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Materials Science and Engineering B 166 (2010) 180–184

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

ynthesis and spectroscopy of transparent colloidal solution of Gd2O3: Er3+, Yb3+

pherical nanocrystals by pulsed laser ablation

.K. Singh, K. Kumar, S.B. Rai ∗

aser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India

r t i c l e i n f o

rticle history:eceived 27 May 2009eceived in revised form 13 October 2009

a b s t r a c t

The synthesis and the spectroscopy of upconverting nanocolloidal solutions have recently generatedconsiderable interest due to their potential application as biolevels and in biological assays. This paper

ccepted 10 November 2009

eywords:olloidal nanoparticleshosphorulsed laser ablationpconversion

reports the synthesis of lanthanides doped transparent colloidal solution via pulsed laser ablation (PLA)which is highly fluorescing. Er3+, Yb3+ co-doped Gd2O3 phosphor has been laser ablated to synthesizethe colloidal solution in triply distilled water. Spherical shaped nanoparticles of the average diameter inthe range of 8–26 nm have been synthesized and characterized. Efficient multicolor upconversion (UC)emission has been observed and possible UC mechanism has been suggested. This approach will providea method to synthesize highly UC efficient, non-agglomerated, pure transparent nanocolloidal solutionfor biological applications from already reported efficient phosphors.

. Introduction

The synthesis and the spectroscopy of upconverting nanocol-oidal solutions have recently generated considerable interest dueo their potential application as biolevels and in biological assays.xcitation of these upconversion (UC) luminescent materials withhe near infrared (NIR) light make them more suitable for biologicalpplications as it induces only a very weak autofluorescence back-round and avoids photo-degradation in biotagging applications1,2]. Moreover, upconverted visible radiation in different wave-ength region has higher efficiency and photochemical stabilityompared to conventional fluorescent organic molecules excitedia two photon absorption (TPA).

Recently few results have been published on the synthesisnd UC luminescence of the transparent colloids synthesizedhrough chemical route [2–4]. However, the report is very limitednd has no means of controlling size and shape of nanoparti-les. Several rare earths (RE) doped phosphors with low phononnergies are also well known to show highly efficient UC lumines-ence [5–8]. However, these crystalline materials normally consistf sub-micrometer to micrometer size grains and nanocrystals

NCs) present in them are too large to substitute for molecu-ar dyes in biological tagging applications [9]. Therefore, it isf great importance to synthesize rare earth doped upconvert-ng nanocrystals which can be transparently dispersed in the

∗ Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 2369889.E-mail address: sbrai49@yahoo.co.in (S.B. Rai).

921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2009.11.018

© 2009 Elsevier B.V. All rights reserved.

solution with a control on the shape and size of the nanoparti-cle.

In the present work, we propose an under water pulsed laserablation (PLA) based procedure to prepare the spherical nanopar-ticles transparently dispersed in the solution (without addition ofthe surface active surfactant) from the micro size phosphor mate-rial. To our knowledge, synthesis of colloidal nanoparticle of highlyfluorescing phosphor material via PLA has not been yet adequatelyaddressed in the literature. Advantage of PLA includes the abilityto produce materials with complex stoichiometry, narrower par-ticle size distribution, reduced porosity and most importantly thecontrol of the shape and size of nanoparticle along with reducedlevel of impurities and defects [10,11]. Also, it has feasibility of pro-ducing colloidal solution in different liquid media such as water,hexane etc. Several authors [12,13] have reported the synthesis ofnanoparticle via PLA with the average diameter lying in 5–20 nmrange.

Er3+, Yb3+ co-doped Gd2O3 phosphor has been chosen forthe purpose because gadolinium and its compounds are promis-ing luminescent system (sharp emission spectra, large Stokeshift, and long luminescence lifetime) in bio-analysis as well ashigh photo-stability and low cost synthesis [14,15]. Er3+, Yb3+

co-doped Gd2O3 phosphor has been synthesized through com-bustion route and further laser ablated to prepare the nano-sized

particle transparently dispersed in the solution. Structural char-acterization has been carried out using transmission electronmicroscope (TEM). UV–visible absorption and UC fluorescencehas been recorded for the optical characterization. MulticolorUC fluorescence has been observed in the colloidal solution

and Engineering B 166 (2010) 180–184 181

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S.K. Singh et al. / Materials Science

hich shows its promising application in the field of biolevel-ng.

. Experimental

.1. Synthesis of Er3+, Yb3+ co-doped Gd2O3 phosphor

The solution combustion method was used for the synthe-is of the phosphor material. Reagent grade Gd(NO3)3·6H2O,r(NO3)3·6H2O and Yb(NO3)3.·6H2O were used while urea wassed as the organic fuel. The composition of the compounds usedas

6.7Gd2O3 + 2Yb2O3 + 0.3Er2O3,

The detailed synthesis procedure is given in our previous work5].

