Lw

Ja

b

c

d

a

ARR1A

KCPLS

1

ulo(capvp[smd

trr

gL

0d

Materials Science and Engineering B 176 (2011) 436–441

Contents lists available at ScienceDirect

Materials Science and Engineering B

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

uminescence enhancement of Y2O3:Eu3+ and Y2SiO5:Ce3+,Tb3+ core particlesith SiO2 shells

.K. Hana, G.A. Hiratab, J.B. Talbota,c, J. McKittricka,d,∗

Material Science and Engineering Program, University of California, San Diego, La Jolla, CA 92093, USACentro de Nanociencias y Nanotecnolgía, Universidad Nacional Autónoma de México, Ensenada, Baja California, MX CP 22860, MexicoDepartment of Nanoengineering, University of California, San Diego, La Jolla, CA 92093, USADepartment of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093, USA

r t i c l e i n f o

rticle history:eceived 20 July 2010eceived in revised form7 November 2010ccepted 10 January 2011

a b s t r a c t

This paper reports on the luminescence and microstructural features of oxide nano-crystalline(Y2O3:Eu3+) and submicron-sized (Y2SiO5:Ce3+,Tb3+) phosphor cores, produced by two different synthesistechniques, and subsequently coated by an inert shell of SiO2 using a sol–gel process. The shells mitigatethe detrimental effect of the phosphor particle surfaces on the photoluminescence emission properties,

eywords:ore/shellhosphoruminescence

thereby increasing luminous output by 20–90%, depending on the core composition and shell thickness.For Y2O3:Eu3+, uniformly shaped, narrow particle size distribution core/shell particles were successfullyfabricated. The photoluminescence emission intensity of core nanoparticles increased with increasingEu3+ activator concentration and the luminescence emission intensity of the core/shell particles was20–50% higher than that of the core particles alone. For Y2SiO5:Ce3+,Tb3+, the core/shell particles showed

nesce

iO2 shell thickness enhancement of the lumithe SiO2 shell thickness.

. Introduction

Nanoparticles are a topic of extensive research due to theirnique applications in many areas, especially in the area of

uminescence (i.e., nanophosphors) [1–7]. Among the numer-us nanophosphors, europium-doped yttrium oxide (Y2O3:Eu3+

Y1−xEux)2O3) has been widely used as a red-phosphor in fluores-ent lamps, high resolution projection devices and displays, suchs cathode ray tubes, plasma display panels and field emission dis-lays [8–12]. Y2O3:Eu3+ nanophosphors have been prepared byarious methods, such as sol–gel techniques [13,14], homogeneousrecipitation [15], spray pyrolysis [16], laser-heated evaporation17], microemulsion [18], combustion synthesis [19], and flamepray pyrolysis [20]. The sol–gel method, especially the Pechiniethod, is a relatively easy technique that controls the size and

istribution of phosphor particles [21,22].

Nanocrystalline samples produce low-luminescence intensity

hat can be explained by considering the poor intake of the excitingadiation due to pronounced reflectance losses coupled with non-adiative relaxation at the surface states. The surface states arising

∗ Corresponding author at: UC San Diego, Material Science and Engineering Pro-ram and Department of Mechanical and Aerospace Engineering, 9500 Gilman Dr.,a Jolla, CA 92093-0411, USA. Tel.: +1 858 534 5425; fax: +1 858 534 5698.

E-mail address: jmckittrick@ucsd.edu (J. McKittrick).

