Anisotropic dielectric properties and ensemble inhomogeneity of cesium-doped tungsten oxide nanoparticles studied by electron energy loss spectroscopy

Abstract

Cs-doped hexagonal tungsten bronze (CsxWO3–y) nanoparticles (NPs) constitute an infrared-absorbing material with high visible transparency. In this study, anisotropic dielectric responses of individual NPs were evaluated by employing high-energy-resolution electron energy loss spectroscopy (EELS) operated in the aloof-beam mode. EELS data were obtained by setting an electron nanoprobe just aside NP edges either parallel or perpendicular to the c-axis to realize an anisotropic interaction condition with NPs. In the EELS spectra, peaks were observed at around 1.3 or 0.9 eV, respectively, depending on beam conditions parallel (∥c) or perpendicular (⊥c) to the c-axis. Those peaks dominantly reflected the dielectric tensor components of either ɛzz or ɛxx and evidenced the anisotropic responses of individual NPs. The peak energy in the ∥c condition was larger than that in the ⊥c condition; this finding qualitatively agrees with optical reflection measurements using polarized light. The experimental values of peak energies and widths were, respectively, smaller and larger than the theoretically evaluated values. These differences could be attributed to a damping of dipole oscillation by electronic excitations owing to oxygen vacancies. In addition, the peak energies of ⊥c and ∥c interaction conditions varied in ranges of 0.7–1.0 eV and 1.1–1.6 eV, respectively. Those variation ranges are larger than the measurement error of about 0.1 eV. Such variation in peak energies supports the presence of ensemble inhomogeneity in dielectric responses of NPs. Those results indicate that the anisotropy and inhomogeneity of dielectric response are important factors for understanding the optical properties of nanoparticulate dispersions.