Physica B 406 (2011) 4188–4194

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Physica B

0921-45

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physb

Effects of dopant concentrations and firing temperatures on decaykinetics of manganese doped willemite nanopowders

Poonam Sharma n, Harbhajan S. Bhatti

Department of Physics, Punjabi University, Patiala 147002, Punjab, India

a r t i c l e i n f o

Article history:

Received 10 June 2010

Received in revised form

28 July 2011

Accepted 30 July 2011Available online 16 August 2011

Keywords:

Nanocrystalline material

Sol–gel method

Laser-induced phosphorescence decay

Optical parameters

26/$ - see front matter & 2011 Elsevier B.V. A

016/j.physb.2011.07.054

esponding author. Tel.: þ91 998 804 6328.

ail address: poonam.vats@gmail.com (P. Shar

a b s t r a c t

Nanocrystallline willemite, Zn2�xMnxSiO4 (0.5rxr5 mol%), doped with variable concentration of

divalent manganese ions, phosphor powders were prepared using the simple wet-chemical sol–gel

method combined with furnace firing at 800, 900, and 1000 1C. X-ray diffraction (XRD) and high

resolution X-ray photoelectron (HR-XPS) scans confirm the presence of willemite phase of Zn2SiO4.

Laser-induced phosphorescence decay measurements of Zn2�xMnxSiO4 nanophosphors were investi-

gated using high peak power pulsed UV nitrogen laser (l¼337.1 nm). The decay curves show non-

single exponential behavior with long term decay rate. Various parameters describing the strength of

optical transitions in atoms and molecules such as, Einstein’s A and B coefficients, ‘f’, integrated cross-

section, and transition dipole moment values have been calculated. The long term decay rate of optical

transition parameters was found to be somewhat temperature and concentration dependent.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Phosphors are widely used in emissive display devices. How-ever, presently used phosphors still need considerable improve-ment such as, in lower current saturation, high efficiency, andbetter chromaticity [1]. Ceramic oxide based phosphors, includingsilicates phosphors, are more chemically stable and environmentfriendly compared to sulfate or polymer-dye based phosphors.Nanophosphor powders or thin films doped with various transi-tion metals and rare-earth ions have recently been recognized tohold special position in photonic and biophotonic industry [2–7].Their potential applications are still very much in designingphase. Further, fundamental research in this field remains achallenge [8]. This is very important because the transition metalsand rare-earth ions doped nanophosphors have proven to be verydifficult and been the subject of debate in the recent years. Theimportant issue will be whether the dopant impurity ions arereally been incorporated into the lattice sites of nanophosphors(doping) or adsorbed on the nanoparticles surface (activation) andtheir effects on optical properties [8]. The phosphor layer servesas a host matrix for various intentional impurities, known asluminescent impurity centers, which ultimately are responsiblefor emission of light in visible region. Typically, luminescentimpurities substitute cationic species in the lattice, for instanceMn2þ replaces Zn2þ to form Zn2SiO4:Mn2þ .

ll rights reserved.

ma).

Metal silicates have recently been reported [9] as an idealphosphor host matrix, for various transition metals and rare-earthions, with magnificent luminescence properties in blue, green, andred spectral regions. Zinc orthosilicate (willemite), Zn2SiO4, is used asa chemically stable transparent dielectric host for a number ofluminescent dopants [10–13]. For example, manganese doped zincsilicate is a well-known photoluminescent, cathodoluminescent,green light-emitting phosphor with high luminescent efficiency,better chemical stability, and splendid color purity. It has been usedin fluorescent lamps, cathode ray tube devices, thin film electro-luminescent panels, field emission display panels, plasma devices forlighting, televisions, projection displays, copy machines, flat paneldisplays, medical imaging, and portable communication equipments[11,14–19]. Recently, zinc silicate is reported to be used as a lasercrystal [20] and upconversion luminescent material [21]. Zn2SiO4

