Low temperature synthesis and photoluminescence properties of red emitting Mg2SiO4:Eu3+ nanophosphor for near UV light emitting diodes

Sensors and Actuators B 195 (2014) 140–149

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Low temperature synthesis and photoluminescence properties of red emitting Mg2 SiO4 :Eu3+ nanophosphor for near UV light emitting diodes Ramachandra Naik a , S.C. Prashantha b,∗ , H. Nagabhushana c,∗∗ , S.C. Sharma d , B.M. Nagabhushana e , H.P. Nagaswarupa b , H.B. Premkumar f a

Department of Physics, New Horizon College of Engineering, Bangalore 560103, India Department of Science, East West Institute of Technology, Bangalore 560091, India c Center for Nano Research (CNR), Tumkur University, Tumkur 572103, 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 f Department of Physics, Acharya Institute of Technology, Bangalore 560107, India b

a r t i c l e

i n f o

Article history: Received 9 November 2013 Received in revised form 29 December 2013 Accepted 6 January 2014 Available online 17 January 2014 Keywords: Mg2 SiO4 :Eu3+ Combustion technique Nanophosphor Photoluminescence CIE

a b s t r a c t A simple and low-cost solution combustion method was used to prepare Eu3+ (1–11 mol%) doped Mg2 SiO4 nanophosphors at 350 ◦ C using metal nitrates as precursors and ODH (Oxali di-hydrazide) as fuel. The final products were well characterized by powder X-ray diffraction (PXRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and UV–visible absorption (UV–Vis). The PXRD patterns of the as-formed products show single orthorhombic phase. The crystallite size was estimated using Scherrer’s method and found to be in the range 20–25 nm. The effect of Eu3+ cations on the luminescence properties of Mg2 SiO4 :Eu3+ nanoparticles were understood from the luminescence studies. The phosphors exhibit bright red emission upon 393 nm excitation. The characteristic emission peaks recorded at ∼577, 590, 612, 650 and 703 nm (5 D0 → 7 FJ=0,1,2,3,4 ) were attributed to the 4f–4f intra shell transitions of Eu3+ ions. The intensity of red emission was found to be related with the concentration of intrinsic defects, especially oxygen-vacancies, which could assist the energy transfer from the Mg2 SiO4 host to the Eu3+ ions. The Commission International De I-Eclairage (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. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Nanomaterials find wide range of applications due to their unique chemical, physical, electrical, magnetic, optical and mechanical properties. Because of these properties, they were useful as catalyst, sensors, coating materials, tunable lasers, and memory devices. The unique spectroscopic properties of rare-earth ions in different host lattices was prompted the development of rare-earth luminescent materials for lamps, cathode ray tubes,

∗ Corresponding authors at: East West Institute of Technology, Department of Science, Bangalore, Karnataka 560 091, India. Tel.: +91 9886021344. ∗∗ Center for Nano Research (CNR), Tumkur University, Tumkur 572103, India. Tel.: +91 9945954010. E-mail addresses: [email protected] (S.C. Prashantha), [email protected] (H. Nagabhushana). 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2014.01.018

radiation monitoring systems, lasers, scintillators, bio sensors and white light-emitting diodes (WLEDs) [1–3]. White light-emitting diodes (LEDs) were the potential materials for significantly improving lighting efficiency, resulting in reduction of the excitation energy and also reduction in pollution from fossil fuel power plants [4]. In order to enhance the quality of white-light, one of the current academic interest is the research for phosphors of singlecomponent and cost effective preparation methods. The demand for developing efficient luminescent materials such as rare earth activated powders were attracted researchers due to their possible photonic applications, good luminescent characteristics, stability in high vacuum, biosensors and absence of corrosive gas emission under electron bombardment when compared to currently used sulfide based phosphors. Among all the rare earth ions, Eu3+ was usually adapted as a red emitting center because of its unique 4f6 structure that can be activated by ultraviolet rays effectively and emit high purity color of the red light [5–7]. Near-ultraviolet (NUV)

R. Naik et al. / Sensors and Actuators B 195 (2014) 140–149

conversion was the most suitable method which can achieve WLED. In this method, red/green/blue tricolor phosphors were pumped by NUV to generate white light. So phosphors play a crucial role in these solid-state lighting devices. However, the luminescent efficiency was low in this system owing to the strong re-absorption of the blue light by the red and green phosphors [8–10]. Silicate family was an attractive class of materials among inorganic phosphors for wide range of applications due to their special properties such as water, chemical resistance and visible light transparency. In particular, inorganic nanophosphors with the incorporation of trivalent rare earth cations revealed major luminescence effects. Further, various vacancies and defects present in host matrix results in different luminescence features [3]. Enhanced electrical, luminescent and optical properties of nano phosphors 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 and the high surface-to-volume ratio [10]. Among silicate family, the Mg2 SiO4 (forsterite) host doped with rare earth ions exhibit some interesting applications such as long lasting phosphor, X-ray imaging, light emitting display (LED), environmental monitoring, etc. Further luminescent materials, doped with Eu3+ ions were widely studied for their high efficiency and proper CIE chromaticity coordinates. Eu3+ doped phosphors were effectively excited by near-UV and blue light, as a result it emits a strong red color which attributes to 4f–5d transitions. It involves broad spectral line width as occurred for low valence rare-earth ions which are crystal field related and can be tuned by the size and the crystal structure [11–14]. Forsterite was prepared preferably through solution based methods in order to get high chemical homogeneity and small crystallite size compared to conventional solid state reaction, which needs higher calcination temperatures to obtain phase pure crystals. However, the synthesis of pure and doped nanocrystalline forsterite with controlled particle size still remained challenging. Therefore, many alternative synthesis techniques were reported for the synthesis of forsterite including the citrate–nitrate method [15], molten-salt approach [16], combined mechanical activation [17], polymer precursor method [18], Flame Spray Pyrolysis [19], mechano-thermal synthesis [20], combustion synthesis [3,21], mechano-chemical synthesis [22] and sol–gel techniques [23,24]. In this paper, we report on low temperature (350 ◦ C) synthesis, structural characterization and photoluminescence studies of Eu3+ (1–11 mol%) doped as prepared Mg2 SiO4 .

