1 Transparency and color-determining factors shown in the imaginary component ε 2 of the dielectric function of Cs 0.32WO 3− y.Ĭs-HTB is usually prepared by the solid-phase reaction in a reductive atmosphere. 13,20 Therefore, the blue-tint problem in Cs-HTB nanoparticles reduces to the problem of controlling the band gap and the conduction-band electrons.įig. On the blue side of the spectrum, the ε 2 is enlarged by band-edge interband transition on the red side, strong absorption occurs by LSPR and polaronic electronic transition. 21 The optical absorption of Cs-HTB nanoparticles is known large at both frequency ends of the visible region. This prohibition of certain transitions between metal-d and ligand-p orbitals further amplifies the visible transparency, enabling it even when the conduction and valence bands overlap in specific directions, as occurs in LaB 6. Although the band gap of Cs-HTB is below 3.3 eV, the calculated electronic structure of Cs-HTB 17,18 suggests that Fermi's selection rule prohibits certain electronic transitions such as W-5d → W-5d and O-2p → O-2p, which are involved in the hybridized W-5d/O-2p orbitals. The small ε 2 is caused mainly by a wide band gap. The imaginary part ε 2 of the dielectric function, which represents the absorption of a photon by an electron, is small at visible frequencies in Cs-HTB (see Fig. In addition, the slight bluish tint accompanying the transmitted light must be resolved in actual applications. 19,20 On the other hand, the visible transparency of Cs-HTB nanoparticles, which is highest among the HTBs, has not been considered in detail, and its origin remains unclear. 17,18,40 The V Os have been analyzed to play a major role in the LSPR and polaronic excitation. 6–16 In Cs-doped HTB (Cs-HTB), recent analyses indicate that W-5d orbitals in the lower conduction band are occupied by the free and trapped electrons originating from Cs + and oxygen vacancies (V O), respectively. The origin of the strong NIR absorption in reduced tungsten oxides and tungsten bronze nanoparticles has been attributed to localized surface plasmon resonance (LSPR) of free electrons and polaronic excitation of trapped electrons. I Introduction Recently, hexagonal tungsten bronze (HTB) nanoparticles have demonstrated a high-level compatibility of luminous transparency and near-infrared (NIR) absorption, 1–3 prompting their application in automotive and architectural windows, laser welding of resins, cancer therapies, 4,5 and related technologies. The comparatively narrow bandgap of Cs 4W 11O 35 was identified as the origin of the less-bluish tint of the produced Cs tungsten bronzes. The high-temperature reduction of Cs 4W 11O 35 is concluded to decrease the number of W deficiencies and produce oxygen vacancies, releasing both free and trapped electrons into the conduction band and thereby activating the near-infrared absorption. The high-temperature reduction caused an orthorhombic-to-hexagonal phase transformation and a nonmetal–metal transition, which was monitored by spectrophotometry, X-ray diffraction, and X-ray photoelectron spectroscopy measurements, assisted by a first-principles analysis using a DFT+U method. The high-temperature reduced Cs 4W 11O 35 crystals absorbed strongly in the near-infrared, providing an improved luminous transparency with a less-bluish tint than normal Cs 0.32WO 3− y synthesized in a reductive atmosphere. Revisiting Wöhler's method (1824), Cs-doped tungsten bronzes were synthesized by reducing Cs-polytungstate at high temperature, and were pulverized into nanoparticles for determining their optical properties.
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