possesses a rhombohedral structure (R3, space group) having wideforbidden band gap of E5.5 eV. Silicate materials are very useful inmany applications of technological importance [22]. Zn2SiO4:Mn2þ

was used as a green component in the first tri-color lamp [23].Additionally, the light emitted by Zn2SiO4:Mn2þ seems to becompatible with the spectral sensitivity of optical detectors (film,photocathodes, and photodiodes) used in X-ray or nuclear imaging[14]. Photostimulated luminescence in silicates is used for develop-ing X-ray storage phosphors [22]. Traditionally, solid state diffusionmethod has been employed for the synthesis of Zn2SiO4, whichinvolves crushing, grinding, ball milling, and sintering of sourcematerials at very high temperatures. But, nowadays, sol–gel, forcedprecipitation, pulsed laser deposition (PLD), organometallic complexroute, combustion methods, dry reaction, spray-pyrolysis, polymer

P. Sharma, H.S Bhatti / Physica B 406 (2011) 4188–4194 4189

assisted methods, etc. [22,24–29] are widely used to synthesizeZn2SiO4 nanophosphor powders or thin films. Zeng et al. [10]synthesized Zn2SiO4 using hydrothermal method with possible low-est crystallization temperature. Lukic et al. [30] synthesized polymerassisted Zn2SiO4 phosphor powders using the sol–gel method, wherepolymer polyethylene glycol is utilized to produce gel.

In the present paper, Zn2SiO4:Mn2þ nanophosphor matrix issynthesized using the wet-chemical sol–gel method [31]. Variousphosphorescence decay parameters such as, Einstein’s A and B

coefficients, ‘f’ values, integrated cross-section, and transitiondipole moment values are calculated and reported. These para-meters are obtained by laser-induced phosphorescence decaycurves, which show non-single exponential decay behavior. Con-siderable percussion of dopant ion concentrations and firingtemperatures are observed in the optical transition parameters.

Fig. 1. Experimental setup used for time-resolved laser induced spectroscopy of

Zn2�xMnxSiO4 (0.5rxr5 mol%).

Fig. 2. Decay kinetics of Zn2�xMnxSiO4 (0.5 mol%) phosphor particles fired at 800,

900, and 1000 1C (showing three overlapping exponentials).

2. Experimental details

Precursors used for the preparation of Zn2SiO4:Mn2þ nano-phosphor powders are tetraethylorthosilicate (TEOS) Si(OC2H5)4,ammonium hydroxide NH4OH, zinc acetate dihydratedZn(OOCCH3)2 �2H2O (99.9%), manganese acetate tetrahydrateMn(OOCCH3)2 �4H2O (99.9%), nitric acid HNO3, and ethanolC2H5OH (absolute, 99.99%). All chemicals are of analytical gradeand used without further purification. The reaction solutions areprepared in high purity deionized water.

2.1. Preparation of Zn2SiO4:Mn nanoparticles

The target gel composition, for preparing the required samples is60 ZnO:40 SiO2 (mol%). The precursors used are zinc acetate,Zn(OOCCH3)2 �2H2O, and tetraethylorthosilicate Si(OC2H5)4, used asthe silica source. At first, silica sol is prepared by hydrolysis andcondensation reaction of tertraethylorthosilicates (TEOS). Si(OC2H5)4

is hydrolyzed and stirred for 1 h. The molar ratio of TEOS:H2O:-C2H5OH:HNO3(3N) is 1:1:6:0.0025, used in the experiment. Requiredamount of Zn(OOCCH3)2 �2H2O followed by Mn(OOCCH3)2 �4H2O isdissolved in 40 ml ethanol–water (4:1 v/v). The doping concentrationof Mn2þ ions varies from 0.5 to 5 mol%, substituting for Zn2þ inZn2SiO4. These two solutions are then mixed together and stirred fornext 1 h. NH4OH (35 %) solution is slowly added drop-by-drop withcontinuous stirring for next 1 h. Gelation of the sol droplets resulteddue to the controlled addition of base. The resultant solution is leftundisturbed for next 3 days till it is gelled properly. It is then dried at110 1C for 24 h to obtained xerogel. Powder phosphors are preparedby firing the preheated gel particles in an open atmosphere for 1 h atdifferent temperatures ranging from 800 1C to 1000 1C. The basicchemical equation involved is as follows:

