Tunable excitation dynamics, fluorescence decay and microstructural analysis of Eu 2 O 3 , Nb 2 O 5 nanoparticles doped birefringent liquid crystals

Abstract In this investigation, liquid crystalline nanocomposite (LCNCs) of azobenzene derivatives 3a (methyl 4-[( E )-(4-butoxy-2,5-difluorophenyl)diazenyl]-3-methoxybenzoate), and 3b (methyl 4-{( E )-[2,5-difluoro-4-(hexyloxy)phenyl]diazenyl}-3-methoxybenzoate) were doped with 0.5 wt.% europium oxide (Eu 2 O 3 ), niobium oxide (Nb 2 O 5 ) nanoparticles (NPs) and named as 3a @Eu 2 O 3 , 3b @Nb 2 O 5 LCNCs. The powdered x-ray diffraction confirms the purity and integration of Eu 2 O 3 , Nb 2 O 5 NPs. Field emission scanning electron microscope studies demonstrate a non-uniform worm/flake-like morphology with agglomeration of LCNCs. A slight shift in optical band gap was observed for 3a, 3b LCNCs, and broad absorption bands were observed in ultraviolet–visible studies. The existence of birefringent textures of 3a @Eu 2 O 3 and 3b @Nb 2 O 5 LCNCs is confirmed using a polarizing optical microscope. Room temperature photoluminescence (PL) studies demonstrate pronounced luminescence maxima with sharp emission peaks of violet, blue, green emissions under excitation at 380 nm, 460 nm, 360 nm, and 375 nm. Steady state PL (SSPL) of 3a @Eu 2 O 3 LCNC exhibits luminescence maxima associated with vibrant red, blue, and greenish-yellow colors, while SSPL of 3b @Nb 2 O 5 LCNC shows emission peaks of brilliant blue, yellow, green, orange, red colors at various λ excitations . Furthermore, the large Stokes shift and full width at half maximum of 3a @Eu 2 O 3 and 3b @Nb 2 O 5 LCNCs are attributed to the presence of additional/virtual energy levels during the photoexcitation process. The fluorescence decay studies of 3a @Eu 2 O 3 LCNC display average lifetime dwell between 19.8 ns and 0.276 ns, while 3b @Nb 2 O 5 exhibits 9.69 ns to 10.20 ns, at various excitation wavelengths. The quantum yield varies between −7.77 × 10 −2 % to 5.23 × 10 −2 % and 0.86% to 0.14% for 3a @Eu 2 O 3 and 3b @Nb 2 O 5 LCNCs, respectively, upon increase in λ excitations . Polarization studies of 3a @Eu 2 O 3 LCNC at λ excitation/emission angle (0°) showed intense defect-centered blue, red emission bands (445 nm, 613.8 nm) attributed to the excited state virtual orbitals that undergo non-radiative decay with higher energy content. Thus, the results obtained for synthesized 3a, 3b LCs doped with Eu 2 O 3 and Nb 2 O 5 NPs make these usable for advanced optoelectronics, photonic, fluorescence/photo-luminescence device applications.