Journal of Luminescence 131 (2011) 988–993

Contents lists available at ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

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

Dual mode emission and harmonic generation in ZnO–CaO–Al2O3: Er3+

nano-composite

R.K. Verma a, K. Kumar a,b, S.B. Rai a,n

a Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, Indiab Nano-technology Application Centre, University of Allahabad, Allahabad, India

a r t i c l e i n f o

Article history:

Received 3 September 2010

Received in revised form

17 January 2011

Accepted 20 January 2011Available online 31 January 2011

Keywords:

Optical properties

Nano-composite

Upconversion

Energy transfer

SHG

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

016/j.jlumin.2011.01.009

esponding author. Tel.: +91 542 230 7308; fa

ail address: sbrai49@yahoo.co.in (S.B. Rai).

a b s t r a c t

Er3 + doped ZnO–CaO–Al2O3 nano-composite phosphor has been synthesized through combustion

method and its emission and harmonic generation properties have been studied. The X-ray diffraction

and thermal analysis techniques have been used to prove the dual phase (ZnO and CaO–Al2O3) nature

of the phosphor. The phosphor has shown up-conversion emission on near-infra-red (976 nm)

excitation and down-conversion emission on 355 nm excitation in presence of Er3 + and thus behaves

as a dual mode phosphor. On excitation with 976 nm diode laser, material shows color tunability

(calcination of composite material at different temperatures). Formation of ZnO nanocrystals on heat

treatment of as-synthesized sample has shown its characteristic emission at 388 nm and also the

energy transfer from ZnO to Er3 + ions. The low temperature emission measurements have been carried

out and the results have been discussed. Phosphor has shown strong second harmonic generation (SHG)

at 532 nm on 1064 nm and at 266 nm on 532 nm excitation.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Rare earth doped luminescent materials (also known asphosphors) have attracted much attention due to their interestingoptical properties in UV–vis.-NIR regions. Now-a-days there aremany application areas, where the luminescence properties ofrare earth ions have been efficiently utilized [1,2]. Currentlyavailable optical devices are efficient enough but to fulfill theever increasing demand of our society, these optical devices needto be improved in terms of luminescence efficiency, color selec-tion, material stability, ease of applicability, etc. [1–6]. Theluminescence efficiency of rare-earth ions in a solid host materialhas been achieved near to the theoretical limits, so there seemslittle room for further enhancement in luminescence efficiency ofrare earth ions doped single compounds. The color selectivity isalso a problem. These limitations have induced researchers tothink on new kinds of materials like multifunctional, hybridmaterials, etc. with multioptical functionality [3–6]. Hybridmaterials containing at least one inorganic phase may beinorganic–organic or inorganic–inorganic based. Composite mate-rial is nothing but a class of hybrid material containing two ormore phases in the same and each phase has its own property. Bychoosing the composite material one can tune/enhance the

ll rights reserved.

x: +91 542 236 9889.

optical properties than that in single phase materials. The inten-sive research in this direction is going on worldwide [3–9].

Triply ionized erbium is a convenient rare earth ion whichefficiently converts the infrared radiation into visible (Up-con-version) due to its favorable electronic energy levels. ZnO on theother hand is a well known semiconductor luminescent materialthat gives UV-violet emission through band edge transition andbroad green/red emission from defect states. Due to its wide bandgap of �3.37 eV at room temperature, ZnO has potential applica-tions such as UV-violate light-emitting diode, laser diode,etc. [10–12]. It is a direct band gap semiconductor, so thephotoluminescence efficiency of band edge transition is veryhigh. To increase the applicability of ZnO emission in lumines-cence devices one needs to dope rare-earth ions in it either totune the emission of ZnO or to couple/combine emissions fromion and ZnO both. The luminescence studies on rare earth ionsdoped in ZnO material has attracted much attention [13–15].However, doping of rare earth ions in ZnO is difficult task due tocharge imbalance and radius mismatch of rare earth ions. In mostcases, rare earth ions take positions on the grain boundaries of theZnO crystal surface, which results in weak energy transfer fromZnO to rare earth ions. Rare earth ions doped composite materials,on the other hand, are easy to synthesize and also give betteremission properties than the previous one.

As an example, when Eu3 + is doped into ZnO–SiO2 glassmatrix, after suitable heat treatments two emission centers areformed one ZnO and the other rare earth in SiO2 phase itself [14].

R.K. Verma et al. / Journal of Luminescence 131 (2011) 988–993 989

Bang et al. [16] have also prepared Eu3 + doped ZnO–SiO2

composite and observed about 10 times enhancement in emissionintensity of Eu3 + ions. These groups have used Eu3 + ion whichdoes not give upconversion emission through 980 nm excitation.

