Title: Spectrally Selective Designs for Optical and Thermal Management
Description: Spectrally selective designs (SSDs), which selectively reflect, transmit, absorb or radiate light depending on the wavelength, impact our lives in many ways. For instance, precisely designed metasurfaces on silicon offer unprecedented control of light in the visible and infrared wavelengths. A less sophisticated example, white paints, simultaneously reflect sunlight and radiate heat to passively cool buildings. SSDs like these are meaningful scientific pursuits as well as socially impactful in their applications. However, the latter is not always the case, as prioritization of novelty and performance in research have often led to SSDs whose sophistication and cost restricts their use. Furthermore, given increasing concerns about cost, eco-friendliness and applicability in the developing world, designs that overcome such issues are becoming increasingly sought-after. The works presented here aim to address this gap between high performance and applicability by combining scientific principles with the use of common materials and simple techniques to create SSDs for optical and energy applications.

The work is categorized under three chapters. The first of these involve solution-derived nanostructured metal surfaces as a plasmonic platform for solar, thermal and optical applications. The second is concerned with porous polymers for passive daytime radiative cooling. The third and last chapter involves porous polymer coatings for switchable optical and thermal management. Prior to these sections, a general introduction to the fundamentals related to the topics – e.g. solar and thermal radiation, plasmon resonances in nanoparticles and electromagnetic scattering of light – are presented. The works in the three aforementioned sections are briefly summarized below. For the work on plasmonic nanostructured metal surfaces, a galvanic-displacement-reaction-based, room-temperature “dip-and-dry” technique is demonstrated for fabricating plasmonic-nanoparticle-coated foils (PNFs). The technique involves simply dipping a metal (M1) foil onto an aqueous salt of a less reactive metal (M2), and allowing the spontaneously resulting chemical reaction to form plasmonic nano or microparticles of M2 to form on M1.

By controlling reaction parameters such as time, temperature and salt concentration, the reflectance spectrum of the PNFs can be tuned across the solar to far infrared wavelengths (0.35 – 20 μm). Consequently, the technique can tune the PNFs solar absorptance (~0.35 to 0.98) and thermal emittance (~0.05 to ~0.95). This is promising for applications such as selective solar absorption, selective thermal infrared emission, super-broadband thermal absorbers and emitters, and radiative cooling. The potential for selective solar absorption is investigated in detail, with the technique tuned to yield copper nanoparticle-coated Zinc substrate with excellent, wide-angle solar absorptance (0.96 at 15°, to 0.97 at 35°, to 0.79 at 80°), and low hemispherical thermal emittance (< 0.10). Issues important for applications, such as mechanical and thermal stability of the PNFs, are also investigated. The work on porous polymers for radiative cooling investigates the effect of porosity on the optical properties of polymers.

Typically, polymers are intrinsically non-absorptive in the solar (0.35-2.5 μm), and emissive in one or more bands within the thermal infrared (2.5-20 μm) wavelengths. When made porous, the voids within the polymer can lead to different optical behaviors depending on their size. For instance, air voids with sizes (~1 μm) similar to solar wavelengths scatter sunlight due to the refractive index contrast between the polymer and air, leading to a high solar reflectance. Nanoscale (~0.1 μm) air voids, which are much smaller than longer thermal wavelengths (> 2.5 μm), lower the effective refractive index of the polymer in those wavelengths and increase thermal emittance. Porous polymer coatings (PPCs) with such air voids and optical properties can be made by scalable, solution-based and paint-like processes such as phase inversion. For example, phase-inverted poly(vinylidene fluoride-co-hexafluoropropene) (P(VdF-HFP)) exhibit an exceptional solar reflectance of up to 0.99 and hemispherical long-wave infrared emittance ~ 0.97.

This allows the P(VdF-HFP) PPCs to achieve a net heat loss and reach sub-ambient temperatures of 6˚C even at noon. This passive radiative cooling performance, which surpasses those of notable designs in the literature, is obtained with a paint like convenience – making it promising as a sustainable cooling solution for buildings. The work on switchable optical and thermal management is related to the work above, and shows that optical performance of PPCs can also be altered by replacing the air in the pores with commonly available liquids. For instance, wetting PPCs with a liquid having the same solar refractive index as the polymer reduces optical scattering and turns the PPCs from white to transparent. Thermally transparent PPCs, meanwhile, turn absorptive or emissive when wetted with infrared-absorptive liquids. Both of these transitions can be reversed by drying – yielding a scalable and low-cost optical switching paradigm for solar and thermal wavelengths. The switchable optical transmittance can be useful in a wide variety of applications, such as controlling daylight in buildings, tunable solar heating and radiative cooling, water responsive systems and thermal camouflage.

The works presented above attempt to achieve a desirable balance between scientific novelty, performance, simplicity and cost, with the intention of bringing high-performing optical designs to low-resource settings in the developing world. While this dissertation is a small step towards that goal, the author hopes that the readers will find the content to be of value.