2ZnOþSiO4-Zn2SiO4 ð1Þ

3. Time-resolved phosphorescence decay kinetics

The experimental setup shown in Fig. 1 is used to fullycharacterize the decay parameters. High peak-power (E10 kW),intense UV pulsed-nitrogen (N2) laser (337.1 nm) is employed asan excitation source for phosphorescence decay measurements.Luminescence in visible region is observed from doped Zn2SiO4

phosphor. The nanophosphor powder is mounted on the sampleholder using xylene, placed at a right angle to the laser beam, andthe emission in the visible region is recorded by the monochro-mator. After the selection of the emission wavelength (425 nmand 528 nm) from the sample, the signal is passed through a fastphotomultiplier tube [RCA8053PMT]. A glass slab or CuSO4 solu-tion is introduced in the path of emitted visible light to filter out

all scattered UV radiations. The signal is then transmitted to digitalstorage oscilloscope (DSO) [TektronicsTDS1012] and finally signalis sent to computer assembly to analyze the observed data. Threecomponents of transition probability have been peeled-off fromdecay curves (Fig. 2) using peeling-off method of Bubes [32–36] toget the average life-time values [31]. Various optical parameters ofinterest like Einstein’s A and B coefficients, ‘f’ (oscillator strength),integrated cross-section, and transition dipole moment values ofcorresponding radiative transitions are calculated.

4. Theoretical background

When samples are irradiated using high energy laser (UV)beam, the electrons present in the valence band raise to theexcited states. On returning back to the valence band, there is an

Fig. 3. X-ray powder diffraction patterns obtained from Zn2�xMnxSiO4 (0.5 mol%)

fired at 1000 1C for 1 h.

P. Sharma, H.S Bhatti / Physica B 406 (2011) 4188–41944190

emission of low energy (visible light) radiations with intensity ofphosphorescence expressed by

logI

Io

� �¼�pt ð2Þ

where ‘I’ is the intensity of phosphorescence radiation at time ‘t’,‘Io’ is the intensity of radiation at cut-off position, and theconstant ‘p’ is the Einstein’s A coefficient value of correspondingradiative transition. A plot of ln (I) vs. t (not shown here) will be astraight line in the case of single value of ‘p’. The calculations havebeen performed separately for each luminescence decay curve.Two or more straight lines can be fitted to this graph and theslope of each component gives the value of ‘p’ [31].

However in most of the cases, when one comes across theinteraction of radiation with solids, there are various trap levels atdifferent depths that give rise to the different exponential decayssuperimposed with each other. Such type of decay curves,obtained due to the superposition of number of exponentialdecay curves, is popularly known as hyperbolic decay curves(shown in Fig. 2). In the present case, a plot of ln (I) vs. t does notshow a linear relationship because of the superposition of thenumber of exponential decays. The different components of thedecay curve may be considered as an emission due to trapspresent at different depths. Thus, it is reasonable to consider thatas the number of exponentials increases, the nature of decaycurve changes to multiexponential. The electromagnetic radia-tions interact with an electronic center through an electric ormagnetic fields of the radiation. ‘f’ value is a measure of therelative strength of the electronic transition within atomic andmolecular systems [22]. It is particularly a useful method forcomparing transition strengths between different types of quan-tum mechanical systems. The oscillator strength, ‘f’, of a transitionis a dimensionless quantity. The ‘f’ value of electric dipole allowedtransition [33–35] is given by

fedðnÞ ¼ 1:5� 104l2 9

ðvÞ2Zp ð3Þ

where ‘l’ is the emission wavelength and ‘Z’ is the index ofrefraction of zinc silicate.

