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Kriti Agarwal1 Chunhui Dai1 Jeong-Hyun Cho1

1, University of Minnesota, Minneapolis, Minnesota, United States

A surface-tension driven self-folding mechanism has led to the realization of diverse self-assembled 3D architectures from patterned 2D ribbons, such as graphene-based pyramids, tube, and cubes. The 3D graphene geometries exhibit distinct plasmon hybridization that cannot be excited in 2D graphene ribbons as a result of plasmon coupling over an extra spatial degree of freedom. However, a detailed systemic study of the plasmon coupling in 3D graphene geometries needs to be performed in order to efficiently utilize the hybrid plasmon modes for desired optoelectronic applications. We have investigated the plasmon coupling and found thathe coupled plasmonic field enhancement in these 3D geometries is strongly dependent on the 3D shape, number of 3D edges, and surface angle of inclination. The uniform coupling in graphene nanocubes gives rise to large circular areas of constant field enhancement. Moreover, if the edges of the nanocubes are spatially separated by nanometer distances, circular field interference patterns are obtained with alternate rings for constructive and destructive coupling. The circular surface enhancement modes in 3D graphene nanocubes can be leveraged for novel optoelectronic applications. In contrast, the square graphene pyramids undergo a strong point enhancement at the apex of the pyramid arising from the inclined tapering faces meeting at the apex. The strong point enhancement propagates throughout the nanopyramid resulting in a volumetric field that is several orders of magnitude higher than in 2D graphene ribbons. Furthermore, as the number of faces and edges are increased to form pentagonal to octagonal pyramids, the point based enhancement can be transformed to uniform surface enhancement at the base of the pyramids. Thus, allowing geometrical parameters in the nanopyramids to be utilized for designing high-sensitivity plasmonic sensors that can assess low concentration analytes in the bulk volume of targeted solution or the molecular surface binding properties at higher analyte concentrations. The graphene tube consisting of only 2 edges existing at the openings of the tube, a virtual hotspot area of extremely high near-field enhancement is created due to radial plasmon coupling at the small opening of the tube. The completely sealed tube demonstrates a strong hotspot, but, the hotspot covers only 13% of the volume of the tube. However, if the nanotubes are fabricated with small slits, the strongly coupled field exists throughout the tube structure giving rise to strong volumetric field. The volumetric field in nanotubes is especially desirable due to their open-ended geometry that can be leveraged for assessing targets flowing through the 3D tube. The self-assembled 3D graphene structures can be varied geometrically to achieve diverse point, edge, surface, and volumetric enhancement modes for achieving plasmonic devices exhibiting increased sensitivity and efficiency, and a high packaging density.

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