Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 511–516

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Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

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Green and NIR sensitized luminescent nanophosphor: Preparation, characterizationand application

Gagandeep Kaur a, Y. Dwivedi a, A. Rai b, S.B. Rai a,⇑a Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, Indiab Department of Chemistry, PPN College, Kanpur, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 August 2011Received in revised form 27 March 2012Accepted 7 April 2012Available online 23 April 2012

Keywords:Luminescent inkPhosphorUpconversionDownconversionPVA

1386-1425/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.saa.2012.04.040

⇑ Corresponding author. Tel.: +91 542 230 7308; faE-mail address: sbrai49@yahoo.co.in (S.B. Rai).

Luminescent nanophosphor of Er, Yb: Gd2O3 has been synthesized by well known combustion synthesis.Along with strong UV–Vis upconversion emission, sensor for temperature and magnetic field, nano-hea-ter etc., it can also be used as luminescent ink by dispersing the nanophosphor in aqueous polyvinyl alco-hol solution. The stability of the ink has been found to depend strongly on the mixing process of thephosphor powder in the polymer solution. The X-ray diffraction results and TEM images with diffractionpattern show that the particles are in cubic phase of Gd2O3 with particles of average size <40 nm. SEMimages show flaky structure which makes it suitable for dispersing in PVA and useful for luminescentink purposes on NIR excitation. The FT-IR and the thermal analysis support the presence of PVA. Upcon-version and downconversion luminescence has also been observed with 532 nm excitation and energytransfer mechanism has been explained. The NIR pumping gives strong UC emission bands in red andgreen regions extending up to extreme UV (240 nm) in this host.

� 2012 Elsevier B.V. All rights reserved.

Introduction

Rare earth activated oxide nanophosphors have been the mostexplored area of research during the last few years as these possessexcellent luminescence characteristics, high thermal stability, longdurability, negligible photobleaching, etc. Due to these merits,these are in high demand for various technological applicationsincluding optoelectronic devices, high definition televisions, bio-logical imaging and tagging, MRI, luminescent paints and inks forsecurity codes etc. [1–6]. Luminescent inks, paints and dyes haveproven themselves useful both as performance wise as well as costeffective in protecting important documents and products againstfraud, detecting any forgery, falsification, unauthorized trading etc.Fluorescent dyes have been used to define security features inmany applications such as currency, identity cards, and bank ac-counts etc. However, due to development in technologies, now,their easy replication is possible. Non aqueous colloids from semi-conductor nanoparticles have recently been used for security inkapplications due to their quantum size effect exhibiting efficientphotoluminescence [7–9]. Their use show numerous problemsi.e. photobleaching, toxicity of cadmium and harmful additivesviz. sulfides, amines etc., though their inorganic counterparts arecapable of tolerating a stronger irradiation but their efficiency ispoor. In this context, upconverting nano-particles (UCNPs) of rareearth may be of great help. These nanoparticles are made water-

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soluble by encapsulating with low molecular weight amphiphilicpolymers to get well-dispersed aqueous nanoparticles with undi-minished photophysical characteristics. Polymer-wrapped UCNPsshow strong upconverted luminescence and suit for biologicalimaging, labeling experiments and other applications [10–15]. Inthis continuation to look for safer UCNPs, development of anti-fakeluminescent ink (colloid) using oxide nanophosphors in polymersas luminescent entities is the issue now-a-days for security reasons[16–19]. Gupta et al. [20] have reported a simple method for thesynthesis of ultra-fine Eu3+-Y2O3 nanophosphors with an averagediameter of �5 nm for development of a transparent colloid thatcould be used as a luminescent security ink. Another group ofresearchers have been able to produce stable luminescent ink frompowder of Y2O3: Eu dispersed in PVA solution which emittedstrong red luminescence under UV illumination and could act asluminescent ink [21]. Recently, Singh et al. [22] have developedinorganic–organic hybrid nanostructure of Gd2O3:Yb3+/Er3+ phos-phor and Eu(DBM)3Phen organic complex dispersed in liquid med-ium for security ink purposes. Over the past decades, the materialsemitting single-narrowband fluorescence are used widely in thefields of anti-fake ink.

