1. Introduction

Lanthanide (Ln) doped materials have been extensively investigated owing to their attractive luminescence characteristics and magnetic properties. The spectroscopic characterization of ions containing f-shell electrons remains an interesting topic as they span vast commercial applications and help to understand basic mechanisms of several peculiar emission dynamics. A number of phenomena exclusive to Ln ions have been reported, such as cooperative emission, quantum cutting, multiphoton absorption, photon avalanche and so on [1–3]. Recently, the unification of lanthanide spectroscopy with nanotechnology expanded their applications from display devices and sensors to the biomedical field.

Among lanthanide elements Europium (Eu) is considered as one of the best known red emitting lanthanide ions having quantum efficiency (QE) up to ~90% in the red region [4] and for this reason, the Eu³⁺ ion is a well-known activator, widely used as red phosphor in television screens and fluorescent lamps etc [5]. The use of other lanthanide ions such as Er, Ho, Sm, Gd and Pr as source of red color is limited by their small absorption cross-section and consequently, low QE. Other red emitting materials as Mn²⁺ (QE~80% in Mg₄GeO_5.5F:Mn) and Sn²⁺ (QE~80% in (Sr,Mg)₃(PO₄)₂:Sn) [6], possess high quantum yield but their applications are limited because of the poor thermal stability, broader emission and large Stokes shift.

A limitation to the optical properties of Eu³⁺ ions is that they do not present upconversion emission because of the unavailability of suitable energy levels or poor excited-state absorption probability in the infrared region. This limits their application for bio-imaging purposes, since visible radiation has limited penetration depth in the skin. However, upconversion emission in Eu ions was reported in the presence of Yb ions under NIR (~1 µm) excitation [7–9] via cooperative energy transfer.

In the present work, a novel yttrium vanadium oxide (Y₈V₂O₁₇) nanophosphor was synthesized using the co-precipitation method. The doping with Eu and Yb allowed the observation of back (VO₄^3-→Eu→Yb) and forward (Yb + Yb→Eu) energy transfer following UV (325 nm) and NIR (976 nm) excitations, respectively. These processes lead to a NIR emission from the Yb³⁺:²F_5/2→²F_7/2 transition at ~1000 nm and frequency upconversion emission from Eu³⁺:⁵D_J→⁷F_J transition, which were inferred by time-resolved spectroscopy and power dependence techniques.

2. Materials and methods

2.1 Material and synthesis

Rare earth oxides (Y₂O₃, Eu₂O₃, Yb₂O₃: 99.9%,), V₂O₅ (99%), glycerol, ethylene glycol, nitric acid, ammonium and sodium hydroxide were purchased from Sigma-Aldrich. All reagents were standard grade and used as received, without any further purification. Eu and Yb codoped Y₈V₂O₁₇ nanophosphors were synthesized using the co-precipitation synthesis method. Salt solutions of Y, Eu, Yb and basic vanadate solutions were used following the reaction:

Stock solutions of appropriate amounts of rare earth oxides Y₂O₃ (0.4 g) and required amount of Eu₂O₃/Yb₂O₃ were dissolved separately in a HNO₃ (2.5 mmol) solution and crystallized under constant evaporation, separately. 0.12 g of V₂O₅ was dissolved in 5 ml of aqueous solution of sodium hydroxide to form a colorless Na₃VO₄ solution. Ultra-pure water (18.0 MΩ) from a Milli-Q deionization unit was used for the sample synthesis wherever it required. Crystallized Y(NO₃)₃.6H₂O, Eu(NO₃)₃.6H₂O and, Yb(NO₃)₃.6H₂O were mixed to the Na₃VO₄ solution (pH = 12) whilst stirring with a magnet at a temperature of 50 °C. A red-yellow precipitate immediately appeared which dissolved latter on after vigorous stirring at room temperature for about 30 min and the reaction solution became clear, acquiring a light green color. The solution color change from yellow to light green suggests the vanadium valence reduction from + 5 to + 3. 10 ml of NH₄OH was added soon after the addition of 5 ml of ethylene glycol under constant stirring. The white precipitate obtained was washed three times with ethanol and distilled water, and was then isolated by centrifugation at 5000 rpm for 15 minutes in ethanol solution. The resulting product was collected and washed with ethanol and deionized water three times and was subsequently aged overnight at 70 °C in air. The resulting white powder was found to be well dispersible in various nonpolar organic solvents, such as hexane, toluene, and cyclohexane solutions. The powder obtained consisted of a mixed phase of YVO₄ and Y₈V₂O₁₇ nanocrystals. To eliminate the YVO₄ phase and to promote the growth of the Y₈V₂O₁₇ phase, the samples were annealed at 1200 °C for 5 hours. For simplicity, Y₈V₂O₁₇:1Eu:xYb doped samples will be referred as YVExY. A given amount (5 mg) of heated phosphor material was dispersed in a mixed solution of glycerol and water (3 ml: 2 ml) and ultrasonicated for 30 minutes. The colloidal solution obtained after the ultrasonication process was found to be well dispersed in solution for at least two months. The nanophosphor was covered by glycerol ligands and acquired positive charges on the surface, which improved the dispersibility in solution.

