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One-Pot Solvothermal Synthesis of Highly Emissive, Sodium-Codoped, LaF 3 and BaLaF 5 Core-Shell Upconverting Nanocrystals

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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.

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Transmission electron microscopy (TEM) images of 0.5%Tm, 20%Yb codoped Upconverting Nanocrystals (UCNCs) in host lattices: (a) LaF3 (12.6 ± 2.5 nm); (b) (Na)LaF3 (7.7 ± 1.9 nm); (c) (Na)LaF3@(Na)LaF3 (22.7 ± 4.5 nm); (d) BaLaF5 (6.0 ± 1.4 nm); (e) Ba(Na)LaF5 (7.5 ± 2.3 nm); (f) Ba(Na)LaF5@Ba(Na)LaF3 (9.6 ± 2.3 nm): (200 kx magnification; 50 nm scale bars).
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nanomaterials-04-00069-f001: Transmission electron microscopy (TEM) images of 0.5%Tm, 20%Yb codoped Upconverting Nanocrystals (UCNCs) in host lattices: (a) LaF3 (12.6 ± 2.5 nm); (b) (Na)LaF3 (7.7 ± 1.9 nm); (c) (Na)LaF3@(Na)LaF3 (22.7 ± 4.5 nm); (d) BaLaF5 (6.0 ± 1.4 nm); (e) Ba(Na)LaF5 (7.5 ± 2.3 nm); (f) Ba(Na)LaF5@Ba(Na)LaF3 (9.6 ± 2.3 nm): (200 kx magnification; 50 nm scale bars).

Mentions: Sodium codoped LaF3 and BaLaF5 (herein denoted as (Na)LaF3 and Ba(Na)LaF5) UCNCs were generated via a solvothermal synthesis, similar to that reported by Wang et al. (see Section 3.1) [34]. Both (Na)LaF3 and Ba(Na)LaF5 materials were doped with 0.5%Tm and 20%Yb, concentrations, versus total R.E., optimized to produce the highest emission intensity from the 1I6 → 3F4 (349 nm) and 1D2 → 3H6 (362 nm) Tm3+ transitions. LaF3 and BaLaF5 UCNCs, synthesized in the absence of sodium in the same size regime, were similarly doped with 0.5%Tm and 20%Yb (Figure 1) to minimize any possible surface defect site variability as a cause of luminescence quenching (Figure S1). Shells of (Na)LaF3 and Ba(Na)LaF5 host crystals were solvothermally grown following initial preparation of core seed crystals, in the presence of excess fluoride, in the same Teflon®-lined autoclavable bombs through addition of aqueous La(NO3)3 and Ba(NO3)2 salts with no intermediate purification required (see Section 3.1). These shells were applied, congruent with earlier studies, to reduce the number of surface defect trap sites capable of quenching surface presenting R.E. ion emission or quenching of core-residing R.E. excited states through R.E.-to-R.E. energy migration to the surface, thereby increasing the overall luminescence output (Figure 2) [8,18,35,36,37].


One-Pot Solvothermal Synthesis of Highly Emissive, Sodium-Codoped, LaF 3 and BaLaF 5 Core-Shell Upconverting Nanocrystals
Transmission electron microscopy (TEM) images of 0.5%Tm, 20%Yb codoped Upconverting Nanocrystals (UCNCs) in host lattices: (a) LaF3 (12.6 ± 2.5 nm); (b) (Na)LaF3 (7.7 ± 1.9 nm); (c) (Na)LaF3@(Na)LaF3 (22.7 ± 4.5 nm); (d) BaLaF5 (6.0 ± 1.4 nm); (e) Ba(Na)LaF5 (7.5 ± 2.3 nm); (f) Ba(Na)LaF5@Ba(Na)LaF3 (9.6 ± 2.3 nm): (200 kx magnification; 50 nm scale bars).
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nanomaterials-04-00069-f001: Transmission electron microscopy (TEM) images of 0.5%Tm, 20%Yb codoped Upconverting Nanocrystals (UCNCs) in host lattices: (a) LaF3 (12.6 ± 2.5 nm); (b) (Na)LaF3 (7.7 ± 1.9 nm); (c) (Na)LaF3@(Na)LaF3 (22.7 ± 4.5 nm); (d) BaLaF5 (6.0 ± 1.4 nm); (e) Ba(Na)LaF5 (7.5 ± 2.3 nm); (f) Ba(Na)LaF5@Ba(Na)LaF3 (9.6 ± 2.3 nm): (200 kx magnification; 50 nm scale bars).
Mentions: Sodium codoped LaF3 and BaLaF5 (herein denoted as (Na)LaF3 and Ba(Na)LaF5) UCNCs were generated via a solvothermal synthesis, similar to that reported by Wang et al. (see Section 3.1) [34]. Both (Na)LaF3 and Ba(Na)LaF5 materials were doped with 0.5%Tm and 20%Yb, concentrations, versus total R.E., optimized to produce the highest emission intensity from the 1I6 → 3F4 (349 nm) and 1D2 → 3H6 (362 nm) Tm3+ transitions. LaF3 and BaLaF5 UCNCs, synthesized in the absence of sodium in the same size regime, were similarly doped with 0.5%Tm and 20%Yb (Figure 1) to minimize any possible surface defect site variability as a cause of luminescence quenching (Figure S1). Shells of (Na)LaF3 and Ba(Na)LaF5 host crystals were solvothermally grown following initial preparation of core seed crystals, in the presence of excess fluoride, in the same Teflon®-lined autoclavable bombs through addition of aqueous La(NO3)3 and Ba(NO3)2 salts with no intermediate purification required (see Section 3.1). These shells were applied, congruent with earlier studies, to reduce the number of surface defect trap sites capable of quenching surface presenting R.E. ion emission or quenching of core-residing R.E. excited states through R.E.-to-R.E. energy migration to the surface, thereby increasing the overall luminescence output (Figure 2) [8,18,35,36,37].

View Article: PubMed Central - PubMed

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.

No MeSH data available.


Related in: MedlinePlus