Open Access
Add to BibSonomyAbstract
In this study, glass-compositional dependence of Tm3+ blue up-conversion photoluminescence (UCPL), which is known to be obtained via three-steps’ energy transfers from Yb3+ to Tm3+ ions under near-infrared light excitation at ~980 nm, is investigated for Tm3+/Yb3+ co-doped TeO2-TlO0.5-ZnO glasses. The third step of energy transfer from Yb3+ to Tm3+ ions is particularly important ((Yb3+, Tm3+); (2F5/2, 3H4)→(2F7/2, 1G4)) since it determines the final blue UCPL intensity from 1G4 level compared to red and near-infrared UCPLs, and so then estimated with varied TlO0.5 and ZnO contents at the expense of TeO2 in the fixed Tm3+ and Yb3+ contents ([Yb3+]/[Tm3+] = 5). The substantial energy transfer rate (ETR) in the third step is evaluated from excitation power dependence of the blue UCPL intensity in comparison with near-infrared UCPL of Tm3+ ions with an aid of analytical method of PL lifetime and Judd-Ofelt theory. It is here revealed that the highest ETR is achieved to be 3.54 × 10−17 cm3/s for the glass composition of 70TeO2-10TlO0.5-19.4ZnO-0.1Tm2O3-0.5Yb2O3, and that the transfer rate is possibly related with the length of TeO2 glass network because a long tellurite glass network can cause segregation of rare-earth elements inducing effective Yb3+-Yb3+ energy migration and less quenching centers like dangling bonds of isolated TeO32-, resulted in the enhancement of the energy transfer for blue UCPL.
© 2014 Optical Society of America
1. Introduction
Tm3+/Yb3+ co-doped phosphors [1–6] is well-known to present blue up-conversion photoluminescence (UCPL) by irradiation of near-infrared (NIR) light around at 980 nm, which is expected for applications to biolabels, display devices (3D display), sensors, solar cells, and so on [7–11]. Tm3+ ion receives excitation energy from Yb3+ ion, resulting in the excitation to higher energy levels, such as 1G4 and 3H4 levels, via non-radiative energy transfers. When the excited 4f electrons are relaxed to lower energy levels, blue (1G4→3H6 at ~480 nm), red (1G4→3F4 at ~650 nm, 3F2,3→3H6 at ~700 nm), or NIR (3H4→3H6 at ~800 nm) emission can be observed as shown in Fig. 1 (The emission and ground levels are also expressed as |5>, |4>, |3>, |2>, |1> and |0> for 1G4, 3F2,3, 3H4, 3H5, 3F4 and 3H6, respectively). Tm3+ blue UCPL from 1G4 level (|5>→|0>) as well as red UCPL (|5>→|1>) requests three steps of energy transfers from neighboring Yb3+ ions in 2F5/2 excited state, while NIR and other red UCPLs from 3H4 levels (|3>→|0>) and 3F2,3 levels (|4>→|0>), respectively, are enough to be excited by two steps of energy transfers. A low efficiency of blue UCPL of Tm3+ ions is therefore an issue to be solved for optical applications. For the purpose of improving the blue UCPL, the third step of energy transfer from Yb3+ ions to the excited Tm3+ in 3H4 ((Yb3+, Tm3+); (2F5/2, 3H4)→(2F7/2, 1G4)) is particularly important for blue UCPL efficiency and emission purity.
Fig. 1 Energy level diagrams, energy transfer and UCPL mechanism of Tm3+/Yb3+ system under excitation at 975 nm.
As the host material of phosphor, fluoride materials are widely considered because of their low phonon energy [1–3, 7–9]. In the study, a TeO2-TlO0.5-ZnO glass system was selected as a host material [12]. TeO2-based glasses are attractive for optical host materials because of their low phonon energy among oxide materials (650-750 cm−1), high refractive index, high non-linear optical properties, high transparency, and so on [13–15]. TeO2-TlO0.5-ZnO glass system is characterized by good chemical stability, homogeneous distribution of lanthanide ions, as well as lower phonon energy. Tuyen et al. [12] reported fluorescence line narrowing (FLN) spectra of Eu3+-doped TeO2-TlO0.5-ZnO glass and discussed on the ligand-field parameters associated with Eu3+ ions in the glass system. Judd-Ofelt parameters of Eu3+ ions was also investigated [16]. In this paper, we have focused ourselves on the glass-compositional dependence on Tm3+ blue UCPL for Tm3+/Yb3+ co-doped TeO2-TlO0.5-ZnO glasses. Since the optimized rare-earth ratio for blue UCPL was found to be [Yb3+]/[Tm3+] = 5 in our previous work [17], the Tm2O3 and Yb2O3 content were fixed to 0.1 and 0.5 mol%, respectively. In this study, glass structures of Tm3+/Yb3+ co-doped TeO2-TlO0.5-ZnO glasses were investigated by Raman technique in order to elucidate their relationship with energy transfer from Yb3+ to Tm3+. Judd-Ofelt parameters for Tm3+ for different compositions of TeO2-TlO0.5-ZnO shall be also reported.
