On the abnormal efficiency of the luminescence of submicron-sized phosphor Y2O3 : Eu3+

ISSN 1063�7834, Physics of the Solid State, 2014, Vol. 56, No. 12, pp. 2496–2506. © Pleiades Publishing, Ltd., 2014.Original Russian Text © T.A. Pomelova, V.V. Bakovets, I.V. Korol’kov, O.V. Antonova, I.P. Dolgovesova, 2014, published in Fizika Tverdogo Tela, 2014, Vol. 56, No. 12, pp. 2410–2419.

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1. INTRODUCTION

Inorganic luminophores are the main stable phos�phors used in information light panels displays, TVscreens, and other devices [1–5]. The development ofrelatively low�cost modern liquid�crystalline andlight�emitting diode screen systems have not excludeddemands in inorganic phosphors, because it is some�times necessary to transform the background radiationspectrum, for example, of blue and UV radiation oflight�emitting diodes [6] to improve color characteris�tics of reproduced images. Inorganic phosphors con�sist of a crystalline matrix doped with activators, forexample, rare�earth metal elements [7–9]. At present,the development of mini� and microscreens for pro�viding the operator service with visual informationunder conditions of hampered view in space, under�water, or in an operative fighting situation is an impor�tant problem. Such screens are suggested to be dis�posed at distances of 1–2 cm from the operator eyepupil [10, 11]. In this case, the light and color effi�ciency of the screen should be retained at the level ofthe efficiency of modern macrodisplays. In order tosolve these problems and to develop high�definitionTV systems, it is necessary to form phosphors withsubmicron� and nanometer�sized particles. Similarproblems have been investigated by a number ofresearch groups [12–15]. However, the obtainedinformation on the efficiency of submicron� andnanometer�sized phosphors is insufficient to designthe corresponding optimal engineering visualizationsystems. One of the most widely used crystal matricesof phosphors is the cubic yttrium oxide doped with

Eu3+, Tb3+, Sm3+, etc. [7–9, 16]. The most studiedmicron�sized phosphor is Y2O3 : Eu3+ in the red emis�sion region. Therefore, the results of investigation ofthis material can be used as the base information foranalyzing the characteristics and mechanism of lumi�nescence when studying the size effects on the lumi�nescence efficiency of submicron� and nanometer�sized phosphors, which is the purpose of the presentwork.

2. SAMPLE PREPARATION

The Y2O3 : Eu phosphor was synthesized by thesol–gel method [17, 18], which makes it possible toform powdered materials of different dispersions,including nanostructured materials. The reagents usedwere the nitrates Y(NO3)3 · 6H2O and Eu(NO3)3 ·5H2O with the main component content no less than99.9 wt %, which were obtained from the correspond�ing oxides. To obtain yttrium hydroxides, we prepareda 0.2 M yttrium nitrate solution with the correspond�ing content of the europium nitrate dopant at calcu�lated concentrations of 5, 7, 9, and 11 at % Eu3+ in theobtained oxide. As a precipitator of yttrium andeuropium hydroxides, we used a twofold excess of the1.44 M solution of NaOH (special�purity grade) inbidistillated water. The synthesis was performed on anapparatus with sputtering of initial reagents to providehomogeneity in the working solution volume. As aresult of the sputtering, yttrium hydroxide precipitatedin submicron regions of the mother solution, whichled to the decrease in sizes of the formed particles [18].

IMPURITYCENTERS

On the Abnormal Efficiency of the Luminescence of Submicron�Sized Phosphor Y2O3 : Eu3+

T. A. Pomelova, V. V. Bakovets, I. V. Korol’kov, O. V. Antonova, and I. P. Dolgovesova

Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Lavrentieva 3, Novosibirsk, 630090 Russia

e�mail: [email protected] March 19, 2014; in final form, June 10, 2014

Abstract—Samples of the Y2O3 : Eu3+ (5–11 at %) phosphor with a high luminescence efficiency have beensynthesized by the sol–gel method. The chemical, substructural, and optical properties of the material havebeen investigated using the thermal analysis, X�ray powder diffraction, electron microscopy, vibrational andRaman spectroscopy, and diffuse reflection methods. The size effect of particles and crystallites on the lumi�nescence parameters of the phosphor has been analyzed. It has been found that the submicron�sizedY2O3 : Eu3+ phosphor containing 9–11 at % Eu3+ exhibits an abnormally high luminescence intensity. It hasbeen shown that these effects are associated with the enrichment of the surface of particles and crystalliteswith dopant ions due to the decrease in the local symmetry of their environment.