2.2 Characterization

Initially, the size and phase of the synthesized nanophosphor material were identified using powder X-ray diffraction (XRD) patterns obtained in a Rigaku D/max-γB diffractometer equipped with a rotating anode and a Cu Kα source (λ = 0.15418 nm) in the range from 20 to 80°. The surface morphologies were confirmed with a JEOL JSM-6330F field emission scanning electron microscope (SEM) while high resolution transmission electron microscopy (HRTEM) was carried out with a FEI Tecnai G² F20 electron microscope with an accelerating voltage of 200 kV.

The absorption spectra of the aqueous suspended samples were recorded with a UV-Vis spectrometer (Shimadzu, UV-1800) in the range 200-1100 nm. Excitation spectra were recorded in a SpexFluorolog F2121 spectrofluorimeter. The samples were ultrasonicated for about 30 minutes, resulting in a well dispersed and homogeneous suspension. The photoluminescence spectra were monitored by exciting the samples with 325 nm (5 mW) and 976 nm (5 W) light from He-Cd and diode lasers, respectively. The laser intensity was controlled by using a set of neutral density filters. The fluorescence signal was recorded by a portable spectrometer (Ocean Optics, HR 4000). Photoluminescence decay measurements were performed with a pulsed 532 nm radiation of Nd:YAG laser (~10 ns) and the cw diode laser (976 nm) as the excitation sources. The later was modulated with a mechanical chopper (model no. SR-540, Stanford Research Systems Inc) and the collected signal was fed into a 1 GHz digital oscilloscope (Tektronix TDS 3032), allowing to obtain the temporal evolution of the photoluminescence. Lifetimes of the radiative levels were estimated by fitting an exponential function to the recorded decay curves.

3. Results and discussion

3.1 Nanostructure analysis

The identification and purity of the phases precipitated in the nanophosphor were determined with the X-ray diffraction (XRD) technique and are presented in Fig. 1 .

 

Fig. 1 X-ray diffraction patterns of as-synthesized and heated nanophosphor samples.

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The peaks observed in the patterns can be indexed with the tetragonal zircon structure of Y₈V₂O₁₇ [JCPDS: 22-1002] [10]. The average crystallite sizes were found to be in the range from 50 to 70 nm by using the Debye-Scherrer equation. Scanning electron micrographs of the annealed samples, as that presented in Fig. 2(a) , reveal the formation of spherical shaped nanoparticles (NPs) that were partially linked due to the high temperature annealing. These spheroidal shaped NPs were also visible in the transmission electron image of Fig. 2(b) and their sizes were found to be distributed mostly in the range from 40 to 80 nm. The selected area electron diffraction (SAED) pattern from an individual nanocrystal, shown in Fig. 2(c) indicates that the synthesized nanocrystals are of single crystalline nature, while the high-resolution TEM of Fig. 2(d) shows lattice fringes with an observed d-spacing of 0.229 nm, which is in good agreement with the lattice spacing in the (321) planes of tetragonal Y₈V₂O₁₇ (0.2283 nm).