2. Experimental 2.1. Synthesis Eu3+ (1–11 mol%) doped Mg2 SiO4 nanophosphors were prepared by the low temperature (350 ◦ C) solution combustion route in a very short time (less than 5 min). In this route, suitable amount of magnesium nitrate (Mg (NO3 )2 ·6H2 O (Sigma Aldrich) and fumed silica (SiO2 (Sigma Aldrich) were mixed in stoichiometric amounts with laboratory prepared oxalyl dihydrazide (ODH; C2 H6 N4 O2 ) fuel and dissolved in a minimum quantity of doubled distilled water in a cylindrical pyrex dish and mixed thoroughly using magnetic stirrer for about 5 min. Eu3+ (1–11 mol%) dopant was added in the form of nitrate Eu(NO3 )2 :4H2 O into the above combination. The details of the stoichiometric calculations of the redox mixtures (oxidizer and fuel) were given elsewhere [3,14]. The pyrex dish containing this solution was placed in a pre-heated muffle furnace maintained at 350 ± 10 ◦ C. The solution boiled resulting in a transparent gel. The gel then formed white foam, which expanded to fill the vessel. Thereafter, the reaction was initiated somewhere in the interior a flame appeared on the surface of the foam and proceeded

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rapidly throughout the entire volume, leaving a white powder with an extremely porous structure. The energy released from the reaction was produced temperature greater than 1200 ◦ C. The reaction was self-propagating and able to sustain high temperature to form Mg2 SiO4 :Eu3+ phosphor. The entire combustion process was over less than ∼5 min. The final product was grinded into a fine powder by agate and mortar. Theoretical equation assuming complete combustion of the redox mixture used for the synthesis of Mg2 SiO4 can be written as 2Mg[NO3 ]2 + SiO2 + 2C2 H6 N4 O2 [ODH] → Mg2 SiO4 + 6N2 + 6H2 O + 4CO2

(1)

2.2. Characterization The crystalline nature of the powder sample was characterized ˚ by PXRD using Philips X-ray diffractometer using Cu-K␣ (1.541 A) radiation with nickel filter. FTIR studies of the samples were performed with a Perkin Elmer FTIR spectrophotometer (Spectrum1000). The surface morphology of the product was examined by Hitachi table top (SEM) (Model TM 3000). The optical absorption studies of the samples were performed in the range 200–800 nm using Elico SL-150 spectrophotometer. Photoluminescence spectra were recorded with a Horiba, (model fluorolog-3) spectrofluorimeter. 3. Results and discussion Fig. 1 shows the PXRD patterns of undoped and Eu3+ (1–11 mol%) doped Mg2 SiO4 powder. The as-formed sample prepared by low temperature combustion synthesis using ODH as fuel show pure orthorhombic phase without any post calcinations. Normally silicate samples required high calcinations temperature to get single pure phase, Table 1 shows various silicate hosts prepared by different methods [25–35]. In our earlier studies, the Mg2 SiO4 sample was prepared using urea as a fuel at furnace temperature was maintained at 500 ± 10 ◦ C and after that it was calcined at 800 ± 10 ◦ C for 3 h to get pure phase [3]. This was attributed to fuel effect and it confirms that fuel plays an important role in combustion synthesis. All the X-ray diffraction peaks of the samples at (0 3 1), (1 3 1), (2 1 1), (2 2 1), (1 4 0) etc. . . were well indexed and well matched with JCPDS card No. 78-1371 with space group pbnm (No.62). Further, very small impurity peak related to MgO phase are detected at 44.5◦ and it can be removed after calcined the sample at 800 ◦ C [3]. The lattice parameters and unit cell volume for orthorhombic Mg2 SiO4 were estimated using the following relations and for (2 1 1) plane was found to be 2.471 A˚ and 289.92 × 10−30 m3 , respectively. 2dsin  = n dhkl =



