Blue shift in ultraviolet absorption spectra of monodisperse CeO2-x nanoparticles

Blue shifts corresponding to the absorption edge in the UV-A (310-400 nm) r ange are obtained from ultraviolet (UV) absorption spectra of monodisperse CeO2-x (0 < x < 0.5) nanoparticles, which are produced as toluene suspensio ns by a successive microemulsification method. The blue shift of the nanopa rticles is inversely proportional to the 2.2 power of the particle size for a direct transition, but no relation is found for an indirect transition. In contrast, the shift for thin films of 100-200 nm thickness exhibits a re lation nearly inverse proportional to the square root of the thickness for the direct transition. These relations are different from that for the quan tum size effect, though the difference is small for the nanoparticles. A po ssible explanation for the blue shift is a valence change of the Ce ions. ( C) 2000 American Institute of Physics. [S0021-8979(00)00803-3].