An integrated cross-section value gives the probability oftransition from one level to another level (radiative transition).The Einstein’s A coefficient value of corresponding radiativetransition is related to the integrated cross-section of the transi-tion [33–35] as

Ics ¼

Zsdv¼

l2

8p p ð4Þ

The corresponding values of dipole-moment [33–35] for var-ious transitions are calculated using the relation:

9m9¼ 3heol3

16p3Zp

!1=2

ð5Þ

Dipole moment values of doped zinc silicate phosphors arecalculated, using Eq. (5), in terms of emission wavelength ‘l’,index of refraction ‘Z’, and the Einstein’s spontaneous coefficientvalue ‘p’.

Einstein’s B coefficient [27–29] will be calculated using thefollowing relation:

B¼2p2m2

3n2eoh2ð6Þ

where ‘m’ is the dipole moment of the transitions from theshallow trapping states of zinc silicate, ‘eo’ is the absolutepermittivity of free space, and ‘h’ is Planck’s constant.

5. Results and discussions

Fig. 3 shows the X-ray diffraction scan of Zn2�xMnxSiO4

(x¼0.5 mol%) nanoparticles synthesized at 1000 1C in an openatmosphere. None of the impurity signals attributed to freereactants or other compounds such as MnO2, SiO2, and ZnO areobserved. The peak positions agree well with the standard patternreported by the Joint Committee on Powder Diffraction Standards(JCPDS, 37�1485) for Zn2SiO4 (willemite). The reflections corre-sponding to planes /300S,/220S,/113S,/140S,/223S,/333S,and /713S indicate the presence of pure willemite phase withrhombohedral structure and R3 space group of Zn2SiO4. Theaverage crystallite size of the powder estimated from the diffrac-tion peak width is 25 nm, using the Debye–Scherrer equation:

D¼0:9l

bð2yÞcosyð7Þ

where ‘l’ and ‘b (2y)’ are the wavelength of Cu Ka radiation(1.5406 A) and corrected half-width of the diffraction peak(rad), ‘D’ is the average diameter of the crystallite, and ‘y’ is thediffraction angle.

Transmission electron micrograph (TEM) of Zn2SiO4:Mn2þ

(0.5 mol%) fired at 1000 1C has been shown in Fig. 4(a) and theinset presents corresponding selected area electron diffractionpattern (SAED). A dark field image of the same sample is shown inFig. 4(b). The fired phosphor particles show an irregular roundedmorphology with agglomerated solid structure. Fig. 5 shows theparticle size distribution histogram. It is evident from theobserved distribution curve that the particle size is 19.84 nm,which is in accordance with the observed diffraction results.

The atomic state and chemical composition of the atoms in thepowders are analyzed by high resolution X-ray photoelectronspectroscopy (HRXPS). XPS is very sensitive to the oxidationstates of element. Signals from Zn, Si, and O are observed in theXPS scans between energy range 90 and 1040 eV. But, the valencestates of Mn are not detected in the present study due to its lowconcentrations in the host matrix. The core level O(1s), Zn(2p),and Si(2p) high resolution X-ray photoelectron scan ofZn2�xMnxSiO4 (0.5 r x r 5 mol %) nanophosphors powder at1000 1C is shown in Fig. 6 and the atomic composition and thechemical state analysis are summarized in Table 1. The calculatedvalues are in good agreement with the standard reported values(NIST XPS data), which confirm pure willemite phase of Zn2SiO4.In Zn2SiO4, the O(1s) intensities are derived from the Si(2p) levelas well from the Zn(2p) line.

Fig. 4. (a) Bright field transmission electron micrograph of Zn2�xMnxSiO4

(0.5 mol%) fired at 1000 1C for 1 h (inset shows the related SAED pattern) and

(b) dark field TEM image of same sample.

