Up-conversion luminescence properties of Y 2 O 2 S:Yb 3+ ,Er 3+ nanophosphors Iko Hyppänen a,b , Jorma Hölsä a,c , Jouko Kankare a,c , Mika Lastusaari a,c , Laura Pihlgren a,d, * a University of Turku, Department of Chemistry, FI-20014 Turku, Finland b Graduate School of Chemical Sensors and Microanalytical Systems (CHEMSEM), Espoo, Finland c Turku University Centre for Materials and Surfaces (MatSurf), Turku, Finland d Graduate School of Materials Research (GSMR), Turku, Finland article info Article history: Received 17 July 2008 Received in revised form 20 November 2008 Accepted 15 December 2008 Available online 2 June 2009 PACS: 78.55.Hx Keywords: Yttrium oxysulfide Nanomaterial Flux method Up-conversion luminescence abstract Up-converting yttrium oxysulfide nanomaterials doped with ytterbium and erbium (Y 2 O 2 S:Yb 3+ ,Er 3+ ) were prepared with the flux method. The precursor oxide materials were prepared using the combustion synthesis. The morphology of the oxysulfides was characterized with transmission electron microscopy (TEM). The particle size distribution was 10–110 nm, depending on the heating temperature. According to the X-ray powder diffraction (XPD), the crystal structure was found hexagonal and the particle sizes estimated with the Scherrer equation agreeded with the TEM images. Upon the 970 nm infrared (IR) laser excitation, the materials yield moderate green (( 2 H 11/2 , 4 S 3/2 ) ? 4 I 15/2 transition) and strong red ( 4 F 9/2 ? 4 I 15/2 ) luminescence. The green luminescence was enhanced with respect to the red one by an increase in both the crystallite size and erbium concentration due to the cross-relaxation (CR) processes. The most intense up-conversion luminescence was achieved with x Yb and x Er equal to 0.10 and 0.005, respectively. Above these concentrations, concentration quenching occurred. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The field of up-conversion luminescence where the absorption of two or more low energy photons is followed by the emission of a higher energy photon has witnessed numerous breakthroughs during the past decades. Up-converting phosphors have several po- tential applications as e.g. lasers and displays [1,2] and in clinical diagnostic assays [3]. Up-converting nanophosphors with high luminescent efficiency are required for coupling to biological compounds. These phos- phors are also needed in the development of novel homogeneous label technology for quantitative all-in-one whole-blood immuno- assay which uses low-cost measurement devices [3]. Whole blood has no capability for up-conversion [3]. Neither there is autolumi- nescence nor absorption of luminescence in the red spectral region (635–710 nm). This enables immunoassays with low background luminescence signals. Whole-blood immunoassays are suitable for point-of-care applications and in resource-limited areas where the pretreatment of blood samples would be inconvenient. The most efficient up-converting phosphors operate with the combination of a trivalent rare earth (R 3+ ) sensitizer (e.g. Yb 3+ , Er 3+ or Sm 3+ ) and an activator (e.g. Er 3+ , Ho 3+ or Tm 3+ ) ion in an optically inactive host lattice [4]. The sensitizer is excited with an infrared (IR) radiation source and transfers this energy to the activator that emits a visible photon. In this work, the nanocrystalline Y 2 O 2 S:Yb 3+ ,Er 3+ materials were prepared with the flux method. Crystal structure as well as phase purities were analyzed and the crystallite sizes were estimated. Up-conversion luminescence was obtained at room temperature with NIR excitation. The effect of the crystallite size and Er 3+ con- centration on the intensity of the green and red luminescence are presented and discussed. 2. Experimental 2.1. Materials preparation 2.1.1. Combustion synthesis of oxide precursors The Yb 3+ and Er 3+ co-doped Y 2 O 3 nanocrystals with nominal Yb 3+ concentrations of 10 and 20 and Er 3+ concentrations of 0.5, 1, 2, 3, and 4 mol% of the yttrium amount were prepared with the combustion synthesis (Eq. (1)) [5]. 6RðNO 3 Þ 3 ðsÞþ 10NH 2 CH 2 COOHðsÞþ 18O 2 ðgÞ ! 3R 2 O 3 ðsÞþ 5N 2 ðgÞþ 18NO 2 ðgÞþ 20CO 2 ðgÞþ 25H 2 OðlÞ ð1Þ where R is Y, Yb and Er. The R(NO 3 ) 3 solutions were prepared by dis- solving Y 2 O 3 (99.99%), Yb 2 O 3 and Er 2 O 3 (both 99.9%) in hot, dilute 0925-3467/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.12.034 * Corresponding author. Address: University of Turku, Department of Chemistry, FI-20014 Turku, Finland. Fax: +358 2 3336700. E-mail address: laerle@utu.fi (L. Pihlgren). Optical Materials 31 (2009) 1787–1790 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat