Infrared photoluminescence of sol–gel spin-coated films of rare-earth activated lanthanum silicate H.K. Juwhari a,1 , M.H. Kailani b , B.I. Lahlouh a , S.A. Abedrabbo a , K.A. Saleh a , W.B. White c,n a Physics Department, University of Jordan, Amman 11942, Jordan b Chemistry Department, University of Jordan, Amman 11942, Jordan c Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA article info Article history: Received 16 June 2012 Accepted 24 July 2012 Available online 2 August 2012 Keywords: Phosphors La-silicate Sol–gel processing Luminescence Er 3 þ -activator abstract A series of phosphors with composition La 9.33x y z Si 6 O 24 O 2 (x ¼Er, y ¼Ce, z ¼Yb) with the silicate oxyapatite structure were synthesized by a sol–gel spin-coating technique. The products formed as well-crystallized laminae 6–8 nm thick. Spectroscopic measurements in the near-infrared showed that excitation through Ce 3 þ enhances 1.53 mm emission of Er 3 þ by a factor of 50, while the effect of Yb 3 þ is to quench the luminescence. & 2012 Elsevier B.V. All rights reserved. 1. Introduction The incorporation of a single rare-earth activator including Er 3 þ , Ce 3 þ , Tb 3 þ , or Eu 3 þ in silicon or silicate complexes has recently received wide attention [1–4]. Nevertheless, preparing single phase RE–silicate complexes with high photoluminescence (PL) efficiency remains difficult. Preparation methods to avoid and/or overcome multi-phase formation include hydrothermal, precipitation and co-precipitation, molten flux, sol–gel, and phy- sical and chemical vapor deposition techniques [5–8]. Er 3 þ -activated phosphors have been of interest as up-converters [9], as mid-infrared lasers [10,11] but most recently in fiber-optic and other communications applications because of the emission peak at 1.53 mm [12]. The luminescent emission spectra of Ce 3 þ , Tb 3 þ , and Eu 3 þ - activated lanthanum silicate compounds have been reported [13]. In the present work, we extend the investigation to Er 3 þ - activated/co-activated La 9.33 Si 6 O 24 O 2 and consider the influence of Ce 3 þ and Yb 3 þ . 2. Phosphor host structure The prototype compound for the apatite structure is Ca 5 (PO 4 ) 3 F also written as the doubled formula, Ca 10 P 6 O 24 F 2 . The structure is hexagonal, space group P6 3 /m with one doubled formula unit/unit cell. There are two cation sites with 4Ca 2 þ on 4f sites, site symmetry C 3 , and 6Ca 2 þ on 6h sites, site symmetry C s (C 1h ). The phosphate ions are isolated tetrahedra but are slightly distorted from their ideal tetrahedral symmetry. The F  ions occupy the special 2a sites with S 3 site symmetry. The apatite structure is extremely tolerant of many types of ionic substitution with only the requirement that local charge balance be maintained [14]. The silicate oxyapatites are derived by replacing the PO 3 4  ions with SiO 4 4  ions and the F  ions by O 2  ions [15]. The excess negative charge can be compensated by replacing part or all of the cations by trivalent rare earths. Partial replacement leads to the family of compounds NaLn 3 Ln 6 Si 6 O 24 O 2 and Sr 2 Ln 2 Ln 6 Si 6 O 24 O 2 [16]. Complete replacement by rare earths requires cation vacan- cies to maintain charge balance. In the silicate apatites, the 4f sites have 9-fold coordination while the 6h sites have 7-fold coordination so that the formula may be written Ln 3.33 IX & 0.67 Ln 6 VII Si 6 IV O 24 IV O 2 III [17]. 3. Experimental methods Compounds with the general composition La 9.33 x y z Ln x Ln y ’Ln’’ z Si 6 O 26 where Ln, Ln’, and Ln’’ ¼ Er 3 þ , Ce 3 þ , and Yb 3 þ respectively, were prepared by the sol–gel method accompanied by a spin-coating technique that has proved useful for other rare- earth apatite phosphors [18]. While maintaining the intended compositions, the precursor solutions were prepared by mixing stoichiometric amounts of La(NO 3 ) 3  6H 2 O powder (Schalau Che- mical 99.9% purity) into a solution of 3 mL acetic acid used as Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.095 n Corresponding author. Tel.: þ1 814 667 2709. E-mail addresses: h.juwhari@ju.edu.jo (H.K. Juwhari), wbw2@psu.edu (W.B. White). 1 Tel.: þ965 6 5355000x22062. Materials Letters 87 (2012) 80–83