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Effects of dopant concentration and annealing temperature on the phosphorescence from Zn2SiO4: Mn2+ nanocrystals
ARTICLE IN PRESS
Journal of Luminescence 111 (2005) 105–111 www.elsevier.com/locate/jlumin
Effects of dopant concentration and annealing temperature on the phosphorescence from Zn2SiO4: Mn2+ nanocrystals A. Patraa,, Gary A. Bakerb, Sheila N. Bakerb a Sol–Gel Division, Central Glass & Ceramic Research Institute, Jadavpur, Calcutta 700032, India Los Alamos National Laboratory, Bioscience and Chemistry Divisions, Los Alamos, NM 87545, USA
b
Received 3 February 2004; received in revised form 30 June 2004; accepted 30 June 2004 Available online 23 August 2004
Abstract The sol–emulsion–gel method is used for the preparation of Mn2+-doped Zn2SiO4 nanoparticles. The luminescence spectra at 520 nm (4T1g-6A1g) and lifetime of the excited state of Mn2+ ions-doped Zn2SiO4 nanocrystals are also found to be sensitive to the annealing temperature (750–1000 1C) and concentration (0.25–5 mol%) of ions. We found that at the lowest dopant levels (0.25 mol%, 750 1C) the intensity decay kinetic is perfectly described by a single rate. However, with increasing concentration and annealing temperature, the decay was found to be biexponential. The fast component decay is due to the pair or cluster formation and the slow component decay is due to isolated ions at higher concentration. r 2004 Elsevier B.V. All rights reserved. PACS: 77.84.Nh; 73.61.Tm; 78.55.m; 78.47.+p Keywords: Sol–Gel; Nanocrystals; Photoluminescence; Time-resolved luminescence
1. Introduction In the new millennium, there has been a renaissance in the study of photonic materials, such as in optical amplification, lasing, optical data storage, upconversion and phosphors [1–9]. Nanoparticles have recently been recognized to hold tremendous potential in the area of photonic Corresponding author. Fax: +91-33-24730957.
E-mail address: [email protected] (A. Patra).
applications [10]. Combining the promising optical properties of transition metal (TM) ions and nanoparticles, the study of excited state dynamics of TM ions in nanoscale environment is important. In case of transition metals ions, the electronic d–d transitions involve electrons which are localized in atomic orbitals of the ions. Therefore, no size-dependent quantization effect (due to confinement of delocalized electrons) is found in these transitions. However, confinement effects may be induced by inter-ionic electronic
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.06.008
ARTICLE IN PRESS 106
A. Patra et al. / Journal of Luminescence 111 (2005) 105–111
interaction and particularly, through electron–phonon interaction, which has important manifestations in influencing the optical properties. Thus, a nanostructure controls either by a judicious choice of the nanoenvironment of the species to be excited or by utilizing a nanoconfined structure to manipulate the excitation dynamics. Nanoscopic interactions play key roles in controlling the excitation dynamics. Used in early luminescent lighting, manganese-doped zinc silicates are well known as green phosphors [1]. Important for color rendering, a host lattice of Zn2SiO4 containing Eu3+, Mn2+ or Ce3+ dopant ions covers the red, green, and blue portions of the visible spectrum, respectively [11–17]. Nanophosphors based on the rare-earth and transition metals are receiving considerable attention as potential coatings in lamps, cathode ray tubes [1], flat panel displays, electroluminescent and optoelectronic devices [11] and more recently, radiation detectors in medical imaging systems [18]. The emission band at 525 nm is due to the spin-forbidden 4T1g-6A1g transition of the 3d5 electronic configuration of isolated Mn2+ [12]. The efficiency of a phosphor material is often limited by the dynamics of the dopant ion which depends on crystal nature, concentration, annealing temperature and phonon frequency. Recently, we reported that upconversion emission depends on crystal nature [8], concentration and nature of oxide host [5,7]. Therefore, time-resolved luminescence study is very important to understand the effect of concentration and annealing temperature on the excited state of Mn2+ ion in Zn2SiO4 nanocrystals. This report includes the synthesis and investigations of how the annealing temperature and concentration affect the photoluminescence properties of the Mn2+ ions in Zn2SiO4 nanocrystals.
