Structural Characterization and Luminescence Properties of Nanostructured Lanthanide-Doped Sc2o3 Prepared by Propellant Synthesis
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Archivio istituzionale della ricerca - Università degli Studi di Venezia Ca' Foscari INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY Nanotechnology 17 (2006) 2805–2812 doi:10.1088/0957-4484/17/11/013 Structural characterization and luminescence properties of nanostructured lanthanide-doped Sc2O3 prepared by propellant synthesis RKrsmanovic´1,OILebedev1,ASpeghini2,MBettinelli2,SPolizzi3 and G VanTendeloo1 1 EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium 2 Dipartimento Scientifico e Tecnologico, Universit`adiVerona and INSTM, UdR Verona, Strada le Grazie 15, 37134 Verona, Italy 3 Dipartimento di Chimica Fisica, Universit`adiVenezia and INSTM, UdR Venezia, ViaTorino 155/b, 30172 Venezia, Mestre, Italy E-mail: [email protected] Received 15 February 2006, in final form 3 April 2006 Published 16 May 2006 Online at stacks.iop.org/Nano/17/2805 Abstract Nanocrystalline powders of undoped and lanthanide-doped scandium oxide were prepared by propellant synthesis and characterized by x-ray powder diffraction, electron microscopy, EDX spectroscopy and luminescence spectroscopy. The obtained material has the Sc2O3 cubic structure (space group Ia3)¯ with unit cell parameter increasing with the size of the dopant. Thecrystallite size is in the range 20–40 nm. The lanthanide-doped samples form Sc2−x Lnx O3 solid solutions with x ≈ 0.2(Ln= Eu or Er). No inhomogeneity was found by microanalysis on the micron scale. The 3+ emission spectrum of the Eu doped Sc2O3 sample shows strong bands in the visible region assigned to 4f–4f transitions of the lanthanide ions. 1. Introduction temperature are employed to prepare Sc2O3 powders, such as an electrochemical method (at 950 ◦C) [8], a reverse-strike Rare earth doped sequioxides yttria (Y2O3),lutetia (Lu2O3) precipitation technique [9]and hydrothermal synthesis [10]. and scandia (Sc2O3) have been found to be interesting These methods provide crystalline materials with a high materials for many technological applications in the field surface area. of optical devices, such as luminescent displays, optical On the other hand, the study of the luminescence amplifiers and solid state lasers [1–4]. Scandium oxide, properties of lanthanide doped nanocrystalline oxide particles in particular, has recently attracted the attention of many has become of paramount importance, due to the fact that researchers for its interesting physical and chemical properties. the optical and electronic properties of these nanocrystalline Its high chemical stability, together with a high bulk refractive materials differ from those of the bulk samples and can be index value (1.90 at 400 nm) and a high ultraviolet cut-off influenced by the particle size [11]. Many investigations (215 nm) [5], makes it interesting for numerous applications have been carried out on lanthanide doped nanocrystalline in the field of photonics and optoelectronics. Moreover, Y2O3 and Lu2O3 [12, 13], but only a few papers have been due to its high thermal conductivity, trivalent rare earth published on rare earth doped nanocrystalline Sc2O3 [14, 15]. doped scandia is a very suitable host material for high Oneofthe recently employed preparation techniques of power solid-state lasers [6]. The growth of a Sc2O3 single nanocrystalline materials is based on a combustion (propellant) crystal by the conventional thermal heating–cooling method synthesis. This technique is very useful for the preparation is difficult due to the extremely high melting temperature of rare earth doped luminescent materials in the form of (2430 ◦C) [7]. On the other hand, efficient methods at lower nanocrystalline powders [15–18], and makes it possible to 0957-4484/06/112805+08$30.00 © 2006 IOP Publishing Ltd Printed in the UK 2805 RKrsmanovi´c et al optimize the parameters such as particle size and uniform procedures [23, 24]: each peak is described by a couple of distribution of activators, which influence the luminescence constrained pseudo-Voigt functions (Kα1andKα2profiles) properties. Activators concentrated on the surface or on and the background by a polynomialfunction; parameters are the grain boundaries are thought to be non-luminescent or optimized by the simplex method. In accordance with the quenching centres [19]. In particular for low voltage excitation, Wagner method [25, 26], cell-edge values were extrapolated luminescent intensities are sensitive to the presence of by applying a weighted least-square linear fit to the lattice contamination on the phosphor surface [20]. Hence, obtaining parameter values calculated from the position of 20 reflections defect-free and surface-contaminant-free, homogeneously in the 2 range of 20◦–86◦ as a function of cos cot . doped phosphor nanocrystalsisvery important. From the profile analysis of the XRD lines, average crystallite In this paper, we show that scandium oxide in size and microstrains were determined by using the Warren– nanocrystalline form can be