Up-conversion luminescence of Er3+-Yb3+ co ... - OSA Publishing

Up-conversion luminescence of Er3+-Yb3+ codoped CaIn2O4 Ting Li, Chongfeng Guo,* and Lin Li National Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base) in Shaanxi Province, National Photoelectric Technology and Functional Materials & Application of Science and Technology International Cooperation Base, Institute of Photonics & Photon-Technology and Department of Physics, Northwest University, Xi’an 710069, China * [email protected]

Abstract: Up-conversion emission spectra of Er3+ singly and Er3+/Yb3+ codoped CaIn2O4 were investigated under a 980 nm diode laser excitation. Double intense UC emission bands in green and red region are observed in Er3+/Yb3+ co-doped CaIn2O4, resulting in the tunable ratio of red to green and the chromaticity coordinates by adjusting the concentration of Er3+ and Yb3+. Based on the pump power dependence, the possible UC mechanism in Er3+/Yb3+ co-doped CaIn2O4 was proposed, and lifetime measurements were also carried out to support our proposal. ©2013 Optical Society of America OCIS codes: (190.7220) Upconversion; (300.6170) Spectra; (160.5690) Rare-earth-doped materials; (160.4760) Optical properties.

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Received 17 Apr 2013; revised 8 Jun 2013; accepted 12 Jul 2013; published 23 Jul 2013 29 July 2013 | Vol. 21, No. 15 | DOI:10.1364/OE.21.018281 | OPTICS EXPRESS 18281

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1. Introduction Up-conversion (UC) is the spectroscopic process that converts two or more lower energy photons with longer wavelength into one higher energy photon with shorter wavelength through multi-photon process [1]. Rare earth ions (REI), such as Er3+, Tm3+, Ho3+, Pr3+ and Nd3+, are suitable activator candidates for UC processes owing to their long-lived excited states and ladder-like energy levels [2]. Driven by the popularization of low-cost efficient continuous wave (CW) infrared diode lasers (e. g. 980 nm InGaAs and 808 nm GaAs/GaAs) and the potential versatile applications of UC phosphors in biomedicine, lighting, displays and solar cells, considerable attention has been focused on the trivalent REI doped UC materials, in which the multi-photon absorption UC occurred through excitation state absorption (ESA) and energy transfer (ET) between ladder-like levels of REI dopants [3–6]. The best host for the UC phosphors should provide phonon energy as low as possible in order to achieve high-efficiency radiative emissions, in which the competitive phononassisted non-radiative deactivation could be inhibited to the utmost. Therefore, fluorides (~355 cm−1) and chlorides (~260 cm−1) are popular UC hosts for their low phonon matrices, which minimizes multi-phonon de-excitation probabilities [7–9]. Up to now, NaYF4 is still in the highest flight in the UC hosts and NaYF4: Er3+, Yb3+ is the most efficient UC materials. Unfortunately, the fluorides and chlorides often exhibit inferior stabilities and pollution to environment [10]. Oxide-based materials are widely recognized as promising hosts for UC applications because they usually offer excellent chemical and physical stabilities. In previous investigations for searching the single-phased white light emitting phosphor, it is found that

