Color tunable YF₃: Ce³⁺/Ln³⁺ (Ln³⁺: Eu³⁺, Tb³⁺, Dy³⁺, Sm³⁺) luminescent system: role of sensitizer and energy transfer study

Abstract

Nano-fluorides with attractive optical properties have been proposed as potential phosphors for solid state lighting and bio-applications. In the present work, redispersible YF₃: Ln³⁺ and YF₃: Ce³⁺/Ln³⁺ nanocrystals were obtained by a simple one step ethylene glycol mediated soft chemical synthesis. Uniformly distributed nanoparticles with a leaf-like morphology and particle size of 10–15 nm were observed. X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transformed infrared spectroscopy (FT-IR), photoluminescence (PL) and decay studies were employed to characterize the samples. The highly intense emissions of various activators (Dy³⁺, Tb³⁺, Eu³⁺ and Sm³⁺) in the presence of Ce³⁺, their long lifetimes (∼ms) and significant reduction in the lifetime of Ce³⁺ proved an efficient energy transfer operating in the nanocrystals. The time resolved emission studies indicated towards the presence of two different sites for the activator ions. Under ultraviolet irradiation, the emission color of YF₃: Ce³⁺/Ln³⁺ nanophosphors can be easily tuned by changing the Ln³⁺ ions. Bright blue (Dy³⁺), green (Tb³⁺), red (Eu³⁺) and purple (Sm³⁺) emissions were observed from corresponding YF₃: Ce³⁺, Ln³⁺ nanocrystals under ultraviolet light.

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1. Introduction

Research on fluorescent nanomaterials continues to be a key area of research in the recent years. In particular, controllable energy transfer in the nanomaterials has been the focus of great interest because it leads to luminescence signals of outstanding selectivity and high sensitivity, which are important factors for optoelectronics, optical sensors and bio-applications. The need to develop future generation mercury-free lamps for solid state lighting is dependent on discovering new and efficient phosphors that are capable of transforming UV light into visible one. The conditions that ought to be satisfied by these novel phosphors are efficient transformation of vacuum UV radiation into visible light as well as a high radiation resistance against vacuum UV irradiation and particle bombardment during operation in the dielectric barrier discharge lamp.

In this context, fluoride nanoparticles doped with rare-earth ions have attracted tremendous interest over last few years because of their unique luminescence properties with useful applications in optical telecommunication, lasers, diagnostics and biological labels.^1,2,3 The fluoride lattice enables the high coordination numbers for guest rare-earth ions and the high ionicity of the rare-earth to fluorine bond results in a very wide band gap and low phonon energies. It is very well established that high phonon energies of the host lattices are primarily responsible for non-radiative relaxation in cases of rare-earth ions. The fluoride lattices are superior in comparison to oxygen-based systems as fluorides possess very low vibrational energies and therefore the quenching of the excited states of the rare earth ions is minimal.⁴ This leads to a low probability of non-radiative decay for rare-earth ions and consequently, the luminescence quantum yields are higher than in the oxides and most other inorganic matrices.⁵

