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J. Korean Ceram. Soc. > Volume 54(1); 2017 > Article
Park and Kim: Up- and Down-Conversion Luminescence of LuNbO4:Yb3+, Er3+ Phosphors


Up-conversion (UC) and down-conversion (DC) luminescence of LuNbO4:0.18Yb3+, xEr3+ (x = 0.01-0.07) powders were investigated. Post-annealed powders were composed of a single LuNbO4 phase with a monoclinic fergusonite structure, whereas as-calcined powders contained a small amount of the Li3NbO4 impurity phase. Under near infrared radiation, the UC spectra of the post-annealed powders exhibited the strong green and weak red emission peaks assigned to the transition of 2H11/2/4S3/2 and 4F9/2 to the ground state (4I15/2) of Er3+ ions, respectively; the green and red emission intensities were approximately 330 and 270% stronger, respectively, than those of the as-calcined powders. A two-photon UC process was involved in the emission as a result of an energy transfer from Yb3+ to Er3+. Under ultraviolet radiation, the DC spectra exhibited broad blue and sharp green emission bands. The DC mechanism was explained using self-activated [NbO4]3- niobates and an energy transfer from [NbO4]3− to Er3+.

1. Introduction

LuNbO4 is a niobate compound with an MNbO4 (M: La3+, Y3+, and Lu3+) formula. YNbO4 and LaNbO4 phosphors doped with rare-earth (RE) elements have been widely investigated for use in various optical applications.1-5) On the other hand, only a few studies on the luminescence of LuNbO4 doped with RE ions have been carried out. For instance, the photoluminescence (PL) spectra of LuNbO4:Eu3+ exhibit strong red emission peaks, which are assigned to f-f transitions of the Eu3+ ions.6-8) Furthermore, the up-conversion (UC) luminescence of LuNbO4 has been rarely studied.
For UC luminescence, sensitizer and activator ions are co-doped into the host materials. Yb3+ ions are typically used as sensitizers owing to their strong absorption of 980 nm, while Er3+, Tm3+, and Ho3+ ions are co-doped as activators, resulting in various UC spectra.9,10) In the case of the Yb3+-Er3+ system, under infrared (IR) radiation (980 nm), the energy absorbed by the sensitizers (Yb3+ ions) is transferred to the activators (Er3+ ions), leading to a multistep excitation of the Er3+ ions: ground-state absorption (GSA) and excited-state absorption (ESA).11) For UC phosphors co-doped with the Yb3+ and Er3+ ions, the UC emission spectra typically exhibit multiple-emission peaks in the green and red regions; the emission ratio of green to red closely depends on the host materials.12-16)
In this study, we prepared LuNbO4 powders co-doped with Yb3+ and Er3+ and then investigated their UC and down-conversion (DC) luminescence. Under near IR (NIR) radiation, the UC spectra exhibited strong green and weak red emission bands, whose intensities dramatically increased after a post-annealing process. Under ultraviolet (UV) radiation, the DC spectra consisted of blue and green emission bands. This dual-mode emission (DC and UC) suggests that LuN-bO4:Yb3+,Er3+ has high potential for use in various optical applications including sensing devices, security printing, electronic displays, etc.

2. Experimental Procedure

Lu0.82-xYb0.18ErxNbO4 (LYExNO) powders (x = 0.01-0.07) were prepared using a conventional solid-state reaction process with Lu2O3 (Molycorp, 99.99%), Nb2O5 (Kojundo Chemical Lab., 99.9%), Yb2O3 (Kojundo Chemical Lab., 99.99%), and Er2O3 (Kojundo Chemical Lab., 99.9%). Stoichiometric starting mixtures with flux of 7 wt% LiCl were calcined at 1300°C for 12 h under a nitrogen atmosphere. Thereafter, the as-calcined powders were ground in an agate mortar and post-annealed under the same conditions as used in the calcination process. An X-ray diffractometer (XRD, Rigaku, Miniflex II) using Cu radiation (λ = 1.5406 A) was used to determine the crystal structure. The UC and DC PL spectra were measured at room temperature using a PL (PSI, Darsa-5000) system with an external 200 mW NIR laser diode (λ = 980 nm) and a 500 W xenon lamp, respectively.