In the present work we have selected Er3 + ion, which givesdownconversion as well as upconversion emission, when dopedin ZnO–CaO–Al2O3 matrix on pumping with 355 and 980 nm,respectively. Moreover, aluminate based phosphors are very goodfor display device applications due to its high stability againstharsh environment [2]. To the best of our knowledge there is noreport on dual phase formation with ZnO in this material. Theheat treatment has been done to convert it into a compositematrix. Under suitable excitations the same material acts as dualphosphor (i.e. shows downconversion as well as upconversionemission in the same materials [17–19]). This material also showsSHG at 532 nm when pumped with 1064 nm radiation and at266 nm when irradiated with 532 nm. The widths of the secondharmonic were 0.124 and 0.204 nm, respectively.

2. Experiment

Following series of the phosphor were synthesized usingcombustion technique:

50CaO+(49�x)Al2O3+xEr2O3, (S1)

50CaO+20Al2O3+30ZnO, (S2)

50CaO+30ZnO+19.5Al2O3+0.5Er2O3 (S3)

The x was varied from 0.25 to 1.0 molwt% to optimize Er3 +

emission. ZnO was kept fixed at 30 molwt% for all cases. Thesolution combustion method [20] was used for the synthesis ofthe phosphor material. Reagent grade nitric acid was used toprepare nitrates of all above mentioned compounds and used asprecursor materials. The stoichiometric ratios of these com-pounds in above composition were taken and dissolved in mini-mum amount of miliQ water. Urea was then added to thesolution, and the mixture was stirred in a beaker for more than2 h to get the homogeneous solution. Solution was then heated at60 1C to evaporate the water. As the water content in the solutiongoes down, the solution changes into a transparent gel. The gelwas then transferred into a closed furnace maintained at a fixedtemperature (�400 1C). After few seconds sample burns andfinally a white foamy powder remains in crucible.

Fig. 1. (a) XRD patterns of as-synthesized and calcinated samples of 1473 K for 50C

To produce crystallization in the sample, as-synthesized sampleswere calcinated at 1073, 1273 and 1473 K temperatures separatelyfor 3 h. The development of crystallinity was checked by powderX-ray diffraction (XRD) patterns using CuKa radiation (1.5406 A).Surface morphology was characterized by ZEISS scanning electronmicroscopy (SEM), Model: SEM-Supra 40 operated at 5 kV. Differ-ential thermal analysis (DTA) and thermo gravimetric analysis (TGA)were carried out using Perkin Elmer Diamond TGA/DTA instrument.The 976 nm radiation from an InGaAs continuous diode laser with800 mW power was used for excitation for upconversion emission.The 355 nm radiation with 120 mJ energy and pulse width 7 ns froma pulsed Nd: YAG laser (Spitlight600, Innolas, Germany,) was used forrecording the down-conversion emission spectra. The spectra wererecorded using iHR320 spectrometer (Horiba Jobin Yuon, USA,resolution �0.06 nm with PMT and �1.0 nm with CCD camera).The spectra were measured at different temperatures starting fromroom temperature to 10 K. For recording second harmonic generation(SHG) spectra, 1064 nm radiation with 400 mJ energy and 300 mJenergy at 532 nm from the same Nd: YAG laser was used. Appro-priate filter was used to avoid the unnecessary signal. Excitationpower at sample surface was kept between 5 and 50 mJ/cm2 forpower dependence measurements. The laser at its fundamentalwavelength has line width o1.0 cm�1 (�0.113 nm).

3. Results and discussion

3.1. Structural studies

Fig. 1(a) shows the XRD patterns of the samples of series S3;the as-synthesized one and the sample calcinated at 1473 K.As-synthesized sample shows crystalline peaks due to the ZnOphase only. There is no clear peak of the CaO–Al2O3 phase.However, sample calcinated at 1473 K shows weak additionalpeaks due to the formation of extra crystalline phase of CaO–Al2O3. The weak crystalline peaks observed in the sample calci-nated at 1473 K match well with Ca12Al13O33 crystal phase. In ourprevious studies of CaO–Al2O3 combination (without ZnO) for-mation of Ca12Al13O33 crystal phase at 1473 K was much clearerwhereas the as-synthesized sample was totally amorphous innature [21]. Thus present calcinated sample contains two phasescomposite form one ZnO and the other Ca12Al13O33 (as-synthe-sized crystalline ZnO and amorphous Ca–Al2O3 and calcinated at1473 K, crystalline ZnO as well as Ca12Al13O33). No other changewas seen in sample. The ZnO crystallite size was calculated using

aO+30ZnO+19.5Al2O3+0.5Er2O3 composition and (b) SEM image of the same.