921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2011.01.003

nce emission intensity of 35–90% that of the core particles, depending on

© 2011 Elsevier B.V. All rights reserved.

from discontinuity in lattice periodicity leading to numerous bro-ken chemical bonds are considerably different from the bulk states.[23,24]. To alleviate these problems, core/shell structured nanopar-ticles have been used to stabilize the surface of the nanoparticles[25–28]. These processes of luminescence quenching could be sup-pressed if one were able to grow a shell of an inert material aroundeach doped nanoparticle, composed of a material through whichenergy cannot be transferred. The idea of surface modification arosefrom semiconductor nanocrystal research to improve quantum effi-ciency by inhibiting energy transfer loss at the surface [29–34].Recently, this concept has been extended to rare-earth-doped inor-ganic luminescent materials, such as CePO4:Tb3+ core particles witha surrounding shells of LaPO4 (Ce, Tb = 20%)PO4·xH2O/LaPO4·xH2O(x = 0.7) and LaPO4:Eu3+/LaPO4 core/shell particles [35–37]. Theseresults indicate that photoluminescence intensity was abouttwo times higher for the CePO4:Tb3+ core/LaPO4 shell mate-rials than for the bare cores and the luminescence yield of(Ce, Tb = 20%)PO4·xH2O/LaPO4·xH2O (x = 0.7) core/shell particlesincreased from 41% to 55% for the Ce3+ emission, indicating thatthis concept could be extended to other luminescent core/inertshell nanoparticles. Moreover, the preparation of core/shell parti-

cles using SiO2 as the shell by the Stöber method [38] is widely usedfor metal and iron oxide nanoparticles [39–42], since silica can beeasily and controllably deposited. Several groups have fabricatedcore/shell nanophosphors with a SiO2 core with the nanophosphordeposited as the shell [43–45]. Although this method has shown

and Engineering B 176 (2011) 436–441 437

sta

ipncseds

2

S(ewt(

2p

p0rSf(lasacm

2

e(ppsnaefdb

2

hppt0tps

J.K. Han et al. / Materials Science

pherically shaped core/shell particles with a narrow size distribu-ion and no agglomeration, the surface of the phosphor layer is stillregion of atomic disorder and thus, of luminescence quenching.

We have previously reported the enhancement of luminescencentensity of Y2O3:Eu3+ nanophosphors coated by SiO2 [46]. Thisaper focused on the synthesis and characterization of Y2O3:Eu3+

anophosphors and SiO2 shells. In the present work, the lumines-ence properties are examined in more detail, the morphology andize distribution of core/SiO2 shell particles are reported and theffect of the SiO2 shells on the luminescence properties of twoifferent core particles, nanocrystalline Y2O3:Eu3+ and submicron-ized Y2SiO5:Ce3+,Tb3+ are investigated.

. Experimental

The starting materials were Y(NO3)3·6H2O (99.8%,igma–Aldrich), Eu2O3 (99.999%, Alfa Aesar), citric acidC6H8O7·H2O, EMD), ethylene glycol (C2H6OH, Fischer Sci-ntific), polyethylene glycol (PEG (C2H4O)nH2O, moleculareight = 20,000, Sigma–Aldrich), nitric acid (68–70%, EM Science),

etra-ethoxysilane (TEOS, 99.9%, Sigma–Aldrich) and NH4OH28–70%, EMD).

.1. Preparation of (Y1−xEux)2O3 (x = 0.01, 0.03 and 0.05) corearticles

The (Y1−xEux)2O3 particles were prepared by a Pechini sol–gelrocess: 7.66 g Y(NO3)3·6H2O and Eu2O3 (molar ratio Eu/Y = 0.01,.03 and 0.05) were dissolved in a nitric acid solution (volumeatio of water and nitric acid = 1:1) to form an aqueous solution.ubsequently, 4.2 g citric acid (CA), which acted as chelating agentor metal ions, and 2.23 ml ethylene glycol (EG) were introducedmolar ratio of metal:CA:EG = 1:1:2). Subsequently, 5 g polyethy-ene glycol (PEG) as a cross-linking agent was added into thequeous solution. The concentration of PEG was 0.05 g/ml. Theolution was then continuously stirred for 10 h at 80 ◦C to formtransparent gel. The gel was preheated at 300 ◦C for 1 h, and then

alcined at 800 ◦C for 1 h in a furnace in air to remove organicaterials and to obtain the nanophosphor powders.

