Under 980 nm excitation, multiple ultraviolet and visible upconversion luminescence from Ho3+ and Eu3+ ions were observed in Yb3+/Ho3+/Eu3+ tri-doped NaYF4 microcrystals (MCs). The high-energy states (5H3-7, 5L6, 5D3 and 5D2) of Eu3+ ions could be efficiently populated by two-step energy transfer (ET) processes of Yb → Ho → Eu. Four-, three-, two-photon UC processes of Eu3+ ions were confirmed by the dependence of 5H3-7, 5L6 and 5D0 levels emission intensities on the pumping power.
©2011 Optical Society of America
Rare earth ions have closely spaced energy levels and unique intra 4f transitions, which afford them the ability to absorb one or more low-energy near-infrared (NIR) photons and subsequently convert them to high-energy emissions . This anti-Stokes emission process is well known as upconversion (UC). In Yb3+ sensitized rare earth ions doped materials, efficient NIR-to-ultraviolet (UV) UC emissions have widely been investigated due to the need for developing short-wavelength solid-state lasers and photodynamic therapies in biomedicine [2–4]. Therefore studies of new approaches to obtain efficient UV UC luminescence are very necessary and valuable.
Recently, by using Yb3+ and Tm3+ (Er3+) as double sensitizers and 980 nm NIR diode laser as pump light, Eu3+ doped fluoride nanocrystals have been demonstrated to exhibit the unusual UC spectrum from visible to UV [5, 6]. Analysis suggested upper levels of Eu3+ ions could be populated efficiently through internal ET between the optically active ions, while they could not be populated even in downconversion (DC) schemes of Eu3+ ions under vacuum UV excitations and at low temperatures. On the other hand, Eu3+ ions are excellent red emitters and famous structural probes under UV light excitation. Thus, it is attractive to explore the impact of ET routes on the spectra of Eu3+ ions. Particularly, Chen et al. have reported that Yb3+/Ho3+ doped NaYF4 is also an ideal model for studying NIR-laser-induced UV UC radiation of Ho3+ ions . It would be of great interest to extend the study and determine whether Ho3+ ions can sensitize other ions such as the Eu3+ ions and to uncover their fundamental optical UC mechanisms.
In this letter, we employed Ho3+ as the bridging ions between Yb3+ and Eu3+ ions to go on investigating the unusual radiative transitions of Eu3+ ions in NaYF4 MCs under NIR 980 nm excitation. From a fundamental point of view, more detailed information about the ET mechanism and interactions between the optically active ions can be obtained by changing the bridging ions from Tm3+, Er3+ to Ho3+, or changing the excitation light sources. Hexagonal NaYF4 was selected for embedding active ions, since it had been known to be an efficient host lattice for UC emissions [8–11]. Hexagonal micropillars of Yb3+/Ho3+/Eu3+ tri-doped β-NaYF4 were synthesized via the ethylenediaminetetraacetic acid (EDTA)-assisted hydrothermal method. Mechanisms for UV-visible UC emissions of Eu3+ by two-step ET processes of Yb3+ → Ho3+ → Eu3+ have been proposed and demonstrated. The impact of energy populating routes on the spectra of Eu3+ in NaYF4:Yb3+/Ho3+/Eu3+ MCs was also investigated based on experimental data and analysis.
Analytical grade Y(NO3)3 • 6H2O, Ho(NO3)3 • 6H2O, Yb(NO3)3 • 6H2O, Eu(NO3)3 • 6H2O, NaF, ethanol, and EDTA were obtained from Beijing Chemical Reagents, China. Deionized water was used throughout. All other chemical reagents were of analytical reagent grade.
In a typical synthesis, 1 mL of 0.5 M Ln(NO3)3 aqueous solution and 0.5 mmol of EDTA were dispensed into 20 mL of deionized water and magnetically stirred for 1 h, forming a chelated Ln-EDTA complex. Then 16 mL of 0.5 M NaF aqueous solution was added to the solution. After vigorous stirring for 1 h, the mixture was transferred into a 50-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained in an oven at 160 °C for 18 h, and then cooled down slowly to room temperature. Subsequently, the suspension was centrifuged at 8000 rpm for 10 min. The resultant product was then washed thoroughly and dried in vacuum at 80 °C.
