KLaF 4 :Er an efficient upconversion phosphor Neetu Tyagi a , A. Amarnath Reddy b , R. Nagarajan a, * a Materials Chemistry Group, Department of Chemistry, University of Delhi, New Delhi 110 007, India b Department of Physics, Indian Institute of Technology, HauzKaus, New Delhi, India article info Article history: Received 8 April 2010 Received in revised form 20 July 2010 Accepted 23 July 2010 Available online 23 August 2010 Keywords: Photoluminescence Upconversion Diffuse reflectance spectra Excited state absorption Energy Transfer Upconversion abstract Er 3+ doped cubic KLaF 4 system is reported as a promising green-emitting phosphor, obtained from a sin- gle step non-aqueous medium based room temperature synthesis. The optical properties of the doped systems were evaluated by diffuse reflectance spectroscopy and photoluminescence spectroscopy under laser excitation. The green to red emission ratio (GRR) of 4.9 with a decay time range of 35–70 ms (for green emission at 522 nm, under 980 nm excitation) has been achieved in the KLaF 4 :Er for the first time. This study proves beneficial for many applications including in vivo bio-labeling, 3D flat panel displays and provides a broader scope for further improvement in its upconversion properties through site selec- tive doping. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Momentous progress has been made in the field of nanomateri- als synthesis, owing to their potential usage in nanodevices com- posed of 2D and 3D assemblies, bioprobes [1]. Of these, the preparation of complex rare earth fluoride nanocrystals with de- sired properties is a challenging task due to the difficulties encoun- tered in their synthesis. Also the fluorides draw extensive interest due to the following interlinked facts of high electro negativity, low polarizability, highly ionic system with large band gap, high refrac- tive index and weak phonon loss resulting in minimum quenching of emissive Ln 3+ ions. Thus, the fluorides provide ideal matrices for doping rare earth ions suitable for various applications including photoluminescence [2]. Among the much focused materials for the luminescent applications, doped rare earth fluorides of the type AREF 4 (A, alkali metal and RE, rare earth) have been investigated extensively [3,4]. Phosphors find applications in vivo imaging of tissues and cells with different colored emissions using NIR irradi- ation, lightening applications, laser diodes, amplifiers, scintillators and TSL dosimetry [5–8]. NaYF 4 doped with Er 3+ /Yb 3+ has been re- ported as an analyte for DNA detection as it has the advantage of weak autoflourescence, low background signals, high penetration depth and high sensitivity [9]. The hexagonal form of NaYF 4 prepared by Hund [10] is known to be the most efficient host for green and blue upconversion until now [11]. Apart from NaYF 4 , the energy upconversion processes of Er 3+ ion in crystals of CaF 2 [12], LiYF 4 [13], KYF 4 [14], and YAlO 3 [15] have been investigated earlier. a form of NaYF 4 exhibits CaF 2 type crystal structure wherein the Na + and Y 3+ randomly oc- cupy the cationic crystallographic sites [16]. On the other hand, the hexagonal structure of NaYF 4 consists of 1.5 formula units (NaLn(Na 0.5 Ln 0.5 )F 6 ) having three cationic sites 1a, 1f and 2h, with the rare earth ion accommodating two sites, the fully occupied site 1a, site 1f randomly shared between Na + and Ln 3+ and the third cationic site 2h fully occupied by Na + . The sites 1a and 1f are nine- fold coordinated with C 3h symmetry and the site 2h is sixfold coor- dinated with C s symmetry [17]. Phosphors emitting lower energy photons on excitation with higher energy photons are downconversion ones and those that emit higher energy photons via multiple absorptions or energy transfer processes are upconversion phosphors. The upconversion efficiency of a phosphor is strongly determined by the multipho- non processes and the concentration of the dopant ion. Thus, a careful selection of a host with low phonon energy is essential for efficient upconversion since fluorescence intensity is decreased by the multiphonon relaxation processes which are dependent upon host optical phonon branches. NaLaF 4 (290 cm À1 ) [18] has been reported to have lower phonon energy than NaYF 4 (360 cm À1 ) [19] and has been studied recently for its upconversion effi- ciency on doping Er 3+ ion. We chose KLaF 4 as a host system for studying upconversion since further lowering of phonon energy is expected for it as compared to NaYF 4 . The effect of the increase in the alkali metal (A-site) ion and the rare earth (B-site) ion results in the increased stability for KLaF 4 as compared to NaYF 4 based on the theoretical calculations by Groen and Oskam [20]. Although co-doped systems have been studied using sensitizer ion Yb 3+ along with the upconversion Er 3+ , we have restricted our studies 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.07.014 * Corresponding author. Tel.: +91 11 2766 2650; fax: +91 11 2766 6605. E-mail address: rnagarajan@chemistry.du.ac.in (R. Nagarajan). Optical Materials 33 (2010) 42–47 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat