Mechanism of photoluminescence enhancement and quenching in Nd2O3 nanoparticles–ferroelectric liquid crystal nanocomposites

Abstract

The mechanism of photoluminescence enhancement and quenching in np-Nd₂O₃:FLC nanocomposites has been explored in the current study by UV-Vis and photoluminescence (PL) spectroscopy techniques. UV-Vis absorption spectra of Nd₂O₃ NPs in the 200–800 nm range shows two absorptions at 248 nm and 292 nm whereas pure FLC gives a broad absorption in the 265 nm to 348 nm region. PL emission intensity of np-Nd₂O₃:FLC composites recorded at 248 nm excitation wavelength where Nd₂O₃ NPs show intense emission, increases with gradually increasing the concentration of NPs up to 8 μl. This enhancement in PL intensity without blue/red shifting the FLC's emission band was attributed to the up-conversion of doped Nd³⁺ ions and transfer of excitation energy to liquid crystal molecules. When excited with 248 nm, the Nd³⁺ ground state ⁴I_9/2 absorption excites electrons to a higher excited state ²G_5/2. The excited Nd³⁺ ions in ²G_5/2 level relax non-radiatively to the metastable ²H_11/2 state and then re-excites to an unstable ⁴G_11/2 level. The electrons populated in ⁴G_11/2 release energy either radiatively to different defect energy levels in the visible region or transfers this excitation energy to liquid crystal molecules which resulted into the enhancement in PL emission intensity. On the other hand, emission spectra at 303, 323, 333, 343 nm excitation wavelengths exhibit quenching of all emission bands in np-Nd₂O₃:FLC composites due to the stress induced structural disordering by the Nd₂O₃ NPs in the FLC matrix and creation of non-radiative channels in the system.