Abstract
The quality of lithographically prepared structures is intimately related to the properties of the metal film from which they are fabricated. Here we compare two kinds of thin gold films on a silicon nitride membrane: a conventional polycrystalline thin film deposited by magnetron sputtering and monocrystalline gold microplates that 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. The structural and optical properties of the nanorods were characterized by analytical transmission electron microscopy including electron energy loss spectroscopy. The dimensions of the nanorods in both substrates reproduced well the designed size of 240×80 nm2 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, monocrystalline nanorods featured inclined boundaries while the boundaries of the polycrystalline nanorods were upright. Q factors and peak loss probabilities of the modes in both structures are within the experimental uncertainty identical. We demonstrate that the optical response of the plasmonic nanorods is not deteriorated when the polycrystalline metal is used instead of the monocrystalline metal.
Highlights
Plasmonic antennas are metallic particles widely studied for their ability to control, enhance, and concentrate electromagnetic field [1]
Focusing of the field stems from the excitation of localized surface plasmons (LSP) – quantized oscillations of the free electron gas in the metal coupled to the evanescent electromagnetic wave propagating along the boundary of the metal
Most pronounced differences have been identified regarding the vertical interfaces of nanorods, inclined for the monocrystalline nanorods but upright for polycrystalline ones
Summary
Plasmonic antennas are metallic particles widely studied for their ability to control, enhance, and concentrate electromagnetic field [1]. Focusing of the field stems from the excitation of localized surface plasmons (LSP) – quantized oscillations of the free electron gas in the metal coupled to the evanescent electromagnetic wave propagating along the boundary of the metal Thanks to this light-concentrating ability, plasmonic antennas have found applications in energy harvesting [3], construction of metasurfaces [4,5], luminescence enhancement [6], or biodetection [7]. In combination with other approaches, like template stripping, the layer quality might be improved even further [18], and increased grain size can allow antenna fabrication from a single grain This can be achieved readily by chemical synthesis where each particle can be made as single crystal grain with great shape variability [19] which should provide the optimum optical properties [20], but their precise placement on the substrate is hard to achieve. We characterize the material properties of the input substrates as well as the resulting nanorods, correlate the nanorod shape with its plasmonic properties and visualize the supported plasmonic modes by scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS) with nanometer spatial resolution not achievable by means of other methods
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