3. Results and Discussion

Almost all the XRD patterns of the as-calcined LYExNO powders (Fig. 1(a)) corresponded to those of ICSD #98-010-9182 (LuNbO4 with a monoclinic fergusonite structure); very weak peaks of an impurity phase of Li3NbO4 were also observed. However, after the post-annealing process, the XRD peaks of the Li3NbO4 impurity phase disappeared completely (Fig. 1(b)), indicating that the synthesized powders were composed entirely of single phase LuNbO4, and that the Yb3+ and Er3+ ions were incorporated completely into the Lu3+ sites. With regard to the ionic radius (r), it is reasonable to infer that the Yb3+ (r = 0.985 A) and Er3+ (r = 1.004 A) ions can be substituted for the Lu3+ ions (r = 0.977 A).
Under NIR radiation, the UC spectra of the post-annealed LYExNO powders were measured, with results shown in Fig. 2(a); these spectra consisted of strong green and weak red peaks. The emission intensities (Iem) at 556 and 673 nm were measured as a function of the input (excitation) power P; the measurement results for x = 0.05 are representatively plotted on a log-log scale, as shown in the inset of Fig. 2(a). In the linear region, Iem is proportional to Pn, where n is the number of photons required for the UC process.11) For x = 0.01, 0.03, 0.05, and 0.07, the slopes n of the 556 nm peak were 2.33, 2.31, 2.26, and 2.32, while those of the 673 nm peak were 2.56, 2.09, 2.32, and 2.64, respectively. These findings demonstrate that a two-photon process was involved in the UC emission. The two-photon UC process is a well-known mechanism,11-13) and we only briefly explain it as follows.
The Yb3+ ions in the ground state (2F7/2) absorb the energy of the 980 nm radiation and switch into the excited state (2F5/2). Then, the absorbed energy of the Yb3+ ions is transferred to the Er3+ ions; this is an energy transfer (ET) process. As a result, a two-photon UC process, involving the 4I15/24I11/2 (ground-state absorption, GSA) and 4I11/24F7/2 (excited-state absorption, ESA) transitions of the Er3+ ions, occurs. Subsequently, nonradiative multiphoton relaxation from 4F7/2 to 2H11/2, 4S3/2, and 4F9/2 levels of the Er3+ ions occurs, resulting in the green (526/556 nm) and red emission (673 nm) assigned to the transitions of the 2H11/2/4S3/2 and 4F9/2 levels to the ground state (4I15/2) of the Er3+ ions, respectively. Besides the main emission peaks at 526, 556, and 673 nm, weak emission peaks were also observed owing to large Stark splitting, because one Er3+ ion has a coordination of eight oxygen atoms in a distorted cube.12)
The variations of the UC emission intensities at 556 and 673 nm for the LYExNO powders are shown in Fig. 2(b). The strongest emission intensity was obtained at x = 0.05; there-after, the intensity decreased owing to a concentration quenching effect. In comparison with the emission intensities of the as-calcined powders, those of the post-annealed powders dramatically increased to approximately 330 and 270% for the 556 and 673 nm emissions, respectively. This behavior could be explained by the fact that the post-annealing process led to an elimination of the impurity phase, as shown in Fig. 1, and a diminution of internal defects of the as-calcined powders that could act as luminescence killers. SEM micrographs of the as-calcined and post-annealed LYE0.05NO powders, which exhibited the strongest green emissions, are shown in Fig. 3(a) and 3(b). There were no significant differences in particle shape or size between the two samples; the powders consisted of irregularly shaped-particles of different sizes. However, nano-sized particles on the surface of the large particles of the as-calcined powders almost disappeared after the post-annealing treatment. This phenomenon might contribute partly to the enhancement of the emission intensity of the post-annealed powders. In addition, the full width at half maximum (FWHM) values of the as-calcined (post-annealed) powders were obtained for 021̄ at 2θ = 28.864°; they were approximately 0.17 (0.15), 0.18 (0.15), 0.17 (0.15), and 0.17 (0.15) for x = 0.01, 0.03, 0.05, and 0.07, respectively; these values were found to be independent of the Er content. However, the FWHM values of the post-annealed powders were apparently lower than those of the as-calcined powders. This revealed that the crystallinity of the post-annealed powders was higher than that of the as-calcined powders, causing the enhancement of the emission intensity in addition to other effects. The intensities of the green emissions of the synthesized powders far surpassed those of the red emissions, resulting in the Commission Internationale de l’Eclairage (CIE) chromaticity coordinates in the green region, as shown in Fig. 4.
In addition to the UC emissions, the DC luminescence of the LYExNO powders was also investigated, with results shown in Figs. 5(a) - (c). Under UV (271 nm) excitation, the PL spectra exhibit strong broad blue emission bands peaking at approximately 412 nm, as shown in Fig. 5(a); weak emission peaks are observed in the green region as well. Corresponding PL excitation (PLE) spectra are shown in Fig. 5(b). The PLE spectra for the 412 nm emission (blue lines) consist of strong and weak bands at 271 and 309 nm, respectively; these bands are called charge transfer bands (CTBs); they occur through a CT mechanism between a ligand (oxygen 2p-like states) and a metal (Nb 4d-like states) of [NbO4]3−. As shown in Fig. 5(c), the PL spectrum of un-doped LuNbO4 exhibits a blue emission band peaking at 412 nm; a corresponding PLE band was observed at 271 nm. This finding confirmed that the blue emission bands and the CTBs of the LYExNO powders (Figs. 5(a) and 5(b), respectively) originated from [NbO4]3− niobates. As shown in Fig. 5(b), the PLE spectra (red lines) for the 556 nm emission exhibit strong and sharp peaks at 380 nm; these peaks originate from the Er3+ ions. In addition, weak broad bands were also observed at 271 nm; these bands corresponded to CTBs. These behaviors indicate that the green emission (556 nm) by 271 nm (CTB) excitation (Fig. 5(a)) originated from the Er3+ ions through an ET process from [NbO4]3− to Er3+. Accordingly, it was evident that the CTB excitation mainly caused the blue emission (412 nm) associated with [NbO4]3− and partly activated the Er3+ ions through the ET process, resulting in the green emission (556 nm). The DC PL spectra activated by 380 nm radiation are shown in the inset of Fig. 5(a): the sharp emission peaks in the green region are assigned to the characteristic energy transitions of the Er3+ ions; the strongest peak appears at 556 nm.
Variations of the blue (412 nm) and green emission intensities (556 nm) for the LYExNO powders activated by CTB (271 nm) are shown in Fig. 6. Increasing x can lead to an increase in the ET probability from [NbO4]3− to Er3+, resulting in a continuous decrease in the emission intensity at 412 nm, which is associated with [NbO4]3−. On the other hand, with increasing x values, the number of Er3+ ions, which can absorb the transferred energy from [NbO4]3−, increased. As a result, the emission intensity at 556 nm, which originated from the Er3+ ions, increased to x = 0.03; it then decreased at x = 0.05 and at higher values of x due to the concentration quenching effect. Accordingly, corresponding CIE chromaticity coordinates (Fig. 7) shifted from the blue region to the green region as the x values increased.

