Optical Properties of Microgeodes: Microcapsules Containing Silicon Nanowires

The tunable optical response of silicon nanowires makes them well-suited for applications such as paints and coatings. However, these applications require bulk collections of nanowires that can be suspended in formulations. Microgeodes, silica shells encapsulating disordered arrangements of silicon nanowires, are a promising way to incorporate nanowires in such applications because they can be dispersed in a fluid and assembled into a coating. However, the optical properties of these composite materials have not yet been investigated. We use a combination of experiment and theory to characterize the visible and infrared response of both individual and bulk assemblies of microgeodes. In the visible region, the response is similar to that of disordered nanowire mats: nanowire absorption and scattering control the observed reflectance, and scattering from the silica shells does not contribute strongly. In the infrared, however, we find that scattering from the silica shells appears to contribute significantly to the reflectance. Also, multiple scattering in bulk assemblies of microgeodes leads to a broadening of the reflectance peak compared to that of individual microgeodes. We explain these results in terms of the relationships among absorption length, scattering length, transport length, and sample thickness. We discuss how the hierarchical structure of microgeodes can be leveraged to engineer materials with prescribed optical properties.