White emission phosphors based on Dy3+-doped into anhydrous rare-earth benzenetricarboxylate complexes
Highlights
► Aqueous synthesis. ► White-light emitting phosphors. ► Photoluminescence of dysprosium. ► Rare-earth benzenetricarboxylate complexes.
Introduction
The carboxylate complexes have been gaining ground in the area of coordination compounds in the past decades due to the variety of their molecular structures which enables different chemical applications in solid state as gas storage molecular devices [1], nanostructured magnetic materials [2]. These properties allow the construction of a wide range of coordination compounds and their widespread use in many areas of knowledge, such as synthesis precursor materials [3], biomarkers [4], and optical markers [5].
Rare earth ions (RE) have been widely used as emitting centres in luminescent applications, such as organic light emitting devices (OLEDs) [6], emergency signalling [7], luminescent markers [8] and display panels [9]. Some of the RE3+ ions exhibit visible emission, for example, the Eu3+, Tb3+ and Tm3+ ions emit in the red, green and blue region, respectively [10]. The RE3+ luminescence properties depend mainly on the electronic energy level structures that are little affected by their chemical environments owing to the effective shielding of the 4f electrons by the filled 5s and 5p external sub-shells [11]. Furthermore, to overcome the small molar absorptivity coefficients (<1 L mol−1 cm−1) of the RE3+ ions, the coordination ligands such as carboxylate and β-diketonate groups are widely used, mainly because of their high absorptivity coefficient and luminescence sensitising abilities. This phenomenon is often denoted as the antenna effect, largely used in the design and synthesis of luminescent complexes [12].
From the point of view of RE3+ optical characteristics, Dy3+ (4f9) ion has relatively large energy gap between their 4F9/2 emitting and 6HJ ground levels. The first triplet state (T1) position of TMA ligand is around 24,000 cm−1, which is above the emitting 4F9/2 level (∼21,000 cm−1) of the Dy3+ ion [13], permitting the ligand-to-metal energy transfer in the TMA complexes. On the other hand, Y3+ (4d0) and Lu3+ (4f14) ions exhibit no luminescence originated from the 4f intraconfigurational transitions. In addition, the Y(TMA) and Lu(TMA) complexes present no water molecules in the coordination sphere as a result of the smaller ionic radii of Y3+ and Lu3+ cations [14]. Therefore, trivalent dysprosium ion and RE(TMA) complexes were selected in the present study as the optical activator and host matrices, respectively.
It is reported the synthesis, characterization and optical properties of the non-doped RE(TMA) and doped RE(TMA):Dy3+(x%) materials (RE3+: Y and Lu; x: 0.1%, 0.5%, 1.0%, 5.0% and 10% mol) in this work. Photoluminescence data were obtained from the excitation and emission spectra and lifetimes measurements. The non-radiative energy transfer from the excited triplet state (T) of TMA ligand to the emitting 4F9/2 level of the Dy3+ ion as function of concentration (x% mol) were also investigated. The RE(TMA):Dy3+(x%) phosphors yield white-light emission.
Section snippets
Synthesis of the doped RE(TMA) system
The RE3+ chlorides were obtained from their respective oxides RE2O3 (Cstarm, 99.99%) by digestion of its aqueous suspension with the addition of concentrated hydrochloric acid until pH reaches 6. RECl3·(H2O)6 crystals were obtained and dried by water bath and afterwards stored under reduced pressure.
The trimesic acid (benzene-1,3,5-tricarboxylic acid – Fluka, 97%), H3(TMA) (Fig. 1), was used without further purification. Considering the RE3+ ions as hard acids, the binding of these ions with
Results and discussion
The chemical composition of the RE3+–TMA system determined by elemental analysis and calculated values are presented in Table S01. These results confirm the general 1:1 ratio between the TMA ligand and RE3+ ions and the absence of the coordinated water molecules in the RE(TMA):Dy3+(x%) system.
The IR spectras were registered at room temperature and presented in Fig. S01. The symmetric νs(CO) and asymmetric νas(CO) stretching modes of the carboxylate group of the TMA ligand are observed in the
Photoluminescent study
It is noteworthy to mention that the Dy3+ ion has an odd-electron configuration (4f9). Therefore, it is labelled as a Kramer ion due to its electronic states that are at least doubly degenerate for any crystal-field perturbation. The maximum number of the Stark components for Kramer ions with 2S+1LJ states is J + 1/2 for any symmetry lower than cubic [22].
The excitation spectra of the Y(TMA):Dy3+(x%) system with emission monitored at 577 nm corresponding to the 4F9/2→6H13/2 transition of the Dy3+
Conclusion
Dy3+-doped anhydrous Y(TMA) and Lu(TMA) inorganic matrices were synthesized using an aqueous reaction. Under the UV excitation at the ligand absorption band, the resulting insoluble luminescent materials exhibit strong near-white emission arising from the 4F9/2→6H15/2 and 4F9/2→6H13/2 intraconfigurational transitions of the Dy3+ ions. Combined with their excellent thermal stability, these complexes can be considered as promising candidates for white-light emitting phosphors.
Acknowledgements
The authors thank Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Instituto Nacional de Ciência e Tecnologia de Nanotecnologia para Marcadores Integrados (Inct-INAMI) for financial support.
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