Infrared Plasmonic Metamaterials Based on Transparent Nanoparticle Films of In₂O₃:Sn for Solar-Thermal Shielding Applications

Abstract

Three-dimensional nanoparticle (NP) assemblies show interesting optical responses that differ from naturally occurring materials, such as metals, oxides, and semiconductors. In this study, we investigate the optical response of thin films comprising Sn:In2O3 NPs (ITO NP films) based on the correlation between complex permittivity and infrared (IR) reflectance for solar-thermal shielding applications. IR ellipsometry measurements are conducted to clarify the presence of Lorentz resonances in plasmonic metamaterials. The Lorentz resonances are correlated to the electric field strength at interparticle gaps by varying the Sn dopant concentration, as confirmed using finite-difference time-domain (FDTD) simulations. High solar-thermal shielding performance was obtained owing to selective near-IR reflection based on strong Lorentz resonances as the ITO NP films were electrically polarizable but magnetically inactive. Thermal shielding efficiency was demonstrated via a comparison of the air temperature change in a simulated box used as a model house. Additionally, we demonstrate the significance of NP packing density on the enhancement of the near-IR reflectance. The role of interparticle spacing for high near-IR reflectance was revealed by comparing effective medium approximation analyses and FDTD simulations. This relationship was also demonstrated by the reduction of solar-thermal shielding performance when using aggregated ITO NPs. Our work confirmed that the control of complex permittivity in plasmonic metamaterials must be considered in the structural design of transparent and reflective materials for solar-thermal shielding applications.

Hierarchically Doped Plasmonic Nanocrystal Metamaterials

“…Tin-doped indium oxide (ITO) nanocrystals are promising building blocks owing to their compositiontunable LSPR frequency, 35−37 and ITO nanocrystal films have been observed to reflect infrared light due to plasmonic coupling, contrasting with the nearly pure absorption of light by isolated ITO nanocrystals. 20 The Re(ε eff ) value of ITO nanocrystal films crosses zero for sufficiently high Sn%, 20 suggesting opportunities to tune effective optical properties, including ENZ characteristics, by hierarchical doping in wellorganized ITO nanocrystal superlattices. We fabricated 2-D superlattices of ITO nanocrystals where variable atomic-scale doping (Sn%) was used to tune the optical properties of each component, and superlattice doping (i.e., varying the mixing ratio of different component nanocrystals) controlled the effective infrared optical response of the metasurface.…”

“…Optical metamaterials are composed of strongly subwavelength substructures, or “meta-atoms,” so as to realize and tune effective properties that may be less accessible or even unachievable in homogeneous materials. − Often, component materials are selected that complement or offset each other’s characteristics. For example, in multilayer metal–dielectric stacks, a real part of the effective permittivity, Re­(ε eff ), of zero can be achieved at a frequency controlled by adjusting the ratio of layer thicknesses so that the negative Re­(ε) of the metal is balanced by the positive Re­(ε) of the dielectric. , The frequency range where Re­(ε) is near zero (ENZ region) is of interest because it can give rise to strong absorption, high near-field enhancement (NFE), strong nonlinear optical response, and properties foundational for transformation optics. − More generally, two-dimensional (2-D) arrays of optical elements give rise to metasurfaces with a broad variety of effective linear and nonlinear optical properties enabling emergent applications in focusing, steering, or converting the frequency of incident light. − To make metasurfaces with components sized well below the wavelength of light and develop scalable fabrication strategies, bottom-up assembly of 2-D superlattices from colloidal nanoparticle building blocks is an appealing alternative to lithographic patterning. − A rich diversity of tunable, emergent optical properties can be envisioned when nanoparticles are assembled in dense superlattices that enable strong coupling and when the characteristics of the component building blocks, as well as their mixing ratio, can be continuously varied.…”

“…Mixed superlattices of Au and Au-seeded Ag nanocubes were fabricated that showed nonmonotonic tuning of the collective plasmon resonance extinction with mixing ratio, but these offer no straightforward tunability of optical properties of the constituent nanoparticles. Tin-doped indium oxide (ITO) nanocrystals are promising building blocks owing to their composition-tunable LSPR frequency, − and ITO nanocrystal films have been observed to reflect infrared light due to plasmonic coupling, contrasting with the nearly pure absorption of light by isolated ITO nanocrystals . The Re­(ε eff ) value of ITO nanocrystal films crosses zero for sufficiently high Sn%, suggesting opportunities to tune effective optical properties, including ENZ characteristics, by hierarchical doping in well-organized ITO nanocrystal superlattices.…”

MoS2 Nanoplatelets on Hybrid Core-Shell (HyCoS) AuPd NPs for Hybrid SERS Platform for Detection of R6G

“…Finally, the HyCoS AuPd NPs demonstrated a significantly increased MLEI value of 22.3 in Figure 3 (f-1), which was 3.7 times higher than the pure Pd NP. The large MLEI enhancement can be due to the strongly localized e-field in the nanogaps via EM coupling between the BG Au NPs [ 38 ]. Moreover, the core-shell configuration demonstrated increased LSPR as the maximum intensity of e-field distribution was found at the edge of AuPd core-shell NPs as seen from the side-view in Figure S16 (e-1).…”

Preparation of Flexible Wavelength-Selective Metasurface for Infrared Radiation Regulation

“…Conventional infrared camouflage techniques are based on low-emissivity coating materials, which obtain low detectability via their low emittance in the whole band (3–14 μm). − Such materials hold a series of advantages, such as low consumption, straightforward manufacturability, and prominent fitness, making them particularly promising in conventional camouflage techniques. − However, radiative heat transfer will be blocked in traditional infrared camouflage with broad-band low emittance, introducing heat instability. , To mitigate the severe heat instability and reduce surface temperature, high emissivity is required in the nonatmospheric window 5–8 μm for radiative cooling, , and many wavelength-selective metasurfaces that could achieve thermal regulation by transferring the radiation in the atmospheric windows to 5–8 μm band have been proposed. − For instance, Liu et al designed and fabricated a broadband absorber by employing an Fabry–Perot resonator on top of a narrowband metal-disk-array absorber. The metasurface could realize high emissivity in the 5–8 μm range as well as low emissivity in the 3–5 and 8–14 μm ranges, which satisfied both infrared suppression and radiative cooling.…”