Fig. 3. Up-conversion spectra of: (a) as-synthesized and 1473 K calcinated

50CaO–48.5Al2O3–0.5Er2O3 samples (S1). (b) as-synthesized and 1473 K calci-

nated 50CaO–30ZnO–19.5Al2O3–0.5Er2O3 samples (S3). Inset of (b) shows FIR

(Igreen/Ired) of two emission bands with temperature.

R.K. Verma et al. / Journal of Luminescence 131 (2011) 988–993990

the Scherer equation:

t¼0:9lbcosy

where the terms have their usual meanings. Average crystallitesize in the case of ZnO was found to be in the range �9–13 nm.

In Fig. 1(b) SEM image of the 50CaO–30ZnO–19.5Al2O3–0.5Er2O3 sample calcinated at 1473 K is shown. The image clearlyshows the presense of two separate crystal phases; one ZnO andthe other of CaO–Al2O3 composition (Ca12Al13O33). ZnO particlesappear like disk shape having diameter �1 mm. Size of ZnOparticles in SEM image appears very large when compared toXRD analysis above. The reason is that SEM image shows theparticle size whereas XRD calculation gives crystallite size.

3.2. Thermal analysis

Thermal analysis is one of the important characterizationmethods to understand the solid state reaction processes (stabi-lity and crystallization) of materials. The thermal analysis hasbeen used to prove the composite phase. TGA and DTA curves ofthe as-synthesized sample (series S3) are presented in Fig. 2.In Fig. 2 endothermic peaks are visible one around 740 1C and theother around 1280 1C. The first is associated with weight loss (4%)in TG curve but the second endothermic peak is without weightloss. It is therefore concluded that first endothermic peak occursdue to the evaporation of water of crystallization and also may beorganic compounds associated with host lattice. A similar obser-vation has been reported in our earlier paper [22]. The secondendothermic peak actually consists of two peaks which can beassociated to the crystal phase formation temperature ofCaO–Al2O3 matrix. The weak endothermic peak around 165 1Ccorresponds to the transition temperature of ZnO phase. This peakis weak in intensity, since our as-synthesized ZnO phase isalready been formed during the preparation. We have also takenthe TGA/DTA spectra of samples S1 and S2. Both the samplesshow similar pattern (particularly at 740 1C) with slight change inpeak position at higher temperature.

3.3. Upconversion emission and effect of calcination

The upconversion spectra of the samples were recorded using976 nm excitation wavelength and are shown in Fig. 3(a). This figureclearly shows the effect of calcination on the upconversion emission

Fig. 2. DTA and TGA spectrum of as-synthesized 50CaO+30ZnO+19.5A-

l2O3+0.5Er2O3 sample.

intensity of S1 series samples. Fig. 3(b) shows the effect of calcinationon the upconversion emission intensity of S3 series samples. Twogroups of upconversion bands viz. one centered in the green(510–570 nm) and the other in red (640–700 nm) are observed. Thebehavior of two emission groups is, however, different in two figures.In Er3+ doped CaO–Al2O3 phosphor (S1) the red emission appearsstronger than the green one. The intensity increases in same propor-tion when the sample is calcinated at different temperatures(see Fig. 3(a)). However in case of Er3+ doped ZnO–CaO–Al2O3

composite material (S3), the emission intensity of green and redbands are equal and much larger than the intensity (y scale) of anyindividual banding CaO–Al2O3 sample.

The green emission between 510 and 570 nm arise due to thetransition from excited 2H11/2 and 4S3/2 states to ground 4I15/2

state, whereas the red emission between 640 and 700 nm arisedue to the transition from excited 4F9/2 state to ground 4I15/2 stateof Er3 + ion. The emission intensity of green band was found toincrease while the emission intensity of red band was found todecrease with the increase of calcination temperature and thesample glows with green color. For the sample calcinated at1473 K the green band is almost 15 times intense in comparison

Fig. 4. Schematic diagram illustrating the mechanism of the energy transfer from ZnO semiconductor to Er3+ ion and corresponding emissions from Er3 + ion.

Fig. 5. 355 nm excited emission spectra of: (a) as-synthesized 50CaO+

30ZnO+20Al2O3 (S2) and (b) as-synthesized and 1473 K calcinated 50CaO�

30ZnO+19.5Al2O3�0.5Er2O3 (S3). Inset of (b) shows enlarged band of Er3 +.

R.K. Verma et al. / Journal of Luminescence 131 (2011) 988–993 991

to emission from the as-synthesized material. In case ofas-synthesized sample of S3 series the emission intensity of greenand the red emissions at room temperature is approximatelyequal and so the phosphor glows with attractive pink color. So, achange in calcination temperature results in a color tunabilityfrom pink to green. The fluorescence intensity ratio (FIR¼ Igreen/Ired) of the two emission bands with temperature is shown in theinset of Fig. 3(b).

The mechanism involved in upconversion emission is schemati-cally shown in Fig. 4. The incident infrared photon populates 4I11/2

level of Er3+ ion through ground state absorption. The ions excited to4I11/2 level reabsorb second incident photon and promoted to higherlying 4F7/2 state. The Er3+ ions in this state decay non-radiatively tolower excited states 2H11/2 and 4S3/2. The excited ions from thesestates decay radiatively to ground state via 2H11/2-

4I15/2 and4S3/2-

4I15/2 transitions and non-radiatively to 4F9/2 state followedby radiative emission corresponding 4F9/2-

4I15/2 transition.The increase in intensity of green band and decrease in

intensity of red band with calcination temperature (Fig. 3(a)and (b)) can be explained on the basis of reduction of averagephonon vibration of the sample. As-synthesized sample containwater and organic impurities which decreases with the increasein calcinations temperature. Sample having larger phonon vibra-tion has large non-radiative transition from 4S3/2 to 4F9/2 level andthus has large population in 4F9/2 level. The probability of thisnon-radiative transition decreases as phonon frequency decreasesand this results in an increase in intensity of green band. A largeenhancement in upconversion emission intensity in green sam-ples S3 (addition of ZnO) compared to samples S1 is due to furtherdecrease in average phonon frequency of the host as Zn is wellknown luminescence enhancer metal [23]. Also emission bandsin Fig. 3(b) have large crystal field splitting due to the largeratomic number/(radius)2 of Zn++ ion.

3.4. Downconversion emission and energy transfer from ZnO to

Er3+ ion

The photoluminescence spectra under 355 nm excitation exhibitstwo emission peaks in ZnO–CaO–Al2O3 phosphor, a strong one in UV

region centered at 385 nm and the other broad weak one centered at520 nm. The former emission is ascribed as band edge emission fromZnO and the later one due to various defect centers present inZnO [24]. In Fig. 5(a) the spectrum of as-synthesized S2 series sampleis shown. However, if Er3+ is also present in ZnO–CaO–Al2O3

phosphor only the band edge along with weak Er3+ emission is seen.In this case defect emission does not appear, because it hastransferred its energy to the Er3+ ions (Fig. 5(b)). The ZnO defectemission intensity increases with the increase in calcination tem-perature and accordingly the Er3+ bands intensity also increases.Sample of S1 series also shows weak Er3+ emission (not shown)

R.K. Verma et al. / Journal of Luminescence 131 (2011) 988–993992

which is much smaller than the intensity obtained when ZnO is alsopresent with Er3+(S3). A mechanism which accounts for the energytransfer in between ZnO and Er3+ is summarized in Fig. 4. When Er3+

doped composite ZnO–CaO–Al2O3 sample is pumped with 355 nmlaser beam, the incident energy is absorbed by ZnO which promotesthe ZnO from the valence band (VB) to conduction band (CB). Thisresults in an emission from band edge of conduction band to thevalance band at 385 nm. Some of the excited ZnO are trapped bybroad defect levels and a part of the recombination energy istransferred to the Er3+ ions to promote the Er3+ ions from groundstate to excited states, which generates the additional Er3+ emissionin green region.

It is well known that optical band gap of the ZnO depends on theparticle size. The band gap of the material simultaneously contributesa shift of valance band and the conduction band edge. Thesecompression or extensions are not necessarily in the same amountand therefore it will result a change in the band gap. As thecalcination temperature increases, the band edge emission due toZnO increases. There are two probable reasons for this enhancement.First is increase in density of ZnO crystal phase and the other isdecrease in quenching centers. In as-synthesized case only a part ofZnO is phase separated and form ZnO crystal phase but increase incalcinations induces phase separation. We have seen by thermalanalysis that when the temperature of the sample is near 1073 Kthere is nearly 4% weight loss which is the removal of water ofcrystallization. In the absence of these quenching centers the emis-sion intensity from the sample increases.