 

Fig. 2 (a) Scanning electron image (bar: 1 μm), (b) transmission electron images (bar: 50 nm), selected area diffraction pattern (c) and high resolution TEM image (d) of the annealed YVE9Yb nanophosphor sample.

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3.2 Optical properties

The UV-Vis transmission spectrum of the annealed sample was monitored in aqueous suspension with a 4.6 × 10⁻⁴ molL⁻¹ concentration. The transmission spectrum shown in Fig. 3(a) contains mainly a broad absorption peak centered at 280 nm, similar to the peak reported by Wu et al. [11] at 272 nm (for 10-20 nm particle size) in the case of YVO₄:Eu. The small red shift is due to the larger particle in our case (50-70 nm). The absorption band was assigned to the overlapping of ¹A₁ → ¹T₂ and ¹A₁ → ¹T₁ transitions of the VO₄^3- ion [12–14].

 

Fig. 3 (a) Transmission spectrum of annealed YVEu nanophosphor in aqueous suspension. (b) Photo-luminescence excitation (PLE) spectrum of the same sample for emission wavelength of 615 nm. The inset shows an enlarged portion of the spectrum, with the excitation peaks of Eu³⁺ ions.

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3.2.1 Infrared cascade emission

Photoluminescence spectra of mono Eu³⁺, Yb³⁺ and codoped Y₈V₂O₁₇ nanophosphors (as-synthesized and annealed) were recorded under excitation with 325, 488 and 532 nm laser radiations. The singly Yb-doped sample, excited by the 976 nm diode laser, does not present any emission peak while the two other samples yield red-dominated emission spectra as shown in Fig. 4 , that were ascribed to 4f-4f transitions of the Eu³⁺ (4f ⁶) ion. Annealed samples had ~5 times intensified and sharper bands than the as-synthesized samples, which present peaks at 538 nm (⁵D₁→⁷F₀), 587, 594.5 nm (⁵D₀→⁷F₁), 609, 615, 619 nm (⁵D₀→⁷F₂), 651 nm (⁵D₀→⁷F₃), 698, 704 nm (⁵D₀→⁷F₄) and 813 nm (⁵D₀→⁷F₆). Due to the intensified transitions of the annealed sample, peaks not visible in the as-synthesized sample could be observed at 511.7 (⁵D₂→⁷F₃), 556.3 nm (⁵D₁→⁷F₁), 580 nm (⁵D₀→⁷F₀), 613.4 and 622 nm of (Stark’s components of the ⁵D₀→⁷F₂ transition), 791 nm (⁵D₁→⁷F₆) and 750 nm (⁵D₀→⁷F₅). It is noticeable that several Stark’s components are visible in the annealed sample while they can hardly be seen in the as-synthesized sample. The appearance of several well defined Stark’s components in the spectrum suggests identical sites of Eu³⁺ ions in nanocrystals. Red-dominated visual images of annealed nanophosphors in on 325 nm excitation are displayed in Fig. 5 . The CIE coordinates of the heated sample (1 mol% Eu, 1200 °C/5 h) are estimated to be (0.61, 0.36), which are located in the reddish-orange region.

 

Fig. 4 Lower panel: Photoluminescence spectra of as-synthesized (YVEu9Yb) and annealed (YVEu and YVEu9Yb) nanophosphors on excitation with 325 nm laser at 500 mW/cm² irradiance. Upper panel: magnified (x 18) spectrum with the wavelength region 585-715 nm excluded for a better visualization of the low intensity peaks.

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Fig. 5 (a) Visual images of annealed nanophosphor (Eu) in ambient light, (b) in aqueous colloidal solutions of Eu phosphor (1 x 10⁻⁴ M) under excitation with 325 nm (500 mW/cm⁻²) and (c) transparent aqueous colloidal solution of 1Eu:9Yb (1x10⁻⁵ M in 1 mm cuvette) on 976 nm (15 W/cm⁻²) laser radiations. Due to the mixing of red (Eu³⁺) and blue scattered laser radiation from beaker solution (b) looks pink.

