No abstract

In the present work, BaTiO3 nanoparticles of four different size ranges were prepared by sol-gel method. The optical band gap of these particles at some size ranges has come down to 2.53 eV from 3.2 eV, resulting in substantial increase in optical absorption by these ferroelectric nanoparticles making them potential candidates for light energy harvesting. XRD results show the presence of higher compressive strain in 23 nm and 54 nm size particles, they exhibit a higher band gap narrowing, whereas tensile strain is observed in 31 nm and 34 nm particles, and they do not show the marginal band gap narrowing. The 23 nm and 54 nm particles also show a coupling of free carriers to phonons by increasing the intensity of LO phonon mode at 715 cm−1. The higher surface charge density is expected in case of enhanced surface optical Raman modes (638 cm−1) contained in 31 and 34 nm size particles. In addition to this, the red shift in an LO mode Raman spectral line at 305 cm−1 with decrease in particle size depicts the presence of phonon confinement in it. The enhanced optical absorption in 23 nm and 54 nm size particles with a narrowed band gap of 3 eV and 2.53 eV is due to exchange correlation interactions between the carriers present in these particles. In 31 nm and 34 nm range particles, the absorption got bleached exhibiting increased band gaps of 3.08 eV and 3.2 eV, respectively. It is due to filling up of conduction band resulting from weakening of exchange correlation interactions between the charge carriers. Hence, it is concluded that the band gap narrowing in the nanoparticles of average size 23 nm/54 nm is a consequence of multiple effects like strain, electron-phonon interaction, and exchange correlation interactions between the carriers which is subdued in some other size ranges like 31 nm/34 nm.

K 0.5 Na 0.5 )NbO 3 (KNN) thin films have been deposited onto Pt/Ti/SiO 2 /Si and quartz substrates by RF magnetron sputtering. The films were deposited at 400°C with the variation in oxygen mixing percentage (OMP) ratio from 0% to 100% and annealed at 700°C in oxygen atmosphere. The crystallinity of the films is found to be increased with increased OMP. Dielectric properties of the films were examined over the frequency range from 1 kHz to 1 MHz and the temperature range of 30°C to 400°C. The Curie temperature of the films was found to be in the range 369°C-373°C. For the first time, the split postdielectric resonator (SPDR) method was used to measure the microwave (10-20 GHz) dielectric properties of KNN thin films. The optical properties of as-deposited and annealed KNN thin films were investigated by means of transmittance spectra. The optical bandgap is calculated by using the Tauc relation, and found to be in the range 4.34-4.40 eV and 4.29-4.37 eV for the as-deposited and annealed films, respectively. The refractive index (n 700nm ) of the films found to be in the range 1.98-2.01 and 1.99-2.07 for as-deposited and annealed films, respectively. The refractive index dispersion is analyzed by using Wemple-DiDomenico (W-D) single-oscillator model. The effect of annealing and OMP on the refractive index, packing density and W-D parameters has been investigated. The average single oscillator energy (E o ) and dispersion energies (E d ) of the annealed KNN thin films are in the range of 6.17-7.16 eV and 18.77-22.19 eV, respectively. AC-conductivity of the annealed films was analyzed by using double power law. Ag/KNN/Pt thin films followed the ohmic conduction (J / E a , where a~1) and the low leakage current density obtained for the deposited at 100% O 2 is 3.14 3 10 -5 A/cm 2 at 50 kV/cm. J. Jones-contributing editor Manuscript No. 35693.

An excellent electromagnetic interference shielding effectiveness of ∼19.5 dB was measured for a 1 mm thick flexible few-layered graphene (FLG)/PVA composite sheet owing to the formation of network-like features by FLG in the PVA matrix.