Thermally enhanced photoluminescence in a lanthanide metal-organic framework

Abstract

MOFs (metal-organic frameworks), whose investigation originates from the study in the feasibility of synthesis of the copper-based architecture, are typically a hybrid porous system consisting of metal ions and organic ligands. The harmonic fusion breaks the convention barrier between two material categories, enabling the system to equip the intrinsic functionalization of the metal ions and the flexibility of the organic ligand simultaneously. Meanwhile, different assembly methods endow the porous structure feasibility in designing the morphology of the cavities. They are conducive to maintaining stability under moderate thermal or chemical erosion. Furthermore, the sizeable specific area would provide a solid foundation for sufficient attachment sites, especially in wide catalysis applications. With the base of the flexible structure, the lanthanide metal ions are introduced to inlay into the framework for particular luminescence functionalization. The system could be an ideal candidate for the integrated characteristics in morphology tunability and the luminance property. Meanwhile, the coordinated ligands could alleviate the sensitization efficiency of the ions. Through the "antenna effect", they could behave as the forerunners to absorb the energy to realize the luminescence and transfer part of that to facilitate the photon conversion of the lanthanide ions from violet to infrared. However, resembling most luminescent materials, the thermal quenching effect dominates, which would cause a reduction in efficiency and damage to the lightning devices. A number of researchers tried to explore that in the dimension of the pure inorganic or organic system. Eu-BTC was investigated in this project report to seek the possibility of a novel system, MOF, for thermal enhancement and its luminescence mechanisms. 365nm and 254nm light resources are employed for excitation variance in the energy pathway. The structure and luminescence properties of the synthesized sample with the hydrothermal method under different temperatures are monitored. Gd-BTC and Tb-BTC are joined into the discussion to help the mechanism exploration. Eu-NDC with a negative correlation between temperature and emission intensity is added to the temperature-sensitive pattern design. The influence inspection of nitric acid and sodium acetate are studied to change the nucleation and size of the crystal bar, respectively. X-ray Diffraction, FTIR, OM, SEM, decay, and photoluminescence spectrometers are employed for assistance. The results show that about 2.7 times enhancement in emission intensity could be detected under 365nm between 303K to 413K. The performance in this temperature region could be ascribed to the more competing enhancement effect owing to negative thermal expansion and partial removal of the molecules than the quenching effect associated with organic ligand and stmctural destruction at elevated temperatures. The behavior ofTb-BTC could be similar to that of the Eu-based system. Moreover, Tb ions substituted framework would have a similar luminescence behavior. The emission intensity of Eu-NDC would monotonically decrease as temperature increases, which is different from that for Eu-BTC. The pattern used in the difference could be observed to have a luminance difference at low temperanire but harmonic at high temperature. This report aims to assist further exploration in the detailed study of properties and the feasibility in temperature-related applications, such as thermometers or anti­counterfeit label designing.