2. Experiments
2.1 Sample preparation
(90-x-y)TeO2-xTlO0.5-(9.4 + y)ZnO-0.1Tm2O3-0.5Yb2O3 (x = 10, 20, 30, y = 0, 10) glasses were prepared by a conventional melt quenching method at ambient atmosphere using commercially-available chemicals, Tl2CO3, ZnO, Tm2O3, and Yb2O3. Tellurium dioxide TeO2 was only prepared by decomposing commercial telluric acid (H6TeO6, Aldrich) at 550 °C for 24 hours [18,19]. Telluric acid is unstable against thermal treatment in air so that the heat-treatment even at such a moderate temperature can decompose it to TeO2 [19], which was checked by X-ray diffraction data before glass synthesis. The mixture of these powders with stoichiometry ratio was melted in a platinum crucible at 800 °C for 8 hours in air. During melt preparation, a part of component in melt is possibly to be evaporated. In the case of this study, ZnO comparatively tended to be evaporated somewhat among TeO2, Tl2O and ZnO, however, there was no significant difference between the nominal composition and final composition, which was confirmed by X-ray fluorescence (XRF) technique. The obtained glasses were annealed for 10 hours at 40 °C below each Tg, and then cooled down to room temperature [18]. Planar surfaces of these glasses were polished to optical-flat. After polishing, the thickness of each glass was ~1 mm. In the sample preparation, we attempted to synthesize six different glass samples of TeO2-TlO0.5-ZnO (See Table 1). However, a composition of 80TeO2-10TlO0.5-9.4ZnO-0.1Tm2O3-0.5Yb2O3 could not be obtained because the sample was not vitrified (out of the glass forming region). Thus, five glass samples were examined and are referred as approximate ratios of the elements for Te, Tl, and Zn in the latter part of the paper. For example, 60TeO2-30TlO0.5-9.4ZnO-0.1Tm2O3-0.5Yb2O3 glass is named by 6-3-1 because the approximate ratio of Te, Tl, and Zn is 6:3:1. The mean distance of Tm3+ and Yb3+ ions is estimated to be ~1.2 nm for all the samples investigated because of the fixed dopant concentration of 0.1Tm2O3 and 0.5Yb2O3 and here we have focused ourselves on compositional dependence of blue UCPL of Tm3+/Yb3+ doped TeO2-TlO0.5-ZnO.
Table 1. Glass compositions of the prepared glasses: (90-x-y)TeO2-xTlO0.5-(9.4 + y)ZnO-0.1Tm2O3-0.5Yb2O3 (x = 10, 20, 30, y = 0, 10) glasses (mol%); 6-3-1 (x = 30, y = 0), 7-2-1 (x = 20, y = 0), 5-3-2 (x = 30, y = 10), 6-2-2 (x = 20, y = 10), 7-1-2 (x = 10, y = 10).
2.2 Characterizations
UCPL spectra were measured using the excitation beam from a CW semiconductor laser (JDSU, 27-7552) operated at 975 nm. The excitation power density was varied from 1.67 kW/cm2 to 8.68 kW/cm2. Optical absorption spectra were measured by UV-vis-NIR spectrophotometer (Jasco, V-570). The wavelength range was 300-2500 nm, which could be observed for Tm3+ and Yb3+ ions absorption peaks needed for Judd-Ofelt analysis [20,21]. Lifetime of 1G4→3H6 transition in Tm3+ ion was measured by dye laser (USHO, KEC-160) using a coumarin 460 dye whose wavelength was tuned to 462 nm. For estimation of glass structure, Raman spectra were measured by micro Raman spectrometer (Jasco, NRS-2000) whose wavelength was 532 nm.