Following this, studies have been done on yttrium trifluoride (YF₃) as a host lattice for lanthanide-doped phosphors. YF₃ doped with rare-earth ions is capable of producing efficient visible emission under vacuum UV irradiation^6,7 and is therefore a good candidate for a potential phosphor in this region. This material provides the possibility for doping with rare earth ions due to its wide band gap ( > 10 eV) and suitable Y³⁺ sites where other trivalent rare-earth elements can be easily substituted without additional charge compensation. The potential applications for YF₃ include new laser materials or an up-conversion biolabel.^8,9,10 Lemyre et al.¹¹ reported hexagonal and quadrilaterally-shaped YF₃ nanoparticles. Li and co workers also prepared fullerene-like,¹² and rice-like Ln³⁺-doped YF₃ nanoparticles^13,14 and investigated their fluorescence and up-conversion luminescence properties. The multicolor upconversion and photoluminescence has also been studied in detail in Yb³⁺/Tm³⁺, Yb³⁺/Ho³⁺ and Yb³⁺/Er³⁺ doped YF₃ sub-microflowers when excited by a 980 nm laser diode by Wang et al.¹⁵ Recently, Tao et al.¹⁶ reported uniform and monodisperse YF₃:Eu³⁺ nanosized and sub-microsized truncated octahedra and observed Eu²⁺ emission when excited at 393 nm. For certain applications such as multiplex labelling, which is of special interest presently in the biological field, nanocrystals of different emission colors should be readily available and the whole group of nanocrystals should be excited at a single wavelength. Although lanthanide-doped nanocrystals of different emission colours can be made by varying the dopants, it is usually difficult to excite all these nanocrystals by single wavelength irradiation since each lanthanide activator has a special set of energy levels. A possible way to circumvent this problem is to use a suitable sensitization, that is to say, codoping the same sensitizing ion for all the lanthanide activators. In this case, a single-wavelength irradiation can be used to excite the sensitizers, followed by energy transfer to and emission from the lanthanide activators. Moreover, for biological labelling and for ease of processing the nanocrystals for various applications, the nanocrystals should be water-dispersible/soluble. Conventionally, lanthanide-doped nanocrystals are made at high temperatures^17,18 or in organic environments^19,20 yielding products that are not soluble in aqueous media. In recent years, much effort has been made to prepare lanthanide-doped nanocrystals that are water-soluble and have functional chemical groupsvia post³ or in situ^21,22 surface modifications of the nanocrystals. However, little research work dealing with multicolor lanthanide-doped nanocrystals has been done so far.²³

In view of this, redispersible YF₃ nanoparticles, both singly doped as well as co-doped with Ce³⁺ (YF₃: Ce³⁺/Ln³⁺) and various activator ions, have been synthesized by a relatively low temperature polyol mediated synthesis using ethylene glycol as a solvent and stabilizing ligand. Ce³⁺ has been used as the sensitizer in combination with different activators (viz.Tb³⁺, Eu³⁺, Sm³⁺ and Dy³⁺). These nanocrystals have been studied for their room temperature luminescence behavior and the energy transfer mechanism is also discussed.

2 Experimental

2.1 Reagents

High purity (99.9%) yttrium oxide [Y₂O₃], europium nitrate [Eu(NO₃)₃·6H₂O], terbium nitrate [Tb(NO₃)₃·4H₂O], dysprosium nitrate, [Dy(NO₃)₃·5H₂O] and sodium fluoride [NaF] were obtained from commercial sources (Aldrich). All chemicals were used as obtained, without further purification.

2.2 Synthesis of doped yttrium fluoride nanoparticles; YF₃: Ln³⁺ (Ln³⁺: Ce³⁺, Tb³⁺, Eu³⁺, Dy³⁺, Sm³⁺)

In a typical synthesis, a stoichiometric amount of Y₂O₃ was dissolved in a minimum amount of HNO₃ to get Y(NO₃)₃. It was evaporated to almost dryness and was re-dissolved in 25 ml of ethylene glycol. The stoichiometric amounts of respective rare-earth nitrates were also added and the solution was heated to 100 °C. At this temperature, NaF dissolved in 25 ml of ethylene glycol and heated to 100 °C was added drop wise to the reaction mixture. The temperature was raised to 160 °C and refluxed for two hours. The resultant precipitate was washed several times with acetone and ethanol and dried under ambient conditions. The powder samples were found to be readily dispersible in solvents like methanol and water.

2.3 Material characterization

The starting materials and all the nominal compositions were characterized by powder X-ray diffraction. X-Ray diffraction (XRD) measurements were carried out on a Philips instrument, operating with Cu-Kα radiation (λ = 1.5406 Å) with a scan rate of 0.02° s⁻¹ in the scattering angular range (2θ) of 10⁰ to 70⁰. Silicon was used as an external standard for correction due to instrumental broadening. The XRD patterns were analyzed by comparing with the reported ones. The average crystallite size was calculated from the diffraction line width based on Scherrer's relation: d = 0.9λ/Bcosθ, wherein λ is the wavelength of X-rays and B is the corrected full width at half maxima (FWHM). High resolution TEM micrographs were recorded on a JEOL 2000FX. The samples were dispersed in methanol solution and then deposited on the carbon coated copper grids for TEM/SAED studies. Photoluminescence (PL) and lifetime investigations were carried out on an Edinburgh F-LSP920. A microsecond 100 W Xe flash lamp was used as the excitation source. The FTIR spectra of the solid samples were recorded in KBR using a Bomem MB102 FTIR (model 610) in the mid IR region (4000–400 cm⁻¹) equipped with a DTGS detector having a resolution of 4 cm⁻¹. For this purpose about 200 mg of dry KBr was mixed with 10 mg of the sample. It was ground and pressed into a transparent, thin pellet at 5 Tons cm⁻². These pellets were used for IR spectral measurements.

