Z. Gong, J.X. Shi, M.L. Gong, H.B. Liang, Mater. Res. Bull. 43 (2008) 2295–2299. [11] T.S. Chan, R.S. Liu, I. Baginskiy, N. Bagkar, B.M. Cheng, J. Electr. Soc. 155 (2008) J284–J286. [12] Z.J. Zhang, S. Chen, J. Wang, X.X. Yang, J.T. Zhao, Y. Tao, H.H. Chen, Y. Huang, Opt. Mater. 32 (2009) 99–103.
[13] P. Zhang, L.X. Li, M.X. Xu, L. Liu, J. Alloy. Compd. 456 (2008) 216–219. [14] P. Zhang, M.X. Xu, Z.T. Zheng, B. Sun, Y.H. Zhang, Mater. Sci. Eng. B. 136 (2007) 159–164. [15] N. Dhananjaya, H. Nagabhushanan, B.M. Nagabhushana, B. Rudraswamy, C. Shivakumara, K.P. Ramesh, R.P.S. Chakradhar, Physica B. 406 (2011) 1645– 1652. [16] Ganngam. Phaomei, W. Rameshwor Singh, R.S. Ningthoujam, J. Lumin. 131 (2011) 1164–1171. [17] Feng Zhang, Yuhua Wang, Yan Wen, Dan Wang, Ye Tao, Opt. Mater. 33 (2011) 475–479. [18] H. Shabir, B. Lal, M. Rafat, J. Sol–Gel Sci. Tech. 53 (2010) 399–404. [19] S. Cho, R. Lee, H. Lee, J. Kim, C. Moon, S. Nam, J. Park, J. Sol–Gel Sci. Tech. 53 (2010) 171–175. [20] Juan Wang, Xu Yunhua, Mirrabos Hojamberdiev, Jianhong Peng, Gangqiang Zhu, J. Noncryst. Solid. 355 (2009) 903–907. [21] Yonghu Chen, Xuerui Cheng, Miao Liu, Zeming Qi, Chaoshu Shi, J. Lumin. 129 (2009) 531–535. [22] S. Ekambaram, K.C. Patil, J. Mater. Chem. 5 (1995) 905–908. [23] H.B. Premkumar, D.V. Sunitha, H. Nagabhushana, S.C. Sharma, B.M. Nagabhushana, C. Shivakumara, J.L. Rao, R.P.S. Chakradhar, J. Lumin. 135 (2013) 105–112. [24] S.C. Prashantha, B.N. Lakshminarasappa, B.M. Nagabhushana, J. Alloy Comp. 509 (2011) 10185–10189. [25] K.C. Patil, M.S. Hegde, Tanu Rattan, S.T. Aruna, Chemistry of Nanocrystalline Oxide Materials, Combustion Synthesis, Properties and Applications, World Scientific Publishing Co., Pt., Ltd., UK, 2008. [26] Gu Feng, Chun Zhong Li, Hai Bo Jiang, J. Cryst. Grow. 289 (2006) 400–404. [27] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435–445. [28] S.C. Prashantha, B.N. Lakshminarasappa, Fouran Singh, J. Lumin. 132 (2012) 3093–3097. [29] G.K. Williamson, W.H. Hall, Acta. Metal. 1 (1953) 22–31. [30] Latha Kumari, W.Z. Li, Charles H. Vannoy, Roger M. Leblanc, D.Z. Wang, Ceram. Intern. 35 (2009) 3355–3364. [31] T. Selvamani, T. Yagyu, S. Kawasaki, I. Mukhopadhyay, Catal Commun. 11 (2010) 537–541. [32] S.M. Borghei, S. Kamali, M.H. Shakib, A. Bazrafshan, M.J. Ghoranneviss, J. Fusi. Enery 30 (2011) 433–436. [33] K. Kviyarasu, P.A. Devarajan, Adv. Appl. Sci. Res. 2 (2011) 131–138. [34] G.A. Kumar, C.W. Chen, J. Ballato, R.E. Riman, Chem. Mater. 19 (2007) 1523– 1528. [35] S. Atalay, H.I. Adiguzel, F. Atalay, Mater. Sci. Eng. A 304 (2001) 796–799. [36] J. Tauc, R. Grigorovici, A. Vancu, Phys. Stat. Sol. 15 (1966) 627–638. [37] Lingling Peng, Yuhua Wang, Zhaofeng Wang, Qizheng Dong, Appl. Phys. A 102 (2011) 387–392. [38] M. Sterrer, T. Berger, O. Diwald, E. Knozinger, J. Am. Chem. Soc. 125 (2003) 195–199. [39] E. Garrone, A. Zecchina, F.S. Stone, Philos. Mag. Part B. 42 (1980) 683–703. [40] D.V. Sunitha, C. Manjunatha, C.J. Shilpa, H. Nagabhushana, S.C. Sharma, B.M. Nagabhushana, N. Dhananjaya, C. Shivakumara, R.P.S. Chakradhar, Spectrosc. Acta Part A: Mole. Biomole. Spectrosc. 99 (2012) 279–287. [41] Atsushi Ishizumi, Satoshi Fujita, Hisao Yanagi, Opt. Mater. 33 (2011) 1116– 1119. [42] A. Bahadur, Y. Dwivedi, S.B. Rai, Spectrosc. Acta Part A: Mole. Biomole. Spectrosc. 91 (2012) 217–221. [43] Hong-Sheng Huang, Zhi-Hong Liu, J. Lumin. 140 (2013) 114–119. [44] Vu Nguyen, Tran kim Anh, Gyu-chul Yi, W. Trek, J. Lumin. 122 (2007) 776–779. [45] Oleg Viagin, Andrey Masalov, Irina Ganina, Yuriy Malyukin, Opt. Mater. 31 (2009) 1808–1810. [46] S.Y. Yang, S.G. Kim, Powd. Tech. 146 (2004) 185–192. [47] Changhyun Jin, Hyunsu Kim, Sunghoon Park, Chongmu Lee, J. Alloy. Comp. 541 (2012) 163–167. [48] I.M. Nagpure, Subhajit Saha, S.J. Dhoble, J. Lumin. 129 (2009) 898–905. [49] W. Stambouli, H. Elhouichet, B. Gelloz, M. Férid, J. Lumin. 138 (2013) 201–208. [50] G. Blasse, J. Sol. Sta. Chem. 62 (1986) 207–211. [51] M. Catti, G. Gazzoni, G. Ivaldi, Acta Crystall. C 39 (1983) 29–34. [52] M. Chandrasekhar, H. Nagabhushana, S.C. Sharma, K.H. Sudheer kumar, N. Dhananjaya, D.V. Sunitha, C. Shivakumara, B.M. Nagabhushana, J. Alloy. Comp. doi.org/10.1016/j.jallcom.2013.08.149. [53] N. Dhananjaya, H. Nagabhushana, B.M. Nagabhushana, B. Rudraswamy, C. Shivakumara, R.P.S. Chakradhar, J. Alloy. Comp. 509 (2011) 2368–2374. [54] L.G. Van Uitert, J. Electr. Soc. 114 (1967) 1048–1053. [55] Fu Yang, Zhiping Yang, Yu Quanmao, Xu Yufeng Liu, Fachun Lu Li, Spectrosc. Acta Part A: Mole. Biomole. Spectrosc. 105 (2013) 626–631. [56] Devayani Chikte, S.K. OmanwarOmanwar, S.V. Moharil, J. Lumin. 142 (2013) 180–183.