.2. Preparation of colloidal solution via pulsed laser ablationPLA) in distilled water

Colloidal nanoparticles were synthesized by the laser ablationf the pellet target of Er3+–Yb3+ co-doped Gd2O3 phosphor materialsing third harmonic (355 nm) of a Nd: YAG pulse laser (Spitlight00, Germany) with a repetition rate of 10 Hz, pulse width of 7 nsnd a maximum output of 130 mJ/pulse. The laser beam is focusedertically on the pellet with the help of a focusing lens of the focalistance 12 cm and 0.4 mm spot size (fluence ∼81 J/cm2). The depthf the liquid layer above the target was about 20 mm. To avoid theormation of deep holes in the pellet, the sample holder was trans-ated mechanically so that laser pulse falls on the fresh surface.lso, the solution is stirred continuously during ablation to avoid

he intra-pulse self absorption. In intra-pulse self absorption parti-le produced by the earlier part of one pulse immediately absorbshe photon of the later part of the same pulse. It is important tovoid this in case of nano second pulse laser because the ejectionf the ablated matter becomes in picoseconds scale. The setup isimilar to that adopted by Zhu et al. [16]. The phosphor pellet wasblated for about 30 min.

It is well documented that shape, size and the efficiencies of col-oid formation depend on the different laser parameters. Semerokt al. have reported the wavelength dependence of various met-ls (Al, Cu, Mo, Fe, Pb and Ni) in the atmospheric circumstancessing laser light at different wavelength [12]. The ablation efficien-ies were higher at shorter wavelength for all metal species. In ourase best efficiency along with the spherical shaped nanoparticlesormation was obtained with 355 nm wavelength of Nd: YAG laserhaving 1064, 532, 355 and 266 nm). The efficiency decreases inase of 266 nm wavelength due to self absorption. Further, laserower in terms of laser fluence has also been optimized (81 J/cm2)o find out the completely spherical shaped nanoparticle. The phos-hor pellet was ablated for different time interval upto 30 min.

.3. Structural and optical characterization

X- Ray diffraction (XRD) patterns were recorded using CuK�adiation (1.5406 Å) with nickel filter. High resolution electronicrographs of the phosphor and colloidal solution were taken

sing a Technai 20G2 (Philips) transmission electron microscopequipped with the charge coupled device (CCD). Sample for theEM measurement of phosphor was dispersed by sonication while

or colloidal solution sonication was not used. Optical absorptionpectrum of the colloidal sample was recorded by a UV–visiblepectrophotometer (UV-1700, Pharmaspec, Shimadzu). UC emis-ion spectrum of the colloidal sample was measured in cuvettef 1 cm path length. Samples for UC fluorescence were excited by

Fig. 1. Transmission electron micrograph of Er , Yb co-doped Gd2O3 phosphorshowing the presence of sub-micrometer sized agglomerated polygonal nanocrystal[inset (A) single polygonal nanocrystal of sub-micrometer size, inset (B) selectedarea electron diffraction (SAED) pattern].

976 nm diode laser and the spectra were recorded using a iHR320(Jovin Yvon) spectrometer equipped with R928 photon countingphoto multiplier tube (PMT).

3. Results and discussion

X-ray diffraction study of the bulk phosphor was carried out toconfirm the phase present in the phosphor. Presence of the mono-clinic Gd2O3 phase in the bulk phosphor material was obtained.TEM Studies were further carried out for the structural char-acterizations. Fig. 1 shows the TEM micrographs of Er3+, Yb3+

co-doped Gd2O3 phosphor material. Polygonal nanocrystals of sub-micrometer size are clearly visible. [Inset (A) in Fig. 1 shows the sin-gle polygonal crystal of sub-micrometer size (∼250 nm).] From theimage it is also clearly visible that crystallites present in the phos-phor material are highly agglomerated. The selected area electrondiffraction pattern (SAED) (inset B) shows the bright spots indicat-ing the high crystalline nature of the phosphor material. Indexingof the SAED pattern also confirms the presence of the monoclinicGd2O3 phase. This phosphor material has been used as a target inpellet form and further laser ablated to get colloidal solution.