3. Results and discussion

As-prepared YF₃ nanocrystals were characterized by detailed powder X-ray diffraction, high resolution transmission electron microscopy and infra-red spectroscopy. The corresponding results are discussed below.

The XRD patterns for the characteristic as-prepared undoped as well as doped YF₃ nanopowders are given in Fig. 1. YF₃ is known to crystallize in an orthorhombic structure [space group: Pnma (62)]. As demonstrated in Fig. 1 all the diffraction peaks can be easily indexed to a pure orthorhombic structure of YF₃ without the presence of any extra phase. The site symmetry of the Y³⁺ ion in YF₃ lattice is C_S.

Fig. 1 XRD pattern of the characteristic undoped, singly-doped and co-doped YF₃ where (a) YF₃, (b) YF₃: 2.5 mol% Tb³⁺, (c) YF₃: 2.5 mol% Ce³⁺, 2.5 mol% Tb³⁺.

The lattice parameters have been refined and the lattice constants of pure YF₃ were calculated to be, a = 6.352 Å, b = 6.850 Å, and c = 4.398 Å, which are in good agreement with those reported in the literature (JCPDS Card No. 74-0911). The powder patterns of rare-earth doped YF₃ are similar to that for the undoped YF₃ nanopowders, which indicate that the rare earth dopants are occupying the Y³⁺ site in the lattice and single-phasic products are obtained. Based on the peak width, the particle size was evaluated to be 12–15 nm from Scherrer's formula for different samples. Doubly-doped YF₃ samples containing activator ions co-doped with Ce³⁺ were also synthesized and their XRD patterns were found to be similar to undoped and singly doped YF₃ thus showing that the orthorhombic structure is retained in all the samples (Fig. 2c).

Fig. 2 FT-IR spectrum as recorded on as-prepared YF₃: 2.5 mol% Tb³⁺ sample.

To explore the surface of the nanoparticles, infrared spectroscopic (IR) studies were performed on the representative YF₃ and doped YF₃ nanopowders. FT-IR spectrum recorded on the as-prepared YF₃: the 2.5 mol% Tb³⁺ sample is shown in Fig. 2. The prominent peaks were observed at 770, 1082, 1462, 1620 and 3360 cm⁻¹. Peaks at 1662 cm⁻¹ and 3360 cm⁻¹ can be ascribed to the bending and stretching vibrations of the –OH group of ethylene glycol which is used as the solvent as well as capping agent in the polyol synthesis.

It is known that free –OH has a stretching frequency at ∼3650 cm⁻¹ which is found to be absent. This indicates the presence of hydrogen bonds in ethylene glycol molecules. Careful examination of the IR spectra shows that on the broad hump (due to –OH stretching vibrations), a small shoulder at ∼2902 cm⁻¹ is present which can be ascribed to stretching vibrations of a –CH₂group of ethylene glycol molecules and its bending vibration is observed at 1462 cm⁻¹. All these observations support the presence of ethylene glycol molecules on the surface of nanoparticles. The advantage of the ethylene glycol based method is that the nanoparticles get stabilized with ethylene glycol moieties which are easily dispersible in solvents like water and methanol. This facilitates the use of such nanoparticles for biomedical applications and imaging.

The structure and the morphology of the nanoparticles was further explored by the high resolution transmission electron microscopy (HR-TEM) studies. HR-TEM images and selected area electron diffraction image of the as-prepared doubly-doped YF₃: 2.5 mol% Ce³⁺, 2.5 mol% Tb³⁺ sample are shown in Fig. 3. Uniformly distributed nanoparticles with particle size 10–15 nm could be observed (as measured by XRD line broadening also). The particles possess a very peculiar leaf-like morphology. The selected area electron diffraction (SAED) image of the sample shows highly crystalline nature of this nanopowder.