2.3 Judd-Ofelt analysis
Judd-Ofelt (JO) theory [20,21] is a powerful tool which can theoretically calculate f-f transition probability of rare-earth ions. There is below shown the Krupke’s formulation [22, 23] to estimate phenomenological parameters, called JO parameters (or omega parameters), which can be determined from oscillator strengths calculated from optical absorption spectra by the following equation:
where ε (ν) is the molar absorption coefficient at wavenumber. This should be obtained as a sum of the electric dipole and magnetic dipole oscillator strengths for the transition band of interest, fexp = fed + fmd, where ν is the wavenumber of transmission J→J’, n is the refractive index, h is Plank’s constant, c is light velocity in vacuum, m is electron mass, and Sed and Smd are line strengths for the electric dipole and magnetic dipole transitions, respectively, as given in [24,25]:The Judd-Ofelt parameters, Ωλ (λ = 2, 4, 6), are closely related to the active ion environment, and can be considered as phenomelogical parameters. The spontaneous emission probabilities (A) of the different electronic transitions are given by
The terms |<aJ||Uλ||bJ’>|2 is the square of the matrix elements of the tensorial operator Uλ, which connects states |aJ> to |bJ’> and is considered to be independent of the host matrix. The values of |<aJ||Uλ||bJ’>|2 were previously listed by Carnall et al. [26]. In fact, the estimation of JO parameters is a problem of solving simultaneous linear equations. With our nonlinear Gauss-Seidel routine based on the least square fitting, JO parameters Ωλ were obtained with a degree of the calculation accuracy for the least square fitting defined by [27,28],
where N is the number of absorption band involved in the JO calculation. The rms error will be listed with the obtained results, which is given in the following [25],where3. Results
Figure 2 displays one typical optical absorption spectrum (6-3-1 sample) for (90-x-y)TeO2-xTlO0.5-(9.4 + y)ZnO-0.1Tm2O3-0.5Yb2O3 (x = 10, 20, 30, y = 0, 10) glasses because the obtained absorption spectra for all the samples had no large difference. The absorption peaks centered at 690, 797, 1215, and 1690 nm were attributable to 3H6→3F2,3, 3H6→3H4, 3H6→3H5, and 3H6→3F4 excitation of Tm3+ ions, respectively [29, 30]. The strong absorption band peaking at 980 nm was assignable to 2F7/2→2F5/2 of Yb3+ ions [31]. The oscillator strengths of the Tm3+ absorption transitions and Judd-Ofelt parameters Ωλ (λ = 2, 4, 6) are shown in Table 2.The results of JO calculation have reasonable agreement between experimental and theoretical (recalculated) oscillator strengths with small rms errors δRMS of at most 27% and the minimum of 12.4% for sample 5-3-2. The large Ω2 parameters of 4~5 pm2 in comparison with Ω4 and Ω6 [16] indicate asymmetric ligand structures around Tm3+ ions in tellurite glasses [32].
Fig. 2 Optical absorption spectra of 60TeO2-30TlO0.5-9.4ZnO-0.1Tm2O3-0.5Yb2O3 glass (6-3-1). The insertion shows a semi-logarithmic plot of the optical absorption spectrum. The optical absorption edge is seen to be ~420 nm.
Table 2. Refractive indices, experimental and theoretical oscillator strengths, Judd-Ofelt parameters (Ω2, 4, 6, and δRMS (with rms error defined by Eqs. (7)-(9)) of (90-x-y)TeO2- xTlO0.5-(9.4 + y)ZnO-0.1Tm2O3-0.5Yb2O3 (x = 10, 20, 30, y = 0, 10) glasses. 6-3-1 (x = 30, y = 0), 7-2-1 (x = 20, y = 0), 5-3-2 (x = 30, y = 10), 6-2-2 (x = 20, y = 10), 7-1-2 (x = 10, y = 10).
UCPL spectra of 60TeO2-30TlO0.5-9.4ZnO-0.1Tm2O3-0.5Yb2O3 glass (6-3-1) with different excitation power density are shown in Fig. 3.Three emission peaks centered at 480, 650 and 800 nm were due to 1G4→3H6 (|5>→|0>), 1G4→3F4, (|5>→|1>), and 3H4→3H6 (|3>→|0>) transitions of Tm3+ ions, respectively. The insertion of Fig. 3 shows UCPL integrated intensity ratio (I480 / I800) of 60TeO2-30TlO0.5-9.4ZnO-0.1Tm2O3-0.5Yb2O3 glass as a function of the excitation power density, Iexc, whose curve is fitted by the following equation given from the rate equation analysis [6]:
where IS = hνexc/τdσd (τd is the experimental lifetime of the donor, and σd is the ground-state absorption cross-section at the excitation wavelength) and means an excitation power density at r = a/2. The saturated intensity ratios a are obtained and shown in Table 3.The highest saturated intensity ratio is 0.47 for sample 6-2-2 and 7-1-2. According to Silva et al. [6], the rate of the third step’s energy transfer γd5 for blue UCPL, which is defined as being a probability that excitation energy on Yb3+ ion is transferred to Tm3+ ion in the excited state. ((Yb3+,Tm3+):(2F5/2, 3H4)→(2F7/2, 1G4)), is proportional to the saturated intensity ratio, a. Thus, the glass with the highest saturated intensity ratio is expected to show the highest energy transfer rate. (The details are given in Discussion.)Fig. 3 UCPL spectra with different excitation power densities of 60TeO2-30TlO0.5-9.4ZnO-0.1Tm2O3-0.5Yb2O3 glass (6-3-1). λex = 975 nm. Each spectrum was offset for eyes guide. Inset shows up-conversion emission intensity ratio (r = I480/I800) of same sample as a function of the excitation power density.
