Ordering and optical properties of monolayers and multilayers of silica spheres deposited by the Langmuir–Blodgett method

Abstract

Monodisperse spherical silica particles (357, 450 and 550 nm in diameter) prepared by the method of Stöber were used to construct two-dimensional and three-dimensional structures with photonic bandgaps with the Langmuir–Blodgett technique. Floating monolayers of silica particles on water were made by using ionic surfactants, hexadecyltrimethylammonium bromide, decyltrimethylamonium bromide, sodium dodecylsulfate and octylbenzenesulfonic acid sodium salt. These monolayers were transferred onto glass microscope slides via vertical deposition. The effect of the type, concentration, and chain length of the surfactant, and the composition of the dispersion medium (chloroform, methanol, or a mixture of both) on the quality of particle ordering was investigated. The solvent was the most important parameter and the largest hcp crystalline areas were obtained with methanol. Up to 6 layers could be deposited. The photonic bandgaps for both monolayers and multilayers are observed at the wavelength predicted by theory. The height of the gap increases and the width decreases gradually as the number of layers increases. The incidence angle dependence of the transmission minimum of these structures also coincides with that predicted by theory: the position of the bandgap shifts with the angle of incidence [from 90 to 40° with respect to the (111) crystal plane] according to Bragg's law.