Fig. 2 shows the TEM micrographs of the nanoparticles (Fig. 2aand b) present in the colloidal solution (after 10 days of synthesis)along with the SAED pattern (Fig. 2c) and histogram (particle sizedistribution graph) (Fig. 2d). The TEM measurements were carriedout at different points of the grid and almost similar results wereobtained each time. TEM image in Fig. 2 shows the presence ofspherical nanoparticles with average diameter lying in between 8and 26 nm. During the PLA, particles grow in the plume ejectedby the target material and are not directly extracted from the pel-let and so in water, the individual particles are then aggregated tosome extent and particles tend to form web like structure over a

long range but the contrast is high enough and primary particles areeasily distinguishable. One can see from the histogram that morethan 87% of particles lie in the 12 ± 3 nm range. Variation of theaverage diameter of nanoparticles in between such a small rangecan be considered roughly as the nearly uniform distribution. Par-

182 S.K. Singh et al. / Materials Science and Engineering B 166 (2010) 180–184

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at 523, 546 and 670 nm wavelength on excitation with 976 nm.Inset in Fig. 4b shows the digital photograph of the clearly visible UCluminescence in transparent Yb3+, Er3+ co-doped Gd2O3 colloidalsolution on excitation with 976 nm diode laser. Strong yellowishgreen color of the colloidal solution (inset a) results from the two

ig. 2. Transmission electron micrographs (a and b) of the colloidal solution of Er3+

c) and histogram for the particle size distribution (d).

icularly, in case of PLA synthesis the uniformity lies in the shortange. Particle size in this range is particularly interesting becauseange is big enough to retain the optical properties of the bulk mate-ials and small enough to find potential applications in bio-labelingnd bioimaging etc. Fig. 2b shows the complete spherical nature ofhe colloidal nanoparticle. The crystalline nature of the materials clear from the SAED pattern (Fig. 2c). Similarity in the opticalpectroscopy (discussed in next paragraphs) of bulk and colloidalaterial confirms that in the colloidal solution the nanoparticles

rystallize in the same phases and stoichiometries as the bulk ones.hus from TEM observation it is concluded that the laser ablation ofhe phosphor changes the micro-sized polygonal nanocrystal intohe spherical nanoparticles.

Fig. 3 represents the typical absorption spectrum of the as pre-ared colloidal solution in the range from 200 to 1100 nm. Thepectrum of the aqueous suspension exhibits a broad absorptionand edge in the UV spectral range (at ∼240 nm) and it coulde attributed to the plasmon level of the gadolinium oxide [17].urther, another strong absorption band is observed near 850 nmhich may be attributed to the absorption of Er3+ ion from ground

evel to 4I9/2 level. The absorption spectrum of the colloid in theegion of interest (500–700 nm) is rather weak. This is perhapsue to low concentration of Er3+ ion in the colloidal solution. Evenhough the absorption peak at ∼660 nm, 550 nm and 530 nm is vis-ble and they may be attributed due to the absorption of Er3+ ionrom ground level to 4F9/2, 4S3/2 and 2H11/2 levels, respectively.

The UC emission spectra of phosphor and its nanocolloidal solu-ion are given in Fig. 4a and b, respectively. Spectra were recordedn the 500–700 nm wavelength region with 976 nm excitation. Themission spectra of the phosphor and its nanocolloidal solution on

xcitation with 976 nm diode laser were found to be nearly theame. Only a small blue shift in the position of the different peaks∼2 nm) in colloidal nanoparticle was observed. Since, it is a wellnown fact that in case of bulk to nano transformation a blue shifts observed in the emission spectrum therefore a blue shift of 2 nm

co-doped Gd2O3 phosphor (after 10 days of synthesis) along with the SAED pattern

was predicted obvious due to bulk to nano transformation of thephosphor nano material. This may also be considered as the crys-tal field effects. Slight modifications in the surrounding of erbiumions takes place when the crystal size is reduced to the nanometricscale.

The visible emission from the nanocolloidal solution is observed

Fig. 3. UV–vis absorption spectrum of colloidal solution of Er3+, Yb3+ co-dopedGd2O3 phosphor produced by pulsed laser ablation (PLA).

S.K. Singh et al. / Materials Science and Engineering B 166 (2010) 180–184 183

Fig. 4. Upconversion (UC) spectrum in 500–700 nm region excited at 976 nm withan input power of 240 mW (a) phosphor material [inset shows the input powerdependence showing the involvement of two input photons in a particular UC tran-sition] (b) nanocolloidal solution [inset shows the digital photograph of the UClit

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uminescence (a) without filter (b) with green filter and (c) with red filter] (Fornterpretation of the references to color in this figure legend, the reader is referredo the web version of the article).

ajor constituent emissions (i.e. green and red) which has beeneparately observed with the aid of green and red color filters (insetand c, respectively). All these peaks are due to Er3+ ion and are

scribed to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transi-ions, respectively. The input power dependence of the emissionntensities helped in clarifying the excitation mechanisms and tohe number of incident photons involved in a particular UC tran-ition. Two most common excitation mechanisms involved in UCre energy transfer (ET) and excited state absorption (ESA). Theelation Iupc ∝ (Pexc)n (where Iupc is the intensity of upconvertedorescence, Pexc is the excitation power and n is the number of pho-ons involved in the UC process) seems to hold well with n ∼ 2. The

wo green bands (523 nm and 546 nm) and the red band (670 nm)ield n = 2.37, 2.41 and 2.12, respectively (inset in Fig. 4a), indicat-ng that two incident NIR photons are involved for each emittedhoton.