Fig. 5. Particle size distribution curve of Zn2�xMnxSiO4 (0.5 mol%) fired at 1000 1C

for 1 h.

Fig. 6. X-ray photoelectron (XPS) spectrum of O(1s), Zn(2p), and Si(2p) levels for

Zn2�xMnxSiO4 (0.5 mol%) nanophoshors fired at 1000 1C for 1 h.

P. Sharma, H.S Bhatti / Physica B 406 (2011) 4188–4194 4191

Non-single exponential behavior of decay curves indicates thepresence of multiple trapping states below the conduction bandor above the valence band, which is introduced by the impurityions at different depth. Graph ‘ln (I)’ vs. ‘t’ (not shown here) isplotted from the obtained decay curves, which have been peeled-off into three components [31] and the slope of each componentgives the three spontaneous values. Thus using Eqs. (3)–(6),

P. Sharma, H.S Bhatti / Physica B 406 (2011) 4188–41944192

corresponding Einstein’s A and B coefficients, ‘f’ (oscillatorstrength), integrated cross-section, and transition dipole momentvalues are calculated. Table 2 shows the Einstein’s A coefficientvalues, which varies from 0.132�105 s�1 to 1.499�105 s�1 at800 1C, 900 1C,and 1000 1C. The Einstein’s A coefficient value ismaximum, 1.499�105 s�1, for sample doped with 5 mol%of Mn2þ ions fired at 800 1C and the minimum value,0.132�105 s�1, is observed for sample doped with 0.5 mol% ofMn2þ ions fired at 1000 1C. An increasing trend of Einstein’s A

coefficient values are observed with the decrease in firing tem-perature and these values are also found to be dopant ions’concentration dependent. Table 3 show the variation of ‘f’ values atthree temperatures for different dopant ions concentration. The ‘f’

Table 1Atomic composition and chemical state of Zn2MnxSiO4 (x¼0.5%) phosphor powder

at 1000 1C (determined by XPS analysis).

Element XPS XPS peak binding Energy (eV)

This work References

Zn Zn 2p3/2 1022.73 Willemite, Zn2SiO4 1022.6a

Si Si 2p 103.05 SiO2 103.0b

O O 1s 533.1 SiO2 533.1c

a L.S. Dake, D.R. Baer, J.M. Zachara, Surf. Interface Anal. 14 (1989) 71.b M.L. Miller, R.W. Linto, Anal. Chem. 57 (1985) 2314.c T.L. Barr, Appl. Surf. Sci.15 (1985) 1.

