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

Authors: Lefu Mei; Jing Xie; Libing Liao; Ming Guan; Haikun Liu
Journal: Advances in Materials Science and Engineering
 (2015)

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## 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 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 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.


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## Body

## 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: 2F7/2 ground state and 2F5/2 excited state around 10,000 cm−1 in NIR region [7]. Additionally, the similar value of energy level of excited 2F5/2state of Yb3+ is close to the 4I11/2 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 (λ
=
0.15406 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. Figure1 shows the XRD patterns of the BaLa1.9−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 2θ side owing to the small size of Yb3+ ion and Er3+ ion substituting for La3+ in the compound.Figure 1
XRD patterns of the BaLa1.9−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° ≤ 2θ ≤ 70° and (b) 29.5° ≤ 2θ ≤ 32°.
(a)
(b)Figure2 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].Figure 2
SEM images of the BaLa2ZnO5:0.05Er3+/0.10Yb3+ plotting scale ×1000 (a) and plotting scale ×5000 (b).
(a)
(b)Upon the 980 nm laser excitation, strong visible emission was observed in codoped crystals due to the result of the upconversion process. Figure3(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 4F9/2 → 4I15/2 transition and the green band centered at 553 nm assigned to the mixed transition 2H11/2 + 4S3/2 → 4I15/2 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.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).
(a)
(b)The physical mechanism involved in the upconversion processes can be elucidated by analyzing the dependence of the integrated upconverted intensity (I
em) as a function of the pumping intensity (P
pump), which is suggested to obey the following empirical equation [19–21]:(1)
I
em
∝
P
pump
n
,where n is the number of pump photons required for the transition from ground state to the upper emitting state. A plot of log I
em versus log P
pump yields a straight line with slope n. 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: 4F9/2 → 4I15/2) and 2.10 for the green emission (553 nm: 4S3/2 → 4I15/2), 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  2F7/2 to the only excited state of  2F5/2 and then transfers the energy to Er3+ ion and promotes the transition of  4I15/2 → 4I11/2 of  Er3+ ion. Furthermore, Er3+ in 4I11/2 level is easy to reach a lower excited level of 4I13/2 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 4I13/2 to the 4F9/2 level. In this case, the emission band located in the red region which associated with the transition of 4F9/2 → 4I15/2. The third energy transfer step (ET3) are followed as: Er3+ ions at 4F9/2 level relax nonradiatively again and back to another lower excited level of 4I9/2, and then the Er3+ are excited from 4I9/2 to 4F7/2 by another ESA process. In this case, the emission band located in the green region which associated with the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of  Er3+ ions, respectively. Accordingly, green emission concerted at 527 and 549 nm was detected in the UC spectra.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).
(a)
(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 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 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.



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*Source: 380936-2015-03-22.xml*