Colloidal Dispersion of Subquarter Micrometer Silicon Spheres for Low-Loss Antenna in Visible Regime

This work reports the development of an agglomeration-free colloidal solution of silicon (Si) spheres exhibiting the electric and magnetic dipole Mie resonances in the visible region as an alternative to the optical nanoantenna based on plasmonic nanoparticles. Size-controlled crystalline Si spheres with 20-250 nm in diameters are grown by a bottom-up process. The Si spheres have heavily boron (B) and phosphorus (P) codoped surface layers, which induce negative surface potential and make the spheres dispersible in alcohol almost perfectly due to the electrostatic repulsions. Formation of agglomeration-free colloidal dispersion allows to deposit Si spheres on an arbitrary substrate and to produce a dielectric nanoantenna for tailored wavelengths. This study demonstrates that, thanks to the almost perfect spherical shape of the developed Si spheres, the forward and backward scattering spectra of single Si spheres can be well-explained by the Mie resonance in a wide size range. It is demonstrated that the Si spheres work as nanoantennas for fluorescence enhancement and a single Si sphere can enhance dye fluorescence at maximum 200-fold.