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Abstract
Here, we study the Er3+ NIR 4I9/2-4I15/2 photoluminescence peaking at 800 nm. It can be detected with a good signal-to-noise for the prepared CaWO4:Yb3+,Er3+ phosphors upon excitation at 980 nm. When directly exciting the Er3+ green and red emitting states over the 333-773 K temperature range, the 800 nm photoluminescence for the samples is undetectable. It shows that the non-radiative relaxation from the upper excited states to the 4I9/2 emitting state is extremely inefficient. Moreover, the 800 nm photoluminescence decay curve is measured at high temperatures. It is found that the 800 nm emission always has a similar lifetime with the Er3+ 4I11/2-4I15/2 transition. This reminds us that the Er3+ 4I9/2 state is mainly populated by the adjacent lower 4I11/2 state by a thermally coupled way.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Among the 17 kinds of rare earth ions, Er3+ is especially attractive for its abundant energy levels and unique optical property [1,2]. Yb3+ is always co-doped along with Er3+ because of its simple energy level structure and excellent energy transfer from Yb3+ to Er3+ [3–7]. With the help of Yb3+, Er3+ can efficiently emit photoluminescence ranging from the deep UV to the NIR spectral range. Li et al. once observed the 4G11/2/2H9/2-4I15/2 transitions of Er3+ in Y2O3:Yb3+,Er3+ upconverted nanoparticles and studied the influence of temperature on these two transitions [8]. Many groups have investigated the 2H11/2/4S3/2-4I15/2 emissions in detail and confirmed that these two green emissions of Er3+ could be used not only for bio-imaging but also for thermal detection [9–14]. Our group successfully detected the 4F7/2-4I15/2 emission [15]. Based on this emission and the 4S3/2-4I15/2 transition, we proposed a novel luminescence ratiometric strategy for a more sensitive temperature measurement. The 4F9/2-4I15/2 red emission also has been intensively studied [16]. Rakov et al. and Lei et al. studied the Stark split components of the 4I13/2 state in different luminescent materials [17,18]. Both the two groups confirmed that the Boltzmann distribution dominated the populations among these Stark states.
In fact, in addition to the above-mentioned excited states, there is also a 4I9/2 state above the 4I11/2 state and below the 4F9/2 state. From this state, the 4I9/2-4I15/2 radiative transition occurs, peaking at ∼800 nm. Investigating this transition is obviously meaningful. However, there are little reports on this transition so far, although the 4I15/2 -4I9/2 absorption line of Er3+ has been observed [19,20]. Recently, our group observed this transition successfully in the scheelite-type host [21]. It was found to undergo a giant enhancement as the temperature increased gradually. Pump power dependence and steady state rate equation were then used to disclose this optical phenomenon. Nonetheless, there is still doubt about the origin of the 4I9/2-4I15/2 transition and more hard evidences are desperately needed.
Different from the previous work, here we are committed to disclosing the origin of the 4I9/2-4I15/2 emission by providing two strong evidences. The first evidence depends on the excitation and emission spectra of the samples. By monitoring at 800 nm corresponding to the 4I9/2-4I15/2 emission, no excitation bands can be found over the 200-700 nm wavelength range. Moreover, it is unlikely to observe the 4I9/2-4I15/2 emission by directly exciting the green emitting state 2H11/2 and the red emitting state 4F9/2. These experimental results demonstrate undoubtedly that it is difficult to populate the 4I9/2 state through the higher excited states. The second evidence is based on the decay curve of the 4I9/2-4I15/2 emission. It is found that this emission always has a similar lifetime with the 1000 nm emission originating from the 4I11/2-4I15/2 transition of Er3+ at relatively high temperatures. This key evidence reveals that the 4I9/2 state of Er3+ is mainly populated by the adjacent lower 4I11/2 state via a thermally coupled way.
2. Materials and methods
The CaWO4:Yb3+,Er3+ (10,1 mol %) phosphors were prepared by high-temperature solid-state method. CaO, WO3, Yb2O3 and Er2O3 powders were used as raw materials. The stoichiometric CaO, WO3, Yb2O3 and Er2O3 powders were mixed fully in an agate mortar. Then, these mixtures were pressed into a slice for the following convenient optical test. In the next step, the slice, put on a corundum crucible, would be sintered at 1473 K for four hours to form the finally samples.