Table 3. Saturated intensity ratio, a, transition probabilities Ai0 and the ratio of products for transition probabilities Ai0 and transition energy hνi0 Lifetime τ5, substantial energy transfer rate (ETR) <γd5> for 1G4 level of Tm3+ ions of (90-x-y)TeO2-xTlO0.5-(9.4 + y)ZnO-0.1Tm2O3-0.5Yb2O3 (x = 10, 20, 30, y = 0, 10) glasses. 6-3-1 (x = 30, y = 0), 7-2-1 (x = 20, y = 0), 5-3-2 (x = 30, y = 10), 6-2-2 (x = 20, y = 10), 7-1-2 (x = 10, y = 10).
The obtained Ωλ parameters allowed us to calculate theoretically emission probabilities A via Eqs. (4) and (6), which are also shown in Table 3. Comparing the transition probabilities A30 (3H4→3H6) and A50 (1G4→3H6), all of A50 values are higher than the corresponding A30 values, meaning that blue PL of Tm3+ ions can potentially be intensified in comparison with NIR PL in Tm3+/Yb3+ doped TeO2-TlO0.5-ZnO glass system.
PL decay curves of 1G4→3H6 transition in Tm3+ ion for all the synthesized glasses were examined. Figure 4 shows PL decay curves with 1G4 lifetime for the respective glass samples synthesized. The 1G4 lifetimes were almost constant with the glass composition altered (See Table 3, too).
Fig. 4 1G4–3H6 photoluminescence decay curves of (90-x-y)TeO2-xTlO0.5-(9.4 + y)ZnO-0.1Tm2O3-0.5Yb2O3 (x = 10, 20, 30, y = 0, 10) glasses (λex = 462 nm, λem = 480 nm). Each curve was offseted for eyes guide. 6-3-1 (x = 30, y = 0), 7-2-1 (x = 20, y = 0), 5-3-2 (x = 30, y = 10), 6-2-2 (x = 20, y = 10), 7-1-2 (x = 10, y = 10).
To acquire the information related with glass structures of the glasses synthesized, Raman investigation was carried out and the results are shown in Fig. 5. All the spectra were deconvoluted in such a way as Fig. 6, where the blue line is the measured Raman spectrum, the red lines are deconvoluted Gaussian curves, and the red dotted line is a reconstructed curve from the obtained Gaussian curves. It have been reported that TeO2 based glass is composed of three unit structures, TeO4 trigonal bipyramids (tbp), TeO3 trigonal pyramid (tp), and TeO3+1 intermediate structure whose peaks are located in the red area on Fig. 6 [13–15]. TeO4 structure is a main unit structure with two equitorial and two axial positioned oxygens, which connect glass networks with Te-eqOax-Te bond (See No.1 peaks in Fig. 6). TeO3 structure is a terminal structure of glass networks (No.3 peak in Fig. 6). TeO3+1 structure is an intermediate structure of TeO4 and TeO3 structures, appearing as No.2 peaks in Fig. 6. Two peaks located around 270 and 310 cm−1 in the blue area in Fig. 6 represent TeO32- island structures [33], which are isolated from glass network. Four peaks located around 380, 420, 450, and 500 cm−1 in the green area represent Te-eqOax-Te bond. The areas of Gaussian functions assigned to the respective glass-structural units above were analyzed by dividing a sum of [TeO4], [TeO3+1], and [TeO3] (expressed as [TeOn]) and shown in Table 4.Only the overall features are here given: (I) the increase in TeO2 content at the constant ZnO content (the decrease in TlO0.5 content) induces higher [Te-O-Te] ratio and lower isolated TeO32- ratio, meaning longer TeO2 glassy networks. (II) Larger ZnO content at the expense of TlO0.5 in constant TeO2 content results in higher [Te-O-Te] ratio (6-3-1 vs 6-2-2; 7-2-1 vs 7-1-2). (III) Larger TlO0.5 content makes TeO32- structures to be increased (6-3-1 and 5-3-2). From the results, it can be seen that ZnO is an intermediate oxide and plays a role of glass network former in TeO2-TlO0.5-ZnO system. On the other hand, TlO0.5 is a network modifier which breaks tellurite glassy network so as to increase TeO32- isolated structures.