DOI: 10.1134/S1063783414120269

PHYSICS OF THE SOLID STATE Vol. 56 No. 12 2014

ON THE ABNORMAL EFFICIENCY OF THE LUMINESCENCE 2497

The obtained precipitate of the hydroxides was washedoff to a neutral value of pH of washing waters, dried at50°C in air, and, then, annealed in air at different tem�peratures in the range of 200–1300°C for 1 h. Accord�ing to the atomic emission spectroscopy (PGS�2 spec�trometer, direct�current arc), the total content ofmetal impurities (37 elements) in the hydroxide didnot exceed 1.5 × 10–2 wt %. The Eu3+ concentration xin the final product Y2 – xEuxO3 corresponded to thechosen Eu3+ concentrations in the initial solutions(x = 0.10, 0.14, 0.18, and 0.22), although, in somecases, the europium content was 7–10% less than thespecified value.

3. CHARACTERIZATION OF SUBMICRON� AND NANOMETER�SIZED SAMPLES

Figure 1 presents the results of the thermal analysisof the Y(OH)3 : Eu · nH2O samples obtained by thesol–gel method in the combination with evolved gasanalysis mass spectrometry. The stages of hydroxidedecomposition were studied using the synchronousthermal analysis (STA) including the simultaneousthermogravimetric (TG) measurements, differentialscanning calorimetry (DSC), and evolved gas analysismass spectrometry (EGA�MS). The STA analysis wasperformed on a NETZSCH STA 449A1 Jupiter® ana�lyzer (the heating rate was 10°C/min in a helium flowof 30 mL/min; Al2O3 crucible) integrated with a qua�drupole mass spectrometer QMS 403B Aëolos®. Theresults demonstrate the temperature ranges of the pro�cesses of dehydration (Fig. 1, local minima 1 and 2)and dehydroxylation (Fig. 1, local minimum 3) withthe release of CO2 and H2O (decomposition of traceamounts of Y and Eu hydrocarbonates; no nitrogenoxides were detected), as well as the phase transforma�

tion of Y(OH)3 : Eu3+ into crystalline Y2O3 : Eu3+

(Fig. 1, local minimum 4), which is indicated by theexoeffect in the DSC curve.

The phase composition and the crystallite size weredetermined using X�ray powder diffraction. The X�raypowder diffraction (XRD) analysis of the samples wasperformed on a Shimadzu XRD�7000 diffractometer(CuK

α radiation, Ni filter, 2θ = 5°–70° in steps of

0.03°, exposure time 2 s). Figure 2 shows the X�ray dif�fraction patterns of the samples measured afterannealing at different temperatures.

Below 700°C, the sample is amorphous with diffusereflections of Y(OH)3. At 400°C, the sample trans�forms into the nanostructured state with the diffusereflection (222) of Y2O3 [19]. The elevated crystallinityof cubic c�Y2O3 with the lattice parameter 10.6239 Å isobserved at temperatures higher than 700°C. Theinfrared (IR) spectroscopy performed on a ScimitarFTS 2000 IR�Fourier spectrometer shows (Fig. 3) theexistence of the absorption bands of the OH�groupbonds at 3500 cm–1 and CO group bonds at 1500–1700 cm–1, which disappear when the annealing tem�

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perature increases to 700°C; in this case, at frequen�cies of 400–500 cm–1, there are absorption bands ofthe stretching vibrations of the Y–O and Eu–O bondsin the crystal lattice [20, 21].

At different annealing stages, the sample dispersionD was determined using the known relationship D =6000/(Sρ) in the approximation of spherical particles,where S [m2 g–1] is the specific surface area found bythe Brunauer–Emmett–Taylor (BET) nitrogen absorp�tion method at 77 K on a KNGU Sorbtometer�M, andρ [g cm–3] is the pycnometric density of the sample[22]. The results of the measurements and the calcula�tions are presented in Fig. 4.