2. Experimental section 2.1. Nanoparticle synthesis Tetraethylorthosilicate (TEOS) was added to a mixture of 1-propanol and 2-butanol and the resultant solution was stirred for 30 min. The
molar ratio of TEOS: H2O:1-propanol:2-butanol =1:3:3:2 is used in this experiment. Therefore, the addition of water and HCl led to a partially hydrolyzed sol after 1 h. Next, aqueous zinc acetate Zn (CH3COO)2?2H2O followed by Mn(CH3COO)2 4H2O was added to the sol. Sol was then slowly added to the supporting solvent consisting of 5% by volume Span 80 in cyclohexane (sol:cyclohexane was 1:4, v/v) to achieve emulsified sol droplets. Gelation of the emulsified sol droplets resulted from controlled addition of a base after which centrifugation (10 000 rpm, 30 min) allowed for facile gel particle collection. The particle was then washed twice with acetone and then twice with methanol. After preliminary drying at 60 1C for 12 h in a vacuum oven, annealing of the as-prepared gel particles was carried out at 750, 850, or 1000 1C in air for 1 h. 2.2. Microstructural characterization Transmission electron microscopy (TEM, JEOL Model 200)) was used to study the morphology and particle size of the phosphor powders. The crystalline phases of annealed powders were identified by X-ray diffraction (XRD) using a Siemens model D500 powder X-ray diffractometer using a CuKa source (1.5418 A˚ radiation). Crystallite sizes (D, in A˚) were estimated from the Scherrer equation D ¼ Kl=b cos y;
ð1Þ
where l is the wavelength of CuKa radiation, b is the corrected half-width of the diffraction peak, y is the angle and K is equal to 0.9. 2.3. Photoluminescence instrumentation Static luminescence data were collected using a SPEX Industries Fluorolog 2 System consisting of a Model 1681 single-stage 0.22 m excitation monochromator, a Model 1680 two-stage 0.22 m emission monochromator, and a high-intensity 450 W Xe arc lamp for excitation. Time-resolved luminescence data were collected on the same instrument with a Model 1934D phosphorimeter attachment and a modular pulsed
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50 W Xe flash lamp. Although the main component of the lamp pulse has a ca. 3 ms decay, the afterglow from the flashlamp results in a tail extending to well beyond 0.02 ms, precluding the reliable collection of time-resolved data at shorter times. Therefore, the detection window was gated from 0.05 to 19 or 30 ms (the lamp decay intensity at 0.05 ms is below 0.2% of the peak lamp intensity). The following expression was used to analyze the experimental time-resolved phosphorescence decays, PðtÞ: PðtÞ ¼ b þ
n X
ai expðt=ti Þ:
ð2Þ
i
Here, n is the number of discrete emissive species, b is a baseline correction (‘‘dc’’ offset), and ai and ti are the pre-exponential factors and excited-state phosphorescence lifetimes associated with the ith component, respectively. For biexponential decays (n ¼ 2), the average lifetime, /tS, was calculated from hti ¼
2 X i¼1
ai t2i =
2 X
ai t i :
Fig. 1. X-ray powder diffraction patterns obtained from Zn2SiO4:Mn (0.25 mol% Mn) annealed for 1 h at (a) 750 1C (b) 850 1C and (c) 1000 1C.
ð3Þ
i¼1
3. Results and discussion 3.1. Microstructural characterization Fig. 1 shows XRD patterns for Zn2SiO4:Mn nanocrystals prepared at different temperatures. The differences in peak intensity and width are consistent with a higher degree of crystallinity as the firing temperature is increased from 750 to 1000 1C. The average crystallite sizes (calculated from Eq. (1)) are 20, 40 and 60 nm for 750, 850 and 1000 1C, respectively. The Zn2SiO4 phase (JCPDS Card 37-1485) is obtained when the sample annealed at 750 1C with minor quantity of ZnO phase (JCPDS Card 36-1451). However, only Zn2SiO4 exists for samples annealed at 850 and 1000 1C. Fig. 2 shows the TEM micrograph of Mn-doped Zn2SiO4 phosphors after firing at 1000 1C and the diameter of particle size is 50 nm.
Fig. 2. TEM micrographs of Zn2SiO4:Mn (0.25 mol%) powders heated at 1000 1C.
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Excitation and emission spectra for nano-sized Zn2SiO4:Mn is provided in Figs. 3 and 4. The rather broad green emission centered near 520 nm corresponds to the 4T1(4G)-6A1(6S) transition for tetrahedral-coordinated Mn2+ (weak crystal field). Significant changes in luminescence result from sintering as shown in Fig. 3 and Table 1. Likely due to the increasing crystallinity of the host matrix and crystallite size with higher treatment temperature, luminescence intensity increased by a factor of about 30 from 750 to 1000 1C, in accordance with the observations of Copeland et al. [19]. Most notably (Table 1), the observed peak shape narrows and blue-shifts (534–520 nm) with increasing annealing temperature from 750 to 1000 1C for 0.25 mol% Mn2+-doped Zn2SiO4 particles. According to Tanabe–Sugano diagram for d5 ions that decreasing crystal field strength, a blue-shift of the 4T1-6A1 optical transition is expected. For example, the PL full-width at halfmaximum (FWHM) significantly decreases from 87 to 47 nm for samples annealed at 750 and 1000 1C. However, with increasing the concentration of Mn2+ ions (0.25–5.0 mol%), the redshift (520–523 nm) of the 4T1-6A1 optical transition occurred for 1000 1C heat-treated samples (Table 1). The red-shift of the emission band with the increasing concentration is due to the exchange interactions between ions with the increasing concentration of Mn2+ ions [20].