.2. Preparation of (Y0.9625Ce0.0075Tb0.03)2SiO5 core particles

In order to investigate the effect of a SiO2 coating on a differ-nt luminescent material with a submicron size, white-emittingY0.9625Ce0.0075Tb0.03)2SiO5 (Y2SiO5:Ce3+,Tb3+) particles were pre-ared by combustion synthesis and then coated with SiO2. Theowders were produced via combustion synthesis. Combustionynthesis involves a highly exothermic reaction between metalitrates and an organic fuel. A detailed explanation of the reactionnd procedure for producing this phosphor can be found in Boszet al. [47]. The as-synthesized powders were annealed at 1350 ◦Cor 1 h to improve the luminescence emission and produce pow-ers with the high temperature X2–Y2SiO5 phase, as determinedy X-ray diffraction.

.3. Fabrication of the SiO2 shells

The silica shells were prepared by hydrolysis of TEOS in an alco-ol solvent in the presence of water and ammonia by the Stöberrocess [38]. First, 1 g of the core particles was added into 30 ml 1-ropanol. The mixture was agitated using ultrasonification for 1.5 h

o disperse the core particles. Then 0.5 ml of deionized water and.4 ml NH4OH were added into the solution followed by an addi-ion of 0.1 ml TEOS. The ratio between the concentrations of corearticles and reagents (H2O, NH4OH and TEOS) was chosen to avoidelf-nucleation of silica and thus, the formation of core-free silica

Fig. 1. XRD patterns of (Y0.95Eu0.05)2O3 core particles.

spheres. The reaction was allowed to continue in the ultrasonicatorto obtain more dispersed core and uniform SiO2 coating on the core.In order to prevent heating of the solution and to better disperse thenanoparticles, ice was frequently added into ultrasonification bathand the bath temperature was fixed at 20 ◦C. The reaction productswere centrifugally separated from the suspension and rinsed withethanol four times.

For the Y2SiO5:Ce3+,Tb3+ particles, an alternative method to con-trol the SiO2 shell thickness was employed for comparison. Theconcentration of H2O and NH4OH in the 1-propanol solution wascontrolled, instead of lengthening the deposition time, because thechange of concentration of H2O and NH4OH is better way to depositthicker SiO2 shells than that of deposition time in terms of timeefficiency of the whole synthetic process.

2.4. Characterization

The particle morphology and size were inspected using a fieldemission scanning electron microscope (FESEM, XL30, Philips) anda transmission electron microscope (TEM, JEOL-2010) operated at200 kV accelerating voltage. The crystalline phases were identifiedby X-ray diffraction (XRD). Photoluminescence (PL) measurementswere taken using a Jobin-Yvon Triax 180 monochromator andSpectrumOne charge-coupled device detection system, which wasshared with the PL system that uses a 450 W Xe lamp as the exci-tation source. The core and core/shell particles were stored inthe form of suspension in ethanol and each mixture has identi-cal core particle densities (0.033 g/ml) since the quantity of coreparticles (1 g) before depositing the shells is the same as afterdepositing the shells. After 1 h sonification, 2 ml of the core andcore/shell suspensions was removed for PL spectroscopy. Particlesizing measurements were performed at 25 ◦C using a Zeta PlusParticle Analyzer (Brookhaven Instruments Corp., Holtsville, NY).

3. Results and discussion

Fig. 1 shows the XRD patterns of (Y0.95Eu0.05)2O3 core parti-cles and they match well with JCPDS 41-1105 for cubic Y2O3 andconfirm our samples as having a cubic structure with Ia3 spacegroup.

The emission spectra of (Y1−xEux)2O3 core particles with dif-ferent activator concentrations are shown in Fig. 2. The emissionspectra are also composed of 5D0–7Fj (j = 0–3) emission lines of

Eu3+, dominated by the strongest red emission 5D0–7F2 transitionof Eu3+ at 611 nm. The PL intensity increases with higher Eu3+ con-centration in the range of 1–5 at.%. Higher Eu3+ concentrations areknown to increase the luminescent intensity in a relatively smallactivator concentration range of 1–10 at.% [48].

