Structural and optical properties of monocrystalline and polycrystalline gold plasmonic nanorods

Plasmonic structures are often fabricated by lithographic patterning of a thin metallic film. Properties of the thin film are intimately related to the quality of the resulting structures. Here we compare two kinds of thin gold films on silicon nitride membrane: a conventional polycrystalline thin film deposited by magnetron sputtering, and monocrystalline gold microplates which were chemically synthesised directly on the membrane's surface for the first time. Both pristine metals were used to fabricate plasmonic nanorods using focused ion beam lithography. Structural and optical properties of the nanorods were characterized by analytical transmission electron microscopy. The dimensions of the nanorods in both substrates reproduced well the designed size of $240 \times 80\ \mathrm{nm^2}$ with the deviations up to 20 nm in both length and width. The shape reproducibility was considerably improved among monocrystalline nanorods fabricated from the same microplate. Interestingly, monocrystal nanorods featured inclined boundaries while the boundaries of the polycrystal nanorods were upright. Q factors and peak loss probabilities of the modes in both structures are within the experimental uncertainty identical. We conclude that the optical response of the plasmonic antennas is not deteriorated when the polycrystalline metal is used instead of the monocrystalline metal.