Angle-dependent optical studies on size-reduced monolayer of self-assembled polystyrene spheres

Abstract

Engineering and controlling colloidal system size, shape, and ordering are crucial for tailoring their optical properties. They are desirable in many applications that rely on a template-based approach. This study focuses on a direct method for self-assembling polystyrene (PS) spheres monolayer into a hexagonal lattice at the air–water interface and at the air–silicon interface in the close-packed (CP) state. The self-assembled polystyrene spheres exhibit vivid structural coloration, primarily influenced by colloidal size and crystalline order. The monolayer of the PS spheres at the air–silicon interface was subjected to reactive ion etching for different durations to achieve size reduction, and the etch rate was determined to be −0.37 nm/s. The transformation of the spheres from the CP to a non-close-packed (NCP) state is, thus, achieved while maintaining the intrinsic lattice spacing. This was confirmed through field emission scanning electron microscope (FESEM) studies. Furthermore, using the FESEM images, a pair correlation function is computed, which shows evidence for a decrease in the degree of ordering of the hexagonal lattice. The optical reflectance studies were carried out on CP and size-reduced NCP states of polystyrene spheres on a silicon substrate. For normal incidence spectra, a blue shift in peak positions was observed with a reduction in sphere size, which was interpreted using an effective medium approximation. These findings establish a direct correlation between the size reduction and the optical response, providing insights into designing tunable photonic materials for various optical applications.