Preparation and characterization of isostructural lanthanide Eu/Gd/Tb metal-organic framework thin films for luminescent applications

Highlights: • Series of new mixed LnMOFs crystals were synthesized by solvothermal synthesis. • Transparent LnMOF thin films were prepared by dropping on Pt/SiO₂/Si substrates. • XPS established Eu³⁺, Eu²⁺, Gd³⁺, Tb³⁺ and Tb⁴⁺ valence states in LnBTC films. • Luminescence spectra confirmed 4f–4f transitions of Ln³⁺ ions for thin films. • Eu_0.5Gd_0.25Tb_0.25MOF film presented emission color tuning (green-yellow to blue). The preparation of the lanthanide metal-organic frameworks LnMOFs (Ln = Eu, Gd, Tb, Eu_0.5Gd_0.25Tb_0.25, Eu_0.25Gd_0.5Tb_0.25 and Eu_0.25Gd_0.25Tb_0.5) by solvothermal synthesis is reported. Three synthesis routes (A, B, C) using different solution (N,N-dimethylforamide (DMF)/H₂O/EtOH (A) and with addition of modulator (sodium acetate) to DMF/H₂O (B) or EtOH/H₂O (C) are presented. Structural characterizations reveal that the coordination effect between Eu³⁺, Gd³⁺, Tb³⁺ ions and 1,3,5-benzenetricarboxylic acid (BTC) ligand in LnMOFs is confirmed. The advantage of green modulation (C) method is creation of rod-like nanocrystals (~50–150 nm). Transparent LnMOF thin films are prepared by dropping of colloidal solution of LnBTC nanocrystals on SiO₂/Si substrates with Pt interlayer. Three new ratios of lanthanides (Eu_0.5Gd_0.25Tb_0.25, Eu_0.25Gd_0.5Tb_0.25 and Eu_0.25Gd_0.25Tb_0.5) in mixed films with bidoped concentration of Eu³⁺/Tb³⁺ ions in Gd³⁺ matrix are select. Comparing luminescence properties of Eu_0.25Gd_0.5Tb_0.25BTC powder and film (C method), film provide the quantum efficiency value 8.6 times lower than powder. Eu_0.5Gd_0.25Tb_0.25BTC film present increase of europium mean lifetime by more than 5 times and improvement for quantum efficiency of about 300%, regarding to Eu_0.25Gd_0.5Tb_0.25BTC film and bicolor emission profile depending only of excitation wavelengths. The resulting transparent luminescent LnMOF films can be used in the fabrication of different optical devices.