438 J.K. Han et al. / Materials Science and En

(tscfcetSsdia

TsFFedd

bare core particles. This indicates that the inert shells play a

Fig. 2. Effect of Eu3+ concentration on the photoluminescence intensity.

The morphology and size of the (Y0.95Eu0.05)2O3 core andY0.95Eu0.05)2O3 core/SiO2 shell particles with different SiO2 shellhickness are shown in the SEM images in Fig. 3(a)–(d). The depo-ition of SiO2 shells on Y2O3:Eu3+ core particles involved the baseatalyzed hydrolysis of TEOS and condensation of SiO2 onto the sur-aces of the core particles. There are several parameters, such as theoncentrations of ammonia catalyst, water and TEOS, which can bemployed to control the thickness of the SiO2 shell. We adjustedhe shell thickness by changing deposition time. Fig. 3(a) showsEM images of the (Y0.95Eu0.05)2O3 core particles and Fig. 3(b)–(d)how SEM images of the core/shell particles with 1 h, 2 h and 5 heposition time, respectively. From the micrographs, the core size

s ∼40 nm and the shell thickness is estimated to be 20 nm, 30 nmnd 50 nm for the 1 h, 2 h and 5 h deposition times, respectively.

To examine the dimension of the SiO2 shells more in detail,EM micrographs of (Y0.95Eu0.05)2O3 core particles and core/SiO2hell particles with 1 h, 2 h and 5 h deposition time are shown inig. 4(a)–(d). The size of single core particle is ∼40 nm as shown in

ig. 4(a) although the particles are not mono-dispersed but agglom-rated. The thickness of silica is approximately 15–20 nm for a 1 heposition, 25–30 nm for a 2 h deposition and 45–50 nm for a 5 heposition as shown in Fig. 4(b)–(d). The inset in Fig. 4(b) shows

Fig. 3. SEM micrographs of (a) (Y0.95Eu0.05)2O3 core and core/SiO2 shell partic

gineering B 176 (2011) 436–441

high magnification images of core/SiO2 shell particle for 1 h coat-ing time. The core size and thickness of the SiO2 shells measuredby TEM images in Fig. 4(a)–(d) are correspond with that obtainedby SEM micrographs as shown in Fig. 3(a)–(d). The morphology ofthe core/shell particles is not perfectly spherical, and multiple coreparticles aggregated and then were coated. However, as the thick-ness of the SiO2 shells increased, the core/shell particles becamemore monodispersed (Figs. 3(d) and 4(d)). By decreasing the con-centration of core particle in the mixture and using longer time inthe sonicator it may be possible to prevent agglomeration [40].

Histograms of the particle diameter of (Y0.95Eu0.05)2O3 core and(Y0.95Eu0.05)2O3 core/shell particles with 1 h, 2 h and 5 h deposi-tion times obtained from the particle size analysis are shown inFig. 5(a)–(d). Although the particle size distribution of core particleis slightly different from the SEM and TEM micrographs due to themeasure of the aggregates along with the single core particles, thesize distributions of core/shell particles are good agreement withthe SEM and TEM micrographs. The average core size is 60 nm andthe average core/shell size is 85, 95 and 150 nm, respectively, forthe 1 h, 2 h and 5 h deposition times, respectively.

The luminescence spectra of (Y0.95Eu0.05)2O3 core particles andcore (Y0.95Eu0.05)2O3/shell (SiO2) particles with shell thicknesses of15–20 nm, 25–30 nm 45–50 nm are shown in Fig. 6 at an excitationwavelength of 254 nm. The inset shows normalized integrated PLintensity (divided by intensity of core particle) as a function of shellthickness. The PL measurements of both the core and core/shellparticles show strong red emission peaks at 611 nm (5D0 → 7F2)and the crystal field splitting of Eu3+ 5D0–7F1,2 transitions can beseen clearly, which implies that the (Y0.95Eu0.05)2O3 core particlesare well crystallized.

