Band gap narrowing in BaTiO < inf > 3 < /inf > nanoparticles facilitated by multiple mechanisms

Abstract

In the present work, BaTiO3nanoparticles 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 2.53eV from 3.2eV, 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 23nm and 54nm size particles, they exhibit a higher band gap narrowing, whereas tensile strain is observed in 31nm and 34nm particles, and they do not show the marginal band gap narrowing. The 23nm and 54nm particles also show a coupling of free carriers to phonons by increasing the intensity of LO phonon mode at 715cm-1. The higher surface charge density is expected in case of enhanced surface optical Raman modes (638cm-1) contained in 31 and 34nm size particles. In addition to this, the red shift in an LO mode Raman spectral line at 305cm-1with decrease in particle size depicts the presence of phonon confinement in it. The enhanced optical absorption in 23nm and 54nm size particles with a narrowed band gap of 3eV and 2.53eV is due to exchange correlation interactions between the carriers present in these particles. In 31nm and 34nm range particles, the absorption got bleached exhibiting increased band gaps of 3.08eV and 3.2eV, 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 23nm/54nm 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 31nm/34nm. © 2014 AIP Publishing LLC.