Fig. 3 High Resolution Transmission Electron Microscopy (HR-TEM) of YF₃: 2.5 mol% Ce³⁺, 2.5 mol% Tb³⁺.

Photoluminescence studies

The luminescence studies were performed on as-synthesized YF₃ nanoparticles. The photoluminescence spectra of YF₃: 2.5 mol% Ce³⁺ is given in Fig. S1, ESI.† The YF₃:Ce³⁺ system shows a broad excitation hump peaking at 260 nm and the corresponding emission peak is observed at 313 nm which is the characteristic Ce³⁺ 5d→4f emission and is in agreement with that reported in literature. The emission spectrum of YF₃: 2.5 mol% Tb³⁺ is dominated by peaks at 487 nm, 543 nm, 580 nm and 620 nm which are due to transitions from ⁵D₄ to ⁷F_J energy states of Tb³⁺ (Fig. 4a). The excitation spectrum corresponding to the main emission at 543 nm (⁵D₄→⁷F₅) showed broad peaks at 285 nm and 313 nm (f-d transitions of Tb³⁺) (Inset, Fig. 4a). Whereas in the case of Eu³⁺ as the dopant, characteristic Eu³⁺ features at 590 nm (⁵D₀→⁷F₁) and at 613 nm (⁵D₀→⁷F₂) were observed (Fig. 4b).

Fig. 4 Emission spectra of (a) YF₃: 2.5 mol% Tb³⁺ (λ_ex = 318 nm), (b) YF₃: 2.5 mol% Eu³⁺ (λ_ex = 320 nm). Insets give the corresponding excitation spectra. Insets give the corresponding excitation spectra.

The corresponding excitation spectrum is characterized by broad bands peaking at 280 nm and 320 nm and a sharp but a relatively smaller peak at 393 nm (which is the direct excitation for Eu³⁺) (Inset, Fig. 4b). Because of the presence of significant amount of Eu³⁺ on the surface, which is exposed to more non-centrosymmetric environment, the parity forbidden transition (⁵D₀→⁷F₂) at 613 nm has greater intensity in the steady state emission spectrum which is quite unlike the emission spectra observed for bulk/micron-sized YF₃ powders. Interestingly, for emission spectra recorded at 280 nm and 320 nm, 613 nm peak (⁵D₀→⁷F₂) has higher intensity than 590 nm peak (⁵D₀→⁷F₁). Since, the ⁵D₀→⁷F₂ transition is electric dipole allowed, implies that Eu³⁺ ions occupy lower symmetry site. However, on exciting the samples at 393 nm, the 590 nm peak (⁵D₀→⁷F₁) was found to have higher intensity. This shows that there are two types of Ln³⁺ sites in the YF₃ nanoparticles synthesized by this route. Since the orthorhombic space groupPnma to which YF₃ belongs possesses only one Y³⁺ site which is symmetric, the other Eu³⁺ site could be the one present on nanoparticles surface which is exposed to the asymmetric environment. Also, the Y³⁺ site in YF₃ is centrosymmetric so this also implies that the fraction of Eu³⁺ ions in bulk is more than that on the surface. Further, the transitions from ⁵D₁ are also observed in addition to those normally observed from ⁵D₀ and this can be attributed to the absence of self quenching and low phonon energies of the host YF₃ lattice. YF₃: 2.5 mol% Dy³⁺ shows traces of emission peaks at 470 nm (⁴F_9/2→⁶H_15/2) and 571 nm (⁴F_9/2→⁶H_13/2) which are characteristics of Dy³⁺ emission spectrum (Fig. 5a). Singly doped YF₃: Dy³⁺ are weakly emitting nanopowders as is also visible from PL spectrum shown in (Fig. 5a). YF₃: 2.5 mol% Sm³⁺ did not show any noticeable emissions (Fig. 5b).

Fig. 5 Emission spectra of (a) YF₃: 2.5 mol% Dy³⁺ (λ_ex = 288 nm), (b) YF₃: 2.5 mol% Sm³⁺ (λ_ex = 240 nm). Insets give the corresponding excitation spectra.