Normalized PL Intensity (Offset)
3.2. Photoluminescence 2.0
(c)
1.5
(b)
1.0
(c) (b)
(a)
(a)
0.5 λem = 530 nm
0.0 200
250
300
λex = 248 nm
350 450
500
550
600
650
Wavelength (nm)
Fig. 4. PL excitation and emission spectra of Zn2SiO4:Mn phosphor particles annealed at 1000 1C for 1 h. Profiles a–c corresponds to doping levels of 0.25, 1, and 5 mol% Mn, respectively.
Table 1 Emission maxima (lmax/nm) and centers-of-gravity (COG/nm) for Zn2SiO4: Mn nanophosphors X
mol%
T (1C)
lmax (COG)a
Mn2+
0.25 0.25 0.25 1 5
750 850 1000 1000 1000
534 526 520 520 523
(544) (544) (525) (531) (532)
a
COG were calculated from the raw spectra using the 2 expression COG=[SI(li)l3 i ]/[SI(li)li ] for a wavelength i range of 450–650 nm.
3.3. Time-resolved photoluminescence (c)
Normalized PL Intensity (Offset)
2.0
(c) (b)
1.5
(b)
1.0
(a)
(a)
0.5
λ em = 530 nm
0.0 200
250
300
350 450
λ ex = 248 nm
500
550
600
650
Wavelength (nm)
Fig. 3. PL excitation and emission spectra of Zn2SiO4:Mn (0.25 mol% Mn) phosphor particles annealed at (a) 750 1C, (b) 850 1C, and (c) 1000 1C for 1 h.
For Zn2SiO4:Mn containing 0.25 mol% Mn calcinated at 750 1C, the PL decay was single exponential with a lifetime of 10.98 ms (Table 2). It should be noted that this sample is substantially longer lived than a commercially available Zn2SiO4:Mn for which a 10% decay time (I=I 0 ¼ 0:1) of 14 ms was reported (assuming first-order decay, this is akin to a 6 ms 1/e lifetime) [16]. This result suggests that this emission decay is due to isolated manganese ions. All other PL decays in Figs. 5 and 6 are characterized by two discrete decay times. With increasing annealing temperatures, /tS are 10.67 and 19.48 ms for 850
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Table 2 Recovered PL intensity decay kinetics for Zn2SiO4:Mn nanophosphors X 2+
T (1C)
a1
t1 (ms)
a2
t2 (ms)
/tS(ms)
0.25 0.25 0.25 1 5
750 850 1000 1000 1000
1603 61.6 140.9 2345 14800
10.98 0.49 1.01 0.17 7.14
— 514.7 3024 16800 22900
— 10.73 19.52 18.20 13.01
— 10.67 19.48 18.18 11.47
1.4
1.4
1.2
1.2
1.0
1.0
PL Intensity (Offset)
PL Intensity (Offset)
Mn
mol%
(c)
0.8 0.6
(b)
0.4
(a)
0.2
4
8
12
16
(b)
0.8
(a)
0.6 0.4 0.2
0.0 0
(c)
20
Time (ms)
0.0 0
6
12
18
24
30
Time (ms)
Fig. 5. PL decay kinetics of Zn2SiO4:Mn (0.25 mol% Mn) phosphor particles annealed at (a) 750 1C, (b) 850 1C, and (c) 1000 1C for 1 h.
Fig. 6. PL decay kinetics of Zn2SiO4:Mn phosphor particles annealed at 1000 1C for 1 h. Profiles a–c corresponds to doping levels of 0.25, 1, and 5 mol% Mn, respectively.
and 1000 1C annealed samples, respectively. It may be due to the increasing crystallinity of the host matrix and also the decreasing hydroxyl groups. Another reason is that the surface-related nonradiative decay decreases with an increase in the particle size. Therefore, we can say that smaller size phosphors are not efficient. For 850 1C annealed sample, the tf ¼ 0:49 ms and ts ¼ 10:73 ms are obtained, whereas for 1000 1C annealed sample, the fast and slow components are 1.01 and 19.52 ms (Table 2), respectively. The only willemite lattice of zinc silicate is formed at higher temperatures (850 and 1000 1C) which is a rhombohedra structure [13] consisting of cornerjoined tetrahedral group of [ZnO4] and [SiO4]. This structure houses two in equivalent Zn sites both having four nearest-neighbor oxygens in a slightly
distorted tetrahedral configuration. Therefore at higher temperatures, the Mn ions reside in two different Zn sites in the Zn2SiO4 nanocrystals. This result clearly indicates that decay time arises from two different sites with increasing the annealing temperature (at 0.25 mol% Mn). For 1.0 mol% Mn2+-doped samples (1000 1C), the fast and slow components are tf ¼ 0:17 ms and ts ¼ 18:20 ms; whereas for 5.0 mol% the fast and slow components are 7.14 and 13.01 ms, respectively (Table 2). These results indicate that the lifetime decreases with an increase in the concentration. Therefore, the emission decay depends on both the concentration and annealing temperature. At higher concentrations (1.0 and 5.0 mol%, but annealing temperature is 1000 1C), the Mn ions also reside in two different Zn sites in the Zn2SiO4 nanocrystals.
