Brossmann, U.

Hofer, F.

Kautsch, A.

Krenn, H.

Szabo, D. V.

Würschum, R.

English

KITopen

Structural and optical properties of nanoparticulate Y2O3:Eu2O3made by microwave plasma synthesisA. Kautsch · U. Brossmann · H. Krenn · F. Hofer ·D.V. Szabó · R. WürschumAbstract Nanoparticulate Y2O3:Eu2O3 with a small, uni-form particle size and a well-defined composition was syn-thesized using a low temperature microwave plasma pro-cess. The structural evolution and the luminescence prop-erties were studied in different states of annealing andEu2O3 addition using X-ray diffraction, transmission elec-tron microscopy, and UV-photoluminescence spectroscopy.As synthesized, the samples were amorphous and showedonly weak luminescence. Subsequent annealing steps fromA. Kautsch · U. Brossmann () · R. WürschumInstitute of Materials Physics, Graz University of Technology,8010 Graz, Austriae-mail: brossmann@tugraz.atFax: +43-316-8738980A. Kautsche-mail: andreas.kautsch@tugraz.atR. Würschume-mail: wuerschum@tugraz.atPresent addressA. KautschInstitute of Experimental Physics, Graz University of Technology,8010 Graz, AustriaH. KrennInstitute of Physics, University of Graz, 8010 Graz, Austriae-mail: heinz.krenn@uni-graz.atF. HoferInstitute of Electron Microscopy and Fine Structure Research,Graz University of Technology, 8010 Graz, Austriae-mail: ferdinand.hofer@tugraz.atD.V. SzabóKarlsruhe Institute of Technology, Institute of AppliedMaterials—Materials Process Technology, 76344Eggenstein-Leopoldshafen, Germanye-mail: dorothee.szabo@kit.edu500°C to 800°C lead to the formation and growth of cu-bic Y2O3:Eu2O3 nanocrystals (5–20 nm) and a concomitantstrong increase of the luminescence yield already at smallgrain sizes in the range of 10 nm. No self-quenching effectswere observed up to 11 mol% Eu2O3.1 IntroductionLuminescent rare earth oxides are in the focus of both tech-nical application and ongoing research as key materials foruse in display technology and lighting [1, 2]. Presently,Eu2O3, particularly in form of Y2O3:Eu2O3, constitutes thebest red phosphor for fluorescence tubes and plasma displaypanels to convert UV light from the plasma discharge to redlight [1, 3].Y2O3 and Eu2O3 preferably form cubic mixed crystalswith a high luminescence yield [1–5]. Both, the Y2O3 hostlattice, and O2− ions in the vicinity of Eu3+ ions, absorbUV-light with maxima in the range of 210 nm and 250 nm,respectively. Subsequent energy transfer to the Eu3+ lumi-nescence centres finally populates the 5D0 level [1, 3–5].Red light is emitted upon transition to one of the 7F lev-els, with the main emission line at 613 nm (5D0 → F2) andweaker lines between 580 and 595 nm [6].The effect of a very small crystallite size on the lumines-cence properties, however, remains an open question. On theone hand, some studies report a noticeable blue shift of boththe absorption range and the main emission line for particlesas large as 40 nm [3, 7]. On the other hand, there are also ob-servations of a red shift of the absorption spectra [4]. Sim-ilarly, studies in literature report observations of a reduced[1] as well as an increased luminescence of nanoparticlesversus the bulk [8]. In addition, lattice and surface defectswere found to significantly impair the luminescence yieldof Y2O3:Eu2O3, particularly for very small particles [1, 3].Therefore, Y2O3:Eu2O3 nanoparticles with a low degree ofinternal strains and agglomeration are expected to show im-proved luminescence.The present work aims at synthesizing and studying thestructural and luminescence properties of such Y2O3:Eu2O3nanoparticles with a small initial particle size. For this goal,microwave plasma (MWP) synthesis provides an ideal fab-rication route [9]. The low reaction temperature of about400◦C and the electric charging in the nonequilibriumplasma leads to the formation of nanoparticles with botha small mean size of d ∼4–5 nm and a narrow size distribu-tion.2 ExperimentalThe nanoparticulate Y2O3:Eu2O3 was produced by co-evaporation of the chemically homologous tetramethylhep-tanedionate Y(TMHD)3 and Eu(TMHD)3 precursors in anAr stream at about 180◦C. The precursor vapor was mixedwith a reaction gas of 20% oxygen in argon and then decom-posed in the microwave induced plasma (2.45 GHz, 500 W)under similar conditions as described for the synthesis ofYSZ [9]. Electric charging induced by the plasma togetherwith low reaction temperatures prevents the formation ofhard agglomerates. The