Unidirectionally Aligned Single Bottom Faceted Nanoparticle-Based Plasmonic Nanogap Structures—Design Idea, Fabrication Methods and Applications to Surface-Enhanced Raman Scattering Platform

The significant advantage of utilizing plasmonic nanostructures is strong light–matter interaction on the subwavelength scale. The plausibility of achieving such extreme nano-optical properties can be realized with the help of a nanoparticle-on-mirror (NPOM) system [1]. NPOMs provide highly efficient platforms in the field of nano-plasmonic sensors or energy devices, and they have emerged as strong candidate building blocks. NPOMs were optimized with gaps between nanoparticle and mirror less than 1 nm, which can yield extremely high plasmonic properties. To achieve strong plasmonic properties, the size of the active material to be coupled with the NPOMs platforms is limited to less than 1 nm. As with the simple electromagnetism example problem, the gap size and plasmonic enhancement of NPOMs are inversely proportional to each other. Due to this limitation, the use of large active materials (10 nm or larger, ex: biomolecule, quantum dots) in NPOMs resulted in low plasmonic enhancement.<br/>Herein, we propose an unidirectionally aligned single bottom faceted NPOMs (FNPOM) nanostructure with enhanced plasmonic enhancement, even in the presence of a large gap size exceeding 20 nm. The design started with the simple idea that the relation of the distance between two parallel plane charges and the electric field strength are more gently decrease compared to point charges. Nanoparticle fabrication was performed with vacuum furnace system. Single bottom faceted sphere shape nanoparticles are fabricated under conditions where the surface tension between the substrate and the molten metal is properly balanced. The diameter of the gold nanoparticles formed is about 100 nm, and the plasmonic resonance wavelength is in visible light. FNPOM was successfully fabricated on a 4-inch substrate, and the statistical uniformity based on optical properties is within 10%. The successful demonstration of FNPOM is meaningful in confirming the critical correlation between nanoparticle micromorphology and optical properties.<br/><br/>[1] R. Chikkaraddy, B. de Nijs, F. Benz, S. J. Barrow, O. A. Scherman, E. Rosta, A. Demetriadou, P. Fox, O. Hess and J. J. Baumberg, Nature, 2016, 535, 127–130