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In order to understand the excitation mechanism and origin of the emissions, the excitation spectrum was measured by monitoring the emission of the Eu³⁺ ion at 615 nm and is depicted in Fig. 3(b). It consists of a broad band (Δλ ~100 nm) centered at 310 nm (¹A₁ → ¹T₁) and a shoulder at 272 nm (¹A₁ → ¹T₂). In addition to these broad bands, at least four sharper peaks of Eu³⁺ ions were assigned to electronic transitions from the ⁷F₀ ground-state to ⁵G₂, ⁵L₆, ⁵D₃ and ⁵D₂ levels. The observation of a broad band and the absence of VO₄^3- emission in Fig. 4 is a direct evidence of energy transfer from the VO₄^3- group to the Eu³⁺ ion. The pathways for excitation, energy transfer and subsequent emission are represented in the schematic diagram of Fig. 6 . According to this diagram, the vanadate group exhibits ground-state ¹A₁ (with configuration t₁⁶e⁰t₂⁰) and excited-states ³T₁, ³T₂, ¹T₁, and ¹T₂ (with configuration t₁⁵e¹t₂⁰). The transition from ¹A₁ to ¹T₁ and ¹T₂ are allowed by an electric dipole mechanism [15]. When the phosphor sample is exposed to 325 nm, the VO₄^3- group absorbs light resonantly through the ¹A₁ → ¹T₁ transition. Due to the overlap of the Eu³⁺ ion energy level (⁵H₆) and VO₄^3- bands, an efficient nonradiative energy transfer occurs. A similar transfer was reported in the literature [16]. The possibility of resonant absorption of 325 nm photons directly by the Eu³⁺ ion itself cannot be ruled out because energy of ⁵H₆ level matches the excitation energy. However, the ¹A₁ → ¹T₁ transition is expected to be much stronger than the partially allowed ⁷F₀→⁵H₆ transition of Eu³⁺. Following the energy transfer from the VO₄^3- group, Eu³⁺ ions excited to the ⁵H₆ level rapidly relax nonradiatively to ⁵D_J levels, from where they decay radiatively to the lower lying ⁷F_j (j = 0 to 5) levels and generate a multicolor spectrum. The concentration of Eu ions was optimized by maximizing the emission intensity at 615 nm and was found to be best at a concentration of 1 mol% (17.1 × 10²⁰ ions). The weak ⁵D₀→⁷F₀ transition, which is forbidden by both electric- and magnetic-dipole selection rules in the case of the free ion, occurs due to a weak J mixing by the crystalline field [17, 18].

 

Fig. 6 Energy level diagram and various optical processes [energy transfer (I and II); cooperative upconversion (III) processes] in Yb and Eu ions.

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The transition ⁵D₀→⁷F₂ is of hypersensitive character and is expected to be absent in centrosymmetric sites for which the odd crystal field Hamiltonian is null [19] but it was nevertheless observed due to the forced electric dipole mechanism. The full width at half maximum (FWHM) of the bands reduced appreciably (615.2 nm: Δλ ~3 nm and for 619 nm: Δλ ~5 nm) after annealing, indicating identical distribution of Eu sites in the annealed sample. Moreover, the ratio of the intensities (R) of the hypersensitive electric dipole transition (⁵D₀→⁷F₂) and the magnetic dipole transition (⁵D₀→⁷F₁) are different for the as-synthesized (ΔR = 6.67) and the annealed sample (ΔR = 4.8), and decreased by ~28%. As the intensity of the magnetic dipole transition does not depend on the local site symmetry, it can be considered as a reference to estimate any structural variation. Reduction in the peak ratio ΔR demonstrates that Eu³⁺ ions are at more symmetric sites when compared to the as-synthesized sample.