In general, for singly-doped YF₃ it can be inferred that except for Tb³⁺ and Eu³⁺, the rare earth ions show extremely weak emission spectra (e.g.Dy³⁺ and Sm³⁺ did not show any significant peak). This could be due to very weak (f–f) absorption coefficients of lanthanide ions.

One way to overcome this drawback for useful applications as visible emitters is by increasing the absorption by activator lanthanide ion through efficient energy transfer. For this purpose, doubly doped YF₃ nanopowders were synthesized by co-doping Ce³⁺ along with activator ions (YF₃: 2.5 mol% Ce³⁺, 2.5 mol% Ln³⁺). To study the influence of the sensitizer Ce³⁺ on the luminescence behavior of activators, detailed photoluminescence studies were carried out on doubly doped YF₃ samples. A significant increase in the emission intensity of all the activators was observed as the consequence of using Ce³⁺ as sensitizer/codopant. Fig. 6(a–d) gives emission spectra for YF₃ : 2.5 mol% Ce³⁺, 2.5 mol% Ln³⁺. The insets in Fig. 6(a–d) show the excitation spectra corresponding to the characteristic emissions of respective activators (the corresponding emission wavelengths are mentioned in the Figure). On exciting these nanopowders by UV radiation (250–256nm), different colors are emitted by these nanopowders depending on the activator present (Fig. 6(a–d)). Hence the color properties of these YF₃: Ce³⁺/Ln³⁺ nanopowders can be tuned by choosing different Ln³⁺ and these nanoparticles could have potential applications in multiplex labelling.

Fig. 6 Emission spectra for YF₃ : 2.5 mol% Ce³⁺, 2.5 mol% Ln³⁺ (a) YF₃ : 2.5 mol% Ce³⁺, 2.5 mol% Tb³⁺ , (b) YF₃ : 2.5 mol% Ce³⁺, 2.5 mol% Dy³⁺ , (c) YF₃ : 2.5 mol% Ce³⁺, 2.5 mol% Sm³⁺ , (d) YF₃ : 2.5 mol% Ce³⁺, 2.5 mol% Eu³⁺.

In all the excitation spectra recorded at the emission wavelength of the respective activator, a broad peak at 240–260 nm was observed apart from other peaks. It is well documented that this peak at 240–260 nm corresponds to the transitions from the ground state ²F_5/2 of Ce³⁺ to the different components of the excited Ce³⁺ 5d states split by the crystal field.²⁴ Thus it exhibits an energy transfer operating from Ce³⁺ (which is acting as the sensitizer) to the activators.

In case of YF₃: Ce³⁺, Tb³⁺, a closer look at the emission spectrum (Fig. 6a) shows that there are emission peaks from both ⁵D₃ (385 nm, 415 nm) and ⁵D₄ (489 nm, 543 nm, 586 nm, 619 nm) levels to ⁷F_J levels. However, the appearance of Ce³⁺ emission peak at ∼320 nm along with characteristic Tb³⁺ peaks indicates that the energy transfer from sensitizer to the luminescent centres is incomplete which is getting manifested as the emission from sensitizer Ce³⁺. In the case of the excitation spectrum of YF₃: Ce³⁺, Dy³⁺, in addition to the a hump at 256 nm, also present in the Tb³⁺ doped sample, a smaller hump at 287 nm is also observed which can be attributed to the direct excitation of Dy³⁺ (Fig. 6b). It is noticeable that the emission spectrum of YF₃: Ce³⁺, Dy³⁺, shows well defined peaks at 478 nm, 571 nm and 658 nm (due to transitions from ⁴F_9/2 level) which otherwise had very weak intensities in the emission spectrum of singly doped sample, YF₃ : Dy³⁺ (Fig. 5a). Similarly, as was mentioned earlier also, in the case of YF₃ : Sm³⁺, very weak emission peaks were observed (Fig. 5b) whereas co-doping Ce³⁺ with Sm³⁺ causes a manifold increase in the emission intensity of Sm³⁺ (Fig. 6c). The maximum effect of codoping Ce³⁺ was observed on Dy³⁺ and Sm³⁺ emission spectra since in these two cases the characteristic emission peaks were so weak that they were almost hidden in the background in the singly doped samples. On the contrary, codoping Ce³⁺ with Eu³⁺ does not lead to any marked improvement in the emission characteristics of europium. It has been postulated that in the presence of Ce³⁺, quenching of the emission in Ce³⁺ and Eu³⁺ co-doped systems occur. The Ce³⁺ electron excited to the lowest 5d state can jump to Eu³⁺ when the unoccupied Eu²⁺ ground state is located at a lower energy than the occupied lowest Ce³⁺ 5d excited state. After the jump, Eu²⁺ and Ce⁴⁺ are formed. The Eu²⁺ electron can jump back to Ce⁴⁺ if the unoccupied Ce³⁺ ground state is located below the occupied Eu²⁺ ground state. The original situation is restored without emission of a photon.²⁵

