Sub-10-nm Plasmonic Nanogaps for Raman and Photoluminescence Modulation

Abstract

With the rapid progress of nanophotonics and nanofabrication technologies, surface plasmons have demonstrated unique capabilities in enhancing light–matter interactions, enabling energy confinement and spectral control at subwavelength scales. Among the various plasmonic phenomena, surface-enhanced Raman scattering (SERS) stands out as a prototypical example, capable of achieving high sensitivity and molecular specificity down to the single-molecule level. Owing to these features, SERS has found extensive applications in diverse areas including bioanalysis, environmental monitoring, molecular analysis, and spectroscopic studies of two-dimensional (2D) materials. The performance of SERS substrates, however, is critically determined by the intensity and spatial distribution of localized electromagnetic fields, particularly the strong coupling that occurs within metallic nanogaps. Both theoretical and experimental studies have confirmed that when the nanogap width is reduced below 10 nm, the electromagnetic field can be tightly confined, leading to orders-of-magnitude enhancement of the Raman signal. Nevertheless, the precise and reproducible fabrication of such sub-10-nm gaps remains a formidable challenge due to intrinsic limitations in the resolution of conventional lithography, resist morphology stability, and proximity effect control. This technical bottleneck not only restricts the large-scale realization of high-performance SERS substrates but also hinders the broader implementation of plasmonic structures for multispectral optical modulation and on- chip photonic integration. To address these challenges, this dissertation systematically investigates the feasibility and underlying mechanisms of a top-down three-step nanofabrication approach—comprising metal deposition, electron-beam lithography (EBL) patterning, and argon ion milling—for constructing metallic nanogap structures and achieving enhanced optical responses. The research is divided into two parts. In the first part, we developed a reproducible fabrication process for square grid nanogap arrays with tunable gap widths ranging from 7 nm to 60 nm and array periods of 150 nm, 200 nm, and 300 nm. The fabricated structures exhibited high uniformity and reproducibility across large areas. Combined experimental measurements and finite-element simulations revealed that the electromagnetic field enhancement and SERS intensity increase exponentially with decreasing gap width, yielding an overall Raman enhancement factor of approximately 10³ under 633 nm excitation. Furthermore, systematic comparison of different array periods revealed a trade-off between hotspot density and proximity effects: while shorter periods increase the number of gaps per unit area, excessive proximity-induced pattern distortion can reduce local field intensity. In addition, few-layer molybdenum disulfide (MoS₂) was transferred onto the nanogap arrays to explore the interplay between plasmonic enhancement and mechanical strain. Raman spectra showed progressive redshifts of characteristic peaks with increasing gap width, indicating that the suspended MoS₂ regions experience tunable tensile strain induced by the underlying nanogap geometry. These results demonstrate that geometric engineering of nanogaps not only optimizes SERS performance but also provides a route to strain control in 2D materials. In the second part, we propose and experimentally verify vertically stacked metal– insulator–metal–insulator–metal (MIMIM) nanogap metasurfaces that support dual Fabry–Pérot (FP) gap-plasmon resonances, enabling tunable multispectral photoluminescence (PL) enhancement. The structures were fabricated by alternately depositing Au and Al₂O₃ layers, defining nanoscale line masks via EBL, and performing directional ion milling to form trapezoidal stacked arrays. Finite-element simulations and reflection spectroscopy confirmed that the upper and lower nanogaps exhibit independent FP resonances at distinct spectral ranges—the visible and near-infrared, respectively—due to their different lateral lengths. PL measurements using Rhodamine 6G (R6G) and IR-820 dyes further demonstrated that each resonance selectively modulates emission within its respective wavelength region. The R6G emission was enhanced and spectrally tuned near the visible FP resonance, whereas the IR-820 emission was modified by the lower-gap resonance in the near-infrared. By systematically varying the EBL exposure dose, we precisely tuned the lateral dimensions of the stacked nanogaps and observed predictable redshifts of both reflection dips and PL peaks, in excellent agreement with simulation results. These findings establish stacked nanogaps as an effective platform for multiwavelength emission control within a single nanostructure. Overall, this dissertation constructs a unified framework that bridges the design principles of planar nanogap arrays for SERS optimization and vertically stacked nanogaps for multimodal PL modulation. Through rigorous integration of deterministic nanofabrication, optical characterization, and electromagnetic modeling, the research elucidates the quantitative relationships among gap geometry, localized field enhancement, and spectral response. The outcomes not only provide reproducible fabrication strategies for sub-10-nm plasmonic cavities but also expand their functional versatility from Raman enhancement and strain engineering in 2D materials to multispectral light-emitting and sensing devices. The results presented herein offer new design concepts and practical methodologies for high-sensitivity molecular detection, tunable photonic interfaces, and multi-band plasmonic integration at the nanoscale. Keywords: Surface plasmon, Nanogap array, SERS; Electron-beam lithography, Strain engineering, Photoluminescence, MIMIM nanogap metasurface.