Fig. 5 Raman spectra of (90-x-y)TeO2-xTlO0.5-(9.4 + y)ZnO-0.1Tm2O3-0.5Yb2O3 (x = 10, 20, 30, y = 0, 10) glasses. 6-3-1 (x = 30, y = 0), 7-2-1 (x = 20, y = 0), 5-3-2 (x = 30, y = 10), 6-2-2 (x = 20, y = 10), 7-1-2 (x = 10, y = 10).
Fig. 6 Raman spectrum (blue line), deconvoluted Gaussian curves (red lines), and reconstructed with Gaussian curves (dotted red line) for 60TeO2-30TlO0.5-9.4ZnO-0.1Tm2O3-0.5Yb2O3 glass (6-3-1).
Table 4. Ratio of glass structure units (Te-O-Te, TeO4, TeO3+1, TeO3, TeO32-(isolated structures) divided by of [TeO4], [TeO3+1], and [TeO3] (expressed as [TeOn])) for (90-x-y)TeO2-xTlO0.5-(9.4 + y) ZnO-0.1Tm2O3-0.5Yb2O3 (x = 10, 20, 30, y = 0, 10) glasses. 6-3-1 (x = 30, y = 0), 7-2-1 (x = 20, y = 0), 5-3-2 (x = 30, y = 10), 6-2-2 (x = 20, y = 10), 7-1-2 (x = 10, y = 10). (n.d. = not detected)
4. Discussion
In this study, we carried out several experiments to obtain saturated intensity ratio a, transition probabilities A50, A30, lifetime τ5, and so on. Using these data, energy transfer rate, γd5 was calculated by the following equation according to Silva et al. [6]:
where Nd is donor concentration of Yb3+ ions. As discussed in our previous paper [17], it is plausible that Nd is equal to the Yb concentration doped in glasses, NYb ( = 1.17 × 1020 cm−3, in this study), due to the saturation condition for blue UCPL obtained. However, possible relaxation processes for the excited Yb3+ ions, such as Yb3+-Yb3+ energy transfer (or energy migration), Yb concentration quenching and multiphonon relaxation can reduce the population of the excited state 2F5/2 of Yb3+. Moreover, the population of Yb ions in the excited state would be influenced by back transfer from Tm3+ in 1G4 level to Yb3+ in ground state. Thus, here we take a substantial energy transfer rate <γd5> defined as follows,where ηYb is the population yield of Yb3+ ions in the excited state (2F5/2). Nd was not determined by the experiments employed in this study, while we could instead estimate the substantial energy transfer <γd5>, which is directly related with blue UCPL efficiency. The substantial calculated energy transfer rates <γd5> are shown in Table 3. The highest energy transfer rate is found to be 3.54 × 10−17 cm3/s for the sample 7-1-2 (70TeO2-10TlO0.5-19.4ZnO-0.1Tm2O3-0.5Yb2O3), which is higher than the reported value by Silva et al. that is 1.21 × 10−17 cm3/s for Tm3+/Yb3+ co-doped Al2O3-CaO-SiO2-MgO glass [6] and also than our previous result (2.07 × 10−17 cm3/s) for 60TeO2-30TlO0.5-8.8ZnO-0.2Tm2O3-1.0Yb2O3 [17].Figure 7 shows the obtained substantial energy transfer rate and the ratios of glass unit structures, estimated from Raman spectra, which were normalized by [TeOn] (a sum of [TeO4], [TeO3+1], and [TeO3]) for standardization. The dependence of the transfer rate on the glass matrix composition behaves similarly as that of the Te-eqOax-Te bond. On the other hand, the isolated structure TeO32- behaves oppositely to the behavior of the transfer rate and Te-eqOax-Te bond. (The ETR and Te-eqOax-Te ratio are increased with TeO2 content (6-3-1→7-2-1 and 5-3-2→6-2-2→7-1-2), while TeO32- is decreased with TeO2 content.) These results indicate that the energy transfer rate <γd5> is related with length of tellurite glass network. Firstly, it is clear that the energy transfer rate increases with increasing length of glass network, which must result in segregation of rare-earth elements (Yb3+ and Tm3+ ions) and decreasing mean distance between Yb3+ and Tm3+ ions, or increasing energy transfer rate γd5. However, shorter mean distance can induce back transfer from Tm3+ in 1G4 to Yb3+ in ground state 2F5/2. From PL lifetime measurement, 1G4 levels had a constant lifetime regardless of the various glass compositions, resulting from the constant Tm concentration. Thus, back transfer is not very significant in this system. Additionally to be mentioned, Tm PL concentration quenching is not significant as well. It is because relatively high Yb3+ concentration compared to Tm3+ ions ([Yb3+]/[Tm3+] = 5) made mean distance between Tm3+ ions larger enough than Yb3+-Yb3+ and Yb3+-Tm3+ ion-pairs.
