Transparent Low Emissivity In₂O₃: Sn Nanoparticle Films Based on Plasmonic Metamaterials for Solar-Thermal Shielding Applications: A Theoretical Study

Plasmonic metamaterial architecture is applied to three-dimensionally stacked nanoparticle (NP) films composed of Sn-doped In2O3 (ITO) for the simultaneous shielding of solar and heat radiation in the near- and mid-infrared (IR) regions, respectively. This study theoretically investigates the optical properties of ITO NP films using the finite-difference time-domain method, and demonstrates that these films are promising low emissivity (Low-E) materials. By controlling the structural parameters, such as the NP size, shape, and stacking arrangement, of the ITO NP films, plasmonic optical dispersions are clearly obtained in the near-IR region. As a result, a high refractive index is noted in the mid-IR region. In particular, the ITO NP films that are densely packed with cubic NP shape exhibit a high refractive index exceeding 5.0 in the mid-IR region, and this value is higher than that of compound semiconductors, such as Si and Ge. This high refractive index is achieved by maximizing the effective permittivity while minimizing the effective permeability owing to the relatively lower electron density. Consequently, the optical responses of the ITO NP films are predominantly governed by the effective permittivity because of the low effective permeability. The high refractive index of the ITO NP films provides high mid-IR reflectance based on the optical interference mechanism, contributing to the high Low-E performance. Additionally, the plasmonic optical dispersions of the ITO NP films further provide strong light reflections in the near-IR region, which resemble metallic characteristics. This effectively shields the near-IR light in solar radiation. These findings suggest that the ITO NP films, which are low-E films capable of transmitting visible and microwave radiations, are promising new materials that simultaneously block solar and heat radiation in the near- and mid-IR regions, respectively.