4. Conclusions

Under NIR radiation, the UC spectra of the post-annealed powders consisted of strong green and weak red peaks assigned to the transition of 2H11/2/4S3/2 and 4F9/2 to the ground state (4I15/2) of the Er3+ ions, respectively. The UC emission occurred through a two-photon ET process from Yb3+ to Er3+. The post-annealing process caused a dramatic increase in the green and red emission intensities to approximately 330 and 270%, respectively, compared to those of the as-calcined powders. These phenomena were attributed to the phase purity and the diminution in the internal defects of the as-calcined powders. Under UV excitation, the DC spectra consisted of broad blue and sharp green emission bands, which originated from [NbO4]3− niobates and the Er3+ ions, respectively. The DC spectra and the variation of the emission intensities indicated an ET process from [NbO4]3− to Er3+. The effective UC and DC emission of the LYExNO powders suggest that they have a high possibility for use in various optical applications.


This work was supported by a Kyonggi University Research Grant 2015.

Fig. 1
XRD patterns of (a) as-calcined and (b) post-annealed LYExNO powders.
Fig. 2
(a) UC spectra and (b) variations of the emission intensities of LYExNO powders under 980 nm radiation. Inset of (a) shows the emission intensity (Iem) of LYE0.05NO as a function of the input power (P).
Fig. 3
SEM micrographs of (a) as-calcined and (b) post-annealed LYE0.05NO powders.
Fig. 4
CIE chromaticity coordinates of LYExNO powders activated by 980 nm radiation.
Fig. 5
(a) PL (λex = 271 nm) and (b) PLE spectra of LYExNO powders. (c) PLE (red line) and PL spectra (black line) of un-doped LuNbO4 powders. Inset of (a) shows the PL spectra activated by 380 nm radiation.
Fig. 6
Variations of the emission intensities at 556 (green line) and 412 nm (blue line) of LYExNO powders activated by 271 nm (CTB) radiation.
Fig. 7
CIE chromaticity coordinates of LYExNO powders activated by 271 nm radiation.


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