The Judd-Ofelt intensity parameters (Ω₂ and Ω₄) for the annealed sample were determined from the emission spectrum by using the method described in Refs [20–22]. and were found to be Ω₂ = 8.3 and Ω₄ = 1.2. These values were used to estimate the spontaneous Einstein’s coefficients (A) of various transitions, from which a radiative lifetime of 3.1 ms could be estimated according to Ref [9]. The experimental lifetime of the ⁵D₀→⁷F₂ transition was measured in the annealed sample with 10 ns pulses at 532 nm and the results are shown in Fig. 7(a) . By taking the ratio between the estimated and experimental (~401 ± 1 µs) lifetimes, the QE of the sample was estimated to be ~13%. This QE is comparable to the efficiency of YVO₄:Eu (30 nm particle size) [23] and Zr_0.9Eu_0.05Ta_0.05O₂ [24] which are 15% and 13.2%, respectively, while QE is higher than Gd₂O₂S:Eu³⁺ [25] and Zr_0.9Eu_0.05Nb_0.05O₂ [24] which are 7.7 and 8.7%, respectively.

 

Fig. 7 (a) Semilog plot of decay curves of ⁵D₀→⁷F₂ transition at different Yb concentrations for 532 nm laser excitation. (b) Normalized decay curves corresponding to ²F_5/2→²F_7/2 transition of Yb³⁺ ions (9 mol%) with and without Eu (1 mol%) under 976 nm laser excitation.

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As seen in Fig. 4, when Yb³⁺ ions coexist with Eu³⁺ ions, all peaks present in the singly doped Eu sample spectrum appear, together with a broad NIR band centered at 980 nm. The appearance of this band is unexpected since Eu³⁺ has no radiative transitions with this energy and the only possibility is through the ²F_5/2 → ²F_7/2 transition of Yb ions, which is confirmed by similar absorption optical band shape of Yb³⁺ ion. Recently, Wei et al. [26] reported the quantum cutting phenomena from the VO₄^3- unit to the Yb ion. However, the occurrence of such process must be ruled out in the present case because no emission was observed in the spectrum of singly Yb doped sample, even at larger laser irradiance. It is worth to mention that the emission intensity of the Eu³⁺ ion was found to reduce monotonically with the Yb concentration, as seen in Fig. 8 . The presence of the Yb³⁺ emission indicates an efficient non-resonant energy transfer from Eu to Yb ions. The energy difference between Eu: ⁵D₀→⁷F₆ transition and Yb: ²F_5/2→²F_7/2 is ~1900 cm⁻¹, while the energy of the maximum energetic phonon is ~840 cm⁻¹ (V-O; from VO₄^3- group). According to the Miyakawa and Dexter theory [7], the probability of a phonon-assisted energy transfer process depends on the energy separation of transitions and the energy of lattice phonons. The energy difference of ~1900 cm⁻¹ can be provided by the emission of at least two lattice phonons. The monotonic reduction in the emission intensity of Eu ions as the concentration of Yb increases, indicates an efficient nonresonant energy transfer process through ⁵D₀ (Eu³⁺) + ²F_5/2 (Yb³⁺) → ⁷F₆ (Eu³⁺) + ²F_7/2 (Yb³⁺) + 2 phonons. However, the band at 980 nm was absent when ⁵D₁ and ⁵D₂ levels of Eu³⁺ ions (in Eu:Yb sample) were resonantly excited with 532 and 488 nm light. The nonappearance of the NIR emission can be explained on the basis of oscillator strength corresponding to these transitions. Level ⁵H₆ is heavily populated as its oscillator strength is ~80 times stronger than ⁵D₀ and ⁵D₁ transitions (⁷F₀→⁵D_J) and efficient energy transfer from VO₄^3- group to Eu³⁺ [27]. The observation of similar NIR emission in Eu:Yb codoped system has been first reported by Yamada et al. [28] and later on observed by Jubera et al. [29]. The schematic diagram showing possible energy transfer pathways is shown in Fig. 6.

 

Fig. 8 Effect of Yb concentrations on Eu:615 nm and Yb:976 nm emission intensities.

