Activation of plasmons and polarons in solar control cesium tungsten bronze and reduced tungsten oxide nanoparticles
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Tsuyoshi Asahi Department of Applied Chemistry, Ehime University, Matsuyama Ehime 790-8577, Japan (Received 14 October 2011; accepted 28 December 2011)
Dispersions of reduced tungsten oxide and tungsten bronze nanoparticles are known to show a remarkable absorption of near-infrared (NIR) light applicable to solar control filters for automotive and architectural windows. Origin of the NIR absorption has been investigated by analyzing dielectric constants of CsxWO3 (x 5 0.15, 0.25, and 0.33) and WO2.72, and using Mie scattering theory. The optical analysis and Mie scattering theory analysis indicate that a localized surface plasmon resonance and polarons of localized electrons contribute alongside to the observed NIR absorption at different wavelengths.
I. INTRODUCTION
Scattered arrays of conductive nanoparticles have been shown to manifest a strong absorption of near-infrared (NIR) light and applied to solar control filters for automotive and architectural windows.1–5 Nanoparticles of transparent conductors such as tin-doped indium oxide (ITO) and antimony-doped tin oxide (ATO) are wellknown examples of those conductive nanoparticles, while nontransparent deeply colored powders such as lanthanum hexaboride (LaB6), reduced tungsten oxide (rTO), and hexagonal tungsten bronze (HTB) have recently been reported as a new material.4,5 Especially Cs-doped HTB nanoparticles are shown to have the highest visible light transmittance with the strongest NIR absorption among materials known so far.4 With a growing application of these NIR-active nanoparticles, the origin of the NIR absorption has been focused and investigated. Detailed studies of LaB6 by analyzing the optical properties with Mie scattering theory6 and by employing high-energy resolution electron energy-loss spectroscopy (EELS)7 have clarified the absorption mechanism to be due to localized surface plasmon resonance (LSPR) of free electrons. For ITO and ATO nanoparticles, LSPR has been considered as responsible as well.2,8 For tungsten oxides, WO6 octahedron comprises a basic structural unit, where a strong electron–phonon interaction is known to activate polarons to absorb visible and infrared lights.9–11 Niklasson et al.11 studied the optical absorption of evaporated films of aggregated WO2.7–2.8 nanoparticles and analyzed in terms of polaron absorptions, although a contribution of free electrons was not considered. For a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.25 J. Mater. Res., Vol. 27, No. 6, Mar 28, 2012
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Cs-doped HTB, there have not been any reports treating the origin of NIR absorption of its nanoparticle assembly. Thus, this work has been conducted to consider the NIR absorption mechanism of Cs-doped HTB and rTO nanoparticles by measuring the optical constants and analyzing with Mie scattering theory. II. EXPERIMENTAL
Let us express Cs-doped HTB as CsxWO3 since a Cs ion is incorporated in the regular vacant space in the hexagonal array of t
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