Based on the above discussion it can be inferred that there exists strong energy transfer between Ce³⁺ donor to acceptor ions (except in the case of Eu³⁺) and that is responsible for the bright luminescence from the nanoparticles on UV excitation.

To study the dynamics of energy transfer, lifetime measurements were done on all the singly and doubly doped YF₃ systems. In all the samples doped with Eu³⁺, Dy³⁺ and Sm³⁺ and Tb³⁺ (singly or co-doped with Ce³⁺) for which life-time could be determined, it was observed that decay-curves could be fitted as bi-exponential decays with occupancy of ∼20% with a shorter life-time and rest with a longer life-time. The observation of two life-times indicates that these ions are occupying two sites with differences in local environment. However, XRD studies have revealed that rare earths are incorporated substitutionally at Y³⁺ site. Hence, in the host lattice, there is only one site with a centre of symmetry C_s for substitutional incorporation. It is probable that majority of ions are incorporated at substitutional Y³⁺ site while the ions at surface of the nanoparticles may be experiencing different local environment and hence shorter life-time due to presence of surface defects.

Table 1 and 2 enlist the lifetimes observed for various singly doped and doubly doped samples. Dy³⁺ and Sm³⁺ show extremely weak peaks in the absence of Ce³⁺ as the sensitizer and hence their lifetimes (in singly doped samples) are not reported. It can be seen that there is a marked increase in the lifetime of Tb³⁺ in presence of Ce³⁺. Similarly, Dy³⁺ and Sm³⁺ also show appreciable lifetimes. On the contrary, there were no significant changes in the lifetime of Eu³⁺ in the presence of sensitizer (Ce³⁺), as also evident from the Tables 1 and 2, which again indicates lack of energy transfer from Ce³⁺ to Eu³⁺. Furthermore, it is worth noting that the lifetime of Ce³⁺ in YF₃, when singly doped, is ∼30 ns as reported in the literature.²⁶ In almost all the co-doped samples, it has been reduced to 15 ns or less, thereby indicating that the lifetime for non-radiative processes undergone by Ce³⁺ reduces and the energy is being transferred to the activator ions. This is accompanied by a significant enhancement in the lifetime values of the activator ions in presence of the Ce³⁺. This supports the energy transfer operating from Ce³⁺ to activator (Ln³⁺) which leads to the reduction of Ce³⁺ lifetime. This is expected to have a positive bearing on the emission properties and quantum efficiency of the activators.

Table 1 Lifetimes of singly doped samples YF₃: 2.5 mol% Ln³⁺ (Ln³⁺: Ce³⁺, Tb³⁺, Dy³⁺, Sm³⁺ and Eu³⁺)

Composition	τ 1(μsec)	τ 2(μsec)
YF3 : 2.5 mol% Ce^3+	30 ns	—
YF3 : 2.5 mol% Tb^3+	1500	5500
YF3 : 2.5 mol% Eu^3+	860	6290
YF3 : 2.5 mol% Sm^3+	weak emission	weak emission
YF3 : 2.5 mol% Dy^3+	weak emission	weak emission

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Table 2 Lifetimes of Ln³⁺ (Tb³⁺, Dy³⁺, Sm³⁺ and Eu³⁺) and Ce³⁺ in co-doped samples, YF₃: 2.5 mol% Ln³⁺, 2.5 mol% Ce³⁺ (Tb³⁺, Dy³⁺, Sm³⁺ and Eu³⁺)