(2) 1

h2 a2

+

k2 b2

(3) +

l2 c2

No diffraction peaks from Eu2 O3 (2 = 29.49◦ ) or other impurities detected up to 11 mol% indicating that Eu3+ ions were obviously homogeneously mixed and effectively doped in the host lattice in Mg2+ sites (REu3+ = 0.095 andRMg2+ = 0.072 nm). It was observed that, the position of the main diffraction peaks shift to the lower angle side (Fig. 1b), which indicates that Eu3+ ions were strongly capped in to the crystal lattice of Mg2 SiO4 . The Eu3+ ions doped into Mg2 SiO4 matrix causes expansion of the unit cell resulting in tensile stress, as a result the PXRD peaks shifted toward lower angle side [36]. From the analysis of PXRD, it was evident that the introduction of an activator (Eu3+ ) did not influence the crystal structure of the phosphor matrix but certainly modified the lattice

142

20

30

50

(101)

22

60

(111)

(120)

(b)

(332)

(222)

40

Undoped Eu-1mol% Eu-5mol% Eu-9mol% Eu-11mol% (142) (160) (331) (031) (400) (260) (033)

(132)

(221) (140) (150) (202)

(041) (210)

(031) (121)

(101) (111)

(020)

Intensity (a.u)

(120)

(a)

(131) (211)

R. Naik et al. / Sensors and Actuators B 195 (2014) 140–149

2 (degree)

24

26

2 (degree)

Fig. 1. PXRD patterns of un-doped and 1–11 mol% Eu3+ doped Mg2 SiO4.

parameters due to the difference in the ionic radius between the dopant and the substituted magnesium ion [37,38]. The average crystallite size (D) was estimated from the line broadening in X-ray powder using Scherrer’s formula [39] K ˇ cos 

(4)

[where, ‘K’; constant, ‘’; wavelength of X-rays, and ‘ˇ’; FWHM] was found to be in the range 15–25 nm. Further, strain present in the Mg2 SiO4 :Eu3+ (1–11 mol%) nanoparticles prepared by combustion method was calculated using the W–H plots. ˇ cos  =

0.9 + 4ε sin  D

5 mol %

9 mol %

11 mol %

(5)

where ‘ε’; the strain associated with the nanoparticles [40]. The above equation represents a straight line between ‘4sin ” (x-axis) and ‘ˇcos ”(y-axis) (shown in Fig. 2). The slope of the line gives the strain and intercept of this line on y-axis gives grain size (D). The detailed values were given in Table 2. The acceptable percentage difference in ionic radii between doped and substituted ions should not exceed 30%. The calculations of the radius percentage difference (Dr ) between the doped ions (Eu3+ ) and substituted ions (Mg2+ ) in Mg2 SiO4 :Eu3+ was calculated based on the formula: Dr =

1 mol %

cos

d=

Undoped

Rm(CN) − Rd(CN) Rm(CN)

(6)

where CN; co-ordination number, Rm(CN); radius of host cations and Rd(CN); radius of dopant ion and it was found to be 24.2%. Thus,

0.8

1.0

1.2

1.4

1.6

1.8

4Sin Fig. 2. W–H plots of Mg2 SiO4 :Eu3+ (1–11 mol%).

it clearly indicates that the Eu3+ ionic radius (r = 0.095 nm, CN = 9) was close to that of Mg2+ (r = 0.072 nm, CN = 9), making it unlikely that Eu3+ ions would substitute with Mg2+ in the Mg2 SiO4 host. Hence, it was believed that the Mg2+ sites were replaced by Eu3+ in this lattice [41]. The room temperature infrared spectra of pure and Eu3+ (1–11 mol%) doped Mg2 SiO4 samples synthesized via combustion method was recorded in the range 400–4000 cm−1 using KBr pellets (shown in Fig. 3). The peaks at 4 2 2, 5 2 0, 6 2 0,6 8 0, 8 8 0,

Table 1 Calcination temperatures of different silicates by different authors. Sl. No

Sample

Preparation technique

Calcination temperature

Reference

1 2 3 4 5 6 7 8 9 10 11 12 13

Mg2 SiO4 :Eu3+ Mg2 SiO4 :Eu3+ Mg2 SiO4 :Eu3+ Mg2 SiO4 Mg2 SiO4 :Dy3+ CdSiO3 :Pr3+ CdSiO3 :RE3+ CdSiO3 :Sm3+ CdSiO3 :Dy3+ Y2 SiO5 Zn2 SiO4 :Eu3+ Zn2 SiO4 :Eu3+ ZrSiO4

Combustion Combustion Polyacrylamide gel method Sol gel Combustion Combustion Solid state Combustion Solid state Sol gel Combustion Solid state Solid state

No calcination 800 ◦ C 900 ◦ C 800 ◦ C 800 ◦ C 800 ◦ C 1100 ◦ C 800 ◦ C 1100 ◦ C 1200–1400 ◦ C 1000–1200 ◦ C 1200 ◦ C 1687 ◦ C

Ramachandra Naik et al. Prashantha et al. [3] Tabrizi et al. [25] Kharaziha et al. [26] Lakshminarasappa et al. [27] Sunitha et al. [28] Kaung et al. [29] Manjunatha et al. [30] Bing Fu et al. [31] Huang et al. [32] Sunitha et al. [33] Ru-Yuan Yang et al. [34] Arno Kaiser et al. [35]