Room temperature X–ray diffraction (XRD) patterns of the phosphors were measured by PANalytical X’Pert Powder X-ray diffractometer (Cu Kα radiation, λ=1.5406 Å). Scanning electron microscope (SEM) image of the phosphors was obtained by Hitachi S-4300. Raman measurement of the pure samples without any doping was performed by Nanobase XperRam-Compact equipped with a diode pumped solid state laser emitting at 532 nm. The 980 nm laser diode (Thorlabs, ITC-4005) and xenon lamp (Zolix Instruments Co., Ltd, LHX150) were used as excitation sources. The monochromator (Zolix Instruments Co., SBP–300), connected with the photomultiplier tube (PMTH–S1–CR131, Zolix Instruments Co., Ltd) and the data acquisition card, was utilized to collect the photoluminescence of the samples. A home-made heating stage, whose temperature accuracy is ± 0.3 K, was used to heat and cool the samples over the 333-773 K temperature range.
3. Results and discussion
3.1 Crystal structure and photoluminescence property
Figure 1(a) presents the measured XRD patterns of the as-prepared CaWO4:Yb3+,Er3+ samples (bottom panel) and the standard patterns with number 41-1431 from the JCPDS (top panel). One can see that the positions of the measured XRD patterns are in good agreement with the reference. Moreover, no redundant peaks could be found for the samples, suggesting the samples hold the single crystal phase. What’s more, the measured XRD peaks are quite sharp. All these experimental results show that the scheelite-type CaWO4:Yb3+,Er3+ phosphors were prepared successfully and were crystallized well. Because the ionic radius of Ca2+ (0.099 nm) is larger than that of Yb3+ (0.0858 nm) and Er3+ (0.0881 nm), a slight shrinkage occurred when the Yb3+ and Er3+ ions were doped into the host. The position of the measured peak slightly moved to the right, as shown in Fig. 1(b). Figure 1(c) depicts the crystal structure of the samples. Due to the similar ionic radius, the Yb3+ and Er3+ ions occupy the Ca2+ ion site in the CaWO4 host. The W site is surrounded by four O, and the Ca,Yb,Er sites are coordinated by eight O. Figure 1(d) presents the SEM image of the as-prepared CaWO4:Yb3+,Er3+ samples. As can be observed, the morphology of the samples is irregular, and the size of these particles is at the micro-meter scale. Figure 1(e) displays the room temperature Raman spectrum of the pure samples without doping. Over the Raman shift range between 50 and 1000 cm-1, there are eight emission bands, peaking at 87, 119, 215, 336, 403, 800, 841 and 914 cm-1, respectively. The positions of these eight Raman-active modes agree well with the previously reported data and the assignments of them have been well identified in the literature [22].
Fig. 1. (a),(b)XRD patterns, (c)Crystal structure and (d)SEM image of the CaWO4:Yb3+,Er3+ sample; (e)Raman spectrum of the pure CaWO4 sample.
As depicted in Fig. 2(a), Er3+ has abundant energy levels. Most of the previous literatures concerning on the luminescence property of Er3+ focused on the green and red emissions, corresponding to the 2H11/2/4S3/2-4I15/2 and 4F9/2-4I15/2 transitions, respectively. Little attention is paid on the Er3+ NIR emission peaking at ∼800 nm, corresponding to 4I9/2-4I15/2 transition. It is ascribed to the fact that this NIR emission is difficult to detect at room temperature. The same problem also exists for our samples. The signal of this NIR emission is relatively weak at low temperatures. However, with increasing temperature from 333 to 773 K, the 800 nm emission increases sharply. As shown in Fig. 2(b), there is a good signal-to-noise for the 800 nm emission at 773 K. One can see from the inset in Fig. 2(b) that a giant thermal enhancement occurs for this emission, which is totally contrary to the popular cognition of thermal quenching effect.
Fig. 2. (a)Energy level diagrams of Er3+-Yb3+ system; (b)Emission spectrum of the samples at 773 K upon excitation at 980 nm; note that the interference light from the laser had been subtracted; the inset shows the integrated intensity of this spectrum as a function of temperature from 333 to 773 K.
In our previous work, we have studied the dependence of the 800 nm emission on power density. It was confirmed that one-photon process should be the leading populating mechanism for this emission. Based on steady state rate equation, we showed that the giant thermal enhancement of the 800 nm emission was mainly assigned to the thermal equilibrium (the Boltzmann distribution) between the 4I9/2 and 4I11/2 excited states. Due to this thermal equilibrium, there are more populations in the 4I9/2 state at the expense of the depopulation of the adjacent lower 4I11/2 state. Obviously, more hard and experimental evidences, in addition to the previous theoretical analyses, are needed to disclose the origin of the 800 nm emission. In the following parts, we will present two experimental evidences.
