Optical absorptions of nanoscaled CoTiO3 and NiTiO3

Photoacoustic spectroscopy has been used to study various sizes of nanoscaled CoTiO3 and NiTiO3 powders. Three absorption bands (or peaks) are observed from ultraviolet to near-infrared (350–1200 nm) in the photoacoustic spectra. The analyses indicate that the absorptions located around 525 nm for CoTiO3 and 445 nm for NiTiO3 are induced by the transition of free excitons, but the absorptions located around 590 nm for CoTiO3 and 505 nm for NiTiO3 are related to that of bound excitons. Besides, the absorptions located around 840 nm both for CoTiO3 and NiTiO3 are corresponding to the electronic transition of additional energy levels caused by defects and impurities. The behavior of the characterized absorptions strongly depends on the grain sizes.