The present study reports the synthesis of multifunctional Er3+,Yb3+: Gd2O3 phosphor material capable of producing intense visi-ble emission in red, green and blue regions by well known combus-tion technique. This method allows the preparation ofhomogeneous phosphor material at a large scale with high lumi-nescence yield and cost-effectiveness. SEM, TEM and XRD have alsobeen done to analyze the structural aspects of the phosphor. Up

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and down conversion luminescence has also been observed with532 nm excitation. Using 976 nm laser excitations, an efficientUC emission bands in UV, blue, green and red regions have beenobserved. This paper also reports an observation of UC emissionbands of Er in Gd2O3 host extending up to extreme UV due to2I11/2 ?

4I15/2 transition. For the application as luminescent markeror security ink, the phosphor was dispersed in an aqueous solutionof an amphiphilic polymer, Polyvinyl Alcohol (PVA) for the produc-tion of transparent stable colloid preventing the agglomeration andprecipitation. PVA in the dispersion plays a role as separator be-tween particles as well as stabilizer and this is not surprising be-cause PVA has been frequently used as particle stabilizer inchemical synthesis of metal colloid [19,21,23]. This approach pro-vides a method to synthesize transparent nanocolloidal solutionfor biological applications as well as luminescent red and greensecurity ink applications.

Experimental

Materials

Analytical reagent (AR) grade gadolinium oxide (Gd2O3, 99.99%,Aldrich), ytterbium oxide (Yb2O3, 99.99%, Aldrich), erbium oxide(Er2O3, 99.99%, Aldrich), urea (Qualigens) as fuel, and nitric acid(Merck, 99.9%) were used for the synthesis of nanophosphor whilePVA, (CH2CHOH)n with average n � 22000 (99.95%, SigmaAldrich),was used for the dispersing the phosphor.

Preparation of Gd2O3: Er3+, Yb3+ nanophosphor

The solution combustion technique has been employed for thesynthesis of Er3+, Yb3+: Gd2O3 nanophosphor. The compositionsused for synthesis of the phosphor are as follows:

ð100� x� yÞGdðNO3Þ36H2Oþ xEr2O3 þ yYb2O3

A composition with x = 0.3 mol% and y = 1.0 and 2.0 mol%,respectively was found to give optimum green and red fluores-cence. All ingredients were taken and dissolved in concentratedHNO3 under constant stirring conditions at 50 �C in a minimumquantity of distilled water. An equimolar amount of urea was thenadded to the solution and allowed to stir for 3–4 h to obtain atransparent gel. Further, gel was burnt at 600 �C to get white foamynanophosphor. The phosphor thus synthesized was annealed at800 �C to remove the excess of organic impurities.

Polyvinyl Alcohol (PVA, (CH2CHOH)n) was dissolved in distilledwater at molar concentration of 15%. After complete dissolution ofPVA, 1 mg of Gd2O3: Er, Yb nanophosphor was added and stirred atroom temperature for around 3–4 h, repeatedly. Further it waskept in an ultrasonic bath for an hour to get properly homogenizedsolution to develop an aqueous-stable transparent colloid ofGd2O3: Er, Yb in PVA. The viscosity of the polymer solution withthe nanophosphor is very less as preferred precursors are the ni-trates, such as gadolinium nitrate, Gd(NO3)3�6H2O, for the produc-tion of phosphor particles. Nitrates are typically highly soluble inwater and the solutions maintain a low viscosity, even at higherconcentrations obeying the reaction mechanism:

2GdðNO3Þ3 þH2Oþ heat! Gd2O3 þ NOX þH2O

Fig. 1. X-ray diffraction (XRD) pattern of Er, Yb: Gd2O3 phosphor.

Materials characterization

X-ray diffraction (XRD) patterns were recorded using 18 kWrotating anode (Cu) based Rigaku powder diffractometer fittedwith a graphite monochromator. Scanning electron micrographsof the samples were taken using Technai 20G2 (Philips) scanning

electron microscope (SEM) equipped with the charge coupled de-vice (CCD). Transmission electron micrographs were taken usinga Technai 20G2 (Philips) transmission electron microscope (TEM)equipped with the charge coupled device (CCD). Fourier transforminfrared (FTIR) spectra of the samples were recorded using Spec-trum RX-I spectrophotometer (Perkin Elmer). TGA measurementswere performed on Pyris Diamond Thermal Analyzer at a heatingrate of 10 �C/min under N2 atmosphere. For up and down conver-sion studies, samples were excited with 976 nm radiation from adiode laser and 532 nm radiation from Nd-YAG laser and spectrawere recorded using an iHR320 (Horiba Jobin Yvon) spectrometerequipped with R928P photon counting photomultiplier tube(PMT).