Composition	τ 1 (μsec)	τ 2 (μsec)	τ(Ce^3+ lifetime)
YF3:2.5 mol% Ce^3^+,2.5 mol% Tb^3	2880	5980	13.5 ns
YF3:2.5 mol% Ce^3^+,2.5 mol% Eu^3+	1000	6600	15 ns
YF3:2.5 mol% Ce^3^+,2.5 mol% Sm^3+	200	4000	15 ns
YF3:2.5 mol% Ce^3^+,2.5 mol% Dy^3+	400	1400	14 ns

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In order to understand the local environment for the two sites, detailed investigations were carried out by recording time resolved emission spectra for YF₃: Eu³⁺ and YF₃:Ce³⁺, Eu³⁺ samples (Fig. 7). At smaller delay times, the intensity of 613 emission peak was found to be greater than 590 nm peak and vice versa was observed for larger delay times, i.e. at larger delay times, 590 nm emission is more intense than the 613 nm emission peak. Now at larger delay times the smaller lifetime component has already decayed and the emission profile is primarily due to larger lifetime component. Also, the 590 emission peak is magnetic dipole allowed which is not sensitive to the site symmetry. From these above-mentioned observations, it can be concluded that larger lifetime component is in a more symmetric environment compared to smaller lifetime component. These findings further support that the majority of ions are incorporated at substitutional sites with a symmetric environment, while the rest of ions at the surface are in an asymmetric environment.

Fig. 7 Time resolved emission spectra of (a) YF₃: 2.5 mol% Eu³⁺ nanoparticles at the delay time of 0.65 ms and 7.25 ms; (b) YF₃: 2.5 mol% Ce³⁺, 2.5 mol% Eu³⁺ nanoparticles at the delay time of 0.13 ms and 8.13 ms after the pulse.

Owing to the strong absorption in the UV range (200–300 nm) due to the allowed transitions of Ce³⁺, followed by efficient energy transfer, intense emissions are observed corresponding to the respective activators. The Ce³⁺ to Ln³⁺ energy transfer efficiency can also be estimated based on the equation η = 1 − I_d/I_d0 = 1 − τ_d/τ_d0, where I_d, I_d0 and τ_d, τ_d0 are the intensity of the emission and excited state lifetimes of the donor in the presence and absence of the acceptor, respectively. In a powder pattern it is quite possible that the intensities of the peaks may not be true representations of the luminescence efficiency, hence the lifetime values are used for calculating the energy transfer efficiency. It is exemplified by calculating the energy transfer efficiency for Ce³⁺ to Sm³⁺. The lifetime of Ce³⁺ in the absence of any activator in YF₃: 2.5 mol% Ce³⁺ was found to be 30 ns. In YF₃: 2.5 mol% Ce³⁺, 2.5 mol% Sm³⁺, the lifetime of Ce³⁺ was found to be 15 ns. Hence, the energy transfer efficiency from Ce³⁺ to Sm³⁺ is 50%. However, in the case of YF₃: Ce³⁺, Eu³⁺, a reduction in the lifetime of Ce³⁺ is also observed even though there is no increase in the luminescence properties on co-doping with Ce³⁺ which indicates that the non-radiative mechanisms are also operating in the lattice.

4. Conclusions

To conclude, redispersible YF₃: Ln³⁺ (Ln³⁺: Eu³⁺, Tb³⁺, Dy³⁺, Sm³⁺) and YF₃: Ce³⁺/Ln³⁺ nanoparticles could be obtained by a simple polyol route. Formation and stabilization of the nanoparticles by ethylene glycol (used as the solvent) could be confirmed by X-ray diffraction and FT-IR spectroscopy respectively. By introducing Ce³⁺ as a sensitizer in YF₃: Ln³⁺, these nanopowders efficiently absorb UV-light (∼256 nm) due to Ce³⁺ absorption and produce various luminescent colors on single wavelength excitation. The time resolved studies and the change in luminescence behavior of Eu³⁺ doped samples on exciting at different wavelengths indicated the presence of two different sites for the rare earth ions in YF₃ nanoparticles. Reduction in lifetime of Ce³⁺ emission when doped along with activators and increase in lifetimes of activators (except Eu³⁺), supported the energy transfer taking place. Thus these results suggest that these nanoparticles can be explored for their potential use as tags in multiplexing biological labelling as well as phosphors for solid state lighting.

