UPCONVERSION LUMINESCENCE AND OPTICAL TEMPERATURE SENSING OF ER3+/YB3+/W6+ DOPED BABI2NB2O9 FERROELECTRIC CERAMIC

Abstract

The host material doped with various trivalent rare earth ions (RE3+) shows upconversion luminescence in which lower energy NIR radiation is converted into high energy visible radiation. This phenomenon can be stimulated using an inexpensive laser source. The upconversion luminescent materials have wide applications in the fields of solid-state lasers, white light-emitting diodes, biological imaging systems, infrared detectors, and diverse medical diagnostic techniques. Ferroelectric materials possess advantageous properties that make them well-suited as host materials for luminescence. This thesis investigates the simultaneous effects of doping the BaBi2Nb2O9 (BBN) system with Er3+, Er3+/Yb3+, and Er3+/Yb3+/W6+ on its structural, upconversion luminescence, temperature sensing, dielectric, ferroelectric and energy storage properties. In the Er3+ doped BaBi2Nb2O9 system, the symmetry (orthorhombic geometry) and phase group (Fmmm) of prepared samples are confirmed from XRD analysis. The Raman spectra are recorded under the excitation of 785 nm, and four intense modes at 164, 225, 560, and 860 cm-1 are identified for undoped BaBi2Nb2O9. The modes suppressed in undoped BaBi2Nb2O9 ceramics can be easily observed in Er3+ doped compositions. However, in Er3+ doped BaBi2- xErxNb2O9 composition, some less intense modes near 370, 389, 419, 437, 691, 730, and 771 cm-1 have been observed that might be associated with the degree of the structural disorder. The light upconversion luminescence (UCL) spectra are traced under 980 nm excitation. For an Er3+ content x = 0.04, the highest UCL emission intensity is obtained; beyond this content, the quenching concentration occurs. The sensitivity of the optimized prepared ceramic (BaBi2-xNb2ErxO9: x = 0.04) is measured using the FIR technique. The absolute sensitivity (Sa) comes out to be 0.99% K-1 at 483 K, and the relative sensitivity (Sr) is approximately 0.40% K-1 at 300 K. Furthermore, the ferroelectric properties of Er3+ doped BBN ferroelectric ceramic degrades as compared to undoped BBN ceramic. Ankita Banwal vi Further, in Er3+/Yb3+ co-doped BaBi2-x-yNb2ErxYbyO9 ferroelectric ceramic, the Er3+ content is fixed at an optimized value (x = 0.04), and Yb3+ is varied from y = 0.00 to 0.12 at A-site of the perovskite structure. The SEM images of prepared ceramics resemble plate-like formations and comprise non-uniform grains with irregular orientation. An increase in grain size with Yb3+ content up to y = 0.10 was found to promote upconversion luminescence. Two bright green bands at 535 nm and 557 nm and a detectable red spectrum near 672 nm were observed in UCL spectra, corresponding to an excitation wavelength of 980 nm. The effective energy transfer process from Yb3+ to Er3+ ions is supported by decay time measurements, which increase with increasing Er3+/Yb3+ content because of the non radiative transition at higher doping concentrations. The pump power dependence upon the UCL intensity for an optimum Yb3+ content y = 0.10 reveals that green and red UC emissions involve two photons. Er3+/Yb3+ co-doped BaBi2-0.04-yNb2Er0.04YbyO9 ceramic system with y = 0.06 and 0.10 exhibit a Sa of 0.69% K-1 and 0.58% K-1 at maximum absolute temperature (Ta) = 523 K and 463 K, respectively, and Sr of 1.10% and 1.01% at maximum relative temperature (Tr) = 303 K. Moreover, the structural and electrical properties are investigated after exploring the optical properties of Er3+/Yb3+ co-doped BaBi2-x-yNb2ErxYbyO9 ferroelectric ceramic. FTIR spectra showed characteristic peaks of the Aurivillius phase at 619 cm-1 and 822 cm-1 . Four Raman modes are visible in undoped BBN, whereas 12 modes have been observed in doped BBN compositions. Temperature-dependent dielectric tests reveal significant dispersion below and above the maximum temperature (Tm), and the dielectric constant (ε') decreases with increasing frequency. The dielectric loss (ɛ") curves are diffused, and variations in the maxima with frequency have been detected, demonstrating the relaxor behavior of all produced BBN compositions. The thinner PE loops were achieved under applied electric fields between 75 kV/cm and 100 kV/cm. The remnant polarization (Pr) and maximum polarization (Pm) were used to calculate the energy storage parameters (W, Wrec, η) of each ceramic composition. The energy storage parameters improve as the applied electric field increases. It can be concluded that the efficiency (η) of undoped, Er3+ doped, and Er3+/Yb3+ co-doped BaBi2-x-yNb2ErxYbyO9 ceramics increased with doping concentrations. In contrast, the energy storage density (W) and recoverable energy storage density (Wrec) of Er3+ doped and Er3+/Yb3+ co-doped BBN ceramic are comparable with undoped BBN. Lastly, the effect of transition metal ion (W6+) in the tri-doped BaBi2-0.04-0.10Nb2- zEr0.04Yb0.10WzO9 ferroelectric ceramic on the structural, upconversion luminescence, temperature sensing, ferroelectric and energy storage density characteristics are discussed. The Er3+ and Yb3+ concentrations are fixed at the optimized values, and the transition metal ion W6+ varies from z = 0.01 to z = 0.04. The UCL emission intensity increases with the incorporation of W6+ ions and increases up to z = 0.02. Time decay analysis is conducted for the green and red emission bands, corresponding to transitions from 2 H11/2 to 4 I15/2 and 4 S3/2 to 4 I15/2 energy levels, respectively. The Sa of the Er3+/Yb3+/W6+ tri-doped BBN system yields a result of 0.91% K-1 at a temperature of 573 K, which is higher than the Er3+/Yb3+ co-doped BBN system and comparable with the Er3+ doped BBN system. The Sr achieved for the Er3+/Yb3+/W6+ tri-doped BBN system is 1.32% at 303 K, the highest among the three BBN systems. The PE loops have improved and reached their maximum Pm value at 9.719 µC/cm2 for z = 0.02. The W, Wrec, and η for undoped BBN ceramics are 0.662 J/cm3 , 0.518 J/cm3 , and 78.25% and for tri-doped BBN ceramics are 0.874 J/cm3 , 0.792 J/cm3 , and 90.53%. In conclusion, the synthesized BaBi2Nb2O9 (BBN) ceramic demonstrates promising properties for multifunctional applications, including optical sensors, energy storage devices, and tunable light sources.