© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 856 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Shyam Sarkar, Brahmaiah Meesaragandla, Chanchal Hazra, and Venkataramanan Mahalingam* Sub-5 nm Ln 3 + -doped BaLuF 5 Nanocrystals: A Platform to Realize Upconversion via Interparticle Energy Transfer (IPET) S. Sarkar, B. Meesaragandla, C. Hazra, Dr. V. Mahalingam Department of Chemical Sciences Indian Institute of Science Education and Research (IISER-Kolkata) Mohanpur Campus, Nadia, WB 741252, India E-mail: mvenkataramanan@yahoo.com DOI: 10.1002/adma.201203641 Upconverting nanomaterials, which possess the ability to con- vert low-energy photons (e.g., near-IR (NIR)) into high-energy photons (visible to ultraviolet) via the multiphoton process, have been gaining enormous research attention in the recent past. [1,2] This interest is principally attributed to the growing importance of these materials in areas such as bioimaging, [3] detection and sensing, [4] solar cells, [5] etc. These upconverting nanomaterials are generally composed of lanthanide (Ln 3 + ) ions which are spatially distributed in a suitable host matrix. [6] The Ln 3 + ions possess interesting optical characteristics, such as sharp emissions, and long luminescence life times, and exhibit multiple emissions spanning a wide region (UV to NIR), which are exploited in several applications like developing phosphors, biomarkers, and optoelectronic devices, to name only a few. [7] Amongst the various host matrices for Ln 3 + ions, fluorides are widely studied, as their phonon energies are generally <400 cm -1 , leading to good luminescence quantum efficiencies. In addition, there are several synthetic methods available to make fluoride nanocrystals in colloidal forms with different sizes and shapes. [8] In addition, the synthetic routes to fluoride nanocrys- tals, provide enough flexibility to design novel structures (e.g., core/shell) that facilitate tuning of the optical properties of Ln 3 + ions for targeted applications. For example, the core/shell nanos- tructure with composition of NaGdF 4 :Yb 3 + -Tm 3 + /NaGdF 4 :Eu 3 + has been designed to obtain dual-mode luminescence from Eu 3 + ions via a 980 nm excitation. [9] Recently, core/shell nano- structures of the type NaGdF 4 :Tm 3 + ,Yb 3 + /NaGdF 4 :Ln 3 + have been designed, and they show that Gd 3 + ions present in the core can, in principle, transfer their energy (gained from Tm 3 + ions in the core) to acceptor Ln 3 + ions (Tb, Sm, etc.) that are located in the shell structure. [10] In another study, an active-core/active-shell (NaGdF 4 :Yb 3 + -Er 3 + /NaGdF 4 :Yb 3 + ) strategy has been adopted where the additional sensitizer ions (Yb 3 + ) present in the shell enhance the upconversion efficiency of the Er 3 + ions present in the core. [11] However, to our knowledge, upconversion via inter- particle energy transfer (IPET) (i.e., placing the sensitizer and activator ions in different nanocrystals), has not been reported. This can be achieved provided the size of the nanocrystals is reduced to a size where the distance covering two (neighboring) nanocrystals falls within the fluorescence resonance energy transfer (FRET) distance ( ≈5 to 7 nm). [12] The upconversion (UC) via IPET, and particularly with ligand stabilized nanocrystals, has multiple advantages. To just mention a few, firstly, it allows one to tune the FRET (or) the energy-transfer efficiency between the Ln 3 + ions by simple tuning of the ligands (or) their length. Secondly, the size and charge incompatibility between the dopant and the matrix ions, which is one of the major issues in materials science, can be avoided by placing the different ions in separate nanoparticles, yet linked spatially. Thirdly, by choosing appropriate systems (e.g., Q-dots and dyes), it might be possible to sensitize lantha- nide emissions (Ln 3 + ), as they generally suffer with low absorp- tion coefficients. There have been few reports on the synthesis of Ln 3 + -doped ultrasmall nanocrystals in the recent past. NaGdF 4 nanocrys- tals with an average size of 3 nm have been synthesized and their potential has been investigated for magnetic resonance imaging (MRI) studies. [13] Similarly, 3.7 nm water-dispersible KGdF 4 has been synthesized, and its upconverting properties have been studied. [14] Recently, ultrasmall Ln 3 + -doped NaLuF 4 and NaYF 4 nanocrystals have been synthesized and their bio- imaging potential has been explored. [15] However, to the best of our knowledge there have been no reports on the application of these ultrasmall nanocrystals to envisage IPET. In this study, we demonstrate the upconversion (UC) proc- esses via interparticle energy transfer in Ln 3 + -doped BaLuF 5 (Ln = Tm and Yb) nanocrystals for the first time. To achieve this, BaLuF 5 nanocrystals with an average size less than 5 nm were prepared as a matrix for Ln 3 + ions. Both Tm 3 + and Yb 3 + ions were independently doped into BaLuF 5 nanocrystals capped with oleic acid (OA) ligands. A colloidal mixture containing the above two nanocrystals exhibits strong blue, red, and near-IR (NIR) emissions upon excitation with a 980 nm laser diode. This IPET is possible, as the distance covering two nanocrystals falls well below the FRET distance for energy transfer between the Ln 3 + ions ( ≈7 nm). [12a] The Ln 3 + -doped BaLuF 5 (Ln = Tm, Yb) nanocrystals capped with oleic acid were synthesized via the thermal decomposition of the corresponding lanthanide trifluoroacetates. The forma- tion of a pure single-phase compound is evident from the XRD pattern of the resulting nanocrystals, shown in Figure 1A. The experimental pattern clearly matches with the standard pattern for cubic phase BaLuF 5 . The HR-TEM images of the nanocrystals dispersed in toluene are shown in Figure 1B. The attachment of OA onto the surface of the nanocrystals is confirmed by the appearance of strong carbonyl stretching Adv. Mater. 2013, 25, 856–860