Enhanced quantum efficiency for Dy³⁺ Emissions in water dispersible PbF₂ nanocrystals

Abstract

PbF₂ nanocrystals found to be a better host for Dy³⁺ ions to exhibit better quantum efficiency. The optimum dopant concentration is found to be higher than that observed in several bulk materials. The nanocrystals are rendered water dispersible by coating poly(acrylic acid) over their surface.

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The Lanthanide (Ln³⁺)-doped nanomaterials are currently getting much attention due to their rich optical properties which arise from their inner 4f electrons.¹ These 4f electrons undergo transition within the 4f orbitals which are generally forbidden, but allowed due to partial admixing of states. The forbidden nature of these 4f transitions lengthens the excited state life time resulting in narrow emission signal.² Moreover, the 4f orbitals are less influenced by the ligand orbitals causing hardly any shift in the peak position. A wide range of emission signals from UV to NIR can be obtained by choosing suitable Ln³⁺ ions.³ In addition, the Ln³⁺ ions can exhibit the upconverting property, which is the conversion of low energy radiation into high energy radiation.⁴

Among the various Ln³⁺-doped materials, literature reports on Dy³⁺-doped nanomaterials are quite scarce. One of the reasons is that Dy³⁺ is quite prone to concentration quenching in several hosts.⁵ For example, the optimum concentration of Dy³⁺ is only 2 % in Dy³⁺-doped oxides such as La₃Ga_5.5Nb_0.5O₁₄, ZrO₂ nanocrystals, etc.⁶ Similarly, Dy³⁺-doped GdVO₄ nanoparticles exhibit the highest luminescent intensity for 2 % Dy³⁺ doping.⁵ In addition to the concentration quenching, the luminescence efficiency is further reduced due to presence of defects near the surface of the nanoparticles. However, they exhibit several interesting optical characteristics. First, they display two strong emissions one in the blue region (479 nm) and the other in the yellow (580 nm) region of the electromagnetic spectrum.⁷ The appropriate combination of these two emissions will lead to the production of white light emission without the addition of any other lanthanide ions.⁸ Second, Dy³⁺ possesses several excitation levels in the 340–480 nm spectral range which allow it to be excited easily with the commercially available ultraviolet or blue light emitting diodes (LEDs). Finally, Dy³⁺ exhibits multiple emissions in the near infrared region like 0.9, 1.1, 1.3, 1.7 and 3 μm which are very important for telecommunication applications.⁹

All the above points demand an improvement in the luminescence efficiency of Dy³⁺-doped nanomaterials, as Dy³⁺ ions are very prone to concentration quenching and enhancement is still a challenge. In fact, achieving this in aqueous medium is particularly interesting as this would widen the scope of their application particularly in the development of sol–gel based glasses.¹⁰ We chose PbF₂ as the host matrix for Ln³⁺ ions for the following reasons. First, PbF₂ is a highly transparent material allowing a wide range of light to pass through without much attenuation in luminescence. Second, the phonon energy is below 250 cm⁻¹, which is much lower than that of other lanthanide fluorides (350–400 cm⁻¹).¹¹ Third, PbF₂ undergoes a phase transition from α→β at temperatures above 335 °C, making it interesting for structure–property studies.¹² In addition, to the best of our knowledge, only very few reports are available on Ln³⁺-doped PbF₂ nanoparticles.¹³ For example, Tikhomirov et al., have reported the synthesis of Ln³⁺-doped PbF₂ nanoparticles through chemical etching of the bulk glass ceramic.¹³ Yb/Er-doped PbF₂ nanoparticles have been made via the microemulsion method. However, the synthetic routes provided nanoparticles devoid of any capping ligands for stabilization or dispersion in various medium. All these factors motivate us to develop a synthetic route for Dy³⁺-doped PbF₂ nanocrystals.

In this paper, we report the synthesis of water dispersible Dy³⁺-doped PbF₂ nanocrystals via the hydrothermal method (see ESI† for more details). The nanocrystals are stabilized with poly(acrylic acid)(PAA) which render them water dispersible. The nanocrystals show strong Dy³⁺ luminescence signals in water. Moreover, the optimum concentration of Dy³⁺ in PbF₂ nanocrystals is about 5 mol%, which is more than that observed for Dy³⁺ in other superior matrices such as Gd₂O₃, GdVO_4,etc.

Fig. 1 (A and B) shows the TEM and FESEM images of the PAA-coated PbF₂ nanocrystals. The images indicate the formation of spherical shape nanocrystals with an average size of 50 nm. Additional microscopic images are shown in Fig. S1 and S2 (see ESI†). The phase analysis of the series of PAA-coated PbF₂:Dy³⁺(x %), [x = 0, 1, 2, 5, 8] nanocrystals was performed using X-ray diffraction (XRD). The XRD pattern for 5 % Dy³⁺-doped PbF₂ is shown in Fig. 1C. The pattern clearly indicates the formation of pure cubic phase. All diffraction peaks are perfectly indexed to the cubic PbF₂ systems which are in good agreement with the standard pattern of cubic PbF₂ crystals (PDF card No. 00-006-0251). Furthermore, the calculated Rietveld refinement of the resulting pattern matches with the experimental one. The PXRD patterns for PbF₂ with different Dy³⁺ doping concentrations are shown in Fig. S3, ESI†. The crystallite size, calculated using the Scherer equation, t = (0.9λ/βcosθ), is found to be 48 nm. This is in agreement with the size measured from the microscopic analyses.

