Tunable Upconversion Luminescence and Energy Transfer Process in BaLa₂ZnO₅:Er³⁺/Yb³⁺ Phosphors

Abstract

BaLa₂ZnO₅:Er³⁺/Yb³⁺ has been synthesized via a high temperature solid-state method, and the tunable upconversion luminescence and energy transfer process between Yb³⁺ and Er³⁺ in this system have been demonstrated. Upon 980 nm laser excitation, the intense green and red emission around 527, 553, and 664 nm were observed for BaLa₂ZnO₅:Er³⁺/Yb³⁺, which can be assigned to the characteristic energy level transitions of ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 of Er³⁺, respectively. The critical Er³⁺ quenching concentration (QC) was determined to be about 5 mol%, and the power studies indicated that mixture of 2- and 3-photon process was responsible for the green and red upconversion luminescence.

1. Introduction

It is believed that the appearance of photon upconversion (UC) luminescence phenomenon has attracted numerous attentions focused on the UC phenomena and UC luminescent materials because of their potential applications in medical labels, multicolor displays [1–4]. The upconversion (UC) luminescence process is achieved through the sequential absorption of two or more excitation photons, which is accorded with the anti-Stokes emission phenomenon. Consequently, UC processes can be induced by low power, continuous wave lasers, obviating the need for high-cost, high-intensity pulsed lasers that are required for simultaneous multiphoton absorption experiments such as simultaneous two-photon absorption and second harmonic generation [5, 6]. Up to now, efficient photon UC process has been observed to occur primarily in the rare-earth elements, namely, those of the Ln series. Lanthanide ions are very suitable to be used in UC process as they have the rich energy level structure that allows for efficient spectral conversion. Among all the lanthanides, Er³⁺ ion has abundant energy level structures, and always acting as the luminescent center can emit intense green and red light. In contrast, Yb³⁺ ion has a strong and broad near-infrared absorption cross section around 980 nm with relatively simple electronic structure of two energy level manifolds: ²F_7/2 ground state and ²F_5/2 excited state around 10,000 cm⁻¹ in NIR region [7]. Additionally, the similar value of energy level of excited ²F_5/2 state of Yb³⁺ is close to the ⁴I_11/2 levels of Er³⁺ ions. Accordingly, cooperative UC process has also been reported for Tm³⁺/Yb³⁺, Ho³⁺/Yb³⁺, and Tm³⁺/Ho³⁺/Yb³⁺ couples in many host materials [8–11].

The ternary oxides XY₂ZO₅ (X = Ba, Y = rare-earth, and Z = Cu, Zn) are receiving much attention because of their very interesting structural, excellent physical, and chemical stability and special magnetic, optical, and superconducting properties. As a member of these compounds, BaY₂ZnO₅ and BaGd₂ZnO₅ have been proved to be efficient UC hosts [12–15]. However, the UC properties of BaLa₂ZnO₅ based phosphor have not been investigated. Based on the effective ionic radii and charge balance of cations with different coordination number (CN), the rare-earth Er³⁺/Yb³⁺ ions are expected to occupy the La³⁺ sites randomly in the BaLa₂ZnO₅ host. Therefore, BaLa₂ZnO₅ can be an excellent host doped with various ions, and they are promising candidates for practical applications. In this paper, Er³⁺/Yb³⁺ codoped BaLa₂ZnO₅ UC materials are synthesized via a solid-state reaction process, and the structure and UC luminescent characteristics of these phosphors have been discussed in detail.

2. Experimental

A series of polycrystalline phosphors BaLa₂ZnO₅:xEr³⁺/Yb³⁺ were synthesized by a solid-state reaction technology. The raw materials were Ba₂CO₃(AR), La₂O₃ (99.99%), ZnO (99.99%), Er₂O₃ (99.99%), and Yb₂O₃ (99.99%), which were used directly without any treatment. The selected starting materials were mixed and ground thoroughly. The homogeneous mixtures were calcined at 1250°C for 3 hours, with the heating rate of 5°C/min, and then the samples were cooled to room temperature naturally. After that, the samples were washed three times by the deionized water and dried for the following measurement.

The phase and crystal structure of the samples were recorded by X-ray diffraction (XRD, D8 Advance diffractometer, Bruker Corporation, Germany) with Cu-Ka radiation ( nm, 40 kV, 30 mA). The morphology of the as-prepared samples was characterized by a field emission scanning electron microscopy (FE-SEM, JSM-7001F). The UC luminescence spectra were recorded on a Hitachi F-4600 spectrophotometer (Hitachi High Technologies Corporation, Tokyo, Japan) equipped with an external power-controllable 980 nm semiconductor laser (Beijing Viasho Technology Company, China) as the excitation source. All the measurements were carried out at room temperature.

3. Results and Discussion

The crystallization and morphology of the as-prepared samples were checked by XRD and SEM measurements. Figure 1 shows the XRD patterns of the BaLa_1.9−xZnO₅:x%Er³⁺/0.10Yb³⁺ (x = 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 7%, and 10%) and the standard PDF diffraction lines of BaLa₂ZnO₅ as a reference. It can be seen that all of the diffraction peaks are matched well with the standard data of BaLa₂ZnO₅ (JCPDS number 52-1670) indicating the introduction of Er³⁺/Yb³⁺ ions into the BaLa₂ZnO₅ lattice is completely dissolved in the BaLa₂ZnO₅ host lattice by substitution for the La³⁺ owing to their similar ionic radii and properties. Moreover, the diffraction peaks of the as-prepared samples shift toward the larger 2 side owing to the small size of Yb³⁺ ion and Er³⁺ ion substituting for La³⁺ in the compound.

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Figure 1 

XRD patterns of the BaLa_1.9−xZnO₅:x%Er³⁺/0.10Yb³⁺ (x = 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 7%, and 10%) and the standard PDF diffraction lines of BaLa₂ZnO₅ as a reference: (a) 10° ≤ 2 ≤ 70° and (b) 29.5° ≤ 2 ≤ 32°.

Figure 2 shows the SEM micrographs of the typical BaLa₂ZnO₅:0.05Er³⁺/0.10Yb³⁺ powders prepared at 1250°C with different plotting scale. SEM result shows sheet-like phosphor grains with an average diameter of about 30 μm. Compared to the as-prepared samples via high temperature solid-state technology, we can find that the current samples have smooth surface and better crystallinity in the form of two-dimensional flaky states, which indicate that they should own better luminescence properties because of the decreased particle surface defects [16].

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Figure 2 

SEM images of the BaLa₂ZnO₅:0.05Er³⁺/0.10Yb³⁺ plotting scale ×1000 (a) and plotting scale ×5000 (b).

Upon the 980 nm laser excitation, strong visible emission was observed in codoped crystals due to the result of the upconversion process. Figure 3(a) shows the comparison of UC luminescence spectra of the as-prepared BaLa₂ZnO₅:xEr/Yb10% (x = 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 7%, and 10%) samples. The emission consists of two strong bands: the red one peaked at 664 nm associated with ⁴F_9/2 → ⁴I_15/2 transition and the green band centered at 553 nm assigned to the mixed transition ²H_11/2 + ⁴S_3/2 → ⁴I_15/2 of the acceptor Er³⁺ ion [17]. To date, a wide variety of Er³⁺ doped BaLa₂ZnO₅ have been generated that are capable of emitting a wide range of colors within the visible spectral region. The above result testified the UC process in the Er³⁺/Yb³⁺ doped BaLa₂ZnO₅. Additionally, as shown in Figure 3(b), it can be seen that either the green (553 nm) or the red (664 nm) emission band intensities of BaLa₂ZnO₅:xEr³⁺/10%Yb³⁺ increase first and then decrease with the increasing concentration of Er³⁺ ion, which is attributed to the concentration quenching effect [18]. With increasing the Er³⁺ contents, the distance between Er³⁺ and Yb³⁺ (or Er³⁺) ions decreased to promote nonradiative ET approach and decreased the luminescent intensity of Er³⁺ ions. After a critical concentration, 5.0 mol% of Er³⁺, both bands are quenched but the intensity of the green one is quenched stronger than that of the red one.

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Figure 3 

Comparison of UC luminescence spectra of the BaLa₂ZnO₅:xEr³⁺/0.10Yb³⁺ (x = 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 7%, and 10%) under 980 nm laser excitation (a), and the variation of green and red emission intensities corresponds to different Er³⁺ doping concentration (b).

The physical mechanism involved in the upconversion processes can be elucidated by analyzing the dependence of the integrated upconverted intensity () as a function of the pumping intensity (), which is suggested to obey the following empirical equation [19–21]: where is the number of pump photons required for the transition from ground state to the upper emitting state. A plot of log  versus log  yields a straight line with slope . Figure 4(a) shows the UC emission spectra of BaLa₂ZnO₅:5%Er³⁺/10%Yb³⁺ with different pumping power, and dependence of green and red UC luminescence intensities upon pumping power is shown in Figure 4(b). With the increasing pumping power, UC emission intensities of BaLa₂ZnO₅:5%Er³⁺/10%Yb³⁺ increased. The calculated slopes were 2.14 for the red emission (664 nm: ⁴F_9/2 → ⁴I_15/2) and 2.10 for the green emission (553 nm: ⁴S_3/2 → ⁴I_15/2), indicating that both red and green emission are the mixture of 2- and 3-photon process which were responsible for the green and red upconversion luminescence. The corresponding energy levels scheme for the infrared excitation and upconversion emission is demonstrated in the schematic energy level diagram of Er³⁺ and Yb³⁺ ions, as shown in Figure 5. According to the above-mentioned three-photon process in BaLa₂ZnO₅:Er³⁺/Yb³⁺, the Yb³⁺ ions act as sensitizer and the Er³⁺ ions as activators. Under 980 nm laser excitation, a Yb³⁺ ion can be excited by one of the near-infrared photons and transited from the ground state of  ²F_7/2 to the only excited state of  ²F_5/2 and then transfers the energy to Er³⁺ ion and promotes the transition of  ⁴I_15/2 → ⁴I_11/2 of  Er³⁺ ion. Furthermore, Er³⁺ in ⁴I_11/2 level is easy to reach a lower excited level of ⁴I_13/2 by a nonradiative relaxation as the similar value of energy level. Then, the second step of ET₂ from Yb³⁺ can promote an excited state absorption (ESA) of Er³⁺ from ⁴I_13/2 to the ⁴F_9/2 level. In this case, the emission band located in the red region which associated with the transition of ⁴F_9/2 → ⁴I_15/2. The third energy transfer step (ET₃) are followed as: Er³⁺ ions at ⁴F_9/2 level relax nonradiatively again and back to another lower excited level of ⁴I_9/2, and then the Er³⁺ are excited from ⁴I_9/2 to ⁴F_7/2 by another ESA process. In this case, the emission band located in the green region which associated with the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of  Er³⁺ ions, respectively. Accordingly, green emission concerted at 527 and 549 nm was detected in the UC spectra.

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Figure 4 

UC emission spectra of BaLa₂ZnO₅:0.05Er³⁺/0.10Yb³⁺ with different pumping power (a) and dependence of green and red UC emission intensities upon pumping power (b).

Figure 5 

Energy level diagram of Er³⁺ and Yb³⁺ ions and the proposed UC luminescence mechanisms in BaLa₂ZnO₅:Er³⁺/Yb³⁺.

4. Conclusions

UC phosphors of Er³⁺/Yb³⁺ codoped BaLa₂ZnO₅ were synthesized by a traditional solid-state reaction method. Under 980 nm near-infrared laser excitation, both green (527 nm and 553 nm) and red (664 nm) emission bands have been found in the UC spectra, and these emission peaks are assigned to the characteristic level transition of ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 of Er³⁺, respectively. The influence of Er³⁺ doped concentration on UC luminescence intensities has been studied, which depicts that the optimum Er³⁺ doped concentration is 5%. The dependence of the UC luminescence on pumping power indicates that the energy transfer from Yb³⁺ to Er³⁺ in the BaLa₂ZnO₅ host is a three-photon process. The mechanisms for the green and red UC luminescence were discussed in detail.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This present work was supported by the National Natural Science Foundations of China (Grant nos. 51202226, 41172053, and 51172216), the Fundamental Research Funds for the Central Universities (Grant nos. 2652013043 and 2652013128), Science and Technology Innovation Fund of the China University of Geosciences (Beijing), and the Fundamental Research Funds for the Central Universities (53200959276).