nanoparticles were collected on acold finger. This powder was subsequently scraped off with arazor blade and placed in an Al2O3 boat for annealing treat-ments at temperatures between 350◦C and 800◦C in air.X-ray diffraction studies of the structural evolution ofnanoscaled (n)-Y2O3:Eu2O3 powders were made using aBruker D8 Advance instrument with a Cu tube. For trans-mission electron microscopy (TEM) studies using a PhilipsCM20 microscope, the n-Y2O3:Eu2O3 powder was dis-persed in 2-Propanol and deposited on a Cu grid.Photoluminescence (PL) studies on n-Y2O3:Eu2O3 andthe pure constituents were carried out on both powder sam-ples on glass and dispersions in ethanol (0.5 g/l). The lumi-nescence spectra were measured with a Perkin-Elmer LS55spectrometer using a high-energy pulsed Xe lamp as lightsource. A filter with a cut-off wavelength of 570 nm pre-vented spurious light from entering the analyzer.Two types of PL spectra were recorded for samples in allconditions of annealing and compositions: emission spectraof luminescence induced by excitation at fixed wavelengthsof 211 and 234 nm and so-called excitation spectra, whichshow the intensity of the main PL emission line at 613 nm asa function of the excitation wavelength. A wavelength scanrate of 100 nm/min and bandwidths of 5 nm and 2.5 to 5 nmfor excitation source and analyzer, respectively, were typi-cally used for these measurements.Fig. 1 X-ray diffraction spectra of Y2O3:Eu2O3 in different stagesof annealing. The symbols F and ! denote literature data forY2O3:Eu2O3 (5 mol%) and Al (specimen holder) from the PDF-2database, respectively [10]. No noticeable change is observed for sam-ples with Eu2O3 addition of 5.6 and 11 mol%3 Results and discussionUnder as-synthesized condition, both Eu2O3 doped (Fig. 1)and undoped Y2O3 nanoparticles show broad X-ray diffrac-tion lines. In complementary TEM studies a small parti-cle size and loose agglomerates of particles with nearlymissing crystalline features were observed (Fig. 2a). Hence,an amorphous state with an average particle size of about4.5 nm is deduced. Annealing at 350◦C did not induce asignificant change in the structural properties. Annealing at500◦C for 8 h lead to the formation of cubic nanocrystals ofan Y2O3:Eu2O3 solid solution (Fig. 1) or Y2O3, as shownby XRD. These data are consistent with literature data forbulk Eu0.1Y1.9O3 (JCPDS #25-1011) and Y2O3 (JPCDS#88-1040) in the PDF-2 database [10], respectively. No evi-dence for the formation of pure cubic Eu2O3 or monoclinicphases was found.From the XRD data, a small average crystallite size of4.6 ± 0.5 nm is determined based on the Scherrer formula[11], whereas a detailed analysis using the approach ofWilliamson–Hall [11] yields a value of 6.5 ± 0.5 nm andindicates a noticeable amount of microstrains. Additionalannealing at 650◦C or 800◦C (4 h) lead to crystallite growthreaching values of 9.5±0.5 nm and 18±1 nm, respectively.For samples annealed at 800◦C, the crystallite size and thecrystalline state was confirmed by TEM studies (Fig. 2b).Experimental data of the luminescence properties ofY2O3:Eu2O3 in dispersions are shown in Figs. 3, 4, and 5.In condition as synthesized and after annealing at 350◦C,the n-Y2O3:Eu2O3 specimens showed only weak lumines-cence due the reduced energy transfer in the amorphousstructure [12]. After annealing at 500◦C, the typical lu-minescence spectrum of Eu3+ was observed (Figs. 3, 4).Fig. 2 TEM micrographs of(a) Y2O3:Eu2O3 as synthesizedand (b) after the annealing stepat 800◦C (4 h, air)Fig. 3 Luminescence spectra of n-Y2O3:Eu2O3(11 mol%) in disper-sion upon excitation with wavelength λex = 234 nm in different statesof annealing. The plots are mutually shifted in vertical direction. Thecrystallite size is deduced from XRD line broadening (Scherrer)Higher annealing temperatures of 650◦C and 800◦C lead toa strong increase of the luminescence yield, but to no quali-tative change in the excitation and emission spectra such asshifts of the peaks with the grain size.Taking into account the structural data (s. Figs. 1, 2) itmay be concluded in agreement with literature data that acrystalline state is essential for the photoluminescence andUV conversion processes in Y2O3:Eu2O3 [3–7]. In fact, thePL yield is reduced for ultra small grain sizes (Figs. 3–5).However, we observed a significant PL yield already at crys-tallite sizes in the range of 5–10 nm (Fig. 3), smaller thanpreviously reported [3].Fig. 4 