No significant difference in the position of the peaks betweenthese spectra is observed, but the PL emission intensity of core/shellparticle is ∼20% higher for a shell thickness of 45–50 nm and50% higher for a shell thickness of 15–20 nm over that of the

significant role in reducing the surface defects that cause lumines-cence quenching in nanophosphors, which has been observed withCePO4:Tb3+/LaPO4 core/shell and LaF3:Ce3+,Tb3+/LaF3 nanoparti-cles [35,49].

les for (b) 1 h coating time (c) 2 h coating time and (d) 5 h coating time.

J.K. Han et al. / Materials Science and Engineering B 176 (2011) 436–441 439

Fig. 4. TEM micrographs of (a) (Y0.95Eu0.05)2O3 core and core/SiO2 shell particles for (b) 1 h coating time (c) 2 h coating time and (d) 5 h coating time.

Fig. 5. Histograms of the particle diameter of (a) (Y0.95Eu0.05)2O3 core and (Y0.95Eu0.05)2O3 core/shell particles for (b) 1 h coating time (c) 2 h coating time and (d) 5 h coatingtime.

440 J.K. Han et al. / Materials Science and Engineering B 176 (2011) 436–441

F(i

aeYrnbns

R

woYSitkcctatamti

wSd

I

wa˛plmot

biosa

3+ 3+

ig. 6. Photoluminescence emission spectra of (Y0.95Eu0.05)2O3 cores, andY0.95Eu0.05)2O3 core/SiO2 shell particles. The inset shows normalized integratedntensity as a function of shell thickness.

In evaluating the reflectivity of UV light (�ex = 254 nm) fromparticle surface, the indices of refraction must be considered:

thanol (suspension medium) (n ≈ 1.367), SiO2 (n ≈ 1.505), and2O3 (n ≈ 2.162) [50–52]. Scattering of the excitation light will beeduced when the index of refraction of the suspension media isear that of the particle, or index of refraction of the shells isetween that of the suspension media and core particle. Using Fres-el’s equation and letting the incident and refracted ray angle at theurface = 0,

=(

n1 − n2

n1 + n2

)2,

here R is reflection coefficient, n1 and n2 are refractive indicesf each medium [53]. The reflectivity of UV between ethanol and2O3:Eu3+ is 5.1% while the total reflectivity of ethanol–SiO2 andiO2–Y2O3:Eu3+ is 3.4%. Therefore, more photons are transmittednto the Y2O3:Eu3+ particles by virtue of the SiO2 coating [54], andhus the PL emission intensity is increased. Compared to other well-nown inert coating such as TiO2 (n = 2.35) [55], the PL intensity ofore/TiO2 shell would be reduced because the total reflectivity ofore/shell particles (7%) is much larger than that of the core par-icles (5.1%). A similar analysis can be performed for MgO, whichlso shows that SiO2 shells are also enhanced in terms of reducinghe total reflectivity. Therefore, coating SiO2 on the core particles isn optimal material to increase PL emission intensity. Although oureasurements were performed in ethanol (n ≈ 1.367), the reduc-

ion in scattering of incident photons should be more pronouncedf measured in air (n ≈ 1).

The luminescence intensity of core/shell particles decreasesith thicker shells. This is likely due to the absorption of light by

iO2 shells at a higher ratio of thickness of shell to core particleiameter. By the Beer–Lambert law:

= I0 exp(−˛x),

here I is the measured intensity of the transmitted light throughlayer of material with thickness x, I0 is the incident intensity andis the absorption coefficient, the measured intensity is inversely

roportional to the thickness of the material through which theight is transmitted. Thus, as the SiO2 shell thickness increases,

ore photons are absorbed in the SiO2 shell, resulting in a decreasef luminescence intensity, which would eventually become lowerhan the bare core particles if thick enough.