Results and discussion

Structural characterization

X-ray diffraction (XRD) analysisX-ray diffraction (XRD) pattern of Er, Yb: Gd2O3 phosphor is

shown in Fig. 1. The figure shows the crystalline cubic phase ofGd2O3 which its low-temperature phase at ordinary pressure,(JCPDS (Joint Committee on Powder Diffraction Standards) FileNo: 43-1014). The diffraction peaks at 2h = 28.54�, 33.04�, 39.02�,42.59�, 47.52�, 56.40�, 57.79� and 59.14� which correspond to(222), (400), (332), (134), (440), (622), (631) and (444) planes,respectively. No diffraction peak corresponding to impurity ionswas detected. The particle size was calculated using Debye–Scher-rer relation and the particle size was found to lie in range 30–40 nm. Particles of this diameter range are preferably required asan inkjet material for printing, which results in line thickness<1 lm [24].

(Scanning electron microscopy) SEM analysisFig. 2 shows a typical scanning electron microscope (SEM) im-

age of the Er, Yb: Gd2O3 nanophosphor. The SEM image exhibitsmicroflaky structure. The flakes show the weakly bound agglomer-ation of the nanoparticles. This gives a hint that these nanoparti-cles could be easily dispersed by ultrasonication. Inset to thefigure shows the magnified SEM image of the phosphor powder.Scanning electron micrograph of the colloidal sample was also ta-ken which also shows the formation of spherical nanocrystals withunburnt organic residuals of diameter in same range.

Fig. 2. Scanning electron microscopy (SEM) micrograph of Er, Yb: Gd2O3 phosphor.Inset shows the magnified SEM image of the same.

Fig. 3. Transmission electron microscopy (TEM) micrograph of Er, Yb: Gd2O3: PVA.Inset shows the electron diffraction pattern of the cubic phase of Gd2O3.

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(Transmission electron microscopy) TEM analysisFig. 3 shows the microstructure analysis of the Eu3+ and Yb3+ co-

doped Gd2O3 phosphor. The TEM image shows the presence ofnearly spherical nanocrystals of average diameter in the range of30–40 nm which is in good agreement with the particle size deter-mined using XRD measurement. The nanocrystals seem to beweakly bound resulting in aggregated nanoparticles, in some cases.This could be due to the presence of an organic part in betweeninorganic particles. The inset of image shows the electron diffrac-tion pattern showing the cubic phase of Gd2O3 as observed in thecase of XRD.

Fig. 4. Fourier transform infra-red (FTIR) spectra of PVA, Er, Yb: Gd2O3 and (Er, Yb:Gd2O3) PVA colloid.

Optical characterization

Fourier transform infra-red (FT-IR) analysisThe Fourier transform infra-red (FTIR) spectra of PVA, Er, Yb:

Gd2O3 and (Er, Yb: Gd2O3) PVA colloid was recorded in order toknow different molecular species present in the phosphor along

with the existence of the polymer with the phosphor in the colloidand is shown in Fig. 4. The IR spectra of PVA exhibits a broad peakat �3421 cm�1 is due to O–H stretching in hydroxyl groups, at�2986 cm�1 due to C–H stretching. The band in the range 2000–1667 cm�1 may be due to overtone or combination band. Absorp-tion observed at 1510–1313 cm�1 may be from C–H bending andCC stretching. C–O stretching was observed at �1023 cm�1

[25,26]. We were unable to detect any absorption peaks resultedby bonding of metal ions which usually appear at short wave num-bers. The combustion synthesis makes use of urea as an organicfuel which on heating decomposes mainly into CO2, H2O and NO3

and these vibrations are present in FTIR spectrum of Er, Yb:Gd2O3 phosphor material. The broad absorption bands at3450 cm�1 and at 1600 cm�1 belong to OH vibration originatingdue to water adsorbed in the sample. The peak at 1264 cm�1 orig-inates due to characteristic nitrate (NO3)� vibrations because ofdissolving of the rare earth salts in nitric acid. The third peak near785 cm�1 is probably due to C–O asymmetric stretching. Theabsorption bands in the FTIR spectra of (Er, Yb: Gd2O3) PVA colloidresemble much to the ingredients with a difference that a signifi-cant peak at 934 cm�1 is observed (not in PVA) which may bedue to bonding of PVA chain with the phosphor particles [21].