R. Naik et al. / Sensors and Actuators B 195 (2014) 140–149

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Table 2 Estimated crystallite and strain values of Mg2 SiO4:Eu3+ nanophosphor. Mg2 SiO4 :Eu3+

Crystallite size (nm) [W–H plot]

Crystallite size (nm) [D–S approach]

Strain (×10−4 )

0 1 3 5 7 9 11

37.98 35.05 24.37 24.37 24.37 21.10 27.62

25.47 22.62 25.95 26.32 21.56 18.43 16.04

26.10 24.00 17.20 17.20 17.20 12.60 21.20

(e)

% Transmitance

(d) (c) (b)

(a) Un-doped Mg2SiO4

(a)

1400 620 1010 520 880 422

(b) Eu 1 mol% (c) Eu 5 mol% (d) Eu 9 mol% (e) Eu 11 mol%

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 3. FT-IR spectra of un-doped and 1–11 mol% Eu3+ doped Mg2 SiO4.

1 0 1 0, 1 2 5 0 and 1 4 0 0 cm−1 were assigned to MgO6 octahedral, Si–O (4 ), Si–O (bending), Mg–O, Si–O (3 , stretching), (CO and Si–O), C–H and NO3 , respectively [3,21], where 3 was asymmetric stretching and 4 was asymmetric deformation vibrations [42].

The surface morphology of pure and Eu3+ (1–11 mol%) doped Mg2 SiO4 nanophosphors prepared via combustion method were studied using SEM and the results were shown in Fig. 4. It was clearly observed from SEM images that, the powders show highly porous, many agglomerates with an irregular morphology, large voids, cracks, pores and shape. This type of morphology was due to escape of large amount of gases during combustion process [43,44]. Further, the dopant concentration does not influence the morphology of the sample. The UV–Visible absorption spectra of pure and Eu3+ (1–11 mol%) doped Mg2 SiO4 nanophosphors were shown in inset of Fig. 5. It was well known that nano materials have large surface-to-volume ratio which results in the formation of voids on the surface as well inside the agglomerated nanoparticles. Such voids can cause fundamental absorption in the UV region. Several defects such as dangling bonds, regions of disorder and absorption of impurity species that result in the absorption of nanocrystals. Thus, the absorption band at 350 nm corresponds to oxygen to silicon (O–Si) ligand-to-metal charge-transfer (LMCT) in the SiO3 2− group. The broad bands in the range 300–500 nm were attributed to the intra configurationally 4f–4f transitions from the ground 7 F0 level which corresponds to the excitation spectra. Also, they were identified with the excitation of 4-fold and 3-fold co-ordinated O2− anions in edges and corners (also called F+ , F centers) of the nanoparticles. In case of smaller size nanoparticles, it was found that increase of defect distribution

Fig. 4. SEM pictures of un-doped and Eu3+ doped Mg2 SiO4 [(A) un-doped, (B) 1 mol% Eu, (C) 3 mol% Eu, (D) 5 mol% Eu, (E) 7 mol% Eu, (F) 9 mol% Eu].

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R. Naik et al. / Sensors and Actuators B 195 (2014) 140–149

2x10

5

7

F0

4.60p

G4 5

D3

F0

5

7

F0

5

G3

emi = 612 nm Mg2SiO4:Eu3+ (1 mol%)

L6

5

7

F0

5

D2

7

1x10

F0

7

9.20p

PL Intensity (a.u)

)

13.80p

(

2 -1 2 ( h )2 eV cm

18.40p

5

393 nm

0.00

464 nm

4.0

4.5

5.0

5.5

6.0

Energy gap (eV)

350

Fig. 5. UV–Visible absorption spectra of un-doped and 1–11 mol% Eu3+ doped Mg2 SiO4 .

400

450

500

Wavelength (nm) Fig. 6. Excitation spectra of Mg2 SiO4 (emission at 612 nm).

on the surface exhibit strong and broad absorption bands due to large surface to volume ratio [45,46]. The optical energy gap (Eg ) of pure and doped Mg2 SiO4 samples prepared through combustion technique were estimated using Wood and Tauc relation [47] hv˛ ∝ (hv − Eg )

k

(7)

where ˛; the absorption coefficient, h; the Planks constant, ; the frequency, Eg ; the optical band gap and k; the constant associated to the different types of electronic transitions k = 1/2, 2, 3/2, 3 for direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions, respectively. The Eg values were estimated by 1/k

extrapolating the linear portion of the curve or tail [(hv˛) = 0] in the UV–Vis absorbance spectra and their values were in the range 5.70–5.85 eV (Fig. 5). It was observed from the spectra the Eg values were shifted with increase of Eu3+ concentrations, which was attributed to quantum confinement effect [48]. A conceivable explanation for the variations observed in the Eg values may be related to the degree of structural order–disorder in to the lattice, which is able to change the intermediary energy level distribution within the band gap. The variations in the band gap values might also be due to higher degree of structural defects. The energy gap values were mainly depends on the preparation methods and experimental conditions (heat-treated and processing time). The excitation spectra of Eu3+ (1–11 mol%) doped Mg2 SiO4 monitored at 612 nm was studied and shown in Fig. 6. A broad excitation band was observed in the wavelength range 300–550 nm, which can be attributed to the intra-configurational (f–f) transitions of Eu3+ . It can be clearly observed that the excitation spectrum consists of a broad band between 250 and 400 nm. The broad band in this range was related to O2− to Eu3+ charge transfer band (CTB), which was caused by the electron transfer from 2p orbits of O2− ions to 4f shells of Eu3+ ions. 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 silicate host becomes stronger which facilitates the electron transfer from O2− to Eu3+ [49]. In the present study, the weak excitation peak observed at 334 nm suggests that the interaction between O2− and Eu3+ was stronger; hence the efficiency of the energy transfer process from the charge transfer band (256 nm) to Eu3+ emitting levels increases. Further, the intra-configuration 4 f6 excitation line (393 nm) was very stronger than the other transitions of Eu3+ ions. The excitation peak observed at 393 nm suggests that the interaction between O2− and Eu3+ was stronger; as a result the efficiency

