C-dot sensitized Eu³⁺ luminescence from Eu³⁺-doped LaF₃–C dot nanocomposites

Abstract

Very strong Eu³⁺ luminescence is achieved via sensitization by carbon dots (C-dots) in Eu³⁺-doped LaF₃–C-dot nanocomposites for the first time. This energy transfer via C-dots leads to the broadband excitation of Eu³⁺ ions which can have potential use in phosphor based LEDs.

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The lanthanide (Ln³⁺)-doped nanoparticles have been widely studied because of their sharp electronic transitions occurring within the 4f orbitals. These 4f–4f transitions are forbidden in nature and possess long luminescence lifetimes and narrow emission peaks. These parity forbidden electronic transitions are partly allowed due to mixing of 4f energy states with 5s and 5p orbitals.^1–3 The sharp transitions from Ln³⁺-doped materials have potential applications in phosphors, luminescence displays, light emitting diodes (LEDs), scintillators, bioimaging, etc.^4–6 For example, Eu³⁺-doped Y₂O₃ is a red phosphor, finding application in the cathode-ray tubes (CRT).⁷ Similarly, Ce³⁺-doped YAG is used in phosphor based LEDs.⁸ However, Ln³⁺-doped materials suffer from low luminescence quantum efficiency due to their low molar absorptivity and narrow absorption band.^9,10 To increase the absorption cross section of Ln³⁺ ions some strategies have been developed. One of these strategies is the sensitization of the Ln³⁺ emission via organic ligands (antenna effect).^11–13 In this approach, the organic ligands possessing a high absorption co-efficient are first excited followed by transfer of the excited energy to the Ln³⁺ ions via the triplet state of the organic molecules.^14,15 Another approach is sensitization of Eu³⁺ emissions through energy transfer from Bi³⁺ ions which have broad UV excitation. However, energy transfer from Bi³⁺ doping is generally observed in the solid state and mostly restricted up to 3 or 4% doping.^16,17 Thus it is an important challenge to find a strategy to enhance the luminescence efficiency of the Ln³⁺ ions particularly in water dispersible colloidal materials as they find interesting use in bio-imaging.¹⁸ In this work we have developed an approach where carbon dots (C-dots) have been used as a broadband sensitizer for the lanthanide ions such as Eu³⁺ as well as Tb³⁺ ions.

C-dots are a new class of carbon-based nanomaterials which have unique photoluminescence (PL) properties. They are first obtained during purification of single-walled carbon nanotubes through preparative electrophoresis in 2004.¹⁹ Several strategies have been reported for the preparation of C-dots.^20–27 The optical properties of the C dots are quite interesting. They show strong excitation dependent PL in the visible region. Though the exact reason for this is debatable, it is generally believed to happen due to a difference in size as well as the presence of several surface related energy states.^28–31 Our idea is to explore this broad excitation to enhance the luminescence quantum efficiency of Eu³⁺ ions in Eu³⁺-doped LaF₃–C dot nanocomposites.

In this communication we report the preparation of Eu³⁺-doped LaF₃–C dot nanocomposites through a hydrothermal process (see the ESI† for more details). Eu³⁺-doped LaF₃–C dot nanocomposites have strong luminescence of C-dots. Upon exciting in a C-dot band (340 nm), strong Eu³⁺ emission peaks were observed due to energy transfer from C-dots to Eu³⁺ ions present in LaF₃. The importance of the nanocomposites in achieving the energy transfer is studied.

Before preparing the nanocomposites, C-dots were prepared independently by the hydrothermal method and characterized. The X-ray diffraction (XRD) pattern of the C-dots shows a broad peak centered at 2θ = 28° which is characteristic of C-dots.³² The broad nature of the peak implies the amorphous nature of the C-dots. Transmission electron microscopy (TEM) indicates that the average size of the C-dots is about 3–4 nm using triethylene glycol as the carbon source. The TEM image and the XRD pattern of the C-dots are shown in Fig. 1.

Fig. 1 TEM image (A) and XRD pattern (B) of C-dots prepared using triethylene glycol as the carbon source.

The photoluminescence (PL) results of the C-dots are shown in Fig. S1 (ESI†). The C-dots show broad emissions in the region 400 nm and 560 nm when excited between 280 nm and 360 nm. A gradual shift towards longer wavelengths is noted with the increase in the excitation wavelength. These excitation dependent emission maxima are characteristic of C-dots and are mostly related to surface bound energy states.^30,31 The lifetime of the C-dots is about 4.02 ns by measuring the emission at 420 nm. The decay curve is shown in Fig. S2 (ESI†).

