Effect of the Dielectric Properties of Substrates on the Scattering Patterns of Gold Nanorods

Understanding the electromagnetic interactions between plasmonic noble metal nanocrystals and different substrates is very important because a number of plasmon-based devices and applications, such as chemical and biological sensing, metamaterials, subwavelength waveguides, plasmon-enhanced photovoltaic cells, plasmon-enhanced spectroscopy, and photo-switches, require the attachment of metal nanocrystals onto various substrates. In this study, we focused on the effect of the dielectric properties of substrates on the far-field scattering patterns of supported Au nanorods. Seven types of substrates, including metals, semiconductors, and insulators, were examined. The far-field scattering patterns from individual Au nanorods were found to exhibit a doughnut shape when the modulus of the dielectric constant of the substrate is above similar to 7, while they appear as solid bright spots when the modulus is below similar to 7. This finding was validated by numerical electrodynamic calculations. Moreover, the doughnut-shaped scattering patterns are very sensitive to the spacing between the nanorod and substrate. For the nanorods supported on silicon wafers, when the spacing is increased above similar to 14 nm, the scattering patterns change from the doughnut shape to the solid bright spot. These results will be useful for the understanding of the plasmonic properties of noble metal nanocrystals supported on substrates and the development of a number of plasmon-based optical and optoelectronic devices at different size scales.