Investigation Of The Graphene Induced Shift In Surface Plasmon Resonance On Gold Film
Alam, Md Kamrul
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With the emergence of graphene and other two-dimensional (2D) nanomaterials, it was proposed that capping Au film with graphene could improve the performance of sensors based on its surface plasmon resonance (SPR) due to an increased local field and surface chemical adsorption. Such an atomically thin film enhanced SPR sensitivity has been further modeled and tested by many groups, but the SPR angle shift induced by 2D materials and its physics has been overlooked. In this work, we systematically investigated the graphene induced SPR shifts by comparing experimental results with that of models. Experimentally, a large SPR shift of ~0.30° is observed, and it is three times larger than that from conventional modeling. Graphene surface roughness and cleanliness can contribute to the shift in SPR resonance angle. Therefore, we develop a polymer free graphene transfer technique to have a contamination free surface. After discussing the deficiencies of this oversimplified model, which treats both graphene and Au films as continuous and smooth isotropic media, we show steps to build more realistic models and evaluate the effect on SPR from surface morphologies of Au and graphene films. Three models: Finite Difference Time Domain (FDTD), Effective Medium Theory (EMT), and Averaged Index Method (AIM) are used to estimate the shift based on the actual surface morphology obtained with atomic force microscopy (AFM) images. The effect of the anisotropic dielectric constant of graphene and the charge transfer between graphene and Au films are also investigated. This study illustrates the challenges in understanding the SPR of noble metal films modified by atomic scale 2D materials and calls for more advanced and realistic modeling in broad applications involving metal film and 2D materials.