Exploiting eutectic formation and phase separation to produce plasmonic metamaterials

Novel Au mesostructures with a polyhedron shape, henceforth referred to as pyramids, are produced by annealing in vacuo an Au thin film on a Si(100) substrate. Gold diffusion and incorporation into pyramids is a function of the thickness of the Au film, the annealing temperature, and the vacuum pressure. The Au pyramids have unique surface morphologies due to the presence of channels and plateaus, where channels are cut into the surface of the pyramids and plateaus are the surfaces between adjacent channels. The bulk of the pyramids consists of Au with cavities that are devoid of Au or Si. Normalized energy dispersive spectroscopy of intact regions of the surface are 98.1 wt. % Au and 1.9 wt. % Si, while the bottom of the channels are 85.7 wt. % Au and 14.3 wt. % Si. Therefore, one step in the growth process is the formation of an Au–Si eutectic. The low concentration of Si in the solid regions of the pyramid and its high concentration in the walls of the cavities are indicative of phase separation of the Au–Si eutectic. The pyramids are oriented in the same direction relative to one another and are a consequence of eutectic formation-induced etching of the Si(100) surface. The scattering spectrum (non-specular reflectivity) of the Au pyramids consists of two very strong surface plasmon polariton states that correspond to excitation from the Au d-bands to the sp conduction bands. The surface morphology produces linearly polarized reflected light.