This work reports on the efficient cooperative upconversion and infrared cascade downconversion emissions in a novel Y8V2O17:Eu:Yb nanophosphor. The excitation with UV light produces emission in the 950-1000 nm region, corresponding to the Yb3+:2F5/2→2F7/2 transition, as well as visible emissions of the Eu3+ ion. Time-resolved spectroscopy measurements revealed that the mechanism responsible for this transition is the efficient cascade nonresonant energy transfer from VO43-→Eu→Yb. When the same nanophosphor is excited with 976 nm radiation, bright reddish upconversion emission of the Eu3+:5DJ→7FJ transition is observed as consequence of the Yb + Yb→Eu cooperative energy transfer mechanism, which was established by analyzing the emission power dependence and the time-resolved spectroscopy of radiative transitions.
©2013 Optical Society of America
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  and for this reason, the Eu3+ ion is a well-known activator, widely used as red phosphor in television screens and fluorescent lamps etc . 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 Mg4GeO5.5F:Mn) and Sn2+ (QE~80% in (Sr,Mg)3(PO4)2:Sn) , 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 (VO43-→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+:2F5/2→2F7/2 transition at ~1000 nm and frequency upconversion emission from Eu3+:5DJ→7FJ 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.
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 .
The peaks observed in the patterns can be indexed with the tetragonal zircon structure of Y8V2O17 [JCPDS: 22-1002] . 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).
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.  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 VO43- ion [12–14].
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.
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 VO43- emission in Fig. 4 is a direct evidence of energy transfer from the VO43- 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 . When the phosphor sample is exposed to 325 nm, the VO43- group absorbs light resonantly through the 1A1 → 1T1 transition. Due to the overlap of the Eu3+ ion energy level (5H6) and VO43- bands, an efficient nonradiative energy transfer occurs. A similar transfer was reported in the literature . 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 VO43- group, Eu3+ ions excited to the 5H6 level rapidly relax nonradiatively to 5DJ levels, from where they decay radiatively to the lower lying 7Fj (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].
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  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 . 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)  and Zr0.9Eu0.05Ta0.05O2  which are 15% and 13.2%, respectively, while QE is higher than Gd2O2S:Eu3+  and Zr0.9Eu0.05Nb0.05O2  which are 7.7 and 8.7%, respectively.
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 2F5/2 → 2F7/2 transition of Yb ions, which is confirmed by similar absorption optical band shape of Yb3+ ion. Recently, Wei et al.  reported the quantum cutting phenomena from the VO43- 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: 2F5/2→2F7/2 is ~1900 cm−1, while the energy of the maximum energetic phonon is ~840 cm−1 (V-O; from VO43- group). According to the Miyakawa and Dexter theory , 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+) + 2F5/2 (Yb3+) → 7F6 (Eu3+) + 2F7/2 (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→5DJ) and efficient energy transfer from VO43- group to Eu3+ . The observation of similar NIR emission in Eu:Yb codoped system has been first reported by Yamada et al.  and later on observed by Jubera et al. . The schematic diagram showing possible energy transfer pathways is shown in Fig. 6.
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 . We have calculated the effective decay time of the 5D0→7F2 transition using the relation:, 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 , where τD-A and τD are the lifetimes of donor ion (Eu) in the presence and absence of acceptor ions (Yb), respectively. The ET efficiency, ηYb, 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:, where ID-A and ID 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 = ηEu (1–ηYb) + 2ηYb, where the QE for Eu3+ ions, ηEu, was set to 0.7 . 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. . However, an efficient nonradiative energy transfer process was certainly established.
3.2.2 Cooperative Upconversion luminescence
Yb3+ ion contains only two states and has strong absorption (~11.7 × 10−21 cm2)  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 . 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→7Fi (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 . 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).
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 . 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 7Fj (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.
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+:2F5/2→2F7/2 transition was reported on excitation with 325 nm laser. An efficient cascade nonresonant energy transfer from VO43-→Eu→Yb was established from time-resolved spectroscopy.
We acknowledge the financial support from FAPESP and CNPq. We also thank C2NANO - Center for Nanoscience and Nanotechnology/MCT for the use of TEM-HR (JEOL −3010) microscope of LME (Laboratório de Microscopia Eletrônica) for TEM measurements.
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