Monodispersed Nanocrystalline Fluoroperovskite Up-Conversion Phosphors Jing Hui Zeng, Ting Xie, Zhi Hua Li, and Yadong Li* Department of Chemistry, Tsinghua UniVersity, 100084 Beijing, People’s Republic of China
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2774–2777
ReceiVed May 24, 2007; ReVised Manuscript ReceiVed September 16, 2007
ABSTRACT: A series of monodispersed fluoroperovskite nanocrystals with the size 20 nm were synthesized and rare earth doped. A series of KMF3:Yb3+,Er3+ (M ) Mn, Zn, Cd, or Mg) up-conversion phosphor nanocrystals were synthesized. The emissions can be directly observed using the naked eye. Different red-to-green luminescent ratios for different host materials were discussed. Powder X-ray diffraction, transmission electron microscopy, and luminescence spectroscopy were used to characterize the products. The up-conversion mechanism was discussed.
1. Introduction Interest in the up-conversion (UC) phosphors has increased in recent years. They have found applications in infrared to visible up-conversion lasers and fiber amplifiers.1,2 Threedimensional display was also proposed using UC luminescence materials.3 A white light originated from the up-conversion process for sol–gel matrices made with Ln3+-doped LaF3 nanoparticles codoped with Yb3+ ions was reported.4 Besides these applications, they can be used as biological labeling materials and for two-photon imaging in confocal microscopy.5,6 As biological labeling materials, the high luminescence efficiency UC phosphors must be monodispersed with a narrow size dispersion in the nanometer region. However, when the size of the phosphors decreases to the nanometer range, the surface defects cause luminescent extinction. As a result, the overall luminescent intensity will decrease accordingly. Furthermore, for biological study, the phosphors must show enough luminescent intensity at room temperature. Otherwise, the biologic reactions will cease due to the deactivation of enzymes at low or high temperatures. The color is another consideration. When multicolored phosphors are applied, in ViVo study of more complex processes in live cells is allowed.7 How to find a phosphor with a strong enough luminescent intensity at room temperature when its size is reduced to the nanometer scale and how to tune the color of the UC luminescence become the key challenges for its applications. Different host materials doped with rare earths for UC phosphor processes were prepared.8–10 Fluorides, when compared with their oxide counterparts, possess lower phonon energy, which will decrease the multiphonon relaxation and increase the UC efficiency.11 Intensive research is focused on fluorides such as NaYF4, KYF4, etc.12,13 Among them, fluoroperovskite compounds are of great scientific interest because most of the perovskites are ferroelectrics and may be desirable for certain applications in electronics and optics. Fluoroperovskites are of stable structure and are inert to oxygen and moisture,14 which will make them satisfactory candidates as hosts for UC phosphors. Fluorides with perovskite structure, or fluoroperovskites, have thus attracted much attention recently. In this paper, we present a synthetic route to fluoroperovskite nanocrystals (NCs) and rare earth doped UC phosphors based on these fluoroperovskite NCs. The cubic shaped phosphor NCs * To whom correspondence should be addressed. Fax: (+86) 10 6278 8765. E-mail:
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are monodispersed within 20 nm in size. The UC luminescence was detectable with the naked eye using a
