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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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Emission intensity vs. 980 nm laser power density (W/cm2), expressed in ln-ln IvP plots of 0.5% Tm, 20% Yb codoped (Na)LaF3 core@shell in the (a) UV/blue and (b) NIR regimes, noting low power saturation of Tm3+3H4 state and onset of saturation (diamond markers); (c) Emission spectrum recorded at 20 W/cm2 and (d) ETU transition diagram of Yb3+-Tm3+. Experimental conditions: 1 mg/mL solutions in toluene at 23 °C; 980 nm CW laser excitation varying from 2.5 to 100 W/cm2.
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nanomaterials-04-00069-f006: Emission intensity vs. 980 nm laser power density (W/cm2), expressed in ln-ln IvP plots of 0.5% Tm, 20% Yb codoped (Na)LaF3 core@shell in the (a) UV/blue and (b) NIR regimes, noting low power saturation of Tm3+3H4 state and onset of saturation (diamond markers); (c) Emission spectrum recorded at 20 W/cm2 and (d) ETU transition diagram of Yb3+-Tm3+. Experimental conditions: 1 mg/mL solutions in toluene at 23 °C; 980 nm CW laser excitation varying from 2.5 to 100 W/cm2.

Mentions: The ln-ln IvP plot of the (Na)LaF3 core@shell system (Figure 6a,b) shows a further departure from the standard, sequential ETU model, including a low power density saturated 1G4 state (479 nm transition) in addition to a saturated 3H4 state. The onset of saturation of these UV and blue emitting states at decreased laser power densities (20–30 W/cm2) suggests a higher probability of a 1G4 → 3H6:3F3 → 1I6 Tm-Tm CR. Shelling of UCNCs has also been seen to increase the lifetimes of emitting states, allowing for further complex radiative and CR pathways to occur [18,36,39]. The appearance of NaYbF4 inclusions in the (Na)LaF3 core@shell as indicated in the XRD data (Figure 3a), an UCNC composition established to have a high efficiency UC mechanism into NIR emitting 3F3 and 3H4 states [5,7], serves in part to explain the significant 803 nm emission intensity seen in the ln-ln IvP plot.


One-Pot Solvothermal Synthesis of Highly Emissive, Sodium-Codoped, LaF 3 and BaLaF 5 Core-Shell Upconverting Nanocrystals
Emission intensity vs. 980 nm laser power density (W/cm2), expressed in ln-ln IvP plots of 0.5% Tm, 20% Yb codoped (Na)LaF3 core@shell in the (a) UV/blue and (b) NIR regimes, noting low power saturation of Tm3+3H4 state and onset of saturation (diamond markers); (c) Emission spectrum recorded at 20 W/cm2 and (d) ETU transition diagram of Yb3+-Tm3+. Experimental conditions: 1 mg/mL solutions in toluene at 23 °C; 980 nm CW laser excitation varying from 2.5 to 100 W/cm2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5304608&req=5

nanomaterials-04-00069-f006: Emission intensity vs. 980 nm laser power density (W/cm2), expressed in ln-ln IvP plots of 0.5% Tm, 20% Yb codoped (Na)LaF3 core@shell in the (a) UV/blue and (b) NIR regimes, noting low power saturation of Tm3+3H4 state and onset of saturation (diamond markers); (c) Emission spectrum recorded at 20 W/cm2 and (d) ETU transition diagram of Yb3+-Tm3+. Experimental conditions: 1 mg/mL solutions in toluene at 23 °C; 980 nm CW laser excitation varying from 2.5 to 100 W/cm2.
Mentions: The ln-ln IvP plot of the (Na)LaF3 core@shell system (Figure 6a,b) shows a further departure from the standard, sequential ETU model, including a low power density saturated 1G4 state (479 nm transition) in addition to a saturated 3H4 state. The onset of saturation of these UV and blue emitting states at decreased laser power densities (20–30 W/cm2) suggests a higher probability of a 1G4 → 3H6:3F3 → 1I6 Tm-Tm CR. Shelling of UCNCs has also been seen to increase the lifetimes of emitting states, allowing for further complex radiative and CR pathways to occur [18,36,39]. The appearance of NaYbF4 inclusions in the (Na)LaF3 core@shell as indicated in the XRD data (Figure 3a), an UCNC composition established to have a high efficiency UC mechanism into NIR emitting 3F3 and 3H4 states [5,7], serves in part to explain the significant 803 nm emission intensity seen in the ln-ln IvP plot.

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