Table 2

Einstein’s A coefficient values ð�105 s�1Þ of Zn2SiO4: M (M¼Mn2þ) nan

Zn2SiO4: M (mol%) 800 1C 900 1

A1 A2 A3 A1

Mn (0.5) 1.171 0.542 0.158 0.846

Mn (1) 1.256 0.556 0.160 0.985

Mn (2) 1.300 0.597 0.166 1.028

Mn (3) 1.259 0.599 0.168 1.111

Mn (4) 1.435 0.624 0.165 1.183

Mn (5) 1.499 0.630 0.171 1.323

Table 3Oscillator strength values (�10�6) of Zn2SiO4: M (M¼Mn2þ) nanopho

Zn2SiO4: M (mol%) 800 1C 900 1

f1 f2 f3 f1

Mn (0.5) 111.43 51.58 15.05 78.7

Mn (1) 118.65 52.53 15.08 91.2

Mn (2) 121.88 55.02 15.53 94.5

Mn (3) 116.26 55.31 15.51 102.5

Mn (4) 134.48 58.51 15.44 109.6

Mn (5) 139.46 58.61 15.95 123.5

Table 4

Integrated cross-section values ð�10�9 m2 s�1Þ of Zn2SiO4: M (M¼Mn2þ

Zn2SiO4: M (mol%) 800 1C 900 1

I1 I2 I3 I1

Mn (0.5) 1.309 0.606 0.177 0.925

Mn (1) 1.394 0.617 0.177 1.073

Mn (2) 1.432 0.658 0.183 1.111

Mn (3) 1.366 0.650 0.182 1.205

Mn (4) 1.580 0.688 0.181 1.289

Mn (5) 1.639 0.689 0.187 1.451

values lie between 12.08�10–6 and 139.46�10–6. The maximumvalue, 139.46�10–6, is observed for sample doped with 5 mol% ofMn2þ ions fired at 800 1C and the minimum value, 12.08�10–6, isobserved for sample doped with 0.5 mol% of Mn2þ ions fired at1000 1C. ‘f’ values follow the same trend as that of Einstein’s A

coefficient values, that is, increase in ‘f’ values with increase inMn2þ concentration and decrease with increase in firing tempera-ture. Table 4 presents the integrated cross-section values ofcorresponding radiative transitions. The values lie between0.142�10–9 m2 s�1 and 1.639�10–9 m2 s�1. 1.639�10–9 m2 s�1

is the maximum value observed for Zn2�xMnxSiO4 (x¼5 mol%)fired at 800 1C and 0.142�10–9 m2s�1 is the minimum valueobserved for Zn2�xMnxSiO4 (x¼0.5 mol%) fired at 1000 1C. As theintegrated cross-section values increase, the corresponding transi-tion probability decreases. This results in increase in quantumefficiency of phosphors. Table 5 presents the dipole moment valuesof transition from certain levels present within the forbidden gap ofhost matrix. The values vary from 6.76�10–31 C m to 22.9�10–31 C m. The maximum value, 22.92�10–31 C m, is observedfor Zn2�xMnxSiO4 fired at 800 1C with 5 mol% of impurity concen-tration and the minimum value, 6.76�10–31 C m, is observed forZn2�xMnxSiO4 fired at 1000 1C with 0.5 mol% of impurity concen-tration. Table 6 shows the Einstein’s B coefficient values, calculatedthe from obtained dipole-moment values, which vary from0.24�1017 m3 (rad/s)/(J s) to 2.76�1017 m3 (rad/s)/(J s). The max-imum value is observed to be 2.76�1017 m3(rad/s)/(J s) for samplefired at 800 1C with 5 mol% of impurity concentration and the

ophosphors at different crystalline temperatures.

C 1000 1C

A2 A3 A1 A2 A3

0.419 0.146 0.766 0.350 0.132

0.454 0.148 0.777 0.362 0.134

0.489 0.149 0.799 0.379 0.138

0.509 0.157 0.853 0.412 0.149

0.545 0.159 0.864 0.423 0.152

0.611 0.165 1.000 0.460 0.143

sphors at different crystalline temperatures.

C 1000 1C

f2 f3 f1 f2 f3

0 39.00 13.59 70.19 32.05 12.08

9 42.02 13.69 71.18 33.20 12.25

1 44.92 13.74 73.51 34.83 12.72

7 46.95 14.53 76.76 37.92 13.69

6 50.47 14.75 80.33 39.35 14.18

1 57.07 15.36 93.47 42.95 13.38

) nanophosphors at different crystalline temperatures.

C 1000 1C

I2 I3 I1 I2 I3

0.458 0.160 0.825 0.337 0.142

0.494 0.161 0.836 0.390 0.144

0.528 0.161 0.864 0.409 0.149

0.552 0.171 0.902 0.446 0.161

0.593 0.173 0.944 0.462 0.167

0.671 0.181 1.098 0.505 0.157

Table 5Dipole moment values (�10�31 Cm) of Zn2SiO4: M (M¼Mn2þ) nanophosphors at different crystalline temperatures.