The decrease in S and the increase in ρ indicate thesintering of submicron� and nanometer�sized sampleswith increasing temperature. We revealed the repro�ducible distortions of the monotony of these depen�

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Fig. 4. Dependences of (a) specific surface area S, (b) pycnometric density ρ, (c) particle size D, and (d) crystallite size d on theannealing temperature of the samples with Eu3+ concentrations of (1) 5, (2) 7, (3) 9, and (4) 11 at %.



dences, which is a result of morphological and struc�tural transformations hydrogel–xerogel–aerosol dur�ing annealing of the samples. However, the calculationof D by the above noted dependence gives a relativelymonotonic and progressive increase in the averageparticle size with temperature (Fig. 4c). Figure 4ddepicts the monotonically increased dependence ofcrystallite size d (the coherent scattering region (CSR)size) on the annealing temperature found from theknown Debye–Scherrer equation by the broadeningof X�ray diffraction peaks d = 0.9λ/(βcosθ), where λ isthe wavelength equal to 0.154184 nm (CuK

α radia�

tion), β is the peak width at the half height; and θ is thebeam incidence angle.

Figure 5 shows the particle morphology examinedusing electron scanning microscopy (SEM) on aJEOL ISM 6700F microscope and high�resolutiontransmission electron microscopy (HRTEM) on anHRTEM JEM�2010 JEOL transmission electronmicroscope.

The micrographs confirm an isometric particleshape close to spherical shape. The curves of the sizedistribution of particles shown in Fig. 5c were obtainedby a statistical processing of the HRTEM micro�graphs. The average particle sizes D coincide fairly wellwith the sizes obtained by the BET absorption methodfor annealing at 700 and 1300°C (Figs. 4c, 4d, 5a). Themicrograph (Fig. 5b) shows the crystallite coalescentinto particles.

4. LUMINESCENT PROPERTIES OF THE SAMPLES

The results presented above show that the Y2O3 : Eu3+

oxide samples annealed at temperatures higher than700°C with marked crystallinity and without hydro�carbonates are interesting for studying the propertiesof phosphors. Figure 6 depicts the excitation (λem =612 nm) and luminescence (λex = 240 nm) spectra ofthe obtained phosphors with Eu3+ concentrations of5 and 11 at %. The luminescence spectra were studiedusing a VARIAN Cary Eclipse fluorescence spectro�photometer with the excitation wavelength 240 nm atroom temperature. The characteristic changes in theintensities and the maximum positions are intermedi�ate for 7 and 9 at % Eu3+, and, because of this, they arenot shown.

As is seen, an increase in the Eu3+ concentrationand the annealing temperature leads to an increase inthe intensity of the excitation band and the emissionpeak λem = 612 nm. Similar variations with the Eu3+

concentration are due to the increase in the number ofthe luminescence centers, and the effect of tempera�ture is associated with the known effect of increase inthe degree of crystallinity of the phosphor [12–15](the intensity of the diffraction reflections and theCSR size increase (Figs. 2 and 4d)). As the Eu3+ con�

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Fig. 5. (a) SEM micrograph of the sample containing 9 at %Eu, 1300°C. (b) HRTEM micrograph of the samples con�taining 7 at % Eu, 1000°C. (c) Size distribution of particleswith 7 at % Eu at different annealing temperatures Tann

and quadratic regression coefficient R2: (1) Tann = 700°C,

R2 = 0.900; (2) Tann = 900°C, R2 = 0.847; and (3) Tann =

1000°C, R2 = 0.811. Arrows show the FWHM of thepeaks.

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POMELOVA et al.

centration increases, the blue shift of the excitationspectrum maximum for λem = 612 nm increases at lowannealing temperatures. By analogy with the knownexplanation of the blue shift with an increase in thesample crystallinity [12], the increase in the Eu3+ con�centration also causes the increase in the sample crys�tallinity, which follows from the increase in the CSRsize with the Eu3+ concentration. For example, in thecase of annealing at 1000°C, the CSR sizes of the sam�ples are 32, 42, 52, and 54 nm at the Eu3+ concentra�tions of 5, 7, 9, and 11 at %, respectively, according tothe XRD data. This fact agrees with the known phe�nomenon of accelerating the crystal nucleation in thepresence of impurities (in this case, Eu3+ ions). There�fore, under the same conditions of crystallization, thecrystallite size will be larger in the presence of higherimpurity content.