Excitation spectra for the luminescence at λem = 613 nm(5D0 → 7F2) measured on dispersed Y2O3:Eu2O3 particles. BulkEu2O3 is shown as a referenceA detailed look at the excitation spectra of n-Y2O3:Eu2O3specimen (Fig. 4) shows two maxima for wavelengths ofabout 211 nm and 234 nm in the far UV range, which canbe attributed to the host-lattice and charge transfer mecha-nisms, respectively [3–5]. The PL yield for both excitationwavelengths increases with the grain size (Fig. 4, Fig. 5)as well as with the Eu2O3 content (Fig. 5). Hardly any PLis observed for the as prepared n-Y2O3:Eu2O3 specimen atλex < 220 nm (Fig. 4). With higher annealing temperaturesand crystallite size the excitation at λex = 211 nm, corre-sponding to the host lattice transfer, becomes more domi-nant in comparison to λex = 234 nm, which may reflect amore efficient host-lattice energy transfer in larger grains[1, 3–5]. It is also interesting to note, that the relative inten-sity of the main emission line, which is attributed to Eu3+Fig. 5 Variation of photoluminescence yield in Y2O3:Eu2O3 withannealing treatment and Eu2O3 content for regions of host lattice(λex = 211 nm) and charge transfer excitation (λex = 234 nm)emission centres in an undistorted cubic lattice environment[13], increases for higher annealing temperatures (Fig. 3).These improved luminescence properties may be at-tributed to a reduced density of lattice and surface defectsas a result of the synthesis method used [12].No evidence for self-quenching was observed for anystate of annealing or Eu2O3 content up to the highest dopinglevel of 11 mol% studied. For bulk material, such a quench-ing effect was reported for Eu2O3 additions to Y2O3 aslow as 6% [14]. Assuming an isotropic distribution of Eu3+in the nanocrystals, the reduced self-absorption may be at-tributed to the large fraction of luminescence centers locateddirectly at the nanoparticles’ surface, which is expected toreduce this doping-dependent critical effect.Based on these results, it can be concluded that mi-crowave plasma synthesis combined with appropriate an-nealing is a powerful technique for the fabrication of ox-ide rare earth nanoparticles, which combine a well-definedsmall size and composition with good luminescence prop-erties. Nevertheless, the present study confirms that the lu-minescence yield of Y2O3:Eu2O3 is reduced for very smallgrain sizes. Slightly larger grains (d ≥ 9.5 nm) with a highdegree of crystallinity show a promising luminescence yieldtendency.Acknowledgements The authors thank A. Meingast, Graz Univer-sity of Technology, for TEM micrographs and S. Draxler, Universityof Graz for technical support.References1. R. Schmechel, M. Kennedy, H. von Seggern, H. Winkler, M.Kolbe, R.A. Fischer, Li Xaomao, A. Benker, M. Winterer, H.Hahn, J. Appl. Phys. 89, 1679 (2001)2. P.K. Sharma, M.H. Jilavi, R. Nass, H. Schmidt, J. Lumin. 82, 187(1999)3. A. Konrad, T. Fries, A. Gahn, F. Kummer, U. Herr, R. Tidecks, K.Samwer, J. Appl. Phys. 86, 3129 (1999)4. Q. Meng, C. Qingyu, B. Chen, X. Zhao, X. Wang, W. Xu,J. Nanosci. Nanotechnol. 8, 1443 (2008)5. A. Konrad, U. Herr, R. Tidecks, F. Kummer, K. Samwer, J. Appl.Phys. 90, 3516 (2001)6. G. Wakefield, E. Holland, P.J. Dobson, J.L. Hutchison, Adv.Mater. 13, 1557 (2001)7. Q. Li, L. Gao, D. Yan: Nanostruct. Mater. 8, 825 (1997)8. E.T. Goldburt, B. Kulkarni, R.N. Bhargava, J. Taylor, M. Libera,J. Lumin. 72–74, 190 (1997)9. U. Brossmann, M. Sagmeister, P. Pölt, G. Kothleitner, I. Letofsky-Papst, D.V. Szabó, R. Würschum, Phys. Status Solidi RRL 1, 107(2007)10. PDF-2, The Powder Diffraction File Rel. 2001 (The InternationalCenter for Diffraction Data, Newtown Square, 2001)11. H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Poly-crystalline and Amorphous Materials, 2nd edn. (Wiley, New York,1974)12. R. Schmechel, H. Winkler, Li Xaomao, M. Kennedy, M. Kolbe,A. Benker, M. Winterer, R.A. Fischer, H. Hahn, H. von Seggern,Scr. Mater. 44, 1213 (2001)13. S.C. Cho, H.S. Uhm, C.U. Bang, D.K. Lee, C. Soo, Thin SolidFilms 517, 4052 (2009)14. X. Ye, W. Zhuang, Y. Hu, T. He, X. Huang, C. Liao, S. Zhong, Z.Xu, H. Nie, G. Deng, J. Appl. Phys. 105, 064302 (2009)

Structural and optical properties of nanoparticulate Y₂O₃:Eu₂O₃ made by microwave plasma synthesis

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Structural and optical properties of nanoparticulate Y₂O₃:Eu₂O₃ made by microwave plasma synthesis

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