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It is well known that the pair or cluster formation is easily obtained at high concentration. The exchange interaction between ions in high concentration results in a shortening of the decay time. Therefore, at higher concentration, the fast component arises from ion pair formation due to concentration quenching as suggested by Morrell et al. [21] and the slow component is from isolated ions [14,20]. Generally, in normal crystal, the surface does not influence the pair formation because the number of nearest-neighbors is lower at the surface. However, it is shown [22] that the probabilities for dopant pair-state for a given concentration are higher in nanocrystal and dopant pair formation also depends on the size of particles. This result indicates that the easy formation of dopant ion pairs occurs at low concentration due to high surface area of the nanocrystal. Therefore, we can say, small concentration of dopant in nanocrystal also forms dopant pair.
4. Conclusions In conclusion, chemical synthesis through the sol–emulsion–gel method is a promising route for the preparation of nanocrystalline Zn2SiO4:Mn powders having particle diameter in the range 50 nm. We have compared the luminescent efficiencies of the nanoparticle of zinc silicate doped by manganese with those of commercial phosphors and found that the intensities are comparable. We observed an increase in emission decay time with an increase in the annealing temperature and decrease in the concentration of dopant ions. In case of 750 1C annealed sample (0.25 mol% Mn2+) showed the single component decay of 10.98 ms. However, with increasing concentration and annealing temperature, the decay was found to be biexponential. The exchange interaction between ions in high concentration results in a shortening of the decay time. The fast component decay is due to the pair or cluster formation and the slow component decay is due to isolated ions. Our results highlight the importance of sintering temperature and dopant concentration of the material. In this regard, time-
resolved luminescence study is essential to unraveling the origin of the observed quenching behavior.
Acknowledgments A. Patra thanks Dr. H. S. Maiti, Director of CGCRI for his kind interest in this work. The Department of Science and Technology (NSTI, No.SR/S5/NM-05/2003) is acknowledged for financial support. We thank David Morris for kindly providing access to and technical assistance with his phosphorimeter. GAB was generously supported by a Reines Fellowship from Los Alamos National Laboratory. References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [2] R. Reisfeld, C.K. Jorgensen (Eds.), Laser and Excited State of Rare-Earths, Springer, Berlin, 1977. [3] A. Polman, J. Appl. Phys 82 (1999) 39. [4] S. Heer, K. Petermann, H.U. Gudel, J. Lumin. 102 (2003) 144. [5] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, J. Phys. Chem. B 106 (2002) 1909. [6] J. Silver, M.I. Martinez-Rubio, T.G. Ireland, G.R. Fern, R.J. Withnall, Phys. Chem. B. 105 (2001) 948. [7] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, Chem. Mater. 15 (2003) 3650. [8] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, Appl. Phys. Lett. 83 (2003) 284. [9] A. Patra, E. Sominska, S. Ramesh, Yu. Kolypin, Z. Zhong, H. Minti, R. Reisfeld, A. Gedanken, J. Phys. Chem. B 103 (1999) 3361. [10] R.N. Bhargava, D. Gallaghar, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [11] T. Minani, T. Miyata, S. Takata, I. Fududa, Jap. J. Appl. Phys. 30 (1990) L117. [12] D.T. Palumbo, J.J. Brown, J. Electrochem. Soc. 117 (1970) 1184. [13] H. Hess, A. Heim, M. Scala, J. Electrochem. Soc. 130 (1983) 2443. [14] C. Barthou, J. Benoit, P. Benalloul, A. Morell, J. Electrochem. Soc. 141 (1994) 524. [15] H.X. Zhang, Y. Zhou, C.H. Kam, Y.L. Lam, Y.C. Chan, Mater. Res. Soc. Symp. Proc. 560 (1990) 9. [16] K.S. Sohn, B. Cho, H.D. Park, J. Am. Ceram. Soc. 82 (1999) 2779. [17] Q.Y. Zhang, K. Pita, W. Ye, W.X. Que, Chem. Phys. Lett. 351 (2002) 163.
ARTICLE IN PRESS A. Patra et al. / Journal of Luminescence 111 (2005) 105–111 [18] I. Kandarakis, D. Cavouras, P. Prassopoulos, E. Kanellopoulos, C.D. Nomicos, G.S. Panayiotakis, Appl. Phys. A 67 (1998) 521. [19] T.S. Copeland, B.I. Lee, J. Qi, A.K. Elrod, J. Lumin. 97 (2002) 168.
