IP: 79.133.107.131 On: Mon, 26 Mar 2018 16:40:18 Copyright: American Scientific Publishers Delivered by Ingenta RESEARCH ARTICLE Copyright © 2005 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 5, 1480–1486, 2005 Strong Visible Cooperative Up-Conversion Emission in ZrO 2 :Yb 3+ Nanocrystals E. De la Rosa, 1 ∗ P. Salas, 2 L. A. Díaz-Torres, 1 A. Martínez, 3 and C. Angeles 2 1 Centro de Investigaciones en Óptica, A.P. 1-948, León Gto., 37160 México 2 Instituto Mexicano del Petróleo, Ingeniería Molecular, A.P. 14-805, D.F., 07730 México 3 Universidad Autónoma Metropolitana—Azc.,DCBI, Sn Pablo 180, D.F., 02200 México Blue, green, and red emission was observed under infrared excitation in ZrO 2 :Yb 3+ nanocrystals prepared by the sol–gel process. The structural characterization was performed by using XRD and HRTEM, suggesting that the crystalline phase of the nanoparticles is controlled by the active ion concentration being mainly tetragonal for 2 mol% of dopant and mainly monoclinic for 0.5 mol%. The blue emission was explained in terms of the cooperative deexcitation of an Yb–Yb pair, while the green and red bands were associated with the up-conversion of traces of Er ion. The number of photons involved in the luminescence process is analyzed in order to confirm that cooperative emission is produced by the interaction of an Yb pair and that the green and red emission are the results of energy transfer between Yb–Er ions. The high efficiency of all bands is explained in terms of the high surface area of the nanoparticles. Keywords: Nanocrystals, Phosphor, Cooperative Up-Conversion, Luminescence, Fluorescence. 1. INTRODUCTION Enormous interest has been shown in the literature on the up-conversion process by materials, crystals, and glasses, doped with trivalent rare earth ions. The up-conversion (UC) is a well-known phenomena used in different appli- cations, such as visible solid state lasers, high-density opti- cal data storages, color displays, sensors, and security used in both bulk and powder depending on the application. 1–3 These doped materials have been sensitized with Yb 3+ ion, taking advantage of its mayor absorption cross section, and the only excited state matches perfectly the emission of a laser diode (LD), enhancing the population on the upper level of the codopant by nonradiative energy transfer from ytterbium ion. The special electronic configuration of Yb 3+ makes the 4f electrons less shielded than other ions of the lanthanide series, showing a higher tendency to interact with neighboring ions. Such an interaction is not restricted to different ions. It has been reported that the Yb–Yb pair interaction produces visible emission. That is, when there is neither an intermediate nor a final energy level (from the ∗ Author to whom correspondence should be addressed. codopant) to be populated in order to emit in the visible. Such emission is named cooperative up-conversion (CUC) and was first observed by Nakazawa in YbPO 4 . 4 In this case, two excited Yb 3+ ions decay simultaneously to the ground state, which subsequently emits a photon with the sum of energies. This process has been reported for dif- ferent materials in bulk as a weak signal, 5 but recently it was reported that such CUC emission could be enhanced if nanosize approximation was considered. 6 Nanoparticles have attracted much attention in recent years because of the strong dependence of the optical properties on the crystallite size that promise high perfor- mance photonics application. Electron confinement effect is not expected due to localization of electrons in atomic orbitals of active ions. However, excitation dynamics is influenced by the nanoscopic interaction that can induce an enhancement of the fluorescence emission. In fact, Bhar- gava et al. 7 reported that doped nanocrystalline phosphor yielded high luminescence efficiency. Since this report, luminescence properties of rare earth doped nanocrystals have been extensively investigated. 8–11 Nanocrystals have been doped with different lanthanide ions such as Eu 3+ , Tb 3+ , and Ce 3+ in order to obtain visible emission by 1480 J. Nanosci. Nanotech. 2005, Vol. 5, No. 9 1533-4880/2005/5/1480/007 doi:10.1166/jnn.2005.320