3. Results and discussion
3.1 Structure and morphology of NaYF4:Yb3+/Ho3+/Eu3+ microcrystals
The crystal structure was analyzed by a Rigaku RU-200b X-ray powder diffractometer (XRD) using a nickel-filtered Cu-Ka radiation (λ = 1.4518Å). The size and morphology were investigated by scanning electron microscope (SEM, KYKY 1000B). A power-adjustable laser diode (980 nm, 0 to 2W) with a lens making the beam parallel was employed as the UC pump source and a 100 W xenon lamp equipped in the Hitachi F-4500 fluorescence spectrophotometer was used as the DC pump source. The UC and DC luminescence spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer (1.0 nm for spectral resolution (FWHM) of the spectrophotometer and 400 V for PMT voltage) at room temperature. A 10-ns Raman laser running at 953.6 nm was used as the pulsed excitation source for temporal investigations. Low temperature emission spectra were obtained at 10 K with the samples mounted in a helium exchange gas chamber of a closed cycle refrigeration system. Figure 1 shows the XRD pattern of the MCs. All the diffraction peaks can be indexed to the pure hexagonal NaYF4 (JCPDS 16-0334). No other impurity peaks were detected. The corresponding SEM image (Fig. 1) shows that the NaYF4:Yb3+/Ho3+/Eu3+ MCs are hexagonal pillars.
3.2 Upconversion emissions of Ho3+ and Eu3+ ions
For discernible distinction between the emissions of Ho3+ and Eu3+ ions, the enlarged UC spectra of NaYF4: 20%Yb3+, 1.5%Ho3+, 3%Eu3+ (a, b) and NaYF4: 20%Yb3+, 1.5%Ho3+ (c, d) MCs in the range of 230-660 nm are shown in Fig. 2 and Fig. 3 . Most emissions of Ho3+ ions correspond well to what observed in Yb3+/Ho3+ codoped crystals . Figure 2 shows UV UC luminescence of the samples under 980 nm excitation from a diode laser. As can be seen in Fig. 2c, the shortest UV UC emission of Ho3+ ions centered at 241 nm was observed, corresponding to 3F4/5D4 → 5I8 emission. Besides them, characteristic peaks of Eu3+ ions can be found explicitly. The wide range UC luminescence of Eu3+ ions includes: 5D0 → 7FJ, 5D1 → 7F3 (red emissions, J = 1, 2), 5D2 → 7FJ (blue-green emissions, J = 0, 3), 5D3 → 7FJ (violet emissions, J = 2, 3) and 5L6 → 7F0, 5H3-7 → 7FJ (ultraviolet emissions, J = 0, 1, 2, 3). Although, compared with our previous results based on YF3:Yb/Tm/Eu samples, Ho3+ is not as effective as Tm3+ serving as a bridge to transfer energy from Yb3+ to Eu3+ under 980 nm excitation . To the best of our knowledge, such efficient UC luminescence of Eu3+ has not been reported through Ho3+ serving as a bridge to transfer energy from Yb3+ to Eu3+ under 980 nm excitation.
3.3. UC mechanisms for Eu3+ and Ho3+ions
Figure 4 describes schematically energy level diagrams of Eu3+, Ho3+, and Yb3+ ions and possible UC processes under 980 nm excitation . Yb3+ ions continuously absorb 980 nm photons and then transfer the energy to populate the 5I6, 5F2/5F4 and 5F2/3F2/5G2 levels of Ho3+ in turn. Different from Ho3+ ions, the population of the levels of Eu3+ ions can only originate through the ETs from excited Ho3+ ions. This can be confirmed from our experimental result: we have not observed any emissions of Eu3+ by UC in NaYF4: 20%Yb3+, 3%Eu3+ MCs. Multiple ET processes may exist since the trivalent Ho3+ ion has abundant energy levels and their small energy mismatches between levels of Eu3+ and Ho3+ ions. For briefness, four possible and representative ET processes should be considered in our paper: ET1 5S2/5F4 → 5I8 (Ho3+): 7F0 → 5D0 (Eu3+); ET2 5G4 → 5I8 (Ho3+): 7F0 → 5D3 (Eu3+); ET3 5F2/3F2/5G2 → 5I8 (Ho3+): 7F0 → 5D4 (Eu3+); ET4 5G4/5D4/3G4 → 5I8 (Ho3+): 7F0 → 5IJ/3P0/5FJ (Eu3+). Simultaneously, the low energy levels 5D3,2,1,0 of Eu3+ ions could be populated through a series of nonradiative relaxations from the neighboring higher excited levels.
