Plasmon-band subpeak and oxidation of solar-control LaB6 nanoparticles

Near-infrared (NIR) absorption in solar-control LaB6 nanoparticles (NPs) is derived from the localized surface plasmon resonance (LSPR) at 1.3 eV, and accompanies an unclarified subpeak at 1.8 eV. As an origin of this subpeak, a disk-like particle shape of LaB6 NP has recently been proposed, besides the previously-proposed, milling-derived LaO phase. A series of heating experiments at 200-850 degrees C in air for LaB6 NPs pulverized with different media beads have been made, followed by x-ray diffraction and transmission electron microscopy observations, to clarify that LaB6 NPs oxidizes to amorphous phases B2O3 and La-B-O at 450-600 degrees C, and crystallize to LaB3O6 at 650-750 degrees C, without forming LaO or La2O3. Dielectric functions of LaO have been derived by first-principles calculations using sX-LDA, and Mie scattering calculations have been made for various sizes, shapes, and the ensembles, showing that LaO NPs, if existed, should exhibit an excessively-broadened absorption band with a blunt LSPR peak at 2.1 eV buried in several interband-transition absorptions at 1.2-4.0 eV. These analyses confirm that the observed 1.8 eV subpeak could not originate from LaO and support the nonspherical shape of NPs as the origin of the subpeak.