Published on Web 03/27/2007
Multinuclear Solid-State NMR Spectroscopy of Doped Lanthanum Fluoride Nanoparticles Andy Y. H. Lo,† Vasanthakumaran Sudarsan,‡ Sri Sivakumar,‡ Frank van Veggel,*,‡ and Robert W. Schurko*,† Contribution from the Department of Chemistry and Biochemistry, UniVersity of Windsor, Windsor, Ontario, Canada N9B 3P4, and Department of Chemistry, UniVersity of Victoria, Victoria, British Columbia, Canada V8W 3V6 Received November 30, 2006; E-mail:
[email protected];
[email protected] Abstract: Multinuclear solid-state NMR spectroscopy and powder X-ray diffraction (XRD) experiments are applied to comprehensively characterize a series of pure and lanthanide-doped LaF3 nanoparticles (NPs) that are capped with di-n-octadectyldithiophosphate ligands (Ln3+ ) diamagnetic Y3+ and Sc3+ and paramagnetic Yb3+ ions), as well as correlated bulk microcrystalline materials (LaF3, YF3, and ScF3). Solidstate 139La and 19F NMR spectroscopy of bulk LaF3 and the LaF3 NPs reveal that the inorganic core of the NP retains the LaF3 structure at the molecular level; however, inhomogeneous broadening of the NMR powder patterns arises from distributions of 139La and 19F NMR interactions, confirming a gradual change in the La and F site environments from the NP core to the surface. 139La and 19F NMR experiments also indicate that low levels (5 and 10 mol %) of Ln3+ doping do not significantly change the LaF3 structure in the NP core. Similar doping levels of paramagnetic Yb3+ ions severely broaden 19F resonances, but only marginally effect 139La powder patterns, suggesting that the dopant ions are uniformly distributed throughout the NP core and occupy vacant La sites. Measurements of 139La T1 and T2 relaxation constants are seen to vary between the bulk material and NPs and between samples with diamagnetic and paramagnetic dopants. 45Sc NMR experiments confirm that the dopants are integrated into the La sites of the LaF3 core. Solid-state 1H and 31P magic-angle spinning (MAS) NMR spectra aid in probing the nature of the capping ligands and their interactions at the NP surface. 31P cross-polarization (CP)/MAS NMR experiments identify not only the dithiophosphate head groups but also thiophosphate and phosphate species which may form during NP synthesis. Finally, 19F-31P CP/MAS and 1H MAS experiments confirm that ligands are coordinated to the NP surface.
Introduction
Lanthanide(III)-doped nanoparticles (NPs) with inorganic cores and outer organic stabilizing ligands are of great interest due to their unique luminescent properties which have made them important in the manufacture of light-emitting diodes (LEDs),1 liquid-crystal displays,2 biological assays,3 nanometerscale optoelectronic devices,4 and light sources in zero-threshold lasers.5 NPs doped with different Ln3+ ions produce spectral emissions across visible light (Eu3+ and Tb3+) and near-infrared (NIR; Pr3+, Nd3+, Er3+, and Yb3+) regions. NIR-emitting NPs are of particular interest because they are the luminescent elements in polymer-based telecommunication components, lasers, displays, and polymer LEDs. Luminescence in Ln3+ ions † ‡
University of Windsor. University of Victoria.
(1) Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner, M. F. Appl. Phys. Lett. 1995, 66, 1316. (2) Justel, T.; Nikol, H.; Ronda, C. Angew. Chem., Int. Ed. 1998, 37, 3085. (3) Dahan, M.; Laurence, T.; Pinaud, F.; Chemla, D. S.; Alivisatos, A. P.; Sauer, M.; Weiss, S. Opt. Lett. 2001, 26, 825. (4) Gonsalves, K. E.; Carlson, G.; Rangarajan, S. P.; Benaissa, M.; JoseYacaman, M. J. Mater. Chem. 1996, 6, 1451. (5) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314. 10.1021/ja068604b CCC: $37.00 © 2007 American Chemical Society
arises from radiative relaxation after the parity-forbidden transitions within the partially filled 4f orbitals that are shielded by the filled 5s and 5p orbitals. Consequently, f-f transitions lead to low molar absorption coefficients and slow emission rates, resulting in long luminescent lifetimes.6 Polymeric materials are advantageous in the fabrication of optical components because they are easy to process by spincoating and photolithography.7 However, direct doping of the Ln3+ ions into polymeric materials is not practical because the largely spaced vibrational and/or electronic transitions present in polymers can often quench f-f transitions associated with the luminescent process. Initially, organic complexes as luminescent materials were studied, but lifetimes of NIR emitters are very short with low ( thiophosphate > phosphate, which suggests that La3+ surface sites interact with the O-containing head groups more strongly. Conclusions
Multinuclear solid-state NMR data, in combination with powder X-ray diffraction measurements, have been used to gain insight into the local atomic and molecular structure in LaF3 NPs. 19F and 139La NMR experiments give insight into the molecular structure of the core and effects of paramagnetic doping and, in combination with 45Sc NMR experiments, into the dopant ion environments. The LaF3 core of the NPs is relatively unperturbed compared to the bulk, but inhomogeneously broadened powder patterns indicate distributions of La and F sites from the core of the NP to its surface. Comparison of powder pattern shapes and relaxation time constants adds new insights into the differences between the structures of NPs
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and bulk parent materials. 1H and 31P NMR experiments identify an interesting surface chemistry, and the combination of paramagnetically doped samples and 19F-31P VACP/MAS experiments reveals the interface between the LaF3 core and the dithiophosphate surface ligands. We anticipate that this study will encourage future solid-state NMR investigations on this important class of NPs. Acknowledgment. The Natural Sciences and Engineering Research Council (NSERC, Canada), the Canadian Foundation for Innovation (CFI), the Ontario Innovation Trust (OIT), and the British Columbia Knowledge Development Fund (BCKDF) are gratefully acknowledged for financial support. R.W.S. and A.Y.H.L. also acknowledge the Centre for Catalysis and Materials Research (CCMR) at the University of Windsor, Ontario Graduate Scholarship (OGS) program, and Imperial Oil and NSERC CRD for additional funding. Supporting Information Available: Tables of solid-state NMR experimental parameters, 139La relaxation curve fits utilizing mono- and biexponential functions, and additional 139La, 89Y, 19F, and 31P NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. JA068604B
