Cs doped tungsten oxide nanoparticles such as Cs0.33WO3 nanoparticles are usually applied to produce highly transparent heat insulation products as a near-infrared absorber. It has been reported that the optical and dielectric properties of Cs0.33WO3 bulk crystal and nanoparticles can be investigated by high energy-resolution electron energy-loss spectroscopy (HR-EELS).
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According to this study, the EELS spectrum of a Cs0.33WO3 crystal showed a prominent peak at 1.2?eV and a subpeak at 1.7?eV. On the basis of the dielectric ding derived from the EELS spectrum, the peaks at 1.2?eV and 1.7?eV were assigned to the volume plasmon excitation of the carrier electrons and electronic excitation, respectively. The EELS spectra of the edge region of individual Cs0.33WO3 nanoparticles exhibited a surface dipole plasmon peak at 0.88?eV and a shoulder structure at 1.4?eV, which corresponded with the optical scattering spectrum. Therefore, it was confirmed that the optical scattering of the filter in the near-infrared region resulted from a surface dipole mode of the plasmon oscillation of the nanoparticles. The peak energies in the experimental spectra of the nanoparticle could not be reproduced by the derived dielectric ding of the Cs0.33WO3 crystal. On the basis of the EELS and annular dark-field scanning transmission electron microscopy observations, this inconsistent result was attributed to Cs atom deficiency in the surface region of the nanoparticles.