Fig. 1 TEM (A) and FESEM (B) images of Dy³⁺-doped PbF₂ nanocrystals. (C) Experimental, Rietveld refined pattern of PbF₂ nanocrystals along with the standard XRD pattern for PbF₂.

The binding of the PAA molecules to the PbF₂ nanocrystals is confirmed by FTIR spectrum shown in Fig. 2 along with that of pure PAA. For the free PAA, five major peaks are observed near 3448, 2952, 1727, 1455 and 1407 cm⁻¹. The strong and broad stretching vibration band centered at 3448 cm⁻¹ is due to O–H in a –COOH group and the peak at 2952 cm⁻¹ is attributed to the methylene (CH₂) stretching vibrations of the alkyl chains of PAA. The band at 1727 cm⁻¹ is assigned to the C=O stretching vibration of the –COOH group, and the bands at 1456 and 1407 cm⁻¹ are due to asymmetric and symmetric stretching vibrations, respectively, of the C–O in –COOH group of the PAA coated over PbF₂.¹⁴ It is quite clear, that the intensity of the carbonyl stretching near 1727 cm⁻¹ for the free PAA is reduced and a new peak appears near 1556 cm⁻¹. This clearly suggests that –COO groups are attached strongly to the Pb²⁺ of PbF₂. The weak intensity of 1727 cm⁻¹ peak indicates that most of the –COO⁻ are attached to the Pb²⁺ ions in the PbF₂ nanocrystals. Moreover, for the free PAA molecules, the region between 3500 and 2800 cm⁻¹ is quite broad indicating the presence of different types of OH groups (i.e. some are hydrogen bonded). However, upon binding to the PbF₂, the OH peaks are comparatively narrowed suggesting that the OH groups of the PAA are in a structured environment. It is this PAA coating that makes these nanocrystals water dispersible. In addition, the PAA coating protects the Pb²⁺ ions from leaching out from the nanocrystals surface which is important for their potential for any biological study. A schematic of the PAA binding to the PbF₂ nanocrystals is illustrated in Fig. S2.

Fig. 2 FTIR spectra of PAA alone and the same coated over PbF₂ nanocrystals.

To further confirm the PAA coating onto PbF₂ nanocrystals, TGA analysis was conducted. The TGA curves for both PAA coated PbF₂ and free PAA are shown in Fig. S3, ESI†. For the free PAA the major weight loss is noted from 200 to 550 °C due to the decomposition of PAA molecules. For the PAA coated PbF₂ nanocrystal, the onset of decomposition is shifted to higher temperature (∼340 °C). This substantiates the FTIR result that the PAA molecules are attached strongly to the PbF₂ surface. In addition, the narrow decomposition range indicates that all the nanocrystals are more or less evenly coated with the PAA.

The optical properties of the Dy³⁺-doped PbF₂ nanocrystals show interesting results. Upon UV excitation, the nanocrystals exhibit two strong emissions which are shown in Fig. 3. The blue emission peak at 478 nm is assigned to the ⁴F_9/2→⁶H_15/2 transition while the one near 580 nm is ascribed to the ⁴F_9/2→⁶H_13/2 transition. The latter transition is hypersensitive and its intensity strongly depends on the host, whereas the ⁴F_9/2→⁶H_15/2 transition is less sensitive to the host matrix. The transitions and the energy levels are illustrated in the inset of Fig. 3. To get the optimum concentration of Dy³⁺ ions in PbF₂ nanocrystals, a series of PbF₂ nanocrystals with different Dy³⁺ concentration is prepared. The emission intensities of both 479 nm and 580 nm peaks of Dy³⁺ ions have been plotted as a function of Dy³⁺ concentration and shown in Fig. 4A. It has been found that the PL intensities of both 478 and 580 nm peaks increase with increase in Dy³⁺ concentration up to 5 mol% doping and then show a decrease

Fig. 3 Emission spectrum of 5 % Dy³⁺-doped PbF₂ nanocrystals (2 wt% dispersion in water). Inset illustrates the energy levels and the transitions.

Fig. 4 (A) Plot of emission intensity verses Dy³⁺-doping concentration and (B) luminescence decay curve observed for 579 nm emission. The red trace is the fitted curve.