Thermogravimetric analysis (TGA) in nitrogen atmosphere witha rise in temperature of 10 �C min�1 up to 700 �C was also done toinvestigate the percentage of PVA in (Er, Yb: Gd2O3) PVA colloidand 25–30% PVA was present in the colloid.

Upconversion and downconversion emission using 532 nm excitationThe emission spectrum of the Er, Yb: Gd2O3 were recorded in

200–1100 nm region using 532 nm radiation and the spectrumthus obtained is shown in Fig. 5. The UC emission bands are ob-served at 287, 309, 332, and 353 nm corresponding to 2D5/2 ?4I15/2, 4G7/2 ?

4I15/2, 2P3/2 ?4I15/2 and 4G9/2 ?

4I13/2 transitions,respectively. Strong violet blue upconversion emission peaks

Fig. 5. Upconversion and downconversion emission spectra of Er, Yb: Gd2O3 on excitation with 532 nm.

Fig. 6. Power dependence for different transitions on excitation with 532 nm andthe number denotes the photons involved in that particular transition.

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centered at 370, 383, 412, 430, 454, 472, and 491 nm are due to2G9/2 ? 4I15/2, 4G11/2 ?

4I15/2, 2P3/2 ? 4I13/2, 2H9/2 ? 4I15/2, 4F3/2 ?4I15/2, 2F5/2 ?

4I15/2, and 2F7/2 ? 4I15/2 transitions. Two peaks seenin green region at 524 and 552 nm are assigned as 2H11/2 ?

4I15/2,4S3/2 ? 4I15/2 transitions, respectively. An intense peak is alsoobserved in red region at 672 nm for transition 4F9/2 ?

4I15/2.Few weak peaks in NIR region centered at 814, 852, 860,and 963 nm are also observed due to 4I9/2 ?

4I15/2, 2H11/2 ?4I13/2,

4S3/2 ? 4I13/2 and 4I11/2 ?4I15/2 transitions, respectively.

The observed emission spectrum can be understood on the ba-sis of two successive photon absorption processes. Initially,532 nm(�18,800 cm�1) laser photon is absorbed by Er3+ ion inthe ground state (4I15/2) and promoted to 2H11/2 and 4S3/2 states lo-cated at �19,200 cm�1 and �18,350 cm�1, respectively. The twostates are very close to each other and a thermalization is possibleeven at room temperature. The lifetime of these states are of theorder of 102 ls, hence the excited ions in these states reabsorb an-other 532 nm laser photon via ESA and promoted to 4G9/2 state. Theexcited ions in this state relax to lower states and result in ob-served upconversion emissions. The pump power dependenceanalysis has also been carried out and the slope for the curveswas found to be 1.97 ± 0.02, 1.88 ± 0.001, 1.75 ± 0.01 and1.56 ± 0.04 for the 2D5/2 ?

4I15/2 (287 nm), 4G9/2 ?4I13/2

(353 nm), 2P3/2 ? 4I13/2 (412 nm), and 2F7/2 ?4I15/2 (491 nm) tran-

sitions, respectively (see Fig. 6). These values indicate that twopump photons are involved in UC process.

Upconversion emission using 976 nm excitationUpconverted photoluminescence spectra of Er, Yb: Gd2O3 were

recorded using 976 nm as excitation wavelength (see Fig. 7). Weobserve almost similar spectra as in case of 532 nm excitationhowever it is highly sensitive to the Yb ion concentration. Along-with the earlier observed emission peaks, two new bands, one at240 nm (2I11/2 ?

4I15/2) and the other at 335 nm (4G9/2 ? 4I15/2)are also observed. The power dependence measurements using976 nm laser has also been carried out indicating the involvementof four and three photons, respectively.

The 4I11/2 level of Er3+ ion is in resonance with the energy of976 nm laser photon. The absorption cross section of this level ishowever small and it is in-sufficient to give any upconversionemission. However, in presence of a trace amount of Yb3+ togetherwith Er3+, an intense upconversion emission is seen. Actually, theincident 976 nm photon is strongly absorbed by Yb3+ ions (absorp-tion cross section of Yb3+ is nearly 20 times larger than Er3+) andexcites them to 2F5/2 level along with the direct absorption ofEr3+ ions. The excited Yb3+ ions transfer their excitation energy tounexcited Er3+ ions, promoting them to 4I11/2 level thus enhancingthe population of 4I11/2 level further. Er3+ ions in 4I11/2 level reab-sorb 976 nm photons through excited state absorption and popu-late 2H11/2 and 4S3/2 levels. The ions in this state reabsorb theincident photons and promote them to 2G7/2 level. Due to small en-

Fig. 7. Frequency upconversion emission spectrum of Er, Yb: Gd2O3 on excitation with 976 nm (A). Parallel lines with (Er, Yb:Gd2O3) PVA colloid showing intense green (B)and red emission (C), respectively on illumination with a NIR source demonstrating its use as luminescent marker and security ink applications.