successfully obtained with the Averbach method [27, 28]onthe 222/444 pairs of reflections. above-mentioned propellant technique. We investigated the By using the Fourier coefficients of two different peaks from structure and morphology of the synthesized powders, in the same family of crystallographic planes, the distribution particular the aggregation and particle size distribution, as of crystallite dimensions and the corresponding distribution well as the luminescence properties for the lanthanide-doped of microstrains ε2(D) were obtained. Both distributions nanocrystalline scandia samples. refer to the direction perpendicular to the (hkl)family of crystallographic planes. The value of the volume-weighted ε2( ) 2. Experimental details average crystallite size, D v,andthevalue of D at Dv/2, taken as a measure of the average microstrains, 2.1. Synthesis were calculated. The instrumental broadening was previously deconvolved by using the Stokes’s method adapted to Nanocrystalline powders of Sc2O3 undoped and doped with analytically defined profiles [23, 24]. 3+ 3+ = 10 mol% of Eu or Er ,Sc1.8Ln0.2O3 (Ln Eu or Er), Scanning electron microscopy (SEM) observations were were prepared using a propellant synthesis [21, 22]. An performed with a JEOL JEM-5510 equipped with an INCA aqueous solution containing appropriate quantities of glycine x-ray microanalysis unit. The average composition of the ( ) ( ) ( ) NH2CH2COOH and metal nitrates, Sc NO3 3 and Ln NO3 3 samples was determined by x-ray energy dispersion (EDX) (Ln = Er or Eu) was prepared starting from high purity spectroscopy acquiring a spectrum for 600 s (live time) at an (99.9%) reagents. The glycine serves as a fuel for the accelerating voltage of 30 kV. For this analysis, powders were combustion reaction, being oxidized by the nitrate ions. A cold-pressed into pellets of 3mmdiameter under a load of glycine-to-metal nitrate molar ratio of 1.2 was employed. The about 2 tons of pressure andthen left uncoated. mixture was heated with a Bunsen flame until the solvent Transmission electron microscopy (TEM) was used to evaporated and auto-combustion occurred with the evolution obtain detailed information about the local structure and size of a brown fume. The synthesisreaction can be described by distribution of the crystalline scandia nanoparticles. Electron the following stoichiometric equation: diffraction and EDX analysis were carried out on a Philips ( ) + + → 3 CM20 microscope, equipped with a LINK-200, operating at 3M NO3 3 5NH2CH2COOH 9O2 2 M2O3 200 kV. High resolution electron microscopy (HREM) was + 5 N ↑+9NO ↑+10CO ↑+25 H O↑ 2 2 2 2 2 2 performed on a JEOL 4000EX electron microscope (0.17 nm where M stands for Sc or the rare earth dopant (Eu or Er). point resolution) operating at 400 kV. TEM specimens After a few seconds, a porous voluminous powder was formed, were prepared by ultrasonically dispersing the finely crushed occupying the entire volume of the reaction vessel. powdered sample in ethanol and depositing drops of this The resultant powder of the combustion reaction was suspension on a holey carbon film grid. The CrystalKitX and ◦ fired for 1 h at 500 Cinordertodecompose any residual MacTempas software packages were used to simulate HREM carbohydrazide and nitrate ions. All nanocrystalline samples images and electron diffraction patterns. were kept in air without any further precaution. 2.3. Spectroscopic characterization 2.2. Structural investigation The 488.0 nm line of an argon laser was used to excite the Themorphology, microstructure and local composition of luminescence spectra. A fibre optic probe was employed the obtained scandia powders were studied by x-ray powder to collect the emission signal. The scattered signal was diffraction, electron microscopy and EDX analysis. analysed using a half-meter monochromator equipped with APhilips X’Pert vertical goniometer with Bragg– a CCD detector. A 1200 lines mm−1 grating was used Brentano geometry, connected to a highly stabilized generator, to collect the luminescence spectrum in the region of the 5 7 3+ was used for x-ray diffraction (XRD) measurements. Co Kα D0 → F0 transition of the Eu ion (578–583 nm, resolution Fe-filtered radiation (λ = 1.788 97 A),˚ a graphite of about 0.02 nm), whilst the laser-excited luminescence monochromator on the diffracted beam and a proportional spectrum in the wider 500–700 nm range was measured using counter with pulse height discriminator were used. The a 150 lines mm−1 grating (resolution of about 0.15 nm). XRD data were collected at room temperature in the 2 All the spectroscopicmeasurements were performed at room (Bragg angle) range of 20◦–100◦ with a step size of 0.05◦ temperature. and acquisition time of 10 s per point for three runs The Eu3+ ion emission decay curve was measured and then averaged. Diffraction data were analysed by using the second harmonic radiation (532.0 nm) of a Nd– fitting limited angular regions using previously described YAG pulsed laser as the excitation source. The scattered 2806 Nanostructured lanthanide-doped Sc2O3 Figure 2. SEM image of the Sc2O3 sample. Figure 1.