Zn2SiO4: M (mol%) 800 1C 900 1C 1000 1C

m1 m2 m3 m1 m2 m3 m1 m2 m3

Mn (0.5) 20.27 13.77 7.26 17.28 12.14 7.01 16.44 11.09 6.76

Mn (1) 21.03 14.02 7.49 18.64 12.57 7.26 16.55 11.24 6.76

Mn (2) 21.36 14.39 7.73 19.02 13.12 7.26 16.76 11.55 7.01

Mn (3) 21.03 14.51 7.73 19.74 13.38 7.49 17.28 12.00 7.25

Mn (4) 22.49 14.75 7.73 20.35 13.90 7.50 17.38 12.14 7.26

Mn (5) 22.92 15.31 7.74 20.41 14.10 7.50 17.46 12.39 7.26

Table 6

Einstein’s B coefficient values ð�1017 m3 ðrad=sÞ=ðJ sÞÞof Zn2SiO4: M (M¼Mn2þ) nanophosphors at different crystalline temperatures.

Zn2SiO4: M (mol%) 800 1C 900 1C 1000 1C

B1 B2 B3 B1 B2 B3 B1 B2 B3

Mn (0.5) 2.13 0.98 0.27 1.55 0.76 0.25 1.40 0.64 0.24

Mn (1) 2.29 1.02 0.29 1.80 0.82 0.27 1.42 0.66 0.24

Mn (2) 2.37 1.07 0.31 1.88 0.89 0.27 1.46 0.69 0.25

Mn (3) 2.49 1.09 0.31 2.02 0.93 0.29 1.55 0.75 0.27

Mn (4) 2.62 1.13 0.31 2.15 1.00 0.29 1.57 0.76 0.27

Mn (5) 2.76 1.16 0.32 2.34 1.07 0.30 1.61 0.78 0.27

P. Sharma, H.S Bhatti / Physica B 406 (2011) 4188–4194 4193

minimum value is observed to be 0.24�1017 m3(rad/s)/(J s) forsample fired at 1000 1C with 0.5 mol% of impurity concentration.

The increasing trend of Einstein’s A and B coefficients, inte-grated cross-section, ‘f’, and dipole moment values with increasein the concentration of dopant ions and decrease in the crystallinetemperature attributed to the fact that the impurity ions madeconsiderable effect at higher dopant concentration. This indicatesthe presence of shallow trapping states at higher value ofimpurity ions concentration. Also, it is observed that all thetransition parameters are dependent, which describe the strengthof optical transitions in the atoms and molecules of interest. Thisin turn increases the quantum efficiency of phosphors, whichmakes them a suitable and efficient material for photonic appli-cations especially in display industry. As for high-quality PDPs,high quantum efficiency, color purity, chemical stability, andproper life-time [28] of phosphors are required. Zn2�xMnxSiO4

accomplishes all the requirements for being used as a fast and anefficient phosphor in futuristic PDPs.

6. Conclusions

Mn doped Zn2SiO4 nanophosphor powders are successfullysynthesized using the wet-chemistry sol–gel method. X-raydiffraction scan confirms the polycrystalline nature of finalproducts. TEM investigation reveals the irregular rounded mor-phology of particles with agglomerated solid structure. HR-XPSdata also confirms the formation of zinc silicate. Different con-centrations of dopant ions in the Zn2SiO4 host matrix appreciablyalter the optical parameter values of the excited state. Thesechanges are observed due to the presence of shallow trappinglevels, introduced in the host matrix by the impurity ions or dueto the perturbation of the localized levels of host, which areresponsible for fast emission from the synthesized samples. Thesevalues also demonstrate appreciable crystalline temperaturedependence, as they increase with the decrease in the crystallinetemperature. The improved efficiency of these fast transitions indoped nanopowder makes them a suitable material for informa-tion storage and opto-electronics industry.

Acknowledgment

The author P.S. is very thankful to Dr. N.P. Lalla and Dr. T.Shripati of UGC-DAE, Inter University Consortium, Indore forproviding electron microscopy images, and XPS data. Financialsupport from University Grant Commission, New Delhi, is highlyacknowledged.

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