G. Kaur et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 511–516 515

ergy separation between 2G9/2, 2K15/2 and 4G11/2 levels, Er3+ ionsrapidly relax to 4G11/2 level. Excited state absorption of pump pho-tons by ions in 4G11/2 level finally populates 2I11/2 level which isagain a long lived level. Availability of excess of Er3+ ions in 4S3/2

and 2H11/2 levels results in enhanced emission of Er3+ ions through-out the region. Thus, the emission from 4F9/2 level populatedby relaxation from 4S3/2 and 2H11/2 levels in red region [675 nm(4F9/2 ?

4I15/2)] is enhanced up to 14 times as compared to theintensity obtained in the absence of Yb3+ ions. We have measuredthe quantum efficiency of the Er, Yb: Gd2O3 nanophosphor and itcomes out to be 58.2%. The nanoparticles formed are of dimensionsranging from 30–40 nm showing intense luminescence and it istrue that the optical emissions efficiency decreases with particlesize. It is perhaps below this value in the present case.

Applications as luminescent marker and invisible security ink

One of the important applications of Er3+, Yb3+: Gd2O3 in PVAcolloid is that it can be used as luminescent marker and for invis-ible security ink in red and green colors that are highly effectiveand economical for the protection of valuable documents and con-sumer products against fraud. It has potential for detecting anycounterfeiting, alteration and unauthorized trading. It also has col-or flexibility. Luminescent ink also finds applications in optoelec-tronics, such as in the development of efficient light-emittingdevices. An important point is that these inks can be easily adaptedto inkjet or screen printing technologies for printing security doc-uments in different colors at the same time. The inkjet printingprocess offers a number of additional advantages, such as precisematerial deposition on paper or on substrate at a well defined po-sition, low material consumption and thus less material waste. Thetime duration for its stability is more than 3 months. The stickingof the nanoparticles containing liquid with paper or any othermaterial i.e. glass slides is attributed to the adhesive property ofour water soluble polymer, Polyvinyl Alcohol. PVA has been foundto be a suitable polymer for aqueous inks useful for use in theprinting. The polymer acts as a carrier liquid and also it does not

exhibit significant absorbance at either the excitation or emissionwavelengths of our interest and interfere with the intensity ofthe observed luminescence. We have successfully prepared atransparent and stable (Er3+, Yb3+:Gd2O3) PVA colloid and the writ-ing test was performed by drawing parallel lines with (Er, Yb:Gd2O3) PVA transparent colloid and illuminating with a NIR source(Fig. 7B and C) to show its applicability as luminescent marker andinvisible security ink upon 976 nm excitation.

Conclusions

Luminescent Er3+, Yb3+ co-doped Gd2O3 nanopowder was syn-thesized by combustion route. Transmission electron microscopeimages show that the particles were in cubic phase of Gd2O3 withparticles of average size <40 nm which was further validated by X-ray diffraction patterns. SEM images show microflaky structurewhich makes the phosphor material suitable for dispersing in poly-vinyl alcohol matrix. Thus the obtained non-agglomerating trans-parent solution was useful for biological applications(luminescence marker) as well as luminescent security ink usingNIR excitation. The FTIR and the thermal analysis support the pres-ence of PVA with nanopowder. UC and DC luminescence was ob-served with 532 nm excitation and the energy transfermechanism has been explained. Multicolor UC emission bands inthe UV, blue, green, red and NIR regions were observed on excita-tion with 976 nm wavelength and possible UC mechanism hasbeen suggested.

Acknowledgements

Authors are grateful to the Alexander von Humboldt founda-tion, Germany for providing pulsed Nd:YAG laser. One of theauthors (G. Kaur) is grateful to CSIR, New Delhi for the award ofSenior Research Fellowship. Authors would also like to acknowl-edge Prof. O.N. Srivastava, B.H.U. Varanasi, for TEM and SEMmeasurements.

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