The optical absorption as a function of thickness SiO2 fabricated

y layer-by-layer assembly on Fe3O4 nanoparticles has been exam-

ned by Aliev et al. [56], which shows absorption is a linear functionf thickness. This corroborates our results that show the PL emis-ion intensity decreases linearly as a function of coating thickness,s shown in the inset in Fig. 6.

Fig. 7. SEM micrographs of Y2SiO5:Ce ,Tb core/SiO2 shell particles with differentconcentration of reagents to deposit SiO2 shells: (a) 0.5 ml H2O, 0.5 ml NH4OH and0.05 ml TEOS and (b) 1 ml H2O and 1 ml NH4OH and 0.05 ml TEOS in 50 ml 1-propanolsolution.

SEM images of Y2SiO5:Ce3+,Tb3+ cores with different silicashell thicknesses are shown in Fig. 7(a) and (b). Fig. 7(a) showsY2SiO5:Ce3+,Tb3+ particles with the SiO2 shell prepared by adding0.5 ml H2O, 0.5 ml NH4OH and 0.05 ml TEOS for 1 h in a 50 ml 1-propanol solution. The size of the core particles is ∼1 �m and theSiO2 shell thickness is ranged from 25 to 30 nm. Fig. 7(b) displayscore/shell particles synthesized by adding 1 ml H2O, 1 ml NH4OHand 0.05 ml TEOS for 1 h in a 50 ml 1-propanol solution to depositSiO2 shells on the core particles. The SiO2 shell thickness rangedfrom 80 to 90 nm, much thicker than that prepared by smallerconcentration of H2O and NH4OH. The SiO2 shells with 80–90 nmthickness are deposited in only 1 h with higher concentration ofH2O and NH4OH. Thus, changing the concentration of H2O andNH4OH is better way to deposit thicker SiO2 shells than that ofdeposition time in terms of a reduction in the processing time.

Luminescence spectra of Y2SiO5:Ce3+,Tb3+ cores andY2SiO5:Ce3+,Tb3+ core/SiO2 shells at an excitation wavelengthof 380 nm are shown in Fig. 8. The spectra consist of two emissionbands at 440 and 490 nm, which are assigned to the Y2SiO5:Ce3+

transitions from the level 5d to the levels 2Fj (j = 5/2, 7/2) andY2SiO5:Tb3+ has an intense and well-defined green emission peaklocated at 544 nm (5D → 7F ). The inset figure shows normalized

4 5integrated PL intensity as a function of shell thickness. No differ-ences in position of the peaks between these spectra are detected,but the PL emission intensity of the Y2SiO5:Ce3+,Tb3+ core particleswith 25–30 nm and 80–90 nm SiO2 shell is 90% and 35% higher

J.K. Han et al. / Materials Science and En

FYi

trwscac

4

PSotciipocttl

A

ag

R

[[[[

[[[

[[[[[[[[[[

[[[[

[

[

[

[

[

[[[[[[[[[[[[[

[[[[

ig. 8. Photoluminescence emission spectra of Y2SiO5:Ce3+,Tb3+ core and2SiO5:Ce3+,Tb3+ core/SiO2 shell particles. The inset shows normalized integrated

ntensity as a function of shell thickness.

han that of core only particles, respectively. This is an unexpectedesult in that even for micron-sized powders, the surface effectsere mitigated by an inert shell. In the case of the thicker SiO2

hell (80–90 nm), the luminescence intensity of Y2SiO5:Ce3+,Tb3+

ore/SiO2 shell particles is lower than the core/shell particles withthinner shell thickness (25–30 nm), but is still higher than the

ore only.