To understand the UC processes well, we investigated the excitation power dependence of UC luminescence intensities. For an unsaturated UC process, the integrated UC luminescence intensity If is proportional to Pn , where P is the pumping laser power, and n is the number of laser photons required in populating the upper emitting state. Figure 5 shows the typical pump-power dependence of UC luminescence of NaYF4: 20%Yb3+, 1.5%Ho3+, 3%Eu3+. The values of photon number n are 1.81 for the 5D0 → 7F0 transition of Eu3+, 2.15 for the 5D2 → 7F0 transition of Eu3+, 2.75 for the 5L6 → 7F0 transition of Eu3+, 2.98 for the 5G4 → 5I8 transition of Ho3+, 3.38 for the 3H4/5G4/1D4 → 5I8 transition of Ho3+, and 3.67 for the 5H3-7 → 7F0 transition of Eu3+, indicating that these transitions are of two-, three- and four-photon UC processes, respectively. Power dependence analyses illustrate that these levels of Eu3+ have the same multi-photon UC characters with the corresponding levels of Ho3+ ions and confirm that they are populated by the ETs from the corresponding levels of Ho3+ ions.
The energy transfer from excited Ho3+ to Eu3+ ions can be further proved by the dynamical analysis on Ho3+ excited states. The decay curves for the representative emissions from the 5F2/3F2/5G2, 5G4, 5S2/5F4 levels of Ho3+ ions in NaYF4: 20%Yb3+, 1.5%Ho3+ and NaYF4: 20%Yb3+, 1.5%Ho3+, 3%Eu3+ MCs were recorded under 953.6 nm pulsed Raman laser, as shown in Fig. 6 . Each of the decay curves can be fitted well into a single-exponential function as I = I0exp(−t/τ) (I0 is the initial emission intensity, τ is the lifetime of the level). The best-fitted results were listed in Table 1 . As can be seen clearly from Table 1, with the addition of Eu3+ ions, all the lifetimes of the 5F2/3F2/5G2, 5G4, 5S2/5F4 levels decrease greatly which explicitly demonstrate the energy transfer from Ho3+ to Eu3+ ions.
3.4. The spectral difference and analysis of Eu3+ ions under 980 nm and 394 nm excitation
Figure 7 presents the absorption spectra of the NaYF4: Yb/Ho/Eu around 980 nm and 394 nm. The absorption of 5L6 level of Eu3+ at 394 nm and the 2F5/2 level of Yb3+ at 980 nm can be observed, clearly. Consequently, the different excitation lights associate with different energy populating processes of Eu3+ in NaYF4: 20%Yb3+, 1.5%Ho3+, 3%Eu3+ MCs. For comparison, Fig. 8 shows the luminescence integral intensity ratios of (5D1 → 7F3)/(5D0 → 7F2) and (5D2 → 7F3)/(5D0 → 7F2) at the different doping concentration of Eu3+ under 394 nm (Fig. 8a) and 980 nm (Fig. 8b) excitation, respectively. It can be found clearly that the ratios exhibit gradual decrease with increasing the doping concentration of Eu3+. The decreases can be associated with the quenching processes of 5D1 and 5D2 emissions. According to previous research, the hexagonal NaYF4 matrix has phonon energies (~520 cm−1). The energy gaps of 5D2 → 5D1 and 5D1 → 5D0 are estimated to be 1771 and 2464 cm−1, respectively. Correspondingly, it needs 3 and 5 phonons to bridge the gaps of 5D2 → 5D1 and 5D1 → 5D0 in Eu3+ doped NaYF4 MC. The emissions from the levels 5D1 and 5D2 of Eu3+ may be quenched through thermally enhanced multiphonon relaxation which can be related to the temperature through , where is the nonradiative relaxation rate at 0 K [14,15]. On the other hand, the concentration quenching occurs at higher doping concentrations of Eu3+ . Besides, the 5D1 emissions can be quenched by the cross relaxation between two Eu3+ ions: 5D1(Eu1) + 7F0(Eu2) → 5D0(Eu1) + 7F3(Eu2). This can be confirmed from the fact in Fig. 8b that the ratio of the (5D1 → 7F3)/(5D0 → 7F2) decreases faster than ratio of (5D2 → 7F3)/(5D0 → 7F2) with increasing the doping concentration of Eu3+ ions under 980 nm excitation.
