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Nanomaterials 2014, 4(1), 69-86; doi:10.3390/nano4010069
Article

One-Pot Solvothermal Synthesis of Highly Emissive, Sodium-Codoped, LaF3 and BaLaF5 Core-Shell Upconverting Nanocrystals

,
 and
*
Department of Chemistry, Duke University, French Family Science Center, 124 Science Drive, Durham, NC 27708, USA
* Author to whom correspondence should be addressed.
Received: 17 December 2013 / Revised: 2 January 2014 / Accepted: 2 January 2014 / Published: 8 January 2014
(This article belongs to the Special Issue Current Trends in Up-Converting Nanoparticles)
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Abstract

We report a one-pot solvothermal synthesis of sub-10 nm, dominant ultraviolet (UV) emissive upconverting nanocrystals (UCNCs), based on sodium-codoped LaF3 and BaLaF5 (0.5%Tm; 20%Yb) and their corresponding core@shell derivatives. Elemental analysis shows a Na-codopant in these crystal systems of ~20% the total cation content; X-ray diffraction (XRD) data indicate a shift in unit cell dimensions consistent with these small codopant ions. Similarly, X-ray photoelectron spectroscopic (XPS) analysis reveals primarily substitution of Na+ for La3+ ions (97% of total Na+ codopant) in the crystal system, and interstitial Na+ (3% of detected Na+) and La3+ (3% of detected La3+) present in (Na)LaF3 and only direct substitution of Na+ for Ba2+ in Ba(Na)LaF5. In each case, XPS analysis of La 3d lines show a decrease in binding energy (0.08–0.25 eV) indicating a reduction in local crystal field symmetry surrounding rare earth (R.E.3+) ions, permitting otherwise disallowed R.E. UC transitions to be enhanced. Studies that examine the impact of laser excitation power upon luminescence intensity were conducted over 2.5–100 W/cm2 range to elucidate UC mechanisms that populate dominant UV emitting states. Low power saturation of Tm3+ 3F3 and 3H4 states was observed and noted as a key initial condition for effective population of the 1D2 and 1I6 UV emitting states, via Tm-Tm cross-relaxation.
Keywords: near-infrared to ultraviolet upconversion; photoluminescence; upconversion nanocrystals; impurity doping; lanthanide; LaF3; BaLaF5; core-shell near-infrared to ultraviolet upconversion; photoluminescence; upconversion nanocrystals; impurity doping; lanthanide; LaF3; BaLaF5; core-shell
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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Stecher, J.T.; Rohlfing, A.B.; Therien, M.J. One-Pot Solvothermal Synthesis of Highly Emissive, Sodium-Codoped, LaF3 and BaLaF5 Core-Shell Upconverting Nanocrystals. Nanomaterials 2014, 4, 69-86.

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