of the energy transfer process from the charge transfer band to Eu3+ emitting levels increases. The f–f transitions within the Eu3+ , 4 f configuration in longer spectral region with 7 F → 5 L (393 nm) 6 0 6 was the most prominent group. A series of other excitation peaks at 3 7 0, 3 8 5, 4 1 0 and 4 7 0 nm were mainly attributed to the characteristic absorption transitions of 7 F0 ground state of Eu3+ ions from 7 F0 → 5 G3 , 7 F0 → 5 G4 , 7 F0 → 5 D3 and 7 F0 → 5 D2 , respectively [28,50,11]. Further, the phosphor was effectively excited by radiations of wavelength in the near-UV region as a result the phosphor was useful in white LEDs. The emission spectra of Mg2 SiO4 : Eu3+ (1–11 mol%) excited at 393 nm was shown in Fig. 7. The spectra consist of emission peaks at 5 7 7, 5 9 0, 6 1 2, 6 5 0 and 7 0 3 nm which were attributed to Eu3+ transitions 5 D0 → 7 F0 , 5 D0 → 7 F1 , 5 D0 → 7 F2 , 5 D0 → 7 F3 and 5 D → 7 F , respectively [3]. In addition, the PL properties under 0 4 resonant (254, 393 and 465 nm) excitations were also investigated and shown in Fig. 8. Emission peaks were attributed to the 5D → 7F ( J J=1,2,3,4 ) transitions [51]. The emission due to 393 nm 0 show enhanced intensity compared to other wavelengths. The result was the crucial evidence that the energy of photo excited carriers in Mg2 SiO4 transfer to Eu3+ ions, resulting in the 5 D0 → 7 FJ

Fig. 7. Emission spectra of Mg2 SiO4 : Eu3+ (1–11 mol%) (excited at 393 nm).

R. Naik et al. / Sensors and Actuators B 195 (2014) 140–149 3+

254 nm 393 nm 464 nm PL Intensity (a.u)

6

8.0x10

PL Intensity (a.u)

Mg2SiO4:Eu (11 mol%) excited with different wavelength

250

6

300

350

400

450

500

Excitation wavelength (nm)

4.0x10

550

600

650

700

750

Wavelength (nm) Fig. 8. Comparison of emission spectra, excited with 254, 393 and 464 nm.

transitions. However, the PL intensities under the different excitations were of different order, as seen in Fig. 8. The PL intensity of 5 D → 7 F transition under 393 nm excitation was more stronger 0 2 than that under 256 and 464 nm excitations, indicating that an efficient energy transfer from host to Eu3+ takes place under 393 nm excitation [52]. Whereas, in other cases there exists a weak energy transfer between the host and lanthanide ions. On the other hand, after direct excitation of Eu3+ ion at 393 nm, sharp emission peaks were observed not only at 590 and 612 nm but also at 576 nm. The peak at 576 nm was due to the 5 D0 → 7 F0 transition of Eu3+ ions. However, such transition was forbidden in the case of the free Eu3+ ions because of parity conservation rule [53]. At room temperature emission sites were practically indistinguishable. Consequently, co-ordination difference between Mg2+ non-equivalent crystallographic sites were neglected, as a result identical crystal field was acting on Eu3+ ions. In particular, emission peak at 612 nm corresponds to 5 D0 → 7 F2 and occurs through the forced electric dipole (FED) which was allowed on the condition that the Eu3+ ion occupies a site without an inverse center. Transition centered at 612 nm was the characteristic red emission confirms energy transfer from Mg2 SiO4 to Eu3+ [54]. Their intensities depend upon both odd even parity state mixing and J–Jo mixing within the 4f-electron state manifold [55], while the 5 D0 → 7 F1 at 590 nm was the allowed magnetic dipole transition which has no electric dipole contribution. The peak around 612 and 590 nm denote the Eu3+ ions occupy the Mg2+ sites with C2 or S6 symmetry. These two emissions were of particular interest because they represent actually the local environment of the Eu3+ ions. The relative PL intensity of the emission peaks depends on the symmetry of the local environment of the Eu3+ activator ions, and it can be described in terms of Judd–Ofelt theory [56,57]. Actually, the magnetic dipole transition of 5 D0 → 7 F1 was permitted and the electric dipole transition of 5 D0 → 7 F2 was forbidden by the parity selection rule. However, in most cases, the local environment of the Eu3+ ions does not exist inversion symmetry and the parity-forbidden transition was partially permitted. This relaxation of the selection rule occurs in non-inversion symmetry sites, such as C2 or S6 sites occupied by the Eu3+ ions in Mg2 SiO4 . The Mg ion occupies two non equivalent octahedral sites in the crystalline structure of Mg2 SiO4 : one (M1) with inversion symmetry (Ci), and the other (M2) with mirror symmetry (Cs). When RE3+ ions were doped into the host, they could probably occupy both the sites. However, the PL results show that RE3+ ion in Mg2 SiO4 was mainly situated more at the low symmetry sites. Because of large difference in ionic radius of Mg2+ and Eu3+ , it was suggested

