Tailoring Light-Particle Interactions: Significance of Nanostructure and Morphology in Radiative Cooling Nanocomposites

<p dir="ltr">Rising global heat driven by climate change has made efficient cooling a critical priority. Conventional air conditioning systems, while effective, contribute significantly to electricity consumption and greenhouse gas emissions. Radiative cooling has emerged as a promising passive alternative, capable of cooling surfaces and buildings by emitting thermal radiation directly into space without consuming electricity. To achieve strong radiative cooling under direct sunlight, materials must simultaneously exhibit high solar reflectance and strong sky window emissivity. %a balance that requires careful engineering of optical properties and nanostructure.Despite rapid advances in radiative cooling technologies, many existing solutions rely on complex multilayered photonic structures that are difficult to fabricate and scale for real-world applications. Recently, composite paints made of dielectric nanoparticles embedded in a polymeric matrix have gained attention due to their manufacturability and effectiveness in lowering surface temperatures. Yet, examining natural systems can reveal additional insights for passive cooling design. For example, the desert snail Sphincterochila boissieri uses a layered calcium carbonate shell to reflect sunlight and emit heat. Inspired by such biological systems, this thesis explores whether natural morphologies can guide the design of more efficient radiative cooling materials.To address this, we investigate the following research questions: (1) How do natural nanolayer structures compare with engineered nanocomposites in radiative cooling efficiency? (2) What lessons can we draw from nature to improve paint-based radiative coolers? (3) How does particle morphology, particularly platelet-shaped, affect spectral reflectance? and (4) What is the role of platelet orientation and size distribution in determining cooling performance?To answer these questions, this thesis develops and applies a multiscale optical modeling framework that captures the performance of radiative cooling materials. The approach is organized across three physical scales. At the atomic scale, we obtain the complex refractive index spectra of materials either from experimental data or, when unavailable, from first-principles electronic structure calculations. At the nanoscale, we calculate how individual particles interact with light, including scattering, absorption, and directional (angular) behavior. For spherical particles, we apply classical Mie theory, while for anisotropic platelets such as hBN, we use finite-element simulations to capture geometry- and orientation-dependent effects. At the microscale, we treat bulk nanocomposites with an effective‐medium approximation coupled to a Monte Carlo radiative-transfer solver, while multilayer structures are modeled separately with the Transfer Matrix Method (TMM) to predict solar reflectance and mid-infrared sky-window emissivity. This three-scale modeling pipeline enables rigorous comparison of diverse architectures, including spherical CaCO₃ particles, bioinspired multilayers mimicking snail shells, and high-performance hBN nanoplatelet-based paints. Our findings reveal that nanolayered structures, inspired by the morphology of desert snail shells, outperform spherical-particle nanocomposites in radiative cooling performance under comparable conditions. Simulations based on real snail shell geometry confirm that nature's design operates near optimal solar reflectance, offering valuable bioinspired insights. Additionally, we designed and experimentally validated an hBN-acrylic nanoporous paint, which achieves a high solar reflectance of 97.9% and sky window emissivity of 0.83 at only 150 microns of thickness, demonstrating substantial sub-ambient cooling with minimal weight. Our subsequent optimization of hBN platelet morphology revealed the importance of particle size, angular orientation, and backscattering characteristics, identifying key parameters for maximizing performance. Together, these results provide a comprehensive framework for designing next-generation radiative cooling materials, bridging nature-inspired structures with manufacturable, high-efficiency coatings and pointing the way toward exploring even bolder morphologies that could push performance further. The outcomes of this work hold promise for energy savings, carbon footprint reduction, and deployment across applications from buildings and vehicles to aerospace and wearable technologies.</p>