Fig. 2 shows the XRD pattern of Eu³⁺-doped LaF₃–C dot nanocomposites. The pattern clearly indicates the formation of hexagonal LaF₃. The peak positions and intensities match well with the standard pattern for pure hexagonal LaF₃ crystals (ICSD PDF Card No.; 01-082-0690). The large width of the diffraction peaks indicates the small size of particles. The lattice parameters are a = 7.16 and c = 7.34 Å. The average crystallite size calculated from the (111) diffraction peak of the XRD data based on the Debye–Scherrer formula is 11.5 nm. We emphasize that no characteristic peak of C dots near 2θ = 25° is noticed, which is most likely obscured by the LaF₃ peaks.

Fig. 2 XRD pattern of (A) Eu³⁺ doped LaF₃–C dot nanocomposites and (B) standard pattern for LaF₃.

To understand the shape and size of the Eu³⁺-doped LaF₃ –C dot nanocomposites, a TEM image was taken (shown in Fig. 3). The image indicates the formation of slightly distorted nanocomposites with an average diameter of 13–14 nm. This value is in agreement with that calculated from the Debye–Scherrer formula. We observed that some agglomeration of particles leading to bigger particles was also observed. We emphasized that the presence of C-dots in the nanocrystals is not clear. This might be due to the very small size of the C-dots (3–4 nm) as shown in Fig. 1A. However, energy dispersive X-Ray (EDX) spectroscopy of the nanocomposites shows the presence of C dots. EDX measurements indicate the presence of C, La, F and Eu³⁺ in the nanocomposites. To verify that the observed carbon peak is indeed due to C dots, we heated the Eu³⁺-doped LaF₃–C dot nanocomposites at 400 °C to remove the C dots. The resulting sample shows very less intense carbon peaks, mostly arising from the carbon present in the glass slide (see Fig. S3, ESI†). Further support for the presence of C-dots in the nanocomposites comes from zeta potential measurements. Zeta potential values of Eu³⁺-doped LaF₃–C dot nanocomposites, Eu³⁺-doped LaF₃ and free C dots are −18.7 mV, 5 mV and −13 mV at pH 7, respectively. These results indicate the presence of C-dots in the nanocomposites and also suggest the stability of nanocomposites in water. In addition, UV absorbance peaks of the nanocomposites show a characteristic peak which is similar to the C dots absorbance spectra reported in the literature (see Fig. S4, ESI†).³³

Fig. 3 TEM image of Eu³⁺-doped LaF₃–C dot nanocomposites.

The PL studies confirm the presence of C-dots in the nanocomposites. Fig. 4 shows the PL spectrum of the water dispersed Eu³⁺-doped LaF₃–C dot nanocomposites at different excitation wavelengths from 280 to 360 nm. Strong C-dot emission peaks are observed from 360 to 550 nm when excited between 280 and 360 nm. They show maximum emission intensity at 420 nm when excited at 340 nm. The corresponding excitation spectrum (see the inset of Fig. 4) indicates excitation maxima at 340 nm. In addition, strong Eu³⁺ emission peaks are observed near 590 and 612 nm upon C-dot excitation (i.e. at 340 nm). These transitions are assigned to ⁵D_o → ⁷F₁ and ⁵D_o → ⁷F₂ transitions, respectively. The PL spectrum of the nanocomposites is shown in Fig. 5 along with the PL spectrum of Eu³⁺-doped LaF₃ nanoparticles. The strong Eu³⁺ emissions from the nanocomposites are believed to occur via energy transfer from C-dots. This is verified from the excitation spectrum of the nanocomposites which shows a broadband characteristic of C-dots along with sharp peaks from Eu³⁺ due to ⁵L₀ ← ⁷F₀ (see the inset of Fig. 5). In addition, under identical conditions the PL analysis of Eu³⁺-doped LaF₃ nanoparticles shows only very weak Eu³⁺ emission intensity (see Fig. 5). This strongly suggests that the strong Eu³⁺ emissions at 340 nm excitation are due to C-dot sensitization.

Fig. 4 PL spectra of Eu³⁺-doped LaF₃–C dot nanocomposites at different excitations. Inset shows the excitation spectrum of Eu³⁺-doped LaF₃–C dot nanocomposites obtained by measuring at 420 nm emission.

Fig. 5 PL spectra of Eu³⁺-doped LaF₃–C dot nanocomposites (black trace) and Eu³⁺-doped LaF₃ (red trace). Inset shows the corresponding excitation spectra (right) and the digital image (left) of the nanocomposites under UV light using a 550 nm cut off filter.