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[20] C.R. Ronda, T. Amrein, J. Lumin. 69 (1996) 245. [21] A. Morell, N. Elkhiati, J. Electrochem. Soc. 140 (1993) 2019. [22] J.F. Suyver, R. Meester, J.J. Kelly, A. Meijerink, Phys. Rev. B. 64 (2001) 235408.
Journal of Luminescence 111 (2005) 105–111 www.elsevier.com/locate/jlumin
Effects of dopant concentration and annealing temperature on the phosphorescence from Zn2SiO4: Mn2+ nanocrystals A. Patraa,, Gary A. Bakerb, Sheila N. Bakerb a Sol–Gel Division, Central Glass & Ceramic Research Institute, Jadavpur, Calcutta 700032, India Los Alamos National Laboratory, Bioscience and Chemistry Divisions, Los Alamos, NM 87545, USA
b
Received 3 February 2004; received in revised form 30 June 2004; accepted 30 June 2004 Available online 23 August 2004
Abstract The sol–emulsion–gel method is used for the preparation of Mn2+-doped Zn2SiO4 nanoparticles. The luminescence spectra at 520 nm (4T1g-6A1g) and lifetime of the excited state of Mn2+ ions-doped Zn2SiO4 nanocrystals are also found to be sensitive to the annealing temperature (750–1000 1C) and concentration (0.25–5 mol%) of ions. We found that at the lowest dopant levels (0.25 mol%, 750 1C) the intensity decay kinetic is perfectly described by a single rate. However, with increasing concentration and annealing temperature, the decay was found to be biexponential. The fast component decay is due to the pair or cluster formation and the slow component decay is due to isolated ions at higher concentration. r 2004 Elsevier B.V. All rights reserved. PACS: 77.84.Nh; 73.61.Tm; 78.55.m; 78.47.+p Keywords: Sol–Gel; Nanocrystals; Photoluminescence; Time-resolved luminescence
1. Introduction In the new millennium, there has been a renaissance in the study of photonic materials, such as in optical amplification, lasing, optical data storage, upconversion and phosphors [1–9]. Nanoparticles have recently been recognized to hold tremendous potential in the area of photonic Corresponding author. Fax: +91-33-24730957.
E-mail address: [email protected] (A. Patra).
applications [10]. Combining the promising optical properties of transition metal (TM) ions and nanoparticles, the study of excited state dynamics of TM ions in nanoscale environment is important. In case of transition metals ions, the electronic d–d transitions involve electrons which are localized in atomic orbitals of the ions. Therefore, no size-dependent quantization effect (due to confinement of delocalized electrons) is found in these transitions. However, confinement effects may be induced by inter-ionic electronic
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.06.008
ARTICLE IN PRESS 106
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interaction and particularly, through electron–phonon interaction, which has important manifestations in influencing the optical properties. Thus, a nanostructure controls either by a judicious choice of the nanoenvironment of the species to be excited or by utilizing a nanoconfined structure to manipulate the excitation dynamics. Nanoscopic interactions play key roles in controlling the excitation dynamics. Used in early luminescent lighting, manganese-doped zinc silicates are well known as green phosphors [1]. Important for color rendering, a host lattice of Zn2SiO4 containing Eu3+, Mn2+ or Ce3+ dopant ions covers the red, green, and blue portions of the visible spectrum, respectively [11–17]. Nanophosphors based on the rare-earth and transition metals are receiving considerable attention as potential coatings in lamps, cathode ray tubes [1], flat panel displays, electroluminescent and optoelectronic devices [11] and more recently, radiation detectors in medical imaging systems [18]. The emission band at 525 nm is due to the spin-forbidden 4T1g-6A1g transition of the 3d5 electronic configuration of isolated Mn2+ [12]. The efficiency of a phosphor material is often limited by the dynamics of the dopant ion which depends on crystal nature, concentration, annealing temperature and phonon frequency. Recently, we reported that upconversion emission depends on crystal nature [8], concentration and nature of oxide host [5,7]. Therefore, time-resolved luminescence study is very important to understand the effect of concentration and annealing temperature on the excited state of Mn2+ ion in Zn2SiO4 nanocrystals. This report includes the synthesis and investigations of how the annealing temperature and concentration affect the photoluminescence properties of the Mn2+ ions in Zn2SiO4 nanocrystals.