The increase in the annealing temperature leads tothe increase in the particle sizes D and crystallite sizesd (Figs. 4c and 4d) due to the processes of their grow�ing and sintering. In this case, the luminescence inten�sity of samples I612 increases monotonically (Fig. 7).For comparison, the dashed line in Fig. 7 indicates thelevel of the relative luminescence intensity of the samespectrum band for commercial phosphor Y2O3 : Eu3+

(Toshiba, production of 2005) with optimal concen�tration 4.3 at % Eu3+, and the solid straight line B(Fig. 7a) indicates the relative luminescence intensityof an Eu2O3 powder at a level of 25 arb. units.

The intensity I612 monotonically increases with D andd, which is also characteristic of the micron�sized phos�phors with the Eu3+ concentration lower than 5 at %. Themicron�sized samples with a concentration higherthan 5–6 at % Eu3+ demonstrate the luminescencequenching [12, 23]. As is seen, the submicron�sized

Fig. 6. (a) Luminescence excitation spectra (λem = 612 nm) and (b) luminescence spectra (λex = 240 nm) of the samples with5 at % Eu. (c) Luminescence excitation spectra and (d) luminescence spectra for the samples with 11 at % Eu.

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samples with the nanocrystallite structure have higherluminescence intensity I612 at lower D and d for theEu3+ concentrations higher than 9 at %. The quench�ing in these samples is not observed at such Eu3+ con�centrations. These facts agree with the known concen�trations of quenching of nanostructured Y2O3 : Eu3+

phosphors (13 at % for 40�nm particles and 18 at % for5�nm particles [12, 23]. It is interesting to elucidatethe nature of the size effect of increase in the lumines�cence intensity of the submicron�sized phosphor atthe Eu3+ concentrations higher than the quenchingconcentration of the micron�sized phosphor.

5. SIZE EFFECTS AND LUMINESCENCE

It is known [2, 8, 24] and confirmed by our resultsthat the phosphor luminescence intensity increases asthe degree of crystallinity characterized by theincrease in the CSR size (crystallite size) increases.Figures 4d and 7b show that the CSR size increaseswith annealing temperature and Eu3+ concentration(for the samples with the same luminescence intensity)and, according to estimations, is ~300 nm at 11 at %Eu3+ and Tann = 1300°C. We take estimation values ofthe CSR size for the XRD reflections whose widths atthe half�height are close to this parameter for theinternal standard, namely, polycrystalline silicon. Theincrease in the CSR size means the increase in thelong�range order of the crystal lattice of the Y2O3

phosphor matrix. Along with this, the study of the X�ray diffraction pattern of the Toshiba phosphor showsthat the CSR size reaches 400 nm; i.e., its crystallinityis substantially higher. Therefore, the degree of long�range order of the crystal structure of submicron�sizedparticles is not a dominant factor for increase in theluminescence intensity.

Another factor of changing the luminescenceintensity with change in the submicron sizes of thephosphors can be the change in the short�range orderin the environment of the Y2O3 matrix cations. Thechange in the short�range order of the crystal structurecan be measured by Raman spectroscopy. Figure 8shows the spectra of the samples with different Eu3+

concentration annealed at different temperatures(Spex Triplemate spectrometer).

For Eu3+ concentrations of 9 and 11 at %, thedegree of short�range order of the Y3+ ion environ�ment increases with annealing temperature, which ismanifested as the rectification of the basis line due tothe decrease in the background luminescence. As isclear from the chemical analysis date presented above,the system does not contain foreign impurities thatcould be responsible for the background lumines�cence. This implies that the background luminescenceis associated with the imperfection of the local struc�ture. The quantitative estimations of the short�rangeorder were performed measuring the full width at half�

maximum (FWHM) of the most intense mode at372 cm–1 (the Ag term, Y–O bond vibrations). It isfound (Fig. 8) that FWHM increases with the Eu3+

concentration (samples containing 7–11 at % Eu3+),and this fact demonstrates the increase in the numberof regions with distorted short�range order in the envi�ronment of Y3+ ions in crystallites. In this case, thesamples with distorted short�range order show lowerluminescence intensity. Recall that the increase inCSR, i.e., the increase in the crystallite sizes leads tothe increase in luminescence intensity I612. It is thecompetition of these effects that determines the inte�gral luminescence efficiency. For comparison, westudied the Raman spectrum of the Toshiba sample,and found that FWHM for the Ag term is 9.76 that isslightly lower than FWHM of the submicron�sizedphosphor samples with the Eu3+ concentration 9–

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Fig. 7. Dependences of the luminescence intensity atλem = 612 nm on (a) particle size D and (b) crystallite size

d for Eu3+ concentrations of (1) 11, (2) 9, (3) 7, and(4) 5 at %. All the dependences are normalized to theintensity.