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down-conversion white light emission could be realized by doping low concentration Eu3+ ions in some special hosts with low phonon energy, such as BaY2ZnO5, BaGd2ZnO5 and CaIn2O4 [11–13]. BaY2ZnO5 and BaGd2ZnO5 have been proved to be efficient UC hosts [14,15], it is deduced that CaIn2O4 could be an expectant host for UC. CaIn2O4 is a semiconductor with low phonon energy about 475 cm−1, which is much lower than those of other typical oxide hosts, such as Y2O3 (~600 cm−1), silicate (~1100 cm−1), tellurite (700~800 cm−1) and tungstates (~900 cm−1), and so on [2,13]. However, no UC luminescence properties have been opened in REI doped CaIn2O4. Trivalent erbium (Er3+) ions are considered as the potential up-convertors for red, green and blue emissions because of its rich energy levels, which match well with the excitation of commercial infrared laser diodes [14–19]. However, it is not effective because of its forbidden 4f-4f transitions and it is necessary to add efficient sensitizer in order to improve the UC intensity of Er3+. Yb3+ only possesses one excited f-electron level and has a strong broad absorption band at around 980 nm, which make it always used as an efficient sensitizer of Er3+ in UC system under the excitation of 980 nm [20–23]. In Yb3+ and Er3+ co-doped UC phosphors, PL spectra usually include red and green double color emission bands and the intensity ratio of red to green emission are tunable and controllable through adjusting the Er3+ and Yb3+ concentration. Here, the UC luminescence of Er3+/Yb3+ co-doped CaIn2O4 phosphors was investigated under excitation of the CW 980 nm infrared LD, aiming at the development of stable and efficient REI doped oxide UC phosphors for displaying and lighting devices. 2. Experimental CaIn2-x-yErxYbyO4 (x = 0.005 ~0.03 and y = 0 ~0.2) phosphors and CaIn2O4 without doping were prepared by conventional high temperature solid-state reaction method. Stoichiometric amounts of raw materials CaCO3 (analytical reagent, AR), In2O3 (spectrum pure, SP), high pure Yb2O3 (99.99%) and Er2O3 (99.99%) were thoroughly ground in an agate mortar. The homogeneous mixture was transferred to a corundum crucible and preheated at 500 °C for 5 h in a muff furnace under ambient atmosphere. After being cooled to room temperature, the sample was reground and placed in an alumina crucible to heat at 1300 °C for 16 h with 5 °C/min heating rate to obtain the samples. The structure and the phase purity of the samples was identified by powder X-ray diffraction (XRD) using a Rigaku-Dmax 3C powder diffractometer with Cu-Kα (λ = 0.154056 nm) radiation in the range of 10°≤2θ≤60°. The infrared absorption spectra were recorded in the range of 4000 ~400 cm−1 with EQUINOX55 Fourier transform infrared (FTIR) spectrometer (Bruker Optics, Germany). The morphology of the phosphor particles was examined by field emission scanning electron microscope (FE-SEM, JSM-6390LV, JEOL). The measurements of the visible UC luminescence were performed on a Hitachi F-7000 spectrophotometer (Hitachi high technologies corporation, Japan) with commercial powercontrollable 980 nm diode laser (Xi’an Juguang Technology Company, China) as the pump source. All spectra were collected under identical experimental conditions. The fluorescence lifetimes were measured by using a 980 nm pulsed laser of an optical parametric oscillator (OPO) as excitation source (pulse width = 7 ns), and the signals were detected by a Tektronix digital oscilloscope (TDS 3052). All of the measurements were performed at room temperature.

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3. Results and Discussion

Fig. 1. (a) XRD patterns of CaIn2O4, CaIn2O4: Er3+, CaIn2O4: Yb3+, Er3+ and the standard data of CaIn2O4 (JCPDS 17-0643); (b) FT-IR spectra of CaIn2O4. Inset shows the SEM photograph of CaIn2O4: Yb3+, Er3+.

Figure 1(a) representatively displays the XRD patterns of blank CaIn2O4, CaIn1.99Er0.01O4, CaIn1.89Yb0.1Er0.01O4 and the standard data of CaIn2O4 (JCPDS 17-0643). It is observed that diffraction peaks of all the samples coincide well with the standard data and no any peaks from the starting materials or impurities are observed, which indicates that all samples are single orthorhombic phase CaIn2O4 with Pca21 or Pbcm space group [13] and the lanthanide doping does not influence the structure of CaIn2O4 in our experiments. Because the phonon energy of the host has important influence on the UCL efficiency, the FT-IR spectra of CaIn2O4 was measured and shown in Fig. 1(b). The strong absorption bands at 400 cm−1, 473 cm−1 and 637 cm−1 are assigned to the stretching vibrations of the In-O-In and In-O-O-In bridge bond, and the stretching vibration of the InO6 octahedron [24,25], respectively. It is found that the absorption band at 473cm−1 is the strongest, which suggests that the phonon energy of CaIn2O4 is about 473 cm−1 and is similar to result in [13]. Inset of Fig. 1(b) shows the FE-SEM micrograph of CaIn1.89Yb0.1Er0.01O4 phosphor annealed at 1300 °C for 16 h. It can be observed that the phosphors is composed of grains of ~2 μm size with smooth surface and most of the grains agglomerate together to form some large particles larger than 10 μm. The agglomeration of the phosphors is due to the high sinter temperature and the long during time.