2. Experimental section 2.1. Nanoparticle synthesis Tetraethylorthosilicate (TEOS) was added to a mixture of 1-propanol and 2-butanol and the resultant solution was stirred for 30 min. The
molar ratio of TEOS: H2O:1-propanol:2-butanol =1:3:3:2 is used in this experiment. Therefore, the addition of water and HCl led to a partially hydrolyzed sol after 1 h. Next, aqueous zinc acetate Zn (CH3COO)2?2H2O followed by Mn(CH3COO)2 4H2O was added to the sol. Sol was then slowly added to the supporting solvent consisting of 5% by volume Span 80 in cyclohexane (sol:cyclohexane was 1:4, v/v) to achieve emulsified sol droplets. Gelation of the emulsified sol droplets resulted from controlled addition of a base after which centrifugation (10 000 rpm, 30 min) allowed for facile gel particle collection. The particle was then washed twice with acetone and then twice with methanol. After preliminary drying at 60 1C for 12 h in a vacuum oven, annealing of the as-prepared gel particles was carried out at 750, 850, or 1000 1C in air for 1 h. 2.2. Microstructural characterization Transmission electron microscopy (TEM, JEOL Model 200)) was used to study the morphology and particle size of the phosphor powders. The crystalline phases of annealed powders were identified by X-ray diffraction (XRD) using a Siemens model D500 powder X-ray diffractometer using a CuKa source (1.5418 A˚ radiation). Crystallite sizes (D, in A˚) were estimated from the Scherrer equation D ¼ Kl=b cos y;
ð1Þ
where l is the wavelength of CuKa radiation, b is the corrected half-width of the diffraction peak, y is the angle and K is equal to 0.9. 2.3. Photoluminescence instrumentation Static luminescence data were collected using a SPEX Industries Fluorolog 2 System consisting of a Model 1681 single-stage 0.22 m excitation monochromator, a Model 1680 two-stage 0.22 m emission monochromator, and a high-intensity 450 W Xe arc lamp for excitation. Time-resolved luminescence data were collected on the same instrument with a Model 1934D phosphorimeter attachment and a modular pulsed
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107
50 W Xe flash lamp. Although the main component of the lamp pulse has a ca. 3 ms decay, the afterglow from the flashlamp results in a tail extending to well beyond 0.02 ms, precluding the reliable collection of time-resolved data at shorter times. Therefore, the detection window was gated from 0.05 to 19 or 30 ms (the lamp decay intensity at 0.05 ms is below 0.2% of the peak lamp intensity). The following expression was used to analyze the experimental time-resolved phosphorescence decays, PðtÞ: PðtÞ ¼ b þ
n X
ai expðt=ti Þ:
ð2Þ
i
Here, n is the number of discrete emissive species, b is a baseline correction (‘‘dc’’ offset), and ai and ti are the pre-exponential factors and excited-state phosphorescence lifetimes associated with the ith component, respectively. For biexponential decays (n ¼ 2), the average lifetime, /tS, was calculated from hti ¼
2 X i¼1
ai t2i =
2 X
ai t i :
Fig. 1. X-ray powder diffraction patterns obtained from Zn2SiO4:Mn (0.25 mol% Mn) annealed for 1 h at (a) 750 1C (b) 850 1C and (c) 1000 1C.
ð3Þ
i¼1
3. Results and discussion 3.1. Microstructural characterization Fig. 1 shows XRD patterns for Zn2SiO4:Mn nanocrystals prepared at different temperatures. The differences in peak intensity and width are consistent with a higher degree of crystallinity as the firing temperature is increased from 750 to 1000 1C. The average crystallite sizes (calculated from Eq. (1)) are 20, 40 and 60 nm for 750, 850 and 1000 1C, respectively. The Zn2SiO4 phase (JCPDS Card 37-1485) is obtained when the sample annealed at 750 1C with minor quantity of ZnO phase (JCPDS Card 36-1451). However, only Zn2SiO4 exists for samples annealed at 850 and 1000 1C. Fig. 2 shows the TEM micrograph of Mn-doped Zn2SiO4 phosphors after firing at 1000 1C and the diameter of particle size is 50 nm.
Fig. 2. TEM micrographs of Zn2SiO4:Mn (0.25 mol%) powders heated at 1000 1C.
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Excitation and emission spectra for nano-sized Zn2SiO4:Mn is provided in Figs. 3 and 4. The rather broad green emission centered near 520 nm corresponds to the 4T1(4G)-6A1(6S) transition for tetrahedral-coordinated Mn2+ (weak crystal field). Significant changes in luminescence result from sintering as shown in Fig. 3 and Table 1. Likely due to the increasing crystallinity of the host matrix and crystallite size with higher treatment temperature, luminescence intensity increased by a factor of about 30 from 750 to 1000 1C, in accordance with the observations of Copeland et al. [19]. Most notably (Table 1), the observed peak shape narrows and blue-shifts (534–520 nm) with increasing annealing temperature from 750 to 1000 1C for 0.25 mol% Mn2+-doped Zn2SiO4 particles. According to Tanabe–Sugano diagram for d5 ions that decreasing crystal field strength, a blue-shift of the 4T1-6A1 optical transition is expected. For example, the PL full-width at halfmaximum (FWHM) significantly decreases from 87 to 47 nm for samples annealed at 750 and 1000 1C. However, with increasing the concentration of Mn2+ ions (0.25–5.0 mol%), the redshift (520–523 nm) of the 4T1-6A1 optical transition occurred for 1000 1C heat-treated samples (Table 1). The red-shift of the emission band with the increasing concentration is due to the exchange interactions between ions with the increasing concentration of Mn2+ ions [20].