145

that only a minor fraction of the total amount of RE3+ goes into Mg substitution sites and a larger amount may be precipitated into Mg2 SiO4 :RE3+ clusters, or even separated as a rare earth oxide phase. The excess amount of Eu2 O3 will likely reside on either surface or grain boundaries of the nanocrystals to yield optimum strain relief. In the present study no diffraction peaks from Eu2 O3 (2 = 29.49◦ ) detected even at 11 mol% indicating that Eu3+ ions enter into the host lattice and replace magnesium ion located on the surface of the nano crystals because of the porosity of Mg2 SiO4 . On the other hand, Yang et al. reported that the Eu3+ ions only replaces the Mg2+ ions in M2 (Cs) site [58,59]. Therefore, Eu3+ ions in the forsterite structure exhibit a bright red emission at 612 nm. The asymmetric ratio and variation of PL intensity at different europium mol concentration was shown in Fig. 9(a) and (b). As dopant concentration of Eu3+ increases, 5 D0 → 7 F2 transition dominates and the emission intensity increases. This may be attributed to the increase in distortion of the local field around the Eu3+ ions [60,61]. Moreover, there was charge imbalance in the host lattice due to doping of trivalent Eu3+ cations. It was shown that relative intensity of the emission lines of Eu3+ depends on the doping concentration of Eu3+ in Mg2 SiO4 phosphor. As Eu3+ concentration increases the emission transition centered at 5 D0 → 7 F2 (612 nm) shows an enhanced emission. Further increase in Eu3+ concentration more than 11 mol% the PL intensity might be decrease due to concentration quenching. The concentration quenching might be explained on the basis of following two factors: (i) the excitation migration due to resonance between the activators was enhanced when the doping concentration was increased, and thus the excitation energy reaches quenching centers, and (ii) the activators were paired or coagulated and were changed to quenching centers. The energy can also be transferred non-radiatively by the radiative reabsorption or multipole-multipole interaction. The energy transfer mechanism in phosphors was essential in order to obtain the critical distance (Rc) i.e. the critical separation between Eu3+ and the quenching site [62]. While discussing the mechanism of energy transfer in phosphors, Blasse [63] suggested that if the activator was introduced solely on one crystallographic site (here Mg2+ site), the critical energy transfer distance (Rc) was approximately equal to twice the radius of a sphere with this volume. In order to further discuss the mechanism of energy transfer between the activators in the Mg2 SiO4 host, the critical energy transfer distance (Rc) was calculated by the following equation:

 3V 1/3

Rc ≈ 2

4Xc N

(8)

where Xc ; the critical concentration, N; the number of cation sites in the unit cell, and V; the volume of the unit cell. For Mg2 SiO4 :Eu3+ nanophosphor the values of N, V and Xc were 4289.92 A´˚ 3 and 0.11, respectively. Using these parameters, the esti´˚ Since Rc was not less than 5 A, ˚ mated Rc was found to be 10.8 A. exchange interaction was not responsible for non radiative energy transfer process from one Eu3+ ion to another Eu3+ ion in this host. According to Blasse theory [64] non radiative transfer between different Eu3+ ions in Mg2 SiO4 phosphor may occur by radiative re-absorption/exchange interaction/multipole–multipole interaction. Usually, radiative re-absorption mechanism comes into effect only when there was broad overlap of emission peaks of the sensitizer and activator. In the present case, radiative–reabsorption was completely ruled out as there were no broad overlapping peaks. Therefore, multipolar interaction was used to explain the concentration quenching mechanism. Multipolar interaction involves several types of interaction such as dipole–dipole (d–d), dipole–quadropole (d–q), quadropole–quadropole (q–q) interaction. As a result, the energy transfer process of Eu3+ in Mg2 SiO4

146

R. Naik et al. / Sensors and Actuators B 195 (2014) 140–149 7

4.0

1.2x10

(a)

(b)

393 nm 464 nm

393 nm

PL Intensity (a.u)

I 612 nm / I 590 nm

3.5

3.0

2.5

6

8.0x10

464 nm

6

4.0x10

2.0

0.0 0

2

4

6

8

10

12

0

2

4

6

8

10

12

3+

Eu Concentration

3+

concentration (Eu )

Fig. 9. (a) and (b)). Variation of asymmetric ratio with Eu3+ concentration in Mg2 SiO4 : Eu3+ (1–11 mol%) nanophosphors and the effect of Eu3+ on the 590 and 612 nm emission peaks.

phosphor would be due to multipolar interaction [41]. Another important parameter, which was sensitive to the nature of the Eu3+ ions environment in the host lattice, was the asymmetric ratio (A21 ) [65,66]. This gives a measure of the degree of distortion from inversion symmetry of the local environment surrounding the Eu3+ ions in the host matrix.