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Fig. 2. (a) UC luminescence spectra, (b) the UC intensity ratio of red to green as a function of Er3+ concentrations and (c) the pump power dependence of red and green UC emissions in CaIn1-0.1-xErxYb0.1O4 under 980 nm excitation.

Figure 2(a) shows the UC luminescence spectra of the Er3+-Yb3+ co-doped CaIn2O4 with variable Er3+ contents (x) ranging from 0.005 to 0.03 and fixed 0.1Yb3+ under excitation at 980 nm. It is observed that the UC spectra include two primary emissions in green and red regions with considerable intensity. The strong green emission consists of two bands centered at 525 and 550 nm, which are assigned to 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+, respectively. The intense red emission at around 660 nm is assigned to the 4F9/2 → 4I15/2 transition [14–16]. With the increase of Er3+ concentration in the range of 0.005 ~0.03, the UC luminescence integrated intensity increases first and decreases after reaching the maximum at x = 0.025 due to the concentration quenching effect. The concentration quenching effect could be explained by the ET between the nearest Er3+ and Yb3+ (or Er3+) ions. With increasing the contents of Er3+, the distance between Er3+ and Yb3+ (or Er3+) ions decreases, which promotes non-radiative ET and leads to the decrease of UC emission intensity [26]. As shown in Fig. 2(b), the integrated intensity ratio of red (defined as the area from 630 to 710 nm) to green (defined as the area from 510 to 590 nm) shows the similar trend to that of UC intensity with the increase of Er3+ concentration. This indicates that the red emission becomes relatively stronger as Er3+ concentration increases, which is in agreement with previous reports in other systems [14,27,28]. This phenomenon is attributed to the higher probability of cross relaxation (CR) process between adjacent Er3+ ions: (4F7/2, 4 I11/2) → (4F9/2, 4F9/2), (4I15/2, 4S3/2) → (4I13/2, 4I9/2) and (4I11/2, 4I13/2) → (4I15/2, 4F9/2) at higher concentration. The CR processes can result in the population of the red emitting state or depopulation of the green emitting states, which leads to a relative enhancement of the red emission at higher Er3+ concentration. It is well known that the UC emission intensity (I) depends on the laser pumping power (P) following the relationship [29,30] I ∝ P n , where the n value is the number of pumping photons absorbed by REI at ground state to transit to the upper emitting state. In order to investigate the UC mechanism of Er3+/Yb3+ co-doped CaIn2O4, the pumping power dependences of red (660 nm) and green (550 nm) emission intensities were examined. Figure 2(c) shows the decadic logarithmic plotted diagram of the green and red UC emission intensities as a function of laser pumping power. The calculated slopes (n) are 2.37 for green (4S3/2 → 4I15/2) and 2.31 for red (4F9/2 → 4I15/2) emissions, respectively. The n values are much larger than 2, indicating more than two (possibly three) photons are involved to generate one up-converted photon. This implies that the red and green emissions may come from both twophoton and three-photon processes. The detailed UC mechanisms are discussed in the following part.

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Fig. 3. Energy level diagram of the Yb3+, Er3+ ions and the proposed UC mechanisms in Er3+/Yb3+ co-doped CaIn2O4.