Fig. 5. A schematic energy level diagram of Er3+ and Yb3+ representing the observedtransitions and possible upconversion mechanism in Er3+, Yb3+ co-doped Gd2O3

nanocolloidal solution.

Fig. 5 shows a schematic energy level diagram of Er3+ and Yb3+

representing the observed transitions and possible UC mechanismin Er3+, Yb3+ co-doped Gd2O3 nanocolloidal solution. Yb3+ ion hasnearly seven times greater absorption cross-section at 976 nm exci-tation in comparison to the Er3+ ion and acts as the sensitizer forthe UC process. Yb3+ ion absorbs the incident photon and promotedto its excited state 2F5/2. It transfers its excitation energy to Er3+ ion(energy transfer UC process) and plays an important role in pop-ulating the nearby lying 4I11/2, 4F9/2 and 4F7/2 levels of Er3+ ion.The 4I11/2 level of Er3+ ion is also populated by self absorption ofincident photon. The Er3+ ion in 4I11/2 level further absorbs the sec-ond photon (excited state UC process) and populates 4F7/2 level.A nonradiative relaxation of the ions from 4F7/2 level populates2H11/2, 4S3/2 and 4F9/2 levels and thus the transitions correspondingto 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, emitting at 525,546, 670 nm wavelengths are observed.

It is also notable that the crystallinity and fluorescence inten-sity is higher in case of bulk phosphor material. Emission bandsof the bulk phosphor show fairly large Stark splitting in compari-son to the colloidal solution. This is obviously due to large crystalfield of Gd2O3 matrix in bulk samples. Change in shape, size, crys-tallinity of the material and the intensity of the fluorescence iscompletely correlated to PLA. PLA led to the formation of stoi-chiometric nanoparticles i.e. atomic compositions of the producednanoparticles remains identical to those of the source materials.During PLA the laser pulse interacts with the target surface, whichleads to the ejection of nanoparticles to the liquid where they arecondensed and cool by rapid quenching and thus crystallinity goesdown. Further, crystallization of the material occurs due to local-ized high temperature in the edge of the ablation shock wave inthe liquid and by the interaction of laser pulses to the nanoparticleswhich have already been formed in the liquid. These interactionsare also responsible for the modification of the shape and size of thematerial. Further relative decrease in the intensity of the fluores-cence is due to small concentration of the material in the solution.It is notable that by increasing the ablation time the concentrationof the nanoparticle in the solution increases and in terms intensityof the fluorescence increases. A four time enhancement in the fluo-

rescence intensity of green band (at 546 nm) was observed for the30 min ablation time compared to 5 min ablation time.

It is worth while here to compare our TEM measurements withthe other earlier works. Heer et al. [2] has synthesized nanoparticle

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ith mean fraction between 10 and 17 nm, Boyer et al. [4] got theanoparticle of the size between about 15 and 30 nm and Matsuurat al. [18] of nearly 50 nm via different synthesis techniques. Thusur reported particle in the mean range from 8 to 26 nm is smallern size to the other reported works and so can be applied to otherpecific biological applications.

Underwater laser ablation method is feasible for the synthesisf fluorescent nanoparticle with spherical shapes (isotropic prop-rties. However, much more effort is needed in this direction to findut the optimized condition for each specific phosphor materials.

. Conclusions

The main outcome of the present study is the synthesis methodor the transparent nanocolloidal solution via PLA. Nano-sizedaverage diameter in the range of 8–26 nm) particle transparentlyispersed in the colloidal solution has been synthesized. Multicolorgreen and red) visible UC fluorescence has been observed in theolloidal solution which shows its promising application in the fieldf bioleveling. We strongly believe that this approach will providemethod to synthesize highly efficient transparent nanocolloidal

olution in different media for biological applications from alreadyeported efficient bulk phosphors.

cknowledgements

Authors are thankful to Alexander Von Humboldt foundation,ermany for providing Nd: YAG laser and Dr. N.P. Lalla of UGC-DAE

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ngineering B 166 (2010) 180–184

Consortium, Indore for carrying out the TEM measurements. Finan-cial assistance from DST, New Delhi is gratefully acknowledged. Mr.Sunil Kumar Singh thankfully acknowledges the financial supportas SRF from CSIR New Delhi.

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