Normalized PL Intensity (Offset)
3.2. Photoluminescence 2.0
(c)
1.5
(b)
1.0
(c) (b)
(a)
(a)
0.5 λem = 530 nm
0.0 200
250
300
λex = 248 nm
350 450
500
550
600
650
Wavelength (nm)
Fig. 4. PL excitation and emission spectra of Zn2SiO4:Mn phosphor particles annealed at 1000 1C for 1 h. Profiles a–c corresponds to doping levels of 0.25, 1, and 5 mol% Mn, respectively.
Table 1 Emission maxima (lmax/nm) and centers-of-gravity (COG/nm) for Zn2SiO4: Mn nanophosphors X
mol%
T (1C)
lmax (COG)a
Mn2+
0.25 0.25 0.25 1 5
750 850 1000 1000 1000
534 526 520 520 523
(544) (544) (525) (531) (532)
a
COG were calculated from the raw spectra using the 2 expression COG=[SI(li)l3 i ]/[SI(li)li ] for a wavelength i range of 450–650 nm.
3.3. Time-resolved photoluminescence (c)
Normalized PL Intensity (Offset)
2.0
(c) (b)
1.5
(b)
1.0
(a)
(a)
0.5
λ em = 530 nm
0.0 200
250
300
350 450
λ ex = 248 nm
500
550
600
650
Wavelength (nm)
Fig. 3. PL excitation and emission spectra of Zn2SiO4:Mn (0.25 mol% Mn) phosphor particles annealed at (a) 750 1C, (b) 850 1C, and (c) 1000 1C for 1 h.
For Zn2SiO4:Mn containing 0.25 mol% Mn calcinated at 750 1C, the PL decay was single exponential with a lifetime of 10.98 ms (Table 2). It should be noted that this sample is substantially longer lived than a commercially available Zn2SiO4:Mn for which a 10% decay time (I=I 0 ¼ 0:1) of 14 ms was reported (assuming first-order decay, this is akin to a 6 ms 1/e lifetime) [16]. This result suggests that this emission decay is due to isolated manganese ions. All other PL decays in Figs. 5 and 6 are characterized by two discrete decay times. With increasing annealing temperatures, /tS are 10.67 and 19.48 ms for 850
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Table 2 Recovered PL intensity decay kinetics for Zn2SiO4:Mn nanophosphors X 2+
T (1C)
a1
t1 (ms)
a2
t2 (ms)
/tS(ms)
0.25 0.25 0.25 1 5
750 850 1000 1000 1000
1603 61.6 140.9 2345 14800
10.98 0.49 1.01 0.17 7.14
— 514.7 3024 16800 22900
— 10.73 19.52 18.20 13.01
— 10.67 19.48 18.18 11.47
1.4
1.4
1.2
1.2
1.0
1.0
PL Intensity (Offset)
PL Intensity (Offset)
Mn
mol%
(c)
0.8 0.6
(b)
0.4
(a)
0.2
4
8
12
16
(b)
0.8
(a)
0.6 0.4 0.2
0.0 0
(c)
20
Time (ms)
0.0 0
6
12
18
24
30
Time (ms)
Fig. 5. PL decay kinetics of Zn2SiO4:Mn (0.25 mol% Mn) phosphor particles annealed at (a) 750 1C, (b) 850 1C, and (c) 1000 1C for 1 h.
Fig. 6. PL decay kinetics of Zn2SiO4:Mn phosphor particles annealed at 1000 1C for 1 h. Profiles a–c corresponds to doping levels of 0.25, 1, and 5 mol% Mn, respectively.