3.5. Effect of cooling on the fluorescence intensity of the bands

We also monitored the emission spectra of Er3 + and ZnO atlower temperatures in order to know the effect of temperature onthe emission intensity of Er3 + and ZnO. A low temperature studyreveals how the emission intensity of the ZnO and Er3 + bands areaffected by lattice vibration and its distortion. We recorded theupconversion emission of the hypersensitive green emission ofEr3 + from room temperature to upto 30 K (see Fig. 6). It is found

Fig. 6. Effect of cooling on the emission spectra of Er3+ ion doped composite

material heat treated at 1473 K.

that FWHM of bands decreases with the decrease of temperature(Fig. 6). However, bands do not show any wavelength shift. Thereason behind this is that the energy levels of Er3 + ion do not shiftwhen the temperature is changed. Of course at lower tempera-tures the lattice vibration decreases; hence, the energy lossthrough non-radiative channel decreases. Thus the radiativeefficiency of Er3 + ion is increases at lower temperature. We alsorecorded the fluorescence spectra in reverse way, i.e. by increas-ing the temperature from 30 K to room temperature. It wasobserved that it follows exactly the same path as in earlier case.

However, at lower temperatures ZnO behaves in a complex wayand gives information about the host as well as emitting material. Theemission intensity of ZnO band increases and it becomes sharper atlower temperatures as shown in Fig. 7. It also shifts towards blue side(� upto 10 nm). There may be several reasons for sharpening andshifting of the peak such as crystal defect, inhomogeneous strain,asymmetric field, etc. [25]. The amount of shift and the sharpnessdepends on the emitting material and the lattice structure. At lowertemperatures the vibrational frequency of the host decreases and thematerial shrinks. Due to this shrinking the ZnO is strained and theband gap is modified. However, shift is not permanent and it regainsits original position as the temperature is raised to room temperature.

3.6. Second harmonic generation (SHG)

SHG is a second order nonlinear process in which fundamentalwavelength at l interacts with nonlinear medium and anotherlaser beam of wavelength l/2 comes out from the material [26,27].SHG properties were studied with 1064 and 532 nm from aQ-switched Nd: YAG laser with pulse width 7 ns and full widthat half maximum (FWHM) around 0.113 nm. When the laser lightof wavelength 1064 nm (an interference filter was used in thepath of the beam which permits only 1064 nm radiation) isincident on S3 sample calcinated at 1473 K, green radiation ofwavelength 532 nm comes out from the material along with1064 nm which can be seen even with naked eye. Though, wehave not observed third or fourth harmonic emission at 355 or266 nm on 1064 nm pumping. However, if we focus 532 nm laser

Fig. 7. Effect of cooling on the emission spectra of ZnO in the heat treated

sample (S3).

Fig. 8. SHG signal in 50CaO–30ZnO–19.5Al2O3–0.5Er2O3 composite calcinated at

1473 K: (a) For incident radiation at 1064 nm and (b) for incident radiation at

532 nm.

R.K. Verma et al. / Journal of Luminescence 131 (2011) 988–993 993

beam on the same material an emission at 266 nm is againobserved. These emissions were further verified by passing thebeams through quartz prism spectrograph and detecting theemissions at 532 and 266 nm. The SHG spectra with 1064 and532 nm excitations for sample S3 calcinated at 1473 K are shownin Fig. 8. Intensity of SHG using 1064 nm is very strong. TheFWHM of the SHG peaks corresponding to the 1064 and 532 nmare �0.124 and �0.205 nm, respectively. The bandwidths arevery near to the laser bandwidth �0.113 nm. The compositematerial is thus unique in many sense (color tunability, energytransfer from ZnO to Er3 + ions and SHG). The harmonic genera-tion occurs due to the presence of non-centrosymmetric ZnOphase [27] in composite. Samples without ZnO do not show anysuch emission.

4. Conclusions

The optical properties of Er3 + doped ZnO–CaO–Al2O3 compo-site phosphor material have been studied. Formation of ZnOcrystal phase has shown its characteristic emission and energytransfer from ZnO to Er3 + ions. The material possesses dual mode

emission, i.e. upconversion and downconversion on suitableexcitations. The efficiency of the emission has been found toincrease with decrease in sample temperature. ZnO band edgeemission also shows a blue shift at lower temperatures due toincrease in lattice strain. The composite material also shows goodSHG on pumping with 1064 and 532 nm wavelengths.

Acknowledgments

The authors are grateful to DST New Delhi for financial assistanceand Alexander Von Humboldt foundation Germany for donatingNd: YAG laser. One of the author K. Kumar is also thankful to UGC,New Delhi, India for the Dr. D.S. Kothari Fellowship.

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