II. Fabrication process Observation of Plasmon Tunneling in Vertically stacked Graphene Spacer System RPGR 2016 PT42_04_1030 Khang June Lee, Tae Keun Kim, Woonggi Hong, Sung Yoon Min, and Sung-Yool Choi School of Electrical Engineering and Graphene Research Center, KAIST ABSTRACT The plasmon can undergo the quantum tunneling when the gap is smaller than 0.5 nm. Especially, the thickness of a single layer of graphene (SLG), 0.34 nm, is so thin that the quantum tunneling can occur directly through the film even without applying external bias. Such a tunneling phenomenon leads to the weakening of the plasmonic coupling. Here, we reveal the plasmon tunneling using Surface enhanced Raman spectroscopy in the well-defined graphene spacer system. As we expect, plasmon tunneling is observed only in the single layer graphene spacer, not in multilayer graphene spacer. I. Introduction III. Results & Analysis The plasmonic coupling, the enhanced electromagnetic field occurring through a uniform and small separation between metallic particles, is required for better application to localized surface plasmon resonance (LSPR). To achieve a high electromagnetic field near the noble metals, a nanoscale separation is required because the enhanced field is strongly inversely proportional to the metallic gap. For this reason, graphene have attracted as a good spacer candidate because of its precise controllability at sub-nanoscale. Here, we experimentally and simulatively investigate the enhancement of plasmonic coupling through the vertical graphene spacer system. Graphene layer number is split by the repeated dry transfer process. Final structure makes plasmonic coupling through the graphene spacer (Figure 1). Graphene layer number control the thickness between the Au nanoparticles and the Au film. As layer number increase, enhanced field by plasmonic coupling become weaker and Raman signal also become smaller. Especially, in the case of single layer graphene spacer, the enhanced field do not follow the classical simulation which can be the evidence of plasmon tunneling. (a) (b) (c) (d) (a) (b) Au-NP SLG stack Image charge Au film Fig. 4. (a) Simulation result of Au-NP on Au film (b) Enhancement factor (EF) was strongly inverse proportional to spacer thickness (c) Raman intensities in different spacer thickness (1~4 SLG layer) (d) Comparison between experiment and simulation result SiO2 Fig. 1. Schematic illustration of graphene spacer system Cross-section view of graphene spacer system and image charge of Au-NP In Fig 4. (d), the experimental EF in single layer case is less than that of double layer spacer case, and appears to be inconsistent with simulation result (classical approach) EF become weaker when the quantum tunneling occurs Indicating plasmon tunneling is occurred in the SLG spacer case. II. Fabrication process (a) (b) (c) (a) (b) Fig. 2 (a, b) Au nanoparticles (Au-NPs) deposition on graphene, top view (a) and tilt view (b) (c) Diameter distribution of Au-NPs, average diameter is 24.1 nm Fabrication process Raman spectroscopy with different concentration of BCB Raman signals observed at 15 points randomly selected throughout the film. Au film deposition (thermal evaporator) Repeated Graphene dry transfer on Au film Au nanoparticle deposition (thermal evaporator) Annealing for spherical shape of Au-NPs Test molecule (BCB) dropping Raman signal detection Fig. 5. We can detect the BCB concentration as low as 10 nM, indicating that our structure is so sensitive. Our graphene spacer system shows good homogeneity. IV. Conclusion In conclusion, we fabricated graphene spacer system which was sandwiched with Au film and Au nanoparticles. Unlike in the case of multilayer graphene (2~4 layer), the single layer graphene shows less enhancement factor than others and inconsistent with the simulation result (classical simulation). For this reason, we finally made a decision that the plasmon tunneling is prevalent in the case of single layer spacer. Fig. 3 Schematic process of dry transfer of single layer graphene which was used in our experiment Small, 11, 2, 175 (2015) MNDL (Molecular & Nano Device Lab.), School of Engineering and Graphene Research Center, KAIST 291 Daehak-ro, Yuseong-gu, Daejeon, 305-701, Korea *E-mail : sungyool.choi@kaist.ac.kr Phone:+82-42-350-7627, 3477 Fax:+82-42-350-7283