Robust Nanogap Cluster Formation via Plasmonic Moiré Superlattices for Surface-Enhanced Raman Spectroscopy

Author: Hwang, Chiyoung

Year: 2026

Degree: Dissertation (Ph.D.)

Advisor: Scherer, Axel

Committee Members: Tai, Yu-Chong; Gao, Wei; DeRose, Guy A.; Scherer, Axel

Option: Medical Engineering

Abstract

Plasmonic nanogaps enable strong confinement of electromagnetic fields and are central to a wide range of applications, including molecular sensing, spectroscopy, and quantum photonics. Achieving nanogaps with dimensions below 10 nm is particularly important for maximizing field enhancement, yet their reproducible fabrication remains difficult due to the sensitivity of conventional nanofabrication processes to small variations.

This thesis introduces a design strategy based on moiré superlattice geometry to generate nanogap clusters in a robust manner. By stacking two periodic nanodisk arrays with a small twist angle, a large-scale superlattice is formed, within which numerous ultranarrow gaps naturally arise from geometric effects. Numerical analysis shows that these gaps can be considerably smaller than the original structural features and can be formed consistently even in the presence of fabrication imperfections.

The proposed concept is validated experimentally through the fabrication of stacked nanodisk arrays using a sequential lithographic process. Voltage-contrast scanning electron microscopy is employed to verify the presence of electrically isolated nanogaps at the sub-10 nm level, providing clear evidence beyond conventional imaging methods. These results indicate that ultranarrow nanogaps can be realized without requiring extreme control over lithographic parameters.

Building upon this concept, elevated plasmonic moiré superlattices are further developed to enhance accessibility to field-enhancing regions, thereby improving sensing performance. The practical relevance of the proposed structures is demonstrated through surface-enhanced Raman spectroscopy measurements, which show consistently high signal enhancement across different fabrication conditions.

Taken together, this work presents a scalable and geometry-driven approach for the reliable creation of nanogap-rich plasmonic systems. The framework provides a versatile platform for exploring nanoscale light–matter interactions and enables future developments in nanophotonic and sensing technologies.