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POMELOVA et al.

11 at %. This fact indicates on a more ordered envi�ronment of Y3+ ions as the basis of the crystal matrix inthe commercial phosphor. Thus, the submicron�sizedphosphor Y2O3 : Eu3+ (9–11 at %) has a worse crystal�linity and lower degree of short�range order of the lat�tice, but its luminescence efficiency is higher.

The luminescence of inorganic crystalline materi�als doped with specific impurities can be caused by thecharge transitions with a radiative recombinationbetween the conduction band and the valence band [1,2] or transitions in the energy gap with participatingimpurity levels, or charge transfer from the excitedlevel to the ground stable level of active ions, for exam�

ple, 5D0–7Fj of the Eu3+ ion. Figure 9 shows the diffuse

reflectance spectra for the phosphor containing twolimiting Eu3+ concentrations of 5 and 11 at %. Forother studied concentrations (7–9 at % Eu3+), theresults are similar. The found optical energy gap widthis ~5 eV, and it is not changed with an increase in theannealing temperature and the Eu3+ concentration:for all the samples, the fundamental band edge inter�sects axis x in the same point at the complete absorp�tion level. This means that the energy band transitionsof yttrium oxide do not participate in the lumines�cence process. The absence of absorption bands in thehigh transmission region indicates the absence of

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Fig. 8. Raman spectra of (a) samples with different Eu3+ concentrations of (1) 11, (2) 9, (3) 7, and (4) 5 at % after annealing at900°C and (b) samples containing 9 at % Eu and annealed at different temperatures of (1) 1300, (2) 1100, (3) 900, and (4) 700°C.(c) Dependences of FWHM for the Ag term of the Y–O bond vibrations on the Eu3+ concentration after annealing at tempera�tures of (1) 700, (2) 900, (3) 1100, and (4) 1300°C. (d) Dependences of the luminescence intensity I612 on the degree of short�

range order (FWHM for the Ag term) at Eu3+ concentrations of (1) 5, (2) 7, (3) 9, and (4) 11 at %.



intraband transitions. Therefore, the luminescence ofthe submicron�sized Y2O3 : Eu3+ samples is only dueto the charge transfer between the energy levels of Eu3+

ions.As is known, the unit cell of c�Y2O3 consists of 24

Y3+ cations in the noncentrosymmetric C2 environ�ment with O2– ions and 8 Y3+ cations in the cen�trosymmetric (inversion center) C3i environment withO2– ions. In both states, the yttrium cation is sur�rounded with 6 oxygen anions [2]. The ionic radii ofY3+ and Eu3+ are 0.90 and 0.94 Å, respectively, andthey are fairly close in order for the matrix cubic struc�ture to be conserved with insignificant change in thelattice parameter as Eu3+ replace in part Y3+ [25]. It isknown as well that the characteristics of the lumines�cence spectrum of phosphor Y2O3 : Eu3+ can bedescribed by so�called “asymmetric ratio” of theintensities I612/I596 of the luminescence peaks at 612and 596 nm, which is identical to the ratio of the num�bers of cations Eu3+ in two states of the local symmetryNC2/NC3i [15, 25–28]. The luminescence band inten�sity I612 is determined by dipole�electrical 5D0 7F2

transition that is hypersensitive to the nearest environ�ment of the Eu3+ ion. The luminescence band at596 nm is determined by the dipole�magnetic 5D0 7F1 transition insensitive to the nearest environment[12, 25, 28, 29]. The ratio of the amplitude or integralintensities of the luminescence bands I612/I596 are usedto estimate ratio NC2/NC3i [26, 29, 30] and to explainthe effect of the Y2O3 : Eu3+ phosphor particle sizes onthe luminescence spectra [15, 31, 32]. Unfortunately,the results of those studies inadequately agree and par�tially contradict to the experimental results of thiswork. This fact is associated with the insufficientlycomplete physicochemical and chemical (impurity)characterization of the materials under study and alsoto different temperatures (room, cryogenic) of theluminescence spectroscopy and different wavelengthsof the luminescence spectrum excitation. Because ofthis, we continued the analysis of the origin of theabnormal increase in the luminescence intensity of thesubmicron�sized Y2O3 : Eu3+ (Fig. 7) in the absence ofthe concentration quenching of the luminescence at9–11 at % Eu3+ characteristic of the commercialmicrostructured Toshiba phosphor with the averageparticle size 6 μm and the crystallite size 400 nm. Theanalysis of the optical characteristics was mainly per�formed for the samples annealed at temperatureshigher than 1000°C, because the annealing at thesetemperatures completely removes OH groups and CO2