To better comprehend the populating processes of the emitting states and the radiative transitions of green and red UC luminescence, the energy level diagram and the UC mechanisms of Er3+/Yb3+ co-doped CaIn2O4 system are schematically given in Fig. 3. Under 980 nm excitation, Yb3+ absorbs an infrared photon and transits from 2F7/2 ground state to excited state 2F5/2 and Er3+ ions are initially excited from ground state to the excited state 4I11/2 through the ground state absorption (GSA): Er3+ (4I15/2) + a photon (980 nm) → Er3+ (4I11/2) or the energy transfer (ET1): Er3+ (4I15/2) + Yb3+ (2F5/2) → Er3+ (4I11/2) + Yb3+ (2F7/2). For above mentioned two processes, the ET1 process plays a dominant role because the absorption cross section of 2F7/2 → 2F5/2 transition of Yb3+ (~34.07 × 10−19 cm2) is about eight times higher than that of the 4I15/2 → 4I11/2 transition of Er3+ (~4.43 × 10−19 cm2) [22]. Then the 4F7/2 level of Er3+ is populated via ET2: Yb3+ (2F5/2) + Er3+ (4I11/2) → Yb3+ (2F7/2) + Er3+ (4F7/2) and the excited state absorption (ESA1): Er3+ (4I11/2) + a photon (980 nm) → Er3+ (4F7/2). The green emitting states 2H11/2 and 4S3/2 levels are populated by rapid non-radiative relaxation (NR) from the upper 4F7/2 level due to the small energy gap between these levels. Finally, 2H11/2 → 4 I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions produce 525 and 550 nm green emissions, respectively. As the results in Fig. 2(c), the n value for the green UCL is 2.37, a three-photon process related to the red emitting level 4F9/2 is also involved in the green UCL, which is described as: Er3+ (4F9/2) + Yb3+ (2F5/2) → Er3+ (2H9/2) + Yb3+ (2F7/2)(ET3) followed by Er3+ (2H9/2)→Er3+ (2H11/2, 4S3/2)(NR) [16,17]. The integrated intensity of red emission is larger than green emission, indicating a larger population of the 4F9/2 level than 2H11/2/4S3/2 levels, which is favorable to enhance the third energy transfer process (ET3). The existence of weak blue emission at 410 nm (it is too weak to be shown in the spectra) arising from the 2H9/2 → 4I15/2 transition can also confirm the ET3 process. For red emission (4F9/2 → 4I15/2), the long-lived intermediary 4I13/2 level is populated by NR from 4I11/2 level. The population of emitting state 4 F9/2 involves following possible processes: Er3+ (4I13/2) + a photon (980 nm) → Er3+ (4F9/2) (ESA2), Yb3+ (2F5/2) + Er3+ (4I13/2) →Yb3+ (2F7/2) + Er3+ (4F9/2) (ET4) and NR from 4S3/2 to 4 F9/2. Since the n value of the red emission is 2.31, a third energy transfer: Yb3+ (2F5/2) + Er3+ (4S3/2) → Yb3+ (2F7/2) + Er3+ (4G7/2) (ET5) pumping to the high lying 4G7/2 level also contributes to the population of 4F9/2 level. After reaching the 4G7/2 state, the Er3+ decay to the 4 G11/2 state and then undergo a cooperative decay to the red emitting 4F9/2 state, by passing the green emitting ones [20].