. Conclusions

Y2O3:Eu3+ nanophosphor cores (∼40 nm) were prepared by theechini method and subsequently coated with SiO2 shells by thetöber process. SEM and TEM images show that 15–50 nm shellsf SiO2 were homogenously coated onto the nanophosphors, withhe thickness controlled by the deposition time. Core/shell parti-les show significant enhancement of the luminescence emissionntensity over that of the core only particles. The luminescencentensity with thinner SiO2 shells (15–20 nm) was greater com-ared with thicker shells (25–50 nm). Additionally, coating SiO2n submicron-sized Y2SiO5:Ce3+,Tb3+ cores (∼1 �m) produced byombustion synthesis resulted in a photoluminescence intensityhat was 90% higher than that of the core only particles for a shellhickness of 25–30 nm. Thicker shells (80–90 nm) increased theuminescence intensity to 35% of the core only particles.

cknowledgements

We gratefully thank Francisco Ruiz at CNYN-UNAM for the TEMnalysis. This work is supported by the U.S. Department of Energy,rant DE-EE002003.

eferences

[1] R.P. Rao, J. Electrochem. Soc. 143 (1996) 189.[2] C. Suryanarayana, Int. Mater. Rev. 40 (1995) 41.

[[

[[

gineering B 176 (2011) 436–441 441

[3] R.N. Bhargava, J. Lumin. 70 (1996) 85.[4] F.E. Kruis, H. Fissan, A. Peled, J. Aerosol Sci. 29 (1998) 511.[5] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994)

416.[6] M. Nyk, R. Kumar, T.Y. Ohulchanskyy, E.J. Bergey, P.N. Prasad, Nano Lett. 8 (2008)

2834.[7] H. Chander, Mater. Sci. Eng. R 49 (2005) 113.[8] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994.[9] S.M.J. Smets, Mater. Chem. Phys. 16 (1989) 283.10] C.R. Ronda, J. Lumin. 72 (1997) 49.11] C.R. Ronda, J. Alloys Compd. 225 (1995) 534.12] O.A. Serra, S.A. Cicillini, R.R. Ishiki, J. Alloys Compd. 303 (2000) 316.13] J. Zhang, Z. Zhang, Z. Tang, Y. Lin, Z. Zheng, J. Mater. Process. Technol. 121 (2002)

265.14] C. He, Y. Guan, L. Yao, W. Cai, X. Li, Z. Yao, Mater. Res. Bull. 38 (2003) 973.15] Y.C. Kang, S.B. Park, I.W. Lenggoro, K. Okuyama, J. Mater. Res. 14 (1999) 2611.16] N. Joffin, J. Dexpert-Ghys, M. Verelst, G. Baret, A. Garcia, J. Lumin. 113 (2005)

249.17] H. Eilers, B.M. Tissue, Chem. Phys. Lett. 251 (1996) 74.18] Q. Pang, J. Shi, Y. Liu, D. Xing, M. Gong, N. Xu, Mater. Sci. Eng. B 103 (2003) 57.19] Y. Tao, G.W. Zhao, W.P. Zhang, S.D. Xia, Mater. Res. Bull. 32 (1997) 501.20] Y.C. Kang, D.J. Seo, S.B. Park, H.D. Park, Jpn. J. Appl. Phys. 40 (2001) 4083.21] M.P. Pechini, U.S. Patent 3,330,697 (1967).22] H. Wang, C.K. Lin, X.M. Liu, J. Lin, M. Yu, Appl. Phys. Lett. 87 (2005) 181907.23] T. Ye, G.W. Zhao, W.P. Zhang, S.D. Xia, Mater. Res. Bull. 32 (1997) 501.24] K. Riwotzki, M. Haase, J. Phys. Chem. B 105 (2001) 12709.25] L.M. Liz-Marzán, M. Giersig, P. Mulvaney, Langmuir 12 (1996) 4329.26] Z. Zhang, R.C. Patel, R. Kothari, C.P. Johnson, S.E. Friberg, P.A. Aikens, J. Phys.