References

1. M. Dejneka, E. R. Snitzer and E. Riman, J. Lumin., 1995, 65, 227–245.
2. S. Tanabe and T. Tamaoka, J. Non-Cryst Solids, 2003, 283, 326–327.
3. G. Yi, H. Lu, S. Zhao, G. Yue, W. Yang, D. Chen and L. H. Guo, Nano Lett., 2004, 4, 2191–2196.
4. C. M. Bender and J. M. Burlitch, Chem. Mater., 2000, 12, 1969–1976.
5. Z. L. Wang, H. L. W. Chan, H. L. Li and J. H. Hao, Appl. Phys. Lett., 2008, 93, 141106.
6. P. S. Peijzel, R. T. Wegh, A. Meijerink, J. Holsa and R. J. Lamminmaki, Opt. Commun., 2002, 204, 195–202.
7. W. W. Piper, J. A. DeLuca and F. S. Ham, J. Lumin., 1974, 8, 344–348.
8. K. J. Guedes, K. Krambrock and J. Y. Gesland, J. Phys. Chem. Solids, 2001, 62, 485–489.
9. M. M. Lage, A. Righi, F. M. Matinaga, J. Y. Gesland and R. L. Moreira, J. Phys.: Condens. Matter, 2004, 16, 3207–3218.
10. Y. W. Zhang, X. Sun, R. Si, L. P. You and C. H. Yan, J. Am. Chem. Soc., 2005, 127, 3260–3261.
11. J. L. Lemyre and A. M. Ritcey, Chem. Mater., 2005, 17, 3040–3043.
12. R. X. Yan and Y. D. Li, Adv. Funct. Mater., 2005, 15, 763–770.
13. X. Wang and Y. D. Li, Angew. Chem., Int. Ed., 2003, 42, 3497–3500.
14. X. Wang, J. Zhuang, Q. Peng and Y. Li, Inorg. Chem., 2006, 45, 6661–6665.
15. S. Wang, S. Song, R. Deng, H. Guo, Y. Lei, F. Cao, X. Li, S. Su and H. Zhang, CrystEngComm, 2010, 12, 3537–3541.
16. F. Tao, Z. J. Wang, L. Z. Yao, W. L. Cai and X. G. Li, J. Phys. Chem. C, 2007, 111, 3241–3245.
17. L. H. Slooff, M. J. A. de Dood, A. Van Blaaderen and A. Polman, Appl. Phys. Lett., 2000, 76, 3682–3684.
18. St J. John, J. L. Coffer, Y. D. Chen and R. F. Pinizzotto, J. Am. Chem. Soc., 1999, 121, 1888–1892.
19. K. Riwotzki, H. Meyssamy, H. H. Schnablegger, A. Kornowski and M. Haase, Angew. Chem., Int. Ed., 2001, 40, 573–576.
20. J. W. Stouwdam and F. Van Veggel, Nano Lett., 2002, 2, 733–737.
21. P. R. Diamente and F. Van. Veggel, J. Fluoresc., 2005, 15, 543–551.
22. F. Wang, Y. Zhang, X. P. Fan and M. Q. Wang, Nanotechnology, 2006, 17, 1527–1532.
23. P. Schuetz and F. Caruso, Chem. Mater., 2002, 14, 4509–4516.
24. M. Yu, J. Lin, J. Fu, H. J. Zhang and Y. C. Han, J. Mater. Chem., 2003, 13, 1413–1419.
25. F. N. Sayed, V. Grover, K. A. Dubey, V. Sudarsan and A. K. Tyagi, J. Colloid Interface Sci., 2011, 353, 445–453.
26. M. Yamaga, E. Hayashi, N. Kodama, K. Itoh, S. Yabashi, Y. Masui, S. Ono, N. Sarukura, T. P. J. Han and H. G. Gallagher, J. Phys.: Condens. Matter, 2006, 18, 6033–6044.

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c1ra00651g/

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