Investigation of the Optical Properties of hBN Nanoparticles for High Solar Reflection

The development of materials with efficient radiative cooling properties is increasingly important as the technology emerges as a passive cooling technique that can help to combat the effects of global warming. In particular, high reflectance of solar irradiation is desirable. Because of this, the numerical calculation of the optical properties of nanomaterials has been crucial the past years for optimizing radiative cooling performance. Past studies have focused on the effect of the particle morphology on the optical properties, but none of them has previously studied a platelet morphology. Mie theory, an analytical solution of optical properties, is mainly used for spherical particles and cylindrical particles with infinitely long thickness. This indicates a need for a better approach for a nanoplatelet morphology. In this work, we focus on calculating the optical properties of hBN nanoplatelets by using Finite Element Method (FEM) and COMSOL Multiphysics. The scattering coefficient and the phase function of hBN nanoplatelets are compared with those of BaSO₄, a spherical nanoparticle that has shown high total solar reflectance due to its high refractive index of 1.66 and high bandgaps of 7.27 eV. hBN has been identified due to having a higher refractive index of 2.1-2.3 when optimally oriented due to its lower bandgap of 5.96 eV. The higher refractive index should contribute to greater total solar reflection. Furthermore, it is predicted that the high aspect ratio of the nanoplatelets will affect the light scattering compared to a spherical nanoparticle shape. The effect of both the refractive index and the morphology, i.e. platelets vs spheres, is taken into account and our first hypothesis that hBN has better optical performance than BaSO₄ is valid, mainly due to the higher permittivity of hBN. The nanoplatelet morphology has shown higher scattering coefficient than spherical nanoparticles at the ultraviolet and visible wavelengths, in contrast at the near-infrared wavelengths. Moreover, the total reflectance of all the samples is calculated by using Monte Carlo simulations to determine the morphology with the greatest reflectance when refractive index controlled for. Finally, the relation of the optical properties with the orientation and the aspect ratio of the hBN nanoplatelet is studied in order to observe the effect of its anisotropic refractive index. This work improves understanding of the individual contributions of morphology and refractive index to the optical properties needed for optimized radiative cooling.