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Kriti Agarwal1 Chunhui Dai1 Jeong-Hyun Cho1

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

Graphene plasmonics has attracted widespread attention in recent years due to their rapid frequency tunability, long plasmon lifetime, and strong light confinement. However, the two-dimensional (2D) nature of graphene imposes stringent requirements on the underlying substrate and limits the plasmon enhancement modes that can be excited. One of the solutions to overcome the limited performance of 2D graphene plasmons is through the fabrication of self-assembled graphene nanotubes. We have realized the 3D graphene nanotubes using the plasma in a reactive ion etching (RIE) system to trigger a surface tension based self-folding caused by grain coalescence in a tin sacrificial hinge. The parameters in the RIE process have also been tuned to control the gap size within nanotubes, and also achieve extremely high aspect ratio (length/radius ~ 10,000) tubes. The radial coupling of plasmons at the opening of the tube creates a virtual uniform cross-section area of extremely strong field enhancement as compared to the rapidly decaying fields in 2D graphene. If small longitudinal gaps are introduced in the nanotubes, propagating surface and edge modes are achieved with intensity based on the gap length between the edges. The propagating modes in the high aspect ratio nanotubes can be utilized for plasmonic waveguides with extremely high propagation lengths. Furthermore, the self-assembled multi-layered (Al2O3/Graphene/Al2O3) nanotubes can be fabricated with an array of horizontally short discontinuous graphene segments as long as the outer layers are maintained as continuous. The smaller aspect ratio graphene segments undergo not only radial plasmon coupling but also a longitudinal coupling between the discontinuous segment openings, creating strong uniform field throughout the volume of the multi-layered tube. The lower contact area of the 3D graphene tubes with the underlying substrate leads to a lower capacitive coupling that causes significant ohmic losses through non-radiative plasmon relaxation due to coupling between plasmons and substrate carriers in metallic and semiconducting substrates. The lower plasmon damping leads to a 13 times stronger field that extends throughout the completely closed graphene tubes as opposed to corner confined weaker field in 2D ribbons on a silicon substrate. The stronger fields throughout the tube structure exhibit a fourfold enhancement in sensitivity and overcome the need to bind targeted molecules to the surface of graphene as exhibited by the example of A/G protein. The 3D graphene nanotubes achieve distinct edge and volumetric enhancement modes while simultaneously increasing the compatibility of graphene with diverse substrates without deterioration in plasmonic properties for application in novel optical and electronic devices.

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