It is interesting to note that the observed optimum concentration (5 %) of Dy³⁺ in PbF₂ is higher compared to many of the bulk oxide materials where the optimum concentration of Dy³⁺ is found to be 2 % (vide supra). Furthermore, the observed lifetime for the 579 nm emission (⁴F_9/2→⁶H_13/2) of Dy³⁺ is about 0.68 ms as calculated from the decay analysis (Fig. 4B). This value is longer than the 0.4 ms observed for the similar emission in Dy³⁺-doped NaYF₄ microcrystals.⁸ The increased optimum concentration and the lifetime are further supported by the quantum efficiency of the resulting Dy³⁺-doped PbF₂ nanocrystals. The calculated quantum yield is about 7 % which is measured using quinine sulphate as the reference standard. The details of the measurement and the calculations are presented in the supplementary information, Fig. S5, ESI†). This value is higher than the calculated 1.5 % for colloidal Dy³⁺(5 %)-doped LaVO₄ nanocrystals with an average size of 40 nm.¹⁵ Although the size of LaVO₄ nanocrystals is slightly smaller , it is reasonable to presume that the high phonon energy of vanadates (∼850 cm⁻¹) compared to PbF₂ may not lead to very high quantum yield. The above points clearly suggest that PbF₂ is a better host for the Dy³⁺ ions.

To understand the reason for the increased quantum efficiency for Dy³⁺ in PbF₂ nanocrystals, we compared the distance between the neighbouring Pb atoms in PbF₂ with that of the LaF₃ which is one of the superior hosts for the Ln³⁺ ions. The calculated Pb-Dy distance in our PbF₂:Dy nanocrystals is about 3.924 Å. This is done by considering the Pb-F distance (2.571 Å) and F-Pb-F angles (109.47°) from the refined parameters. This value is supported by the ∼4.0 Å Pb-Pb distance observed for the bulk PbF₂.¹⁷ Whereas the reported value for the La-Pr in LaF₃:Pr is close to 4.1 Å.¹⁸ The slightly larger distance for Pb-Dy compared to La-Pr implies that the distance between the neighbouring ions may not be the reason for the observed higher optimum dopant concentration. To verify whether PAA coating has any influence on the luminescence, we prepared Dy³⁺-doped PbF₂ without PAA coating under identical conditions. The observed lifetime for the Dy³⁺ is about 0.47 ms which is lower compared to the PAA coated PbF₂ nanocrystals. This implies that one of the reason for the enhanced quantum yield for Dy³⁺ in the PbF₂ matrix is the strong PAA coating which probably reduces the number of defects near the surface. The measured decay curve is displayed in Fig. S6 (ESI†).

Finally, to verify that PbF₂ is a good host for other Ln³⁺ as well, we prepared Eu³⁺-doped PbF₂ nanocrystals using a similar method. The emission spectrum of 5 % Eu³⁺-doped PbF₂ displayed in Fig. S7 (ESI†) exhibited strong Eu³⁺ signals in water. The calculated quantum yield for the Eu³⁺-doped sample is 16.21 % with reference to the quinine sulphate. This value is close to that of 16 % observed for Ce/Tb³⁺-doped LaF₃ nanocrystals.¹⁶ The fact the Ce³⁺ has higher absorption co-efficient suggests that observed quantum yield (16 %) for Eu³⁺ in PbF₂ is a good value. The high quantum yield is also supported by the long luminescence lifetime (1.21 ms) observed for the PAA coated Eu³⁺-doped PbF₂ nanocrystals. The decay curve is shown in Fig. S8 (ESI†).

Conclusions

In this article we report the hydrothermal synthesis of poly(acrylic acid) coated Dy³⁺-doped PbF₂ nanocrystals for the first time. The nanocrystals are in the size range of 50 nm. The strong binding of the polymer to the PbF₂ surface not only protects the Pb²⁺ from leaching in to the solution but enhances their dispersibility in aqueous solvents such as water. Upon UV excitation, the nanocrystals display strong blue and yellow luminescence characteristics of Dy³⁺ ions. Interestingly, the observed optimum Dy³⁺ concentration in PbF₂ nanocrystals is about 5 mol% which is higher compared to even some of the bulk oxide materials. This led to the enhanced Dy³⁺ luminescence quantum yield in PbF₂ nanocrystals. The better quantum efficiency, water dispersibility together with the high transparency of Ln³⁺-doped PbF₂ nanocrystals can be envisioned for bio-marking as well as in developing sol–gel based glasses.

VM thanks the Department of Science and Technology (DST), India and IISER-Kolkata for the funding. SS and CH thank UGC and IISER-K, respectively for the scholarship. VS thanks BARC, Mumbai for the financial support. We thank Dr. Aparna Laskar for helping us with TEM measurements. We thank the reviewers for the very valuable comments which helped us improving our article.

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Footnote

† Electronic Supplementary Information (ESI) available: Details of the synthesis, XRD patterns, TGA, quantum yield calculations, PL of Eu³⁺-doped PbF₂ nanocrystals. See DOI: 10.1039/c2ra21113k/

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