We believe the energy transfer from the C-dots to the Eu³⁺-doped LaF₃ is likely of the FRET type. This is supported by the overlap between the emission spectrum of the C-dots and the excitation spectrum of the Eu³⁺-doped LaF₃ (shown in Fig. S5, ESI†). We emphasize that the emission intensity of Eu³⁺ ions in the nanocomposites upon 340 nm excitation is more intense compared to 394 nm excitation (see Fig. S6, ESI†). The C-dot sensitization of the Eu³⁺ ions is further verified using lifetime measurements. The lifetimes were measured by exciting the sample at 340 nm and monitoring the emission at 612 nm. This value is 0.32 ms for the Eu³⁺-doped LaF₃–C dot nanocomposites, whereas it is about 0.27 ms for the Eu³⁺-doped LaF₃ nanoparticles at 394 nm excitation. The results are shown in Fig. S7 (ESI†). To evaluate the energy transfer efficiency, we measured the donor (C-dots) lifetime with and without the acceptor (nanocomposites) using the equation 1 − τ₀/τ where τ is the lifetime of the C-dot emission in Eu³⁺-doped LaF₃–C dot nanocomposites and τ₀ is the lifetime of the C-dots. The values of τ and τ₀ are 3.6 ns and 4.02 ns respectively. The calculated energy transfer efficiency is about 11%. The lifetime decay curves of both C-dot emission and the same from nanocomposites are shown in Fig. S8 (ESI†). Though this efficiency seems to be low, it is good as lanthanide ions generally possess low absorption efficiency.

To understand the importance of the nanocomposite formation in the energy transfer, we performed the following control experiment. A physical mixture of Eu³⁺-doped LaF₃ nanoparticles and C-dots was prepared by simple mixing the corresponding dispersions. This physical mixture shows strong C-dots emission and only very weak Eu³⁺ emission upon excitation at 340 nm. This suggests that formation of nanocomposites is important to observe C-dot sensitized Eu³⁺ emission (see Fig. S9, ESI†). Furthermore, a simple mixture containing the C-dots and Eu(NO)₃ in water shows only an intense C-dot emission (see Fig. S10, ESI†). All these imply the importance of doping Eu³⁺ ions in a suitable matrix and the formation of nanocomposites.

We have extended a similar study to Tb³⁺ ions by preparing Tb³⁺-doped LaF₃–C dot nanocomposites under similar experimental conditions. The PL study shows strong Tb³⁺ emission upon C-dot excitation with a background emission from C-dots. The PL spectrum of Tb³⁺-doped LaF₃–C dot nanocomposites is shown Fig. S11 (ESI†) along with Tb³⁺-doped LaF₃ nanoparticles. Here again the C-dot sensitization leads to higher Tb³⁺ emission intensity compared to Tb³⁺-doped LaF₃ nanocrystals alone. Extending the study to other Ln³⁺ ions such as Dy³⁺ and Tm³⁺ is obscured by strong C-dot emission in their emission region.

In conclusion we reported the hydrothermal synthesis of Eu³⁺-doped LaF₃–C dot nanocomposites and observed very strong Eu³⁺ luminescence via sensitization by carbon dots (C-dots) for the first time. This energy transfer via C-dots leads to broadband excitation. The maxima of this broadband excitation are toward far UV (340 nm) compared to direct excitation of Eu³⁺ (394 nm) which can have potential use in phosphor based LEDs.

Experimental

In a typical procedure, 15 mL of triethylene glycol (TREG) was added to La(NO₃)₃ (0.95 mmol) and Eu(NO₃)₃ (0.05 mmol) and heated at 40 °C with vigorous stirring to make a homogeneous dispersion. After 1 h of stirring, solution was transferred to the Teflon-lined stainless-steel autoclave with a total volume of 100 mL. The autoclave was sealed and maintained at 200 °C for 10 h. After the autoclave was cooled to room temperature naturally, the products were collected by centrifugation, washed several times with absolute ethanol, and then dried in a vacuum desiccator. Supernatant carbon dots (C dots) were also collected after centrifugation and stored at 4 °C.

VM thanks the Department of Science and Technology (DST), India, Council for Scientific and Industrial Research (CSIR), and IISER-Kolkata for the funding. TS and CH thank UGC and IISER-K, respectively, for the scholarship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthesis of Eu³⁺-doped LaF₃–C dot nanocomposites, Eu³⁺-doped LaF₃–C dot nanocomposites, Tb³⁺-doped LaF₃–C dot nanocomposites, Dy³⁺-doped LaF₃–C dot nanocomposites and Tm³⁺ doped LaF₃, TEM, EDAX, the lifetime of nanocomposites, UV-Vis spectra, PL of a physical mixture of Eu³⁺-doped LaF₃ and C dots and PL of Tb³⁺ doped LaF₃–C-dot nanocomposites and Tb³⁺ doped LaF₃. See DOI: 10.1039/c4nj01647e

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