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To confirm the energy transfer (ET) process, the concentration of Yb ion was varied while keeping the concentration of Eu³⁺ ions constant, and monitoring the decay time of the strongest emission Eu:⁵D₀→⁷F₂ transition. From the luminescence decay curves shown in Fig. 7 (a), the energy transfer probability, energy transfer efficiency and QE can be determined. At higher concentrations, the non-radiative relaxation energy transfer process takes place and thus, the fit with an exponential function will not give an accurate lifetime value. In this situation, only a mean value of lifetime and an effective relation rate can be calculated [30]. We have calculated the effective decay time of the ⁵D₀→⁷F₂ transition using the relation:${\tau }^{eff}=\frac{{\displaystyle {\int }_0^{\infty }I(t)dt}}{I(0)dt}$, where I(t) represents the emission intensity at time t after the incident beam was turned off. The calculated values are presented in Table 1 . One can see that the decay time decreases by increasing the Yb³⁺ concentration and this behavior can be attributed to an extra decay pathway due to the Yb³⁺ doping: ET from Eu:⁵D₀ to Yb³⁺. The energy transfer rate can be estimated from $W_{D-A}=\frac{1}{{\tau }_{D-A}^{eff}}-\frac{1}{{\tau }_D^{eff}}$, where τ_D-A and τ_D are the lifetimes of donor ion (Eu) in the presence and absence of acceptor ions (Yb), respectively. The ET efficiency, η_Yb, is defined as the ratio of Eu³⁺ ions that depopulate by ET to Yb³⁺ ions over the total number of excited Eu³⁺ ions. The ET efficiency was calculated using the relation:${\eta }_{Yb}=1-\frac{{\displaystyle \int I_{D-A}}dt}{{\displaystyle \int I_D}dt}$, where I_D-A and I_D are the integrated intensities of Eu:⁵D₀→⁷F₂ decay curves in the presence and the absence of Yb ions. The total QE, η, is defined as the ratio of the number of emitted photons to the number of absorbed photons, assuming that all excited Yb³⁺ ions decay radiatively. This assumption leads to an upper limit for the QE. Total QE can be defined as η = η_Eu (1–η_Yb) + 2η_Yb, where the QE for Eu³⁺ ions, η_Eu, was set to 0.7 [31]. The ET probability, ET efficiencies, and QE are tabulated in Table 1. To calculate these values, the nonradiative losses were not taken into account. It can be seen that the energy transfer efficiency increases up to a maximum value of 17% only, for YVE9Y nanophosphors and the corresponding total QE has been found to be about 94%, which is far from existence of quantum cutting process as suggested by Luo et al. [32]. However, an efficient nonradiative energy transfer process was certainly established.

Table 1. Values of effective decay times τ^eff (Eu:⁵D₀), energy transfer probabilities (W_D-A), energy transfer efficiencies (η_Yb), and quantum efficiencies (η) as a function of the Yb³⁺ doping concentration.

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3.2.2 Cooperative Upconversion luminescence

Yb³⁺ ion contains only two states and has strong absorption (~11.7 × 10⁻²¹ cm²) [33] at ~976 nm and a resonant fluorescent emission is observed at this wavelength. However, depending on the relative distance between two neighboring Yb³⁺ ions, a short range dipole-dipole interaction can take place, leading to a cooperative emission of radiation at ~488 nm, i.e., a pair of Yb³⁺ ions loses its excitation energy by emitting a photon at 488 nm. This process is known as cooperative emission [7]. On the other hand, Eu³⁺ ions do not absorb 976 nm radiation and so, it does not show any fluorescence when excited with this wavelength. However, in the presence of a trace amount of Yb³⁺ ions (0.1 mol%), the emission spectrum exhibits a broad blue emission centered at ~488 nm due to the cooperative emission of Yb³⁺ ions and a strong orange/red emission of Eu³⁺ ion corresponding to ⁵D₀→⁷F_i (i = 0 to 5) transitions, as depicted in Fig. 9 . However, in case of as-synthesized sample, no emission from Yb and only a weak emission from the Eu³⁺ ion are observed. This may happen due to the remaining organic species in the lattice which may act as quenching centers by promoting non-radiative relaxation. The emission intensity of Eu³⁺ ion increases with Yb³⁺ ion concentration till 9 mol%, however beyond this value the emission intensity of Eu³⁺ ions shows saturation and thereafter, it gradually reduces. The reduction in the emission intensity is expected because of the energy migration among Yb-Yb ions or photodarkening [34]. The red upconversion emission from Eu³⁺ ions was strong enough to be easily seen by naked eye even in a dilute aqueous solution, as presented in Fig. 5(c).