 5 I ( D → 7 F2 )d  2 0

(9)

I1 (5 D0 → 7 F1 )d

where I2 ; intensity of electric dipole transition at 616 nm (5 D0 → 7 F2 ) and I1 ; intensity of the magnetic dipole transition at 590 nm (5 D0 → 7 F2 ). The values of A21 decrease with increase of Eu3+ ions. However, it was 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 asymmetry ratio of Mg2 SiO4 :Eu3+ decreases with the increase of doped Eu3+ concentration and as shown in Fig. 9(b) [67]. In order to determine the type of interaction involved in the energy transfer Vanuitert’s [68] proposed an equation:



I = k 1 + ˇ(X)Q/3 X

−1

(10)

where I; the integral intensity of emission spectra from 550–750 nm, X; the activator concentration, I/X; the emission intensity per activator (X), ˇ and k; constants for a given host under same excitation condition. According to above equation, Q = 3 for the energy transfer among the nearest neighbor ions, while Q = 6, 8 and 10 for d–d, d–q and q–q interactions, respectively [41,69]. Assuming that ˇ(X)Q/3  1, above equation can be written as log

I X

= K −

Q log X 3

(K = log K − log ˇ)

(11)

From Eq. (11), the multipolar character (Q) can be obtained by plot log (I/X) v/s log (X) as shown in Fig. 10. The slope and multipolar character Q was found to be ∼ −0.5265 and 8.311 which was close to 8. Therefore, the concentration quenching in Mg2SiO4:Eu3+ phosphor occurred due to dipole to quadropole interaction. The Eu3+ energy level diagram corresponding to the 5D → 7F o J (J=0,1,2,3,4) emission peaks was shown in Fig. 11. The energy levels of Eu3+ ion arise from the 4fn configuration. Its emission mainly comes from the transition from excited 5 D0 level to the 7 FJ (J=0–4) levels. Variation of the local environment will cause the shift and splitting of the energy level. At the same time,

8.4

Slope: -0.5265 Q : 8.311

8.3 8.2 8.1

log (I/x)

A21 =

the selection rules and transition probabilities between states strongly depend on the crystal field strength. When Eu3+ ions were embedded in a site with inversion symmetry, the 5 D0 → 7 F1 magnetic dipole transition was dominating, while in a site without inversion symmetry the 5 D0 → 7 F2 electric dipole transition was the strongest [70]. When the phosphor was excited by 393 nm wavelength, Eu3+ ions were raised to 5 L6 level from the ground state. Since, the separation between 5 D0 → 7 FJ (J=0,1,2,3,4,5) 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 center of symmetry of the crystal lattice, then it will give both magnetic and electric dipole transitions. When the rare earth ions located at the center of symmetry, magnetic dipole transitions 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 was first excited from the ground state (5 D3 configuration) to the singlet state of the 5 D3,2,1,0 configurations. Then the electrons pass to the triplet state, mainly to level 4 because of symmetry reasons.

8.0 7.9

Equation

y = a + b*x

Weight

No Weightin 0.00935

Residual Sum of Squares

7.8 7.7

Pearson's r Adj. R-Square

-0.97795 0.94547

O

Intercept

Value Standard Erro 8.31192 0.04257

O

Slope

-0.52652

0.0

0.2

0.05622

0.4

0.6

0.8

1.0

log (x) Fig. 10. Relation between log(x) and log (I/x) in Mg2 SiO4 :Eu3+ (1–11 mol%) nanophosphor.

R. Naik et al. / Sensors and Actuators B 195 (2014) 140–149

5D

Non-radiative transition

F4

5D

4

5L 7 5 L6

8 0

= 577 nm

= 590 nm

= 612 nm

16

= 703 nm

3 5D 0

= 650 nm

Energy x 103 (cm-1)

28

Red emission

Eu-O Charge Transfer band

40

7

3 2 1 0

Fig. 11. The energy level diagram of Eu3+ ion showing the states involved in the luminescence process and the transition probabilities.

The last transition 5 Do 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 5 D4 → 5 D0 energy level and radiative transitions occur between 5 Do → 7 FJ states. The increase in PL intensity observed might be due to the decrease of cross relaxation between Eu3+ ions in this process [3,71,72]. The Commission International De I-Eclairage (CIE) 1931 chromaticity coordinates for Mg2 SiO4 :Eu3+ (1–11 mol%) phosphors as a function of Eu3+ concentration for the luminous color was depicted by the PL spectra. The CIE chromaticity coordinates calculated from the PL spectra were shown in Fig. 12. According to our knowledge, the CIE coordinates of red emission of Eu3+ ions not only depend upon the asymmetric ratio but also depend upon the higher energy 5 D (=3, 2, 1) emission levels. Literature reveals that the Eu3+ doping J effect becomes stronger in the case of particles with higher crystallinity, resulting in an improved activation degree of Eu3+ (hence the improved red color emission) [73,74]. It was observed that the CIE co-ordinates of 11 mol% Eu3+ activated Mg2 SiO4 :Eu3+ phosphor was very close to the National Television System Committee (NTSC) standard values. Their corresponding location was marked in Fig. 12 with star in red region. Therefore, the present phosphor was highly

Fig. 12. CIE diagram of Mg2 SiO4 :Eu3+ (1–11 mol%) nanophosphor. [Inset: (x,y) coordinate values of 1–11 mol% Mg2 SiO4 :Eu3+ nanophosphor].