Comparing Fig. 8a and Fig. 8b, we can observed obviously that the luminescence integral intensity ratios of (5D1 → 7F3)/(5D0 → 7F2) and (5D2 → 7F3)/(5D0 → 7F2) under 980 nm excitation are much larger than those under 394 nm excitation at the same doping concentration of Eu3+. Such results confirm again that the ETs from Yb3+ to Ho3+ and then from Ho3+ to Eu3+ play more efficient roles in populating the high-energy states of Eu3+ ions under 980 nm excitation than pumping them directly under 394 nm excitation .
Besides, we can found clearly in Fig. 8 that the intensity ratios of the (5D1 → 7F3)/(5D0 → 7F2) and (5D2 → 7F3)/(5D0 → 7F2) decrease dramatically with increasing the doping concentration of Eu3+ ions under 980 nm excitation comparing with those under 394 nm excitation at 300 K. The fast decrease of the ratios under 980 nm excitation can be attributed to the thermal effect of 980 nm infrared light, which increase the local temperature of samples. Cross relaxation, concentration quenching, and multiphonon relaxation rates increased sharply at higher temperatures under 980 nm excitation . The 5D1 and 5D2 emissions were quenched more and more with increasing the doping concentration of Eu3+ ions. Accordingly, the intensity ratios of the (5D1 → 7F3)/(5D0 → 7F2) and (5D2 → 7F3)/(5D0 → 7F2) decreased dramatically under 980 nm excitation, which is demonstrated in Fig. 8b. This can be further confirmed by the experimental data in Fig. 8b that when the samples were cooled from room temperature to 10 K, the intensity ratios of the (5D1 → 7F3)/(5D0 → 7F2) and (5D2 → 7F3)/(5D0 → 7F2) decrease gently with increasing the doping concentration of Eu3+ ions under 980 nm excitation, which are analogous to those under 394 nm excitation at a room temperature.
In summary, Yb3+/Ho3+/Eu3+ tri-doped NaYF4 MCs were fabricated through a simple hydrothermal process. It was found that not only unusual UV emissions of Ho3+ ions, but also unusual UV emissions of Eu3+ ions could be observed in this tri-doped system. Power-dependence analysis and dynamical analysis on Ho3+ excited states confirm that Ho3+ is an important dopant that served as a “bridging ion” in the efficient UC excitation processes of Eu3+. Besides, the spectra of NaYF4: Yb3+/Ho3+/Eu3+ MCs under 980 nm and 394 nm excitation were compared. The luminescence integral intensity ratios of (5D1 → 7F3)/(5D0 → 7F2) and (5D2 → 7F3)/(5D0 → 7F2) under 980 nm excitation are much larger than those under 394 nm excitation at the same doping concentration of Eu3+. These results indicate that a UC scheme is a better way to populate the high-energy levels of Eu3+ than a DC scheme.
This work was supported by the National High Technology Research and Development Program of China (863 program: 2009AA03Z309) and the National Natural Science Foundation of China (NNSFC) (grants 10874058, 51072065, and 60908031).
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