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Since the ET from Yb3+ to Er3+ plays a primary role in UC luminescence of Er3+/Yb3+ codoped CaIn2O4, it is necessary to investigate the effect of Yb3+ concentration on the UC emissions. The UC luminescence spectra of CaIn2-0.01-yEr0.01YbyO4 (y = 0 ~0.20) as a function of Yb3+ contents (y) are displayed in Fig. 4(a). In case of the Er3+ singly doped CaIn2O4, its UC spectrum includes strong green emission and very weak red emission, while intense green and red emissions appeared simultaneously in the Yb3+/Er3+ co-doped CaIn2O4. This is due to the larger energy gap between 4F7/2 and 4F9/2 levels than that of 4F7/2 and 4S3/2/4H11/2 levels. With the increase of Yb3+ concentration and invariable Er3+ concentration, the UC intensity of sample first grows gradually and reaching the maximum at y = 0.15. In addition, the UC intensities in green and red regions become stronger with the introduction of sensitizer Yb3+ in comparison with that of Er3+ singly doped CaIn2O4. These results may be due to the more populated green and red emitting levels of Er3+, which thanks to the larger absorption cross section of Yb3+ at 980 nm and larger energy overlap between Yb3+ and Er3+. The energy gap between the Yb3+ ground state 2F7/2 and excited state 2F5/2 is about 10225 cm−1, and a similar energy gap of about 10350 cm−1 exists between 4I15/2 and 4I11/2 states, 4I11/2 and 4F7/2 states as well as 4I13/2 and 4F9/2 states of Er3+ levels [2], which will lead to an efficient ET from Yb3+ to Er3+. The reason for the UC emission intensity decreases as y beyond 0.15 is concentration quenching and the energy back transfer (EBT) from Er3+ to Yb3+: Er3+ (4S3/2) + Yb3+ (2F7/2) → Er3+ (4I13/2) + Yb3+ (2F5/2) [16].

Fig. 4. UC luminescence spectra of CaIn2-0.01-yEr0.01YbyO4 (a), the ratio of red to green emission intensities (b) and the color coordinates (c) as a function of Yb3+ concentrations.

In comparison with green emission, the UC intensity of red emission was enhanced greatly with the increase of Yb3+ concentration. As shown in Fig. 4(b), the intensity ratios of red to green emission increase monotonously from 0.073 to 2.376 with increasing the Yb3+ contents from 0 to 0.20. The emission color appears green for Er3+ solely doped CaIn2O4, but the emission color gradually changes to yellowish color with the addition of Yb3+ due to a combination of green and red emissions. The chromaticity coordinates of CaIn2-0.01yEr0.01YbyO4 (y = 0 ~0.20) are shown in Fig. 4(c), it is observed that the CIE coordinates gradually shift from (0.284, 0.693) to (0.412, 0.565) with the increase of Yb3+ concentration due to remarkable enhancement of the red emission intensity. It can be deduced that the predominant way to populate the red emitting level is the ET4 process among the NR relaxation from 4S3/2/2H11/2 to 4F9/2. Before concentration quenching, the average distance between Er3+ and Yb3+ become shorter and the ET from Yb3+ to Er3+ become more convenient with the increase of Yb3+ concentrations. Moreover, the ET4 process is more favorable than the ET2 process because the intrinsic lifetime of the 4I13/2 level is much longer than that of #189010 - $15.00 USD (C) 2013 OSA


4

I11/2 [15], which also explains the increase of red to green emission intensity ratio with increasing Yb3+ concentration.

Fig. 5. The temporal evolution of the green 4S3/2 → 4I15/2 (a) and red 4F9/2 → 4I15/2 (b) emissions in CaIn2-0.01-yEr0.01YbyO4 (y = 0, 0.10, 0.15, 0.20) after 980 nm pulsed laser excitation. (Insets show the enlarged curve of ascent stage)

In order to further investigate the influence of the ET from Yb3+ to Er3+ on the UC luminescence, temporal evolution of the green (4S3/2 → 4I15/2, 550 nm) and red (4F9/2 → 4I15/2, 660 nm) luminescence in samples CaIn2-0.01-yEr0.01YbyO4 (y = 0 ~0.20) were recorded after 980 nm pulsed laser excitation, which are shown in Fig. 5. The transient curves of Yb3+/Er3+ co-doped samples exhibit obvious delayed rise and decay parts in comparison with that of the Yb3+-free sample, which confirms the existence of ET and suggests that the de-excitation dynamics are also dominated by the ET processes. As shown in the insets of Fig. 5, the rise time become shorter with the increase of Yb3+ contents, which is due to the faster ET from Yb3+ to Er3+. Theoretically, the ET probability (WET) and the distance between the donor and the acceptor ions (r) follow the relationship [31]: 1 . (1) r6 Closer ionic distance could be obtained at higher REI concentration and more efficient ET from Yb3+ to Er3+ could be expected, resulting in a shorter rising time in the temporal evolution at higher Yb3+ concentration. It can be seen that the decay curves of Yb3+/Er3+ codoped CaIn2O4 samples are remarkably different from that of Er3+ single doped sample, which indicates that the introduction of Yb3+ ion modifies the UC dynamics of the Er3+ ion. The effective lifetimes of the green and red levels can be calculated by Eq. (2) [32]: WET ∝