and 1000 1C annealed samples, respectively. It may be due to the increasing crystallinity of the host matrix and also the decreasing hydroxyl groups. Another reason is that the surface-related nonradiative decay decreases with an increase in the particle size. Therefore, we can say that smaller size phosphors are not efficient. For 850 1C annealed sample, the tf ¼ 0:49 ms and ts ¼ 10:73 ms are obtained, whereas for 1000 1C annealed sample, the fast and slow components are 1.01 and 19.52 ms (Table 2), respectively. The only willemite lattice of zinc silicate is formed at higher temperatures (850 and 1000 1C) which is a rhombohedra structure [13] consisting of cornerjoined tetrahedral group of [ZnO4] and [SiO4]. This structure houses two in equivalent Zn sites both having four nearest-neighbor oxygens in a slightly
distorted tetrahedral configuration. Therefore at higher temperatures, the Mn ions reside in two different Zn sites in the Zn2SiO4 nanocrystals. This result clearly indicates that decay time arises from two different sites with increasing the annealing temperature (at 0.25 mol% Mn). For 1.0 mol% Mn2+-doped samples (1000 1C), the fast and slow components are tf ¼ 0:17 ms and ts ¼ 18:20 ms; whereas for 5.0 mol% the fast and slow components are 7.14 and 13.01 ms, respectively (Table 2). These results indicate that the lifetime decreases with an increase in the concentration. Therefore, the emission decay depends on both the concentration and annealing temperature. At higher concentrations (1.0 and 5.0 mol%, but annealing temperature is 1000 1C), the Mn ions also reside in two different Zn sites in the Zn2SiO4 nanocrystals.
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It is well known that the pair or cluster formation is easily obtained at high concentration. The exchange interaction between ions in high concentration results in a shortening of the decay time. Therefore, at higher concentration, the fast component arises from ion pair formation due to concentration quenching as suggested by Morrell et al. [21] and the slow component is from isolated ions [14,20]. Generally, in normal crystal, the surface does not influence the pair formation because the number of nearest-neighbors is lower at the surface. However, it is shown [22] that the probabilities for dopant pair-state for a given concentration are higher in nanocrystal and dopant pair formation also depends on the size of particles. This result indicates that the easy formation of dopant ion pairs occurs at low concentration due to high surface area of the nanocrystal. Therefore, we can say, small concentration of dopant in nanocrystal also forms dopant pair.
4. Conclusions In conclusion, chemical synthesis through the sol–emulsion–gel method is a promising route for the preparation of nanocrystalline Zn2SiO4:Mn powders having particle diameter in the range 50 nm. We have compared the luminescent efficiencies of the nanoparticle of zinc silicate doped by manganese with those of commercial phosphors and found that the intensities are comparable. We observed an increase in emission decay time with an increase in the annealing temperature and decrease in the concentration of dopant ions. In case of 750 1C annealed sample (0.25 mol% Mn2+) showed the single component decay of 10.98 ms. However, with increasing concentration and annealing temperature, the decay was found to be biexponential. The exchange interaction between ions in high concentration results in a shortening of the decay time. The fast component decay is due to the pair or cluster formation and the slow component decay is due to isolated ions. Our results highlight the importance of sintering temperature and dopant concentration of the material. In this regard, time-
resolved luminescence study is essential to unraveling the origin of the observed quenching behavior.
Acknowledgments A. Patra thanks Dr. H. S. Maiti, Director of CGCRI for his kind interest in this work. The Department of Science and Technology (NSTI, No.SR/S5/NM-05/2003) is acknowledged for financial support. We thank David Morris for kindly providing access to and technical assistance with his phosphorimeter. GAB was generously supported by a Reines Fellowship from Los Alamos National Laboratory. References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [2] R. Reisfeld, C.K. Jorgensen (Eds.), Laser and Excited State of Rare-Earths, Springer, Berlin, 1977. [3] A. Polman, J. Appl. Phys 82 (1999) 39. [4] S. Heer, K. Petermann, H.U. Gudel, J. Lumin. 102 (2003) 144. [5] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, J. Phys. Chem. B 106 (2002) 1909. [6] J. Silver, M.I. Martinez-Rubio, T.G. Ireland, G.R. Fern, R.J. Withnall, Phys. Chem. B. 105 (2001) 948. [7] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, Chem. Mater. 15 (2003) 3650. [8] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, Appl. Phys. Lett. 83 (2003) 284. [9] A. Patra, E. Sominska, S. Ramesh, Yu. Kolypin, Z. Zhong, H. Minti, R. Reisfeld, A. Gedanken, J. Phys. Chem. B 103 (1999) 3361. [10] R.N. Bhargava, D. Gallaghar, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [11] T. Minani, T. Miyata, S. Takata, I. Fududa, Jap. J. Appl. Phys. 30 (1990) L117. [12] D.T. Palumbo, J.J. Brown, J. Electrochem. Soc. 117 (1970) 1184. [13] H. Hess, A. Heim, M. Scala, J. Electrochem. Soc. 130 (1983) 2443. [14] C. Barthou, J. Benoit, P. Benalloul, A. Morell, J. Electrochem. Soc. 141 (1994) 524. [15] H.X. Zhang, Y. Zhou, C.H. Kam, Y.L. Lam, Y.C. Chan, Mater. Res. Soc. Symp. Proc. 560 (1990) 9. [16] K.S. Sohn, B. Cho, H.D. Park, J. Am. Ceram. Soc. 82 (1999) 2779. [17] Q.Y. Zhang, K. Pita, W. Ye, W.X. Que, Chem. Phys. Lett. 351 (2002) 163.
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