One-Pot Solvothermal Synthesis of Highly Emissive, Sodium-Codoped, LaF(3) and BaLaF(5) 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 LaF(3) and BaLaF(5) (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 La(3+) ions (97% of total Na(+) codopant) in the crystal system, and interstitial Na(+) (3% of detected Na(+)) and La(3+) (3% of detected La(3+)) present in (Na)LaF(3) and only direct substitution of Na(+) for Ba(2+) in Ba(Na)LaF(5). 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/cm(2) range to elucidate UC mechanisms that populate dominant UV emitting states. Low power saturation of Tm(3+) (3)F(3) and (3)H(4) states was observed and noted as a key initial condition for effective population of the (1)D(2) and (1)I(6) UV emitting states, via Tm-Tm cross-relaxation.