∞

 τ= 

0 ∞ 0

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I (t )tdt I (t )dt

,

(2)


in which I(t) represents the luminescence intensity at a time t, and the corresponding lifetimes are listed in Fig. 5, respectively. It is found that the green and red emission lifetimes in Yb3+/Er3+ co-doped samples are longer than those of the Yb3+-free sample, which confirms the presence of ET from Yb3+ to Er3+ in present system. In addition, the green emitting level lifetimes decrease from 363 to 223 μs with the increase of Yb3+ from 0.10 to 0.20. Usually, the excited state is depopulated by the radiative and non-radiative transition processes, and the non-radiative processes include phonon-assisted non-radiative process and ET process. With the increase of Yb3+ concentration, more efficient ET from Yb3+ to Er3+ could be expected. Correspondingly, the EBT from Er3+ to Yb3+: Yb3+ (2F7/2) + Er3+ (4S3/2) → Yb3+ (2F5/2) + Er3+ (4I13/2) also become convenient in much higher Yb3+ concentration, which accelerates the depopulation of 4S3/2 level and results in the reduction of the green emission lifetime. This result agrees with the emission intensity variation trend shown in Fig. 4. However, with the increase of Yb3+ concentration from 0.10 to 0.20, the lifetime of red emitting level hardly change within a permitted error limit. As mentioned above, the red level is mostly populated by ET3: Yb3+ (2F5/2) + Er3+ (4I13/2) → Yb3+ (2F7/2) + Er3+ (4F9/2), in which the energy mismatch is about 1600 cm−1 between 2F5/2 (Yb3+) and 4F9/2 (Er3+). Therefore the EBT process from Er3+ (4F9/2) to Yb3+ (2F7/2) could not occur in red emission process, which results in the invariable lifetime of 4F9/2 at high Yb3+ concentration. 4. Conclusion Er3+ solely and Er3+/Yb3+ co-doped CaIn2O4 UC phosphors were prepared by solid state reaction technology. Under the excitation of a 980 nm laser diode, the UC luminescence spectra of Er3+ singly and Er3+/Yb3+ co-doped sample consist of intense green and red emissions from 2H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ ions, respectively. The integrated intensity ratios of red to green could be tuned by adjusting the concentration of Er3+ or Yb3+, which results in the tunable UC emission for CaIn2O4: Er3+, Yb3+. Comparing with Er3+ solely doped CaIn2O4, the intensities of green and red emissions are enhanced greatly in Er3+/Yb3+ co-doped CaIn2O4 owing to the efficient energy transfer form Yb3+ to Er3+, which was confirmed by the lifetime measurements. Consequently, CaIn2O4 is an excellent UC host and CaIn2O4: Er3+, Yb3+ serves as a new efficient UC luminescence material with potential applications in lighting, displaying, security ink, and so on. Acknowledgments This work was supported by the high-level talent project of Northwest University, National Natural Science Foundation of China (No. 11274251), Natural Science Foundation of Hubei Province (2010CDB01607), Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (excellent), Foundation of Key Laboratory of Photoelectric Technology in Shaanxi Province (12JS094), Foundation of Shaanxi Educational Committee (11JK0528), the Open Foundation of Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base) in Shaanxi Province (ZS11010, ZS12020).

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