Fig. 7 Ratios of glass unit structures estimated from Raman spectra and substantial energy transfer rates (ETR) <γd5> of (90-x-y)TeO2-xTlO0.5-(9.4 + y)ZnO-0.1Tm2O3-0.5Yb2O3 (x = 10, 20, 30, y = 0, 10) glasses. 6-3-1 (x = 30, y = 0), 7-2-1 (x = 20, y = 0), 5-3-2 (x = 30, y = 10), 6-2-2 (x = 20, y = 10), 7-1-2 (x = 10, y = 10).
The decreasing quenching centers like dangling bonds on TeO32- are another plausible cause to explain the increasing transfer rate <γd5> as defined in Eq. (12). The estimate <γd5> = includes the influence of several relaxation processes of the excited state of Yb3+ ions, energy migration, concentration quenching, multiphonon relaxation and so on. The energy migration does not always depopulate the excited state but spatial location of the excitation energy on Yb3+ (as 2F5/2) is needed to be near Tm3+ ions to give the energy to Tm3+ ions. On the other hand, if it is combined with quenching centers, the excitation energy would be lost (Yb concentration quenching). Not only the concentration quenching but also other relaxation process for Yb3+ ions must degrade the population yield ηYb in Eq. (12). Therefore, as stated above, the decrease in the dangling bonds is other one of the most plausible reasons of increasing substantial energy transfer rate <γd5> for Tm3+ blue UCPL in possible segregates of rare-earth elements (Yb3+/Tm3+ clusters) produced as a by-product of the formation of longer TeO2 glass networks. Moreover, as discussed in our previous paper [17], the relatively high Yb/Tm concentration ratio of not [Yb3+]/[Tm3+] = 3 but = 5 gave a positive effect to the energy migration effect: The excitation energy was traveled via several Yb3+ ions and finally reached Yb3+ ions located close to Tm3+ ions on excited/ground state, which was enabled to promote the capture of the excitation energy by Tm3+ ions.
In addition, all the samples exhibited yellow color regardless of the glass matrix compositions. And no significant difference in the UV absorption edge wavelength was observed. As shown in the insertion of Fig. 2, the absorption edge of the synthesized glass was positioned around 420 nm with a small tail up to 500 nm. Since the central position of the emission peak of blue UCPL of Tm3+ ion was 476 nm, it is supposed that the blue UCPL would be more or less decreased by the tail absorption of host glasses. Hence, if the optical absorption near the band-gap of host glasses were reduced, the intensity of blue UCPL might be increased, resulting in a further increase of saturated intensity ratio a and substantial energy transfer rate <γd5>. For furthermore improvement of Tm3+ blue UCPL, reducing hydroxyl groups in glasses is also effective as reported for Tm3+/Yb3+ co-doped Al2O3-CaO-SiO2-MgO glass by Silva et al. [6]. More investigations are required and now in progress.
6. Conclusion
Blue UCPL properties of Tm3+/Yb3+ co-doped (90-x-y)TeO2-xTlO0.5-(9.4 + y)ZnO-0.1Tm2O3-0.5Yb2O3 (x = 10, 20, 30, y = 0, 10) glasses synthesized by a conventional melt quenching method were reported. The optimized glass matrix composition for the highest substantial energy transfer rate <γd5>, defined as <γd5> = ηYbγd5 (γd5: energy transfer probability of the third step from Yb3+ to Tm3+ ions, ηYb: Yb population yield of the excited state 2F5/2) was 70TeO2-10TlO0.5-19.4ZnO-0.1Tm2O3-0.5Yb2O3 and its energy transfer rate <γd5> was found to be achieve 3.54 × 10−17 cm3/s. The glass compositional dependence of the energy transfer rate exhibited the similar behavior as Te-eqOax-Te bond, and the decreasing isolated structures per [TeOn] resulted in an increase of the transfer rate <γd5>. These results indicated that the increasing energy transfer rate <γd5> was possibly related with longer tellurite glass networks providing less dangling bonds of TeO32- isolated structures which might cause quenching of excited Yb3+ population ηYb for the blue photoluminescence.
Acknowledgments
This work was supported by the JSPS International Training Program (ITP), “Young Scientist-Training Program for World Ceramics Networks” and by grant from Institute of Ceramics Research and Education (ICRE) in Nagoya Institute of Technology.