from the product; as known, OH groups and CO2

entering in the compositions of hydrates, hydrocar�bonates, and yttrium carbonates suppress the phos�phor Y2O3 :Eu3+ luminescence [15, 25].

Figure 10 shows the results of processing of theluminescence spectra (Fig. 7) with the determination

of the change in the asymmetric ratio of luminescenceI612/I596 in the dependence on the average sizes of theparticles and crystallites at the constant relative inten�sity 250 arb. units of the band at 612 nm (section A),the concentration of the Eu3+ dopant, and also thedopant concentration at the constant crystallite size60 nm (section C).

We note a high value of ratio I612/I596 = NC2/NC3i =(7–9)i = (8–10)a for the Eu3+ ions in the studied sam�ples, where index i indicates the integral peak height,and index a is the amplitude value with respect toequilibrium ratio 3/1 for the Y3+ ions in the oxidematrix [2]. This result indicates the formation of irreg�ular solutions of yttrium and europium oxides in theprocess of synthesizing the phosphor under consider�ation.

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Fig. 9. Diffuse reflectance spectra of the submicron�sizedsamples containing (a) 5 and (b) 11 at % Eu3+ for differentannealing temperatures of (1) 700, (2) 900, (3) 1100, and(4) 1300°C.

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POMELOVA et al.

As is seen, the ratio I612/I596 has the tendency toincrease as the particle size D and crystallite size ddecrease. This ratio increases with increasing NEu. AsNEu increases in the range of 7–11 at %, ratio I612/I596

remains almost constant at the constant luminescenceintensity I612 = 250 arb. units. With the allowance forthe data of Fig. 7, it means that, to maintain a constantluminescence intensity with a decrease in the particleand crystallite sizes, it is necessary to increase the Eu3+

dopant concentration. In this case, the concentrationluminescence quenching in the band at 612 nm is notobserved in the submicron�sized samples at the dopantcontents lower than, at least, 11 at %.

As the particle and crystallite sizes decrease at aconstant Eu3+ ion concentration, the number of theions in the C2 state at the surfaces of the particles andcrystallites increases due to the reduction of the localsymmetry of surrounding the Eu2+ ions by oxygen ionsO2–. It implies that intensity I612 should increase as thecrystallite and particle sizes decrease. However, Fig. 7demonstrates the decrease in the luminescence inten�

sity with a decrease in the crystallite and particle sizes.Even the increase in the effective emitting surface areaproportional to the increase in the specific surface areaof the powder sample with a decrease in the particlesize (Fig. 4a) does not compensate the noted decreasein the luminescence intensity. The noted specific fea�tures can be associated with the following variants ofchanging the luminescence efficiency of the submi�cron�sized phosphor.

(1) At the same Eu3+ ion concentrations on the sur�face or in the bulk of the particle or crystallite, one partof these ions on the surface is in the state C2, another,smaller, part is in the state C3i, and the third part ofthese ions is distributed in the states with a completeloss of the symmetry of the local environment Cd. Thethird part ions do not have luminescence properties orthey are strongly suppressed. Among these ions can beEu3+ ions existing in a specific surface state [31],which provides the radiative charge transition 5D0 7F2 with the emission peak at 620 nm. However, in ourcase, such peak is not observed even as a shoulder.