 

Fig. 9 Emission spectra of 9Yb and 1Eu:9Yb (as-synthesized and annealed) codoped Y₈V₂O₁₇ nanophosphors under 976 nm excitation at a power of 15 W/cm². The lower inset shows a log-log plot of the irradiance dependence of the upconversion intensity in the 1Eu:9Yb annealed sample.

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The emission observed in the spectrum could be explained on the basis of the cooperative energy transfer from Yb³⁺ ions to Eu³⁺ ions. The Yb³⁺ ion may transfer its energy by two ways: either by sequential sensitization or cooperative sensitization. In our case, the cooperative sensitization is expected to be dominant due to the unavailability of energy levels in Eu³⁺ ions for sequential sensitization [7]. Furthermore, the energy of cooperative emission (~20450 cm⁻¹) of Yb³⁺ ions lies between the excitation energies of the ⁵D₂ (~21552 cm⁻¹) and ⁵D₁ (~19011cm⁻¹) levels of Eu³⁺ ion. Thus, the ⁵D₁ level of Eu³⁺ can be non-resonantly excited by the transfer of this total excitation energy (of a pair of Yb³⁺ ions). This level relaxes non-radiatively to the metastable ⁵D₀ state of Eu³⁺ ions. According to Fig. 6, the ⁵D₀ level depopulates through various radiative transitions to the different component of the ground state. Examples of cooperative energy transfer from a pair of Yb³⁺ ions to other RE ions have already been reported [7–9]. An enhancement of at least two times in the emission intensity of the Eu³⁺ ion transitions was observed and several transitions from the excited ⁵D₁ level to the ground ⁷F_{j (j = 1,2)} levels also appear. The observation of sharp and intensified transitions indicates segregation of Eu³⁺ and Yb³⁺ ions in the crystalline environment. The integrated area under the corresponding upconversion spectral profile (Eu³⁺:615 nm) was found to be a function of the incident light irradiance, as shown in the inset of Fig. 9. A fit to the curve clearly reveals a quadratic dependence, i.e., the involvement of two photons. It is noted that when the pump laser power exceeds 2.8 W, the graph shows a saturation behavior.

Though the energy transfer from Yb³⁺→Eu³⁺ is evident from the visually observed red emission under excitation with 976 nm radiation, we monitored decay curves of the 488 nm emission of Yb³⁺-doped (9 mol%) annealed samples in order to understand the energy transfer dynamics with and without Eu³⁺ (1 mol%) ions, under 976 nm laser excitations. Both curves shown in Fig. 7(b) were fit by mono-exponential functions, although a significant reduction in the cooperative emission lifetime was observed in the presence of Eu³⁺ ions. The decay times were estimated to be 962 ± 2 and 710 ± 1 μs without and with 1 mol% of Eu³⁺, respectively, while the energy transfer rate was estimated to be 370 s⁻¹. The reduction in the lifetime reflects the non-radiative energy transfer from Yb → Eu ions. Such multifunctional material which can effectively sense UV/blue and infrared radiations may find sensor applications. Observation of IR emission is useful in solar cells to enhance conversion efficiency.

4. Conclusions

In summary, we synthesized Y₈V₂O₁₇:Eu and Y₈V₂O₁₇:Eu:Yb nanophosphor materials using the co-precipitation method and explored their optical properties with the aid of different laser excitations and time-resolved spectroscopy. We demonstrated an efficient cooperative upconversion emission of Eu ions through the Yb + Yb→Eu process, which was confirmed by studying the power dependence and time-resolved spectroscopy. Yet, a wide infrared cascade emission, in the 950-1000 region, of Yb³⁺:²F_5/2→²F_7/2 transition was reported on excitation with 325 nm laser. An efficient cascade nonresonant energy transfer from VO₄^3-→Eu→Yb was established from time-resolved spectroscopy.