References and links
1. A. Braud, S. Girard, J. L. Doualan, M. Thuau, R. Moncorge, and A. Tkachuk, “Energy-transfer processes in Yb:Tm-doped KY3F10, LiYF4, and BaY2F8 single crystals for laser operation at 1.5 and 2.3 μm,” Phys. Rev. B 61(8), 5280–5292 (2000). [CrossRef]
2. J. Liao, Z. Yang, H. Wu, D. Yan, J. Qiu, Z. Song, Y. Yang, D. Zhou, and Z. Yin, “Enhancement of the up-conversion luminescence of Yb3+/Er3+ or Yb3+/Tm3+ co-doped NaYF4 nanoparticles by photonic crystals,” J. Mater. Chem. C 1(40), 6541–6546 (2013). [CrossRef]
3. M. Quintanilla, N. O. Núñez, W. Cantelar, M. Ocaña, and F. Cussó, “Energy transfer efficiency in YF3 nanocrystals: Quantifying the Yb3+ to Tm3+ infrared dynamics,” J. Appl. Phys. 113(17), 174308 (2013). [CrossRef]
4. N. Ch, R. Babu, C. Srinivasa Rao, M. G. Brik, G. Naga Raju, I. V. Kityk, and N. Veeraiah, “Manifestation of up-conversion in Yb3+/Tm3+ doped Li2O-Y2O3-SiO2 glass system,” Appl. Phys. B 110(3), 335–344 (2013).
5. D. Li, Y. Wang, X. Zhang, H. Dong, L. Liu, G. Shi, and Y. Song, “Effect of Li+ ions on enhancement of near-infrared upconversion emission in Y2O3:Tm3+/Yb3+ nanocrystals,” J. Appl. Phys. 112(9), 094701 (2012). [CrossRef]
6. W. F. Silva, F. G. Rego-Filho, M. T. de Araujo, E. A. Gouveia, M. V. D. Vermelho, P. T. Udo, N. G. C. Astrath, M. L. Baesso, and C. Jacinto, “Highly efficient upconversion emission and luminescence switching from Yb3+/Tm3+ co-doped water-free low silica calcium aluminosilicate glass,” J. Lumin. 128(5–6), 744–746 (2008). [CrossRef]
7. F. Wang, Y. Han, C. S. Lim, Y. Lu, J. Wang, J. Xu, H. Chen, C. Zhang, M. Hong, and X. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature 463(7284), 1061–1065 (2010). [CrossRef] [PubMed]
8. S. Gai, P. Yang, C. Li, W. Wang, Y. Dai, N. Niu, and J. Lin, “Synthesis of magnetic, up-ponversion luminescent, and mesoporous core-shell-structured nanocomposites as drug carriers,” Adv. Funct. Mater. 20(7), 1166–1172 (2010). [CrossRef]
9. X. Guo, W. Song, C. Chen, W. Di, and W. Qin, “Near-infrared photocatalysis of β-NaYF4:Yb3+,Tm3+@ZnO composites,” Phys. Chem. Chem. Phys. 15(35), 14681–14688 (2013). [CrossRef] [PubMed]
10. H.-Q. Wang, M. Batentschuk, A. Osvet, L. Pinna, and C. J. Brabec, “Rare-earth ion doped up-conversion materials for photovoltaic applications,” Adv. Mater. 23(22-23), 2675–2680 (2011). [CrossRef] [PubMed]
11. J. Chang, Y. Ning, S. Wu, W. Niu, and S. Zhang, “Effective utilizing NIR light using direct electron injection from up-conversion nanoparticles to the TiO2 photoanode in dye-sensitized solar cells,” Adv. Funct. Mater. 23(47), 5910–5915 (2013). [CrossRef]
12. V. P. Tuyen, T. Hayakawa, M. Nogami, J.-R. Duclère, and P. Thomas, “Fluorescence line narrowing spectroscopy of Eu3+ in zinc-thallium-tellurite glass,” J. Solid State Chem. 183(11), 2714–2719 (2010). [CrossRef]
13. E. R. Barney, A. C. Hannon, D. Holland, N. Umesaki, M. Tatsumisago, R. G. Orman, and S. Feller, “Terminal oxygens in amorphous TeO2,” J. Phys. Chem. Lett. 4(14), 2312–2316 (2013). [CrossRef]
14. H. M. Oo, H. Mohamed-Kamari, and W. M. Wan-Yusoff, “Optical properties of bismuth tellurite based glass,” Int. J. Mol. Sci. 13(12), 4623–4631 (2012). [CrossRef] [PubMed]
15. D. Linda, J.-R. Duclère, T. Hayakawa, M. Dutreilh-Colas, T. Cardinal, A. Mirgorodsky, A. Kabadou, and P. Thomas, “Optical properties of tellurite glasses elaborated within the TeO2-Tl2O-Ag2O and TeO2-ZnO-Ag2O ternary systems,” J. Alloy. Comp. 561(5), 151–160 (2013). [CrossRef]
16. V. P. Tuyen, T. Hayakawa, M. Nogami, J. -R. Duclère, and P. Thomas, “Photoluminescence properties and Judd-Ofelt analysis of Eu3+ ions in zinc-thallium-tellurite glasses,” J. Non-Crystal Solids, to be submitted (2014).