Chem. B 104 (2000) 1176.27] G.S. Yi, G.M. Chow, Chem. Mater. 19 (2007) 341.28] M. Darbandi, W. Hoheisel, T. Nann, Nanotechnology 17 (2006) 4168.29] H. Yang, P.H. Holloway, Adv. Funct. Mater. 14 (2004) 152.30] J.S. Steckel, J.P. Zimmer, S. Coe-Sullivan, N.E. Stott, V. Bulovic, M.G. Bawendi,

Angew. Chem. Int. Ed. 43 (2004) 2154.31] M. Brumer, A. Kigel, L. Amirav, A. Sashchiuk, O. Solomesch, N. Tessler, E. Lifshitz,

Adv. Funct. Mater. 15 (2005) 1111.32] S. Wang, B.R. Jarrett, S.M. Kauzlarich, A.Y. Louie, J. Am. Chem. Soc. 129 (2007)

3848.33] J.P. Zimmer, S.W. Kim, S. Ohnishi, E. Tanaka, J.V. Frangioni, M.G. Bawendi, J. Am.

Chem. Soc. 128 (2006) 2526.34] D. Valerini, A. Creti, M. Lomascolo, L. Manna, R. Cingolani, M. Anni, Phys. Rev. B

71 (2005) 235409.35] K. Kompe, H. Borchert, J. Storz, A. Lobo, S. Adam, T. Moller, M. Haase, Angew.

Chem. Int. Ed. 42 (2003) 5513.36] O. Lehmann, K. Kompe, M. Haase, J. Am. Chem. Soc. 126 (2004) 14935.37] V. Buissette, M. Moreau, T. Gacoin, J.P. Boilot, Adv. Funct. Mater. 16 (2006) 351.38] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62.39] Y. Lu, Y. Yin, B.T. Mayers, Y. Xia, Nano Lett. 2 (2002) 183.40] Y. Lu, Y. Yin, Z.Y. Li, Y. Xia, Nano Lett. 2 (2002) 785.41] F. Caruso, Adv. Mater. 13 (2001) 11.42] Y. Deng, D. Qi, C. Deng, X. Zhang, D. Zhao, J. Am. Chem. Soc. 130 (2008) 28.43] H. Wang, M. Yu, C.K. Lin, X.M. Liu, J. Lin, J. Phys. Chem. C 111 (2007) 11223.44] H. Feng, Y. Chen, F. Tang, J. Ren, Mater. Lett. 60 (2006) 737.45] S.E. Lin, B.Y. Yum, J.J. Shuve, W.C.J. Wei, J. Am. Ceram. Soc. 91 (2008) 3976.46] J.K. Han, J.B. Talbot, J. McKittrick, ECS Trans. 28 (2010) 183.47] E.J. Bosze, G.A. Hirata, L.E. Shea-Rohwer, J. McKittrick, J. Lumin. 104 (2003) 47.48] H. Huang, G.Q. Xu, W.S. Chin, L.M. Gan, C.H. Chew, Nanotechnology 13 (2002)

318.49] J.W. Stouwdam, F. Van Veggel, Langmuir 20 (2004) 11763.50] J. Rheims, J. Köser, T. Wriedt, Meas. Sci. Technol. 8 (1997) 601.51] I.H. Malitson, J. Opt. Soc. Am. 55 (1965) 1205.52] M. Bass, Handbook of Optics, vol. 2, second ed., McGraw-Hill, 1994.

53] E. Hecht, Optics, second ed., Addison Wesley, 1987.54] I.Y. Jung, Y. Cho, S.G. Lee, S.H. Sohn, D.K. Kim, D.K. Lee, Y.M. Kweon, Appl. Phys.

Lett. 87 (2005) 191908.55] J.S. King, E. Graugnard, C.J. Summers, Adv. Mater. 8 (2005) 1010.56] F.G. Aliev, M.A. Correa-Duarte, A. Mamedov, J.W. Ostrander, M. Giersig, L.M.

Liz-Marzán, N.A. Kotov, Adv. Mater. 11 (1999) 1006.