7

8

9

10

11A

sym

met

ric

rati

o

0 200 400 600 800 1000 1200 1400

Particle size D, nm

I612 = 250 arb. unitsIntensityIntegral

7

8

9

10

11

0 40 80 120 160

Crystallite size d, nm

IntensityIntegral

(a) (b)

I612 = 250 arb. units

7

8

9

10

11

Asy

mm

etri

c ra

tio

4 6 8 10 12

Asy

mm

etri

c ra

tio

Concentration Eu, at %

I612 = 250 arb. units

IntensityIntegral 7

8

9

10

11

Asy

mm

etri

c ra

tio

4 6 8 10 12Concentration Eu, at %

d = 60 nm

(c) (d)

IntensityIntegral

Fig. 10. Dependences of the asymmetric ratio I612/I596 at a constant luminescence intensity I612 on (a) the particle size, (b) the

crystallite size, and (c) the Eu3+ concentration. (d) Dependence of the ratio I612/I596 on the Eu3+ concentration at a fixed crys�tallite size.



Thus, in this variant, the decrease in the crystallite andparticle sizes will favor the increase in the Eu3+ contentin state C2 in the surface layer.

(2) If the Eu3+ ions are surface�inactive impuritiesin the Y2O3 matrix, their concentration on the surfaceis lower than that in the bulk. In this case, with adecrease in the particle and crystallite sizes, thedopant concentration will mainly increase in the par�ticle bulk and will cause the concentration lumines�cence quenching. The increase in the Eu3+ concentra�tion in the sample will cause even stronger lumines�cence quenching, and this fact makes it impossible tomaintain or increase the luminescence intensity.

(3) If the Eu3+ ions are surface�inactive impuritiesin the Y2O3 matrix, their concentration on the surfaceis higher than that in the bulk. In this case, thedecrease in the particle sizes with the same Eu3+ con�centration in the sample will be associated with thetransition of Eu3+ ions in states C2 and C3i from thebulk to the surface layer, predominantly, to state C2. Itis like that this redistribution is insufficient for theluminescence efficiency to be conserved or increased(Fig. 7).

Figure 10d demonstrates the increase in ratioI612/I596 as the Eu3+ dopant concentration increases inthe samples with one crystallite size (for example,60 nm; section B in Fig. 7). Similar tendency is alsoobserved at a constant particle size, but it is not shownhere. This implies that ratio NC2/NC3i increases. In[27], it is shown that the ratio is almost unchanged inthe samples with the particle size ~10 nm: it is 4.7, 4.3,and 5.2 at the Eu3+ concentrations of 3.0, 5.0, and10.0 at %, respectively. The obtained results for thesubmicron�sized samples demonstrate that theincrease in the Eu3+ dopant concentration results inthe increase in the luminescence intensity, and, there�fore, the content of Eu3+ ions in state C2 increases.These facts and the increase in NC2/NC3i should beassigned to the redistribution of Eu3+ ions on the par�ticle or crystallite surfaces. In the opposite case, weshould observe the concentration quenching, as it wasobserved in the known case [27], at least, for the con�centration 10 at % Eu3+. The conclusion gives thegrounds to consider Eu3+ as surface�active ions in theY2O3 matrix (Section 3). In the bulk of the crystallites,the Eu3+ content also slightly increases, because theprecise XRD method demonstrates an insignificantincrease in the unit cell volume [27], which is prima�rily associated with the bulk of the crystallites.

Thus, the increase in luminescence intensity I612 inthe submicron�sized samples is due to the structuralredistribution of dopant Eu3+ in surface layers of theparticles and crystallites. The complete description ofthis effect mechanism needs more detailed study of thechemical and structural composition of the phosphorsurface layers.

6. CONCLUSIONS

It was found that the powder phosphor Y2O3 : Eu3+

has an abnormally high luminescence efficiency in theband at 612 nm for particle sizes of 250–1000 nm,crystallite sizes of 60–160 nm, and Eu3+ concentra�tions higher than 9 at %. The luminescence intensityof these samples increases by 50% as compared to thatof micron�sized commercial phosphors.

The nature of increase in the luminescence inten�sity is associated with the chemical and structuraltransformations of surface layers of submicroparticlesand nanocrystallites with the enrichment in the Eu3+

dopant with respect to the bulk of the crystallites. Thisredistribution is due to the increase in the number ofEu3+ ions in the reduced symmetry state C2. Theunderstanding of the mechanism of these transforma�tions requires precise crystal chemical studies of sur�face layers of particles and crystallites.

ACKNOWLEDGMENTS

This study was supported by the Siberian Branch ofthe Russian Academy of Sciences within the frame�work of Integration Project No. 40 (2012).

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Translated by Yu. Ryzhkov

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On the abnormal efficiency of the luminescence of submicron-sized phosphor Y2O3 : Eu3+