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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.

4. Conclusions In the present work, Mg2 SiO4 :Eu3+ (1–11 mol%) nanophosphors were prepared by simple and low cost solution combustion method at very low temperature (350 ◦ C). The PXRD patterns confirmed single phase, orthorhombic structure and the particle size was in nano range. The UV–Vis absorption of un-doped and doped phosphor show an intense absorption band in the range 300–400 nm corresponding to ligand-to-metal charge-transfer (O2− to Eu3+ ) band. Porous nature of Mg2 SiO4 :Eu3+ product was confirmed from SEM micrographs. The phosphor exhibits different emission (in the range 550 to 750 nm) due to Eu3+ corresponding to 5D → 7F 0 J (J=0,1,2,3,4) transitions upon 393 nm excitation. The transition centered at 612 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. It was observed that the emission spectrum excited at 393 nm showed prominent spectral lines with higher order of intensity compared to other excitation wavelengths owing to efficient energy transfer takes place from host to Eu3+ ions at 393 nm and was close to visible region (NUV) which may be useful for LED applications. Further, the phosphor showed excellent CIE chromaticity co-ordinates (x, y) as a result it was quite useful for display applications.

Acknowledgments 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 RN, SCP and HPN thanks to VGST, Govt. of Karnataka, India, (No:VGST / CISEE /2012-13 / 282) for extended to carry out this research work.

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Biographies Ramachandra Naik M.Sc.: Physics (Electronics)—2011, Bangalore University. Ph.D.: Pursuing (Material Science, Luminescence) Presently working as assistant Professor in the department of Physics, New Horizon College of Engineering, Bangalore560103, India. Area of Interest: Nanomaterials, Luminescence. Dr. S.C. Prashantha M. Sc.: Physics—2000, Bangalore University. M.Phil.: Physics—2008, Bharathidasan University, Tamilnadu. Ph.D.: Solid state Physics—2012 (Material Science), Bangalore University, Presently working as Associate Professor and Head in the Department of Physics, East West Institute of Technology, Bangalore-560091, India. Area of Interest: Nanomaterials, Luminescence, defects studies, SHI irradiation. Dr. H. Nagabhushana M.Sc.: Physics—1997, Bangalore University. M.Phil.: Physics—2000, Bangalore University. Ph.D.: Solid state Physics—2002 (Material

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Science), Bangalore University D.Sc.: Material Science, Tumkur University. Presently working as Associate Professor in Centre for Nano Research (CNR), Tumkur University, Tumkur-572103, India. Area of Interest: Nanomaterials, Luminescence, defects studies, SHI irradiation, Space Science, etc. Dr. S.C. Sharma B.E.: Mechanical Engineering, BMSCE, Bangalore University. M.Tech.: Metal Casting Science and Engineering, MSRIT, Bangalore University. Ph.D.: Mechanical Engineering, Mysore University. D.Sc.: Material Science, Mangalore University. Computer Science Engineering, Kuvempu University. Material science, Deakin University, Australia. D.Eng., Mechanical Engineering, Avinashlingam University for Women, Coimbatore, Tamilnadu. Ex-Vice Chancellor, Tumkur University, Tumkur-572103, India. Presently working as Chief mentor and Director for B.S. Narayan center of excellence for advanced materials, Department of Mechanical Engineering, B.M.S. Institute of Technology, Yelahanka, Bangalore-560 064, India. Area of Interest: Nanomaterials, Luminescence, material science, advanced materials and composites. Dr. B.M. Nagabhushana M. Sc.: Chemistry—1986, Gulberga University. Ph.D.: Chemistry—2008, Bangalore University Presently working as Professor in the department of Chemistry, M.S. Ramaiah Institute of Technology, Bangalore-560 054, India. Area of Interest: Nanomaterials, Luminescence, dye degradation, composite materials etc. Dr. H.P. Nagaswarupa M. Sc.: Chemistry-2002, Bangalore University. M. Phil.,: Chemistry-2006, Bharathidasan University, Tamilnadu. Ph.D.: Chemistry-2012, Bharathidasan University, Tamilnadu. Presently working as Professor and Head in the Department of Chemistry, East West Institute of Technology, Bangalore-560091, India. Area of Interest: Nanomaterials, corrosion studies, etc. Dr. H.B. Premakumar M.Sc.: Physics—1999, Bangalore University. M.Phil.: Physics—2008, Bharathidasan University, Tamil Nadu. Ph.D.: Solid state Physics—2013 (Material Science), Tumkur University. Presently working as Associate Professor in the department of Physics, Acharya Institute of Technology, Bangalore-560107, India. Area of Interest: Nanomaterials, Luminescence, SHI irradiation.