3.2 The first evidence
The first evidence is discussed first. Figure 3(a) presents the excitation spectrum of the samples at 773 K by monitoring at 800 nm. One can see that in the wavelength range from 200 to 700 nm, no excitation bands can be found, demonstrating that it is difficult to populate the 4I9/2 state by exciting the higher states. As shown in Fig. 3(b), the samples only emit the 4S3/2-4I15/2 transition upon excitation at 520 nm, corresponding to the 4I15/2-2H11/2 absorption line of Er3+. As can be observed, there are no emissions at ∼650 and 800 nm, being an indication that the non-radiative relaxation channels from the 2H11/2 and 4S3/2 states to the lower 4F9/2 and 4I9/2 states are quite inefficient (see the inset in Fig. 3(b)). Figure 3(c) presents the emission spectra of the samples upon excitation at 650 nm, corresponding to the 4I15/2-4F9/2 absorption line of Er3+. Obviously, no NIR emission at 800 nm could be found for the samples. Therefore, the non-radiative relaxation from the 4F9/2 state to the 4I9/2 state is hard to occur (see the inset in Fig. 3(c)). These experimental results reveal that the NIR emission at 800 nm is not likely to be from the excited states higher than the 4I9/2 state of Er3+. In other words, the non-radiative transition of the 4F9/2 state, although present, should not be the leading way to populate the 4I9/2 state.
Fig. 3. (a)Excitation spectrum of the samples at 773 K by monitoring at 800 nm; Emission spectra of the samples and the energy level diagrams of Er3+-Yb3+ system upon excitations at (b)520 nm and (c)650 nm in the temperature range from 333 to 773 K.
3.3 The second evidence
The second evidence is then discussed. We measured the decay curves for the 800 and 1000 nm emissions at 693, 733 and 773 K, which are presented in Figs. 4(a), 4(b) and 4(c), respectively. Obviously, the two emission bands always follow the same decay. Moreover, both the two emission bands are in good agreement with the following double-exponential formula:
where I(t) is the emission intensity of the luminescence decay curve at time t, A1 and A2 are two pre-exponential coefficients, t1 and t2 are two time constants (the so-called lifetimes), and B is the luminescence background. It is easy to know that for a simple system made up of a ground state and an excited state, the decay of the excited state follows the single-exponential formula due to the radiative and non-radiative transitions. However, most of the lanthanide ions generally own complex energy levels, making there a number of physical mechanisms in addition to the basic radiative and non-radiative transitions, such as the cross-relaxation, energy transfer, back energy transfer, energy migration, photon avalanche, and so forth. At this point, the decay of the excited state no longer follows the single-exponential formula as these other mechanisms also are expected to be reflected. In our work, the 4I9/2 and 4I11/2 states are likely to be influenced by other states, such as the 4I13/2 state, although the mechanism is quite weak. In this case, the decay curves of the two measured emissions satisfy the double-exponential formula, rather than the single-exponential one. The effective lifetime teff can be written as:Fig. 4. Luminescence decay curves for the 800 and 1000 nm emissions at (a)693 K, (b)733 K, and (c)773 K, respectively.
By using this equation, we have calculated the effective lifetime teff for the 800 and 1000 nm emission bands. At 693 K, the lifetimes for these two emissions bands are ∼0.9 ms. With the rise of temperature, the lifetime goes up to ∼1.1 ms 733 K. While as the temperature further increased to 773 K, the lifetime for them becomes ∼1.4 ms. The 800 and 1000 nm emission bands always share the same lifetime. This reminds us that the Er3+ 4I9/2 state is indeed mainly populated by the adjacent lower 4I11/2 state by a thermally coupled way.
4. Conclusions
To summary, we prepared the CaWO4:Yb3+,Er3+ phosphors and successfully detected the Er3+ NIR 4I9/2-4I15/2 photoluminescence peaking at 800 nm upon excitation at 980 nm. Then, we present two experimental evidences in this work to disclose the origin of this 800 nm emission. The first evidence is that there is no emission found at 800 nm for the as-prepared samples when the samples are excited to the states (the Er3+ 2H11/2 and 4F9/2 states) higher than the 4I9/2 state. It indicates that the non-radiative relaxation from the upper states to the 4I9/2 emitting state is extremely inefficient. We then present the second evidence by measuring the decay curve of the 800 nm photoluminescence at relatively high temperatures. It is found that the 800 nm emission always has a same decay law with the Er3+ 4I11/2-4I15/2 transition. This reminds us that the Er3+ 4I9/2 state is mainly populated by the adjacent lower 4I11/2 state by a thermally coupled way. Our work has successfully disclosed the origin of the 800 nm photoluminescence of Er3+, which is expected to make us better understand the luminescent property of Er3+ and then use it in the future.
Funding
National Natural Science Foundation of China (61505045, 81571720).
Disclosures
The authors declare no conflicts of interest.
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