17. M. Uchida, T. Hayakawa, T. Suhara, J. -R. Duclère, and P. Thomas, “Dependence of rare-earth concentration on blue up-conversion photoluminescence properties for Tm3+/Yb3+ co-doped TeO2-TlO0.5-ZnO glasses,” J. Appl. Glass Sci. (to be submitted) (2013).
18. M. Soulis, J.-R. Duclère, T. Hayakawa, V. Couderc, M. Dutreilh-Colas, and P. Thomas, “Second harmonic generation induced by optical poling in new TeO2-Tl2O-ZnO glasses,” Mater. Res. Bull. 45(5), 551–557 (2010). [CrossRef]
19. J. C. J. Bart, A. Bossi, P. Perissinoto, A. Castellan, and N. Giordano, “Some observations on the thermochemistry of telluric acid,” J. Them. Analys. 8(2), 313–327 (1975). [CrossRef]
20. B. R. Judd, “Optical absorption intensities of rare-earth ions,” Phys. Rev. 127(3), 750–761 (1962). [CrossRef]
21. G. S. Ofelt, “Intensities of crystal spectra of rare earth ions,” J. Chem. Phys. 37(3), 511–520 (1962). [CrossRef]
22. W. F. Krupke, “Radiative transition probabilities within the 4f3 ground configuration of Nd:YAG,” IEEE J. Quantum Electron. 7(4), 153–159 (1971). [CrossRef]
23. W. F. Krupke, “Induced-emission cross sections in neodymium laser glasses,” IEEE J. Quantum Electron. 10(4), 450–457 (1974). [CrossRef]
24. W. F. Krupke and J. B. Gruber, “Optical-absorption intensities of rare-earth ions in crystals: the absorption spectrum of thulium ethyl sulfate,” Phys. Rev. 139(6A), A2008–A2016 (1965). [CrossRef]
25. N. Spector, R. Reisfeld, and L. Boehm, “Eigenstates and radiative transition probabilities for Tm3+ (4f12) in phosphate and tellurite glasses,” Chem. Phys. Lett. 49(1), 49–53 (1977). [CrossRef]
26. W. T. Carnall, P. R. Fields, and K. Rajnak, “Electronic energy levels in the trivalent lanthanide aquo ions. I. Pr3+, Nd3+, Pm3+, Sm3+, Dy3+, Ho3+, Er3+, and Tm3+,” J. Chem. Phys. 49(10), 4424–4442 (1968). [CrossRef]
27. G. H. Jia, C. Y. Tu, J. F. Li, Z. J. Zhu, Y. You, Y. Wang, and B. C. Wu, “Spectroscopy of GdAl3(BO3)4:Tm3+ crystal,” J. Appl. Phys. 96(11), 6262–6266 (2004). [CrossRef]
28. W. Guo, Y. Chen, Y. Lin, Z. Luo, X. Gong, and Y. Huang, “Spectroscopic properties and laser performance of Tm3+-doped NaLa(MoO4)2 crystal,” J. Appl. Phys. 103(9), 093106 (2008). [CrossRef]
29. O. A. Blackburn, M. Tropiano, T. J. Sørensen, J. Thom, A. Beeby, L. M. Bushby, D. Parker, L. S. Natrajan, and S. Faulkner, “Luminescence and upconversion from thulium(III) species in solution,” Phys. Chem. Chem. Phys. 14(38), 13378–13384 (2012). [CrossRef] [PubMed]
30. C. Reinhard and H. U. Güdel, “High-resolution optical spectroscopy of Na3[Ln(dpa)3].13H2O with Ln = Er3+, Tm3+, Yb3+,” Inorg. Chem. 41(5), 1048–1055 (2002). [CrossRef] [PubMed]
31. Z. Shen, M. Nygren, and U. Halenius, “Absorption spectra of rare-earth-doped α-sialon ceramics,” J. Mater. Sci. Lett. 16(4), 263–266 (1997). [CrossRef]
32. R. Balda, L. M. Lacha, J. Fernández, M. A. Arriandiaga, J. M. Fernández-Navarro, and D. Munoz-Martin, “Spectroscopic properties of the 1.4 µm emission of Tm3+ ions in TeO2-WO3-PbO glasses,” Opt. Express 16(16), 11836–11846 (2008). [CrossRef]
33. O. Noguera, T. Merle-Méjean, A. P. Mirgorodsky, P. Thomas, and J.-C. Champarnaud-Mesjard, “Dynamics and crystal chemistry of tellurites. II. Composition- and temperature-dependence of the Raman spectra of x(Tl2O)-(1-x)Te2O glasses: evidence for a phase separation?” J. Phys. Chem. Solids 65(5), 981–993 (2004). [CrossRef]
