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

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.