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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.

No MeSH data available.


X-ray powder diffraction (XRD) data acquired for: (a1) LaF3 (Green); (a2) (Na)LaF3 (Blue); (a3) (Na)LaF3@(Na)LaF3 (Violet); (a4) line spectrum of hexagonal LaF3, JCPDS: 72-1435 (Black); hexagonal NaYbF4 inclusions denoted in (Na)LaF3@(Na)LaF3: (b1) BaLaF5 (Green); (b2) Ba(Na)LaF5 (Blue); (b3) Ba(Na)LaF5@Ba(Na)LaF3 (Violet); (b4) displays JCPDS: 43-0394 (cubic BaCeF5 comparative, Black); and (a5,b5) line spectrum of hexagonal NaLaF4, JCPDS: 75-1923 (Red).
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nanomaterials-04-00069-f003: X-ray powder diffraction (XRD) data acquired for: (a1) LaF3 (Green); (a2) (Na)LaF3 (Blue); (a3) (Na)LaF3@(Na)LaF3 (Violet); (a4) line spectrum of hexagonal LaF3, JCPDS: 72-1435 (Black); hexagonal NaYbF4 inclusions denoted in (Na)LaF3@(Na)LaF3: (b1) BaLaF5 (Green); (b2) Ba(Na)LaF5 (Blue); (b3) Ba(Na)LaF5@Ba(Na)LaF3 (Violet); (b4) displays JCPDS: 43-0394 (cubic BaCeF5 comparative, Black); and (a5,b5) line spectrum of hexagonal NaLaF4, JCPDS: 75-1923 (Red).

Mentions: Unlike previous reports of Ca2+ and Ba2+ codoping of R.E.F3, wherein the crystal structure is changed from hexagonal to cubic and a resultant shift in both unit cell dimensions and crystal field environment is seen [21], Na-codoping into (Na)LaF3 does not drive a compositional or phase change to NaLaF4, nor does Ba(Na)LaF5 show hexagonal NaLaF4 impurities (Figure 3). Most notably, Na-codoping of LaF3 results in a decrease in 2θ values of all crystallographic peaks (Figure S2) compared to the undoped control, indicating an increase in unit cell dimension most likely due to Na+ insertion into interstitial sites in the hexagonal unit cell, though direct La3+ to Na+ substitution is also expected [19,22,40,41]. The reverse trend can be seen in Ba(Na)LaF5, where an increase in 2θ values (Table S1) indicates a reduction in unit cell volume, likely through substitution of Ba2+ ions for Na+ [19,20,42,43]. Inductively coupled plasma optical emission spectroscopy (ICP-OES) elemental analysis of (Na)LaF3 and Ba(Na)LaF5 (Table 1) indicates a high level of Na-codopant (>20%) in both UCNC systems, and a marked replacement of Ba2+ ions for Na+ in the Ba(Na)LaF5 UCNC cores. In contrast to the expected 1:1 ratio of Ba:rare earth (R.E.) elements (La, Tm, and Yb) in BaLaF5, a 0.74:1 (Ba:R.E.) ratio was determined, along with a 0.60:1 Na:R.E. molar ratio. Either by substitution of La3+ or Ba2+, insertion into interstitial sites, or a combination of the two, the change in unit cell dimensions would decrease the crystal field symmetry surrounding emitting Tm3+ ions and facilitate luminescence enhancement [19,40].


One-Pot Solvothermal Synthesis of Highly Emissive, Sodium-Codoped, LaF 3 and BaLaF 5 Core-Shell Upconverting Nanocrystals
X-ray powder diffraction (XRD) data acquired for: (a1) LaF3 (Green); (a2) (Na)LaF3 (Blue); (a3) (Na)LaF3@(Na)LaF3 (Violet); (a4) line spectrum of hexagonal LaF3, JCPDS: 72-1435 (Black); hexagonal NaYbF4 inclusions denoted in (Na)LaF3@(Na)LaF3: (b1) BaLaF5 (Green); (b2) Ba(Na)LaF5 (Blue); (b3) Ba(Na)LaF5@Ba(Na)LaF3 (Violet); (b4) displays JCPDS: 43-0394 (cubic BaCeF5 comparative, Black); and (a5,b5) line spectrum of hexagonal NaLaF4, JCPDS: 75-1923 (Red).
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nanomaterials-04-00069-f003: X-ray powder diffraction (XRD) data acquired for: (a1) LaF3 (Green); (a2) (Na)LaF3 (Blue); (a3) (Na)LaF3@(Na)LaF3 (Violet); (a4) line spectrum of hexagonal LaF3, JCPDS: 72-1435 (Black); hexagonal NaYbF4 inclusions denoted in (Na)LaF3@(Na)LaF3: (b1) BaLaF5 (Green); (b2) Ba(Na)LaF5 (Blue); (b3) Ba(Na)LaF5@Ba(Na)LaF3 (Violet); (b4) displays JCPDS: 43-0394 (cubic BaCeF5 comparative, Black); and (a5,b5) line spectrum of hexagonal NaLaF4, JCPDS: 75-1923 (Red).
Mentions: Unlike previous reports of Ca2+ and Ba2+ codoping of R.E.F3, wherein the crystal structure is changed from hexagonal to cubic and a resultant shift in both unit cell dimensions and crystal field environment is seen [21], Na-codoping into (Na)LaF3 does not drive a compositional or phase change to NaLaF4, nor does Ba(Na)LaF5 show hexagonal NaLaF4 impurities (Figure 3). Most notably, Na-codoping of LaF3 results in a decrease in 2θ values of all crystallographic peaks (Figure S2) compared to the undoped control, indicating an increase in unit cell dimension most likely due to Na+ insertion into interstitial sites in the hexagonal unit cell, though direct La3+ to Na+ substitution is also expected [19,22,40,41]. The reverse trend can be seen in Ba(Na)LaF5, where an increase in 2θ values (Table S1) indicates a reduction in unit cell volume, likely through substitution of Ba2+ ions for Na+ [19,20,42,43]. Inductively coupled plasma optical emission spectroscopy (ICP-OES) elemental analysis of (Na)LaF3 and Ba(Na)LaF5 (Table 1) indicates a high level of Na-codopant (>20%) in both UCNC systems, and a marked replacement of Ba2+ ions for Na+ in the Ba(Na)LaF5 UCNC cores. In contrast to the expected 1:1 ratio of Ba:rare earth (R.E.) elements (La, Tm, and Yb) in BaLaF5, a 0.74:1 (Ba:R.E.) ratio was determined, along with a 0.60:1 Na:R.E. molar ratio. Either by substitution of La3+ or Ba2+, insertion into interstitial sites, or a combination of the two, the change in unit cell dimensions would decrease the crystal field symmetry surrounding emitting Tm3+ ions and facilitate luminescence enhancement [19,40].

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.